WO2010077614A2

WO2010077614A2 - Liquid natural gas processing

Info

Publication number: WO2010077614A2
Application number: PCT/US2009/067014
Authority: WO
Inventors: Stephen Weinberg; Kenneth Reddick
Original assignee: Howe Baker Engineers LLC
Current assignee: Howe Baker Engineers LLC
Priority date: 2008-12-08
Filing date: 2009-12-07
Publication date: 2010-07-08
Anticipated expiration: 2011-06-08
Also published as: WO2010077614A3

Abstract

A process for the recovery of LPG from liquefied natural gas (LNG) is disclosed. The LNG feed stream is subjected to a two stage separation process where the bottoms from the first stage separation containing C₂+ hydrocarbons is split into three portions, one portion is fed directly to the top of the second separation stage as a reflux stream, a second portion is warmed and partially vaporized by cross heat exchange with overhead vapors from the second separation stage before being fed to the middle of this second separation stage, and the third portion is heated and vaporized by cross heat exchange with bottom liquids from the second separation stage prior to being fed into the bottom of this same separation stage.

Description

LIQUID NATURAL GAS PROCESSING RELATED APPLICATIONS This application is a continuation-in-part application of co-pending application

U.S. Ser. No. 11/188,961 , filed July 25, 2005, which is a continuation-in-part application of U.S. Ser. No. 10/115,150, filed April 3, 2002, issued on September 13, 2005 as U.S. Patent No. 6,941,771.

FIELD OF THE INVENTION The present invention is directed toward the high recovery of hydrocarbons heavier than methane from liquefied natural gas (LNG) and in particular to a two step separation process where the C₂+ hydrocarbons (NGL) recovered in the first separation stage are split into three fractions before entering the second separation stage to aid in the recovery of LPG (C₃+ hydrocarbons) and to decrease the required compression horsepower needed to compress the overhead methane-rich stream from the first separation stage. Propane recoveries of up to 99+% can be achieved with up to a 45% reduction in horsepower needed for compression of the overhead LNG vapors.

BACKGROUND OF THE INVENTION Natural gas typically contains up to 15 vol. % of hydrocarbons heavier than methane. Thus, natural gas is typically separated to provide a pipeline quality gaseous fraction and a less volatile heavier liquid hydrocarbon fraction. These valuable natural gas liquids (NGL) are comprised of ethane, propane, butane, and minor amounts of other heavy hydrocarbons. In some circumstances, as an alternative to transportation in pipelines, natural gas at remote locations is liquefied and transported in special LNG tankers to appropriate LNG handling and storage terminals. The LNG can then be revaporized and used as a gaseous fuel in the same fashion as natural gas. Because the LNG is comprised of at least 80 mole percent methane it is often necessary to separate the methane from the heavier natural gas hydrocarbons to conform to pipeline specifications for heating value. In addition, it is often desirable to recover the NGL or LPG (C3+ hydrocarbons) because its components have a higher value as liquid products, where they are used as petrochemical feedstocks, compared to their value as fuel gas.

NGL or LPG is typically recovered from LNG streams by many well-known processes including "lean oil" adsorption, refrigerated "lean oil" absorption, and condensation at cryogenic temperatures. Although there are many known processes, there is always a compromise between high recovery and process simplicity (i.e., low capital investment). The most common process for recovering NGL or LPG from LNG is to pump and vaporize the LNG, and then redirect the resultant gaseous fluid to a typical industry standard turbo-expansion type cryogenic NGL recovery process. Such a process requires a large pressure drop across the turbo-expander or JT. valve to generate cryogenic temperatures. In addition, such prior processes typically require that the resultant gaseous fluid, after LPG extraction, be re-compressed to attain the pre-expansion step pressure. Alternatives to this standard process are known and two such processes are disclosed in U.S. Pat. Nos. 5,588,308 and 5,114,451. The NGL recovery process described in the '308 patent uses autorefrigeration and integrated heat exchange instead of external refrigeration or feed turbo-expanders. This process, however, requires that the LNG feed be at ambient temperature and be pretreated to remove water, acid gases and other impurities. The process described in the '451 patent recovers NGL from a LNG feed that has been warmed by heat exchange with a compressed recycle portion of the fractionation overhead. The balance of the overhead, comprised of methane-rich residual gas, is compressed and heated for introduction into pipeline distribution systems. The present invention described herein provides another alternative LPG recovery process that produces a low-pressure, liquid methane-rich stream that can be directed to the main LNG export pumps where it can be pumped to pipeline pressures, and eventually routed to the main LNG vaporizers. Moreover, the present invention uses a two step separation process where the C₂+ hydrocarbons recovered in the first separation stage are split into three portions, one portion is fed directly to the top of the second separation stage as a reflux stream, a second portion is warmed and partially vaporized by cross heat exchange with overhead vapors from the second separation stage before being fed to the middle of this second separation stage, and the third portion is heated and vaporized by cross heat exchange with bottom liquids from the second separation stage prior to being fed into the bottom of this same separation stage. This process flow scheme, as well as variations to the same, are described in the specification below and defined in the claims which follow.

