Micromixing Efficiency Enhancement in a Rotating Packed Bed

Jan 22, 2015 - There is an emerging trend toward the packing research in various reactors. Lévêque et al.(18) made a detailed investigation of .... ...
27 downloads 3 Views 7MB Size
Download PDF
Research Note pubs.acs.org/IECR

Micromixing Efficiency Enhancement in a Rotating Packed Bed Reactor with Surface-Modified Nickel Foam Packing Guang-Wen Chu,†,‡ Yin-Jiang Song,‡ Wen-Jie Zhang,‡ Yong Luo,*,‡ Hai-Kui Zou,‡ Yang Xiang,‡ and Jian-Feng Chen†,‡ †

State Key Laboratory of Organic−Inorganic Composites and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Micromixing, the mixing at the molecular scale, is believed to play a substantial role in many industrial processes. In this work, a novel surface-modified nickel foam packing (SNP) loaded in a rotating packed bed (RPB) reactor was first proposed to improve the micromixing efficiency studied by adopting the iodide−iodate reaction as a working system. Experimental results showed that the RPB reactor with the SNP presented better micromixing efficiency, compared to the RPB reactor with the nonmodified nickel foam packing (NNP) or traditional stainless steel wire mesh packing (SWP). It could be found that the segregation index (XS) decreased as the rotational speed and liquid flow rate each increased, while it increased with increasing acid concentration and volumetric ratio. Moreover, the micromixing time (tm) was calculated using an incorporation model based on the experimental data. Compared with NNP or SWP, the minimum tm value of SNP (tm = 3.28 ms) is less than that of NNP (tm = 5.68 ms) or SWP (tm = 8.67 ms).

1. INTRODUCTION The mixing process can be divided into macromixing, mesomixing, and micromixing, which are on different levels. Macromixing and mesomixing are coarse-scale processes. Micromixing, which is defined as mixing at the molecular scale, is believed to play a substantial role in many industrial processes, such as precipitation, consecutive reactions, parallel reactions, combustion, catalytic reactions, etc.1−5 Micromixing may influence the conversion and reaction selectivity, which affects the yield and quality of final products. In recent years, there has been a growing interest in the research of micromixing. Wang et al.6 characterized the micromixing efficiency of a new microporous tube-in-tube microchannel reactor by using a parallel competing reaction system. Yang et al.7 investigated the micromixing efficiency of viscous media in a Y-type microchannel reactor using the iodide−iodate test reaction as a working system. Nikolas et al.8 experimentally investigated the micromixing characteristics of a spinning disk reactor. Krupa et al.9 adopted the characterization of micromixing in T-jet mixers and defined a characteristic mixing time. A rotating packed bed (RPB) reactor is a novel reactor that takes advantage of the centrifugal force to intensify the processes limited by mass transfer and/or mixing rate.10 RPB reactors have been successfully applied to many chemical processes, such as desorption,11 absorption,12 distillation,13 ozone oxidation,14 etc. In the high-gravity field of a RPB reactor, the liquid is disintegrated to liquid filament, film, and droplets, thus generating a large mass-transfer interfacial area. In addition, it weakens the influence of surface tension, thus enhancing mass transfer and mixing. Compared with other mixing devices, the RPB reactor has a distinct advantage in improving the micromixing efficiency. Chen et al.15 indicated that the micromixing efficiency in a RPB reactor was shown to be higher than that in other reported mixing devices. Yang et al.16 studied the micromixing efficiency in a special designed © 2015 American Chemical Society

