bluuebunny/crossref_metadata_2025 huggingface dataset:
Calculation of primary bubble volume in gravitational and centrifugal fields
Authors: Voit Harald , Zeppenfeld Rolf , Mersmann Alfons
Published: August 2005

A one‐stage model of the formation of primary bubbles is presented in which the bubble volume is deduced from an equilibrium of buoyancy, viscosity, inertia and surface tension forces. In contrast to the two‐stage model, presented by Kumar and Kuloor, it was not assumed that the drag coefficient in bubble expansion can be described by the same constants as in the steady‐state bubble ascent. The constants were adapted in such a way that the introduction of an additional bubble volume was not necessary. It was demonstrated that this model describes the bubble formation in gravitational and centrifugal fields equally well and, furthermore, is also applicable to structurally viscous liquids, provided that the effective shear gradient

is calculated from the equilibrium of shearing and buoyancy forces. The model is based on the assumption of a constant volumetric flow rate during bubble formation and, for this reason, a minimum Froude number is necessary in analogy to the weeping limit for sieve plates. The normalized presentation permits simple operation. The possibility of applying the model to drop formation was confirmed by comparison of experimental values with those, predicted by the model.
Influence of electrolytes on the absorption of nitrogen oxide components N2O4 and N2O3 in aqueous absorbents
Authors: Weisweiler Werner , Deiß Karl‐Heinz
Published: August 2005

The influence of electrolytes, which are dissolved in the aqueous absorbent and do not react with nitrogen oxides, on the absorption kinetics of both these components was investigated experimentally. In addition to demineralized water, various salt solutions of different concentrations as well as sodium hydroxide solution were used as absorbents. The term H \documentclass{article}\pagestyle{empty}\begin{document}$ H\sqrt {k_1 D} $\end{document} for N2O4 and N2O3, which is important for the design of industrial absorbers, was determined as a function of composition and concentration of the absorbents. In the case of N2O4, the chosen measuring and evaluation methods permitted a separate determination of the rate constant k of the pseudo first order reaction and of the solubility H. The diffusion coefficient D of the gas in the absorbent can be obtained only by calculation. Experimental results showed that \documentclass{article}\pagestyle{empty}\begin{document}$(H\sqrt {k_1 D} )\,_{{\rm N}_{\rm 2} {\rm O}_{\rm 4} } $\end{document} decreases with increasing ionic strength I, however, without a clear indication of any ion‐specific effects. This decrease does not appear to be caused simply by a reduction in solubility (salting out effect), or in diffusion coefficient, but at least, to the same extent, through a decrease of the rate constant k with increasing electrolyte content in the absorbent. The measurements permitted the determination of the gas‐based salting out parameter for N2O4. The investigations on the absorption of N2O3 in water and in an Na2SO4 solution showed no experimentally detectable influence of dissolved salts on \documentclass{article}\pagestyle{empty}\begin{document}$(H\sqrt {k_1 D} )\,_{{\rm N}_{\rm 2} {\rm O}_{\rm 3} } $\end{document}. The numerical value of \documentclass{article}\pagestyle{empty}\begin{document}$(H\sqrt {k_1 D} )\,_{{\rm N}_{\rm 2} {\rm O}_{\rm 3} } $\end{document} is six times that of \documentclass{article}\pagestyle{empty}\begin{document}$(H\sqrt {k_1 D} )\,_{{\rm N}_{\rm 2} {\rm O}_{\rm 4} } $\end{document}.
New equipment for the determination of virial coefficients of pure substances and binary mixtures
Authors: Spiske Jürgen , Gaube Johann
Published: August 2005

A new equipment for the measurement of virial coefficients of pure substances and binary mixtures is described. It consists of an improved modification of the set‐up proposed by Eucken and Meyer in 1929. The development is based on a critical review of the existing methods particularly with respect to adsorption effects. In order to reduce the influence of adsorption on the accuracy of virial coefficients, an optimum range for gas density is recommended. The accuracy of the virial coefficients is better than 20 cm3mol−1. The set‐up was employed for the measurement of virial coefficients of acetone and hexane in the range from 352.95 to 393.85 K. The results are in excellent agreement with data published in literature. Furthermore, cross‐virial coefficients of the binary system acetone/hexane were measured and interpreted.
Fluid dynamics and mass transfer in a bubble column with suspended particles
Authors: Sauer Thomas , Hempel Dietmar‐Christian
Published: August 2005

