Separation membranes

Separations based on the use of membranes have gained significance In the context of Analytical Chemistry. The large variety of commercially available membranes facilitates their use for a variety of applications. Their functioning Is based on the selective passage of a substance or a group of substances with given characteristics. 

Membrane separation Is generally Implemented In a continuous fashion. The essential components of a continuous module are shown In Fig. 4.16. A carrier solution containing the sample —to which reagents facilitating the separation 

Depending on the nature of the substances to be separated, one can distinguish between two chief types of continuous membrane separation  

Fig- 4.17 (A) Automatic determination of residual ozone in water by liquid diffusion with two pumps (Pt and P2) working intermittently under the control of a computer. (B) Functioning of the peristaltic pumps and the injection valve. (Reproduced from [13] with permission of the American Chemical Society). 

The membrane separation process involves several elementary steps, which include the solution of hydrogen and its diffusion as atomic hydrogen through the membrane bulk material. Polymeric membranes are frequently applied in industrial 

Inlet CO [mol%] Parameter Stage 1 Stage 2 stages Overall 

Generally, the membranes need to be protected against rapid temperature and pressure changes and they are subject to poisoning by carbon monoxide, hydrogen 

Membrane separation of reformate is usually operated at elevated pressure, as the driving force for the permeation process. Thus, steam reforming is the preferred procedure, because only liquid pumps are required instead of large compressors for air pressurisation, which draw unacceptable parasitic losses [105], especially for small mobile systems with an electrical power equivalent of less than lOkWei. 

Another measure to increase the hydrogen partial pressure difference between permeate and retenate is to use sweep gas on the retenate side. Because the hydrogen requires humidification for low temperature PEM fuel cells to prevent membrane dry-out, steam is the preferred sweep gas [405]. OHany et al. highlighted the effect of steam as the sweep gas for the permeate in a methane steam reforming membrane reactor [406]. Higher methane conversion was observed, which originated from back-diffusion of steam from the permeate to the reaction side of the membrane, which increased the S/C ratio and consequently the conversion. 

As we discussed earlier for the solid-liquid separation technique, filtration separates particles by forcing the fluid through a filtering medium on which solids are deposited. The conventional filtration involves the separation of large particles (dp 10 pm) by using canvas, synthetic fabrics, or glass fiber as filter medium. 

We can use the same filtration principle for the separation of small particles down to small size of the molecular level by using polymeric membranes. Depending upon the size range of the particles separated, membrane separation processes can be classified into three categories microfiltration, ultrafiltration, and reverse osmosis, the major differences of which are summarized in Table 10.2. 

ProcessSize Cutoff Molecular Wt. Cutoff Pressure Drop (psi) Material Retained 

Micro- filtration Ultra- 0.02 - 10/zm 10 Suspended material including microorganisms 

Membrane filters are usually made by casting a polymer solution on a surface and then gelling the liquid film slowly by exposing it to humid air. The size of the pores in the membranes can be varied by altering the composition of the casting solution or the gelation condition. Another common technique is to irradiate a thin polymeric film in a field of a-particles and then chemically etch the film to produce well-defined pores. 

The performance of a membrane in separation can be described in terms of permeation rate or permeation flux (mol m s ) and permselectivity. The permeation flux is usually normalized per unit of pressure (mol m s Pa bj called the permeance, or is further normalized per emit of thickness (mol m m s called the permeability, if the thickness of the separation layer is known. In many cases only a part of the separation layer is active, and the use of permeability gives rise to larger values than the real intrinsic ones. Therefore, in case of doubt, the flux values should always be given together with the (partial) pressure of the relevant components at the high-pressure (feed) and low-pressure (permeate) sides of the membrane as well as the apparent membrane thickness. 

For the pressure-driven convective flow, which is most commonly used to describe flow in a capillary or porous medium, the transport equation may be described by Darcy s law  

The perm-selectivity of a membrane toward a mixture is generally expressed by one of two parameters the separation factor and retention. The separation factor is defined by 

The retention is defined as the fraction of solute in the feed retained by the membrane, which is expressed by 

Membrane separations are driven by pressure, concentration, or electric field across the membrane and can be differentiated according to type of driving force, molecular size, or type of operation. Common membrane processes include microfiltration, ultrafiltration, nanofiltration/ reverse osmosis, gas separation, pervaporation, and dialysis/electrodialysis [3,4]. Some processes have been applied extensively for separation and purification of gas and liquid mixtures in industry. 

So far, the separation of azeotropic systems has been restricted to the use of pressure shift and the use of entrainers. The third method is to use a membrane to alter the vapor-liquid equilibrium behavior. Pervaporation differs from other membrane processes in that the phase-state on one side of the membrane is different from the other side. The feed to the membrane is a liquid mixture at a high-enough pressure to maintain it in the liquid phase. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, maintaining it in the vapor phase. Dense membranes are used for pervaporation, and selectivity results from chemical affinity (see Chapter 10). Most pervaporation membranes in commercial use are hydrophyllic19. This means that they preferentially allow 

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Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

Donnan membrane equilibrium This concerns the distribution of ions on each side of a membrane separating two portions of a solution of... 

Membrane separator. A separator that passes gas or vapor to the mass spectrometer through a semipermeable (e.g., silicon) membrane that selectively transmits organic compounds in preference to carrier gas. Membrane separator, membrane enricher, semipermeable membrane separator, and semipermeable membrane enricher are synonymous terms. 

Separator GC/MS interface. An interface in which the effluent from the gas chromatograph is enriched in the ratio of sample to carrier gas. Separator, molecular separator, and enricher are synonymous terms. A separator should generally be defined as an effusion separator, a jet separator, or a membrane separator. 

Membrane separation Membrane s vitches Membrane technology... 

R. W. Baker and co-workers. Membrane Separation Systems, Recent Developments and Future Directions, Noyes Data Corp., Park Ridge, N.J., 1991. 

In Eigure 1, the lower edge of the drawing is the flesh side of the hide. The hide, as it is removed from an animal, has body fat and a thin membrane separating the hide from the fat and flesh of the body of the animal. The area near the inside of the hide is made up of the heaviest fibers of the hide. 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

Dense Symmetrical Membranes. These membranes are used on a large scale ia packagiag appHcations (see Eilms and sheeting Packaging materials). They are also used widely ia the laboratory to characterize membrane separation properties. However, it is difficult to make mechanically strong and defect-free symmetrical membranes thinner than 20 p.m, so the flux is low, and these membranes are rarely used in separation processes. Eor laboratory work, the membranes are prepared by solution casting or by melt pressing. 

Module Selection. The choice of the appropriate membrane module for a particular membrane separation balances a number of factors. The principal factors that enter into this decision are Hsted in Table 2. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

Highly pure perchloric acid can also be produced by a patented electrochemical process ia which 22% by weight hypochlorous acid is oxidized to chloric acid ia a membrane-separated electrolyzer, and then additionally oxidized to perchloric acid (8,84). The desired electrochemical oxidation takes place ia two stages ... 

One unique appHcation area for PSF is in membrane separation uses. Asymmetric PSF membranes are used in ultrafiltration, reverse osmosis, and ambulatory hemodialysis (artificial kidney) units. Gas-separation membrane technology was developed in the 1970s based on a polysulfone coating appHed to a hoUow-fiber support. The PRISM (Monsanto) gas-separation system based on this concept has been a significant breakthrough in gas-separation... 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

Factors affecting RO membrane separations and water flux include feed variables such as solute concentration, temperature, pH, and pretreatment requirements membrane variables such as polymer type, module geometry, and module arrangement and process variables such as feed flow rate, operating time and pressure, and water recovery. 

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