Dr. Darren Broom of Hiden Isochema, (United Kingdom), presenting his talk “Challenges in characterizing adsorbents for gas storage and separation,” on February 21st, 2024 (IAS Webinar 5.1).
Some resources that were mentioned during the talk that may be useful for viewers:
https://link.springer.com/article/10.1007/s10450-023-00424-9
https://hidenisochema.com/content/uploads/2021/08/Hiden-Isochema-white-paper-Characterizing-adsorbents-for-gas-separations-0821.pdf
https://pubs.acs.org/doi/full/10.1021/ie051056a
https://pubs.acs.org/doi/abs/10.1021/acs.iecr.9b04243
Welcome to the first presentation in the fifth webinar series presented by the international absorption Society the IAS is an international organization dedicated to advancing absorption as a solution to Scientific engineering and human welfare challenges through the promotion of absorption research and education we thank everyone for attending the webinar today the webinar
Series has been an immense success and the recordings of previous webinars continue to be viewed on YouTube and bilb this is the first webinar of our fifth series which we’ll continue to have monthly throughout the year we intend to continue with a variety of speakers from industry Academia and
Other research institutes as well as PhD students in early career researchers announcements regarding the fifth series will be distributed through the IAS mailing list and the IAS Twitter feed today’s webinar will be given by Dr Darren brw at hien isima in wton in the United Kingdom I am bobes mapen at the
University of Alberta in Canada today’s webinar will be moderated by David daachi at Imperial College London in the United Kingdom we are required to remind attendees and future viewers that the views expressed by the speaker host or other moderators are not necessarily those of the IAS or the institutions associated with those IND
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Help us expand our YouTube channel by liking this video And subscribe I’ll now hand it over to David who will moderate the YouTube Q&A thank you bash uh and thank you uh to to Darren or Dr Darren broom for joining us for today’s webinar session uh Darren broom is currently a
Product manager for heyen isoka in the UK he received a PhD in materials physics from the University of sford also in the UK in 2003 and was a post-doctoral research fellow at the European commission’s Institute for energy in the Netherlands from 2004 to 2006 7 where he was working on hydrogen storage
Materials in 2007 he returned to the UK to join hyen isoka in 2011 Springer published his book on the characterization of hydrogen storage materials and since 2015 he has been a UK representative on the international en energy agency hydrogen technology collaboration program tasks 32 and 40 on hydrogen based energy
Storage more recently he has been focusing on characterizing absorbance for gas separations including the measurement of binary gas adsorption uh during the webinar questions can be submitted to the speaker as comments on the YouTube live stream and um I will moderate those and and present them to Darren at the end of the
Presentation uh with that I welcome Dr Darren broom again and um pass over the presentation to him should I share my screen uh yes D go can you see that okay and hear me yeah we able to see that we can hear you as well okay great thank you for the
Introduction David um so today I’m going to be talking to you about challenges in characterizing absorbance for gas storage and separation this is actually an accompanying talk to a um article I published at the end of last year um in the journal absorption in a special issue in honor of um Dr Oran
Tallow so I’m going to follow the basic structure of the um the article I’m actually cutting out one of the um sections but I’ll begin with an introduction I’ll then talk about the requirements for absorbance of gas separations and for gas storage I’ll then run through some measurement
Methods um focusing on multicomponent um gas absorption equilibria I’ll then talk about challenges for high pressure gas absorption and for um multicomponent gas absorption I’ll discuss some um issues around absorption capacity definitions and then finish with a conclusion so um porous absorbance have been used for many years um for
Separating gases um PSA that’s pressure swing absorption was actually first commercialized back in the 1960s these um applications um commercially use absorbance such as activated carbons or carbon molecular cves in the case of kinetic separations um as well as zeolites and silica gels um but more recently there
Have been a number of new materials reported um that have been investigated quite widely for um gas storage and separation these include moths and coughs but also different types of um porous organic polymer now I’ve got some examples sep operations here I won’t go into too much
Detail but um I’ve got um the processing of industrial gases so that’s the production and purification of gases we have air separation for nitrogen and oxygen production helium production um and hydrogen purification which was actually one of the early um commercial uses for PSA then we have some energy and
Environmental separations that can include biogas purification natural gas upgrading so the removal of contaminants from that natural gas and then a big area at the moment is carbon dioxide capture um from various gas streams but porous absorbance can also be used for um storing gases two of the
Most um investigated um gases are the storage of hydrogen and methane and that’s um due to their use um as energy carriers or fuels in transportation the use of absorption potentially allows um greater storage capacity of the gas um at higher temperatures than liquid storage um which requires quite low
Temperatures particularly in the case of hydrogen but also at lower pressures than Compressed Gas storage um in the case of hydrogen this can be used as a energy carrier in hydrogen fuel cell cars and in the case of methane or natural gas that can be used um in internal combustion
Engines now a crucial step um in developing this technology is to characterize the absorbance that’s for both um assessing materials for the applications and for process design various um types of information are required from an absorption perspective that includes working capacity and isotherm shape um the selectivity for gas
Separations um enthalpies or Heats of absorption um but also the resorption kinetics so in this talk I want to cover these requirements um and the methods used to determine such data I’ll focus on multicomponent um equilibria and then I’ll also look at the challenges associated with determining both high pressure absorption and multicomponent
Gas absorption equilibria and as I said at the start this is a presentation associated with this um article published near the end of last year in um absorption so beginning with gas separations um various performance parameters are required I actually covered these in a white paper published by Hy nisaka I think in
2021 um these include the working capacity and isotherm shape selectivity Heats or enthalpies of absorption absorption kinetics all of these um well sorry the first three of those um can be determined by measuring gas absorption equilibria the kinetics is almost a um a separate point so in terms of working capacity I
Have a um schematic over on the right hand side here um showing two isotherms measured at different temperatures the working capacity is the reversible capacity of the absorbent between absorption and regeneration conditions so over on the right hand side here I’m showing pressure swing uh between an upper pressure and a lower pressure
Pressure but also the working capacity for our for temperature swing absorption between the two temperatures of the absorption isotherms most important Point here is that that the working capacity for um separations is more important than the total uptake working capacity can also be affected by isotherm
Shape um if you have a steeper rise in this kind of type one or um favorable isoform um then you actually reduce the working capacity so a more linear isotherm can provide a more um a larger working capacity um assuming equal um saturation capacities now working capacity is
