SOLVENT-BASED RECOVERY AND PURIFICATION PROCESS

FIELD OF THE INVENTION

The field of this invention relates to use of solvent aided fractionation in one or more of several sequential distillation units for separation and/or purification of useful components from a mixture containing a plurality of volatile organic compounds. More particularly, this invention concerns processes in which both distributive separation and purification operations are integrated through a single preselected solvent. Processes according to this invention are particularly useful in recovery and purification of olefins, such as are typically produced by thermal or catalytic cracking of suitable petroleum derived feedstocks. Beneficially, processes of the invention are used where the olefins being recovered and purified are ethylene and/or propylene.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under United States Department of Energy Cooperative Agreement No. DE-FC07-01 ID 14090.

BACKGROUND OF THE INVENTION

As is well known, olefins, or alkenes, are a homologous series of hydrocarbon compounds characterized by having a double bond of four shared electrons between two carbon atoms. The simplest member of the series, ethylene, is the largest volume organic chemical produced today. Olefins including, importantly, ethylene, propylene and smaller amounts of butadiene, are converted to a multitude of intermediate and end products on a large scale, mainly polymeric materials.

Commercial production of olefins is, almost exclusively, accomplished by pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Thermal cracking feed stocks include streams of ethane, propane or a hydrocarbon liquid ranging in boiling point from light straight-run gasoline through gas oil.

Commercial olefins plants utilize distillation-based separation processes to recover and purify high-value products from a cracked gas furnace effluent. While such plants are capable of producing very high-purity products, their conventional distillation systems are relatively inefficient, requiring large reflux ratios and/or cryogenic conditions to effect the required separations.

A number of absorber-based processes have, therefore, been proposed which utilize physical solvents to recover ethylene from cracked gas. Exemplary of these are processes described in U.S. Pat. No. 5,019,143 in the name of Yuv R. Mehra; U.S. Pat. No. 5,220,097 in the name of Wilfred K. Lam, Yuv R. Mehra and Dow W. Mullins; U.S. Pat. No. 5,326,929 in the name of Yuv R. Mehra, Wilfred K. Lam and Dow W. Mullins; U.S. Pat. No. 5,453,559 in the name of Christopher L. Phillips and Vijender K. Verma; and U.S. Pat. No. 6,340,429 in the name of Air Minkkinen, Jean-Herve Le GaI and Pierre Marache. While these systems have, at least in part, eliminated the need for the very low temperatures that conventional distillation employs, their use of absorption has generally been suggested for only the initial ethylene recovery (e.g. the demethanization) step. As a result, the capabilities of the circulating solvent have not been fully utilized.

More particularly, processes described in U.S. 5,326,929, U.S. 5,220,097, and U.S. 5,019,143 are a combination of an absorber demethanizer, a methane absorber and an auto-refrigerated recovery section to recover ethylene from a cracked gas.

U.S. Pat. No. 5,453,559 (Phillips et al.) describes a condensation-absorption process for the recovery of olefins. The Phillips et al. process uses pre-condensation and a demethanizer pre-stripper distillation tower that is said to make the absorber operation more efficient. They claim that their process substantially reduces the solvent recirculation rate to the absorption unit and eliminates hydrogen expansion in the cold box.

U.S. Pat. No. 5,520,724 (Bauer et al.) describes an absorption-based process for the recovery of light hydrocarbons from a fluid catalytic cracker waste gas. In the Bauer et al. process, light hydrocarbons are scrubbed from the waste gas using an organic, preferably paraffinic solvent. Particularly suitable as solvents are pentane, isopentane, or mixtures thereof. The process consists of a scrubbing tower, a regeneration tower, and distillation columns for purification of the light hydrocarbons that are recovered.

U.S. Pat. No. 6,340,429 (Minkkinen et al.) describes an absorption-based process for separating ethane and ethylene from a hydrocarbon stream. In the Minkkinen et al. process, more highly unsaturated compounds such as acetylene, methylacetylene and propadiene are at least partially hydrogenated before being separated from the solvent stream. The partially purified olefins are then separated from the solvent and purified using traditional distillation.

