Measurement of the Diffusivity and Solubility of Methane into Aqueous TBAB
Creators: name: Ojo, Olajumoke Ifeoluwa
Publisher: name: Graduate Studies
Year: 2018
Subjects: subject: Solubility
subject: Diffusivity
subject: Semiclathrates
subject: Natural gas storage
subject: Methane
subject: Tetra-n-butyl Ammonium Bromide
subject: Engineering--Chemical
subject: Engineering--Petroleum

description: Natural gas is a relatively clean burning fuel that can be used for cooking, heating and electricity generation. To adequately meet the developing world’s growing demand needs for natural gas it is necessary to have adequate infrastructure for gas transportation and storage. Historically, natural gas has been stored either in underground geological formations or above ground as either Compressed Natural Gas (CNG) or Liquefied Natural Gas (LNG). All of the previously noted storage technologies are mature technologies, but also technologies that require large capital investments. As a lower cost alternative to CNG storage, a number of researchers [1] have proposed using gas hydrates and, more recently, semiclathrates as a medium for above-ground natural gas storage. Gas hydrates are ice-like compounds, formed in the presence of water and light gases that have the ability to entrap large volumes of gas at conditions that are much milder than those needed to store natural gas as either CNG or LNG. Semiclathrates are similar compounds to gas hydrates. However, the presence of a quaternary ammonium salt leads to semiclathrates being able to form at conditions that are even milder than those required for gas hydrate formation. Several experimental analyses have been carried out in order to determine the three-phase (solid-liquid-vapour) equilibrium conditions at which semiclathrates form. However, in order to more thoroughly evaluate the feasibility of using semiclathrates for gas storage purposes it will be necessary to know the solubility of the gas in the solution as well as the diffusivity of the gas in the solution. Originally, the aim of the study was to modify an existing experimental apparatus and to measure the solubility and diffusivity of methane gas in aqueous solutions of tetra-n-butyl-ammonium bromide (TBAB) at conditions close to semiclathrate formation conditions. However, as the experiments proceeded, it was found that maintaining a constant pressure in the reactor was not straight forward. Thus, the main focus of the thesis shifted to establishing an adequate control strategy. Three control strategies were tried; PID control, on-off control and on-off control plus an inline microflow metering valve. For each strategy, several runs were carried out in an attempt to measure the solubility and diffusivity. Each of the experimental runs took close to a month to complete, after which time it was concluded that only an on-off controller with an inline microflow metering valve was capable of providing the appropriate control needed to maintain a constant pressure in the reactor. Following the experiments, the diffusivity and solubility of methane in aqueous TBAB solutions were regressed from the experimental data using the finite acting model and the infinite acting model as utilized in the original work of Etminan [2]. From the analysis of the experimental data, the diffusivity and solubility were seen to follow the expected trend; the diffusivity increases and the solubility decreases with temperature, at a constant pressure.