SOLVENT-AIDED FRACTIONATION PROCESS

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.

FIELD OF THE INVENTION

The field of this invention relates to use of solvent aided fractionation, separation and/or purification of useful components from a mixture containing a plurality of volatile organic compounds. More particularly, this invention concerns processes using a preselected liquid solvent in a fractionating unit including stripping and rectifying sections.

Processes according to this invention are particularly useful for at least partial separation of components from a mixed gas stream of three or more components, that include a light class of components which have a relatively low boiling point and/or a low affinity for the solvent, a heavy class of components which have a relatively high boiling points and/or a high affinity for the solvent, and a intermediate class of components which have an intermediate boiling points and/or an intermediate affinity for the solvent. Fractionating units of the invention generate effluent streams including at least an overhead stream comprising at least one member of the light class of compounds, but essentially free of solvent and compounds of the heavy class; a bottom stream comprising a portion of the solvent and at least one member of the heavy class of compounds, but essentially free of compounds of the light class, and at least one sidedraw stream of liquid comprising a portion of the solvent and at least one member of the intermediate class of compounds.

Beneficially, processes of the invention are used for fractionation of mixed gas stream containing olefins. Such streams are typically produced by thermal or catalytic cracking of suitable petroleum derived feedstocks, and the olefins being recovered and purified are typically ethylene and/or propylene.

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 hydrocarbon liquids ranging in boiling point from light straight-run gasoline through gas oil.

In a typical ethylene plant the cracking represent about 25 percent of the cost of the unit while the compression, heating, dehydration, recovery and refrigeration sections represent the remaining about 75 percent of the total. This endothermic process is carried out in large pyrolysis furnaces with the expenditure of large quantities of heat which is provided in part by burning the methane produced in the cracking process. After cracking, the reactor effluent is put through a series of separation steps involving cryogenic separation of products such as ethylene and propylene. The total energy requirement for the process is thus very large and ways to reduce it are of substantial commercial interest.

Several methods are known for separation of an organic gas containing unsaturated linkages from gaseous mixtures. These include, for instance, cryogenic distillation, liquid sorption, membrane separation and the so called "pressure swing adsorption" in which sorption occurs at a higher pressure than the pressure at which the sorbent is regenerated. Cryogenic distillation and liquid sorption are common techniques for separating carbon monoxide and alkenes from gaseous mixtures containing molecules of similar size, e.g., nitrogen or methane. However, both techniques have disadvantages such as high capital cost and high operating expenses. For example, liquid sorption techniques suffer from high solvent circulation rates and associated high energy costs.

Impurity refers to compounds that are present in the olefin plant feedstocks and products. Well-defined target levels exist for impurities. Common impurities in ethylene and propylene include: acetylene, methyl acetylene, methane, ethane,

propane, propadiene, and carbon dioxide. Listed below are the mole weight and atmospheric boiling points for the light products from thermal cracking and some common compounds potentially found in an olefins unit. Included are some compounds which have similar boiling temperatures to cracked products and may be present in feedstocks or produced in trace amounts during thermal cracking.

Recently the trend in the hydrocarbon processing industry is to reduce commercially acceptable levels of impurities in major olefin product streams, i.e., ethylene, propylene, and hydrogen. The need for purity improvements is directly related to the increasing use of higher activity catalysts for production of polyethylene and polypropylene, and to a limited extent other olefin derivatives.

Mole Compound Weight Normal Boiling

Point, ⁰ C

Hydrogen 2.016 -252.8

Nitrogen 28.013 -195.8

Carbon monoxide 28.010 -191.5

Oxygen 31.999 -183.0

Methane 16.043 -161.5

Ethylene 28.054 -103.8

Ethane 30.070 -88.7

Phosphine 33.970 -87.4

Acetylene * 26.038 -84.0

Carbon dioxide * 44.010 -78.5

Radon 222.00 -61.8

Hydrogen sulfide 34.080 -60.4

Arsine 77.910 -55.0

Carbonyl sulfide 60.070 -50.3

Propylene 42.081 -47.8

Propane 44.097 -42.1

Propadiene (PD) 40.065 -34.5

Cyclo-propane 42.081 -32.8

Methyl acetylene 40.065 -23.2

Water 18.015 100.

