Hydrocarbon Gas Processing

This invention relates to a process and apparatus for improving the separation of a gas containing hydrocarbons. Assignees S.M.E. Products LP and Ortloff Engineers, Ltd. were parties to a joint research agreement that was in effect before the invention of this application was made. The applicants claim the benefits under Title 35, United States Code, Section 119(e) of prior U.S. Provisional Application No. 62/513,851 which was filed on Jun. 1, 2017 and prior U.S. Provisional Application No. 62/667,833 which was filed on May 7, 2018.

Ethylene, ethane, propylene, propane, and/or heavier hydrocarbons can be recovered from a variety of gases, such as natural gas, refinery gas, and synthetic gas streams obtained from other hydrocarbon materials such as coal, crude oil, naphtha, oil shale, tar sands, and lignite. Natural gas usually has a major proportion of methane and ethane, i.e., methane and ethane together comprise at least 50 mole percent of the gas. The gas also contains relatively lesser amounts of heavier hydrocarbons such as propane, butanes, pentanes, and the like, as well as hydrogen, nitrogen, carbon dioxide, and/or other gases.

The present invention is generally concerned with improving the recovery of ethylene, ethane, propylene, propane, and heavier hydrocarbons from such gas streams. A typical analysis of a gas stream to be processed in accordance with this invention would be, in approximate mole percent, 78.6% methane, 12.5% ethane and other C₂components, 4.9% propane and other C₃components, 0.6% iso-butane, 1.4% normal butane, and 1.1% pentanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present.

The historically cyclic fluctuations in the prices of both natural gas and its natural gas liquid (NGL) constituents have at times reduced the incremental value of ethane, ethylene, propane, propylene, and heavier components as liquid products. This has resulted in a demand for processes that can provide more efficient recoveries of these products, for processes that can provide efficient recoveries with lower capital investment, and for processes that can be easily adapted or adjusted to vary the recovery of a specific component over a broad range. Available processes for separating these materials include those based upon cooling and refrigeration of gas, oil absorption, and refrigerated oil absorption. Additionally, cryogenic processes have become popular because of the availability of economical equipment that produces power while simultaneously expanding and extracting heat from the gas being processed. Depending upon the pressure of the gas source, the richness (ethane, ethylene, and heavier hydrocarbons content) of the gas, and the desired end products, each of these processes or a combination thereof may be employed.

The cryogenic expansion process is now generally preferred for natural gas liquids recovery because it provides maximum simplicity with ease of startup, operating flexibility, good efficiency, safety, and good reliability. U.S. Pat. Nos. 3,292,380; 4,061,481; 4,140,504; 4,157,904; 4,171,964; 4,185,978; 4,251,249; 4,278,457; 4,519,824; 4,617,039; 4,687,499; 4,689,063; 4,690,702; 4,854,955; 4,869,740; 4,889,545; 5,275,005; 5,555,748; 5,566,554; 5,568,737; 5,771,712; 5,799,507; 5,881,569; 5,890,378; 5,983,664; 6,182,469; 6,578,379; 6,712,880; 6,915,662; 7,191,617; 7,219,513; 8,590,340; 8,881,549; 8,919,148; 9,021,831; 9,021,832; 9,052,136; 9,052,137; 9,057,558; 9,068,774; 9,074,814; 9,080,810; 9,080,811; 9,476,639; 9,637,428; 9,783,470; 9,927,171; 9,933,207; and 9,939,195; reissue U.S. Pat. No. 33,408; and co-pending application Ser. Nos. 11/839,693; 12/868,993; 12/869,139; 14/714,912; 14/828,093; 15/259,891; 15/332,670; 15/332,706; 15/332,723; and 15/668,139 describe relevant processes (although the description of the present invention in some cases is based on different processing conditions than those described in the cited U.S. Patents and co-pending applications).

In a typical cryogenic expansion recovery process, a feed gas stream under pressure is cooled by heat exchange with other streams of the process and/or external sources of refrigeration such as a propane compression-refrigeration system. As the gas is cooled, liquids may be condensed and collected in one or more separators as high-pressure liquids containing some of the desired C₂+ components. Depending on the richness of the gas and the amount of liquids formed, the high-pressure liquids may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion of the liquids results in further cooling of the stream. Under some conditions, pre-cooling the high pressure liquids prior to the expansion may be desirable in order to further lower the temperature resulting from the expansion. The expanded stream, comprising a mixture of liquid and vapor, is fractionated in a distillation (demethanizer or deethanizer) column. In the column, the expansion cooled stream(s) is (are) distilled to separate residual methane, nitrogen, and other volatile gases as overhead vapor from the desired C₂components, C₃components, and heavier hydrocarbon components as bottom liquid product, or to separate residual methane, C₂components, nitrogen, and other volatile gases as overhead vapor from the desired C₃components and heavier hydrocarbon components as bottom liquid product.

If the feed gas is not totally condensed (typically it is not), the vapor remaining from the partial condensation can be split into two streams. One portion of the vapor is passed through a work expansion machine or engine, or an expansion valve, to a lower pressure at which additional liquids are condensed as a result of further cooling of the stream. The pressure after expansion is essentially the same as the pressure at which the distillation column is operated. The combined vapor-liquid phases resulting from the expansion are supplied as feed to the column.

The remaining portion of the vapor is cooled to substantial condensation by heat exchange with other process streams, e.g., the cold fractionation tower overhead. Some or all of the high-pressure liquid may be combined with this vapor portion prior to cooling. The resulting cooled stream is then expanded through an appropriate expansion device, such as an expansion valve, to the pressure at which the demethanizer is operated. During expansion, a portion of the liquid will vaporize, resulting in cooling of the total stream. The flash expanded stream is then supplied as top feed to the demethanizer. Typically, the vapor portion of the flash expanded stream and the demethanizer overhead vapor combine in an upper separator section in the fractionation tower as residual methane product gas. Alternatively, the cooled and expanded stream may be supplied to a separator to provide vapor and liquid streams. The vapor is combined with the tower overhead and the liquid is supplied to the column as a top column feed.

