Membrane Based GasâLiquid Dispersion Integrated in Fixed-Bed

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Membrane Based Gas−Liquid Dispersion Integrated in Fixed-Bed Reactor: A Highly Efficient Technology for Heterogeneous Catalysis Miaomiao Hou, Hong Jiang,* Yefei Liu, Changlin Chen, Weihong Xing, and Rizhi Chen* State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210009, China

ABSTRACT: A fixed-bed reactor with an external multichannel ceramic membrane for generation of gas−liquid microdispersion system is developed for heterogeneous catalysis. The reactor performance is evaluated by using glycerol hydrogenolysis to 1,2-propanediol over Cu−ZnO as the model reaction. Results highlight that the introduction of membrane with suitable pore size and channels is highly efficient for glycerol hydrogenolysis. Compared to the traditional feeding, the use of ceramic membrane allows a uniform microbubble-in-liquid dispersion system with higher gas holdup, and as a result, the glycerol conversion significantly increases from 84.9% to 97.4% with only half of the hydrogen consumption. The glycerol conversion of 92% and the 1,2-propanediol selectivity of 93% can be achieved in a 52 h continuous run. Our study demonstrates the advantages of the membrane dispersion based fixed-bed reactor in a continuous heterogeneous catalytic reaction.

1. INTRODUCTION Heterogeneous catalysis has been considered as a key technology for a sustainable alternative for chemical, pharmaceutical, cosmetic, etc. industries,1−3 which strongly depends on the highly efficient mass transfer. However, the present reactors exhibit severe mass-transfer limitations when they are applied to heterogeneously catalyzed gas−liquid reactions.4 During the past decades, there has been growing interest in the development of new technologies to intensify gas−liquid mass transfer.4,5 Microdispersion technology offers considerable attraction due to its high mass-transfer efficiency and good controllability,6−8 which has been used to improve the efficiency of multiphase reactions.9−11 Membrane dispersion technology has been reported as a promising microdispersion technology for intensifying the mass-transfer performance and reaction process in liquid− liquid8,12 and gas−liquid13,14 heterogeneous systems as well as liquid−liquid homogeneous systems.15 Generally, one phase can be uniformly dispersed into another phase in microscale size through a porous membrane. To date, membrane dispersion technology has been applied in many aspects, such as extraction separation, emulsion preparation, preparation of nanoparticles, and heterogeneous catalytic reactions.16−19 For example, Tan et al.5 applied a flat-sheet stainless steel membrane with a pore size of 5 μm to disperse © XXXX American Chemical Society

hydrochloride in the hydrogenation of ethylanthraquinone to produce H2O2. The microbubbles with the size less than 100 μm were generated, and the mass-transfer coefficient was more than 2 orders of magnitude larger than the values in normal gas−liquid trickle-bed reactors. The ethylanthraquinone conversion as much as 35% was obtained in less than 3.5 s. In our previous works,20−23 a slurry reactor with a single tubular ceramic membrane as the dispersion medium of reactants has been successfully developed for gas−liquid−solid and liquid−liquid−solid heterogeneous catalytic reactions. In these studies, the ceramic membranes could produce uniform distributed reactants in microscale size; meanwhile, the controlled addition of reactants inhibited the side reactions and thereby enhanced the product selectivity. Although these studies showed excellent mass-transfer efficiency and reaction performance with the assistance of membrane dispersion, deep insights in the effects of membrane structure and operation conditions on the heterogeneous catalysis still keep a great challenge. Received: Revised: Accepted: Published: A

October 9, 2017 December 13, 2017 December 13, 2017 December 13, 2017 DOI: 10.1021/acs.iecr.7b04184 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Membrane dispersion enhanced glycerol hydrogenolysis setup.

Table 1. Ceramic Membrane Specifications

Recently, hydrogenolysis of glycerol has been identified as a preferably alternative route to produce 1,2-propanediol, which can promote the biodiesel industry in accordance with the current sustainable development strategy.24−30 To achieve better performance, many studies primarily focus on the development of novel catalysts, such as the preparation methods,25 selection of support,31 catalyst active ingredient components,32 and condition optimization.33 However, there

are few investigations on the reaction process as well as the gas−liquid transfer. Glycerol hydrogenolysis to 1,2-propanediol is a typical gas−liquid−solid multiphase reaction, in which efficient gas−liquid mixing and high mass-transfer performance are urgently demanded. The microdispersion of hydrogen will possibly lead to further intensification of glycerol hydrogenolysis. B

DOI: 10.1021/acs.iecr.7b04184 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research In this work, a continuous fixed-bed reactor system was developed by using multichannel ceramic membranes as the dispersion medium for heterogeneous catalysis. The gas−liquid microdispersion system flowed through the reactor to realize the heterogeneous catalysis. The present work aimed at the better understanding of the effects of membrane structure and operation conditions on the microscale distribution and the intensification of heterogeneous catalysis. Liquid phase hydrogenolysis of glycerol to 1,2-propanediol over a Cu−ZnO catalyst was chosen as a model reaction. The influences of membrane microstructure and operation conditions on the glycerol hydrogenolysis were investigated in detail. For comparison, the hydrogen was fed directly into the fixed-bed reactor. A continuous long-time operation was performed to estimate the feasibility of the membrane dispersion based fixedbed reactor for the hydrogenolysis of glycerol to 1,2propanediol.

