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.

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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 shows a flow diagram of a natural gas liquefaction plant with the addition of the dense fluid expander.

[0018] 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

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.