bluuebunny/crossref_metadata_2025 huggingface dataset:
The cocurrent downflow contactor–a novel reactor for gas‐liquid‐solid catalysed reactions
Authors: Boyes Adrian P. , Raymahasay Sugat , Tilston Mike W. , Lu Xia X. , Sarmento Sandra , Chugtai Arshad , Winterbottom John M.
Published: August 2005

The Cocurrent Downflow Contactor (CDC) has been developed as a mass transfer and reactor device, with and without addition of tangential (swirl) flow, giving gas hold‐up (Eg) values of 0.5–0.75, interfacial areas in the range 1000–6000 m2m−3 liquid and kLa values in the range of 0.15–1.55 s−1 for absorption using the O2/H2O system. It has been studied as a catalytic slurry reactor for the hydrogenation of (i) itaconic acid and (ii) triglycerides catalysed by Pd and Ni catalysts. The reactions were observed to be largely surface‐reaction rate controlled, due to the very efficient mass transfer (kLa up to 11.75 s−1 under reaction conditions) and application of swirl flow‐enhanced reaction rates. The CDC has recently been found to be capable of operating as a fixed bed reactor, thus eliminating a downstream catalyst separation problem (therefore more cost effective), and is superior in its mass transfer characteristics to other known devices. Scale‐up can be undertaken without loss of performance efficiency.
Basic characteristics of spinel type manganese mixed oxide/titanium composite anodes for electroorganic redox catalysis
Authors: Lodowicks Egbert , Beck Fritz
Published: August 2005

Manganese mixed oxide composite layers of about 1 μm thickness on titanium sheet as substrate were fabricated by firing of the corresponding nitrates at a typical temperature of 400°C in air. The activity of these anodes was evaluated by cyclic voltammetry (10 mV/s) and the stability was determined by chronopotentiometry (2.5 mA/cm2) in 1M H2SO4. The oxidation of 2‐propanol was examined as a simple electroorganic model reaction. The quality of a first category of mixed oxides with general composition MnMe2O4 decresed in the order Me = Co, Ni, Fe. In a second group with the general formula MeMn2O4 a decrease in the order Me = Co, Cu, Fe, Ni, Ti, Zn, Cd, Ca, Mg, (Zn, Ge), Li was observed. The corresponding candidates of the second group were superior to those of the first. The anode service life τ of the optimum spinel anode CoMn2O4/Ti is dependent on the current density, according to jλ τ = const. (λ = 1.7). Thus high current densities are precluded. The mechanism has been discussed in terms of a heterogeneous redox catalysis: surface Mn(VII) states are formed in a slow electrochemical step. In a subsequent fast chemical oxidation of the organic molecule the original reduced state is regenerated. This also explains the comparatively good service life of these anodes.
Recovery of zinc from pickling baths with amberlite LA‐2. Influence of modifier on the extraction equilibrium
Authors: Martín Marta San , Bart Hans‐Jörg
Published: August 2005

The extraction of zinc chloride by Amberlite LA‐2, a long chain secondary alkylamine, dissolved in Shellsol K, a naphthenic solvent, has been studied at room temperature. Isodecanol was used as modifier. The aqueous phases were chloride solutions of Zn(II) and Fe(II) in the concentration range commonly encountered in pickling baths, viz. about 1 and 1.5 mol/l, respectively. The ionic strength was kept constant at 4 mol/l in all experiments. Measurement of the distribution equilibrium of zinc chloride was carried out over wide concentration ranges of amine, zinc chloride, free chloride and isodecanol. It was assumed that the zinc chloride was extracted according to the following reaction

On the basis of this reaction, an easy to use model was developed to describe the extraction behaviour of Zn in typical pickling baths. Prediction and optimization of a solvent extraction plant performance under changing industrial feed conditions is then possible. This model takes account of the influence of isodecanol used as modifier.
Liquid velocity and dispersion coefficient in an airlift reactor with inverse internal loop
Authors: Kochbeck Birgit , Hempel Dietmar C.
Published: August 2005

An airlift reactor with inverse internal loop (annulus sparged) is investigated with regard to liquid velocity and dispersion coefficient for the purpose of reactor design. To distinguish between the influence of the individual parts of the reactor (downcomer, riser, bottom) on the liquid velocity and mixing, several draft tubes with different geometries and draft tube bottom clearances are successively installed.
Mathematical simulation of bioseparation in an affinity packed column
Authors: Sridhar P. , Sastri N. V. S. , Modak J. M. , Mukherjee A. K.
Published: August 2005

Affinity chromatography (biospecific adsorption) relies on specific interactions of biological molecules such as enzymes, antigens, antibodies, and proteins. The process consists of three steps: adsorption, washing, and elution. A mathematical model including convection, diffusion, and reversible reaction is formulated to analyse the breakthrough behaviour of the solute. A moving finite element orthogonal collocation method is applied with respect to the space variables of the governing partial differential equations of the model to evaluate the breakthrough of the solute. Danckwerts' boundary conditions are considered for the column. The validity of the numerical scheme is checked by comparison with an analytical solution for a simplified model. The results obtained from model simulation show that the breakthrough time of the solute is significantly influenced by the axial dispersion coefficient, solute concentration, ligand content, reaction kinetics, particle porosity, particle size, and flow rate. Solute recovery and bed utilisation efficiencies are evaluated for different values of the above parameters.
Economics of the oxidative coupling of methane as an add‐on unit for naphtha cracking
Authors: Hoebink Jozef H. B. J. , Venderbosch Henny M. , van Geem Paul C. , Van den Oosterkamp Paul F. , Marin Guy B.
Published: August 2005

