Packed Bed Tubular Reactor Research Articles

Pulsating wave propagation in reactive flows: flow-distributed oscillations

Stationary waves in a reactive flow with equal transport coefficients were recently generated by passing the oscillating Belousov-Zhabotinsky reaction medium through a tubular packed bed reactor while keeping the concentrations constant at the inflow boundary [M. Kaern and M. Menzinger, Phys. Rev. E 60, 3471 (1999)]. Here we study the effects of oscillatory boundary conditions, and observe traveling wave fronts that propagate in either a pulsating or a steady manner. The present experiment is isothermal and conditions are such that all species have identical transport properties. This excludes rapid thermal or activator diffusion and the wave pulsation appears to be induced by an essentially kinematic mechanism. Our experimental findings are supported by numerical simulations of the full reaction-diffusion-advection system using a realistic kinetic model. Finally, the kinematic essence of the mechanism inducing wave pulsation is captured in an iterative one-variable map.

NO reduction, CO and hydrocarbon oxidation over combustion synthesized Ag/CeO2 catalyst

The combustion technique produces ionically dispersed Ag on a nano-crystalline CeO2 surface. The catalysts thus produced were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Catalytic properties towards NO reduction, CO and hydrocarbon oxidation have been investigated using the temperature programmed reaction technique in a packed bed tubular reactor. These results are compared with α-Al2O3 supported finely divided Ag metal particles synthesized by the same method. Both oxidation and reduction reactions over Ag/CeO2 have been observed to occur at lower temperatures compared to Ag/Al2O3. The rate and turnover frequency of the NO+CO reaction over 1% Ag/CeO2 are 56.3 μmol g−1 s−1 and 0.97 s−1 at 225°C respectively. Activation energy (Ea) values are 71 and 67 kJ mol−1 for CO+O2 and NO+CO reactions, respectively, over 1% Ag/CeO2 catalyst.

Control of Axial Temperature Distribution in a Packed-Bed Reactor.

Control of temperature profiles of a tubular packed-bed reactor with our proposed design method for a parabolic distributed parameter system is investigated. The oxidation reactor of carbon monoxide over a platinum-catalyst is used as a controlled process. The reactor dynamics are modeled by a simple parabolic partial differential equation in one-dimensional space, on the assumption that temperature distribution in the radial direction of the reactor is negligible. A state feedback controller was designed by use of a reduced order lumped parameter system, which was obtained by applying the finite integral transform technique to the phenomenological model of the process. It is demonstrated that hot-spot temperatures can be eliminated and the axial temperature profile of the reactor can be made to agree well with a specified target profile.

Performance of Zeolite-Supported Catalysts for Selective Catalytic Reduction of Nitric Oxide and Oxidation of Methane

The selective catalytic reduction of nitric oxide by methane over copper ion-exchanged natural zeolites was investigated in a packed-bed tubular reactor. The catalytic activity of Cu−N and Cu−H−N catalysts was confirmed as NO, CH4, and O2 displayed very little reaction in the absence of any catalyst and zeolite without ion exchange was totally inactive. A maximum NO conversion of 33% at 650 °C for Cu−N-66 was achieved with 2% NO and 1% CH4 and a contact time of 0.9 g s cm-3, but the introduction of 2% O2 reduced the NO conversion to only 12%. Ion exchange for the H-form prior to copper ion exchange was essential for oxygen to promote catalytic activity over the temperature range 250−650 °C, with a maximum conversion of 30% at 450 °C with O2 present. The direct reduction of NO by methane was ruled out as a possible reaction pathway.

Controller performance assessment by frequency domain techniques

Abstract A system identification based method for assessing the performance of closed-loop systems is proposed, utilizing measures which coincide naturally with classical and modern frequency domain design specifications. Standard robust control system design methodologies seek to maximize closed-loop performance, subject to strict robustness requirements and include specifications for bandwidth and peak magnitude of the sensitivity and complementary sensitivity functions. Estimates of these transfer functions can be obtained by exciting the reference input with a zero mean, pseudo random binary sequence, observing the process output and error response, and developing a closed-loop model. Performance assessment is based on the comparison between the observed frequency response characteristics and the design specifications. Selection of appropriate model structures, experiment design, and model validation which will ensure reasonable estimates of the closed-loop transfer functions are considered in this paper. A case study involving the performance assessment of a packed bed tubular reactor control system is presented.

Temperature correction for packed bed multitubular reactors with concentric axial thermowells

The following simple empirical correlation was developed to relate the thermowell temperature, T tw , in tubes containing concentric axial thermowells to the centerline and mean temperatures, T a and T m , in tubes without thermowells at the same axial location in packed bed multitubular reactors: (T a−T tw)(T a−T m=36.51( d tw d t ) 1.9 − ( 34.05 − 3.955 Bi )( d tw d t ) 2 0 ⩽ (d twd t ⩽0.3, 1 ⩽ Bi ⩽ 10. In conjunction with a reactor model of the tubes without thermowells, use of this correlation provides an easy, accurate method of predicting the thermowell temperature for comparison with data. This correlation was developed from computer simulations of pseudo-homogeneous, plug flow, packed bed tubular reactors with and without thermowells. T tw is always lower than T a for exothermic reactions; thus, stable operation in the instrumented tubes does not guarantee that the tubes without thermowells will not experience runaway. At industrial operating conditions, thermowell sheath conduction has a negligible effect on the temperature measurement of the surrounding bed.

