THE USE OF A SIMULATION MODEL OF THE TORULA YEAST PROCESS FOR INVESTMENT EVALUATION

LEONEL GoNzALEzt Institute Cubano de Investigaciones de 10s Derivados de la Cana de Azircar (ICIDCA), Ministerio de la Industria

Azucarera, Habana, Cuba

and

PAUL FRIEDMAN Ministerio de la Industria Quimica, Avenida Salvador Allende 666, Habana, Cuba

(Received 23 April 1979)

Abstract-A simulation model for the production of torula yeast from sugar cane molasses was developed as a tool for investment evaluation. The mode1 consists of about 20 rather specific equipment and economic modules along with the GEMCS executive program. The simulation mode1 was used to analyze different operating and design conditions for the process. Solutions were found which permitted savings in raw material and energy requirements as well as reductions in the volume of effluents and lower production and investment costs, when compared to the scheme originally considered.

Scope-The use of mathematical models to describe chemical plants is now widespread. Many examples of the steady-state simulation of chemical processes have appeared in the literature during the last fifteen years (see HlavBEek[22] for an excellent review of the field). However the majority of the cases reported have been of existing plants and generally have been carried out within an academic environment. With the exception of the work of Klumpar[lbl71 few articles have appeared on the use of steady-state simulation for project evaluation.

Although the computational tools used and the models developed in this paper are relatively simple as compared to work carried out in highly developed countries, it is felt that this study is of interest as an example of how chemical plant simulation can be successfully used in a developing country.

The process simulated in this study, the production of torula yeast from sugar cane molasses, is one of increasing importance worldwide because of the problems of protein scarcity in many countries. However, the decisions which must be made regarding the different technological alternatives are quite difficult due to many interactions among the process and design variables. In the situation where offers are being considered and discussions are taking place between the investor and various possible suppliers a simulation model which can rapidly calculate material and energy balances, equipment size, raw material and utilities consumption, effluent flow along with investment and production costs can be a very useful tool. In this study, fairly simple but specific models were developed for the process equipment. The best available technological and economic data were incorporated. The influence of important process and design variables such as effluent recirculation rate, fermentor temperature, plant size and type of refrigeration scheme were studied. The results proved to be useful during the stage of project evaluation after which a number of plants were acquired.

Conclysions and Significance-The simulation model developed for the production of torula yeast from sugar cane molasses proved to be useful during the stage of project evaluation. Several important process and design variables were studied. It was found that the partial recirculation of effluent from the 1st separation reduced both water consumption and effluent volume, with an optimum value of about 60%. The effect of fermentor temperature was also shown to be significant, the optimum value being 34°C. Runs at different plant sizes showed that production and investment costs per ton increase considerably for capacities of less than 30 ton/day and decrease very slowly for capacities of more than 60 ton/day, with the range of 40-60 ton/day being the most attractive. Two different refrigeration systems were analyzed: ammonia compression with and without auxilliary cooling towers. The system using auxilliary cooling towers seemed to be preferable. On comparing the results of the most attractive solution to the original scheme under consideration (see Table 4) savings were achieved in the consumption of water, electricity and molasses, along with lower investment and production costs.

tAuthor to whom all correspondence should be sent.

123

124 L. GONZALU and P. FRIEDMAN

INTROWCTION

The problem of finding a stable supply of protein for animal feed has become an important problem in the world. In the case of Cuba, with its rapid development of live stock and the characteristics of its trade, this prob- lem is especially important.

The existence of the sugar industry with its high production of sugar byproducts such as bagasse and final molasses constitutes a very favorable source for protein production.

Because of the large amount of protein required, on the order of more than lOO,ooO tons/year, one must examine all the potential sources, particularly those which would permit high levels of production. One of the sources which could supply a large part of these requirements, and where some experience in its use is available, is the production of single cell protein by the fermentation of carbohydrates.

