Permeation retention

Membrane Crude Oil Retentate Permeate Retentate Crude Oil Permeate... 

Permeate flux y 1/h/m X 5(water removed Y Swater added Permeate Retentate ... 

Permeate, retentate Permeate = stream that goes through the membrane and leaves the membrane module. Retentate = stream that has been depleted of permeate. Retentate is the part of the feed that leaves the membrane module without passing through the membrane... 

Peak or peak ratio Surface Permeate Retentate % Recovery... 

Fig. 4. Diagram of a hoUow-fiber ultrafilter filtration system where A corresponds to the retentate reservoir B, circulation pump C, pressure gauge at module inlet D, ultrafilter module E, permeate reservoir F, pressure gauge at module outlet G, value to control module outlet pressure and H, drain...

Bentone-34 has commonly been used in packed columns (138—139). The retention indices of many benzene homologues on squalane have been determined (140). Gas chromatography of C —aromatic compounds using a Ucon B550X-coated capillary column is discussed in Reference 141. A variety of other separation media have also been used, including phthaUc acids (142), Hquid crystals (143), and Werner complexes (144). Gel permeation chromatography of alkylbenzenes and the separation of the Cg aromatics treated with zeofltes ate described in References 145—148. 

The three streams and associated variables of the RO membrane process are shown in Figure 2b the feed the product stream, called the permeate and the concentrated reject stream, called the concentrate or retentate. The water flow through the membrane is reported in terms of water flux, J. ... 

Because a preconcentration step is probably needed to make the final sequence more economical, it is logical to start with the opportunistic separation. This separation produces one of the products, pure water, as the underflow and a concentrated distillate appropriate for feed into either strategic separation. Arbitrarily choosing pervaporation first, the retentate has a composition on the 2-propanol-rich side of the azeotrope, whereas the permeate is pure water. No further strategic separations are required. 

Cascade with recycle required to obtain one (relatively) pure permeate and enriched retentate. 

Membrane-retained components are collectively called concentrate or retentate. Materials permeating the membrane are called filtrate, ultrafiltrate, or permeate. It is the objective of ultrafiltration to recover or concentrate particular species in the retentate (eg, latex concentration, pigment recovery, protein recovery from cheese and casein wheys, and concentration of proteins for biopharmaceuticals) or to produce a purified permeate (eg, sewage treatment, production of sterile water or antibiotics, etc). Diafiltration is a specific ultrafiltration process in which the retentate is further purified or the permeable sohds are extracted further by the addition of water or, in the case of proteins, buffer to the retentate. 

If the solute size is approximately the (apparent) membrane-pore size, it interferes with the pore dimensions. The solute concentration in the permeate first increases, then decreases with time. The point of maximum interference is further characterized as a minimum flux. Figure 4 is a plot of retention and flux versus molecular weight. It shows the minimum flux at ca 60—90% retention. 

Flux is maximized when the upstream concentration is minimized. For any specific task, therefore, the most efficient (minimum membrane area) configuration is an open-loop system where retentate is returned to the feed tank (Fig. 8). When the objective is concentration (eg, enzyme), a batch system is employed. If the object is to produce a constant stream of uniform-quahty permeate, the system may be operated continuously (eg, electrocoating). 

When fouling is absent, the optimum concentration is 0.37 C. If the permeate soHds are of primary value, it is usually preferable to diafilter at the minimum retentate volume to minimize permeate dilution. 

Pervaporation. Vapor arbitrated pervaporation is used to remove ethanol from whiskey by selective passage of the alcohol through a membrane. Whiskey flows on one side of a membrane. A water-vapor stream flows on the other side and sweeps away the ethanol that permeates the membrane. Thus alcohol reduction and selective retention of flavor and aroma components can be achieved usiag membranes with a particular porosity. The ethanol can be recovered by condensing or scmbbiag the vapor stream. Pervaporation systems operate at or slightly above atmospheric pressure (Fig. 

Reverse Osmosis. A reverse osmosis (RO) process has been developed to remove alcohol from distilled spirits without affecting the sensory properties (14). It consists of passing barrel-strength whiskey through a permeable membrane at high pressure, causing the alcohol to permeate the membrane and concentrating the flavor components in the retentate. 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

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. 

Membrane thickness is a factor in microbial retention, Tortiioiis-pore membranes rated at 0,22 jlrn tvpicallv have surface openings as large as 1 jlrn (Fig, 22-71), Narrovv er restrictions are found beneath the surface. In challenge tests, P. diminnta organisms are found vv ell beneath the surface of an 0,2 jlrn membrane, but not in the permeate. 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Process Retentate Permeate Common range of feed pressure (atm) Membrane type Typical iqrplications/ species... 

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