CO2 Sorption & Diffusion under Humid Conditions: Competitive Adsorption Under Real World Conditions

Thanks very much Brad, and welcome to this afternoon's or the morning's presentation, depending where you are on CO2 absorption and diffusion under humidity conditions and particularly competitive absorption under what I describe as real world conditions. This is a part of a research project that's being run in the Department of Chemical Engineering and paola is the PhD student whose work I'll be talking about today. So first he just gives you a little bit of a background around carbon capture. It's a topic of great importance to societies and economies around the world. And the general acceptance of most countries are that we are emitting too much CO2 into the environment. And one strategy for reducing this is to capture at source CO2 when it's emitted. And one of the major emission sources are fossil fuel powered stations. So stations that use either coal or use gas fire that reducing power, but in the same process they release enormous amounts of CO2 which are detrimental to environment. And we're really moving forward towards the potential larger scale deployment of large scale technology and plants for capturing CO2. And current technology is very much based around use of liquid amine solutions. So liquid based adsorption is most commonly practiced as it is in the Petronova plant in Texas, usa, which one of the largest carbon capture plants in the world. World. But we're seeing a lot of increasing interest in the use of solid state adsorbents. In other words, capturing CO2 using solids rather than liquids. And certainly it's quite probable the next five, 10 years we're going to see a substantial deployment of solid state materials as the media for capturing CO2 from a range of applications, both from both from the emissions from power stations as well as the capture of CO2 directly from air. In both application spaces you need adsorbents that are going to be designed so that they can capture CO2 effectively and readily. Now it turns out that when you're capturing CO2, it is a bit more complicated than that. Certainly a lot of people are interested in developing new materials, new porous materials, specifically to allow for CO2 capture to be optimized. However, if you want to design new materials and first printables, you need good experimental data. And that data is in generally rather lacking in availability. And you also need good data on both mass transfer, resistance and diffusion for these new absorbents. So if you're going to design materials from first principles, you need good sets of experimental data to help you feed back into your material design. And to do that you need to have fast and accurate characterization techniques that give you the Right sort of information which can then be used to feed into molecular simulations or other models for by which allowing you to develop smarter materials. One of the complications operationally is that CO2 sorption is a bit more complicated than people often realize. And if you look at the papers published on CO2 sorption and solid state materials, and there are hundreds of papers, certainly over 95% of those papers simply report on the amount of CO2 adsorbed by the material of interest as a function of pressure and and what we describe as a single component absorption study, that single component being carbon dioxide. However, if you look at the real world, if you look at the materials being emitted from a flue gas from a power station, or you simply look at the gases in the air for direct air capture, it's much more complicated because in fact you're typically got between 5 and 15% water vapor present in the emissions from a firestor from a coal fired power station. For example, even if you're capturing water vapor directly from the air, if you're capturing CO2 direct from the air, you've got 1 to 2% of water vapor contain with and if you're Capturing Flue Gas CO2, you've got to deal with the presence of no 2 and no 3 as well as so 2 and so 3. And both of those have significant concentrations. They're both chemically very active, generally very acidic. So they also compete for surface sites with water with CO2. So actually you've got quite a complicated absorption problem. So really as well as well as needing fast and accurate characterization techniques, you need techniques can replicate the real world industrial scenario. So if you want to choose the best materials for CO2 capture, you need to do those capture experiments in conditions that mimic the industrial environment they'll ultimately deploy. Otherwise you're optimizing the material for the wrong problem set. And today we're going to talk about experimental methods by which you can measure adsorption prices where there's more than one component present. And we're going to talk about two experimental techniques. We won't go into experimental detail enormously in terms of the materials, but we will spend a bit of time discussing techniques and the sort of data sets that you can generate from those experimental methods. So the study we're going to talk about today, it's only a fairly brief presentation, it's just over about half an hour. We're going to look at the use of dynamic vapor absorption as a rapid characterization technique which allows US to measure CO2 uptake in the presence of other vapours such as water vapor. We're going to look at some examples on some complex metal organic framework materials and MOF materials. So we'll look at CO2 absorption and watersorption in the presence of in each other's presence using some MOF substrates. And welcome to calculate some diffusion constants. Obviously, if