Method for upgrading products of Fischer-Tropsch synthesis

#1. Field of the Invention

#This invention is concerned with a process for improving products of converting synthesis gas, i.e., mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures and oxygenates. In one aspect, this invention is concerned with a process for increasing the yield of hydrocarbon mixtures rich in aromatic hydrocarbons over that obtained in a known Fischer-Tropsch synthesis process. In still another aspect, this invention is concerned with utilizing a novel class of crystalline zeolite catalysts for improving the products obtained by the conversion of synthesis gas.

#2. Prior Art

#FIG. I is a condensed schematic block flow arrangement of a typical Fischer-Tropsch synthesis process directed to the conversion of coal to synthesis gas comprising carbon monoxide and hydrogen, the reduction of carbon monoxide by the Fischer-Tropsch Process to form a product mixture comprising hydrocarbon and oxygenates and the recovery of these products for further use.

#FIG. II is a condensed schematic block flow arrangement for improving the process of FIG. I by employing the concepts of the present invention.

#Referring now to FIG. I, there is shown in block flow arrangement a substantially reduced process flow arrangement of a Fischer-Tropsch syngas conversion process. A coal gasifier section 2 is provided to which pulverized coal is introduced by conduit 4, steam by conduit 6 and oxygen by conduit 8. The products of gasifier section 2 are then passed by conduit 10 to a gas scrubber section 12. In scrubber section 12, carbon monoxide and hydrogen are separated from hydrogen sulfide removed by conduit 14, carbon dioxide removed by conduit 16, tars and phenols removed by conduit 18 and ammonia removed by conduit 20. The carbon monoxide-hydrogen rich gas is passed from section 12 by conduit 22 to a partial combustion zone 24 supplied with steam by conduit 26 and oxygen by conduit 28. Recycle C.sub.2 fuel gas product of the combination process separated from carbon dioxide is recycled by conduit 30 to the partial combustion section 24. In the partial combustion operation of section 24, a carbon monoxide-hydrogen synthesis gas of desired ratio is formed for passage and use in a downstream Fischer-Tropsch synthesis gas conversion operation.

#The synthesis gas prepared as above briefly identified is passed by conduit 32 to a Fischer-Tropsch synthesis reaction section 36 containing a synthesis catalyst in admixture with C.sub.2 recycle gas introduced by conduit 34. Generally the synthesis gas feed is introduced at a temperature of about 160.degree. C. and at an elevated pressure of about 365 psig. The temperature of the synthesis gas passed in contact with catalyst in a fluid catalyst operation rapidly rises by the exothermic heat liberated so that the Fischer-Tropsch and water gas shift reactions take place. The products of the Fischer-Tropsch synthesis reactions are conveyed by conduit 38 to a primary cooling section 40 wherein the temperature of the mixture is reduced to within the range of 280.degree. to about 400.degree. F. In the primary cooling section a separation is made at a temperature which permits the recovery of a slurry oil with entrained catalyst fines as by conduit 42, and a decant oil stream boiling above about 400.degree. F. by conduit 44. In one typical operation, the decant oil stream will have an ASTM/95% boiling point of about 490.degree. F. A light oil stream boiling below about 500.degree. F. and more usually below about 450.degree. F. including lower boiling gaseous components and water containing oxygenates is passed by conduit 46 to a second or final cooling and separating section 48 maintained at a temperature of about 100.degree. F. and an elevated pressure of about 265 psig. In Section 48 a separation is made to recover a water phase comprising water soluble chemicals and oxygenates withdrawn by conduit 50, a light hydrocarbon oil phase comprising hydrocarbon and entrained oxygenates boiling from about C.sub.5 hydrocarbons up to 400.degree. F. and as high as about 500.degree. F. boiling hydrocarbons withdrawn by conduit 52. A vaporous phase rich in C.sub.5 hydrocarbon and comprising lower boiling gaseous components including C.sub.2 to C.sub.4 hydrocarbon are withdrawn by conduit 54. The vaporous phase also comprising unreacted carbon monoxide and hydrogen may be recycled by conduit 34 to the Fischer-Tropsch unit 36. In a typical operation about one volume of fresh feed is used with two volumes of the recycle gas. The lighter hydrocarbons do not completely condense and an absorber system is used for their recovery. Methane and LPG hydrocarbons are blended with other components for use in a pipe line system where desired. On the other hand they may be passed to a gas reforming section for conversion to synthesis gas and recycle as feed gas in the synthesis operation. The light oil hydrocarbon phase with entrained oxygenates in conduit 52 is passed through a water wash section 56 to which wash water is added by conduit 58 to remove water soluble oxygenate components. In wash section 56, water soluble oxygenates are removed and withdrawn therefrom by conduit 60. The water phases in conduits 50 and 60 are combined and passed to a chemicals recovery operation 62. The washed hydrocarbon phase is removed by conduit 64 and passed to a clay treater 66 along with a hydrocarbon fraction boiling below about 600.degree. F. recovered from the decanted oil phase in conduit 44 and a heavy oil product fraction recovered as hereinafter described. The hydrocarbon phase thus recovered and passed to this clay treating section is preheated to an elevated temperature of about 600.degree. F. before contacting a silica/alumina catalyst or clay composition retained in treater 66. This operation converts retained oxygenates and some alpha olefins in the hydrocarbon product thereby imparting a higher octane rating to gasoline material in these materials. The treatment also operates to remove harmful acids and other oxygenates retained in the hydrocarbon phase after the water wash. The clay treated hydrocarbon product is then passed by conduit 68 to a hydrocarbon separation reaction 70. A portion of the hydrocarbon vapors in conduit 54 not directly recycled to the Fischer-Tropsch conversion operation by conduit 34 is also passed to the hydrocarbon separation reaction 70. In the hydrocarbon separation section 70, a separation is made to recover a flue gas stream comprising C.sub.2 hydrocarbons and other components suitable for reforming to CO and H.sub.2 withdrawn by conduit 72. A portion of this material is passed through a CO.sub.2 scrubber 74 before recycle by conduit 30 to the partial combustion zone 24. A portion of the fuel gas may be withdrawn for other use by conduit 76. In separation section 70, a C.sub.2 olefin rich stream is recovered by conduit 78 for chemical processing as desired. A C.sub.3 to C.sub.4 rich hydrocarbon stream is withdrawn by conduit 80 and passed to catalytic polymerization in section 82. Polymerized material suitable for blending with gasoline product is withdrawn by conduit 84. A synthetic gasoline product fraction or naphtha of low octane is recovered by conduit 86 and a light fuel oil product such as a No. 2 fuel oil is withdrawn by conduit 88 and a heavier oil fraction is withdrawn by conduit 90 for admixture with the decant oil fraction in conduit 44. The blend of hydrocarbon product thus formed in conduit 44 and passed to separator 92 will boil in the range of about 400.degree. F. to about 1000.degree. F. This material blend is passed to separator section 92 wherein a separation is made to recover a fraction boiling in the range of from about 400.degree. to 650.degree. F. withdrawn by conduit 94 from a heavier waxy oil withdrawn by conduit 96.

#In this relatively complicated synthesis gas conversion operation and product recovery system, it is not unusual to recover a product distribution comprising 2% ethylene, 8% LPG, 70% gasoline boiling material, 3% fuel oil, 3% waxy oil and about 14% of materials defined as oxygenates.