Industrial Synthesis of Hydrocarbons from Hydrogen
and Carbon Monoxide A. J. V. UNDERWOOD 38, Victoria Street, London S. W. 1, England
T
I n 1925 Fischer and Tropsch (3) showed that a reaction yielding hydrocarbons could be carried out with almost negligible formation of oxygenated compounds. This resulted from the use of a suitable catalyst and low operating pressures. I n 1934 Ruhrchemie A.-G. undertook the industrial development of the Fischer process. Laboratory, pilot-plant, and semicommercial investigations were followed by the construction of several commercial plants. The first Fischer industrial plant went into operation in Germany in 1936. Over 1,000,000 tons of Fischer liquid per year are being produced in Germany at present by this process. Plants have also been built in France, Japan, and Manchukuo, and a license has been purchased by a group which contemplates a plant in South Africa.
HE Fischer-Tropsch synthesis is a process in which liquid hydrocarbons are produced from a mixture of carbon monoxide and hydrogen, and may be represented by the following reactions: catalyst n CO 2n H2 C,Hz, n H20 catalyst 7t CO (2n 1) Hz --+ CnHln+2 n H20
+ + +
-
+
+
The early work of Sabatier showed that methane could be produced catalytically from a mixture of hydrogen and carbon monoxide. Later the Badische Anilin- und Soda-Fabrik found that mixtures of hydrocarbons and oxygenated compounds could be produced from hydrogen and carbon monoxide a t high pressures.. 449
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The Synthesis The most effective catalysts for this synthesis are cobalt, nickel, and iron. It is significant that these three metals can all react with carbon monoxide and hydrogen t o form metal carbides. The theory of the synthesis is that metal carbides are formed with subsequent reduction by hydrogen t o form methylene radicals, which then polymerize to higher hydrocarbons. The hydrocarbons thus formed may be hydrogenated to saturated compounds.
0.25
0.1
0.5
1.0
2.5
5.0
IO
15 20
PRESSURE IN ATM. ABSOLUTE
OF PRESSURE ON TOTAL YIELD FIGURE1. EFFECT AND YIELDOF VARIOUS FRACTIONS
Various promoters have been tried with the catalyst, including thoria, magnesia, manganese oxide, and alumina. Several supports have been experimented with, but the most satisfactory is kieselguhr. I n the present commercial installations a cobalt catalyst with a promoter, supported on kieselguhr, is used because cobalt gives the highest yield of hydrocarbons and has the longest life. The theoretical yield of hydrocarbons from one cubic meter of ideal gas (two parts hydrogen plus one part carbon monoxide) is 208 grams. Yields of 130 to 140 grams have been reached commercially in one-stage operation. These yields can be increased by the use of lower space velocities or by multistage operation. The synthesis reaction is carried out at temperatures around 190-200" C. Investigations have shown that the operating pressure has an effect on the yield and also on the character of the product. As pressure is increased the yield a t first increases, but a point is reached where a further increase in pressure brings about a decreased yield. The maximum yield is obtained in the range of 5 t o 15 atmospheres. Table I shows the results of some experiments a t various pressures.
l2
-
1
I
I
I
I
I
ATM PRESSURE SYNTHESIS PRESSURE SYNTHESIS
-,._"._MEDIUM
+ g
p 3 7 g 6 ; 5 * 4 K
E 3 2 I
0 I
3
5
7 9
12 14 16 IE 20 CARBON NUMBER
24
28
32
OF HYDROCARBONS PRODUCED FIGURE2. AMOVNT AT Two PRESSURES
Increasing the time of contact, i. e., lower space velocities, TABLEI. YIELDSOF HYDROCARBONS AT DIFFERENT PRESSURES results in a lower catalyst operating temperature and a higher yield, with less formation of methane. Figure 4 shows Yield'. Grams per Cubic Meterb Gaseous the yield us. space velocity for the medium-pressure synthesis Gasoline hydrocarGage Solid and Oil fraction bons, inin the two-stage operation. (above (below cluding Pressure, liquid hy- Paraffin There is a great deal of flexibility in the character of prodAtm. drocarbons wax 200' C.) 200' C.) CJ and C4 ucts that can be obtained from this process. Figure 5 shows 69 38 117 10 38 0 15 43 73 50 1.5 131 an example of the variation in the composition of products 60 51 39 33 150 5 by changing operating conditions in the medium pressure 70 36 39 33 15 145 54 37 47 21 50 138 synthesis. 27 34 43 31 150 104 7 -
a Average yields per cubic meter of ideal gas over 4 weeks' o eration with a single passage of the gas over the catalyst and without reviviication of the catalyst. b Under normal conditions.
