Rein: Circulation in Vacuum Pans CIRCULATION IN VACUUM PANS1 Peter W. Rein1, Luis F. Echeverri1 and Sumanta Acharya2 Audubon Sugar Institute, Louisiana State University AgCenter 2 Mechanical Engineering Dept., Louisiana State University Baton Rouge, LA 70803 ABSTRACT The crystallization of sucrose involves complex processes that require the correct design of vacuum pans and precise operation. Numerous parameters, such as tube dimensions, downtake size and pan geometry, determine the quality of the sugar produced and the throughput, which results in certain vacuum pans giving better performance than others. A particularly important factor is the circulation, which is strongly interrelated with the convectiveboiling heat transfer in the calandria and determines, to a large extent, the velocity of crystallization and the uniformity of the conditions inside the vessel. In this paper several factors that affect boiling and circulation in vacuum pans are identified. The use of Computational Fluid Dynamics in the analysis of circulation and in the evaluation of alternatives for hydrodynamic optimization is discussed. Options for making changes to batch pans to improve the characteristics of the pan are identified, either to change the geometry or assist circulation. Some results with steam-assisted circulation (steam jiggers) are given, which show how the performance of a pan can be improved. Keywords: Circulation, vacuum pans, crystallization. INTRODUCTION It is widely accepted that the circulation of the massecuite within vacuum pans must be as high as practically possible in order to achieve the maximum throughput of the equipment and produce a good quality sugar. The benefits associated with good circulation can be listed as: • Quality: Improved crystal size distribution and lower sugar color due to uniformity of fluid conditions within the vacuum pan. Good circulation prevents stagnant regions, hot and cold zones that can induce differences in the super saturation level and crystal growth rate. • Recovery: A higher massecuite dry substance concentration is possible, leading to a higher crystal yield and a lower purity of molasses. • Capacity: Improved heat transfer leads to a higher crystallization rate and increased throughput. • Energy efficiency: Use of vapor from later evaporator effects becomes possible (e.g. vapor 2 or vapor 3). The design of batch vacuum pans has evolved to a practically uniform basic geometry (central downtake, straight side walls, fixed calandria, tubes of diameter 100 mm and length 800 - 1300 mm, bottom angle 18-25°), which may be assisted mechanically with an impeller (stirrer) or by injecting a gas to improve circulation. This design has evolved essentially by trial and error. 1 The circulation mechanism in vacuum pans has not been completely understood due to the complexity of performing measurements inside the vessels (Wright. The evaporation rate changes dramatically during each strike. Table 1. Ideally. while the liquid moves in a radial direction towards the center and goes back to the bottom through the downtake. This paper discusses some factors that affect the circulation in vacuum pans. 1966) and the numerous operational regimes that are possible. The balance between the buoyancy forces and the resistance to flow determines the circulation rate. any reduction in the massecuite velocity will be detrimental to heat transfer (Bosworth. 1959). 1959). following a strong interaction between heat transfer and circulation. to enter into the calandria tubes again.2 3. decreasing progressively and finding its minimum value at the last stage of each cycle (Table 1). Any mechanism that reduces the evaporation rate will lower the circulation. and therefore circulation. HEAT TRANSFER AND CIRCULATION Convective-boiling heat transfer in the calandria tubes is the driving force for the circulation of massecuite. 2004 Although valuable experimental work on the sugar boiling process has been done and technological development has led to proven designs of batch and continuous vacuum pans. which should be smooth and offer minimum obstruction to flow (Webre. and conversely. The vapor formed crosses the free massecuite surface before going to the condenser. higher strike levels and high concentrations reduce the evaporation rate. Type of Boiling Grain / Seed A B C Maximum kg /(m2 h) 61 38 25 18 Minimum kg /(m2 h) 8. are increased as the heating steam pressure. Average evaporation rates measured in South Africa. However. Vol.Journal American Society of Sugar Cane Technologists. Using experimental measurements. the vacuum and the purity are raised. there is still a lack of understanding of the important issues affecting vacuum pans design.5 DESIGN ISSUES AFFECTING CIRCULATION 2 . alternatives for the analysis of flow characteristics and options for enhancing circulation. The hydraulic resistance or friction depends essentially on the massecuite viscosity and the geometry of the passages within the pan. longer tubes. 