USE OF DENSE FLUID EXPANDERS IN CRYOGENIC NATURAL GAS LIQUIDS RECOVERY

Abstract

A system and a process for processing a natural gas liquid feed is provided. In the system, a dense fluid expander is positioned downstream from a subcooler heat exchanger. The subcooler heat exchanger cools one or more reflux streams against an overhead vapor stream and a distillation column positioned downstream from the dense fluid expander. An increase in recovery of natural gas liquids is provided through the use of this system having at least one dense fluid expander.

Description

This invention provides improved recovery of natural gas liquids from natural gas streams. More particularly, the invention uses a dense fluid expander to result in increased recovery of ethane or propane from natural gas streams.

BACKGROUND OF THE INVENTION

This invention relates to a process for processing natural gas or other methane-rich gas streams to produce a liquefied natural gas (LNG) stream that has a high methane purity and a liquid stream containing predominantly hydrocarbons heavier than methane.

Natural gas is typically recovered from wells drilled into underground reservoirs. It usually has a major proportion of methane, i.e., methane comprises at least 50 mole percent of the gas. Depending on the particular underground reservoir, the natural gas also contains relatively lesser amounts of heavier hydrocarbons such as ethane, propane, butanes, pentanes and the like, as well as water, hydrogen, nitrogen, carbon dioxide, and other gases.

Most natural gas is handled in gaseous form. The most common means for transporting natural gas from the wellhead to gas processing plants and then to the natural gas consumers is in high pressure gas transmission pipelines. In some circumstances, however, it has been found necessary and/or desirable to liquefy the natural gas either for transport or for use. In remote locations, for instance, there is often no pipeline infrastructure that would allow for convenient transportation of the natural gas to market. In such cases, the much lower specific volume of LNG relative to natural gas in the gaseous state can greatly reduce transportation costs by allowing delivery of the LNG using cargo ships and transport trucks.

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 present invention is generally concerned with the liquefaction of natural gas while producing as a co-product a liquid stream consisting primarily of hydrocarbons heavier than methane, such as natural gas liquids (NGL) composed of ethane, propane, butanes, and heavier hydrocarbon components, liquefied petroleum gas (LPG) composed of propane, butanes, and heavier hydrocarbon components, or condensate composed of butanes and heavier hydrocarbon components. Producing the co-product liquid stream has two important benefits: the LNG produced has a high methane purity, and the co-product liquid is a valuable product that may be used for many other purposes. A typical analysis of a natural gas stream to be processed in accordance with this invention would be, in approximate mole percent, 84.2% methane, 7.9% ethane and other C2 components, 4.9% propane and other C3 components, 1.0% iso-butane, 1.1% normal butane, 0.8% pentanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present.

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 C2 components, C3 components, and heavier hydrocarbon components as bottom liquid product, or to separate residual methane, C2 components, nitrogen, and other volatile gases as overhead vapor from the desired C3 components 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 C2 components, C3 components, C4 components, 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 C2 components, C3 components, C4 components, and heavier hydrocarbon components from the vapors.

There are a number of methods known for liquefying natural gas. For instance, see Finn, Adrian J., Grant L. Johnson, and Terry R. Tomlinson, “LNG Technology for Offshore and Mid-Scale Plants”, Proceedings of the Seventy-Ninth Annual Convention of the Gas Processors Association, pp. 429-450, Atlanta, Ga., Mar. 13-15, 2000 and Kikkawa, Yoshitsugi, Masaaki Ohishi, and Noriyoshi Nozawa, “Optimize the Power System of Baseload LNG Plant”, Proceedings of the Eightieth Annual Convention of the Gas Processors Association, San Antonio, Tex., Mar. 12-14, 2001 for surveys of a number of such processes. U.S. Pat. Nos. 4,445,917; 4,525,185; 4,545,795; 4,755,200; 5,291,736; 5,363,655; 5,365,740; 5,600,969; 5,615,561; 5,651,269; 5,755,114; 5,893,274; 6,014,869; 6,062,041; 6,119,479; 6,125,653; 6,250,105 B1; 6,269,655 B1; 6,272,882 B1; 6,308,531 B1; 6,324,867 B1; and 6,347,532 B1. These methods generally include steps in which the natural gas is purified (by removing water and troublesome compounds such as carbon dioxide and sulfur compounds), cooled, condensed, and expanded. Cooling and condensation of the natural gas can be accomplished in many different manners. “Cascade refrigeration” employs heat exchange of the natural gas with several refrigerants having successively lower boiling points, such as propane, ethane, and methane. As an alternative, this heat exchange can be accomplished using a single refrigerant by evaporating the refrigerant at several different pressure levels. “Multi-component refrigeration” employs heat exchange of the natural gas with one or more refrigerant fluids composed of several refrigerant components in lieu of multiple single-component refrigerants. Expansion of the natural gas can be accomplished both isenthalpically (using Joule-Thomson expansion, for instance) and isentropically (using a work-expansion turbine, for instance).

