Method and system to prevent carry-over of hydrocarbon mist from an ngl column of an lng plant

Abstract

The present disclosure is directed to a method and system for preventing carry-over of C 2+ hydrocarbon mist from an NGL recovery column in an LNG plant. A wash loop of recirculating liquid hydrocarbon is provided at the upper portion of the NGL column. A sidestream of liquid hydrocarbons collected within the column is removed from the column and pumped to sufficient height to be returned to the column at a temperature substantially equivalent to the temperature of liquid with the upper portion of the column. In one embodiment, liquid hydrocarbons of a desired composition can be loaded and reloaded to a working medium holding drum in fluid communication with the top of the column. The working medium can be selected to be sufficiently heavy not to be lost from the column and to have a suitable freezing point to avoid freezing within the column.

1 . A method for reducing hydrocarbon mist from escaping the top of an NGL recovery column in an LNG plant, the method comprising:
 a. flowing a hydrocarbon gas stream including a hydrocarbon mist upwardly in an NGL column;   b. spraying droplets of a hydrocarbon liquid into the gas stream to strip at least some of the hydrocarbon mist from the upwardly flowing gas stream;   c. collecting the droplets of the hydrocarbon liquid in a tray within the column;   d. removing a liquid sidestream of the collected hydrocarbon liquid from the column; and   e. returning at least a portion of the liquid sidestream to form the droplets which are sprayed in step b.   
     
     
         2 . The method of  claim 1 , wherein the at least a portion of the liquid sidestream does not change phase when returned to form the droplets. 
     
     
         3 . The method of  claim 1 , further comprising cooling the at least a portion of the liquid sidestream to a temperature substantially equivalent to the temperature within an upper portion of the column prior to returning the at least a portion of the liquid sidestream to the column. 
     
     
         4 . The method of  claim 1 , further comprising supplying the liquid sidestream to a working medium holding drum in which the liquid sidestream is separated into a methane depleted sidestream and a gas stream including methane. 
     
     
         5 . The method of  claim 4 , wherein the liquid sidestream is heated prior to supplying the liquid sidestream to the working medium holding drum. 
     
     
         6 . The method of  claim 4 , further comprising adding a makeup hydrocarbon liquid to at least one of the liquid sidestream, the methane depleted sidestream or the working medium holding drum so that the methane depleted sidestream is of a predetermined composition. 
     
     
         7 . The method of  claim 2 , wherein the at least a portion of the liquid sidestream is cooled such that the droplets that are sprayed are at a temperature of between about −50° C. and −110° C. 
     
     
         8 . The method of  claim 6 , wherein the makeup hydrocarbon liquid is added to the working medium holding drum as needed to maintain sufficient liquid for spraying droplets in the column. 
     
     
         9 . The method of  claim 5 , further comprising cooling the at least a portion of the liquid sidestream to a temperature substantially equivalent to the temperature within an upper portion of the column prior to feeding the at least a portion of the liquid sidestream to the column, wherein the cooling of the at least a portion of the liquid sidestream is conducted in cross exchange with the heating of the liquid sidestream prior to supplying the liquid sidestream to the working medium holding drum. 
     
     
         10 . The method of  claim 9 , further comprising subjecting the at least a portion of the liquid sidestream to further cooling prior to being returned to the column. 
     
     
         11 . The method of  claim 6 , wherein the makeup hydrocarbon liquid comprises C 2-10  hydrocarbons having a freezing temperature between about −182.8° C. and about −29.7° C. 
     
     
         12 . A system for preventing hydrocarbon mist from escaping the top of an NGL recovery column in an LNG plant, comprising:
 a. a column for recovering NGL, the column comprising:
 i. a natural gas inlet for feeding natural gas to the column; 
 ii. an NGL outlet at a lower portion of the column for dispensing natural gas liquids from the column; 
 iii. a gas outlet at an upper portion of the column for removing gaseous components from the column; 
 iv. at least one tray for collecting liquids within the column; 
 v. a spray mechanism for generating a spray of droplets which are collected as liquids in the at least one tray; 
 vi. a liquid inlet for receiving and feeding liquid to the spray mechanism; 
 vii. a liquid outlet for receiving liquid from the at least one tray; and 
   b. a loop for fluidly connecting the liquid outlet with the liquid inlet to transport a sidestream of liquid, the loop including a pump in fluid communication with the liquid outlet for receiving and pumping the at least a portion of the sidestream to the liquid inlet.   
     
     
         13 . The system of  claim 12 , wherein the loop comprises:
 a heat exchanger in fluid communication with the pump for cooling the at least a portion of the sidestream.   
     
     
         14 . The system of  claim 12 , further comprising:
 a working medium holding drum in fluid communication receiving the at least a portion of the sidestream which separates the at least a portion of the sidestream into a methane depleted sidestream and a gas stream including methane, the methane depleted sidestream being pumped by the pump to the liquids inlet.   
     
     
         15 . The system of  claim 12 , further comprising:
 a. a controller for controlling the flow rate of the at least a portion of the sidestream;   b. a valve in communication with the controller for increasing or decreasing the flow rate of the at least a portion of the sidestream;   c. a flow meter in communication with the controller for measuring the flow rate of the at least a portion of the sidestream; and   d. a liquid level detector in communication with the controller for measuring the liquid level in the tray.