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Development of a New Microreactor Based on Annular Microsegments for Fine Particle Production Hideharu Nagasawa†,‡ and Kazuhiro Mae*,‡ AdVanced Core Technology Laboratories, Fuji Photo Film Company, Ltd., 210, Nakanuma, Minamiashigara-shi, Kanagawa 250-0193, Japan, and Department of Chemical Engineering, Kyoto UniVersity, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
A new type of microreactor for fine particle production was developed to achieve stable continuous production, precise control of particle properties (such as particle size, distribution, and shape), and high throughput. The proposed microreactor, the annular multi-lamination microreactor, had an annular five-layer structure in a column at the center of the microreactor. By utilizing the microreactor, the crystallization of fine silver halide particles was examined under various operational conditions. From these results, optimum structures and operation methods for the high-throughput production of fine particles, which do not cause clogging of the microchannel, were determined. In addition, we give a simple method of equaling-up for industrial production. Thus, it is expected that the concepts of design, operational method, and equaling-up shown here can be incorporated into practical microreaction technology in the industrial field of fine particle production. 1. Introduction Fine particles are widely used as materials of many types of chemical industry products, for example, photofunctional inorganic fine particles for photographs and electronic displays. The performance of these products is strongly affected by particle properties, such as the particle sizes, the distributions of the sizes, and the shapes. Therefore, to keep up with the trend of the fine chemical industry toward industrial production of fine particles, it is essential to control the particle properties precisely. Fine particle are produced by two methods: one is a breakdown method and the other is a buildup method. The breakdown method produces fine particles mainly via physical operations, such as milling raw materials mechanically. On the other hand, the buildup method produces fine particles mainly by chemical reaction or crystallization. Generally, the buildup method is more suitable than the breakdown method for producing fine particles with a narrow shape distribution. In the buildup method, large-scale batch reactors are used in the conventional industrial production of fine particles. However, the particle size and the distribution of particle size formed in the conventional manner do not satisfy the requirements for the fine particles. This is because the rate of forming fine particles is much faster than the mixing rate of the reactant. In particular, this problem is great when using a large-scale batch reactor. To overcome this problem, microreaction technology has been attracting attention as a method for fine particle production. It is expected that a microreactor will be able to control particle properties, using its inherent advantages, such as precise operability of mixing and reaction. Many investigations into the formation of inorganic and organic fine particles have been made from this viewpoint.1-13 Nakamura et al. studied the formation of cadmium selenide fine particles, utilizing a simple microreaction system that consisted of syringe pumps, a capillary tube, and an oil bath. They then showed that single nanoparticles were formed and the nano* To whom correspondence should be addressed. Tel:+81-75-3832668. Fax: +81-75-383-2658. E-mail:
[email protected]. † Advanced Core Technology Laboratories, Fuji Photo Film Company, Ltd. ‡ Kyoto University.
particle sizes were precisely controlled by accurate adjustment of reaction temperature.1 Wang et al. produced multilayer nanoparticles with a core-shell structure. They covered cadmium selenide with zinc sulfide, using a multistage microreaction system, and showed that the fluorescence efficiency of the prepared particles was better than that of only cadmium selenide.2 Further, Wille et al. produced fine pigment particles, utilizing a multilamination-type microreactor, and showed that the size of fine particles formed by the microreactor was smaller than that produced by a conventional batch method. Nagasawa et al. formed polystyrene fine particles, utilizing a new micromixer, and showed that the particle size could be made smaller and its distribution narrower by instant mixing, based on the collision of microsegments.4 Lo¨b et al. presented a new concept of micromixing, using the separation-layer micromixer, which deliberately postpones and controls mixing and prevents particle precipitation on the reactor’s wall by having an inert fluid layer. Also, they realized two types of the separation-layer micromixer: a circular type and a planar type, with concentric and stacked lamellae, respectively.12 Thus, it has been demonstrated that the microreactor is a promising tool for improving particle properties, in light of its mixing and temperature control. The microreactor is advantageous, especially because mixing is the most crucial factor in control of nucleation formation and particle growth. In the microreaction technology, mixing is classified into two categories: instant mixing and precise mixing controlled by molecular diffusion. The instant mixing by a micromixer is a useful way to produce fine particles, as shown by Nagasawa et al.;4 however, it has limitations in its ability to achieve desired particle size by extremely rapid nucleation and uniform growth of particles. The other mixing type, precise mixing via interface by controlled molecular diffusion, has the potential to control the nucleation and the growth rate of nanoparticles. Generally, the diffusion between two fluids is easy to control in a microchannel. However, the microchannel has a risk of clogging by the precipitated particles, depending on such particle synthesis conditions as the microchannel dimensions and the type of processing. Stable continuous production, which does not cause clogging of a microchannel and has high throughput, is also important for industrial production. In this paper, a new type
10.1021/ie050869w CCC: $33.50 © 2006 American Chemical Society Published on Web 03/04/2006
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2. Materials and Experiment
Figure 1. Overview of the microreactor.
