Backmixing Characterization of a Bubble Column with Short Venturi

Article pubs.acs.org/IECR

Backmixing Characterization of a Bubble Column with Short Venturi Throats by Multipoint Internal Tracer Injections Zi-Bin Huang,† Zhen-Min Cheng,*,† Jian-Ding Chen,‡ Xiang-Chen Fang,§ and Tao Yang§ †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China § Fushun Research Institute of Petroleum and Petrochemicals, Fushun, Liaoning Province, 113001, China ‡

ABSTRACT: In reduction of liquid backmixing in bubble columns, Wu et al. [Wu, Y; Cheng, Z. M.; Huang, Z. B. Backmixing reduction of a bubble column by interruption of the global liquid circulation. Ind. Eng. Chem. Res. 2009, 48, 6558− 6563] have shown that the backmixing could be substantially reduced by interrupting the global liquid circulation using four identical short Venturi throats (SVTs). Considering the advantages of such structures, it is necessary to further investigate the backmixing characteristics of a bubble column with SVTs. In the present work, a multipoint tracer injection and detection device was employed to enhance the insight into where and how the liquid tracer was backmixed in different stages of a 50-cm diameter column. Tracer response profiles were measured to track the liquid backmixing trajectory in different stages at various operating conditions. The results showed that the backmixing of the column was sensitive to the throat diameter of SVTs. Based on the observed physical phenomena, a tanks-in-series with backflow (TISB) model was adopted to interpret the tracer experiment data, which shows an excellent agreement between the prediction and experimental measurements.

1. INTRODUCTION The staging of conventional bubble columns by reduction of the column diameter in a periodical way along the flowing direction has been demonstrated in our previous work to be an efficient way to interrupt the global liquid circulation by elimination of the downward flow of the liquid.1 The results showed that the liquid backmixing could be reduced by 60% and the number of CSTR stages could increase from 1.4 to 3.2 according to the tanks-inseries model. To reduce liquid serious backmixing of a bubble column, partition sieve plates or trays are reported to be widely used for staging a bubble column.2−4 Joshi and Sharma,5 and Urseanu et al.6 have shown that the liquid phase backmixing can also be substantially reduced by providing radial baffles or packed elements. However, a short Venturi throat (SVT), because of its smooth contracting and expanding sections, has an inherent advantage over the sieve plates or trays, in that it can generate a higher velocity and prohibit the downward liquid flow at the throat section for a given pressure drop across it. In spite of our previous efforts on the reduction of liquid backmixing by employing four identical SVTs, it is clear that a more exhaustive investigation of the liquid mixing hydrodynamics in the column is necessary to facilitate the design and scale up of bubble columns with such structures. Published experimental studies on backmixing reduction of liquid phase in bubble columns have shown that a singlestage bubble column behaves like a continuous stirred tank reactor. Although the axial dispersion model lacks a proper physical basis for bubble columns, the popularity of the model is due to the fact that it contains only a single parameter.7−11 Here we represent the multistage bubble © 2012 American Chemical Society

Figure 1. Schematic diagram of the bubble column with SVTs (including details of SVTs). (1) N2 cylinder, (2) KCl tracer container, (3) electromagnetic valve, (4) liquid inlet, (5a−5f) tracer injection positions, (6A−6D) tracer detection positions, (7) gas inlet, (8) SVT, (9) liquid outlet, (10) conductivity meter, (11) data acquisition system, (12) computer.

column by tanks-in-series with backflow (TISB) model. In this model, the nonideality of the liquid flow is described by Received: Revised: Accepted: Published: 9733

January 12, 2012 June 8, 2012 July 3, 2012 July 3, 2012 dx.doi.org/10.1021/ie3001138 | Ind. Eng. Chem. Res. 2012, 51, 9733−9741

Industrial & Engineering Chemistry Research

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the number of mixed stages and the backflow rates of the streams between them. Although several conductivity probes were placed downstream to detect the response of a pulse injection in our earlier work,1 the tracer was only injected from the entrance of the column rather than within a certain stage; therefore, the whole column was only treated like a black box and the information of liquid backmixing characteristics between and within stages was still not available. In the present work, tracer studies were conducted in a pilot plant scale unit by using a multipoint tracer injection and detection device to further understand where and how the liquid tracer was backmixed in different stages of the column. Furthermore, the TISB model has been used to interpret the results of the experiments.

