The UNIT PROCESS-THERMAL DECOMPOSITION* W. L. FAITH Kansas State College, Manhattan, Kansas
T
HE UNIT process, thermal decomposition or pyrolysis, has been largely neglected in treatises dealing with the teaching of chemical engineering unit processes. The only recent publications are one by Keyes (I) on a specific phase of the subject and a chapter on dry distillation of coal in Olsen's "Chemical Engineering Unit Processes and Principles" (2). The probable reason for this neglect is the complex nature of the chief raw materials involved: coal, wood, and petroleum. The few cases in which a simple transformation takes place such as dehydrogenation and dehydration of alcohols, and decarboxylation of phthalic acid are of only relatively minor importance. On the other hand, the r61e of thesmal decomposition in other unit processes cannot be overemphasized. Its importance in the catalytic oxidation of organic compounds in the vapor phase is an excellent example of this. At Kansas State College, the required unit process course is called "Organic Chemical Technology" which is the offspring of a former course called "Industrial Organic Chemistry." The course still includes descriptive material on the process industries (paper and pulp, rubber, petroleum refining, and so forth) but is outlined as nearly as possible according to the chemical engineering unit processes. The course is given to second-semester seniors and is preceded by inorganic chemical technology (one-half year) and a year's study each of elements of chemical engineering, organic chemistry, and physical chemistry along with the other usual courses. Thermal decomposition is the first unit process studied in this course. It is introduced by a brief review of the pyrolysis of organic compounds. In this review, emphasis is placed on the effect of time, temperature, pressure, concentration and catalysis on equilibrium, rate of reaction, type of reaction occurring, and so forth. Paraffin hydrocarbon decomposition is considered first. The chief reactions are dehydrogenation and condensation. Considerable data are available on the effect of the variables mentioned above in Hurd's "Pyrolysis of Carbon Compounds" (3). A study of the pyrolysis of olefins, naphthenes, and aromatic hydrocarbons follows. Next, pyrolysis of the alcohols offers an excellent example of the effect of catalysts on the
direction and extent of the reaction, through a consideration of dehydration and dehydrogenation. The aldehydes and ketones may he used to elaborate on the complexity of thermal decomposition, the acids to show decarboxvlation and dehvdration. Various miscellaneous decompositions of industrial importance, such as the decomposition of acetone to methane and ketene, are also considered. THERMAL DECOMPOSITION OF COAL
The pyrolysis of coal is introduced by a study of the raw material itself. An understanding of the various theories of coal formation enables the student to grasp more easily the significance of solvent extraction and other research methods on the chemical composition of coal. The physical structure should also he mentioned, since it has been shown that the banded ingredients (clarain, durain, fusain, and vitrain, or their American equivalents (4)) are important in blending coal for the coke oven (5,6). Knowledge of the type of molecules in coal, or those formed when the coal is heated to the plastic state, logically leads to a probable mechanism of pyrolysis during destructive distillation. This thermal decomposition can be divided into two types: (1) Fusion of coal and evolution of vapors. (2) Decomposition of these vapors in contact with hot coke or hot oven walls. A very reasonable explanation of these reactions (6, 7, 8) then leads to the compounds found in the tar and light oils subsequently extracted from the gas, as well as to the composition of the gas itself. In discussing this chemistry of coal carbonization, the d e c t of the five v a r i a b l e t i m e , temperature, pressure, concentration and catalysis-should be emphasized. Gluud in his "International Handbook of the ByProduct Coal Industry" (6), presents excellent material on this subject. Some of the other references a t the end of this article are also pertinent (4,9,10,11). The distribution of nitrogen and sulfur in the products of coal carbonization may also he predicated from simple reactions (6,7). Of course, in all this discussion, as well as that which follows, it is well to remember the remark made by Curtis (a),"Generalities in discussing * Presented before the Division of Chemical Enpineering of the coal carbonization are seldom ~ermissible." With this Society for the Promotion of Engineering Edication a t the in mind, however, one can diScuss coal carbonization, forty-fifth annual meeting in Cambridge, Massachusetts, June and yet realize the limitations of his theories. 29 to July 2, 1937. 279
After a discussion of the chemistry of a unit process the chemical engineer usually looks next for material and heat balances. Generalized balances (7, 12) may be given a t this point followed by a discussion of the effect of raw material, equipment design, and operation methods on product variation. Suggested outlines for this part of the class work are as follows:
