Process Analysis on Preparation of Cyclobutanetetracarboxylic

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2018, 57, 2476−2485

Process Analysis on Preparation of Cyclobutanetetracarboxylic Dianhydride in a Photomicroreactor within Gas−Liquid Taylor Flow Wenhua Xu,† Yuanhai Su,*,† Yang Song,† Minjing Shang,† Li Zha,† and Qinghua Lu*,†,‡ †

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China School of Chemical Science and Engineering, Tongji University, Shanghai 200092, P. R. China

‡

ABSTRACT: Cyclobutanetetracarboxylic dianhydride (CBDA) was prepared by photodimerization of maleic anhydride in capillary microreactors within Taylor flow under UV irradiation. Both inert gas and ultrasound were introduced into this photochemical process to avoid the potential channel clogging resulting from the CBDA deposition. Higher input power of the Hg lamp, lower reactant concentration, longer residence time, and smaller inner diameter of the capillary were found to result in higher CBDA yield. The yield of CBDA could reach as high as 66.3% at an actual residence time of 30 min in the capillary microreactor with 0.5 mm inner diameter. According to the analysis of energy conversion, only low effective photoelectric transformation efficiencies could be achieved in this photochemical process. However, the photonic efficiency for this photodimerization carried out in capillary photomicroreactors was much higher compared to batch photoreactors and was comparable with that for other photochemical transformations using similar photomicroreactors and light sources.

1. INTRODUCTION Polyimide has been applied in a variety of fields such as aerospace and microelectronics for its excellent combined properties, such as good mechanical performance, outstanding thermostability, controlled thermal expansion behavior, and superior electrical/insulating properties.1−3 However, higher demands for aromatic polyimides are required due to rapid progress of electro-optical device technologies and the aromatic polyimides are not always perfect regarding to the optical properties mainly resulting from poor light transmission because of its brown color. Recently, alicyclic polyimides with higher transparency, lower coefficient of thermal expansion, and satisfied thermostability have been synthesized for optical and optoelectronic device applications.4−9 The superior properties of alicyclic polyimides mainly originate from the low polarity, low electron density, and low probability of undergoing interor intramolecular charge transfer of alicyclic polyimides.10 CBDA as an important aliphatic anhydride is typically applied to prepare aliphatic polyimide, and currently most relevant research works have been devoted to the optimization of reaction conditions. Schenk et al. prepared cis-CBDA in dioxane through the photochemical reaction of maleic anhydride with the use of immersion-well photoreactor irradiated by the ultraviolet (UV) light from a high-pressure mercury (Hg) lamp.11 With a similar reactor system, Suzuki et al. optimized the reaction conditions and found that ethyl acetate was a proper solvent for the preparation of CBDA and its cis-trans-cis configuration could be determined by X-ray.12 However, drawbacks are associated with this traditional batch © 2018 American Chemical Society

processing, such as obvious concentration gradient and inhomogeneous light radiation distribution inside the reactor. Moreover, the product (CBDA) can precipitate and adhere to the lamp cooling jacket, and scatter the irradiated light, thus further deteriorating the photon transport. These drawbacks weaken the stability of reactor system and render the irradiation time for this photochemical process to be rather long with low yield of CBDA, significantly limiting the application of this traditional preparation method. In recent years, the application of microreactors for photochemical transformations has attracted considerable attention from both academia and industry due to its obvious advantages compared with conventional batch processing, such as continuous-flow operation, larger specific surface area, faster mixing efficiency, enhanced heat and mass transfer rates, increased radiation homogeneity of entire reaction medium, easier process control, reduced safety hazards, and the ease of increasing throughput by numbering-up, etc.13−19 These advantages allow for shorter reaction/residence times, higher reaction selectivity, and lower catalyst loading for photochemical processes conducted in photomicroreactors as compared with conventional batch photoreactors. Furthermore, the photon flux density in microreactors was reported to be 150 times that in the batch reactor, which may explain clearly why Received: Revised: Accepted: Published: 2476

November 4, 2017 February 1, 2018 February 2, 2018 February 2, 2018 DOI: 10.1021/acs.iecr.7b04572 Ind. Eng. Chem. Res. 2018, 57, 2476−2485

