Retentate and permeate

Diafiltration If a batch process is run so that the permeate is replaced by an equal volume of fresh solvent, unretained solutes are flushed through the system more efficiently. A major use of UF is fractionation, where a solvent, a retained solute and an unretained solute are present. An example is whey, containing water, protein, and lactose. If the retention of protein is I and the retention of lactose is 0, the concentration of protein in the retentate rises during UF. The ratio of protein to lac tose rises, but the feed concentration of lactose is unchanged in retentate and permeate. Diafiltration dilutes the feed, and permits the concentration of lactose to be reduced. Diafiltration is used to produce high-purity products, and is used to fractionate high-value products. R is always 0 for eveiy component. 

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

Even in this case, the use of a hybrid process combining NF, IE, and ED appears to improve the economics and performance of the tartaric stabilization of wines. For instance, Ferrarini (2001) proposed to split raw wine into a retentate and permeate by NF. The permeate, being richer in minerals, was processed by using in sequence cationic and anionic exchange resins and ED to reduce its potassium, calcium, and tartrate ion contents. By recombining the de-ashed permeate with the NF retentate, Ferrarini (2001) asserted to obtain a stabilized wine retaining almost all the flavor and aroma compounds originally present in raw wine. 

Subsequent studies of these proton-transport membranes will be conducted over a wide range of hydrogen partial pressures on the permeate side. This will allow assessment of whether the driving force for hydrogen permeance is more accurately represented as the natural logarithm of the ratio of the hydrogen partial pressures on the retentate and permeate sides, as observed in ion transport oxygen membranes. 

Both retentate and permeate from membrane separation techniques have become important starting materials in producing novel products and ingredients from milk of unique functional properties and organoleptic quality. Henning et al. [7] enumerated the current and new applications of membrane technologies in the dairy industry, which include... 

Figure 1. Schematic representation of a semi-equilibrium dialysis cell showing composition of retentate and permeate solutions after --20 hrs. Metal ions complexed by the micelle-solubilired ligand are reject by the membrane.

Three basic quantities are defined to describe membrane performance. Flux is the permeate flow rate normalized to total membrane filter area. For protein recovery in the cell separation step, instantaneous protein transmission can be measured by determining enzyme concentration simultaneously on the retentate and permeate sides of the filter during cell concentration. Percent transmission is calculated as ... 

Sousa et al [5.76, 5.77] modeled a CMR utilizing a dense catalytic polymeric membrane for an equilibrium limited elementary gas phase reaction of the type ttaA +abB acC +adD. The model considers well-stirred retentate and permeate sides, isothermal operation, Fickian transport across the membrane with constant diffusivities, and a linear sorption equilibrium between the bulk and membrane phases. The conversion enhancement over the thermodynamic equilibrium value corresponding to equimolar feed conditions is studied for three different cases An > 0, An = 0, and An < 0, where An = (ac + ad) -(aa + ab). Souza et al [5.76, 5.77] conclude that the conversion can be significantly enhanced, when the diffusion coefficients of the products are higher than those of the reactants and/or the sorption coefficients are lower, the degree of enhancement affected strongly by An and the Thiele modulus. They report that performance of a dense polymeric membrane CMR depends on both the sorption and diffusion coefficients but in a different way, so the study of such a reactor should not be based on overall component permeabilities. 

Since the concentrations change along the length of a separator, a more fundamental definition of R is based on the local retentate and permeate compos-tions, Cj and Cj (see Fig. 30,26) ... 

Ib/ft also, concentration of solids in suspension, kg/m, g/L, or Ib/ft Cp, in feed Cp, in permeate c, critical concentration in thickener concentration at which a gel layer forms in ultrafiltration c , in pores of medium Cj, at surface in ultrafiltration Cq, in feed to sedi-menter Cj, Cj, local retentate and permeate concentrations D Overflow from screen, kg/h or m/h also, diameter or pore size, m, /im,... 

The diversity of applications for UF in the pharmaceutical industry is unequaled in any other industry group. Concentration, purification, desalting, fractionation and sterilization are all practiced in one form or another. In some cases, the product is in the retentate in others, it is in the permeate. Occasionally, both retentate and permeate contain valuable products. The fact that all of these operations are possible with UF at ambient temperatures without phase change or addition of chemicals/solvents makes it an ideal separation tool for labile drugs and biologicals. Proteins, polysaccharides, vitamins, hormones, viruses, vaccines and antibiotics are all processed with UF. Even the water to make up these pharmaceutical solutions is often sterilized and depyrogenated by UF (see the discussion on ultrapure water in an earlier section of this chapter). 