The decision to invest in this type of process, on the scale required, was not made easily. Some figures will give an idea of the magnitudes involved. For example, in order to produce 100,000 tons/yr of protein from final molasses, !JOO,OOO-l,OOO,OOO tonslyr of final molasses must be provided. The investment required for at least 10 factories is of the order of 150-200 million dollars with additional requirements of about 50 W of electricity and 160,000 m3/day of water.

The above mentioned numbers correspond to some of the most critical problems for the investment analyst. The search for processes with high efficiencies is man- datory since final molasses is a product which has many competing uses such as: to produce alcohol and rum, direct use for animal feed, or, for export. The high consumption of fresh water and the large number of factories needed constitute a serious problem of plant location, to which must be added the problem of water pollution.

Because of the many complex problems involved in the evaluation of different investment alternatives for torula yeast factories it was decided to develop a simu- lation model of the process which might serve as a tool for the analysis of different options in : technologies being offered, changes in process flow sheet and opti- mization of process parameters. Hopefully, this type of study would result in lower consumptions of molasses, water and electricity, as well as lower production and investment costs.

PROCRSSDRSCRIPTION

The flow diagram for the manufacture of Torula Yeast (Candida Utilis) from final molasses is shown in Fig. 1.

The final molasses from the sugar factory is pumped to storage and then to dilution tanks where its density is adjusted to a desired value. It is then sterilized in heat exchangers and cooled to the fermentor temperature.

Phosphorus and nitrogen are supplied by urea, am- monium sulfate and diammonium phosphate solutions which are kept in storage tanks. Their concentrations are then adjusted in balance tanks before entering the fer- mentor.

The sterilized and cooled final molasses along with the nutrients are pumped to the fermentor. Air is supplied by a blower. Since the process of biological growth is exo- thermic and the temperature must be maintained at its optimal value (32-36°C) a refrigeration system is required to cool the fermentor. The stream which leaves the fermentor contains cellular material, secondary fermen- tation products, CO,, Hz0 and air in the form of an emulsion. The stream is first de-emulsified and then sent to centrifuges where the yeast concentration is increased from %2Og/l to 70-15OgP. The cream from the first separation is then washed with water in order to reduce the salt content of the final product. It is then sent to a second centrifuging stage where its concentration is set

Fig. 1. Flow diagram for torula yeast production.

The use of a simulation model of the torula yeast process for investment evaluation 125

at 130-16Og/l. The effluent from the first separation can be partially returned to the fermentor.

The cream of yeast from the separation section is pumped to an evaporator where thermolysis takes place and its concentration is increased to 200-22Og/l. It is then sent to a dryer where a solids content of 92-95% is achieved and finally loaded in sacks for sale.

Various types of services are required such as: refri- geration for the fermentor; steam for sterilizing the final molasses, evaporating the cream and cleaning; and others such as instrument air, laboratory, workshop, etc.

Objectives MODELLING STRATEGY

As was previously mentioned, the general objective of this study was to develop a simulation model of the Torula Yeast production process which could be used as a tool for the analysis of the different techno-economic options available. Thus, with this model one should be able to:

(i) Calculate material and energy balances so that the equipment sizes, raw material and utilities consumption and environmental effects could be analyzed and modified.

(ii) Study the economic effects of varying process parameters such as: recirculation of efftuent, fermentor temperature and factory size.

(iii) Analyze changes in the flow diagram, especially in the areas of fermentation, refrigeration, and drying.

(iv) Carry out repetitive calculations of production and investment costs, under different economic con- ditions and in a rapid manner, so that these results could be used in the discussion of offers from foreign firms.

(v) Easily extend the model in accordance with future needs, based on its flexible structure.

Modelling decisions In order to carry out the proposed objectives several

decisions were made regarding the executive program and the models to be developed.

Utilize GEMCS[20], an executive program which was available and had the advantages of using little memory and having a flexible structure so that changes could easily be made in the model.

