Abstract:
The present invention relates to a process for separating a hydrocarbon gas into a fraction containing a predominant portion of the methane or ethane and lighter components and a fraction containing a predominant portion of the C 2  or C 3  and heavier components in which process the feed gas is treated in one or more heat exchange, and expansion steps; partly condensed feed gas is directed into a separator wherein a first residue vapor is separated from a C 2  or C 3 -containing liquid; and C 2  or C 3 -containing liquids, at substantially the pressure of separation, are directed into a distillation column wherein said liquid is separated into a second residue is separated to recover a C 2  or C 3 -containing product. The foregoing process is improved by cooling said second residue to partially condense it.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method and apparatus for the improved recovery of C 2  or C 3  and heavier components from hydrocarbon gases. 
         [0002]    In conventional processes for extracting ethane or propane and heavier components from hydrocarbon gases, the C 2  and/or C 3  bearing gases are treated by a combination of expansion (or compression followed by expansion) heat exchange and refrigeration to obtain a partially condensed stream which is collected in a feed separator having a pressure typically in the order of 50 to 1200 psia and a temperature in the order of −50° to −200° F. These conditions of course can vary substantially, depending on the pressure and temperature conditions necessary to achieve partial condensation for a particular gas, and the pressure and temperature at which the feed is available to the process. The liquid resulting from partial condensation is supplied to a fractionation column called a heavy ends fractionation column (HEFC) as a mid-column feed while the vapor from the feed separator is further cooled via heat exchange, expansion or other means and then enters a light ends fractionation column (LEFC) as a feed. The overhead stream from the LEFC is used to generate reflux by partially condensing the overhead vapors from the HEFC through appropriate heat exchange means. In a typical system the HEFC column will operate at a pressure less than or substantially equal to that of the HEFC feed separator (possibly allowing for a small pressure drop as the partially condensed liquid passes from the separator to the HEFC) and the HEFC overhead vapors leave at a temperature in the order of 0° to −170° F. The heat exchange of these overhead vapors against the residue vapors from the LEFC provides partial condensate which is used as a reflux to the LEFC. 
         [0003]    Pre-cooling of the gas before it is expanded to the LEFC pressure will commonly result in formation of a high-pressure condensate. To avoid damage to the expander, the high pressure condensate, if it forms, is usually separated, separately expanded through a Joule-Thomson valve and used as a further feed to the mid-portion of the HEFC column. Refrigeration in such a process is sometimes entirely generated by work expansion of the vapors remaining after partial condensation of the high pressure gas to the column operating pressure. Other processes may include external refrigeration of the high pressure gases to provide some of the required cooling. 
         [0004]    When processing natural gas, feed is typically available at line pressure, of 600-1000 psia. In such case expansion to a pressure in the order of 150-300 psia is common. In an alternate process, facilities may be designed to extract ethane or ethylene or propane or propylene from refinery gases. Refinery gases commonly are available a pressure of 150 psia-250 psia. In this case, at the convenience of the process designer, the LEFC may be designed to operate at a pressure below the pressure of the refinery gas which is available, i.e., perhaps 50-100 psia, so that work expansion can be used to supply refrigeration to the process. This will result in lower LEFC temperatures and will increase potential heat leakage and other engineering problems associated with cryogenic temperatures. It is also possible in this case to compress the refinery gas to a higher pressure so that it may be thereafter expanded in a work-expansion machine to afford refrigeration to the overall process. 
         [0005]    A typical flow plan of a process for separating C 3  and heavier hydrocarbons from a gas stream is illustrated in U.S. Pat. No. 4,251,249 to Jerry G. Gulsby. 
