Page 358 - Chemical Process Equipment - Selection and Design

11.7. APPLICATIONS AND PERFORMANCE OF EQUIPMENT 321
expelled before the bulk of the cake is deposited. Moreover, when
Depth
Sulficlenl the cake is finally deposited from a thickened slurry, it does so with
To Hold Slurry
Wol. For Ome Tes an open structure that allows rapid filtration. A similar factor is
operative in belt or top feed drum filters in which the coarse
particles drop out first and thus form the desirable open structure.
A review of such methods of enhancement of filtration rates is by
Svarovsky (1981).
The relative suitability of the common kinds of solid-liquid
separation equipment is summarized in Table 11.3. Filtration is the
To Gas most frequently used operation, but sedimentation as a method of
Meter pretreatment and centrifugation for difficulty filterable materials has
many applications. Table 11.15 gives more detail about the kinds of
filters appropriate to particular services.
Representative commercial sizes of some types of pressure
filters for operation in batch modes are reported in Table 11.11.
Some of these data are quite old, and not all of the equipment is
currently popular; thus manufacturers should be consulted for the
latest information. Commercially available size ranges of continuous
Gas or air belt, rotary drum, rotary disk, and horizontal rotary filters are listed
pressure
in Table 11.12. For the most part these devices operate with vacua
of 500 Torr or less.
Sedimentation equipment is employed on a large scale for
mineral and ore processing. These and other applications are listed
in Table 11.9(a). The clarification operations of Table 11.9(b) are
Jacket of water cleaning and sewage treatment. The sludges that are
ou+lek- formed often are concentrated further by filtration. Such
I-
applications are listed in Table 11.16 along with other common
applications of plate-and-frame filter presses. Sludge filter cakes are
compressible and have high resistivity so that the elevated pressures
at which presses can be operated are necessary for them. Among
the kinds of data given here are modes of conditioning the slurries,
slurry concentrations, cake characteristics, and cycle times.
Clarification of a great variety of industrial liquids is
accomplished on smaller scales than in tank clarifiers by application
of cartridge filters; some of these applications are listed in Table
Therm 11.10.
Cycle times, air rates, and minimum cake thicknesses in
operation of rotary drum filters are stated in Table 11.13. A few
special applications of horizontal belt filters are given in Table 11.14,
but in recent times this kind of equipment is taking over many of
the traditional functions of rotary drum filters. Belt filters are
favored particularly for freely filtering slurries with wide range of
'Filter cake I Fil particle sizes.
The applications listed in Table 11.17 and 11.18 are a few of
s . 1 Jackist inlet those of rotary drum, rotary disk, and tipping or tilting pan filters.
(b)
Drain The last type employs a number of vacuum pans on a rotating
circular track; after the cake is formed, the pans are blown back
Figure 11.7. Two types of laboratory filter arrangements. (a) with air and then tipped to discharge the cake. The data of these
Vacuum test filter arrangement; standard sizes are 0.1, 0.05, or tables include particle size range, moisture content of the cake,
0.025 sqft (Dahlstrorn and Siluerblutt, 1977). (b) Laboratory filtering rate, solids handling rate, vacuum pump load and degree of
pressure filter with a vertical filtering surface and a mechanical vacuum. Clearly a wide range of s0me of these variables occurs in
agitator; mild air agitation may be preferred (Bosley, 1977).
practice.
Characteristics of centrifugal filters and sedimentation centri-
manufacturers' literature. Representative data are collected in this fuges are in Table 11.19. The filtering types are made to handle
section and summarized in tabular form. One of the reasons why from less than 5 tons/hr to more than 100 tons/hr of solids, with
more performance data have not been published is the difficulty of g-levels ranging from 30 to 3000. For sedimentation types, the
describing each system concisely in adequate detail. Nevertheless, g-levels listed range up to 18,000, but high values can be used only
the limited listmgs here should afford some perspective of the with small diameter equipment because of metal strength
nature and magnitude of some actual and possibly potential limitations. Capacity of sedimentation types is measured in terms of
applications. liquid rates, the maximum listed here being 100,00Q&/hr. An
Performance often is improved by appropriate pretreatment of outstanding feature of centrifugal separators is the small sizes of
the slurry with flocculants or other means. An operating practice particles that can be handled satisfactorily; the values in the table
that is finding increasing acceptance is the delaying of cake cover the range 1-400pm. Short retention time is a feature of
deposition by some mechanical means such as scraping, brushing, centrifuge operation that may be of interest when unstable materials
severe agitation, or vibration. In these ways most of the filtrate is need to be processed.