Abstract:
Disclosed herein is a methanol production process that includes at least two membrane separation steps. Using the process of the invention, the efficiency of methanol production from syngas is increased by reducing the compression requirements of the process and/or improving the methanol product yield. As an additional advantage, the first membrane separation step generates a hydrogen-rich stream which can be sent for other uses. An additional benefit is that the process of the invention may debottleneck existing methanol plants if more syngas or carbon dioxide is available, allowing for feed of imported carbon dioxide into the synthesis loop. This is a way of sequestering carbon dioxide.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. application Ser. No. 13/175,399, filed Jul. 15, 2011, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a methanol production process that includes at least two membrane separation steps, using a hydrogen-selective membrane followed by a carbon dioxide-selective membrane, to improve the efficiency of methanol production from natural gas. Hydrogen recovered during the membrane separation step can be sent for other uses. The process of the invention may debottleneck existing methanol plants, allowing for feed of recycled carbon dioxide into the synthesis loop, resulting in sequestration of the carbon dioxide and production of additional methanol. 
     BACKGROUND OF THE INVENTION 
     Methanol, the simplest alcohol, with a chemical formula of CH 3 OH, is a light, volatile, colorless, flammable liquid. A polar liquid at room temperature, methanol finds use as an antifreeze, solvent, fuel, and as a denaturant for ethanol. It is also used for producing biodiesel via a transesterification reaction. 
     The largest use of methanol, however, is in the manufacture of other chemicals. About forty percent of methanol is converted to formaldehyde, and from there into products as diverse as plastics, plywood, paints, explosives, and permanent-press textiles. 
     Methanol is also used on a limited basis as fuel for internal combustion engines. The use of methanol as a motor fuel received attention during the oil crises of the 1970&#39;s due to its availability, low cost, and environmental benefits. However, due to the rising cost of methanol and its corrosivity to rubber and many synthetic polymers used in the auto industry, by the late 1990s automakers had stopped building vehicles capable of operating on either methanol or gasoline (“flexible fuel vehicles”), switching their attention instead to ethanol-fueled vehicles. Even so, pure methanol is required as fuel by various auto, truck, and motorcycle racing organizations. 
     In 1923, German chemists Alwin Mittasch and Mathias Pier, working for BASF, developed a process for converting synthesis gas (a mixture of carbon monoxide, carbon dioxide, and hydrogen) into methanol. The process used a chromium and magnesium oxide catalyst and required extremely vigorous conditions—pressures ranging from 50 to 220 bar, and temperatures up to 450° C. A patent (U.S. Pat. No. 1,569,775) covering this process was issued on Jan. 12, 1926. 
     Modern methanol production has been made more efficient through the use of catalysts (typically copper) capable of operating at lower pressures. The modern low-pressure methanol (LPM) production process was developed by ICI in the late 1960&#39;s, with the technology now owned by Johnson Matthey (London), a leading licensor of methanol technology. 
     The production of synthesis gas (“syngas”) via steam reforming of natural gas is the first step in many processes for methanol production. At low to moderate pressures and at high temperatures around 850° C., methane reacts with steam on a nickel catalyst to produce syngas according to the following reactions:
         CH 4 +H 2 O→CO+3H 2      CO+H 2 O→CO 2 +H 2  
 
This process, commonly referred to as “steam methane reforming” (SMR) is highly endothermic, and maintaining reaction temperature by external heating is a critical part of the process.
       

