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Journal of Membrane Science 328 (2009) 219–227

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

ntensification of micromixing efficiency in a ceramic membrane reactor withurbulence promoter

ong Wu a,c, Chao Hua a, Wangliang Li b, Qiang Li b,c, Hongshuai Gao a,c, Huizhou Liu a,∗

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, ChinaNational Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, ChinaGraduate University of Chinese Academy of Sciences, Beijing 100049, China

r t i c l e i n f o

rticle history:eceived 26 September 2008eceived in revised form 1 December 2008ccepted 2 December 2008vailable online 10 December 2008

eywords:eramic membrane reactor

ncorporation model

a b s t r a c t

Micromixing efficiency in a ceramic membrane reactor (CMR) by using turbulence promoters was studiedwith iodide–iodate test reaction. The existing incorporation model did not take into account concentrationgradient. To elucidate the micromixing efficiency, a new incorporation model coupled with a continuousstirred tank reactor (CSTR) in series model was established to determine the ratio of micromixing time(tm) to segregation index (XS). The ratio of tm to XS can be affected significantly by low flux ratio. Basedon the ratio, the experimental results indicated that membrane pore diameter has significant influenceon micromixing efficiency at high superficial Reynolds number (ReS) under the same transmembranepressure drop. However, the micromixing efficiency changed slightly at low ReS. In addition, increasing

icromixing timeurbulence promoters

permeation flow rates and bulk flow rates or decreasing membrane pore sizes could enhance micromixingefficiency. The introduction of turbulence promoters can further intensify the micromixing efficiency ofthe reactor. The micromixing efficiency of CMR with turbulence promoters was observed in a sequenceof entry tube < cylindrical insert < helical insert < KenicsTM static mixer insert. The order of the energyconsumption was similar to the order of micromixing efficiency. The micromixing time of the reactorwas in the range from 0.7 to 300 ms. Based on its performance, CMR might be used as a promising reactor

ms.

for the fast reaction syste

. Introduction

Effective micromixing operations are dominant in chemicalndustry, such as fast complex reaction [1,2], polymerization [3],recipitation [4,5] and catalysis [6]. Optimal micromixing at aolecular scale can improve the contact of reactants and their

electivity, yield and quality of final products [7]. The developmentf new reactors for better micromixing efficiency is important.he micromixing efficiency of many reactors has been reported,uch as stirred tank reactor [8], impinging jets reactor [9], staticixer [10], rotating packed bed [11], Couette flow reactor [12],

ltrasound reactor [13], microfluidic devices [14]. However, theseeactors need long time for construction, high energy consump-ion and operational complexity. Micro-dispersed technology cane used to enhance mixing and mass transfer as an alternative

o classical reactors [15–17]. As one of microporous membranes,eramic membrane has many advantages such as chemical stability,specially for alkaline, acid or organic solvent, favorable mechanicaltrength, continuous operations and easily scaling up, and has been

∗ Corresponding author. Tel.: +86 10 62554264; fax: +86 10 62554264.E-mail address: hzliu@home.ipe.ac.cn (H. Liu).

376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2008.12.010

© 2008 Elsevier B.V. All rights reserved.

widely applied to filtration [18], solvent distillation [19], multiphasereaction [20], adsorption [21], etc. However, the micromixing per-formance of CMR was little reported. In addition, Kieffer et al. [22]studied the mass transfer in the lumen side of a membrane contac-tor using computational fluid dynamics (CFD). The results indicatedthat the mixing and reaction occurred in a very small part of thelumen side. As a result, turbulence promoters are used to intensifythe micromixing efficiency and reduce concentration polarizationby the enhancement of turbulence [23]. This study also aims tointroduce a new application of turbulence promoters for CMR.

