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Catalyst emissions from fluidising catalytic cracking units have the potential to impact significantly on the environmental compliance of oil refineries. Traditionally it has been assumed that gas velocity and fine particles significantly impact on emission levels. Through the use of a simple fluidised bed model, sensitivity analysis was conducted to identify the key operating parameters that influence emission rates. It was found that in addition to velocity, density and mid sized particles are the most influential factors for emission rates. Further work is needed to identify how these parameters can be altered during normal operations to reduce catalyst emissions.

The petroleum industry currently employs fluidising catalytic cracking units (FCCUs) as the major tool in producing gasoline from crude oil (Fig. 1). FCCUs typically consist of a rising main where the chemical reactions between catalyst and hydrocarbons occur, a reactor to separate the product and catalyst, and a regenerator to recharge the used catalyst. The regenerator is a fluidised bed used to combust coke from the used catalyst, with cyclones to remove particles from the flue gas stream before venting to the atmosphere. The recharged catalyst then recirculates through the rising main and the process is repeated [1]. Full-size image (6 K) Fig. 1. FCCU stylised schematic. Figure options In recent years, fine particle emissions from industry have been identified as important contributors to poor environmental and health standards across the United States [2]. With increasing demands for cleaner air, catalyst emissions from FCCUs have the potential to impact significantly on the environmental efficiency of the overall refining operation [3]. Currently, FCCUs are designed and operated in such a way as to maximise output and profitability of the refinery, while using end of pipe control technology to clean emission gas before they reach the atmosphere [4] and [5]. Thus there is a need for the relationships between current operational strategies and air pollution to be better understood allowing cheaper operational changes to be used to modify emission levels. The aims of this paper are twofold, firstly to identify the key operating parameters in terms of catalyst emissions from FCCUs, and secondly, to provide the foundation to develop a more detailed FCCU emission model to confirm the sensitivity results.

The common belief that only fines and velocity affect emission rates from FCCUs is not supported by this work. The model has shown that by increasing bed velocity, emission rates can actually be lowered through a gain in cyclone efficiencies. The interactions of other parameters such as catalyst density along with the concentration of medium to large particles are significant. The use of sensitivity analysis coupled with the trends visible from the model output has enabled a better understanding of how emission is influenced by operational parameters. Further work is needed to identify exactly how and why these parameters are so influential. In addition, the model needs to be expanded, taking into account feedback loops and interparticle relationships such as attrition. Once this is completed the sensitivity analysis can be repeated to confirm the results mentioned in this paper. This work has also shown the benefits achieved through the use of higher order sensitivity analysis techniques on industrial scale projects. It is hoped that oil refineries can use this information to alter operational conditions in such a way as to lower particle emissions without expensive end of pipe control measures being utilised.