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New Technique Promises to Accelerate Materials Development for Gas Separations

In this interview, Darren Broom, product manager at Hiden Isochema, talks to AZoM about a new technique that promises to accelerate materials development for gas separations. 

To begin, can you give us a brief introduction to gas separation and why it is important?

Industrial gases have an extensive range of applications, from preserving food and carbonating fizzy drinks, through to welding, steelmaking, and oil refining. Natural gas, meanwhile, is used across the globe as an energy source, for household cooking and heating, while the semiconductor industry is a major consumer of speciality gases, for manufacturing the electronic components used in computers, smartphones, and many other devices.

In every case, pure gas must be produced, and this typically involves a gas separation or purification process. Gas separation therefore affects many aspects of modern life, and without it our lives would be very different.

What methods are typically used to separate gases?

Gases can be separated in different ways, and the choice often depends on the required purity and scale. Distillation is commonly used, particularly for large scale production, but it is energy intensive. Cryogenic air distillation, for example, is used to produce nitrogen, oxygen and argon. More energy-efficient methods of separating gases, however, include the use of membranes and porous materials in processes such as Pressure Swing Adsorption (PSA) and Temperature Swing Adsorption (TSA). PSA and TSA use nanoporous materials to either extract the desired gas from a mixture or remove unwanted contaminants. Such materials, which include activated carbons, silicas and zeolites, selectively adsorb different gases, depending on the temperature and pressure, and the nature of the gas-solid interactions.

Traditionally, what techniques are used to analyze new materials for separating gases?

An important aspect of developing new nanoporous materials for PSA or TSA is the characterization of their adsorption properties. For separation processes, the behavior of materials in the presence of gas mixtures must be understood. This involves measuring multicomponent gas adsorption equilibria and kinetics. The most important information is probably equilibrium adsorption behavior – the amount of each gas adsorbed under equilibrium at a given temperature and pressure, and gas phase composition.

The two main types of measurements use open or closed systems. Traditionally, volumetric techniques are used for this purpose, although there are several variants, some of which include gravimetric measurement – that is, the use of a microbalance to determine the change in the weight of a sample, in response to gas adsorption from the mixture.

Open volumetric systems involve flowing a gas mixture over the sample and measuring the outlet gas composition and flow rate. Integration of the inlet and outlet flow rates and compositions allows calculation of the amount of each gas adsorbed by the sample. This is sometimes called a dynamic column breakthrough measurement. There are limits to the accuracy of this approach, however, because of the difficulty of accurately measuring the total flow rate of a gas of changing composition.

Closed volumetric systems, meanwhile, are more accurate, but the measurements are laborious. In this case, a gas mixture is circulated round a closed circuit, using a recirculation pump. The gas mixture is forced through the adsorbent bed until equilibrium is achieved. Under equilibrium, the gas mixture composition is analyzed, using a Gas Chromatograph (GC) or Mass Spectrometer (MS), and a molar balance calculation is performed using the initial and final gas compositions and pressures, in the system of known volume, to allow calculation of the amount of each gas adsorbed. Although accurate, the difficulty with this approach is that the measurement of each point can take a day or so, meaning that weeks of experimental time are often required to collect a good dataset.

Can you tell us about the new ‘integral mass balance’ (IMB) method?

Open system volumetric measurements are quicker than their closed system counterparts, but less accurate due the outlet flow rate measurement accuracy issue. The IMB method uses an open system for speed, but eliminates the need to measure the outlet flow rate, by instead determining the total weight change of the sample as a function of time. A gas mixture of controlled composition is flowed over the sample, which is suspended from a microbalance, while the outlet composition is determined using a mass spectrometer. Integration of the mass balance equations for this system then allows calculation of the amount of each gas adsorbed. This calculation process is why we have called it the ‘integral mass balance’, or IMB, method.


The IGA-003-MC instrument used to implement the IMB method. Image Credit: Hiden Isochema

We have been able to measure a 20 point binary gas adsorption isotherm in only 4 hours using this technique. In comparison, closed systems of comparable accuracy would require at least 20 days of experimental time. It also requires only a few grams of sample. It therefore represents a significant advance in the characterization of adsorbents for gas separations.

How has this technique been demonstrated? Can you tell us a bit about this process?

In a recent paper, we have demonstrated the practical implementation of the IMB method by replicating data previously measured on zeolite 5A using an IGA-003-MC, one of the models in our Intelligent Gravimetric Analyzer (IGA) range of gas and vapor sorption instruments. The measurements used a mixture of nitrogen (N2) and oxygen (O2) at a pressure of 9.15 bar. This adsorbent and these conditions are relevant to air separation for the production of pure O2 for various applications, including the production of oxygen for medical purposes.

To both demonstrate and validate the technique, we needed to select a set of measurements for which reliable prior data were available. For this purpose, we collaborated with Professor Orhan Talu of Cleveland State University in the US. Professor Talu provided us with some of the same material used in a study he published back in 1996. We then replicated the measurements, under identical conditions, using the IGA-003-MC instrument. We found good agreement, which convinced us of the accuracy of the IMB method.

How did this collaboration between Hiden Isochema Ltd and Cleveland State University come about?

