- Email: [email protected]
Manufacture of controlled emulsions and particulates using membrane emulsification
Desalination 224 (2008) 215–220
Manufacture of controlled emulsions and particulates using membrane emulsification Qingchun Yuana*, Ruozhou Houa, Nita Aryantia, Richard A. Williamsa, Simon Biggsa, Simon Lawsonb, Helen Silgramc, Manish Sarkarc, Richard Birchd a
Institute of Particle Science and Engineering, bParticlesCIC, University of Leeds, Houldworth Building, Leeds, LS2 9JT, UK Tel. +44 (113) 343-7811; Fax: +44 (113) 343-2781; email: [email protected] c ICI Paints, Wexham Road, Slough Berkshire SL2 5DS, UK d Quest International, Ashford, Kent TN24 0LT, UK Received 28 November 2006; Accepted 7 February 2007
Abstract Crossflow and rotating membrane emulsification techniques were used for making oil-in-water (O/W) emulsions. The emulsions produced from a variety of oils and monomers (viscosity 7–528 mPas) exhibited narrow size distributions over a wide droplet size range, with the average droplet size ranging from less than 1 µm up to 500 µm. The monomer emulsions were further encapsulated to produce microcapsules through subsequent polymerisation reactions. The monodispersity feature of the primary emulsions was retained after the encapsulation. In comparison with other homogenisation methods, our experimental results demonstrated that the membrane emulsification technique is not only superior in emulsion droplet size controls, but also advantageous in energy efficiency and industrial-scale productions. Keywords: Membrane emulsification; Size control; Microcapsules; Industrialisation
1. Introduction Emulsion manufacturing is an important process in food, pharmaceutical, cosmetics as well as many other chemical industries. Membrane emul*Corresponding author.
sification is a technique which is based on a novel concept of generating droplets “drop-by-drop” to produce emulsions [1]. Using a membrane of well defined pore structure, this technique is capable of manufacturing size controlled, mono-disperse products with high efficiency, low energy con-
Presented at the 11th Aachen Membrane Colloquium, 28–29 March, 2007, Aachen, Germany. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.095
216
Q. Yuan et al. / Desalination 224 (2008) 215–220
sumption and excellent reproducibility. To date, membrane emulsification has been applied to create a variety of particulate products with novel structures and functionalities[2]. In the membrane emulsification, emulsions are typically produced by forcing a disperse phase to permeate through the uniform micropores of a membrane into a continuous phase. Droplets, which are formed at the pore openings on the membrane surface, are dispatched by the relative shear motion between the membrane and continuous phase. Using this concept, different technologies have been developed for commercial production [3–6]. The current work featured the use of crossflow membrane emulsification (XME) and rotating membrane reactor (RMR) techniques for precision production of monomer/ oil in water emulsions. Effects of disperse phase viscosity and operational parameters on process throughput and droplet monodispersity were discussed. Preliminary attempts to encapsulate the monomer droplets obtained were successfully carried out through interfacial polymerisation. (a)
Some exploratory emulsification results on RMR were also introduced. 2. Experimental Fig. 1(a) and (b) illustrate photographic images of the pilot-scale XME and batch RMR apparatus used in the current investigation and schematic operating principles, respectively. The two apparatus differ in the way the droplets detachment is initiated. In the XME, droplet detachment is stimulated by circulating the continuous phase along the membrane surface in cross flow [2], whereas the RMR works on a relatively novel idea in which the droplet detachment is initiated by rotating the membrane itself [4]. This has a potential advantage of eliminating the pump circulation of the continuous phase, thus creates a low shear and well controlled flow environment to enable formulation of delicate and fragile structured particulate products. Table 1 lists some of the key system and process parameters adopted in the current investigation. (b)
Fig. 1. (a) Mechanism of crossflow membrane emulsification and a pilot rig. (b) Mechanism of rotating membrane emulsification and a laboratory-scale reactor.