Slurry bubble columns are widely used in biotechnology. Therefore, the effects of solid particles on fluidization characteristics, gas hold‐up and volumetric liquid‐side mass transfer coefficient were measured in a slurry bubble column (i.d. 0.14 m). The density and diameter of the suspended particles were similar to those applied in biotechnology with immobilized bacteria. Based on models of turbulence and of liquid circulation induced by rising gas bubbles, equations for critical gas velocity, gas hold‐up and volumetric liquid‐side mass transfer coefficient were obtained by dimensional analysis.
Investigation of entrainment in tray columns
Authors: Puppich Peter , Goedecke Ralf
Published: August 2005

This paper describes a pilot plant for entrainment measurements on various types of column trays. The data obtained from different trays differ by up to a factor of 10. The measurements were compared with the theoretical model of Stichlmair. The Stichlmair entrainment diagram permits a fairly accurate prediction for most types of trays if the height of the two‐phase layer is known. Prediction of the height and relative liquid hold‐up in this two‐phase system should be modified by a term reflecting the vapour flow profile, which requires further testing. These tests should examine the effect of tray spacing, fractional free area and, in particular, the influence of different media.
Heat transfer in boiling of multicomponent mixtures
Authors: Schmitt Dieter W.
Published: August 2005

Pool boiling data of multicomponent mixtures show that the heat transfer coefficients can be considerably lower than those of a corresponding pure fluid with the same physical properties as the mixture. The proposed model, which assumes maximum mass transfer resistance in the liquid boundary layer, is confirmed by the comparison between calculated and experimental data. The model allows the prediction of heat transfer coefficients in pool boiling of mixtures using only single component properties and vapour‐liquid equilibrium data. Judging by experience with a number of different systems, the method should be sufficiently accurate for most practical applications.
Pressure pulsations in the piping of reciprocating pumps
Authors: Vetter Gerhard , Schweinfurther Friedrich
Published: August 2005

Pressure pulsations in hydraulic systems, generated by reciprocating pumps, can cause serious problems with regard to plant safety and reliability. In particular, fatigue problems arise in high‐pressure piping systems. The available knowledge is not sufficient for an accurate computation of pressure peaks in the piping of reciprocating pumps. A number of calculation models are available which, however, neglect both fluid compressibility and friction. This contribution presents a calculation method which allows a precise modelling of various pump installations. Comparison of calculated and experimental data shows a good agreement and provides a validation of the computational model.
Influence of temperature and properties of solids on the size and growth of bubbles in gas fluidized beds
Authors: Hilligardt Klaus , Werther Joachim
Published: August 2005

In the present paper, models are developed for the description of coalescence and splitting of bubbles in gas fluidized beds. The distinction between slow and fast bubbles, which originates from the Davidson model of gas flow in and around a rising bubble, was found to be decisive for the modelling of coalescence processes. On the basis of the respective models, it is shown that a change in bed temperature does influence the mechanisms of both bubble coalescence and splitting. In cases of both slow and fast bubbles undergoing splitting, the theory predicts a decrease of bubble size with temperature whereas in the case of a non‐splitting system with fast bubbles, typically encountered with Geldart type B particles, no significant temperature effect on bubble size is expected. The theoretical predictions are shown to be in agreement with own measurements as well as with the results of other workers.
Particle velocity in a stratified two‐phase flow between inclined parallel plates
Authors: Meon Walter , Blass Eckhart
Published: August 2005

Fundamental investigations of coalescence processes in settlers with plate packages for the separation of liquid‐liquid dispersions prompted an analysis of the drop motion on the interface of a stratified two‐phase flow. Therefore, a physical model was developed, which permits the calculation of drop velocity along the interface of an inclined trickling film. Starting from a balance of forces around a single particle, the model provides the equation of motion for a steady motion of single particles in a stratified two‐phase flow. Several assumptions and considerations were necessary to solve this equation, such as taking into account the influence of the wall and of flow resistance. The model calculation assumes Newtonian liquids and laminar, smooth trickling films. A comparison with experimental results confirms that the drop motion behaviour is well reproduced.
Rheology and flow of phosphate slurries (mine tailings) in pipes
Authors: Fam Doudou , Dodds John , Scrivener Olivier
Published: August 2005

