Darren Broom explains how to extend the operational range of automated dynamic vapour sorption analysers
Dynamic vapour sorption (DVS) typically involves the measurement of moisture uptake at near ambient temperature in the relative humidity (RH) range 2 to 98%. Applications include the determination of transport rates in food and packaging, and amorphous phase detection and quantification, hydrate analysis and hygroscopicity studies of pharmaceuticals. In some fields, there are significant advantages to the measurement of moisture sorption outside the above regime; however, this adds additional complications.
In DVS, water vapour is usually delivered to the sample by mixing wet (saturated) and dry gas streams to a varying extent. At low RHs the problem is the control of the flow rate. DVS analysers invariably operate using thermal mass flow controllers (MFCs), which provide very stable control but have limitations both in absolute accuracy and the control of low flow rates, typically below 2% of their full-scale range. This corresponds to 2% RH if the flow ranges are matched. MFCs of different ranges can be used but this in turn limits the achievable RH range unless additional flow streams are added, which increases instrument cost and complexity.
The difficulties at high RH originate from the problem of bulk condensation. A high level of temperature control is required to prevent this occurring near 100% RH because relatively small changes in the temperature can result in significant shifts in the saturation vapour pressure – or the partial pressure at which bulk condensation will occur. See the inset of Fig. 1, which shows the large shift at 50°C caused by an error of ± 1°C. This sensitivity poses significant challenges to designers of DVS instruments.
With regard to temperature, the saturated stream is typically generated using a reservoir held at the same temperature as the sample. This helps maintain a stable, isothermal environment but places limitations on the achievable measurement range. The minimum is typically 5°C because temperatures lower than 0°C will result in freezing while the boiling point of water defines the maximum.
Overcoming challenges
Each of these difficulties, however, can be overcome by appropriate instrument engineering. For high temperature measurements, for example, the sample can be independently heated. This approach is used in the Hiden Isochema IGAsorp-HT, a DVS analyser that can perform measurements up to 300°C (see Fig. 1). This configuration also allows control below 1% RH at ambient temperature.
The main benefit of extending the range of operation is the increase in the number of applications to which DVS can be applied. Two examples are the analysis of water sorption by microporous materials for gas and air drying, which requires low RH studies, and the study of new materials for water sorption-based thermochemical energy storage, which requires high temperatures.
Water sorption by microporous materials such as zeolites occurs at low RH due to its increased interaction with small pores compared to mesopores or macropores. Once the pores of a hydrophilic material are filled, as required for gas drying, the sorption isotherm saturates, ie, reaches a plateau – see Fig.1. The information contained below this, in the Henry’s Law region, is inaccessible without low RH measurement capability. Full in-situ drying of the sample must also be possible to reliably measure reproducible isotherms or sorption kinetics.
Thermochemical energy storage systems typically operate at high temperatures. Current demonstration and pilot plants use silica gels and zeolites as sorbents, and characterisation of their moisture sorption properties under operational conditions is clearly critical, which is a key benefit of DVS. The alternative is to measure sorption with respect to vacuum but the measurement of water sorption at ambient pressure in a dynamic system more closely resembles the situation in real sorption systems.
These examples illustrate the need to expand the operational range of DVS analysers beyond standard (near-ambient) conditions; however, the characterisation of materials for a range of other applications will also benefit from such increased functionality, which is subject to the practical restrictions outlined above. Further expansion through improved instrument design will lead to yet more applications for the already widely applied DVS technique.
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Darren Broom is with Hiden Isochema.