Q. Yuan et al. / Desalination 224 (2008) 215–220
217
Table 1 Some key system and process features of the pilot-scale XME rig and laboratory-scale RMR system used
Membrane type Pore shape and size Dimensions Disperse phase Continuous phase
XME
RMR
Sintered ceramic Random shaped, 0.2, 1 and 5 µm Tubular, Ø20×600 mm with 7 channels of Ø4 mm Oils/monomers Aqueous solutions of 2 wt% effective surfactants
Laser-drilled stainless steel Close to round, 100 µm Tubular, Ø10×85 mm Mineral oil Aqueous solution of 2 wt% surfactant and 0.1 wt% thickener
3. Results and discussion 3.1. XME Table 2 summarises the experimental conditions and results obtained in the XME investigation. Fig. 2 illustrates both the number and volume based droplet size distributions of the emulsions made in run 1, 2 and 3. The inset images are SEM/optical micrographs of microcapsules produced from the corresponding emulsions. It can be seen that despite considerable differences in the disperse phase viscosity (ηdisperse) and transmembrane pressure (ΔP) applied, relatively monodisperse emulsions, with coefficient of variations (CV) at around 30%, can be steadily produced from different pore size membranes. This indicates that XME is a fairly robust technology capable of manufacturing size controlled emulsions over a range of different process conditions. The relative monodisperse emulsions of monomers have been successfully encapsulated through interfacial polymerisation followed. The inset images show that individual solid microcapsules were produced, and the size characteristics of the original emulsions have largely been retained. In an XME system containing fast absorbing emulsifiers, the emulsion droplet size has been reported to be mainly depending on membrane pore size and crossflow velocity only. This has been the case in our investigation as well. As we
can see from Table 1 in runs 2, 4, and 5, emulsions of similar Sauter diameter D3,2 were produced from the same crossflow velocity and membrane pore size. The disperse phase viscosity and transmembrane pressure, although differing significantly in these runs, had very limited effect on the mean droplet size. For industrial applications, the process throughput has been a concern, especially when small pore membranes are used. The achieved oil fluxes in the current system are in the range of 0.002 to 0.08 m3/m2·h while keeping the CV value below the monodispersity threshold of 35%. The figures agree with the general notion that in a typical XME operation, the maximum disperse phase influx has to be restrained to below 0.1 m3/m2·h in order to enable uniform droplets being formed on the membrane surface in a “size-stable” or “dripping” mode [2]. However, our experiments also revealed that although further increasing the throughput (by using a higher ΔP) might lead to some widening of the droplet size distribution, the thus produced emulsions were still showing notably better qualities than that produced using the conventional rotorstator emulsification technology. Fig. 3(a) and (b) compares the droplet size distributions of a low and high viscosity oil emulsions produced by the two methods respectively. Clearly in both cases the XME products (Table 1, runs 4 and 5) have exhibited significantly narrower size distributions than those produced by homogenisation. The
218
Q. Yuan et al. / Desalination 224 (2008) 215–220
Table 2 Experimental conditions and results of the XME No.
1 2 3 4 5
Experimental conditions
Experimental results
Figures
Membrane pore, µm
ηdisperse, mPas
Vcf, m/s
ΔP, MPa
D3,2, µm
CV, %
Oil flux, ×10!3 m3/m2 h
0.2 1 5 1 1
528 49 18 7 528
1.75 1.75 1.75 1.75 1.75
0.5 0.05 0.015 0.5 0.5
0.75 6.0 45.5 5.7 5.2
27.4 30.5 35.5 45.5 37.8
2.2 2.6 84.3 364 2.4
2 2 2 3a 3b
Fig. 2. Size-distributions of the emulsions made on ceramic membranes of 0.2, 1 and 5 µm using the XME. Inset of SEM/optical micrographs show the microcapsules made from the emulsions of 0.75 and 45.5 µm.
