AIChE T4 174869Turboexpander-Compressor Technology for Ethylene Plants Radjen Krishnasing Senior Lead Process Engineer The Shaw Group Gabriele Mariotti Engineering Manager GE Oil & Gas Florence, Italy Kara Byrne Applications Engineer & Commercial Manager GE Oil & Gas Houston, TX, USA Radjen Krishnasing Senior lead process Engineer The Shaw Group Prepared for Presentation at the 2010 Spring National Meeting San Antonio, TX, March 21-25, 2010 AIChE and EPC shall not be responsible for statements or opinions contained in papers or printed in its publications Turboexpander-Compressor Technology for Ethylene Plants Radjen Krishnasing Senior lead process Engineer The Shaw Group Gabriele Mariotti Engineering Manager GE Oil & Gas Florence, Italy Kara Byrne Applications Engineer & Commercial Manager GE Oil & Gas Radjen Krishnasing Senior lead process Engineer The Shaw Group Abstract Today’s ethylene plants incorporate Turboexpander Systems to optimize cryogenic recovery and reduce the energy demand. The molecular weight and flow rate of the residue gas depend directly on the selected upstream feedstock gas composition, conversion, and feedrates. Various recent ethylene units have generated residue gas volumetric flow ranges from approximately 100-200%. Hence, the Turboexpander system is designed and manufactured accordingly. As we are aware, the typical naphtha cracker produces a methane rich residue gas (bulk hydrogen is recovered, treated, and delivered as a high pressure co-product). On the other hand, the typical ethane or E/P cracker and hence. Depending on the plant fresh feedstock and the potential hydrogen pre-recovery. In order to meet the above demands. integrated into the cold fractionation cryogenic section of an ethylene unit. we will discuss the typical performance of one. Key mechanical design recommendations (e. multistage control. a correspondingly wide range of residue gas composition and quantity. producing a cryogenic stream that can be 40oC to 50oC lower than the lowest level of ethylene refrigerant. therefore. Most ethylene units are designed to crack either a light feedstock.produces a very high hydrogen content residue gas. Part A Radjen Krishnasing Introduction Turbo-expanders/re-compressors play a crucial role in the recovery of both ethylene and hydrogen from cracked gas in steam cracking units.. back to refrigeration at the lowest temperature levels. Turbo-expanders take the tail gas (mixture of hydrogen and methane) at high pressure and low temperature and drop the pressure over the expander with isentropic efficiencies of well more than 80%. the Turboexpander solution must be flexible. As an overview. or a heavy feedstock. Based on the demand from the different feedstocks and the industry requirements for feedstock flexibility. such as ethane/propane. high head wheels) will be outlined. the warmed up tail gas is compressed by the re-compressor to fuel gas pressure level. magnetic bearings. The driver of the re-compressor is the expander that conveys the energy liberated by the expansion through a common shaft. the tail gas can be very rich in methane for one feed or very rich in hydrogen for another. we will then discuss the technology and mechanical solutions.and two-stage Turboexpander solutions for the expansion and recompression of the residue gas. A turboexpander converts energy that has been incorporated into the cracked gas. These cryogenic streams are then used for refrigeration to retrieve the last minor portion of ethylene from the tail gas that otherwise would have been lost.g. This presentation will also include related design improvements that have been successfully utilized in other Turboexpander applications. Current designs and revamps require a wider range of feedstocks. such as naphtha or heavier . by the cracked gas compressor and by the ethylene/propylene refrigeration systems. Turbo-expanders are. to further enhance the recovery of ethylene and hydrogen. After providing refrigeration. variable nozzles. Effects ethylene plant feedstock A critical parameter in the integration and design of turbo-expanders is the composition of the tail gas (mixture of hydrogen and methane). It produces a tail gas that has a close resemblance to naphtha or gasoil.14 Tail gas H2 / CH4 ratio 4. which means that the . Table 1: The molar ratio of key components / ethylene in cracker effluent for typically used feedstocks.22 0.44 0. in particular ethane.53 0.02 0. Table 1 below demonstrates the yield patterns of different feedstock. However.83 0.30 Cracked Gas CH4 / C2H4 0. produces a high ratio of hydrogen to methane. To the contrary. Hydrogen is recovered at high pressure (3000 kPa).19 0.07 0.24 0. It shows a noticeable difference between ethane feed and any other feedstock: .07 0. - Naphtha and gasoil as feed produce a relatively low ratio of hydrogen to ethylene. Feedstock type Ethane Propane Naphth Gasoil a Cracked Gas H2 / C2H4 1.01 0. therefore requiring very high recovery of hydrogen as product. there are units with a much wider range of feedstock. Recovery of hydrogen as product can be as high as 90%. However. Cracking a light feedstock. - Propane as feedstock has a very interesting mid-position. It produces very low ratio of methane leaving a tail gas high in hydrogen. The need is limited to the hydrogenation of acetylenes and small quantities of high purity