J-350A真空乳化机传动系统和搅拌系统设计
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Desalination 144 (2002) 167172Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France,July 712, 2002.0011-9164/02/$ See front matter 2002 Elsevier Science B.V. All rights reserved*Corresponding author.Preparation and analysis of oil-in-water emulsionswith a narrow droplet size distributionusing Shirasu-porous-glass (SPG) membranesGoran T. Vladisavljevica,*, Helmar SchubertbaInstitute of Food Technology and Biochemistry, Faculty of Agriculture, University of Belgrade, P.O. Box 127,YU-11081 Belgrade-Zemun, YugoslaviaTel. +381 (11) 615-315/327; Fax +381 (11) 199-711; email: gtvladisafrodita.rcub.bg.ac.yubInstitute of Food Process Engineering, Faculty of Chemical Engineering, University of Karlsruhe (T.H.),Kaiserstrasse 12, D-76128 Karlsruhe, GermanyTel. +49 (721) 608-2497; Fax +49 (721) 694-320; email: Helmar.Schubertlvt.uni-karlsruhe.deReceived 30 January 2002; accepted 13 February 2002AbstractShirasu-porous-glass (SPG) membranes with a mean pore size from 0.46.6 m were used to produce O/Wemulsions consisting of vegetable (rape seed) oil as the dispersed phase and Span 80 dissolved in demineralizedwater as the continuous phase. The emulsion droplets with a mean droplet size 3.5 times larger than the mean poresize and the span of the droplet size distribution between 0.26 and 0.45 were produced using 2% emulsifier at atransmembrane pressure slightly exceeding the capillary pressure. Under these conditions the dispersed phase fluxthrough the membrane was in the range of 0.77 lm2h1 and only about 2% of the pores were active. However, ifthe transmembrane pressure was considerably higher than the capillary pressure, the dispersed phase flux stronglyincreased and droplets with a broad droplet size distribution were produced. The hydraulic resistance of the SPGmembrane was inversely proportional to the square of the mean pore size, which is in agreement with the Hagen-Poiseuille law. The membrane porosity is independent on the pore size and ranged from 5360%.Keywords: Membrane emulsification; Porous glass membrane; Oil-in-water emulsions; SPG membrane; Hydraulicmembrane resistance; Membrane porosity.168G.T. Vladisavljevic, H. Schubert / Desalination 144 (2002) 1671721. IntroductionMembrane emulsification (ME) is a newemulsification technique, especially suitable forthe production of highly uniform particles or dropletsof controlled mean size 1. In conventional emul-sification devices, such as high-pressure homo-genizers and rotor-stator systems, interfacial areais increased by disruption of larger droplets in apremix using high energy inputs 2. In a MEsystem, however, small droplets are directlyformed by permeation of dispersed phase througha porous membrane into moving (flowing or stirring)continuous phase. In that way, droplet size can bemore effectively controlled and the required shearstress is much smaller. The additional advantageis the possibility to obtain uniform droplets overa much wider range of mean droplet sizes.Due to uniform pores, a wide range of availablemean pore sizes (0.0530 m) and the possibilityof surface modification, the Shirasu-porous-glassmembrane developed by Nakashima and Shimizu3 is a potentially suitable membrane for mecha-nical emulsification. The aim of this work is toinvestigate the influence of pore size of the SPGmembrane and some operating parameters on theemulsification result at the transmembrane pressuresslightly exceeding the capillary pressure.2. TheoryThe permeation of pure water through a porousmembrane with a thickness of m whose pores areassumed to be capillaries of diameter dp and lengthlp, can be explained in terms of the Hagen-Poiseuille law 4:2/32pwpwmwwtmdvlRJp= (1)where ptm is the transmembrane pressure, Rm thehydraulic membrane resistance, w the waterviscosity, Jw the water flux through the membrane,and vw the mean water velocity in the pores. Thesubstitution of vw = Jw/ and lp = m into Eq. (1)gives:)/(32)/(2pmwwtmmdJpR=(2)where is the mean tortuosity factor of the poresand the membrane porosity.During ME process droplets are usually formedsimultaneously at only 240% of the pores 5.The fraction of active pores at any moment is givenby:tmmddpRJk=/ (3)where Jd is the dispersed phase flux through themembrane and d is the viscosity of dispersedphase. If the active pores are arranged over themembrane surface in a square array, the distancebetween the centers of adjacent active pores canbe expressed as:5 . 0)/)(2/(kdzp=(4)Neglecting droplet deformation in the directionof continuous phase flow, droplets do not toucheach other at the pore openings if z ddrop. Thus,the condition for unhindered droplet growth inthe case of uniform pore arrangement and rigiddroplets reads:2)/(5 . 