Continuous-flow synthesis of Pigment Red 146 in a microreactor system

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Applied Chemistry

Continuous-flow synthesis of Pigment Red 146 in a microreactor system fajun Wang, yuncheng Ding, and Jianhong Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03045 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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Industrial & Engineering Chemistry Research

Continuous-flow synthesis of Pigment Red 146 in a microreactor system Fa-Jun Wang, Yun-Cheng Ding, Jian-Hong Xu＊ The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

＊

Corresponding author: [email protected] 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract In this study, a continuous-flow coupling reaction method has been developed for the synthesis of Pigment Red 146 (C.I. PR 146). A CFD simulation was used to evaluate the effects of the micromixer structures on the mixing process. After the optimization of the microreactor structures and the reaction conditions, the conversion of the coupling reaction reached more than 99% using microsieve pore dispersion reactor (two microsieve pores arranged side by side), and a smaller particle size and narrower particle size distribution were obtained than were obtained in conventional batch reactors. Furthermore, the lightness and the green and the blue hues of these pigment products were more intense than those of the C.I. PR 146 standard. The largest conformity of the hue of the pigment products compared with the C.I. PR 146 standard was obtained using a microsieve pore dispersion reactor under the optimized reaction conditions. Finally, a scaling-up study of the pigment synthesis process in the microreactor system was initially explored. In conclusion, the microreactor system led to an improvement of the pigment products, and this method has a good industrial value.

1 Introduction Organic color pigments have been used as colorants in many applications, such as color printing, cosmetics, coatings and plastics. Among the organic colored pigments, the azo pigments are the most well-known class pigments. One example is the Pigment Red 146 (C.I. PR 146). In general, azo pigments are synthesized by two reaction steps, namely, diazotization and coupling reactions (Scheme 1). Due to its strong blue light absorption, bright colored, strong heat and solvent resistance, C.I. PR 146 is mainly used as a pigment in printing paste, waterborne coatings and inks. As reported in the literature

, the color properties, which are primarily related to the conversion of the reaction, the

1,2

particle size and the particle size distribution, are key indicators for the azo pigments.

2


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Scheme 1. The coupling reaction resulting in C.I.PR 146. In industry, the azo pigments are often synthesized by batch processes using stirred tanks with volumes varying from 20 m3 to 80 m3. However, batch processes are more likely to lead to a board particle size distribution, a larger particle size and a lower conversation of the coupling reaction, which result in poor color properties1. Therefore, an alternative technology is needed to solve these problems. Recently, microreactor technology has attracted the attention of many researchers. The mixing, mass and heat transport performances can be enhanced significantly, which can improve the efficiency and yield of the reactions3,4. As reported in the literature, many kinds of microreactors have been developed to study various reactions 2-12. Over the past two decades, the microreactor technology has been used to synthesize azo pigments. Wille et al.13 reported the first successful synthesis of azo pigments in microfluidic devices, and two azo coupling processes were carried out with a high flow rate (80 mL/min). The azo pigments showed improvements in their color properties compared with those of pigments produces by batch processes, and the mean particle size was significantly reduced from 600 nm to 90 nm. Wille et al.2 also reported on the high efficiency of microdevices used in their laboratory and on a pilot-plant scale. The scaling-up efforts were not found by reproducing the laboratory-scale parameters based on the number-up concept. This research also illustrated that the product properties such as color strength and color shade were preserved on the pilot-scale. In contrast, the traditional batch process had serious scaling-up efforts. The synthesis of Pigment Yellow 12 in a micromixer

3



apparatus was developed by Pennemann et al.1 The mixing was enhanced by using a micromixer with multilamination of the thin lamellae. Compared with that of a batch process, a more rapid mixing of the reactants was achieved, which led to a narrower particle size distribution and a smaller particle size. The micromixer apparatus also increased the glossiness by 73% and the transparency by 66% compared with those of the Yellow 12 standard, while maintaining the tinctorial power. The continuous-flow synthesis of azo dyes in a microreactor system was also studied. The synthesis of the azo dye was the same as that of azo pigments, and as reported in the literature, excellent results, such as a high product yield and selectivity, and a short reaction time were obtained by using microreactor technology.14-17 However, to date, there are few reports about the continuous-flow synthesis of C.I.PR 146 in a microreactor system. In this work, we first focused on the process intensification of the C.I. PR 146 synthesis process. Microsieve dispersion reactors with different structures were studied to improve the conversion of the coupling reaction. A CFD simulation was also carried out to study the concentration distributions in different microsieve dispersion reactors to optimize the structures of the reactors. Then, the effects of flow rates, reaction temperatures and residences time on the conversion of the coupling reaction, the particle size and the particle size distribution were studied to optimize the reaction conditions. The rheological properties and colorant properties were studied and discussed in comparison with the C.I. PR 146 standard. Finally, a scaling-up study of the pigment synthesis process in the microreactor system was initially explored.