hydrogen product. but a very high ratio of heavy byproducts to ethylene.24 C2H4 Cracked Gas Pygas / C2H4 0. a typical ethylene complex based on ethane (or ethane/propane) needs very little hydrogen as product.63 0. an ethylene unit cracking naphtha or heavy liquid feedstock produces a lower ratio of hydrogen to methane but demands much more hydrogen co-product for the hydrogenation of unsaturated by-products that have been produced. for use by downstream polymer units.51 0.Ethane as feed produces the highest ratio of hydrogen to ethylene. These fractions often require hydrogenation to either serve as recycle feed to the cracking furnaces or as finished product of the ethylene plant. expressed in component molar ratio with respect to ethylene. while the ratio of heavier byproducts to ethylene is the lowest.62 Cracked Gas (C4 & C5) / 0. A typical ethylene unit cracking liquid feed is therefore characterized by a very high recovery of hydrogen to balance this need.15 0.liquid feedstock.48 Ethylene plants cracking primarily liquid feedstock produce relatively high ratios of unsaturated C4 and heavier fractions.23 1.05 0. Propane can act as a buffer for the heavy feedstock in ethylene plants designed with a broad range of feed slate such as a unit to crack a combination of ethane and heavy feeds. As a result. as a lighter tail gas will have a richer ethylene content. On the contrary. a low ratio of methane and an insignificant amount of C 4 and heavier fractions. A minimal variation of composition and flow rate to the turboexpander is then often caused by the extent of hydrogen recovery. It is also not very sensitive to the cracking severity because the high hydrogen recovery results in a residue gas feeding the turbo-expander that is very rich in methane. are not affected if the feedstock cracked by the ethylene unit does not vary over a wide range from heavy to light naphtha. the recovery of hydrogen as a product is little to none.000 KTA ethylene (1 million metric tons per year). . The variations in composition. producing a nominal product rate of 1. The first case is for an ethylene plant where the predominant feedstock is naphtha. or the overall plant capacity.recovered hydrogen can no longer be part of the tail gas that feeds the turbo-expander. However. maximizing the available tail gas for the turbo-expander is a critical parameter in reducing the loss of ethylene. and frequently the flow rate of the tail gas to the turbo-expander. only a single stage is needed. The challenge in the integration and design of the turboexpander is to find the optimal balance between maximizing hydrogen recovery while maintaining a reasonable flow to the turbo expander to minimize loss of ethylene. Case study The following two cases are presented to further emphasize the design challenges when specifying and selecting a turbo-expander. Hydrogen recovery is maximized while minimizing the loss of ethylene in the tail (or residue) gas. an ethylene plant cracking ethane or a combination of ethane/propane is characterized by a very high ratio of hydrogen to ethylene. This case will demonstrate that with the integration of a turbo-expander. meaning that virtually all of the tail gas is available as feed to the turbo-expander. 746 kilowatts Expander Inlet Flow rate (kmol/hr) Molecular weight Pressure (kPA) Temperature( oC) Expander outlet Naphtha feed cracking Higher Hydrogen recovery (less flow through turboexpander) Lower Hydrogen recovery (more flow through turbo.0145 psi / kPA) kW Hp = 0.5 3050 -97 13. Key item clarifications: Am3 / min Actual cubic meters / minute dHs Isentropic enthalpy difference between inlet & outlet kmol / hr 1000 moles per hour kPA kilo pressure atmospheric Ξ (0.2 3050 -103 .expander) 2400 2176 2850 14.0 3050 -100 14.Table 2: Overview liquid (Naphtha) feedstock cracking. ethane or ethane/propane. it produces a high ratio of hydrogen and a low ratio of methane.52 356 -3 194 13.99 354 -3 214 14. . resulting in a low ratio of hydrogen and a high ratio of methane.2 357 -4 250 604 604 604 111 105 118 28.640 30. It is based on 1. require two single-stage expanders in series. The opposite is true if propane is cracked. A turbo-expander designed for a hydrogen rich feed will.5 million metric tons per year).500 KTA ethylene production rate (1. that when ethane is cracked. in general. The limitation is imposed by the recompressor section as is discussed in the second part of this paper.000 970 870 1140 86 86 85 The second case (Tables 3A and 3B) is for an ethylene unit cracking light feedstock.Flow rate (Actual 69 m3/min) Re-compressor Inlet Flow rate (kmol/hr) Mole weight Pressure (kPA) Temperature ( oC) Flow rate 3 (Am /min) Re-compressor Outlet Pressure (kPA) Expander dHs (kJ/kg) Turbo-expander RPM Expander power (kW) Expander efficiency (%) 63 83 2400 2176 2850 13. As can be seen from Table 1.630 27. . ethane/propane) feedstock cracking (Low-Pressure Machine) 100% C2 feed crackin g 50/50 C2/C3 feed cracking HP Expander Inlet Flow rate (kmol/hr) Mole weight Pressure (kPA) Temperature( oC) 7935 4.91 1165 -135 7742 7. ethane/propane) feedstock cracking (HighPressure Machine) Key Item Clarifications: Refer to Table 2 Table 3B: Overview light (ethane.72 532 43 647 7567 7.340 1355 1265 89 86 .17 636 60 550 740 130 740 80 20.66 2131 -118 HP Expander outlet Flow rate (Am3/min) LP Expander Inlet