The Chemical Engineering Journal最新文献

A model has been developed for the removal of sulphur dioxide from flue gas by absorption into a limestone slurry. The flue gas desulphurization unit consists of an absorber tower and an oxidation tank. Flue gas enters the absorption tower at the bottom and meets the limestone slurry. There are five important chemical reactions with a finite rate. The rate-limiting reactions are limestone dissolution, calcium sulphite precipitation and dissolution, gypsum precipitation, sulphur dioxide absorption and sulphite oxidation in the slurry. The model also accounts for the presence of chloride ions, magnesium ions and organic acids in the limestone slurry. The absorption rates of sulphur dioxide and carbon dioxide in the tower are calculated according to the two-film model. A non-uniform set of limestone particles is also included in the model. The model was tested against literature data and the agreement between the data and the model was satisfactory. A sensitivity analysis of the desulphurization process was carried out, the inputs to the model were changed and the results from the calculations were compared with the expected results. The response to the change in the inputs agreed well with the expected results.

Downward (inverse) fluidization can be achieved when the density of the particles is less than that of the liquid and the liquid is the continuous phase. This technique is mainly used in biochemical engineering operations, e.g. for fermentation and waste water treatment. Experiments were conducted to study the hydrodynamics of inverse gas—liquid—solid fluidized beds using very light particles. The experimental data for the minimum liquid velocity at the onset of fluidization are correlated in terms of the physical properties of the fluids, particle characteristics and system variables. A correlation for the friction factor is also proposed.

Two novel experimental methods are used. Vertical uniaxial stretching is obtained by attaching a perspex rod to the lower end of a silicone putty cylinder; the rod then descends into water of constant depth. The stress and rate of extension change little during each test, but the rate of extension may be varied from 0.005 to 0.10 s⁻¹ by modifying the experimental conditions. Biaxial stretching is acchieved by placing a disc of silicone putty across the top of an open glass cylinder which is lightly pressurized. The sample expands as a spherical cap, the height of the centre above the cylinder being timed. The stress in the cap passes through a shallow minimum as it expands (at constant pressure) and the slowly varying rate of biaxial extension may be readily determined. This lies in the range 0.003–0.06 s⁻¹. For low rates of uniaxial or biaxial extension, it is possible to plot the extension against time and to show how the extensional viscosity varies with the strain rate (or principal extension ratio). For high rates of extension, a ‘single point’ determination of the extensional viscosity may be made, with the stress and strain rate averaged at the mid-point of the sample's extension. The temperature is 26.5 ± 1.5 °C. The following is shown under the experimental conditions:

(a) the extensional viscosity (uniaxial or biaxial) is in the range 1.0 × 105 to 3.0 × 10⁵ Pa s;

(b) for extensional strain rates between 0.01 and 0.04 s⁻¹, the uniaxial and biaxial extensional viscosities are of comparable value;

(c) both forms of the extensional viscosity tend to decrease with increased extensional strain rate, the biaxial extensional viscosity falling more rapidly and being higher than the uniaxial viscosity at low strain rates and lower at high strain rates;

(d) there are no signs of rupture in uniaxial extension (principal extension ratios up to 1.8 and extensional strain rate up to 0.1 s⁻¹);

(e) in biaxial extension, the sample tends to rupture more easily as the strain rate is increased. (The sample fails at the principal extension ratio of 2.0 at an extensional strain rate of 0.02 s⁻¹ and fails at a principal extension ratio of 1.3 at an extensional strain rate of 0.07 s⁻¹.)

Studies on moist wheat bran medium, inoculated with Aspergillus niger CFTRI 1105, indicate steep gradients in the temperature and enzyme levels at different depths in deep bed rectangular fermenters with different loads. An increase of about 2.5 °C over a bed depth of 40 mm in a fermenter with a load of 49.7 kg m⁻² was due to metabolic heat generation in the initial fermentation period. This was found to result in a doubled fermentation time to attain enzyme levels which were comparable, although still lower by 4.6%, with those in a fermenter with a load of 17.8 kg m⁻². The temperature variations were about 10–18 °C for a bed depth of 80 mm and the highest enzyme levels were lower by 23.4%, even after 48 h, with a load of 49.7 kg m⁻² compared with those for a load of 17.8 kg m⁻² . The metabolic heat and enzyme biosynthesis functions were found to be significantly affected by an increase in bed depth. The maximum temperature variations recorded in the fermenter with a load of 126.1 kg m⁻² were 19.5 °C at 12 h and 21.2 °C at 60 h for 80 and 160 mm bed depths respectively. Consequently, the maximum enzyme levels were lower by 81%–86% and required a 2.5-fold increase in fermentation time compared with a fermenter with a load of 17.8 kg m⁻².

