Toluene methylation to para-xylene

To investigate the effect of operational parameter and transport phenomena on para-xylene production from toluene methylation with methanol, a fixed bed tubular reactor packed with Al-HMS-5 mesoporous catalyst was numerically studied. A mechanistic Longmuir–Hinshelwood-type kinetic study has been implemented on a proposed reaction network based on former experimental observation and theoretical background. Kinetic parameters and activation energy related to proposed reaction network for toluene methylation were evaluated using nonlinear regression and Arrhenius plot, respectively. In addition, heat transfer, fluid flow, and chemical reaction equations consisting of toluene methylation and xylene isomerization were solved using finite element method. In order to optimize toluene methylation process, reaction temperature and residence time were investigated. The results showed that uniform distribution of temperature exists at the reactor. There is only deviation from uniform temperature at the reactor entrance, but in other places, the temperature distribution is uniform. As a result, fluid temperature quickly becomes the same as the wall temperature, making the toluene methylation reaction highly efficient. Finally, the residence time of 60 s and wall temperature of 425 K were recommended as optimum working values.

     

Computational Fluid Dynamic Design of Jet Stirred Reactors for Measuring Intrinsic Kinetics of Gas‐Phase and Gas‐Solid Reactions

Nonreactive and reactive computational fluid dynamic simulations were applied to optimize the design of a laboratory scale jet stirred reactor for measuring intrinsic kinetics of gas‐phase and gas‐solid reactions, i.e. kinetics determined by chemical steps only and not by heat or mass transfer. In the past these reactors were designed and tested based on empirical design criteria and residence time distribution experiments. This work shows that these do not always capture important local effects that are vital for kinetic studies. First the degree of macro–mixing was evaluated for three different geometries (down case, 45° case and 90° case) by performing in silico residence time distribution experiments at 900 K, showing that with these type of experiments only minor differences are observed. However, the ethane steam cracking simulations revealed major differences, with the 45° case being the most uniform in terms of temperature and the 90° case being by far the worst. The species nonuniformity in all geometries was acceptable and was in some cases even partly masked by important shortcut streams such as those observed in the 90° case. The existing gradients on the substrate surface are sufficiently small to be neglected in modeling efforts. As temperature is the major parameter determining the rate of the surface reactions, the 45° case is suggested as the best geometry for measuring intrinsic kinetics.     

Thermal performance improvement in water nanofluid/GNP–SDBS in novel design of double-layer microchannel heat sink with sinusoidal cavities and rectangular ribs

In this numerical study, laminar flow of water nanofluid/GNP–SDBS (graphene nanoplatelet–sodium dodecylbenzene sulfonate) for 0–0.1% solid nanoparticles mass fraction was investigated for Reynolds numbers of 50–1000 in 3D space via finite volume method. In the newly proposed microchannel design, the cooling fluid is moving in countercurrent in the upper and lower layers of the microchannels, and there are cavities and sinusoidal routes on the solid walls of the microchannel, and the presence of rectangular ribs on the flow centerline along the fluid path enhances mixing for cooling fluid and creates better heat transfer for warm surfaces. The results of this study show that this special design of the microchannel can have a substantial increase in Nusselt number and heat transfer so that in the considered geometry by adding solid nanoparticles mass fraction it is possible to increase average Nusselt number for each Reynolds number by approximately 20%. Also, the mixing of the fluid because of formation of secondary flows has a strong effect on making the temperature distribution uniform in the cooling fluid and solid bed (wall) of the microchannel, especially in the lower layer. The upper layer of the microchannel always has a lower temperature due to indirect contact with heat flux compared with the lower layer. In this study, by increasing Reynolds number and mass fraction of solid nanoparticles the Nusselt number is increased and heat resistance of the lower wall of the microchannel is reduced. Based on the investigation of flow field and heat transfer, the use of the proposed design of the microchannel is recommended for Reynolds number less than 300.

     

A new approach for isothermal calorimetric technique

Using flexible heat flux sensors mounted on the lateral and bottom of outside reactor wall, a new approach is developed for isothermal calorimetric technique to overcome the disadvantages of heat flow calorimetric methods. Although the proposed system needs a calibration procedure before or after the reaction completion to evaluate the lateral heat transfer area, the measurement is versatile and totally independent of the reaction media, jacket fluid, and the variations of heat transfer coefficient. Knowledge of the variations of the heat transfer coefficient is essential for the effective control and scale up of a reactor and can be inferred by the new method during the reaction. The stirrer power and the heat loss can be determined easily as well. No pre-calibration is needed for the sensors and no heating element is applied inside the reactor for temperature control. Experiments are carried out to validate the performance of the new proposed technique. With the help of a heater, the heat generated in the reactor is measured at various levels of power input. The predicted heater power inputs are in good agreement with the corresponding heat inputs. The relative detection limit in the range of 0.8–1 W L⁻¹ is expected for this technique. Using the hydrolysis of acetic anhydride, the heat of reaction at 25°C is determined, which is within the range reported in the literatures. The capability of the system to deal with the variations in the overall heat transfer coefficient is also demonstrated using a simulated reaction.     

