Preparation of highly monodispersed emulsions by swirl flow membrane emulsification using Shirasu porous glass (SPG) membranes – A comparative study with cross-flow membrane emulsification

Journal Pre-proof Preparation of Highly monodispersed Emulsions by Swirl Flow Membrane Emulsification using Shirasu Porous Glass (SPG) Membranes – A Comparative Study with Cross-flow Membrane Emulsification Jophous Mugabi, Shunji Tamaru, Karatani Naohiro, Roberto A. LemusMondaca, Noriyuki Igura, Mitsuya Shimoda

PII:

S0255-2701(18)31326-6

DOI:

https://doi.org/10.1016/j.cep.2019.107677

Reference:

CEP 107677

To appear in:

Chemical Engineering and Processing - Process Intensification

Received Date:

30 October 2018

Revised Date:

30 September 2019

Accepted Date:

30 September 2019

Please cite this article as: Mugabi J, Tamaru S, Naohiro K, LemusMondaca RA, Igura N, Shimoda M, Preparation of Highly monodispersed Emulsions by Swirl Flow Membrane Emulsification using Shirasu Porous Glass (SPG) Membranes – A Comparative Study with Cross-flow Membrane Emulsification, Chemical Engineering and Processing - Process Intensification (2019), doi: https://doi.org/10.1016/j.cep.2019.107677

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Preparation of Highly monodispersed Emulsions by Swirl Flow Membrane Emulsification using Shirasu Porous Glass (SPG) Membranes – A Comparative Study with Cross-flow Membrane Emulsification

Jophous Mugabi, Shunji Tamaru, Karatani Naohiro, Roberto A. Lemus-Mondaca, Noriyuki Igura* and Mitsuya Shimoda

Kyushu University; 744 Motooka, Nishi-Ku, Fukuoka city, Fukuoka 819-0395, Japan *corresponding author; [email protected], Tel. /Fax: (81)-92-802-4805

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Graphical abstract

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Laboratory of Food Process Engineering, Graduate school of Bioresource and Bioenvironmental Science, Faculty of Agriculture,

Highly monodispersed emulsion

F(Y, X, Z) FX

FZ

[FD] = [ F(Y,X,Z) ]

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FD

0

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Swirling flow of continuous phase

Volume fraction (%)

Extremely high radial drag force (FD)

FI

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Extremely high disperse phase flux (FI)

SPG Membrane

Droplet diameter (µm)

Highlights 

Swirl flow exerts an extremely high radial drag force on the membrane wall which enabled preparation of highly monodispersed emulsions at high disperse phase fluxes.

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Emulsion with highly uniform droplet size were prepared at very low SDS surfactant concentrations.

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The emulsions were stable throughout the period of storage, although were prepared at very

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low SDS surfactant concentrations.

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Abstract

Highly monodispersed oil-in-water (O/W) emulsions with narrow droplet size distribution (span) were

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prepared using swirl flow membrane emulsification at high dispersed phase fluxes (up to 15.6 m3/m2h) greater than the droplet dripping mode of droplet formation and at very low concentrations of sodium dodecyl

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sulphate (SDS) surfactant (as low as 0.01 wt.%). The swirl flow membrane emulsification method involved the generation of a centrifugal kind of flow in the continuous phase. This exerted higher radial shear stresses on

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the membrane wall which overcame the higher kinetic energy of the dispersed phase emerging from membrane pores when high dispersed phase fluxes were applied. The emulsions droplet size (d50) was in the

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narrow range of 30.2 to 35 and span of 0.239 to 0.34 for swirl flow ME as compared to the highly polydispersed emulsions with d50 in the range of 38.8 to 43.5 µm and span in range of 0.65 to 2.32 for the cross-flow ME

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method, for the 9.6 µm SPG membrane. Keywords: swirl flow; cross-flow; membrane emulsification; oil-in-water emulsions; high dispersed phase fluxes; sodium dodecyl sulphate (SDS) surfactant

1. Introduction

The membrane emulsification (ME) method is a drop-by-drop method of emulsion preparation which involves the permeation of the to-be dispersed phase fluid through the membrane pores to form droplets into the continuous phase fluid flowing at the membrane`s permeate side (Liu et al., 2011; Piacentini et al., 2014; Vladisavljevic and Williams, 2005). The emulsion droplets grow at the pore openings and detach upon reaching a certain size which is determined by the balance between the shear drag force acting on the droplets due to the flow of the continuous phase, the buoyancy of the droplet, the interfacial tension and the inertial force