RPB reactor that could obtain the liquid samples along the radial direction of the rotor. Yang et al.17 designed a premixed liquid distributor for the liquid−liquid macromixing before the liquids flowed into the rotor. The micromixing efficiency of the RPB reactor with the premixed liquid distributor was better than that of the RPB reactor with nonpremixed liquid distributor. For a RPB reactor, the most important section for mass transfer and chemical reaction is the packing zone. There is an emerging trend toward the packing research in various reactors. Lévêque et al.18 made a detailed investigation of ceramic foam used for a distillation packing material. The hydraulic characteristics of the foam packing were experimentally determined by a gas−liquid counter-current flow. The performance, in terms of pressure drop per unit height and flooding behavior, was quite low, comparing to these classical distillation packing materials. Li et al.19 utilized two detailed three-dimensional geometric models for the CFD calculations of two types of SiC structured packings. Results indicated that these novel SiC packings have much better mass-transfer efficiency than Mellapak packings. Zhang et al.20 measured the hydrodynamic performance and mass-transfer efficiency of nickel foam packing in a distillation column. It was shown that the mass-transfer efficiency of nickel foam packing is 40% higher than that of traditional stainless steel wire mesh packing (SWP) and pressure drop is 20% of the SWPs. Stemmet21 made comparisons between the solid foam packing and other random packings and found that the fractional pressure drop was 10 times lower. Received: Revised: Accepted: Published: 1697

November 6, 2014 December 18, 2014 January 22, 2015 January 22, 2015 DOI: 10.1021/ie504407a Ind. Eng. Chem. Res. 2015, 54, 1697−1702

Research Note

Industrial & Engineering Chemistry Research Micromixing would be affected by the interaction of liquid elements and packing surfaces in a RPB reactor, since liquid elements in the rotor would collide with the packing surface again and again. However, few investigators have studied the influence of surface modification packing on micromixing performance. In this work, we first obtained a surface-modified nickel foam packing (SNP) via a chemical method and then conducted the micromixing experiments in a RPB reactor with the above SNP by using the iodate−iodide reaction system. In order to make comparisons, the micromixing efficiency of a nonmodified nickel foam packing (NNP) and a SWP loaded in the same RPB reactor were also studied by using the same system. The micromixing time (tm) was calculated using an incorporation model, based on experimental results.

2. EXPERIMENTAL SECTION 2.1. Experimental System. The well-known Dushman22,23 reaction between iodide and iodate coupled with a neutralization reaction is one of the parallel competing reactions. The involved reactions are as following: H 2BO3− + H+ ⇔ H3BO3

(instantaneous)

5I− + IO3− + 6H+ ⇔ 3I 2 + 3H 2O I− + I 2 ⇔ I3−

(fast)

(1) (2) (3)

Reaction 1 is instantaneous while reaction 2 is fast, but on the same order of magnitude as the micromixing process. The segregation index (XS), which has been defined as the relative amount of consumed acid, is a criterion for the evaluation of the micromixing quality. The XS has been defined as follows:

Y YST

XS =

Y=

(4)

2(n I2 + n I3−)

YST =

n H+ 0

(5)

6(IO3−)0 + (H 2BO3−)0

6(IO3−)0

Figure 1. Photographs of nickel foam packing: (a) 5 ppi and (b) 50 ppi. [Note: ppi denotes pores per inch.]

(6)

Under perfect mixing conditions, the injected acid is instantaneously dispersed and consumed by borate, according to reaction 1. So XS would be equal to zero. In contrast, when the micromixing time is of the same or larger order of magnitude as the reaction time of reaction 2, the local excess of acid would induce iodine formation and, consequently, I3− production. Therefore, in the case of perfect I3− formation, the value of XS would be equal to one. The analysis and calculation process of eq 4 has been reported in previous literature.7 2.2. Nickel Foam Packing and Its Surface Hydrophobic Treatment. Nickel foam, which has the advantages of high porosity, wide pore size controllable range, superior performance of fluid penetration, and good mechanical performance, is a new type of porous metal material that is used for packings loaded in a RPB reactor. Photographs of the nickel foam packings (Jilin Zhuoer Technology Co., Ltd.) used in this work are shown in Figure 1, and their specifications are given in Table 1.24 There are numerous hydrophobic surface modification methods.25,26 An effective method, developed by Lu et al.,27 was employed for hydrophobic modification of a nickel foam packing. In this methodology, a nickel foam packing was first immersed in the dilute NaOH solution for 30

Table 1. Specification of RPB Reactor item

value

inner diameter of the rotor outer diameter of the rotor axial thickness of packing premixed feed angle feed tube diameter

