Article pubs.acs.org/IECR
Micromixing Efficiency of Viscous Media in Novel Impinging StreamRotating Packed Bed Reactor Weizhou Jiao,*,† Youzhi Liu,† and Guisheng Qi† †
Research Center of Shanxi Province for High Gravity Chemical Engineering and Technology, North University of China, Taiyuan 030051, People’s Republic of China ABSTRACT: Micromixing efficiency of viscous media in novel IS-RPB reactor was studied by using iodide-iodate test reaction as working system. Experiments were carried out in glycerol-water solution with different viscosities. The effects of high gravity factor, liquid flow rate, acid concentration, liquid flow ratio, and viscosity on segregation index and micromixing time were investigated. The experimental results showed that the IS-RPB was effective in enhancing micromixing when the liquid viscosity increased. Furthermore, the micromixing time tm of the IS-RPB reactor was determined based on the incorporation model, and its value was in the range of 10−5−10−4 s. Compared with other mixing devices, IS-RPB reactor has better micromixing characteristics than other reactors, which has a good application prospect. fields with higher dispersion and turbulence, which has been applied to extraction, liquid membrane separation, emulsion, and reaction. Considering that various chemical processes (including polymeric reaction, precipitation reaction, and crystallization) involve the mixing of viscous fluids, the effect of viscosity on micromixing should be taken into account when evaluating the micromixing efficiency of IS-RPB devices.19 In this work, the micromixing efficiency of viscous solution in IS-RPB reactor was studied by using the parallel competing reaction. In addition, the micromixing time was estimated in terms of the experimental data and incorporation model.
1. INTRODUCTION Processes involving mixing are profoundly concerned with the growth of various fields such as polymerization, crystallization, precipitation, and so on.1−4 If the reaction rate is much slower than the mixing rate, the reactants in the solution can be homogeneously mixed before the reaction takes place. While for fast reactions, reactions happen before a homogeneous environment is achieved, proceed in an inhomogeneous solution, and are sensitive to the micromixing efficiency. In this case, micromixing has a large influence on the product distribution, the selectivity of the chemical reactions, and the molecular weight distribution of polymer molecules. Process intensification is a relatively new concept that has been gaining strong momentum because of its ability to significantly reduce capital and operating costs while improving process yield/selectivity and enhancing the inherent safety of many industrial operations. Ramshaw5 invented rotating packed bed (RPB), a novel gas−liquid reactor that utilizes centrifugal acceleration to intensify mixing and mass transfer. 1−2 orders of enhancement in mass transfer can thus be achieved in a rotating packed bed. Based on the above advantages, the feasibility of the RPB has been extensively studied in many systems such as distillation,6,7 absorption,8−10 stripping,11,12 chemical reaction,13,14 and ozone oxidation.15,16 In recent years, there has been an emerging trend toward the application of RPB in liquid−liquid precipitation processes such as the production of drug nanoparticles and inorganic nanoparticles. These pioneering efforts show that RPB is a promising novel reactor for fast liquid−liquid reactions because of excellent micromixing efficiency. However, the first visual evidence of liquid no uniform distributions was presented by Burns and Ramshaw17 in a RPB with reticulated PVC packing. The experiment indicated that liquid had little ability to wander laterally through the packing in comparison with its radial motion. As mentioned above, Liu18 developed a novel reactor for intensifying liquid/liquid mixing, named Impinging StreamRotating Packed Bed (IS-RPB). Therefore, the high gravity technology is expanded from the gas/liquid to liquid/liquid © 2012 American Chemical Society
2. EXPERIMENTS 2.1. The Principle of IS-RPB Reactor. The IS-RPB reactor used in this study is shown in Figure 1, which consists of rotor,
Figure 1. Schematic of the impinging stream-rotating packed bed unit.