The figures in Table I indicate that a t pressures of 5 to 15 atmospheres, there is less formation of hydrocarbons in
Industrial Application by Ruhrchemie A.-G. The industrial process as used by Ruhrchemie A.-G. consists of the following steps: The production of synthesis gas with a hydrogen to carbon monoxide ratio of 2 to 1. Purification of the synthesis gas.
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Synthesis of the hydrocarbons (manufacture and reclaiming of catalyst). Recovery of hydrocarbons. Treatment of hydrocarbons. One pound of hydrocarbons requires 110 to 125 cubic feet of synthesis gas containing about 13 per cent inerts. The character of t,he products obtained from the Fischer process is independent of the raw material. The following are some of the raw materials which can be used for making synthesis gas: Caking coals, by carbonization, and gasification of the resulting coke. Noncaking bituminous and brown coals in lump or bri uet form: (a) with preliminary carbonization, ( b ) by direct gasixcation in externally heated retorts, or ( c ) by direct gasification in internally heated generators. Small size noncaking bituminous and brown coals, and coke. Coke-oven gas or natural gas.
451
THEORETICAL YIELD
I
co
200
- v)
c? J
.
2E2 -150
5
0 K
100
I
I 0
I
I
2
I
3
GAS VELOCITY (LITERS PER HOUR PER GRAM COBALT)
FIGURE 4. YIELDOF IDEAL GAS us. SPACEVELOCITY FOR MEDIUM PRESSURE SYNTHESIS IK TWO-STAGE OPERATION OPERATING PERIOD (WEEKS)
FIGURE 3. VARIATION OF YIELD WITH TIMEA T VARIOUS PRESSU~ES
There is a variety of processes for the production of synthesis gas from coal or coke, which are the raw materials utilized in all the plants in Germany and France. Smong the processes which can be used are the following: Gasification of high-temperature or low-temperature coke in water gas generators, which may be accompanied by cracking of coke oven gas in the generators. Direct gasification of noncaking coals with the Bubiag-Didier system. Gasification of noncaking bituminous coals in water gas generators. Production of synthesis gas from brown coal briquets in a Koppers generator.
Winkler generator, using brown coal coke. Lurgi process for gasification of noncaking bituminous coal, brown coal, or coke under pressure with the use of oxygen. Gasification of noncaking coal by the Wintershal-Schmalfeldt process. When coal or coke is used for synthesis gas production, there is a deficiency of hydrogen. Carbon is discarded as carbon dioxide in order to adjust the ratio of hydrogen to carbon monoxide. In the United States a t present the most interesting ram material for the process is methane. Large quantities are available and in some cases have substantially no value. It is less difficult to produce synthesis gas from methane than from solid fuels, and a smaller investment is required for equipment. Bmong the methods by which methane can be converted to synthesis gas with a 2 t o 1 hydrogen to carbon monoxide ratio are the follolTing:
ACTIVATEDCARBON PLANT
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20 I-
15
B
L
5
IO
2 5
0
1
3
5
7
9
1 1 195'C
15
19 320°C
25
31-CARBON 460T
NO.
