Vacuum drum filters performance

Under all conditions, it is essential to operate with combinations of drum speed, submergence and vacuum, which, for the feed conditions that apply, will ensure that a fully dischargeable cake is formed. If this is not done then progressive deterioration in the effectiveness of the filter medium will occur and this will adversely affect the performance of the machine. Typical filtration cycles for drum filters are given in Table 7.4. 

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... 

Commercially, separation is mostly performed by vacuum filtration on belt or drum filters or by pressure in membrane filters (Illingworth, 2002). Evenly-sized well-developed spherical crystals, such as those obtained in a well-executed Tirtiaux process, can be easily recovered using vacuum filters. For optimal filtration, the air on top of the filter may be cooled or heated to the filtration temperature to avoid melting or crystallization on the filter and clogging of the filter cake. This results in increasing entrainment and reducing filter speed. A considerable amount of oil is entrained in the filter cake, which can be as much as 60-70% for milk fat. 

As previously noted, there are two available modules for eqvfipment simulation, namely vacuum filters and pressure filters (which include variable volvune filters). Within either module there are several options available from the Start Menu (see, e.g. Figure 5.8), each of which facilitates the simulation of a particular type of filter. All simulations are performed in the same general manner, the principal differences arise as a consequence of the operational limitation(s) of filter type. Figure 5.15 shows an example screen display for the simulation of a bottom fed rotary drum filter fitted with a knife discharge. 

Rotary drum vacuum filters are a widely used piece of equipment for this filtration step, due to its simple operation and low investment costs. The drum is typically precoated (e.g., with diato-maceous earth, sUica, perlite) to improve the process performance [7]. Additional filter aid is often added to improve the filtrate qnality hence, the amount of slndge generated from this unit operation is relatively large. Several other filtration devices such as filter automates and cross-flow filtration devices are also nsed for the filtration. One of the benefits of the latter is that they can be operated withont addition of filter aids and thus, the amount of generated sludge can be reduced. 

The SulFerox sulfur-cake matrix tends to be quite compressible and reportedly lends itself well to filtration via plate-and-fiame filters or automated batch filter presses with recessed membranes for post-filtration sulfur-cake squeezing. It is claimed that only small amounts of wash water are needed to displace the residual process solution from the filtercake. Rotary drum vacuum filters were used in all early SulFerox applications, but variations in feed slurry characteristics due to differing inlet gas contaminants and required additive levels made their performance inconsistent. It is repotted that filtration by pressurized-feed filter presses has eliminated the problem of cake quality variation and has substantially reduced iron chelate losses (Anon., 1994). The sulfur filtercake from pressurized-feed filter presses is reported to contain 10 to 23 wt% moisture (Allen, 1995). 

Vacuum filtration typically is performed using a mechanically supported cylindrical rotating drum covered by a filter medium (cloth, coil springs, or wire mesh fabric). Water is drawn into the center by vacuum while the solids are scraped off the drum. 

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