35 mm 82 mm 19 mm 45° 4 mm

min and then in H2SO4 solution to eliminate surface contaminants and oxides for 20 min; then, it was steeped in a 0.01 mol/L ethanol solution of monoalkyl phosphonic acid at 40 °C for 46 h. The modified packing was taken out, rinsed with deionized water and ethanol thoroughly, and dried in air finally. In order to know the changes of the physical properties of the surface-modified packing, the contact angles of SNP and NNP were measured by a contact angle determinator and the evolution of the nickel surface morphology was investigated via scanning electron microscopy (SEM). Figure 2 shows the water contact angle on the surface of NNP and SNP. It indicated that 1698

DOI: 10.1021/ie504407a Ind. Eng. Chem. Res. 2015, 54, 1697−1702

Research Note

Industrial & Engineering Chemistry Research

Figure 3. SEM photographs of nickel foam packing: (a) NNP and (b) SNP. Figure 2. Photograph of water contact angle for (a) the NNP surface and (b) the SNP surface.

the contact angle of NNP is 108.50°, and it reaches a value of 134.76° when treated hydrophobically. Compared with NNP, the water contact angle of SNP was increased by ∼30°. The packings were cut into two halves, and two thin sheets from each half were obtained for SEM analysis. The NNP has an average hole diameter of 0.51 mm, as described in Table 1, and Figure 3a shows the SEM photograph of its surface. As shown in Figure 3b, the surface of NNP has some holes with an average pore size of ∼0.5 μm. It can be seen that the holes became finer and a structure consisting of many twisted nanoholes appeared after modification. It could be clearly observed that numerous micropores constituted a continuous slipcover over the nickel surface. These binary microstructures are believed to play a crucial role in the hydrophobic performance of the SNP. 2.3. Experimental Procedure. A RPB reactor unit generally consists of the rotor filled with a packing, casing, liquid inlets, a liquid distributor, a liquid outlet, a gas inlet, a gas outlet, and a motor. Figure 4 shows the structure of a RPB reactor for a liquid−liquid reaction used in this work, and specifications of the RPB reactor are shown in Table 2. A premixed distributor, which is shown in Figure 5, was designed for this RPB reactor. The experimental flowchart is displayed in Figure 6. Solutions A and B were prepared first. The concentrations of solution A are shown in Table 3, and solution B is H2SO4 with a certain concentration of 0.12−0.24 mol/L. Different liquids were introduced into the RPB from the corresponding inlets.

Figure 4. Sketch of RPB reactor for liquid−liquid reaction. [Legend: 1, shaft; 2, distributor; 3, shell; 4, inlet of A; 5, inlet of B; 6, packing; and 7, liquid outlet.]

Table 2. Specification of Nickel Foam Packing

1699

item

value/comment

color hole diameter inside diameter outer diameter axial thickness

silver gray 5 ppi (5.08 mm), 50 ppi (0.51 mm) 35 mm 80 mm 19 mm

DOI: 10.1021/ie504407a Ind. Eng. Chem. Res. 2015, 54, 1697−1702

Research Note

Industrial & Engineering Chemistry Research

Figure 7. Comparison of micromixing efficiency in RPB reactor with NNP and SWP.

Figure 5. Y-type distributor.

ppi had been obviously improved. This can be explained by the fact that the structure of multiple holes forces the liquid to be dispersed as smaller elements. The coalescence and dispersion frequency of liquid particles is accelerated, thus effectively enhancing the performance of the micromixing. Since the micromixing efficiency of NNP was better than that of SWP, additional work focused solely on the NNP. 3.2. Comparison of Micromixing Efficiency between NNP and SNP. The comparison of the micromixing efficiency of the RPB reactor with NNP and SNP is shown in Figure 8. It

Figure 6. Schematic diagram of the experimental setup.

Table 3. Concentrations of Reactant reactant

concentration (mol/L)

H3BO3 NaOH KIO3 KI

0.1818 0.0909 0.0023 0.0117

The fluid then could be torn into small liquid elements in the centrifugal field, producing rapid renewal surface area. Samples would be obtained at the liquid outlet of the RPB reactor, and then immediately analyzed by a spectrophotometer. Combined with the measured data, XS can be calculated.7

Figure 8. Comparison of micromixing efficiency of RPB reactor with NNP and SNP.