Important for separations but um also um selectivity is crucial too this can be equilibrium selectivity or kinetic SE selectivity these are defined um in these ways so the um equilibrium selectivity is the ratio of the ratio of the two um components um absorbed and the partial pressure of those components in the gas
Phase while the kinetic selectivity um can be calculated using this expression um so it’s a ratio of the um of the diffusivities of the two gases so that’s kinetic selectivity is basically used when um gases um diffuse at significantly different rates through the pores of the
Material now moving on to the heat of absorption or the enthalpy of absorption this defines the um interaction strength um between the absorbate and the absorbent to determine this experimentally we can measure isotherms at two or more closely space temperatures it’s it’s ideal to it’s
Better to use more than two but two has um become um fairly standard unfortunately and in this case we apply the acais claperon type relation which is given by this expression here where Qi is the isosteric heat of absorption and R is the universal gas constant p
And t of course are pressure and temperature so we plot in this case the natural logs of the pressure for points of equal uptake that’s isosteric points um against um one over T the inverse temperature and then the gradient gives us the isosteric heat of absorption in terms of absorption
Kinetics this determines the rate at which absorption occurs this is actually critical alongside equilibrium information in Practical terms if the kinetics are too sluggish uh that could prevent the use of a certain absorbent in uh um an equilibrium um separation different gases tend to diffuse at um different rates that’s why
We get a kinetic selectivity in some cases the absorption kinetics can be affected by various factors it’s actually quite complicated this can include micropore diffusion um so that’s um fian diffusion or diffusion that follows fix law uh there are surface barriers those are barriers to the entry of molecules into the um surface
Particles can be affected by nonisothermal condition um but also macropore diffusion here macropores actually refers to um anything in the misore range in the IUPAC classification and and higher so now I’ll move on to requirements for gas storage um as I’ve said before gas absorption um in Porous absorbance can
Potentially allow a greater storage density at a higher temperature than required for liquid storage and at a lower pressure uh than required for Compressed Gas storage if we can increase the temperature um versus liquid storage that reduces the energy required for liquefaction um which can be quite High particularly for
Hydrogen in the case of pressure um we reduce the energy required to compress the gas um before it’s um put into the uh storage container um a critical practical Point here is that both High Press gas storage tanks and low temperature liquid storage tanks are are expensive gas storage tanks for hydrogen
Typically go up to um 700 bar they’ve got multiple layers and they use um quite expensive carbon fiber um for strength so they’re not particularly cheap so um gas storage tanks at lower pressures can be made much cheaper and in case in the case of liquid storage tanks you have layers of insulation
Which again is not um cheap so basically that allows us to reduce the storage tank cost ultimately the aim um with any of these cases is to provide the um sufficient amount of gas storage capacity uh to provide a long driving range for any um vehicle um but also at an affordable
Cost so hydrogen storage using porous absorbance as attracted probably the most attention um unfortunately uh molecular hydrogen interacts weakly um with Solid Surfaces or poor walls I’m talking here about phys physisorption or physical absorption um so we really need to use low temperatures and in terms of characterization this is typically 77
Kelvin um due to the convenience of using liquid nitrogen as a cryogen storage at ambient temperature is the ideal um and there’s been a lot of attempts to um to find porous absorbance that will absorb significant amounts of hydrogen at ambient temperature um but this has had limited success so
Far so to characterize absorbance um we’ll tend to determine isotherms at 77 Kelvin um but also ambient temperature a recent trend has also been to look at temperature swings this can increase the deliverable amount of hydrogen so you’re actually using a temperature increase to drive hydrogen
Um from the the um the the absorbent bed um a typical range has become for some reason 77 Kelvin to 160 Kelvin um and in that case we also require hydrogen absorption isms at variable low temperatures to calculate this deliverable um hydrogen capacity um but actually studies looking at variable low
Temperature ads or hydrogen absorption are fairly scarce moving on to Natural Gas storage the aim of research here is to reduce the pressure required to um store a sufficient amount of gas um in compressed natural gas vehicles which are um actually quite widely used there’s millions of vehicles
In use um worldwide they typically use 200 bar um which gives a sufficient um amount of capacity um so we’re aiming to lower the lower the pressure um which will reduce um the cost of compression now natural gas mostly consists of methane so in terms of um natural gas storage studies we
Tend to assess materials using um methane absorption isotherms now methane absorbs more strongly than hydrogen um we don’t need to go to the low temperatures required for hydrogen um so typical studies look at um isotherms at ambient temperature so around 300 Kelvin and the focus in the case of
Natural gas storage tends to be on volumetric capacity which is different to hydrogen there’s been a traditional focus on gravimetric capacity um in the case of hydrogen even though volumetric capacity is also very important so as for separations working capacity is is the most um important factor here not not total uptake the
Working capacity here is between the charge and discharge discharge pressure of the store in the case of hydrogen storage this is typically between 100 bar um and the lower pressure of three to eight bar in in quite a few recent cases I’ve seen five bar being used um but I think it
Depends on the exact um setup the important point to note here in terms of studying hydrogen storage is the uptake um at one bar is mostly irrelevant actually if you go back maybe um 20 years you’ll find quite a few papers um on hydrogen storage that actually measure isotherms only up to
One bar and in fact you actually want a low uptake at one bar because that will increase your working capacity for both hydrogen storage and methane storage um both gravimetric and volumetric capacities are important despite the traditional focus on gravimetric capacities in the hydrogen case um so in this case we want to
Determine both gravimetric and volumetric isotherms just two more points um I want to note here the first is that natural gas is actually a mixture it also contains other hydrocarbons such as ethane and smaller amounts of C3 to C7 alanes and gas species such as nitrogen and carbon
Dioxide so although we measure methane as an initial way of characterizing a material real world testing must include these other components the main practical um outcome of this is that the more strongly absorbing hydrocarbons in natural gas can lead to capacity loss with cycling because those hydrocarbons will absorb
And will actually stay in the material um unless they’re really sort of driven off um using higher temperatures or some something like that so we get a capacity loss of the absorbent with cycling in the um case of pure of of proper real natural gas and then also absorption kinetics
For um gas storage are are still important um the um in the case of hydrogen hydrogen generally absorbs very rapidly it’s quite often too quick to actually measure um methane is sort of similar but can be can be slower so the kinetics are generally not a not a
Problem but but but they still do need to be um looked at in the case of methane um the large enthalpy or heat of absorption can lead to large temper temperature increases during charging so um rapid charging maybe heat transfer limited so the crucial Point here is that you have a number of
Different stages you want to rap rapidly charge a store when you’re refueling your vehicle you then want to slowly discharge as the vehicle is being um um being used um and so these different speeds um mean that different Pro properties are important at different stages and it’s the the large
