Ind. Eng. Chem. Fundam. 1981, 20, 1 1 1
This has been confirmed (Lockhart, 1980b) by microscopic observation, using direct and alternating electric fields applied to crude solvent-refined coal contained in cells having both parallel and coaxial electrode geometries. For the particular solvent-refined coal and experimental conditions used (Lockhart, 1980b),dielectrophoresis was much stronger than electrophoresis. Secondly, Lee et al. (1980) and Lee (1978) refer only to electrophoresis, which relates to the intrinsic surface charge on the particles. However, at the electric field strengths used by these authors charge injection from the electrodes might be expected, similar to the coronacharging step observed in electrostatic precipitation. Such an extrinsic charge injection and the accompanying attraction and repulsion effects at the electrodes have in fact been observed (Lockhart, 1980b) along with the intrinsic electrophoretic and dielectrophoretic phenomena. These extrinsic processes became more prominent with increasing Sir: We agree with Lockhart that dielectrophoresis might be developed into a practical separation technique. However, in our application the electrophoretic separation depends upon the surface charge produced by the addition of the surfactant. As shown by Lee et al. (1979), the { potential and therefore the electrophoretic mobility measured with our Zeta meter is a strong function of the concentration of the surfactant added to the slurry. The original small charges present in the coal-derived slurries are overwhelmed by the surfactant. This gives us a means of controlling the separation independent of the original
voltage and decreasing frequency of the field, but were significantly reduced by insulating one electrode. The design and optimization of apparatus for electrical separation in coal liquefaction products may need to take account of all these factors.
Literature Cited Lee, C. H. Ph.D. Thesis, Illinois Institute of Technology, 1978. Lee, C. H.; Gldaspow, D.; Wasan, D. T. Ind. E-. Chem. Fundam. 1980, 19, 166. Lockhart, N. C. J . Appl. Phys. 198Oa, 57, 2085. Lockhart, N. C. fuel 198Ob, 59, 389. Pohl, H. A. "Dielectrophoresis", Cambridge University Press: Cambridge, 1978.
CSIRO Institute of Earth Resources Physical Technology Unit Ryde, N.S.W. 2112 Australia
Neville C. Lockhart
properties of the slurries that depend upon the type of coal used, the process conditions employed, possible oxidation of the liquids, etc.
Literature Cited Lee, C.; Gidaspow, D.; Wasan, D. T. Proceedings of the Technlcal Program. International Powder and Bulk Solids Handling and Processing, May 15-17, 1979, pp 529-534; Figure 1.
Department of Chemical Engineering Illinois Institute of Technology Chicago, Illinois 60616
CORRECTION The Viscosity and Thermal Conductivity of Simple Dense Gases, Y. Cohen and S. I. Sandler, Ind. Eng. Chem. F u n d a m . 1980,19,186.
Page 187. A factor of All2 is missing from both the ordinate and abscissa of Figure 3. These should be yX/A'12 and byplA1I2,respectively. Also, the denominator in eq 14 should be b e rather than p*.
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0 1981 American Chemical Society
D. Gidaspow* D. T.Wasan
