RETENTION TIME DISTRIBUTION AND SUCROSE HYDROLYSIS AS CRITERIA FOR THE

OPTIMISATION OF THE EVAPORATION PROCESS

E. C. Wittwer and W. Mauch Technische Universitat Berlin, Department of Sugar Technology, GDR

ABSTRACT

For a more intensive use of the raw materials at the disposal of a cane sugar factory, the latter must try to save as much bagasse as possible. To achieve this goal, some energy conservation measures must be taken, such as switching from vacuum to pressure evaporation. However, this change causes some problems like an increase in the sucrose hydrolysis and a higher tendency to colour formation. The retention time in the evapora- tors and the retention time spectra are important processing parmeters which affect the different reactions taking place in'the juice. In this paper a way is shown to determine the ideal retention time spectra for an evapo- ration station formed by Robert-type evaporators. These ideal spectra can then be compared with the real retention time spectra obtained by giving some tracer material into the evaporators and measuring the concentration of the tracer material at the exit of the evaporators from time to time. The comparison gives us information about flow conditions in the appara- tus. The hydrolysis reaction of sucrose is a pseudo first-order reac- tion. The fraction of splitted sucrose at different conditions can be calcu- lated by using an experimental expression for the determination of the hydrolysis rate constant. In the paper at hand, the effect of increasing the temperatures and varying the mean retention time in the evaporators on the losses of sucrose by hydrolysis was examined. The article closes with a brief discussion about the problem of colour formation at higher tempera- tures.

INTRODUCTION

One of the keys to economic development is the optiinum use of available resources, which includes natural resources like raw materials, energy potentials, land use, and others. The cane sugar industry can play an important role, provi- ded it takes some appropriate measures, not only as energy producer for regional consumption, but also as basis for a diversified by-products industry.

Although in the last years significant efforts have been made in order to reduce the energy consumption in the factory process, thus obtaining more bagasse for alternative uses, yet there are other energy saving potentials which could substan- tially improve the energy efficiency of a cane sugar factory. There are three main areas in a factory where energy conservation measures can be adopted: 1. The steam generation unit; 2. The power generation and distribution and 3. The process heat circuits. We want to focus our attention on the last of these areas because there large energy saving potentials exist, which do not necessarily require great investments in new equipment.

Today, the process steam requirement of a cane sugar factory, working with

1548

traditional heat schemes and equipment, fluctuates between 45 and 65 kg per 100 kg cane, depending on product quality, crystallisation scheme and optimisation of steam distribution, among other factors. It has been reported that a factory on the Ivory Coast achieved a steam consumption of 38 kg1100 kg cane (Cremoux and Lombard I ) . Some factory constructors have proposed high efficiency heat sche- mes, with which a steam consumption of 34 kg1100 kg cane could be achieved (Garcia L6pez and Clark 2 ) . Investigations carried out at our Institute by Wardhana led to the design of a 4 000 tld cane sugar factory for the production of white sugar, which needs 31,5 kg steam per 100 kg cane from the steam generator. The factory in question produces a bagase surplus of 46,2% and, in addition, has a surplus power capacity of 2,5 MW. It has two boilers producing steam at 73 bar and 476°C. Figure 1 shows a simplified scheme of the evaporation station which consists of five stages. To achieve such low figures, it is necessary to change some process parameters, the most important of them being the tempe- rature (and pressure respectively) in the evaporators. Working here with higher temperatures produces vapours with higher exergy (or availability of work). Fur- thermore, the exergy losses in the condenser are drastically reduced.

It must be underlined that, in a usual factory, the evaporation station and the vacuum pans consume together up to 80% of the process heat. As is shown in Figure 1, the evaporators could supply almost all other steam-consuming equip- ment in the factory with process heat. From this point of view, the evaporation station should be regarded as the actual steam generator for process heat.

In middle and long-range terms, the cane sugar industry, as a whole, has the possibility to improve its energy consumption figures, if appropriate energy saving measures are undertaken patiently and systematically. Let us look, for instance, at the consumption of primary energy in the sugar industry of the Federal Repu- blic of Germany between the campaigns 1950-1951 and 1978-1979. The Annual Report of the Verein der Zuckerindustrie 1979-1980 shows the following figures:

I Primary Energy Consumption

1950-1951: 103 kWh1100 kg beet. 1978-1979: 59 kWh1100 kg beet.

