Showing papers in "Chemical Engineering & Technology in 1991"

Thermally Safe Operation of a Semibatch Reactor for Liquid-Liquid Reactions - Fast Reactions

Abstract: Accumulation of the reactant supplied to a cooled semibatch reactor (SBR) will occur if the mass transfer rate across the interface is insufficient to keep pace with the supply rate. Then, due to a low starting temperature or supercooling, the reaction temperature does not rise fast enough to the desired value. This accumulation may eventually lead to a temperature runaway. We investigated the possibility of such an event for reactions of the type "chemically enhanced mass transfer" or "fast" and found that only low distribution coefficients, i.e. 10-4 or lower, can lead to accumulation. At higher distribution coefficients, the mass transfer rate across the interface of a well-mixed dispersion is generally sufficient to prevent accumulation. A thermal runaway in the fast regime exerts a moderate effect, because the effective activation energy is halved. Calculations for the instantaneous reaction regime, regarded as a special case of fast reactions, show that there is no runaway possible.

Modelling of pressure drop in packed columns

Abstract: The correct choice of packing is of decisive importance for optimum process efficiency in the operation of two-phase countercurrent columns. An important criterion for this choice is the pressure drop in the gas flow. Theoretical relationships are derived for calculating the pressure drop in beds with dry and trickle packings. It has been demonstrated by comprehensive experiments that these relationships allow the pressure drop to be determined more accurately than by previous methods. The experiments were performed at the Department of Thermal Separation Processes of Bochum University on 54 different packed beds, using 24 different systems.

Studies of drop break‐up in liquid‐liquid systems in a rotating disc contactor. Part I: Conditions of no mass transfer

Abstract: Observations on the break-up of single drops in a rotating disc contactor show that there is a critical rotor speed below which drops of a given size do not break. Weber and Reynolds numbers are required to correlate the data and not simply a Weber number as for turbine agitated systems. The probability of break-up of a drop at the rotor edge is expressed in terms of a Weber number with a lower limit for critical conditions. The mean number of daughter drops produced on break-up is correlated as a simple function of drop diameter based on the critical diameter and agreement is found with data from other types of agitated equipment.

Hydrodynamics in a pressurized cocurrent gas‐liquid trickle‐bed reactor

Abstract: The influence of gas density on total external liquid hold-up, pressure drop and gas-liquid interfacial area, under trickle-flow conditions, and the transition to pulse flow have been investigated with nitrogen or helium as the gas phase up to 7.5 MPa. It is concluded that the hydrodynamics depends on the gas density and not on the reactor pressure. At higher gas densities, the operating region for trickle flow becomes larger while the gas-liquid interfacial area increases only slightly. Pressure gradient and liquid hold-up are considerably affected by gas density and correlations have been derived for these parameters.

Coke removal from a zeolite catalyst by supercritical fluids

Abstract: In the study of the possible coke removal from zeolite catalysts, the solubility of polyaromates in supercritical fluids (SCF) was determined, showing that it becomes drastically reduced with increasing molar mass. Experiments with supercritical ethyl benzene and benzene showed that only coke precursors can be dissolved and, therefore, complete coke removal by SCF is impossible but it has been confirmed that, under supercritical conditions, deactivation of the catalyst could be slowed down.

Effect of mixing behaviour on gas‐liquid mass transfer in highly viscous, stirred non‐newtonian liquids

Abstract: On etudie l'effet du melangeage sur le transfert de masse gaz-liquide dans des solutions aqueuses de xanthane et de polyacrylamide agitees dans un reacteur de 50 l (diametre=0,4 m)

Modelling of selective catalytic denox reactors – strategy for replacing deactivated catalyst elements

Abstract: A simulation model for denox reactors (high dust) containing honeycomb catalysts and operating at temperatures between 300 and 400 °C has been developed. The model predicts the NOx reduction as a function of process variables (temperature, gas velocity, NOx concentration, NH3/NOx ratio) as well as a function of reactor and catalyst geometry (pitch diameter, length of catalyst layer, number of layers). Model parameters such as the rate constant and the adsorption equilibrium constant of NH3 on the catalyst surface were determined by curve fitting of experimental data obtained in a pilot test unit. Based on model calculations, it is shown that the effect of catalyst poisoning (deactivation) on NOx reduction is not the same for every catalyst layer. The lifetimes of catalyst elements located at the inlet to the reactor appear shorter than those of the elements positioned at the outlet from the reactor. Experimental verification of this theoretical finding is required.