In the ideal operation of such a separation process, the residue gas leaving the process will contain substantially all of the methane in the feed gas with essentially none of the heavier hydrocarbon components, and the bottoms fraction leaving the demethanizer will contain substantially all of the heavier hydrocarbon components with essentially no methane or more volatile components. In practice, however, this ideal situation is not obtained because the conventional demethanizer is operated largely as a stripping column. The methane product of the process, therefore, typically comprises vapors leaving the top fractionation stage of the column, together with vapors not subjected to any rectification step. Considerable losses of C₂, C₃, and C₄+ components occur because the top liquid feed contains substantial quantities of these components and heavier hydrocarbon components, resulting in corresponding equilibrium quantities of C₂components, C₃components, C₄components, and heavier hydrocarbon components in the vapors leaving the top fractionation stage of the demethanizer. The loss of these desirable components could be significantly reduced if the rising vapors could be brought into contact with a significant quantity of liquid (reflux) capable of absorbing the C₂components, C₃components, C₄components, and heavier hydrocarbon components from the vapors.

In recent years, the preferred processes for hydrocarbon separation use an upper absorber section to provide additional rectification of the rising vapors. For many of these processes, the source of the reflux stream for the upper rectification section is a recycled stream of residue gas supplied under pressure. The recycled residue gas stream is usually cooled to substantial condensation by heat exchange with other process streams, e.g., the cold fractionation tower overhead. The resulting substantially condensed stream is then expanded through an appropriate expansion device, such as an expansion valve, to the pressure at which the demethanizer is operated. During expansion, a portion of the liquid will usually vaporize, resulting in cooling of the total stream. The flash expanded stream is then supplied as top feed to the demethanizer. Typical process schemes of this type are disclosed in U.S. Pat. Nos. 4,889,545; 5,568,737; 5,881,569; 9,052,137; and 9,080,811 and in Mowrey, E. Ross, “Efficient, High Recovery of Liquids from Natural Gas Utilizing a High Pressure Absorber”, Proceedings of the Eighty-First Annual Convention of the Gas Processors Association, Dallas, Tex., Mar. 11-13, 2002. Unfortunately, in addition to the additional rectification section in the demethanizer, these processes also require surplus compression capacity to provide the motive force for recycling the reflux stream to the demethanizer, adding to both the capital cost and the operating cost of facilities using these processes.

Another means of providing a reflux stream for the upper rectification section is to withdraw a distillation vapor stream from a lower location on the tower (and perhaps combine it with a portion of the tower overhead vapor). This vapor (or combined vapor) stream is compressed to higher pressure, then cooled to substantial condensation, expanded to the tower operating pressure, and supplied as top feed to the tower. Typical process schemes of this type are disclosed in U.S. Pat. No. 9,476,639 and co-pending application Ser. Nos. 11/839,693; 12/869,139; and Ser. No. 15/259,891. These also require an additional rectification section in the demethanizer, plus a compressor to provide motive force for recycling the reflux stream to the demethanizer, again adding to both the capital cost and the operating cost of facilities using these processes.

However, there are many gas processing plants that have been built in the U.S. and other countries according to U.S. Pat. Nos. 4,157,904 and 4,278,457 (as well as other processes) that have no upper absorber section to provide additional rectification of the rising vapors and cannot be easily modified to add this feature. Also, these plants do not usually have surplus compression capacity to allow recycling a reflux stream. As a result, these plants are not as efficient when operated to recover C₂components and heavier components from the gas (commonly referred to as “ethane recovery”), and are particularly inefficient when operated to recover only the C₃components and heavier components from the gas (commonly referred to as “ethane rejection”).

The present invention is a novel means of providing additional rectification that can be easily added to existing gas processing plants to increase the recovery of the desired C₂components and/or C₃components without requiring additional residue gas compression. The incremental value of this increased recovery is often substantial. For the Examples given later, the incremental income from the additional recovery capability over that of the prior art is in the range of US$710,000 to US$4,720,000 [€590,000 to €3,930,000] per year using an average incremental value US$0.10-0.58 per gallon [€22-129 per m³] for hydrocarbon liquids compared to the corresponding hydrocarbon gases.

The present invention also combines what heretofore have been individual equipment items into a common housing, thereby reducing both the plot space requirements and the capital cost of the addition. Surprisingly, applicants have found that the more compact arrangement also significantly increases the product recovery at a given power consumption, thereby increasing the process efficiency and reducing the operating cost of the facility. In addition, the more compact arrangement also eliminates much of the piping used to interconnect the individual equipment items in traditional plant designs, further reducing capital cost and also eliminating the associated flanged piping connections. Since piping flanges are a potential leak source for hydrocarbons (which are volatile organic compounds, VOCs, that contribute to greenhouse gases and may also be precursors to atmospheric ozone formation), eliminating these flanges reduces the potential for atmospheric emissions that may damage the environment.

In accordance with the present invention, it has been found that C₂recoveries in excess of 99% can be obtained. Similarly, in those instances where recovery of C₂components is not desired, C₃recoveries in excess of 96% can be maintained. The present invention, although applicable at lower pressures and warmer temperatures, is particularly advantageous when processing feed gases in the range of 400 to 1500 psia [2,758 to 10,342 kPa(a)] or higher under conditions requiring NGL recovery column overhead temperatures of −50° F. [−46° C.] or colder.

In the following explanation of the above figures, tables are provided summarizing flow rates calculated for representative process conditions. In the tables appearing herein, the values for flow rates (in moles per hour) have been rounded to the nearest whole number for convenience. The total stream rates shown in the tables include all non-hydrocarbon components and hence are generally larger than the sum of the stream flow rates for the hydrocarbon components. Temperatures indicated are approximate values rounded to the nearest degree. It should also be noted that the process design calculations performed for the purpose of comparing the processes depicted in the figures are based on the assumption of no heat leak from (or to) the surroundings to (or from) the process. The quality of commercially available insulating materials makes this a very reasonable assumption and one that is typically made by those skilled in the art.