1966).2 22 6. The buoyancy that results from the density difference between the vapor-phase and the surrounding massecuite is the main driving force. 24. Conversely. it is expected that a flow of massecuite and vapor rises after passing through the calandria tubes and reaches the surface. Rouillard (1985) identified that evaporation rates. evidence exists which indicates that serious variations from this ideal circulation pattern could occur (Wright. Tube diameter and length The high viscosities of the massecuites demand the use of large diameters to overcome the friction with the available buoyancy.5 to get a pan with reasonable circulation. where evaporation rates are higher. 1959) Disengagement vapor 1. with vertical tubes arranged around it.4*Dcalandria Tubes D 0.5-3. The exchange area is normally around 6 m2/m3. defined as the ratio of the cross sectional area of the tubes to the area of the downtake.25 m 58° 10-25° L 0. Floating calandria: The downtake is located in the outer annular region. The combination of the annular downtake with a small central downtake has also been tried. A single central downtake is normally used.1m Inclined plate calandria Conical enlargement Conical bottom Floating calandria Straight side Saucer bottom Horizontal flat calandria Straight sides "W" bottom Figure 1. but for high strike grades.7-1. 1966). The diameter of the downtake is 30-50 % of the calandria diameter. Although it was developed in an attempt to improve the circulation characteristics.2 m 18° 0..0 m Strike Height 1. Tubes above this size exhibit less resistance and promote circulation. The downtake is sized to give a circulation ratio.Rein: Circulation in Vacuum Pans Calandria The calandria is essentially a tube and shell heat exchanger. Vacuum pan designs. less than 2. The top plate is made horizontal in some cases. The different variations are shown in Figure 1. measurements involving the use of a radioisotope tracer showed that the conventional central downtake performs better (Wright. prone to the formation of false grain (Webre. 1998). resulting in an inefficient design. but the lower area/volume ratio is 3 . Horizontal flat plate calandria: This is the most common design. Inclined plate calandria: The plates of the calandria are angled between 10º and 25º to the horizontal. The most common diameter is 100 mm (4 in). it can be increased to 9 m2/m3 (van der Poel et al. usually around 40 % for natural circulation pans. while smaller downtakes can be used when a stirrer is provided. constituting an efficient design of simple construction and maintenance. “W” bottom: While smaller angles of the bottom reduce the footing volume. and probably there is no justification for tubes longer than 900 mm. Conical enlargement (flared pans): Some designers adopted a conical enlargement of the body above the calandria to increase the capacity of the vessel without increasing the strike height and give a lower graining volume to final strike volume. In the case of calandria pans.Journal American Society of Sugar Cane Technologists. 1985). The disengagement volume should represent between 65-100 % of the strike volume. ensuring a smooth discharge of the massecuite. the available temperature difference becomes smaller. Tubes with a length between 900 and 1500 mm (36-60 in) have been used. 1959). even if it results in a decrease in the heat exchange area (Webre. 2004 unfavorable (Webre. after which it starts reducing progressively as the massecuite level increases. Numerous comparisons have shown that straight-sided pans perform better than “low-head conically-enlarged” pans (Chen and Chou. they reduce the discharge velocity and lengthen the strike time. this shape is believed to lead to a significant stagnant zone. Strike height As the strike height increases. all parameters that impair circulation. Disengagement height above the massecuite: Enough space above the massecuite must be provided for disengagement of the liquid from the vapor. but currently they are designed normally between 700 and 1200 mm. leading to a reduction in evaporation rate and consequently reduced circulation. The bottom angle does not need to be greater than 20º (Bosworth. which occurs often on starting a boiling or on cutting over. this was largely discredited after a negative effect on circulation was recognized (van der Poel et al. minimizing entrainment and allowing for froth explosion. Pan shape Bottom: The bottom shape should promote an even distribution of the massecuite across the calandria. being normally between 1. raising the boiling temperature. 1993). Two discharge valves are recommended. However. without restricting circulation or providing stagnant areas. 