COURSEWORK

Coursework



Coursework

Question 1

(a)

The rotary vacuum filter is very basic in design, application and operation. It is also an extremely effective solid/liquid separation device due to its unique methods of handling (discharging) formed cake solids, relatively low operating labor requirements and its ability to filter continuous or batch process flows. The filter is rated by the size [filter area] of the drum and its potential output [typically expressed as pounds per hour of dry solids per square foot of filter area; precoat filters are rated by hydraulics instead of by solids]. The sizing of the auxiliary package components is dependent upon the area of the filter and the type of application. Since rotary vacuum filters can handle such a wide range of materials, expected solids output can range from 5# to 200# per hour per square foot and 2 gallons to 40 gallons per hour per square foot for precoat discharge. Any RVF utilizing a scraper, endless belt, string or roll discharge must have a drum with (1) filtrate pipes and a (2) valvebody with bridge blocks. A filter with a precoat discharge can use (1) a drum with filtrate pipes, (2) a drum with a valvebody, (3) a valveless drum or (4) a drum without filtrate pipes. For this reason, precoat discharge filters have a wide array of designs, specialty features and varying requirements for successful operation (Buscall, 1987, 873). The valvebody on universal drum designs allows vacuum and air flow to be controlled to each radial position of the drum, a requirement for all of the discharge designs except precoat.

(b)

Without a valvebody, the drum can only be used for precoat discharge applications. Universal and valveless drum designs have applied vacuum only to the surface of the drum by means of the filtrate pipes. All liquid and air are contained within the filtrate pipes. The interior of the drum is dry and at atmospheric pressure. Precoat discharge applications are typically high foam generators. Since foam does not easily separate out from the air/liquid stream coming out of the filter drum, there is a high tendency for the foam to be swept through the vacuum receiver, into the vacuum pump and out of the filtration system with the vacuum pump seal water. This can cause environmental problems, vacuum pump operation problems and a loss of filtered product. By adding a second receiver with a diameter sufficient to reduce the air flow velocity to 1 ft/sec (or less), most foam can be dropped out of the air stream. Foam carry-over can also be eliminated by reducing the filter operating vacuum level.

Sketch 1: Universal drum design schematic

Sketch 2: Valveless drum schematic

(c)

The theory had been developed from an assumption that the removal of particles from the flow was proportional to their concentration in the flow, this assumption being expressed mathematically as C=C0e. Use of this basic equation, allowing for the change in the value of h with time, to calculate values of C and hence U at any interval of time ...