PRODUCTION OF BIOETHANOL AND ALCOHOL.
Bioethanol is dehydrated ethyl alcohol, that is used as a gasoline substitute for motor vehicles. Bioethanol fuel is mainly produced in the process of sugar fermentation, although it can also be produced chemically.
The main sources of sugar required for the production of ethanol are fuel or energy crops: corn, wheat, sugarcane and sorghum.
Ethanol or ethyl alcohol (C2H5OH) is a clear, colorless, biodegradable liquid. It has low toxicity and does not pollute the environment when spilled. Ethanol burns to form carbon dioxide and water and is a high-octane fuel replacing lead as an octane booster in gasoline.
When mixing ethanol with gasoline, the fuel mixture is saturated with oxygen, so it burns more fully – the amount of emissions into the atmosphere decreases.
Ethanol fuel mixtures are widely used in the United States, Europe, and Brazil. The most common mixture is E10 – 10% ethanol and 90% gasoline. The car’s engines do not require any modifications to run on E10. E85 can already be used in modern cars – up to 85% ethanol and 15% gasoline.
To create bioethanol production, we do:
- conduct a pre-design study of future production (Feasibility study)
- determine the main technical and economic indicators
- we develop a hardware and technological scheme of a distillation and concentration plant for producing raw alcohol and an adsorption plant for dehydration
- we calculate all material and energy flows
- develop specifications and requirements for equipment
- we order the production of equipment and monitor its manufacture
- develop automation and production process by management schemes
- we install equipment and automation systems
- we control the commissioning of plants into commercial operation
- educate plant management staff
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BIOETHANOL AND ALCOHOL PRODUCTION TECHNOLOGY.
The starting product for the production of bioethanol and alcohol is starch-containing raw materials (grain, potatoes) or sugar-containing raw materials (molasses, rarely sugar beet). Starch raw material is converted into sugar by enzymatic treatment. Under the impact of yeast enzymes, sugar turns into ethyl alcohol (C2H5OH) and carbon dioxide (SO2), the result is a mature brew. The brew can be made from grain, potato, beet, melyasna, and also can be mixed: melyasno-grain-potato, melyasno-beet, beet-potato, melyasno-sugar and sugar-grain-potato.
The brew is a multicomponent system consisting of water (82… 90% by weight), dry substances (4… 13% by weight) and ethyl alcohol with accompanying volatile impurities (5… 9% by weight. or 6… 14% vol.). The brew always contains some carbon dioxide. Its content in the brew selected directly from the fermentation machine is 1… 1.5 g/dm³. When submitting the brew to the rectification compartment 35… 45% of carbon dioxide is lost.
Dry substances of the brew are represented by unfermented sugar, dextrins, fiber, amino acids and other nitrogenous substances, mineral salts. The solids are largely in dissolved form and partly in suspended particles (peel, yeast cells, etc.).
Dry substances of brew (yeast mass, shot, fiber, gypsum, calcium phosphate, caramel, melanoidins) are deposited on the walls of the equipment, disrupting its work.
Grain-potato brew contains many suspendedparticles. It is more viscous than molasses. Mineral salts and acids in the brew cause its acid reaction (pH = 3.5… 5.6).
The beet brew and brew that was obtained from young potatoes strongly foam. The molasses brew strongly foams too sometimes. With an excess of limestone in molasses and acidification of the dispersion with sulfuric acid, a plentiful gypsum precipitate can be released from the brew.
The equipment for the production of bioethanol allows you to distill the brew, but distillation of brew with an alcohol concentration of less than 7.5% is undesirable, because the productivity of rectification plants is sharply reduced, the steam consumption increases and the loss of alcohol increases.
The brew contains more than 70 accompanying alcohol of various volatile substances (impurities). Their content is small, usually not more than 1.0% of the amount of ethyl alcohol, but sometimes reaches 1.5%, mainly due to methanol.
All volatile impurities by chemical nature can be divided into four groups: alcohols (except ethyl), aldehydes, acids and esters. In addition, a group of nitrogenous substances (ammonia, amines, amino acids), sulfur-containing substances (hydrogen sulfide, sulfurous anhydride, sulfonic acids, mercaptans), etc. are isolated.
The composition and content of volatile impurities depends on the quality of raw materials of the technological modes of its processing. Impurities partially pass from raw materials, water, auxiliary materials, partially formed during the preparation of wort, but mainly appear during fermentation.
Most impurities (0.4… 0.6% of the amount of ethyl alcohol) are alcohols, which are the basis of fusel oil. The content of its main components: isoamyl alcohols 60-90%, isobutyl 8-27%, n-propyl 3-20% of their total. They are formed during fermentation.
