Enhanced Sepration Of Dual-membrane Module For CO₂ Gas Mixture

Oil-associated gas, raw natural gas and raw hydrogen products are the common CO₂ gas mixture processed by membrane technology. However, current commercial-available membrane materials have limited hydrocarbon/CO₂ or H₂/CO₂ selectivities; high-performance materials could not achieve commercialization in recent future due to stability issues. Therefore, intensifying the membrane module or membrane process becomes the most effective and industry-oriented method. Studies have been made to optimize the module by introducing another simultaneously-operating reverse-selective membrane into the same module, which is denoted dual-membrane module ?DMM?. However, the DMMs are limited by the inaccurate mathematical models, insufficient investigations on the flow patterns and impacts of membrane selectivities. To improve the separation performance of DMMs, this work proposed several dual-membrane-based processes and methods to solve the separation problems of natural gas/CO₂ and H₂/CO₂ gas mixtures, aiming to employ dual-membrane module to improve the separation performance with current-available membranes and reduce the operational cost of natural gas pretreatment and H₂/CO₂ separation.This work proposed a calculation method for the non-ideal multi-comoponent separation process of rubbery polymeric membranes. The method integrated small-scale molecular dynamic simulation with free volume theory. By correlating the results of molecular dynamic simulation, the parameters, such as specific free volume, plasticization potential and gas penetrability, are derived. A non-ideal model of membrane module is proposed by integrating several non-ideal effects such as polymer plasticization, competitive sorption, swelling, gas fugacity, porous support resistance and pressure build-ups. Through the proposed model, the membrane vapor recovery was able to be calculated accurately. The comparisons between the proposed model and experimental data indicated that, the prediction error of product purity was decreased form 10%of the ideal model to 1%of the proposed model; the difference of permeate/residue flow rates between ideal model and proposed model was reduced from 50% to 4%. The proposed method is able to acquire the key parameters of free volume theory by small-scale molecular dynamic simulation, and is able to be expanded to other rubbery polymeric membrane systems. A mathematical model based on finite-difference method was proposed for dual-membrane module. The discrete protocol of the continuity equation enabled the model to be unconditionally stable; the modularity of the model was also improved comparing to conventional differential models, and the non-ideal factors can be integrated with the model simply and effectively. The comparisons of model predictions and experimental data indicated the accuracy of the proposed model. The model was then embedded into commercial process simulation platform, providing a powerful simulation tool for the process design and optimization of dual-membrane module-based processes.Oil-associated gas is a hydrocarbon-rich by-product of oil-fields. However, the CO₂ content highly affects the recovery of hydrocarbons. This work porposed a novel dual-membrane process for oil-associated gas recovery. The proposed process utilized dual-membrane module as an efficient dew-pointing equipment with simultaneous separation of CO₂. The dew-pointed hydrocarbons were refed to condenser, while the separated CO₂ was reinjected into reservoir for enhanced oil recovery. The method reduced the capture cost of CO₂, and improved the separation performance of hydrocarbons, resulting in mutural decrease in operational cost and natural gas loss. The dual-membrane process was able to achieve 8% and 16%lower investment and operational cost, comparing to conventional single-membrane-based processes. The low cost also enabled lower hydrocarbon production cost, a 9%or more reduction in hydrocarbon production cost can be found for the dual-membrane process, which indicated the high separation efficiency of the dual-membrane module.Natural gas fields are also endured with high CO₂ concentration, which increases the pretreatment cost of amine scrubbing. This work proposed two dual-membrane-based process for natural gas pretreatment. The processes contained a two-stage CO₂ separation membrane cascade to separated CO₂ with 90 mol%purity, and a condensation-dew-pointing loop for vapor recovery. Through optimizations and comparisons, the results showed that the processes with front locating condensation were the most effective separation sequences, and the condensation-dual-membrane module process high highest separation performance for each individual unit, therefore resulted in lowest process investment ?3%or more lower than other separation sequence or single-membrane-based processes?, and the CO₂ capture cost is also 2%less than other processes. When membrane selectivities increased, the dual-membrane module was able to provide higher separation performance through two-way enrichment effect. When membrane selectivities was raised to 50, the dual-membrane module-condensation configuration became the optimal separation sequence, while the conunterpart single-membrane process cannot provide similar effects by surpassing other separation sequence, indicating the dual-membrane module is the key to develop the full potential of high-performance membrane materials.H₂/CO₂ gas mixture is a common raw product for hydrogen production process, the separation of it depends the production cost of hydrogen. This work proposed a novel flow pattern for dual-membrane module, which is able to enhance the H₂ separation performance. The flow pattern is co?H₂?:counter?CO₂? current, and the co-current flow pattern of the H₂-selective membrane avoided the position where bulk CO₂ concentration was highest, which reduces the CO₂ permeation in H₂ product. With the aid of high pressure gradient in the H₂-selective membrane, the bulk CO₂ was futher rejected by the H₂-selective membrane, and the H₂ could be separated in CO₂-lean environment. The analysis of separation performance showed that the co?H₂?:counter?CO₂? dual-membrane module could increase the product purity of H₂ by 2-8 mol%, comparing to conventional counter?H₂?:counter?CO₂? or single-membrane modules. When feed CO₂ concentration was increased to 40mol%, the co?H₂?:counter?CO₂? dual-membrane module could provide 22 mol% higher purity for H₂ product than single-membrane module. The co?H₂?:counter?CO₂? dual-membrane module also provided an alternative rounte for the separation of H₂/CO₂. When the selectivity of CO₂-selective membrane was increased, the separation performance of the co?H₂?:counter?CO₂? dual-membrane module was improved more than conventional dual-membrane modules, and the optimal range of the selectivity of the CO₂-selective membrane was 20-40. The effects indicated that the co?H₂?:counter?CO₂? dual-membrane module is and will be an effective module configuration at present and future.