Method for preventing the formation of deposits downstream of a mixer and apparatus therefor

Abstract: A method of preventing the formation of deposits on surfaces downstream of a mixer in which possibly supersaturated mixtures issuing from the mixer are surrounded by a sheath of unsaturated solution. In an arrangement described, the sheath of unsaturated mixture is obtained by bleeding off some of the mixture issuing from the mixer sufficiently downstream of the mixer definitely to be unsaturated and returning this portion of the mixture to surround that issuing from the mixer.

Description: The present invention relates to the mixing and transport of reactive substances. In processes which involve the mixing and transport of reactive substances, the formation of unwanted precipitates on internal walls of reactor vessels and pipework can cause problems. Such unwanted precipitates can arise from the formation of localised regions of supersaturation in the mixture and it is an object of the present invention to provide a method of and apparatus for mixing reactive substances in which the output from a mixing device such as a vortex mixer is isolated from contact with any solid surfaces in the vicinity of the vortex mixer. According to one aspect of the present invention there is provided a method of mixing reactive substances according to claim 1. Preferably, the output flow from the mixer is directed axially along an axial diffuser and the pressure difference which occurs in use between the ends of the diffuser is used to return a portion of the flow from the axial diffuser to the outlet from the mixer to envelop the output flow from the mixer. A suitable mixing device is a vortex mixer. Also according to the present invention there is provided an apparatus for mixing reactive substances as defined in claim 5. Preferably the apparatus includes an axial diffuser so arranged that the output from the vortex mixer flows axially along it and part of the mixture issuing from the axial diffuser is bled off and returned to the inlet to the axial diffuser so as to envelop the output flow from the vortex mixer so as to isolate it from the structure of the axial diffuser. Preferably, the vortex mixer comprises a vortex chamber having one or more inlets arranged to direct an inlet flow substantially tangentially into the vortex chamber and an axial outlet in an end wall of the chamber. The end wall of the chamber can be planar or conical in form. The invention will be described further, by way of example, with reference to the accompanying drawings in which:- Fig 1 is a schematic longitudinal sectional view of an embodiment; and Fig 2 is a graph of supersaturation levels plotted along the length of the embodiment in Fig 1. In Fig 1, a vortex mixer 1 is serured by bolts 2 to the flanged end of a housing 3. The vortex mixer 1 comprises a vortex chamber 4 having one or more inlets 5, preferably at the periphery of the chamber 4, to direct an inlet flow tangentially or substantially tangentially into the chamber 4 and a central axial outlet 6 in an end wall of the chamber 4. In Fig 1, the chamber 4 is provided with a pair of diametrically opposed tangentially directed inlet ports 5. Liquids introduced through the ports 5 swirl through the chamber and in so doing become thoroughly mixed before exiting at the outlet 6. The housing 3 contains a centrally located tube 7 coaxial with the housing 3 and held in position, for example, by a spider assembly 8. A cooling jacket 9 can be provided about the housing 3. The tube 7 functions as an axial diffuser. In operation, reagents introduced at the separate inlets 5 are mixed in the vortex chamber 4 whereby the product of the reaction becomes supersaturated in solution. This takes place very quickly after mixing and a further short time interval, known as the precipitate induction time and which can be of the order of milliseconds in duration, elapses before the commencement of precipitate formation. Thereafter precipitate forms rapidly and supersaturation levels decrease until at equilibrium solubility, where supersaturation is negligible, precipitate formation comes to an end. Precipitation takes place if the supersaturated mixture from the vortex chamber 4 contacts a solid surface. This can cause blockage and fouling in the flow line from the vortex mixer. The presence of the coaxial tube 7 within the housing 3 serves to avoid such blockage and fouling. The supersaturated mixture emerging at the outlet 6 from the vortex chamber 4 is in the form of a jet which expands radially with distance from the outlet. In Fig 1 the envelope or confines of the stream issuing from the outlet 6 and passing along the tube 7 is indicated by the reference numeral 10. The jet emerging from the outlet 6 along the centre line of the tube 7 entrains liquid mixture, which is no longer supersaturated and is known as aged, from the downstream end of the tube 7. The flow directions are indicated by the arrows in Fig 1. A portion of the aged flow at the downstream end of the tube 7 is drawn between the exterior of the tube 7 and the interior of the housing 3 to surround the jet emerging from the outlet 6. This aged flow serves as a jacket or shield about the supersaturated mixture from the outlet 6 to prevent the mixture encountering a solid surface during its travel through the tube 7 and housing assembly (3, 8, 9). Fig 2 illustrates how supersaturation levels decrease with time in the axial direction of flow. In a correctly dimensioned assembly the residence time within the tube 7 is sufficient for the supersaturated mixture to become fully aged so that no fouling or blockage takes place in pipework downstream of the assembly comprising the vortex mixer 1 and axial diffuser 7, 8, 9. The tube 7 allows the precipitate residence time in which the solution solubility can reach equilibrium without the supersaturated flow from the vortex mixer 1 coming into contact with solid surfaces. In this way the flow from the vortex mixer 1 can age without solid deposits fouling surfaces of the tube 7 and the walls of downstream pipework.