CONDITIONS ON CoiwosIFIGURE5 . EFFECTOF OPERATIKG TION OF PRODUCTS 3CH4
+ '202 + 2H.0 +4CO + 8Hz CHI + '/io*+CO + 2Hz
(1)
(2)
Equation 1 can be effectively carried out in the presence of
a nickel catalyst a t temperatures as low as 1400" F. (760" C.). A similar method with methane and steam is used for the commercial manufacture of hydrogen by the Standard Oil Company of New Jersey in their refinery a t Baton Rouge, La. The carbon dioxide for reaction 1 can be obtained by recovery from flue gas or other carbon-dioxidebearing gases. There are also certain localities in the United States where carbon dioxide occurs naturally. It is necessary that the synthesis gas be practically free from sulfur compounds, because sulfur causes rapid deterioration of the synthesis catalyst. The maximum amount of sulfur permissible is about 0.1 grain per 100 cubic feet. Several processes for hydrogen sulfide removal are available, but in the German plants the hydrogen sulfide is removed by the iron oxide process. The problem of organic sulfur removal led to the development of a new process. The catalytic reduction of organic sulfur to hydrogen sulfide and subsequent removal is expensive. Roelen and Feisst ( 2 ) developed a process which involves passing the synthesis gas over a suitable catalyst a t temperatures of 350" to 570" E'. (177" to 299" C.). The organic sulfur compounds are broken down and fixed in the catalyst. The synthesis gas leaves the purifier a t substantially the temperature a t which the synthesis is carried out SO that no great amount of cooling or heating is necessary. The catalyst used is a specially prepared form of alkaline iron oxide. Another process which shows promise utilizes a specially prepared nickel catalyst for the oxidation of sulfur compounds to sulfur dioxide, which is subsequently removed. In the synthesis step, to obtain high yields and maintain the activity of the catalyst, extremely accurate temperature control is necessary. Large quantities of heat must be removed. The heat evolved
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in the reaction amounts t o about 20 per cent of the heat of combustion of the synthesis gas or approximately 7200 B. t. u. per pound of product. There are several possible methods for removing the heat of reaction. The method now used is indirect cooling with water to form steam. Other possible methods which may be developed in the future are direct cooling by oil, water, or gas, or some other method of indirect cooling, such as with oil. When cooling with water, a large aniount of steam is generated which can be used in operating the process. The steam thus produced can have a pressure as high as 225 pounds per square inch. In the atmospheric operation the synthesis gas passes downward over horizontal tubes provided with fins. The catalyst is outside of the tubes, and water circulates inside of the tubes. In the pressure synthesis the catalyst is placed inside of specially designed vertical tubes, with water circulating around the tubes. In the latter type the cooling is very effective. In both cases the temperature of reaction is controlled by regulation of the steam pressure. These catalyst chambers are shown in Figures 6 and 7. Each chamber has a gas throughput of 37,000 to 56,000 cubic feet of gas per hour. The cooling systems of several units can be connected together with one common steam system. For such an arrangement the quality of the catalyst must be uniform, since this system results in
CHAJIBERS FOR ATMOSPHERIC PRESSURE SYNTHESIS FIGURE 6 . CATALYST
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the same temperature for all catalyst chambers in the group. When a fresh batch of catalyst is used for the synthesis, the temperature and gas throughput must be carefully regulated according to a definite schedule, since the activity and life of the catalyst may be greatly affected by its initial treatment. Two-stage operation is being used a t present. A carbon monoxide conversion of 70 to 80 per cent is effected in one stage and the remainder in the second stage. The over-all carbon monoxide conversion is 90 to 92 per cent. As the reaction proceeds, the inert constituents increase in percentage so that the gas entering the second stage is much lower in hydrogen and carbon monoxide content. Therefore the production per unit of catalyst is not so great in the second as in the first stage. The yield can be still further increased by the use of more than two stages, but its advisability is a question of economics. The product from the pressure synthesis contains a higher proportion of heavier hydrocarbons than that from the atmospheric synthesis. The following table shows the percentage composition of the products from the two types of synthesis; the same catalyst was used in both cases: Atm. Pressure Synthesis
a
Medium Pressure synthesis”
About 7 atmospheres.
b Figures in parentheses indicate per cent olefins in the fraction.
