Effect of liquid wetting on catalyst effectiveness in trickle-bed reactors

The liquid in trickle-bed reactors falls in rivulets down over the catalyst particles. Heretofore, studies of the effect of fractional liquid coverage of the outer surface of the particle on reaction rate have ignored the distribution of the rivulets. A model has been developed in this paper for predicting the effect of number and size of the liquid rivulets. The system of equations has been solved numerically for a first order reaction for two types of reactions: a), one where the rate is determined by the reactant concentration in the liquid, and b), a reaction where the rate is controlled by reactant concentration in the gas phase, as in some hydrogenations. The results show that intraparticle concentration profiles are strongly affected by the distribution and size of the rivulets. However, for both types of reactions the overall rate, as measured by an overall effectiveness factor, is not significantly affected. This result is important since it suggests that distribution and size of the liquid rivulets may be safely ignored in evaluating the effect of liquid coverage, or wetting efficiency, on the rate of reaction     

Effect of char gasification reaction order on bounding solutions for char combustion

The rate of oxidation and surface temperature of a char particle during combustion is determined by the combined action of reactions of carbon with oxygen and CO₂ at the surface and carbon monoxide with O₂ in the gas phase. Limiting values for the rates and temperature were determined by Amundson et al. assuming that the CO₂ and CCO₂ reactions were first order with respect to the gas reactant concentration. Results are presented here for the cases in which the CO₂ reaction is zeroth order and the CCO₂ reaction is both first and zero order to bound the reaction orders found in practice. Allowance for zeroth order kinetics is found to have a profound effect on the shape of the bounding solutions for burning rates and temperatures, most notably by showing that diffusion-limited combustion can occur at much lower temperatur than would be predicted using first order kinetics.     

Effects of reaction order and convection around gas-bubbles in a gas-liquid reacting system

The mass transfer of gaseous reactant into liquid for chemical reaction is significantly affected by relative flow at the interface of gas and liquid. Two extreme cases are for a bubble behaving like a solid particle due to absorbed surfactant impurities and for a freely-internally circulating bubble with a relative interfacial velocity; the present calculations indicate a ratio of mass-transfer rates of a factor up to 1·4–2·45. The factor decreases with increasing reaction rate, becoming negligible for values of K > 2000 sec^?1. It is larger for a $\text{3}\text{2}$ order reaction than for a 1st order reaction.If there is internal circulation, the relative flow changes depending on whether the bubble is alone or in a rising bubble swarm. For small reaction rates the effect of this change in the mass transfer rate has been calculated to be 7–9% at typical bubble sizes of 0·1–0·2 cm radius. The mass transfer rate for a freely-circulating bubble is about 15% larger for a $\text{1}\text{2}$ order reaction than for a 1st order reaction at steady state. Transient and time averaged values of the Sherwood number were obtained. Shrinkage of bubbles from loss of reactant was also considered.     

Phenomenological approach to interpret the effect of liquid flow modulation in trickle bed reactors at the particle scale

This work analyzes the influence of liquid flow modulation on the behavior of a reaction occurring in a spherical porous particle within a trickle bed reactor. A single first-order reaction between a gaseous reactant and a non-volatile liquid reactant is considered. Non-steady-state mass balances for gas and liquid reactants are formulated and solved under isothermal conditions in order to focus the analysis on the mass transport effects. Dynamic reactant profiles inside the catalytic particle are obtained for different cycling and system conditions. The enhancement factor (ε) due to periodic operation is defined to evaluate the impact of induced liquid flow modulation on reaction rate. Influence of cycling and system parameters on the enhancement factor is also reported for a wide range of conditions. Experimental trends observed by several authors can be explained with this approach.     

Cyclohexene Reactivity over Palladium Acetate Supported in Liquid Phase

The competitive reactions of cyclohexene: disproportionation and dehydrogenation were studied over a supported-liquid phase catalyst (SLPC) of palladium acetate solved in a stationary phase employing two different techniques: pulsed and flow micro-reactor, where the products of reaction are analysed by gas chromatography. The effect of the liquid phase (comparing an aliphatic phase, squalane, with a polar phase, Triton), the reaction environment (comparing He, N₂ and air) and the process conditions (temperature and reactant concentration) have been studied. Although continuous deactivation was observed in all the cases, the catalyst supported on the apolar phase presents better performance, whereas the disproportionation reaction only takes place at no oxidant conditions and the operation conditions only affect to the selectivity of the reaction. A kinetic model for the reaction, taking into account the catalyst deactivation has been proposed, and kinetic and deactivation constant were calculated by fitting the model to the experimental data. Arrhenius parameters were obtained from measured data at several temperatures.     

