Hydrodynamics in a Rotating Packed Bed. I. A Novel Experimental

Article pubs.acs.org/IECR

Hydrodynamics in a Rotating Packed Bed. I. A Novel Experimental Method Zuo-yi Yan, Cheng Lin,* and Qi Ruan Department of Chemical Engineering, Fuzhou University, Fuzhou 350108, Fujian, P. R. China ABSTRACT: A novel experimental method is proposed to investigate the fluid flow in rotating packed beds (RPB). Experimental results show that the novel method is simple and effective and can be used not only in the lab experimental equipment but also in the industrial grade equipment. By using this method, the hydrodynamic performances of the RPB can be easily studied, which include actual wetting and false wetting, the flow pattern of the liquid film flow, the number and width of liquid trajectories, etc., and consequently, their changing rules are obtained. The results have significant effect on the gas−liquid two phase transfer behavior and are the basis of establishing accurately hydrodynamic mathematical model for RPB.

1. INTRODUCTION Rotating packed bed (RPB) was first invented by Ramshaw and Mallinson to intensify gas−liquid mass transfer efficiency.1 Under RPB operation, the centrifugal force is larger far than gravity. Due to the larger centrifugal acceleration, thinner films and tiny droplets are produced for enhancing gas−liquid mass transfer. In addition, the high centrifugal force permits the use of packings with a large surface area in the range of 1000−4000 m2/m3, which is 5−10 times higher compared to the packings used in conventional columns, which would lead to a much higher mass transfer rate.2−7 The gas−liquid mass transfer is greatly intensified, thus reducing the physical size of the equipment more than that with the conventional packed bed and, as a result, reducing the capital and operating costs. Based on these benefits, rotating packed beds have been widely used in many fields of industry, especially in those relating to gas− liquid mass transfer and heat transfer processes, such as distillation,3−5 absorption,6,7 and extraction operation, etc.8,9 Since the fluid flow characteristics have great impact on gas− liquid transfer behavior, many researchers have done a great deal of studies on this issue. Munjal et al.10,11 and Thomas et al.12 studied the liquid mechanics in the high gravity field by investigating the liquid flow on the rotating disk and rotating blade. However, the status for the liquid flow on the rotating disk or rotating blade is far different from liquid flow in the rotating bed packed with random packings. Burns et al.,13 Zhang et al.,14 and Guo et al.15 observed the state of the liquid flow in the RPB by using the high-speed stroboscopic photography. However, this method requires some special camera device and equipment structure. In addition, the cost of this method is quite high, and it is only used in the lab as experimental equipment. In this paper, we propose a novel experimental method that can be used simply and effectively to investigate the fluid flow in RPB.

Figure 1. Experimental setup of an PRB: 1, pump; 2, gas flowmeter; 3, gas inlet; 4, manometer; 5, gas outlet; 6, liquid distributor; 7, water tank; 8, 15, valve; 9, fluid flowmeter; 10, packing; 11, liquid outlet; 12, rotor; 13, electromotor; 14, frequency converter.

distributor and sprays into the inner edge of the packed bed. To study conveniently, the structure of the inner edge of the bed is designed as shown in Figure 2. The obstruction divides the inner edge into two parts: noncirculation area and circulation area. Inside the bed, the liquid from the circulation area moves outward through the packings because of the centrifugal force action. The liquid is then splashed onto the stationary housing and is collected at the bottom. The gas is introduced from the stationary housing, flows inward through the packings, and leaves the PRB through a pipe. The packing used in this study is a triangular spiral packing, as shown in Figure 3. Table1 presents the details of the rotating packed bed and the physical properties of the triangular spiral packings. 2.2. Experimental Method. To capture liquid trajectories in RPB and obtain the pressure drops of the bed, the following experimental steps are considered. (1) Radial cross section: Several ring stainless steel plates were machined. A type of paper and colored water were Received: Revised: Accepted: Published:

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. A self-made small PRB is shown in Figure 1. The liquid enters the packed bed from a liquid © 2012 American Chemical Society

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October 2, 2011 June 23, 2012 July 13, 2012 July 13, 2012 dx.doi.org/10.1021/ie202257t | Ind. Eng. Chem. Res. 2012, 51, 10472−10481

Industrial & Engineering Chemistry Research

Article

Figure 4. Structure of the bed for experiment step 1.

the square stainless steel plates by the same method as in experimental step 1. The square stainless steel plates were inserted into the packings along the radial direction (shown in Figure 5).

