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Controlled Production of Emulsions Using a Crossflow Membrane
0263±8762/98/$10.00+0.00 Institution of Chemical Engineers Trans IChemE, Vol 76, Part A, November 1998
CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE Part II: Industrial Scale Manufacture R. A. WILLIAMS (FELLOW), S. J. PENG, D. A. WHEELER*, N. C. MORLEY*, D. TAYLOR**, M. WHALLEY * and D. W. HOULDSWORTH² Particle and Colloid Engineering Group, Camborne School of Mines, University of Exeter, Redruth, Cornwall, UK *Disperse Technologies Ltd, Dorking, Surrey, UK **Fairey Industrial Ceramics Ltd, Stone, Staffordshire, UK ² Memtech (UK) Ltd, Swansea, UK
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his paper presents the results of an investigation into the practical application of an innovative emulsion manufacturing methodÐ cross¯ ow membrane emulsi® cation on a pilot-plant scale. This method produces emulsions by breaking up the discontinuous phase in a controlled mannerÐ ¯ owing from one side to another of a well de® ned porous membrane. The droplets formed on the surface of the other side of the membrane are scoured away by the cross¯ ow of the continuous phase. In this way the emulsion can be manufactured under lower shear conditions than with conventional emulsi® cation techniques and the droplet size and size distribution can be controlled through judicious choice of the process ¯ ow conditions and the pore size and size distribution of the membrane. Experiments have been carried out to demonstrate the effects of the process parameters and membrane structure on emulsion droplet size. Batch and semi-continuous manufacture of oil/water dispersions is demonstrated. Keywords: emulsi® cation; energy utilization; cross¯ ow membrane; on-line size measurement
INTRODUCTION
can be obtained. The droplet size distribution is often wide and this, in turn affects the emulsion characteristics and stability. Secondly, the energy utilization for large scale production of emulsions using traditional (rotor-stator) methods is very poor and gets worse as vessel size increases. This adds signi® cantly to the cost of manufacture. Thirdly, reproducibility on a single piece of equipment is often poor and the quality of the product can vary from one manufacturing vessel design to another even on the same manufacture scale. Scale-up is therefore a common and dif® cult problem: the transformation from laboratory to small scale to full scale manufacture is dif® cult. This can lead to in¯ exibility of manufacturing apparatus and consequent additional cost arising from the under-utilization of expensive equipment. The aim of this work is to investigate the application of cross¯ ow membrane emulsi® cation (XME), at a pilot scale, with a view to addressing the above limitations of conventional emulsi® cation methods. The principles of XME have been described in part I of the paper1 and are ampli® ed elsewhere2 ± 6 . However, important practical issues need to be addressed before the method can be widely employed to manufacture industrial emulsions. For example, how to select the correct membrane pore size and porosity and to set process conditions for a given product speci® cation (size and size distribution).
Emulsion manufacturing is a very important process in the food, chemical, mineral processing, cosmetics and pharmaceutical industries. Increasingly there is a need to produce emulsions in which the droplets have a de® ned particle size distribution (e.g. bimodal) or a very narrow size distribution. Conventional methods for manufacturing emulsions are based on: · rotor-stator systems such as tooth-disc high speed homogenizers, or colloid mills in which a high shear is generated between a rotor and a stationary smooth, roughened or grooved surface. Here turbulence is the primary cause of ¯ uid disruption leading to the formation of droplets · high pressure homogenizer systems in which the emulsion mixture is passed through a narrow ori® ce, or inject dispersion in which two jets of different components are made to collide head-on. These processes may be assisted with use of power, ultrasound or electrical ® elds. Pressures in the range 5.0 ´ 106 ±3.5 ´ 107 Pa are common. Here ¯ uid separation is caused by turbulence and cavitation effects and high rates of energy dissipation occur. However it is well known that a number of problems may be associated with these existing methods of production: First, sometimes the droplet size and size distribution cannot easily be controlled. Even if the mean droplet size 902
CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II
DROPLET FORMATION THROUGH A MEMBRANE TUBE Droplet formation from a membrane tube is believed to be similar to that from a single pore if the membrane tube can be considered to be composed of a large number of individual capillary pores (Figure 1). In previous work1 it has been reported that for given conditions the droplet size is proportional to the diameter of the pore from which it emerges. Usually, the droplet diameter is between 2 and 8 times the pore diameter. It is therefore important that the pore size distribution is narrow in order to produce a product that exhibits a narrow droplet size distribution. As the hold-up (or concentration) of the discontinuous phase in the emulsion product increases, the properties of the bulk emulsion phase changes. For example, the viscosity increases and the hydrodynamic conditions near the membrane surface may be substantially in¯ uenced, thus changing the shear force and therefore the size of the detached droplet. The following discussion will focus on certain important conditions including the porosity of membrane surface, transmembrane pressure and pore size. Effect of Porosity The porosity of the membrane surface can be important for a successful XME process because it determines the distance between two adjacent pores. This distance is critical to ensure that two adjacent droplets do not come suf® ciently close to allow contact with each other, which may lead to coalescence. Therefore, the pores are preferred to be uniformly located on the surface to ensure maximum distance between any two adjacent pores for a given porosity. To study the relationship between the porosity and the distance between two adjacent pores, an ideal membrane is assumed for which the pore size is uniform, the pores are straight cylinders and are located on the surface uniformly. The ideal membrane can be represented by a cubic unit (Figure 2) of size a, each unit containing one cylinder of diameter dp (dp # a). The porosity of the cubic unit is then calculated as: Volume of the pore space in the unit e Volume of unit p4 2 dp a p dp 2 p 1 1 a3 4 a 4 c2
Figure 1. Schematic diagram showing droplet formation from a membrane tube.
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where c is de® ned as the maximum possible droplet size ratio: a c 2 dp Here the size a can be considered the average distance between the centres of two adjacent pores for a given porosity. For example, if the droplet size ratio produced by the membrane is greater than c, then it is likely that two adjacent droplets will contact each other. Therefore, to ensure that no coalescence can occur, the c value of the membrane must be greater than the droplet size ratio of the ® nal product desired. Figure 3 illustrates the relationship between the porosity of the membrane and the critical droplet size ratio c for the above ideal membrane. Equation (1) is valid for an ideal membrane. In reality, however, the effect of non-ideal pore openings and the size of the pore passage implies that the porosity of a membrane would need to be smaller than the value given in equation (1). This is because the diameter of the pore opening determines the droplet size. For instance, the porosity according to equation (1) is 0.049 for a droplet size ratio 4. If the pore passage is not cylindrical and has a small neck inside, then the porosity will be smaller than 0.049. A further constraint may be imposed by the manufacturing process, since if the porosity is too low the membrane surface area required to achieve a realistic throughput may be excessive. Effect of Pressure Previously it has been demonstrated that there is an optimum range of applied transmembrane pressures ($ Pc ) for successful emulsi® cation. If the pressure is too low (
Figure 2. Two representative cubic units of an ideal membrane.
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WILLIAMS et al. Based on the results of droplet formation from a single pore, it is possible to predict the droplet size distribution for a given pore size distribution under particular conditions during the droplet formation from a membrane tube. Figure 5 demonstrates the droplet size distributions (D1, D2) calculated by using two membrane tubes having pore size distributions (P1, P2 respectively): It is evident from the above result that a small number proportion (1%) of `large’ (2 mm) pores in D2 yields a signi® cant proportion of large droplets in the ® nal product. This serves to demonstrate that the quality of membrane is of paramount importance to the success of the membrane emulsi® cation process.
