WO2007018509A1

WO2007018509A1 - Cryogenic fractionation process

Info

Publication number: WO2007018509A1
Application number: PCT/US2005/026762
Authority: WO
Inventors: Rian Reyneke
Original assignee: Innovene Usa Llc
Priority date: 2005-07-28
Filing date: 2005-07-28
Publication date: 2007-02-15

Abstract

Processes using cryogenic distillation columns in a front-end fractionation unit that more efficiently recover one or more useful component from a mixture containing a plurality of volatile organic compounds are disclosed. Beneficially, processes of the invention are used for initial fractionation of cracked gas stream (100) containing olefins such as are typically produced by thermal cracking of suitable hydrocarbon feedstocks, by chilling a portion of the cracked gas stream by indirect heat exchange (106) with the column overheads stream (110), directing the chilled portion to the cryogenic distillation tower (105), and directing a second portion (101) of the cracked gas stream to separate point on the cryogenic distillation tower.

Description

CRYOGENIC 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 a cryogenic distillation column in a fractionation unit to more efficiently recover one or more useful component from a mixture containing a plurality of volatile organic compounds. Processes according to this invention are particularly useful for at least partial separation of components from a mixed gas stream in an olefins manufacturing process. Beneficially, processes of the invention are used for initial fractionation of cracked 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 requirements for the process are 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 is a common technique for separating alkenes, such as ethylene, from gaseous mixtures containing molecules of similar size, e.g., ethane or methane. However, cryogenic techniques have disadvantages such as high capital cost and high operating expenses.

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.

*Sublimation temperature

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.

U.S. Pat. No. 6,354,105 in the name of Rong-Jwyn Lee, Pallav Jain, Jame Yao, Jong Juh Chen and Douglas G. Elliot (Lee et al.) describes a split feed compression process for recovery of ethylene, ethane and heavier components using a cryogenic distillation column which involved dividing the feed gas into a first gaseous stream and a main gaseous stream. The first gaseous stream is compressed and cooled, and then expanded and introduced into the top of a cryogenic distillation column as a main reflux stream.

Lee et al. described a process specific to demethanizer towers, but did not address use of towers in which the ethane or ethylene is recovered to the overhead from the tower. Lee et al. state that a beneficial aspect of their process is high recovery of C2 hydrocarbons to the tower bottoms stream.

De Cintio et al (Hydrocarbon Processing, July 1991 , pp 83-90) describe a front-end deethanizer tower which utilizes a two-feed arrangement, one vapor and one liquid feed. They did not disclose or suggest reducing energy requirements of the process by heat integrating tower overheads with feed chilling.

Ethylene separation and purification is typically done via cryogenic distillation. Many different process designs have been suggested for the separation and purification of ethylene from the cooled and compressed reactor effluent gas. A summary of such designs which are currently commercially available has been published recently (Hydrocarbon Processing, March 2003, pp 96-98).

These designs have typically been grouped in terms of the initial rectification operation that is performed. For example, in a "front end demethanizer" design the compressed, cooled cracked gas is directed to a demethanizer feed chilling train in which it is successively chilled and partially condensed. The condensed liquids are separated and directed to a demethanizer tower. In this tower the methane and lighter components are recovered into the demethanizer overhead stream and the ethylene and heavier components are recovered into the demethanizer bottoms stream. The ethylene and heavier components are typically sent to a deethanizer column where any components heavier than ethane are recovered into the deethanizer bottoms stream. The deethanizer overhead stream contains primarily ethylene, ethane and acetylene and is directed to an acetylene removal step, such as a hydrogenation reactor. The essentially acetylene-free stream from the hydrogenation reactor is then directed to a C2 splitter column where the ethylene and ethane are separated. Front end demethanizer designs are often used when the ethylene is produced from a relatively heavy feedstock, such as a naphtha or gas oil.

"Front-end deethanizer" methods include those in which a deethanizer tower is the initial rectification operation that is performed. In these methods the cracked gas is chilled and directed to a deethanizer tower. The overhead of the deethanizer contains primarily methane, hydrogen, ethylene and ethane, while components heavier than ethane are recovered to the deethanizer bottoms Front- end deethanizer methods are often utilized when the steam cracker feed is a light gas such as ethane, propane, butane, or a mixture of these.

