TITLE OF THE INVENTION:

Method for selective extraction of natural gas liquids from "rich" natural gas

FIELD OF THE INVENTION The present invention relates to a method for selective extraction of natural gas liquids from "rich" natural gas

BACKGROUND OF THE INVENTION

Natural gas coming from a producing well contains many natural gas liquids (NGLs) that are commonly removed. The removal of natural NGLs usually takes place in a relatively centralized processing plant. The objective is to reduce the hydrocarbon dew point to prevent problems in the pipelines from liquid fallout. To remove NGLs, there are three common processes; Refrigeration, Lean Oil Absorption and Cryogenic.

With Refrigeration, a refrigeration plant is employed to provide cold to lower the temperature of the natural gas. Refrigeration is able to extract a large percentage of propane and most of the butane and heavier components.

With Lean Oil Absorption, an absorbing oil with an affinity for NGLs is brought into contact with natural gas in a contact tower where it soaks up a high proportion of

NGLs. The "rich" absorption oil, now containing NGLs exits the absorption tower. This "rich" mixture of absorbing oil and NGLs is chilled to -30 F to separate the NGLs and absorbing oil. This process can extract 90% of the propane and heavier hydrocarbons and about 30% of the ethane.

The cryogenic process enables higher recoveries of ethane. The first generation cryogenic plants were able to extract up to 70% of the ethane from the gas, since the early 1990s, modifications to the cryogenic process have allowed ethane recoveries up to 99% extraction level. This increase in recovery comes with higher operating costs. There are a number of different ways to chill the gas the one most commonly used is the turbo expander process. In this process external refrigerants are used to cool the natural gas stream, then an expansion turbine is used to rapidly expand the chilled gases, which causes the temperature to drop significantly. This rapid temperature drop condenses ethane and

other hydrocarbons in the gas stream while maintaining methane in a gaseous form. Operations of gas processing plants in reduced recovery modes is difficult, the plants are typically designed to achieve high recoveries of all the NGLs and are not designed to recover only pentanes and heavier or only butanes.

SUMMARY OF THE INVENTION

There is provided a method for selective extraction of natural gas liquids from "rich" natural gas. The method involves the step of effecting a heat exchange between a rich natural gas stream and a refrigerant fluid to lower a temperature of the rich natural gas stream. The heat exchange is controlled to lower the temperature of the rich natural gas stream to a selected hydrocarbon dew point in order to condense at least one selected hydrocarbon liquids carried in the rich natural gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention 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: FlG. 1 is a schematic diagram of a facility equipped with indirect cooling in accordance with the teachings of the present invention.

FIG. 2 is a schematic diagram showing a variation of the indirect cooling illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred method will now be described with reference to FIG. 1.

Referring to FIG. 1, "rich" natural gas stream 20 is straddled into stream 30 for indirect pre-cooling in heat exchanger 1. The cooling is provided by the countercurrent flow of stream 40. The now colder stream 31 then enters separator 2 where water and heavy hydrocarbons are condensed and separated from lighter fractions (hydrocarbons, carbon dioxide, nitrogen, etc.). The separated heavier fractions exit separator vessel 2 through line

60. The lighter fractions exit through line 32 which then enters heat exchanger cold box section 6 for further cooling. The colder stream 33 now enters separator vessel 3. The condensed and separated propane exits separator vessel 3 through line 61. The separated lighter fractions leave separator 3 through line 34 for further cooling in heat exchanger cold box section 7. The colder stream 35 now enters separator vessel 4. The condensed and separated carbon dioxide exits separator vessel 4 through line 62. The separated lighter fractions leave separator 6 through line 36 for further cooling in heat exchanger cold box section 8. The colder stream 37 now enters separator vessel 5. The condensed and separated ethane exits separator vessel 5 through line 63. The now "lean" gas leaves separator 5 through line 38 for pre-heating in cold box section 7 and cold box section 6. The now warmer "lean" gas exits cold box through stream 39 and mixes with the vaporized LNG stream 52 to form the mixture stream 40. The mixed "lean" gas now enters heat exchanger 1 for pre-heating, it then exits it at or near transmission line temperature through stream 41. The refrigerant is LNG supplied from tank 9 and pressurized by pump 10 into stream 51. The pressurized LNG flows first into heat exchanger cold box section 8 where it begins to pick up heat, then into heat exchanger cold box section section 7 and finally into heat exchanger cold box section 6. This vaporized stream exits the cold box as stream 52 and is mixed with the "lean" gas stream 49 to form a "lean" mixture as stream 40.