Figure 3. The maximum porosity for the membrane surface vs. the droplet size (ratio) requirement of the product.
where c is the o/w interfacial tension and dp is the capillary pore diameter. For a membrane tube, the critical pressure of the membrane could lie in a range (if the pore size is not uniform). Figure 4 illustrates the minimum transmembrane pressure as a function of the pore size for two interfacial tensions. There is no reliable theoretical approach to estimate the useable maximum transmembrane pressure. It is believed that the maximum transmembrane pressure would be affected by the membrane itself (e.g. pore structure) and the process conditions (e.g. cross¯ ow velocity). Generally, the highest transmembrane pressure should be applied in order to achieve the highest emulsi® cation production rate. An experimental approach must be adopted for optimising individual process conditions. It is believed that pressures 2±10 times higher than the minimum transmembrane pressure are practical. Effect of Pore Size
EXPERIMENTAL Preparation of Membranes Experiments were conducted using several types of membrane composed, for example, from ceramic, stainless steel and polymeric (track-etched) materials. The discussion here is focused upon the development and use of a high speci® cation ceramic membrane. The ceramic membrane element is a tubular sintered alumina substrate with one or more alumina coatings on the internal surface. Both the substrate and the membrane coating have a tightly-controlled pore size distribution, the average pore size of the substrate being 5±10 times that of the membrane coating. The substrate provides the mechanical strength necessary to support the ® ner surface membrane, and can be varied to suit the requirements of the particular process. The substrate itself was composed of a blend of fused aluminas and a ® ne-grained calcined alumina. These were combined with extrusion aids and water to produce a body of suitable consistency. The body was extruded via a ram extruder through a die to give single- or multi-channel tubes. After drying, the tubes were ® red to a high temperature and
In the discussion above, an ideal membrane that has uniform pore size has been assumed. In practice, however, the membrane contains pores in a narrow size range or perhaps possesses a few extremely large pores. The consequence of the existence of few large pores is that the XME process may generate droplets having a bimodal distribution.
Figure 4. The minimum transmembrane pressure as a function of pore size for two interfacial tensions.
Figure 5. Predicted product droplet size distributions (D1, D2) as a function of pore size distributions (P1, P2) respectively. (Membrane: length 600 mm; inside diameter 5 mm; porosity 0.1; pore tortuosity 1.0. process parameters: cross¯ ow velocity 1.0 m s ±1; transmembrane pressure 1.0 ´ 105 Pa; o/w interfacial tension 6 mN m ±1; viscosity of continuous phase 1.0 cP and viscosity of product 10.0 cP.)
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CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II
Figure 6. Surface pore size distribution of a typical ceramic emulsi® cation membrane tube.
clipped to the required length. All substrate tubes were proof tested by measuring their maximum pore size and pore size distribution via bubble point test equipment (described below). The internal surfaces of the substrate tubes were then coated with a thin layer of a calcined alumina, or a blend thereof, by controlled immersion in an aqueous suspension. After drying the coating was ® red. The process was repeated if multiple coatings were required. The ends of the tubes were then glazed to seal the exposed substrate at the ends. The membranes were again proof tested by measuring their maximum pore size via bubble point test equipment. The pore size distribution of the membrane layer was adapted from two measurement techniques. The basic measurement of pore size is based on the bubble point technique of British Standard 1752:1983. The technique for measuring the pore size distribution, using the bubble point technique was adapted from the method of Gelinas and Angers7 . For the bubble point technique, the porous ceramic is saturated with a liquid (water, isopropanol or other liquid depending on the pore size of the ceramic). One side of the liquid-saturated sample is subjected to an increasing pressure whilst the other side is submerged under a known depth of the liquid medium. The pressure at which the ® rst air bubbles emerge from the ceramic is noted. The maximum pore size can then be calculated using established equations. Some explanation is needed of the term `maximum pore size’ . Assuming that the pores are isolated
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capillaries passing through the ceramic from the air side to the liquid side (as in the case of the test) and where each capillary has discrete variations in diameter along its length. The established expression (see BS 1752:1983) for pore diameter gives the smallest diameter along the length of the capillary, since this diameter determines the pressure at which the liquid can be forced from the capillary. The `maximum pore size’ in this context is the largest of the small diameters of each capillary. If a coarse material is coated with a ® ne pore material the pore size determined by the bubble point method will relate to the ® ner pores of the coating and ignores the pores of the substrate. Therefore this method is ideally suited to the speci® c determination of the pore size distribution of the coating layer of the ceramic cross¯ ow membranes. For the bubble point technique applied to pore distribution analysis of whole membranes, the membrane is saturated with a medium and inserted in a jig which holds it a ® xed distance below the surface of the liquid. The ends are enclosed in caps that seal the internal channels of the membrane from external liquid but allow the channels to be pressurized. The air pressure in the channels is increased in stages until the ® rst bubbles are observed. Above this pressure, the air pressure is increased in steps. At each pressure increments, the air is allowed to pass through the pores that are open at that pressure until a constant `input’ pressure is achieved. The air supply is then shut off and the air pressure in the channels falls to a constant `residual’ pressure (effectively when air bubbles cease to ¯ ow the pores). The input and residual pressures are recorded for each pressure step. The pressure is increased until the liquid has been driven from all the pores. The pressure is then reduced in ten steps to ambient pressure, recording the input and residual pressure at each step. The pore size distribution is calculated from the above data because the difference between the input pressure and the residual pressure at each step is proportional to the volume of the pores whose size correspond to each input pressure. Figure 6 shows a typical surface pore size distribution on a multiple-coated membrane having a surface pore size of 0.2 mm. It is evident that multiple coatings can be used to reduce the pore size and sharpen the size distribution signi® cantly, thus creating a high performance membrane. Figure 7 (a) shows a SEM photograph of a cross-section through a ceramic membrane of 0.2 mm nominal pore size. It is clear that the surface was ® rst coated by 0.5 mm particles and then by 0.2 mm particles.
Figure 7. (a) SEM micrograph of cross-section through view of ceramic membrane tube with multiple coatings. (b) SEM photograph of membrane surface.
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Table 1. Details of membrane tubes used for emulsi® cation process. Single Channel Length (mm) Inside diameter of each channel (mm) Tube outside diameter (mm) Porosity of coating layer (Ð ) Pore size of coating layer (mm) Porosity of supporting layer (Ð ) Pore size of supporting layer (mm)
600 5 10 <0.1 0.2±0.5 0.36 2±3
7-Round Channel 600 3.5 20 <0.1 0.2±0.5 0.36 2±3
Figure 7 (b) shows a SEM photograph of the coated membrane surface. It is evident that the surface is not smooth on a micrometer scale, but this can be varied and controlled to some degree by varying the manufacturing process. Membranes having properties illustrated in Figure 6 can be made routinely using the procedures described above. Alternative properties can be obtained by varying the substrate or coating materials. Results from two typical ceramic membrane tubes will be presented here, which consist of a single tube element and a multiple-channel element which contains seven identical tubes. The details are summarized in Table 1.
(90±500 l/hr), membrane module, pressure transmitters before and after the membrane module, heat exchanger and a stirrer. Two sampling valves are installed after the storage tank and before returning back to the tank. The discontinuous phase is held in a storage tank (5 l) ® tted with heating tapes, temperature indicator, one way valve for air inlet, safety valve and air pressure regulator. The discontinuous phase tank is capable of pressurising to 7.0 ´ 105 Pa. The ® nal product loop contains a heat exchanger, temperature indicator, a jacketed cooling receiver tank and a stirrer. A control system is used which determines the power source for the above three major systems and speed control of the lobe pump plus 4±20 mA signals from the ¯ owmeter, pressure transmitters and the temperature transmitters. In addition, if required, a droplet size detector (described below) in the continuous phase tank sends information to the computer and the process can be adjusted automatically. The computer and the interface software are essential parts of the apparatus if automatic ¯ ow control and monitoring of critical process parameters are required. For simple batch operation the process can also be run manually without the computer. Emulsion Characterization
Emulsi® cation Apparatus Figure 8 shows the major components of the XME apparatus, as described below. The continuous phase system contains the continuous phase storage tank (10 l) ® tted with heating tapes and temperature indicator, a three lobe rotary pump, magnetic ¯ ow meter
In-process measurements of the droplet number and size were made in the continuous phase tank (Figure 8) using a focused beam re¯ ectance measurement instrument (FBRM, Lasentec, USA) which is based on scanning a highly focused laser beam (that emanates from a dip-in probe) over the droplets pass the probe tip, as described in detail elsewhere 8 . When the laser beam intercepts the surface of a
Figure 8. (a) Schematic ¯ ow sheet of the XME apparatus and (b) details of the membrane module.
