Phase-Transfer Catalysis in Segmented Flow in a Microchannel: Fluidic Control of Selectivity and Productivity

Precise control over the interfacial area of aqueous and organic slugs in segmented flow in a microchannel reactor provides an attractive means to optimize the yield and productivity of a phase-transfer-catalyzed reaction. Herein, we report the selective alkylation of phenylacetonitrile to the monoalkylated product in a microchannel of 250-mu m internal diameter operated in a continuous and solvent-free manner in the slug-flow regime. The conversion of phenylacetonitrile increased from 40% to 99% as a result of a 97% larger slug surface-to-volume ratio when the volumetric aqueous-to-organic phase flow ratio was raised from 1.0 to 6.1 at the same residence time. The larger surface-to-volume ratio significantly promoted catalyst phase transfer but decreased selectivity because of the simultaneous increase of the rate of the consecutive reaction to the dialkylated product. There exists all Optimum flow ratio with a maximum productivity. Conversion and selectivity in the microchannel reactor were both found to be significantly larger than in a stirred reactor.

Numbered-up gas-liquid micro/milli channels reactor with modular flow distributor

Gas-liquid processing in microreactors remains mostly restricted to the laboratory scale due to the complexity and expenditure needed for an adequate numbering-up with a uniform flow distribution. Here, the numbering-up is presented for multi-phase (gas-liquid) flow in microreactor suitable for a production capacity of kg/h. Based on the barrier channels concept, the barrier-based micro/milli reactor (BMMR) is designed and fabricated to deliver flow non-uniformity of less than 10%. The BMMR consists of eight parallel channels all operated in the Taylor flow regime and with a liquid flow rate up to 150. mL/min. The quality of the flow distribution is reported by studying two aspects. The first aspect is the influence of different viscosities, surface tensions and flow rates. The second aspect is the influence of modularity by testing three different reaction channels type: (1) square channels fabricated in a stainless steel plate, (2) square channels fabricated in a glass plate, and (3) circular channels (capillaries) made of stainless steel. Additionally, the BMMR is compared to that of a single channel regard the slug and bubble lengths and bubble generation frequency. The results pave the ground for bringing multi-phase flow in microreactor one step closer for large scale production via numbering-up. © 2012 Elsevier B.V.

Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re-U), axial flow Reynolds number (Re-W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers. (c) 2006 Elsevier Ltd. All rights reserved.

Late Pleistocene Interactions of East and West Antarctic Ice-Flow Regimes: Evidence from the McMurdo Ice Shelf

We present new interpretations of deglaciation in McMurdo Sound and the western Ross Sea, with observationally based reconstructions of interactions between East and West Antarctic ice at the last glacial maximum (LGM), 16 000, 12 000, 8000 and 4000 sp. At the LGM? East Antarctic ice from Mulock Glacier split, one branch turned westward south of Ross Island but the other branch rounded Ross Island before flowing southwest into McMurdo Sound. This flow regime, constrained by an ice saddle north of Ross Island, is consistent with the reconstruction of Stuiver and others (1981a). After the LGM, grounding-line retreat was most rapid in areas with greatest water depth, especially along the Victoria Land coast. By 12 000 sp, the ice-now regime in McMurdo Sound changed to through-flowing Mulock Glacier ice, with lesser contributions from Koettlitz, Blue and Ferrar Glaciers, because the former ice saddle north of Ross Island was replaced by a dome. The modern flew regime was established similar to 4000 BP. Ice derived from high elevations on the Polar Plateau but now stranded on the McMurdo Ice Shelf, and the pattern of the Transantarctic Mountains erratics support our reconstructions of Mulock Glacier ice rounding Minna Bluff but with all ice from Skelton Glacier ablating south of the bluff. They are inconsistent with Drewry's (1979) LGM reconstruction that includes Skelton Glacier ice in the McMurdo-Sound through-flow. Drewry's (1979) model closely approximates our results for 12 000-4000 BP. Ice-sheet modeling holds promise for determining whether deglaciation proceeded by grounding-line retreat of an ice sheet that was largely stagnant, because it never approached equilibrium flowline profiles after the Ross Ice Shelf, grounded, or of a dynamic ice sheet with flowline profiles kept low by active ice streams that extended northward from present-day outlet glaciers after the Ross Ice Shelf grounded.

