The present invention relates to a rotating surface of revolution reactor or spinning 5 disc reactor for mass and heat transfer applications, and in particular to such a reactor provided with a porous or semi-permeable element on its reaction surface so as to allow for filtration or solvation of reactants on the reaction surface.

Rotating reactors or spinning disc reactors (SDRs) for mass and heat transfer

10 applications are known from the present applicant's International patent applications

WO00/48731, WO00/48729, WO00/48732, WO00/48730 and WO00/48728, the full contents of which are hereby incorporated into the present application by reference.

Rotating reactors generally comprise a rotating or spinning surface, for example a disc or a cone, onto which one or more liquid reactants are supplied. Centrifugal

15 forces cause the reactants to pass outwardly across the surface (i.e. centrifugal acceleration is aligned with a surface radius vector) in the form of a thin, generally wavy film, the film then being thrown from a circumference of the surface for collection. High turbulence and shear stresses in the film cause excellent mixing and mass transfer, and the low thickness of the film allows for excellent heat transfer to

20 and from the film. It is to be appreciated that the generation of a thin, generally wavy and radially outwardly-movmg film of reactant on the spinning surface is a key feature of SDR technology, including the present invention.

It is known, for example from EP 0 020 055, to provide an apparatus for effecting

25 mass transfer in which there is provided a relatively thick disc or annulus of a permeable material with a voidage of at least 80%. The disc or annulus is rotated about its central axis, and two fluid phases are passed radially through the disc or annulus by way of centrifugal forces resulting in radial accelerations of more than about 5000ms ^"2 . It is to be appreciated that both phases pass through the bulk of the

30 permeable material in a radial direction, and there is no generation of a thin wavy film on a surface of the permeable material.

It is also known, for example from US 4,549,998, to provide a contacting device comprising at least one porous circular plate, and preferably several such plates coaxially stacked on one another. The plate is rotated at high speed, and a liquid reactant is supplied to one side of the plate, generally at an axial part thereof, and passes radially across the plate as a thin film when the plate is rotated. The plate is preferably provided surface features that cause perturbations in the thin film so as to improve mixing and shearing. The pores in the plate are intended to allow the liquid reactant to pass through the plate from the said one side to an opposed side, thereby enabling a radially-travelling thin film to be formed on both sides of the plate, even when the liquid reactant is fed only to the said one side of the plate. The pores in the plate are relatively large (1.5mm), and are provided for the sole purpose of enabling liquid communication between the two sides of the plate.

Industrial crystallisation generally involves the generation of supersaturation in an appropriate volume so that the excess solute can be discharged as crystals having a desired size distribution. The means for creating the supersaturation may involve any of the following techniques: 1) cooling a saturated solution where the solubility varies significantly with temperature; 2) evaporating a saturated solution when the solubility in insensitive to solution temperature; 3) reacting two (or more) fluids (e.g. liquid/liquid or liquid/gas) to yield an insoluble component which then "precipitates"; and 4) adding an anti-solvent to a saturated solution to reduce the solubility of the solvent.

A perennial problem which is encountered when performing crystallisation is the occurrence of crystal scale deposition on solid surfaces within a crystalliser. This may be particularly severe at those surfaces involved in the generation of supersaturation, e.g. where cooling or heating takes place. However, scaling may also occur on any surface that is exposed to the supersaturated solution, thereby altering the surface characteristics and impeding heat transfer. Crystal scale deposition is particularly troublesome in continuously operated crystallisers, since the

operation of the crystalliser has to be interrupted from time to time in order to dissolve or otherwise remove the accumulated scale. In this regard, the use of polished and/or non-stick surfaces may only be a temporary expedient rather than a strategy for the long term.

In other applications, for example the production of drugs or pigment products comprising small particles, a key requirement is that a primary particle size distribution should be narrow and that the mean particle size should be small, for example lOμm or less. These desirable characteristics are imposed by the product function, whether as an active agent in a drug formulation or as a pigment in a printing ink. A fine particle size promotes rapid dissolution and hence rapid drug action. It is also conducive to the generation of strong colours in a printing ink. Conventionally, such particles are produced batchwise in a stirred vessel. An appropriate reaction between two or more fluids generates a high degree of supersaturation and causes particles to be precipitated from the mother liquor as a fine suspension, typically having a solid mass fraction of several percentage points. Unfortunately, the mixing environment within the stirred vessel is usually insufficiently intense compared with the rapid crystal nucleation rates. This results in non-uniform supersaturation zones within the vessel and a consequent broadening of the particle size distribution.