Use only information for modelling which existed or could be easily obtained and not start an intensive experi- menial program which could significantly delay the use of the model.,

Emphasize the use of the model as an economic tool so that it could be used for the analysis of offers from foreign firms. Include the economic information within the. GEMCS structure without developing a separate program for that function.

Modify the GEMCS executive so that many case studies could be run at one time, but not include an optimization subroutine.

Calculate the sizes of some equipment even though this design function would require the reversal of in- formation flow.

Emit a summary which would report the consumption of raw materials and utilities as well as the production and investment costs.

Sources of information The theory of fermentation in single or multiple stages

with limiting substrate is well known; the elementary

kinetic theory used in this study can be found in several texts [ 1.21 and articles [3,4].

The description of the torula yeast process is given in G6mez[5] and is supplemented by offers from various suppliers of these installations [6].

The determination of the kinetic parameters for the growth of yeast in sugar cane molasses or similar sub- strates has been studied by Almazin et a1.[7-101, Fernindez [ 111, EstCvez et al. [ 121.

The unit operations have been modelled in a very simple manner as described in conventional chemical engineering textbooks, while specific aspects have been included from equipment catalogues, offers from sup- pliers, and technical information.

The properties of sugar cane molasses, torula yeast, and the nutrients have been studied by Fernandez et al.[13].

The work of Klumpar[l617] was found useful in the development of the economic aspects of the model al- though a different method was used in this study. Economic information was obtained from Popper [ 181 and cost files of ICIDCA[19].

information flow diagram Based on the process flow diagram of Fig. 1 and the

objectives proposed for the study, the information flow diagram, shown in Fig. 2, was developed.

The unit computations or modules shown in this diagram do not directly correspond to the plant equip- ment and, in some cases, the information flow is reversed because of modelling requirements.

A description of the streams and modules are given in Tables 1 and 2. A complete description of the modules, including the computer programs in FORTRAN, can be found in Gonzilez[211.

Implementation of process model The complete simulation program consisting of the

process modules and the GEMCS executive, both in FORTRAN, was run on a French-made IRIS50 com- puter (similar to the IBM 360). About thirty runs were required for debugging.

The program utilizes 43 Kbytes of memory and about 15 CPU seconds per run.

The different case studies analyzed correspond to two information flow diagrams, the only difference between them being a small change in the refrigeration system. The number of modules varied from 28 to 32, with 46-48 streams and 60&650 process parameters.

About 1.5 man years were required for the study.

RESULTS AND DISCUSSION

Influence of recirculation One of the most important factors of plant location is

the availability of fresh water. Because of this problem, the cooling systems are closed, with recirculation of water.

The partial recirculation of the clear stream from the 1st separation, to dilute the nutritive medium that enters the fermentor, is a way to reduce water consumption. Other advantages of recirculation are a reduction in the amount of effluents and an increase in the yield since the losses of product due to the separation are reduced.

On the other hand, recirculation will require an in- crease in the refrigeration capacity since the cooling effect due to dilution with fresh water is eliminated.

126 L. GONZALEZ and P. FRIEDMAN

1 6

3 3 IX

Fig. 2. Information flow diagram.

Recirculation also causes an increase of the solids con- centration in the fermentor, mainly non-fermentable products or metabolites, which are harmful to growth. This increase in concentration can result in a product with a higher ash content if the same rate of washing is used, or a higher consumption of water for washing if the same ash content is required. The above mentioned reasons, along with small increases in the make-up water for the cooling system, may reduce or even eliminate the possible savings, as well as reduce the yeast concen- tration due to increased flow to the 2nd separation. Finally, the possibility of contamination is increased by the use of recirculation, a factor which must also be considered.

The reduction in water consumption due to recir- culation is shown in Fig. 3. The product with recir- culation must have the same ash content as the product would have without recirculation. A saving of about

20m’/ton can be obtained with a recirculation rate of about 50%.