An experimental study of the flow of phosphate mine tailings was carried out in order to characterize these suspensions of fine particles for pipeline transport. Rheological measurements with a rotating cylinder viscometer indicate a viscoplastic behaviour which can be represented by a Herschell‐Bulkley or by a Casson model. The variation of the parameters of these models with solids concentration reveals a change in the behaviour at a concentration of 10 to 15%. The pressure drops were measured during flow in capillary tubes, in both laminar and turbulent regimes. The variation of the pressure drop coefficient (Fanning friction factor) as a function of the generalized Reynolds number confirms the validity of the employed rheological models. The results are in good agreement with measurements made in pipes of industrial size. The velocity profiles measured with a novel ultrasonic velocimeter demonstrate the influence of the rheological behaviour on the yield and wall stresses.
Dynamics of distillation with high product purities
Authors: Wozny Günter , Witt Werner , Jeromin Lutz
Published: August 2005

For the simulation of steady‐ and non‐steady‐state behaiour of separation columns, a simulation program was developed and employed for the calculation and optimization of methanol‐water separation. The demand for increasingly higher purities and energy‐saving optimization of plants results in complex column configurations and thus in complex control schemes. The number of possible control schemes was reduced on the basis of heuristic rules. By simulating the process, the optimum control and manipulative variables and measuring points were determined from the reduced number of control concepts. A control scheme was introduced. The model was verified by simulation of the dynamic behaviour of the process. The investigation included the effects of computing technique, time interval, model reduction and tray hold‐up on the results. Thus, various effects and interactions were verified theoretically with respect to the robustness of the model. This was followed by a simulation of the non‐ontrolled and controlled column. By implementing the controllers in the existing column program, the computer can predict the control parameters. These parameters were checked and corrected when the metahnol‐ater column was put in operation. In the start‐up phase, major problems were caused by the sensitivity and failure rate of density, throughput and pump measuring sensors. Feed impurities were another source of problems. Operation with high product purities is possible, with a side stream to discharge impurities.
Bubble flow in pressurized gas/solid fluidized beds
Authors: Schweinzer Jürgen , Molerus Otto
Published: August 2005

This contribution presents the measurements of bubble behaviour in presence of A, B and D powders in a semi‐industrial scale pressurized fluidized bed. Local measurements were taken at static pressures of 0.2, 1.0 and 2.5 MPa, using capacitance probes. Quartz sand with mean particle diameter of 157 μm was fluidized in small “two‐dimensional” transparent equipment with Frigen R 115. In addition, pressure fluctuations in the bulk of the fluidized bed with inner diameter of 0.4 m were studied experimentally.
Vapour‐liquid and vapour‐liquid‐liquid equilibria in ternary and quaternary systems of n‐hexadecane, benzene, water and hydrogen, at elevated temperatures and pressures
Authors: Dohrn Ralf , Brunner Gerd
Published: August 2005

Phase equilibria in the n‐hexadecane‐water‐hydrogen and n‐hexadecane‐benzene‐water‐hydrogen systems were determined experimentally at temperatures between 200 and 350°C and pressures between 100 and 300 bar. At high water concentrations, three‐phase equilibria were observed. Two‐phase regions could be correlated with a modified Redlich‐Kwong equation of state. The influence of interaction parameters on the calculated miscibility gaps was investigated. On application of mean interaction parameters, it is possible to calculate phase equilibria at different pressures or temperatures with the same set of parameters. In the ternary system, the three‐phase regions could be calculated from the correlated binodal curves of two‐phase regions. In the quaternary system, cross‐sections through the vapour‐liquid miscibility gap could be successfully correlated.
Scale‐up of power consumption in agitated ball mills
Authors: Weit Herbert , Schwedes Jörg
Published: August 2005

Previous investigations have shown that the specific energy input is the overall parameter of influence on product size during communition in agitated ball mills, from laboratory up to industrial scale. The specific energy input is the introduced energy related to the amount of comminuted material. This parameter can be used for mill scale‐up. Consequently, a method had to be found of introducing power into the mill so as to obtain a given specific energy input. For this purpose, stirring tests with purely Newtonian liquids were carried out in absence of solids and hence, without comminution. Mathematical models are presented which describe the power consumption in agitated ball mills in absence of grinding beads. In addition, tests with grinding beads filling were also performed, leading to scale‐up guidelines with respect to power consumption. Finally, the influence of size and material of grinding beads was investigated.
Progress in the kinetics of coal and char gasification
Authors: van Heek Karl Heinrich , Mühlen Heinz‐Jürgen , Jüntgen Harald
Published: August 2005

