Liquid filtration treatment processes

Cross-flow filtration is also referred to as tangential flow filtration or microfiltration, but all three terms refer to a process by which membranes are used to separate components in a liquid solution (or suspension) on the basis of their size. The development of robust membranes in polymeric and ceramic materials has provided a powerful new technology for bioseparations, which is already widespread in the process industries as well as for water treatment processes. 

An example of a solid-liquid phase separation - often referred to as a mechanical separation - is filtration. Filters are also used in gas-sohd separation. Filtration may be used to recover liquid or sohd or both. Also, it can be used in waste-treatment processes. Walas [6] describes many solid-hquid separators, but we will only consider the rotary-drum filter. Reliable sizing of rotary-drum filters requires bench and pilot-scale testing with the slurry. Nevertheless, a model of the filtering process will show some of the physical factors that influence filtration and will give a preliminary estimate of the filter size in those cases where data are available. 

The term post-treatment refers to processes vdiich are used after the principal solid-liquid separation has been achieved. The final step in the latter operation is usually some type of cake filtration which will leave a cake that may be fully saturated with liquid or partially drained and it is extremely unlikely that the solids are in an acceptable condition for any subsequent operation such as thermal drying. Thus the post treatment processes are washing and deliquoring which are concerned req>ectively with removing the soluble solids from the liquid remaining in the cake and then purging the major proportion of the liquid fi om the cake pores. 

Major constituent of paper, cardboards, and textiles is the cellulose. Some of the products of cellulose are cellophane, rayon fiber, nitrocellulose, water-soluble adhesives, and binders. Powdered cellulose is used as inactive fillers in tablets and as a thickener and stabilizer in processed foods. In the laboratories, cellulose is used in TLCs as a stationary phase, liquid filtration, as highly hydrophilic and absorbent sponges. These days cellulose insulation made from recycled paper is becoming popular as an environment-fi-iendly material. On treatment with boric acid cellulose can be used as a fire retardant. 

The recovery of dissolved inorganic materials can be achieved by evaporation and crystallization, although membrane filtration is probably a more economic process. If the materials are unwanted, then membrane separation will still remove them from the liquid wastes, but only as a concentrated solution, still in need of treatment. Chemical reaction, probably also involving pH adjustment, to convert the dissolved salts into an insoluble precipitate, is an attractive treatment process, with filtration of the precipitate from the resultant suspension. 

Filtration is more normally used as a post-treatment process, further to improve the water quality. However, oil-water separators are also designed on a filtration basis. When two non-miscible liquids of different densities flow by gravity through a porous medium, they wiU have different penetrability rates. In such a filter, the oil-water mixture first enters a sedimentation tank, where heavier particles settle out. The oil-water mixture then enters an inlet basket and flows into the porous medium, where the oil separates from the water. The water flows on down through a screen at the bottom, then up and over a water weir, which creates the water table supporting the oil longer. The oil travels horizontally on top of the water into the oil trap and then over an adjustable oil weir, which is set at a position slightly above the maximum water level. 

With powder activated earbon, in most cases, the carbon is dosed into the liquid, mixed and then removed by a filtration process. In some cases, two or more mixing steps are used to optimise the use of powder carbon. Powder activated carbon is used in a wide range of liquid phase applications and some specific gas phase applications such as Incinerator flue gas treatment and where it is bonded into filters sueh as fabrics for personnel protection. 

Flocculation and sedimentation arc two processes used to separate waste streams that contain both a liquid and a solid phase. Both are well-developed, highly competitive processes, which arc oflcii used in the complete treatment of waste streams. They may also be used instead of, or in addition to, filtration. Some applications include the removal of suspended solid particles and soluble heavy metals from aqueous streams. Many industries use both processes in the rcmowal of pollutants from their wastewaters. These processes work best when the waste stream contains a low concentration of the contaminating solids. Although they are applicable to a wide variety of aqueous waste streams, these processes arc not generally used to treat nonaqueous or semisolid waste streams such as sludges and slurries. 

Regarding the color, we only see a need for colorless ionic liquids in very specific applications (see above). One easy treatment that often reduces coloration quite impressively, especially of imidazolium ionic liquids, is purification by column chromatography/filtration over silica 60. For this purification method, the ionic liquid is dissolved in a volatile solvent such as CFF2C12. Usually, most of the colored impurities stick to the silica, while the ionic liquid is eluted with the solvent. By repetition of the process several times, a seriously colored ionic liquid can be converted into an almost completely colorless material. 

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