0/kddpdrop(5)In order to avoid contact between two neigh-boring droplets at the pore openings, the fractionof active pores must be kept below kmax = (/4)(ddrop/dp)0.5.The dependence of kmax on droplet/pore diameterratio and membrane porosity is shown in Fig. 1.The porosities of SPG membranes are in the rangeof 0.50.6 6 and for O/W emulsions ddrop/dpranges typically from 2.58 7. Therefore, toensure that no coalescence can occur at the surfaceof the SPG membrane, the percentage of activepores must be kept below 225%, dependingmostly on the droplet/pore diameter ratio.The average droplet formation time tf can becalculated as 89:G.T. Vladisavljevic, H. Schubert / Desalination 144 (2002) 167172169)/)(/)(3/2(33 , 42dpfJddkt=(6)Although the above expressions are onlyapproximate, they are useful in an attempt to givesome quantitative explanations of the experimentalresults.3. ExperimentalThe O/W emulsions have been prepared usingvegetable (rape seed) oil (Floreal, Germany) witha viscosity of 58 mPas as the dispersed phase and2% (w/w) Tween 80 (Merck) dissolved in demine-ralized water as the continuous phase. Micro-porous glass membranes were supplied from SPGTechnology (Miyazaki, Japan) with a mean poresize of 0.4, 1.4, 2.5, 5.0, and 6.6 m, determinedby a Shimadzu model 9320 mercury porosimeter.The SPG membrane tube (125 mm length 10 mmOD 0.7 mm wall thickness) was installed insidea laboratory made stainless steel module pro-viding the effective membrane area of 31.4 cm2.The continuous phase was recirculated in aclosed loop between the membrane module and acontinuous-phase reservoir using a Netzsch model10110100Droplet/pore diameter ratio ddrop/dp =0.1 =0.2 =0.4 =0.62Fig. 1. Maximum percentage of active pores for unhindereddroplet growth as a function of droplet/pore diameter ratioand membrane porosity.NL 20 Mohno-pump (Fig. 2). In the most expe-riments, the continuous phase flow rate was305 lh1, which corresponds to a mean velocityin the membrane tube of 1.4 ms1 and tubeReynolds number of 8500. Under these conditionsthe shear stress at the membrane surface was 8 Pa.The oil phase was placed in the pressure vesseland was introduced at the module shell side withcompressed air. The weight of oil permeated throughthe membrane was measured by a digital balance.The droplet size distribution was determined bya light scattering particle size analyser using PIDStechnology (Coulter LS 230).4. Results and discussion4.1. Properties of the SPG membranes used inthis studyThe hydraulic and morphological propertiesof the SPG membranes used are listed in Table 1.The hydraulic membrane resistances werecalculated by measuring the pure water fluxthrough the membrane and using Eq. (2). Themembrane resistance was inversely proportionalto the square of the mean pore size (Fig. 3):emulsion orcontinuousphaseCompressedairMohnopumpCleaningpumpBalancePIPIPImVentCleaning cycleOilcleaningsolutiontModuleV1Fig. 2. Schematic view of the experimental set-up forcrossflow membrane emulsification. The V1 valve is closedduring emulsification and open during cleaning-in-place(CIP) cycle.170G.T. Vladisavljevic, H. Schubert / Desalination 144 (2002) 167172Table 1The hydraulic resistances, Rm, the wall porosities, , andthe mean pore tortuosity factors, for the SPG membranesused in this workPore size, dp, m 0.41.4 2.5 5.0 6.6 Rm 109, m1 350 29 9.0 3.2 1.5 Porosity, 0.60 0.53 0.60 0.58Tortuosity, 1.7 1.3 2.1 1.7 0.1110110100103Mean pore size dp / mFig. 3. Hydraulic resistance of the SPG membrane as afunction of mean pore size.2056. 0=pmdR(7)where Rm and dp are in m1 and m, respectively.Eq. (7) is in accordance with Eq. (2). However,the membrane porosities measured by the pycno-metric method 10 and the mean pore tortuosityfactors calculated from Eq. (2) were independenton the mean pore size (Table 1). The valueslisted in Table 1 are within the range of 5060%,reported earlier for a typical SPG membrane 6.As a comparison, the porosity of coating layer ofcomposite ceramic membranes ranges typicallyfrom 3060% 5.4.2. Preparation of O/W emulsions using the SPGmembranesThe experimental results obtained at ptm1.1pcap are given in Table 2. Except for the 6.6 mmembrane, the experimentally determined pcapvalues are higher than the theoretical values cal-culated from the Laplace equation. Under givenconditions the emulsions with a very narrowdroplet size distribution were prepared over a widerange of mean droplet sizes (Fig. 4). The span ofthe droplet size distribution of 0.260.45 (Table 2)was much lower than that