Investigation of Operation Regimes in a Multistage Internal-Loop Airlift

Oct 13, 2010 - Multiphase internal-loop airlift reactor is effective in reducing liquid back-mixing. To further improve the distribution of solid part...
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Ind. Eng. Chem. Res. 2010, 49, 11752–11759

Investigation of Operation Regimes in a Multistage Internal-Loop Airlift Reactor Wei Yu, Tiefeng Wang,* Malin Liu, and Feifei Song Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China

Multiphase internal-loop airlift reactor is effective in reducing liquid back-mixing. To further improve the distribution of solid particles in different stages, a novel interstage internal, i.e., a perforated plate with several long tubes, was used to provide a separate flowing path for gas and slurry phases. With this novel internal, a gas layer was formed between the two adjacent stages and had a significant effect on the operation regimes. In this work, the effects of the opening ratio of the internal and superficial gas and liquid velocities on the operation regimes were experimentally studied for both cocurrent and countercurrent flows. The experimental results show that the superficial gas and liquid velocities have significant effects on the operation regimes. The effect of the opening ratio for gas channels on the operation regimes was more significant than that of the opening ratio for liquid channels. Compared with the traditional perforated plate internal, the novel internal showed better performance for operation flexibility. The regime map was obtained from the calculated gas layer height with the mathematical model proposed in our previous work and was in good agreement with the experiential results. The results in this work can provide guidance for design and scaleup of the multistage reactors. 1. Introduction Bubble columns and airlift loop reactors are widely used in chemical and biochemical processes, especially in the gas-toliquid processes of Fischer-Tropsch synthesis, methanol synthesis, and dimethyl ether synthesis. Compared with the conventional stirred tank reactor, bubble columns and airlift loop reactors have the advantages of simple construction, good heat transfer, and feasible scaleup.1,2 Most works in the literature focused on the single-stage reactor. However, the single-stage reactor has the disadvantageous feature of intense liquid backmixing, thus very inefficient for a process that requires a higher conversion of the liquid reactants. The use of tanks-in-series can effectively decrease the liquid back-mixing, but this will result in more complexity of the operation and control. In our previous work,3,4 a novel multistage internal-loop airlift reactor was proposed by analogy with the tanks-in-series concept. This multistage internal-loop airlift reactor achieved excellent performances in decreasing the interstage liquid back-mixing,5 especially showing better performance in the aspect of the uniform distribution of the solid particles than the multistage bubble column,6-11 multistage internal-loop airlift reactor with perforated plate internal,12 and multistage external-loop airlift reactor13 reported in the literature. The good performance of the multistage reactor in our work is closely related to the liquid circulation in an airlift reactor and the special structure of the novel interstage internal. The novel internal used in our work was a perforated plate with several long tubes, in which the gas flowed through the orifices and the liquid or liquid-solid suspension flowed through the long tubes. However, when such an internal is used, a gas layer will be formed below it, which in turn affects the liquid level and liquid circulation in the stage below the internal.14 The presence of the gas layer resulted in three operation regimes of the multistage reactor for cocurrent flow, as illustrated in Figure 1. Normal operation regime is characterized by the gas layer between the bottom of the interstage internal and the top edge * To whom correspondence should be addressed. Tel.: 86-1062797490. E-mail: [email protected].

