Ind. Eng. Chem. Res. 2009, 48, 10643–10649
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Studies on Performance of Crossflow Concentric-Ring Rotating Bed Guang Q. Wang, Yun Q. Jiao,† Zhi C. Xu, and Jian B. Ji* Institute of Chemical Engineering and Materials Science, Zhejiang UniVersity of Technology, Hangzhou, Zhejiang 310014, Peoples Republic of China
A new kind of high-gravity devicescrossflow concentric-ring rotating bed was developed, of which the rotor contains a series of perforated concentric rings. In operation, the gas could repeatedly crosscurrently make contact with the dispersed liquid flow between the concentric rings. The rotor structures and flow arrangements determine the device’s potential features, such as lower pressure drop and power consumption, which, together with the multiple-contact mechanism, are favorable to gas-liquid stripping processes. In a pilot crossflow concentric-ring rotating bed, the performance of the rotor was investigated under different operation conditions. The experimental results show that the pressure drop ∆p, the power consumption P, and the mass transfer coefficient kLa fall within the range of 0.03-1.02 kPa, 0.65-1.08 kW, and 0.027-0.071 s-1, respectively. Empirical correlations based upon the experimental data were proposed to describe the dependence of pressure drop and power consumption on operation parameters. Compared with packed columns and spray contactor, the crossflow concentric-ring rotating bed has great advantages in mass transfer, equipment size, and gas/ liquid loading despite the additional energy consumption. Moreover, the crossflow concentric-ring rotating bed shows lower pressure drop, power consumption, and mass transfer coefficient than that of counterflow or crossflow rotating packed beds. However, considering that stripping makes no high demand on mass transfer, crossflow concentric-ring rotating bed can be a promising option in performing stripping processes. Introduction As a promising alternative to enhancing heat and mass transfer processes, high gravity technology (abbreviated to HIGEE), which appeared along with the concept of process intensification in the 1970s, is used to replace a gravitational field with a high centrifugal field produced by rotating a doughnut-shaped rigid bed with an eye in its center.1 In this process, the liquid phase in a rotating bed is generally subjected to an adjustable acceleration of several hundred g’s and easily dispersed into fine droplets, which then interact closely with the gas stream. This intensifies the micromixing and interfacial mass transfer between the gas and the liquid by 1 or 2 orders of magnitude; thus, the equipment size and consequent capital costs can be greatly reduced compared with the process’s conventional counterpart. Originally, the flow arrangement of the gas and liquid phases in a rotating bed is “counterflow”;2,3 that is, liquid enters at the eye of the bed and flows radially outward due to centrifugal force while gas is introduced at the outer periphery of the rotor and forced to flow radially inward by pressure driving force. The counterflow mode results in two obvious disadvantages. One is larger pressure drop owing to centrifugal force and variable flow area of gas phase. The other is relative low capacity limited by the inner area of the bed. To overcome these disadvantages, the crossflow rotating bed was developed,4,5 the rotor of which typically is a packing bed. The crossflow differs from counterflow in that the gas flows axially through the bed perpendicularly to the liquid flow. This flow mode leads not only to higher gas velocity but also to a lower pressure drop. Considering that the crossflow rotating bed has the unique feature that there is no critical gas velocity limitation as flooding, it would be more useful in absorption or stripping with a higher flow rate than would a counterflow rotating bed. However, the * To whom correspondence should be addressed. Tel.: +86 571 88320053. Fax: +86 571 88320053. E-mail:
[email protected]. † Present address: Department of Control Science and Engineering, Zhejiang University, Hangzhou, PRC.