^* Sublimation temperature

Absorption and extractive distillation involve the contacting of a suitable liquid solvent with a gas mixture in order to enhance the separation of one or more components from the gas mixture. The interaction of the solvent and gas components may be purely physical in nature (as in absorption systems). In this case the components within the absorber are separated based on their pure component boiling points, with the lower-boiling materials exiting at the top of the absorber column.

There may also be selective chemical interactions between one or more gas-phase components and the solvent which can impact the relative volatility of the gas-phase components (as in extractive distillation systems). In this case the separation of components within the extractive distillation column depends both on the pure component boiling points of the various gas phase components and on the relative affinities of the gas phase components for the solvent. Components with the higher affinities for the solvent will typically exit at the bottom of the extractive distillation column while components with lower affinities for the solvent will typically exit at the top of the extractive distillation column.

Typical commercial applications of absorption technology within the chemical and petrochemical industries include the separation of carbon dioxide, H2S, and ammonia from hydrocarbon and other gases, removal of SO2 from various gas streams, including flue gas, removal of CO from gas streams, recovery of chlorinated compounds from a mixed stream, and recovery of relatively heavy hydrocarbons from a mixed hydrocarbon stream. Numerous reviews of the theory and practice of industrial absorption technology are available, including "Gas- Liquid Reactions" by P. V. Dankwerts (McGraw-Hill, 1970), "Absorption, Distillation, and Cooling Towers" by W. S. Norman (Wiley, 1961 ), and "Chemical Engineers' Handbook", by R. H. Perry and C. H. Chilton, editors (McGraw-Hill, 2001 ).

Typical commercial applications of extractive distillation technology for separating close-boiling components from a feed stream include the separation of butadiene from mixed C4 streams, the removal of heptane isomers from cyclohexane, the separation of propylene and propane, and separating toluene from non-aromatics. A summary of theory and technology for distillation, azeotropic and extractive, can be found in the Kirk-Othmer "Encyclopedia of Chemical Technology" fourth edition (John Wiley & Sons, Volume 8, pp. 358-98).

It should be noted that the process of this invention is applicable to both absorption systems, in which separation is based primarily on the pure component boiling points of the gas phase components, and to extractive distillation, in which separation is also impacted by the chemical interactions between gas-phase components and the solvent.

The equipment used for contacting the gas mixture with the solvent can be a tower that contains mechanical means for enhancing the contacting of the vapor and liquid within the tower. These means can include structured or unstructured packing and contacting trays such as bubble-cap, sieve, or valve-type trays. An

empty vessel into which the solvent is sprayed and through which the gas flows can also be used. By far the most common means of enhancing the contacting is a packed or trayed tower in which the solvent and gas streams flow counter- currently in at least a portion of the tower. In a typical design the lean solvent is introduced at the top of the contacting tower and the gas mixture is introduced either at the very bottom or in a middle section of the tower. The rich solvent (containing components absorbed by the solvent) exits at the bottom of the tower, and the overhead product exits at the top of the tower.

The section of the contacting tower that lies between the gas mixture inlet and the solvent inlet is the absorber or rectification section of the tower in which the gas mixture and solvent are contacted in a counter-current fashion. Optionally there can be a stripping section within the tower situated between the inlet of the gas mixture and the bottom of the contacting tower in which the rich solvent is stripped of one or more components of the gas mixture. In this case stripping vapor is introduced into the bottom of the tower.

Solvent recovery can be carried out in a separate vessel and typically involves separating the absorbed component or components from the rich solvent in order to produce a relatively pure stream of the separated component or components and a regenerated lean solvent stream. The solvent can then either be discarded or, more commonly, recycled and reused in the contacting tower.

The patent literature for absorption technology is voluminous, and is generally specific to a certain desired separation. A recent example of a more generic advance is described in Witzko et al. (US 2002/0014154). Witzko et al. describe a membrane-based unit that can be used in place of a standard packed or trayed absorber tower.

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.

Processes of Mehra utilize a combination of an absorber demethanizer, a methane absorber and an auto-refrigerated recovery section to recover ethylene from a cracked gas.