2 EXPERMENTAL 2.1 Materials Technical-grade Red base KD (3-Amino-p-anisanilide) with a mass fraction purity of ≥98% and 4


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Technical-grade naphthol AS-LC (N-(4-chloro-2,5-dimethoxyphenyl)-3-hydroxy-2-naphthamide) with a mass fraction purity of ≥98% were supplied by Shenyang Research Institute of Chemical Industry. Analytical reagent-grade hydrochloric acid with a mass fraction purity of 36% ~ 38%, and analytical reagent-grade concentrated sulfuric acid with a mass fraction purity of 95% ~ 98% were supplied by Beijing Tong Guang Fine Chemicals Company. Analytical reagent-grade sodium nitrite and sodium hydroxide with mass fraction purities of ≥99% were supplied by Shanghai Macklin Biochemical Co., Ltd. Chemically pure Turkey Red oil with a mass fraction purity ≥50% was supplied by Sinopharm Chemical Reagent Beijing Co., Ltd. The C.I. PR 146, a standard commercial products was supplied by the Shenyang Research Institute of Chemical Industry.

2.2 Preparation of the reactant solution Solution 1 (Diazonium salt): Red base KD (3-Amino-p-anisanilide) (0.0151 mol) was dissolved in deionized water (100 mL) in a 200 mL beaker, and then concentrated hydrochloric acid (4.420 g) was added into the beaker with stirring. Next, the solution was brought to a volume of 150 mL with deionized water, and the solution was continuously stirred at room temperature until it transformed into a white slurry. The transformation process is shown in Figure 1.

Figure 1. The slurrying process of Red base KD

The white slurry of Red base KD was placed in an ice-water bath (0~5 OC) with stirring, and 30%

5



sodium nitrite (1.066 g) was added dropwise until the white slurry was completely dissolved. Then, the reaction solution was tested with potassium starch iodide paper. When the potassium iodide test paper turned blue, it indicated that the Red base KD reacted completely. Furthermore, because nitrous acid affects the coupling reaction, sulfamic acid was introduced to react with the excess sodium nitrite to avoid the generation of nitrous acid. The synthetic diazonium salt solution is shown in Figure 2.

Figure 2. The diazonium salt solution Solution 2 (Coupling component): Naphthol AS-LC (N-(4-chloro-2,5-dimethoxyphenyl)-3hydroxy-2-naphthamide) (0.0151 mol) was dissolved in deionized water (100 mL) in a 200 mL beaker, and then Turkey Red oil (1.00 g) and sodium hydroxide （ 3.75 g) were added into the beaker with stirring. Next, the solution was brought to a volume of 150 mL with deionized water and heated to 90 C until the naphthol AS-LC was dissolved completely. Finally, the solution was filtered with suction

O

and the filter liquor was placed in a beaker for the next reaction.

2.3 Experimental procedure for the continuous process The experimental setup of the C.I. PR 146 synthesis microreactor system is shown in Figure 3. Two constant-flow pumps were used to pump solution 1 (dazonium salt) and solution 2 (coupling component) into the microreactor system, and then the two solutions were mixed by the micromixer. A delay loop tube was connected to the outlet of the micromixer. Both the micromixer and the delay loop tubes were submerged into the water bath to control the reaction temperature. The micromixer was made of stainless steel, and the delay loop tube was made from PTFE (Poly tetra fluoroethylene) with 6


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an outer diameter of 3 mm and an inner diameter of 2 mm. The residence of the reaction was controlled by changing the length of the PTFE coiled tubes. The pigment products were obtained at the exit of the experimental setup. After the collection was compete, the products were stirred for 5 min. To stop the pigment crystallization process, a resin solution was added into the collection tank1. This stirring step was complete after 15 min. Then, the reaction mixture was heated to 90 OC to break up a mass of aggregated pigments and then cooled to room temperature. Finally, the products were filtered by air pump filtration, washed by water and dried for 12 h. As shown in Figure 4, the micromixer used in this work included different forms of mixing, such as membrane dispersion with a microsieve pore of 50 μm and microsieve dispersion with a microsieve pore of 300 μm. The dimensions of the microchannel were 10 mm (length)×2 mm (wide)×0.33 mm (height). As reported in the literature, this kind of micromixer has been used in various chemical synthesis processes 18-21, and its scale-up was reported in the patent 22.

