Superficial gas velocitj (m/5)
Figure 1. Modes of operation of a packed column. Curves are taken from Musil et al. (1968). Air rate as shown; superficial water velocity 0.0035 m/sec; glass Raschig rings size 10.3 mm in a glass column of diameter 100 mm, packed height 1.75 m.
that way, a column operates in the bubbling or flooded mode (colonne noy6e), with high holdup and pressure drop. When the bottom chamber contains mainly gas, and liquid drains away freely, operation is in the ordinary trickling mode (colonne arros6e) with lower holdup and pressure drop. No instability was reported in either mode. These observations may be used to explain the results reported by Leung et al. They used a packing support grid with voidage less than that of the packing-a grid that would flood at flow rates less than the flooding rates for the packing. Then, after the gas rate was set, using the pressure-actuated flow controller, liquid was added at a rate greater than the grid flooding rate until, with backup of liquid in the packing, the preset pressure drop was reached. The flow was then brought back to a steady value by the controller. At this point the tower would be operating in the two modes a t the same time, trickling in the upper part, bubbling below. The surface of separation would necessarily be somewhere between the differential pressure sensing points. The restricting grid was essential to the operation if the tower were to operate without the bottom chamber flooded. Without this restriction, more flexible operation could have
been achieved with less trouble and expense by the use of an adjustable liquid seal. In an earlier report of this work, Brooks et al. (1974) implicitly propose this kind of explanation, when they write of “flooding flowrates” and “flooded operation.” But now they specifically reject it. Part of the problem arises from an ambiguous use of the words “flooding” and Wooded.” At one point they are used to describe bubbling operation, at another to describe operation at limiting flows. But that is not the whole story. In the later article they assert, “In the incipient flooding mode both liquid and gaseous phases are continuous. For a given column, higher flow rates can be achieved in the ‘incipient flooding mode’ than in the ‘emulsification operating mode.’ ” But neither in their article nor in the thesis on which it is based (Hutton 1974) do they adduce the slightest experimental evidence for these claims. They have not shown how they established whether the gas phase was continuous. They have not reported any contrasting behavior of the column in the simple bubbling mode. They have not furnished any photographs or even given a verbal description of the appearance of the column by which the reader might judge. I t is a commonplace of the scientific method that assertations made without evidence may be rejected without apology. And again, that new explanations can be ignored if the conventional ones are sufficient. That is my position. I believe that the results can be satisfactorily explained in terms of the well-known operating modes. No evidence has been presented to support the claim of novel phenomena.
Nomenclature g = local gravitational acceleration, m/sec2 h = total holdup, dimensionless p = pressure, N/m2 z = column height, m e = dry voidage, dimensionless p~ = liquid density, kg/m3 Literature Cited Brooks, P. C., Hutton, B. E. T., Leung, L. S., Nicklin, D. J., Ch8m. Eng., 81, (4) 152 (1974). Hutton, B. E. T., Ph.D. Thesis, University of Queensland, 1974. Leung, L. S., Hutton, B. E. T., Nicklin, D. J., Ind. Eng. Chem. Fundam., 14, 63 (1975). Musil, L.. Prost, C., Le Goff, P., Chim. Ind., Gen. Chim., 100, 674 (1968)
Department of Chemical Engineering Uniuersity of New South Wales Kensington, New South Wales 2033 Australia
John E.Buchanan
A Second Mode of Operating Packed Columns and Wetted Wall Columns Sir: In his letter Buchanan asserted that our “incipient flooding” mode of operation is the same as the “bubbling column” operation. Operation of a bubbling column is of course well known, and we apologize for not mentioning such operation in our paper. In a bubbling column, the gas travels up the bed in the form of bubbles; i.e., the gas phase is discontinuous. In the incipient flooding mode described in our paper, we claim that the gas phase is continuous and 88
Ind. Eng. Chem., Fundam., Vol. 15, No. 1, 1976
is therefore different from the bubbling mode. The three different modes of countercurrent operation are summarized in Figure 1.Figure l a shows the normal mode of operation, Figure l b the bubbling mode, and Figure ICthe “incipient flooding” mode of operation. The existence of the incipient flooding mode was first demonstrated in a wetted wall column where we can observe readily that both phases are continuous. The experi-
Gas
Liaui6
Orifice plate f o r stabalizing i n c i r i e n t f1oodir.g operation
%as (a)
Noma1 w e t t e d w a l l o p e r a t i o n - iow h o l d u p , b o t h p h a s e s continuous
(b)
Liquid
Bubbles L i v u i d Bubble c a l u n n - h i g h 1Lcuid h o l d u p , cas p h a s e d i s p e r s e d .
