Showing papers in "Chemical Engineering & Technology in 1989"

A new model of the effect of stirring intensity on the rate of secondary nucleation

Abstract: Secondary nucleation in industrial crystallizers depends on both supersaturation and mechanical stress by stirring. Most models which consider mechanical stress assume that nucleation is proprotional to the energy transferred to the crystals during collisions. This is not based on any physical relationship and, in addition, the models do not satisfactorily reproduce the experimental results. Own model, based on the theory of Hertz/Huber, which accounts for the stress of the crystals caused by impact, gave better results. This well-known and proven theory allows the calculation of the volume abraded during collisions between crystals and stirrer or walls. Introducing a nucleate efficienncy, the effect of mechanical stress on the rate of secondary nucleation, due to stirring intensity and crystallizer size, can be determined.

Flow phenomena and local heat and mass transfer in corrugated passages

Abstract: En se basant sur une methode de mesure des transferts locaux de masse et de chaleur developpee dans une etude anterieure, une procedure a ete formulee pour la determination des transferts locaux de masse et de transfert dans des structures ondulees. En utilisant l'analogie existant entre les transferts de masse et de chaleur, cette technique permet la determination des distributions de transfert de chaleur dans chaque structure a resolution locale elevee

Coupled simulation of heat transfer and reaction in a steam reforming furnace

Abstract: For design and simulation of chemical process furnaces in accordance with present-day standards, the temperature distributions in the firebox and in the reactors must be generated simultaneously. This calls for coupling of the simulations of the firebox and of the reactor. For the calculation of heat transfer in the firebox, a generalized furnace simulations program, based on the zone method, is applied. Monte Carlo simulation techniques are used to determine the view factors. The location of the burners is explicitly accounted for. Absorption and emission of radiation by the flue gas are calculated by considering band contributions for carbon dioxide and water. For the simulation of the steam reforming reactors, a one-dimensional heterogeneous model, which accounts for the presence of intraparticle partial pressure gradients, is used. The simulated temperature distribution in the furnace is in excellent agreement with industrial results, as is the simulated product distribution.

Increase in viscosity and its influence on polymerization processes

Abstract: In the course of polymerization in homogeneous systems, the viscosity of the reaction mass increases by several orders of magnitude. The viscosity increase is affected by reaction conditions, concentration and properties of the formed polymer. Empirical correlations for homogeneous and heterogeneous polymerizations are given. Viscosity significantly affects the polymerization kinetics as well as heat, mass and momentum balances of the polymerization reactor. The influence of viscosity and its increase on conductive heat transfer, stirrer power input and cooling capacity, molecular diffusion and mass transfer coefficients, mixing time and residence time distribution in homogeneous and heterogeneous polymerizations in stirred tank and tubular reactors is reviewed.

Micromixing limits in an MSMPR crystallizer

Abstract: The effect of micromixing limits on a process of crystallization in an MSMPR crystallizer is studied with respect to power law growth and nucleation kinetics. Three limiting cases corresponding to maximum mixedness and complete segregation in an MSMPR crystallizer and plug flow configuration were analyzed for processes in which supersaturation is generated by conventional techniques. The sensitivity of these three limiting cases to the supersaturation generation term in each mode of operation was investigated using several numerical examples. The study demonstrates the effects of mixing on the overall crystallizer performance and, in particular, the enormous micromixing influence at high supersaturation generation rates. The difference in the product CSD arise from the variations of supersaturation profiles experienced by the elementary volumes throughout their sojourn. Characterization of mixing in a real crystallizer at some intermediate levels is emphasized with the aid of relevant industrial examples.

A study of gas and liquid distributions in structured packings

Abstract: Etude des ecoulements de liquide et gaz dans une colonne (diametre: 0,5 m) et leurs interactions pour differents types de garnissages structures (Sulzer, Mellapak 250 Y et 500 Y, Sulzer plastic BX, Julius Montz B1-250 et B5-450, Raschig Rapulak 250 YC). Discussion des resultats en fonction du facteur de maldistribution et du facteur d'ecoulement a la paroi

Evaporation rate and separation factor of molecular distillation in a falling film apparatus

Abstract: During molecular distillation in a falling film apparatus, temperature and concentration of the more volatile components undergo a marked decrease in axial and radial directions, due to the distillation in vacuo. The gradients produce changes in the rate of surface evaporation and separation efficiency along the flow path. A theoretical study of this problem is presented for binary mixtures. The respective differential equations for heat and mass transfer in the liquid film are solved numerically. As the results demonstrate, the aspect ratio of an apparatus should be large when a high separation is required and small when a high distillation rate is more important.