1-Diazonium salt tank; 2-Coupling component tank; 3-Pump 01; 4-Pump 02; 5-Micromixer; 6-Delay loop tube; 7-Pigment Red 146 tank.

Figure 3. Experimental setup of the C.I. PR 146 synthesis microreactor system

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Figure 4. The schematic diagram and the photographs of the micromixer 2.4 Sample analysis The conversion of the coupling reaction was calculated using the filter paper percolation ring test. According to the chemical reaction equation of coupling reaction, the reaction is complete when the concentrations and volume flow rate are in equilibrium. When the reaction mixture was dropped onto the filter paper, a percolation ring was detected by H-acid to indicate whether the diazonium salt was completely consumed. When the percolation ring turned purple, it indicated that there was diazonium salt remaining, and the coupling component was then added to the reaction mixture until the purple of the percolation ring disappeared within 5 s. The amount of the coupling component that was added dropwise was weighed to calculate the conversion of the coupling reaction. The formula is as follows:

xc 

m1  m2  100% m1

（1）

xc: the conversion of the coupling reaction; m1: the total mass of the coupling component participating in the reaction;

8


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m2: the mass of the coupling component added after the reaction. The particle size distribution of the pigment was investigated using laser diffraction (Mastersizer 2000, Malvern Instruments Ltd., Malvern, UK). For the characterization experiments, the azo pigments were dispersed with adhesive varnish. Both the azo pigment (0.2000 g) and the adhesive agent (0.8000g) were put into a disk mill and milled four times for 140 s, 50 laps each time. Finally, the pigment sample was scraped onto sulfuric acid paper, and the relevant data for the hue was measured using a colorimeter (colorimeter SC-10, Shenzhen 3nh Technology Co., Ltd., CHN). To characterize the purity of the azo pigments, a quantity of azo pigment was dissolved in the concentrated sulfuric acid (98%). and the absorbance of the sample was measured using a UV spectrophotometer (UV-2450, Shimadzu, JPN)

2.5 Batch reaction The pigments were also synthesized by a batch process. First, the coupling component was put into a 1 L beaker and then the diazonium salt was added slowly with stirring. After the addition was complete, the reaction mixture was stirred for 3 min. Finally, the resinating step was carried out.

3 RESULTS AND DISCUSSION 3.1 The study of the microreactor structures 3.1.1 The effect of micromixer structures on the conversion of the coupling reaction This section discusses the conversion of the coupling reaction using different micromixing structures, including one microsieve pore dispersion reactor in which two microsieve pores were arranged side by side and one in which the microsieve pores were arranged back and forth in the continuous phase flow direction. Membrane dispersion was also considered. The volume flow rates of the reactant solutions were set at 30 mL/min. The resident time of the coupling reaction was 40 s, and 9



the reaction temperature was 25 OC. One of the difficulties of the coupling reaction was the poor water solubility of naphthol AS-LC. This material dissolves in strong base solution at high temperatures and tends to precipitate in neutral and acidic environments. When the coupling reaction is not uniformly mixed, the precipitation of naphthol AS-LC will occur. In addition, the precipitated components will be coated with pigment particles, and the conversion of the coupling reaction will decrease, causing the quality of the pigment to deteriorate. Therefore, it is important to improve the conversion of the coupling reaction. The particle size and the particle distribution were measured by laser diffraction. The definition of the size distribution is based on the assumption that the size is defined within multiple subsets. The number of particles and the size of the particles are then found in these subsets. Depending on the cumulative percentage of this distribution, various diameters can be defined. For example, d(0.1) is a volume-based diameter with a cumulative percentage of 10%, and d(0.5) is a volume-based diameter with a cumulative percentage of 50%, and is equivalent to the median diameter. The diameter d(0.9) is a volume-based diameter with a cumulative percentage of 90%. D[4.3] is the volume average particle size. According to Table 1, the particle size was not obviously changed. Thus, there was not a significant effect on the particle size from the different micromixing structures. As shown in Figure 5, the conversion of the coupling reaction using membrane dispersion was better than that of the other mixing forms. Unfortunately, the membrane dispersion was not suitable for a long-time continuous operation because the pigment products accumulated on the surface of the membrane, leading to an increase in the pump pressure from 0.2 Mpa to >1 Mpa. A considerable amount of pigment was found adhering to the surface of the membrane and clogging the micromixer. Using microsieve pore 10