Experimental investigation of porous media combustion in a planar micro-combustor

The micro-combustor (emitter) is a key component of the micro-thermophotovoltaic (TPV) system. In order to improve the system efficiency, higher wall temperature and uniform distribution along the combustor wall is desirable. Porous media combustion of premixed H₂-air in a planar micro-combustor with the channel width of 1 mm is experimentally studied. The wall temperature is measured by using an infrared thermometer under the flow conditions of Ф = 0.6-1.0 and U₀ = 2-3 m/s. The effects of flow conditions and position of the porous media on both the wall temperature distribution and the emitter efficiency are investigated. The experimental results indicate that the wall temperature increases with increasing mixture velocity, and higher emitter efficiency is achieved for mixtures with Ф ≈ 0.8. In addition, the flames are found to be effectively anchored by the inserted porous media, despite the change of flow conditions. The emitter efficiency was noted to be significantly influenced by the position of the porous media, for which the mixture preheating (by the combustor wall) is believed to be a main reason.     

Bubble size effect on low liquid input drift-flux parameters

We investigated the effect of bubble size on the drift-flux parameters at low liquid flow conditions by measuring the radial profiles of void fraction and phase velocities in a vertical bubbly pipe flow of diameter and height . To study the effect of the bubble size we used two different types of bubble inlets. We measured the local bubble fraction and velocity U_g by using single and four-point-optical fibre probes, and we used Laser Doppler Anemometry to determine the liquid velocity U_l. The distribution parameter C₀ and the weighted mean drift velocity |U_drift| were directly computed from the local measurements at a height on our experimental set-up. Both parameters were influenced by the bubble size. Provided no liquid flow reversal occurred at the near wall region, the distribution parameter reached a below unity minimum plateau value of C₀=0.95 for wall peaking void fraction profiles. At low liquid input conditions both the liquid input and bubble size had an influence on the distribution parameter. Extreme values such as C₀>2 were measured. From these measurements we developed models for the drift-flux parameters to take into account the effect of bubble size and input-flow conditions for our intermediate pipe diameter value. These models were tested and validated with separately collected experimental data.     

Dissociation and heat transfer in laminar flow

An analytic study of heat transfer and temperature and composition distribution was conducted for a reacting fluid in laminar flow in a tubular reactor. The reaction chosen was reversible and homogeneous of the form A ? 2U. Numerical solutions were obtained by machine computation for conditions of chemical equilibrium and of finite kinetics. For endothermic, dissociative reactions the radial energy transfer due to diffusion adds to the contribution due to a temporary difference. Nusselt numbers up to 10 fold greater than those for the corresponding inert system are obtained. The diffusion contribution is a maximum when the radial mass transfer is not subject to a reaction resistance; i.e. at equilibrium.     

Heat and mass fluxes in presence of superficial reaction in a not completely developed laminar flow

A mathematical model has been developed in order to study the effect of superficial reaction, the catalytic propane combustion, and its coupling with the fluid flow on mass and energy fluxes between gas and solid phases in a monolithic reactor. Simulations have been performed at different values of the gas inlet temperature and propane mass fraction. The effect of heat produced at the wall by the chemical reaction on fluid flow is studied separately from the entrance effect. A new interpretation of the real driving force is proposed, which is able to correctly predict, in a very simple manner, the values of fluxes independently of the presence of Graetz entrance effects. Also, the dependency of the Nu and Sh numbers with the gas inlet temperature and propane mass fraction is correctly included. This result leads to a different definition of the Nu and Sh numbers and, for the cases investigated numerical correlations are also given.     

Catalytic hydrotreatment of water contaminated by chlorinated aromatics

The catalytic hydrotreatment at low temperature of water contaminated by chlorobenzene and o-chlorobiphenyl has been studied experimentally using a Pd/C catalyst. Reaction runs have been carried out in a stirred reactor at constant temperature (T=30 °C) and pressure (P1 bar). Liquid phase concentration of chlororganic reactant and hydrogenated products, chloride ions concentration and pH have been measured during reaction time. Experimental results have been modelled assuming gas–liquid and liquid–solid equilibrium and the kinetic constants of the HDCl surface reactions have been evaluated.     