SUMMARY OF THE INVENTION This application is directed to an improved process for the recovery of LPG from LNG which avoids the need for dehydration, the removal of acid gases and other impurities. A further advantage of the instant invention is that it significantly reduces the overall energy and fuel requirements because the residue gas compression horsepower requirements associated with a typical LPG or NGL recovery facility are reduced. Another advantage of the present invention is that it does not require a large pressure drop across a turbo-expander or JT. value to generate cryogenic temperatures. This, in turn, reduces the total capital investment to construct our process by 30 to 50% compared to a typical cryogenic LPG or NGL recovery facility. One embodiment of the present invention recovers hydrocarbons heavier than methane using low pressure LNG (for example, directly from an LNG storage system) by using a two step separation process where the NGL recovered in the first separation (recovery) stage are split into three portions before entering the second separation stage. A first portion is fed directly to top of a deethanizer as a reflux. A second portion of the liquid C₂+ hydrocarbons is warmed and partially vaporized by cross heat exchange with overhead vapors from the deethanizer before being fed to the middle of the deethanizer tower. Finally, a third NGL portion is heated and vaporized by cross- exchange with the bottom liquids from the deethanizer prior to being fed into the deethanizer tower. The deethanizer is reboiled with an external heating medium in a bottom reboiler.

The hot liquids from the deethanizer bottom constitute the recovered LPG product and are sufficiently chilled by cross-exchange with the third portion of the first separation stage bottom liquids. The chilled LPG product is then sent for storage in an atmospheric tank. The overhead vapors from the first separation stage are compressed to a slightly higher pressure using a low-temperature, low head compressor prior to being cross exchanged with cold LNG feed in a cold box. The boost in pressure is desired to maintain an adequate temperature differential between this warmer overhead vapor stream, which constitutes the LPG free product LNG gas, and the incoming cold LNG feed stream. This enables the warmer natural gas stream to be substantially re- liquefied and sub-cooled so that it may be directed back to the storage terminal high- pressure LNG send-out and pipeline vaporization system. In addition, boil-off gas from the storage terminal LNG tanks may also be combined with the compressed overhead vapors from the first separation stage and re-condensed in the cold box by heat exchange with the incoming LNG feed. Additionally, an external refrigerant stream or cold, high-pressure LNG send-out may be used in the cold box to remove the heat of compression and pre-chill product LNG gas from the cold compressor discharge. This feature provides the capability of increasing the amount of LNG product sub-cooling. The overhead vapors from the deethanizer are cooled and partially condensed by cross heat exchange with the second stream of the first separation stage bottom liquids. The liquid feed to the deethanizer is preheated, which reduces the reboiler heat load. The partially condensed deethanizer overhead stream may now be routed in two different manners as follows: the entire stream may be passed through the cold box where it is further cooled and condensed; however, it is not necessary to completely condense this stream. This stream is then fed to the top of the first separation stage as a reflux to hold down additional propane vapors; alternatively, a portion of the stream of overheads from the deethanizer may be passed through the a cold box, where it is substantially condensed and substantially sub-cooled, and then routed to the top of the first separation stage as a reflux to hold down additional propane vapors. The remaining portion of the warmer, partially condensed deethanizer overhead stream is fed to the middle of the first separation stage. The above-described process can achieve propane recoveries as high as 98%. Another embodiment of the present invention involves passing the entire deethanizer overhead stream through the LNG exchanger cold box to cool and partially condense the overhead vapors and then feeding this stream to the middle of the first separation stage. In addition, this variation involves separating a portion of the compressed product LNG that is substantially condensed and substantially sub-cooled and recycling it to the top of the first separation stage as a reflux to enhance propane recovery. This embodiment improves the propane recovery from about 98% to over 99%. The additional cooling of the deethanizer overhead stream in the cold box in conjunction with removing a portion of the compressed LNG product for use as a reflux allows for the increase in propane recovery. In this embodiment, the amount of additional cooling and condensing required for the deethanizer overhead stream does not create a build-up of the internal recycle that would significantly increases the reboiler duty. This embodiment in the process flow scheme also decreases the required compressor horsepower by as much as 45% due to the reduced actual volume of overhead vapors from the first separation stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of one embodiment of the present invention.