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dispersion for mixing solved the clogging problems. The pore size of the microsieve reactor used in this study (300 μm) was two orders of magnitude larger than the particle size of the pigments. Therefore, the pigment particles passed through the pores more easily. First, the microsieve pore dispersion reactor with two microsieve pores arranged back and forth was studied, and the conversion of the coupling reaction ranged from 87% to 89%. The reason for the relatively low conversion was the poor mixing effect. The two microsieve pores arranged back and forth may have led to a long mass transfer distance for the diazonium salt passing through the microsieve pores and diffusing to the boundary of the microchannel. The coupling reaction was not complete immediately, and the mixed solution in the microchannel was converted from alkaline to acidic. The coupling component precipitated easily in acidic solution, leading to an incomplete utilization of the coupling component and a low conversion of the coupling reaction. To solve this problem, the microsieve pore dispersion reactor with two microsieve pores arranged side by side was used, which resulted in a higher conversion of the coupling reaction (96.99%) and the absence of blocking phenomena during a long-time operation. The equal weight products that were synthesized by different reactions of the coupling components were dissolved in concentrated sulfuric acid (98%). The absorbance was measured using a UV-visible spectrophotometer. As shown in Figure 6, product 1 was synthesized using the microsieve pore dispersion reactor with two microsieve pores arranged side by side; and product 2 and product 3 were synthesized using the microsieve pore dispersion reactor with two microsieve pores arranged back and forth. The absorbance decreased as the conversion of the coupling reaction decreased from 100% to 81.15%. Therefore, the conversion of the coupling reaction needs to be improved to obtain a better product quality. 11



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Table 1. The particle sizes of azo pigments and the conversion of the azo coupling reactions using different mixing methods Particle size/μm Equipment

d(0.1)

d(0.5)

d(0.9)

D[4,3]

Microsieve pore dispersion

2.934

5.757

12.097

7.397

（arranged back and forth）

3.079

6.071

12.374

7.465

3.477

7.671

18.877

10.012

3.163

6.499

14.449

8.291

Microsieve pore dispersion

2.695

5.677

12.995

7.610

(arranged side by side)

3.190

7.623

21.080

10.305

2.601

5.234

11.291

7.247

2.829

6.178

15.122

8.387

4.317

7.790

13.736

9.801

Membrane dispersion

3.393

6.311

11.953

9.068

（50 μm）

3.245

6.467

11.908

9.673

3.652

6.856

12.412

9.514

Average

Average

Average

Figure 5. The conversion of coupling reaction with different micromixing structures

12


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Figure 6. The U-V absorption curves of the pigments with different conversions of the coupling reaction. (product 1 synthesized by the microsieve pore dispersion reactor with two microsieve pores arranged side by side; product 2 and product 3 synthesized by the microseive pore dispersion reactor with two microsieve pores arranged back and forth ) 3.1.2 The characterization of the micromixing performance in the microsieve pore dispersion reactor To gain a better understanding of the mixing process, a CFD simulation was carried out using commercial software, Fluent 6.3. The purpose of the simulation was to gain an understanding of the flowing and mixing processes, therefore, the reactions were not considered in the simulations. The dimensions used in the simulation were the same as those of the microsieve pore dispersion reactors and the volume flow rates of the two solutions were 60 mL/min. Furthermore, steady state, incompressible liquid flow, and isothermal conditions were employed and the gravity was ignored. At the exit, a fixed reference pressure (p=0Pa) was specified. In the computational domain, no-slip boundary conditions were set at all walls. Although the Reynolds number of our system was approximately 900, it corresponded to laminar flow for traditional equipment. However, the critical Reynolds number from laminar flow to turbulence flow can be reduced in microchannel3,23-24. Considering the small scale of the microchannel and the mixing enhanced by the rapid reactions, the K-Epsilon Model was selected for the simulations. The convergence condition was set as the RMS Res

13



less than 1×10-4. A three-dimensional grid was created using GAMBIT 2.4.6. As shown in Figure 7, to verify the grid independence, the number of cells was varied from 100,000 to 600,000. The results showed that the mass fraction of reactant 1 and the flow rate of the reactants were not obviously changed as the number of cells varied from 400,000 to 600,000; the grid independence of the CFD simulation was verified as well. In this study, we selected the 500,000 cells of grid. As shown in Figure 8, the inside of the microreactor was chosen as the simulation region. The dimensions of the microchannel were 10 mm (length) × 2 mm (wide) × 0.33 mm (height), and the diameter of the micropore was 0.33 mm.

