THE EFFECT OF FINAL EFFECT OPERATING … Peacock...Peacock, S.D. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 _____ THE EFFECT OF FINAL EFFECT OPERATING PRESSURE ON SUCROSE DEGRADATION

Peacock, S.D. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 ________________________________________________________________________________________________

THE EFFECT OF FINAL EFFECT OPERATING PRESSURE ON SUCROSE DEGRADATION IN EVAPORATOR STATIONS

By

S.D. PEACOCK

Tongaat Hulett Sugar Limited

Private Bag 3, Glenashley, 4022, South Africa [email protected]

KEYWORDS: Evaporators, Vacuum, Sucrose Degradation, Syrup Temperature.

Abstract THE CHOICE of the operating pressure of the final effect vessel of an evaporator station has an effect on the heat transfer coefficient achieved in this vessel. This impacts on the evaporative capacity of the station. The selected final effect pressure also impacts on the rate of sucrose degradation in the evaporator station, due to its direct effect on syrup temperature and to a secondary effect on the residence time of the juice in the individual vessels. Changing the final effect pressure will change the distribution of pressures across all effects, which will affect the rate of sucrose degradation in all vessels, not only those in the final effect. In order to study these phenomena, it is therefore necessary to model heat transfer and inversion in all vessels, rather than focusing on the final effect vessels alone. By carrying out computer simulations, the effect of varying the final effect pressure on the degradation of sucrose across the station can be evaluated. Energy efficiency effects may also be investigated. Based on the results obtained, recommendations can be made on the optimal choice of operating pressure for a final effect evaporator.

Introduction The fundamental principles of multiple effect evaporation, as originally formulated by Rillieux, are reasonably simple and easy to understand. However, complex interactions occur within an evaporator station under varying operating conditions that may not be readily apparent when using �common sense� sugar engineering. Effective evaporator station design therefore requires detailed modelling to ensure that the correct design decisions are made. This was particularly illustrated by Love et al. (1999) in their investigation of the robustness of an evaporator station when faced with variations in vapour bleed flow rate. The objective of the current study is to determine the optimum final effect pressure to be employed in an evaporator station, based on the consideration of such aspects as heat transfer capacity, residence time, sucrose degradation and energy efficiency. A literature survey was carried out to investigate each of these portions of the problem in detail. The individual components were then combined in an overall model of a multiple effect evaporator station. Final effect pressure The capacity of a multiple effect evaporator station is dependent on the overall temperature difference driving force that is available to carry out the evaporation process. Typically, this total driving force is of the order of 65!C (from exhaust steam at around 200 kPa(a) to a final effect operating pressure of around 15 kPa(a)). While the available temperature difference can be

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increased by lowering the final effect operating pressure, this may not always be advisable. As the temperature and pressure within the final vessel decrease, so too does the heat transfer coefficient in this vessel (which is strongly affected by the increased syrup viscosity under these conditions). An optimum vacuum level appears to exist (from a pure capacity point of view) as a result of the conflicting influences of the decreased heat transfer coefficient and the increased temperature difference driving force.

Based on factory trials at the Racecourse mill, Crawford (1953) found the optimum level of vacuum in the final effect to be around 25� Hg (equivalent to 17 kPa(a)). Smith and Taylor (1981) found that very low final effect pressures were detrimental to evaporative capacity. A final effect vapour temperature of between 55 and 60!C was considered to be optimal. This temperature is achieved at vessel pressures between 16 and 20 kPa(a). Outside of this pressure range, the total evaporative capacity of the station was found to decrease rapidly. A further complicating factor in the analysis of the optimum final effect pressure in an evaporator station is the control system that is employed. It is common practice to install a throttle valve in one of the vapour lines in the station (typically in the vapour line feeding the vessel immediately after the last bleed vapour stream has been withdrawn). In order to reduce the rate of evaporation in the station, this valve is throttled, resulting in a pressure drop in the vapour line which reduces the available driving force to the remainder of the evaporator vessels. This results in a decrease in evaporative capacity without substantially affecting the energy efficiency of the evaporator station. When an increase in capacity is required, the throttle valve is opened. In designing an evaporator station, some degree of surplus capacity will usually be included in order to allow for fouling of the heating surface during operation and for a degree of conservatism in the design. Under normal operating conditions, it is therefore likely that the vapour throttle valve will be partially closed. The resulting pressure drop across the valve may be viewed as an indicator of the excess capacity that is available in the evaporator station above the evaporative capacity that is actually required. Because of the resulting pressure drop, the boiling temperatures of the latter vessels in the station will be reduced. However, the pressures of all vessels upstream of the throttle valve may be slightly increased.