In all of these processes, the absorber column or columns recover all of the ethylene and ethane that is contained in the feed into the bottoms product of the absorber column. There is no disclosure or suggestion of separating ethylene and ethane in the absorber column.

There remains a need to utilize the solvent as efficiently as it could be used. In addition, the ethylene/ethane splitter columns in each of these prior art processes are based solely on distillation.

Also, in all of these known processes the solvent that is utilized is not necessarily selective to any single component in the feed gas. The interaction of the solvent and the feed gas is purely physical, and the components of the feed gas are separated according to their relative pure component boiling points. The purpose of the solvent in the prior art processes is therefore only to recover all C2+ components into the absorber column bottoms. In contrast, the concept of extractive distillation, well known to those skilled in the art, employs a solvent that is selective to one or more of the components in the column feed. That is, the solvent preferentially absorbs one or more of the components over others that are present in the feed. In this way it increases the relative volatility of key components in the feed, enabling an easier separation to be carried out. In extractive distillation, therefore, the separation of the components of the feed gas is based upon solvent affinity rather than pure component boiling points. It should be noted that this invention is applicable to both absorption-based systems, where the separation is based on relative pure component boiling points, and extractive distillation-based systems, where separation is based on the affinity of the solvent for the various components.

It is therefore a general object of the present invention to provide an improved process which overcomes the aforesaid problem of prior art methods, for example in production of olefins from thermal cracking of hydrocarbon feed stocks.

More particularly, it is an object of the present invention to provide an improved method for separation and/or purification of useful components from a mixture containing a plurality of volatile organic compounds, that use solvent aided fractionation in one or more of several sequential distillation units.

It is another object of the present invention to provide an improved aforesaid method in which both distributive separation and purification operations are integrated through a single preselected solvent.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.

SUMMARY OF THE INVENTION

Economical processes are disclosed for separation and/or purification of useful components from a mixture containing a plurality of volatile organic compounds by use of solvent aided fractionation in one or more of several sequential distillation units.

In broad aspect the invention is a process for recovery of one or more useful component from a mixture containing a plurality of volatile organic compounds, which process comprises: a solvent aided fractionation of a petroleum derived fluid feedstream comprising a plurality of volatile organics including a light class of compounds having relatively low boiling points, a heavy class of compounds having relatively high boiling points, and an intermediate class of compounds having intermediate boiling points, in a first distillation unit using a preselected liquid solvent, to thereby obtain a first overhead stream, essentially free of solvent, comprising at least one member of the light class of compounds, a first bottom stream comprising a portion of the solvent, at least one member of the intermediate class of compounds, and at least one member of the heavy class of compounds, and a first sidedraw stream of liquid from a stage of the fractionation between an inlet of the feedstream and the effluent overhead, the first sidedraw liquid comprising a portion of the solvent and at least one member of the intermediate class of compounds, but essentially free of at least one member of the heavy class of compounds in the feedstream; and a solvent aided fractionation of all or a portion of the first sidedraw stream in a second distillation unit using the preselected liquid solvent, to thereby obtain a second overhead stream, essentially free of solvent, comprising at least one member of the light class of compounds, and a second bottom stream comprising the solvent and at least one member of the intermediate class of compounds. Advantageously, a portion of the first sidedraw stream of liquid is cooled and directed into the first distillation unit. Suitable mixtures may also contain dihydrogen.

Processes according to the invention beneficially further comprises partial condensation of at least one member of the light class of compounds from the first overhead stream to thereby obtain a condensate stream and a dihydrogen enriched gaseous stream, and recovery of purified dihydrogen from at least a portion of the dihydrogen enriched stream, and optionally further comprises counter current contacting of all or a portion of the condensate with at least a portion of the second overhead stream in a distillation column to thereby obtain a bottom effluent and introducing all or a portion of the bottom effluent into the second distillation unit.