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CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II
droplet, part of the light is scattered backwards and detected by a photo diode. Because the laser beam scans particles in a random manner the width (not the intensity) of the signal represents the chord length (not necessarily the true diameter) of that droplet. Due to the high speed scanning, the chord length distribution and the number of droplets particles scanned in a particular size range of the dispersion can be calculated statistically from the signals in a very short time (over 1000 particles can be counted in a second). It is convenient to use the FBRM to monitor the dispersions for the chord length distribution and the number fraction of the particles in a selected size range. This was used to detect any gross variations of particle size in the emulsion production process with time. Ex situ analyses of emulsions were made using the standard method of forward light scattering9 using a Malvern MasterSizer. Samples were removed from the process, diluted and presented for careful analysis using the standard circulating ¯ ow cell. The results give the particle volume distribution of the dispersion. Two expressions which will be used to characterize the droplet size distribution are span and uniformity of the distributions: d x, 0.9 d x, 0.1 Span 4 d x, 0.5 Xi d x, 0.5 di Uniformity 5 d x, 0.5 Xi
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The formulation of two o/w emulsions at room temperature will be discussed here, as shown in Tables 2 and 3. These are model systems for room temperature emulsions pertaining to a cosmetics application. Tests were performed in batch and semi-continuous modes. For a batch operation, the whole system (including the continuous phase, the connection line between discontinuous phase tank and the membrane module, the membrane module itself, the circulation loop and the continuous phase
tank) was thoroughly cleaned before the membrane module was dismantled from the circulation loop. The membrane surface was wetted to render it hydrophilic for an o/w emulsion (w/o emulsions need a hydrophobic surface) before it was mounted in the membrane module and re® tted back into the circulation loop. The continuous phase tank and circulation loop were further washed with de-ionized water before the appropriate amount of continuous phase was poured into the continuous phase tank and the circulation pump motor started at a minimum speed. The appropriate amount of discontinuous phase was poured into the discontinuous phase tank and the air inside the membrane chamber and connection line was gently purged. With the valve beneath the discontinuous phase tank (V23) shut, the discontinuous phase tank was pressurized to the desired transmembrane pressure and the circulation pump was adjusted to achieve the desired cross¯ ow rate and controlled by the PC afterwards. The FBRM was then started to monitor the number counts of the particles in selected size ranges and the droplet size every 1 minute continuously and the emulsi® cation process was started by opening the valve beneath the discontinuous phase tank (V23). Samples of the emulsion were regularly removed through the sampling valves and analysed. When the emulsion concentration had reached the desired hold-up (which could be calculated from the amount of discontinuous phase injected into the continuous phase), the process was then stopped. The system was drained and cleaned using pressurized hot water and cleaning agents. For a semi-continuous operation, similar procedures to the batch process were performed, however, just before the emulsion concentration had reached the desired level (25 wt %), instead of stopping the process, the emulsi® cation was continued but a ® xed volume of product was bled out and appropriate amount of continuous phase was added to the continuous phase tank to dilute the emulsion concentration (20 wt %). Once the emulsion concentration had again reached the desired product level (25 wt %), these procedures were repeated by bleeding and adding appropriate amounts of product and continuous phase. In this case, the discontinuous phase tank was large enough to hold the appropriate amount of discontinuous phase for the whole process. The droplet size distribution was sampled and measured periodically using the Malvern MasterSizer. During the whole process, the FBRM was used to monitor the number and size of the droplets in a given size interval continuously. For a full-scale continuous operation, the procedures described in the semi-continuous operation would be replaced by two well-controlled pumps. One of them would be used to bleed the product out and the other to add the continuous phase into the continuous phase tank. If required, the number and size of the droplets could
Table 2. Emulsion formulation 1.
Table 3. Emulsion formulation 2.
The x is replaced by any of the letters n, s, l, n that de® ne the distribution type of volume, surface, length or number. The span gives a description of the width of the distribution which is independent of the median size. Here d(x,0.5) is the median size of the distribution and di and Xi are respectively the mean diameter of, and result in, size class i. The uniformity is independent of the median size and is a measure of the absolute deviations from the median. Samples of ® nal product were removed and analysed using re¯ ected and transmitted light microscopy and also for cryogenic scanning electron microscopy. Experimental Procedures
Materials Continuous phase Discontinuous phase
Concentration w/w%
Deionized water Triethanolamine Sodium Nipastat
66.7 3.0 0.3
Mineral oil Isostearic Acid
27.0 3.0
Trans IChemE, Vol 76, Part A, November 1998
Materials Continuous phase Discontinuous phase
Deionized water Sorbitol (70%) Dobanol 91-8 Formalin Mineral oil (Marcol 172)
Concentration w/w% 36.40 36.40 2.16 0.04 25.00
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Figure 9. Effect of increasing cross¯ ow velocity on the droplet size distribution (A, B, C, D) produced from a membrane having the pore size distribution (P).
Figure 11. Photomicrograph of the emulsion droplet obtained by XME (Product D2, Figure 10).
Effect of Pore Size be monitored directly in situ by FBRM as for the semi-continuous mode of operation described above.
RESULTS AND DISCUSSIONS Effect of Cross¯ ow Velocity As reported elsewhere1 the droplet size is proportional to the pore size but it can be tuned by the magnitude of the cross¯ ow, particularly when the droplet sizes are large (>1 mm). Experiments using emulsion formulation 1 (Table 2) were conducted for four different cross¯ ow velocities using a single channel membrane tube. Figure 9 shows the droplet size distribution (labelled A, B, C, D) of the product as a function of cross¯ ow velocity and pore size distribution (P). Four cross¯ ow velocities were chosen: 1.12, 2.49, 4.34 and 5.09 m s ± 1 , respectively, which correspond to tube Reynolds numbers given in the ® gure. Figure 9 clearly demonstrates that the droplet size decreases (A, B, C, D) as the cross¯ ow velocity increases. As the droplet size becomes smaller the effect of the cross¯ ow velocity may become less signi® cant depending upon the hydrodynamic conditions near the surface of membrane, the droplet size, the sublayer thickness and stability.
To further exploit the results of the effect of pore size on the droplet size during the droplet formation from a single pore, experiments were conducted using two 7-round channel membrane tubes of nominal pore sizes of 0.2 and 0.5 mm (Table 1). The cross¯ ow velocity for both tests were maintained at 0.6 m/s. The transmembrane pressure was 2.8´105 Pa. Figure 10 shows the droplet size distributions (D1, D2) as a function of pore size distribution (P1, P2). The average droplet size of the two distributions (D1, D2) were 0.56 mm and 1.41 mm. The ratio of droplet size to the pore diameter is 2.80 and 2.82, respectively. The corresponding span and uniformity were 0.83, 0.83 and 0.43, 0.43. Table 4 summarizes the results for the two membrane tubes. Figure 11 shows a photomicrograph of the product produced using the 0.5 mm membrane (D2 in Figure 10). Semi-continuous Production Approach A continuous run to produce 12 kg of product based on the formulation listed in Table 3 was completed successfully using the procedures described previously. A 7-round channel ceramic membrane of 0.5 mm nominal pore size was used. The transmembrane pressure was 1.4 ´ 105 Pa. The average cross¯ ow velocity was maintained at 1.0 m s± 1 (tube Reynolds number of approximately 750). Figure 12 summarizes the results of a typical production batch. The ® gure plots the emulsion concentration in the continuous phase tank (a), the discontinuous phase ¯ ux rate (b), the average droplet size (dn5 0 ), the span and the uniformity of the droplet size distribution (c) and the percentage of particles in a selected apparent size range as a function of time (d). The results shown in (c) and (d) were Table 4. Summary of the effect of pore size on the droplet size. membrane 1 Average pore size (mm) Average droplet size (mm) Ratio of droplet size over pore size (-) Span (-) Uniformity (-)
0.2 0.56 2.80 0.82 0.43
membrane 2 0.5 1.41 2.82 0.82 0.43
Figure 10. Effect of pore size on the droplet size distribution.
