Daniel O. Borio

Non-adiabatic radial-flow reactor for styrene production

A non-adiabatic radial reactor is proposed to carry out the dehydrogenation of ethylbenzene to styrene. Radial flow and continuous heating (using superheated steam) are the main features of the new design. Steam used as heating medium flows through tubes, which are radially installed in the catalyst bed. By means of steady-state simulations, this new design has been compared with two adiabatic beds with radial flow and reheating between stages (similar to those used in industry). For equal steam consumption, the proposed design leads to higher selectivity to styrene than the industrial adiabatic design. This enhancement in selectivity (which is observed for different conversion levels) would significantly improve the economics of the styrene production process.

Kinetic analysis of enzymatic esterification of fatty acids and ethanol

Abstract The esterification of oleic acid and ethanol with immobilised lipase in a solvent-free media is presented in this work. The reaction has been carried out at a laboratory scale in a stirred tank reactor operating batchwise. The evolution of oleic acid conversion to ester was determined by using two analysis techniques: measurement of the acid index and gas chromatography. The influence of the operating conditions on the equilibrium conversion and conversion-time profiles was analysed. The amount of enzyme, temperature, initial alcohol/oleic acid molar ratio and initial water content were considered. A kinetic model has been developed for the enzymatic esterification of oleic acid and ethanol catalysed by Candida Antartica immobilised lipase. Some kinetic models were considered and a second order reaction kinetic was selected. The parameter adjustment was carried out by using a computer program based on Marquardt algorithms. The results from the simulated experiments by using the proposed model showed a satisfactory agreement with the experimental data.

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.

A new cocurrent reactor for ammonia synthesis

An alternative design for the industrial multitubular shell-side fixed-bed ammonia converter is presented. It can be run either under cocurrent or countercurrent cooling flow. The behaviour of these two alternatives is compared with that of the existing fully-autothermal co- or countercurrent designs. The comparison among the four cooling schemes is drawn by means of a heterogeneous two-dimensional mathematical model. The results indicate that the cocurrent alternative of the new design is capable of producing a significant increase in the ammonia yield. This higher value could also be obtained at lower maximum temperatures, which would lengthen the catalyst life.

Thermal regimes in cocurrently cooled fixed-bed reactors

Abstract Seven different thermal regimes can be achieved when an exothermic first-order reaction takes place in a cocurrently cooled fixed-bed reactor. All the regimes are feasible in reactors where the inlet temperature of the coolant is different from that of the reacting stream. New thermal regimes were found; they are qualitatively very different from those commonly reported in the literature (e.g. maximum-minimum, or minimum-maximum profiles, cold-spot regime). Based on a pseudohomogeneous plug-flow model, the conditions of existence for all the regimes, as well as guidelines to select the operating conditions to achieve the desired profile properly are presented. The diversity of feasible regimes can be very useful to track optimum thermal profiles required to solve practical optimization problems.

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.

Optimization of Steam Reformers: Heat Flux Distribution and Carbon Formation

Optimal heat flux distributions along the axial position of steam reformer tubes are analyzed, aiming to maximize the outlet methane conversion without violating the upper bounds specified for the tube skin temperature, Tw(z) and the local heat flux, Q(z). The possible occurrence of the carbon deposition phenomenon is considered as an additional constraint in the optimization problem by means of a kinetic criterion which accounts for the rate of carbon formation (rC,net).The problem of finding the axial heat flux profile that leads to maximum methane conversion (or minimum tube skin temperature for specified production) without violating the upper limits imposed for Tw(z), Q(z) and rC,net is in principle a complex optimization process. In this work, a relatively simple semi-analytical method, that requires only iterative reactor simulations, is proposed.At conditions of fresh catalyst and for operations with typical feed compositions and temperatures, the carbon formation constraint is not active. For low values of the maximum allowable local heat flux (Qall), the optimal heating policies are monotonically increasing Tw(z) trajectories. Conversely, for higher Qall values the optimal Tw(z) shows an initial increase in the first tube section followed by an isothermal section. Decreasing axial wall temperature profiles or Tw(z) curves with maximum are clearly not optimal.When the catalyst is strongly deactivated, the optimal manipulation of Q(z), or Tw(z), is an appropriate procedure to achieve carbon free conditions. As this heating policy consists basically in reducing the firing in the top section of the reformer tube, it leads to unavoidable production losses.The shape of the catalyst activity axial profile has a considerable influence on the risk of carbon formation. Increasing activity distributions (similar to those found when sulfur poisoning takes place) may result in severe carbon formation in the reformer top. An optimal control of the heat input can contribute to minimize this practical operation problem.

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.

Thermal regimes in cocurrently cooled fixed-bed reactors for parallel reactions: application to practical design problems

Abstract The conditions of existence of the seven feasible thermal regimes for exothermic first-order parallel reactions occurring in cocurrently-cooled fixed-bed reactors are derived here. The diversity of thermal regimes offered by the cocurrent scheme was applied to solve practical optimization problems. Theoretical optimal temperature profiles inside the reactor tubes which maximize the outlet yield of the desired product were determined. These ideal thermal trajectories were very well tracked by attainable thermal profiles generated by simulation of a cocurrently-cooled fixed-bed reactor represented by a plug-flow pseudo-homogeneous model. In fact, the objective function values found at feasible operation conditions were very close to the optimal ones. These results indicate that the conventional designs such as constant coolant temperature or the countercurrent scheme (where only hot-spot or decreasing profiles can be generated) could not handle the optimization problems as efficiently as the cocurrent configuration. Moreover, the cocurrent design allows, among other advantages, to reach the specified level of conversion (or yield) with higher selectivity and lower maximum temperature values than the common constant coolant temperature configuration. These findings imply the possibility of a significant improvement in the economics of the plant.