Figure 2. Structure of the inner edge of the bed.


(3) Radial cross section: On the basis of the experimental step 1, a flabellate stainless steel plate was machined and inserted into the packings with a certain angle α between two ring stainless steel plates (shown in Figure 6). The top ring plate is slightly smaller than the bottom ring plate. The appropriate gap between the outer edge of the flabellate plate and the top ring plate makes sure that the liquid could flow out smoothly. (4) The pressure drops of the bed were measured at different liquid and gas flow rates and rotational speeds by installing pressure taps close to the inner edge and outer edge of the bed, as shown in Figure 1.

Figure 3. Triangular spiral packing.

Table 1. Details of the Rotating Packed Bed and the Physical Properties of the Triangular Spiral Packings details of the rotating packed bed

physical properties of the packing 2 × 2 mm

100 mm 30 mm

dimension of the packing specific surface area packing porosity

100 mm

packing density

1160 kg/m3

outer diameter

300 mm

inner diameter axial length of the rotor radial length of the rotor

3. RESULTS AND DISCUSSION Figure 7 shows the results of the experimental steps 1 and 2, which exhibit the moving trajectories of the liquid on the radial cross section and axial cross section. The RPB is clearly divided into two different parts: entry region and development region. There exists no blank area on the entry region because it is fully filled with the colored water. However, there are lots of blank areas on the development region for the liquid moves along the trajectories. It is seen that the trajectories appear in the form of straight line from overall- and macro-angles. The reasons are as follows: the forces acting on the liquid particle are analyzed, as shown in Figure 8, which includes radial shear stress by packings, radial resistance by gas, centrifugal force, Coriolis force, gravitational force, circumferential shear by packings, and surface tension of liquid particle. As the liquid first enters the inner of the bed, it hits the packings and causes most momentum loss in the radial direction. There is a great relative movement between the liquid and the packings in the

2500 m2/m3 0.84

selected to make sure that there were no diffusion and no teeth-like-shapes formed on paper when the colored water passed through the paper surface. The paper was stuck onto the surface of the ring stainless steel plates, which were put into the RPB and divided the RPB into several equal parts along the axial direction (shown in Figure 4). The colored water was added into the tank when the rotor ran stably. (2) Axial cross section: Several square stainless steel plates were machined. The paper was stuck onto the surface of 10473

dx.doi.org/10.1021/ie202257t | Ind. Eng. Chem. Res. 2012, 51, 10472−10481


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Figure 7. Result of experiment steps 1 and 2.

circumferential direction at the beginning. Thus, the paper is full of the colored water, and there exists nearly no blank area in the entry region. In this process, the entry region acts as a role of redistribution of liquid and the energy is transferred from the packings to the liquid continuously. Finally, the liquid is accelerated to match the same circumferential velocity as the packings when the liquid just enters the development region. In development region, Coriolis force counterbalances with the circumferential shear by packings. Therefore, the circumferential velocity of the liquid tends to zero and there is hardly any relative movement along the circumferential direction between the liquid and the packings. Thus, the moving trajectory of the liquid in the development region tends to straight line, and there exist lots of blank areas on the paper. Figure 7 also shows that the moving trajectory of liquid is intermittent. It is only when the liquid passes through the packings near the surface of the plate that the colored water leaves marks on the paper. The process can be shown in Figure 9a. In fact, the moving trajectories are not a strict axial symmetry. The reason is that the distribution of the liquid includes a certain randomness, which is caused by the external disturbances. However, it is found that the liquid trajectories are an axial symmetry to a great extent by repeated experiments. This implies that the randomness does exist but not influence much. Thus, it is possible to study the performance of the liquid in the whole RPB by a flabellate plate.

Figure 8. Forces act on the liquid particle.

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Figure 9. Comparison of the plate with different placements.