That is to say, in three decades there has been a reduction in energy consump- tion to nearly 57% of the original values. According to calculations by Baloh, ' it is possible to reduce the consumption of primary energy to a figure of 29 kWh1100 kg beet or 28% of the energy consumed three decades ago, with the utilisation of vapour compression in the evaporation station and in the sugar house, whilst producing electrical energy with a gas turbine at the same time. But cane sugar

1 technologist have already heard such suggestions before, i e increasing the tempe- ratures in their evaporators, So far they have reacted sceptically, due to the fact that at higher temperatures higher sucrose losses through hydrolysis must be expected. The same is valid for an increase in colour formation in the juices and also an acceleration of scale formation. The question of scale formation, which needs further experimental work, will not be discussed in this paper. Now, it is a proven fact that if all other parameters are maintained unchanged, then an increa- se in the temperature of a sugar solution causes an increase in the hydrolysis of sucrose and an increase of colour formation as well.

It must be remembered however, that the hydrolysis rate of sucrose and colour formation are not only a function of temperature. It also depends on the pH- value, sucrose concentration, on the kind of non-sugars present (Kelly and Brown ') and last but not least, on retention time and retention time distribution (retention time spectra), which is a function of construction parameters of the

1549

to pre-heaters C

h 2 4

m

4,

7

equipment. In this paper we want to discuss how variations in the retention time in the evaporators affect the hydrolysis of sucrose. In this respect some brief words must be said about retention time distribution in an evaporation station. ~

I I RETENTION TIME DISTRIBUTION IN THE EVAPORATORS

The evaporation station can be seen as a cascade of ideally stirred tanks, without backmixing between stages. 'Not all the sucrose molecules which come into one evaporator at a certain instant will leave the evaporator later at the same time, but some of them will pass through the equipment faster than others, which will stay longer inside the evaporator. Therefore one must use the concept of mean retention time, which is a statiscal value that can be applied theoretically for all molecules in the evaporator. It must be emphasized that knowing the mean retention time does not give any information on how the retention times are distributed among the molecules passing through the tank. For the analysis of retention time distribution, it is useful to define a distribution funqtion E(t) which shows the distribution of the mass stream leaving the balance boundary on diffe- rent retentions times t. The distribution function E( t ) is determined mathemati- cally for an ideal tank or tank cascade and then compared with the experimentally obtained distribution function. In the next pages it is shown how E(t) can be determined for the ideal case. Let us imagine an ideally stirred tank, where a certain amount Mu of a tracer material is given at the nlet in form of an impulse signal (Fig. 2). Then E(t) is given by

where M is the mass stream (kglmin) leaving the tank. E(t) can also be expressed as a function of the tracer concentration at the tank outlet, w(t). In this case

where w(t) is the mass fraction of the tracer material in the leaving stream (M,, being the total mass of tracer material).

I Figure 2. Ideally s t~ r r ed t ank .

~ E(t) dt is often called the "external age" and represents the fraction of tracer material leaving the tank between time t and t + dt or, with other words, it is the fraction of tracer particles which has resided in the apparatus for a time between t 1 and t + dt. Therefore

(3) By mass balance it can be shown easily that the distribution function at the

i outlet of the first tank of an ideally stirred tank cascade results (Grassmann 6).

I 1 t ~ El (t) = -=- exp - t t (4)

where 7 is the mean retention time in the tank and can be obtained from the volume V in the tank and the flow rate 9, or from the mass content M and the mass stream Ah.

The distribution at the outlet of a cascade with n tanks of equal size, is given by (Grassmann 6):

Where 5 is the common mean retention time for all tanks. In the sugar industry we can apply equation (6) only rarely (eg. the pre-liming

equipment, where each division is almost the same size). In the evaporation station however, we have to deal with a cascade made up by tanks of different sizes.