For convenience, process parameters are reported in both the traditional British units and in the units of the Système International d'Unités (SI). The molar flow rates given in the tables may be interpreted as either pound moles per hour or kilogram moles per hour. The energy consumptions reported as horsepower (HP) and/or thousand British Thermal Units per hour (MBTU/Hr) correspond to the stated molar flow rates in pound moles per hour. The energy consumptions reported as kilowatts (kW) correspond to the stated molar flow rates in kilogram moles per hour.

FIG. 1 is a process flow diagram showing the design of a processing plant to recover C₂+ components from natural gas using prior art according to U.S. Pat. No. 4,157,904 or 4,278,457. In this simulation of the process, inlet gas enters the plant at 120° F. [49° C.] and 815 psia [5,617 kPa(a)] as stream 31. If the inlet gas contains a concentration of sulfur compounds which would prevent the product streams from meeting specifications, the sulfur compounds are removed by appropriate pretreatment of the feed gas (not illustrated). In addition, the feed stream is usually dehydrated to prevent hydrate (ice) formation under cryogenic conditions. Solid desiccant has typically been used for this purpose.

The feed stream 31 is cooled in heat exchanger 10 by heat exchange with cool residue gas (stream 39a), pumped liquid product at 20° F. [−7° C.] (stream 42a), demethanizer reboiler liquids at 0° F. [−18° C.] (stream 41), demethanizer side reboiler liquids at −45° F. [−43° C.] (stream 40), and propane refrigerant. Stream 31a then enters separator 11 at −29° F. [−34° C.] and 795 psia [5,479 kPa(a)] where the vapor (stream 32) is separated from the condensed liquid (stream 33).

The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37. The liquid (stream 33) from separator 11 is optionally divided into two streams, 35 and 38. (Stream 35 may contain from 0% to 100% of the separator liquid in stream 33. If stream 35 contains any portion of the separator liquid, then the process of FIG. 1 is according to U.S. Pat. No. 4,157,904. Otherwise, the process of FIG. 1 is according to U.S. Pat. No. 4,278,457.) For the process illustrated in FIG. 1, stream 35 contains about 15% of the total separator liquid. Stream 34, containing about 30% of the total separator vapor, is combined with stream 35 and the combined stream 36 passes through heat exchanger 12 in heat exchange relation with the cold residue gas (stream 39) where it is cooled to substantial condensation. The resulting substantially condensed stream 36a at −158° F. [−106° C.] is then flash expanded through expansion valve 13 to the operating pressure (approximately 168 psia [1,156 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 1, the expanded stream 36b leaving expansion valve 13 reaches a temperature of −176° F. [−115° C.] and is supplied to separator section 17a in the upper region of fractionation tower 17. The liquids separated therein become the top feed to demethanizing section 17b.

The remaining 70% of the vapor from separator 11 (stream 37) enters a work expansion machine 14 in which mechanical energy is extracted from this portion of the high pressure feed. The machine 14 expands the vapor substantially isentropically to the tower operating pressure, with the work expansion cooling the expanded stream 37a to a temperature of approximately −126° F. [−88° C.]. The typical commercially available expanders are capable of recovering on the order of 80-85% of the work theoretically available in an ideal isentropic expansion. The work recovered is often used to drive a centrifugal compressor (such as item 15) that can be used to re-compress the residue gas (stream 39b), for example. The partially condensed expanded stream 37a is thereafter supplied as feed to fractionation tower 17 at an upper mid-column feed point. The remaining separator liquid in stream 38 (if any) is expanded to the operating pressure of fractionation tower 17 by expansion valve 16, cooling stream 38a to −85° F. [−65° C.] before it is supplied to fractionation tower 17 at a lower mid-column feed point.

The demethanizer in tower 17 is a conventional distillation column containing a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing. As is often the case in natural gas processing plants, the fractionation tower may consist of two sections. The upper section 17a is a separator wherein the partially vaporized top feed is divided into its respective vapor and liquid portions, and wherein the vapor rising from the lower distillation or demethanizing section 17b is combined with the vapor portion of the top feed to form the cold demethanizer overhead vapor (stream 39) which exits the top of the tower. The lower, demethanizing section 17b contains the trays and/or packing and provides the necessary contact between the liquids falling downward and the vapors rising upward. The demethanizing section 17b also includes reboilers (such as the reboiler and the side reboiler described previously and supplemental reboiler 18) which heat and vaporize a portion of the liquids flowing down the column to provide the stripping vapors which flow up the column to strip the liquid product, stream 42, of methane and lighter components.

The liquid product stream 42 exits the bottom of the tower at 7° F. [−14° C.], based on a typical specification of a methane concentration of 0.5% on a volume basis in the bottom product. It is pumped to higher pressure by pump 21 (stream 42a) and then heated to 95° F. [35° C.] (stream 42b) as it provides cooling of the feed gas in heat exchanger 10 as described earlier. The residue gas (demethanizer overhead vapor stream 39) passes countercurrently to the incoming feed gas in heat exchanger 12 where it is heated from −176° F. [−115° C.] to −47° F. [−44° C.] (stream 39a) and in heat exchanger 10 where it is heated to 113° F. [45° C.] (stream 39b). The residue gas is then re-compressed in two stages. The first stage is compressor 15 driven by expansion machine 14. The second stage is compressor 19 driven by a supplemental power source which compresses the residue gas (stream 39d) to sales line pressure. After cooling to 120° F. [49° C.] in discharge cooler 20, the residue gas product (stream 39e) flows to the sales gas pipeline at 765 psia [5,272 kPa(a)], sufficient to meet line requirements (usually on the order of the inlet pressure).

