Hydrate phase equilibrium data of CO2+H2O+Tetrabutyl ammonium salts (chloride and fluoride).
The UNFCC’s (United Nations Framework Convention on Climate Change) goal, to stabilize greenhouse gas concentrations through energy technology and prevent anthropogenic interference with the climate system, has led to interest in mitigation strategies such as carbon dioxide capture and storage (CCS). CCS involves the capturing of CO2, emitted from large industrial sources, before release into the atmosphere and storing it in safe underground rock formations. Gas hydrate or clathrate hydrate crystallization is a promising technology for the capturing of CO2. Clathrate hydrates are non-stoichiometric crystalline compounds that consist of a lattice of water molecules that physically encage molecules of another component i.e. CO2. Clathrate hydrates made of CO2 gas are formed at high pressures and relatively low temperatures. Previous hydrate phase equilibrium data has been measured for temperature and pressure ranges of (273 - 283) K and (1200 - 4500) kPa. Industrial flue gas exits coal-fired power plants at atmospheric pressure. Hence, the operative costs of compressing such flue gas to the necessary hydrate formation pressure would be significantly high. The pressure at which a clathrate hydrate is formed increases exponentially with temperature. Hence, the lowest possible formation pressure is desired, in order to minimize the flue gas compression costs. High operating pressures can be reduced by use of additives/hydrate promoters which will lower the hydrate formation conditions without affecting the kinetics or the efficiency of CO2 recovery. Tetra-n-butyl ammonium chloride (TBAC) and tetra-n-butyl ammonium fluoride (TBAF) are additives that can reduce hydrate formation pressures to feasible industrial conditions, however, there is insufficient phase data available on the systems comprising of (CO2 + H2O + TBAC) and (CO2 + H2O + TBAF). Due to the current interest in CO2 capture and storage by gas hydrate crystallization; there is a demand in experimental phase equilibrium data for the relevant CO2 hydrate systems. In order to design efficient CCS processes, reliable and accurate phase equilibrium data is required. The present study involved the measurement of hydrate phase equilibrium data for systems CO2 + H2O + TBAC and CO2 + H2O + TBAF. CO2 + H2O + TBAC was measured at a range of (795 -4520) kPa and (281.25- 293.55) K at 4.21 wt%, 10.05 wt% and 30.13 wt% concentrations of TBAC. CO2 + H2O + TBAF was measured at a range of (680-2396) kPa and (285.8– 304.3) K at 4.01 wt% and 30.26 wt% concentrations of TBAF. The hydrate phase equilibrium measurements were taken at a maximum pressure of 5 MPa to limit the nominal compression costs and achieve a feasible method to capture CO2. The phase equilibria data was measured using an isochoric pressure-search method (isochoric cooling from the vapour-liquid region to the hydrate-vapour-liquid region). Once hydrate formation was confirmed by a sudden drop in pressure (due to gas encapsulation), the system temperature was then increased to dissociate the hydrates. The hydrate-vapour-liquid equilibrium point was recognized as the point of complete decomposition of the hydrate. A visual high-pressure autoclave, consisting of a 316 stainless steel cylindrical cell (60cm3) and two sapphire windows, was operated to measure the hydrate equilibrium phase data. Pt-100 platinum resistance thermometers and P-10 pressure transducers measured the temperature and pressure conditions respectively during the isochoric pressure-search procedure. The uncertainty of the hydrate equilibrium data point is ±0.3 K and ±1.71 kPa, with a confidence of approximately 95%. This uncertainty consisted of the uncertainty of calibration of the devices, uncertainty of the instruments (specified by the supplier), uncertainty of data repeatability and the uncertainty of the isochoric pressure-search method. Vapour pressure measurements for carbon dioxide and ethane were performed to verify the pressure and temperature calibrations. The test systems, compromising of CO2 + H2O, CO2 + H2O + TBAC (4 wt%) and CO2 + H2O + TBAF (4 wt%) were measured to validate the accuracy of the experimental apparatus, experimental methodology and ensure the measured hydrate phase equilibrium data were correct and reliable. An increase in the tetrabutyl ammonium salt concentration resulted in an improved promoting effect of the equilibrium pressure. With reference to a pure CO2 hydrate, average temperature shifts of approximately 6 K, 9 K and 15 K were noted for 4.21 wt%, 10.05 wt% and 30.13 wt% concentrations of TBAC. An average temperature increase of 11 K, 16 K and 29 K was observed at 4.01 wt%, 8.27 wt% and 30.26 wt% concentrations of TBAF respectively. A comparison of the effects of TBAC and TBAF validated TBAF to be a stronger promoter for CO2 hydrate formation. The hydrate formation rate was observed to have decreased as the hydrate formation region was approached. This phenomenon validated the work of Servio and Englezos(2011), which illustrates that the solubility of CO2 in water decreases with system temperature, as the system approaches the conditions at which hydrates commence to form. Hydrates formed in the CO2 + H2O + TBAC and CO2 + H2O + TBAF systems had a slower hydrate formation rate than the CO2 + H2O system. Hence, the presence of TBAC and TBAF further reduces CO2 solubility during hydrate formation. The modelling approach of Eslamimanesh (2012) was used to model semi-clathrate phase equilibrium conditions of the CO2 + H2O + TBAC and CO2 + H2O + TBAF systems. The predicted results of the Eslamimanesh (2012) model demonstrated reasonable accuracy at the experimental pressure range. The model was used to predict semi-clathrate phase equilibrium data for each crystal structure of TBAC (i.e. types A, B and C) and TBAF (i.e. types A and B). The predicted results clarified the type of semi-clathrate crystal structure at each experimental condition.