Stirred-cell filtration, membrane

Ultrafiltration was carried out in an Amicon stirred cell, 65 ml capacity, at 4 C with continuous stirring under C02 pressure until 25 ml filtrate had been collected. MWC0 = nominal molecular weight cut-off of the membrane. 

Prq[iaration of clarified digests sludge. Samples were removed under anaerobic conditions and moved immediately into the anaerobic chamber. Before analysis, the samples were clarified by centrifugation and filtration with 0.22-jLtm Acrodisc filters. Concentrated digester supernatant was also examined, and it was prepared by ultrafiltration in the anaerobic chamber with an Amicon stirred cell ultraconcentrator and Amicon PMIO membranes. 

Humic Acid Fractionation. The two different size fractions of humic acid used in this study were obtained by ultrafiltration of previously characterized HA from Lake Bradford, FL. An Amicon ultrafiltration cell (model 8050) was used for fractionation with a filtration membrane of the desired pore size. Molecules exceeding that pore size were retained above the membrane. The nominal molecular weight cut-off (MWCO), or pore sizes, were reported by the manufacturer (Amicon), and are based on globular protein standards. Membranes used in this study were XM300 (MWCO 300,000 daltons, lot number AI), YMIOO (MWCO 100,000 daltons, lot number AT), and XM50 (MWCO 50,000 daltons, lot number AG). A 5 liter fiberglass reservoir (Amicon model RG5) was filled with the solution to be fractionated. The solution was ultrafiltered under pressure (50 psi) with constant stirring. 

Ultrafiltration was used to determine the extent of europium binding to humic acid. While stirring, Eu(III) + Eu- 152 tracer was added to a 10 ml solution of humate at pH 4.2 (ace tate buffer) in an Amicon model 8050 ultrafiltration cell. After a 500 /xL aliquot was withdrawn for counting, the cell was immediately pressurized. An Amicon YM2 filtration membrane (1000 molecular weight cut-off) would retain the humic acid plus the bound europium while unbound Eu(III) would elute. One ml of filtrate was collected and tested for the presence of humate by measuring the visible spectrum of the solution with a Milton Roy Spectronic 1201. On cessation of filtration, 500 /xL aliquots were removed from the filtrate and the residual solution above the membrane. The samples were counted for the presence of Eu-152 in a well-type Nal(Tl) crystal connected to a single channel analyzer. 

The early attempts at membrane filtration of blood were disappointing. Stirred-cells were used to reduce the accumulation of blood cells on the membrane, but the stirring hemolyzed the red cells and the flux was low even at high stirring rates. 

Stirred cell systems were selected for the experimental work for a number of reasons (1) volumes are small which is required for the use of IHSS reference material, (ii) membrane samples are small which allows the use of a new membrane for each experiment, (iii) the solution chemistry can be precisely controlled, (iv) experiments are relatively short and thus the investigation of a great number of parameters is possible, and (v) the concentration in the cell represents the concentration in a crossflow module (recovery about 70%). A comparison of mass transfer values was demonstrated in the case of NF in Chapter 7. Drawings of the filtration equipment are shown in Appendix 2, A hydrodynamic analysis is also shown in Appendix 2. 

For fractionation experiments, the perspex stirred cells (see Chapter 4 for equipment description) were operated directly from the nitrogen bottle without a reservoir. Membranes were floated in a beaker of MilliQ water, skin side down, for at least one hour to remove the glycerin coating. Then at least 300mL of MilliQ water were filtered through the membrane. The filtrate was analysed with UV and DOC to confirm full removal of glycerin. The membranes were reused up to 5 times and stored in 0.1 % sodium azide at 4 C. Pure water flux was measured after the filtration of 500 mM of MiUiQ water prior to each experiment. The filtration protocols for serial and parallel fractionation were described in Chapter 4. In this Chapter parallel fractionation results will be shown. 

All experiments were stirred at 270 rpm unless otherwise indicated. A feed reservoir of 1.5 L was connected to the stirred cell to provide extended filtration volume. Pure water flux was measured after the filtration of 1 L of MilliQ water for both membranes. 

