Browsing by Author "Brown, D. E."

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    The distribution of solids in a mechanically stirred liquid
    (2000-09) Taylor, David J.; Brown, D. E.
    The literature available on the effect of vessel geometry, particle properties and liquid properties on the distribution of solids in a mechanically stirred vessel has been examined. This has shown that there are wide discrepancies between different authors on each of these areas. Solids concentrations have been measured using an electrical conductivity probe at various positions in two stirred vessels 0.61m and 1.83m in diameter. Different impeller designs were examined using A 310s and Pitched Blade Turbines to distribute the solids in the vessel. Pitched Blade Turbines with different diameter ratios of D=T/3 and D=T/2 were used. The sand particles used-had diameters from 150 pm - 1050 pm. It has been shown that constant power per unit volume is a reliable design rule for: 1. The effect o f vessel scale 2. The effect o f impeller diameter 3. The effect o f impeller design on the distribution of solids in a mechanically agitated stirred standard vessel. It has also been shown that the solids concentration data presented here can be correlated on the basis of N mod = N (C )da{yPo~lD~5 J and the vessel operating conditions can be characterised by a design methodology based on its use. Two methods of modelling the data are presented which describe the system in terms of: 1. An array of oscillating spheres and 2. The 1-d sedimentation dispersion model. These methods were found to model the data with different degrees of success and were generally disappointing.
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    Fluid mixing in rotor/stator mixers
    (Cranfield University, 1996-12) Sparks, T.; Brown, D. E.
    An industrial rotor-stator mixer was fitted into a flow loop to carry out overall power balance, flow visualisation and residence time distribution experiments. These were performed on various rotor-stator geometries and a half-scale unit. The overall power measurements showed that a large amount of power was given to the fluid by the rotor and estimates of the local turbulent energy dissipation rate per unit mass, e, were made using these data. It was found that the pumping efficiency of rotor- stator mixers is - 10 to 20% and an expression for the motor power (when the flow rate is controlled) was found. The flow pattern was characterised by a high tangential velocity in the rotor followed by an abrupt transformation to radial flow through the stator. It was suggested that the kinetic energy of the fluid in the rotor is transformed to pumping, ftiction and turbulence in the stator and that this is the region of greatest importance for mixing. The residence time distribution is characterised by a region of plug flow in series with a region of mixed flow. The flow through the volute has a dominant effect on the overall residence time distribution and the RTD is insensitive to operating condition (flow rate, rotor speed) or geometry. The knowledge gained from the above experiments was used to design diazo-coupling experiments (a mixing-sensitive competitive chemical reaction with well known kinetics) such that they gave qualitative information (e. g. best feed position) and quantitative information (e. g. turbulent energy dissipation rate) on the performance of a rotor-stator mixer. e was found to be proportional to the power given the fluid by the rotor and estimates for e of order 500 W kg-' were made using a micro-mixing model.
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    Gas/liquid separation in an I-SEP cyclone
    (Cranfield University, 2002-07) Allstaff, E. J.; Brown, D. E.
    An extensive experimental programme tested an l-SEP unit with air and water. This provided data on the separation efficiency and pressure drop across the l- SEP, as they varied with changing inlet conditions, geometry and outlet restrictions. The main conclusion from this work is that the resistance on the outlets directly affects the efficiency of the separator. Although it is possible to optimise the performance by changing 'the geometry, a small change in inlet conditions, common in oil and gas applications, causes the performance to change. _By using an identified relationship between the differential pressure and the operating conditions, an optimum performance over a much wider range of inlet conditions can be achieved. This work has contributed to the field of gas/liquid separation in axial-flow cyclones by demonstrating that manipulating the differential pressure between the outlets can control the separation efficiency. The relationship developed between the optimum differential pressure and the inlet conditions has not been identified in any of the literature available at the time of this work.
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    Liquid-liquid dispersions from in-line rotor-stator mixers
    (Cranfield University, 2004-07) Thapar, N.; Brown, D. E.
