Clarifying Filtration

The usual objective of clarifying filtration is to separate solids at a very low concentration fi om a liquid stream. The liquid may be drinking (potable) water, wine, beer, oil, etc. and it is usually the liquid which is the valuable product. The techniques used in clarification processes include deep-bed, precoat, candle and cartridge filtration all of which involve capture of particles inside the porous mass of the filter. Such techniques produce clearer filtrates than those obtained in clarification by sedimentation. The filtration techniques listed are ofiioa complementary they are eirqrloyed for similar duties, but usually operate over different conditions of feed flow rate, feed concentration and process economics. These operating conditions are summarised in Table 6.1. 

Technique Typical face velocity Volume filtrate before regeneration per unit filter area at 0. Ig T sohds Relative running cost per unit mass removed Best filtrate quality achievable  

All clarifying filtration techniques include at least one of the following mechanisms  

The following mechanisms should also be considered in view of their importance in particle attachment/detachment mechanisms  

In addition to the above mechanisms orthokinetic flocculation may be induced due to the shear produced inside the porous media. This may encourage entrapment as collection efiddmcy usually inqiroves with increase in suspmded particle size. The collection mechanisms are described in greater detail below. 

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By filtration mechanism. Although the mechanism for separation and accumulation of solids is not clearly understood, hvo models are generally considered and are the basis for the apphcation of theoiy to the filh ation process. When solids are stopped at the surface of a filter medium and pile upon one another to form a cake of increasing thickness, the separation is called cake filtration. When solids are trapped within tne pores or body of the medium, it is termed depth, filter-medium, or clarifying filtration. 

Concentration. Clarified filtrates, centrates, or column eluates are usually too dilute for use in their specific applications, thus, substantial amounts of water must be removed. This can be achieved by evaporation or by ultrafiltration. Concentration methods used in industrial settings, such as evaporation, which is done under vacuum, and solvent extraction, may or may not be suitable for dewatering proteins because of their potential for thermal or chemical denaturation, and due to high energy costs associated with evaporation. The benefit of evaporation is that nonvolatile compounds that may stabilize the proteins are retained. 

Polarize a portion of the clarified filtrate after inversion in a 200 mm. jacketed tube at 87°, as directed under 24. In calcu-... 

Mechanisms and design models for filtration other than that forming a filter cake are also described, and so is non-Newtonian filtration. It is well known that a filtration cycle displays characteristics of these other filtration modes prior to the occurreace of cake filtration. This period is usually very short even at low concentrations of slurry. The other filtration modes are particularly relevant to clarifying filtration, at low to very low concentrations (less than 1% solids by volume). This type of filtration is inq>ortant industrially, and is typical of the deep-bed filtration discussed in Chapter 6. Filtration of non-Newtonian liquids is somewhat more specialised, but can be found in the filtration of oils and polymer melts. 

The cake filtration mechanism has already been explained in Section 2.4. The following discussion refers mainly to other types of filtration which can occur very early on in the otherwise cake filtration cycle, or over a prolonged period during clarifying filtration. 

Table 6.1 Comparison between clarifying filtration techniques...

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