Showing papers in "Canadian Journal of Chemical Engineering in 1990"

The yield stress of fibre suspensions

Abstract: Yield stresses were determined for commercial wood pulp suspensions and synthetic fibre suspensions of low and medium mass concentration. The yield stresses measured represent interfibre failure of the network rather than failure between the suspension and a solid surface. The measurements were carried out in a rotary viscometer at low yield stresses and in a concentric rotary shear tester at yield stresses in excess of 2500 Pa. The experimental results were correlated with the volumetric concentration Cv in equations of the form τy = aCbv where a and b are constant for a given fibre type. It was found that b ≃ 3, in agreement with the predictions of a theoretical model of fibre network strength based upon the interlocking of elastically bent fibres. The dependence of the yield stress on the fibre properties of aspect ratio and modulus of elasticity was not adequately predicted by the model, suggesting that fibre bending alone did not account for the network strength over the concentration range studied.

The electrochemical oxidation of aqueous phenol at a glassy carbon electrode

Abstract: The electrooxidation of phenol is of interest as a model compound for the treatment of aqueous organic wastes. The effect of voltage, concentration and temperature on the electrochemical oxidation of acidic dilute aqueous solutions of phenol was studied. Electrolysis was carried out by recirculating phenol solutions through a flow-by electrochemical reactor employing a reticulated glassy carbon anode. Concentrations of phenol and some breakdown products were monitored using HPLC analysis. Increased voltage was found to shift the product distribution to favour more oxidized products but also to increase electrode corrosion and decrease current efficiency. Higher phenol concentrations (over the range of 5-20 mmol/L) showed a shift in product distribution to favour less oxidized, mostly insoluble products. Elevated temperatures (about 50°C and higher) showed a marked ability to reduce electrode passivation and increase the phenol oxidation rate.

Reaction kinetics over iron catalysts used for the fischer-tropsch synthesis

Abstract: The kinetics of the Fischer-Tropsch (FT) and water-gas shift (WGS) reactions were studied in a stirred tank slurry reactor, using a 100 Fe/0.3 Cu/0.2 K catalyst and a commercial catalyst, Ruhrchemie LP 33/81. The 100 Fe/0.3 Cu/0.2 K catalyst was more active than the Ruhrchemie catalyst for both reactions. FT kinetics which accounted for water inhibition was the best of the rate forms considered for the 100 Fe/0.3 Cu/0.2 K catalyst. First order in H2 kinetics was sufficient for the Ruhrchemie catalyst, probably due to the low conversions obtained during the test of this catalyst. WGS kinetics, which included product inhibition, were also studied for each catalyst.

Sparged loop reactors

Abstract: Circulating bubble columns or loop reactors form one of the important classes of modified bubble columns. The present paper analyses the performance of external loop air lift reactors (EL-ALR). The EL-ALR has many advantageous features especially at large scale. These arise from its feature of having controlled liquid circulation which is the key parameter for the design and operation of EL-ALR contactors. Therefore, a reliable and generalised circulation model has been presented. The reliability of the model depends on the accuracy of the predictive methods for the gas hold-up and the two-phase fractional pressure drop. Detailed analysis of these fundamental parameters of gas-liquid flows has been presented. The effects of design (area ratio of riser to downcomer, height to diameter ratio and volume of reactor) and operating (gas flow rates and sparging locations) parameters on the performance of the EL-ALR have been analysed in detail. A rational basis has been developed for the estimation of pressure drop, mixing time and mass transfer coefficient. An optimum combination of design and operating parameters has been suggested and a criterion has been developed for the optimum location of spargers. An attempt has been made to provide critical analysis of the published information and to construct a coherent picture of EL-ALR.

