Showing papers in "Chemical Engineering Research & Design in 1991"

The modelling of coalescence processes in fluid-liquid dispersions : a review of current understanding

Abstract: The review addresses current understanding of the processes of collision and coalescence in pure gas-liquid and liquid-liquid dispersions. Attention is restricted to flow-driven collisions, apart from brief reference to related gravity-driven phenomena at free-interfaces. Plane-film and full numerical solutions of drainage are compared in the immobile, partially-mobile and fully-mobile cases and related expressions developed for the coalescence probability. Finally, the perspectives and problems involved in the use of such experssions are examined, in conjunction with flow simulation codes for the mathematical modelling of particle-size evolution in dispersed flows.

A LDA study of turbulent flow in a baffled vessel agitated by a pitched blade turbine

Abstract: Two components of mean and fluctuating turbulent velocities were measured in water by a laser Doppler anemometer in a flat-bottomed, baffled vessel agitated by a 45° pitched blade turbine impeller of 1/3rd of the vessel diameter. Using two impeller off-bottom clearances of T/4 and T/2, these measurements were carried out at many points in the vessel. Overall flow patterns have been obtained along with quantitative information on mean flows and the distribution of velocity fluctuations. The main findings are: (1) Near the base, the impeller off-bottom clearance influences decisively the flow pattern. In the case of the mid-height impeller location, a low intensity flow reversal prevails over its entirety. (2) A two-region model is proposed for the distribution of the fluctuating and mean velocities within the agitated vessel, viz., the impeller discharge zone and the remaining bulk. (3) Except in the impeller outflow stream, the fluctuating velocity measurements indicate that the turbulence is locally isotropic and virtually homogeneous. (4) The maximum turbulence intensity produced by the pitched blade turbine impeller is similar to that reported in the literature for the disc turbine. The results are broadly in agreement with the similar study of Ranade and Joshi except that a distinct region of flow reversal is found beneath the impeller at the off-bottom clearance of T/4 and the impeller discharge and total entrained flow are about 20% less

Interfacing heat exchanger network synthesis and detailed heat exchanger design

Abstract: Current heat exchanger network synthesis targeting and design procedures involve the use of assumed stream heat transfer coefficients. However, during detailed heat exchanger design, allowable pressure drops are often the most critical factors. The result can be big differences between the exchanger sizes and costs anticipated by the network designer and those realised by the exchanger designer. This in turn prejudices any optimisation attempted at the network design stage. In the paper it is shown how allowable pressure drop can be used as a basis of network design and consistency between expectation and realisation achieved

Targeting flue gas emissions

Abstract: Methods are well established to target the minimum energy consumption for a given process. However, such targets do not give a true indication of the resulting flue gas emissions from the utility system. This paper attempts to relate energy targets to the resulting flue gas emissions. In this way targets are set for flue gas emissions which allow different utility options to be compared on the basis of simple thermodynamic models. Such utility options can be compared on a local or global basis. Considering global emissions gives a broader view of the problem and one we should universally adopt. (author).

A new process for the microbial removal of sulphate and heavy metals from contaminated waters extracted by a geohydrological control system

Abstract: TLe underlying groundwater of many well established metal-refining sites are contaminated with sulphate and heavy metals. Geohydrological control systems are being developed to prevent the spreading of the contamination. These systems produce an effluent which has to be purified before it can be discharge to the environment or reused as process water. An an aerobic sulphate-reducing microbial process to treat such waters has been evaluated in a 9 m 3 pilot plant after a 2 year research programme. The process uses ethanol as the microbial growth substrate and energy source for sulphate reduction which leads to precipitation of heavy metal sulphides. The pilot plant successfully demonstrated that this process removes sulphate and a wide range of heavy metals to acceptable levels allowing the aqueous effluent to be discharged safely

An experimental study of electroacoustic crossflow microfiltration

Abstract: Results from an experimental study of the effects of the principal process and suspension characteristics on crossflow microfiltration augmented by electrical and ultrasonic force fields are presented. It has been found that both fields, either in isolation or in combination, can reduce membrane fouling: the extent of filtration improvement is affected by field strengths, acoustic frequency, suspension concentration, liquid viscosity, and particle size and surface charge. Synergistic effects were observed when the fields were applied simultaneously. As well as increasing filtrate fluxes, use of either or both force fields allows lower crossflow velocities to be used. This implies that smaller equipment can be used for a given throughput, reduced energy consumption is possible by the recirculation pump, there is a lesser tendency to degrade shear sensitive streams, and heat transfer duties may be reduced for recirculation loop exchangers.

