CO2-selective membranes

Fuel ceUs membrane systems are present not only as proton-exchange membrane (PEM), which aUows protons to pass from the anode to the cathode, to combine with oxygen and electrons producing water, but also for the production and purification of the H2 (see Fig. 9.4). This system provides a CO2-selective membrane process for the purification and water-gas shift reaction of a reformed gas, generated from on-board reforming of a fuel, for example, hydrocarbon, gasoline, diesel, methanol, or natural gas, to hydrogen for fuel ceU vehicles [74]. 

Ho WSW, CO2 selective membrane process and system for reforming a fuel to hydrogen for a fuel cell, USP No 6,579,331. BI. 

We have developed a mathematical model for the countercurrent WGS membrane reactor with a CO2-selective membrane in the hollow-fiber configuration using air as the sweep gas. With this model, we have elucidated the effects of system parameters on the novel WGS membrane reactor for synthesis gases from steam reforming and autothermal reforming. The modeling results show that H2 enhancement via CO2 removal and CO reduction to 10 ppm or lower are achievable. For comparison and the completeness of the modeling work, we have also developed a similar model for the cocurrent WGS membrane reactor. 

Figure 1. Schematic of a Countercurrent CO2 Selective Membrane Reactor Containing Catalyst Particles...

Based on the membrane properties, water-gas shift (WGS) membrane reactors are classified into two categories, namely, CO2 selective membrane reactors and H2 selective membrane reactors. In the CO2 selective membrane reactors, CO2 was removed from the catalytic membrane reactor and the reaction mixture becomes H2-rich steam. This may cause over reduction of Fe- or Cu-based catalysts. However, in the H2-selective membrane, CO2 will be present at a higher concentration in the reaction medium, affecting the reaction rate. 

Reports are also available on CO2 selective membrane reactors for WGS reaction. Zou et al. [40] first time synthesized polymeric C02-selective membrane by incorporating fixed and mobile carriers in cross-linked poly vinyl alcohol. Micro-porous Teflon was used as support. They used Cu0/Zn0/Al203 catalyst for low temperature WGS reaction. They investigated the effect of water content on the CO2 selectivity and CO2/H2 selectivity. As the water concentration in the sweep gas increased, both CO2 permeability and CO2/H2 selectivity increased significantly. Figure 6.18 shows the influence of temperature on CO2 permeability and CO2/H2 selectivity. Both CO2 permeability and CO2/ H2 selectivity decrease with increasing reactimi temperature. After the catalyst activation, the synthesis gas feed containing 1% CO, 17% CO2, 45% H2 and 37% N2 was pumped into the membrane reactor. They are able to achieve almost 100% CO conversion. They also developed a one-dimensional non-isothermal model to simulate the simultaneous reaction and transport process and verified the model experimentally under an isothermal condition. 

Membrane technology has often been mentioned as the next technological generation for the prtrification of natural gases. Indeed, membrane systems are operated successfully for gas sweetening for decades. The best known examples include CO2 selective membranes that are based on pure polymers, e.g., cellulose acetate (Cynara membranes by Natco or Separex membranes by UOP) and polyimide (Ube). Despite their popularity, their performance at high pressures deteriorates as a result of CO2 induced plasticization. 

Kumar et al. [40] reported interesting data for membranes where MCM-48 supports (pore size 2.4 mn) were modified with polyethyleneimine (PEI). They reported an N2/CO2 selectivity of 1.31 ( 293 K) in the absence of water, 17.6 ( 293 K) in the presence of water, and 1.35 ( 363 K) in the presence of water for a feed mixture of 80/20 N2/CO2 and feed pressure of 20 psi (103.4 cm Hg). In the presence of water, the size of the diffusing unit (CO2) increased due to the clustering of water molecules, which in turn reduced the CO2 diffusivity at room temperature, and hence, the PEI-MCM 48 membranes were highly N2 selective in the presence of water. This is opposite to what we and others [10, 12, 13] observe (CO2 selective membrane), and it may be due to the fact that in our case, the amine groups are readily accessible to the CO2 molecules (since they form a brush-like structure) for reactive separation whereas the PEI approach, in contrast, may be dominated by a solution-diffusion mechanism rather than reactive or facilitated transport. 

Following the conclusion of the authors, the best process designs are obtained using Hj-selective membranes, in configuration counter-current Nj sweep gas that operates warm for bulk Hj recovery, combined with low-temperature COj-selective membranes that assist CO2 purification and liquefaction they have the potential to restrict the increase of levelized cost of the electricity (LCOE) caused by 90% CO2 capture to about 15%. Of course, it is also expected that larger cost reductions are also possible if higher permeance, and especially higher H2/CO2 selectivity membranes, are developed in the next future. 

We have been studying the novel process for CO2 separation named membrane/absorption hybrid method. The advantages of this process are that high gas permeance and selectivity were obtained. The concept of this process is shown in Fig. 1. Both feed gas and absorbent solution are supplied to the inside of hollow fibers. While Ae liquid flows upward inside the hollow fibers, absorbent solution absorbs CO2 selectively and it becomes a rich solution. Most of rich solution permeates the membrane to the permeate side maintained at reduced pressure, where it liberated CO2 to become a lean solution. Compared to a conventional gas absorption... 

CO2 Selective Ceramic Membrane for Water-Cas-Shift Reaction with Simultaneous Recovery of CO2. This project aims to develop a high temperature C02-selective membrane to enhance... 