Another aspect of special significance is recovery from the first and second bottom streams of a product stream using a third distillation unit to thereby obtain a third bottom stream, essentially free of members of the intermediate class of compounds, comprising solvent and at least one member of the heavy class of compounds, and third overhead stream, essentially free of solvent, members of the light and heavy classes of compounds, but comprising at least one member of the intermediate class of compounds, wherein the first bottom stream is introduced into the third distillation unit at a stage below the level of introduction of the second bottom stream.

In another aspect, processes of the invention further comprise withdrawing a side stream of vapor from the third distillation unit at a level above that of the introduction of the first bottom stream, and using this vapor sidedraw stream for stripping vapor in the second distillation unit, and may further comprises recovery of a substantial portion of the solvent from the third bottom stream using a forth distillation unit to thereby obtain a forth bottom stream comprising the solvent.

Yet another aspect of special significance is the withdrawing of a side stream of vapor from the forth distillation unit, and using this vapor sidedraw stream for stripping vapor in the third distillation unit.

Advantageously, the preselected liquid solvent is a mixture predominantly comprising volatile organic compounds having boiling points at least as high as one or more member in the heavy class of compounds.

Processes of the invention are particularly useful where the olefins being recovered and purified are ethylene and/or propylene wherein the light class of compounds comprises methane, the heavy class of compounds comprises ethane and

hydrocarbons heavier than ethane, and the intermediate class of compounds comprises ethylene.

In one aspect the invention is a process for recovery of one or more useful olefin from a mixture containing a plurality of volatile organic compounds, which process comprises: a solvent aided fractionation of a petroleum derived fluid feedstream comprising dihydrogen and methane, ethylene, ethane, and optionally components heavier than ethane, in a first distillation unit using a preselected liquid solvent, to thereby obtain a first overhead stream, essentially free of solvent, comprising dihydrogen and at least a portion of the methane in the feedstream, a first bottom stream comprising a portion of the ethylene and solvent, and optionally components heavier than ethane, and a first sidedraw stream of liquid from a stage of the fractionation between an inlet of the feedstream and the effluent overhead, the first sidedraw liquid comprising methane, a portion of the solvent and ethylene, but essentially free of ethane; and a solvent aided fractionation of all or a portion of the first sidedraw stream in a second distillation unit using the preselected liquid solvent, to thereby obtain a second overhead stream, essentially free of solvent, comprising methane, and a second bottom stream comprising the ethylene and solvent.

Processes according to the invention may further comprise recovery from the first and second bottom streams of a purified ethylene product stream using a third distillation unit to thereby obtain a third bottom stream, essentially free of ethylene, comprising the solvent, ethane, and optionally components heavier than ethane, and a third overhead stream of purified ethylene product, wherein the first bottom stream is introduced into the third distillation unit at a stage below the level of introduction of the second bottom stream.

In another aspect of the invention, a portion of the first sidedraw stream of liquid is cooled and directed into the first distillation unit.

Processes according to the invention may further comprise partial condensation of the first overhead stream to thereby obtain a condensate stream and a dihydrogen enriched gaseous stream, and recovery of purified dihydrogen from at least a portion of the dihydrogen enriched stream.

Beneficially, all or a portion of the condensate is contacted counter-currently with a portion of the second overhead stream in a distillation column to thereby obtain a

bottom effluent, and all or a portion of the bottom effluent is introduced into the second distillation unit. For best results, a portion of the dihydrogen enriched gaseous stream is distributed into a cryogenic system for recovery of dihydrogen, and another portion to fuel gas.

Another aspect of special significance is where the preselected liquid solvent is a mixture predominantly comprising volatile organic compounds having from 4 to 8 carbon atoms, and essentially free of compounds having relatively low boiling points. Processes according to the invention, wherein the light class of compounds comprises methane, the heavy class of compounds comprises ethane and hydrocarbons heavier than ethane, and the intermediate class of compounds comprises ethylene, advantageously use in the preselected liquid solvent a mixture comprising hydrocarbon compounds having from 3 to 4 carbon atoms, essentially free of compounds having relatively low boiling points, and wherein the amount of C3 compounds in the mixture is less than about 30 percent by weight.