Two-phase flow and mass transfer in scrubbers†

Abstract: Experimental investigations were carried out in spray scrubbers of different sizes with cocurrent flow of gas and liquid. Of special interest were the local processes in the mass transfer zone. The scrubber was operated with warm water/air system (cooling tower) to obtain detailed information about mass transfer. Air is being humidified with water vapour, which in turn leads to a temperature drop in the liquid. The liquid temperatures are relatively easy to measure and are shown as liquid isotherms. In the case of plug flow, the liquid isotherms should be straight horizontal lines. In reality, significant deviations from plug flow are caused by the transfer of liquid to the walls. A large part of the liquid forms a film flow at the wall. Furthermore, nearly all the mass transfer is completed in the zone of liquid atomization immediately beneath the nozzle. The number of measured transfer units was between 0.5 and 2.0 and was significantly influenced by the liquid flow rate. Based on the improved knowledge of the proceses inside the scrubber, a simplified model has been developed. Since the model simulates all the essential processes inside the mass transfer zone, liquid distribution and mass transfer efficiency can be approximately predicted.

Fluid mechanics of circulating fluidized beds

Abstract: On developpe un modele de mecanique des fluides pour un ecoulement gaz-solides vertical. A l'aide de ce modele, des equilibres de masse et de force sont determines et un diagramme adimensionnel d'etat et de perte de charge est calcule. On discute le comportement operatoire d'un lit fluidise circulant avec un siphon dans le trop-plein de solides

Role of sparger design in mechanically agitated gas‐liquid reactors. Part I: Power consumption

Abstract: Power consumption (Part I) and liquid phase mixing time (Part II) were measured in 0.57, 1.0 and 1.5 m i.d. vessels. A pitched blade downflow impeller (PTD) was used. Design details of the PTD impeller such as diameter (0.22T to 0.5T), blade width (0.25D to 0.4D) and blade thickness (2.8, 4.3 and 6.4 mm) were studied. The effect of sparger type and geometry on power consumption has been investigated. For this purpose, pipe, ring, conical and concentric ring sparger were employed. Design details of the ring sparger such as ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important parameter and was therefore varied for nearly all the spargers studied. A correlation for the power consumption has been developed.

A New Pneumatic Bearing Dynamometer for Power Input Measurement in Stirred Tanks

Abstract: A turntable dynamometer has been constructed for the accurate measurement of power input and mixing applications in bench stirred tank reactors. The main feature of this device is a pneumatic bearing with complementary conical parts. The conical pneumatic bearing permitted to apply eccentric loads without affecting its stability. The static friction torque in the pneumatic bearing was very small, 4 x Nm, and can be neglected in the experimental ranges of measured torques, i.e. from 5 x to 2.21 Nm. In accordance with the instrumentation used, the deviations obtained with the apparatus are less than 10% at low torque readings. At moderate torques, deviations lower than 1 % are routinely obtained. Several power input measurements show that the obtained data scatter is lower than 2.5 % . The power input response in the turbulent regime is in agreement with dimensional analysis: the power input depends on the cube of the impeller speed. In addition, data obtained with a turbine impeller under ungassed conditions agree with the predictions of a published correlation, which takes into account several geometrical parameters.

Dynamic behaviour and simulation of a liquid‐liquid extraction column

Abstract: Dynamic behaviour of a stirred liquid-liquid extraction column was studied experimentally. Various input variables of the column were varied stepwise and the resulting variations in the system and output variables were measured. In addition to experimental work, a computer model was developed on the basis of the dispersion model to simulate the dynamic behaviour of the extractor. This model forms a component program of the dynamic process simulator DIVA, developed at the TU Stuttgart. The experiments showed that the hydrodynamic parameters exhibit no significant dynamic behaviour of their own. Therefore, changes occurring in these parameters closely follow variations in input and system variables. As a result, steady-state relationships for the calculation of flow parameters could be used in the simulation program. The simulator satisfactorily reproduced the experimental results for a number of disturbances. However, this was not always the case. As shown in the following, the model did not take into account the column level controller which, under certain conditions, exerts a very strong influence on the column's dynamic behaviour. As a result, larger differences occurred between experimental and simulated data. This influence on the extractor's dynamic behaviour can, however, be eliminated by a simple modification of the level controller arrangement.