Temperature increase um during charging that’s that’s the main point here so now I’ll move on to the measurement methods um I’ll start with single component manometric and gravimetric measurements um but then look at a series of M of multicomponent measurements starting with closed volumetric systems then move on to open volumetric or um
Breakthrough or dynamic column breakthrough systems um method quite often called total desorption and then combined volumetric gravimetric systems and the new integral mass balance method that was introduced fairly recently it’s interesting to note that some of these multicomponent methods have have been using for a long time since at least the 1920
I think that’s mainly the closed volumetric approach and total desorption the apparatus used in those days though was um mostly sort of elaborate glassware the more modern methods um or modern versions date back more to the 1980s and 1990s so beginning with gravimetric technique in this case we use a
Microbalance which is housed in a vacuum and pressure compatible chamber have a a schematic over on the left hand side where we have a sample suspended from the balance arm with counter weight on the other side we have a pressure measurement and then a way of introducing gas and evacuating the
System so the measurement begins under vacuum so we Degas the sample we then apply a pressure to the microbalance chamber and then we monitor the weight change as a result of absorption by the sample and then the buoyancy corrected value of the um sample weight at equilibrium is used to determine the
Amount of gas um absorbed um this process is then repeated until a full isotherm has been measured in contrast with the ma the manometric technique uses the real gas law uh to perform molar balance calculations in an instrument of known internal volume so I have another schematic over
On the left hand side of a calibrated dosing volume V1 a sample cell containing the sample V2 um with a valve separating the two we first Evacuate the instrument again we’re degassing the sample in that case and then we hold the volume V2 the sample cell under
Vacuum we pressurize volume V1 um to an ini ini initial pressure and then dose gas to the sample by open opening the separating valve between those two volumes and then we measure the final pressure in order to calculate the amount of gas absorbed then to measure an entire isotherm we
Repeat these that um the the pressurization and dosing process calculating a molar amount absorbed at each stage um and we sum those up through the course of an ISO um to measure the total amount absorbed at the highest pressure both of these techniques are um good for measuring Pure Gas absorption
But when we’re looking at multicomponent um absorption um the mixtures involved require an additional method to determine the composition of the gas and the absorbed phase there are some methods that can be used to study gas mixture absorption um but in some cases it’s not possible to check results for
Thermodynamic consistency this is a point I wanted to make um because for example um an article um published in 2022 um by by Danny shade Crystal Walton David Shaw um covered 18 different techniques for measuring mixed gas absorption but several of these produce results that cannot be tested um for
Thermodynamic consistency so it’s a subset of of methods that that can be tested for thermodynamic consistency um the importance of this was emphasized by cirar in 2006 when he wrote it is essential that pure and B binary gas absorption equilibrium data used for absorptive process design obey these consistency tests for Reliable
Multicomponent data extrapolation and interpolation I also want to mention that some of the very early multicomponent measurements going back maybe to the 1920s and 1930s were later looked at in terms of thermodynamic consist consistency test um and it was found that they were unlikely to be measured either under
Equilibrium um or that they contained too few data points to actually test the thermodynamic consistency so there the thermodynamic consistency tests are also important um to confirm the reliability of data whether you’re using that for process design or just for materials assessment so I’ll now run through um each of the main multicomponent
Measurement methods so we have closed volumetric systems I have a schematic over on the right hand side it has two calibrated volumes V1 and V2 an absorbent column and then a circulation pump for circulating um the gas mixture through the bed note that a system like this
Requires lots of well a number of Separation valves isolation valves um which are removed from the um diagram just for clarity so a mixture of known composition from the two molar quantities um put into V1 and V2 is circulated through the bed once equilibrium has been achieved
We use a material balance of this form so it’s from the um the internal volume of the system and and the initial pressure at the charge pressure and the um that same volume but also the equilibrium pressure after it’s achieved equilibrium we can calculate the the amount of each um component absorbed
Here the V is um includes the two um calibrated volumes and the um gas phase volume in the absorbent column this method um can be quite accurate um but it’s laborious to perform and it had there’s no real control over the final state by that I
Mean that you you pressurize it um to an initial pressure and then the the absorption process leads to a pressure drop which you require for the measurement um to occur both the gas phase composition and the pressure will change during that process and and that you have very little control over of
Course you can make calculations back calculate it um but it does mean there’s a certain Randomness to the data that you obtain from this technique then we have open volumetric systems these are also called Dynamic column breakthrough systems we have another schematic over on the right hand side
Again it’s simplified but we can flow a gas mixture into a column um the back pressure is regulated for measurements at non-ambient pressure and then the MFM is a mass flow meter for measuring the total Outlet flow rate then we can also analyze the um gas composition
Position so we pass a mixture of a known composition through the bed basically measuring a breakthrough curve and then we can integrate the um Inlet and Outlet molar quantities um using this integral in the in the the first term here and then correct it for the accumulation in the
Column so here f is the molar flow rate in and out of the column and Yi is the molar fraction of compon component I this is less laborious than the closed volumetric um technique and we have control over the final State because you’re controlling the gas phase composition using the mass flow
Controllers um but it tends to be less accurate due to the need to measure the total Outlet flow rate which is difficult to measure accurately then we have um a technique called to desorption sometimes called flow desorption again we have a schematic over on the right hand side we have um
Very similar setup to the open volumetric system but we have a separate collection volume that the contents of the absorbent um bed the the absorbed um molecules can be um desorbed into so we flow a mixture of known composition through the bed until equilibrium has been achieved
We then seal off the column um and then desorb it using a um a temperature increase into the collection volume once um the all of the contents of the absorbent column has been desorbed into the collection volume the um the contents of the collection volume are analyzed in terms of the gas
Composition and the pressure um and that allows us to calculate the equilibrium amount um absorbed this method is accurate um but it’s even more laborious than closed system measurements there’s multiple stages to um to the measurement of each data point um these systems I think tend to be manually
Operated think they’ve be quite difficult to automate and note that the collection volume is typically cooled um so you quite often see in the literature that the um collection volume is called using liquid nitrogen to to um to help colle collect all the all the gas um so