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Industrial & Engineering Chemistry Research

capillaries was resistant to UV irradiation. The capillary was connected with a T-micromixer to form a microreactor system. This capillary microreactor was then wrapped around the Pyrex immersion well that was immersed inside an ultrasonic bath (SB-5200 DTD, SCIENTZ, 400W, 40 kHz, China). The reactant solution and the inert gas (nitrogen) with various volumetric flow rates (Vl and Vg) were delivered to the Tmicromixer by a syringe pump (Fusion 200, Chemyxlnc) and a gas mass flow controller (Bronkhorst), respectively. The volumetric flow rate ratio of the gas to the liquid phase (r) was easily varied by controlling the syringe pump and the gas mass flow controller. A Taylor flow regime was established in the T-micromixer and then flowed through the capillary microreactor, in which the photodimerization took place with the UV irradiation. Two heating−cooling circulators (DLSB-5/ 20, Yuhua Instrument Equipment Co., Ltd., China) were applied to form a cooling unit to respectively remove the heat produced by the Hg lamp and the ultrasonic bath with ethanol as the cooling medium. The surface tension (γ) and viscosity (μ) of the reaction solution were measured by a tensiometer (Datephysics DCAT11, Gmbh, Germany) and a viscometer (DV2TLVTJ0, Brookfield, United States), respectively. 2.2. Synthesis of CBDA. Preparation of CBDA was realized through the maleic anhydride photodimerization with the UV irradiation (see Figure 1a). According to literature, ethyl acetate

photochemical reactions can be substantially accelerated in microreactors.20−22 Recently, Dario et al. summarized nine reasons for carrying out photochemistry in microreactors and gave an overview of application examples in various fields, including organic synthesis, material science, and water treatment.23 UV irradiation has been predominantly used to enable photochemical transformations due to its high-energy content. UV light can directly activate organic molecules or indirectly by means of homogeneous or heterogeneous photocatalysis. In direct photochemical activation, UV light can be absorbed directly by organic molecules bearing chromophoric groups, allowing the excited organic molecules to undergo chemical transformations. A number of literature reports have proven the successful combination of microreactors with direct UV activation for photochemical transformations to produce fine chemicals.15,23 Fukuyama et al. performed a photochemical [2 + 2] cycloaddition reaction in continuous-flow microreactors with direct UV activation (300 W, Hg lamp) and obtained cycloaddition products in high yield at a residence time of 2 h.24 The photocycloaddition of maleimide and n-hexyne to produce 3,4-dimethyl-1-pent-4-enylpyrrole-2,5-dione was realized in a microreactor constructed by fluorinated ethylene propylene (FEP) tubing (2.7 mm ID) with three layers, which was coiled around a Pyrex immersion well containing a 400 W medium-pressure Hg lamp.25 Excellent yield (82%) of the target product was obtained at a short residence time. Horie et al. prepared CBDA by direct UV activation of MA in a recycle microreactor system containing a filter unit for solid product separation and up to 69% conversion of MA could be obtained with the recycle number of 4.5 for the reaction solution flowing in the microreactor system.26 However, in these UV-triggered photochemical processes the effects of some important process parameters such as the power input of UV light sources and the reactant concentration usually were not concerned. In particular, some parameters relevant to the energy utilization efficiency (e.g., the effective photoelectric transformation efficiency and the photonic efficiency) have not been evaluated. In this work, we report continuous-flow preparation of CBDA through photodimerization in capillary microreactors within gas−liquid Taylor flow (slug flow) under direct UV activation. Effects of various influencing factors such as input power of the high-pressure Hg lamp, reactant concentration, inner diameter of the capillary, residence time, and flow rate ratio of the gas to the liquid phase on the reaction performance were investigated in detail. A heat balance analysis for the whole photomicroreactor system was carried out in order to guide stable and sustained operations with the UV irradiation. Furthermore, the effective photoelectric transformation efficiency and the photonic efficiency were evaluated to reveal the energy utilization efficiency in this photochemical process and the importance of the compatibility between the microreactor and the UV light source.

Figure 1. (a) Scheme of maleic anhydride photodimerization, (b) internal circulations within gas−liquid Taylor flow, and (c) schematic representation of the photomicroreactor system.

was chosen as an optimized solvent and a low set temperature for the immersion well was preferred in order to maintain the regular work of the high-pressure Hg lamp.12 Since the solubility of target product (CBDA) in ethyl acetate was poor, a combined method was used to avoid the channel clogging. Gas−liquid Taylor flow was formed in the capillary with various volumetric flow rate ratios of the gas to the liquid phase (r), and the whole photomicroreactor system was immersed inside an ultrasonic bath in order to prohibit the aggregation of CBDA (Figure 1, panels b and c). In Taylor flow, toroidal vortices were established due to the friction between the gas−liquid two phases and the slip velocity, which provided enhanced mixing, increased radial mass transfer and minimal axial dispersion in the liquid phase.27−32 As observed, the CBDA suspension could be formed inside the capillary microreactor as the maleic anhydride photodimerization proceeded. However, with the

2. EXPERIMENTAL SECTION 2.1. Materials. Ethyl acetate (EA) and maleic anhydride (MA) were purchased from Aladdin and used without further purification. A high-pressure Hg lamp with a Pyrex immersion well (CEL-LAM500, 55 mm outer diameter and 350 mm height) was purchased from CEAULIT, China. FEP capillaries with various inner diameters (di = 0.5, 0.75, and 1.00 mm) and length (L), T-micromixers, and union fittings were purchased from IDEX Health and Science. The fabrication material of 2477