you're interested in processes of absorption, there are two things you want to know about. You want to know about how much CO2 is capture by your material, but you also know about the rate of capture because the rate of capture will be a critical design issue when you're designing your adsorption towers. So you also want to measure CO2 diffusion constants. We'll talk look at that. And we'll also look at some data obtained using a DVS breakthrough analyzer. And there are some problems where simple gravimetric data sets are not sufficient. And you need a slightly more sophisticated method which allows you to study true competitive adsorption and cosorption phenomena. And we're going to talk about this newer technique as well later in the presentation today. So just to spend a minute or two to remind you about vapor absorption science, just to sort of give you a very brief refresher here, we're going to be interested in looking at how much our samples increase in mass. And the first techniques I'm going to talk about are gravimetric techniques. The dynamic vapor absorption method is a gravimetric method. So we're going to take a sample, we're going to increase the humidity or the partial pressure of CO2 around the sample. As we increase the partial pressure around the sample of our adsorbate, its mass will increase. So we're going to get adsorption as molecules adsorb on the surface. And as we decrease the pressure going back down the isotherm, we're going to get desorption. And we'll sometimes get a gap between adsorption and desorption called hysteresis. And one of the reasons we're doing the adsorption process is twofold. Firstly, we want to understand how much CO2 or water vapor, or both, taken up by a sample. So there's a fundamental material information we want to get, but we also want to understand about the mechanism. When the water goes into my mof, where is it? And here are listed beneath some of the five most common locations for our water or indeed our CO2 molecules. They can be absorbed on the active sites. On the surface, they can form multi layers. In some cases they can absorb into the pores in the material, they can condense. Occasionally we get dissolution into the bulk and sometimes we have hydration reactions occurring. So as well as understanding the amount of CO2 or water vapor taken up by the sample, the isothermal also allows us to understand the mechanism by which water is taken up or CO2 is captured. And that allows us to better engineer, better design our material. So just again to just give a bit more background about the DVS technique, we're going to take a sample. Typically anywhere between 10 and 50 milligrams would be common. We're going to take that sample and we're going to weigh it. As I said, these are gravimetric techniques. They use a micro balance to weigh the sample. I'm going to do that under typically temperature controlled conditions. And we're going to vary the concentration solid. As a sample absorbs water or absorbs CO2, the weight will change and we'll monitor that. We will monitor increasing concentrations of humidity or CO2. We will look for the changes in sample mass, which is shown by the red lines. And we'll then decrease the concentration of vapor or gas present and look for decreases in mass. And in the end from the kinetic information we get here, we'll end up getting the isotherm information here. So whether it's relative humidity or the partial pressure of CO2, we will measure that gravimetrically. That's the basis of of the DVS method. Here's just an image of the instrument and a sample resides inside here. This controls the temperature on the outside here and the microbiomes is at the top here. So it's quite a simple instrument, but it can provide quite important data sets as far as understanding the behavior of adsorbent materials. So just a little more information about this particular machine. The machine we'll be using in the first part of the study is called a devious resolution and is specifically designed to do cosorption phenomena. So what that means is the instrument can generate two different gas streams, mix them and expose them to the sample. And in the study we're going to look at today, we're going to be generating water vapor, typically in a nitrogen carry gas. And these experiments are all conducted at ambient temperature and pressure. So the composition of the gas varies depending on the experiment. But it's all done at atmospheric temperature and pressure. And of course there are reasons for that are actually quite pragmatic. In the real world a lot of gas option will be done at room temperature and room pressure. So there's no need to do it under vacuum in some cases. So we have two gas streams in our system. So the gas streams, as I said, is going to be a humidity and nitrogen gas stream, so water vapor, and that's generated by these devices on this side of the instrument here. And there's a humidity probe that confirms the humidity of the gas that we've generated. And in the other part of the instrument here, we generate a CO2, a carbon dioxide gas tube and nitrogen. Again, we've got an ultrasonic sensor here that measures the gas composition so it tells us how much CO2. So we know how much CO2 we've got, we know how much water vapor got. And we mix those together and we pass those over the sample. So here's a sample over here with our 15 milligrams of material. We're going to look at the change in mass as we change the composition of these streams of gas that's fed through the instrument. So it's quite a simple experiment. And the important thing is we've got very good control of the solvent to go in, we've got very good temperature control and