Those are the multiple stages involved in these measurement types then we have closed volumetric gravimetric measurements so again this is just a schematic we have a very similar setup to the closed volumetric system um but we have a m microbalance chamber in which the sample is held so
That you can also measure sample weight so we circulate a known gas mixture prepared in V1 and V2 through the microbalance chamber um and then there are two main options for making the measurements um if the molar masses of a binary mixture differ sufficiently the amount absorbed can be calculated by solving
Simultaneous mass balance equations that’s not possible if the um if the molar masses are very similar um in the mixture or alternatively we can analyze the the um final gas composition um to determine the amount absorbed from the composition um change that’s um sometimes or quite often actually called gravimetric chromatographic um
Measurement um but both of these approaches have limitations I’ll say a little bit more about the the challenges involved in each of these methods um in a later section and then we have the IMB method which um we introduced in 2020 this is essentially similar to an open
Volumetric system but we measure the sample weight um in situ so again we have a schematic here we’ve got control over the gas mixture gas mixture can flow into the microbalance chamber this has to be equipped with a a special reactor in this case we control the back
Pressure um and analyze the composition of the gas mixture on the outlet so a known gas mixture is passed up through the bed until we achieve equilibrium in the same way as the open volumetric methods um and then we measure the sample weight change as a function of
Time DW by DT um in situ using the microbalance The crucial Point here is that eliminates the need to measure the total um Outlet flow rate this um seems to be fairly accurate and also allows multi-point measurements which is quite crucial in terms of measurement time just want to say a bit about
Kinetic measurements although I’m not going to expand on the kinetic side um much um later basically with Kinetic measurements the situation is rather complicated you have a a range of different methods that can be um categorized as either microscopic or macroscopic the case of microscopic techniques that includes quens or quasi
Elastic Neutron scattering and also Pulse Field gradient NMR spectroscopy whereas macroscopic techniques include um the use of manometric or gravimetric um instruments as I um introduced earlier but also other specialist techniques such as um frequency response but these different techniques measure absorption and or diffusion over different length
Scales um actually quen and NMR spectroscopy measure diffusion over over different length scales themselves quen looks at a um a really quite um the the sort of nanometer scale sort of over the course of a a unit cells or something whereas NMR um looks slightly longer length scales um but macroscopic
Techniques goes up to the um the um sort of the particle size or or the in in fact the entire bed um but they also look at um diffusion under different conditions so microscopic techniques tend to measure diffusion under under equilibrium so the gas is still
Diffusing um on through the um the poor Network um but um but there isn’t there are there aren’t transient conditions um whereas macroscopic techniques um tend to measure diffusion under these transient condition conditions so we apply a step change in pressure for example and and then look at the um the
Kinetics as a fun function of time so with macroscopic Kinetic measurements which are um more common um most of these macroscopic techniques involve perturbing the system in some in some way then we’re basically just measuring the response to that perturbation in order to determine the absorption or desorption
Kinetics so when we using the gravimetric technique we change the pressure we ramp the pressure up in the microbalance chamber and then we measure the weight response um to look at Kinetics um with a manometric instrument we change the pressure dosing pressure and then we measure the pressure response that gives us kinetic
Information um when it comes to frequency response methods we change the volume um or the flow rate cyclically and measure the pressure or flow rate response so in in terms of changing volume we change the volume that leads to a pressure change um which is affected by the the
Kinetics and each CA in each case this the response um provides information um regarding the kinetics of the system we quite often have to um fit data to models in order to extract um information such as diffusion coefficients from this data so now I’ll move on to high pressure um absorption challenges
Um low pressure absorption measurements are very common so the classic example is nitrogen absorption at 77 Kelvin to determine bet area po size distribution Poe volume and so forth um but for absorption applications we quite often require higher pressure measurements in the case of storage um
We might be um looking at pressures of up to 100 bar um but in terms of gas separation most will um involve some kind of over pressure I know there are um separations particularly with natural gas that can go up to tens of bars um but even sort of air separation or
Something you might be looking at Seven bar and so forth now you have to take a lot of care with low pressure measurements because um you can get errors um in measurements from various um various factors um but when we go to higher um pressures the difficulties increase
Significantly so looking at the two techniques now in the case of the manometric technique um we’re summing molar quantities through the course of an absorption isotherm measurement um and this can lead to large overall measure um errors from small errors in some of the measured parameters I’ve got an
Expression here which um the details don’t really matter um but it’s basically um exess expressing the number of moles absorbed at absorption isostep isotherm step M from three real gas terms so you have pressure um volume um the compressibility of the gas um temperature but also the the density of
The solid and the the um the volume of the sample cell and eror errors in any of these parameters can result result in large errors in the calculated molar amount absorbed so essentially if you if you imagine you have a dens your the um density of your solid in this expression
Um has a um a reasonable error on it um that will create quite a large error in the calculated number of moles in the gravimetric technique you don’t get that same accumulation of of um error um but that doesn’t mean there aren’t um problems the main source of
Error is from the buoyancy correction so I mentioned that briefly during the my original description of the gravimetric technique got a simpler expression here um this is excess Mass absorb this is the experimentally measured absorb um experimentally measured Mass this is the mass of the solid under um vacuum and
Then this is the buoyancy correction here um on the right hand side so we can have errors in the solid density row solid could have errors in the temperature and pressure measurements used um to calculate the gas density or um problems with the equation of State used to calculate that
Gas density um and those can affect the um final calculation this is Amplified by the high gas densities at high pressure so basically as you go to high high pressures your gas density here increas es that increases the size of the buoyancy correction and that’s the origin of the of of the difficulties
That can occur at um High pressures now as a result of some of these difficulties um or certainly part part of the problem um has caused reproducibility problems in the past so hydrogen storage material research is a um a good example U myself and my co-author Michael hersher summarized um
Some of the cases in this 2016 article um we were motivated to write this by continuing problems in the literature um but there was also no clear overview of of the story really so newcomers uh must work quite hard um to untangle the story from um the um
Original Publications so there are a number of examples including different carbon Nano structures boron nitride nanot tubes conducting polymers uh moths and um hydrogen storage by spillover all affected um and some of the studies we cover actually uh containing irreproducible data are still being um cited regularly so you have to be quite