higher the disperse phase viscosity, the more predominant the improvement in the uniformity of the droplet size. It is worth pointing out that the oil flux of run 4 has reached 0.36 m3/m2·h with a membrane of 1 µm at ΔP= 0.5 MPa. This implies that it takes just over 1 min to prepare 1 L of 5 µm emulsion containing 25 v/v% of the low viscosity oil by using an Ø20×600 mm tubular membrane. Such a throughput is well comparable to a
medium-sized laboratory-scale homogeniser device. The output capacity of an industrial homogeniser, on the other hand, typically lies around 1 m3/h [7]. For the production of similar concentrations of O/W emulsions, an XME system would require merely an array of 13 tubular membranes of Ø20×600 mm. Consider that membrane emulsification only requires a tiny fraction of energy consumptions demanded by the conventional homogenisation methods [8], and the
Q. Yuan et al. / Desalination 224 (2008) 215–220
219
Fig. 3. Size distributions of emulsions of a low viscosity oil (7 mPas, a) and highly viscous oil (528 mPas, b) prepared by the XME (see Table 1 for details) and a mini-homogeniser (5 ml capacity, operated at 19,000 rpm for 2 min in the case of low viscosity oil, and 30,000 rpm for 5 min in the case of high viscosity oil).
(a)
(b)
Fig. 4. (a) Size distribution and (b) droplet microscopic image of a low viscosity mineral oil-in-water emulsion produced with RMR at a membrane rotational speed of 1500 rpm.
process throughput can be easily scaled up by simply increasing the membrane area that produces droplets. It shows that membrane emulsification will be a both attractive and promising technology for large-scale applications, not only in terms of superior product qualities, but also in terms of production scales and operation economics.
3.2. RMR Only exploratory results of the RMR are presented here. Fig. 4 illustrates the size distribution and a droplet micrograph of a low viscosity mineral oil in water emulsion produced at a membrane rotational speed of 1500 rpm. The emulsion has a number mean droplet size of 133 µm and CV of 13%.
220
Q. Yuan et al. / Desalination 224 (2008) 215–220
It can be seen that the emulsion droplet size is fairly uniform. Other preliminary experiments have shown that monodisperse mineral oil in water emulsions with average droplet size ranging from 80 to 500 µm and CV varying from 10% to 35% can be readily produced on the RMR with the membrane rotational speed from 0 to 1500 rpm. Higher membrane rotational speeds generate stronger detaching shear force on the membrane surface, thus, produce smaller emulsion droplets. 4. Conclusions The XME experiments indicate that monodisperse monomer/oil in water emulsions can be readily formulated over a wide range of different process conditions. The monomer emulsion droplets obtained have been successfully polymerised into microcapsules while retaining the size characteristics in the primary emulsions. Although running the XME at significantly higher disperse fluxes can lead to droplet CVs exceeding the monodispersity threshold of 35%, the XME products would still make significantly favourite comparisons on size distribution characteristics with those produced from homogenisation. This makes the XME a highly attractive technology even for general emulsification appli-
cations, given its other distinctive advantages such as simplicity, low energy consumption, easy scale-up and reproducibility prospects. Exploratory experiments on RMR show that highly uniform emulsion droplets can be produced, with the droplet size being dictated by the membrane rotational speed.
References [1] R.A. Williams, Making the perfect particle. Ingenia, 7 (2001) 26–32. [2] G.T. Vladisavljevic and R.A. Williams, Adv. Coll. Interf. Sci., 113 (2005) 1–20. [3] R.A. Williams, S.J. Peng, D.A. Wheeler, N.C. Morley, D. Taylor, M. Whalley and D. Houldsworth, Trans IChemE, 76 (1998) 902–910. [4] G.T. Vladisavljevic and R.A. Williams, J. Coll. Interf. Sci., 299 (2006) 396–402. [5] R.A. Williams, Controlled dispersion using a spinning membrane reactor, UK Patent Application No. PCT/GB00/04917, University of Leeds, 2001. [6] V. Schroder, O. Behrend and H. Schubert, J. Coll. Interf. Sci., 202 (1998) 334–340. [7] M.J. Lynch and W.C. Griffin, Food emulsions, in: Emulsions and Emulsion Technology, Surfactant Science Series, Vol. 6(1), Marcel Dekker, New York, 1974, pp. 249–289. [8] H. Schubert, Emulsification—New developments and trends, AIChE Meeting, Los Angeles, 2000.
