Example 7.2 Rotary drum filter

A high value catalyst is to be separated from aqueous suspension at a temperature of 50°C using a bottom fed, knife discharge, rotary drum filter. The drum is 2.5 m in diameter, 1.4 m in width and rotates at 0.2 rpm. The filter cycle follows the conventional sequence of cake formation, displacement washing (also at 50°C) and gas deliquoring, and all phases take place at an applied vacuum of 70 kPa. The characteristics of the catalyst and suspension are shown in Table 7.6 along with other operational parameters of the filter. 

Determine the throughputs of solids, liquid and solutes and other pertinent values at all points in the filter cycle and comment on the feasibility of operation. 

With reference to the outline calculation sequence in Section 7.2, the cycle is divided into consecutive phases such that tj= tf- - t - - 

To calculate a value for the effective feed concentration (c), the mass fraction of solids in the feed (5) needs to be evaluated. Referring to Appendix C, which details conversion factors for alternative expressions of suspension concentration, equation (C.9) gives 

From the bottom row of Table 7.7, the sohds production rate = M/t) = 0.51 kg s and the cake moisture content (M) is given by equation (7.22) where 

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Figure 7.9 Schematic representation of the example rotary drum filter cycle. Cake formation phase 0 122°, rise phase 122 -> 198°, washing phase 198 252°, deliquoring phase 252 324° and cake discharge 324 360°.

Internal Rotary-Drum Filters An example of an internal rotary-drum filter is illustrated in Figure 14. The filter medium is contained on the inner periphery. This design is ideal for rapidly settling slurries that do not require a high degree of washing. Tankless filters of this design consist of multiple-compartment drum vacuum filters. 

From the viewpoint of accommodation to the filter-supporting structure, some cloths cannot be used, even though the filtering characteristics are excellent. For rotary drum filters, for example, the cloth is pressed onto the drum by the caulking method, which uses cords that pass over the drum. In this case, the closely woven cloths manufactured from monofilament polyethylene or polypropylene fiber are less desirable than more flexible cloths of polyfilament fibers or staple cloths. 

An example of a solid-liquid phase separation - often referred to as a mechanical separation - is filtration. Filters are also used in gas-sohd separation. Filtration may be used to recover liquid or sohd or both. Also, it can be used in waste-treatment processes. Walas [6] describes many solid-hquid 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 available. 

The equations for sizing rotary-drum filters are summarized in Table 6.18. Equation 6.18.1 is the liquid mass balance. In this procedure, y is a mass fraction. Because the cake is wet, the liquid entering the filter will be less then the liquid leaving. Equation 6.18.2 is the solids mass balance, assuming that all the solids in the slurry are removed. Solve Equation 6.18.2 for the cake formation rate, me. Then, solve Equation 6.18.1 for the filtrate volumetric flow rate, V2. Next, calculate the filtration area from Equation 6.18.5 and the dmm area from Equation 7.18.6. Finally, select a standard rotary filter from Table 6.20. The calculation procedure for sizing a rotary filter is outlined in Table 6.19. Example 6.5 illustrates the sizing procedure. 

Various types of filtration equipment are available commercially and can be operated in batch, semicon-tinuous, or continuous modes. Among the commonly used types are the plate and frame filter, rotary drum filter, leaf filter, plate filter, and tray filter. Apart from the plate and tray filters, all other are enclosed and therefore are easy to work with when sterility of the solids is an important issue. Moreover, all these filters are examples of dead-end filters. Cross-flow filtration is mostly used in the purification stage through membranes with very low pore sizes and is discussed later. 

Example 30.3. A rotary drum filter with 30 percent submergence is to be used to filter a concentrated aqueous slurry of CaCOj containing 14.7 lb of solids per cubic foot of water (236 kg/ra ). The pressure drop is to be 20 in. Hg. If the filter cake contains 50 percent moisture (wet basis), calculate the filter area required to filter 10 gal/min of slurry when the filter cycle time is 5 min. Assume that the specific cake resistance is the same as in Example 30.2 and that the filter-medium resistance R is negligible. The temperature is 20°C. 

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. 