"Front-end depropanizer" methods are similar to front-end deethtanizer methods, except that the first step is to remove the propane and lighter components from the cracked gas stream, In this case the depropanizer overhead stream contains propane and lighter components, while components heavier than propane are recovered to the depropanizer bottoms.

In general the optimal separation method will depend on many factors, including feed type, product requirements, energy cost, and feed cost, among others. However, all of these methods are cryogenic in nature, that is troy operate at temperatures below ambient, and therefore require significant energy in the form of refrigeration power

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 recovery, but does not require appreciable increases in capital and operating costs.

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 from a cracked gas mixture.

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

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 recovery of one or more useful component from a mixture containing a plurality of volatile organic compounds, especially for cryogenically recovering components of a relatively impure olefins stream produced by thermal cracking of hydrocarbons. Processes of this invention comprise: providing a fractionation unit comprising a cryogenic distillation column having an overhead outlet and reflux inlet near the top of the column, two or more feed inlets below the reflux inlet, and an outlet for liquid residue from the bottom of the column; fractionating a mixture of organic compounds to thereby obtain at least an overhead stream comprising a portion of the more volatile components and a liquid residue comprising a portion of the less volatile components of the mixture; heating at least a portion of the overhead stream by indirect heat exchange with a first portion of the mixture, processing the heated overhead stream to obtain liquid condensate, and using a sufficient portion of the condensate to reflux the top of the cryogenic distillation column; feeding the first portion of the mixture effluent from the indirect heat exchange into the column at level below the reflux inlet; and feeding a second portion of the mixture directly or indirectly into to the column at level below the inlet of the first portion.

Suitable feed mixtures of organic compounds include any gaseous stream comprising hydrogen, methane, ethylene, C3 hydrocarbons, and optionally acetylene and/or C4 hydrocarbons. Particularly suitable are mixtures comprising olefins produced by thermal cracking of hydrocarbons.

In one aspect of the present invention, the overhead stream is essentially free of C4 hydrocarbons. In another, the overhead stream is essentially free of C3 hydrocarbons. In yet another, the liquid residue from the bottom of the column is essentially free of ethylene. In some cases, it is beneficial that the overhead stream and the liquid residue each contain C3 hydrocarbons.

In a particularly useful aspect of the present invention, the second portion of the mixture is chilled, and the resulting chilled second portion is fed directly or indirectly into to the column at level below the inlet of the first portion. The first portion of the mixture effluent from the indirect heat exchange advantageously is chilled to a temperature at least 5° F lower than the chilled second portion, and the resulting chilled stream is fed directly or indirectly into to the column at level below the reflux inlet. Another aspect of the present 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: providing a fractionation unit comprising a cryogenic distillation column having an overhead outlet and reflux inlet near the top of the column, two or more feed inlets below the reflux inlet, and an outlet for liquid residue from the bottom of the column; fractionating a gaseous mixture comprising hydrogen, methane, ethylene, acetylene, and C3 hydrocarbons to thereby obtain at least an overhead stream comprising a portion of the more volatile components and a liquid residue comprising at least a portion of the less volatile components of the mixture; heating at least a portion of the overhead stream by indirect heat exchange with a first portion of the mixture, compressing at least a portion of the heated overhead stream to an absolute pressure at least 50 percent higher than that in the top of the column; processing the compressed stream to obtain liquid condensate, and using a sufficient portion of the condensate to reflux the top of the cryogenic distillation column; feeding the first portion of the mixture effluent from the indirect heat exchange into the column at level below the reflux inlet; and feeding a second portion of the mixture directly or indirectly into to the column at level below the inlet of the first portion.

Another aspect of special significance is the separation of acetylenic impurities, if any, from the overhead stream. In theses cases, processes of the present invention further comprises treating the compressed overhead stream by a process of selective hydrogenation to obtain a compressed stream essentially free of acetylene, and thereafter processing the compressed stream to obtain the liquid condensate.

In yet other aspects of the present invention the gaseous mixture is an effluent stream from a steam cracking unit. In most cases liquid residue from the bottom of the column advantageously is essentially free of ethylene.

