HEAT INTEGRATION OF FERMENTATION AND RECOVERY STEPS FOR FUEL ETHANOL PRODUCTION FROM LIGNOCELLULOSIC BIOMASS R. Grisales 1, C.A. Cardona 1 *, O.J. Sánchez 1,2, Gutiérrez L.F. 1 1 Department of Chemical Engineering - National University of Colombia at Manizales, Colombia. 2 Department of Engineering - University of Caldas, Manizales, Colombia. Abstract. The task of heat integration has become one of the main issues in process synthesis. Heat integration allows considerable energy savings in process industry, particularly in the production of bulk chemicals from oil. Current prices of oil have done even more imperative the problem of optimal energy utilization. For this reason is of great interest the synthesis of energyefficient flowsheets for ethanol production from different feedstocks. In this work, a heat integration approach for the analysis of fermentation and recovery steps is presented. The analyzed technological schema includes: a) a continuous bioreactor which is fed with sugar-containing lignocellulosic hydrolyzate; b) a distillation train where dehydrated alcohol is obtained by conventional and azeotropic distillation; c) an evaporation system to remove water from the stillage in order to reduce the amount of wastewater for the treatment step. Low ethanol content in stream from fermentor increases energy cost in the distillation train, and therefore, in the evaporators. Heat integration is necessary to make the industrial process economically attractive and environmentally friendly. Aspen Plus is used to calculate the preliminary mass and energy balances. Through complex graphical representation of the energy requirements of the process, the heat exchanged is identified taking into account the steam and cooling utilities and the possible combined heat of the system. Each process is represented by its respective hot and cold profiles that are defined by the upper and lower composite curves corresponding to the process. For this aim, we apply method based on the thermodynamic analysis (pinch technology) and the utilization of special software developed by our group. A cost function involving energy expenditures is defined for the evaluation of the proposals. The performed heat integration analysis for the energysaving fuel ethanol production demonstrated to be a powerful tool for process synthesis involving biotechnological transformations. Keywords: Process Integration, Pinch Analysis, Bioethanol. 1. Introduction The task of heat integration has become one of the main issues in process synthesis. Heat integration allows considerable energy savings in process industry, particularly in the production of bulk chemicals from oil. Current prices of oil have done even more imperative the problem of optimal energy utilization. For this reason is of great interest the synthesis of energy-efficient flowsheets for ethanol production from different feedstocks. The importance of the lignocellulosic biomass as a material raw for the production of fuel ethanol is fully recognized due to the renewable character of the obtained by this way bioethanol (Cardona and Sánchez, 2004). In this work, a heat integration approach for the analysis of fermentation and recovery steps for fuel ethanol production from lignocellulosic biomass is presented. Pinch technology provides tools to design the heat exchanger network and utilities, being based on the design of the reaction, separation and recirculation sections, for which the mass and energy balances of the plant were * To whom all correspondence should be addressed. Address: Department of Chemical Engineering, National University of Colombia at Manizales, Cra. 27 No. 64-60, Of. F505, Manizales - Colombia. E-mail: ccardona@nevado.manizales.unal.edu.co 1
established. In a preliminary design of the heat exchanger network, the pinch technology allows to obtain the minimum values for several parameters of the process such as: utilities types and their levels, minimum number of heat exchange units and their areas and estimation of the operating and capital costs. These minimum values can be obtained without making a detailed design of the topology of heat exchange network or the exchangers that form it. It is only necessary to know the thermal data of the process streams (Shenoy, 1985). For this reason, the pinch technology is useful to obtain information that allows proposing modifications to the preliminary design of heat exchanger networks and comparing several alternatives without having to complete the design of each one of them. 2. Methods 2.1. Process Simulation The bioethanol production from lignocellulosic biomass can be described as a process composed of five main steps: Feedstock pretreatment, biological transformation, separation