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.

Characterization of the hairpin vortex solution in plane Couette flow

Quantitative evidence that establishes the existence of the hairpin vortex state (HVS) in plane Couette flow (PCF) is provided in this work. The evidence presented in this paper shows that the HVS can be obtained via homotopy from a flow with a simple geometrical configuration, namely, the laterally heated flow (LHF). Although the early stages of bifurcations of LHF have been previously investigated, our linear stability analysis reveals that the root in the LHF yields multiple branches via symmetry breaking. These branches connect to the PCF manifold as steady nonlinear amplitude solutions. Moreover, we show that the HVS has a direct bifurcation route to the Rayleigh-Bénard convection. © 2010 The American Physical Society.

Rapid aquaporin translocation regulates cellular water flow:the mechanism of hypotonicity-induced sub-cellular localization of the aquaporin 1 water channel

The control of cellular water flow is mediated by the aquaporin (AQP) family of membrane proteins. The family's structural features and the mechanism of selective water passage through the AQP pore are established, but there remains a gap in our knowledge of how water transport is regulated. Two broad possibilities exist. One is controlling the passage of water through the AQP pore, but this has only been observed as a phenomenon in some plant and microbial AQPs. An alternative is controlling the number of AQPs in the cell membrane. Here we describe a novel pathway in mammalian cells whereby a hypotonic stimulus directly induces intracellular calcium elevations, through transient receptor potential channels, that trigger AQP1 translocation. This translocation, which has a direct role in cell volume regulation, occurs within 30s and is dependent on calmodulin activation and phosphorylation of AQP1 at two threonine residues by protein kinase C. This direct mechanism provides a rationale for the changes in water transport that are required in response to constantly-changing local cellular water availability. Moreover, since calcium is a pluripotent and ubiquitous second messenger in biological systems, the discovery of its role in the regulation of AQP translocation has ramifications for diverse physiological and pathophysiological processes, as well as providing an explanation for the rapid regulation of water flow that is necessary for cell homeostasis.

Transition of planar Couette flow at infinite Reynolds numbers

An outline of the state space of planar Couette flow at high Reynolds numbers (Re<105) is investigated via a variety of efficient numerical techniques. It is verified from nonlinear analysis that the lower branch of the hairpin vortex state (HVS) asymptotically approaches the primary (laminar) state with increasing Re. It is also predicted that the lower branch of the HVS at high Re belongs to the stability boundary that initiates a transition to turbulence, and that one of the unstable manifolds of the lower branch of HVS lies on the boundary. These facts suggest HVS may provide a criterion to estimate a minimum perturbation arising transition to turbulent states at the infinite Re limit. © 2013 American Physical Society.

Human aquaporins:regulators of transcellular water flow

Background - Emerging evidence supports the view that (AQP) aquaporin water channels are regulators of transcellular water flow. Consistent with their expression in most tissues, AQPs are associated with diverse physiological and pathophysiological processes. Scope of review - AQP knockout studies suggest that the regulatory role of AQPs, rather than their action as passive channels, is their critical function. Transport through all AQPs occurs by a common passive mechanism, but their regulation and cellular distribution varies significantly depending on cell and tissue type; the role of AQPs in cell volume regulation (CVR) is particularly notable. This review examines the regulatory role of AQPs in transcellular water flow, especially in CVR. We focus on key systems of the human body, encompassing processes as diverse as urine concentration in the kidney to clearance of brain oedema. Major conclusions - AQPs are crucial for the regulation of water homeostasis, providing selective pores for the rapid movement of water across diverse cell membranes and playing regulatory roles in CVR. Gating mechanisms have been proposed for human AQPs, but have only been reported for plant and microbial AQPs. Consequently, it is likely that the distribution and abundance of AQPs in a particular membrane is the determinant of membrane water permeability and a regulator of transcellular water flow. General significance - Elucidating the mechanisms that regulate transcellular water flow will improve our understanding of the human body in health and disease. The central role of specific AQPs in regulating water homeostasis will provide routes to a range of novel therapies. This article is part of a Special Issue entitled Aquaporins.