In previous studies, competitive parallel reaction system [1] (e.g.iodide–iodate test reaction) and competitive consecutive reactionsystem [24] (e.g. diazo coupling method) were developed to studythe micromixing phenomena. Segregation index can be used tomeasure the micromixing efficiency by chemical selectivity. Gen-erally, the micromixing efficiency could be determined using XS,when the volume ratio of KIO3 and KI solution to acid solution ishigh. However, the comparison of the micromixing efficiency of the

ceramic membrane reactor is difficult to determine by XS as low fluxratio of KIO3 and KI solution to acid solution lead to the variationof the concentrations along the direction of bulk flow. Moreover,it is not easy to compare other reactors with XS. To quantify themicromixing phenomena, many models, such as engulfment model

2 brane

[imicbwmtbaut

TcTswCb

2

2

b[l

H

5

I

wbttma

r

w

r

w

20 Y. Wu et al. / Journal of Mem

25]; probability density function (PDF) model [26] and micromix-ng model coupled with CFD [8,27], have been used to describe the

icromixing phenomena. However, it was complex to character-ze the mixing process by these models. Especially, hydrodynamicsonditions varied drastically. Incorporation model, as proposedy Fournier et al. [1], overcame these drawbacks. Moreover, itould be favorable to use the incorporation model for both laminaricromixing [28] and turbulence micromixing [29]. Micromixing

ime (tm), a parameter of the incorporation model, was a necessaryase of the models mentioned above. At the same time, it was useds the scale of micromixing efficiency because tm was kept samender the similar hydrodynamic condition. As a result, micromixingime of different reactors can be quantified and compared easily.

In this work, micromixing efficiency of CMR was investigated.he characteristic micromixing time was calculated, based on theoupled model of incorporation model and CSTR in series model.he effects of ReS, transmembrane pressure drop, membrane poreize and the configurations of turbulence promoters were discussedith tm. Additionally, energy consumption of CMR was also studied.

MR with turbulence promoters was compared with other reactorsy tm.

. Experimental

.1. Parallel competitive reaction system

Reaction system is based on the parallel competitive reactionsetween iodide and iodate coupled with a neutralization reaction1,30]. It is also called the Dushmen reaction, consisting of the fol-owing three chemical reactions:

2BO3− + H+ → H3BO3 (1)

I− + IO3− + 6H+ → 3I2 + 3H2O (2)

− + I2 ↔ I3− (3)

here neutralization (1) is a quasi-instantaneous reaction followedy a fast redox reaction (2). The production distribution is con-rolled by the chemical kinetics. The iodine can further react withhe iodide ions to yield I3

− ions. The amount of I2 depends on theicromixing efficiency. At 25 ◦C, the kinetics of the three reactions

re expressed as [31]

1 = k1[H+][H2BO3−] (4)

here k1 = 1 × 1011 mol−1 dm3 s−1.

2 = k2[H+]2[I−]2[IO3−] (5)

here k2 depends on the ionic strength I of the medium.

Fig. 1. The geometric configuratio

Science 328 (2009) 219–227

• I < 0.166 mol dm−1

log10 k2 = 9.28105 − 3.664√

I (6)

• I ≥ 0.166 mol dm−1

log10 k2 = 8.383 − 1.5112√

I + 0.23689I (7)

r3 = r3+ − r3

− = k3+[I−][I2] − k3

−[I3−] (8)

where k3+ = 5.9 × 109 mol−1 dm3 s−1 and k3

− = 7.5 × 106 s−1.

The amount of I3− is measured by the spectrophotometer. The

equivalent iodine concentration can be determined by the equa-tions:

− 53

[I2]2 +(

[I−]0 − 83

[I3−]

)− [I3

−]KB

= 0 (9)

log10 KB = 555T

+ 7.355 − 2.575 log10 T (10)

The amount of I2 is a measure of the segregation state of the fluid.The micromixing time can be extracted from the segregation indexby the incorporation model. The segregation index XS is defined as

XS = Y

YST(11)

where Y can be considered as the ratio of moles of acid consumedby the reaction (2) to the total moles of acid injected:

Y =2(nI2 + nI3

− )

nH0+

(12)

The quantity of iodine is due to the stoichiometric ratio of thereactants. YST is the maximum value of Y, when the micromixingprocess is poor mixing. YST is defined as followed:

YST =6nIO3

−,0

6nIO3−,0 + nH2BO3

−,0(13)

Therefore, XS = 0 and 1 indicate that perfect micromixing and totalsegregation, respectively. Generally, micromixing of actual reactorsis partially segregated. The value of XS should be between 0 and 1.