In yet another aspect the invention is a process for recovery of one or more useful component from a mixture containing a plurality of volatile organic compounds, which process comprises: a solvent aided fractionation of a petroleum derived fluid feedstream comprising dihydrogen and a plurality of volatile organics including a light class of compounds having relatively low boiling points, a heavy class of compounds having relatively high boiling points, and an intermediate class of compounds having intermediate boiling points, in a first distillation unit using a preselected liquid solvent, to thereby obtain a first overhead stream, essentially free of solvent, comprising dihydrogen and at least one member of the light class of compounds, a first bottom stream comprising a portion of the solvent, at least one member of the intermediate class of compounds, and at least one member of the heavy class of compounds, and a first sidedraw stream of liquid from a stage of the fractionation between an inlet of the feedstream and the effluent overhead, the first sidedraw liquid comprising a portion of the solvent and at least one member of the intermediate class of compounds, but essentially free of at least one member of the heavy class of compounds in the feedstream; and a fractionation of all or a portion of the first sidedraw stream in a second distillation unit to thereby obtain a second overhead stream comprising at least one member of the light class of compounds, and a second bottom stream comprising the solvent and at least one member of the intermediate class of compounds.

Processes according to this aspect of the invention advantageously further comprise introducing all or a portion of the second overhead stream into the first distillation unit.

Processes according to according to the invention may further comprises recovery from the first and second bottom streams of a product stream using a third distillation unit to thereby obtain a third bottom stream, essentially free of members of the intermediate class of compounds, comprising solvent and at least one member of the heavy class of compounds, and third overhead stream, essentially free of solvent, members of the light and heavy classes of compounds, but comprising at least one member of the intermediate class of compounds, wherein the first bottom stream is introduced into the third distillation unit at a stage below the level of introduction of the second bottom stream. These processes, optionally, further comprises withdrawing a side stream of vapor from the third distillation unit at a level above that of the introduction of the first bottom stream, and using this vapor sidedraw stream for stripping vapor in the second distillation unit.

The process shall be described for the purposes of illustration only in connection with certain embodiments. However, it is recognized that various changes, additions, improvements and modifications to the illustrated embodiments may be made by those persons skilled in the art, all falling within the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE FIGURES

The appended claims set forth those novel features which characterize the present invention. The present invention itself, as well as advantages thereof, may best be understood, however, by reference to the following brief description of preferred embodiments taken in conjunction with the annexed drawing, in which:

FIGURE 1 depicts the recovery light olefins using distributed absorption based techniques.

FIGURE 2 depicts the liquid molar concentration profiles of ethylene, ethane and methane in column 102 of FIGURE 1.

FIGURE 3 depicts the vapor/liquid composition equilibrium for an ethylene/ethane system in the presence of a selective and a non-selective solvent, and in the absence of solvent.

FIGURE 4 depicts a design for the cryogenic section associated with the recovery process depicted in FIGURE 1.

For a more complete understanding of the present invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawing and described below by way of examples of the invention.

BRIEF DESCRIPTION OF THE INVENTION

This invention consists of a novel process for the recovery and purification of a component from a mixture of components in a feed gas. The concept of the invention is very general and could be applied to many separations of commercial interest in which there is need to first recover a desired component from a multi-component mixture, and then to carry out a final purification of that component. Such examples include but are not limited to the recovery and purification of ethylene from a cracked gas stream, recovery and purification of propylene from a refinery fuel gas stream, recovery and purification of C4 materials from a mixed hydrocarbon stream. In order to clarify the concept of this invention, a single commercially relevant example will be described, namely the recovery and purification of ethylene from a cracked gas stream. Those skilled in the art will readily see how the concept of this invention could be applied to other desired separations.