Table 1. Operating Conditions of Tracer Experiments for SVT #2 uL uG tracer injection tracer detection experiment (cm s−1) (cm s−1) position (see Figure 1) position (see Figure 1) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.45 0.75 1.06 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

7.5 7.5 7.5 0 1.5 4.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5 7.5

5d 5d 5d 5d 5d 5d 5d 5a 5a 5a 5a 5b 5c 5d 5f 5f 5f 5f

6D 6D 6D 6D 6D 6D 6D 6A 6B 6C 6D 6A 6B 6C 6A 6B 6C 6D

2. EXPERIMENTAL SECTION Figure 1 shows the bubble column of 50 cm diameter and 600 cm height, which is divided into five stages by the use of four identical SVTs. The SVTs, which essentially consist of a contracting section, a cylindrical throat, and an expanding section, are positioned at various heights of 1.5, 2.5, 3.5, and 4.5 m measured from the bottom of the column. Two

Figure 2. Tracer response curves for injections at the top stage of the column (uL = 0.75, uG = 7.5 cm·s−1). 9734

dx.doi.org/10.1021/ie3001138 | Ind. Eng. Chem. Res. 2012, 51, 9733−9741


Article

Table 2. Characteristics of Response Curves for Injections at the Top Stage of the Column (uL = 0.75, uG = 7.5 cm·s−1) dimensionless variance (σ2)

mean residence time (tm/s) probe

A

B

C

D

A

B

C

D

BC SVT #1 SVT #2

578 412

539 393

520 279 210

548 210 104

0.732 0.242

0.782 0.408

0.819 0.831 0.372

0.964 0.989 0.972

Figure 3. Tracer response curves at different gas velocity (experiments 4−7).

To systematically characterize the degree of liquid backmixing reduction of the bubble column with SVTs and liquid backmixing behavior between and within stages, we will employ the multiple-point tracer injection and detection device developed in our earlier work,12 which could inject the liquid tracer into any stage and track the tracer trajectories on a certain stage. As depicted in Figure 1, six pulse injections and four detections for the tracer were provided, among which injection point 5b and detection point 6A were positioned at the first stage, 5c and 6B at the second stage, 5d and 6C at the third stage, and 5f and 6D at the topmost stage. In addition, the injection point 5a was located at the bottom of the column, which is slightly below the gas

different types of SVTs are studied; their design and geometric dimensions are also shown in Figure 1. For convenience, the empty bubble column without any SVTs is denoted as “BC”, the bubble column with four identical SVTs of throat diameter 23 cm is denoted as “SVT #1”, and the bubble column with four identical SVTs of throat diameter 12 cm is denoted as “SVT #2”. Air and tap water were used as the gas and liquid phase, respectively. The gas was pumped into the reactor from the bottom through a bubble cap gas distributor with 85 tubes of diameter 4 mm. The superficial liquid velocity used in the experiments varied from 0.45 to 1.06 cm s−1 while the superficial gas velocity was ranged from 0 to 7.5 cm s−1. 9735



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Figure 4. Characteristics of response curves versus gas flow rate (experiments 4−7).

distributor. KCl aqueous solution with a certain concentration was used as the tracer, and the experimental details have been described in our previous works.1,12 Table 1 summarizes the operating conditions of one-third of the total 54 tracer experiments for the bubble column with SVT #2 in the present work.

3. RESULTS AND DISCUSSION 3.1. Tracer Injections at the Top Stage of the Column. We first characterized the effect of SVTs on the degree of liquid backmixing reduction of the bubble column by tracing the liquid flowing trajectory. For all these cases, the tracer was introduced at the topmost stage of the column, i.e., the injection position 5f in Figure 1. From Figure 2, one can immediately see that both probe A and B located at the first and second stage, respectively, could not detect tracer concentrations for the bubble column with SVT #2. Although they can monitor the variation of tracer concentrations for the bubble column with SVT #1, the mean liquid residence time and the variance of the response curves of these two probes are smaller than that for the empty column, especially for the dimensionless variance, as listed in Table 2. Since the strong backmixing of the liquid phase in a bubble column is originated from the liquid circulation due to the upward flow

Figure 5. Tracer response curves at different liquid velocities (experiments 1−3).

in the column central region and the downward flow in the annular wall region.13−16 Therefore, the above phenomena mean that the liquid backmixing of the column is substantially reduced by the introduction of SVT #2 through preventing the downward liquid circulation flow. In addition, the probes C and D respectively located at the middle and top stages of the column can monitor the tracer response signals for the column 9736