Claims: A method of mixing reactive fluids as part of a precipitation reaction, wherein there is included the operations of admitting the reactants to a mixer (1) to produce a supersaturated solution of a reaction product, directing the output from the mixer (1) along a conduit and surrounding the output flow from the mixer (1) with an unsaturated solution of the reaction product. A method according to claim 1 wherein the output flow from the mixer (1) is directed axially along an axial flow diffuser (7) and a portion of unsaturated solution of the reaction product leaving the axial flow diffuser (7) is used to form an annulus of unsaturated solution of the reaction product surrounding the supersaturated solution entering the axial flow diffuser (7) so as to isolate the supersaturated solution of reaction product from the structure of the axial flow diffuser (7). A method according to claim 2 wherein the pressure difference which occurs in use between the ends of the axial flow diffuser (7) is used to return the portion of unsaturated solution of the reaction product to the input end of the axial flow diffuser (7) so as to form the annulus of unsaturated solution of the reaction product surrounding the supersaturated solution of the reaction product entering the axial flow diffuser (7). A method according to any of claims 1 to 3 wherein the reactive substances are mixed in a vortex mixer. An apparatus for mixing reactive fluids as part of a precipitation reaction wherein there is provided a vortex mixer (1) for mixing the reactants to provide a supersaturated solution of a reaction product, an axial outlet (6) from the mixer (1), an axial flow diffuser (7) positioned to receive the output from the mixer (1) and means for injecting into the input end of the axial flow diffuser (7) an annulus of an unsaturated solution of the reaction product so as to surround the supersaturated solution of the reaction product issuing from the mixer (1) and isolate it from the structure of the axial flow diffuser (7). An apparatus according to claim 5 wherein the axial flow diffuser (7) is so dimensioned that fluid leaving the axial flow diffuser (7) does so in an unsaturated state and a proportion of the unsaturated fluid is recirculated to the input end of the axial flow diffuser (7) so as to provide the said annulus of unsaturated solution of the reaction product. An apparatus according to claim 6 wherein the internal form of the distal end of an housing (3) within which the axial flow diffuser (7) is mounted is shaped to divert a portion of the unsaturated fluid back to the input end of the axial flow diffuser (7) and provide the annulus of unsaturated solution of the reaction product surrounding the supersaturated solution issuing from the mixer (1).

Inventors: BOWE MICHAEL JOSEPH, FALLOWS PAUL, BOWE, MICHAEL JOSEPH, FALLOWS, PAUL
Applicants: ATOMIC ENERGY AUTHORITY UK, UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1992
Main classification: B01F5
IPC codes: B01F5, B01F3, B01F13