     The syngas is then compressed and reacted on a second catalyst to produce methanol. Today, the most commonly used catalyst is a mixture of copper, zinc oxide, and alumina first used by ICI in 1966. At 50-100 bar and 250° C., it can catalyze the production of methanol from syngas with high selectivity:
         CO+2H 2 →CH 3 OH   CO 2 +3H 2 →CH 3 OH+H 2 O       

     The production of syngas from methane produces 3 moles of hydrogen gas for every mole of carbon monoxide (and 4 moles of hydrogen per mole of carbon dioxide), while the methanol synthesis reaction consumes only 2 moles of hydrogen gas per mole of carbon monoxide (and 3 moles of hydrogen gas per mole of carbon dioxide). In both reaction pathways, one more mole of hydrogen is generated than is needed for methanol synthesis. This excess hydrogen occupies capacity in both the compressor train and the methanol reactor. As a result, the methanol production process is inefficient, resulting in unnecessary costs due to increased compressor power requirements and less than optimum methanol yields. Reactants are lost when excess hydrogen is purged from the synthesis loop and used as fuel for the reformer. 
       FIG. 1  is a schematic showing a conventional process for methanol production. Feed streams of natural gas  101  and steam  102  are fed into reformer  103 , resulting in the production of syngas stream  104 . Syngas stream  104  is then passed to compression chain  105  (typically comprising at least make-up compressor  105   a  and recycle compressor  105   b ) to produce high-pressure gas stream  106 . High-pressure stream  106  is then passed to methanol synthesis reactor  107  to produce reaction product stream  108 , containing methanol and unreacted syngas. This stream  108  is then routed to condenser  109 , from which condensed stream  110 , containing methanol and water, drops out. Overhead stream  111 , containing unreacted syngas and enriched in hydrogen and inerts (methane and possibly nitrogen), is then split into purge stream  112  and recycle stream  113 , which is routed back to the recycle compressor  105   b , where it is combined with fresh feed. 
     It would be desirable to provide an improved methanol production process that is more efficient, with reduced compressor power requirements and/or improved methanol product yield. 
     SUMMARY OF THE INVENTION 
     In our earlier application, U.S. Ser. No. 13/175,399, filed Jul. 15, 2011, which has been allowed, we disclosed processes for the production of methanol from syngas which removed excess hydrogen from the syngas before it reaches the methanol synthesis loop. 
     We have since discovered an even more efficient process, in which excess hydrogen is removed after the methanol synthesis loop, and carbon dioxide is recycled back to the synthesis loop. 
     Accordingly, disclosed herein is a methanol production process including the following steps: 
     (a) providing a source of syngas, wherein the syngas has a first composition parameter R 1 , where R 1 &gt;2; 
     (b) passing the syngas to a methanol synthesis loop to produce a condensed methanol product stream; 
     (c) withdrawing a purge stream from the methanol synthesis loop to limit the concentration of inerts and excess hydrogen; 
     (d) providing a first membrane having a first feed side and a first permeate side, where the first membrane exhibits a selectivity to hydrogen over carbon dioxide of at least about 5, and a selectivity to hydrogen over carbon monoxide of at least about 20; 
     (e) passing at least a portion of the purge stream across the first feed side; 
     (f) withdrawing from the first permeate side a hydrogen-rich first permeate stream, wherein the first permeate stream has a second composition parameter R 2 , where R 2 &lt;R 1 ; 
     (g) withdrawing from the first feed side a hydrogen-depleted first residue stream; 
     (h) providing a second membrane having a second feed side and a second permeate side, where the second membrane is selective for carbon dioxide over hydrogen and methane; 
     (i) passing the first residue stream across the second feed side; 