Processes of this invention are particularly suitable for use in purification of aliphatically unsaturated organic compounds produced, generally, by thermal cracking of hydrocarbons.

Aliphatically unsaturated compounds of most interest, with regard to purification by the method of the present invention, have two to about eight carbon atoms, preferably two to about four carbon atoms, and more preferably ethylene and propylene.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

While this invention is susceptible of embodiment in many different forms, this specification and accompanying drawing disclose only some specific forms as an example of the use of the invention. In particular, preferred embodiments of the invention for purification of a gaseous mixture comprising an olefin, preferably an olefin of from two to about eight carbon atoms having a single double bond, acetylenic impurities having the same or similar carbon content and optionally alkanes (paraffin hydrocarbons) and/or alkenes having more than one double bond (di- or tri- olefin hydrocarbons) produced by thermal cracking of hydrocarbons are illustrated and described. The invention is not intended to be limited to the embodiments so described, and the scope of the invention will be pointed out in the appended claims.

The apparatus of this invention is used with certain conventional components the details of which, although not fully illustrated or described, will be apparent to those having skill in the art and an understanding of the necessary function of such components.

More specifically invention is described below in relation to a process for the recovery and purification of ethylene from a cracked gas stream. This invention would be beneficial in any system where the separation of at least three components is desired, and these components have differing pure component boiling points and/or different affinities for a solvent. It is particularly useful for recovering and purifying a desired component from a mixture which contains one or more other components which have low relative volatilities with respect to the desired component and which are therefore difficult or costly to separate using conventional distillation. The following description is made in terms of ethylene recovery in order to facilitate understanding of the process of this invention. It in no way limits the scope of the invention.

In FIGURE 1 , a petroleum derived fluid feedstream enters distributor absorber column 102 through conduit 101. In this embodiment of the invention the feedstream contains very few C4 hydrocarbons and is essentially free of butadiene. For example, the feedstream can be derived from an overhead product of a depropanizer column (not shown). A supply of lean solvent enters the top of distributor absorber column 102 through conduit 103 and flows down the column thereby aiding in removal of ethylene from the overhead gas. The overhead vapor, containing predominately methane and dihydrogen with little, if any, ethylene, is transferred to a cryogenic cold section through

conduit 104 for best results. Conditions suitable for solvent aided fractionation are maintained within distributor absorber column 102, in part, by reboiler heat exchanger 105. Column 102 may optionally employ one or more side condensers or liquid pump- around circuits and/or side reboilers (not shown) in order to strip the liquid bottoms stream of methane and dihydrogen.

The bottoms liquid stream 107 containing only a portion of the ethylene that enters the column, along with ethane, C3 hydrocarbons and solvent, because a sidedraw stream of liquid is withdrawn from column 102 through conduit 108. A portion of the sidedraw stream is cooled in exchanger 109 for return to column 102 as an additional reflux liquid through conduit 110. Another portion of the sidedraw stream containing the remainder of the ethylene that is fed into 102 (but does not exit in the bottom stream) is transferred into distillation column 112 through conduit 111.

The sidedraw stream is drawn from column 102 at a stage of fractionation where it contains only a small amount of ethane, so small that the ratio of ethane to ethylene in the stream is very close to the ratio of ethane to ethylene in product-quality ethylene.

The sidedraw stream therefore contains ethylene and solvent, as well as some methane and dissolved dihydrogen.

FIGURE 2 depicts the liquid molar concentration profiles of ethylene, ethane, and methane within column 102. The section labeled "Top" in FIGURE 2 lies between the lean solvent inlet through conduit 103 and the sidedraw through conduit 108. There is essentially no ethane in this section, and the primary mass transfer that is occurring is capture of ethylene from the up-flowing gas by the down-flowing lean solvent. The section labeled "Middle" in FIGURE 2 lies between the sidedraw stream 108 and the feed stage. The primary mass transfer that is occurring in this section is solvent- assisted separation of ethylene from ethane. Note that at the sidedraw stage there is essentially no ethane remaining in the liquid. The section labeled "Bottom" in FIGURE 2 lies between the feed stage and the bottom stage. The primary mass transfer that is occurring in this section is the stripping of methane from the down-flowing liquid by the up-flowing stripping vapor. The liquid at the bottom of column 102 contains essentially no methane.