Methyl alcohol is not included in the group of “fusel” alcohols. It is formed mainly during the heat treatment of raw materials, a small amount of it is formed during fermentation. The source of methanol may be an excess of formalin used for antisepting raw materials and equipment. Its (methanol) content in crude alcohol is 0.005… 0.5% vol., But in some cases (for example, by processing sugar beets) reaches 1.2% vol. There is practically no methyl alcohol in molasses brew (or not more than 0.0001% vol.).
Crude alcohol contain mostly acetic aldehydes. The distillate of molasses brew of aldehydes contains about 0.05% of the amount of ethyl alcohol, which is 10… 50 times higher than their content in grain-potato distillate. The amount of aldehydes grows with increased wort aeration during yeast generation. Aldehydes are preferably formed during fermentation. There can be propionic, oily, valerian aldehydes aside from acetic one.
Esters are mainly acetic-ethyl (up to 350 mg/dm ³), formic-ethyl, acetic-methyl, isobutyric-ethyl, acetic-isobutyl, isovalerian-propyl, acetic-isoamyl, etc.
Volatile acids (acetic, oily, propionic, valerian, etc.) are usually not more than 0.1% of the amount of alcohol. Among the nitrogenous compounds may be ammonia, trimethylamine, triethylamine, ethanolamine; from sulfur-containing – hydrogen sulfide, sulfurous anhydride, ethyl mercaptan and ethyl sulfur aldehyde. These sulfurous compounds are present in molasses brew and brew from other raw materials, if it is defective.
Raw alcohol has the most acetic-ethyl alcohol, the content of which reaches 200… 400 mg/dm ³ of alcohol; the other more than 20 esters account for only 100… 200 mg/dm ³. Ethyl esters of propionic, oily, isovaleric, capronic acids, as well as isoamyl acetate, isobutyl acetate and isoamyl valerate prevail.
The production of bioethanol from brew can be carried out by a simple distillation and by distillation in rectification brew columns. In the first case, the process is carried out in devices of periodic action, in the other – continuous. In both cases, alcohol is released from the brew during boiling, along with the accompanying volatile impurities, and a brew distillate (low-grade raw alcohol) is obtained, which (depending on the concentration of alcohol in the brew) contains 30… 50% alcohol, related impurities and water.
Ethyl alcohol is isolated and purified from coarse distillate or raw alcohol by rectification (multiple distillation). The distillation and rectification processes are based on the difference in volatility (boiling point) of the substances to be separated.
All alcohol-related substances by volatility are divided into main (volatile than ethyl alcohol), tail (less volatile than alcohol), intermediate and terminal one (the volatility of which in local conditions can be more or less than ethyl alcohol).
The production of bioethanol implies, that in the process of isolation and purification of alcohol, by-products appear: bard, luther water, head fraction, fusel oil and fusel alcohol. With bard and lutheran water, a non-volatile part of the brew is displayed; alcohol-related volatile impurities are excreted with the main fraction, with fusel oil and fusel alcohol.
The brew-bard, that was freed from alcohol, part of water and accompanying volatile impurities contains all the dry substances of the brew and part of the water. The quality of the bard is fully determined by the quality of the original brew, so there are no special conditions for its composition except for the maximum permissible content of ethyl alcohol not exceeding 0.015% vol. (0.012 wt% or 0.0047% mol.).
Dry substances of the brew and non-volatile products of alcoholic fermentation remain in the bard: glycerol, pyruvic acid, etc. The content of dry substances in the bard varies no more than 4… 12%.
Grain bard in its natural form is a valuable feed product. It is dehydrated and dried and used as an additive to feed.
Molasses bard contains a significant amount of mineral compounds (28… 32% of the total mass of dry substances). Now it is thickened and used as fertilizers, plasticizers in the production of concrete and for feed purposes after partial desalination. The yield of bard depends on the alcohol content in the brew and is 8… 12 times the volume of alcohol.
Luther water is the residue after purification of raw alcohol. Heavy alcohol impurities are removed: acids, some esters, aldehydes, alcohols and other volatile impurities with a high boiling point and preponderance. Luther water has an acidic reaction, aggressive against carbon steels.
Luthern water is used for preparation of cereal kneads, molasses sprays, fusel oil washing. The output of luthern water is determined by the conditions of alcohol purification.
Light volatile impurities in the process of alcohol purification are isolated with the main fraction. The main fraction (MF) is a mixture of ethyl alcohol and main impurities.