Mass transfer in external-loop airlift bioreactors using static mixers

Abstract: The influence of static mixers on the overall gas-liquid volumetric mass transfer coefficient (KLaL) was examined in an external-loop type airlift bioreactor (approximately 15 L volume, 1.8 m static liquid height, Ar/Ad = 0.444). The study was conducted with aqueous salt solution (0.15 kmol ṁ m−3 NaCl) and with pseudoplastic solutions of carboxymethyl cellulose (0.2 − 0.6 wt./vol. % (g/100 mL) CMC). Over a broad range of power law parameters K (10−3 − 10 Pa ṁ sn) and n (0.5 − 1.0), the presence of static mixers in the riser was found to enhance the KLaL relative to mixer-free mode of operation. The extent of increase in KLaL depended on the fluid “thickness”, K: the higher the K, the greater the KLaL intensification due to static mixers. For otherwise identical conditions, the presence of static mixers improved KLaL by 30-500%, depending on the fluid. The boost in KLaL was associated with increased gas holdup and gas-liquid interfacial area, which arose due to bubble breakup accomplished by the static mixing elements. Potential advantages of static mixers in upgrading the performance of oxygen-limited fermentations were pointed out. On a etudie l'influence des melangeurs statiques sur le coefficient de transfert de matiere volumetrique global gas-liquide (KLaL) dans un bioreacteur ascendant a boucle externe (environ 15 L de volume, 1,8 m de hauteur de liquide statique, Ar/Ad = 0,444). L'etude a ete menee avec une solution de sel aqueuse (0,15 kmol ṁ m−3 NaCl) et des solutions pseudo-plastiques de carboxymethyl cellulose (0,2 - 0,6 en poids/% volume (g/100 mL) CMC). Pour une large gamme de parametres de loi de puissance K (10−3 - 10 Pa ṁ sn) et n (0,5 - 1,0), on a trouve que la presence de melangeurs statiques dans la colonne montante ameliorait le KLaL par rapport au mode de fonctionnement sans melangeurs. L'importance de l'augmentation du KLaL depend de la consistance du fluide, K: plus K est grand, plus l'augmentation de KLaL par les melangeurs statiques est importante. Pour des conditions identiques, la presence de melangeurs statiques augmente KLaL de 30 a 500% selon le fluide. Cette importante augmentation de KLaL est associee a une plus grande retention de gaz et une plus grande surface interfaciale gaz-liquide qui sont dues a la rupture des bulles provoquee par les elements melangeurs. On souligne les avantages potentiels des melangeurs statiques dans l'amelioration de la performance des fermentations avec peu d'oxygene.

Flow regimes, hold–up and pressure drop for two phase flowin helical coils

Abstract: Experimental results on flow pattern, hold–up and pressure drop are presented for cocurrent upward and downward air water flow in helical coils. A tube of 0.01 m internal diameter was used and the ratio of coil to tube diameter was varied from 11 to 156.5. Water flow rate was varied from 4.9 × 10-6 m3/s to 92 × 10-6 m3/s while the range of gas flow rate covered was 83 × 10-6 m3/s to 610 × 10-6 m3/s. A new mechanistic approach is proposed to correlate pressure drop data in coils. The proposed model retains the identity of each phase and separately accounts for the effects of curvature and tube inclination resulting from the torsion of the tube. This makes it possible to use a single model to predict pressure drop for both upward and downward two–phase flow in coiled tubes. Required correlations for hold–up, interfacial friction factor and friction factors for individual phases are provided. On presente des resultats experimentaux sur le diagramme d'ecoulement, la retention et la perte de charge pour un ecoulement air eau a cocourant ascendant et descendant dans des serpentins helicoidaux. Un tube de 0,01 m de diametre interne a ete utilise et on a fait varier le rapport entre le serpentin et le diametre du tube entre 11 et 156,5. On a fait varier le debit d'eau de 4,9 × 10-6 m3/s a 92 × 10 6 m3/s a tandis que la gamme du debit du gaz etudiee variait de 83 × I0-6 m3/s a 610 × I0-6 m3/s. On propose une nouvelle approche mecanistique pour correler les donnees de perte de charge dans les serpentins. Le modele propose identifie chaque phase et tient compte separemment des effets de la courbure et de l'inclinaison du tube dues a la torsion du tube. Il est ainsi possible d'utiliser un modele simple pour predire la perte de charge a la fois pour l'ecoulement diphasique ascendant et descendant dans les tubes en spirales. On presente les correlations requises pour la retention, le facteur de friction interfaciale et les facteurs de friction pour chaque phase.

A correlation for predicting diffusion coefficients in alkanes

Abstract: A correlation was developed for predicting limiting mutual diffusion coefficients of gaseous and liquid solutes in normal alkane solvents with carbon numbers ranging from 7 to 28 at temperatures to 570 K. The functional form of the correlation is based on the rough hard sphere theory. The parameters needed in the correlation are the molecular weights and hard sphere diameters of the solute and solvent molecules and molar volume of the solvent. The data used in the development of the correlation included 143 measurements with solute/solvent molecular mass ratios ranging from 0.005 to 2.26 and size ratios ranging from 0.3 to 1.3. The average absolute error in predictions was only 6.3%. When the correlation was used to predict literature values for binary n-alkane systems, the average absolute error was 16.7%.

Rotary cylinders: Solid transport prediction by dimensional and rheological analysis

Abstract: A semi-experimental model for predicting the axial transport of the granular bed in a rotating cylinder is proposed. It is based on dimensional analysis and on the determination of an apparent viscosity characterizing the flow behaviour of the bed. Unknown constants in the model are determined either by analysis or by tuning with experimental data. An example of such tuning is shown to work well. The model is capable of giving as a function of filling angle and friction angle, the axial velocity that varies along the cylinder axis. This is important for the control of the process taking place inside the cylinder.