Figure 7. Grid independence study of the CFD simulation

Figure 8. The simulation model of the microreactor: (a) two micr-sieve pores were arranged back and forth；(b) two microsieve pores were arranged side by side. 14


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As shown in Figure 9, the velocity profiles indicated that a regular velocity distribution was obtained. Figure 10 shows the concentration profiles of solution 1 for the different reactor structures, and an X-Y coordinate system was established to evaluate the concentration distribution of solution 1 at different locations.

Figure 9. Velocity profiles of different microsieve pore arrangements

Figure 10. Concentration profiles of different microsieve pore arrangements Figure 11 and Figure 12 respectively show the concentration distributions of the micromixing processes with two microsieve pores arranged back and forth and arranged side by side, respectively. The figures show that a higher concentration distribution development can be obtained during the mixing process using the microreactor with two microsieve pores arranged side by side. To compare 15



the difference in the concentration distribution intuitively, we define X=0.8 mm (X= -0.8 mm) to X=1.5 mm (X=-1.5 mm) as the edge part of the channel. As shown in Figure 13, there is clearly a higher concentration distribution for two microsieve pores arranged side by side than for two microsieve pores arranged back and forth at X=0.8 mm\X=1.1 mm\X=1.5 mm and Y=0~0.5 mm. The higher concentration distribution leads to a better mixing effect and a faster reaction. A high conversion of the coupling reaction can also be obtained.

Figure 11. Concentration profiles of different locations wtih two micropores arranged back and forth.

Figure 12. Concentration profiles of different locations wtih two micropores arranged side by side.

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Figure 13. Concentration profiles of different microsieve pore arrangements at X=0.8 mm\X=1.1 mm\X=1.5 mm and Y=0~0.5 mm (X1: two micropores arranged side by side; X2: two micropores arranged back and forth) 3.2 The optimization of the reaction conditions using the microsieve pore dispersion reactor (two microsieve pores arranged side by side) 3.2.1 The effect of the residence time on the coupling reaction The coupling reaction between phenol and diazonium salts is generally carried out under strong alkaline conditions and the reaction was carried out slowly under strong acidic conditions. To study the effect of the resident time on the coupling reaction, the pigment mixture solution at the outlet of the microreactor system was directly passed into a strong sulfuric acid solution (pH=1~2) to quench the coupling reaction. The flow rates of the reactants were set at 30 mL/min and the reaction temperature was 20 OC. Then, the pigment products, which were synthesized using different resident times, were filtered by air pump filtration, washed by water, and dried for 12 h. The dried pigments were dissolved in concentrated sulfuric acid (98%), and the absorbance of the sulfuric acid solution of the pigment was detected by the UV-visible spectrophotometer. As shown in Figure 14, the C.I. PR 146 standard was used to obtain the linear relationship between the concentration and the absorbance, which can be used as standard working curve. As shown in Figure 15, the conversion of the coupling reaction increased with increasing the resident time. When the resident time is more than 30 s, a good conversion (>99%) can be achieved . 17



Figure 14. The sulfuric acid solution of the azo pigment red 146: Concentration and absorbance linear relationship.

Figure 15. The conversion of the coupling reaction at different resident times

3.2.2 The effect of the temperature on the coupling reaction In this section, the effect of the reaction temperature on the coupling reaction is discussed. The flow rates of the reactants were set at 30 mL/min, and the reaction time was 30 s. The purity of the pigment product was obtained using the standard working curve (Figure 14). As shown in Figure 16, a high purity of the pigment products was obtained when the reaction temperature was 25 OC. When the temperature was higher than 25

C, the purity decreased because of the decomposition of the

O

diazonium salts. The effects of the reaction temperature on the particle size and particle size distribution were studied. Figure 17 shows that d(0.1) and d(0.5) were essentially unchanged at various temperatures (10 OC~50 OC). However, d(0.9) showed an increasing trend as the temperature increased.

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As shown in Figure 18, the particle size distribution at different temperatures were essentially the same. Noted that it was possible for the diazonium salt to decompose as the reaction temperature increased, so room temperature (25 OC) is an appropriate temperature for the reaction.