The main fraction of ethyl alcohol is a clear liquid, colorless, slightly yellowish or greenish. The concentration of alcohol in it (by alcohol meter) should be 92% vol. In its composition is allowed the content of acids 1 g/dm³; esters 30 g/dm³; fusel oil 2 g/dm³; aldehydes: from starchy raw materials 10 g/dm³, from molasses 35 g/dm³; methanol: from molasses 0.5% vol., from grain 1.5% vol., from potatoes 2.5% vol., from mixed raw materials 6% vol.
The composition of MF contains about 90% ethyl alcohol, 2… 6% impurities and about 5% water. The composition and content of impurities and the yield of MF largely depend on the type and quality of raw materials, the conditions of fermentation and purification of alcohol.
The content of esters and aldehydes in MF increases sharply (up to 7%) with increased aeration of wort during yeast generation. The main fraction obtained from potato raw materials (and especially in the processing of beets) contains up to 4% methanol. The yield of the head fraction is usually 1.5… 3% when processing grain and potatoes and 3… 5% when processing molasses.
The main fraction is dispersing now in special distillation plants in order to extract ethyl alcohol from it.
By processing the head fraction, its concentrate is obtained – a yellow-green liquid with a sharp suffocating odor, which is usually partially mixed with water. Anhydrous part of concentrate contains 15-20 wt%. ethers, 15… 45% aldehydes, 20… 45% ethyl alcohol and 0.1… 0.5% wt. acids. Concentrate of the head fraction (KGF) is used as a carbon-containing feedingin the production of feed yeast, but it can be a source of solvents and pure organic products: acetic aldehyde, acetic-ethyl ether.
Intermediate impurities during alcohol purification are isolated mainly in the form of fusel oil and fusel alcohol. Commodity fusel oil according to DSTU 17071 – 71 must meet the following requirements: density (at 20 0S relative to water) is not higher than 0.837; refractive index – not less than 1.3950; distillation limits – up to 120 0S should be distilled a little more than 50% of the original volume. Clear liquid from light yellow to red-brown. During shaking it should not form turbidity.
The composition of the commodity fusel oil includes: ethyl alcohol 7… 15% by weight, water 8… 15% by weight, the rest falls mainly on S3, S4 and S5 alcohols and some other intermediate impurities with limited solubility in water (high-molecular esters, aldehydes, acetals, acids, nitrogenous and sulfurous compounds).
Alcohols in fusel oil are contained in approximately the following ratio: isoamyl – 40… 75%, isobutyl – 18… 22%, propyl – 10… 15%. The composition and yield of fusel oil is not constant and varies depending on the type and quality of raw materials, the yeast race used, the technological conditions for fermentation and purification of alcohol. The yield is usually 0.25… 0.4% from the release of alcohol.
Fusel oil is a valuable product. It is customary to disperse it into constituent components that are used in organic synthesis for the manufacture of medicines and odorous substances, solvents, extractants, flotation reagents.
Fusel alcohol is a colorless liquid with an ethanol concentration (by alcohol meter) of 75… 85% vol. with the smell of pear essence due to the presence of acetic-isoamyl ether in it. It contains water 25… 30% vol, ethanol 45… 60% vol, alcohols (S3…S5) – 5… 20% vol. (mainly propanol and isobutanol), esters 0.3… 0.8% vol., A small amount of volatile nitrogenous substances, aldehydes and acids.
Fusel alcohol is selected, if necessary, in an amount of 0.5… 1.5% of the total amount of alcohol and used for technical purposes, for example, for the preparation of denatured alcohol, or dispersed to isolate rectified alcohol and fusel oil components.
BIOETHANOL DEHYDRATION.
DEHYDRATION BY ADSORPTION ON MOLECULAR SIEVES.
Water-alcohol liquid (strong raw alcohol) with ethanol content of 93… 95% vol. or water-alcohol steam, the condensate of which has the same strength.
The bioethanol equipment is installed in such way, that the crude alcohol enters the upper tray of the regeneration column operating under increased pressure (usually about 0.2 MPa). The column is heated by closed steam, water is removed from the cube.
Dry saturated water-alcohol steam from the top of the column enters the superheater, in which its temperature rises to 85… 95 0S. Superheated steam enters the upper part of the adsorber filled with granules of artificial 3A zeolite, usually 2.5… 5 mm in diameter. Passing through the pellet bed, the water-alcohol vapor gives off water, which is sorbed in the pores of the granules, and the dehydrated vapor exits at the bottom of the adsorber and is sent to the condenser. From the condenser, dehydrated bioethanol is obtained, which is accounted for and transferred to a container designed for daily production, and from it to the warehouse.