Gas entrainment rate and flow pattern of vertical plunging liquid jets

Abstract: For a vertical plunging liquid jet system using various liquids, the effects of the operating conditions including the nozzle length-to-diameter ratio on the gas entrainment rate Qg were evaluated experimentally. The differences in Qg were related to the changes in the jet shape before plunging and the velocity of the plunging liquid jet at the point where the gas sheath breaks up. Empirical relationships were also presented to predict the maximum depth Z of bubbles entrained by the diffusing jet. The changes of Z were further discussed in terms of the downward liquid velocity distribution in the submerged two-phase region, which depends mainly on the size of entrained bubbles.

Dilute acid hydrolysis of lignocellulosics: An application to medium consistency suspensions of hardwoods using a plug flow reactor

Abstract: A one-stage process for the high temperature rapid dilute acid hydrolysis of hardwoods, to produce simultaneously furfural and fermentable sugars, is described. A continuous plug flow reactor unit designed to hydrolyse medium consistency wood suspensions (up to 13% w/w in solids) has been operated at capacities up to 90 kg/h, with temperatures ranging from 190°C to 240°C and 20-120 s as residence times. The final products, mainly furfural and sugars, have been analyzed using high performance liquid chromatography. Yields of 15 kg of hexoses (30% of the theoretical potential) and 10 kg of furfural (65% of the theoretical potential) for 100 kg of sawdust (dry basis) have been obtained. Two pseudokinetic modeis were used to simulate the pentosan and hexosan acid hydrolysis reactions. A Flowtran Simulation has been conducted resulting in mass and energy balances for an eventual industrial unit. On decrit un processus monoetage pour l'hydrolyse rapide a l'acide dilue a haute temperature de bois feuillus, pour produire simultanement du furfural et des sucres fermentescibles. On a fait fonctionner un reacteur en continu a ecoulement piston concu pour l'hydrolyse de suspensions de bois de consistance moyenne (jusqu'a 13% en poids des solides) a des capacites allant jusqu'a 90 kg/h, avec des temperatures comprises entre 190 et 240°C et des temps de sejour de 20 a 120 s. Les produits finaux, principalement du furfural et des sucres, ont ete analyses par chromatographie liquide a haute performance. On a obtenu des rendements de 15 kg d'hexoses (30% du potentiel theorique) et de 10 kg de furfural (65% du potentiel theorique) pour 100 kg de sciure de bois (base seche). On a utilise deux modeles pseudo-cinetiques afin de simuler les reactions d'hydrolyse acide des pentosanes et des hexosanes. Une simulation par Flowtran a ete effectuee donnant des bilans massiques et d'energie pour une unite industrielle eventuelle.

Scale-up of liquid-liquid dispersions in stirred tanks

Abstract: A study of local drop size, drop size distribution and holdup fraction was conducted for liquid-liquid dispersions in three geometrically similar stirred tanks of standard configuration over a range of stirrer speeds and dispersed phase volume fractions. The tanks had diameter ratios 1:2:4, the smallest diameter being 11 cm. Two liquid-liquid systems were used: n-heptane-water and n-heptane containing different concentrations of di-(2-ethylhexyl) phosphoric acid dispersed in an aqueous sulphate solution. No significant local variations of the three parameters investigated were observed at constant stirrer speed and constant dispersed phase volume fraction. Measurements in the three tanks indicated that the rule of equal impeller tip speed provides the best scale-up criterion for equal interfacial areas per unit volume of dispersion. A correlation was also proposed for the Sauter mean diameter.

Effect on surface charges of bubbles and fine particles on air flotation process

Abstract: The adhesion of fine particles onto bubbles in flotation was studied on the basis of surface charge measurements of the bubbles and particles The surface charges of the bubbles were measured by the use of a micro-electropheresis apparatus devised in our previous study and the mechanism of the bubble charging was studied under various experimental conditions In distilled water, the bubbles were negatively charged and the iso-electric point appeared at pH = 25 The surface charges of the bubbles in the surfactant solution were determined by the surfactant molecules adsorbed at the surface and depend strongly on the values of pH The flotation efficiency of latex particles (0923 μm) was found to be strongly influenced by the surface charges of both the particles and the bubbles The force between the particle and the bubble was estimated from the observation of the particle attachment to the bubble surface, and a simple equation including the effects of the hydrodynamic and surface charge interactions was proposed to determine the floatability limit

Development of Mixing Rules for Predicting the Viscosity of Bitumen and Its Fractions Blended with Toluene

