Temperature effects on separate values of adsorption and surface reaction rates by dynamic studies

The study of a three-phase catalytic slurry reactor for SO₂ oxidation on activated carbon has been conducted with a dynamic response technique. The analysis of zero and first temporal moments of the gas for temperatures between 10 and 26°C provides individual measurements of the adsorption, desorption and surface reaction rate constants and their energies of activation. A parallel first-moment analysis was carried out for O₂ adsorption onto carbon to obtain the necessary equilibrium constants for temperatures used in the SO₂-oxidation calculations. With second moment analysis of adsorption data, we find the adsorption kinetic constants with and without reaction are in quantitative agreement. By evaluating the separate temperatu effect on the adsorption/desorption, and surface reaction rates, we conclude that neither step is controlling at temperatures up to 26°C. The values the several kinetic and equilibrium constants agree well with those found by other authors.     

Heterogeneous catalysis rate parameters in a gradientless reactor by the method of moments

The transient behaviour of an internally-recycled catalytic reactor has been studied accounting for the contributions from intraparticle diffusion, adsorption and surface reaction. Equations for the zeroth, first and second absolute moments of the break-through curves are given in terms of the rate constants for the three elementary steps (first order processes are assumed throughout). The moments can be obtained from experimental break-through curves and the rate parameters for adsorption and surface reaction then evaluated from such data. Simplified equations also are derived for the case of insignificant chemical reaction and this case is used to test the method experimentally. Values of the hydrogen adsorption equilibrium constant for γ-alumina supported Co Mo-Ni catalyst are reported.     

Dynamic analysis of a trickle bed reactor by moment technique

The performance of a trickle bed reactor is investigated by the moment technique. Residence time distributions of SO₂ tracer in both gas (Helium) and liquid (distilled water) effluents are used to predict zero reduced and first absolute moments and these values are compared with the derived theoretical expressions. Correlations are suggested for gas-liquid mass transfer coefficient, liquid hold up, and extent of axial mixing in liquid phase.True adsorption equilibrium constant of the system is estimated as 0.378 from liquid full bed experiments and contacting efficiency of the trickle bed reactor is found as 0.987.Effect of axial dispersion is not significant on gas-liquid mass transfer coefficient since absorption factor is small, but is found to be quite important on the true estimation of adsorption factor.     

Dynamics of three-phase slurry reactors

The dynamic behavior of three-phase slurry systems is presented in terms of the response characteristics of the reactor to a pulse or step input. Equations are derived for the steady-state conversion and the first moment of the response in the gas phase to pulse input of concentration. Overall irreversible and reversible first-order reactions, and adsorption-reaction processes at the catalyst surface have been considered from the viewpoint of evaluating rate coefficients from measured response data. Analysis of two-step adsorption-reaction processes is particularly interesting since dynamic data offer the possibility of separate evaluation of adsorption and surface reaction rate constants, which is not possible from steady-state measurements.     

Modeling of Kinetic Expressions for the Reduction of NOx by Hydrogen in Oxygen‐Rich Exhausts Using a Gradient‐Free Loop Reactor

The reduction of NO_x by hydrogen under lean conditions is investigated in a gradient‐free loop reactor. Using this computer‐controlled reactor, the reaction rates can be measured under exact isothermal conditions. Systematic variation of the input concentrations of hydrogen, nitric oxide, oxygen as well as reaction temperature provides a complete data set of reaction rates for the given reaction system. A number of kinetic rate expressions were evaluated for their ability to fit the experimental data by using toolboxes of MATLAB. The temperature influence on reaction rate constants and adsorption equilibrium constants were correlated simultaneously using Arrhenius and van’t Hoff equations, respectively. The kinetic rate expression based on a Langmuir‐Hinshelwood‐type model describes the data and the model can be improved by introducing a correction term in square root of hydrogen partial pressure over the range of conditions investigated.     