for 10 h. At the reactor outlet, the gas−liquid mixture were cooled and recovered in the gas−liquid separator. The products were analyzed by using a gas chromatograph (Shimadzu, GC-2014), and 1,4-butanediol was used as an internal standard. 1,2-Propanediol was the target product for the hydrogenolysis of glycerol, and acetol, n-propanol, ethylene glycol, and methanol were the byproducts. The glycerol conversion was defined as the ratio of the C-based mole of all products to initially added glycerol, whereas the selectivity of 1,2-propanediol was described as the C-based mole ratio of the 1,2-propanediol in all products.33 In fact, to ensure the accuracy of the results, the glycerol hydrogenolysis was repeated at least two times. We even did four or five repeated experiments of a point where a particular trend occurred. The data in the figures are the average values calculated according to the repeated experiments. 2.3. Measurement of Bubbles and Gas Holdup. The size distribution and gas holdup of microbubbles were measured using the microbubble generation device built by our group.34 In these tests, oxygen (99.99%) was used as the model gas phase regarding the security and convenience, and aqueous glycerol solution (40 wt %) was applied as the liquid phase. The experimental procedure of bubble generation was the same as section 2.2, except that a cylindrical plexiglas pipe was fixed on top of the membrane distributor to observe the bubbles, and all the experiments were carried out under atmospheric pressure and ambient temperature. The bubble size and its distribution were measured using a high speed photographic technique (Phantom Miro C110, Vision Research Inc.) with a maximum speed of 800 fps and 1280 × 1024 pixels. In this work, the middle position of the plexiglas cylinder pipe was selected as the point for measuring the bubble size distribution. In each measurement, five groups of 300 bubbles were evaluated from the images, and then the bubble diameter distribution was obtained by calculating the average value. The overall gas holdup (εg) in the bubble column was analyzed by the pressure drop method using two pressure sensors (Nanjing Hongmu Technology Co. Ltd.) with a measurement accuracy of 0.5%, and the vertical distance between the two pressure sensors was 0.4 m. The gas holdup was calculated on the basis of the following equation:

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. A schematic view of the experimental setup is shown in Figure 1. The membrane dispersion module was made of stainless steel. The porous ceramic membranes (Jiangsu Jiuwu High-Tech Co., Ltd., China) with 500 mm length and 30 mm outer diameter were used as the dispersion medium. Different pore sizes and channels of ceramic membranes were used to examine the effect of ceramic membrane structure on the reaction performance, as shown in Table 1. The asymmetric ceramic membrane was composed of a fine membrane layer of α-Al2O3 (pore size of 50, 200, and 500 nm, respectively) on the inside of a tubular α-Al2O3 porous support with an average pore size of 3000 nm. A Szweico 2ZB-2L20A double-plunger micropump purchased from Beijing Spacecrafts, China, was used to feed the aqueous glycerol solution, and a D08-1F mass flow controller provided by Beijing Sevenstar Huachuang Electronics Co., Ltd., China, was applied to deliver the gas. The fixed-bed reactor consisted of a stainless steel tube (length, 600 mm; outer diameter, 25 mm; inner diameter, 20 mm) and a thermocouple with three-stage temperature control. The CuO−ZnO particles (35 mL, 10−14 mesh) were placed between two layers of quartz sands. A gas−liquid separator was located at the reactor outlet where gas is separated from the liquid. 2.2. Hydrogenolysis Experiment. The CuO−ZnO catalyst was prepared by a coprecipitation method according to our previous work24 and pulverized into particles with 10− 14 mesh. Before reaction, the CuO−ZnO sample was activated in a 300 mL·min−1 H2 flow from ambient to 250 °C at the rate of 2 °C·min−1 at atmospheric pressure for 3 h, followed by natural cooling in the N2 atmosphere to room temperature. The activated catalyst was marked as Cu−ZnO. Then, the aqueous glycerol solution (40 wt %) was continuously pumped into the membrane channels and the pure hydrogen (99.99%) was delivered into the annular space between the module and the ceramic membrane at a given flow rate, and the reactor was heated to the target temperature (200 °C, 2 °C·min−1). Under the pressure, a microdispersion system was generated when the hydrogen passed through the ceramic membrane into the liquid phase, which flowed out of the membrane dispersion module and into the fixed-bed reactor, and the hydrogenolysis reaction started. The hydrogenolysis reaction was performed

εg = 1 −

Δp g Δhρl

(1)

where ρl is the liquid density, Δp is the pressure drop between the two pressure ports, Δh is the vertical distance between two pressure sensors, and g is the gravitational acceleration. 2.4. Characterization of Ceramic Membrane and Cu− ZnO Catalyst. The microstructure of the surface and crosssection of porous ceramic membrane was examined by field emission scanning electron microscope (FESEM, Hitachi S4800II). The crystal structures of the fresh and recovered Cu− ZnO catalyst powders were characterized by X-ray diffraction (XRD) conducted with a Rigaku SmartLab X-ray diffractometer using Cu Kα radiation operating at 40 kV and 15 mA. The samples were scanned in a 2θ range of 20−80° with 2θ step-scan interval of 0.02°.

3. RESULTS AND DISCUSSION The formation of the microdispersion system is mainly dependent on the gas dispersion through the micropores of membrane to the liquid reactants; hence, the structure of the C


Article


Figure 4. Effect of pore size of porous ceramic membrane on the gas holdup.

Figure 2. Effect of porous ceramic membrane pore size and hydrogen/glycerol volume ratio on the glycerol hydrogenolysis to 1,2-propanediol. Reaction conditions: reaction temperature 200 °C, hydrogen pressure 4.0 MPa, LHSV 1.0 h−1.

process of glycerol hydrogenolysis over Cu−ZnO in the fixedbed reactor intensified by multichannel ceramic membrane was studied in detail by investigating the effects of membrane structure and operation conditions including hydrogen/ glycerol volume ratio, liquid hourly space velocity (LHSV) and hydrogen pressure on the glycerol conversion and 1,2propanediol selectivity.

ceramic membrane, i.e., pore size and channel number, probably brings about a significant effect on the membrane dispersion performance. At the same time, some operation conditions such as gas velocity, liquid velocity, and pressure can affect the gas−liquid mixing.35−37 Therefore, the catalytic

Figure 3. Bubble photographs captured and size distributions at different pore sizes of porous ceramic membranes: (a and a′) 50 nm; (b and b′) 200 nm; (c and c′) 500 nm; (d and d′) 3000 nm. D