3. RESULTS AND DISCUSSION 3.1. Comparison of the Micromixing Efficiency of Different Packings. With the purpose of investigating the influence of packing characteristics, three types of packings (SWP, NNP with a pore size of 5 ppi, and NNP with a pore size of 50 ppi), were packed in the rotor of the RPB reactor, which operated with the rotational speed varying from 200 rpm to 1600 rpm. [Note: ppi denotes pores per inch.] Figure 7 presents the effect of rotational speed on the XS and shows that higher speed resulted in a better micromixing efficiency, as expected. Comparisons of the SWP and two types of NNP indicated that the micromixing efficiency of the RPB reactor with NNP was better than that with SWP. Moreover, the micromixing efficiency of the RPB reactor with the NNP of 50

can be seen that the micromixing efficiency of the RPB reactor with SNP is better than that with NNP. The possible reasons are as follows. Since the surface was modified, the water contact angle increased significantly with the change of surface characteristics, implying that a water droplet hardly sticks to the surface but easily rolls off. In this situation, more liquid interfacial area would be generated. This caused a decrease of the contact area between the liquid droplets and surfacemodified packing and resulted in a large liquid−liquid contact area. In addition, the structure, consisting of many twisted nanoholes, led to the increase of the packing’s surface hydrophobicity. The liquid was cut by these nanoholes into smaller microelements. It could accelerate the film update frequency and the liquid elements’ collision probability was increased, thus improving the efficiency of liquid−liquid mixing. 1700

DOI: 10.1021/ie504407a Ind. Eng. Chem. Res. 2015, 54, 1697−1702

Research Note

Industrial & Engineering Chemistry Research 3.3. Effects of Different Operating Conditions on Micromixing Efficiency. The functional trends of XS with the operation parameters, such as liquid volumetric rate and volumetric ratio, are presented in Figure 9.

uneven mixing of reactants, making the reaction selectivity and yield decrease. For tm < tR, the reaction is conducted under uniform micromixing conditions, resulting in high selectivity and an easy way to scale up. Especially for fast reactions, tm of the reactor is very important and the ideal reactor should have a better micromixing efficiency, which requires tm < tR. The micromixing time was calculated by using an incorporation model,28,29 based on experimental results. Figure 10a presents the tm of three types of packings: SWP, NNP with

Figure 9. Effect of (a) volumetric flow rate and [H+] (b) volumetric flow ratio on micromixing efficiency.

Figure 9a shows that XS decreased as the liquid volumetric rate increased, which means that a better micromixing efficiency was achieved at higher liquid volumetric rate; while XS increased sharply when the acid concentration ([H+]) increased from 0.16 mol/L to 0.24 mol/L. A higher liquid relative velocity between liquid elements and the rotation packing was induced by increasing liquid volumetric rate, while higher [H+] shifts the reaction equilibrium toward I2, causing a larger XS. Figure 9b shows how XS increased as the volumetric ratio (R) is increased, which indicated that the micromixing efficiency decreased with increasing R when the feed stoichiometric ratio was fixed. With increasing R, the total volumetric rate was reduced accordingly, resulting in the decrease of impact strength of the liquid in the packing zone in the rotor of the RPB reactor. At the same time, [H+] was also increased. All of the above-mentioned reasons would lead to the increase of XS, i.e., an attenuation of the micromixing performance. 3.4. Micromixing Time. Reaction processes have two concepts of time, which called micromixing time (tm) and chemical reaction characteristic time (tR). For tm > tR, it is illustrated that the reaction is carried out under a condition of

Figure 10. Comparison of micromixing time of (a) different NNP and SWP and (b) different NNP and SNP.

a pore size of 5 ppi, and NNP with a pore size of 50 ppi. Figure 10b shows the comparison of tm between NNP and SNP. The range of tm values for SNP is varied from 3.28 ms to 4.63 ms. Compared with NNP or SWP, the minimum value of the tm value of the SNP (3.28 ms) is less than that of the NNP (5.68 ms) or SWP (8.67 ms). It means that nickel foam packing has a better micromixing performance than the traditional packing and the surface modification achieves the effect of promotion.

