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3. Organic chemicals from carbohydrates (biomass). The main constituents of plants are carbohydrates which comprises the structure part of the plant. They are polysaccharides such as cellulose ans starch. Starch occurs in the plant kingdom in large quantities in foods such as cereals constructed and therefore occurs very widely and may be obtained from wood, cotton, etc. Thus, not only is the potential for chemicals considerable, but the feedback is renewable.
The major route from biomass to chemicals is via fermentation processes. However, thses processes cannot utilize polysaccharides like cellulose ans starch, and so the latter must first be subjected to acidic or enzymic hydrolysis to form the simpler sugars (the mono- or disaccharides, e.g. sucrose), which are suitable starting materials.
Fermentation processes utilize single-cell micro-organisms typically yeasts, fungi, bacteria or moulds to produce particular chemicals. Some of these processes have been used in the domestic situation for many thousands of years, the best-known example being fermentation of grains to produce alcoholic beverages. Indeed up until about 1950 this was the most popular route to aliphatic organic chemicals, since the ethanol produced could be dehydrated to give ethylene, which is the key intermediate for the synthesis of a whole range of aliphatic compounds. Although chemicals production in this way has been declining there is a lot of interest in producing automobile fuel in this way.
Disadvantages reflected in this can be divided into two parts (i) raw materials (ii) the fermentation process. Raw-material costs are higher than that of crude oil, because biomass is an agricultural materials and therefore in comparison its production and harvesting is very labor-intensive. Also, being a solid materials transportation is more difficult and expensive. Major disadvantages of fermentation compared with petrochemicals processes are, firstly the time scale, which is usually of the order of days compared to literally seconds for catalytic petrochemical reactions, and secondly, the fact that the product is usually obtained as a dilute aqueous solution (< 10% concentration). The separation and purification costs are therefore very high indeed. Since the micro-organism is a living system, little variation in process conditions is permitted. Even a relatively small increase in temperature to increase the reaction rate may result in death of the micro-organism and termination of the process.
On the other hand particular advantages of fermentation methods are that they are very selective and that some chemicals which are structurally very complex, and therefore extremely difficult to synthesize, and/or require a multi-stage synthesis, are early made. Notable examples are various antibiotics, e.g., penicillins, cephalosporins and streptomycins.
Proyided that the immense practical problems associated with the rapidly developing field of genetic engineering, where micro-organisms such as bacteria are “tailor-made” to produce the requires chemical, can be overcome, then the interest in the fermentation methods will be very considerable. However it seems unlike that bulk chemicals, i.e. those required in very large quantities such as ethylene and benzene, will be produces in this way in the foreseeable future because of the slow reaction rate ans the very high product separation costs.
Organic chemicals from animal and vegetable oils and fats. Animal and vegetable oils and fats--commomly known as lipids--are composes of mixtures of glycerides, which are esters of the trihydric alcohol, glycerol (propane-1,2,3-triol). There are many different sources of these oils. Some popular sources are soya, corn, palm-kernel, rapeseed and olive, animals fats and even sperm whales. The oils are isolated by solvent extraction and considerable quantities are used in the food industries as cooking oils and fats, and for production of butter, margarine and various other foodstuffs such as ice-cream. There is still controversy about the effect of the R-groups in these foodstuffs on human health, particularly on high cholesterol levels in blood which may lead to high blood pressure and heart disease.
Opinion now seems to favor a high proportion of unsaturated groups as being beneficial in lowering cholesterol levels and reducing the risk of heart attacks. This has led to a trend away from cooking fats and ordinary butter or margarine (which are all in rich in saturated R-groups) to cooking oils and the use of margarine rich in polyunsaturates.
Being esters, the uses of lipids for chemicals production starts with hydrolysis. Although this can be either acid- or alkali-catalyzed, the latter is preferred since it is an irreversible reaction, and under those conditions the process is known as saponification.
It is important to note that asponification, hydrolysis (fat splitting) and hydrogenolysis reactions do not each use a single glyceride (or methyl ester). in practice, the vegetable oil which is used is a mixture of various glycerides and the product is therefore a mixture which requires separating.
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