deficiency of a rotating packing bed is a radially nonuniform density of packing after a long-term operation due to poor dynamic balance, which has an obviously adverse effect on mass transfer performance. In view of these considerations, recently a new nonpacking rotating bed termed concentric-ring rotating bed was developed. In this study, the structures and features of the concentric-ring rotating bed were illustrated in detail, and its hydrodynamics and power consumption as well as mass transfer performance were also experimentally investigated. In addition, on the basis of the data in this work and in the literature, the overall performance of the crossflow concentric-ring rotating bed was evaluated by comparison with other contactors. The results of this study could provide further insight into the feasibility of its application in chemical process industries. Construction of the Rotating Bed The concentric-ring rotating bed is a novel kind of nonpacking HIGEE-based contactor and is characterized by a rotor combining a rotating disk with a series of concentric rings fixed on it as shown in Figure 1. The rotational disk perforated with large holes as gas channels was driven by a motor through a vertical shaft. The concentric rings, the upper parts of which were
Figure 1. Simplified sketch of crossflow concentric-ring rotating bed.
10.1021/ie900956r CCC: $40.75 2009 American Chemical Society Published on Web 10/07/2009
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Table 1. Comparison between Concentric-Ring Rotating Bed and Rotating Zigzag Bed in Characteristics, Performance, and Applications concentric-ring rotating bed rotor structure rotation dynamic seals in casing liquid distributor intermediate feed multirotor in one casing gas flow mode liquid flow mode
rotating zigzag bed
concentric rings wholly rotating one yes no yes and very difficult axially, linearly dispersed, radially
concentric rings partially rotational no no yes yes but very easy radially, zigzag dispersed-collected, radially gas velocity in rotor constant constant or varying liquid holdup smaller larger liquid residence time shorter, nonadjustable longer, adjustable pressure drop lower higher power consumption higher lower mass transfer capacity lower higher best application (one unit) absorption and stripping continuous distillation
perforated with small holes as liquid channels, were coaxially installed on the rotating disk with equal radial space. To prevent the gas from bypassing the rotational disk, a labyrinth seal between the rotational disk and the stationary casing is indispensable. A liquid distributor rotating with the shaft was installed at the center of the rotor to obtain a spatial uniform liquid distribution. The gas is introduced by the gas inlet into the casing and flows upward axially through the rotating disk driven by a pressure difference. The liquid is fed by the liquid inlet into distributor and travels radially outward through the small holes at the concentric rings in form of fine droplets due to centrifugal force. The liquid leaves from the rotor as a shower of droplets, which is collected on the casing wall and flows through the guide tubes into liquid tanks. In the rotor the continuous gas phase flows perpendicularly to the dispersed liquid phase, which makes the liquid be in repeated contact with the fresh gas in the space between the adjacent concentric rings. This multiplecontact mechanism is particularly advantageous to stripping processes, which involve removal of one or more volatile components from a liquid by contacting it with a gas stream. The concentric-ring rotating bed has some excellent features compared with other kinds of rotating beds owing to its unique structures and flow arrangements. One striking advantage is lower gas pressure drop. On one hand, the gas flow does not need to overcome the centrifugal force because the gas travels axially through the rotor. On the other hand, the pressure loss due to flow area changes in counterflow mode does not exist. Therefore the pressure drop of a crossflow concentric-ring rotating bed only results from form drag exerted by the rotational disk and the friction between the gas and liquid phases. The other advantage is lower power consumption because of lower liquid holdup in the rotor with a small weight. These advantages further make crossflow concentric-ring rotating bed applicable to stripping processes with a large gas flow rate. The same authors have developed another kind of rotating bed6,7 called “rotating zigzag bed” (RZB), in which the gas and liquid contacts in a complicated mode. Structurally a concentricring rotating bed can be considered as RZB from which the stationary parts were removed. This determines the similarities and differences between them. The characteristics, performance, and application of the concentric-ring rotating bed were compared with those of RZB in Table 1. It is can be seen that the concentric-ring rotating bed exhibits obvious advantages over the RZB in pressure drop and power consumption despite a little poorer mass transfer which, however, usually can meet the
Figure 2. Schematic diagram of experimental setup.
