Abstract: A review of the diagnostical methods of the flow patterns which largely determine the performance of gas—liquid systems encountered widely in chemical engineering practice is presented. The need for objective and reliable diagnostical criteria has been confirmed by the analysis of published studies; confidence in the flow pattern maps used in design and operation is limited. The methods of stochastic signal analysis as applied to the fluctuations of some structural (voidage) and energetic (pressure) parameters are found to be promising. The increasing availability of suitable hardware facilitates on-line diagnostics aimed at the optimization and safety control of industrial equipment.

Abstract: Owing to the mechanisms of bubble formation and detachment during boiling, the concentration of impurities contained in the liquid can increase locally by several orders of magnitude. This may cause severe fouling even for low foulant concentrations. As the nucleation site density increases with increasing heat flux, a similar trend is expected for the deposition rates. High heat fluxes, however, are desirable as they allow smaller equipment. Using a precision pool boiling test apparatus in conjunction with a microprocessor-controlled camera and video equipment, the effects of heat flux and of CaSO 4 concentration on the formation and growth of deposits are investigated. The transient change in heat transfer is closely related to changes in bubble departure diameter, bubble frequency and bubble site density.

Abstract: In pulse-jet filters, bag cleaning is accomplished by a compressed air pulse being injected into the clean side of a filter bag. This causes an abrupt increase in pressure within it, leading to a flow reversal as well as to acceleration of the filter cloth. Investigations with a 2.5 m long filter bag have shown that it is not the forces arising from the fabric acceleration, but those arising from the reverse air flow which play the major role in bag cleaning, and that the dust-cake release is related to the peak pressure built up inside the bag. In order to achieve complete dust dislodgement, a critical overpressure is necessary at every location along the length of a bag for a certain period of time. This overpressure depends on the dust-cake-fabric interaction. Experiments with flat-geometry fabric samples have shown that the critical overpressure can be found from a laboratory-scale experiment.

Abstract: Interfacial areas and gas hold-ups have been determined at pressures up to 1.85 MPa in a bubble column with a diameter of 85.5 mm and for superficial gas velocities between 1 and 10 cm s−1. In some experiments the bubble column was packed with glass cylinders of length 5.0 mm and diameter 4.0 mm. The interfacial areas were determined by the chemical method using the model reaction between CO2 and aqueous diethanolamine (DEA) and hold-ups by observation of height differences. The interfacial areas in the packed bubble column are unaffected by pressure. The gas hold-ups as well as the interfacial areas in the bubble column increase with increasing operating pressure. The magnitude of the pressure influence depends on the superficial gas velocity. The positive influence of pressure on the gas hold-ups and the interfacial in the bubble column originates from the formation of smaller bubbles at the gas distributor.

Abstract: Previous experimental and theoretical work has shown that the numerous one- and two-phase packed bed dispersion models can be regarded to be equivalent in describing important aspects of fixed bed behaviour. This paper is concerned with the problem of how thermal model coefficients for any other model are obtained if a set of reliable coefficients is known for one particular model from experimental results. The results of this analysis agree with observations made during the past decades that the Nusselt numbers are particularly sensitive to small changes of the axial dispersion coefficients and that—depending on the model—they differ considerably. While this is the case for the range of lower Reynolds numbers (say Re

Abstract: The mixing of a packed bulk material consisting of non-cohesive mechanically identical particles during shearing is modelled mathematically with the aid of a diffusion equation. In this paper, an equation relating the diffusion coefficient proportionally to the shear rate is derived. This prediction was verified experimentally by shearing experiments using a Couette apparatus. The model bulk consisted of vertically packed cylindrical metal rods, marked by colouring.

Abstract: The paper presents definitions of various types of uncertainty and examples of their applicability in chemical engineering. Special attention is given to such problems as control, modelling, process synthesis, multi-objective optimization and expert systems.

Abstract: All important studies on the influence of pressure on mass transfer phenomena in gas—liquid systems and reactors are reviewed critically. Points of agreement and conflict are indicated and discussed. It is concluded that: (1) the initial bubble size at a single orifice decreases with increasing pressure; (2) the gas-phase mass transfer coefficient k G is inversely proportional to the pressure to the power n , where n depends on the mass transfer mechanism; (3) the liquid-phase mass transfer coefficient k L is not influenced by pressure; (4) the gas hold-up e G in bubble columns increases with increasing pressure. However, insufficient data on the influence of the operating pressure on the interfacial areas in gas—liquid contactors are available.

