Investigation of coupling dehydrogenation and hydrogenation reactions in a fixed bed catalytic reactor with well-mixed catalyst pattern

AbstractThe enhancement of ethylbenzene conversion by further displacement of the thermodynamic equilibrium via the influence of the dual-functionality of a well-mixed catalyst pattern has been investigated. A rigorous steady state mathematical model based on the dusty gas model is implemented for the simulation. The simulation results reveal that the introduction of the concept of the reaction coupling has significant effect on the displacement of the thermodynamic equilibrium and considerable enhancement of simultaneous production of styrene and cyclohexane. Almost 100% conversion of the ethylbenzene and benzene is achieved through the application of this approach. It is also found that considerable decrease in the reactor length is achieved by employing a reactor catalyst bed with different bed compositions. Effective operating regions with optimal conditions are observed. An effective reactor length criterion is used to evaluate the performance of the reactor under these optimal conditions. The effective reactor length is found to be sensitive and favored by high feed temperature and pressure. The sensitivity analysis shows that the key parameters of feed temperature, pressure, and the bed composition play an important role on the reactor performance. The results also show that almost 100% conversion of ethylbenzene and benzene at low temperature and shorter reactor length can be achieved by maintaining the reactor beds at different temperatures. This temperature switching policy may result in appreciable energy saving. Moreover, operating the reactor at low temperature protect the catalyst from the excessive temperatures which have destructive effects on the catalysts and the mechanical stability of the reactors. Also, the low temperature operation has significant contribution to the reduction of the operating cost

DETERMINATION OF MIXING HEIGHT IN RIYADH, SAUDI ARABIA

Twice-daily mixing heights, one morning and one afternoon were calculated by using the computational program MIXHTS. MIXHTS utilizes the meteorological data collected from the King Khalid International Airport (KKIA) surface and upper air stations in Riyadh city during the year 2002. Climatic data shows that the city of Riyadh has in general, a hot and dry weather in summer and cold and dry in wintertime, whereas strong insolation is dominant all over the year. Monthly average afternoon mixing heights are ranged from 1629 m to 3971 m, whereas the morning mixing heights are ranged from as low as 935 m to 2920 m. Estimation of mixing heights obtained by this work were found in a good agreement with the daily maximum value obtained from the dry adiabatic temperature method

Influence of Hydrodynamic Flow Regimes on the Prediction of Gas Hold-up and Liquid Circulation in Airlift Reactors

The influence of the recently identified hydrodynamic flow regimes on the prediction of gas hold-up and liquid circulation velocity in airlift reactors has been investigated. Some hydrodynamic models based on momentum balance are considered. Experiments were conducted on a pilot plant, external loop airlift reactor using air-water system. The generalised equation of Joshi and Lali for the homogeneous regime predicts the overall gas hold-up in this region with satisfactory accuracy and the difference between the experimental and predicted values is lower than ± 8%. Hsu and Dudukovic model predicts the liquid circulation velocity based on the gas hold-up correlations proposed for the homogenous, transition and heterogeneous flow regimes and two-phase friction factor with satisfactory accuracy (within 4.9 %). The results also show that the friction factor has pronounced effect on the model predictions. It would appear, also that the characterisation of various flow regimes has profound effect on the predictions of these important design parameters. Superposition approach is implemented to develop a correlation for the prediction of the axial gas hold-up in the riser as a function of the height and superficial gas velocity. The proposed correlation gives an average error of 4.7%. Keywords: Airlift reactors, flow regimes, gas hold-up, liquid circulatio

Development of exotic geometrical structures in complex dynamics of a forced bioethanol reactor

In this short communication, the impact of the influence of a dynamic membrane on a bioreactor forced by high amplitude oscillations is investigated. The chaotic state of the reactor bifurcates to unprecedented complex dynamics. Very interesting exotic geometrical structures embedded in a high dimensional phase space are developed. Despite the complexity of the dynamics of this system, it can produce an increase of average ethanol yield up to 19.62% compared to unforced system. Keywords: Bioethanol, Chaos, Dynamic membranes, Exotic geometrical topologies, Forced bioreacto

Parametric Investigation of Industrial Methanators Using the Dusty Gas Model

A parametric investigation is carried out for industrial methanators in order to get a deeper insight into this complex gas-solid catalytic system. The dusty gas model is used to account for internal diffusion. The results of the rigorous simulation model developed are in a good agreement with industrial plant data. The performance of the industrial methanator is shown to be quite sensitive to change in catalyst pellet size, feed temporature and feed pressure

Effect of Ion Sizes on Separation Characteristics of Nanofiltration Membrane Systems

The effect of the ion size on salt rejection is investigated using the Donnan steric pore model (DSPM). This model accounts for the transport phenomena occurring inside the membrane through the appropriate ion sizes. Four hydrated ions (Na+, Mg2+, Cl-, SO42-) are considered. The ion radius is determined from Stokes-Einstein relationship (Stokes radius), the Born’s theory (Born’s effective radius), and the strength of hydration (Pauling radius). Experimental data validate predictions of ion rejections. It was found that ionic hydration has strong influence on the diffusion properties of ions, which affects significantly the ion rejections. The charge density across the surface of the membrane varies according to the choice of radius of solute, causes remarkable variations in the model prediction. It was also found that the charge density should be predicted accurately to obtain the surface potential which will aid in the improvement of membrane performance and also give approximate index for fabrication of thin film composite layer membrane. Keywords: Membrane, Mathematical modeling, Nanofiltration, Ion sizeCorresponding Author: Mr. M. E. E. Abashar Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia, E-mail address: [email protected]

Prediction of physical properties of nanofiltration membranes using experiment and theoretical models

Two commercial nanofiltration (NF) membranes, viz., Desal-HL and NF 700 MWCO were investigated experimentally using neutral and charged solutes, viz., glucose, sodium chloride and magnesium chloride. Effect of pH was studied for sodium chloride rejection and isoelectric point of the membrane was deduced. Experimental results were analyzed using Donnan steric pore and dielectric exclusion models. Dielectric exclusion arises due to the difference in dielectric constant between the bulk and the nano-pore. Born dielectric effect was used as dielectric exclusion phenomena in the present investigation. Stokes-Einstein, Born effective and Pauling radii were used for theoretical simulation, which accurately predicted different charge densities. Empirical correlations were proposed between charge density, concentration and pH for each radius. Charge density decreased drastically when dielectric exclusion term was included in the theoretical model, which showed the real physical characteristics of the membranes employed. Charge density and radius of pore was found to be an important surface parameter in predicting the separation effects in NF membranes. . .