circulacion en tachos de vacio - rein[1]

Rein: Circulation in Vacuum Pans

CIRCULATION IN VACUUM PANS

Peter W. Rein1, Luis F. Echeverri1 and Sumanta Acharya2 1Audubon Sugar Institute, Louisiana State University AgCenter

2Mechanical 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 convective-boiling 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.

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Journal American Society of Sugar Cane Technologists, Vol. 24, 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, there is still a lack of understanding of the important issues affecting vacuum pans design. This paper discusses some factors that affect the circulation in vacuum pans, alternatives for the analysis of flow characteristics and options for enhancing circulation. HEAT TRANSFER AND CIRCULATION

Convective-boiling heat transfer in the calandria tubes is the driving force for the circulation of massecuite. The buoyancy that results from the density difference between the vapor-phase and the surrounding massecuite is the main driving force, following a strong interaction between heat transfer and circulation. Any mechanism that reduces the evaporation rate will lower the circulation, and conversely, any reduction in the massecuite velocity will be detrimental to heat transfer (Bosworth, 1959). The balance between the buoyancy forces and the resistance to flow determines the circulation rate. The hydraulic resistance or friction depends essentially on the massecuite viscosity and the geometry of the passages within the pan, which should be smooth and offer minimum obstruction to flow (Webre, 1959).

The circulation mechanism in vacuum pans has not been completely understood due to the complexity of performing measurements inside the vessels (Wright, 1966) and the numerous operational regimes that are possible. Ideally, it is expected that a flow of massecuite and vapor rises after passing through the calandria tubes and reaches the surface. The vapor formed crosses the free massecuite surface before going to the condenser, while the liquid moves in a radial direction towards the center and goes back to the bottom through the downtake, to enter into the calandria tubes again. However, evidence exists which indicates that serious variations from this ideal circulation pattern could occur (Wright, 1966).

Using experimental measurements, Rouillard (1985) identified that evaporation rates, and therefore circulation, are increased as the heating steam pressure, the vacuum and the purity are raised. Conversely, longer tubes, higher strike levels and high concentrations reduce the evaporation rate. The evaporation rate changes dramatically during each strike, decreasing progressively and finding its minimum value at the last stage of each cycle (Table 1).

Table 1. Average evaporation rates measured in South Africa.

Type of Boiling Maximum kg /(m2 h) Minimum kg

/(m2 h) Grain / Seed 61 8.2

A 38 22 B 25 6.2 C 18 3.5

DESIGN ISSUES AFFECTING CIRCULATION

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Calandria

The calandria is essentially a tube and shell heat exchanger. The exchange area is normally around 6 m2/m3, but for high strike grades, where evaporation rates are higher, it can be increased to 9 m2/m3 (van der Poel et al., 1998). The different variations are shown in Figure 1.

Horizontal flat plate calandria: This is the most common design. A single central downtake is normally used, with vertical tubes arranged around it, constituting an efficient design of simple construction and maintenance. The diameter of the downtake is 30-50 % of the calandria diameter, usually around 40 % for natural circulation pans, while smaller downtakes can be used when a stirrer is provided.

Inclined plate calandria: The plates of the calandria are angled between 10 and 25 to the horizontal. The top plate is made horizontal in some cases.

Floating calandria: The downtake is located in the outer annular region. Although it was developed in an attempt to improve the circulation characteristics, measurements involving the use of a radioisotope tracer showed that the conventional central downtake performs better (Wright, 1966). The combination of the annular downtake with a small central downtake has also been tried, resulting in an inefficient design, prone to the formation of false grain (Webre, 1959)

Inclined plate calandriaConical enlargement

Conical bottom

Floating calandriaStraight side

Saucer bottom

Horizontal flat calandriaStraight sides"W" bottom

0.4*DcalandriaHeight 1.25 m

10-2558

18

L 0.7-1.2 m

Tubes D 0.1m

Strike

Disengagementvapor 1.5-3.0 m

Figure 1. Vacuum pan designs.