requirement of unidirectional mass transfer services, such as absorption and desorption. Experimental Setup and Procedures The experiments were carried out in a pilot crossflow concentric-ring rotating bed, and a simplified schematic diagram of the experimental system is shown in Figure 2. The rotor, installed in a casing with 0.4 m in diameter, has an inner and outer diameter, di and do, of 0.11 and 0.35 m, respectively. The axial height H of the concentric rings is 6.5 cm and the radial distances between the adjacent concentric rings are 2 cm. In this device the rotor was driven by a motor and can be operated from 800 to 1400 rpm tuned by a frequency modulator, which provides centrifugal force of 80 to 250g based on the arithmetic mean diameter. The hydrodynamics performance and power consumption were studied using an air-water system, while acetone-water was chosen in mass transfer measurements. In the experiments, the air at room temperature from a blower is introduced tangentially into the casing after measuring the flow rate by an orificemeter. The air then flows axially upward through the rotational disk and leaves the rotor by the gas outlet. The liquid at a temperature of 20 °C was pumped into the center of the rotor after the flow rate was measured by rotameter and then travels radially outward through the concentric rings. The discharged liquid enters into a storage tank and is fed to the pump again. The pressure drop across the rotating bed was measured using a U-tube manometer installed between the inlet and outlet pipe with both inner diameters of 21 cm. The pressure taps were vertically located around the pipe circumference. The power consumption of the motor was determined by a threephase watt-hour meter with the aid of a stopwatch. For evaluation of the mass transfer performance, the acetone-water samples were taken at the inlet and outlet liquid stream, and then their composition was measured by gas chromatography. On the basis of overall mass balance, the composition of the outlet gas can also be easily determined as presented later. To obtain the design equation for a crossflow concentricring rotating bed, assuming that the gas-side mass transfer resistance can be neglected for the process of stripping acetone from aqueous solution, the mass transfer performance in terms of liquid-phase volumetric mass transfer coefficient can be estimated from eq 1 derived by Kelleher et al.:8 kLa )
4LNL FLπ(do2 - di2)H
(1)
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where L and NL is mass flow rate and number of transfer units of liquid phase, respectively. The latter can be calculated by the logarithmic mean concentration difference (LMCD) method. NL ) (xi - xo)/∆xm
(2)
The term ∆xm is the logarithmic mean of concentration difference of liquid phase at the outlet, ∆xo, and that at the inlet, ∆xi, which was defined as ∆xm )
∆xo - ∆xi ln(∆xo /∆xi)
(3)
where ∆xo and ∆xi can be respectively expressed as ∆xo ) xo - x*o
(4)
∆xi ) xi - x*i
(5)
where x* is the fictitious liquid mass fraction that is in equilibrium with gas mass fraction y. The equilibrium line is given by a straight line for a highly dilute solution. y ) mx*
(6)
The mass fraction at the outlet gas stream yo can be obtained from overall mass balance equation. yo ) yi + (L/V)(xi - xo)
(7)
The experimental procedures are measuring the pressure drop, power consumption, and composition of liquid samples under different gas flow rates at constant liquid flow rate and rotational speed. Steady state was taken to occur when the composition of consecutive samples varied less than 1% and pressure drop remained constant. After a change in an operating condition, it takes approximately 10 min to attain steady state, and the same procedures were followed repeatedly in order to study the pressure drop, power consumption, and liquid-phase volumetric mass transfer coefficient dependence on gas and liquid flow rate as well as the rotational speed. Results and Discussion Pressure Drop. The dry pressure drop is measured in the absence of liquid flow and mainly results from the friction force and form drag when the gas travels across the rotating bed. Considering comparison between rotating bed and conventional column, the superficial gas velocity was defined as follows: ug )
4V FgπDi2
(8)
where V, Di denotes mass flow rate of gas and the inner diameter of the casing, respectively. The effects of the superficial gas velocity and the rotational speed on the dry pressure drop are shown in Figure 3. As indicated by Figure 3a that the dry pressure drop markedly increases with the increase of gas velocity. This is as expected considering that both friction and drag force increase with gas velocity increase. On the other hand, Figure 3b shows that the dry pressure drop almost keeps constant regardless of the rotational speed increase, which can be attributed to the fact that centrifugal force has almost no influence on gas flow and the friction between the gas and concentric rings because the gas flows perpendicularly to the centrifugal field.
