Continuous-membrane filtration

Fig. 3 Schematic representation of batch-wise passive membrane dialysis (A) and continuous membrane filtration dead-end-filtration (B) and loop reactor (C)...

Plasmapheresis. The separation of plasma from whole blood by continuous membrane filtration represents an improvement over conventional centrifugation techniques in terms of efficiency, safety and cost. In the past, plasmapheresis was carried out with blood donors by collecting their whole blood in plastic bags which were then centrifuged to separate the red cells from the plasma. The supernatant plasma was then decanted and the red cells returned to the donorenabling plasma to be drawn from the same person as frequently as three times per week. Most of this plasma is then processed to yield purified components such as albumin or anti-hemophilic factor (Factor VIII). 

By adjusting the pore size, one can allow the passage of buffer ions and small molecules but exclude larger molecules of interest. With the formation of nanofilters or nanoporous membranes within the microfluidic systems, this strategy can be implemented easily. Membrane (filtration)-based preconcentration will not have any chemical bias (mainly dependent on the size of the molecule), but continuous membrane filtration could generate eventual clogging of the system, which is one of the main problems in this technique. 

Unimmobilized Corynebacterium propinquum (CGMCC No. 0886) cells containing a cobalt-dependent NHase were employed in either batch or continuous reactions for the production of nicotinamide from 3-cyanopyridine [24]. In the continuous process, membrane filtration separated precipitated product (>5 wt%) and the microbial cell catalyst from the reaction mixture, where the catalyst was then recovered and returned to the reactor using a continuous addition of aqueous 3-cyanpyridine to maintain substrate concentration at <20% (w/v), a final conversion of >99% was obtained. 

Membrane filtration using a polyaramide membrane [56] showed a retention of more than 99.8%. Application of this catalyst in a continuously operated membrane reactor showed conversion for more than 150 h. The ee dropped from 80% in the beginning (non-bonded analogue 97%) to about 20% after 150 h. The average ee for the first 80 h was 50%. 

Two types of continuous membrane reactors have been applied for oligomer- or polymer-bound homogeneous catalytic conversions and recycling of the catalysts. In the so-called dead-end-filtration reactor the catalyst is compartmentalized in the reactor and is retained by the horizontally situated nanofiltration membrane. Reactants are continuously pumped into the reactor, whereas products and unreacted materials cross the membrane for further processing [57]. 

Membrane absorbers are continuous chromatographic supports, which circumvent some of the above-mentioned problems of particulate stationary phases. They were originally derived from membrane (filtration) technology. The immobilization of interactive (ionic, hydrophobic, or biospecific) groups on the surface of microfiltration membranes was found to increase the selectivity of certain separation procedure. Ideally such activated membranes, or membrane adsorbers, allow the selective adsorption of certain substances and substance classes, which may subsequently be eluted by means of a stepwise change of the mobile phase (elution buffer). More complete information on the various types of modern membrane technology can be found in some recent reviews [e.g., 31-33]. 

A. Plum, G. Braun and A. Rehorek, Process monitoring of anaerobic azo dye degradation by high-performance hquid chromatography-diode array detection continuously coupled to membrane filtration sampling modules. J. ChmmatogrA, 987 (2003) 395-402. 

Bubble Point Constancy. Although the exact relationship between the bubble point and the "pore size" of a microfiltration membrane is a matter of dispute (11, 12, 13, 14), nevertheless, it remains the quickest and most convenient means for demonstrating the continuing integrity of a membrane filtration system. It is consequently important that the bubble point be both reproducible (within a given range) and constant. It was, therefore, of considerable interest to discover that the bubble points of both conventional and poly(vinylidene fluoride) membranes increased with immersion time in deionized water whereas those of Tyrann-M/E and polyamide remained essentially constant (Figure 6). 

The critical issue for a successful RO plant is pretreatment. Long-term operating experience proves the viability of continuous MF/UF pretreatment of RO for the desalination of a wide variety of water sources. MF/UF has proven to simplify and reduce the costs of traditional pretreatment, comprised of deep-bed media filters combined with chemical treatment. MF/UF produces filtrate of a consistent quality almost irrespective of fluctuations in feed-water quality. In the last five years, RO-membrane improvements, combined with the use of membrane filtration for pretreatment, have halved the cost of advanced treatment and are now more widely used for the reuse of municipal wastewater. 