of the draft tube, with a liquid circulation in the stage below the internal. As the liquid level decreases below the draft tube, the flow enters the abnormal operation regime I where liquid circulation was interrupted and the solid particles cannot be uniformly suspended. As the liquid level increases to the surface of the internal plate, the flow enters the abnormal operation regime II where the liquid and particles flow through both the orifices and long tubes and may cause an unsteady flow pattern and nonuniform distribution of solid particles. In contrast, the multistage reactor with countercurrent flow has only two operation regimes with respect to the gas layer height, namely, the normal operation regime and abnormal operation regime I. In the multistage reactor, the gas layer height should be controlled in a reasonable range for the normal operation. Although the presence of a gas layer between the two stages in a multistage reactor has been reported in some published works,12,15,16 all these studies focused on the perforated plate internals and did not systematically study the operation regimes. The normal operation regime with the novel internal is important to homogeneously suspend the solid particles and avoid the flooding problem that was encountered in trickle beds with countercurrent flow for deeper hydrodesulfurization.17,18 In this work, we studied the operation regimes in the multistage internal-loop airlift reactor with a novel interstage internal for both cocurrent and countercurrent flows. The effects of the opening ratio of the internal and the superficial gas and liquid velocities were experimentally investigated. In addition, the operation regimes with the novel internal were compared with those with the traditional perforated plate. The regime map was obtained from the calculated gas layer height with the mathematical model proposed in our previous work.14 2. Experimental Section 2.1. Apparatus. The schematic of the experimental apparatus is shown in Figure 2. The reactor was a vertical Plexiglas column with 0.20 m outer diameter, 0.19 m inner diameter, and 2.85 m height. Two draft tubes were installed inside the reactor. Each draft tube was of 0.12 m outer diameter, 0.11 m inner diameter, and 1.0 m height. The experimental apparatus was divided into two

10.1021/ie100746z  2010 American Chemical Society Published on Web 10/13/2010

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Figure 1. Illustration of the operation regimes in the multistage reactor for cocurrent flow.

Figure 2. Scheme of the experimental apparatus and interstage internal.

stages by an interstage internal. Both stages were internal-loop airlift sections with the annular region being the riser and the draft tube being the downcomer. Two types of internals, named internal I and internal II, were used in this work. The structures of the interstage internals are shown in Figure 3. Internal I was a traditional perforated plate. The orifices were Φ8 and had opening ratios of 1.77 and 5.31%. When internal I was used, both gas and liquid flowed through the orifices. Internal II was a novel interstage internal, which was a perforated plate with several long tubes. The orifices were Φ2 and had an opening ratio of 0.11 and 0.22%. The tubes were Φ8 and 300 mm height and had an opening ratio of 0.53, 1.06, and 1.60%. When internal II was used, the gas flowed mainly through the orifices, and the liquid flowed through the tubes. Air and tap water were used as the gas and liquid phases, respectively. Air had the same flowing manner for both cocurrent and countercurrent flows. It was pumped into the system from the bottom of the reactor, distributed by a perforated gas distributor with 30 holes of Φ3. Air in the bottom stage then flowed through the interstage internal into the top stage and flowed out of the system from the top of the gas-liquid separator. The difference between the gas holdups in the riser and downcomer drove the liquid circulation in each stage. The liquid flowing manner was different for the cocurrent and countercurrent flows. When the

reactor was operated in cocurrent mode, as shown in Figure 2a, the liquid phase pumped from a stirred tank into the bottom stage, flowed through the interstage internal to the top stage and then flowed out of the top stage into the stirred tank. When the reactor was operated in countercurrent flow, as shown in Figure 2b, the liquid phase pumped by pump 7a from a stirred tank into the top stage of the reactor. Some liquid in the top stage flowed through the interstage internal into the bottom stage and then pumped by pump 7b back into the stirred tank. The rest of liquid in the top stage overflowed into the stirred tank through the overflow line 13. 2.2. Measuring Method. The operation regime was closely related to the gas layer height, which was defined as the average height below the internal and the liquid surface. The gas layer height was measured by visual observation. Three parallel measurements were carried out at a given operating condition, and the average value was used as the final result, with an average discrepancy within (5%. 3. Results and Discussion 3.1. Operation Regimes with the Novel Internal. 3.1.1. Effect of Superficial Gas Velocity. The effect of the

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Figure 5. Effect of superficial liquid velocity on the operation regimes for both cocurrent and countercurrent flows.

Figure 3. Structure of the interstage internals.