A summary of stream flow rates and energy consumption for the process illustrated in FIG. 1 is set forth in the following table:

FIG. 2 is a process flow diagram showing one manner in which the design of the processing plant in FIG. 1 can be adjusted to operate at a lower C₂component recovery level. This is a common requirement when the relative values of natural gas and liquid hydrocarbons are variable, causing recovery of the C₂components to be unprofitable at times. The process of FIG. 2 has been applied to the same feed gas composition and conditions as described previously for FIG. 1. However, in the simulation of the process of FIG. 2, the process operating conditions have been adjusted to reject nearly all of C₂components to the residue gas rather than recovering them in the bottom liquid product from the fractionation tower.

In this simulation of the process, inlet gas enters the plant at 120° F. [49° C.] and 815 psia [5,617 kPa(a)] as stream 31 and is cooled in heat exchanger 10 by heat exchange with cool residue gas stream 39a and flashed separator liquids (stream 38a). (One consequence of operating the FIG. 2 process to reject nearly all of the C₂components to the residue gas is that the temperatures of the liquids flowing down fractionation tower 17 are much warmer, to the point that side reboiler stream 40 and reboiler stream 41 are too warm to be used to cool the inlet gas, so that all of the column reboil heat must be supplied by supplemental reboiler 18. The pumped bottom product (stream 42a) is also too warm to be used to cool the inlet gas. In the FIG. 2 process, the flashed separator liquids are used in heat exchanger 10 in lieu of the side reboiler liquids in order to provide some cooling of the inlet gas while simultaneously reducing the duty required from supplemental reboiler 18.) Cooled stream 31a enters separator 11 at −14° F. [−26° C.] and 795 psia [5,479 kPa(a)] where the vapor (stream 32) is separated from the condensed liquid (stream 33).

The vapor (stream 32) from separator 11 is divided into two streams, 34 and 37, and the liquid (stream 33) is optionally divided into two streams, 35 and 38. For the process illustrated in FIG. 2, stream 35 contains about 36% of the total separator liquid. Stream 34, containing about 33% of the total separator vapor, is combined with stream 35 and the combined stream 36 passes through heat exchanger 12 in heat exchange relation with the cold residue gas (stream 39) where it is cooled to partial condensation. The resulting partially condensed stream 36a at −72° F. [−58° C.] is then flash expanded through expansion valve 13 to the operating pressure (approximately 200 psia [1,380 kPa(a)]) of fractionation tower 17. During expansion some of the liquid in the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 2, the expanded stream 36b leaving expansion valve 13 reaches a temperature of −138° F. [−94° C.] and is supplied to fractionation tower 17 at the top feed point.

The remaining 67% of the vapor from separator 11 (stream 37) enters a work expansion machine 14 in which mechanical energy is extracted from this portion of the high pressure feed. The machine 14 expands the vapor substantially isentropically to the tower operating pressure, with the work expansion cooling the expanded stream 37a to a temperature of approximately −103° F. [−75° C.] before it is supplied as feed to fractionation tower 17 at an upper mid-column feed point. The remaining separator liquid in stream 38 (if any) is expanded to slightly above the operating pressure of fractionation tower 17 by expansion valve 16, cooling stream 38a to −61° F. [−51° C.] before it is heated to 103° F. [39° C.] in heat exchanger 10 as described previously, with heated stream 40a then supplied to fractionation tower 17 at a lower mid-column feed point.

Note that when fractionation tower 17 is operated to reject the C₂components to the residue gas product as shown in FIG. 2, the column is typically referred to as a deethanizer and its lower section 17b is called a deethanizing section. The liquid product stream 42 exits the bottom of deethanizer 17 at 137° F. [58° C.], based on a typical specification of an ethane to propane ratio of 0.020:1 on a volume basis in the bottom product. The residue gas (deethanizer overhead vapor stream 39) passes countercurrently to the incoming feed gas in heat exchanger 12 where it is heated from −91° F. [−68° C.] to −29° F. [−34° C.] (stream 39a) and in heat exchanger 10 where it is heated to 103° F. [39° C.] (stream 39b) as it provides cooling as described previously. The residue gas is then re-compressed in two stages, compressor 15 driven by expansion machine 14 and compressor 19 driven by a supplemental power source. After stream 39d is cooled to 120° F. [49° C.] in discharge cooler 20, the residue gas product (stream 39e) flows to the sales gas pipeline at 765 psia [5,272 kPa(a)].

A summary of stream flow rates and energy consumption for the process illustrated in FIG. 2 is set forth in the following table:

Co-pending application Ser. No. 15/332,723 describes one means of improving the performance of the FIG. 1 process to recover more of the C₂components in the bottom liquid product. FIG. 1 can be adapted to use this process as shown in FIG. 3. The operating conditions of the FIG. 3 process have been adjusted as shown to reduce the methane content of the liquid product to the same level as that of the FIG. 1 process. The feed gas composition and conditions considered in the process presented in FIG. 3 are the same as those in FIG. 1. Accordingly, the FIG. 3 process can be compared with that of the FIG. 1 process.

Most of the process conditions shown for the FIG. 3 process are much the same as the corresponding process conditions for the FIG. 1 process. The main difference is the disposition of substantially condensed stream 36a and column overhead vapor stream 39. In the FIG. 3 process, column overhead vapor stream 39 is divided into two streams, stream 151 and stream 152, whereupon stream 151 is compressed from the operating pressure (approximately 174 psia [1,202 kPa(a)]) of fractionation tower 17 to approximately 379 psia [2,616 kPa(a)] by reflux compressor 22. Compressed stream 151a at −81° F. [−63° C.] and substantially condensed stream 36a at −81° F. [−63° C.] are then directed into a heat exchange means in cooling section 117a of processing assembly 117. This heat exchange means may be comprised of a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The heat exchange means is configured to provide heat exchange between stream 151a flowing through one pass of the heat exchange means, substantially condensed stream 36a flowing through another pass of the heat exchange means, and a further rectified vapor stream arising from rectifying section 117b of processing assembly 117, so that stream 151a is cooled to substantial condensation (stream 151b) and stream 36a is further cooled (stream 36b) while heating the further rectified vapor stream.