Figure 16. The purity of the pigment products at different reaction temperatures

Figure 17. Various percentiles of the volume-base particle diameter at various temperature

Figure 18. The particle size distributions of the different concentrations of reactants particle diameters at various temperatures

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3.2.3 The effect of the volume flow rates on the coupling reaction The volume flow rate was also considered in this work and was varied from 10 mL/min to 50 mL/min to study its effect on the conversion of the coupling reaction and the particle size and the particle size distribution of the pigment products. The resident time of the coupling reaction was 30 s and the reaction temperature was 25 OC. As shown in Figure 19, it shows that the conversion of the coupling reaction increased as the volume flow rate increased from 10 mL/min to 30 mL/min, and the conversion was essentially unchanged as the volume flow rate increased from 30 mL/min to 50 mL/min. The results in Figure 20 show that the particle size generally decreased as the volume flow rate increased. As the volume flow rate was increased from 10 mL/min to 30 mL/min, d(0.9)and d(0.5) decreased significantly, and they decreased slowly when the flow rate was greater than 30 mL/min. Additionally, d(0.1) was essentially unchanged as the volume flow rate increased from 10 mL/min to 50 mL/min. Figure 21 shows that the low volume flow rate resulted in a broad particle size distribution, and narrower particle size distributions were achieved at higher volume flow rates. When the flow rates were increased, the mixing performance was improved in the micromixer system, and a high volume flow rate can reduce the backmixing and prevent the pigment particles from agglomeration. Based on these results, the volume flow rate should be greater than 30 mL/min.

Figure 19. The particle size distribution of the different concentrations of reactants particle diameter using various temperatures

20


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Figure 20. Various percentiles of the volume-base particle diameters using various flow rates

Figure 21. The particle size distribution using various flow rates

3.3 The comparison of the particle size distribution and colorant properties between the PR 146 standard and the pigment products, and the rheological properties of the pigment products. The C.I. PR 146 was synthesized using the microsieve pore dispersion reactor (two microsieve pores arranged side by side) under the optimized conditions. The volume flow rates of the reactants were greater than 30 mL/min. The resident time of the coupling reaction was more than 30 s and the reaction temperature was 25 OC. To compare the microreactor system with the batch process, the pigment products were also synthesized by the batch process. As illustrated in Table 2 and Figure 22, smaller particle size and narrower particle size distribution were obtained using the microreactor system. When the pigment products were synthesized by the batch process, they were highly agglomerated, which led to a broad particle size distribution with a maximum at approximately 60 µm. 21



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Table 2. Comparison of the volume-based particle diameters of the azo pigments synthesized using the microreactor system and the batch process. Equipment

d(0.1)

Microreaction system

3.320μm

7.318μm

15.843μm

9.084μm

0.661μm

11.682μm

61.987μm

25.195μm

Batch reactor

d(0.5)

d(0.9)

D[4,3]

Figure 22. The particle size distribution of the microreactor system and the batch process The dates for the hue of the pigment products were measured using a colorimeter. The coordinates of the CIELab color space ( L*, a*, b*) were obtained to evaluate for the pigment products. The L* represents the brightness of the sample, L* is the lightness axis, a* represents the green (-) / red (+) axis, and b* represents the blue (-) / yellow (+) axis. The differences in these coordinates between the pigment products and the C.I. PR 146 standard were calculated, which led to the color difference △Eab* as a measure of the Euclidean distance between the two colors. From Table 3, the coordinates for the redness/greenness a* of the pigment products were smaller than those of the C.I. PR 146

standard, which indicates that the green hue of these pigment products was slightly more intense. The differences can also be found for the the lightness L* and the yellowness/blueness b*. The lightness L* of the pigment products was slightly larger than that of the C.I. PR 146 standard with increasing volume flow rate and the blue hue of these pigments was larger with a higher volume flow rate. The overall differences of the pigment products were characterized by the color differences ∆Eab * listed in 22


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Table 3, which indicate that the largest conformity of the pigment products was obtained compared with the hue of the C.I. PR 146 standard with a volume flow rate of 30 mL/min. Figure 23 depicts sulfuric acid paper scraped with two pigment samples. The left section is the Pigment Red 146 standard, and the right section is the product synthesized using the microreactor system. In our experiment, the data for the hue of the pigments was measured by a colorimeter. However, for qualitative research, the method of visual detection can be used to measure the color saturation and transparency of the pigment products. Figure 23 shows that the yellow hue of the Pigment Red 146 standard was larger than that of the pigment products produced by the microreactor system, and the black part can also be seen in the sulfuric acid paper, which is used to observe the differences in transparency between the pigments standard and the products. The pigment standard can be clearly observed on the surface of the black part (part ①), while the pigment products can not be seen on the surface of the black part (part ②). Therefore, the color saturation and transparency can be increased compared with the pigment standard using the microreactor system.