The Phillips et al. patent describes a hybrid condensation-absorption process for the recovery of olefins. This process uses pre-condensation and a demethanizer pre-stripper distillation tower to make the absorber operation more efficient. They state 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 in the name of Heinz Bauer and Hans Becker, describes an absorption-based process for recovery of hydrocarbons from a fluid catalytic cracker waste gas. The 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.

The Minkkinen et al. patent describes an absorption-based process for separating ethane and ethylene from a hydrocarbon stream. In this process more highly unsaturated compounds such as acetylene, methylacetylene and propadiene are at least partially hydrogenated before being separated from the solvent stream. The at least partially purified olefins are then separated from the solvent and purified using traditional distillation.

In all of these processes, the absorber tower or towers recover all of the ethylene and ethane that is contained in the feed into the bottoms product of the absorber tower. In the terminology of distillation and absorption, the key components of the separation are methane and ethylene. There is no separation of ethylene and ethane (non-key components) in the absorber tower and so the solvent is not used as efficiently as it could be. This is general for the state of absorber design in general - advantage is not taken of any separation of non-key components that may occur within the absorber tower itself.

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 recovery and separation of desirable components from gaseous mixtures.

An improved method for recovery of one or more useful components from mixtures containing a plurality of volatile organic compounds should exhibit greater energy efficiency, thereby providing lower variable costs of operation.

More particularly, it is an object of the present invention to provide an improved method for recovery and at least partial purification of ethylene and/or propylene for a cracked gas mixture.

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 preselected liquid solvents in a fractionating unit including stripping and rectifying sections. Processes according to this invention are particularly useful for at least partial separation of components from a mixed gas stream of three or more components, for example, one or more desired olefins such as are typically produced by thermal cracking of suitable hydrocarbon feedstocks.

In one aspect the invention is a process for recovery of one or more useful components from a mixture containing a plurality of volatile organic compounds, which process comprises: (a) providing a 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; and (b) a solvent aided fractionation of the fluid feedstream using a preselected liquid solvent in a fractionating unit including stripping and rectifying sections, to thereby obtain at least three effluent streams. The effluent streams comprise a first overhead stream comprising at least one member of the light class of compounds, but essentially free of solvent and compounds of the heavy class, a first bottom stream comprising a portion of the solvent and at least one member of the heavy class of compounds, but essentially free of compounds of the light class, and at least one sidedraw stream of liquid comprising a portion of the solvent and at least one member of the intermediate class of compounds, with the proviso that

compounds of one or more of the classes are distributed into at least two of the effluent streams. More particularly, a compound of one or more class is distributed into a sidedraw stream and into one other effluent stream.

Depending upon the feedstream and desired separation, the invention provides several useful limitations for the sidedraw. At least one sidedraw stream of liquid, essentially free of compounds of the light class of compounds in the feedstream, for example is withdrawn from the stripping section of the fractionating unit. Also, at least one sidedraw stream further comprises at least a portion of the heavy class of compounds in the feedstream, and is essentially free of compounds of the light class of compounds in the feedstream. In other cases at least one sidedraw stream, essentially free of compounds of the heavy class of compounds in the feedstream, is withdrawn from the rectifying section of the fractionating unit. In another, at least one sidedraw stream further comprises at least a portion of the light class of compounds, and is essentially free of compounds of the heavy class of compounds in the feedstream.

In one aspect of the invention, the bottom stream further comprises at least a portion of the intermediate class of compounds in the feedstream.

Advantageously, the bottom stream and the sidedraw stream each containing at least 20 percent of the intermediate components in the mixed gas which is fed into the fractionating unit.

In yet another aspect of the invention, at least one sidedraw stream of liquid, essentially free of compounds of the light class of compounds in the feedstream, is beneficially withdrawn from the stripping section of the fractionating unit, and wherein at least one sidedraw stream, essentially free of compounds of the heavy class of compounds in the feedstream, is withdrawn from the rectifying section of the fractionating unit. Advantageously, a portion of at least one sidedraw stream of liquid is cooled and directed into the fractionating unit.

Processes of the invention can further comprise maintaining a differential in absolute operating pressure in a portion of the rectifying section of the fractionating unit which is at least 50 percent higher than the absolute pressure in the stripping section of the fractionating unit.