24. when the highest level coincides with the maximum density and viscosity of the massecuite. A height around 1. Conical bottom: When coil pans dominated. while longer tubes are chosen for high-grade pans or when a stirrer is provided. Vol. As a result. Short tubes are selected for low-grade pans. Tests performed by Austmeyer (1986) suggested that a maximum heat transfer coefficient is reached when the height of the strike is 800 mm above the top tube plate.. 1998).5–3. 1998). Usually the angle of the bottom is between 17 and 25º (van der Poel et al.2 – 4 . Short tubes give the best heat transfer coefficients. 1959).0 m but preferably as high as possible. the hydrostatic pressure in the calandria increases. a conical bottom was the preferred option. and allow the discharge of massecuite within an acceptable time. The situation is particularly critical at the end of the strike. 1959) and the graining volume increases (Rouillard.. flashing. leading to a reduction in temperature gradient between massecuite and the heating vapor. thus improving capacity. It was proposed that the massecuite remains stagnant within the tubes until enough superheat is reached. and then a large burst of vapor erupts and expels the fluid upwards inside the vessel. Its installation gives all the advantages associated with good circulation mentioned previously. coalescence. temperature differences. An intense evaporation occurs. other theories involving nucleation. 5 . and stirrers do not have much effect in this period. The height may be higher with high grade or refined boilings. Unsteady nature of boiling Boiling heat transfer or evaporation is an intrinsically unsteady process. This is shown in Table 2. etc. eruptions. surface tension. and promotes circulation at the end of the strike. It has also been shown that stirrers improve the quality of high-grade sugar produced (Rein. while massecuite penetrates the tube from above and below. In a similar way. formation of slugs or columns of vapor. The results showed an intermittent flow in the tubes and downward currents over the top calandria plate. performance and capacity. There is no general agreement about the cost effectiveness of a stirrer. Forced or assisted circulation Stirrers: Pan stirrers if properly designed can significantly improve the performance of a pan. The vapor escapes. The bubbles in general display random behavior. densities.6 m could give the best balance between quality. Wright (1966) proposed an “eruptive” mechanism after performing measurements of the circulation patterns in high-grade pans. the hydrostatic pressure of the massecuite in the calandria increases the massecuite temperature. characterized by intermittent nucleation and a fluctuating generation of vapor. As the strike height is increased. particularly at the end of the strike. coalescence of bubbles. which is influenced by many factors like bubble size. it is chaotic and difficult to characterize in detail. and hydrostatic head. This suggests that the effect of buoyancy forces due to temperature differences and mechanical circulation are small in comparison to the circulation induced by vapor bubbles. The effect of the forced circulation becomes important at this stage. but the lack of information has not enabled reasonable conclusions to be drawn. 1988). In contrast. work against the use of stirrers. the evaporation is minimal. The assisted circulation improves heat transfer and so shortens the duration of the batch boiling. the slip velocity. resulting in a low bubble flow. The first stage of a strike is characterized by a high evaporation rate. viscosity. During the last stages of the strike. Evaporation is usually considered a steady state phenomenon. but in reality. This is a consequence of the better circulation leading to more homogeneous crystallization conditions within the pan. have been proposed to describe the boiling in vacuum pans. air leakage and high power consumption. and is reflected in higher heat transfer coefficients with respect to natural circulation vacuum pans (Austmeyer.Rein: Circulation in Vacuum Pans 1. high capital costs. 1986). making the analysis complex. This approach is known as Computational Fluid Dynamics (CFD) and has proved to be a valuable tool for research and analysis of engineering problems in many different industries. 1995). if steam economy is important. Jigger steam: The installation of a sparge pipe under the calandria to blow low pressure steam is another option to assist the circulation. The simplicity.. Injection of compressed air at 750 kPa preheated to 65-70º C beneath the calandria of A pans was reported to be effective in reducing the boiling times by 11 % and to increase recovery without an appreciable effect on the pan vacuum (Stobie. 