A variety of hydrocarbon or non-hydrocarbon solvents are useful for any number of desired separations. For best results according to this embodiment of the invention, suitable solvents should (but are not required to) selectively absorb ethane over

ethylene. The difference between a selective solvent and a non-selective solvent is demonstrated in FIGURE 3 which depicts the pseudo-binary phase diagram for ethylene and ethane in the presence of a selective and a non-selective solvent. A pseudo-binary x-y phase diagram is one in which the vapor/liquid phase equilibrium data for the constituents to be separated (in this case ethylene and ethane) are plotted on a solvent-free basis. Also shown in FIGURE 3 is a traditional ethylene/ethane phase diagram (representing the vapor/liquid equilibrium in the absence of solvent). In the presence of the selective solvent the relative volatility of ethylene/ethane binary is increased and the pseudo-binary phase diagram shifts further away from the x-y diagonal line as compared with the solvent-free phase diagram. Such a shift represents enhancement of the separation of ethylene and ethane in the presence of the selective solvent. With a non-selective solvent, there is no shift in the pseudo- binary phase diagram, and it overlays the solvent-free binary phase diagram. Thus a non-selective solvent does not increase the relative volatility of the ethylene/ethane binary. It should be noted that the process of this invention is applicable to either a selective or a non-selective solvent.

Referring again to FIGURE 1 , a portion of the sidedraw is fed to distillation column 112, which acts as an absorptive demethanizer. Another supply of lean solvent enters the top of column 112 through conduit 113 and flows down the column thereby aiding in removal of ethylene from the overhead gas.

If present, condensed liquid from the cryogenic cold section can also be charged into column 112. Likewise, column 112 may have one or more side condensers or liquid pump around circuits and/or side reboilers (not shown). The overhead gas stream, containing primarily methane with small amounts of dihydrogen, is transferred through conduit 116 into the cryogenic cold section. A liquid bottom stream, comprising ethylene and solvent with negligible amounts of methane or dihydrogen, is withdrawn from distillation column 112 through conduit 117, and fed into distillation column 118. In a preferred embodiment of the invention, columns 112 and 118 are thermally coupled via a vapor sidedraw stream through conduit 119 thereby providing at least a portion of the stripping vapor to the bottom of column 112.

In FIGURE 1 the cryogenic cold section is shown schematically as a single unit which processes streams 104 and 116 in order to produce a purified hydrogen stream and a fuel gas stream, and optionally other streams. Those skilled in the art will

recognize that in practice this system will be made up of a multiplicity of heating, cooling and separations operations arranged to produce the desired products.

It should be noted that, in accordance with the invention, alternative arrangements (not shown) exists for columns 112 and 102. For example, column 112 may be configured as a side stripper to column 102. In this arrangement a single stream of solvent is directed into the top of 102. The sidedraw stream 111 is directed to the top of column 112 and the overhead stream 116 is directed back to column 102.

Under this arrangement, the cryogenic system depicted in FIGURE 4 would no longer be viable. Advantageously, the arrangement shown in FIGURE 1 allows for columns 102 and 112 to operate at different pressures, which may allow more optimal operation.

Liquid streams from the bottoms of columns 102 and 112, containing solvent and essentially all of the ethylene originally contained in the fluid feedstream to distributor absorber column 102, are transferred into distillation column 118, which operates as a solvent-assisted C2 splitter. Liquid from the bottom of column 102 is transferred through conduit 107, exchanger 120, and conduit 123 and into column 118, and liquid from the bottom of column 112 is transferred through conduit 117 into column 118. Because liquid from the bottom of column 112 contains essentially no ethane, it enters near the top of column 118. There are a relatively small number of rectifying stages above this feed, the purpose of which is to separate the relatively heavy solvent from the ethylene product. Column 118 is refluxed using condenser 121. A liquid overhead stream of product-quality ethylene is removed through conduit 122.