In another aspect, the invention is a process for recovery of one or more useful components from a mixture containing a plurality of volatile organic compounds, which process comprises: (a) providing a 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; and (b) a solvent aided fractionation of the fluid feedstream using a preselected liquid solvent in a fractionating unit including stripping and rectifying sections, to thereby obtain at least three effluent streams. These three effluent streams comprise a first overhead stream comprising at least one member of the light class of compounds, but essentially free of solvent and compounds of the heavy class, a first bottom stream comprising a portion of the solvent and at least one member of the heavy class of compounds, but essentially free of compounds of the light class, and at least one sidedraw stream of liquid comprising a portion of the solvent, a portion of the heavy class and one or more members of the intermediate class of compounds. Advantageously, the bottom stream and the sidedraw stream each contain at least 20 percent of the heavy components in the mixed gas which is fed into the fractionating unit.

In several cases, the first overhead stream further comprises at least a portion of the intermediate class of compounds in the feedstream.

In some cases, processes of the invention further comprises maintaining a differential in absolute operating pressure in a portion of the rectifying section of the fractionating unit which is at least 50 percent higher than the absolute pressures in the stripping section of the fractionating unit.

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) providing a 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; and (b) a solvent aided fractionation of the fluid feedstream using a preselected liquid solvent in a fractionating unit including stripping and rectifying sections at suitable preselected conditions of operation, to thereby obtain at least three effluent streams. The effluent streams comprise a first overhead stream comprising at least one member of the light class of compounds, but essentially free of solvent and compounds of the intermediate and heavy classes; a first bottom stream comprising a portion of the solvent and at least one member of the heavy class of compounds, but essentially free of compounds of the light class, and at least one sidedraw stream of liquid comprising a portion of the solvent, a portion of compounds of the light class, and at least one member of the intermediate class of compounds, but essentially free of compounds of the heavy

class. In several cases, the bottom stream further comprises at least a portion of the intermediate class of compounds in the feedstream.

Another aspect of special significance is processes according to the invention, wherein the fluid feedstream comprises methane, ethylene and ethane, and portions of the ethylene in the feedstream are distributed into the bottom stream and at least one sidedraw stream. Preselected liquid solvents advantageously comprise a mixture of at least two members selected from a group consisting of organic compounds having 3 or more carbon atoms.

In a process of the invention for recovery of ethylene from a cracked gas, a particularly useful liquid solvent is a mixture comprising hydrocarbon compounds having from 3 to 4 carbon atoms, essentially free of compounds having relatively low boiling points. The amount of C3 compounds in the mixture is less than about 30 percent by weight, for best results.

In other cases, processes according to the invention provide conditions of operation wherein the absolute operating pressure in a portion of the rectifying is at least 50 percent higher than the absolute pressure in the stripping section of the fractionating unit. For best results in some cases, an effluent stream of vapor from the lower-pressure section is compressed and directed into the rectification section. In yet another aspect, the invention is a process for recovery of one or more useful components from a mixture containing a plurality of volatile organic compounds, which process comprises: (a) providing a 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; and (b) a solvent aided fractionation of the fluid feedstream using a preselected liquid solvent in a fractionating unit including stripping and rectifying sections at suitable preselected conditions of operation, to thereby obtain at least three effluent streams. The three effluent streams comprise a first overhead stream comprising at least one member of the light class of compounds, but essentially free of solvent and compounds of the intermediate and heavy classes, a first bottom stream comprising a portion of the solvent and at least one member of the heavy class of compounds, but essentially free of compounds of the light and intermediate classes; at least one sidedraw stream of liquid, from the rectifying section of the fractionating unit, comprising a portion of the solvent, a portion of compounds of the light class, and at least one member of the intermediate class of compounds, but essentially free of compounds of the heavy

class, and at least one sidedraw stream of liquid, from the stripping section of the fractionating unit, comprising a portion of the solvent, a portion of the heavy class and one or more members of the intermediate class of compounds, but essentially free of compounds of the light class. In some separations according the invention, an effluent stream of vapor is withdrawn from the stripping section, compressed, and directed into the rectification section.

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 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 drawings, in which:

FIGURE 1 is a schematic diagram of a solvent aided fractionation according to the invention in which a light component A is distributed between two of three effluent streams.