24. allowing reductions in the factory steam requirements. low cost and absence of moving parts of this alternative make it particularly straightforward to put into practice. The use of lower pressure vapors becomes possible (e. 1998). Vol. Currently there is a strong trend towards CFD assisted design. 2004 Table 2. coming from the 2nd or 3rd evaporator effect). allowing an increase in circulation with low energy consumption (15 % of that required by stirrers). 1999). 6 .Journal American Society of Sugar Cane Technologists. Air injection: The injection of air instead of vapor has been adopted in some Spanish factories. It should be noted that the vapor admitted through the ring does not condense. However this option requires a much larger vacuum pump or ejector to remove the additional air and is not recommended here.g. unlike a stirrer. Typical heat transfer coefficients in cane sugar vacuum pan boiling (Bubnik et al. but passes straight through the massecuite without causing any superheating or dissolution of crystal. compared to at least 45-50º C in the absence of a stirrer. The advantage of jigger steam is that it can be shut off at any time. particularly for the most complicated cases. thus enabling a larger heating area to be installed for a given pan diameter. A stirrer can be used with a smaller diameter downtake.. although experimentation still remains important. Venting of incondensables into the jigger arrangement can be done to reduce the consumption of vapor. reducing the massecuite specific gravity in the calandria and reducing the effect of hydrostatic head. making it possible to resolve complex problems through the discretization of the geometry and numerical solution of the governing equations. low costs. APPLICATION OF CFD IN THE SUGAR INDUSTRY The development of numerical techniques and codes for the solution of fluid mechanics and heat transfer problems has evolved particularly rapidly in the last few years. and can be adjusted to give the required degree of circulation. Natural circulation (W / m2K) 570 32 With stirrers (W / m2K) 640 224 Start End Mechanical agitators provide the option of achieving an acceptable heat transfer with a temperature difference as low as 20° C. and without breakage of crystals (van der Poel et al. as well as an important reduction in circulation when the strike height and the viscosity are increased. which is the most appropriate considering the complex interaction of the phases and the high volumetric fraction of the secondary phase (vapor) in the tubes and above the calandria. and the computational domain has been restricted to the zone below the massecuite free surface. 1982). A correct numerical approximation of the flow patterns inside the vessel could give valuable information for the optimization of the design of vacuum pans. which is an acceptable approach considering the high viscosity of the massecuite (Bunton. Analysis of the fluid flow inside vacuum pans A first attempt to analyze numerically the flow pattern inside vacuum pans was developed by Bunton (1982). The main characteristics of the CFD model and some initial results are presented here. in view of the importance of massecuite circulation and the complexity of the problem. for convenience the case can be analyzed as a two-phase flow (liquid and vapor). A two-dimensional axisymmetric problem has been considered to simplify the geometry.Rein: Circulation in Vacuum Pans The application of CFD in the sugar industry has much potential and has been already used to some extent in the analysis of juice clarifiers (Steindl. CFD Model In a vacuum pan there are three phases present (liquid: mother liquor . The Eulerian-Eulerian multiphase model was selected. Chetty. 2001. who using the rings approach developed a numerical solution for boiling in the calandria. The massecuite has been assumed to be Newtonian. where important improvements have been reported. 2001). finding that between 90. For the grid generation and CFD solution.gas: water vapor solid: sugar crystals). Brown (1986) obtained results with a commercial CFD code for a stirred crystallizer. A grid independence study was performed. The numerical results have suggested undesirable recirculation zones. alternatives to reduce the resistance to the flow or improve the fluid circulation have not been explored or reported. representing the calandria tubes as rings and neglecting the vapor phase. Nonetheless. However.000 and 180. which was coupled with a commercial CFD code (CFX) for the solution of the flow in the rest of the domain. a commercial code has been used (FLUENT). It is important to use a grid fine enough so that the accuracy of the results is independent of the grid points used.000 quadrilateral elements the results were the same. A more refined model was developed by Stephens (2002). requiring some considerable assumptions in the process. a Eulerian 7 . A promising application of CFD is the analysis of flow in vacuum pans. vacuum pans have been modeled using two-dimensional axisymmetric isothermal two-phase models. With this model. 