Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-12-08
2002-06-11
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S623000
Reexamination Certificate
active
06401486
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to methods and apparatus for high recovery of hydrocarbon liquids from methane-rich natural gases and other gases, e.g., refinery gases. More particularly, the present invention provides methods and apparatus for more efficiently and economically achieving high recovery of ethane, propane, propylene and heavier hydrocarbon liquids (C
2+
hydrocarbons) in association with liquified natural gas production.
II. Description of the Background
Due to its clean burning characteristics and the implementation of more stringent environmental regulations, the projected demand for natural gas has been increasing during recent years. In addition to methane, natural gas includes some heavier hydrocarbons and impurities, e.g., carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas may be further processed to separate and recover heavier hydrocarbons as natural gas liquids (NGL) and produce pipeline quality methane. The pipeline quality methane is then delivered to gas pipelines as the sales gas ultimately transmitted to end-users.
In the case of remote gas production or distant gas markets, transportation of produced natural gas via gas pipeline might not be economical or even feasible. Accordingly, liquifaction of natural gas has become a viable and widely adopted scheme. The economics of liquifying natural gas is feasible due mainly to the great reduction in volume as the gas is converted to a liquified state, making it easy to store and transport. Another advantage of converting the produced natural gas to a liquified form is that the produced LNG can be economically stored to supplement energy suppliers during seasonal peak demand periods. Liquified natural gas, typically stored at atmospheric pressures and at temperatures of approximately −260° F., is transported to distant markets via refrigerated tankers.
Processes for the liquifaction of natural gas are well known in the art. Natural gas comprising predominantly methane enters an LNG plant at elevated pressures and is pretreated to produce a purified feed stock suitable for liquifaction at cryogenic temperatures. The pretreatment typically includes removal of acid gases, e.g., hydrogen sulfide and carbon dioxide, together with other contaminants, including moisture and mercury. The purified gas is thereafter processed through a plurality of cooling stages using indirect heat exchange with one or more refrigerants to progressively reduce its temperature until total liquifaction is achieved. The pressurized liquid natural gas is sub-cooled to reduce flashed vapor through one or more expansion stages to final atmospheric pressure suitable for storage and transportation. The flashed vapor from each expansion stage, together with the boil off gas produced as a result of heat gain, are collected and used as a source of plant fuel gas with any excess recycled to the liquifaction process.
Because a significant amount of refrigeration energy is required for liquifying natural gas, the refrigeration system becomes one of the major units in an LNG facility. Mechanical refrigeration cycles mostly in closed circuit are often employed in LNG projects. A number of liquifaction processes have been developed with the differences mainly found in the refrigeration cycles used. The most commonly used LNG processes can be classified into three categories as follows:
1) The cascade process presenting the benefits of easy start-up or shutdown. The cascade process consists of successive refrigeration cycles using propane, ethane or ethylene, and methane. The thermal efficiency can be readily enhanced by the use of multi-compressor stages. U.S. Pat. No. 5,669,234, incorporated herein by reference, represents an exemplary cascade process.
2) The propane pre-cooled mixed refrigerant process involves the use of a multi-component mixture of hydrocarbons, typically comprising propane, ethane, methane, and optionally other light components in one cycle, and a separate propane refrigeration cycle to provide pre-cooling of natural gas and the mixed refrigerant to approximately −35° F. The propane mixed refrigerant process advantageously provides improved thermal efficiency. However, a significant disadvantage results from the use of extremely large spiral wound exchangers. Such exchangers are a long lead item requiring special facilities in the field to manufacture. Examples of the propane mixed refrigerant process include those disclosed in U.S. Pat. Nos. 4,404,008 and 4,445,916, incorporated herein by reference.
3) The single, mixed refrigerant process includes heavier hydrocarbons, e.g., butanes and pentanes, in the multi-component mixture and eliminates the pre-cooled propane refrigeration cycle. It presents the simplicity of single compression in the heat exchanger line and is particularly advantageous for small LNG plants. U.S. Pat. No. 4,033,735, incorporated herein by reference, represents an exemplary single, mixed refrigerant process.
The use of a turbo expander in combination with mechanical refrigeration cycles has also been adopted in many LNG processes. Examples of the use of a turbo expander are disclosed in U.S. Pat. Nos. 3,724,226; 4,065,278; 5,755,114; 4,970,867, 5,537,827; and Int'l Patent No. WO 95/27179.
In addition to methane, natural gas typically contains various amounts of ethane, propane and heavier hydrocarbons. The composition varies significantly depending on the source of the gas and gas reserve characteristics. Hydrocarbons heavier than methane need to be removed from LNG for various reasons. Hydrocarbons heavier than pentane, including aromatics, having high freezing points must be reduced to an extremely low level to prevent the freezing and plugging of process equipment in the course of cooling and liquifaction steps. After separation of these heavy components from LNG, they provide excellent gasoline blending stock. Many patents have been directed to methods for removal of these heavy hydrocarbons. For instance, U.S. Pat. No. 5,325,673 discloses the use of a single scrub column in the pretreatment step operated substantially as an absorption column to remove freezable C
5+
components from a natural gas stream feeding to an LNG facility. The heavy liquid recovered subsequently can be fractionated into various fractions for use as make-up refrigerants. U.S. Pat. No. 5,737,940 describes an exemplary system incorporated in a cascade process.
Besides being liquified as part of LNG and used as fuel, lighter natural gas liquid (NGL) components, e.g., hydrocarbons having 2-4 carbon atoms, can also be a source of feedstock to refineries or petrochemical plants. Therefore, it is often desirable to maximize the recovery of NGL to enhance revenue. To achieve high recovery of these components, it is common practice to design an NGL recovery plant so that the tail gas produced by the NGL recovery plant and comprising primarily methane is delivered to the LNG facility for liquifaction. U.S. Pat. Nos. 5,291,736 and 5,950,453 are typical examples of such combined facilities.
Among several different NGL recovery processes, the cryogenic expansion process has become the preferred process for deep hydrocarbon liquid recovery. In a conventional turbo-expander process, the feed gas at elevated pressure is pretreated for the removal of acid gases, moisture and other contaminants to produce a purified feed stock suitable for further processing at cryogenic temperatures. The purified feed gas is then cooled to partial condensation by heat exchange with other process streams and/or external propane refrigeration, depending upon the richness of the gas. The condensed liquid after removal of the less volatile components is then separated and fed to a fractionation column, operated at medium or low pressure, to recover the heavy hydrocarbon constituents desired. The remaining non-condensed vapor portion is turbo-expanded to a lower pressure, resulting in fur
Chen Jong Juh
Elliot Douglas G.
Lee Rong-Jwyn
Yao Jame
Brookhart, Esq. Walter R.
Doerrler William C.
Jensen, Esq. William P.
Shook Hardy & Bacon L.L.P.
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