GB653740A - Improvements in or relating to the cracking of heavy hydrocarbons - Google Patents

Abstract

Heavy hydrocarbon oils, such as crude oil distillation-residues and tarry by-products of thermal cracking, are heated to a temperature in the range 800-900 DEG F. and applied to the upper part of a preheated, downwardly-moving, unagitated column of closely-packed particles of solid material having either an inert or a catalytic-cracking character, the rate of application of the oil being adjusted so that a temperature between 900 DEG and 1100 DEG F. is maintained in the column below the oil feed-level, and the contact time being adjusted, by controlling the rate of movement of the column, so that the oil is converted into non-tacky coke, which is deposited on the solid particles, and vapours of lower boiling hydrocarbons. A column of gravel (5 mesh, U.S. standard) initially at 970 DEG F. moves by gravity down chamber 1 (Fig. 1) which is divided into a coking zone 14, 15, 16, a purging zone 17, and a combustion <PICT:0653740/III/1> zone 18, 19, 20. A reduced oil of flash point 330 DEG F. preheated to 840 DEG F. is supplied to the coking zone through heater 26 and one of the spargers 21; the rate of feed being controlled, either manually or thermostatically, so that 6.7 lb. of oil is spread upon 24 lb. of gravel. The contact time is 8 1/2 minutes; and 87 per cent of the feed-oil is converted into lower boiling hydrocarbon vapours which are withdrawn through outlet 4, while 7 1/2 per cent is converted into coke which is deposited on the gravel. The vapours withdrawn through outlet 4 may be catalytically cracked to form gasoline. The coked-gravel is purged in zone 17 with steam or other gas supplied through distributer 17a. This purging zone also acts as a sealing zone by preventing vapour-flow between the coking and combustion zones. The coke is burned off the purged gravel with air supplied to the combustion zone through line 35 and distributers 33; the products of combustion being withdrawn through collectors 28 and passed to gas/solids separator 30. Coils 36 supplied through line 37 with a heat-exchange medium such as water, a molten salt, or the feed-oil itself, are disposed in the combustion zone. The heated water (steam) or salt may be used to heat the heater 26. In the case of water, the steam generated may be used to supply feed water to a boiler. The temperature in the combustion zone is controlled by regulating the admission of air or by means of coils 36. The temperature of the gravel leaving the combustion zone is controlled by the lower-most coil 36; and the hot gravel is returned to the top of the reaction chamber by elevator 7. When coke is a desired end-product, the use of coke as the column material is advantageous; <PICT:0653740/III/2> and in this case the combustion zone is only used to remove volatile matter; and the nett coke-make is removed from the system through pipe 10. The feed-oil may be introduced through several independently - controlled spargers 21. In another modification (Fig. 2), the reaction chamber 102 has three feed-oil spargers 108 by means of which both the temperature and rate of supply of the oil injected through each sparger may be independently controlled. Each sparger is provided with a thermostatically-controlled valve 110 responsive to an element 112 disposed in the coking zone; and each sparger is associated with a heat-exchanger 118 also controlled by thermostatic element 112. The speed of the column is controllable through variation of the speeds of conveyers 103 and 105. With proper control of the variables, the coke produced in the chamber may be applied in three superposed layers.