The second portion of the mixture beneficially can be chilled, and the resulting chilled second portion is fed directly or indirectly into to the column at level below the inlet of the first portion. In yet another aspect of the present invention, the mixture effluent from the indirect heat exchange is chilled to a temperature at least 5° F lower than the chilled second portion, and the resulting chilled stream is fed directly or indirectly into to the column at level below the reflux inlet.

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 comparative process for initial fractionation of cracked gas stream containing olefins.

FIGURE 2 is a schematic diagram of an embodiment of this invention in which a cryogenic distillation column in a fractionation unit is used to remove C4 and heavier hydrocarbons from a cracked gas.

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 one or more side condensers or side reboilers could be used on any of the embodiments 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

The commercial manufacture of ethylene has taken place for decades, and processes for the production of purified ethylene products have been the subject of much academic and commercial interest. In particular the separation of ethylene from other byproducts produced in its manufacture has been exhaustively studied. There are many commercial methods for recovering and purifying ethylene from a mixed hydrocarbon stream.

The vast majority of ethylene is produced commercially through the steam cracking of hydrocarbons. In a steam cracking furnace a relatively low-pressure hydrocarbon feed is mixed with steam and this mixture is subjected to high temperatures. The hydrocarbons are converted into a furnace effluent gas mixture, also known as cracked gas, which typically comprises ethylene, methane, hydrogen, acetylene and unconverted feed, as well as some hydrocarbons heavier than the feed. The hot furnace effluent gas is cooled by raising high pressure steam and also typically by direct contact with circulated cooled quench oil and/or circulated cooled water. These cooling steps typically condense and at least partially remove relatively heavy hydrocarbons, typically in the naphtha range and heavier. The uncondensed cooled effluent gas is then directed to a compressor section in which the gas is compressed in one or more stages (typically 3-5 stages) to an elevated pressure. The effluent from each stage is typically cooled against an ambient temperature medium and any condensed liquids removed before entering the subsequent compression stage. Acid gases such as H2S and CO2 are generally removed after one of these stages of compression, for example through the use of a caustic contacting tower or an amine scrubbing system. Once compressed, scrubbed and dried, the furnace effluent gas enters the separation section.

The process of this invention provides for a more energy-efficient method for performing the initial rectification operation for removing relatively heavy hydrocarbons from a mixed gas, for example mixtures containing ethylene and components heavier than ethane. The feed to the initial rectification column is split into at least two streams. At least one of the feed streams is cooled by heat exchange with the rectification column overhead stream. The two or more streams are then separately fed to the initial cryogenic distillation column.

The equipment used for fractionation of the mixture of volatile organics generally includes a cryogenic distillation column that contains mechanical means for enhancing the contacting of the vapor and liquid within the column. These means can include structured or unstructured packing and contacting trays such as bubble-cap, sieve, or valve-type trays. By far the most common type of distillation column is a packed or trayed column in which the liquid and gas streams flow counter-currently in at least a portion of the column. In a typical design, a liquid reflux is introduced at the top of the distillation column and the gas mixture is introduced into a middle section of the column. The liquid residue (containing heavier, less volatile components) exits at the bottom of the column, and the overhead vapor exits at the top of the tower.

In "front-end deethanizer" methods of the invention, the cracked gas is chilled and then directed to a deethanizer distillation column. Within this column components heavier than ethane are removed in the bottoms stream, and the ethane and lighter components, including the desired ethylene product, are recovered in the overhead. The overhead stream can be further compressed if desired, and then is typically further processed in a demethanizer tower and a C2 splitter tower to recover a purified ethylene product. Acetylene impurities can be removed either before or after processing the stream in a demethanizer column. Front-end deethanizer flowsheets are often utilized when the ethylene is produced from relatively light feedstocks, such as ethane, propane or butane. "Front-end depropanizer" flowsheets are similar in concept to front-end deethanizer flowsheets, except that the initial rectification column is operated so that essentially all of the C4 and heavier hydrocarbons are recovered in the column bottoms stream. In a "partial depropanizer" column some C3 hydrocarbons can also be recovered in the column bottoms stream. The depropanizer overheads stream therefore contains C3 and lighter components, including ethylene. This stream can be further compressed if desired and is typically further processed within a deethanizer tower, a demethanizer tower, and a C2 splitter tower to recover a purified ethylene product. The order of these subsequent separation steps can vary, but typically the C2 splitter column is the final separation step within the ethylene purification train. Acetylene impurities can be removed either before the subsequent purification steps, or they may be removed from the C2 splitter feed before final purification of the ethylene product. The optimal separation method will depend on many factors, including feed type, product requirements, energy cost, and feed cost, among others.

