Mass transfer studies under gas phase controlled conditions.

Abstract

Gas-phase controlled mass-transfer for the system, hexane-nitrogen-dodecane has been studied. Three different contacting apparatuses were used: (1) a wetted wall column; (2) a bubble column; (3) a slug-flow absorber. The outlet gas streams from these apparatuses were analysed and the effect of gas-flow rate, liquid flow rate and contact time on overall mass-transfer efficiencies, ET, were investigated. For the wetted wall column mass-transfer efficiencies were found to be a function of column diameter and contact time. The results are found to be consistent with the data of previous investigators. Practically no end-effect mass-transfer took place. For the bubble column, ET, was found to be very high and end-effect mass-transfer predominated. The EE values fell between 0.98 and 0.99. ET was observed to be the function of gas flow rate and contact time. For the slug-flow absorber, mass-transfer efficiencies were again high and end-effect mass-transfer also predominated EE ranging from 0.94 to 0.98. ET was observed to be the function of liquid flow rate as well as gas flow rate, and contact time. Asymtopic values of E T less than 1.0 were observed and are considered to be as the result of incomplete liquid mixing. The effect of liquid flow rate was further studies by varying the position of liquid loading, top or-bottom of the absorber. ET, values were slightly reduced when the liquid was introduced at the top of the column. The overall gas phase mass transfer coefficients were calculated for the rise period mass-transfer. Highest values were found with the bubble column, but are considered to be low in view of the high ET, values observed. Lowest values were found with the slug flow absorber and suggested that mass-transfer was inhibited when bubble movement is restricted. The KOG values for wetted wall column, bubble column and slug-flow absorber, at 50 cc. per min. gas rate, 7 cc. per min. liquid rate and 0.5 cm. I. D. tube, were 0.15 x 10-4, 0.35 x 10-4 and 0.029 x 10-4 g-mole/(sec.) (atm.) (cm.2), respectively. Photographs of bubbles rising in a slug-flow absorber were taken. Slug size was found to be dependent on both gas and liquid flow rate. Variations in gas and liquid rates resulted in the slugs changing in form from an elongated gas slug to a hemispherical cap.