       SUMMARY OF THE INVENTION 
       [0006]    In one embodiment of the invention, there is described a process for separating a hydrocarbon gas containing at least methane, ethane and C 3  components into a fraction containing a predominant portion of the ethane and lighter components and a fraction containing a predominant portion of the C 3  and heavier components or a predominant portion of the methane and lighter components and a fraction containing a predominant portion of the C 2  and heavier components, in which process 
         [0007]    (a) the feed gas is treated in one or more heat exchangers, and expansion steps to provide at least one partly condensed hydrocarbon gas, providing thereby at least one first residue vapor and at least one C 2  or C 3 -containing liquid which liquid also contains lighter hydrocarbons; and 
         [0008]    (b) at least a portion of the C 2  or C 3 -containing liquids is directed into a distillation column wherein said liquid is separated into a second residue containing lighter hydrocarbons and a C 2  or C 3 -containing product; comprising: 
         [0009]    (1) cooling said second residue to partially condense it; 
         [0010]    (2) intimately contacting at least part of one of said first residue vapors with at least part of the liquid portion of the partially condensed second residue in at least one contacting stage and thereafter separating the vapors and liquids from said contacting stage; 
         [0011]    (3) supplying the liquids thereby recovered to the distillation column as a liquid feed thereto; and 
         [0012]    (4) directing the vapors thereby recovered into heat exchange relation with said second residue from the distillation column, thereby to supply the cooling of step (1), and thereafter discharging said residue gases; the improvement further comprising: 
         [0013]    (5) recovering a recycle gas stream from an expander-compressor or residue gas compressor; 
         [0014]    (6) cooling and partially condensing the recycle stream in said one or more heat exchangers; 
         [0015]    (7) expanding the recycle stream thereby further condensing a portion of and cooling the recycle stream; 
         [0016]    (8) feeding the expanded recycle stream to a subcooler, whereby the expanded recycle stream is heat exchanged in the subcooler with gases from top of the heavy-ends fractionation column thereby providing colder temperatures to the vapors from the heavy ends fractionation column, 
         [0017]    The contacting step (2) is carried out in a feed separator/absorber which includes fractionation means for vapor/liquid counter-current contact and 
         [0018]    (i) wherein said partly condensed second residue is introduced into said separator/absorber above or at an intermediate point in said fractionation means, whereby the liquid portion of it passes downwardly through said fractionation means; and 
         [0019]    (ii) wherein said partly condensed portion of the first residue is introduced into said separator/absorber above or at an intermediate point in said fractionation means, whereby the liquid portion of it passes downwardly through said fractionation means; and wherein said portion of the cooled C 2  or C 3 -containing liquid from the separator is introduced into said separator/absorber above or at an intermediate point in said fractionation means, whereby the liquid portion of it passes downwardly through said fractionation means; and 
         [0020]    (iii) said at least part of one of said first residue vapors is supplied to said separator/absorber below said fractionation means, whereby the first residue vapor rises through said fractionation means in counter-current contact with the liquid portion of the partly condensed second residue, 
         [0021]    The fractionation means in said separator/absorber provide the equivalent of at least one theoretical distillation stage arranged to contact at least part of one of said first residue vapors with the liquid portion of the partly condensed second residue. 
         [0022]    The fractionation means in said separator/absorber provide the equivalent of at least one theoretical distillation stage arranged to contact at least part of one of said first residue vapors with the liquid portion of the partly condensed second residue. 
         [0023]    The recycle gas stream recovered may further pass through expander-compressor discharge cooler or other compression discharge cooler prior to it being partially condensed in the one or more heat exchangers. The one or more heat exchangers where the recycle stream is partially condensed may have other liquid and gas flows present therein which can further be used, in addition to the gases from the top of the light-ends fractionation column to partially condense the recycle stream. For example, the liquid product from the light-ends fractionation column, the reboiler fluid, the side heater fluid and/or the residue gas streams may all pass through the one or more heat exchangers. 
         [0024]    The one or more heat exchangers may be shell and tube, plate-fin exchangers or other means of heat exchange. The expansion of the recycle stream may be through a flow control valve or additional turboexpander. 
         [0025]    The cold expanded recycle stream that is fed to the subcooler will combine with the overhead stream from the light-ends fractionation column resulting in a cooler reflux stream that is fed into the light-ends fractionation column thereby promoting increased reflux and thus, a greater recovery from the light-ends fractionation column. 
         [0026]    Further, there is described an apparatus for separating a hydrocarbon gas containing at least ethane and C 3  components into a fraction containing a predominant portion of ethane and lighter components and a fraction containing a predominant portion of the C 3  and heavier components in which apparatus 
         [0027]    (a) one or more heat exchange means and one or more expansion means are provided which are cooperatively connected to provide at least one partly condensed hydrocarbon gas, providing thereby at least one first residue vapor and at least one C 3 -containing liquid which liquid also contains lighter hydrocarbons and 
         [0028]    (b) a distillation column connected to receive at least one of said C 3 -containing liquids which is adapted to separate the C 3 -containing liquids into a second residue containing lighter hydrocarbons and a C 3 -containing product; 
         [0029]    the improvement comprising 
         [0030]    (1) heat exchange means connected to said distillation column to receive said second residue and to partially condense it; 
         [0031]    (2) contacting and separating means connected to receive at least part of one of the first residue vapors and at least part of the liquid portion of the partially condensed second residue and to comingle said vapor and liquid in at least one contacting stage, which means include separation means for separating the vapor and liquid after contact in said stage; 
         [0032]    (3) said means (2) being further connected to supply the liquids separated therein to the distillation column as a liquid feed thereto, and 
         [0033]    (4) said means (2) also being connected to direct the vapors separated therein into heat exchange relation with said second residue from the distillation column in said heat exchange means (1); the improvement further comprising 
         [0034]    (5) Product cooler means connected to said distillation column to receive said second residue from said distillation column and to feed said second residue to said heat exchange means. 
