Desorbing hydrocarbons from a molecular sieve with two different desorbing mediums

摘要

C 10-24 -straight chain hydrocarbons are isothermally separated in the vapour phase from mixtures thereof with branched chain hydrocarbons by selective adsorption on a molecular sieve, preferably of Type 5A, the adsorbed straight chain hydrocarbons being subsequently desorbed in two stages, firstly with at least one straight chain hydrocarbon having an average number of carbon atoms in the molecule in the range exhibited by the five lowest components of the feed, and secondly with a straight chain hydrocarbon having an average of 1-4 carbon atoms less in the molecule than are present either in the lowest component of the feed or in the first desorbing medium. As shown, a vaporized mixture of fresh feed and purge recycle is charged by line 3 to adsorbent vessel 30 containing a Type 5A calcium sodium aluminosilicate molecular sieve, the effluent mixture of light desorbent displaced (from the previous cycle second stage desorption) and unadsorbed branched chain hydrocarbons passing by line 5 to fractionator 35, separated light desorbent passing by line 20 and line 7 to subsequent stages. When the adsorption stage is complete, the vessel 30a is depressured by venting through line 9, and vessel 50a is then purged in counterflow with light desorbent introduced by line 8 to remove surface adsorbed hydrocarbons, purge effluent being recycled by lines 9 and 2 with feed. After purging, vessel 30b is repressured and heavy desorbent fed in counterflow by line 10, effluent leaving by line 13 to fractionator 50 where the required straight chain hydrocarbons are recovered as the bottom product by line 24, and heavy and light desorbent fractions are recycled by lines 25 and 23 respectively. Thereafter, light desorbent is passed by lines 7 and 6 in counterflow into vessel 30c, effluent from which is passed together with light desorbent from line 23 by line 27 to fractionator 45, where it is separated into light and heavy desorbents, both of which are then returned to the process by lines 21 and 22 respectively. Vessels 30, 30a, 30b and 30c may be separate vessels operated through successive stages, or may comprise one vessel worked cyclically. The process is suitably performed at 575-700 F., the adsorption stage at 5-80 p.s.i.g., and the desorption stages at 1-20 p.s.i.g. above the highest adsorption pressure. When the feed is kerosene and/or gas oil, the heavy desorbent is suitably n-decane and the light desorbent n-heptane or n-octane.