On the concentration dependence of the UNIQUAC/UNIFAC models

The UNIQUAC/UNIFAC models can not in general represent vapor-liquid and liquid-liquid equilibria simultaneously with satisfactory results. Recent modifications of the models take into account association and solvation effects by means of chemical theory. However, this approach leads to complicated expressions for the activity coefficients and the improvements are only moderate. A modification of the UNIQUAC/UNIFAC models is presented which takes into account solvation and association by allowing the interaction energies to vary with composition. This approach leads to explicit expressions for the activity coefficients. It is shown that the modified models in simultaneous correlations lead to quantitative representation of vapor-liquid equilibrium data and semi-quantitative representation of binary and ternary liquid-liquid equilibrium data.     

A liquid-liquid extractor model based on UNIFAC

A steady state liquid-liquid extractor model using the UNIFAC group contribution method for phase equilibria has been developed. A heuristic method, which limits the number of stage calculations per iteration to a low number is used, according to the residuals calculated in the previous iteration, the method either selects or rejects a stage for the calculations. With this approach, the same accuracy in results have been obtained at about half the computing time with respect to without use of heuristic method. The model has been applied to simulate staged extractors as well as continuous extractors (plate columns). The model is based on a very robust flash calculation routine and on a versatile package (UNIFAC) for the prediction of thermodynamic properties.     

On the dynamics of distillation processes-V: The topology of the boiling temperature surface and its relation to azeotropic distillation

It is shown that ridges and valleys in the boiling temperature surface do not coincide with azeotropic distillation region boundaries. Thus, the prevai and valleys is shown to be false. This allows for a greater flexibility in design and a better understanding of the behavior of azeotropic distillation     

Process optimisation using linear models

A method is introduced for the optimisation of problems represented by linear models. It is applied to chemical process situations, and its superiority, in some respects, to Linear Programming is demonstrated. Its application to the general optimisation problem through quasilinearisation is also discussed.     

On controlling an autothermal fixed-bed reactor at an unstable state-II: Discrimination among rival models to achieve suitable internal structure

Shortcomings of the traditional approach to modeling fixed-bed reactors are presented. These include a very oscillatory approximation to the initial dead time in the temperature response to boundary forcing, as well as the extreme resistivity of the large-scale lumped system to model reduction by modal techniques. These problems are shown to be directly related to an ill-conditioned internal structure of the approximate o.d.e. model, as a result of both an inadequate formulation of the original p.d.e. model and the discretization technique used in lumping the axial coordinate.A new technique for structural dominance analysis is described and shown to be well-suited for investigating the characteristics of a large-scale state-variable model. Five alternative lumped models of an autothermal tubular reactor are analyzed with the new technique; dispersive models are found to be well-behaved with respect to the shortcomings mentioned above.Finally, a set of guidelines is proposed for modeling fixed-bed reactors when an accurate low-order linear model of the system is desirable as, for example, in control applications.     

Periodic capillary models of diffusion in porous solids

Periodic capillary networks are proposed as models for describing diffusion in porous solids. The capillary network is analyzed by means of the nodal c     

The dependence of coefficients in the relation of velocity ratio on parameters of a capsule

The velocity of a capsule in a pipeline is usually plotted in most papers in graphs as a dependence of velocity ratio U_k/U on the mean liquid velocity U. In order to compare various velocity measurements there had been derived the equation U_k/U = f(U) which in general case is hyperbolic containing three parameters. However for evaluation of most published results a simplified linear form of the equation with only two coeflicients is good enough. Both empirical coefficients are influenced by parameters of the system like capsule-pipe diameter ratio, capsule-length diameter ratio or apparent density.     

Fluidized bed char combustion kinetic models

The effect of the kinetics of the heterogeneous reaction between the char and oxygen, and the importance of the subsequent homogeneous oxidation of carbon monoxide oil the performance of an atmospheric fluidized bed combustor have been examined by a comparative analysis of three models. Numerical calculations illustrating the effect of operating variables such as bed temperature, fuel and sorbent particle feed size and excess air were performed for values of the parameters which are pertinent to the design of large scale utility boilers. It is found that the kinetic models predict existence of multiple steady states over a wide range of operating conditions and values of model parameters. The model predictions of combustion efficiency, carbon loading and CO concentrations are in qualitative agreement with experimental data reported in the literature.     