reported for ceramicmembranes. As an example, Joscelyne andTrgrdh 11 obtained the span of 0.891.6 usingceramic membranes with a mean pore size of 0.1Droplet diameter d / m11010010-210-11020406080100Cumulative volume Q3 / vol %Fig. 4. Droplet size distribution curves at ptm 1.1pcap:, mean pore size dp = 6.6 m; , dp = 5.0 m; , dp =2.5 m; , dp = 1.4 m; , dp = 0.4 m.Volume frequency q3 / m-1Cumulative volume Q3/vol%G.T. Vladisavljevic, H. Schubert / Desalination 144 (2002) 167172171Pore size, dp, m0.41.4 2.5 5.0 6.6 pcap, kPa 185 48 24 10 5 d3,2, m 1.44.6 8.5 14.7 23.9 Span 0.450.300.440.37 0.26Jd, l m2 h1) 0.72.3 2.9 4.3 6.6 Table 2The experimentally found capillary pressures, pcap, andemulsification results at ptm 1.1pcap and = 12 vol%for different mean pore sizes0.5 m. However, the dispersed phase flux in theirexperiments was 15270 kgm2h1. Williams etal. 12 obtained the span of 0.83 at the oil flux of8 l m2h1 using ceramic membrane with a meanpore size of 0.5 m.The mean droplet size was found to be 3.5 timeslarger than the mean pore size, if ptm 1.1pcap(Fig. 5). The droplet/pore diameter ratio of 3.5 issimilar to 3.25 reported by Nakashima et al. 1 forSPG membrane. Using Eq. (3), it was found thatdroplets were formed simultaneously at only 1.62.6% of the pores (Table 3). Taking a mean valueof 0.58 and k = 0.02, one obtains from Eq (5):ddrop/dp 8. Thus, the condition for unhindereddroplet growth is satisfied in the given case. Theaverage droplet formation times of 0.61.8 s(Table 3) were similar to 11.5 s found bySchrder and Schubert 8 for Tween 20 and aceramic membrane. The longest droplet formationtime for the 6.6 m membrane is a consequenceof largest droplet volume in that case.The mean droplet size was almost independentof the dispersed phase content up to 20 vol%(Fig. 6), although the dispersed phase flux throughthe membrane increased with time. This trend wasalso found by Katoh et al. 13 in the preparationof food emulsions using SPG membranes. Thesmall decrease of the mean droplet size in theexperiments with the 6.6 m membrane can beexplained by the disruption of droplets duringrecirculation. At the smaller cross-flow velocityof the continuous phase (0.9 ms1), the disruptionwas less pronounced but the mean droplet sizewas higher.Table 3Calculations of the fraction of active pores, k and the averagedroplet formation time, tf at ptm 1.1pcap and = 12 vol% for different mean pore sizesPore size, dp, m 0.4 1.4 2.5 5.0 6.6 k 0.019 0.020 0.016 0.020 0.026tf , s 0.6 0.7 0.8 0.9 1.8 0123456704812162024Mean pore size dp / md3,2 = 3.5dp Fig. 5. Relationship between mean pore size and meandroplet size at ptm 1.1pcap.05101520101418222630Dispersed phase concentration / vol %2.5 m-Membrane5 m-Membrane6.6 m-Membrane, 1.4 m/s6.6 m-Membrane, 0.9 m/sFig. 6. The influence of dispersed phase content on meandroplet size at ptm 1.1pcap for different mean pore sizes.172G.T. Vladisavljevic, H. Schubert / Desalination 144 (2002) 1671725. ConclusionsThe SPG membrane can be successfully usedto produce O/W emulsions with a very narrowdroplet size distribution, on the condition that thetransmembrane pressure is not much higher thanthe capillary pressure. Under these conditions themean droplet size is 3.5 times larger than the meanpore size and not significantly affected by thedispersed phase content up to 20 vol%. However,the transmembrane flux is very low, because atany moment only about 2% of the pores are active.AcknowledgementThis work was financially supported by theAlexander von Humboldt Foundation, Bonn,Germany.Symbolsdp Mean pore size of the membrane, mddrop Droplet diameter, md3,2 Mean Sauter diameter, md4,3 Volume-weighted mean droplet diameter,mJd Dispersed phase flux through the mem-brane, ms1Jw Pure water flux through the membrane,ms1k Fraction of active poreslp Pore length, mpcap Cappilary pressure, PaRm Hydraulic membrane resistance, m1tf Average droplet formation time, svw Mean water velocity in the pores, ms1z Distance between the centres of adjacentactive pores, mGreekptm Transmembrane pressure, Pam Membrane thickness, m Membrane porosityd Viscosity of dispersed phase, Pasw Viscosity of water, Pas Mean tortuosity factor of pores Dispersed phase concentration in emul-sion, vol %References1T. Nakashima, M. Shimizu and M. Kukizaki, Mem-brane Emulsification, Operation Manual, IndustrialResearch Institute of Miyazaki Prefecture, Miyazaki,Japan, 1991.2H. Karbstein and H. Schubert, Developments in thecontinuous mechanical production of oil-in-watermacro-emulsions, Chem. Eng. Process., 34 (1995)205211.3T. Nakashima and M. Shimizu, Porous
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