METHOD TO PRODUCE PLNG AND CCNG AT STRADDLE PLANTS

Abstract

There is provided a method to produce PLNG and CCNG at Straddle Plants. In contrast to present practice at Straddle Plants, there is added a slipstream of high pressure, pre-treated, pre-cooled natural gas stream to feed a new PLNG and or CCNG production section. This slipstream is further cooled in a heat exchanger by a counter-current vapour fraction of the expanded gas before entering an expander apparatus. The cooled gas is expanded into a separator. The cold vapour fraction from the separator is routed and expanded to the Straddle Plant fractionator. The produced PLNG is routed to storage.

Description

FIELD

This relates to a method for producing PLNG (Pressurized Liquid Natural Gas) and CCNG (Cold Compressed Natural Gas). The method was developed with straddle plants in mind, but has broader application.

BACKGROUND

Canadian Patent Application No. 2,813,260 (Lourenco et al) entitled “Producing LNG at Straddle Plants using a NG high pressure stream,” which is hereby incorporated by reference, describes a process addition to straddle plants which are used to capture and recover natural gas liquids (NGL's) from gas transmission pipelines. The described process allows these plants to be retrofitted to also produce Liquefied Natural Gas (LNG).

There will hereinafter be described an alternative to the method described in the Canadian Patent Application No. 2,813,260 to also produce PLNG and CCNG. The method can be used wherever pressurized gas flows and supporting infrastructure exist to deal with the process streams, such as at straddle plants.

SUMMARY

According to an aspect, there is provided a method to produce Pressurized Liquid Natural Gas (PLNG) and Cold Compressed Natural Gas (CCNG), comprising passing a dewatered natural gas stream at pressures of between 450 psig and 1200 psig through one or more heat exchangers to pre-cool the natural gas stream, passing the pre-cooled natural gas stream through a gas column where natural gas liquid fractions and natural gas fractions are separated, passing the pre-cooled natural gas fractions at pressures of between 450 psig and 1200 psig through one or more heat exchangers to further cool the natural gas fractions, passing the further cooled natural gas fractions at pressures of between 450 psig and 1200 psig through a gas expansion apparatus where pressure of the natural gas fractions is lowered to a pressure of less than 300 psig, and passing the natural gas fractions at a pressure of less than 300 psig through a separator where they are separated into a PLNG stream and a gaseous stream at a pressure of less than 300 psig.

According to another aspect, this process may be a retro-fit to existing straddle plants.

According to an aspect, there is provided a method to produce PLNG where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus, the PLNG section comprising providing heat exchangers on a high pressure, pre-treated, pre-cooled natural gas PLNG feed line to a gas expansion apparatus, providing a gas liquid separator downstream of the expansion apparatus, and providing a pressure reducing device for the separated gaseous stream into the straddle plant fractionator.

According to another aspect, the gas expansion apparatus may be a JT valve or a gas expander turbine.

According to another aspect, further cooling may be provide by the vapour fraction of the expanded gas.

According to another aspect, the gaseous stream from the PLNG separator may be returned to the straddle plant fractionator.

According to another aspect, the gas to be liquefied may be pressurized liquid natural gas (PLNG).

According to an aspect, there is provided a method to produce CCNG where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus.

According to an aspect, there is provided a straddle plant CCNG section, comprising a gas expansion apparatus, a feed line feeding gas to the gas expansion apparatus, a CCNG receiver to separate the vapour and liquid fractions, and a liquids return line to the straddle plant fractionator.

According to another aspect, the liquefied gas production plant may include a PLNG line to storage.

According to another aspect, the CCNG liquid fraction may be routed to the straddle plant fractionator for NGL's recovery.

According to an aspect, there is provided a method to produce Pressurized Liquid Natural Gas (PLNG), the method comprising the steps of passing a dewatered natural gas stream at pressures of between 450 psig and 1200 psig through a first phase separator to obtain a natural gas liquid fraction and a natural gas fraction, the separated natural gas fractions being at a pressure of between 450 psig and 1200 psig, passing at least a portion of the natural gas fraction through a gas expansion apparatus to reduce the pressure of the natural gas fractions to less than 300 psig, and passing the pressure-reduced natural gas fraction through a second phase separator to obtain at least one of a PLNG stream and a CCNG stream at a pressure of less than 300 psig.

