Article pubs.acs.org/IECR
A New Crossflow Rotating Bed, Part 2: Structure Optimization Guang Q. Wang, Cheng F. Guo, Zhi C. Xu, Yun L. Yu, and Jian B. Ji* Zhejiang Province Key Laboratory of Biofuel, College of Chemical Engineering & Materials Science, Zhejiang University of Technology, Hangzhou, Zhejiang, 310014, People’s Republic of China
ABSTRACT: Based on the framework of the rotating zigzag bed (RZB), the crossflow concentric-baffle rotating bed (CRB) was proposed, with the rotor containing a set of perforated concentric baffles. In the first paper in this series, the distillation performance of the CRB was studied and compared with that of the RZB. It was found that the CRB showed the poorer masstransfer efficiency than the RZB, which led to an obvious loss in pressure drop and shaft power. Three possible main factors affecting the mass-transfer performance of the CRB (i.e., liquid distribution, seal, and flow pattern) were identified and experimentally investigated. The results revealed that the CRB had a good liquid self-distribution and could dispense with the liquid distributor. The elimination of the liquid distributor significantly enhanced the mass-transfer performance despite a moderately higher pressure drop. For the CRB, the gland seal was preferred due to its great advantage over labyrinth seal in mass transfer, although it caused an additional power consumption, which could be reduced by the selection of proper seal materials. The introduction of counter-current contact in the CRB rotor could not improve its mass-transfer performance but, instead, increased the pressure drop and shaft power required. The favorable gas−liquid contact pattern in the CRB rotor was crosscurrent. The optimized CRB was obtained based on the above results and compared with the RZB. The comparison demonstrated that, when the size of the rotor was same, the optimized CRB showed no poorer mass-transfer performance, much lower pressure drop, and shaft power than the RZB especially at the higher loads. Therefore, the optimized CRB could be potentially applied in gas−liquid contacting processes with a larger throughput.
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INTRODUCTION The crossflow concentric-baffle rotating bed (CRB) was proposed based on the framework of the RZBa combined rotating and stationary rotor. In Part 1 of this series,1 the distillation performance of the CRB was investigated and compared with that of the RZB. The comparison results showed that, although the CRB had obvious advantages in pressure drop and shaft power for one contacting stage, its mass-transfer performance was greatly inferior to that of the RZB. It was apparent to all that the mass-transfer performance is the most important parameter, which, to a great extent, determined the capital investment of mass-transfer apparatus. How to improve the mass transfer performance of the CRB was another topic that should be investigated. Usually, many factors can result in poor mass-transfer performance, but the three main factors can be identified and analyzed as follows. The most common reason for the poor mass-transfer performance of contactor was the initial liquid maldistribution. For the conventional contactors, tray or packed column, the exhaustive studies on liquid distribution have been published in the literature and various liquid (re)distributors were invented and, at present, are widely used.2 However, for high-gravity technology (HIGEE) devices, the liquid distribution was not studied sufficiently and the initial liquid distributor was proprietary and almost not disclosed to the public. The analysis showed3 that the common liquid distributors for a trayed or packed column cannot be used for a rotating bed. In addition, the rotating bed was more sensitive to the initial liquid distribution than the conventional column, because of the much poorer self-compensation effect and the more difficult vertical distribution on the cylindrical periphery. © 2014 American Chemical Society
For the rotating bed, the seal system was crucial to ensure that the fluid flowed along the preset path and contacted with each other in a required mode in the rotor. Generally, two types of seals were used in the rotating bed, to prevent leakage from the casing to the environment (shaft seal) and to prevent the (partial) gas from noncontacting with the liquid (inner seal). The shaft seal was common, because of its wide use in other rotating devices, such as pumps and compressors. The inner seal was necessary but was a trouble spot, because of contacting with fluid and invisibility. The poor sealing performance would lead to the bad contacting between two phases, which caused an adverse effect on mass-transfer performance. In addition, as stated in Part 1 of this series,1 the contacting pattern between the gas phase and the liquid phase in the CRB rotor was cross-current, which only made a minor contribution to the mass transfer in the RZB rotor. The better mass-transfer performance of the RZB rotor lay in gas−liquid counter-current contacting, which was eliminated in the CRB rotor. It seemed to be the most important reason for the poor mass-transfer performance of the CRB rotor, compared with the RZB rotor. The continuous and full counterflow contacting pattern in the CRB rotor was a feasible means to improve its mass-transfer performance. Obviously, the initial liquid distribution, the seal system as well as the fluid contacting manner was dependent on the internal structure of the CRB rotor. Therefore, it was necessary Received: Revised: Accepted: Published: 4038
October 22, 2013 February 10, 2014 February 18, 2014 February 18, 2014 dx.doi.org/10.1021/ie403571u | Ind. Eng. Chem. Res. 2014, 53, 4038−4045
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Figure 1. Schematic diagram of double annular tubes distributor. 1−liquid receiver; 2−connecting pipe; 3−liquid guide disc; 4−annular tubes; 5− holes.
