Selectivity membranes

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Because the facilitated transport process employs a specific reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far higher than those achieved by other membrane processes. This one fact has maintained interest in facilitated transport since the 1970s, but the problems of the physical instability of the liquid membrane and the chemical instability of the carrier agent are yet to be overcome. 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

Membrane Sep r tion. The separation of components ofhquid milk products can be accompHshed with semipermeable membranes by either ultrafiltration (qv) or hyperfiltration, also called reverse osmosis (qv) (30). With ultrafiltration (UF) the membrane selectively prevents the passage of large molecules such as protein. In reverse osmosis (RO) different small, low molecular weight molecules are separated. Both procedures require that pressure be maintained and that the energy needed is a cost item. The materials from which the membranes are made are similar for both processes and include cellulose acetate, poly(vinyl chloride), poly(vinyHdene diduoride), nylon, and polyamide (see AFembrane technology). Membranes are commonly used for the concentration of whey and milk for cheesemaking (31). For example, membranes with 100 and 200 p.m are used to obtain a 4 1 reduction of skimmed milk. 

More economically competitive if ideal permselectivity is >15 (highly dependent on membrane selection) indication of feasibiUty obtained with information on critical temperature and van der Waals volume. 

Boundary layer effects Membranes (selective permeability for ions, gases etc.), ion exchangers, controlled release of pharmaceuticals. 

A critical consideration with UF technology is the problem of fouling. Foulants interfere with UF by reducing product rates- sometimes drastically-and altering membrane selectivity. The story of a successful UF application is in many respects the story of how fouling was successfully controlled. Fouling must be considered at every step of UF process development in order to achieve success. 

Concentration of Electrolyte Myer and Sievers"" applied the Donnan equilibrium to charged membranes and developed a quantitative theory of membrane selectivity. They expressed this selectivity in terms of a selectivity constant, which they defined as the concentration of fixed ions attached to the polymer network. They determined the selectivity constant of a number of membranes by the measurement of diffusion potentials. Nasini etal and Kumins"" extended the measurements to paint and varnish films. 

Brackish water. Usually associated with salty water, brackish water TDS levels range from 2,000 to 20,000 ppm or more. Most industrial sources of RW supply may be well water, surface waters, or the like, but do not specifically have to contain high levels of sodium chloride. The RO applied pressure required is from 250 to 600 psig, and the permeate recovery rates are typically 60% down to perhaps 40%. There is a tremendous variety in so-called brackish water sources, and correct membrane selection and other design criteria are critical to manufacturing an efficient RO plant. 

The conclusion above is valid for ideally selective membranes. Real membranes in most cases have limited selectivity. A quantitative criterion of membrane selectivity for an ion to be measured, relative to another ion M +, is the selectivity coefficient The lower this coefficient, the higher the sefectivity wifi be for ions relative to ions An electrolyte system with an imperfectly selective membrane can be described by the scheme (5.16). We assume, for the sake of simplicity, that ions and have the same charge. Then the membrane potential is determined by Eq. (5.17), and the equation for the full cell s OCV becomes... 

Table 20-24 compares properties of commonly used polyethersulfone (PES) or regenerated cellulose membranes. Membrane selection is based on experience with vendors, molecular weight rating for high... 

The design and capacity of an RO unit is dependent upon the type of chemicals in the plating solution and the dragout solution rate. Certain chemicals require specific membranes. For instance, polyamide membranes work best on zinc chloride and nickel baths, and polyether/amide membranes are suggested for chromic acid and acid copper solutions. The flow rate across the membrane is very important. It should be set at a rate to obtain maximum product recovery. RO systems have a 95% recovery rate with some materials and with optimum membrane selection.22... 

Fig. 5-9. Total number of stages and total membrane surface area versus membrane selectivity for the separation of 1 kg s-1 of a racemic mixture at a membrane permeability of 1.6 x 10 2 kg m 2.s, yielding both enantiomers at 95 % purity [55].

Aside from the factors accounted for in the simplified Eq. (9), a deeper consideration of the membrane selectivity reveals other influential parameters. The membrane polarity, for example, which depends mainly on the nature of membrane plasticizer, may give a selectivity modifying influence because of the improved solvation of high valence ions by more polar media. 

With appropriate membrane pore size and a narrow distribution, membrane selectivity for smaller gas molecules can be high but the overall permeability is generally low due to a high flow resistance in fine pores. Several studies are being conducted to develop molecular sieve-type membranes using different inorganic materials, for example, those based on carbon (Liu, 2007), silica (Pex and van Delft, 2005), and zeolites (Lin, 2007). 

Approaches to make a polymeric membrane selective to C02 attempt to enhance the solubility selectivity of the polymer material for C02 and reduce the diffusivity selectivity of the polymer that favors smaller hydrogen molecule. The permeability of a polymer membrane for species A, PA, is often expressed as (Ghosal and Freeman, 1994)... 

It follows from Equation 8.13 that aA/B can be expressed as the product of the diffusivity selectivity, DA/DB, and the solubility selectivity, SA/SB. Diffusion (or mobility) selectivity is governed primarily by the size difference between gas molecules and always favors smaller gas molecules. Solubility selectivity is controlled by the relative condensability of the gases in the polymer and their relative affinity for the polymer. Solubility selectivity typically favors larger, more condensable molecules. From Equation 8.13, it is seen that the product of gas mobility and solubility selectivity determines the overall membrane selectivity. It is clear that for a membrane to be C02 selective, it must have high diffusivity selectivity based on the affinity for C02 but it should be flexible enough to permeate larger molecules such... 

The membrane selectively rejects oxygen and nitrogen. The field test showed a selectivity for chlorine over nitrogen of about ten. That this is so much lower than that obtained in the laboratory is attributed to concentration polarisation. Increasing the rate of flow through the module can alleviate this. At the same time, chlorine recovery can be maintained by adding modules in series. This is precisely what would be done in a commercial unit, and so one can reasonably expect better results in full-scale operation. 

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