Alcohol from Corn
BASE CASE PRODUCTION OF ALCOHOL
The alcohol plant is sized to produce 50 MM gal per yr of alcohol (motor fuel grade) from corn. The overall alcohol product yield is 2.57 gallons per bushel of corn thereby requiring 19.43 MM bushels of corn per year. In addition to the alcohol product, the plant produces 117,111 T per yr of Distillers Dark Grains by product. Illinois coal which is to be used as the fuel for the plant.
The alcohol plant, in general, uses existing process technology currently employed in grain alcohol plants. The plant operates as a continuous flow process, except for the fermentation and fungal amylase sections which are operated batch wise in order to allow for frequent sterilization of the equipment. The distillation system employs a two pressure concept which significantly improves its steam economy. The two pressure concept is used in other chemical processing fields and in industrial and beverage alcohol production but has not, to our knowledge, been employed in a commercial grain motor fuel alcohol distillery. This process concept, along with other heat economy measures, results in a total steam usage of 31.7 lbs per gallon of alcohol. The distillation system uses 21.4 lbs per gallon of which 2.8 lbs per gallon is obtained as flash vapours from mash cooking.
All of the utility requirements, with the exception or electricity, are produced within the boundaries of the plant. Water is obtained from a well field located close to the plant. The boiler burns relatively low cost, high sulfur coal. Flue gas from the boiler is used to dry the stillage residue in producing the Distillers Dark Grains (DDG) by product. Waste water is treated in a two stage, activated sludge, treatment facility. The sludge is dewatered and fed to the boiler. Cooling water is recycled from a two cell cooling tower.
Approximately one third of the plant power requirement is obtained from steam turbines. The coal fired boiler produces 600 psig 600°F steam which is used to drive turbines for large energy users. The turbines exhaust the steam at 150 psig which is suitable for process requirements.
The corn is received, stored, and milled in section 100. After milling, the feed material is pneumatically conveyed to Section 200 for mash cooking and starch conversion to sugar (Saccharification). The enzyme, fungal amylase, required for saccharification is produced in Section 300. The saccharified mash is cooled and sent to fermentation, Section 400. The sugars in the mash are fermented to alcohol in the batch fermenters. The fermented mash is then transferred to Section 500 for alcohol recovery and dehydration. The stillage residue from the alcohol stripper rectifier (Section 500) is sent m Section 600 for dry grains recovery. The alcohol product and by products are delivered to Section 700 for storing and shipping.
The three major energy inputs are corn, coal, and electricity. The two primary outputs are alcohol and Distillers Dark Grains. A breakdown of electrical power and steam usage is shown in Table 1.
Another method for determining the thermal efficiency is to consider the energy required to produce the corn crop, and credit for the energy required to produce the DDG on the bases of the corn, as an additional energy input item. In Illinois, the average usage is 95,500 Btu bu of corn which includes energy for field preparation, harvesting, fertilizer supplies, transportation and other miscellaneous usages.
A third method of calculating the thermal efficiency is to include the total thermal energy contained in the corn and dried grains. The method yields an overall plant efficiency of 68.9 percent.
EXCURSIONS ON FEEDSTOCK MATERIAL
In considering the production of motor fuel grade alcohol, the most logical primary feedstock choice is corn, since it is produced voluminously in major growing areas of the United States and since producers of industrial grade alcohol prefer corn because of its economic advantages.
The Department of Energy requested that we evaluate feedstock materials other than corn. The objective is to compare the required alcohol selling price for the excursions to the bese case (corn) alcohol selling price. The alcohol selling price is the market price required to recover production cost and to return to the investor a 15percent discounted cash flow interest rate of return.
Wheat and milo can be processed in essentially the same equipment as the corn feed material. Sweet sorghum requires new front end equipment, more steam generating capacity, and bagasse storage and handling facilities; consequently the investment is considerably higher than for the base case. The sweet sorghum excursion is designed for corn as the feed material during the dead season. (We assumed sweet sorghum could be harvested for 165 days of the year). This becomes very expensive since a large portion of the plant is idle during either season. In addition, the sweet sorghum raw material is more expensive than the corn; and the Distillers Dark Grain by product from sweet sorghum is less valuable than the corn feed materials by product.
Another alternative would be to design the sweat sorghum plant to process double the sweet sorghum feed material during the active season, and concentrate approximately one half the syrup for storage and use during the dead season. This alternative was not considered in detail because it became obvious that the investment would be higher than the combination corn, sorghum alternative due to the high cost of doubling the size of the front end equipment, and energy use would increase.
The wheat excursion results in a relatively high alcohol selling price; whereas the milo excursion results in the lowest alcohol selling price. Milo production in this country is substantially less than corn, and a major new market for milo could cause the milo market price to increase. This would reduce the advantage for production of alcohol from milo.
BACKGROUND AND JOB SCOPE
Contract EJ 78 C 01 6639 of August 31, 1978 between the Department of Energy and Raphael Katzen Associates was set up to permit development of realistic estimates of investment, production costs, and sales prices for motor fuel alcohol from grain. This assessment was to be based on the most advanced current fermentation and distillation technology, which has been proved on a commercial basis.
With a base case defined for production of 50 million gallons per year of motor fuel alcohol (199° proof) from corn, and with optional feedstock variations on wheat, milo and sweet sorghum; grass roots site requirements and definitions were developed between the Department of Energy and Raphael Katzen Associates. A nominal Central Illinois location was selected for the estimate, which would be suitable for any of the feed materials except sweet sorghum, where a southern location might be advantageous from the raw material supply standpoint.
Variations of plant size, and effect on investment and operating costs, were evaluated for facilities of 10 million gal per yr alcohol capacity and 100 million gal per yr alcohol capacity.
The facility was defined as a grass roots facility, complete with all utilities and services required for operation of a grain alcohol plant. In addition to the alcohol product, it was agreed that an animal feed byproduct was essential to the operation, not only from a maximization of product sales, but also to minimize waste disposal problems. Feed markets for this type of grain alcohol plant residue are already well established, and the animal feed processing section was designed to yield a conventional marketable product defined as Distillers Dark Grains (DDG).
With coal as the essential fossil fuel (thus to eliminate the need for oil or natural gas) efforts were made to minimize energy requirements and maximize the ratio of fuel output value (alcohol) versus the fuel input value (coal). An option for use of corn stover as a fuel was also estimated, in efforts to eliminate the use of fossil fuel. However, in either of these cases, it was still found necessary to purchase electrical energy, and energy charges for fossil fuel production of this electrical energy were made against the operation.
In the fermentation of sugar produced from the starch contained in grain, certain extraneous materials other than ethyl alcohol are produced. These are fusel oils, a complex mixture of higher molecular weight alcohols, and yeast (saccharomyces). In Table 7 are listed the assumptions used for this study with regard to fusel oil formation. Table 8 lists the assumptions made with regard to the formation of yeast.
ALCOHOLS, HIGHER ALIPHATIC SURVEY AND NATURAL ALCOHOLS MANUFACTURE
The monohydric aliphatic alcohols of six or more carbon atoms are generally referred to as higher alcohols. Historically, the higher alcohols, particularly those of 12 or more carbon atoms, were derived from natural fats, oils, and waxes and were called fatty alcohols; but now similar alcohols are widely available from synthetic processes using petrochemical feedstocks (qv). Although the natural and synthetic alcohols are used interchangeably for many applications, for some applications the distinction still remains. The higher alcohols can be separated into the plasticizer range alcohols, generally 6 11 carbon atoms, and the detergent range alcohols, 12 or more carbon atoms. There is, however, considerable overlap in use. Production of higher alcohols in North America, Europe, and Japan in 1985 was about 2,600,000 tons and United States production was 35percent of that total. About three fourths of the U.S. output was plasticizer range alcohols, which are used primarily as ester derivatives in plasticizers (qv) and lubricants. The detergent range alcohols are used mainly as sulfate, ethoxy and ethoxysulfate derivatives in a wide variety of detergent, and surfactant applications.
