Abstract:
A process and system is provided for separating a feed gas stream containing methane, at least one C 2  component, at least one C 3  component, and optionally heavier components, into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  and heavier components. The feed stream is cooled, at least partially condensed, and fed to a fractionation column wherein the feed stream is separated into an overhead vapor stream comprising primarily the lighter components of the feed stream and a bottoms liquid stream comprising primarily the heavier components of the feed stream. The introduction of a reboiler onto the fractionation column assists in removing co-absorbed C 2  and lighter components from the fractionation column bottoms thereby facilitating more efficient operation of a downstream deethanizer column. Addition of residue recycle can further supplement recovery of desired components.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/360,753, filed Jul. 1, 2010, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is directed toward processes and systems for recovering liquefied petroleum gas (LPG) from a hydrocarbon gas stream, especially a natural gas stream or a refinery gas stream. Particularly, the processes and systems described herein may be utilized to enhance LPG recovery, particularly when processing higher pressure or leaner feed streams thereby providing broader applicability compared to previous processes. 
         [0004]    2. Description of the Prior Art 
         [0005]    Natural gas comprises primarily methane, but can also include varying amounts of heavy hydrocarbon components such as ethane, propane, butane, and pentane, for example. It is well known that natural gas streams can be separated into their respective component parts. Such processes involve a combination of chilling, expansion, distillation and/or like operations to separate methane and ethane from C 3  and heavier hydrocarbon components. Typically the separation made is of methane and ethane from propane and heavier components. If economically desirable, the ethane could also be recovered and similarly, it is desirable in many instances to further fractionate the recovered C 3  (or alternatively C 2 ) and heavier components. 
         [0006]    One process that has been devised for separating a natural gas stream into light and heavy component streams is shown in U.S. Pat. No. 5,771,712, incorporated by reference herein its entirety. The &#39;712 Patent demonstrates a typical process wherein an overhead stream from a deethanizer is passed into heat exchange with an exit stream from an absorber to cool the overhead stream from the deethanizer to a temperature at which it is partially liquefied. This partially liquefied stream is then introduced into the absorber wherein the liquid portion of the stream passes downwardly through the absorber to contact a gaseous stream passing upwardly through the absorber. While this processing system has been effective to separate C 2  and lighter components from C 3  and heavier components, it is relatively inefficient when processing lower pressure feed gas streams. It is also relatively inefficient when processing rich feed gas streams with respect to their C 3  and heavier content. It is particularly ineffective when large amounts of very light gases, such as hydrogen, may be present in the feed gas stream charged to the process. Hydrogen in gaseous streams recovered from refinery operations, which may be desirably separated in such processes, is not uncommon. While the occurrence of hydrogen in significant quantities in natural gas is rare, the presence of hydrogen in similar streams from refinery operations is common. 
         [0007]    U.S. Pat. No. 6,405,561 discloses a process for recovering C 3  and heavier components from low-pressure natural gas or refinery gas streams. The &#39;561 patent teaches the improvement of cooling and partially condensing a deethanizer overhead gas stream to produce a deethanizer liquid stream that is further cooled and directed into an upper portion of a separator/absorber, which separates the inlet feed stream into a liquid bottoms stream comprising primarily C 3  and heavier components and an overhead gas stream comprising primarily C 2  and lighter components. The process of the &#39;561 patent is particularly effective for treatment of feed gas streams at lower pressure that contain substantial amounts of very light components, including hydrogen that is often found in refinery applications. The process of the &#39;561 patent is also effective for treatment of feed gas streams rich with respect to recoverable C 3  and heavier components. 
         [0008]    However, as feed gas pressure increases, or if feed gas streams with higher quantities of C 2  and lighter components are used, the process of the &#39;561 patent becomes less effective due to co-adsorption of these lighter components in the separator/absorber bottoms stream. As a result, these lighter components tend to reduce the temperature required to partially condense the deethanizer overhead gas stream. Thus, the refrigerant medium used in this condensation operation must be changed from propane to a colder, more horsepower-intensive refrigeration media. As a result, the investment in equipment and operating cost is increased substantially. 
