Chemistry of hydrocarbon compounds – Saturated compound synthesis – By isomerization
Reexamination Certificate
2001-10-09
2004-07-06
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Saturated compound synthesis
By isomerization
C585S734000, C585S738000, C585S826000
Reexamination Certificate
active
06759563
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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 C
4
and heavier hydrocarbons having boiling points of less than 215° C. (419° F.) at atmospheric pressure. This range of hydrocarbons includes C
4
-C
6
paraffins and especially the C
5
and C
6
normal paraffins that have relatively low octane numbers. The C
4
-C
6
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 C
6
and heavier hydrocarbons to aromatic compounds. Normal C
5
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 C
6
and heavier hydrocarbons can be upgraded into aromatics through hydrocyclization, the conversion of C
6
'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 C
6
paraffins to an isomerization unit to obtain C
6
isoparaffin hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert C
6
and lighter boiling hydrocarbons to higher octane C
6
isoparaffin hydrocarbons and reforming to convert C
7
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 C
6
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 r
Griffin Walter D.
Maas Maryann
Molinaro Frank S.
Tolomei John G.
UOP LLC
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