Should an attempt be made to reduce the quantity of sucrose destroyed in the evaporator station by reducing the final effect operating pressure, then the heat transfer coefficient in this final vessel will decrease. Consequently, the overall capacity of the evaporator station will be reduced. This may be acceptable if the intention is to achieve a reduction in inversion at the expense of evaporative capacity.

In order to maintain the required rate of evaporation, however, the control system will compensate by opening the vapour throttle valve. This may lead to an increase in boiling temperature in some of the latter vessels of the station, while possibly decreasing the temperatures in the earlier vessels of the set. The overall impact of these changes on the total level of sucrose degradation experienced is difficult to predict without carrying out the relevant calculations. From this brief survey, it is apparent that no firm recommendations can be made concerning the optimum final effect pressure to be employed without carrying out detailed analysis of the evaporator station as a whole. This is particularly true if the objective is to achieve a reduction in sucrose degradation, as a �common sense� approach to the problem may perhaps achieve the wrong result. Heat transfer

Numerous attempts have been made over the years to correlate the heat transfer coefficient (HTC) for individual evaporator vessels against their operating conditions, with the most important of these being the temperature and concentration of the juice within the vessel. Although juice

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temperature and brix are strongly correlated in most sugar industry evaporator stations (with decreasing temperatures being associated with increasing brix levels), it is widely accepted that evaporator HTC values increase with increasing temperature and decrease with increasing concentration. This is demonstrated by considering the temperature and concentration dependence of a small sample of published correlations, as summarised in Table 1. In some cases, the concentration dependence of the HTC is expressed in terms of viscosity (") which increases with increasing brix and is also itself strongly dependent on temperature.

Table 1�Some literature correlations for heat transfer coefficient prediction.

Reference Temperature dependence Concentration dependence Macdonald and Rodgers (1947) HTC # T2 HTC # "�0.5

Saranin and Jenkins (1954) HTC # T1.93 HTC # "�0,.36

Batstone and Prince (1969) HTC # T2 HTC # Brix�0.5

Zagrodzki and Kubasiewicz (1977) HTC # T1.5 HTC # "�0.6

Hoffman (1990) HTC # T1.54 HTC # Brix�0.7

Van der Poel et al. (1998) HTC # T1 HTC # Brix�1

All of the correlations in the table, along with a number of others from the literature, were tested against measured heat transfer data from Chang (1964), Hussey (1973) and Taylor and Smith (1981)1. This comparison showed the correlation of van der Poel et al. (1998) to be the most suitable for characterising the experimental data, both in terms of numerical accuracy and in terms of correctly reflecting the �shape� of the data.

The correlation of van der Poel et al. (1998) is based on the well-known �Swedish formula�, as presented by Macdonald and Rodgers (1947) and other authors. It has the following form:

DS

j

wTk

U$

% ... (1)

where U is the overall heat transfer coefficient for the evaporator [kW/(m2.K)], k is a constant parameter (with a value of 0.465), Tj is the boiling point temperature of the juice within the evaporator vessel [!C] and wDS is the concentration of the boiling juice leaving the evaporator vessel [%(m/m)]. For the purposes of the current study, the k coefficient in equation (1) was adjusted to a value of 0.573518 in order to better fit the available experimental data. The results are shown in Figure 1, where the measured and predicted heat transfer coefficient values are shown as a function of viscosity (on a logarithmic scale)2. While the above correlation can be used to predict the heat transfer coefficients in the majority of evaporator vessels in the station, it was decided not to use this relationship for the final effect evaporator vessel. Instead, the correlation of Smith and Taylor (1981) was used, as it was specifically developed for this purpose based on a dedicated set of experimental data generated at a number of factories. 1 Taylor, L.A.W. and Smith, I.A. (1981). Influence of downtake sealing and last effect vapour temperature on

evaporator HTC. Tongaat Hulett Sugar Limited - unpublished internal report. 2 For the purposes of producing the graph in Figure 1, solution viscosities were calculated using the correlation of

Genotelle (1978) for pure sucrose solutions. Work carried out by Tongaat Hulett Sugar has shown that actual syrup viscosities do not substantially differ from those of the equivalent pure sucrose solutions.

