The efficient estimation of rates of multicomponent condensation by a film model

The calculation of rates of multicomponent mass transfer by a film model in situations where the interfacial composition is not defined is shown to be equivalent to the calculation of the vapour fraction and phase compositions of a mixture of known overall composition. A method of calculating vapour fraction is adapted to the calculation of rates of multicomponent condensation.     

Performance of a large-cone-angle hydrocyclone—I: Hydrodynamics

As opposed to “normal” hydrocyclones, large-cone-angle hydrocyclones can be used to separate solids with equal free-fall velocities but with different densities. One of the applications of these cyclones is the recovery of heavy minerals from sand.In order to investigate the hydrodynamical behaviour of such a cyclone the following experiments have been carried out on two cyclones of diferent dimensions: determination of solids distribution at steady state operation; measurement of the residence time distribution of the liquid; pressure gradient and pressure drop measurements.From the experimental results it follows that at the bottom a bed of suspended solids rotates, which more or less seems to be fluidized. Similar to the derivation of an expression of the pressure gradient in a one-phase vortex flow from the Navier-Stokes equation, an expression of the pressure gradient in such a solid-liquid vortex flow has been obtained from two-phase momentum equations. From this theory it follows that in the rotating bed a secondary solids flow has to occur; solids are moving towards the centre at the bottom and away from the centre at the top.     

The pinch design method for heat exchanger networks

A novel method is presented for the design of heat exchanger networks. The method is the first to combine sufficient simplicity to be used by hand with near certainty to identify “best” designs, even for large problems. “Best” designs feature the highest degree of energy recovery possible with a given number of capital items. Moreover, they feature network patterns required for good controllability, plant layout, intrinsic safety, etc.     

Some aspects of limonene oxidation at Pt electrodes in aqueous and acetonitrile solutions

Limonene oxidation does not occur at reasonable rates in aqueous media, but it is found that in acidified acetonitrile solutions containing some water, steady-state currents can be maintained. Obtained data suggest that water addition to a double bond happens before electrochemical oxidation in the acetonitrile solution takes place.     

Heat transfer in laminar flow of non-Newtonian fluids

The main objective of this work is the consideration of local heat transfer coefficients for non-Newtonian power-law pseudoplastic liquid in laminar flow in circular conduits. The wall boundary conditions chosen are cases involving uniformly constant heat flux and step change in heat flux.Analytical solutions are developed for the wall temperature profile and compared with experimental data. Additionally, the experimental data have been correlated for comparison with existing relationships, hitherto not verified adequately. The limits of experimental data are:

     

Minimum utility usage in heat exchanger network synthesis A transportation problem

This paper formulates the minimum utility calculation for a heat exchanger network synthesis as a “transportation problem” from linear programming, allowing one to develop an effective interactive computing aid for this problem. The approach is to linearize cooling/heating curves and partition the only at potential pinch points. In this formulation user imposed constraints are readily included, the latter permitting selected stream/stream matches disallowed in total or in part.By altering the formulation of the objective function, the paper also shows how to solve a minimum utility cost problem, where each utility is available at a single temperature level. A procedure is included to handle a utility which passes through a temperature change when being used.Extending the partitioning procedure permits the formulation to accommodate match dependent approach temperatures, an extension which is useful when heat transfer through a third fluid only is allowed for some matches.     

Mass transfer in laminar liquid jets: Measurement of diffusion coefficients

Laminar liquid jets used in mass transfer studies have been generally discharged from convergent nozzles or orifices. The use of long straight capillary tubes to discharge the liquid jet is studied in this work and numerical values of the jet diameter and surface velocity are obtained. These computed values are applied to predict the actual absorption rates in the laminar jet for different values of N_J= N_Re/N_Fr, N_we and the axial coordinate. The effect of N_we is slight and can be disregarded in analysing experimental data. Experimental values of absorption of CO₂ in water and water-glycerol solutions are obtained using a laminar liquid jet discharged from capillary tube and good agreement is found between the computed values and the measured absorption rates. The results are applied to the determination of diffusion coefficient for the CO₂ in water glycerol solutions.     

Designing heat exchanger networks for existing chemical plants

New targets for the heat exchanger network synthesis problem, which account for both heating and power needs, are defined. Especially useful when updating existing facilities, the proposed targets stand for both the maximum heat flow to be recovered and the types and number of heating utilities to be allocated if all plant energy requirements are to be satisfied at the lowest fuel cost. The new goals are computed by solving a small-scale nonlinear programming problem introduced in this paper. Accounting for them, a heat exchanger network involving a low number of units is synthesized using the transportation formulation¹. The results clearly show the economic disadvantages of recycling process heat beyond a certain limiting value, unless bottlenecks in the current steam-power system, precluding further fuel savings, are removed.     

A two-parameter model for partial segregation: Application to flow reactors with pre- and unmixed feed

A two-parameter (TP) mixing model for partial segregation has been applied to continuous stirred tank reactors (CSTR) for cases with premixed and unmixed feed. It was found that in the case of perfectly mixed, homogeneous feed the model predicts similar or better results than earlier one-paramete models. However, a significant advantage of the TP model was found in the case of unmixed feed. Its two parameters are suitable for representing two se situations existing in the case of unmixed feed, namely, imperfect mixing of the feed and partial segregation of the reacting mixture. The single-param models cannot be used to represent both phenomena. Reversible and irreversible reactions were studied, and deviations from a perfectly mixed CSTR were into account by considering the RTD expression of Wolf and Resnick. The parameters of the model were determined from experimental data, when available, from arbitrary values within their permissible range, otherwise. Good agreement was found between experimental and predicted conversions.     