Most higher alcohols of commercial importance are primary alcohols; secondary alcohols have more limited specialty uses. Detergent range alcohols are apt to be straight chain materials and are made either from natural fats and oils or by petrochemical, processes. The plasticizer range alcohols are more likely to be branched chain materials and are made primarily by petrochemical processes. Whereas alcohols made from natural fats and oils are always linear, some petrochemical processes produce linear alcohols and others do not.
Detergent Range Alcohols. Natural or synthetic detergent range alcohols are usually described as middle cut (12 15 carbon atoms) or heavy cut (16 18 carbon atoms), corresponding to the distillation fractions of coconut alcohol from which these alcohols were first derived. Because middle cut alcohols are preferred for most detergent applications, manufacturers maximize this production through feedstock choice (natural alcohols), or by manipulating processing conditions (synthetic alcohols). The co product light cut (6 11 carbon atoms) and heavy cut alcohols are also valuable products. Only a small percentage of detergent range alcohols are sold as pure single carbon chain materials.
The higher alcohols occur in minor quantities primarily as the wax ester (ester of a fatty alcohol and a fatty acid) in many oilseed and marine sources. Free alcohols octacosanol, C28H58O, and triacontanol, C32H66O, have been isolated in very small amounts from sugarcane and its products. Oil from the sperm whale is rich in wax esters of hexadecanol, octadecenol, and eicosenol; this oil was formerly a major commercial source of these alcohols. The oil of the North Atlantic barracudina fish contains 85percent wax esters that consist mainly of hexadecanol and octadecenol. Minor amounts of alcohols having 12 26 carbon atoms have been found in both ancient and recent marine sediments, probably having their origin in ocean marine life. Wool grease from sheep also contains higher alcohols as wax esters, and is a minor commercial source of alcohol. The seeds of the shrub jojoba which grows in the North American desert give an oil which contains esters of eicosenol and docosenol and the natural waxes such as carnauba wax and candelilla wax contain wax esters with alcohols of 26 34 carbon atoms. Although higher alcohols could be obtained from any of these plant sources by saponification of the esters, they are not commercially important sources.
Table 1 provides physical property data for selected pure alcohols. The homologous series of primary normal alcohols exhibits definite trends in physical properties for each additional CH2 unit the normal boiling point increases by about 20°C, the specific gravity increases by about 0.003 units, and the melting point increases by about 10°C in the lower end of the range and about 4°C in the upper end. The water solubility decreases with increasing molecular weight and the oil solubility increases. In general, the higher alcohols are soluble in lower alcohols such as ethanol and methanol and in diethyl ether and petroleum ether. The solubility of water in 1 hexanol and 1 octanol is appreciable, but drops off rapidly as alcohol molecular weight increases. Enough solubility remains, however, to make even 1 octadecanol slightly hygroscopic. Mixtures of alcohols, such as 1 octadecanol and 1 hexadecanol, are considerably more hygroscopic. Below C12 the normal alcohols are colourless, oily liquids with light, rather fruity odours. At room temperature pure 1 dodecanol solidifies to soft, crystalline platelets and the physical form of higher molecular weight alcohols progresses from these soft platelets to crystalline waxes. Although 1 dodecanol has a slight odour, the higher homologues are essentially odourless. The secondary and branched primary alcohols are oily liquids at room temperature and have light, fruity odours. They are soluble in alcohol solvents and diethyl ether, and also show less affinity for water as molecular weights increase. The members of this group do not have well defined freezing points; they set to a glass at very low temperatures. Physical properties are often ill defined because of difficulties in obtaining pure samples.
The higher alcohols undergo the same chemical reactions as other primary or secondary alcohols. Similar to other chemicals having long carbon chains, however, reactivity decreases as molecular weight or chain branching increase. This lower reactivity and concommitant decreased solubility in water and in other solvents means that more rigorous reaction conditions, or even use of different reaction schemes as compared to shorter chain alcohols, are generally required.
Shipment and Storage
Detergent range alcohols are available in 208 L (55 gal) drums ot approximately 160 kg or 23.000 L (6000 gal) tank trucks, in tank cars of 75.000 L (20,000 gal) containing about 60,000 kg, and in marine barges. The tank trucks and cars are usually insulated and equipped with an external heating jacket; the barges have coils for melting and heating the alcohols. High melting alcohols such as hexadecanol and octadecanol are also available as flaked material in three ply, polyethylene lined 22.7 kg (50 lb) bags. Detergent range alcohols have a U.S. Dept. of Transportation classification as nonhazardous for shipment. The perfume grade alcohols, such as specially purified octanol and decanol, are available in bottles and cans other plasticizer range materials are available in 208 L drums, 23,000 L tank trucks. 75,000 L tank cars, and in marine barges. Because of low melting points, most of these materials do not require transports having heating equipment. Bulk shipments are usually described by the commercial name of the material, such as methyliso butylcarbinol for 4 methyl 2 pentanol. The names hexyl octyl, or decyl alcohol are used as freight descriptions for the linear or branched alcohols of corresponding carbon number. Linear and branched alcohols of 6 9 carbon atoms, and mixtures containing them, are classified as combustible for shipment by the U.S. DOT because of their low flash points. Alcohols of 10 carbons and above are classified as non hazardous.
The higher alcohols are not corrosive to carbon steel, and equipment suitable for handling solvents or gasoline is also suitable for the alcohols. However, special storage conditions are often needed to maintain alcohol quality. Lined carbon steel tanks having nitrogen blankets to exclude both moisture and oxygen are recommended for storage of detergent range alcohols. Preferred storage temperature is no higher than 10°C above the alcohol melting point and repeated cycles of melting and solidifying must be avoided. Low pressure steam is generally used for heating; for the high melting hexadecanol and octadecanol, hot water can be used in order to reduce exposure to high temperature heating surfaces. Although they are generally considered quite stable, alcohols which are stored either for long periods of time or under improper conditions can undergo such subtle changes as deterioration of colour, increase in carbonyl level, or a decrease in acid heat stability. It is sometimes preferable to store high melting alcohols as flakes in bags at ambient temperature rather than melted in a tank at higher temperature.
To prevent rusting and moisture pickup resulting from the hygroscopic nature of plasticizer range alcohols, tanks should be protected from moisture by such devices as a drying tube on the tank or a dry air blanket; nitrogen is usually not needed because ambient storage temperature is adequate for these lower melting materials. In general, plasticizer range alcohols are more storage stable than the detergent range alcohols. However, to avoid the danger of fire resulting from the low flash points of plasticizer range alcohols, tanks should be grounded, have no interior sources of ignition, be filled from the bottom or have a filling line extending to the bottom to prevent static sparks, and be equipped with flame arrestors.
Because the higher alcohols are made by a number of processes and from different raw materials, analytical procedures are designed to yield three kinds of information the carbon chain length distribution, or combining weight, of the alcohols present; the purity of the material; and the presence of minor impurities and contaminants that would interfere with subsequent use of the product. Analytical methods and characterization of alcohols have been summarized.
For the detergent range alcohols, capillary gas chromatography, fast, accurate, and simple to use, is by far the most useful method for determining composition and purity. By the proper choice of the capillary stationary phase, carbon chain distribution and the amount of unsaturated, chain branched, or secondary alcohols, as well as the level of minor materials such as esters and hydrocarbons, can be determined.