Abstract: A comprehensive model is presented for predicting the liquid-mixture viscosity of bitumen and its fractions diluted with toluene. The proposed model is based on the Walther two-parameter viscosity-temperature correlation and a simple viscosity mixing rule. New viscosity data at 22-100”C are presented for 14 blends of Cold Lake bitumen and its fractions with toluene. All of these data are well correlated by the Walther correlation with AAD < 2%. The predictions from a recently proposed simple mixing formula for toluene-bitumen blends viscosity are well within an order of magnitude of the data. The blend viscosity predictions are improved substantially by use of a temperature-dependent viscous interaction parameter. For predicting the viscosity of toluene-diluted whole bitumen, the bitumen is characterized both as a “black-oil” and as a mixture of five fractions or pseudocomponents. The calculation procedure for the viscosity of toluene-diluted bitumen is outlined and the two characterization approaches are evaluated. On presente un modkle complet pour pridire la viscositi liquide-melange pour un bitume et des fractions diluees avec du tolukne. Le modkle propose est base sur la correlation viscositk-temperature a deux paramktres de Walther et une rkgle de melange de viscosite simple. De nouvelles donnees de viscosite B des temperatures entre 22 et 100°C sont presentees pour 14 melanges de bitume de Cold Lake et des fractions diluees avec du toluene. Toutes ces donnees sont correlees par la correlation avec AAD < 2%. Les predictions venant d’une formule de melange simple rkcemment propsee pour la viscosite de melanges tolukne-bitume sont bien dans I’ordre de grandeur des donnees. On peut grandement amiliorer les predictions de viscosite du melange en utilisant un paramktre d’interaction visqueux dependant de la temperature. Pour predire la viscosite du bitume brut dilue avec du toluene, le bitume est caractirise comme une <

Gas-Liquid mass transfer in a three-phase fluidized bed with floating bubble breakers

Abstract: The effects of liquid and gas velocities, particle size and volume ratio of floating bubble breakers to solid particles (Vf/Vs) on both the volumetric mass transfer coefficient, kla, and the gas-liquid interfacial area, a, have been determined in three-phase fluidized beds with floating bubble breakers. Beds having a volume ratio (Vf/Vs) of about 0.15 showed a maximum increase in both kla and a of about 30% in comparison to that in the corresponding bed without floating bubble breakers. The volumetric mass transfer coefficient in three-phase fluidized beds with or without floating bubble breakers can be estimated from the surface renewal frequency of liquid microeddies and the particle size.

Three-dimensional turbulent flow in agitated vessels with a nonisotropic viscosity turbulence model

Abstract: On a resolu les equations de mouvement comprenant un modele non isotrope k-e et moyennees dans le temps pour etudier l'ecoulement tridimensionnel dans des reservoirs agites par une turbine. Les resultats ont ete valides par des mesures des trois composantes de vitesse a l'aide d'un anemometre a fil chaud et par des donnees publiees. Le modele de turbulence non isotrope considere l'effet de rotation et de courbure de la turbulence avec un terme turbulent du nombre de Richardson et tient compte des effets tridimensionnels importants par la non isotropie de la viscosite. De meme, on a trouve que le modele de turbulence isotrope k-e frequemment utilise ne convenait pas pour decrire cet ecoulement turbulent tridimensionnel

Oil recovery from oil in water emulsions using a flotation column

Abstract: A flotation column was used to recover oil from oil in water emulsions The feed oil concentrations investigated were relatively high, in the range of 025 to 8 percent by volume Previous studies using conventional flotation cells dealt with very dilute systems where the oil concentration was less than 01 percent by volume The oil recovery was found to decrease with an increase in the feed oil concentration The addition of wash water to the froth zone of the column had little effect on the oil recovery The oil recovery decreased with increasing feed flow rate and surfactant concentration The effect of increasing the gas flow rate was to initially increase the oil recovery The oil recovery data were analyzed using a kinetic model The order of the flotation kinetics was found to be 06 The gas hold-up behaviour of the recovery and the froth zones of the flotation column is found to correlate well with the drift-flux model On a utilise une colonne de flottation pour la recuperation d'huile a partir d'emulsions d'huile dans I'eau Les concentrations d'huile d'alimentation etudiees etaient relativement elevees, dans la gamme de 0,25 a 8 pour cent par volume Les etudes anterieures utilisant des cellules de flottation classiques utilisaient des systemes tres dilues avec une concentration d'huile inferieure a 0, l pour cent par volume On a trouve que la recuperation d'huile diminuait avec I'augmentation de la concentration d'huile d'alimentation L'ajout d'eau de lavage dans la zone de moussage de la colonne a peu d'effet sur la recuperation d'huile La recuperation d'huile diminue avec l'augmentation du debit d'alimentation et la concentration de surfactant L'effet de I'augmentation du debit du gaz est d'augmenter au depart la recuperation d'huile Les donnees de rCcuperation d'huile ont ete analysees avec un modele cinetique On a trouve que I'ordre de la cinttique etait de 0,6 On a trouve que le comportement de retention du gaz de la recuperation et les zones de rnoussage de la colonne de tlottation se correler de facon satisfaisante au modele de flux-derive