2002) and bagasse boilers (Dixon. In general. who wrote a program to solve the flow field. The Audubon Sugar Institute in conjunction with the LSU mechanical engineering department is working currently on the study of the flow in vacuum pans. knowing that tubes with less circulation evaporate less. Thus the calculations are limited to small temperature differences.s) 1. Solving the transport equations for both phases requires substantial computational effort. Strike height (m) Absolute pressure (kPa) Density massecuite (kg/m3) Dynamic viscosity massecuite (Pa. It is assumed that the same evaporation takes place across the calandria. Massecuite properties and evaporation used to represent a B-strike.Journal American Society of Sugar Cane Technologists.0 1490 52. 2004 treatment is used for each phase.1 11 0. 8 . The created pressure difference promotes the up flow of massecuite in the calandria and down flow in the downtake. and rises due to its density difference with the surrounding massecuite. Vol. have been undertaken in this study.12E-05 RESULTS AND DISCUSSION CASE 1. which falls below the pressure in the downtake. 24. As a case study. Since the vapor bubbles are the main driving force for the circulation of massecuite. In addition three other cases. Reference case: W-bottom. However. 1992. a 90 m3 pan was selected. the calculations are expected to be representative of the real situation. and can require several days of processing before a solution is reached.0 14. The temperature field is assumed isothermal. The steam is injected without any momentum. steam is assumed to be injected in the zone corresponding to the calandria tubes.183 1. The properties of the massecuite and the amount of steam to be injected in the calandria were determined using figures considered normal for conditions between the middle and the end of a B-strike (Table 3). 2001). a pan with a steam jigger installed. a pan with a conical enlargement. which is considered a modern well-designed pan giving a good performance. As driving force for the circulation of massecuite.h) Density vapor (kg/m3) Dynamic viscosity vapor (Pa. and a pan with a conical bottom. Table 3. Stephens.s) Evaporation Coefficient (kg/m2. resulting in coupled transport equations for the liquid and the vapor phase. The heat transfer phenomena associated with the process have been studied extensively by Rouillard (1985) and Austmeyer (1986). straight sides The flow simulations have illustrated how the acceleration of vapor rising through the calandria tubes reduces the local static pressure. a velocity profile across the calandria smoother than in real vacuum pans is expected. The calandria is represented as a set of concentric rings (Brown. Contours volume fraction of vapor (%) b. An undesirable large vorticity over the top tube plate was found for all the runs. which has a smaller flow and suggests that the tubes located in this region have less massecuite flowing through them.Rein: Circulation in Vacuum Pans a. there is an important reduction in the useful area of the downtake. vacuum pan. W-bottom. Massecuite velocity vectors Figure 2.b). This vorticity seems to originate in the region where the flow coming up from the tubes meets tangentially the down flow at the wall of the downtake (Figure 2.d). CFD results for straight-sided. Vapor velocity vectors d. Consequently. resulting in practically the same static pressure in the inlet of all the tubes (Figure 2. There is also a narrower area for the liquid that travels in radial direction towards the center. which is desirable from the heat transfer point of view and to have uniform conditions within the vessel. unless a blatantly wrong geometry is chosen for the bottom. Contours of static pressure (Pa) c. and some 9 . The resultant distribution of the flow across the calandria is even. with exception of the ring located closest to the downtake. it will not have any important implication on the even distribution of the massecuite across the calandria. The velocities below the calandria are low enough to let the hydrostatic head determine the pressure field. As a consequence of this vorticity. Contours volume fraction of vapor (%) b. and could lead to different crystallization rates for crystals that are trapped there. 2004 recirculation of part of the liquid in the zone over the top calandria plate. The outer rings represent precisely the zone of the calandria where more tubes are present. The results indicated that the conical enlargement leads to another large vorticity over the top tube plate. which has a negative momentum in the vertical direction. This vorticity does not contribute at all to the desired circulation. However. 