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Fig. 1�Measured and predicted HTC values. The concentration dependence of the HTC is not of primary importance in final effect vessels, as the liquor leaving these evaporators is generally at the final syrup brix (typically between 65 and 68%). It is rather the temperature dependence of the HTC which is of more importance. The relationship used for final effect vessels is therefore:

& ' 13.1T034.0U v ($% ... (2)

where Tv is the saturated vapour temperature within the final effect vessel [!C]. A comparison between equation (2) and the experimental data of Chang (1964), Hussey (1973) and Taylor and Smith (1981)1 is given in Figure 2.

Fig. 2�Final effect HTC correlation compared against experimental data.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.1 1.0 10.0 100.0

Viscosity [cP]

Hea

t Tra

nsfe

r Coe

ffici

ent

[kW

/(m2.

K)]

Chang (1964)

Hussey (1973)

Taylor & Smith (1981)

Predicted HTC

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Final Effect Pressure [kPa(a)]

Hea

t Tra

nsfe

r Coe

ffici

ent

[kW

/(m2.

K)]

Chang (1964)

Hussey (1973)

Taylor & Smith (1981)

Correlation

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Residence time The residence time of juice in an evaporator is a function of the design and type of the vessel

(Kestner, Robert, falling film, plate, etc.), the flow rate of liquid through the vessel and the liquid holdup/operating volume within the unit itself. For the purposes of the current study, only Kestner (climbing film) and Robert vessels were considered as these still make up the bulk of the evaporators in the cane sugar industry.

The normal operating volume of boiling juice in a Kestner evaporator is equivalent to a static liquid holdup within the tubes of between 0.5 and 1.0 m above the bottom tubeplate (Allan, 1975; James et al., 1978). Average residence times of two to three minutes are typical (Allan, 1975) although mean retention times of up to five minutes have been occasionally measured in tests carried out by Tongaat Hulett Sugar. For the purposes of the current study, an operating level of 1.0 m within the tubes was assumed.

As a result of the natural circulation patterns in Robert vessels, they can be expected to behave in a similar way to well-mixed tanks (Wittwer and Mauch, 1983). This is borne out in practice by the shape of the residence time distribution curves published in the sugar literature. Average residence times of between five and 20 minutes are typical, with longer residence times often being experienced towards the tail of the evaporator station (Reva et al., 1975; Wong Sak Hoi, and Tse Chi Shum, 1996; Van der Poel et al., 1998; Krook, 2001; Eggleston et al., 2004). Numerous tests carried out by Tongaat Hulett Sugar and by others have confirmed these trends and have shown that the theoretical calculation of residence times for Robert vessels based on their expected liquid holdup yields reasonably accurate values (Purchase et al., 1987).

Optimum heat transfer in Robert vessels is achieved by maintaining static liquid levels of between 25 and 50% of the tube length of the vessel, with lower levels being optimal when larger temperature differences are employed across the individual evaporator vessel (Guo et al., 1983). Under normal operating conditions, a holdup equivalent to a static liquid height of 30 to 40% of the length of the tubes can be considered typical (Watson, 1987).

Van der Poel et al. (1998) report the typical liquid holdup in a Robert evaporator to be between 5 and 7 litres/m2 of heating surface area, with higher values being experienced in vessels with relatively short tubes. For the purposes of the current study, an operating level of 35% of the length of the tubes was assumed.

In all cases studied here, the actual liquid holdup in individual evaporator vessels was determined by means of construction drawings (given the static liquid holdups as discussed above). The liquid inventory within the Kestner vessels was found to range between 3.2 and 4.2 L/m2 of heating surface, while the liquid inventory in the Robert vessels was found to vary between 9.9 and 10.5 L/m2.