Because essentially the entire cracked gas stream passes through the front-end rectification operation, it is beneficial to carry out this operation with the highest possible efficiency. We have found a simple yet effective design which significantly improves the energy efficiency of front-end deethanizer and front-end depropanizer columns. This design consists of splitting the front-end rectification column feed into at least two streams. At least one of the streams is chilled through indirect heat exchange with the column overhead. The two or more streams are directed to different points on the front-end rectification column. This design is particularly useful when the overhead stream from the front-end rectification column is to be compressed and/or heated before processing it further.

A benefit of this invention is that it requires less energy than prior-art processes to effect the same desired separation, in particular the removal of C3+ or C4+ hydrocarbons from a cracked gas stream containing ethylene. A further benefit of this invention is that it warms the column overhead stream, thereby allowing a downstream compressor to be constructed of a lower-grade metallurgy than in prior-art processes.

The present invention consists of a new method for cooling and introducing the cracked gas feed to a front-end rectification column while heat-integrating at least one of the feed streams with the column overhead stream. There has now been found a surprisingly large improvement in energy efficiency when combining a split-feed arrangement with feed/overhead heat integration for a rectification column which recovers the ethylene to the column overhead.

This invention is applicable to common front-end deethanizer and front-end depropanizer designs for the initial rectification of cracked gases produced during ethylene manufacture. It is particularly applicable in designs where the overhead of the front-end rectification tower is compressed or heated before being further processed.

This invention represents an improved method for removing relatively heavy hydrocarbons from the cracked gas produced in an ethylene manufacturing process. This invention relates to the initial ("front-end") rectification operation to which is directed the dried chilled cracked gas from an ethylene cracking furnace. In particular this invention relates to front-end deethanizer and front-end depropanizer operations. This invention does not relate to front-end demethanizer operations.

It is to be noted that while the comparative process and the process of this invention are described in terms of a front-end partial depropanizer operation, those skilled in the art will recognize that the concepts of this invention can be applied to the operation of front-end full depropanizer and front-end deethanizer columns. In these cases the composition of the overhead and bottoms streams will be different from those described herein under, and certain optimal operating conditions may be different. However, the basic concepts of split-feed operation and heat integration of the feed stream with the column overhead are generally applicable to front-end deethanizer front-end full depropanizer, and front-end partial depropanizer columns.

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 fractionation of dried, chilled gaseous mixtures from the thermal cracking of hydrocarbons, for example, cracked gas from a steam cracking furnace.

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, a preferred embodiment of the invention for recovery and partial separation of components from a mixture of volatile organic compounds is illustrated and described. The invention is not intended to be limited to the embodiment 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. Various values of compositions, flow rates, temperatures, and pressures are given in association with a specific example described below; those conditions are approximate and merely illustrative, and are not meant to limit the invention.

For purposes of comparison only, an exemplary process is described below with reference to FIGURE 1 and compared with the inventive process depicted in FIGURE 2. More specifically with reference to FIGURE 1 , which illustrates a cryogenic distillation column of a fractionation unit in which a single- feed, front-end, partial depropanizer is utilized to remove C3 and heavier hydrocarbons from a cracked gas.

A gaseous mixture comprising hydrogen, methane, ethylene, ethane, acetylene, C3 hydrocarbons, and C4+ hydrocarbons, from a source of cracked gas (not shown), enters exchanger 2 in stream 1. The mixture is chilled in exchanger 2, and a chilled effluent therefrom in stream 3 is fed into the partial depropanizer column 4. In practice exchanger 2 could represent a series of exchangers in which the cracked gas is chilled in stages through heat exchange with cold process streams and/or various levels of external refrigeration. Liquid residue is withdrawn from the column in bottoms stream 5. The liquid residue comprises C4+ hydrocarbons and a portion of the C3 hydrocarbons that enter in stream 3. Stream 5 is essentially free of C2 and more volatile, lighter, components. It can be processed further to recover desired products such as propylene, butenes, butadiene, and naphtha if desired. Stripping vapor is provided to column 4 with reboiler 6.

