structure engineering of multiphase food systems by...

Structure Engineering of Multiphase Food Systems by FlowProcessing

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Swiss Federal Institute of Technology Zürich (ETH), Laboratory of Food Process Engineering; CH-8092 Zürich

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For fine dispersing of disperse structures like solid particles/biological cells, emulsion drops, gas cellsand aggregates thereof in concentrated emulsion/suspension systems, the volumetric power andenergy inputs as integral processing parameters are not sufficient to optimize the dispersingefficiency. The flow field characteristics are of additional importance and the ratio of shear andelongational flow contributions influences the dispersing efficiency strongly. It is shown that in welldefined shear, elongational or mixed dispersing flow fields the disperse particle/drop size distributionand -shape can be adjusted to generate specific rheological and other quality properties of relatedfood systems. It is also demonstrated how the synergistic application of dispersing flow experimentsand Computational Fluid Dynamics (CFD) allow to optimize such processes and related productproperties.

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Micro-structuring in shear- and elongational dispersing flow fields links the areas of processengineering and material engineering. The latter includes the structure- rheology relationships.Rheologically the viscous behaviour of multiphase food systems is in general non-Newtonian, mostlyshear-thinning and time-dependent. In many cases elastic properties are also not negligible.Dispersing as a flow structuring unit operation regarded here, generates the product microstructuredue to shear- and normal stresses which act in the dispersing flow velocity field. A detaileddescription of the velocity field can be received from flow visualization experiments and fromComputational Fluid Dynamics (CFD). The coupling of local velocity field information and rheologicalmaterial functions which are received from rheometric shear and elongation experiments, allow fordetermining the related local shear and normal stress distributions. Micro-structuring is triggered bythe stresses acting locally in the flow field and occurs in general if critical stresses of the "structuringunits" like droplets and particle aggregates are exceeded.

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Laminar flow fields are of uni-/multiaxial shear or elongational nature or a mixture of both. Withinsuch flows, the rheological behaviour mirrors the dynamic behaviour of structural units which canchange by orientation, deformation and aggregation or dispersing (structuring mechanisms). Figure(1) gives a schematic overview on such structure-rheology relationships related to the viscousbehaviour in shear flow. The flow induced structuring generates non-Newtonian behaviour even fordispersions with Newtonian continuous fluid phases. The shear rate-, disperse phase concentration-and time dependent viscous behaviour can be described by the structure-related fluid immobilization

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Structure Engineering of Multiphase Food Systems by Flow Processingwysiwyg://menu.4/file:/F|/HOME/ARCHIV-S...THZ/Tagungs-CDs/ECCE_2001/pages/474.htm

behaviour, which depends strongly on the structuring mechanisms described before and investigatedin previous work [1]. Elongational flow fields induce more efficiently micro-structural changes due tothe fact that there is no rotational contribution of the velocity field which, like in shear flows, causesthe structuring units to rotate. This is demonstrated in figure (2) for the break-up of droplets withdiameter x in uniaxial shear- or elongation flows by the well known critical Weber number We c(equation (1)) dependency on viscosity ratio λ (= η disperse / η continuous) [2,4].

Droplets are preferably used as "model structuring units" due to their isotropy, ho-mogeneity andsymetry. In uni-axial shear the drop break-up is limited to the do-main λ <= 4, in uniaxial elongationalflow fields, this limitation does not exist (fig.2)[3,5].