10 . CFD results for vacuum pan with conical enlargement. Massecuite velocity vectors Figure 3. This vorticity interacts with the vorticity located toward the downtake. The results indicate a small vorticity over the calandria plate close to the sides of the pan (Figure 2. this does not show any effect on the circulation through the outer rings. CASE 2. opposing the rise of massecuite. suggesting that it does not influence significantly the flow of the massecuite through the calandria tubes. Vol. The additional vorticity involves a large downflow near the sides. between the sides and roughly the middle of the calandria width (Figure 3). and its size exceeds expectation. Conical enlargement. a.d). and that results in a reduction of 19 % in the circulation in the outer rings (Figure 4).Journal American Society of Sugar Cane Technologists. 24. 8 2. a.Rein: Circulation in Vacuum Pans 0. The steam injected below the calandria increased substantially the circulation in the pan.8 3.14 0. A simplified steam jigger system is considered. It also appears to reduce the recirculation in the downtake and that it may be better to introduce the vapor uniformly below the calandria to induce a higher flow in all the tubes. Liquid velocity results across the calandria for different pan geometries. CFD results for vacuum pan assisted with steam jiggers.6 1. Contours volume fraction of vapor (%) b.6 2.4 2.16 Liquid velocity (m/s 0.00 1. Massecuite velocity vectors Figure 5. but no significant reduction in the circulation in the other tubes was observed.4 Radius (m) W-bottom W-jigger W-flared V-bottom Figure 4. CASE 3.10 0. This indicates that the tubes located closer to the points where the gas is injected will have a very high circulation.4 1. The average circulation increased 24% respect the same geometry without steam injection.12 0. Circulation assisted with steam jiggers.0 2.2 3.0 3.18 0.08 0.2 2. consisting of a single sparge ring under the calandria.20 0. accelerating the flow particularly in the rings located over the steam jigger (Figures 4 and 5).04 0. Vorticity over the top tube plate increase. 11 .02 0.06 0. and that the flared sides or conical enlargement impairs the circulation. Contours volume fraction of vapor (%) b.7 kW/m3. while the use of steam jiggers improves circulation. or else just below the downtake in the case of a radial flow impeller. 2004 CASE 4. 12 . the CFD will be applied to explore alternatives for improving the circulation in batch vacuum pans. the results obtained agree with experience. A scale model designed and constructed specifically to study the circulation in vacuum pans with laser anemometry techniques (PDPA) will be used to verify the computational results. Finally. viscosity. Conical or “V” bottom. Vol. In spite of the assumptions made in modeling the problem. The flow field over the top tube plate and the flow distribution across the calandria were similar to that found for the W-bottom reference case. PRACTICAL OPTIONS FOR IMPROVING CIRCULATION Stirrers The stirrer itself is located in the downtake if it is an axial flow impeller. Massecuite velocity vectors Figure 6. and extended to the analysis of continuous vacuum pans. but with a massecuite flow 5 % higher.Journal American Society of Sugar Cane Technologists. Tip speed has to be kept below 7 m/s (23 ft/s). downtake area.g. The expected stagnation zone at the lowest part of the pan is small. and steam jigger design). pan geometry. strike height. or else false grain will form. 24. Installed power should be in the range 1 to 1. The higher circulation is attributed to the smaller resistance of the bottom to the downflow and smoother change of direction. type of calandria. This work will continue using CFD to evaluate the factors affecting circulation in vacuum pans (e. CFD results for straight-sided with conical or V-bottom. indicating that the W-bottom is efficient in distributing the massecuite. a. The Lula conversion is reported in detail at this conference (Bergeron and Carline. from more than one outlet. This flaring of pans was done to increase the strike/seed ratio of pans. This ensures a positive purging of incondensables. pan volume increased by 25 % and the heating surface by 52 %. Incondensables need to be purged by the steam flow and are generally located at a point furthest from the steam inlet. Conversion of flared to straight-sided pans As shown earlier. Condensate and incondensables removal Adequate arrangements for the removal of condensate and incondensable gases must be made. 2003). but this design is now largely obsolete. The circulation ratio is affected adversely. It is important that these details are given proper attention. Attempts should be made to get rid of any unnecessary ironware in the massecuite. It is good to be generous in sizing these outlets. As a result of the conversion. The size of the condensate drains should be based on liquid outlet velocities of less than 0. The two incondensable offtakes should be separately vented. Thus the total quantity to be vented should be about 1 % of the steam flow to the calandria. The best arrangement is a radial flow of steam from a vapor belt around the calandria. If good results are to be obtained.Rein: Circulation in Vacuum Pans Each stirrer should be designed for the particular pan and its duty individually. 