Experimental study on the reaction rate of a second‐order chemical reaction in a planar liquid jet

Instantaneous concentrations of reactive species are simultaneously measured in a planar liquid jet with a second‐order chemical reaction to investigate the statistical properties of the chemical reaction rate and the validity of models which have been proposed for concentration correlation. The jet flow contains the reactant A, and the ambient flow contains the reactant B. The results show that the concentration correlation of the reactants makes a negative contribution to the mean reaction rate, and this contribution is important in the downstream direction. The concentration correlation changes owing to the chemical reaction. The effects of the chemical reaction on the concentration correlation change with the flow location and the Damköhler number. The concentration correlation predicted by the Toor's model and the three‐environment model are compared with the experimental results. The results show that these models fail to accurately estimate the concentration correlation. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3969–3988, 2014     

Gas absorption with an unusual chemical reaction: the chlorination of toluene

The rate of absorption of chlorine in a laminar jet of toluene, and the corresponding bulk liquid temperature increases, have been measured as a function of contact time. Since the reaction rate is very highly dependent on the concentration of dissolved chlorine, bulk mixing of the jet on entering the receiver effectively ‘freezes’ the reaction and enables experimental determination of the ratio

This ratio is independent of the mass diffusion coefficient for 1st order kinetics, and numerical studies show this is also true for nth order reactions. The ratio therefore should be particularly valuable for abstracting kinetic information from absorption rate data in systems where interfacial turbulence may occur. A numerical analysis, which takes into account the increasing interface temperature, indicates that the overall concentration dependency of the chlorine reaction rate can be fitted by an empirical fifth order equation.     

Flow regimes for adiabatic gas-liquid flow in microchannels

The patterns forming during adiabatic gas-liquid flows in single microchannels are reviewed and the parameters influencing pattern transitions are discussed. Six major patterns were identified: the surface tension dominated bubbly and Taylor flows, the transitional churn and Taylor-annular flows, and the inertia dominated dispersed and annular flows. From the various parameters that have been studied in the literature, channel size, phase superficial velocities, liquid phase surface tension, wall wettability and inlet conditions were found to affect the flow pattern formed while channel cross sectional geometry affected the patterns but to a lesser degree. Liquid viscosity and flow orientation with respect to gravity also seemed to play some role but the results were not conclusive. A universal flow regime map does not seem to exist and this is attributed to a lack of consistency in the inlets used in the various studies as well as to the effects of wall properties, such as wettability, contamination and roughness which are not usually varied systematically or reported. From the different flow regime maps suggested, those using U_GS-U_LS as coordinates represented better the transitions between patterns.     

Pyrolytic characteristics of Jatropha seedshell cake in thermobalance and fluidized bed reactors

Pyrolytic kinetic parameters of Jatropha seedshell cake (JSC) were determined based on reaction mechanism approach under isothermal condition in a thermobalance reactor. Avrami-Erofeev reaction model represents the pyrolysis conversion of JSC waste well with activation energy of 36.4 kJ mol^?1 and frequency factor of 9.18 s^?1. The effects of reaction temperature, gas flow rate and feedstock particle size on the products distribution have been determined in a bubbling fluidized bed reactor. Pyrolytic bio-oil yield increases up to 42 wt% at 500 °C with the mean particle size of 1.7 mm and gas flow rate higher than 3U _mf , where the maximum heating value of bio-oil was obtained. The pyrolytic bio-oil is characterized by more oxygen, lower HHVs, less sulfur and more nitrogen than petroleum fuel oils. The pyrolytic oil showed plateaus around 360 °C in distribution of components’ boiling point due to high yields of fatty acid and glycerides.     

Hydrodenitrogenation of pyridine over a Co-Mo-AI2O3 catalyst

The vapour phase hydrogenation of pyridine was investigated in an integral flow reactor at a pressure of 5.62 MPa over an alumina-supported cobalt-molybdate catalyst. The effect of various process variables, viz. temperature, reactant ratio and space time, on the conversion of pyridine was studied. The kinetics of the hydrogenation of pyridine to form piperidine have been investigated. The reaction showed pseudo-second order behaviour and the order of the reaction rate in the initial pyridine concentration was found to vary with temperature.     