The effect of recirculation on the total etliuent volume is also shown in Fig. 3. A reduction of about 23% is obtained at a recirculation rate of about 5040%.

Influence of temperature

As mentioned previously, a decrease in molasses con- sumption is very desirable, not only because of the decrease in the cost of production, but also because the unused molasses could then be used for other purposes.

The fermentation temperature is a factor which has a strong effect on the yield. The size of the effect varies with different yeast cultures and is also interrelated with other process variables such as pH, sugar concentration, etc.

The analysis of how a decrease in fermentation tem- perature can increase yield is complex. The theory of

Recirculation, %

Fig. 3. Effects of effluent recirculation.

The use of a simulation model of the torula yeast process for investment evaluation

Table 1. Description of streams in information flow diagram

127

Wt.mber I.P.D. DWCdDtiOtl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

Fictitious, Properties of Molasses Final Molasses Diluted Molasses Dilution Water (R) * Hot Sterilized Molasses Steam to Sterilization (R) Cold Stsriliaed Molasses CWlkrg Water t0 SteriliZeItim (R) Fictitious, Properties of Urea Fictitious, Properties of Am. Phos. Fictitious, Properties of Dism. Solid Diasawniuu Phosphate Solid Urea Solid Ihrmoniuu Sulfate Dilution Water Diam. Phos. (R) Dilution Water Urea (R) Dilution Water Am. Sulf. (R) DiamnoniuoPhosphateSolution Urea Solution kanoniuo Sulfate Solution Solution Molasses and Nutrients Idem plus Recirculation Solution Medim to Fermentor Mlution Water of Mediuo (R) Fenaenteddediun

Air (RI Cooling Water to Fezmentor (R) Cooling Water from Fenuentor

Anti-foaw Agent (R) Air to De-emulsifier (R) Creem from lst Separation Clear Strean from lst Separation Recirculation from lst Separation Effluent from lst Separation Washed Creau from lst Separation Wash Water (R) Creaw from 2nd Separation Effluent from 2nd Separation Fictitious, Properties Tower Water Steaw to Evaporator (R) Cooling Water to Condenser (R) Concentrated Creaw Dry Yeast Fuel to Dryer (R) Cooling Water to Refrigeration (R) Total Consumption Tower Water Wake-up Tower Water (R) Cooling Water Fuel to Boiler (R) Treated Water (R) Cooling Water to Blower (R) Fictitious, Total Fwl Consuaption Fictitious, Partial Sus Process Water Fictitious, Total ConsrPlption Process Water Water to Tower, System 2

* (R): reversal of information flow

128 L. GONZALEZ and P. FRIEDMAN

Table 2. Description of modules in information flow diagram

MO&h

Nmae Calculates:

mlsm

DISCLV

SlMIX

CCCLRR

NuTRl!I

DISCLN

MIXER

BWWER

SEPA

RVAP

DRWIR

requirement of final molasses, size of tanks

and investment wst

dilution water required, sixe of tanks, energy

needed for mixing and investment wst

amount of steam required, volume of equipment

and inveshneot cost

amocmt of cooling water raqulred, exchanger

area and investment cost

the consumption of nutrients for a given

production

amomt of mter required to dissolve a solid

product, size of tanks and investment cost

the characteristics of au output stream with

up to 4 input streams

the aswmt of yeast produced, air required,

heat to be removed, size of equipentand investmeut wst

the flow of air per blower, energy consunption

and investment cost

the consunption of anti-foam agent, equipment

size and investmsnt wst

the flow of cream aud clarified strems, nunber

of equipment needed, energy consused and

investmeot wst

stemn consumption, cooling water requirements

and investment cost

fuel consunption, electricity wnsunption and

investment cost

energy consmnption, cooling water requirenents

and investment wst

energy wnsunption, malcevp water requiremnts,

air wnsunption and investment cost

fuel consmption, treated water required for

boiler, equipent size and investment Cost

energy consumption and investment cost for all

WPS

v&xx3 of tanks and investment wst

total consuuptions of energy and raw materials,

equipment costs, labor wsts, total production

cost and total investmsnt cost

when convergence is obtained for recycle

streams

*Not shown in information flow diagrm since these

modules donotuse the standard systmofinput and

output streams

The use of a simulation model of the torula yeast process for investment evaluation 129