The downtake is sized to give a circulation ratio, defined as the ratio of the cross sectional area of the tubes to the area of the downtake, less than 2.5 to get a pan with reasonable circulation. Tube diameter and length

The high viscosities of the massecuites demand the use of large diameters to overcome the friction with the available buoyancy. The most common diameter is 100 mm (4 in). Tubes above this size exhibit less resistance and promote circulation, but the lower area/volume ratio is

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unfavorable (Webre, 1959) and the graining volume increases (Rouillard, 1985). Tubes with a length between 900 and 1500 mm (36-60 in) have been used, but currently they are designed normally between 700 and 1200 mm. Short tubes are selected for low-grade pans, while longer tubes are chosen for high-grade pans or when a stirrer is provided. Short tubes give the best heat transfer coefficients, and probably there is no justification for tubes longer than 900 mm, even if it results in a decrease in the heat exchange area (Webre, 1959). Pan shape

Bottom: The bottom shape should promote an even distribution of the massecuite across the calandria, without restricting circulation or providing stagnant areas, and allow the discharge of massecuite within an acceptable time.

Conical bottom: When coil pans dominated, a conical bottom was the preferred option, ensuring a smooth discharge of the massecuite. In the case of calandria pans, this shape is believed to lead to a significant stagnant zone. The bottom angle does not need to be greater than 20 (Bosworth, 1959). W bottom: While smaller angles of the bottom reduce the footing volume, they reduce the discharge velocity and lengthen the strike time. Two discharge valves are recommended. Usually the angle of the bottom is between 17 and 25 (van der Poel et al., 1998).

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. However, this was largely discredited after a negative effect on circulation was recognized (van der Poel et al., 1998). Numerous comparisons have shown that straight-sided pans perform better than low-head conically-enlarged pans (Chen and Chou, 1993).

Disengagement height above the massecuite: Enough space above the massecuite must be provided for disengagement of the liquid from the vapor, minimizing entrainment and allowing for froth explosion, which occurs often on starting a boiling or on cutting over. The disengagement volume should represent between 65-100 % of the strike volume, being normally between 1.53.0 m but preferably as high as possible. Strike height

As the strike height increases, the hydrostatic pressure in the calandria increases, raising the boiling temperature. As a result, the available temperature difference becomes smaller, leading to a reduction in evaporation rate and consequently reduced circulation. The situation is particularly critical at the end of the strike, when the highest level coincides with the maximum density and viscosity of the massecuite, all parameters that impair circulation.

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, after which it starts reducing progressively as the massecuite level increases. A height around 1.2

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1.6 m could give the best balance between quality, performance and capacity. 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, characterized by intermittent nucleation and a fluctuating generation of vapor. The bubbles in general display random behavior, which is influenced by many factors like bubble size, coalescence, the slip velocity, densities, viscosity, surface tension, and hydrostatic head, making the analysis complex. Evaporation is usually considered a steady state phenomenon, but in reality, it is chaotic and difficult to characterize in detail.

Wright (1966) proposed an eruptive mechanism after performing measurements of the circulation patterns in high-grade pans. The results showed an intermittent flow in the tubes and downward currents over the top calandria plate. 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. The vapor escapes, while massecuite penetrates the tube from above and below. In a similar way, other theories involving nucleation, coalescence of bubbles, formation of slugs or columns of vapor, temperature differences, flashing, eruptions, etc. have been proposed to describe the boiling in vacuum pans, but the lack of information has not enabled reasonable conclusions to be drawn. Forced or assisted circulation

Stirrers: Pan stirrers if properly designed can significantly improve the performance of a pan. The assisted circulation improves heat transfer and so shortens the duration of the batch boiling, thus improving capacity. It has also been shown that stirrers improve the quality of high-grade sugar produced (Rein, 1988). This is a consequence of the better circulation leading to more homogeneous crystallization conditions within the pan.

The first stage of a strike is characterized by a high evaporation rate. An intense evaporation occurs, and stirrers do not have much effect in this period. 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. As the strike height is increased, the hydrostatic pressure of the massecuite in the calandria increases the massecuite temperature, leading to a reduction in temperature gradient between massecuite and the heating vapor. During the last stages of the strike, the evaporation is minimal, resulting in a low bubble flow. The effect of the forced circulation becomes important at this stage, and is reflected in higher heat transfer coefficients with respect to natural circulation vacuum pans (Austmeyer, 1986). This is shown in Table 2.

There is no general agreement about the cost effectiveness of a stirrer. Its installation

gives all the advantages associated with good circulation mentioned previously, and promotes circulation at the end of the strike. In contrast, high capital costs, air leakage and high power consumption, particularly at the end of the strike, work against the use of stirrers.

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Table 2. Typical heat transfer coefficients in cane sugar vacuum pan boiling (Bubnik et al., 1995).

