Noemi S. Schbib

Modeling a Silicon CVD Spouted Bed Pilot Plant Reactor

This study reports a comprehensive multiphase gas-solid dynamic mathematical model that successfully describes the batch growth of silicon particles in a CVD submerged spouted bed reactor. This multiphase reactor model takes into account the hydrodynamics and interphase mass exchange between the different fluidized bed regions (spout or grid zone, bubbles and emulsion phase) and uses applicable kinetic rate models to describe both heterogeneous and homogeneous reactions. The model also incorporates a population balance equation representing particle growth and agglomeration.The CVD submerged spouted bed reactor operation is simulated by means of a sequential modular procedure, which involves the solution of the reactor model and the population balance equation.It is shown that the proposed CVD multiphase reactor model successfully simulates experimental data obtained from batch operation in a pilot scale reactor at REC Silicon Inc. The modeling of experiments obtained for different operating conditions allows correlating the scavenging factor as a function of the silane concentration for short- and long-term operations.

Simulation and optimization of a set of catalytic reactors used for dehydrogenation of butene into butadiene

Abstract The simulation and optimization of a set of industrial fixed bed catalytic reactors is presented. The reactors are operating in deactivation-regeneration cycles. Dynamic mathematical models for the four stages of the process are included, i.e. dehydrogenation (deactivation by coking), steam purge, oxidative regeneration and evacuation. An iterative method was used to simulate an autothermal process, which is common in the industrial practice. To prevent the permanent loss of the catalyst activity by sintering, an upper limit of temperature has been imposed. The cycle time, temperature and composition of the feed during the regeneration stage are selected as optimization variables. Under autothermal conditions, the four stages of the cycle start with markedly non-uniform thermal profiles in the catalytic bed, which have considerable influence on the maximum temperature of the cycle. In this way, the production rate of butadiene has been substantially improved as both the maximum allowable temperature and the inert-catalyst ratio increase. The higher the oxygen molar fraction at the regeneration stage, the shorter is the optimal duration of the cycle.

Steady-state analysis and optimization of a radial-flow ammonia synthesis reactor

Abstract The steady, state simulation and optimization of a large-scale ammonia converter is considered. The reactor consists of two adiabatic radial-flow catalyst beds in series with interstage cooling. The feed stream is preheated in an interbed heat exchanger using the hot gases leaving the first catalyst bed (autothermal operation). The first-bed inlet temperature is controlled using a cold by-pass stream. A heterogeneous one-dimensional model is used to simulate the catalyst beds. The feedback of heat, inherent in autothermal processes, is a source of reactor instability. The influence of the manipulated variables on the reactor stability is analyzed in the present paper, aiming to investigate the steady-state multiplicity phenomenon and its connection with the optimal operating points.

Dynamics and control of an industrial front-end acetylene converter

The dynamic simulation of a frontal industrial acetylene converter and its closed-loop performance under three different control strategies are discussed in the present work. The reactor is used for the selective hydrogenation of acetylene in the presence of large amounts of ethylene. It consists of a condenser followed by three catalytic beds with intermediate cooling. In the first step, the steady-state and the open-loop dynamic modeling and simulation of the converter are described. An analysis of the closed-loop behavior follows, starting with the existing control scheme, i.e. three conventional feedback loops located at the condenser and at the two intermediate heat exchangers respectively. The second control configuration studied is formed by adding feedforward scheme, which acts on the set points of the feedback loops. This configuration proves to diminish the peaks in the reactor temperature produced by changes in the total reactants flowrate and/or in the CO or the C2H2 concentration. The last scheme studied is the previous one plus an external feedback composition loop. To keep the acetylene concentration in the final ethylene below 3 ppm, this controller should ensure around 1 ppm C2H2 at the reactor outlet (approximately 1.4 10−4 wt% for a typical process stream composition).

Dynamics and control of a radial-flow ammonia synthesis reactor

Abstract The steady- and non steady-state simulation of an industrial ammonia converter is presented. The reactor includes two adiabatic radial-flow catalyst beds in series. An interbed (gas-gas) heat exchanger is used to preheat the feed stream. The steady-state results showed good agreement with plant data. The influence of different disturbances (feed composition and temperature, reactor pressure) on the dynamic evolution of the main variables is analysed. The open-loop and closed-loop operation is compared from the standpoint of the reactor stability.

Effect of Catalyst Deactivation on the Dynamics of Cyclic Reactive Processes

Abstract The transient behaviour of a set of industrial fixed-bed catalytic reactors is presented in this paper. These reactors, used for the dehydrogenation of 1-butene into 1-3 butadiene, operate in reaction-regeneration cycles. Unlike previous contributions, the influence of catalyst sintering on the reactor operation is analysed in this work. The dynamic model presented includes a mechanism for catalyst deactivation due to fouling and the loss of activity due to thermal degradation.

Formulation of Reduced-Order Models for the Dynamic and Stability Analyses of Autothermal Radial Flow Reactors

Autothermal radial flow reactors typically consist of a reactor setup of multiple catalyst-beds with internal heat exchange. These reactors are widely used because of their high efficiency due to the internal heat exchange, and radial flow arrangements are preferred due to their low pressure drops. Although an efficient multi-functional reactor arrangement, this setup has shown to provide for an additional destabilizing mechanism via the heat feedback. Thus, additional stability considerations are necessary when operating autothermal or non-adiabatic reactors at high conversions. This work proposes the formulation of a simplified model to investigate the effect of the heat transfer feedback on the stability of autothermal radial flow reactors. The present work focuses on a lumping approach to reduce the order of a complex distributed parameter system. The model is complex enough so as to preserve the intricacies of this reactor arrangement, but still yield a tractable dynamic formulation. The industrial ammonia synthesis process has been chosen as a case study to illustrate the proposed methodology. The lumped model predictions are qualitatively compared against numerical simulations of a detailed mathematical model.

Reversal Flow in Fixed-Bed Reactors Operating Under Reaction-Regeneration Cycles

The unsteady state simulation of a set of industrial fixed-bed reactors is presented. The catalytic dehydrogenation of 1-butene into 1,3-butadiene is selected as case study. These reactors operate under reaction-regeneration cycles. Each stage of the process, i.e., dehydrogenation (deactivation by coking), steam purge, oxidative regeneration and evacuation, is simulated by means of the corresponding dynamic model. The kinetic parameters used in the dehydrogenation and regeneration stages for a Cr 2 O 3 /Al 2 O 3 catalyst are taken from the literature. The performance of the reactors is investigated for two different operation modes: conventional (CO) and periodic flow reversal (PRFO). The PRFO mode shows significantly lower values for the average bed temperature and residual coke concentrations than those corresponding to the CO mode. This behaviour has a favourable effect on the production rate of butadiene. In addition, the influence of the catalyst sintering is analysed in this paper for both operation modes. The deactivation rate by thermal degradation is lower in the case of PRFO due to the lower average temperature of the catalyst bed. Periodic flow reversal strategies provide an economical and efficient alternative to optimise cyclic processes.