Ind. Eng. Chem. Process Des. Dev. 1982, 21, 273-276
273
Power Consumption and Pumping Characteristics in a Loop Reactor Yasuhlro Murakaml, Tsutomu Hlrose, Shlnlchl Ono, Hlroto Eltoku, and Toru Nlshljlma Department of Chemlcal Engineering, Kyushu University, Fukuoka 8 12, Japan
Measurements of power consumption in a loop reactor of 10 cm i.d. with pitched paddle impellers were carried out in the range of mixing Reynolds number Re,,, = lo2 to 2 X io5 using water and corn syrup solution (p = 1-60 mPa*s)as working fluid. The effect of pitched angle of the impeller blade, I#J , upon power consumption is expressed by the equation N P a (sin $)2 at Re,,, > 200 where flow is turbulent. There exists no influence of baffle condition and no difference between batch and continuous operations. The circulation velocity was controlled by changing the opening size of throttle rings and length of tubing loop as well as changing the impeller speed. The results were rearranged into the pumping characteristic curves based on the idea that the impeller region and the tubing loop behave independently. The characteristics of the impellers tested are similar to that of the axial-flow pump. By use of the resultant characteristic curves, an approach to estimated power consumption is proposed.
Introduction The recent development of highly active catalysts enables olefin polymerization to proceed toward a bulk polymerization. As frequently discussed, some difficulties have been pointed out such as deposition of polymer on the reactor wall and impeller and insufficient heat transfer capacity in a conventional stirred tank reactor process. To solve these problems, the loop reactor was proposed by Norwood (1962) in a slurry polymerization of olefins. Furthermore, the loop reactor has potential applications to biochemical reaction and processing of highly viscous liquids. The advantages of the loop reactor are summarized as follows. (1)Deposition of polymer on the reactor wall may be prevented under high Reynolds number operation. (2) Heat transfer area per unit reactor volume is larger than that of a stirred tank reactor. (3) Scale-up of a loop reactor is probably easy, because this structure is mainly composed of straight tubes. In the application to a slurry polymerization of olefins, the loop reactor is desirably operated at high Reynolds number to prevent polymer deposition. However, power consumption under such high Reynolds number operation seems to be large. Thus, the information about the power consumption is one of the most important factors for suitable design and operation. In this study, power consumption in the loop reactor was measured under various conditions, and the influences of baffle, impeller, and tubing geometries and the feed rate are examined. These results are discussed and referred to the results (Nagata et al., 1956) obtained in the stirred tank reactor. The loop reactor mainly consists of impeller and closed loop tubing systems. As it is very similar to a pump and tubing system, the pumping characteristic test is expected to give some useful suggestions for interpretation of power consumption in the loop reactor. The pumping characteristic tests carried out under various circulation velocities and impeller speeds will be applied to the proposed method to estimate the power consumption in the loop reactor. Experimental Section Figure 1 shows the schematic diagram of the experimental apparatus to measure the power consumption. Water or corn syrup solution (1= 1-60 mPa.s) was circulated in a loop reactor of 10 cm i.d. and 22 L (partly 31 0196-4305/82/1121-0273$01.25/0
L) volume by pitched blade paddle impellers of four different pitched angles. Further detailed dimensions of the impellers are shown in Figure 2. The pitched blade paddle instead of the marine screw was selected for the following reasons. In our preliminary experiment,the pitched paddle was found to be as efficient as the marine screw owing to high pumping capacity in spite of high power consumption, indicating the potential application of the pitched paddle to practice. Another reason is that the pitched blade paddle is very simple in geometry and the most suitable to investigate the geometrical effects (pitched angle) on efficiency. The resistance to flow, hence the flow rate, was varied under constant impeller speed by inserting a throttle ring of different sized openings ( m = 0, 0.4, 0.6, 0.8, and 1.0) far downstream of the impeller as shown in Figure 1or by changing the loop length and the number of bends named I- and L-shaped loop reactors as shown in Figure 3. The flow rate was measured by pulse response method with KC1 solution as a tracer, details of which were reported previously (Sato et al., 1979). Torque exerted on the impeller shaft was measured by torque transducers with capacities of 0.2 and 1.0 J. The discharge pressure of the impeller was evaluated by measuring the pressure distribution along the wall. These three measurements were used to interpret the power consumption in the light of the pumping characteristics of the impeller. Effect of Pitched Angle on Power Consumption The effect of the pitched angle, 4, on the power consumption was first examined in the I-shaped loop reactor without a throttle ring. Power number, Np = P/pn3d,5,obtained from the results of torque measurement are plotted against mixing Reynolds number, ReM = pnd12/p, in Figure 4. This figure shows the results obtained under four different pitched angles of impeller blades (i.e., 4 = 20, 30,40,and 60") all in unbaffled. In small mixing Reynolds number region (ReM< 200), Np decreases with increase of ReMand this region is considered as a laminar flow region. As the mixing Reynolds number exceeds 200, the power number begins to depend greatly upon the pitched angle but little upon ReM,and thus this region is considered as a turbulent flow region. In this region, the value of Npincreases with the pitched angle of the impeller. These facts are similar to the behavior in a stirred tank reactor with the pitched blade paddle impeller. 0 1982 American Chemical Society
274
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982 10
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To examine the effect of pitched angle, the power numbers in the loop reactor at ReM = 104-106 are plotted against sin 4 in Figure 5 from which the following expression is derived for the loop reactor.
