Distillation Efficiency for Removal of Solved Minute Water in a Light Hydrocarbon

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The purpose of this work is to clarify the reason why it is said that the efficiency of dehydrating distillation in hydrocarbon-water system is extremely low. The research results obtained so far are summarized as follows. 1.Distillation experiment with a single-tray column In dehydrating distillation experiments for three systems of benzene-water,toluene-water,and hexane-water,it was shown that in a range of relatively high water concentration the tray efficiency reached as high as those for heptane-toluene system(about 70%),but in a low water concentration range the efficiency became low as the water concentration decreases. 2.Distillation experiment with a five-tray column In this experiment,we examined a concentration profile in a five-tray column by measuring the water concentration on each stage. The results showed that the water concentration below the third or fourth stage from the top of the column does not change and hence the limit of the dehydration exists.The tray efficien … More cy in item 1 above is the calculated result based on the vapor-liquid equilibrium (VLE) relation obtained in a range of relatively high water concentration,where the VLE ratio was assumed to be constant. Taking into account also the above results,it was considered that the reason of the efficiency drop may be largely due to the VLE relation in a range of low water concentration, which have not been yet measured. Therefore we conducted careful measurement of the VLE relation mainly in that range. 3.Measurement of the VLE relation in a range of low water concentration Using the liquid on the bottom stage of the five-tray column and molecular sieves as a dehydrating agent,we could measure the VLE relation up to about 2wt-ppm in water content. For benzene-water and hexane-water systems it was found that in a range above 100wt-ppm in water content the VLE ratio is almost constant,but in a range of the lower concentration it decreases gradually with decrease of the water concentration and tends to approach unity abruptly below 5wt-ppm. The calculated values of the tray efficiency based on this VLE data became higher than those obtained in item 1, but still gave its low value in a low water concentration range. Consequently,we could not reduce the reason of the efficiency drop to the VLE relation only. So we intend to clarify this questionable point in future. Less