A. Effect of raw material on products formed (factors to be considered) 1. Swelling tendency 2. Particle size 3. Moisture 4. Ash (13) 5. Banded ingredients (vitrain, clarain, fusain, durain) 6. Softening temperature 7. Degree of plasticity reached 8. Temperature of initial coke formation 9. Characteristics of volatile fraction 10. Rate of evolution of volatile products B. Effectof operation on products formed Consider the following factors: 1. Heating temperature a. High temperature carbonization b. Low temperature carbonization c. Middle temperature carbonization 2. Heating rate 3. Vacuum distillation 4. Time of heating below pasty stage (11) 5. Steaming C. Effectof equipment design on products formed (Factors to be considered) (14) 1. Method of applying heat 2. Path of travel of gases 3. Materials of construction of oven walls, and so forth 4. Height of oven (overheating of top) 5. Design of heatingflues (for uniform heating) 6. Length of flame (delayed combustion to prevent overheating of bottom) Obviously there is a certain amount of overlapping and variation of importance between the several items listed in the outline. The individual instructor is the best judge of the proper method of presentation. Continuing:
D. Heat flow and fuel economy 1. Path of heat flow 2. Resistances to heat flow a. Selection of refractories 3. Exo- or endothermic character of reaction 4. Type of heating gas 5. Recovery of sensible heat a. Waste heat boilers b. Coal tar stills c. Regeneration, recuperation, and so forth d. Dry quenching of coke 6. Design of heating flues
Commercial Coking Eguipment (for High Tempera1ure Carbonieation).-In discussing commercial equipment. it is difficult to determine just what one should include, or rather what one should exclude. Time available and interest in the subject are usually deciding factors. A suggested outline is: 1. Reauirements of coke ovens 2. ~ e & ttrends in coke oven design (15) 3. Vertical flue ovens a. Becker oven 4. Horizontal flue ovens a. Semet-Solvay oven 5. Gas retorts 6. Combination ovens a. Koppers combination coke and gas oven 7. Starting the coke oven.
A considerable amount of information is available concerning the three ovens mentioned (2, 6, 7, 14, 16, 17). For that reason they were selected in preference to others. The last item, "Starting the coke oven," is quite important. It has been found that students are very much interested in how all continuous or semicontinuous processes are started. In this particular case discussion of this phase aids in the understanding of coke oven construction. Material on this subject may be found in Gluud (106. cit., p. 335). Only general material is presented to the class on doors, pushers, automatic valves, and so forth. Emphasis is placed on the reasons for certain details of construction. Technical Processes.-This consists of a description of commercial by-product coke processes, of much the same nature as is presented in the usual textbooks on "Industrial Chemistry." Economic considerations and a brief history of the development of coal distillation should be included. LOW TEMPERATURE CARBONIZATION
(18)
Suggested outline: 1. Purpose a. Production of im~roveddomestic fuel b. Complete gasification c. More nearly complete utilization of coal 2. Difficulties and suggested remedies a. Cost b. Poor heat transfer c. Sticking of partially coked coal d . Quenching e. Poor coke i. Lack of uniform size ii. Lack of strength iii. Poor combustion characteristics 3. Oven types a. Static b. Semi-static c. Continuous 4. Description of technical oven a. Salerni system
1. Purposes 2. Types of ovens a. Atmospheric i. Knowles oven (20) b. Pressure i. Bluemner or Carbon01 COAL-TAR
DISTILLATION
The thermal decomposition of coal has not been adequately treated until one discusses the distillation of one of its derivatives, coal-tar. The purpose of coal-tar distillation is to obtain as high a yield of oil as possible. Of course, if coal-tar distillation was a simple physical separation, i t would have no place in this paper. The distillation is not this simple since both gas and pitch are formed a t the expense of the oil yield, evidence in itself of thermal decomposition. Weiss (21) has published an excellent paper on this subject and the following teaching outline was derived from his article.
A. Variation of oil yield (effect of important variables) 1. Temperature 2. Rate of heating 3. Time 4. Agitation a. Steam b. Air 5. Vacuum 6. Inert gas recirculation B. Types of stills 1: Simple batch still still 2. Steam-agitated " 3. Vacuum still 4. Gas re-circulation still 5. Pipe still 6. Reilly coke still 7. Barrett coke-oven still
I. H a r d 4 Distillation Recent; literature on the destructive distillation of wood is practically nil, particularly in regard to retort construction and thermal reactions. This is undoubtedly due to the uncertain status of the industry. Nevertheless, as long as there is a demand for charcoal, the industry will not die, and a general knowledge of the problems involved should be a part of the equipment of every student chemical engineer. This subject should be introduced by a consideration of the economics of the wood distillation industry, if for no other reason than to increase student interest. Next, the chemistry of the thermal decomposition of wood is introduced by a consideration of the composition of the raw material.
