Membranes separation factor

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

Separation Factor The separation factor, a, is defined consistent with other separation methods. It is important to recall that in membranes, a is the result of differing rates, and that it has no eqm-librium implications. The convention in membrane separations is to define the separation so that a > I. 

Recently, high-quality SOD membranes for water separation have been developed by Khajavi etal. [21, 52]. These zeolite membranes should allow an absolute separation of water from almost any mixture since only very small molecules such as water, hydrogen, helium, and ammonia can theoretically enter through the six-membered window apertures. Water/alcohol separation factors 10 000 have been reported with reasonable water fluxes up to 2.25 kg nr h at 473 K in pervaporation experiments. 

Furthermore, the application of the SOD membrane in a FT reaction has been investigated. The advantages of water removal in a FT reaction are threefold (i) reduction of H20-promoted catalyst deactivation, (ii) increased reactor productivity, and (iii) displaced water gas shift (WGS) equilibrium to enhance the conversion of CO2 to hydrocarbons [53]. Khajavi etal. report a mixture of H2O/H2 separation factors 10000 and water fluxes of 2.3 kg m h under... 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

However this is not dways the case, especially when the two components weakly interact with the surface When using the membrane to separate a H2/ isobutane mixture, the permeation of isobutane, due to its size, is restricted over the entire temperature range and the transmembrane fluxes of the two components of the mixture better follow the permeabilities of the pure gases. Separation factors are here much higher (factors up to 80 have been measured). 

Membranes offer a format for interaction of an analyte with a stationary phase alternative to the familiar column. For certain kinds of separations, particularly preparative separations involving strong adsorption, the membrane format is extremely useful. A 5 x 4 mm hollow-fiber membrane layered with the protein bovine serum albumin was used for the chiral separation of the amino acid tryptophan, with a separation factor of up to 6.6.62 Diethey-laminoethyl-derivatized membrane disks were used for high-speed ion exchange separations of oligonucleotides.63 Sulfonated membranes were used for peptide separations, and reversed-phase separations of peptides, steroids, and aromatic hydrocarbons were accomplished on C18-derivatized membranes. 

Fig. 5-4. (a) Separation of d,1-phenylalanine by an amino acid immobilized in the pores of a polysulfone ultrafiltration membrane, (b) Effect of volume flux on the separation factor, Jv = volume flux, T = 37 °C [32]. 

A very high separation factor has been obtained in phenol dehydration by using pervaporation process and PVA/PAA as membranes. The membrane composition and the process characteristics are presented in table 1. 

Membrane composition PVA/PAA Composition of liquid mixture Permeation rate / / (g m"2 h"1) T/ C Separation factor... 

Hybrid membranes composed of poly(vinyl alcohol) (PVA) and tetraethylorthosilicate (TEOS), synthetised via hydrolysis and a co-condensation reaction for the pervaporation separation of water-isopropanol mixtures has also been reported [32], These hybrid membranes show a significant improvement in the membrane performance for water-isopropanol mixture separation. The separation factor increased drastically upon increasing the crosslinking (TEOS) density due to a reduction of free volume and increased chain stiffness. However, the separation factor decreased drastically when PVA was crosslinked with the highest amount of TEOS (mass ratio of TEOS to PVA is 2 1). The highest separation selectivity is found to be 900 for PVA TEOS (1.5 1 w/w) at 30°C. For all membranes, the selectivity decreased drastically up to 20 mass % of water in the feed and then remained almost constant beyond 20 mass %, signifying that the separation selectivity is much influenced at lower composition of water in the feed. 

Composition of the membrane PAA.HC1/PVA/GA Composition of liquid mixture Water flux / / (kgm 2h 1) T/°C Separation factor... 

The PVA/PAAm IPN membranes were found to have pervaporation separation factors ranging from 45 to 4100 and permeation rates of about 0.06-0.1 kg m 2 h 1, for 95 % ethanol aqueous solution, at 75 °C [46], For a concentration of 10 wt% ethanol, the permeation rates were as large as 9 kg m 2 h 1 and the separation factors were about 20 [46],... 

