Cake thickness measurement

Figure 15.2 A schematic diagram of cake thickness measurement by a photointerrupter sensor [18].

Enclosed agitated filters are useful when volatile solvents are in use or when the solvent gives off toxic vapor or fume. Another significant advantage is that their operation does not require any manual labor. Control can be manual or automatic, usually by timers or by specific measurements of the product. Most filters are made of mild steel, with the exposed surfaces protected by lead, tile, mbber lining, or by coating or spraying with other substances as necessary. Filtration areas up to 10 m are available and the maximum cake thickness is 1 m. Apphcations are mainly in the chemical industry for the recovery of solvents. 

Cake Thickness Control Sometimes the rate of cake formation with bottom feed-type filters is rapid enough to create a cake too thick for subsequent operations. Cake thickness may be controlled by adjusting the bridge-blocks in the filter valve to decrease the effective submergence, by reducing the slurry level in the vat, and by reducing the vacuum level in the cake formation portion of the filter valve. If these measures are inadequate, it may be necessaiy to use a toploading filter. 

At the end of the run, measure and record the filtrate volume (and weight, if appropriate), cake thickness, final cake temperature (if appropriate), wet cake weight, and note the cake discharge characteristics (roU, sticks to media, etc.). 

It is known that the specific resistance for centrifuge cake, especially for compressible cake, is greater than that of the pressure or vacuum filter. Therefore, the specific resistance has to be measured from centrifuge tests for different cake thicknesses so as to scale up accurately for centrifuge performance. It cannot be extrapolated from pressure and vacuum filtration data. For cake thickness that is much smaller compared to the basket radius, Eq. (18-116 7) can be approximated by... 

Thickness of the filler cake is measured and reported in of an inch. Also, the cake is visually examined and its consistency reported using such notations as hard, soft, tough, rubbery, or firm. ... 

No serious attempt has yet been made to standardize filtration tests and to categorize filtration behavior in generally accepted terms. A possibly useful measure of filterability, however, has been proposed by Purchas (1977 1981). The time in minutes required to form a cake 1 cm thick when the cell is operated with a differential of 500Torr (0.67 bar) is called the Standard Cake Formation Time (SCFT), tF. The pressure of 500Torr is selected because it is obtained easily with common laboratory equipment. The procedure suggested is to make a series of tests at several cake thicknesses and to obtain the SCFT by interpolation, rather than to interrupt a single test to make observations of cake thickness. A direct relation exists, of course, between the SCFT and resistivity a some examples are... 

Coating thickness was measured both wet and dry. Wet thickness measurements were taken using a Gardner s Company Precision Hot Cake, which can measure between 125 and 500 pm films with a 2a uncertainty of +10%. Dry film thickness measurements were taken using an ElektroPhysik s Exacto FN gauge, which can measure between 0.1 and 2000pm with a 2a uncertainty of 3%. 

On-line measurements of cake thickness during filter-system operation have not been feasible in most systems. However, when the pressure drop across the filter system, the gas flow rate, and the particle concentration in the incoming gases are measured, these data can be used to calculate the mass of filter cake on each filter. The overall pressure drop can be modeled as ... 

Constants in Eqs. (30,13) and (30.16i ), respectively Distance in cake measured from filter medium, m or ft also, thickness of selective layer in ultrafiltration L, filter cake thickness Molecular weight... 

Pikal et al studied a 5%-povidone (polyvinyl pyrrolidone, PVP) solution, dried from an 8-ml fill volume, of 2-cm depth, and from a 4-ml fill volume at a 1-cm fill depth. In this instance, the fill depth had little effect on the specific surface area of the dried product (2.5 m g compared to 2.3 m g )- It was found that the rates of secondary drying (as measured by the mean ratio of the (1 - F) values), the normalised water contents of the two products were essentially identical. A mean ratio of (1 - F) of 1.19 0.17 was found, indicating that cake thickness did not significantly affect secondary drying kinetics. It can thus be concluded that material at the top of the cake dries at a similar rate to material at the bottom, provided overheating is avoided. Similar experiments were carried out on an amorphous, formulated moxalactam product, as shown below. 

Tlie discussions of the basic features of filtration given thus far illustrate that the unit operation involves some rather complicated hydrodynamics that depend strongly on the physical properties of both fluid and particles, as well as interaction with a complex porous medium. The process is essentially influenced by two different groups of factors, which can be broadly lumped into macro- and micro-properties. Macrofactors are related to variables such as the area of a filter medium, pressure differences, cake thickness and the viscosity of the liquid phase. Such parameters are readily measured. Micro-factors include the influences of the size and configuration of pores in the cake and filter medium, the thickness of the electrical double layer on the surface of solid particles, and other properties. 

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