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Abstract: Micromixers have been considered in numerous recent studies with the aim of mixing different liquid streams for the common circumstance of non-inertial flow, i.e., in the Stokes flow regime. Under such conditions, the diffusion of momentum is dominant but the diffusion of species remains weak because the Schmidt number of liquids is large. Most mixers that have potential for application in the Stokes regime make use of a folding flow pattern that approximates the baker''s transformation. In the work presented here, the general scaling of mixers of this type is developed from the exact equation for species transport and computations are made for a specimen mixer geometry to test the effectiveness of the resulting scaling. The scaling relation developed is found to give an excellent representation of the actual mixing characteristics of the specimen mixer over the entire range of Péclet number of practical interest. Finite volume computations are employed to solve the governing equations up to around . At higher Péclet numbers, where finite volume numerical solution becomes inaccurate with affordable mesh sizes, the species equation is solved using a Monte Carlo method instead. Finally, the scaling relation is used to develop the design relations needed to determine the number of mixing elements, the pressure drop incurred and the Péclet number of operation to achieve a given mixture uniformity within a specified mixing time. [Copyright &y& Elsevier]

Abstract: The separation of one component from a multicomponent fluid solution is commonly achieved by bringing the solution into contact with a second immiscible phase, as in liquid–liquid or gas–liquid contactors. Counter-current flow of the phases allows high purity separations and existing approaches typically force one of the phases to disperse into the other, producing the close contact necessary for the solute species to be distributed by diffusion throughout the volume of each phase. This phase mixing leaves the contacting strongly dependent on fluid and interface mechanical properties. Thus an approach that suits one phase and solute system will not necessarily suit others. In addition, it is then necessary to separate the dispersed phase which can be a major drawback of the operation sometimes posing substantial difficulties in easily emulsified liquid–liquid systems or in easily foamed gas–liquid systems. A new approach, which has recently been demonstrated experimentally for microchannels, uses a rotating spiral channel to allow controlled contacting giving a very high ratio of interfacial surface area to fluid volume but avoids phase mixing. Its application to larger channels, up to millimetres in size, is considered here. The two phases are forced to flow side by side in parallel layers along the narrow spiral channel. Selection of spiral parameters, rotation rate and pressure gradient along the channel controls the flow rate ratio of the phases and the relative thickness of the phase layers. This allows adjustment to reach the optimum mass transfer (limited strongly neither by one phase nor the other) and adaptation to phase and solute systems having widely differing fluid viscosities, densities, solute diffusivities and interface equilibria. With appropriate control over these parameters, a single device is, in principle, capable of application to a wide range of separation requirements. This rotating spiral contacting is, however, a new technology and remains to be investigated and tested in detail. The present work develops a model yielding both quantitative prediction of flow and mass transfer in the contacting channel and a framework for determining suitable designs, operating conditions and mass transfer performance for liquid–liquid and gas–liquid operations. [Copyright &y& Elsevier]

Abstract: A prototype device using a rotating spiral microchannel to produce multistage distillation has been designed and used to separate 2,2-dimethylbutane from an initial 50:50 mixture with 2-methyl-2-butene. Distillation or other phase-contacting processes in microchannels requires a strategy for handling surface forces so that the phases can interact in a controlled manner and efficient mass transfer can take place. The rotating spiral approach uses centrifugal force to maintain segregation of the phases into parallel-flowing liquid and vapour layers. When centrifugal force is opposed by pressure gradient along the channel, these layers can flow counter currently. Changing rotation rate and pressure allows adjustment of the phase flow rates and the contacting layer thicknesses. Thus, it is possible to control the phase contacting and take full advantage of the rapid mass transfer that is potentially available for small microchannel dimensions. This efficient contact can be maintained over long channel distances, allowing a large number of separation stages to be reached. A further advantage of the rotating spiral approach is that the mass transfer is augmented by secondary motions produced by Coriolis force. The paper describes the design of the prototype device, including flow and thermal aspects, and presents results for a simple binary distillation. The results obtained demonstrate the practical feasibility of rotating spiral contacting and provide initial quantitative data that are used to evaluate the performance achieved by the prototype device. [Copyright &y& Elsevier]