A new method for in situ measurment of the interaction of vapour mixtures is providing new insight into the behaviour of real world systems. M J Benham
and K M Thomas report on the implications.Solids materials are exposed to a range of different vapours in the real world and any or all of these may be able to interact with the solid surface.
These might include atmospheric pollutants as well as ever-present water vapour and, when more than one species is present, they will compete with each other.
The conditions, including the sequence of exposure together with their concentration levels can vary considerably. Atmospheric gases like nitrogen and oxygen may also be involved in these competitive processes.
For industrial applications such as separation or filtration, the measurement of both the kinetics of interaction as well as the equilibrium uptake of the competitive components is fundamental.
One area where understanding of these amulticomponent' interactions has great significance is in the abatement of environmentally unfriendly species such as volatile organic compounds (VOCs) from air or process streams by adsorbent materials. Adsorbents such as activated carbons have a strong physical interaction with many species, including VOCs, at specific sites of the large internal surface of their pore structure.
This provides the mechanism to concentrate the pollutant and so reduce emissions.
The performance of the adsorbent material will be, broadly speaking, dependent on the relative strength of interaction for a given species together with the kinetics of molecular transport. The complex mixture of gases and vapours that are present in real situations demands experimental methods to resolve how the material will behave and to enable understanding of both these performance governing factors.
In this context, any reduction in efficiency of the adsorbent due to the presence of water vapour is a specific major problem given the variation of weather conditions. Degradation in the lifetime of activated carbon adsorbents is sometimes observed in the presence of water vapour even though water molecules are assumed not to compete with organic vapours at the principle adsorption sites which are hydrophobic. There are however other sites where water will more readily interact and, in principal, build isolated clusters at sufficiently high humidity that could affect the interaction with VOCs.
Experimental methods
The most common experimental method is determination of the so-called breakthrough times for a given species in a mixture flowing through a bed of adsorbent.
Here, the breakthrough time is how long it takes for the species to materialise downstream and this is a combined function of both kinetics and the adsorbed concentration.
The most accurate and direct method for measuring the total uptake and combined kinetics is the gravimetric method: The solid is continuously weighed using an electronic sorption balance during exposure to a controlled flow and concentration of each component.
However, measurement of the interaction properties of each vapour component is not straightforward.
The method evaluated here uses a close-coupled dynamic sampling HPR20 mass spectrometer (DSMS) to measure downstream concentrations within a gravimetric analyser. This retains the absolute accuracy of total uptake and kinetics measurement and adds the means to study transient response of each species as well as their individual concentrations. The recently developed IGA-200 instrument is used for these experiments.
Results
Two examples of transient response of an active carbon in the presence of water vapour and octane vapour at fixed concentrations in a helium stream are shown in Figs. 1 and 2. In these experiments the carbon is exposed to one component initially until equilibrium is established and then the companion is added.
The plots show the reaction rate against time where the total reaction rate is the gravimetric differential rate of weight change (DTG) and this is superposed with the individual rates for each species measured by the DSMS.
Fig. 1 shows that water molecules are continuously displaced and released from the solid during the adsorption of octane. Fig. 2 shows that the reverse is also true at the high humidity value used but to a lesser extent as a small burst of octane is released. However, the reaction rates are quite similar for the pure component and the mixture.
The amount of water involved in the displacement of octane can be separately verified by in situ thermal desorption of the solid as shown in Fig. 3. Octane vapour requires a much higher temperature for desorption but the small residual water content is also apparent. Numerical integration of the thermal desorption results for a series of exposure conditions shows how the concentration of each component varies. u
ENQUIRY No 17
M J Benham is with Hiden Isochema Ltd, Warrington, UK. www.HidenIsochema.com. K M Thomas is with Northern Carbon Research Laboratories, University of Newcastle-upon-Tyne, UK.
