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Backmixing Reduction of a Bubble Column by Interruption of the Global Liquid Circulation Yang Wu, Zhen-Min Cheng,* and Zi-Bin Huang State Key Laboratory of Chemical Engineering, UNILAB Research Center of Chemical Reaction Engineering, East China UniVersity of Science and Technology, Shanghai 200237, P. R. China
The strong backmixing of the liquid phase which is prevalent in a bubble column should be ascribed to its global circulation which is upward in the central region while downward near the wall on the reactor scale. To suppress the backmixing behavior, interruption of the liquid circulation route should be effective. To verify this presumption, an investigation was conducted with a bubble column 550 cm in height and 50 cm in diameter, which was equipped with four channels of the same size installed at different heights so as to cut the downward motion of the liquid and therefore prevent the global circulation the liquid. It shows that although the radial velocity distribution of the liquid remains parabolic in the short column constituted by two neighbor channels, the velocity profile tends to be flat in the upward direction as the channel is approached. The residence time distribution analysis shows that the backmixing of liquid could be reduced by 60% when four channels 12 cm in diameter were applied. 1. Introduction Backmixing has been known to have a substantial influence on both the reaction rate and product selectivity to nonzeroorder reactions. For most gas-liquid reactions carried out in a bubble column, the influence of backmixing is not severe on reaction rate since the reaction orders for many liquid reactants are approaching zero. However, the influence on the product selectivity is usually remarkable since most of the reactions are consecutive, such as paraffin oxidation and Fischer-Tropsch synthesis. In a bubble column, the liquid flow tends to be upward in the central region while being downward near the wall, which forms a global circulation of the liquid through the column1-3 and thereby leads to serious liquid backmixing as large as that in a continuously stirred tank reactor (CSTR).4-6 It has been shown that the extent of liquid backmixing in a bubble column is proportional to the product of the time-averaged center-line liquid velocity and the column diameter for both the small and large columns.5 Moreover, for operations at high pressures, the liquid backmixing could also increase with an increase of gas density.7 In the past decades, the reduction of backmixing in a bubble column has been performed by installation of perforated partition plates to divide the whole column into multiple stages so that the column could be converted into a multistage CSTR, besides, the liquid velocity could become more uniform through the distribution of the perforated partition plates.8 The backmixing reduction is primarily dependent on a number of factors, i.e., the number and the position of the plates, the diameter and open area of the plates, and so on. Magnussen et al.9 and Alvare´ and Al-Dahhan10 have studied the effects of plate open area, the diameter of holes, and the gas and liquid superficial velocities in a countercurrent plated bubble column. Dreher and Krishna11 applied partition sieve plates with an open area of 18.6% and 30.7% in columns of different diameters. As a result, they concluded that the extent of liquid backmixing is strongly dependent on the open area of plates and independent of the column diameter. Alvare´ and Al-Dahhan10 obtained a 60% * To whom correspondence should be addressed. E-mail: zmcheng@ ecust.edu.cn. Telephone: +86-21-64253529. Fax: +86-21-64253528.