For this case the retention time distribution function is given by (Baloh and Wittwer 7):

n again being the number of (unequal) tanks and i, the mean retention time in each tank. The constants K, can be obtained from the following equations:

i,( '- 2) K1 =

(Il - f2) (fl - 73) . . . (Il - 7,)

Let us apply equation (7) to calculate the ideal distribution function E(t) for I the evapdration station given by Honig. " The station in question is a quadruple with pre-evaporator, that is to say, from the juice side it can be regarded as a quintuple. The mean retention times in each stage are:

Stage: I I I I11 IV V t (min): 4,3 6,6 8,O 10,5 . 17

Using equation (7) we can draw the ideal retention time distribution curve at the outlet of each stage. So, for state I we have

El(t) = K1 . exp (8)

1 + \ - - -

4 a e x ( - 4,3) One obtains K1 from the equation set (7a) when putting n=5, letting all Ti other

than Tl being 0. That is: ~ ~ ( 5 - 2 )

Kt = 1 - -

(El - 0 ) (El -())(El - O ) ( t l - 0 ) - El

At the outlet of the second stage we have:

E2(t) =Kl exp (-+) + K2 exp (-+) - - 1 1 . exp (- +) + - . exp - - t

tl - t2 t2 - t l t 2 (9)

= - 0,435 . exp (-4k) + 0,435 exp (-iF) As we see from equation (9), K1 is not the same as K1 in equation (8). The

value of the K, is a function of the stage under consideration. Following the same procedure we can obtain E3(t), E4(t) and Es(t). For the distribution function at the outlet of the evaporation station, Es(t), we have:

t Es(t) =K, exp (- t) + K2 . exp (- T) + K3 - exP (- *) +

+K4 . exp (- l) + Ks . exp (10)

and after putting the values of the mean residence times in (7a), with n = 5:

Es(t) = 0,119 - exp (- &) + (-2,201) . exp (- -&) +

4,393 . exp (- $) + (2,946) . exp (- 6) +

0,636 exp (- -&) The plot of the equations (Fig. 3) snows the retention time distribution curves

for each stage. These curves correspond to a cascade of ideally stirred tanks of unequal size. The real curves for an existing evaporation station must be deter- mined by giving some tracer material into the first stage and taking samples every few minutes at the outlet of the stages. Following this procedure, one can obtain, at first, a concentration distribution as a function of time and using equation (2), one can obtain the real distribution function. These curves can then be compared with the ideal curves obtained through equation (7) and given in Fig. 3, presenting us with additional information about flow conditions in the equipment. If a great deviation is found between the curve sets and, especially, if the real curves show long "tails" at the tank outlet (that is, the curves tend to run parallel to the

, I

Figure 3. Retention time spectra o f an evaporation station.

abcissa) that could be a sign of "dead volumes" and unfavourable design in the respective stage. The wide retention spectra that result are very damaging with regard to sucrose hydrolysis and colour formation.

Furthermore, we see that what matters in not only the value of the mean retention time in the evaporators, but also the shape of the retention time distri- bution functions. For this reason, it can occur that the sucrose hydrolysis and the colour formation are different for two apparatus, which have the same mean retention time and work at the same conditions. A narrow retention time distri- bution function is an additional criterium for the good performance of an appara- tus (like high heat transfer coefficients). At tracer material for the sugar industry we use sorbitol. As sugar alcohol it belongs to the carbohydrate family, can be detected easily and specifically by an enzymatical analysis and does not disturb the manufacturing process like salts (Mosich 9).

SUCROSE HYDROLYSIS IN THE EVAPORATORS

Now we should like to discuss how variations in retention times and temperatu- res do affect the sucrose losses in the evaporators. The hydrolysis of sucrose is a pseudo firstorder reaction for which it applies

where x is the concentration of splitted sucrose at time t , Co is the concentration at the beginning of the reaction and k (min-l) is the hydrolysis rate constant. The fraction of hydrolysed sucrose at time t is obtained by integrating the differential equation (11). One obtains:

