David E. Clough

Intrinsic kinetics for rapid decomposition of methane in an aerosol flow reactor

Abstract A one-dimensional nonisothermal model is developed for the high-temperature rapid dissociation of methane in a fluid-wall graphite aerosol flow reactor. Intrinsic reaction kinetics are identified through simulation of the model and comparison to experimental results. The dissociation rate can be described by d X d t =6×10 11 exp −25000 T (1−X) 4.4 s −1 over the temperature range 1533 K and residence time range 0.9 s . This rate expression is useful for describing high-temperature short residence time processes for producing hydrogen and carbon black by methane decomposition (CH4→C+2H2).

Modeling a low pressure steam-oxygen fluidized bed coal gasifying reactor

Abstract A comprehensive model accounting for the jetting region and homogeneous dilute phase reactions is developed for the adiabatic and continuous gasification of coal particles in a fluidized bed. The division of flows in the bed is determined by means of a modified two-phase theory which considers inlet gas jets, bubbles, free of particles, which develop at the top of the jets and grow in size as they rise, and an emulsion phase consisting of particles and the surrounding interstitial gas. The model describes the gasification of coal particles by pyrolytic devolatilization and three heterogeneous chemical reactions: oxidation by oxygen and steam, and reduction of carbon dioxide. Carbon monoxide and hydrogen produced by the heterogeneous reactions can be oxidized to carbon dioxide and steam by incoming oxygen within the dilute phase jets and bubbles. Furthermore, the water-gas shift reaction can occur in the dilute phase and interstitial gas. Simulations both with and without homogeneous reactions occurring in the jets and bubbles indicate that dilute phase homogeneous reactions have considerable influence on carbon conversion, bed temperature, and product gas composition. It has also been found that the jetting-emulsion mass and beat interchange has a substantial effect on overall bed performance and the temperature of the bed close to the inlet gas distributor. Results indicate that water-gas shift equilibrium is established rapidly and significant quantities of hydrogen and carbon monoxide and a nonuniform steam concentration are present within the combustion zone.

Application of a gamma-radiation density Gauge for determining hydrodynamic properties of fluidized beds

Abstract A modified, off-the-shelf, gamma-radiation density gauge has been interfaced into a microprocessor and mounted to a movable assembly mechanism which allows fluctuating bed density to be monitored on-line at any axial height in a fluidized bed. The signal processing required to determine expanded bed height L f , dense phase voidage e D , dense phase superficial gas velocity u Do , cross-sectional centerline bubble phase volume fraction 〈δ B 〉 t c , bubble size d b , frequency f b , is described in detail. Example measurements are presented as made on both a cast-acrylic model column and an industrial, pilot-scale, high-pressure, fluidized-bed reactor. Accuracy of the measurements is discussed.

Distributed parameter estimation and identification for system with fast and slow dynamics: A tubular, fixed-bed catalytic reactor to form styrene

Abstract A simple and efficient on-line scheme is developed to estimate temperature and compositions along a packed bed reactor in which styrene is being produced by the dehydrogenation of ethylbenzene. Slowly varying catalyst activity is also identified. The system is distributed in time and axial position and is nonlinear in the states: temperature and nine compositions. The dehydrogenation rate is augmented with a catalyst activity parameter which is assumed to undergo a long-term exponential decay. Since the decline in catalyst activity is slow when compared to state dynamics, a quasi-steady-state approach is used to derive a state filter equation neglecting process state dynamics and assuming spatially uncorrelated measurements and model uncertainty. For this filter, temperature measurements are available from four locations along the reactor and compositions are measured only at the reactor exit. A second dynamic, Kalman filter is used to identify the slowly varying catalyst activity. The two filters, one for distributed, steady-state, state estimation and the other for dynamic catalyst activity identification, are tested by computer simulation using measurements with added white noise. Several cases for numbers of sensors and noise levels are studied. The overall scheme is efficient and useable for on-line implementation. The steady-state filter is readily extended to distributed systems in more than one spatial variable such as reactor models with axial and radial dependencies. For steady-state or static models, multiple measurements yield significant improvements in the quality of the optimal estimates. Internal measurement locations allow for the subdivision of the spatial domain for the problem and improved profile estimates.

Modeling of char particle size/conversion distributions in a fluidized bed gasifier: non-isothermal effects

Abstract Mass and thermal energy balances are developed to describe the time-dependent behavior of reacting char particles distributed in both size and conversion in continuous, steam—oxygen, fluidized-bed gasifier. These relations are solved numerically utilizing the method of orthogonal collocation to simulate both steady-state and time-dependent behavior. The influences of superficial gas velocity, mechanical removal of solids, and reactants gas concentration on steady-state bed characterisctics and particle size/conversion distributions are determined. An investigation is made of particle heating times, and the validity of neglecting temperature profiles in reacting char particles is confirmed. These heating times are compared to the dynamics governing solids flow and particle size/conversion distributions. The transient responses of solids holdup and the distributions are presented to show the dynamic effects of step changes in solids feed rate, superficial gas velocity, and particle-size distribution in the feed.