The catalyst life is now 4 t o 6 months. The valuable constituents are recovered from the spent catalyst and used for the preparation of fresh catalyst which has an activity equal to that prepared from new material. A laboratory is maintained for the testing of new catalysts and the control of industrially prepared catalysts. The products leaving the catalyst chambers are cooled. I n the atmospheric operation direct cooling is used; in pressure operation cooling is indirect. The hydrocarbons can be recovered from the gas by oil absorption or activated carbon. Activated carbon is used in the atmospheric operation. The absence of sulfur and gum-forming compounds makes the product particularly suitable for this type of recovery system. A plant operating on the Fischer-Ruhrchemie process is shown on page 451. Oil absorption is used in the pressure operation. The lean oil used is a fraction of the synthesis product. The tail gas from the plant contains inert constituents, notably nitrogen and carbon dioxide. It has a calorific value of about 200 to 250 B. t. u. per cubic foot and may be used for fuel. The tail gas may be utilized for conversion into hydrogen and carbon monoxide, and the gas mixed with fresh synthesis gas. The use of the tail gas in this manner, however, depends on the quantity of gas and the amount of inerts. Since the primary products are practically free from sulfur, the only treatment necessary is a light alkaline wash to remove traces of organic acids. Further treatment is necessary to convert the primary products into marketable materials. The products are straight-chain hydrocarbons. The gasoline fraction is therefore of low octane number and the Diesel oil fraction of high cetane number. The paraffin wax fraction can be converted into waxes of various melting points. A wax melting a t 195’F. (90.6’ C.) is being produced commercially on a large scale. Waxes with even higher melting points can be made for special purposes. Synthetic paraffin wax is being used in Germany to obtain synthetic fatty acids by oxidation. The first processes for converting the product to gasoline were the thermal cracking processes. Egloff, Nelson, and
CHAMBERS FOR MEDIEMPRESSURE FIGERE7. CATALYST SYNTHESIS
hlorrell(1) report a yield of 84 per cent by volume of 66 octane gasoline from the liquid products of the synthesis. This yield includes polymerization of the cracked gases but does not include the additional gasoline obtainable by polymerization of the unsaturates in the CI-Cd fraction from the synthesis. Snodgrass and Perrin (4) report a yield of 82 per cent by weight of 68 octane gasoline from the liquid primary products by the T. V. P. (true vapor phase) process. This yield does not include polymerization of any gases. I n the light of recent developments in refinery technology, it is probable that thermal processes are not the most suitable means of processing the synthesis product. The following are possible ways of treating the material: 1. The “Gaso1”-i.
e., the CS and Ca fractions together with
gases from cracking or reforming o erations-could be utilized
by catalytic dehydrogenation a n j catalytic polymerization. iilkylation could also be used on these components, If extraneous isobutane were available. 2. The light gasoline of about 175’ F. (SO0 C.) end point could be used as straight-run gasoline for blending. The heavy gasoline fraction could be catalytically reformed. This offers a possible source of benzene and toluene. 3. The fractions above 392” F. (200”C.) could be catalytically cracked. By these processes it is possible to convert the primary products into gasoline with a yield of about 85 per cent by volume. Of the gasoline produced, about 60 per cent could be used for aviation fuel after the addition of lead. The remaining 40 per cent is motor gasoline. A high-quality Diesel oil can be produced from the primary product. In addition to the straight-run Diesel oil a further production of Diesel oil is possible by a mild oracking of the wax fraction. The Diesel oil fraction has a cetane value of approximately 85, which permits it to be blended with lowgrade tar oils to produce high-grade Diesel oils. Tar oils such
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as creosote and anthracene oil are unsuitable for use as Diesel fuels but when blended in approximately equal amounts with the synthetic Diesel oil a high-grade fuel is produced. Part of the Fischer product can be utilized for the production of synthetic lubricating oil. There are two mays in which this can be done. Suitable paraffin hydrocarbons can be chlorinated and these compounds coupled with aromatic hydrocarbons. The other method involves the polymerization of heavy olefins by the use of aluminum chloride. The lubricating oils by polymerization are of high quality and are being produced commercially in Germany. When these oils are subjected to an oxidation test, they show a smaller formation of carbon but a larger increase in viscosity than lubricating oils from natural sources. The resistance t o oxygen can be improved by mild hydrogenation. The most interesting aspect of this process in the United
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States is its application to synthetic liquids produced from methane. This may be an important factor for the conservation of natural resources, since there is a large amount of natural gas available a t present.