TIME-DOMAIN DYNAMICS OF A THREE-PHASE SLURRY REACTOR

In this article the dynamics of a slurry reactor for the carbon-catalysed oxidation of S0₂ at 293 K, is studied using time-domain analysis of 0_.2 concentrations in the exit gas after step inputs. The slurry is modelled by a linear system of ordinary differential equations and the dead volume associated with the detector is described empirically with a two-parameter, second-order system. Using prior reliable values for the non-reaction parameters, it is possible to simultaneously determine the rate constants for surface reaction and adsorption onto the carbon as well as the equilibrium constant for adsorption of 0₂, hence under reacting conditions. This compares very well with the equilibrium constant determined from adsorption of pure O₂     

Experiments and modeling on the deacidification of agglomerates of nanoparticles in a fluidized bed

The deacidification of the fumed silica AEROSIL® 200 was studied experimentally in a batch fluidized bed in the temperature range from 250 °C to 400 °C. For a well fluidized bed, the temperature and the steam concentration in the fluidizing gas are the determining parameters for the overall rate of deacidification. If the bed is not well fluidized, e.g. because it is too shallow, or it is fluidized near the point of minimum fluidization velocity, the rate of deacidification drops because channeling and bypassing occur. The adsorption equilibrium of steam and HCl on AEROSIL® 200 was measured for a wide temperature range and the temperature dependency of the Henry coefficient for steam is given. A mathematical reactor model was developed for the adsorption and for the surface reaction on highly agglomerated nanoparticles in a fluidized bed. In applying this model to the experimental data for the deacidification, a simple kinetic rate expression could be derived for the deacidification reaction, which is otherwise not obtainable. The temperature dependency of the rate constant was also determined. All other parameters for the model can either be found through independent measurements (e.g. adsorption equilibrium or fluidizing characteristics) or in literature. The model can be used for sizing and optimizing of fluidized bed reactors in the production of fumed oxides.     

Nickel-catalyzed hydrogenation of soybean oil: I. Kinetic,equilibrium and mass transfer determinations

The kinetic and equilibrium constants were determined for the hydrogenation of soybean oil on a commercial nickel catalyst in a 300-ml Parr batch reactor. These constants were used to calculate the hydrogen gas absorption coefficients by coupling mass transfer with reaction rate based on a Langmuir Hinshelwood model. The activation energy for the rate-determining step was 23 kcal/g mol whereas the adsorption energy for hydrogen was −12.5 kcal/g mol. The gas absorption coefficients varied between 0.3 to 0.7 min⁻¹ as the temperature ranged between 140–180 C.     

Dynamic and pseudodynamic modelling of transient and periodic CSTR operation

Using the simple catalytic reaction A+B°C, catalytic surface dynamics can be modelled either by assuming finite rates of adsorption and desorption (termed the dynamic model) or by assuming adsorption equilibrium between the gas and the surface (the pseudodynamic model). In general, the responses of the two models are quite different, the pseudodynamic model being unable to predict rate overshoot for reactor start-up. Both models appear able to describe rate enhancement and simple (i.e. single frequency) resonance for forced feed composition cycling. Reasons for preferring the dynamic model over its pseudodynamic counterpart are discussed.     

Transient response of three-phase slurry reactors

Analytical time domain solutions are provided for the transient response of three-phase slurry reactors, with diffusion and first-order irreversible ch input of the reactant in the inlet gas stream; and (b) when the catalyst particles are suddenly introduced into the reactor which has otherwise reached obtained for the general case of continuous slurry reactors but can be easily reduced for application to semi-batch slurry reactors in which there is n and are expected to be useful for evaluating the rate and equilibrium constants from dynamic response experiments.Asymptotic solutions have also been obtained for the case of large Thiele parameter (φ ≥ 3) and some other simpler expressions have been derived as only for gaseous reactants of moderate or high solubility in the liquid phase and are not applicable to those with low solubility.     

Effect of hydrogenation catalyst activity on adsorption and surface reaction rates

Dynamic studies of the hydrogenation of α-methyl styrene in a three-phase slurry reactor showed that catalyst reduction temperature appreciably affects individual values of adsorption equilibrium constants, adsorption rate coefficients and surface reaction rate coefficients. Measurements were made in a well-stirred slurry reactor with Pd/Al₂O₃ catalyst particles at 22, 38 and 50°C. Increasing the temperature for reducing, in hydrogen, PdCl₂ to Pd from 250 to 420 to 510°C decreased the palladium dispersion, and caused a reduction in equilibrium adsorption, adsorption rate and surface reaction rate coefficients. The heat of adsorption and the activation energy for surface reaction, 3.8 and 10 kcal/mole, respectively, were constant with reduction temperature. The relative magnitudes of metal dispersion at the three catalyst reduction temperatures were estimated from equilibrium adsorption coefficients, and found to agree with an experimental study by Boitiaux et al. (1983a).     