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CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II obtained from ex situ forward light scattering analysis and in situ FBRM, respectively. In Figure 12 (a), once the emulsion concentration reached 25%, 1 kg of product was removed and 0.75 kg of continuous phase was added (hence bulk oil hold-up fell to 20%). This procedure was repeated seven times at approximately 45 minutes intervals. In Figure 12 (b), it is seen that the measured discontinuous phase ¯ ux rate was nearly constant throughout the process, which is expected because the transmembrane pressure was kept constant. In Figure 12 (c), the results from the Malvern MasterSizer show the average droplet size, the span and the uniformity of the droplet size distribution as a function of time. It is clear that the average droplet size (d 5 0 1.916 0.02 mm), the span (0.836 0.01) and the uniformity (0.266 0.00) are remarkably constant throughout the process. This demonstrates that the droplet size and size distribution are not affected by the semi-continuous operation. Figure 12 (d) presents the number fraction of the particles in the selected size range of <3.3 mm. Corresponding to Figure 12 (a), the fraction does vary to some degree due to the bleeding of product and the addition of continuous phase but the general trend does not change and keeps at a constant level. It may be postulated that the ¯ uctuations of the trend of the number fraction would disappear if the process is carried out as in a continuous operation. Nevertheless these results and subsequent trials
Figure 12. Semi-continuous production of cosmetic emulsion at room temperature.
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demonstrate that FBRM could be used as a means of control during the XME operations. SCALE-UP AND PROCESS CONSIDERATIONS One of the major potential advantages of the XME is the scale-up capability for large scale industrial operations. It is evident that the process can be scaled-up freely by adding more membrane tubes into the module. The membrane tubes are usually installed in parallel, however, if the membrane area required is very large, they could be installed in series. This could reduce the capacity requirement of the circulation pump. The production rate (or discontinuous phase ¯ ux rate) depends upon many factors: e.g. the droplet size of the product designed, the appropriate membrane porosity, pore size, the viscosity of the discontinuous phase, etc. Experiments have shown that using a ceramic membrane, an oil ¯ ux rate of about 20 m ± 2 h ± 1 for a double-coated layers of 0.2 mm nominal pore size can be achieved using a transmembrane pressure of 1.4´105 Pa. Figure 13 shows a pilot-scale XME plant under trial evaluation, which has been used to con® rm these ® ndings. In this contribution the emulsi® cation performance at room temperature has been discussed. The process can also be operated at elevated temperatures and can use different types of (coarser) membrane, depending on the required speci® cation of the product1 0 . The use of two or more different membrane tubes allows the manufacture of multiple emulsions. Depending upon the nature of the emulsion product, the rate of XME manufacture can also be enhanced, sometimes by an order of magnitude, by premixing the phases and forcing this crudely mixed product through an in-line membrane or membrane tube. In this case the mechanism of formation is to use the membrane to further break the pre-formed emulsion. The lower shear production environment may also afford some bene® ts in cosmetic and food applications since the orginal structure of the continuous phase tends to be
Figure 13 Front view of a laboratory-scale pilot plant for XME process.
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WILLIAMS et al. NOMENCLATURE a c d dp Pc R s x e w c
size of a unit cube ratio of distance between two pores over the pore diameter diameter of particle diameter of pore minimum transmembrane pressure rotor radius gap size variable porosity oil volume fraction interfacial tension
REFERENCES Figure 14. Comparison of relative energy consumption for different emulsi® cation processes. HPH: high-pressure homogenizer; RS: rotorstator; XME: cross¯ ow membrane emulsi® cation. (Assuming viscosity of the continuous and the dispersed phase 100 mPa s; density of the continuous phase and the dispersed phase 1000 kg m ±3; interfacial ±1 tension 10 mN m ; w oil volume fraction; R/s ratio of rotor radius to gap size.)
better preserved. A survey of the approximate energy requirements for the different types of continuous emulsi® cation processes is summarized in Figure 14, based on data derived from Schubert1 1 . It is evident that XME has a signi® cantly lower energy demand, and this may be an additional obvious bene® t.
CONCLUSIONS The experiments reported here have demonstrated the following characteristics of the cross¯ ow membrane method: · Droplet size is controllable and size distribution can be very narrow through judicious choice of process conditions, such as cross¯ ow velocity, transmembrane pressure, pore size, etc. · Manufacturing process is repeatable and hence products can be made reproducibly; · Process lends itself to reliable operation and is independent of scale-up (by adding more membrane modules); · Process can be operated in batch or continuous mode; · The continuous phase structure may be conserved by virtue of the lower shear process conditions compared with conventionally-used methods; Practical consideration of process ¯ exibility, guaranteed product quality9 and energy utilization would appear favourable for such processes. Further experiments are in progress to investigate the industrial scale production of emulsions.
1. Peng, S. J. and Williams, R. A., 1998, Controlled production of emulsion using a cross¯ ow membrane: Part IÐ Droplet formation from a single pore, Trans IChemE, 76(A8): 894±901. 2. Morley, N. C. and Wheeler, D. A., 1991, private communication. 3. Kandori, K., Kishi, K. and Ishikawa, T., 1991, Preparation of monodispersed W/O emulsions by Shirasu-porous-glass ® lter emulsi® cation technique, Colloids and Surfaces, 55: 73±78. 4. Morley, N. C., Williams, R. A. and Wheeler D. A. 1995,1996,1997, Dispersions of immiscible phases, UK Patent 9606738.4, International Patent Application No. PCT/GB97/00910. 5. Omi, S., 1996, Preparation of monodisperse microspheres using the Shirasu porous glass emulsi® cation technique, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 109: 97±107. 6. Katoh, R., Asano, Y., Furuya, A., Sotoyama, K. and Tomita, M, 1996, Preparation of food emulsions using a membrane emulsi® cation system, J Membrane Sci, 113: 131±135. 7. Gelinas, C. and Angers, R., 1986, Improvement of the dynamic waterexpulsion method for pore size distribution measurement, Am Ceram Soc Bull, 65: 1297±1300. 8. Peng, S. J. and Williams, R. A., 1994, Direct measurement of ¯ oc breakage in ¯ owing suspensions, J Colloid Interface Sci, 116: 321±332. 9. Malvern MasterSizer Manual, Version 2.15, 1992±1994. 10. Williams, R. A., Peng, S. J., Wheeler, D. A., Morley, N. C., Taylor, D., Whalley, M. and Houldsworth, D.W., 1998, Controlled production of emulsions using XME, World Congress on Particle Technology 3, Brighton, UK, July 1998, published on CD-ROM, Paper No 374 (IChemE, UK). 11. Schubert, H., 1997, Advances in the mechanical production of food emulsions, Paper presented in the Int Congress on Food Engineering, Brighton UK, 13± 18 April 1997.
ACKNOWLEDGEMENT The authors acknowledge with thanks the support of this research programme given by Engineering and Physical Research Council (GR/J 41987) and industrial partners including Memtech (UK) Ltd, Fairey Industrial Ceramics Ltd, Open DAW Consultants and Disperse Technologies Ltd.
ADDRESS Correspondence concerning this paper should be addressed to Professor R. A. Williams, Particle and Colloid Engineering Group, Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE, UK. The manuscript was received 12 June 1997 and accepted for publication after revision 3 February 1998.