continuous fermentation considers that the process is limited by substrate, in this case the sugar content of the fermentation medium. But the reality is that the produc- tivity @I-’ h-‘) and production of a fermentor are limited by air. This means that in a specific installation it is not possible to increase the productivity by lowering the fermentation temperature since the oxygen transfer is generally limiting. However, it is possible to maintain a certain production level with a lower consumption of molasses by correctly adjusting the operation of limiting substrate to one of limiting air, by design.

The effect of fermentation temperature on molasses consumption is shown in Fig. 4. It can be seen that the rate of the decrease in molasses consumption is lower at 32°C where the yields approach the theoretical limit.

Since the consumption of molasses is the most im- portant element in the cost of production, even though the refrigeration cost increases as the fermentation tem- perature is lowered, the total cost decreases, as is shown in Fig. 4, where a 7.6% reduction is obtained at 32°C.

Effects of plant size The economic advantage of scale-up is well known. In

the case of yeast production, however, this factor is closely related with that of plant location. The problems of transportation of molasses, supply of manpower and other factors, make plant location next to a sugar mill very attractive.

The most expensive piece of equipment in the yeast plant is the fermentor, not only because of its base cost, but also due to the related costs of blowers, instruments, and piping. So in order to achieve the lowest investment cost per ton of product one must design for a maximun fermentor size. Then, any higher production rate can be met by installing more fermentors in parallel.

The variation of investment and production costs per ton of product are shown in Fig. 5. Both costs increase considerably for capacities of less than 30 ton/day and decrease very slowly for capacities of more than 60 ton/day, so that the size range of 40-60 ton/day seems to be the most attractive.

Analysis of an alternate refrigeration system It has already been shown that the use of lower

fermentation temperatures and the partial recirculation

4.2 *

4.0 .

- 1.0. @

I - 1.04 3

9 - 1.0.2 P

5

- 1.00 0.

8 -0.6. ._

-0.66 a

o..o _ 0.0 4

10 PO so 40 *o *cl 70 BO DO

PlD”, If,. , ton I4w

Fig. 5. Effects of plant size.

of effluents are advantageous. However, both of these operating conditions require increases in the capacity of the refrigeration system.

In the information flow diagram of Fig. 2 all the fermentor cooling is done by ammonia compressors. However the temperature required in the fermentor permits the use of cooling towers for part of the fer- mentor cooling duty, which should represent a significant saving in the consumption of electricity and in the in- vestment cost.

To analyze this possibility, the scheme of Fig. 2 was modified, as shown in Fig. 6. The remaining modules are the same as in the original information flow diagram. In this system the cooling water for the fermentors first passes through a forced draft cooling tower and then through the ammonia compressors.

The 2 refrigeration systems are compared in Table 3. The second system shows a marked advantage at 36°C with a saving in energy consumption of about 2% and a reduction in investment cost of about 12%. The ad- vantage is not as great at 34°C since the possibility of atmospheric cooling is less and the decrease in the electricity requirement is 12% while the investment cost is reduced about 7.5%.

32 34 36

F6rmrniation T6m~K!tUr6,°C

Fig. 4. Effects of fermentation temperature.

130 L. GONZALEZ and P. FRIEDMAN

Fig. 6. Modification for refrigeration system 2.

General analysis Fourteen different case studies were analyzed. The 3

most favorable ones are shown in Table 4.

The first case is most favorable in regards to produc- tion cost, consumption of molasses and saving of water, but shows disadvantages in a higher consumption of electricity and a higher investment cost.