Figure 2. Schematic diagram of seals: (a) labyrinth seal and (b) gland seal.
Distributor. For the CRB, the flow space of the liquid is the axially middle section of the rotor (corresponding to liquid holes on the baffles); therefore, an initial liquid distributor should be installed to ensure the liquid to be uniformly distributed in this space. In previous experiments, a selfdesigned double annular tubes distributor was employed and schematically shown in Figure 1. The distributor could rotate at the center of the CRB rotor. The liquid entered the cylindrical receiver and, because of rotation, was thrown into four short connecting pipes, which were connected with two annular tubes located at different vertical positions. Each of the two annular tubes was installed with 18 exit holes designed to be centered at an angle of 60° from horizontal. These holes were divided into two groups, each of which had 9 holes with a circumferentially uniform distribution. The movement direction of liquid droplets exiting from the holes was changed to horizontal by the guide disks fixed on the connecting pipes. To check the validity of this type of distributor, a CRB rotor without the distributor was also used for the purpose of
to make some structural modifications to enhance the masstransfer performance of the CRB. In this part, the performance of the CRB rotor with and without a distributor was experimentally studied and compared to justify the presence or absence of the distributor. The effects of different seals (i.e., labyrinth and gland seal) on the distillation performance of the CRB rotor were also investigated. The CRB with a crosscurrent plus counter-current gas−liquid contacting pattern was derived from the original CRB and experimentally studied. Based on these experimental results, an optimized structure of the CRB rotor was developed. The distillation performance of the optimized CRB rotor was also examined and compared with that of the RZB.
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EXPERIMENTAL WORK
In this study, the experimental setup system and operation procedures were the same as those described in Part 11 and are not duplicated here. If necessary please refer to ref 1. 4039
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Figure 3. Gas and liquid flows in different CRB rotors.
comparison. This was based on the consideration that the prototype of the CRB (i.e., RZB) can dispense with the liquid distributor because the perforated baffles can act as good liquid (re)distributors. It was worth noting that other types of liquid distributors for HIGEE device were almost kept secret and not known to the public. They were not considered here. The measurement of the distributor performance focused on its effects on the pressure drop, shaft power, and mass-transfer performance of the CRB rotor. Seal. In the CRB rotor, a dynamic seal must be used, because of the presence of both stationary and rotating parts. The stationary part of the CRB rotor was a stationary disk with a series of concentric grooves on its lower side. The rotational part was a rotating disk that had a series of concentric rotational baffle installed on the upper side. The upper parts of the rotational baffles were extended into the grooves, and a labyrinth seal was formed between them, as shown in Figure 2a. The labyrinth seal was nonwearing but easy to leak, because of the inadequate machining precision, improper assembly, and malfunction.4 To check the potential leakage and its influence, the gland seal was used for comparison. The gland seal was a static seal and shown in Figure 2b, where the stationary disk was fixed with the rotational disk by bolts and became rotating (i.e., the entire rotor was rotating). There was no clearance between the baffles and the grooves. It should be noted that the additional dynamic seal between the gas outlet and the casing was required to prevent the gas from bypassing the rotor. By the way, the rotor with the gland seal can be regarded as a configuration with the dynamic seal (labyrinth) shifted from the rotor to the gas outlet. Liquid Flow Pattern. Flow routes and contacting manner were decisive on the performance of the HIGEE devices. For the CRB, the liquid traveled straight in the radial direction and contacted with the gas in a cross-current pattern. Generally speaking, the cross-current flow regime was inferior to the counter-current flow, in that the latter can ensure maximal driving force in the case of equilibrium processes such as distillation and absorption. To introduce counter-current flow in the CRB rotor, three rotors with different baffles were selected in this study. The gas and liquid flow paths and contacting manner of the three rotors are shown in Figure 3.