(c)
3 s
Liqui?
I n c i T i e n t flooding operatiori
- h i g h lipuie h l d q , t o ? h p k s e s can?inuo“s.
Figure 1. T h r e e modes of countercurrent gas-liquid flow in a column.
ments in wetted wall columns are described in great detail in the paper and the visual observation presented in two figures (Figures 4 and 5). Buchanan made no mention of it in his letter. Given the mode exists in wetted wall columns, we would expect it to exist also in certain packed columns; e.g., it would surely exist in a column of carefully packed Raschig rings. We surmised that the incipient flooding mode of operation does occur in packed columns. Our observation a t the wall of the packed column operating in the “incipient flooding mode” suggested that the gas does form continuous channels indicating that the gas phase is continuous. This is not to say that bubbling operation is not possible. Figure 1 clearly shows that both modes of operation are possible depending on the gas flow rate and the arrangement. Buchanan claimed that we have been trapped into a wrong conclusion and that all our results can be explained in terms of the bubbling mode. But what about the wetted wall column? This is the equipment that demonstrates most effectively the incipient flooding mode. Unfortunately, Buchanan did not even bother to mention the wetted wall column in his criticism of our work. As the wetted wall column is the key to the whole story, Buchanan’s omission is a pity! An indirect proof of operating a packed column with a continuous gas phase in the incipient flooding mode is given in a paper recently submitted to this journal for pubiication (Leung et al., 1976). We presented there a theoretical analysis based on a continuous gas phase for countercurrent gas liquid flow in a packed column. The analysis confirms that two modes of operation with a continuous gas
phase are possible. A similar analysis for a wetted wall column (Nicklin and Koch, 1969) also shows that in theory two modes of operation with a continuous gas phase are possible. We are grateful to Buchanan for drawing our attention to the work of Musil et al. (1968). We were not aware of the French reference before and have now examined their paper. There is some similarity between their work and ours. The key difference is in their interpretation that they are operating a bubbling column and our interpretation that we are operating in the “incipient flooding mode.” It is of course possible, as indicated in Figure 1, to have either type of operation. We believe that Musil et al., like Kafarov (1960), might even be operating in the “incipient flooding mode” in some of their experimental runs without realizing that the gas phase might be continuous. The main advantage of having a continuous gas phase is low liquid axial mixing, an important point in convincing practicing engineers of the merits of operation in the “incipient flooding” mode. Literature Cited Kafarov, V. V., International Symposium in Distillation, Institution of Chemical Engineers, pp 153-58, 1960. Leung, L. S., Hutton, B. E. T., Nicklin, D. J., Brooks, P. C.. submitted to Ind. Eng. Chem., Fundam., 1976. Musil, L., Prost, C., Le Goff, P., Chem. Ind., Gen. Chem., 100,674 (1968). Nicklin. D. J , Koch. C.T., “A Model of Annular Flow,” Cocurrent Gas Liquid Flow.” p 239, E. Rhodes and D. S. Scott, Ed., Plenum Press, New York, N.Y., 1969.
Department of Chemical Engineering uniuersity of Queensland St. Lucia, Queensland 4067 Australia
Ind. Eng. Chem.. Fundam., Vol.
L. S.Leung* D. J. Nicklin B. E.T. Hutton
15, No. 1 , 1976
89