The feasibility of methane coupling as an add‐on unit to a naphtha cracker was studied. The existing cold box was used for separation of ethylene and unconverted methane, but arrangements were made for separation of the coupling by‐products. The cracker's methane was used as a feedstock. The concept is technically and economically feasible at spring 1992 prices, provided the catalyst meets the assumed 30% conversion and 80% C2 selectivity.
Selective oxidation of o‐xylene to phthalic anhydride over vanadia/titania: A comparative study of integral and recycle reactor operation
Authors: Nobbenhuis Marc G. , Barnickel Peter , Wokaun Alexander , Baiker Alfons
Published: August 2005

Two vanadia/titania catalysts, containing 1 and 8 wt% vanadia on anatase, have been investigated for the selective oxidation of o‐xylene to phthalic anhydride. In a comparative study, activity and selectivity in o‐xylene oxidation were measured using an integral reactor and a reactor with external product recirculation, with the objective of studying the influence of backmixing in the latter on the reaction behaviour. Adsorbed surface species were investigated by means of in‐situ diffuse reflectance FT‐IR spectroscopy, using an environmental chamber which corresponds to an integral reactor. No difference in activity and selectivity in o‐xylene oxidation was observed between the two catalysts. FT‐IR studies showed adsorbed o‐xylene, benzoate and a high concentration of phthalic anhydride on the surface of both catalysts. o‐Xylene oxidation follows a sequential reaction path, with o‐tolualdehyde as the first intermediate and phthalide, phthalic anhydride and maleic anhydride as sequential products. No evidence was found for a direct oxidation‐path of o‐xylene to phthalic anhydride. Total oxidation products CO and CO2 are formed by direct oxidation of the o‐xylene and by oxidation of the reaction products. Due to greatly reduced concentration and temperature gradients, the recycle reactor proved to be highly suitable for studying the extremely exothermic oxidation of o‐xylene.
Three‐Stage wiped‐film molecular evaporator: Design and application
Authors: Cvengroš Ján
Published: August 2005

This paper describes the design principles of multistage molecular evaporators with a wiped film which are used for gentle distillation of low‐volatility blends of thermolabile materials. The conditions of distillation in an evaporator with a convex evaporation surface are discussed, along with problems relating to the function of the segmented wiper in the formation of the retarded liquid film and the functional principles of closing elements in a multistage evaporator with a single evaporating cylinder and of the built‐in entrainment separator in the distillation space, which is designed to retain splashing spray. A TMO 65 three‐stage molecular evaporator is described, with an evaporating cylinder having a diameter of 65 mm and a total evaporating area of 0.19 m2. This pilot‐plant evaporator can be used in the small‐capacity production of specialty chemicals or in technological research as a verification unit for establishing data necessary for the operation of large evaporators. These possible applications are documented by the results obtained on fractionation of various important materials. Fractional distillation of crude monoacylglycerols (M) containing 35 to 55% M afforded concentrates containing over 90% M, at yields of 78 – 95% M in the product fraction. Fractionation of oligomers of higher α‐olefins gave distillate cuts with high contents of the respective n‐mer, suitable for use as synthetic lubricating oils. Fractionation of polyalkylaromatic residue from the production of monoalkylbenzenes produced a distillate fraction with excellent electrical insulation properties. Finally, purification of technical diane‐bis‐glycidyl ether yielded over 75% of a product fraction which was found suitable for production of insulating material for electrical equipment operating at voltages of 6 to 15 kV.
Vortex depth in unbaffled single and multiple impeller agitated vessels
Authors: Markopoulos Joannis , Kontogeorgaki Eleni
Published: August 2005

Although vortex formation is usually an undesirable phenomenon in the process industry, satisfactory process conditions and results can also be obtained in unbaffled agitated vessels in the presence of a vortex. This fact and especially the low power requirements in these systems, with their immediate relevance to the energy problem in the process industry, show the true importance of vortex formation in agitated vessels. This article reviews the literature results and the correlations proposed for the prediction of vortex depth in unbaffled agitated vessels with various types of single and multiple impeller systems and presents a critical discussion on the basis of a theoretical analysis.
New and efficient high‐temperature processes with circulating fluid bed reactors
Authors: Reh Lothar
Published: August 2005

