Membrane reactors feed pressure

After preformation, the substrates and carbon dioxide were supplied continuously. The membrane reactor was pressurized at the feed side up to 20 MPa with the reaction mixture. A trans-membrane pressure was created by opening a needle valve on the permeate side after which the continuous process started. 

In a catalytic membrane reactor the pressure difference between feed and permeate could be adjusted such that high selectivity and high conversion in a once through process is obtained (Fig. 29d). The amount of catalyst necessary and the required residence time of the gas would be less than in conventional fixed bed reactors since the diffusional resistance is overcome by the external pressure gradient. The above advantages are already partly exploited by the use of macro-porous catalyst pellets, mentioned in Section 10.1.2.3 [19, 20]. 

Fig. 5. Methane conversion and oxygen flux during partial oxidation of methane in a ceramic membrane reactor. Reaction conditions pressure, 1 atm temperature, 1173 K, feed gas molar ratio, CH Ar = 80/20 feed flow rate, 20 mL min-1 (NTP) catalyst mass, 1.5 g membrane surface area, 8.4 cm2 (57).

Fig. 6. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane disk was prepared by pressing Bao.5Sro.5Coo.8Feo.2O3-s oxide powder in a stainless steel module (17 mm inside diameter) under a pressure of (1.3-1.9) X 109 Pa. The effective area of the membrane disk exposed to the feed gas (CH4) was 1.0 cm2 (72).

Figure 13.9 MSR reaction. CH4 equilibrium conversion for both traditional and membrane reactors. I is the ratio ofthe sweep flow rate to the CH4 feed flow rate. H20/CH4 feed molar ratio = 3, permeate pressure = 100 kPa.

The permeate is continuously withdrawn through the membrane from the feed sueam. The fluid velocity, pressure and species concentrations on both sides of the membrane and permeate flux are made complex by the reaction and the suction of the permeate stream and all of them depend on the position, design configurations and operating conditions in the membrane reactor. In other words, the Navier-Stokes equations, the convective diffusion equations of species and the reaction kinetics equations are coupled. The transport equations are usually coupled through the concentration-dependent membrane flux and species concentration gradients at the membrane wall. As shown in Chapter 10, for all the available membrane reactor models, the hydrodynamics is assumed to follow prescribed velocity and sometimes pressure drop equations. This makes the species transport and kinetics equations decoupled and renders the solution of... 

The top product recycle mode in Figure 11.12 brings part of the permeate stream at a lower pressure to join the feed suream at a higher pressure. Thus, additional energy external to the membrane reactor will be required to recompress the recycled permeate. On the contrary, in the bottom product recycle, also shown in Figure 11.12, only the transmembrane pressure difference and the longitudinal pressure drop need to be overcome between the recycled portion of the bottom product (or retentate) and the feed. Therefore, the required pressure recompression is expected to be small compared to the top product recycle mode. 

In many situations the conversion of a membrane reactor increases as the total permeate rate increases. This is to be expected if the membrane has a perfect or very high pcimselcctivity. In many commercially available porous inorganic membranes, however, the permselectivity is moderate and some reactants as well as products other than the most selective species "leak through the membrane. This leakage rate often increases with the total permeate rate, for example, as the feed side pressure increases. This has an important consequence on the reactor performance. 

A membrane reactor preceded or followed bv a conventional reactor. Consider a typical commercial porous membrane currently available that exhibits a moderate to low gas separation factor and a high gas permeance even for the gas intended to be the retentate. Much of this less permeable gas in the feed stream is lost to the permeate (low pressure) side in the entrance section of the reactor due to its high partial pressure difference across the membrane layer. This leads to the undesirable effect of a low reactant conversion in that section. An effective way of reducing this reactant loss is to have a membrane enclosed section preceded by an impermeable reaction zone. To achieve a maximum total conversion, the impermeable length relative to the membrane length needs to be optimized. 

Saracco G., Veldsink J.W., Versteeg G.F. and van Swaaij W.P.M., Catalytic combustion of propane in a membrane reactor with separate feed of reactants. I. Operation in absence of trans-membrane pressure gradients, Chem. Eng. Sci., 56 2005 (1995). 