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When the liquid flow was introduced in the rotating bed, the wet pressure drop can be measured. By analogy with trayed or packed column, the specific liquid rate was employed and expressed by uL )
3600L πdiFLH
(9)
The dependence of the wet pressure drop on the superficial gas velocity and the specific liquid rate as well as the rotational speed was given in Figure 4. The variation of wet pressure drop with superficial gas velocity at different rotational speeds and a specific liquid rate of 44.5 m/h was shown in Figure 4a, from which it can be seen that with a superficial gas velocity increase the wet pressure drop obviously increases. This variation is similar to that of the dry pressure drop and can be interpreted likewise. Figure 4b depicts the dependence of wet pressure drop on the rotational speed at different specific liquid rate and a superficial gas velocity of 3.54 m/s. The rotational speed has a moderate effect on the wet pressure drop; that is, an increase in the rotational speed yields an increase in the wet pressure drop. Furthermore, by comparing Figure 3b with 4b, it can be observed that the increase of wet pressure drop with the rotational speed increase is larger than that of dry pressure drop. A possible explanation for this phenomenon is that the interaction between gas and liquid would be enhanced by increasing rotational speed. Figure 4c describes the wet pressure drop as a function of specific liquid rate at different superficial gas velocities and a rotational speed of 973 rpm As shown in Figure 4c, the wet pressure drop is nearly independent of the specific liquid rate. This can be possibly interpreted that the liquid holdup in rotor is small and almost unchanged in spite of liquid flow rate increase. Moreover, assuming that the wet pressure drop is dependent on the three operation parameters mentioned above, the following correlation with power function form is obtained from the nonlinear regression of experimental data: ∆p ) 0.019n0.35uL0.018ug0.95
(10)
Analysis shows that most experimental data lie within (20% of the values estimated by eq 10. It is found that the wet pressure drop varies with the gas velocity, the rotational speed, and liquid flow rate to the 0.95, 0.35, and 0.018 power. The gas has the most significant influence on the wet pressure drop and the liquid has the least. Power Consumption. For a rotating bed, power consumption is an important measure of energy expenditure. Usually power is used to accelerate the liquid and to overcome bearing friction and frictional losses as the liquid passes through the rotor. Moreover, whether a rotating bed is energy-saving depends on its power consumption besides pressure drop. In addition, the power consumption is a dominant factor in the choice of motor. The power consumption of rotating bed depends on gas, liquid flow rate, rotational speed, and structures of rotor. In this study, the effects of gas and liquid flow rate as well as rotational speed on power consumption were experimentally investigated and presented below. The effects of the gas and liquid flow rate as well as the rotational speed on the power consumption were presented in Figure 5. The dependence of the power consumption on the superficial gas velocity at different rotational speeds and a specific liquid rate of 44.5 m/h was given in Figure 5a, which shows that increasing the gas velocity only brings a slight increase of the power consumption. A gas flow rate increase usually leads to more liquid holding in the rotor and more power
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from the motor to accelerate it. At lower rotational speed, gas flow rate has no effect on power consumption, whereas this effect becomes a little pronounced at a higher rotational speed. Figure 5b describes the variation of the power consumption with the rotational speed at different specific liquid rates and a superficial gas velocity of 3.54 m/s. It reveals that the higher rotational speed can yield a noticeable increase of the power consumption. This can be attributed to that a higher rotational speed gives a larger energy loss that arises from bearing friction and liquid impingement on concentric rings. Figure 5c illustrates the variation of the power consumption with a specific liquid rate at different superficial gas velocities and a rotational speed of 973 rpm. It suggests that increasing liquid flow rate can provide an appreciable increase of the power consumption. The increase of liquid flow rate can increase the energy losses due to impingement