Abstract

The regeneration of FCC catalysts leads to significant NOx emissions requiring the development of novel additives that catalyze in situ the reduction of NOx to N2 in the regenerator of the FCC unit in order to fulfill existing and anticipated logistic demands. The identification of reaction intermediates is of utmost importance for understanding the mechanisms by which NOx is formed and reduced. Therefore, characterization of coke, using a wide range of physicochemical techniques (i.e., IR and NMR spectroscopy, elemental analysis, LD-/ MALDI-TOF-MS spectroscopy) has been carried out. The surface chemistry during the FCC regeneration process was investigated by temperature programmed desorption and oxidation experiments. From coke loaded spent FCC catalysts NH3 and HCN were formed via pyrolysis at temperatures above 350°C. The amount of NH3 released was significantly influenced by the concentration of water in the samples. Higher water contents favor the formation of NH3, which supports the hypothesis that nitrogen containing aromatic compounds such as pyridine can react to NH3 and CO2 via hydrolysis. TPO experiments indicated that polyaromatic derivatives of pyrrole (carbazole) are cracked to CO and HCN, which can be subsequently oxidized to NO. Nitrogen and carbon containing species in the coke are oxidized sequentially during the regeneration process (C-species between 450 and 700°C; N-species above 650°C).

Introduction

Fluid catalytic cracking (FCC) is a key process in modern refineries.1 Worldwide approximately 300 FCC units are operated, converting vacuum gas oil and high boiling residues into lighter fuel products and petrochemical feedstock. Due to its central function in modern integrated refineries, a range of technological improvements has been implemented, to increase the economical benefits from FCC units.2 In addition to investments concerning the process design, new catalysts and additives have been developed to fulfill the economic demands of the market.3 However, refiners are bound to invest also in eco-efficient technologies for the production of fuels and petrochemicals with significantly reduced emissions of environmental pollutants. This is imposed by various stringent national and international regulations addressing emissions from a range of refinery processes and especially FCC regenerators, such as NO, SOx, CO and CO2 emissions from regenerator flue gases.1 Approximately 2000 t/yr NOx are released from a typical refinery. The FCC units contribute to approximately 50% of that. The concentrations of the NOx emissions from regenerator flue gases vary in the range of 50–500 ppm 4,5 depending on the nature of the feed, the operating conditions of the FCC unit and the amount of CO promoter added. In the fluid catalyti

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