Ultrafiltration processing time

There are two other aspects of importance in practical ultrafiltration processing time and the membrane area. A brief treatment of processing time with a view to minimizing it will be considered now. The processing time depends on a variety of factors the total volume to be processed the concentration of retained solids, especially if it leads to gel polarized operation (equation (6.3.143)) the mode of operation (continuous, batch, etc.) (Cherytm, 1986). One approach suggested by Ng et al. (1976) employs the gel-polarized condition and perfect rejection of the protein i for continuous diafiltration operation ... 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

Then, water is added continuously while the filtration continues at nearly constant flux. This latter filtration stage, when water is added to maintain a constant flux, is referred to as diafiltration. Proper choice of the diafiltration starting time can minimize the required membrane area, which is often the major part of the capital cost in an ultrafiltration process. 

In this paper, complete mathematical formulations for correlating the time cycles with other operating conditions are presented. The optimum diafiltration cycle (in terms of volume fraction), and the total cycle time are solved as functions of membrane area, flux, initial volume and recovery. Convenient charts, which can be used as a guide in designing or modifying an ultrafiltration process, are provided. 

The optimum time cycle and the relative diafiltration volume in the ultrafiltration-diafiltration process can be expressed as a function of three variables, P, Q, and R. P and Q are simple functions of the initial volume, membrane area, and flux (P = mA/Vo, Q = bA/Vo), and R is the solute recovery. From these, the time cycle and relative diafiltration volume (Vd/Vo) can be solved at various values of m, b, Vo, A, and R (m and b are respectively the slope and intercept of the flux, J = m In Vo/V + b). At a fixed recovery, the optimum time cycle and the relative diafiltration volume become functions of only two variables P and Q. Thus, the optimum operating condition can be simply plotted as function of P and Q. These plots, providing convenient and sufficient information, can be used as a guide in the design and operation of the ultrafiltration process. 

EFFECT OF MEMBRANE OPERATION TIME ON THE EFFICIENCY OF THE ULTRAFILTRATION PROCESS... 

A theoretical analysis has been carried out for galvanostatic and potentiostatic pulse regimes [27]. The idea that developed is a bit the same as backflushing with pressure driven-membrane operation such as microfiltration or ultrafiltration. The time dependencies of the extent of the concentration polarization near the membrane surface during the pulse are described theoretically for both pulse regimes and a qualitative discussion of the pause duration is presented. The main characteristic of the non-stationary process is the transition time between the state without polarization and the state with stationary polarization. 

Ultrafiltration was used to speed up the concentration step and as a substitute for dialysis in washing the concentrated virus. In this modified centrifugation process, the total number of centrifuges was reduced almost in half and the processing time with each centrifuge lowered from 8 hours to k hours. Final concentration of a 30 liter batch was accomplished with 30 ft of 100,000 NMWCO ultrafiltration membrane. Inlet pressure was 20 psi and the outlet pressure was 10 psi (Table V). 

There are two main issues in an ultrafiltration process productivity and selectivity. Productivity is quantified in terms of the permeate flux, this being defined as the permeation rate per unit membrane surface area. Factors that affect permeate flux are solute type, solute concentration, membrane type, solution pH, solution ionic strength, apphed pressure (also called the transmembrane pressure), and the hydrodynamic conditions on the feed side. The volumetric permeate flux, which is the volume of permeate collected per unit time per unit membrane area is given by the following generalized equation, based on a resistance model ... 

Cross-Flow Filtration in Porous Pipes. Another way of limiting cake growth is to pump the slurry through porous pipes at high velocities of the order of thousands of times the filtration velocity through the walls of the pipes. This is ia direct analogy with the now weU-estabHshed process of ultrafiltration which itself borders on reverse osmosis at the molecular level. The three processes are closely related yet different ia many respects. 

The seminal discovery that transformed membrane separation from a laboratory to an industrial process was the development, in the early 1960s, of the Loeb-Sourirajan process for making defect-free, high flux, asymmetric reverse osmosis membranes (5). These membranes consist of an ultrathin, selective surface film on a microporous support, which provides the mechanical strength. The flux of the first Loeb-Sourirajan reverse osmosis membrane was 10 times higher than that of any membrane then avaUable and made reverse osmosis practical. The work of Loeb and Sourirajan, and the timely infusion of large sums of research doUars from the U.S. Department of Interior, Office of Saline Water (OSW), resulted in the commercialization of reverse osmosis (qv) and was a primary factor in the development of ultrafiltration (qv) and microfiltration. The development of electro dialysis was also aided by OSW funding. 

Ultrafiltration has the advantage that there is removal of low molecular weight fermentation products and medium components during concentration of the exopolysaccharide. In addition, biological degradation is minimised because fluid is held only for a short time during the filtration process. Other advantages lie in file fact that there is no requirement for solvent recovery and the process is carried out at ambient (not elevated) temperature. 

Diafiltration is a process whereby an ultrafiltration system is utilized to reduce or eliminate low molecular mass molecules from a solution and is sometimes employed as part of biopharmaceuti-cal downstream processing. In practice, this normally entails the removal of, for example, salts, ethanol and other solvents, buffer components, amino acids, peptides, added protein stabilizers or other molecules from a protein solution. Diafiltration is generally preceded by an ultrafiltration step to reduce process volumes initially. The actual diafiltration process is identical to that of ultrafiltration, except for the fact that the level of reservoir is maintained at a constant volume. This is achieved by the continual addition of solvent lacking the low molecular mass molecules that are to be removed. By recycling the concentrated material and adding sufficient fresh solvent to the system such that five times the original volume has emerged from the system as permeate, over 99... 

It must be kept in mind, however, that CE data resemble a macroscopic effect of complexation and, like ultrafiltration or size exclusion chromatography CE, do not give information on specific binding sites. Despite the existence of a well-established theoretical basis, it is not possible to get reliable complexation constants, for practical reasons, if too many complexation processes are involved at the same time. Often the combination with spectroscopic investigations, especially with NMR and fluorimetry, may provide more details. 

In both media water and w/o-microemulsions the enzymes denaturate with time. The complete immobilisation of the enzyme system in a continuous process leads to a limitation caused by the denaturation of the enzymes only. In order to obtain a complete immobilisation of the enzyme the transmembrane pressure of the ultrafiltration unit should often not exceed 1 bar [120]. 

After the mild-hydrolysis step at 70°, the sialic acids liberated are removed from the sample by dialysis or ultrafiltration at 2°, and the macromolecular material is rehydrolyzed, using, however, the stronger acidic conditions of 0.1 M acid. The dialysis time ranges between 6 and 24 h, depending on the volume and viscosity of the hydrolysis mixture. Therefore, the optimum dialysis time should be evaluated by determinations of sialic acid in the eluate, or by addition of a trace of radioactive Nen5Ac. The dialyzates, or filtrates, are combined, and processed as will be described. By using this procedure, the overall yield of purified sialic acids is 70-80%, and the loss of O-acetyl groups107 is 40%. 

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