Article


instance, for the ceramic membrane with a pore size of 200 nm, as the hydrogen/glycerol volume ratio is increased from 200 to 300, the conversion of glycerol is improved from 91.5% to 97.4%, and a further increase in hydrogen/glycerol volume ratio does not obviously enhance the overall conversion. The same trend is found for the other three membranes. In this study, the change of hydrogen/glycerol volume ratio is realized by changing the flow rate of hydrogen with the fixed glycerol aqueous solution. Therefore, the increase in hydrogen/glycerol volume ratio results in the increase in hydrogen feed, and so the amount of hydrogen dissolved in the glycerol aqueous solution first increases and then reaches saturation, which is the main reason for the change of the glycerol conversion with the hydrogen/glycerol volume ratio. The results in Figure 2 hightlight that the membrane pore size can significantly affect the glycerol conversion. The glycerol conversion is improved when the membrane pore size decreases from 3000 to 200 nm. Interestingly, with further reducing the membrane pore size, the conversion was decreased. For example, when the hydrogen/glycerol volume ratio is 300, the glycerol conversion for the ceramic membranes with pore sizes of 50, 200, 500, and 3000 nm are 89.6%, 97.4%, 92.9%, and 86.9%, respectively, which indicates the ceramic membrane with too large or too small pore size is detrimental to the glycerol hydrogenolysis. It is generally acknowledged that the decrease in membrane pore size enhances the micromixing performance, and then increases the conversion, yield, and quality of target products.38 To explain our experimental phenomena, the effect of membrane pore size on the bubble size and gas holdup was investigated, because the two parameters determine the dispersion quality and gas−liquid mass-transfer efficiency.34 The bubble photographs and the corresponding bubble size distributions, and the gas holdup obtained using membranes with different pore sizes are displayed in Figures 3 and 4, respectively. It is noted that the bubble size decresases with reducing the membrane pore size from 3000 to 200 nm. As expected, the gas holdup can significantly rise, leading to the efficient interphase mass transfer, thereby higher glycerol conversion (Figure 2). However, as the membrane pore size is further reduced to 50 nm, the bubble size almost remains stable, and the gas holdup significantly decreases and is evenly lower that that of the membrane with a pore size of 500 nm, resulting in a lower glycerol conversion (Figure 2). A possible explaination is as follows. When a membrane with a very small pore size (50 nm) was used as a gas distributor, smaller bubbles can be formed on the membrane surface, but only when the detaching force becomes larger than the holding force due to the bubble expansion, the formed smaller bubbles can detach from the pore opening and then enter into the liquid phase.39 However, Peng and Williams suggested that with very small droplets the laminar sublayer thickness might start to limit the effect of cross-flow velocity.40 That is, near the membrane surface the flow velocity of the water phase is relatively small to the internal water phase. Thus, it is more difficult for the smaller bubbles to detach from the membrane surface under the same shear force, and the unstripped bubbles get together with adjacent bubbles, leading to a decrease in gas holdup and no obvious decrease in bubble size (Figure 3a), thereby lower glycerol conversion. If larger cross-flow velocity and superficial gas velocity are applied, the formed smaller bubbles might detach from the membrane surface, thereby a higher gas holdup, which will be investigated in further work.

Figure 5. Effect of porous ceramic membrane pore size and liquid hourly space velocity on the glycerol hydrogenolysis to 1,2propanediol. Reaction conditions: reaction temperature 200 °C, hydrogen pressure 4.0 MPa, H2/glycerol solution 300 (volume ratio).

Figure 6. Effect of porous ceramic membrane pore size and pressure on the glycerol hydrogenolysis to 1,2-propanediol. Reaction conditions: reaction temperature 200 °C, H2/glycerol solution 300 (volume ratio), LHSV 1.0 h−‑1.

Figure 7. Effect of porous ceramic membrane channel number on the glycerol hydrogenolysis to 1,2-propanediol. Reaction conditions: reaction temperature 200 °C, hydrogen pressure 4.0 MPa, H2/ glycerol solution 300 (volume ratio), LHSV 1.0 h−1.

3.1. Effect of Ceramic Membrane Pore Size and Operating Conditions. 3.1.1. Hydrogen/Glycerol Volume Ratio. The effect of the hydrogen/glycerol volume ratio on the glycerol hydrogenolysis to 1,2-propanediol was investigated using 19-channel porous ceramic membranes with four different pore sizes (50, 200, 500, and 3000 nm) as the gas distributor, and the results are shown in Figure 2. The 1,2propanediol selectivity remains stable irrespective of the hydrogen/glycerol volume ratio and membrane pore size, whereas the glycerol conversion varies significantly with the hydrogen/glycerol volume ratio and membrane pore size. For E


Article


Figure 8. Bubble photographs captured and size distributions at different channel numbers of porous ceramic membranes: (a and a′) 7; (b and b′) 19; (c and c′) 37.

significantly varies as the LHSV and membrane pore size change. For the ceramic membrane with a pore size of 500 or 3000 nm, the glycerol conversion shows a noticeable rise as the LHSV reduces from 2.0 to 1 h−1, and then a slight increase in conversion is observed with further decreasing the LHSV to 0.5 h−1. For the ceramic membrane with a pore size of 200 and 50 nm, when the LHSV is between 1.0 and 2.0 h−1, the glycerol conversion exhibits a significant rise with decreasing LHSV, but as the LHSV is in the range of 0.5 and 1.0 h−1, the glycerol conversion shows a slight decrease with the decrease in LHSV. The decrease in LHSV means a rise of residence time of the microdispersion system over the Cu−ZnO catalyst, so the glycerol hydrogenolysis is more sufficient, resulting in the enhancement of glycerol conversion. Hence, it is generally accepted that the smaller LHSV is favorable for the hydrogenolysis reaction.41−43 But for the membrane dispersion intensified fixed-bed reactor, an optimum LHSV exists. In this study, the hydrogen/glycerol volume ratio is fixed, so both the liquid cross-flow velocity and the gas velocity decrease synchronously to realize the decrease in LHSV. In the dispersion process intensified by the ceramic membrane, these two parameters play important roles in the micromixing.44,45 Lower liquid cross-flow velocity and gas velocity are not conducive to the formation of uniform microbubbles and the improvement of gas holdup,45,46 which is the main reason for the smaller rise of the glycerol conversion with the decrease in LHSV for the membrane with a pore size of 500 or 3000 nm (Figure 5). As the membrane pore size is much

Figure 9. Effect of channel number of porous ceramic membrane on the gas holdup.