This contribution reviews the work performed mainly at Bergbau‐Forschung during the past few years. First, it deals with the quantitative description of gasification kinetics, taking into account the pressure of the gasifying agent and partial pressures of product gases. Thereby, a method and quantitative data are provided for the calculation of reaction rates for uncatalyzed and potassium‐catalyzed gasification, on the example of chars from high volatile bituminous coals. In addition, a method for characterization of internal surface area by oxygen chemisorption with respect to char reactivity has been improved. Progress has also been achieved in the assessment of the impact of pyrolysis conditions, especially the time‐temperature profile during char formation, on its chemical activity during subsequent use.
Studies of drop break‐up in liquid‐liquid systems in a rotating disc contactor. Part I: Conditions of no mass transfer
Authors: Bahmanyar Hossein , Slater Michael J.
Published: August 2005

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.
Modelling of pressure drop in packed columns
Authors: Billet Reinhard , Schultes Michael
Published: August 2005

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.
A new pneumatic bearing dynamometer for power input measurement in stirred tanks
Authors: Reséndiz Roberto , Martínez Alfredo , Ascanio Gabriel , Galindo Enrique
Published: August 2005

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 × 10−4 Nm, and can be neglected in the experimental ranges of measured torques, i.e. from 5 × 10−3 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.
Development of rate expressions for the catalytic decomposition of nitric oxide with ammonia on iron oxide
Authors: Claußen Michael , Kainer Hartmut , Kunz Ulrich , Hoffmann Ulrich
Published: August 2005

The reduction of nitric oxide with ammonia on iron oxide catalysts has been studied in a continuous‐flow recycle reactor using simulated flue gas in the temperature range from 573 to 673 K. NO and HN3 concentrations were varied between 0 and 1000 vpm, O2 and H2O concentrations between 0 and 9 vol.‐%, the remainder being nitrogen. In the presence of oxygen, the formulated reaction rate equation describes the measured rates of the main reaction NO + 2/3 NH3 ⇄ 5/6 N2 + H2O. Its form corresponds to the Langmuir‐Hinshelwood type. The rate equation well fits the data, which cover the whole industrial temperature and concentration range. In the absence of oxygen, the measured reaction rates can be best described by a power law.
Vibration induced pneumatic coveying
Authors: Molerus Otto , Siebenhaar Wolfgang
Published: August 2005

A new transport system has been developed which combines vibration of the conveying pipe with amplitudes of the order of fractions of a millimetre with rather low superficial velocities of the carrier gas. This new system was specially designed for gentle transport of fragile particulate materials.
Safe operation of a batch reactor: Safe storage of organic peroxides in supply vessels
Authors: Steensma Metske , Westerterp K. Roel
Published: August 2005

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.
Hydrodynamics, axial mixing and mass transfer in open turbine rotating disc contactor (OTRDC)
Authors: Jia‐Wen Zhu , Shou‐Hua Zhang , Xiao‐Kui Zhou , Xiao‐Xiang Chen , Yuan‐Fu Su , Vogelpohl Alfons
Published: August 2005

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.
Modelling of selective catalytic denox reactors – strategy for replacing deactivated catalyst elements
Authors: Lefers Jan B. , Lodder Peter , Enoch Gerd D.
Published: August 2005

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.
Effect of doping with nickel and copper on catalytic behaviour of wide‐pore zeolites with different pore structures
Authors: Exner Herbert , Fetting Fritz
Published: August 2005

The catalytic properties of wide‐pore zeolites, namely Ni‐ and Cu‐doped faujastie H‐Y and H‐mordenite (H‐M) are compared on the example of conversion of n‐hexane. After thermal pretreatment of ion‐exchanged zeolites, reduction with hydrogen (P ≤ 40 bar) leads to a large number of metallic clusters and crystallites as well as ionic species in the zeolites. Notwithstanding this variety, the different types of catalysts can be divided into three classes: classical bifunctional catalysts (for example NiHM, NiHY) with ion exchange degrees > 30%, monofunctional catalysts, such as CuHM and CuHY with high copper contents (ion exchange degree > 40%) and metal‐doped (Ni and Cu) zeolites which, depending on the conditions of pretreatment, act as bifunctional and/or monofunctional compounds and, therefore, show very unstable reaction behaviour. In addition to the extent of reduction, the cation distribution affects the activity and deactivation behaviour and, to a lesser extent, the product selectivity. The reaction mechanism of bifunctional catalysis is similar to that observed in platinum‐doped catalysts, while the mechanism of monofunctional catalysis resembles the process on undoped zeolites.
Infinite‐dilution activity coefficients of organic solutes in methyl phenyl ether, butyl ether and methyl iso‐butyl ketone
Authors: Campanella Enrique A.
Published: August 2005