After the adsorbent is saturated with water, the superheated steam is switched to a second adsorber, where the dehydration process proceeds in a similar manner. The first adsorber at this time is put on regeneration – restoration of the water-absorbing ability of the adsorbent. This process is carried out by passing through it from the bottom up a part (about 25… 30%) of the dewatered steam leaving the other adsorber. The dewatered steam displaces water from the adsorbent and together with its vapors leaves the upper part of the adsorber, which is in the regeneration mode. Water-alcohol steam leaving this adsorber is sent to the condenser. A condensate (regenerate) having a strength of about 70% vol is fed to the regenerator. Water extracted from raw alcohol vapors is withdrawn from the bottom of the regeneration column.
If in the sorption mode the adsorber is under increased pressure, the regeneration mode is under vacuum. Switching adsorbers from mode to mode is carried out every 2… 15 minutes by a system of valves controlled according to a given program, which is the know-how of the authors of the technology.
The described process is called Pressure Swing Adsorption (PSA). Water is adsorbed on the pore surface of the molecular sieve, so called because of the ability to “weed out” molecules of a certain size. This sieve is an artificial zeolite – an alkaline aluminosilicate in sodium form. The structure of zeolite A-type, it has pores with a diameter of about 0.3 nm. In pores of this size, a water molecule (0.28 nm) is retained, but an ethanol molecule (0.44 nm) does not fit. Equipment for bioethanol is calculated for a specific capacity of the enterprise.
The most famous manufacturer of artificial zeolites for this process is the Swiss company Zeochem AG, whose ZEOCHEM® adsorbent Z3-03 designed to dehydrate ethanol and methanol, other polar gases and liquids. It can be regenerated by reducing the partial pressure of the adsorbent or increasing the temperature of the adsorbent to 200… 230 °C.
MEMBRANE DEHYDRATION OF BIOETHANOL.
The method is based on different permeabilities of polymer or ceramic membranes for water steam and ethanol.
Equipment for the production of bioethanol (membrane dehydration of bioethanol) consists of an evaporator of non-dehydrated alcohol and a steam separator, a superheater and a series of filters, each of which is a package of membranes, their ends attached to steel tube grids. The membrane is a porous inert ceramic tube, on the inner surface of which there is a layer of artificial zeolite (type NaA) with a pore diameter of 0.4 nm, through which only water molecules penetrate. The location of the layer on the inner surface of the tube protects it from damage during installation.
The design of the filter is similar to the design of a one-way shell and tube heat exchanger. Water-alcohol steam moves in the tube space under pressure (0.6 MPa), and the tube side is under vacuum. Passing successively through low-angle filters, water-alcohol steam is freed from water vapor up to the required residual concentration.
Dehydrated alcohol vapor and water vapor condense in heat exchangers. Steam condensate containing a small amount of ethanol (2… 5%) is returned for regeneration.
Membrane dehydration has several advantages over the PSA process. First, it is a small amount of regenerate – 4… 6% of the volume of bioethanol produced (about 25% in the PSA process). Secondly, a slightly lower steam consumption due to its lower need for ethanol regeneration.
The disadvantage of membrane dehydration plants is about twice the cost of equipment (including membranes) compared to the PSA process. A refrigeration unit is also needed to produce low temperature (5 °C) water used to condense the regenerate, and a higher heating steam potential is required.
The production of bioethanol by membrane plants has another significant drawback – increased requirements for the purity of water-ethanol pairs; the presence of higher alcohols, aldehydes and other related impurities is permissible in limited quantities.
BIOETHANOL DEHYDRATION, AZEOTROPIC RECTIFICATION.
In the CIS countries and in many old factories abroad, alcohol dehydration and absolution of alcohol is carried out using the properties of triple indivisible boiling (azeotropic) mixtures. Such a mixture is formed when added to an alcohol with a concentration of 95… 96.5% vol. a third component, for example benzene, cyclohexane, trichloroethane or other substances forming azeotropic mixtures with alcohol and water.
In most cases, now the hydrocarbon cyclohexane (S6N12) is used as the third (water-collecting) component – a transparent, colorless light-colored liquid with a boiling point of 80.8 °C, a melting point of 6.6 °C, a density of 0.778 kg/dm³, which is practically insoluble in water, dissolves indefinitely in ether, acetone, benzene, alcohol.
Pairs of cyclohexane with air form an explosive mixture. Flash point 18 °C, ignition point 260 °C. Concentration limits of ignition 1.2… 10.6% vol. Cyclohexane is toxic, the maximum permissible concentration (MPC) of vapors in the air in the zone of production premises is 80 mg/m³, but it is less toxic than benzene, due to which it displaces the latter in the technology of alcohol absolution.