Starting with a brief review of the development of the circulating fluid bed (CFB) reactor principle, its main features as an important tool for efficient and thus environmentally acceptable processing of fine particulate solids, typically less than 150 μm diameter, are discussed. Various new industrial processes and high temperature process options starting with alumina calcination are described. Developing applications in the cement, metallurgical and chemical industries are approaching large‐scale industrial importance. Efficient combustion or pressure gasification of fossil fuels and carbon‐containing residues, fulfilling increasingly strict environmental legislation, is by far the most rapidly expanding field of application. In support of these developments our fundamental understanding of the fluid dynamics of CFB riser and of cyclone precipitation at high solid loadings has to be steadily improved. The reaction behaviour of aggregating fine particulate solids of varying size distributions also remains an open question.
The role of Cl− in a Li+‐ZnO‐Cl− catalyst on the oxidative coupling of methane and the oxidative dehydrogenation of ethane
Authors: Wang Dingjun , Rosynek Michael P. , Lunsford Jack H.
Published: August 2005

The effect of Cl− ion addition to a Li+‐ZnO catalyst has been studied with respect to the oxidative coupling of CH4 and the oxidative dehydrogenation (OXD) of C2H6. Increasing the Cl/Li ratio from 0.65 to 0.90 had relatively little effect on the CH4 conversion, whereas the C2H4/C2H6 ratio was enhanced significantly as a result of an increase in the OXD reaction rate. Conversely, loss of Cl− from the catalyst during the reaction had a much more deleterious effect on ethane OXD activity than on methane coupling activity. Addition of Cl− ions at a Cl/Li ratio of 0.9 caused a decrease both in the number of basic sites and in the basic strength of these sites, as determined by temperature‐programmed desorption of CO2. The similarities between the results obtained over Li+‐ZnO‐Cl− catalysts and those previously reported for Li+‐MgO‐Cl− catalysts confirm that basicity of the host oxide plays only a minor role in determining the properties of these chlorided catalysts.
An inverse correlation of the enhancement effect of tetrachloromethane as a feedstream additive in the oxidative coupling of methane on silica‐supported alkaline earth and alkali‐alkaline earth catalysts with the polarizing ability of the alkaline earth cations
Authors: Ahmed Shamsuddin , Moffat John B.
Published: August 2005

Results are reported for the oxidative coupling of methane on silica‐supported alkaline earths prepared from either their acetates or chlorides with and without alkali metals as dopants and in the absence or presence of carbon tetrachloride (TCM). The addition of small quantities of TCM to the methane feedstream produces increases in the conversion of methane and the selectivities to C2 hydrocarbons which correlate with the increase in cation size and thus are inversely related to the polarizing abilities of the cations.
The effect of gas injection on the flow of immiscible liquids in horizontal pipes
Authors: Nädler Martin , Mewes Dieter
Published: August 2005

The flow of two immiscible liquids and the influence of an additional inserted gas phase in horizontal pipes is investigated. The experiments are carried out in a transparent horizontal pipe with an inner diameter of 59 mm and a total length of 48 m. Experimental results are presented for the flow regimes of the two phase and three phase flow of oil, water and gas mixtures. The effect of phase inversion on the pressure drop is measured. The experimental results obtained for the three phase flow of oil, water and air indicate that drag reduction is possible by injecting gas in laminar flowing mixtures of oil and water. In the aerated slug flow regime of oil, water and air a water dominated and an oil dominated flow system can be distinguished. The pressure drop of the three phase flow system is of the magnitude as the pressure drop of the two phase flow of gas and the dominating liquid phase.
Performance of a venturi jet scrubber for H2S removal by iron‐complex solution
Authors: Nasr Mohammad Reza Jaffari , Abedinzadegan M.
Published: August 2005

Venturi jet scrubbers are widely used in chemical industries as an efficient gas‐liquid contacting equipment. They are simple in design, have no moving parts, operate with circulating liquid and are reliable in service. Moreover, due to high relative velocity of liquid and gas, considerable overall mass transfer can be produced. In spite of their wide application, readily usable design data are not available in literature, particularly for units producing a gas flow by their own pumping action. The purpose of this paper is to introduce the equipment and its application for H2S removal. The effects of various parameters such as gas and liquid flow rates as well as pressure on hydrodynamics and mass transfer performance of the equipment are investigated. Results of experiments have been shown graphically.
Heat transfer in turbulent falling films
Authors: Numrich Reiner
Published: August 2005

In condensers with high condensation rates or in falling film evaporators the falling film at the vertical tube wall shows a turbulent flow pattern. For optimal dimensioning of these apparatuses the knowledge and the description of the heat transfer in this turbulent falling film is necessary. Especially in fluids with a low thermal conductivity in liquid phase (i.e. hydrocarbons) the main heat transfer resistance is in the falling film, also if there are several components in gas phase. For calculating the heat transfer usually dimensionless Nusselt equations were used. These equations mainly base on experimental results which were found with water vapour as test fluid. It is shown that these equations fail for fluids with low thermal conductivity resp. high Prandtl numbers. Based on a Nusselt equation for turbulent tube flow a new equation for turbulent film flow is derived. A comparison between calculated and measured data for Prandtl numbers up to 50 shows good agreement.
On‐Line mass spectrometer analysis of gasification gas
Authors: Karlegärd Åsa , Götz Adolf , Bjerle Ingemar
Published: August 2005

