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Abdelhamid Ajbar
Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia

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Journal article
Published: 26 May 2021 in Symmetry
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In many countries the COVID-19 pandemic seems to witness second and third waves with dire consequences on human lives and economies. Given this situation the modeling of the transmission of the disease is still the subject of research with the ultimate goal of understanding the dynamics of the disease and assessing the efficacy of different mitigation strategies undertaken by the affected countries. We propose a mathematical model for COVID-19 transmission. The model is structured upon five classes: an individual can be susceptible, exposed, infectious, quarantined or removed. The model is based on a nonlinear incidence rate, takes into account the influence of media on public behavior, and assumes the recovery rate to be dependent on the hospital-beds to population ratio. A detailed analysis of the proposed model is carried out, including the existence and uniqueness of solutions, stability analysis of the disease-free equilibrium (symmetry) and sensitivity analysis. We found that if the basic reproduction number is less than unity the system can exhibit Hopf and backward bifurcations for some range of parameters. Numerical simulations using parameter values fitted to Saudi Arabia are carried out to support the theoretical proofs and to analyze the effects of hospital-beds to population ratio, quarantine, and media effects on the predicted nonlinear behavior.

ACS Style

Abdelhamid Ajbar; Rubayyi Alqahtani; Mourad Boumaza. Dynamics of a COVID-19 Model with a Nonlinear Incidence Rate, Quarantine, Media Effects, and Number of Hospital Beds. Symmetry 2021, 13, 947 .

AMA Style

Abdelhamid Ajbar, Rubayyi Alqahtani, Mourad Boumaza. Dynamics of a COVID-19 Model with a Nonlinear Incidence Rate, Quarantine, Media Effects, and Number of Hospital Beds. Symmetry. 2021; 13 (6):947.

Chicago/Turabian Style

Abdelhamid Ajbar; Rubayyi Alqahtani; Mourad Boumaza. 2021. "Dynamics of a COVID-19 Model with a Nonlinear Incidence Rate, Quarantine, Media Effects, and Number of Hospital Beds." Symmetry 13, no. 6: 947.

Original research article
Published: 13 May 2021 in Frontiers in Physics
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The paper studies the dynamics of the classical susceptible-infectious-removed (SIR) model when applied to the transmission of COVID-19 disease. The model includes the classical linear incidence rate but considers a nonlinear removal rate that depends on the hospital-bed population ratio. The model also includes the effects of media on public awareness. We prove that when the basic reproduction number is less than unity the model can exhibit a number of nonlinear phenomena including saddle-node, backward, and Hopf bifurcations. The model is fitted to COVID-19 data pertinent to Saudi Arabia. Numerical simulations are provided to supplement the theoretical analysis and delineate the effects of hospital-bed population ratio and public awareness on the control of the disease.

ACS Style

Abdelhamid Ajbar; Rubayyi T. Alqahtani; Mourad Boumaza. Dynamics of an SIR-Based COVID-19 Model With Linear Incidence Rate, Nonlinear Removal Rate, and Public Awareness. Frontiers in Physics 2021, 9, 1 .

AMA Style

Abdelhamid Ajbar, Rubayyi T. Alqahtani, Mourad Boumaza. Dynamics of an SIR-Based COVID-19 Model With Linear Incidence Rate, Nonlinear Removal Rate, and Public Awareness. Frontiers in Physics. 2021; 9 ():1.

Chicago/Turabian Style

Abdelhamid Ajbar; Rubayyi T. Alqahtani; Mourad Boumaza. 2021. "Dynamics of an SIR-Based COVID-19 Model With Linear Incidence Rate, Nonlinear Removal Rate, and Public Awareness." Frontiers in Physics 9, no. : 1.

Journal article
Published: 09 April 2021 in Energy
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In this paper, the energy performance and economic feasibility of conventional wood dryer systems integrated with a heat recovery unit and solar air heaters are numerically investigated. Four different designs of wood dryers, composed of an insulated drying chamber, a heat recovery unit (HRU), a flat plate air collector (FPC), a photovoltaic/thermal air collector (PVT) and an auxiliary heater, are presented and compared. Annual dynamic simulations were carried out using a heat and mass transfer model with realistic meteorological data from two different climatic zones as boundary conditions. The effect of recovering waste heat and integrating each type of solar air heaters on the wood dryer energy consumption are presented and analyzed. Results show that recovering waste heat from conventional wood dryers in cold climate is more beneficial than in hot climate with an annual energy consumption reduction up to 41%. It is also shown that using a combined heat recovery unit and PVT collector improved significantly the energy efficiency of conventional dryers and reduced by about 67.5% and 49.5% the yearly energy consumption in hot and cold climates, respectively. Based on the economic analysis, the use of the HRU is beneficial in both climatic zones with a maximum payback period of 2 years. However, the use of the PVT collector is recommended only for wood dryers in hot climates where the payback period is about 3.5 years.

