Random Packing Article.pdf

1.0 Introduction Fossil fuels supply more than 90% of the world’s energy needs. However, the emission gas such as carbon dioxide will cause the greenhouse effect to the environment due to the combustion process of fossil fuels. CO2 gas is produced during the reaction contributes to additional absorption and emission of thermal infrared in the atmosphere and eventually results in the global climate changes. As a result, the elimination of CO2 from combustion process is an important issue for now. There are a number of technologies introduced for the CO2 capture such as absorption, membrane separation, cryogenic and etc. Other than fossil fuels, the CO2 capture of remaining commercial available fuel such as biofuel, coal biomass and natural gas is also our main concern. The main challenge according to CO2 capture technology is to reduce the overall cost by lowering both the energy and the capital cost requirements. Therefore, compromise between cost and efficiency for an available CO2 capture technology is very important. Commercial CO2 capture technology that exists today is very expensive and large energy usage. Besides, they are a number of relatively low cost CO2 mitigation technologies included improving energy supply and end-use efficiency by switching coal or oil to gas where possible, forestation, and inexpensive renewable energy application. Definitely they are sufficient for short term goals, but they will not be the final solution for longterm. 1.1 Objective Owing to the greenhouse gas emits mainly because of mankind activity, especially combustion, which is always happen in industries. Therefore the major effect of this will cause harm to human and also affect the climate change. Hence, the purposes of gas scrubber system introduced to the industries are shown as below: - Minimize the greenhouse effect - Meet the requirement of Clean Air Act - Minimize the losses of solvent - Maximize the efficiency of plant and design integrity 1 The Realization of a Packed-bed Tower - Minimize the consumption of utilities 2.0 Packing There is a variety of different packings in shape, size and performance are available and they can be classified into three categories: • Random or dumped packings • Structured packings • Grid packings Random packings are just dumped into the shell to give the packing pieces a random orientation. Structure packings are stacked in the shell to take the shape of a packed bed. Characteristics of tower packings Besides low cost, the desirable characteristics of packings are described below (Kister, 1992). a) A large surface area: Interfacial area of contact between the gas and the liquid is created in a packed bed by spreading of the liquid on the surface of the packing. Smaller packings offer a larger area per unit packed volume, but the pressure drop per unit bed height becomes more. b) Uniform flow of the gas and the liquid: The packed bed must have a uniform voidage so that a uniform flow of the gas and of the liquid occurs. The shape of the packing should be such that no stagnant pocket of liquid is created in the bed. A stagnant liquid pool is not effective for mass transfer. c) Void volume: A packed bed should have a high fractional voidage so as to keep the pressure drop low. d) Mechanical strength: The packing material should have sufficient mechanical strength so that it does not break or deform during filling or during operation under the weight of the bed. 2 The Realization of a Packed-bed Tower e) Fouling resistance: Fouling or deposition of solid or sediment within the bed is detrimental to good tower operation. Bigger packings are less susceptible to fouling. Also, the packings should not trap fine solid particles that may be present in the liquid. Types of tower packings Tower packings are made of ceramics, metals or plastics. Kister (1992) and Larson (1997) identified three generations of the evolutionary process of the random packings. Random Structured Grid First generation Second generation Third generation Raschig ring, Lessing ring, Cross-partition ring, Berl saddle, Spiral ring. Pall ring (plastic and metal) and its modified versions (like Flexiring, Hy-pac, etc), Intalox saddle and its modifications, etc. IMPT (Norton), CMP (Norton, Glitsch), Nutter ring, Jaeger Tripac, Koch Flexisaddle, Fleximax, Norpac, Hiflow,etc a) First generation random packings (1907 to mid-1950s): These included three types of packings— Raschig rings, Lessing rings and other modifications of the Raschig ring and Berl saddles. These are mostly packed randomly; ‘stacked’ packings are used in only a few cases. i. Raschig ring: This is the oldest type of tower packing introduced by the German chemist F. Raschig in 1907. It is a hollow cylinder having a length equal to its outer diameter. The size of the Raschig ring ranges from ¼ inch to 4 inches. These rings are made of ceramic materials (unglazed porcelain), metals or plastics (e.g. high-density polyethylene, HDPE). 3 The Realization of a Packed-bed Tower Metal or plastic rings are made by cutting tubes of a suitable size. The Raschig ring is probably the most rugged packing and can be used even when a severe bumping or vibrating condition may occur. Other members of the Raschig ring family are: (1) ‘Lessing ring’, which is similar to the Raschig ring except that it has a partition along the axis of the ring. The partition increases the surface area but the advantage is rather small in practice. This packing has not been quite popular. (2) The ‘cross-partition ring’ that has two partitions instead of one in a Lessing ring. (3) The ceramic ‘spiral ring’ that has an internal helix which creates internal whirl of the gas and of the liquid and enhances the rate of mass transfer. The latter two types are sometimes stacked in one or two layers on the support grid of a randomly packed tower. Although Raschig rings are still in use, the other variations of them are rarely used. ii. Berl saddle: The berl saddle is the first modern packing developed in the late 1930s. It is so called because it has the shape of a saddle. A packed bed of Berl saddles has a larger specific surface area (i.e. surface area per unit packed volume) and a smaller voidage than the Raschig ring. Compared to the Raschig ring, the pressure drop is substanitially less because of its ‘aerodynamic shape’. It has a rib on one surface that prevents possible overlapping of the surfaces of two adjacent pieces. Berl saddles offer higher capacity and a better performance than Raschig rings but are more expensive. Raschig ring Cross-partition ring 4 The Realization of a Packed-bed Tower The area of the packing is almost fully utilized for effective contact and mass transfer between the gas and the liquid phases. Intalox saddles are better packing than the Berl saddles. It is made by cutting windows on the wall of a metal Raschig ring and bending the window tongues inwards. higher capacity and efficiency. The smooth edges of the Intalox saddle are scalloped and holes inserted to make the super Intalox. Pall rings: The pall ring and its modifications evolved from Raschig ring. Koch-Glitsch offers a similar ceramic packing under the trade name ‘Flexisaddle’. in 1987. Similar to the Berl saddle. The Intalox saddle and its modified varieties are of ceramic or plastic make.Lessing ring Berl saddle Figure 1: Various types of first generation random packing ring b) Second generation random packings (mid 1950s to mid-1970s): The ‘Intalox saddle’ may be considered to be the first member of the second generation random packing developed by the Norton Chemical Products Corporation in the early 1950s. This design promotes quick drainage of the liquid. it offers a larger specific interfacial area and a smaller pressure drop compared to the Raschig ring. It is an improved version of the Berl saddle and offers lesse ‘form friction’ resistance to gas flow. While a bed of saddles offers reduced ‘form friction’ or drag because of the 5 The Realization of a Packed-bed Tower . Because of its particular shape two adjacent pieces of the packing do not ‘nest’ and hence a stagnant pool of liquid is not created between them. ‘Intalox snowflakes’. is a plastic packing of unique shape having a large number of liquid drip points. introduced by the Norton Corp. eliminates stagnant pockets and provides more open area. However. causing continuous renewal of the liquid surface and superior mass transfer performance. aerodynamic shape. Ceramic Pall rings. The metal ‘Hy-Pak Tower Packing’ of the Norton Corp. have not been very popular. pall rings do so by allowing ‘through flow’ of the gas. Similar packings are marketed by other companies under different trade names. Since the interior surface is much more accessible to gas and liquid flow. because direct passages on the wall are available. which are Raschig rings with a few windows on the wall. the capacity and efficiency of the bed are enhanced. a slightly modified version of the Pall ring..) 6 The Realization of a Packed-bed Tower . Intalox saddle Pall ring Plastic Pall ring Metal ‘Flexiring’ (Koch Engg. has two bent tongues in each window and is claimed to have better efficiency. The ‘Chempak’ or ‘Levapak’ ring is made by cutting the Pall ring in two halves. NOR PAC (from Nutter Engineering). combines the high volume and even distribution of surface area of a Pall ring and the aerodynamic shape of the intalox saddle. exposing the tongues and promoting better performance. Because of low height. ‘Nutter rings’ have somewhat similar characteristics and are available in both metal and plastic. The Jaeger ‘Tri-Packs’ (metal or plastic) resembles the Pall ring but has a spherical shape. less weight. HcKp (from Koch). The ‘Fleximax’ is an open saddle type packing from Koch-Glitsch. Several third generation random packings have been the offshoots of the Pall ring. larger interfacial area and lesser liquid retention in the bed.Norton ‘Hy-Pak’ ring (metal) Figure 2: Variouss type of second generation random packing ring c) Third generationrandom packings (mid-1970s-): A pretty large number of metal and plstic tower packings have been developed since mid-seventies that offer improved performance in terms of lower pressure drop. This packing offers more void volume and better distribution of surface area. 7 The Realization of a Packed-bed Tower . The Cascade Mini-Ring (CMP) is similar to the Pall ring but has a height-todiameter ratio (aspect ratio) of 1:3 compared to 1:1 of the latter. such a packing element has a lower centre of gravity and therefore tends to orient with the circular open end facing the vapour flow.. This reduces friction and enhances the mass transfer coefficient and effective surface area. The ‘Intalox Metal Tower Packing’ (IMTP). It also prevents interlocking of the pieces in the bed. LANPAC (from Lantec Products) are a few other third generation random packings. Many of these packings evolved from the intalox saddle. a random packing developed by the Norton Corp. Glitsch Inc. called Panapak. 1994. made from thin metal strips to form a honeycomb-like structure did not gain much popularity because if severe maldistribution of liquid. Intalox high performance corrugated structured packing (made from thin metal sheets). The first structured packing. 8 The Realization of a Packed-bed Tower .Intalox metal tower packing (IMTP) Nutter ring Cascade Miniring Figure 3: Various types of third generation random packing ring d) Structure packings: Structured packings have emerged as the formidable competitor of random packings since the 1980s (Helling and DesJardin. These are made from woven wire mesh or corrugated metal or plastic sheet. Their major advantages are low gas pressure drop (because of ‘through flow’ of the gas) and improved capacity and efficiency. Bennett and Kovac.. 2000). Sulzer and Nutter Engineering came up with acceptable high efficiency structured packings made of corrugated metal sheets or wire mesh. Since the late 1970s and the early 1980s. ‘Flexeramic’ corrugated structured packing (Koch Engg. Corrugated metal sheet structured packing: There are quite a few tower packings of this category. keeping a suitable gap between the adjacent members to make a packing piece.). ‘Montz B1’ Structured packing Sulzer wire-gauge packing CY (Nutter Engg. Figure 4: Various types of structure packing i. textured or grooved to promote mixing and turbulence in the falling liquid film and thereby to increase the mass transfer coefficient and efficiency. A bunch of corrugated sheets are arranged parallelly. These are fabricated from thin corrugated (or crimped) metal sheets. A number of such pieces are arranged and stacked one after another. The corrugation angle 9 The Realization of a Packed-bed Tower . A piece of packing above is rotated at a certain angle relative to the piece immediately below it. Corporation). The height of a piece is typically 8 to 12 inches. The surface of a sheet is often made embossed. Wire mesh structured packings: Sulzer supply three types of such packing marked AX. It has a surface area above 550ft2/ft3. A larger corrugation angle increases the capacity in terms of the liquid load but reduces the mass transfer efficiency. grooved and 250m2/m3 perforated Flexipak Similar to Mellapak - Gempak Smooth or lanced 45o Montz Metals. smooth. A cylindrical tube made by knitting multifilament wires is flattened into a ribbon and then made in to a packing by corrugation. plastics. and by Norton Corp under the name ‘Intalox High-Performance Wire Gauge Packing’. Table 1: Characteristics of a few structured packings Structured packing Material and surface Crimp angle Area Mellapak 45o or 60o About Metals. Similar packings are marketed by Glitsch under the trade name Gempok.. plastics. iii. This packing is similar to the Sulzer wire mesh packing. 