| 摘要 |
A method for the separation of para-xylene from an 8C aromatic charge by means of an adsorption stage, a liquid phase isomerization stage without addition of hydrogen and a vapor phase isomerization of ethylbenzene.- DETAILED DESCRIPTION - The initial charge contains ortho, meta and para-xylenes and ethylbenzene. The adsorption stage (18) comprises a mobile bed containing zeolite with a charge from which ethylbenzene has been separated by distillation (3) or adsorption. A liquid phase isomerization without hydrogen (26) is carried out on the raffinate after which the isomerate is distilled (27) to recover the toluene (29) which is re-cycled. The separated isomerate passes to a xylene distillation column (9) then re-cycled to the adsorption stage. The ethylbenzene, separated in column (2) is separately isomerized (40) with hydrogen at a higher temperature then stabilized and distilled (5,2,9) and re-cycled to the adsorption stage. The residue from (5) is mixed with incoming fresh 8C charge (1). Pure para-xylene is recovered as extract (22) which can then be further purified by crystallization (7).TF - TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The ethylbenzene content of the second fraction from the adsorption stage (depleted in para-xylene and containing toluene, meta and ortho xylenes and ethylbenzene) is preferably 5 - 8% wt. The first, liquid phase isomerization, is carried out at 200 - 260degreesC., 20 - 30 bar and toluene/isomerization charge less than 15%, preferably 10 - 12%. The zeolite catalyst is ZSM5 and the VVH is less than 10h-1, preferably 2 - 4h-1 The column (2) contains a mixture of xylenes and ethylbenzene and is regulated so that at least 75% of ethylbenzene is recovered as distillate : the residue passes to a column (9) and a distillate containing the xylenes is returned to the adsorption stage and a second residue (containing C9+ hydrocarbons and ortho-xylene) passes to a rerun column (12) from which a distillate of ortho-xylene is re-cycled to the first isomerization zone. The distillate from (2) is isomerized in the vapor phase with H2 and the isomerate obtained is distilled in a stabilization column (5) to eliminate light fractions, then re-cycled to (2). The isomerization is effected at 350 - 480degreesC., 5 - 20 bar and a VVH less than 10h-1, preferably 0.5 - 4 h-1. The catalyst contains an acid phase and a group VIII metal and the H2/hydrocarbons ratio is less than 10, preferably 3 - 6.Alternatively, a catalytic dealkylation can be carried out at 420 - 460degreesC., 10 - 25 bar, preferably 14 - 20 bar, VVH of 10 - 12, and with a zeolite catalyst having an acid function and a hydro-dehydrogenating metal, preferably of group VIII. The H2/hydrocarbons ratio is 1.2 - 2.2, preferably 1.5 - 2.0. The mobile adsorption bed comprises a zeolite containing an element from K, Rb, Cs, Ba,Ca and Sr, and optionally water. The distillate from (2) or the fraction from adsorption contains at least 85% ethylbenzene, preferably 90%
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