Bubble breakup in stirred vessels : predicting the sauter mean diameter

Abstract: The size of the bubbles generated in gas-liquid stirred vessels is mainly a function of the agitation level but the prediction of the equilibrium bubble size is not possible from theory. A few correlations have been suggested based on the concept of a critical Weber number, the validity of which have not been confirmed. We report experiments where bubbles of known initial sizes were introduced in non-coalescing conditions, and the subsequent changes in their size were monitored as a function of the agitation level or power input. The Sauter mean bubble diameters are predicted well by an equation developed using Kolmogoroff's theory of local isotropic turbulence, provided the energy dissipation rate per unit mass is calculated on the basis of impeller swept volume

Pervaporation-esterification coupling. II: Modelling of the influence of different operating parameters

Abstract: A previous paper showed that the coupling of esterification (reversible) reactions to pervaporation (membrane process) can lead a quasi-complete conversion of reactants. The experimental conditions used were not the best to reach high conversion during a length of time. The influences of different parameters, such as the catalyst concentration, the ratios of membrane area to volume of solution, the ratios of initial number of mole of acid to alcohol, and the reaction and pervaporation temperature on the rate of conversion were studied experimentally and theoretically on the basis of the previously described model

Pervaporation-esterification coupling. I, Basic kinetic model

Abstract: The coupling of an equilibrated reaction to a membrane process is an alternative way to reach complete conversion of reactants. The selective pervaporation of water, a product of the esterification reactions of 1-propanol and 2-propanol by propionic acid, was used to drive the reaction to completion. A model describing the kinetics of the system was established and reaction rates constants were determined experimentally. Experimental and calculated curves are in good agreement

Process innovation using oscillatory flow within baffled tubes

Abstract: The inclusion of periodic baffles ino a tube coupled with fluid oscillation can result in a range of fluid mechanics where excellent fluid mixing can be achieved. This configuration can give high heat and mass transfer in both batch and continuous modes of operation. Potential applications include plus flow mixers, reactors, bioreactors, filtration units and crystallizers

Oscillations flow within tubes containing wall or central baffles

Abstract: Periodic baffles are either posioned at the wall or centre of the tube. In both cases numerical simulations are presented for steady axial flow within the tube up to a net flow Reynolds number of 200. In addition, experimental and numerical results are presented for oscillatory flows and the increasing complexity of the developing eddy motion is followed to the point where the axial symmetry of the flow is broken.

Mass transfer around carbon particles burning in fluidised beds

Abstract: A study is described of combustion of coke and char particles in fluidised beds of sand and of catalyst beads. The carbon particles studied ranged in size from 1.3 to 22 mm and the beds of sand were made of closely sized particles with average diameters of 0.22, 0.32, 0.46 and 0.99 mm. The beds were operated at pressures in the range 1 to 5 bar and at a temperature of 1223 K; under those conditions carbon combustion is «diffusion controlled». The experimental data obtained clearly support the mechanism of direct carbon oxidation to CO, followed by combustion of the monoxide in the gas phase. During combustion in beds of sand the monoxide formed seems to be consumed outside the concentration boundary layer for oxygen; in beds of active catalyst combustion of CO to CO 2 takes place close to the carbon surface. Mass transfer coefficients for diffusion of oxygen to burning particles are shown to be well predicted by equation (6) in the paper, which is based on the theory of Coelho and Guedes de Carvalho

The influence of gas and liquid axial dispersion on determination of kLa by dynamic method

Abstract: The dynamic pressure step methods, for measuring oxygen transfer rates in aerated, agitated vessels, is analysed from the point of view of error in k L a value. The errors incurred by the use of simple models (plug flow and perfect mixing) are estimated with respect to the two-phase axial dispersion model (this model is assumed to be an accurate representation of the gas and liquid mixing in equipment with a large height to diameter ratio). It is shown that the error in k L a value with the dynamic pressure step method is several times lower than that for the standard dynamic gassing out technique over a wide range of hydrodynamic conditions

Analogy between the modelling of pullout in solution spinning and the prediction of the vortex size in contraction flows

Abstract: Solutions of high molecular weight polymers in suitable solvents show a pronounced shear thinning behaviour combined with strong elastic effects and wall slippage During spinning, these solutions are pulled out of the spinneret at a certain draw ratio This process is studied After surveying the equations governing the spinning process, attention is focused on the position of the detachment point in dependence of the solution properties and the processing conditions