Flow rate (kmol/hr) Mole weight Pressure (kPA) Temperature( oC) 127 121 LP Expander outlet Flow rate 215 (Am3/min) HP Compressor Inlet Flow rate (kmol/hr) Mole weight Pressure (kPA) Temperature ( oC) Flow rate (Am3/min) HP Compressor Outlet Pressure (kPA) Expander dHs (kJ/kg) Turbo-expander RPM Expander power (kW) Expander efficiency (%) 7837 4.240 1360 1255 86 85 LP Compressor Inlet Flow rate (kmol/hr) Mole weight Pressure (kPA) Temperature ( oC) Flow rate (Am3/min) LP Compressor Outlet Pressure (kPA) Expander dHs (kJ/kg) Turbo-expander RPM Expander power (kW) Expander efficiency (%) 100% Ethane feed cracking 50/50 Ethane/propa ne feed cracking 7923 4.17 541 43 612 630 124 636 82 20.Table 3A: Overview light (ethane.72 630 63 580 7567 7.000 16.42 1175 -134 207 7836 4.94 2131 -114 7883 7.000 16. As can be seen in the second column in Table 2. The naphtha case demonstrates the effects of higher or lower hydrogen recovery than the design recovery of the turbo-expander. As the demand for raw C 4 and perhaps also raw C5 as finished co-products without hydrogenation increase. the overall isentropic drop in that case would be 250 kJ/kg. A single-stage design is therefore very common for naphtha (or other liquid/LPG feedstock) based ethylene plants. but it will provide less refrigeration because of the reduced flow rate through the turbo-expander. A higher recovery of hydrogen can be desired in plant operations as a way to produce more product hydrogen. it is ultimately letdown to the fuel gas header and combusted in the cracking furnaces. Instead of letting the product hydrogen across a control valve (isenthalpic). This will reduce the total flow through the expander. As a general guideline. For our ethane cracking case. while at the same time increasing the molecular weight. The isentropic enthalpy drop across each expander stage is kept around 125kJ/kg. the isentropic enthalpy drop is in the order of 110kJ/kg – a number that falls in this range and does not provide unusual constraints to the design of the turbo-expander. an enthalpy drop of up to 180kJ/kg is considered to set an optimal basis for the turbo-expander design. the constraint is not the expander side but the compressor side. this number is indicative of the expander or re-compressor wheel tip speed. As can be seen from the tables. If there is no other output for product hydrogen. the turbo-expander is still within its operable range. For our naphtha case. Although using a single stage expander is not impossible. This will have to be taken into consideration when deciding on increasing recovery of hydrogen. becoming the limiting factor in the design. it would be more beneficial to pass this . In general. an ethylene plant cracking liquid feedstock can end up with excess hydrogen product.Further evaluation/observations An important turbo-expander design parameter is the isentropic enthalpy drop (dHs) across the expander. As discussed in the second part of this paper. The re-compressor rotor is therefore the larger of the two wheels. a two-stage turbo-expander/re-compressor design is used. the volumetric flow of gas flowing into the recompressor is nearly five times higher than the expander outlet flowrate. While the naphtha case turboexpanders use a 225mm expander wheel and the gas case a 350mm wheel. When it comes to turbo-expanders however. hardly has affected the high (isentropic) efficiencies the industry has relied upon. a mixed feed case of ethane and propane is less stringent to the operation of the turbo-expander. The gas cracker case evaluation demonstrates the simple fact that in case a turbo-expander is designed for the tail gas of an ethylene plant cracking ethane (tail gas very rich in hydrogen). This feature continues to make turbo-expanders a very important choice in maximizing the economics of ethylene plants. these are by far not the largest sizes used in other branches of the industry for turbo-expanders. which has been seen since the early use of turbo-expanders. from small ethylene units to today’s mega-size plants. The first column of Table 3A and Table 3B are for pure ethane feedstock. It is also interesting to note that the scale-up. the sizes are far from reaching their maximum.excess of hydrogen through the expander. it will reduce the refrigeration demand from ethylene/propylene refrigeration systems. Table 2 shows that the increased flow rate combined with a reduced molecular weight will increase the RPM of the turbo-expander. Part B Gabriele Mariotti Kara Byrne . while the second column of each is for a 50/50 ethane/propane case. In these days of mega-size steam cracking units. The third column of Table 2 (the naphtha case) demonstrates the effects this will have. such as separation columns. How much hydrogen can be diverted to the turboexpander is a function of how much room is available in the design of the turbo-expander. serious challenges are presented to the sizes of major compressors and other equipment. More hydrogen across the expander will result in more cryogenic duty from the turbo-expander. and as an overall effect. A typical design comfortably will accommodate an increase such as demonstrated in the table. This paper. Dr. Dr. Judson Swearingen. is greatly improved by replacing the JT Valve with a simple and reliable machine that expands a single-phase stream in a nearly isentropic method. it