we've got very good mass resolution. So that allows us to get very good quality adsorption information. So let's start by looking at some data. So this is a experiment, this is a water absorption experiment. So it's a single component. It's this water vapor, 25 degrees atmospheric pressure. And this particular material is a MOF, it's MIL101. And this one has actually got some fluorination in it as well. But it's basically a fluorinated mof. And what we're doing here is the following. We asked in the blue lines, we are stepping up the humidity in 5% steps. So the blue line is the humidity steps and the red line is the mass increase. So as I increase the humidity, I increase the amount of water that's taken up by the sample. And what you can see here is by the time I get up to about 90% humidity, my sample has about 80 to 84% water. So in other words, by mass, if I started with 15 milligrams of sample, I probably now have about, probably now have about 27 milligrams of material. And of that I have a lot of water. 80% of the sample mass is now water vapor that's adsorbed in my material. And this experiment takes a couple of days to run. So we can see the adsorption and we see the desorption. And in a moment we'll look at the isotherm for this material, but this is just water vapor by itself. So now we're moving on to a more complicated experiment. Now we're doing a two component experiment. So in this experiment what we're going to do is we're going to take the same material, we're going to expose it to assume humidity, and once the sample is equilibrated, or close to equilibrated, we are then going to expose it to CO2. So let me just describe it a bit more detail. So initially the sample is dry. So we can see on these axes here, this is the concentration and the green line is the concentration of water vapor. And if you take the line across here, we've got about 5% water vapor. So the first thousand minutes we expose a sample to 5% of water vapor and the sample picks up about 4 and a half percent of water. And that's quite consistent with the data you saw on the previous slide. So now, well, now what we do is we maintain the concentration of humidity at about 5% and we now introduce CO2. So the blue line represents the CO2 concentration. So we now step in a pulse of CO2. It's about 10% CO2 concentration. And you can see here that the concentrate that the amount of mass of the sample increases when we do that by about half a percent. So in other words, this sample now has about 4 1/2% by mass in it. It has about 4 and a 4%, just over 4% of water vapor and about half a percent or just under of CO2 vapor. Now in these experiments we are assuming that water vapor is very strongly absorbed and in many cases is true, and that CO2 absorbs, but the water vapor does not desorb. So in other words, when the CO2 goes in, the water vapor concentration in the sample is unchanged. And for some materials that's a reasonable assumption. But as we'll see later, there are some materials where that is not a good assumption. And we have an alternate technique to deal with the case where there's true competitive absorption occurring. So essentially we can do this experiment. And you notice now that when I go from 10% CO2 back to 0%, then my data drops back onto this line here. And you can see here, when I turn the water vapor off, I'm starting to dry the sample out. So from these two component experiments we can work out how much CO2 is taken up by the sample. But what you can see is that for this material, water vapor dominates adsorption behavior even at 5% humidity, nearly 90% of the uptake can be ascribed to the water vapor. So this particular MOF is very sensitive to water vapor. So what we can construct from experiments like this, these are single component experiments. Now so this is adsorption in the blue and desorption for this is the MIL101 material here at basically 25 degrees C from 0 to 90% RH. So you can see here the absorption is at them and there's a bit of, a little bit of hysteresis there, which implies a little bit of mesoporosity in that material. And on the right hand side here we can see these are CO2, these are carbon dioxide adsorption isotherm for two different mill materials. There's MIL101 and a MIL101 variant, again at 298 Kelvin up to 100% PP Nord. And you see here that the initial mill material has a slightly higher uptake compared to the modified material. But again we can measure at ambient temperature and pressures, both water vapor absorption and CO2 sorption using the DVS technique. Of course, what we're really interested in is what happens in terms of the adsorption behavior for CO2 at different humidities. This is what's shown on this overhead. So let me explain in more detail what we're looking at here. So in these experiments what we're doing is we would take the sample in question and here are four fairly common moth type materials. Porous solids are potential materials for CO2 capture. And we essentially pre equilibrate the sample of different communities, 0, 5, 10, 15 or 20. And then at once with the materials established at that humidity, we actually. Let me just get rid of this thing here. So essentially, as you can see here for this HKust1, at 0% humidity, that material took up about 0.3738 millimoles per gram of CO2. But as the humidity increases, the amount of CO2 it can capture goes down. So in other words, not surprisingly for that particular material, the higher the moisture content, the less efficient it is at CO2 capture. And indeed we see a similar behavior for MIL53. In fact, Mule53 probably dropped by about a factor of, probably a factor of six from when it's been tested in a dry state to when