According to equation (12) sucrose hydrolysis is an exponential function of time. But it is true indeed that for short periods and at normal factory conditions (low k-values), the reaction behaves like if it were a linear function of time. If one knows the value of the hydrolysis rate constant-k and the retention time t, then the hydrolysis losses can be easily calculated from equation (12). For practi- cal calculations, the mean retention time in the evaporator can be used as expo- nent in equation (12). It should be remembered however, that the mean reten- tion time does not apply to all sucrose molecules in the evaporator. The correct mathematical solution can be found with the help of the distribution function E (t) by solving the integral p

for each stage (Danckwerts lo). This means, however, that one must dispose of a mathematical expression for the real distribution function of an evaporator. IFor k Parker has given the following expression:

where C is molar concentration and T the absolute temperature. The concentra- tions can be expressed as mass percent sucrose (S), which is simpler for the factory technician. We obtain:

where g is the density of the solution and k is in (min-I). Using equation (14a) we can calculate the sucrose losses due to hydrolysis in

the evaporation station. It must be said, however, that Parker's equation was obtained from laboratory measurements and it can be expected that the sucrose losses are higher at factory conditions.

Kelly and Brown reported that Brown found k-values in complex solutions, which were 1,5-2 times greater than tbose predicted from Parker's equation.

In Table 1 the sucrose losses calculated by Honig for an evaporation station like Fig. 4 are shown. We have calculated the losses for the same station using equations (12, 14a). Our values are in Table 1, together with the values for the hydrolysis rate constant at each evaporator.

As Table 1 shows, with equations (12, 14a) we obtained somewhat lower values (17% lower) than the losses given by Honig. 8a. However, the differences can be regarded as slight. Let us now look at the consequences in respect to hydrolysis,

Table 1. Sucrose Losses (Hydrolysis, % (rel.) of Sucrose in Juice) in a Usual Evaporation Station

Retention Hydrolysis k (min-') Hydrolysis Stage

Times (min) (Honig '") (eq. 14a) (eq. 12)

I 4,3 0,012 2,01 x lo-5 0,0087 I1 6,6 0,009 9,70 x 0,0064

111 8,O 0,001 3,77 x 1 0 P 0,0030 IV 10,5 0,001 6,94 x lo-' 0,0007 V 17,O - " 6,lO x lo-' 0,0001

<o ' Total 0,023

if we increase the temperatures in the evaporators. Applying the temperatures proposed by Wardhana (see Fig. 1) to the evaporation station in Fig. 4, this leads to the hydrolysis rates in Table 2, where the results for two different retention time sets were calculated.

Table 2. Sucrose Losses (Hydrolysis, %(rel.) of Sucrose in Juice) During Pressure Evaporation for Different Retention Times.

--

k - - t Hydrolysis t

Stage Hydrolysis (min- ') (min) ("/.I (min) (x)

Total 46,4 0,045 23 , 0,024

From Tables 1 and 2, we can conclude that increasing the temperature in the evaporators without changing the mean retention times, led to an increase in the sucrose hydrolysis from 0,019% (Table 1) to 0,045% on sucrose in juice (Table 2). If together with increasing the temperature a reduction in the retention times is undertaken from a total retention time in the evaporators of 46,3 min (Table 1) to 25 min (Table 2), sucrose losses of 0,024% are obtained. This figure is still somewhat higher than the losses with the conditions of Fig. 4 (0,019%) but we are of the opinion that a brief analysis would show that the benefits of using pressure evaporation are higher than the costs of increased sucrose hydrolysis. As an example we can take a factory with a grinding capacity of 4 000 t/d and a campaign lasting 150 days. A .sucrose content of 14% in cane brings 84 000 t in the campaign. With the assumption that 94% (78 960 t) of the sucrose reaches the evaporators, the sucrose losses due to hydrolysis in the evaporators are:

Case 1: usual temperatures (Table 1): 0,019% = 15 t . Case 2: higher tempera tures and lower retent ion times (Table 2): 0,024% = 19 t.

So, we obtain excess losses of 4 t sugar between case 1 and 2 (with a sugar price of U.S. $0,15 per pound, the excess losses would be equivalent to U.S. $1 320 in the campaign).