DOI: 10.1021/acs.iecr.7b04572 Ind. Eng. Chem. Res. 2018, 57, 2476−2485

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Industrial & Engineering Chemistry Research combined effect of ultrasound and Taylor flow, the deposition of CBDA on the inner wall of the capillary and the channel clogging were not observed in the experiments. A high-speed CCD camera (PhantomLab110−12G, United States) installed with a stereo microscope (Olympus, SZ2-CLS, Japan) was used for capturing snapshots of the gas−liquid Taylor flow inside the capillary microreactor. The temperature (T) in the ultrasonic bath was less than 0 °C controlled by a heating−cooling circulator. The initial MA concentration (C0) was in the range of 5%−15% (wt). After the experiment ran steadily, the CBDA suspension was collected, centrifuged, and then purified with acetic anhydride at 120 °C for 12 h. Finally, the obtained solid powder was dried in a vacuum drying oven (60 °C) overnight. The final product (CBDA) was weighed and the yield (Y) was calculated. 2.3. Characterization. A UV/visible spectrophotometer (UV-1800, Shimadzu, Japan) was used to measure the light absorption properties of the MA solution. In order to determine and characterize the product, Fourier transform infrared spectra (FT-IR) were measured with a Spectrum 100 FT-IR spectrometer (PerkinElmer, Inc., United States), and 1H nuclear magnetic resonance spectra (1H NMR) were measured with a Varian Mercury-400 spectrometer (Varian, Palo Alto, CA) using deuterated dimethyl sulfoxide ([D6]DMSO) as the solvent at room temperature. The emission spectrum of the high-pressure Hg lamp was detected by optical power meter of CEL-NP2000 (see Figure 2). UPLC-Q/TOF MS analyses were

Table 1. Gradient Procedure of UPLC Condition 1 2 3 4 5 6 7 8 9

time (min)

flow rate (mL/min)

A%

B%

Initial 1.50 3.50 5.00 7.00 8.00 11.00 11.50 13.00

0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300 0.300

95.0 80.0 60.0 40.0 15.0 0.0 0.0 95.0 95.0

5.0 20.0 40.0 60.0 85.0 100.0 100.0 5.0 5.0

L/h at 350 °C. The source temperature was 115 °C. The cone gas flow was 50.0 L/h. The mass range was m/z 50 to 1500 in the positive mode. To ensure accuracy and reproducibility, all analyses were conducted using an independent reference spray via the Lock Spray interface; leucine-encephalin (200 pg/μL) was used as a lock mass (m/z 556.2771) under positive-ion conditions for real-time calibration (flow rate of 30 μL/min). Before the experiment, a single-point calibration was performed against the lock mass compound (leucine-encephalin). A multiple-point calibration was then performed over the range of m/z 50−1500 using sodium formate solution (prepared from 10% formic acid/0.1 mol/L sodium hydroxide solution/acetonitrile, 10 mL/10 mL/80 mL). All points fell within 1 ppm during the calibration. The resolving power of the instrument was 8000. Accurate masses and compositions of the precursor and fragment ions were calculated using the MassLynx 4.1 software supplied with the instrument.

3. RESULTS AND DISCUSSION 3.1. Determination and Characterization of CBDA. The FT-IR spectra and 1H NMR spectra of the product are shown in Figure 3 (panels a and b, respectively). The C−C stretching vibration peak is at 1242 and 1196 cm−1. The carbonyl (CO) absorption peaks of asymmetrical stretching and symmetrical stretching are at 1856 and 1780 cm−1, respectively. The peaks of 1450 and 3013 cm−1 are C−H inplane bending vibration peak and tensile vibration peak. The asymmetrical stretching and the symmetrical stretching C−O− C peaks accord with 1096, 964, and 933 cm−1. The disappearance of the CC bond and the formation of fourmembered ring structure of the C−C bond demonstrate the synthesis of CBDA. In Figure 3b, it can be seen that the chemical shift of the methine group (−CH) appeared at 3.86 ppm as a single peak, which further demonstrates the successful preparation of CBDA. The chromatography diagrams of the product prepared in our work and commercial CBDA (standard sample) in the positive ion mode are shown in Figure 4a. The retention times of these two samples were consistent with each other. The mass spectra of the chromatographic effluent peaks in the positive ion mode are shown in Figure 4b. [M + H]+ peaks of these two samples were detected with the same m/z at 197.008, which again confirms the synthesized product. 3.2. Effects of the UV Source Power Input and the Reactant Concentration on the Reaction Performance. Photochemical transformations belong to a kind of important energy conversion modes, which utilize photons to overcome activation barriers. The photochemical activation can provide pathways for the syntheses of new products, which are often

Figure 2. Emission spectrum of the high-pressure Hg lamp with 500 W power input.