It is important to remember that sucrose degradation is not a linear function of reaction time, but rather an exponential function (pseudo first order reaction). Consequently, it is not only the average residence time within an evaporator vessel that is relevant, but also the shape of the retention time distribution function (Wittwer and Mauch, 1983; Wittwer, 1984).

However, given that the degree of sucrose inversion is usually very low in sugar production applications, many authors have simplified their calculations by assuming a linear (zero order) rate of reaction. This simplifying approach has also been used in the current study. Sucrose degradation

The degradation of sucrose in aqueous solutions is catalysed by both hydrogen ions and hydroxyl ions, but is much greater with the former. The rates of these two reactions were investigated by Vukov (1965) and found to be additive. We may thus write the composite rate of sucrose degradation as:

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& ' WSOHH kkr )$)$*% (* ... (3)

where r is the overall reaction rate [kg sucrose destroyed/(m3.h)], βS is the volumetric concentration of sucrose in the solution [kg/m3], βW is the volumetric concentration of water in the solution [kg/m3] and the values of the two reaction rate constants [m3/(kg.h)] are given by:

& ' pHT

567069.15klog H10 ((%* ... (4)

& ' pOH30.0T

534017.9klog OH10 $((%( ... (5)

(after conversion from the original units used by Vukov (1965) to more conventional SI units). In equations (3) through (5), kH+ and kOH� are the reaction rate constants for the reactions catalysed by hydrogen ions and hydroxyl ions, respectively, T is the solution temperature [K] and pH and pOH have their conventional meanings (Peacock and Starzak, 1995). The validity of these equations was experimentally confirmed for sucrose concentrations from 0 to 900 kg/m3, temperatures from 20 to 130!C, pH values greater than 1 and pOH values greater than 1. A number of other researchers have also investigated sucrose degradation and their published reaction rates are in reasonable agreement with equations (3) through (5) above (Schäffler, 1984; Edye and Clark, 1995). These relationships may therefore be considered suitable for modelling the degradation of sucrose in factory juices and syrups and have been used in the current study.

Sucrose degradation in aqueous solutions may also be catalysed by the presence of inorganic salts. However, while Vukov (1965) and other authors have studied the rate of this reaction, their work was based solely on the inorganic salt profile of beet juices and is unlikely to apply to lower-pH cane streams. For the purposes of the current study, therefore, the salt-catalysed degradation of sucrose has been ignored. As a consequence of this simplifying assumption, it is expected that the rates of sucrose degradation predicted in the study will be lower than those actually experienced in industrial practice. It should be noted that the pH and pOH values used in equations (4) and (5) do not refer to the values as usually measured in a laboratory at room temperature, but rather to the actual operating conditions experienced by the aqueous solution. The value of pH declines as the temperature rises, due to the increased dissociation of water at higher temperatures. It is critical to take this change into account when modelling inversion, because the logarithmic nature of the pH scale means that a small change in pH results in a substantial change in hydrogen ion concentration (and hence inversion). The pH at elevated operating temperatures may be predicted by means of the following relationship (Schäffler, 1987):

& ' & ' 2pH0017.0pH015.00339.0dT

dpH $($*(% ... (6)

In addition, it is important to note that the dissociation constant (pK) of water also changes with temperature (Honig, 1963; Schäffler, 1987; Edye and Clark, 1995; Eggleston and Amorim, 2006). We may use the fact that pK = pH + pOH to determine a value for pOH from a known pH value, by means of the following relationship from Kell (1975):

& 'W102

5

3

7

2

6

logT

105641.8T

3.1262957.15

T10984.3

T103456.2

T7.541969.51pK

+$⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ ,*⎟⎠⎞

⎜⎝⎛(

(⎟⎟⎠

⎞⎜⎜⎝

⎛ ,*⎟⎟⎠

⎞⎜⎜⎝

⎛ ,*⎟⎠⎞

⎜⎝⎛(%

... (7)

where T is the temperature [K] and ρW is the density of pure water [kg/m3].