According to another aspect, the dewatered natural gas stream may be a natural gas stream in a straddle plant.

According to another aspect, the method may further comprise the step of passing a natural gas stream through a dewaterer and a heat exchanger to cool the natural gas stream prior to obtaining the dewatered natural gas stream.

According to another aspect, the method may further comprise the step of cooling at least a portion of the separated natural gas fraction in one or more heat exchangers.

According to another aspect, the method may further comprise the step of collecting each of the at least one of the PLNG stream and the CCNG stream separately in a storage vessel.

According to another aspect, the gas expansion apparatus may comprise a JT valve or a gas expander turbine.

According to another aspect, at least a portion of an output of the first or the second phase separator may be used to cool the natural gas stream.

According to another aspect, at least a portion of the output of the second phase separator may be used to cool a fractionator of the saddle plant.

There is provided a method to produce also PLNG and CCNG at straddle plants. A first step involves passing a dewatered natural gas stream at pressures of between 400 psig and 1200 psig, through one or more heat exchangers to pre-cool the natural gas stream. A second step involves passing the dewatered and pre-cooled natural gas stream through a gas separator where natural gas liquids and gaseous fractions are separated. A third step involves passing a portion of the gaseous fraction at pressures of between 400 psig and 1200 psig through one or more heat exchangers for further cooling. A fourth step involves passing the cooled gaseous fraction at pressures of between 400 psig and 1200 psig through a gas expansion apparatus where the gas pressure is lowered to pressures less than 300 psig. A fifth step involves passing the expanded gas into a separator where the expanded gas is separated into a PLNG stream and a gaseous stream at pressures less than 300 psig.

Where there is a high pressure stream of natural gas (i.e. at pressures in a range of 400 psig to 1200 psig) that can be tapped, the above method can operate without external power inputs, resulting in substantial savings in both capital and operating costs.

The input temperature of a high pressure stream of natural gas is relatively constant. This means that once steady state is achieved, the ratio of cold gas vapour is constant relative to a flow rate of the natural gas.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:

FIG. 1 labelled as PRIOR ART is a schematic diagram of a typical straddle plant equipped with a gas pre-treatment, heat exchangers (cold box), an expander-compressor and a main compressor for re-compression to gas transmission pipeline.

FIG. 2 is a schematic diagram of a typical straddle plant with the addition of a PLNG production unit facility equipped with heat exchangers, an expander, a separator, a JT valve and associated process instrumentation controls.

FIG. 3 is a schematic diagram of a typical straddle plant with the addition of a PLNG production unit facility equipped with a JT valve in lieu of an expander.

FIG. 4 is a schematic diagram of a typical straddle plant with the addition of a PLNG production unit facility with an alternate flow path to the PLNG production facility.

FIG. 5 is a schematic diagram of a typical straddle plant with the addition of PLNG and CCNG production unit facilities equipped with a JT valve and an expander.

FIG. 6 is a schematic diagram of a typical straddle plant with the addition of PLNG and CCNG production unit facilities equipped with JT valves in lieu of an expander.

FIG. 7 is a schematic diagram of a typical straddle plant with the addition of PLNG and CCNG production unit facilities with an alternate flow path to the production facilities.

FIG. 8 is a schematic diagram of a typical straddle plant with addition of PLNG and CCNG production unit facilities and a pump back to the fractionation column.

FIG. 9 is a schematic diagram of a typical straddle plant with addition of PLNG and CCNG production unit facilities that are not connected to the fractionation column.

FIG. 10 is a schematic diagram of a typical straddle plant with addition of a CCNG production unit facility.