The second case is favorable in all aspects since its product cost is comparable with the first case, although the consumptions of molasses and water are a bit higher. However, the electricity consumption is lower as is the investment cost. This seems to be the best choice.

The third case is the most favorable as to electricity savings and lower investment cost, but is inferior in the saving of molasses and production cost.

CONCLUSIONS

The simulation model for the torula yeast process was used as a tool for the analysis of different operating and design conditions which permitted savings in the con- sumptions of molasses, water and electricity, as well as reductions in the volume of effluents, production cost and total investment cost, when compared to the scheme originally considered.

It was possible to calculate the material and energy balances, equipment sizing, production and investment cost for many different process conditions by the com- puter using about 15 CPU seconds for each case.

Table 3. Comparison of alternate refrigeration systems

CONDITIONS

36'C, 602 Rec. 34'C, 60% Rec.

RSl RS2 RSl BS2 ---

Energy for Canpre- ssion, ton 1.0 0.270 1.0 0.760

Total Electricity, kw-h

Total Investment, $

1.0 0.707 1.0 0.876

1.0 0.880 1.0 0.925

Table 4. Savings as compared to base case* (plant size: 40 ton/day)

% SAVINGS

Elec- Invest- Prod. Conditions Water tricitv Molasses mont cost.

RS1,32°C,60% Rec. 44.5 - 6.0 14.7 - 2.8 9.8

BS2,34°C,60% Rec. 34.1 9.1 11.1 0.5 9.7

RS2.36°C.60% Rec. 34.8 20.8 2.4 9.4 6.4

* Base case: RSl, 36OC, 0% Rec.

The use of a simulation model of the torula yeast process for investment evaluation 131

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for forage from Cuban sugar cane molasses, Boletin ICIDCA sobre 10s Lkrivados de la Caria de Azucar 2, I (1%8). (In Spanish).

8. 0. Almazan & A. Saenz, A comparison of single and dual stage systems for the production of torula yeast from Cuban sugar cane molasses, Boletin ICIDCA sobre 10s Lkrivados de la Caria de Azucar 6,3 (1972), (In Spanish).

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10. 0. Almazan & D. Fernlndez, A preliminary report on the efficiency and productivity of torula yeast from a-strike molasses. Boletin ICIDCA sobre 10s Lkrivados de la Catia de Aztkar 4, I (1970). (In Spanish).

I I. D. Femandez, The influence of recirculation on the produc-

tion of biomass in a single stage. Cuba Azricar, July-Sept. (1970), (In Spanish).

12. R. Estevez, The kinetics of the growth of torulopsis utilis in sugar cane juice. Internal Rep. ICIDCA (1971), (In Spanish).

13. M. Fernandes & J. Biart, et al., Properties of Cuban molas- ses. Internal Rep. ICllXA (l%S-73), (In Spanish).

14. I. V. Klumaar. Process feasibilitv bv simnlified Simulation. Chemtech. February (1973). . . _

15. I. V. Klumpar, Process predesign. Chemical Engineering,76, 20 (1%9).

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18. H. Popper, Modem Cost Engineering Techniques. McGraw- Hill, New York (1970).

19. H. Noa & 0. Argudin, Internal Rep. ICILX’A (1%5-70), (In Spanish).

20. A. I. Johnson, et al., GEMCS, A General Engineering and Management Computation System. McMaster University, Hamilton (1970).

21. L. Gonzalez, A techno-economic analysis of the tech- nological options for the production of biomass with the use of the digital computer. Masters Thesis, La Universidad de la Habana, Habana (l974), (In Spanish).*

22. V. HlavaSek, Analysis of a complex plant-steady state and transient behavior. Camp. Chem. Eng. 1, I (1977).

*Interested individuals can obtain a copy of the L. Gonzalez Masters Thesis by writing to the following address: Biblioteca, I Instituto Superior Polittcnico. “Jose Antonio Echevarrfa”. Marianao, Habana, Cuba.