US 3140240 A
Description (OCR text may contain errors)
y 7, 1954 D. FOWLER, JR-
PROCESS FOR CARBONIZING COAL Filed July 21, 1960 m Y -m E MR w mm E T llllll F mv w D M 0m( S ow NN 8 mm United States Patent 3,140,240 PRUCESS FOR CARBONIZING COAL Donald Leigh Fowler, Jr., Pittsburgh, Pa., assignor to Consoiidation Coal Company, Pittsburgh, Pa, a corporation of Pennsylvania Filed July 21, 1960, Ser. No. 44,405 5 Claims. (Cl. 202-44) This invention relates to a process for carbonizing coal and more particularly to a process for introducing raw coal into a distillation vessel and removing volatilized constituents of coal from the distillation vessel.
This invention in certain aspects is concerned primarily with the introduction of raw coal and the removal of tar vapors from a distillation vessel that employs twin mixing screws to rapidly transfer heat from an inert heat carrier to the raw coal and autogenously evolve tar vapors from the coal. For example, a combustion process of this type is described in British Patent 793,518, entitled Method of Degasifying or Degasifying and Gasifying Pulverulent or Finely Granular Fuels.
Briefly, the above referred to British patent describes a process wherein coal, transported in a conventional manner, and an inert heat carrier, are intimately mixed in a distillation vessel in the absence of oxidizing gases. The intimate mixing of the inert heat carrier and the coal rapidly increases the temperature of the coal above the lower distillation temperature and tar vapors are rapidly evolved from the coal.
Throughout the present specification the terms tar and tar vapors are intended to include any of the constituents of the coal that are autogenously evolved as a vapor during the carbonization process. The term distillation temperature is intended to mean any temperature at which tar vapors are evolved from raw coal. The tar vapors evolved in the distillation vessel are highly concentrated and form the vapor phase in the distillation vessel.
The distillation vessel includes a pair of conveying Worms which both intimately mix the inert heat carrier and the raw coal and convey the same to an after-devolatilizer vessel. The volatile constituents in vapor form are withdrawn separately from the distillation vessel by means of a conduit extending upwardly therefrom.
Considerable operating difliculty has been encountered with the above described process due to plugging of the withdrawal conduit for the tar vapors. It is believed that substantial polymerization of certain fractions of the tar vapors takes place in the withdrawal conduit to form nondesirable liquids or solid coke particles therein. In addition to the polymerization of certain fractions of the tar vapors, substantial amounts of the inert heat carrier and partially devolatilized coal are entrained with the vaporized volatile constituents and deposited in the withdrawal conduit. The entrained solids being at an elevated temperature promote polymerization of certain of the volatile constituents in the withdrawal conduit so that the withdrawal conduit plugs with solid material and the apparatus is inoperative. In the above described process the inert heat carrier that is intimately mixed with the raw coal in the distillation vessel is at a temperature above the normal temperature employed in fluidized low temperature carbonization processes. For example, the inert heat carrier maybe at a temperature between 1000 F. and
3,149,240 Patented July 7, 1964 1400 F. It has been found when raw coal is intimately mixed with the inert heat carrier at this elevated temperature, rapid heat transfer occurs and the tar vapors literally flash off the coal. The tar vapors so evolved have different physical and chemical properties than the tar vapors evolved by either low temperature carbonization fluidization processes or high temperature carbonization processes. The liquid viscosity of the tars obtained by the previously described process is higher than the liquid viscosity of the tars obtained by low temperature carbonization. Further, it has been found that there is a larger fraction of the material that has a boiling point above 400 F. in the tars obtained by the previously described process. The difference in the physical and chemical properties of the tars obtained is believed to contribute substantially to the operating difficulties encountered in this process.
In addition to the operating difficulties previously described, there is a substantial heat loss through the walls of the distillation vessel. It has been discovered by positioning the distillation vessel within the dense phase of the fluidized bed of inert material at an elevated temperature in the treating vessel that not only is the previously discussed heat loss eliminated, but a beneficial heat transfer between the inert material in the dense phase of the fluidized bed and the walls of the distillation vessel is obtained.
