Filtration filter cycle time

Typically, a filter cake or precoat is built up on the filter septa to prevent blinding, short filter cycle times, and cosdy cleaning of the septa. Then diatomite is added as body feed to the Hquid to be filtered so that the permeabiHty of the filter cake may be maintained. Filler aid permeabiHty of diatomite ranges from 0.06 to 30 lnF. At the end of the filter cycle the filtrate is clear and the soHds are retained in the soHd or semisoHd diatomite filter cake. The type and amount of diatomite for precoat and body feed are normally deterrnined by pilot studies (18,19). 

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. 

After washing is completed, additional time is needed to remove the cake, clean the filter, and reassemble the filter. The total filter cycle time is the sum of the filtration time, plus the washing time, plus the cleaning time. 

EXAMPLE 14.2-3. Rate of yVashing and Total Filter Cycle Time At the end of the filtration cycle in Example 14.2-2, a total filtrate volume of 3.37 m is collected in a total time of 269.7 s. The cake is to be washed by through washing in the plate-and-frame press using a volume of wash water equal to 10% of the filtrate volume. Calculate the time of washing and the total filter cycle time if cleaning the filter takes 20 min. 

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. 

Rotary vacuum filtration of a fermentation broth is to be carried out at the rate of5000 liter/hr. The properties of the dilute suspension, e.g. (ij, p, p and the cake properties, c,RcS are provided in Example 6.3.7. The vacuum based applied AP is 55 cm Hg. Assume an incompressible cake whose resistance dominates filtration resistance. The filter cycle time is 80 s the filtration time duration is 20 s. Determine the total filter area of the rotary filter needed for this operation. The temperature of operation is the same as that in Example 6.3.7 so that the physical properties are identical. 

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. 

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 filtration area of large table filters is more than 200 m and having few moving parts can rotate at a cycle time of 1.5 minutes. These machines can handle thick cakes and may be operated at high vacuum levels. The major subassemblies of the machine include ... 

Various rules of thumb exist for standard water filtration rates and cycle time before backwashing. Higher filtration rates may appear to be economically justified, however, when the filter loading is within conventional limits. In this example, we examine the issues involved for constant-rate filtration for a dual-media bed. Dual- and mixed-media beds result in increased production of water in a filter for two reasons. First, the larger grains (say charcoal approximately 1-mm size) as a top layer help reduce cake formation and deposition within the small (150-mm) top layer of the bed. Second, the head loss in the region of significant filtration is reduced. 

Each filter has demonstrated the capacity to filter the full brine flow of 195 m3 h 1. The pressure drop through the filter medium is measured and monitored continuously. Typically, it is nearly constant over a 2-h filtration at 195 m3 h 1. Back-pulse cleaning restores the initial pressure drop from cycle to cycle, with only a slow increase over time. After 12 months running time, the initial pressure drop at the beginning of the filter cycle had increased by 0.6 bar. The filter membranes were chemically cleaned with 5 % hydrochloric acid. After a cleaning time of 2 h the filter was started again and the pressure drop was less than 0.1 bar greater than that of new filter socks. 

The next step.was the removal of ammonia from the crude soln , which was done by steam-stripping in an evaporator to an ammonia recovery system, where NH, was absorbed in w. The resultant NH,-free crude soln was filtered through a Nutsch type filter to remove the catalyst and other insol impurities. The filtrate referred to as clear liquor was stored in a 1000 gal tank from which it could be transferred by suction into either of two 280 gal jacketed evaporators. The evaporation was conducted under 24" vacuum with 50 lb steam press in the jacket. A total of 425 gal of "clear liquor was concentrated until a sample of its "mother liquor showed the strength of 35% NaOH. During this operation the bulk of NaN, being less sol in w than NaOH, pptd. Then the mixt was cooled to 80-90°F (27-32°) (to cause the pptn of addnl NaN,) and dropped to a wringer. The yield was ca 75% NaN, and the overall cycling time was 5 6 hrs... 

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 ... 

McCabe and Smith [23] state that the cycle time for filtering CaCOs is 5 min. According to Table 6.20, 37.5 % of the drum is submerged during filtration. Because the drum is only partially submerged, the filtering time,... 

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. 

Example 7.9 A CaCOs sludge with cake filtration parameters determined in the previous example is to be dewatered in a vacuum filter under a vacuum of 630 mmHg. The mass of filtered solids per unit volume of filtrate is to be 60 kg/m The filtration temperature is determined to be 25°C and the cycle time, half of which is form time, is five minutes. Calculate the filter yield. 

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... 

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