Effect of Filter Medium Resistance on Continuous Rotary-Drum Filter. Repeat Example 14.2-4 for the continuous rotary-drum vacuum filter but do not neglect the constant R , which is the filter medium resistance to flow. 

Throughput in Continuous Rotary Drum Filter. A rotary drum filter having an area of 2.20 m is to be used to filter the CaCO slurry given in Example 14.2-4. The drum has a 28% submergence and the filter cycle time is 300 s. A pressure drop of 62.0 kN/m is to be used. Calculate the slurry feed rate in kgslurry/s for the following cases. 

As previously noted, there are two available modules for eqvfipment simulation, namely vacuum filters and pressure filters (which include variable volvune filters). Within either module there are several options available from the Start Menu (see, e.g. Figure 5.8), each of which facilitates the simulation of a particular type of filter. All simulations are performed in the same general manner, the principal differences arise as a consequence of the operational limitation(s) of filter type. Figure 5.15 shows an example screen display for the simulation of a bottom fed rotary drum filter fitted with a knife discharge. 

In Section 7.1 the principal features of common continuous filter cycles are described, while Section 7.2 presents the equations required to model these cycles. Section 7.3 provides detailed example calculations for the horizontal belt filter and the rotary drum filter as these are representative of typical continuous cycles. Section 7.4 shows how computer simulations can be used to examine in detail the effects of process variables on... 

Coagulants and flocculants can enable a more open, faster medium to be employed, and still retain fines capture, thus providing an increase in throughput whilst achieving acceptable cake moisture contents [24,25]. Examples of rotary drum filters and rotary disc filters are shown in Figures 6.15 and 6.16 respectively. 

Rotary disc vacuum filters have the advantage, compared with rotary drum filters, of giving a much larger filter area per unit of floor area. They are thus particularly suitable for the processing of bulk products, for example in coal preparation, ore dressing, pulp and paper processing, and so on. 

Other examples of rotary-drum vacuum filters are ... 

FIGURE 14.9 Rotary vacuum drum filter (Example 14.6). 

Raw fermentation broth is an example of a large volume production. Rotary drum vacuum filters (RVF s) have traditionally been found in this service. Slow-settling materials or more difficult filtrations with large scale production requirements are typical applications for this type of equipment. For an overview of filter selection versus filtering rates, see Table 3, which is excerpted by special permission from Chemical Engineering/Deskbook Issue, February 15, 1971, by McGraw Hill, Inc., New York, NY 10020. 

CONTINUOUS FILTRATION. In a continuous filter, say, of the rotary-drum type, the feed, filtrate, and cake move at steady constant rates. For any particular element of the filter surface, however, conditions are not steady but transient. Follow, for example, an element of the filter cloth from the moment it enters the pond of slurry until it is scraped clean once more. It is evident that the process consists of several steps in series—cake formation, washing, drying, and discharging—and that each step involves progressive and continual change in conditions. The pressure drop across the filter during cake formation is, however, held constant. Thus the foregoing equations for discontinuous constant-pressure filtration may, with some modification, be applied to continuous filters. 

The process system consists of two long-tube vertical evaporators, a draft-tube baffled crystallizer, a rotary-drum vacuum filter, and a direct-heat rotary dryer. Also, pumps are needed to move the solution from evaporator 1 to evaporator 2, to recycle the filtrate from the filter to the crystallizer, and to move the magma from the crystallizer to the filter and a heat exchanger is needed to heat the recycle filtrate. However, the purchase costs for the three pumps and the heat exchanger are not considered here because examples for these types of equipment are presented in Section 16.5. For the equipment considered here, assume fabrication from stainless steel, with a material factor of 2 for the ratio of stainless steel cost to carbon steel cost. For the process, using the following size factors and the equations in Table 16.32, the estimated f.o.b. equipment purchase costs at a CE index of 394 are included in the following table. 

Figure 5.15 Example screen display for a rotary vacuum drum filter simulation using EDS. The data are the same as those in the worked example shown in Section 7.3.2 (with no deliquoring during the rise phase).

Example 13.1 Sizing of a rotary drum vacuum filter... 

Fig. 6 Examples of downstream process equipment (A) filter press (B) rotary vaccum drum (C) microfiltration unit. (View this art in color at www.dekker.com.)...

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. 

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