is increasingly common to find a turboexpander as a key component for the overall production in a hydrocarbon gas separation plant. Carl von Linde. After World War II.Foreward The importance of turboexpanders has increased significantly over the past few decades since the first application of a turboexpander in the oil and gas industry by the founder of Rotoflow. turboexpanders were used to replace a Joule-Thompson (JT) valve in order to increase the overall efficiency of air separation plants. when it is directly coupled to a compressor the interaction of the two machines must be taken into account. . Turboexpander History The turboexpander is a reaction type radial turbine originally developed to replace the Joule-Thompson (JT) valve in air separation plants. The fact that the radial inflow turbine could handle two-phase flow at the discharge made the machine perfect for heavy hydrocarbon removal. further developed and improved the turbines for many other applications. He realized the overall cooling capacity of the plant and. including Dr. will focus on some typical turboexpander compressor selections showing the interaction between the selection of the turboexpander and re-compressor. cannot be optimized beyond the mechanical limitations of each machine. Judson Swearingen began to develop the turboexpander for natural gas processing applications (Photo-1). This is especially important for designing a more efficient and competitive ethylene plant. utilized the first radial turbine for air liquefaction in the early 1900s. George Claude. therefore. Driven by increased competition in the oil and gas market. the cost and performance. Typically. The French Engineer. such as refrigeration and jet propulsion engines. therefore. While the turboexpander alone can easily reach isentropic efficiencies of up to 90%. after a brief discussion of current technologies and the characteristics of GE Oil & Gas Turboexpanders. The turboexpander efficiency is limited by the compressor (and vice versa) and. German engineers. turboexpanders were used in ethylene projects and then naturally progressed into several other markets such as liquefied natural gas. . In the 1960s.The turboexpander continues to date to develop in the natural gas industry. and gas-to-liquids. geothermal. Depending on the service required. and Geothermal/Waste Heat Energy Recovery. This is able to achieve much lower temperatures than throttling the fluid through a JT valve by isenthalpic expansion. Photo-2: Turboexpander-Generator General Arrangement Turboexpander-Compressor Mechanical power drives a compressor impeller either coupled to the same shaft as the turboexpander or driven via a gearbox (Photo-3). . Typical applications covered by GE Oil & Gas Turboexpanders are: Natural Gas Processing/Dew Point Control Plants.Turboexpander Applications Turboexpanders are predominantly used in refrigeration/liquefaction processes and power generation applications. The lower temperatures considerably increase the overall refrigeration cycle efficiency. The refrigeration/liquefaction process utilizes the Turboexpander for cooling fluids through nearly isentropic expansion from a higher pressure to a lower one. Pressure Let Down Energy Recovery. mechanical power produced by expansion of flow in the radial turbine can be recovered or dissipated through three main machine configurations: Turboexpander-Generator Mechanical power is converted into electrical power through a reduction gear and a generator (Photo-2). since the same service can be covered through either a Turboexpander-Generator or a Turboexpander-Compressor. . Table-1 lists the pros and cons of both solutions.Photo-3: Turboexpander-Compressor General Arrangement Turboexpander-Dyno Mechanical power is dissipated through an oil brake if it is not economical to convert the excess power into another form of energy (Photo-4). Photo-4: Turboexpander-Dyno Often it is not clear which turboexpander configuration is suitable for an ethylene plant. or similar. Simpler machine control can easily be set up for a fully automatic control system. Parts that normally need to be customized for each project are the wheels (both turboexpander and compressor). and other auxiliaries. auxiliaries and controls. A fixed speed machine can typically perform better in off-design condition when the enthalpy drop is maintained constant with process controls. and blower configurations have not been included in the comparison table because they are not typically applied to medium and large sized machines that are commonly found in ethylene plants. shaft. The stiff shaft design improves the operating range and the capability to withstand very high imbalances. nozzle assembly. The frame size is directly linked to the casing and. Units may be arranged in series. It should be noted that dyno. compressor follower. Perfect for oil free applications with the use of active magnetic bearings (AMB). Recompressor is designed independently from the turboexpander. If the plant throughput (flow) is decreased while the pressure ratio is kept constant. The wheel can be optimized to achieve the best aerodynamics by freely changing the RPM without other machinery constraints. This reduces the size of required anti-surge systems to manage unbalances in flow between the turboexpander and compressor. seals and the pressurized auxiliaries