         [0035]    The contacting and separating means includes fractionation means for countercurrent vapor/liquid contact and wherein said means is connected to receive the portion of one of first residue vapors to be treated therein below said fractionation means and to receive the portion of said liquids from the partially condensed second residue to be treated therein above said fractionation means said fractionation means thereby being adapted so that the first residue vapors rise therethrough in countercurrent contact with partially condensed second residue. 
         [0036]    The fractionation means includes vapor/liquid contacting means which are the equivalent of at least one theoretical distillation stage. 
         [0037]    The contacting and separating means (2) comprise means for comingling at least part of one of said first residue vapors with the liquid portion of the partially condensed second residue. 
         [0038]    The contacting and separating means (2) comprise means for comingling at least part of one of said first residue vapors with both the liquid and vapor portion of said partially condensed second residue. 
         [0039]    The contacting and separating means includes fractionation means for countercurrent vapor/liquid contact and wherein said means is connected to receive the portion of one of first residue vapors to be treated therein below said fractionation means and to receive the portion of said liquids from the partially condensed second residue, portion of the partially condensed first residue and portion of the cooled C 3 -containing liquid from the separator to be treated therein above or at an intermediate point in said fractionation means said fractionation means thereby being adapted so that the first residue vapors rise there-through in countercurrent contact with partially condensed second residue and portion of the partially condensed first residue and being further adapted so that the portion of the C 3 -containing liquid from the separator is cooled by the liquids exiting the fractionation means. 
         [0040]    The fractionation means includes vapor/liquid contacting means which are the equivalent of at least one theoretical distillation stage. 
         [0041]    The contacting and separating means ( 2 ) comprise means for comingling at least part of one of said first residue vapors with the liquid portion of the partially condensed second residue, liquid portion of the partially condensed portion of the first residue and portion of the cooled C 3 -containing liquid from the separator. 
         [0042]    The contacting and separating means (2) comprise means for comingling at least part of one of said first residue vapors with both the liquid and vapor portion of said partially condensed second residue, said partially condensed portion of the first residue and portion of the cooled C 2  or C 3 -containing liquid from the separator, 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1A  is a partial schematic representation of a hydrocarbons separation process according to the invention which shows half the process due to scaling constraints. 
           [0044]      FIG. 1B  is a partial schematic representation of the other half of a hydrocarbons separation process according to the invention which shows the other half of the process due to scaling constraints. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0045]    The present invention provides an improved process for recovering C 2  or C 3  and heavier components from hydrocarbon-bearing gases. In the improved process of the present invention the overhead vapor from the HEFC column is partly condensed and then at least the liquid condensate is combined with at least the vapor from the partially condensed feed gases described above in the LEFC which, in the present invention, also acts as an absorber. The LEFC is designed to afford one or more contacting stages. Usually such stages are assumed for design purposes to be equilibrium stages, but in practice this need not be so. Vapor from the feed separator/absorber passes in heat exchange relation to the overhead from the HEFC, thereby providing partial condensation of that stream, and liquid from the LEFC is supplied to the HEFC as an upper or top liquid feed to the column. 
         [0046]    If the LEFC contains an absorption section, such as packing, or one or more fractionation trays, these stages will be assumed to correspond to a suitable number of theoretical separation stages. Our calculations have shown benefits with as few as one theoretical stage, and greater benefits as the number of theoretical stages is increased. We believe that benefits can be realized even with the equivalent of a fractional theoretical stage. The partially condensed HEFC overhead is supplied above this section, and the liquid portion of it passes downward through the absorption section. The partially condensed feed stream is usually supplied below the absorption section, so that the vapor portion of it passes upwardly through it in countercurrent contact with the liquids from the partially condensed HEFC overhead. The rising vapor joins the vapors which separate from partially condensed HEFC overhead above the absorption section, to form a combined residue stream. 