     (j) withdrawing from the second feed side a carbon dioxide-depleted second residue stream; 
     (k) withdrawing from the second permeate side a carbon dioxide-enriched second permeate stream, wherein the second permeate stream has a third composition parameter R 3 , where R 3 &lt;R 2 ; and 
     (l) passing the second permeate stream to the methanol synthesis loop. 
     Membranes for use in the first membrane separation step (d) preferably exhibit a selectivity to hydrogen over carbon dioxide of at least about 5 and, more preferably, at least about 10, and to hydrogen over carbon monoxide of at least about 20. Hydrogen permeance of the first membrane is typically at least 100 gpu and, preferably, at least 200 gpu. 
     Preferred first membrane materials include polymers, such as polyimides, polyamides, polyurethanes, polyureas, polybenzimidazoles, and polybenzoxazoles; metals, such as palladium; zeolites; and carbon, by way of example and not by way of limitation. 
     First membrane operating temperature is typically within the range of about 50° C. to about 150° C.; preferably, within the range of about 100° C. to about 150° C. The feed side of the first membrane is typically maintained at a pressure within the range of about 45 bar to about 100 bar, with the permeate side typically maintained at a pressure within the range of about 2 bar to about 10 bar. 
     Any membrane that exhibits a selectivity to carbon dioxide over hydrogen of at least about 5, and over methane of at least about 10, may be used in the second membrane separation step (h). Carbon dioxide permeance of the second membrane is typically at least 200 gpu and, preferably, at least 400 gpu. 
     Any membrane with suitable performance properties may be used in second membrane separation step (h). Many polymeric materials, especially elastomeric materials, are very permeable to carbon dioxide. Preferred membranes for separating carbon dioxide from other gases often have a selective layer based on a polyether. 
     Second membrane operating temperature is typically within the range of about 0° C. to about 80° C.; preferably, within the range of about 20° C. to about 60° C. The feed side of the second membrane is typically maintained at a pressure within the range of about 45 bar to about 100 bar, with the permeate side typically maintained at a pressure within the range of about 10 bar to about 30 bar. 
     By practicing the process of the invention, existing methanol plants may be made more efficient by recovering carbon dioxide from the purge gas and recycling it back to the synthesis loop. This results in additional methanol production, and is also a way of sequestering carbon dioxide, thereby preventing its release to the environment. In addition, the process of the invention generates a hydrogen-rich stream from the first membrane separation step. This hydrogen-rich stream can be used for other purposes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of a conventional methanol production process (not in accordance with the invention). 
         FIG. 2  is a schematic drawing of a basic embodiment process of the invention for methanol production that involves two membrane separation steps to treat a purge stream from the methanol production process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The terms “natural gas” and “methane” are used interchangeably herein. 
     Gas percentages given herein are by volume unless stated otherwise. 
     Pressures as given herein are in bar absolute unless stated otherwise. 
     For any gas stream herein, the composition may be expressed in terms of a composition parameter, R, where: 
               R   =       (       molar   ⁢           ⁢   flow   ⁢           ⁢   of   ⁢           ⁢     H   2       -     molar   ⁢           ⁢   flow   ⁢           ⁢   of   ⁢           ⁢     CO   2         )       (       molar   ⁢           ⁢   flow   ⁢           ⁢   of   ⁢           ⁢   CO     +     molar   ⁢           ⁢   flow   ⁢           ⁢   of   ⁢           ⁢     CO   2         )         ,         
Specific composition parameters are referred to herein as R 1 , R 2 , and R 3 .
 