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CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE Part II: Industrial Scale Manufacture R. A. WILLIAMS (FELLOW), S. J. PENG, D. A. WHEELER*, N. C. MORLEY*, D. TAYLOR**, M. WHALLEY * and D. W. HOULDSWORTH² Particle and Colloid Engineering Group, Camborne School of Mines, University of Exeter, Redruth, Cornwall, UK *Disperse Technologies Ltd, Dorking, Surrey, UK **Fairey Industrial Ceramics Ltd, Stone, Staffordshire, UK ² Memtech (UK) Ltd, Swansea, UK
T
his paper presents the results of an investigation into the practical application of an innovative emulsion manufacturing methodÐ cross¯ ow membrane emulsi® cation on a pilot-plant scale. This method produces emulsions by breaking up the discontinuous phase in a controlled mannerÐ ¯ owing from one side to another of a well de® ned porous membrane. The droplets formed on the surface of the other side of the membrane are scoured away by the cross¯ ow of the continuous phase. In this way the emulsion can be manufactured under lower shear conditions than with conventional emulsi® cation techniques and the droplet size and size distribution can be controlled through judicious choice of the process ¯ ow conditions and the pore size and size distribution of the membrane. Experiments have been carried out to demonstrate the effects of the process parameters and membrane structure on emulsion droplet size. Batch and semi-continuous manufacture of oil/water dispersions is demonstrated. Keywords: emulsi® cation; energy utilization; cross¯ ow membrane; on-line size measurement
INTRODUCTION
can be obtained. The droplet size distribution is often wide and this, in turn affects the emulsion characteristics and stability. Secondly, the energy utilization for large scale production of emulsions using traditional (rotor-stator) methods is very poor and gets worse as vessel size increases. This adds signi® cantly to the cost of manufacture. Thirdly, reproducibility on a single piece of equipment is often poor and the quality of the product can vary from one manufacturing vessel design to another even on the same manufacture scale. Scale-up is therefore a common and dif® cult problem: the transformation from laboratory to small scale to full scale manufacture is dif® cult. This can lead to in¯ exibility of manufacturing apparatus and consequent additional cost arising from the under-utilization of expensive equipment. The aim of this work is to investigate the application of cross¯ ow membrane emulsi® cation (XME), at a pilot scale, with a view to addressing the above limitations of conventional emulsi® cation methods. The principles of XME have been described in part I of the paper1 and are ampli® ed elsewhere2 ± 6 . However, important practical issues need to be addressed before the method can be widely employed to manufacture industrial emulsions. For example, how to select the correct membrane pore size and porosity and to set process conditions for a given product speci® cation (size and size distribution).
Emulsion manufacturing is a very important process in the food, chemical, mineral processing, cosmetics and pharmaceutical industries. Increasingly there is a need to produce emulsions in which the droplets have a de® ned particle size distribution (e.g. bimodal) or a very narrow size distribution. Conventional methods for manufacturing emulsions are based on: · rotor-stator systems such as tooth-disc high speed homogenizers, or colloid mills in which a high shear is generated between a rotor and a stationary smooth, roughened or grooved surface. Here turbulence is the primary cause of ¯ uid disruption leading to the formation of droplets · high pressure homogenizer systems in which the emulsion mixture is passed through a narrow ori® ce, or inject dispersion in which two jets of different components are made to collide head-on. These processes may be assisted with use of power, ultrasound or electrical ® elds. Pressures in the range 5.0 ´ 106 ±3.5 ´ 107 Pa are common. Here ¯ uid separation is caused by turbulence and cavitation effects and high rates of energy dissipation occur. However it is well known that a number of problems may be associated with these existing methods of production: First, sometimes the droplet size and size distribution cannot easily be controlled. Even if the mean droplet size 902
CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II
DROPLET FORMATION THROUGH A MEMBRANE TUBE Droplet formation from a membrane tube is believed to be similar to that from a single pore if the membrane tube can be considered to be composed of a large number of individual capillary pores (Figure 1). In previous work1 it has been reported that for given conditions the droplet size is proportional to the diameter of the pore from which it emerges. Usually, the droplet diameter is between 2 and 8 times the pore diameter. It is therefore important that the pore size distribution is narrow in order to produce a product that exhibits a narrow droplet size distribution. As the hold-up (or concentration) of the discontinuous phase in the emulsion product increases, the properties of the bulk emulsion phase changes. For example, the viscosity increases and the hydrodynamic conditions near the membrane surface may be substantially in¯ uenced, thus changing the shear force and therefore the size of the detached droplet. The following discussion will focus on certain important conditions including the porosity of membrane surface, transmembrane pressure and pore size. Effect of Porosity The porosity of the membrane surface can be important for a successful XME process because it determines the distance between two adjacent pores. This distance is critical to ensure that two adjacent droplets do not come suf® ciently close to allow contact with each other, which may lead to coalescence. Therefore, the pores are preferred to be uniformly located on the surface to ensure maximum distance between any two adjacent pores for a given porosity. To study the relationship between the porosity and the distance between two adjacent pores, an ideal membrane is assumed for which the pore size is uniform, the pores are straight cylinders and are located on the surface uniformly. The ideal membrane can be represented by a cubic unit (Figure 2) of size a, each unit containing one cylinder of diameter dp (dp # a). The porosity of the cubic unit is then calculated as: Volume of the pore space in the unit e Volume of unit p4 2 dp a p dp 2 p 1 1 a3 4 a 4 c2
Figure 1. Schematic diagram showing droplet formation from a membrane tube.
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where c is de® ned as the maximum possible droplet size ratio: a c 2 dp Here the size a can be considered the average distance between the centres of two adjacent pores for a given porosity. For example, if the droplet size ratio produced by the membrane is greater than c, then it is likely that two adjacent droplets will contact each other. Therefore, to ensure that no coalescence can occur, the c value of the membrane must be greater than the droplet size ratio of the ® nal product desired. Figure 3 illustrates the relationship between the porosity of the membrane and the critical droplet size ratio c for the above ideal membrane. Equation (1) is valid for an ideal membrane. In reality, however, the effect of non-ideal pore openings and the size of the pore passage implies that the porosity of a membrane would need to be smaller than the value given in equation (1). This is because the diameter of the pore opening determines the droplet size. For instance, the porosity according to equation (1) is 0.049 for a droplet size ratio 4. If the pore passage is not cylindrical and has a small neck inside, then the porosity will be smaller than 0.049. A further constraint may be imposed by the manufacturing process, since if the porosity is too low the membrane surface area required to achieve a realistic throughput may be excessive. Effect of Pressure Previously it has been demonstrated that there is an optimum range of applied transmembrane pressures ($ Pc ) for successful emulsi® cation. If the pressure is too low (
Figure 2. Two representative cubic units of an ideal membrane.
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WILLIAMS et al. Based on the results of droplet formation from a single pore, it is possible to predict the droplet size distribution for a given pore size distribution under particular conditions during the droplet formation from a membrane tube. Figure 5 demonstrates the droplet size distributions (D1, D2) calculated by using two membrane tubes having pore size distributions (P1, P2 respectively): It is evident from the above result that a small number proportion (1%) of `large’ (2 mm) pores in D2 yields a signi® cant proportion of large droplets in the ® nal product. This serves to demonstrate that the quality of membrane is of paramount importance to the success of the membrane emulsi® cation process.
