Eng2nTring
Rectifying Column Perfor mance
Process development
EFFECT OF INTERMITTENT REFLUX
P. T. O’LEARY
AND
JOHN
R. BOWMAN
M E L L O N INSTITUTE, PITTSBURGH, P A .
JAMES COULL UNIVERSITY
r
OF PITTSBURGH, PITTSBURGH, P A .
T h e performance of rectifying columns under intermittent reflux, whereby fixed intervals of operation at zero and total reflux are alternated in time, is calculated theoretically, assuming that both the decay of distillate composition under zero reflux and the growth at total reflux depend exponentially on time. The resulting formulas indicate that the mean distillate composition from such operation is always heavier than that obtained by continuous reflux at the same ratio, but only slightly so under ordinary conditions. Experimental results check the theoretical predictions satisfactorily.
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EVERAL “intermittent reflux” rectifying column heads have been described in the literature (3, 5 ) . In all of these the condensate is alternately wholly removed as product or returned to the column as reflux, rather than continuously divided into reflux and product streams as in conventional heads. The column therefore is operated periodically with intervals of zero and infinite reflux ratio. Such operation can be characterized mathematically by the mean reflux ratio and the period of a complete cycle. calcuIation of the performance of a columIl lvith such operation requires knoq+dge of the manner in which equilibrium is approached at zero and at total reflux. Relatively little information of this kind is available, but some studies have been made on the total reflux case ( 2 , 4 , 6 ) . The assumption to be made here is that both the decay under zero reflux and the growth under total reflux are exponential in time. This is a usual first-order approximation for a periodic process in general, where a variable approaches a limit asymptotically. I t s application to the present problem is simple because the limiting distillate compositions are easily obtained. The growth and decay constants for a given column are not necessarily equal. Their ratio, however, can be calculated from the equilibrium distillate composition a t partial continuous reflux. This is an application of the general relationship that always exists between the ratio of forward t o ba2kward rates of a reversible reaction and the equilibrium constant of the reaction.
I n the zero reflux interval, the initial distillate composition is, say, VI, and the asymptotic limit is the composition of the feed, y/. The exponential decay assumption therefore gives
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