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In order to get experimental results from laminar dispersing flow fields which are close to real flows indispersing apparatus, the experimental set up should allow to investigate pure shear- andelongational flows but also well defined mixed flows. Locally these conditions are ideally fulfilled in aFour-Roller Apparatus (FRA) or a Cross-Slot Channel (CSC). Within the FRA the velocity ratio α ofthe two diagonal roller pairs determines whether pure shear (α = 1), pure elongation (α = 0) or mixedflows (0< α <= 0) are generated (fig. 3). Figure (3) shows the FRA built in a laser optic arrangementfor optical rheometry. Figure (4) demonstrates drop deformation and break-up in the stagnation flowdomain of the FRA for two different viscosity ratios λ under pure elongational flow conditions. It hasbeen shown that mono- or bimodal drop size distributions result depending on the rheological andinterfacial properties of the two fluid phases. Consequently drop dispersing in well-definedelongational flow fields is a basis for processing techniques of narrowly size distributed drops orcapsules (fig. 5) [13].

Another field of interest in micro-structure engineering is particle shaping. If during drop deformationa solidifying mechanism for the drop fluid is superimposed (e.g. crystallization, gelation, networking)and the solidifying kinetics is adjusted to the time scale of drop shape deformation, well defined butcomplex particle/capsule shapes can be created (fig. 6) [10].

Within multiple drop systems (emulsions) in flows with a shear component a dynamic equilibriumbetween drop break-up and re-coalescence has to be taken into account depending strongly on thepresence of surfactants and their specific interface stabilization properties (e.g. adsorption kinetics,interfacial movability). Pure elongational flow does not support drop re-coalescence. As discussedabove for single drops, particle shaping and superimposed shape fixing (solidification of drop phase)can also be applied to multi-drop systems. A typical example is shown in figure (7) for a bio-polymerblend system [11,12].

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In order to get information about the locally acting shear- and normal stresses as well as the relatedshear- and elongational rates, Computational Fluid Dynamics (CFD) was applied to model flows inthe geometries described before.

Using a finite element code (SEPRAN), the shear- and elongation rate fields were calculated bysolving the equations for mass-, energy- and momentum conservation. Results are shown exemplaryfor the FRA in figure (8). Figure (9) demonstrates additionally elongation rate fields in three differentexperimentally investigated cross-slot flow channels. From such calculations the stress/strain -history (SSH) of droplets or other structuring elements respectively along particle tracks within theapparatus flow, can be derived.

Using an adapted boundary integral method (BIM) basically developed by Löwenberg et al.[6], thedrop deformation in superimposed shear-/elongation flows was calculated, assuming clean interfaces

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(no surfactants) and Newtonian flow behaviour of the continuous and disperse fluid phases. Someresults from drop deformation simulations are demonstrated in figure (10) [ 10]. If critical capillarynumbers Ca.c (= critical Weber numbers We c) are derived from such calculations, the expected

increase from pure elongation (Ca.c = 0.12) to pure shear (Ca.c = 0.42) is found as shown in figure

(11). These results fit nearly perfectly to the experimental results (fig.2).

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As a consequence derived from the results of fundamental model dispersing flow investigations likedescribed before, laminar dispersing flows to be applied in dispersing apparatus should have anelongation component. It is obvious that close to apparatus walls, at which the continuous phaseadheres, shear always dominates elongation. However from practical experience and also from theBIM calculations demonstrated before it can be stated that even a minor elongation contribution ofabout 10-20%, has a relevant impact on the dispersing efficiency in particular at higher viscosityratios (λ > 2)..

In narrow annular gap reactors (NAGR) which consist basically of a concentric cylinder gap geometrywith rotating inner cylinder, the treated fluid systems experience a rather narrow range of shearstresses [8]. Commonly dispersing/mixing elements are adapted to the rotating inner and /or theouter cylinder wall. In the case of in-built wall-scraping rotor- or stator blades or pins, which are ingeneral inclined relative to a tangential plane to the cylinder wall (fig. 12), high elongation rates actlocally. This has been proved experimentally using Particle Imaging Velocimetry (PIV) as well as byCFD calculations which applied a particle tracking method [ 11]. For the investigated flow gapbetween a rotating dispersing blade which is mounted to the inner cylinder scraping the surface ofthe outer cylinder, and the inner cylinder surface, three particle tracks are shown in figure (13). Forvarious flow gap ratios of rS = 0.25, 0.4, 0.6 and a constant blade angle of β = 30°, the calculated

shear and elongation rates ( , ) along the three selected particle tracks are shown in figure (14). -Beside maximum shear- and elongation rates, the shearing and elongation times and consequentlythe total shear- and elongational deformations are relevant to generate a flow induced (i.e.dispersed) microstructure.