Radiotracer investigation of the pulp flow characteristics in the superphosphate production chamber

By means of radiotracer⁵¹Cr impregnated in the solid phase the residence time distribution of the pulp in the mixing chamber was obtained. The mathematical model constructed on the basis of the experimental distribution showed an unsatisfactory functioning of the reactor. Ways were proposed to correct this shortcoming, both in the existing chamber and in that which is being designed for a new plant.     

Radiotracer investigation in a rotary fluidized bioreactor

A rotary fluidized bioreactor (RFBR) designed for treatment of wastewater was required to be investigated for its hydrodynamic behaviour and validation of design. A radiotracer investigation was carried out to measure residence time distribution (RTD) of wastewater in the RFBR using ⁸²Br as a radiotracer. The radiotracer was instantaneously injected into the inlet feed line and monitored at the inlet and outlet of the reactor using collimated scintillation detectors connected to a data acquisition system. The measured RTD data was treated and simulated using a tanks-in-series model and model parameters i.e. number of tanks describing the degree of mixing was obtained. The results of the investigation showed no flow abnormalities and the reactor behaved as an ideal continuously stirred-tank reactor at all the operating conditions. Based on the results, the design of the reactor was validated.     

Assessing the flow characteristics of nanofluids during turbulent natural convection

High-performance cooling is of vital importance for the cutting-edge technology of today, from nanoelectronic mechanical systems to nuclear reactors. Advances in nanotechnology have allowed the development of a new category of coolants, termed nanofluids that have the potential to enhance the thermal performance of conventional heat transfer fluids. At the present time, nanofluids are a controversial research theme, since there is yet no conclusive answer to explain the underlying physical mechanisms of heat transfer. The current study investigates experimentally the heat and mass transfer behaviour of dilute Al₂O₃–H₂O nanofluids under turbulent natural convection—Rayleigh number of the order of 10⁹—in a cubic Rayleigh–Bénard cell with optical access. Traditional heat transfer measurements were combined with a velocimetry method to obtain a deeper understanding of the impact of nanoparticles on the heat transfer performance of the base fluid. Particle image velocimetry was employed to quantify the resulting mean velocity field and flow structures in dilute nanofluids under natural convection, at three parallel planes inside the cubic cell. All the results were compared with that for the base fluid, i.e. deionised water. It was observed that the presence of a minute amount of Al₂O₃ nanoparticles in deionised water, φ_v =?0.00026 vol.%, considerably modifies the mass transfer behaviour of the fluid in the bulk region of turbulent Rayleigh–Bénard convection. Simultaneously, the general heat transport, as expressed by the Nusselt number, remained unaffected within the experimental uncertainty.

     

Residence time distribution study in a pilot-scale gas–solid fluidized bed reactor using radiotracer technique

This paper describes a radiotracer study carried out to measure residence time distribution (RTD) of coal particles in a pilot-scale gas–solid fluidized bed reactor (FBR). Gold-198 labelled on coal particles was used as radiotracer. RTD measurements were conducted for selected operating conditions and mean residence times (MRTs) of the coal particles were determined. Gamma function model was used to simulate the measured RTD data and mixing of coal particles in the reactor was investigated. Based on the results, the performance of the air distributor used in the reactor was evaluated.     

Main chemical engineering parameters affecting radical polymerization processes in heterogeneous media

The main parameters which may affect both reactor performance and polymerization processes in heterogeneous media are described. Viscosity and its influence on heat and mass transfer, residence time distribution in tubular reactor, as well as, influence of agitation on coagulum formation and of monomer feed rate on polymerization rate in semi-batch processes are examined.     

Mean residence time of Markov processes for particle transport in fludized bed reactors

A new approach for studying the particle dynamics and RTD (residence time distribution) in processes is to formulate stochastic models. A common question to all models for RTD is whether Danckwerts’ law for mean residence time holds. In this paper we revisit a Markov process that has been proposed by Dehling et al. (1999) as a stochastic model for particle transport in fluidized bed reactors. Under the volumetric flow balance conditions, we deduce different boundary conditions at the entrance and the exit of the reactor, and in both discrete model and continuous model we show that processes satisfy Danckwerts’ law, stating that the mean residence time of particle transport in fluidized bed reactors equals V/v, where V denotes the volume of the reactor occupied by the fluid and v the volumetric inflow rate.     