Long-term flow rates and biomat zone hydrology in soil columns receiving septic tank effluent

Soil absorption systems (SAS) are used commonly to treat and disperse septic tank effluent (STE). SAS can hydraulically fail as a result of the low permeable biomat zone that develops on the infiltrative surface. The objectives of this experiment were to compare the hydraulic properties of biomats grown in soils of different textures, to investigate the long-term acceptance rates (LTAR) from prolonged application of STE, and to assess if soils were of major importance in determining LTAR. The STE was applied to repacked sand, Oxisol and Vertisol soil columns over a period of 16 months, at equivalent hydraulic loading rates of 50, 35 and 8 L/m(2)/d, respectively Infiltration rates, soil matric potentials, and biomat hydraulic properties were measured either directly from the soil columns or calculated using established soil physics theory. Biomats 1 to 2 cm thick developed in all soils columns with hydraulic resistances of 27 to 39 d. These biomats reduced a 4 order of magnitude variation in saturated hydraulic conductivity (K.) between the soils to a one order of magnitude variation in LTAR. A relationship between biomat resistance and organic loading rate was observed in all soils. Saturated hydraulic conductivity influenced the rate and extent of biomat development. However, once the biomat was established, the LTAR was governed by the resistance of the biomat and the sub-biomat soil unsaturated flow regime induced by the biomat. Results show that whilst initial soil K. is likely to be important in the establishment of the biomat zone in a trench, LTAR is determined by the biomat resistance and the unsaturated soil hydraulic conductivity, not the K, of a soil. The results call into question the commonly used approach of basing the LTAR, and ultimately trench length in SAS, on the initial K, of soils. (c) 2006 Elsevier Ltd. All rights reserved.

Further studies on flow distribution on distillation sieve trays

The thesis describes experimental work on the possibility of using deflection baffles in conventional distillation trays as flow straightening devices, with the view of enhancing tray efficiency. The mode of operation is based on deflecting part of the liquid momentum from the centre of the tray to the segment regions in order to drive stagnating liquid at the edges forward. The first part of the work was a detailed investigation into the two-phase flow patterns produced on a conventional sieve tray having 1 mm hole size perforations. The data provide a check on some earlier work and extend the range of the existing databank, particularly to conditions more typical of industrial operation. A critical survey of data collected on trays with different hole sizes (Hine, 1990; Chambers, 1993; Fenwick, 1996; this work) showed that the hole diameter has a significant influence on the flow regime, the size of the stagnant regions and the hydraulic and mass transfer performance. Five modified tray topologies were created with different configurations of baffles and tested extensively in the 2.44 m diameter air-water pilot distillation simulator for their efficacy in achieving uniform flow across the tray and for their impact on tray loading capacity and mass transfer efficiency. Special attention was given to the calibration of the over 100 temperature probes used in measuring the water temperature across the tray on which the heat and mass transfer analogy is based. In addition to normal tray capacity experiments, higher weir load experiments were conducted using a 'half-tray' mode in order to extend the range of data to conditions more typical of industrial operation. The modified trays show superior flow characteristics compared to the conventional tray in terms of the ability to replenish the zones of exceptionally low temperatures and high residence times at the edges of the tray, to lower the bulk liquid gradient and to achieve a more uniform flow across the tray. These superior flow abilities, however, tend to diminish with increasing weir load because of the increasing tendency for the liquid to jump over the barriers instead of flowing over them. The modified tray topologies showed no tendency to cause undue limitation to tray loading capacity. Although the improvement in the efficiency of a single tray over that of the conventional tray was moderate and in some cases marginal, the multiplier effect in a multiple tray column situation would be significant (Porter et al., 1972). These results are in good agreement with an associated CFD studies (Fischer, 1999) carried out by partners in the Advanced Studies in Distillation consortium. It is concluded that deflection baffles can be used in a conventional distillation sieve tray to achieve better liquid flow distribution and obtain enhanced mass transfer efficiency, without undermining the tray loading capacity. Unlike any other controlled-flow tray whose mechanical complexity impose stringent manufacturing and installation tolerances, the baffled-tray models are simple to design, manufacture and install and thus provide an economic method of retrofitting badly performing sieve trays both in terms of downtime and fabrication. NOTE APPENDICES 2-5 ARE ON A SEPARATE FLOPPY DISK ONLY AVAILABLE FOR CONSULTATION AT ASTON UNIVERSITY LIBRARY WITH PRIOR ARRANGEMENT