Activity coefficients of hydrocarbons and chloroparaffins at infinite dilution in methyl phenyl ether, butyl ether and methyl iso‐butyl ketone at 293.15 K have been obtained by the retention‐time method in a modified gas chromatograph. The experimental data are compared with predictions of the UNIFAC and MOSCED models. Selectivities for the 1,1,1‐trichloroetane‐1,2‐dichloroetane separation process are reported.
A multi‐parameter model for absorption
Authors: Guha Chandan , Roy Subhash Chandra
Published: August 2005

A three‐parameter model for mass transfer by absorption, based on a combined effect of non‐steady‐state molecular and eddy diffusion through a resistance zone has been suggested, which encompasses all the existing theories on mass transfer at the gas‐liquid interface. King's approach [5] to describing gas‐liquid mass transfer phenomena through an eddy‐diffusivity model can explain some experimental results, but does not provide a general mathematical solution of the problem. The shortcomings of the King's model are overcome in the present work and a general solution is presented. This solution confirms the experimental findings of Versteeg et al. [10] for liquid phase mass transfer. The steady‐state solution of the present theory, valid for the dimensionless number α > 0, shows a close agreement with experimental data of Luk [22], the only source of all the information necessary for model verification.
A critical study of multivariable self‐tuning algorithms for distillation control
Authors: Radhakrishnan Thota K. , Gangiah Kota
Published: August 2005

A control algorithm which has been acclaimed as the best algorithm for a real system may not be the best algorithm for a different real system. Therefore, various self‐tuning algorithms for real distillation columns have been evaluated, in order to compare their performances. A variable forgetting factor algorithm is modified using a filter which permits the employment of one instead of two covariance matrices for distillation control. A cautious self‐tuning control of SISO system is extended to MIMO system of distillation control. Multivariable self‐tuning regulator, multivariable self‐tuning controller and multivariable cautious self‐tuning controller are implemented with modified variable forgetting factor for linear transfer function model, Waller et al. column, and rigorous non‐linear model, Wood and Berry column. For distillation control, a multivariable cautious self‐tuning algorithm with modified variable forgetting factor is much simpler than earlier reported algorithms. This has produced better results and demonstrated its effectiveness, even in the presence of noise when other adaptive controllers give unsatisfactory performance.
Dynamic modelling of multitubular catalytic reactors
Authors: Stankiewicz Andrzej , Eigenberger G. Gerhart
Published: August 2005

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.
A learning fedforward control strategy for the concentration control of recycle reactors. Part I: Set‐point control
Authors: Schnitzlein Klaus , Löwe Arno
Published: August 2005

Unknown reaction rates make concentration control in laboratory recycle reactors a difficult task, particularly when gas chromatographs or other analytical equipment with long analysis times from part of the control system. For such time‐delay systems, a model‐based feedforward control technique is developed which uses some kind of learning linear mapping to provide estimates of the reaction rates. Regarding the number of time steps necessary to satisfy a new set point, in a variety of simulation studies, this technique has proven superior to similar but simpler control strategies.
Investigation on pure shearing of cohesive limestone with true biaxial shear tester
Authors: Nowak Marek , Schwedes Jörg
Published: August 2005

Pure shear deformation of a bulk solid sample was performed with the True Biaxial Shear Tester. The influence of stress history on pure shearing was examined by three different consolidation procedures. Apart from the influence of the consolidation procedure, the effect of volume change, i.e. the change in the bulk solid density during consolidation, on pure shear deformation was investigated. Cohesive limestone with x50 = 5 μm was used for the experiments.
Mixing times in stirred suspensions
Authors: Kraume Matthias
Published: August 2005

In stirred systems, the presence of solid particles leads to a pronounced lengthening of mixing time, sometimes to over 10 times that for the single‐phase state. The mixing behaviour is strongly heterogeneous, since the slurry is mixed comparatively rapidly, while markedly slower homogenization occurs in the solid‐free zone. This is the consequence of different fluid velocities in the two regions. For particle settling velocities in excess of 5 cm/s, mixing times assume maximum values on reaching the state of complete suspension. By contrast, at lower settling velocities, maximum mixing times occur before suspension is complete. Mixing times are influenced only be the state of suspension and not by the mode of its generation. Consequently, for the fulfilment of the 90% suspended slurry height criterion, mixing times are independent of stirrer speed, solid concentration, type of agitator or diameter ratio d/D. The effects of particle diameter, viscosity and equipment dimensions on mixing time, when the 90% slurry height criterion is fulfilled, are reflected by the ratio of the vessel diameter to a representative liquid velocity.
Significance of axial heat dispersion for the description of heat transport in wall‐cooled packed beds
Authors: Borkink J. G. Harold , Westerterp K. Roel
Published: August 2005