We report here on the structure and gas transport properties of asymmetric membranes produced by the LB deposition of a polymeric lipid on porous supports. The effects of temperature on the structure and gas transport is described. The selectivity of CO2 over N2 permeation through the LB polymer films is determined. The polymerized lipid used in this study contains tertiary amines which may influence the CO2 selectivity over N2. The long term objective of our work is to understand how structure and chemistry of ultrathin films influence the gas permeation. 

Acid gases CO2/CH4 H2S/CH4 co2/n2 Enhanced oil recovery recover CO2 for reinjection Natural gas and landfill gas sweetening Sour gas sweetening Digester gas treatment Successful Successful Feasible, but no known installation Successful Must remove condensable hydrocarbons More robust and higher selectivity membranes are needed... 

The very high H2/CO2 permselectivity for the 825°C fired standard silica membrane is remarkable. It is even more remarkable that the H2/CH4 selectivity is lower, which is contrary to the common observation that the H2/CO2 selectivity is lower than the H2/CH4 selectivity. 

Due to the environmental focus on CO2 emissions around the world, there are numerous CO2 selective materials under development—several hundred polymers are reported (articles and patents). The main challenge for bringing these membranes into commercialization is to document durability over time (maintaining separation properties) during real operating conditions. 

Polymeric C02-selective membranes consisting of both mobile and fixed carriers in cross-linked PVA were synthesized. The membranes showed good C02/H2 and CO2/ CO selectivities, and high C02 permeability up to 170 °C. The effects of feed pressure, water content, and temperature on transport properties were investigated. The C02 permeability and C02/H2 selectivity decreased with increasing feed pressure,... 

It has been shovm that membranes can enhance the conversion of a water-gas shift membrane reactor and concurrently separate hydrogen from carbon dioxide. The efficiency of CO2 control using the membrane reactor with a H2/CO2 selectivity of 15 is significantly higher compared to a conventioncd technique (i.e. wet washing with a sorbent). It is not necessary to exceed a selectivity of approximately 40 for H2/CO2 for the process under consideration, because further increase in reactor performance seems marginal. Enlargement of the permeation is an important aspect on the other hand, so that the total surface area necessary for the full-scale application can be reduced. 

In this reaction, it would be extremely interesting to remove the CO2 via selective membranes, but it is a challenge to find membranes stable at the harsh reaction conditions. Another important reaction with MR potential is the Fisher-Tropsch reaction to form higher alkanes ... 

T.M. Nenoflf F. Bonhomme "Zeolite Membranes with High CO2 Selectivity". US Patent submitted to Sandia National Laboratories, August 2002. 

Develop a mathematical model for the novel water-gas-shift (WGS) membrane reactor with a carbon dioxide (C02)-selective membrane to elucidate the effects of system parameters on the reactor and to show the feasibility of achieving hydrogen (H2) enhancement via CO2 removal and carbon monoxide (CO) reduction to 10 parts per million (ppm) or lower from the modeling study. 

We have developed a one-dimensional non-isothermal model for the countercurrent WGS membrane reactor with a C02-selective membrane in the hollow-fiber configuration using air as the sweep gas. Figure 1 shows the schematic of each hollow-fiber membrane with catalyst particles in the reactor. The modeling study of the membrane reactor is based on (1) the CO2 / H2 selectivity and CO2 permeance reported by Ho [1, 2] and (2) low-temperature WGS reaction kinetics for the commercial catalyst copper oxide, zinc oxide, aluminum oxide (CuO/ZnO/ AI2O3) reported by Moe [3] and others [4]. In this modeling study, the model that we have developed has taken into account critical system parameters including temperature, pressure, feed gas flow rate, sweep gas (air) flow rate, CO2 permeance, CO2 /H2 selectivity, CO concentration, CO conversion, H2 purity, H2 recovery, CO2 concentration, membrane area, water (H20)/C0 ratio, and reaction equilibrium. 

The use of piezoelectric crystals for measuring dissolved CO2 was investigated (22). Crystals were coated with didodecylamine (DDDA) or dioctadecylamine (DODA) and placed in a chamber separated from the sample solution by a Teflon membrane, a Polyvinyl chloride membrane, or a Fluoropore filter with polyethylene web backing. The DDDA was 33 times more sensitive to H2O vapor than to CO2. The membranes were not sufficient for a selective measurement of CO2, and a differential mode of measurement was used to circumvent H2O vapor interference. 

Clearly, the application of any type of membrane in the hot-end of the IGCC process can improve the conversion of CO if the membrane can selectively remove H2 or CO2 from the syngas. A highly selective membrane for H2 relative to the other syngas components is required to enhance equilibrium as well as to prevent the loss of valuable reactants and to minimize the need for further purification of the H2-rich permeate. 

WGS reactors with the membrane reactors. A combination of membrane designs such as hydrogen-selective sweep membranes for bulk hydrogen recovery and C02-selective membranes for the cold end of the process to support the CO2 liquefacticMi system has been shown to improve the IGCC s commercial aspects. Future developments on membrane technology are required to improve the short lifetime, selectivity and permeability coefficients of these systems. 

It has be en found that in some situations electrically neutral species can penetrate through the polymeric membrane of the ISFET and change the value of the interfacial potential at the insulator/membrane interface. This applies especially to the acidobasic species such as CO2, lower carboxylic acids, ammonia etc. when the pH of the solution is far from their pK and they exist essentially in undissociated form. In other words, the ion selective membrane of the ISFET cannot reject electrically neutral species. 

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