The present applicant has been researching the use of SDR technology for crystallisation and precipitation techniques, for example as described in WO03/008083. Use of an SDR helps to address the aforementioned problems by creating a highly sheared liquid film on a rotating surface, thereby dramatically enhancing the mixing intensity in the liquid phase. This feature is critically important in allowing an SDR to generate product having the desired characteristics of a narrow particle size distribution and a small particle size.

Although a reactor, whether an SDR or a stirred vessel, can be regarded as the heart of any process system, downstream work-up/separation is usually responsible for

most of the system capital cost. A dilemma faced by a designer of a conventional system is that the isolation of fine particles from their mother liquor is inherently difficult. Normally, separation is achieved by batch or continuous filtration, using pressure or elevated acceleration to force liquid through a permeable filter medium, leaving a cake of wet product upon the filter medium. Typical devices are pressure filters (using gas pressure), centrifuges (using centrifugal acceleration) or filter presses (using pumped liquid pressure). However, the build up of a cake of fine product particles on the filter medium gives rise to a growing pressure drop which steadily reduces the flow of the mother liquor, and hence reduces the corresponding filter performance. As manufacturing scale increases, the depth of the filter cake required increases, and the selection and design of filtration equipment so as to maintain acceptable liquor throughput becomes more and more difficult in order to collect desirable and finely divided products. Under these conditions, the effective filter becomes the developing cake rather than the actual filter medium, and the finer the particles, the more difficult it is to collect useful cake depths in acceptable timeframes using economic pressure conditions. Once a wet filter cake has been produced, it is generally removed from the filter medium and is then deposited in a heated and/or evacuated chamber in order to drive off the remaining solvent and to produce a friable dry powder product. As the powder dries, it is usually mixed so as to facilitate the drying process and to reduce agglomeration.

An alternative approach for the disengagement of fine particles from their mother liquor is to employ a cross flow technique. This requires a pressurised dilute slurry to pass over a permeable surface, the surface retaining the particles while allowing the liquid to be abstracted. Conditions are arranged so that a substantial slurry/wall shear stress is maintained, thereby ensuring that a filter cake is never established. Instead, the slurry merely becomes more concentrated as it proceeds along the permeable surface, a final slurry concentration being chosen so as to be suitable for a subsequent drying operation. Cross flow is also beneficial for facilitating reverse osmosis where a high differential pressure across a suitable semi-permeable membrane can be used to remove solvent from solutions. A high wall shear stress

ensures that the surface solute concentration does not deviate significantly from the bulk value (i.e. suppresses polarisation) and thereby maximises the permeate flux. In this way, the principal basis for the polarisation of the filter and the consequent loss of performance is avoided. While the operating intensity of a cross flow filter is much higher than that of its conventional equivalent, it is acutely influenced by the intensity of agitation/mixing within the slurry boundary layer. Current examples of cross flow filters usually comprise lengths of permeable tube which run full and this limits the degree of liquid shear and the corresponding flow rate which may be generated through the permeable surface. US 6,165,365 discloses an alternative cross flow filter arrangement comprising a stack of rotating membranes in close proximity to a coaxial stack of stationary separator elements. It is to be noted that the flow path in this device always runs full.

According to a first aspect of the present invention, there is provided a reactor apparatus including a support element rotatable about an axis, the support element having an exposed, external surface generally centred on the axis and an internal surface opposed to the exposed surface, the exposed surface being adapted for outward flow of a thin film of a fluid phase component thereacross when supplied thereto as the support element is rotated, and wherein at least a portion of the support element is permeable or semi-permeable or porous so as to allow passage of liquid or gas phase components therethrough between the external and internal surfaces but to prevent passage of micrometre-scale particles therethrough.

Generally, the thin film will be in the form of a thin wavy film, the waves being important for enhanced mass transfer and shear within the film. The waves are not generated as a result of vibration, but are generally inherent in SDR applications where a thin film passes across a rotating surface.