system makes it very difficult for gas to escape from the machine in case of failure. This limits the maximum tip speed of the wheel and tripping devices need to be redundant for safety reasons. the overall dimension of . The naming convention for machine standardization is the “Frame” size. therefore. Labyrinth. Efficiencies are sometimes lower than turboexpander-generator due to the balancing of the turboexpander and compressor performance and limitations. gear. The machine is typically more complex than a TurboexpanderCompressor due to the presence of a gearbox. pump. the machine speed will reduce with a significant loss in efficiency.COMPRESSORTURBOEXPANDER. Very robust and simple machine. For a well-balanced machine. the turboexpander flow and re-compressor flow are linked. The machine has a tendency to speed up in case of electric load rejection.GENERATOR TURBOEXPANDER- Table-1: Comparison of Various Turboexpander Machinery Configurations PROS CONS Very high efficiencies can be achieved. Simpler plant layout: reduced number of piping interconnections. merging more stages into a single machine with higher efficiency. increasing the complexity and tuning of the control system. diffuser cone. Cost per unit is higher and oil free solutions are not yet economically feasible. GE Oil & Gas Product Line The GE Oil & Gas Turboexpanders product line is standardized so that most of the components are pre-designed. generator. Table-2: GE Oil & Gas Frame Size vs. and turboexpander-generators (EG) single stage or multistage integrally geared types. the active magnetic . The design temperatures typically set the materials of construction for the components. 2. For cryogenic applications the turboexpander casing is typically stainless steel. turboexpander-multistage compressors (ECC). there are standard comments and exceptions to all of the industry specifications listed above. While there are no size limitations for turboexpander-generators and turboexpander-compressors with traditional oil bearings. The design pressure sets the flange ratings. Typical design limitations are as follows: Power up to 35 MW Wheel diameter up to 1800mm Design temperature from –270oC to +315oC Mechanical design in accordance with API 617 Chapter 4 Lube oil system in accordance with API 614 Chapters 1. by using a fixed nozzle instead of a variable nozzle. Other components are also affected mechanically. but if warm enough low temperature carbon steel can be used. Each of the Frame Sizes are clarified further in Table-2. The compressor casing and bearing housing are typically carbon steel due to the warmer temperatures. For example. Frame sizes are also distinguished by the design pressure and flow rate. and 4 Turbine operability in accordance with IEC45 or API 612 Chapter 12 As with most turbomachinery designs.the machine. Flange Ratings & Flow FRAME # TURBOEXPANDER RATING ACCORDING TO ANSI (PSI) 150 10 15 20 25 30 40 50 60 80 100 130 160 180 x x x x x x x x x X 300 x x x x x x x x x x x x 600 x x x x x x x x x x 900 x x x x x x x x x OUTLET FLOW (ACMH) 1500 x x x x x 450 1000 4000 5500 9000 16000 25000 36000 45000 65000 100000 160000 200000 TURBOEXPANDER GENERATOR FRAME SIZE AVAILABLE TURBOEXPANDER COMPRESSOR FRAME SIZE AVAILABLE Table-2 is applicable to turboexpander-compressors (EC). the design temperature limitations can exceed the values given above. Each standard frame can accommodate a specific diameter range of turboexpander wheels. This recently applied technology is able to ensure the highest quality pressure-containing components while also minimizing any potential defects during the manufacturing of the unit.bearing (AMB) units need to be checked versus the standard bearing size from AMB suppliers. ratings. and nozzle loads. allows the AMB to operate without being contaminated or harmed by the aggressive gas. Photo-1 shows a machine currently installed with this technology. . Photo-3: Turboexpander-Compressor with “Canned” Active Magnetic Bearing The GE Oil & Gas product line offers a fabricated casing design. In particular. the re-compressor discharge volute can be manufactured with a variable section scroll and a tangential nozzle to provide the best efficiency and range. Moreover. This design. The internal parts made by castings can now be aerodynamically shaped for the best efficiency. GE Oil & Gas has additional experience with special “canned type” magnetic bearings that are suitable for aggressive and sour gases typically not tolerated by standard electrical devices. This design encapsulates traditional electrical components of the AMB within a metal “can” made of Inconel material that prevents any contact with process gas. mainly used in natural gas applications. as shown in Figure-1. the use of a fabricated casing ensures the flexibility to design for a wide range of applications. in addition to the traditional Rotoflow cast casing design. GE Oil & Gas can provide patented solutions with a traditional Rotoflow slot and pin mechanism. Figure-2: Slot and Pin Inlet Guide Vane (Nozzle) Assembly . which is very effective on turboexpander-compressors. shown in Figure-2. which adjusts the guide vanes using a “progressive” opening law for precision flow control and minimal actuating forces. shown in Figure-3.Figure-1: Turboexpander-Compressor Cross-Sectional Drawing The control of the turboexpander is