4. CONCLUSIONS In this work, the micromixing efficiency of a RPB reactor with SWP, NNP, and SNP was studied. Experimental results 1701

DOI: 10.1021/ie504407a Ind. Eng. Chem. Res. 2015, 54, 1697−1702

Research Note

Industrial & Engineering Chemistry Research

(6) Wang, Q.; Wang, J. X.; Yu, W.; Shao, L.; Chen, J. F. Experimental Study of Micromixing Efficiency in Channel Reactor. J. Beijing Univ. Chem. Technol. 2009, 36, 1. (7) Yang, K.; Chu, G. W.; Shao, L.; Xiang, Y.; Zhang, L. L.; Chen, J. F. Micromixing Efficiency of Viscous Media in Micro-channel Reactor. Chin. J. Chem. Eng. 2009, 17, 546. (8) Jacobsen, N. C.; Hinrichsen, O. Micromixing Efficiency of a Spinning Disk Reactor. Ind. Eng. Chem. Res. 2012, 51, 11643. (9) Krupa, K.; Nunes, M. I.; Santos, R. J.; Bourne, J. R. Characterization of Micromixing in T-jet Mixers. Chem. Eng. Sci. 2014, 111, 48. (10) Liu, H. S.; Lin, C. C.; Wu, S. C.; Hsu, H. W. Characteristics of a Rotating Packed Bed. Ind. Eng. Chem. Res. 1996, 35, 3590. (11) Tan, C. S.; Lee, P. L. Supercritical CO2 Desorption of Activated Carbon Loaded with 2,2,3,3-Tetrafluoro-1-Propanol in a Rotating Packed Bed. Environ. Sci. Technol. 2008, 42, 2150. (12) Lin, C. C.; Liu, W. T.; Tan, C. S. Removal of Carbon Dioxide by Absorption in a Rotating Packed Bed. Ind. Eng. Chem. Res. 2003, 42, 2381. (13) Kelleher, Y.; Fair, J. R. Distillation Studies in a High-Gravity Contactor. Ind. Eng. Chem. Res. 1996, 35, 4646. (14) Lin, C. C.; Liu, W. T. Ozone Oxidation in a Rotating Packed Bed. J. Chem. Technol. Biotechnol. 2003, 78, 138. (15) Chen, Y. S.; Liu, H. S.; Lin, C. C.; Liu, W. Z. Micromixing in a RPB. J. Chem. Eng. Jpn. 2004, 37, 1122. (16) Yang, H. J.; Chu, G. W.; Zhang, J. W.; Shen, Z. G.; Chen, J. F. Micromixing Efficiency in a Rotating Packed Bed: Experiments and Simulation. Ind. Eng. Chem. Res. 2005, 44, 7730. (17) Yang, K.; Chu, G. W.; Shao, L.; Luo, Y.; Chen, J. F. Micromixing Efficiency of Rotating Packed Bed with Premixed Liquid Distributor. Chem. Eng. J. 2009, 153, 222. (18) Julien, L.; David, R.; Michel, P.; Michel, M. Hydrodynamic and Mass Transfer Efficiency of Ceramic Foam Packing Applied to Distillation. Chem. Eng. Sci. 2009, 64, 2607. (19) Li, X. G.; Gao, G. H.; Zhang, L. H.; Sui, H.; Li, H.; Gao, X.; Yang, Z. M. Multiscale Simulation and Experimental Study of Novel SiC Structured Packings. Ind. Eng. Chem. Res. 2012, 51, 915. (20) Zhang, W. J.; Zhang, B.; Shi, Z. Q. Study on Hydrodynamic Performance and Mass Transfer Efficiency of Nickel Foam Packing. Procedia Eng. 2011, 18, 271. (21) Stemmet, C. P. Gas−Liquid Solid Foam Reactors: Hydrodynamics and Mass Transfer; Ph.D. dissertation. Eindhoven University of Technology, Eindhoven, The Netherlands, 2008. (22) Villermaux, J.; Falk, L.; Fournier, M. C.; Detrez, C. Use of Parallel Competing Reactions to Characterize Micromixing Efficiency. AIChE Symp. Ser. 1992, 286, 6. (23) Dushman, S. The Rate of the Reaction between Iodic and Hydriodic Acids. J. Phys. Chem. 1904, 8, 453. (24) Zou, H. K.; Chu, G. W.; Zhao, H. Higee Reactor Enhancement Technology for Environmental Applications: From Theory to Industrialization. Chin. Sci: Chem. 2014, 44, 1413. (25) Tian, F. F.; Hu, A. M.; Li, M. The Electrochemical Deposition of Super Hydrophobic Ni Thin Films. J. Fudan Univ., Nat. Sci. 2012, 51, 163. (26) Gao, N. W.; Li, M.; Jing, W. H.; Fan, Y. Q.; Xu, N. P. Improving the Filtration Performance of ZrO2 Membrane in Non-polar Organic Solvents by Surface Hydrophobic Modification. J. Membr. Sci. 2011, 375, 276. (27) Li, M.; Xu, J.; Lu, Q. Creating Superhydrophobic Surfaces with Flowery Structures on Nickel Substrates through a Wet-ChemicalProcess. J. Mater. Chem. 2007, 17, 4772. (28) Fournier, M. C.; Falk, L.; Villermaux, J. A New Parallel Competing Reaction System for Assessing Micromixing Efficiency Determination of Micromixing Time by a Simple Mixing Model. Chem. Eng. Sci. 1996, 51, 5187. (29) Bourne, J. R. Comments on the Iodide/Iodate Method for Characterising Micromixing. Chem. Eng. J. 2008, 140, 638.