reduction in liquid backmixing with three kinds of plates of various open areas and hole diameters. However, the application of partition plates is not free from any drawbacks. In the case of high temperature and pressure and especially with solid catalysts, the plates are subjected to a series of mechanical or safety problems due to the large impacting forces by the gas and liquid phases at high superficial velocities. Therefore, further investigations should be taken in finding more feasible internal structures to satisfy the rigid industrial operating condition. The present work is aimed at the development of a suitable internal of better flexibility, which is not only effective enough in reducing the backmixing but also to be applied easily. Since the serious backmixing in a bubble column is originated from the liquid circulation due to the upward flow in the column central region and the downward flow in the wall region, therefore, the interruption of such a global liquid circulation should be the solution to the problem. In this work, an attempt will be made by elimination of the downward flow of the liquid rather than by dividing the column with partition plates to make the velocity uniform. Since only the upward flow is required and the upward flow generally prevails in the central area of the column, therefore, a reduction of the column diameter in a periodical way along the flowing direction would be a feasible solution. In the paragraphs that follow, detailed experimental work along with results and discussions will be presented to demonstrate the effect of this column configuration on the reduction of backmixing. 2. Experimental Section The experimental work was conducted with the air-water system in a cylindrical plexiglass column 50 cm in diameter and 550 cm in height. Both the gas and liquid phases are flowing upward through a gas distributor consisting of 85 stainless steel tubes 4 mm in outside diameter, and the flow rates of the two phases are controlled by flowmeters. To eliminate downward flow of the liquid phase, four channels 12 and 23 cm in diameter were respectively installed at different heights of 1.5, 2.5, 3.5 and 4.5 m measured from the bottom of the column, which is shown in Figure 1.
10.1021/ie900128j CCC: $40.75 2009 American Chemical Society Published on Web 06/17/2009
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two phases were varied from 0.15 to 0.76 cm · s for the liquid and from 1.5 to 7.6 cm · s-1 for the gas which covers the flow regime of uniform bubbling flow and churn turbulent flow. 3. Results and Discussion
Figure 1. Schematic diagram of the experimental setup: 1 N2 cylinder; 2 KCl tracer container; 3 air outlet; 4 channel; 5a, 5b, 5c, 5d differential pressure ports; 6 liquid inlet; 7 air inlet; 8 liquid outlet; 9 conductivity meter; 10 A/D converter; 11 data acquisition system; 12 Pavlov tube; 13 differential pressure transducer; 14 bubble column.
The liquid velocity was measured by a modified Pavlov tube based on the work of Hills.12 The two ends of the Pavlov tube are linked to a differential pressure transducer. Before experiments, the Pavlov tube was first calibrated in a turbulent water flow to determine the probe coefficient and was then corrected in a series of gas-liquid flow rates to ensure its reliability on the measurement of liquid velocity under different two-phase flow conditions. The differential pressure was sampled every 0.1s, and the radial velocity profile was obtained by moving the tube along the column diameter. In view of the high static water pressure at the bottom of the column, 2500 mL aqueous KCl tracer solution of 20 wt % in concentration was injected into the liquid feed stream as an impulse by high-pressure nitrogen of 7.0 MPa through the switch of an electromagnetic valve. The pressure of nitrogen is believed to be large enough to inject the KCl tracer into the liquid feed stream instantaneously. The conductivity probes were placed at the outlet or in the middle of the column for the residence time distribution (RTD) measurements. The pressure drop for the two phases flowing through the channel was measured with a differential pressure transducer at positions “5b” and “5c” shown in Figure 1, which are corresponding to the inlet and outlet of the channel. Within the capacity of the pump facility, the superficial velocities of the