From the HV the combining weight can be calculated for subsequent chemical reactions. Carbonyl content is important, especially for those alcohols manufactured from aldehydes by the oxo process. It is often expressed similarly to HV as the mg of KOH equivalent, to the carbonyl oxygen in 1 g of sample. Acidity, expressed in terms of the equivalent weight percent of acetic acid, is used to determine the quality of the alcohol, as are moisture and APHA colour. As with the detergent range alcohols, tests which measure colour stability in the presence of sulfuric acid are employed to predict the colour changes that may occur in subsequent reactions utilizing acid catalysts.
Additionally, analytical determinations such as odour, chloride level, hydrocarbon content, and trace metal content, are required for specific uses.
Specifications and Standards
Most of the detergent range alcohols used commercially consist of mixtures of alcohols, and a wide variety of products is available. Table 2 shows the approximate carbon chain length composition of both the commonly used mixtures and single carbon materials; typical properties are given in Table 3. Although only even carbon alcohols are available from natural fats and oils and the Ziegler process, the development of the oxo process for linear alcohols has made odd carbon alcohols a commercial reality, albeit with some chain branching. Commercial mixtures of these latter alcohols contain both odd and even numbered chain lengths. The major production of detergent range alcohols is in the 12 18 carbon range.
Manufacture from Fats and Oils
Fats and oils from a number of animal and vegetable sources are the feedstocks for the manufacture of natural higher alcohols. These materials consist of triglycerides glycerol esterified with three moles of a fatty acid. The alcohol is manufactured by reduction of the fatty acid functional group. A small amount of natural alcohol is also obtained commercially by saponification of natural wax esters of the higher alcohols, such as wool grease.
The carbon chain lengths of the fatty acids available from natural fats and oils range from 6 22 and higher, although a given material has a narrower range. Each triglyceride has a random distribution of fatty acid chain lengths and unsaturation, but the proportion of the various acids is fairly uniform for fats and oils from a common source. Any triglyceride or fatty acid may be utilized as a raw material for the manufacture of alcohols, but the commonly used materials are coconut oil, palm kernel oil, lard, tallow, rapeseed oil, and palm oil, and to a lesser extent soybean oil, corn oil and babassu oil. Coconut and palm kernel oil are the primary sources of dodecanol and tetradecanol; lard, tallow, and palm oil are the primary sources of hexadecanol and octadecanol. Producers of natural fatty alcohols typically make a broad range of alcohol products having various carbon chain lengths. They vary feedstocks to meet market needs for particular alcohols and to take advantage of changes in the relative costs of the various feedstock materials.
The first commercial production of fatty alcohol in the 1930s employed the sodium reduction process using a methyl ester feedstock. The process was used in plants constructed up to about 1950, but it was expensive, hazardous, and complex. By about 1960 most of the sodium reduction plants had been replaced by those employing the catalytic hydrogenolysis process.
Hydrogenolysis Process. Fatty alcohols are produced by hydrogenolysis of methyl esters or fatty acids in the presence of a heterogeneous catalyst at 20,700 31,000 kPa (3000 4500 psi) and 250 300°C in conversions of 90 98percent. A higher conversion can be achieved using more rigorous reaction conditions, but it is accompanied by a significant amount of hydrocarbon production.
To prepare methyl ester feedstock for making fatty alcohols, any free fatty acid must first be removed from the fat or oil so that the acid does not react with the catalyst used in the subsequent alcoholysis step. Fatty acid removal may be accomplished either by refining or by converting the acid directly to a methyl ester. Refining is done either chemically, by removal of a soap formed with sodium hydroxide or sodium carbonate (alkali refining), or physically, by steam distillation of the fatty acids (steam refining). In the case of chemical refining, the by product soap is acidified to give a fatty acid and these foots are used as animal feed or upgraded for industrial fatty acid use. The by product fatty acid from steam refining is of a higher grade than acidifed foots and is used directly as an industrial fatty acid or as animal feed. In either case, the fatty acid can also be converted to the methyl ester find used as additional alcohol feedstock. Refined oil is dried to prevent the reaction of water with the catalyst during alcoholysis.
Alcoholysis (ester interchange) is performed at atmospheric pressure near the boiling point of methanol in carbon steel equipment. Sodium methoxide, CH3ONa, the catalyst, can be prepared in the same reactor by reaction of methanol and metallic sodium, or it can be purchased in methanol solution. Usage is approximately 0.3 1.0 wt percent of the triglyceride.
Monohydric alcohols can be considered to be hydrocarbons in which one of the hydrogens is replaced by an OH group.
If the hydrocarbon consists of an unbranched carbon chain, the equivalent primary alcohol is called normal, indicated by the prefix n.
Depending on the location of the OH group along the hydrocarbon chain, and the number of replaceable hydrogens in the same carbon atom, it is possible to have three types of alcohols.
LOWER SATURATED ACYCLIC (ALIPHATIC) ALCOHOLS
CH3OH (methanol, wood alcohol, carbinol, Columbian spirit, wood spirit) is the simplest of the saturated monohydric alcohols, with a molecular weight of 32.04. At room temperatures, this alcohol is a colourless, neutral mobile, flammable, volatile liquid with a characteristic odour.
Methyl alcohol is rarely found naturally in the Free State e.g., trace amounts found in essential oils and fermented liquors. However, it exists in the plant kingdom as a part of complex organic substances, one of which is responsible for the name, wood alcohol. Methyl comes from the Greek words for wine and wood, krasi and xulon, indicating the original use and source. Several plant oils contain methyl esters; oil of wintergreen has methyl salicylate, C6H4(OH) COOCH3, and oil of jasmine contains methyl anthranilate, C5H4(NH2) COOCH3. Alkaloids and natural pigments also contain the methyl radical in the form of complex ethers. The natural product (wood alcohol) obtained from hard wood is a crude solvent with an offensive odour and many impurities. By contrast, synthetic methanol is an extremely pure product with a characteristic odour and a water white colour.
The first chemist to recognize methanol is said to be Robert Boyle in 1661, who found a neutral substance in the liquor obtained from wood distillation. Taylor in 1812 gave identity to methanol and called this substance pyroligneous ether, and Dumas and Peligot isolated and identified the alcoholic compound in 1834. Berthelot first synthesized methanol in 1858 by saponification of methyl chloride. Pyroligneous acid was the only commercial source of methanol for several decades, but in the last 35 years or so, pressure synthesis from carbon oxides and hydrogen has superseded the wood distillation method.
When hardwoods such as maple, birch, beech, and oak are heated in the absence of air to temperatures of 160 430°C, thermal decomposition takes place and produces non condensable gases, a watery distillate (known as pyroligneous acid), wood tar, and charcoal. The aqueous distillate is refined by extraction and/or distillation to produce acetic and, methanol, and acetone.
Most of today s methanol is produced by the catalytic reduction of either carbon monoxide, carbon dioxide, or mixed carbon oxides plus hydrogen in the presence of zinc and chromium oxides. Operating conditions range from 100 600 atmospheres and 250 400°C. Commercial Solvents Corporation became the first American company to produce and market synthetic methanol in 1927.
Other manufacturing methods of lesser importance are as follows direct oxidation of hydrocarbons, saponification of methyl chloride, and preparation of methyl formate from sodium methoxide and carbon monoxide followed by low pressure catalytic hydrogenation.
Physical Properties. The more important physical properties of methyl alcohol are given in Table 2. This compound burns with a blue flame and no soot to CO2 and H2O, and may form explosive mixtures with air. The first stage of methanol oxidation is formaldehyde (HCHO), which then proceeds to formic acid (HCOOH), carbonic acid (HOCOOH), and finally to carbon dioxide and water, Methyl alcohol is labeled a poison under federal and state statutes, and is quite toxic to humans.