of microreactor that has a cylindrical contracted flow channel and an annular microreaction channel for fine particle production, as concretely described later, is presented based on the above considerations and also in consideration of the fabrication cost of the microreactor, and several distinct operational methods are investigated for stable continuous production of fine particles with high throughput.
2.1. Proposed Microreactor. A new microreactor, the annular multi-lamination microreactor, has been developed, as shown in Figure 1, to realize the aforementioned goals. Figures 2a and 2b show the separate parts, and the internal structure and a schematic of the flow, in the cylindrical microreactor, respectively. The external appearance is almost rectangular, and this microreactor has a thermal jacket to allow control of the reaction temperature. The reaction section consists of some stainless steel parts, a column, a cylindrical outer pipe, and four inner cylindrical pipes with different diameters, forming partitions for injecting five different fluids. These parts are all arranged coaxially and create a 5-fold nested annular multilamination structure. The operation of this microreactor are summarized as follows: Five different fluids of any type are injected in each of five annular microspace layers from the left side of inlet zone to be formed by the 5-fold nested annular multi-lamination structure. These fluids then take the form of five annular segments at the end of the four cylindrical partition pipes, position A, where all five fluids contact, and go through a microchannel in the mixing and reaction zone while keeping this segment formation. The microchannel has a contracted flow section that joins the cylindrical segments, just after position A. In this section, the microchannel becomes narrow while keeping the annular segments separated, and the mixing and reaction is promoted by reducing the diffusion distance. The five annular fluid segments have different roles. Outermost and innermost streams have the function of protection of the three middle fluids. When inactive fluids are flown in the outermost and innermost streams, these inactive fluids prevent the clogging of the passage with the particles in the middle streams. The
Figure 2. Details of the microreactor: (a) internal parts and (b) schematic of flow in the microreactor.
Ind. Eng. Chem. Res., Vol. 45, No. 7, 2006 2181 Table 1. Dimensions of Microreactors Fabricated Value parameter
A-ML-R-1
A-ML-R-2
A-ML-R-3
partial cross section of inlet zone
outer diameter of microchannel, OD inner diameter of microchannel, ID width of microchannel, W1 length of microchannel, L total width of inlet, W2 taper angle of contracted flow section, θ width of each segmenta A B C D E volume of microchannel, V
16.0 mm
16.0 mm
8.0 mm
15.8 mm
15.8 mm
7.8 mm
100 µm 150 mm 9.0 mm 32.9°
100 µm 150 mm 5.0 mm 18.6°
100 µm 150 mm 8.4 mm 45.0°
1 mm (3.11) 1 mm (2.03) 1 mm (1.51) 1 mm (1.20) 1 mm (1.00) 5.00 mL
0.80 mm (0.98) 0.65 mm (1.00) 0.55 mm (1.02) 0.50 mm (1.00) 0.45 mm (1.00) 2.59 mL
0.40 mm (1.56) 0.32 mm (1.30) 0.27 mm (1.16) 0.23 mm (1.09) 0.21 mm (1.00) 1.85 mL
a The value in brackets shows the ratio of the velocity of each segment to the velocity of segment E when the volume flow rates of each segment are the same.