The mass spectrometer is capable of making rapid, repetitive multicomponent analyses with measurement times as short as a second or less. Mass spectrometers used to be too expensive to be used for process analyses, but simpler and cheaper versions have been developed during the last decade. Mass spectrometers are commonly used within the petrochemical industry, but their use for fuel gas streams is not so widespread. This paper describes tests on a quadrupole mass spectrometer in three different gasification projects. Calibration methods and analysis accuracies obtained are accounted for. On the basis of the tests, it can be concluded that the quadrupole mass spectrometer is very suitable for continuous analysis of complex process streams. An important result is that the separation of CO and N2, an analysis problem that is traditionally regarded as difficult, worked very well. The analysis frequency was approximately once per minute, which should be enough for most control loops.
Synthesis of tertiary amyl methyl ether (TAME): Influence of internal and external mass transfer resistance
Authors: Oost Carsten , Hoffmann Ulrich
Published: August 2005

An experimental set‐up is presented for the measurement of reaction rates in the liquid phase synthesis of tertiary amyl methyl ether (TAME) from methanol and a mixture of 2‐methyl‐1‐butene and 2‐methyl‐2‐butene using an acidic ion exchange resin (Lewatit SPC 118) as catalyst. The reaction was performed in a continuous‐flow recycle reactor at a temperature of 333.15 K and a pressure of 1.6 MPa. Determination of the age distribution (Fcurve) of the reactor showed that mixed flow is approached at a recycle ratio of R = 10. Experiments were performed to reveal the limits of operating conditions where the various resistances do not affect the rate. In agreement with estimation of Reynolds numbers, hindrance by external mass transfer can be excluded at a volumetric flow rate of 10 ml/min and a recycle ratio of R = 10. The maximum temperature increase of the whole particle resulting from limitation of external heat transfer was estimated to be about 3 K. Variation of the size of resin particles revealed the existence of a critical methanol concentration, above which reaction rates can be determined without influence of internal mass transport phenomena. The maximum temperature increase in the particle was estimated to be 0.3 K.
An improved system for automated peptide synthesis
Authors: Bicciato Silvio , Bagno Andrea , Dettin Monica , Buso Orfeo , Di Bello Carlo
Published: August 2005

This paper describes an original system for automated solid‐phase peptide synthesis (SPPS) that was developed in our laboratories. The synthesizer is equipped with separate activator and batch reactors, and was designed to operate under either manual or computer control. Two metering pumps, which constitute the core of the entire apparatus, provide for the delivery and transfer of reagents and solvents in the appropriate quantities and in the correct sequence, and permit simultaneous condensation and activation operations that result in considerable time saving. Various applications to the synthesis of biologically important peptides, utilizing the most widely used strategies in SPPS, are presented as examples of this improved technology.
Modelling of agglomeration in industrial crystallization from solution
Authors: David René , Marchal Philippe , Marcant Bruno
Published: August 2005

A phenomenological approach to evaluating the agglomeration rate of growing crystals is presented. Crystals larger than the Kolmogoroff microscale collide because of the fluctuating velocity field. Bridges between the crystals form as a result of competition between crystalline growth and the turbulent velocity fluctuations on the opposite sides of the agglomerate.

Crystals smaller than the Kolmogoroff microscale collide because of viscous laminar microshear stresses within the same turbulent eddy, are kept close together by Van der Waals forces and are joined together by crystalline bridges that grow between them. The strength of these bridges depends on the supersaturation level during the moments following the collision.

These two sets of phenomena lead to two different preferential generation mechanisms for agglomerates. One example involving important agglomeration effects is developed for each case: the crystallization of adipic acid belongs to the first group (large crystals), whereas the precipitation of calcium oxalate monohydrate is typical of the second case (small crystals).
Study of the runaway characteristics of suspension polymerisation of styrene
Authors: Nemeth Sandor , Thyrion Fernand C.
Published: August 2005

The safe operation of a batch polymerisation reactor assumes sufficient knowledge about all the possible reactions. An investigation has been made in order to predict, with the aid of a mathematical model based on a detailed reaction mechanism, the runaway ability of a styrene batch suspension polymerisation reactor. The results of the simulation were compared with the experimental data obtained from a bench‐scale test cell.
Adiabatic gas absorption in packed columns with large thermal effects
Authors: Swaminathan Rama , Sadasivam M. , Balakrishnan A. R.
Published: August 2005

The performance of packed bed adiabatic gas absorbers has been analyzed using the enthalpy and material balance equations incorporating a new parameter, fr, defined as the fraction of the heat of absorption that is accounted for in the gas stream. An experimental programme to evaluate fr was carried out and the data correlated. It was found that fr was dependent on the thermo‐physical properties and the hydrodynamic conditions. It is postulated that the mechanism by which this heat transfer occurs is by turbulence at the interface and by entrainment.
Modelling of a woodchip refiner using artificial neural network
Authors: Qian Yu , Tessier Patrick J. C.
Published: August 2005