Elongation- and shear rate integrated over time provide the elongational and shear deformation ( ε,γ).Equations (2) and (3) describe the time integrals for ε1 and γ, which are approximated numerically

applying a trapezoidal rule according to Burden and Faires [9] using a time step ∆t, according to thetime increments used for the numerical particle tracking (NPT).

For optimization of a structuring process the total energy density rates for the elongational and for theshear component are of further interest. The time integral of the elongational dissipation componentin primary flow direction e1 describes the elongational energy density eE , given by equation (4),

whereas the shear dissipation component contributes to the integral of the shear energy density e S,

as given by Equation (5). The velocity vector is formulated in polar coordinates (r,θ). Applying therotational coordinate transformation to the velocity gradient , eE and eS are found in the local

coordinate system (e1,e2). Both Equations (4) and (5) are formulated in terms of the shear viscosity

ηS( ) as a function of the shear rate , which is transformed onto the local coordinate system

(e1,e2) ( is the magnitude of the rate-of-strain tensor ). For the investigated non Newtonian model

fluid system with a watery CMC solution as the continuous fluid phase, the viscosity function ηS(

) was described by a Carreau-Yasuda (CY) model defined by Equation (6). Approximating the timeintegrals between t = tstart and t = tend applying a trapezoidal rule, the expressions eE and eS can

be found numerically. The flow structuring shear- and elongation related volumetric energy inputsaccording to Equations 5 and 6 are also compared for the three representative particle tracks,positioned at 25%, 50% and 75% of the scraper blade gap width h gap. Figure (15) shows the

increase of eE and eS as a function of time between t = tstart and t = tend along the three

representative particle tracks for a shear-thinning fluid (n=0.65) with β = 150° at Re=80. To express

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the flow structuring energy contributions of the two distinguished flow types (shear, elongation),Equations (7,8) define the flow type contribution factors , with regard to the flow around theinvestigated scraper blades, for elongational flow and shear flow , respectively.- Within the workreported here the maximum value of = 0.35 was received for Re=10, β=150°, rS = 0.6 for a given

non-Newtonian continuous fluid phase (CMC solution, 2%, power-law exponent from CY modeln=0.65).Disperse shaping / structuring experiments using a disperse protein containing phase withina polysaccharide solution as continuous phase have been already carried out successfully [ 14] andare also part of the ongoing research work.

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�� ) structure - rheology relationships for multiphase systems with disperse components (droplets, particles, fibres,macromolecules) [3]

�� ") critical Weber number as a function of the viscosity ratio λ = ηdisperse / ηcontinuous for uniaxial shear-

and elongational flows

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�� #) Four Roller Apparatus (FRA) connected to a laser optical set-up and stream line patterns for various diagonalroller pair speed ratios

�� $) Droplet break-up under pure elongational flow conditions in a Four Roller Apparatus (FRA); left: tip-stream drop

break-up for ε ≈ 5 s-1, λ ≈ 10-4 (water /silicon oil), right: filament break-up for ε ≈ 5 s-1, λ ≈ 1 (water + PEG/silicon oil)

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�� %) Drops / Micro-capsules generated in elongational dispersing flow

�� &) Shaped drops / micro-capsules - Dispersing in Four Roller Apparatus with superimposed solidification for shapefixing

�� ') Shape-adjusted biopolymer particles (gellan) in another continuous watery biopolymer solution(?-carraghenan); shear / elongation induced structuring in excentric cylinder gap at increasing shear and elongation rates