FIG. 2 is a schematic flow diagram of another embodiment of the present invention.

FIG. 3 is a schematic flow diagram of yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

LPG is recovered from low-pressure liquefied natural gas (LNG) without the need for external refrigeration or feed turboexpanders as used in prior processes. Referring to FIG. 1 , process 100 shows the incoming LNG feed stream 1 enters pump 2 at very low pressures, typically in the ra3ge of 0-5 psig and at a temperature of less than minus 200⁰F. Pump 2 may be any pump design typically used for pumping LNG provided that it is capable of increasing the pressure of the LNG several hundred pounds to approximately 100-500 psig, preferably to the process range of 300-350 psig. The resultant stream 3 from pump 2 is physically fed to cold box 4 where it is cross- exchanged with substantially LPG-free gas in stream 9 obtained from the discharge of compressor 8. In those circumstances where additional cooling is necessary in cold box 4, an external refrigerant line 32 may be employed to increase the cooling capacity. Although the exact nature of the external refrigerant is not critical to the invention, another separate LNG stream may be the most convenient to use. Alternatively, The heated stream of LNG feed is removed from cold box 4 as stream 5.

After being warmed and partially vaporized, the LNG stream 5 can be further warmed, if needed during process start-up, with an optional heat exchanger (not shown) and then fed to the first separator or recovery tower 6. Separator 6 may be comprised of a single separation process or a series flow arrangement of several unit operations routinely used to separate fractions of LNG feedstocks. The internal configuration of the particular separator(s) used is a matter of routine engineering design and is not critical to our invention. Stream 5 is separated in separator 6 into an NGL (C2+) rich bottom stream 11 which is removed via pump 12 and stream 13. Stream 13 is split into three portions to create streams 14, 15 and 40. The relative portions of these three streams are dependent on the level of LPG recovery desired.. A preferred split would be 35 to 75 % in stream 14, 5 to 35 % in stream 15, and 10 to 45 % in stream 40. Stream 14 is eventually heated before being routed via line 31 as feed to deethanizer tower 16. A preferred method of heating stream 14 is to cross exchange it with the deethanizer bottoms liquid in heat exchanger 43. The deethanizer bottoms is the LPG product stream 19. After heat exchange with stream 14 the cooled LPG product 42 can be sent to atmospheric storage. Stream 15 is used directly as a reflux stream in deethanizer 16 to increase the recovery of the desired heavy components. Deethanizer 16 may be heated by a bottom reboiler (not shown) or a side reboiler 27.