of liquid droplets on concentric rings and acceleration of these droplets again. In the load/total power consumption of a rotating bed, a portion is idle power consumption, which is used to overcome various friction losses in the absence of gas and liquid flow. The idle power consumption mainly depends on the type of bearing and shaft seal, the diameter and the rotational speed of rotor, and the frictional coefficient between air and rotor. Another portion is effective power consumption used to accelerate the liquid. The effective power consumption was largely influenced by flow rates of liquid and gas and rotational speed of the rotor. Using the experimental results and regression analysis, we can obtain the correlation of effective and total power consumptions as follows: Pe ) 3.38 × 10-4nuL0.16ug0.33
(11)
Pt ) 5.5 × 10-3n0.67 + 3.38 × 10-4nuL0.16ug0.33
(12)
Although the (load) power consumption is relatively small (ca. 1.0 kW) based on the experimental results, it still seems that compared with conventional columns, a rotating bed is at a disadvantage due to this power consumption. But the reduction in size and consequent lower capital costs might be sufficient to offset the additional costs. However, this issue needs an overall evaluation. Mass Transfer Coefficient. The crossflow concentric-ring rotating bed is inherently applicable to stripping processes because of its structural arrangements that enable liquid to repeatedly contact with fresh gas; thus, a larger concentration driving force can be achieved. Therefore, in this study the mass transfer performance of the crossflow concentric-ring rotating bed was investigated in a liquid-phase controlled process-
stripping of acetone from water by air. And its liquid-phase volumetric mass transfer coefficients were experimentally measured and assessed in this section. Figure 6 depicts the effects of the gas and liquid flow rate as well as the rotational speed on the liquid-phase volumetric mass transfer coefficients kLa. The variation of kLa with superficial gas velocity at different rotational speeds and a specific liquid rate of 44.5 m/h was given in Figure 6a, which shows that kLa decreases slightly and then increases again with superficial gas velocity increase, which is more obvious at a rotational speed above 1000 rpm. This result can be interpreted by the dual effect of gas velocity on the mass transfer process. The increase of gas velocity puts the dispersed liquid in a danger to be entrained at a lower rotational speed. However at a higher rotational speed, the liquid can resist the entrainment and the beneficial effect of gas velocity increase is dominant. Figure 6b describes the dependence of kLa on the rotational speed at different gas velocities and a specific liquid rate of 44.5 m/h. It can be seen from Figure 6b that the increase of rotational speed decreases kLa at a lower rotational speed and increases kLa at a higher rotational speed. Perhaps this is owing to the fact that on the one hand, higher rotor speed results in shorter gas-liquid contact time that is disadvantageous to mass transfer; on the other hand, rotational speed increase can lead to finer liquid droplets and better dispersion of liquid and improves mass transfer. Figure 6c illustrates the variation of kLa with specific liquid rate at different superficial gas velocities and a rotational speed of 1082 rpm. Figure 6c suggests that a higher liquid flow rate yields a decrease of kLa. This is because a higher liquid flow rate brings an increase of xo, and thus kLa decreases as indicated by eq 1 and eq 2. Comparison with Other Contactors. As is well-known, there exist several alternatives available for the stripping processes. Conventional contactors, such as trayed and packed column, are well recognized now and preferred when stripping is considered. Compared with trayed column, packed column is preferable owing to lower pressure drop and higher efficiency. Another kind of contactor, spray column, is an open vessel where spray is injected down into upflowing gas to form a rain of liquid. The advantages of spray contactor are simple: low gas pressure drop and tendency to be nonfouling. Recently, a new kind of contactor-rotating packed bed (RPB), which makes use of centrifugal field to intensify transport processes and micromixing, was developed and applied in some unit operations such as absorption, stripping, distillation, and reactive precipitation. According to the gas and liquid flow mode, RPB can be
Figure 3. Variation of the dry pressure drop (a) with superficial gas velocity (b) with rotational speed.