Careful when looking at hydrogen storage data now I didn’t show this data in the um in the recent absorption paper um but in 2009 14 European Laboratories measured hydrogen absorption by a carbon molecular Civ um and found somewhat surprising results on the left hand side you have the 77 Kelvin measurements and
On the right hand side you have the ambient temperature measurements um so you can see at the at the lower pressures there was um probably reasonable agreement but by the time you’ve got to about 10 bar or one megap pascal there’s a really big spread in
The data um and in fact on the um the ambient temperature measurements are to some extent worse because of these two data sets here sorry I keep um moving that on yeah so we we have two data sets that are clear outliers and then once you get to
The higher pressures although there’s a cluster of of data sets that are roughly in agreement um there’s still quite a wide spread in data 10 years later in 2019 a uh another inter liberat exercise was published this showed better agreement so C ly at ambient temperature and low low
Pressures so these are the plots over on the on the left hand side for ambient temperature um but the 77 Kelvin data um still showed um problems once you get above say about 20 bar in this case the gray line for sample one in the top plot is a known
Outlier that was due to a dead volume calibration error um in that case and then the light blue data reported an unusually low sample density so it looks like they were they were outliers with known problems um but there’s still quite a spread in the data now very recently um this year a
New interlaboratory exercise or the results of an interlaboratory exercise were published and this was performed on hydrogen absorption in the metal organic framework Ze 8 and this showed much better high pressure agreement um on the left hand side we have 77 Kelvin data up to 100 bar and on
The right hand side the data up to 20 bar still note I’m not sure how clear it is but there are were um a couple of outliers in this case that show a difference of 0.5 weight percent only about sort of 20 bar so it still wasn’t um there still wasn’t complete agreement
But this is looking a lot better than than previous um data um and I’m wondering whether it was mainly the crystallinity of the sample um that that helped here so we have a more certain idea of its density whereas the previous um data sets were for carbons now a fairly um significant
Advance in terms of other absorbates was made in 2018 and 2020 um with two interlaboratory exercises coordinated by nist um these were for carbon dioxide and methane absorption in reference zeolites um so if you’re studying these gases you can use these materials to um um to check your apparatus and
Measurement protocols but it is worth noting that it took work to get this level of agreement so the this is not sort of the original raw raw data um and in fact with the CO2 results I think it was five sample sets um Were Somehow remeasured or corrected or whatever to
Get them to get better agreement um so this is very useful reference data um but you wouldn’t expect necessarily to get this level of agreement on the first try with a reference material so moving on now to multicomponent challenges in a 1998 review on binary gas absorption experiments Oran tallo stated that
Compared to Pure Gas absorption measurements binary measurements are at least an order of magnitude more complicated and time consuming the reason for this is the difficulties in determining the amount of each component absorbed um from measurements of the gas phase um composition basically it’s difficult to achieve um sufficient accuracy um when
You’re um making these types of measurements there’s also a difficulty with the need to reach equilibrium um with the different gases in a mixture thoroughly mixed throughout um the system and throughout an absorbent column um so it’s a combination of um of these factors that makes multicomponent measurements so much more challenging
And the more components you have the more difficult it becomes so Turner and qu Turner or even higher measurements um are even more difficult and it’s actually um data is quite rare um on sort of turnery and qu erary um mixtures now one one factor in this lack
Of data is the lack of commercial instruments so commercial instruments for low press pure gas absorptions so nitrogen argon and so forth have been available for um at least 50 years um for Pure Gas absorption high pressure instruments have been available since I think at least the um the early
90s um but this is not definitely not the case for multicomponent absorption measurements now Christ Walton stated in a 2019 article that um or concluded that until access to commercial multicomponent absorption systems is comparable to that of pure component instruments on the market the knowledge gap on absorption of complex mixtures is
Likely to persist this is a point I um definitely agree with um and there are a number of reasons um for this sort of lack of um commercial instruments and that’s mostly associated with um the difficulty of the measurements and I’ll go through um each of the techniques um
Now so in terms of closed volumetric systems we go back to the schematic that I showed earlier this typically requires a few grams of sample you’ve got quite a large volume in in the system um and that requires a large sample for the change in um composition um to be measurable
Um a key point is that the gas mixture must be circulated quite slowly through the system and that’s to avoid the buildup of pressure and composition gradients across the bed so you might think that you could speed up a measurement by by um pumping this the gas round very quickly but actually if
You have a tightly tightly packed bed you may end up with a a pressure gradient across that bed and that would also result in a composition gradient as well which would really mess up the measurement so you require several hours really of recirculation per per data
Point um and then in between each each measurement um the bed must um really be regenerated there was a recent example that I mentioned in the paper um um from Danny shade and Christal Walton where I think they measured S three points um in succession but that’s actually not um not a common
Approach so in each case you have like maybe a day of measurement time or something like that because you need to regenerate the absorbent um between each data point and you also have the point that I made earlier the final state of the system in terms of the total
Pressure the gas phase composition and the absorbed um phase composition is uncontrolled moving on to open volumetric systems this typically requires large samples you can find um sample sizes in the range of about 20 um to 30 grams used in the um in the literature um the final State um is
Controlled in this case and the measurement is fairly quick compared compared to the slow recirculation process in the closed volumetric systems however um high accuracy is quite difficult to achieve um and that’s because of the um the requirement to measure the total Outlet flow rate the key Point here is
That the outlet flow um composition is changing throughout a measurement so if you use a thermal mass flow meter you need to cross-calibrate with the um the gas analysis on the outlet um because the the um the thermal conductivity of the of the mixture will depend on on the
Composition um so that’s one of the reasons why um larger samples tend to be used to to get the accuracy that’s um that’s required in terms of total desorption again this requires large samples just like open volumetric systems the final state is controlled because it’s dictated by the the gas
Mixture that you’re th flowing through the the column um and in this case higher accuracy is easier to achieve um because we’re not relying on that that measurement of the total um Outlet flow rate however the key Point here is it’s very time consuming um so multiple stages are involved in the measurement
Of um each data point um the um I mentioned previously that the collection volume is typically cooled so you’ve got a you’ve got an equ you you regenerate the absorbent you um you flow the gas mixture through the absorbent column until equilibrium is achieved you then