Natural circulation (W / m2K)

With stirrers (W / m2K)

Start 570 640 End 32 224

Mechanical agitators provide the option of achieving an acceptable heat transfer with a

temperature difference as low as 20 C, compared to at least 45-50 C in the absence of a stirrer. The use of lower pressure vapors becomes possible (e.g. coming from the 2nd or 3rd evaporator effect), allowing reductions in the factory steam requirements.

A stirrer can be used with a smaller diameter downtake, thus enabling a larger heating area to be installed for a given pan diameter.

Jigger steam: The installation of a sparge pipe under the calandria to blow low pressure steam is another option to assist the circulation, reducing the massecuite specific gravity in the calandria and reducing the effect of hydrostatic head. The simplicity, low cost and absence of moving parts of this alternative make it particularly straightforward to put into practice. Venting of incondensables into the jigger arrangement can be done to reduce the consumption of vapor, if steam economy is important.

It should be noted that the vapor admitted through the ring does not condense, but passes straight through the massecuite without causing any superheating or dissolution of crystal. The advantage of jigger steam is that it can be shut off at any time, unlike a stirrer, and can be adjusted to give the required degree of circulation.

Air injection: The injection of air instead of vapor has been adopted in some Spanish factories, allowing an increase in circulation with low energy consumption (15 % of that required by stirrers), low costs, and without breakage of crystals (van der Poel et al., 1998). Injection of compressed air at 750 kPa preheated to 65-70 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, 1999). However this option requires a much larger vacuum pump or ejector to remove the additional air and is not recommended here. 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, making it possible to resolve complex problems through the discretization of the geometry and numerical solution of the governing equations. 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. Currently there is a strong trend towards CFD assisted design, although experimentation still remains important, particularly for the most complicated cases.

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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, 2001; Chetty, 2002) and bagasse boilers (Dixon, 2001), where important improvements have been reported. A promising application of CFD is the analysis of flow in vacuum pans, in view of the importance of massecuite circulation and the complexity of the problem. A correct numerical approximation of the flow patterns inside the vessel could give valuable information for the optimization of the design of vacuum pans. 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), who wrote a program to solve the flow field, requiring some considerable assumptions in the process. Brown (1986) obtained results with a commercial CFD code for a stirred crystallizer, representing the calandria tubes as rings and neglecting the vapor phase. A more refined model was developed by Stephens (2002), who using the rings approach developed a numerical solution for boiling in the calandria, which was coupled with a commercial CFD code (CFX) for the solution of the flow in the rest of the domain.

In general, vacuum pans have been modeled using two-dimensional axisymmetric isothermal two-phase models. The massecuite has been assumed to be Newtonian, and the computational domain has been restricted to the zone below the massecuite free surface. The numerical results have suggested undesirable recirculation zones, as well as an important reduction in circulation when the strike height and the viscosity are increased. Nonetheless, alternatives to reduce the resistance to the flow or improve the fluid circulation have not been explored or reported.

The Audubon Sugar Institute in conjunction with the LSU mechanical engineering department is working currently on the study of the flow in vacuum pans. The main characteristics of the CFD model and some initial results are presented here. CFD Model

In a vacuum pan there are three phases present (liquid: mother liquor - gas: water vapor - solid: sugar crystals). However, for convenience the case can be analyzed as a two-phase flow (liquid and vapor), which is an acceptable approach considering the high viscosity of the massecuite (Bunton, 1982).

For the grid generation and CFD solution, a commercial code has been used (FLUENT). A two-dimensional axisymmetric problem has been considered to simplify the geometry. A grid independence study was performed, finding that between 90,000 and 180,000 quadrilateral elements the results were the same. It is important to use a grid fine enough so that the accuracy of the results is independent of the grid points used.

The Eulerian-Eulerian multiphase model was selected, 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. With this model, a Eulerian

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treatment is used for each phase, resulting in coupled transport equations for the liquid and the vapor phase. Solving the transport equations for both phases requires substantial computational effort, and can require several days of processing before a solution is reached.

The temperature field is assumed isothermal. Thus the calculations are limited to small temperature differences. Since the vapor bubbles are the main driving force for the circulation of massecuite, the calculations are expected to be representative of the real situation. The heat transfer phenomena associated with the process have been studied extensively by Rouillard (1985) and Austmeyer (1986).

