Fase 4. Desarrollo de propuesta
5.1 Trabajo a futuro
During the past fifty years, ethanol acquired economic value as a fuel, a solvent, a substrate in chemical synthesis, and in the fortification of neutral spirits (gin and vodka). In general, the production of fermentation (or non-synthetic) ethanol consists of two steps: ethanolic fermentation of a sugar-rich solution, followed by concentration and purification of the ethanol by distillation. The usual substrates are sugar-cane syrup, starch from various grains (corn, rice, oats, rye, etc), and lactose from whey.
In New Zealand, all industrial and potable ethanol required for the domestic market is produced from deproteinated whey by fermentation. The technology was established in
1 980 when the first distillery was commissioned by the New Zealand Co-operative Dairy Co. Ltd. at Reporoa (Mawson, 1 987) and as of 1 994 there were three maj or distilleries (Table VII. I ) producing over 1 7 M1 of absolute ethanol per annum (Mawson, 1 994).
Table VU.I Production sites of whey ethanol in New Zealand (Mawson, 1 9 87).
DistiUery Reporoa Tirau Edgecumbe
Date commissioned 1 980 1981 1 982
Substrates processed Deproteinated lactic Deproteinated lactic Sulfuric acid whey
whey whey permeate
Maize
Grades of alcohol Industrial Industrial Industrial
produced 95% (v/v) 95% (v/v) 99.5+% (v/v)
99.5+% (v/v) Potable
Whey, the fluid obtained by separating coagulum from milk, is a dilute, aqueous solution of lactose (approx. 5% w/w), proteins (approx. 0. 8% w/w), and ash (approx. 1 % w/w) (Reesen, 1 978; Short, 1 978). Until recently whey was treated as a waste product because of its low quantity of total solids, but tougher waste disposal legislation forced the industry to seek alternative uses for whey (Zadow, 1984). Whey proteins make an attractive product for the food industry, particularly in the form of whey protein concentrate (WPC), because of their high nutritional and functional value. The deproteinized whey which results from the production of WPC contains mainly lactose, which can be purified by crystallisation and used in the food industry for flavour enhancement, protein stabilization, controlled browning, or rearranged into lactulose (Short, 1 978). From demineralized whey, the lactose can be fermented into methane, lactic acid, or ethanol (Short, 1 978).
The product from whey fermentation is an approximately 2.5% (v/v), dilute mixture of ethanol and fermentation by-products. A mixture of higher alcohols and esters, known as fusel oils, constitutes the main by-product and is present in a concentration of less than one percent of total ethanol produced. The composition and concentration of the fuse I oils vary depending on the substrate, the microorganisms used, and the fermentation conditions. Drucker ( 1 9 8 1 ) presented a summary of the typical compositions of fusel oils for the most
common fermentation conditions. Fusel oils impart an aroma which is generally desirable in the beverage industry (wine, brandy, whisky), but not in the production of ethanol for industrial purposes or for neutral spirits production (gin, vodka).
Distillation, the next step in the production of alcohol, must remove most of these by products and raise the ethanol content to the required specification. In general, the purification and concentration of ethanol to very near its azeotropic point can be achieved with three columns (Figure VIT. 1) and the energy for distillation is provided by direct steam
injection. In the first column the beer is concentrated to 90% (v/v) ethanol and it is possible to remove from this column a side stream rich in fusel oils. In the second column, the ethanol-rich stream is refined by extractive distillation: the ethanol-rich feed is diluted to allow for the separation of the fusel oils, which concentrate at the top of the column and are removed from the reflux drum. The bottom product of the extractive distillation
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column, a dilute ( 8- 1 0 % v/v), purified ethanol stream, IS brought to the final concentration of 96.5% (v/v) ethanol in the third column.
F 2.5% (v/v) 90% (v/v) s s Extractive Distillation 96.5% (v/v) s
Figure VU.I Schematic drawing of an ethanol purification plant. F: feed stream; S: steam stream.
Due to its low lactose content, the fermentation of whey results in a beer containing 2.5% (v/v) ethanol. According to Maiorella et al. ( 1 984), distillation costs increase significantly if the concentration of the fermentation broth fed to the column is below 6% but no further savings are realized in increasing the concentration over 9%. In order to increase the ethanol concentration of the fermentation broth, pre-concentration of whey by reverse osmosis or evaporation is an established procedure (Mawson, 1 987). Another option under study is continuous fermentation with ethanol removal by pervaporation (Mulder et al. , 1 983; Younaian et al. , 1 990; Shabtai & Mandel, 1 993). Improvements in this process are still required to solve problems related to the concentration of salts in the fermentation broth (Shabtai & Mandel, 1 993), fouling of the membrane (Younian et al. , 1 990; Shabtai
& Mandel, 1993) and increased consumption of electricity (Neel, 1 990). Mulder et al. ( 1 983) have suggested that an ultrafiltration unit installed before pervaporation could separate the solids from the fermentation broth and thus reduce the possibility of fouling
of the pervaporation membrane. The yeast cells would then be recycled to the fermentation vessel.
It has been estimated that steam consumption constitutes a quarter of the total production cost of whey ethanol (Mawson, 1 987) and that, worldwide, distillation accounts for 1 5% of the total industrial energy usage (Fell, 1 997). In general, traditional process changes aimed at reducing the energy input during distillation include tray retrofitting (Mix et al. ,
1 978), efficient use of insulation, introduction of heat recovery schemes and vapour recompression, and multi-effect pressure distillation (Mawson, 1 987). It has also been suggested that energy savings and process simplification could be achieved by direct removal of ethanol from the fermentation broth with either selective membranes, extractive fermentation or flash fermentation, liquid extraction, adsorption or absorption with either water or ethanol selective materials, or carrier gas distillation (Douglas & Feinberg, 1 983; Maiorella et al., 1 9 84).
In the removal of fuse I oils, the wine industry uses a spinning cone column (SCC) that i s supposed to have a higher efficiency i n the mass transfer between the vapour and liquid phases than tray columns (Harders et al. , 1 995). Whilst these wine flavours are similar to the composition of by-products from ethanolic fermentation of whey, detailed data on the separation ability of the SCC is not available in the public domain.
Larsson & Zacchi ( 1 996) compared the concentration and purification step for the production of 94% (w/w) ethanol from a dilute glucose solution of approximately 5% (w/w) using distillation with heat integration, distillation with an absorption heat transformer, distillation with mechanical vapour recompression, phase separation with potassium carbonate, and extraction of ethanol from the fermentation broth with Aldol 85. It was concluded that distillation with heat integration was the most economical option.
Douglas & Feinberg ( 1 983) reported on a series of Solar Energy Research Institute (SERl) sponsored developments (Table VII.2) for ethanol separation using non-distillation techniques. Their conclusions were based on the energy requirements for the production
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o f dehydrated ethanol