Overhead stream 7 from the top of the column comprises hydrogen, methane, ethylene, ethane, acetylene, and C3 components. Stream 7 is essentially free of C4 and less volatile, heavier hydrocarbons. Stream 7 is directed to compressor 8 to produce compressed overhead stream 9. Because this compressed stream contains both hydrogen and acetylene, it can be directed into an acetylene hydrogenation unit 10. The detailed design and operation of an acetylene hydrogenation unit is well-known to those skilled in the art and is not described in detail herein. An acetylene hydrogenation unit will generally involve heating or cooling of stream 9 to achieve a desired temperature, selective hydrogenation of the acetylene over a Pd-based catalyst in one or more reactors, and the subsequent cooling of the essentially acetylene-free reactor effluent.

The essentially acetylene-free reactor effluent in stream 11 is chilled and partially condensed in exchanger 12. In practice exchanger 12 could represent a series of exchangers in which the acetylene-free reactor effluent is chilled in stages through heat exchange with cold process streams and/or various levels of external refrigeration such as a propylene refrigeration system. The chilled and partially condensed stream 13 is directed to separation drum 14 in which vapor and liquid are separated. The vapor exits in stream 15 and is processed further to recover ethylene and other desired products. The condensed liquid in stream 16 is apportioned into two streams. Stream 17 is directed as reflux into the top of column 4. The pressure of stream 17 is reduced through valve 18 to a suitable level before entering column 4 as stream 17a. If there is more liquid in stream 13 than is required as reflux to column 4, a portion of the condensed overhead liquid is withdrawn in stream 19. This stream would be further processed to recovery ethylene and other desired products.

FIGURE 2 depicts an embodiment this invention in which a split-feed, front- end, partial depropanizer is utilized to remove C3 and heavier hydrocarbons from a cracked gas. The overhead stream is essentially free of C4+ hydrocarbons, and the liquid residue from the column bottom is essentially free of C2 and lighter compounds. C3 components are present in both the column overhead stream and bottom residue stream. The overhead stream from the top of the column is compressed before being processed and condensed to provide reflux liquid to the top of the column.

A gaseous mixture comprising hydrogen, methane, ethylene, ethane, acetylene, C3 hydrocarbons, and C4+ hydrocarbons, from a source of cracked gas (not shown) in stream 100 is divided into streams 101 and 102. Stream 101 is chilled in exchanger 103 to produce stream 104 which enters at a lower location on the partial depropanizer column 105. Stream 102 is directed to exchanger 106 where it is chilled by indirect heat exchange with the column overhead stream. The chilled stream 107 is typically at a lower temperature than stream 104. Stream 107 is directed to an upper location on the partial depropanizer column 105. A liquid residue stream 108 is withdrawn from the bottom of column 105, and comprises C4+ hydrocarbons and optionally a portion of the C3 hydrocarbons in stream 100. Stream 108 is essentially free of C2 or lighter components. It can be processed further to recover desired products such as propylene, butenes, butadiene, and naphtha if desired. Stripping vapor is provided to column 105 with reboiler 109.

Overhead stream 110 comprising hydrogen, methane, ethylene, ethane, acetylene, and C3 components is essentially free of C4 and heavier hydrocarbons. It is directed into exchanger 106 where it is warmed by heat exchange with feed stream 102. The warmed overhead stream 111 is directed to compressor 112 to produce compressed stream 113 which is directed to the acetylene hydrogenation unit 114. The selective acetylene hydrogenation process of unit 114 will generally involve heating or cooling of stream 113 to achieve a suitable temperature for selective hydrogenation of the acetylene over a Pd-based catalyst in one or more reactors, and the subsequent cooling of the essentially acetylene-free reactor effluent.