Table 2. Experimental Conditions for the Crystallization of Silver Halide Using the Proposed Microreactor segment
solution
concentration (mol/L)
temperature (K)
flow rate (mL/min)
inactive fluid 1 reactant A reactant B reactant C inactive fluid 2
distilled water silver nitrate solution gelatin-halogen solution halogen solution distilled water
N/A 0.05 gelatin, 6 g/L; halogen, 0.0025 mol/L 0.05 N/A
293 293 293 293 293
2-50 2-50 2-50 2-50 2-50
inactive streams also have a role to promote stable transportation with low pressure drop when highly viscous fluids such as polymer solutions are flown in the middle streams. On the other hand, when inactive fluid is flown in a stream between reactant streams, the reaction starts after both reactants diffuse and encounter each other in the inactive fluid. This prevents the deposition of particles at the edge of the cylindrical partition pipe. The middle stream can be used to control the reaction field with certainty by selecting the kind of fluid. To clarify the specific nature of the aforementioned feature in the microreactor, we fabricated three types of annular multi-lamination microreactors, as listed in Table 1, and then examined the effect of the arrangement of microsegments, the flow rate, the dimensions of the microreactor, etc. on the particle size of silver halides. 2.2. Experimental Method. Two types of silver halide crystallization described by eqs 1 and 2 were conducted as model reactions for fine particle production. These crystallizations are suitable for checking the functions of the proposed microreactor, such as clogging or reaction controllability, because these reactions are fast and their products easily precipitate.
AgNO3 + NaCl f AgClV + Na+ + NO3-
(1)
AgNO3 + KBr f AgBrV + K+ + NO3-
(2)
Table 2 lists the experimental conditions for silver halide crystallization using the developed microreactor. An aqueous solution of 0.05 mol/L AgNO3 and an aqueous solution of 0.05 mol/L halide, such as NaCl or KBr, were used as reactants. An
aqueous solution of 6 g/L low-molecular gelatin (molecular weight of 20 000 g/mol), including 0.0025 mol/L NaCl or KBr, was used for the middle layer between the reactants. Also, distilled water was used for the inactive fluid of the outermost and innermost streams. The volume flow rate of each fluid was varied in the range of 2-50 mL/min, using five nonpulsation plunger pumps (HYM-06, Fuji Techno Industries Co.). The reaction temperature was kept at 293 K by controlling the thermal jacket temperature. On the other hand, silver halide crystallizations using a conventional batch method were also conducted for comparison. A 50-mL glass vessel and a magnetic stirrer were used for this experiment. The vessel was filled with a certain quantity of protective colloid solution and stirred well, then AgNO3 and halide were added into the solution, using submerged addition tubes at a fixed volume flow rate, using two syringe pumps (PHD2000, Harvard Co.). Table 3 shows the experimental conditions for silver halide crystallization using the batch vessel. The average particle size of crystallized silver halide fine particles was calculated from the absorbance measured by an ultraviolet (UV) spectrometer to be 700 nm for AgCl and at 750 nm for AgBr. The fine particles were also observed via transmission electron microscopy (TEM) to verify the average particle size and determine the particle size distribution and crystal habit. Generally, the silver halide grows by Ostwald ripening immediately, just after crystallization; therefore, a depressor of growth was added to the crystallized fine silver halide particle dispersion with rapid mixing. Here, the absorbance should be measured within 30 s to avoid a measurement error. A 2 wt % 4-hydroxy-6-methyl-1,3,3a,7-tetrazaindene
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Table 3. Experimental Conditions for Conventional Batch Reactor solution
concentration (mol/L)
temperature (K)
rotation speed (rpm)
injection rate (mL/min)
injection time (min)
gelatin-sodium chloride solution silver nitrate solution sodium chloride solution
a 0.05 0.05
293 293 293
900 N/Ab N/Ab
N/A 2 2
N/A 2.5 2.5
a The gelatin-sodium chloride solution in the glass vessel was a solution of 10 mL distilled water with 6 g/L low-molecular gelatin (molecular weight of 20 000 g/mol) and 0.0025 mol/L NaCl. b Not available.
