Browsing by Author "Asili, Vahid"
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- ItemOpen AccessMechanistic Model for Ultraviolet Degradation of Light Hydrocarbons in Waste Gas(2018-08-30) Asili, Vahid; De Visscher, Alex; Roberts, Edward (Ted); Mohamad, Abdulmajeed; Salahub, Dennis; Ray, Madhumita; Azaiez, JalelA 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.
- ItemOpen AccessUltraviolet Degradation of H2S in Waste Gas: A Comprehensive First–Principles Model(2013-07-24) Asili, Vahid; De Visscher, Alex; Azaiez, JalelIn upstream oil and gas operations, a considerable amount of hydrogen sulfide (H2S) is being emitted every year. Health issues associated with H2S that might be very serious vary dependent on the length of exposure. Furthermore, the gas is highly corrosive, which is a concern in the industry. A promising technique that can be used to remove this air pollutant from waste gas is photolysis, which has been successfully used in wastewater treatment processes. However, to the best knowledge of the author, no inclusive model has yet been developed for H2S gas phase photolysis. In this research study, a sophisticated simulation model was developed successfully to describe ultraviolet degradation of H2S from waste gas. The photochemical reactor has been modeled with 19 chemical species and a total of 47 chemical and photochemical reactions which includes a light field model, a chemical model, a flow pattern model and a mass transfer model. Simulation shows that the UV degradation of H2S in waste gas is a highly efficient process. Simulations were run to investigate the process efficiency is a function of initial concentration, gas flow rate, and relative humidity. The model was also validated with some literature experimental data by applying the model to those experimental conditions, and comparing the results. Comparison of simulation results and the experimental data indicates that the model overestimates the removal efficiency to some extent; however, observing the same trends in two different cases of the effect of initial H2S concentration and the effect of gas flow rate validates the model with sufficient accuracy to establish the feasibility of the process. Previously, it was found experimentally that the main photolysis (or photocatalysis) product is SO42- (or H2SO4) at very low concentration of H2S. The model also agrees with this result, and furthermore predicts SO2 as another photolysis product which is predominant at high iii concentration of H2S. Hence altogether, it can be concluded based on the model that the UV degradation technique is effective, and it decomposes H2S to less harmful products which are also easier to be treated. Moreover, the capability of the model for modeling the degradation of multiple pollutants at the same time was tested by model extension with NOx reactions. According to the simulation results, adding H2S in the NOx photolysis system has positive effects on the degradation efficiency for both H2S and NOx. The extended model indicates that the proposed model can be used as the basis of a modular comprehensive model to predict removal efficiency for each species and moreover product analysis when more than one pollutant is present.