For the avoidance of doubt, the term "exposed, external surface" is used in the context of the present application to mean a surface that is not in such close proximity to another surface or obstruction so as to prevent formation of a thin, wavy

film of a fluid phase component when the support element is rotated and the fluid phase component supplied to the surface. In particular, the term is intended to distinguish over the arrangement of US 6,165,365 in which the surfaces of the membrane packs are not exposed, but are located in close proximity to separator elements so as to define channels which run full when fluid is supplied thereto. Because the channels of US 6,165,365 always run full (this is required for the recirculation of fluid), there can be no formation of a thin, wavy film, this being of key importance in the present invention.

According to a second aspect of the present invention, there is provided a method of processing a fluid phase component by way of a reactor apparatus including a support element rotatable about an axis, the support element having an exposed, external surface generally centred on the axis and an internal surface opposed to the exposed surface, at least a portion of the support element being permeable or semi-permeable or porous so as to allow passage of liquid or gas phase components therethrough between the external and internal surfaces but to prevent passage of micrometre-scale particles therethrough, wherein the fluid phase component is supplied to the exposed, external surface while the support element is rotated about the axis, the fluid phase component being caused to flow outwardly across the exposed, external surface in the form of a thin, wavy film as a result of centrifugal forces, and wherein liquid is caused to pass through the support element from the external to the internal surface, or liquid or gas is caused to pass through the support element from the internal to the external surface.

The internal surface of at least the porous part of the support element advantageously opens into or faces one or more channels or a chamber where liquid that has passed through from the exposed, external surface can be collected, or to which liquid or gas can be supplied, optionally under pressure, so as to pass from the channel or chamber to the exposed, external surface. Liquid or gas can be supplied to, or liquid collected from, the channel or chamber along an axial pipe that can also serve as a rotatable axle on which the support element is mounted.

The support element may be formed as a generally flat, hollow disc-shaped member with the exposed, external surface being a circular external surface of the disc and internal surface facing the hollow interior of the disc. Alternatively, the internal surface may be supported on a metal plate or the like provided with radial or spiral grooves or other surface structures which serve to define the channels or chamber.

Alternatively, the support element may include a spiral member centred on the axis, the spiral member having an exposed, external surface (facing the axis and generally parallel therewith) and an internal surface facing a channel or chamber. The spiral member may be elongate (in the manner of a Swiss roll or a roulade), or may be mounted on a disc-shaped member. General reactor configurations of this type, but without the porous, permeable or semi-permeable feature, are disclosed in more detail in WO 2004/004888, the full disclosure of which is hereby incorporated into the present application by way of reference.

Alternatively, the support element may be configured a concave cone or bowl or a cylinder or the like with the exposed, external surface being a surface that generally faces the axis of rotation.

The apparatus and method of the present invention may be used in a number of different ways.

Firstly, by supplying liquid or gas to the internal channel or chamber and causing the liquid or gas to flow from the inner surface to the exposed, outer surface, a gas/liquid or liquid/liquid interface can be formed in the immediate vicinity of the exposed, external surface, thereby helping to prevent or at least reduce surface nucleation on the exposed, external surface when the reactor is used for crystallisation processes.

In this case, the fluid phase component forming the thin film will generally be a crystal slurry.

This mode of operation may be considered to be analogous to film boiling, where a thin film of steam acts as an effective insulator at a heating surface and thereby causes a marked reduction in the heat transfer rate. The fluid permeation rate needed to establish a film of liquid or gas between the exposed, external surface and the thin film of fluid phase component will be greater with higher levels of shear stress in the neighbourhood of the exposed, external surface. hi some applications, the permeating gas or liquid is immiscible with the crystal slurry, thereby avoiding undue dilution and negation of the tendency for the fluid phase component to crystallise (i.e. the supersaturation in the thin film would become negative). However, provided that care is taken to avoid undue dilution, a miscible fluid may be employed as the permeating fluid, and it is envisaged that most applications will use a miscible fluid.

In systems where crystallisation from solution is driven by cooling (i.e. where solubility varies significantly with temperature), it is advantageous to supply a cooled liquid at an appropriate temperature to the internal channel or chamber and to cause this to flow from the inner surface to the exposed, outer surface. The cooled liquid will help to cool the surfaces of the reactor, thereby promoting heat transfer from the fluid phase component of the thin film so as to reduce solubility and to promote crystallisation. Moreover, the cooled liquid will also decrease the solute concentration at the boundary layer between the thin film and the exposed, outer surface, thereby reducing the surface supersaturation and helping to prevent crystal scale formation on the surface. It is preferred to balance the cooling and the dilution effects such that the dilution effect just overcomes the extent to which cooling of the exposed, outer surface generates supersaturation in the given heat transfer environment. Providing too great a flow of from the inner surface to the exposed, outer surface will tend to overdilute the fluid phase component of the thin film and thereby cause unwanted dissolution of crystals at locations other than directly at the exposed, outer surface.

