Page 324 - Chemical process engineering design and economics

Separator Design 303

Aj 6.916m 2
L D = — = ————— = 5.674 m (18.5 ft)
D 1.219m

Thus, the total decanter length is

L = L s + L D = 0.8180 + 5.624 = 6.442 m (21.14 ft)

Rounding the length off in 3 in increments, L = 21.25 ft (6.447 m). We should
increase the decanter length to account for the diffuser plates at the entrance of the
decanter. There appears to be no rule on the needed length except that the plates
are closely spaced. We will assume six inches will be needed. Thus, L = 21.75 ft
(6.63 m).
The length to diameter ratio is

L 6.63
— = ——— = 5.44
D 1.219

The ratio recommended by Barton [20] is five for settlers without considering the
coalescence time for the droplets.

Solid-Liquid Separators

An example of a solid-liquid phase separation - often referred to as a mechanical
separation - is filtration. Filters are also used in gas-solid separation. Filtration
may be used to recover liquid or solid or both. Also, it can be used in waste-
treatment processes. Walas [6] describes many solid-liquid separators, but we will
only consider the rotary-drum filter. Reliable sizing of rotary-drum filters requires
bench and pilot-scale testing with the slurry. Nevertheless, a model of the filtering
process will show some of the physical factors that influence filtration and will
give a preliminary estimate of the filter size in those cases where data are avail-
able.

Rotary-Drum Filters

As shown in Figure 6.8, a rotary-drum filter consists of three parts: a drum with
an automatic filter valve, a filter tank with a slurry agitator, and a scraper for re-
moving the cake. The drum rotates from 0.1 to 2 rpm about its horizontal axis
[23]. Other characteristics are: drum diameters from 4 to 14 ft (1.22 to 4.27 m),
drum length from 1.5 to 18 ft (0.427 to 5.49 m), and drum surface area from 18 to
2
2
783 ft (1.67 to 72.7 m ) [24]. A filter cloth is wrapped around the drum, which

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