Now let us look at the energy saving. Wardhana has shown that using the evaporation scheme of Fig. 1 a cane sugar factory (even producing white sugar) could achieve a bagasse surplus of 46%. For a 4 000 t/d factory, working 150 days and with a yield of 28% bagasse (50% humidity), the surplus amounts to 77 280 t bagasse. If we make the assumption that the factory of case 1 (usual temperatures, Table 1) reaches a bagasse surplus of 20% (an optimistic assump- tion) then we would have a surplus of 33 600 t for case 1. That is to say, in changing to pressure evaporation we achieve an excess of 43 680 t of bagasse (50% humidity). This excess can be expressed in fuel oil equivalent. The low calorific value of bagasse (50% humidity) is 7 600 kJ/kg and that of fuel oil is 39 000 kJ/kg. 1 t of humid bagasse corresponds to 0,195 t of fuel oil. So we have an excess equivalent to 8 500 t fuel oil. (If we take a fuel oil price of US$300/t we save US$ 2,55 millions).

Thus we see, that the losses due to excess hydrolysis are uncomparably lower than the gains due to fuel saving. This remains valid even if the sucrose losses at factory conditions are 2 or 3 times greater than predicted by equation (14a).

A pressure evaporation scheme like in Fig. 1 is especially recommendable in the case of new or reconstructed factories but, as was shown, the substantial savings in bagasse make it also possible for an existing factory to invest a little in evaporators, heat exhangers, or even in a new boiler, if necessary. It must be borne in mind that not all of the excess hydrolysed sucrose must be regarded as lost sucrose, because at least a part of the invert sugar will re-appear in the molasses.

Many factories use quadruple effect evaporators with no more than 110°C in the juice side of the first stage and around 60 "C in the last stage. We have investigated the consequences of varying ,the temperatures in a quadruple, which works at conditions given by Spencer-Meade, l2 assuming the values for pH and retention times shown in Table 3. In Table 3 the results of the calculations are presented.

Table 3. Sucrose Losses (Hydrolysis, % (rel.) of sucrose in Juice) in a Quadruple Effect Evaporator Under Different Conditions. I

Stage I I1 I11 IV sucrose (weight %) 21,8 27,7 39 70 Total

pH 7, l 7,o 6,9 6 3

t (min) 5 7 8 10 30 Temp. ("C) 107 97 83 57 -

loss (%) 0,002 0,001 - - 0,003

Temp ("C) 125 118 109 96 -

loss (%) 0,008 0,008 0,004 0,001 0,021 Temp. ("C) 125 118 109 96 -

t (min) 2 4 8 11 25 loss (%) 0,003 0,004 0,004 0,001 0,012 t (min) 1,5 4 6 8,5 20 loss (%) 0,002 0,004 0,003 0,001 0,010

THE PROBLEM O F COLOUR FORMATION i I From an economic viewpoint increased temperatures could cause more pro-

blems in relation to colour formation than to the dydrolysis of sucrose. This is not the place for making a complete analysis of colour formation (see for instan- ce, Kelly and Brown, Zagrodzki and Kubasiewicz, l3 Madsen et a1 '1, we only want to remark on some aspects in an outlined form.

Although colour formation is a result of multiple and complex reactions and not all of them are fully understood, one can separate the colour forming substan- ces as follows:

1. Coloured substances present in the cane such as chlorophyll, carotenoids and others. -

2. Substances present in the cane which are not coloured but can be involved in colour forming reactions.

3. Coloured substances formed through chemical reactions during processing (ca-

ramels and products of the Maillard reaction) under participation of invert sugar.

Therefrom it is deducible that colour formation is not only a question of the presence of invert sugar or of variations in its concentration. A part of colour formation can be successfully diminished by adopting careful juice purification measures.

Retention times and especially, retention time spectra, are of substantial signi- ficance to colour formation in the evaporators. Honig 8b has already noted that if the mean retention time lasts no longer than 2 min, the juices could be evaporated 1 at 120°C. According to experiments carried out by Krug l5 in a German sugar factory, the mean retention time for a Robert evaporator was clearly below 90 s for the first stage. Athenstedt l6 (1961) studying colour formation and hydrolysis under basic conditions, found that although above 120°C the hydrolysis reaction increases further with temperature, this was not the case regarding colour forma- tion. It seems to be a temperature region where colour formation does not run parallel to sucrose hydrolysis. The same author found that the inclusion of oxy- gen in the presence of iron sharply increases the juice colour. This means that passing from vacuum to pressure evaporation can lead to reduced colour for such evaporators which do not work totally hermetical at vacuum conditions. Rubio compared the colour formation in a factory during the campaign 1976, which was working at normal conditions with the values of the campaign 1979 when the pressures were increased to 2,7 bar (130°C, saturated steam) in the calandria of the first stage. The conclusions of the comparisons are meaningful, namely, no significant differences were found in relation to colour formation between the two case.