Methanol is a highly polar compound and is the closest alcohol in structure to water when considered as an organic derivative (R OH) of it. Consequently, methanol is a powerful solvent for many substances, including synthetic coatings and adhesives like nitrocellulose, ethyl cellulose, and polyvinyl butyral; natural gums and resins, which include shellac, rosin, lauri, and manila; dyes; and most organic liquids.
Among the many important physical properties of methanol, only a few will be shown in graphic form. The vapour pressure of pure methanol between 10 and 240°C has been given by Timmermans as a logarithmic plot against the reciprocal of absolute temperature. Figure 1 presents vapour pressure data for methanol, ethanol, isopropanol, l propanol, and 1 butanol in terms of the above coordinates in order to obtain data as a straight line.
Since methanol is so frequently used in industry as an aqueous solution, studies by Carr and Riddick have been condensed and presented in Table 3. These physical properties are also of importance for analyzing simple mixtures.
The wide usage of methanol as an anti freeze gives practical importance to the freezing point curve shown in Figure 5. The data that correspond to percent by weight make up the lower curve, since methanol has a density of less than 1. Therefore, methanol is more effective as antifreeze when formulated on a weight percent basis rather than by a numerically equivalent volume percent basis.
Acetone, benzene, carbon disulfide and chloroform are some of the more than 110 compounds with which methanol forms a constant boiling mixture (Horsley). Methanol does not form an azeotrope with water; hence it may be recovered from aqueous solutions by distillation.
Chemical Properties. The chemical activity of methyl alcohol is closely related to other saturated alcohols, particularly with respect to reactions of the hydroxyl group. However, the methyl group deviates on occasion from typical alkyl group reactions since it is unique in having only one carbon atom. Examples are presented for the more important reactions of methanol.
Commercially, these reactions are conducted in a continuous flow system in the presence of a dehydrating catalyst, such as alumina gel, at an elevated temperature and pressure. The ratio of the three amines depends upon the ratio of the reactants and the reaction conditions. Pressure distillation is employed to separate the amines. The formation of di and trimethylamine may be suppressed by recycling these products, by introducing water into the feed, or by the use of a large excess of ammonia. It may be increased by recycling monomethylamine.
Toxicology. The principal physiological effect of methanol on human beings is damage of the central nervous system with a specific deleterious effect on the optic nerve. Additional effects may also occur depending on the exposure; these include degenerative changes in the kidney, liver, heart, and other organs. The human body eliminates methanol very slowly. A widely accepted belief is that the toxicity of methanol is due mostly to the metabolites, formaldehyde and formic acid, which are powerful nerve poisons in their own right. Oral ingestion of methanol leads to physiological effects which are similar to those arising from inhalation, but symptoms are likely to appear sooner and to be more severe in nature. An oral dose of 2 4 oz is usually fatal, although 1 2 oz has been reported to be a lethal dose in a number of cases. This picture is further complicated in that effects on humans are not constant small amounts of methanol will not affect some individuals and seriously harm others. The vapour toxicity of methanol is low compared to many common solvents at low concentrations. At the 1958 Conference of Governmental Industrial Hygienists, a maximum allowable concentration of 200 ppm for continuous 8 hr per day working exposure was adopted as the limit of safety. This same limit was adopted for toluene, amyl acetate, and propyl acetate. However, a person may be subject to an excess of methanol vapours or take an excess internally without a severe initial reaction. Even if the dosage is not lethal, blindness occurs in many instances. Consequently, methanol is labeled a poison under federal and state statutes.
A brief exposure to high concentrations of methanol vapours may develop acute poisoning. For example, 1000 ppm in air may cause irritation of the eyes and mucous membranes; 5000 ppm may produce stupor or sleepiness; and 50,000 ppm may result in profound narcosis in 1 2 hrs and will probably lead to death. Repeated exposure to high concentrations for brief periods, or a continuing exposure to low concentrations for a long time may produce chronic poisoning. Again, this effect is related to the accumulation of methanol in the human system caused by slowness in elimination.
Skin contact with methanol, in general, gives no adverse effects if gross exposure is avoided. Methanol has an additional physical action on skin since it is a solvent for natural skin oils, and can induce a drying effect by dissolving fats and oils from the upper skin layers.
Methyl alcohol finds numerous applications, as a cleaner of steel, metal and plastic surfaces; as a component in glass cleaners and special dry cleaners; and as a reducing agent in copper cleaning, brass annealing, and soldering fluxes.
Formation of gas hydrates and ice in natural gas pipelines is inhibited by this alcohol. Methanol coagulates latex rubber, and is used to manufacture dipped rubber goods. Miscellaneous applications are as a taxidermy agent, and an ingredient of embalming fluids.
Ethanol, CH3CH2OH, is probably the best known of all alcohols, with such common names as alcohol, grain alcohol, wine spirit, Cologne spirit, and ethyl alcohol. This compound is a colourless, neutral, mobile liquid of molecular weight 46.07, which has an agreeable but pungent odour, and a sharp burning taste.
Ethyl alcohol resembles methanol in that it is seldom found in nature. These rare occurrences are in the unripe seeds of Heracleum giganteum and Heracleum spondylium, in the urine and blood of men who have consumed an excessive amount of alcoholic beverages, and in the urine of diabetic people.
As mentioned previously, intoxicating liquids obtained from the fermentation of saccharine plant juices were known by the ancient Egyptians and Greeks, and contained ethanol in impure form. It was not until the late eighteenth century that the purification of mixtures to a pure anhydrous product was achieved. Saussure, in 1808, determined the constitution of ethanol. However, it was not until 1899 that Berthelot first synthesized ethyl alcohol by reacting ethylene with sulfuric acid, and by hydrolyzing the resultant ethyl sulfuric acid with boiling water to yield sulfuric acid and ethanol.
The above equations indicate that the starting material for the fermentation process may be any raw material containing hexose sugar, or materials that can be transformed into hexose sugars. France and Belgium use sugar beets; Germany utilizes potatoes; other European countries have converted sulfite liquor and sawdust, while corn, sugar, and cane molasses are the most popular in the United States. Sawdust and wood flour require the conversion of cellulose to fermentable sugars by acid hydrolysis, but grains can be converted by the action of malt. The cheapest source of fermentable sugar is blackstrap molasses which is a by product of sugar cane manufacture, and contains 50 to 60percent sugar by weight. The United States used to import considerable molasses from Cuba, but in recent years a hostile climate between these countries has halted the import of Cuba s molasses. Hence, its disposition as a mixed animal feed has become significant in other countries for reasons of economics. Most of the industrial alcohol produced by fermentation in the United States comes from blackstrap molasses, of which Puerto Rico has become an important source.
Trihydric and Polyhydric Alcohols
TRIHYDRIC ALIPHATIC ALCOHOLS (GLYCEROLS)
Alcohols with three or more hydroxyl groups are called polyhydric alcohols or polyols. Just as we find the generic name of glycol applied to the dihydric alcohols, in a similar manner the trihydric alcohols are named glycerols after their most important member. A special grouping of straight chain alcohols containing four or more hydroxyl groups on an equivalent number of carbon atoms have the name of sugar alcohols, and are discussed in a later section on Polyhydric Alcohols.
Glycerols are derived from hydrocarbons by the substitution of three hydroxyl groups for three hydrogen atoms which had been linked to different carbon atoms.
As the number of carbon atoms increases, so does the number of isomeric trihydric alcohols. With the additional configurations, any combination of primary, secondary, and tertiary alcohol groups can be obtained. Furthermore, the alcohol groups may be oxidized to aldehyde, keto, and carboxylic acid groups, thereby leading to 19 possible combinations of the four different functions.
A number of routes leading to the formation of trihydric alcohols have been studied, and several general methods are illustrated below. Specific methods for 1, 2, 3 propanetriol will be presented in a later section.