ACS Style

Bilal Lamrani; Frédéric Kuznik; Abdelhamid Ajbar; Mourad Boumaza. Energy analysis and economic feasibility of wood dryers integrated with heat recovery unit and solar air heaters in cold and hot climates. Energy 2021, 228, 120598 .

AMA Style

Bilal Lamrani, Frédéric Kuznik, Abdelhamid Ajbar, Mourad Boumaza. Energy analysis and economic feasibility of wood dryers integrated with heat recovery unit and solar air heaters in cold and hot climates. Energy. 2021; 228 ():120598.

Chicago/Turabian Style

Bilal Lamrani; Frédéric Kuznik; Abdelhamid Ajbar; Mourad Boumaza. 2021. "Energy analysis and economic feasibility of wood dryers integrated with heat recovery unit and solar air heaters in cold and hot climates." Energy 228, no. : 120598.

Journal article
Published: 04 March 2021 in Processes
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This paper proposes and analyzes a mathematical model for the production of bioethanol in a continuous bioreactor with recycling. The kinetics correspond to the use of Saccharomyces bayanus for the fermentation of sugars found in wastewater from soft drinks. The proposed model considers product growth latency, which was experimentally found in batch studies of ethanol production. Furthermore, the inhibition effect of ethanol is expressed by a modified version of the classical Andrew’s model for substrate inhibition. The proposed model consists of only three ordinary differential equations containing a minimal number of operating parameters, which include the bioreactor residence time, glucose feed concentration, recycle ratio and the fraction of biomass removed from the reactor by the flow. The positivity and the boundedness of solutions of the model were confirmed under reasonable restrictions of parameters. The stability analysis showed that there is a value of residence time at which an exchange of stability occurs between the trivial washout and non-washout solutions. This critical value depends only on the substrate feed concentration, biomass death rate, recycle ratio and purge fraction. Dynamic simulations of the model were carried out for substrate concentration in the range of 100–250 g/L, commonly used for the production of ethanol. An inverse response due to the inhibition effects of ethanol was observed in the time evolution of substrate and biomass concentrations. Parametric studies showed that ethanol concentration increases with the recycle ratio, with the inverse of residence time and with the inverse of purge fraction. The effect of ethanol latency has, on the other hand, a substantial effect on ethanol concentration. Despite its unstructured nature and the fact that some parameters such as temperature and acidity were not taken into consideration, the proposed model managed to provide useful results on the bioreactor-settler stability and the effect of key parameters on its dynamic behavior, which could pave the way for future optimization studies.

ACS Style

Rubayyi Alqahtani; Samir Bhowmik; Abdelhamid Ajbar; Mourad Boumaza. Stability and Performance Analysis of Bioethanol Production with Delay and Growth Inhibition in a Continuous Bioreactor with Recycle. Processes 2021, 9, 461 .

AMA Style

Rubayyi Alqahtani, Samir Bhowmik, Abdelhamid Ajbar, Mourad Boumaza. Stability and Performance Analysis of Bioethanol Production with Delay and Growth Inhibition in a Continuous Bioreactor with Recycle. Processes. 2021; 9 (3):461.

Chicago/Turabian Style

Rubayyi Alqahtani; Samir Bhowmik; Abdelhamid Ajbar; Mourad Boumaza. 2021. "Stability and Performance Analysis of Bioethanol Production with Delay and Growth Inhibition in a Continuous Bioreactor with Recycle." Processes 9, no. 3: 461.

