US2761825A

US2761825A - Fluidized coking

Info

Publication number: US2761825A
Application number: US259854A
Authority: US
Inventors: Theodore H Schultz
Original assignee: Sinclair Refining Co
Current assignee: Sinclair Refining Co
Priority date: 1951-12-04
Filing date: 1951-12-04
Publication date: 1956-09-04
Anticipated expiration: 1973-09-04

Description

Sept. 4, 1956 T. H. scHuLTz 2,761,825

FLUDIZED COKNG Filed Dec. 4, 1951 ATTORN EYS patented sept. 4, 1956 tic FLUrnrznn como Theodore H. Schultz, Munster, Ind., assignor to Sinclair Refining Company, New York, N. Y., a corporation of Maine Application December 4, 19571, Serial No. 259,854

1 Claim. (Cl. 2in-15) My invention relates to improvements in the art of coking hydrocarbon oils, such as those derived from crude petroleum to convert them into distillate stocks and a minirnum of residue in the form of coke. Application Serial No. 121,575, tiled October 15, 1949, of Kenneth M. Watson, now Patent No. 2,707,702, deals with coking operations in which a body of coke particles is maintained in a liuidized state in a reaction vessel, in which hydrocarbon oil is introduced into the uidized body of coke particles in the reaction vessel, in which a coking ternperature is maintained in the reaction vessel, in which vapors are taken off overhead from the reaction vessel, and in which large coke particles are selectively removed as formed from the lluidized body of coke particles by elutriation to maintain equilibrium. In these coking operations, the hydrocarbon oil charge stock is preheated to about 800 to 975 F. depending upon the nature of the stock and heater limitations, before injection into the coke bed. The discharge temperature from the heater coil must be limited to about 925 to 975 F. due to the increasing rate of formation of coke and coking materials in heavy oil charge stocks at high temperature. Since the economics of the coking process require relatively high temperatures in the reaction vessel, e. g. 950 to 1100 F., for good rates of reduction of residual charge stock and maximum production of distillate stock, and since the coking reaction is an endothermic reaction, heat from an extraneous source must be supplied to the reaction vessel. 'Ihe heat advantageously is supplied by employing superheated steam as a uidizing medium and dispersion medium for the heavy oil charge but at the higher coking levels, the volume of steam required may become uneconomical. Heated hydrocarbon stocks such as the eiiiuent from thermal cracking or reforming units may be utilized as heat carriers but this increases the size of equipment required and imposes additional burdens on the fractionating equipment. Also where the primary purpose of the coking operation is to produce charge stocks for catalytic cracking, it is undesirable to admix cracked distillates with the coker gas oil.

In the coking process of my invention the heavy oil charge stock is coked in the presence of a body of coke particles maintained in a uidized state at high coking temperature, and a substantial portion of the heat for the coking reaction is supplied by a hot olefin rich gaseous hydrocarbon stream.

According to my invention, a stream of gaseous hydrocarbons is separately preheated to an elevated cracking temperature for a restricted period of time to provide an lolefin rich gas stream which'is introduced into a coking reactor at a ow rate and in suhcient amount to maintain a high coking temperature in the coking reactor in which the coking stock is contacted with a body of coke particles maintained in a iluidized state. Large cokea'r'ticles lare selectively withdrawn from the uidized bed by elutriation and a vapor mixture is taken 'oli overhead from the coking reactor. Advantageously, the olen rich gas stream is employed as the uidizing medium. With particular advantage, the gaseous hydrocarbons comprise hydrocarbon gases other than methane, e. g, ethane to butane, and are subjected to rapid pyrolysis to produce a gas stream of maximum olefin content and minimum aromatic content. Within the coking reactor, time is prov vided for polymerization of olens to aromatics by passage :of the olen rich stream through the coke bed, which aromatics are recoverable from the condensed coking reaction products. Y

The pyrolysis ,of gaseous hydrocarbons to produce aromatics takes place thermodynamically in two stages after the gaseous hydrocarbons are heated to a cracking temperature. A rapid primary decomposition takes place with absorption of heat which is followed by slower aromatic forming reactions with evolution of heat. For example, when butane is heated at a cracking temperature under 2 atmospheres pressure for 0.15 second, vthe exothermic heat developed between 0.15 and 4.8 seconds is about 340 B. t. u. per pound.

In the process of my invention the gaseous hydrocarbons are cracked in a separate cracking zone where heat is absorbed to the limit of a theoretical maximum heat content corresponding to theY formation of the maximum concentration of gaseous olelins at which point they are injected into the coking reactor to provide suiiicient soaking time for the exothermic aromatics forming reaction. vhus the needfor employing large amounts of heated hydrocarbon vapors or Aexcessive quantities of high temperature steam is avoided by utilizing the exothermic heat of polymerization of oleins to aromatics to fuliill the requirement for extraneous heat which must be supplied s the coking reactor.