Studies of drop break‐up in liquid‐liquid systems in a rotating disc contactor. Part II: Effects of mass transfer and scale‐up

Abstract: The number fraction of drops of a given size which break up at rotor level in a rotating disc contactor has been observed during mass transfer in either direction to or from solvent or aqueous drops. Critical rotor speeds for a given drop size undergoing mass transfer can be used to find an effective interfacial tension. Using this interfacial tension value, the break-up fractions are correlated within experimental uncertainties in the same manner as for no mass transfer. Drop break-up fractions depend on column size and relevant empirical correlations of the data are presented. The results may be used to estimate the effect of mass transfer on drop size distributions in an RDC.

Impact of particle size dispersity on thermal conductivity of packed beds: Measurement, numerical simulation, prediction

Abstract: The influence of particle size dispersity on thermal conductivity of packed beds without fluid flow is investigated. For this purpose, new experimental data for bi- and polydispersed packings are presented and data from the literature reviewed. All data are tabulated. The results of numerical calculations carried out for regular arrangements of spheres are also discussed. The impact of particle size dispersity on thermal conductivity is shown to be primarily due to the change in bed porosity. Thus, prediction can be carried out in the same way as for monodispersed packings if based on the actual bed porosity. A model developed by Schlunder and co-workers is revised and recommended for practical application.

Non‐steady‐state catalytic decontamination of waste gases

Abstract: The non-steady-state behaviour of a fixed bed reactor controlled by periodical direction reversal of reactant feed, applied in catalytic oxidation of industrial waste gases, containing organic compounds and carbon monoxide, has been investigated. The effects of the type of oxidized compound, its initial feed concentration, linear gas velocity, inerts-to-catalyst ratio and inerts thermophysical characteristics on the formation of reactor concentration and temperature fields were elucidated. It was shown that autothermal reactor behaviour is guaranteed by concentrations of the oxidized component which ensure an adiabatic temperature increase in excess of 20 °C. Deviations of either flow rate or initial concentration of the oxidized compound do not disturb the operational stability.

Role of sparger design in mechanically agitated gas‐liquid reactors. Part II: Liquid phase mixing

Abstract: Liquid phase mixing time was measured in 0.57, 1.0 and 1.5 m i.d. mechanically agitated gas-liquid reactors. Transient conductivity technique was used for the mixing time measurement. Pitched blade downflow turbine was employed. The design details of PTD impellers such as diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.35 D) were studied. The influence of sparger types and their design on mixing time has been investigated. For this purpose, pipe, ring, conical, and concentric ring spargers were employed. The design details of the ring sparger, i.e. ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important variable and, therefore, it was varied for practically all the spargers studied in this work. It was found that the liquid phase mixing time depends on the impeller design, sparger design, sparger location, impeller speed and superficial gas velocity. Correlations have been developed for the dimensionless mixing time.

Electrochemical impedance spectroscopy of oxygen and hydrogen evolution on amorphous alloys in 1 M KOH

Abstract: The mechanisms of oxygen and hydrogen evolution on amorphous alloys G 14 (Fe60Co20Si10B10) and G 16 (Co50Ni25Si15B10) in 1 M KOH at T = 298 K and 333 K were studied by electrochemical impedance spectroscopy (EIS) Comparative measurements were carried out on polycrystalline Pt electrodes Impedance spectra in the frequency range 10−3 Hz ≤ f ≤ 104 Hz were analyzed to determine the kinetic behaviour of amorphous alloys by application of transfer function analysis, using non-linear fit routines The EIS-data are interpreted in terms of consecutive reaction mechanisms for both oxygen and hydrogen evolution

Simulation of fluidized‐bed reactors for catalytic oxidative coupling of methane to C2‐hydrocarbons

Abstract: The “bubble assemblage model” of Kato and Wen was applied to simulate the catalytic oxidative coupling of methane to C2-hydrocarbons in a fluidized bed reactor. Simulation results were compared to experimental data obtained in a laboratory-scale fluidized bed reactor. To improve the accuracy of predictions, the influence of fluid bed hydrodynamic and kinetic submodels was investigated by applying a sensitivity analysis. It was shown that the most important element in the model is the applied reaction scheme; the consecutive reactions of C2 hydrocarbons occurring most probably in the gas phase should be considered.