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due to the flow of the dispersed phase in the membrane pores (Joscelyne and Trägårdh, 2000). Although, even in the absence of shear flow at the membrane surface, droplets can be spontaneously detached from a membrane whose pore openings have non-circular cross sections, entirely due to the action of interfacial

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tension but at lower production rates (Kukizaki, 2009; Kukizaki and Goto, 2007; Sugiura et al., 2001; Yasuno et al., 2002). In ME method, the resulting emulsion droplet size is primarily controlled by the membrane pore

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size and therefore, emulsions with small droplet size and narrow droplet size distribution can be easily

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prepared at lower shear stresses, and low energy input (104–106 J/m3), by choice of the appropriate membrane pore size and process parameters (De Luca et al., 2004; Gijsbertsen-Abrahamse et al., 2004; Joscelyne and Trägårdh, 2000). This renders the ME a suitable method for preparing functional emulsions

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containing heat and shear sensitive ingredients such as proteins, starches and vitamins (Joscelyne and Trägårdh, 2000; Schröder and Schubert, 1999). However, the ME method has not been widely adopted for

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commercial emulsion productions because of its low dispersed phase throughput, which greatly lowers the

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overall emulsion production rate (Charcosset, 2009; Gijsbertsen-Abrahamse et al., 2004; Joscelyne and Trägårdh, 2000; Vladisavljevic and Williams, 2005). In order for the ME method to be feasible for commercial scale emulsion production, the dispersed phase fluxes should be atleast above 0.1 m3/m2h (Wagdare et al., 2010). However, in the conventional ME methods, the dispersed phase flux is typically restrained within the range of 0.001–0.1 m3/m2h depending on the membrane pore size in order to prevent the emulsification process from transitioning from the size-stable zone to the continuous outflow or jetting zone (Charcosset et al., 2004; Joscelyne and Trägårdh, 2000; Katoh et al., 1996; Vladisavljević et al., 2012) and to avoid steric

hindrance among droplets that may be formed simultaneously at the adjacent pores (Abrahamse et al., 2002). This is because, in the conventional cross-flow ME method, the continuous phase is made to flow parallel to the axis of the porous membrane exerting weaker shear forces along the membrane wall, and thus as the dispersed phase flux is increased, the flow of the continuous phase liquid is gradually pushed away from the membrane wall by the higher kinetic energies of the radially extruding droplets of the dispersed phase from the tubular membrane pores. The wall shear stress exerted by the continuous phase flow gradually becomes weaker resulting in droplet coalescence and wetting of the membrane by the dispersed phase and eventual

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termination of the process due to membrane fouling (Joscelyne and Trägårdh, 2000). However, if sufficient wall shear stresses are generated in the system, it is possible to prepare emulsion at high dispersed phase throughput, above 0.1 m3/m2h, which is essential for the preparation of emulsions at commercial scale

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(Charcosset, 2009; Holdich et al., 2013).

A novel method of swirl flow ME (figure 1) was developed by Shimoda et al., (2011), for preparation of

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emulsions at high dispersed phase throughput, owing to the higher wall shear stresses generated by the

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swirling flow. In this method, the continuous phase was introduced in a tangential direction to the axis of the tubular membrane, thereby creating a highly turbulent centrifugal flow along the axis of the membrane tube (Pruvost et al., 2000). Basically, swirl flow is a flow having a rotational velocity added to the linear velocity

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component by use of a swirl flow generator, causing the fluid particles to move in spiral trajectories where their velocities vary continuously with respect to the axial and radial coordinates (Eldrainy et al., 2009; Najafi

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et al., 2011; Vaidya et al., 2011). The tangential velocity component of swirl flow is the main velocity