Characteristics of liquid slugs in gas–liquid Taylor flow in microchannels

The hydrodynamics of liquid slugs in gas–liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (L_s/w<1.5) by evaluating the recirculation rate, velocity and time. The recirculation velocity was found to increase linearly with the two-phase superficial velocity U_TP. The product of the liquid slug residence time and the recirculation rate is independent of U_TP under the studied flow conditions. These results suggest that the amount of heat or mass transferred between a given liquid slug and its surroundings is independent of the total flow rate and determined principally by the characteristics of the liquid slug.     

Flow regime transition identification in three phase co‐current bubble columns

Bubble columns have wide applications in absorption, bio‐reactions, catalytic slurry reactions, coal liquefaction; and are simple to operate, have less operating costs; provide good heat and mass transfer. Experiments have been performed for identifying transition regime in a 15 cm diameter bubble column with liquid phase as water and air as the gas phase. Glass beads of mean diameter 35 µm have been used as solid phase. The superficial gas velocity is in the range 0 ≤ U_g ≤ 16.3 cm/s and superficial liquid velocity in the range of 0 ≤ U_l ≤ 12.26 cm/s. Solid loading up to 9% (w/v) has been used. Pressure signals have been measured using differential pressure transducers (DPTs) at four different axial locations. Classical analysis (Wallis approach and Zuber–Findlay approach), Statistical analysis and Fractal analysis have been used for regime transition identification. Statistical analysis and Fractal analysis have shown almost the same transition points for all the liquid and gas velocities. Effect of solid concentration, liquid velocity and gas velocity over transition regime has also been studied. As the solid concentration is increased it has insignificant effect over transition regime for lower values (<1%), while transition values decrease for higher solid concentration (>1%). © 2012 Canadian Society for Chemical Engineering     

A STUDY ON THE PERFORMANCE OF THE CATALYTIC POROUS-WALL THREE-PHASE REACTOR

A theoretical investigation of a catalytic porous-wall reactor in which gaseous and liquid reactants approach each other from opposite sides of the catalyst is undertaken. Equations for the annular liquid-channel are coupled with those for the catalytic wall and solved numerically and analytically using a simplified model. For the model reaction under study, the main design and operation parameters which affect reactor performance are the Thiele modulus, Peclet number, width of the liquid channel and the inlet concentration of the reactant in the liquid phase.

The effect of reactor configuration is peculiar to the cylindrical geometry because the thickness and relative location of the catalytic wall as well as the selection of the liquid and gas channels can influence the reactor performance. Thin-walled catalyst tubes have larger effectiveness factors and as the tube radius approaches that of the reactor, conversion in the reactor increases especially when the liquid is saturated with the gaseous reactant. Concentration of the liquid reactant in the feed has a significant effect if the reactant is depleted at some point inside the catalyst wall. Since the reaction zone width can be adjusted by changing the feed composition, this might have important implications with respect to selectivity.     

Theoretical study of the application of porous membrane reactor to oxidative dehydrogenation of n-butane

This paper is the theoretical study of the oxidative dehydrogenation of n-butane in porous membrane reactors. Performance of the membrane reactors was compared with that of conventional fixed-bed reactors. The porous membrane was employed to add oxygen to the reaction side in a controlled manner so that the reaction could take place evenly.Mathematical models for the fixed-bed reactor and the membrane reactor were developed considering non-isothermal condition and both radial heat and mass dispersion. From this study, it was found that the hot spot problem was pronounced particularly near the entrance of the conventional fixed-bed reactor. In addition, the assumption of plug flow condition did not adequately represent the reaction system. The effect of radial dispersion must be taken into account in the modelling.The use of the porous membrane to control the distribution of oxygen feed to the reaction side could significantly reduce the hot spot temperature. The results also showed that there were optimum feed ratios of air/n-butane for both the fixed-bed reactors and the membrane reactors. The membrane reactor outperformed the fixed-bed reactor at high values of the ratio. In addition, there was an optimum membrane reactor size. When the reactor size was smaller than the optimum value, the increased reactor size increased the reaction and heat generation and, consequently, the conversion and the selectivity to C₄ increased. However, when the reactor size was larger than the optimum value, oxygen could not reach the reactant near the stainless steel wall. It was consumed to react with the product, C₄. As a result, the yield dropped. Finally, it was found that the increase of wall temperature increased the yield and that the feed air temperature could help control the temperature profile of the reaction bed along the reactor length.