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Apart from the changes in pH and pOH that occur with temperature, the pH of juice also decreases over the evaporator station as a result of the increase in concentration and subsequent drop in water dissociation. Many other chemical reactions and precipitations also occur that contribute to the drop in pH, including the sucrose degradation reaction itself. Typical clear juice pH values of between 7.0 and 7.3 usually result in a syrup with a pH in the range between 6.3 and 6.5 (Eggleston et al., 2004). Purchase et al. (1987) showed a decrease of 0.01405 in pH for every 1% increase in juice brix, when a juice sample was evaporated under laboratory conditions. For the purposes of the current study, a clear juice pH of 7.0 has been used. A linear decrease in pH with brix has been assumed, based on the work of Purchase et al. (1987). This results in syrup pH values of 6.3 to 6.4.

The rate of sucrose degradation is greatest at low pH (under acidic conditions, a one-unit drop in pH increases the rate tenfold), high temperatures (a 10!C temperature increase more than doubles the rate), high water concentrations (i.e. low brixes) and long residence times (Schäffler, 1984; Eggleston and Amorim, 2006). In standard multiple effect evaporator stations, the greatest sucrose losses occur in the vessels towards the front of the set where the brixes are lowest and juice temperatures are at or above 100!C (Reva et al., 1975; Hugot, 1986; Eggleston et al., 2004). Up to 90% of the total sucrose losses can occur in the first two effects (Purchase et al., 1987). The lower temperatures and increasing brix values towards the tail result in dramatically reduced losses towards the latter effects, even though the juice residence times in each vessel tend to increase.

It is of interest to note that Eggleston and her co-workers found the presence of evaporator scale to increase the rate of sucrose degradation in an evaporator station, through the impact of decreased heat transfer coefficients on residence times and the temperatures of the heating surfaces (Eggleston et al., 2004; Eggleston and Amorim, 2006). This is of particular interest in the current study, where it is expected that lower heat transfer rates will be achieved at decreased final effect evaporator pressures.

It is assumed that any sucrose lost through degradation in the evaporator station would have been sold on the world export market at an average realisation of 13 US cents per pound. This is equivalent to a value of US$286.60 per tonne of raw sugar.

Although colour formation across the evaporator station is not explicitly considered in this study, it is expected that conditions which lead to an increase in sucrose degradation will also lead to increased colour formation (Wittwer and Mauch, 1983; Wittwer, 1984; Smejkal et al., 2005). As with sucrose degradation, the majority of the colour formation in an evaporator station occurs in the first vessels, with negligible colour increase being experienced in the final two effects (Honig, 1963).

Evaporator station modelling

Evaporator station modelling was carried out using an updated version of the software developed by Hoekstra (1981). The operation of two separate evaporator stations was simulated, namely a quadruple effect processing 300 t/h of clear juice and a more energy-efficient quintuple effect processing 650 t/h of clear juice. The configurations of the two evaporator stations are described in Table 2. The quadruple effect evaporator station is installed in a small factory of reasonably low energy efficiency. The steam requirements of the rawhouse are met through the use of V1 vapour bleed and the capacity of the station is controlled by means of a vapour throttle valve installed in the V1 line entering the second effect Robert vessel. By contrast, the quintuple effect station is installed in a larger, more energy-efficient factory which exports bagasse fibre and cogenerates a small amount of electrical power. Extensive V2 vapour bleeding is used to meet the steam demand of the rawhouse and control is exercised through a vapour throttle valve installed in the V2 vapour line

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feeding the third effect Robert vessels. These two mills are fairly typical of the conditions prevalent in the sugar industry worldwide.

Table 2�Evaporator station configurations to be modelled.

Station A Station B

First Effect 3 x Kestner vessels (1982, 1666 and 1666 m2)

2 x Kestner vessels (5500 and 5500 m2)

Second Effect 1435 m2 Robert vessel 2 x Kestner vessels (5500 and 5500 m2)

Third Effect 1435 m2 Robert vessel 2 x Robert vessels (1000 and 1000 m2)

Fourth Effect 2 x Robert vessels (1435 and 1435 m2)

2 x Robert vessels (1000 and 1000 m2)

Fifth Effect 2 x Robert vessels (1430 and 1430 m2)