Convective instability induced by differential transport in the tubular packed-bed reactor

Convective (or spatial) instability, which manifests itself as the tuned amplification of perturbations in the course of their propagation along a non-isothermal packed-bed tubular reactor, is shown to occur in the exothermic standard reaction A → B + heat. The instability is caused by the interplay of the differential transport of heat and matter and of the activator-inhibitor kinetics inherent in non-isothermal, exothermic reactions (where heat plays the role of autocatalytic species or activator, and matter represents the inhibitor). The differential transport is caused by the inert reactor packing which acts as a thermal reservoir and slows down the diffusive and advective transport of heat relative to that of matter. This instability appears to be relevant to an earlier observation (Puszynski and Hlavacek, 1980, Chem. Engng Sci. 35, 1769–1774) of sustained temperature oscillations in a packed-bed reactor at high Lewis number.

Absolute instability of a tubular packed-bed reactor with recycling

The stationary state of a non-isothermal tubular packed-bed reactor with partial recycling of the product flow or simply of the reaction heat, is shown to be absolutely unstable. This instability is closely related to the convective instability of a fixed-bed reactor without recycling [Yakhnin et al., 1994, Chem. Engng Sci. (in press)] which manifests itself as the ability of the system to amplify perturbations in the course of their spatial propagation. Recycling causes these amplified perturbations to be reinjected, thereby keeping the reactor from settling into its stationary state. This results in self-sustained periodic waves that circulate through the reactor.

Oxidation of ethene in a wall-cooled packed-bed reactor

The selective oxidation of ethene over a silver on α-alumina catalyst was studied in a pilot plant with a wall-cooled tubular packed bed reactor. Gas and solid temperatures in the catalyst bed were measured at different axial and radial positions as well as concentrations at different axial position. Total pressure, inlet mole fraction ethene, mass flow rate and wall temperature were varied. The experiments are compared with model calculations. A pseudo-homogeneous model and a heterogeneous two-dimensional model were used for the calculations. Kinetics and heat transport parameters in the model have been determined independently in separate studies. The work is distinctive in the accuracy paid to a large number of experimental details. The models correspond reasonably well with the experiments at all operating conditions. The main problem in the description of the reactor is the inability of the kinetic expressions to predict the reaction rates over the entire range of partial pressures. As a result some systematic deviations are found, in particular in the predicted temperature profiles. The heterogeneous model gives better agreement with the experiments although systematically larger temperature differences are calculated on the basis of available correlations in literature than has been measured. On the basis of this work it is difficult to determine directly whether an influence of chemical reaction is present on the values of the heat transport parameters.

An experimental study of the selective oxidation of ethene in a wall cooled tubular packed bed reactor

The selective oxidation of ethene over a silver on α-alumina catalyst was studied in a wall cooled tubular reactor. Temperatures were measured inside the bed at different axial and radical positions as well as the overall conversion and selectivity. Locally measured temperatures vary after repacking the bed whereas the global properties do not vary. Angular variations in temperature cannot be described by present day models. The steady state temperature profiles in the packed bed and overall conversion and selectivity as function of different operating conditions are discussed.

Numerical singularity analysis

A numerical scheme is presented for classifying the different static bifurcation behaviors exhibited by a nonlinear system in its remaining parameter space. This numerical technique differs from previously published schemes in that the application of singularity theory is done numerically and requires no explicit differentiations of the system in question. It also does not require the reduction of the mathematical model to a scalar equation. The utility of this multivariable scheme will be demonstrated through an application to a seven PDE tubular packed-bed reactor model.

Cracking of n-heptane on fluorinated γ-alumina catalysts in the presence of hydrogen: Catalytic activity and nature of acid active sites

The cracking of n-heptane over fluorinated γ-alumina catalysts with different fluorine contents was studied using a tubular packed-bed reactor at 723 K and 2400 kPa. Maximum activity was obtained for the catalyst containing 2.5 wt.-% fluorine. Cracking and isomerization of n-heptane are the primary reactions. Hydrogen can be activated on the surface of fluorinated aluminas and the olefins formed during cracking are hydrogenated by hydride transfer from hydrogen. Two mechanisms for cracking, viz., β-scission and protolytic cracking, can explain the initial selectivities obtained, which means that both Lewis and Brønsted sites could be active in this process. The original Lewis sites present in γ-alumina have a very low cracking activity.