Characterization of hydrodynamic parameters in a multistage column contactor

Abstract: Accurate knowledge of hydrodynamic parameters is of major importance for the performance study of liquid-liquid column extractors. The effects of operating parameters on dispersed phase holdup profiles, drop size distributions, and axial mixing in both phases were investigated in a 127 mm diameter multistage contactor of pilot plant scale for the toluene-water physically equilibrated system. Correlations for the mean holdup, the mean drop size, and the continuous phase backmixing were obtained. A stronger dependence of holdup and drop size on the operating conditions and especially on the agitation speed was observed as compared to previous investigations for the same type of contactor. The axial mixing for the single phase flow was found to follow adequately an existing correlation, while the continuous-phase axial mixing in two-phase flows showed some deviations from other existing correlations. Also, flooding criteria, important for the control of the extraction process, were determined based on the shape of the holdup profiles. II est tres important de connaitre exactement les parametres hydrodynamiques pour I'etude de performance des extracteurs a colonne liquide-liquide. Les effets des parametres d'operation sur les profils de retention de la phase dispersee, les distributions de taille des particules et le melange axial dans les deux phases, ont ete etudies dans un contacteur pilote multietage de 127 mm de diametre pour le systeme equilibre physiquement toluene-eau. On a obtenu des correlations pour la retention moyenne, la taille moyenne des gouttes et le retromelange en phase continue. On a observe une forte dependance de la retention et de la taille des gouttes par rapport aux conditions d'operation et en particulier par rapport e la vitesse d'agitation comparativement e des recherches anterieures pour le meme type de contacteur. On a trouve que le melange axial pour I'ecoulement de la phase simple suivait bien une correlation existante, tandis que le melange axial en phase continue dans les ecoulements diphasiques montrent quelques ecarts avec les autres correlations existantes. En outre, on a determine des criteres d'engorgement, importants pour le controle du processus d'extraction, en se basant sur la forme des profils de retention.

Control of crystal size distribution in a batch cooling crystallizer

Abstract: A control scheme for crystal size distribution (CSD) in a batch crystallizer, based on indirect measurement of fines suspension density and manipulation of fines dissolving rate, is proposed and implemented on a 27 L laboratory batch cooling crystallizer using the potash alum-water system. The measured variable was a temperature difference related to the fines suspension density detected by a new fines sampling/suspension density measuring device proposed by Rohani and Paine (1987). Servo-control of the fines suspension density was achieved using a conventional PI control mode. Two different cooling policies, namely, linear cooling and isothermal operation were examined and improvement in the final CSD was observed in both cases. The weight-mean crystal size and the coefficient of variation of the end product showed a maximum improvement (larger mean size and smaller coefficient of variation) of 80% and 31% over the uncontrolled experiments, respectively. The weight fraction of fines (smaller than 150 μm) in the end product was decreased by a maximum of 99% over the uncontrolled run. Higher overall rates of fines dissolution led to a more uniform product with a larger weight-mean crystal size at the expense of a small reduction in the rate of solids make.

Experimental study of sawdust and coal particle mixing in sand or catalyst fluidized beds

Abstract: This paper presents an experimental study in order to establish mixing and segregation conditions of wood sawdust and coal particles in a fluidized bed of sand or alumina particles. Experiments were performed at room temperature under atmospheric pressure. Simple equipment was used to divide the bed into layers and useful results could be obtained in a short period of time. The influence of fluidization velocity, time of an experiment, size and concentration of sawdust or coal particles was investigated. Uniformity of mixing was found to increase with an increase in the fluidization velocity. For u/umf less than 2.5, strong segregation occurred between sawdust and sand particles. Determination of good mixing conditions will be useful in gasification studies of sawdust and coal.

A kinetic investigation of styrene/butyl acrylate copolymerization

Abstract: An experimental investigation of the kinetics of the bulk free radical copolymerization of styrene/butyl acrylate initiated with 2,2′-azobisisobutyronitrile (AIBN) was conducted at 50°C using various initiator concentrations and initial monomer ratios including the azeotropic composition. Reactivity ratios were determined by analyzing composition data using the error-in-variables model. The experiments, designed using the Tidwell-Mortimer criterion, were conducted in sealed ampoules at 50°C to low conversion levels (< 7%). From the calculated reactivity ratios, the azeotropic composition was determined and further experiments were run at these conditions as well as at two other initial monomer feed compositions in order to collect composition, rate and molecular weight data. The experiments were run through the full conversion range at two different initiator concentrations.

Devolatilization and combustion of large coal particles in a fluidized bed

Abstract: Devolatilization and combustion of large particles of Eastern Canadian coals (Evans and Minto), 5-50 mm dia., were studied in a bench-scale atmospheric fluidized bed reactor at 1023-1173 K with 0.5 mm sand particles as the bed material. The devolatilization time, mass loss history, changes in proximate volatiles content and C/H mass ratio, and temperature history at the centre of the particle during devolatilization were determined. The mass loss during devolatilization is correlated with the proximate volatiles content of the parent coal. The devolatilization time is correlated with the initial particle diameter by a power-law relation with an exponent of 1.54-1.64. The results show insignificant effect of superficial velocity on devolatilization. On a etudie la devolatilisation et la combustion de larges particules de charbon de I'Est du Canada (Evans et Minto), dc 5 a 50 mm de diametre, dans un reacteur a lit fluidise atmospherique de laboratoire, a des temperatures de 1023 a I173 K et avec des particules de sable de 0,5 mm de diamere comme materiau du lit. On a determine le temps de devolatilisation, l'histoire de la perte de masse, les changements dans la teneur des volatils voisins et le bilan massique C/H ainsi que I'histoire de la temperature au centre de la particule durant la devolatilisation. La perte de charge durant la dkvolatilisdtion est correlee a la teneur des volatils voisins d'un chdrbon apparente. Le temps de devolatilisation est correle au diametre initial des particules par une relation de loi de puissance avec un exposant de 134- I,64. Les resultats montrent un effet non significatif de la vitesse superficielle sur la devolatilisation.