Abstract: Freeze-drying experiments were carried out for milk and water in a vial, and the influence of the heating plate temperature was examined. A simple simulation based on a 'uniformly retreating ice front' type model was performed. A numerical model enables the changes in both water content and temperature of these products to be described satisfactorily during the experiment. Three transfer parameters (water vapour diffusivity in the dry layer, the external mass transfer coefficient, and the contact resistance to the heat transfer) were determined. The contact resistance between the product and the heating plate is the most important resistance to heat transfer, even controlling the dehydration kinetics.

Abstract: Mixtures with very many components are often best described by continuous distribution functions. While the continuous analysis of the chemical kinetics of a mixture in which individual components undergo first-order reactions dates back to 1968, the case where the intrinsic kinetics are nonlinear has been analyzed only recently. This paper presents an overview of many recent results obtained in the area, and it discusses two converse issues: first, given the intrinsic kinetics, what are the observable overall lumped kinetics; second, given the observable overall lumped kinetics, which intrinsic kinetics can justify the observed behavior. While most of the results are for a batch or plug flow reactor, some results relevant to other reactor configurations are also discussed.

Abstract: A heterogeneous fluid dynamic model has been developed to describe the complex flow structure of two-phase flow in bubble columns. The equation of continuity and the momentum balances are the basis of the model. The coupling of the two phases is performed by a force of interaction which is deduced by a force balance around a single rising bubble. Multiphase flow mixing processes are taken into consideration by introducing the turbulent viscosities of the two phases involved. The model equations were implemented successfully by applying a tridiagonal matrix algorithm.

Abstract: Experimental data are presented concerning the local solid-liquid mass transfer coefficients in a packed column operated with a cocurrent downflow of gas and liquid. The experiments were carried out for two diameters of spherical particles, with the flow rates of both phases and the physicochemical properties of the liquid varied over a wide range. The experimental results are correlated and compared with the appropriate literature data.

Abstract: Dimensional, asymptotic and speculative analysis is used to identify the many regimes of motion of a single bubble. This structure, together with existing analytical solutions for individual regimes, is used to formulate a single correlating equation for the entire gamut of behavior. The only significant empiricism in the expression is that which characterizes the sharpness of the transitions between regimes. Prior correlations have not incorporated all of this theoretical structure, and are thereby necessarily in error functionally.

Abstract: Simple experiments yield general correlations for the interaction force experienced by a sphere of any diameter that is submerged in a liquid fluidized bed. The results confirm the well-known relationship for submerged objects substantially larger than the fluidized particles and approach the appropriately modified ‘unhindered’ limit for very small submerged spheres.

Abstract: In the paper a procedure for the design of a fluidized bed dryer of biosynthesis products is presented. In this method a simplified mathematical model of heat and mass transfer in a fluidized bed dryer and deactivation kinetics of biosynthesis products during drying are utilized. The procedure is illustrated by the drying of yeast Saccharomyces cerevisiae. The effect of longitudinal dispersion on the process and results of fluidized bed drying in a continuous system are analysed.

Abstract: The nucleation and early growth characteristics of K2SO4 crystals have been studied under metastable zone conditions relevant to salting-out from aqueous solution by the addition of aqueous acetone. Primary nuclei are formed well before the metastable zone boundary is reached. The crystals then develop by a surface-reaction growth mechanism with a growth order higher than 2, the precise value depending on the actual supersaturation created and/or solution composition. The metastable zone width and the mechanisms of nucleation and early crystal growth for salting-out precipitation are similar to those relevant to cooling crystallization of unseeded solutions. Particular attention is paid to the case where acetone diluted with water is used as the salting-out agent. This mode of operation, which avoids excessive nucleation in the local regions of primary contact, appears to offer considerable advantages for industrial processing.

Abstract: For vacuum contact drying of mechanically agitated, coarse, hygroscopic bulk material the drying rate and final moisture content can be controlled by a proper choice of drying conditions, mixing intensity and granular bed properties. This is demonstrated by means of theoretical and experimental investigations.