         [0047]    While described above with respect to a preferred embodiment in which overhead vapors are condensed and used to absorb valuable ethane, ethylene, propane, propylene, etc, from the expander outlet vapors, we point out that the present invention is not limited to this exact embodiment. Advantages can be realized, for instance, by treating only a part of the expander outlet vapor in this manner, or using only part of the overhead condensate as an absorbent in cases where other design considerations indicate that portions of the expander outlet or overhead condensate should bypass the LEFC. We also point out that the LEFC can be constructed as either a separate vessel, or as a section of the HEFC column. 
         [0048]    In the practice of this invention there will necessarily be a slight pressure difference between the LEFC and the HEFC which must be taken into account. If the overhead vapors pass through the condenser and into the separator without any boost in pressure, the LEFC will assume an operating pressure slightly below the operating pressure of the HEFC. In this case the liquid feed withdrawn from the LEFC can be pumped to its feed position in the HEFC. An alternative is to provide a booster blower in the vapor line to raise the operating pressure in the overhead condenser and LEFC sufficiently so that the liquid feed can be supplied to the HEFC without pumping. Still another alternate is to mount the LEFC at a sufficient elevation relative to the feed position of the liquid withdrawn therefrom that the hydrostatic head of the liquid will overcome the pressure difference. 
         [0049]    In still another alternate, all or a part of the partially condensed HEFC overhead and all or part of the partially condensed feed can be combined, such as in the pipe line joining the expander output to the LEFC and if thoroughly intermingled, the liquids and vapors will mix together and separate in accordance with a relative volatility of the various components of the total combined streams. In this embodiment the vapor-liquid mixture from the overhead condenser can be used without separation, or the liquid powder thereof may be separated. Such co-mingling is considered for purposed of this invention as a contacting stage. 
         [0050]    In still another variation of the foregoing, the partially condensed overhead vapors can be separated, and the all or a part of the separated liquid supplied to the LEFC or mixed with the vapors fed thereto. 
         [0051]    The present invention provides improved recovery of ethane or ethylene, propane or propylene per amount of power input required to operate the process. An improvement in operating power required for operating a HEFC process may appear either in the form of reduced power requirements for external refrigeration, reduced power requirements for compression or recompression, or both. Alternatively, if desired, increased C2 or C3 recovery can be obtained for a fixed power input. 
         [0052]      FIG. 1A  and  FIG. 1B  represent a schematic of a hydrocarbon separation process according to the invention. A hydrocarbon bearing gas natural gas is fed through line  20  to a warm gas/gas exchanger  22 -E 3000  and then to a chiller  22 -E 3400 . Refrigeration is supplied through line  52  and  53 . The chiller has a line  54  which will withdraw refrigeration for recompression and liquefaction. The cooled gas stream is fed through line  21  through a cold gas/gas exchanger  22 - 3100  to a cold separation vessel  22 -D 1000 . 
         [0053]    The hydrocarbon gas stream will be separated into two streams with the vapor leaving through line  22  and the bottoms through line  25  to line  16 . The bottoms will pass through a valve in line  26  for flow control and will rejoin line  26  to line  35  where they will enter subcooler  22 -E 3200 . These cooled hydrocarbon gases leave the subcooler through line  36  and enter light ends fractionation column  22 -T 2000 . The hydrocarbon gas stream that is not diverted will continue through line  37  to the light ends fractionation column  22 -T 2000  at the top of the column. 
         [0054]    The vapor from the cold separation vessel  22 -D 1000  will leave through line  22  and reach a junction with line  24 . Line  24  will also contain a valve assembly PV which is used to control the flow of the stream in Line  24 . The remainder of the vapor from the cold separation vessel flow through line  23  through an expander/compressor  22 -X 6000 . This expanded hydrocarbon gas stream will be fed through line  29  into the light ends fractionation column  22 -T 2000 . 
         [0055]    The vapor from the light ends fractionation column  22 -T 2000  will leave through line  39  and pass through line  40  where they will pass through cold gas/gas exchanger  22 -E 3100  and warm gas/gas exchanger before passing through line  55  to an expander/compressor  22 - 06000  where the compressed gas stream will enter and expander/compressor discharge cooler  22 -E 4100  through line  59 . The discharged gas stream will exit through line  58  and for sales or further processing as required. 
         [0056]    Line  56  contacts line  55  and some of the hydrocarbon gas will be drawn off before entering the expander/compressor  22 -C 6000  and recovered for use as fuel gas. A valve assembly is present in line  56  for controlling the quantity of the material to be used as fuel gas. 
         [0057]    The bottoms from the light ends fractionation column  22 -T 2000  will exit through line  31 . These bottoms comprise an intermediate liquid stream that required further fractionation. Line  31  is in fluid communication with a transfer pump  22 -P 5000 NB which directs the bottoms from the light ends fractionating column to line  33  and into the top of a heavy ends fractionation column  22 -T 2100 . 
