Cellulose to ethanol

Ethanol can also be produced from cellulose (qv) or biomass such as wood (qv), com stover, and municipal soHd wastes (see Euels frombiomass Euels FROMWASTe). Each of these resources has inherent technical or economic problems. The Tennessee Valley Authority (TVA) is operating a 2 t/d pilot plant on converting cellulose to ethanol. 

Emert Also known as the Gulf process, the University of Arkansas process, and SSF. A process for converting cellulose to ethanol by simultaneous saccharification and fermentation. Invented by G. H. Emert. 

Cellulosic Materials. Over 900 x 106 metric tons of carbohydrate-containing cellulosic wastes are generated annually. The technology for converting this material into ethanol is available, but the stoichiometry of the process is disadvantageous. Even if each step in the process of the conversion of cellulose to ethanol proceeded with 100% yields, almost two-thirds of the mass would disappear during the sequence, most of it as carbon dioxide in the fermentation of glucose to ethanol. This amount of carbon dioxide leads to a disposal problem rather than to a raw material credit (209). 

A great amount of time, money and effort is being devoted to the use of cellulose as a feedstock for the production of ethanol. The studies incorporate chemical or enzymatic conversion of the cellulose to glucose and the conversion of this to ethanol with yeast (Saccharomyces) or bacteria (Zymomonas). However, a third process is presently under development at Massachusetts Institute of Technology whereby the direct conversion of cellulose to ethanol is being attempted without a separate hydrolysis step. ... 

Process or cellulose to ethanol and single-cell protein (99,100). 

In designing an efficient SSF system for the conversion of cellulose to ethanol, the fermentation temperature should be compatible with the saccharification temperature that is generally between 45 and 55 °C. The optimal temperature for the most commonly available cellulase is about 50 °C. Therefore, the use of high-temperature-tolerant microbes is desirable for the application of the SSF process to ethanol production. Typical industrial ethanol-producing yeast strains are mesophiHc with an optimal fermentation temperature of 30-37°C. Only a few yeast strains that are thermotolerant, as well as good ethanol fermenters, have been described. However, some thermophilic bacterial species are known to produce ethanol from cellulosic-derived carbohydrates [68,69]. 

Hahn-Hagerdal, B. Mattiasson, B. Albertsson, P.-A. "Extractive Bioconversion in Aqueous Two-Phase Systems. A Model Study on the Conversion of Cellulose to Ethanol" Biotech. Letters. 1981, 3, 2, pp 53-58. 

Subsequent research led to the construction of a cellulose-to-ethanol pilot plant at Pittsburgh, Kansas. This pilot plant has a capacity of one ton of feedstock per day and has been in operation since January, 1976. Information gained from work done in our Merriam laboratories and the operation of the pilot plant has led us to recognize that renewable resources in the form of carbohydrates are an excellent source of a variety of chemicals now derived from fossil fuels. 

The cellulose-to-ethanol process has five basic steps as shown in Figure I. They are feedstock handling and pretreatment, enzyme production, yeast production, simultaneous saccharification/fermentation (SSF) and ethanol recovery. Cellulose is the most abundant organic material on the earth. It is annually renewable, and not directly useful as a foodstuff. It is a polymer of glucose linked /8-1,4 as compared with the a-1,4 linked polymer starch which by contrast is easily digestible by man. There are three basic classes of potential cellulose feedstocks. These are agricultural by-products, industrial and municipal wastes, and special crops. The availability of these materials in the U.S. is shown in Table I. For economic reasons, we are concentrating our efforts on those materials that are collected for some other reason. 

Doran, J.B. and Ingram, L.O. (1993) Fermentation of crystalline cellulose to ethanol by Klebsiella oxytoca containing chromosomally integrated Zymomonas mobilis genes. BiotechnoL Progr., 9, 533-538. 

In direct microbial conversion of lignocellulosic biomass into ethanol that could simplify the ethanol production process from these materials and reduce ethanol production costs, Clostridium thermocellum, a thermoanaerobe was used for enzyme production, hydrolysis and glucose fermentation (755). Cofermentation with C thermosaccharolyticum simultaneously converted the hemicellulosic sugars to ethanol. However, the formations of by-products such as acetic acid and low ethanol tolerance are some drawbacks of the process. Neurospora crassa produces extracellular cellulase and xylanase and has the ability to ferment cellulose to ethanol 139). 

Much current research focuses on the formation of bioethanol from cellulosic plants, plants that contain the complex carbohydrate cellulose. Cellulose is not readily metabolized and so does not compete with the food supply. However, the chemistry for converting cellulose to ethanol is much more complex than that for converting corn. Cellulosic bioethanol could be produced from very fast-growing nonfood plants, such as prairie grasses and switchgrass, which readily renew themselves without the use of fertilizers. 

Zhou S, Ingram LO. (2001). Simultaneous saccharification and fermentation of amorphous cellulose to ethanol by lecximbmant Klebsiella oxytoca SZ21 without supplemental cellulase. Biotechnol Lett, 23,1455-1462. 

Wen F, Sun J, Zhao H. (2010). Yeast surface display of trifunctional minicellulosomes for simultaneous saccharification and fermentation of cellulose to ethanol. Appl Environ Microbiol, 76, 1251-1260. 

All the strains were found to utilize cellulose and produce extracellular cellulase and xylanase enzymes. F. oxysporum 841 was found to be a potential mould for direct conversion of cellulose to ethanol/acetic acid. Though other species, e.g. F. oxysporum 2018, 62287 and 62291 also produced comparable amounts of cellulase and xylanase enzymes, only traces of ethanol and acetic acid were detected in non-aerated cultures. Maximiun cellulase activity was observed after 96 h of bioconversion process. F. oxysporum VTT-D-80134, however, was not found to produce sufficient cellulolytic and xylanolytic activities to convert cellulose or hemicellulose directly into ethanol [81]. 

D. C., Lynd, L.R., and van Zyl, W.H. (2007) Functional expression of cel-lobiohydrolyases in Saccharomyces cerevisiae towards one-step conversion of cellulose to ethanol. Enzyme Microb. Technol, 40, 1291-1299. 

A hybrid process that combines membrane separation and distillation for bioethanol and biobutanol production is being worked on by MTR [88, 89]. The membrane units use either vapor permeation or PV. The BioSep processes offer more than 50% energy savings and are cost competitive with respect to conventional distillation-molecular sieve technology. They are attractive when the ethanol concentration in the fermentation step is low, such as in cellulose-to-ethanol and algae-to-ethanol. In the case of biobutanol production, the membrane systems concentrate and dehydrate the acetone, butanol and ethanol mixture, saving up to 87% of the energy required to recover biobutanol by conventional separation techniques. 

---