Neural networks can be an attractive alternative to mathematical modelling of complex and poorly understood processes if input/output data can easily be obtained. Woodchip refining falls into this category. The mechanism of the refining process is still being studied and no thorough models have yet been developed. A feed‐forward neural network is proposed for modelling of woodchip refiners. The outputs predicted by the neural network are compared with industrial refiner data. It is also shown that a modified neural network structure can be used to optimize refiner operation and product quality. The advantages and disadvantages of neural network model application in simulation and optimization of industrial processes are discussed.
Amine plant simulation by mass transfer and kinetic approach
Authors: Fathi J. , Baheri H. R.
Published: August 2005

A computer program has been developed for the simulation of gas‐liquid absorption columns. The calculations are based on known assumptions and methods – the two‐film theory. Mass and energy balances together with chemical reactions are carried out for each tray of the column. The results obtained from the simulation procedure are compared with those measured at industrial plants. For the items “rich solution temperature” and “calculated number of trays” the agreement of calculated and measured results are satisfactory. However, for the CO2 and H2S content of the streams only few comparisons could be made due to the fact that no measured values are available.
Experimental determination and simulation of explosion limits of benzene‐inert gas‐air mixtures
Authors: Gündüz Gönül , Stickling Johannes , Rennhack Rolf
Published: August 2005

In this work, the influences of inert diluents, N2 and CO2, on the explosion limits of benzene‐air mixtures were investigated at two different initial temperatures. The model of constant flame temperature was tested for the simulation of the experimental data. It was seen that the model fitted the data of the lower explosion limits, but didn't correlate with the data of the upper explosion limits.
Polydispersity effect in facilitated transport through liquid surfactant membranes
Authors: Datta Siddhartha , Sanyal Shyamal K.
Published: August 2005

An advancing reaction‐front model has been proposed in the present study of facilitated transport through liquid surfactant membranes by taking into consideration a size distribution of emulsion drops. The model takes into account the continuous and outer liquid membrane phase resistances along with diffusion through composite emulsion drops. The effect of various dispersion phenomena on the mass transfer rate of solute has been discussed for two different operating conditions corresponding to different drop size distributions but having the same Sauter mean diameter. Membrane leakage is assumed to be negligible.
Conversion of ethane on modified zsm‐5 zeolites: A study of aromatization as a function of reaction media
Authors: Hagen Anke , Roessner Frank , Reschetilowski Wladimir
Published: August 2005

Ethane was converted into aromatic hydrocarbons on H‐ZSM‐5 and Zn‐ZSM‐5 at 773 K in a flow reactor system using nitrogen and hydrogen as carrier gases. Residence time was varied by changing catalyst weights to obtain information about primary and secondary reaction steps. On H‐ZSM‐5 ethane conversion proceeds via formation of ethene and subsequent oligomerization. Oligomers undergo fast isomerization, cracking and cyclization leading to a broad spectrum of aromatics. Methane is a secondary product. Dehydrogenation reactions proceed irreversibly. Introduction of zinc species significantly enhances the aromatization activity of H‐ZSM‐5 on the one hand and causes a change of reaction mechanism on the other. Oligomerization of olefinic intermediates can take place at zinc sites. In nitrogen, aromatization of C6 olefins is faster than isomerization and cracking of these oligomers. Dehydrogenation steps proceed reversibly. On using hydrogen as the carrier gas over Zn‐ZSM‐5, the conversion of ethane as well as the formation of aromatics decrease. Furthermore, aromatics richer in hydrogen (especially C8 aromatics) are then formed preferentially.
Comparison of methods for characterization of complex mixtures
Authors: Quiroga Mónica E. , Campanella Enrique A.
Published: August 2005

Comparative calculations of several methods of characterization for three multicomponent mixtures that include paraffin, naphthene, and aromatic fractions are shown. Each method requires different experimental information about the mixtures but all use the numerical Gauss‐Legendre quadrature. The results show that characterization making direct use of the TPB distillation curve is the most suitable approach for characterizing complex multicomponent mixtures.
Three‐step heterogeneous catalytic reaction mechanism with coverage dependent parameters
Authors: Murzin Dmitry Yu.
Published: August 2005

A three‐step catalytic reaction (TSCR) mechanism is considered. It involves the steps of adsorption of two different reactants and the bimolecular reaction between adsorbed species. For this mechanism the rates of elementary reactions were assumed to be dependent on the surface coverages of two adspecies due to the repulsive mutual interactions between adsorbed species. The surface electronic gas model was chosen for the description of surface inhomogeneity. It is demonstrated that for such a TSCR mechanism sustained oscillations cannot exit. The application of the model to evaluation of kinetic data is discussed.
A study of gas hold‐up, liquid velocity, and mixing time in a complex high viscosity, fermentation fluid in an airlift bioreactor
Authors: Wang Yuchun , McNeil Brian
Published: August 2005

Whole mean gas hold‐up εg, liquid recirculation velocity U1d and mixing time tm were measured in a 0.12 m3 airlift reactor, with an external recirculation loop (ALR‐EL), which was used for the production of the exopolysaccharide, Scleroglucan, by cultivation of the filamentous fungus, Scleroticum glucanicum. Under identical fermentation conditions, the effects of air flowrate and the viscosity of the culture fluid on εg, U1d, and tm were investigated. In the range studied, empirical correlation for εg, U1d and tm with both superficial velocity Ugr and consistency coefficient K were obtained and expressed separately.
Optimization strategies and their application to heat exchanger network synthesis
Authors: Roosen Peter , Groß Bernd
Published: August 2005

Heat exchanger network synthesis (HENS) is a frequently encountered class of chemical engineering optimization targets. Besides its immediate relevance to practical, industrial applications, the HENS problem presents an adequate complexity class to test new technique of numerical optimization and benchmark them.

