Considering Condensation

Abbildung 1: Zwei theoretische Biodekontaminationszyklen (beide mit einer Temperatur von 23 °C) mit unterschiedlichem Feuchtegehalt zu Beginn der Konditionierung. Während der Dekontaminationsphase wird ein Teil von vH2O2 zersetzt. In diesem Fall werden 10 % von vH2O2 vom Anfangswert zersetzt und mehr H2O2 zum Ausgleich verdampft. Eine ähnliche Situation zeigt sich unter Verwendung zweier verschiedener H2O2-Lösungskonzentrationen: 12 %-m in den oberen Grafiken und 59 %-m in den unteren Grafiken.

Figure 1: Two theoretical bio-decontamination cycles, (both with Temperature at 23 °C) with different humidity levels at the onset of conditioning. During the decontamination phase, some vH2O2 will decompose. In this case, 10% of vH2O2 has decomposed from its initial value and more H2O2 is vaporized to compensate. A similar situation is shown using two different concentrations of H2O2 solution: 12 %-m in upper graphs and 59 %-m in lower graphs.

Abbildung 1: Zwei theoretische Biodekontaminationszyklen (beide mit einer Temperatur von 23 °C) mit unterschiedlichem Feuchtegehalt zu Beginn der Konditionierung. Während der Dekontaminationsphase wird ein Teil von vH2O2 zersetzt. In diesem Fall werden 10 % von vH2O2 vom Anfangswert zersetzt und mehr H2O2 zum Ausgleich verdampft. Eine ähnliche Situation zeigt sich unter Verwendung zweier verschiedener H2O2-Lösungskonzentrationen: 12 %-m in den oberen Grafiken und 59 %-m in den unteren Grafiken.

Figure 1: Two theoretical bio-decontamination cycles, (both with Temperature at 23 °C) with different humidity levels at the onset of conditioning. During the decontamination phase, some vH2O2 will decompose. In this case, 10% of vH2O2 has decomposed from its initial value and more H2O2 is vaporized to compensate. A similar situation is shown using two different concentrations of H2O2 solution: 12 %-m in upper graphs and 59 %-m in lower graphs.

Abbildung 2: Zwei Biodekontaminationszyklen mit verschiedenen Lösungen an flüssigem H2O2 (T = 23 °C). In diesem Fall werden 10 % von vH2O2 vom Anfangswert zersetzt und mehr H2O2 zum Ausgleich und zur konstanten Aufrechterhaltung des vH2O2-Gehalts verdampft.

Figure 2: Two bio-decontamination cycles with different solutions of liquid H2O2 (T = 23 °C). In this case, 10% of vH2O2 has decomposed from its initial value and more H2O2 is vaporized to compensate and keep the vH2O2 level constant.

Abbildung 3: Zwei Biodekontaminationszyklen mit unterschiedlichen Temperaturen. In diesem Fall werden 10 % von vH2O2 vom Anfangswert zersetzt und mehr H2O2 zum Ausgleich und zur konstanten Aufrechterhaltung des vH2O2-Gehalts verdampft.

Figure 3: Two bio-decontamination cycles with different temperatures. In this case, 10% of vH2O2 has decomposed from its initial value and more H2O2 is vaporized to compensate and keep the vH2O2 level constant.

Abbildung 4: vH2O2-ppm als Funktion von rS/rF-Sensormesswerten bei 5 °C / Figure 4: ppm vH2O2 as a function of RS/RH sensor readings at 5.0 °C

Abbildung 5: vH2O2-ppm als Funktion von rS/rF-Sensormesswerten bei 50 °C / Figure 5: ppm vH2O2 as a function of RS/RH sensor readings at 50.0 °C

Abbildung 6: Maximale vH2O2-ppm bei verschiedenen Temperaturen erzeugt mit 35 % und 59 % an H2O2.

Figure 6: Maximum ppm vH2O2 at various temperatures produced with 35% and 59 vol-% H2O2.

Abbildung 7: Kondensationspunkte bei gegebenen Temperaturen und vH2O2-ppm (an jedem Punkt rS = 100 %, maximale %rF variiert gemäß den Kurven)

Figure 7. Condensation points at given temperatures and ppm vH2O2 (at each point RS = 100 %, maximum %RH varies according to curves)

In this article the key parameters are discussed that influence the condensation point and maximum ppm H2O2 in vaporized hydrogen peroxide applications. Four rules are proposed that can guide repeatable, effective, bio-decontamination processes.

Vaporized hydrogen peroxide (vH2O2) is widely used to perform biodecontamination in isolators, test chambers, and transfer hatches. