Process for cooling methane-rich gas

Abstract

A process for liquifying a methane-rich gas (1) cools the gas in first heat exchanger (A) by evaporation of a refrigerant fluid (4), particularly as the first stage of the liquefaction process. The cooled gas feed (2) is then passed through a heat exchange liquefaction unit (F) to produce a liquified product (7). The liquid refrigerant fluid (3) at ambient temperature is delivered from a compressor (D) and condenser (E) arrangement to the first heat exchanger (A) where it is further cooled and delivered to a second heat exchanger (F) that provides further heat exchange with the methane-rich gas stream that is delivered as a liquified product (7). The refrigerant liquid is further cooled by heat exchange with the second heat exchanger (F) and cycled via a pressure reducing valve (C) to the first heat exchanger (A) for use as a coolant, and during the pressure reduction of the liquid refrigerant the temperature and pressure is such that evolution of refrigerant vapour is avoided. This has the advantage that the requirement for vapour / liquid separators and associated pipework is unnecessary. The refrigerant fluid is typically ammonia but can be carbon dioxide or propane.

Description

Field of the Invention The invention relates to a method of cooling methane-rich gas, particularly as a first stage of a liquefaction process. Background to the Invention Natural gas liquefaction processes typically include a first stage of cooling the feed natural gas (after removal of impurities and water vapor) to around -30 / -40 degC by means of heat exchange with an evaporating refrigerant. This first stage of cooling may also be used to recondense refrigerant recycled from a colder second stage of the liquefaction process. Propane is the most widely used refrigerant in this first stage of cooling. Use of other refrigerants (ammonia, carbon dioxide) has been described. Drawing 1 / 4 represents one typical refrigeration arrangement. The feed gas stream (1) flows to a heat exchanger (A) contained in a kettle-like vessel (B) and emerges at a lower temperature as stream (2). Vessel (B) contains a sufficient volume of evaporating refrigerant to submerge the heat exchange surface. A stream of liquid refrigerant (3) at near-ambient temperature is let down in pressure into vessel (B) through valve (C), forming stream (4) with significant evolution of vapor. The vapor content of (4) together with the vapor evolved by cooling of stream (1) accumulates in the upper part of the vessel (5), is compressed by compressor (D), condensed in condenser (E) and recycled as stream (3). Drawing 2 / 4 shows another typical arrangement. The feed gas stream (1) is cooled in multi-pass heat exchanger (A), having outlet stream (2). A stream of liquid refrigerant (3) at near-ambient temperature is let down in pressure into vapor-liquid separator vessel (B) through valve (C), forming stream (4) with significant evolution of vapor. The vapor outlet stream (4-1) from separator (B) is reheated in heat exchanger (A), having outlet stream (5-1). The liquid outlet stream (4-2) from separator (B) is revaporised and heated in a second passage of heat exchanger (A) having outlet stream (5-2). The combined stream (5-3) is compressed by compressor (D), condensed in condenser (E) and recycled as stream (3). In order to ensure even distribution of vapor and liquid across multiple parallel passages in the heat exchanger (A), several separators (B) in parallel may be provided. In both the examples outlined above, a significant volume of vapor is evolved in letting down in pressure of the refrigerant to the pressure at which the heat transfer takes place, with resulting need for large and heavy vapor-liquid separation equipment and piping. Summary of the Invention The main aspect of the invention relates to the liquefaction of methane-rich gas and discloses an improved first stage of cooling of the methane-rich feed gas (and of cooling and recondensing of recycled refrigerant from a second colder stage of the liquefaction process if applicable) to a temperature of from -30 degC to -70degC depending on the refrigerant selected. Where pressures are stated anywhere in this application as “bar”, these are bar absolute. According to the invention, illustrated in Drawing 3 / 4, the methane-rich feed gas (1) of a liquefaction process is first cooled in heat exchanger (A) to a temperature of from -30 degC to -70 degC in stream (2). The cooled stream (2) then enters a colder second stage of the liquefaction process (F) and emerges as stream (7) of liquefied product. (The internal details of the colder second stage (F) of the liquefaction process are not part of this main aspect of the Application). A stream of liquid refrigerant (3) at near-ambient temperature is first cooled in heat exchanger (A) having outlet stream (3-1) with the temperature of the said stream (2). The cooled liquid refrigerant is then further cooled by heat exchange within second stage of the liquefaction process (F). The emerging further cooled outlet stream (3-2) is let down in pressure through valve (C) to form stream (4). Due to the above-said step of further cooling within the liquefaction unit (F), stream (4) is entirely in the liquid phase, avoiding the evolution of vapor on letting down in pressure described above in relation to existing practice. Stream (4) flows to heat exchanger (A), in which it is evaporated and reheated to form stream (5) with near-ambient temperature. The cooling process may be configured with multiple stages, so that the evaporated coolant enters compressor (D) at multiple pressure levels. The Applicant respectfully submits that the further stage of cooling of the refrigerant between streams (3-1) and (3-2) in the colder second stage of the liquefaction process (F), by means of which the evolution of vapor on the letting down of pressure across valve (C) is avoided, is both novel and inventive, and is of significant practical value in avoiding requirement for vapor / liquid separators with associated pipework. Accordingly, there is provided as follows a description of a process for cooling a stream of methane-rich gas, particularly as part of a liquefaction process (reference is made to Drawing 3 / 4 and the equipment tags and stream numbers shown thereon): - providing a stream (1) of feed methane-rich gas at a pressure of from 30 to 150 bar and at ambient temperature; - cooling the feed gas in a first hot passage of heat exchanger (A) to a temperature of from -30 degC to -70 degC; - passing the cooled feed gas (2) through liquefaction unit (F) to produce liquefied product stream (7); - providing a stream (3) of liquid refrigerant at near ambient temperature; - cooling said refrigerant stream in a second hot passage of heat exchanger (A) to form stream (3-1) with the said temperature range of stream (2); - cooling stream (3-1) in a heat exchange passage within the liquefaction unit (F) to form stream (3-2); - reducing the pressure of stream (3-2) by passage through valve (C) to a pressure of from 0.9 bar to 10 bar; - providing that stream (4) is fully condensed to the liquid state by adjusting the temperature of stream (3-2); - reheating stream (4) in a cold passage of heat exchanger (A) - compressing the said reheated stream (5); - condensing the compressed vapour (6), and then - recycling the condensed refrigerant vapor (3). Embodiment of the Invention In a particularly advantageous second aspect of the invention, illustrated in Drawing 4 / 4, the cooled vapor Stream (2) is liquefied through the agency of a gas expander. Stream 2 is divided into two parts. A first part Stream (2-1) is cooled and condensed by passage through a second heat exchanger (G), emerging as a first liquefied product stream (7). 5 A second part Stream (8) flows to a gas expander (H), having an outlet stream (9) with a pressure of from 3 to 50 bar. Stream (9) contains both vapor and liquid, which are separated in vapor / liquid separator (I) The liquid outlet from (I) then forms a second liquefied product stream (10). 10 The vapor outlet from separator (I) stream (11) is successively reheated in the heat exchangers (G) and (A), recompressed and recycled to the feed gas (1). The Applicant respectfully submits that the efficient and simple process illustrated in Drawing 4 / 4, comprising a first stage of cooling without evolution of 15 flash vapor together with a second stage of cooling and liquefaction by agency of a partially liquefying expander, is both novel and inventive.