2.2. Materials and methods

2.2.1. Equipments and reagentsSingle channel ceramic microfiltration and ultrafiltration mem-

brane were used for investigations and were provided kindly by

the Membrane Science and Technology Research Center, NanjingUniversity of Technology, PRC. Inner diameter of the channel is7.7 mm and the length is 200 mm. This membrane contains a finelayer of zirconia (ZrO2), having superficial pore size of 20 nm,0.2 �m and 0.8 �m, on �-Al2O3 porous support. Configuration and

n of turbulence promoters.

Y. Wu et al. / Journal of Membrane Science 328 (2009) 219–227 221

Table 1Reactant concentrations of the experimental systems.

Reactant H2SO4 (mol dm−3) NaOH (mol dm−3) H3BO3 (mol dm−3) KIO3 (×10−3 mol dm−3) KI (×10−2 mol dm−3)

S 0.1818 2.333 1.167S 0.1818 2.333 1.167S 0.1818 2.333 1.167S 0.1818 1.167 0.5835

pciK1ct

dwiTbtT

2

rSmsidcmCd

R

wa

2

watbuwt

Ft

ystem 1 0.025 0.0909ystem 2 0.05 0.0909ystem 3 0.1 0.0909ystem 4 0.05 0.0909

arameter for turbulence promoters were shown in Fig. 1 ((a)ylindrical inserts, outer diameter: 5 mm and 6.35 mm; (b) helicalnserts, outer diameter: 7.6 mm, spiral pitch: 12 mm and 18 mm; (c)enicsTM static mixer insert, outer diameter: 7.6 mm, spiral pitch:2 mm). These promoters were made of stainless steel. The energyonsumption per unit volume of permeate EP was used to comparehe performance of various turbulence promoters [32].

KI and KIO3 powders were dissolved in the defined volume ofeionized water, respectively. Aqueous H3BO3 and NaOH solutionere added to KI and KIO3 solution, respectively. Finally, deion-

zed water was added into the volumetric flask to graduation.his sequence of the mixing operation must be followed carefullyecause I− and IO3

− coexist in a basic solution, which may preventhe iodine formation. The reactant concentrations were shown inable 1. All the reagents were of analytic grade.

.2.2. Residence time distribution (RTD) of CMRRTD curves can provide the macromixing characteristic in the

eactor. A stimulus-response method was used to determine RTD.aturated KCl solution was selected as tracer. An experimentalodule of the combination of CMR and turbulence promoters is

hown in Fig. 2. One milliliter of saturated KCl solution was injectednto the ceramic reactor from the front inlet. Meanwhile, the con-uctivity was in situ measured at reactor’s inlet and outlet. Theonductivity signal was acquired by digital conductivity instru-ent (DDS-302B, Tianjing Shengbang Instrument Co. Ltd., China).

onsidering the experiments under the similar hydrodynamic con-itions, the superficial dimensionless ReS can be defined as

eS = du�

�(14)

here d and u are the inner diameter of tube ceramic membranend the slip flow velocity in the tube side, respectively.

.2.3. Micromixing of CMRThe experimental setup is shown in Fig. 2. KI and KIO3 solution

as injected through the ceramic membrane tube side. Sulfuriccid solution permeated from the shell side of the module into

he tube side, and then got mixed with KI and KIO3 solution. Theulk flow rate and the transmembrane pressure were adjusted bysing peristaltic pump with buffer tank. The samples were takenhen the flow rate was stable and total exchange volume was five

imes in contrast to the volume of the module. The amount of I3−

ig. 2. Schematic figure of the experimental set-up. (1) Bulk flow; (2) permeation flowurbulence promoter; (6) mixed solution reservoir and samples outlet.

Fig. 3. Residence time distribution curves of CMR (ReS = 700).

produced was measured immediately by the spectrophotometer at353 nm (Lambda Bio 40, PerkinElmer, USA). All experiments werecarried out at 25 ◦C. The module was washed with KI solution priorto each experiment. In order to further enhance the micromixing inCMR, the turbulence promoters were introduced in the tube side.At downstream, the membrane reactor was maintained under theatmospheric pressure.

3. Determination of micromixing time

3.1. Macromixing of CMR

The curves of RTD are shown in Fig. 3. The inlet response curveswere similar to the outlet response curves. This indicated that theflow regime is close to plug flow mode at the same ReS. The flowregime of the combination of CMR and turbulence promoters wasalso close to plug flow condition by interrupting the parabolic veloc-ity profile characteristic of the open pipe. Back mixing coefficientwas also small. Therefore, axial mixing of CMR was not influencedby the turbulence promoters. In addition, observed conductivity

values, using turbulence promoters, were lager than those withoutturbulence promoters at the peak place. The result indicated thatradial mixing was intensified by the turbulence promoters. The sim-ilar results were also obtained under the different permeation flowrates and ReS.

