Dead-end cake filtration

Figure 3.4 The effect of pH on the dead-end cake filtration of china clay suspensions.

We observed earlier in dead-end cake filtration (equation (6.3.135k)) that Rcs varies inversely with the square of the particle radius therefore, in effect, the filtration flux varies with the square of the particle radius for cake dominated filtration. The larger the particle radius, the higher the filtration flux. As shown in equation (7.2.145), in cross-flow microfiltration also the averaged filtration flux increases with particle radius, here as Romero and... 

Difference between dead-end (conventional) filtration and cross-flow filtration. Rq is the resisitance of the cake formed on the membrane by the impermeable solutes, is the resistance of the membrane and J is the flux. 

Tarleton E.S. and Morgan S.A., 2001. An experimental study of abrupt changes in cake structure during dead-end pressure filtration. Filtration, 1(4), 93-100. 

In a dead-end membrane filtration process, the number of particles instantaneously arriving at the surface of membrane or a formed fouling layer is mainly controlled by the filtration rate and slurry concentration, while the packing structure of the particles depends on their size, shape, physical and chemical properties and so forth. Since the solid compressive loading on a thin layer of the cake surface is limited, the porosity of the surface cake layer, e (that is Si i, 2,2 and ii in Figure 15.6), during a membrane filtration process can be assumed to be a constant value and can be preliminarily estimated by a low-head filtration experimental system proposed by Haynes [41]. 

The solid-liquid separation of shinies containing particles below 10 pm is difficult by conventional filtration techniques. A conventional approach would be to use a slurry thickener in which the formation of a filter cake is restricted and the product is discharged continuously as concentrated slurry. Such filters use filter cloths as the filtration medium and are limited to concentrating particles above 5 xm in size. Dead end membrane microfiltration, in which the particle-containing fluid is pumped directly through a polymeric membrane, is used for the industrial clarification and sterilisation of liquids. Such process allows the removal of particles down to 0.1 xm or less, but is only suitable for feeds containing very low concentrations of particles as otherwise the membrane becomes too rapidly clogged.2,4,8... 

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. 

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

Tangential filtration is distinguished from conventional filtration, also known as dead-end filtration, by the fact that the main flow is tangential to the membrane surface (Figure 11.11). In dead-end filtration a fast drop in the permeate flux is observed due to cake formation over the filter... 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

In dead-end filtration, a cake forms on the surface of the pad as the filtration proceeds. The cake permeability is the most important physical property of a porous medium and the hydraulic properties of the flow and the specific cake resistance are described by Darcy s Law ... 

In filtration unit operation, especially in microfiltration, one usually differentiates between dead-end filtration (with cake formation) and cross-flow filtration [25] (Fig. 5). The cross-flow filter can have different geometries (Fig. 6) phase membranes, tubular membranes, or pleated membranes, of which the tubular and pleated ones are already accepted as cross-flow geometries in reactor technology, as mentioned above. In filtration engineering the cross-flow term means that the filtrate flows perpendicularly to the suspension stream. Cross-flow may not be considered a sufficiently illustrative term here [25]. A better term would be parallel filtration, but the term cross-flow filtration has been accepted generally and may be difficult to change at present. 

Figure 5 Dead-end filtration with cake formation and cross-flow cakc-frcc filtration. (From...

Microfiltration membranes are similar to UF membranes but have larger pores. Microfiltration membranes are used to separate particles in the range of 0.02-10 pm from liquid or gas streams. Commercial MF membranes are made from a wide variety of materials including polymers, metals, and ceramics. A wide variety of membrane module designs are available including tubular, spiral wound, pleated sheet, hollow fiber, and flat sheet designs. Some modules are best suited for crossflow filtration, and others are designed for dead-end filtration. In dead-end filtration, the feed liquid flows normal to the surface of the membrane, and retained particles build up with time as a cake layer on the membrane surface or within the pores of the membrane. 

Membrane Formation. In earlier work. 2.) it was found that fumed silica particles could be dispersed in aqueous suspension with the aid of ultrasonic sound. Observations under the electron microscope showed that the dispersion contained disc-like particles, approximately 150-200 1 in diameter and 70-80 1 in height. Filtration experiments carried out in the "dead-end" mode (i.e., zero crossflow velocity) on 0.2 urn membrane support showed typical Class II cake formation kinetics, i.e., the permeation rate decreased according to equation (12). However, as may be seen from Figure 7, the decrease in the permeation rate observed during formation in the crossflow module is only t 1, considerably slower than the t 5 dependence predicted and observed earlier. This difference may be expected due to the presence of lift forces created by turbulence in the crossflow device, and models for the hydrodynamics in such cases have been proposed. 

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 estimation of a steady-state value of the CFF filtrate flux in general is difficult, because it is affected by many factors, including the type of membrane module, the characteristics of the membranes and suspensions, and the operating conditions. For example, it has been reported that the specific cake resistance of cocci in CFF was equal to the value obtained by dead-end filtration. However, the cake resistance became much larger in the CFF of bacillar microorganisms... 

Hermia (1982) introduced the Filtration Laws, which aim to describe fouling mechanisms. The models are valid for unstirred, dead-end filtration (deposition without cake dismrbance due to shear and no gravity settling) and complete rejection of solute by the membrane (but obviously allowing pore penetration). Under conditions where permeate drag dominates, the effect of stirring may be negligible. The constant pressure filtration law is shown in equation (3.11). 

---