Flow patterns, performance and scale-up of distillation trays

This work is concerned with the nature of liquid flow across industrial sieve trays operating in the spray, mixed, and the emulsified flow regimes. In order to overcome the practical difficulties of removing many samples from a commercial tray, the mass transfer process was investigated in an air water simulator column by heat transfer analogy. The temperature of the warm water was measured by many thermocouples as the water flowed across the single pass 1.2 m diameter sieve tray. The thermocouples were linked to a mini computer for the storage of the data. The temperature data were then transferred to a main frame computer to generate temperature profiles - analogous to concentration profiles. A comprehensive study of the existing tray efficiency models was carried out using computerised numerical solutions. The calculated results were compared with experimental results published by the Fractionation Research Incorporation (FRl) and the existing models did not show any agreement with the experimental results. Only the Porter and Lockett model showed a reasonable agreement with the experimental results for cenain tray efficiency values. A rectangular active section tray was constructed and tested to establish the channelling effect and the result of its effect on circular tray designs. The developed flow patterns showed predominantly flat profiles and some indication of significant liquid flow through the central region of the tray. This comfirms that the rectangular tray configuration might not be a satisfactory solution for liquid maldistribution on sieve trays. For a typical industrial tray the flow of liquid as it crosses the tray from the inlet to the outlet weir could be affected by the mixing of liquid by the eddy, momentum and the weir shape in the axial or the transverse direction or both. Conventional U-shape profiles were developed when the operating conditions were such that the froth dispersion was in the mixed regime, with good liquid temperature distribution while in the spray regime. For the 12.5 mm hole diameter tray the constant temperature profiles were found to be in the axial direction while in the spray regime and in the transverse direction for the 4.5 mm hole tray. It was observed that the extent of the liquid stagnant zones at the sides of the tray depended on the tray hole diameter and was larger for the 4.5 mm hole tray. The liquid hold-up results show a high liquid hold-up at the areas of the tray with low liquid temperatures, this supports the doubts about the assumptions of constant point efficiency across an operating tray. Liquid flow over the outlet weir showed more liquid flow at the centre of the tray at high liquid loading with low liquid flow at both ends of the weir. The calculated results of the point and tray efficiency model showed a general increase in the calculated point and tray efficiencies with an increase in the weir loading, as the flow regime changed from the spray to the mixed regime the point and the tray efficiencies increased from approximately 30 to 80%.Through the mixed flow regime the efficiencies were found to remain fairly constant, and as the operating conditions were changed to maintain an emulsified flow regime there was a decrease in the resulting efficiencies. The results of the estimated coefficient of mixing for the small and large hole diameter trays show that the extent of liquid mixing on an operating tray generally increased with increasing capacity factor, but decreased with increasing weir loads. This demonstrates that above certain weir loads, the effect of eddy diffusion mechanism on the process of liquid mixing on an operating tray to be negligible.

