This invention relates generally to the isomerization of hydrocarbons.  This invention relates more specifically to the isomerization of light paraffins using a solid catalyst and the separation of more highly branched paraffins from less highly branched and non-branched paraffins by liquid phase adsorptive separation and distillation. 
High-octane gasoline is required for modern gasoline engines.  Formerly it was common to accomplish octane number improvement by the use of various lead-containing additives.  As lead is phased out of gasoline for environmental reasons, it has become increasingly necessary to rearrange the structure of the hydrocarbons used in gasoline blending in order to obtain high-octane levels.  Catalytic reforming and catalytic isomerization are two widely used processes for this upgrading. 
A gasoline blending pool normally includes C4 and heavier hydrocarbons having boiling points of less than 215xc2x0 C. (419xc2x0 F.) at atmospheric pressure.  This range of hydrocarbons includes C4-C6 paraffins and especially the C5 and C6 normal paraffins that have relatively low octane numbers.  The C4-C6 hydrocarbons have the greatest susceptibility to octane improvement by lead addition and were formerly upgraded in this manner.  Octane improvement can also be obtained by using isomerization to rearrange the structure of the paraffinic hydrocarbons into branched-chain paraffins or reforming to convert the C6 and heavier hydrocarbons to aromatic compounds.  Normal C5 hydrocarbons are not readily converted into aromatics, therefore, the common practice has been to isomerize these lighter hydrocarbons into corresponding branched-chain isoparaffins.  Although the C6 and heavier hydrocarbons can be upgraded into aromatics through hydrocyclization, the conversion of C6""s to aromatics creates higher density species and increases gas yields with both effects leading to a reduction in liquid volume yields.  Therefore, it is common practice to charge the C6 paraffins to an isomerization unit to obtain C6 isoparaffin hydrocarbons.  Consequently, octane upgrading commonly uses isomerization to convert C6 and lighter boiling hydrocarbons to higher octane C6 isoparaffin hydrocarbons and reforming to convert C7 and higher boiling hydrocarbons to higher octane aromatic and isoparaffin hydrocarbons. 
The effluent from an isomerization reaction zone will contain a mixture of more highly branched and less highly branched paraffins.  In order to further increase the octane of the products from the isomerization zone, normal paraffins, and sometimes less highly branched isoparaffins, are typically recycled to the isomerization zone along with the feed stream in order to increase the ratio of less highly branched paraffins to more highly branched paraffins entering the isomerization zone.  A variety of methods are known to treat the effluent from the isomerization zone for the recovery of normal paraffins and monomethyl branched isoparaffins for recycling these less highly branched paraffins to the isomerization zone. 
U.S. Pat.  No.  2,966,528 B1 issued to Haensel discloses a process for the isomerization of C6 hydrocarbons and the adsorptive separation of normal hydrocarbons from branched chain hydrocarbons.  The process adsorbs normal hydrocarbons from the effluent of the isomerization zone and recovers the unadsorbed hydrocarbons as product, desorbs straight chain hydrocarbons using a normal paraffin desorbent, and returns the desorbent and adsorbed straight chain hydrocarbons to the isomerization zone. 
U.S. Pat.  No.  3,755,144 B1 shows a process for the isomerization of a pentane/hexane feed and the separation of normal paraffins from the isomerization zone effluent.  The isomerization zone effluent is separated by an adsorbent separation zone that includes facilities for the recovery of desorbent from the normal paraffin containing stream that is recycled to the isomerization zone.  An extract stream that contains isoparaffins is sent to a deisohexanizer column that separates isopentane and dimethyl butane as a product stream and provides a recycle stream of less branched isohexane that is returned to the isomerization zone. 
U.S. Pat.  Nos.  4,717,784 B1 and 4,804,802 B1 disclose processes for the isomerization of a hydrocarbon feed and the use of adsorptive separation to generate normal paraffin and monomethyl-branched paraffin recycle streams.  The effluent from the isomerization zone enters an adsorbent separation zone that contains a 5A-type sieve and a ferrierite-type sieve that adsorb normal paraffins and monomethyl-branched paraffins, respectively. 
U.S. Pat.  No.  4,804,802 B1 discloses steam or hydrogen as the desorbent for desorbing the normal paraffins and monomethyl-branched paraffins from the adsorption section and teaches that steam or hydrogen may be recycled with the normal paraffins or monomethyl-branched paraffins to the isomerization zone. 