The results suggest that the membrane with a very small pore size is not in favor of the formation of microbubbles and the heterogeneous catalysis. A suitable membrane pore size is necessary for the heterogeneous catalysis. Therefore, the ceramic membrane with a pore size of 200 nm and the hydrogen/glycerol volume ratio of 300 are chosen for the subsequent experiments due to a higher conversion of glycerol is achieved with less hydrogen consumption. 3.1.2. Liquid Hourly Space Velocity. Figure 5 presents the effects of LHSV and membrane pore size on the glycerol conversion and 1,2-propanediol selectivity. The selectivity of 1,2-propanediol is also not affected by the LHSV and membrane pore size. However, the glycerol conversion F


Article


Figure 10. (a) Effect of feeding mode on the hydrogenolysis of glycerol. (b) Bubble photographs for the traditional feeding. Reaction conditions for (a): reaction temperature 200 °C, hydrogen pressure 4.0 MPa, LHSV 1.0 h−1. Ceramic membrane specifications: 19 channel, 200 nm.

Table 2. Comparison of Glycerol Conversion and 1,2-Propanediol Selectivity in Various Reactors reactor

catalyst

reaction conditions

hydrogen/ glycerol (mL/mL)

conversion (%)

selectivity (%)

ref

membrane dispersion based fixed-bed reactor fixed-bed reactor

Cu/ZnO

catalyst 35 mL, 40 wt % glycerol solution, 200 °C, 4.0 MPa, LHSV 1.0 h−1

300

97.8

92.2

this work

Cu/ZnO

600

84.9

92.5

this work

slurry-bed reactor

Cu−ZrO2−MgO

62

97

Rekha et al.51

fixed-bed reactor

Cu/ZnO

85.7

82.3

Zheng et al.56

fixed-bed reactor

Ni−Cu−SiO2

fixed-bed reactor

Ni−Ag/γ-Al2O3

slurry-bed reactor

Cu/Al2O3

slurry-bed reactor

Cu/SiO2

slurry-bed reactor

Pt/Nb2O5

catalyst 35 mL, 40 wt % glycerol solution, 200 °C, 4.0 MPa, LHSV 1.0 h−1 catalyst 0.6 g, 20 wt % glycerol solution, 180 °C, 4.0 MPa, stirring speed 500 rpm, reaction time 8h catalyst 0.10 g, 20 wt % glycerol solution, 250 °C, 2.0 MPa, WHSV 12 h−1 catalyst 1.0 g, 80 wt % glycerol solution, 220 °C, 3.0 MPa, WHSV 0.5 h−1 catalyst 0.5 g, 20 wt % glycerol solution, 200 °C, atmospheric pressure, WHSV 2.01 h−1 80 wt % glycerol solution, 200 °C, 4.0 MPa, stirring speed 500 rpm, reaction time 24 h catalyst 0.35 g, 240 °C, 8.0 MPa, weight ratio of catalyst/glycerol 0.006, stirring speed 1000 rpm, reaction time 5 h catalyst 0.75 g, 20 wt % aqueous glycerol solution, 140 °C, 5.0 MPa, stirring speed 800 rpm, reaction time 10 h

Figure 11. Stability of glycerol hydrogenolysis with membrane dispersion and without membrane. Reaction conditions: reaction temperature 200 °C, hydrogen pressure 4.0 MPa, H2/glycerol solution 300 (volume ratio), LHSV 1.0 h−1. Ceramic membrane specifications: 19 channel, 200 nm.

4900 2000

100

92

Lee et al.57

3600

80

58

Rekha et al.58

75.7

95.8

Wolosiak-Hnat et al.59

51.9

96.6

Vasiliadou et al.60

78

98

Rodrigues et al.61

Figure 12. XRD patterns of Cu−ZnO catalysts: (a) fresh; (b) used with membrane; (c) used without membrane.

Table 3. Crystallite Size of Catalysts Calculated from XRD Diffractions

smaller (50 or 200 nm), smaller bubbles can be produced (Figure 3), and then a larger shear force is needed for the smaller bubbles to move from the membrane surface into the liquid phase.39 Furthermore, for the smaller bubbles, the effect of cross-flow velocity might be limited by the laminar sublayer thickness, and larger shear force can alleviate this limitation.40 G

catalysts

Cu (nm)

ZnO (nm)

fresh used with membrane used without membrane

11.6 31.5 32.7

6.2 23.6 23.0


Article


Figure 13. FESEM images of the surface and cross sections of fresh membrane (a, c) and used membrane (b, d).

Thus, as LHSV is lower, the provided shear force is not enough to completely make the smaller bubbles detach from the membrane surface into the liquid phase, leading to less amount of dissolved hydrogen in the glycerol solution, thereby lower glycerol conversion (Figure 5). According to the above results, the ceramic membrane with a pore size of 200 nm and a LHSV of 1.0 h−1 are the optimum operating conditions for the membrane dispersion to intensify the glycerol hydrogenolysis to 1,2-propanediol. 3.1.3. Reaction Pressure. Figure 6 illustrates the variation of glycerol conversion and 1,2-propanediol selectivity under different hydrogen pressures and membrane pore sizes. The 1,2-propanediol selectivity exhibits little dependence on the hydrogen pressure and membrane pore size. The increase in hydrogen pressure provides a positive effect on the glycerol conversion. For each ceramic membrane in this experiment, the glycerol conversion increases almost linearly as the hydrogen pressure increases from 2 to 4 MPa and then tends to remain constant. This is possible because the dissolution of hydrogen increases with the hydrogen pressure until it reaches saturation at a certain hydrogen flow.47 The highest conversions is also obtained when the porous ceramic membrane with a pore size of 200 nm is applied in the fixedbed reactor under different hydrogen pressures, further indicating that a suitable membrane pore size can take some responsibility for the process intensification. According to the above discussion, a hydrogen pressure of 4 MPa and a membrane pore size of 200 nm are selected. 3.2. Effect of Ceramic Membrane Channel Number. Figure 7 gives the effect of ceramic membrane channel number on the hydrogenolysis of glycerol. As can be seen, the selectivity of 1,2-propanediol remains unchanged, indicating that the number of the ceramic membrane channel has no influence on the 1,2-propanediol selectivity. However, the highest glycerol conversion is achieved for the 19-channel porous ceramic membrane. A cold model experiment is also conducted to explore the reasonable reason for the phenomenon. As can be seen from the bubble photograph and bubble size distribution in Figure 8, the bubble sizes are