performed using a Primer UPLC-Q-TOF mass spectrometer (Waters Corp., Milford, MA) equipped with an electrospray ionization source for determining the composition and molecular weight of the product. Data acquisition, handling, and instrument control were performed using MassLynx 4.1 software. The chromatographic conditions were as follows: Waters BEH C18 (2.1*100 mm,1.7 μm), the mobile phase consisted of A (water with 0.1% formic acid) and B (CH3CN with 0.1% formic acid) linear gradient system, and the gradient procedure (see Table 1). The flow rate was 0.300 mL/min, the column temperature was 45 °C, and the injection volume was 1 μL. For electrospray ionization positive-ion mode, the voltages of capillary, sample cone, and extraction cone were set at 3000, 35, and 3 V, respectively. The desolvation gas flow rate was 600 2478


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Figure 4. (a) Chromatogram and (b) mass spectra of product in the positive ion mode.

Figure 3. (a) FT-IR spectra of the product and (b) NMR spectra of the product.

difficult to realize through the thermochemical activation.33−35 As well-known, the proper choice of light sources for photochemical reactions strongly depends on the absorption wavelength of reactants or photocatalysts, which should overlap with the emission spectra of the light sources. If this is satisfied, higher power input of the light source usually leads to higher photochemical transformation rate. Figure 5 illustrates the impact of the power input of the highpressure Hg lamp on the reaction performance in the capillary microreactor at various initial reactant concentrations when the volumetric flow rates of the gas and liquid phases were maintained constant. It can be seen that the yield increased with raising the power input of the high-pressure Hg lamp. Higher power input resulted in higher photon flux, which can subsequently excite more reactant molecules for the dimerization leading to the desired CBDA. Moreover, the yield of CBDA increased significantly with the decrease of the initial reactant concentration at the same power input. With the 500 W power input of the high-pressure Hg lamp and an initial MA concentration of 5%, the yield of CBDA reached 28% when the theoretical residence time (tth) was 17.6 min. However, it should be noted that higher concentrations will be beneficial for achieving higher throughput from a productivity standpoint. 3.3. Effect of the Residence Time on the Reaction Performance. The residence time (reaction time) is one of the most important process parameters in continuous-flow synthesis using microreactors, which can be controlled easily by

Figure 5. Effects of the power input of the Hg lamp and the initial reactant concentration on the reaction performance (di = 1 mm, L = 9.4 m, r = 1, Vg = 0.25 mL/min, and Vl = 0.25 mL/min).

changing flow rates of reactive fluids, inner diameter, and length of microreactors. The following equation (eq 1) is usually used to calculate the theoretical residence time of the gas−liquid two phases in capillary microreactors without considering the gas compressibility: t th =

Vc Vc π × (di /2)2 × L = = Vt Vg + Vl Vg + Vl

(1)

where Vc is the volume of the capillary microreactor and Vt is the total superficial volumetric flow rate of the gas−liquid two phases. However, the actual residence time of the gas−liquid two phases in microreactors is different from the theoretical residence time calculated by eq 1, especially when obvious 2479


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Industrial & Engineering Chemistry Research pressure drop exists along a long channel. So far, there have not been general models available to precisely predict the residence time in microchannels or capillaries for gas−liquid two-phase systems. Here, a straightforward method was used to obtain the residence time of the gas−liquid two phases in capillary microreactors.36 The syringe pump for introducing the MA solution into the capillary microreactor was stopped, while the gas mass flow controller was kept running at a constant value of the superficial gas volumetric flow rate. After the liquid phase inside the capillary microreactor was evacuated by the gas phase, the syringe pump was turned on again to introduce the MA solution. As soon as the gas−liquid flow entered the capillary microreactor, the time was recorded with a stop watch until the liquid phase exited the microreactor. A comparison between the measured residence times (ta) and the theoretical residence times (tth) shows that the ratio of the actual residence time to the theoretical residence time (ta/tth) was in the range of 1.07−1.18 under involved experimental conditions (see Table 2). As reported in literature, the critical capillary number

Figure 6. Effect of the residence time on the reaction performance in the capillary microreactor (di = 1 mm, L = 9.4 m, r = 1, and 500 W power input of the Hg lamp).