Figure 4. Effect of superficial gas velocity on the operation regimes for cocurrent and countercurrent flows.

superficial gas velocity on the operation regimes was experimentally studied. The results for both cocurrent and countercurrent flows are shown in Figure 4. The gas layer height increased with an increase in the superficial gas velocity for both cocurrent and countercurrent flows. The reason was that increased superficial gas velocity resulted in a significant increase in pressure drop through the orifices of the internal, which in turn resulted in an increase in the gas layer height. As the gas layer height increased to the height of 0.20 m (the distance from the bottom of the interstage internal to the top edge of the draft tube), the flow entered the abnormal operation regime I. This was undesirable for the reactor performance because liquid circulation was interrupted. The superficial gas

velocity corresponding to transition from the normal operation regime to the abnormal operation regime I was larger for cocurrent flow than for countercurrent flow at the same operation conditions. It increased slightly with an increase in the superficial liquid velocity. Thus, the superficial gas velocity had a more significant effect on the operation regimes, and the maximum superficial gas velocity should be controlled within the normal operation regime for both cocurrent and countercurrent flows. 3.1.2. Effect of Superficial Liquid Velocity. Figure 5 shows the effect of the superficial liquid velocity on the operation regimes for both cocurrent and countercurrent flows. With an increase in the superficial liquid velocity, the gas layer height increased for countercurrent flow but decreased for cocurrent flow. With a further increase in the superficial liquid velocity, the countercurrent flow entered the abnormal operation regime I, while the cocurrent flow entered the abnormal operation regime II where the liquid level approached the internal. In the abnormal operation regime II, the mixture of gas and liquid flowed through both the orifices and long tubes and could enhance the interstage liquid back-mixing between the two stages. The reason for the different effect of the superficial liquid velocity on the cocurrent and countercurruent flows was that the increased superficial liquid velocity resulted in an increase in pressure drop through the vertical long tubes, which had a positive effect on the normal operation regime for cocurrent flow, but a negative effect on the normal operation regime for countercurrent flow. This makes that the cocurrent flow has a wider normal operation regime than that of the countercurrent flow. The critical superficial liquid velocity corresponding to transition from the normal operation regime to abnormal operation regime was lower for cocurrent flow than that for countercurrent flow at the same operation conditions. At a higher superficial liquid velocity, the normal operation regime was wider for cocurrent flow but narrower for countercurrent flow. Thus, a reasonable range of superficial liquid velocity should be controlled for both cocurrent and countercurrent flows. By comparison, a higher superficial liquid velocity was favorable for cocurrent flow, and a lower superficial liquid velocity was favorable for countercurrent flow. 3.1.3. Effect of Opening Ratio for Gas Channels. Figure 6 illustrates the effect of the opening ratio for the gas channels on the operation regimes for cocurrent flow. The opening ratio for the gas channels, ξg, had an important effect on the operation regimes in terms of the range of both superficial gas and liquid velocities. Increased ξg resulted in a significant decrease in the gas layer height and a wider range of superficial gas velocity

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Figure 6. Effect of opening ratio for gas channel on the operation regimes for cocurrent flow, with variation of the (a) superficial gas velocity and (b) superficial liquid velocity.

for the normal operation regime. The critical superficial gas velocity for transition from the normal operation regime to the abnormal operation regime was larger for ξg ) 0.22% than for ξg ) 0.11%, as shown in Figure 6b. The reason was that increased ξg resulted in a decrease in pressure drop through the orifices, which in turn led to a decrease in the gas layer height. The effect of ξg on the operation regimes for countercurrent flow was similar to the results for cocurrent flow with a change in the superficial gas velocity, as shown in Figure 7a. However, this effect was different with a change in the superficial liquid velocity, as shown in Figure 7b. As the superficial liquid velocity increased, the countercurrent flow entered the abnormal operation regime I, while the cocurrent flow entered the abnormal operation regime II. It can be seen from Figure 7b that the critical superficial liquid velocity for transition from the normal operation regime to abnormal operation regime I is lower for ξg ) 0.22% than for ξg ) 0.11%, which indicated that increased ξg was favorable for a wider normal operation regime. However, a larger ξg may cause liquid leakage from the upper stage to the lower stage and increase the interstage back-mixing. Therefore, the proper value of ξg should be determined by a combined consideration of the operation range and interstage back-mixing. 3.1.4. Effect of Opening Ratio for Liquid Channels. The effect of the opening ratio for the liquid channels, ξl, on the operation regimes for cocurrent flow is shown in Figure 8. Compared with the opening ratio for the gas channels, the opening ratio for the liquid channels had much less significant effect on the gas layer height and operation regime. The gas layer height increased only slightly with an increase in the ξl at a higher superficial liquid and gas velocities and remained almost unchanged at lower superficial liquid and gas velocities. The reason was that increased ξl resulted in a decrease in the flowing

