Ind. E n g . Chem. Res. 1987,26, 638
638
consideration of their reactions for the design of industrial reactors. Registry No. CH,OH, 67-56-1. Literature Cited
Chem. Process Des. Dev. 1983a,22,532. Mihail, R.; Straja, S.; Maria, Gh.; Musca, G.; Pop, Gr. Chem. Eng. Sci. 1983b,38, 1581. Ono, Y.; Mori, T. J . Chem. Soc., Faraday Trans. 1 1981,77,2209. Deceased.
Hutchings, G. J.; Gottschalk, F.; Hunter, R. Ind. Eng. Chem. Res. 1987,preceding paper in this issue. Kaliaguine, S.; Mahaya, A.; Simard, F.; Lemay, G. Post-Conference Catalysis Symposium, Kyoto, Japan, 1986 Aug 22-26. Mihail, R.; Straja, S.; Maria, Gh.; Musca, G.; Pop, Gr. I n d . Eng.
Raul Mihail,+Sorin Straja,* Gheorghe Maria Gavril Musca, Grigore Pop Chemical and Biochemical Energetics Institute Bucharest, Splaiul Independentei 202, Romania
Comments on “Clear Liquid Height and Froth Density on Sieve Trays” Sir: The recent article by Colwell (1981) on the correlation of clear liquid height and froth density on sieve trays was claimed to have been based on data which cover only the froth regime. The froth/spray transition correlation due to Porter and Wong (1969) had been used in screening out the spray regime data. The Porter and Wong (1969) correlation for the froth/spray transition suffers two shortcomings. Firstly, the correlation is suitable only for sieve trays with free areas of about 5%. For data obtained from sieve trays with free areas other than 5%, the correlation is well-known to be inapplicable (see, for example: Wong and Kwan, 1979; Chen et al., 1982). It is evident from Table I of Colwell (1981) that a major portion of the data used to develop the correlation were derived from sieve trays with free areas that are not about 5%. Thus, the screening-out process carried out by Colwell in eliminating the spray regime data is unreliable in that an inappropriate froth/spray transition correlation had been used. Secondly, the other shortcoming inherent in the use of the correlation due to Porter and Wong is the fact that this correlation gives the height of the froth at transition, not the clear liquid height. Thus, to obtain the clear liquid height at transition, one needs to know the froth density which must be obtained from some other correlation. To overcome the first shortcoming, one may use the empirical correlation given by Wong and Kwan (1979) which had been analytically derived by Chen et al. (1982) ~ M T
- = 3.09(Uh/UJ
+ 2.06
dh
where h M T is the height of the gas-liquid mixture at transition, d h is the hole diameter, Uh is the hole velocity, and Ut is the “large drop” terminal velocity. The clear liquid height (hLT) is related to h M T by h M T ( 1 - t) = h L T , where t is the voidage. The large drop terminal velocity may be calculated by
Ut = 0.317(pL/p~)0.5 (m se1)
(2)
where pL and pG are the liquid and gas densities, respectively. Equation 1is also applicable to sieve trays with free areas other than about 5%. However, eq 1 still suffers the shortcoming that the froth density, which is related to 6 , must be available before the clear liquid height may be determined. Lockett (1981) gave a correlation based on a wide range of data including those of Porter and Wong (1969). This correlation gave h L T instead of h, and therefore was done without the need for a correlation for the froth density. The data fell within a rather wide band, and the best-fit
regression line for the froth/spray transition was given as (3) Dhulesia (1983), in analyzing a rather limited range of data for the valve trays, made distinctions between the transitions froth mixed and mixed spray, rather than the usual froth/spray transition. Using similar reasonings based on Dhulesia’s work on the valve tray, Chen (1985) showed that for sieve trays the scatter in the data plotted by Lockett (1981) was enveloped by the two straight lines
(4) ~ L T
- = 3.79U(,(pG /pL)0’5 dh
(5)
and that data in this region may be considered as one of “mixed” and is one of general uncertainty for the various methods of flow pattern determination as employed by the different experimenters. Equation 4 gives the transition for mixed-spray regimes, and eq 5 gives the transition for the froth-mixed regimes. The froth/spray transition due to Lockett given here as eq 3 in fact lines almost exactly halfway between eq 4 and 5. Thus, to be completely certain that data did not contain those in the spray regime, one should use eq 5, which also has the benefit that it gives directly the clear liquid height at transition. Unfortunately, the data used by Colwell(1981) were not readily available to me. It would be interesting to check if the use of eq 5 in screening out the spray regime data would in fact have any effect on the correlation of Colwell (1981). Literature Cited Chen, J. J. J. Chem. Eng. Res. Des. 1985,63,206. Chen, J. J. J.; Wong, P. F. Y.; Kwan, W. K. Int. J. Multiphase Flow 1982,8,565. Colwell, C. J. Ind. Eng. Chem. Process Des. Deu. 1981, 20, 298. Dhulesia, H. Chem. Eng. Res. Des. 1983,61, 329. Lockett, M. J. Trans. Inst. Chem. Eng. 1981,59,26. Porter, K.E.; Wong, P. F. Y. Distillation 1969, 2:22 (Inst. Chem. Eng. Symp. Ser. 32). Wong, P. F. Y.; Kwan, W. K. Trans. Inst. Chem. Eng. 1979,57,205.
John J. J. Chen Chemical and Materials Engineering Department T h e University of Auckland Auckland, N e w Zealand
0888-5885/87/2626-0638$01.50/0 0 1987 American Chemical Society