it's tested and at something like 20% humidity, which is not a very high humidity, but it's a high enough humidity to undermine the performance. So these two materials have quite sensitive performance to the humidity in the environment. If we look at the mill 101 materials which are shown here. So although mill 101 has a lower performance at 0% humidity, at higher humidity, its performance is actually better than mill 53, for example. And here's a variant of mil 101, and we'll see in a moment, we look more carefully, it has slightly better performance at intermediate humidities. So in other words, for many porous materials, the humidity can have a substantial effect on the CO2 capture performance. So if we're optimizing materials for CO2 capture, we need to keep in mind that in many application spaces, water vapor will be competing and can fundamentally change the performance of the material that we think is is best. So this looks a little more careful. This is the previous data, just enlarged a bit. And it's just a little more subtlety to the data here. What you can see here, if we look at these data here at this at 5% humidity and also here at 10% humidity and also at 15% humidity, this modified Mil 101 gave slightly better CO2 uptake compared to the untreated material here. So these are not substantial, but they're real and they're measurable. And again, if you are optimizing your material, checking its performance at a range of humidities gives you an idea as to whether it's going to have the sort of real world performance you might like. That might mean it could be used in an industrial CO2 capture problem. So obviously, as well as the equilibrium, we're also interested in the kinetics, how fast the molecules diffuse into the pores, and also if they diffuse inside the molecular structures as well. So essentially we're interested in measuring the rate which molecules go into our materials. And one of the nice things about the DVS techniques is we get mass as a function of time, and we get mass as a function of time. It's not very difficult for us to compute the rate of diffusion. So the models for surface diffusion are fairly straightforward and they're really based on the size of the particles and the sample thicknesses. So essentially we're able to work out from our kinetic, from our kinetic data, first order diffusion constants. So here's some sample data I've got here. And what we're looking at is the following. So here is the mass of the sample, here is mass as a function of time. So you can see the increase in mass in this case is CO2 absorbs in the sample. And we can reanalyze that in terms of a Sanifikian plot, which is taking T to the half Plotting mass of the function mass at time T versus mass at the final time. Essentially, this line here allows us to explicitly determine the diffusion constant. And of course, nice thing about doing things such as CO2 diffusion in the presence of water vapour is we can measure the diffusion constant at different humidities and in some cases that humidity may impact on the diffusion behavior. So here We've got some CO2 diffusion constants as a function of relative humidity. These are for Mil101 and from modified Mil101. And we can see here is that we go up in humidity, we seem to be getting larger diffusion constants. So in this particular case we're seeing some increased rates of diffusion for CO2 at higher humidities. And that potentially is important because high humidities are much closer to the industrial world. And also we are interested in establishing equilibrium as quickly as we can in our material, so faster diffusion content. And that's good news for us. So, as I said before, if we've got experiments where the water vapor is effectively very strongly absorbed and it's not impacted by the CO2 presence, then the DVS experiments I discussed are really quite well suited. But there'll be some materials and there'll be some problems where the CO2 and the water vapor compete for the same site. And if you do that, one of the challenges with the gravimetric method is that we are measuring the mass, I.e. the total mass of the sample. So we can't be certain exactly how much is CO2 absorbed and how much is H2Osorbed. And if we feel that they might change during the, during the process of CO2 absorption, we need a different technique. The technique I'm going to describe now is called, It's a breakthrough analysis technique. Now, breakthrough analysis is not new, but we've done breakthrough analysis in a slightly different fashion here. And the novelty around the method I'll describe here is firstly, we're able to deal with very small samples, 20 to 100 milligrams. Secondly, we've got both CO2 and water vapor centers in our system. So we can measure the concentration of CO2 and water vapor that are going to go into the sample and the actual concentration that goes out. So the breakthrough analysis technique is a little different to the gravimetric technique. In the DVS technique, we measure change in sample mass. This technique is different. In this technique, we're going to take a sample, we're going to pack into a small column, we're going to measure the concentration of CO2 and whatever going into the column, and. And we're going to measure the concentration exits the column. It's a bit more like an IGC method, if you're familiar with that method. So that's the basis of the method. And with this drawing here, it'll probably help us understand how it works. So the key part here is the sample is over here in a small column. It's packed, and essentially we're going to pass through the column a gas. That gas contains CO2 and it contains H2O. So we're going to present both gases