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with and without SVTs. One can also see from Table 2 that the dimensionless variances in the column with SVT #1 are larger than those obtained with SVT #2 for the probes C and D. For the bubble column with SVT #1, all four probes could detect tracer concentrations in each stage of the column, as shown in Figure 2. These results suggest that both the internals of SVT #1 and SVT #2 could not interrupt the global liquid circulation in the column completely, but the liquid flow in the column with SVT #2 is closer to a model of 5 CSTR in series from an overall perspective. For the empty bubble column, without any SVTs, it can be clearly observed that all four response curves are similar to each other and demonstrate the obvious long tail phenomena, which means the existence of liquid strong backmixing in the whole column. Such an observation is consistent with the findings of Myers et al.17 It can be explained that in the empty bubble column, the liquid travels upward in the central core and downward near the wall region, which forms a global liquid circulation through the column and thereby makes the whole column behave like a continuous stirred tank reactor. Thus, the response signals detected by the four probes at various positions in the empty column all demonstrate similar long tail phenomena, which imply the liquid flow pattern in an empty bubble column is analogous to completely backmixing flow. 3.2. Tracer Injections at the Middle Stage of the Column. To see where and how the liquid tracer is backmixed in different stages of the bubble column, a series of experiments with different superficial gas and liquid velocities have been conducted for tracer injections at the middle stage of the column. 3.2.1. Backmixing Characteristics at Different Gas Flow Rates. As shown in Figure 3, at constant uL, if uG is 0, the spreading of probe response signals is small for the bubble column with and without SVTs; if uG is greater than 0, the spreading tracer curves for the bubble column with SVTs drift significantly far from the case without SVTs, even under uniform bubbly flow condition. Moreover, the mean liquid residence time and dimensionless variance for the bubble column change little with increasing gas velocity, as illustrated in Figure 4. This suggests that the liquid backmixing is practically independent of superficial gas velocity. This is consistent with the study reported by Gupta et al.18 and Grasemann et al.19 Although the impulse responses of probe D in these experiments are very similar, the concentration profiles for the empty bubble column exhibit a peak overshoot after the injection compared to the response signals for the column with SVTs, as can be seen from Figure3 (2)−(4). This reveals that most of the injected liquid tracers could be carried upward to the outlet of the column in a very short time by the upward flowing part of the strong global liquid circulation existing in the empty column. Meanwhile, since the global liquid circulation pattern is restricted in the bubble column with SVTs, the upward liquid flowing part in the bubble column with SVTs is less vigorous than that in the empty column, thus the peak overshoot of the response curve only exhibits in the case for the empty bubble column. 3.2.2. Backmixing Characteristics at Different Liquid Flow Rates. The effect of liquid flow rate on concentration profiles is shown in Figure 5. It is evident that the influence of liquid velocity on the tracer response signals is remarkable, which is reflected in the spread of these curves. With an increase in liquid flow rate, the peak height increases and the residence time of the

Figure 6. Characteristics of response curves versus liquid flow rates (experiments 1−3).

tracer in the system decreases as expected for the bubble column with and without SVTs. Such result is in agreement with the work of Wiemann and Mewes20 and Al-Dahhan et al.21 It can be interpreted that the backflow of liquid phase is suppressed by the bulk liquid flow. For a constant superficial gas and liquid velocity, the dimensionless variance of response signals for the column with SVT #2 is less than that with SVT #1, which is also less than the case without any SVTs, as depicted in Figure 6. This indicates that the backmixing of liquid phase decreases with decreasing the throat diameter of SVT, which is reflected in the spread of tracer response signals. It can also be seen from Figure 6 that the liquid mean residence time decreases with an increase in the superficial liquid velocity. For high superficial liquid velocities, the tracer is carried out of the column with and without SVTs more quickly. Thus, for a high liquid flow rate, the mean residence time of the tracer is reduced and a narrow residence time distribution is obtained. 3.3. Tracer Injections at Different Stages of the Column. To further understand the backmixing characteristic of liquid phase on a certain stage, tracer experiments were carried out at the same operating condition using four different 9737



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Figure 7. Effect of tracer injection positions on response signals (uL = 0.75, uG = 7.5 cm·s−1).

3.4. Tracer Injections at the Bottom Stage of the Column. Figure 8 shows the tracer response curves for the four probes utilized in studying the interstage backmixing for tracer injections at the bottom of the column in the presence and absence of SVTs. Immediately after the injection, the tracer is transported upward along the column center. Within a short time, the tracer is distributed over the whole empty column because of the intense global liquid circulation flow pattern. Therefore, the response times of the all four probes for the empty column are short, and the peak locations of the corresponding curves are more to the left compared to that for the bubble column with SVTs. When the internals of SVTs were adopted in the system, the bubble column diameter would be reduced in a periodical way along the column axis. Accordingly, the global liquid circulation which encompasses the entire column would be confined to some degree. Hence, the time delay after the injection for each case increases with an increase in the distance between the probe position and the tracer injection position. All tracer response curves reach a maximum value and then decrease. Under the same operating condition, the maximum is shifted toward longer times and the maximum value is reduced.