DESCRIPTION OF A PREFERRED EMBODIMENT

A straddle plant is a natural gas processing plant constructed near a transmission pipeline downstream from the fields where the natural gas in the pipeline has been produced, also called an “on-line” plant. The straddle plant removes natural gas liquids, the C₂⁺ gas fractions, from the transmission natural gas stream. This is done by first pre-treating the gas stream, pre-cooling it and then reducing the transmission gas high pressure stream in a range of 400 to 1200 psig, typically about 1000 psig, through a gas expander to pressures typically about 275 psig, to cool, condense and separate the C₂⁺ gas fractions in a distillation column. The bottoms of the distillation column exit the plant as the recovered natural gas liquids (NGL's). The distillation column overhead stream, primarily C₂⁻ gas fractions, are pre-heated in a countercurrent heat exchange by the straddle plant pre-treated feed gas stream and re-compressed in two steps back to the same transmission pipeline gas pressure. The major operating cost of these straddle plants are the re-compression costs. The re-compression is typically done in two steps. The first step is done through a booster compressor, which typically is a direct drive compressor connected to the gas expander, the energy recovered by expanding the gas from the transmission gas pipeline high pressure is directly used to compress the distillation gas overhead stream from distillation column pressure to an intermediate gas pressure. The main re-compressor then compresses this intermediate gas pressure to transmission gas pipeline pressure. The economics of a straddle plant are based on the quantities and revenues of natural gas liquids (C₂⁻ gas fractions) produced against the re-compression and maintenance costs.

In Canadian Patent Application No. 2,813,260 (Lourenco et al.) entitled “Producing LNG at Straddle Plants using a NG high pressure stream” the objective was to produce LNG by diverting a portion of a pre-treated and pre-cooled high pressure natural gas, cooling it further, treating it for carbon dioxide removal, followed by further cooling and expansion to low pressures to produce LNG at −160 C. The objective of this new process is to produce PLNG and CCNG, the process differs from the above LNG process since it does not require the removal of carbon dioxide. The key point of PLNG technology is that it condenses and stores at pressures between 150 and 300 psig corresponding to a temperature range of approximately −100 to −120 C. At these higher temperatures, the solubility of carbon dioxide in PLNG increases up to 2 mol %, thus eliminating the need for carbon dioxide treatment as in the production of LNG which requires a minimum carbon dioxide concentration of 50 ppm to avoid carbon dioxide freeze-out and process pluggage. This is an important feature since carbon dioxide treatment requires the use of molecular sieves which are capital intensive and contribute significantly to the operating costs of a LNG plant. Therefore, in terms of capital and operating costs the production of PLNG provide a more economical alternative to the use of natural gas in lieu of LNG where applicable.

The main feature of CCNG is its density when compared to CNG (Compressed Natural Gas). The density of natural gas is best achieved by controlled cooling and pressure. A Straddle Plant is an ideal location to produce CCNG since its normal operating process conditions allows for a simple retrofit to route a portion of the pre-cooled, high pressure gas stream, to a controlled CCNG production unit. Allowing CCNG to be produced at Straddle Plants, on demand, at a client's required density. This retrofit will allow existing Straddle Plants to generate a new product commodity and associated new revenues.

Referring to FIG. 1, a pressurized pipeline natural gas stream 1 is routed to a straddle plant through valve 2. Valve 38, allows the transmission gas pipeline to bypass the straddle plant. High pressure gas stream 3 enters the straddle plant and is first pre-treated in unit 4 to remove the water content. The de-watered stream 5 is then routed to cold box 6 where it is pre-cooled in coil 7 by counter current gas streams is series, first by gas coil 24, then gas coil 31 and finally gas coil 21. The high pressure, pre-cooled gas stream 8 enters separator 9 where the liquids and gaseous fractions are separated. The liquid fraction is routed through stream 18 to expansion valve 19, where the pressure is reduced to column 26 operating pressure, this pressure expansion generates more coolth energy and the now expanded and cooler gas is routed through stream 20 to coil 21 in the cold box, pre-cooling the high pressure gas stream in coil 7. The now warmer stream 22 enters distillation column 26 for fractionation and NGL recovery. The gaseous fraction exits separator 9, through stream 10 which divides into two streams, 11 and 14. Stream 11 enters expander compressor 12 where the high pressure gas is expanded to column 26 operating pressure, generating torque in shaft A, which drives booster compressor 33, and the colder gas stream exits the expander-compressor 12 through stream 13 into column 26 for NGL's recovery. The gaseous stream 14 is routed through heat exchanger 29 for further cooling. The colder high pressure stream 15 is flows through expansion valve 16, where the high pressure gas is expanded to column 26 operating pressure and the cooler expanded gas enters column 26 through stream 17 as a reflux stream to control column 26 overhead operating temperature. The control of column overhead 26 operating temperature determines the recovery of NGL's from the feed gas stream. The distillation column bottoms temperature is controlled by reboiler stream 23, which obtains heat through coil 24 and returns it through stream 25 to the bottom section of distillation column 26. The control of column 26 bottoms operating temperature determines the quality of the NGL's recovered. The recovered NGL's exit column 26 bottoms through line 27. The stripped gas exits column 26 through stream 28 and is pre-heated in heat exchanger 29, the warmer stripped gas enters the cold box through coil 31 for further pre-heating. The warmer gas stream 32 enters booster compressor 33 which is connected through shaft A to the expander 12, thus recovering the mechanical work produced by the expander and boosting stream 32 pressure to stream 34. The boosted pressure stream 34 enters main compressor 35, where the pressure is increased to transmission pipeline pressure and routed through stream 36, through straddle plant block valve 37 and into pipeline gas distribution stream 39.