       SUMMARY OF THE INVENTION 
       [0009]    In one embodiment of the present invention, there is provided a process for separating a feed gas stream containing methane, at least one C 2  component, and at least one C 3  component into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  component. The process comprises first cooling the feed gas stream to a temperature sufficient to condense the majority of the at least one C 3  component in the feed gas stream to produce a cooled feed stream. The cooled feed stream is introduced into a separator vessel to separate the cooled feed stream into a separator gas stream and a separator liquid stream. At least a portion of both of the separator gas and liquid streams from the separator vessel is introduced into a fractionation column to produce a fractionation column bottoms product and a fractionation column overhead residue gas stream. The fractionation column bottoms product is introduced into a deethanizer tower which produces a deethanizer bottoms stream comprising a majority of the at least one C 3  component and a deethanizer overhead gas stream. 
         [0010]    In another embodiment of the present invention, there is provided a process for separating a feed gas stream containing methane, at least one C 2  component, and at least one C 3  component into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  component. The process comprises cooling the feed gas stream to a temperature sufficient to condense the majority of the at least one C 3  component therein to produce a cooled feed stream. The cooled feed stream is passed to a fractionation column to produce a liquid fractionation column bottoms product and a fractionation column overhead residue gas stream. The fractionation column including a reboiler operable to vaporize at least a portion of the fractionation column liquid which is taken from the bottom or near the bottom of the column. The vaporized portion is then reintroduced into the fractionation column. The fractionation column bottoms product is introduced into a deethanizer tower which produces a deethanizer bottoms stream comprising a majority of the at least one C 3  component and a deethanizer overhead gas stream. The deethanizer overhead gas stream is cooled and at least partially condensed thereby producing a deethanizer liquid reflux stream and a deethanizer residue gas stream. Optionally, the deethanizer residue gas stream is combined with at least a portion of the overhead residue gas stream to form a combined residue gas stream. At least a portion of the combined residue gas stream is compressed and cooled to produce a residue gas reflux stream. The residue gas reflux stream is introduced into the fractionation column. 
         [0011]    In a further embodiment of the present invention, there is provided a process for separating a feed gas stream containing methane, at least one C 2  component, and at least one C 3  component into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  component. The process comprises cooling the feed gas stream to a temperature sufficient to condense the majority of the at least one C 3  component in the feed gas stream to produce a cooled feed stream. The cooled feed stream is passed to a fractionation column to produce a liquid fractionation column bottoms product and a fractionation column overhead residue gas stream. The fractionation column bottoms product is introduced into a deethanizer tower, which produces a deethanizer bottoms stream comprising a majority of the at least one C 3  component and a deethanizer overhead gas stream. The deethanizer overhead gas stream is cooled and at least partially condensed thereby producing a deethanizer liquid reflux stream and a deethanizer residue gas stream. At least a portion of the fractionation column overhead residue gas stream is compressed and cooled to produce a residue gas reflux stream. The residue gas reflux stream is then introduced into the fractionation column. 
         [0012]    In yet another embodiment of the present invention, there is provided a system for separating a feed gas stream containing methane, at least one C 2  component, and at least one C 3  component into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  component. The system comprises a feed stream heat exchanger configured to cool the feed gas stream to a temperature sufficient to condense the majority of the at least one C 3  component in the feed gas stream to produce a cooled feed stream. A separator vessel is located downstream from the first heat exchanger and configured to separate the cooled feed stream into a separator gas stream and a separator liquid stream. A fractionation column is located downstream from the separator vessel and configured to receive at least a portion of both the separator gas and liquid streams and produce a fractionation column bottoms product and a fractionation column overhead residue gas stream. A deethanizer tower is located downstream from the separator vessel and configured to receive at least a portion of the fractionation column bottoms product and to produce a deethanizer bottoms stream comprising a majority of the at least one C 3  component and a deethanizer overhead gas stream. 