The stream of gaseous hydrocarbons is preheated to an elevated cracking temperature of about 1100 to 1600 F. under pressures of about 10 to 250 p. s. i. g. for a restricted period of time of less than about 0.2 second, preferably about 0.08 to 0.1 second. The resulting olen `rich gas stream Vis introduced into the -coking reactor at a ow rate and in sufficient proportion to the quantity of coking charge stock to maintain a coking temperature of about 950 to `ll00 F. Temperature control in the coking reactor may be elected in several ways, alone or in combination. For example, the proportions of olen rich gas and coking feed may be varied, or the residence time of the hydrocarbon gases in the heater and their soaking time in the coking reactor maybe regulated to vary respectively the amount of superheat and heat of polymerization given up by the hydrocarbon gases. In addition, the soaking time of the olefin rich gas stream in the coking reactor may be regulated to produce the maximum amount of benzene or a larger amount of tar with a corresponding increase in exothermic heat. Also the temperature at which the coking charge stock enters the coking reactor may be varied, as may be the level in the uidized coke bed at which the olefin rich gas stream is introduced. Gr another gas such as steam may be employed to furnish all or a part of the iiuidizing gas requirement. Thus temperature control may be readily adjusted after start up with, for example, superheated steam as the fluidizing medium.

Conventional gas cracking operations present the prob- Y lem of efficient recovery of the high heat content of the cracked gases. Handling and separation of the product mixture presents additional difficulties including disposal of by-product tars and carbon. My invention has definite economic advantages with respect to heat and product recovery in gas cracking operations. For example, the

excess heat including the heat of polymerization of the oletns in the gas stream to aromatics is recovered in the coking reactor and the heat content of the vapor eiiiuent from the coking reactor may be used to preheat the coking charge stock or to generate` steam for use as a uidizing or elutriating medium. The carbon and tars produced in v the gas cracking operation are collected in the coking reactor or reduced to coke in the coldng reactor by recycle of heavy ends from the fractionatingcolumn. Theqdesirable aromatics may be removedfrom the liquid coking reaction products by fractionation, For example, the vapor effluent may be fractionated to obtain a fraction containing mostly benzene which may berecovered therefrom as by extractive distillation with a selective solvent for aromatics. Valuable gases such as propylene and butylene also may be recovered from the fractionating units.

By way of further illustration, .I will describemyrimproved coking process as lcarried outfinthe equipment illustrated in the accompanying drawing.

The illustrated apparatus comprises a lreaction vessel 10, separate heaters 11 and 12 and fractionating

columns

14 and 15. Heater 11 may be a red heater of any conventional construction embodying oneor more series of connected tubes, the deFlorez heater 'for-example or it may be a conventional preheater of the heat exchange type. Heater 12 may be a conventional cracking heater containing a high temperature resistant tube coil of, for example, chromium-alloy steel or carbomndum. Conical grid 17 opening into the upper end of elutriator 18 is spaced just above the lowerend offreaction vessel r10. Reaction vessel is with advantagerelatively high with respect to itsdiarneter, as illustrated.

particles maintained in the reaction vessel above grid 17 is discharged from cracking 'heater 12 through connection 19 into the space below the 4grid and vpasses upwardly through the grid into the body o f lcoke particles. A hydrocarbon gas stream is introduced into cracking heater 12 by means of line 20. Elutriatingsteam, superheated, is introduced into the lower partof lutriator 18 through connection 21. The lower end of Velutriator 18 opens into coke hopper 22 from which coke is discharged through connection 23. Residual charge stock is introduced into reaction vessel 10 through heater 11 and connection 24. The overhead from Yreaction vessel 10 passes to the lower part of ractionating column 14 through connection 25. Pressures of 10 to 80 p. s. i. g., for example, may be maintained in the fractionating columns. Higher pressures maintained in the reaction vessel are reduced by means of valve 26 located at the discharge end of connection 25. Pressure in heater 11 may be regulated and reduced to reaction vessel pressure, if higher pressure is maintained in heater 11, by means of valve 27 in connection 24. Pressure in cracking heater 12 may be regulated and reduced to reaction vessel pressure, if higher pressure is maintained in the heater, by means of valve 28 in connection 19. The relatively high boiling distillate fraction derived from the residual stock supplied through connection 29 by the coking in reaction vessel 10 is recovered as high boiling stock discharged as a sidestream from fractionating column 14 through connection 30 andas a bottom stream from the lower end of fractionating column 14 through connection 31. lf any signiiicant amount of coke fines is carried over into column ,14 `from reaction vessel 10, the heavy stock discharged from the lower end of column 14 containing such nes may be reintroducedinto reaction vessel 10 through heater 11.V Noncondensable gas is withdrawn from fractionating column 14 byrmeans of v line 32. A hydrocarbon fraction boiling in the range of about 130 to 250 F. containing gasoline or naphtha and aromatics is withdrawny from column 14 as a sidestream through connection 33 and introduced into the lower part of fractionating column 15. A heavy naphtha fraction is Withdrawn from column by means of line34.

