Production of ethanol from sweet sorghum is more direct compared to starch and cellulosic feedstock as the carbohydrates produced by sweet sorghum are sugars which can be directly fermented to ethanol. This benefit of sweet sorghum is partially offset by the need to initiate fermentation immediately (on-farm) to minimize the loss of microbial conversion of the sugars to non-ethanol products. Researchers in the School of Biosystems & Agricultural Engineering at Oklahoma State University have demonstrated the ability to harvest, express, and ferment sweet sorghum on-farm. The fermentation product is an aqueous 6 – 10 wt % ethanol product. The resulting challenge is to devise a means to economically deliver fuel-grade ethanol to the marketplace.
The high concentration of water (90+ wt %) in the fermentation product provides a strong incentive to perform on-farm dewatering in lieu of costly transport to a central processing facility. This paper describes the unique challenges associated with design of an on-farm distillation system capable of producing a near azeotropic (95 vol%) ethanol product. Final on-farm processing using a membrane pervaporation process to produce the final fuel-grade ethanol is under investigation but not addressed in this paper. The on-farm or small scale production of ethanol presents a set of novel technological challenges and opportunities from a design and engineering stand point, the success of which lies in it being economical and easy to operate by on-farm personnel.
Existing centralized bio-ethanol separation systems are large scale process plants capable of producing 100+ gpm of fuel grade ethanol. The design of the beer still, rectifier, and side stripper columns used to produce the azeotropic ethanol intermediate product for this service are well established. Unfortunately, these designs cannot be applied without significant modification to the on-farm case. The two main factors that drive the design of an on-arm operation are its scale and operational environment. Unlike industrial scale facilities, on-farm units must operate at low feed rates of about 250 gallons per hour. The corresponding ethanol production rate of less than 1 gpm is more than two orders of magnitude less than a commercial facility. It is also unlikely that an on-farm facility will have access to traditional utilities such as steam or cooling water. Therefore, the design of an on farm unit will differ from its industrial counterpart.
This paper focuses on the design and simulation of a distillation system for ethanol separation which is representative of an ‘on-farm' scenario. A two column distillation configuration with associated ancillary equipment was designed for the above purpose. The column configuration, feed stream ethanol concentration, product ethanol concentration and recovery ethanol quantities were chosen to realistically represent on-farm operation. The first column, generally referred to as the beer still was designed to handle a heavy solid content. The second column known as the rectifying column can use either trays or packing for separation. Propane or natural gas is assumed to be the only available source of heat, requiring non-standard design of the column reboilers. An air-cooled condenser is employed due to the lack of availability of cooling water. Simulation results, preliminary mechanical design, and costs are presented along with special maintenance and operational requirements. A preliminary costing analysis of the on-farm ethanol water separation unit was also undertaken and compared to industrially successful large scale units.