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Industrial
Alcohol
Production -
Ethanol used for industrial, laboratory and fuel purposes is called industrial alcohol. Ethanol has now become synonymous with energy. The various substrates used for ethanol production and their processing is briefly summarised below.
1. Sugar crops, e.g., sugarcane, sugarbeet, sorghum, etc. provide a good substrate. Juices from these crops contain sugar, e.g., sugarcane juice has about 12% fermentable sugars, and can be used directly for fermentation.
2. Bye product sugars from crop processing, e.g., molasses, sweet sorghum syrup, and spent sulphite liquor are the most common substrates. Molasses ('C' grade) obtained after sugar recovery contains about 50-55% total fermentable sugars. Molasses is first suitably diluted before being used for fermentation.
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3. Cereals like maize, wheat sorghum, etc. contain 60-75% w/w starch, which on hydrolysis produces glucose in the ratio 9: 10. Generally, starch is a mixture of a-amylose (20-30%; water soluble linear polymer) and amylopectin (70-80%; water insoluble branched polymer).
Conversion of starch into glucose is usually achieved by a cooking process aided by enzymes. Seeds are powdered and the resulting meal is mixed with water in the ratio of 1 : 2.5-3. α-Amylase is then added and the temperature is raised by steam injection to 105-110°C and held for about 20 min.
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During this period, starch grains become dissolved to form a viscous suspension (gelatinisation) due to the cooking effect of heat. At the same time α-amylase digests partially some of the starch molecules; this reduces the viscosity of the suspension; this is called liquefaction. The suspension contains solubilized starch molecules, and dextrins
The suspension called mash, is cooled to 85-90°C, and more α - amylase is added to complete the process of liquefaction (for 90 min). The total dose of α-amylase is about 1.5 kg/ton starch. The mash is cooled to 55-60°C, and 1.5-3.5 I glucoamylase/ton of starch is added to produce glucose from dextrins and maltose.
The reaction duration is usually 2 days, and it achieves the dextrose equivalent of 99. Saccharification by glucoamylase continues during the fermentation.
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4. Tubers like cassava, yams, potato, etc. are rich in starch (Ca. 30% on fresh weight basis). Cassava tubers are washed, mashed to pulp and subjected to liquefaction and saccharification in a manner similar to cereals. Cassava tuber roots can be air dried to 15% moisture and stored; tuber chips or meals are better for storage.
5. Cellulosic substrates are the most abundant. Acid hydrolysis of cellulose is technically possible and was used during the war. Enzyme hydrolysis has presented problems. A more promising approach appears to be the direct conversion of suitably pretreated cellulose into ethanol by mixed cultures of celluloytic and fermenting bacteria.
Some thermophilic Clostridium species are rather promising. The pretreatment of cellulosic biomass may be either physical or chemical and is aimed at reducing the size of the substrate to help in cellulose hydrolysis.
Typically, fermentation of 100 g glucose by selected strains of Saccharomyces cerevisiae and Saccharomyces carlsbergensis yields 45-49 g ethanol, the theoretical limit being 51.1 g (C6H12O6 --> 2C2H2OH + 2CO2). Both batch and continuous fermentation processes are used, and often yeast cells are recycled to save the substrate used up as cell matter.
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Continuous process uses 3-5 closed vessels; ethanol concentration increases from Ca. 4% in the first vessel to Ca. 10% in the final vessel, the productivity ranging from 10 to 20 kg ethanol/m2/hr. In contrast, batch process productivity ranges between 1.5 and 3 kg ethanoVm2/hr. However, final yields are much higher in the batch process, which is the most commonly used.
Generally, ammonium sulphate or urea (N source) and a salt of phosphoric acid (P source) are added to the substrate. Most commonly the temperature during fermentation is 32-38°C and pH is between 4.5 - 5.0; in large fermenter vessels, some form of cooling is necessary.
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As the ethanol level rises, both ethanol production and yeast cell growth are progressively inhibited, and eventual1y cells begin to die. In general, yeast growth stops at 6-9% w/v ethanol, but ethanol production, at least in some strains, may continue upto 15% ethanol or higher. Ethanol inhibition can be relieved by continuous removal of ethanol.
The commercial process, called Biostill technique, passes cell free broth through an evaporation chamber for ethanol removal and this medium is then fed back into the fermenter. Ethanol recovery is based on distillation.
(i) The broth is distilled in a beer column to yield 85% v/v ethanol.
(ii) The next step of rectification gives 96.5% ethanol, which is then
(iii) dehydrated to 99.4% using benzene or cyc1ohexane if the ethanol is to be used as a fuel blend.
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