To achieve the counter-current contacting between gas and liquid phase, some rows of the holes for draining liquid were blocked. In the original CRB rotor (rotor 1), there were 14 rows of holes for liquid on the middle parts of the baffles. For rotor 2, half of the holes were blocked and with regard to rotor 3, only three rows of holes were active. For three rotors, the gas flow path was similar but the liquid flow path was different. In rotors 2 and 3, the liquid droplets leaving a certain baffle would be collected by the next baffle and then they flowed downward along the inner cylindrical surface. In the course of liquid dropping, the counterflow between the gas and liquid was obtained. Once the dropping liquid reached the highest row of holes, it would be thrown off. The liquid from the next baffle would climb on the next but one baffle, counterflow contacting with the gas again. When the climbing liquid reached to the lowest row of holes it would be flung away again. Compared with rotor 2, the counter-current pattern accounted for a more share in the contacting process in rotor 3.
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RESULTS AND DISCUSSION Presence and Absence of Liquid Distributor. The distillation performance of the CRB rotor with and without initial liquid distributor was compared and shown in this section. Figure 4 showed the stage efficiency of the CRB rotor at 1000 and 1200 rpm, in the presence and absence of a liquid distributor. This figure revealed that the removal of the liquid distributor can bring an increase of stage efficiency. This stage efficiency increase could be up to 35% at higher F-factors. This result was unexpected, which can be interpreted by the liquid distribution profile. The liquid distribution profile in the absence of a distributor was different from that in the presence of a distributor, which is shown in Figure 5. When the distributor was installed, the liquid was dispersed into a limited number of liquid droplet lines by the holes on the distributor. Because the liquid distributor and the rotor were rotated synchronously, there were almost no liquid droplets in the regions between two adjacent lines. But when the liquid distributor was removed, the innermost baffle acted as an initial liquid distributor and dispersed the liquid into a very large number of liquid droplet lines. Therefore, compared with the 4040
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Figure 4. Stage efficiency of the CRB rotor at 1000 and 1200 rpm in the presence (and the absence) of a liquid distributor.
Figure 6. Pressure drop per theoretical stage of the CRB rotor at 1000 and 1200 rpm in the presence and absence of a liquid distributor.
contrary to intuition! On one hand, the liquid distributor located at the center of the rotor could hinder the vapor flow and its absence would slightly reduce the total pressure drop measured between the gas inlet and outlet. On the other hand, the liquid distribution profile in the absence of a distributor, shown in Figure 5b, would lead to more pressure loss due to the friction between the gas phase and the liquid phase. Figure 7 described the shaft power per theoretical stage of the CRB rotor at 800 and 1000 rpm in the presence and absence of a liquid distributor. As expected, the shaft power per theoretical stage in the absence of a liquid distributor was lower than that in the presence of a liquid distributor. This is because of the fact that, when the distributor was removed, the number of theoretical stages was larger but the shaft power remained
Figure 5. Top view of the liquid distribution profile (a) in the presence and (b) in the absence of a liquid distributor.
liquid distributor, the innermost baffle was a much better distributor, which could produce more uniformly distributed liquid droplets. This clearly showed that the initial liquid distribution was of great concern for HIGEE devices. Moreover, the larger increase of stage efficiency at higher Ffactors was perhaps due to the fact that a higher liquid flow rate could overload the distributor, because the number of exit holes for liquid on the distributor was less compared to that on the baffle. Figure 6 indicated the pressure drop per theoretical stage of the CRB rotor at 1000 and 1200 rpm in the presence and absence of a liquid distributor. It can be seen that the removal of liquid distributor had almost no effect on the pressure drop per theoretical stage. This implied that the elimination of liquid distributor would cause a pressure drop increase because it brought a better mass-transfer performance. This result was
Figure 7. Shaft power per theoretical stage of the CRB rotor at 800 and 1000 rpm in the presence and absence of a liquid distributor. 4041
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almost constant, if only the F-factor and the rotational speed remained the same. The results showed that the CRB rotor had a good selfdistribution and could dispense with the liquid distributor. The removal of the liquid distributor could not only significantly enhance the mass-transfer efficiency with a lower shaft power but also simplify the interior structures of the CRB. In view of this result, all the next runs of experiments were carried out without the liquid distributor. Labyrinth Seal and Gland Seal. The distillation performance of the CRB rotor with a labyrinth and gland seal was compared and given in this section. Figure 8 represents the stage efficiency of the CRB rotor with a labyrinth and gland seal at 1000 and 1200 rpm. It was shown
Figure 9. Pressure drop per theoretical stage of the CRB rotor with a labyrinth and gland seal at 1000 and 1200 rpm.