         [0013]    In still another embodiment of the present invention, there is provided a system for separating a feed gas stream containing methane, at least one C 2  component, and at least one C 3  component into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  component. The system comprises a feed stream heat exchanger configured to cool the feed gas stream to a temperature sufficient to condense the majority of the at least one C 3  component therein to produce a cooled feed stream. A fractionation column is located downstream from the feed stream heat exchanger and is configured to receive the cooled feed stream and produce a fractionation column bottoms product and a fractionation column overhead residue gas stream. The fractionation column includes a reboiler configured to vaporize at least a portion of the fractionation column liquid and reintroduce the vaporized fractionation column liquid back into the fractionation column. A deethanizer tower is located downstream from the fractionation column and configured to receive at least another portion of the fractionation column bottoms product and produce a deethanizer bottoms stream comprising a majority of the at least one C 3  component and a deethanizer overhead gas stream. A deethanizer heat exchanger is provided and configured to receive and cool the deethanizer overhead gas stream. A deethanizer separation vessel is located downstream from the deethanizer heat exchanger and is configured to separate the cooled deethanizer overhead gas stream into a deethanizer liquid reflux stream and a deethanizer residue gas stream. Optionally, the system further includes a conduit configured to merge at least a portion of the deethanizer residue gas stream with at least a portion of the fractionation column overhead residue gas stream to form a combined residue gas stream. A residue gas heat exchanger is provided and configured to condense at least a portion of the combined residue stream to form a residue gas reflux stream. Conduit is configured to deliver at least a portion of the residue gas reflux stream from the gas condensation unit to the fractionation column. 
         [0014]    In even a further embodiment, there is provided a system for separating a feed gas stream containing methane, at least one C 2  component, and at least one C 3  component into a volatile gas stream containing a major portion of the methane and at least one C 2  component and a less volatile stream containing a major portion of the at least one C 3  component. The system comprises a feed stream heat exchanger configured to cool the feed gas stream to a temperature sufficient to condense the majority of the at least one C 3  component in the feed gas stream to produce a cooled feed stream. A fractionation column is located downstream from the heat exchanger and is configured to receive the cooled feed stream and produce a fractionation column bottoms product and a fractionation column overhead residue gas stream. A deethanizer tower is located downstream from the fractionation column and configured to receive at least a portion of the fractionation column bottoms product and produce a deethanizer bottoms stream comprising a majority of the at least one C 3  component and a deethanizer overhead gas stream. A deethanizer heat exchanger is provided and configured to receive and cool the deethanizer overhead gas stream. A deethanizer separation vessel is provided and configured to separate the cooled deethanizer overhead gas stream into a deethanizer liquid reflux stream and a deethanizer residue gas stream. Conduit is provided and configured to deliver at least a portion of the deethanizer liquid reflux stream to the fractionation column. A residue gas heat exchanger is provided and configured to condense at least a portion of the fractionation column overhead residue gas stream. Conduit is provided and configured to deliver at least a portion of the condensed fractionation column overhead residue gas stream to the fractionation column. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic diagram of a process according to one embodiment of the present invention; and 
           [0016]      FIG. 2  is a schematic diagram of a process according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Turning to  FIG. 1 , an embodiment of the present invention is shown that is particularly adapted for the recovery of C 3  and heavier components from a hydrocarbon-containing gas stream, such as a natural gas or refinery gas stream. In particular embodiments, the inlet feed gas stream  10  comprises methane, at least one C 2  component, at least one C 3  component, and optionally heavier components. In still other embodiments, inlet feed gas stream  10  comprises methane as the predominant component, with C 2 , C 3 , and heavier components being present in lesser amounts. In refinery applications, the feed may also contain significant quantities of lighter components such as hydrogen. Particularly, in certain applications, the feed stream may comprise as much as 10%, or even as much as 50%, hydrogen. 