Table 2. Measured residence time and the theoretical residence time of the gas-liquid two phases in the capillary microreactor di (mm)

L (m)

Vt (mL/min)

tth (min)

ta (min)

ta/tth

1.00

9.4

0.27 0.38 0.50 0.60 0.90 1.50

27.33 19.42 14.76 12.30 8.20 4.92

30.44 21.63 17.55 14.19 9.48 5.81

1.10 1.11 1.12 1.07 1.14 1.18

this photodimerization. In order to evaluate the attenuation effect of photon transport inside photomicroreactors, the Lambert−Beer law was applied: I A = log10 0 = εAV Clp (2) I where A is the absorbance, I0 is the intensity of emission light, I is the intensity of transmission light, C is the MA concentration in the solution, εav is the average molar extinction coefficient with respect to the wavelength in the range of 220−350 nm, and lp is the transport distance of photons, respectively. The UV−vis absorption spectra of the reactant solutions with different MA concentrations can illustrate the photon absorption properties of the reactant solutions clearly, as shown in Figure 7a. The MA solution absorbed the photons within an UV region of 220−350 nm, with a characteristic wavelength (λmax) of 251 nm. As shown in Figure 7b, by measuring the slope of the absorbance versus the concentration of the MA solution at λmax, the molar extinction coefficient (ε) could be determined to be 122.44 M−1 cm−1 according to eq 2, and the value of εav was considered to be 61.22 M−1 cm−1. On the basis of the average molar extinction coefficient, the attenuation effect of photon transport in this photochemical process was evaluated. It is assumed that the distribution of all chemicals in the reaction medium was homogeneous, and there was no light scattering resulting from the formation of the gas− liquid two-phase flow. The capillary with a circular cross section applied in these experiments is supposed to be equivalent to a capillary with a square cross section for the evaluation simplification of the attenuation effect. The light intensity distribution along the transport direction can be calculated inside this equivalent capillary based on eq 2. Figure 8 shows the light intensity distribution as a function of transport distance with the 5 mmol/L MA reaction solution. It can be predicted that about 8% photons are absorbed by the reaction medium within a distance of 1 mm, and about 5% photons are absorbed when the distance decreases to 0.5 mm. It seems that the light intensity distribution can almost reach homogeneity when the characteristic distance of photon transport is less than 1 mm. However, small difference on the light intensity distribution might lead to different reaction performance in the photochemical process when the power input of the light source is large.

(Ca*) can serve as a criterion of transition from the circulation flow to the bypass flow in capillaries, below which a steady droplet/slug flow with internal circulations can be obtained.37 The value of Ca* was 0.7378 for capillaries placed horizontally, and it was slightly higher than 0.7378 for the downward flow in capillaries.37 The value of Ca was lower than 6.65 × 10−3 in our experiments, which was much lower than the critical capillary number. So the flow pattern in the capillary microreactor for photochemical transformations was maintained to be Taylor flow for most of flow rates involved in this work. The effect of the actual residence time on the reaction performance in the capillary photomicroreactor with various initial MA concentrations is shown in Figure 6. The superficial volumetric flow rate ratio of the gas to the liquid phase was 1, and the power input of the Hg lamp was 500 W. As shown in Figure 6, the yield of CBDA increased with the increase of the actual residence time for different initial MA concentrations. It is worth noting that the increasing trend was more pronounced when the initial MA concentration was lower. With an initial MA concentration of 5%, the yield of CBDA increased from 13.3% to 35.2% as the actual residence time increased from 5.8 to 21.6 min. Longer residence time resulted in more chances for the dimerization of activated reactant molecules, leading to more CBDA. 3.4. Effect of the Capillary Dimension on the Reaction Performance. The characteristic dimensions of reactors play an important role in the irradiation distribution in photoreactors. A smaller characteristic dimension will be beneficial for achieving a homogeneous irradiation of the entire reaction medium. Here, three capillary microreactors with different inner diameters (i.e., 1, 0.75, and 0.50 mm) were applied for 2480


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Figure 9. Effect of the inner diameter of the capillary microreactor on the reaction performance (C0 = 5%, r = 1, L = 9.4 m, and 500 W power input of the Hg lamp).

be seen in Figure 9, a higher yield was obtained in the capillary with a smaller diameter at the same actual residence times. This was mainly attributed to enhanced mixing and improved irradiation distribution. The microreactor with a smaller dimension had a higher specific surface area, which can result in the improved mixing and mass transfer performance within the gas−liquid Taylor flow regime and finally the improved product yield.38 Therefore, the yield of CBDA obtained in the capillary photomicroreactor with 0.5 mm inner diameter was higher than that obtained in the capillary photomicroreactor with 0.7 mm or 1 mm inner diameter. The yield of CBDA could reach 66.3% in the capillary photomicroreactor with 0.5 mm inner diameter, which was much higher than 26.0% obtained from a batch photoreactor.26 Meanwhile, the reaction time was significantly shortened from 6 h to 30 min. This comparison indicates the importance of a homogeneous irradiation over the reaction medium and the enhanced mixing, and the advantages of photomicroreactors over batch photoreactors for photochemical transformations. 3.5. Effect of the Gas to Liquid Flow Rate Ratio on the Reaction Performance. The hydrodynamic characteristics of gas−liquid Taylor flow in microreactors or microchannels depend on numerous factors, such as liquid properties, surface properties of channel walls, dimensions and structures of channels, and flow rates of gas and liquid phases, etc.39−41 In general, a higher flow rate ratio of the gas to the liquid phase will lead to longer gas bubbles and shorter liquid slugs in a microchannel within Taylor flow when the total flow rate of the gas−liquid two phases is maintained constant. Figure 10 shows the effect of the flow rate ratio of the gas to the liquid phase on the flow pattern in the capillary microreactor at a constant total superficial flow rate of the gas−liquid two phases (Vt = 1 mL/min). It can be seen that the flow pattern in the capillary microreactor changed from bubbly flow (Figure 10, panels a and b) to Taylor flow (Figure 10, panels c and d) with the increase in the flow rate ratio of the gas to the liquid phase. The length of Taylor bubbles increased with the increase of the superficial gas flow rate, while the length of liquid slugs (Ls) was just opposite (Figure 10, panels c and d). As the superficial gas velocity was large enough, the gas phase occupied most of the space in the capillary microreactor, and the liquid phase existed in the liquid slugs with short length and the liquid films with long length (Figure 10e). The thickness of liquid films (δ) can be calculated to be in the range