I have known Professor Talu for a number of years. We have met previously at various specialist adsorption conferences, perhaps most notably the Fundamentals of Adsorption (FOA) conference held in Baltimore in May 2013, and a workshop on ‘Measurement Needs in the Adsorption Sciences’ held at the US National Institute of Standards and Technology (NIST) in November 2014. Professor Talu has an excellent reputation in the field, and served as President of the International Adsorption Society (IAS) from 2007 to 2010.

We have talked in the past about collaborating, but when our team at Hiden Isochema first established the IMB method as a viable way of measuring multicomponent gas adsorption we realized we would need help to refine and validate the technique. Professor Talu was the obvious choice, as he has decades of experience in measuring and characterizing multicomponent gas adsorption by porous materials. We were very pleased indeed when he then agreed to work with us on this project. Amongst other things, he encouraged us to revise the way in which we were calculating the adsorbed quantities, and the resulting approach is presented in detail in our recent paper.

What are the advantages of being able to analyze more materials?

At the moment, there is a overreliance on theoretical models, such as the Ideal Adsorbed Solution Theory (IAST), for calculating multicomponent gas adsorption isotherms from single component data. The results of such calculations always need to be checked using experiments, because the models are not always applicable or accurate, depending on the adsorbate-adsorbent system. Existing experimental techniques, however, are very time-consuming.

A quicker approach, as provided by the IMB method, would allow more materials to be screened in a practical timeframe, using experimental measurements rather than relying on theory. Chemists, for example, developing new nanoporous materials, will therefore be able to directly characterize the performance of a material for a given separation, without the need to rely on potentially inaccurate models. Meanwhile, theoretical studies of multicomponent adsorption, using molecular simulations, are also restricted, to some extent, by the lack of accurate, validated data on different adsorbate-adsorbent systems. Wider adoption of the IMB method should significantly increase the amount of experimental data available for such studies. This would help theoretical work on multicomponent adsorption and the development of new separation processes by chemical engineers, as well as aiding experimental screening of new adsorbents for different types of gas separation, using PSA and TSA.

What are some of the current and future applications for this technique?

The measurements we made to validate the technique are relevant to air separation for oxygen production. In medicine, where purified O2 is widely used, this technology is vital. Small-scale and portable medical O2 generators are widely available for personal use, but larger PSA O2 generators, filled with zeolites, have been installed at emergency field hospitals constructed to cope with the current coronavirus crisis. Reliable O2 supplies have been critical for treating patients.

However, there are many other important applications for gas separation, for which we think the IMB method will be able to provide accurate data. Capturing CO2 from power plant flue gases, for example, is of great interest, as this will help tackle the difficult and serious problem of climate change due to increasing greenhouse gas emissions. Other future targets include separations used for natural gas upgrading and biogas purification, as well as hydrogen (H2) production and purification. Both H2 and natural gas are important for the transition to a low carbon energy future, in which fossil fuel use will be gradually phased out.

Can you tell us a bit more about Hiden Isochema?

Hiden Isochema is a world leader in the design and manufacture of gas and vapor sorption instrumentation for research, development and production applications in surface chemistry and materials science.

We have been producing sorption measurement systems since 1992, when Hiden Analytical first began manufacturing the Intelligent Gravimetric Analyzer (IGA). After a decade of continued success, Hiden Isochema was then formed as a wholly-owned subsidiary of Hiden Analytical, in order to further specialize in the development and manufacture of sorption-specific instrumentation. The two companies are now members of the Hiden Instruments Group.

Since then, we have expanded our product range to include manometric sorption systems, dedicated breakthrough analyzers, and membrane permeation analyzers. In 2013, we introduced a new type of sorption microbalance, called the XEMIS, which offers high pressure operation and compatibility with corrosive species. We have also continued to strengthen our reputation for delivering high quality and versatile instrumentation while providing industry-leading levels of technical support.

Our instruments are found in high profile academic and industrial research laboratories around the world at universities, research institutes, and R&D labs, as well as numerous process and quality control installations.

Where can readers find more information?

You can view the full paper via the link below:
Integral Mass Balance (IMB) Method for Measuring Multicomponent Gas Adsorption Equilibria in Nanoporous Materials

And you can read more about the IGA-003 MC on our website using this link: IGA-003 MC

For more details about the products and services Hiden Isochema offers, visit www.hidenisochema.com. You can also follow us on Twitter.

About Darren Broom

Darren Broom is a product manager for Hiden Isochema. He obtained a PhD in materials physics in 2002 from the University of Salford in the UK, and then spent three years as a postdoctoral research fellow at the European Commission’s Institute for Energy in the Netherlands, working on hydrogen storage materials. He then returned to the UK to join Hiden Isochema in 2007.

In 2011, Springer published his book on the characterization of hydrogen storage materials, and he has since broadened his interest in the analysis of gas-solid interactions to include the characterization of porous adsorbents for gas separations. Since 2015, he has also been one of six UK representatives on the International Energy Agency (IEA) Hydrogen Technology Collaboration Program (TCP) Tasks 32 and 40 on hydrogen-based energy storage (https://www.ieahydrogen.org/).

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Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

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