Model-Based Analysis of a Highly Efficient CO₂ Separation Process Using Flexible Metal–Organic Frameworks with Isotherm Hysteresis

Abstract

To realize large-scale CO 2 separation to prevent global warming, energy and space saving separation technologies are required. Flexible metal−organic frameworks (flexible MOFs) with gate-opening properties have been found to be promising adsorbents, but the unconventional sigmoidal shapes and hysteresis of the isotherms have hindered quantitative evaluation of their applications in separation processes. This paper is the first to evaluate ELM-11, a flexible MOF that exhibits sigmoidal isotherms with hysteresis, in a process model and quantify its advantages over a conventional adsorbent. Based on the experimental uptake data, isotherm models for adsorption and desorption were developed, and their parameters were estimated. The resulting isotherm models are incorporated into a rigorous dynamic model of partial differential algebraic equations (PDAEs) to simulate a vacuum pressure swing adsorption (VPSA) process to evaluate the process performance. Numerical challenges to solve the PDAEs, including sigmoidal and hysteresis isotherm models, were resolved with the proposed numerical approaches. Sensitivity analysis for feed pressure and temperature was performed to identify the optimal operating strategy. A comparison with a conventional adsorbent, zeolite 13X, showed that high selectivity and sigmoidal isotherms of ELM-11 give higher productivity and CO 2 product purity exceeding 99% without rinse and purge operations as well as lower power consumption for the compressor and vacuum pump.

Modeling, parameter estimation, and uncertainty quantification for CO2 adsorption process using flexible metal–organic frameworks by Bayesian Monte Carlo methods

“…Assumption 5 can be supported by Kondo et al [35] which reported that the double exponential model is in good agreement with the time profile data of adsorption volume for ELM-11. Assumption 6 is considered reasonable based on the results of Yang et al [14] and Takakura et al, [36] where nitrogen adsorption on ELM-11 has been confirmed to be negligible We verified their study by measuring the N 2 adsorption isotherms of ELM-11 in the pressure range of 0-20 bar at temperatures of 273 and 298 K, which show that adsorption of N 2 on ELM-11 is negligible. Assumption 8 can be found in Casas et al, [37] where only the delay due to the internal pipe volume is considered.…”

“…A model for sigmoidal isotherms has been proposed by Hefti et al [20] in the form of a weighted dual-site Langmuir equation, which was employed for ELM-11 in our recent study. [36] In this model, the adsorption isotherm of ELM-11 is modeled by a weighted sum of the two isotherm equations for two different mechanisms: an isotherm n L , which occurs only when the gate is closed, and the other adsorption isotherm n U , which occurs only when the gate is open:…”

Evaluating the CO2 capture performance of a “phase-change” metal–organic framework in a pressure-vacuum swing adsorption process

“…As for other adsorbents with stepped isotherms, the step pressure increases to a higher pressure with temperature. 6,7,9,20,22 Like MIL-53(Al), 36 a prototypical “phase-change” adsorbent, N 2 does not show a stepped isotherm under the same measurement conditions as CO 2 . However, a key difference to MIL-53(Al) is that when the step occurs, F4_MIL-140A(Ce) does not show any hysteresis during desorption (Fig.…”

“…60,[62][63][64] Our process model does not currently have the ability to consider desorption hysteresis and hysteresis loop scanning, which explains why we did not include MIL-53(Al) in this comparison. However, given the recent work by Takakura et al, 20 this could present an opportunity for future work. Another factor is that the step in the CO 2 isotherm of MIL-53(Al) does not occur until very high CO 2 partial pressures at the temperatures considered in this work (6-8 bar a at 298 K), 7 which would not be achieved even under the PSA conditions investigated here.…”

“…[12][13][14][15][16][17][18][19] Until very recently, a "phase-change" MOF had not been investigated in this scenario. 20 Amine-appended MOFs with isoreticularly expanded MOF-74 structure are perhaps the most widely known class of "phasechange" MOFs for CO 2 capture. 6,[21][22][23][24] The general formula for these MOFs is M II 2 (dobpdc), where M II is a divalent metal (Mg, Mn, Fe, Co, Ni, Zn) and dobpdc 4− is 4,4′-dioxido-3,3′biphenyldicarboxylate.…”