FIGURE 2 is a schematic diagram of a solvent aided fractionation according to the invention in which a heavy component C is distributed between two of three effluent streams.

FIGURE 3 is a schematic diagram of a solvent aided fractionation according to the invention in which a light component A is distributed between two of three effluent streams, and an intermediate component B is distributed between two of three effluent streams.

FIGURE 4 is a schematic diagram of a solvent aided fractionation according to the invention in which an intermediate component B is distributed between two of three effluent streams, and a heavy component C is distributed between two of three effluent streams. FIGURE 5 is a schematic diagram of a solvent aided fractionation according to the invention in which a light component A is distributed between two of four effluent streams, intermediate component B is distributed between two of four effluent streams, and heavy component C is distributed between two of four effluent streams.

FIGURE 6 is a schematic diagram of a solvent aided fractionation according to the invention in which two portions of the fractionation operate under different absolute pressures.

FIGURE 7 is a graph depicting solvent-free liquid molar_concentration profiles of methane, ethylene and ethane within the fractionation column of the Example.

It should be noted that only essential separation and heating/cooling steps are shown in these schematic diagrams. Those skilled in the art will recognize that individual streams in any of the embodiments may be heated or cooled in order to improve the overall efficiency or operability of a given apparatus. Additionally, one or more side condensers or side reboilers could be used on many of the apparatuses that are described below. These practices are well understood by those skilled in the art and do not constitute an essential part of this invention.

BRIEF DESCRIPTION OF THE INVENTION

Processes of this invention are suitable for use in recovery and separation of organic compounds from a mixture comprising volatile organic compounds. 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 or propylene.

Processes of the invention include recovery and partial separation of components from a gaseous mixture through use of a suitable solvent. In particular it encompasses the principle of "Distributed Absorption" in which at least one of the components of the mixture is allowed to leave the fractionation unit in two distinct streams. This is contrary to prior art in the fields of absorption and extractive distillation, which teach essentially quantitative separation of each component into a single effluent stream.

For the purpose of following descriptions, three generic components, i.e.,

Component A, Component B, and Component C are described. Each of these generic components may itself be a mixture of compounds and so the descriptions given here can relate to the separation of mixtures that contain more than three

compounds. In absorption-based systems, the interaction between the solvent and components A, B, and C is purely physical and the components are therefore separated according to their relative pure component boiling points. In an absorption-based system, generic Component A has the lowest boiling point, Component C has the highest boiling point, and Component B has a boiling point that is intermediate between that of Component A and Component C. In extractive distillation-based systems an interaction exists between the solvent and the various components in the gas stream which affects the relative volatility of the components to be separated. In such an extractive distillation-based system, generic Component A has the lowest affinity for the solvent and is therefore least absorbed by the solvent. Component C has the highest affinity for the solvent, and is therefore most strongly absorbed by the solvent. Component B has an affinity for the solvent that is intermediate between that of Component A and Component C.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS OF THE

INVENTION

While this invention is susceptible of embodiment in many different forms, this specification and accompanying drawings disclose only some specific forms as an example of the use of the invention. In particular, preferred embodiments of the invention for the solvent assisted recovery and partial separation of components from a mixture of volatile organic compounds 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 process 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 with reference to FIGURE 1 , which illustrates a fractionating unit, including stripping and rectifying sections, wherein a solvent aided fractionation according to the invention distributes light component A between two of three effluent streams, i.e., an overhead stream, a bottoms stream, and a sidedraw stream from the rectification section. In this embodiment component A is distributed into the overheads and a sidedraw stream. The mixture of components A, B and C is directed as stream 11 into column 12. A lean solvent stream enters near the top of column 12 via stream 13. Overhead

effluent stream 14 contains essentially pure component A. Sidedraw stream 15 is taken from the column at a point between the column feed inlet stream 11 and the lean solvent inlet stream 13. This sidedraw stream contains components A, B, and solvent. The bottoms stream 16 contains only component C and solvent.

While not limiting the methods by which the column could be controlled and operated, one practical method for operating the column is described below. The amount of component B in the overhead stream can be controlled by adjusting the flow or temperature of the lean solvent entering the top of the column. The amount of component C in the sidedraw stream can be controlled by adjusting the sidedraw flow rate. Optionally, a portion of the sidedraw can be cooled and returned to the column to further control the amount of component C entering the sidedraw stream. The amount of component B in the bottoms stream can be controlled by adjusting the amount of stripping vapor generated by the reboiler 17. Those skilled in the art would be able to develop similar control strategies for each of the embodiments described below.