solution (up to pH 9) was used as the depressor of growth for average particle size measurement and a 0.1 wt % 3-carboxymethyl-5- [2-(3-ethylthiazolidine-2-yliden)ethylidene] rhodanine solution was thus used for TEM measurement. 3. Results and Discussion 3.1. Performance of Preventing Clogging (Stable Continuous Production). To check the efficacy of prevention of clogging of the microchannel by arranging inactive fluids in the outermost and innermost streams, the AgCl crystallization test was conducted using one type of the annular multilamination microreactor, A-ML-R-2, which is listed in Table 1. The volume flow rate was 15 mL/min for each stream and other experimental conditions were as listed in Table 2. Figure 3 shows the pressure loss and the average particle size against elapsed time during AgCl crystallization when utilizing A-ML-R-2. The pressure loss per second was measured automatically by a pressure sensor, and the average particle sizes were measured approximately every 15 minutes by the UV spectrometer method. The pressure loss was kept at a small value, and the average particle sizes were constant at ca. 70 nm during continuous crystallization, indicating that a stable continuous production was attained by the proposed microreactor. This is due to the distinct flow regimes for preventing the contact of the particles on the wall of the microreactor, in which there are inactive fluids in the outermost and innermost streams, and the slower mixing between each fluid layer is caused by the fact that the fluid layer themselves are thicker. The annular flow was determined to be stable with laminar flow and stable under all the experimental conditions. Thus, it was confirmed that the microreactor’s structure and the arrangement of the inactive fluids on the outermost and innermost streams were extremely effective in preventing the clogging of the microchannel and the increase in pressure loss. 3.2. Control of Particle Size. 3.2.1. Control of Particle Size by the Middle Layer between Reactants. To examine the effect of a middle layer in preventing precipitation on the edge of the cylindrical partition pipe at position A and controlling the particle size, two types of operation for AgBr crystallization were conducted using A-ML-R-2. The experimental conditions are as shown in Table 2. Test 1 was performed in the
Figure 3. Demonstration of stable continuous production.
arrangement in which AgNO3 and KBr in protective colloid solutions were placed in direct contact, without a middle layer. Test 2 was performed under the arrangement in which protective colloid solution flowed between AgNO3 and KBr solutions as a middle layer. Figure 4a compares the average particle sizes of AgBr produced by test 1 and test 2. The average particle size decreased as the flow rate for each test increased. The average particle size produced by test 2 was much smaller than that by test 1 under all experimental conditions. The average particle size in test 2 was 13 nm at the flow rate of 50 mL/min. These results could be also confirmed from the TEM photographs, as shown in Figure 4b. The photograph in Figure 4c shows the edge of the cylindrical partition pipes after test 1. The precipitation of AgBr was observed at the edge of the cylindrical partition pipe between the AgNO3 and KBr solutions. There was no precipitation in test 2. The interpretation of these results is discussed later in detail. Next, to confirm the advantage of this operational method, crystallizations of AgCl and AgBr were conducted using A-MLR-2 and the conventional batch reactor. The experimental conditions for A-ML-R-2 and the batch method are listed in
Figure 4. Effect of the middle layer segment: (a) comparison of the average particle size in test 1 and test 2, (b) transmission electron microscopy (TEM) photographs of fine AgBr particles produced in test 1 (left) and test 2 (right) with and without a separating layer at 50 mL/min, and (c) photograph showing the precipitation at the edge of cylindrical partition pipe.
