On the controllability and energy sensitivity of heat-integrated desiccant adsorption dryers

This work studies the controllability of heat-integrated zeolite adsorption dryers. Mean product moisture content, temperature and vitamin C concentration (representative of product quality) are considered as controlled variables. Set-point tracking and disturbance rejection controllability metrics are considered in addition to energy performance sensitivity. In adsorption dryers, the adsorption system introduces extra degrees of freedom of which some input-output pairs are promising. For corresponding inputs, adsorption dryers are shown to have higher steady-state gains than equivalent conventional dryers due to correlation between dehumidification, adsorption heat and the controlled variables. They also show improved resilience to ambient air disturbances due to adsorbent subsystem-induced self-regulation properties. The encouraging mechanisms of the self-regulation are adsorption heat, kinetic and equilibrium properties of the adsorbent. Due to the high correlation between product moisture content and temperature, improved controllability is observed when vitamin C concentration is used as an output variable instead of product temperature. It is thus proposed that on the availability of reliable soft sensors or state estimators, instead of product temperature, vitamin C or some other temperature-dependent quality measure should be controlled in addition to product moisture in decentralised drying system control. Under perfect rejection of unfavourable disturbances like ambient temperature drop and humidity rise, the energy performance of adsorption dryers is not significantly degraded, whereas, it is for conventional systems. (C) 2012 Elsevier Ltd. All rights reserved.