Received: Revised: Accepted: Published: 7113
November 10, 2011 April 15, 2012 May 3, 2012 May 3, 2012 dx.doi.org/10.1021/ie202586f | Ind. Eng. Chem. Res. 2012, 51, 7113−7118
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value of Y in the total segregation case when the micromixing process is infinitely slow. The value of XS is within the range of 0 < XS < 1 for partial segregation, and XS = 0 and XS = 1 indicate the perfect micromixing and the total segregation, respectively. 2.3. Experimental Section. The glycerol−water solution was used to simulate the behavior of the viscosity reaction system. The kinetics of the chemical test reactions in a glycerol−water solution have been well discussed by Guichardon.24 The viscosity and glycerol concentration of the solution are listed in Figure 2. In our experiments, solutions A
shell, liquid inlet and outlet, motor, etc. The specification of ISRPB is given in Table 1.The packing used in this study is Table 1. Details of Rotor and Casing packing material
stainless steel
outer diameter inner diameter axial width of packing casing material casing geometry diameter of casing height of casing power of motor
150 mm 40 mm 80 mm stainless steel column box 320 mm 120 mm 0.75 kW
stainless steel wire meshes of 0.3 mm diameter, of 0.96 porosity. The axial height of the bed is 30 mm. The inner and outer radii of the packing are 30 mm and 50 mm, respectively. The rotating speed varies from 500 to 2900 rpm, providing the centrifugal acceleration range from 26.5 g to 370.2 g. The principle of IS-RPB is to bring the two streams flowing along the same axis (the impinging angle α is 180°) in the opposite direction into collision to change the liquid nonuniform distribution of RPB, and its impinging spray surface enters the inner brim of RPB along the radial direction. In this experiment, the impinging distance d and the impinging angle α are 30 mm and 180°, respectively. As a result of such a collision, the worse impinging spray surface is further mixed by the aid of RPB. The predominant mixing characteristics of the IS-RPB reactor is that the end effect in the inner radius of RPB offsets the fringe effect of IS, which makes the fluid micromixed second. Finally, the liquid expelled from the bottom of the ISRPB. 2.2. Parallel Competing Reaction System. The parallel competing reaction system was proposed by Fournier,20,21 which is described by the following scheme H2BO3− + H + ↔ H3BO3 (quasi‐instantaneous)
(1)
5I − + IO3− + 6H + ↔ 3I2 + 3H2O (fast)
(2)
−
I + I2 ↔
I3−
Figure 2. Influence of the glycerin concentration on the viscosity of glycerin/water solutions.
and B were prepared first. Solution A contains H2BO−3 , I−, and IO−3 , while solution B contains H+. The reactants were prepared in two solutions. For solution A, the boric acid and sodium hydroxide were first added into glycerol−water solution, whereafter the potassium iodate and the potassium iodide were introduced into the solution, which ensured the iodide and iodates ions coexist in a basic solution and prevented iodine from forming. The initial concentration of H2BO−3 was 0.09 mol·L−1, while for I−, 1.2 × 10−2 mol·L−1, and for IO−3 , 2.3 × 10−3 mol·L−1. The other solution, solution B, was H2SO4 solution(glycerol−water) (0.05−0.08 mol·L−1, the corresponding H+ concentration was 0.10−0.16 mol·L−1). The flow rate ratio of stream A to stream B was adjusted to be between 1 and 15. Figure 3 shows the diagram of the experimental setup. Solutions A and B were pumped simultaneously into the IS-
(3)
Reaction 1 is instantaneous, while reaction 2 is fast but much slower than reaction 1. Reaction 3 is the further reaction between iodine formed from reaction 2 with iodide to form triiodide. The amount of I3− produced depends on the micromixing efficiency and can be detected via the spectrophotometer at 353 nm. Further details have been reported in the literature.22,23 The micromixing “efficiency” is quantified by the segregation index XS, the value of which varies from 0 to 1 to indicate the fraction of unwanted product, i.e., iodine. Therefore, the smaller the value of XS, the better micromixing efficiency is. The segregation index is defined as XS = Y/YST, where Y=
2(n I2 + n I3−)
YST =
n H0+
(4) − 6CIO3,0
− + C −) (6CIO3,0 H2BO3,0
(5)
Figure 3. Schematic diagram of the experimental setup: 1: stirred tank A; 2: IS-RPB; 3: liquid outlet; 4: rotameter; 5: valve; 6: pump; 7: stirred tank B.
where Y is the ratio of acid mole number consumed by reaction 2 divided by the total acid mole number injected, and YST is the 7114
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RPB after conforming the valves 5 opened and mixed with each other in the bed. If micromixing is homogeneous, then almost all of the injected H+ reacts with H2BO−3 to form H3BO3. So reactions 2 and 3 may not occur. Otherwise, the three reactions may all occur. The flow rates of solutions A and B varied from 42 to 140 L·h−1. All experiments were conducted at a fixed temperature of 20 ± 1 °C. The concentration of I−3 in the outlet stream was measured by a spectrophotometer at 353 nm.