Gas-liquid-solid flow modelling in a bubble column

An alternative approach to the modelling of solid-liquid and gas-liquid-solid flows for a 5:1 height to width aspect ratio bubble column is presented here. A modified transport equation for the volume fraction of a dispersed phase has been developed for the investigation of turbulent buoyancy driven flows (Chem. Eng. Proc., in press). In this study, a modified transport equation has been employed for discrete phase motion considering both solid-liquid and gas-liquid-solid flows. The modelling of the three-phase flow in a bubble column was achieved in the following case: injecting a slug of solid particles into the column for 10 s at a velocity of 0.1 m s-1 and then the gas phase flow was initiated with a superficial gas velocity of 0.02 cm s-1. © 2003 Elsevier B.V. All rights reserved.

The modelling of buoyancy driven flow in bubble columns

Using the analogy between lateral convection of heat and the two-phase flow in bubble columns, alternative turbulence modelling methods were analysed. The k-ε turbulence and Reynolds stress models were used to predict the buoyant motion of fluids where a density difference arises due to the introduction of heat or a discrete phase. A large height to width aspect ratio cavity was employed in the transport of heat and it was shown that the Reynolds stress model with the use of velocity profiles including the laminar flow solution resulted in turbulent vortices developing. The turbulence models were then applied to the simulation of gas-liquid flow for a 5:1 height to width aspect ratio bubble column. In the case of a gas superficial velocity of 0.02 m s-1 it was determined that employing the Reynolds stress model yielded the most realistic simulation results. © 2003 Elsevier B.V. All rights reserved.

Heterogeneous catalysis in an oscillatory baffled flow reactor

The first demonstration of heterogeneous catalysis within an oscillatory baffled flow reactor (OBR) is reported, exemplified by the solid acid catalysed esterification of organic acids, an important prototypical reaction for fine chemicals and biofuel synthesis. Suspension of a PrSOH-SBA-15 catalyst powder is readily achieved within the OBR under an oscillatory flow, facilitating the continuous esterification of hexanoic acid. Excellent semi-quantitative agreement is obtained between OBR and conventional stirred batch reaction kinetics, demonstrating efficient mixing, and highlighting the potential of OBRs for continuous, heterogeneously catalysed liquid phase transformations. Kinetic analysis highlights acid chain length (i.e. steric factors) as a key predictor of activity. Continuous esterification offers improved ester yields compared with batch operation, due to the removal of water by-product from the catalyst, evidencing the versatility of the OBR for heterogeneous flow chemistry and potential role as a new clean catalytic technology. © The Royal Society of Chemistry 2013.

Multi-level genetic algorithm for the resource-constrained re-entrant scheduling problem in the flow shop

The re-entrant flow shop scheduling problem (RFSP) is regarded as a NP-hard problem and attracted the attention of both researchers and industry. Current approach attempts to minimize the makespan of RFSP without considering the interdependency between the resource constraints and the re-entrant probability. This paper proposed Multi-level genetic algorithm (GA) by including the co-related re-entrant possibility and production mode in multi-level chromosome encoding. Repair operator is incorporated in the Multi-level genetic algorithm so as to revise the infeasible solution by resolving the resource conflict. With the objective of minimizing the makespan, Multi-level genetic algorithm (GA) is proposed and ANOVA is used to fine tune the parameter setting of GA. The experiment shows that the proposed approach is more effective to find the near-optimal schedule than the simulated annealing algorithm for both small-size problem and large-size problem. © 2013 Published by Elsevier Ltd.

Measuring the flow properties of small powder samples using an avalanche tester

The feasibility of using a small-scale avalanche tester to measure the flow properties of pharmaceutical lactose powders was examined. The modes of behaviour observed in larger systems were displayed and showed a clear distinction between angular, free-flowing particles and more spherical particles of similar flow characteristics. Angular Lactohale LH100 particles showed slumping behaviour at a rotational frequency of 0.33Hz which disappeared at higher frequencies. Spherical lactose powder with a similar flow function to LH100 only showed rolling behaviour under the same conditions, as did more cohesive powders LH200 and LH300. Further investigation of the LH100 data using Fast Fourier analysis showed that the slumping frequency was 1/10th of the rotational frequency.

Platinum-catalysed cinnamaldehyde hydrogenation in continuous flow

Platinum is one of the most widely used hydrogenation catalysts. Here we describe the translation of batch reactions to continuous flow, affording tunable C=O versus C=C hydrogenation over a Pt/SiO2 catalyst, resulting in high steady state activity and single-pass yields in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions. Negligible catalyst deactivation occurs under extended flow operation due to removal of reactively-formed poisons from the reaction zone. Process intensification imparts a four-fold enhancement in cinnamyl alcohol productivity.