Figure 7. (a) Absorption spectra of the solution with different MA concentrations and (b) relationship of the maximum absorbance and the MA concentration.

Figure 8. Light intensity distribution (I/I0) as a function of photon transport distance with the 5 mmol/L MA reaction solution according to the Lambert−Beer law with the average molar extinction coefficient of 61.22 M−1 cm−1 with respect to the wavelength in the range of 220−400 nm.

Figure 9 demonstrates the effect of the capillary dimension on the photochemical performance with the 500 W power input of the Hg lamp. These three different capillaries had the same length (L = 9.4 m) with various inner diameters (di = 0.5, 0.75, and 1.00 mm), and the actual residence times in them were controlled by varying the total flow rate of the gas−liquid two phases at a constant superficial flow rate ratio of the gas to the liquid phase. The initial MA concentration was 5%. As can 2481


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Figure 11. Effect of the liquid flow rate percentage over the total volumetric flow rate of the gas−liquid two phases on the reaction performance in the capillary microreactor (C0 = 5%, di = 1 mm, L = 9.4 m, Vt = 1 mL/min, and 500 W power input of the Hg lamp).

Figure 10. Flow pattern of the nitrogen and MA solution two-phase system in the capillary microreactor with inner diameter of 1 mm with Vt of 1 mL/min: (a) r = 1/9, (b) r = 3/7, (c) r = 5/5, (d) r = 7/3, and (e) r = 9/1.

when the ultrasonic bath was applied for this MA photodimerization. However, we did not observe the channel clogging phenomena for the utilization of the capillary microreactor in this photochemical process by applying both the ultrasonic bath and Taylor flow with an appropriate superficial volumetric flow rate ratio of the gas to the liquid phase. 3.6. Energy Conversion and Efficiency. Substantial heat is produced during the irradiation of the UV and visible light from the high-pressure Hg lamp, which should be removed timely. Otherwise, the accumulated heat will significantly raise the temperature and destroy the lamp, and thus the whole photoreactor system will completely fail to work. Therefore, a proper cooling unit should be integrated into the photoreactor system in order to achieve stable operations. In this work, a heat balance analysis for the whole photomicroreactor system was conducted in order to establish a proper heat management for the MA photodimerization to produce CBDA. The heat balance for the photochemical process in the capillary microreactor can be expressed by the following equation:

of 1.2−4.9 μm under the involved experimental conditions according to the following correlation: δ 0.66Ca 2/3 = di 1 + 3.33Ca 2/3

(3)

where Ca is capillary number and it can be calculated as μUs/γ (Us is the superficial velocity of liquid slug). Therefore, the zones of thin liquid films provided extremely short diffusion distance and large effective interfacial area for the mixing and photon transport processes. Abiev developed a three-layer model for the mass transfer within Taylor flow in microchannels and the correlation of the internal circulation time in the Taylor vortex with the liquid slug length.42 It was pointed out that the internal circulations could occur even in very short liquid droplets/slugs (e.g., about 0.17 times of the tube diameter).42,43 For the liquid slugs in the capillary microreactor within Taylor flow, the internal circulations were beneficial for achieving a homogeneous mixing and thus prohibited the obvious MA concentration gradient. As shown in Figure 10, the liquid slug length was about 0.5−3 times of the inner diameter of the capillary, depending on the volumetric flow rate ratio of the gas phase to the liquid phase. Abiev’s work has shown the relationship between the dimensional internal circulation time and Ls/di. With the increase of Ls/di, the dimensional internal circulation time increased, and accordingly the Taylor vortex in droplets/slugs became weaker.42,44 Furthermore, the light refraction phenomena may become more serious with the increase of the gas bubble length or the decrease of Ls/di due to the formation of the gas−liquid interface, which was not beneficial for the light absorbance of the reaction solution. As can be seen in Figure 11, the yield of CBDA slightly decreased from 13.0% to 12.6%, and then increased to 20.7% as the liquid flow rate percentage over the total volumetric flow rate of the gas−liquid two phases increased from 10% to 90% (i.e., r decreased from 9 to 0.11), at a constant total flow rate of the gas−liquid two phases. These results indicated that the irradiation but not the mixing dominated over this MA photodimerization when the gas was introduced into the capillary microreactor to form gas−liquid two-phase flow. It should be noted that without introducing the gas phase the channel clogging easily occurred in the photomicroreactor even