Substantially condensed stream 151b at −171° F. [−113° C.] is then flash expanded through expansion valve 23 to slightly above the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 3, the expanded stream 151c leaving expansion valve 23 reaches a temperature of −185° F. [−121° C.] before it is directed into a heat and mass transfer means in rectifying section 117b of processing assembly 117. This heat and mass transfer means may also be comprised of a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The heat and mass transfer means is configured to provide heat exchange between a partially rectified vapor stream arising from absorbing section 117c of processing assembly 117 that is flowing upward through one pass of the heat and mass transfer means, and the flash expanded substantially condensed stream 151c flowing downward, so that the partially rectified vapor stream is cooled while heating the expanded stream. As the partially rectified vapor stream is cooled, a portion of it is condensed and falls downward while the remaining vapor continues flowing upward through the heat and mass transfer means. The heat and mass transfer means provides continuous contact between the condensed liquid and the partially rectified vapor stream so that it also functions to provide mass transfer between the vapor and liquid phases, thereby providing further rectification of the partially rectified vapor stream to form the further rectified vapor stream. This further rectified vapor stream arising from the heat and mass transfer means is then directed to the heat exchange means in cooling section 117a of processing assembly 117 to be heated as described previously. The condensed liquid from the bottom of the heat and mass transfer means is directed to absorbing section 117c of processing assembly 117.

The flash expanded stream 151c is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, and exits the heat and mass transfer means in rectifying section 117b at −178° F. [−117° C.]. The heated flash expanded stream discharges into separator section 117d of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with the remaining portion (stream 152) of overhead vapor stream 39 to form a combined vapor stream that enters a mass transfer means in absorbing section 117c of processing assembly 117. The mass transfer means may consist of a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing, but could also be comprised of a non-heat transfer zone in a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The mass transfer means is configured to provide contact between the cold condensed liquid leaving the bottom of the heat and mass transfer means in rectifying section 117b and the combined vapor stream arising from separator section 117d. As the combined vapor stream rises upward through absorbing section 117c, it is contacted with the cold liquid falling downward to condense and absorb C₂components, C₃components, and heavier components from the combined vapor stream. The resulting partially rectified vapor stream is then directed to the heat and mass transfer means in rectifying section 117b of processing assembly 117 for further rectification as described previously.

The liquid phase (if any) from the heated flash expanded stream leaving rectifying section 117b of processing assembly 117 that is separated in separator section 117d combines with the distillation liquid leaving the bottom of the mass transfer means in absorbing section 117c of processing assembly 117 to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 24 (stream 154a at −170° F. [−112° C.]). Further cooled stream 36b at −169° F. [−112° C.] is flash expanded through expansion valve 13 to the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream to −177° F. [−116° C.]. Flash expanded stream 36c then joins with pumped stream 154a to form combined feed stream 155, which then enters fractionation column 17 at the top feed point at −176° F. [−116° C.].

The further rectified vapor stream leaves the heat and mass transfer means in rectifying section 117b of processing assembly 117 at −182° F. [−119° C.] and enters the heat exchange means in cooling section 117a of processing assembly 117. The vapor is heated to −96° F. [−71° C.] as it provides cooling to streams 36a and 151a as described previously. The heated vapor is then discharged from processing assembly 117 as cool residue gas stream 153, which is heated and compressed as described previously for stream 39 in the FIG. 1 process.

A summary of stream flow rates and energy consumption for the process illustrated in FIG. 3 is set forth in the following table:

A comparison of Tables I and III shows that, compared to the FIG. 1 process, the FIG. 3 process improves ethane recovery from 96.69% to 98.70%, propane recovery from 99.84% to 100.00%, and butane+ recovery from 99.99% to 100.00%. Comparison of Tables I and III further shows that these increased product yields were achieved without using additional power.

The process of co-pending application Ser. No. 15/332,723 can also be operated to reject nearly all of the C₂components to the residue gas rather than recovering them in the liquid product. The operating conditions of the FIG. 3 process can be altered as illustrated in FIG. 4 (including the idling of the heat exchange means in cooling section 117a of processing assembly 117) to reduce the ethane content of the liquid product to the essentially the same level as that of the FIG. 2 process. The feed gas composition and conditions considered in the process presented in FIG. 4 are the same as those in FIG. 2. Accordingly, the FIG. 4 process can be compared with that of the FIG. 2 process.

Most of the process conditions shown for the FIG. 4 process are much the same as the corresponding process conditions for the FIG. 2 process. The main differences are again the disposition of substantially condensed stream 36a and column overhead vapor stream 39. In the FIG. 4 process, substantially condensed stream 36a is flash expanded through expansion valve 23 to slightly above the operating pressure (approximately 200 psia [1,381 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream is vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 4, the expanded stream 36b leaving expansion valve 23 reaches a temperature of −156° F. [−104° C.] before it is directed into the heat and mass transfer means in rectifying section 117b of processing assembly 117.

The flash expanded stream 36b is further vaporized as it provides cooling and partial condensation of the combined vapor stream, and exits the heat and mass transfer means in rectifying section 117b at −83° F. [−64° C.]. The heated flash expanded stream discharges into separator section 117d of processing assembly 117 and is separated into its respective vapor and liquid phases. The vapor phase combines with overhead vapor stream 39 to form the combined vapor stream that enters the mass transfer means in absorbing section 117c as described previously, and the liquid phase combines with the condensed liquid from the bottom of the mass transfer means in absorbing section 117c to form combined liquid stream 154. Combined liquid stream 154 leaves the bottom of processing assembly 117 and is pumped to higher pressure by pump 24 so that stream 154a at −73° F. [−58° C.] can enter fractionation column 17 at the top feed point. The further rectified vapor stream leaves the heat and mass transfer means in rectifying section 117b and discharges from processing assembly 117 at −104° F. [−76° C.] as cold residue gas stream 153, which is then heated and compressed as described previously for stream 39 in the FIG. 2 process.

