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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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Eq. (24)
Eq. (25) jD = nRe"1^
2
. _ ^x,locP 3
cG0 vpD^)
where
a = solid surface area per unit bed volume Ga = fluid superficial velocity, g/s/cm2
Combining Eqs. (24), (25) and (26) yields:
Joe .T/2 Ga
acD^ \pDabJ
From this expression we see that for scaleup purposes the prototype and model have to be dynamically similar with respect to solvent flow and thermally similar with respect to solvent temperature. Both of these conditions can be easily satisfied by maintaining geometrical similarity. However, for this case, it is not necessary to scale up this process based on the design of the Berty reactor. Our test results show a relationship between the desired results and system agitation as measured by turbulence through a specific volume element. The prototype could be of a different design as long as the criteria for dynamic and thermal similarity are met. Other considerations like shape factors and solute concentrations are fixed by the desired final operation and, as stated, will have to be determined empirically. Diffusivity of the solute into the solvent is primarily fixed by solvent viscosity and density, and specific properties of the solute. Once again, these values can be determined empirically.
The scaleup process is not just a matter of plugging values into prescribed equations, nor can exact scaleup criteria be obtained from generalized correlations for certain types of equipment. Instead, in a good scaleup approach, all variables that describe the process are determined; desired process conditions and the magnitude of the scaleup taken into account; and a design selected. Scaleup designs are based almost exclusively on the principle of similarity.
In the example shown above, the process is fairly simple (no chemical reaction, a relatively small magnitude of scaleup, and small variations in solvation chemistry, temperature and pressure). For more complicated systems, the approach is somewhat more involved and can be less reliable. For instance, if the system relies heavily on strategic placements of parts because flow patterns inside the model give optimal cleaning in a particular region, then entrance effects, density gradients, etc., can not be ignored when designing the prototype. For these cases, the affecting variables may not necessarily correspond geometrically between the model and the prototype. It is also more involved for very large scale ratios where pressure and density gradients in the prototype become a major factor because they can no longer be assumed small enough to ignore.
Ultimately, the success of the scaleup approach is determined by comparing data obtained from both the model and the prototype. If performance data are similar for the two systems, the scaleup process can be considered a success. Mild variations in performance are to be expected however. These variations are due to the tradeoffs the design engineering team have to make to complete the scaleup process. Successful scaleup approaches start with understanding the existing process, knowing what is expected from the prototype design, and then making the right decisions with the right tools. Winning approaches are always a combination of good teamwork, communication, and use of tools like the principle of similarity.
ACKNOWLEDGMENTS
Pacific Northwest Laboratory is operated for the U. S. Department of Energy by Battelle Memorial Institute under Contract DE-ACQ6-76RLO 1830.
REFERENCES
1. Fedors, R. F., A Method for Estimating Both the Solubility Parameters and Molar Volumes of Liquids, Polymer Engineering Science, 14:2 (Feb. 1974)
2. Fleming, R., Scaleup in Practice, Reinhold Publishing, New York (1958)
3. Fogler, H. S., Elements of Chemical Reaction Engineering, Prentice-Hall, Englewood Cliffs, New Jersey (1986)
4. Johnstone, R. E., Thring, M. W., Pilot Plants, Models, and Scaleup Methods in Chemical Engineering, McGraw-Hill, New York (1957)
5. Lee, J. ., Biochemical Engineering, Prentice-Hall, Engelwood Cliffs, New Jersey (1992)
6. McCabe, W. L., Smith, J. C., andHarriott, P. Unit Operations of Chemical Engineering, McGraw-Hill, New York (1985)
7. McHugh, M. A. and Krokonis, V. J., Supercritical Fluid Extraction:
Principles and Practice, Butterworth Publishers, Stoneham, Massachusetts (1986) '
8. Motyl, . ., Cleaning Studies of Metal Substrates Using Liquid/ Supercritical Fluid Carbon Dioxide, Rockwell International, Rocky Flats Plant, Golden, Colorado (1988)
9. , R. H., Green, D. W., and Maloney, J. O., Perry's Chemical Engineers Handbook, McGraw-Hill, New York (1984)
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