Secondly, and again with reference to crystallisation processes, by supplying liquid to the internal channel or chamber and causing the liquid to flow from the inner surface

to the exposed, outer surface, a standing surface supersaturation can be reduced to zero or less.

This mode of operation is expected to require a much lower rate of liquid transpiration from the internal surface to the exposed, external surface than is required to form an interface in the first mode of operation, and may therefore be more practical than the first mode of operation. A liquid solvent or anti-solvent, chosen for compatibility with the system of crystallisation, can be transpired into the boundary layer between the thin firm of crystal slurry and the exposed, external surface.

The standing concentration of solute in the liquid immediately in contact with the exposed, external surface (Cw) is the result of two opposing influences. On the one hand, solute diffuses from the bulk solution at a concentration (C _B ) over the boundary layer of thickness (Δ). On the other hand, the wall concentration (Cw) is diluted by the transpiring solvent arriving through the permeable part of the support element from the internal channel or chamber at a superficial velocity (U). Noting that a steady state is desired, the necessary transpiration rate is given by a solute mass balance over a time (dt) as follows:

Cw-U.dt - (D/ A) (C _B - C _w ).dt = 0

where D is the solute diffusion coefficient. Noting that the mass transfer coefficient is given as ho = D/Δ, this leads to:

U = h _D (C _B - C _w )/C _w

A typical value for hp on s spinning surface is around lO^ms ^"1 . In order to prevent or at least reduce scale deposition, Cw must be less than the saturation concentration. (CB - Cw)/C _w is likely to be between 0.1 and 1. Hence the probable values of U needed to suppress scale will generally (in this example) be in a range of 10 ^"4 to

10 ^"3 InS ^"1 . For a disc of 15 cm diameter, this corresponds to a total permeation rate of 2 x 10 ^"3 to 2 x lO^dmV ¹ . It will be appreciated that these figures are given by way of example only, and will naturally vary depending on the crystallisation system under consideration. It is important to balance the scale inhibition requirement against undue dilution of the crystal slurry.

Where the permeable support element acts as an osmotic membrane, the effective pressure differential causing transpiration is many atmospheres and operates provided that there is a significant solute concentration difference across the membrane. This renders the flux relatively uniform across the support element despite the radial pressure gradient within the channel. A further beneficial feature of the osmotic membrane in this context is that the flow is self-driven and presents no pressure control problems with varying rotational speed.

In a third mode of operation, embodiments of the present invention may be used for cross flow filtration. In this mode, the relevant part of the support element includes pores that are small enough to block passage of the finest particle size fraction that is desired to be filtered out of a slurry or the like. Shear stresses developed in the thin film of slurry on the exposed, external surface when the support element is rotated help to prevent the build up of a filter cake which would otherwise slow the filtration process. A slurry feed is provided to a central part of the exposed, external surface, and the slurry travels towards a periphery of the exposed, external surface as a thin film or progressively thickening slurry, with liquid filtrate passing through the pores to the internal surface and thus to the channel or chamber which can act as a return manifold and allow the filtrate to be collected, for example by way of an axial pipe as hereinbefore described. By way of example, particle volume fractions of at least 60% may be achieved with feed concentrations in a range of 2 to 5%. Preferably, the slurry is concentrate sufficiently by way of passage across the exposed, external surface so that it can be sent directly to a fluid bed drying unit. If required, the concentrated slurry or dried product may be washed in a wash solvent and then passed again over a reactor apparatus of embodiments of the present invention for

reconcentration, thus mimicking the repeated washing of conventional filter cakes in existing filtration systems.

It is to be appreciated that there already exist known devices that use rotational forces to thicken slurries of finely divided solids in liquids. Such known devices include cyclones and solid-bowl centrifuges (in which particles are spun into a sedimentary layer and discharged, at intervals, with a liquid flush). These known devices all suffer from the fundamental flaw of allowing a proportion of the product to exit to waste with the solvent liquor, which is unacceptable for high value materials. Embodiments of the present invention avoid this disadvantage.

In all of the modes of operation described hereinabove, it will be appreciated that a pressure gradient must be established between the exposed, external surface and the inner surface so as to cause liquid to pass in one direction or the other.