A summary of stream flow rates and energy consumption for the process illustrated in FIG. 4 is set forth in the following table:

A comparison of Tables II and IV shows that, compared to the FIG. 2 process, the FIG. 4 process improves propane recovery from 89.20% to 96.50% and butane+ recovery from 98.81% to 100.00%. Comparison of Tables II and IV further shows that these increased product yields were achieved without using additional power.

In those cases where it is desirable to maximize the recovery of C₂components in the liquid product (as in the FIG. 1 prior art process described previously, for instance), the present invention offers significant efficiency advantages over the prior art process depicted in FIG. 1 and the process of co-pending application Ser. No. 15/332,723 depicted in FIG. 3. FIG. 5 illustrates a flow diagram of the FIG. 1 prior art process that has been adapted to use the present invention. The operating conditions of the FIG. 5 process have been adjusted as shown to increase the ethane content of the liquid product above the level that is possible with the FIGS. 1 and 3 processes. The feed gas composition and conditions considered in the process presented in FIG. 5 are the same as those in FIGS. 1 and 3. Accordingly, the FIG. 5 process can be compared with that of the FIGS. 1 and 3 processes to illustrate the advantages of the present invention.

Most of the process conditions shown for the FIG. 5 process are much the same as the corresponding process conditions for the FIG. 1 process. The main difference is the disposition of partially condensed stream 36a and column overhead vapor stream 39. In the FIG. 5 process, column overhead vapor stream 39 at −169° F. [−112° C.] and 192 psia [1,322 kPa(a)] (the operating pressure of fractionation tower 17) is directed to separator section 117d inside single equipment item processing assembly 117. A heated combined stream 152 from cooling section 117a inside processing assembly 117 is divided into two streams, stream 153 and stream 151. Stream 151 is heated to 114° F. [46° C.] in heat exchanger 25 and then compressed to approximately 370 psia [2,549 kPa(a)] by reflux compressor 22. Compressed stream 151b is cooled to 120° F. [49° C.] (stream 151c) in discharge cooler 26, and then to −65° F. [−54° C.] (stream 151d) in heat exchanger 25 as it heats stream 151 as described previously. Cooled compressed stream 151d and partially condensed stream 36a at −70° F. [−56° C.] are then directed into a heat exchange means in cooling section 117a inside processing assembly 117. This heat exchange means may be comprised of a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The heat exchange means is configured to provide heat exchange between cooled compressed stream 151d flowing through one pass of the heat exchange means, partially condensed stream 36a flowing through another pass of the heat exchange means, and a combined stream arising from rectifying section 117b inside processing assembly 117, so that stream 151d is cooled to substantial condensation (stream 151e) and stream 36a is further cooled and substantially condensed (stream 36b) while heating the combined stream.

Absorbing section 117c inside processing assembly 117 contains a mass transfer means. This mass transfer means may consist of a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing, but could also be comprised of a non-heat transfer zone in a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The mass transfer means is configured to provide contact between cold condensed liquid leaving the bottom of a heat and mass transfer means in rectifying section 117b inside processing assembly 117 and column overhead vapor stream 39 arising from separator section 117d inside processing assembly 117. As the column overhead vapor stream rises upward through absorbing section 117c, it is contacted with the cold liquid falling downward to condense and absorb C₂components, C₃components, and heavier components from the vapor stream. The resulting partially rectified vapor stream is then directed to the heat and mass transfer means in rectifying section 117b inside processing assembly 117 for further rectification.

Substantially condensed stream 151e at −178° F. [−117° C.] is flash expanded through expansion valve 23 to slightly above the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 5, the expanded stream 151f leaving expansion valve 23 reaches a temperature of −184° F. [−120° C.] before it is directed into the heat and mass transfer means in rectifying section 117b inside processing assembly 117. This heat and mass transfer means may also be comprised of a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers. The heat and mass transfer means is configured to provide heat exchange between the partially rectified vapor stream arising from absorbing section 117c inside processing assembly 117 that is flowing upward through one pass of the heat and mass transfer means, and the flash expanded substantially condensed stream 151f flowing downward, so that the partially rectified vapor stream is cooled while heating the expanded stream. As the partially rectified vapor stream is cooled, a portion of it is condensed and falls downward while the remaining vapor continues flowing upward through the heat and mass transfer means. The heat and mass transfer means provides continuous contact between the condensed liquid and the partially rectified vapor stream so that it also functions to provide mass transfer between the vapor and liquid phases, thereby providing further rectification of the partially rectified vapor stream to form a further rectified vapor stream. The condensed liquid from the bottom of the heat and mass transfer means is directed to absorbing section 117c inside processing assembly 117.

The flash expanded stream 151f is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, and exits the heat and mass transfer means in rectifying section 117b inside processing assembly 117 at −182° F. [−119° C.]. The heated flash expanded stream then mixes with the further rectified vapor stream to form a combined stream at −181° F. [−119° C.] that is directed to the heat exchange means in cooling section 117a inside processing assembly 117. The combined stream is heated as it provides cooling to streams 151d and 36a as described previously.

The distillation liquid leaving the bottom of the mass transfer means in absorbing section 117c discharges from the bottom of processing assembly 117 (stream 154) and is pumped to higher pressure by pump 24 (stream 154a at −172° F. [−113° C.]). Further cooled substantially condensed stream 36b at −160° F. [−107° C.] is flash expanded through expansion valve 13 to the operating pressure of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream to −172° F. [−114° C.]. Flash expanded stream 36c then joins with pumped stream 154a to form combined feed stream 155, which enters fractionation column 17 at the top feed point at −172° F. [−114° C.].