Journal article
Published: 01 October 2020 in Advances in Difference Equations
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In this paper, the dynamical behavior of a SEIR epidemic system that takes into account governmental action and individual reaction is investigated. The transmission rate takes into account the impact of governmental action modeled as a step function while the decreasing contacts among individuals responding to the severity of the pandemic is modeled as a decreasing exponential function. We show that the proposed model is capable of predicting Hopf bifurcation points for a wide range of physically realistic parameters for the COVID-19 disease. In this regard, the model predicts periodic behavior that emanates from one Hopf point. The model also predicts stable oscillations connecting two Hopf points. The effect of the different model parameters on the existence of such periodic behavior is numerically investigated. Useful diagrams are constructed that delineate the range of periodic behavior predicted by the model.

ACS Style

Abdelhamid Ajbar; Rubayyi T. Alqahtani. Bifurcation analysis of a SEIR epidemic system with governmental action and individual reaction. Advances in Difference Equations 2020, 2020, 1 -14.

AMA Style

Abdelhamid Ajbar, Rubayyi T. Alqahtani. Bifurcation analysis of a SEIR epidemic system with governmental action and individual reaction. Advances in Difference Equations. 2020; 2020 (1):1-14.

Chicago/Turabian Style

Abdelhamid Ajbar; Rubayyi T. Alqahtani. 2020. "Bifurcation analysis of a SEIR epidemic system with governmental action and individual reaction." Advances in Difference Equations 2020, no. 1: 1-14.

Journal article
Published: 09 July 2020 in Processes
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The processing of fine and ultrafine particles using a fluidized bed is challenging in view of their unpredictable hydrodynamic behavior due to interparticle forces. The use of assisted fluidization techniques in such cases can be effective in improving the bed hydrodynamics. This work investigates the dynamics of pulsed fluidized bed of ultrafine nanosilica subjected to square-wave flow pulsations. The pulse duration used in this study is sufficient to allow the complete collapse of the pulsed fluidized bed between two consecutive flow pulsations. The proposed pulsation strategy is carefully implemented using electronic mass flow controllers with the help of analog output signals from data acquisition system. Given that the different regions of the fluidized bed exhibit varying dynamics, which together contribute to overall bed dynamics, the bed transients in the upper, central, and lower regions of the fluidized bed are monitored using several sensitive pressure transducers located along the height of the bed. The effect of the flow pulsation on the hydrodynamics of the fluidized bed is rigorously characterized. A significant reduction in the minimum fluidization velocity was obtained and an increase in the bed homogeneity was observed due to flow pulsations. The frequency domain analysis of the signals clearly delineated the frequency of the various events occurring during the fluidization.

ACS Style

Mohammad Asif; Ebrahim H. Al-Ghurabi; Abdelhamid Ajbar; Nadavala Siva Kumar. Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed. Processes 2020, 8, 807 .

AMA Style

Mohammad Asif, Ebrahim H. Al-Ghurabi, Abdelhamid Ajbar, Nadavala Siva Kumar. Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed. Processes. 2020; 8 (7):807.

Chicago/Turabian Style

Mohammad Asif; Ebrahim H. Al-Ghurabi; Abdelhamid Ajbar; Nadavala Siva Kumar. 2020. "Hydrodynamics of Pulsed Fluidized Bed of Ultrafine Powder: Fully Collapsing Fluidized Bed." Processes 8, no. 7: 807.

Journal article
Published: 30 October 2019 in Nanomaterials
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Decoration of 2D semiconductor structures with heterogeneous metal quantum dots has attracted considerable attention due to advanced optical, electrical, and catalytic properties that result from the large surface-to-volume ratio associated with these structures. Herein, we report on silver quantum dot decorated 2D SnO2 nanoflakes for the photocatalytic abatement of water effluents, the synthesis of which was achieved through a straightforward and mild hydrothermal procedure. The photocatalysts were systematically investigated using UV–Vis, XRD, electron microscopy (SEM, HR-TEM), EDX, XPS and FTIR. The photocatalytic activity of the nanostructures was evaluated for the abatement of water pollutant rhodamine B (RhB), under light irradiation. The mild hydrothermal synthesis (100 °C) proved highly efficient for the production of large scale Ag quantum dot (QD)/SnO2 nanoflakes for a novel photocatalytic application. The decoration of SnO2 with Ag QDs significantly enhances the synergetic charge transfer, which diminishes the photo-induced electron-hole reunion. Moreover, the plasmonic effect from Ag QDs and 2D-SnO2 structures acts as an electron tank to collect the photo-induced electrons, generating a Schottky barrier between the SnO2 structures and quantum dots. Overall, this resulted in a facile and efficient degradation of RhB, with a rate double that of pristine SnO2.