The manner in which the raw coal is conveyed in the above described process is by conventional mechanical handling means. I have discovered by providing a slurry consisting essentially of particulate raw coal and water that the raw coal may be introduced into the distillation vessel with conventional pumping means employed to transport liquids. Further, the water in the slurry is vaporized in the distillation vessel and serves as a substantially inert diluent gas to lower the partial pressure of the tar vapors and further serves as a sweep gas to strip the tar vapors from the solids within the distillation vessel to thereby increase the operating efficiency of the carbonization process.
It has been discovered that by diluting the concentrated tar vapors within the distillation vessel with a substantially inert diluent gas and conveying the mixture of tar vapors and inert diluent gas into a treating chamber, where the entrained solids are separated from the tar vapors and diluent gas, and rapidly withdrawing the tar vapors from the treating chamber, that the operating difliculties encountered in the heretofore described process are minimized. It has further been discovered that by diluting the tar vapors with an inert diluent gas in the distillation vessel that polymerization of certain fractions of the tar vapors is minimized and coking or plugging of the transport conduits is substantially eliminated. The dilution of the tar vapors with an inert diluent gas lowers the partial pressure of the tar vapors to further reduce the possibility of tar condensation and increase the velocity of the tar vapors through the treating chamber and cyclone. The increase in velocity through the treating chamber reduces the residence time of the tar vapors at the high carbonization temperature and thereby minimizes polymerization of certain fractions of the tar vapors.
It is, therefore, a primary object of this invention to provide an improved process for carbonizing coal wherein polymerization of the tar vapors is minimized.
Another object of this invention is to provide an improved means for introducing raw coal into a distillation vessel wherein the raw coal is intimately mixed with inert heat carriers to rapidly evolve tar vapors from the coal.
Another object of this invention is to provide an improved process for carbonizing finely divided coal wherein a substantially inert diluent gas is generated in the distillation vessel to serve as a diluent for the tar vapors evolved therein.
These and other objects and advantages of this invention will be more completely disclosed and described in the following specification, the accompanying drawings and the appended claims.
The figure is a schematic representation of my improved process.
Referring to the figure, there is shown schematically apparatus for carbonizing finely divided coal which includes a distillation vessel 10, a treating vessel 12, a condensing vessel 14 and a combustion vessel 16. The distillation vessel has an outer housing 18, and a pair of mixing worms and 22 extending longitudinally therein. The mixing worms 20 and 22 are driven by a motor 24 and are arranged to rotate in the same direction and to intimately mix material fed into the distillation vessel 12. The worms 20 and 22 are self-cleaning and remove any material adhering to the blades as they rotate during the distilling or carbonizing operation.
A storage chamber 26 is connected to the distillation vessel 10 by means of a conduit 28 that has a regulating means such as slide valve 30 therein. The storage chamber 26 is provided for the inert heat carrier and serves as a hopper for the inert heat carrier which is at a temperature of between 1000 and 1400 F. A storage tank 32 contains an inventory of a stabilized coal-water slurry which may be prepared in a manner described in co-pending patent application Serial No. 9,884, now abandoned. The stabilized slurry consists essentially of particulate raw coal and water. The slurry exhibits the static property of retaining the larger sized particles in suspension under quiescent conditions and the dynamic property of being readily pumpable by means of conventional pumping apparatus. The percent solids concentration in the stabilized slurry can range from 60% solids by weight to 70% solids by weight or greater depending upon the amount of water vapor desired in the distillation vessel 10 and also depending upon the desired pressure drop in the transport conduits through which the stabilized slurry is pumped. The stabilized slurry is withdrawn from storage tank 32 through conduit 34 and introduced into the inlet side of pumping means 36. Valve means 38 in conduit 34 controls the rate of stabilized slurry fed to pump 36. The outlet of pump 36 is connected to conduit 40 which in turn transports the stabilized raw coal-water slurry to the distillation vessel 10.