tracer injection positions. In each configuration, the tracer was injected and detected at the same stage. Figure 7 apparently shows that the injected pulse of KCl from each stage causes a sharp peak in the response signal of the corresponding conductivity probe for each experiment. This is because the tracer injection and detection positions are located at the same stage. In either case, the tracer injected into a certain stage will always first be detected by the conductivity probe that is in the same stage. Thus each response curve exhibits an obvious peak overshoot. By comparing the three curves in each part of Figure 7, it is evident that the peak height of the response curve for the column with SVT #2 is always higher than that for the column with SVT #1, which is also higher than that for the empty bubble column under the same operating condition. This finding indicates that the global liquid circulation existing in the empty column is restricted by reducing the column diameter periodically. Moreover, the reduction degree of the global circulation increases with decreasing the column diameter. In accordance with the investigation above, the liquid backmixing is significantly reduced for the bubble column with SVT #2, which is reflected in the spread of concentration signals. 9738



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Figure 8. Tracer responses for injections at the bottom stage of the column (experiments 8−11).

present study, N = 5), the backflow coefficient k, which describes liquid exchange between two adjacent stages, can be evaluated by minimizing the root-mean-square deviation between experimental and model values of E(θ) at each interval of time. Figure 9 shows the comparison of model predictions with the experimental tracer responses for injections at the bottom of the column. The responses have been normalized with respect to their maximum for the sake of comparison. Clearly, the agreement between the predicted and the experimental data can be seen to be excellent. For the empty bubble column, the backflow ratio is larger than 1 which means that an intense global liquid circulation does exist in the empty column. For the bubble column with SVT #1, the liquid backflow rate is nearly the same as the net liquid flow rate, which indicates the internals SVT #1 can restrict the downward flow of liquid phase to some extent in the whole column. In spite of the throat diameter of SVT #1 being smaller than 0.7 times the column diameter, it still cannot break the global liquid circulation completely. When the throat diameter of SVTs reduces to 12 cm, the liquid backflow ratio becomes very small, which suggests that the global liquid circulation could be interrupted practically.

From the comparison of three curves in each part of Figure 8, one can also clearly see that the shapes of tracer responses for the bubble column with SVT #2 are narrower than those of the other two cases. In addition, the response curve of probe D is found to be almost symmetrical in relation with the mean residence time, as shown in Figure 8 (4). It can be concluded that SVTs #2 provide the desired effect of staging for the liquid phase in the bubble column, which inherently results in reduced backmixing. When SVT #2 was used, because the global liquid circulation was effectively interrupted, each stage was assumed to be perfectly mixed and the whole column was treated like a tanks-in-series reactor. Therefore, the liquid flow in the overall column with SVT #2 can be considered approximately as a model of 5 CSTR in series. 3.5. Model Interpretation of Response Signals. According to the agreement of the physical model conception with the real reactor arrangement in this study, we employed the TISB model to describe the time evolution of a liquid tracer, which is injected into the column as an ideal pulse. In this model, the fluid within a stage is assumed to be well mixed and the degree of backmixing is characterized by the number of stages N and the backflow coefficient k (the ratio of the liquid backflow rate to the net liquid flow rate).22,23 For a given stage N (in the 9739



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characteristic between and within stages of a 50 cm diameter column with and without SVTs. First, the main influence of the SVTs on the backmixing degree of liquid phase was investigated with tracer injected at the top of the column by tracing the liquid flowing trajectory. It has been found that the introduction of SVT #2 drastically reduces the liquid backmixing by preventing the downward circulation flow of liquid phase. In addition, the effect of superficial liquid and gas velocities on liquid backmixing characteristic was also investigated, which reveals that the liquid backmixing is practically independent of gas velocity, while the liquid mean residence time decreases with an increase in the liquid velocity. To enhance the understanding of the characteristic of the conductivity probe response profile of liquid phase, a modified version of the tanks-in-series model, which captures the essence of the observed physical phenomenon in the bubble column with SVTs, has been adopted for the description of liquid phase backmixing behavior. The unknown parameter k, which describes liquid exchange between the adjacent stages, can be obtained by matching the model prediction to the experimentally measured tracer response. Comparison of the developed model with experimental results is satisfactory. It also shows that the backflow coefficient for the empty bubble column is 8.842 while it is 1.158 and 0.062 for the column with SVT #1 and SVT #2, respectively.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-21-64253529. Fax: +86-21-64253528. Notes

The authors declare no competing financial interest.

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SYMBOLS E(t) = E-curve, s−1 E(θ) = dimensionless E-curve N = stage number k = liquid backflow coefficient t = time, s tm = mean liquid residence time, s uG = superficial gas velocity, cm·s−1 uL = superficial liquid velocity, cm·s−1

Subscripts

■

G = gas phase L = liquid phase

REFERENCES

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Figure 9. Comparison of model prediction with experimental probe responses (uL = 0.75, uG = 7.5 cm·s−1).

5. CONCLUSIONS A developed multipoint tracer injection and detection device has been employed to gain a deep insight into the liquid backmixing 9740



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Backmixing Characterization of a Bubble Column with Short Venturi

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