Manufacture of controlled emulsions and particulates using membrane emulsification Qingchun Yuana*, Ruozhou Houa, Nita Aryantia, Richard A. Williamsa, Simon Biggsa, Simon Lawsonb, Helen Silgramc, Manish Sarkarc, Richard Birchd a
Institute of Particle Science and Engineering, bParticlesCIC, University of Leeds, Houldworth Building, Leeds, LS2 9JT, UK Tel. +44 (113) 343-7811; Fax: +44 (113) 343-2781; email: [email protected] c ICI Paints, Wexham Road, Slough Berkshire SL2 5DS, UK d Quest International, Ashford, Kent TN24 0LT, UK Received 28 November 2006; Accepted 7 February 2007
Abstract Crossflow and rotating membrane emulsification techniques were used for making oil-in-water (O/W) emulsions. The emulsions produced from a variety of oils and monomers (viscosity 7–528 mPas) exhibited narrow size distributions over a wide droplet size range, with the average droplet size ranging from less than 1 µm up to 500 µm. The monomer emulsions were further encapsulated to produce microcapsules through subsequent polymerisation reactions. The monodispersity feature of the primary emulsions was retained after the encapsulation. In comparison with other homogenisation methods, our experimental results demonstrated that the membrane emulsification technique is not only superior in emulsion droplet size controls, but also advantageous in energy efficiency and industrial-scale productions. Keywords: Membrane emulsification; Size control; Microcapsules; Industrialisation
1. Introduction Emulsion manufacturing is an important process in food, pharmaceutical, cosmetics as well as many other chemical industries. Membrane emul*Corresponding author.
sification is a technique which is based on a novel concept of generating droplets “drop-by-drop” to produce emulsions [1]. Using a membrane of well defined pore structure, this technique is capable of manufacturing size controlled, mono-disperse products with high efficiency, low energy con-
Presented at the 11th Aachen Membrane Colloquium, 28–29 March, 2007, Aachen, Germany. 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.095
216
Q. Yuan et al. / Desalination 224 (2008) 215–220
sumption and excellent reproducibility. To date, membrane emulsification has been applied to create a variety of particulate products with novel structures and functionalities[2]. In the membrane emulsification, emulsions are typically produced by forcing a disperse phase to permeate through the uniform micropores of a membrane into a continuous phase. Droplets, which are formed at the pore openings on the membrane surface, are dispatched by the relative shear motion between the membrane and continuous phase. Using this concept, different technologies have been developed for commercial production [3–6]. The current work featured the use of crossflow membrane emulsification (XME) and rotating membrane reactor (RMR) techniques for precision production of monomer/ oil in water emulsions. Effects of disperse phase viscosity and operational parameters on process throughput and droplet monodispersity were discussed. Preliminary attempts to encapsulate the monomer droplets obtained were successfully carried out through interfacial polymerisation. (a)
Some exploratory emulsification results on RMR were also introduced. 2. Experimental Fig. 1(a) and (b) illustrate photographic images of the pilot-scale XME and batch RMR apparatus used in the current investigation and schematic operating principles, respectively. The two apparatus differ in the way the droplets detachment is initiated. In the XME, droplet detachment is stimulated by circulating the continuous phase along the membrane surface in cross flow [2], whereas the RMR works on a relatively novel idea in which the droplet detachment is initiated by rotating the membrane itself [4]. This has a potential advantage of eliminating the pump circulation of the continuous phase, thus creates a low shear and well controlled flow environment to enable formulation of delicate and fragile structured particulate products. Table 1 lists some of the key system and process parameters adopted in the current investigation. (b)
Fig. 1. (a) Mechanism of crossflow membrane emulsification and a pilot rig. (b) Mechanism of rotating membrane emulsification and a laboratory-scale reactor.