The heated combined stream 152 is discharged from the heat exchange means in cooling section 117a inside processing assembly 117 at −80° F. [−62° C.]. It is divided into the previously described stream 151, and into cool residue gas stream 153 which is then heated and compressed as described previously for stream 39 in the FIG. 1 process.

A summary of stream flow rates and energy consumption for the process illustrated in FIG. 5 is set forth in the following table:

A comparison of Tables I and V shows that, compared to the prior art of FIG. 1, the present invention improves ethane recovery from 96.69% to 99.60%, propane recovery from 99.84% to 100.00%, and butane+ recovery from 99.99% to 100.00%. The economic impact of these improved recoveries is significant. Using an average incremental value $0.10/gallon [€21.9/m³] for hydrocarbon liquids compared to the corresponding hydrocarbon gases, the improved recoveries represent more than US$710,000 [€590,000] of additional annual revenue for the plant operator. Comparison of Tables III and V shows that the present invention is also an improvement over co-pending application Ser. No. 15/332,723, increasing the ethane recovery from 98.70% to 99.60%. Comparison of Tables I, III, and V further shows that these increased product yields were achieved using essentially the same power as the FIGS. 1 and 3 processes. In terms of the recovery efficiency (defined by the quantity of C₂components and heavier components recovered per unit of power), the present invention represents more than a 1% improvement over the prior art of the FIG. 1.

The improvement in recovery efficiency provided by the present invention over that of the prior art of the FIG. 1 process is primarily due to the supplemental indirect cooling of the column overhead vapor provided by flash expanded stream 151f in rectifying section 117b inside processing assembly 117, in addition to the direct-contact cooling provided by stream 36b in the prior art process of FIG. 1. Although stream 36b is quite cold, it is not an ideal reflux stream because it contains significant concentrations of the C₂components, C₃components, and C₄+ components that demethanizer 17 is supposed to capture, resulting in losses of these desirable components due to equilibrium effects at the top of column 17 for the prior art process of FIG. 1. For the present invention shown in FIG. 5, however, the supplemental cooling provided by flash expanded stream 151f has no equilibrium effects to overcome because there is no direct contact between flash expanded stream 151f and the column overhead vapor stream to be rectified.

The present invention has the further advantage of using the heat and mass transfer means in rectifying section 117b to simultaneously cool the column overhead vapor stream and condense the heavier hydrocarbon components from it, providing more efficient rectification than using reflux in a conventional distillation column. As a result, more of the C₂components, C₃components, and heavier hydrocarbon components can be removed from the column overhead vapor stream using the refrigeration available in flash expanded stream 151f than is possible using conventional mass transfer equipment and conventional heat transfer equipment.

The present invention offers two other advantages over the prior art in addition to the increase in processing efficiency. First, the compact arrangement of processing assembly 117 of the present invention incorporates what would normally be three separate equipment items (the heat exchange means in cooling section 117a, the heat and mass transfer means in rectifying section 117b, and the mass transfer means in absorbing section 117c) into a single equipment item (processing assembly 117 in FIG. 5 of the present invention). This reduces the plot space requirements and eliminates the interconnecting piping, reducing the capital cost of modifying a processing plant to use the present invention. Second, elimination of the interconnecting piping means that a processing plant modified to use the present invention has far fewer flanged connections, reducing the number of potential leak sources in the plant. Hydrocarbons are volatile organic compounds (VOCs), some of which are classified as greenhouse gases and some of which may be precursors to atmospheric ozone formation, which means the present invention reduces the potential for atmospheric releases that may damage the environment.

One additional advantage of the present invention is how easily it can be incorporated into an existing gas processing plant to effect the superior performance described above. As shown in FIG. 5, only three connections (commonly referred to as “tie-ins”) to the existing plant are needed: for partially condensed stream 36a (represented by the dashed line between stream 36a and stream 36b that is removed from service), for column feed line 155 (represented by the connection with stream 154a), and for column overhead vapor stream 39 (represented by the dashed line between stream 39 and stream 152 that is removed from service). The existing plant can continue to operate while the new processing assembly 117 is installed near fractionation tower 17, with just a short plant shutdown when installation is complete to make the new tie-ins to these three existing lines. The plant can then be restarted, with all of the existing equipment remaining in service and operating exactly as before, except that the product recovery is now higher with no increase in compression power.

The main reason the present invention is more efficient than our co-pending application Ser. No. 15/332,723 depicted in FIG. 3 is that it removes nearly all of the heat of compression added by reflux compressor 22 via discharge cooler 26. In the FIG. 3 process, compressor discharge stream 151a is much hotter than compressor suction stream 151 (−81° F. [−63° C.] for stream 151a versus −167° F. [−110° C.] for stream 151). This additional heat in the compressed stream must be removed in cooling section 117a of processing assembly in the FIG. 3 process, meaning less cooling is available for streams 36a and 151a. Contrast this with the FIG. 5 embodiment of the present invention, where the cooled compressed stream 151d is nearly the same temperature as compressor suction stream 151 (−65° F. [−54° C.] for stream 151d versus −80° F. [−60° C.] for stream 151). This means more cooling is available in cooling section 117a inside processing assembly 117 of the present invention, which in turn allows more reflux flow to the top of demethanizer 17 (16% higher flow for stream 155 in FIG. 5 compared to stream 155 in FIG. 3).

The present invention also offers advantages when product economics favor rejecting the C₂components to the residue gas product. The present invention can be easily reconfigured to operate in a manner similar to that of our U.S. Pat. Nos. 9,637,428 and 9,927,171 as shown in FIG. 6. The operating conditions of the FIG. 5 embodiment of the present invention can be altered as illustrated in FIG. 6 to reduce the ethane content of the liquid product to the same level as that of the FIG. 2 prior art process and of co-pending application Ser. No. 15/332,723 depicted in FIG. 4. The feed gas composition and conditions considered in the process presented in FIG. 6 are the same as those in FIGS. 2 and 4. Accordingly, the FIG. 6 process can be compared with that of the FIGS. 2 and 4 processes to further illustrate the advantages of the present invention.

