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...

AI Technical Summary

Benefits of technology

The invention relates to a process for separating ethane and heavier components from natural gas. The technical effect of the invention is achieved by using two columns operated at different pressures, where the absorbent column operates at a high pressure and the fractionator operates at a low pressure. This reduces the overall compression power required, and the separation of methane from ethane and heavier components can be achieved with less heating requirement due to the favorable relative volatility between components. The fractionator overhead stream can be efficiently utilized for stripping in the absorbent, and only the liquid portion of the expander discharge is used as reflux to the absorbent, reducing the size of the absorber. The second fractionator operates at a lower pressure and temperature, resulting in more efficient separation and the use of residue gas compression heat for reboiling the fractionator, eliminating the requirement for steam or hot oil heating.

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 over 90% C3 recovery is excessive and often cannot be justified from an economical perspective.

While all of these processes can achieve very high NGL recovery, several difficulties still remain.

Moreover, when processing a rich gas stream with relatively high levels of C5+ hydrocarbons, additional external refrigeration is often required.

Typically, such process configurations are complex and are difficult to operate.

While such complex configurations are suited to achieve high C2 and C3 recovery to over 95%, they tend to be cost prohibitive and not suitable for offshore applications.

Therefore, although various configurations and methods are known to recover NGL from a feed gas, all or almost all of them suffer from one or more disadvantages when dewpointing and moderate C3 recovery is required.

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