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Figure 5. Change in the average particle size of silver halide with the flow rate.
Figure 6. Effect of the inactive fluid flow rate on the average particle size of AgBr.
Tables 2 and 3, respectively. Figure 5 shows the average particle size of silver halide fine particle crystallization, plotted against the volume flow rate of each stream using A-ML-R-2. The average particle sizes produced by the conventional batch reactor were 75 nm for AgCl and 20 nm for AgBr, respectively, under fully mixing conditions. On the other hand, the average sizes of AgCl (54 nm) and AgBr (13 nm) produced when using A-MLR-2 at a flow rate of 50 mL/min were smaller than those of the batch reactor. The reason for these differences may be as follows: generally, the crystallization process consists of a nuclei formation process and a growth or aggregate process of nuclei. In the case of the batch reactor, both processes occur at the same time and space under a circulation flow in the vessel, so it is difficult to control the particle size. On the other hand, the proposed microreactor can separate the nuclei process and the growth or aggregate process by precise molecular diffusion control in a laminar flow, so it should be able to provide a fast reaction for particle formation. We can easily control the rates of nuclei formation and growth of particles just by changing the width of the middle layer, the concentration of reactants, and the flow rates. This suggests that the final particle size can be produced according to the needs of products by this proposed operational method with the annular multi-lamination microreactor. Summarizing these results, it was verified that the arrangement of a middle layer between reactants is an attractive method for controlling particle properties and achieving stable continuous production. 3.2.2. Controlling the Particle Size by Changing the Volume Flow Rate of Inactive Fluids. To examine the controllability of the average particle size by changing the volume flow rate of inactive fluids, fine AgBr particle crystallization was performed. In the experiment, the ratio of the inactive fluid volume flow rate to the reactant and protective colloid volume flow rate (QRatio) was set to be either 1 or 5. Other experimental conditions, such as segment layout, concentration, and temperature, are listed in Table 2. Figure 6 shows the average particle size of AgBr crystallization versus the mean residence time, which was calculated by dividing the microchannel volume by the total flow rate. Comparing the results for the same mean residence time, the AgBr average particle size produced at QRatio ) 5 was smaller than that at QRatio ) 1. 3.3. Discussion of Mechanism of Crystallization in the Proposed Microreactor. 3.3.1. Mechanism of Crystallization. In this type of the microreactor, diffusion of the reactant species is closely related to the crystallization. Therefore, in this section, we will discuss the mechanism of crystallization considering the kinetic transport phenomena in the microchannel.
The Reynolds number of the flow in the microchannel ranged from 3 to 83 in this study. Therefore, the flow of reactant solutions in the microchannel should be laminar, maintaining annular microsegments. Free silver ions (Ag+) and free halogen ions (X-, where X ) Br- or Cl- in each reactant solution) should diffuse into each other, according to their concentration gradients, and precipitate as AgX particles beyond a certain concentration. A schematic expression of this phenomenon is shown in Figure 7. In the figure, the concentration profiles of the Ag ion, [Ag+], the halogen ion, [X-], and [Ag+][X-] along the radius r are shown schematically at five points in the z-direction: the confluent point (z ) Z1), corresponding to position A; the start point of the microreaction zone (z ) Z2); the start point of nucleation (z ) Z3); the start point of particle growth (z ) Z4); and the finish point of particle growth (z ) Z5). At Z1, the profiles of [Ag+] and [X-] are rectangular. However, as the solution flows toward the outlet, the profiles broaden gradually in the direction