Patent Publication Number: US-2019184332-A1

Title: Helium purity adjustment in a membrane system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application No. 62/599,571 filed Dec. 15, 2017, the contents of which cited application are hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a membrane system for recovering helium from a gas stream. More particularly, the invention relates to recovering and producing helium from natural gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a two-stage membrane system for removal and recovery of helium from a gas stream. 
         FIG. 2  shows a two-stage membrane system in which one permeate stream is sent to a second membrane block while a second permeate stream bypasses the second membrane block. 
     
    
    
     DETAILED DESCRIPTION 
     A two stage membrane system is shown as one of the possible configurations of a multistage membrane system. A gas feed  5  (such as natural gas) is sent to a first-stage membrane block  10  consisting of several membrane elements. A permeate stream  16  is sent to compressor  18  with compressed gas  20  sent to second-stage membrane block  22 . Second stage permeate  26  has a high level of helium. A residue stream  12 , also referred to as export gas, contains a low level of helium. Recycle stream  14  is in a typical two stage membrane scheme with recycle, also referred to as a closed loop configuration. 
     In relation to the present invention it is important to understand two variations that are also shown in  FIG. 1 . The first variation is using stream  25  instead of, or as a combination with stream  14 . A membrane system with a stream  25  is referred to as an open loop configuration. There are two scenarios. First, if the target component concentration in stream  25  is higher than the requirement in the residue stream  12 , then the first membrane block  10  will need to remove more of the target component. However, if the target component concentration in stream  25  is equal to the requirement in the residue stream  12 , then the first membrane block will also target the same export gas requirement. 
     A second variation is introducing a second permeate stream from the 1 st  stage membrane block  10 . As shown with stream  24 . This is called a pre-membrane configuration. The concentration and pressure level of stream  24  can be different to the concentration and the pressure level of stream  16  that is going to the compressor unit  18 . This scheme is mainly used in CO2 and H2S removal applications when one of the following conditions apply. A first condition is that the duty of compressing all of the CO 2  rich permeate gas  16  coming from membrane block  10  is too high and is not beneficial in overall NPV (compression duty vs hydrocarbon recovery). A second condition can be the situation where the second stage permeate gas  27  is flared or incinerated (not reinjected or vented) and hence requires a minimum amount of hydrocarbons to be burned without the need for using assist fuel gas. 
     Variations can be membrane systems that have features of “two-step”, “three stage”, combinations with downstream separation technologies (Pressure Swing adsorption (PSA), . . . ) with recycle streams from the downstream separation technologies integrated back in the membrane system. 
     Application: natural gas treating for the removal of components like CO 2 , H 2 S, water, He, H 2 , . . . . Other applications can include the separation of CO 2  from ethane streams. 
     The invention is a variation to the two-stage membrane system described above. 
     The invention involves the recovery of helium, the flow adjustment vs. the traditional premembrane configuration and a purity adjustment vs. the prior art traditional configuration. 
     Unlike acid gas removal applications of carbon dioxide, hydrogen sulfide and other gases, the focus in helium removal/recovery applications is two-fold. Both the natural gas stream (1 st  stage membrane residue stream  12 ) and the concentrated helium stream (2 nd  stage membrane permeate stream  32 ) are important to the customer 
     In a traditional premembrane configuration, the flow rate of stream  30  is fixed by the choice of number of premembrane elements in block  10  (typically 1 or 2 membrane elements per tube, sometimes more). As such, the operator has few options to control flow  16  to compressor  18 . Once the number of premembrane elements in membrane block  10  is fixed and other degrees of freedom are selected (membrane operating temperature, membrane permeate pressure), the flow of streams  16  and  30  are set. 
       FIG. 2  shows the differences from the prior art of  FIG. 1 . A first difference from the prior art is the introduction of a flow control device  52  that will allow to fine tune the operation and match the optimal membrane operation with the optimal compressor  18  operation by controlling the flow  50 . This is important since membrane properties are not always easy to predict and tend to change over time. 
     The membrane can be operated with the same pressure for streams  16  and  30  or with different pressures for streams  16  and  30 . In a traditional two stage system with premembrane configuration, the composition of stream  32  is fixed once the parameters (degrees of freedom) in the upstream system have been set (operating temperature and pressure in membrane blocks  10  and  22 , the number of premembrane elements in membrane block  10 ). As such the operator has few options left to control the composition of stream  32  (in this specific application, the composition refers to the helium purity in stream  32  which is feeding the downstream reinjection compressor  36  or other purification units through stream  40 . The downstream compression  36  or other purification units  40  may have specific stringent requirements on the helium purity to achieve their performance or avoid operating problems (like condensation during reinjection compression). 
     A second important feature of the present invention is the introduction of a composition control device  60  that will measure the composition in stream  31  or  32  and control valve  50 . This allows the operator to fine tune the operation and match optimal membrane operation with the purity requirements for the permeate  32  which has a high helium concentration. Stream  32  is shown as either proceeding in stream  34  to compressor  36  to possibly being reinjected as stream  38  or sent in stream  40  for further purification such as by pressure swing adsorption or cryogenic treatment. 
     There are other variations in the operation of the process of this invention.  FIG. 2  is shown as an open loop configuration but the principles of the invention can be extended when a closed loop configuration is selected.  FIG. 2  is shown as a two-stage membrane system but the ideas can be extended when a single or multistage system is selected. The two-stage membrane system may be integrated with downstream purification steps. 
     SPECIFIC EMBODIMENTS 
     While the following is described in conjunction with specific embodiments, it will be understood that this description is intended to illustrate and not limit the scope of the preceding description and the appended claims. 
     A first embodiment of the invention is a process of treating a gas stream comprising sending the gas stream through a first membrane module to produce a first and a second permeate streams comprising a higher level of helium than the gas stream and a first residue stream comprising a low level of helium wherein the ratio size of the first permeate stream to the second permeate stream is controlled according by predetermined factors; sending at the first permeate streams to a compressor to produce a compressed permeate stream and sending the second permeate stream to be a helium product stream; sending the compressed permeate stream to a second membrane module to produce a third permeate stream and a second residue stream; and combining the third permeate stream and the helium product stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the predetermined factor is the capacity of the compressor. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the predetermined factor is the desired helium concentration in the combined third permeate stream and the helium product stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the gas stream is sent through a third membrane module. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the residue stream has a helium level that is about one tenth of the helium level of the gas stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the gas stream is natural gas. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the process is at a temperature of about 40° C. 
     Without further elaboration, it is believed that using the preceding description that one skilled in the art can utilize the present invention to its fullest extent and easily ascertain the essential characteristics of this invention, without departing from the spirit and scope thereof, to make various changes and modifications of the invention and to adapt it to various usages and conditions. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limiting the remainder of the disclosure in any way whatsoever, and that it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims. 
     In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.