Design of Gas Separation Processes Using Type II Porous Liquids as Physical Solvents

Abstract

Solvent-based absorption systems are frequently used for industrial-scale CO2 capture from point sources. Physical and chemical solvents are well understood; however, there are drawbacks such as low gas capacity, high regeneration energy, and large operating units. Porous liquids (liquids with intrinsic microporosity) present an opportunity to improve the effectiveness of solvent-based separation processes, especially physisorption-based systems. Type II porous liquids are developed by dissolving a discrete, microporous material in a sterically hindered solvent that cannot penetrate the pores of the porous material. These liquids exhibit dual-mode sorption (i.e., Henry + Langmuir) and show excellent potential for gas separations. In this work, a first-order high-pressure CO2/CH4 separation process is modeled using a porous liquid comprised of a discrete porous material dissolved in a sterically hindered solvent. Gas–liquid phase equilibria of the porous liquids are modeled using gas-cage equilibrium data. A McCabe–Thiele approach is employed to estimate the amount of solvent and size of the absorption tower needed compared to an industrial solvent, revealing that porous liquids can decrease the capital needed to adequately separate a gaseous mixture. The energy requirements for a variety of regeneration scenarios are calculated, highlighting that porous liquids can lower the energy burden necessary for gas separation processes. The porous liquid shows the potential to significantly reduce the solvent consumption and size of operating units for this gas separation, with the potential to lower the overall cost and energy required to capture CO2.

Design Principles Guiding Solvent Size Selection in ZIF-Based Type 3 Porous Liquids for Permanent Porosity

“…From density measurements of the PLs, the partial molar volumes of ZIF−8 were calculated, Figure 5b, and are useful as a simple comparison method. 56 The results showed that the volume contribution of ZIF−8 in solvents possessing a d W greater than 5.4 Å is similar to that of bulk ZIF−8 and decreased less than 15%, Figure S23. This further confirms that these solvents are excluded from the pores of the framework, as the volume contribution of ZIF−8 is unchanged in the PLs compared to the bulk solid.…”

“…Partial molar volume: Partial molar volumes of formed PLs were calculated following previously reported methods. 56 In general, the measured density (g/mL) of the formed dispersions was plotted as a function of the ZIF concentration (g/mL). The data was fitted linearly with a fixed slope, and the slope and y-intercept of the fitted line were used to calculate the partial molar volume of the ZIFs.…”

“…Recent work has shown that density analysis can be used as a simple method to evaluate the availability of host porosity within PL systems. 31,56 The density of ZIF−8 (0.943 g/mL), which was determined using crystallographic data following previously reported methods, 31,56 is smaller than the densities of the many typical solvents. Therefore, the relative density of a ZIF−8 PL is expected to decrease with increasing ZIF loading as the solvent molecules are too large to be adsorbed.…”

“…Through modification of previously reported methods, 30,31,56 the densities of ZIF samples were measured while dispersed in various solvents at concentrations between 0 and 20 wt % ZIF. Lower-loading ZIF dispersions were made through serial dilution of the higher-concentration dispersions.…”

Impact of Vertex Functionalization on Flexibility of Porous Organic Cages

“…20 Additionally, the CC13 POC was a component of one of the first reported PLs in 2015 6 and has been subsequently used in preparation of Type 2 PLs. 21 Altering the POC in the PLs has resulted in size and shape selectivity for gas capture along with substantial increase in gas adsorption capacity. 7,22,23 For example, a Type 2 PL formed from an imine-based POC and a 15-crown-5 ether had an 8-fold increase in CH 4 adsorption over the neat liquid, 6 while the use of scrambled POCs in Type 2 PL doubled the amount of CH 4 adsorbed per POC.…”