Yang Kuang (楊曠), Chu Guangwen (初廣文), Shao Lei (邵磊), Xiang Yang (向陽), Zhang Liangliang (張亮亮) and Chen Jianfeng (陳建峰)

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Micromixing Efficiency of Viscous Media in Micro-channel Reactor*

Yang Kuang (楊曠)1, Chu Guangwen (初廣文)1, Shao Lei (邵磊)1, Xiang Yang (向陽)1, Zhang Liangliang (張亮亮)1and Chen Jianfeng (陳建峰)2,**

1Research Center of the Ministry of Education for High Gravity Engineering and Technology, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China2Key Lab for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China

micro-channel reactor, micromixing, incorporation model, viscosity

1 INTRODUCTION

Micromixing theory is concerned with those features of mixing which cause the attainment of homogeneity on the molecular level,.. the reduction of the scale of unmixed blobs of fluid by breakage and deformation, and the final mixing by molecular diffusion [1]. A reaction, which happens in molecular level, is said to be slow or fast when the time required for the micromixing step is, respectively, shorter or longer than the time required for the chemical reaction. In the slow reaction regime, mixing at the molecular scale is realized before the reaction, whose rate is controlled by intrinsic kinetics. While for fast reactions, reaction happens before a homogeneous environment achieved, proceeds in an inhomogeneous solution and is sensitive to micromixing efficiency. In this case, micromixing may influence the selectivity of the chemical reactions [2, 3], the size distribution of particles [4-8] or the mass distribution of polymer molecules [9].

There has been a growing interest in micro-channel reactor for its large area-to-volume ratio resulting in better yield and selectivity than conventional devices [10-12]. Zhao. [13]experimentally studied the immiscible liquid-liquid two-phase flow patterns in a T-junction rectangular micro-channel by using a CCD camera. The mass transfer characteristics of immisciblefluids in opposing-flow and cross-flow T-junctions were also investigated [14]. Recently, mixing efficiency in micro-channel reactor has been investigated widely by chemical methods. Wolfgang. [15] adopted the parallel competing reaction to determine mixing efficiency of micromixer devices for the first time. Kockmann. [16] studied mixing quality of three T-mixers by parallel competing reaction. Soleymani. [17] investigated liquid mixing in a T-type micromixer and using a method called the fourth Bourne reaction system to determine the performances of different micromixers. Bothe. [18] investigated the flow and mixing behaviors of a T-type micromixer and evaluated the mixing efficiency by numerical simulations. Panic. [19] applied the well-known “Villermaux/ Dushman method” to continuous processes and described the mixing performance of five micromixers differing in mixing principle and internal geometry. Ying. [20] characterize the micromixing efficiency of T-type micro reactor by parallel competing reaction system. Chen. [21] characterize the micromixing efficiency of a microstructured device called membrane dispersion minireactor by a typical Dushman reaction (iodide-iodate) coupled with a neutralization and precipitation reaction of BaSO4. Most of the experiment and simulation results were based on aqueous solutions, while data concerning viscous solutions are scare. Considering that various biomedical and biochemical process (including DNA purification, polymeric reaction, enzyme reaction and protein folding) involve the mixing of viscous fluids, the effect of viscosity on micromixing should be taken into account when evaluating the micromixing efficiency of micro-channel devices [22].

In this work, the micromixing efficiency of viscous solution in a Y-type micro-channel reactor was studied by using the parallel competing reaction. In addition, the micromixing time was estimated in terms of the experimental data and the incorporation model.

2 EXPERIMENTAL

2.1 Micro-channel reactor

The Y-shaped micro-channel reactor used in this study is shown in Fig. 1, which consists of inlet channels 1, 2 and outlet channel. Details of the micro-channel reactor are shown in Table 1.

2.2 Parallel competing reaction system

A novel parallel competing reaction system was proposed by Fournier [23]. Later on, its experimental procedure and reaction kinetics were also presented [24, 25]. This reaction scheme consists of the following three chemical reactions:

Reaction (3) is an equilibrium reaction and the equilibrium constantBis a function of temperature:

Figure 1 Photo and schematic of the Y-type micro-channel reactor

Table 1 Specification of the micro-channel reactor used in this study

2.3 Experimental procedure

The glycerin-water solution was used to mimic the behavior of the viscous reaction systems, as reported by Guichardon[26]. In our experiments, the boric acid solution was added to sodium hydroxide solution to obtain a mixture buffer solution. Then, potassium iodide and potassium iodate solution were added to the buffer solution in sequence and reached a certain concentration by supplementing water and glycerin. And it was denoted solution ‘A’, which was a mixture of iodate (0.00233 mol·L－1), iodide(0.01167 mol·L－1) and borate ions(0.1818 mol·L－1). The viscosity of A can be adjusted by changing the mass fraction of glycerin and 7 different viscosities (1 mPa·s, 1.16 mPa·s, 1.85 mPa·s, 2.6 mPa·s, 5.59 mPa·s, 20.46 mPa·s, 46.3 mPa·s, 57.83 mPa·s respectively) were investigated in the present work. Sulfuric acid solution was denoted ‘B’. The initial concentration of H2SO4solution is 0.025-0.1 mol·L－1, corresponding concentration of H+is 0.05-0.2 mol·L－1.