This may be achieved by applying a negative pressure at the internal surface, for example by -pumping liquid out of the channel or chamber through an axial pipe. This will consequently draw liquid from the exposed, external surface in cross flow filtration applications.

Alternatively, a positive pressure can be applied at the internal surface, for example by supplying liquid or gas to the channel or chamber, and the fluid in the channel or chamber will then transpire through to the exposed, external surface for crystal scale inhibition applications. The liquid or gas may be pumped into the channel or chamber at an overpressure. In some embodiments, centrifugal forces alone (due to rotation of the support element) may be sufficient to create the necessary pressure gradient. It is generally preferred that the channel or chamber is configured so as to run full during operation of the reactor.

Alternatively or in addition, the support element as a whole may be disposed within a sealed housing, and a positive or negative pressure can be applied at the exposed,

external surface by way of supplying a pressurised gas phase component to the housing or by at least partially evacuating the housing. The gas phase component may be substantially chemically inert with respect to any liquid or slurry on the exposed, external surface, or may react therewith so as to form predetermined products.

Alternatively or in addition, where the support element includes a semi-permeable membrane, osmotic pressure may be used to effect transpiration between the exposed, external surface and the internal surface, the direction of transpiration being determined by the relative concentrations of fluid on the exposed, external surface and the internal surface.

Not all of the support element need be porous between the exposed, external surface and the internal surface, hi some embodiments, for example, a central part of the support element (or at least a central part of the exposed, external surface) near the axis may be non-porous so as to define a zone in which the thin film simply undergoes high shear mixing and optional reaction. One or more annular porous, permeably or semi-permeable regions may then be provided towards the periphery of the support element for filtration or scale inhibition purposes.

Various different porous, permeable or semi-permeable structures may be employed in embodiments of the present invention. It will be appreciated that the structure must be permeable between the exposed, external surface and the internal surface at least in the permeable region of the support element.

In one embodiment, the support element or an annular part thereof is made out of a sintered metal powder or fibres, or alternatively a fine fibrous mesh, for example of metal or polymeric fibres. Suitable materials are, for example, available from Porvair® pic. Alternatively or in addition, porous ceramic materials may be used.

The sinter, mesh or ceramic may be substantially uniform between the exposed, external surface, in which case it preferably has a relatively fine pore size (i.e. small enough to block passage of the finest relevant particle fraction). In some embodiments, this may be in a range of 0.1 μm to 5μm, preferably 0.5μm to 2μm, typically around lμm.

Alternatively, two or more layers of sinter, mesh or ceramic may be provided, with the pore size increasing towards the internal surface. For example, there may be provided a relatively thin layer of sinter, mesh or ceramic as the exposed, external surface, having a pore size as described above, with this layer being stacked on a relatively thick layer of a coarser sinter, mesh or ceramic having a larger pore size, for example in a range of lOμm to 50μm or up to lOOμm or even more. The layers may be made out of different materials, for example a fine layer of sinter on a coarser layer of mesh, or a fine layer of ceramic on a coarser layer of sinter etc.

It is important that the layer or layers of sinter, mesh or ceramic is or are capable of withstanding the pressure differential between the exposed, external surface and the internal surface, and are also capable of withstanding the high forces generated by rotation of the support element. For example, it is preferred that the layer or layers can withstand a pressure differential of 3 bar (3x10 ⁵ Pa) across a 20cm diameter without breakage (although some central deflection may be acceptable).

Alternatively, the exposed, external surface (or at least an annular portion thereof) may be formed as an osmotic membrane or a nano- or ultrafiltration membrane, which may be made out of suitable polymeric, synthetic or other materials, hi some embodiments using nano- or ultrafiltration, the membrane may have a pore size in a range of O.lμm to 5μm, preferably 0.5μm to 2μm ₅ typically around lμm. Generally, osmotic membranes tend to have smaller pore sizes than ultrafiltration membranes, generally much less than O.lμm, and are permeable only to water and other low molecular weight species. Because osmotic and ultrafiltration membranes tend to be very thin and elastically flexible, it is desirable to provide structural support the

osmotic membrane. This may be provided by way of a perforated plate or one or more sintered layers or a mesh of fibres or porous ceramic, preferably with a pore size of 1 Oμm to 50μm or up to lOOμm or even more.