As driving force for the circulation of massecuite, steam is assumed to be injected in the zone corresponding to the calandria tubes. The calandria is represented as a set of concentric rings (Brown, 1992; Stephens, 2001). It is assumed that the same evaporation takes place across the calandria. However, knowing that tubes with less circulation evaporate less, a velocity profile across the calandria smoother than in real vacuum pans is expected. The steam is injected without any momentum, and rises due to its density difference with the surrounding massecuite.

As a case study, a 90 m3 pan was selected, which is considered a modern well-designed pan giving a good performance. In addition three other cases, a pan with a conical enlargement, a pan with a steam jigger installed, and a pan with a conical bottom, have been undertaken in this study.

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). Table 3. Massecuite properties and evaporation used to represent a B-strike.

Strike height (m) 1.0 Absolute pressure (kPa) 14.0 Density massecuite (kg/m3) 1490 Dynamic viscosity massecuite (Pa.s) 52.1 Evaporation Coefficient (kg/m2.h) 11 Density vapor (kg/m3) 0.183 Dynamic viscosity vapor (Pa.s) 1.12E-05

RESULTS AND DISCUSSION CASE 1. Reference case: W-bottom, straight sides

The flow simulations have illustrated how the acceleration of vapor rising through the calandria tubes reduces the local static pressure, which falls below the pressure in the downtake. The created pressure difference promotes the up flow of massecuite in the calandria and down flow in the downtake.

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a. Contours volume fraction of vapor (%) b. Contours of static pressure (Pa)

c. Vapor velocity vectors d. Massecuite velocity vectors Figure 2. CFD results for straight-sided, W-bottom, vacuum pan.

The resultant distribution of the flow across the calandria is even, with exception of the ring located closest to the downtake, which has a smaller flow and suggests that the tubes located in this region have less massecuite flowing through them.

The velocities below the calandria are low enough to let the hydrostatic head determine the pressure field, resulting in practically the same static pressure in the inlet of all the tubes (Figure 2.b). Consequently, unless a blatantly wrong geometry is chosen for the bottom, it will not have any important implication on the even distribution of the massecuite across the calandria, which is desirable from the heat transfer point of view and to have uniform conditions within the vessel.

An undesirable large vorticity over the top tube plate was found for all the runs. 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). As a consequence of this vorticity, there is an important reduction in the useful area of the downtake. There is also a narrower area for the liquid that travels in radial direction towards the center, and some

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recirculation of part of the liquid in the zone over the top calandria plate. This vorticity does not contribute at all to the desired circulation, and could lead to different crystallization rates for crystals that are trapped there.

The results indicate a small vorticity over the calandria plate close to the sides of the pan (Figure 2.d). However, this does not show any effect on the circulation through the outer rings, suggesting that it does not influence significantly the flow of the massecuite through the calandria tubes. CASE 2. Conical enlargement.

The results indicated that the conical enlargement leads to another large vorticity over the top tube plate, between the sides and roughly the middle of the calandria width (Figure 3). This vorticity interacts with the vorticity located toward the downtake, and its size exceeds expectation. The additional vorticity involves a large downflow near the sides, which has a negative momentum in the vertical direction, opposing the rise of massecuite, and that results in a reduction of 19 % in the circulation in the outer rings (Figure 4). The outer rings represent precisely the zone of the calandria where more tubes are present.

a. Contours volume fraction of vapor (%) b. Massecuite velocity vectors

Figure 3. CFD results for vacuum pan with conical enlargement.

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0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

Radius (m)

Liqu

id v

eloc

ity (m

/s

W-bottom W-flared

W-jigger V-bottom

Figure 4. Liquid velocity results across the calandria for different pan geometries. CASE 3. Circulation assisted with steam jiggers.

A simplified steam jigger system is considered, consisting of a single sparge ring under the calandria. The steam injected below the calandria increased substantially the circulation in the pan, accelerating the flow particularly in the rings located over the steam jigger (Figures 4 and 5). This indicates that the tubes located closer to the points where the gas is injected will have a very high circulation. Vorticity over the top tube plate increase, but no significant reduction in the circulation in the other tubes was observed. The average circulation increased 24% respect the same geometry without steam injection. 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.


Figure 5. CFD results for vacuum pan assisted with steam jiggers.

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CASE 4. Conical or V bottom.

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, but with a massecuite flow 5 % higher. The higher circulation is attributed to the smaller resistance of the bottom to the downflow and smoother change of direction. The expected stagnation zone at the lowest part of the pan is small.


Figure 6. CFD results for straight-sided with conical or V-bottom.