Figure 10 shows the result of the experimental step 3, which compares with that of experimental step 1. There are more

unbroken trajectories from the inner edge to the outer edge on the paper than that of the experimental step 1. It implies that the method for the experimental step 3 can capture more complete liquid trajectories, and the process can be shown in Figure 9b. It is also observed from Figure 10 that the number of the liquid trajectories on the outer edge is much more than that on the inner edge of the flabellate paper. The reason is that the stainless steel plate acts as a guiding role when the liquid reaches the surface of the plate. At the inner edge of the plate, there is only a part of the liquid reaches the plate’s surface and turns into the surface flow. More and more liquid reaches the plate surface with the increasing of the radial distance. Finally, all the liquid is captured by the plate on the outer edge. Thus, the number of moving trajectories on the outer edge of the paper equals to that of the whole liquid trajectories in the flabellate region. Figure 10 also illustrates that the width of trajectory in the experiment step 3 is about the same as that in experiment step 1, basically. This implies that the tilted flabellate plate can only change the direction of the liquid trajectories when the tilt angle is small. The reason is that the high rotating speed causes the strong centrifugal force that is a predominant factor, and the tilt surface had little effect on the liquid movement when

Figure 10. Comparison result of the experiment steps 3 and 1.

Figure 11. Comparison result of the different total input variable of the liquid (rotor speed, 900 rpm; liquid flow rate, 100 L/h; total input, 45 and 500 mL). 10475



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Consequently, there are two moving trajectories on the paper at t + Δt moment, and the wetting region increases seemingly. The ABC path is wetted from t to t + Δt for the existing of liquid film and liquid particles. The ABC path at this moment is defined as a trajectory of the actual wetting. At the t + Δt moment, the liquid no longer passes the ABC path. However, there is no dry on the path ABC because there still exists little liquid, which adheres to the surface of solid by the molecular force and this part of the liquid could not be thrown away from the rotor by the centrifugal force. Although the ABC path at this moment is wetted, it has little contribution to the gas− liquid transfer process. Thus, the ABC path at this moment could be defined as the trajectory of the false wetting. The wetting efficiency is related to the number and width of the trajectories of the actual wetting. Under a given operation condition, the number and width of the trajectories of actual wetting is a fixed value. It is an interesting phenomenon that the number of the liquid trajectories is practically kept in the same value when the total input of the liquid is limited in a small range (about 10− 150 mL). Figure 13 shows the experimental result for the total input of the liquid varying from 15 to 75 mL (rotor speed, 900 rpm; liquid flow rate, 100 L/h). This result implies that the value of Δt is about few seconds and the position of liquid trajectory is not changed if the input time less than Δt. Therefore, the number and width of the trajectories of the actual wetting can be obtained as long as the total input of the liquid is kept within a certain value. Figure 14 shows a comparison of the shooting photograph13 by using the high-speed stroboscopic photography and the results by the method in this paper. As shown in Figure 14b, the highlighted area reflects the moving trajectories of liquid. There are also entry region and development region on the shooting photograph. The entry region is covered by the highlight. However, there is some dark area on the development region, and the liquid trajectories are straight along the radial direction. These results are consistent with that of the novel experimental method, which proves the exactness and feasibility of the novel experimental method. By comparison, the shooting photograph seems to have a larger highlighted area. This is because there is still a little liquid that adheres to the surface of the packings by the molecular force in the false wetting area. The highlighted area includes the false wetting area and the actual wetting area based on the imaging principle of the high-speed stroboscopic photography.

the tilt angel is small. Thus, the number of all the liquid trajectories on the flabellate paper can reflect the wetting efficiency in the flabellate region indirectly. Obviously, the wetting efficiency of the RPB is determined by the number and width of liquid trajectories. Figure 11 shows a comparison result of the different total input variable of the liquid (rotor speed, 900 rpm; liquid flow rate, 100 L/h; total input, 45 and 500 mL). It is found that the total input of the liquid has great influence on the number of liquid trajectories. It implies that the wetting region increases with the input time, since the total input liquid volume is proportional to the input time. This phenomenon contradicts the theory of the steady flow. This is because the position of the liquid trajectories changes with time, which is shown in Figure 12. At the t moment, liquid from the distributor enters the

Figure 12. Position of the liquid trajectory changed with the time.

entry region at mark A. The moving trajectory of the liquid is curved in the entry region for the circumferential relative movement. Therefore, the liquid flows along a curve path from A to B. Then, the liquid enters the development region and flows along an approximate straight path from B to C. In this process, the entry region acts as a role of redistribution of the liquid, which may change with the external disturbances. The external disturbance changes with time. Thus, at the t + Δt moment, the liquid flows along another curve path from A to B′ and finally along an approximate straight path from B′ to C′.