primarily accomplished by means of adjustable guide vanes (nozzles). Also available is a newly patented multilink mechanism. Nozzle segments are subjected to severe working conditions as shown in the Finite Element Analysis of Figure-3. These conditions are due to the high velocities of the gas at this location (similar to the wheel tip speed) and because of the presence of solid particles and liquid droplets passing through the turboexpander. . For this reason. The improved mechanical design of the nozzle mechanism is associated with increased aerodynamic performance design. Antifriction and anti-wear coatings on the nozzle segments minimize the losses during the first isenthalpic expansion. the turboexpander and compressor wheels need to be carefully designed in order to avoid excessive stresses. harmful resonances.Precise flow regulation is useful in turboexpander-generators in order to minimize the speed fluctuations at low load and synchronize the generator to the grid without using an external control valve. GE Oil & Gas designs and manufactures open and closed wheel designs up to 1800 mm diameters in various materials. As shown in Figure-4. and erosion by liquid droplets. The wheel and wheel attachment has a strong influence on the rotor dynamics of the machine. Another key component of the turboexpander-compressor is the wheel. To ensure the reliability of the machine. tungsten carbide coatings or surface induction hardening are typically applied to the nozzles to minimize erosion problems. where the compressor head requirements are very severe (Figure-5). improves the maximum tip speed and head capability. the maximum head is determined by a compromise between the mechanical aspects (tip speed) and aero design (blade loading). a splined fit. the most common material in ethylene plants is 7050 Aluminum. which is required to reach very high tip speeds. Each wheel is analyzed by means of a finite element analysis (FEA) tool to assess the stress and modal behavior. The modal behavior is assessed to avoid possible resonances between the stimuli from the nozzle segments and natural modes of the wheel. This material has a very good weight to strength ratio. Figure-5: Finite Element Analysis of a Compressor Wheel In ethylene plants. Titanium with superior properties is not typically used when there is hydrogen in the tail gas. TIE ROD KEYS Figure-6: Hirth Serration HIRT . to attach the wheel to the shaft. but is commonly used in many other turboexpander applications. GE Oil & Gas uses hirth serration (Figure-6).In general. This solution minimizes the centrifugal stresses on the wheel and. therefore. as shown in Figure-7. The specific speed is the key parameter for the assessment of the efficiency of a radial turbine at the design point. The optimal range of specific speed for turboexpander design. is from ~1800 to ~2000. Figure-8 further explains this idea pictorially.Turboexpander Performance Turboexpander Selection The turboexpander performance is computed as a function of a nondimensional factor called specific speed (Ns) defined as: N Q2 Ns 3/ 4 his where Q2 is volumetric flow at the discharge.2 BTU/lbm) High Specific Speed (2000 < Ns < 2500): 180 kJ/kg (76. and N is the rotating speed of the machine selected.is . Turboexpander Specific Speed The specific speed is related to the maximum enthalpy drop that one stage can handle. Figure-7: Normalized Efficiency vs. SPOUTING VELOCITY: Co hts . This is similar to converting the potential energy in a water tower into a velocity at the exit of the tower. his is the isentropic enthalpy drop through the turboexpander. Typical numbers for the maximum enthalpy drop are: Low Specific Speed (500 < Ns < 1000): 350 kJ/kg (148. In other words. This is a nondimensional parameter where u1 is the tip speed of the wheel and C o is the spouting velocity.2 BTU/lb m) A second important parameter to consider is the u 1/Co factor. with H being the potential energy and w being the speed at the water tower exit. it is the speed that is created from putting work into the system. The spouting velocity is the fluid speed that would be achieved if the entire isentropic enthalpy drop were to be converted into speed. the inlet of the turboexpander wheel is radial. The u1/Co factor becomes important during the testing of a turboexpander. In an ethylene plant. Figure-10: Sample Correlation Curves for Efficiency Correction Factors . The efficiency of the machine in off-design conditions considers the effect of variation of flow rate and u 1/Co ratio.Figure-8: Spouting Velocity Pictorially Represented The u1/Co factor determines the degree of reaction of the turboexpander stage and is selected during the design phase (Figure-9). based on experience (Figure-10). formula correction factors are provided in correlation curves. The optimum u 1/Co is around 0.7. The turboexpander efficiency is affected by the change in two main parameters: u1/Co and Q2/N (the flow coefficient). the gas conditions are never constant. Current API 617 practices call for it to be one of the measured values in the machine final testing. It is important to predict the behavior of the turboexpander in off-design conditions. In this configuration. improving the ability to withstand liquid at the inlet. After the calculations have been completed. corresponding to approximately a 50% degree of