Alternatively, the exposed external surface may be formed as a perforated plate and/or one or more sintered layers or a mesh of fibres or porous ceramic, preferably with a pore size of lOμm to 5 Oμm or up to lOOμm or even more, and an osmotic membrane can be provided as the internal surface.

The osmotic or nano/ultrafiltration membrane may be very thin, for example around lμm, and may be deposited from solution onto a layer of sinter, mesh or porous ceramic as a thin semi-permeable layer. This asymmetric structure is advantageous in that flux can be kept high through use of a very thin membrane, while- structural integrity is maintained by way of the layer of sinter, mesh or porous ceramic.

For a reverse osmosis duty, the membrane may comprise a polymer cast in the form of an asymmetric film with a very thin (less than 1 μm) separation surface backed by a coarser structure.

These types of membranes are known to those skilled in the art.

For a better understanding of the present invention and to show how it may be carried into effect, reference shall now be made by way of example to the accompanying drawings, in which:

FIGURE 1 shows a first embodiment of the invention in schematic form;

FIGURE 2 shows a second embodiment of the invention in schematic form; and

FIGURES 3 to 5 show porous structures for use with embodiments of the present invention.

Figure 1 shows a sealed housing 1 in which is mounted a rotatable disc member 2 mounted on an axle 3. The axle 3, as well as serving to rotate the disc member 2, is hollow and defines a pipe through which fluid can be abstracted and removed by way of collector 10. The axle 3 passes out of the housing 1 by way of a rotary seal 11 and is supported by bearings 12, 13. A drive unit (not shown in Figure 1) serves to rotate the axle 3 at high speed. The disc member 2 includes a support element 4 comprising a permeable membrane 5 mounted on a coarser porous disc of support gauze or sinter 6. The support element 4 has an exposed, external surface 7 and an opposed surface 8 that is internal to the disc member 2. The disc member 2 includes a cavity 9 that faces the internal surface 8. The sealed housing 1 includes a viewing port 14 and an axially located feed pipe 15 for supplying a slurry to a central part of the exposed surface 7. During use, the disc member 2 is rotated at high speed and slurry is supplied by way of the feed pipe 15. The slurry then passes radially across the exposed surface 7 as a thin wavy film before being thrown from a periphery of the disc member 2 and then removed from the housing by way of outlet 16. The sealed housing is pressurised, for example by way of supplying a gas that may be relatively inert with regard to the components of the slurry, or may react therewith so as to generate predetermined reaction products, hi this way, at least part of a liquid phase component of the slurry is forced through the membrane 5 and the porous disc 6 from the exposed surface 7 to the internal surface 8, and thus into the cavity 9, from whence the liquid filtrate passes down the pipe in the axle 3 and is collected at collector 10. The cavity 9 thus serves as part of a return manifold for the liquid phase component. Accordingly, as the slurry passes radially across the exposed surface 7, it becomes progressively thickened, the thickened slurry being collected by way of outlet 16. The thickened slurry can then be dried in a conventional dryer, or may be passed one or more times through further reactors similar to that shown in Figure 1.

It will be appreciated that the permeable membrane 5 should have a pore size no larger than the largest particle fraction that is to be retained in the thickened slurry.

The porous disc 6 serves mainly to provide structural stability to the membrane 5, and can therefore have a larger pore size.

Instead of the sealed housing 1 being pressurised, a negative pressure or vacuum may be applied at the collector 10 so as to such the liquid phase component from the exposed surface 7.

The embodiment of Figure 1 is particularly suited for use in cross flow filtration applications. As previously discussed, liquids (and slurries) supplied to an inner radial position on a rotating surface experience a substantial radial acceleration which generates a thin, highly sheared film which then leaves the surface at its periphery. The shear stresses developed in the film help to prevent the build-up of a filter cake on the exposed surface 7, which would otherwise slow filtration. The cavity 9 serves as a mother liquor abstraction chamber allowing the liquid phase component to be removed by way of the pipe in the axle 3. The cavity 9 is designed so that it runs full, thereby giving rise to a significant radial pressure gradient being established in the liquid phase component by way of rotation of the disc member 2. The radial pressure gradient is given by:

PEDGE - PAXIS = ¹ A PVT ²

where p is the liquid density and V _T is the disc tip speed.

Clearly the standing pressure within the sealed housing must exceed ¹ A pVτ ² in order to ensure a positive pressure differential across the whole of the exposed surface 7.