    Experiments were performed on an industrial scale Silverson in-line rotor-stator mixer to investigate its liquid-liquid dispersion capabilities. A non-coalescing kerosene-water system was used in the tests and the effect of stator geometry, rotor-speeds, flowrates and dispersed phase concentrations on the droplet size distribution was investigated. The rotor-speed and the dispersed phase concentration were found to have the most influence on droplet size. Drop sizes were also seen to increase with an increase in open area. No differences were observed in the droplet sizes or power draw of the stators with the smallest and similar open areas (Emulsor Screens and Square Hole High Shear screen). The power supplied to the fluid was proportional to N3 indicating the mixer was operating under turbulent conditions. d32 was correlated against the rotor speed and dispersed phase and the relationship was found to be close to that described by Chen and Middleman (1967). This analysis suggested that inertial stresses in the viscous sub-range were mainly responsible for drop break-up. d32 = 2x109 (1 + 20(D)(We Re4)-'" An estimation of the average energy dissipation rate was made in order to determine the Kolomogov length scale. The droplet sizes ranged from below the Kolmogorov length scale to significantly higher, suggesting that droplet break-up is due to inertial and viscous sub-range eddies. The Re could be defined in different regions within the mixer these values were used along with the nominal residence times in each region to determine where in the mixer the main drop break-up was occurring. The residence times for each region were in the following order: Shear Gap < Stator < Inlet < Rotor < Volute. The relatively long residence times and the magnitude of the stresses indicated that droplet break-up in a single pass through the in-line rotor-stator mixer is predominately determined by the viscous and inertial stresses in the rotor region.
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    Operational characteristics of rotor/stator mixers
    (Cranfield University, 1993-08) Sparks, T.; Brown, D. E.
    Fast chemical reactors require rapid mixing, and literature shows that rotor/stator mixers can provide this. Results have been published, at small scale, showing rotor/stator mixers to be promising as chemical reactors. An industrial size rotor/stator mixer has been procured and fitted into a test loop. Instrumentation was used to measure shaft torque, shaft speed, differential pressure and differential temperature. Measurements were used to produce an energy balance for the mixer that could be used to assess its mixing performance, based upon general descriptions of mixing. Residence time distribution (RTD) curves were obtained and it was shown that the mixer exhibited substantial backmixing. Modifications to the mixer are proposed that may produce a RTD that better approximates plug flow. It is concluded that if the mixer could be made to better approach plug flow, as mixing rates would be very high, use of such a device could improve the product quality from a competitive chemical reaction.
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    Production of a lipase from a pseudomonad species
    (Cranfield University, 1991-03) Smith, Catherine J.; Brown, D. E.
    A lipase-producing micro-organism was found to be a strain of Pseudomonas aeruginosa. When it was grown batchwise in a stirred 2 dM3 fermenter in a simple defined minimal salts medium containing yeast extract and glucose, it produced lipase at a level of 0.173 LU/cm3 without the need for a lipid substrate. Batch cultures of cells were fed glucose at constant rates. An optimum rate was found to be 0.50 g/dm3/h and this produced a maximum lipase activity of 0.80 LU/cm3. Medium composition was changed until it was possible to produce 25 g/dm3 dry weight of cells. Evidence of linear growth due to nutient, limitations was obtained. A microcomputer controlled protocol was used successfully to feed glucose solutions at exponentially increasing rates. Problems were encountered with the production of lipase from the interaction with pyocyanin production and from possible trace nutrient limitations.
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    Tertiary nitrification of wastewater in trickling filters
    (Cranfield University, 1994-10) Biddle, Jeremy R.; Wheatley, A. D.; Brown, D. E.
    This study was designed to investigate, at laboratory and pilot scale, the effects of various loading and climatic factors on the nitrification perfon-nance of four media, and to undertake a comparative assessment of the media. The media used were blast furnace slag and three random plastic media: Flocor RS, Etapak 160 and a new medium, Etapak 210. Laboratory experiments using pure cultures of Nitrosomonas europaea have determined the effect on nitrification of temperature, pH and substrate, BOD and inhibitor concentrations. Optimal values have been resolved for the temperature and pH and half- saturation constants for the substrate and inhibition are calculated for pure culture conditions. The presence of glucose and glutamic acid has been shown to have a beneficial effect on nitrification, although this observation could not be adequately explained. Pilot scale research, conducted over a two year period, has provided an accurate representation of a nitrifying trickling filter by using effluent from Cranfield STW in filters exposed to the full climadc variation. Data obtained from these filters have indicated the superior performance of the blast furnace slag media due mostly to its ability to maintain a large active bacterial Population without excessive accumulation. Results have been used to support or challenge previous publications, and to generate a set of desion curves. Conclusions from this research have been used in the design of a full scale nitrifying trickling filter which has been constructed at Cranfield STW, early data from which are presented.