Figure 2 Experimental set-up

Figure 3SagainstAin aqueous solution and viscous solution (A/B10, [H+]0.2 mol·L－1)■1.0 mPa·s;●46.3 mPa·s

3 RESULTS AND DISCUSSION

3.1 Effects of volumetric flow rate of A on XS

For different viscosities of the solution, the Reynolds number based on equivalent diameter of the main channel (width of 0.8 mm and depth of 0.5 mm) in our research is in the range of 12-1420 and the flow in the outlet channel is laminar. To consider the effect ofon micromixing, volumetric flow rate of A was varied. It can be seen from Fig. 3 that segregation indexSdecreased and approached a constant value asAincreased, which means that micromixing efficiency was enhanced with an increasing liquid flow rate. The reason is that increasing volumetric flow rate (higher) results in decreasing residence time and increasing energy dissipation rate. Although the first phenomenon does not confer any benefit to the mixing, the second phenomenon is of substantial benefit to micromixing. It should be kept in mind that only micromixing rate, depends on, whereas intrinsic reaction rate is fixed once reagent concentration and temperature are fixed. WhenAincreased further, the micromxing rate of micro-channel reactor increased substantially because of more violent impinging between A and B at the Y-junction area. When the micromxing rate is much lager than reaction rate, reaction will happen in a perfect uniform environment and controlled by intrinsic kinetics. The theoretical lowest value ofSthat would be attained under the perfectly mixed limit controlled by the reaction kinetics is to be close 0 as Fig. 3 shows. That is to say, the iodide-diodate test reaction is not sensitive to the mixing any more whenis large.

3.2 Effects of volumetric flow ratio on XS

3.3 Effects of viscosity on XS

Effects of viscosity onSare presented in Fig. 5, whereSis plotted against the viscosities for differentA. It was found thatSincreased with a decreasingAand increasing viscosity, indicating poor micromixing efficiency. Magnetic resonance imaging (MRI) has been used for the first time to obtain both cross-sectional velocity and concentration maps of flow through an optically opaque Y-shaped microfluidic sensor [27]. The flow images indicate a velocity gradient across the micro-channel, and the more viscous stream flows more slowly. As the two inlet streams are of equal flow rate, the viscous stream of glycerol must occupy a greater fraction of the channel. The above analysis can be applied to our system considering that the geometries and two inlet streams investigated in our work are similar to that in Ref. [27]. WhenAandare fixed, the increasing of viscosity will make the solution of A flow slower and occupy a greater fraction of cross section of the channel, resulting in longer diffusion distance for the ions for reaction. Also, diffusion coefficients of ions will become lower when viscosity of A increases. The above two reasons will cause micromixing efficiency decreased and segregation index increased. The results demonstrate thatSin pure aqueous solution (1 mPa·s) is nearly two orders of magnitude smaller than in the 60% glycerin solution (10 mPa·s). The impact of the glycerin addition is quite significant in this study, while the influence of the viscosity is relatively smaller in the stirred vessel [26], where the mixing efficiency is only roughly double times worse in 60% glycerine solution.

4 DETERMINATION OF MICROMIXING TIME

From Eq.(8), mass balance equations of the ionic species can be written as follows:

5 CONCLUSIONS

The micromixing efficiency of viscous fluids in Y-type micro-channel reactor is tested by iodide-iodate test reaction. The results show thatSincreased with a decrease in volumetric flow rate and an increase in flow ratio in viscous fluids, exhibiting the same rule as in aqueous solution. It was also observed thatSincreased with an increase of viscosities.

Based on the incorporation model, micromixing time is estimated and lies in the order of 10－3-10－4s, which means that micro-channel reactor has excellent micromixing efficiency compared to traditional reactors (.., stirred tank). Further work should be directed to the investigation on the mixing efficiency of higher viscous fluid and industrial applications of micro-channel reactor.

NOMENCLATURE

A0initial concentration of A, mol·L－1

B0initial concentration of B, mol·L－1

Cconcentration of species, mol·L－1

C10environmental concentration of species, mol·L－1

ratio ofAtoB

Rrate of reactantion, mol·L－1·s－1

1rate of Reaction (1)

2rate of Reaction (2)

3rate of forward Reaction (3)

4rate of backward Reaction (3)

mmicromixing time, s

Rcharacteristic reaction time, s

Avolumetric flow rate of A, ml·min－1

Bvolumetric flow rate of B, ml·min－1

Ssegregation index

selectivity of iodine

STvalue ofin the total segregation case

viscosity, mPa·s

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2009-01-14,

2009-07-12.

the National Natural Science Foundation of China (20821004, 20806004) and the National High Technology Research and Development Program of China (2007AA030207, 2006AA030202, 2006AA030203).

** To whom correspondence should be addressed. E-mail: chenjf@mail.buct.edu.cn; chugw@mail.buct.edu.cn