An application of the network-of-zones model to solids suspension in multiple impeller mixers

Abstract: A modified version of the two-dimensional network-of-zones model was applied to the case of solid suspensions stirred with multiple, radial impellers. The configuration investigated was characterized by a high aspect ratio (H/T=4); the tank was equipped with standard baffles and four Rushton turbines with an impeller to tank diameter ratio D/T=1/3

Rapid design algorithms for shell-and-tube and compact heat exchangers

Abstract: We present rapid design algorithms for the design of both shell-and-tube heat exchangers and compact heat exchangers. The principal features of these algorithms is that they make the full use of stream pressure drop a design objective. The algorithms presented only consider thermal design and consequently do not replace existing design programs

On the role of elasticity on mixing with a helical ribbon impeller

Abstract: Power consumption as well as mixing and circulation times have been studied in the laminar region (usually accepted to be defined by Re≤10 for helical ribbon impellers) using Newtonian and elastic constant viscosity fluids. Elasticity effects are negligible for Re 1.0. The impact of the elasticity level is discussed by considering several representations of the power data. A shift of the upper limit for the laminar region towards lower values of Re is suggested. No significant effect of elasticity is observed for mixing and circulation times. The role of rheological parameters like the Weissenberg number is also analysed

Effect of water vapour on the calcination of limestone and raw meal in a suspension reactor

Abstract: Samples of Cauldon limestone and Cauldon cement raw meal have been calcined in a suspension reactor simulating the dynamic and thermal conditions within a commercial precalciner. Batches of +40-90 μm diameter stone have been injected into a hot air stream, containing controlled levels of water vapour, at three temperature levels, viz. 1074, 1106 and 1126 K. Water vapour injected at the 2.22% level enhanced the conversion achieved in the calcination process, whereas injection at the 6.09% level produced a retarding effect

Some aspects of the practical application of pinch technology methods

Abstract: The theory of pinch technology is now well developed. However, the more recent methods often require complex computer programs not available to many users. This paper suggests alternative ways of applying the theory to practical situations. Current methods are briefly reviewed, and difficulties with stream data extraction, area and cost targeting and time-dependent situations (including batch processes) are highlighted. Practical short-cut solutions are proposed, based mainly on the straightforward Problem Table algorithm. Three worked examples are included: data extraction for a dryer, {Delta} T{sub min} optimisation in a topology trap with phase changes present, and start-up of a continuous process. (author).

Some factors determining the critical feed time of a semi-batch reactor

Abstract: The product distribution of fast reactions can be strongly influenced by mixing on various scales. When, for reasons of selectivity and safety, such reactions are conducted in a semi-batch reactor, the feed addition time t f , has to be specified. If it exceeds a critical value, t crit , product distribution becomes independent of t f and depends on micromixing. When, however, t f

Robust dynamic simulation of chemical engineering processes

Abstract: Modelling of chemical engineering processes operating under transient conditions often gives rise to mixed systems of differential and algebraic equations (DAEs). There are currently reliable and efficient numerical method for the solution of DAEs provided that the simulation does not depart too far from a region of normal operation. Outside this region, existing method for the solution of the DAEs may become inefficient, or even fail. We examine the changes that must be made to allow a DAE solver to overcome some of these difficulties. Particular problems that are considered are the problems of imposing physical bounds on the numerical solution, integrating partially determined systems and solving systems containing fast transients

Simulation of gas separation in an internally staged permeator

Abstract: Mathematical models have been developed for an internally staged permeator for the cases of cocurrent, countercurrent and perfect mixing conditions. The problem of oxygen enrichment from air in a cellulosic membrane has been considered in the simulation

N,O-Carboxymethyl Chitosan, A Water Soluble Derivative and Potential "Green" Food Preservative

Abstract: Carboxymethyl Chitosan (NOCC) was prepared from the reaction between chitosan and monochloroacetic acid under alkaline conditions. It has numerous desirable features. It is water soluble, derived from a naturally occurring polysaccharide and is biodegradable. NOCC provides a valuable use for crustacean shells e.g. crabs, lobsters and shrimps which are waste materials and pollutants in many parts of the world

