Gas-Bubble Columns for Gas-liquid Contacting HERMAN L. SHULMAN’
AND
M.
c. MOLSTAD
UNIVERSITY O F PENNSYLVANIA, PHILADELPHIA, PA.
T h e performance of gas-bubble columns (in which bubbles, formed by passing a gas t h r o u g h a porous plate a t t h e base of t h e column, rise up t h r o u g h a descending continuous liquid phase) has been investigated by determ i n i n g t h e pressure drop and liquid-film mass-transfer characteristics of such columns. Rate of carbon dioxide absorption and desorption and hydrogen desorption, w i t h water as t h e liquid, has been measured. T w o distinctly different regions were found in t h e investigation of masstransfer rates. A t low gas rates (the streamline region) t h e rate of mass transfer i s a f u n c t i o n of gas rate and a t high gas rates (the t u r b u l e n t region) it is not. In addition, t h e mass-transfer rate was found t o be a f u n c t i o n of liquid rate, water temperature, column height, and l i q u i d diffusivity and independent of t h e column diameter,
T
HIS work deals with an investigation of the performance of gas-bubble columns for gas-liquid contacting operations. A gas-bubble column is a column in which bubbles, formed by passing a gas through a porous plate a t the base of the column, rise up through a descending continuous liquid phase. In 1945 one of the authors was associated with some unpublished industrial work in which gas-bubble columns were used in a pilot plant to chlorinate hydrocarbons. The gas-bubble columns were found to be extremely effective for this purpose and the possibility of extending their use to other gas-liquid contacting problems appeared very promising. As there was no published work directly applicable, a program was outlined for the investigation of this type of equipment. The work on the absorption of pure chlorine and the physical behavior of the gas-bubble column, which revealed its ability to handle high liquid rates a t low gas rates, pointed to fields of application where the liquid-film resistance may be the controlling factor in mass transfer. This, combined with the availability of good methods for the determination of liquid-film resistances, led to the decision tg employ carbon dioxide desorption and absorption and hydrogen desorption for the investigation of the mass-transfer characteristics of the columns. In planning the program for the investigation it was realized that a large number of variables might influence the masstransfer rates and that the findings of previous mass-transfer studies with packed columns and liquid-liquid extraction spray columns would not serve to limit the number of variables to be studied. Accordingly the customary variables and several variables which are not usually investigated in mass-transfer studies were included in the experiments. The items subject to investigation included the effect of: 1. Gasrate 2. Liquid rate 3. Water temperature 4. Column diameter 5. Column height 6. Plate orosity 7. Liquii diff usivity 8. Desorption, compared with absorptitm
The results obtained in the study of these variables are discussed in the following sections and equations are presented to,correlate the data obtained. J
Present address, Clsrkson College of Technology, Potsditm, N. Y.
plate porosity, and t h e direction of mass transfer-that is, absorption or desorption. The data have been correlated by means of three equations, t w o for t h e lLstreamline” region and one for t h e “turbulent” region. It can be concluded t h a t gas-bubble columns exhibit mass-transfer rates of t h e same order of magnitude as packed columns a t low l i q u i d rates and m u c h greater mass-transfer rates a t h i g h liquid rates where packed columns are inefficient. The pressure drop across t h e gas-bubble columns is m u c h greater t h a n t h a t across packed columns of t h e same height. The fields of application of t h e gas-bubble columns would include chemical reactions where gases are brought i n t o i n t i m a t e contact w i t h liquids and absorbers for applications where t h e pressure drop would n o t make t h e operation uneconomical. APPARATUS AND PROCEDURE
The apparatus is shown in Figure 1. Compressed air from a line a t 70 pounds per square inch is passed through a filter, A , consisting of a 12-inch section of 2-inch pipe, packed with glass wool and equipped with a drain valve, and then through a 0.25inch reducing valve, B. This valve maintains a constant pressure at the 0.25-inch needle valve, C, which controls the flow through a rotameter, D. The temperature and pressure of the air are measured in the line leading to the gas-bubble column by means of a thermometer, T,and water manometer, E. Water is obtained from the city mains and is passed through a a/&oh bronze reducing valve, F, which maintains a constant pressure at the a/a-inch bronze globe valve, G, which controls the flow through the rotameter, H . For the desorption tests, the water enters column Z packed with 0.5-inch stoneware Raachig rings where i t contacta the solute gas, carbon dioxide or hydrogen, supplied from cylinder J. The water which is almost saturated with the solute gas then enters column K through a distributor designed to introduce the water horizontally as a number of small jets at the liquid surface. The columns used were of 1-inch, 2-inch, and 4-inch inside diameter and were &foot sections of flanged glass pipe. A conical end design was adopted, based on the experience of Blanding and Elgin ( 1 ) with liquid-liquid spray columns. The conical end, made of sheet copper, is 2 inches long and increases 1 inch in diameter from the narrow to the wide end. Several small holes drilled in the side of the conical end prevent the collection of air from stray bubbles a t the higher gas rates. The base of the column consists of two IO-inch diameter brass plates bolted together so as to enclose a 7-inch diameter by 6inch glass cylinder. A short piece of flanged brass tubing, of 4inch inside diameter, is soldered to the lower brass plate. The porous plates are bolted to the flange which is located so that there is a free space of 1 inch between the bottom of the conical section and the porous plate. For the 1- and 2-inch columns, brass washers, s / Q 2 inch thick, with openings cut to match the column diameter are placed above and below the porous plate with gaskets to make an airtight fit. In this way the exposed area of the porous plate is always equal to the cross-sectional area of the column. The air enters through the lower brass plate into the 4-inch diameter section and passes through the porous plate into the liquid as bubbles. The water leaves the column through two diametrically opposed openings in the lower brass plate and passes through an adjustable vented takeoff 1058
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 19SO
IOSQ
by the manufacturers. The oarbolt plate used WM cut from a cylinder oi porous carbon, Grade 10, supplied by the National Carbon Company, Imc., Cleveland, Ohio. Photographs of
