Energy intensive cryogenic distillation is the standard practice at petrochemical facilities which deliver thousands of consumer products – from food additives to plastics and packaging. New techniques involving molecular sieving through membranes are being studied where the differential in molecule size can be exploited by porous adsorbents in membranes. One important distillation-based petrochemical gas separation is olefin and paraffin separation, which is particularly challenging due to the relatively close boiling points and kinetic diameters between these molecules: And of these, the separaton of propylene from propane is the hardest to solve.
Channel geometry (left) and scan electronic microscope image (right) of layered AYRSORB™ F250. Image Credit: framergy, Inc.
To reduce the need for cryogenic distillation and accomplish less energy-intensive separation with a solid sorbent, framergy examined metal organic frameworks (often referred to as MOFs). MOFs are crystalline 3D structures with an extraordinarily high internal surface areas and unique, tailorable chemical properties. Physical analysis suggests that AYRSORB™ F250, which is the trademark for MOFs known as PCN-250 and MIL-127, is geometrically promising as a propylene/propane separator. The distinctive hierarchically micro- and mesopores of AYRSORB™ F250 in a single crystal presents an advantage over the diffusion-limited MOFs, since the mesopores of the MOF function as channels, which enhance the diffusion of molecules.
framergy investigated the utilization of AYRSORB™ F250 in selective propylene/propane separations by first testing the single component gas uptake isotherms. These two results were used to calculate gas selectivity by using the ideal adsorbed solution theory (IAST), a thermodynamic model which predicts mixed-gas adsorption. The most remarkable difference between propylene and propane is the presence of carbon to carbon double bond (π bond) of propylene which enables better interaction with aromatic linkers of unique MOF structures and thus increasing the isosteric heat of adsorption. The difference in isosteric heat of adsorption between the targeted olefin/paraffin pair suggests selective separation.
Propylene and propane weight uptake results at 1.2 bar pressure range was over 25 wt.% and 22 wt.%, respectively. This far exceeds common adsorbents such as zeolites and activated carbons. The results were compiled by integrating the partial pressure isotherm data for both propylene and propane gases at room temperature. Following, the experimental data was modelled using Langmuir and Freundlich analytical isotherm equations and as a result, the IAST selectivity values were identified. The results showed that selective propylene/propane separation can be achieved with AYRSORB™ F250 based on extraordinarily high IAST selectivity values of over 12.
Unfortunately, finding the right MOF for the particular gas separation is only a part of the problem. One has to address the platform for the application, and understand if the MOF will be stable enough for industrial use. MOF crystals themselves cannot be constructed as membranes as they lack mechanical strength to overcome pressure differences across the membrane. Therefore, they are dispersed in porous polymers in the form of mixed matrix membranes (MMM). A series of MOF-based MMMs have displayed superior separation efficiency over MMMs with traditional fillers (e.g. zeolite and metal oxides), yet to meet industrial requirement, chemical stability and higher performance must be achieved. MOF-based MMMs’ poor permeability is due to the presence of polymer components and the interfacial defects created by inadequate adhesion between the polymer and MOF.
To address this problem, framergy validated a MOF-based membrane configuration based on a continuous layer of MOF crystals grown on microporous inorganic substrates such as alumina, silica, or porous titanite. Adhering AYRSORB™ F250 onto a porous substrate as a continuous, functional layer can eliminate the interfacial defects. Production of continuous layer membranes has only been achieved for a few MOFs, but chemical and thermal stability related requirements still apply. In fact, framergy’s early bench-top tests dating back to 2015 have already identified the preferential interaction between inorganic substrates and AYRSORB™ F250 during synthesis. When fused silica substrates were immersed into a solvothermal reactor, under specific seeding conditions, AYRSORB™ F250 was continuously grown on the substrate surface.
Bench scale to pilot scale testing of petrochemical separations is a specialty at framergy, where its lab and experience are unparalleled for industrial scale up of MOF technology. Considering that AYRSORB™ F250 is already known as the most stable MOF, framergy’s work on testing propylene/propane suggests that the MOF as a layered membrane would be an ideal medium for olefin and paraffin separations in general. AYRSORB™ F250 is available for research purposes through Strem Chemicals and pilot demonstrations are pending in 2021.
This information has been sourced, reviewed and adapted from materials provided by framergy, Inc. For more information on this source, please visit framergy, Inc.