Heated (or cooled) liquid from the bottom of column 102 is directed into column 118 at a point below that of stream 117. Rectifying stages within column 118 between the two feed stages operate to separate ethane from the ethylene. Solvent from the higher feed stage flows down to thereby enhance separation of ethane from ethylene in the middle rectifying section. This solvent-assisted separation beneficially allows column 118 to have relatively fewer rectifying stages and a relatively smaller condenser duty than conventional distillation-based ethylene/ethane splitters known in the prior art. In FIGURE 1 , a vapor sidedraw stream is withdrawn from column 118 through conduit 119, and provides stripping vapor into column 112. Stripping stages in the lower part of column 118, below the lower feed stage, operate to strip ethylene from the bottoms product. One or more side reboilers can be used on column 118 (not shown). Stripping vapor in the bottom of column 118 is supplied by a reboiler 125.

A liquid bottom stream, containing ethane, C3s, and solvent, is transferred from column 118 into a solvent recovery section through conduit 128.

A stream containing ethane and C3 hydrocarbons essentially free of solvent is transferred through conduit 129 to storage and/or additional processing (not shown). A stream of lean solvent for reuse is transferred through conduit 130 and would typically be recycled for use in the process (not shown). Solvent makeup and purge streams are understood to be present, but are not shown. Additionally, solvent purification steps such as filtering or contaminant removal may be included in a solvent circulation loop.

Depicted in FIGURE 4 is the cryogenic cold section of this embodiment that beneficially provides a relatively high recovery of purified hydrogen product. Streams that are common between FIGURE 1 and FIGURE 4 have the same number designator. It should be noted that the design of the cryogenic section may be different if recovery of a purified hydrogen product were not desired. It should be further noted that, in order to simplify FIGURE 4, exchangers are shown as separate and distinct exchangers (for example, 200, 203, 206, 211, 213, 217, and 228). Those skilled in the art will recognize that in actual practice these exchanger duties would typically be combined into one or more multi-pass heat exchangers. This is depicted in FIGURE 4 by indicating that multiple other process streams, including some shown in FIGURE 4, pass through and are heated or cooled in multi-pass exchanger 228.

The overhead stream 116 from column 112 in FIGURE 1 is first chilled in exchanger 200 to form a chilled stream 201 and thereafter enters the bottom of absorber column 202 which serves as an absorber to recover ethylene and heavy components from the stream. Column 202 contains a relatively small number of theoretical contacting stages, for example less than 10 theoretical stages. The overhead stream 104 from column 102 in FIGURE 1 is cooled and partially condensed in exchanger 203. The vapor and liquid resulting in stream 204 are separated in drum 205. Liquid from drum 205 is heated in exchanger 206 before entering absorber column 202 as solvent to the absorber. Uncondensed vapor from 205 is split into two portions. A first portion, stream 207, and the column 202 overhead vapor stream 208 are combined. Another portion, stream 209, enters the hydrogen recovery section.

Solvent rich liquid from column 202, stream 210, is heated in exchanger 211 , and returned, in stream 114, as recycle solvent to the process of FIGURE 1.

The combined stream 212 is heated in exchanger 213, and stream 214 therefrom is sent to an expander 215. The expanded vapor in stream 216 is reheated in exchanger 217 to provide refrigeration to the overall process. The reheated vapor, stream 218, is combined with reheated vapor, stream 219, from the hydrogen recovery section before being re-compressed in compressor 220. The recomposed vapor, stream 221, contains primarily methane and some dihydrogen, and would typically be used as fuel. Note that the expander 215 and compressor 220 could be combined into a coupled "compander" set if desired.

Stream 209 enters the hydrogen recovery unit. It is first cooled and partially condensed in exchanger 222. Liquid and vapor are separated in drum 223, and the liquid, stream 224, is expanded across valve 225 to produce stream 224a, which joins stream 226 from exchanger 227. The combined stream provides refrigeration for the overall process by being reheated in exchangers 222 and 228.

The vapor, stream 229, is further cooled and partially condensed in exchanger 227. The vapor and liquid exiting exchanger 227 are separated in drum 230. The liquid, stream 231 , containing primarily methane, is expanded across valve 232 to form stream 231a. The vapor, stream 233, containing mostly dihydrogen, is split into two streams. Stream 234 is reheated in exchangers 227, 222 and 228, after which it is removed as the purified dihydrogen product, stream 235. The other portion, stream 236, is expanded across valve 237, and joins stream 231a. The combined stream is reheated in exchanger 227 to form stream 226. Stream 226 is combined with 224a and further reheated in exchangers 222 and 228 to produce stream 219.

Other process streams, including some of those shown in FIGURE 4, can be heated or cooled in 228. For example, in practice any of the exchangers may be integrated into a multi-pass heat exchanger. In this case the stream labeled "Other

Process Streams" in FIGURE 4 would represent a number of streams to be heated or cooled, including (but not limited to) streams 116, 210, 212, and 216.

EXAMPLE OF THE INVENTION

The following Example will serve to illustrate specific embodiment of the herein disclosed invention. This example should not, however, be construed as limiting the scope of the novel invention as there are many variations which may be made thereon

without departing from the spirit of the disclosed invention, as those of skill in the art will recognize.

Example 1

An ethylene recovery process based on the embodiment of FIGURES 1 and 4 was simulated using commercially available process simulation software. The mixed hydrocarbon feed is derived from the cracked gas of a steam cracking plant from which essentially all components heavier than propane have been removed. The mixed hydrocarbon feed therefore contains primarily hydrogen, carbon monoxide, methane, ethylene, ethane, propylene, propane, and minor amounts of propadiene, methyl acetylene, and other contaminants. The solvent employed is a mixture of C3 and C4 hydrocarbons. The composition of key streams is given in Table 1. The individual heat exchanger duties are given in Table 2. In all cases the stream and equipment numbers relate to those shown in FIGURES 1 and 4.

Note that several heat exchangers, used in the design of this example, are not shown in FIGURE 1. Two side coolers are used on column 102 above the location of the sidedraw stream 108. The total duty of these side coolers is 23.5 MMBTU/hr. A side cooler with a duty of 4.5 MMBTU/hr is utilized on column 112 above the location of stream 111. Finally, a side reboiler with a duty of 44.0 MMBTU/hr is utilized on column 118 below the location of feed stream 123.

The embodiment of FIGURES 1 and 4 can be shown to recover and purify ethylene using less energy than prior art absorption systems. Table 3 compares the cracked gas compressor and refrigeration compressor energy required for the prior art process of Phillips '559 and the process of this invention as described in Example 1.

It is clear that the process of this invention accomplishes the recovery and purification of ethylene from a cracked gas stream with significantly less energy than the prior art process.

For the purposes of the present invention, "predominantly" is defined as more than about fifty percent. "Substantially" is defined as occurring with sufficient frequency or being present in such proportions as to measurably affect macroscopic properties of an associated compound or system. Where the frequency or proportion for such impact is not clear, substantially is to be regarded as about twenty per cent or more. The term "a feedstock consisting essentially of is defined as at least 95 percent of the

feedstock by volume. The term "essentially free of" is defined as absolutely except that small variations which have no more than a negligible effect on macroscopic qualities and final outcome are permitted, typically up to about one percent. The term "solvent" is defined as one or a plurality of volatile chemical compounds which aid in fractionation of volatile organic compounds as described herein.

Table 1 Flows and Conditions for Streams in Figure 1

OO

Table 1 (Continued) Flows and Conditions for Streams in Figure 4

CD

TABLE 2 Heat Exchanger Duties for Example 1

Table 3 Compressor Energy Comparison