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component, which affects the swirling flow field and its distribution depends on the method swirl flow generation (Najafi et al., 2011). The swirl tangential velocity increases the composite velocity within the membrane, thins the boundary layer, enhances the tangential and radial turbulent fluctuation, and therefore increases the inertial force towards and around the membrane wall (Durmus et al., 2002). This inertial force component overcomes the high kinetic energy of the dispersed phase emerging from membrane pores and effectively scoops away the dispersed phase from the membrane surface so that the surface remains clean, and thereby prevents wetting of the membrane wall and forming of fouling layers. For this reason,

monodispersed emulsions could be prepared, in the jetting mode of droplet formation, at high dispersed phase fluxes in the range of 0.3 to 3 m3/m2h in the previous study by Shimoda et al., (2011). And the characterisation of the effect of process parameter on emulsion droplet size and monodispersity in swirl flow ME were made in the subsequent study by Mugabi et al., (2018). In this study we compared the generation monodispersed emulsions using the novel swirl flow ME method and the conventional cross-flow ME method at high dispersed phase fluxes. The emulsions were prepared using both methods at high dispersed phase fluxes in the range of 2.0 to 15.6 m3/m2h, in which the mode of

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droplet formation had fully transitioned from the dripping mode to the continuous out flow mode.

2. Methods and Materials

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2.1. Materials

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Oil-in-water (O/W) emulsions were prepared using de-ionised water containing anionic sodium

Fig. 1 Swirl flow membrane emulsification process.

Fig. 2 Schematic diagram of swirl flow ME experimental

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setup showing the tangential inlet of the continuous phase in the membrane module.

dodecyl sulphate (SDS) or non-ionic Tween 20 (Polyoxyethylene (20) sorbitan monolaurate) surfactants at

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various concentrations as the continuous water phases and Low viscosity type liquid paraffin and methyl laurate as the dispersed oil phases. All chemicals were bought from Nacalai Tesque Inc., Kyoto, Japan. The

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microporous membranes were cylindrical hydrophilic Shirasu porous glass (SPG) membranes (Nakashima et al., 2000) of inner diameter 8.4×10-3 m, length of 1.5×10-1 m, the thickness of 7×10-4 m and pore sizes of 1.9,

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without surface pre-treatment.

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5.2, 9.6 and 20 µm. The membranes were purchased from SPG technology, Miyazaki, Japan and were used

2.2. Preparation of O/W Emulsion Using the Swirl Flow ME Method O/W emulsions were prepared using the swirl flow ME setup shown in figure 2. The dispersed oil phase was

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permeated through the outer surface of the cylindrical membrane at flow rates of 50 to 400 mL/min using a

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high flow, high pressure, pulseless Plunger pump NP-GXL-1000U (Japan Precision Instruments Inc., Tokyo, Japan). The formed droplets were detached from the membrane wall by the shear forces generated by the swirl flowing continuous phase containing surfactants at various concentrations, to form the O/W emulsion. The continuous water phase was introduced into the inner space of the membrane from a tangential direction through an inlet orifice tube attached on the membrane module, perpendicular to the axis of the tubular membrane in order to generate a decaying type of swirl flow within the membrane tube (Pruvost et al., 2000;

Yilmaz et al., 1999), as indicated in figure 2. The intensity of the swirl flow was controlled by adjusting the flow rate of the inlet continuous phase liquid. The throttling ratio (T) of inlet orifice diameter (2.5×10−3 m) to the membrane inner diameter was 11 as was calculated from equation 1 and this high throttling ratio was responsible for the creation of the highly vigorous swirl flow velocity in the membrane tube. 2 𝐷 𝑇 = ( 𝑡⁄𝐷 ) … … … … … … … … … … … . … … 1 𝑖

Herein, 𝐷𝑡 [m] is the inner diameter of the membrane tube and 𝐷𝑖 [m] is the diameter of the inlet orifice. The swirl flow velocity of continuous water phase at the membrane entrance was varied between 1.7 to 13.6 m/s

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using a gear pump, MG-2 SP (Kyowa Chemical Industry Co., Ltd, Kagawa, Japan) and thus creating swirling flows of 3,900 to 31,200 rpm revolutions (R) in the tubular membrane, as calculated from equation 2. 𝑉 𝑅 = ( 𝑖⁄𝜋𝐷 ) × 60 … … … . . … … … … … … . . … 2 𝑡

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Herein, 𝑉𝑖 [m/s] is the inlet velocity of the continuous phase. Unlike in the conventional cross-flow method,

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the swirling flow exerted stronger inertial force towards and around the inner surface of the membrane which facilitated faster detachment of the emulsion droplets from the membrane pores and prevented fouling of

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the membrane surface.