When operating the present invention in this manner, many of the process conditions shown for the FIG. 6 process are much the same as the corresponding process conditions for the FIG. 2 process, although most of the process configuration is like the FIG. 5 embodiment of the present invention. The main difference relative to the FIG. 5 embodiment is that the flash expanded stream 36b directed to the heat and mass transfer means in rectifying section 117b inside processing assembly 117 for FIG. 6 originates from substantially condensed stream 36a, rather than from heated combined stream 152 as in FIG. 5. As such, reflux compressor 22 and its associated equipment are not needed and can be taken out of service (as indicated by the dashed lines), eliminating the power consumption of this compressor when operating in this manner.

For the operating conditions shown in FIG. 6, combined stream 36 is cooled to −92° F. [−69° C.] in heat exchanger 12 by heat exchange with cool residue gas stream 153. The substantially condensed stream 36a is flash expanded through expansion valve 23 to slightly above the operating pressure (approximately 200 psia [1,381 kPa(a)]) of fractionation tower 17. During expansion a portion of the stream may be vaporized, resulting in cooling of the total stream. In the process illustrated in FIG. 6, the expanded stream 36b leaving expansion valve 23 reaches a temperature of −156° F. [−104° C.] before it is directed into the heat and mass transfer means in rectifying section 117b inside processing assembly 117.

The flash expanded stream 36b is further vaporized as it provides cooling and partial condensation of the partially rectified vapor stream, and exits the heat and mass transfer means in rectifying section 117b inside processing assembly 117 at −83° F. [−64° C.]. The heated flash expanded stream 36c is then mixed with pumped liquid stream 154a to form combined feed stream 155, which enters fractionation column 17 at the top feed point at −82° F. [−64° C.].

The further rectified vapor stream leaves the heat and mass transfer means in rectifying section 117b inside processing assembly 117 at −104° F. [−76° C.]. Since the heat exchange means in cooling section 117a inside processing assembly 117 has been idled, the vapor simply discharges from processing assembly 117 as cool residue gas stream 153, which is heated and compressed as described previously for stream 39 in the FIG. 2 process.

A summary of stream flow rates and energy consumption for the process illustrated in FIG. 6 is set forth in the following table:

A comparison of Tables II and VI shows that, compared to the prior art, the FIG. 6 process improves propane recovery from 89.20% to 96.50% and butane+ recovery from 98.81% to 100.00%. Comparison of Tables II and VI further shows that these increased product yields were achieved without using additional power. The economic impact of these improved recoveries is substantial. Using an average incremental value $0.58/gallon [€129/m³] for hydrocarbon liquids compared to the corresponding hydrocarbon gases, the improved recoveries represent more than US$4,720,000 [€3,930,000] of additional annual revenue for the plant operator. A comparison of Tables IV and VI shows that the FIG. 6 process has essentially the same performance as co-pending application Ser. No. 15/332,723 when rejecting C₂components to the residue gas product.

In the embodiment of the present invention shown in FIG. 5, heat exchanger 25 and discharge cooler 26 are used to remove the heat of compression produced in reflux compressor 22. Some applications may favor eliminating this capital expense by supplying compressor discharge stream 151a directly to the heat exchange means in cooling section 117a inside processing assembly 117 as shown in FIG. 7. The choice of which embodiment is best for a given application will generally depend on factors such as plant size and the cost of heat exchange equipment.

Some circumstances may favor mounting the liquid pump inside the processing assembly to further reduce the number of equipment items and the plot space requirements. Such embodiments are shown in FIGS. 8, 9, 14, and 15, with pump 124 mounted inside processing assembly 117 as shown to send the distillation liquid stream from separator section 117d via conduit 154 to combine with stream 36c and form combined feed stream 155 that is supplied as the top feed to column 17. The pump and its driver may both be mounted inside the processing assembly if a submerged pump or canned motor pump is used, or just the pump itself may be mounted inside the processing assembly (using a magnetically-coupled drive for the pump, for instance). For either option, the potential for atmospheric releases of hydrocarbons that may damage the environment is reduced still further.

Some circumstances may favor locating the processing assembly at a higher elevation than the top feed point on fractionation column 17. In such cases, it may be possible for distillation liquid stream 154 to flow by gravity head and combine with stream 36c so that the resulting combined feed stream 155 then flows to the top feed point on fractionation column 17 as shown in FIGS. 10, 11, 16, and 17, eliminating the need for pump 24/124 shown in the FIGS. 5 through 9 and 12 through 15 embodiments.

Some circumstances may favor eliminating cooling section 117a from processing assembly 117, and using a heat exchange means external to the processing assembly for feed cooling, such as heat exchanger 27 shown in FIGS. 12 through 17. Such an arrangement allows processing assembly 117 to be smaller, which may reduce the overall plant cost and/or shorten the fabrication schedule in some cases. Note that in all cases exchanger 27 is representative of either a multitude of individual heat exchangers or a single multi-pass heat exchanger, or any combination thereof. Each such heat exchanger may be comprised of a fin and tube type heat exchanger, a plate type heat exchanger, a brazed aluminum type heat exchanger, or other type of heat transfer device, including multi-pass and/or multi-service heat exchangers.

The present invention provides improved recovery of C₂components, C₃components, and heavier hydrocarbon components per amount of utility consumption required to operate the process. An improvement in utility consumption required for operating the process may appear in the form of reduced power requirements for compression or re-compression, reduced power requirements for external refrigeration, reduced energy requirements for supplemental heating, or a combination thereof.

While there have been described what are believed to be preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto, e.g. to adapt the invention to various conditions, types of feed, or other requirements without departing from the spirit of the present invention as defined by the following claims.