Mobecs: Model-Object Based Expert Control Systems

An expert control system that operates at the supervisory level is developed to tailor the control of a chemical reactor in response to process faults and changes in operating conditions. Heuristic and quantitative process model information are utilized in this model-object based expert control system (MOBECS). A prototype of MOBECS has been developed to control a bench-scale packed-bed tubular reactor operated autothermally.

STEADY AND NON-STEADY STATE MODELING OF TUBULAR FIXED BED REACTORS

Abstract In this work a detailed non-steady state two dimensional heterogeneous model for a non-isothermal, non-adiabatic packed bed tubular reactor is discussed. The model has the following features: i) axial and radial heat and mass dispersion, ii) heat transfer balance for the cooling fluid, iii) comprehensive treatment of the convective term, iv) spatially-dependent physical parameters, v) radial porosity and velocity profiles. Results are given, for both steady and non-steady state situations, in the context of an industrial unit for phthalic anhydride production. Co-current flow of reactive and cooling mediums is assumed.

Dynamic model of packed-bed tubular reactors

A mixing cells model to simulate the dynamic behaviour of fixed bed catalytic reactors, operating either autonomously or with a heat recuperator is presented. Comprehensiveness and consistency of such a model are discussed by comparison with other models and checked against experimental data. A few examples are reported for illustration purposes,also covering the case of an autothermal loop where the thermal feedback can cause instability problems.

Use of nonlinear transformations and a self‐tuning regulator to develop an algorithm for catalytic reactor temperature control

AbstractPrior to developing a multivariable control scheme for conversion and production rates in a packed bed tubular reactor carrying out highly exothermic reactions, it was important to stabilize the reactor temperature. In this paper we illustrate the use of a self‐tuning regulator to develop an inner loop controller that satisfies some necessary conditions on temperature response, and that is capable of controlling the reactor over a wide range of operating conditions. The importance of using a nonlinear transformation of the reactor hot‐spot temperature is demonstrated. Varying the input constraining parameter in the “one‐step optimal” self‐tuning controller is shown to be a very effective way of achieving a controller with the desired properties for response smoothness.

The bifurcation behavior of an autothermal packed bed tubular reactor

A model describing the dynamic behavior of a laboratory scale, autothermal tubular CO oxidation reactor with countercurrent coolant flow is presented. After a discussion of the solution to this model, the steady state version of the same model is subjected to an application of static bifurcation theory. Numerical difficulties encountered and overcome in this study include steady state reactor profiles accessible only by the use of spline collocation, a reactor model whose large number of differential equations requires efficient computational procedures for mapping the steady state behavior, and isolated solution branches reachable only by an algorithm developed to create three-dimensional solution surfaces. The reactor behavior uncovered will be cataloged over the relevant range of reactant inlet conditions by the use of a numerical singularity technique. Among the unusual solution branches found is a reactor behavior that will be shown to be a characteristic of not only this particular model, but of nonadiabatic tubular reactors in general.

The Bifurcation Behavior of an Autothermal Packed Bed Tubular Reactor

Abstract The analysis of a chemical reactor by numerical bifurcation techniques can completely define the multiplicity and stability of its equilibrium states and can give insights into the reactor’s parametric sensitivity and dynamical behavior in the neighborhood of the chosen operating points. In this paper, we will present some of the techniques used and developed in analyzing an autothermal packed bed tubular reactor. Bifurcation diagrams will be presented for the uncontrolled system to assess the effects of the variable parameters (such as feed flowrate, feed temperature, etc.) and for a case where the reactor was stabilized at an open-loop unstable state by feedback control.

The unsteady-state solution for a finite dispersion-type catalytic packed-bed tubular reactor

The unsteady-state solution is obtained by an operator theory in functional analysis setting for a general finite catalytic packed-bed tubular reactor model with axial dispersion in the bed, mass transfer between the catalyst particles and the flowing phase, and intraparticle diffusion and surface reaction in the catalyst particles.

Design of a Control Scheme for a Catalytic Fixed-Bed Reactor

The model of a catalytic packed-bed tubular reactor is used for designing and evaluating the performance of a suboptimal controller. Disturbances consist of random variations in temperature and/or concentration of feed stream. The effect of disturbances is monitored by a single temperature measurement and the control command generated regulates the temperature or concentration of an intermediate stream. The measurement signal is corrupted by white noise. The intermediate stream is introduced to the reactor right after the measurement point.The control problem is formulated as a linear-quadratic-gaussian problem. Covariance matrices for particle and wall temperatures, filter and controller gain matrices are obtained from the solution of Riccati equations. Standard deviations of fluid temperature and concentration along the reactor are then computed. The deviation of the fluid temperature and concentration from the initial steady state values are used for evaluating the effect of the control scheme. The design procedure is helpful in selecting the signal to noise ratio of the measurement signal, the objective function, the manipulated variable and the location for measurement/injection point.For the reactor model used, the temperature of the intermediate stream is selected as the preferred manipulated variable. The best location for the measurement/injection point falls in the region between 40% to 50% of the reactor length from the reactor entrance. An objective function including the deviations along the reactor gives better results than an objective function based on the deviations at reactor exit.