Figure 8. Stage efficiency of the CRB rotor with a labyrinth and gland seal at 1000 and 1200 rpm.
that the introduction of the gland seal enhanced the masstransfer performance by 30%−70%. The enhancement can be attributed to two factors. One was the elimination of potential gas leakage and the good contacting between the gas phase and the liquid phase. The other was that the gland seal prevented the possible backmixing in the labyrinth channel due to the liquid droplet entrainment by the gas. Figure 9 demonstrated the pressure drop per theoretical stage of the CRB rotor with a labyrinth and gland seal at 1000 and 1200 rpm. It indicated that the pressure drop of the CRB rotor with a gland seal was 10%− 20% lower than that with a labyrinth seal, especially at higher Ffactors. This result, together with the higher mass-transfer efficiency, suggested that the gland seal would cause a slight increase of pressure drop. The reason for this was a possible gas leakage in the labyrinth channel, because of the improper assembly and nonsmooth operation. The leakage would cause a small quantity of gas to bypass the liquid and slightly reduced the pressure drop. Figure 10 illustrated the shaft power per theoretical stage of the CRB rotor with a labyrinth and gland seal at 1000 and 1200 rpm. It was evident that the gland seal brought a slight decrease in shaft power per theoretical stage than the labyrinth seal, especially at higher F-factors. This was the result of higher stage efficiencies. It should be pointed out that the gland seal would lead to an additional shaft power,
Figure 10. Shaft power per theoretical stage of the CRB rotor with a labyrinth and gland seal at 1000 and 1200 rpm.
resulting from additional dynamic seal between the gas outlet and the casing (see Figure 2), which was included in the idle power and could not be reflected here. However, this additional power consumption could be reduced only if good material with lower friction factor was used to manufacture seals. The results showed that the gland seal was better than the labyrinth seal in mass transfer when they were used in the CRB rotor. Although the gland seal would cause the additional power consumption, this power can be reduced by the selection of proper seal materials. Therefore, the gland seal was adopted in the next runs of experiments. Fluid Flow Pattern. The distillation performance of three CRB rotors was compared and is given in this section. For rotor 1, all rows of holes for draining liquid were active. Moreover, for rotors 2 and 3, 7 and 11 rows were blocked, respectively. 4042
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Figure 11 depicts the stage efficiency of three CRB rotors at a speed of 1000 rpm. It was shown that the mass transfer
Figure 12. Pressure drop per theoretical stage of three CRB rotors at a speed of 1000 rpm. Figure 11. Stage efficiency of three CRB rotors at a speed of 1000 rpm.
deteriorated as some rows of holes for liquid became blocked. As was illustrated in Figure 3, blocking some rows of holes for liquid would weaken the cross-current contacting but introduce the counter-current contacting between the gas phase and the liquid phase. Although the counter-current pattern was superior to the cross-current pattern, with respect to driving force in gas−liquid equilibrium processes, the mass-transfer coefficient of counter-current contacting was small, because of the small relative velocity between the gas phase and the liquid phase in this study. Therefore, the introduction of counter-current contacting perhaps cannot offset the weakening of the crosscurrent contacting. Ultimately, the mass-transfer performance became poorer, compared with simplex cross-current contacting. Figure 12 displayed the pressure drop per theoretical stage of three CRB rotors at a speed of 1000 rpm. Although more liquid drainage holes were blocked, the pressure drop increased. In this study, the number of liquid drainage holes was dependent on the innermost baffle (minimum radii) at the minimum rotational speed. Therefore, the holes on the baffles for liquid would not be fully occupied with liquid under normal conditions. The additional holes for liquid provided the extra channels for the gas flow, and the actual gas flow area was larger than the nominal area. The actual gas flow decerased as more rows of holes for liquid became blocked and the pressure drop increased. Figure 13 showed the shaft power per theoretical stage of three CRB rotors at a speed of 1000 rpm. Generally, the descending order of the shaft power of three rotors was rotor 3 > rotor 2 > rotor 1. On one hand, the higher shaft power of rotors 2 and 3 was from the collisions between the liquid droplets and the blocked zone on the baffles. On the other hand, the liquid climbing higher on the baffle led to higher liquid holdup and higher shaft power before being dispersed by the holes. The results showed that the introduction of counter-current contacting in the CRB rotor could not improve its masstransfer performance but, increase the pressure drop and shaft
Figure 13. Shaft power per theoretical stage of three CRB rotors at a speed of 1000 rpm.
power required. The favorable gas−liquid contacting manner in the CRB rotor was considered to be cross-current. Optimized CRB and Its Performance. Based on the above results, it could be concluded that the optimized CRB was a totally rotating rotor without a liquid distributor, where the gas phase is cross-currently contacted with the liquid phase. Figures 14−16 present the distillation performance comparison between the optimized CRB and the RZB. It could be seen from Figure 14 that the stage efficiency of the optimized CRB was ∼60% of that of the RZB and twice as much as that of CRB with original structure. Figure 15 reveals that the pressure drop per theoretical stage of the optimized CRB is greater than that of the RZB at lower F-factors, but at higher F-factors, the former was lower than the latter. This suggested that the optimized CRB has the advantage over the RZB for the 4043
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Figure 16. Shaft power per theoretical stage comparison between the optimized CRB and the RZB.