This sensory property was used to probe the suitability of metalloden-drimers for nanofiltration membrane techniques in homogeneous systems. During continuous-flow membrane filtration, any leaching of a metalloden-... 

However, it can be assumed for most electrochemical applications of ionic liquids, especially for electroplating, that suitable regeneration procedures can be found. This is first, because transfer of several regeneration options that have been established for aqueous solutions should be possible, allowing regeneration and reuse of ionic liquid based electrolytes. Secondly, for purification of fiesh ionic liquids on the laboratory scale a number of methods, such as distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous adsorption in a chromatography column, have already been tested. The recovery of ionic liquids from rinse or washing water, e.g. by nanofiltration, can also be an important issue. 

This study focuses firstly on the transfer of regeneration principles as they have been developed in the field of water-based electroplating and of purification options for ionic liquids as they are experienced in other fields of ionic liquid application. A number of purification procedures for fresh ionic liquids have already been tested on the laboratory scale with respect to their finishing in downstream processing. These include distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous chromatography. But little is known yet about efficiency on the technical scale. Another important aspect discussed is the recovery of ionic liquids from rinse or washing water. 

This chapter continues the discussion on hltration started in Chapter 7, except that it deals with advanced hltration. We have dehned filtration as a unit operation of separating solids or particles from huids. A unit operation of hltration carried out using membranes as hlter media is advanced hlhation. This chapter discusses advanced hlhation using elechodialysis membranes and pressure membranes. Filtration using pressure membranes include reverse osmosis, nanohlhation, microhltration, and ultrahltration. 

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

Briefly, the use of membrane filtration by breweries has been slowed down by the difficulties in filtering this very special beverage made from natural ingredients. To overcome these problems intensive research and resources have been spent, mostly because the market is promising. The bottom line is that membrane filtration produces a high-quahty filtered product without the use of filter aids that need to be disposed of, membrane processes are easy to automate, and their operation is more continuous-like as compared to the filter-aid methods. 

Aquatic suspended particles are usually characterized by a continuous particle size distribution. The distinction between particulate and dissolved compounds, conventionally made in the past by membrane filtration, does not consider organic and inorganic colloids appropriately. Colloids of iron(IIl) and manganese(III,IV) oxides, sulfur, and sulfides are often present as submicron particles that may not be retained by membrane filters (e.g., Buffle et al., 1992). Recent measurements in the ocean led to the conclusion that a significant portion of the operationally defined dissolved organic carbon may in fact be present in the form of colloid particles. 

Sedimentation and/or filtration (26,28) will be feasible for separating the insoluble chromium hydroxide precipitates (or chemical floes) from a wastewater. Other feasible solid-water separation processes for removing the insoluble chromium hydroxide include membrane filtration (such as ultrafiltration and microfiltration), continuous DAF, PC-SBR-sedimentation, PC-SBR-DAF. The following is a summary of the solid-water separation processes feasible for the combined application of chemical reduction and precipitation. 

Membrane filtration is useful where the usage is moderate and a continuous circulation of water can be maintained. Thus, with the exception of that drawn off for use, the water is continually being returned to the storage tank and refiltered. As many waterborne bacteria are small, it is usual to install a 0.22-pm pore-size membrane as the terminal filter and to use coarser prefilters to prolong its life. Membrane filters require regular sterilization to prevent microbial colonization and growthrough . They may be treated chemically with the remainder of the storage/distribution system or removed and treated by moist heat. The latter method is usually the most successful for heavily contaminated filters. 

Solomon, B. A. Colton, C. K. Friedman, L. 1. Castino, F. Wiltbank, T. B. Martin, D. M. "Microporous Membrane Filtration for Continuous-Flow Plasmapheresis" In Ultrafiltration Membranes and Applications Vol. 3 of Polymer Science and Technology Cooper, A. R., Ed. Henum Press New York, N.Y., 1980, pp 489-505. Zydney, A. L. "Cross-flow membrane plasmapheresis an analysis of flux and hemolysis PhD Thesis, Massachusetts Institute of Technology, 1985. 

---