Cellulose acetate permeation rates

A commercially available cellulose acetate film which we would now describe as homogeneous or isotropic, gave the results shown in Row 2 of Table I. The volumetric permeation rate of water per unit membrane area, called the water permeation flux Jl mVm day, and the water permeation constant. A, m m day atm were both very low, but a salt rejection of 94 percent was obtained. We define ... 

Gas Dehydration. It has been found that water vapor permeates cellulose acetate membranes at a rate approximately 500 times that of methane (Ref. 2). This exceptionally high selectivity for water vapor make cellulose acetate membrane systems attractive for dehydration of hydrocarbon gas streams to pipeline specifications on either a pure gas stream or while simultaneously removing contaminating acid gases. For these applications the small size, low weight and low maintenance of the SEPAREX system is particularly advantageous for offshore installations. 

Deposition of polyelectrolytes Lajimi et al. [56] explored the surface modification of nanofiltration cellulose acetate (CA) membranes by alternating layer-by-layer deposition of acidic chitosan (CHI) and sodium alginate (AEG) as the cationic and anionic polyelectrolyte, respectively. The supporting CA membranes were obtained by a phase separation process from acetone/formamide. The permeation rate of salted solutions was found to be higher than that of pure water. The rejection of monovalent salt was decreased, while that of divalent salt remained constant so that the retention ratio increased. Increasing the concentration of feed solutions enhanced this selectivity effect. 

The physicochemical criteria approach to reverse osmosis separations Involving the surface excess free energy of solvation for ionized and nonlonized solutes has been demonstrated by this work to include nonaqueous solutions. The parameters and correlations presented in this work permit the prediction of reverse osmosis separations and permeation rates for different alkali metal halides for cellulose acetate OEastman E-398) membranes of different surface porosities from only a single set of experimental data for a sodium chloride-methanol reference feed solution system. 

With support from the OSW, Reid at the University of Florida pursued an alternative design in the mid 1950 s based on filtration equipment available at the time. His design used pressurized air to drive water across polymeric films. Of the commercially available films, cellulose acetate was the most attractive due to its high salt rejections. Unfortunately, product water permeation rates were low. Since permeation rate was inversely proportional to film thickness, solution casting techniques were developed to form films as thin as possible [10]. Around the time of Hassler s and Reid s work, the term reverse osmosis was adopted to describe membrane desalination [8]. 

The osmotic pressure which has to be overcome depends on the concentration of the solution and on the nature of the solutes. As the osmotic pressure increases with concentration, the rate of permeation of the solvent diminishes as the concentrate thickens and it is not economically viable to aim at above a certain concentration (5% for NaCl and 30% for a sugar solution, for example). The usual operating pressures are 40-50 bar giving solvent flux anywhere between 2 and 401m h, depending on the solution and the membrane used. Most membranes are made of cellulose acetate or its close relatives and they usually allow a limited solute passage in order to obtain reasonable permeate flow rates. 

Permeation rates for specific components and membranes are not constants. They vary with temperature, pressure, and the presence of other components in the gas. Detailed permeation rate data for commercial membranes are normally considered proprietary however, some comparative data have been published. Table 15-2 lists typical permeation rate data for a number of membranes and gases. The cellulose acetate data are from Mazur and Chan (1982), and the other data are based on a paper by Tomlinson and Finn (1990). 

The permeation data given in Table 15-2 are relative values based on a permeation rate of 1.0 for oxygen in cellulose acetate and polysulfone. However, data presented by Schell and I loemschemeyer (1982) for the permeation rates of various gases in cellulose acetate indicate that actual values, expressed as (scf)/(ft )(hr)(IOO psi), are close to the relative values listed in the table. For example, they show the permeation rate for oxygen, measured at room temperature and 100 psig, to actually be about 1.0 (scf)/(ft )(hr)(l00 psi). and the actual permeation rates for other gases in cellulose acetate to be similar to those listed. 

Permeation rates are relative, based on I for oxygen in polysulfone and cellulose acetate. 

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