of r with the elapsed time, t, because of both the progress of diffusion and the increase in [Ag+][X-]. Nucleation then occurs at Z3, where the value of [Ag+][X-] becomes larger than the concentration of critical supersaturation, Ccrit. In the case of conducting a fast reaction, such as silver halide precipitation under laminar flow in this study, it is determined whether the nucleation process is governed by the diffusion-controlling step or the reaction rate controlling step, by varying the degree of gradient of [Ag+] and [X-], which dominates the mass-transfer rate of Ag+ and X- to the nucleation zone. When fluxes of Ag+ and X- are small, because of slow concentration gradients, the nucleation process should be governed by the diffusion-controlling step. The value of [Ag+][X-] in the nucleation zone then remains small, despite diffusion transportation of Ag+ and X- from the reactant solution segments on both sides of the nucleation zone, and nucleation is terminated. On the other hand, when fluxes of Ag+ and X- are large, because of high concentration gradients, the nucleation process should be governed by the reaction-rate-controlling step. In that case, the zone of high [Ag+][X-] value widens in the direction of r and z, and nucleation will continue further downstream than in a diffusion controlling regime. For both cases, when the value of [Ag+][X-] in the nucleation zone becomes lower than Ccrit, because of the consumption of Ag+ and X-, the nucleation is terminated, and the growth of nuclei proceeds until the value of [Ag+][X-] becomes lower than the saturation concentration, C∞, which is determined by the solubility product (Ksp) of AgX, during the period from Z4 to Z5 in the lower part of figure. Finally, when the value of [Ag+][X-] becomes lower than C∞ at position Z5, because of the consumption of Ag+ and X-, the growth of fine particles is terminated.
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Figure 7. Sketch of the flow and concentration profiles of [Ag+], [X-], and [Ag+][X-] in the reactor.
Figure 8. Schematic diagram of concentration profiles for [Ag+][Br-] and [Ag+][Cl-] at the middles of the cross sections of the two reactant solution layers.
In following sections, the discussion will focus on the effects of (i) the solubility product, (ii) the presence of a middle layer, and (iii) the width of segments, given the average size and size distribution of particles, on the basis of the aforementioned mechanism. 3.3.2. Influence of Solubility Product (Ksp). The average sizes of the fine particles of AgBr and AgCl to be crystallized were quite different, as shown in Figure 5. This is attributable to the difference of the values of the solubility product of the two compounds. Figure 8 shows a schematic diagram of concentration profiles for [Ag+][Br-] and [Ag+][Cl-] at the middle of the cross section of the two reactant solution layers. In these experiments, the mass-transfer rates of [Ag+], [Cl-], and [Br-] are almost the same because physical properties such as coefficient of kinematic viscosity and diffusion coefficient of reactant species in water are almost the same. However, the solubility product of AgBr, Ksp(AgBr) ) 4.9 × 10-13 mol2/L2 at 298 K, is three orders smaller than that of AgCl, Ksp(AgCl) ) 1.8 × 10-10 mol2/L2 at 298 K. Therefore, even if the values of [Ag+][Br-] and [Ag+][Cl-] are almost the same, the AgBr system attains critical supersaturation state earlier and has a larger degree of supersaturation than the AgCl system. Therefore, a large number of nuclei are generated within a short time in the AgBr system. In addition, the particle growth proceeds under the condition of supersaturation, because of the high concentration of ions. On the other hand, in the AgBr system, the nucleation zone is narrow, because the critical concentration is low, so the amount of reactants per one nucleus is small in the particle growth zone, and, as a result, the average size of
Figure 9. Schematic diagram of concentration profiles for [Ag+][Br-] at the middles of the cross sections of the two reactant solution layers of test 1 without a middle layer and test 2 with a middle layer.