Particle structure change of CaO during high temperature sulphatization

Abstract: The effects of particle size (layer thickness), BET surface area and pore size distribution on the reactivity of CaO derived from limestone were studied by using a pH-stat and the BET method. Visual analysis was obtained with SEM, and X-ray diffraction was used to determine the crystalline components present. It was found that the pore size distribution, including pore volume and pore surface distribution, affected the sulphatization behaviour. The larger pores (D > 50 A) with their pore volume and pore surface were to some extent responsible for both initial reactivity and ultimate capacity of CaO.

Fluidized bed dryers: Dynamic modelling and control

Abstract: Fluidized bed drying finds important applications in the chemical industry on account of the following advantages: (a) rapid exchange of heat and mass between drying media and particles yields the desired product quality and reduces the overall drying time and (b) easy handling of feed and product. There is much scope for improving the existing modelling techniques as applied to predicting the performance characteristics of the dryers. This should provide a sound basis for the implementation of appropriate control strategies. This report is a review of prevailing modelling, identification techniques and control strategies. Hopefully, the up to date information in this paper will be found useful for research work on fluidized bed dryIng.

Drop coalescence on inclined plates in liquids

Abstract: Inclined plates are often installed as mechanical separation aids in gravity settlers for the separation of liquid-liquid systems The authors investigated the process of coalescence of single drops on trickling films flowing off inclined plates The method of investigation was developed by Blass and Rautenberg It ensures that, under certain conditions, only hydrodynamic parameters influence the coalescence process to any significant extent The coalescence time of the drops characterizes the process of coalescence Short time between 1 and 3 seconds provide favourable conditions for coalescence whereas long times, often ten times as long or even longer are unfavourable This contribution introduces a criterion for clear demarcation between these two ranges and also a determination of separation parameters for favourable conditions for coalescence Furthermore, the function of the plates is described, which is determined by hydrodynamic and wetting processes The characteristic operating regions of a plate are shown in a working diagram

Dynamic modelling of multitubular catalytic reactors

Abstract: This paper presents a study on modelling and simulation of transient operational characteristics in multitubular fixed-bed reactors. The dynamic model of the reactor is based on a “porous body” approach which regards the intertubular space as a pseudo-homogeneous environment. Such an approach permits to take into account most factors in the geometrical design of the unit and thus to study the influence of various shell-side geometrical and operational parameters on the reactor behaviour. Based on the model, the dynamic responses of the two most common industrial reactor designs, i.e. the parallel flow unit with distributing plates and the crossflow reactor with disk-and-doughnut baffles have been investigated and compared. In addition, some problems of correct space discretization and use of time-dependent regridding procedures, are discussed.

Safe operation of a batch reactor: Safe storage of organic peroxides in supply vessels

Abstract: In this study, we investigated the limits of safe operation for a cooled reactor, operated batchwise. As an example of a single-phase reaction, we studied the decomposition of t-butyl peroxypivalate, a well-known organic peroxide, undergoing self-heating at relatively low temperatures. If sufficiently diluted, it can be supplied to a polymerization process from large, cooled but unstirred vessels. We present a number of extensions to the existing homogeneous explosion theory, namely a practical definition of the critical condition, its calculation, and expressions for the available time before runaway in the case of a supercritical condition, taking into account the effects of natural convection inside the vessel and the reactant conversion. The extensions of the theory were confirmed by adiabatic and non-adiabatic runaway experiments on bench scale, and natural convection cooling experiments with liquids in various packages.

Calculation methods for multicomponent adsorption equilibria

Abstract: The knowledge of multicomponent adsorption equilibria is of great importance for the industrial design and application of adsorptive purification and separation processes. As the experimental determination of the required data is time-consuming and costly, theoretical help is desirable. This contribution presents and assesses the most common theoretical approaches to the calculation of multicomponent adsorption equilibria. The models are based on the assumption of an adsorbed solution, extension of the volume filling of micropores theory or statistical thermodynamics. These models are applied to different adsorptive/adsorbent systems with ideal and non-ideal equilibrium behaviour. It is established that, at low coverages of adsorbent surfaces, all theories provide good estimates of the adsorbed phase composition and total loading while, at high saturations, no model is completely satisfactory.