2.3. Preparation O/W Emulsions Using the Cross-Flow ME

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For comparison purpose, O/W emulsions were also prepared using the cross-flow ME method. The tubular membrane was fixed into the membrane module in a horizontal orientation as shown in figure 3. The dispersed

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phase was permeated through the SPG membrane using a high flow, high pressure, pulse less Plunger pump NP-GXL-1000U at flow rates of 50 to 400 mL/min. The emulsion droplets formed on the permeated side of the

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membrane were detached from the membrane pores by shear forces exerted by the cross flowing continuous phase at flow rates of 500—4000 mL/min, to form the oil in water emulsion which corresponded to flow velocity of 1.7—13.6 m/s. In all experiments, the SPG membranes were thoroughly wetted by the continuous phase liquid by sonicating them in the continuous phase liquid for about 5 minutes before assembling them in the module and after the

experiment, the membrane module and membranes were first cleaned-in-place with ethanol, followed by soapy water.

Fig. 3 Schematic diagram of cross-flow ME experimental

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setup for preparing O/W emulsion.

Then dismounted and cleaned by ultrasonication in ethanol and water for about 5 minutes. The emulsions were not recirculated through the membrane module and dispersed phase content of the emulsions was measured from the ratio of the dispersed phase to the continuous phase flow rates. O/W emulsions of up to

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44.4 wt.% dispersed oil phase content were prepared. The dispersed phase fluxes (J) was calculated using

𝐽 =

𝑄 … … … … … … … … … … … … … … … .3 𝜌𝑑 𝐴

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equation 3.

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Herein, Q [kg/h] is the mass flow rate of the dispersed oil phase through the membrane of effective surface area A [m2] and 𝜌𝑑 [kg/m3] is the density of the dispersed oil phase. The weight of oil phase permeated through

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the membrane pores was measured using an electronic balance on which the oil phase vessel was placed. 2.4. Measurement of the Mean droplet size and size distribution

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The mean droplets size of the prepared emulsions was measured using a laser diffraction particle size analyser

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(SALD2100, Shimadzu Co. Ltd., Kyoto, Japan). The mean droplet diameter was defined as the median droplet diameter (d50) corresponding to the value at which the cumulative volume percentage of droplets is 50%. The droplet size distributions, denoted as the span, was used to define the uniformity of the emulsion droplets and was determined from equation 4. 𝑆𝑝𝑎𝑛 =

(𝑑90 − 𝑑10 ) ………….…… ………..4 𝑑50

Herein, 𝑑90 , 𝑑10 and 𝑑50 are the droplet diameters corresponding to 90, 10 and 50 wt.% on a relative cumulative droplet size distribution curve of the emulsion droplets. The monodispersed (uniform droplet size) emulsions were represented by smaller span values in the range of 0.2 to 0.5.

3. Results and Discussion

3.1. Membrane Surface Area and Swirl Flow Intensity In this study, the tangential inlet method which produces a decaying type of swirl flow whose intensity varies

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along the membrane tube was used to generate the swirling flow in the cylindrical SPG membrane. For this reason, the shear stress was not constant throughout the membrane length. In this section we determined in which section of the membrane length was the swirl flow shear stress most effective and produced the most

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stable emulsions.

The membrane tube was divided using a non-porous Teflon tape covering, as illustrated in figure 4 into 3 equal

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portions; the lower portion A, closer the module entrance, the central portion B and the upper portion C,

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closer to the module exit. The droplet diameter and monodispersity of the emulsion prepared using the whole membrane area (D) and each of membrane portion, A, B and C were compared. Emulsions were prepared using SPG membrane of pore size 20 μm, methyl laurate at 3.1 m3/m2h as the dispersed phase and 1.0 wt.%

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Tween 20 at 11.9 m/s as the continuous phase. The pores of one part of the cylindrical SPG membrane were open and used to prepare emulsions while the rest of the membrane was covered with non-porous Teflon

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tape. As shown in figure 5, the most monodispersed emulsions were prepared using the central membrane