Figure 13. Comparison result of the different total input variable of the liquid (rotor speed, 900 rpm; liquid flow rate, 100 L/h; total input, 15, 45, and 75 mL). 10476



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Figure 14. Comparison of the shooting photograph11 by using the high-speed stroboscopic photography and the result by the method in this paper: (a) result by the method in this paper; (b) shooting photograph11 by high-speed camera.

Figure 15. Results of the experiment step 3 under different operating conditions.

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Therefore, it is difficult to distinguish actual wetting and false wetting area on the shooting photograph. Figure 15 shows the results of the experiment step 3 under different operation conditions. In the process of the experiment, it was found that the width of the liquid trajectories is changeless under a certain operation conditions. However, the number of the liquid trajectories may change. This phenomenon may be caused by external disturbances that affect the redistribution ability of the entry region. By repeating the experiments, it was found that the instability of the rotor and the loose or asymmetrical packings may cause low repeatability of experimental results. In the conditions of the stable running rotor with the substantial and symmetrical packings, the repeatability of experiment is quite good and the difference of the number of the liquid trajectories is about 1−3. Figure 16 displays the number of liquid trajectories of the actual wetting as a function of the liquid flow rates ranging from Figure 17. Effect of the rotor speed on the number of the liquid trajectories.

Figure 16. Effect of the flow rate of the liquid on the number of the liquid trajectories. Figure 18. Effect of the rotor speed on the width of the liquid trajectories.

0 to 300 L/h. As shown in Figure 16, the number of trajectories increases with the liquid flow rates but not linearly. The reason is that the thickness of liquid film on the packings surface, the number of liquid trajectories, and the number of liquid particles among the packings space increase with the liquid flow rates at the same time. Figures 17 and 18 display the number and width of liquid trajectories of the actual wetting as a function of rotor speed ranging from 600 to 1200 rpm. As shown in Figure 17, the number of trajectories increases with the rotor speed. This is because the number of trajectories is related to the redistribution ability of the entry region, which has the close relationship to the rotor speed. The faster the rotor speed is, the greater the dispersion ability of the entry region is. As shown in Figure 18, the width of the liquid trajectories decreases with the rotor speed. This is because the size of the liquid particles and liquid film decreases with the centrifugal force. As the liquid trajectories are the channels for the liquid film flow and the liquid particles, the faster the rotor speed is, the narrower the passage is. Figure 19 shows the effect of rotor speed ranging from 450 to 1200 rpm on pressure drop for the unirrigated bed. The results show that the pressure drop significantly increases with the rotor speed. Figure 20 shows the effect of gas flow rate on the

Figure 19. Effect of the rotor speed on the pressure drop.

pressure drop for an unirrigated bed. The pump provides a gas flow rate variation from 5 m3/h to 45 m3/h, and the results show that the pressure drop significantly increases with the gas 10478