and effluent treatment. As a base case of process flowsheet, a simulation of biomass-to-ethanol process with ethanol production of 26.000 kg/h ethanol designed in a previous work was used (Cardona and Sánchez, 2004). Lignocellulosic complex is broken down by means of dilute acid pretreatment including the addition of sulfuric acid at high pressure. In this step, the polymers of the pretreated biomass are separated from the lignocellulosic matrix and the hemicellulose is hydrolyzed yielding a mixture of soluble pentoses (mainly xylose) and a small amount of hexoses (mainly glucose). The soluble fraction is then directed to the detoxification stage where inhibitors are removed by means of ion exchange. Then, the detoxified soluble fraction is unified with the solid fraction from the pretreatment reactor and sent to the biological transformation stage. This stage comprises the simultaneous saccharification and co-fermentation process (SSCF) where a combined fermentation of pentoses and hexoses is carried out together with the enzymatic hydrolysis of cellulose as proposed by the US National Renewable Energy Laboratory (Wooley et al. 1999). The conversion is considered to be done by genetic-engineered strain of Zymomonas mobilis. The culture broth contains 6-8% (w/w) ethanol that should be recovered in the separation step that includes two distillation columns for the concentration of ethanol near the azeotropic point. From the first column (beer column) top is produced a distillate with a 45% (w/w) ethanol content. The bottoms (stillage) are directed to evaporation stage. In the second column, aqueous solution of ethanol is rectified until 90% (w/w) ethanol content. The bottoms are used as process water for washing the hydrolyzate from the pretreatment reactor. The dehydration of the ethanol is achieved through azeotropic distillation. The distillate of the rectification column is fed to azeotropic column where benzene (the entrainer) is added in such a way, that the formation of a ternary azeotrope allow the ethanol withdrawal with more than 99% (w/w) purity from the column bottoms. The distillate is directed to a separator where the heterogeneous azeotrope is fractioned: one fraction is taken back to azeotropic column as the reflux stream and the other one is fed to stripper, where most of water is collected and the distillate is recycled to azeotropic column. The evaporation of stillage is performed by an evaporation train. After the first evaporator, the concentrated liquid is directed to a centrifuge where the flow is cooled and the solids (mainly lignin) are removed. Part of the 2
liquid stream exiting the centrifuge is mixed with the bottoms of the rectification column in order to recycle these flows for washing the hydrolyzate from the pretreated reactor. The other part is directed to the second and third stages of evaporation. The steam from the first evaporator has a pressure of 0.6 atm and a temperature of 85.5 C and is utilized for heating the second evaporator. Concentrated syrup is removed from the third evaporator working under vacuum conditions. The steam from second and third evaporator is condensed and combined with the condensate from the second evaporator. All these streams are combined and recycled as process water for the pretreatment reactor, which works under high pressure (11.2 atm). The generated steam of the pretreatment reactor is used for heating the broth feeding the beer column. The flowsheet of the biological transformation, distillation and evaporation steps is shown in Figure 1. The flowsheet configuration for the fuel ethanol production from biomass was simulated using Aspen Plus (Aspen Technologies, Inc., USA). For the synthesis of the distillation train were used the principles of the topological thermodynamics (analysis of the statics). The simulation of the phase equilibria was performed using software designed by our research group. This simulation utilized the NRTL model for the calculations of activities of liquid phase and the Hayden-O Connell equation of state for the gas phase. 2.2. Process Data Based on the flowsheet of fuel ethanol production, hot and cold stream were selected taking into account their potential for heat recovery, composition changes and stability. Interface between Aspen Plus and Aspen Pinch v. 11 was used to carry out stream discrimination analysis. The process stream data required for the pinch analysis consists of the supply (Ts) and target (Tt) temperatures, the thermal load and the heat transfer coefficients of the streams. The process stream temperatures and thermal loads where extracted from the simulated process flowsheet. The enthalpy change of each stream was identified to determine the thermal load and variations in heat capacity (C p ). Aspen Pinch makes the segmentation of the streams where the heat capacity is not constant. Heat transfer coefficients were estimated from literature (Uldrich, 1984; Woods, 1995) assuming shell and tube heat exchangers. The calculations of the heat transfer coefficients requires the fluid data of the stream, namely the velocity, density, viscosity and the geometry of the heat transfer surface. 