A two‐dimensional pseudo‐homogeneous model with axial dispersion of heat has been solved numerically with different boundary conditions at the inlet and outlet of the packed bed. The model solutions are fitted to experimental temperature profiles, determined in a wall‐cooled packed bed in which a hot gas is cooled down, and best fit values of the effective axial and radial thermal conductivities and the wall heat transfer coefficient are obtained. In the range of Reynolds numbers employed, Re > 50, the axial dispersion of heat is found to be of no significance for the description of heat transport in wall‐cooled packed beds without reaction, provided that the inlet boundary conditions are chosen appropriately. If a radially flat inlet temperature profile is assumed, while the actual profile is curved, an apparent improvement in the description of heat transport is observed when axial dispersion is incorporated into the heat balance and high effective axial thermal conductivities are obtained. If a Danckwerts type inlet boundary condition is used, assuming a flat temperature profile immediately in front of the inlet, an apparent improvement is also found on incorporation of axial dispersion of heat. This is caused by the temperature jump at the inlet, compensating for the overestimation of inlet temperature, in the case of cooling. The latter also explains why the inclusion of axial dispersion may eliminate the so‐called “length effect”, often related to the effective radial thermal conductivity and the wall heat transfer coefficient. It is shown for the outlet boundary condition that deletion of the axial dispersion term from the heat balance at the outlet is a convenient boundary condition for the model being solved numerically.
Electrical conductivities of pure organic liquids
Authors: Horvath Ari L.
Published: August 2005

This contribution presents a method of evaluating electrical conductivities of pure organic liquids. Emphasis is placed on the methodology of selection, correlation and analysis of experimental data. The proposed method brings order out of conflicting, confusing and chaotic experimental observations. It is also shown that the consistent and evaluated data are generated at a fraction of the cost and time required for producing the original raw experimental data.
Micromixing effects on a parallel reaction in flow reactors
Authors: Atiqullah Muhammad , Hassan Mirza M. , Beg Shafkat A.
Published: August 2005

Using the micromixing concepts of Danckwerts and Zwietering, the Peclet number Pe has been correlated mathematically to the degree of segregation J for the axial dispersion model. The results were applied to compare the micromixing effects on a model, mixed‐order parallel reaction system in continuous flow reactors. Axial dispersion model, and Ng and Rippin's two‐environment model were used to find the micromixing effects in tubular and stirred tank reactors, respectively. The performance of these reactors, with varying geometries, has been evaluated in terms of overall conversion, selectivity, and yield under identical operating and reaction conditions. The overall conversion increases in a tubular reactor with the increase in J, irrespective of the kinetic orders. However, in a stirred tank reactor, the conversion is found to be micromixing‐sensitive, depending on the order of reaction. For m = 1 and n = 2 (case 1), the conversion is fairly insensitive to micromixing effects while it decreases for m = 0.5 and n = 1 (case 2) with increasing J. For the same extent of micromixing, a tubular reactor gives, in both cases, a higher conversion than a stirred tank reactor. The selectivity, in either case, decreases in both reactors with increasing segregation effects. However, in each case, the selectivity of a tubular reactor was fairly close to that of a stirred tank reactor at the same value of J. As far as the yield is concerned, both reactors achieve nearly the same value, without significant micromixing effects.
Drying characteristics of starch in an inert medium fluidized bed
Authors: Lee Dong‐Hyun , Kim Sang‐Done
Published: August 2005

The effects of gas velocity (0.32 to 0.67 m/s), inlet gas temperature (25 to 100 °C) and the mass ratio of starch to inert particles (0.1 to 0.4) on the drying rate of starch in a 0.083 m‐ID × 0.80 m‐high inert medium fluidized bed were investigated. The drying mechanism in an inert medium fluidized bed can be classified into adhesion‐dispersion, evaporation and disintegration steps. The drying rate increases with the increasing inlet gas temperature and velocity; the rates being about 10 times those reported for an agitated pan dryer. However, the drying rate decreases slightly as the mass ratio of starch to inert particles increases. Also, the drying rate exhibits a maximum at an optimum bed porosity. The drying rate data obtained in an inert medium fluidized bed have been correlated with the relevant dimensionless groups, i.e. Stefan and particle Reynolds numbers based on the theory of isotropic turbulence.
Techno‐Economic investigation of an air humidification‐dehumidification desalination process
Authors: Khedr Maaly
Published: August 2005

