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Composition control for a mixed refrigerant liquefaction system

IP.com Disclosure Number: IPCOM000242075D
Publication Date: 2015-Jun-17
Document File: 3 page(s) / 54K

Publishing Venue

The IP.com Prior Art Database

Abstract

When a hydrocarbon mixture such as a mixed refrigerant (MR) is used as a refrigerant for a Liquefied Natural Gas (LNG) process, it is necessary to control the composition of the mixture to ensure efficient operation. Typically this is done by controlling the addition of the individual components to the mixture since several make-ups are generally used, each of a pure or nearly pure constituent of the overall mixture. The addition of the make-up streams normally occurs to compensate for losses or to increase the overall inventory of the mixed refrigerant. There are times, however, when it is necessary to decrease the inventory of the mixed refrigerant, so refrigerant has to be released. Refrigerant can be released from a number of locations in the refrigerant loop and the choice of the release point can have different results for the remaining mixture. This paper covers various methods for adjusting the inventory.

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Composition control for a mixed refrigerant liquefaction system

When a hydrocarbon mixture such as a mixed refrigerant (MR) is used as a refrigerant for a Liquefied Natural Gas (LNG) process, it is necessary to control the composition of the mixture to ensure efficient operation.  Typically this is done by controlling the addition of the individual components to the mixture since several make-ups are generally used, each of a pure or nearly pure constituent of the overall mixture.  The addition of the make-up streams normally occurs to compensate for losses or to increase the overall inventory of the mixed refrigerant.  There are times, however, when it is necessary to decrease the inventory of the mixed refrigerant, so refrigerant has to be released.  Refrigerant can be released from a number of locations in the refrigerant loop and the choice of the release point can have different results for the remaining mixture.

One of the reasons that mixed refrigerants are utilized for the liquefaction process is that a single refrigerant stream can be divided easily into multiple streams to provide multiple levels of refrigeration within the process.  This is accomplished by partial condensation and separation of the liquid fraction of the refrigerant from the vapor fraction of the refrigerant.  A single separation will produce two streams which can then be used to deliver two different temperatures of refrigeration to the process.  Additional partial condensations can produce additional separations and refrigerants with different properties.  All of this is part of the system as designed.  The number of separations involved is generally related to the complexity of the refrigerant mixture.  Since there are multiple streams available, there is a composition difference between the streams.  If there is a need to adjust the composition of the overall inventory for the refrigerant, then by choosing from which stream to release the excess refrigerant the operator can manipulate the composition of the system.

For a precooled MR process, the typical precooling refrigerant is propane and typical liquefaction refrigerant will consist of nitrogen, methane, ethane, propane and other hydrocarbons.  There is one separation step occurring after the propane precooling section where the high pressure mixed refrigerant separator allows for the MR liquid and vapor streams to be separated.  The liquid and vapor streams are independently cooled in the main exchanger and sent to two separate expansion devices, such as Joule Thomson (J‑T) valves to partially or fully condense them.  The composition of the MR liquid stream will have most of the propane and heavier components, a significant fraction of the ethane and a small amount of methane with a trace of mostly dissolved nitrogen.  The composition of the MR vapor stream will have nearly all of the nitrogen, the greater portion of the methane and the remaining ethane.  The ethane separation between the...