     A schematic drawing of a basic embodiment process of the invention for methanol production is shown in  FIG. 2 . It will be appreciated by those of skill in the art that this, like  FIG. 1 , is a very simple block diagram, intended to make clear the key unit operations of the process of the invention, and that an actual process train will usually include many additional steps of a standard type, such as heating, chilling, compressing, condensing, pumping, various types of separation and/or fractionation, as well as monitoring of pressures, temperatures, flows, and the like. It will also be appreciated by those of skill in the art that the details of the unit operations may differ from product to product. 
     Referring to the figure, feed streams of natural gas,  201 , and steam,  202 , are fed into, for example, a steam reformer,  203 , resulting in the production of syngas,  204 . Although  FIG. 2  illustrates an example in which syngas is produced using a steam methane reforming process, any source of syngas can be used to provide syngas for use in the process of the invention. 
     The invention is particularly designed for syngas sources having an excess of hydrogen for methanol production. Expressed quantitatively, the invention is particularly directed to the manufacture of methanol from syngas having a composition parameter, R 1 , that is greater than 2; that is, R=R 1 &gt;2. 
     Syngas stream  204  is then passed to a compression chain,  205  (typically comprising at least a make-up compressor,  205   a , and a recycle compressor,  205   b ), to produce a high-pressure gas stream,  206 . High-pressure stream  206  is then passed to a methanol synthesis reactor,  207 , to produce a reaction product stream,  208 , containing methanol and unreacted syngas. 
     Methanol synthesis reactors are known in the art and typically rely on a catalyst bed to catalyze the reaction of carbon oxides and hydrogen to produce methanol. As discussed in the Background of the Invention, the most common catalyst in use today is a mixture of copper, zinc oxide, and alumina first used by ICI in 1966. At 50-100 bar and 250° C., it can catalyze the production of methanol from carbon oxides and hydrogen with high selectivity. 
     Reaction product stream  208  is then routed to a condenser,  209 , from which a condensed stream,  210 , containing methanol and water, drops out. An overhead stream,  211 , containing unreacted syngas and enriched in hydrogen and inerts (methane and possibly nitrogen), is then split into a purge stream,  212 , and a recycle stream,  213 , which is routed back to the recycle compressor  205   b , where it is combined with fresh feed. 
     In accordance with the present invention, at least a portion of purge stream  212  is then passed as a feed stream to a first membrane unit,  214 , that includes membranes,  215 , that exhibit a selectivity to hydrogen over carbon dioxide of at least about 5; preferably, at least about 10; more preferably, at least about 15. In addition, the membranes  215  should exhibit a selectivity for hydrogen over carbon monoxide of at least about 20. Hydrogen permeance of the first membrane is typically at least 100 gpu and, preferably, at least 200 gpu. 
     Any membrane with suitable performance properties may be used in the first membrane separation step. Examples of such membranes include the polybenzimidazole (PBI) based membranes taught by K. O&#39;Brien et al. in “Fabrication and Scale-Up of PBI-based Membrane System for Pre-Combustion Capture of Carbon Dioxide” (DOE NETL Project Fact Sheet 2009) and polyimide-based membranes taught by B. T. Low et al. in “Simultaneous Occurrence of Chemical Grafting, Cross-linking, and Etching on the Surface of Polyimide Membranes and Their Impact on H 2 /CO 2  Separation” ( Macromolecules , Vol. 41, No. 4, pp. 1297-1309, 2008). 
     Preferred first membrane materials include polymers, such as polyimides, polyamides, polyurethanes, polyureas, polybenzimidazoles, and polybenzoxazoles; metals, such as palladium; zeolites; and carbon, by way of example and not by way of limitation. 
     The membrane may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art. 
     The membranes may be manufactured as flat sheets or as fibers and housed in any convenient module form, including spiral-wound modules, plate-and-frame modules, and potted hollow-fiber modules. The making of all these types of membranes and modules is well-known in the art. 
     Flat-sheet membranes in spiral-wound modules is the most preferred choice for the membrane/module configuration. A number of designs that enable spiral-wound modules to be used in counterflow mode, with or without sweep on the permeate side, have been devised. A representative example is described in U.S. Pat. No. 5,034,126, to Dow Chemical. 
     Membrane unit  214  may contain a single membrane module or bank of membrane modules or an array of modules. A single unit or stage containing one or a bank of membrane modules is adequate for many applications. If the residue stream requires further hydrogen removal, it may be passed to a second bank of membrane modules for a second processing step. If the permeate stream requires further concentration, it may be passed to a second bank of membrane modules for a second-stage treatment. Such multi-stage or multi-step processes, and variants thereof, will be familiar to those of skill in the art, who will appreciate that the membrane separation step may be configured in many possible ways, including single-stage, multistage, multistep, or more complicated arrays of two or more units, in serial or cascade arrangements. 