         [0058]    A stream comprising a cooler, intermediate product liquid is withdrawn from the heavy ends fractionation column  22 -T 2100  through line  41  which is fed to a side heater  22 -E 3800  which will heat the stream and return it through line  42  to a point lower in the heavy ends fractionation column from which it was withdrawn. Another side steam is withdrawn from the heavy ends fractionation column  22 -T 2100  through line  43  which is fed to a heavy ends fractionation column reboiler  22 -E 3700  which will heat the side stream. This stream is fed to a trim reboiler  22 -E 4000  where it will be further heated before being returned through line  44  to a point lower in the heavy ends fractionation column from which it was withdrawn. Line  45  will supply heating media (not shown) to the trim reboiler  22 -E 4000  while line  46  will return heating media from the trim reboiler. 
         [0059]    A line at the bottom of the heavy ends fractionating column will remove some of the hydrocarbon comprising mainly of C2s and less volatile hydrocarbons or C3s and less volatile hydrocarbon and direct it to a valve in line  51 . Line  51  receives bottoms from the heavy ends fractionating column  22 -T 2100 . Line  47  feeds the bottoms from the heavy ends fractionating column and feeds them to a heavy ends fractionating column bottoms pump  22 -P 5100 A/B which feeds the bottoms through line  49  to a product exchanger  22 -E 3600  which feeds the bottoms through line  50  to the product pump  22 -P 5200 A/B. This pump directs the bottoms through line  51  where they can be directly fed to a pipeline. A valve in line  49  will allow bypass of the product exchanger  22 -E 3600  and divert the flow to an air or water cooled heat exchanger when the plant is operated in the C3 and heavier recovery mode. After cooling, these bottoms can be fed back into line  49  for feeding to the product exchanger  22 -E 3600 . 
         [0060]    The vapor from the heavy ends fractionation column  22 -T 2100  will exit through line  34  and pass through a subcooler  22 -E 3200 . Line  38  exits the subcooler  22 -E 3200  and connects to a valve. The vapor from the heavy ends fractionation column will be fed through line  30  into the light ends fractionation column  22 -T 2000  where they will be further fractionated for reentry back into the heavy ends fractionation column as a reflux stream. 
         [0061]    A portion of the compressed residue gas from stream  58  is recycled through the overall cryogenic process not only to increase ethane and heavier hydrocarbon component recoveries, but also to reduce the energy consumption of the overall system. 
         [0062]    The improved process utilizes the recycle stream  1  in which a portion of the residue gas is cooled and may be partially liquefied in via heat exchange, expanded reducing its temperature and thus increasing the reflux in the light-ends fractionation column,  22 -T 2000 . This recycle stream  1  is fed downstream from the expander-compressor,  22 -X 10600  and expander-compressor discharge cooler,  22 -E 4100  or downstream of the residue gas compressor aftercooler. The recycle stream  1  is cooled and partially condensed in the inlet plate-fin heat exchanger,  22 -E 3000  where the recycle stream  1  can be cross-exchanged with an inlet stream  20 , liquid product stream  49 , the reboiler fluid stream  43 , the side heater fluid stream  41  and the residue gas stream  40  together. The recycle stream leaves the heat exchanger  22 -E 3000  through line  2  and is expanded across a flow-control valve V 2  where further liquefaction and cooling to the recycle stream will occur. This further cooled and liquefied recycle stream passes through flow-control valve V 2  and enters line  3  which is fed into the subcooler  22 -E 3200 . The subcooler  22 -E 3200  provides additional refrigeration by mixing with the vapor from the light-ends fractionation column  22 -T 2000 . By reaching these cold temperatures, additional liquefaction occurs thus providing more reflux to the light ends fractionation column  22 -T 2000 . Said reflux will result in more ethane adsorption as well as increasing ethane and heavier component recoveries. 
         [0063]    The recycle stream having provided more cooling to the subcooler  22 -E 3200  and subsequently cooler reflux for the light-ends fractionation column  22 -T 2000  flows through subcooler  22 -E 3200  and enters line  4  where it will flow to line  40  where it will be fed through heat exchanger  22 -E 3000  where it will be further heated and then fed through line  55  to expander/compressor  22 -C 6000 . The compressed stream will be fed through line  59  to expander/compressor discharge cooler  22 -E 4100  where it will be recompressed and fed into line  1  where it will recycle ultimately to subcooler  22 -E 3200 . 
         [0064]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.