Process for the removal of acidic contaminants from a natural gas stream

Abstract

Acidic contaminants are removed from a natural gas stream comprising hydrocarbons and acidic contaminants in a process comprising the steps of: (a) expanding the natural gas stream, thereby cooling the natural gas stream and allowing at least part of the acidic contaminants to liquefy with the proviso that no solid acidic contaminants are formed, thereby obtaining a cooled natural gas stream that contains liquefied acidic contaminants; (b) separating at least part of the liquefied acidic contaminants from the cooled natural gas stream in a separator comprising centrifugal separation means, to obtain a contaminants-depleted natural gas stream; and (c) treating the contaminants-depleted natural gas stream with a liquid physical solvent to obtain a sweet natural gas stream and a liquid solution of acidic contaminants.

1 . A process for the removal of acidic contaminants from a natural gas stream comprising hydrocarbons and acidic contaminants, which process comprises:
 (a) expanding the natural gas stream, thereby cooling the natural gas stream and allowing at least part of the acidic contaminants to liquefy such that no solid acidic contaminants are formed, thereby obtaining a cooled natural gas stream that contains liquefied acidic contaminants;   (b) separating at least part of the liquefied acidic contaminants from the cooled natural gas stream in a separator comprising centrifugal separation means, to obtain a contaminants-depleted natural gas stream; and   (c) treating the contaminants-depleted natural gas stream with a liquid physical solvent to obtain a sweet natural gas stream and a liquid solution of acidic contaminants.   
     
     
         2 . Process as claimed in  claim 1 , in which the natural gas stream is expanded from a pressure ranging from 70 to 130 bar to a pressure ranging from 5 to 30 bar. 
     
     
         3 . Process as claimed in  claim 1 , in which the natural gas stream is cooled by expansion from a temperature ranging from −20 to 50 C to a temperature ranging from −40 to −70 C. 
     
     
         4 . Process as claimed in  claim 1 , wherein the natural gas stream is expanded using at least two expansion devices and the operating parameters of the expansion devices are chosen such that the liquefied acidic contaminants have a certain droplet size distribution. 
     
     
         5 . Process as claimed in  claim 1 , in which the separator comprises two trays between which open-ended swirl tubes extend, each from an opening in one tray to some distance below the a coaxial opening in the other tray, each swirl tube having been provided with swirl means to impart a rotary movement to gas entering the swirl tube. 
     
     
         6 . Process as claimed in  claim 5 , in which the separator further comprises a coalescer, upstream and/or downstream of the swirl-tubes. 
     
     
         7 . Process as claimed in  claim 5 , in which the separator comprises an assembly consisting of a box-like structure extending parallel to the trays having at least one open side with a grid of vanes disposed one behind the other, which assembly has been arranged upstream of the trays. 
     
     
         8 . Process as claimed in  claim 1 , in which the natural gas stream is pre-cooled by means of heat exchange. 
     
     
         9 . Process as claimed in  claim 1 , in which the separator comprises a housing with a gas inlet for the cooled natural gas stream at one end of the housing, 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, wherein the separating body comprises a plurality of channels over a part of the length of the axis of the housing, which ducts have been arranged around a central axis of rotation. 
     
     
         10 . Process as claimed in  claim 9 , in which the gas inlet is tangential and in which the separator is preferably provided with an additional liquid outlet upstream of the separating body. 
     
     
         11 . Process as claimed in  claim 1 , wherein in step (b) a separator is used comprising:
 1) 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;   2) tangentially arranged inlet means to introduce the mixture into the first separation section;   3) means to remove liquid from the first separation section;   4) means to remove liquid from the third separation section; and   5) means to remove a gaseous stream, lean in liquid, from the third separation section.   
     
     
         12 . Process as claimed in  claim 1 , in which the contaminants-depleted natural gas stream is compressed, to a pressure ranging from 20 to 120 bar, before being treated with the liquid physical solvent. 
     
     
         13 . Process as claimed in  claim 1 , in which the liquid physical solvent is selected from the group consisting of tetramethylene sulphone (sulpholane) and derivatives, dimethyl sulphoxide, amides of aliphatic carboxylic acids, N-alkyl pyrrolidone, in particular N-methyl pyrrolidone, N-alkyl piperidones, N-methyl piperidone, methanol, ethanol, ethylene glycol, polyethylene glycols, mono- or di(C 1 -C 4 )alkyl ethers of ethylene glycol or polyethylene glycols, having a molecular weight from 50 to 800, and mixtures thereof.