The essentially acetylene-free reactor effluent stream 115 is chilled and partially condensed in exchanger 116. In practice exchanger 116 could represent a series of exchangers in which the acetylene-free reactor effluent is chilled in stages through heat exchange with cold process streams and/or various levels of external refrigeration. The chilled and partially condensed stream 117 is directed into separation drum 118 wherein vapor is separated from the liquid condensate. The vapor exits as stream 119 and is processed further to recover ethylene and other desired products. The liquid condensate in stream 120 is divided into two streams. Stream 121 is directed as reflux to the top of column 105. The pressure of stream 121 is reduced through valve 122 before entering column 105 as stream 121a. If there is more liquid condensate in stream 117 than is required as reflux to column 105, a portion of the liquid condensate can be withdrawn as stream 123. This stream would be further processed to recovery ethylene and other desired products.

It will be further recognized by those skilled in the art that once the general concept of the front-end partial depropanizer column operation depicted for the embodiment of this invention in FIGURE 2 is grasped, it can also be implemented on the other embodiments of this invention, for example front-end full depropanizer, and front-end deethanizer configurations as well. EXAMPLE OF THE INVENTION

The following Example will serve to illustrate a certain 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.

General

To demonstrate several beneficial aspects of the present invention, both the comparative process depicted in FIGURE 1 and the embodiment of FIGURE 2 were simulated using commercially available process simulation software. The mixed hydrocarbon gas feed was derived from the effluent from a battery of steam cracking furnaces cracking a mixture of ethane, propane, and naphtha. The furnace effluent was quenched, cooled, dried, purified of acid gases, and chilled to 25°F against propylene refrigerant and cold process streams before entering the simulations.

In both simulations the partial depropanizer column had 31 theoretical stages, the single-stage compressor outlet pressure was 530 psig, and the liquid product from the partial depropanizer (stream 18 of FIGURE 1 and stream 123 of FIGURE 2) was about 20,000 Ib/hr.

Feed stream 1 was chilled to approximately negative 1 °F before entering the partial depropanizer column at theoretical stage 15 (as numbered with stage 1 at the top of the column and stage 31 at the bottom of the column). Stream compositions of the process are given in Table 1. Table 2 presents a summary of the compressor and heat exchanger duties for the process of FIGURE 1. Stream and unit numbers in Tables 1 and 2 correspond to those of FIGURE 1.

For the process of this invention as embodied in FIGURE 2, the feed was divided such that the upper feed stream 102 comprises 60 percent of stream 100. The 60 percent portion was cooled to negative 35°F before entering the partial depropanizer at theoretical stage 10. The 40 percent portion in lower feed 101 is cooled to 0⁰F before entering the partial depropanizer at theoretical stage 16. The stream compositions of the process of this invention are given in Table 3. Table 4 presents a summary of the compressor and heat exchanger duties for process of this invention. Stream and unit numbers in Tables 3 and 4 correspond to those of FIGURE 2. Note that heat exchanger 12 and exchanger 116 are shown as single heat exchangers in FIGURE 1 and FIGURE 2, respectively, but were modeled as a series of heat exchangers cooled by various levels of propylene refrigerant. The duties presented for these exchangers in Tables 2 and 4 correspond to the sum of these various individual exchangers.

The total compressor horsepower, as calculated for each process, included the power required by the respective compressors shown in the Figures, and also for the propylene refrigerant compressor which is used to deliver the required chilling duties for exchangers 2 and 12 in FIGURE 1 , and exchangers 103, and 116 in FIGURE 2. The total compressor horsepower requirement for the comparative process was 22,382 HP. The total compressor horsepower requirement for the process of this invention was 21 ,291 HP. This result was an energy savings of over 1 ,000 HP for this invention.

The energy savings provided by the process of this invention was clearly unexpected and counter-intuitive, because the energy requirement of the compressor 112 of the present invention is actually 1 ,308 HP higher than the energy requirement of compressor 8 of the comparative process. Even though this process compressor energy requirement was higher for the process of this invention, it is more than offset by a reduction in energy requirement for the propylene refrigeration compressor brought about by the more efficient heat integration of this invention. This more efficient heat integration allows for a much smaller reflux stream in the process of this invention (compare stream 121 of

Table 3 with stream 17 of Table 1 ).

It is also clear from the data in Tables 1 and 3 that the feed to the compressor 112 of the process of this invention (see stream 111 in Table 3) was warmer than the feed to compressor 8 (see stream 7 in Table 1 ). Advantageously, compressor 112 of the present invention could be made of a lower-grade metallurgy than that of compressor 8.