; (3) peristaltic pumps; (4) manometers; (5) membrane module with or without

222 Y. Wu et al. / Journal of Membrane

Fva

3

iabitFiaIrcatarwacabf

3

tvl

ig. 4. Effect of reactant concentration on absorbance and XS. (a) Results with theariation of H+ concentration in entry ceramic membrane; (b) results with the vari-tion of I− and IO3

− concentration in entry ceramic membrane.

.2. Effect of reactant concentration on absorbance and XS

The variations of reactant concentrations play a crucial rolen the micromixing experiments. Fig. 4 illustrates the absorbancet 353 nm and XS for different reactant concentrations. It cane observed in Fig. 4a that the absorbance at 353 nm and XS

ncreased with the increase of H+ concentration. According to reac-ion kinetics, reaction (2) is sensitive to the reactant concentrations.urthermore, a higher concentration means a higher production ofodine at the same ReS. As seen in Fig. 4b, the absorbance and XSlso increased with the increasing of I− and IO3

− concentrations.− and IO3

− concentrations markedly influenced the iodate–iodideeaction. Consequently, the absorbance at 353 nm can limit theomparison of micromixing efficiency of the reactor. The XS valuend the sensitivity of the micromixing experiments can be adjusthrough various reactant concentrations. H+ concentration vari-tion had more significant influence on micromixing than othereactant concentration (Fig. 4a and b). If reactant concentrationsere not appropriate, the relationship between absorbance and

ctual I3− concentration would not be in the linear range that

an result in a wrong interpretation of micromixing efficiency. Inddition, the results for system 2 were similar with other systemsecause the micromixing time was related to hydrodynamic per-

ormance. Therefore, system 2 was used as a model system.

.3. Modeling

Incorporation model assumes that an incoming reactant solu-ion can be divided into a series of isolate aggregates at a definiteolume of reactor, which interact with the surrounding liquid. Iso-ated aggregates grow progressively by incorporating surrounding

Science 328 (2009) 219–227

liquid. The equation is followed below:

dci

dt= 1

g(t)dg(t)

dt(ci0 − ci) + Ri,j (15)

The available growth law of aggregates is a function of tm [33]:

g(t) = exp(

t

tm

)(16)

then

dci

dt= ci0 − ci

tm+ Ri,j (17)

In this study, sulfuric acid solution was injected from the shellside into the tube side. Due to tube membrane, the concentrationsof reactants differ drastically along the axial direction of ceramicmembrane tube, which may cause the variation in the reactionkinetics. A numerical model is developed to compare accuratelythe micromixing efficiency using the segregation index. In addi-tion, due to plug flow regime, the reactor is discretized into a seriesof unit cells along the direction of bulk flow. The unit cell arrange-ment is shown in Fig. 5. A series of unit cells along the direction ofbulk flow represent the CSTR in series model [16] and preserves theconcentration gradients of the bulk flow in the tube side. Therefore,the coupled model of incorporation model and CSTR in series modelis more close to the reality. As shown in Fig. 4, the reactant concen-trations have much more significant influence on the segregationindex than the hydrodynamic condition at the low ReS. The segre-gation index remained almost constant when the turbulence existsat the high ReS. The difference of hydrodynamic conditions can benegligible along the axial direction of the bulk flow. Therefore, thedeveloped numerical model is based on the following assumptions:

(1) Isothermal operation;(2) The pseudo steady hydrodynamic state along the axial direction

of the bulk flow;(3) The aggregates are of the same size and interact with the sur-

rounding liquid but not each other;(4) Each unit cell is assumed to be ideally mixed and no back mixing

with each other.