3. RESULTS AND DISCUSSION 3.1. Effect of High Gravity Factor β. The high gravity factor β10 is a ratio of the average high gravitational acceleration in the IS-RPB and the gravitational acceleration g. Figure 4
Figure 5. Effect of liquid flow rate on the segregation index XS.
furthermore larger relative velocity between liquids and packing and more homogeneous mixing. Consequently, the micromixing efficiency enhanced with the increasing liquid flow rate. Second, the residence time distribution (RTD) also decreases with increasing liquid flow rates, which may increase the coalescence-redispersion frequency during liquid elements, resulting in the intensified mixing. 3.3. Effect of Liquid Flow Ratio η. Figure 6 shows the changes of the segregation index XS in the flow rate ratio η at
Figure 4. Effect of high gravity factor on segregation index XS.
displays the effect of high gravity factor β ranging from 26.5 to 373.2 on the segregation index XS under the condition of different acid concentration. The initial liquid flow ratio, liquid flow rate, and liquid viscosity were 7 (by volume), 70 L·h−1, and 10 mPa·s, respectively. As shown in Figure 4, the segregation index XS decreased with the increasing high gravity factor β under experimental conditions. As expected, the increasing high gravity factor enhanced the micromixing efficiency. The reasons for this may be as follows. As the liquid sprayed to the packed bed in the radial direction, it is vigorously fragmentized by the fast rotating packing into small droplets and thin films. Some of them may be captured by the packing and some may keep traveling through the void space. This would create high relative velocity among all kinds of liquid elements. Consequently, the frequency of the vigorous impingement and coalescence-dispersion of the liquid would occur, which achieve good micromixing (small XS) of IS-RPB. However, when the high gravity factor was further increased, only a small effect on XS was observed, which was similar to the results of Jiao et al.19 At the same time, Figure 4 shows the distributions of XS at different acid concentrations. It can be appreciated that XS is sensitive to the acid concentration. The high acid concentration will mean a higher production of iodine, which lead to a larger XS. 3.2. Effect of Liquid Flow Rate VA. Figure 5 illustrates the effect of the liquid flow rate VA on the segregation index XS in the range of 42 to 140 L·h−1. As expected, increasing the liquid flow rate could enhance the micromixing efficiency owing to the following major contributions. First, the higher liquid flow rates causes higher injecting velocity from the liquid distributor,
Figure 6. Effect of liquid flow ratio on the segregation index XS.
VA = 70 L·h−1. It is clear that the segregation index XS increases with the increasing liquid flow ratio. This is probably due to the fact that the increasing liquid flow ratio at fixed liquid flow rates of A results in less intensive impinging between A and B, which lead to the worse mixing (bigger XS). Meanwhile, the acid reacts in reactions 1 and 2 simultaneously, the rate of the reaction 2 is more sensitive to acid concentration than that of reaction 1 because its reaction order with respect to acid is higher. The higher concentration of acid will make reaction 2 faster notably and more iodine is formed, meaning an increasing of XS. 3.4. Effect of Liquid Viscosity. Figure 7 illustrates the effect of high gravity factor on XS in different viscous media. Under a given experimental condition, segregation index XS 7115
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where Cj denotes the concentration of reactants (H3BO−3 , H+, I−, IO−3 , I2, I−3 ), and Cj0 represents the initial concentration of j ion in solution A. Rj is the net production rate of species j for the reaction. The available growth law of aggregates is a function of tm
g (t ) = exp(t /tm)
(7)
Then
dCj dt
=
Cj0 − Cj tm
+ Rj
(8)
The detailed process of the above equations have been reported by Yang et al.29 Here we also determine tm of IS-RPB with this model. Figure 8 gives the relationship between tm and XS based on the incorporation model at the same condition with the Figure 7. Effect of liquid viscosity on the segregation index XS.