Extending potential flow modelling of flat-sheet geometries as applied in membrane-based systems

The efficiency of chemical reactors can be analysed using the residence time distribution. This research focusses on flat-sheet geometries applied in membrane-based systems. The residence time distribution depends mainly on the 2D velocity field, parallel to the membrane. The velocity average over the transversal direction is calculated using potential flow theory. A combination of real and virtual sources and sinks are used to model the internal inlets and outlets. Furthermore, a novel method is presented to calculate the residence time distribution. By ignoring diffusion and dispersion, every streamline is modelled to have a fixed residence time, which can be calculated with a simple quadrature based on a coordinate transformation. The model predicts the impact of the two-dimensional geometry on the residence time distribution, but it is demonstrated that large zones of nearly stagnant flow have only a limited impact on the residence time distribution. The new model can predict the travelling time from the inlet to each interior location, providing a better tool to analyse spatially distributed chemical reactions. The models agreed highly with pressure measurements (R² = 0.94–0.98) and they agreed well with tracer experiments for the residence time (R² = 0.73–0.99).     

Material‐Efficient Microfluidic Platform for Exploratory Studies of Visible‐Light Photoredox Catalysis

We present an automated microfluidic platform for in‐flow studies of visible‐light photoredox catalysis in liquid or gas–liquid reactions at the 15 μL scale. An oscillatory flow strategy enables a flexible residence time while preserving the mixing and heat transfer advantages of flow systems. The adjustable photon flux made possible with the platform is characterized using actinometry. Case studies of oxidative hydroxylation of phenylboronic acids and dimerization of thiophenol demonstrate the capabilities and advantages of the system. Reaction conditions identified through droplet screening translate directly to continuous synthesis with minor platform modifications.     

Flow and heat transfer in non-Newtonian nanofluids over porous surfaces

In the present study, heat transfer and fluid flow of a pseudo-plastic non-Newtonian nanofluid over permeable surface has been solved in the presence of injection and suction. Similarity solution method is utilized to convert the governing partial differential equations into ordinary differential equations, which then is solved numerically using Runge–Kutta–Fehlberg fourth–fifth order (RKF45) method. The Cu, CuO, TiO₂ and Al₂O₃ nanoparticles are considered in this study along with sodium carboxymethyl cellulose (CMC)/water as base fluid. Validation has been done with former numerical results. The influence of power-law index, volume fraction of nanoparticles, nanoparticles type and permeability parameter on nanofluid flow and heat transfer was investigated. The results of the study illustrated that the flow and heat transfer of non-Newtonian nanofluid in the presence of suction and injection has different behaviors. For injection and the impermeable plate, the non-Newtonian nanofluid shows a better heat transfer performance compared to Newtonian nanofluid. However, changing the type of nanoparticles has a more intense influence on heat transfer process during suction. It was also observed that in injection, contrary to the other two cases, the usage of non-Newtonian nanofluid can decrease heat transfer in all cases.

     

Lattice Boltzmann study of multi-walled carbon nanotube (MWCNT)-Fe3O4/water hybrid nanofluids natural convection heat transfer in a Π-shaped cavity equipped by hot obstacle

In the present paper, the effect of nanofluid and the hot obstacle in a Π-shaped cavity is investigated. Lattice Boltzmann method is used to simulate the fluid flow and heat transfer. The effects of the parameters such as the nanoparticle solid volume fraction, the Rayleigh number, aspect ratio of cavity and hot obstacle position on the flow pattern and heat transfer parameters are studied. The numerical results are compared with previous results for validation, and a good agreement obtained. It is found that the average Nusselt number is increased by increasing the nanoparticle solid volume fraction, the Rayleigh number and the aspect ratio of cavity. Moreover, the effect of Rayleigh number on the average Nusselt number at high Rayleigh numbers (10⁵–10⁶) is more pronounced than that at low Rayleigh numbers (10³–10⁴) due to the different heat transfer mechanisms. The position of the hot obstacle affects the heat transfer significantly. When the hot obstacle is located on the center, the heat transfer is more effective.