The physical, chemical, and toxicological properties of the glycerols will be illustrated by using 1, 2, 3 propanetriol as an example. Because of the great industrial importance of 1, 2, 3 propanetriol, it will be referred to as glycerol, and its properties will be indicated in some detail in the early portion of the section on trihydric aliphatic alcohols. The typical chemical reactions of glycerol will generally apply to other trihydric alcohols where they involve the reactivity of one, two, or three hydroxyl groups.
Almost all the applications of the trihydric alcohols are included in the more than 1500 uses of glycerol (1,2,3 propanediol) e.g., as a humectant solvent, plasticizer, a component of pharmaceuticals, and as a derivative employed in plastics, coatings, explosives, and foods.
1, 2, 3 Propanetriol, CH2OHCHOHCH 2OH (glycerol, glycerin, glycerine) is a clear, water white, simple trihydric alcohol of molecular weight 92.09. It exists as a viscous, hygroscopic, odourless liquid with a sweet taste.
Glycerol is a component part of all animal and vegetable fats and oils (triglycerides). A Swedish chemist, Karl Scheele, discovered glycerol in 1779 when he saponified olive oil with litharge to make lead plaster. He found a clear, sweet tasting syrup had formed on the surface of the mixture, and he call this liquid the sweet principle of oils. ChevreuI, in 1813, recognized ester like glycerol derivatives in fats and oils, and discovered the saponification process. He studied the trihydric alcohol further, and gave it its present name of glycerol (derived from the Greek glykeros meaning sweet). Pelouze, Berthelot, Lucca, and Wurtz later established the chemical composition as that of a trihydric alcohol which could be separated from animal and vegetable fats. Tilghman discovered that glycerin could be produced by splitting fats with heat. A current practice is to use the term glycerol for the pure chemical compound, whereas glycerin denotes commercial grades with variable glycerol contents. The spelling glycerine is considered incorrect because the ending ine applies to a base, and glycerol is not a base.
Occurrence. Although glycerol is present as a triglyceride in all animal and vegetable fats and oils, it is rarely found in a free state in these fats unless rancidity or decomposition has occurred during storage or handling. Oils from vegetable sources such as coconut, olive, and soybean yield larger amounts of glycerol that the higher molecular weight animal fats, which include tallow, mutton, and lard. Glycerol is also widely found in nature as the triglyceride in fatty substances like lecithin (found in eggs, soybeans, and brain, nerves, liver, kidneys and other animal organs) and cephalin (found in brain, liver, and other organs as above).
Production. The major portion of the worlds production of glycerol comes from the manufacture of soaps by the saponification of fats and oils, (of which glycerol is a coproduct), and from the hydrolysis of fats and oils into fatty acids and glycerin.
The aqueous liquid stream or layer is called glycerol sweet water and may contain up to 20 percent glycerol. Molten fatty acids are neutralized with lime, then the mixture is filtered. After preliminary evaporation to a half crude state, excess lime is precipitated by carbonates or sulfates. followed by filtration and evaporation to a saponification crude that contains about 88percent glycerol. Additional purification by vacuum distillation yields the assorted commercial grades such as high gravity, dynamite USP, etc.
A recent development in the refining of glycerol sweet water is the use of ion exchange resins in fixed bed columns. The good quality (low ion content) of a glycerol stream from the splitting process is the main reason for the commercial success of the ion exchange approach.
There are several secondary sources of glycerol from fats and oils, and these come from the production of ester type chemicals such as the methyl and ethyl esters of fatty acids, and from fatty alcohols made by the sodium reduction process. Both appear to be declining sources, since modern industrial practice favours the formation of fatty acids directly from fats and oils by splitting, followed by direct conversion to esters or fatty alcohols by catalytic methods.
The production of glycerol by fermentation of sugars has only been studied in the United States on a pilot plant scale, although it has been a production operation in Europe for many years.
The addition of sodium sulfite to the fermenting liquid ties up acetaldehyde and favours glycerol formation. The crude glycerin obtained by this method is very poor in quality; consequently, expensive refining methods would be required to compete with the saponification, splitting, or synthetic methods of manufacture.
The production of synthetic glycerol on a commercial scale was a major chemical achievement of the past two decades, and earned Shell Chemical Company the Chemical Engineering Achievement Gold Medal Award in 1948. This important development was stimulated to some extent by the impact of synthetic detergents on the laundry soap market, whose introduction subsequently led to a reduced processing of fats and oils, thereby producing less glycerin for refining.
The above route has the advantage of producing allyl alcohol as an intermediate. The unsaturated alcohol, along with allyl chloride, is widely used chemicals with their own commercial importance.
The second process (2) is less lime consumming than the first (I), and can be employed to yield epichlorohydrin by treating the mixed chlorohydrins with calcium hydroxide.
Both procedures result in a dilute solution containing about 5percent glycerol plus sodium chloride. The yield of dilute glycerol is about 90percent when based on aliyl chloride. Since a 20 fold concentration has to be achieved, it is essential that all synthesis steps be carried out with considerable care to reduce impurities to a minimum. The solution is concentrated to about 80percent glycerol in multiple effect evaporators, and sodium chloride is removed by centrifugation. Vacuum distillation and further desalting gives a crude distillate which is odourous, coloured, and generally unsatisfactory as a finished product. Fortunately, the colour bodies and some of the odorous materials are easily extracted from hot raffmate with a hydrocarbon solvent. Steam vacuum distillation of the raffinate removes light esters and chlorides as a top cut, and yields glycerol of 99percent purity or better from a second column. Synthetic glycerol produced in this manner meets the specifications of the United States Pharmacopeias.
Boiling Point. The boiling point of pure glycerol at atmospheric pressure (760 mm Hg) is 290°C. Since some decomposition takes place at this temperature, boiling points at reduced pressures have been determined and calculated. Duhrings rule is found to apply to the boiling point of glycerol water solutions when they are compared to a similar reference liquid such as water.
Boiling points of various compounds, and mixtures of glycerol with this component, are given in Table 3. Azeotropes and a number of non azeotropes are listed. A ternary mixture, ethyl alcohol, water glycerol, is particularly useful to distill anhydrous ethanol, since the affinity of glycerol for water prevents distillation of an alcohol ,water azeotrope. The addition of hygroscopic salts further improves the efficiency of this procedure, and provides the basis for manufacturing absolute alcohol.
Hygroscopicity. Anhydrous glycerol is highly hygroscopic and can absorb about 50percent of its weight of water. This ability to attract moisture and hold it is one of the most valuable properties of glycerol, and is the basis for its applications as a humectant and as a conditioning and plasticizing agent. Aqueous solutions of glycerol at any concentration will gain or supply moisture until a concentration is reached which is in equilibrium with the moisture of the air. Several determinations have been made of the relative humidity maintained over aqueous glycerol solutions, and Figure 2 is a composite plot of percent relative humidity versus per cent glycerol by weight in water solution for a temperate range of 20 100°C. The plot can be used in two ways (1) an open dish of aqueous glycerol in a large space will lose or gain moisture until its composition is in equilibrium with the relative humidity of the atmosphere, and (2) an open dish of aqueous glycerol in a cabinet or other enclosure of limited space will produce a relative humidity that corresponds to the glycerol concentration. The reservoir of glycerol solution must be sufficiently large, of course, so that it can take up or supply the required amount of moisture.
Refractive Index. The refractive index n2D0 of pure glycerol is 1.47399. This determination can be easily made with good precision, ind is quite sensitive to dilution with water. However, specific gravity is a little more precise and supersedes this method for analytical purposes.
Solubility and Solvent Power. Glycerol, with its three hydroxyl groups, has solubility characteristics similar to those of water and the simple aliphatic alcohols.