Experimental tracking of particle-size distribution in a fluidized bed

Abstract Particle-size distribution is a fundamental parameter in fluidization. It affects fluidization behavior, transport and kinetic properties in the bed, and process considerations for operation. Many factors affect particle-size distribution, including solids feed, solids discharge, elutriation, particle growth and particle shrinkage. We have developed a method to track this important parameter via an on-line process computer. The on-line particle-size distribution measurements, made automatically and with little time-delay after sampling, are obtained every 60 s in the experimental demonstrations. Such a frequency is high enough to properly track particle-size distribution even during extreme distribution dynamics. Particle sampling is continuous in our method, and the sample system is closed-loop so that all sampled particles are returned to the fluidized bed. The experimental demonstrations were performed on a large-particle, sand-air fluidized bed. In this paper we detail the experimental equipment and procedures used. The experimental results presented include the tracking of particle-size distribution during two different particle-size distribution transients. Validity of the on-line measurements were verified via sieve analysis of grab samples. The on-line measurements are shown to be representative of actual conditions in the bed. They are somewhat noisy (corrupted with random error), however, with the measurement noise increasing as a function of particle size.

STOCHASTIC INFERENTIAL CONTROL OF THE FLUIDIZED STATE VIA DIFFERENTIAL PRESSURE MEASUREMENTS

Abstract Differential pressure is measured across some portion of most industrial fluidized beds. Average differential pressure is related to the weight of material suspended between the measurement locations, but higher frequency noise of the measurement is usually not considered. In this work stochastic variations in differential pressure were modeled as a time series and related to the degree of mixing and turbulence within the bed (fluidized stale). Specifically, mean values of recursively estimated model parameters changed regularly with fluidizing-air flow and, consequently, with the character of the fluidized state. Control of the fluidized state was inferred through closed loop control of model parameters. Both a simple floating algorithm and more complex adaptive algorithm were successful based on controller and fluidized bed performance.

Dynamics of particle size/conversion distributions in fluidized beds: application to char gasification

Abstract Dynamic relations are given which describe the time-dependent behavior of the distribution of reacting particles which are distributed in both size and in their degree of conversion in a fluidized bed. These relations are solve by the method of orthogonal collocation. The adequacy of the orthogonal collocation formulation is determined by comparing steady-state results to steady-state solutions obtained directly by a semi-analytical method. Transient responses of bed weight and particle size/conversion distributions to a step increase in the feed rate are given to exemplify results of the dynamic solution algorithm.

Worst-case losses from a cylindrical calorimeter for solar simulator calibration.

High-flux solar simulators consist of lamps that mimic concentrated sunlight from a field of heliostats or parabolic dish. These installations are used to test promising solar-thermal technologies for commercial potential. Solar simulators can be calibrated with cylindrical calorimeters, devices that approximate black body absorbers. Calorimeter accuracy is crucial to solar simulator characterization and maintenance. To discover the worst-case performance of a cylindrical calorimeter during flux measurement Monte Carlo ray tracing was coupled to finite volume simulations. Results indicated that the calorimeter can exhibit an observer effect that distorts the solar simulator flux profile. Furthermore, the proposed design was sensitive to changes in calorimeter optical properties, changes that can result from oxidation and/or photobleaching over time. Design fidelity and robustness were substantially improved through the use of a beveled (conical) calorimeter aperture.

Use of Image-Based Direct Normal Irradiance Forecasts in the Model Predictive Control of a Solar-Thermal Reactor

A model predictive control (MPC) system for a solar-thermal reactor was developed and applied to the solar-thermal steam-gasification of carbon. The controller aims at rejecting the disturbances in solar irradiance, caused by the presence of clouds. Changes in solar irradiance are anticipated using direct normal irradiance (DNI) forecasts generated using images acquired through a Total Sky Imager (TSI). The DNI predictor provides an estimation of the disturbances for the control algorithm, for a time horizon of 1 min. The proposed predictor utilizes information obtained through the analysis of sky images, in combination with current atmospheric measurements, to produce the DNI forecast. The predictions of the disturbances are used, in combination with a dynamic model of the process, to determine the required control moves at every time step. The performance of the proposed DNI predictor-controller scheme was compared to the performance of an equivalent MPC that does not use DNI forecasts in the calculation of the control signals. In addition, the performance of a controller fed with perfect DNI predictions was also evaluated.