CONCLUSIONS

Changing from vacuum to pressure evaporation in the cane sugar industry is accompanied by some problems of which the increased sucrose hydrolysis and colour formation are the most important. The actual increase in hydrolysis and colour formation for a given factory depends on different parameters, some of them being process parameters and others being determined by the quality and characteristics of the raw materials. So, in each case, the effect of changing the evaporation conditions must be determined. Taking appropriate measures, the negative effects of increased temperatures can be diminished. Besides such mea- sures as improving the cleaning of the cane and the juice purification process, the retention times in the evaporators can be varied and the retention time spectra can be optimised through appropriate equipment. A cost-benefit analysis will show that even if the mean retention times in the evaporators are not changed, the benefits of using high temperatures are higher than the costs, provided of course, that some use is found for the excess bagasse.

REFERENCES

1. Cremoux, J . e t Lombard, P. (1980) Complexe sucr~kre de Borotou K o r o Un exemple d ' icono- 1

mle d'Cnergle en sucrerle de cannes Ind Allmentalres e t Agr~coles , pp 747-749 2 Garcia Ldpez, F y Clark, J . A . (1969) Comb~naciones de evaporac16n de alta e f ~ c ~ e n c l a y

producc~dn de bagazo y energia elCctr~ca Cuba Azucar, Oct -DIG , pp 17-31 3. Wardhana, T. (1978). S t u d ~ e elner ~ u j k ' e i f a b r ~ k mlt Druckverdampfung Zuckerlng 103, I - .

pp. 467-476 1 I

Baloh, T . (1981): Study of a Beet Sugar Factory where Vapour Compression is Applied. Unpu- blished. Kelly, F. and Brown, D . (1978-1979): Thermal Decomposition and Colour Formation in Aqueous Sucrose Solutions. Sugar Techn. Reviews 6, pp. 1-48. Grassmann, P. (1967): Introduction to Thermal Process Technique. W. de Gruyter, Berlin (inGerman), Chapter 15. Baloh, T . and Wittwer, E . (1981): Verweilzeitspektren der Riihrbehalterkaskaden. Zuckering. 106, pp. 678-682. Honig, P. (1963): Principles of Sugar Technology. Elsevier Publ. Co., Amsterdam, Vol. 3., Chapter 3, a) p. 118, b) p. 146. Mosich, K. (1976): Untersuchungen an einer Verdampfanlage wahrend der Kampagne. Z. Zuc- kering. 26, pp. 312-316. Danckwerts, P. (1953): Continuous Flow Systems. Distribution of Residence Times. Chem. Engng. Sci. 2, pp. 1-13. Parker, K. J . (1970): Chemical Problems in the Sucrose Industry. La Sucrerie Belge 89, pp. 119- 126. Spencer, G. L. and Meade, G. P. (1963): Cane Sugar Handbook, 9th edition, John Wiley & Sons Inc. New York, London, pp. 155. Zagrodzki, S. and Kubasiewicz, A. (1977-1978): Heat Economy in Beet Sugar Factory Evapora- tion. Sugar Techn. Reviews 5, pp. 1-154. Madsen, R . et a1 (1978-1979) Formation of Colour Compounds in Production of Sugar From Sugar Beet. Sugar Techn. Reviews 6, pp. 49-115. Krug, S. (1979): Investigating the Evaporation Station in a Sugar Factory: Determination of Heat Transfer Coefficients and Retention Time Spectra in Each Evaporator. Diploma-Thesis, Technical University of Berlin, Department of Sugar Technology. Athenstedt, M. (1961): i jber die Zersetzung der Saccharose in alkalischer Losung. Z. Zuckering. 11, pp. 661-668. Rubio, E . (1979): Desarrollo de color en la evaporaci6n. CubaAzdcar, 0ct.-Dic. , pp. 27-31.