Abstract: In connection with the drying of materials wetted with mixtures, the question of how mass transfer in the wet pores depends on the pore saturation arises. The analysis reveals that, similar to the thermal conductivity of composite materials, diffusion may take place quantitatively in both the pore liquid and the penetrating gas phase. The overall or effective diffusivity, defined with the gradient of one component in the liquid, ranges between two limiting values: the binary liquid diffusion coefficient at 100% saturation and a so-called effective gas-side diffusivity in the nearly dry material, which strongly depends on the gas-liquid equilibrium. At moderate temperatures, the limiting values are, in general, of the same order of magnitude; they may, however, differ considerably if one component is non-volatile. It can be shown that the effective diffusivity should follow a function of the pore saturation limited by the curves obtained for a parallel and a serial arrangement of the liquid and the gas phase.

Abstract: For the selection and design of a drying process a certain amount of information is required. Thermodynamic equilibria as well as heat and mass transfer are the classical tools with which to describe the drying operation. In the development of a drying process further specific properties of the product to be dried must be known. These properties, such as, for example, explosion characteristics or sticking behaviour, reduce in most cases the number of drying processes possible. Specific desired qualities of the final product, such as low dust level, also create restrictions to the process design. Therefore the properties of the moist product, the dry product, and even the product during drying must be characterized. Some laboratory methods for this characterization are discussed. Their application is explained with the help of two examples. Many of the laboratory methods in use are not physically based but are of a more empirical character. The reason for this is that the physical relationships are often not well understood. Intensive research is required in this field. A physical basis for product characterization is suggested in this paper.

Abstract: In this work a way of calculating effective transport coefficients from the microgeometry of a porous medium is presented. The model material consists of a random packing of uniform spheres, and by applying the Voronoi—Delaunay tessellation technique the void between the spheres is simulated as a network of cylindrical pores. The tessellation yields all the necessary information for the structural characterization, such as the pore diameter, pore angle and pore length distribution functions and the topological interconnection. The effective transport coefficients of ordinary diffusion, Knudsen flow and viscous flow are calculated numerically by mass balancing at each network node and over all nodes of the system. The results obtained agree very well with the experimental ones, especially for ordinary diffusion. For Knudsen and viscous flow, inaccuracies in the estimation of the pore overlapping volume cause a relative error between the numerical and experimental results of the order of 16%–33%.

Abstract: Diffusion coefficients in eight binary liquid mixtures were determined as a function of temperature and composition with the dispersion method. Simultaneously, the density, viscosity and refractive index were measured.

Abstract: The Voronoi-Delaunay tessellation technique for subdividing the space between entities is used in this paper in order to characterize a porous medium. The model material consists of a bundle of cylinders with uniform diameter, randomly arranged so that they are closest to each other. The resulting network consists of quadrilaterals of different width and height and infinite depth. The width is defined as the closest distance between two adjacent cylinders (narrowest neck). On the other hand, the height is defined as the length of a Voronoi polygon's edge (distance between two nodes). The height vector lies perpendicular to the width vector. The coordination number, denoting the number of edges meeting at a node, was always found to be 3. The porosity of the packing was 0.47 and of the resulting network 0.41. The simulation consists of about 150 realizations of a model network, which, on average, accommodated about 89 cylinders, 180 nodes and 270 slits. Topologically, the slits do not show any preference for a specific orientation, so that in a large-scale network all possible orientations of the slits will be equally represented. The relation between the slit width and the slit height, of great importance for calculating dynamic properties, is very satisfactorily approximated by a linear expression. A way to predict the saturation-dependent transport coefficient of ordinary diffusion, Knudsen diffusion and viscous flow is proposed. The network showed a percolation threshold at a number based saturation of 0.67, the same as for the honeycomb regular lattice. The tortuosity factor was found to be 2.95.

Abstract: New experimental values of the liquid-side mass transfer coefficient k L were obtained for the absorption of helium, nitrogen and propane into a film of a highly viscous aqueous solution of ethylene glycol (kinematic viscosity v = 1.4 × 10 m 2 s −1 ), flowing down the vertical surface of an expanded metal sheet and down a smooth wetted wall. The purpose of the experiments performed in the range of Reynolds numbers 8 m in the relationship between k L and the molecular diffusivity of a gas, k L ≈ D m . In the case of a smooth wetted wall the mean value of the exponent was close to 0.5 and the results were interpreted by the surface renewal model. In the case of an expanded metal sheet the mean value of the exponent was m = 0.64 and the film-penetration model was the most suitable for the interpretation of the experimental results.

