Statistical optimization of process conditions for photocatalytic degradation of phenol with immobilization of nano TiO<Subscript>2</Subscript> on perlite granules

Response surface methodology (RSM) using D-optimal design was applied to optimization of photocatalytic degradation of phenol by new composite nano-catalyst (TiO₂/Perlite). Effects of seven factors (initial pH, initial phenol concentration, reaction temperature, UV irradiation time, UV light intensity, catalyst calcination temperature, and dosage of TiO₂/perlite) on phenol conversion efficiency were studied and optimized by using the statistical software MODDE 8.02. On statistical analysis of the results from the experimental studies, the optimum process conditions were as follows: initial pH, 10.7; initial phenol concentration, 0.5 mM; reaction temperature, 27 °C; UV irradiation time, 6.5 h; UV light intensity, 250 W; catalyst calcination temperature, 600 °C; and TiO₂/perlite dosage, 6 g/L. Analysis of variance (ANOVA) showed a high coefficient of determination (R²) of 91.8%.     

Dynamic setup scheduling and flow control in manufacturing systems

We propose a method for flow control of parts in a manufacturing system with machines that require setups. The setup scheduling problem is investigated in the context of a multilevel hierarchy of discrete events with distinct frequencies. The higher level of the hierarchy calculates a target trajectory in the surplus/backlog space of the part types which must be tracked at the level of setups. We consider a feedback setup scheduling policy which usescorridors in the surplus/backlog space of the part types to determine the timing of the set-up changes in order to guide the trajectory in the desired direction. An interesting case in which the trajectory leads to a target point (e.g., a hedging point) is investigated in detail. It is shown that in this case the surplus/backlog trajectory at the setup level can lead to a limit cycle. Conditions for linear corridors which result in a stable limit cycle are determined.     

Production control of a manufacturing system with multiple machinestates

The production control of a single-product manufacturing system with arbitrary number of machine states (failure modes) is discussed. The objective is to find a production policy that would meet the demand for the product with minimum average inventory or backlog cost. The optimal production policy has a special structure and is called a hedging-point policy. If the hedging points are known, the optimal production rate is readily specified. Assuming a set of tentative hedging points, the simple structure of the optimal policy is utilized to find the steady-state probability distribution of the surplus (inventory or backlog). Once this function is determined, the average surplus cost is easily calculated in terms of the values of the hedging points. The average cost is then minimized to find the optimum hedging points     

Stability and performance of distributed production control methodsbased on continuous-flow models

Investigates the stability and performance of a two-level production control method for part release, routing, and machine scheduling in manufacturing systems. At the first level (off line), a continuous-flow (fluid) approximation of the production control problem is formulated and solved as a linear program. At the second level (on line), simple distributed control policies are used to track the solution of the continuous-flow model of level one. A major research issue concerns the potential instability of decentralized tracking policies due to the discreteness of the decision space (caused by the sequential nature of operations and their discrete processing times). The author considers a broad class of distributed tracking policies, which is called nonidling-nonexceeding (NINE) and finds a sufficient condition for their stability. The results show the NINE policies potential instability for nonacyclic systems due to machine starvation, which is caused by tracking delays induced by the discrete nature of operations. The sufficient stability condition takes a familiar form, namely that, the largest eigenvalue of a matrix-which captures the dynamics of tracking delays in the system-must be less than one. It ensures the contraction of tracking delays in the feedback loops of material flow in the system. When this condition is met, an upper bound on the performance of the policy is readily obtained. It is further shown that any nonidling dispatching policy can be considered as a special NINE policy, and thus the same stability and performance results apply to the class of all nonidling policies. The author also investigates NINE policies with buffer priorities and shows that a NINE policy with certain buffer priority ordering is always stable. Simulation experiments show that near-zero work-in-process and finished-parts' inventory can be achieved with the method even for demands that are very close to the production capacity of the system     

A visible-light-active BiFeO<Subscript>3</Subscript>/ZnS nanocomposite for photocatalytic conversion of greenhouse gases

Given the changes in environmental conditions in the world, photocatalytic conversion of greenhouse gases is of great interest today. Our aim was to increase the photocatalytic efficiency of BiFeO₃/ZnS (p-n heterojunction photocatalyst) by varying the molar ratio of ZnS to perovskite structure of BiFeO₃ using hydrothermal synthesis. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), FT-IR spectroscopy showed the small crystal size and suitable distribution of ZnS particles on the BiFeO₃ structure. The results of UV-visible, and photoluminescence (PL) spectroscopy analyses showed the good behavior of p-n heterostructure in absorption of visible light and lowering electron-hole recombination. The best visible light photocatalytic efficiency of CO₂ reduction, 24.8%, was obtained by an equimolar ratio of BiFeO₃/ZnS.     

