389246 Mixalco Fermentations and Continuum Particle Distribution Modeling of Shock Pretreated Corn Stover

Thursday, November 20, 2014: 5:20 PM
International C (Marriott Marquis Atlanta)
Pratik Darvekar and Mark Holtzapple, Chemical Engineering, Texas A&M University, College Station, TX

There is a huge discrepancy between the rate of discovery of new oil reserves and the rate of oil consumption; this will eventually lead to an energy crisis. Also, burning fossil fuels increases atmospheric carbon dioxide levels, a greenhouse gas that leads to global warming. Today, the transportation sector is almost entirely dependent on petroleum-based fuels and accounts for 70% of global carbon monoxide emissions and 19% of global carbon dioxide emissions.  Biofuels is an attractive solution to these problems because it is renewable and carbon neutral.

             Lignocellulose is the world’s fourth largest energy source behind oil, coal, and natural gas, respectively. It is present in large quantities as crop residues and has the potential for high crop yields per acre. Biochemical conversion of lignocellulosic biomass through saccharification (breakdown of cellulose to sugars using enzymes) followed by fermentation to produce ethanol is a major pathway for biofuel production from biomass. However, this process requires the use of extracellular enzymes and sterile conditions to grow a particular bacteria or fungi, which make it expensive and difficult to control. An alternative process to convert biomass into fuels and chemicals is the MixAlcoTM process, which uses the carboxylate platform to convert biomass in a mixture of carboxylic acids salts.  The carboxylate salts obtained after the fermentation step are concentrated and are thermally converted  to ketones, hydrogenated to alcohols, and catalytically converted to alkanes/hydrocarbons (gasoline, jet fuel etc.).

          The carboxylate platform has the highest product yields in literature. Carboxylic acids are thermodynamically favored over ethanol; hence, no sterile conditions are required. The process is inexpensive and robust, no sophisticated controls or stainless steel fermenters needed, use of farm-grade plastic fermenters and PVC piping keeps the capital costs extremely low. It allows the use of variety of feedstocks:  agricultural residues (lignocellulosic biomass), energy crops, food scraps, animal manure, sewage sludge and municipal solid wastes. Use of undefined mixed cultures is vital as it can digest almost any organic matter like cellulose, hemicellulose, starch, fats or proteins. No external enzymes or use of genetically modified organisms are required.

         In its natural state, lignocellulosic biomass suffers from low digestibility because of its structural characteristics, such as lignin content and cellulose crystyllanity. Prior to fermentation, an alkaline pretreatment process is employed to remove a large percentage of lignin, which binds the cellulose and hemicellulose together, rendering the biomass much more digestible. Lime is the most suitable choice because it is the least expensive alkali, safe to handle and easily recoverable. The delignification of biomass using lime pretreatment highly depends on temperature and availability of oxygen. Two equally effective lime pretreatment methods have been developed:

(1)     Short-term oxidative lime pretreatment (OLP), which uses pure oxygen at a pressure of 6.9 bar and high temperature (110°C) for 4 hours; and

(2)     Submerged lime pretreatment (SLP) that uses air as the oxidizing agent at atmospheric pressure and 50°C for 28 days.

 SLP pretreatment is more economical as it does not need pure oxygen, high pressures, and temperatures.

         The efficacy of these alkaline pretreatments is further enhanced by a recently developed mechanical treatment called shock pretreatment. This process uses a shockwave, or rapid pressurization to render the biomass more amenable to biological and enzymatic digestion.      

         The main purpose of this research is to study the effects of shock pretreatment on mixed -acid fermentations and compare the benefit of shock pretreatment over submerged lime pretreatment in continuous counter current fermentations using the CPDM model.

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