Analysis of particle trajectories of small particles in flotation when the particles and bubbles are both charged

Abstract: On a analyse la trajectoire d'une petite particule se deplacant vers la surface d'une bulle en tenant compte des effets des charges de surface de la bulle et de la particule ainsi que de l'interaction hydrodynamique a courte distance pres de la surface de la bulle, dans un processus de flottation. L'efficacite de capture des particules, η S , par bulle, a ete estimee par l'analyse de la trajectoire et l'efficacite de la flottaison, η T , a ete calculee. La dependance du diametre de particule sur η T a egalement ete examinee par l'analyse de la trajectoire des particules

Unified entry length for newtonian and power–law fluids in laminar pipe flow

Abstract: A numerical method based on finite differencing is used for investigating the steady–state entrance region laminar flow of incompressible Newtonian and power–law fluids in a circular pipe. The Solution method is validated by comparing the results for Newtonian fluids with those reported in the literature. For power–law fluids, the entry length results are compared with other approximate solutions in the literature. On the basis of the calculated results, a generalized entry length ξ99 = 0.056 is shown to be valid for the laminar flow at Re > 200 of both Newtonian and power–law fluids with 0.75 < n < 1.5. On a utilise une methode numerique basee sur les differences finies pour etudier l'ecoulement laminaire permanent dans la region d'entree pour des fluides incompressibles newtoniens et de loi de puissance dans un tuyau. La methode de resolution est validee en comparant les resultats obtenus pour les fluides newtoniens a ceux indiques dans la litterature. Pour les fluides de loi de puissance, les resultats sur la longueur d'entree sont compares a d'autres resolutions approximatives publiees. En s'appuyant sur les resultats calcules, on montre que la longueur d'entree generalisee ξ99 = 0,056 est valide pour l'ecoulement laminaire tant des fluides newtoniens ( Re > 200) que des fluides de loi de puissance si 0,75 < n < 1,5.

Modelling of equilibrium solubility of CO2 and H2S in aqueous diglycolamine (DGA) solutions

Abstract: Information on acid gas solubility in solvents utilized is needed for the design of gas plants. A mathematical model for the prediction of equilibrium solubility of CO2 and H2S in aqueous 2-amino-2-(ethoxy)ethanol (DGA) solutions is presented. The equilibrium constants, K1 and K2, representing a simple proton transfer reaction and the carbamate formation reaction, respectively, were found to be functions of temperature and free acid gas concentration. In addition, K2, was affected by DGA concentration as well. Model predictions agree favourably with experimental data. Des donnees sur la solubilite des gaz acides dans les solvants sont necessaires pour la conception des usines a gaz. On presente un modele mathematique pour la prediction de la solubilite d'equilibre du CO2 et du H5 dans des solutions aqueuses de 2-amino-2(ethoxy)ethanol (DGA). On a trouve que les constantes d'equilibre, K1 et K2, representant une reaction de transfert de protons simples et une reaction de formation de carbamate, respectivement, etaient des fonctions de la temperature et de la concentration de gaz acide libre. De plus, la concentration de DGA a influe sur K2. Les predictions du modele concordent favorablement avec les donnees experimentales.

A simplified approach to the operation of a batch crystallizer

Abstract: A general model for a seeded cooling batch crystallizer based on first principles is developed, incorporating either size-dependent or size-independent growth rates. A simple approach is proposed to obtain temperature-time trajectories at constant supersaturation or nucleation rate, without resorting to optimization techniques. Cooling curves at constant supersaturation, which lead to a substantial improvement (a smaller coefficient of variation and a larger mean size) of the terminal crystal size distribution, can be determined even in the absence of precise nucleation and growth kinetics, whereas properties related to the crystal size distribution are sensitive to such kinetics. Experimental results for the potassium sulfate-water system, potash alum-water system, and hexamethylenetetramine in ethanol, methanol, and 2-propanol/water are predicted reasonably well by the model. Extension to any type of batch crystallization with super-saturation induced by means other than cooling, such as reactive precipitation and salting out, is briefly discussed. On a mis au point un modele general pour un cristalliseur discontinu inocule a refroidissement base sur les premiers principes, qui utilise des vitesses de croissance dependant ou non de la taille. On propose une methode simple pour obtenir des trajectoires temps-temperature a sursaturation ou a vitesse de nucleation constantes, sans faire appel a des techniques d'optimisation. Des courbes de refroidissement a sursaturation constante, qui entra ine une amelioration substantielle (coefficient de variation plus petit et taille moyenne plus grande) de la distribution terminale de taille des cristaux, peuvent etre determinees meme en l'absence de nucleation precise et de cinetique de croissance, alors que les proprietes liees a la distribution de taille des cristaux sont sensibles a ces cinetiques. Les resultats experimentaux pour le systeme sulfate de potassium-eau, alun de potasse-eau et hexamethylenetetramine dans l'ethanol, le methanol et le 2-propanol-eau sont predits raisonnablement bien par le modele. On analyse brievement la possibilite d'etendre le modele a n'importe quelle methode de cristallisation discontinue ou la sursaturation est provoquee autrement que par refroidissement, comme la precipitation reactive et le desalaee.