Seal it off you then heat up the bed cool The Collection volume volum um and then finally you seal off the collection volume and then you perform the gas analysis um and it’s probably this laboriousness that’s um led to the technique falling out of favor it’s um I
Don’t think there’s any data Maybe in the last 20 years that’s been produced using this technique which is quite an interesting point I think terms of closed volumetric gravimetric systems we again require large samples due to the volumetric aspect um but the sample size is limited by the balance
Capacity in the same CA same way as the closed volumetric systems the final state is uncontrolled because you’re relying on on adsorption um by the sample to change the the pressure and the gas composition um to make the measurement itself um and because of the need to recirculate the gas it’s as time
Consuming as closed volumetric measurements um and there’s actually quite a tricky tradeoff between the need for large samples for the volumetric aspect and the The Limited balance capacity and um there’s been examples where an auxiliary volume of absorbent has been used um to hold more sample in
Order to get um to get a good measurement then we have the IMB method and this requires relatively small samples um although we still Ed 3.4 five grams to um to demonstrate the technique which is um still enough certainly to frighten moth chemists um the final State um is
Controlled and you can actually make multipoint um isotherm measurements which speeds up the measurement process considerably um but the accuracy depends on the um buoyancy correction um and the uncertainty in the measurement depends on the difference in the molar masses of the components so actually as you go to larger um
Differences um in the molar masses of of of the um of the absorbates you get a larger uncertainty on the the buoyancy correction so basically we need to make more measurements um to to to to look at the importance of of this now I want to make a note of the lack of
Um reference data for multicomponent measurements um no data is basically currently available um this would require interlaboratory exercises of the same kind as those performed by nist for single component U measurements and these may be quite challenging to implement due to the um relatively small number of groups actually working on uh multicomponent
Absorption um and really there’s been very few tests of interlaboratory agreement published in the literature um one one example is the oxygen and nitrogen binary absorption data that we published um to demonstrate the IMB method and that used Zite 5A we found good agreement using three different
Techniques on the same Zite 5A sample actually the the plot is shown on the right hand side I hope it’s clear enough but these open points are the original um data measured um by um talo and co-workers back in and published back in 1996 these are actually measured using
Two different techniques open volumetric and closed volumetric and then the the closed points um are the me um results from the IMB um method and in terms of reproducibility um there was a paper published by David scha and co-workers in um 2021 compiled a um a database of
Binary absorption data and then they used it to look at reproducibility in cases where they had enough data and these are um the plots for the comparison of CO2 and methane absorption Zite 13x at Round ambient temperature um and they found that the selectivities for CO2 varied considerably so it’s worth
Looking at the the axis over here on the on the left hand side so we had selectivities um in a um in a pressure range of going up to 13 bar that ranged from um I think these Blue Points were actually about three up to nearly 90 so um this should
Really be quite alarming I think um there’s a we really need to need to look at this um it’s not clear where that that uncertainty is is coming from you get just just to um emphasize you do get differences in selectivity as a function of composition and total pressure um but
Some of these variations if you look down at one bar um if I can use the pointer here you’ve got the N the nearly 90 value um going down to just over 20 at um ambient pressure in three different studies so this is this is really um quite poor agreement final section
Um I realize time is getting on but um I’d like to go through this um um reasonably carefully if I can um so once once you’ve measured made an absorption measurement um you need to calculate the amount of gas absorbed and in principle this is very straightforward amount gas absorbed is
Just the amount of the absorbed species in the absorbed um phase however um the Practical issue is there’s no way of actually distinguishing between absorbed and nonabsorbed um molecules um so we typically use the concept of excess absorption um to um get round this and that differs to what is known as
Absolute absorption which is the total quantity of absorbate in the absorbed um phase now these two um two um definitions of absorption excess and absolute are essentially call it low pressure So Below a bar so for liquid nitrogen um so nitrogen absorption at liquid nitrogen temperatures um the
Difference is not important but as you go to higher pressures um they can differ um considerably so I’ve got a schematic plot over on the right hand side here um that was that was used in an article um we published back in 2013 um it shows um absolute absorption
As you go up in pressure and this reaches a a saturation level um it’s monotonic as well it always increases you get no decrease in the absolute amount absorbed um but the excess initially increases but then decreases as you go to higher pressures um the excess is basically the
Um the amount um absorbed over and above the amount that would or the amount present sorry in the system over and above the amount of absorbate that would be present if there were no um gas solid interactions that’s determined by an expression like the one on the left hand
Side here so the the um absolute amount absorbed is the excess plus this gas phase term what you can see what we were attempting to show on the right hand side here is that this difference between excess and absolute um increases with pressure because this gas phase
Term increases as the gas phase density increases so we’ve just got some points here at Point a down at low pressure excess is equal to Absolute but by the time you’ve got to high pressures at Point D um the absolute is is much greater than the the
Excess we’ve got a third definition here um which is net absorption um to explain the differences between the different absorbed quantities it’s quite useful to consider the reference states that you use these are volume reference States used for the calculation there are actually three possibilities um two of
These Define the excess and absolute and then the third um was term net absorption by Gummer and Tallow in back in 2010 so the excess absorption just uses the the volume of the solid it ignores the poe volume or the absorbed phase volume the absolute absorption includes poor volume in the case of
Microporous materials but the absorbed phase volume in in the case of materials with a poor size distribution but the net absorption just uses the empty um sample container and that that actually removes all the UN uncertainty in Sample volume or poor volume or any of those uncertainties and this choice of
Reference state has a dramatic effect on the calculated calculated absorb quantities of elevated pressures so I have an example this is quite high pressure data this is going up to 500 bar um but the absolute is monotonic as shown on the on one of the earlier slides the excess
Decreases um it’s not straight As in the um in the cartoon that I showed earlier but that’s due to gas compressibility but the net um absorption um go actually goes negative um so you can see the significance of the of um being precise about these definitions now one of the difficulties is the
Differing opinions expressed in the literature on which it’s best to um use so I’ve got just some example quotes here so from 2018 cirar said the true and only experimental variable to measure the extent of absorption from a pure gas or a gas mixture is the giban surface
Excess in 2010 gar and talo when presenting net absorption said net absorption is the natural theoretical framework for quantifying micropore absorption which is separate from and different than excess absorption the natural framework for absorption on flat surfaces while Alan Myers and Peter Monson in 2014 said the choice is either