The above described process in FIG. 1 is the operation of a traditional straddle plant, there are various straddle plant modes of operation to improve the recovery of the NGL's, in all cases its objective is to produce NGL's.

Referring to FIG. 2, the difference from FIG. 1, is the addition of a PLNG production section to a conventional straddle plant which as described above produces NGL's. A pressurized pipeline natural gas stream 1 is routed to a straddle plant through valve 2. Valve 38, allows the transmission gas pipeline to bypass the straddle plant. High pressure gas stream 3 enters the straddle plant and is first pre-treated in unit 4 to remove the water content. The de-watered stream 5 is then routed to cold box 6 where it is pre-cooled in coil 7 by counter current gas streams is series, first by gas coil 24, then gas coil 31 and finally gas coil 21. The high pressure, pre-cooled gas stream 8 enters separator 9 where the liquids and gaseous fractions are separated. The liquid fraction is routed through stream 18 to expansion valve 19, where the pressure is reduced to column 26 operating pressure, this pressure expansion generates more coolth and the now expanded and cooler gas is routed through stream 20 to coil 21 in the cold box, pre-cooling the high pressure gas stream in coil 7. The now warmer stream 22 enters distillation column 26 for fractionation and NGL recovery. The gaseous fraction exits separator 9, through stream 10 divides into two streams; 11 and 14. Stream 11 enters expander-compressor 12 where the high pressure gas is expanded to column 26 pressure, generating torque in shaft A, which drives booster compressor 33 and, the colder gas stream exits expander-compressor 12 through stream 13 into column 26 for NGL's recovery. Stream 14 is routed through heat exchanger 29 where it is further cooled, the colder stream 15 is split into two streams; 40 and 41. Stream 41 is expanded through valve 16 to distillation column 26 operating pressure as a reflux stream to control distillation column overhead temperature of stream 28. The control of column 26 overhead operating temperature determines the recovery of NGL's from the feed gas stream. The distillation column bottoms temperature is controlled by reboiler stream 23, which obtains heat through coil 24 and returns it through stream 25 to the bottom section of distillation column 26. The control of column 26 bottoms operating temperature determines the quality of the NGL's recovered. The recovered NGL's exit column 26 bottoms through line 27. The stripped gas exits column 26 through stream 28 and is pre-heated in heat exchanger 29, the warmer stripped gas stream 30, enters the cold box 6 through coil 31 for further pre-heating. The warmer gas stream 32 enters booster compressor 33 which is connected through shaft A to the expander 12, thus recovering the mechanical work produced by the expander and boosting stream 32 pressure to stream 34. The boosted pressure stream 34 enters main compressor 35, where the pressure is increased to transmission pipeline pressure and routed through stream 36, through straddle plant block valve 37 and into pipeline gas distribution stream 39.

The high pressure gaseous stream 40 is the PLNG section feed stream, it is routed through heat exchanger 42 where it is further cooled by gaseous stream 47, the colder stream 43 enters expander-generator 44, where and is expanded to separator 46 operating pressure, the expanded stream 45 enters separator 46, where the liquid fraction PLNG is separated from the its gaseous fraction. The gaseous stream 47 exits separator 46 and enters heat exchanger 42 where it gives up some of its coolth to stream 40, the warmer stream 48 flows through pressure control valve 49 into column 26. The pressure control valve 49 controls separator 46 operating pressure. The produced PLNG stream 51 is routed to storage.

The teachings described herein permit straddle plants to produce PLNG in addition to NGL's by adding a PLNG skid to an existing straddle plant. The benefit of producing PLNG versus LNG is the elimination of a carbon dioxide treatment unit which is a pre-requisite to produce LNG. A straddle plant is an ideal plant to retrofit and produce PLNG in addition to NGL's, it has the front end and back end infrastructure to easily incorporate a PLNG production skid using standard operating equipment. The novelty of the proposed teachings is in the integration of a process to produce PLNG into an existing straddle plant. The production of PLNG provides an alternative to LNG as a natural gas supply in a liquid form at lower operating and capital costs. By adding a PLNG skid to an existing straddle plant, it simplifies the process and reduces capital, maintenance and operations costs. In the preferred method, a pre-treated, pre-cooled high pressure natural gas stream is further cooled in a counter-current second heat exchanger with the produced very cold gaseous stream and then expanded through a gas expander. The gas expander produces torque and therefore shaft power that can be converted into mechanical compression power or electricity. In the preferred application the shaft power is used for electricity. The expanded gas is separated into a gaseous and a PLNG stream. The gaseous stream is first routed to a heat exchanger where it further cools the pre-treated, pre-cooled high pressure gas stream and then discharged into a distillation column. This gaseous flow stream controls the separator pressure. The liquid stream, PLNG is routed to storage. Using these teachings, a straddle plant may improve its economics by also producing PLNG in addition to NGLs.

A main feature of this method is the flexibility of the process to meet various operating conditions since the ratio of PLNG production is proportional to the cold gaseous stream generated and returned to the distillation column. The method also provides for a significant savings in energy when compared to other PLNG processes since the process uses existing straddle plant infra-structure. The teachings can be used in any straddle plant size.

Variations:

It should be noted that the motive force generated by the expanders can be connected to a gas compressor to boost gas pressure versus a power generator that produces electricity as proposed.

Referring to FIG. 3, the main difference from FIGS. 2 and 3 is the use of a JT expansion valve 52 in lieu of an expander-generator. The use of a JT valve versus an expander-generator is an alternative mode of PLNG production at a lower capital cost but resulting in a lower production of PLNG.

Referring to FIG. 4, the main difference from FIGS. 2, and 3, is the temperature of the stream to the PLNG skid is upstream of heat exchanger 29. This stream is warmer and hence the production of PLNG for this mode of operation will be less than in FIGS. 2 and 3.

Referring to FIG. 5, the difference from FIGS. 2, 3 and 4 is the production of Cold Compressed Natural Gas (CCNG) in addition to production of PLNG. The main feature of CCNG is its density when compared to CNG (Compressed Natural Gas). The density of natural gas is best achieved by controlling both; cooling and pressure. A Straddle Plant is an ideal location to also produce CCNG since its normal operating process conditions allows for a simple retrofit to route a portion of the pre-cooled, high pressure gas stream, to a controlled CCNG production unit. Allowing CCNG to be produced at Straddle Plants, on demand, at a client's required density.