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portion. At the module entrance, the swirl flow intensity was very high and unstable leading to the production of smaller droplets but with a broader span while towards the module exit the swirl flow shear forces have greatly reduced leading to the production of bigger droplets with broader span. The swirl flow intensity was most stable in the central portion of the membrane surface area. And emulsification using the whole membrane surface area (D) produced emulsion with widest droplet size distribution due to the unpredictable wall shear stresses along the membrane length. Therefore, the central membrane surface area, where swirl

flow intensity was most stable, was adopted as the effective membrane length (about 5x10-2 m) for use in most of the experiments. In the tangential entry swirl flow generators, the continuous phase liquid is introduced into the membrane tube through a tangential inlet duct or nozzle attached or drilled at the entrance of the membrane tube module (Pruvost et al., 2000; Shimoda et al., 2011). The single tangential inlet adds a circumferential velocity

with

non-axisymmetric

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flow

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dimensional

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component to the main axial flow of the liquid thereby creating a swirling motion characterized by a three-

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Fig. 4 Partitioning of the SPG membrane using Teflon tape; A lower portion of membrane pores open, B the central

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portion pores open, C upper portion pores open and D whole membrane pores open.

Fig. 5 Comparison of the droplet size distribution of emulsions prepared using the lower (A), central (B), and upper (C) membrane area portions and the whole membrane area (D).

behaviour, at the entrance of the membrane tube module.

The swirl flow intensity is usually quantified by the swirl flow number Sn, which is defined as the ratio of axial flux of tangential momentum to the axial flux of axial momentum times the effective nozzle radius (Eldrainy et al., 2009; Najafi et al., 2011; Vaidya et al., 2011), as shown in equation 5.

𝑆𝑛 =

Axial flux of tangential momentum Axial flux of axial momentum × Radius 𝑅

𝑆𝑛 =

∫0 𝜌𝑈𝑊𝑟 2 𝑑𝑟 𝑅

𝑅 ∫0 𝜌𝑈 2 𝑟 𝑑𝑟

……………………..….5

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Herein U and W are the axial and tangential velocity components respectively, R is the swirler radius and 𝜌 is the density. The circumferential velocity decays along the flow path leading to the decrease in the swirling motion, tending towards an axial flow with vanishing radial and tangential velocities, and the turbulence

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intensities decays into an axial flow (Pruvost et al., 2000; Vaidya et al., 2011).

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The generation of swirling flow using the tangential inlet method gives an advantage of easy control of the swirling speed and simplicity of design. The development of decay in the swirling flow was prevented by use

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of a shorter emulsification distance and the swirl flow intensity was controlled by adjusting the velocity of the continuous water phase admitted through the inlet nozzle. However, for industrial application, the continuous

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kind of swirl flow in which the swirl flow intensity is maintained throughout the membrane tube length, would be of a higher advantage. This kind of swirl flow can be generated by inserting coiled wires, twisted tapes or

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helical vanes along the length of the membrane tube, or by making of helical grooves in the inner surface of the membrane tube (swirl flow tube) or by direct membrane rotation (Eiamsa-ard and Promvonge, 2005; Imao

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et al., 1996; Schietekat et al., 2014; Sheikholeslami et al., 2015). 3.2. Effect of Surfactant Concentration on Emulsion’ d50 and Span O/W emulsions were prepared using SDS surfactant in the range of 0.01–0.5 wt.% at 10.2 m/s continuous phase velocity. The dispersed phase liquid was low viscosity type liquid paraffin at 3.9 m3/m2h and the SPG membrane was of pore size 9.6 µm. The relationship between the emulsions’ d50 and span with surfactant concentration was studied for O/W emulsions prepared using the swirl flow and the cross-flow ME methods.

In both methods, it was observed that the emulsions’ d50 and span generally decreased with increase in SDS surfactant concentration as shown in Figures 6. However, smaller and highly monodispersed emulsion droplets were prepared by swirl flow ME method even at very low emulsifier concentration of less than 0.1 wt.%. The d50 was moderately constant varying in the range of 32.5 – 37.0 µm while the span value varied in a close range of 0.40 – 0.24 for the swirl flow ME while for the cross-flow ME method the emulsions had larger droplet diameter and were highly polydispersed with span values above 1.0 at all surfactant concentration (figure 6).