1990. BX and CY. Olujic et al. W-shaped Sharp crimp perforations angle 10 The Realization of a Packed-bed Tower . Sulzer packing type CY has a surface area of about 200ft2/ft3. Glitsch also developed Goodloe for which the knitted wire-mesh is used. 2001). Montz A packing (Nutter Engineering) is made from perforated wire mesh sheets with a specially contoured corrugation. The surface area is about 150ft2/ft3. Perforations are sometimes made on the sheets to provide channels of communication between the two surfaces of a corrugated sheet and to improve wetting of the surfaces. ii. The packing elements are made of corrugated layers of wire mesh. embossed Sinusoidal MAX-PAC Metals.of the sheets varies from 28o to 45o (Fair and Bravo. The packings have high efficiency (low ‘HETP’) as well. these packings have been very popular for use in vacumn distillation columns. Montz series Snap-Grid series Jaeger Products MaxPak series Materials for tower packings The common materials can be use for fabrication of tower packings are ceramic.Although developed in the late 1970s. The higher initial cost of such packings is amply compensated by the lesser operating costs because of lesser pressure drop across the bed. Also. particularly of those made from corrugated sheets. the structured packings are being used for near-atmosphere services as well (Bravo. Table 2: Common structured packings Supplier Structured packing Metal grid packing Sulzer Chemtech Mellapak series Mellagrid series Koch Engineering Flexipak series Flexigrid series Glitsch Inc. metals. so called ‘grid packings’. The first major application was in air separation columns (Parkinson and Ondrey. and plastics. 1998). There are few factors to be considered for the selection of a material for tower packings: • Ease of fabrication • Mechanical strength • Corrosion resistance 11 The Realization of a Packed-bed Tower . 1997). Now. As a result. the structured packings made visible inroads to separation technology in the late 1980s. the well-defined geometric shape. makes them amenable to theoretical analysis. Gempak series C-Grid and EF-25 Grid series Nutter Engineering Co. have been in use since long for high gas/ vapour capacities at a low pressure drop. Another class of packings. 1997). modeling and scale-up (Fair and Sticklemayer. tendency to degrade in an oxidative environment or when exposed to UV. a suitable type of stainless steel is used. For corrosive services. It is rather easy to fill them and clean them in situ by water or even stream. Capacity and Efficiency of Random and Structured Packing 12 The Realization of a Packed-bed Tower .• Wet ability • Ease of cleaning • Cost Ceramic packings declined in popularity since the advent of plastic packings. Plastic packings may be made into a large number of shapes. thus reducing the downtime to a tenth of that for ceramic packings. removal and cleaning. Common materials are polyethylene. Metal random packings offer higher capacity. They are preferred for highly corrosive services. Metal packings are unbreakable and have higher compression resistance but have less wet ability than that of ceramic rings. for example. PVC. are prone to breakage. these have limited shapes (normally rings and saddles only). brittleness at low temperature or on aging. the air-drying tower and SO3 absorption tower in a H2SO4 plant as well as for the operation at elevated temperatures. However. polypropylene. light. efficiency and turndown ratio because of a smaller wall thickness and more open area. and require more ‘downtime’ for filling. Plastic packings are more expensive than the ceramic packings. unbreakable. Plastic packings are cheap. The disadvantages of plastics packings are: poor wet ability. and corrosion-resistance. and poly-yinylidine fluoride. structured packing is more preferable to achieve this performance.Figure 5: Comparison of Structured packing and random packing (COLUMN INTERNALS n. Low High Structured and random packing can be used under the requirement when the packing factor increase and specific area increase too. Table 3: Results of Capacity and Efficiency of Random and Structured Packing Capacity Efficiency Type of packing used High Low Structured and random packing can be used under the requirement when the packing factor and specific surface area decrease. Low Low From the figure above. an increase in packing factor and decrease in specific surface will achieve this performance.d. random packing is more preferable under this performance.) From the diagram above. Thus. it shows that the capacity of packing will increase when the packing factor decrease while the efficiency will increase when specific surface area of packing increase. However. Therefore. from the figure above. the result of capacity and efficiency of these two types of packing are shown as below. 13 The Realization of a Packed-bed Tower . High High In order to achieve this performance.0 Design and specification of a packed-bed tower 3. structured packing is more preferable under this performance.1 Packed-bed tower overview 14 The Realization of a Packed-bed Tower . an increase in specific surface area and decrease in packing factor will do. from the figure above. Thus. 3. Figure 6(a): Schematic diagram of a typical packed-bed tower Figure 6(b): Diagram of a vapor distributor with packing support 15 The Realization of a Packed-bed Tower . whenever low holdup is necessary. and whenever plastic or ceramic construction is required. Especially in large diameter towers. packed towers are desirable whenever low pressure. such as liquid 16 The Realization of a Packed-bed Tower . the cost of packing and other required internals. In addition. liquid and vapor distribution are important considerations where others type of towers are lacking of. In large towers.Figure 6(c): Diagram of a trough-type distributor Figure 6(d): Diagram of a typical perforated pipe distributor Figure 6(e): Diagram of a liquid redistributor Figure 6(f): Diagram of a hold-down grib Generally. some of the newer structured (ordered) packings can provide more theoretical stages per unit of tower height than other tower such as tray tower. because of the tendency of the phases to lose the function of distribution. Also. 17 The Realization of a Packed-bed Tower . (e) is a device to redistribute liquid. this height can be 20-25 ft. or metal materials. (b) is a combination packing support and vapor distributor used for beds of random packings. Trough distributors can be fabricated form ceramic. spray nozzles may replace the perforations. It is sometimes called a wall wiper.distributors and redistributors can be the cheapest if compared to others tower. the height between redistributors is limited to 20-25 ft. this type of distributor can be used in very large towers where caution must be taken not to ensure levelness of the device. Depth of plastic random packings may be limited by the deformability of the packing elements to 10-15 ft. for example. For both random and structured packings. In some large towers. which has a tendency to flow toward the wall. the individual parts of which are described one-by-one: (a) is an example of a typical packed-bed column showing the inlet and outlet connections and a variety of possible packings. which is available in a variety of shapes. Note that both random and dumped packings are present as well as structured (ordered) packings. plastic. In specialized forms. cross-flow tray tower. There are various kinds of internals of a generalized packed-bed tower are represented in Figure XX. (d) is an example pf a perforated pipe distributor. It is a very efficient type over a wide range of liquid rates but suffers from its likelihood of plugging from even minute-size solids in the liquid feed. and for structured packing elements. For metal random packings. (c) is a trough-type distributor that is suitable for liquid rates in excess of 2gpm/sqft in towers 1ft in diameter and larger. it is an integral part of the element. care must be taken not to starve the far notches from their equal share of liquid flow. The serrated shape is used to increase the area for vapor flow. There are two features that should be maximized in packed-bed towers are: (a) Open area—the average percentage of the cross-sectional area of the tower not blocked by the packing. from disintegrating because of mechanical disturbances at the top of the bed. such as those made of carbon. a packing consisting of fine sand would have great separation efficiency but very low capacity. Hence. Procedure of packing a tower 18 The Realization of a Packed-bed Tower . structure packing has largely replaced packing in the form of rings in many packed towers. This is comparable condition where the vapor-liquid contact occurs only on the 5 or 6 in. the will be greater the capacity of a tower. above the tray deck in a tray tower and the majority of the tower’s volume is not used to exchange heat or mass between vapor and liquid. Therefore. per cubic foot of tower volume. (b) Wetted surface area—the number of square feet of packing surface area available for vapor-liquid contacting. a packing consisting of empty space would have lots of capacity but awful separation efficiency. For instance. For the larger in the open area of packing. for instance. Structured packing has about 50% more open area than random packings and two or three times their wetted-surface area. In a packed-bed tower. the area used for the downcomer that feeds the liquid to a tray and the area used for fraining liquid from a tray are unavailable for vapor flow. and hence available for the flow of vapor and liquid. In another way. the entire cross-sectional area of a packed-bed tower is available for vapor flow while in a tray tower. the selection of packing for a column is a compromise between maximizing open area and maximizing the wetted surface area. the entire packed volume is used for the vapor-liquid contacting. The greater the wetted surface area of a packing. Also. the higher the separation efficiency of the tower.(f) is a hold-down grid to keep low density packings in place and to prevent fragile random packings. Figure 47: Techniques of filling a tower with random packings: (a) wet-packing by filling the tower with water. The flow channels in such a bed are regular and the gas pressure drop becomes less as a result. (b) dry-packing by lowering buckers filled with the packing. 3. Plastic packings cannot be filled in this way because they will float in the liquid. the tower is filled with water or a suitable liquid and the packing is dumped into it. they may form a heap at the centre. One rare occasion. The chute-and-sock method is also used (this technique is very useful for loading a solid catalyst in a reactor). Ceramic packing may break if dumped from above. popular with ceramic packings.It is not advisable to pack a tower by dropping the packings into the tower from the top. ‘Dry packing’ may be done by lowering the packing in a wire bucket that is led into the column through a manhole (Figure XX). 1984). the following factors have to be taken into account. 19 The Realization of a Packed-bed Tower . and (d) packing through a chute only (Chen. (c) the chute-and-sock method of packing. Structured packings are made in pieces to fit a column of given diameter and are stacked in an appropriate way. In the ‘wet packing technique’. There are a few common techniques (Figure XX) of installation of random packings.2 Design of a packed-bed tower In order to choose a mechanically and commercially feasible scrubber system. A ring-type packing may roll down the heap and get a preferential horizontal orientation. Also the packings may not get spread uniformly. a random packing like the Raschig ring is stacked in a column in layers. etc. the actual design of the tower (diameter.