Mechanistic Model for Ultraviolet Degradation of Light Hydrocarbons in Waste Gas

Abstract

A mechanistic simulation model was developed to describe ultraviolet waste gas treatment of the light hydrocarbons methane, ethane, and ethylene. The presented model can be used in both environmental and chemical engineering applications. Efforts were made to include all the possible chemical and photochemical reactions between air components and each hydrocarbon. The most comprehensive model consists of 199 reactions (165 chemical reactions, and 34 photochemical reactions) with 69 reactive species.

Trials indicated that most of the computation time was spent on calculating reactions that have insignificant effects on the effluent concentration. Hence, model variants were developed that include only the most relevant chemical and photochemical reactions, without loss of accuracy, while maintaining the lowest possible run-time. NOx is included in one model version as well. This work may benefit Eulerian air quality models, where most of the computation time is spent resolving the complicated chemistry.

Simulation results confirmed that removal efficiency (i.e., conversion) of ethylene is significantly higher than ethane, followed by methane, as expected. Sensitivity analysis of the models indicated that the water content, ozone premixing, and reactor cross-section are the main contributing factors affecting the removal efficiency, while changing the temperature and flow pattern do not influence the conversions by much. The proposed model is able to predict the reaction products of the photolysis process in the gas phase. The predicted effluent has a composition in general agreement with literature research.

COMSOL simulations of the photoreactor showed that the assumptions made in the original model development were justified, since the simulated flow pattern was consistent with the fully-developed laminar assumption in the base model. Also, thermal analysis indicated a noticeable temperature gradient in the gas phase photoreactor, but removal efficiencies are not impacted meaningfully by the temperature rise. Hence it can be concluded that the gas phase UV photolysis is mostly photon-limited, kinetically- and diffusion-controlled. This research reverses the conventional wisdom of waste gas photolysis where it is believed that turbulent flow is essential, a thin gap between lamp and wall is preferred, sophisticated light field modeling is essential, and detailed chemical modeling is unnecessary.