FIGURE 2 illustrates a fractionating unit, including stripping and rectifying sections, wherein a solvent aided fractionation according to the invention distributes heavy component C between two of three effluent streams, i.e., an overhead stream, a bottoms stream, and a sidedraw stream from the stripping section. A feed mixture comprising A, B and C is directed as stream 21 to column 22. A lean solvent stream enters near the top of column 22 via stream 23. Overhead effluent stream 24 contains essentially pure component A. A sidedraw stream 25 is taken from the stripping section of column 22 at a point below the feed inlet stream 21 , and the column bottoms stream 26. This sidedraw stream contains essentially all of component B entering the fractionating unit, as well as a portion of the component C entering the unit. The sidestream also contains solvent. The bottom stream 26 contains the balance of component C and solvent. As shown, the column is reboiled with exchanger 27.

FIGURE 3 illustrates a fractionating unit, including stripping and rectifying sections, wherein a solvent aided fractionation according to the invention distributes light component A between two of three effluent streams, i.e., an overhead stream, and a sidedraw stream from the rectification section, and an intermediate component B is distributed between two of the three effluent streams, i.e., a bottoms stream, and a sidedraw stream from the rectification section. The feed mixture comprising A, B and C is directed as stream 31 into column 32. Lean solvent enters near the top of column 32 via stream 33. Overhead effluent stream

34 contains essentially pure component A. A sidedraw stream 35 is taken from the rectification section of column at a point above the column feed inlet stream 31 and the lean solvent inlet stream 33. This sidedraw stream contains component A, a portion of the component B entering the unit, and solvent. The bottoms stream 36 contains the balance of component B, essentially all of component C entering the unit, and solvent. As shown, the column is reboiled with exchanger 37.

FIGURE 4 illustrates a fractionating unit, including stripping and rectifying sections, wherein a solvent aided fractionation according to the invention distributes intermediate component B between two of three effluent streams, i.e., an overhead stream, and a sidedraw stream from the stripping section, and a heavy component C is distributed between two of the three effluent streams, i.e., a bottoms stream, and a sidedraw stream from the stripping section. The feed mixture comprising A, B and C is directed as stream 41 into column 42. Lean solvent enters near the top of column 42 via stream 43. Overhead effluent stream 44 contains essentially all of component A entering the column, as well as a portion of the component B entering the column. A sidedraw stream 45 is taken from the stripping section of column at a point below the column feed inlet stream 41. This sidedraw stream contains component B, a portion of the component C entering the unit, and solvent. The bottoms stream 46 contains the balance of component C entering the unit, and solvent. As shown, the column is reboiled with exchanger 47.

FIGURE 5 illustrates a fractionating unit, including stripping and rectifying sections, wherein a solvent aided fractionation according to the invention distributes: light component A between two of four effluent streams, i.e., an overhead stream and a sidedraw stream from the rectification section; intermediate component B between two of four effluent streams, i.e., the sidedraw stream from the rectification section and a sidedraw stream from the stripping section; and heavy component C between two of four effluent streams, i.e., the sidedraw stream from the stripping section, and the bottom stream.

The feed mixture comprising A, B and C is directed as stream 51 into column 52. Lean solvent enters near the top of column 52 via stream 53. Overhead effluent stream 54 contains essentially pure component A. A sidedraw stream 55 is taken from the rectification section of column 52 at a point above the column feed inlet stream 51 and the lean solvent inlet stream 53. This sidedraw stream contains the balance of component A, and a portion of component B entering the unit, and a portion of the solvent entering the unit. A sidedraw stream

56 is taken from the stripping section of column 52 at a point below the column feed inlet stream 51. This sidedraw stream contains the balance of component B, and a portion of heavy component C entering the unit, and a portion of the solvent entering the unit. The bottoms stream 57 contains the balance of component C and solvent entering the unit. As shown, the column is reboiled with exchanger 58.

The fractionating units depicted in figures 1 through 5 are useful elements of larger separation and purification processes. In such larger processes other elements would typically carry out the tasks of further separating and purifying each of the components, reclaiming and purifying the solvent for re-use, and re- circulating the reclaimed solvent back to the recovery column or columns. There are many ways in which the larger process could be configured, depending on the nature of the components, the nature of the solvent, and the desired product recovery and purity specifications. The precise configuration of the larger process can de developed using methods well known to those skilled in the art, and does not affect the nature of this invention.

In some cases it may be desirable to divide the fractionating unit of this invention into two or even more subsections. This may be desirable, for example, in order to operate one section of the fractionation unit at one pressure and another section of the fractionation unit at a second pressure. As depicted in FIGURE 6, a particularly useful implementation of this invention uses a dual- pressure configuration. This dual-pressure configuration can be used in place of the single fractionation devices shown in FIGURE 1 and FIGURE 3. In the implementation of FIGURE 6, column 12 of FIGURE 1 has been divided into two subsections - a low-pressure subsection 102 and a higher-pressure subsection 108.

In the configuration of FIGURE 6, the mixture of components A, B and C is directed as stream 101 to the low-pressure subsection 102. Subsection 102 is reboiled with exchanger 103 so that components A and B are stripped from the bottoms stream 104. Stream 104, therefore, comprises component C and solvent and is substantially free of components A and B. The overhead vapor stream 105 from subsection 102 comprises components A and B and solvent vapor. It is compressed in compressor 106 and directed as stream 107 to the higher pressure, rectification subsection 108.

A lean solvent stream 109 enters near the top of the higher-pressure subsection 108. The overhead stream 110 contains essentially pure component

A. The bottoms liquid stream 111 from the higher-pressure subsection comprises

components A and B and solvent, and is substantially free of component C. Stream 111 is divided into two streams. Stream 112 is withdrawn as a product stream from the system. Stream 113 is flashed across valve 114 and directed as reflux liquid stream 115 to the low-pressure subsection 102. In the implementation shown, enough reflux liquid stream 115 is directed to the low-pressure subsection 102 to ensure that stream 105 is substantially free of component C.

This two-pressure implementation depicted in FIGURE 6 allows the stripping operation in subsection 102 and the rectification operation in subsection 108 to occur at different pressures. This would be particularly beneficial in cases where there are maximum temperature limitations within the stripping sections. Such would be the case, for example, when the solvent and/or one or more of the components were prone to thermal degradation or caused fouling of the equipment when exposed to high temperatures. In this case subsection 102 could be operated at a lower pressure and therefore a lower temperature, while carrying out a higher-pressure rectification in subsection 108.

It should be noted that other dual-pressure implementations of the invention are possible. For example, the product stream shown as stream 112 in FIGURE 6 could be taken as a sidedraw product from subsection 108 or subsection 102, rather than as a bottoms product from 108. Likewise more complex couplings of the two subsections could be configured. For example, reflux for subsection 102 could be taken as a side-draw stream from subsection 108, while at least a portion of the bottoms stream from subsection 108 would enter subsection 102 at a point between the feed location of stream 101 and the top of the subsection. These and other various implementations are all within the scope of the two-pressure aspect of the invention.

It will be further recognized by those skilled in the art that once the general concept of dual-pressure operation depicted for the second embodiment of this invention in FIGURE 6 is grasped, it can also be implemented on the other embodiments of this invention.

EXAMPLES OF THE INVENTION

The following Examples will serve to illustrate certain specific embodiments of the herein disclosed invention. These Examples 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 FIGURE 3 was simulated using commercially available process simulation software. A gas mixture consisting of the C3- fraction of a cracked gas stream (for example, the overhead stream from a front-end depropanizer column) from a steam cracking furnace is first chilled to negative 15° F, and then introduced into a fractionating unit, including stripping and rectifying sections. In this example the cracked gas stream consists primarily of hydrogen, methane, ethane, ethylene, acetylene, propane, propylene, methylacetylene and propadiene.