ACS Style

Nadavala Siva Kumar; Mohammad Asif; T. Ranjeth Kumar Reddy; Gnanendra Shanmugam; Abdelhamid Ajbar. Silver Quantum Dot Decorated 2D-SnO2 Nanoflakes for Photocatalytic Degradation of the Water Pollutant Rhodamine B. Nanomaterials 2019, 9, 1536 .

AMA Style

Nadavala Siva Kumar, Mohammad Asif, T. Ranjeth Kumar Reddy, Gnanendra Shanmugam, Abdelhamid Ajbar. Silver Quantum Dot Decorated 2D-SnO2 Nanoflakes for Photocatalytic Degradation of the Water Pollutant Rhodamine B. Nanomaterials. 2019; 9 (11):1536.

Chicago/Turabian Style

Nadavala Siva Kumar; Mohammad Asif; T. Ranjeth Kumar Reddy; Gnanendra Shanmugam; Abdelhamid Ajbar. 2019. "Silver Quantum Dot Decorated 2D-SnO2 Nanoflakes for Photocatalytic Degradation of the Water Pollutant Rhodamine B." Nanomaterials 9, no. 11: 1536.

Journal article
Published: 27 August 2018 in Applied Sciences
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The present study proposes a cost-effective assisted fluidization technique of particle mixing to improve the carbon capture effectiveness of a fluidized bed containing fine adsorbent powder. Using activated carbon as the adsorbent, we mixed external particle of Geldart group B classification in different fractions to examine the effectiveness of the proposed strategy of particle mixing. Four different particle-mixing cases were considered by varying the amount of added particle—0, 5, 10, and 30 wt %—on external particle-free basis. The inlet flow of the nitrogen was fixed, while two different flows of carbon dioxide were used. The adsorption experiment consisted of a three step procedure comprising purging using pure nitrogen, followed by adsorption with fixed inlet CO2 concentration, and finally the desorption step. Inlet flows of both nitrogen and CO2 were separately controlled using electronic mass flow controllers with the help of data acquisition system (DAQ). The CO2 breakthrough was carefully monitored using the CO2/O2 analyzer, whose analog output was recorded using the DAQ. Best results were obtained with 10% external particles. This is in conformity with the results of our previous study of bed hydrodynamics, which pointed to clear improvement in the fluidization behavior with particle mixing.

ACS Style

Ebrahim H. Al-Ghurabi; Abdelhamid Ajbar; Mohammad Asif. Enhancement of CO2 Removal Efficacy of Fluidized Bed Using Particle Mixing. Applied Sciences 2018, 8, 1467 .

AMA Style

Ebrahim H. Al-Ghurabi, Abdelhamid Ajbar, Mohammad Asif. Enhancement of CO2 Removal Efficacy of Fluidized Bed Using Particle Mixing. Applied Sciences. 2018; 8 (9):1467.

Chicago/Turabian Style

Ebrahim H. Al-Ghurabi; Abdelhamid Ajbar; Mohammad Asif. 2018. "Enhancement of CO2 Removal Efficacy of Fluidized Bed Using Particle Mixing." Applied Sciences 8, no. 9: 1467.

Journal article
Published: 27 August 2018 in Applied Sciences
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We have developed a new particle-mixing strategy for improving the fluidization hydrodynamics of Geldart group C powders by mixing with small proportions of group B particles. Two different group C particles with widely different physical properties, i.e., 1 μm calcium hydroxide powder and 27 μm porous activated carbon, were selected for investigation in the present work. A carefully sieved sample of inert sand was used as external group B particles for mixing. Fluidization experiments were carried out, and the quality of the fluidization was assessed using the fluidization index. For the monocomponent fluidization of fine calcium hydroxide powder, pressure drop was sometimes as much as 250% higher than the effective weight of the bed. The proposed strategy of particle mixing substantially improved its fluidization hydrodynamics. On the other hand, the development of channels and cracks during the monocomponent fluidization of the activated carbon led to gas bypassing, resulting in low pressure drop and poor contact of phases. Particle mixing was found to improve fluidization behavior, and the chi-squared test showed that the best results were obtained with 13 wt% particle mixing.

ACS Style

Ebrahim H. Al-Ghurabi; Abdelhamid Ajbar; Mohammad Asif. Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles. Applied Sciences 2018, 8, 1469 .