Figure 3. The maximum porosity for the membrane surface vs. the droplet size (ratio) requirement of the product.
where c is the o/w interfacial tension and dp is the capillary pore diameter. For a membrane tube, the critical pressure of the membrane could lie in a range (if the pore size is not uniform). Figure 4 illustrates the minimum transmembrane pressure as a function of the pore size for two interfacial tensions. There is no reliable theoretical approach to estimate the useable maximum transmembrane pressure. It is believed that the maximum transmembrane pressure would be affected by the membrane itself (e.g. pore structure) and the process conditions (e.g. cross¯ ow velocity). Generally, the highest transmembrane pressure should be applied in order to achieve the highest emulsi® cation production rate. An experimental approach must be adopted for optimising individual process conditions. It is believed that pressures 2±10 times higher than the minimum transmembrane pressure are practical. Effect of Pore Size
EXPERIMENTAL Preparation of Membranes Experiments were conducted using several types of membrane composed, for example, from ceramic, stainless steel and polymeric (track-etched) materials. The discussion here is focused upon the development and use of a high speci® cation ceramic membrane. The ceramic membrane element is a tubular sintered alumina substrate with one or more alumina coatings on the internal surface. Both the substrate and the membrane coating have a tightly-controlled pore size distribution, the average pore size of the substrate being 5±10 times that of the membrane coating. The substrate provides the mechanical strength necessary to support the ® ner surface membrane, and can be varied to suit the requirements of the particular process. The substrate itself was composed of a blend of fused aluminas and a ® ne-grained calcined alumina. These were combined with extrusion aids and water to produce a body of suitable consistency. The body was extruded via a ram extruder through a die to give single- or multi-channel tubes. After drying, the tubes were ® red to a high temperature and
In the discussion above, an ideal membrane that has uniform pore size has been assumed. In practice, however, the membrane contains pores in a narrow size range or perhaps possesses a few extremely large pores. The consequence of the existence of few large pores is that the XME process may generate droplets having a bimodal distribution.
Figure 4. The minimum transmembrane pressure as a function of pore size for two interfacial tensions.
Figure 5. Predicted product droplet size distributions (D1, D2) as a function of pore size distributions (P1, P2) respectively. (Membrane: length 600 mm; inside diameter 5 mm; porosity 0.1; pore tortuosity 1.0. process parameters: cross¯ ow velocity 1.0 m s ±1; transmembrane pressure 1.0 ´ 105 Pa; o/w interfacial tension 6 mN m ±1; viscosity of continuous phase 1.0 cP and viscosity of product 10.0 cP.)
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CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II
Figure 6. Surface pore size distribution of a typical ceramic emulsi® cation membrane tube.
clipped to the required length. All substrate tubes were proof tested by measuring their maximum pore size and pore size distribution via bubble point test equipment (described below). The internal surfaces of the substrate tubes were then coated with a thin layer of a calcined alumina, or a blend thereof, by controlled immersion in an aqueous suspension. After drying the coating was ® red. The process was repeated if multiple coatings were required. The ends of the tubes were then glazed to seal the exposed substrate at the ends. The membranes were again proof tested by measuring their maximum pore size via bubble point test equipment. The pore size distribution of the membrane layer was adapted from two measurement techniques. The basic measurement of pore size is based on the bubble point technique of British Standard 1752:1983. The technique for measuring the pore size distribution, using the bubble point technique was adapted from the method of Gelinas and Angers7 . For the bubble point technique, the porous ceramic is saturated with a liquid (water, isopropanol or other liquid depending on the pore size of the ceramic). One side of the liquid-saturated sample is subjected to an increasing pressure whilst the other side is submerged under a known depth of the liquid medium. The pressure at which the ® rst air bubbles emerge from the ceramic is noted. The maximum pore size can then be calculated using established equations. Some explanation is needed of the term `maximum pore size’ . Assuming that the pores are isolated
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capillaries passing through the ceramic from the air side to the liquid side (as in the case of the test) and where each capillary has discrete variations in diameter along its length. The established expression (see BS 1752:1983) for pore diameter gives the smallest diameter along the length of the capillary, since this diameter determines the pressure at which the liquid can be forced from the capillary. The `maximum pore size’ in this context is the largest of the small diameters of each capillary. If a coarse material is coated with a ® ne pore material the pore size determined by the bubble point method will relate to the ® ner pores of the coating and ignores the pores of the substrate. Therefore this method is ideally suited to the speci® c determination of the pore size distribution of the coating layer of the ceramic cross¯ ow membranes. For the bubble point technique applied to pore distribution analysis of whole membranes, the membrane is saturated with a medium and inserted in a jig which holds it a ® xed distance below the surface of the liquid. The ends are enclosed in caps that seal the internal channels of the membrane from external liquid but allow the channels to be pressurized. The air pressure in the channels is increased in stages until the ® rst bubbles are observed. Above this pressure, the air pressure is increased in steps. At each pressure increments, the air is allowed to pass through the pores that are open at that pressure until a constant `input’ pressure is achieved. The air supply is then shut off and the air pressure in the channels falls to a constant `residual’ pressure (effectively when air bubbles cease to ¯ ow the pores). The input and residual pressures are recorded for each pressure step. The pressure is increased until the liquid has been driven from all the pores. The pressure is then reduced in ten steps to ambient pressure, recording the input and residual pressure at each step. The pore size distribution is calculated from the above data because the difference between the input pressure and the residual pressure at each step is proportional to the volume of the pores whose size correspond to each input pressure. Figure 6 shows a typical surface pore size distribution on a multiple-coated membrane having a surface pore size of 0.2 mm. It is evident that multiple coatings can be used to reduce the pore size and sharpen the size distribution signi® cantly, thus creating a high performance membrane. Figure 7 (a) shows a SEM photograph of a cross-section through a ceramic membrane of 0.2 mm nominal pore size. It is clear that the surface was ® rst coated by 0.5 mm particles and then by 0.2 mm particles.
Figure 7. (a) SEM micrograph of cross-section through view of ceramic membrane tube with multiple coatings. (b) SEM photograph of membrane surface.
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Table 1. Details of membrane tubes used for emulsi® cation process. Single Channel Length (mm) Inside diameter of each channel (mm) Tube outside diameter (mm) Porosity of coating layer (Ð ) Pore size of coating layer (mm) Porosity of supporting layer (Ð ) Pore size of supporting layer (mm)
600 5 10 <0.1 0.2±0.5 0.36 2±3
7-Round Channel 600 3.5 20 <0.1 0.2±0.5 0.36 2±3
Figure 7 (b) shows a SEM photograph of the coated membrane surface. It is evident that the surface is not smooth on a micrometer scale, but this can be varied and controlled to some degree by varying the manufacturing process. Membranes having properties illustrated in Figure 6 can be made routinely using the procedures described above. Alternative properties can be obtained by varying the substrate or coating materials. Results from two typical ceramic membrane tubes will be presented here, which consist of a single tube element and a multiple-channel element which contains seven identical tubes. The details are summarized in Table 1.
(90±500 l/hr), membrane module, pressure transmitters before and after the membrane module, heat exchanger and a stirrer. Two sampling valves are installed after the storage tank and before returning back to the tank. The discontinuous phase is held in a storage tank (5 l) ® tted with heating tapes, temperature indicator, one way valve for air inlet, safety valve and air pressure regulator. The discontinuous phase tank is capable of pressurising to 7.0 ´ 105 Pa. The ® nal product loop contains a heat exchanger, temperature indicator, a jacketed cooling receiver tank and a stirrer. A control system is used which determines the power source for the above three major systems and speed control of the lobe pump plus 4±20 mA signals from the ¯ owmeter, pressure transmitters and the temperature transmitters. In addition, if required, a droplet size detector (described below) in the continuous phase tank sends information to the computer and the process can be adjusted automatically. The computer and the interface software are essential parts of the apparatus if automatic ¯ ow control and monitoring of critical process parameters are required. For simple batch operation the process can also be run manually without the computer. Emulsion Characterization
Emulsi® cation Apparatus Figure 8 shows the major components of the XME apparatus, as described below. The continuous phase system contains the continuous phase storage tank (10 l) ® tted with heating tapes and temperature indicator, a three lobe rotary pump, magnetic ¯ ow meter
In-process measurements of the droplet number and size were made in the continuous phase tank (Figure 8) using a focused beam re¯ ectance measurement instrument (FBRM, Lasentec, USA) which is based on scanning a highly focused laser beam (that emanates from a dip-in probe) over the droplets pass the probe tip, as described in detail elsewhere 8 . When the laser beam intercepts the surface of a
Figure 8. (a) Schematic ¯ ow sheet of the XME apparatus and (b) details of the membrane module.
