CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase under 35 U.S.C. § 371 of International Application No. PCT/IB2017/057970, filed Dec. 14, 2017, which claims the benefit of priority of European Patent Application No. 17150017.6, filed Jan. 2, 2017, the entire contents of each of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to an ethylene plant refrigeration system.

BACKGROUND OF THE INVENTION

In an ethylene plant, a charge gas such as a pyrolysis gas is typically processed to remove methane and hydrogen by a demethanizer and the remainder is processed in a known manner to separate ethylene. The separation of the gases in an ethylene plant through condensation and fractionation at cryogenic temperatures requires refrigeration over a wide temperature range. The capital cost involved in the refrigeration system of an ethylene plant can be a significant part of the overall plant cost. Therefore, capital savings for the refrigeration system will significantly affect the overall plant cost.

U.S. Pat. No. 5,979,177 discloses a process for the production of ethylene from a charge gas containing hydrogen, methane, ethylene and other C2 and heavier hydrocarbons by a low pressure demethanizer in a refrigeration system. A binary refrigerant comprising a mixture of methane and ethylene is used for the cooling. The binary refrigerant is progressively expanded and cooled through a series of heat exchangers. The charge gas is brought into contact with the cooled binary refrigerant in the heat exchangers to be cooled. The streams of binary refrigerants which have been used in the heat exchangers are compressed by a single compressor and subsequently expanded to be cooled for reusing in the series of heat exchangers.

The compression of the binary refrigerant which has been used in the heat exchangers requires a large amount of energy. It is desirable to be able to provide a refrigeration system which requires less energy.

SUMMARY

In the context of the present invention, fifteen embodiments are now described. Embodiment 1 is a refrigeration system for cooling a charge gas by a binary refrigerant, the refrigeration system comprises n heat exchangers (H-201,H-202,H-203,H-204) for progressively cooling the charge gas (2001) by the binary refrigerant (2501), wherein n is an integer of at least 2, wherein the refrigerant (2501) is successively fed to the first to the nth heat exchanger (H-201,H-202,H-203,H-204), wherein a portion of the refrigerant is expanded to lower the temperature after each of the n heat exchangers to provide first to nth expanded refrigerants (2502,2503,2504,2505), wherein each of the expanded refrigerants is fed back to the series of heat exchangers such that the kth expanded refrigerant (2502,2503,2504,2505) is successively fed back to the kth to the first heat exchangers (H-204,H-203,H-202,H-201) to provide cooling and result in kth heated refrigerant (2410,2308,2206,2104), wherein k is an integer of 1 to n, wherein the heated refrigerants (2410,2308,2206,2104) have temperatures of 0° C. to 25° C.; n compressor stages (K-111, K-113, K-113, K-114, K-121, K-122, K-123K-211, K-212, K-213, K-214, K-311, K-312) for compressing the heated refrigerants (2410,2308,2206,2104) arranged such that the output from the mth compressor stage (K-211,K-212,K-213) is fed to the (m+1)th compressor stage (K-212,K-213,K-214) after being cooled by a respective interstage cooler (H-211, H-212, H-213), wherein m is an integer of 1 to (n−1), and the output from the nth compressor stage is fed to the nth interstage cooler (H-214); at least one separator (V-101, V-102, V-103, V-104, V-105, V-110, V-111, V-112, V-113, V-114, V-121, V-122, V-123, V-124, V-201,V-202,V-203, V-204, V-205, V-210, V-211, V-212, V-213, V-214, V-221, V-222, V-224, V-311, V-312) following one of the heat exchangers (H-101, H-102, H-103, H-104, H-105, H-106, H-114, H-115, H-116, H-117, H-123, H-202, H-203, H-204, H-311A, H-311B) for separating the cooled charge gas from the heat exchanger to produce an overhead (2005,2008,2011) to be fed to the subsequent heat exchanger and a bottoms (2004,2007,2010); and a demethanizer (C-201) for separating the bottoms (2004,2007,2010) from the at least one separator into an overhead comprising methane and a bottoms comprising C2+ hydrocarbons.