Preparation of hemolysate-filled hexamethylene sebacamide microcapsules with controlled diameter

Abstract: Hemolysate-filled hexamethylene sebacamide microcapsules (nylon-6, 10 membranes) were prepared by interfacial polymerization. Sheet lattice, frame lattice, turbine and marine impellers were tested to control the microcapsule mean diameter and size distribution parameters. Mean diameters ranging from 40 to 200 μm were obtained depending on the impeller type, rotational speed and emulsifier concentration. The size dispersion of microcapsule preparations was best controlled with a frame lattice impeller, achieving standard deviation values less than 40% of the mean diameter. The wire diameter and mesh dimensions of the impeller lattice had little effect on the distribution parameters. Des microcapsules de sebacamide d'hexamethylene remplies d'hemolysat (membranes de nylon de 6.10) ont ete preparees par polymerisation interfaciale. On a teste des treillis en feuilles, des treilles encadre, des turbines et des helices marines pour controler le diametre moyen des microcapsules et les parametres de distribution de taille. Des diametres moyens compris entre 40 et 200 μm ont ete obtenus en fonction du type de mobile. de la vitesse de rotation et de la concentration d'emulsifiant. La dispersion de taille de preparations de microcapsules etait mieux controlee avec un mobile a treillis encadre, les valeurs d'ecart-type obtenues etant inferieures a 40 % du diametre moyen. Le diametre du fil et les dimensions de la maille du treillis du mobile avait peu d'effet sur les parametres de distribution.

Rheology of oil in water emulsions with added solids

Abstract: The effect of added solids on the rheology of oil in water emulsions was investigated. The range of the oil concentration, solids free basis, was (0-70%) and the solids volume fraction was (0-0.16). The solids mean diameter was 45 μm and it was about four times larger than the oil droplets. In the absence of added solids, non-Newtonian behaviour was observed for oil concentrations above 40%. The added solids increased the emulsion viscosity in a manner similar to the addition of solids to a homogeneous fluid. The rheological data of all the emulsion-solids mixtures investigated were correlated as relative viscosity versus solids volume fraction, where the relative viscosity is defined as the ratio of the emulsion-solids mixture viscosity to the solids-free emulsion viscosity. In the case of non-Newtonian systems, the emulsion-solids mixture viscosity and the solids-free emulsion viscosity were calculated at the same shear stress. The Barnea and Mizrahi viscosity correlation was found to fit the data well. On a etudie l'effet de l'addition de solides sur la rheologie de l'huile dans des emulsions aqueuses. Les concentrations d'huile, sur une base libre de solids, vont de 0 a 70% et la fraction volumique des solides est comprise entre 0 a 0,16. Le diametre moyen des solides est de 45 μm et est environ quatre fois plus grand que celui des gouttelettes d'huile. Sans addition de solides, on observe un comportement non newtonien pour toutes les concentrations d'huile superieures a 40%. L'addition de solides augmente la viscosite de l'emulsion de la meme maniere que si on ajoute des solides a un fluide homogene. Les donnees rheologiques de tous les melanges emulsion-solides etudies ont ete correlees sous forme d'une relation viscosite relative-fraction volumique des solides, la viscosite relative etant definie comme le rapport de la viscosite des melanges emulsion-solides sur la viscosite de l'emulsion libre de solides. Dans le cas de systemes non newtoniens, la viscosite des melanges emulsion-solides et la viscosite de l'emulsion libre de solides ont ete calculees a la meme contrainte de cisaillement. On a trouve que la correlation de viscosite de Barnea et Mizrahi calait bien les donnees.