This trihydric alcohol is miscible in all proportions with water, lower alcohols and glycols, and phenols. It has a limited miscibility with ether, acetone, ethyl acetate, dioxane, and aniline. Glycerol is practically insoluble in hydrocarbons, chlorinated hydrocarbons, higher alcohols, and fatty oils. However, it will dissolve many organic and inorganic compounds to some extent. Aliphatic and aromatic hydrocarbons show an improved miscibility with glycerol as they gain hydroxyl and amine groups. The introduction of alkyl groups, as may be expected, results in a decreasing miscibility with glycerol. Table 4 shows the miscibility of a variety of organic solvents with glycerol.
Thermal Conductivity. The thermal conductivity of glycerol solutions increases with rising temperature, as well as with increasing water content.
Vapour Pressure. The vapour pressure of glycerol is lower than one would expect from its molecular weight, as a result of the molecular association characteristic of alcohols, water, and other polar compounds. Since many important applications of glycerol make use of its relative nonvolatility, this property has been determined by various investigators for both the pure compound and aqueous solutions.
The vapour pressure of 100percent glycerol is about 0.00018 mm Hg at 20°C and about 0.195 mm Hg at 100°C.
When glycerol is dissolved in water, it causes a greater reduction in vapour pressure than can be related to molar concentration. This effect is due to the formation of hydrates. vapour pressure for aqueous glycerol solutions may be calculated by Duhrings rule through the use of an allied reference liquid, such as an aliphatic alcohol or water.
Viscosity. The high viscosity of glycerol is one of its distinctive properties and is the basis of several applications. Segar and Oberstar worked with 99.97percent pure glycerol and Ostwald viscometers to obtain viscosity data over a 0 to 100°C temperature range, in concentrations from 0 to 100percent. Some of their data are plotted in Figure 3, which consists of a family of curves at several temperatures showing increasing viscosity with a rise in glycerol concentration and a fall in temperature. Glycerol polymers of the di and polyglycerol variety exhibit higher viscosities with increasing molecular weight.
In general, electrolytes increase the viscosity of anhydrous glycerol and aqueous solutions, but several compounds have the opposite effect, e.g., potassium iodide, ammonium iodide and bromide, rubidium chloride and bromide, and cesium chloride and nitrate. The viscosity of glycerol rises with increasing pressure.
When glycerol is super cooled, its viscosity increases gradually until the temperature range of 70 to 110°C is reached. In this region, glycerol changes from a viscous liquid to a rigid glassy form called the vitreous state. and its physical constants approach those of crystalline glycerol rather than liquid glycerol.
Taste. The taste of glycerol is predominantly sweet, although tests made by Cameron show it to be somewhat less sweet than sucrose. When concentrated glycerin is taken into the mouth, it produces a sensation of warmth.
Chemical Properties. As mentioned previously, glycerol is the simplest trihydric alcohol and contains two primary and one secondary hydroxyl groups. The molecule is symmetrical; hence, the two CH2OH groups are identical in their properties. The chemical nature of glycerol is characteristic of primary and secondary alcohols, but the presence of multiple hydroxyl groups contributes to additional reactions and derivatives as compared to mono and dihydric alcohols. In general, the primary hydroxyl groups are more reactive than the secondary hydroxyl. Even though a reaction has been initiated on one hydroxyl group, there is some reaction with the second and third hydroxyl groups before the most reactive position has been completely utilized. As a result of this generalization, glycerol derivatives are almost always obtained as mixtures containing isomers and products of different degrees of reaction.
The pure chemical compound (1,2,3 propanetriol), is commonly called glycerol and the ol ending indicates the presence of a hydroxyl group. The term glycerin is applied to technical grades which contain moisture and organic impurities.
METHANOL FROM COAL
Clearly, the one stage gasification process represents only one family of processes for producing synthesis gas, which can ultimately be utilized to produce methanol. A brief description of the characteristics of various types of one stage gasification processes is presented in Table 1 and Figure 2. However, any one of the many processes capable of converting coal into a synthesis gas of acceptable composition is, in principle, a viable link to the production of methanol. A brief diagram of the alternative routes that may be considered is presented in Figure 3.
The manner is which methanol is ultimately produced from synthesis gas is depicted schematically in figure 4. As is typical for low conversion processes of this nature, a substantial recycle stream is necessary to obtain high yields. Of further note are the entry points of feed at both ends of the converter. The lower point is, in reality, a bypass stream that allows fine adjustments of flow through the reactor and thus enables precise temperature control in the converter bed. This control is vital to successful process operation.
In Figure 5, methanol production by partial oxidation of methane is depicted schematically. The oxidation furnace may or may not contain a catalyst. Catalytic agents claimed to be successful are iron, nickel, copper, palladium, etc., their oxides, mixtures of their oxides, mixtures of their oxides and metals, aluminum sulfate, and alkyl ethers.
The three major methanol synthesis processes that are marketed today are the ICI process (Imperial Chemical Industries), the Lurgi low pressure process (Lurgi Corporation), and the CPI Vulcan process (Vulcan Cincinnati).
The Lurgi Process (a) and (b) is coal sized to 1½ to 4 inch mesh lock hopper fed to piled bed on grate.
The Koppers Totzek Process is pulverized coal, 02, and steam introduced through burners; particles entrained in gas stream, ash slagged.
The Winkler Process (a) and (b) is pulverized coal screw fed to fluidized bed.
The U Gas Process is pulverized coal pretreated in fluidized bed and gasified in fluidized bed.
Methanol Fuel Product
The coal gasification processes considered in the preliminary screening are those that have been in commercial use in Europe, Asia, and Africa for making town gas and synthesis gas (usually for ammonia production) Lurgi (moving bed), Koppers Totzek (entrained flow) and Winkler (fluidized bed) and the Texaco partial oxidation process which has been commercially demonstrated with a variety of feedstocks from natural gas to vacuum resid and petroleum coke.
Texaco has operated a 10 tpd pilot unit for a series of runs with different coals during the past few years, and it is reported that a 200 tpd commercial scale unit is being refitted for coal in a German plant site. It is DuPonts opinion that the Texaco process is sufficiently demonstrated to meet the requirements of this study.
High Spot Process Evaluation
Using information obtained, high spot estimates were made for chemical grade methanol from coal processes based upon Koppers Totzek, Lurgi, Texaco and Winkler coal gasification units. The amount of coal and oxygen required to produce a nominal 5,000 tpd of chemical grade methanol, the thermal efficiency and the capital investment were estimated and the results Shown in Table 2.
Lurgi and Texaco processes have higher thermal efficiency than the other two processes because of the above atmospheric operating pressures and lower requirement for compression.
Texaco and Lurgi appear to require the lowest investment although the differences are well within the accuracy of the estimates.
Textco technology is well adapted to eastern bituminous coals as is Koppers Totzek; there is some question about the ability of Lurgi and Winkler gasifiers to handle swelling caking coals in a trouble free manner.
Texaco gasifiers handle larger throughputs per unit than Lurgi gasifiers and this reduces the number of gasifiers needed.
In the following sections a more detailed consideration of process scope and definitions will be presented to provide a basis for estimating capital requirements. It should be emphasized that although it is DuPonts opinion that the assumption of Texaco gasifiers and the other process units in this study will result in representative results for production of methanol fuel from high sulfur eastern bituminous coals, it is recognized that for a given installation with specific coal, other technologies and processes should be evaluated to achieve optimum design.
A grid (8 squares) over the hopper prevents large tramp material from entering the hopper. If the grid becomes plugged, the train must be stopped to remove such material. Communication between the receiving hopper and the locomotive is required to regulate train speed and stop start.
Stage 2 Coal is fed from the day pile at the rate of 1,600 tph through two vibrating feeders. At this rate, the 10,000 ton day pile can be transferred to the storage pile(s) in 6¼ hours (one shift or less). A recording belt scale in the transfer conveyor between the feeders and the tripper gives a check on the tonnage delivered by each unit train and tonnage into storage piles.