Regardless of the method used to liquefy the natural gas stream, it is common to require removal of a significant fraction of the hydrocarbons heavier than methane before the methane-rich stream is liquefied. The reasons for this hydrocarbon removal step are numerous, including the need to control the heating value of the LNG stream, and the value of these heavier hydrocarbon components as products in their own right. Unfortunately, little attention has been focused heretofore on the efficiency of the hydrocarbon removal step.

The Gas Subcooled Process (GSP) was developed by Ortloff Engineers, Ltd. in the late 1970's to recover higher yields of ethane and propane from natural gas streams than the previous industry standard design. The GSP design incorporates the addition of a reflux stream generated from a portion of the inlet gas which is fed as reflux to the top of the demethanizer. With the GSP design, the expander feed separator operates at a warmer temperature, which eliminates instabilities when operating too close to the phase envelope. GSP also incorporates an additional reflux stream feeding the demethanizer column above the expander feed. This enables GSP to achieve significantly higher recoveries than the conventional ISS design. In ethane recovery mode the GSP process produces a mixed NGL product stream, typically meeting a maximum methane in ethane liquid product specification. In propane recovery mode a mixed LPG product stream is produced, typically meeting a maximum ethane in propane liquid product specification. The residue gas product stream will contain methane or methane and ethane, depending on the mode of operation.

Ortloff's Recycle Split Vapor (RSV) process is an enhancement of the original GSP technology. The RSV process can provide ultra-high ethane or propane recovery from natural gas streams. It can also be operated to recover only a portion of the ethane. The RSV design incorporates the addition of a small reflux stream generated from residue gas which is used to supplement the usual reflux stream. An additional rectification section is installed above the typical top feed point of the GSP process. The liquefied residue gas stream is then fed as reflux to the top of this new section. The lower section of the tower provides bulk recovery of the desired liquid product while the top section provides the “polishing” step. The RSV technology is extremely flexible, and can operate as either an ethane recovery or a propane recovery process. This flexibility allows an operator to maximize plant profits based on ethane economics. In addition, an RSV plant can operate at flow rates significantly different than design. In the case of lower flow, higher recoveries can be achieved; for flow rates higher than design, high product recoveries can be maintained.

In the prior art, a natural gas feed which contains a mixture of hydrocarbons is cooled and partially liquefied in a feed heat exchanger. The resulting two-phase mixture is then separated into vapor and liquid. A portion of the vapor is expanded into a distillation column while another portion is liquefied and subcooled in a subcooler heat exchanger with the resulting liquid expanded through a valve into the same distillation column referenced above. The liquid from the phase separator is also expanded into the column through a valve.

SUMMARY OF THE INVENTION

The invention is a system and process for treating a natural gas liquid feed. In one embodiment, the invention is a system for processing a natural gas liquid feed comprising a dense fluid expander positioned downstream from a subcooler heat exchanger, wherein the subcooler heat exchanger cools one or more reflux streams against an overhead vapor stream and a distillation column positioned downstream from said dense fluid expander.

In another embodiment, the invention is a process for separating a hydrocarbon mixture into a liquid stream and a vapor stream, said process comprising the steps of first cooling a hydrocarbon feed to produce a two-phase mixture; separating the two-phase mixture into a vapor stream and a liquid stream; sending at least a portion of said vapor stream through a subcooler heat exchanger to produce a liquefied stream; sending the liquefied stream through a dense fluid expander to produce an expanded liquefied stream; and sending said expanded liquefied stream to a distillation column to produce a liquids stream and a residual gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram of a natural gas liquefaction plant with the addition of the dense fluid expander.

