Thermodynamic analysis of an alcohol distillery

Energy Vol. 13, No. 5, pp. 455-459,1988 Printedin Great Britain. All rights reserved

0360-5442/88 $3.00 + 0.00 Copyright @ 1988 Pergamon Press plc

THERMODYNAMIC

MARCELO

ANALYSIS OF AN ALCOHOL DISTILLERY

CASTIER

and

KRISHNASWAMY

RAJAGoPALt

Programa de Engenharia Quimica-COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, Rio de Janeiro, RJ 21944, Brazil (Received 11 July 1986)

Abstract-An energy analysis is performed on an autonomous distillery in Brazil for the production of alcohol from sugar cane. Selected instantaneous operational data complemented by mean operational values and design values are used to calculate the mass and energy balances. The second-law efficiency is evaluated in important sectors of the plant and in the overall process. The sectors of steam generation (.54.7%), fermentation (32.0%) and distillation (7.2%) are found to account for most of the exergy losses in the plant. The fraction of the exergy output to that of the input is 33.8%, which is comparable to the effectiveness of small power plants.

INTRODUCTION

In recent years, alcohol produced from sugar cane has become an important alternative to gasoline as motor fuel and as feedstock for the chemical industry in Brazil. Although the fall in

oil prices in the beginning of 1986 may slow down the rise in the number of new distilleries, it is likely that existing distilleries will continue to operate and expand. An energy analysis of an operating autonomous distillery for producing alcohol from sugar cane in Brazil is presented. Autonomous distilleries, where alcohol is not produced as a by-product of sugar manufacture, are in a peculiar position with respect to fuel conservation. All of the energy necessary for the process is provided by burning bagasse obtained while crushing the sugar cane and, usually, there is an excess of bagasse. There is little motivation for fuel conservation, although there are many potential applications for this bagasse. For example, it can be used as fuel in other industrial plants or as feedstock for the production of paper and chemicals. The production of fuel alcohol from sugar cane has been studied by several authors.‘-’ The scopes and methodologies vary considerably, ranging from purely economic analyses to energy analyses based on the first law of thermodynamics. Detailed energy balances or analyses of the industrial process are not made in any of these published studies. Second-law analyses of distillery sectors have been presented by Bessa and Rajagopal.s3y It has been shown’ that the effectiveness in the steam-generation sector can be improved with bagasse predriers. Our energy analysis is based on the exergy approach. Operational and design data of a typical autonomous distillery (Agua Limpa Distillery in Monte Aprazivel, State of Sao Paulo, Brazil) are used to compute the mass and energy balances. The energetically inefficient points of the process, which can in principle be improved, are identified.

METHODOLOGY

We assess the equipment and distillery performances using the effectiveness, which is the relation between the exergy of the output and input streams to an equipment or process. The reference state proposed by Petit and Gaggioli” is adopted since it represents well the ambient conditions around the distillery. The liquid streams in a distillery are nonideal solutions. Formulas for the exergy of multicomponent nonideal mixtures derived by Rajagopal and Castier” are employed. tTo whom correspondence

should be addressed. 455

*

(I411LW)

ELEIRICITY FROY PueLlC SUPPLY ___.~_._.

i

.-.-.-.-._.

IYBlBlllONWATER (514741 c

SUGAR CANE (125000)

b

MILLING

------_----~ 133663)

14174)

T

---------(468991

@-=I

1 ETHANOL

BROTH

ETHANOL

LOSS (19151

HYDRATED

(140863 t'/hI

fl408311

FERMENTED

FERMENTATION

(399831

CONDENSATES

Fig. 1. Schematic of the Agua Limpa distillery.

I

1

CONCENTRATION

PRECONCENTRATED JUICE (101626)

(19489

----------

I’

I

I

II

l-

MIXED JUICE (1439561

4 VAPOUR FROM : THE 3'dEFFECTi

HIGH PRESSURE STEAM LOW PRESSURE STEAM

-------

OTHERWISE

ELECTRICITY

(ALL FLOWS ARE IN kq/h,UNLESS

-.-.-

......... VEGETAL VAPOUR

PROCESS STREAM

HOT WATER -

0

COOLING WATER

SYMBOLS :

WATER (6848)

SPECIFIED)

Thermodynamic

analysis of an alcohol distillery

457

Sugar cane and bagasse streams are modeled as containing dry fiber and a solution of sucrose in water. The chemical contribution to the exergy of the dry fiber is evaluated using a formula proposed by Szargut and Styrylska’* for fuels whose lower heating value (LHV) and absolute entropy are available. Correlations proposed by Hugot13 are utilized to compute the LHV. The absolute entropy of the dry bagasse at 298.15 K is estimated to be equal to 0.293 kJ/(kg.K). Streams charcterized as sugar cane juice are modeled as solutions of sucrose in water. Data reported by Taylor and Rowlinson’4 for sucrose-water mixtures are utilized. Density data from Arqued,” a correlation for specific heat from Hugot13 and formation data for sucrose from Honig16 are used. The main process streams after the fermentation sector are considered as ethanol-water mixtures. The van Laar model with parameters from Gmehling and Onken” is used to compute the excess Gibbs free energy. The exergy of the steam and water streams is computed by using steam-table data. The exergy of the electricity streams is considered to be equal to their electric power. MASS