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for this phenomenon. However, the possible reason may be expressed by the result of the liquid trajectories. On the one hand, the height of the liquid trajectories (Le is equal to the width of the liquid trajectories because the packing in the RPB is basically isotropy) increases with the liquid flow rates according to the previous analysis. The process can be expressed by Figure 22. Figure 22a represents the situation of gas movement in a unirrigated bed. The equivalent absolute roughness of the disorderly and unsystematic packings is H. The flow resistance of the gas increases with the value of H. However, the exposed roughness is H1 (H1 = H − Le) when the gas passes an irrigated bed. In other words, the liquid trajectories smooth the roughness, which reduces the flow resistance of the gas. On the other hand, the passing section of gas decreases with the liquid flow rates, which has an adverse effect on the gas flow. Both of these two factors counterbalance each other, and the pressure drop does not vary with the liquid flow rates in a certain range basically. Figure 23 shows the shape of the liquid trajectories at higher magnification. The trajectories consist of many short lines, nodes, and some confluences and branches. As shown in Figure 23, the short lines are not strictly along the radial direction. This is because the Coriolis force counterbalances with the circumferential force exerted by the packings when the liquid travels along the packings surface. However, once the liquid enters the packings space, it is not exerted by the acting force of the packings; thus, the direction of the liquid trajectories is changed. The distance for the liquid particles flying through the packings space is very short, and the direction of the liquid is corrected soon, which results in lots of nodes, confluences, and branches. Therefore, on the whole, it is seen that the trajectories appear in the form of straight line along the radial direction. Indeed, the frequency of the hitting is so high that the intermittent force exerted by the packings can be seen as a body force from the overall- and macro-angle. It is generally considered that the thickness of liquid film on the surface of packings is very thin (about 10−5 m, which was reported by Munjal et al.,10,11 Zhang et al.,14 Guo et al.,15 and Guo et al.22) and the liquid flow on the surface of packings is in laminar flow. However, according to the analysis of Figure 23, the liquid is constantly hitting the packings surface and the liquid reclothes the film on the solid surface after each hitting; the liquid film flow on the packings surface is unlikely to remain as laminar flow. This process can be described by the phenomenon of the boundary layer development shown in Figure 24. As shown in Figure 24, the boundary-layer develops from the position A and reaches the full development at the position B. The thickness of the boundary-layer is zero at the position A because the liquid just reaches the packings surface. The thickness of the boundary-layer is increasing for the viscous force of the packings surface. The turbulence flow happens between A and B, and the laminar flow happens between B and C. The length of the AB may be very short because the thickness of the liquid film is very thin. However, the length of the AC may also be very short for the frequency of hitting is very high. Thus, the turbulence flow occupies a certain proportion in the liquid film flow and should not be neglected. Moreover, the liquid channel is formed by the disorderly and unsystematic packings which makes the liquid flow tends to be turbulence at lower flow rate.

Figure 20. Effect of the gas flow rate on the pressure drop.

flow rate. Both of these two behaviors are very common in many places in the relevant literature; thus, it has not been discussed in this paper. Figure 21 shows the effect of liquid flow rates ranging from 0 to 300 L/h on the pressure drop at different rotor speeds and

Figure 21. Effect of the liquid flow rate on pressure drop.

gas flow rates. As is observed in Figure 21, the law of pressure drop in RPB is very different from that in conventional packed towers. The pressure drop increases obviously with the liquid flow rates in conventional packed towers; however, the pressure drop has not any obvious change trend in the RPB. This result is consistent with that in the work of Liu et al.,16 Lin et al.,17 and Lin et al.18 (In the work of Chandra et al.19 and Keyvani et al.,20 the pressure drop in irrigated beds is slightly higher than in unirrigated beds. In the work of Zheng et al.,21 the pressure drop in irrigated beds is slightly lower than in unirrigated beds. In fact, the change value of the pressure drop in the cited literature is very small). There is still no satisfactory explanation 10479



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Figure 22. Schematic of the gas flow in the irrigated bed.

transfer process. The flow pattern of the liquid is not a simple laminar flow. The turbulence flow occupies a certain proportion in the whole liquid film flow, which can not be neglected.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the fund projects of Fujian Province Education Office (serial number: JB08001 and JB06047)

Figure 23. Shape of the liquid trajactories at higher magnification.