Q. Yuan et al. / Desalination 224 (2008) 215–220
217
Table 1 Some key system and process features of the pilot-scale XME rig and laboratory-scale RMR system used
Membrane type Pore shape and size Dimensions Disperse phase Continuous phase
XME
RMR
Sintered ceramic Random shaped, 0.2, 1 and 5 µm Tubular, Ø20×600 mm with 7 channels of Ø4 mm Oils/monomers Aqueous solutions of 2 wt% effective surfactants
Laser-drilled stainless steel Close to round, 100 µm Tubular, Ø10×85 mm Mineral oil Aqueous solution of 2 wt% surfactant and 0.1 wt% thickener
3. Results and discussion 3.1. XME Table 2 summarises the experimental conditions and results obtained in the XME investigation. Fig. 2 illustrates both the number and volume based droplet size distributions of the emulsions made in run 1, 2 and 3. The inset images are SEM/optical micrographs of microcapsules produced from the corresponding emulsions. It can be seen that despite considerable differences in the disperse phase viscosity (ηdisperse) and transmembrane pressure (ΔP) applied, relatively monodisperse emulsions, with coefficient of variations (CV) at around 30%, can be steadily produced from different pore size membranes. This indicates that XME is a fairly robust technology capable of manufacturing size controlled emulsions over a range of different process conditions. The relative monodisperse emulsions of monomers have been successfully encapsulated through interfacial polymerisation followed. The inset images show that individual solid microcapsules were produced, and the size characteristics of the original emulsions have largely been retained. In an XME system containing fast absorbing emulsifiers, the emulsion droplet size has been reported to be mainly depending on membrane pore size and crossflow velocity only. This has been the case in our investigation as well. As we
can see from Table 1 in runs 2, 4, and 5, emulsions of similar Sauter diameter D3,2 were produced from the same crossflow velocity and membrane pore size. The disperse phase viscosity and transmembrane pressure, although differing significantly in these runs, had very limited effect on the mean droplet size. For industrial applications, the process throughput has been a concern, especially when small pore membranes are used. The achieved oil fluxes in the current system are in the range of 0.002 to 0.08 m3/m2·h while keeping the CV value below the monodispersity threshold of 35%. The figures agree with the general notion that in a typical XME operation, the maximum disperse phase influx has to be restrained to below 0.1 m3/m2·h in order to enable uniform droplets being formed on the membrane surface in a “size-stable” or “dripping” mode [2]. However, our experiments also revealed that although further increasing the throughput (by using a higher ΔP) might lead to some widening of the droplet size distribution, the thus produced emulsions were still showing notably better qualities than that produced using the conventional rotorstator emulsification technology. Fig. 3(a) and (b) compares the droplet size distributions of a low and high viscosity oil emulsions produced by the two methods respectively. Clearly in both cases the XME products (Table 1, runs 4 and 5) have exhibited significantly narrower size distributions than those produced by homogenisation. The
218
Q. Yuan et al. / Desalination 224 (2008) 215–220
Table 2 Experimental conditions and results of the XME No.
1 2 3 4 5
Experimental conditions
Experimental results
Figures
Membrane pore, µm
ηdisperse, mPas
Vcf, m/s
ΔP, MPa
D3,2, µm
CV, %
Oil flux, ×10!3 m3/m2 h
0.2 1 5 1 1
528 49 18 7 528
1.75 1.75 1.75 1.75 1.75
0.5 0.05 0.015 0.5 0.5
0.75 6.0 45.5 5.7 5.2
27.4 30.5 35.5 45.5 37.8
2.2 2.6 84.3 364 2.4
2 2 2 3a 3b
Fig. 2. Size-distributions of the emulsions made on ceramic membranes of 0.2, 1 and 5 µm using the XME. Inset of SEM/optical micrographs show the microcapsules made from the emulsions of 0.75 and 45.5 µm.