Fluid dynamic description of jet‐loop reactors in multiphase operation

Abstract: A superposition of simple physical equations allows a satisfactory, comprehensive fluid dynamic description of jet-loop reactors in multiphase operation up to a dispersed phase hold-up of 15%. In the case of higher concentrations, there are further interactions which are not yet included in the model description. Apart from the drag coefficient of the single-phase jet loop, information on the particle swarm behaviour, i.e. on the slip velocity of the dispersed phase is required. Explicit formulation of the influence of the relevant geometrical parameters permits a reliable scale-up without additional adaptation parameters.

Advances in modelling and design of multitubular fixed‐ bed reactors

Abstract: This paper presents a method mathematical modelling of cocurrent multitubular fixed-bed reactors. The method is adapted to the most widespread type of industrial reactor design, i.e. the system fitted with disk-and-doughnut baffles. The proposed model makes it possible to estimate the influence of non-uniformity of the heat carrier flow on the reactor characteristics, taking into account the coolant streams by-passing via the tube-to-baffle clearances. Based on the model, the effect of the operating parameters as well as of some more important geometrical parameters of the reactor on an exemplary highly exothermic process of partial benzene oxidation to maleic anhydride is discussed.

Thermally stabilized combustion

Abstract: Thermally stabilized combustion has a number of unique characteristics which permit the generation of steam or other forms of process energy from the heat of combustion of a gaseous or clean liquid fuel in remarkably compact, integrated apparatus while truly minimizing the concentrations of NOx, CO and unburnt fuel in the effluent. These characteristics, which have been identified by a long-range program of research, are described and the advantages and limitations of this process are discussed.

Venturi scrubber calculation and optimization

Abstract: Collection efficiency and pressure loss of a venturi scrubber can usually be determined from the calculation models of Barth and Calvert. However, this only leads to reasonable results if the droplets are uniformly distributed throughout each cross-section of the washing zone. In this work, a two-zone models is developed which allows the calculation of the venturi scrubber at operating conditions with a non-uniform droplet distribution. The scrubber geometry is also taken into account. A venturi scrubber was developed on the basis of this new model. Its energy consumption is significantly lower than the well-Known optimum characteristic of Wicke and Holzer. For ideal operating conditions, a new optimum characteristic is given which should be the aim of future optimizations.

Effect of transfer on droplet size in liquid‐liquid dispersions

Abstract: Mean droplet diameter of the dispersed phase in pulsed plate extraction columns was measured for binary and ternary systems. A change in the droplet size in the presence of solute, dependent on such factors as the direction of mass transfer, droplet size in the binary system, radio of diffusion coefficients, viscosity of the continuous phase, interfacial tension and driving force for extraction was observed. Two new empirical correlations of mean droplet sizes in mass transfer conditions are proposed. These correlations have been successfully applied to other extraction columns.

Measurement of aeration capacity of fermenters

Abstract: Important aspects of K1a measurement in agitated aerated vessels are briefly characterized from the standpoint of reliability of the measured data. An analysis of the shortcomings of dynamic methods for measuring the K1a is given. A new modification of the dynamic method is proposed, which excludes these inadequacies through a special experimental arrangement. This was verified by comparing the data with those obtained by an independent steady-state method. The new method can serve as a standard of reference, suitable for testing newly developed methods for K1a measurement. The initial response method was tested in this way in non-coalescent liquids, and shown to lead to errors of the order of hundreds of percent. K1a values obtained with the initial response method were found to depend strongly on the probe's location in the vessel.

Measurement of heat transfer coefficients in tubes by temperature oscillation analysis

Abstract: An experimental technique and evaluation method is described for the determination of local heat transfer coefficients in tubes or other ducts. By means of a rotary mixing valve, cold and warm fluid flows are mixed in order to generate an arbitrarily shaped but periodically oscillating inlet temperature profile at the test tube. The propagation of the fundamental harmonic oscillation from the fluid to the outer surface of the tube wall is calculated analytically. Comparison of fluid and wall oscillations yields the heat transfer coefficient to be measured. The inaccurate measurement of fluid bulk temperature in the centre of the cross-section is compensated by an additional correction. Experiments were carried out with turbulent water flow through a copper tube. Measured heat transfer coefficients were compared to values calculated using Hausen's equation and good agreement was obtained.

Heat transfer enhancement in fin‐plate heat exchangers by wing type vortex generators

Abstract: Heat transfer and fluid mechanical data were computed for laminar channel flows containing strong longitudinal vortex pairs. The strong vortices are generated by thin delta wings and delta winglet pairs of low aspect ratios and large angles of attack. These wings are attached to the channel walls. The data show that longitudinal vortices cause high local peaks in heat transfer and marked increases in the overall channel heat transfer. These increases occur over a wide region of channel wall, compared to the vortex generating wing area. The results are of special interest for compact heat exchangers. The heat transfer enhancement allows a considerable reduction in the heat transfer area which, in turn, reduces the manufacturing and operating costs.