Predetermined amounts of stabilized coal-water slurry and heated inert material are fed through respective conduits 40 and 28 into the distillation vessel 10. The mixing worms 20 and 22 are rotating at a predetermined speed and maintain the mixture within the distillation vessel for a predetermined period of time. The pitch of the mixing worms 20 and 22 is so designed that there is an intimate mixing between the inert heat carriers from chamber 26 and the raw coal in the coal-water slurry from the storage vessel 32. The inert heat carrier is at a temperature between 1000 and 1400 F. and rapidly transfers heat to the coal-water slurry within the distillation vessel 10 to first vaporize the water into steam and then to autogenously evolve a substantial portion of the volatile constituents or tar in the coal. The steam and the vaporized tar are discharged from the distillation vessel 10 through a conduit 42 that extends upwardly therefrom into the upper portion of the treating vessel 12. The partially devolatilized coal and the inert heat carrier are discharged from the distillation vessel 10' through conduit 44 extending downwardly therefrom.
The distillation vessel 10 has a portion positioned inside of the treating vessel 12. The treating vessel 12 has a gas inlet 46 adjacent its bottom portion to which a conduit 48 is connected. The conduit 48 admits inert gas such as recycle gas or the like at about between 700 and 900 F. into the treating vessel 12. An inventory of solids is maintained within the treating vessel 12. The solids include the inert heat carrier employed to devolatilize the raw coal in the distillation vessel 10 and the portion of the partially devolatilized coal withdrawn from distillation vessel 10. A dense phase fluidized bed of the solids is maintained within the treating vessel 12 and the inert gas entering through conduit 48 serves as the fluidizing gas. The partially devolatilized coal and inert heat carrier withdrawn through conduit 44 from distillation vessel 10 is introduced into the bottom of treating vessel 10 by means of the fiuidizing gas in conduit 43; The inert material and partially devolatilized coal is maintained within treating vessel 12 at an elevated temperature such as between 700 and 900 F. to further volatilize the volatile constituents in the partially devolatilized coal and to obtain char therefrom. A portion of the distillation vessel 10 is positioned within the dense phase of the fluidized bed in vessel 12 and the outer housing 18 is maintained at substantially the same or a higher temperature than the solids in the dense phase of the fluidized bed 12. As previously stated, the dense phase fluidized bed is maintained at an elevated temperature of about 700 to 900 F. to after-devolatilize the partially devolatilized coal withdrawn from the distillation vessel 10.
The treating vessel 12 has a first solids outlet 50 connected to a conduit 52 which conveys solids withdrawn from treating vessel 12 to combustion vessel 16. A regulating means such as a slide valve 54 is provided in conduit 52 to regulate the withdrawal of solids from treating vessel 12. A source of air at an elevated temperature of about 750 F. is introduced under pressure into the lower portion of combustion vessel 16 through conduit 56. The air conveys material entering combustion vessel 16 through conduit 52 to the storage chamber 26. In addition, the air supports combustion of a portion of the inert solids within the combustion vessel 16 to elevate the temperature of the solids within storage chamber 26 to a temperature of between 1100 F. and 1400 F.
A cyclone separator 70 is positioned in storage chamber 26 and separates the solids entrained in the combustion supporting gases. The solids are discharged through conduit 72 into the storage chamber 26 and the gas is discharged through conduit 74. Both the solids and the gas are at a temperature of between 1000 F. and 1400" F.
Where it is desired to minimize the ratio of inert heat carrier to raw coal in the distillation vessel 10, the slurry being transported through conduit 40 may be heated to an elevated temperature below the distillation temperature of the coal by means of the heat exchanger 76. The gases withdrawn from chamber 26 may be conveyed through conduit 74 to the heat exchanger 76 and discharged therefrom through conduit 78. With this arrangement the raw coal-water slurry enters heat exchanger '76 and may be heated to any desired temperature below the distillation temperature of the raw coal. The material entering heat exchanger 76 through conduit 40 is then conveyed from heat exchanger 76 to the distillation vessel 10. Where the slurry is heated above the vaporization temperature of the water, the raw coal and the water, as steam, is conveyed to the distillation vessel 10. Where desired a portion of the steam may be withdrawn in a conventional manner before the raw coal and steam are introduced into the distillation vessel 10.