13 . Old pans at Lula and Alma factories have been re-tubed and at the same time have been changed from flared to straight-sided pans by increasing the calandria diameters. it is seldom possible to transfer a stirrer from one pan to another without redesigning the arrangement. Incondensable quantity is estimated as about 100 mg/kg of steam. In order to ensure total venting of incondensables. and that sufficient quantity is vented. there are a large number of these pans still installed in sugar mills. However. one at the top and one at the bottom of the calandria. Thermostatic valves can be used to bleed incondensables if steam economy is important. Louvers or any other steel devices installed in the pan in the mistaken idea that they will improve circulation should be removed. Removing Resistance to Circulation Any unnecessary steel inside the pan represents a resistance to circulation. the incondensables offtake arrangements should consist of two rings around the downtake. the conical enlargement above the calandria has a negative effect on pan circulation. As the steam flow is radially inwards. There is no universal stirrer for all occasions. but the circulator and the absence of the flared body lead to a significant net improvement in pan performance.5 ft/s) at maximum evaporation rate. Care needs to be given to the size and number of holes in the incondensables offtake rings to ensure uniform offtake around the downtake. the amount vented should be 100 times the quantity of incondensables. as they can be the cause of under-performance if not properly designed. C pan boiling times dropped by 2 hours. Condensate is generally removed from the lowest point of the shell.45 m/s (1. one-third of the width of the calandria from the pan periphery. This compares with estimated steam flows to the pan of 17 ton/hr dropping to about 3. so that the flash will aid circulation. Before the 2002 season. and heat transfer drops even further. As the heat transfer rate reduces. 1981). If the feed is conditioned and at a higher temperature than the boiling massecuite. giving a maximum vapor 1 rate of 1. fitted under the calandria. the rate of movement of massecuite drops. If the pan is badly designed and/or the massecuite level gets too high.6 m3 (1400 ft3) pans. The sizing of the ring has to be calculated so that the pressure drop through the ring is less than 10 % of the pressure drop through each hole. It had 32 x 9.7 ton/hr on average. but it was still considered that the circulation in these pans could be improved. Steam-assisted circulation Sparging of steam underneath the calandria has often been used as a means of improving circulation. steam jiggers were fitted to three C massecuite pans at Raceland. These pans already had circulators fitted. The syrup or molasses feed system should also represent minimum obstruction to the circulating massecuite. Thus the effect of steam assistance. facing downwards. thus increasing the rate of heat transfer to the massecuite and further improving circulation.5 ft). The number of holes and their size is determined by the required flow rate of vapor. and not up through the massecuite. Steam assisted circulation can get such a pan to start boiling again. It has the effect of increasing the velocity of the massecuite through the tubes.4 m (13 ft) and above the calandria 0. with the diameter at the calandria 0. condensate outlets should be positioned at the periphery of the pan. The steam jigger consisted of a 51 mm (2”) ring.15 ton/hr. 14 . These pans were all nominally 39. Likewise. and high Brix C massecuites could not easily be achieved. where a different sparging arrangement should be used to give a more uniform distribution of vapor under the calandria.47 m (15. This simple arrangement may not be satisfactory in a larger pan. often called “steam jigger” is to enable a higher strike height to be achieved as well as a reduced boiling time. The feed should be introduced through the periphery of the pan below the calandria or through minimum sized pipes or channels on the floor of the pan. and should not run from the bottom tube plate down through the massecuite. 24. The sizing of the system was based on vapor injection at a maximum rate of 25 kg/m3 hr. there comes a time with a high viscosity massecuite when the pan stops boiling. This is a vicious cycle. as it was not possible to run them continuously on vapor 1. 