B.-B. Li et al. [64] have studied the separation of EtOH-H20 solutions by pervaporation (PV) using chitosan (CS), poly (vinyl alcohol)-poly(acrylonitrile) (PVA-PAN) and chitosan-poly(vinyl alcohol)/poly(acrylonitrile) (CS-PVA/PAN) composite membranes. It was found that the separation factor of the CS-PVA/PAN composite membrane increased with an increase of PVA concentration in the CS-PVA polymer from 0 to 40 wt%. With an increase in the membrane thickness from 12 to 18 pm, the separation factor of the CS-PVA/PAN composite membrane increased and the permeation flux decreased. With an increase of ethanol-water solution temperature, the separation factor of the CS membrane decreased and the permeation flux of the CS membrane increased while the separation factor and the permeation flux of PVA/PAN and CS-PVA/PAN composite membranes increased. 

Another recent work presents the possibility to use a membrane made by PVA-g-acrylonitrile (AN) to separate acetic acid/water mixtures by pervaporation [70], The best separation factor (14.6) has been obtained by using PVA-g-AN (52 %) membrane, at 30°C, 90 % acetic acid in the feed. The permeation rate was 0.09 kg m"2h 1. 

To attain this goal, a pervaporation technique has been proposed, using a PVA composite membrane, made by casting of a mixture of PVA aqueous solution and a GA one on a polyethersulfone (PES) porous support, solvent evaporation and thermic curing [72], Excellent dehydration performance has been obtained (separation factor 320 and permeation flux 1.5 kg m 2 h 1, for 90 wt% TFEA in the feed and 80 °C). 

The obtained results show that the prepared membranes are methanol selective, but the performance of these membranes (separation factor=30, for PVA/Pacr.Ac.=80/20, 5 wt% methanol in the feed, 25 °C) is lower than those reported by J.W. Rhim and Y.K. Kim [75] (separation factor 1250 for PVA/Pacr.Ac.=75/25, 20 wt% methanol in the feed, 30 °C). 

Solid PVA-Co2+ composite asymetric membranes have been prepared starting from PVA and two different salts Co(N03)2 and Co(CH3COO)2, respectively, in order to separate cyclohexene/cyclohexan mixtures. A facilitated transport mechanism has been evidenced, due to the capacity of Co2+ ions to coordinate the olefin molecules [82], The authors reported stronger complexation of Co2+ ions with cyclohexene in the case of PVA/ Co(CH3COO)2 mixtures then in the case of PVA/ Co(N03)2 mixtures. It was found that for a concentration ratio of ([Co2+]/[OH]) by 0.75 mol/mol, the permeation flux of PVA membrane containing Co2+ increases 2-3 times and the separation factor increses 50 times compared with pure PVA membrane. 

Composition of the membrane PVA/PAcr.Ac Composition of liquid mixture fusel oil Permeation rate/ /(gm-2h- ) T/°C Separation factor Permeate activation energy Ea / (kJ mol 1)... 

Evapomeation is a new membrane-separation technique for liquids mixtures, which eliminates some disadvantages of the pervaporation technique such as the decreasing of membrane permselectivity, due to its swelling by the direct contact with the feed solution. In evapomeation technique the membrane is not in direct contact with the feed solution, only with the solution s vapors. In this way the swelling of the membrane could be suppressed and consequently, the permeation rates in evapomeation are smaller than those in pervaporation, but the separation factor is greater [83],... 

The influence of the CD content in the membrane and the n-PrOH respectively p-xylene content in the feed mixture on the separation factors and sorption and diffusion selectivities of the CD/PVA membranes for the n-PrOH/I-PrOH and p-xylene and o-xylene mixtures by evapomeation are presented in tables 12 and 13. 

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