This article presents a humidification‐dehumidification (H.D.) process suitable for desalination of sea‐ and brackish water. The work correlates performance characteristics of the combination of different components in such a loop. Water yield, energy requirement and design data are evaluated as functions of different parameters. The investigation covered water to air mass ratios ranging from 60 to 80, concentration ratios between 2 and 5, at a temperature difference of between 10 and 16 °C, along the liquid for dehumidification. The results showed that 76% of energy consumed in the humidifier is recovered by condensation. Increase of the concentration ratio to 5 can reduce the make‐up water and rejected brine by about 58% and 24% respectively. Cost calculations show that the H.D. process has significant potential as an alternative for small capacity desalination plants and permits to operate systems as small as 10 m3/day in output.
Cooling water treatment by ozonization
Authors: Lin Sheng H. , Yeh Kuo L.
Published: August 2005

Ozonization of water in a cooling tower was investigated. A laboratory experimental set‐up was employed to study changes in water quality during ozonization. Measured data were analyzed in the context of cooling water's resistance to corrosion, scaling and biofouling. A method is proposed for determining the suitable dosage of ozone for maintaining the necessary water quality. It has been found that ozonization is very effective in maintaining the correct water quality for the required purpose and provides an attractive alternative to the conventional multiple chemical method of cooling water treatment.
Activity evaluation of a catalytic distillation packing for MTBE production
Authors: Sundmacher Kai , Hoffmann Ulrich
Published: August 2005

The octane enhancer methyl tertiary butyl ether (MTBE) can be produced very efficiently from methanol and isobutene in a reactive distillation column where the heterogeneous catalyst also acts as distillation packing. Some mathematical models have been published for the simulation of such a process but they focus on the physical transport processes between the vapour and liquid phases. However, the aim of this paper is to analyze the importance of the internal and external multicomponent mass and heat transfer phenomena on the catalyst under boiling conditions. Therefore, experiments were carried out in a reactive distillation column at different compositions of feed, column pressures and reflux ratios using a Raschig ring shaped acidic ion exchange resin as the catalyst. The temperature and composition of the liquid phase entering and leaving the catalytic column section were measured. These data were used to evaluate the effectiveness factor of the catalyst with a rigorous macrokinetic model. It is shown that the effectiveness factor varies significantly along the column length. Under certain operating conditions, decomposition of MTBE can occur accompanied by boiling processes inside the catalyst macropores.
Transient behaviour of shell and tube heat exchangers
Authors: Roetzel Wilfried , Xuan Yimin
Published: August 2005

Based on the dispersion model rather than on the conventional pluf‐flow model, a method of predicting the transient behaviour of parallel and counterflow heat exchangers is developed, whereby the effects of shell‐side flow maldistribution and the finite heat capacities of both fluids, tube bundle and shell are considered. The final solution has been obtained by numerical inversion of the Laplace transform, to determine the transient responses to arbitrary inlet temperature variations. The experiment was carried out in a one pass tube heat exchanger. The comparison between therotical and experimental results is presented and the feasibility of this method is discussed.
Industrial fires and explosions due to electrostatic ignition
Authors: Mittal Manju , Guha Barun K.
Published: August 2005

Static electricity is a frequent source of fires and explosions in industry. A variety of operations may generate static electricity leading to such fires and explosions. This requires adequate preventive and protective measures against this hazard. The present paper describes the theory and mechanism of electrostatic sparking, parameters needed to assess the respective hazard in a plant, safety measures to combat electrostatic problems, common operations where such problems exist and measures to eliminate or mitigate these problems.
Estimation of kinetic parameters of benzene hydrogenation in a trickle bed reactor
Authors: Sharma Sunil Dutt , Gadgil Kasturi , Sarkar Manoj Kumar
Published: August 2005

A high pressure and high temperature catalytic hydrocracking unit based on trickle‐bed reactors was constructed. Data obtained under conditions simulating those of industrial processes, were evaluated. Partial wetting characteristics of trickle beds were especially taken into account in the interpretation of results together with combined contributions of gas and liquid phase reactions.
The cyclone scrubber – a high efficiency wet separator
Authors: Krames Jörg , Büttner Helmut
Published: August 2005