• Efficiency • Design variables • Sizing • Operation and maintenance • Material used • Costs Efficiency The efficiency of an absorption process in part of the following: • The solubility of the pollutant(s) in the scrubbing solvent • Pollutant(s) concentration in the airstream being treated • Temperature and pressure of the system • Flow rates of gas and liquid (liquid/air ratio) • Gas-liquid contact surface area • Stripping efficiency of the solution and recycling of the solvent In order to increase the higher absorption efficiency in a wet scrubber. The absorption efficiency will also be improved in the scrubber if the temperature can be reduced meanwhile the liquid-to-air ratio increased.) will generally depend on the given vapor-liquid equilibrium for the specific pollutant/ scrubbing solvents. As if the data are available. In addition. depth of packed bed. such data are not always available for all pollutants encountered in industry today. The type of tower used as mentioned before will affect the equilibrium as well. However. the ability to increase gas-liquid contact will always significant. height. empirical data will always be superior to theoretical 20 The Realization of a Packed-bed Tower . As previously defined. In any absorption process. There is an inverse relationship between m and driving force. the solubility of the given pollutant in the gas and liquid phases will determine the equilibrium concentration of the pollutant in the given example. = 2.54cm) and the scrubbing liquor (absorbent) used is water. the tower is packed with 2 in. the slope m of the equilibrium curve is low. If a pollutant is readily soluble in the scrubbing liquor. Theoretical models of flow through a packed tower There have been a number of attempts to develop simplified models of two-phase (the gas and the liquid) flow through a packed bed for a better understanding of the flow phenomena as well as to theoretically determine the pressure drop and the flooding capacity. this once-through method has the consequence of sending a large flow of water to a treatment facility. Therefore. Any such model visualizes a simplified picture of the bed and of flow through it so as to make it amenable to theoretical analysis. The water is sprayed from top and the slurry is collected at the bottom. the smaller the slope. this concentration gradient is the driving force to mass transfer between the phases. Design variables Packed tower wet scrubber is commonly implemented in air pollution control installations. This example is applicable for either organic or inorganic air pollutant control. possible removal efficiency is controlled by the concentration gradient of the pollutant being treated between the gas and the liquid phases.data for design purposes. The configuration used is somewhat simplified. the more readily the pollutant will dissolve into the scrubbing liquor. The scrubbing liquor spray system is described as a once-through process with no recirculation. we can apply a similar type of pollutant having available data to model another system if such empirical data are not available for the corresponding pollutant with an added safety factor built into the design. It should be noted that in a field installation. ceramic Raschig Rings (note: 1 in. For example. There will be three models cited here. 21 The Realization of a Packed-bed Tower . Therefore. This represents a high-driving-force system. the void volume in the bed decreases.4). an early version of the model was used by Ergan back in 1952 to develop an equation for pressure drop across a packed-bed of solid.e. With increasing liquid flow. (1989) used this model to predict pressure drop and flooding for both random and structured packings. In fact. the hypothetical spheres may not even touch each other (as if they remain ‘suspended’ but stationary). The liquid flows as a film along the walls of the cylinders and is subject to shear force at the gas-liquid interface because if the upflowing gas. Billet (1995) used this model to develop equations for gas-phase pressure drop and the condition of beginning of loading of liquid in the bed. When the voidage is large (i.a) The particle model: The packed bed is visualized as consisting of a number of spheres of a size calculated on the basis of the void volume of the bed and the surface area of the packing (Figure XX). the size of the hypothetical spheres increases and the pressure drop also increases. The pressure drop across the bed is a result of drag of the following gas on the spheres. The flooding conditions were also analytically laid down as and [uL=uLfl and uG=uGfl] 22 The Realization of a Packed-bed Tower . b) The channel model: The packed bed is considered to act like a cylindrical block with a number of uniformly distributed vertical channels in it. Stichlmair et al. The hypothetical channel diameter can be calculated from the voidage and specific area of the packings. >0. Loading starts when the shear force at the interface is large enough to reduce the liquid and vapor velocities at the interface to zero. So. the concentration of both phases change continuously. Also. The model has been criticized by some people because it visualizes the packing material to form a continuous medium. (iii) determination of the minimum and the actual solvent rate.g. c) Percolation model: This model (Hanley. the liquid flowing down through the packing remains in contact with the up-flowing gas at every point of the packed section. and (vi) design of the liquid distributor and redistributor (if necessary). Rocha et al. Sizing of a packed column basically includes the following steps: (i) selection of the solvent. it has been used by other people too (e. (iv) determination of the column diameter. 1994) assumes that a part of the liquid flowing through the bed gets accumulated in certain locations of the bed causing local blockage or ‘localized flooding’. The number of flooded locations increases with the increasing liquid rate. (ii) selection of packing. This creates the enhanced pressure drop. (v) determination of the packed height. etc. design of shell. 1993) to predict packed-bed pressure drop. The following items and variables should be known or available for design purpose: 23 The Realization of a Packed-bed Tower . (including selection of the materials to be used for the tower internals and to build the tower). nozzles.Here uL is the superficial liquid velocity and hL is the liquid holdup in the bed. packing support and the gas distributor. column support. Nevertheless. Billet (1995) and the German group used these criterion to determine the flooding capacity of the bed. In a packed tower. on the other hand. a packed column is called ‘continuous differential contact equipment’.. mol/ (time) (area)] and the specific interfacial area of contact between the gas and the liquid phases. KGā. KLā. Kyā. Design method based on the individual mass transfer coefficients Figure 48: The parameters on the sizing of the packed tower Consider the packed-bed tower shown above. we use the mole fraction unit of the gas and the liquid-phase concentrations. is taken on the basis of unit packed volume and has the unit of m2/m3 or ft2/ft3. sometimes called ‘capacity coeffecients’ (kyā. kxā. The flow rates (G’ and L’) are taken on the basis of the unit cross-sectional area [i. ā. etc. this is intrinsically negative in the case of absorption. the packed volume in the differential section for unit cross-sectional area of the bed= (1) (dh) 24 The Realization of a Packed-bed Tower . Then.e. The rate of flow of the solute (with the carrier gas) = G’y mol/ (time) (area). The change in the solute flow rate over the section= d(G’y). Let NA be the local flux and ky be the individual gas-phase mass transfer coefficient.).(a) Equilibrium data (b) Flow rates and terminal concentrations of the gas and liquid phases (c) Individual or overall volumetric mass transfer coefficients. We make a steady state mass balance over a small section of the column of thickness dh. the rate of flow per unit cross-sectional area) is given. (2) into Eq. 1994): (a) Draw the equilibrium curve on the x-y plane for the particular gas-liquid system.Interfacial area of contact in the differential section= (ā) (1) (dh) Rate of mass transfer of the solute= (ā) (dh) (NA) A mass balance over the elementary section of the bed yields (ā) (dh) (NA)= -d(G’y)= -G’dy.ydG’ (1) Since the carrier gas is not soluble.e.e. The 25 The Realization of a Packed-bed Tower . Otherwise. rearranging and putting NA= ky(y-yi). (1). (ā) (dh) NA (1-y)= -G’dy (3) Thus. Ls is known. since the interfacial concentration yi is not explicitly known as a function of the variable y. the minimum liquid rate on solute-free basis (Ls)min is to be determined following the procedure detailed in previous section. -dG’= (ā) (dh) (NA) (2) Substituting Eq. (4) Integrating within the appropriate limits. (6) If the liquid mass flow rate (i. The integration is not straight-forward. i. the change in the total gas flow rate is also equal to the rate of mass transfer of the solute. (b) Draw the operating line from the material balance equation. we get (5) Evaluation of the integral above gives the height of the packing. The following steps should be followed in general (McNulty. a set of lines parallel to the one drawn in step (c) may be constructed. (c) Take any point (x.y) on the operating line. The outlet liquid concentration x1 is obtained from the overall material balance. draw a line of slope –kx/ky from the point S(x. The line SR is called a ‘tie’ line’. (7) The height of the packing for a stripping column can be obtained in a similar way. (5) graphically or numerically. The design equations given below can be derived following the above procedure.). etc.] Now we have a set of (y. yi) pair for y2≤ y ≤ y1. So the design equation corresponding to Eq. Evaluate the integral in Eq. are given or known. So yi is known for the particular value of y. Ky. Note that Gs can be calculated from the given feed gas flow rate. kL. Figure XX. Kx. kxā and kyā).2 to 2 times) of the minimum rate.e. kG. (e) Calculate G= Gs (1+y) at each point. The height of the packing can also be determined using other types of individual mass transfer coefficients (kx. If kx and ky or their ratio are constant.y) to meet the equilibrium curve at R(xi. Using the known values of kx and ky (or kxā and kyā).actual liquid rate Ls is taken as a suitable multiple (commonly 1. rather than kx and ky. (5) becomes (8) 26 The Realization of a Packed-bed Tower . But here y2>y1 and the gas-phase driving force at any point is yi-y. [Note that very often the mass transfer coefficients combined with the specific interfacial area (i. (f) Calculate the value of the integrand fro a set of suitably spaced values of y. (d) Repeat step (c) for a number of other points on the operating line. yi). (10) Graphical or numerical integration of the right-hand side of the above equation is simpler than that of Eq. draw a vertical line through it and extend up to equilibrium curve to reach the point y*.y) on the operating line. Eq. (5). (4) becomes (9) Here y* is the gas-phase concentration (in mole fraction) that is capable of remaining in equilibrium with a liquid having a bulk concentration x. 27 The Realization of a Packed-bed Tower . Design equations similar to Eq. The required packed height is obtained by integration of the equation between the two terminal concentrations. Plot the operating line. (7) can be obtained when the overall coefficient is given.Figure 49: The flooding curve Design method based on the overall mass transfer coefficient If we express NA in terms of the overall mass transfer coefficient [NA= Ky(y-y*)]. If the values of the integrand for suitably spaced values of the variable y are calculated. the integral can be evaluated graphically or numerically. take any point (x. As a result. y. x* are schematically shown in Figure XX. varies. xi. the suffix M means ‘log mean’. yi. Taking this quantity out of the integral sign. Design method based on height of a transfer unit By using Eq. Design equations based on the overall coefficients for a stripping operation can be easily derived from above. and yB= (1-y). Chilton and Colburn (1935) called this quantity ‘height of a transfer unit’ based on the individual gas-phase coefficient or the ‘height of an individual gas-phase transfer unit’. (12) as (14) Where 28 The Realization of a Packed-bed Tower . we may rewrite Eq.] The gas-phase mass transfer coefficient often varies as (G’)0. y*. the quantity G’/kyā(1-y)iM remains fairly constant over the packed section of the bed although the total gas mass flow rate.8. remains independent of the prevailing driving force (but the coefficient ky depends upon the concentration through yiBM).