Application of a variable-order semi-implicit Runge-Kutta method to chemical models

A semi-implicit Runge-Kutta method is proposed and application is made to models of chemical kinetics. The presented algorithm GRKV4 is based on an A-stable formula with variable order up to four and possesses an automatic stepsize control. The range of application of GRKV4 is discussed.     

Local thermodynamic models for high pressure process calculations

This paper discusses the local thermodynamic model concept in the context of high pressure chemical processes where equations of state are usually required for performing thermodynamic property evaluations. Functional forms for local K-value models are developed by simplifying the rigorous equations governing fluid phase equilibria. The local models have imbedded adjustable parameters that are evaluated by regression against either experimental data or thermodynamic properties calculated with more rigorous theoretically based models. During the course of the calculations the parameters in the local models are updated periodically, thereby ensuring that the local models continually provide accurate values for thermodynamic properties. Use of the concepts for process calculations is demonstrated in an example problem.     

Effect of temperature on the tensile strength of lactose coated with fatty acids. Part 2. Tablets

A study has been made of the changes produced in the tensile strengths of tablets made from powdered lactose coated with fatty acids when alterations are made to the temperatures employed during tableting, storage and testing.The strengths at a fixed packing fraction of 0.86 depend on the temperature at which they are measured: they decrease with the amount of acid employed; they initially increase as the temperature during compression is increased, then pass through minima. They also depend on the storage conditions and on the melting points of the fatty acids concerned.The results conform to the Arrhenius equation and are explained in terms of the masking of Van der Waals' forces between lactose particles and the formation of welded bonds by the coatings.     

Steady-state analysis of high temperature fuel cells

A mathematical model is presented to describe the steady state behavior of high temperature solid electrolyte fuel cells. The resulting equations are solved for the case of anodic oxidation of hydrogen and carbon monoxide. Similar to chemical reactors, fuel cells are found to exhibit steady-state multiplicity over a wide range of parameters. The paper discusses the relative importance of the pertinent design and operating parameters in order to maintain ignited steady states corresponding to high current densities and nearly complete fuel conversion.     

Experimental observation of temperature gradients occurring in a single zeolite pellet

An experimental study has been made of temperature profiles incident within a zeolite adsorbent particle in the course of adsorption of n-heptan. Using thermocouple wires 0.1 mm in diameter it has been possible to measure simultaneously the temperatures of the surface and in the centre. It was observed that temperature differences between center of the pellet and ambient stagnant gas may be more than 20°C. The results of this work indicate that the overheating of an adsorbent particle is a very rapid process which is followed by a slow cooling of the hot particle. Experimental data are compared with predictions calculated from the theoretical model.     

Optimal temperature profiles for heterogeneous catalytic parallel reactions

The problem of determination of the optimal temperature profiles in a fixed-bed catalytic reactor for the catalytic parallel reactions including transport phenomena (external or internal diffusion) has been worked out.Special attention was given to predict the shapes of the optimal temperature profiles. The general considerations are illustrated by some numerical examples. It has been found that for every studied case the optimal temperature profile need not to end by the isotherm T = T_max.     

On the mechanism of enzyme-catalysed gas—liquid reactions: Absorption of CO2, into buffer solutions containing carbonic anhydrase

The gas absorption with chemical reaction data of Lee and Tsao [1, 2] (i.e. oxidation of glucose solutions containing glucose oxidase or platinum deposited on activated carbon and absorption of carbondioxide into buffer solutions in the presence of carbonic anhydrase) were reexamined and various reasons for their apparent inconsistency with the classical gas absorption models were suggested.The rate of absorption of carbondioxide into 0.5 gmol/l Na₂CO₃ + 0.5 gmol/l NaHCO₂ in the presence of carbonic anhydrase was measured in a stirred cell and also in a laminar jet and a wetted wall column aparatus. Enhancement in the absorption rate due to enzyme was observed in all three absorbers and the results were found to be compatible with classical mass transfer models (such as Higbie's and Danckwerts' models). On the contrary to Tsao[2], who proposed a two-zone model, which assumes a surface-excess of the enzyme at the gas—liquid interface, these results suggest that the carbonic anhydrase distribution in the liquid is probably uniform.