In spite of the assumptions made in modeling the problem, the results obtained agree with experience, indicating that the W-bottom is efficient in distributing the massecuite, and that the flared sides or conical enlargement impairs the circulation, while the use of steam jiggers improves circulation.

This work will continue using CFD to evaluate the factors affecting circulation in vacuum pans (e.g. pan geometry, strike height, viscosity, type of calandria, downtake area, and steam jigger design). 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, the CFD will be applied to explore alternatives for improving the circulation in batch vacuum pans, and extended to the analysis of continuous vacuum pans. PRACTICAL OPTIONS FOR IMPROVING CIRCULATION

Stirrers

The stirrer itself is located in the downtake if it is an axial flow impeller, or else just below the downtake in the case of a radial flow impeller. Installed power should be in the range 1 to 1.7 kW/m3. Tip speed has to be kept below 7 m/s (23 ft/s), or else false grain will form.

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Each stirrer should be designed for the particular pan and its duty individually. There is no universal stirrer for all occasions. If good results are to be obtained, it is seldom possible to transfer a stirrer from one pan to another without redesigning the arrangement. Conversion of flared to straight-sided pans

As shown earlier, the conical enlargement above the calandria has a negative effect on pan circulation. This flaring of pans was done to increase the strike/seed ratio of pans, but this design is now largely obsolete. However, there are a large number of these pans still installed in sugar mills. 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. The Lula conversion is reported in detail at this conference (Bergeron and Carline, 2003). As a result of the conversion, pan volume increased by 25 % and the heating surface by 52 %. The circulation ratio is affected adversely, but the circulator and the absence of the flared body lead to a significant net improvement in pan performance. C pan boiling times dropped by 2 hours. Condensate and incondensables removal

Adequate arrangements for the removal of condensate and incondensable gases must be made. It is important that these details are given proper attention, as they can be the cause of under-performance if not properly designed. Condensate is generally removed from the lowest point of the shell, from more than one outlet. It is good to be generous in sizing these outlets. The size of the condensate drains should be based on liquid outlet velocities of less than 0.45 m/s (1.5 ft/s) at maximum evaporation rate.

Incondensables need to be purged by the steam flow and are generally located at a point furthest from the steam inlet. This ensures a positive purging of incondensables. The best arrangement is a radial flow of steam from a vapor belt around the calandria. As the steam flow is radially inwards, the incondensables offtake arrangements should consist of two rings around the downtake, one at the top and one at the bottom of the calandria. Care needs to be given to the size and number of holes in the incondensables offtake rings to ensure uniform offtake around the downtake, and that sufficient quantity is vented. The two incondensable offtakes should be separately vented. Incondensable quantity is estimated as about 100 mg/kg of steam. In order to ensure total venting of incondensables, the amount vented should be 100 times the quantity of incondensables. Thus the total quantity to be vented should be about 1 % of the steam flow to the calandria.

Thermostatic valves can be used to bleed incondensables if steam economy is important. Removing Resistance to Circulation

Any unnecessary steel inside the pan represents a resistance to circulation. Attempts should be made to get rid of any unnecessary ironware in the massecuite. Louvers or any other steel devices installed in the pan in the mistaken idea that they will improve circulation should be removed.

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Condensate and incondensable venting lines should not be run through the massecuite. Incondensable gases should be vented through the outside of the calandria, and not up through the massecuite. Likewise, condensate outlets should be positioned at the periphery of the pan, and should not run from the bottom tube plate down through the massecuite.

The syrup or molasses feed system should also represent minimum obstruction to the circulating massecuite. 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. If the feed is conditioned and at a higher temperature than the boiling massecuite, the feed must be directed under the calandria, so that the flash will aid circulation. Steam-assisted circulation

Sparging of steam underneath the calandria has often been used as a means of improving circulation. It has the effect of increasing the velocity of the massecuite through the tubes, thus increasing the rate of heat transfer to the massecuite and further improving circulation.

If the pan is badly designed and/or the massecuite level gets too high, there comes a time with a high viscosity massecuite when the pan stops boiling. Heat transfer in the tube is insufficient to cause significant boiling and movement of massecuite slows. This is a vicious cycle. As the heat transfer rate reduces, the rate of movement of massecuite drops, and heat transfer drops even further. Steam assisted circulation can get such a pan to start boiling again. Thus the effect of steam assistance, often called steam jigger is to enable a higher strike height to be achieved as well as a reduced boiling time.

