Rotary vacuum drum filtration

Whole-broth clarification Rotary vacuum drum filtration Centrifugation Crossflow MF, UF Maintains high throughput Does not require filter aids... 

Filtration is a unit operation commonly employed nowadays in biotechnological processes. In this unit operation, a filter medium acts as a physical barrier to particles larger than its pores. Traditional filtration devices such as filter presses and rotary vacuum drum filters have so far found no application for the separation of animal cells. Nevertheless, membrane filters are commonly employed, as well as some alternative filter designs such as spin-filters. In the next sections, the most common types of filters used for animal cell separation will be discussed. 

The filter press is to be replaced by a rotary vacuum-drum filter with negligible filter-medium resistance. This rotary filter can deliver the filtrate at a rate of 1000 lb/h when the drum speed is 0.3 r/min. Assuming the fraction submerged and the pressure drop are unchanged, what drum speed in r/min is necessary to make the amount of filtrate delivered in 24 h from the rotary filter exactly equal to the maximum amount of filtrate obtainable per 24 h from the plate-and-frame filter ... 

Filtration can include filter presses, rotary vacuum drum filters, and variations on synthetic membrane... 

Precoated rotary vacuum drum filters (Fig. 6) are used by filtering a slurry of filter aid and water first, then subsequent product filtration. Difficult filtering materials, which have a tendency to blind, are removed with a doctor blade. Precoat is removed along with the slurry to expose a new filtration surface each cycle. 

Several of the eontinuous filtration units discussed below may be operated in pressure and vacuum conditions thus the continuous rotary vacuum drum, disc or hnmimtsjl belt filter can be enclosed in a pressure v seL Separation may then ensue by use of a gas pressure blanket in the vessdL This technique is oflen used in solvent fihrations. Some processes m the simultaaeous use of pressure upstream, with vacuum downstream of tire filter in this maimer, the continuous filtration characteristic can be coiqiled with fairer filtration dif ential pressures. 

Figure 11.4 Rotary vacuum drum filter (Courtesy of Filtration Services Lt4 Macclesfield, U.K.)...

EXAMPLE 14.2-4. Filtration in a Continuous Rotary Drum Filter A rotary vacuum drum filter having a 33% submergence of the drum in the slurry is to be used to filter a CaCO slurry as given in Example 14.2-1 using a pressure drop of 67.0 kPa. The solids concentration in the slurry is = 0.191 kg solid/kg slurry and the filter cake is such that the kg wet cake/kg dry cake = m — 2.0. The density and viscosity of the filtrate can be assumed as that of water at 298.2 K. Calculate the filter area needed to filter 0.778 kg slurry/s. The filter cycle time is 250 s. The specific cake resistance can be represented by a = (4.37 x 10 ) (—Ap) , where —Ap is in Pa and a in m/kg. 

Figure 1.26 Representations of rotary vacuum drum filters, (a) bottom fed, knife/scraper discharge (Filtration Services) (b) bottom fed, roller discharge (c) bottom fed, string discharge (d) bottom fed, belt discharge (Dorr-Oliver Eimco) (e) top fed (Filtration Services) (f) internal drum.

The precoat drum shown schematically in Figure 1.43 is similar in basic form and operation to the bottom fed rotary vacuum drum filter (see Section 1.4.1.5). Prior to filtration of the feed, a suspension of precoat is filtered under vacuum onto the drum surface to a depth between 4 and 15 cm. The feed is then introduced and filtration proceeds to produce thin, sometimes... 

A further point which is often ignored with rotary filters is the necessity to have a certain ratio between pick-up time and filtration time. Figure 17.10 shows, for example, that in a case where the pick-up time is 60% of the total time an ordinary rotary vacuum drum or belt filter could run at, for instance. 

A specific design of the rotary vacuum drum filter is shown in Figure 3.14. The drum rotates at 10 to 60 revolutions per hour and a vacuum of approximately 400 to 160 torr is built up with a liquid seal pump that is connected to the drum cells via the control head and filtrate pipes. This sucks the liquid through the filter cloth, and the solids contained in the suspension are deposited in a uniform layer on the cloth on that part of the drum that is submerged in the feed suspension. 

Figure 3.14 Rotary vacuum drum filter. 1, Suspension 2, filter trough 3, pendulum agitator 4, filter cells 5, drum 6, filter cloth 7, control head 8, filtrate pipes 9, filter cake 10, washing device 11, wash fluid 12, mother filtrate 13, wash filtrate 14, discharge device (e.g. scraper discharge) 1 5, solid...

The rotary vacuum drum filters described so far utilize a filtrate flow from outside the drum to the inside, where the vacuum is applied. Rotary drum filters also exist with filtrate flow from inside the drum. A simple version is little more than a rotating drum strainer, used to clarify raw water, as filters for the recovery of fibres in pulp and paper mills, and as polishers after final effluent treatment. The filter works on the hydrostatic pressure of the head of suspension, and there is no need for a vacuum pump. The filter medium is pleated so as to increase the total filtration area, and the drive is usually by means of a variable speed motor. 

Rotary vacuum drum and vacuum belt filters are extensively used on many process filtration operations, to provide filtration, clarification, dewatering or concentration. Moulded thermoplastic vacuum drum filters are particularly suited to providing continuous... 

For finely ground ores and minerals, rotary drum filtration rates may be 1500 lb/(day)(sqft), at 20 rev/hr and 18-25 In. Hg vacuum. 

Table 7.4. Filtration cycles for drum-type rotary vacuum filter (times in seconds)... 

In commercial operations of the dewaxing step, the mixture of wax-bearing oil and solvent is heated to ensure complete solution of the wax and is then passed through exchangers and chillers to totally enclosed rotary drum filters at a temperature about 10° to 15° F. below the desired pour point of the dewaxed oil. Vacuum for filtration is applied to the filtrate receiving vessel. The wax cake on the filter is washed with cold solvent, to remove the occluded oil-solvent solution. Sulfur dioxide superheated gas is used for stripping the solvent from the oil-solution and the wax phase, and the last traces of solvent are removed under vacuum. 

Filtration. Filtration can include filter presses, rotary drum vacuum filters (RDVF), belt filters, and variations on synthetic membrane filtration equipment, such as filter cartridges, pancake filters, or plate and frame filter presses. These processes typically operate in a batch mode when the filter chamber is filled up or the vacuum drum cake is exhausted, a new batch must be started. This type of filtration is also called dead-end filtration because the only fluid flow is through the membrane itself. Due to the small size of cells and their compressible nature, typical cell cakes have low permeability and filter aids, such as diatomaceous earths, perlite, or other mined materials are added to overcome this limitation. Moreover, the presence of high solids and viscous polymeric fermentation byproducts can limit filtration fluxes without the use of filter aids. 

The salient features of rotary-drum filtration are illustrated in Fig. 14.9, where a cylindrical drum having a permeable surface is revolving counterclockwise partially submerged in a slurry. A pressure differential is usually maintained between the outer and inner surfaces by means of a vacuum pump. However, the drum might be enclosed and operated under pressure. In addition to the vacuum or pressure, each point on the periphery of the drum is subjected to a hydrostatic head of slurry. 

Figures 7.5 to Figure 7.7 pertain to the use of rotary vacuum filters in vacuum filtration. In vacuum filtration, a drum wrapped in filter cloth rotates slowly while the lower portion is submerged in a sludge tank (Figure 7.7a). A vacuum applied in the underside of the drum sucks the sludge onto the filter cloth, separating the filtrate and, thus, dewatering the sludge.

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