A highly exothermic nitrogen generation system (NGS) can be achieved by mixing solutions of sodium nitrite and ammonium chloride, a process used by the oil and gas industry to dissolve paraffin wax and gas hydrates. Although its main products are nitrogen gas and a sodium chloride brine, the NGS has a side reaction that produces nitrogen oxides. To optimize this process to ensure the greatest and fastest heat generation with the lowest oxide production, this reaction was checked by infrared spectroscopy and calorimetry. The factors temperature, pH, and initial concentration of nitrite and ammonium were evaluated, and the optimal conditions of the NGS were determined by the constructed models to predict heat and NOx generation. These conditions were a ratio of ammonium/nitrite equal to 1 and a catalyst concentration of 0.07 mol·L–1 (for a case in which the temperature is 5 °C).

No abstract

Summary The highly exothermic reaction between ammonium chloride (NH4Cl) and sodium nitrite (NaNO2) has an important application in the area of flow assurance. Because of the high heat generation, this reaction has been used as a heat source for the fluidization of low-melting-point deposits formed during oil and gas production. Because this reaction is strongly pH dependent, the incorrect choice of pH can result in an uncontrollable temperature increase caused by the system’s inability to dissipate the large amount of heat generated in a short time, causing accidents such as structural damage and explosions. Thus, the aim of this work was to study a method that involved adjusting the pH over time to ensure controlled heat generation, with high calorimetric conversion, and avoid the development of a thermal-runaway reaction (pH-based control of the kinetics and process safety). The kinetics and thermodynamics of this reaction were studied using heat-flow reaction calorimetry and attenuated total reflection (ATR)-Fourier-transform infrared (FTIR) (ATR-FTIR) spectroscopy. Following a semiempirical approach, calorimetric and spectroscopic data were fitted to a kinetic equation using nitrite, ammonium (NH4+), and hydronium concentrations. The molar enthalpy calculated was –322.92 kJ/mol, and the Arrhenius parameters were determined as the frequency factor [ln(A)] = 22.21 and the apparent activation energy (Ea) = 63.40  kJ/mol. The kinetic model constructed made it possible to properly evaluate the pH profile that should be maintained to control the kinetics (heat-generation rate) and process safety [time to maximum rate under adiabatic conditions (TMRAD)] of the reaction. The strategy of adjusting the pH over time ensured controlled heat generation and high calorimetric conversion, which cannot be achieved by simply adding catalyst at the beginning of the reaction, and minimized the risk of developing a runaway reaction. However, in real applications, the pH control must be made using the balance between the thermal risk (TMRAD) and the performance of the method (qr), because although it is possible to decrease the thermal risk (increase the value of TMRAD) by increasing the pH, this increase is accompanied by a decrease in the heat-generation rate. Thus, from the proper balance of these factors (qr and TMRAD), pH control can ensure adequate levels of heat production within an acceptable thermal risk. Supplementary materials are available in support of this paper and have been published online under Supplementary Data at https://doi.org/10.2118/205389-PA. SPE is not responsible for the content or functionality of supplementary materials supplied by the authors.

No abstract

Summary Nitrogen-generating systems (NGSs) are mainly used in the oil industry to fluidize low melting point organic deposits and gas hydrate buildups. They are exothermic reactions between two nitrogenous salts in acidic catalytic media. This work investigates the use of CO2 to promote NGS reactions instead of commonly used acids such as acetic and citric acids, which can be problematic for corrosion control. Sodium nitrite and ammonium chloride were the reactants, and CO2 performance was evaluated for up to 4 hours at 5 and 25°C, and either under autogenous pressure at 10, 25, and 50 bar of CO2 or pressurized at 10 bar of CO2 by adding 40 bar of nitrogen (totaling 50 bar). The nitrite conversion was determined by measuring the concentration of residual nitrite using titration. Thus, it was verified that the CO2 effectively promoted the NGS at various experimental conditions. The nitrite conversion increased with increasing CO2 pressure and increasing temperature. Moreover, the nitrite conversion was enhanced in the pressurized system (PS) because the high pressure enabled the dissolution of CO2 in the aqueous medium, and therefore, the constant formation of carbonic acid, favoring the acidic catalytic medium at the reaction. This advantage was confirmed by carrying out an NGS catalyzed by acetic acid, in which the pH increases as reagents are consumed, and therefore, a lower nitrite conversion is achieved. The use of CO2 also converts the NGS in a process more suitable for flow assurance applications in offshore oil production, particularly in the Brazilian presalt fields where the coproduced CO2 can be used.