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Power consumption in the loop reactor may be divided into two contributions. One is consumed in the agitation of the impeller and the other is consumed in the tubing system of the loop reactor. The former contribution may be expressed by the following relation which was shown by Nagata et al. (1956) for the turbulent stirred tank reactor.
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On the other hand, the relation in the latter could be expressed by the following equation. NP a: aZ3a: (sin 4)3 (3) In this equation, the previous finding (Sato et al., 1979) that the average circulation velocity, a,, is proportional to sin 4 is incorporated. Since power consumption in the loop reactor may be described as the sum of eq 2 and 3, the present result expressed by eq 1 is consistent with these facts. The most desirable impeller is probably one which generates the maximum axial velocity with the minimum power consumption. To find the most effective impeller, the modified power number, Np* = P/pa,3dl2,in terms of average circulation velocity is plotted vs. the tube Reynolds number, Re = pii,d,/p, as shown in Figure 6. The value of Np* at the pitched angle of 40' is the smallest of the four impellers tested. Thus, the 40O-pitched blade paddle is recommended.
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982 I
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can be considered to behave independently, just as in the conventional pump-tubing system. Thus, the pumping characteristic test could be extended to the case of the loop reactor. The effect of loop geometry can be represented by a single parameter of flow resistance or pressure loss in the tubing loop, which is balanced by the discharge pressure of the impeller. In the present study, the flow resistance was controlled by inserting throttle rings. Some examples of the pressure distribution in the impeller section are shown in Figure 9 for the case of an I-shaped loop reactor with m = 0.8. The discharge pressure of the impeller, Ap, was obtained from the pressure jump at the impeller position by the extrapolation of the pressure distribution. The pumping efficiency, 9, was defined as the ratio of the power transformed into discharge pressure to the total power consumption, i.e. v = AP*Q/P (4) Obtained values of torque, T , pumping efficiency, 1,and discharge pressure, Ap, are plotted against the flow rate, Q, according to the standard method of the pumping characteristic test. An example for the impeller with a pitched angle of 40' is shown in Figure 10. The values obtained under different impeller speeds have similar dependence on the flow rate. Besides, these pumping
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fluence of pitched angle of the impeller blade, the presence of baffle plates, and the flow system (i.e., batch and continuous operations) are summarized as follows. (1) The relation between power number and mixing Reynolds number in the loop reactor is similar to that of the stirred tank reactor. (2) The influence of pitched angle of the impeller upon the power consumption is very small at low mixing Reynolds number region (ReM< 200). In the region of high mixing Reynolds number, which corresponds to the turbulent flow region, the influence of pitched angle is remarkable as presented by eq 1. (3) The impeller with the pitched angle of 40' has the most excellent discharging capacity when the power consumption is the same. (4) The influence of baffle and the difference between batch and continuous operations can be neglected. (5) An approach to estimate the power consumption in the loop reactor is proposed by applying the pumping characteristic curves which were prepared.
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Nomenclature do = opening diameter of the throttle ring, m dl = outer diameter of the impeller, m d, = inner diameter of the tube, m f = friction factor L = total length of the loop reactor, m 1 = axial length, m
1, = total length of straight tubes, m m = ratio of opening area of the throttle ring to cross-sectional area of the reactor tube (do2 d:) Np = power number ( P / p n 3 d l ) Np*= modified power number in terms of average circulation velocity (P/pa?dl2) n = impeller speed, 1/s P = power, J/s p = static pressure, Pa p o = reference static pressure, Pa A p = discharge pressure of the impeller, Pa Q = flow rate, m3/s R = curvature radius of a bend, m Re = tube Reynolds number (pn,d,/K) ReM = mixing Reynolds number ( p n d 1 2 / p ) T = torque, J n, = average circulation velocity, m/s V = total volume of the loop reactor, m3
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(5) In this equation, the first and the second terms of the right-hand side are the contributions of straight portion and bends, respectively, and the necessary parameters, f and {, can be evaluated by Drew's (Drew et al., 1936) and Ito's (Ito,1959) formulas. This additive contribution was confirmed in the previous paper (Sato et al., 1979). Then, the corresponding values of Np, q, and Q / n d 1 3 can be obtained from the value of Ap/(paZ2/2)on the characteristic curve. Finally, the required impeller speed and resultant power consumption are estimated. Conclusions Power consumption in the loop reactor was measured over a wide range of mixing Reynolds number. The in-
Greek Letters
loss coefficient for a bend pumping efficiency = viscosity of fluid, Pa-s p = density of fluid, kg/m3 6 = pitched angle of the impeller blade, deg {=
9 =
Literature Cited Drew, T. 0.; Koo, E. C.; McAdams, W. H. Trans. AIChE 1936, 28, 56. Ito, H. J . Jpn. SOC.Mech. Eng. 1959, 62,1634. Nagata, S.; Yokoyama, T.; Maeda, H. Kagaku Kogaku 1956, 20, 582. Norwood, D. D. Jpn. Patent, Showa 37-10087, 1962. Sato, Y . ; Murakami, Y . ; Hirose, T.; Hashiguchi, Y . ; Ono, S.; Ichlkawa, M. J . Chem . Eng . Jpn . W79* 12 448. I
Received for review October 21, 1980 Accepted December 1, 1981
