Production of porous silica microparticles by membrane emulsification

A method for the production of near-monodispersed spherical silica particles with controllable porosity based on the formation of uniform emulsion droplets using membrane emulsification is described. A hydrophobic metal membrane with a 15 μm pore size and 200 μm pore spacing was used to produce near-monodispersed droplets, with a mean size that could be controlled between 65 and 240 μm containing acidified sodium silicate solution (with 4 and 6 wt % SiO2) in kerosene. After drying and shrinking, the final silica particles had a mean size in the range between 30 and 70 μm. The coefficient of variation for both the droplets and the particles did not exceed 35%. The most uniform particles had a mean diameter of 40 μm and coefficient of variation of 17%. By altering the pH of the sodium silicate solution and aging the gel particles in water or acetone, the internal structure of the silica particles was successfully modified, and both micro- and mesoporous near-monodispersed spherical particles were produced with an average internal pore size between 1 and 6 nm and an average surface area between 360 and 750 m2 g–1. A material balance and particle size analysis provided identical values for the internal voidage of the particles, when compared to the voidage as determined by BET analysis

Novel membrane emulsification method of producing highly uniform silica particles using inexpensive silica sources

A membrane emulsification method for production of monodispersed silica-based ion exchange particles through water-in-oil emulsion route is developed. A hydrophobic microsieve membrane with 15 :m pore size and 200 :m pore spacing was used to produce droplets, with a mean size between 65 and 240 :m containing acidified sodium silicate solution (with 4 and 6% wt. SiO2) in kerosene. After drying, the final silica particles had a mean size in the range between 30 and 70 :m. Coefficient of variation for both the droplets and particles did not exceed 35%. The most uniform particles had a mean diameter of 40 :m and coefficient of variation of 17%. The particles were functionalised with 3-aminopropyltrimethoxysilane and used for chemisorption of Cu(II) from an aqueous solution of CuSO4 in a continuous flow stirred cell with slotted pore microfiltration membrane. Functionalised silica particles showed a higher binding affinity toward Cu(II) than non-treated silica particles

Low pressure microfilter design aspects and filtration performance

A microfilter should retain micron sized material yet provide minimal resistance to liquid flow. A slotted pore surface microfilter was oscillated whilst filtering yeast cells under constant rate. At shear rates over 7760 s-1, a pore blocking model fitted the data. The operating pressure was very low (<1000 Pa), but particle retention was limited by the 4 micron pore slot width. A sintered glass micro-bead coating improved yeast rejection: 95% at 1.7 microns at a shear rate of 5000 s-1, with a 1.2 kPa transmembrane pressure. Two models were validated to assist with the design of future micro-bead coatings constructed from spherical particles

Preparation and characterization of PLGA particles for subcutaneous controlled drug release by membrane emulsification

Uniformly sized microparticles of poly(DL, lactic-co-glycolic) (PLGA) acid, with controllable median diameters within the size range 40 to 140 microns, were successfully prepared by membrane emulsification of an oil phase injected into an aqueous phase, followed by solvent removal. Initially, simple particles were produced as an oil-in-water emulsion, where dichloromethane (DCM) and PLGA were the oil phase and water with stabiliser was the continuous phase. The oil was injected into the aqueous phase through an array type microporous membrane, which has very regular pores equally spaced apart, and two different pore sizes were used: 20 and 40 microns in diameter. Shear was provided at the membrane surface, causing the drops to detach, by a simple paddle stirrer rotating above the membrane. Further tests involved the production of a primary water-in-oil emulsion, using a mechanical homogeniser, which was then subsequently injected into a water phase through the microporous membrane to form a water-in-oil-in-water emulsion. These tests used a water soluble model drug (blue dextran) and encapsulation efficiencies of up to 100% were obtained for concentrations of 15% PLGA dissolved in the DCM and injected through a 40 micron membrane. Solidification of the PLGA particles followed by removal of the DCM through the surrounding aqueous continuous phase. Different PLGA concentrations, particle size and osmotic pressures were considered in order to find their effect on encapsulation efficiency. Osmotic pressure was varied by changing the salt concentration in the external aqueous phase whilst maintaining a constant internal aqueous phase salt concentration. Osmotic pressure was found to be a significant factor on the resulting particle structure, for the tests conducted at lower PLGA concentrations (10 and 5% PLGA). The PLGA concentration and particle size distribution influence the time to complete the solidification stage and a slow solidification, formed by stirring gently overnight, provided the most monosized particles and highest encapsulation efficiency

Stirred cell membrane emulsification and factors influencing dispersion drop size and uniformity

