Hydroextraction
Engrn7ring
Techniques for Permeability Measuremeirt
f L S S
development I
M. M. HARUNI AND J. ANDERSON STORROW Chemical Engineering laborafory, Applied Chemisfry Department, College of Technology, Manchesfer, England
T
Theoretical Bases
HIS paper describes the experimental and analytical tech-
niques developed to assess the validity of the assumed flow mechanism and equations proposed for hydroextraction (3). I n the first section of this paper flow rate equations have been checked both by testing the effect of major variables and by observing the applicability of functions derived from the flow equations. In so doing, various methods of testing hydroextractors have been assessed and have provided satisfactory techniques t h a t are available in various circumstances. The second section of the paper presents permeability data obtained from cakes formed and test>edin a small cell acting as part of a centrifuge basket under both centrifugal and hydrostatic heads. Filtration permeabilities were found for samples cut from hydroextractor cakes. Results showed a maximum difference of 20% between hydroextraction and filtration permeabilities. The previous discrepancy ( I , @ was caused mainly by faulty technique in the visual detection of the critical wetting rate. Drainage volume measurements for use when more precise techniques are not available are discussed in the final section.
Figure 1.
FLOWRATEEQUATIONS. The flow equations for washwater within the pores of a hydroextractor cake were developed ( I ) on the following assumptions: 1. The effect of the gravitational field may be neglected, since it is only 0.02 to 0.2% of the centrifugal force field in the tests described. 2. The interstices a t all positions in the cake are completely filled with water when r L 5 ro (Figure 1) and the water moves in streamline flow through the cake. 3. The liquid flows radially outward relative to the basket in the centrifugal field, tangential components with respect to the basket appearing only because of the liquid filling the expanding cross section to flow as i t moves outward. 4. The kinetic energy changes may be neglected when the liquid enters, is within, and leaves the cake owing to changes in the radial component of the space velocity. 5. The permeability of the cake may be considered as uniform in a preliminary analysis of the system.
Probe Arrangement for rL Measurements
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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6. A highly permeable layer, such as filter cloth behind a cake, does not run with its pores filled with liquid. 7 . Bny resistance to flow associated with the cloth-cake interface may be included in the mean permeability of the cake. 8. The assumed conditions are maintained throughout the experimental range, particularly that the flow remains streamline, with cake permeability independent of variations in flo~vrate and compressive stress owing to the centrifugal field. The laminar flow regime is likely to be maintained in tests on materials of permeability of the order lo-' gram per square second.
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Vol. 44, No. 11
Le., -with vater flowing through a cake of fixed dimensions. 811 the analytical methods listed have been used in this laboratory and found useful. Results exemplifying their use will be quoted here or later.
Analytical Methods 1 f E T H Q D I, BlSED O K 1 ' ?\fE ~ 4SURERIEYTS WHEN
fL
I S INDEPEED-
TIME. I n method Ia, tests were made keeping the feed rate, q, constant and measuring the variation of T L with n, The variation is expected to follon a straight line plot Then expressed as log (ro' - T L ~ )against log n, the gradient of the line being -2.0. The ordinate of the line a t a particular abscissa should yield I
