Cross-flow operation mode, modules

In a general way, most of ceramic membrane modules operate in a cross-flow filtration mode [28] as shown in Figure 6.18. However, as discussed hereafter, a dead-end filtration mode may be used in some specific applications. Membrane modules constitute basic units from which all sorts of filtration plants can be designed not only for current liquid applications but also for gas and vapor separation, membrane reactors, and contactors, which represent the future applications of ceramic membranes. In liquid filtration, hydrodynamics in each module can be described as one incoming flow on the feed side gf, which results in two... 

As cross-flow mode is preferred over a dead-end one, the proper choice of the module is the next crucial step. For a given module design and feed solution, the cross-flow velocity is the main parameter that determines mass transfer in the module. Four cross-flow operations shown in Fig. 17 are used ... 

For industrial applications, a cross-flow operation is preferred because of the lower fouling tendency relative to the dead-end mode (figure VIII - 14b). In the cross-flow operation, the feed flows parallel to the membrane surface with the inlet feed stream entering the membrane module at a certain composition. The feed composition inside the module changes as a function of distance in the module, while the feed stream is separated into two a permeate stream and a retentate stream. The consequences of fouling in dead-end systems are shown schematically in figure VUI - 15. In dead-end filtration, the cake grows with time and consequently the flux decreases with time.Hux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module choice and cross-flow velocities. 

The cross-, co- and counter-flow schemes are illustrated in Figure 4.17, together with the concentration gradient across a median section of the membrane. It follows from Figure 4.17 that system performance can be improved by operating a module in an appropriate flow mode (generally counter-flow). However, such improvements require that the concentration at the membrane permeate surface equals the bulk concentration of the permeate at that point. This condition cannot be met with processes such as ultrafiltration or reverse osmosis in which the permeate is a liquid. In these processes, the selective side of the membrane faces the... 

Boundary layer formulation. Many membrane processes are operated in cross-flow mode, in which the pressurised process feed is circulated at high velocity parallel to the surface of the membrane, thus limiting the accumulation of solutes (or particles) on the membrane surface to a layer which is thin compared to the height of the filtration module [2]. The decline in permeate flux due to the hydraulic resistance of this concentrated layer can thus be limited. A boundary layer formulation of the convective diffusion equation can give predictions for concentration polarisation in cross-flow filtration and, therefore, predict the flux for different operating conditions. Interparticle force calculations are used in two ways in this approach. Firstly, they allow the direct calculation of the osmotic pressure at the membrane. This removes the need for difficult and extensive experi-... 

Membrane modules can be operated in the dead-end or cross-flow modes (see Figure 18.8). Dead-end ultrafiltration is used mostly for laboratory-scale applications and industrial ultrafiltration processes are usually carried out in the cross-flow mode. The main advantage of cross-flow ultrafiltration is the lower extent of concentration polarization. The cross-flow mode also allows recirculation of the retentate stream to the feed tank followed by its mixing with fresh feed that leads to several operational advantages. 

By the use of hollow-fiber membranes, the ideal module comprises short fibers with a wide bore to avoid a high pressure drop in the flow direction thereby disturbing the uniform radial flow pattern and creating channeling. The membranes should also possess thick porous walls with small pore size and a high ligand density. The hollow-fiber modules can be operated in cross-flow mode that makes them especially suitable in the treatment of solutions containing particulate material. 

Dead-end Mode of operation where there is only one feedstream and one outlet stream (the filtrate of permeate). Refers to modules that do not have a means for cross-flow, e.g. stirred cells. Cross-flow modules could be operated in the dead-end mode by shutting off the reject outlet of the module. 

A hollow fibre module can be operated in three different modes co-current, cross, or counter-current flow. In co-current flow, the permeate flows in the same direction as the feed and retentate. In cross-flow, the permeate flows perpendicularly to the feed and retentate while in countercurrent flow the permeate flows in the opposite direction. The countercurrent flow pattern gives the best performance as the driving force for transport is maximized along the module length. One can produce an arbitrarily high purity... 

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