Configurations and methods of integrated NGL recovery and LNG liquefaction

Abstract

Contemplated plants include a NGL recovery portion and a LNG liquefaction portion, wherein the NGL recovery portion provides a low-temperature and high-pressure overhead product directly to the LNG liquefaction portion. Feed gas cooling and condensation are most preferably performed using refrigeration cycles that employ refrigerants other than the demethanizer / absorber overhead product. Thus, cold demethanizer / absorber overhead product is compressed with the turbo-expansion and delivered to a liquefaction portion at significantly lower temperature and higher pressure without net compression energy expenditure.

Description

[0001] This application claims priority to our copending U.S. provisional patent application with the Ser. No. 60 / 697,467, which was filed 7 Jul. 2005.FIELD OF THE INVENTION [0002] The field of the invention is natural gas liquids (NGL) recovery and liquefied natural gas (LNG) liquefaction, and particularly integrated plant configurations for such processes. BACKGROUND OF THE INVENTION [0003] While the crude oil supply in the world is diminishing, the supply of natural gas is still relatively abundant in many parts of the world. Natural gas is typically recovered from oil and gas production wells located onshore and offshore. Depending on the particular formation and reservoir, natural gas also contains relatively low quantities of non-methane hydrocarbons, including ethane, propane, i-butane, n-butane, pentanes, hexane and heavier components, as well as water, nitrogen, carbon dioxide, hydrogen sulfide, mercaptans, and other gases. [0004] Natural gas from the wellheads is commonly ...

AI Technical Summary

Benefits of technology

[0010] The present invention is directed to configurations, plants, and methods for natural gas processing and liquefaction in which a cold separator overhead product is directly compressed in a compressor that is driven by a feed gas vapor expander, and wherein the compressed cold separator overhead product is then liquefied in a liquefaction unit. Most advantageously, such plants integrate NGL processing and LNG liquefaction in an efficient, cost effective, and technically simple manner.

Problems solved by technology

However, in remote locations lacking the necessary pipeline infrastructure, natural gas is commonly transported by liquefying the natural gas and transporting the gas in liquid form (e.g., using LNG cargo carriers).

Unfortunately, liquefaction of natural gas is problematic as natural gas also contains aromatics (e.g., benzene) and heavy hydrocarbons, which solidify when chilled to the cryogenic temperatures.

While these processes improve C2 recovery to at least some degree, the energy spent for residue gas recompression renders the process often uneconomical.

However, as these NGL processes operate independently from the liquefaction plants, they will generally require additional compression and refrigeration prior to LNG liquefaction of the residue gas.

While such process can advantageously be used to eliminate wax formation by removal of C6+ and heavier components, they are not suitable for the removal of C2+ components, especially at high levels (65% or higher C2 recovery) and consequently fail to produce the lean residue gas that can be liquefied for the North American natural gas market.

While such processes attempt to operate the absorption column at high feed gas pressure (e.g., 800 psig or higher) to reduce energy consumption, it should be noted that NGL separation suffers significantly due to decreasing relative volatilities of the NGL components, consequently producing NGL with excessive methane content.

Moreover, such process schemes typically fail to achieve high C2 and C3 recovery (e.g., greater than 60%).

Unfortunately, most known standalone LNG liquefaction processes using single or multiple refrigeration cycles (either cascade refrigeration or mixed refrigerant cycle) have relatively low efficiencies when C2 or C3 recovery is incorporated upstream of the LNG liquefaction plant.

Thus, while numerous plant configurations and methods for NGL recovery and LNG liquefaction are known in the art, all or almost all of them, suffer from various disadvantages.

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