The equation of the incorporation model in the unit cell k forcomponent i can be given by

dci

dt= ci,k−1 − ci

tm+ Ri,k (18)

where ci and Ri,k are the concentration of component i of the aggre-gate and the reaction rate of component i of aggregates in the cellk, respectively. ci,k−1 is the concentration of component i in the cellk−1 and is also the surrounding concentration. i represents H3BO3,H2BO3

−, I−, IO3−, I3

− and I2, respectively.The reactant mass balance of unit cell k is given by the following

equation:

ci,k−1Qb,k−1 = ci,k(Qb,k−1 + Qd) + ciQf (19)

The final volume flow rate of the aggregates

Qf = Qd exp(

t

tm

)(20)

where Qb,k−1, Qd and Qf are volume bulk flow rate, volume flow rateof the permeation flow and volume flow rate of aggregates in thecell k, respectively. t is the lifetime of aggregates.

Based on this mass balance of unit cell, outlet concentration of

the component i can be calculated for each cell. To initiate the cal-culation, the reactants concentrations are assumed to be uniformover the inlet of the cells. The concentrations of component i andflow rate leaving the cells are stored and used as inputs for the nextcells.

Y. Wu et al. / Journal of Membrane Science 328 (2009) 219–227 223

ncorp

tmaioho

cataXbwtrHcat

tdddwar

4

4

ibpaaHtmamvtgam

4

fl

pressure drop could enhance mass transfer of H+. The energy ofacid solution emitted from the membrane pore was controlled bytransmembrane pressure drop. The effect of transmembrane pres-sure drop was not as significant as membrane pore size at the low

Fig. 5. Scheme of the coupled model of i

The ordinary differential equations (18) can be solved by itera-ive method such as 4-order Runge–Kutta method using a simplified

ethod [34]. The initial conditions are shown in Table 1. The iter-tions end in each cell when the concentration of H+ is zero. Thentegration of Eqs. (18)–(20) is stopped when the concentrationsf I2 and I3

+ discretized into n + 1 unit cells is 1 × 10−6 mol dm−3

igher than those discretized into n unit cells. We can obtain a seriesf theoretical ratio between tm and XS.

The results for the theoretical ratio between tm and XS by theoupled model of incorporation model and CSTR in series modelre shown in Fig. 6. As expected, the segregation index increased asm increased. The curves in Fig. 6a demonstrated that tm increasedlmost linearly as XS increased for system 1. For systems 2 and 3,S increased slowly as tm increased (Fig. 6b and c). Particularly, XSecame independent of tm to reach a constant value for system 3,hen tm was long enough. The reason could be that most of reac-

ants are consumed in the ceramic membrane tube, and later theeaction rates become slow. Segregation index has a larger value as+ increased at the same tm. The higher H+ concentration would

ause the larger aggregate to produce more iodine and vice versa,s chemical kinetics affected drastically and the surrounding liquidakes much time to react inside the aggregate.

Fig. 7 shows the differences of XS between the batch model andhe model at the different flow rate ratio ([H+] = 0.1 mol dm−3). Theifference of XS decreased, as tm increased for all flux ratios. Theifference of XS varied slightly when tm was more than 30 ms. Theifference of XS was nearly constant at the definite flow rate ratiohen tm was less than 8 ms. Much difference in XS was observed

t the range from 8 to 30 ms. Meanwhile, higher flow rate ratioesulted in a small change on XS.

. Results and discussion

.1. Effect of membrane pore sizes on tm

Due to membrane pore size, the droplet size of permeation flows different. Furthermore, the aggregate volume would be affectedy the membrane pore size. Fig. 8 shows the effect of the membraneore sizes on tm. The micromixing efficiency of CMR was enhanceds membrane pore size decreased. When the pore size was 0.2 �mnd 0.8 �m, tm was almost same in the whole experimental range.owever, the micromixing efficiency was much improved when

he pore size decreased 20 nm in contrast to 0.2 �m and 0.8 �m. Theicromixing time was 330 ms for 0.2 �m and only 180 ms for 20 nm

t ReS = 750. Moreover, the curves in Fig. 8 also showed that theicromixing efficiency can be enhanced, as the aggregate sizes are

ery small by increasing shear force flow in the tube side. Accordingo the incorporation model, the micromixing is related to the aggre-ate volume. The smaller membrane pore size yields the smallerggregates. As a result, the smaller membrane pore size has bettericromixing efficiency.

.2. Effect of transmembrane pressure drop on tm

Transmembrane pressure drop provides energy of permeationow. The turbulence intensity of the vortexes, formed on the mem-

oration model and CSTR in series model.

brane surface, was enhanced by higher transmembrane pressuredrop. Fig. 9 shows the effect of transmembrane pressure drop ontm using the different pore sizes. An increase in transmembrane

Fig. 6. The theoretical relationship between tm and XS.