         [0018]    The present invention exhibits the flexibility to accommodate a wide variety of feed pressures. In one embodiment, the feed stream  10  can be supplied at a pressure of at least 300 psi, or particularly, between about 350 psi to about 700 psi. Typically, feed stream  10  will be supplied at a temperature that is above the condensation point for the C 3  components present therein; therefore, feed stream will need to be cooled in order to condense these components. In this embodiment, feed gas stream  10  is passed through a heat exchanger  12  where it is cooled to a temperature sufficient to condense the majority of the at least one C 3  component in the feed gas stream to produce a cooled gas feed stream. Note, the use of the word “gas” in the term “cooled gas feed stream” should not be taken as implying that the entirety of the stream is present in the gaseous state. Certain components, particularly the heavier components may be present as liquids. The cooling streams used in heat exchanger  12  are discussed in greater detail below. It will be understood that the heat exchange function shown schematically in heat exchanger  12  may be accomplished in a single or a plurality of heat exchange vessels. 
         [0019]    The cooled inlet gas is passed via a line or conduit  14  to a separation vessel  16  where it is separated into a vapor stream  18  and a bottoms stream  20 . Vapor stream  18  is directed toward an expander  22  to reduce the pressure of and further cool the stream. The expanded vapor stream is passed via a line  24  to a fractionation column  26  containing one or more theoretical stages of mass transfer. In certain embodiments, the fractionation column  26  is a conventional distillation column containing a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing. 
         [0020]    The bottoms stream  20  recovered from separator vessel  16  contains primarily C 3  and heavier components, although the bottoms stream  20  will also contain quantities of lighter materials. As explained further below, ultimately these lighter components will be separated from the C 3  and heavier components in subsequent processing steps. In order to maximize the efficiency of those subsequent processing steps, the present embodiment seeks to control the levels of C 2  and lighter components contained in the liquid, predominantly C 3  stream that will be further processed. Thus, bottoms stream  20  is also passed to fractionation column  26 . Generally, bottoms stream  20  is introduced into fractionation column  26  below the introduction point for the expanded vapor stream carried by line  24 , although the arrangement of the introduction points for the various streams fed to fractionation column  26  can be varied as deemed appropriate. This step of introducing bottoms stream  20  into fractionation column  26  provides an opportunity for the lighter materials co-absorbed in liquid bottoms stream  20  to be separated therefrom. Fractionation column  26  is equipped with an optional reboiler  28  to assist with separation of the C 2  and lighter components from the bottoms of the fractionation column. A portion of the fractionation column liquids taken from the bottom or near the bottom of column  26  are directed to reboiler  28  and at least partially vaporized and then reintroduced into the fractionation column  26 . Accordingly, as the liquid stream exiting fractionation column  26  contains fewer C 2  or lighter components, it has higher condensation temperature than the stream  20 . This permits a propane refrigerant, or similar refrigerant, to be used to condense the overhead stream from a deethanizer  36 , which is discussed in greater detail below. Otherwise, if the bottoms product from fractionation column  26  contained a higher level of C 2  or lighter components, a colder and therefore more expensive refrigeration system would need to be employed. 
         [0021]    In fractionation column  26 , a liquid bottoms stream comprising primarily C 3  and heavier components plus some light components is recovered via a line  30  and a pump  32  and pumped via a line  34  to heat exchanger  12  where it is used to cool the inlet gas stream in line  10 . The stream in line  34  is then passed to a deethanizer  36 . In deethanizer  36  the stream from line  34  is separated by conventional distillation techniques as well known to the art for deethanizers into an overhead vapor stream  38  and a bottoms stream  40 . Deethanizer  36  also comprises a conventional reboiler  42 . The stream recovered from deethanizer  36  through line  40  comprises primarily C 3  and heavier components. An overhead stream is recovered from the deethanizer via line  38 , which is rich in C 2  and lighter components and is passed to a heat exchanger  44  where it is partially condensed and then through a line  46  to a separator  48 . From separator  48 , a liquid stream is withdrawn via a line  50  and passed to a pump  52  from which a portion of the liquid stream is passed via a line  54  into an upper portion of deethanizer  36  as a reflux. The vapor stream recovered from separator  48  is passed via a line  56 . 