3.1. Liquid Velocity Distribution. Influence of the channel size on the bubble column fluid dynamics was first examined via the cross-sectional liquid velocity distribution. In Figure 2, four profiles were obtained at positions 5a, 5b, 5c, and 5d, indicating the different positions with reference to the channels as shown in Figure 1. Profile 1 was obtained at position 5d, which was located on the top of the final stage channel by a distance of 0.2 m and was 1.0 m below the outlet of the column. It shows the velocity profile is parabolic with the highest upward velocity at the centerline and the highest downward velocity in the vicinity of the wall. Profile 2 was obtained at position 5a, which was located below the first stage channel by 0.3 m and was 0.9 m from the bottom of column. It shows a similar parabolic velocity profile was obtained. The similar velocity profiles at 5a and 5d imply that the radial distribution of the liquid axial velocity in the column that is away from the channel is not influenced. However, significantly different results were obtained at positions 5b and 5c which were close to the channel. Profile 3 was obtained at position 5b which was at the inlet of the first stage channel and was 1.2 m from the bottom of column, while profile 4 was obtained at position 5c which was at the outlet of the first stage channel and was 1.6 m from the bottom of column. It is obvious that profile 3 is nearly flat, with almost no velocity gradient over the cross-section of the column; therefore, the downward flow components near the wall has been eliminated and plug flow in the upward direction is generated. As a comparison, profile 4 shows a remarkably nonuniform liquid velocity distribution, which is extremely large in the central area of the column while rather small in the other area as a result of the formation of a jet flow when the fluids are passing through the channel. The effect of channel size could be observed by comparing profile 4 in Figure 2a and b. In Figure 2a with a channel diameter of 12 cm, the velocity of the fluid is upward, except for a very small region near the wall where the velocity is downward. On the contrary, in the case of Figure 2b with a channel diameter of 23 cm, although the velocity of the fluid in the central area is upward, there is a substantial downward flow near the wall starting from dimensionless diameter of 0.6-1.0. Therefore, a
Figure 2. Effect of channels on liquid velocity distribution. Operation condition: ul ) 0.76 cm · s-1, ug ) 6.02 cm · s-1. 1(0) Measured without channels at 5d. 2(O) Measured with four channels at 5a. 3(∆) Measured with four channels at 5b. 4(3) Measured with four channels at 5c.
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Figure 3. Radial distribution of pressure drop between points 5b and 5c as shown in Figure 1. 1(0) ug ) 3.02 cm · s-1. 2(O) ug ) 6.04 cm · s-1. 3(∆) ug ) 7.60 cm · s-1.
channel diameter of 23 cm is too large for interruption of the global liquid circulation and 12 cm is an appropriate size. 3.2. Pressure Drop. Despite the role of the channels in interruption of the global liquid circulation, the pressure drop should be concerned since the channel diameters are 12 or 23 cm which are much smaller compared with the column diameter of 50 cm. Figure 3 presents a pressure drop distribution in the radial direction, which indicates that the increase in pressure drop is negligible even in the case of a small channel diameter of 12 cm, since the largest increase in the pressure drop was only about 1 kPa at gas and liquid superficial velocities of 7.5 and 0.75 cm · s-1. It shows in Figure 3a and b that the pressure drop distribution in the radial direction is uniform without channels; however, a significant pressure drop distribution is observed in Figure 3c and d when the channels are installed. This could be explained from the mechanical conservation equation which gives the lowest pressure in the center of the column where the kinematic energy is the largest, and a radial gradient in pressure will be established from the wall to the center and leads to a transverse mixing of the fluids. 3.3. Tracer Response Experiments. To examine the effect of the channels on the backmixing behavior of liquid phase in the bubble column, the tracer response technique was used to give the residence time distribution profiles.13 In this work, the RTD measurements were taken at the outlet of the column and the exit of the first channel. As shown Figure 4 where three