A flow visualization technique using analytical indicators: Theory and some applications

The fundamental concepts of a direct flow visualization technique, based on chemically and electrochemically induced local pH variations is described. Sharp changes in indicator colour in liquid media permit a highly reproducible observation of convection patterns, illustrated by five applications.     

A new technique for studying dispersion flow,holdup and axial mixing in packed extraction columns

A new technique is described which allows the direct measurement of static and dynamic holdup and axial dispersion coefficients in a packed extraction column. The method is applicable for a particular liquid-liquid system and solid packing which all have the same refractive index. The physical behaviour of the liquids may be observed free of distortions due to refraction. Preliminary results are reported.     

The flow of non-cohesive solids through orifices

The rate of flow of glass ballotini, and fluidisable cracking catalyst through long rectangular orifices has been measured. When these measurements and others reported in the literature were tested against the correlations of Fowler and Glastonbury, and Beverloo et al. a significant scatter of the available 109 data points about the correlations was found.Underlying trends in the set of measurements were analysed to show effects of the shape of orifice, shape of particles and the ratio of orifice width or diameter to particle size. The form of correlation in terms of dimensionless groups is inferred and the best values of constants found by regression. The experimental data were correlated with standard deviation of ± 12% by the equation

Two principal methods of using the correlation are proposed.     

Numerical analysis of a finite cylindrical pellet model in solid-gas reactions

A two-dimensional grain model has been developed to describe solid-gas reactions in a cylindrical pellet of finite length. The grains are assumed to be spherical. Numerical solutions of the model have been compared to infinite cylinder solutions, infinite slab solutions, and the approximate infinite cylinder and infinite slab solutions developed by Szekely et al. (Szekely J., Evans J. W. and Sohn H. Y., Gas—Solid Reactions. Academic Press, New York 1976). Pellet sizes and reactor conditions under which one or more of these solutions is valid have been indicated. The accuracy of the other models depends on whether they are to be used to determine rate constants from initial rate data or whether they are to be used to relate conversion to time.When the Theile-type reaction modulus σ_L = L[3(1 - ε₀)K/D_ear_g]^{$\text{1}\text{2}$}, has a value 2.2 with a cylinder half-length of L = 0.2 cm and a cylinder radius of R_p = 0.8 cm the conversion levels after about 8 min are 40% by an infinite slab model, 50% by the 2-D model and 65% by the approximate infinite slab solution. If one determines a rate constant from the initial slope of the rate curve, then the infinite slab model gives a value 14% high. The approximate infinite slab solution is inappropriate for the latter purpose. Similar results are obtained when the cylinder radius is much smaller than the half-length.Pellets with R_p/L > 4 or <0.25 can be described by infinite slab or infinite cylinder models, respectively, if the diffusional resistance is not large.     

Design of resilient processing plants—IV: Some new results on heat exchanger network synthesis

Simple and reliable methods to synthesize the most efficient network of heat exchangers to heat and cool known process streams between stated temperatuExamples in this paper demonstrate the strong need for systematic design procedures for resilient networks. Networks designed on the basis of intuitiveSome fundamental properties of optimal resilient heat exchanger networks are proven first. On this basis a theoretically rigorous synthesis technique i     

Modelling and parametric sensitivity of an annular photoreactor with complex kinetics

This paper reports the behaviour of a continuous annular reactor irradiated with a tubular ultraviolet source placed at its axis, and analyzes the case of a chemical reaction occurring in the homogeneous phase with complex kinetics. The reacting system is analyzed without resorting to the kinetic steady state hypothesis, thus allowing the computation of concentration profiles for the intermediate activated species.Three different models of radiation sources are considered: the line source model with emission in parallel planes, the line source model with sphericaWe have investigated and evaluated the influence of all significant parameters of the system on the reactant, intermediate species and product concentr     

The effect of pressure on the flow of gas in fluidized beds of fine particles

The division of gas flow between the bubble and dense phases of fluidized beds of six different types of Group A powders has been studied at pressures of up to 20 bar using surface collapse and X-ray absorption measurements. It was found that with these fine powders as pressure increases at constant volumetric gas flowrate so the size and hold-up of bubbles decrease while their frequency increases. Contrary to previous measurements the average bubb velocity appears to decrease with increasing pressure. The dominant mode of bubble break-up in all the powders was found to be division from the rear, contrast to that observed with Group B powders at atmospheric pressure. Interstitial phase voidage was found to increase with increasing pressure.The results are interpreted in terms of a model which assumes a difference between the voidages, and hence the gas flow, of powder in the wakes behind     

Two-phase gas-liquid flow in rectangular channels

The results of investigation of hydraulics in a two-phase gas—liquid flow in vertical and horizontal rectangular channels have been presented in the paper.Flow regime maps have been worked out and a method for calculating the pressure drop in a two-phase mixture flow has been proposed.The methods applied in a two-phase flow in pipes have been used to describe the results of investigations, after adopting the correction for the difference between the flow in rectangular channels and that in pipes.