Not a great deal of information is available which deals strictly with the pyrolytic reactions involved, hut Hawley in Chapter I1 and I11 of his "Wood Distilation" (22) presents an excellent rGsum6 of that which is known. In addition to this, if we consider that methyl alcohol and acetic acid are the primary products of wood distillation, many of the other compounds formed may he predicted as the result of interaction between these molecules and their derivatives. Also, some workers (23) have attempted to show that methyl alcohol is formed from the methoxy groups of the ligno-cellulose, so this possibility should he considered. Conyers (24) and Liddell (9) both present brief reviews of the chemistry of wood distillation. The effect of important variables on the quantity and quality of products formed may then be studied according to the following outline: 1. 2. 3. 4. 5. 6.
Temperature Time Pressure (23) Presence of steam Rate of distillation Moisture 7. Catalysts 8. Composition of wood 9. Retort design
An excellent discussion of these factors may be found in Chapters I1 and 111 of Hawley's book (106. cit.). Liddell (9) also presents pertinent material. Next should be considered "Heat Transfer and Fuel Economy" under the following headings:
1. Exothermic character of the reaction 2. Control of temperature during distillation 3. Mechanism of heating (Hawley, p. 59) (Conduction, convection, radiation) 4. ~ f f e cof t moisture 5. Control of firing a. Appearance of distillate b. Temperature of vapors c. Regular schedule 6. Heat utilization (25) a. Burning retbrt gas b. Tar as boiler fuel c. Sensible heat for me-driers d. Proper distribution of burning gases 7. Importance of temperature control a. Constant and increased by-product yield b. Regularity of operation c. Avoidance of overheating (retort repairs) 8. Typical heat balance (26) A . Commerciel Retorts 1. Horizontal a. Rectangular oven (Jumbo with buggies) b. Cylindrical retort 2. Vertical a. Fixed retort 6. Removable retort c. Fixed retort with removable cage
3. Continuous retorts a. Badger-Stafford retort 4. Materials of construction The classification of retorts listed above was made up from Roger's "Manual of Industrial Chemistry" (7). Of those mentioned, the only ones which are presented to the class in any detail are the Jumbo oven and Badger-Stafford retort (27).
B. Technical Processes Typical commercial wood distillation processes are described. The old lime-acetate process is barely mentioned. Emphasis is placed on the Suida process, and some consideration is given to the method recently described by Othmer (28).
II. Resinous Wood Distillation A typical outline of this subject is as follows: A. Composition of raw material B. Probable course of reactions 1. Effect of rosin content C. Choice of retort 1. Effect of retort size on corrosion 2. Economic considerations (labor, fuel, depreciation, and so forth) 3. Internal flues vs. external heating 4. Vertical vs. horizontal retorts 5. Withdrawal of tar or pitch 6. Type of process a. Destructive distillation b. Steam distillation 6. Steam-solvent process D. Typical commercial process
Since there is so much variation from plant to plant within the industry itself, i t is practically impossible to go into much detail. Information which will fit the outline may he found in Hawley (22), Rogers (7),and others (16, 17). THERMAL DECOMPOSITION OF PETROLEUM
Presentation of a detailed outline on the thermal decomposition of petroleum would be in a great measure only a repetition of Keyes' excellent article previously cited (1). An outline quite similar to those presented for pyrolysis of coal and wood could be prepared very easily from this material. There should, however, be some information added to that presented by Keyes, particularly on polymerization of crackina still Eases. This subiect riphtfullv belongs under therm2 decomposition; since it occurs along with the cracking reaction. By only a slight change of conditions, one can shift from one type of reaction to another. Pertinent articles on polymerization are cited a t the end of this paper (29,30, 31,32, 33, 34). MISCELLANEOUS REACTIONS
When time permits or local interest is sufficient, it is advisable to present several minor processes such as the thermal decomposition of shale (9). At Kansas State College research is being camed out on the thermal decomposition of straw and other farm waste products. Some of the data obtained are usually presented to the class.
