Determination of Reflux Ratio and Boil-Up Rate for Distillation Column with Intermittent Take-Off-Type Reflux Control Yi-Chung Chang,' U. S. Bureau of Mines, Morgantown, W. Va., and Liang-tseng Fan,2 West Virginia University, Morgantown, W. Va. N BATCH
distillation work, accurate
I control of reflux ratio is important. a major factor affecting
Reflux ratio is sharpness of separation and time of distillation. Inaccuracy in its control often leads to w o n g interpretation of data. One of the many reflux regulating devices is an intermittent take-off type [Brandt, P. L., Perkins, R. B., ,Jr., Halverson, L. K., Oil GUS J . 45, KO. 31, 86,102 (1946)], as shown in Figure 1. The condensate from the reflux condenser falls into a small glass cup, to which a distillate take-off tube is connected a t the bottom. The interior of the cup is ground t o form a valve seat. A spherical plug, which fits into this seat, seals the distillate tube when reflux is to be returned to the column, the reflux overflowing the cup. In operation, reflux from the condenser falls into the cup, overflows, and is returned to the column when the valve is seated, closing the distillate line; but the entire reflux is taken overhead when the valve is lifted. The reflux ratio can thus be controlled by suitable timer switch actuation, opening and closing the valve.
variables can be expressed by BT - 3600_ C = ~ Bt 3600 C
+
_ (1)
where
R
= reflux ratio = reflux/product B = boil-up rate a t top of column excluding internal reflux, ml. per hour T = reflux time, seconds per cycle t = discharge time, seconds per cycle C = volume of cup, nil.
not given, they can be determined experimentally. To determine the leakage, the column may be operated under total reflux and the volume of the condensate collected in the receiver over a certain interval of time measured. Boil-up rate usually has no appreciable effect on leakage; the column can be operated a t any convenient rate. Boil-up rate may be determined by a rate device placed between the pot and the column [Nples et al., Znd. Eng. Chem. 43, 1452 (1951)l or by the following formula, if the product, P , collected during 1 hour is known. P ( T t ) - 3600(C TL) B = (3) t
In deriving this equation, it is assumed that the spherical plug fits the seat perfectly, so that during reflux no condensate can leak out. If leakage exists, Equation 1 may be In using Equation 3, C must be known modified to include a leakage term. first. When both B and Care unknown, BT - 3600 ( C T L ) R = Bt (*I the product rate may be measured a t 3600 (C TL) t v o different timer settings using equal boil-up rates, and these two sets of where L = rate of leakage, ml. per values substituted in Equation 3. second. T17ith values for B , C, and L given. B = Pi(T1 t i ) - 360WC T I L ) (4) the reflux ratio can be calculated by tl Equation 2 for each combination of Pz(Tz t z ) - 3600(C TzL) B = (5) timer settings. Khen B, C, and L are tz Note. tl must be different from t ~ . SO.E%OID By simultaneously solving Equations 4 and 5, B and C can be obtained. REFLJX In one test on a 1-inch column, the -CON3EVSE3 leakage through the valve determined under total reflux was 0.71 ml. per hour or 0.0002 ml. per second. The distillates collected were 166.5 ml. per hour a t a timer setting of TI = 20 seconds and tl = 2 seconds, and 120.4 ml. per hour a t Tz= 40 seconds, t z = 3 seconds. On substituting these values in Equations 4 and 5 and solving simultaneously, the value for C was 0.18 nil., and the boil-up rate was 1500 ml. per hour. The boil-up rate determined from the Figure 1. Intermittent take-off refluxheat removed by the cooling water flowcontrol device for distillation column ing through the reflux condenser was 1430 ml. per hour. Considering the loss of some heat due to imperfect insulation, these results agree well. Without this correction, the boil-up rates calculated by employing the apREFLUX . T I M E - 2 0 SEC. parent reflux ratio determined by the O l S C H 4 R G E T I M E - 2 SEC. c 5 C A P 4 C I T Y - 0 2 ML timer settings are 1832 ml. per hour for the first setting and 1726 for the second setting. K i t h the type of control described, the reflux ratio determined by the ratio of the reflux time and discharge time can deviate considerably from the BOIL-UP R 4 T E . M U H R actual value. By the procedures described, accurate reflux ratios can be Figure 2. Effect of boil-up rate on easily determined and controlled. corrected reflux ratio
+
+
+ +
+
+ +
+
+
TO
Depending on the size of the column, the cup has a holdup of from 0.1 to 0.5 ml. and at each lift of the plug, one of these quantities of distillate is taken over, in addition to the proportion of reflux indicated by the timer setting. Vnder conditions of low vapor throughput, this holdup becomes an appreciable part of the total distillate, and the reflux ratio actually realized is considerably different from that determined by the timer setting as shown in Figure 2. If the boil-up rate were calculated from the apparent reflux ratio determined by the setting of the timer and the distillate collected in a certain time interval, considerable error might result. Using the conditions stipulated for Figure 2 for a boil-up rate of 1000 ml. per hour, the distillate collected in five minutes is 10.3 ml. (assuming no leakage). This, when multiplied by 1 plus the apparent reflux ratio, or 11, will give a calculated boil-up rate of 1360 nil. per hour, n hich is 36% off the actual value. Hom-ever, the relationship between the reflux ratio and the other operating 1 Present address, Research Center, IT. S. Steel Corp., Jlonroeville, Pa. Present address, Kansas State College, Rlanhattan, Kaxi.
1
PACK€:.
COLLKk
CUP
VOL. 31, NO. 6, JUNE 1959
1121