Incorporation of gradient into random search optimization

Abstract: To improve the rate of convergence of random search optimization procedures, the incorporation of a gradient-oriented one-dimensional search is investigated. With this modification to the random search procedure based on uniform sampling and region contraction [1], convergence to within 0.01% of the global optimum was obtained substantially faster for typical chemical engineering problems. It was also found that the reliability of obtaining the global optimum was improved.

Advances in modelling and design of multitubular fixed-bed reactors. I: Description of interaction between reactants and coolant

Abstract: Article de synthese fournissant, a l'aide de tableaux, les correlations formees dans la litterature pour le transfert de chaleur (coefficient de transfert et conductivite thermique) et l'hydrodynamique (perte de charge) dans le reacteur

Reduction of energy requirements in distillation

Abstract: Distillation has been, up to now, the standard method for separating fluid mixtures in the process industry and the consensus is that it will remain so for years to come. The only disadvantage of distillation is its high energy requirement. In the following, it will be demonstrated on the example of an ideal ternary mixture, how the energy requirement of distillation process can be reduced. Very effective are some modifications of single columns, optimization of column sequences and direct or indirect column coupling. It will be shown that, just by column coupling, the energy requirement can be reduced to a third.

Selective hydrogenation: Kinetic investigation of 1, 3‐cyclo‐octadiene and cyclo‐octene hydrogenation

Abstract: The vapour phase hydrogenation of 1,3-cyclo-octadiene (COD) over Pd on alumina has been studied in the gas phase, using a recycle reactor. The dependences of COD hydrogenation rates and those of the consecutive cyclo-octene (COE) hydrogenation on the reaction conditions are well described by a model which takes into account mass transfer in the porous layer of palladium. The kinetics of the COE hydrogenation is of a Langmuir-Hinshelwood type. The hydrogenation of COD is of first order with respect to hydrogen. The reaction order with respect to COD is −0.15. If the catalyst surface is completely covered by COD, the hydrogenation of COE is totally suppressed. This indicates a much stronger adsorption of COD than of COE. Hydrogenation of COE can only occur in the innermost part of the pores when the transport of COD by diffusion is insufficient to supply the entire active surface of the catalyst. This happens if the ratio P/PCOD exceeds 2/φ2 with φ being the Thiele's modulus. This relationship permits a simple evaluation of the effective diffusion coefficient in the porous Pd-layer.

Continuous crystallization of potassium carbonate

Abstract: Continuous crystallization of potassium carbonate was investigated in a mixed suspension mixed product removal (MSMPR) crystallizer with 7.0 1 working volume. Highly polydispersed product was obtained, with x50 varying between 320 and 670 μm. Crystal growth rates can be described by the model of Abegg, Stevens and Larson (ASL) (size dependent crystal growth rate). Values of G0 = 8.6 × 10−9 to 3.6 × 10−8 m/s and B0/φ = 2.7 × 108 to 4.2 × 109 1/m3s were obtained for mT = 45 to 92 kg/m3 and e = 0.45 ± 0.05 W/kg. Nucleation kinetics at 27 °C can be described by the equation: B0/φ = kBφse0.73G02.5. Since G0 ∝ σ, convection and/or diffusion rather than surface integration are the crystal growth controlling mechanisms.

Design and analysis of a state observer for the temperature front of a retification column

Abstract: In order to achieve optimal energy-saving, load-dependent operation of rectification columns, development and implementation of model-based control concepts are necessary. This paper reports on the design and step-by-step testing of a state observer for the estimation of two state variables which cannot be measured directly, namely the vapour rate and the location of mass transfer zone (temperature front) in the stripping section of a production column. The procedure is described, and the relevant simulation and operation results are presented and discussed. Initial experience, gained when the concept was put into practical operation, is reported.