The tripper belt conveyor receives the coal from the transfer conveyor and delivers it to either one or both stacker belt conveyors. The tripper and the stacker belt are mounted on a common traveling tower permitting two storage piles to be formed giving a 15 day supply of coal for the boiler and gasifier section. Tripper tower may remain stationary to discharge coal at a given point or travel to blend the newly arrived coal with the coal already in storage.
Stage 3 From each of the piles, vibrating feeders are placed on 40 foot centers to reclaim the coal at the rate of 400 tph. These feeders are actuated in groups of four for the purpose of controlling the rate of withdrawal from a pile, and discharge to a 36 inch recovery belt conveyor which, in turn, feeds a primary (ring mill) crusher. A belt scale in this recovery conveyor controls the variable feeder.
When blending from both piles is required, coal may be recovered from both piles at regulated lower tonnages. Each ring mill crushes 8 0 coal at 3 rate of 400 tph and reduces it to ¾ 0. A 36 inch belt conveyor from each ring mill delivers to a common surge bin to end Stage 3.
Stage 4 From the surge bin, ¾ 0 coal is withdrawn by a variable rate vibrating feeder at a nominal 24 tph onto a 24 inch belt conveyor for delivery as boiler fuel. A second 24 inch belt conveyor is furnished to deliver wet char (60percent solids) to the boiler. Both of these conveyors, discharge into a double shaft paddle mixer from which this boiler fuel drops into a boiler fuel surge bin.
Stage 5 From the surge bin, coal is withdrawn through two regulating feeders at the rate of 179 tph each and discharged into two rod mills. This rate is 15percent greater than is required for the gasification section; therefore, once the system is full, one mill could be down 7 hours, 12 minutes per day or both down 3 hours, 36 minutes per day.
Each rod mill requires a one hour shutdown period every third day to clean out thin and broken rods and then add about 9,000 pounds of new rods. The shutdown time permits this servicing as well as other maintenance in the grinding section.
Water is introduced at the feed end of the rod mills along with the coal for wet grinding. The amount of water is regulated by a density controller located at the discharge of each mill to form a slurry of 50 to 54percent solids by weight. This slurry with its solids ground to 14 mesh 0 is delivered to two sumps from where it is pumped to two ball mills where the solids are reduced to 80percent 200 mesh. Trommel screens at the discharge of these mills prevent balls or other relatively large solids from continuing in the flow of slurry.
A density controller at the discharge of each ball mill adds water, if needed, to maintain the ball mill discharge slurry at 50 to 54percent solids by weight. This completes the preparation of the coal slurry in this open grinding circuit. Slurry from each ball mill falls into a sump and is pumped to two steel tanks having the capacity for storing a 24 hour slurry supply ahead of the gasification section. Each tank is equipped with agitators to keep the slurry in a ready condition.
Two centrifuges (plus a standby) receiving ¾ 28 mesh from the screens discharge solids to the ash conveyor and effluent to thickener. A lime slaker is provided to neutralize an acid ash slurry from the flue gas cleanup process containing 25,000 lb per hr of 200 mesh solids, 480 gpm water and 100 to 200 lb per hr of H2SO4 at 140ºF. This solution has pH of 2 and is delivered to the ash handling building. The lime slaker is automatic and neutralizes the slurry to a pH of 7 prior to releasing it for delivery to the thickeners.
Two 75 foot diameter thickeners are provided complete with bridges, rakes, drives and lifting mechanisms. Both thickeners operate together receiving the aforementioned screen underflow, dryer effluent and slurry totaling 1,480 gpm water plus 14 tph solids. Clarified water from the thickeners shall contain not more than ½ percent solids by weight.
Two pumps (plus a standby) deliver underflow from the thickeners, delivering to one disc filter (plus one standby filter) from which the solids are discharged to the ash conveyor and the effluent to the pump sump along with the overflow from the thickeners. A clarified water pump (plus a standby) returns this water back to the gasification section for reuse. The net water excess is consumed in the coal slurry preparation (Stage 5). The centrifuged ash, filtered ash, and possibly grit from the lime slaker is delivered at about 52 tph on a belt conveyor to a steel silo from which it is loaded out into unit train cars.
Dust suppression and/or collection devices are provided at the receiving hopper, through tunnels, sample building, transfer points, loading points, crushers and surge bin ahead of the rod mills. Two concrete lowering tubes are placed at the discharge points of the belt conveyors feeding the day pile to prevent excessive dusting and a telescopic chute with automatic controls is placed at the discharge of each traveling stacker belt to aid in dust prevention.
Coal Gasification and Air Separation. Coal slurry from the slurry storage tank is pumped to a slurry surge tank. In the surge tank the slurry is preheated (preheat will reduce the oxygen requirement). Oxygen is produced at the air separation plant operating at 92 psig. 6,330 tpd of 99.5percent oxygen is produced and compressed to 935 psig (by centrifugal compressors). Air cooling is used in the inter coolers while water cooling is used in the turbine exhaust steam condensers.
The slurry (50 to 55 weight per cent coal) is then pumped to the Texaco gasifiers via booster pumps and reciprocating charge pumps. The slurry is mixed with O2 and enters the combustion zone where partial oxidation occurs at 800 psig according to the following reaction
The majority of the sulfur is converted to H3S and some to COS. The nitrogen in the coal is converted to free nitrogen with some traces of NH3 and HCN. The ash is melted due to the high temperature of the reaction. It is critical that the gasification temperature exceed the melting point of the ash sufficiently to yield a free flowing molten slag. The slag is assumed to leave the gasification section at 2300°F. It is normally true that the ash with high calcium content will have lower melting temperature. Therefore, by adding small quantities of CaO or other fluxing agents to the coal slurry, the slagging temperature may be reduced. In most cases this procedure will not be necessary.
The synthesis gas flows from the flame chamber to the slag separation section where gravity separation occurs. The molten slag drops to a wet bottom and is washed away. About 5percent of the carbon in the feed is carried away with the synthesis gas as an unburned char containing 40 to 60percent ash.
The synthesis gas is quenched by sour water from the shift. There is no energy penalty because this water will be consumed in the shift section and will eliminate the need for injection of live steam to the shift section (except for control purposes). The quench step should result in less expensive material of construction, and less erosion and scaling from reduction in the T in the waste heat exchanger and the increase in the gas volume.
The bulk of the slag passes from the gasification section to the slag quench section. The quench operation is a semi continuous process with a cycle time of about 15 to 17 minutes. In normal operation, the cycles are timed so the quench section of one gasifier is discharged while the others are being filled with slag and water. By utilizing the cyclic discharge the flow rate is large enough to wash all the slag from the bottom of the gasifier.
The quenched gas from the slag separator enters a waste heat exchanger and is cooled to 633ºF by generating 1,270 psig. 576°F steam. The 633°F exit temperature has been determined by heat and material balances on the entire gas cooling system in which a gas composition of 50percent water vapour at saturation is assumed to exit the gasifier system. This specification is set by the design of the shift section.
The gas then flows to a three stage dust removal section in which adiabatic saturation is also reached. The moisture content of the 633°F gas will increase in the saturation process to maintain 50percent volume water vapour in the gas. The bulk of the dust is removed in the saturation process. The residual dust is collected in the high energy venturi scrubber followed by a final cleanup using a wash tray. The makeup water is added to the scrubbing cycle in the wash tray section.
The char slurry is withdrawn from the quency scrubber with a maximum of 10percent solids slurry. The liquid level in the quench section is maintained by bleeding a side stream from the venturi cycle which contains about 1percent slurry. The liquid level in the venturi cycle is controlled by the makeup of preheated sour water coming from the shift.