A methane-rich overhead stream 17 is removed from deethanizer 16 and routed to overhead condenser 41 where it is cooled and partially condensed by cross heat exchange with the third portion of separator 6 bottoms, stream 40, and creating stream 18. Stream 18 is then routed through the cold box 4 where it is further cooled and condensed to form stream 30, but it is not necessarily completely condensed. Stream 30 is then fed as a reflux feed to separator 6 to hold down additional propane vapors. The heated third portion of the bottoms from separator 6 in stream 44 is fed into the middle of deethanizer 16. As mentioned a recovered LPG product stream 42 is removed from the process and routed to LPG storage or pumped to an LPG pipeline or fractionator (not shown). A methane-rich overhead stream 7, substantially free of LPG, is removed from separator 6 and fed to a low temperature, low head compressor 8 where it forms compressed LNG stream 9. Compressor 8 is needed to provide enough boost in pressure so that exiting stream 9 maintains an adequate temperature difference in the main gas cold box 4 to form re-liquefied methane-rich gas (LNG) exit stream 10. Compressor 8 is designed to achieve a marginal pressure increase of about 75 to 115 psi, preferably increasing the pressure from about 300 psig to about 350-425 psig. The re-liquefied methane-rich (LNG) in stream 10 is directed to the main LNG export pumps (not shown) where the liquid will be pumped to pipeline pressures and eventually routed to the main LNG vaporizers.

Fig. 2 illustrates a different embodiment of the present invention where in process 200 stream 18 is divided into streams 20 and 21, where a portion in stream 21 is then routed through the cold box 4 where it is sub-cooled and substantially condensed to form stream 30, and is then routed to the top of the separator 6 as a reflux to hold down additional propane vapors. The remaining portion of the warmer, partially condensed deethanizer overhead stream 20 is fed to the middle of separator 6. This embodiment helps achieve higher NGL recoveries in the first separator 6. A preferred split would be 5 to 20% in stream 21 and 80 to 95% in stream 22. Fig. 3 also shows another embodiment of the present invention. In process 300 the entire deethanizer overhead stream 18 is passed through the cold box 4 to cool and partially condense the overhead vapors to form stream 30, which is then fed to the middle of separator 6, as opposed to using it as a reflux. Reflux stream 22 is obtained by separating a portion of the compressed product LNG stream 10 that is substantially condensed and substantially sub-cooled as stream 10. By recycling this portion of the compressed LNG product the recovery of propane can be increased from about 98% to over 99%. This embodiment also decreases the horsepower required by compressor 8 by as much as 45% due to the reduced actual volume of overhead vapors from separator 6. The relative portions of stream 22 and 23 are dependent on the LNG feed composition and the amount of ethane recovery required. A preferred split would be 2 to 15% in stream 22 and 85 to 98% in stream 23.

As one knowledgeable in this area of technology, the particular design of the heat exchangers, pumps, compressors and separators is not critical to our invention. Indeed, it is a matter of routine engineering practice to select and size the specific unit operations to achieve the desired performance. The present invention lies with the unique combination of unit operations and the discovery of using untreated LNG as external reflux to achieve high levels of separation efficiency in order to recover NGL. While the applicants have described what is believed to be the preferred embodiments of the invention, those knowledgeable in this area of technology will recognize that other and further modifications may be made thereto, e.g., to adapt the invention to various conditions, type of feeds, or other requirements, without departing from the spirit of our invention as defined by the following claims.

Claims

We claim:

1. A process of recovering hydrocarbons heavier than methane from liquefied natural gas (LNG) comprising, a) pumping liquid, low pressure LNG to a pressure of greater than 100 psia; b) directing the pressurized liquid LNG from step a) to a cold box where it is heat exchanged to increase its temperature; c) directing the heat exchanged pressurized liquid LNG from step b) to a separator where, in combination with a first reflux, a separator overhead is produced along with a separator bottoms; d) pressurizing the separator bottoms and then splitting the pressurized separator bottoms into a first, a second and a third portion; e) directing the first portion of pressurized separator bottoms to a deethanizer as a reflux stream; f) heating the second portion of pressurized separator bottoms by cross heat exchanging with a product stream comprising hydrocarbons heavier than methane; g) directing the heated second portion of pressurized separator bottoms from step f) to the deethanizer; h) heating the third portion of pressurized separator bottoms by cross heat exchanging with a deethanizer overhead stream; i) directing the heated third portion of pressurized separator bottoms from step h) to the deethanizer; j) directing a cooled deethanizer overhead stream formed in step h) to the cold box where it is cross heat exchanged with the pressurized liquid LNG and then using it as a reflux in the separator; and k) removing the separator overhead from the separator and compressing the separator overhead prior to introduction into the cold box and heat exchanging with the pressurized liquid LNG to produce a re-liquefied pressurized LNG.

2. The process of claim 1 further comprising providing an external refrigerant to the cold box.

3. The process of claim 2 further comprising providing a second stream of LNG as the external refrigerant to the cold box. -

4. The process of claim 1 further comprising providing a boiloff gas from an LNG storage tank and combining it with the compressed overhead prior to introduction into the cold box.

5. A process of recovering hydrocarbons heavier than methane from liquefied natural gas (LNG) comprising, a) pumping liquid, low pressure LNG to a pressure of greater than 100 psia; b) directing the pressurized liquid LNG from step a) to a cold box where it is heat exchanged to increase its temperature; c) directing the heat exchanged pressurized liquid LNG from step b) to a separator where, in combination with a first reflux, a separator overhead is produced along with a separator bottoms; d) pressurizing the separator bottoms and then splitting the pressurized separator bottoms into a first, a second and a third portion; e) directing the first portion of pressurized separator bottoms to a deethanizer as a reflux stream; f) heating the second portion of pressurized separator bottoms by cross heat exchanging with a reboiled product stream comprising hydrocarbons heavier than methane; g) directing the heated second portion of pressurized separator bottoms from step f) to the deethanizer; h) heating the third portion of pressurized separator bottoms by cross heat exchanging with a deethanizer overhead stream; i) directing the heated third portion of pressurized separator bottoms from step h) to the deethanizer; j) splitting the cooled deethanizer overhead stream formed in step h) into first and second cooled deethanizer overhead portions, where the first cooled deethanizer overhead portion is fed to the separator and the second cooled deethanizer overhead portion is passed through the cold box where it is cross heat exchanged with the pressurized liquid LNG and then used as a reflux in the separator; and

k) removing the separator overhead from the separator and compressing the separator overhead prior to introduction into the cold box and heat exchanging with the pressurized liquid LNG to produce a re-liquefied pressurized LNG.

6. The process of claim 5 further comprising providing an external refrigerant to the cold box.

7. The process of claim 6 further comprising providing a second stream of LNG as the external refrigerant to the cold box.

8. The process of claim 5 further comprising providing a boiloff gas from an LNG storage tank and combining it with the compressed overhead prior to introduction into the cold box.

9. A process of recovering hydrocarbons heavier than methane from liquefied natural gas (LNG) comprising, a) pumping liquid, low pressure LNG to a pressure of greater than 100 psia; b) directing the pressurized liquid LNG from step a) to a cold box where it is heat exchanged to increase its temperature; c) directing the heat exchanged pressurized liquid LNG from step b) to a separator where, in combination with a first reflux, a separator overhead is produced along with a separator bottoms; d) pressurizing the separator bottoms and then splitting the pressurized separator bottoms into a first, a second and a third portion; e) directing the first portion of pressurized separator bottoms to a deethanizer as a reflux stream; f) heating the second portion of pressurized separator bottoms by cross heat exchanging with a reboiled product stream comprising hydrocarbons heavier than methane; g) directing the heated second portion of pressurized separator bottoms from step f) to the deethanizer; h) heating the third portion of pressurized separator bottoms by cross heat exchanging with a deethanizer overhead stream; i) directing the heated third portion of pressurized separator bottoms from step h) to the deethanizer; j) directing a cooled deethanizer overhead stream formed in step h) to the cold box where it is cross heat exchanged against the pressurized liquid LNG and then used as a feed to the separator; k) removing the separator overhead from the separator and compressing the separator overhead prior to introduction into the cold box and heat exchanging against the pressurized liquid LNG to produce a re-liquefied pressurized LNG; and

I) separating a portion of the re-liquefied pressurized LNG for use as a reflux in the separator.

10. The process of claim 9 further comprising providing an external refrigerant to the cold box.

11. The process of claim 10 further comprising providing a second stream of LNG as the external refrigerant to the cold box.

12. The process of claim 9 further comprising providing a boiloff gas from an LNG storage tank and combining it with the compressed overhead prior to introduction into the cold box.