Marisa N. Pedernera

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.

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.

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.

Green synthesis of nanocrystalline faujasite zeolite

In this work, the synthesis of 28 and 38 nm Na-Y zeolite nanocrystals is reported. The process was performed via green synthesis without any organic structure directing agents (SDAs), at low temperature and applying ultrasound-assisted aging. The zeolite nanoparticles obtained were characterized by X ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N2 adsorption - desorption isotherm at 77 K. A simple, fast, efficient and environmentally friendly procedure to synthetize nanocrystalline FAU zeolite is described in the present study. The sonochemical pretreatment was found to directly affect the Na-Y zeolite properties. The final product obtained via ultrasound pretreatment shows high phase purity and crystallinity degree with crystal size smaller than 38 nm and elevated surface area (SBET ∼ 950 m2 g-1). The hierarchical micro- and mesoporous FAU zeolites exhibit a pore volume of ∼0.4 cm3 g-1.