         [0022]    Deethanizer  36  is maintained at a higher pressure than fractionation column  26 . The increased pressure for deethanizer  36  is supplied by pump  32  and maintained by a valve  57  disposed in line  56 . In certain embodiments, the pressure in deethanizer  36  is at least 25 psi, or at least 100 psi, or at least 200 psi greater than the pressure in fractionation column  26 . 
         [0023]    A second portion of the liquid stream from separator  48  is passed via a line  58 , through a heat exchanger  60 , and into an upper portion of fractionation column  26 . An overhead vapor stream recovered from the upper portion of fractionation column  26  is passed via a line  62 , through heat exchanger  66  and then combined with the stream in line  56 . It is noted that the stream carried by line  56  is flashed across valve  57 . The combined stream contains a residue gas that comprises a major portion of the C 2  and lighter components from the inlet gas feed stream. This stream is passed via line  64  through heat exchanger  12  so as to provide cooling for feed stream  10 . Alternatively, stream  56  and stream  62  can be passed separately through heat exchanger  12  such that stream  56 , which contains a significant quantity of C 2  components, would be available for internal use thus reducing the C 2  content of the residue gas. 
         [0024]    The cooling to heat exchanger  12  provided by the materials carried by lines  34  and  64  can be supplemented by a refrigerant, such as propane, supplied to heat exchanger  12  by line  76 . Next, the residue gas carried by line  64  is passed through a compressor  66 . The residue gas exits compressor  66  via line  68 . Optionally, a portion of the residue gas carried by line  68  is passed via a line  70  to a heat exchanger  72  where it is cooled and condensed. In the embodiment illustrated, the chilled portion of residue gas exiting heat exchanger  72  is refluxed to the top of fractionation column  26 . The other portion of residue gas from line  68  is withdrawn from the system via line  74 . In those embodiments in which streams  56  and  62  are not combined and an additional reflux is desired for column  26 , a portion of the contents of stream  62  are compressed, condensed and refluxed to the column. 
         [0025]    In an illustrative embodiment of the process shown in  FIG. 1 , a dehydrated gas stream is charged to the process at 340 psia and 114° F. The gas stream is cooled in heat exchanger  12  to a temperature of −66° F and 330 psia and charged to separation vessel  16 . In separation vessel  16 , gaseous overhead stream  18  is produced and passed through expander  22  and is carried by line  24  at −99° F. and 150 psia to fractionation column  26 . The liquid stream recovered via line  20  at −1.5° F. and 145 psia and directed through pump  32  where its pressure is increased to 360 psia. The stream carried by line  34  is used to provide refrigeration to heat exchanger  12  and then directed to deethanizer  36  at a temperature of 74° F. and 355 psia. 
         [0026]    In deethanizer  36 , a bottoms liquid stream composed primarily of C 3  and heavier components is recovered via a line  40  at a temperature of 173° F. at 350 psia. The vapor stream recovered via line  56  is at a temperature of 24° F. at 335 psia. In the current simulation, the vapor stream recovered via line  56  was withdrawn from the system and used as fuel gas. However, as illustrated in  FIG. 1 , this stream can be flashed across valve  57  and combined with the gas carried by line  62 . A liquid reflux stream carried by line  58  is withdrawn from the deethanizer at a temperature of 24° F. and 335 psia. This stream is cooled, expanded, and refluxed to fractionation column  26  at −111° F. and 145 psia. 
         [0027]    The overhead vapor from fractionation column  26  carried by line  62  is at a temperature of −117° F. and a pressure of 140 psia and is heat exchanged with the stream carried by line  58  and emerges from heat exchanger  60  at −99°F. and 135 psia and directed to heat exchanger  12  via line  64 . This residue gas stream exits heat exchanger  12  at 95° F. and 125 psia and is directed toward compressor  66  (in this simulation a series of compressor stages with intercooling) where it is boosted to 1265 psia and its temperature raised to 115° F. A portion of this compressed stream is withdrawn via line  70 , cooled and condensed by heat exchanger  72  and refluxed to fractionation column  26  at a temperature of −112° F. and pressure of 1255 psia. 
