Search Results (6)

This work investigates the occurrence, stability, and dynamic accessibility of steady states in isothermal continuous stirred-tank reactors (CSTRs) containing free or immobilized enzymes subject to substrate inhibition. For homogeneous systems, a dimensionless reactor model was employed to identify operating conditions leading to multiplicity and hysteresis. For immobilized enzymes, the reactor mass balance was coupled with an intraparticle diffusion–reaction model to determine substrate concentration profiles and effectiveness factors. Steady-state solutions were obtained using complementary stationary and transient numerical approaches, while stability was assessed through eigenvalue analysis of the Jacobian matrix referring to the linearized system of governing equations. The results show that substrate inhibition may generate multiple steady states at both the reactor and particle scales. For immobilized enzymes, multiple concentration profiles and effectiveness factors may coexist for specific combinations of kinetic and transport parameters. Stability analysis showed that the multiple steady states identified at both the reactor and particle scales are organized into stable lower and upper branches separated by an unstable intermediate branch. Dynamic simulations showed that, in regions where multiple stable states coexist, the steady state attained by the system depends on the initial intraparticle concentration profile and operating trajectory. Moderate intraparticle diffusion limitations were found to alleviate substrate inhibition and substantially improve reactor performance. By combining multiplicity, stability, and dynamic accessibility analyses, this work provides a framework for identifying physically attainable operating states and predicting reactor performance under different reaction–diffusion conditions, thereby supporting the design, start-up, and operation of continuous reactors employing immobilized enzymes under substrate inhibition. Full article

Isothermal constant-strain-rate compression experiments of S280 ultrahigh-strength stainless steel were conducted at 800–1150 °C, 0.001–10 s⁻¹, and 70% height reduction. The flow stress behaviors were analyzed based on the compression data. The strain compensation Arrhenius constitutive relation, multiple linear regression constitutive relation, and back-propagation (BP) neural network constitutive relation of this alloy were established for the first time. The S280 ultrahigh-strength stainless steel is characterized by a positive strain rate and negative temperature sensitivity. Its flow stress at high temperature (1000–1150 °C) and low temperature (800–950 °C) is generally at the steady state and the softening state, respectively. The three new flow stress constitutive relations all meet the requirements for engineering applications in terms of predictive precision. The BP neural network constitutive relation shows the highest predictive precision, with correlation coefficient R of 0.999 and average absolute relative error AARE of 1.04%. The strain compensation Arrhenius constitutive relation shows the lowest predictive precision, with R of 0.994 and AARE of 14.748%. The multiple linear regression constitutive relation shows the modest predictive precision, with R of 0.994 and AARE of 6.24%. Full article

The catalytic oxidation of CO is probably the most investigated reaction in the literature, for decades, because of its extended environmental and fundamental importance. In this paper, the oxidation of CO on La_1−xSr_xMnO₃ perovskites (LSMx), either unloaded or loaded with dispersed Ir nanoparticles (Ir/LSMx), was studied in the temperature range 100–450 °C under excess O₂ conditions (1% CO + 5% O₂). The perovskites, of the type La_1−xSr_xMnO₃ (x = 0.0, 0.3, 0.5 and 0.7), were prepared by the coprecipitation method. The physicochemical and structural properties of both the LSMx and the homologous Ir/LSMx catalysts were evaluated by various techniques (XRD, N₂ sorption–desorption by BET-BJH, H₂-TPR and H₂-Chem), in order to better understand the structure–activity–stability correlations. The effect of preoxidation/prereduction/aging of the catalysts on their activity and stability was also investigated. Results revealed that both LSMx and Ir/LSMx are effective for CO oxidation, with the latter being superior to the former. In both series of materials, increasing the substitution of La by Sr in the composition of the perovskite resulted to a gradual suppression of their CO oxidation activity when these were prereduced; the opposite was true for preoxidized samples. Inverse hysteresis phenomena in activity were observed during heating/cooling cycles on the prereduced Ir/LSMx catalysts with the loop amplitude narrowing with increasing Sr-content in LSMx. Oxidative thermal sintering experiments at high temperatures revealed excellent antisintering behavior of Ir nanoparticles supported on LSMx, resulting from perovskite’s favorable antisintering properties of high oxygen storage capacity and surface oxygen vacancies. Full article