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droplet, part of the light is scattered backwards and detected by a photo diode. Because the laser beam scans particles in a random manner the width (not the intensity) of the signal represents the chord length (not necessarily the true diameter) of that droplet. Due to the high speed scanning, the chord length distribution and the number of droplets particles scanned in a particular size range of the dispersion can be calculated statistically from the signals in a very short time (over 1000 particles can be counted in a second). It is convenient to use the FBRM to monitor the dispersions for the chord length distribution and the number fraction of the particles in a selected size range. This was used to detect any gross variations of particle size in the emulsion production process with time. Ex situ analyses of emulsions were made using the standard method of forward light scattering9 using a Malvern MasterSizer. Samples were removed from the process, diluted and presented for careful analysis using the standard circulating ¯ ow cell. The results give the particle volume distribution of the dispersion. Two expressions which will be used to characterize the droplet size distribution are span and uniformity of the distributions: d x, 0.9 d x, 0.1 Span 4 d x, 0.5 Xi d x, 0.5 di Uniformity 5 d x, 0.5 Xi
907
The formulation of two o/w emulsions at room temperature will be discussed here, as shown in Tables 2 and 3. These are model systems for room temperature emulsions pertaining to a cosmetics application. Tests were performed in batch and semi-continuous modes. For a batch operation, the whole system (including the continuous phase, the connection line between discontinuous phase tank and the membrane module, the membrane module itself, the circulation loop and the continuous phase
tank) was thoroughly cleaned before the membrane module was dismantled from the circulation loop. The membrane surface was wetted to render it hydrophilic for an o/w emulsion (w/o emulsions need a hydrophobic surface) before it was mounted in the membrane module and re® tted back into the circulation loop. The continuous phase tank and circulation loop were further washed with de-ionized water before the appropriate amount of continuous phase was poured into the continuous phase tank and the circulation pump motor started at a minimum speed. The appropriate amount of discontinuous phase was poured into the discontinuous phase tank and the air inside the membrane chamber and connection line was gently purged. With the valve beneath the discontinuous phase tank (V23) shut, the discontinuous phase tank was pressurized to the desired transmembrane pressure and the circulation pump was adjusted to achieve the desired cross¯ ow rate and controlled by the PC afterwards. The FBRM was then started to monitor the number counts of the particles in selected size ranges and the droplet size every 1 minute continuously and the emulsi® cation process was started by opening the valve beneath the discontinuous phase tank (V23). Samples of the emulsion were regularly removed through the sampling valves and analysed. When the emulsion concentration had reached the desired hold-up (which could be calculated from the amount of discontinuous phase injected into the continuous phase), the process was then stopped. The system was drained and cleaned using pressurized hot water and cleaning agents. For a semi-continuous operation, similar procedures to the batch process were performed, however, just before the emulsion concentration had reached the desired level (25 wt %), instead of stopping the process, the emulsi® cation was continued but a ® xed volume of product was bled out and appropriate amount of continuous phase was added to the continuous phase tank to dilute the emulsion concentration (20 wt %). Once the emulsion concentration had again reached the desired product level (25 wt %), these procedures were repeated by bleeding and adding appropriate amounts of product and continuous phase. In this case, the discontinuous phase tank was large enough to hold the appropriate amount of discontinuous phase for the whole process. The droplet size distribution was sampled and measured periodically using the Malvern MasterSizer. During the whole process, the FBRM was used to monitor the number and size of the droplets in a given size interval continuously. For a full-scale continuous operation, the procedures described in the semi-continuous operation would be replaced by two well-controlled pumps. One of them would be used to bleed the product out and the other to add the continuous phase into the continuous phase tank. If required, the number and size of the droplets could
Table 2. Emulsion formulation 1.
Table 3. Emulsion formulation 2.
The x is replaced by any of the letters n, s, l, n that de® ne the distribution type of volume, surface, length or number. The span gives a description of the width of the distribution which is independent of the median size. Here d(x,0.5) is the median size of the distribution and di and Xi are respectively the mean diameter of, and result in, size class i. The uniformity is independent of the median size and is a measure of the absolute deviations from the median. Samples of ® nal product were removed and analysed using re¯ ected and transmitted light microscopy and also for cryogenic scanning electron microscopy. Experimental Procedures
Materials Continuous phase Discontinuous phase
Concentration w/w%
Deionized water Triethanolamine Sodium Nipastat
66.7 3.0 0.3
Mineral oil Isostearic Acid
27.0 3.0
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Materials Continuous phase Discontinuous phase
Deionized water Sorbitol (70%) Dobanol 91-8 Formalin Mineral oil (Marcol 172)
Concentration w/w% 36.40 36.40 2.16 0.04 25.00
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WILLIAMS et al.
Figure 9. Effect of increasing cross¯ ow velocity on the droplet size distribution (A, B, C, D) produced from a membrane having the pore size distribution (P).
Figure 11. Photomicrograph of the emulsion droplet obtained by XME (Product D2, Figure 10).
Effect of Pore Size be monitored directly in situ by FBRM as for the semi-continuous mode of operation described above.
RESULTS AND DISCUSSIONS Effect of Cross¯ ow Velocity As reported elsewhere1 the droplet size is proportional to the pore size but it can be tuned by the magnitude of the cross¯ ow, particularly when the droplet sizes are large (>1 mm). Experiments using emulsion formulation 1 (Table 2) were conducted for four different cross¯ ow velocities using a single channel membrane tube. Figure 9 shows the droplet size distribution (labelled A, B, C, D) of the product as a function of cross¯ ow velocity and pore size distribution (P). Four cross¯ ow velocities were chosen: 1.12, 2.49, 4.34 and 5.09 m s ± 1 , respectively, which correspond to tube Reynolds numbers given in the ® gure. Figure 9 clearly demonstrates that the droplet size decreases (A, B, C, D) as the cross¯ ow velocity increases. As the droplet size becomes smaller the effect of the cross¯ ow velocity may become less signi® cant depending upon the hydrodynamic conditions near the surface of membrane, the droplet size, the sublayer thickness and stability.
To further exploit the results of the effect of pore size on the droplet size during the droplet formation from a single pore, experiments were conducted using two 7-round channel membrane tubes of nominal pore sizes of 0.2 and 0.5 mm (Table 1). The cross¯ ow velocity for both tests were maintained at 0.6 m/s. The transmembrane pressure was 2.8´105 Pa. Figure 10 shows the droplet size distributions (D1, D2) as a function of pore size distribution (P1, P2). The average droplet size of the two distributions (D1, D2) were 0.56 mm and 1.41 mm. The ratio of droplet size to the pore diameter is 2.80 and 2.82, respectively. The corresponding span and uniformity were 0.83, 0.83 and 0.43, 0.43. Table 4 summarizes the results for the two membrane tubes. Figure 11 shows a photomicrograph of the product produced using the 0.5 mm membrane (D2 in Figure 10). Semi-continuous Production Approach A continuous run to produce 12 kg of product based on the formulation listed in Table 3 was completed successfully using the procedures described previously. A 7-round channel ceramic membrane of 0.5 mm nominal pore size was used. The transmembrane pressure was 1.4 ´ 105 Pa. The average cross¯ ow velocity was maintained at 1.0 m s± 1 (tube Reynolds number of approximately 750). Figure 12 summarizes the results of a typical production batch. The ® gure plots the emulsion concentration in the continuous phase tank (a), the discontinuous phase ¯ ux rate (b), the average droplet size (dn5 0 ), the span and the uniformity of the droplet size distribution (c) and the percentage of particles in a selected apparent size range as a function of time (d). The results shown in (c) and (d) were Table 4. Summary of the effect of pore size on the droplet size. membrane 1 Average pore size (mm) Average droplet size (mm) Ratio of droplet size over pore size (-) Span (-) Uniformity (-)
0.2 0.56 2.80 0.82 0.43
membrane 2 0.5 1.41 2.82 0.82 0.43
Figure 10. Effect of pore size on the droplet size distribution.
