Chemical Processing - October 2007 - (Page 51)

>> MAKING IT WORK MAKING IT WORK << Ethanol plant boosts output and saves energy advanced process control enables increased throughput while cutting steam costs By Jacob Duke, Badger State Ethanol, and Lina Rueda, Pavilion Technologies Ethanol production in thE u.S. haS grown rapidly during the past two decades, increasing from 175 million gallons in 1980 to 3.9 billion gallons in 2005. demand, long spurred by the need to replace methyl tertiary-butyl ether, which since 1997 has been under Epa scrutiny as a potential carcinogen, may get a further boost because ethanol now is being considered as a replacement for gasoline. however, this will require improvements in the energy efficiency of ethanol production. currently, argonne national laboratory calculates that it takes 740,000 Btu of fossil fuels to make 1 million Btu of energy from ethanol using corn (1). Badger State Ethanol (BSE) is one of the 97 biomassfeedstock ethanol plants built in the u.S. as of 2006. Based in Monroe, wis., BSE has produced ethanol from corn since 2002. BSE’s plant uses a dry mill process in which the starch in the corn is hydrolyzed into sugar and then fermented into alcohol. the major steps in the dry mill process are: milling, liquefaction, saccharification, fermentation, distillation, dehydration and denaturing. BSE wanted to enhance energy efficiency and so hired pavilion technologies to evaluate options for improvement. after some initial work, pavilion proposed an advance process control (apc) solution focused on the distillation and molecular-sieve-based dehydration steps. BSE set the following main objectives for the apc project: • increase average production capacity by 3.5% to 7% in the distillation and molecular-sieve operations; and • decrease distillation and sieve-vaporizer steam usage on the order of 5% per gallon or reduce Btu/gal numbers by 1.5% to 3%. if meeting the competing goals of boosting production while simultaneously cutting steam costs wasn’t challenging enough, variable feedstock composition also affected optimization. the model wound up more complex than initially anticipated. the solution, besides effectively managing the temperatures and flows in the distillation and dehydration processes, had to be robust, especially as the facility didn’t have any previous apc experience. The process the distillation and dehydration operations consist of three www.chemicalprocessing.com columns and a parallel grouping of molecular sieves (Figure 1). the distillation system is fed with the fermentation process product, a mash that contains 10% to 14% alcohol, water and all non-fermentable solids from the corn and yeast cells (a). the first column, called the beer column, splits the mash into 190-proof (95 vol. %) alcohol and a residue mash called stillage (B). whole stillage is transferred from the base of the beer column to the co-product processing area. the alcohol goes to a second column or rectifier. the bottoms from the rectifier pass to a side-stripper column — it sends alcohol back to the rectifier, while its bottom stream is recycled to the liquefaction process to reduce heat consumption. Meanwhile, overhead from the rectifier goes to the dehydration step to yield product that exceeds 99 vol. % alcohol. without that final drying process, the ethanol produced by fermentation wouldn’t be a viable fuel. BSE’s dehydration process consists of three vessels, each containing type 3a zeolite beads, which require regeneration. the units cycle through a sequence of in service, ready and regeneration. pressurized, superheated 190-proof alcohol from the distillation system is fed into one of the vessels. water is exothermically adsorbed on the beads while ethanol passes through the bed. when the beads near full loading, the feed alcohol is diverted to a second vessel ready to take over. the anhydrous ethanol vapor goes to the process for heat recovery, is cooled back to a liquid and sent to storage tanks. Meanwhile, the molecular sieves in the first vessel are regenerated, removing the water by adding energy with recycled product. the molecular sieves’ water removal ability is impacted by feed temperature, pressure, residence time and composition. the apc application was needed to improve the performance of the system by controlling these variables to achieve maxioctober 2007 • 51 http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - October 2007

Chemical Processing - October 2007 Contents From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor Biofeedstocks See Real Growth Become a Cyber-Security Pacesetter Go Beyond Condition Monitoring Disposable Equipment Earns Lasting Role Improve Control Loop Performance Ethanol Plant Boosts Output and Saves Energy Process Puzzler Plant InSites Equipment & Services Ad Lits Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - October 2007

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