fine particles becomes smaller. Figure 5 shows that the average particle sizes are almost constant in the AgBr system even with change of total flow rate, or in other words, the change of mean residence time at the microreactor. This means that the crystallization process in AgBr system in this experiment consisted of only the nucleation process; that is, the growth process hardly occurred at all. On the other hand, in the AgCl system, the average particle size becomes larger with the decrease in total flow rate (that is, the increase in the mean residence time). This means that the nuclei grow gradually after the nucleation process in the AgCl system. 3.3.3. Role of the Middle Layer. Figure 9 shows a schematic diagram of concentration profiles for [Ag+][Br-] at the middles of the cross section of the two reactant solution layers of both test 1 without a middle layer and test 2 with a middle layer. In test 1 without a middle layer, the reactant species, Ag+ and Br-, diffuse directly into adjacent reactant solution layers, starting from just after the confluent point, Z1. Therefore, the value of [Ag+][Br-] increases abruptly and the degree of supersaturation becomes very large in a short period in the region near the contacting interface just after the confluent point. A large number of nuclei then are generated under the reactionrate-controlling regime. These nuclei aggregate because they lack the protective colloid. The diffusion of Ag+ and Br- then is promoted by the increasing concentration gradients of reactant species, which are due to the diffusion distance being reduced because of the contracted flow, and nucleation occurs again. Through this precipitation process, large and polydisperse particles should be formed in test 1. On the other hand, in test
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Figure 10. Schematic diagram of concentration profiles for [Ag+][Br-] at the middles of the cross sections of the two reactant solution layers at QRatio ) 1 and QRatio ) 5.
2 with a middle layer, Ag+ and Br- are diffused into the middle layer. Because of such a middle layer, these two reactant species contact within a diluted flow after the diffusion of reactant species are promoted by the contracted flow. Therefore, the degree of supersaturation becomes a finite specific value, and nucleation proceeds under the diffusion-controlling regime with a protective colloid. Through this crystallization process, small and monodisperse particles should be formed. 3.3.4. Influence of the Width of Segments. Figure 6 shows that the average particle size formed in the same mean residence time becomes smaller by increasing the volume flow rate of inactive fluids without changing the volume flow rate of reactants solutions and the protective colloid. This is because the segment widths of the reactants and the protective colloid fluid, namely, the diffusion distance, become narrow with the pressure applied to each segment. Figure 10 is a schematic diagram of concentration profiles for [Ag+][Br-] at the middles of the cross section of two reactant solution layers at QRatio ) 1 and QRatio ) 5. In the case of a narrow segment width and with QRatio ) 5, Ag+ and Br- start to contact with a high value of [Ag+][Br-] and high diffusion rates of Ag+ and Br- at an earlier stage, because of the steep concentration gradients of Ag+ and Br-. Thus, the degree of supersaturation rises abruptly in a short time, and a larger number of nuclei are generated. However, the amount of reactants per one nucleus becomes smaller, and, as a result, the average size of the fine particle is reduced, along with the consumption of the two ions. We can say that reducing the volume flow rate and contracting the flow by reducing the channel width are effective for making small particles. 3.4. Effect of Shape in the Cylindrical Contracted Flow Section. One of the important priorities in fabricating a microreactor for production is to make a reactor using conventional machining technique, without the aid of micromachining techniques. With this limitation, the developed microreactor consists of channels of five fluids, whose inlet widths are on the order of millimeters. To create microsegment flows in the millimeter-order inlets, a cylindrical contracted flow section should be designed. To determine the guidelines for designing the shape and dimensions of the cylindrical contracted flow section, two types of the microreactor (A-ML-R-1 and A-MLR-2) were fabricated. The design concepts of these microreactors are as follows: A-ML-R-1 has a high reduction ratio, due to a 32.9° taper angle, from the widths of the five channel inlets to the assembly of annular segments with a width of 100 µm. A-ML-R-2 has a low reduction ratio, because of an 18.6° taper angle, from the same five-section inlet zone to the same assembly of microsegments in A-ML-R-1. Using both reactors, the crystallizations of fine AgBr particles were conducted under the experimental conditions listed in Table 2.
Figure 11. Effect of the reduction ratio at contracted flow section: (a) comparison of the average particle size of fine AgBr particles produced by two types of annular multi-lamination microreactor and (b) a photograph of the flow pattern at a rate of 5 mL/min in the contracted flow section of A-ML-R-1.
Figure 11a shows the average particle size of AgBr crystallization against the volume flow rate for each segment. With A-ML-R-2, the average size of fine AgBr particles was