This paper presents an adaptive control scheme to face the challenge of rejecting input and output disturbances. The research is put on a layer of the design and start-up of chemical plants. The emphasis is on handling disturbances appearing in a narrow band of frequencies, which illustrates standard forms of disturbances in the alluded kind of systems. The controller is made up of a central RS structure that stabilizes the closed-loop plant. A second layer boosts the control law performance by adding the Youla–Kucera (YK) filter or Q parametrization and taking advantage of the internal model principle (IMP). This practice aids in modeling unknown disturbances with online control adjustment. We probe the resultant compensator for three non-isothermal continuous stirred tank reactors connected in series. The plant should conduct a first-order exothermic reaction consuming reactant A, while an isothermal operation stays and the outlet concentration is close to its nominal value. The primary concerns are open-loop instability and steady-state multiplicity in the plant’s first unit. The control objective is to reject input and output disturbances in a band of frequencies of $0.0002\mathrm{Hz}$ to $0.007\mathrm{Hz}$, whether there are variants or not in time. We test the controller with input signals depicting both variations in the auxiliary services and abrupt changes. We then compare the executions of the resultant control law with a model-based predictive control (MPC). We find comparable responses to multiple disturbances. However, the adaptive control offers an effortless control input. We also conclude that the adaptive controller responds well to reference changes, while the MPC fails due to input constraints. Full article

Transporting green hydrogen by existing natural gas networks has become a practical means to accommodate curtailed wind and solar power. Restricted by pipe materials and pressure levels, there is an upper limit on the hydrogen blending ratio of hydrogen-enriched compressed natural gas (HCNG) that can be transported by natural gas pipelines, which affects whether the natural gas network can supply energy safely and reliably. To this end, this paper investigates the effects of the intermittent and fluctuating green hydrogen produced by different types of renewable energy on the dynamic distribution of hydrogen concentration after it is blended into natural gas pipelines. Based on the isothermal steady-state simulation results of the natural gas network, two convection–diffusion models for the dynamic simulation of hydrogen injections are proposed. Finally, the dynamic changes of hydrogen concentration in the pipelines under scenarios of multiple green hydrogen types and multiple injection nodes are simulated on a seven-node natural gas network. The simulation results indicate that, compared with the solar-power-dominated hydrogen production-blending scenario, the hydrogen concentrations in the natural gas pipelines are more uniformly distributed in the wind-power-dominated scenario and the solar–wind power balance scenario. To be specific, in the solar-power-dominated scenario, the hydrogen concentration exceeds the limit for more time whilst the overall hydrogen production is low, and the local hydrogen concentration in the natural gas network exceeds the limit for nearly 50% of the time in a day. By comparison, in the wind-power-dominated scenario, all pipelines can work under safe conditions. The hydrogen concentration overrun time in the solar–wind power balance scenario is also improved compared with the solar-power-dominated scenario, and the limit-exceeding time of the hydrogen concentration in Pipe 5 and Pipe 6 is reduced to 91.24% and 91.99% of the solar-power-dominated scenario. This work can help verify the day-ahead scheduling strategy of the electricity-HCNG integrated energy system (IES) and provide a reference for the design of local hydrogen production-blending systems. Full article

Boundary layer analysis is invoked to clarify the aspects of variable thermal conductivity and thermophoretic forces on a steady state of MHD micropolar fluid flow in the existence of a uniform transverse magnetic field along an isothermal horizontal plate. The micropolar pattern permits the rotational freedom degrees that lead to couple stresses and a non symmetric stress tensor. The initiated PDEs governing the case pattern are mutated into a non-dimensional system due to proper transformations. The transformed mathematical governing equations are solved by implementing a very potent computer algebra software MATLAB code. The plotted graphs analyzed the attitude of multiple physical aspects involving factors on the flow attitude of micropolar velocity and angular velocity and temperature. Through the involved factors, the couple stress, skin friction and Nusselt number are manifested and interpreted amply. A new outcome for drag force and heat gradient experienced by the key factors is portrayed. Augmentation in Ω results in the thermophoretic forces that encapsulate the mass transmission. The local Nusselt number strengthened as the thermal conductivity, heat absorption factors or wall suction velocity were improved, and weakened due to the existence of viscous dissipation or heat generation impacts. As a particular case, the governing field equations of a classical Newtonian liquid are given by dropping the micropolar parameter impacts. Full article