The continuous enzyme membrane reactor (CMR). (1) Temperature-controlled water-bath (2) Feed tanig (3) Stirrer motor for feed tank (4) Feed pump (5) Feed inlet line to the reaction vessel (6) Reaction vessel (7) Magnetic stirring table (8) Prefilter (9) Recycle pum (10) Flowmeter (11) Membrane inlet pressure gauge (12) Hollow fiber membrane cartridge (13) Membrane outlet pressure gaug (1 Pressure adjusbneut valve (15) Retentate recycle line (16) Air bath environment (17) Pemieate (product) line (18) Permeate collection vessel (19) Electronic balance... 

A reference case for the C02-selective WGS membrane reactor was chosen with the C02/H2 selectivity of 40, the C02 permeability of 4000 Barrer, the inlet sweep-to-feed molar flow rate ratio of 1, the membrane thickness of 5jum, 52,500 hollow libers (a length of 61 cm, an inner diameter of 0.1 cm, and a porous support with a porosity of 50% and a thickness of 30jLon), both inlet feed and sweep temperatures of 140 °C, and the feed and sweep pressures of 3 and latm, respectively. With respect to this case, the effects of C02/H2 selectivity, C02 permeability, sweep-to-feed ratio, inlet feed temperature, inlet sweep temperature, and catalyst activity on the reactor behavior were then investigated. 

Complete membrane systems can be operated in a variety of modes with e.g. CO- or counter flow of feed (high pressure side) and permeate (low pressure side) streams and with membrane modules coupled in different ways. Permeation and separation in these complex engineering systems will not be treated in this chapter. Heat and mass transfer limitations on the gas-membrane surfaces or interfaces can be important with high fluxes and/or strongly adsorbing gases as well as in membrane reactors. These effects will not be treated explicitly but are introduced in experimental results, e.g., by variation of sweep rates of permeated gases. 

Other reactor configurations and concepts have also been discussed in the technical literature. Most commonly dted are hybrid concepts, where the membrane reactor is used as an add-on stage to an already existing conventional reactor. This particular configuration has a number of attractive features, especially for applications involving conventional type porous membranes, which are characterized by moderate (Knudsen-type) permselective properties. Staged membrane reactors have received mention and so have reactors with multiple feed-ports and recycle. To facilitate the transport across the membrane in laboratory studies one often applies a sweep gas or a vacuum in the permeate side or a pressure gradient across the membrane. It is unlikely that the first two approaches, effective as they may be in laboratory applications, will find widespread commercial application. 

It is proposed to replace the conventional PER with a membrane reactor in order to impro e the selectivity. As a rule of thumb, a 19f increase in the selectivity to ethylene oxide translates to an increase in profit of about S2 mii-lion/yr. The feed consists of 12% (mole) oxygen. 69f ethylene, and the remainder nitrogen at a temperature of 250 C and a pressure of 2 atm. The total molar How rate is 0.0093 mol/s and to a reactor containing 2 kg of catalyst. 

The electronics needed to operate six temperature sensors on each microreactor channel are located underneath these cards. The ribbon connector, which is also visible in Fig. 12.8, is used to transfer electrical signals directly from the reactor board to the heater circuit board (described below). Serial communications for the Redwood flow manifolds is provided by two, four-conductor RJ-11 jacks on the front. The front of the board has gas inlets for the reactor feed and the purge gas. In addition, there are two gas outlets, one for each reaction channel. The inlet and outlet fittings are 1/16-inch type 316L stainless steel. The tubing assembly component having the greatest pressure sensitivity is the microreactor membrane. 

The same group [2.354] has also recently reported on the performance of a membrane reactor with separate feed of reactants for the catalytic combustion of methane. In this membrane reactor methane and air streams are fed at opposite sides of a Pt/y-A Os-activated porous membrane, which also acts as catalyst for their reaction. In their study Neomagus et al. [2.354] assessed the effect of a number of operating parameters (temperature, methane feed concentration, pressure difference applied over the membrane, type and amount of catalyst, time of operation) on the attainable conversion and possible slip of unconverted methane to the air-feed side. The maximum specific heat power load, which could be attained with the most active membrane, in the absence of methane slip, was approximately 15 kW m with virtually no NO emissions. These authors report that this performance will likely be exceeded with a properly designed membrane, tailored for the purpose of energy production. 

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