M.A. El-Rifai

Mathematical modeling of catalytic fluidized-bed reactors. I: The multistage three-phase model

Abstract The purpose of this paper is to develop a novel model for gas-solid fluidized-bed catalytic reactors. This model is called the “multistage three-phase model”. It considers the fluid bed to be composed of a number of equivalent stages in series. Within each stage exchange of gas takes place between the bubble, cloud-wake and emulsion phases. An exact analytical solution for the concentration profiles along the bed is obtained. All parameters in the model are correlated with known or experimentally obtainable quantities. Model predictions are shown to compare reasonably well with various published experimental data on conversions of gas-fluidized catalytic reactors. A worked numerical example is given to demonstrate the applicability of the proposed model.

Forecasting of ozone pollution using artificial neural networks

Purpose – The objective of this study is to develop and validate a neural‐based modelling methodology applicable to site‐specific short‐ and medium‐term ozone concentration forecasting. A novel modelling technique utilizing two feed forward artificial neural networks (FFNN) is developed to improve the performance of time series predictions.Design/methodology/approach – Air pollution and meteorological data were collected for one year in two locations in Kuwait. The hourly averages of the data were processed to generate a covariance matrix and analyzed to generate the principal component method. A two‐FFNN model is then used to predict the actual data.Findings – The newly developed model improves the prediction accuracy over the conventional method. Owing to the presence of noise and other minor disturbances in the data, shorter‐range modelling gives better modelling results.Originality/value – A novel modelling technique is developed to predict the time series of zone concentration.

An assessment of the air pollution data from two monitoring stations in Kuwait

This article assesses the air pollution data from two monitoring stations in Kuwait. The measurements cover major pollutants, i.e., CO, CO2, methanated and non-methanated hydrocarbons, NO x , SO2, O3, and particulate matter (PM10). The data also includes meteorological parameters, i.e., solar intensity, temperature, wind speed, and wind direction, and has been collected over a period 4 years, from 2001 to 2004. Data analysis includes the assessment of annual hourly averages and 1-h maxima. Typical pollutant concentration trends, similar to those previously reported for Kuwait and for other locations around the world, are observed except for particulate matter measurements, which have higher values because of proximity to the desert. Emissions of nitrogen oxides show a consistent increase over the years. This is caused by the increase in the number of motor vehicles and the expansion in power generation and industrial activities. The data collected is a subset of the air quality criteria, as defined by the US EPA (United States Environmental Protection Agency).

Salt recovery from brine generated by large-scale seawater desalination plants

AbstractWater shortages in the Middle East and North Africa (MENA) region countries mandate the installation of large-scale desalination plants. Concentrate management requires properly operated cost-effective technologies to reduce the environmental impacts arising from brine discharge. Significant improvement in economics may be obtained by the recovery of chemicals from brines. This study addresses the management of modular brine streams generated from large-scale reverse osmosis desalination plants with microfiltration and nanofiltration (NF) as pretreatment stages. Appropriate salt recovery schemes have been identified and analyzed from the performance and environmental points of view. The economics of salt recovery schemes from NF and reverse osmosis (RO) brine based on evaporation ponds, brine evaporator and membrane crystallizer (MCr) are analyzed and compared. Phased application of the salt recovery program is considered. The results indicate that using NF as pretreatment and adopting salt recove...

Composition dynamics in multi-mixer-settler extractive reaction batteries

Abstract Linear dynamic models are developed for countercurrent multistage mixer-settler cascades involving simultaneous solvent extraction and chemical reaction. The analysis assumes a pseudo first order chemical reaction in the extract phase, perfect mixing in the contacting step and allows for various extents of mixing in the separation step. Typical results demonstrating the effects of variables such as the chemical reaction rate constant, extraction factor, residence times and number of stages on the frequency response of the two product streams are presented.

Integrated analysis of coupled separation operations

An integrated system analysis model is developed for processes involving two interacting countercurrent separation cascades coupled through a single-solvent circulation loop. The overall process model reveals interesting trends characterizing the dependence of the strippant (energy) required for achieving a given solute fractional recovery on the system design and operating variables. There is a minimum solvent circulation rate corresponding to infinite stripping agent requirements. The stripping agent requirements are minimized at an optimum solvent circulation rate and an optimum partition of the total system separation capacity between the two cascades. The developed rational design optimization method is compared with existing rule of thumb approach to the process design of coupled separation operations.

Maximization of thermal efficiency of fluidized-bed heat regenerators

Abstract In this work, we develop a two-phase mathematical model that describes the dynamic behavior of fluidized-bed heat regenerators. This model is based upon the two-phase theory of fluidization and accounts for the major aspects of bubbling fluidized beds. The model is first employed to identify operating conditions necessary to attain a constant effluent gas temperature. The devised model is then used to characterize the major parameters which influence the thermal efficiency of regenerative fluidized beds during the regeneration and the working periods. The set of design and operating parameters which leads to an efficient operation is, therefore, identified and accounted for on theoretical grounds. The model results can be used to design efficient fluid-bed heat regenerators or to improve the efficiency of existing units.

MATHEMATICAL MODELING OF FLUIDIZED BED HEAT REGENERATORS

Abstract Based on the two-phase theory of fluidization, a fairly rigorous model is developed to describe the dynamic behavior of fluidized-bed heat regenerators. This model takes into account the major hydrodynamic aspects of bubbling fluid beds. Based on the assumption that the gas leaves the bed in thermal equilibrium with the solids, a rather simplified model is presented. Exact analytical solutions are obtained for both models. The predictions of the more rigorous model are found to be in good agreement with many experimental observations. On the other hand, the simplified model gives satisfactory results only when the value of the parameter m1 is relatively large. Using the rigorous model, it has been shown that the bed thermal efficiency can be improved by reducing the time of operation, increasing the mass of solids or decreasing the bed aspect ratio. Also it has been found that when the gas flow-rate is increased, the amount of heat transferred to the solids displays a non-monotonic behavior and p...

Effect of kinetic regime and axial dispersion on the dynamic response of counter flow extractive reactors

Abstract Plug flow as well as axial dispersion dynamic models for counter-flow extractive reactors are formulated and solved to investigate the frequency response characteristics of such reactors. The reactions considered are taken to be either infinitely fast, taking place at the interface between the two phases or within a thin film in one of the phases, or slow taking place in the bulk of one of the phases. The results demonstrate the effect of the kinetic regime as well as axial dispersion (Peclet number) of the continuous phase on the dynamic behaviour of the two phase reactor. For the plug flow case (P e → ∞) it is shown that for some input-output relations the kinetic regime affects the functional forms of the transfer functions. The complex transfer functions are approximated, using simple fitting techniques, to simpler ones suitable for the design of the reactors control loops. For the axial dispersion case the results show that the continuous phase Peclet number affects the frequency response of the reactor in a complicated manner giving rise, for some input-output relations and small values of the Peclet number, to complex oscillations in both amplitude ratio and phase angle.

Temperature transients in fixed bed heat regenerators

Abstract A numerical solution of the nonlinear distributed parameter dynamics of both parallel-flow and counter-flow fixed bed heat regenerators is presented. The effects of various operating conditions on the fluid and solids temperature transients and on the thermal efficiency of the system are investigated. The character of the response at various levels is found to depend to a large extent on the direction of the initial axial temperature gradient along the solids.