224 Y. Wu et al. / Journal of Membrane Science 328 (2009) 219–227

RsitlodpeWeoptflaasswcd

4

eit

6.35 mm diameter, tm was intensified in contrast to the entry tube.

Fig. 7. Comparison between the batch model and the coupled model.

eS in Fig. 9a. The micromixing time was almost similar under theame transmembrane pressure drop. The micromixing time wasndependent of membrane pore size. The possible reason was thathe surface of ceramic membrane is hydrophilic and the turbu-ence intensity was low. The permeation flow was easy to spreadn membrane surface. Specific surface area per unit volume wasecreased. On the other hand, tm is affected greatly for differentore size at the high ReS in Fig. 9b. The difference of micromixingfficiency was much obvious at low transmembrane pressure drop.

hen transmembrane pressure drop was 50 kPa, the micromixingfficiency was independent of pore size, regardless of the laminarr turbulence regime. It was only related to the transmembraneressure drop. The reason was that the vortexes, formed by emit-ing from membrane pore, enhanced micromixing. The permeationow did not easily spread on the hydrophilic surface. The smallerggregates were formed and mass transfer between the aggregatesnd the surrounding liquid was improved at high ReS. When poreize was 20 nm, tm was not sensitive to the transmembrane pres-ure drop and was observed to be constant value (0.25 s) at thehole experimental range. Therefore, the micromixing efficiency

an be improved and adjusted by the transmembrane pressurerop.

.3. Effect of permeation flux on tm using turbulence promoters

The effect of permeation flux on tm using turbulence promot-rs was studied in Fig. 10. The micromixing time decreased with anncrease in permeation flux. Permeation flux was closely related tohe transmembrane pressure drop. The low permeation flux did not

Fig. 8. Effect of membrane pore sizes on tm (J = 4.2 dm3 m−2 min−1).

Fig. 9. Effect of transmembrane pressure drop on tm. (a) ReS = 1700; (b) ReS = 4100.

have enough energy to improve the micromixing efficiency. Dueto the fast parallel competitive reaction, the reaction was carriedout on the surface of the membrane and most of iodine was pro-duced in this region. In order to enhance the micromixing efficiencyand mass transfer near the membrane surface, some turbulencepromoters were used in the ceramic membrane tube to alter thehydrodynamic characteristics.

In the combination of CMR and cylindrical insert of 5 mm or

However, the curves of tm are similar. The possible reason is thatthe parallel competitive reaction is very fast and the reactionscomplete near the membrane surface. The variation of diametercannot greatly enhance the turbulence in the tube side. There are

Fig. 10. Effect of J on tm using turbulence promoters (ReS = 1700).

Y. Wu et al. / Journal of Membrane

F

osseoitwhmptsfl

4

thmtoswotp

TC

R

R

S

U

T

R

C

C

C

ig. 11. Effect of ReS on tm using turbulence promoters (J = 6 dm3 m−2 min−1).

ther ways to improve hydrodynamic characteristics on membraneurface. The helical inserts of different spiral pitch and KenicsTM

tatic mixer insert were used as the turbulence promoters. Thexperimental results have proved that the micromixing efficiencyf helical inserts was greatly enhanced in contrast to cylindrical

nserts. Meanwhile, micromixing efficiency of the membrane reac-or with a spiral pitch H = 12 mm helical insert was higher than thatith a spiral pitch H = 18 mm helical insert. KenicsTM static mixer

ad the best micromixing efficiency among the turbulence pro-oters. The minimum micromixing time reached 9.71 ms. In the

resence of turbulence promoters, the micromixing efficiency ofhe combination of CMR and turbulence promoters was not sen-itive with the increase in permeation flux when the permeationow was more than 5.5 dm3 m−2 min−1.