Figure 14. Stage efficiency comparison between the optimized CRB and the RZB.
Figure 17. Schematic diagram of contacting stage in the CRB and the RZB.
stage would be reduced by 50% or more. With the consideration of this point, the overall performance of the optimized CRB was equivalent to or slightly better than that of the RZB with the same size rotor. Conclusively, the optimized CRB showed no poorer masstransfer performance but much lower pressure drop than the RZB, especially at higher F-factors if the rotor had the same radius. From the perspective of the principles and performance, the CRB can be capable of gas−liquid contacting processes, such as distillation, absorption, with a larger throughput. However, semi-industrial or industrial-scale field tests must be carried out to check its safety, reliability, and operability before its commercial application.
Figure 15. Pressure drop per theoretical stage comparison between the optimized CRB and the RZB.
applications with a large capacity. Figure 16 indicated that the shaft power per theoretical stage of the optimized CRB was 60% higher than that of the RZB. At first glance, it seemed that the optimized CRB was inferior to the RZB, because of the low stage efficiency and high shaft power per theoretical stage. However, the difference between the contacting stage of CRB and RZB should be noted, as shown in Figure 17. At the same F-factors based on the sectional area of the mass-transfer zone, the radial distance of one contacting stage in the optimized CRB was about the half of that in the RZB. Therefore, for the rotor with the same size, the contacting stages in the optimized CRB were twice as many as those in the RZB. That is to say, the number of theoretical stages of the optimized CRB was equal to or more than that of the RZB. The pressured drop and shaft power per theoretical
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CONCLUSIONS Compared with the RZB, the original CRB showed the poorer mass-transfer efficiency, which caused an obvious loss of advantages in pressure drop and shaft power. To solve this problem, three possible factors resulting in bad mass-transfer performance of the CRB were discussed and identified, i.e., liquid distribution, seal, and flow pattern,. The effects of three factors on the distillation performance of the CRB were 4044
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experimentally investigated. First, the CRB had a good liquid self-distribution and could dispense with the liquid distributor. Its mass-transfer performance even was significantly enhanced when the liquid distributor was absent, despite a moderately higher pressure drop. Second, the gland seal was preferred in the CRB rotor, because of its great advantage over the labyrinth seal in mass transfer, even though it caused additional power consumption, which, however, could be reduced by the selection of proper seal materials. Third, the introduction of counter-current contacting in the CRB rotor, unexpectedly, could not improve its mass-transfer performance but, instead, increased the pressure drop and shaft power required. The favorable gas−liquid contacting manner in the CRB rotor was cross-current. Lastly, the optimized CRB was a rotor with a gland seal and without a liquid distributor, where the gas phase cross-currently contacted with the liquid phase. In comparison with the same size RZB, the optimized CRB showed no poorer mass-transfer performance but the much lower pressure drop and shaft power, which was more remarkable at higher loads. The CRB was a promising HIGEE device applicable to gas− liquid contacting processes with a larger throughput.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 571 88320053. Fax: +86 571 88320053. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Hangzhou Ke-li Chemical Equipment Co., Ltd., for supplying a crossflow concentric-baffle rotating bed and supports in structural modifications.
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REFERENCES
(1) Wang,G. Q.; Guo,C. F.; Xu, Z. C.; Li, Y. M.; Ji, J. B. A New Crossflow Rotating Bed, Part 1: Distillation Performance. Ind. Eng. Chem. Res. 2014, 53, DOI: 10.1021/ie4032296. (2) Green, D. B.; Perry, R. H. Perry’s Chemical Engineers’ Handbook, 8th Edition; McGraw−Hill: New York, 2007. (3) Wang, G. Q.; Xu, Z. C.; Ji, J. B. Progress on Higee Distillation Introduction to a New Device and Its Industrial Applications. Chem. Eng. Res. Des. 2011, 89, 1434−1442. (4) Flitney, R. Seals and Sealing Handbook, 5th Edition; Elsevier: New York, 2007.
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