basically the same when different channel numbers of porous ceramic membranes with the same pore size are applied, indicating that the bubble size is independent of the channel number. It also demonstrates that the main factor affecting the bubble size is the membrane pore size (Figure 3). By comparing the gas holdup at different numbers of channels (Figure 9), one can find that the gas holdup first increases and then decreases with increasing channel number, which can be explained by the following facts. Under the conditions of the same outer diameter and length of the ceramic membrane, the membrane area increases as the number of membrane channels rises (Table 1). A higher membrane area is in favor of the formation of more microbubbles, leading to the increase in gas holdup. But when the number of channels is much higher, like 37, the layout of the channel increases, as shown in Table 1. The gas flows through the membrane pores into the liquid across the channels under the transmembrane pressure, so the pressure gradient exists in the radial direction, and the corresponding gas distribution also has a concentration gradient. Thus, as the layout of the channel increases, the distribution of gas in the inner channels decreases, resulting in a decrease in the effective utilization of the channels,48 thereby lowering the gas holdup. However, under the same hydrogen flow, as the channel number rises, the cross-section area of the channels increases, and the corresponding superficial gas velocity reduces. The works of Han et al.34 and Deng et al.45 suggested that there is a positive correlation between the superficial gas velocity and gas holdup. Therefore, with increasing channel number, the gas holdup will decrease due to the reduction in superficial gas velocity. The trend of gas holdup is consistent with that of glycerol conversion (Figures 7 and 9), which shows that the change of glycerol conversion with membrane channel is related to the gas holdup. Considering the results, a 19-channel ceramic membrane is suitable for the hydrogenolysis of glycerol. As discussed above, the membrane structure and reaction conditions almost have no influence on the 1,2-propanediol selectivity, which might be related to the reaction mechanism. It has been reported that the glycerol hydrogenolysis over Cu− H


Article

Industrial & Engineering Chemistry Research ZnO catalyst follows a two-step reaction mechanism, i.e., first glycerol dehydration to acetol on acidic ZnO surface, followed by hydrogenation on Cu surface.33,49 Thus, the catalysts other than operation conditions may play a key role in determining the 1,2-propanediol selectivity. Similar phenomena also have been reported on the glycerol hydrogenolysis over Cu-based catalysts.33,50,51 3.3. Comparison of Membrane Dispersion with Traditional Feeding. To further evaluate the behavior of the porous multichannel ceramic membrane distributor, hydrogen is fed to the fixed-bed reactor in two different ways for the hydrogenolysis of glycerol to 1,2-propanediol. One is dispersing the hydrogen into the aqueous glycerol solution by the ceramic membrane (average pore size of 200 nm). The other is a traditional feeding, i.e., adding the hydrogen directly by an inlet with a diameter 3 mm and mixing with the aqueous glycerol solution. The effect of feeding mode on the glycerol conversion and 1,2-propanediol selectivity under different hydrogen/glycerol volume ratios are shown in Figure 10. It can be observed that the 1,2-propanediol selectivity almost remains unchanged irrespective of the feeding mode and the hydrogen/glycerol volume ratio. In contrast, the conversion of glycerol varies markedly with these conditions. For the traditional feeding, as the hydrogen/ glycerol volume ratio increases from 200 to 600, the conversion of glycerol is improved from 71.2% to 84.9%, and a further increase in hydrogen/glycerol volume ratio does not significantly improve the conversion. For the membrane dispersion, the glycerol conversion first increases from 91.5% to 97.4% until the hydrogen/glycerol volume ratio of 300 and then remains constant. When that is compared to the traditional feeding, the conversion for the membrane dispersion increases by 14.7%, indicating that the hydrogenolysis of glycerol can be significantly intensified by the porous ceramic membrane. Furthermore, higher conversion with less hydrogen consumption can be obtained for the membrane dispersion, which is of great significance in energy saving. It is mainly because the porous ceramic membrane distributor owns many microchannels to satisfy the dispersion of hydrogen in the aqueous glycerol solution in the way of microbubbles, which increases the mass transfer effectively, and as a result, improves the conversion. This explanation is further confirmed by the bubble photographs (Figures 8b and 10b) and the corresponding gas holdup. As presented in Figure 10b, small amounts of millimeter-sized bubbles are obtained for the traditional feeding, whereas the significant decrease in bubble diameter and noticeable increase in bubble numbers are observed when the membrane is used as a gas distributor. At the same time, the gas holdup for the traditional feeding is only 0.56%, obviously lower than that for membrane dispersion (Figure 9). Furthermore, smaller bubbles with slow rise velocity52,53 have be produced by the membrane dispersion (Figure 8b), thus increasing the hydrogen residence time inside the fixed-bed, then contributing to higher glycerol conversion. In addition, in each case, the glycerol conversion can reach an optimum value with increasing hydrogen/glycerol volume ratio (Figure 10a), which might be explained as follows. Under certain operating conditions, due to the effect of the kinetic diffusional limitation of hydrogen during gas− liquid mixing,54,55 the amount of hydrogen dissolved in the glycerol aqueous solution tends to be stable when hydrogen is continuously increased, leading to an optimum glycerol conversion.

Table 2 lists the experimental results from this work and various reactors under their respective optimal operation conditions as found in the literature.51,56−61 It is noted that the developed novel membrane dispersion based fixed-bed reactor exhibits superior catalytic performance in the hydrogenolysis of glycerol as compared to the reported results. Although the glycerol conversion is slightly lower than that of the fixed-bed reactor with Ni−Cu−SiO2 catalyst,57 the hydrogen consumption is significantly reduced. The results highlight that the membrane distributor can effectively promote the reactant conversion with less hydrogen consumption. 3.4. Long-term Operation of Membrane Dispersion Based Fixed-bed Reactor. The operation stability of the fixed-bed reactor intensified by multichannel ceramic membrane for continuous glycerol hydrogenolysis to 1,2-propanediol is investigated under the optimal operation conditions as discussed above. As shown in Figure 11, the reaction process can be stably operated for 52 h in spite of slight decrease in glycerol conversion. The increase in grain size of Cu and ZnO in the catalyst during the continuous hydrogenolysis (Figure 12 and Table 3) should be responsible for the slight decrease in glycerol conversion.49,62 Meanwhile, similar trends are observed for continuous glycerol hydrogenolysis to 1,2propanediol in the fixed-bed reactor without membrane (Figures 11 and 12, Table 3), suggesting that the membrane dispersion has no obvious effect on the stability of the Cu− ZnO catalyst in the case. In this work, the ceramic membrane is one of the most key parts in the whole system, which controls the addition of the hydrogen and the subsequent interphase mass transfer and hydrogenolysis reaction. To investigate the stability of the ceramic membrane, the microstructures of fresh and used membranes were characterized by FESEM. As shown in Figure 13, there are no obvious difference between the surfaces of the fresh membrane and the used membrane, and the top layer of the membrane remains good intact with the support and its thickness keeps constant after 52 h of continuous reaction. These results suggest that the multichannel ceramic membrane has excellent stability in the reaction systems.