Rate parameters from dynamic experiments with single catalyst pellets

Equations have been derived relating the rate and equilibrium parameters for adsorption and reaction on a single catalyst pellet to measurable properties of pulse-response experiments. In particular, it is shown that intraparticle diffusivities, adsorption equilibrium constants, and adsorption rate constants can be evaluated from the moments of the response peak from one end face of a catalyst pellet when a pulse of diffusing component is passed over the other end face.The method is illustrated with experimental data for helium (in nitrogen) and cyclopropane (in helium) pulses diffusing through alumina pellets at 35–90°C and atmospheric pressure.The results suggest that this dynamic procedure offers a rapid, simple method for establishing the rate and equilibrium parameters for adsorbing systems, provided all processes are linear. An advantage is that the intraparticle diffusivity can be obtained from the first moment of the response peaks, without recourse to the less accurate, second moment. Extension of the method to first-order reaction systems is discussed. A disadvantage is that the results may not be of high accuracy when many parameters are to be evaluated from one set of data; i.e. from one set of experimental conditions.     

Sorption enhanced reaction for high purity products in reversible reactions

Reversible reactions (A + B = C + D) can be performed to near completion using an admixture of catalyst and sorbent that will selectively adsorb one of the reaction products. For an initially clean sorbent and a favorable adsorption isotherm and a long reactor, the adsorbed product, C, will propagate as a sharp, shock‐like front. While the adsorbed product will not move faster than this front, the second, nonadsorbed product, D, will, in principle, leave the reactor, uncontaminated. However, a parametric analysis of the two examples presented in this work, the water gas shift and the cracking of hydrogen sulfide, reveals an unexpectedly complex behavior. While assuming adsorption equilibrium the effect of the equilibrium constant, the reaction kinetics and adsorption isotherm on the reactant and product concentration profiles are simulated. It is found that desired behavior is favored by large equilibrium constants, rapid kinetics, and strong nonlinear adsorption. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5452–5461, 2017     

DYNAMIC ADSORPTION OF S02 ON ZEOLITE MOLECULAR SIEVES†

Sulfur dioxide is one of the major pollutants resulting from fuel combustion. Numerous dry, semi-dry and wet processes have been developed for pollution control of sulfur dioxide. Solid carbonates, natural and synthetic zeolites, ion exchange resins and carbon based sorbents are the most commonly used dry sorbents for sulfur dioxide removal

In this study, measurements of the adsorption properties of sulfur dioxide on zeolites were investigated. The adsorbents used in this work are 5A, 4A and AW300 type molecular sieve zeolites. Adsorption equilibrium parameters were determined from the pulse chromatographic response to injections of low concentrations of sulfur dioxide. The method of moments were used to evaluate the adsorption equilibrium parameters from pulse chromatographic experiments. Data, such as adsorption equilibrium constants or reaction rate parameters are essential in the design of adsorption systems or reactors in which sulfur dioxide is removed

The experiments were conducted in a temperature range of 523-718 K. The relatively strong adsorption properties of sulfur dioxide on zeolites necessitated the use of high carrier gas flow rates and subsequently non-isobaric operation. Non-isobaric pulse chromatography theory was found to describe accurately the adsorption trends." printpubdate="Adsorption equilibrium constants of S0₂ were found to decrease considerably with increasing temperature. It was also found out that adsorption of SO₂ on the adsorbents investigated were found to decrease in the order of AW300 ≤ 4A ≤ 5A. The adsorption equilibrium parameter of S0₂ on 5A was found as 11.78 at 673 K, whereas it has a value of 157.11 at 523 K. The adsorption equilibrium parameter of S0₂ on 4A zeolite was determined to be 8.63 at 718 K and 213.78 at 523 K.     

MODELING THE FISCHER-TROPSCH REACTIONIN A SLURRY BUBBLE COLUMN REACTOR

Reactant (CO and H 2 ) concentration and conversion profiles were determined as a function of axial distance for the Fischer-Tropsch reaction in a slurry bubble column reactor. Model equations were developed from the basic concepts, i.e., conservation of mass and momentum, and combined with iron catalyst reaction kinetics as well as mass transfer coefficients, gas, liquid, and solid phase holdup, Henry's Law constants, minimum fluidization velocity, and terminal velocity obtained from empirical correlations. Concentration profiles and conversion were determined for varying key process variables: liquid-phase velocity and rate constant. Results suggest that the reaction is kinetically limited and that conversion is proportional to liquid velocity. Thus, process improvements can be achieved by either maximizing liquid-phase velocity or increasing the rate constant by modifying the catalyst.     

