Filtration, cost pressure

The cross-flow filtration is often used in combination with the procedure known as backflushing, whereby the filtration flow is reversed for a short period of time so that particles or molecules very attached are removed from the membrane surface. The speed of cross-flow filtration, transmanbrane pressure, and the frequency of backflushing are important process parameters, which are optimized to obtain low fouling, high permeate flux, and low energy costs. In many such... 

The so-called hyperbar vacuum filtration is a combination of vacuum and pressure filtration in a pull—push arrangement, whereby a vacuum pump of a fan generates vacuum downstream of the filter medium, while a compressor maintains higher-than-atmospheric pressure upstream. If, for example, the vacuum produced is 80 kPa, ie, absolute pressure of 20 kPa, and the absolute pressure before the filter is 150 kPa, the total pressure drop of 130 kPa is created across the filter medium. This is a new idea in principle but in practice requires three primary movers a Hquid pump to pump in the suspension, a vacuum pump to produce the vacuum, and a compressor to supply the compressed air. The cost of having to provide, install, and maintain one additional primary mover has deterred the development of hyperbar vacuum filtration only Andrit2 in Austria offers a system commercially. 

Pressure leaf filters are used to separate much the same lands of slurries as are filter presses and are used much more extensively than filter presses for filter-aid filtrations. They should be seriously considered whenever uniformity of production permits long-time operation under essentially constant filtration conditions, when thorough washing with a minimum of hquor is desired, or when vapors or fumes make closed construction desirable. Under such conditions, if the filter medium does not require frequent changing, they may show a considerable advantage in cycle and labor economy over a filter press, which has a lower initial cost, and advantages of economy and flexibility over continuous vacuum filters, which have a higher first cost. 

Natural ventilation is the controlled flow of air through doors, windows, vents, and other purposely provided openings caused by stack effect and wind pressure. Natural ventilation is used in spaces with a significant heat release, when process and hygienic requirements for indoor air quality allow outdoor air supply without filtration and treatment. Natural ventilation cannot be used when incoming outdoor air causes mist or condensation. Natural ventilation allows significant air change rates (20 to 50 ach) for heat removal with ntinimal operation costs. 

Pressure filters or filter presses are commonly of the batch type, and are characterized by smaller floor area, high filtration rates, and lower capital cost. Dryer cakes are produced. The chemical industry uses these filters more widely than mineral processing industries, mainly because of its batch operation. The most common types of pressure filters used are the plate and frame presses. These comprise a series of vertical, alternating parallel frames and plates, with the filter cloth being held against the plate and the formation of cake occurring in the hollow frame. 

This cycle of vaporisation of the solvent, condensation, extraction, and vacuum-filtration may be repeated any number of times in a solid-fluid serial extractor. The occurrence of an extractive material fluid bed as a result of the flow of boiling hot vapour provides for effective extraction, while pressure filtration provides for short cycle times. This functional principle makes it possible to achieve filtration pressures which are 50-100 times more effective than when using the Soxhlet method, where only the low hydrostatic pressure of the extractive fluid operates. Solid-fluid-vortex extraction according to the proprietary FEXTRA (Feststoff Extraktion) principle is low cost. 

Another method, which is even more successful in preventing binding of the septum, is the use of a precoat.53 Before filtration is begun a coating of 2-6 in (5-15 cm) of diatomaceous earth or perlite filter aid is deposited on the surface of the septum. During filtration operations the scraper is set so that it slowly removes the precoat and, of course, with it the materials that would have plugged the filter. Since the precoat causes a considerable pressure drop, the rate of filtration is slowed down. Flow rates may vary from 2 to 50 gal/hr/ft2 (0.025-0.60 m/hr). The precoat material costs around 3 or 40/lb and is used at the rate of 10- 15 lb/1,000 gal of feed (1,200 to 1,800 kg/m3). 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

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