Time cycle, pressure filters

The following argument assumes that tests have provided values of specific and medium resistances and the data is to be used to check, predict, control or design a pressure filter where information on the cycle time to filter a given volume of filtrate, or slurry, is required. The instantaneous flow rate q is assumed to be (neglecting the volume of the cake) ... 

The fundamental case for pressure filters may be made using equation 10 for dry cake production capacity Y (kg/m s) derived from Darcy s law when the filter medium resistance is neglected. Eor the same cycle time (same speed), if the pressure drop is increased by a factor of four, production capacity is doubled. In other words, filtration area can be halved for the same capacity but only if is constant. If increases with pressure drop, and depending how fast it increases, the increased pressure drop may not give much more capacity and may actually cause capacity reductions. 

Optimization of Cycle Times. In batch filters, one of the important decisions is how much time is allocated to the different operations such as filtration, displacement dewatering, cake washing, and cake discharge, which may involve opening of the pressure vessel. Ah. of this has to happen within a cycle time /. which itself is not fixed, though some of the times involved may be defined, such as the cake discharge time. 

The vertical recessed plate automatic press, shown schematically in Figure 15 and described previously, is another example of a horizontal belt pressure filter. Cycle times ate short, typically between 10 and 30 minutes, and the operation is fully automated. The maximum cake thickness is about 35 mm washing and dewatering (by air displacement) of cakes is possible. Apphcations include treatment of mineral slurries, sugar, sewage sludge, and fillers like talc, clay, and whiting. 

A pressure filter is a plate-and-frame press which operates on an intermittent time cycle. Drier cakes are generally attainable from a filter press. Sludge drying beds are usually used for smaller sludge volumes, which drain and dry rapidly. Thein appHcation is usually restricted to the more arid climates. 

Vacuum pump capacity is conventionally based on the total cycle and expressed as mVh-m" (cfi7i/ft ) of filter area measured at pump inlet conditions. Thus, the gas volumes per unit area passing during each dry period in the cycle are totaled and divided by the cycle time to arrive at the design air rate. Since air rate measurements in the test program are based on pressure drop across the cake and filter medium only, allowance must be made For additional expansion due to pressure drop within the filter and auxiliary piping system in arriving at vacuum pump inlet conditions. 

It is advisable to start a constant-pressure filtration test, like a comparable plant operation, at a low pressure, and smoothly increase the pressure to the desired operating level. In such cases, time and filtrate-quantity data shoulci not be taken until the constant operating pressure is reahzed. The value of r calculated from the extrapolated intercept then reflec ts the resistance of both the filter medium and that part of the cake deposited during the pressure-buildup period. When only the total mass of diy cake is measured for the tot cycle time, as is usually true in vacuum leaf tests, at least three runs of different lengths should be made to permit a rehable plot of 0/V against W. If rectification of the resulting three points is dubious, additional runs should be made. 

The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite, or other suitable precoat materials. 

Example 4 Cycle time for maximum amount of production from a plate-and-frame filter press. Tests with a plate-and-frame filter press, operated at constant pressure, have shown that the relation between the volume of filtrate delivered and the time in operation can be represented as follows ... 

Erom this definition, Lf is the amonnt of cake formed per nnit area of filter cloth per unit of time. In vacnnm filtration, the only time that the cake is formed is when the drum is submerged in the tank. In pressure filtration, the only time that the cake is also formed is when the slndge is pumped into the plates. Thns, t in the previons equation is called the form time tf. Also, the filters operate on a cycle. Calling the cycle time as t tf may be expressed as a fraction / of C- Thus, t = tf = fL may be... 

For incompressible cakes, Equation (7.66) is the design cake filtration equation. In vacuum filtration,/is equal to the fraction of submergence of the drum. Also, for the pressure filter,/is the fraction of the total cycle time that the sludge is pumped into the unit. 

The influence of filtration velocity on resulting pressure build up and cycle-time is illustrated in Fig. 4. Note that the displayed offset in the base pressure loss for a clean filter (t ) has been used to enhance readability... 

Equations (30.32) and (30.33) apply both to continuous vacuum filters and to continuous pressure filters. When R is negligible, Eq. (30.33) predicts that the filtrate flow rate varies inversely with the square root of the viscosity and of the cycle time. This has been observed experimentally with thick cakes and long cycle times with short cycle times, however, this is not true and the more complicated relationship shown in Eq. (30.32) must be used. In general, the filtration rate increases as the drum speed increases and the cycle time diminishes, because the cake formed on the drum face is thinner than at low drum speeds. At speeds above a certain critical value, however, the filtration rate no longer increases with speed but remains constant, and the cake tends to become wet and difficult to discharge. 

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. 

A continuous pressure filter is to yield 1400 gal/h of filtrate from the slurry described in Prob. 30.7. The pressure drop is limited to a maximum of 50 Ibyfin. How much filter area must be provided if the cycle time is 3 min and the drum submergence is 50 percent ... 

Several indicators are utilized to determine the point at which the filter space is filled with solids from the bleached oil when the pressure drop across the leaves reaches a predetermined level, when a predetermined decrease in flow rate occurs, or when a calculated load level is reached. Short cycles or premature filter stoppages are usually the result of (i) inadequate body feed (ii) too high flow rate, which can cause the solids to pack (iii) too low flow rate, which can allow the solids to settle and block the flow rate (iv) blinded screens, which reduce the filter surface area or (v) solid load exceeding filter capacity. The perfect circumstance is when the differential pressure is reached, and the flow rate is severely reduced at the same time that the calculated filter capacity is exhausted. Once the filter cycle is complete, the filter cake must be removed and the process repeated all over again (Butterworth, 1978). 

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