Abstract:
An improved process is described for the separation and recovery of substantially all the propane and heavier hydrocarbon components in a hydrocarbon gaseous feedstream. In this process, the vapor stream from a deethanizer is cooled to liquefaction and contacted with a vapor phase from the hydrocarbon gaseous feedstream. The contact takes place within a direct heat exchanger, and the resulting vapor fraction, which is essentially ethane and methane, is the gaseous product of the process.

Description:
FIELD OF THE INVENTION 
     This invention relates to a process for treating a gaseous hydrocarbon containing feedstream such as natural gas, crude oil solution gas or refinery gas to separate and recover propane and heavier hydrocarbon components. 
     BACKGROUND OF THE INVENTION 
     Gaseous streams containing methane and ethane occur naturally, such as in natural gas and crude oil solution gas, and also as byproducts of a variety of refinery processes. In addition to methane and ethane, these gases often contain a substantial quantity of hydrocarbons of higher molecular weight, e.g., propane, butane, pentane and their unsaturated analogs. 
     Recent substantial increases in the market for the propane and heavier hydrocarbon components of natural gas have provided demand for process yielding higher recovery levels of these products. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, refrigerated oil absorption, and the more recent cryogenic process utilizing the principle of gas expansion through a mechanical device to produce power while simultaneously extracting heat from the system. Depending upon the pressure of the gas source, the richness (propane and heavier hydrocarbon content) of the gas and the desired end results, each of these prior art processes or a combination thereof may be employed. 
     Prior to the advent of the cryogenic expansion process, propane and the heavier component hydrocarbons were frequently separated by liquefaction and treatment with an absorption medium. The natural gas streams were contacted with an absorption oil (usually heptane), and the propane and the heavier hydrocarbon components were absorbed and thereafter desorbed and recovered. 
     In most present day refining processes, propane and the higher molecular weight components of natural gas and refinery gas are separated and recovered by liquefaction and cryogenic distillation at temperatures below 0° F. Refrigeration for separation is supplied totally or partially by expansion of the gaseous stream in a turboexpander which produces power that may be used for example in driving a compressor. 
     In a typical cryogenic expansion-type recovery process, a feedstream gas under pressure is cooled by heat exchange with other streams of the process and/or external sources of cooling such as a propane compression refrigeration system. As the gas is cooled, liquids are condensed and are collected in one or more separators as a high pressure liquid feed containing most of the desired propane and heavier hydrocarbons. The high pressure liquid feed is transferred to a deethanizer column after its pressure is adjusted to the operating pressure of the deethanizer. The deethanizer is a fractionating column in which the liquid feed is fractionated to separate residual methane and ethane from the desired produces of propane and heavier hydrocarbon components. 
     If the feedstream is not totally condensed (typically it is not), the vapor remaining from this partial condensation is expanded in a turboexpander to a lower pressure. Additional liquids are condensed as a result of the further cooling of the stream during expansion. The pressure after the expansion is usually the same pressure at which the deethanizer is operated. Liquids thus obtained are also supplied as a feed to the deethanizer. Typically, remaining vapor and deethanizer overhead vapor are combined as a residual methane/ethane product gas. 
     In the ideal operation of such a separation process, the vapors leaving the process will contain substantially all the methane and ethane found in the feed gas to the recovery plant and substantially no propane or heavier hydrocarbon components. The bottoms fraction leaving the deethanizer will contain substantially all the propane and heavier hydrocarbon components and essentially no methane or ethane. In practice, this ideal situation is not obtained because the conventional deethanizer is operated largely as a stripping column. Therefore, the methane and ethane vapors leaving the top fractionation stage of the column will contain vapors not subjected to any rectification step. Substantial losses of propane and heavier hydrocarbons occur because the vapors discharged from the low temperature separation steps contain propane and heavier hydrocarbon components which could be recovered if those vapors were brought to lower temperature, or if they were contacted with a significant quantity of a relatively heavy hydrocarbon, e.g. heptane, capable of absorbing the propane. 
     U.S. Pat. No. 4,272,269 which issued to Hammond, et al on June 6, 1981 described one such process that combines both the cryogenic expansion step and the absorption process to increase the recovery percentage of the propane and hydrocarbon components. The disadvantage with using an absorption oil is that additional refining steps are needed to desorb the propane and prepare the absorption oil for reuse. 
     The problem associated with all types of propane recovery operations is one of efficiency. The main objective is to recover as much of the propane and heavier hydrocarbon components as is economically possible. The conventional systems in operation today are capable of economically recovering, at most, about 95% of the propane in a feedstream. Because of the large volume of gas that is processed, there is a definite need to find efficient methods to recover more of the propane and heavier hydrocarbons in a gaseous feedstream. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a process for treating a gaseous hydrocarbon-containing feedstream where it is desirable to leave substantially all of the methane and ethane in the hydrocarbon gaseous stream and to separate and recover substantially all of the propane and heavier hydrocarbon components. It has been found that increased percentage of propane and heavier hydrocarbon components can be economically recovered by contacting the expanded vapor from a gaseous feedstream with at least a portion of the liquefaction overhead from a deethanizer. 
     The process of the present invention comprises cooling a gaseous hydrocarbon-containing feedstream to form a vapor stream and a liquid stream. The liquid stream is transferred to a deethanizer while the vapor stream is expanded transferred to the bottom of a direct heat exchanger column. The deethanizer overhead, which consists mainly of methane and ethane, is cooled liquefaction and fed to the upper portion of the direct heat exchanger column. The liquid methane and ethane flow downward within the direct heat exchanger column and contact gaseous propane and heavier hydrocarbons that flow upward. The methane and ethane vaporize by absorbing heat from the gaseous propane and heavier hydrocarbons which causes the propane and heavier hydrocarbons to condense at the bottom of the direct heat exchanger column. The gaseous methane and ethane within the direct heat exchanger column are removed from the overhead as a product of the process. The liquid at the bottom of the direct heat exchanger column is removed and fed to the lower portion of the deethanizer. The liquid at the bottom of the deethanizer is removed as a product of the process, and at least a portion of the gaseous overhead is cooled and returned to the top of the direct heat exchanger column. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic flow diagram illustrating the present invention. 