Lattice Boltzmann method for flow and heat transfer in microgeometries

Recent technological developments have made it possible to design various microdevices where fluid flow and heat transfer are involved. For the proper design of such systems, the governing physics needs to be investigated. Due to the difficulty to study complex geometries in micro scales using experimental techniques, computational tools are developed to analyze and simulate flow and heat transfer in microgeometries. However, conventional numerical methods using the Navier-Stokes equations fail to predict some aspects of microflows such as nonlinear pressure distribution, increase mass flow rate, slip flow and temperature jump at the solid boundaries. This necessitates the development of new computational methods which depend on the kinetic theory that are both accurate and computationally efficient. In this study, lattice Boltzmann method (LBM) was used to investigate the flow and heat transfer in micro sized geometries. The LBM depends on the Boltzmann equation which is valid in the whole rarefaction regime that can be observed in micro flows. Results were obtained for isothermal channel flows at Knudsen numbers higher than 0.01 at different pressure ratios. LBM solutions for micro-Couette and micro-Poiseuille flow were found to be in good agreement with the analytical solutions valid in the slip flow regime (0.01 < Kn < 0.1) and direct simulation Monte Carlo solutions that are valid in the transition regime (0.1 < Kn < 10) for pressure distribution and velocity field. The isothermal LBM was further extended to simulate flows including heat transfer. The method was first validated for continuum channel flows with and without constrictions by comparing the thermal LBM results against accurate solutions obtained from analytical equations and finite element method. Finally, the capability of thermal LBM was improved by adding the effect of rarefaction and the method was used to analyze the behavior of gas flow in microchannels. The major finding of this research is that, the newly developed particle-based method described here can be used as an alternative numerical tool in order to study non-continuum effects observed in micro-electro-mechanical-systems (MEMS).

Lattice Boltzmann Method for Flow and Heat Transfer in Microgeometries

Recent technological developments have made it possible to design various microdevices where fluid flow and heat transfer are involved. For the proper design of such systems, the governing physics needs to be investigated. Due to the difficulty to study complex geometries in micro scales using experimental techniques, computational tools are developed to analyze and simulate flow and heat transfer in microgeometries. However, conventional numerical methods using the Navier-Stokes equations fail to predict some aspects of microflows such as nonlinear pressure distribution, increase mass flow rate, slip flow and temperature jump at the solid boundaries. This necessitates the development of new computational methods which depend on the kinetic theory that are both accurate and computationally efficient. In this study, lattice Boltzmann method (LBM) was used to investigate the flow and heat transfer in micro sized geometries. The LBM depends on the Boltzmann equation which is valid in the whole rarefaction regime that can be observed in micro flows. Results were obtained for isothermal channel flows at Knudsen numbers higher than 0.01 at different pressure ratios. LBM solutions for micro-Couette and micro-Poiseuille flow were found to be in good agreement with the analytical solutions valid in the slip flow regime (0.01 < Kn < 0.1) and direct simulation Monte Carlo solutions that are valid in the transition regime (0.1 < Kn < 10) for pressure distribution and velocity field. The isothermal LBM was further extended to simulate flows including heat transfer. The method was first validated for continuum channel flows with and without constrictions by comparing the thermal LBM results against accurate solutions obtained from analytical equations and finite element method. Finally, the capability of thermal LBM was improved by adding the effect of rarefaction and the method was used to analyze the behavior of gas flow in microchannels. The major finding of this research is that, the newly developed particle-based method described here can be used as an alternative numerical tool in order to study non-continuum effects observed in micro-electro-mechanical-systems (MEMS).

Analysis of friction factor reduction in turbulent water flow using a superhydrophobic coating

This study provides experimental and theoretical evidence that the coating of the inner surface of copper pipes with superhydrophobic (SH) materials induces a Cassie state flow regime on the flow of water. This results in an increase in the fluid's dimensionless velocity distribution coefficient, a, which gives rise to an increase in the apparent Reynolds number, which may approach the "plug flow state". Experimental evidence from the SH coating of a classic unsteady-state flow system resulted in a significant decrease in the friction factor and associated energy loss. The friction factor decrease can be attributed to an increase in the apparent Reynolds number. The study demonstrates that the Cassie effects imposed by SH coating can be quantitatively shown to decrease the frictional resistance to flow in commercial pipes.