Due to induced turbulent interactions between particles to be separated and the scrubbing liquid present as droplets, very good collection efficiency down to submicron particle size can be achieved as shown by detailed studies [1]. Assumed are suitable flow conditions, a long residence time and a certain droplet size distribution or a special liquid dispersion system. As the name implies, the cyclone scrubber basically consists of a cyclone. This cyclone serves as a contact space in which the interaction between dust particles and scrubbing liquid droplets takes place, and as a droplet separator. Thus, under normal conditions, a water saturated but dust‐ and droplet‐free gas stream leaves the cyclone. The droplets are generated by a pneumatic atomization nozzle, arranged at the cyclone inlet in the direction of flow. A complete theoretical description of the three‐dimensional, three‐phase, non‐steady turbulent flow in the cyclone scrubber has not been possible up to now. The detailed experimental investigations presented in this work, and covering multiple parameters, led to physically based conclusions about the dominant interactions and separation mechanisms involved. Grade efficiencies which did not fall below 0.75, down to a particle size of 0.5 μm, could be measured by an optical particle counter. Collection efficiency of 99.2% was determined by gravimetric analysis of the dust load, according to VDI‐guideline 2066, in the raw and clean gas sections of the dust load, according to VDI‐guideline 2066, in the raw and clean gas sections of the cyclone scrubber.
A rotating two‐phase gas/liquid flow regime for pressure reduction in underwater plasma arc welding
Authors: Steinkamp Helmut , Mewes Dieter
Published: August 2005

Plasma arc welding processes are used in the off‐shore industry for construction and maintenance of underwater structures and pipelines in a wet environment. At greater water depths the density of the plasma gas increases because of the greater hydrostatic pressure. This causes conductive heat losses to the wet environment to increase. To maintain the energy flux to the workpiece to be welded, the plasma arc has to burn in a local dry area with an inside pressure of 1 bar. This requirement can be fulfilled by a rotating cylinder with a liquid film flowing down the inner wall. The flow around the rotating cylinder is experimentally investigated. The rotating cylinder is placed above the work surface which is simulated by a flat plate. Because of the centrifugal forces of the rotating flow inside the gap between the lower end of the cylinder and the flat plate the water is forced out of the cylinder. The velocity distribution in the flow is measured by laser Doppler anemometry. The phase distribution in the two‐phase flow in the gap is measured by local electrical probes. The static pressure inside the gaseous atmosphere is reduced in comparison to the hydrostatic pressure of the surrounding water. The pressure reduction is given by the void fraction, the phase distribution and the volume flow rates of both phases in the gap as well as by the speed of revolution and the design of the cylinder and the work surface. The influence of these parameters on heat transfer from the workpiece to the two‐phase flow regime is also investigated.
Deactivation of a vinyl acetate catalyst
Authors: Abel Roland , Prauser Günther , Tiltscher Helmut
Published: August 2005

Kinetic studies as well as morphological and physico‐chemical surface studies have been carried out on the deactivation of a palladium carrier catalyst for vinyl acetate synthesis in the ethylene gas phase process. The experiments cover concentration and temperature ranges of technical interest at a total pressure of 900 kPa. The deactivation rate depends on the temperature and the oxygen and acetic acid concentrations. The decline in activity is best described by a rate law of second order relative to the activity number. This behaviour is attributed to the observed aggregation of finely dispersed palladium. It can be concluded that palladium(II) acetate acts as a transport species in a chemically assisted sintering mechanism. At temperatures T ≤ 438 K the more advantageous final texture of the catalyst will only be formed on reaction.
Modelling of coadsorption of different‐sized molecules by using methods of statistical thermodynamics
Authors: Meyer Carsten , Hesse Diethard
Published: August 2005

Using methods of statistical thermodynamics, adsorption isotherms for the description of multicomponent chemisorption of different‐sized molecular species are derived by assuming the adsorbing surface to be energetically homogeneous and by neglecting lateral interactions between the adsorbed particles. In the first part of this contribution, the adsorption equilibria for a mixture of different‐sized chain molecules adsorbed on a lattice of coordination number z are discussed supposing each unit of a given chain to bind to one adsorption site only. In the second part, the coadsorption of molecular species consisting of a different number of segments on a continuous surface is treated. A comparison of the predicted to the measured isotherms for the coadsorption of two freon species on activated carbon shows that the proposed equations describe the results of the coadsorption measurements sufficiently well, provided the adsorption data for the pure components have been determined experimentally.
New concepts for the design of mass polymerization reactors
Authors: Thiele Reiner
Published: August 2005

A novel reactor arrangement for the anionic polymerization of styrene described in this paper comprises alternating adiabatically operated plug‐flow reactors and heat exchangers. A novel tube‐bundle recycle reactor is presented for the bulk polymerization of styrene and acrylonitrile.