FIG. 2 shows a flow diagram of a natural gas liquefaction plant in which a portion of a vent gas is recycled with the addition of the dense fluid expander to the flow scheme.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, a liquid expansion valve may be replaced by a dense fluid expander. In other embodiments of the invention, the dense fluid expander is put on a parallel line to improve efficiency in recovery of natural gas liquids. If the dense fluid expander is placed on a line parallel to a valve, efficiency is improved in that if for some reason the dense fluid expander is out of commission the plant can continue to operate in a manner similar to operation in prior art designs. A dense fluid expander works similar to a pump working in reverse. It isentropically expands liquids where the reflux stream from the subcooler is in two phases. It is often referred to as a hydraulic turbine.

In another embodiment of the invention, a portion of residue gas that is produced by the distillation column is recycled to be cooled in an improved version of the Recycle Split Vapor (RSV) process.

In FIG. 1 is shown one embodiment of the invention which has added the dense fluid expander 60 upstream of J-T reflux valve 64 to the upper vapor stream 66 entering column 22. Both FIG. 1 and FIG. 2 are significantly simplified but show the elements of the claimed system that are relevant to the present invention. A feed stream 10 is cooled in heat exchanger 12 by heat exchange with stream 34 to produce cooled stream 14 that enters separation vessel 16 to be divided into vapor stream 48 and condensed liquid stream 18. Note that in all cases heat exchanger 12 is representative of either a multitude of individual heat exchangers or a single multi-pass heat exchanger, or any combination thereof. (The decision as to whether to use more than one heat exchanger for the indicated cooling services will depend on a number of factors including, but not limited to, inlet gas flow rate, heat exchanger size, stream temperatures, etc.) Vapor stream 48 from separation vessel 16 is divided into two streams, 48 and 50, with stream 48 continuing to heat exchanger 32 in heat exchange relation with refrigerant stream 30, resulting in cooling and substantial condensation of stream 48 to produce cooled stream 58. Stream 50 enters a work expansion machine 52 in which mechanical energy is extracted from this portion of the high pressure feed. The machine 52 expands the vapor substantially isentropically from a pressure of about 1278 psia [8,812 kPa(a)] to the tower operating pressure, with the work expansion cooling the expanded stream 50 a to a temperature of approximately −57° F. [−49° 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 expanded and partially condensed stream 54 a is supplied as feed to distillation column 22 at a mid-column feed point.

The demethanizer in the distillation column, also referred to as fractionation tower 22 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 is a separator wherein the top feed is divided into its respective vapor and liquid portions, and wherein the vapor rising from the lower distillation section combined with the vapor portion (if any) of the top feed 66 to form the demethanizer overhead vapor (stream 30) which exits the top of the tower. The lower section contains the trays and/or packing and provides the necessary contact between the liquids falling downward and the vapors rising upward. The lower section also includes one or more reboilers (such as reboiler 26) which heat and vaporize a portion of the liquids 24 flowing down the column to provide the stripping vapors which flow up the column. The liquid product stream 28 exits the bottom of the tower at 213° F. [101° C.], based on a typical specification of an ethane to propane ratio of 0.020:1 on a molar basis in the bottom product. The overhead distillation stream 30 leaves demethanizer 22 at −73° F. [−59° C.] and is then heated as passing through heat exchanger 32 to provide stream 34 that passes through heat exchanger 12 to produce stream 36 which is compressed by compressors 38 (which may be driven by expander 52) and 42 and cooled by heat exchangers 40 and 44 to produce residual gas stream 46 which may be used as a fuel gas. Stream 48 which is cooled to produce cooled stream 58 passes through dense fluid expander 60 to stream 62, optionally through valve 64 and then stream 66 enters an upper portion of distillation column 22. A portion of stream 58 may bypass dense fluid expander 56 to flow through valve 58 to stream 60 and then to stream 66. Valve 64 may be used to keep the discharge of dense fluid expander 60 single phase (liquid). It may also be eliminated and the discharge of dense fluid expander 60 may be two-phase.

FIG. 2 has the same flow diagram as FIG. 1 except that a portion of stream 46 is recycled as stream 70 to be cooled in heat exchanger 32 with a cooled stream 72 passing through valve 74 to stream 76 to enter an upper portion of distillation column 22. A dense fluid expander may also be placed upstream of valve 74 or to replace valve 74 but, as the flow of reflux stream 72 is lower than the reflux stream 58, the benefit is lower.