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Figure 4. Variation of the wet pressure drop (a) with superficial gas velocity (b) with rotational speed (c) with specific liquid rate.
Figure 5. Variation of the power consumption (a) with superficial gas velocity, (b) with rotational speed, and (c) with specific liquid rate.
classified as counterflow and crossflow, and the latter is superior to the former in that crossflow RPB features uniform gas velocity and no flooding. The concentric-ring rotating bed, as a nonpacking rotating bed, is proposed with a view to intensifying the stripping process. Table 2 summarizes the performance data of the concentric-ring rotating bed applied in stripping processes and compares them with those of packed column (random and structured), spray contactor, and rotating packed bed (counterflow and crossflow) reported in the literature. For the stripping processes, the kLa of concentric-ring rotating bed is an order of magnitude higher than that of packed columns with either random or structured packing. The radial width of the concentric-ring rotating bed and the height of the packed column are inversely proportional to kLa. Hence, it can be inferred that the former would be an order of magnitude smaller
than the latter, just as indicated in the table. On the other hand, the flow area at the inner periphery of the concentric-ring rotating bed and the cross-sectional area of the packed column are inversely proportional to permissible liquid loadings. But the liquid loadings for the concentric-ring rotating bed are much higher than those of packed column despite that the inside area of the rotor is as much as the cross-sectional area of the packed column. This difference of liquid loading is attributed to the action of centrifugal force. For gas loadings and pressure drop, their values of concentric-ring rotating bed are in the same range as those of packed column. Overall, the concentric-ring rotating bed is superior to packed column despite the additional power consumption. In terms of hydrodynamic regime, the spray contactor is similar to the concentric-ring rotating bed. From this point of
Figure 6. Variation of the volumetric mass transfer coefficient (a) with superficial gas velocity, (b) with rotational speed, and (c) with specific liquid rate.
view, the concentric-ring rotating bed can be considered as a spray column in a centrifugal field. The capacity of a spray contactor is usually much lower than that of a concentric-ring rotating bed because it is operated well below flooding velocity to avoid entrainment of spraying droplets. The kLa of a spray contactor is the same order of magnitude as that of packed columns, but is an order of magnitude lower than that of the concentric-ring rotating bed. This difference can be attributed to larger droplets in a spray contactor and lower relative velocity between the gas and liquid phases. In addition, spray contactors have a lower pressure drop than concentric-ring rotating beds. Despite this, a spray contactor is only effective in limited services such as a liquid-controlled system with low concentration, owing to lower capacity and mass transfer rate. Hi-flow ring 0.457 2.90
1.23-3.39 × 10-3 0.01-0.18 10
1.72-2.84 × 10-3 0.7-3.0
9
stripping acetone-H2O counterflow
0.68-3.32 9.7-22.5
Raschig ring 0.14/0.29/0.44 2.07
0.40-0.80 1.8-3.1
2.05-6.60 × 10-3 0.02-0.8dd
Mellapak 0.245 2.80 250 0.96
11
0.55-1.04 3.6-16.2
11
1.15-7.62 × 10-3 0.018-0.75d
Sulzer BX 0.245 2.80 492 0.90
stripping C7H8-H2O/CHCl3-H2O counterflow
0.55-1.04 5.4-14.4
packed column (structured)
12
0.50-9.2 × 10-3 0.04-0.14e
chamber 0.91 × 1.2 × 0.61
stripping CO2-H2O counterflow
n/a 0.06-0.85a
spray contactor
1.0-2.5 2.0-12f 0.3-1.6 14
foam metal 0.457/0.254 0.044 656/1476/2952 0.92
stripping CO2-H2O counterflow
0.55-2.08b 32.4-130 400-1300
0.18-0.90 3.2-15g n/a 15
wire gauze 0.164/0.064 0.20 457
stripping O2-H2O crossflow
5.0-15.0c 12.4-62.2 300-1500
rotating packed bed (RPB)
2.7-7.1 × 10-2 0.03-1.02h 0.65-1.08 this study
0.35/0.11 0.065
stripping Acetone-H2O crossflow
0.44-3.54 17.8-71.3 800-1400
concentric-ring rotating bed
a
Based on cross section 0.91 m × 0.61 m. b Based on the inner periphery area of the rotor. c Based on the cross-sectional area of the rotor. d Estimated from the graphs released by packing vendors. e Estimated according to reference 13. f Based on radial width of the packing rotor. g Based on the axial depth of the packing rotor. h Total pressure drop.
