Filtered particle-phase pressure

Fig. 32. Filtered particle phase pressure (in CGS units) extracted from simulations over 16 x 16 cm domain using 128 x 128 cells. Source Andrews and Sundaresan (2005). The filtered particle-phase pressure includes the Reynolds stress-like fluctuations and the kinetic theory pressure.

When the cake structure is composed of particles that are readily deformed or become rearranged under pressure, the resulting cake is characterized as being compressible. Those that are not readily deformed are referred to as sem-compressible, and those that deform only slightly are considered incompressible. Porosity (defined as the ratio of pore volume to the volume of cake) does not decrease with increasing pressure drop. The porosity of a compressible cake decreases under pressure, and its hydraulic resistance to the flow of the liquid phase increases with an increase in the pressure differential across the filter media. 

A common cause of unusually high pressure is a plugged in-line filter. In-line filters are found at the very beginning of the flow line in the mobile phase reservoir, immediately before and/or after the injector, and just ahead of the column. With time, they can become plugged due to particles that are filtered out (particles can appear in the mobile phase and sample even if they were filtered ahead of time), and thus the pressure required to sustain a given flow rate can become quite high. The solution to this problem is to backflush the filters with solvent or clean them with a nitric acid solution in an ultrasonic bath. 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

Figure 7.18 gives the ratio (K /K)s4 s4 of the calculated equilibrium constants for solution-phase ammonium nitrate compared to the solid salt product at various temperatures and water activities. As the water activity, i.e., water vapor pressure above the solution, increases, the equilibrium constant falls. That is, at higher relative humidities, relatively less HNO, and NH, are found in the vapor phase at equilibrium. This may be why relatively more ammonium nitrate in particles collected on filters evaporates at lower RHs compared to higher ones. 

Step 3—In a separate step, styrene-acrylonitrile (SAN) resin is prepared by emulsion, suspension, or mass polymerization by free-radical techniques. The operation is carried out in stainless-steel reactors operated at about 75°C (167°F) and moderate pressure for about 7 hours. Tlie final chemical operation is the blending of the ABS graft phase with the SAN resin, plus adding various antioxidants, lubricants, stabilizers, and pigments. Final operations involve preparation of a slurry of fine resin particles (via chemical flocculation), filtering, and drying in a standard fluid-bed dryer at 121-132°C (250-270°F) inlet air temperature. 

Temperature fluctuation during the sampling period may be a more serious concern than pressure drop (Umlauf, 1999). The liquid-phase vapor pressure of POPs increases by about three-fold for a 10°C rise in temperature (Falconer and Bidleman, 1994 Hinckley et al., 1990). During a normal 24-hour collection run, particle-sorbed species which are deposited on the filter of a hi-vol sampler at night may be revolatilized in the heat of the next day. In some studies, collection periods have been kept to 11-12 hours to avoid the diurnal temperature cycle (Cotham and Bidleman, 1995 Foreman and Bidleman, 1990 Harner and Bidleman, 1998a). 

The separation of suspensions is the selective removal of suspended solids, say, by the ordinary processes of filtration. Application can also made to the separation of colloidal suspensions of minute or microscopic solid particles, and even of emulsions, the suspension of minute immiscible liquid droplets within another liquid phase. A distinguishing feature of ordinary filtration is usually that the discharged liquid phase does not form a continuum on the downflow or reject side of the membrane, or filter, and more or less exists at atmospheric pressure. If otherwise, if a contiunuum is formed, the process is more that of reverse osmosis, also called hyperfiltration. In common use, notably for the upgrading or desalination of salt water or brackish water, reverse osmosis is a subject for special consideration. 

The problem of the removal of aerosol particles from gas streams has become of increasing importance from the standpoint of public health and the recovery of valuable products. Technology of controlling the aerosol particles or improving the liquid phase of aerosol is very important in many industrial processes such as oil and petroleum, electronic, mining, and food, as well as waste products like noxious emission of aerosol in chemical plants. There are several ways for this purpose among which fibrous filters are more popular so that it is obvious to try to improve their efficiency. The efficiency of collection and the pressure drop are the most important practical considerations in the design of these fibrous filters [2], Various... 

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