RTD profiles were measured, profile 1 was measured at the outlet of column without the internal, profile 2 was at the outlet of column with four channels, and profile 3 was at the outlet of the first channel. This figure shows that profiles 1 and 3 are very similar and are close to the mixing in a CSTR. This is understandable since profile 3 is corresponding to the RTD of a short bubble column 150 cm in height. Differently, profile 2 is more close to the RTD profile of a plug flow reactor with a certain degree of axial dispersion, which indicates a significant reduction in the backmixing degree for the liquid phase was obtained through the installation of the four channels. Since the RTD profiles depicted in Figure 4 for the channel diameters of 12 and 23 cm are quite similar, a pertinent RTD analysis should be provided so as to provide a reliable comparison between them. For a flowing system like a bubble column which has a relatively large extent of backmixing, the tanks-in-series (TIS) model is superior to the axial-dispersion plug flow model. In this work, both the dimensionless variance σθ2 of the RTD and the tank number N were used in combination according to the following formula: σθ2 ) with
σt2 tm2
and N )
1 σθ2
(1)
Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009 ∞
∞
tm )
∑ ct 0 ∞
and σt2 )
∑c 0
∑ ct
diameter is 12 cm has been reduced by 60% from 0.7-0.8 in no. 1 to 0.3 in no. 2, and the stage number N has increased from 1.4 to 3.2. However, the improvement is only marginal for no. 4, which indicates that the backmixing is not effectively reduced when the channel diameter is 23 cm. Moreover, it is observed the backmixing in no. 5 is even worse than in no. 1, which means that the backmixing is more serious in a short column than in a longer one since the column height helps to reduce axial backmixing. An additional explanation for this result is the fact that the gas phase was forced into the center of the column by the channel, thus inducing stronger circulation within the stage between the channels and resulting in a stronger backmixing. 3.4. Backflow Model between Stages. Although the TIS model is one of best one for the cases with serious liquid backmixing, it should be noted that the stage number is usually not an integer and is not equal to the real stage number as is
2
0 ∞
∑c
- tm2
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(2)
0
where θ is the dimensionless residence time, c is the tracer concentration, σ is variance, N is the tank number, and tm is the average residence time. Table 1 summarizes five groups of RTD measurements in different cases. Case no. 1 was measured at the outlet of the column with the constant diameter, no. 2 was at the outlet of the column with four channels 12 cm in diameter, no. 3 was at the outlet of the first stage of the channels 12 cm in diameter, no. 4 was at the outlet of the column with four channels 23 cm in diameter, and no. 5 was at the outlet of the first stage channel 23 cm in diameter. Comparing the results of nos. 1 and 2, it shows that the variance for the liquid flow when the channel
Figure 4. Effect of channels on RTD curves. (profile 1) Measured at outlet of column without channels. (profile 2) Measured at outlet of column with four channels. (profile 3) Measured at outlet of the first stage in column with four channels. Table 1. Backmixing Characterization of the Liquid Flow in the Bubble Columnsa no. 1 -1
ul cm · s 0.30 0.30 0.45 0.45 0.60 0.60 0.60
-1
ug cm · s 3.02 4.53 3.02 4.53 3.02 4.53 6.04
2
no. 2 2
no. 3 2
no. 4 2
no. 5 2
σθ
N
σθ
N
σθ
N
σθ
N
σθ
N
0.662 0.754 0.733 0.812 0.647 0.671 0.695
1.511 1.327 1.365 1.232 1.546 1.49 1.438
0.313 0.349 0.324 0.313 0.307 0.296 0.317
3.193 2.865 3.086 3.199 3.257 3.384 3.159
0.466 0.604 0.522 0.475 0.539 0.773 0.524
2.146 1.655 1.916 2.105 1.857 1.294 1.907
0.566 0.538 0.536 0.496 0.452 0.386 0.556
1.768 1.858 1.866 2.017 2.211 2.593 1.768
0.785 0.859 0.815 0.920 0.736 0.817 0.997
1.274 1.164 1.228 1.087 1.358 1.224 1.274
a Positions of the measurements: no. 1 at the outlet of the bubble column with a constant diameter; no. 2 at the outlet of the bubble column with four channels 12 cm in diameter; no. 3 at the outlet of the first channel 12 cm in diameter; no. 4 at the outlet of the bubble column with four channels 23 cm in diameter; no. 5 at the outlet of the first channel 23 cm in diameter.
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shown in Table 1. Therefore, a real stage model with backflow (RSMB) was considered in this work.10,15 According to the definition of the RSMB model, the injected tracer is supposed to be homogenously mixed with the fluid at a flow rate of QL in the first stage of the series; meanwhile, a stream of liquid backflow is occurring at a flow rate of QB between two neighboring stages, as depicted in Figure 5. According to the transient tracer mass conservation equations for each of the perfectly mixed stages, the following equations are established: For the first stage, dc1 ) kc2 - (1 + k)c1 dt
(3)
for the intermediate stages, dci ) (1 + k)ci-1 - (1 + 2k)ci + kci+1 dt and for the last stage,
Figure 5. Illustration of the RSMB model.