AND

ENERGY

BALANCES

The equipments in the plant are grouped into sectors for analysis (Fig. 1). The mass and energy balances of the plant are reconciled to provide a consistent basis for the energy analysis. Data have been obtained from the following sources: (1) instantaneous measurements made in the plant; (2) production and laboratory reports of the distillery; (3) average operational values, and (4) design information. Data other than design data have been favored whenever these are available and consistent. The reconciled mass and energy flows are also shown in Fig. 1. RESULTS

AND

DISCUSSION

Table 1 and Fig. 2 summarize the second-law analysis for the distillery. Detailed results can be obtained from the authors on request. The overall effectiveness of the plant is 33.8%. The two lowest values of effectiveness are in the sectors where chemical reactions occur, namely, the steam generation sector (SGS) and the fermentation sector (FS). In the SGS, besides combustion, the energy is also degraded because of the large difference between the temperature of the combustion chamber and that of the steam. The effectiveness could be improved by producing higher pressure steam. It can also be improved by predrying bagasse. In the FS, thermal and pressure effects are of little importance. Possibilities for improved performance are the use of better yeast strains and minimization of ethanol losses from the fermentation vats. The energy balance has indicated that the bagasse from the milling sector was insufficient to produce all of the necessary high-pressure steam. In the period of analysis, the sugar cane had higher sugar and moisture contents and a lower fiber content than average operational values. This variation in sugar cane composition causes a high yield of alcohol and a reduced amount of moist bagasse with low heating value. Table 1. Lost exergy and effectiveness in the Agua Limpa distillery.

Milling Steam Generation Electricity Juice

Generation

Concentration

Fermentation Dlstillatlon

458

MARCELO CASTIERand KRISHNASWAMY RAJAGOPAL

_-

-

:

ri 4

Thermodynamic analysis of an alcohol distillery

459

CONCLUSIONS

The sectors of steam generation (54.7%), fermentation (32.0%) and distillation (7.2%) account for most of the lost exergy. In the sector of steam generation, the high humidity and the high residual sugar content in the bagasse from the milling sector may be partly responsible for the inefficiencies. The steam demand is mainly determined by the low pressure steam needed for the distillation (70.6%) and preconcentration (22.7%) sectors. In our opinion, the surplus process steam could be utilized more effectively in preconcentration, thus making it possible to control the concentration of the juice in fermentation. The variation of juice in fermentation seems to be the main reason for the higher productivity of the distillery studied compared to other distilleries without preconcentration. The overall effectiveness compares well with that of small power stations. The value is low compared with oil refineries mainly because, in a distillery, the liquid fuel is produced through chemical reactions during the fermentation process, in a highly irreversible step. Acknowledgements-The European Economic Community (Contract EEC-FINEP-COPPE, No. 11270) has provided financial support. J. L. de Medeiros (Federal University of Rio de Janeiro) and I. G. Bessa (COPPE) contributed through helpful discussions. N. A. Ramella and N. Vieira (Agua Limpa Distillery) have kindly provided operational data and helped in measuring plant data.

REFERENCES

1. D. M. Jenkins,

T. A. McClure,

and T. S. Reddy, “Net Energy Analysis of Alcohol Fuels,” Am.

Petrol. Inst. Publ. No. 4312 (1979).

2. J. A. Polack, H. S. Birkett, and M. D. West, Chem. Engng Prog. 77(6), 62 (1981).

3. S. R. Martin, Chem. Engr. 377, 50 (1982). 4. J. G. da Silva, G. E. Serra, J. R. Moreira, and J. C. Goncalves, Bras. Acuc. 6, 452 (1976). 5. A. S. Khan, and R. Fox, “Net Analyses of Alcohol Production from Sugarcane in the Cariri Region 6. 7. 8. 9. 10. 11.

12.

13. 14. 15. 16. 17.

of Ceara, Brazil,” Working Paper No. 10, Dept. of Chemical Engineering, Federal University of Ceara, Fortaleza, Brazil (1981). W. S. Fong, J. L. Jones, and K. T. Semrau, Chem. Engng Prog. 76, 39 (1980). H. Perez-Blanco, and B. Hannon, Energy 7, 267 (1982). I. G. Bessa, and K. Rajagopal, Anais do II1 Congress0 Brasileiro de Energia 3, 1142. Rio de Janeiro, Brazil (1984). K. Rajagopal, Anais do XI Encontro sobre Escoamento em Meios Porosos VI, P. 1.13/l, Rio de Janeiro, Brazil (1983). P. J. Petit, and R. A. Gaggioli, Am. Chem. Sot. Symp. Ser. 122, 15 (1980). K. Rajagopal and M. Castier, “Thermodynamic Energetic Analysis of Alcohol Distillery,” Final Report, Contract EEC-COPPE-FINEP 11270, COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil (November 1984). J. Szargut and T. Styrylska, Brennst.-Waerme-Kraft 16, 589 (1964). E. Hugot, Manual da Engenharia Acucareiru, Mestre Jou. Sao Paula (1977). J. B. Taylor and J. S. Rowlinson, Trans. Faraday Sot. 51,1183 (1955). A. P. Arqued, Fabricazion de1 Azucar, Salvat, Barcelona (1955). P. Honig, Principios de Tecnologia Azucarera, Continental, Mexico City (1969). Equilibrium Data Collection,” I/l, DECHEMA J. Gmehling and U. Onken, “Vapour-Liquid Series, Frankfurt. F.R.G. (1977).