One method of separating normal paraffins from isoparaffins uses adsorptive separation under liquid phase conditions.  In such methods, the isomerization effluent contacts a solid adsorbent having a selectivity for normal paraffins to effect the selective adsorption of normal paraffins and allow recovery of the isoparaffins as a high-octane product.  Contacting the normal paraffin-containing adsorbent with the desorbent material in a desorption step removes normal paraffins from the adsorbent for recycle to the isomerization zone.  Both the isoparaffin and normal paraffin-containing streams undergo a separation for the recovery of desorbent before the isoparaffins are recovered as a product and the normal paraffins recycled to the isomerization zone.  Liquid phase adsorption has been carried out in conventional swing bed systems as shown in U.S. Pat.  No.  2,966,528 B1.  The use of simulated moving bed systems for the selective adsorption of normal paraffins is also known and disclosed in U.S. Pat.  No.  3,755,144 B1.  Simulated moving bed systems have the advantage of increasing recovery and purity of the adsorbed and non-adsorbed components in the isomerization zone effluent for a given unit of adsorbent material. 
In liquid phase adsorption systems, the adsorbent contains selective pores that will selectively adsorb at least one component in the feed mixture.  The selective pore volume is limited and the quantity of such pores must accommodate the desired volume of components to be adsorbed from the feed mixture.  The desorbent material is also a selectively adsorbed component.  Therefore, an extract column is typically used to recover desorbent, otherwise any desorbent that passes through the reactors of the isomerization zone and enters the adsorption section increases the amount of adsorbed component in the feed mixture and requires additional adsorbent.  If the quantity of selectively adsorbed components is increased without increasing the available selective pore volume for a given unit of feed, it was is believed that the purity of the extract and raffinate streams from the adsorption section decreased.  Therefore, the extract column has been viewed as necessary for the desorption stage of the adsorption section since the loaded adsorbent contains normal paraffins and desorbent material as adsorbed components and all of these adsorbed components must be displaced by the desorbent.  Without the extract column, desorbent flow during the desorption step would increase if traditional desorbent to pore volume ratios are maintained thereby placing a greater quantity of desorbent in circulation and increasing the amount of selective pore volume needed during the feed step of the adsorption process.  Under the conventional system, without some method of rejecting desorbent material from the recycled extract stream, the selective pore volume and desorbent requirements would continue to progressively increase. 
Most moving bed adsorption processes also use a desorbent material that has a different composition than the primary components in the feed stream to the adsorption section.  As a result, the desorbent material is typically recovered from the raffinate material that it has desorbed for reuse in the adsorption section.  It has been the usual practice to use a raffinate column to separate the desorbent material from the raffinate stream.  U.S. Pat.  No.  5,043,525 B1 teaches that the combination of an isomerization zone for the isomerization of C5-C6 paraffins and an adsorptive separation section for the recycle of low octane paraffins to the isomerization section can be operated with a single fractionation column for the separation of raffinate, extract, product, desorbent and heavier hydrocarbon components. 
In broad terms, the invention is an arrangement for a combination of an isomerization section for the isomerization of C5 and C6 paraffins and an adsorptive separation section for the separation of the isomerization zone effluent.  This arrangement is structured such that an overall feed stream is separated in a first fractionation zone, preferably a naphtha splitter column, to provide feed and desorbent to an adsorption section.  Effluent from an isomerization zone along with the feed generated in the first fractionation zone enter an adsorption section that is used to separate the adsorption section feed and effluent into a raffinate stream and an extract stream.  The separation is conducted by contacting the adsorption section feed and effluent with an adsorbent and desorbing the adsorbed components from the adsorbent using the desorbent from the first fractionation zone and desorbent from a second fractionation zone.  The raffinate from the adsorption section enters a second fractionation zone, preferably a deisohexanizer, that supplies an overhead isomerate product stream, a bottoms stream of heavy hydrocarbons and a sidecut stream of desorbent material comprising at least normal hexane.  This arrangement also increases the octane of the isomerate product recovered overhead from the deisohexanizer through the recovery of monomethylpentanes with the desorbent material as it is removed from the deisohexanizer column.  The monomethylpentanes are ultimately recycled through the isomerization zone and converted to higher-octane isomers.  The extract stream is passed to the isomerization zone for the generation of isomerized products.  The isomerization zone effluent is recycled to the adsorption section.  In a specific embodiment of the invention, the naphtha splitter column and/or the deisohexanizer may be divided wall columns.  In another specific embodiment of the invention, the overhead stream from the first fractionation zone may be routed past the adsorption zone and be combined with the extract and introduced directly to the isomerization zone.  Other objects, embodiments, and aspects of this invention are described in the following detailed description of the invention.