Abstract: Air pollution is caused to a large extent by submicrometre liquid and solid particles called 'aerosols'. Separation of aerosols from residual gases is a major problem today. Well-known equipment like the venturi scrubber cannot handle submicrometre aerosols successfully. However, enlargement of the aerosols by condensed water vapour and subsequent separation by inertia forces in an air jet containing minute water droplets can lead to an acceptable process with moderate energy consumption. The generation of minute water droplets and their role in the separation of aerosols from gases is described in this paper.

Abstract: An analysis of the structure of the steady-state solutions has been performed for the model, which describes transport phenomena and chemical reaction in a porous catalyst pellet. The model equations have been derived based on the assumption that the temperature of the pellet is uniform but different from that of the surrounding fluid. Applying theorems from the singularity theory a method is presented for dividing the global parameter space of the model into regions with different bifurcation diagrams (pellet temperature versus Thiele modulus) and hence different numbers of steady-state solutions. The analysis, performed for the reaction orders 1 4 ⪯ n ⪯ 3 and for three pellet shapes (slab, cylinder and sphere), has led to very interesting conclusions. A subregion of parameters has been found where five steady-state solutions can exist. This subregion decreases with increasing reaction order and there exists a limiting value of the reaction order for which it disappears: ng = 1.151 for the slab and 0.351 for the cylinder. Another surprising feature revealed by this analysis is the dependence of the size of the parameter region with five solutions on the shape of the pellet. This region decreases, going from the slab through the cylinder to the sphere.

Abstract: Extensive experimental results on the pressure loss associated with Newtonian fluid flow through a screen are reported. It is shown that the use of two dimensionless groups, namely, the loss coefficient and the Reynolds number, provides an adequate representation of the data over wide ranges of conditions of Reynolds number, size of screens, etc. There appears to be no influence of the screen aperture/column diameter ratio on the loss coefficient in the range 100–1200.

Abstract: The separation of a certain mass of water from moist air is a basic operation in air conditioning and drying which, if performed by means of the conventional method of cooling and dehumidification, requires a considerable amount of thermal energy. The separation method by direct contact with a hygroscopic solution appears to be less heat consuming, but in such a case the solution desorption operation becomes the critical unit of the energetic performance of the process. This work presents a solution desorption unit in which ambient air is used as the inert carrier and heat is supplied to the contact surface at the same time as the mass transfer takes place. Two heat exchangers recover some sensible heat from the outgoing fluids, preheating the incoming ones, while the inlet temperature of the heating fluid is modulated to keep the final concentration of the solution at the required value. A mathematical model of the apparatus is presented which allows the circulating fluid states to be forecasted, both at the column ends and locally, and the model output analysis is discussed. A detailed analysis of the sensitivity of the performance of the apparatus to the main operating parameters shows which are the main variables affecting the energetic behaviour and their most suitable values for low heat consumption for each working condition. The unit appears to be able to work with quite low values of both energy consumption and thermal level of heating power, over a wide range of combinations of these two parameters, depending on the operation conditions.

Abstract: In order to investigate the mixing of the liquid phase in bubble columns, the axial and radial distribution of a tracer substance in the water/air system was measured. To this end, the tracer was added at the top of the column as a Dirac pulsed signal evenly distributed over the column cross-section. It was established that the concentration distribution of the tracer was a function of the radial profiles of the mean axial velocity of the liquid and of the axial turbulence intensity. On this basis, a model was developed which takes into account the radial profiles of the mean axial velocity of the liquid as the deterministic parameter of mixing. The axial dispersion coefficient which is then obtained and which is adjusted with respect to the deterministic component can be described as the product of the radius of the bubble column and the stochastic component of the axial velocity component, that is, of the turbulence intensity.

Abstract: The process of partial oxidation of o-xylene into phthalic anhydride carried out in two catalyst beds is investigated for the first time. The activity of the second bed is lower than that in the first. It is shown experimentally in a pilot non-isothermal non-adiabatic integral reactor that the use of two beds of industrial catalyst (fresh catalyst in the first bed and spent in the second), in combination with more than one feed-in of o-xylene into the reactor, improves the selectivity and the quality of the phthalic anhydride, as well as the productivity.