4. CONCLUSION In this work, a fixed-bed reactor with the assistant of a ceramic membrane for forming a microbubble-in-liquid system is developed for continuous hydrogenolysis of glycerol over Cu− ZnO. The membrane structure including the pore size and channel number and the reaction conditions significantly affect the bubble size distribution and gas holdup, thereby influencing the gas−liquid mixing and reaction performance. A superior reaction performance with less hydrogen consumption can be achieved by using a suitable ceramic membrane, whereas the ceramic membrane exhibits excellent stability. Our research verifies the advantages of a fixed-bed reactor with membrane as distributor and will aid the development of multiphase catalytic reactors with high reaction performance.

■

AUTHOR INFORMATION

Corresponding Authors

*H. Jiang. E-mail: [email protected]. *R. Chen. E-mail: [email protected]. ORCID

Rizhi Chen: 0000-0003-4298-483X I


Article

Industrial & Engineering Chemistry Research Notes

(18) Chen, G. G.; Luo, G. S.; Xu, J. H.; Wang, J. D. Membrane Dispersion Precipitation Method to Prepare Nanopartials. Powder Technol. 2004, 139, 180. (19) Tan, J.; Du, L.; Lu, Y. C.; Xu, J. H.; Luo, G. S. Development of a Gas-Liquid Microstructured System for Oxidation of Hydrogenated 2-Ethyltetrahydroanthraquinone. Chem. Eng. J. 2011, 171, 1406. (20) Jiang, H.; Meng, L.; Chen, R. Z.; Jin, W. Q.; Xing, W. H.; Xu, N. P. A Novel Dual-membrane Reactor for Continuous Heterogeneous Oxidation Catalysis. Ind. Eng. Chem. Res. 2011, 50, 10458. (21) Chen, R. Z.; Bao, Y. H.; Xing, W. H.; Jin, W. Q.; Xu, N. P. Enhanced Phenol Hydrogenation with Oxygen Using a Ceramic Membrane Distributor. Chin. J. Catal. 2013, 34, 200. (22) Meng, L.; Guo, H. Z.; Dong, Z. Y.; Jiang, H.; Xing, W. H.; Jin, W. Q. Ceramic Hollow Fiber Membrane Distributor for Heterogeneous Catalysis: Effects of Membrane Structure and Operating Conditions. Chem. Eng. J. 2013, 223, 356. (23) Mao, H. L.; Chen, R. Z.; Xing, W. H.; Jin, W. Q. Organic Solvent-free Process for Cyclohexanone Ammoximation by a Ceramic Membrane Distributor. Chem. Eng. Technol. 2016, 39, 883. (24) Wang, C. C.; Jiang, H.; Chen, C. L.; Chen, R. Z.; Xing, W. H. Solvent Effect on Hydrogenolysis of Glycerol to 1,2-Propanediol over Cu-ZnO Catalyst. Chem. Eng. J. 2015, 264, 344. (25) Bienholz, A.; Schwab, F.; Claus, P. Hydrogenolysis of Glycerol over a Highly Active CuO/ZnO Catalyst Prepared by an Oxalate Gel Method: Influence of Solvent and Reaction Temperature on Catalyst Deactivation. Green Chem. 2010, 12, 290. (26) Li, Y. M.; Liu, H. M.; Ma, L.; He, D. H. Glycerol Hydrogenolysis to Propanediols over Supported Pd-Re Catalysts. RSC Adv. 2014, 4, 5503. (27) Garcia, R.; Besson, M.; Gallezot, P. Chemoselective CatalyticOxidation of Glycerol with Air on Platinum Metals. Appl. Catal., A 1995, 127, 165. (28) Chaminand, J.; Djakovitch, L.; Gallezot, P.; Marion, P.; Pinel, C.; Rosier, C. Glycerol Hydrogenolysis on Heterogeneous Catalysts. Green Chem. 2004, 6, 359. (29) Barbelli, M. L.; Santori, G. F.; Nichio, N. N. Aqueous Phase Hydrogenolysis of Glycerol to Bio-propylene Glycol over Pt-Sn Catalysts. Bioresour. Technol. 2012, 111, 500. (30) Liu, Y. Q.; Pasupulety, N.; Gunda, K.; Rempel, G. L.; Ng, F. T. T. Glycerol Hydrogenolysis to 1,2-Propanediol by Cu/ZnO/Al2O3 Catalysts. Top. Catal. 2014, 57, 1454. (31) Jin, S. H.; Xiao, Z. H.; Li, C.; Williams, C. T.; Liang, C. H. Hydrogenolysis of Glycerol over HY Zeolite Supported Ru Catalysts. J. Energy Chem. 2014, 23, 185. (32) Rode, C. V.; Mane, R. B.; Potdar, A. S.; Patil, P. B.; Niphadkar, P. S.; Joshi, P. N. Copper Modified Waste Fly Ash as a Promising Catalyst for Glycerol Hydrogenolysis. Catal. Today 2012, 190, 31. (33) Pudi, S. M.; Biswas, P.; Kumar, S. Selective Hydrogenolysis of Glycerol to 1,2-Propanediol over Highly Active Copper-magnesia Catalysts: Reaction Parameter, Catalyst Stability and Mechanism Study. J. Chem. Technol. Biotechnol. 2016, 91, 2063. (34) Han, Y.; Liu, Y. F.; Jiang, H.; Xing, W. H.; Chen, R. Z. Large Scale Preparation of Microbubbles by Multi-channel Ceramic Membranes: Hydrodynamics and Mass Transfer Characteristics. Can. J. Chem. Eng. 2017, 95, 2176. (35) Painmanakul, P.; Loubiere, K.; Hebrard, G.; Buffiere, P. Study of Different Membrane Spargers Used in Waste Water Treatment: Characterisation and Performance. Chem. Eng. Process. 2004, 43, 1347. (36) Li, S. W.; Xu, J. H.; Wang, Y. J.; Luo, G. S. Mesomixing Scale Controlling and Its Effect on Micromixing Performance. Chem. Eng. Sci. 2007, 62, 3620. (37) Zheng, C.; Tan, J.; Wang, K.; Luo, G. S. Stability and Pressure Drop of Gas-Liquid Micro-dispersion Flows through a Capillary. Chem. Eng. Sci. 2016, 140, 134. (38) Wu, Y.; Hua, C.; Li, W. L.; Li, Q.; Gao, H. S.; Liu, H. Z. Intensification of Micromixing Efficiency in a Ceramic Membrane Reactor with Turbulence Promoter. J. Membr. Sci. 2009, 328, 219.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial supports from the National Natural Science Foundation (91534110, 21606124), the Jiangsu Province Natural Science Foundation for Distinguished Young Scholars (BK20150044), the National Key R&D Program (2016YFB0301503), and the Jiangsu Province Natural Science Foundation (BK20160978) of China are gratefully acknowledged.