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CONTROLLED PRODUCTION OF EMULSIONS USING A CROSSFLOW MEMBRANE: PART II obtained from ex situ forward light scattering analysis and in situ FBRM, respectively. In Figure 12 (a), once the emulsion concentration reached 25%, 1 kg of product was removed and 0.75 kg of continuous phase was added (hence bulk oil hold-up fell to 20%). This procedure was repeated seven times at approximately 45 minutes intervals. In Figure 12 (b), it is seen that the measured discontinuous phase ¯ ux rate was nearly constant throughout the process, which is expected because the transmembrane pressure was kept constant. In Figure 12 (c), the results from the Malvern MasterSizer show the average droplet size, the span and the uniformity of the droplet size distribution as a function of time. It is clear that the average droplet size (d 5 0 1.916 0.02 mm), the span (0.836 0.01) and the uniformity (0.266 0.00) are remarkably constant throughout the process. This demonstrates that the droplet size and size distribution are not affected by the semi-continuous operation. Figure 12 (d) presents the number fraction of the particles in the selected size range of <3.3 mm. Corresponding to Figure 12 (a), the fraction does vary to some degree due to the bleeding of product and the addition of continuous phase but the general trend does not change and keeps at a constant level. It may be postulated that the ¯ uctuations of the trend of the number fraction would disappear if the process is carried out as in a continuous operation. Nevertheless these results and subsequent trials
Figure 12. Semi-continuous production of cosmetic emulsion at room temperature.
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demonstrate that FBRM could be used as a means of control during the XME operations. SCALE-UP AND PROCESS CONSIDERATIONS One of the major potential advantages of the XME is the scale-up capability for large scale industrial operations. It is evident that the process can be scaled-up freely by adding more membrane tubes into the module. The membrane tubes are usually installed in parallel, however, if the membrane area required is very large, they could be installed in series. This could reduce the capacity requirement of the circulation pump. The production rate (or discontinuous phase ¯ ux rate) depends upon many factors: e.g. the droplet size of the product designed, the appropriate membrane porosity, pore size, the viscosity of the discontinuous phase, etc. Experiments have shown that using a ceramic membrane, an oil ¯ ux rate of about 20 m ± 2 h ± 1 for a double-coated layers of 0.2 mm nominal pore size can be achieved using a transmembrane pressure of 1.4´105 Pa. Figure 13 shows a pilot-scale XME plant under trial evaluation, which has been used to con® rm these ® ndings. In this contribution the emulsi® cation performance at room temperature has been discussed. The process can also be operated at elevated temperatures and can use different types of (coarser) membrane, depending on the required speci® cation of the product1 0 . The use of two or more different membrane tubes allows the manufacture of multiple emulsions. Depending upon the nature of the emulsion product, the rate of XME manufacture can also be enhanced, sometimes by an order of magnitude, by premixing the phases and forcing this crudely mixed product through an in-line membrane or membrane tube. In this case the mechanism of formation is to use the membrane to further break the pre-formed emulsion. The lower shear production environment may also afford some bene® ts in cosmetic and food applications since the orginal structure of the continuous phase tends to be
Figure 13 Front view of a laboratory-scale pilot plant for XME process.
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WILLIAMS et al. NOMENCLATURE a c d dp Pc R s x e w c
size of a unit cube ratio of distance between two pores over the pore diameter diameter of particle diameter of pore minimum transmembrane pressure rotor radius gap size variable porosity oil volume fraction interfacial tension
REFERENCES Figure 14. Comparison of relative energy consumption for different emulsi® cation processes. HPH: high-pressure homogenizer; RS: rotorstator; XME: cross¯ ow membrane emulsi® cation. (Assuming viscosity of the continuous and the dispersed phase 100 mPa s; density of the continuous phase and the dispersed phase 1000 kg m ±3; interfacial ±1 tension 10 mN m ; w oil volume fraction; R/s ratio of rotor radius to gap size.)
better preserved. A survey of the approximate energy requirements for the different types of continuous emulsi® cation processes is summarized in Figure 14, based on data derived from Schubert1 1 . It is evident that XME has a signi® cantly lower energy demand, and this may be an additional obvious bene® t.
CONCLUSIONS The experiments reported here have demonstrated the following characteristics of the cross¯ ow membrane method: · Droplet size is controllable and size distribution can be very narrow through judicious choice of process conditions, such as cross¯ ow velocity, transmembrane pressure, pore size, etc. · Manufacturing process is repeatable and hence products can be made reproducibly; · Process lends itself to reliable operation and is independent of scale-up (by adding more membrane modules); · Process can be operated in batch or continuous mode; · The continuous phase structure may be conserved by virtue of the lower shear process conditions compared with conventionally-used methods; Practical consideration of process ¯ exibility, guaranteed product quality9 and energy utilization would appear favourable for such processes. Further experiments are in progress to investigate the industrial scale production of emulsions.
1. Peng, S. J. and Williams, R. A., 1998, Controlled production of emulsion using a cross¯ ow membrane: Part IÐ Droplet formation from a single pore, Trans IChemE, 76(A8): 894±901. 2. Morley, N. C. and Wheeler, D. A., 1991, private communication. 3. Kandori, K., Kishi, K. and Ishikawa, T., 1991, Preparation of monodispersed W/O emulsions by Shirasu-porous-glass ® lter emulsi® cation technique, Colloids and Surfaces, 55: 73±78. 4. Morley, N. C., Williams, R. A. and Wheeler D. A. 1995,1996,1997, Dispersions of immiscible phases, UK Patent 9606738.4, International Patent Application No. PCT/GB97/00910. 5. Omi, S., 1996, Preparation of monodisperse microspheres using the Shirasu porous glass emulsi® cation technique, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 109: 97±107. 6. Katoh, R., Asano, Y., Furuya, A., Sotoyama, K. and Tomita, M, 1996, Preparation of food emulsions using a membrane emulsi® cation system, J Membrane Sci, 113: 131±135. 7. Gelinas, C. and Angers, R., 1986, Improvement of the dynamic waterexpulsion method for pore size distribution measurement, Am Ceram Soc Bull, 65: 1297±1300. 8. Peng, S. J. and Williams, R. A., 1994, Direct measurement of ¯ oc breakage in ¯ owing suspensions, J Colloid Interface Sci, 116: 321±332. 9. Malvern MasterSizer Manual, Version 2.15, 1992±1994. 10. Williams, R. A., Peng, S. J., Wheeler, D. A., Morley, N. C., Taylor, D., Whalley, M. and Houldsworth, D.W., 1998, Controlled production of emulsions using XME, World Congress on Particle Technology 3, Brighton, UK, July 1998, published on CD-ROM, Paper No 374 (IChemE, UK). 11. Schubert, H., 1997, Advances in the mechanical production of food emulsions, Paper presented in the Int Congress on Food Engineering, Brighton UK, 13± 18 April 1997.
ACKNOWLEDGEMENT The authors acknowledge with thanks the support of this research programme given by Engineering and Physical Research Council (GR/J 41987) and industrial partners including Memtech (UK) Ltd, Fairey Industrial Ceramics Ltd, Open DAW Consultants and Disperse Technologies Ltd.
ADDRESS Correspondence concerning this paper should be addressed to Professor R. A. Williams, Particle and Colloid Engineering Group, Camborne School of Mines, University of Exeter, Redruth, Cornwall TR15 3SE, UK. The manuscript was received 12 June 1997 and accepted for publication after revision 3 February 1998.
Trans IChemE, Vol 76, Part A, November 1998