To accept the potential errors in absolute absorption associated with the poor volume estimates or the clear flaws in the Gibbs excess formalism of absorption thermodynamics um these are not the only examples but they’re quite good examples which seem to in some way be pushing towards excess net or absolute um my
Impression is this could be quite confusing for um newcomers if they were to read um through these papers there are also different definitions of absolute um absorption used by different authors so this in my view adds to the confusion so got an expression here used by canaco and
Co-workers um back in 2001 they expressed excess absorption in terms of the volume of the absorbed phase so we’ve got this expression here where where NX is the excess ABS excess absorption n AB is absolute absorption and then we have this gas phase term where importantly V ad is the volume of
The absorbed phase now the absorbed phase volume can be assumed constant or it can vary um but in the case of the Myers and Monson paper in 2014 they defined absolute as using this expression down here where um where VP is the poe volume so actually absolute absorption was
Detered was defined in terms of Poe volume rather than the absorbed phase volume in order to calculate absolute absorption we can make different assumptions about the absorbed phase um um and that adds further uncertainty so we can assume a constant volume so for example the volume of the absorbed
Phase is equal to the poe volume or we can assume a constant density so for example the density of the absorbed phase is um equal to the density of the liquid um but these are assumptions and um none of them are are likely to be entirely accurate so even if you’re
Using Poe volume which may seem quite reasonable for purely microporous materials and that’s what what Myers and Monson intended that that that definition was for purely microporus materials the the poe volume will depend on how it is defined and determined and I’ve I just put this graphic in from the
IUPAC recommendations on the right hand side that this shows surface area being um depending on how it is defined and the radius of the probe well it will be similar um in terms of poor volume we also have different capacities used for um different applications don’t need to go on about
This too much but um different units are used so weight percent is common in hydrogen storage studies whereas Vol V over V is common in methane storage but this is different to moles per gram which is recommended um in the IUPAC guidelines for storage studies we’re interested in gravimetric and volumetric
Capacities um and crucially as I was trying to emphasize earlier working gravimetric and volumetric capacities and those will be different for different applications and potentially the gravimetric and volumetric capacities could be expressed in terms of net excess and absolute absorption so there are lots of different combinations
Um that um essentially authors are free free to choose um and there’s also a fourth term called total absorption which is used in some hydrogen storage studies and and in my view that just adds to the confusion so just some final comments before concluding I think that actually
Dismissing one or more of net excess or absolute absorption due to the drawbacks um of each of their drawbacks seems unnecessarily um extreme I think net absorption clearly contains the least uncertainty because you don’t have uncertainty over the sample volume which is determined using helium or the or or um uncertainties
Over the absorbed phase volume either the the the Assumption you’re using or the poor volume measurements um but to me absolute absorption seems inappropriate for actually reporting raw experimental data and that’s because you have to um make some assumption about the absorbed phase volume or density so you’re taking the
Experimental data and then adding in this other this other term for the calculation um but I also think it’s fine to provide calculated absolute um absorb quantities alongside sort of the experiment mental net or excess and I’d say it’s perhaps best just to use um the definition most appropriate um to the
Application and I’d say that I didn’t really make this point in the paper but I think reporting experimental data is different to other um uses such as simulating absorption column Dynamics or behavior so I really think um different definitions could be useful in in in different ways in different um um
In different scenarios and and it that makes it um difficult to just really simplify um and finally I think it would be valuable actually if we could produce some formal guidelines on these different definitions and and and how they’re used that would that would clear
Up the the confusion that I see in the literature and with that I’ll conclude so I think there’s a number of challenges associated with characterizing absorbance for gas storage and separation also I think these challenges make um for interesting research questions in the coming years um the measurement of High Press
Gas absorption isms can be um subject to significant error that’s been demonstrated by problems in the literature but also by into laboratory exercises uh where agreement has been difficult to achieve um the measurement of multi component gas absorption remains difficult that’s due to required sample sizes for most of the techniques timec
Consuming nature of the um of the methods U but also the challenge of achieving high accuracy there has been some progress recently which I tried to highlight um in the paper but I think there’s still much to do and then care must be taken when you’re calculating absorbed quantities particularly at high
Pressures um due to the different definitions of absorption capacity and and variations also in in how um each of the definitions are precisely um defined um and I think the development formal guidelines um on absorption definitions would be um really quite valuable um and with that I’ll finish and thank you for
Your attention and I’ll also apologize for um going over time no problem at all thank you Darren uh for the for the presentation um it was uh I think a pretty great summary um and an introduction to these topics for for most people and of and of your paper last year
Um we have a few questions um if you’re happy to to take some oh certainly uh one uh is from a attendee uh on YouTube masier and um I’m just kind of going to going to summarize the the situation a bit um he or or they have an ionic liquid impregnated polymeric
Membrane uh and they only have access to TGA and they would like to measure CO2 uptake of this material and do you have any suggestions or recommendations for them uh in in performing this measurement well I would say that um making gravimetric measurements would be more suitable in the um case of ionic
Liquids um it’s actually if it’s ionic liquid supported in was it porous membrane po polymeric porous membrane yeah yeah so that’s that’s actually more absorption than than adsorption um so you don’t you don’t actually have the same problems with the um with the um sort of definitions of
Absorption that I’ve been talking about but it definitely sounds something that would be more suitable for a gravimetric um instrument rather than the manometric I would have thought yeah and and he has a TGA so I think that satisfies that um criteria I think in general the problem with any
Absorption measurement or TGA or what whatever kind of instrument you have is kind of starting with a clean sample right because you need a you need a a good uh clean sample and a baseline And if your sample is perhaps volatile which it may be in this case
Then achieving that could be difficult and present challenges because you you’re potentially evaporating your a part of your material in the Regeneration step let’s say and that and that’s probably you know a factor to consider here yeah well the the thing with ionic liquids is they have very very low Vapor
Pressures um so you can actually um pump on them and they and they don’t um they won’t evaporate um but one of the important advantages of a gravimetric measurement in particularly in this case is that you can monitor the sample weight or mass while you’re degassing while you’re