■

REFERENCES

(1) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 1688. (2) Dumeignil, F.; Paul, J. F.; Paul, S. Heterogeneous Catalysis with Renewed Attention: Principles, Theories, and Concepts. J. Chem. Educ. 2017, 94, 675. (3) Ernst, J. B.; Schwermann, C.; Yokota, G. I.; Tada, M.; Muratsugu, S.; Doltsinis, N. L.; Doltsinis, N. L.; Glorius, F. Molecular Adsorbates Switch on Heterogeneous Catalysis: Induction of Reactivity by N-heterocyclic Carbenes. J. Am. Chem. Soc. 2017, 139, 9144. (4) Gelhausen, M. G.; Yang, S. Q.; Cegla, M.; Agar, D. W. Cyclic Mass Transport Phenomena in a Novel Reactor for Gas-Liquid-Solid Contacting. AIChE J. 2017, 63, 208. (5) Tan, J.; Zhang, J. S.; Lu, Y. C.; Xu, J. H.; Luo, G. S. Process Intensification of Catalytic Hydrogenation of Ethylanthraquinone with Gas-Liquid Microdispersion. AIChE J. 2012, 58, 1326. (6) Mason, B. P.; Price, K. E.; Steinbacher, J. L.; Bogdan, A. R.; McQuade, D. T. Greener Approaches to Organic Synthesis Using Microreactor Technology. Chem. Rev. 2007, 107, 2300. (7) Cabaret, F.; Fradette, L.; Tanguy, P. A. Gas-Liquid Mass Transfer in Unbaffled Dual-impeller Mixers. Chem. Eng. Sci. 2008, 63, 1636. (8) Chen, G. G.; Luo, G. S.; Sun, Y.; Xu, J. H.; Wang, J. D. A Ceramic Microfiltration Tube Membrane Dispersion Extractor. AIChE J. 2004, 50, 382. (9) Chambers, R. D.; Holling, D.; Spink, R. C. H.; Sandford, G. Elemental Fluorine-Part 13. Gas-Liquid Thin Film Microreactors for Selective Direct Fluorination. Lab Chip 2001, 1, 132. (10) Jähnisch, K.; Baerns, M.; Hessel, V.; Ehrfeld, W.; Haverkamp, V.; Lowe, H.; Wille, C.; Guber, A. Direct Fluorination of Toluene Using Elemental Fluorine in Gas/Liquid Microreactors. J. Fluorine Chem. 2000, 105, 117. (11) Tadepalli, S.; Qian, D. Y.; Lawal, A. Comparison of Performance of Microreactor and Semi-batch Reactor for Catalytic Hydrogenation of O-nitroanisole. Catal. Today 2007, 125, 64. (12) Xu, J. H.; Luo, G. S.; Chen, G. G.; Tan, B. Mass Transfer Performance and Two-phase Flow Characteristic in Membrane Dispersion Mini-extractor. J. Membr. Sci. 2005, 249, 75. (13) Tan, J.; Liu, Z. D.; Lu, Y. C.; Xu, J. H.; Luo, G. S. Process Intensification of H2O2 Extraction Using Gas-Liquid-Liquid Microdispersion System. Sep. Purif. Technol. 2011, 80, 225. (14) Tan, J.; Xu, J. H.; Wang, K.; Luo, G. S. Rapid Measurement of Gas Solubility in Liquids Using a Membrane Dispersion Microcontactor. Ind. Eng. Chem. Res. 2010, 49, 10040. (15) Chen, G. G.; Luo, G. S.; Yang, L. M.; Xu, J. H.; Sun, Y.; Wang, J. D. Synthesis and Size Control of CaHPO4 Particles in a Two-liquid Phase Micromixing Process. J. Cryst. Growth 2005, 279, 501. (16) Grzenia, D. L.; Dong, R. W.; Jasuja, H.; Kipper, M. J.; Qian, X. H.; Wickramasinghe, S. R. Conditioning Biomass Hydrolysates by Membrane Extraction. J. Membr. Sci. 2012, 415, 75. (17) Gijsbertsen-Abrahamse, A. J.; van der Padt, A.; Boom, R. M. Influence of Membrane Morphplogy on Pore Activation in Membrane Emulsification. J. Membr. Sci. 2003, 217, 141. J