A new load cell for measuring normal and shear stresses

Abstract: Developpement d'un dispositif a cinq composantes permettant de determiner les contraintes normales et de cisaillement agissant sur les parois d'un silo. La cellule de charge peut etre utilisee pour mesurer simultanement la contrainte normale ainsi que l'intensite et la direction de la contrainte de cisaillement

Parameters influencing the mean particle size of a crystalline product

Abstract: Lors de la conception d'un cristalliseur, on doit tenir compte des proprietes physiques et chimiques de la solution et des solides aussi bien que de la geometrie, du type et des conditions operatoires du cristalliseur. La propriete la plus importante est la solubilite du systeme a cristalliser. Des solubilites elevees conduisent a des sursaturations relatives faibles et des dimensions moyennes de particules elevees et reciproquement. Des systemes a solubilite elevee peuvent etre sensibles a l'abrasion et l'attrition

Absorption process for the removal of organic solvents from exhaust air

Abstract: It is shown in this report that, under certain conditions, cleaning of exhaust air by absorption presents an interesting alternative to the existing methods. In particular, this process can be used with advantage for small quantities of exhaust air, high and strongly fluctuating solvent concentrations and solvent mixtures such as those occurring in batch operation. A washing agent was employed which had not been previously used for this purpose. Its properties, which were investigated in the laboratory, were confirmed during a 15-months' trial period in a semi-industrial plant. The process and plant were optimized for practical use. Through its option of solvent recovery from exhaust air, this physical absorption process contributes to environmental control.

Conversion of methanol to light olefins over zeolite H‐T

Abstract: The conversion of methanol over zeolite H-T was investigated in a fixed bed reactor. H-T was prepared from Na-T by ion exchange with HCl and NH4Cl solutions. The HCl solution caused appreciable dealumination. High yields of ethene were obtained with NH4Cl exchangeed zeolites Na-T with 45% decationization, high proprne yields with low HCl or NH4Cl Exchanged catalysts. Low methanol partial pressures and short residence times favoured the formation of lower olefins. Lowest coking rates were observed at a reaction temperature of 693 K. The catalytic activity decrased slowly with the number of reaction/regeneration cycles. The distribution of procucts > C2 could be described by the most probable distribution of Schulz and Flory.

Investigation of a wet dust scrubber with a pneumatic nozzle: Dust collection based on turbulent diffusion

Abstract: Performances d'un depoussiereur par voie humide pour l'epuration d'effluents gazeux d'industries chimiques, d'industries du fer et de l'acier et d'incinerateurs de dechets

Vapour‐liquid equilibria of hydrogen/coal liquid and methane/coal liquid systems at elevated temperatures and pressures

Abstract: A cubic equation of state is modified in such a way that prediction of PVT data from 40 model compounds, typical of coal oil, becomes possible with an absolute mean deviation of less than 2% for saturated liquid volumes and vapour pressures > 1 bar. Additional correlations for binary interaction parameters are obtained by an optimization procedure using vapour-liquid euilibrium (VLE) data from known heavy hydrocarbon liquid/light gas systems. When the modified equation is applied to coal-derived liquids, only specific gravity and boiling analysis data of the coal liquids are required, primarily in order to determine the equation-of-state parameters. The proposed equation is shown to allow a good prediction of VLE data for systems consisting of wide-boiling-range coal oils and light gases. Experimental values were obtained at elevated temperatures and pressures with a circulation flow apparatus.

On‐line measurement of partical size distribution and partical concentration in suspensions by ultrasonic spectrometry

Abstract: La spectrometrie ultrasonore se base sur la mesure de l'extinction ultrasonore due a la presence de particules. A partir des mesures a differentes frequences et de la connaissance de l'extinction ultrasonore des particules, un systeme d'equations lineaires peut etre etabli et resolu afin de determiner la distribution des dimensions de particules et la concentration des particules. Avec un appareil couvrant le domaine de frequence compris entre 1,7 MHz et 81 MHz, des particules de diametres compris entre 20 et 1000 μm peuvent etre analysees. Les mesures peuvent etre realisees avec des concentrations en particules jusqu'a 10% en volume

Simulation of primary and secondary reformers for improved energy performance of an ammonia plant

Abstract: The energy consumption of ammonia plants based on steam reforming of naphtha or natural gas can be reduced by shifting the reforming load from the primary to the secondary reformer. It is shown that lowering of the primary reformer operating temperature 20 °C results in 2% increase of unconverted methane content in the outlet stream. The increased methane content can be processed in the secondary reformer if the process air inlet temperature is increased to 400 °C. The lower operating temperature reduces the energy consumption of the primary reformer by about 6% (and the overall consumption by about 2%) and also prolongs the service life of reformer tubes.

Modelling and simulation of monopropellant hydrazine thrusters for spacecraft position control

Abstract: Modelling of the kinetics of a 20-N hydrazine thruster is carried out by considering two different heterogeneous reaction channels for hydrazine decomposition to the intermediate ammonia. The parameters of the reactor model are estimated by making use of temperature profiles at the wall of the decomposition chamber. Simulated ageing of the catalyst revealed an increase of the rate of deactivation with increasing total firing time