One of the more commonly used techniques is that of serum rq)lacement [19]. The latex particles are confined in a stirred cell by a filtration membrane. Washing with water not only cleans the latex but the serum replacement technique can also be used to obtain the senuiL The cleaning process can be followed by monitoring, for example, the ctxiductivity of the serum. 

The efforts that can be made to restrict the amount of flux decline, i.e. to establish an equilibrium flux rate, can be split into two categories, dependent largely on the scale of operation stirred cells and crossflow filtration. The former is of use in the laboratory for small-scale separations, the latter is more appropriate for process applications. Both stirring and crossflow enq>loy the same princ le hi ear at the sur ce of the deport. Further techniques to restrict deposit thickness, minimise fouling or regenerate flux are discussed in Section. 10.7. All the main membrane arrangements are illustrated in Figure 10.4. 

Cytochrome P-450 fractions were pooled and the free Emulgen 913 removed from the enzyme preparation by stirring with Amberlite XAD-2 beads followed by filtration. The filtrate was concentrated in an Amicon ultrafiltration cell using a YM 10 Diaflo membrane. Dialysis was carried out in 2 liters of Buffer I for 24 hr when required. The fractions containing cytochrome P-450 were stored under nitrogen in 0.5 ml aliquots at -62°. 

NMR analysis, the PHA was extracted from freeze-dried cells. For this purpose, 1.0 g freeze-dried cells were stirred in 200 mL of chloroform for 24 hours at 30°C. The extract was filtered to remove cells debris, and the chloroform was concentrated to a volume of about 15 ruL using rotary evaporator. The concentrated solution was then added drop-wise to 150 luL of rapidly stirred methanol to precipitate the dissolved PHA.The precipitated PHA was then recovered by filtration using a 0.45 pm PTFE membrane and dried overnight at room temperature. The purified PHA was dissolved in deuterated chlorofonn (CDCl ) and subjected to H and NMR analyses. 

Kamoshita Y, Ohashi R, and Suuzuki T. Improvement of filtration performance of stirred ceramic membrane reactor and its application to rapid fermentation of lactic acid by dense cell culture of Lactoccus lactis. J. Ferment. Bioeng. 1998 85(4) 422 27. 

Pure water flux was determined for each membrane using 3 L MilliQ water. The last 100 mL of filtrate were analysed for TOC as a control sample for organic contamination. The cell was then filled with the feed solution, the stirring switched to 270 rpm, and the pressure adjusted to 100 kPa (unless indicated otherwise). Two types of filtration protocols were used, a standard and a regck protocol. Pure water flux was determined after the recycle experiments only, using 1 L of MilliQ water. 

Membranes are subject to concentration polarization. During UF, the solute accumulates at the membrane surface and the actual concentration may be much higher than that which corresponds to the bulk concentration. This process is called concentration polarization of the membrane. It can be minimized by use of stirred filtration cells or cross-flow units. In... 

Conventional stirred filtration cells or a specially designed radial-flow cell equipped with a pump can be used. Membranes made of polysulfone, polyamide, cellulose, etc. are suitable. The essential parameters are the molecular mass exclusion rate in wide pH ranges (1-10) and an appropriate permeate flow rate (1-10 ml min ), retentate volume (2-10 ml), and gas pressure (300 kPa is a suitable pressure in most cases). A nominal exclusion rate of lOkgmoH has been shown to be convenient for polymers having a molecular mass between 30 and 50 kg moH. A polymer concentration of 1 % (w/v) in the cell solution is most appropriate for both retention of elements and their subsequent determination in the retentate. 

The silicate solution is somewhat diluted, aged, clarified, and ultrafiltered in a rapidly stirred pressure cell at 50 psi. By following the concentration of SiO- and Na20 in the filtrate one can calculate by difference how much of the silicate is polymerized to a size that will not go through the pores. Also, it can be shown that Si(OH)4 and HSiOj pass freely through the membrane so that the initial concentration of these species in the filtrate is the same as in the initial solution. The commercial sodium silicate solutions were diluted with an cqui volume of water, aged about a week, filtered through fine filter paper to remove traces of suspended matter, and then ultrafiltered. 

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