The dusty liquor withdrawn from the quench section is cooled to 170°F against the thickener overflow. After cool down, the pressure is reduced and it is stripped with CO2 rich gas from the acid gas separation plant. At a temperature of 130°F it then enters the thickener. Due to evaporation in the thickener the temperature is reduced to 120°F, and the underflow concentration is expected to be in the range of 25 to 30percent. The underflow is pumped to a rotary vacuum filter from which a wet char containing 40percent water is discharged and conveyed to the steam plant as a fuel.
The filtrate combines with the thickener overflow and is reheated to 410°F and reinjected to the wash tray of the scrubbing section. Some excess of sour water leaves the system as a fine solids purge stream from the thickener overflow to the slag disposal sump.
The slag liquor from the gasifier underflow and the dusty liquor from the scrubber underflow are stripped with CO2 rich gas from the acid gas treatment unit. The overhead gas from the stripper is preheated from 130º to 220°F by utilizing 20 psig steam and then incinerated in the steam plant. By heating this gas, the energy penalty on the boiler is decreased, and also corrosion in the transfer line is prevented.
Girdler Chemicals cobalt molybdenum catalyst has been assumed for this study. Synthesis gas leaves the gasification section at 430°F and 770 psig, and has a steam to dry gas ratio of 1 1. About one sixth of this gas is bypassed around the shift reactor for control purposes, while the remaining gas is preheated to 50*F above the reaction initiation temperature.
To improve the overall thermal efficiency of the process, it is imperative that the mode of heat recovery from the reactor effluent gas be done in a very efficient manner.
Sour water, condensed as a result of this cool down and heat recovery, is separated and recycled for synthesis gas quench and scrubbing. An additional 73,600 Ib per hr knocked out at 130°F is recycled on level control to the wet grinding mills.
Gas Treating Selective H2S Removal Unit Synthesis gas treating is accomplished in two stages. In the first stage, H2S is selectively removed with respect to CO2 and COS, and the treated gas leaving the H2S absorber has a total sulfur content of 210 ppm. This results in the off gas stream to sulfur plant having an H2S concentration of 23 mol per cent. In the second stage, residual COS and a major portion of the inlet CO2 are removed yielding a treated methanol synthesis gas containing 10 mol per cent CO2 and less than 1 ppm total sulfur.
Although there are several other competitive physical solvent processes in the market, for this study Allied Chemical Corporations Selexol process was chosen. To meet the maximum equipment size requirement imposed by shipping as well as from operational flexibility viewpoint, the entire treating plant has been divided into two identical and parallel trains.
Synthesis gas leaving the shift at 715 psig and 130ºF enters the H2S absorber where it is contacted counter currently with lean solution presaturated with CO2 from CO2 absorber. Due to the higher solubility of H2S relative to CO2 in the solvent only about 20percent of the inlet CO2 is absorbed along with almost 100percent removal of H2S. To minimize the amount of CO, CO2 and H2 losses from the H2S stripper overhead, rich solution is flashed in three successive stages and the flashed vapours are combined, recompressed and recycled back to the absorber after cooling.
Flashed rich solution from the third stage is pumped to the top of the H2S stripper, where stripping is accomplished using 100 psig steam. Stripper reboiler duty is minimized by heating the rich solution to 275°F against the lean solution from the stripper bottom.
CO2 Removal Unit Desulfurized gas is cooled before being contacted with chilled lean solution at 22°F in the CO2 absorber. Lean solution from H2S stripper is chilled to 20°F using propane refrigerant before being combined with lean solution from CO2 stripper bottoms. The lean solution circulation is controlled to give a very close equilibrium at the absorber bottom which allows the CO3 concentration in the residue gas to be held at the 10 mol per cent level desired for the low pressure methanol synthesis process.
Rich solution from absorber bottoms is let down to 275 psia through a hydraulic turbine which provides part of the energy required for driving the lean solution pump. Flashed vapours are compressed and combined with the inlet gas, thus minimizing CO and H2 losses from CO2 stripper overhead. Flashed rich solution is further let down to 16 psia and acts as a refrigerant before being stripped using dry nitrogen from the air separation plant. The stripping rate is set to yield COS content below 1 ppm in the final product gas.
CO2 rich gas is unsuitable from the environmental standpoint for venting to atmosphere. Hence, it is compressed and sent to strip dusty liquor before being routed to the boiler for incineration of residual sulfur compounds.
Methanol Synthesis and Fractionation. The methanol synthesis and fractionation areas produce 5.496 tpd of fuel grade methanol (95 wt percent methanol). ICIs low pressure methanol synthesis technology was assumed in the design of this unit.
Synthesis gas at 680 psig and 85°F from the treating unit is passed through the desulfurizer drums. Each drum contains two packed beds of activated carbon granules which act as a sulfur guard for the methanol synthesis catalyst. The drums are designed to operate on stream for three days. At the end of this period one drum is taken off stream while a fresh bed is put on line. The spent carbon is regenerated with steam and is then returned to service.
From the desulfurizer drums the gas enters the synthesis makeup compressor and is compressed to 1,542 psig. A condensing steam turbine using 1,175 psig, 925°F steam exhausting to 3½ inch Hg absorber is used to drive the makeup compressor. The discharge gas from the compressor is combined with synthesis recycle gas and cooled in an air cooler to 130°F.
The cool gas is divided into two equal streams and then sent to two parallel synthesis loops each consisting of a synthesis converter, heat exchange train, and recycle compressor, in a loop, the gas is further divided with a portion of the gas being sent as quench for the converter and the remainder being sent as feed to the converter. In order to achieve the temperature necessary for reaction to methanol, the feed gas is first passed through the converter feed preheater, where the gas is heated by hot reactor effluent.
The preheated feed gas enters the top of the converter vessel and flows downward through several catalyst beds to the converter outlet. At the exit of each catalyst bed, cold quench gas is injected to control the inlet temperature of the next catalyst bed.
The outlet gas contains about 5.5 mol per cent methanol together with large amounts of unreacted CO, CO2, and H2 inerts such as N2, argon, and CH4; and some quantities of by products such as water, dimethyl ether and higher alcohols.
To separate the unreacted gases from the product methanol, the converter outlet gas is cooled in a heat exchanger train. The resulting two phase mixture from the cool down train is separated and the unreacted gases are sent to the recycle compressor and the condensed liquid containing about 73 mol per cent methanol and 25 mol per cent water is sent to the fractionation unit, where the water content is reduced to about 5percent and low boilers such as dimethyl ether and unreacted gases are taken off the top.
Sulfur Plant. Acid gas from the treating unit leaves the H2S stripper reflux accumulator and enters the inlet scrubber where any entrained sour water is knocked out. Since the H2S concentration in the feed is only 23 mol per cent, the bypass type plant configuration with three stages has been selected. In this configuration 35percent of the acid gas is oxidized in the reactor furnace with a stoichiometric quantity of air. The hot combustion products are cooled in a waste heat boiler, generating saturated 105 psig steam.
Effluent gas is combined with the acid gas which bypassed the combustion step and recycle SO2 rich gas stream from the flue gas cleanup unit and the mixture is heated to 450°F before entering the first converter. It is important that H2S to SO2 ratio at the converter inlet be maintained at 2 to 1 for maximum conversion efficiency.
Conversion takes place in the presence of activated alumina Catalyst. To ensure hydrolysis of COS in the feed, the bottom 12 inches in the catalyst bed contains Co Mo catalyst. All the sulfur formed in the converter is condensed by cooling the reactor effluent gas. The reaction gas is again heated up to 425°F before entering the second converter. Here some more conversion occurs and the sulfur formed is condensed by cooling the reaction gas to 340° F.