(11) The locations of x. the ‘Colburn-Drew mass transfer coefficient’. (5) and rewrite it in the following form (12) Where yiBM is the log mean value of yB [= (1-y)] defined as follows: (13) [Note that we are dealing with binary gas mixture in which B is the carrier gas (nondiffusing). ky’= kyyiBM. denoted by HtG. G’. Also. and The following table summarizes the expressions for the various forms of HTUs and NTUs. Figure 7: The NTUs and HTUs 29 The Realization of a Packed-bed Tower . and the larger will be the NTU. if we put NtOG= 1 in the above equation. Some qualitative physical significance can be attributed to the HTU and the NTU. on the other hand. If the equilibrium relation is linear with slope m (i. the less will be the driving force available (particularly near the top of the column in case of absorption and near the bottom of the column in case of stripping). if the operating and the equilibrium lines are nearly straight and parallel. i. in the case of absorption of a dilute gas [when (1y)*M/ (1-y)= 1]. So (15) If we consider one overall gas-phase transfer unit.e. The greater the extent of separation desired. (y1-y2) (y-y*)av. Thus. indicates the difficulty of separation. For example. (16 a) (16 b) 30 The Realization of a Packed-bed Tower . For a ‘good packing’ (especially the one that provides more specific interfacial area of contact). (y-y*) is approximately constant. The number of transfer units (NTU). a single transfer unit corresponds to the height of packing over which the change in gas concentration is approximately equal to the average driving force. For a particular gas-liquid system. A quantitative significance can be attributed to NTU in certain limiting cases. y *= mx or y= mx*) the heights of the individual and overall transfer units are related as follows (The derivation of these equations is left as an exercise).Packed-bed mass transfer data for gas-liquid systems are often reported in terms of the height of a transfer unit. The HTU indicates inversely the relative ease with which a given packing can accomplish separation for a particular system. HTU depends upon the type of packing and the gas and the liquid flow rates. the value of HTU is less and the packed height required for a specified degree of separation is smaller. The HTU data on typical systems maybe obtained from the manufacturer of a particular packing.e. and the design of column internals. based on the determination of the flooding velocity by using the Eckert’s GPDC chart. the tower diameter and the pressure drop across the bed may be estimated. So far as column internals are concerned. proceeds as follows. It may be done by using the limited information available in the open literature and the manufacturer’s catalogue. There are broadly two approaches. putting and . the selection of the type and size packing. 3. for example. Once the design gas flow rate is fixed. the flow parameter. For example.. the design of a packed-bed tower for a particular service involves a number of things such as the selection of solvent. In this section. the determination of column diameter and height of packing.3 Column diameter of a packed-bed tower Basically. 31 The Realization of a Packed-bed Tower . there is no well-defined procedure.The relations may be considerably simplified if the solute concentrations are low. (2001). The latter is obtained from the same chart.e. One of the approaches. An algebraic correlation fro the Eckert’s flooding curve (and a dozen similar equations) has been given by Piche et al. Flv) is evaluated. we discuss one very important item of design. We have Where Ā= L’/ mG’ (= L/mG) is the absorption factor. (ii) The value of the ordinate is obtained from the flooding curve and the mass flow rate of the gas at flooding is calculated. (iii) The operating gas flow rate is normally taken as 70 to 80% of the flooding velocity to guard against inherent errors in the flooding curve and also to keep some flexibility in the design to take care of any sudden surge in the gas flow rate. (i) From the total liquid and gas flow rates (either specified or calculated by material balance) the abscissa (i. and = mG’/ L’ (= mG/ L) is the stripping factor. the determination of the diameter of a packed column. A step-by-step procedure is outlined in Ludwig (1997).25-0.[G’ in lb/ft2.6.25 for foaming systems. 2001).h..5 inch per foot. ΔP/L [(inch water)/ (ft packing)]. it is 1. atmospheric pressure distillation: 0. 1990. The value of the flow parameter is calculated and the capacity parameter corresponding to the allowable pressure drop is obtained from the chart. dp= 1-3/1 inches. Manufacturers of packings generally supply the pressure drop and flooding characteristics of their products as plots of ΔP versus F s [=μsG(ρG)0. it is desirable that the flooding point or pressure drop is determined by interpolation of the available data.s2] The second approach does not use the flooding curve at all because of its limited accuracy and applicability.5 inch water per foot of packed bed.5 inch per foot. A few practical values of the allowable pressure drop. for Paul rings.lbf/lbm.0 inch. (Ludwig. dp<1 inch. If enough data are available in the company’s catalogue.997) are: low to medium pressure column operation: 0. 1994. The column diameter is now easy to determine. absorption or similar systems: 0. For the first generation random packings. Normally.5 to 1. the flood point pressure drop is about 2-2. ρ in lb/ft3. Strigle. dp/Dc ranges between 1/20 and 1/10. dp= 2-3 inches. The recommended sizes of packing for different column diameters are: Dc<1ft.5-1. For most modern packings.4 for non-foaming systems.1-0. 0.4-0. it is 0. vacuum distillation: 0. gc in ft.1-0.2.5]. μ in cP. Dc= 1-3 ft. Limited data and information on pressure drop calculation for a bed of structured packings are available (Fair and Bravo. Olujic et al. The pressure drop at flooding for the particular packing can be calculated from the Kister and Gill equation. It may be noted that the quantity Fs is also taken as a measure of the ‘capacity parameter’ or ‘factor’ for flow through a packed tower at low-to-moderate pressure when ρG<<ρL. The allowable pressure drop in the bed is taken as a basis of design and the Strigle’s GPDC chart is used directly. Dc>3 ft. Fp in ft-1. The gas velocity at flooding can also be calculated from these results. 32 The Realization of a Packed-bed Tower . 2 gpm/ft2 for structured packings 3. Dc= column diameter Bed voidage 70 to 90% (more for structured packings) Open area of packing support 70 to 85% or more (for gas/ liquid flow) Re-distribution of liquid After 3 to 10 tower diameter (10 to 20 ft) Gas pressure drop Less than 0. 33 The Realization of a Packed-bed Tower . Common ranges of values of the more important packed-bed parameters are given in Table 41. the operating bed pressure drop should not be less than 0.1 to 0. This method is more appropriate because the changes in compositions of the liquid and vapor phases occur differentially in a packed column rather than in stepwise fashion as in tray column. height of packing required can be evaluated either based on the gas-phase or the liquid-phase.In order to maintain proper vapor distribution through the bed.In this method.5 inch water per foot bed depth Operating velocity 70 to 80% of flooding velocity Minimum wetting rate 0. 0. the superficial gas velocity normally remains below 1m/s.1 inch water/ft.4 Packing height of a packed-bed tower The concept of the analysis of a packed column is mainly on the method of transfer units. the liquid velocity remains around 1cm/s. In a column operating near atmospheric pressure. The packed height (z) is calculated using the following formula: z=NxH Where.5 to 2gpm/ft2 for random packings. Table 4: Ranges of a few important packed-tower parameters Random packing nominal size Dc/20 to Dc/10. p. A single transfer unit gives the change of composition of one of the phases equal to the average driving force producing the change.e. we have: z = NOG x HOG 34 The Realization of a Packed-bed Tower . the more efficient the mass transfer (i. Hence. For the gas-phase. it incorporates the mass transfer coefficient that we have seen earlier. The values of HTU can be estimated from empirical correlations or pilot plant tests.N = number of transfer units (NTU) . Foust et al. ["Principles of Unit Operations" 2nd Ed.dimension of length The number of transfer units (NTU) required is a measure of the difficulty of the separation. larger mass transfer coefficient) the smaller the value of HTU. a larger number of transfer units will be required for a very high purity product. The height of a transfer unit (HTU) is a measure of the separation effectiveness of the particular packings for a particular separation process..dimensionless H = height of transfer units (HTU) . but the applications are rather restricted. As such. Basically.391] Figure 8: The mass flow diagram of the packed tower Determination of the packed height can be based on either the gas-phase or the liquidphase. The NTU is similar to the number of theoretical trays required for tray column. other than mole fraction. y1* is the mole fraction of solute in vapor that is in equilibrium with the liquid of mole fraction x1 and y2* is mole fraction of solute in vapor that is in equilibrium with the liquid of mole fraction x2. e. 35 The Realization of a Packed-bed Tower . Since KY has a unit of mole/ (area. etc.g. and "a" has a unit of (area/volume). the combined parameter KY a will have the unit of mole/ (volume. As seen earlier. Table 6.mole fraction).time. such as kg-mole/ (m3.time. packing manufacturers report their data with both K Y and "a" combined as a single parameter. driving force can be expressed in partial pressure (kPa.3) that characterize the wetting characteristics of the packing material (area/volume). Normally.KY is the overall gas-phase mass transfer coefficient.s.driving force).driving force). "a" is the packing parameter that we had seen earlier (recall the topic on column pressure drop. mm-Hg). psi. wt%. Thus. y) as shown is any point in the column. ] NOTE: Both equilibrium line and operating line are straight lines under dilute conditions.y2*) is the concentration difference driving force for mass transfer in the gas phase at point 2 (top of column). equilibrium values y1* and y2* can also be calculated using Henry's Law (y = m x. y2* = m x2 Similarly for the liquid-phase we have: z = NOL x HOL 36 The Realization of a Packed-bed Tower . this time no subscripts are shown.Figure 9: Mole fraction solute in vapor versus mole fraction solute in liquid (y1 . we have: y1* = m x1.y*) as shown previously.y1*) is the concentration difference driving force for mass transfer in the gas phase at point 1 (bottom of column) and (y2 . Alternatively. [Point P (x. where m is the gradient) which is used to represents the equilibrium relationship at dilute conditions. The concentration difference driving force for mass transfer in the gas phase at point P is (y . KX is the overall liquid-phase mass transfer coefficient and "a" is the packing parameter seen earlier. Again, normally both KX and "a" combined as a single parameter. Likewise, x1* is the mole fraction of solute in liquid that is in equilibrium with the vapor of mole fraction y1 and x2* is mole fraction of solute in liquid that is in equilibrium with the vapor of mole fraction y2. Refer to Figure 134 for finding values of x1* and x2* from the equilibrium line. Alternatively, x1* = y1 /m and x2* = y2 /m. (x1* - x1) is the concentration difference driving force for mass transfer in the liquid phase at point 1 (bottom of column) and (x2* - x2) is the concentration difference driving force for mass transfer in the liquid phase at point 2 (top of column). Table 5: Typical example of a packed-bed data-sheet (Basic design information and parameters) 37 The Realization of a Packed-bed Tower 3.5 Consideration Packing factor The Eckert chart contains a parameter Fp that characterizes the packing and is called ‘packing factor’ (another notation Cf can be used to denote the same quantity). The packing factor introduced by Lobo in 1945 used to be taken as a p/ε3 (ap= surface area of the packing per unit volume; ε= void fraction of the packed bed). The packing factor could be calculated from these two properties of a packing. It was later found that the pressure drop and flooding data could be better correlated if the packing factor was taken as an empirical quantity. In fact, it is now taken to be so and is determined by experimental measurement of pressure drop across a packed bed and using the generalized pressure drop correlation discussed below. The values of Fp for different packings are supplied by the manufacturers. The packing factor and a few other characteristics of several random packings are given in the table below. The packing factor inversely indicates the capacity of a packing; the specific surface area indicates its mass transfer efficiency. It is intriguing that the values of the packing factor of the same packing obtained from different soruces are found to vary. Table 6: The information of particular packing 38 The Realization of a Packed-bed Tower Liquid holdup In order to facilitate mass transfer on the packed-bed surface, there must be a reasonable liquid holdup in the bed. However, excessive holdup increases pressure drop over the bed and is also undesirable if the liquid is heat-sensitive. Generally, it ranges from a few percent to about 15% of the bed volume. There are two types of liquid holdup (expressed as volume of liquid per unit bed volume) have been defined. Static holdup: It is the amount of liquid remaining in unit volume of the bed after the bed is drained for a reasonable time. It is insignificant compared to the total holdup. Operating holdup (hLo): It is the difference between the total holdup and the static holdup when the bed is in operation. The is another term ‘dynamic holdup’ to denote the scenario. Several correlations for estimation of the quantity are available (Kister,1992). A recent correlation (Engel et al., 1997) for hLo (volume fraction of the bed) given below is claimed to have an error within 16% for most systems. Minimum wetting rate (MWR) It is the liquid throughput below which the film on the packing surface breaks up reducing the wetted area. A liquid rate below MWR is too small to wet all the packing surface. The effective interfacial area of the gas-liquid contact decreases and the efficiency of mass transfer decreases as a result. Among the many correlations available for its prediction, the one due to Schmidt (1979) has been found to work very well. Minimum liquid rate for random packings is reported to lie in the range 0.5-2 gpm/ft2 (1.25-5 m3/m2h); for structured packing it is 0.1-0.2 gpm/ft2 (0.12-0.25 m3/m2h). 