The invention provides a power improvement of up to 2-3% which can be translated into a similar increase in production of natural gas liquids.

SPECIFIC EMBODIMENTS

While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims.

A first embodiment of the invention is a system for producing natural gas liquid products comprising a dense fluid expander positioned downstream from a subcooler heat exchanger, wherein the subcooler heat exchanger cools one or more reflux streams against an overhead vapor stream and a distillation column positioned downstream from the dense fluid expander. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein a backpressure valve is located between the dense fluid expander and the distillation column. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising a Joules-Thomson valve positioned on a line parallel to the dense fluid expander. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the dense fluid expander is positioned to accept the liquid feed from the dense fluid expander into an upper portion of the distillation column. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the distillation column has an upper exit for a residue gas stream and a lower exit for a liquid stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising a recycle line positioned to return a portion of the residual gas stream to the distillation column. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising a dense fluid expander on the recycle line.

A second embodiment of the invention is a process for separating a hydrocarbon mixture into a liquid stream and a vapor stream, the process comprising the steps of cooling a hydrocarbon feed to produce a two-phase mixture; separating the two-phase mixture into a vapor stream and a liquid stream; sending at least a portion of the vapor stream through a subcooler heat exchanger to produce a liquefied stream; sending the liquefied stream through a dense fluid expander to produce an expanded liquefied stream; and sending the expanded liquefied stream to a distillation column to produce a liquids stream and a residual gas stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising sending the expanded liquefied stream through a backpressure valve to maintain a liquid state and then to the distillation column. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein a portion of the residual gas stream is returned to the distillation column. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the hydrocarbon feed comprises a natural gas stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the liquids stream comprises a natural gas liquids stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the liquids stream comprises a mixture of propane and butane. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the dense fluid expander increases a percentage of liquids in the expanded liquid stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein power produced by the dense fluid expander is recovered in a generator or is allowed to dissipate. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the power is dissipated by being sent to an oil brake.

Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

Claims

1. A system for producing natural gas liquid products comprising

a. a dense fluid expander positioned downstream from a subcooler heat exchanger, wherein said subcooler heat exchanger cools one or more reflux streams against an overhead vapor stream

b. and a distillation column positioned downstream from said dense fluid expander.

2. The system of claim 1 wherein a backpressure valve is located between said dense fluid expander and said distillation column.

3. The system of claim 1 further comprising a Joules-Thomson valve positioned on a line parallel to said dense fluid expander.

4. The system of claim 1 wherein said dense fluid expander is positioned to accept the liquid feed from said dense fluid expander into an upper portion of said distillation column.

5. The system of claim 1 wherein said distillation column has an upper exit for a residue gas stream and a lower exit for a liquid stream.

6. The system of claim 1 further comprising a recycle line positioned to return a portion of the residual gas stream to said distillation column.

7. The system of claim 6 further comprising a dense fluid expander on said recycle line.

8. A process for separating a hydrocarbon mixture into a liquid stream and a vapor stream, said process comprising the steps of:

a. cooling a hydrocarbon feed to produce a two-phase mixture;

b. separating said two-phase mixture into a vapor stream and a liquid stream;

c. sending at least a portion of said vapor stream through a subcooler heat exchanger to produce a liquefied stream;

d. sending the liquefied stream through a dense fluid expander to produce an expanded liquefied stream; and

e. sending said expanded liquefied stream to a distillation column to produce a liquids stream and a residual gas stream.

9. The process of claim 8 further comprising sending said expanded liquefied stream through a backpressure valve to maintain a liquid state and then to said distillation column.

10. The process of claim 8 wherein a portion of said residual gas stream is returned to said distillation column.

11. The process of claim 8 wherein said hydrocarbon feed comprises a natural gas stream.

12. The process of claim 8 wherein said liquids stream comprises a natural gas liquids stream.

13. The process of claim 8 wherein said liquids stream comprises a mixture of propane and butane.

14. The process of claim 8 wherein said dense fluid expander increases a percentage of liquids in said expanded liquid stream.

15. The process of claim 8 wherein power produced by said dense fluid expander is recovered in a generator or is allowed to dissipate.

16. The process of claim 8 wherein said power is dissipated by being sent to an oil brake.