International Journal of Engineering Research in Africa Vol. 49

Abstract: The global use of natural gas is growing quickly, and this is attributed primarily to its environmental advantage over fossil fuels such as crude oil and coal. This natural gas is usually flared in refineries because extra charges would be incurred in collecting and processing it. A country flares about 800 million standard cubic feet (Mmscf) of gas per day, from approximately 144 gas flare points across the nation, losing a huge amount of money per annum. A liquefied natural gas plant has converted about 5.58 trillion cubic feet (Tcf) of associated gas to exports as liquefied natural gas and natural gas liquids (NGLs), thus helping to reduce gas flaring from upstream companies. Natural gas liquids (NGLs) are major contributors to this economic benefit through petrochemical feedstock for industrial purposes, fuel for residential, commercial and agricultural applications, in addition to using other products as propellant, refrigerant and gasoline blending. To contribute to the technology of natural gas liquid system, in this work, a fractionation system has been modelled and simulated using Aspen HYSYS to determine the status of processes involved and the compositions of the NGLs. The results obtained revealed that each of methane, ethane, propane, iso-butane and n-butane could be successfully separated with high purity from natural gas feed stream. Also, it was observed from the validation carried out on the developed model of the system, which was ascertained by its operations that were in line with the theoretical principles of separation involved in the plant, that it can be used for further analyses of the system.

Abstract: Dispersion is one of the fate processes of oil spill. This research has been carried out on the numerical simulation of the dispersion of crude oil using the model obtained from the work of Hamam (1987). The model was solved with the explicit, implicit and Crank-Nicolson methods of solution of partial differentiation equations with the aid of MATLAB, and the concentration of the crude oil dispersed in water was obtained. The results obtained revealed that the three methods could be used to study the process because the profiles given by all of them were very similar. Also discovered from the investigations carried out was that the concentration of crude oil was decreasing with time for a particular spatial point while, for a particular time, it was increasing along the length of the water body. It was also discovered that experiments would be very necessary in order for the validation of the results obtained from the simulations.

Abstract: The purpose of this paper is to investigate the magnetohydrodynamic stability external flow of a viscous, incompressible and electrically conducting fluid over a moving flat plate using temporal linear stability analysis. Using a similarity variables based on the Skan-Falkner transformation, the governing differential equations of mean flow are transformed into a nonlinear ordinary differential equation, which is then solved numerically by the Runge-Kutta method. The MHD stability equation is solved numerically by using the Chebyshev spectral collocation method, which is based on the eigenfunction expansion in terms of Chebyshev polynomials, collocation points, and the subsequent solution of the resulting generalized eigenvalue problem with the QZ algorithm. The influence of the pressure gradient, magnetic field and wall velocity on the dimensionless mean velocity are presented graphically and discussed. For the disturbance flow in the presence of magnetic field and moving wall, the critical Reynolds number increases with increasing the magnetic field parameter and wall velocity, indicating that these parameters stabilize the flow.

Abstract: Pressure drop in a vertical or deviated borehole has been found to be due to hydrostatic changes and friction as a result of the produced fluids flowing to the surface. When oil flows upwards, the flowing pressure along the tubing string drops, and this makes gas to start liberating. Thus, multiphase flow forms in the tubing string. Hence, adequate modelling of vertical lift performance is required to predict the pressure drop and subsequently the wellbore pressure because many factors are involved [1]. In this work, sensitivity analysis of multiphase flow in a well has been carried out with the aid of PROSPER in which the most accurate correlation was chosen from twelve selected built-in correlations present in the program to predict the pressure drop via gradient matching. A sensitivity analysis of the well was further performed to investigate the parameters such as tubing diameter, gas-oil ratio and wellhead pressure that were affecting the vertical lift performance of a high water cut well. The results obtained from the correlation matching showed that Dun and Ros [2] original correlation was the best fit correlation for the well. The results of the sensitivity analysis revealed that reduction of wellhead pressure from 600 psi to 400 psi could increase liquid rate by 41%. An adjustment of wellhead pressure was found to give the most significant impact on the production rate of the well as compared to other two parameters studied.

Abstract: Application of multi-pulse multilevel inverters is considered in this paper for distributed generation. A five-level twelve-pulse neutral point clamped inverter has been combined with a proton exchange membrane fuel cell in order to investigate load following characteristics of the fuel cell. The fuel cell implemented with a three-phase multi-pulse multilevel inverter is adept of delivering single-phase and three-phase loads both in islanded and grid-connected approach. Changes in power demand from no-load to full-load (120 kW) have been applied to study the characteristics of the system from the perspective of how it can follow the load changes in load demand. It has been observed that the fuel cell model is adept of following power request as per requirement; however, there is a response time of few seconds, because the reformer for the fuel cell requires time to generate fuel and the fuel cell requires time for chemical reactions to take place in it. Implementations of six-pulse and twelve-pulse five-level neutral point clamped and flying capacitor inverters show that total harmonic distortions for six-pulse and twelve-pulse five-level neutral point clamped inverters to be 1.066219% and 0.406149% respectively as compared to 2.466889% and 1.5104075% for flying capacitor inverters. It has been observed that with twelve-pulse neutral point clamped inverter, the output voltage waveform is smoother and close to sinewave. The results of the research work is presented with analyses to validate that multi-pulse multilevel neutral point clamped inverter is a better way out for the fuel cell power generation model as this type of inverters produces smoother waveforms to improve power quality with lower harmonics.