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Figure 7. Effect of opening ratio for gas channel on the operation regimes for countercurrent flow, with variation of the (a) superficial gas velocity and (b) superficial liquid velocity.

resistance of the liquid through long tubes and thus an increase in the gas layer height. The effect of the opening ratio for the liquid channels on the operation regimes for countercurrent flow was contrary to the results for cocurrent flow, as shown in Figure 9. The gas layer height decreased slightly with the increase in the ξl at lower superficial liquid velocity and decreased more notably at higher superficial liquid velocity. The reason was that an increase in ξl resulted in a decrease in the flowing resistance of the liquid through the tubes, which thus led to a decrease in the gas layer height. 3.2. Operation Regimes with Conventional Perforated Plate. The effects of the superficial gas and liquid velocities on the operation regimes for countercurrent flows with a conventional perforated plate as the interstage internal are shown in Figure 10. In the normal operation regime, the gas layer height increased with an increase in the superficial gas and liquid velocities, but remained stable with time on-stream. However, when the superficial gas velocity increased over a critical value, the gas layer height became unstable and increased with time on-stream, as shown in Figure 10b. This indicated that the flow entered the abnormal operation regime. In addition, the gas layer height increased faster at a higher superficial liquid velocity. Above the critical superficial gas velocity, the liquid was prevented from flowing from the top stage to the bottom stage through the orifices due to the high through-hole gas velocity. Similar liquid flooding phenomena existed in multistage fluidized beds19 or trickle beds operated in countercurrent mode. Thus, compared with the conventional perforated plate, the novel internal in this work can avoid liquid flooding up to a much higher superficial gas velocity and give a wider normal operation regime.

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Figure 8. Effect of opening ratio for liquid channel on the operation regimes for cocurrent flow, with variation of the (a) superficial gas velocity and (b) superficial liquid velocity.

Figure 9. Effect of opening ratio for liquid channel on the operation regimes for countercurrent flow, with variation of the (a) superficial gas velocity and (b) superficial liquid velocity.

4. Model Prediction for the Operation Regimes The gas layer height is determined by the balance between the pressure drops of the gas and liquid through the internal, as shown in Figure 11. On the basis of this analysis, a mathematical model was proposed to predict the gas layer height successfully in our previous work.14 When the system is operated in cocurrent mode, the gas layer height can be calculated by 2 1 u0 (1 - εg,T)Flgh1 ) Fg ′ 2 2 (C ) d

-1.75

0.1582ξl

Fl0.75d-1.25Ul1.75µl0.25h

()

Ul 1 - kf,CEFl 2 ξl

2

(1)

When the system is operated in countercurrent mode, the gas layer height can be calculated by 2 1 u0 (1 - εg,T)Flgh1 ) Fg ′ 2 + 2 (C ) d

-1.75

0.1582ξl

Fl0.75d-1.25Ul1.75µl0.25h

()

Ul 1 + kf,CEFl 2 ξl

2

(2)

To ensure the system operated in the normal regime, the gas layer height h1 should satisfy the following condition: l > h1 > 0

(3)

where l is the distance from the bottom of the interstage internal to the top edge of the draft tube.

Figure 10. Effects of superficial gas and liquid velocities on the operation regimes for countercurrent flow with the traditional perforated plate: (a) operation regimes; (b) change in gas layer height with time on-stream.

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Figure 11. Schematic of the gas layer height underneath the novel interstage internal.

Figure 12. Comparison of experimental superficial gas velocity corresponding to the critical value from the normal to abnormal operation regime I with those predicted.

Figure 12 shows the comparison between the measured and calculated superficial gas velocity for transition from the normal operation regime to abnormal operation regime I for both cocurrent and countercurrent flows. A satisfactory agreement was obtained. An operation regime map described by transition boundaries in terms of superficial gas and liquid velocities was constructed to investigate the operation flexibility. The flow regimes in the multistage internal airlift reactor were closely related to the gas layer height below the internal. The factors that affect the gas layer height include the operating conditions (Ug and Ul), physical properties of the gas and liquid (viscosity and density), and geometry of the internal (opening manner and ratio). The experimental results were obtained in a specific apparatus; thus the regime map based on these results cannot be directly used for reactor scaleup. However, the model developed in this work included the effects of the above factors and can be used to determine the flow regime for rector design and scaleup.