An Example has 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.

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.

Table 1

Table 2

Table 3

OO

Table 4

Claims

That which is claimed is:

1. A process for recovery of one or more useful component from a mixture containing a plurality of volatile organic compounds, which process comprises: (1-a) providing a fractionation unit comprising a cryogenic distillation column having an overhead outlet and reflux inlet near the top of the column, two or more feed inlets below the reflux inlet, and an outlet for liquid residue from the bottom of the column;

(1-b) fractionating a mixture of organic compounds to thereby obtain at least an overhead stream comprising a portion of the more volatile components and a liquid residue comprising a portion of the less volatile components of the mixture;

(1-c) heating at least a portion of the overhead stream by indirect heat exchange with a first portion of the mixture, processing the heated overhead stream to obtain liquid condensate, and using a sufficient portion of the condensate to reflux the top of the cryogenic distillation column;

(1-d) feeding the first portion of the mixture effluent from the indirect heat exchange into the column at level below the reflux inlet; and

(1-e) feeding a second portion of the mixture directly or indirectly into to the column at level below the inlet of the first portion.

2. The process according to claim 1 wherein the mixture of organic compounds is a gaseous stream comprising hydrogen, methane, ethylene, acetylene, C3 hydrocarbons, and C4 hydrocarbons.

3. The process according to claim 2 wherein the overhead stream is essentially free of C4 hydrocarbons.

4. The process according to claim 2 wherein the overhead stream is essentially free of C3 hydrocarbons.

5. The process according to claim 2 wherein the liquid residue from the bottom of the column is essentially free of ethylene.

6. The process according to claim 2 wherein the overhead stream and the liquid residue each contain C3 hydrocarbons.

7. The process according to claim 1 wherein the second portion of the mixture is chilled, and the resulting chilled second portion is fed directly or indirectly into to the column at level below the inlet of the first portion.

8. The process according to claim 7 wherein the first portion of the mixture effluent from the indirect heat exchange is chilled to a temperature at least

5° F lower than the chilled second portion, and the resulting chilled stream is fed directly or indirectly into to the column at level below the reflux inlet.

9. A process for recovery of one or more useful component from a mixture containing a plurality of volatile organic compounds, which process comprises:

(9-a) providing a fractionation unit comprising a cryogenic distillation column having an overhead outlet and reflux inlet near the top of the column, two or more feed inlets below the reflux inlet, and an outlet for liquid residue from the bottom of the column; (9-b) fractionating a gaseous mixture comprising hydrogen, methane, ethylene, acetylene, and C3 hydrocarbons to thereby obtain at least an overhead stream comprising a portion of the more volatile components and a liquid residue comprising a portion of the less volatile components of the mixture;

(9-c) heating at least a portion of the overhead stream by indirect heat exchange with a first portion of the mixture, compressing at least a portion of the heated overhead stream to an absolute pressure at least 50 percent higher than that in the top of the column;

(9-d) processing the compressed stream to obtain liquid condensate, and using a sufficient portion of the condensate to reflux the top of the cryogenic distillation column;

(9-e) feeding the first portion of the mixture effluent from the indirect heat exchange into the column at level below the reflux inlet; and

(9-f) feeding a second portion of the mixture directly or indirectly into to the column at level below the inlet of the first portion.

10. The process according to claim 9 which further comprises treating the compressed overhead stream by a process of selective hydrogenation to obtain a compressed stream essentially free of acetylene, and thereafter processing the compressed stream to obtain the liquid condensate.

11. The process according to claim 9 wherein the gaseous mixture is an effluent stream from a steam cracking unit.

12. The process according to claim 9 wherein the liquid residue from the bottom of the column is essentially free of ethylene.

13. The process according to claim 9 wherein the second portion of the mixture is chilled, and the resulting chilled second portion is fed directly or indirectly into to the column at level below the inlet of the first portion.

14. The process according to claim 13 wherein the first portion of the mixture effluent from the indirect heat exchange is chilled to a temperature at least 5° F lower than the chilled second portion, and the resulting chilled stream is fed directly or indirectly into to the column at level below the reflux inlet.