Article

Industrial & Engineering Chemistry Research (39) Kukizaki, M.; Goto, M. Size Control of Nanobubbles Generated from Shirasu-porous-glass (SPG) Membranes. J. Membr. Sci. 2006, 281, 386. (40) Peng, S. J.; Williams, R. A. Controlled Production of Emulsions Using Crossflow Membrane. Part. Part. Syst. Charact. 1998, 15, 21. (41) Hao, S. L.; Peng, W. C.; Zhao, N.; Xiao, F. K.; Wei, W.; Sun, Y. H. Hydrogenolysis of Glycerol to 1,2-Propanediol Catalyzed by CuH4SiW12O40/Al2O3 in Liquid Phase. J. Chem. Technol. Biotechnol. 2010, 85, 1499. (42) Abghari, S. Z.; Shokri, S.; Baloochi, B.; Marvast, M. A.; Ghanizadeh, S.; Behroozi, A. Analysis of Sulfur Removal in Gasoil Hydrodesulfurization Process by Application of Response Surface Methodology. Korean J. Chem. Eng. 2011, 28, 93. (43) Wang, H. Y.; Dai, F.; Yang, Y. Q.; Li, Z. X.; Li, C. S.; Zhang, S. J. Catalyst Grading Optimization and Kinetic Simulation of Shale Oil Hydrotreating Process. Energy Fuels 2017, 31, 4353. (44) Loubière, K.; Castaignède, V.; Hébrard, G.; Roustan, M. Bubble Formation at a Flexible Orifice with Liquid Cross-flow. Chem. Eng. Process. 2004, 43, 717. (45) Deng, Z. H.; Wang, T. F.; Zhang, N.; Wang, Z. W. Gas Holdup, Bubble Behavior and Mass Transfer in a 5 m High Internal-loop Airlift Reactor with Non-Newtonian Fluid. Chem. Eng. J. 2010, 160, 729. (46) Yuan, H. S.; Tan, S. C.; Feng, L.; Liu, X. Heterogeneous Bubble Nucleation on Heated Surface from Insoluble Gas. Int. J. Heat Mass Transfer 2016, 101, 1185. (47) Feng, J.; Zhang, Y. Q.; Xiong, W.; Ding, H. Hydrogenolysis of Glycerol to 1,2-Propanediol and Ethylene Glycol over Ru-Co/ZrO2 Catalysts. Catalysts 2016, 6, 51. (48) Doleček, P.; Cakl, J. Permeate Flow in Hexagonal 19-Channel Inorganic Membrane under Filtration and Backflush Operating Modes. J. Membr. Sci. 1998, 149, 171. (49) Wang, S.; Liu, H. C. Selective Hydrogenolysis of Glycerol to Propylene Glycol on Cu-ZnO Catalysts. Catal. Lett. 2007, 117, 62. (50) Wang, C. C.; Jiang, H.; Chen, C. L.; Chen, R. Z.; Xing, W. H. A Submerged Catalysis/Membrane Filtration System for Hydrogenolysis of Glycerol to 1,2-Propanediol over Cu-ZnO Catalyst. J. Membr. Sci. 2015, 489, 135. (51) Rekha, V.; Raju, N.; Sumana, C.; Douglas, S. P.; Lingaiah, N. Selective Hydrogenolysis of Glycerol over Cu-ZrO2-MgO Catalysts. Catal. Lett. 2016, 146, 1487. (52) Tsuchiya, K.; Furumoto, A.; Fan, L. S.; Zhang, J. P. Suspension Viscosity and Bubble Rise Velocity in Liquid-Solid Fluidized Beds. Chem. Eng. Sci. 1997, 52, 3053. (53) Matsuura, A.; Fan, L. S. Distribution of Bubble Properties in a Gas-Liquid-Solid Fluidized Bed. AIChE J. 1984, 30, 894. (54) Pauss, A.; Andre, G.; Perrier, M.; Guiot, S. R. Liquid-to-Gas Mass Transfer in Anaerobic Processes: Inevitable Transfer Limitations of Methane and Hydrogen in the Biomethanation Process. Appl. Environ. Microbiol. 1990, 56, 1636. (55) Peureux, J.; Torres, M.; Mozzanega, H.; Giroir-Fendler, A.; Dalmon, J. A. Nitrobenzene Liquid-phase Hydrogenation in a Membrane Reactor. Catal. Today 1995, 25, 409. (56) Zheng, L. P.; Li, X. W.; Du, W. C.; Shi, D. W.; Ning, W. S.; Lu, X. Y.; Hou, Z. Y. Metal-organic Framework Derived Cu/ZnO Catalysts for Continuous Hydrogenolysis of Glycerol. Appl. Catal., B 2017, 203, 146. (57) Lee, M.; Hwang, Y. K.; Chang, J. S.; Chae, H. J.; Hwang, D. W. Vapor-phase Hydrogenolysis of Glycerol to 1,2-Propanediol Using a Chromium-free Ni-Cu-SiO2 Nanocomposite Catalyst. Catal. Commun. 2016, 84, 5. (58) Rekha, V.; Raju, N.; Sumana, C.; Lingaiah, N. Continuous Hydrogenolysis of Glycerol to 1,2-Propanediol over Bi-metallic Ni-Ag Supported on γ-Al2O3 Catalysts. Catal. Lett. 2017, 147, 1441. (59) Wolosiak-Hnat, A.; Milchert, E.; Grzmil, B. Influence of Parameters on Glycerol Hydrogenolysis over a Cu/Al2O3 Catalyst. Chem. Eng. Technol. 2013, 36, 411. (60) Vasiliadou, E. S.; Eggenhuisen, T. M.; Munnik, P.; de Jongh, P. E.; de Jongc, K. P.; Lemonidou, A. A. Synthesis and Performance of

Highly Dispersed Cu/SiO2 Catalysts for the Hydrogenolysis of Glycerol. Appl. Catal., B 2014, 145, 108. (61) Rodrigues, R.; Isoda, N.; Gonçalves, M.; Figueiredo, F. C. A.; Mandelli, D.; Carvalho, W. A. Effect of Niobia and Alumina as Support for Pt Catalysts in the Hydrogenolysis of Glycerol. Chem. Eng. J. 2012, 198, 457. (62) Du, Y.; Wang, C. C.; Jiang, H.; Chen, C. L.; Chen, R. Z. Insights into Deactivation Mechanism of Cu-ZnO Catalyst in Hydrogenolysis of Glycerol to 1,2-Propanediol. J. Ind. Eng. Chem. 2016, 35, 262.

K


Membrane Based GasâLiquid Dispersion Integrated in Fixed-Bed

Recommend Documents

Membrane Based GasâLiquid Dispersion Integrated in Fixed-Bed