Before the 2002 season, steam jiggers were fitted to three C massecuite pans at Raceland. These pans are fitted with stirrers, but it was still considered that the circulation in these pans could be improved, as it was not possible to run them continuously on vapor 1, and high Brix C massecuites could not easily be achieved. The steam jigger consisted of a 51 mm (2) ring, fitted under the calandria, one-third of the width of the calandria from the pan periphery. It had 32 x 9.5 mm (3/8) holes drilled in the pipe, facing downwards. These pans were all nominally 39.6 m3 (1400 ft3) pans, with the diameter at the calandria 0.4 m (13 ft) and above the calandria 0.47 m (15.5 ft). These pans already had circulators fitted, but still the steam jigger had a marked effect on boiling times and strike heights.

The sizing of the system was based on vapor injection at a maximum rate of 25 kg/m3 hr, giving a maximum vapor 1 rate of 1.15 ton/hr. This compares with estimated steam flows to the pan of 17 ton/hr dropping to about 3.7 ton/hr on average. The number of holes and their size is determined by the required flow rate of vapor. 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. This ensures that roughly equal flows will be achieved through every hole (Knaebel, 1981). This simple arrangement may not be satisfactory in a larger pan, where a different sparging arrangement should be used to give a more uniform distribution of vapor under the calandria.

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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. Nonetheless the following was achieved:

The pans could be boiled throughout the full cycle using vapor 1. In 2001, the pans were run on exhaust during the final stages of the boiling.

Average C massecuite Brix increased from 95.6 to 96.8. After the most severe processing problems in October and November were over, this enabled the mill to achieve their lowest recorded molasses target purity differences.

Boiling times were roughly the same in both years in spite of conditions which in some mills led to pans stopping boiling altogether.

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.

During November, the stirrer in pan 1 stopped working. Instead of stopping to repair the stirrer, the mill was able to continue operating the pan with the stirrer in the downtake representing a restriction to circulation. Figure 7 shows that boiling times increased, but the pan operation continued

Pan #1 Boiling Time For November 2002

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100

Strike #

Hou

rs F

or B

oilin

g

Pan with Circulator and Jigger Steam

Pan without Circulator, Only Jigger

Nov. 18,2002

Figure 7. Boiling times in Raceland pan 1 during November 2002.

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CONCLUSIONS

Circulation in a vacuum pan is one of the most important parameters affecting performance. A number of factors which influence the operation of vacuum pans have been identified. Some relatively minor changes can be made to improve capacity and performance. However it is clear that most designs are far from being optimal. CFD appears to offer a tool for progressing efforts to improve the design and operation of both batch and continuous pans.

REFERENCES

1. Austmeyer, K.E. 1986. Analysis of sugar boiling and its technical consequences: Part II. Heat transfer during sugar boiling. Int. Sugar J. 88(1046):23-29.

2. Bergeron, S., and G. Carline. 2003. Changing the heating surface to volume ratio on a

low grade batch pan. J. Amer. Soc. Sugar Cane Technol. (abstract) 24: In press.

3. Bosworth, R. 1959. Evaporation and circulation in the crystallization process. Pages: 371-393. In: Principles of Sugar Technology. Vol. 2, Crystallization. P. Honig, ed. Elsevier, Netherlands.

4. Brown, D.J., K. Alexander, and F. Boysan. 1992. Crystal growth measurement and

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ABSTRACTINTRODUCTIONGrain / Seed

DESIGN ISSUES AFFECTING CIRCULATION

CalandriaUnsteady nature of boilingStirrers: Pan stirrers if properly designed can significantly improve the performance of a pan. The assisted circulation improves heat transfer and so shortens the duration of the batch boiling, thus improving capacity. It has also been shown that stirJigger steam: The installation of a sparge pipe under the calandria to blow low pressure steam is another option to assist the circulation, reducing the massecuite specific gravity in the calandria and reducing the effect of hydrostatic head. The simplicAir injection: The injection of air instead of vapor has been adopted in some Spanish factories, allowing an increase in circulation with low energy consumption (15 % of that required by stirrers), low costs, and without breakage of crystals (van der APPLICATION OF CFD IN THE SUGAR INDUSTRY

RESULTS AND DISCUSSIONPRACTICAL OPTIONS FOR IMPROVING CIRCULATIONStirrersConversion of flared to straight-sided pans

Condensate and incondensables removalCONCLUSIONS

REFERENCES

circulacion en tachos de vacio - rein[1]

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