Transmembrane pressure filtration

The factors to consider in the selection of crossflow filtration include the flow configuration, tangential linear velocity, transmembrane pressure drop (driving force), separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, low protein-binding ability, and hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

Kempken et al. [113] employed a rotating disc filter to harvest CHO cells, and observed that the filter could be operated at low transmembrane-pressure with high wall shear rates, leading to high filtrate flow rates, high product yields and minimum fouling. They concluded that their system offered a powerful alternative to conventional tangential flow filtration. 

The change of flux velocity with transmembrane pressure can be explained by the concentration polarisation phenomenon. The physical processes at the membrane surface during the filtration procedure may be described by theo-... 

A hemodialysis membrane with an effective area of 0.06 m, thickness of 50 ttm, and permeability of 2.96 x 10 " m is used to filter urea and other impurities from the blood. The viscosity of blood plasma is 1.2 x 10 N s/m. What is the expected filtration rate if the transmembrane pressure is 2.25 Pa ... 

Albumin solutions (1, 2, and 5wt%) are continuously ultrafiltered through a flat plate filter with a channel height of 2 mm. Under cross-flow filtration with a transmembrane pressure of 0.5 MPa, steady-state filtrate fluxes (cm min ) are obtained as given in Table P9.2. 

Figure 14.6 Cross-flow filtration flux of baker s yeast suspension. Cell concentration 7%. Transmembrane pressure 0.49 bar. Flow rate 0.5 ms". ...

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

Under certain conditions, scale-up of membrane reactors is straightforward. Provided that (i) the reactor contents are well mixed so that the reactor is operated as a CSTR, and that (ii) the membrane is configured for filtration in the tangential mode, the pertinent design criterion, besides constant residence time T in the reactor, is constant fluidity F of the substrate/product solution through the membrane at all reactor scales. Fluidity is defined by Eq. (19.36) (V = ultrafiltered volume, AP = transmembrane pressure, t = filtration time, and A = membrane area). 

It is interesting to note that the problem of operation with large pressure differences between the feed/retentate side and the permeate compartment of membrane filtration modules was identified long ago. The concept of operation under low uniform transmembrane pressure (UTMP) was pioneered and first... 

Defiance, L. and Jaffrin, M.Y. (1999) Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux application to a membrane bioreactor used for wastewater treatment. Journal of Membrane Science, 152, 203-210. 

The following processes can be described as selective therapeutic plasmapheresis. In a first step, blood is withdrawn from the patient and separated by crossflow filtration in a hollow-fiber membrane cartridge water and some plasma solutes are transferred through a semipermeable membrane under a convection process. The transmembrane pressure applied from blood to filtrate compartment ensures flow and mass transfers. Then, the filtrate perfuses the adsorption columns where toxins are retained and is finally mixed with blood cells and other plasma components before returning to the patient (Figure 18.11). 

Dynamic filtration modules present a relative movement between the membrane and the module, or between the membrane and a rotor. Thus, it is possible to adjust the shear stress independently of the feed flow rate and of the transmembrane pressure drop. 

Fig. 11. F concentrations in the permeate vs. time during the filtration of the Fatick water on NF90, NF270 and BW30 membranes (Feed composition see Table 9 operation conditions Transmembrane pressure 7.5 bars conversion ratio 0.87).

Fig. 14.6. Filtration flux as a function of time of filtration for the filtration of O.Oi g/L silica particles in 0.001 M NaCI solution at pH 6 at a membrane of mean pore diameter 84 nm. The particle size was very close to the pore size. The critical transmembrane pressure for these conditions was calculated as 130 kPa. Operation below this pressure gives only a gradual decline in filtration flux with time. Operation above this pressure gives an initially higher filtration flux which declines rapidly with time. In the latter case the intial hydrodynamic force exceeds the electrical double layer repulsion between the membrane and the particles, causing the particles to block the membrane pores.

Microfiltration is a unit operation for the separation of small particles. The separation limits are between 0.02 and 10 (jum particle dimensions. Microfiltration can be carried out in a dead-end mode and a cross-flow mode. In downstream processing, the cross-flow filtration is carried out continuously or discontinuously. The most important parameters that determine the productivity of cross-flow microfiltration are transmembrane pressure, velocity, particle size and surface, viscosity of the liquid and additives such as surfactants, and changing the surface and surface tension. 

In this section, we will show the process of the construction of a mathematical model, step by step, in accordance with the procedure shown in Fig. 3.4. The case studied has already been introduced in Figs. 1.1 and 1.2 of Chapter 1. These figures are concerned with a device for filtration with membranes, where the gradient is given by the transmembrane pressure between the tangential flow of the suspension and the downstream flow. The interest here is to obtain data about the critical situations that impose stopping of the filtration. At the same time, it is important to, a priori, know the unit behaviour when some of the components of the unit, such as, for example, the type of pump or the membrane surface, are changed. 

In this situation, if the pressure filtration stays unchanged, the filtrate rate will decrease with time. When unacceptable values of the filtrate rate are reached, the process must be stopped and the membrane cleaned or replaced. This mode of operation is uneconomical. One solution to this problem is to increase the transmembrane pressure in order to maintain the flow rate but, in this case, the pumping flow rate has to be reduced because pumps generally present a pre-established and characteristic flow rate-pressure relation which is, a priori, unchangeable. Consequently, when the pressure is continuously increased, the clogging rate will increase faster than when a high tangential velocity is used in the unit. 

Throughput characterizes how much filtration can be conducted before the operation needs to be stopped due to fouling. As the membrane fouls there is a decrease in the flux or an increase in transmembrane pressure (TMP) depending on the operation mode, and the operation is stopped when either the flux is too low or the TMP is too high. The amount of filtrate that is processed per unit membrane area is called throughput. 

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