Table 3. CIELab-based colorimetric evaluation of the pigments

Sample

L*

30 mL/min

Average 35 mL/min

Average C.I.PR 146 standard

a*

b*

ΔL

Δa

Δb

48.56

53.08

13.39

0.93

-0.12

0.08

△Eab*

48.55

52.59

13.25

0.92

-0.61

-0.06

1.11

47.79

51.77

13.31

0.16

-1.43

0.77

1.63

48.30

52.48

13.32

48.48

52.03

13.28

0.85

-1.17

-0.03

1.45

47.97

51.45

13.26

0.34

-1.75

-0.05

1.79

47.88

51.58

12.79

0.25

-1.62

-0.52

1.72

48.11

51.69

13.11

47.63

53.20

13.31

0.94

1.23

23


1.65


Figure 23. The diagram of the color contrast Finally, the rheological properties of the C.I.PR 146 were investigated by measuring the relationship between the shear stress and the shear rate as well as the relationship between the dynamic viscosity and the shear rate. In addition to the volume flow rates, the other optimized synthesis conditions were utilized. The results are illustrated in Figure 24 and Figure 25. When the shear rate was low (100s-1), the dynamic viscosity was essentially a fixed value, which is typical for Newtonian fluids. There was not significant trend with different flow rates, but a lower dynamic viscosity was obtained with a flow of 50 mL/min. The C.I. PR 146 synthesized with a flow of 50 mL/min exhibited a more obvious shear thinning behavior. Furthermore, as shown in Figure 25, the dynamic viscosity increased significantly at lower shear rates, which is characteristic of a fluid with shear-thinning properties.

Figure 24. Flow curves of the pigment dispersions. 24


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Figure 25. Dependence of the dynamic viscosity on the shear rate

3.4 The scaling-up study of the pigment synthesis process in the microreactor system The scaling-up study of the pigment synthesis process in the microreactor system was initially explored. The volume flow rates of the reactants were increased to 100 mL/min, and as shown in the Figure 26, the number of microsieve pores was increased to four to reduce the pressure of the system. The conversion of the coupling reaction and the pressure of the two pumps are shown in Table 4. A high conversation of the coupling reaction (>99%) was obtained and operating for 30 min, the two pump pressures did not exceed 0.2 Mpa. The number of the microsieve pores can be increased more than four with the volume flow rates increasing, which can be used to enhance the mixing effect and reduce the pressure of the system for the process of scaling-up in industry .

Figure 26. The scale-up process of the number of the micropores

25



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Table 4.The conversion and the pressures with microseive pore dispersion (four microsieve pores) at a flow rate of 100 mL/min. No.

Conversion/%

Pressure/Mpa

Pressure/Mpa

(coupling reaction)

(Pump01)

(Pump02)

1#

99.50%

0.2

0.1

2#

99.60%

0.1

0.1

3#

99.86%

0.2

0.2

4 Conclusion In this work, we demonstrated a significant improvement in the C.I. PR146 synthesis process in the microsieve pore dispersion reactor system with a high conversion of the diazo-coupling reaction (>99%). The CFD simulation was used to evaluate the effects of the micromixer structures on the micromixing processes. After an optimization of the microsieve pore dispersion reactors structures and the reaction conditions, a smaller particle size and a narrower particle size distribution were obtained using the microreactor system than were obtained using the batch process. The rheological properties and colorant properties of the C.I. PR 146 were also studied. The largest conformity of the azo pigments was obtained compared with the hue of the C.I. PR 146 standard under the optimized microreactor structure and the reaction conditions. Finally, the scaling-up study of the pigment synthesis process in the microreactor system was initially explored. In conclusion, the microreactor system leads to an improvement in the pigment products, and this method has a great industrial value.

Acknowledgement The authors gratefully acknowledge the supports of the National Natural Science Foundation of China (21476121,21322604) for this work.