     

Rotating Biological Contactor Reactor with Biofilm Promoting Mats for Treatment of Benzene and Xylene Containing Wastewater

A novel rotating biological contactor (RBC) bioreactor immobilized with microorganisms was designed to remove volatile organic compounds (VOC), such as benzene and xylene from emissions, and its performance was investigated. Gas-phase VOCs stripped by air injection were 98?% removed in the RBC when the superficial air flow rate was 375?ml/h (1,193 and 1,226?mg/l of benzene and xylene, respectively). The maximum removal rate was observed to be 1,007 and 1,872?mg/m³/day for benzene and xylene, respectively. The concentration profile of benzene and xylene along the RBC was dependent on the air flow rate and the degree of microbial adaptation. Air flow rate and residence time were found to be the most important operational parameters for the RBC reactor. By manipulating these operational parameters, the removal efficiency and capacity of the bioreactor could be enhanced. The kinetic constant K _s demonstrated a linear relationship that indicated the maximum removal of benzene and xylene in RBC reactor. The phylogenic profile shows the presence of bacterium like Pseudomonas sp., Bacillus sp., and Enterococcus sp., which belonged to the phylum Firmicutes, and Proteobacteria that were responsible for the 98?% organic removal in the RBC.     

Microcalorimetric performance of the growth in culture of Escherichia coli,Proteus mirabilis and their mixtures in different proportions

Microcalorimetry is a technique that determines the heat flow produced as a result of microbial activities. The heat variations resulting from chemical reactions, which take place during metabolism, can be used to monitor bacterial growth in a culture medium. However, there are very few studies using calorimetry to investigate the relationships between two bacteria. In this work, we studied the interaction between E. coli and P. mirabilis, two bacteria belonging to the family Enterobacteriaceae. We have prepared three samples, mixing both enterobacteria at a concentration of 10³ CFU mL^?1 but in different proportions. Experimental equipment used was a Calvet microcalorimeter, where a constant temperature of 309.65 K was maintained. Then, we compared the shape of the heat flow–time curves of single microorganisms and their mixtures. Also, we calculated the thermokinetic parameters such as growth constant (k), generation time (G), detection time (t _d) and the amount of heat released (Q). The results obtained showed that when E. coli and P. mirabilis were put together in the culture medium, the growth profile of P. mirabilis seemed to dominate, even at low proportions in the sample.     

Computational fluid dynamics simulation and redesign of a screw conveyor reactor

National Renewable Energy Laboratory (NREL) designed a shrinking-bed reactor to maintain a constant bulk packing density of cellulosic biomass. The high solid-to-liquid ratio in the pretreatment process allows a high sugar yield and avoids the need to flush large volumes of solution through the reactor. To scale up the shrinking-bed reactor, NREL investigated a pilot-scale screw conveyor reactor in which an interrupted flight between screws was employed to mimic the “shrinking-bed” effect. In the experiments with the screw conveyor reactor, overmixing and uneven flow occurred. These phenomena produce negative effects on biomass hydrolysis. The flow behavior inside the reactor was analyzed to allow redesign of the screw to achieve adequate mixing and even flow. In the present study, computational fluid dynamics (CFD) was utilized to simulate the fluid flow in the porous media, and a new screw design was proposed. CFD analysis performed on the redesigned reactor indicated that an even flow pattern was achieved.     

Application of 85Kr dating to groundwater in volcanic aquifer of Kumamoto Area, Japan

Groundwater age was determined by ⁸⁵Kr/Kr specific activity of gases dissolved in groundwater at Kumamoto Area, in which newly developed Kr extraction system and liquid scintillation counting technique for ⁸⁵Kr were applied. Apparent mean residence times observed were 8.2 ± 0.7 years and 20.7 ± 0.6 years for the groundwater taken from a well at the recharge area and that from an artesian borehole at the discharge area, respectively showing a consistent age trend estimated from the observed groundwater flow system of Kumamoto Area and also that determined using environmental tritium. Similar mean residence times of ⁸⁵Kr and tritium were observed at the discharge area but the mean residence time of ⁸⁵Kr at the recharge area was younger compared to that of tritium, probably due to mixing of river water containing recent ⁸⁵Kr high in concentration into groundwater flow coming from the northern upland??s recharge area. The mean residence time of the groundwater flow coming from northern upland area and a traveling time of the groundwater between two sampling points were estimated by a simple model assuming mixing the river water with the groundwater and a piston flow between two sampling points.     

Synthesis of thiol-capped gold nanoparticle with a flow system using organosilane as a reducing agent

Synthesis of thiol-capped gold nanoparticle is carried out by mixing of a THF solution of HAuCl₄·4H₂O and ⁿC₁₂H₂₅SH with a THF solution of triethylsilane in a flow system that consists of polytetrafluoroethylene tubing. The effect of residence time and reaction temperature on the particle size is examined.