Preparing the sample so if that if the ionic liquid were to start evaporating when you exposed it to um vacuum you would see a weight decrease in the in the sample um and you typically typically don’t actually because of the the very low I think they sometimes say
Negligible vapor pressure but I think that’s a little bit open to question it’s just the vapor pressure is is so low temperature obviously sorry what’s it also depends on the temperature probably it’s negligible so I’m thinking more of De probably degassing under ambient temperature rather than the sort
Of the higher temperature degassing that you’ll tend to do for for um porous absorbance thanks for that so much sure I hope that kind of uh answered your question um you know measurements are typically material dependent right and you’ll probably have to do some preliminary studies to see what works
Best um I could I could perhaps add that that the um there is a significant difference to the between the gravimetric instruments typically measure for measuring isotherms and and standard TGA so I don’t know whether TGA would be particularly um useful um you don’t have the long-term balance stability that’s required for um
Gravimetric aborption uh measurements with a with a TGA um you you need the balance to be St stable over relatively long periods um and the instruments that are typically used for measuring um absorption or or absorption isotherms um tend to have quite like a clean environment whereas TGA can just
Be done in a in a in a carrier gas um so there is an important distinction there between just standard TGA and and the gravimetric instruments that that um that are required for absorption isotherm measurements great thank you he says thanks um so so I assume that’s good um
A few attendees Timmy and and kajia say great talk um and and another Isabelle says they have had um good success with gravimetric measurements for Ionic liquid Composite Materials uh so there’s some feedback there excellent Um I think uh Nick Wilkins um who was who was here earlier I think he has a question on you know how can somebody who is new to this kind of work can firm or have some kind of certainty that their high pressure hydrogen measurement is is
Reliable yeah that’s a good question um and it’s quite difficult to answer um the zif8 data that I showed that was um published very recently um from that was from a big European project um coordinated by Bam um so Europe’s answer to to nist I
Suppose um and there is a hope I think that that zif8 will be um certified as a reference material um so you have that that possibility um as a way of sort of validating your your instrument so um a starting point is is is basically to make measurements on uh materials for
Which you’re fairly conf confident you know the the um the uptake of and in in fact even in the inter laboratory exercise published back in 2009 where there was quite a spread of data there was actually a cluster of um of isotherms that that that showed pretty
Good agreement um and and so that was a commercial carbon molecular Civ I think it showed about 1.6 weight weight percent of uptake um so that and um we tend to use Zite 13x actually at heyden as a sort of an internal standard because we think we roughly know what
The uptake should be so I think it’s thoroughly testing your instruments on those known materials is the is the is the crucial sort of starting point thank you yeah I think that’s uh pretty good advice for for any measurement in general uh not just necessarily hydrogen but
Related to hydrogen I guess just um adding on or you know related to that um I think the personally the the purity of your hydrogen can also be a big problem in some cases um do you have any comments or or suggestions around that that’s certainly true yes um for a
Good measurement hydrogen needs to be very pure um so I would say a minimum sort of six nines that’s um 99.9999% um but you can actually purify the the hydrogen further with a um with a molecular Civ um trap or some other kind of filter um a liquid nitrogen trap
Is actually very a very good way of freezing out any um impurities in hydrogen so you basically have a coil of tubing that goes down through a liquid nitrogen duer before the gas comes out of the duer and goes into the instrument um and as long as you keep that the Trap
Clean um that can that can purify the hydrogen um and also there’s um a gas from Air Products I think called bip which actually has a filter within the within the cylinder it’s like a column of activated carbon or something that that removes impurities as the gas
Comes out and hydrogen is available with that BP technology and that provides very high Purity okay that’s interesting I hadn’t heard of that uh option before so that could be something to look into um thank you for that um let’s see we’ll probably finish up on this one just because we’re you
Know slightly overtime as well uh this is from from bubes who’s hosting today um I’ll I’ll combine a couple couple into one um do you have any comments on the challenges of measuring uh kind of helium adsorption at high pressure or you know determining the the volume of the gravimetric system using
Helium uh and moving and I guess second question is Um what are the challenges of CO2 water binary measurements and any challenges for off-the-shelf instruments for for that particular measurement those are quite good but quite big questions actually yeah um the helium certainly helium volume calibration has to be done um at ambient temperature or above that’s that’s um perhaps fairly obvious
But but may maybe not you’ll get helium absorption certainly at 77 Kelvin um I think it might be a um point of debate really as to how much helium absorption you will get um at higher temperatures um and higher pressures and it is almost certainly material
Dependent um so if you have larger poor materials um maybe you don’t get any helium absorption but once your pore sizes are down to to the molecular level and you have um absorption potentials clearly overlapping from the opposing poor walls um I think you must get some um helium
Absorption um there have been papers published on ways of um correcting that out or removing that that includes making the volume calibration at at high temperature because the amount absorb decreases with an increase in in temperature and Oran talo actually published with Sashi Gama a method for um for um calibrating with
Um calibrating helium volumes involving me measuring helium isotherms at different temperatures but it’s quite a timec consuming um process I don’t know it’s it’s quite a difficult problem on co2 and water I don’t know how much I’m going to be able to say in a a reasonably concise um answer it’s it’s quite
Um challenging because of um the so with with water you need to um have extremely good um temperature control um to to just to um keep sort of rhes at um at constant um levels um and um CO2 can potentially dissolve in water I don’t know um under conditions that are of interest
For direct air capture that’s CO2 EX extremely low concentrations um so that again is is sort of difficult um to control um I think it’s very much a work in progress yeah especially for the for the deak um situation um thanks again uh for your for answering those questions
Um and again thank you for um today’s webinar um there are another there you know several comments again in the YouTube chat thanking you for your presentation um and with that I’m going to hand over back to bubes who’s going to wrap up the session for today and uh
Let you all know what’s coming up uh next month or the next webinar excellent thank you David thank you Dr Broom for your presentation on challenges and characterizing absorbance for Gast origin separation we also thank all of our attendees for joining this webinar and hope that it was both educational and
Enjoyable an editor version of this webinar will be posted on the IAS YouTube channel in billybilly with an announcement on the IAS Twitter feed the hope is that the work discussed today will be an useful resource for the absorption science community in the future the next confirmed IAS webinar will be
Given by Lee changlin on February 22nd at 5 a.m UTC announcements regarding the next webinar and the other IAS events will be posted on our Twitter feed and through the IAS mailing list with that we thank you for joining us and we hope that you will join us again soon