reaction. The absorbed power determines the operating speed of the turboexpander-compressor. the Mach number is normally not an issue because of the low molecular weight gas.8 BTU/lbm) A well-balanced turboexpander and compressor wheel depends on the process design. Here is a typical range for u1/Co and Q2/N turboexpander off design conditions: % Q2/N: 30 to 140% of design case % u1/Co: from 30 to 135% of design case Compressor Selection The compressor is used as a brake for the turboexpander. and head rise. an .5 BTU/lbm) High flow coefficient (0. The capability for a given wheel to produce power depends on both and u2 squared and the mass flow rate that is handled by the compressor wheel. The compressor selection is very important in ethylene applications.280): 120 kJ/kg (50. The compressor load influences the turboexpander efficiency. Gexp his is Gcomp his It should be noted that the capability for the compressor to act as a load for the turboexpander does not depend on the polytropic efficiency. For this reason. The turboexpander wheel power (including mechanical losses) should be the same as the compressor absorbed power.180 < < 0. The Work Coefficient is limited by the aerodynamic design of the wheel and the peripheral speed affects the static stress on the impeller. Compressors with controllable power absorption characteristics can be supplied to provide more flexibility to the turboexpander.The overall plant control and machine selection should take into account the turboexpander behavior during off-design conditions. Recent developments in ethylene plant design also impose more importance on the re-compressor performance. In ethylene applications.025 < < 0. D 2 is the impeller diameter and u 2 is the wheel peripheral speed. where very often the compressor is required to produce very high head. Typical numbers for the maximum enthalpy change on the compressor wheel are as follows: Low flow coefficient (0. but rather an important plant component that is required to operate with good polytropic efficiency. The compressor is no longer seen as a “by product”.100): 150 kJ/kg (63. The compressor selection is made using three main parameters: 4Q1 Flow coefficient: D22u 2 u2 Compressor Peripheral Mach Number: Mu a 0t h Work Coefficient: 2 u2 where Q1 is the volumetric flow at the inlet. turndown. also reducing the turboexpander speed. The following graph (Figure-12) represents the change of compressor flow coefficient as a function of the rotational speed for two density ratios. Ns > 800). This behavior is exactly the opposite of the turboexpander. . Turboexpander and Compressor Interaction As seen earlier. Ns curve has a flat peak portion ranging from ~1800 to ~2000 (Graph-2). the specific speed (Ns) is one of the main parameters to determine the efficiency of the expander. This is important in order to stay within an acceptable efficiency range as a function of the ratio h to the expander volumetric flow at the outlet (Figure-11). the efficiency of the compressor will drop because of the “internal” recirculation.optional hot bypass around the compressors can be used to artificially increase the absorbed power. Targeting a minimum value of Ns (i. The rotational speed must be limited below a given value in order to limit the compressor flow coefficient and also to increase the capability to produce head and power. The warmer gas at lower density on the compressor side tends to increase the flow coefficient. As a consequence. the rotational speed affects the compressor flow coefficient.e. which has an impact on the expander efficiency as seen in Figure-11. The efficiency vs. This ratio is between the density at the expander outlet and the density at the compressor inlet. This needs to be kept under a given value by reducing the speed. it is possible to determine the minimum rotational speed of the machine. Figure-11: Minimum Rotational Speed of Turboexpander (Assuming Similar Mass Flow Rate Between Turboexpander Compressor) On the other hand. This service can be satisfied either with oil bearings or active magnetic bearings. the turboexpander and the compressor selection have to be balanced. With the margins available in cold boxes. The focus was on the turboexpander-compressor configuration since this is more complex than a turboexpander-generator in conjunction with a stand-alone re-compressor. based on all parameters. In order to do so. therefore. Case Studies Two case studies where analyzed. a larger percentage of ethylene recovery. This could occur for several reasons. The machine selection for this service does not have any issues related to specific speed at the higher range of efficiencies. However. This case reduces the C2 and C3 refrigeration to a certain extent. but the major issue that affects this “balance” is the density ratio imbalance between the turboexpander discharge and the compressor suction. the turboexpander efficiency may be negatively affected. The selection based on compressor efficiency can be further optimized to improve the efficiency. corresponding to a Frame 20 (EC201).Figure-12: Compressor Rotational Speed vs. Higher Hydrogen Recovery: increased rate of hydrogen recovery with decreased flow. the initial selection fits . to provide examples of the trends in today’s ethylene plants: a naphtha cracker producing a methane-rich residue gas and a typical ethane or ethane/propane (EP) cracker producing a light hydrogenrich residue gas were analyzed. Lower Hydrogen Recovery: reduced rate of hydrogen recovery and. Liquid Cracker The liquid cracker evaluation was made considering the following scenarios: Base Case: high percentage of hydrogen recovery. Flow Coefficient In summary. this increased rate of hydrogen does not affect the ethylene recovery or the refrigeration. The turboexpander-compressor is at the lower end of GE Oil & Gas production capabilities. 