The output performance of the reactor will be a complex function of rotational speed, surface permeability, particle size and output particle volume fraction desired. The membrane 5 may be a permeable polymeric membrane with an appropriate pore size, optionally supported on a thin sintered fibrous network of fine metal fibres such as available from Porvair® pic. The courser porous disc 6, which may be a coarse metal mesh, serves primarily to resist the applied pressure forces and to prevent

undue distortion of the membrane 5. The polymeric membrane may be very thin, for example of the order of lμm, and may be deposited on the thin sintered fibrous layer by deposition from solution.

The embodiment of Figure 1 is also particularly suited for crystal scale inhibition applications. Here, solution is applied to the exposed surface 7 by way of feed 15, and crystallisation or precipitation occurs as the solution passes across the exposed surface 7. A fluid, for example a solvent, is passed up the pipe in the axle 3 and caused to transpire through the support element 4 from the cavity 9, through the porous disc 6 and membrane 5, to the exposed surface 7. The embodiment of Figure 1 is designed for applications in which all of the fluid passing up the pipe in the axle 3 is transpired through to the exposed surface.

Figure 2 shows an alternative embodiment of the present invention in more detail. As in the Figure 1 embodiment, there is provided a sealed housing 1, rotatable disc member 2 and axle/pipe 3 passing through a rotary seal 11. The axle 3 is supported by bearings 12, 13 and rotated by a drive unit 17. The axle 3 includes an inner pipe 27 and a coaxial outer pipe 28. A feed pipe 15 supplies slurry or liquid phase component, and an outlet 16 collects material that is thrown from the periphery of the disc member 2 for storage in a vessel 19. A gas inlet 20 allows the sealed housing 1 to be pressurised. The disc member 2 is shown in more detail than in Figure 1. A flange 21 is provided at a top of the axle 3 and provides a mounting surface for a main disc plate 22 with a circumferential wall 23. A spreader plate 24 with grooves or channels on its upper surface defining the cavity 9 is provided on top of the disc plate 22 within the wall 23 and bolted to the disc plate 22. A lower surface of the spreader plate 24 is provided with support fins 25 to prevent vortex formation under the spreader plate 24. A disc of relatively coarse porous sintered material 6 is provided on top of the spreader plate 24, and a fine permeable membrane 5 is provided on top of the material 6. The disc of material 6 and the membrane 5, together defining the support element 4 with its exposed, external surface 7 and

internal surface 8, are clamped on top of the spreader plate 24 by way of a circumferential clamping flange 26.

The embodiment of Figure 2 is particularly suited for crystal scale inhibition with additional cooling or heating. A liquid is supplied upwardly through the inner pipe 27 to the cavity 9, and part of the liquid then passes through the support element 4 from the internal surface 8 to the exposed, external surface 7 so as to form an interface with the slurry or liquid phase component on the surface 7 provided by feed 15, or to mix therewith. The remainder of the liquid passes across the upper surface of the spreader plate 24 and then returns under the spreader plate 24 to be removed by way of the outer pipe 28. The liquid may be recirculated via a temperature- controlled bath (not shown) by way of collector 10 and a rotary union and pipes 18. In this way, the liquid can additionally be utilised as a heat transfer fluid for controlling a temperature of the support element 4.

Figure 3 shows a cross section through, a support element 4 comprising a layer of fine sintered material 6 having a mean pore size of lμm, the upper surface being the exposed, external surface 7 and the lower surface being the internal surface 8.

Figure 4 shows an alternative support element 4 having a main structure of coarse sintered material 6 with a mean pore size in the range 10 to 50μm, and a molecular osmotic membrane 5 provided on top of the material 6 and defining the exposed, external surface 7.

Figure 5 shows another alternative support element 4 having a main structure of coarse sintered material 6 with a mean pore size in the range 10 to 50μm, and a molecular osmotic membrane 5 provided underneath the material 6. In this embodiment, the coarse material 6 defines the exposed, external surface 7 and the membrane 5 the internal surface 8.

The support elements 4 of Figures 4 and 5, by virtue of the osmotic membrane 5, can make use of osmotic pressure to help drive a liquid phase component from one surface of the support element to the other, with optional assistance from a pressurised head space either at the exposed, external surface 7 or the internal surface 8.

The preferred features of the invention are applicable to all aspects of the invention and may be used in any possible combination.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components, integers, moieties, additives or steps.