AMA Style

Ebrahim H. Al-Ghurabi, Abdelhamid Ajbar, Mohammad Asif. Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles. Applied Sciences. 2018; 8 (9):1469.

Chicago/Turabian Style

Ebrahim H. Al-Ghurabi; Abdelhamid Ajbar; Mohammad Asif. 2018. "Improving Fluidization Hydrodynamics of Group C Particles by Mixing with Group B Particles." Applied Sciences 8, no. 9: 1469.

Article
Published: 25 May 2017 in Journal of Applied Polymer Science
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Polypropylene (PP)/Multiwall carbon nanotubes (MWCNTs) nanocomposites were fabricated via melt compounding that utilizes a corotating twin-screw extruder. Two commercially available MWCNTs, Baytubes C150P and C70P, were incorporated into PP matrix at concentration of 3 wt %. The nanocomposites samples were analyzed using scanning electron microscopy, dynamic mechanical analysis (DMA), nanoindentation test, and picoammeter. It was found that both MWCNTs types were well distributed and dispersed in the PP matrix and no agglomeration of MWCNTs was observed. The DMA analysis results showed that the incorporation of MWCNTs enhanced the storage modulus and thermal stability of the PP matrix. Whereas, nanoindentation creep results showed that the creep rate and displacement of the PP/MWCNTs nanocomposites was lower than the neat PP, in which C70P < C150P < PP. The reduction of creep rate and creep displacement was associated to the improvement of creep resistance. There were also improvements on hardness and stiffness of the nanocomposites. Additionally, the electrical resistivity of the neat PP decreased with the incorporation of MWCNTs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45293.

ACS Style

Achmad Chafidz; Wara Dyah Pita Rengga; Rawaiz Khan; Mujtahid Kaavessina; Abdulaziz M. Almutlaq; Waheed Almasry; Abdelhamid Ajbar. Polypropylene/multiwall carbon nanotubes nanocomposites: Nanoindentation, dynamic mechanical, and electrical properties. Journal of Applied Polymer Science 2017, 134, 1 .

AMA Style

Achmad Chafidz, Wara Dyah Pita Rengga, Rawaiz Khan, Mujtahid Kaavessina, Abdulaziz M. Almutlaq, Waheed Almasry, Abdelhamid Ajbar. Polypropylene/multiwall carbon nanotubes nanocomposites: Nanoindentation, dynamic mechanical, and electrical properties. Journal of Applied Polymer Science. 2017; 134 (37):1.

Chicago/Turabian Style

Achmad Chafidz; Wara Dyah Pita Rengga; Rawaiz Khan; Mujtahid Kaavessina; Abdulaziz M. Almutlaq; Waheed Almasry; Abdelhamid Ajbar. 2017. "Polypropylene/multiwall carbon nanotubes nanocomposites: Nanoindentation, dynamic mechanical, and electrical properties." Journal of Applied Polymer Science 134, no. 37: 1.

Journal article
Published: 01 December 2014 in Brazilian Journal of Chemical Engineering
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A low-order collocation method is often useful in revealing the main features such as concentration and temperature profiles and the effectiveness factor for porous catalyst particles. Two modifications are introduced in this paper to make the method more efficient. The first modification is to add an extra collocation point at the center of the particle. It is shown that such extra point introduces a single variable non-linear equation to be solved after obtaining the standard collocation method solution. In the second modification, the polynomial solution obtained from the application of the orthogonal collocation method is transformed to a rational function form. These two modifications are applied to specific examples and it is shown that they can improve the performance of collocation methods in general and the one-point collocation method in particular.

ACS Style

Moustafa Soliman; Y. Al-Zeghayer; A. Ajbar. A modified orthogonal collocation method for reaction diffusion problems. Brazilian Journal of Chemical Engineering 2014, 31, 967 -975.

AMA Style

Moustafa Soliman, Y. Al-Zeghayer, A. Ajbar. A modified orthogonal collocation method for reaction diffusion problems. Brazilian Journal of Chemical Engineering. 2014; 31 (4):967-975.

Chicago/Turabian Style

Moustafa Soliman; Y. Al-Zeghayer; A. Ajbar. 2014. "A modified orthogonal collocation method for reaction diffusion problems." Brazilian Journal of Chemical Engineering 31, no. 4: 967-975.

