Rubbery membrane

For rubbery membranes (hydrophobic), the degree of swelling has less effect on selectivity. Thus the permeate pressure is less critical to the separation, but it is critical to the driving force, thus flux, since the vapor pressure of the organic will be high compared to that of water. 

Pollution Control Pervaporation is used to reduce the organic loading of a waste stream, thus effecting product recovery and a reduction in waste-treatment costs. An illustration is a waste stream containing II percent (wt) n-propanol. The residue is stripped to 0.5 percent and 96 percent of the alcohol is recovered in the permeate as a 45 percent solution. This application uses a hydrophobic, rubbery membrane. The residue is sent to a conventional waste-treatment plant. 

Another series of papers by Paul and co-workers [4-6,19,22] focuses on the same phenomenon using rubbery membranes and permeation of organic solvents such as hexane, benzene and carbon tetrachloride. Such membranes are highly swollen by the organic solvents and, when operated in reverse osmosis mode,... 

Figure 2.17 Flux of n-hexane through a rubbery membrane as a function of the hexane concentration difference in the membrane. Data taken from both reverse osmosis ( ) and pervaporation (O) experiments. Feed-side and permeate-side membrane concentrations, Ci0 m) and Cie m), calculated from the operating conditions through Equations (2.26), (2.36) and (2.76). Maximum flux is obtained at the maximum concentration difference, when the permeate-side membrane concentration cit(m)), equals zero [19]. Reprinted from Driving Force for Hydraulic and Pervaporation Transport in Homogeneous Membranes, D.R. Paul and D.J. Paciotti, J. Polym. Sci., Polym. Phys. Ed. 13, 1201 Copyright 1975. This material is used by permission of John Wiley Sons, Inc.

Because the membrane selectivity and pressure ratio achievable in a commercial membrane system are limited, a one-stage membrane system may not provide the separation desired. The problem is illustrated in Figure 8.16. The target of the process is 90% removal of a volatile organic compound (VOC), which is the permeable component, from the feed gas, which contains 1 vol% of this component. This calculation and those that immediately follow assume a feed gas mixture VOC and nitrogen. Rubbery membranes such as silicone rubber permeate the VOC preferentially because of its greater condensability and hence solubility in the membrane. In this calculation, the pressure ratio is fixed at 20... 

Figure 8.30 The relative size and condensability (boiling point) of the principal components of natural gas. Glassy membranes generally separate by differences in size rubbery membranes separate by differences in condensability...

Figure 9.6 Comparative separation factors for toluene and trichloroethylene from water with various rubbery membranes [28]. These experiments were performed with thick films in laboratory test cells. In practice, separation factors obtained with membrane modules are far less because of concentration polarization effects. Reprinted from Nijhuis et al. [28], p. 248 with permission of Bakish Materials Corporation, Englewood, NJ...

C02-selective polar rubbery membranes (a o /h =10 > 1 room temperature) this type of rubbery membrane increases the selectivity at low temperatures at 10°C, Kcoj/Hj = 1 considered and... 

Fig. 40. Permeability as a function of molar volume for a rubbery and glassy polymer, illustrating the different balance between sorption and diffusion in these pol5rmer types. The rubbery membrane is highly permeable the permeability increases rapidly with increasing permeant size because sorption dominates. The glassy membrane is much less permeable the permeability decreases with increasing permeant size because diffusion dominates (85)., 0.335 X 10 i m mol / 10" cm (STP) cm ...

Dioctylphenyl phosphonate 2.2x 10 5 Clear transparent, soft and rubbery membrane with no inclusions. Good operational lifetime of > 2 weeks. 

Dibutyl sebacate/ Dioctylphenyl phosphonate 0.36 0.36 36 2 x5 X 10- 10" -3 X10- Colourless, transparent, soft and rubbery membrane. Lifetime > 2 months. 

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