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�� () CFD simulation of elongation (left) and shear rate fields (right) for a Four Roller Apparatus (FRA); (programSEPRAN) [8]

�� *) CFD simulation of streamlines (top) and elongation rate fields for different cross slot flow channels (programSEPRAN)

�� +) calculated droplet deformations for mixed shear-/elongational flows ( α = 1: simple shear; α = 0 elongation);

simulation with BIM; We = 0.35, λ = 1

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�� ) calculated critical Weber numbers (from BIM simulation); λ = 1

�� ") Cross section of narrow annular gap reactor (NAGR) with wall-scraping dispersing/mixing blade

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�� #) Selected particle tracks around dispersing/mixing blade in Narrow Annular Gap Reactor (NAGR)

Figure 14 a,b: shear (top) and elongational rates (bottom) along selected particle tracks in Narrow Annular Gap Reactor(NAGR) with blade angle β = 30° ; Re = 10 and various scraper blade gap size ratio r S [11]

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�� %) Volumetric elongational energy eE (top) and shear energy eS (bottom) for selected particle tracks from t=t startto t=tend; Re=80, n=0.65; β=150°

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[1] Windhab E. 2000."Fluid Immobilization-A structure related key mechanism for the viscous flowbehaviour of concentrated suspension systems ", Applied Rheology 3 (2000),

[2] Grace, H.P. 1982. "Dispersion Phenomena in High Viscosity Immiscible FluidSystems",

[3] Wolf B.; 1995."Untersuchungen zum Formverhalten mikroskopisch kleiner Fluidtropfen instationären und instationären Scherströmungen", PhD Thesis, ETH Zürich, 1995, Nr. 11068

[4] Cox R.G.;1969. Journal of Fluid Mechanics, 37, pp. 601-623

[5] Windhab, E.; Wolf, B.1992."Influence of Deformation and Break-up for Emulsified Droplets on theRheological Emulsion Properties", Theoretical and Applied Rheology, edited by P. Moldenaers and

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R. Keunings, Brussels, Belgium, August 17-21, vol. 2, 681-683, 1992

[6] Christini V., Blawzdziewicz J., Löwenberg M.1998."Drop break-up in three-dimensional viscousflow", Physics of Fluids 10(8),1998; pp. 1781-1783

[7] Feigl K."Use of Numerical Simulation in Food Processing", in P.Fischer, I.Marti, E.Windhab: Proc.

of 2nd Int. Symp. Of Food Rhreology and Structure; 12-16.3.2000, Zürich; pp. 69-76

[8] Stranzinger M.1999."Numerical and Experimental Investigations of Newtonian andNon-Newtonian Flow in Annular Gaps with Scraper Blades", PhD Thesis, ETH Zürich (1999); Nr.13369

[9] Burden R.L., Faires J.D. 1993."Numerical Analysis, 5th Edition edn. PWS Publishing Company

[10] B. Walther, Emulsion Droplets influenced by different flow behaviour; Diploma Thesis, SIKGothenburg (A. Hermansson, P. Walkenström) / ETH Zürich (E. Windhab, P. Fischer), (2000)

[11] R. Scirocco, B. Wolf, P. Fischer, E. Windhab.2000.", Diploma Thesis, ETH-Zürich

[12] Wolf B. 2000."Biopolymer Suspensions with Spheroidal and Cylindrical Particle Shapes:

Generation and Flow Behaviour", in P.Fischer, I.Marti, E.Windhab: Proc. of 2 nd Int. Symp. Of FoodRhreology and Structure; 12-16.3.2000, Zürich; pp. 404-406

[13] K. Maruyama, E. Windhab, P. Fischer, "Dispersing Flows in Cross Slot Channels", InternalProject work ETH Zürich, 2000

[14] P. Walkenström, E. Windhab, A.-M. Hermansson; "Shear-induced structu-ring of particulatewhey protein gels Food Hydrocolloids 12, 459-468 (1998).

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