Prediction of Metal Organic Frameworks for Vacuum Swing Absorption of Carbon Dioxide.
The paper I'll discuss in this post is this one: Prediction of MOF Performance in Vacuum Swing Adsorption Systems for Postcombustion CO2 Capture Based on Integrated Molecular Simulations, Process Optimizations, and Machine Learning Models (Thomas D. Burns,§ Kasturi Nagesh Pai,§ Sai Gokul Subraveti, Sean P. Collins, Mykhaylo Krykunov, Arvind Rajendran,* and Tom K. Woo,* Environ. Sci. Technol. 2020, 54, 7, 4536-4544.)
The extraction of molecules from very dilute streams requires in general two things, high surface area combined with a high "Kd" - "Kd" referring to the distribution constant, the ratio of the solubility of the molecule being extracted in one phase - the concentrating phase - to its solubility in the dilute phase. If the solubility in both phases is high, the extraction can still take place if a reactant in one of the phases is removed by converting it into another molecule, so that the equilibrium condition results in the continuous flow from one phase to another. In general, with some exceptions, this latter approach requires energy, since the concentration of any particular solute by extraction from one system into another requires overcoming entropy, what is generally known as the "entropy of mixing."
Along with the chemical wastes we have dumped on all future generations - as I often remark in this space - we have dumped entropy on them: It will require energy for future generations - if they can do it at all given the fact that we have also used up most resources - to clean up after the worldwide sybaritic party we've been holding for the last half a century or so.
"By the time we got to Woodstock..." (We never left; what a mess!)
A system that has the appropriate features for the removal of a very dangerous fossil fuel waste, carbon dioxide - which despite its perniciousness is still dilute (416 ppm as of this writing) - is the photosynthetic system of plants and microorganisms. As it is self replicating, it can obviously under the right conditions produce a huge surface area, and it can use this surface area to take up dilute energy, light, to create a continuous flow of carbon dioxide in two phases, either air and quasi solid leaves, or water and seaweeds and other photosynthetic organisms.
This process has been going on for billions of years. The low entropy existence of dangerous fossil fuels represents this process of equilibrium shifting taking place over a very long period: The "fossil" in "fossil fuels" is all carbon that has been captured from a dilute stream, in air and water over many hundreds of millions of years. We have released the majority of this carbon in roughly a century, with enormous consequences for life on Earth. ("By the time we got to Woodstock..." What a party!)
It is obvious that biomass, even without fossilization, can be used in technological settings to remove carbon dioxide from the atmosphere, via the intermediary of combustion. As practiced almost everywhere in the world, the combustion of biomass represents a huge health risk: Slightly less than half of the six to seven million air pollution deaths that take place on this planet involves the combustion of biomass.
This said, it is conceivable, although not to my knowledge widely practiced, to subject biomass to combustion cleanly, in a situation known as "oxyfuel combustion" where the combustion takes place not in air but in pure or nearly pure oxygen. Pure, or nearly pure production of oxygen is also feasibly carried out by thermochemical splitting of either carbon dioxide or water, the latter case being somewhat more familiar. The necessary energy for the thermodynamics of such splitting is available cleanly from nuclear energy. In this case, combustion of biomass in pure oxygen, highly concentrated gases rich in carbon dioxide, with - depending on conditions - some nitrogen oxides and some carbon monoxide, can be obtained. Theoretically this carbon can be captured and reduced to make value added products such as polymers, carbon fibers, carbon materials, and highly refractory, chemically resistant, and very hard metal carbides. These value added products sequester carbon dioxide, as does, to be fair, wood and fabric.
However even in concentrated streams of carbon dioxide separation of the streams is required. This is also true, more true in fact, if biomass is burned in air instead of pure oxygen.
The paper under discussion here refers to MOF's, "metal organic frameworks" which are just what they sound like, metals arranged in particular porous frames constructed with organic (carbon) molecules. The inorganic equivalents of these are well known, they are called "zeolites" and they play a huge role in chemical technology already, primarily as catalysts, but also in purification systems.
The authors here have used molecular modeling techniques and machine learning to try to identify MOF's that separate carbon dioxide with the highest energy efficiency.