This retrofit referring to FIG. 5 is an addition of a CCNG production section to a conventional straddle plant which as described previously produce NGL's. A pressurized pipeline natural gas stream 1 is routed to a straddle plant through valve 2. Valve 38, allows the transmission gas pipeline to bypass the straddle plant. High pressure gas stream 3 enters the straddle plant and is first pre-treated in unit 4 to remove the water content. The de-watered stream 5 is then routed to cold box 6 where it is pre-cooled in coil 7 by counter current gas streams is series, first by gas coil 24, then gas coil 31 and finally gas coil 21. The high pressure, pre-cooled gas stream 8 enters separator 9 where the liquids and gaseous fractions are separated. The liquid fraction is routed through stream 18 to expansion valve 19, where the pressure is reduced to column 26 operating pressure, this pressure expansion generates more coolth and the now expanded and cooler gas is routed through stream 20 to coil 21 in the cold box, pre-cooling the high pressure gas stream in coil 7. The now warmer stream 22 enters distillation column 26 for fractionation and NGL recovery. The gaseous fraction exits separator 9, through stream 10 divides into two streams; 11 and 14. Stream 11 enters expander-compressor 12 where the cold high pressure gas is expanded to column 26 pressure, generating torque in shaft A, which drives booster compressor 33 and, the colder gas stream exits expander-compressor 12 through stream 13 into column 26 for NGL's fractionation and recovery. Stream 14 is routed through heat exchanger 29 where it is further cooled, the colder stream 15 is split into two streams; 54 and 41. Stream 41 is expanded through valve 16 to distillation column 26 operating pressure as a reflux stream to control distillation column overhead temperature of stream 28. The control of column 26 overhead operating temperature determines the recovery of NGL's from the feed gas stream. The distillation column bottoms temperature is controlled by reboiler stream 23, which obtains heat through coil 24 and returns it through stream 25 to the bottom section of distillation column 26. The control of column 26 bottoms operating temperature determines the quality of the NGL's recovered. The recovered NGL's exit column 26 bottoms through line 27. The stripped gas exits column 26 through stream 28 and is pre-heated in heat exchanger 29, the warmer stripped gas stream 30, enters the cold box 6 through coil 31 for further pre-heating. The warmer gas stream 32 enters booster compressor 33 which is connected through shaft A to the expander 12, thus recovering the mechanical work produced by the expander and boosting stream 32 pressure to stream 34. The boosted pressure stream 34 enters main compressor 35, where the pressure is increased to transmission pipeline pressure and routed through stream 36, through straddle plant block valve 37 and into pipeline gas distribution stream 39.

The high pressure gaseous stream 54 is the CCNG section feed stream, it is routed through expander-generator 55, where it is expanded to separator 57 operating pressure, the expanded stream 56 enters separator 57, where the liquid fraction PLNG is separated from its gaseous fraction, CCNG. The gaseous stream 63 exits separator 57 and is routed to CCNG storage. The PLNG fraction exits separator 57 through stream 58 and is split into streams 59 and 60. Stream 59, is an optional PLNG production stream, routed to PLNG storage. Stream 60 is further expanded through valve 61 and routed as a reflux stream 62 into fractionation column 26.

The CCNG process configuration as described in FIG. 5 provides an option for a Straddle Plant to produce CCNG and PLNG in addition to its current mode of operation that produces just NGL's. The availability of CCNG and PLNG provides new markets for the use of natural gas.

Referring to FIG. 6, the main difference from FIG. 5, is the use of a JT expansion valve 64 in lieu of an expander-generator 55. The use of a JT valve 64 versus an expander-generator 55 is an alternative mode of CCNG production at a lower capital cost but resulting in a lower production of CCNG.

Referring to FIG. 7, the main difference from FIG. 5, is the high pressure gaseous stream 65 to the CCNG section feed stream. This routing is done before heat exchanger 29, and prevents changing heat exchanger 29 sizing, since no additional cooling load is required, albeit this produces a less denser CCNG stream than in FIG. 5.

Referring to FIG. 8, a pump 66 is used to return condensed liquids in line 60 from separator 57 to column 26. As line 62 conveys a liquid to column 26, the pressure in line 58 and 60 may be less than the pressure of column 26. As such the production of CCNG and PLNG may not be limited by the pressure of column 26. In other words, the pressure of separator 57 may be less than the pressure in column 26, which allows for greater flexibility in the pressure and temperature characteristics of the produced CCNG and PLNG.

Referring to FIG. 9, the example process has a CCNG stream 63 and a PLNG stream 59, but does not have a return flow to column 26 as in the other examples. While the return flow may be used to increase the efficiency of column 26 in some circumstances, the entire flow along line 65 may be used to produce CCNG or PLNG, the composition of which will be controlled by the temperature and pressure within separator 57.

Referring to FIG. 10, the example process is used to produce a CCNG stream 63 only, with any liquids separated in separator 57 is returned to column 26.

It will be understood that the examples shown in FIGS. 2-10 are non-limiting examples, and that the various features and options discussed and depicted may be combined in other combinations than those depicted where practical. For example, certain examples may be modified to only produce CCNG instead of both CCNG and PLNG; certain examples may be modified to only produce PLNG; JT valves and expanders may be alternately used depending on the circumstances; the point at which the feed stream used to produce CCNG and/or PLNG is taken may be selected based on the preferences of the user; and there may or may not be a return stream to column 26. Other modifications and combinations of features will be apparent to those skilled in the art. While not shown, the CCNR and/or PLNG may be collected in a separate storage vessel that is separate from the normal, or prior art, operation of the straddle plant.