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The most monodispersed emulsions prepared by swirl flow ME method had a span value of 0.239 corresponding to the d50 of 33.4 µm for the 9.6 µm membrane pore size and were likely achievable in the SDS concentrations range of 0.01 to 0.1 wt.%, at various swirl flow velocities and dispersed phase fluxes. The least

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achievable span value for the most monodispersed emulsions was dependent on the membrane pore size and corresponded to a specific d50. In this study, the least achievable span values were 0.231 (d50 = 9.675 µm) for

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20.0 µm membrane, as shown in figure 7.

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the 1.9 µm membrane, 0.255 (d50 = 20.383 µm) for the 5.2 µm membrane and 0.271 (d50 = 69.835 µm) for the

Fig.6 Relationship of SDS concentration with d50 and span of emulsions prepared using swirl flow and cross-flow ME methods at 3.9 m3/m2h dispersed phase flux, 10.2 m/s continuous phase velocity and 9.6 µm membrane pore size.

Fig.

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Droplet

size

distribution

for

the

most

monodispersed emulsions prepared by membranes of pore size 1.9, 5.2, 9.6 and 20 µm using swirl flow membrane emulsification.

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In membrane emulsification, surfactants influence droplet formation and detachment by lowering the interfacial tension, the force that acts against droplet detachment from the membrane pores (Peng and Williams, 1998; Schröder and Schubert, 1999). Lowering of the interfacial tension results in faster droplet formation and

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detachment of smaller droplets with a narrower span (Schröder et al., 1998). Since the surfactants were

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dissolved in the continuous phase, the continuous phase flow rate had a direct effect on the rate of diffusion of the surfactant molecules from the liquid bulk and adsorption on the new oil-water interface (Lepercq-Bost

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et al., 2008; Timgren et al., 2010).

For this reason, the highly turbulent nature of the swirling flow facilitated faster diffusion of the emulsifier to

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the forming droplet interface, which resulted into faster rate of interfacial tension lowering and droplet detachment, leading to formation of smaller and highly monodispersed emulsion droplets even when the SDS

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surfactant concentrations was very low in the range of 0.01–0.1 wt.%. And these emulsions were found to be

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as stable as the emulsions prepared at higher SDS concentration above 0.5 wt.% SDS during storage at room

(about

25oC)

for

a

period

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temperature

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Fig. 8 Comparison of stability of emulsions prepared at low (0.05 & 0.1 wt.%) and at high (0.5 & 1.0 wt.%) SDS

dispersed phase flux using 9.6 µm pore size SPG membrane.

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concentration at 10.2 m/s swirl flow velocity, 3.9 m3/m2h

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130 days, as shown in figure 8. This showed that the

emulsion droplets were adequately covered by surfactant molecules hence stabilising them against

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coalescence and destabilisation during storage. On the other hand, emulsions prepared using the cross-flow ME in the same surfactant range, destabilised with phase separation after a few hours of storage.

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3.3. Effect of Dispersed Phase Flux on Emulsion’ d50 and Span

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The dispersed oil phase was permeated through the porous membrane at flow rates of 50 to 400 mL/min using a high flow, high pressure, pulseless Plunger pump NP-GXL-1000U. The dispersed oil phase flux through SPG membranes pores increased with increase in dispersed phase flow rate and was closely related for all membrane pore sizes used in the study as shown in Figure 9. The disperse phase flux was varied in the range of 2.0 to 15.6 m3/m2h, which was higher than the droplet size stable zone fluxes of 0.001–0.1 m3/m2h, used in the previous studies of ME and far above the minimum

disperse phase flux of 0.1 m3/m2h needed for industrial scale production rate. The emulsion droplets were formed in the continuous outflow (jetting) mode of droplet formation and the emulsion production throughput was higher with emulsions of up to 45 wt. % by prepared. Emulsions formed by the cross-flow and the swirl flow ME methods were compared. From figure 10, it can be observed that emulsion droplet size and span of emulsions prepared by the swirl flow ME method were stable at all disperse phase fluxes while the

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emulsions

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Fig. 9 Relationship of the dispersed phase flow rate with

the dispersed phase flux through SPG membranes of pore

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size 1.9, 5.2, 9.6 and 20 µm.

