The Chemical Engineering Journal最新文献

The effect of alcohol, organic acid and potassium chloride concentration on bubble size, bubble stability, terminal bubble rise velocity and gas hold-up in bubble columns was investigated. The addition of alcohols and organic acids to the water reduced the bubble size and the bubble rise velocity significantly. These organic solutes also changed the coalescence behaviour of aqueous solutions drastically, from high coalescence behaviour of pure water to coalescence restrain of the various solutions.

The mechanism of the coalescence suppression behaviour of electrolytic solutions of potassium chloride is discussed on the basis of ionic forces between ions and water molecules. The gas hold-up for low potassium chloride concentration increased owing to the ions reinforcing the liquid film between bubbles against bubble coalescence. For high potassium chloride concentration and low superficial gas velocity, large but unstable bubbles formed at the gas distributor plate. Increasing the gas velocity causes these bubbles to break into many smaller bubbles, thus increasing the gas hold-up.

A unified similarity transformation has been proposed which can deduce a class of possible similarity solutions during forced-, free- and mixed-convection flow of non-newtonian inelastic fluids through porous media under Darcy and non-Darcy conditions. The velocity scale using the slip velocity at the wall under mixed-convection flow has been utilized for generating a modified Peclet number which correctly transforms to the appropriate Peclet and Rayleigh numbers under limiting conditions. The unified approach is seen to reveal the existence of three limiting regimes lined by forced convection, Darcy free convection and Forchheimer free convection together with three intermediate regimes of Darcy mixed convection, Darcy-Forchheimer free convection and Forchheimer mixed convection. Exact solutions have been found for each of the regimes and the effects of non-newtonian character on the heat transfer characteristics have been deciphered.

In the course of our current interest in designing appropriate reactors for sonochemistry, we have investigated a heterogeneous reaction which is highly improved by ultrasound, with the aim of trying to understand the mechanism of the reaction rate enhancement and the influence of the sonication parameters.

The chosen model reaction is a Michaël reaction: the addition of ethyl malonate to chalcone in toluene under solid-liquid phase transfer conditions. This reaction is very sensitive to sonic irradiation: the initial reaction rate is increased by a factor of 10 under sonication.

In order to separate the physical and chemical effects of ultrasound, we have studied the disruption of the solid catalyst (KOH) in toluene and compared the particle sizes with the values obtained during the reaction with and without ultrasound. It can be seen that sonication prevents particle agglomeration as the reaction proceeds.

The effect of the generator power has been studied using a cuphorn. The reaction yield is dramatically increased near the cavitation power threshold and then remains nearly constant. Finally, different sonication systems have been compared, the cleaning bath giving the poorest results.

An analysis of various models of a continuous stirred-tank bioreactor (CSTB) showed that those based on the specific uptake rate of substrate described the dynamics of the CSTB satisfactorily for controlling the parameters after a disturbance so that all substrate introduced into the bioreactor was utilized.

The maximum values of dilution rate D_max and/or substrate concentration S_{0 max} in the feed after a step increase in D and/or S₀ respectively which could be achieved without causing cell washout were measured and compared with those calculated using a “short-cut” method. It was found that the calculated values of D_max and S _{0 max} were lower by approximately 20% than the experimental results. Thus the values of D_max and S_{0 max} predicted by the short-cut method were not substantial underestimates and can be considered safe for the operation of a CSTB utilizing inhibitory substrates.

Depending on which definition is chosen, biotechnology can be considered to be one of the oldest industrial technologies or one of the newest. For the chemical engineer, however, the major distinction is in terms of the scale of operation. The majority of products of the new biotechnology are extremely high value, low volume biochemicals. Thus recovery (separation) processes for such products may be relatively costly and energy intensive, because of the small quantities involved, but at the same time must be highly efficient in order to minimize loss of valuable product. This contrasts with the more traditional biotechnological processes of the food and beverage industries, antibiotic and medium value pharmaceuticals production, and wastewater treatment. Here, the problems for the chemical engineer are more like those in the chemical or process industries.

In this paper a typical biological process is compared with its counterpart chemical process and the role of the chemical engineer in designing and developing such a process is examined through a series of examples. Just as the chemical industry for decades was dominated by the chemist, so the bioprocess industry is still dominated by the industrial microbiologist. There are consequently many areas in which improvements can be made to established industrial biological processes by the application of simple engineering concepts.

Based on experimental results for the system of dipeptides (CBZ-leucyl-glycine and CBZ-glycyl-phenylalanine) on reverse-phase silica C₁₈, a detailed rate model including the intrinsic adsorption kinetics is proposed for multicomponent liquid chromatography. Simulation of the model was performed to investigate the effects of various parameters (rate constants, adsorption affinity, sorbent capacity, axial dispersion, feed concentration, sample size and velocity) on the band shape and spreading of elution chromatography. For cases with a finite rate of interaction between adsorbate and adsorbent, the mean retention time for the elution profiles of solutes is still determined by the equilibrium parameters, while the separation (resolution) between solute bands is largely influenced by the rate constants of the intrinsic adsorption kinetics. The effect of intrinsic adsorption kinetics becomes pre-eminent when the superficial velocity V_f is high. An optimal resolution diagram is presented to facilitate determination of the optimal operation parameters for touching-band separation.

Isothermal experiments were conducted to elucidate the rate process of stabilization for mesophase petroleum pitch fibre. The rate of oxygen consumption in the fibres was quantified by measuring the time course of the oxygen concentration in the effluent gas from the reactor. A mathematical model of the stabilization, in which the mass transfer of molecular oxygen and the autoxidation reaction with the active sites in the fibre were taken into account, was also constructed and a numerical simulation of the model equations was conducted. Physical parameters such as the effective diffusivity of molecular oxygen in the fibre, the reaction rate constant and the gas—solid equilibrium coefficient of molecular oxygen, which are included in the mathematical model, were determined by comparing the experimental values of oxygen concentration in the exit gas with theoretical values. Physicochemical processes of the stabilization were also discussed on the basis of the theoretical results. It was concluded that the stabilization is complete even when the reaction does not proceed deeply into the fibre.

A method of mathematical modelling of the real distribution of contaminant concentrations in a multistage rinse system of automatic electroplating lines has been presented. In the model of rinsing, it has been assumed that mixing is not perfect. The experimentally determined coefficients of mixing are a measure of the rinsing imperfectness. The assumed method of mathematical modelling has been verified experimentally in an automatic electroplating system with a positive result. A good fit to the experimental values has been obtained. The experimental data have also been compared with the results which were obtained using a perfect mixing model.

The characteristics of the falling film of condensate flowing downwards along the condenser surface of a molecular evaporator are presented. They result from solving the mathematical model of glycerol vapour condensation. The significance of the film surface temperature of the condensate and of the effect of the process parameters determining film temperature such as evaporation rate, condenser temperature and condenser height are discussed. Measures for enhancing the vapour condensation efficiency are also treated.

The dispersed-media granulometric distribution determination is often necessary. We mention the principal methods used nowadays with their own application fields and their limitations. Then we present the computer-assisted image analysis and its advantages.

The different aspects of this new method are developed, especially in the case of emulsions and granular systems. The statistical aspect needed for any results analysis is mentioned, and in particular the precision obtained as a function of the sample size.

Finally, we present a summary of the different advantages of computer-assisted image analysis, applied to the dispersed-media granulometric distribution determination.