Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 577
Comments on: “An Investigation into the Nucleation Kinetics of Urea Crystallization in Water by Means of Crystal-Size Distribution Analysis”
Sir: In response to the above from Keight, it is obvious that mass balance constraints force nucleation rate (J)to be inversely related with growth rate ( r ) to the third power at constant suspension density (MT)and residence time (77. However, we did not consider the case of constant suspension density. As indicated in the article, the crystallizer temperature was varied between 3 and 16 “ C , which are the practical limits between freezing and a minimum supersaturation ratio (since the feed solution was maintained a t room temperature or about 22 O C ) . As a result, suspension density varied from 4.5 g of urea/100 cm3 of slurry at higher temperatures to some runs at 14.7 g/100 cm3 of slurry at lower temperatures. The residence time was held fairly constant for the 36 runs, varying from 30 to 38 min, resulting in a reasonable run length and conditions which could be reproduced.
0019-7882/78/1117-0577$01.00/0
As mentioned in our paper, one would really like to use supersaturation as the primary variable with which to correlate nucleation rate. However, at low supersaturations this is impossible to measure and thus growth rate is used as a measure of supersaturation. Increasing residence time in urea-water systems does not result in larger crystals, since nucleation due to secondary effects dominates and since supersaturation decreases growth rate more than nucleation rate. It would be interesting to extend our studies over a wider range of temperatures, but unfortunately a greater investment in temperature control of transfer lines would be required. Department of Chemical Engineering T h e University of Toledo Toledo, Ohio 43606
0 1978 American Chemical Society
L. E. Lahti* M. L. Jones, Jr. K. D. Lodaya
