Isosteric Heat of Vapor Adsorption: Desiccant Selection

Vapor sorption experiments measure the uptake and attraction of vapors with materials, such as water interacting with desiccants. The isosteric heat of adsorption (i.e., HOA, isosteric enthalpy of adsorption) is a measure of interactive energetic strength. Uptake amount and HOA values guide the proper desiccant selection for a dehumidifier system where considerations of the adsorption capacity and energy requirement for its regeneration are influential.

1 Introduction

Keeping consumables such as foods and therapeutics dry is essential for their long-term storage and distribution. Additional materials for separations, decontamination, catalysis, remediation and construction may also require a dry working environment. Water vapor (humidity) is the most commonly encountered environmental substance that interacts with these materials. In fact, removing humidity in commercial and residential buildings is a major contributor to energy usage in urban areas. Therefore, material interactions with water - acting favorably towards water by displaying hydrophilic “water loving” interactions or acting adversely toward water by producing hydrophobic “water fearing” interactions - need to be understood to allow continuous peak performance at any moisture level.

1.1 Amount Adsorbed

Materials, specifically desiccants, have a working capacity to sorb (adsorb and/or absorb) water vapor from their environment. The amount sorbed at specific conditions, such as temperature and humidity level, can be monitored by static volumetric vapor sorption experiments. The results can be graphically represented by an isotherm (constant temperature, change in pressure) with volume/mass/moles of vapor sorbed per gram of material on the y-axis. The x-axis reports the absolute or relative pressure (p/p0), which may also be reported as percent relative humidity (%RH) or water activity (aw) when water is the vapor source. 

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