Journal article
Published: 01 October 2014 in The International Journal of Electrical Engineering & Education
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This paper presents the main features of a study carried out by the higher education authority in the Kingdom of Saudi Arabia for developing a practical framework for formulating and assessing learning outcomes in electrical engineering education. The proposed learning outcomes were based on a compilation of different international accreditation frameworks. The formulated learning outcomes were grouped into four learning areas: (1) engineering sciences, (2) engineering analysis and investigation, (3) engineering design and (4) engineering practice. The paper also presents the main elements of a proposed standardized exit exam to test the developed learning outcomes. A table of specification was constructed that maps the developed learning outcomes with the test questions distributed over various learning levels. It allows the transformation of the developed learning outcomes into balanced questions to be used in an exit exam. The paper also discusses the implications of the proposed exit exam on electrical engineering education, and presents the results of a trial exam carried out recently.

ACS Style

Mohamed El-Kady; Abdullah Alsadaawi; Saeed Al-Zahrani; Abdelhamid Ajbar. Assessing learning outcomes in electrical engineering education: A case study from Saudi Arabia. The International Journal of Electrical Engineering & Education 2014, 51, 354 -367.

AMA Style

Mohamed El-Kady, Abdullah Alsadaawi, Saeed Al-Zahrani, Abdelhamid Ajbar. Assessing learning outcomes in electrical engineering education: A case study from Saudi Arabia. The International Journal of Electrical Engineering & Education. 2014; 51 (4):354-367.

Chicago/Turabian Style

Mohamed El-Kady; Abdullah Alsadaawi; Saeed Al-Zahrani; Abdelhamid Ajbar. 2014. "Assessing learning outcomes in electrical engineering education: A case study from Saudi Arabia." The International Journal of Electrical Engineering & Education 51, no. 4: 354-367.

Journal article
Published: 27 August 2014 in Arabian Journal for Science and Engineering
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This paper examines the conditions necessary for the emergence of complex dynamic behavior in systems of pure and simple microbial competition in the chemostat under time-invariant feed conditions. In particular, we study the effect of variable yield coefficients and the presence of microorganisms in the inflow on the dynamics of such systems. This is accomplished through the study of a mathematical model of two microbial populations competing for a single nutrient in a chemostat. A numerical investigation is carried out for a particular case for which the yield coefficient associated with one species is linearly dependent on the substrate, while the other species exists in the inflow. Both Monod and substrate inhibition growth rates are examined. The numerical investigation showed the existence of complex behavior in the model, characterized by the existence of stable quasi-periodic states resulting from torus bifurcations of limit cycles. Also, limit cycles may undergo period doubling leading to periodic states of increasing period. It seems that the variability of the yield coefficient of one species and the presence of at least one microorganism in the inflow are necessary conditions for complex dynamics to arise in pure and simple competition in the chemostat.

ACS Style

Abdelhamid Ajbar; Mohamed Asif. On the Existence of Complex Dynamics in Pure and Simple Microbial Competition in Bioreactors. Arabian Journal for Science and Engineering 2014, 39, 7495 -7501.

AMA Style

Abdelhamid Ajbar, Mohamed Asif. On the Existence of Complex Dynamics in Pure and Simple Microbial Competition in Bioreactors. Arabian Journal for Science and Engineering. 2014; 39 (11):7495-7501.

Chicago/Turabian Style

Abdelhamid Ajbar; Mohamed Asif. 2014. "On the Existence of Complex Dynamics in Pure and Simple Microbial Competition in Bioreactors." Arabian Journal for Science and Engineering 39, no. 11: 7495-7501.

Journal article
Published: 22 October 2013 in Asia-Pacific Journal of Chemical Engineering
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ACS Style

Abdelhamid Ajbar; Youssef AlZeghayer. A fundamental analysis of dynamics of waste biodegradation in aerobic processes. Asia-Pacific Journal of Chemical Engineering 2013, 9, 423 -430.

AMA Style

Abdelhamid Ajbar, Youssef AlZeghayer. A fundamental analysis of dynamics of waste biodegradation in aerobic processes. Asia-Pacific Journal of Chemical Engineering. 2013; 9 (3):423-430.

Chicago/Turabian Style

Abdelhamid Ajbar; Youssef AlZeghayer. 2013. "A fundamental analysis of dynamics of waste biodegradation in aerobic processes." Asia-Pacific Journal of Chemical Engineering 9, no. 3: 423-430.