     FIG. 2 is a schematic flow diagram illustrating a preferred embodiment of the present invention. 
     FIG. 3 is a schematic flow diagram illustrating a variation of the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, a gaseous feedstream of natural gas, solution gas or refinery gas, which typically contains hydrocarbons ranging from methane to hexane, is processed to separate and recover the propane and heavier hydrocarbon components. 
     As illustrated in FIG. 1, the gaseous feedstream comes in through line 10. The natural gas feed to a natural gas plant will generally be at about atmospheric temperature and at an elevated pressure substantially above atmospheric pressure. Prior to the initial cooling step, the gaseous feedstream enters an inlet separator 12 for removal of liquid hydrocarbons. Additionally, water vapor in the feedstream is removed in a dehydration unit 16 to avoid the formation of ice throughout the process. These preparatory steps are known to those skilled in the art and, depending on the composition of the gaseous feedstream, may not be necessary. 
     The gaseous feedstream flows through line 18 and is initially cooled to a temperature of about -10° F. One method of cooling the feedstream, as shown in FIG. 1, involves contact with indirect heat exchangers 20 and 24 and a propane refrigerant 22. Other methods are available and known to those skilled in the art. After the gaseous feedstream is sufficiently cooled, it enters an inlet vessel 28 where it is separated into a vapor stream and a liquid stream. The vapor stream is expanded and passes through line 94 into the lower portion of a direct heat exchanger column 74 which is a mixing chamber. Direct heat exchanger column 74 may have a variety of configurations, but its purpose is to allow direct contact between vapor and liquid phases. A packed column is one configuration that can function as a direct heat exchanger. A preferred configuration is similar to a tray-type absorber where liquid enters the top and flows down a series of trays, contacting gas which is flowing upward from the bottom. 
     FIG. 2 represents a preferred embodiment where the vapor stream from inlet vessel 28 is work expanded in turboexpander 96, cooled to a temperature of about -50° F. and partially condensed. This cooled and partially condensed stream flows through line 98 to the upper portion of a separator 100. The liquid from the inlet vessel 28 flowing through line 29 passes through an expansion valve 30 and enters the lower portion of separator 100. The liquid from the bottom of the separator 100 combines with the liquid from the direct heat exchanger column 74 in line 36. The vapor from separator 100 is fed to the lower portion of the direct heat exchanger column 74. 
     This preferred embodiment is most effective on gaseous feedstreams rich in propane and heavier hydrocarbon components. The additional steps separate many of these components early in the process, thus increasing the efficiency of the subsequent separation steps. 
     Referring to FIG. 2, the flow from line 102 into the lower portion of the direct heat exchanger column 74 contains gaseous propane and heavier hydrocarbon components. These gases flow upward, contacting downward flowing liquid methane and ethane which enter the upper portion of the direct heat exchanger column through line 72 (described in more detail later). The liquid methane and ethane descend from tray to tray in the direct heat exchanger column and evaporate. The energy for the evaporization is supplied by the condensation of gaseous propane and heavier hydrocarbons ascending from the bottom of the direct heat exchanger column. 
     A vapor stream consisting essentially of pure methane and ethane is formed within the direct heat exchanger column 74. The overhead from the direct heat exchanger column flows through line 80, passes through a series of indirect heat exchangers 66 and 24, and enters the compressor side 86 of the turboexpander. After this compression step, the gas is further compressed in a residual gas compressor 90 and is removed as a gaseous product of the process. 
     The liquid propane and heavier hydrocarbons are removed from the bottom of the direct heat exchanger column 74 through line 76 and combined with the liquid stream from separator 100. These streams are heated through a series of indirect heat exchangers and can provide some or all of the cooling requirement for gas entering inlet vessel 28. The warmed liquid stream is fed to the deethanizer 444 which is essentially a fractionating column. Liquid from the bottom of the deethanizer 44 is removed through line 46 as a liquid product of the process. The liquid product consists essentially of propane and heavier hydrocarbon components. 
     The gaseous overhead from the deethanizer is cooled and fed to the direct heat exchanger column 75. FIG. 2 shows one embodiment where the deethanizer overhead, which consists essentially of methane and ethane, is cooled by propane refrigeration 50 and fed to separator 54. The liquid portion flows out line 56. Part of the liquid is returned through line 60 to the deethanizer as reflux, and the rest of it flows through line 62 into line 64. Alternatively, line 62 can flow into the direct heat exchanger column 74. In either case, vapor in line 64 is further cooled by indirect heat exchangers 38 and 66 to about -50° F. at which point the vapor liquefies. The liquid flows through line 68 to an expansion valve 70 where the methane and ethane is partially flashed, further reducing the temperature to about -70° F. After the pressure reduction, the cold liquid and gas flow through line 72 into the upper portion of the direct heat exchanger column 74. The cycle is complete with the liquid methane and ethane descending from tray to tray condensing gaseous propane and heavier hydrocarbons and the gaseous methane and ethane flowing out the top of the direct heat exchanger column through line 80. 
     FIG. 3 shows an embodiment where some of the deethanizer overhead is removed from the process. In this embodiment, the deethanizer overhead is cooled by propane refrigerant 50 and indirect heat exchangers 38 and 66 prior to entering separator 54. The gaseous overhead from separator 54 has little, if any, propane and heavier hydrocarbons. The overhead is transferred to line 88 through line 64, and the liquid flows out through line 56. Part of the liquid is returned through line 60 to the deethanizer as reflux, and the rest flows through expansion valve 70, partially flashing the methane and ethane. As previously stated, the flow from line 72 is fed to the upper portion of the direct heat exchanger unit, and the liquid methane and ethane descend from tray to tray condensing gaseous propane and heavier hydrocarbons. 
     EXAMPLE 
     The process of this invention will be further understood by reference to a specific example. For illustrative purposes, a gaseous feedstream having the following composition will be used: 
     