operation conditions gas loading ug (m/s) liquid loading uL (m/h) rotating speed n (rpm) testing processes process type system flow type equipment packing type diameter d (m) axial height H (m) surface area ap (m-1) void fraction ε performance mass transfer coefficient kLa (s-1) pressure drop ∆p (kPa/m) power P (kW) reference
packed column (random)
Table 2. Performance Comparison between Various Contactors for Stripping Processes
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Before the comparison between the concentric-ring rotating bed and the RPB was made, it should be noted that the kLa of counterflow RPB appeared be an order of magnitude higher than that of crossflow RPB. This discrepancy can easily be understood by taking the specific surface area of the packing into account. If the rule of thumb that the kLa is proportional to the specific surface area of packing16 still holds good at high acceleration, the kLa of a counterflow RPB with a specific surface area of 500 m-1, which is commonly used in industrial service, is on the order of 10-2. Therefore, compared with counterflow or crossflow RPB, a concentric-ring rotating bed shows about an order of magnitude lower kLa, and yet its pressure drop is reduced by up to 100-fold. The concentric-ring rotating bed permits higher gas loading than counterflow RPB due to no limit of flooding. Moreover, the power consumption driving the concentric-ring rotating bed is lower than that of counterflow and crossflow RPB because the concentricring rotating bed is nonpacking and has very low liquid holdup. Conclusively, a concentric-ring rotating bed has the virtue of lower pressure drop and power consumption at the cost of a certain loss in mass transfer in comparison with counterflow or crossflow RPB. Considering that stripping usually makes low demands on mass transfer, the concentric-ring rotating bed is nonetheless an attractive choice in stripping processes.
p ) pressure, kPa ug ) superficial gas velocity, m/s uL ) specific liquid rate, m/h V ) gas mass flow rate, kg/s x ) liquid-phase mass fraction, y ) gas-phase mass fraction, -
Concluding Remarks
Literature Cited
A crossflow concentric-ring rotating bed with a rotor containing a series of concentric rings and based on high-gravity technology was developed. In the rotor, the liquid phase is dispersed, stage-by-stage traveling through the concentric rings and the gas flows axially; thus the liquid crosscurrently and repeatedly contacts with the fresh gas. This multiple-contact mechanism, together with the possible advantages of low pressure drop and power consumption resulting from its structures and flow arrangements, is especially favorable to stripping operations. In a pilot experimental system, the pressure drop, power consumption, and mass transfer coefficient were measured and evaluated under different operation conditions. The experimental results show that crossflow concentric-ring rotating bed has as low a pressure drop as 1 kPa and power consumption not higher than 1 kW. In addition, its kLa values are on the order of 10-2, which is an order of magnitude lower than those of counterflow and crossflow RPB. However, the striking advantage of the lower energy consumption arising from low pressure drop and power makes it a promising alternative in chemical process industries.
