Ramees Khaleel Rahman

View Video Presentation: https://doi.org/10.2514/6.2022-1447.vid Sustainable aviation fuels can reduce carbon emissions from jet engines. Farnesane is a biofuel that has properties similar to jet fuel. In this work, shock tube studies are conducted for a farnesane mixture and ethylene mixture under sooting conditions. The soot measurement is done using the laser-induced extinction method to measure soot induction time and soot volume fraction time-histories at 1850 K and ~4 atm. The results presented herein will be a useful validation target for developing kinetic models for soot formation.

Carbonyl sulfide (COS), carbon disulfide (CS₂), and mercaptans (e.g., CH₃SH) are unwanted sulfurous species that are produced in sulfur recovery units (SRUs). They can cause equipment corrosion, reduce catalyst activity, and are pollutants for the environment. This necessitates process optimization to minimize their formation in the SRUs using an accurate kinetic model. In this paper, new reactions of COS and mercaptans are studied using the CBS‐QB3 level of theory, and their kinetics are evaluated using transition state theory. Such reactions are added to a detailed SRU reaction mechanism, and the updated mechanism is validated using the data on COS and mercaptans from lab‐scale experimental setups and an industrial plant. Compared to the previous models, significant improvements in the predicted concentrations of the important species such as COS, carbon monoxide (CO), and sulfur oxides (SO and SO₂) are observed due to the inclusion of the new reactions in the mechanism. The simulation studies in a homogeneous reactor within the temperature range of 800–2000 K revealed that furnace temperatures near 1300 K are required to minimize CO and COS formation, while ensuring the oxidation of methane (CH₄), mercaptans, and aromatics. Mercaptan formation mainly took place at low temperatures near 900 K. Claus furnace simulations are conducted using industrial feed and operating conditions to show that the optimized inlet air temperature and fuel gas flow rate can assist in simultaneously reducing the emissions of CO and harmful sulfurous species such as COS, CH₃SH, and CS₂from the furnace.

The low and fluctuating demand and sales price of sulfur and its increasing production due to the discovery of new sour fields have mandated optimization studies to reduce the production cost in sulfur recovery units. Natural gas (mainly methane) is continuously co-fired with acid gas (H₂S and CO₂) in the Claus furnace to maintain temperatures above 1050°C, which adds to the operating cost. This study aims at optimizing the usage of natural gas in sulfur recovery units and enhancing sulfur yield simultaneously using a detailed reaction mechanism for Claus feed combustion, while ensuring complete BTEX destruction. An optimal combination of feed-preheating using natural gas and its co-firing in the furnace is determined through furnace and fired heater simulations. Through simulation studies by varying natural gas flow rates into the Claus furnace and to the air preheater, it is observed that a considerable amount of natural gas can be saved to maintain the required furnace temperature, if air is preheated to a higher temperature rather than natural gas co-firing (also validated in an operational sulfur recovery plant), (b) a reduction in natural gas to the furnace improves the sulfur recovery efficiency, and (c) the destruction of aromatic contaminants (that are harmful for the catalysts present downstream) in the furnace is enhanced with decreasing natural gas co-firing and increasing feed temperature. With multi-objective optimization, further increase in sulfur yield by 8% and a reduction in fuel gas demand is achieved.

Sulfur compounds present in crude oil and gas are absorbed primarily in the form of acid gas (H₂S and CO₂) and then converted to elemental sulfur in the sulfur recovery units (SRU). To comply with the increasing environmental standards on sulfur emissions, SRUs are required to achieve Sulfur recovery efficiency ranging from 98% to 99.99%. The existing technologies for higher sulfur recovery induce very high cost additions to an already economically deficient SRU. To reduce the SRU costs, a modified thermal section is proposed in this study to increase the sulfur recovery efficiency in the relatively inexpensive Claus furnace in order to reduce the number of expensive catalytic units required to meet the environmental standards. The modified SRU is based on double-stage acid gas combustion taking place in two Claus furnaces, along with the intermediate withdrawal of sulfur and H₂O exiting the first furnace. The SRU's conventional and modified thermal sections are simulated using industrial plant data and a detailed reaction mechanism. Compared to the conventional thermal section, Sulfur recovery efficiency increased from 63% to 75% due to a shift in the Claus equilibrium reactions towards Sulfur production (2H₂S + SO₂ ⇌ 3S + 2H₂O), as both H₂O and sulfur were withdrawn in the modified case. To determine the overall Sulfur recovery efficiency, the 2-stage and 3-stage catalytic sections combined with the conventional or modified thermal sections were simulated. In comparison, the overall Sulfur recovery efficiency (thermal + catalytic sections) in the modified thermal section increased remarkably from 96.5% to 98% with the 2-stage and from 97.6% to 98.6% with the 3-stage catalytic units. The higher amount of sulfur captured in the modified thermal section resulted in the decreased concentrations of H₂S, SO₂, H₂, CO and H₂O in the effluent gas, which helped to reduce the processing load/requirements in catalytic units. These results will assist in providing guidelines to reduce the number of expensive catalytic units to decrease the operating cost of SRUs.

The formation of mercaptans and organosulfur species in Claus process leads to a reduction in the sulfur recovery efficiency, and an increase in the cost of tail gas treatment. While process optimization can be used to minimize their production in plants, the existing models fail to predict their concentrations to a reasonable accuracy. This study provides a detailed and validated model for Claus process including organosulfur species reactions that can be used in process optimization. A Claus mechanism, developed in a previous study, was used as the base mechanism. To this, the organosulfur reaction mechanism, developed using quantum chemistry, was added. The rate constants of the elementary reactions were determined using transition state theory. New rate parameters were validated with experimental data from the literature as well as from the sulfur recovery units in the UAE. The reaction furnace of the Claus process was modelled as an adiabatic plug flow reactor, and the waste heat boiler was modelled as a heat exchanger. New reactions and their rates for COS and CS₂ formation provided a good match between the experimental data and the computed values. The important reactions contributing to COS formation were: i) CO+S₃=COS+S₂; ii) COS+O=CO₂+S; iii) COS+SH=CO+HSS. Similar analyses were done for other organosulfur species to find the reactions responsible for their formation. The process simulations of SRU thermal unit were conducted using Chemkin Pro software to determine the conditions under which their concentrations can be minimized in the furnace. COS concentration was found to be reducing beyond 1500 K while mercaptans were destructed around 1050 K. It is also observed that CO concentration increased due to the thermal cracking of CO₂. The presence of CO₂ and fuel gas in the feed to the Claus process was found to be contributing highly to the formation of organosulfur species, but the furnace temperature played a vital role in their formation and destruction. The model developed in this study successfully predict the concentrations of organosulfur species including mercaptans, COS and CS₂ in Claus process that will help in the optimization of process parameters to minimize their formation, maximize the sulfur recovery efficiency, and reduce the load on downstream catalytic units.