39 The Realization of a Packed-bed Tower Flooding in a packed tower Knowledge of the hydrodynamic and mass transfer characteristics of a packed bed tower such as the influence of the flow rate of the gas and of the liquid on pressure drop, liquid holdup and the gas- and the liquid-phase mass transfer coefficients in the bed is essential for the design of such a device. Bed pressure drop and the phenomena of loading and flooding The liquid distributed on the top of a packed bed trickles down by gravity. Flow of the gas is pressure-driven and the pressure is generated by a blower or a compressor. The gas undergoes pressure drop as it flows through the bed because (i) both skin friction and form friction, (ii) frequent changes in the flow direction, and (iii) expansion and contraction. When the packing is dry (there is no liquid throughput), the maximum area for flow of the gas is available. However, when a liquid flows through the bed, a part of the open space of the bed is occupied by the liquid (called ‘liquid holdup’ in the bed) and the area available for gas flow decreases. This is the reason where the increasing liquid throughput results in the increasing pressure drop of the gas. Typical gas flow rate vs. pressure drop curves on the log-log scale for a dry bed (no liquid flow) and for two constant liquid rates are qualitatively shown in the figure below. Figure 10: Pressure drop curves on the log-log scale The plot is linear for a dry bed. For an irrigated bed, such a curve is nearly linear with a slope of about 2 in the lower region (i.e. ΔP varies nearly as the square of gas rate). The 40 The Realization of a Packed-bed Tower If the gas flow rate is further increased. The visual and physical symptoms of flooding are: (i) accumulation of a layer of liquid at the top of the bed. The liquid holdup in the bed increases as a result. The definition of ‘flooding’. This steady increase in the pressure drop continues till the point B (Figure XX) is reached. Every packing has its own geometrical and surface characteristics. (ii) a sharp rise in pressure drop. Liquid accumulates more in the upper region of the bed almost preventing the flow of gas. it is not very realistic to work out separate carrelations for pressure drop (and for mass transfer) for packings of different 41 The Realization of a Packed-bed Tower . 1992) states that: It is a region of rapidly increasing pressure drop with simultaneous loss of mass transfer efficiency. At the point B and beyond. Prediction of pressure drop and flooding In order to come out with a complete design of packed towers. However. the column operates like a ‘bubble column with gas-liquid upflow’.slope of the straight section. (iii) a sharp rise in liquid holdup in the bed. Over the region BC. and (iv) a sharp fall in mass transfer efficiency. some researchers have reported a reasonably stable operation beyond the point D. While the operation of the column becomes very unstable over the region CD and the mass transfer efficiency drops significantly. the prediction of the flooding point and pressure drop is essential. Pressure drop per unit bed height as well as the flooding characteristics are also different for individual packings even when all other parameters including ‘nominal packing size’ remain the same. Charts. This phenomenon is known as ‘flooding’. however. suggested by Bravo and Fair (Kister. This is because beyond the point D. the drag of the gas impedes the downward liquid velocity. the upflowing gas interferes strongly with the draining liquid. The bed becomes ‘flooded’ (point D) when the voids in the bed become full of liquid and the liquid becomes the continuous phase—a case of ‘phase inversion’. If the gas rate is increased at a constant liquid rate. the liquid accumulation rate increases very sharply. accumulation or ‘loading’ of the liquid starts. correlations and theoretical models have been proposed for this purpose. decreases slightly at higher liquid rates. The point C is called the point of ‘incipient flooding’. Figure 11: Eckert’s curve The GPDCs proposed by Eckert (1975 and before) of the erstwhile Norton Company have been widely used for packed tower design. Some of the correlations have a semi-theoretical basis and include adjustable constants specific to a group of packing. viscosity and surface tension) of the fluids and (ii) the packing type and its features (size. The idea of a GPDC was first introduced by Leva (1954).types and sizes. see Billet. The major variables and parameters that determine the pressure drop and flooding characteristics are: (i) the properties (density. Strigle. voidage and surface area and surface properties). Eckert. Billet and others. Stichlmair. and Kister. Instead efforts were made to develop a ‘generalized pressure drop correlation’ (GPDC) that would be applicable to all kinds of random packings. (2001) reviewed all important correlations proposed for the flooding point coming from US and German Schools. Recently. A number of charts and correlations have been proposed by the US School (led by Leva. These researchers also proposed a new correlation developed by using artificial neural network (ANN) technique to 1019 data sets reported by different workers. see Kister. A second group of charts and correlations have been proposed by the German School (led by Mersman. 1992) during the last fifty years. to name a few. Piche et al. 1995). The 1970-version (Figure XX) gives a 42 The Realization of a Packed-bed Tower . It works well with most first generation packings but not for several second generation packings and smaller modern packings. the abscissa and the ordinate of the Strigle’s chart resemble the corresponding quantities of Fair’s flooding chart for a tray tower. The ordinate 43 The Realization of a Packed-bed Tower . The Strigle’s version (Figure XX) is now most popular for packed tower design (Larson and Kister. 1997). For first generation packings. Figure 12: Strigle’s curve Flow parameter. The flow parameter Flv. in ft/s Interestingly. the ‘packing’ factor’ is high (generally above 60ft-1) and the pressure drop is ΔP/L ≥ 2 inches of water per foot packed height at ‘incipient’ flooding. represents the square root of the ratio of liquid and vapor kinetic energies. Eckert’s chart was further refined by Strigle (1994) using a data bank of 4500 pressure drop measurements on beds having different types and sizes of packings as well as using different liquids and gases. Eckert’s 1975 version omitted the flooding curve because such a curve always has a doubtful accuracy. Eckert’s chart wa). The error in pressure drop prediction is claimed to be within ±11% for normal ranges of operation. It has a ‘flow parameter’ as the abscissa and a ‘capacity parameter’ as the ordinate. Capacity parameter.number of constant pressure drop curves and a flooding curve. μ in cP] Strigle’s chart also excluded the curve for pressure drop at flooding. However.5 inches water per foot is considered to represent the ‘incipient flooding’ condition. may be corrected for changes in interfacial tension and viscosity. he suggested the following equation. (inch water/ft. For dry bed pressure drop at nearly atmospheric pressure. The dry bed packing factor Fpd of any packing. 1992). the curve for ΔP/L= 1. Kister and Gill (1991) proposed the following correlation for flood point pressure drop in terms of the packing factor. FP in ft-1) Robbin (1991) proposed another set of correlations for pressure drop prediction over a wide range of operating conditions.h. [σ in dyne/cm.6 Mass transfer efficiency A parametric study of carbon dioxide (CO2) absorption performance into an aqueous solution of monoethanolamine (MEA) in the spray column was carried out experimentally over wide ranges of process conditions.describes a balance between forces due to vapor flow (that acts to entrain swarms of liquid droplets) and the gravity force that resists entrainment (Kister. the packing factors for dry and irrigated beds are likely to be different. KGae and was found to vary 44 The Realization of a Packed-bed Tower . Here F p is a characteristic parameter of the packing. which is now considered an empirical quantity. can be calculated from the above equation by measuring the pressure drop across an esperimental packed bed. The dry bed pressure drop can also be calculated using the Ergun’s equation. called the ‘packing factor’. if necessary. The performance of the spray was interpreted in terms of the overall mass transfer coefficient. However. and ρG is in lb/ft3. The above equation has an important application. The quantity Cs. 3. (inch water/ ft) Here G’ is in lb/ft2. which is akin to the Souders-Brown constant. The performance of the spray column was compared to that of a packed column and showed a promise for CO2 capture application.hr. Z is column height in m. (a) CO2 partial pressure (kPa) (b) Gas flow rate (MEA concentration) (m3/m2.G is mole ratio of CO2 in gas stream. and size of spray nozzle. CO2 loading.with process parameters. The mass transfer performance was determined in terms of the volumetric overall mass transfer coefficient by using the following equation Where GI is inert gas flow rate in kmol/m2. including gas flow rate. liquid flow rate. P is total pressure on the system in kPa. CO2 partial pressure.G and y*CO2 are mole fraction of CO2 in gas stream and equilibrium mole fraction of CO2. MEA concentration. yCO2. and YCO2.hr) 45 The Realization of a Packed-bed Tower . hr) (f) liquid flow rate (CO2 loading) (m3/m2.(c) Gas flow rate (liquid flow rate) (d) CO2 loading (mole CO2/ mole MEA) (m3/m2.hr) (e) Liquid flow rate (CO2 loading) (m3/m2.hr) Figure 13: Effect of parameters on the performance 46 The Realization of a Packed-bed Tower . thus resulting in only a small change in the amount of CO 2 absorbed as the partial pressure increases. thus resulting in an increase in droplet surface area per unit volume of dispersed liquid and (2) an increase in number of droplets produced by the nozzle and also the surface area available for mass transfer. an increase in CO2 partial pressure leads to an increasing amount of CO2 transferred into liquid phase. as the gas flow increases the amount of CO 2 molecules available for the absorption increases. b) Effect of gas flow rate. This would lead to a higher mass transfer flux. but also the liquid flow rate and availability of reactive MEA in the liquid which as seen in this case controls the rate of mass transfer after the gas flow rate reaches the point. Note that KGae increases more rapidly at low flow rates compared to at high flow rates. the reduction in droplet size by the increasing liquid flow is insignificant. KGae increases with gas flow rate to a certain point and then remains constant. KGae increases with liquid flow rate. KGae decreases with CO2 partial pressure. The mass transfer may be mainly controlled by CO 2 reaction in the liquid. The rapid increase was caused by (1) a reduction in size of spray droplets from larger diameter to smaller diameter. This is because increasing the liquid flow increases effective interfacial area (ae). between liquid and gas. In general. causing KGae to reduce as partial pressure increases. This may be caused by the restricted diffusion and amount of reactive MEA in the liquid phase. This suggests the gas-phase controlled mass transfer takes place at low gas flow rates and the liquid-phase controlled mass transfer takes over at high gas flow rate.Effect of parameters on the performance of the packed-bed tower a) Effect of CO2 partial pressure. the overall rate of gas absorption is not only dependent upon the gas flow rate. leaving the increasing number of spray 47 The Realization of a Packed-bed Tower . the increasing mass flux occurs in a lower extent compared to the change in partial pressure. However. At the high liquid flow rate. c) Effect of liquid flow rate. However. By considering mass flux of CO2 absorption (NCO2). the spray is more fully developed with the smaller liquid droplets that offer higher ae.7 Packed-bed tower internals Bed limiter Table 7: The information of bed limiter Bed limiter Structured packing bed limiter (Noninterfering) Diameter Specification All column • Minimizes interference of high diameters performance liquid distributors Random packing bed limiter (Noninterfering) All column diameters • • Contour Requires no vessel attachments Minimizes interference of drip point from high performance liquid distributors 48 The Realization of a Packed-bed Tower . causing the KGae to decrease. As the liquid flow rate increases. d) Effect of MEA concentration. Such decrease in KGae is caused by an increase in solution viscosity. This is due to the fact that the increasing MEA concentration yields a higher amount of the active MEA available to diffuse towards the gas-liquid interface and react with CO2. resulting in a lower effective area (ae). It was found that KGae of a larger nozzle is lower that of a smaller nozzle at the low end of liquid flow rate. KGae decreases with CO2 loading. f) Effect of nozzle size. This is because the spray of the lager nozzle is not fully developed.droplets to be the primary factor that defined the lower increase in mass transfer performance. This shows that the solution viscosity is more influential on the effective area in the spray column than in the packed column. 