.4. Effect of ReS on tm using turbulence promoters

Higher flow velocity in the tube side can produce strongerurbulence at the same conditions. Different turbulence promotersave different effects on micromixing efficiency at the same ReS. Asentioned above, the micromixing efficiency was affected little by

he permeation flow (J > 5.5 dm3 m−2 min−1). Therefore, the effectf ReS on tm using turbulence promoters was studied under the

ame permeation flux (6 dm3 m−2 min−1). As shown in Fig. 11, itas obvious that tm decreased as ReS increased. ReS had little effect

n tm, when ReS reached a critical value regardless of the type ofhe turbulence promoters. The results indicated the turbulenceromoters had significant influence on the laminar regime rather

able 2omparison of CMR with other reactors.

eactor Experimental condition

usthon turbine tank [33] 1 mol dm−3 H+, 12.1 mmol dm−3 H2.33 mmol dm−3 KIO3, 0.0116 mol

liding surface mixing device [35] 2 mol dm−3 H+, 0.0909 mol dm−3

2.33 mmol dm−3 KIO3, 11.6 mmolltrasound reactor [13] 0.1 mol dm−3 H+, 0.0121 mol dm−

2.33 mmol dm−3 KIO3, 11.6 mmolaylor–Couette reactor [36] 2 mol dm−3 H+, 0.0121 mol dm−3 H

2.33 mmol dm−3 KIO3, 11.6 mmolotor stator reactor [37] 0.16 mol dm−3 H+, 0.0909 mol dm

H2BO3− , 2.33 mmol dm−3 KIO3,

11.6 mmol dm−3 KIompact heat exchanger reactor [38] 0.25 mol dm−3 H+, 0.0909 mol dm

H2BO3− , 2.33 mmol dm−3 KIO3,

11.6 mmol dm−3 KIMR with turbulence promoters (this work) 0.1 mol dm−3 H+, 0.0909 mol dm−

2.33 mmol dm−3 KIO3, 11.6 mmolMR (this work) 0.1 mol dm−3 H+, 0.0909 mol dm−

2.33 mmol dm−3 KIO3, 11.6 mmol

Science 328 (2009) 219–227 225

than the turbulence regime. In other words, a suitable geometrypromotes the transition to turbulence. Micromixing efficiency ofthe reactor with KenicsTM static mixer was at best among the inves-tigated turbulence promoters. Micromixing efficiency of the helicalinserts was better than cylindrical inserts, but a little poorer thanKenicsTM static mixer insert. As mentioned above, the smaller thehelical pitch, the higher the micromixing efficiency. Micromixingtime of helical insert and KenicsTM static mixer insert were similarat the same spiral pitch. The introduction of turbulence promotersintensified micromixing efficiency of CMR in contrast to entry tubebecause the flow field was generated by the turbulence promotersscoured the surface of the membrane and the scouring might formsmaller aggregates. KenicsTM static mixer insert can produce muchmore vortexes than helical inserts, whereas most of vortexes werein the center of the bulk flow rather than on the membrane surface.Meanwhile, more vortexes would consume more energy. Therefore,the introduction of KenicsTM static mixer insert cannot intensifymore greatly the micromixing efficiency than that of helical insert.

Unexpected results were also obtained with the comparison ofvarious diameters of cylindrical insert. The micromixing time wasthe same when ReS was less than 2000. The variation in the diame-ter of cylindrical insert did not alter tm. With the increase of ReS, theflow regime transited into turbulence. 5 mm diameter cylindricalinsert had higher micromixing efficiency than 6.35 mm diametercylindrical insert. The reason can be that the turbulence developedwithout enough space in the tube when larger diameter cylindricalinsert was used. Therefore, the micromixing efficiency of ceramicmembrane can be controlled by using the turbulence promoters.In the following experiments, the energy input was investigated indetails.

4.5. Energy consumption of CMR

Better micromixing efficiency always leads to a higher pressuredrop, but pressure drop is directly related to the energy consump-tion. Furthermore, the energy consumption is the function of fluxand pressure drop. The presence of turbulence promoters can causehigher pressure drop along the direction of bulk flow. Fig. 12 showsthe ceramic membrane with turbulence promoters, which consumemore energy than entry tube at the same ReS. Helical insert of12 mm spiral pitch cause much more energy consumption than that

of 18 mm spiral pitch. Energy consumption of cylindrical inserts,as mentioned above, was close to the entry tube. However, CMRusing a commercial KenicsTM static mixer insert exhibited muchhigher energy consumption in contrast to the other turbulencepromoters. The energy consumption of CMR followed the order