Surface and rheological properties of zirconia suspensions

Abstract: The importance of surface chemistry on the rheological properties of concentrated zirconia suspension is investigated. The pH and nature of anions were found to have a very significant effect on the rheological properties of ZrO{2} suspensions. The variation in rheological properties with pH and anions correlated well with the change in surface properties of the Zr0{2} with pH and anion type and concentration. The isoelectric point (IEP) for monoclinic ZrO{2} determined in 0.001M sodium chloride solution was 6.7. At and near the IEP, ZrO{2} suspensions are highly flocculated and exhibited pronounced yield stress behaviour. At pH a few units either side of the IEP, the magnitude of the mobility is high and a low viscosity dispersed suspension is obtained. The ZrO{2} suspension was dispersed with KOH at high pH while hydrochloric or nitric acid can be used for dispersion at low pH. Sulphuric acid however does not disperse the ZrO{2} suspensions at low pH because of significant sulphate adsorption on the zirconia particle. At low pH, ZrO{2} is positively charged, sulphate adsorption reduces the magnitude of the surface charge which is observed as a low positive mobility. The results clearly show how the surface chemistry of ZrO{2} can be regulated to control the rheology.

Flow of granular material through vertical slots

Abstract: The relationship between the dimensions of a vertical slot and the mass flowrate of granular solids flowing through it has been investigated. Most experiments were with rape seed and casein. The solids were discharged from a rectangular vessel 165mm x 155mm through slots up to 400mm long and 70mm wide. Flow rates varied from less than 0.001 kg s{-1} to more than 2 kg s{-1} and were correlated with the product of an effective flow area and the square root of the hydraulic mean diameter of this area. For a specified material, the dead space corrections to slot length and width that gave a best fit of the experimental data had different numerical values. The weight of a discharge vessel, at conditions for which only part of the slot height was occupied by the solids flow, was found to correlate quite well with flow rate.

Designing a solvent for gas absorption

Abstract: We describe the molecular design of a solvent for removing carbon dioxide and hydrogen sulphide from gas mixtures. The design is arrived at by using recent methods of predicting the solubitity of gases in organic solvents from the functional groups of the solvent molecule. The predicted values of gas solubility and of the other relevant physical properties of the new solvent are then compared with experimentally measured values. Good agreement is found for gas solubitity. Two outline design studies are presented to compare the performance of the new solvent with that of solvents now used in practice. The work as a whole is intended to illustrate the potential and the limitations of the new theories of gas solubility, and to identify the need for future research

Effect of approach velocity on binary and interfacial coalescence

Abstract: A simple model is presented for the prediction of the variation with time in the area of the liquid film and its thickness during the approach of two equisized bubbles (or drops) or a bubble to an interface. The area of the film is found to increase with time until a maximum is reached, and then decrease as the bubble recovers its spherical shape, for given values of the initial approach velocity and external force. The area of the film, net force acting on the bubble and its velocity increase as the initial approach velocity or the applied force increases. However, the times required for the area of the film to attain its maximum and zero values increase with the applied force but decrease with the initial approach velocity. Equations are also derived for the variation in film thickness with time due to inertial and viscous drainage. Since the area is only finite for a short period of time, the initial drainage is always inertial but may become viscous under certain conditions. The model successfully predicts the experimentally measured variation in area of the film above an air bubble during its gravitational approach to a deformable air-water interface before coalescence. It also correctly predicts the observed reduction in coalescence time with decreasing initial approach velocity and allows for the possibility of bubbles rebounding from the interface. Observed coalescence times can be explained in terms of inertial drainage allowing for frictional losses at the surface of the film; viscous drainage is always too slow even when enhanced by surface mobility

The flexible pinch design method for heat exchanger networks: Pt. 1; Heuristic guidelines for free hand designs

Abstract: In the past decade, Pinch Technology designs have achieved significant cost and energy savings on processes worldwide. While Pinch Technology is particularly useful for obtaining energy and area targets, as well as giving valuable insights for beneficial process changes, it has been realised that the minimum approach temperature difference constraint used in the Pinch Design Method (PDM) is too rigid and inevitably limits the flexibility of the designer. In this work, the Flexible Pinch Design Method (FPDM) is outlined, in which the heat exchangers are not so constrained. Using a heuristic procedure, Fixed Energy Designs can be achieved with relatively simple networks, both for problems from the literature as well as realistic industrial case studies. (author).

Pressure swing adsorption in non-isothermal, non-equilibrium conditions : single adsorbate

Abstract: A two bed pressure swing adsorber has been modelled for a one adsorbate system operating non-isothermally and constrained by a non-linear, Langmuir type adsorption isotherm. Using 5A molecular sieves to separate ethane and carbon dioxide from nitrogen, it was found that a linear driving force equation gave predictions of concentrations in both adsorbed and fluid phases, and predictions of temperature that compared well with experimental values. Both the transient region of operation and cyclic steady-state have been considered