A.A. Al-Zahrani

Modeling solids and gas flow through an L-valve

An experimental program was conducted to study the flow of particulate solids through an L-valve with air as the regulating medium. Two different size solid granules were used in L-valves of diameters 25, 36, and 50 mm using two horizontal sections length of 170 and 290 mm. For such a simple system in which the feed hopper and the discharge end of the valve are at atmospheric pressure, a mathematical model, based on a modified Ergun equation written for the moving bed in both the downcomer and horizontal sections of the L-valve, can be used to predict the superficial gas velocity in these two sections at any operating conditions of the L-valve. Correlations are also given from which external aeration and the pressure drop across the L-valve can be predicted for any solids flow rate.

A study of pressure drop fluctuations in a gas-solids fluidized bed

Abstract A model is proposed which relates the amplitude of the pressure drop fluctuations across a gas-solids fluidized bed to geometry and physical properties of the bed and the particles. The model is based on the energy balance around the fluidized bed. The proposed model predictions for amplitude of pressure drop fluctuations agree well with the experimental data. A model for fundamental frequency of pressure fluctuations, which was originally proposed by Baskakov, is verified using experimental data from several sources.

Reaction Mechanism and Deactivation Modes of Heterogeneous Catalytic Systems

Solving the problem of catalyst deactivation is essential in process design. To do this, various aspects of the kinetics of processes with catalyst deactivation, and their different mechanisms, are discussed. Catalyst deactivation often cannot be avoided, but more knowledge on its mechanism can help to find kinetic means to reduce its harmful consequences. When deactivation is caused by coke, the generation of coke precursors is the determining step in the deactivation kinetics. Different types of deactivation were distinguished that lead to different evolution of the process. The phenomenon of non-uniform coking can be linked to catalyst surface non-uniformity. For the class of catalysts with more than one type of active sites, an explanation was suggested for the observed trends in the deactivation modes. For catalytic processes using catalyst particles of industrial size, the influence of intraparticle diffusion resistance is important. The analysis showed that for a number of processes, the decrease of the reaction rate due to deactivation is less under diffusion control. For certain reaction mechanisms, there exist operation conditions where the rate of the process under diffusion control exceeds the rate in the kinetic control regime. A significant problem is the change of selectivity in the course of catalyst deactivation. The selectivity may either decrease or increase, and depends on the reaction mechanism during deactivation. The changes are larger when there is no diffusion resistance. The intentional poisoning of catalysts and its influence on catalyst activity and selectivity for the process of ethylene oxide production was discussed.

Preparation of activated carbon from fly ash and its application for CO2 capture

Power and desalination plants are one of the main anthropogenic sources for CO2 generation, which is one of the key elements to cause greenhouse gas effect and thus contribute to the global warming. Fly ash (FA) generated in desalination and power plants was converted into activated carbon (AC) treated with KOH at higher temperature and tested for CO2 capturing efficiency. Morphological characteristics of FA such as BET specific surface area (SSA), pore volume, pore diameter, and pore size distribution (PSD) were performed using N2 adsorption isotherm. CO2 adsorption capacity and adsorption isotherms of CO2 over AC were measured by performing thermogravimetric analysis at different temperatures. BET SSA of 161 m2g−1 and adsorption capacity of 26mg CO2/g AC can be obtained by activation at KOH/FA ratio of 5 at 700 °C and activation time of 2 h. Therefore, great potential exists for producing AC from FA, which will have the positive effect of reducing the landfill problem and global warming.

Role of Chemical Kinetics in the Heterogeneous Catalysis Studies

Abstract The paper presents shortly some of the important elements of the theory and of the practical applications of the kinetics of heterogeneous catalytic reactions. Discussed are some of the most important concepts of the kinetics of complex heterogeneous catalytic reactions, methodology of building kinetic models and mathematical treatment of experimental data, influence of heat and mass transfer, types of laboratory reactors, kinetics and nanosized catalysts and others. Examples for use of the kinetic studies for the development and application of industrial catalysts and modeling of industrial reactors are presented.

Particle size distribution in a continuous gas-solid fluidized bed

Abstract The size distributions of solids during the continuous operation of a gas–solid fluidized bed were recorded. Samples were taken at different time intervals for measurement of particle size distribution via sieving. The solids used in this work were: sand, iron and manganese dioxide. A single parameter stochastic model is proposed to describe the particle size distribution in a gas–solid fluidized bed at different times of operation. The model parameter is evaluated using experimental data. The variation of the model parameter with time was studied and found to be a function of the solids structure. The results indicate a satisfactory agreement between model predictions and experimental data.

Effect of fixed bed characteristics on the performance of pulsed water flow humidification-dehumidification solar desalination unit

Abstract This work investigates the effect of fixed bed characteristics on the performance a of pulsed flow humidification-dehumidification (HD) solar desalination unit. The results show that the unit performance has been improved by increasing the Raschig ring diameter and increasing the distance between layers up to a certain limit and by decreasing the bed height. The solar collector located in Jeddah Saudi Arabia was investigated under different conditions of feed water flow rate, tilt angle, and area of the collector. An equation for the collector efficiency and performance was derived. The power loss in the condenser was calculated to be 2.125u2009W. The results showed that the productivity of the unit within the range of data studied can be within the range from 1.3 to 1.9u2009l/m2·h and the gain output ratio of the pulsed flow system to be within the range of 1.80–2.74.

Use of Intraparticle Mass Transfer Parameters as a Design Tool for Catalyst Pellets

A chromatographic method and a dynamic Wicke-Kallenbach method (DMWK) were used to determine the diffusion characteristics of two industrial copper containing catalysts. The first catalyst was used in nitrobenzene hydrogenation to aniline and the second was used in a low temperature water-gas shift reaction. Experimental results show that application of these two methods leads to similar results. Experimental data obtained allow for monitoring changes in the texture of the catalyst grains and intraparticle diffusivity of gaseous reagents at different states of the catalyst activity and use, which can be used as criteria for designing optimal industrial catalyst pellets.