Fig. 10 Effect of the dispersed phase flux on droplet size and span of emulsions prepared using the swirl flow and the cross-flow ME methods at 0.1 wt.% SDS surfactant, 10.2 m/s continuous phase velocity and 9.6 µm pore size SPG membrane.

formed

by the cross-flow ME had bigger droplets and were highly polydispersed with spans values above 1.5 at all disperse phase fluxes. This was because the shear drag force exerted by the cross flowing continuous phase was very weak to control droplets formation at higher dispersed phase fluxes. In the jetting mode of droplet

formation, the kinetic energy of the out-flowing dispersed oil phase is very high, and if the wall shear stress exerted by the continuous phase is too low to cut the jet as fast as it exits the pores, the droplet formation will occur by breakdown of the jetted oil-liquid by Rayleigh–Taylor instabilities. In this way the droplet size cannot be controlled, and the emulsions formed are highly polydispersed with very wide span as shown in figure 10, for emulsions formed by cross-flow ME. However, in the swirl flow ME method, the inertial force component overcomes the high kinetic energy of the dispersed phase emerging from membrane pores and effectively cut the out-flowing liquid jets of the dispersed phase into highly monodispersed emulsion droplets.

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Thus, controlling the emulsion droplet size and span within a very narrow range at all disperse phase fluxes.

Fig. 11. Relationship of continuous phase flow velocity

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with d50 and span of emulsions prepared using the swirl flow and the cross-flow ME methods at 0.1 wt.% SDS, 3.9 m3/m2h dispersed flux and 9.6 µm pore size SPG

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membrane.

3.4. Effect of Continuous Flow Velocity on Emulsion’ d50 and Span

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O/W emulsions were prepared using SPG membrane of pore size 9.6 µm using low viscosity type liquid paraffin

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at 3.9 m3/m2h as the dispersed phase and de-ionised water containing 0.1 wt.% SDS surfactant as the continuous phase. As shown in figure 11, it was observed that for both the swirl flow and the cross-flow ME methods, the droplet size (d50) decreased gradually with increase in continuous flow velocity due to increase in the wall shear stress which is responsible for detachment of the emulsion droplet from the membrane pores. Highly monodispersed emulsions with smaller droplet size and span values were prepared by the swirl flow method as compared with the cross-flow ME method. The d50 was in the narrow range of 30.2 to 35 µm and span of 0.239 to 0.34 for swirl flow ME whereas for the cross-flow ME method the emulsions were highly

polydispersed with d50 was in the range of 38.8 to 43.5 µm and span in range of 0.65 to 2.32. For the same continuous flow velocity, the swirling flow exerted higher wall shear stress than the cross flowing continuous phase. The higher wall shear stress in swirl flow ME was due to the swirl tangential velocity component involved the centrifugal flow which increases the composite velocity within the membrane, thins the boundary layer, enhances the tangential and radial turbulent fluctuation, and therefore increases the inertial force towards and around the membrane wall (Durmus et al., 2002). This inertial force component increases the wall shear

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stress in swirl flow ME and enables faster droplet detachment and formation of smaller and highly monodispersed emulsion droplets.

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Conclusion

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The swirl flow ME method involved the generation of the centrifugal kind of flow in the continuous phase

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liquid. This kind of flow exerted higher radial shear stress on the membrane pores, which enabled fast detachment of the emulsion droplets, even when high dispersed phase fluxes, beyond the droplet dripping mode of droplet formation were applied. O/W emulsions with a highly narrow droplet size distribution (span)

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were prepared using an SPG membrane over a wide range of process conditions. The most highly monodispersed were of mean droplet size (d50) of 9.675 µm and span of 0.231 for 1.9 µm membranes, d50 =

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20.383 µm and span = 0.255 for 5.2 µm membrane, d50 = 33.427 µm and span = 0.239 for 9.6 µm membrane, and d50 = 69.835 µm and span = 0.271 for the 20 µm membrane. They were obtained at various ranges of

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operating conditions of swirl flow velocity, high dispersed phase fluxes of 2.0–11.7 m3/m2h, and very low SDS concentration of 0.01–0.1 wt.%. The emulsions, although prepared at very low surfactant concentrations, they were stable without significant change in droplet size and span throughout the 130 days period of storage.

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