This paper first sketches two established approaches to HEN optimization. The fist one is based on thermodynamic insight, while the second one uses MINLP techniques for rigid optimization. Afterwards an alternative approach based on the paradigm of evolutionary optimization is introduced. The newly developed optimizer module is coupled to the well known ASPEN PLUS simulator. The program package allows the optimization of parameters as well as network structure.
Energy system analysis is fertilizer complex – pinch analysis vs. Exergy analysis
Authors: Radgen Peter , Lucas Klaus
Published: August 2005

A critical comparison of the strength and weakness of two optimization strategies for thermochemical process is made, i.e. pinch analysis and exergy analysis. Although both methods have a unique root, i.e. the minimization of the exergy losses, pinch analysis has evolved to become an instrument of direct and optimal design of a heat exchanger network while exergy analysis as such does not give an indication about process optimization. In combination with an adequate simulation code of a process, however, it can help to find the optimum process structure.
Exergy as an objective function to MINLP problems in process synthesis
Authors: Futterer Eberhard , Gruhn Günter , Lohe Bernd , Noronha Santosh , Rücker Andreas
Published: August 2005

The optimal layout of a chemical plant is determined by both economic and environmental aspects. The mathematical approach to this problem is given by representing the process through a superstructure, including any foreseeable process topology. It is described by integer variables, whereas real variables arise from the unit models and the thermodynamic calculations. By including process constraints and an objective function, a Mixed Integer Nonlinear Programming (MINLP) optimization problem is postulated. In this paper MINLP algorithms are combined with a technique for the exergy analysis of chemical processes for the preliminary screening of process alternatives. The exergy analysis as a screening technique is advantageous due to a significantly smaller amount of required data and computing time, compared with true optimization, including cost estimation at an earlier stage of process synthesis and analysis. Thus, a broader solution space can be examined. The subsequent cost analysis has then only to be focused on a reduced set of parameters.
Growth of a liquid film accompanied by a zero‐order reaction
Authors: Sirohi Vivek , Garg Rajeev , Bhaskarwar Ashok N.
Published: August 2005

The growth of a thin liquid film surrounded by gas pocket(s), undergoing absorption with a zero‐order chemical reaction, has been simulated by an apt mathematical model in order to study the influence of various parameters involved. The solution has been obtained semi analytically using Goodman's integral method and solving the resulting differential equation by a fourth‐order Runge‐Kutta numerical integration algorithm. The computations reveal the strong dependence of film growth on reaction rate, diffusivity, and molar volume whereas the effect of gas‐pocket volume and initial film thickness are moderate.
Application of parametric sensitivity to batch process safety: Theoretical and experimental studies
Authors: Alós Miguel A. , Zaldívar Jose M. , Strozzi Fernanda , Nomen Rosa , Sempere Julià
Published: August 2005

The generalised thermal parametric sensitivity criterion is applied to the thermal behaviour of the esterification of 2‐butanol with propionic anhydride in a batch reactor under isoperibolic conditions. The reaction is catalysed by sulphuric acid, and the study covered the effects of both catalyst concentration and, in the absence of catalyst, jacket temperature. The experimental location of regions of parametric sensitivity agreed well with theoretical predictions. However, the process conditions that meet conventional standards for safe process design fall into the supercritical sensitivity region of the parametric model. This apparently contradictory result is discussed. The usefulness of the parametric sensitivity approach is illustrated by practical examples of scale‐up and heat transfer in reactions prone to thermal runaway.
Hybrid knowledge‐based tool for mixing equipment selection – the fuzzy neural network
Authors: Kraslawski Andrzej , Pedrycz Witold , Koiranen Tuomas , Nyström Lars
Published: August 2005

The hybrid knowledge‐based system proposed in this paper consists of a “stiff” segment, viz. the expert system based on the object‐oriented approach, and a flexible part, viz. the neural network. Some of the input parameters of the problem and output parameters of the “stiff” system are presented as the fuzzy numbers. Detailed information is also presented about the development of the neural network. The most evident advantages of the proposed introduction of a hybrid architecture of the knowledge‐based system are a faster evaluation and generation of design alternatives and support of systematic searches and storage of experience. In addition, the resulting ability to extrapolate results would be unattainable with separately acting stiff and flexible systems. A system for the estimation of the parameters of a mixing system for wastewater treatment is presented as an example to illustrate the principles of the hybrid system.
Freezing time predictions of buried pipes: A 3‐D transient simulation
Authors: Hastaoglu Mehmet A. , Hakim Ali Abbas
Published: August 2005