Acknowledgment Acknowledgment is made to Anglo-Transvaal Consolidated Investment Co., Ltd., for permission to publish some of the information in this paper.
Literature Cited (1) Egloff, Nelson, and Morrell, IND.ENO.CHEM.,29, 555 (1937). (2) Fischer. J . Inst. Fuel, 10, 10 (1936); Wilke, Chem. Fabrik, 11, 563 (1938). (3) Fischer and Tropsch, German Patent 484,337 (1925). (4) Snodgrass and Perrin, J . Inst. Petroleum Tech., 24, 289 (1938).
Development of Rancidity in Stoddard Dry Cleaning Solvent ADRIAN C. SMITH, CHARLES S. LOWE,
AND GEORGE P. FULTON
National Association of Dyers and Cleaners, Silver Spring, Md.
The accumulation in Stoddard dry cleaning solvent of substances associated with rancidity was studied by chemical and physical methods. The increase in fatty acid content was found to be due largely to free fatty acids present in soap additions, rather than to fatty acids present in the soil from the garments, and could not be used as a criterion for rancidity without qualifications. The Kreis test, in a modified form, has been found useful in detecting incipient rancidity. Peroxides formed during the cleaning operations decomposed in the drying cabinet and gave rise to aldehydes and low-molecular-weight acids. Potentiometric titration curves on residues from used dry cleaning solvent indicate the buffering effect of these decomposition products.
N
UMEROUS studies have been carried out to determine
the nature, causes, methods of detection, and control of rancid odor developed in edible fats, soaps, textile oils, and similar products. The application of the results of such studies to the odor problem of the dry cleaner, mho must remove cheaply and efficiently soil which may contain any or all of these substances from all types of textile fabrios without permitting any odor to remain on the garments, has not
been undertaken. The necessity for such an investigation has materially increased within the past few years as a result of the marked tendency toward clarification of solvent by pressure filtration alone, without alkali treatment or vacuum distillation. Elimination or less frequent use of the latter methods of purifying the solvent permits a more rapid accumulation of products associated with rancidity, with subsequent odor trouble. The purpose of this paper is to describe a series of experimental cleaning operations carried out under controlled conditions and to correlate chemical and physical tests made on the solvent after each load with detection of objectionable odors on the garments.
The Dry Cleaning Process Owing t o lack of information in the scientific literature regarding the technique of dry cleaning, a brief resume of average cleaning practice with factors pertinent to this investigation is given here. CLEANING OPERATIONS.Mechanical action in a horizontal cylinder-type washer with reversing drive is employed to bring about intimate contact with the dry cleaning solvent and produce force necessary to dislodge loose soil. -4charge of 2 pounds of filter powder for each 100 gallons of solvent is added directly on the garments to increase the efficiency of filtration. This initial run with filter powder requires about 10 minutes and is known as the break. The solvent is circulated through the filter screens, previously coated with filter powder, from the start of the break run, in order to carry away loose soil as quickly as possible and thus minimize any tendency toward reabsorption of soil by the garments. The filter circulation is then shut off and soap is added, usually in the ratio of 1 pound of soap to 25 gallons of solvent. The amount of soap added varies somewhat with the type of