Figure 13. Operation regime map for cocurrent flow with the novel internal: (a) ξl ) 0.53% and ξg ) 0.11%; (b) ξl ) 0.53% and ξg ) 0.22%; (c) ξl ) 1.06% and ξg ) 0.22%.

Figure 13 shows the operation regime maps for cocurrent flows using the novel internal with different opening ratios for the gas and liquid channels. The measured transition gas and liquid velocities were also shown for comparison, and a good agreement was obtained. From the operation regime map, three operation regimes were observed. The opening ratio for the gas channels played an important effect on the operation regime, while the effect of the opening ratio for the liquid channels is much less notable. The normal operation regime can be obtained by adjusting the opening ratio of the internal and superficial gas and liquid velocities. Figure 14 shows the operation regime map for countercurrent flow with the novel internal. The measured transition gas and liquid velocities were in good agreement with the predicted values. It can be seen that two operation regimes existed. The

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regime with this novel internal is much wider than that with the conventional perforated plate. (4) The model for the prediction of the gas layer height can be used to quantitatively describe the operation regime and optimum operation conditions for the multistage reactor with the novel internal. Acknowledgment We gratefully acknowledge the financial support by Foundation for the Author of National Excellent Doctoral Dissertation of the People’s Republic of China (Grant No. 200757) and the National 973 Project of China (Grant No. 2007CB714302). Nomenclature Figure 14. Operation regime map for countercurrent flow with the novel internal.

Cd′ ) resistance coefficient of internal for gas flow D ) diameter of the long tube, m F ) friction factor in the long tube G ) gravitational acceleration, m/s2 h ) length of the long tube, m h0 ) vertical distance between two tapping ports, m h1 ) gas layer height, m kf ) flowing resistance coefficient in the long tube kf,CE ) flowing resistance coefficient of contraction and expansion Ug ) superficial gas velocity, m/s Ul ) superficial liquid velocity, m/s u0 ) through-hole gas velocity, m/s Greek Letters

Figure 15. Comparison of operation regimes for countercurrent flows with novel internal and conventional perforated plate.

opening ratio for both gas and liquid channels also played an important effect on the operation regimes. The operation regimes for countercurrent flows with the novel internal and conventional perforated plate were compared, and the results are shown in Figure 15.The normal operation regime with the novel internal was much wider than that with the perforated plate. Liquid flooding is a main problem that is often encountered in countercurrent operation. The novel internal is favorable to avoid liquid flooding by providing separate paths for flowing of gas and liquid. Such a reactor design has potential application in many processes, such as ultra-hydrodesulfurization process to remove sulfur content from diesel. 5. Conclusions The operation regimes of the multistage reactor for both cocurrent and countercurrent flows were experimentally studied and theoretically analyzed. The conclusions are as follows: (1) For cocurrent flows, three operation regimes exist, the normal operation regime, abnormal operation regime I, and abnormal operation regime II, while, for countercurrent flows, two operation regimes exist, namely the normal operation regime and abnormal operation regime I. (2) The superficial gas and liquid velocities and opening ratio for the gas channels have significant effects on the operation regimes for both cocurrent and countercurrent flows. However, the effect of the opening ratio for the liquid channels is less notable for cocurrent flow than for countercurrent flow. (3) For countercurrent flows, the novel internal in this work is favorable for avoiding liquid flooding. The normal operation

µl ) viscosity of the liquid, Pa · s Fl ) density of liquid, kg/m3 Fg ) density of gas, kg/m3 εg ) gas holdup in the riser of the bottom stage εg,T ) gas holdup in the long tubes of the internal ξ ) open ratio of the perforated plate ξg ) open ratio for gas channel ξl ) open ratio for liquid channel

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ReceiVed for reView March 28, 2010 ReVised manuscript receiVed September 5, 2010 Accepted September 28, 2010 IE100746Z