Results The results of the modelling are presented in Figures 3 through 8. Figures 3 and 4 show the predicted degradation of sucrose in the two evaporator stations as a function of the final effect operating pressure. In station A, the optimum pressure lies at around 14 kPa(a), while the optimum pressure for station B falls at around 12 kPa(a). Figures 5 and 6 show the sucrose degradation occurring in the two evaporator stations as a function of the pressure drop across the vapour throttle valve. This pressure drop is an indication of the degree of spare capacity that exists in the evaporator station. From the results of the calculations, it is apparent that the minimum inversion occurs at the point where the vapour throttle valve is providing the greatest pressure drop (i.e. where the evaporator station is at its maximum capacity point).

Fig. 3�Sucrose degradation as a function of the final effect pressure in station A.

4 800

4 900

5 000

5 100

5 200

5 300

10 11 12 13 14 15 16 17 18 19 20


Sucr

ose

Lost

[kg

/wk]

1 376

1 404

1 433

1 462

1 490

1 519Va

lue

Lost

[U

S$/w

k]Station A

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Fig. 4�Sucrose degradation as a function of the final effect pressure in station B.

Fig. 5�Sucrose degradation as a function of throttle valve pressure drop in station A.

4 800

4 900

5 000

5 100

5 200

5 300

0 2 4 6 8 10 12 14 16 18 20

Throttle Valve Pressure Drop [kPa]

Sucr

ose

Lost

[kg

/wk]

Station A

20 900

21 000

21 100

21 200

21 300

21 400

9 10 11 12 13 14 15 16 17 18


Sucr

ose

Lost

[kg

/wk]

5 990

6 019

6 047

6 076

6 105

6 133

Valu

e Lo

st [

US$

/wk]

Station B

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Fig. 6�Sucrose degradation as a function of throttle valve pressure drop in station B.

The modelling results can be more easily understood with reference to Figures 7 and 8, which show the marginal sucrose loss in each evaporator effect as a function of the final effect pressure3.

Fig. 7�Marginal sucrose degradation in each effect of station A.

3 The marginal sucrose loss at a particular final effect pressure (in a given evaporator effect) is defined as the

sucrose loss at that pressure minus the minimum sucrose loss over all final effect pressures. The marginal sucrose loss is (by definition) zero for at least one pressure within the range studied and reflects the additional sucrose that is lost in a given effect as a result of not operating at the final effect pressure that gives the minimum loss for that particular effect.

20 900

20 950

21 000

21 050

21 100

21 150

21 200

21 250

21 300

21 350

21 400

0 2 4 6 8 10 12 14

Throttle Valve Pressure Drop [kPa]

Sucr

ose

Lost

[kg

/wk]

Station B

0

50

100

150

200

250

300

350

10 12 14 16 18 20


Mar

gina

l Suc

rose

Los

t [k

g/w

k]

First EffectSecond EffectThird EffectFourth Effect

Station A

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Fig. 8�Marginal sucrose degradation in each effect of station B.

Discussion If we examine the curve for the final effect evaporator in each case, it is evident that the

sucrose loss drops as the final effect pressure decreases. This is in line with our expectations. However, we see more unusual behaviour occurring in the other effects.

In terms of the evaporative capacity of station A, the optimum final effect pressure occurs at around 14 kPa(a). At this pressure, the V1 vapour throttle valve is yielding its maximum pressure drop and the throttled pressure of the V1 vapour feeding the second effect vessels is at its lowest. The effect of this lower pressure is propagated down the tail and the third effect pressure is therefore also at its lowest under these conditions.

As the final effect operating pressure moves away from the optimum (either increasing or decreasing), the evaporative capacity of the station decreases. This is to be expected, as final effect pressures above the optimum reduce the available temperature difference driving force for evaporation, while final effect pressures below the optimum result in poor HTC values in the final effect due to the increased syrup viscosity.

To compensate for the decreased capacity of the station, the vapour throttle valve opens, decreasing the imposed pressure drop. As a result, the V1 vapour feeding the second effect is increased in pressure. The increased pressure and temperature in the second and third effect vessels results in increased levels of sucrose degradation in these units. This is evident in Figure 7, where the inversion in the second and third effect vessels is at a minimum at a final effect pressure of 14 kPa(a) and increases at points away from the optimum.