3. This finding differs from the behavior observed in the packed column in that the KGae of packed column decreases by 5% for every molarity of MEA increasing. causing the KGae to increase accordingly. e) Effect of CO2 loading. KGae increases with MEA concentration. This is due to the fact that as the CO2 loading increases the amount of active MEA decreases. structure packings allow itself to be supported by a simple open grid structure due to the inherent construction of the packing.Random packing bed limiter All column diameters • • Fastens to vessel wall For use with traditional distributors Structured packing bed limiter/ Liquid distributor support All column diameters with structured packing • Supports tubular. The specified flow rate at the time of order placement will not limit the capacity of the packing they retain. • It must physically retain and support the packed-bed under operating conditions in the column including but not limited to packing type and size. and material buildup in the bed and surge conditions. a gas-injection type support is available for random packings due to the separation passages for liquid and vapor flow so that the two phases do not compete for the same opening. 49 The Realization of a Packed-bed Tower . there are some factors to be considered in choosing the design of a packing support which is compatible to the corresponding packed-bed. material of construction. corrosion allowance. Generally. design temperature. • It must have a high percentage of free area to allow unrestricted counter-current flow of down coming liquid and upward flowing vapor. On the other hand. bed depth. operating liquid holdup. However. Packing elements are retained with specific slot openings while the contour of the support provides a high percentage of open area. channel or trough type liquid distributors Used for large diameter columns to reduce structural components Provides limited uplift resistance • • Support plates Every packed-bed will need a support. [900 mm] Gas injection design Random packing gas injection support plate 12-48 in. low support strength requirements Table 9: The information of liquid collector Liquid collector Deck style Trough style Chevron vane Diameter All diameters Specification • For total or partial liquid draw-off • Suitable to feed a liquid distributor or trayed section below ≥ 40 in. [1000 • Permits thermal mm] expansion • Total or partial liquid draw • 25-40% open area ≥ 30 in.Table 8: The information of packing support Support plates Diameter Specification Contour Structured packing support grid All column diameters Supports all sheet metal or wire gauze packings Random packing gas injection support plate ≥ 36 in. [100-900 mm] Low hydraulic loading. [760 mm] • High vapor capacity • Low pressure drop • Can be used for drawoff or collection of liquid between packed-beds Contour 50 The Realization of a Packed-bed Tower . [3001200 mm] Gas injection design Light duty random packing support plate 4-36 in. 5:1 Channel distributor with bottom orifices No Contour 2-16gpm/ft2 (5-40 m3/m2h) 0.3-12gpm/ft2 (0.5-10gpm/ft2 (4-25 m3/m2h) 51 The Realization of a Packed-bed Tower .5-20 m3/m2h) Trough distributor with drip tubes No Pipe-arm distributor with orifices No High turndown ratio is available 2:1 (10:1 when using multilevel orifices at each discharge conductor) 2.75-20 m3/m2h) 0.Distributor Low flow rate (≤ 20gpm.75-20 m3/m2h) Trough distributor with enhanced baffle plates Pan distributor with V-Notch risers No 2:1 (5:1 if sufficient column height) - 1-8gpm/ft2 (2.3-20gpm/ft2 (0. 50m3/m2h) Distributor Re-distributor availability Yes Turndown ratio 2:1 Flow rate Channel distributor with drip tubes Yes 2:1 0.75-50 m3/m2h) 1.75-30 m3/m2h) Tubular distributor No 0.3-8gpm/ft2 (0.3-8gpm/ft2 (0. High flow rate (≥ 20gpm.5:1 2-40gpm/ft2 (5-100 m3/m2h) Spray nozzle distributor No 2:1 0. 50m3/m2h) Distributor Turndown ratio 2:1 Flow rate Deck distributor Re-distributor availability Yes Pan distributor Yes 2:1 ≥ 2gpm/ft2 (5 m3/m2h) Pan distributor with bottom orifices No 2.5-75 m3/m2h) Deck distributor with bottom orifices Yes 2.2-50gpm/ft2 (0.5-120 m3/m2h) Trough distributor with bottom orifices No 2:1 1-20gpm/ft2 (2.5:1 1-50gpm/ft2 (2.5-120 m3/m2h) Enclosed channel distributor for offshore applications No - 1-30gpm/ft2 (2.5:1 1-30gpm/ft2 (2.5-75 m3/m2h) Sample 4-80gpm/ft2 (10-200 m3/m2h) 52 The Realization of a Packed-bed Tower .5-50 m3/m2h) Trough distributor with weirs No 2. 4.0 Carbon Dioxide Capture System Equipments Figure 14: Carbon Dioxide Capture System Split flow configuration (Vozniuk 2010) Figure 15: Carbon Dioxide Capture System (Alstom and American Electric Power to Bring CO2 Capture Technology to Commercial Scale by 2011 2007) 53 The Realization of a Packed-bed Tower . it could provide the information about the essential equipment being used in the carbon dioxide capture system. Below are the list of the equipments being used and their function. Table 10: Essential Equipment of Carbon Dioxide Capture System Essential Equipment Absorber (Packed Bed Scrubber) Heat Exchanger Desorber/Stripper/Regenerator Condenser Pump Reboiler Reflux Drum/Liquid Separator Solvent Cooler Table 11: Optional Equipment of Carbon Dioxide Capture System Optional Equipment Semi Lean Flash Drum Semi Lean Cooler Reclaimer Feed gas cooler Table 12: Function of equipments Equipment Function Absorber It is function as the place to allow the feed stream components.By comparing both figures above. such as hydrogen. it is consist of the solvent that already absorb CO2 and leave the absorber. carbon and others come in contact with the solvent that absorb CO2 Solvent cooler It is function as a tools to cool down the solvent that recycle back from desorber in order to allow more absorption of CO2 Heat Exchanger The type of heat exchanger being used is shell and tube. For the hot stream. sulfur. On the other hand. Therefore. it is function as the tools to heat up the solvent leave the absorber but before entering desorber and cool down the solvent before reuse again to absorb CO2 in absorber at the same 54 The Realization of a Packed-bed Tower . the cold stream consists of solvent that required cooling down to allow more absorption of CO2. the solvent that being separated in the desorber will recycle back to the absorber for absorption of CO2 Condenser It is a device to reduce a gas or vapour into liquid. Beside that. Semi Lean Its function same as solvent cooler. it also provides a place from which noncondensable vapors may be vented. the liquefaction will occur. Semi Lean It is a device used to recover solvent under split flow configuration Flash Drum of carbon dioxide scrubbing process. Furthermore. Cooler 55 The Realization of a Packed-bed Tower . Beside that. it also provides more time for the operator to respond if they have exceeded the condenser’s capacity. At the same time. Water wash It is a place to balance water in the system and to remove any solvent droplets or solvent vapour carried over in order to prevent excess emissions of solvent together with vent gas. The purpose of introducing it can reduce the reboiler duty. it is operated by removing the heat from the gas or vapour. Reflux Drum It is a device to separate water from reflux when distilling particular substance.time Flue gas cooler It is the place allows the hot feed stream being cooled by cooling water so as to achieve acceptable absorption efficiency. once the heat being eliminated. Desorber In the desorber. Reclaimer It is a device used to remove the solids and degradation product. The CO2 being released will become concentrated with water before proceed to transportation and storage Reboiler It is function as a tool to boil the water that being removed in the desorber and transfer back to the desorber to drive the separation process. the CO2 will be emerged and also allow removing of water and traces of solvent. supplement change demand. check feed gas for Cause Upstream process entrained hydrocarbon Problem: Low feed gas flow rate Consequence Action Reduced amine Reduce amine flow. liquid Add antifoam (discriminately). Increase amine flow Cause Change in controller possible jet flooding Problem: Low amine Flow rate Consequence Action Potential reduction in Adjust conditions determining rate status or supply acid gas recovery 56 The Realization of a Packed-bed Tower . upset check relative gas/amine temperatures to determine likelihood of hydrocarbon condensation.2 psi/tray) Cause Consequence Action Possibly tray damage Poor efficiency. identify root cause (eg. potential feed gas with recycle or clean gas reduction in mass if warranted Cause Upstream process change transfer due to weeping Problem: High feed gas flow rate Consequence Action Increase amine demand. off-spec Corrosion) treated gas Problem: Differential pressure sudden increase.4. erratic action Cause Consequence Action Foaming. offMechanical repair spec treated gas Problem: Differential pressure gradual increase Cause Consequence Action Possible fouling Flooding. flooding Poor efficiency.1 Troubleshooting Absorber Table 13: Troubleshooting of Absorber(Amine Basic Practices Guidelines 2007) Problem: Differential pressure consistently low (normally 0. general plant reduce gas and/or liquid rates. efficiency. carryover.1-0. poor Cleanout. pressure Cause Change in controller Problem: High amine Flow rate Consequence Action Increased utility Adjust conditions determining rate status of supply consumption pressure Problem: Low feed gas temperature (normally 80-120 °F) Cause Consequence Action Change in upstream Reduced acid gas Increase temperature of feed gas process and/or ambient conditions Cause Change in upstream process and/or recovery in extreme and/or amine cases Problem: High feed gas temperature Consequence Action Potentially reduced acid Decrease feed gas temperature. Excessive moisture in treated gas. with potential downstream condensation and resultant Cause Change in upstream corrosion/fouling Problem: Low lean amine temperature Consequence Action Potentially reduced acid Reduce lean amine cooling or process and/or gas removal from high ambient conditions viscosity or low rate of supply heat reaction Problem: Low lean amine/feed gas temperature Differential (normally lean amine at least 10 °F hotter than feed gas) 57 The Realization of a Packed-bed Tower . or gas recovery increase amine flow rate to ambient conditions improve heat balance Problem: High lean amine temperature (90-130°F) Cause Consequence Action Change in upstream Potentially reduced acid Increase lean amine cooling process and/or gas removal due to poor ambient conditions equilibrium at high absorber temperature. add antifoam. SRU Correct absorber operation.Cause Change in upstream Consequence Condensations of process and/or hydrocarbon. rich amine upset or fouling clean up amine. skim hydrocarbon from flash drum Cause System venting to Problem: Low pressure Consequence May not get into the atmosphere or relief regenerator Action Find the source of leaking system Problem: Negative pressure Cause Consequence Action Relief stack draft causing a Air may be drawn into Adjust relief system vacuum on the system flash drum and pressure to hold positive contaminate the amine or pressure on drum 58 The Realization of a Packed-bed Tower . ambient conditions potentially resulting in Action Increase lean amine temperature foaming and/or Cause Overcirculation emulsification Problem: Low rich amine loading Consequence Action Excessive utility Reduce amine circulation rate Cause Undercirculation