(1) Ramshaw, C. HIGEE distillationsAn example of process intensification. Chem. Eng. 1983, 389, 13–14. (2) Kumar, M. P.; Rao, D. P. Studies on a high-gravity gas-liquid contactor. Ind. Eng. Chem. Res. 1990, 29 (5), 917–920. (3) Liu, H. S.; Lin, C. C.; Wu, S. C.; Hsu, H. W. Characteristics of a rotating packed bed. Ind. Eng. Chem. Res. 1996, 35 (10), 3590–3596. (4) Guo, F.; Zheng, C.; Guo, K.; Feng, Y. D.; Gardner, N. C. Hydrodynamics and mass transfer in crossflow rotating packed bed. Chem. Eng. Sci. 1997, 52 (21/22), 3853–3859. (5) Lin, C. C.; Wei, T. Y.; Hsu, S. K.; Liu, W. T. Performance of a pilot-scale crossflow rotating packed bed in removing VOCs from waste gas streams. Sep. Purif. Technol. 2006, 52 (2), 274–279. (6) Wang, G. Q.; Xu, Z. C.; Yu, Y. L.; Ji, J. B. Performance of a rotating zigzag bedsA new HIGEE. Chem. Eng. Process. 2008, 47 (12), 2131–2139. (7) Wang, G. Q.; Xu, O. G.; Xu, Z. C.; Ji., J. B. New HIGEE-rotating zigzag bed and its mass transfer performance. Ind. Eng. Chem. Res. 2008, 47 (22), 8840–8846. (8) Kelleher, T.; Fair, J. R. Distillation studies in a high-gravity contactor. Ind. Eng. Chem. Res. 1996, 35 (12), 4646–4655. (9) Groenhof, H. C. Scaling-up of packed columns. Part I. Chem. Eng. J. 1977, 14 (3), 181–191. (10) Anvaripour, B.; Yoswathana, N.; Ashton, N.; Arrowsmith, A. Stripping Ethanol and Acetone from Water with Modern Packings; WIT Transactions on Ecology and the Environment Vol. 13, Water Pollution III; Computational Mechanics Publications: Boston, MA, 1995; pp 493-500. (11) Ortiz-Del Castillo, J. R.; Guerrero-Medina, G.; Lopez-Toledo, J.; Rocha, J. A. Design of steam-stripping columns for removal of volatile organic compounds from water using random and structured packings. Ind. Eng. Chem. Res. 2000, 39 (3), 731–739. (12) Yeh, N. K.; Rochelle, G. T. Liquid-phase mass transfer in spray contactors. AIChE J. 2003, 49 (9), 2363–2373. (13) Biswas, S.; Rajmohan, B.; Meikap, B. C. Hydrodynamics characterization of a countercurrent spray column for particulate scrubbing from flue gases. Asia-Pac. J. Chem. Eng. 2008, 3, 544–549. (14) Keyvani, M.; Gardner, N. C. Operating characteristics of rotating beds. Chem. Eng. Prog. 1989, 85 (9), 48–52. (15) Guo, K. Characteristics of Hydrodynamics and Mass Transfer in Crossflow Rotating Packed Bed. Ph.D. Dissertation. Beijing University of Chemical Technology, Beijing, 1996. (16) Harrison, M. E.; France, J. J. Distillation column troubleshooting. Part 2. Packed column. Chem. Eng. 1989, 96, 121.
Acknowledgment The authors thank Hangzhou Ke-li Chemical Equipment Co. Ltd for supplying a crossflow concentric-ring rotating bed and supports in the experiments. Nomenclature a ) gas-liquid interfacial area, m-1 ap ) specific surface area of packing, m-1 d ) diameter of rotor, or packed column, m D ) diameter of the casing of rotating bed, m H ) the height of the concentric-rings, rotor and packing bed, m kL ) liquid-side mass transfer coefficient, m/s L ) liquid mass flow rate, kg/s m ) slope of the equilibrium line, NL ) liquid-phase number of mass transfer units, n ) rotational speed, rpm P ) power consumption, kW
Greek ∆ ) change or drop ε ) void fraction, π ) circular constant F ) density, kg/m3 Subscript e ) effective g ) gas phase i ) inner L ) liquid phase m ) logarithmic mean o ) outer t ) total Superscript * ) equilibrium value
ReceiVed for reView June 12, 2009 ReVised manuscript receiVed September 14, 2009 Accepted September 25, 2009 IE900956R
