Process for removing gaseous contaminants from a feed gas stream comprising methane and gaseous contaminants

Abstract

A process for removing gaseous contaminants from a feed gas stream that comprises a gaseous product and gaseous contaminants, comprising: providing the feed gas stream, cooling the feed gas stream to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product, separating the two phases by means of a gas/liquid separator, and introducing the gaseous phase rich in gaseous product into a cryogenic separation device that comprises a freezing zone and a distillation zone positioned below the freezing zone, and removing from the cryogenic separation device a bottom stream rich in liquid phase contaminant and lean in gaseous product, and a top stream rich in gaseous product and lean in gaseous contaminant. The invention further includes a device for carrying out the present process, the purified gas stream, and a process for liquefying a feed gas stream.

1 . A process for removing gaseous contaminants from a feed gas stream that comprises a gaseous product and gaseous contaminants, the process comprising:
 1) providing the feed gas stream;   2) cooling the feed gas stream to a temperature at which liquid phase contaminant is formed as well as a gaseous phase rich in gaseous product;   3) separating the two phases as obtained in step 2) by means of a gas/liquid separator; and   4) introducing the methane enriched gaseous phase as obtained in step 3) into a cryogenic separation device which comprises a freezing zone and a distillation zone which is positioned below the freezing zone; and   5) removing from the cryogenic separation section a bottom stream rich in liquid phase contaminant and lean in gaseous product, and a top stream rich in gaseous product and lean in gaseous contaminant.   
     
     
         2 . The process according  claim 1  wherein the gas/liquid separator in step 3) comprises a gas/liquid inlet at an intermediate level, a liquid outlet arranged below the gas/liquid inlet and a gas outlet arranged above the gas/liquid inlet, in which vessel a normally horizontal coalescer is present above the gas/liquid inlet and over the whole cross-section of the vessel and in which vessel a centrifugal liquid separator is arranged above the coalescer and over the whole cross-section of the vessel, the liquid separator comprising one or more swirl tubes. 
     
     
         3 . The process according to  claim 1  wherein the gas/liquid separator in step 3) comprises a centrifugal separator which comprises a bundle of parallel channels that are arranged within a spinning tube parallel to an axis of rotation of the spinning tube. 
     
     
         4 . The process according  claim 3  wherein the gas/liquid separator in step 3) comprises:
 a) a housing comprising a first, second and third separation section for separating liquid from the mixture, wherein the second separation section is arranged below the first separation section and above the third separation section, the respective separation sections are in communication with each other, and the second separation section comprises a rotating coalescer element;   b) tangentially arranged inlet means to introduce the mixture into the first separation section;   c) means to remove liquid from the first separation section;   d) means to remove liquid from the third separation section; and   e) means to remove a gaseous stream, lean in liquid, from the third separation section.   
     
     
         5 . The process according to  claim 1  wherein the gas/liquid separator in step 3) comprises a housing with a gas inlet for contaminated gas at one end of the vessel, a separating body, a gas outlet for purified gas at the opposite end of the housing and a contaminants outlet downstream of the separating body or upstream and downstream of the separating body, wherein the separating body comprises a plurality of ducts over a part of the length of the axis of the housing, which ducts have been arranged around a central axis of rotation, in which apparatus the separating body has been composed of a plurality of perforated discs wherein the perforations of the discs form the ducts. 
     
     
         6 . The process according to  claim 1  wherein the gaseous contaminants are carbon dioxide, and/or hydrogen sulphide, wherein the carbon dioxide, if present, is present in the range of from 1 to 90 vol %, based on the total feed gas stream, and wherein which the hydrogen sulphide, if present, is present stream in the range of from 0.1 to 60 vol % based on the total feed gas stream. 
     
     
         7 . The process according to  claim 1  wherein the feed gas stream is a natural gas which comprises between 20 and 80 vol % of methane. 
     
     
         8 . The process according to  claim 1  wherein the feed gas stream in step 1) has a temperature between −20 and 150° C. and a pressure between 10 and 150 bara. 
     
     
         9 . The process according to  claim 1  wherein the cooling in step 2) is performed using at least one technique selected from the group consisting of: expansion over an orifice or a valve; an expander; a turbo expander; and a laval nozzle, and wherein the feed gas stream is pre-cooled to a temperature between 15 and −35° C. 
     
     
         10 . The process according to  claim 1  wherein the feed gas stream is cooled in step 2) to a temperature between −30 and −80° C. 
     
     
         11 . The process according to  claim 1  wherein the freezing zone in step 4) is designed to control the formation and melting of solid contaminant and to prevent the introduction of solid contaminant into the distillation zone. 
     
     
         12 . The process according to  claim 1  wherein step 4) is carried out as follows:
 a) the gaseous phase rich in gaseous product is cooled to a temperature above the freeze out temperature of any contaminant present in the feed gas stream to obtain a cooled gaseous phase rich in gaseous product,   b) the cooled gaseous phase rich in gaseous product as obtained in step a) is introduced into the distillation zone of the cryogenic separation device;   c) gaseous phase rich in gaseous product inside the top section of the distillation is introduced into the freezing zone;   d) the gaseous phase rich in gaseous product that is introduced in the freezing zone is contacted in the freezing zone with a stream of cold liquid at a temperature lower than the freeze out temperature of any contaminant present in the gaseous phase rich in product, for forming solid contaminant and a further gaseous product enriched gaseous phase;   e) the solid contaminant obtained in step d) is melted and a stream of melted solid contaminant is introduced into the distillation zone; and   at least part of the further gaseous product enriched gaseous phase obtained in step d) is condensed to form liquid phase.   
     
     
         13 . The process according to  claim 1  wherein the gaseous phase rich in gaseous product obtained in step 3) is cooled in a cooling step to a temperature at which at least part of the gaseous phase rich in gaseous product is condensed, and the fluid so obtained is introduced into the cryogenic separation device in step 4). 
     
     
         14 . The process according to  claim 1  wherein the gaseous phase rich in gaseous product obtained in step 3) is recompressed in one or more compression steps and the recompressed gaseous phase rich in gaseous product so obtained is cooled by means of expansion to a temperature above the freeze out temperature of any contaminant present in the feed gas stream before it is introduced into the cryogenic separation device in step 4), wherein the cooling between steps 3) and 4) is at least partly be done by means of an external refrigerant. 
     
     
         15 . The process according to  claim 14  wherein the external refrigerant has a higher molecular weight than the gaseous phase rich in gaseous product to be cooled. 
     
     
         16 . The process according to  claim 14  wherein the external refrigerant comprises a propane cycle, an ethane/propane mixed refrigerant or an ethane/propane cascade.