         [0028]    While specific temperatures have been referred to in connection with the embodiment illustrated in  FIG. 1 , it should be understood that a wide range of temperature and pressure variations are possible within the scope of the present invention. Such temperature and pressure variations are readily determined by those skilled in the art based upon the composition of the specific feed streams, the desired recoveries and the like within the scope of the processes disclosed above. 
         [0029]      FIG. 2  illustrates another embodiment of a process in accordance with the present invention. Note, when applicable, the same reference numerals used in the description of  FIG. 1  have been used to identify comparable lines or equipment. In the process of  FIG. 2 , the inlet gas stream is charged to the process via a line  10 . The inlet feed gas is cooled in a heat exchanger  12  and thereafter passed via a line  14  to a heat exchanger  15  where it is further cooled to a selected temperature and passed via line  17  to a fractionation column  26  containing one or more theoretical stages of mass transfer. Fractionation column  26  is equipped with a reboiler  28  to assist with separation of the C 2  and lighter components from the bottoms of the fractionation column. A portion of the tower liquid from fractionation column  26  is directed to reboiler  28  and at least partially vaporized and then reintroduced into the bottom of fractionation column  26 . 
         [0030]    In fractionation column  26 , a liquid bottoms product comprising primarily C 3  and heavier components plus some light components is recovered via a line  30  and a pump  32  and pumped via a line  34  to heat exchanger  12  where it is used to cool the inlet gas stream in line  10 . The stream in line  34  is then passed via to a deethanizer  36 . In deethanizer  32  the stream from line  34  is separated by conventional distillation techniques into an overhead vapor stream  38  and a bottoms stream  40 . A conventional reboiler  42  is shown for with-drawing a portion of the deethanizer tower liquid, at least partially vaporizing the withdrawn portion, and returning the at least partially vaporized stream back to deethanizer  36 . The stream recovered from deethanizer  36  through line  40  comprises primarily C 3  and heavier components. An overhead stream is recovered from the deethanizer via line  38 , which is rich in C 2  and lighter components and is passed to a heat exchanger  44  where it is at least partially condensed and then through a line  46  to a separator  48 . From separator  48 , a liquid stream is withdrawn via a line  50  and passed to a pump  52  from which a portion of the liquid stream is passed via a line  54  into an upper portion of deethanizer  36  as a reflux. The vapor stream recovered from separator  48  is passed via a line  56  and through an expansion valve  57 . The vapor stream is then combined with the residue gas from line  62  and directed toward a compressor  66  via line  64 . 
         [0031]    A second portion of the liquid stream from separator  48  is passed via a line  58  and a heat exchanger  60  into an upper portion of fractionation column  26 . An overhead vapor stream recovered from the upper portion of fractionation column  26  is passed via a line  62  through heat exchanger  60  to combination with the stream in line  26 . The combined stream carried by line  64  contains a major portion of the C 2  and lighter components from the inlet gas feed stream. As noted above, the stream in line  64  is compressed by compressor  66  and passed into line  68 . A portion of the compressed residue gas carried by line  68  is passed via a line  70  to a heat exchanger  72  where it is cooled and condensed. In the embodiment illustrated, the condensed portion of residue gas exiting heat exchanger  72  is refluxed to the top of fractionation column  26 . The other portion of residue gas from line  68  is withdrawn from the system via line  74 . 
         [0032]    It is noted that, as discussed above with respect to  FIG. 1 , in certain embodiments, streams  56  and  62  may be kept separate. When an additional reflux is desired for column  26 , a slip stream of the material carried by line  62  can be compressed, condensed, and refluxed to the column. It is also noted that for any embodiment discussed above, it is within the scope of the present invention for the residue gas reflux carried by line  70  to be used without equipping fractionation column  26  with a reboiler  28 . 
         [0033]    While the present invention has been described by reference to certain of its preferred embodiments, it is respectfully pointed out that the embodiments described are illustrative rather than limiting in nature and that many variations and modifications are possible within the scope of the present invention.