to the sample simultaneously. So prior to the sample in this area here, we have a sensor which measures the gas composition in terms of CO2. Another sensor measures the humidity. So we're checking how much water Vapor, how much CO2, go into the sample. Then we've got the similar sensors at the end of the column to check how much comes out by CO2 and also how much water vapor comes out. So basically, we're doing a sort of mass balance. We check what goes in, we check what goes out. And this is what's called a breakthrough analyzer. And these techniques have been around for many years, and they're quite often used to check the performance of things like activated carbon inside filtration beds. But we're using it in a slightly more sophisticated fashion. We've got multiple sensors and a more sophisticated environment generating the humidity and CO2 partial pressures. So what happens in a breakthrough experiment is that this is the data measured by the sensors after the column. So you imagine I've got a column packed with my activated carbon on my mof. When I first pass some. Oops, sorry. When I first pass CO2 through the column, nothing will come out because it's going to be fully absorbed in the sample. So initially, my detector, after the column, sees no CO2. Then after some period of time after the sites in the solid are absorbed, then my CO2 will appear in my detector. And that's called the breakthrough curve. So we're getting a breakthrough point. And then we get to saturation. At this point here, there are no sites left in the solid. So all the CO2 that enters the column comes out of the column. And of course, if I look at this here, if I know the concentration of CO2, I know the flow rate, I know the amount of material present, I can determine the amount of material, in this case CO2 absorbed. I can do an integration, the number of moles for a particular concentration, for a particular amount of sample, for a particular temperature. So I can determine this, and I could do this for more than one component. So here I've shown you an example that just CO2 present, but because I've got a separate sensor for CO2 and a separate sensor for water vapor, I can do both of these at the same time. And we'll see that in the next overhead. So now we've got a breakthrough curve for a MOF HKUST one. And you can see here is the concentration that my detectors are reading. And I've got two detectors, so I'm measuring two quantities. One detector is measuring water vapor and the other measuring CO2. So let's have a look at the datasets. So the green line is the CO2 line and this is the axis here. So what you can see is after about five minutes, my CO2 has been absorbed by the sample and now my concentration increases. So essentially this area here tells me about the amount of CO2 that's been captured by my material. But do remember also this, this is in the presence of water vapor. So while the CO2 is being kept, so is the water vapor. So the time for water vapor is much longer. So this is given by the blue curve shown here and by the blue axis. So it takes about 1500 minutes before the water vapor breaks through. So in other words, the sample has a low capacity for CO2 but a very large capacity for water vapor. So in other words, water takes a long time to break through. CO2 takes a short time to break through. But basically we can measure the competition between CO2 and water vapor. So we can measure from these areas, we can detect the amount of uptake. And from the shapes of these curves, we can also determine diffusion constant. So diffusion information and equilibrium uptake can all be obtained using breakthrough analysis type methods. And for these samples, we can work out what the capacity is. And for CO2 and water vapor, we can work out that it's 1.15 and 4.59 millimoles per gram. So again, integrating the curves allows us to determine the uptake, we know the concentrations that go in. So effectively we can use a breakthrough analyzer to generate a competitive, a fully competitive absorption isotherm. So you can step your way up the isotherm and determine the adsorption of CO2 and water vapor simultaneously using a breakthrough analyzer. As I said, if there's a competition between water Vapor and the CO2 that is accounted for in the experiment. And to give you an example, if we look here at this data set, you'll notice an unusual anomaly here. So hang on, I noticed my maximum concentration of CO2 is about here, but I've got a bit of a bump here. So how can I have more CO2 coming out than I put in? Well, of course what that little lump tells us is that actually there's been some competition between the CO2 and the water vapor. And in that process the water vapor has displaced some CO2. So that's why we get to. So these sorts of kinetics is a hint that there's some competition occurring between the two species for the surface of mine material. So that's going to be the end of my presentation. Just a couple of conclusions. The DVS is a fast technique for providing researchers with accurate experimental data for rapidly screening soil absorbents for CO2 capture. And that can be done in the presence of water vapor. And we can fine tune our MOFs for CO2 capture by using DVS type data sets as well as obtain the equilibrium uptake, that is the isotherms. We can also obtain diffusion constants, which is also very important in terms of process development. And the breakthrough analysis method that I described here can also be used to measure competitive adsorption and CO absorption for materials such as the moss we've studied here. I thank you for your time and attention and I'll be happy to answer any questions you might have. Thank you.