The essence of the process and equipment for dehydrating alcohol is as follows. A power supply with an alcohol concentration of 95… 96.5% vol. and a certain amount of cyclohexane is added. During the rectification process, an azeotropic mixture is formed in the column, which leads to itself as a light volatile component (LVC) and leaves the column as an upper product in a vapor state. Dehydrated ethyl alcohol flows down the column as a heavy volatile component (HVC); all water entering the column with alcohol is removed with an azeotropic mixture.
The vapors of the azeotropic mixture during condensation give a heterogeneous (heterogeneous) liquid mixture that exfoliates, forming an upper (light) layer containing 93… 94% by weight. cyclohexane, 6… 7 wt% ethanol and a small amount of water; and a lower (heavy) layer which contains 69-71 wt%. ethanol, 21-23 wt% water and 6-8 wt%. cyclohexane.
The initial alcohol is usually fed to the plate of the dewatering column. Some cyclohexane is fed to the column on the upper tray. When heat is supplied to the column, the mixture of ethanol, water and cyclohexane in the column is divided into an azeotropic mixture and dehydrated ethyl alcohol. The latter is removed from the bottom of the column, cooled in a heat exchanger and sent to the absolute alcohol collector.
The azeotropic steam is condensed in the dewatering column reflux condenser and condenser. The condensate in the decanter is delaminated. The top layer, which contains mainly cyclohexane, is supplied for irrigation to the dewatering column (reflux). The lower layer from the decanter, passing the refrigerator, is sent to the second-stage decanter, from where the upper layer enriched with cyclohexane enters the dewatering column, and the lower (alcohol-water) layer enters the regeneration column feed tray through the heater.
The equipment for the production of bioethanol, its dehydration by azeotropic distillation is arranged so that in the regeneration column alcohol with a certain amount of cyclohexane is concentrated and partially directed to the feeding tray of the dehydration column, and partially to the reflux of the regeneration column. Regeneration column is equipped with dephlegmator and condenser; its heating can be carried out both with open and closed steam. The power supplied to it is heated in the heat exchanger. If necessary, fusel fraction is taken from regeneration column.
Dewatering and regeneration columns have 60… 70 multi-cap trays. A certain amount of cyclohexane is constantly circulated in the dewatering column, which acts as a water carrier from the column to the decanter.
Regeneration column in its structure and operating mode practically does not differ from alcohol column of any rectification unit. It is fed by an aqueous-alcoholic liquid with an alcohol concentration of 75… 80% vol. and should ensure its concentration up to 95… 96% vol. Working reflux ratio is taken not more than R = 4… 5. The steam flow to the regeneration column is assumed to be equal to 6-8 kg per decalitre of conventional alcohol introduced into the plant. Column is loaded with alcohol by temperature on feeding plate.
Dewatering column operates with reflux ratio R = 4… 5; specific steam consumption 13… 14 kg/dal of reference alcohol introduced into the column. The column is loaded with alcohol according to the concentration of dehydrated alcohol; when concentration decreases, alcohol supply to the column is reduced.
Column is loaded with cyclohexane at temperature 15… 16th plate counting from below; column loading with cyclohexane is reduced with decrease in temperature, increasing – increasing. The temperature in the feed lead-in area and above the top plate is also recorded. The temperature above the feed tray and the top is largely dependent on the accumulation of associated volatile impurities in the column, especially in the production of fuel ethanol.
The absolution of alcohol consumes 18.5… 20 kg/dal of heating steam, cooling water 0.32… 0.33 m3/dal and cyclohexane 0.01… 0.03 kg/dal.
When receiveng dehydrated ethanol from the brew, the azeotropic rectification unit is connected with a raw- or brew-rectification unit into a single technological line. Raw- or brew-rectification plant in this case ensures the production of concentrated alcohol, which without cooling is immediately introduced into the dehydration column. When feeding the dehydration plant with concentrated raw alcohol or technical alcohol, technical absolute alcohol (bioethanol) is obtained; almost all impurities contained in raw alcohol pass into its composition. Their number may even increase slightly. Such an alcohol is preferably used as a motor fuel component or solvent.
Compared to sorption or membrane plants, the steam consumption for the production of bioethanol of azeotropic distillation is much higher – up to 20 kg/dal of reference alcohol. In addition, constant feeding of the plant with cyclohexane is required, which is lost through alcohol traps and with a dehydrated product. For these reasons, new enterprises are equipped with sorption plants for dehydration of alcohol.