The results indicate that the temperature gradients play a key role in the biosynthesis of the enzyme in a solid state fermentation system involving deep beds.

Optimization of batch distillation has been studied extensively over the last 30 years. Previously, the solution methods were basically derived or computed using short-cut models due to the lack of suitable computation techniques. In this study, a modified approach based on the work of Biegler and coworkers (L. T. Biegler, Comput.Chem.Eng., 8 (1984) 243; J. E. Cuthrell and L. T. Biegler, AIChE J., 8 (1987) 1257) was implemented to determine optimal constrained solutions for a ternary system with various objective functions, such as maximum product, minimum energy required and mininum end time, using a rigorous model. Unlike previous investigations, the optimal control solutions derived in this study were independent of the process model implemented. Therefore, no limitation on the distillation models exists, i.e. any rigorous model can be used to find solutions.

The optimal control problem for the batch distillation of mixtures of benzene, toluene and o-xylene was solved. The solutions were assumed to be continuous or discontinuous polynomials. It was found that a discontinuous solution is superior to continuous results because of the discontinuous nature of the system itself.

The fast Fourier transform technique was introduced as a calculational tool for the estimation of parameters in the time domain. A complex fixed bed enzymatic reactor was selected as the model example. The results show that this is a suitable and effective technique in this regard.

Mass transfer between liquid drops and a surrounding fluid with an accompanying instantaneous chemical reaction is important in a number of industrial processes. For small drops which behave as rigid particles the coupled transport-reaction phenomenon occurs through a moving boundary mechanism. Here we present a theoretical analysis of the problem under non-isothermal conditions by considering the continuous phase diffusional resistance. The relevant transport equations are subjected to a coordinate transformation in order to immobilize the reaction front. Computed results for the enhancement factor and interfacial temperature rise are presented for a wide range of relevant system parameters. The results show enhancement factor and interfacial temperature rise maxima and some other interesting features that can be explained on a physical basis.

An extension of the range of convergence of the classical Newton-Raphson method and modified forms of it by use of the functional transformation method is demonstrated herein by use of numerical examples of difficult- to-solve distillation problems. Variations of the Newton-Raphson method considered are as follows: (1) the 2N Newton-Raphson method with the Broyden modification; (2) the 2N Newton-Raphson method with the Broyden-Bennett modification; (3) the almost-band algorithm with the Broyden-Householder modification; (4) the almost-band algorithm with Schubert's modification; and (5) parametric continuation with step size selection by Gear's method.

The expansion of conical spouted beds has been studied on a wide experimental base (using contactors of different geometries and solids of different diameters and sphericities). The correlation obtained for calculating the global voidage from the operation conditions is of a more general application than those previously proposed in the literature, which are generally centred on the study of voidage in the spout. The global incipient voidages are delimited for both stable regimes corresponding to two voidage ranges: spouting and jet spouting. While the correlation for calculation of expansion can be applied for the calculation of global minimum jet spouting voidage, a correlation is proposed for the calculation of the global minimum spouting voidage. This equation is applicable in a wide range of operation conditions.

The limitations of the few correlations in the literature for the design of conical spouted beds and the non-validity of these conventional correlations proposed for cylindrical spouted beds have been proven. Consequently, original hydrodynamic correlations for spouting and jet spouting, corresponding to conical contactors, have been proposed for the calculation of the maximum pressure drop and of the pressure drop in stable operational conditions. The hydrodynamic study has been carried out with different geometries of the contactor—inlet system (different angles and diameters of inlet) and with solids of different particle sizes, densities and shape factors, so that the correlations obtained are of general applicability.