demonstrate that the surface modification of the nickel foam packing gives a great contribution to improving the micromixing efficiency. This path provides a better selectivity and product for those reactions affected or limited by micromixing. It also shows that XS decreased with an increase of rotational speed and liquid flow rate; while the micromixing efficiency is diminished with an increase of acid concentration and volumetric ratio. The micromixing time was calculated using an incorporation model. The minimum tm value of modified nickel foam packing (3.28 ms) is less than that of nonmodified packing (5.68 ms) or stainless steel mesh packing (8.67 ms). The RPB reactor with SNP has potential applicability in chemical reaction of hydrophobic reactants.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 64446466. Fax: +86 10 64434784. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21406008 and U1462127).

■

NOTATIONS SNP = surface-modified nickel foam packing NNP = nonmodified nickel foam packing SWP = traditional stainless steel wire mesh packing [H+] = initial concentration of H+ ion (mol L−1) A = solution containing H2BO3−, I−, and IO3− B = solution of sulfuric acid VA = liquid flow rate of solution A (mL min−1) VB = liquid flow rate of solution B (mL min−1) R = volumetric ratio (dimensionless) nI2 = mole number of produced I2 (mol) nI3− = mole number of I3− (mol) nH+0 = mole number of initial H+ (mol) [IO3−]0 = initial concentration of IO3− (mol L−1) (H2BO3−)0 = initial concentration of H2BO3− (mol L−1) tR = reaction time (s) tm = micromixing time (s) XS = segregation index (dimensionless) Y = selectivity of iodide (dimensionless) YST = selectivity of iodide in the case of total segregation (dimensionless)

■

REFERENCES

(1) Baldyga, J.; Podgorska, W.; Pohorecki, R. Mixing-Precipitation Model with Application to Double Feed Semibatch Precipitation. Chem. Eng. Sci. 1995, 50, 1281. (2) Bourne, J. R.; Kozicki, F.; Rys, P. Mixing and Fast Chemical Reaction Test Reactions to Determine Segregation. Chem. Eng. Sci. 1981, 36, 1643. (3) Tolgyesi, W. S. Relative Reactivity of Toluene−Benzene in Nitronium Tetrafluoroborate Nitration. Can. J. Chem. Eng. 1965, 43, 343. (4) Pantelides, C. C.; Erickson, W. D.; Longwell, J. P.; Sarofim, A. F. Use of Relative Reaction Rates of CO and H2 as a Measure of Micromixing in Combustion Systems. Chem. Eng. Sci. 1985, 40, 375. (5) Bourne, J. R. Mixing on the Molecular Scale. Chem. Eng. Sci. 1983, 38, 5. 1702

DOI: 10.1021/ie504407a Ind. Eng. Chem. Res. 2015, 54, 1697−1702