         ______________________________________Component        Mole %______________________________________Carbon Dioxide   .900Nitrogen         3.540Hydrogen Sulfide 0.000Methane          65.043Ethane           19.353Propane          7.376I-Butane         0.835Butane           2.121I-Pentane        0.321Pentane          0.320Hexane+          0.191______________________________________ 
    
     By way of illustration, the gaseous feedstream in this instance would be at a temperature of about 70° F. and a pressure of about 515 psia. 
     The following table illustrates the calculated temperatures and pressures at major points as the feedstream passes through the system shown in FIG. 2. 
     
         ______________________________________Line or Unit   Temperature                     PressureNumber         °F. psia______________________________________26             -10.00     50034             -26.21     300102            -41.58     30080             -71.00     30076             -55.00     30042             48.00      49048             47.10      49064             24.04      48560             24.04      49062             24.04      49072             -73.83     30084             53.00      28546             215.00     490______________________________________ 
    
     The composition of the gaseous product of the process from the direct heat exchanger column 74 passing through line 80 and the liquid product of the process from the deethanizer passing through line 46 would be as follows: 
     
         ______________________________________Component        Mole %______________________________________Gaseous ProductComposition (80)Carbon Dioxide   1.014Nitrogen         3.987Hydrogen Sulfide 0.000Methane          73.269Ethane           21.595Propane          0.133I-Butane         0.000Butane           0.000I-Pentane        0.000Pentane          0.000Hexane+          0.000Liquid ProductComposition (46)Carbon Dioxide   0.000Nitrogen         0.000Hydrogen Sulfide 0.000Methane          0.000Ethane           1.627Propane          64.639I-Butane         7.437Butane           18.890I-Pentane        2.857Pentane          2.845Hexane+          1.702______________________________________ 
    
     By way of comparison, the recovery of propane in conventional systems is about 95 mol % of feedstream, whereas this process separates and recovers about 98 to 99 mol % of the propane in the feedstream. 
     The principle of the invention and the best mode contemplated for applying that principle have been described. It is to be understood that the foregoing is illustrative only and that other means and techniques can be employed without departing from the true scope of the invention defined in the following claims.