Multiobjective Optimization Based on “Distance-to-Target” Approach of Membrane Units for Separation of CO2/CH4

Abstract

The effective separation of CO2 and CH4 mixtures is essential for many applications, such as biogas upgrading, natural gas sweetening or enhanced oil recovery. Membrane separations can contribute greatly in these tasks, and innovative membrane materials are being developed for this gas separation. The aim of this work is the evaluation of the potential of two types of highly CO2-permeable membranes (modified commercial polydimethylsiloxane and non-commercial ionic liquid–chitosan composite membranes) whose selective layers possess different hydrophobic and hydrophilic characteristics for the separation of CO2/CH4 mixtures. The study of the technical performance of the selected membranes can provide a better understanding of their potentiality. The optimization of the performance of hollow fiber modules for both types of membranes was carried out by a “distance-to-target” approach that considered multiple objectives related to the purities and recovery of both gases. The results demonstrated that the ionic liquid–chitosan composite membranes improved the performance of other innovative membranes, with purity and recovery percentage values of 86 and 95%, respectively, for CO2 in the permeate stream, and 97 and 92% for CH4 in the retentate stream. The developed multiobjective optimization allowed for the determination of the optimal process design and performance parameters, such as the membrane area, pressure ratio and stage cut required to achieve maximum values for component separation in terms of purity and recovery. Since the purities and recoveries obtained were not enough to fulfill the requirements imposed on CO2 and CH4 streams to be directly valorized, the design of more complex multi-stage separation systems was also proposed by the application of this optimization methodology, which is considered as a useful tool to advance the implementation of the membrane separation processes.

Biopolymer-Based Mixed Matrix Membranes (MMMs) for CO2/CH4 Separation: Experimental and Modeling Evaluation

“…The stage-cut and feed pressure considered for the modelling equations were 0.5 and 4 bar, respectively, in agreement with the experimentally obtained values. The CO2 purity in the permeate increases while the recovery decreases with increasing CO2 concentration in the feed, which indicates that some CO2 goes to the retentate, decreasing the recovery of the CH4 in the retentate [56]. All the membranes observe a similar trend and IL-CS and AM-4:IL-CS composite membranes provide the largest values of CO2 and CH4 purity, closely followed by the commercial PDMS membrane.…”

“…Both parameters are considered as the most important parameters to determine the effectiveness of the simulated membranes and all the specifications are focused on their values so that the optimization of the process aims at obtaining the maximum purities and recoveries of each component in the permeate for the most permeable one and on the retentate for the other. In response to the difficulty of maximizing both process parameters at the same time., a multi-objective problem would be proposed for establishing more global targets [56]. Another aspect that should be fixed before undertaking the simulation is the number of cells, n, required to achieve a determined objective.…”

“…The magnitude of this process parameter determines the precision of the results obtained. In this work, we selected a value of n = 100 as a compromise between the numerical discretization and the model precision [56].…”

Techno-Economic Optimization of Multistage Membrane Processes with Innovative Hollow Fiber Modules for the Production of High-Purity CO2 and CH4 from Different Sources

“…The modeling of membrane separation of gas mixtures in hollow fiber modules has been performed using different approaches. In this work, the model previously presented in a preceding paper from this research group has been applied . The model had been previously validated with experimental data.…”

“…Despite these efforts to improve the performance of commercially available membranes and reduce the environmental impacts of innovative membranes by consideration of environmentally friendly biopolymers as membrane materials, initial process engineering tasks have demonstrated that the separation process performance in terms of purity and recovery would not be enough to obtain streams that fulfill the requirements imposed to CO 2 and CH 4 direct valorization in a single membrane stage. Consequently, further efforts must be taken into consideration to design more complex multistage separation processes that can achieve the imposed requirements . Sometimes, these multistage processes require the design of complex membrane networks, with multiple membrane units in series under multistage and multistep configurations, that require recycle streams.…”