2.3. Pinch Analysis First step in the pinch analysis was targeting. For this, a minimal temperature difference ( T min ) between hot and cold streams was selected. Two values of this difference were used (10 C and 5 C). Hot and cold composite curves were obtained for the selected streams and T min. These curves show how the energy can be transferred from hot streams to cold streams inside the process. Hot (steam at 2 bar) and cold (cooling water entering at 10 C and exiting at 25 C) utilities were utilized for completing the overall heat balance. For the determination of required steam and cooling water, the same as the potential for heat recovery, a grand composite curve was elaborated. Consumed energy in hot and cold utilities for the process was established based on the simulation performed by Aspen Plus. This energy was compared with the utilities consumption for the process when the integration was considered for given T min. For this calculations, costs of 32.952 US$/Gcal for hot utilities and 3.089 US$/Gcal for cold utilities were assumed (Al-Riyami et al., 2001). 3
32-INOCL 30-HZSSF HZCOOLER SSCF 31-HZSSF 34-OUTGS 33-NH3 STEAMRX 35-BROTH 57-DIST4 BEERCLMN 43-CO2 47-REFLX HX1 RECTIFIC AZEODIST 41-DIST1 36-MEDM STRIPPER 48-DIST3 STEAMRXN 44-ETOH DISTSPLT MEDMSPLT FEEDBEER 45-DIST2 52-FEED3 53-FEED4 FEEDBEE MIXER2 DECANTER HEATERFD 51-BENZN 59-WASTW MIXER12 49-ETOH 46-RCBOT 7-RECYWT 76-LIQ 100-ETOH COOLER MIXER10 40-BOTT1 67-VAPOR 63-VAPOR 71-VAPOR 2NDEVAPR CENTRIFG FLSH2EVP 3RDEVAPR SPLIT 66-CENTR 75-LIQ 69-LIQ 72-COND 64-SLURR 68-COND 70-SYRUP 65-WETSL PUMDISC CONDENS1 1STEVAPR CONDENST 10 SPLT MIXER11 73-A 73-COND 74-RCWT2 PUMP 73-B Fig. 1. Flowsheet for fuel ethanol production from biomass (base case). SSCF Bioreactor for SSCF process, BEERCLMN Beer column, RECTIFIC Rectification column, AZEODIST Azeotropic distillation column, STRIPPER Stripper column, DECANTER Decanter, 1STEVAPR First evaporator, 2NDEVAPR- Second evaporator, 3RDEVAPR Third evaporator, CENTRIFG Centrifuge. 2 9 1 4
Second step comprises the design of the heat exchanger network (HEN). Grid diagram where the streams with their respective supply and target temperatures and the position of the pinch are shown was elaborated. Considering the heuristic laws of the pinch (Linnhoff and Townsend, 1982; Shenoy, 1985), different heat exchanger configurations that guaranteed the target temperatures of all the streams were proposed and evaluated in terms of total heat recovery and operation costs. Heat exchange surface was also estimated. 3. Results and Discussion The flowsheet for fuel ethanol production from biomass is shown in Figure 1. In this configuration, an empiric heat integration represented by the use of steam from pretreatment reactor (not shown in the figure) for heating the culture broth feeding the beer column is implemented. In addition, the steam from the first evaporator is used for heating the liquid to be evaporated in the second evaporator stage. For extracting data of the streams for the pinch analysis, top streams of the beer and rectification columns and stripper were considered as an energy sources (hot streams). The heat needed by the reboilers of these columns was considered as heat sinks. Azeotropic column was not taken into account due to the instability of the process. Preliminary analysis showed that the main heat source is the top of the beer column with an available heat duty of 57.163 Gcal/h. Hot and cold streams of the evaporation train were also selected. Fig. 2. Hot and cold balanced composite curves for a minimal temperature difference of 10 C. Red line: hot streams; Blue line: cold streams; green line: pinch point; dotted line: violation of pinch. According to the energy balances for actual process configuration (base case), utilities consumption was of 287.24 Gcal/h for hot utilities and 175.92 Gcal/h for cold utilities. If applying the principles of heat integration by which the hot streams can be utilized for heating cold streams for T min = 10 C, the utilities consumption can be reduced to 219.4 Gcal/h and 107.8 Gcal/h for hot and cold utilities respectively, as shown in Figure 2. 5