Journal article
Published: 11 April 2013 in Ecological Modelling
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This paper investigates the complex dynamics resulting from interactions between one predator and one prey in a chemostat. A standard model is extended by allowing the yield coefficient associated with the prey to vary linearly with the substrate concentration. When this dependence is negligible, the proposed model is reduced to the classical constant yield model which was shown in the literature to produce periodic behavior for a wide range of parameters. In this paper we analyze the proposed model and we show that while the static behavior is relatively simple, the dynamics are complex and involve limit cycles and period doubling sequences leading to chaos. Numerical simulations are also presented to analyze the model equations and to determine the effect of its parameters on the resulting dynamics. The proposed model could serve as a basis to re-examine the importance of variable yield coefficients in predicting complex behavior in predator–prey interactions in the chemostat.

ACS Style

Emad Ali; Mohammed Asif; Abdelhamid Ajbar. Study of chaotic behavior in predator–prey interactions in a chemostat. Ecological Modelling 2013, 259, 10 -15.

AMA Style

Emad Ali, Mohammed Asif, Abdelhamid Ajbar. Study of chaotic behavior in predator–prey interactions in a chemostat. Ecological Modelling. 2013; 259 ():10-15.

Chicago/Turabian Style

Emad Ali; Mohammed Asif; Abdelhamid Ajbar. 2013. "Study of chaotic behavior in predator–prey interactions in a chemostat." Ecological Modelling 259, no. : 10-15.

Articles
Published: 07 March 2013 in DESALINATION AND WATER TREATMENT
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The forecast of long-term water demand is important for the planning of future requirements for water supply, distribution, and wastewater systems. The forecast is particularly important for arid countries such as Saudi Arabia which rely on costly desalination plants to satisfy the growing water demand. This study develops a model for forecasting water demand for Riyadh city, the capital of the country. The development of a sound forecast model is complicated by the uncertainties associated with key factors, such as the population growth and the economic activity, which is largely dependent on fluctuating oil prices. The forecast is also made difficult by the inefficient management of unaccounted-for-water (UFW). All these factors limit the usefulness of any deterministic forecast model. This paper develops a probabilistic forecast model that incorporates explicitly the uncertainties associated with population growth, household size, household income as well as conservation measures, and UFW management. The methodology makes use of historic time series records of water consumption to forecast the future demand, and applies the Monte Carlo sampling to describe the associated uncertainties. Results show that future water demand in the city is governed equally by socioeconomic factors and weather conditions. The study also illustrates the importance of conservation measures and the need for reduction of UFW.

ACS Style

Ibrahim Almutaz; Emad Ali; Yasir Khalid; Abdel Hamid Ajbar. A long-term forecast of water demand for a desalinated dependent city: case of Riyadh City in Saudi Arabia. DESALINATION AND WATER TREATMENT 2013, 51, 5934 -5941.

AMA Style

Ibrahim Almutaz, Emad Ali, Yasir Khalid, Abdel Hamid Ajbar. A long-term forecast of water demand for a desalinated dependent city: case of Riyadh City in Saudi Arabia. DESALINATION AND WATER TREATMENT. 2013; 51 (31):5934-5941.

Chicago/Turabian Style

Ibrahim Almutaz; Emad Ali; Yasir Khalid; Abdel Hamid Ajbar. 2013. "A long-term forecast of water demand for a desalinated dependent city: case of Riyadh City in Saudi Arabia." DESALINATION AND WATER TREATMENT 51, no. 31: 5934-5941.

Journal article
Published: 10 January 2013 in Arabian Journal for Science and Engineering
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This paper investigates the dynamic performance of bioethanol production process. The process is simulated by an experimentally validated model in the literature. The analysis of the open loop system revealed that the maximum productivity occurred at a moderate ethanol production and substrate consumption. Improvement of the process productivity beyond the maximum value is sought through periodic input forcing. Rectangular pulses were used to simulate the periodic input function because they are suitable for discrete control systems. The parameters of the rectangular pulse such as period, amplitude and central value were designed using numerical optimization. Forcing the optimized cycling input into the process revealed substantial increment of the ethanol production and substrate consumption. However, the obtained enhancement of productivity is considered marginal when compared to the existing maximum productivity. On the other hand, enrichment of productivity is considered reasonable when compared to that obtained when the process operates at steady state using the average value for the input periodic function.