Operational mode tm

3BO3,dm−3 KI

Batch, stirring speed = 5–9 rpm 20–80 ms

H2BO3− ,

dm−3 KISemi-batch, stirring speed = 9–50 rpm 25–110 ms

3 H3BO3,dm−3 KI

Semi-batch, J = 123–1950 ml min−1 3–50 ms

3BO3,dm−3 KI

Semi-batch, Ta = 5 × 104 to 2 × 105 8–70 ms

−3 Continuous, stirring speed = 30 rpm 0.1 ms

−3 Continuous, J = 1–350 dm3 h−1 8–150 ms

3 H2BO3− ,

dm−3 KIContinuous, ReS = 700–3800 0.8–100 ms

3 H2BO3− ,

dm−3 KIContinuous, ReS = 700–3800 80–380 ms

226 Y. Wu et al. / Journal of Membrane Science 328 (2009) 219–227

om

biceb

4

rTtilitathcm

5

utmtMdbcflaips

A

SoR

Nomenclature

ci chemical concentration of species i in the aggregate(mol dm−3)

ci,0 initial reactant concentration for surrounding fluid(mol dm−3)

ci,k initial reactant concentration for surrounding fluidin the unit cell k (mol dm−3)

D inner diameter of tube ceramic membrane (m)g(t) growth function of incorporation lawI ionic strength (mol dm−3)J permeation flux of acid solution (dm−3 m−2 min−1)KB equilibrium constant (dm−3 mol−1)ki rate constant of Dushman reactionQb,k−1 bulk flow rate in the unit cell k (dm−3 s−1)Qd flow rate of acid solution in the unit cell k (dm−3 s−1)Qf flow rate of aggregate (dm−3 s−1)ReS superficial Reynolds numberRi,k rate of reaction ‘i’ in the unit cell k (mol dm−3 s−1)T surrounding temperature (K)t lifetime of aggregates (s)tm micromixing time (s)v linear velocity in the tube side (m s−1)XS segregation indexY actual yield of undesired productYST maxium yield of undesired proudt

Greek letters� fluid viscosity (Pa s)� fluid density (kg m−3)�P transmembrane pressure drop (kPa)

Subscripts0 initial timei reactant

Fig. 12. Effect of ReS on energy consumption of CMR.

f entry tube < cylindrical inserts < helical inserts < KenicsTM staticixer insert.

In addition, the permeation flux was significantly influencedy transmembrane pressure drop and by membrane pore size and

nterface tension. The smaller the pore size, the higher the energyonsumption. However, due to small permeation flow rate, thenergy consumption can be considered as a negligible factor. Theulk flow rate is crucial for energy consumption.

.6. Comparison with other mixing devices

The micromixing time of mixing devices such as static mixer,otor stator reactor, ultrasound reactor, has been determined.able 2 shows the comparison of the combination of CMR andurbulence promoters with the other reactors. In addition, forndustrial applications, the superiority of operational mode fol-owed the order of continuous > semi-batch > batch. Mixing processn the combination of CMR and turbulence promoters also belongso a continuous process. Furthermore, micromixing can be adjustednd further enhanced by the choice of turbulence promoters. Even-ually, the combination of CMR and turbulence promoters not onlyad good micromixing performance but also had lower energyonsumption, simpler constructions, more flexible controlling andore productive.

. Conclusions

Micromixing efficiency of the membrane reactor was studiedtilizing iodide–iodate test reaction. Due to the reactant concentra-ion variations of bulk flow, the coupled model of an incorporation

odel and a CSTR in series model was developed to determine tm inhe CMR, which can be used to elucidate the micromixing efficiency.

icromixing efficiency can be enhanced when the membrane poreecreased. tm of three kinds of pore sizes was similar at low ReS,ut tm was obviously different at high ReS. The micromixing effi-iency was enhanced with an increase in ReS and the permeationow rate. Compared with other reactors, the combination of CMRnd turbulence promoters has a distinctive advantage in micromix-ng efficiency. Particularly, the membrane reactor using short spiralitch for helical insert is excellent for considering both energy con-umption and micromixing efficiency.

cknowledgements

This work was financially supported by the National Naturalcience Key Fund (No. 20490200), State Major Basic Research Devel-pment Program of China (Grant 2007CB613507) and by Innovativeesearch Group Science Fund (No. 20221603).

j reactionk unit cell

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