Claims

1 A process for liquefying methane-rich gases comprising- providing a stream (1) of feed methane-rich gas at a pressure of from 30 to 150 bar at ambient temperature;- cooling the feed gas in a first hot passage of heat exchanger (A);- passing the cooled feed gas (2) through liquefaction unit (F) to produce liquefied product (7);- providing a stream (3) of liquid refrigerant at ambient temperature;- cooling said refrigerant stream in a second hot passage of heat exchanger (A) to form stream (3-1) with the temperature of stream (2);- cooling stream (3-1) in a heat exchange passage within liquefaction unit (F) to form stream (3-2);- reducing the pressure of stream (3-2) by passage through valve (C);- providing that stream (4) is fully condensed to the liquid state by adjusting the temperature of stream (3-2);- reheating stream (4) in a cold passage of heat exchanger (A);- compressing the said reheated stream (5);- condensing the compressed vapour (6), and then- recycling the condensed refrigerant vapor (3).2 A process according to Claim 1 in which streams (2) and (3-1) are at a temperature of -30 deg to -70 degC.3 A process according to Claim 1 in which stream (4) is at a pressure offrom 0.8 bar to 10 bar.4 A process as claimed in any preceding claim in which- stream (2) is divided into a first stream (2-1) and a second stream (8);- cooling and condensing said stream (2-1) in a first hot passage of heat exchanger (G) to form a first liquefied product stream (7);- passing said second stream through gas expander (H) to form a mixture of liquid and vapor at a pressure of from 3 bar to 30 bar;- passing the outlet stream (9) of the gas expander (H) into a separator (I) to form a second liquefied product stream (10) and a vapor stream (11);- reheating said vapor stream (10) in a cold passage of heat exchanger (G) and a second cold passage of heat exchanger (A);- compressing the said reheated stream (13) in compressor (J);- cooling the compressed vapor stream (14) in cooler (K); and then- recycling the cooled vapor stream (15) at a pressure of from 30 to 150 bar.5 A process according to the Claim 4 in which the feed gas stream (1) is admitted to compressor (J) at intermediate pressure level between the pressure of stream (13) and the pressure of stream (14).6 A process according to Claims 4 or 5 in which stream (9) comprises vapor only.

Images