2004 Condensate and incondensable venting lines should not be run through the massecuite. Incondensable gases should be vented through the outside of the calandria. This ensures that roughly equal flows will be achieved through every hole (Knaebel. the feed must be directed under the calandria.Journal American Society of Sugar Cane Technologists. but still the steam jigger had a marked effect on boiling times and strike heights. These pans are fitted with stirrers.5 mm (3/8”) holes drilled in the pipe. Heat transfer in the tube is insufficient to cause significant boiling and movement of massecuite slows. Vol. 18. In 2001. Only Jigger Nov. Instead of stopping to repair the stirrer. 15 . this enabled the mill to achieve their lowest recorded molasses target purity differences. Nonetheless the following was achieved: • • • The pans could be boiled throughout the full cycle using vapor 1. but the pan operation continued Pan #1 Boiling Time For November 2002 10 9 8 7 Hours For Boiling 6 5 4 3 2 1 0 0 10 20 30 40 50 Strike # 60 70 80 90 100 Pan with Circulator and Jigger Steam Pan without Circulator. During November. Figure 7 shows that boiling times increased. Boiling times in Raceland pan 1 during November 2002. Boiling times were roughly the same in both years in spite of conditions which in some mills led to pans stopping boiling altogether. After the most severe processing problems in October and November were over.Rein: Circulation in Vacuum Pans Comparison of the performance before and after the installation of the steam jiggers is complicated by the fact that processing conditions in 2002 were the worst experienced in many years as a result of the severe weather conditions. Average C massecuite Brix increased from 95. the mill was able to continue operating the pan with the stirrer in the downtake representing a restriction to circulation.2002 Figure 7. the stirrer in pan 1 stopped working. the pans were run on exhaust during the final stages of the boiling.8.6 to 96. It should be borne in mind too that the effect of installing a steam jigger system can be expected to be most significant in a pan without a circulator. F. 16 . Chou. Int. Crystallization. J. Chetty. 6. Sugar Technol. Soc. Rouillard. Chen. 1993. Assoc. Proc. P. Brown. 23:1-8. 11. Afr. Davis. Netherlands. Bruhns. 2. Pages: 371-393. Amer. Proc. Cane Sugar Handbook. 59:4347. Honig. 1992. Analysis of sugar boiling and its technical consequences: Part II. 1995.E. Urban. 1981. 8th Edition. 24. Raghunandan. Bergeron. 12th Ed.. Proc. Simplified sparger design. Assoc. Austmeyer. K.J. 25:80-85. Zuckerind 117(1):35-39. Steindl. CFD appears to offer a tool for progressing efforts to improve the design and operation of both batch and continuous pans. 2004 CONCLUSIONS Circulation in a vacuum pan is one of the most important parameters affecting performance. 1988. 2001. Sugar Cane Technol. S. Development of the new generation SRI clarifier design. 4. Bubnik. S. 2003. and G.Journal American Society of Sugar Cane Technologists.. D. 7. Sugar Cane Technol. 1959.S. and S. The research process from an engineering perspective. 12. R. A number of factors which influence the operation of vacuum pans have been identified. Sugar Proc. 1985. Maharaj. ed. Soc. Carline. Chem.. S. Z. A successful modification to the Dorr 444 clarifier. 9. 88(3):116-117.. Changing the heating surface to volume ratio on a low grade batch pan. D. 10. The effect of pan design and operation on white sugar quality.E. 8. Afr. pp. 2001. Sugar Technol. Vol. Some relatively minor changes can be made to improve capacity and performance. Sugar Cane Technol. K. John Wiley & Sons. P. Vol. Knaebel. 2. Bartens. 2002. P. Proc. J. 32-40.. Heat transfer during sugar boiling. Conf . and M. and F.A. and C.J. T. Massecuite boiling. Rein. 5. Bosworth. 88(1046):23-29. However it is clear that most designs are far from being optimal. Crystal growth measurement and modeling of fluid flow in a crystallizer. S. Soc. Res. 3. S. Workshop on White Sugar Quality. Elsevier. 76:433-434.B.W. K. Sugar Technologists Manual: Chemical and Physical Data for Sugar Manufactures and Users. (abstract) 24: In press. Sugar J. Proc. In: Principles of Sugar Technology. Alexander. Dixon. Eng. Evaporation and circulation in the crystallization process. REFERENCES 1. Int. A. 1986. Aust. Boysan. E. R. Kadlek. G. Thesis. 2. 33:179-184. PhD. Natural and mechanical circulation in vacuum pans. R. Crystallization.W. D. Netherlands. 17. ed.Rein: Circulation in Vacuum Pans 13.W. Pneumatic agitation of high grade massecuite. Univ. 16. P. Elsevier. 1999. Studies on modeling circulation in sugar vacuum pans.L. Bartens. 14. P. James Cook. Circulation movements in sugar vacuum pans. 1959. Queensland Soc. Stobie. Verlag. 1966. H. Proc. 1998. Wright. Berlin. Sugar J. 17 . and T. Sugar Cane Technol. Vol. In: Principles of Sugar Technology. Stephens. A. 101(1206):302-306. Webre. Int. Honig. 2001. Van der Poel. Schiweck. Schwartz.M.. Pages: 394-452. Sugar Technology. 15. P.