Application of mixture experimental design for photocatalytic ammonia degradation by sunlight‐driven WO3‐Ag3PO4‐ZnO ternary photocatalysts

Using statistical mixture design, the best composition of a heterojunction photocatalyst containing ZnO, Ag₃PO₄, and WO₃ was determined to maximize the sunlight driven ammonia removal from aqueous solution via both photocatalysis and adsorption processes. All samples were prepared by coprecipitation and immobilized over perlite granules as floatable support. X‐ray diffraction (XRD), Fourier‐transform infrared (FTIR), field emission scanning electron microscope (FESEM), Brunauer, Emmett, and Teller (BET), ultraviolet–visible (UV‐vis), and photoluminescence (PL) analyses were used to characterize the catalysts. Three responses of ammonia removal by photocatalysis, adsorption, and the total ammonia removal were modeled by special cubic models, and the ANOVA confirmed the significance of them. The maximum ammonia removal, approximately 88%, was obtained by photocatalyst composed of 32.93‐wt% WO₃, 41.82‐wt% Ag₃PO₄, and 25.26‐wt% ZnO. The contribution of photocatalysis and adsorption was estimated to be 72.74% and 14.44%, respectively, indicating the dominance of photocatalysis process. According to kinetic study, the optimum photocatalyst showed the highest apparent rate constant and lowest half‐life time of ammonia removal. The maximum quantum yield of 1.7% was calculated from the best photocatalyst composite at the maximum intensity of visible light received from sunlight. The reuse ability test revealed that the optimum ternary photocatalyst is suitable for wastewater treatments in practical applications.     

The effect of surfactants on the photocatalytic performance of BiOCl-ZnO nanoparticles in the degradation of an organic pollutant

The effect of surfactants on the performance of BiOCl-ZnO nanoparticles was investigated in the photocatalytic degradation of an organic pollutant. BiOCl-ZnO nanoparticles, modified with cetyl trimethyl ammonium bromide (CTAB) and polyethyleneglycol-20000 (PEG) as two kinds of surfactants, were used as photocatalysts. The photocatalysts were characterized by SEM, XRD, FTIR and DRS analyses. Characterization of the photocatalysts indicated the positive role of the surfactants in increasing the surface area of the photocatalysts. The experimental results demonstrated that PEG had more impressive effect than CTAB on the photocatalytic performance. The effects of important operational parameters, such as initial pollutant concentration, catalyst dosage and pH, on the degradation efficiency were studied. About 96.3% of the organic pollutant removal from synthetic wastewater was obtained at optimal conditions under visible irradiation. Equilibrium data were well fitted to the Langmuir isotherm equation.     

Photocatalytic degradation of ammonia by light expanded clay aggregate (LECA)-coating of TiO2 nanoparticles

Photocatalytic degradation of ammonia on supported TiO₂ nanoparticles was investigated. The TiO₂ nanoparticles used as photocatalyst were coated on light expanded clay aggregate granules (LECA), which is a porous and light weight support. Photocatalytic reaction activity of prepared catalyst was determined by ammonia degradation from water synthetically polluted with ammonia. Experiment results showed significantly high photocatalytic activity for the immobilized catalysts. The ammonia was removed more than 85% within 300 min of the process with optimum calcinations temperature 550 °C and pH 11. Kinetics of the photocatalytic reaction followed a pseudo-first order model. XRF, XRD and SEM analyses revealed a rather uniform coating of TiO₂ on the support. By using floated TiO₂/LECA as a photocatalyst in aqueous solution of NH ₃ ^? , the ammonia was photodegraded into N₂ and H₂ gases, while NO ₂ ^? and NO ₃ ^? were formed at very low concentrations.     

Computational fluid dynamics modeling of hydrogen production in an autothermal reactor: Effect of different thermal conditions

A numerical model was developed and validated to simulate and improve the reforming efficiency of methane to syngas (CO+H₂) in an autothermal reactor. This work was undertaken in a 0.8 cm diameter and 30 cm length quartz tubular reactor. The exhaust gas from combustion at the bottom of reactor was passed over a Ru/γ-Al₂O₃ catalyst bed. The Eddy Dissipation Concept (EDC) model for turbulence-chemistry interaction in combination with a modified standard k-? model for turbulence and a reaction mechanism with 23 species and 39 elementary reactions were considered in the combustion model. The pre-exponential factors and activation energy values for the catalyst (Ru) were obtained by using the experimental results. The percentage of difference between the predicted and measured mole fractions of the major species in the exhaust gas from combustion and catalyst bed zones was less than 5.02% and 7.73%, respectively. In addition, the results showed that the reforming efficiency, based on hydrogen yield, was increased with increase in catalyst bed’s thermal conductivity. Moreover, an enhancement of 4.34% in the reforming efficiency was obtained with increase in the catalyst bed wall heat flux from 0.5 to 2.0 kW/m².