FIG.2 shows another arrangement for indirect contact method for extraction of natural gas liquids where the cold energy supply fluid is one other than LNG, the main difference being that when LNG is used it can be injected into the transmission pipeline while an optional fluid may or may not be injected into the pipeline. "Rich" natural gas stream 20 is straddled into stream 30 for indirect pre-cooling in heat exchanger 1. The cooling is provided by the countercurrent flow of stream 39. The now colder stream 31 then enters separator 2 where water and heavy hydrocarbons are condensed and separated from lighter fractions (hydrocarbons, carbon dioxide, nitrogen, etc.). The separated heavier fractions exit separator vessel 2 through line 60. The lighter fractions exit through line 32 which then enters heat exchanger cold box section 6 for further cooling. The colder stream 33 now enters separator vessel 3. The condensed and separated propane exits separator vessel 3 through line 61. The separated lighter fractions leave separator 3 through line 34 for further cooling in heat exchanger cold box section 7. The colder stream 35 now enters

separator vessel 4. The condensed and separated carbon dioxide exits separator vessel 4 through line 62. The separated lighter fractions leave separator 4 through line 36 for further cooling in heat exchanger cold box section 8. The colder stream 37 now enters separator vessel 5. The condensed and separated ethane exits separator vessel 5 through line 63. The now "lean" gas leaves separator 5 through line 38 for pre-heating in cold box section 7 and cold box section 6. The now warmer "lean" gas exits cold box through stream 39. The "lean" gas now enters heat exchanger 1 for pre-heating, it then exits it at a near transmission line temperature through stream 40. The refrigerant is supplied from tank 9 into stream 51. This refrigerant flows first into heat exchanger cold box section 8 where it begins to pick up heat, then into heat exchanger cold box section section 7 and finally into heat exchanger cold box section 6. This vaporized stream exits the cold box as stream 52. For those skilled in the art, the number of heat exchangers and separators can be re-arranged to achieve the desired separation of hydrocarbons and other components present in the "rich" gas stream.

Liquid Natural Gas (LNG) has been selected for the purpose of illustration. It will be appreciated that other refrigerants such as liquid nitrogen, liquid carbon dioxide, liquid oxygen and the like can be used to condense the "rich" gas stream. It is preferred that the refrigerant fluid be within the cryogenic temperature range, merely because colder temperatures are required in order to condense some of the natural gas liquids, such as ethane. Other hydrocarbon refrigerants can be used such as ethane and propane. For example, liquid carbon dioxide could be used as a refrigerant to condense a number of natural gas liquids, but would not be effective in condensing ethane. There are drawbacks to the use of some refrigerant fluids, such as liquid oxygen. Liquid oxygen could be used, but is not preferred due to safety concerns. Liquid Natural Gas and Liquid Nitrogen are two of the more viable refrigerants which could be used.

In the preferred method, refrigerant fluids provide the "cold energy" required to condense and extract the NGLs . A typical straddle plant is designed to achieve high recoveries of all NGLs and the "turndown" to lower recoveries are difficult to obtain. The above method allows for ease of "turndown" by simply changing the temperature set point controller which then changes the LNG flow rate. As the LNG gives up its cold energy to condense the NGLs in the "rich" stream it becomes a "lean" gas ready for distribution.

Existing plants operate in a mode that recovers at least some percentage of all components, it is not generally possible to operate the plants to achieve a specific hydrocarbon dew point. Control of hydrocarbon dew point for gas transportation is critical due to the influence of ambient temperatures and pressure reductions during transportation that can cause liquid fallout. To reach higher extraction levels more expensive metallurgy, more compression, and more capital investment is required. According to the present invention there is provided a method for liquefaction and extraction of NGLs from natural gas. A pre-cooling takes place in a heat exchanger of the incoming "rich" natural gas stream, containing methane, ethane, propane, butanes, pentanes, other heavier hydrocarbons, water and carbon dioxide with a countercurrent flow of "lean" natural gas. Separation of water and heavier hydrocarbons from lighter hydrocarbons then takes place in a series of separators by controlling the temperature at each separator through a heat exchange with refrigerant fluids. Cooling upstream of each separator through a heat exchange with refrigerant fluids permits selective control of the extraction of NGL's. The method provides for ease of "turndown" to achieve high or low recoveries ratios between and Hydrocarbon Dew Point (HDC) control. The use of the above described method at a straddle plant facility provides a distinct advantage over methods currently in use. Existing systems bring the pressure of the natural gas down to remove the natural gas liquids and then increasing the pressure of the natural gas back up in order to return the natural gas to the pipeline after processing. With the present method, the natural gas can be freed of the natural gas liquids without a change in pressure.

hi 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.

It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.