Continuous crystallization of potash alum: MSMPR kinetics

Abstract: The crystallization kinetics of potash alum (aluminium potassium sulphate) from aqueous and aqueous acetone solutions respectively in a continuous mixed-suspension, mixed-product-removal (MSMPR) crystallizer (capacity 4 L) are reported. Growth and nucleation rates are determined by direct model estimation via the population balance analysis of crystal size distribution measured using laser light scattering and sieve analysis. The overall growth rate during drowning-out precipitation strongly decreases with increasing acetone concentration in the crystallizing solution. Nucleation rates evaluated at zero-size, however, are virtually unaffected while the “effective” nucleation rates are strongly increased by the presence of acetone with a reduction in the supersaturation order. Magma density dependence is approximately first order during cooling crystallization while a somewhat lower dependence is detected during drowning-out precipitation. On presente les cinetiques de cristallisation de l'alun de potasse (sulfate de potassium et d'aluminium) a partir de solutions aqueuses et de solutions d'acetone aqueuses, respectivement, dans un critalliseur (capacite de 4 L) mixte continu a suspension et a extraction de produit (MSMPR). Les taux de croissance et les vitesses de nucleation sont determines par estimation directe du modele; on effectue a cette tin l'analyse de bilans de population de la distribution de taille des cristaux par diffusion de lumiere laser et par analyse granulometrique. Le taux de croissance global lors de la precipitation par noyage diminue fortement avec l'augmentation de la concentration d'acetone dans la solution cristallisante. Cependant, les vitesses de nucleation evaluees a la taille zero sont peu affectees, tandis que les vitesses de nucleation efficaces augmentent fortement en presence d'acetone avec une reduction de l'ordre de sursaturation. La dependance de la densite de magma est environ du premier ordre durant la cristallisation de refroidissement, bien qu'une dependance un peu plus faible soit observee durant la precipitation par noyage.

Experimental investigation of some hydrodynamic factors affecting slurry pipeline wall erosion

Abstract: Using a non-intrusive technique, particle velocities and concentrations have been measured as functions of position on the pipe wall for water slurries of two different sands. The experimental conditions correspond to both low (fine particles) and high contact load flows. Although the velocity and concentration distributions for the two sands were quite different, flux distributions computed from the product of velocity and concentration were similar. The flux distributions are used to interpret wall erosion distributions for sands of these sizes. In the case of the coarser sand, there is evidence that the contact load mechanism contributes significantly to abrasive wear. In this case, scaleup of erosion test results may be possible, using concepts provided by mechanistic flow models. For the finer sand, wall erosion rates appear to be reduced by a lubrication force. On a mesure par une technique non intrusive des vitesses et des concentrations de particules en fonction de leur position a la paroi du tuyau pour des boues aqueuses de deux sables differents. Les conditions experimentales correspondent a un ecoulement de charge de contact faible (particules fines) et eleve. Bien que les distributions de vitesse et de concentration pour les deux sables soient assez differentes, les distributions de flux calculees a partir du produit de la vitesse et de la concentration sont analogues. Les distributions de flux sont utilisees pour interpreter les distributions d'erosion des parois pour des sables de taille comparable. Dans le cas du sable grossier, on a pu verifier que le mecanisme de charge de contact contribue de maniere significative a l'usure abrasive. Dans ce cas, la mise a l'echelle de resultats d'essais d'erosion peut etre possible, en faisant appel a des concepts fournis par les modeles d'ecoulement mecanistiques. Pour le sable fin, les vitesses d'erosion des parois semblent etre reduites par la lubrification.

Influence of particle size and particle/fluid combination on particulate fouling in heat exchangers

Abstract: The influence of particle size on particulate fouling rates during convective sensible heat transfer to liquids has been investigated. Experiments have been carried out using aluminum oxide particles with nominal diameters ranging from 0.06 μm to 15 μm, suspended in an organic solvent for a wide range of concentrations. The observed influence of particle size on the asymptotic fouling resistance is compared with the predictions of the Watkinson–Epstein model. The results of this comparison suggest that, in addition to the changes of mass transfer coefficient with increasing particle size, reduced adhesion forces between particles and wall and increasing removal forces have to be considered as well. Additionally, the influence of different particle/fluid combinations was investigated. Experiments were performed with aluminum oxide particles suspended in isopropanol, isobutanol or water, and with kaolin particles suspended in water. On a etudie l'influence de la taille des particules sur les vitesses d'encrassement durant le transfert de chaleur sensible convectif vers les liquides. Des experiences ont ete menees avec des particules d'oxyde d'aluminium de diametres nominaux compris entre 0,06 et 15 jum en suspension dans un solvant organique pour une large gamme de concentrations. L'influence observee de la taille des particules sur la resistance d'encrassement asymptotique est comparee aux predictions du modele de Watkinson—Epstein. Les resultats de cette comparaison suggerent que, en plus des changements du coefficient de transfert de matiere avec une plus grande taille de particules, on doit considerer egalement une diminution des forces d'adhesion entre les particules et la paroi et une augmentation des forces de retrait. Par ailleurs, on a etudie l'influence de differentes combinaisons particules/fluides. Les experiences ont ete menees avec des particules d'oxyde d'aluminium en suspension dans de l'isopropanol, de l'isobutanol ou de l'eau, ainsi que des particules de kaolin en suspension dans de l'eau.