Configurations and methods for offshore ngl recovery

Abstract

A natural gas two-column processing plant allows for recovery of at least 95% of C4 and heavier hydrocarbons, and about 60 to 80% of C3 hydrocarbons from a rich feed gas stream in which the first column (absorber) operates at a higher pressure than the second column, with the absorber receiving a compressed gas from the second column, and a turboexpander discharging a two-phase stream to the top of the absorber. Most typically, contemplated configurations and methods operate without the use of external refrigeration.

Description

[0001]This application claims priority to U.S. provisional application with the Ser. No. 61 / 694,949, which was filed on Aug. 30, 2012, and which is incorporated by reference herein.FIELD OF THE INVENTION[0002]The field of the invention is removal and recovery of natural gas liquids (NGL) from feed gases to meet pipeline hydrocarbon dew point and heating value specifications, especially for offshore applications.BACKGROUND OF THE INVENTION[0003]Numerous systems and methods are known in the art to recover C2, C3, and heavier components from natural gas, but all or almost all of them are configured for high recovery (i.e., over 90%) of NGL and require use of a turboexpander and deep refrigeration, which are costly and can only be economically justified if there are significant downstream markets. However, this is not case with offshore NGL recovery systems where space is at a premium and economic viability of the installation depends on a relatively small footprint and low operating an...

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Benefits of technology

[0009]In one preferred aspect of the inventive subject matter, two columns are operated at different pressures with the first column (absorber) operating at a relatively high pressure of about 550 psig and with the second column (fractionator) operating at about 450 psig. By operating the absorber at relatively high pressure, the compression ratio of the residue gas is reduced, thereby minimizing the overall compression horsepower. With the fractionator operating at about 450 psig, it should be noted that the separation of methane from the ethane and heavier components can be accomplished with less heating requirement due to the favorable relative volatility between components, resulting in a smaller diameter column.

[0010]In another preferred aspect of the invention, the vapor stream from the fractionator overhead is advantageously utilized for stripping in the absorber. In one embodiment of this process, the fractionator overhead stream is compressed and the “free” heat of compression is used to efficiently remove the methane components from the NGL from the absorber.

[0011]It should also be particularly appreciated that only the liquid portion of the expander discharge is used as reflux to the absorber, which is entirely different from heretofore known configurations and methods, which require the exp

Problems solved by technology

Numerous systems and methods are known in the art to recover C2, C3, and heavier components from natural gas, but all or almost all of them are configured for high recovery (i.e., over 90%) of NGL and require use of a turboexpander and deep refrigeration, which are costly and can only be economically justified if there are significant downstream markets.

However, this is not case with offshore NGL recovery systems where space is at a premium and economic viability of the installation depends on a relatively small footprint and low operating and capital cost.

Therefore, in all or almost all cases, high capital investment and operating costs required for high recovery typically cannot be justified.

In view of the changed demand, the complexity of currently known NGL processing plants that allow o

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