So we will now take the time to answer all the questions that you guys have for. We do have one question for the breakthrough analysis. What is the column size? Does the pelletized sample. Is the pelletized sample necessary? What is the particle size of the sample?

Yeah, fine. So the column we're using at the moment is typically it's about a quarter inch diameter, it's about up to 2cm long. We can use powders or pellets or granules, provided they're not too large. And we have to be careful that the powder particles aren't too fine, otherwise we get larger pressure drops than we would like. But essentially it's a fairly small column, partially because obviously we want to use small samples. One of the points of what we're trying to achieve at the moment is to have small sample sizes which are more compatible with the sample size that people manufacture when they're developing new moths.

Okay. Can the different absorption processes be better understood with heat of absorption desorption information?

I mean, certainly if I was trying to understand sort of complex absorption, we've got water vapor, CO2, maybe SOX or NOX present. Certainly heat of adsorption or desorption management would be helpful. I don't think it's essential but in my view it would be useful and I would if somebody said you could measure those in your experiments I would certainly be able interested in doing that.

Can you please tell more about the temperature when you did the CO absorption And how about high temperatures such as 200 degrees C?

So the experiments we reported on the day were done at 25 degrees but there is an in situ heater. So we can do two things. We can either run the coadsorption experiments at elevated temperatures up to 200 to 300 degrees and we can also. We can also precondition a sample or post condition the sample or regenerate the sample in situ. So once we've done the adsorption experiment we can heat the sample up, drive off the gases and then run the experiment again.

For the next generation of mof. MOF design what will be the key factors to consider for better CO2H2O separation?

I think that's for us that's still working in progress. We are trying to understand what are the surface groups and surface functionalities are going to give you the best performance. Obviously there's work out there in terms of the use of aiming functionalities on the surface or different MOF to do this. There's also interest in making materials more hydrophobic. The more hydrophobic you make materials then there is less competition for water vapor. So potentially there are more sites available for CO2. I think that's an active research question. So I think we don't have the answers to all those things at the moment. But certainly the basicity of a surface in terms of amine functionalities high to prevent water vapor being strongly attractive. They're all things that are issues of interest at the moment.

Someone asks where do we find this DVS software to use it?

The devious software for the resolution instrument, the experiments we run on the resolution that's a standard part of the devious software. So if you've got a devious resolution and you want to get some advice on how to do that experiment if you contact one of the product managers at sms I'm sure they're better help you with that. But that's a standard instrument that does the CO2 water vapor sorption experiments.

Is the absorption fit the Langmuir system.

The. If we go back and look. Let me just find the isotherms here. No, certainly it's. They're certainly not Langmurian shaped isotherm. So the shorter answer is no that they're not. The CO2 zorption isosomes looked more Henry Law type region. They were quite linear and the ones for water vapor were much more, much more consistent with mesoporous material. So they did not, they did not fit the Langmuir system. No.

