Filtration dead-end

Dead end filtration is a batch process. That means that the filter will accumulate and eventually blind off with particulates such that water can no longer pass through. The filtration system will need to be taken off line and the filter will need to be either cleaned or replaced. 

2 is substituted into Equation 9.1, and integration from the start of filtration to time t (s) gives 

A suspension of baker s yeast = 10 kgm // =0.001 kgm s ) is filtered with a dead end filter (filter area= 1 m ) at a constant filtration pressure difference of 0.1 MPa. 

Calculate the volume of filtrate versus time relationship, when the specific cake resistance of baker s yeast a and the resistance of the filtering medium are 7 X lO m kg and 3.5 X 10 ° m , respectively. How long does it take 

In conventional filtration systems used for cell separation, plate filters (e.g., a filter press) and/or rotary drum filters are normally used (cf. Chapter 9). The filtrate fluxes in these filters decrease with time due to an increase in the resistance of the cake Rc (m 1), as shown by Equation 9.1. If the cake on the filtering medium is incompressible, then Rc can be calculated using Equation 9.2, with the value of the specific cake resistance a (m kg-1) given by the Kozeny-Carman equation (Equation 9.3). For many microorganisms, however, the values of a obtained by dead-end filtration (cf. Section 9.3) are larger than those calculated by Equation 9.3, as shown 

The average filtrate flux from the start of filtration to 149 min is 5.6xl(Tsin3nr2s 1. 

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Fig. 28. Schematic representation of dead-end and cross-flow filtration with microfiltration membranes. The equipment used in dead-end filtration is simple, but retained particles plug the membranes rapidly. The equipment required for cross-flow filtration is more complex, but the membrane lifetime is...

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomplished without the use of a membrane. The usual materi s retained by a microfiltra-tion membrane range in size from several [Lm down to 0.2 [Lm. At the low end of this spectrum, very large soluble macromolecules are retained by a microfilter. Bacteria and other microorganisms are a particularly important class of particles retained by MF membranes. Among membrane processes, dead-end filtration is uniquely common to MF, but cross-flow configurations are often used. 

A membrane is defined as an intervening phase separating two phases forming an active or passive barrier to the transport of matter. Membrane processes can be operated as (1) Dead-end filtration and (2) Cross-flow filtration. Dead-end filtration refers to filtration at one end. A problem with these systems is frequent membrane clogging. Cross-flow filtration overcomes the problem of membrane clogging and is widely used in water and wastewater treatment. 

Dead-end filtration through membrane filters is common in some industries where high purity is imperative. When clogged, the membrane has to be replaced. The water is first purified, and the filters serve as a final polisher. They are unsuitable for applications where they have to remove any significant concentration of particulate matter, as the cost of membrane replacement can become very high. 

All experimental setups described in the literature for the separation of homogeneous catalysts by membrane filtration technology can be divided into two general classes Dead-end filtration and cross-flow filtration. The first type of unit is characterized by a product flow perpendicular to the surface of the membrane, while the flow in the case of cross-flow filtration is parallel to the membrane surface (see Figure 4.1). 

The dead-end setup is by far the easiest apparatus both in construction and use. Reactor and separation unit can be combined and only one pump is needed to pump in the feed. A cross-flow setup, on the other hand, needs a separation unit next to the actual reactor and an additional pump to provide a rapid circulation across the membrane. The major disadvantage of the dead-end filtration is the possibility of concentration polarization, which is defined as an accumulation of retained material on the feed side of the membrane. This effect causes non-optimal membrane performance since losses through membrane defects, which are of course always present, will be amplified by a high surface concentration. In extreme cases concentration polarization can also lead to precipitation of material and membrane fouling. A membrane installed in a cross-flow setup, preferably applied with a turbulent flow, will suffer much less from this... 

Figure 4.2. Dead-end filtration reactor described by Vogt et

Commercial dead-end filtration cells are available from Millipore [12] suitable for ultrafiltration (Figure 4.4, e.g. model 8003 and 8010) and from Schleicher Schuell [13] (Figure 4.5) applicable for ultra- and nanofiltration. 

Figure 4.3. Dead-end filtration reactor4 described by Keurentjes et a/.[9,10]...

Dead-block coders, 7 691 Dead-burned dolomite, 15 27, 53 Dead-end filtration, 11 388 15 827, 829 Dead end hydrogenation reactor, 10 811, 812... 

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]. 

The lowest theoretical interparticular volume of perfectly packed uniformly sized spherical beads is calculated to be about 26% of the total available volume. In practice, even the best packed columns still contain about 30-40% void volume in addition to the internal porosity of the beads. The problem of interparticular volume does not exist in systems in which a membrane is used as the separation medium. Both theoretical calculations and experimental results clearly document that membrane systems can be operated in a dead-end filtration... 

There are two principal modes under which deep bed filtration may be carried out. In the first, dead-end filtration which is illustrated in Figure 7.1, the slurry is filtered in such a way that it is fed perpendicularly to the filter medium and there is little flow parallel to the surface of the medium. In the second, termed cross-flow filtration which is discussed in Section 7.3.5. and which is used particularly for very dilute suspensions, the slurry is continuously recirculated so that it flows essentially across the surface of the filter medium at a rate considerably in excess of the flowrate through the filter cake. 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

Figure 9.2 Alternative methods of filtration, (a) Dead-end filtration (b) cross-flow filtration.

Figure 14.4 Specific cake resistance of several microorganisms measured by dead-end filtration.

As stated in Chapter 9, cross-flow filtration (CFF) provides a higher efficiency than dead-end filtration, as some of particles retained on the membrane surface are swept off by the liquid flowing parallel to the surface. As shown by a solid line in Figure 14.6 [3], filtrate flux decreases with time from the start of filtration due to an accumulation of filtered particles on the membrane surface, as in the case of dead-end filtration. The flux then reaches an almost constant value, where... 

Membrane reactors allow a different option for the separation of biocatalysts from substrates and products and for retention in the reactor. Size-specific pores allow the substrate and product molecules, but not the enzyme molecules, to pass the membrane. Membrane reactors can be operated as CSTRs with dead-end filtration (Figure 5.5e) or as loop or recycle reactors (Figure 5.5f) with tangential (crossflow) filtration. 

In the last few years, a third type of microfiltration operating system called semi-dead-end filtration has emerged. In these systems, the membrane unit is operated as a dead-end filter until the pressure required to maintain a useful flow across the filter reaches its maximum level. At this point, the filter is operated in cross-flow mode, while concurrently backflushing with air or permeate solution. After a short period of backflushing in cross-flow mode to remove material deposited on the membrane, the system is switched back to dead-end operation. This procedure is particularly applicable in microfiltration units used as final bacterial and virus filters for municipal water treatment plants. The feed water has a very low loading of material to be removed, so in-line operation can be used for a prolonged time before backflushing and cross-flow to remove the deposited solids is needed. 

It may be possible to do a membrane autopsy to identify the foulant(s) and fouling mechanism. For microporous membranes the blocking law analysis [1], which uses permeate volume (V) vs. time (t) data, can supplement the observations. The generalized relationship at constant pressure and in dead-end filtration mode gives,... 

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

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