The stripping and rectifying sections contain liquid/vapor contacting trays equivalent to 90 equilibrium stages. In this example the column has liquid pump- around circuits at theoretical stages 15, 20, 24 and 30. that chill solvent at each of these stages to negative 30° F (stage 1 is at the top of the column, stage 90 is at the bottom of the column). The feed is introduced at theoretical stage 70. A chilled lean solvent consisting of a mixture of C3 and C4 hydrocarbons is introduced into the top of the column (stage 1 ). A liquid sidestream is removed at theoretical stage 30. This liquid sidestream contains primarily solvent and ethylene, along with some dissolved hydrogen and methane. The amount of ethane in the sidestream is controlled (with the sidestream draw rate, for example) so that the ratio of ethane to ethylene in this stream is less than or equal to that in the product-quality ethylene to be produced. The bottoms of the column contain primarily solvent and C2+ hydrocarbons. The amount of hydrogen and methane in the bottoms stream is controlled by reboiling the bottom of the column using a conventional reboiler exchanger.

Table 1 shows the compositions and conditions of the various streams of this example, and the heat exchanger duties are shown in Table 2. All stream and exchanger numbers are referenced to FIGURE 3. FIGURE 7 depicts the liquid molar concentration profiles of methane, ethylene, and ethane in the column of this example. The section labeled "Top" in FIGURE 7 lies between the lean solvent inlet (theoretical stage 1 ) and the liquid side draw point (theoretical stage 30). There is essentially no ethane in this section, and the primary mass transfer that is occurring is capture of ethylene from the upward flowing gas by the downward flowing lean solvent. The section labeled "Middle" in FIGURE 7 lies between the side draw point (theoretical stage 30) and the feed location (theoretical stage 70). The primary mass transfer that is occurring in this section is solvent-assisted separation of ethylene from ethane. Note that at the side draw point there is essentially no ethane remaining in the liquid. The section labeled

"Stripping" in FIGURE 7 lies between the feed point (theoretical stage 70) and the bottom stream (theoretical stage 90). The primary mass transfer that is occurring in this section is stripping of methane from the liquid by the upward flowing stripping vapors. The liquid at the bottom of the distributor absorber column contains essentially no methane. The reboiler duty for this example is 24.5 million BTUs per hour.

It is clear from the compositions given in Table 1 and shown in FIGURE 7 that ethylene is distributed between the sidedraw stream 35 and bottoms stream 36 of the column, and hydrogen and methane are distributed between the sidedraw and the overheads stream 34 from the column. As was noted, this example is similar to the embodiment of this invention shown in FIGURE 3, where Component A (with a relatively low boiling point or affinity for the solvent) is the combination of hydrogen and methane, Component B (with an intermediate boiling point or affinity for the solvent) is ethylene, and Component C (with a relatively high boiling point or affinity for the solvent) is the combination of ethane and heavier hydrocarbons.

EXAMPLE 2

This example demonstrates the operation of a dual-pressure absorption device similar In nature to the embodiment of FIGURE 6. An ethylene recovery process based on the dual-pressure absorption device of Figure 6 was simulated using commercially available process simulation software. The feedstream into the dual-pressure absorption device is the same as in Example 1 , except that the feed pressure is approximately 245 psia. Like the single-pressure device of FIGURE 1 , the dual-pressure device of this example produces an overhead stream (stream 110 of FIGURE 6) comprising hydrogen and methane, an intermediate stream (stream 112 of FIGURE 6) comprising solvent and ethylene, and a bottoms stream (stream 104 of FIGURE 6) comprising solvent, ethylene, and ethane.

In this example the low-pressure section (corresponding to column 102 in FIGURE 6) is operated at a top pressure of 235 psia and the high-pressure section (corresponding to column 108 in FIGURE 6) is operated at a top pressure of 455 psia. The high-pressure section is therefore operated at a pressure approximately 94% higher than that of the low-pressure section. The reboiler of the low-pressure section of this example operates at a temperature of about 56° F. This is significantly lower than the reboiler temperature of the absorber device of

Example 1 , which operates at around 104 ⁰ F. The lower temperature reboiler

operation provided by the dual-pressure design of FIGURE 6 and demonstrated in this example could be advantageous when one or more compounds in the feed or solvent stream is thermally sensitive.

Examples have been presented and hypotheses advanced herein in order to better communicate certain facets of the invention. The scope of the invention is determined solely by the scope of the appended claims.

TABLE 1 Stream Properties and Compositions for Example

TABLE 2 Exchanger Duties for Example