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REFERENCES

(1) Ramshaw, C.; Mallinson, R. H. Mass transfer process. U.S. Patent 4,283,255, 1981. (2) Dipendu, S. Prediction of mass transfer coefficient in rotating bed contactor (HIGEE) using artificial neural network. Heat Mass Transfer 2009, 45, 451. (3) Ramshaw, C. HIGEE distillation: An example of process intensification. Chem. Eng. 1983, 2, 13. (4) Kelleher, T.; Fair, J. R. Distillation studies in a high-gravity contactor. Ind. Eng. Chem. Res. 1996, 35, 4646. (5) Lin, C. C.; Ho, T. J.; Liu, W. T. Distillation in a rotating packed bed. J. Chem. Eng. Jpn. 2002, 35, 1298. (6) Lin, C. C.; Liu, W. T.; Tan, C. S. Removal of carbon dioxide by absorption in a rotating packed bed. Ind. Eng. Chem. Res. 2003, 42, 2381. (7) Lin, C. C.; Liu, H. S. Adsorption in a centrifugal field: Basic dye adsorption by activated carbon. Ind. Eng. Chem. Res. 2000, 39, 161. (8) Liu, Y. Z.; Qi, G. S.; Yang, L. R. Study on the mass transfer characteristics in impinging stream rotating packed bed extractor. Chem. Ind. and Eng. Pro. 2003, 22, 1108. (9) Li, T. C.; Qi, G. S. Impinging stream rotating packed bed extractor. Petro-chem. Equip. 2004, 33, 26. (10) Munjal, S.; Dudukovic, M. P.; Ramachandran, P. Mass-transfer in rotating packed beds. I. Development of gas−liquid and liquid− solid mass-transfer correlations. Chem. Eng. Sci. 1989, 44, 2245.

Figure 24. Schematic of the liquid film flow.

4. CONCLUSION A novel experimental method is proposed to investigate the fluid flow in rotating packed beds. The experimental results prove that the novel method is simple and effective. The method can be used not only in the lab experimental equipment but also in the industrial grade equipment. Especially, the method can be used to check situations of the fluid flow in the industrial grade beds at will, without any highspeed video camera and equipment reform. By using the novel experimental method, the liquid trajectories are obtained easily. Through the research on the liquid trajectories, the peculiar laws of the liquid flow in RPB can be obtained: There are actual wetting and false wetting in the RPB, and only actual wetting contribute to gas−liquid 10480



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(11) Munjal, S.; Dudukovic, M. P.; Ramachandran, P. Mass-transfer in rotating packed beds. II. Experimental results and comparison with theory and gravity flow. Chem. Eng. Sci. 1989, 44, 2257. (12) Thomas, S.; Faghri, A.; Hankey, W. Experimental analysis and flow visualization of thin liquid film on a stationary and rotating disk. ASME J Fluids Eng. 1991, 113, 73. (13) Burns, J. R.; Ramshaw, C. Process intensification: Visual study of liquid maldistribution in rotating packed beds. Chem. Eng. Sci. 1996, 51, 1347. (14) Zhang. J. Experiment and modelling of liquid flow and mass transfer in rotating packed bed. Ph.D. Dissertation, Beijing University of Chemical Technology, 1996. (15) Guo, K. A study on liquid flowing inside the HIGEE rotor. Ph. D. Dissertation, Beijing University of Chemical Technology, 1996. (16) Liu, H. S.; Lin, C. C.; Wu, S. C.; Hsu, H. W. Characteristics of a rotating packed bed. Ind. Eng. Chem. Res. 1996, 35, 3590. (17) Lin, C. C.; Chen, B. C. Characteristics of cross-flow rotating packed beds. J. Ind. Eng. Chem. 2008, 14, 322. (18) Lin, C. C.; Jian, G. S. Characteristics of a rotating packed bed equipped with blade packings. Sep. Purif. Technol. 2007, 54, 51. (19) Chandra., A.; Goswami., P. S.; Rao, D. P. Characteristics of flow in a rotating packed bed (HIGEE) with split packing. Ind. Eng. Chem. Res. 2005, 44, 4051. (20) Keyvani, M.; Gardner, N. C. Operating characteristics of rotating beds. Chem. Eng. Prog. 1989, 9, 48. (21) Zheng, C.; Guo, K.; Feng, Y.; Yang, C.; Gardner, N. C. Pressure drop of centripetal gas flow through rotating beds. Ind. Eng. Chem. Res. 2000, 39, 829. (22) Guo, F.; Zheng, C.; Guo, K.; Feng, Y.; Gardner, N. C. Hydrodynamics and mass-transfer in cross flow rotating packed bed. Chem. Eng. Sci. 1997, 52, 3853.

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Hydrodynamics in a Rotating Packed Bed. I. A Novel Experimental

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