     The first membrane operating temperature is typically within the range of about 50° C. to about 150° C.; preferably, within the range of about 100° C. to about 150° C. The feed side of the first membrane is typically maintained at a pressure within the range of about 45 bar to about 100 bar, with the permeate side typically maintained at a pressure within the range of about 2 bar to about 10 bar. 
     Referring back to  FIG. 2 , purge stream  212  is passed across the feed side of the membranes  215 . A permeate stream,  216 , is withdrawn from the permeate side. Permeate stream  216  is enriched in hydrogen as compared with purge stream  212 , and has a composition parameter R 2 , where R 2 &gt;R 1 . 
     Hydrogen-rich stream  216  can be used for whatever purpose is desired. It may, for example, be used as reformer fuel gas, or used as a source of hydrogen for another process, such as ammonia production. 
     A hydrogen-depleted first residue stream,  217 , is withdrawn from the feed side of first membrane unit  214 . First residue stream  217  is then routed to a second membrane separation unit,  218 . Second membrane separation unit  218  includes membranes,  219 , that are selective for carbon dioxide over hydrogen, methane, and nitrogen. 
     In particular, the membranes in second unit  218  typically have a selectivity for carbon dioxide over hydrogen of at least about 5; over methane of at least about 10; and, over nitrogen of at least about 20. Carbon dioxide permeance of the second membrane is typically at least 200 gpu and, preferably, at least 400 gpu. 
     Any membrane with suitable performance properties may be used in the second membrane separation step. Many polymeric materials, especially elastomeric materials, are very permeable to carbon dioxide. Such polymeric materials are described, for example, in two publications by Lin et al., “Materials selection guidelines for membranes that remove CO 2  from gas mixtures” ( J. Mol. Struct.,  739, 57-75, 2005) and “Plastization-Enhanced Hydrogen Purification Using Polymeric Membranes” ( Science,  311, 639-642, 2006). 
     Preferred membranes for separating carbon dioxide from other gases often have a selective layer based on a polyether. Not many membranes are known to have high carbon dioxide/hydrogen selectivity. A representative preferred material for the selective layer is Pebax®, a polyamide-polyether block copolymer material described in detail in U.S. Pat. No. 4,963,165. We have found that membranes using Pebax® as the selective polymer can maintain a selectivity of 9, 10, or greater under process conditions. 
     Membrane modules are as discussed above. 
     The second membrane operating temperature is typically within the range of about 0° C. to about 80° C.; preferably, within the range of about 20° C. to about 60° C. The feed side of the second membrane is typically maintained at a pressure within the range of about 45 bar to about 100 bar, with the permeate side typically maintained at a pressure within the range of about 10 bar to about 30 bar. 
     A carbon dioxide-enriched second permeate stream,  220 , is withdrawn from the permeate side of second membrane unit  218 . The carbon dioxide content in second permeate stream  220  has now been built up from about 1-3 vol % in the purge stream  212 , to about 7-30 vol % in permeate stream  220 . 
     Carbon dioxide-enriched second permeate stream  220  is then recycled back to the methanol synthesis loop upstream of compression chain  205 , where it joins syngas stream  204  as feed to the methanol synthesis loop. Second permeate stream  220  has a composition parameter R 3 , where R 3 &lt;R 2 . The addition of carbon dioxide-enriched second permeate stream  220  to the feed stream to the methanol synthesis loop results in additional methanol production. 
     A carbon dioxide-depleted second residue stream,  221 , is withdrawn from the membrane side of second membrane separation unit  218 . This stream can then be sent for use as fuel gas or for any other desired purpose. 
     The invention is now further described by the following examples, which are intended to be illustrative of the invention, but are not intended to limit the scope or underlying principles in any way. 
     EXAMPLES 
     Example 1 
     Conventional Methanol Production Process (not in Accordance with the Invention) 
     The computer calculations in the following Examples were performed using a modeling program, ChemCad 5.6 (ChemStations, Inc., Houston, Tex.) containing code developed by assignee&#39;s engineering group for applications specific to assignee&#39;s processes. 
     The calculation for this Example was performed using the flow scheme shown in  FIG. 1  and described in the Background of the Invention, above. This flow scheme does not include a membrane separation step upstream of the methanol synthesis process (not in accordance with the invention). Syngas flow was assumed to be 106 metric tons per hour (Mt/h). 
     The flow rates and chemical compositions of the streams in the methanol synthesis loop were calculated. The results of this calculation are shown in Table 1. 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Reactor 
                 Reactor 
                   