Embodiment 2 is the refrigeration system of embodiment 1, wherein the kth heated refrigerant (2410,2308,2206,2104) is fed to (n−k+1) th compressor stage (K-211,K-212,K-213,K-214), respectively. Embodiment 3 is the refrigerant system of any of embodiments 1 and 2, wherein the charge gas (2011) from the nth heat exchanger (H-204) is successively fed back to the nth to the 1st heat exchangers without separation, preferably after being cooled. Embodiment 4 is the refrigeration system of any of embodiments 1 and 2, wherein the charge gas (2011) from the nth heat exchanger (H-204) is separated into a stream of H2 and a stream of methane and each of the streams is successively fed back to the nth to the 1st heat exchangers, preferably after the stream of H2 and/or the stream of methane is cooled. Embodiment 5 is the refrigeration system of any of embodiments 1 and 2, wherein the refrigeration system further comprises a charge gas heat exchanger (H-205) for cooling the charge gas (2011) from the nth heat exchanger (H-204) and a separator (V-204) for separating the cooled charge gas from the charge gas heat exchanger (H-205) into a stream of H2 and a stream of methane to be fed back to the charge gas heat exchanger (H-205) and successively to the nth to the first heat exchanger, wherein the stream of methane is expanded to lower the temperature before being fed back to the charge gas heat exchanger (H-205).

Embodiment 6 is the refrigeration system of any of the preceding embodiments, wherein the refrigeration system further comprises a refrigerant heat exchanger (H-206) for cooling and partly condensing the overhead from the demethanizer (C-201) by the refrigerant from the nth heat exchanger (H-204) which has been expanded to lower the temperature before being fed, wherein a vapour fraction of the cooled overhead is successively fed back to the nth to the first heat exchanger and a liquid fraction of the cooled overhead is fed back to the demethanizer (C-201) as reflux, wherein the heated refrigerant from the refrigerant heat exchanger (H-206) is successively fed back to the nth to the first heat exchanger and subsequently to the first compressor stage (K-211). Embodiment 7 is the refrigeration system of any of the preceding embodiments, wherein the refrigeration system further comprises a cooling system for liquefying the binary refrigerant (2561) from the nth interstage cooler (H-223) to provide the refrigerant (2501) to be fed to the first heat exchanger (H-201) as a liquid.

Embodiment 8 is the refrigeration system of embodiment 7, wherein the cooling system for liquefying the binary refrigerant (2561) from the nth interstage cooler (H-223) comprises a series of coolers (H-215, H-216, H-217) for cooling the binary refrigerant (2561) by a propylene refrigerant, a series of compressor stages (K-221,K-222,K-223) for recompressing vapour fractions of the propylene refrigerant used in the coolers and a condenser (H-223) for condensing the propylene refrigerant from the final compressor stage (K-223) to be used by the coolers. Embodiment 9 is the refrigeration system of any of the preceding embodiments, wherein the demethanizer (C-201) is operated at a pressure below 25 bara, for example below 20 bara, for example below 18 bara, for example below 15 bara. Embodiment 10 is the refrigeration system of any of the preceding embodiments, wherein the charge gas (2001) upon entering the first heat exchanger (H-201) has a pressure of at most 30 bara, for example at most 25 bara, for example at most 20 bara, for example at most 18 bara. Embodiment 11 is the refrigeration system of any of the preceding embodiments, wherein each of the interstage coolers (H-221, H-212, H-213, H-214) are cooled by cooling water. Embodiment 12 is the refrigeration system of any of the preceding embodiments, wherein each of the interstage coolers (H-221, H-212, H-213, H-214) are cooled by chilled water originating from an absorption chiller process.

Embodiment 13 is the refrigeration system of any of the preceding embodiments, wherein each of the interstage coolers (H-311A) is followed by a further cooler cooled by chilled water from an absorption chiller (H-311B). Embodiment 14 is the refrigeration system of any of embodiments 12 and 13, wherein the heat required by the absorption chiller is waste heat from a steam cracker process, such as hot quench water from a quench column. Embodiment 15 is a process for cooling a charge gas by a binary refrigerant by the refrigeration system of any of the preceding embodiments.

The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.”

Throughout this application, the term “about” is used to indicate that a value includes the standard deviation of error for the device or method being employed to determine the value.

The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and/or.”

As used in this specification and claim(s), the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

As used in this specification and claim(s), the phrase “successively fed back to the kth to the first heat exchangers” means that the stream is fed to the kth, (k−1)th, . . . , the second (2nd) and the first (1st) heat exchanger in this order to successively provide cooling to each of the heat exchangers.

As used herein, the term “C# hydrocarbons”, wherein “#” is a positive integer, is meant to describe all hydrocarbons having # carbon atoms. C# hydrocarbons are sometimes indicated as just “C#”. Moreover, the term “C#+ hydrocarbons” is meant to describe all hydrocarbon molecules having # or more carbon atoms.

Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

