Volume 16, Issue 1

Despite the importance of natural gas (NG) as an energy source, there is a lot of pressurized landfill gas not exploited so far because it contains high CO 2 concentration. Therefore, this study aimed to develop a 2-D mathematical model to simulate CO 2 removal from NG stream contains high CO 2 concentration up to 70% at high-pressure up to 60 bar using three different dimensions of polyvinylidene fluoride (PVDF) hollow fiber membrane contactors. Aqueous solutions of activated methyldiethanolamine (MDEA) with piperazine (PZ) were adopted. The performance of considered absorbent at high-pressure was evaluated at the non-wetting mode condition of membrane contactor. Moreover, the effect of pressure, contact area, gas flow rate, MDEA concentration into the amine mixture, PZ concentration, temperature and membrane properties were theoretically investigated. The findings stated that activated MDEA had different performance in terms of membrane wetting compared with other amines, which used at high pressure in previous studies. In addition, the simulation results showed that CO 2 removal efficiency was significantly enhanced, when the operating pressure, contact area, PZ concentration and temperature were increased. However, increasing gas flow rate leads to reduce CO 2 removal efficiency. Furthermore, the CO 2 absorption was significantly improved by adding a small amount of PZ to MDEA. The predicted model results showed a good agreement with experimental data obtained from the literature.

A simplified tuning guideline for internal model control (IMC) based modified Smith predictor technique is reported here for unstable lag dominated first-order processes with dead time (UFOPDT). Pole location in right half section of s-plane signifies the unstable behaviour of UFOPDT processes. Mostly, chemical processes like isothermal chemical reactor, bioreactor, dimerization reactor, fluid catalytic cracker etc. are found to be lag dominated and unstable along with considerable time delay. Smith predictor technique based control methodology is considered to be a well-accepted approach for such cases. However, conventional Smith predictor technique is not capable enough for controlling UFOPDT processes. Whereas modified Smith predictor is found to be quite competent in such cases as its design involves more than one controller. Modified Smith predictor structure is capable to provide desirable closed loop response during set point tracking along with the load recovery phases. To mitigate the tuning complexity of multiple controllers involved in modified Smith predictor designing, suggested IMC structure employs single tuning parameter λ i.e. closed loop time constant for all three controllers concerned. Noticeable performance enhancement is reported by the proposed scheme as no overshoot is observed during set point tracking. Moreover, smooth and efficient load rejection behaviour is also obtained. Supremacy of the proposed tuning is established through closed loop performance comparison with others’ reported modified Smith predictor based tuning relations for chemical reactor and bioreactor in terms of performance indices as well as stability margins.

This paper presents the investigation of a two dimensional coupled model of heat and mass transfer in a mixture of AB 2 – AB 5 metal hydride (MH) systems of a cylindrical configuration during hydrogen sorption using COMSOL 5.3a commercial software. The parametric study on the sorption process has been studied with variation of heat transfer coefficient (HTC), and activation energy (AE) to understand the effects they have on the reaction kinetics of the sorption process. The simulation results demonstrate the importance of mutual dependence between the temperature propagation in the body of metal hydride, the absorbed concentration of the hydrogen gas, and the gas pressure for the absorption of hydrogen gas in metal hydrides. The decrease in the activation energy is found to have significant effect on the dynamic performances of hydrogen absorption in the MH reactors with an increased amount of hydrogen conversion, whilst the variation of heat transfer coefficient displayed insignificant change in hydrogen conversion. The simulated results show good agreement with the experimental results obtained from HYSA Systems and were implemented for use in the STILL RX60-30L electric forklift fuel cell applications designed by HYSA Systems in the University of the Western Cape.

The design of flow field greatly influences Proton Exchange Membrane Fuel Cell (PEMFC) performance, as it not only distributes the reactants, also removes the product (water) from the cell. Improper water removal blocks the reaction sites, which results in mass transport losses. A complete 3-D numerical model of PEMFC with a single serpentine (1-S), double serpentine (2-S),triple serpentine (3-S) and 3-2-1 serpentine flow fields with round corner was designed with the help of commercial Computational Fluid Dynamics (CFD) code ANSYS FLUENT. Simulations were carried out to investigate the effect of four flow fields on distribution of pressure, mass fraction of H 2 , O 2 , current flux density distribution, water content of membrane, and liquid water activity in the flow channels as well as the functioning of cells. Performance properties of proposed four designs were evaluated and found that 3-2-1 serpentine flow field performance is better than the 1-S, 2-S, and 3-S flow fields for the given flow rates of reactants and this 3-2-1 serpentine flow field model was validated with literature experimental data. The results also show that the velocity in channels increases with a decrease in the number of flow passes, which improve the reactions in the catalyst layers, reaction product removal from the cell thus reduces the concentration losses and improves the cell performance.

Dead biomass of micro-organisms can be used as biosorbents for the mitigation of heavy metal pollution in the aqueous environment. The aim of this study was to statistically model and optimise the sorption of Cd(II) and Pb(II) by dead biomass of Pseudomonas aeruginosa and to study the interactions between operating conditions. Statistically significant models were obtained for Cd(II) and Pb(II) sorption. The standard deviation for the Cd(II) and Pb(II) models were 0.86 and 1.54 while the coefficient of determination (R 2 ) were 0.9978 and 0.9928 respectively. For both models, the adjusted R 2 was in good agreement with the predicted R 2 as the difference was less than 0.2. Numerical optimisation revealed that optimum Cd(II) removal of 88.6 % can be achieved at 1.172 ppm initial metal concentration, pH of 8.85, temperature of 43.72 °C, agitation time of 125.96 minutes and dead cell mass of 114.8 mg. Also, an optimum Pb(II) removal of 100 % can be achieved at 1.936 ppm initial metal concentration, pH of 6.88, temperature of 37.24 °C, agitation time of 130.57 minutes and dead cell mass of 122.85 mg. The study has revealed that at careful selected operational parameters, dead biomass of Pseudomonas aeruginosa can be valorised for the removal of heavy metals in aqueous media.

This paper presents a comparison between some numerical methods and techniques for solving the nonlinear advection-dispersion equation, which may be used to describe the adsorption of phenol into a granular activated carbon fixed bed under local equilibrium conditions. The adsorption is described by the Langmuir isotherm, which makes the advection-dispersion equation nonlinear. This equation is solved successively by using the approximation and linearization techniques. For each technique, two types of numerical algorithms are used. Concerning the first one, the Implicit and the Runge Kutta schemes are used. As for the second one, the Modified Picard iteration and the Newton Raphson scheme are applied. Simulation results have been compared to each other and to the experimental data as well. Both of the Implicit and the Runge Kutta algorithms have led to superimposed simulated breakthrough curves. Both of the modified Picard and Newton Raphson schemes have given identical results too. However, comparing to the experimental data, the obtained solution, using the approximation technique, has underestimated the retardation of solute and failed in fitting the experimental breakthrough. The Obtained solution, using the linearization technique, has correctly fitted the experimental results under all the conditions of: feed flow rate, activated carbon bed height and the inlet phenol concentration.