Considering the values for energy balances for each of both cases (base case configuration and balanced composite curves), the difference shows a maximal potential of energy recovery that is in accordance with the pinch laws. Expressing this potential in terms of operation costs and assuming an annual operation time of 8000 h, savings in utilities of 18*10 6 US$/year in steam and 1.7*10 6 US$/year in cooling water may be reached that is equivalent to recovery the 24% of the energy consumed as hot and cold utilities under ideal conditions. When T min = 5 C is used in the targeting, hot utilities consumption is reduced to 200.7 Gcal/h and cold utilities consumption is reduced to 89.1 Gcal/h. These data represent savings in steam of 23*10 6 US$/year and in cooling water of 21.5*10 6 US$/year. The corresponding composite curves for this case are shown in Figure 3. Therefore, higher potential of heat recovery can be obtained with a T min of 5 C, but this difference implies larger restrictions during the design of HEN due to the necessity of utilizing greater heat transfer surfaces. Fig. 3. Hot and cold balanced composite curves for a minimal temperature difference of 5 C. Red line: hot streams; Blue line: cold streams; green line: pinch point; dotted line: violation of pinch. The temperature range for the minimal temperature difference of 10 C is within 91 C-101 C. Taking into account this interval, the design of HEN was carried out. The base case configuration shows a violation of the heuristic rule of pinch (see Figures 2 and 4). Hence, the initial alternative of integration corresponded to heating feed stream of the beer column only until 91 C (just below pinch) since the violation of heuristic rule implies the utilization of additional heat exchange surface. In this point, heat exchanger HX1 was located with a heat load of 19.3 Gcal/h (see Figure1). Due to the HEN features for the anhydrous ethanol production, minimal difference of temperature of 10 C does not allow hot streams to be located above pinch. For this reason, the integration should take adventage of hot and cold streams located below pinch to avoid the mentioned violation, limiting the possibilities of integration considerably. However, the use of the hot stream from the top of beer column was analyzed thanks to its available enthalpy (57.16 Gcal/h). It was made the decision of splitting this stream for integrating it with the cold streams of second and third evaporators. A heat duty of 24 Gcal is exchanged with the second evaporator, remaining 5 Gcal/h to 6
complete the required value of heat. For the third evaporator, the duty to be completed is of 15.184 Gcal/h. Thus, exhanger HX2 and HX3 were placed together with two additional heaters to complete the mentioned duties. The new flowsheet configuration is shown in Figure 5. The calculated consumption of hot and cold utilities by the new designed HEN was of 241.3 Gcal/h for hot utilities, and 129.74 Gcal/h for cold utilities. The difference between the utilities required by the base case configuration and the new design gives the recovered heat, which can be expressed as economic benefits. Consequently, the energy savings corresponding to this proposed design of HEN were 46.224 Gcal/h (287.524-241.3 Gcal/h) for hot utilities and 46.224 Gcal/h (175.92-129.74 Gcal/h) for cold utilities. Therefore, the estimated economic benefits of the recovered energy for a minimal temperature difference of 10 C are 12.2*10 6 US$/year in hot utilities and 1.1*10 6 US$/year, that correspond to 67% of the maximum possible energy recovery calculated in the targeting step of pinch analysis. 101 ºC STEAMRXM 101 ºC 100 ºC FEEDBEER 95 ºC LPS 92.8 ºC 0.77 MMkcal/h 91 ºC 19.3 MMkcal/h Fig. 4. Process to process heat exchanger unit that transfer heat across the pinch in the base case configuration for T min =10ºC. FEEDBEER feed stream to the beer column; STEAMRXN condensing steam from the pretreated reactor. For a minimal temperature difference of 5 C, the temperature interval is within 93.4 C-88.4 C. Considering this range, the base configuration again violates the heuristic rule of pinch, as shown in Figure 6. The proposed alternative comprises the heating of the feed stream of beer column only until 88.4ºC. In this point, heat exchanger HX1 was located. Once more, the hot stream of beer column top is involved in the design of HEN through its splitting. In this case, the heat exchanger HX2 that allows the integration between this stream and the second evaporator, entirely supplies the required heat. The exchanger HX3 togheter with a new process heat exchanger supplies the heat requirements of the third evaporator (see Figure 7). These results are illustrated by the grand composite curve shown in Figure 8. 7