consumption Problem: High rich amine loading Consequence Action Reduced acid gas Increase amine circulation rate removal. corrosion Flash drum Table 14: Troubleshooting of flash drum(Amine Basic Practices Guidelines 2007) Problem: High pressure (45-65 psig for no rich amine pump while 0-25 psig with Cause Excessive hydrocarbon in rich amine pump) Consequence Action Regenerator foaming. for gas or liquid or system losses hydrocarbon separation to plant. find leak or loss (SRU upset) Lean/rich exchangers 59 The Realization of a Packed-bed Tower . foaming in due to low residence time absorbers or regenerators. absorber level strength strength problem. Check amine inventory. absorber upset and hydrocarbon carryover in holding up or losing rich amine to regenerator amine. imbalance in amine flows Cause Dehydrating amine Problem: Low amine level Consequence Action Flash gas or liquid Add amine or condensate system. add violation). Problem: High amine level Consequence Action Amine carryover into gas Remove some amine from absorbers returning amine system. check upset gas and liquid absorber operation hydrocarbon carryunder Cause Water leaking into system. skim regenerator (SRU upset) hydrocarbon from flash drum Problem: High hydrocarbon level (normally 0-5 % level above amine) Cause Consequence Action Insufficient skimming Amine foaming. foaming in antifoam. gas absorption (possible clean up amine. diluting amine plant. SRU Increase skim rate.Cause Hydrocarbon carryover from absorber cause an explosive mixture Problem: High flash gas rate Consequence Action Foaming and reduced acid Correct absorber operation. Upset of or shutdown of Determine cause for loss loss of reboiler heat downstream sulfur unit. clean exchangers Cause Fouling. high pressure drop lean amine temperature and low regenerator feed preheat Regenerator/Stripper/Desorber Table 16: Troubleshooting of Regenerator(Amine Basic Practices Guidelines 2007) Problem: Low or decreasing reflux drum pressure Cause Consequence Action Failed pressure controller.Table 15: Troubleshooting of lean/rich exchangers(Amine Basic Practices Guidelines 2007) Cause Low fuel rate Problem: High rich amine temperature Consequence Action Flashing and corrosion in Check amine flows. 60 The Realization of a Packed-bed Tower . source. equipment of demand Problem: High pressure Consequence Reduced circulation. exchangers and Cause Exchanger fouling possibly bypass hot lean regenerator inlet amine flows Problem: Low rich amine temperature Consequence Action Poor stripping in Check all temperatures for regenerator and/or poor performance increased reboiler steam (fouling). Minimize velocities by optimizing steam consumption. loss of feed. of feed or heating medium. unit install reflux purge and containment clean water backup to shutdown control corrosion. loss of release to atmosphere. Action Locate the point of high failure reduced heat transfer. drain line. downstream unit shutdown temperature. increased lean reflux drum. remove degradation products 61 The Realization of a Packed-bed Tower . decreased throughput. drain contamination degradation hydrocarbon liquid from reflux drum. steam overhead line. check control failure. plugging upset contactor level control and from corrosion products or flows.Problem: High or rising reflux drum pressure Cause Consequence Action Downstream unit Relief. drain side. loss of cooling media acceleration of amine drain hydrocarbons from degradation. shutdown and clean overhead line Problem: High top pressure for conventional condenser/drum overhead systems Cause Condenser fouled and (normally 5-15 psig) Consequence Upset of downstream Action Reduce feed. drum. relief. drum. hydrocarbons from flash the cooling media side. unit shutdown. steam out hydrocarbon or upset accelerate amine liquid gas product. reboiler duty. blocked outlet reduced heat input. reduce problems. raise reflux excessive reboiler duty. shutdown and loadings. failed pressure increased lean loadings. flooded vessel. SRU level control. foaming and reboiler duty. check rich amine salts. flow treating capability. condenser fouled on excessive entrainment. hydrocarbons from flash controller. loss of Check rich flash drum contactor levels. unit shutdown. high regenerator feed circuit or plugging in pressure valves. reduce plugged on the process sulfur unit. Remove heat stable salt anions and sodium. Reduce feed. reduce clean overhead line throughput Problem: Sudden loss of rich amine feed rate Cause Consequence Action Loss of flash drum or Loss of throughput. orifices filters or exchangers. downstream unit unit shutdown Steam out of otherwise shutdown. relief. plugged heat overhead line. loss of Restore feed. Restore reboiler heat input. 62 The Realization of a Packed-bed Tower . decrease circulation. Increase reboiler heat input caused by: excessive corrosion insufficient heating media input. loss of return temperature containment. loss of Loss of throughput. Skim rich amine regenerator. Balance reflux purges and makeup water rates. treating capability.(reclaim). Problem: Sudden loss of acid gas product rate Cause Consequence Action Loss of feed. Check for plugging from corrosion contactor foaming or products or salts. pressure reflux or pump around controller failure. equipment draining). treating capability of system. reboiler heating media. raise overhead line. increase amine strength. upset/shutdown of flash/reflux drums for tower internals downstream sulfur unit hydrocarbon. leaks or upset. loss of Balance flows in and out drum level resetting flow. foaming. change malfunction carbon filter Problem: Lean loading exceeds spec/treated gases and liquids fail to meet spec Cause Consequence Action Insufficient reboiler heat Off-spec products. tower internals malfunction Problem: Sudden increase in acid gas product rate Cause Consequence Action Hydrocarbon intrusion into Amine carryover. Check for open drains maintenance activity (filter changes. check for hydrocarbons to regenerator Problem: Gradual decline in rich amine flow rate Cause Consequence Action Declining rich amine flash Loss of throughput. loss of High lean loadings to off. loss of sodium level regenerator level. add antifoam until pressure drop is normal • Monitor carbon filter. high lead to stop amine loss. unit of feed. hydrocarbon normal Consequence Amine and/or hydrocarbon • intrusion from flash drum. change if 63 The Realization of a Packed-bed Tower . remove heat stable salt fouled lean/rich anions and sodium. if tower.Determine reason for loss reboiler heating media. tower spec products. if high in Action Determine hydrocarbon increase purge • Test feed and bottoms for foaming tendency.supply. spec products. clean exchangers. check reboiler. leaking lean/rich exchangers when rich amine exceeds lean amine pressure. tower shutdown downstream for loss of heating media internals malfunction sulfur unit shutdown Problem: occasional sudden rise in tower pressure drop then returning to Cause Foaming. tray purge of reflux to stop internals malfunction blowout amine loss. caustic contamination Problem: Little or no tower pressure drop Cause Consequence Action Loss of feed. over-circulation. fouled fouled exchangers. reboiler heat input loadings leading to off- high in amine stop flunctuations. carryover into downstream composition of reflux hydrocarbon refluxed to sulfur unit. determine reason tray blowout. salts anions and sodium reflux. insufficient heat increase filtration costs • input leading to and increased plugging degradation products Remove amine excessive lean loadings.necessary • Skim rich amine flash drum and reflux drum for hydrocarbon • Ensure proper levels are maintained in rich flash and reflux drum. change carbon filters to damage increased energy prevent antifoam • consumption. or flow • Reduce feed rate • Reduce heat input Problem: Gradual buildup or sudden permanent buildup of tower pressure drop Cause Consequence Action Reduced throughput. off-spec products. • Remove heat stable concentrations in higher corrosion rates. check level instruments • Check reboiler heating medium control for flunctuating pressure. (reclaimer) under-circulation • leading to excessive chemical or water wash Shutdown and 64 The Realization of a Packed-bed Tower . high • Buildup of • Discontinue corrosion products lean loadings leading to antifoam additions and causing tray plugging. reduced buildup and foaming circulation leading to episodes Excessive corrosion rates caused by high ammonia/amine higher rich loadings. temperature. too system Consequence Increased lean loadings Action Raise reboiler heat input. increase 65 The Realization of a Packed-bed Tower . loss system Consequence Overtax overhead system Action Reduce reboiler duty of reflux or pumparound leading to excess water Re-establish cooling media. Raise condenser cooling excessive rich loadings. increase cooler cooling media amine in reflux water by Reduce circulation entrainment and/or vaporization. high heat tower stable salt anion content • • size. reflux products pumparound return level or pumparound flow temperatures.rich loadings. Clean rich amine Excessive particle Reduce filter pore accumulation caused by flash drum bottom poor filtration. filter • replacement. • increased filter pore filtration upstream of size to control filter regenerator replacement cost. keep rich accumulation of loadings at or below particles in rich amine recommended levels Add additional Keep lean loadings flash drum Problem: Low top temperature for conventional condenser/drum overhead Cause Insufficient heat input. minimal. too low loss and poor downstream condenser/pumparound loadings sulfur unit feed. control valve failure. media temperature tower internals malfunction Problem: High top temperature for conventional condenser/drum overhead Cause Too high heat input. cold reflux or pumparound leading to off-spec Raise reflux or return temperatures. pump and throughput unit failure • Action Check controller. fouled or plugged levels reducing circulation lost. Determine if feed Determine if heating media lost. loss of • media.corrosion rates throughout regenerator. pumparound temperature temperature control • problem input Cut cooling media Raise reboiler heat Reflux Pump Table 18: Troubleshooting of Reflux Pump(Amine Basic Practices Guidelines 2007) Cause Control failure. loss of cooling pumping problems. leaks. with hydrates of NH3 readings Action Confirm instrument insufficient heat input. correct upstream overhead exchanger. excess energy costs Reflux Drum Table 17: Troubleshooting of Reflux Drum (Amine Basic Practices Guidelines 2007) Problem: Acid gas product/Reflux temperature reads below 90 °F (normally 90Cause Too much cooling media. 66 The Realization of a Packed-bed Tower . • reflux level control rate or raise its problem. 130 °F) Consequence Plugged overhead line • too cold cooling media. loss of reboiler leading to heat/mass insure correct valve heating media. loss of Problem: Loss of reflux Consequence Increased amine strength • feed. transfer problems or trim and metallurgy plugging. accelerate amine degradation. accelerates corrosion. increase in rich downstream sulfur unit. insufficient reboiler heat Problem: Reflux ratio too low Consequence High lean loadings leading to off-spec product • Action Check/repair controller input. • Compute water too low tower top plugged overhead lines balance and adjust temperature setting. plugging. pump • problems input • Raise reboiler heat Clear plugging in reflux system Problem: Loss of pumparound flow Cause Consequence Action Loss or decline in heat Excess water to • Increase heat input input. raise temperatures which circulation insufficient makeup water. leaking draw tray. erratic amine strengths media flow Increase lean Restore cooling coolant side of exchanger • fouled balance and adjust Compute water purge/makeup • Clean pumparound cooler Problem: Increase in pumparound flow Cause Consequence Action Improper water balance. to reboiler loadings.correct supply problem • Determine if cooling media lost or restricted Cause Controller problems. loss of cooling increase in pressure which • media. Erratic amine strengths. • too large purge rates. purge/makeup rates condenser leak • Raise tower top temperature • Analyze chloride 67 The Realization of a Packed-bed Tower . • Reduced removal exchanger fouling. Close cooler bypass. shutdown and repair condenser Lean amine cooler Table 19: Troubleshooting of lean amine cooler(Amine Basic Practices Guidelines 2007) Problem: High temperature (Normally 90-130 °F) Cause Consequence Action Cooler bypass open. rapid loss of cooling may be an indication of mechanical problems Cause Cooler bypass closed. check cooling liquid/liquid contactors water supply. loss of efficiency calculate heat transfer cooling water • coefficients and clean Hydrocarbon vaporization in exchangers.level of amine. loss of heat to regenerator Problem: Low temperature Consequence Condensation of • Action Open cooler bypass hydrocarbons in absorbers • Check regenerator bottoms temperature for deviation Cause Fouling. reduced heat transfer Reboiler Table 20: Troubleshooting of reboiler(Amine Basic Practices Guidelines 2007) 68 The Realization of a Packed-bed Tower . equipment failure Problem: High Pressure Consequence Reduced circulation or Action Locate the point of high high lean amine pressure drop temperature leading to offspec product. • Isolate leak • Remove heat stable salts and anions • Remove amine degradation products (reclaim) Problem: High reboiler temperature Cause Consequence Action Tower overhead or acid Increased corrosion rates • Clear overhead line gas product line plugged leading to filter plugging pressure restrictions or pressure control and equipment fouling and • problem. too hot heating plugging.Problem: Low reboiler temperature Cause Consequence Loss of reboiler heating High lean loadings leading • Action Confirm initial and media. final state and flows of or lean/rich exchangers emission of toxic gases to heating media and due to excessive corrosion atmosphere calculate duty to rates. corrosion temperature and Confirm 69 The Realization of a Packed-bed Tower . heat balance loss of contaminant • Compute tower Compute required heat requirement and compare to available heat • Compute reboiler and lean/rich exchanger heat transfer coefficients to determine fouling. failure of exchanger reflux/pumparound control • leading to overcooling. fouling of reboiler to off-spec product. Clean exchangers if fouled. • Restore supply of supply. heat input improper design of • reboiler. adjust as needed causing higher pressure. extended Increase steam flow until controller malfunction reclaiming run bumping or violent boiling 70 The Realization of a Packed-bed Tower . high amine salt anions and sodium degradation product levels • Decrease reboiler Remove heat stable Remove amine degradation products • Evaluate heat transfer equipment for coefficient and hydraulics • Clean reboiler and/or lean/rich exchangers Problem: Loss of heating media flow Cause Consequence Action Controller failure. acceleration of • high heat stable salt anion amine degradation drum/reflux drum for Skim rich flash and sodium content. condensate system off-spec product bottleneck heating media • Lower condensate system pressure Reclaimer Table 21: Troubleshooting of reclaimer(Amine Basic Practices Guidelines 2007) Cause Improper setting. hydrocarbons improper circulation of • amine through reboiler. decreased media. too high heat input.media. throughput. loss of Loss of treating capability. or Problem: Low steam rate Consequence Action Low boil up. damage leading to reboiler pressure of heating hydrocarbon incursion failure. • Conduct observation of the stack and areas adjacent to the stack to determine 71 The Realization of a Packed-bed Tower .is heard. bumping of violent boiling is no longer heard Cause Improper set point or inability to clean up MEA Problem: High feed rate Consequence Carryover of salts into controller malfunction regenerator. stop MEA. inability to Cause Improper set point or clean up MEA Problem: Low feed rate Consequence Exposed tubes may cause controller malfunction thermal degradation of Cause High steam pressure (above about 90 psig) or Action Re-adjust level Action Re-adjust level amine on hot tube surface Problem: Low temperature Consequence Action Thermal degradation of End the batch run. or. possible corrosion steam flow. corrective action is recommended to be taken within 8 hours to return the parameter to normal. or Problem: High steam rate Consequence Action Bumping.d.2 Maintenance plan for Carbon Dioxide Capture System Table 22: Maintenance Plan of Carbon Dioxide Capture System(Example Packed Bed Wet Scrubber Agency Operation & Maintenance Plan n. carryover of Decrease steam flow until controller malfunction salts into regenerator.) • Daily Check all the indicators. if steam. temperature indicator and others to ensure that they are not out of the normal operating range. If one of them out of the normal operating range (to be specified by the facility). such as pressure drop indicator. salt concentration in pressure is problem reduce reclaimer too high it as appropriate 4. then reduce slightly Cause Improper setting. If the droplets reentrainment is occurring. Quaterly (quarter of a year) • Conduct a walk around inspection of the entire system to search for leaks. If the liquid pressure is out of normal operating range (to be specified by the facility). header pluggage and nozzle erosion. The sign of droplet reentrainment may include fallout of solid-containing droplets.if droplet reentrainment is occurring. Pluggage problem are indicated by higher than normal pressure and erosion problem indicated by less than normal pressure. the appropriate action is recommended to be taken within 8 hours 72 The Realization of a Packed-bed Tower . corrective action is recommended to be taken within 8 hours. Weekly • Check liquid pressure gauges on supply headers to the scrubber to monitor for problems such as nozzle pluggage. the appropriates action is recommended to be taken within 8 hours • Semi-annually Conduct an internal inspection of the system to search for signs of:  Corrosion and erosion  Solids deposits in the equipments  Plugged or eroded spray nozzles If any of these condition exists. or a mud lip around the stack. corrective action is recommended to be taken within 8 hours. discoloration of the stack and adjacent surfaces. If there is leaking detected in the system. 0 Cost Estimation Scrubbing Reagent Below are the estimated specification and the market price of the scrubbing reagent.90 / kg 73 The Realization of a Packed-bed Tower .5.80 / kg RM 5. Table 23: Estimated specification of Scrubbing Reagent Specification Operating Temperature (°C) Operating Pressure (bar) Inlet Solvent Flow Rate (kgmole/h) MEA content in amine (mass %) Total usage (kgmole/month) Monoethanolamine (MEA) 40 Potassium Carbonate (K2CO3) 40 1 148000 1 148000 29 - 28416000 28416000 Table 24: Market Price of Scrubbing Reagent Scrubbing Reagent Monoethanolamine (MEA) Potassium Carbonate (K2CO3) Primechem Malaysia Sdn Bhd RM 6.50 / kg - Best Chemical Co. (S) Pte Ltd RM 6. On the other hand. combination of membrane technology and absorption technology result in gas absorption membrane. there are some modifications can be made. Furthermore. algae can remove carbon dioxide as well. there are still a lot of improvements to go in order to enhance the performance of Carbon Dioxide Removal Process. there are some research mention that room temperature ionic liquid can be used as the scrubbing reagent as well but still under research phase.0 Recommendation From the technology available to capture carbon dioxide. which is the combination of advantages of two or more different technologies.6. it is the same goes to scrubbing reagent. 74 The Realization of a Packed-bed Tower . Therefore. So that. it got potential to classify as scrubbing reagent in the future. plantation of algae near the source of CO2 can be one of the solutions for the climate change issues. such as produce a hybrid system. Therefore. In addition. adsorption technology and cryogenics need to be developed further in order to increase their performance before being commercialized since it is cheaper compare with absorption technology. Beside that. it may produce better scrubbing reagent for carbon dioxide removal by increasing its capacity. Therefore. For example. packing support and mist eliminator. Therefore. However. which means using of physical solvent for higher concentration of carbon dioxide in feed stream and chemical solvent for lower concentration of carbon dioxide. the scrubbing reagent being used depends on the concentration of carbon dioxide in the feed stream. 75 The Realization of a Packed-bed Tower .7. However. packed bed scrubber is the type of scrubber that most often used in the carbon dioxide capture system.0 Conclusion As a conclusion. it is still under development stage to enhance its performance even there is a lot of CO2 capture already exists in the world. it has met the objective of understanding the purpose of gas scrubber system being used in the industry. the best solution in preventing the greenhouse gas effect effeciently is still an unknown. 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STANFORD. such as pressure vessel and heat exchanger • Learn about the types of machine involved to fabricate a particular product • Learn about the components that used together with fabricating a particular product.0 Appendices Appendix A: Learning Outcome Academic From this industrial training. such as flanges. such as welding tools. the way of welding working. the learning outcome from the aspect of academic are shown as below: • Learn about the process need to be go through in order to fabricate some of the equipments.9. welding consumables parts and others • Learn about the side factors need to be considered in order to accomplish the particular product 84 The Realization of a Packed-bed Tower . gasket and others • Learn about the safety and precaution when working in the engineering factory • Learn more about welding process. • Learn one of the major problem faced by the industry. the learning outcome from the aspect of non-academic are shown as below: • Learn about the culture and principle of the company in working • Learn the way of communicate with supplier • Experience the working life as a research engineer • Experience the working environment of an engineering company. which is team work environment • Learn better way to manage the workload given in order to be punctual • Experience the realistic side of a company during working 85 The Realization of a Packed-bed Tower . packed bed scrubber internals and others • Learn the problem that may occur in the system and the action to fix it Non-academic From this industrial training. such as scrubbing reagent. which is the effect of greenhouse gas • Learn the way available to solve greenhouse gas effect of industry • Learn the flow of carbon dioxide scrubbing process and some different flow configuration in order to achieve cost effective • Learn the transport and storage of carbon dioxide under carbon dioxide capture and storage technology • Learn the consideration that involved in order to choose the best option for the scrubbing process. such as filter vessel and plasma arc welding. such as experience on building up a pilot plant and research on the renewable energy technology. it does help us learn a lot from the aspect of academic and non-academic as an eye opener to the industry.• Expose to the strategy of a company in order to go through numerous challenges and survive in this competitive era • Realize the important of action and planning in the way to achieve the target • Being shared with the success story of the company and realize the process in order to develop the company from nothing become better Comment During this industrial training. Beside that. This is because the tasks given involved different attribute can be learned. looking deep into the equipments already fabricated and process already used in the factory in order to look for further improvement. 86 The Realization of a Packed-bed Tower . overall is good. Then. welding joint and others instead reading on the guideline only. such as the angle of welding. Appendix B: Advanced Research and Development Pathways Below are the future pathways of the research on carbon dioxide capture technology in order to achieve improvements. the company may provide more practical work instead of research work. Solid Adsorbents • Metal-organic Frameworks • Functionalized Fibrous Matrices 87 The Realization of a Packed-bed Tower . For example. there are some recommendations as well. we can perform as well instead of only know the knowledge. Furthermore. we may work in this industry in the future. Firstly. Beside that. so that. we can know better about the proper consequence of welding from the aspect of quality. potential scrubbing reagent and others. which mean work on our own hand under a qualified welder by using the waste material. Hence. arrangement of intern student to experience about the welding process.However. The benefit of this practical work is providing more knowledge for the way of welding. Thank you. that’s all my comment and recommendation regarding to this industrial training and I did appreciate this opportunity to join the training programme provided. such as reduce energy consumption for solvent recovery stage. the complete cost estimation of carbon dioxide capture system. the control loop. which are the controlled system needed for the carbon dioxide capture system. there are some researches still need to be done.• Poly (ionic liquid) Structured fluid absorbents • CO2 hydrates • Liquid crystals • Ionic liquid Non-thermal regeneration method • Electrical Swing Adsorption • Electrochemical methods Remarks: Before proceeding to the advanced research topic. 88 The Realization of a Packed-bed Tower .