Optimization of Water Consumption in Bioethanol Plants

Current industrial practices and population demands are placing extra pressure on natural resources. Concerns related to overconsumption and depletion of these resources have focused on energy sources like crude oil, carbon and natural gas ¹ while water has been overlooked from that list mainly due its wide availability in many regions of the world.  As a result, water is inexpensive compared to any other raw material, in spite of an average annual increase of 6.7% over the last years according to the 2008 GWI/OECD² However, the industrial growth and the development suggests that by 2025 the industrial water usage (including utility cooling and heating, processing, transportation, air conditioning, cleaning, etc.) will account for about 11% of the total world water consumption ^3,4 which can cause water stress in several regions of the world. The increasing concern towards water resources^5,6 and the green policies supported by many governments are making the management of water consumption and wastewater treatment an important topic with new economic incentives for implementing technologies that are more environmentally friendly, and that can ensure efficient use of water resources including the treatment and recycling of wastewater ⁷

In this work we address the water consumption optimization of first and second generation bioethanol production plants. Corn-based ethanol is considered the base case to compare the water consumption with the one resulting from the production of ethanol from switchgrass following thermo-chemical, thermo-biochemical or biochemical routes. In order to optimize the water consumption a three stage method is used. First, energy consumption is optimized in the production processes, which reduces the cooling needs of the processes. We consider the optimal processes described by Karuppiah et al, Martín & Grossmann.^8-10

 Next a number of technologies are implemented to substitute the use of water as cooling agent so that is possible to reduce the water to be cooled down reducing the evaporation and drift losses in the cooling tower¹¹. Finally, in order to synthesize the optimal water networks for the bioethanol production plants together with the cooling water and stream loops, we use the general superstructure of integrated process water networks,  which has been proposed recently by Ahmetović and Grossmann ¹². We define the processes units that generate contaminants like washing stages, cooling tower and boiler, sources of water like distillation columns or gas-liquid flash separators, sinks like fermentation processes, cooling towers, boilers, pretreatment tanks or gasifiers. In this way it is possible to indentify the main contaminants. We assume that total suspended solids, organics and total dissolved solids are the most important contaminants generated. Thus, we propose a number of treatment units to handle them such as screens, combined aerobic and anaerobic treatment and reverse osmosis, respectively. With this information we design the optimal water network for each of the energetically optimized production processes of ethanol.

By energy optimization of the flowsheet together with the implementation of multieffect columns and the design of optimal heat exchanger networks, it is not only possible to reduce the energy consumption by one half but also to reduce the cooling needs by two thirds. As a result, the water to be treated in the cooling tower is reduced and so are the water loses by evaporation and drift. Furthermore, the use of air cooling to reduce the use of water as cooling agent has an impact  only in case of thermo-chemical processes, where the intercooling of the compression stages has a large contribution in the cooling needs. Finally, by coupling energy optimization, air cooling and the optimal design of water networks, all the ethanol production processes considered consume less than 3 gal water per gal ethanol in the range of the needs for the production of gasoline ¹³. Moreover, it turns out that corn ethanol is not as water intense as has been reported so far and even consumes lees freshwater than any other process (1.46 gal/gal). Zero discharge is not achieved for all the production processes but small values are reported.

References

(1) Hayward, T.  (2008) BP Statistical Review of World Energy June 2008

(2) GWI/OCDE (2008)   http://www.globalwaterintel.com/archive/9/9/analysis/world-water-prices-rise-by-67.html

(3) Rosegrant, M.W.  Cai, X. Cline, (2002) Global Water Outlook to 2025 – Averting an Impending Crisis. International Food Policy Research institute, Washington

(4) Sparks Companies Inc. (2003) Global water and food outlook (2003)

(5) Elcock, D. (2008) Baseline and Projected Water Demand Data for Energy and Competing Water Use Sectors, ANL/EVS/TM/08-8,

(6) Chiu, Yi-Wen,  Walseth^‡, B.,  Suh, S. Water Embodied in Bioethanol in the United States Environ. Sci. Technol., 2009, 43 (8), pp 2688–2692

(7) Petrakis S. Reduce cooling water consumption. New closed loop cooling method improves process cooling tower operations. Hydrocarbon Processing, 2008, December, 95-98

(8) Karuppiah et al. 2008 AICHE J. 54, 1499-1525

(9) Martin, M., Grossmann, I.E: Process Optimization of Bioethanol Production via Gasification of Switchgrass to be submitted AIChE J.  2010

 

(10) Martin, M., Grossmann, I.E: Process Optimization of Bioethanol Production via Hydrolysis of Switchgrass   to be submitted AIChE J. (2010b)

 

(11)  Phillips, S., Aden, A., Jechura, J. and Dayton, D., Eggeman, T (2007) Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis of Lignocellulosic Biomass Technical Report, NREL/TP-510-41168, April 2007

 

(12) Ahmetović, E., & Grossmann, I. E. General superstructure and global optimization for the design of integrated process water networks Submitted to AIChE J. 2010

(13) Andy Aden Water Usage for Current and Future Ethanol Production. Southwest Hydrology, September/October 2007.  22-23