Ceramic membranes liquid-solid separation

Separation of manufactured sohds from process liquids and recycling of these liquids (water or organic solvents) is an interesting way to valorize by-products and to minimize the production of liquid effluents in a number of industries. Microfiltration ceramic membranes have been aheady used for the recovery of particles in the ceramic industry and in drilling operations, of pigments in paint and ink industries, and have potential applications in a wide variety of liquid-solid separation... 

The third main class of separation methods, the use of micro-porous and non-porous membranes as semi-permeable barriers (see Figure 2c) is rapidly gaining popularity in industrial separation processes for application to difficult and highly selective separations. Membranes are usually fabricated from natural fibres, synthetic polymers, ceramics or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibres or spiral-wound sheets. For the micro-porous membranes, separation is effected by differing rates of diffusion through the pores, while for non-porous membranes, separation occurs because of differences in both the solubility in the membrane and the rate of diffusion through the membrane. Table 2 is a compilation of the more common industrial separation operations based on the use of a barrier. A more comprehensive table is given by Seader and Henley.1... 

There are also techniques involving the use of nonporous, solid or liquid membranes that separate the donor phase from the receiving phase by an evident phase boundary. Most often used are three-phase systems (donor phase, membrane, and acceptor phase) or two-phase systems, in which one of the surrounding phases is the same as the membrane. Solid membranes are made of chemically resistant, hydrophobic polymers (PTFE, PVDF, PS, PP, silicates), metals (Pd alloys), or ceramic materials. Channels of membrane modules have a volume ranging from 10 to 1000 pL and, according to their geometry, can be classified as planar or fibrous. For setting up a membrane system, two modes can be used the membrane can be immersed in a sample (membrane in sample, MIS) or the sample can be introduced into a membrane (sample in membrane, SIM). In both systems, only a small amount of sample is in direct contact with membrane, because ratio of the membrane surface area to the sample volume is small. 

The dual-phase (DP) membrane used in analytical separation usually consists of a polymer, or in some cases a ceramic solid-phase support impregnated with a fluid (i.e., gaseous or liquid phase). If the fluid is air the DP membrane is known as a gas-diffusion membrane. DP membranes incorporating a liquid phase can be considered in a broader sense as liquid membranes. The liquid phase in a liquid DP membrane can be identical to the feed and/or receiver solution (e.g., dialysis membranes, membrane-assisted LLE (MALLE)) or it can form a third immiscible liquid phase in the membrane separation system (e.g., supported and polymer liquid membranes). Membranes incorporating a liquid phase immiscible with the feed and receiver solutions are usually referred to as liquid membranes. This narrower definition of liquid membranes, currently accepted in the literature, will be used in subsequent discussions. The... 

In many processes, including those in nature, transport proceeds via diffusion rather than convection. Substances diffuse spontaneously from a high to a low chemical potential. Processes which make use of a concentration difference as the driving force are gas separation, vapour permeation, pervaporation, dialysis, diffusion dialysis, carrier mediated processes and membrane contactors (In pervaporation, gas separation and vapour permeation it is preferred to express the driving force as a partial pressure difference or an activity difference rather than concentration difference). On the basis of differences in structure and functionality it is possible to distinguish between processes that use a synthetic solid (polymeric or sometimes ceramic or zeolitic) membrane (gas separation, dialysis and pervaporation) and those that use a liquid (with or without a carder) as the membrane. 

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