Vacuum leaf test

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. 

Various small scale test units and procedures are available to determine slurry characteristics and suitability for a particular application. Buchner funnel, and vacuum leaf test units can be purchased or rented fi om vendors to perform in-house tests, or one can have tests conducted at the vendor s facility. Pilot testing on the actual equipment would be the optimum with a rental unit in the plant. In either case, slurry integrity must be maintained to ensure accurate filtration data. 

Sizing of filtration equipment is impossible from a theoretical approach because of the vaiying nature of the solid particles. Almost all applications are determined with the aid of test work. Typical test arrangements are a vacuum leaf test, a buchner funnel, and a pressure bomb method. 

Vacuum filters are usually simulated with a Buchner funnel test or filter leaf test (54). The measured parameters are cake weight, cake moisture, and filtration rate. Retention aids are usually evaluated using the Britt jar test, also called the Dynamic Drainage Jar, which simulates the shear conditions found on the paper machine and predicts performance (55). 

Figure 18-95 also contains a schematic layout of the equipment which is required for all bottom-feed leaf tests. Note that there are no valves in the drainage line between the test leaf and the filtrate receiver, nor between the filtrate receiver and the vacuum pump. 

At the start of the leaf test run, the hose between the test leaf and filtrate receiver should be crimped by hand to bring the filtrate receiver to the operating vacuum level. The use of a valve at this point is not only less convenient but very frequently results in a hydraulic... 

In any leaf test program there is always a question as to what vacuum level should be used. With very porous materials, a vacuum in the range of 0.1 to 0.3 bar (3 to 9 in Hg) should be used, and, except for thermal-drying apphcations using hot air, the vacuum level should be adjusted to give an air rate in the range of 450 to 900 mVm h (30 to 40 cfm/ft measured at the vacuum. 

FILTRATION LEAF TEST DATA SHEET - VACUUM AND PRESSURE... 

A sample of the slurry was tested, using a vacuum leaf filter of 0.05 m2 filtering surface and a vacuum equivalent to a pressure difference of 71.3 kN/m2. The volume of filtrate collected in the first 300 s was 250 cm3 and, after a further 300 s, an additional 150 cm3 was collected. It may be assumed that cake is incompressible and the cloth resistance is the same in the leaf as in the filter press. 

Figure 11.3. Laboratory test data with a vacuum leaf filter, (a) Rates of formation of dry cake and filtrate, (b) Washing efficiency, (c) Air flow rate vs. drying time, (d) Correlation of moisture content with the air rate, pressure difference AP, cake amount W Ib/sqft, drying time 6d min and viscosity of liquid Dahlstrom and Silverblatt, 1977).

For scale-up, certain tests, such as test for settling rate to evaluate flocculation, design of thickeners, and pressrue-bomb or vacuum-leaf experiments to determine the rate of cake buildup, are easy to perform on a bench scale. On the other hand, for design of centrifuges, small-scale test results are not sufficient, and pilot or full-scale units are necessary for final design. 

Cheape, D. W., Jr., Leaf tests can establish optimum rotary-vacuum-filter operation. Chemical Engineering, 5 141-148 (1982)... 

FIGURE 22.59a Vacuum horizontal leaf test filter. 

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