Model-based development of high-pressure membrane contactors for natural gas sweetening
Creators: name: Quek, Ven Chian
Publisher: name: Imperial College London
Year: 2019

description: Membrane separation and chemical solvent absorption technologies are both widely employed for Natural Gas (NG) Sweetening. A new hybrid process combining the advantages of both technologies, called membrane contactor (MBC), has been drawing significant attention over the past decade. MBC is considered a promising process for intensification purposes, as it can provide high specific surface areas, independent control over the gas and liquid flow rates, modularity, and compactness. Nevertheless, no literature to date has conducted a process-wide assessment of using MBC for NG sweetening, and therefore, its intensification potential cannot be systematically quantified. This challenge is the motivation of the research presented in this thesis. The main objective of this work is to develop process-wide modeling of NG sweetening using MBC to enable MBC process design and performance assessment. A predictive mathematical model of high-pressure MBC for NG sweetening with alkanolamines as the chemical solvent was developed. The model explicitly accounts for the rates of mass transfer through the membrane, diffusion, and chemical reaction in the liquid phase. A combination of 1-d and 2-d mass-balance equations to predict the CO2 absorption flux was considered, whereby the degree of membrane wetting itself is calculated using the Laplace-Young equation based upon knowledge of the membrane pore-size distribution, fluid flow configuration, and operating condition. The MBC model can also predict the solvent evaporative losses and the hydrocarbon (HC) absorption into amine solvent, which is important to maintain the CO2 absorption performance in MBC and to quantify the potential solvent make-up and product loss. Then, a full-scale steady-state MBC-based NG sweetening process was developed whereby the MBC model (absorption section) is integrated with the conventional solvent regeneration model (desorption section) in the gPROMS ProcessBuilder environment. The predictive capability of the model was tested using two sets of membranes with different characteristics, against data from two experimental settings; a lab-scale MBC module, where the purification is conducted using binary gas mixtures of CH4/CO2 and N2/CO2; and a pilot-scale MBC module operated under industrially relevant conditions at a NG processing plant in Malaysia. The important operation and design parameters such as the CO2 partial pressure, gas and liquid flowrates, pressure, temperature, liquid CO2 loading, membrane area and fiber length were varied to determine the performance of MBC. The model results clearly showed the change in the CO2 absorption performance and the energy consumption related to the variables. All model predictions showed a close agreement with the measured CO2 absorption fluxes, energy consumptions and the HC absorption and recovery from the solvent. The MBC model provided valuable research recommendations, whereby approximately 80% increase of CO2 absorption performance was achieved in MBC pilot plant by operating a smaller diameter of hollow fiber membrane in a horizontal orientation. The integrated process model is used to analyse the effects of various design configuration and operating conditions, such as the lean and semi-lean operations to meet sales gas specification. The experimental data and model analysis has confirmed the advantages of semi-lean operation in terms of energy reduction and physical footprint. Finally, a model based scale-up of a commercial MBC for NG sweetening was conducted to gauge its intensification potential. Overall, the scaled-up MBC commercial module showed promising prospects, whereby (i) the predicted reboiler energy per ton of CO2 removed is lower than the conventional amine absorption column by 12 - 50%; (ii) the predicted amine loss rate per treated gas and per ton of CO2 removed are lower compared to the typical loss rates reported in a conventional amine-based processes; and (iii) a potential savings of US$ 0.96-1.1 million 〖"yr" 〗^"-1" from the HC recovery can be realized, subject to further sweetening of the flash gas. Nevertheless, the MBC may require larger footprint than conventional absorption column due to horizontal operation. Going forward, the MBC model can be used to provide research and development targets, such as the improvement in membrane specific surface area and the hydrophobicity to enable commercial MBC modules to be stacked, thereby improving the intensification potential in terms of footprint.