                 Overhead 
                   
                 Recycle 
               
               
                   
                 Syngas 
                 Feed Gas 
                 Output 
                 Condensate 
                 Stream 
                 Purge Gas 
                 Gas 
               
               
                 Parameter/Stream 
                 104 
                 106 
                 108 
                 110 
                 111 
                 112 
                 113 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Total Flow (Mt/h) 
                 106 
                 185 
                 185 
                 92.0 
                 93.4 
                 14.0 
                 79.4 
               
               
                 Temperature (° C.) 
                 150 
                 65 
                 280 
                 40 
                 40 
                 40 
                 40 
               
               
                 Pressure (bar) 
                 16.5 
                 103 
                 95 
                 90 
                 88 
                 88 
                 88 
               
             
          
           
               
                 Component (mol %) 
               
             
          
           
               
                 Hydrogen 
                 73.4 
                 79.2 
                 71.1 
                 0.24 
                 83.2 
                 83.2 
                 83.2 
               
               
                 Carbon monoxide 
                 14.9 
                 6.6 
                 0.80 
                 0.01 
                 0.93 
                 0.93 
                 0.93 
               
               
                 Carbon dioxide 
                 7.8 
                 3.7 
                 0.91 
                 0.43 
                 0.99 
                 0.99 
                 0.99 
               
               
                 Methane 
                 3.7 
                 9.7 
                 11.8 
                 0.45 
                 13.7 
                 13.7 
                 13.7 
               
               
                 Nitrogen 
                 0.20 
                 0.54 
                 0.65 
                 0 
                 0.76 
                 0.76 
                 0.76 
               
               
                 Methanol 
                 0 
                 0.23 
                 11.1 
                 74.0 
                 0.39 
                 0.39 
                 0.39 
               
               
                 Water 
                 0 
                 0.04 
                 3.7 
                 24.9 
                 0.06 
                 0.06 
                 0.0 
               
               
                   
               
             
          
         
       
     
     In this “no membrane” example (not in accordance with the invention), approximately 96% of the carbon oxides in the syngas are converted to methanol. Most of the balance, approximately 3% of the carbon oxides in the feed syngas, is lost in the purge gas. The make-up compressor compresses 24,000 Ibmol/h, with a power consumption of 29,000 hp. The recycle compressor compresses 50,000 Ibmol/h, with a power consumption of 5,400 hp. 
     Example 2 
     Methanol Production Process in Accordance with the Invention 
     The calculation for this Example was performed using the flow scheme shown in  FIG. 2  and described in the Detailed Description, above. This flow scheme includes two membrane separation steps downstream of the methanol synthesis loop. 
     The membranes,  215 , in first membrane separation unit,  214 , were assumed to have the properties shown in Table 2, at a membrane operating temperature within the range of about 50° C. and about 150° C. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Gas 
                 Permeance (gpu)* 
                 H 2 /Gas Selectivity** 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Hydrogen 
                 300 
                 — 
               
               
                   
                 Carbon monoxide 
                 &lt;2 
                 &gt;100 
               
               
                   
                 Carbon dioxide 
                 20 
                 15 
               
               
                   
                 Methane 
                 &lt;2 
                 &gt;100 
               
               
                   
                 Nitrogen 
                 &lt;2 
                 &gt;100 
               
               
                   
                 Water 
                 500 
                 0.6 
               
               
                   
                   
               
               
                   
                 *Gas permeation unit; 1 gpu = 1 × 10 −6  cm 3 (STP)/cm 2  · s · cmHg 
               
               
                   
                 **Estimated, not measured 
               
             
          
         
       
     
     The membranes,  219 , in second membrane separation unit,  218 , were selective for carbon dioxide over hydrogen and were assumed to have the properties shown in Table 3, at a membrane operating temperature within the range of about 0° C. and about 40° C. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Gas 
                 Permeance (gpu)* 
                 CO 2 /Gas Selectivity** 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Carbon dioxide 
                 600 
                 — 
               
               
                   
                 Hydrogen 
                 60 
                 10 
               
               
                   