Figure 6. RTD curve approximation with TIS and RSMB models.
(4)
dcN ) (1 + k)cN+1 - (1 + k)cN dt
(5)
The initial condition is given as follows: t ) 0, c1 ) c0, c2 ) c3 ) · · · cN ) 0
(6)
where ci is the tracer concentration in the ith stage. In this model, the first model parameter N is an integer from 2 to infinity. Another model parameter k is the back-flow coefficient defined as k ) QB/QL. In this work, N is selected to be a positive integer more than 1, and the other parameter to be determined is k. Through parameter estimation, the values of k under different gas and liquid flow rates and column channel diameters were obtained. It shows that under all conditions, the backflow model RSMB approximates the experimental RTD profiles better than the simple TIS model (Figure 6). For the bubble column with channel diameter of 23 cm, the value of k is 0.48, and it is 0.11 for channel diameter of 12 cm; therefore, the backflow of liquid is four times larger in the case of a channel diameter of 23 cm
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than that in the case of 12 cm, which is consistent with the velocity distribution presented in Figure 2. 4. Conclusions In reduction of liquid backmixing in the bubble columns, the present work proposed that the strong circulation of the liquid phase that is prevalent in such equipments should be interrupted. On the basis of the velocity distribution of the liquid over the column cross section, a column with periodic constrictions in diameter was developed to prevent the downward flow of the liquid near the wall and it was accomplished by the installation of channels along the column. Channels with two diameters of 12 and 23 cm were examined experimentally in terms of their effects on the velocity distribution and residence time distribution. This analysis shows that both of them have improved the fluid dynamics in the bubble column; however, the channel diameter of 12 cm is better than 23 cm. With four channels 12 cm in diameter installed at different heights along the column, the backmixing could be reduced by 60% in terms of the dimensionless variance of the residence time; meanwhile, the number of CSTR stages could increase from 1.4 to 3.2 according to the tanks-in-series model. A backflow model was further applied to identify the different behavior of the two channels, it shows that the backflow coefficient for the 23 cm channel diameter is 0.48 while it is 0.11 for the 12-cm one. It is concluded that a channel diameter of 23 cm is too large for the column 50 cm in diameter, since it leads to a substantial backflow of the liquid. Acknowledgment This work was carried out under support from the State Key Laboratory of Chemical Engineering in China via an open project (No. SKL-ChE-08C03). Fruitful discussions with Professor G. B. Marin and assistant Professor J. Thybaut from the Laboratory of Chemical Technology at Ghent University of Belgium are invaluable in completion of this work. Partial support from the Natural Science Foundation of China (No. 20876043) and the Shanghai Scientific and Technical Committee on Fundamental Research Project (No. 07DJ 14002) are also gratefully acknowledged. Nomenclature c ) tracer concentration (g · L-1) CSTR ) continuously stirred tank reactor d ) diameter of channels (m) D ) diameter of the column(m) k ) model parameter N ) number of tanks QL ) net liquid flow rate (cm3 · s-1) QB ) liquid backflow rate (cm3 · s-1) r ) radial coordinate (m) RSMB ) real stage model with backflow RTD ) residence time distribution t ) time (s)
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tm ) the average residence time (s) TIS ) tanks-in-series u ) linear velocity (cm · s-1) V ) volume of the reactor (m3) Greek Letters ∆P ) differential pressure (Pa) θ ) dimensionless time E(θ) ) dimensionless exit age distribution function σt2 ) variance σθ2 ) dimensionless variance Subscripts i ) data points l ) liquid phase g ) gas phase B ) liquid backflow L ) liquid flow
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ReceiVed for reView January 24, 2009 ReVised manuscript receiVed May 1, 2009 Accepted May 27, 2009 IE900128J