Analysis of heat transfer coefficient in silica nanofluids with water as base fluid under transitional flow

The effect of Reynolds number variation in a vertical double pipe counterflow heat exchanger due to the changes in viscosity can cause the change in flow regime, for instance, when heats up and cools down, it can convert from turbulent to laminar or inversely, that can have significant effect on heat transfer coefficient and pressure drop. Mainly, the range of transition phase has been studied in this study with the investigation of silica nanofluid dispersed in water in three different concentrations. The results have been compared with distilled water sample and showed a remarkable raise in heat transfer coefficient while pressure drop has been increased respectively, as well. Although pumping power has to go up at the same time and it is a drawback, heat transfer efficiency grows for diluted samples. On the other hand, for the most concentrated sample, effect of pressure drop dominates which leads to decline in the overall efficiency.

A novel vortex flow reactor for the purification of B-phycoerythrin from Porphyridium cruentum

The purification of B-phycoerythrin from a concentrated extract of disrupted Porphyridium cruentum cells was carried out using a new vortex flow reactor design for protein purification. The reactor behaved as an expanded bed in the laminar vortices flow regime where the Streamline DEAE resin was expanded by the axial flow and stabilized by the vortex flow. After the broth culture was centrifuged and resuspended in the adsorption buffer, the concentrated extract of disrupted cells was directly loaded into the vortex flow reactor. The purification of B-phycoerythrin was carried out in two steps: adsorption in the expanded bed and elution from the settled bed. 142.0 mg of B-phycoerythrin was eluted representing a total recovery yield of 86.6%. Prior to B-phycoerythrin purification, the protein adsorption of the vortex flow reactor was characterized through hydrodynamic studies and a dynamic capacity measurement using a standard protein.

A STUDY OF HEAT TRANSFER AND FLOW CHARACTERISTICS OF RISING TAYLOR BUBBLES

Practical application of flow boiling to ground- and space-based thermal management systems hinges on the ability to predict the system’s heat removal capabilities under expected operating conditions. Research in this field has shown that the heat transfer coefficient within two-phase heat exchangers can be largely dependent on the experienced flow regime. This finding has inspired an effort to develop mechanistic heat transfer models for each flow pattern which are likely to outperform traditional empirical correlations. As a contribution to the effort, this work aimed to identify the heat transfer mechanisms for the slug flow regime through analysis of individual Taylor bubbles. An experimental apparatus was developed to inject single vapor Taylor bubbles into co-currently flowing liquid HFE 7100. The heat transfer was measured as the bubble rose through a 6 mm inner diameter heated tube using an infrared thermography technique. High-speed flow visualization was obtained and the bubble film thickness measured in an adiabatic section. Experiments were conducted at various liquid mass fluxes (43-200 kg/m2s) and gravity levels (0.01g-1.8g) to characterize the effect of bubble drift velocity on the heat transfer mechanisms. Variable gravity testing was conducted during a NASA parabolic flight campaign. Results from the experiments showed that the drift velocity strongly affects the hydrodynamics and heat transfer of single elongated bubbles. At low gravity levels, bubbles exhibited shapes characteristic of capillary flows and the heat transfer enhancement due to the bubble was dominated by conduction through the thin film. At moderate to high gravity, traditional Taylor bubbles provided small values of enhancement within the film, but large peaks in the wake heat transfer occurred due to turbulent vortices induced by the film plunging into the trailing liquid slug. Characteristics of the wake heat transfer profiles were analyzed and related to the predicted velocity field. Results were compared and shown to agree with numerical simulations of colleagues from EPFL, Switzerland. In addition, a preliminary study was completed on the effect of a Taylor bubble passing through nucleate flow boiling, showing that the thinning thermal boundary layer within the film suppressed nucleation, thereby decreasing the heat transfer coefficient.