This phenomenon also explains the behaviour shown in Figure 5, where the minimum level of sucrose degradation coincides with the maximum capacity of the evaporator station. Since the level of sucrose degradation in the earlier vessels of the station is far greater than that occurring in the final effect, the increased loss in these vessels far outweighs the impact of the reduced loss that can be achieved in the final effect by decreasing the syrup temperature. It is interesting to note that changes in the pressure drop across the vapour throttle valve do not have a substantial impact on the operating pressure of the first effect vessel upstream of the valve. As a result, the quantity of sucrose destroyed in the first effect vessel is effectively independent of the final effect pressure.

0

50

100

150

200

250

300

9 10 11 12 13 14 15 16 17 18


Mar

gina

l Suc

rose

Los

t [k

g/w

k]

First EffectSecond EffectThird EffectFourth EffectFifth Effect

Station B

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Similar behaviour can be observed in the graphs for station B. However, in this case the throttle valve is located in the V2 vapour line feeding the third effect vessels. The evaporative capacity of station B is optimised at a final effect pressure of around 12 kPa(a) and the pressure drop across the throttle valve is maximised at this point. As the final effect pressure moves away from this optimum, the vapour throttle valve opens to compensate for the reduced capacity and this leads to an increase in temperature (and hence sucrose degradation) in the third and fourth effect vessels. As with station A, the sucrose degradation in the vessels upstream of the throttle valve (i.e. the first and second effect vessels) is essentially independent of the final effect operating pressure. In fact, the results obtained are reasonably intuitive. The minimum level of sucrose degradation occurs where the vessels in the tail of the station (i.e. downstream of the vapour throttle valve) are at their �coldest�. This occurs whenever the pressure drop across the vapour throttle valve is at its greatest. It is therefore best to operate an existing evaporator station at the final effect pressure that yields the optimum evaporative capacity. This is in contrast to the �common sense� solution to the problem that only considers the impact of the final effect pressure on the final effect vessels themselves. An analysis of the problem on this basis would call for reducing the final effect pressure to the minimum achievable value (given the installed vacuum-raising equipment). In considering the evaporator station as a whole, however, it is apparent that this would lead to a poor solution. It is interesting to note the value of the sucrose lost through inversion in the two evaporator stations, as shown in Figures 3 and 4. From the range of the two graphs, it is evident that no more than US$150 worth of raw sugar per week could be saved from degradation through the optimisation of the final effect evaporator pressure. This highlights the fact that the majority of the sucrose degradation in an evaporator station occurs in the earlier vessels. Manipulation of the conditions in the latter vessels of the tail can therefore only have a small impact on the total value of the loss. In order to minimise sucrose degradation, far more attention should be paid to the vessels in the front of the station (perhaps by reducing the pressure of the exhaust steam, if possible).

Further comments From the work carried out in this study, it is apparent that the optimum final effect pressure in an existing evaporator station is a function of the specific design and operating characteristics of that particular station. Although a useful technique has been developed for determining the optimum final effect pressure, no general recommendation can be made as to the location of this optimum for a particular set of evaporators.

Since the level of sucrose degradation is minimised when the evaporative capacity of the station is at its maximum, it should be possible to optimise existing evaporator stations by experimental means. By varying the final effect operating pressure over a suitable range and determining the position of the vapour throttle valve, it should be possible to locate the optimum in an empirical way.

The techniques described here for modelling sucrose degradation in evaporator stations can also be applied to the study of the energy efficiency of the stations, as a function of such parameters as the final effect pressure. The calculations carried out in this study have shown that the energy efficiency of a station is optimised at a final effect pressure similar to that which optimises the capacity of the station. However, since energy efficiency is highly dependent on the details of the station design (e.g. condensate flash arrangements, etc.), no generalised recommendations can be made.