Heat transfer from a buried pipe carrying a flowing liquid is analysed using a 3‐dimensional geometry, transient modeling and numerical methods. The pipe is buried horizontally in soil whose initial surface temperature is assumed to be below that of the bulk soil and the pipe, and also below the freezing temperature of the liquid. The problem is solved using string‐intersected‐boundaries and a three‐level alternating‐direction‐implicit finite‐difference method. It is possible to predict the time taken for the fluid anywhere in the pipe to fall to its freezing point. The minimum burial depth needed so that the pipe does not freeze is also predicted. The simulation was run on an ordinary mainframe computer with very small computation times. The package developed can be used by designers such as pipeline, plant and water distribution engineers.
A general integral method for the determination of empirical reaction rates and parameters
Authors: Neuhaus Gerd W. , Maier Wilhelm F.
Published: August 2005

A general procedure for the determination of empirical rates and kinetic parameters of irreversible, constant volume reactions is presented which is clearly superior to the conventional methods of kinetic analysis. By nonlinear regression of the integrated potential rate equation for stoichiometrically independent reactions of n the order, the empirical pre‐exponential factor, activation energy, and reaction order are obtained. The regression equation relates explicitly the final concentration of the reference component not only to the time parameter of the reaction but also to the initial concentration. This includes an integral method for the determination of reaction orders in concentration. Two limiting cases are treated ‐ the stoichiometric mixture of reactants and a mixture with one reactant in large excess. By analysis of standard Hougen‐Watson rate models the dependence of empirical reaction orders on experimental conditions and simulated rate parameters is evaluated. By grouping the data into ranges of monotonous concentration change, conversion rates and rate parameters of reactions with changing volumes are calculated in a less rigorous deduction. The simplified interpretation of published experimental data is demonstated for the total oxidation of methane on Pd‐catalysts and the dehydrogenation of cyclohexanol on ZnO.
On the activity and stability of platinum/alumina catalyst during multiple deactivation and regeneration
Authors: Manga Ngomo H. , Susu Alfred A.
Published: August 2005

Catalyst surface characterizations have been carried out to investigate the role of dispersion on catalyst activity and to probe the occurrence of oscillations in coking levels with cycle number generally observed during multiple deactivation and regeneration schemes. The titrations were done, cycle by cycle, at 430°C after oxidation and at the same temperature (430°C) after reduction at 500°C. Results show the usually observed trend ‐ that the dispersions after oxidation are higher than those after reduction. The average decline in dispersion from oxidation to reduction was calculated to be 39.25%. It was observed that the cycles with high toxic coke removal were characterised by high deactivation times. The deactivation times were still high even for cycles subsequent to those with low dispersion. At high dispersions the catalyst had short deactivation times, that is the small crystallites deactivate faster than large ones. The nature of reducebale coke and the efficiency of its removal is a much more determinant factor of catalyst activity than the level of metal disperision. Thus prolonged toxic coke reduction at the high temperature of 500°C, though resulting in an apparent lowering of dispersion, does not affect the quality of the catalyst. The dispersion before reduction could be retained on oxidation. Hence reduction at 500°C did not introduce sintering.
A model for primary and heterogeneous secondary reactions of wood pyrolysis
Authors: Ahuja Pradeep , Kumar Surendra , Singh Prem Chand
Published: August 2005

A chain growth model for heterogeneous secondary reactions is developed for the pyrolysis of large wood particles and the parameters determined by nonlinear optimization. The model takes both the volatile retention time and cracking and repolymerization reactions of the vapours with the decomposing solid as well as sutocatalysis into consideration. The extent of the secondary reactions is strongly influenced by the time and the ratio of the autocatalytic (propagation) reaction rate to noncatalytic (initiation) reaction rate. The wood which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exothermic heat of reaction and yields a higher amount of final char residue. This fact confirms the heterogeneous secondary reactions lead to carbon enrichment of the final residue and are accompanied with an exothermic heat of reaction. The lower activation energies of the initiation and propagation reactions as compared to primary reactions (competitive reaction model consisting of weight loss and char forming reactions) confirm autocatalysis in large particles. The sealed reactor studies of small quantities of fine wood samples show that heterogeneous secondary reactions and not lower heating rates in large particles are the main source of char formed during the thermal decomposition of large wood particles. The model predictions are in agreement with the weight loss and temperature versus time curves over a wide range of particle size and furnace temperatures.
Distributed control suboptimal operating policies for ethane thermal cracking reactors
Authors: Bozga Grigore , Tsakiris Cristian
Published: August 2005

A nonlinear unsteady state optimal control problem for a classical, constant diameter ethane thermal cracking reactor is formulated and solved. The process is represented by a pseudo steady state mathematical model, consisting of mass and heat balance, pressure drop, and coking equations. The performance index is of an economical nature, representing the global benefit of reactor operation over a constant operating time. As control variables, the space and time dependent skin tube temperature, the time dependent steam to hydrocarbon ratio, and the time dependent feed flow rate were considered. The results are in agreement with the process physicochemical and technological fundamentals.
Does the fluid elasticity influence the dispersion in packed beds?
Authors: Westerterp K. Roel , Wijngaarden Ruud J. , Nijhuis Niek B. G.
Published: August 2005