decreased sharply and then gradually reaches a plateau with the increase of the high gravity factor. The centrifugal acceleration makes liquid film thickness decreased due to the decrease of the diffusion distance at low rotating speed. In a centrifugal field, thin liquid films and tiny liquid droplets are generated, which result in a decrease of mass transfer resistance and an increase in micromixing efficiency (small XS). When the high gravity factor was further increased (>208.1), only a small effect on XS was observed. This was probably due to the fact that the liquid film thickness remained almost unchanged with the increasing high gravity factor at high rotating speed.25 At the same time, it was found that XS increased with the increasing viscosity, indicating poor micromixing efficiency. This phenomenon can be summarized as follows. For the low high gravity factor, higher viscous media weakened the molecule diffusion. That is to say, lower molecule diffusion velocity could result in worse micromixing efficiency. However, when the high gravity factor was further increased, only a small effect on XS was observed. This was probably due to the fact that the larger centrifugal force enhanced the molecule diffusion. 3.5. Micromixing Time tm Determination. Segregation index XS cannot be directly used to describe quantificationally the micromixing performance. For a fast chemical reaction, their characteristic reaction time tr is very small. The selectivity, quality, or distribution of the final product can be controlled only by decreasing the micromixing time tm less or close to tr. The micromixing time tm cannot be calculated directly. Many models had been proposed to calculate the micromixing time from the segregation index, such as the IEM model,26 the droplet erosion and diffusion model,27 the engulfment deformation diffusion model, 28 and the incorporation model.23 Among these models, the incorporation model is believed to be simple and widely used to estimate the micromixing time in many kinds of reactors. In the incorporation model the acid aggregates are dispersed in a large volume of iodate, iodide, and borate ions. Acid aggregates grow by progressively incorporating the surrounding medium where reactions 1 and 2 take place. The growing law of aggregates is a function of the micromixing time dCj dt
= (Cj0 − Cj)
1 dg + Rj g dt
Figure 8. Relationship between tm and XS.
experimental results of Figure 2-Figure 6. According to the experimental values of XS, it was observed that tm was in the range of 10−4−10−5 s for the IS-RPB reactor, corresponding to the value of XS (results of our experiments are between 0.0008 and 0.226). Compared with the conventional devices, the micromixing time tm of stirred tanks21 in aqueous solution ranged from 0.01 s to 0.2 s. In 2009, Yang et al.29 measured the micromixing efficiency of the microchannel reactor. They found that the micromixing time tm was in the range of 10−3−10−4 s at different viscosities. In 2005, Yang et al.30 measured the micromixing efficiency in a rotating packed bed with aqueous solution on the basis of a parallel competing iodide-iodate reaction system. Through both the micromixing theory with appropriate assumptions on the energy dissipation rate in the RPB and an incorporation model, tm is approximately evaluated to be about 10−4 s. In 2000, Monnier et al.31 found the micromixing efficiency in a continuous flow cell with viscous media (1−4.5 mPa) was enhanced by ultrasound in a similar operational condition. They found that the increasing ultrasound power could improve the micromixing efficiency, where tm varied from 0.002 to 0.01 at the experimental condition. From the above discussion, we can conclude that the IS-RPB, with the aid of the centrifugal force and the initial liquid impinging, could improve the micromixing efficiency significantly not only for aqueous solution but also for viscous media here.
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4. CONCLUSIONS A novel concept of a liquid−liquid contact reactor, the so-called impinging stream-rotating packed bed (IS-RPB), was presented for improving micromixing of iodide and iodate. Employing an iodide−iodate reaction system, the micromixing efficiency of IS-RPB was investigated. In this system, the XS decreased with the increasing high gravity factor and liquid flow rate. The decrease of the liquid flow ratio, liquid viscosity, and acid concentration results in the decrease of XS. Based on the incorporation model, the micromixing time in an IS-RPB reactor was also evaluated, and its value was in the range of 10−5−10−4 s. The extremely low tm also provides a new approach for studying the ultrafast reaction kinetics. These results demonstrated that IS-RPB could be used as an ideal intensifying liquid−liquid micromixing reactor for the ultrafast reaction kinetics in the viscous media.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 86-351-3921986. Fax: 86-351-3921497. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the Youth Science and Technology Foundation of Province Shanxi of China (No. 2010021007-2) for financial support of this work.
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NOTATIONS CH+ initial concentration of H+ ion, mol·L−1 d impinging distance, m g acceleration of gravity, 9.8 m·s−2 KB equilibrium constant, L·mol −1 L liquid flow rate, L/h r1 inner diameter of IS-RPB, m r2 outer diameter of IS-RPB, m T temperature, K tr reaction time, s tm micromixing time, s VA liquid flow rate of A solution, L·h−1 VB liquid flow rate of B solution, L·h−1 XS segregation index, dimensionless Y selectivity of iodide, dimensionless YST selectivity of iodide in the case of total segregation, dimensionless
Greek symbols
μ viscosities, mPa·s α impinging angle, ° β high gravity factor, dimensionless η η = VA/VB liquid flow ratio, dimensionless
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REFERENCES
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