32-INOCL 30-HZSSF HZCOOLER SSCF 31-HZSSF 34-OUTGS SPLITBEE MIXBEER 33-NH3 TOP-BEER BEERCLM STEAMRX 41-DIST1 35-BROTH 57-DIST4 43-CO2 47-REFLX HX1 RECTIFIC AZEODIST FEEDBEER 36-MEDM STRIPPER 48-DIST3 STEAMRXN 44-ETOH DISTSPLT MEDMSPLT 45-DIST2 52-FEED3 53-FEED4 FEEDBEE HX2-BEER HEATERFD MIXER2 DECANTER 51-BENZN MIXBEHX2 MIXER12 49-ETOH 46-RCBOT 7-RECYWT 76-LIQ MIXBEHX3 100-ETOH COOLER 67-VAPOR MIXER10 HX3-BEER 71-VAPOR HX3 40-BOTT1 CENTRIFG SPLIT 66-CENTR FLSH2EVP 3RDEVAPR HX3--BEE 75-LIQ 72-COND HX2 70-SYRUP 64-SLURR HX4 CONDENS1 65-WETSL HX3-BEE PUMDISC 1STEVAPR 63-VAPOR 13 12 CONDENST 10 SPLT MIXER11 73-A 73-COND 74-RCWT2 PUMP 73-B Figure 6: Flowsheet for fuel ethanol production with heat integration for Tmin 5 C. SSCF Bioreactor for SSCF process, BEERCLMN Beer column, RECTIFIC Rectification column, AZEODIST Azeotropic distillation column, STRIPPER Stripper column, DECANTER Decanter, 1STEVAPR First evaporator, 2NDEVAPR- Second evaporator, 3RDEVAPR Third evaporator, CENTRIFG Centrifuge. 59-WASTW 3 2 1 8
1.5 MMkcal/h 93.4 ºC STEAMRXN 101 ºC 100 ºC FEEDBEER 95 ºC LPS 92 ºC 88.4 ºC 18.2 MMkcal/h 30 ºC Fig. 6. Process to process heat exchanger unit that transfer heat across the pinch in the base case configuration for T min =5ºC. FEEDBEER feed stream to the beer column; STEAMRXN condensing steam from the pretreated reactor. Similar calculations for the determination of energy savings give the following values for energy consumption: hot utilities used by the new designed HEN (at T min of 5 C): 221.98 Gcal/h; cold utilities used by the new HEN: 111.04 Gcal/h. These results lead to recovered energy in hot utilities of 65.544 Gcal/h (287.524-221.98 Gcal/h) and in cold utilities of 65.544 Gcal/h (175.92-111.04 Gcal/h). The economic benefits for this minimal temperature difference were estimated in 17.3*10 6 US$/year for hot utilities and 1.6*10 6 US$/year in cold utilities. The achieved recovery is 75.5% of the maximum possible energy recovery calculated in the targeting step. 4. Conclusions The utilization of a minimal temperature difference of 5ºC, despite the utilization of greater heat exchange surface, favors a more effective heat integration of hot and cold process streams. The composite curves for both studied cases were different in their structure. For a minimal temperature difference of 5ºC is presented a pockets of additional heat recovery that is not present for a temperature difference of 10ºC. The heat recovery potential is greater in the former case because of the hot and cold streams that can exchange heat not only below pinch (as in the case of 10ºC), but above it. The energy saving achieved in the design step with respect to targeting step was superior for a minimal temperature difference of 5ºC than for 10ºC. Evidently, a decision about the more appropriate configuration of HEN that were optimal from economic point of view requires the evaluation of both operation and capital costs of the proposed configurations derived from the pinch analysis. For this task, the calculation of heat exchanger surface is quite important. Undoubtedly, process simulators and algorithmic approach like pinch analysis, are a powerful tools for optimizing heat exchanger networks in such complex processes as biotechnological ones. 9
Fig. 8. Grand composite curve for the design of heat exchanger network at T min =5ºC. References Al-Riyami, B. A., Klemes, J., Perry, S. (2001). Heat Integration Retrofit Analysis of a Heat Exchanger Network of a Fluid Catalytic Cracking Plant. Applied Thermal Engineering, 21, 1449. Cardona, C. A., Sánchez, O. J. (2004). Analysis of Integrated Flow Sheets for Biotechnological Production of Fuel Ethanol. In: PRES 2004, 16th International Congress of Chemical and Process Engineering, CHISA 2004, Prague, Czech Republic. Linnhoff, B., Townsend, D. W. (1982). A User Guide on Process Integration for the Efficient Uses of Energy. Warwick Printing Company, England. Shenoy, U. V. (1985). Heat Exchanger Network Synthesis: The Pinch Technology-Based Approach. Gulf Publishing Company, Houston. Uldrich, G. D. (1984). A Guide to Chemical Engineering Process Design and Economics. Wiley, New York. Woods, D. A. (1995). Process Design and Engineering Practice. Prentice Hall, Englewood Cliffs, NJ. Wooley, R., Ruth, M., Sheehan, J., Ibsen, K., Majdeski, H. Galvez, A. (1999). Lignocellulosic biomass to ethanol process design and economics utilizing co-current dilute acid prehydrolysis and enzymatic hydrolysis. Current and futuristic scenarios. Golden, Colorado: National Renewable Energy Laboratory, Technical Report NREL/TP-580-26157. Acknowledgments The author wish to express their acknowledgments to the National Institute for the Development of Science and Technology of Colombia, Colciencias, the National University of Colombia at Manizales and the University of Caldas (Colombia) for economic support of this work. 10