Reasons are given why the axial dispersion in a gas flowing through a packed bed may be influenced by the elasticity ‐ or compressibility ‐ of the fluid. To support this hypothesis, experiments have been done in a packed column at pressures from 0.13 to 2.0 MPa. The elasticity E of a gas is proportional to the pressure P and the compressibility to 1/P. The axial dispersion coefficients as determined were found to be a function of the pressure in the packed bed in the turbulent flow region of 3 < Rep < 150 if the Bodenstein number is plotted as a function of the particle Reynolds number. This is shown to be an artifact. The pressure influence is eliminated, if Bom, ax is plotted versus the ratio of the kinetic forces over the elastic forces ϱu2/E. Regrettably, Bom, ax seems to be independent of ϱu2/E. For the moment we only can conclude that Bom, ax in the turbulent region is a unique function of the velocity of the gas which flows through the packed bed. Although the fact that a constant Bo value is obtained when plotted against ϱu2/E, the experimental results are so intriguing we wanted to make them public already now. The experimental work proceeds.
Energy transfer in hydrogen/air and methane/air mixtures
Authors: Aretz Wilfried , Metz Stefan W. , Wilhelmi Herbert
Published: August 2005

This article presents gas kinetic calculation methods for the energy transport in hydrogen/ air and methane/ air mixture. The total molecular heat conductivity as well as its various shares are calculated and discussed for a temperature range of 400 to 3500 K and for a pressure of 105 Pa. The variation of the air/ fuel ratio under the conditions of chemical equilibrium is also investigated. As opposed to our previous article, an extension of the Chapman‐Knskog method which goes beyond the classical 1st approximation for elastic collisions is applied for the evaluation of suitable calculation methods. This is carried out following the method for strong relaxing thermal nonequilibrium according to Brun. The equations applied in this work are simplified formulations for the proximity to equilibrium. The method for the evaluation of the parameters of the inelastic collision and some chosen results are presented. A discussion of the various shares of the molecular heat conductivity emphasizes the considerable influence of the diffusion and the thermal diffusion processes in fuel/ air mixtures compared to the Fourier heat conductivity.
Modelling of affinity separation by batch and fixed bed adsorption – a comparative study
Authors: Sridhar P.
Published: August 2005

Affinity separation can be achieved by batch and fixed bed modes. Mathematical models including film mass transfer, intraparticle diffusion, and reversible reaction are formulated for both fixed bed and batch adsorption processes. Orthogonal collocation method is used to numerically evaluate the performance of batch and fixed bed adsorption. The efficiencies are evaluated in terms of both the solute recovery and adsorbent utilization. The effect of the following parameters is simulated for the comparison purpose: solute concentration, reaction kinetics, ligand content, and particle size. Fixed bed mode is found to be efficient.
Kinetics of calcium carbonate decomposition
Authors: Rao T. Rajeswara
Published: August 2005

Experiments on thermal decomposition of calcium carbonate were carried out in a thermogravimetric analyser under non‐isothermal conditions of different heating rates (10 to 100°C/ min). A new technique for determining the kinetic parameters from non‐ isothermal thermogravimetric data was described. The activation energy and frequency factors were determined from the proposed method and also by the widely used Coats and Redfern method. The kinetic compensation effect between the activation energy and frequency factors obtained from both the methods were found to be very consistent and are in very good agreement with the literature values. The activation energy and frequency factors were also determined from isothermal experiments in the temperature range from 680 to 875°C. The activation energy and frequency factors determined from isothermal data using initial rate method were also found to be in very good agreement with the above results. It is also found that the kinetic parameters determined by isothermal analysis were consistent with the values determined by non‐isothermal analysis.
Numerical simulation of contraction flow of a dilute polymer solution
Authors: Bschorer Sabine , Brunn Peter O.
Published: August 2005

Numerical results are presented for a contraction flow of a non‐Newtonian fluid. In this work a new finite‐volume algorithm was used and various non‐Newtonian fluid models (with and without elasticity) were studied. The results are discussed and compared to experimental results obtained with a dilute polymer solution.
Dual substrate limitations in upflow packed‐bed biofilm reactors — a theoretical study
Authors: Beg Shafkat Ali , Hassan Mirza Manirul , Chaudhry Muhammad Arshad Saeed
Published: August 2005

The performance of an upflow packed‐bed biofilm reactor has been analyzed under dual substrate limitation conditions. The numerical solution of the proposed equations defining the system has been obtained for a wide range of operating conditions for a case of practical significance involving glucose and oxygen as dual substrates. The results show that the inlet glucose concentration defines the limiting substrate at a position near the inlet of the reactor. For inlet glucose concentrations up to 300 mg/l, glucose acts as the limiting substrate. However, for inlet concentrations of 400 mg/l of glucose or higher, oxygen assumes the role of the limiting substrate at that position. For all other positions in the reactor, glucose acts as the limiting substrate, irrespective of its inlet concentration. Extensive computations were performed in order to define regions where glucose, oxygen or both are limiting. The predicted results have been found to be in agreement with the theoretical criteria, proposed in the literature, of determining the limiting substrate.