In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.

The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given a broad purposive interpretation consistent with the description as a whole.

Claims

1. A method to produce pressurized liquid natural gas (PLNG) and cold compressed natural gas (CCNG), comprising the steps of:

passing a dewatered natural gas stream at pressures of between 450 psig and 1200 psig through one or more heat exchangers to pre-cool the natural gas stream;

passing the pre-cooled natural gas stream through a gas column where natural gas liquid fractions and natural gas fractions are separated;

passing the pre-cooled natural gas fractions at pressures of between 450 psig and 1200 psig through one or more heat exchangers to further cool the natural gas fractions;

passing the further cooled natural gas fractions at pressures of between 450 psig and 1200 psig through a gas expansion apparatus where pressure of the natural gas fractions is lowered to a pressure of less than 300 psig; and

passing the natural gas fractions at a pressure of less than 300 psig through a separator where they are separated into a PLNG stream and a CCNG stream at a pressure of less than 300 psig.

2. The method of claim 1, further comprising the step of modifying an existing straddle plant to perform the steps.

3. A method to produce pressurized liquid natural gas (PLNG) where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus, comprising the steps of:

providing heat exchangers on a high pressure, pre-treated, pre-cooled natural gas PLNG feed line to a gas expansion apparatus;

providing a gas liquid separator downstream of the expansion apparatus; and

providing a pressure reducing device for a separated gaseous stream into a straddle plant fractionator.

4. The method of claim 3, wherein the gas expansion apparatus is a Joule-Thompson (JT) valve or a gas expander turbine.

5. The method of claim 3, wherein cooling is provided by a vapour fraction of the expanded gas.

6. The method of claim 3, wherein the gaseous stream from the gas liquid separator is returned to the straddle plant fractionator.

7. The method of claim 3, wherein the gas to be liquefied is PLNG.

8. A method to produce cold compressed natural gas (CCNG) where a high pressure, pre-treated, pre-cooled natural gas stream from a straddle plant is routed to a gas expansion apparatus.

9. A cold compressed natural gas (CCNG) section of a straddle plant, the CCNG section comprising:

a gas expansion apparatus;

a feed line feeding gas to the gas expansion apparatus;

a CCNG receiver to separate vapour and liquid fractions; and

a liquids return line to a straddle plant fractionator.

10. The CCNG section of claim 9, further comprising a pressurized liquid natural gas line to storage.

11. The CCNG section of claim 9, wherein the CCNG liquid fraction is routed to the straddle plant fractionator for NGL's recovery.

12. A method to produce Pressurized Liquid Natural Gas (PLNG), the method comprising the steps of:

passing a dewatered natural gas stream at pressures of between 450 psig and 1200 psig through a first phase separator to obtain a natural gas liquid fraction and a natural gas fraction, the separated natural gas fractions being at a pressure of between 450 psig and 1200 psig;

passing at least a portion of the natural gas fraction through a gas expansion apparatus to reduce the pressure of the natural gas fraction to less than 300 psig; and

passing the pressure-reduced natural gas fraction through a second phase separator to obtain at least one of a PLNG stream and a CCNG stream at a pressure of less than 300 psig.

13. The method of claim 12, wherein the dewatered natural gas stream is a natural gas stream in a straddle plant.

14. The method of claim 12, further comprising the step of passing a natural gas stream through a dewaterer and a heat exchanger to cool the natural gas stream to obtain the dewatered natural gas stream.

15. The method of claim 12, further comprising the step of cooling at least a portion of the separated natural gas fraction in one or more heat exchangers.

16. The method of claim 12, further comprising the step of collecting each of the at least one of the PLNG stream and the CCNG stream separately in a storage vessel.

17. The method of claim 12, wherein the gas expansion apparatus comprises a Joule-Thompson valve or a gas expander turbine.

18. The method of claim 12, wherein at least a portion of an output of the first or the second phase separator is used to cool the natural gas stream.

19. The method of claim 12, wherein at least a portion of the output of the second phase separator is used to cool a fractionator of the saddle plant.