Exxon Research and Engineering Company

Frank J Healy

Paul R Geissler

Dual desorbent composition and dual temperature techniques are provided as improvements for a simulated moving bed adsorption-desorption separation process which help reduce total desorbent requirements. The invention describes a process for continuously separating, in a liquid phase, components of a C.sub.8 aromatic feed mixture by contacting said feed mixture with a solid sorbent and utilizing a simulated countercurrent flow system wherein a liquid stream flows through serially and circularly interconnected desorption, rectification and sorption zones. One of the embodiments envisioned in the present improved process is directed to the employment of two desorbent streams of different strengths. This embodiment minimizes total amount of desorbent requirements by using a strong desorbent stream in the desorption zone and a suitable weaker desorbent stream in the rectification zone to achieve the desired separation. In another embodiment, the concept of temperature gradient is utilized to increase the desorbing power of the desorbent employed. Such an embodiment is carried out by providing a means of heating the desorbent stream prior to its introduction to the desorption zone and thereafter removing any residual heat before the desorbent stream passes to the rectification zone by means of a heat exchanger. Further, both the dual desorbent composition and the dual temperature techniques can be also simultaneously employed as another embodiment.

Dual desorbent composition and temperature separation processes

Dual desorbent composition and/or dual temperature technique, when employed in a simulated moving-bed adsorption-desorption separation system embedded with a solid sorbent comprising certain sieves, e.g., Rb-exchanged Type X crystalline aluminosilicate sieve, which preferentially adsorb ethylbenzene, entails an improved separation and recovery of ethylbenzene from C.sub.8 aromatic feedstreams as sorbate product. Further, certain specific sieves, which preferentially adsorb xylene isomers, i.e., orthoxylene, metaxylene and paraxylene, may also be employed, in said simulated process utilizing the dual desorbent composition and/or the dual temperature concept, in order to effect the improved separation and recovery of ethylbenzene from its C.sub.8 aromatic isomers as raffinate product.

Separation and recovery of ethylbenzene by dual desorbent composition

Dual desorbent composition and dual temperature techniques as improvements for simulated moving-bed adsorption-desorption separation processes whereby certain hydrocarbons can be separated from mixtures containing various unsaturated and/or saturated aliphatic and/or aromatic hydrocarbons. The dual desorbent composition technique is implemented by employing a strong desorbent stream in the desorption zone and a weaker desorbent stream in the rectification zone of a simulated moving-bed countercurrent adsorption-separation system; while the dual temperature concept envisions the temperature differentiation between the desorption zone and the remaining zones of said separation system in order to both minimize the desorbent requirement and enhance the purity of the separation products. A combined use of both the dual desorbent composition and the dual temperature techniques can further improve the performance of said simulated moving-bed system.

Separation of hydrocarbons

Simulated moving bed processes for the separation of C.sub.8 aromatic isomers using molecular sieve adsorbents are improved by the use of meta-diisopropylbenzene as the eluting agent. The invention describes a process for continuously separating, in a liquid phase, components of the feed mixture by contacting said feed mixture with a solid sorbent and utilizing a simulated countercurrent flow system wherein a liquid stream flows through serially and circularly interconnected desorption, rectification and sorption zones. In such a process the flow of liquid stream is interrupted between both the desorption and rectification zones and a first portion of effluent from the desorption zone, a portion being rich in sorbate may be directly circulated and a second portion, the same comprising sorbate of high purity and desorbent may be fed to a distillation apparatus from which one portion of the sorbate distillate may be circulated as reflux to the rectification zone to maximize the sorbate product purity at reduced costs and another portion is recovered as product. The present improved process is conducted using meta-diisopropylbenzene as a desorbent either alone or in combination with inert diluents such as paraffins. This embodiment provides a more effective separation of paraxylene and ethylbenzene peaks, thus increasing paraxylene recovery and at the same time reducing the amount of recycle and further ethylbenzene-paraxylene separations such as distillations and the like. This better separation allows more efficient use of the adsorbent bed and/or lower ratios of eluting materials to C.sub.8 aromatic feed which results in a more economic process.

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Morristown, NJ, United States of America

Frank J Healy