While the modelling of an existing evaporator station can be readily carried out, it is far more difficult to consider the effect of final effect pressure on the design of a new evaporator station (with particular reference to the impact on sucrose degradation). Further work will be

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required in order to integrate the requirement of minimising inversion into the design procedure for evaporator stations, while also meeting the many objectives that already need to be considered (as described by Love et al., 1999). Conclusions The choice of final effect operating pressure in an evaporator station has an impact on its capacity, as well as on the level of sucrose degradation that will occur. These effects are not confined to the final effect vessels, but influence the entire station through a change in the distribution of pressures across the vessels. It is essential to model the entire evaporator station in detail in order to study these phenomena, as conclusions reached by considering only the final effect vessels themselves will not be correct.

By modelling two separate evaporator stations that are representative of typical conditions in the sugar industry, it was found that the level of sucrose degradation in an evaporator station is minimised when the pressure drop across the vapour throttle valve is at its greatest. This occurs when the evaporative capacity of the station is maximised relative to the final effect pressure. The optimal final effect pressure may either be established by calculation using the techniques described in the current study, or by experiment using an existing evaporator station.

REFERENCES

Allan, G.N. (1975). Long tube evaporators in South African sugar industry. Rev. Agric. Suc., 55: 265�274.

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LA PRESSION DANS LE DERNIER CORPS ET SON EFFET SUR LA DEGRADATION DU SACCHAROSE A L�EVAPORATION

Par S.D. PEACOCK

Tongaat Hulett Sugar Limited Private Bag 3, Glenashley, 4022, South Africa

[email protected] MOTS CLEFS: Évaporateurs, Vide, Dégradation du Saccharose, Température du Sirop.

Résume LE VIDE que l�on choisit pour le dernier corps des évaporateurs a un effet sur le transfert de chaleur dans cet évaporateur. Cela affecte la capacité de la station d�évaporation. On note aussi que le vide du dernier effet control la vitesse de la dégradation chimique du saccharose, directement en contrôlant la température du sirop et indirectement en affectant le temps de séjour du jus dans les évaporateurs. Un changement dans la pression du dernier corps affecte les pressions dans tous les autres corps, et donc la vitesse de la dégradation chimique du saccharose dans chaque corps. Pour comprendre tous ces effets il faut étudier le transfert de chaleur et l�inversion dans tous les corps, et non dans le dernier seulement. On a évalué les effets de différentes pressions dans le dernier corps en se servant de simulations sur ordinateur. En même temps on a pu étudier l�efficience énergétique. Les résultats obtenus permettent de choisir la pression optimale dans le dernier corps.

EL EFECTO DE LA PRESIÓN DE OPERACIÓN DEL ÚLTIMO EFECTO DE UNA ESTACIÓN DE EVAPORADORES EN LA DEGRADACIÓN DE SACAROSA

Por S.D. PEACOCK

Tongaat Hulett Sugar Limited Private Bag 3, Glenashley, 4022, South Africa

[email protected] PALABRAS CLAVE: Evaporadores, Vacío, Degradación De Sacarosa, Temperatura De La Meladura.

Resumen LA ESCOGENCIA de la presión de operación del último efecto de una estación de evaporadores tiene efecto en el coeficiente de transferencia alcanzado en ese cuerpo. Esto a su vez afecta la capacidad evaporativa de la estación. La presión escogida para el efecto final también afecta la tasa de degradación de sacarosa en la estación de evaporadores debido a su efecto directo en la temperatura de la meladura e indirecto en el tiempo de residencia del jugo en los cuerpos individuales. El cambio de la presión del efecto final cambia la distribución de presiones a lo largo de la serie lo cual afectará la tasa de degradación en todos los cuerpos no sólo los del último efecto. Para estudiar este fenómeno, es necesario modelar la transferencia de calor y la inversión en todos los cuerpos en lugar de hacerlo sólo para los últimos cuerpos. Por medio de simulaciones computacionales pueden evaluarse tanto el efecto de variar la presión del último efecto en la degradación de la sacarosa a lo largo de la serie de evaporadores como los efectos en la eficiencia energética. Con base en los resultados obtenidos se establecen recomendaciones sobre la escogencia óptima de la presión de operación para el evaporador del último efecto.

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THE EFFECT OF FINAL EFFECT OPERATING … Peacock...Peacock, S.D. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 _____ THE EFFECT OF FINAL EFFECT OPERATING PRESSURE ON SUCROSE DEGRADATION