Membranes permeation flow, zeolite

Figure 9. H2 ( ) / n-butane ( ) separaticm with the ccxnposite zeolite-alumina membrane (fluxes in the permeate as a function of the tenq>erature). A mixture of hydrogen, n-btitane and nitrogen (12 14 74) was fed in the tube (Fig. 2) with a flow rate of 4.8 1/h. Sweep gas (N2), countercurrent mode, flow rate 4.3 1/h.

Transfer mechanisms involved in SC CO2 permeation through such porous membranes can be convection (Poiseuille law), diffusion (Knudsen flow), and surface membrane interaction by adsorption, capillary condensation, etc. [11]. Mechanisms have been specifically investigated for nanofiltration and zeolite membranes. With a nanofilter presenting a pore diameter of about 1 nm, Sarrade [11] mentioned a Poiseuille flow associated with an irreversible CO2 adsorption on the micropore wall. Transfer... 

Using a similar approach, Masuda et al. [219] employed a ZSM-5 zeolite membrane in a flow-through configuration for the methanol-to-olefins process. As in the previous work, permeating molecules would ideally have a uniform residence time... 

A schematic picture of different t5q)es of pores is given in Fig. 9.1 and of main types of pore shapes in Fig. 9.2. In single crystal zeolites the pore characteristics are an intrinsic property of the crystalline lattice [3] but in zeolite membranes other pore types also occur. As can be seen from Fig. 9.1, isolated pores and dead ends do not contribute to the permeation under steady conditions. With adsorbing gases, dead end pores can contribute however in transient measurements [1,2,3]. Dead ends do also contribute to the porosity as measured by adsorption techniques but do not contribute to the effective porosity in permeation. Pore shapes are channel-like or slit-shaped. Pore constrictions are important for flow resistance, especially when capillary condensation and surface diffusion phenomena occur in systems with a relatively large internal surface area. 

FIGURE 6.24 CO conversion and H2 recovery (a), and H2 permeance and H2/CO2 reparation factor (b) versus time-on-stream for WGS reaction in the modified bilayer MFI zeolite membrane (total feed gas flow rate 100 ml/min (STP), feed side pressure 2 atm, helium sweeping 20 ml/min (STP), permeate side pressure 1 atm). (Taken from Figure 6 of H. Wcmg, X. Dong, YS. Lin, J. Membr. Sci. 450 (2014) 425.)... 

RO membranes including polymeric (PA-TFC membrane) and molecular sieve zeolite membranes were investigated for ion removal from the water produced at oil field and coal bed methane sites by a cross-flow RO process [78]. Pretreatments including NF and adsorption by active carbon were implemented. The study revealed that (1) most of permeation tests lasted only 3 months due to severe fouling, (2) multistage pretreatment is crucial to extend membrane life, and (3) only NF treatment could extend the membrane life to 6 months. 

It is known that zeolite membranes essentially contain intercrystalline non-zeolitic pores (defects). This irregular nature of zeolite membranes with intercrystalline pores adds to the complexity of the transport process in addition to the contribution of a support layer to the permeation resistance. For zeolite membranes, selectivity similar to that expected for Knudsen flow generally indicates the presence of intercrystalline pores. Separation based primarily on adsorption differences, which is generally true in the separation of liquid mixtures by pervaporation, may have tolerance to the intercrystalline pores. However, in order to obtain high perm-selectivity, the zeolite membranes must have negligible amounts of intercrystalline pores and pinholes of larger than 2nm so as to reduce the gas flux from these defects [3]. 

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