Filtros

SELECTION OF FILTRATION EQUIPMENT

If a process developer who must provide the mechanical separation of solids from
a liquid has cleared the first decision hurdle by determining that filtration is the way
to get the job done or that it must remain in the running until some of the details of
equipment choice have been settled—choosing the right filter and right filtration conditions maystill be difficult. Much as in the broader determination of which unit operation to employ, the selection of filtration equipment involves the balancing of process specifications and objectives against capabilities and characteristics of the various equipment choices (including filter media) available.

The important process-related factors are slurry character, production throughput, processconditions, performance requirements, and permissible materials of construction.

The important equipment-related factors are type of cycle (batch or continuous), driving force, production rates of the largest and smallest units, separation sharpness, washing capability, dependability, feasible materials of construction, and cost.

The estimated cost must account for installed cost, equipmentlife, operating labor, maintenance, replacement filter media, and costs associated with product-yield loss (if any). In between the process and equipment factors are considerations of slurry preconditioning and use of filter aids.

Slurry characteristics determine whether a clarifying or a cake filter is appropriate; and if the latter, they determine the rate of formation and nature of the cake.They affect the choice of driving force and cycle as well as specific design of machine. There are no absolute selection techniques available to come up with the “best” choice since there are so many factors involved, many of them difficult to make quantitative and, not uncommonly, some contradictory in their demands. However, there are some published general suggestions to guide the thinking ofthe engineer who faces the selection of filtration equipment.
Figure 18-133 is a decision tree designed by Tiller [Chem. Eng., 81(9), 118 (1974)] to show the steps to be followed in solving a filtration problem. It is erected on the premise that rate of cake formation is the most important guide to equipment selection. A filter-selection process proposed by Purchas (op. cit., pp. 10–14) employsadditional criteria and is based on a combination of process specifications and the results of simple tests. The application is coded by use of Figs. 18-134, 18-135, and 18-136, and the resulting codes are matched against Table 18-11 to identify possible filters. Information needed for Fig. 18-137 can be obtained by observing the settling of a slurry sample (Purchas suggests 1 L)


in agraduated cylinder. Filter-cake-growth rate (Fig. 18-136) is determined by small-scale leaf or funnel tests as described earlier. Almost all types of continuous filters can be adapted for cake washing. The effectiveness of washing is a function of the number of wash displacements applied, and this, in turn, is influenced by the ratio of wash time to cake-formation time. Countercurrent washing,particularly with three or more stages, is usually limited to horizontal filters, although a two-stage countercurrent wash sometimes can be applied on a drum filter handling freely filtering material, such as crystallized salts.


Cake washing on batch filters is commonly done, although, generally, a greater number of wash displacements may be required in order to achieve the same degree of washingobtainable on a continuous filter.
Continuous filters are most attractive when the process application is a steady-state continuous one, but the rate at which cake forms and the magnitude of production rate are sometimes overriding factors.



A rotary vacuum filter, for example, is a dubious choice if a 3-mm (0.12-in) cake will not form under normal vacuum in less than 5 min and if less than...