630 84-88% 936 14. The same case study was analyzed by increasing the flow rate by 25%.800 33. In fact. Table-3 shows an overview of the machine performance.000 1.630 36.into a very standard unit.200 (mm) 200 230 (%) 84-88% 72-76% (hp) 1039 1035 (%) 15. a standard configuration for the 100% and 111% flows.000 35. From a machinery design point-of-view. Table-2 provides a summary of the machinery sizing for the Liquid Cracker case to highlight the important turboexpander factors. Variation occurs due to co-cracking of propane or other feedstock.7 72-76% 933 71-74% 1219 GAS CRACKER Gas crackers produce a very large residue gas stream with high concentrations of hydrogen. The gas does not vary with hydrogen product demand. this service is considered to be more difficult due to the high enthalpy change involved. and both the mechanical and aerodynamic characteristics are well within proven experience. . the demand of hydrogen product is very low.500 3. Both units are sized into a Frame 40 (EC401) with good efficiencies and with well-referenced mechanical and aerodynamic parameters. A first selection was made with a 2-stage expander compressor. such as specific speed (Ns). Since the gas conditions remain unchanged. This reference is based on 100% ethane cracking (the base case) with the option of 50/50 Ethane/Propane cracking. Table-2: Liquid Cracker Turboexpander-Compressor Sizing at 100% Flow Case Description UNIT Condition RPM Ns Diameter Efficiency Wheel Power Weight Liquid Frame size BASE H2 RECOVERY Exp Comp Design 35.800 Exp Comp Off-Design 36. scaled up to the Frame 25 (EC251).3 EC0201 LOWER H2 RECOVERY HIGHER H2 RECOVERY Exp Comp Off-Design 33.7 82-86% 1223 15. the machine selection resulted in a similar unit design. Table-4: Gas Cracker Turboexpander-Multistage Compressor Sizing at 100% Flow Case Description UNIT Condition RPM Ns Diameter Efficiency Wheel Power Weight Liquid Frame size 100% Ethane BASE Exp (mm) (%) (hp) (%) 23.000 20. the efficiency is highly penalized with respect to the traditional design at nearly the same specific speed.890 18.900 3. GE Oil & Gas has selected for this service a single Frame 40 (ECC401) machine.000 1. with twostage compressors directly coupled to a single expander wheel.200 (mm) 325 425 350 425 83868384(%) 87% 73-77% 89% 72-76% 87% 70-74% 88% 70-74% (hp) 1629 1626 1630 1627 1509 1507 1528 1526 (%) 0.5 EC0401 EC0401 If the flow is increased by 11%.000 350 78-82% 2396 0. or by increasing the diameter and reducing the rotational speed to keep the same peripheral speed. . This arrangement would also be considered if the turboexpander enthalpy drop per stage were lower.000 16.8 4. the design remains basically the same.890 18.000 20.360 16.Table-3: Gas Cracker Turboexpander-Compressor Sizing at 100% Flow Case Description UNIT Condition RPM Ns Diameter Efficiency Wheel Power Weight Liquid Frame size 100% Ethane BASE 100% Ethane BASE 50/50 Ethane/Propane 50/50 Ethane/Propane Exp_HP Comp_HP Exp_LP Comp_LP Exp_HP Comp_HP Exp_LP Comp_LP Design Design Off-Design Off-Design 20. the selected wheels are larger in terms of flow capability (larger flow coefficient).360 1. The second option is required to handle the different enthalpy change.000 20.000 23.6 4.270 16. With the intent of simplifying the plant layout and reducing cost. The flow capacity of a turboexpander can be increased by either using a wheel design with a higher flow coefficient/specific speed.890 3. The turboexpander-compressor-compressor solution (Figure-13) could be considered as a low cost alternative solution.000 18.6 50/50 Ethane/Propane Comp_LP Comp_HP Exp Comp_LP Comp_HP Design Off-Design 23.270 16.100 3.9 5.9 ECC401 Due to the very high enthalpy drop across the expander stage. The rotor dynamics of this arrangement needs to be analyzed carefully to ensure a robust design without harmful expander wheel-overhung modes throughout the operating range.700 350 350 74-78% 74-78% 77-81% 71-75% 71-75% 1466 1466 27244 1386 1386 4.000 1. However. This type of unit is referenced with oil bearings and can also be developed with AMB.400 3. The results show that there are no issues with increasing the machine capacity. . However. due to the scalability of the unit frame sizes. large enthalpy drops per stage and optimization trade-offs between the expander and compressor wheels need to be carefully evaluated to find the best compromise between cost and performance.BRG Expander Compressor Figure-13: Turboexpander-Compressor-Compressor (ECC) Arrangement Conclusions This paper presents an overview of current turboexpander technology to provide information for the selection of the best machine configuration and thermodynamic design for ethylene plant applications. GE Oil & Gas has analyzed potential selections for turboexpander-compressors for large ethylene plants.
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