ACS Style

Emad Ali; A. Ajbar; K. AlHumaizi. Enhanced Ethanol Reactor Operation Through Periodic Forcing of the Feed Rate. Arabian Journal for Science and Engineering 2013, 38, 741 -750.

AMA Style

Emad Ali, A. Ajbar, K. AlHumaizi. Enhanced Ethanol Reactor Operation Through Periodic Forcing of the Feed Rate. Arabian Journal for Science and Engineering. 2013; 38 (4):741-750.

Chicago/Turabian Style

Emad Ali; A. Ajbar; K. AlHumaizi. 2013. "Enhanced Ethanol Reactor Operation Through Periodic Forcing of the Feed Rate." Arabian Journal for Science and Engineering 38, no. 4: 741-750.

Journal article
Published: 31 January 2012 in Journal of Process Control
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ACS Style

Ali Emad; A. Ajbar; I. Almutaz. Periodic control of a reverse osmosis desalination process. Journal of Process Control 2012, 22, 218 -227.

AMA Style

Ali Emad, A. Ajbar, I. Almutaz. Periodic control of a reverse osmosis desalination process. Journal of Process Control. 2012; 22 (1):218-227.

Chicago/Turabian Style

Ali Emad; A. Ajbar; I. Almutaz. 2012. "Periodic control of a reverse osmosis desalination process." Journal of Process Control 22, no. 1: 218-227.

Journal article
Published: 30 November 2011 in Mathematical and Computer Modelling
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The paper investigates the static and dynamic behavior of a continuous flow bioreactor for the aerobic biodegradation of municipal and industrial waste. The unstructured kinetic model for the bioreactor accounts for oxygen limitation in the bioreactor model while the growth rate is described by the biomass-dependent Contois model. This kinetic model is known to explain the aerobic biodegradation of solid municipal organic waste well. The yield coefficient is assumed to depend linearly on the substrate. The analysis of the unsteady-state model is carried out both analytically and through numerical simulations. The results show the ability of the model to predict a wide range of behavior, including oscillations for some range of kinetic and operating parameters. The analysis also showed that a variability of the yield coefficient is necessary for the existence of Hopf points in the model. The effects of the model parameters on the performance of the bioreactor are also investigated.

ACS Style

Abdelhamid Ajbar; Malik AlAhmad; Emad Ali. On the dynamics of biodegradation of wastewater in aerated continuous bioreactors. Mathematical and Computer Modelling 2011, 54, 1930 -1942.

AMA Style

Abdelhamid Ajbar, Malik AlAhmad, Emad Ali. On the dynamics of biodegradation of wastewater in aerated continuous bioreactors. Mathematical and Computer Modelling. 2011; 54 (9-10):1930-1942.

Chicago/Turabian Style

Abdelhamid Ajbar; Malik AlAhmad; Emad Ali. 2011. "On the dynamics of biodegradation of wastewater in aerated continuous bioreactors." Mathematical and Computer Modelling 54, no. 9-10: 1930-1942.

Journal article
Published: 13 September 2011 in Korean Journal of Chemical Engineering
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This paper presents a detailed modeling and simulations of a reformer unit used in the direct reduction of iron (DRI) process. A one-dimensional heterogeneous model for the catalyst tubes which takes into account the intraparticle mass transfer resistance was developed, while the furnace was modeled with bottom firing configuration. Validation against data from a local iron/steel plant showed satisfactory results. The performance variables of the unit were the process gas temperature, wall temperature and conversions of hydrogen, methane and carbon dioxide. The profiles of these output variables along the distance were calculated. The effect of operating parameters such as inlet temperature, natural gas flow rate and gas composition was also determined.

ACS Style

Abdelhamid Ajbar; Khalid Alhumaizi; Moustafa Soliman. Modeling and simulations of a reformer used in direct reduction of iron. Korean Journal of Chemical Engineering 2011, 28, 2242 -2249.

AMA Style

Abdelhamid Ajbar, Khalid Alhumaizi, Moustafa Soliman. Modeling and simulations of a reformer used in direct reduction of iron. Korean Journal of Chemical Engineering. 2011; 28 (12):2242-2249.

Chicago/Turabian Style

Abdelhamid Ajbar; Khalid Alhumaizi; Moustafa Soliman. 2011. "Modeling and simulations of a reformer used in direct reduction of iron." Korean Journal of Chemical Engineering 28, no. 12: 2242-2249.