All right, I believe that answers all the questions. Unless there are any other questions.

Hold on, I've got one more on the chat here. Brad. The data I showed before, it said, and the question is, you mentioned the CO2 diffuses faster in the presence of water. That's right. I mean, the data suggested to us that at higher humidities the CO2 did seem to diffuse faster. And that's something we're interested in understanding. But certainly the preliminary data suggested at higher humidities we had faster CO2 diffusion, which from a process development perspective is an interesting observation.

All right, I believe now that concludes all the questions. Okay, wait, hold on. We got one more. Is the absorption better used for microporous materials only.

You can use it for microporous or you can use it for mesoporous materials. The real challenge around, I mean, obviously a lot of moths have got very small pores, but if you've got, if you're using, if you're using materials that are microporous, the problem with those materials is that if you want to regenerate them, you have to heat them to quite high temperatures to clear the micropores. So that is a bit of a complication, really. So I think micropores are interesting, but because the pores are very small, they're quite difficult to clear and they take quite a bit of energy to clear. And that might, and that might undermine the cost of regeneration from an industrial process.

What is the smallest particle size we can use for the breakthrough experiment?

I'm trying to think, how small did we go? I mean, it's only really around the pressure drops. We just don't want too large a pressure drop. I mean, there's no reason why you can't use particles of a few microns. But you've got to think also if you're planning to use these materials to make large scale absorption columns, then, you know, as much as it might be fun to make, you know, 50 nanometer particles, you aren't going to pack a column and make a product with those. So we tend to use particles that are, that are a few micron, up to a couple hundred microns. But turning around a pressure drop, that's all another question here. Which RH is more important for really industrial applications? I think which humidity does depend on a little bit around the particular process you're working On I think the point we're working on at the moment is that it's quite false to Simply look at CO2 absorption efficiency when you're doing CO2 by itself. When water vapor is in, many industrial pros are going to be there. So I think whether it's 5% humidity or 50% humidity, my view is it's just, it's only sensible that you take into account what is the humidity relevant to the process you're trying to replicate. So for example, if you're doing capture, if we're doing direct airborne captures, just imagine you want to capture CO2 directly from the air. You're typically looking at about 40% relative humidity, which is about 1% water molecules in the air. Whereas in combustion processes you tend to end up with, if you're doing flue gas capture, it's tend to have higher moisture content in the air. So I think the moisture content should very much reflect the sort of process you're trying to optimize.

There's a very interesting observation in regards to diffusion enhancement. There must be an interaction between water and CO2 that enhances diffusion, right?

Yes, I mean, yes, that would seem to be the case. I mean, we're obviously interested in that, but there's certainly something going on there because that there was, there is quite a substantial enhancement in the diffusion processes. So that is interesting. So the answer, it does seem to be, but I can't give you a better explanation at the moment. But in a few months time, when we've got this work a bit more understood in more detail, we'll probably come back with some answers to that interesting observation. Just got one more here, sorry, from Moonis. How may hysteresis complicate your inferences or conclusions? Hysteresis is often a sign as what the adsorption mechanism is in your material. So if you see hysteresis, it's often a sign or an evidence for the nature of the absorption process. So I wouldn't say necessarily it's a complication. I would generally say hysteresis is sometimes some very good evidence to understand fundamentally what the adsorption process is and specifically where your molecules are. So if we look at the data, when we had a single component experiment where there's some hysteresis between adsorption and desorption, and that was in a region of sort of 30 to 60%, that pretty well tells us that the molecules are living in mesopotes pores between 2 and 10 nanometers. So I think the hysteresis can tell us a bit about the place where the molecules are absorbed, which can help us understand how we might design and to be better in the future. And we will be sharing slides as we do normally. Yes.

All right. I believe that concludes all the questions. I want to thank everyone for joining today. If you have any other questions, feel free to contact one of your local sales reps and they'd be happy to answer any questions that you have.

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Email us at science surface measurementsystems.com Email.

Me directly if you want or email some question you want not to talk about publicly. Do feel free to email directly. All right.

And thank you all for coming.

Okay, take care everybody. Bye bye.