The treating vessel 12 has a second withdrawal outlet 58 connected to conduit 60 with regulating means 62 through which inert material may be withdrawn through treating Fr! 9 vessel 12. The steam and tar vapors withdrawn through conduit 42 from distillation vessel enter the treating vessel 12 above the dense phase of the fluidized bed. The steam and tar vapors are diluted by the fluidizing gas passing upwardly through vessel 12 and further dilute the tar vapors evolved from the raw coal in distillation vessel 10. The inert gas, steam and tar vapors enter cyclone separator 64 where entrained solids are separated therefrom. The gases and tar vapor pass through conduit 66 and are transported to distillation vessel 14. The entrained solids pass downwardly through dip leg 68 through cyclone 64 and are discharged into the dense phase of fluidized bed 12.
The condensing vessel 14 may be a spray condenser which is adapted to separate the volatile constituents of the coal from the steam and inert gas.
It will be noted from the above description and the apparatus set forth in the figure that a novel means of introducing raw coal into distillation vessel 10 is disclosed. The raw coal is introduced into the distillation vessel in the form of a stabilized coal-Water slurry. An inventory of the slurry is maintained in storage tank 32 and transported through a conduit by conventional pumping means and introduced as a slurry into the distillation vessel 10. The raw coal, being introduced in slurry form, also provides an inert gas diluent in the distillation vessel 10. The water of the slurry is vaporized with the inert heat carrier in distillation vessel 10 and serves as a diluent gas for the tar vapors autogenously envolved from the raw coal by means of the rapid heat exchange between the raw coal and the inert heat carrier. The steam in distillation vessel 10 serves to reduce the partial pressure of the tar vapors and thereby minimize condensation of the tar vapors in the conduits. The steam in distillation vessel 10 serves as a sweep gas to remove tar vapors which may be entrapped within the particles of inert heat carrier or on the surface of the partially devolatilized coal. The steam serves to accelerate the withdrawal of the tar vapors from the distillation vessel 10 and the treating vessel 12 so that the residence time for the tar vapors at the elevated carbonization temperatures is minimized. This reduces the possibility of certain fractions of the tar vapors being polymerized and solidifying within the transport conduits.
The partially devolatilized coal and the inert heat carrier are withdrawn from the distillation vessel 10 and are introduced into the treating vessel 12 where further devolatilization of the partially devolatilized coal takes place. The partially devolatilized coal entering treating vessel 12 has been subjected to sufficiently high temperature in the distillation vessel 10 to deagglomerize the partially devolatilized coal. The agglomeration problem is, therefore, not present within treating vessel 12. The tar vapors evolved from the partially devolatilized coal in treating vessel 12 are entrained with the inert fluidizing gas passing upwardly through treating vessel 12 and quickly conveyed to cyclone separator 64 where entrained solids are separated therefrom. The tar vapors are thereafter conveyed with the diluent inert fluidizing gas through conduit 66 to condensing vessel 14.
The tar obtained by means of the apparatus illustrated in the figure has different physical properties than the tar obtained by fluidized low temperature carbonization. The tar obtained by the heretofore described process contains a substantial fraction that boils above 400 C. The viscosity of the tar is much higher than that obtained by fluidized low temperature carbonization. Set forth below for exemplary purposes only is Table I comparing the analysis of whole dry solids-free tar obtained by (a) flash carbonization employing a distillation vessel with twin screw mixers wherein an inert heat carrier is intimately mixed with the raw coal at a temperature between 1000 and 1400 F., and (b) a fluidized low temperature carbonization process.
Table I Analyses on Whole, Dry, Solids-Free Tar Flash Low Tem- TBP, C. Carbouperature ization Carbonizatlon Wt. Percent:
230 0. (147 C. at 50 mm. Hg) 10.8 14.3 230 x 300 (141 C. at 5 mm. Hg) 4. 4 7. 4 300 x 350 (185 C. at 5 mm. Hg)-.- 6. 3 10.4 350 x 400 (223 C. at 5 mm. Hg) 5.9 9. 5 400 x 425 (240 C. at 5 mm. Hg) 4. 9 58 4 +425 67. 7 G./cc. at 25 C 1. 210 1.104 Viscosity, Centipoises:
303 F 320, 000 21 3l4 88,000 18 342 12, 000 14 Yields, Wt. Percent MAF 00 Dry, Solids-Free Tar 35.0 13. 5 230 C. Tar Acids 1.79 .87 360 C. Distillate 7. 88 4. 56 +360 C. Pitch 27.1 8.9
Table II compares the size of the various fractions of coke oven tar, flash carbonization tar and low temperature carbonization tar.