In the first step, the enzyme is concentrated six-fold from 3 to 18%. (Notice that even though salt is removed in the permeate, the salt concentration remains at 3% in retentate and permeate since it is freely permeable to the membrane.) In the second step, the process stream is diluted by a factor of three to reduce the salt concentration from 3 to 1%. (As a consequence, the enzyme is now also diluted by a factor of three from 18 to 6%). In the third step, the enzyme is reconcentrated three-fold from 6 to 18%. The net result is the enzyme has been concentrated six-fold while the salt concentration has been reduced three-fold. 

Figure 3.99 shows Wang s data on the hydrolysis of starch by a-amylase using the same MWCO membrane. Here the difference in carbohydrate concentrations between retentate and permeate is much larger because the products of a-amylase hydrolysis are predominantly maltose and larger dextrins. Eventually the dextrins are hydrolyzed to glucose, but at a slower rate. Thus, the carbohydrate concentration in the reactor increases with time. These results suggest that the selection of membrane MWCO can control the products of the reaction. 

The effects of pressure on lactose retention and permeate flux of the HL membrane were shown in Fig. 5. Permeate fluxes of 18.0, 33.2, and 40.6 L m h were obtained at pressure of 1.4, 2.1, and 2.8 MPa, respectively. The effect of pressure on lactose retention was not significant (p>03). The lactose retention kept almost constant at 97 1%. These observations are in agreement with literature data [10]. 

The material balance equations for components B and D on the retentate and permeate sides become ... 

The amount M of solute or colloid deposited (in m on the membranes was calculated by using mass balance (see equation (6.1)), where Vf, Vr, and Vpi, are the volumes of feed, retentate and permeate (sample i), respectively, and cf, cr, and cpi the concentrations of feed, retentate and permeate. 

Combining Eqs. 10.3 and 10.4 gives the governing membrane permeation equation in terms of the hydrogen partial pressures of the retentate and permeate, the Richardson Eq. 10.5 [102]. 

Equations 9.10 and 9.11 present very interesting results they indicate that there remains a constant difference in flows and composition between any two points along a horizontal cross-section in the MCS. This occurs despite the continual change in retentate and permeate flowrates down the length of the MCS. 

The goal of the model for membrane unit for gas separation is to predict the flow rate and composition of retentate and permeate streams, for a given feed stream containing n components, membrane type and area, and permeate pressure. Here, the process boundary and variables are limited to one of the membrane modules shown in Figure 4.5. In this section, the solution-diffusion mechanism is used to predict the separation behavior of the membrane. In the development of a membrane model, it is assumed that the process is at steady state, pressure is constant on feed side, and permeability of a component through the... 

Mole Fraction Summation for Feed, Retentate and Permeate ... 

Here, F, Zf and h are, respectively, the molar flow rate, mole fraction of component of i and total enthalpy, all in cell k their subscripts, ret and perm, refer to retentate and permeate streams. Equations (10.4) and (10.5) are mass balances and mass-transfer equations for each of the components present in the membrane feed. The cross-flow model [Equations (10.3)-(10.7)] was implemented in ACM v8.4 and validated against the experimental data in Pan (1986) and the predicted values of Davis (2002). The Joule-Thompson effect was validated by simulating adiabatic throttling of permeate gas through a valve in Aspen Hysys. Both these validations are described in detail in Appendix lOA. 

The most generic distinction in the wide variety of membrane reactors can be made according to the possible functional roles of the membrane in the reactor, being controlled addition of reactants, separation of products from the reaction mixture and retention of the catalyst. Additionally, membrane processes can be divided based on the physical state of the retentate and permeate, respectively ... 

The results of catalytic activity tests carried out with the INOCERMIC membrane at 1 atm are reported in Fig. 6.9 in terms of CH4, CO, CO2, H2 concentration (vol%, dry basis) at both retentate and permeate sides as function of time. During the start-up phase only CO2 was detected at the reactor outlet at the retentate side, due to the methane total combustion, while at the permeate side any gaseous component was detected. After the reactor start-up the more reducing conditions allowed to obtain a mixture of CH4, CO, CO2 and H2 at both retentate and permeate sides, with a remarkable amount of H2 with respect to the other components. The H2 concentration at the permeate side was about 2 vol%, while the temperature close to the membrane surface was about 490°C. 

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