Ql + Q u = Qc + Qm + Qe

(4)

where Ql is the heat released by the high-pressure Hg lamp, Qu is the heat released by the ultrasonic bath, Qc is the heat taken by the cooling unit, Qm is the heat absorbed by the medium (ethanol) inside the ultrasonic bath, and Qe is the heat transferred to the environment. Qm can be further calculated as Q m = kM ΔT

(5)

where k, M, and ΔT are the specific heat capacity, mass, and temperature variation of the medium inside the ultrasonic bath, respectively. The medium for the external cooling unit was also ethanol, and its volumetric flow rate was 0.5 mL/min. Combining eqs 4 and 5, the theoretical temperature variation of the medium (ΔTth) inside the ultrasonic bath for the process without photochemical transformations can be predicted by the following: ΔTth = (Q l + Q u − Q c − Q e)/(kM) = [t(Wl × ηl + Wu × ηu − Wc × ηc) − Q e]/(kM)

(6)

where t is the running time, Wl is the power input of the Hg lamp, Wu is the working power of the ultrasonic bath, Wc is the working power of the cooling unit, ηl, ηu, and ηc are the thermal 2482


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was higher than that for the 300 W power input. When the photochemical transformations were carried out in the capillary photomicroreactor with the 300 or 500 W power input, the temperature of the medium inside the ultrasonic bath gradually tended to reach stable after a running time of 16 min, especially for the 300 W power input, implying the operational stability of this photochemical process. Concerning the effective luminous energy (Er) for the photochemical transformations, the working power of the cooling unit should be satisfied by the following inequality in order to maintain a controlled set temperature and protect the high-pressure Hg lamp applied for the photomicroreactor for a long running time:

efficiencies of the Hg lamp, the ultrasonic bath and the cooling unit, respectively. It is assumed that most energies emitted by the Hg lamp and the ultrasonic bath were converted to heat finally and the values of ηl, ηu, and ηc were equal to 1 for the simplification of the calculation. Moreover, the value of Qe was considered to be zero considering the low thermal conductivity of air, and thus adiabatic conditions were assumed for these devices. Therefore, the prediction of the temperature increase for the medium inside the ultrasonic bath can be realized by applying eq 6. A thermocouple was applied to measure the actual temperature variations of the medium inside the ultrasonic bath with and without photochemical transformations (ΔTa,r and ΔTa,nr). For the case without photochemical transformations, the solvent (ethyl acetate) and the gas were introduced into the photomicroreactor system to form Taylor flow while keeping the UV irradiation. The temperature of the medium inside the ultrasonic bath increased with a function of time for different power input of the Hg lamp (300 and 500 W), as shown in Figure 12a. For the process without

Wc ≥ Wl + Wu − Er

(7)

Moreover, the difference between the temperature variations of the medium inside the ultrasonic bath for the processes with and without photochemical transformations could be attributed to the fact that the reaction solution absorbed a part of energy emitted by the Hg lamp during the photochemical transformations. This part of energy was mainly comprised of the effective luminous energy, which can be calculated by the following equation: Er = kM(ΔTa,nr − ΔTa,r) − Q r

(8)

where Qr is the heat released from the reaction during the running time and it can be neglected due to the much lower weight of the collected reaction solution compared with that of the medium inside the ultrasonic bath. For the photochemical transformations with the 500 W power input of the Hg lamp and 32 min running time, the value of Er was determined to be 42 KJ, which occupied 4.4% of the total energy emitted by the Hg lamp (960 KJ). Such a percentage can be considered as the effective photoelectric transformation efficiency. 95.6% of the total energy emitted by the Hg lamp was not absorbed by the reaction solution. For the 300 W high-pressure Hg lamp, a bit higher percentage (4.8%) of the total emitted energy was used for the photochemical transformations conducted in the capillary microreactor. This analysis indicated that the energy utilization efficiency in this photochemical process was rather low even when the microreactor was applied. In fact, the highpressure Hg lamp produced a great amount of heat when it emitted the UV and visible light. Furthermore, the mismatching between the volumes of the high-pressure Hg lamp and the capillary microreactor partially accounted for the low-energy utilization efficiency. Compact light sources such as UV lightemitting diodes (UV LEDs) can be integrated into photomicroreactor systems for UV-triggered photochemical processes, which may improve the energy utilization efficiency. However, a proper cooling design is still a prerequisite for the photochemical processes in order to prolong the lifetime of light sources. Actually, the emission spectrum of the 500 W high-pressure Hg lamp and the absorption spectra of the reactant solution demonstrate that lots of light with the wavelength outside the range of 220−350 nm could not be absorbed by the reaction solution (see Figure 2 and Figure 7a). Furthermore, the Pyrex immersion well could act as a wavelength filter cutting the UV wavelength less than 275 nm. Therefore, only the photons with the wavelength of 275−350 nm could be absorbed by the reaction solution. Moreover, not all MA molecules excited by photons participated in the photodimerization. Instead, some of them could return to the ground state by radiation or