Water-in-oil (w/o) and oil-in-water (o/w) emulsions were generated using 30-μm pore diameter surface membranes to investigate the factors influencing drop size, and the degree of uniformity of drop size distribution, using a stirred cell employing a simple paddle mounted above a circular disc membrane. The importance of the transitional radius, which is the radius at which the vortex around the unbaffled paddle stirrer changes from a forced vortex to a free vortex and the shear stress at the membrane surface below the stirrer is at its greatest, is demonstrated. Monosized emulsions were produced, with drop size distribution coefficient of variation values of 10% for o/w emulsions and 13.5% for w/o emulsions. These tests demonstrated that a membrane of reduced annular operating area (ringed membrane) produced a more monosized o/w emulsion than a membrane where the full area was used to generate the emulsion, without affecting the mean drop size. The improved size distribution was achieved while the transitional radius was located within the ringed annular section of the membrane. The force balance model, applied to drops formed at the surface of the membrane during emulsification, predicted the droplet diameter provided further drop break up within the stirred cell did not occur. Drop break up occurred at Reynolds numbers below 300 for both oil-in-water and water-in-oil dispersions. Therefore, for Reynolds numbers greater than this, an annular radial ring membrane can be designed to produce monosized droplets using the stirred cell at known continuous phase viscosities with predictable mean droplet size. This knowledge can be used as a design tool to produce monosized droplets of a specified size for various applications using simple stirred cell emulsification

Production of silica particles by membrane emulsification

Production of silica particles by membrane emulsificatio

Novel method of producing highly uniform silica particles using inexpensive silica sources

Novel method of producing highly uniform silica particles using inexpensive silica source

Liquid-liquid membrane dispersion in a stirred cell with and without controlled shear

Oil was passed through membranes into a continuous water phase containing a surfactant (Tween 20) to form oil dispersions with drop diameters between 40 and 400 μm. Two types of stirred equipment were used:  a Weissenberg rheometer (cone and plate geometry) providing constant shear stress at all radial positions which was modified to include a membrane instead of the plate and a simple stirred cell, with a paddle rotating above the membrane, providing variable shear with radial position. Experiments show that the simple paddle-stirred cell provided an oil drop dispersion that was as monosized as that produced by the controlled shear device, if not better. An analysis indicated that only the section of the membrane close to the radius of the highest shear under the paddle stirred membrane produced oil drops. The membranes used in the experiments contained a regular array of nontortuous pores uniformly spaced and provided oil injection rates up to 1000 L m-2 h-1, which is much higher than reported fluxes for the alternative tortuous pore channel membranes made by sintering

Membrane surface modification: techniques, properties and applications

Conventional membranes are used in membrane emulsification to generate highly uniform droplets over a desired range of droplet sizes, and in microfiltration for the removal of solid particles down to 0.01 microns in size from liquid streams. However, the productivity and operating costs for both of these processes may suffer because the internal pore structure of these membranes can be become blocked during operation, leading to a large pressure drop across the membrane. Such problems can be avoided by using a special type of membrane called a microsieve, and unlike conventional membranes, microsieves do not have an internal pore structure. The microsieves developed at Micropore Technologies consist of a thin metal sheet with uniformly sized and spaced pores, where each pore forms a direct channel from one side of the membrane to the other. In this thesis, two surface coatings have been developed to optimise the performance of the microsieves used in membrane emulsification and microfiltration, resulting in two new composite membrane types. The first surface coating was required to improve the uniformity of aqueous droplets formed during the preparation of water-in-oil (w/o) emulsions. The second surface coating was required to remove yeast cells from aqueous feed suspensions. Simple criteria were used in the decision-making process to select the best type of coating, which included performance (droplet uniformity or yeast cell rejection achieved), durability and material costs. This thesis explores the development work for both coatings, and includes brief notes on those coatings that did not meet the criteria.</p

Novel yeast and oil drop microfiltration equipment

The conventional microfiltration of yeast and oil is problematic due to irreversible fouling on the membrane surface and within the internal pore structure. These problems result in the need for high shear at the membrane surface, entailing higher operating costs, the periodic replacement of the filter and cell or drop damage making filtration more difficult. A new type of filter is commercially available, which is a true surface filter with no internal structure, where each pore forms a direct channel of uniform size from one side of the membrane to the other. These membranes can be used in an oscillating filtration system, which provides a high peak shear directly at the membrane surface. The benefits of this system are high permeate fluxes, low operating pressures, long-life membranes and lower operating costs. Yeast filtration tests have been performed to validate this equipment. The frequency and amplitude of oscillation have been investigated using yeast (deformable) and calcium carbonate (non-deformable) challenge suspensions. Initial tests have shown that a higher shear provides a higher critical flux and a lower operating pressure. However, a higher shear resulted in a lower grade efficiency in the permeate. Simultaneous development work has been performed on the membrane surface coating in order to maintain a high grade efficiency at high shear, and to minimise any biological adhesion. A case will be presented to highlight the significant process benefits obtained by those industries where the microfiltration of biological material is necessary, including beer filtration and oil filtration, where biological material present in seawater easily fouls existing filtration systems