higher the disperse phase viscosity, the more predominant the improvement in the uniformity of the droplet size. It is worth pointing out that the oil flux of run 4 has reached 0.36 m3/m2·h with a membrane of 1 µm at ΔP= 0.5 MPa. This implies that it takes just over 1 min to prepare 1 L of 5 µm emulsion containing 25 v/v% of the low viscosity oil by using an Ø20×600 mm tubular membrane. Such a throughput is well comparable to a
medium-sized laboratory-scale homogeniser device. The output capacity of an industrial homogeniser, on the other hand, typically lies around 1 m3/h [7]. For the production of similar concentrations of O/W emulsions, an XME system would require merely an array of 13 tubular membranes of Ø20×600 mm. Consider that membrane emulsification only requires a tiny fraction of energy consumptions demanded by the conventional homogenisation methods [8], and the
Q. Yuan et al. / Desalination 224 (2008) 215–220
219
Fig. 3. Size distributions of emulsions of a low viscosity oil (7 mPas, a) and highly viscous oil (528 mPas, b) prepared by the XME (see Table 1 for details) and a mini-homogeniser (5 ml capacity, operated at 19,000 rpm for 2 min in the case of low viscosity oil, and 30,000 rpm for 5 min in the case of high viscosity oil).
(a)
(b)
Fig. 4. (a) Size distribution and (b) droplet microscopic image of a low viscosity mineral oil-in-water emulsion produced with RMR at a membrane rotational speed of 1500 rpm.
process throughput can be easily scaled up by simply increasing the membrane area that produces droplets. It shows that membrane emulsification will be a both attractive and promising technology for large-scale applications, not only in terms of superior product qualities, but also in terms of production scales and operation economics.
3.2. RMR Only exploratory results of the RMR are presented here. Fig. 4 illustrates the size distribution and a droplet micrograph of a low viscosity mineral oil in water emulsion produced at a membrane rotational speed of 1500 rpm. The emulsion has a number mean droplet size of 133 µm and CV of 13%.
220
Q. Yuan et al. / Desalination 224 (2008) 215–220
It can be seen that the emulsion droplet size is fairly uniform. Other preliminary experiments have shown that monodisperse mineral oil in water emulsions with average droplet size ranging from 80 to 500 µm and CV varying from 10% to 35% can be readily produced on the RMR with the membrane rotational speed from 0 to 1500 rpm. Higher membrane rotational speeds generate stronger detaching shear force on the membrane surface, thus, produce smaller emulsion droplets. 4. Conclusions The XME experiments indicate that monodisperse monomer/oil in water emulsions can be readily formulated over a wide range of different process conditions. The monomer emulsion droplets obtained have been successfully polymerised into microcapsules while retaining the size characteristics in the primary emulsions. Although running the XME at significantly higher disperse fluxes can lead to droplet CVs exceeding the monodispersity threshold of 35%, the XME products would still make significantly favourite comparisons on size distribution characteristics with those produced from homogenisation. This makes the XME a highly attractive technology even for general emulsification appli-
cations, given its other distinctive advantages such as simplicity, low energy consumption, easy scale-up and reproducibility prospects. Exploratory experiments on RMR show that highly uniform emulsion droplets can be produced, with the droplet size being dictated by the membrane rotational speed.
References [1] R.A. Williams, Making the perfect particle. Ingenia, 7 (2001) 26–32. [2] G.T. Vladisavljevic and R.A. Williams, Adv. Coll. Interf. Sci., 113 (2005) 1–20. [3] R.A. Williams, S.J. Peng, D.A. Wheeler, N.C. Morley, D. Taylor, M. Whalley and D. Houldsworth, Trans IChemE, 76 (1998) 902–910. [4] G.T. Vladisavljevic and R.A. Williams, J. Coll. Interf. Sci., 299 (2006) 396–402. [5] R.A. Williams, Controlled dispersion using a spinning membrane reactor, UK Patent Application No. PCT/GB00/04917, University of Leeds, 2001. [6] V. Schroder, O. Behrend and H. Schubert, J. Coll. Interf. Sci., 202 (1998) 334–340. [7] M.J. Lynch and W.C. Griffin, Food emulsions, in: Emulsions and Emulsion Technology, Surfactant Science Series, Vol. 6(1), Marcel Dekker, New York, 1974, pp. 249–289. [8] H. Schubert, Emulsification—New developments and trends, AIChE Meeting, Los Angeles, 2000.