Subsea Natural Gas Treatment Using Membranes: Experimental and Modeling Study
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Subsea natural gas reservoirs are usually saturated with water vapor and impurities such as CO2 and H2S, which are the main contributors to the pipeline blockage, hydrate formation, and corrosion. Dehydration is usually the first stage in natural gas processing; a safe and reliable dehydration process close to the wellhead is desired to be installed subsea to replace the continuous chemical injection for hydrate prevention. This thesis is based on a subproject in the SUBPRO centre, aiming at dehydration of natural gas using a novel closed-loop membrane process to bring down the impurity levels to meet the pipeline specifications for the direct transportation of natural gas, with no further purification onshore. At first stage, the feasibility of subsea natural gas dehydration by membrane was evaluated experimentally in a non-porous membrane contactor using a hydrophobic material. A flat sheet membrane contactor was employed as the membrane interface, which is in contact with a glycol solvent (triethylene glycol (TEG)). A structured packing turbulence promoter was used in the gas phase to improve the efficiency of the system. The water flux and outlet dew point were measured at different operating conditions (i.e., pressures, liquid and gas flow rates). A systematic modeling approach was then introduced, and the membrane permeability and overall mass transfer coefficient were precisely estimated. Novel correlations were also introduced for the axial dispersion and boundary conditions due to the presence of turbulence promoter. Continuous regeneration of the TEG is performed by using membrane thermopervaporation (TPV) unit, which is energy-efficient membrane process due to the use of free cooling energy offered by the cold subsea water. The performance of a composite membrane in thermopervaporation for TEG regeneration was investigated experimentally. Permeation characterizations of a thin film composite (TFC) membrane were carried out in the in-house made membrane testing rig and module. A 3D computational fluid dynamics simulation was then performed over the entire module to characterize the membrane performance and to correct the driving force. Concentration polarization phenomena was investigated and the effect of operating temperature and concentration on the process and membrane performance was studied by experiment and modeling. Membrane materials suitable for natural gas processing were also studied. A mixed matrix membrane (MMM) with a metal-organic framework (MOF) was fabricated to remove water and CO2 simultaneously from natural gas. This membrane was also found to be a good candidate for other CO2 separation applications, such as CO2 capture from the humid flue gas. The MMMs were tested for the humid gas streams to understand the efficiency of the MOF in the mixed matrix for CO2 removal in the presence of water vapor. In this study, a water harvesting metal-organic framework, MOF-801, was synthesized and dispersed in Pebax matrix to fabricate MMMs. A molecular simulation (Monte Carlo and Molecular Dynamics) study was performed to reveal the potential adsorption sites in the MOF-801 lattice and favorable interactions with gases.