Retention, membrane separation

Retention Rejection and Reflection Retention and rejection are used almost interchangeably. A third term, reflection, includes a measure of solute-solvent coupling, and is the term used in irreversible thermodynamic descriptions of membrane separations. It is important in only a few practical cases. Rejection is the term of trade in reverse osmosis (RO) and NF, and retention is usually used in UF and MF. 

The combination of diafiltration and batch concentration can be used to fractionate two macrosolutes whose retentions differ by as little as 0.2. It is possible in principle to achieve separations that are competitive with chromatography. When tanks and other equipment are considered, as well as the floor space they occupy, the economics of membrane separation of proteins may be attractive [R. van Reis, U.S. Patent 5,256,294 (1993)]. 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

The pore size of a membrane is manifested in the permeabilities and separation (retention) characteristics of the membrane. For gases, the pore size of the membrane also affects the prevailing transport mechanisms through the pores (see Chapter 6). 

A crucial characteristic of a membrane is the molecular weight cutoff (MWCO) value, which is defined as the molecular weight at which 90% of the solutes are retained by the membrane. The retention factor R of solute A to be separated by the membrane is defined by the ratio of the concentration of A in the permeate to that in the retentate, as expressed in the following equation ... 

When looking for an economically feasible enzymatic system, retention and reuse of the biocatalyst should be taken into account as potential alternatives [98, 99]. Enzymatic membrane reactors (EMR) result from the coupling of a membrane separation process with an enzymatic reactor. They can be considered as reactors where separation of the enzyme from the reactants and products is performed by means of a semipermeable membrane that acts as a selective barrier [98]. A difference in chemical potential, pressure, or electric field is usually responsible from the movement of solutes across the membrane, by diffusion, convection, or electrophoretic migration. The selective membrane should ensure the complete retention of the enzyme in order to maintain the full activity inside the system. Furthermore, the technique may include the integration of a purification step in the process, as products can be easily separated from the reaction mixture by means of the selective membrane. 

Figure 19.2 summarises the most important features of the casein micelle and P-lg. Both the micellar casein and the globular p-lg depend in size and structure on pH and thermal conditions. Processing and compositional factors thus induce a considerable variability in molecule size, which needs to be taken in account when trying to establish stmcture-function relationships or in membrane separation, where the molecule size influences retention. 

The generic permselectivity of a membrane can be described by the retention coefficient for liquid phase or the separation factor for gas phase. Separation factor will be defined and discussed in Chapter 7. In the case of liquid-phase membrane separation, the retention coefficient of the membrane can be characterized by some commonly used model molecules such as polyethylene glycol (PEG) polymers which have linear chains and arc more flexible or dextians which arc slightly branched. The choice of these model molecules is due to their relatively low cost. They are quite deviated from the generally... 

The majority of gas separation applications use pressure difference as the driving force for the membrane separation. As such, the issues of sealing the ends of membrane elements and connecting the elements and the module or process piping are critical in providing gas-tight or essentially leakproof conditions. The seals and connections are necessary to prevent remixing of the permeate and the retentate streams. 

As in the case of gas separation discussed in Chapter 7, which reaction component(s) in a membrane reactor permeates through the membrane determines if any gas recompression is required. If the permeate(s) is one of the desirable products and needs to be further processed downstream at a pressure comparable to that before the membrane separation, recompression of the permeate will be required. On the other hand, if the retentate(s) continues to be processed, essentially no recompression will be necessary. Recompressing a gas can be rather expensive and its associated costs can be pivotal in deciding whether a process is economical. 

Membrane and Membrane Design Most membranes are polymers in nature, but some inorganic membranes have become available. The most common membranes are based on polysulfone, cellulose acetate, polyamide, fluoropolymers, and other compounds. Formation of a symmetric membrane structure is an important element in the success of UF/NF membrane separation (16). The other considerations for membrane separation are as follows (1) separation capabilities (retention or selectivity), (2) separation rate (flux), (3) chemical and mechanical stabilities, and (4) membrane material cost. 

FIGURE 4.23 A three stage membrane separation process—the recycled gas from stage three need to be recompressed. This configuration could typically be used for removal of CO2 from gas stream where the retentate is the product. (From Spillman R.W., Chem. Eng. Progr., 85, 41, 1989. With permission.)... 

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... 

Schaep, J. et al.. Modeling the retention of ionic compounds for different nanofiltration membranes, Separ. Purif. Technol, 22-23,169,... 

From the beginning, in the subsequent membrane separation step the first test with the re-immobilized catalysts yielded a much better Rh retention of 96 % as compared to the Rh/TPP-system with only 56 % [28]. 

With respect to succeeding membrane separation it was found that generally an increase of the molecular mass of the amines leads to improved retention of rhodium, of phosphorus ligand and, last but not least, of the amines. It can be demonstrated that an increase in molecular mass does have a contradictoryeffect on the overall efficiency. A high amount of permeate corresponds to a lower flux of permeate due to the higher concentration of compounds within the retentate (osmotic pressure). Traces of amine in the permeate are the result of a very low temperature-dependent dissociation of the ammonium salts into amine and free acid according to eq. (7). 

In order to stabilize the ammonium salts, the presence of free amine in the system is recommended. With respect to this additional requirement, an ideal system must have the same good retention for amines as well as for phosphorus and rhodium. Therefore the choice of distearylamine could only be regarded as a good compromise of hydroformylation requirements and membrane separation properties. 

A membrane-separation system separates an influent stream into two effluent streams the permeate and the retentate or concentrate, as shown in Fig. 2. The permeate is the portion of the fluid that has passed through the membrane. The retentate (concentrate) contains the constituents that have been rejected by the membrane. 

As in the case of all membrane separation processes, the choice of an appropriate module type is based on feed type (viscosity, suspended solids content, and particle size), required membrane packing density (based on flux, total throughput, and available floor space), good flow hydrodynamics (for minimization of concentration polarization, effective cleaning and sanitation), and module cost. The various types of membrane modules and their fabrication have been reviewed by Strathmann.f Hollow fiber modules, which have the highest membrane packing density of all module types, are the most suitable for use in OD because of the inherently low flux of this process. However, the membranes that have provided the best fluxes and volatiles retention because of their relatively large pore diameters and porosities, that is those fabricated from PTFE, have not yet become available in hollow fibers with an acceptably low wall thickness. 

The use of interfacially formed thin films for membrane separations was, therefore, obvious. Reference to such membranes in Morgan s book characterizes them as retentive to dyes but permeable to salts. Early attempts to use interfacially formed membranes at the North Star Research Laboratories in the late 1960 s tended to confirm the low salt retention described by Morgan. In these early attempts to prepare composite membranes polysulfone support films were saturated in solutions of various diamines, acid acceptors and surfactants. After draining, the films were contacted with hexane solutions of various diacyl chlorides. In this initial work, salt rejections of these membranes were generally too low for practical use in seawater or brackish water applications. Similarly poor results were described in a patent by Scala and co-workers, dating to that era, wherein interfacial condensation membranes were produced (34). 

A drug is absorbed through diffusion across a series of separate barriers where the single layer of epithelial cells is the most significant barrier to absorption. Many in vitro methods have been developed for the study of this phenomenon. These methods include small animal gut studies, cell culture (i.e., Caco-2 cell culture model), octanol-water partition coefficients, measures of hydrogen bonding and desolvation energies, immobilized artificial membranes, and retention time on reversed-phase HPLC columns. 

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