Non-ideal modelling of polymeric hollow-fibre membrane systems: Pre-combustion CO2 capture case study
Introduction
Polymeric membrane gas separation is an attractive technology for many industrial applications; because the process is conceptually straightforward and energy-efficient, using a semi-permeable polymeric membrane that enables some gases to pass through easily, while other gases experience a barrier [1,2]. This provides greater versatility in separation applications compared to other technologies. Membrane gas separation is commercialised in natural gas sweetening and air enrichment [3,4], and research efforts are focused on applying membranes to other gas separation applications. One area in which membrane gas separation has strong potential is CO2 capture; a strategy proposed to reduce CO2 emissions from large point sources. Pre-combustion CO2 capture in particular holds promise. In this case, an integrated gasification combined cycle (IGCC) process generates syngas (mainly CO and H2) through partial oxidation, which is further converted through the water-gas shift reaction into CO2 and H2. The CO2 is separated from H2 to yield a low-carbon high heating value fuel in combined cycle plants for power generation [5].
The driving force for membrane gas separation is the chemical potential difference across the polymeric membrane [6]. However, in the majority of the membrane gas separation literature, difference between the partial pressure of components (i.e. ideal gas behavior) in upstream and downstream is considered the driving force for permeation [7]. Given that chemical potential is a complex function of temperature and fugacity, a rigorous approach should be employed to better describe permeation through membranes. For most industrial applications, the systems under consideration are complex, given the high-pressure feed streams, mixture of gases and vapours, as well as, significant pressure drop through the membrane. Robust membrane models are therefore needed to characterise membrane processes under high pressure and temperature, to better assist in the development of membrane gas separation technology and ensure accurate process simulations are undertaken. Currently, membrane models are severely limited for industry, as the majority of commercial process simulators do not include membrane process unit operations in their software suites, or the membrane simulation is relatively basic. This is also the case in the academic literature, where membrane models are often simplistic [[8], [9], [10]]. For these models, the permeance of components is considered constant, whereas it is inherently dependent on temperature, pressure and concentration of individual components [11]. The conventional models often use simple correlations for required physical and transport properties and neglect the variations of the properties as a result of change in concentration, temperature and pressure [12].
Pre-combustion CO2 capture represents a good model system to investigate more complex behavior in membrane processes through simulations, since in this CO2 capture scenario, high temperature and pressure gas is fed to the CO2/H2 separation unit. Xu et al. [13] conducted a parametric analysis of pre-combustion CO2 capture using single and double stage membrane separation but the model assumed isothermal operation and ignored both concentration polarization and real gas behavior. Similar models which neglect some or all of these effects have been also developed by Choi et al. [14] and Giordano et al. [15]. Franz and Scherer developed a more detailed Aspen model to evaluate the separation and energy efficiency loss of an IGCC power plant but the model still assumed constant permeance of components [16].
A number of robust mathematical models capable of simulating membrane operations have been presented in the literature. Scholz et al. [17] provided a detailed membrane gas separation model that included effects such as real gas behavior, non-isothermal operation (the Joule-Thomson effect) and concentration polarization. They showed these non-ideal effects adversely influence membrane gas separation. However, physical and transport properties were taken as constant at feed and permeate conditions. In reality, these properties can change, as temperature, pressure and concentration of the gas vary along the membrane module.
Here, a rigorous membrane model incorporating non-isothermal operation, real gas behavior, concentration polarization, pressure loss along the module, temperature-dependent permeance and variable physical and transport properties is presented and applied to a membrane process undertaking pre-combustion CO2 capture. The rigorous model is validated against experimental data presented in the literature and compared with a simplistic model to demonstrate the deviation in performance due to individual effects of real gas behavior, non-isothermal operation and concentration polarization.
Section snippets
Methodology
The mathematical model is based on an asymmetric hollow-fibre membrane module, i.e. a dense polymeric membrane layer on a porous support. To build the rigorous model, as illustrated in Fig. 1, the membrane module is discretized into N cells for each of which the mass, energy and momentum balance equations are simultaneously solved with the membrane transport equation.
The model can simulate co-current and counter-current operations, shell-side or lumen-side feed and constant or variable gas
Model validation
The proposed membrane model is validated with experimental data published by Pan [32], Sidhoum [33] and Feng et al. [34]. The performance of the membrane modules is represented by the dimensionless parameter stage-cut which is the ratio of feed flow rate to permeate flow rate. The activation energies of permeation have not been reported for the systems described by Pan [32] and Sidhoum [33], and hence, constant permeabilities have been used in these cases. Temperature-dependent permeabilities
Conclusion
A model of single-stage membrane gas separation has been developed for hollow-fibre modules. The model utilises non-isothermal operation with temperature-dependent permeance, concentration polarization, real gas behavior, frictional pressure loss and variable physical and transport properties along the membrane module. The model has been coded in Aspen Custom Modeller enabling end-users to have access to the massive component database and thermodynamic properties of Aspen Properties. It is also
Acknowledgements
Ehsan Soroodan Miandoab acknowledges The University of Melbourne for the Melbourne Research Scholarship.
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