GB1428174A - Production of cracked gases from liquid hydrocarbon - Google Patents

Abstract

1428174 Fluidized bed thermal cracking UBE INDUSTRIES Ltd 14 March 1973 [7 April 1972] 12219/73 Heading C5E Gases, e.g. fuel gas, hydrogen or olefins such as acetylene, ethylene and propylene, are obtained by the thermal cracking of liquid hydrocarbon, e.g. crude, heavy or residue oil, in a unitary fluidized bed of solid particles having a central zone for cracking a liquid hydrocarbon starting material to gases, an upper zone for quenching cracked gases and a lower zone for cracking liquid hydrocarbon recovered from the cracked gases, the central zone communicating with the upper and lower zones through narrow passages or neck zones; oxygen, a first portion of steam as a fluidizing gas, a second portion of steam as a jet stream and a first portion of recovered liquid hydrocarbon is fed into the lower zone from the bottom to fluidize the particles and partially burn the recovered liquid hydrocarbon to produce cracked gases; a first portion of the liquid hydrocarbon starting material is sprayed and a third portion of steam is fed as a fluidizing gas into the central zone while spouting steam, cracked gases and solid particles from the lower zone into the central zone to crack the liquid hydrocarbon starting material to gases; a second portion of the liquid hydrocarbon starting material and/or a second portion of the recovered liquid hydrocarbon is sprayed into the upper zone while spouting the mixture of steam, cracked gases and solid particles from the central zone into the upper zone to quench the gases and cause a part of the carbon, tar and heavy oil associated with the gases to adhere to the solid particles; the resultant gas mixture is removed from the upper zone and the remaining tar, carbon and heavy oil which together constitute the recovered liquid hydrocarbon which is recycled to the process are separated, and contaminated solid particles are returned from the upper zone to the lower zone. The particles are inert and refractory particles with sufficient mechanical strength, have an average diameter of 0À5 to 5À0 mm., and some of the particles in the central zone are forced to recycle to the lower zone. The amount of contaminated particles under forced circulation from the upper zone to the lower zone per hour is about 15% of all the particles. Oxygen may be fed to the central zone to effect a partial combustion of the liquid hydrocarbon starting material to crack or promote the cracking of the liquid hydrocarbon. The gas mixture from the upper zone may be separated by a cyclone into purified gases and carbon, and the purified gases further separated in a distillation column into refined gases and heavy oil fraction and tar, the separated carbon, heavy oil fraction and tar being recycled to the fluidized bed as the recovered liquid hydrocarbon. The partial combustion of the recovered liquid hydrocarbon in the lower zone is carried out at 1050‹ to 1400‹ C., the thermal cracking of the liquid hydrocarbon starting material in the central zone takes place at 700‹ to 900‹ C., and the cracked gas mixture is quenched in the upper zone to 400‹ to 550‹ C. The superficial velocity (defined) of the whole gas in the lower zone, and possibly the central zone, is adjusted to within 1À3 to 6À0 times, and in the upper zone to 1À3 to 4À0 times the minimum fluidizing velocity of the solid particles; the gas velocities (defined) in the narrow neck zones are 1À1 to 4 times the terminal velocity of the solid particles; and the gas velocity of the second portion of steam as a jet stream is adjusted to 2 to 10 times the superficial velocity of the whole gas in the lower zone, and the steam is 10 to 30% by volume of the whole amount of the gas supplied to the lower zone. The quantity of the first and second portions of steam used as a fluidizing gas in the lower zone is 0À5 to 1À2 parts and the quantity of oxygen supplied to the lower zone for effecting fluidization of the solid particles and a partial combustion of the recovered liquid hydrocarbon is 0À5 to 2 parts/l part recovered liquid hydrocarbon sprayed into the lower zone; and the quantity of the third portion of steam fed to the central zone is 0À1 to 0À5 parts/1 part liquid hydrocarbon starting material; all parts being by weight. The effluent gas may be hydrogenated over a nickel catalyst to produce town gas. A furnace for performing the cracking has a central chamber 1b with constricted upper and lower portions communicating with the constricted lower portion of the upper chamber la and the constricted upper portion of the lower chamber 1c; the upper chamber is provided with an upper gas exhaust port 18, a solid particle outlet 21 at an upper region and an injection opening 20 for liquid hydrocarbon at a lower region of the chamber; the central chamber has a first perforated conical plate distributor 25 for fluidizing gas forming the constricted lower portion of the chamber and having an injection opening 27 for liquid hydrocarbon starting material; the lower chamber has a second perforated conical plate distributor 29 forming a constricted lower portion of the lower chamber, for a first portion of steam as a fluidizing gas, the apex of which issues a jet stream of a second portion of steam into the lower chamber, an injection nozzle 31 for recovered liquid hydrocarbon in the region of the apex, and a solid particle inlet 37 connected by passage 8 to the outlet 21 of the upper chamber for recycling solid particles. A passage 7 for recycling solid particles may be provided between the central and lower chambers via outlets 23 and inlet 35. Communication between the constricted portions of the chambers may be by narrow pipe A, B of height, e.g. 0À2 to 0À4 times the height of the lower chamber. The constricted lower portions of the chambers are conical with a conical angle from 60 to 120 degrees. The injection opening in the central chamber may be a two concentric pipe construction, the inner for liquid hydrocarbon starting material and the outer for additional steam as means for spraying the liquid hydrocarbon starting material, or a three concentric pipe construction, the inner for feeding liquid hydrocarbon, the intermediate for steam, and the outer for additional steam. The injection opening in the upper chamber and the injection nozzle in the lower chamber may be of single pipe or two concentric pipe construction, the inner for liquid hydrocarbon and/or recovered liquid hydrocarbon respectively and the outer for additional steam. The first portion of steam and oxygen may be fed from the second conical distributor while the third portion of steam is fed from the first conical distributor, and oxygen may be additionally fed from the first conical distributor.