(731f) 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-...

(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