The overhead from fractionating column 15 containingr the products of the coking operation and the aromatics ofthe polymerization reaction boiling in the range of about Anolelin y hydrocarbon gas stream for uidizing the body of ,coke

150 to 185 F. pass through connection 16 to appropriate solvent extraction and recovery equipment not shown.

In carrying out the process of my invention in the apparatus illustrated, operation is initiated by charging reaction vessel 10 with coke particles, for example, from a previous run or ground to pass a screen of from 6 to 10 mesh per inch. This charge of coke particles may be, for example, about 9 feet in diameter and 22 feet high. The supercial velocity through the coke charge may approximate from 11/2 to 5 feet per second. A straight-rouvresidual stock, for example, a reduced crude is heated to a temperature short of that at which coke deposition in the heaterbegins within the time factor in the heater, '800 to 975 F., usually not above about 920 F., under a discharge pressure of l0 to 250 p. s. i. g., in heater 11. A stream of hydrocarbon gas other than methane, and advantageously containing olens is heated to a high cracking temperature, l100 to 1600 F., under a pressure of 10 `material produced by the cracking of the gaseous hydroto 250 p. s. i. g., during an exposure time of about 0.1 second in heater 12. The time factor in reaction vessel 10 approximates 10 to 50 seconds. The coking temperature is maintained in the reaction vessel without heating the residual stock to a temperature exceeding about 975 F., or that at which coke deposition within the heater begins, by means of the superheat of the olen rich gaseous hydrocarbon ei'lluent from high temperature cracking heater 12 and by means of the exothermic heat of polymerization of the olens to form aromatics. Good heat economy is eifected by this Autilization of the superheat and the heat of polymerization of the olefin rich gas stream to supply the substantial heat required for coking the Aresidual stocks and t0 maintain a highjreaction temperature in the reaction vessel. Coke building material derived from the residual stocks, and the small amount of such carbon, deposits on the coke particles, which as a result, grow as deposited material is coked. As the coke particles grow, larger particles are selectively discharged through elutriator 18 and connection 23, the admission of elutriating steam, superheated, through connection 21 being regulated with respect to discharge of coke pellets to maintain a substantially constant volume of growing coke particles in reaction vessel 10. The vapor mixture taken overhead from fractionating column 15 is a fraction boiling within the range of about to 185 F. This fraction is conventionally treated to Vremove aromatics therefrom as by extractive distillation with a selective solvent-for aromatics such as liquid sulfur dioxide.

Iclaim:

In -a method for ycokin'g'hydrocarbon oils by contact-` ing the coking stock at a high colting temperature with a body of coke particles maintainedv in a uidized staterin a coking reactor from which large coke particles are selectively withdrawn byfelutriation, the improvement which comprises separately preheating a stream of normally gaseous hydrocarbons to an elevated cracking tempera.

Iture of about 1l00 to 1600" F. under pressures of about l0 to 250 p. s. i. g. for a restricted period of time of less than about 0.2 second to provide an olefin rich gas stream,

introducing into the coking reactor the olefin rich gas stream at a ow rate and in sufficient amount to maintain by means of its sensible heat and it exothermic heat of ypolymerization a high coking temperature in the coking reactor, and withdrawing an overhead stream from the coliing reactor.

References 'Cited in the ille of this patent UNITED `STATES PATENTS 1,963,264 Egloi-et al Juue1'9, 1934 1,963,265 Fisher et al. June 19, 1934 1,972,944 Morrell Sept. 11, 1934 1,995,005 Morrell et al. Mar. 19, 1935 (0111er references on followingvpage) 5 UNITED STATES PATENTS Nelson Mar. 17, 1936 Angell July 28, 1936 Eglo et al. Nov. 24, 1936 Atwell Nov. 24, 1936 Hu Feb. 8, 1938 Angell Apr. 12, 1938 6 2,445,328 Keith July 20, 1948 2,511,088 Whaley June 13, 1950 2,707,702 Watson May 3, 1955 OTHER REFERENCES TY, Page