                 Carbon monoxide 
                 20 
                 30 
               
               
                   
                 Methane 
                 20 
                 30 
               
               
                   
                 Nitrogen 
                 30 
                 20 
               
               
                   
                 Water 
                 2000 
                 0.3 
               
               
                   
                   
               
               
                   
                 *Gas permeation unit; 1 gpu = 1 × 10 −6  cm 3 (STP)/cm 2  · s · cmHg 
               
               
                   
                 **Estimated, not measured 
               
             
          
         
       
     
     Syngas flow for this calculation was assumed to be 106 Mt/h. First membrane  215  area was assumed to be 1,343 m 2 ; second membrane  219  area was assumed to be 1,427 m 2 . 
     The flow rates and chemical compositions of the streams in the methanol synthesis loop were calculated. The results of this calculation are shown in Table 4. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
                 Reactor 
                   
                   
                   
                   
                 First 
                 Second 
                   
               
             
          
           
               
                   
                   
                 Feed 
                 Reactor 
                 Product 
                 Overhead 
                 Purge 
                 Recycle 
                 Mem. 
                 Mem. 
                 Fuel 
               
               
                   
                 Syngas 
                 Gas 
                 Output 
                 Stream 
                 Stream 
                 Gas 
                 Gas 
                 Perm. 
                 Perm. 
                 Gas 
               
               
                 Parameter/Stream 
                 204 
                 206 
                 208 
                 210 
                 211 
                 212 
                 213 
                 216 
                 220 
                 221 
               
               
                   
               
             
          
           
               
                 Total Flow (Mt/h) 
                 106 
                 177 
                 177 
                 92 
                 85 
                 20 
                 65 
                 7.4 
                 6.2 
                 6.0 
               
               
                 Temperature (° C.) 
                 150 
                 70 
                 280 
                 50 
                 50 
                 50 
                 50 
                 53 
                 50 
                 43 
               
               
                 Pressure (bar) 
                 16.5 
                 103 
                 95 
                 90 
                 88 
                 86 
                 86 
                 2.1 
                 16.5 
                 103 
               
             
          
           
               
                 Component (mol %) 
               
             
          
           
               
                 Hydrogen 
                 73.4 
                 73.5 
                 61.0 
                 0.27 
                 75.6 
                 75.6 
                 75.6 
                 95.1 
                 15.2 
                 5.0 
               
               
                 Carbon monoxide 
                 14.9 
                 8.2 
                 1.1 
                 0.01 
                 1.3 
                 1.3 
                 1.3 
                 0.20 
                 4.5 
                 5.5 
               
               
                 Carbon dioxide 
                 7.8 
                 4.7 
                 1.2 
                 0.53 
                 1.4 
                 1.4 
                 1.4 
                 0.55 
                 8.0 
                 0.23 
               
               
                 Methane 
                 3.7 
                 12.6 
                 16.2 
                 0.70 
                 19.9 
                 19.9 
                 19.9 
                 3.0 
                 68.4 
                 84.5 
               
               
                 Nitrogen 
                 0.2 
                 0.71 
                 0.91 
                 0.01 
                 1.1 
                 1.1 
                 1.1 
                 0.17 
                 3.4 
                 4.8 
               
               
                 Methanol 
                 0 
                 0.31 
                 14.7 
                 73.5 
                 0.65 
                 0.65 
                 0.65 
                 0.84 
                 0.02 
                 0 
               
               
                 Water 
                 0 
                 0.05 
                 4.9 
                 25.0 
                 0.11 
                 0.11 
                 0.11 
                 0.14 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     In this “two membrane” example (in accordance with the invention), approximately 98% of the carbon oxides in the syngas are converted to methanol. Most of the balance, approximately 2% of the carbon oxides in the feed syngas, is lost in the purge gas. The make-up compressor compresses 24,800 Ibmol/h, with a power consumption of 29,800 hp. The recycle compressor compresses 50,000 Ibmol/h, with a power consumption of 5,400 hp.