LITERATURE CITED
(1) KEYES,D. B.. "Cracking of petroleum-an example of the methods of teaching the fundamentals of unit processes," Tmns. Am. Inst. C h m . E n"m . .. 32. 472-92 (1936). , OLSEN,J. C., "Unit processes and principles of chemical engineering." D. Van Nostrand Co., Inc., New York City,
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HURD, L. C., "Pyrolysis of ckbon compounds," The Chemical Catalog Co.. New York City, 1929. CURTIS,H. A., "Present day knowledge of coal,'' J. Soc. C h . Ind.. 51. 350-6 (19321. ROBERTS.J.; "~lending: w i t h special reference to the Davidson Rotary Retort, PROCEEDINGS OF THB T ~ R INTERNATIONAL CONPERENCE ON BITUMINOUS COAL, 334-51 ... . . (1!231>~ \----,. G ~ u u n , W., "By-product coke industry." (American The Chemical Catalog Co., Edition by D. L. JACOBSON), New York Citv. 1932. ROGERS,A,, "hianual of industrial chemistry," 5th Ed., D. Van Nostrand Co., Inc., New York City, 1931. LOWRY,H. H., "Thermal decomposition of coal hydrocarban," Ind. Eng. C h . , 26, 3 2 0 4 (1934). LIDDELL,D. M., "Handbook of chemical engineering," McGraw-Hill Book Co., Iuc., New York City, 1922, Vol. 11. LOWRY, H. H., "The chemical: coal," Ind. Eng. C h . , 26, 133-9 (1934). PARR, S. W., "Low-temperature carbonization of coal," ibid., 21, 1 6 4 4 (1929).
(12) BAUM.K.. "Fuel economy in Germsn coke plants," T m m INTERNATIONAL CONFERENCE oa BITUM~NOUS COAL, 50740 (1931). ~ - ~ - - , ~ LAVINE, I., "Progress in low-rank coals," Ind. Eng. Chem., 26, 151-64 (1934). PORTER.H. C:, "Iiprovement of design of coal-carbonizing equipment," ibid., 24, 1363-8 (1932). PORTER, H. C.. "Progress in coal carbonization, gas-making and by-product recovery," ibid., 26, 1 5 0 4 (1934). RIEGEL, E. R., "Industrial chemistry," 3rd Ed., The Chemical Catalog Co.. New Y a k City, 1937. READ,W. T., "Industrial chemistry," 2nd Ed., John Wiley D & Sons. Inc.. New York Cifv. 1938. "Symposium i n low-temperafure carbonization of coal." CONFERENCE BITUMINOUS COAL, THIRDINTERNATIONAL pp. 272494, 1931. BROWNLIE.D., "Carbonization of coal-oil mixtures, Ind. Enz. C h . , 28. 629-35 (1936). M c B ~ D ER. , S., i ' ~ r o ~ e ~ s coal i d g in Knowles coke oven," C h m . Met. Eng., 42, 3 0 0 3 (1935). WEISS. J. M., "The distillation of coal tar," J. Soc. Chem. Ind., 51, 219-23, 246-50 (1932). L. F.,''Wood distillation." The Chemical Cataloe HAWLEY, Co., New York City, 1923. Fnoucn, P. K., H. B. SPALDING, AND T. S. BACON, "Destructive distillation of wood and cellulose under pressure," Ind. Ene. C h . . 20. 3 6 4 0 (19281. F. G.; "The dirtiilation bf G o d " (excerpts CONYERS. from address) J. Soc. Chem. Ind., 48, 88-9 (1929). ~~
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MCBRIDE,R. S., "Chemical engineering problems in hardwood distillation," C h . Met. Eng.. 39, 604-8 (1932). BENSON,H. K.. T. G. THOMPSON, AND G. S. WILSON, "The chemical utilization of wood in Washineton." Bull.No. 19, Uniu. of Washington Eng. Exfit. Sla.(1523): NELSON,W. G., "Waste-wwd utilization by the BadgerStafford proees-he Ford woad-distillation plant at Iron Mountain," Ind. Eng. Chcm., 22, 312-5 (1930). OTHMER. D. F.. "Acetic acid and a orofit from wwd disti~~atidn."Chkm. Met. Eng., 42, 356-61 (1935). KEITH, P. C., JR., AND J. T., WARD,"Motor fuels of high octane-blending value produced by thermal process," Oil Gas J., 34, No. 28,3640 (1935).
"Symposium on polymerization," Ind. Eng. C h . ,27,105581 (1935). . . COOKE.M. B., H. R. SWANSON, AND C. R. WAGNER, "Thermal process for polymerizing oleiin-bearing gases." Ref.Natural Gasoline Mfr.. 14, 497-505 (1935) IPATIEPP. V. N. AND H. PINES, "Propylene palymerization," Ind. Eng. Chem., 28, 684-6 (1936). V N.AND B. B. CORWS,"Gasoline from erhylcnc IPATIEPF. hy catalytic polyrneri~atiun,"ihid.. 28, Rfi0-l (1936). EOLOPP, G ''Polymer gasoline," ibid.. 28, 1461-7 (1936).
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