A Study of N2 and CH4 Separation by Dual Reflux Pressure Swing Adsorption

Abstract

Natural gas is expected to have an increasing importance in global energy consumptions due to its clean combustion and low carbon emissions. However, there are increasing number of sub-quality gas fields that are contaminated with excess nitrogen and such nitrogen has to be removed in order to meet sale gas heating values and prevent from stratification and roll-over when natural gas is liquefied and transported. This poses significant challenges on the separation of nitrogen and methane due to their similar molecular size and physical properties. Cryogenic distillation is the only viable technology to separate nitrogen from natural gas for large scale production (>25 MMscfd), however the final nitrogen vent still contains as much as 2 mol% methane, which is a greenhouse gas 21 times more potent than carbon dioxide. While main competing technologies, including slow oxidization, membrane separation and absorption are currently not economically viable, pressure swing adsorption (PSA) is ideal to recover such a dilute methane from the nitrogen vent due to its non-chemical nature, low energy consumptions and is able to perform separation at near ambient conditions. However due to the low selectivity of adsorbents for methane and nitrogen, conventional PSA processes which only incorporate single reflux streams are not able to simultaneously achieve high nitrogen purity and high methane enrichment. A new and more advanced configuration of PSA which incorporates two reflux streams, namely dual reflux pressure swing adsorption (DR-PSA) was experimentally proved to be able to produce two relatively pure products even at low pressure ratios. In this research work, a numerical model that incorporates full material and energy balances of DR-PSA was developed and it was validated through the experiments previously conducted by Saleman et al (2014). The model was able to accurately predict product compositions and describe the dynamics of the DR-PSA unit, such as gas concentration and temperature profiles. The numerical simulation was subsequently extended to all the four basic configurations of DR-PSA and the differences between them were investigated which provided significant insight on future optimizations. A novel DR-PSA cycle which incorporates a total reflux step was developed in order to further increase product purities and recoveries, at a cost of higher energy consumption. The theory was validated by both experiments and numerical simulations which proved its superior separation ability compared to conventional DR-PSA cycles. The last section of this thesis focused on practical application of DR-PSA units, by estimating the size and number of units required to recover dilute methane from nitrogen vent in a typical industrial practice. A new adsorbent: TMA-Y which showed higher selectivity for methane over nitrogen than activated carbon was adopted. The simulation results suggested that with sufficient optimization, a dilute methane can be recovered with a product containing more than 0.3 mol fraction of methane while a pure nitrogen product with less than 100 ppm of methane can simultaneously be produced. Economic estimations suggested that the energy consumption of the recovery process was similar to the most economical CO2 capture technologies in terms of greenhouse gas emission reductions.