Hydrocarbon conversion processes are often performed in the presence of hydrogen. This is done for various reasons such as to aid the vaporization of the hydrocarbon, to provide hydrogen which is necessary for the desired reaction or to prolong the life of catalyst used in the reaction zone. In many cases, the hydrogen is recovered from the reaction zone effluent and recirculated. Often this recycle hydrogen stream is purified before being returned to the reaction zone. In another mode of operation the hydrogen is not recycled, or if recycled it is only after having passed through other processing units or purification steps. This is most commonly practiced in processes which consume only minor amounts of hydrogen or which produce hydrogen. These include, for example, isomerization processes, alkylation and dealkylation processes, hydrogenation and dehydrogenation processes, reforming processes and mild desulfurization or denitrification processes.
It is not uncommon for hydrocarbon conversion processes to produce effluents which need to be thereafter separated into hydrocarbon products. Frequently, hydrocarbon conversion processes produce light hydrocarbons as by-products which need to be removed from the process to avoid a buildup. While it is possible to purge the system to remove light hydrocarbons, such action can be undesirable due to the loss of desired components along with the by-products. Hence, it is often desired to separate light hydrocarbons, e.g., C.sub.4 -, from heavier hydrocarbons, e.g., C.sub.5 +, and hydrogen, e.g., from a purge stream or from a reactor effluent separator overhead stream. This type of separation is required, for example, in the thermal dealkylation of alkylaromatic hydrocarbons, such as for the production of benzene from toluene. Toluene is produced in large quantities, often as the by-product of thermal cracking, extraction, reforming or isomerization operations, or directly from petroleum or coal derived naphtha fractions. However, the market for toluene can be limited, and there is a significant economic incentive for its conversion to benzene since benzene is in demand as a basic starting material in the production of many petrochemicals. A variety of separation techniques for performing such separations have been proposed.
U.S. Pat. No. 4,058,452, issued to Laboda, relates to the dealkylation of aromatic hydrocarbons and discloses a process wherein a hydrogen-containing feedstream stream is purified in an absorber to remove light paraffins and produce a hydrogen-rich gas stream which is passed through the reaction zone on a once-through basis. The gas separated from the reaction zone effluent by partial condensation is passed into a stripper as the stripping media used to remove these same light paraffins from the liquid used in the absorber.
A similar type of separation is disclosed in U.S. Pat. No. 4,547,205, issued to Steacy, which sets forth processes for the recovery of hydrogen and C.sub.6 + product hydrocarbons from the effluent stream of a hydrocarbon conversion reaction zone. The effluent stream is partially condensed to remove the bulk of the heavy hydrocarbons, which are sent to a fractionation zone. The remaining vapor is compressed to a substantially higher pressure. The vapor then passes into an autorefrigeration zone in which it is cooled and partially condensed by indirect heat exchange against flashed fluids. The still pressurized uncondensed compounds are transferred to a pressure swing adsorption zone, which produces a high purity hydrogen product.
As noted in the above cited U.S. Pat. No. 4,547,205one stage in the separation process is performed using pressure swing adsorption. Both thermal swing adsorption processes and pressure swing adsorption processes are generally known in the art for various types of adsorptive separations. Generally, thermal swing processes utilize the process steps of adsorption at a low temperature, regeneration at an elevated temperature with a hot purge gas and subsequent cooling down to the adsorption temperature. One process for drying gases generally exemplary of thermal swing processes is described in U.S. Pat. No. 4,484,933, issued to Cohen. The patent describes basic thermal swing processing steps coupled with the use of an auxiliary adsorber bed for improving the regeneration step. Thermal swing processes are often used for drying gases and liquids and for purification where trace impurities are to be removed. Often, thermal swing processes are employed when the components to be adsorbed are strongly adsorbed on the adsorbent, i.e., water, and thus, heat is required for regeneration.
Pressure swing adsorption (PSA) provides a means for adsorption that does not require heat for regeneration. Instead, regeneration is accomplished by reducing the pressure in the adsorber bed to below the pressure at which adsorption had occured. PSA process typically include the steps of adsorption at an elevated pressure, desorption to a lower pressure and repressurization to the adsorption pressure. The process also often include a purge step at the desorption pressure to enhance desorption.
Such PSA processing is disclosed in U.S. Pat. No. 3,430,418 issued to Wagner and in U.S. Pat. No. 3,986,849 issued to Fuderer et al., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system. Such cycles are commonly based on the release of void space gas from the product end of each bed in one or more cocurrent depressurization steps upon completion of the adsorption step. In these cycles, the released gas typically is employed for pressure equalization and for subsequent purge steps. The bed is thereafter countercurrently depressurized and often purged to desorb the more selectively adsorbed component of the gas mixture from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
PSA processes have been employed for both purification and bulk separations. Some PSA processes such as set forth in the above-described U.S. Pat. No. 3,430,418 are particularly well suited for providing a single high purity product gas such as hydrogen and a waste, or fuel gas. Other PSA processes have been disclosed to recover more than one product quality gas. This is often desired when there are two or more desired components in the feedstream.
A PSA process is disclosed in U.S. Pat. No. 4,813,980, issued to Sicar, which relates to the separation of hydrogen, and carbon dioxide from mixtures with methane and other light gases and utilizes two groups of adsorber beds connected in series. The adsorbers in the first group undergo a cycle sequence comprising the following steps, (a) adsorption, (b) high pressure rinse, i.e., copurge, (c) depressure, (d) evacuation, i.e, vacuum, (e) equalize pressure, and (f) final pressurization. The adsorbers in the second group undergo a cycle sequence comprising the following steps (1) adsorption, (2) pressure equalization, (3) depressurizing, (4) purge, and (5) repressurization. The above-identified process can be severely deficient when separating feedstreams that contain heavy hydrocarbons because of the difficulty in desorbing heavy hydrocarbons by pressure swing methods.
Combined thermal swing-pressure swing processes have been proposed for dehydration and carbon dioxide removal, particularly in the purification of air and natural gas streams. U.S. Pat. No. 3,738,084, issued to Simonet, et al., discloses a process for the adsorption of moisture and carbon dioxide that employs thermal swing adsorption in one adsorber and both pressure swing and thermal swing in another adsorber. U.S. Pat. No. 3,841,058, issued to Templeman, discloses a method of purifying natural gas or the like to render it suitable for liquefaction. The method consists essentially of absorbing water and methanol from a natural gas stream also containing carbon dioxide in a first bed of absorbent material and subsequently absorbing carbon dioxide in a second bed of absorbent material, regenerating the absorbent material of the first bed by passing gas therethrough at an elevated temperature and regenerating the absorbent material in the second bed by pressure reduction at a temperature not exceeding 100.degree. C. U.S. Pat. No. 4,249,915, issued to Sicar, et al., discloses a process employing both thermal swing and pressure swing adsorption to remove moisture and carbon dioxide from air. The patent discloses that the air stream is passed to the pressure swing adsorber to remove moisture and the effluent therefrom is passed to the thermal swing adsorber to remove carbon dioxide.
Although integrated thermal swing-pressure swing processes have been proposed for air and light gas purification, there is no specific direction in the disclosures of these processes of how to separate light hydrocarbons from mixtures with hydrogen and heavy hydrocarbons. Accordingly, in view of the above-described need to separate light hydrocarbons from mixture with hydrogen and heavy hydrocarbons, processes are sought which can utilize thermal swing and pressure swing adsorption technology to accomplish the desired separation.