Abstract: Renewable energy sources, such as photovoltaic, fuel cell and wind energy are becoming a sustainable alternative to non-renewable sources like fossil fuel. However, to integrate these energies into the grid, power electronic converters plays major role due to their power conditioning capability, reliability and effectiveness. In this paper, design, modeling and analysis of a DC-DC boost converter with robust controlling technique, fuzzy sliding mode controlling strategy has been developed and a brief comparison has been performed with a sliding mode controller and a clasical PID controller which employed both current and a voltage control loop. The system is designed to achieve a fast dynamic response, zero steady-state error, and satisfactory stability. To realize that a detailed mathematical derivation of sliding mode fuzzy logic controller and a linearized small signal model of the power electronic converter around its DC steady state operating point is performed. Finally, in order to evaluate the designed system, a software simulation based on MATLAB/ Simulink environment is developed and results of the simulation shows the effectiveness of the proposed techniques.

Abstract: The main purpose of this work is to introduced power quality improvement in low voltage distribution networks with the application of Improved Unified Power Quality Conditioner (I-UPQC). Ordinarily, in the normal UPQC, the series inverter handles active injection while the shunt inverter provides load imaginary power injection. However, in the case of I-UPQC, the series inverter of the UPQC performs two functions concurrently as a sag and swell compensator and assists the shunt inverter in load reactive power requirements. The reactive power sharing is achieved by the integration of the Power Angle Control (PAC) of UPQC to coordinate imaginary power-sharing between the two inverters. With the view that the series inverter produces active and reactive power, this concept is named I-UPQC. Full mathematical analysis to extend the PAC method to I-UPQC has been carried out in this work. The simulation and results produced in the MATLAB / SIMULINK environment and discussion to support the developed concept are also presented. The result from the proposed concept is confirmed by comparing the concept with operation unified power quality conditioner in steady-state.

Abstract: This paper incorporates Flexible AC Transmission System (FACTS) device loss with the general loss sensitivity equation for the determination of optimum location for its placement in deregulated power networks, with objectives of Available Transfer Capability (ATC) enhancement, bus voltage improvement and loss reduction. A detailed mathematical model in terms of circuit system parameters is presented based on FACTS loss amalgamation approach. Thyristor Controlled Series Capacitor (TCSC) FACTS device is considered for simulation and analysis because of its capability to control active power among other parameters. The TCSC location is established based on analysis of sensitivity factors obtained from partial derivatives of the resultant loss equations (including FACTS) with respect to control parameters. ATC values are obtained using ACPRDF method and with TCSC in place, these values are enhanced for different bilateral and multilateral power transactions. IEEE 5 Bus system is used for the demonstration of the effectiveness of this approach. Placement with this method resulted into ATC enhancement of more than 60% well above the values obtained when TCSC was placed with thermal limit method. In addition, a substantial bus voltage improvement of up to 3% deviation minimization as well as up to 10% active power loss reduction was recorded with this placement.

Abstract: In this paper, the evolution of texture in the ferrite phase and mechanical behavior of cold-drawn pearlitic steel wires produced for strand manufacturing at Trefisoud company was investigated. Wire drawing induces the development of dislocation density, reduction of interlamellar spacing and the refinement of grains size which leads to a strong hardening of the wires. That explains the increase of the tensile strength from 1242 MPa to 2618 MPa with higher deformation. Also, the cementite lamellae are rotated toward the drawing axis and the thickness of lamellae further decreases when strain level increases, this phenomenon leads to a somewhat fibrous structure. The quantitative analysis obtained by EBSD data shows the development of a strong (<110> // ND) texture of the ferrite phase leading to a structural transformation from isotropic to anisotropic.

Abstract: In recent years, diesel engine is developing rapidly in the direction of high power and super long stroke, which requires higher strength of its key moving parts. Connecting rod is one of the key moving parts of diesel engine which is subjected to complex alternating load during the working process. This loading condition has a great influence on its structural strength and reliability. In the proposed study, the strength and fatigue of a low-speed diesel engine con-rod made of 42CrMoA are analyzed. The 3-D model of the con-rod assembly built in the proposed study. The stress distribution and deformation of the con-rod assembly under the maximum explosive pressure are presented and studied. In the present paper, fatigue safety factor of all parts of con-rod assembly under the maximum explosive pressure condition is checked. According to the results carried out from the proposed work, the corresponding alternating stress is 340MPa, while the fatigue limit of 42CrMo material is above 430-540MPa, which means that the con-rod parts work under the alternating stress far below the fatigue limit. The kirasushvili method is adopted in the present paper as the standard of safety factor evaluation of con-rod. According to the allowable safety factor table of kirasushvili method, the minimum safety factor of the big and small ends of the con-rod and rod body can meet the requirements without fatigue damage.