On the falling-rate period

Abstract: A fixed bed of glass particles, wetted with water, was dried from above The measured moisture profiles within the sample show that its surface is not completely dry during the falling-rate period Hence, the current view, according to which the moisture front retreats into the sample directly after reaching the critical moisture content, has to be revised Assuming that parallel dry and wet channels form within the sample during the constant-rate period, the onset of the falling-rate period depends only on the diameter of these channels If the moisture is distributed very finely, a large number of channels with a small diameter have to be considered The falling-rate period for this system occurs at low moisture contents For a coarse distribution of moisture, fewer channels with a large diameter should be assumed Their drying rates show the beginning of the falling-rate period already at high moisture contents A good agreement between measured and calculated drying rates could be achieved by fitting the channel diameter Only for very low moisture contents, is the agreement poor This is to be expected, because the measured moisture profiles show a completely dry sample surface for these moisture contents, indicating that the moisture front has already retreated into the sample

Hydrodynamics, axial mixing and mass transfer in open turbine rotating disc contactor (OTRDC)

Abstract: An experimental study of hydrodynamics, axial mixing and mass transfer has been carried out in a newly developed liquid-liquid extraction contactor, namely the open turbine rotating disc contactor (OTRDC). It has been established that the OTRDC can be operated with larger holdups of the dispersed phase, larger interfaces and, hence, more efficient mass transfer than the conventional RDC. In correlating axial mixing data, a combined model has been applied in which both the forward mixing due to drop size distribution and the backmixing of droplets are taken into account. The RTD curves of dispersed phase predicted by the model agree well with the experimental data. Comparison of experimental mass transfer data with those predicted by the proposed axial mixing model and the theoretical single drop model shows that they are in good agreement.

Decrease of diffusion coefficients near binodal states of liquid‐liquid systems

Abstract: This work investigates the thermodynamically based assumption that diffusion at binodal states of unstable fluid systems vanishes. It is shown experimentally that a scaling law, which describes the fall in diffusion near critical points, may also be applied in an extended form to its abatement at the limits of solubility. These results may profoundly affect the modelling of mass transfer between phases whose bulks exhibit states of saturation.

Three‐component mass transfer in liquid‐liquid extraction with the system glycerol‐acetone‐water. Part 2: Calculation and experimental investigation of three‐component mass transfer in a countercurrent extraction column

Abstract: On decrit les effets se deroulant au cours du transfert de masse ternaire entre les phases saturees et insaturees du systeme glycerol-acetone-eau

Three-component mass transfer in liquid-liquid extraction with the system glycerol-acetone-water. Part 1: The influence of vanishing diffusion coefficients in binodal states on the mechanisms of mass transfer†

Abstract: This contribution deals with the consequences of the decrease in diffusion coefficients at the boundaries of miscibility in fluid systems and its effect on the mechanisms of mass transfer. The liquid-liquid (1–1) extraction involving several components is taken as an example for the discussion of this topic. A short review of Hampe's theory on transport mechanisms in the transition regions of interfaces [1] explains the intrinsic interdependence between mass and momentum transfer. Finally, a discussion of the peculiarities of resistances to mass transfer in boundary layers, based on the changes in molecular transport properties, is presented.

Flow characteristics of large oscillating drops in liquid-liquid systems

Abstract: A study was made of the flow characteristics of large oscillating drops of pure liquid-liquid systems, using a thermostatically-controlled, rising drop column, 50 mm in diameter and 1000 mm in length. Mirrors in the jacket enabled front and side views of drops to be photographed simultaneously. Single drops in the size range 5–10 mm were investigated with both mutually-saturated phases and when the solute was being transferred from the dispersed phase. The systems studied were (1) toluene and acetone (dispersed)-water (continuous), and (2) n-heptane and acetone (dispersed)-water (continuous). Acetone concentrations were varied up to 3.75 kmol/m3. The oscillations of a travelling drop were asymmetrical; therefore, the amplitude cannot be expressed accurately in terms of only two axes. The area change of the drop compared to that of a sphere of equal volume ‘e’, was shown to represent the amplitude accurately. The periods of droplet oscillation were uniform for the mutually saturated systems of constant physical and flow properties but changed when mass transfer was taking place. The interfacial tension exerted a marked effect on the amplitude, which also depended upon the oscillation frequency. The amplitude changed with droplet size in a similar manner to the terminal velocity, i.e. it increased with increasing size until it reached a maximum, subsequently decreasing less rapidly. The drag coefficient increased with increasing rate of mass transfer from the drop. Correlation of the results and the area eccentricity ‘e’ by dimensional analysis embracing all possible parameters and physical properties affecting drop oscillation, resulted in the correlation e = 0.22 Sr0.42 We−0.53M0.13 with a mean deviation of ± 14%. This will facilitate more accurate prediction of the interfacial area for mass transfer calculations, relating to equipment containing droplets in the oscillating regime.