LA REPARTITION DU TEMPS DE RETENTION ET L'HYDROLYSE DU SACCHAROSE COMME

CRITERES POUR L'OPTIMISATION DU PROCESSUS D'EVAPORATION

E. C. Wittwer et W. Mauch . UniversitC technique de Berlin, Allemagne, Service de technologie sucrikre

I I

Pour utiliser au mieux la rnatikre premiere disponible dans une sucrerie, il faut Cconomiser autant de bagasse que possible. A cette fin, il faut prendre des mesures pour la conservation de l'energie, para exemple, remplacer l'evaporation sous vide par l'evaporation sous pression. Nean- 1 moins, ce changement pose certains problkmes tels que l'augmentation de l'hydrolyse du saccharose et une tendance plus marquee vers la formation de la couleur. Le temps de retention dans les evaporateurs et les spectres de tempes de retention sont des paramktres importants affectant les diff6- rentes r6actions qui se produisent dans le jus. Cet expose montre comment

dkterminer les spectres idCaux de temps de retention pour une station d'kvaporation dotCe d'appareils de type Robert. Ces spectres idkaux peu- vent alors Ctre comparks avec les spectres rkels de temps de rktention obtenus a la suite de l'introduction d'un traceur dans les caisses d'kvapora- tion et en mesurant de temps en temps la concentration du traceur a la sortie des appareils. La comparaison permet d'obtenir des donnkes sur les conditions d'kcoulement dans les caisses.

La rkaction de l'hydrolyse du sacchmose est une pseudo-rkaction de premier ordre. La fraction de saccharose skparke dans diffkrentes condi- tions peut Ctre calculke en employant une expression expkrimentale pour dkterminer la constante du taux d'hydrolyse. Les auteurs de l'exposk ont analysk les effets qu'exerceraient 1'Clkvation des tempkratures et la varia- tion du temps moyen de refention dans les kvaporateurs sur les pertes de saccharose par haydrolyse. Pour terminer, ils se penchent sur le problkme de la formation de la couleur a de hautes tempkratures. ,

1 DISTRIBUCION DEL TIEMPO DE RETENCION I

E HIDROLISIS DE LA SACAROSA COMO CRITERIOS i PARA LA OPTIMIZACION DEL PROCESO DE

EVAPORACION

E. C. Wittwer y W. Mauch Universidad TCcnica de Berlin, Departamento de Tecnologia Azucarera, RDA

It RESUMEN

Para un uso mas intensivo de la materia prima disponible en una fabrica de azucar, ksta debe tratar de ahorrar la mayor cantidad posible de bagazo. Para alcanzar este objetivo deben tomarse algunas medidas de conservaci6n de la energia, tales como sustituir la evaporacion a1 vacio por la evapora- cion a presion. Sin embargo, este cambio ocasiona algunos problemas, tales como un aumento de la hidrolisis de la sacarosa y una mayor tendencia a la formacion del color. El tiempo de retencion en 10s evaporadores y 10s espectros de tiempo de retencion son importantes parametros del proceso, 10s cuales afectan las diferentes reacciones que se producen en el jugo. En este trabajo se expone un medio para determinar 10s espectros ideales de tiempo de retencion para una estacion de evaporadores del tipo Robert. Estos espectros ideales pueden ser despuks comparados con 10s espectros reales de tiempo de retencion obtenidos mediante la introduction en 10s evaporadores de algun material trazador y midiendo, de cuando en cuando, la concentracion del material trazador a la salida de 10s evaporadores. Esta comparacion nos permitira conocer las condiciones del flujo en 10s evapo- radores. La reaccion de la hidrolisis de la sacarosa es una seudo reaccion de primer orden. La fracci6n de la sacarosa desdoblada, en diferentes

condiciones, se puede calcular usando una expresion experimental para la I determinaci6n de la constante de velocidad de hidr6lisis. En este trabajo se 1 examin6 el efecto del aumento. de las temperaturas y del cambio del tiempo

medio de retenci6n en 10s evaporadores en las perdidas de sacarosa por la hidrolisis. El articulo concluye con una breve discusion sobre el problema de la formaci6n del color a temperaturas mhs altas.