How may i measure an adsorption isotherm of VOCs on adsorbents.
The principle is clear for gases like N2. This may be done by a device from quantachrome or micromeritics. But this may not be applicable for analytes (e.g. VOCs) that have a vapor pressure of way below 1 atm at room temperature.
I was thinking of placing the adsorbent in a headspace vial together with a certain atmosphere of analyte (e.g. 30 ppm of benzene in nitrogen). After some time one could determine the gasphase concentration of benzene via headspace-GCMS?
Of course this measurement describes only one point of the isotherm. This is a time-consuming one ;)
Any better ideas out there?
Maybe from packed-bed chromatography? I was recording breakthrough curves recently...
EDIT:
https://www.researchgate.net/publication/7307185_Determination_of_gas_phase_adsorption_isotherms--a_simple_constant_volume_method
i found this one. similar to aboves ideas...
Article Determination of gas phase adsorption isotherms-a simple con...
When did this in the past in our lab, we were using a sorption balance. The adsorbent was placed in a small crucible (just like in a thermobalance) and the weight was continuously recorded with time. When the mass increase reached a constant value after a given time, we were increasing the vapour pressure of the VOC inside the balance.
Note that now, commercial sorption analyzers can do the same automatically, based on a manometric principle. Micromeritics 3Flex and Bell Aqua3, for example, can do that perfectly. At the present time, we are measuring water adsorption isotherms with a 3Flex, and it is no problem to replace water by any another organic solvent of higher vapour pressure (hence a VOC). The major advantage is that it is fully automatic.
Alain
I utilized Gas Bag of 10 L, which of one made one point of the isotherm. When prepare over 5 bag that have different concentration of VOCs with different mass of adsorbent, GC-FID analysis with direct inject mode can be easily applied because ppm level of VOCs will be still remained in the bag after reaching the equilibrium. That was my application that you read in my paper.
We use dynamic vapor sorption for measurements like that. Adsorption is measured gravimetrically. Adsorbate vapor pressure is regulated by mixing a dry carrier gas stream (N2) with a stream saturated with the adsorbate. Our balance has a resolution of 0.1 µg, p/p0 resolution is better than 1% and monitored in real-time by a dew point analyser. The system is in an incubator for precise temperature control (up to 90°C). For most adsorbents, definitely MOFs or zeolites, some 10 mg is sufficient for a good isotherm measurement.
Pieter
I think that you should consider a simpler method tan those descriged above. If you are using a pure VOC as the adsorbate then if the pores are wide enough the adsorption capacity will be the same as that of nitrogen at the same relative pressure, no more, no less. However, in real life the VOC is a contaminant of air. Thus, to measure the adsorption capacity of several VOC's in a static aparatus over several different samples simultaneously I accurately weighed the adsorbents in small bottles, placed them in a dessicator and then placed a Petri dish with the VOC in the same dish and weighed the increase in mass on a daily basis. My VOC,s were o-DCB, acetone, TCE, Toluene and Water. The weight adsorbed of each of these was in direct proportion to their vapour densities and to the pore size distribution of my materials determined from combination of the results of nitrogen adsorption and mercury porosimetry. When I used a spiked air stream in a dynamic adsorption aparatus I found that the useful adsorption capacity before the breakthrough of the VOC was at best equal to the micropore volumen. Obviously this was seriously reduced when the adsorption temperatura was raised above ambient of the contact time reduced. Please see the following publications:
M. Yates, J. Blanco, P. Ávila and M.P. Martín. Honeycomb monoliths of activated carbons for effluent gas purification. Microporous and Mesoporous Materials; 37 (2000) 201-208.
M. Yates, J. Blanco, M.A. Martín-Luengo and M.P. Martin. The dynamic adsorption behaviour of volatile organic compounds on activated carbon honeycomb monoliths. Studies in Surface Science and Catalysis; 144 (2002) 569-576.
M. Yates, J. Blanco, M.A. Martin-Luengo and M.P. Martin. Vapour adsorption capacity of controlled porosity honeycomb monoliths. Microporous and Mesoporous Materials; 65 (2003) 219-231.
M. Yates, J.A. Martin, M.A. Martin-Luengo and J. Blanco. Study of the efficiency of monolithic activated carbon adsorption units. Studies in Surface Science and Catalysis; 160 (2006) 583-590.
Hello
Like Dr. Celzard, we also have a good experience with sorption balances (from Rubotherm in our case). The method has good accuracy, but may be more time consuming for longer processes, as you measure each point individually. For more information on the method and some issues which have to be considered, you can have a look at our recent paper (link below). We studied sorption in polymers, but the methodology would not change much.
https://www.researchgate.net/publication/271539730_Equilibrium_sorption_of_ethylene_in_polyethylene_Experimental_study_and_PC-SAFT_simulations
(http://www.sciencedirect.com/science/article/pii/S0032386115000518)
Regards
Josef
Article Equilibrium sorption of ethylene in polyethylene: Experiment...
Dr Pieter Bertier uses a good dynamical method. Another one is a method of ideal chromatography (de Vault's method). Any information you can take from article:
Adsorption behavior of Vulkan-7H graphitized carbon black. T.V. Baikova et al// Russian Chemical Bulletin, Vol. 42, No. 9, P. 1477-1479
Determination of sorption isotherm requires measurement of VOC relative vapor pressure or thermodynamic activity. Being direct method, headspace GC analysis is the best for it, especially for sorbates with low volatility and relatively high activity. The same is valid for slow adsorption processes. Look, for example in our paper http://pubs.rsc.org/en/content/articlelanding/2000/p2/b003477k#!divAbstract
Gravimetric and dynamic methods are compromising for this purpose if the pressure measuring valve and/or vapor source are separated from adsorption cell even by some short tubing. Still, these two methods are better for determination of sorbate uptake.
Hello
To the last comment. In the case of gravimetric methods, there is usually a pressure sensor directly inside the measuring cell, which provides the real pressure values. If pure substances are to be measured, which I think is the case here based on the original question, then this solution is sufficient,
Regards,
Josef
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Atmosphere
- Air