Rejection membranes, ability

Selectivity Membrane ability to pass certain species while rejecting others. 

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

The ability to generate and use an H+ gradient across the membrane to supply energy for uptake and rejection of wanted and unwanted ions and... 

Figure 24 shows the rejections of polymer solutes, polyethylene glycols) (PEG) with monodispersed molecular weights. From Fig. 24, it is apparent that the composite membrane can find application for ultrafiltration. The molecular weight cut-off drastically decreased by more than 10 fold from the swollen state at 25 °C to the shrunken state at 45 °C. Thus the switching ability of the gel was demonstrated in the permeation experiments. 

The rejection coefficient is a measure of the ability of the membrane to separate salt from the feed solution. 

The ability of a reverse osmosis membrane to withstand chlorine attack without showing significant loss in rejection is measured in ppm h. This is the product of chlorine exposure expressed in ppm and the length of exposure expressed in hours. Thus, 1000 ppm - h is 1 ppm chlorine for 1000 h or 10 ppm chlorine for 100 h or 1000 ppm chlorine for 1 h, and so on. 

The main challenge of the first separation involves development of a viable membrane. An economical highly H2 selective membrane with the ability to reject both N2 and C02 is required for this stage, and such a membrane does not yet exist. Polymer-zeolite or ceramic-zeolite hybrid membranes may provide the required... 

Rejection is a property of the specific feed water component and the membrane of interest. Table 3.2 lists the general rejection ability of the most common polyamide composite RO membranes. Note that ionic charge of the component of interest plays a role its rejection by an RO membrane the rejection of multi-valent ions is generally greater than for mono-valent ions. 

The performance of a given membrane may be characterized according to its product flux and purity of product. Flux, which is a rate of flow per unit area of membrane, is a function of membrane thickness, chemical composition of feed, membrane porosity, time of operation, pressure across membrane, and feedwater temperature. Product purity, in turn, is a function of the rejection ability of the particular membrane. 

The major practical difference between UF, NF, and RO membranes is their respective abilities to reject solutes. Rejection is generally defined as... 

Rejection rate measures the ability of the membrane to retain a certain molecule. The observed solute rejection rate R, for a given specie i is given by ... 

Polysulfone membranes were prepared from 12.5, 13.75, and 15% (wt. %) polysulfone solution in dimethylformamide and formed on the surface of porous, sintered polymethyl methacrylate bars. An effective surface of each membrane was 49.2 cm. The effect of some casting parameters (composition and the temperature of the casting solution, time of solvent evaporation) and the pressure applied on the transport and separation properties of the membranes were analyzed. The experiments were carried out in a 1.2 dm pressure apparatus with continuous circulation of the permeate between feeding tank and the apparatus. It was found that membranes cast from 12.5% polysulfone solution of a temperature of 298 K with no solvent evaporation displayed the best properties. After 160 hours of operation at 0.18 MPa, the membranes in question showed an ability of a 97 to 99% rejection of 781.2 molecular-weight dye. The volume flux of the dye solution varied from 0.6 to 0.8m /m per day. 

It must be re-emphaslzed at this point that the solubility parameter and Its use for quantifying physicochemical Interactions Is based on enthalplc considerations only entropic considerations are neglected. Consequently, while the Ideas outlined In this section provide a basis for membrane material selection (or at least for narrowing the number of possible choices), prediction of permeation and rejection remains elusive until the ability to predict effective transport corridor size 4 Is more firmly established. At that time the differences - 4, and ik, - i > can be used In conjunction with A and A for meiftrane Mterral... 

The ability to separate cells from a high molecular weight extracellular product is one of the touted advantages of using microporous membranes over ultrafiltration membranes. Microporous membranes compete with centrifuges and rotary vacuum filters for this type of recovery. High yield protein/cell separations with membranes, however, have not been well established in the literature. Rejection coefficients for extracellular proteins in bacterial cell broth have been reported to vary widely from 5 to 100% for microporous membranes of 0.1 to 0.6 micron... 

Rejection or retention coefficient This describes the ability of the membrane to retain the desired species from the feed on the membrane surface. Since the rejection is often dependent on membrane characteristics and operating parameters, these must be clearly stated so that a fair comparison can be made between different types of membranes for a given application. It is defined as / = 1- Cp/Cn, where, Cp is the concentration of the species in permeate and Q is its concentration in the retentate. If a significant passage of the species occurs, then an average concentration is used. 

During all the membrane separation experiments, feed and permeate were sampled periodically. Concentration of anserine and carnosine was analyzed with high-performance liquid chromatography (HPLC). Concentration of creatinine and sodium ion was determined with HPLC and ICP-AES (inductively coupled plasma-atomic emission spectroscopy), respectively. Then, rejection ability of a membrane against each component was calculated with Eq. 22.1 ... 

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