Figure 12. (a) Measured temperature variation of the medium in the ultrasonic bath with and without photochemical transformations, (b) comparison among the theoretical temperature variation and actual temperature variations of the medium inside ultrasonic bath with and without photochemical transformations at the 500 W power input of the Hg lamp.

photochemical transformations, the predicted values accord well with experimental data. For example, the experimental value was 8.0 °C when the operation was running for a period of 32.0 min with the 500 W power input, which was close to the predicted value (8.7 °C, see Figure 12b). Obviously, the temperature increase of the medium for the 500 W power input 2483


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and visible light. On the other side, only a small part of the emitted light could be absorbed by the reaction solution, and the mismatching between the volumes of the high-pressure Hg lamp and the capillary still existed. All these aspects led to a low effective photoelectric transformation efficiency (e.g., 4.4%). The photonic efficiency for this photodimerization carried out in capillary photomicroreactors was much higher compared to batch photoreactors, and it was comparable with that for other photochemical transformations using similar photomicroreactors and light sources.

nonradiative processes without triggering any transformations. Typically, the photonic efficiency (ξ) is applied to describe the utilization efficiency of the luminous energy for photochemical transformations, which can be calculated by eq 9: r ξ= a q (9) ra = Y × C0 × Vl

(10)

350

q=

■

∑275 Wλ/Eλ NA

Eλ = h × υλ = h ×

(11)

c λ

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Tel.: +86 21-54738710. E-mail: [email protected].

(12)

where ra is the reaction rate and q is the photon flux in the range 275−365 nm. In eqs 11 and 12, Wλ is the emission energy flux distribution, Eλ is the energy of a photon at a wavelength of λ, NA is Avogadro’s constant, h, υλ, and c are Plank’s constant, the photon frequency, and the light velocity, respectively. The maximum yield of 66.3% was obtained in the capillary photomicroreactor (0.5 mm inner dimeter) at the residence time of 30 min with the irradiation of the 500 W high-pressure Hg lamp. In this case, the photonic efficiency was calculated to be 0.0155 according to eqs 9−12. Such a value is close to that obtained in other photomicroreactors with traditional lamps as light sources and is much higher compared to batch photoreactors (0.0086−0.0042).22,45 Using LEDs as light sources may be an effective means to improve the efficiency of optical conversion, and relevant research is in progress.

ORCID

Yuanhai Su: 0000-0002-0718-301X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We would like to acknowledge financial support from the National Natural Science Foundation of China (Grants 21676164, 51733007, and 21706157), the Basic Research Project of Shanghai Science and Technology Commission (16JC1403900), National Program on Key Basic Research Project of China (2014CB643605), and the Shanghai Academy of Spaceflight Technology.

■

■

CONCLUSION The preparation of CBDA through photodimerization of maleic anhydride in capillary microreactors within gas−liquid Taylor flow was realized with the UV irradiation. With the combined effect of the inert gas, Taylor flow, and ultrasound, the potential clogging of the capillary photomicroreactor could be avoided. A proper cooling unit was integrated into this photomicroreactor system in order to control the temperature and protect the high-pressure Hg lamp. Effects of various factors such as the power input of the high-pressure Hg lamp, the reactant concentration, the residence time, the capillary dimension, and the flow rate ratio of the gas to the liquid phase on the photochemical process were studied. Higher power input of the Hg lamp, lower reactant concentration, and longer residence time were beneficial for improving the CBDA yield. Both thin liquid films and internal circulations inside the liquid slugs within the Taylor flow regime in the capillary microreactors were found to be favorable for achieving homogeneous mixing. However, the light refraction phenomena possibly became more serious with the increase in the flow rate ratio of the gas to the liquid phase, which was not beneficial for the light absorbance of the reaction solution. The product yield could reach as high as 66.3% in the capillary microreactor with 0.5 mm inner diameter at a reaction time of 30 min with the use of the 500 W power input Hg lamp. Furthermore, the heat balance in the whole photomicroreactor system for the processes with and without photochemical transformations was analyzed, which can guide the optimized design of the cooling unit for stable operations of the photomicroreactor system. The high-pressure Hg lamp produced a large amount of heat when it emitted the UV

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Process Analysis on Preparation of Cyclobutanetetracarboxylic

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