Modeling the fischer-tropsch reactionin a slurry bubble column reactor

Reactant (CO and H 2 ) concentration and conversion profiles were determined as a function of axial distance for the Fischer-Tropsch reaction in a slurry bubble column reactor. Model equations were developed from the basic concepts, i.e., conservation of mass and momentum, and combined with iron catalyst reaction kinetics as well as mass transfer coefficients, gas, liquid, and solid phase holdup, Henry's Law constants, minimum fluidization velocity, and terminal velocity obtained from empirical correlations. Concentration profiles and conversion were determined for varying key process variables: liquid-phase velocity and rate constant. Results suggest that the reaction is kinetically limited and that conversion is proportional to liquid velocity. Thus, process improvements can be achieved by either maximizing liquid-phase velocity or increasing the rate constant by modifying the catalyst.     

Co‐current and Countercurrent Configurations for a Membrane Dual Type Methanol Reactor

A dynamic model for a membrane dual‐type methanol reactor was developed in the presence of catalyst deactivation. This reactor is a shell and tube type where the first reactor is cooled with cooling water and the second one with feed synthesis gas. In this reactor system, the wall of the tubes in the gas‐cooled reactor is covered with a palladium‐silver membrane which is only permeable to hydrogen. Hydrogen can penetrate from the feed synthesis gas side into the reaction side due to the hydrogen partial pressure driving force. Hydrogen permeation through the membrane shifts the reaction towards the product side according to the thermodynamic equilibrium. Moreover, the performance of the reactor was investigated when the reaction gas side and feed gas side streams are continuously either co‐current or countercurrent. Comparison between co‐current and countercurrent mode in terms of temperature, activity, methanol production rate as well as permeation rate of hydrogen through the membrane shows that the reactor in co‐current configuration operates with lower conversion and also lower permeation rate of hydrogen but with longer catalyst life than does the reactor in countercurrent configuration.     

Comparison of Concentration Profiles for Boundary Layers in Absorption with Chemical Reaction Processes

An analysis of five different systems of absorption‐with‐chemical‐reaction in gas‐liquid reactors, commonly encountered in various industrial processes, is presented. To analyze the interphase mass transfer from gas to liquid, the rate limiting parameters and the concentrations at the gas‐liquid interface were determined on the basis of pertinent theories. The calculations presented, are based on the Whitman theory for gas and liquid phase mass transfer coefficients and Henry equilibrium constants. The necessary diffusion coefficients were calculated from existing correlations, and the corresponding chemical reaction rate constants were obtained from the literature, assuming pseudo first order chemical reaction. The process parameters required (pressure, temperature, and the gas‐liquid contact time) were within the values that occur in industrial processes. The results presented, are the concentration profiles in the boundary layers for the systems studied, calculated and graphically presented, together with the gas and liquid film thicknesses and Hatta numbers, obtained from calculations for the liquid phase mass transfer. The results may contribute to a better understanding of the absorption‐with‐chemical‐reaction processes in industrial plants, thus lowering the operational costs of these processes and alleviating the ecological problems of existing technologies.     

Model for predicting the performance of membrane bioadsorber reactor process in water treatment applications

This research focuses on the development of a mathematical model for performance prediction/simulation of the membrane bioadsorber reactor (MBR) process. The MBR process integrates biodegradation, adsorption and membrane filtration, for water or wastewater treatment and water reclamation. The model sub-processes include the following phenomenological aspects: (a) biological reaction in bulk liquid solution, (b) film transfer from bulk liquid phase to the biofilm, (c) diffusion with biological reaction inside biofilm, (d) adsorption equilibrium at the biofilm-adsorbent interface, and (e) diffusion within the adsorbent (powder activated carbon) particles. The model exhibited good simulative capability for three model organic compounds, namely, phenol, para-nitrophenol, and toluene, chosen based on their varying adsorption and biodegradation characteristics. A phenomenological approach was employed to examine the relative contributions of adsorption and biodegradation to contaminant removal, and to obtain insights into adsorbent bioregeneration. Therefore, simulation studies were conducted under three different scenarios: (a) adsorption and biodegradation in biofilm and liquid phase suspension are operative, conforming to the assumptions of the generalized model; (b) biodegradation is operative in biofilm as well as liquid phase suspension, but adsorption is absent; and (c) adsorption alone is operative without biodegradation. Sensitivity studies were preformed to investigate the dependence of process dynamics on model parameters pertaining to adsorption equilibrium and kinetics, liquid film transport, biofilm diffusion, biochemical reaction kinetics, influent concentration, and reactor flow conditions.