Humidity, condensation point, and maximum achievable vH2O2

This blog is the first in a 4-part series where we describe how process parameters influence condensation and the maximum achievable concentration of hydrogen peroxide vapor during vH₂O₂ bio-decontamination applications.

In this series we propose four basic process parameter rules.  But before we address the first parameter that affects condensation and the maximum achievable vH₂O₂ ppm, we will review two important values: relative humidity and relative saturation.

Because water (H₂O) and hydrogen peroxide (H₂O₂) have similar molecular structures, both affect the condensation point of the air. However, relative humidity (RH) indicates only the level of water vapor in the air at a given temperature, whereas relative saturation indicates the level of water vapor as well as hydrogen peroxide vapor in the air.  In air that contains hydrogen peroxide vapor, condensation will occur before 100% relative humidity. Therefore, the condensation point can reliably be anticipated with the relative saturation (RS) measurement.

When relative saturation reaches 100 %RS, the vapor mixture will condense. Relative humidity and relative saturation differ whenever there is vH₂O₂ present and the difference between RH and RS is further affected by the amount of vH₂O₂ present.  Once condensation occurs (relative saturation has reached 100% RS), the ppm vH₂O₂ can no longer increase. In fact, H2O2 vapor concentration will often decrease as some vH₂O₂ will decompose into water and oxygen upon condensation. When this occurs, more vH₂O₂ must be injected to compensate.

If there is dripping condensation by the end of the decontamination phase, vH₂O₂ ppm readings may initially increase during aeration because the droplets release vH₂O₂ back into the air.

This brings us to the first rule:  Lowering the initial level of humidity increases the amount of H2O2 vapor that can be used before condensation.  

The graphs below illustrate that when relative humidity is higher at the onset of the conditioning phase (because de-humidification was not performed), condensation occurs sooner during decontamination. Therefore, the lower the relative humidity when conditioning begins, the higher the maximum achievable ppm vH₂O₂ before condensation occurs.

During the decontamination phase, some vH₂O₂ will decompose into water and oxygen. The amount of vH₂O₂ that decomposes will depend on conditions such as: materials, temperature, humidity, and air flow. The actual decomposition expected under certain conditions must be measured.  In following graphs, we have assumed that 10% of vH₂O₂ has decomposed from its initial value and more H₂O₂ is vaporized to compensate.