Table II Comparison 0 Tar Properties Flash Low Tem- Coke Oven Carboniza- Tempera- Tar tion Tar Garbonization Tar 100. 0 100. 0 100. 0 -230 C. Tar Acids, Vol. Percent. 1 20-25 48 45 Specific Gravity:
230 C 0.983 0. 985 0.952 1. 047 1. 047 0.995 1. 098 l. 1. 2o 1. 317 214 1 ZOO/270 0. Fraction.
According to the provisions of the patent statutes, I have expained the principle, preferred construction, and mode of operation of my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.
I. The method of carbonizin g finely divided coal to produce therefrom a liquid distillate which comprises intro-- ducing said coal in the form of a slurry consisting essentially of finely divided coal and water into a distillation vessel comprising two worms rotating in the same direction and mutually cleaning one another, concurrently introducing an inert finely divided solid heat carrier into said distillation vessel, said inert heat carrier being at a temperature above the carbonization temperature of said coal, intimately mixing said slurry and said inert heat carrier by means of said two worms in said distillation vessel to generate a diluent gas therein by vaporizing the water in said slurry into steam and also by intermixing said coal With said inert heat carrier evolving a substantial portion of said distillate in vapor form, said steam serving as a substantially inert diluent gas for said distillate vapors autogenously evolved in said distillation vessel, withdrawmg said mixture of steam and distillate vapor from said distillation vessel, and separating said distillate vapor from said steam.
2. The method of carbonizing finely divided coal as set forth in claim 1 which includes positioning said distillation vessel in a treating vessel, said distillation vessel having a vapor withdrawal conduit extending upwardly therefrom, withdrawing a first stream comprising an admixture of treated coal and said inert heat carrier from said distillation vessel and introducing said admixture into said treating vessel, introducing an inert gas into said treating vessel at a suflicient velocity to fluidize said admixture and maintain a dense phase fluidized bed of said admixture in said treating vessel and further devolatilize said coal in said admixture, said distillation vessel positioned within said dense phase fluidized bed of said admixture in heat exchange relation With said admixture so that said admixture supplies heat to the external surface of said distillation vessel positioned in said dense phase fluidized bed, said vapor'withdrawal conduit extending above the dense phase of said dense phase fluidized bed, withdrawing said mixture of steam and distillate'vapor from said distillation vessel through said vapor withdrawal conduit, diluting said last named vapor mixture with said inert gas in said treating vessel, transporting said mixture of said inert gas, steam and distillate vapor from said treating vessel to a condensing vessel, and separating said distillate vapor from said steam and inert gas.
3. The method of carbonizing finely divided coal as set forth in claim 1 which includes providing a storage vessel for a stabilized slurry consisting essentially of particulate raw coal and water, transporting said stabilized slurry from said storage vessel by means of pumping said slurry through a conduit, and introducing said stabilized slurry from said conduit into said distillation vessel.
4. The method of carbonizing finely divided coal as set forth in claim 2 which includes maintaining said inventory of said mixture in said treating vessel for a suflicient length of time to evolve a quantity of said liquid distillate in vapor form from said treated coal, and transporting said distillate vapor evolved in said treating vessel therefrom with said inert fluidizing gas to said condensing vessel.
5. The method of carbonizing finely divided coal as set forth in claim 4 which includes withdrawing as a vapor mixture from said treating vessel said distillate vapors evolved in said distillate vessel, said steam, said distillate vapors evolved in said treating vessel and said inert fluidizing gas, transporting said last named mixture to a condensing vessel and separating said distillate vapor from said steam and said inert fluidizing gas.
References Cited in the file of this patent UNITED STATES PATENTS 776,171 Caldwell Nov. 29, 1904 1,478,864 Trent Dec. 25, 1923 1,554,587 Lamplough Sept. 22, 1925 1,573,824 Griffiths Feb. 23, 1926 2,382,809 Otto Aug. 14, 1945 2,474,251 Howe June 28, 1949 FOREIGN PATENTS 793,517 Great Britain Apr. 16, 1958 793,518 Great Britain Apr. 16, 1958