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References (1) Pennemann, H.; Forster, S.; Kinkel, Improvement of Dye Properties of the Azo Pigment Yellow 12 Using a Micromixer-Based Process. Org. Process Res. Dev. 2005, 9, 188. (2) Wille, Ch.; Gabski, H.- P.; Haller, Th. Synthesis of pigments in a three-stage microreactor pilot plant—an experimental technical report. Chem. Eng. J. 2004, 101, 179. (3) Zhang, J. S.; Wang, K.; Lu, Y. C.; Luo, G. S. Characterization and modeling of micromixing performance in micropore dispersion reactors. Chem. Eng. Prog. 2010, 49, 740. (4) Jähnisch, K.; Hessel, V.; Löwe, H.; Baerns, M. Chemistry in Microstructured Reactors. Angew. Chem. Int. Ed. 2004, 43, 406. (5) Long, M.; Sprague, M. A.; Grimes, A. A.; Rich, B. D. Khine, M.; A simple three-dimensional vortex micromixer. Appl. Phys. Lett. 2009, 94, 133501. (6) Wong, S. H.; Ward, M. C. L.; Wharton, C. W. Micro T-mixer as a rapid mixing micromixer. Sens. Actuators B. 2004, 100, 359. (7) Greenway, G. M.; Haswell, S. J.; Morgan, D. O.; Skelton, V. The use of a novel microreactor for high throughput continuous flow organic synthesis. Sens. Actuators B. 2000, 63, 153. (8) Huang, J. P.; Sang, F. N.; Luo, G. S.; Xu, J. H. Continuous synthesis of Gabapentin with a microreaction system. Chem. Eng. Sci. 2017, 173, 507. (9) Miyazaki, M.; Honda,T.; Nakamura, H.; Maeda, H. Development of a Microreactor for Amino Acid Polymerization. Chem. Eng. Technol. 2007, 30, 300. (10) Xia, S.T.; Ding, X. F.; Wang, Y. J. Continuous-Flow Synthesis of an Important Liquid-Crystal Intermediate Using a Microreaction System. Ind. Eng. Chem. Res. 2018, 57, 12113. (11) Chen, G. G.; Luo, G. S.; Xu, J. H.; Wang, J. D. Membrane dispersion precipitation method to prepare nanoparticles. Powder Technol. 2004, 139, 180. (12) Ying, Y.; Chen, G.; Zhao, Y.; Li, S.; Yuan, Q. A high throughput methodology for continuous preparation of monodispersed nanocrystals in microfluidic reactors. Chem. Eng. J. 2009, 135, 209. (13) Wille, C.; Autze, V.; Kim, H.; Nickel, U.; Oberbeck, S.; Schwalbe, T.; Unverdorben, L. In proceedings of the 6th international conference on microreaction technology. IMRET 6, March 11-14, New Orleans, LA, U.S.A. 2002, 7. (14) Wootton, R. C. R.; Fortt, R.; De Mello, A. J. On-chip generation and reaction of unstable 27



intermediates—Monolithic nanoreactors for diazonium chemistry: Azo dyes. Lab Chip. 2002, 2, 5. (15) Akwi, F. M.; Bosma, C.; Watts, P. A facile optimization of diazotization and phase transfer catalyzed azo-coupling reactions in microreactors. J. Flow Chem. 2016, 6, 73. (16) Akwi, F. M.; Watts, P. Study on the scale-up of phase-transfer-catalyzed azo coupling reactions in flow reactors. Chem. Heterocycl. Compd. 2016, 52, 943. (17) Wang, F. J.; Huang, J. P.; Xu, J. H.; Continuous-flow synthesis of azo dyes in a microreactor system. Chem. Eng. Prog. 2018, 127, 43. (18) Du, Ch. C.; Zhang, J. S.; Luo. G. S. Organocatalyzed Beckmann rearrangement of cyclohexanone oxime in a microreactor: Kinetic model and product inhibition. AIChE J. 2018, 64, 571. (19) Zhang, J. S.; Wang, K.; Lin, X. Y.; Lu, Y. C.; Luo, G. S. Intensification of fast exothermic reaction by gas agitation in a microchemical system. AIChE J. 2014, 60, 2724. (20) Kockmann, N.; Kiefer, T.; Engler, M.; Woias, P. Convective mixing and chemical reactions in microchannels with high flow rates, Sens. Actuators B. 2006, 117, 495. (21) Zha, L. Pu, X.; Shang, M. J.; Li, G. X.; Xu, W. H.; Lu, Q. H.; Su, Y. H. A study on the micromixing performance in microreactors for polymer solutions. AIChE J. 2014, 60, 2724. (22) Xu, J. H.; Luo, G. S.; Lu, Y. C.; Wang, Y. J.; Chen, G. G. Multi-channeled micro-structured reactor, China Patent CN 1736577; Tsinghua Unv.: CN, issued July 08, 2005. (23) Peng, X. F.; Peterson, G. P.; Wang, B. X. Heat transfer characteristics of water flowing through microchannels. EXP HEAT TRANSFER. 1994, 7, 265. (24) Mala G. M.; Li, D.; Dale, J. D. Heat transfer and fluid flow in microchannels. Int. J. Heat Mass Transfer, 1997, 40, 3079.

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Abstract graphic

29


Continuous-flow synthesis of Pigment Red 146 in a microreactor system

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