From the paper's introduction:
Several technologies are being explored as energy-efficient alternatives to current solvent-based CO2 scrubbing systems. At the forefront are solid sorbent-based technologies that use porous materials within pressure and/or temperature swing adsorption (P/TSA) systems. Technoeconomic studies suggest that P/TSA technologies have the potential to substantially reduce the cost of carbon capture if the right solid sorbents can be found.(4) Metal–organic frameworks (MOFs), which are crystalline nanoporous materials that are constructed from inorganic and organic building units,(5,6) have attracted significant attention as possible sorbents. Due to the seemingly endless combination of building units that can be combined to construct them, a dizzying range of MOFs is possible such that they can potentially be tuned for any given application. Indeed, it is estimated that nearly 70 000 different MOFs have been synthesized and characterized to date.(7)
MOFs are often introduced as outstanding materials for postcombustion CO2 capture by highlighting a few targeted adsorption properties, such as high CO2 uptake capacity or CO2/N2 selectivity,(8?12) without a clear understanding of how these properties affect their performance in a real industrial P/TSA process. Large databases, some containing millions of hypothetical materials,(13) have been screened computationally via detailed atomistic simulations for their potential to be used as solid sorbents for postcombustion CO2 capture.(14?17)
If the phrase, "...industrial-scale CCS projects exist that capture and store more than a million tons of CO2 per year..." impresses you, it shouldn't. The fraction that "a million tons of carbon dioxide" represents of what we dump each year, roughly 35 billion tons, is 0.00003. All of the carbon dioxide capture and sequestration "industrial plants" on the planet are essentially useless.
Carbon capture involves "parasitic energy" losses which the authors call "PE." The authors note that the US DOE, a standard set presumably before the organization entered into the process of being decimated by one of the most stupid leaders the world has ever seen, has set a standard for the recovery of between 90% to 95% of the carbon dioxide in a gas stream.
The authors further write:
...In this work, we have screened 1632 MOFs and related materials with an advanced PSA process simulator that has been experimentally validated at the pilot scale using a four-step light-product pressurization (LPP)(34) cycle shown in Figure 1. This cycle has been shown to be the most energy-efficient cycle for postcombustion CO2 capture.(34) While a costing algorithm could be used as the objective function, there are large uncertainties in the costing of MOFs to estimate the capital expenditures. Hence, the goal here was to obtain high-reliability results at a scale where our models have been validated. Therefore, the process conditions have been optimized for each material to minimize the PE or to maximize the productivity while meeting the 95/90 purity–recovery targets (PRTs). The productivity of a material or how much CO2 the sorbent can extract per unit volume of the material per unit time is not only important for determining how much material is required for CO2 capture but also vital for determining the complexity of the VSA system required and more suitable yet essential considerations such as the capacity of the vacuum pumps that are required.
Here is a cartoon showing the outline of how a VSA (Vacuum Swing Absorption) system works. (It is similar to "Pressure Swing Absorption" systems which produce commercial streams of concentrated (nearly pure) nitrogen or oxygen (for portable oxygen supplies for medical use).
The caption:
The authors screen 1584 MOF's after eliminating those that required the use of rare or toxic metals or could not be industrialized by other means.
This graphic shows some results of their screening efforts.
The caption:
This table also refers to some results:
Another figure showing results for selectivity:
The caption:
Note that the scale on the abscissa is logarithmic, a value of "3" is 1000 times more selective than a value of "1".
And so on...
Excerpts from the conclusion:
...Advanced process simulations of a four-step VSA system, which have been validated at the pilot scale, have been integrated with atomistic simulations allowing for the screening of 1632 experimentally characterized MOFs for postcombustion CO2 capture. A total of 392 MOFs were found to meet the 95/90 purity–recovery targets, while a dozen materials, including IISERP-MOF2, UTSA-16 and zeolite NaA, were able to simultaneously achieve PEs <250 kWhe/MT CO2 and productivities greater than those of zeolite-13X. Although we have assumed a dry flue gas, a handful of MOFs have been reported, whose CO2 adsorption properties are nearly unchanged in high-humidity conditions.(19,55) In those cases, the results of these simulations remain applicable. For MOFs whose adsorption properties are substantially changed in humid conditions, the flue gas stream can be dried at an energetic cost estimated to be as low as 24 kWhe/MT CO2.(40) In total, 97 MOFs were found to have PEs <250 kWhe/MT CO2, which makes them highly competitive with advanced solvent-based scrubbing systems even when the cost of drying the flue gas is included...
These sorts of papers suggest the world can be saved. This is not to say that it will be easy or cheap and that it will not take extreme effort, particularly in a time of rising stupidity, but if we have left all future generations a physiochemical environmental disaster, at least we have left them with something, that something being insight.
I trust you will do your best to enjoy all that can be enjoyed under the circumstances in the coming days.
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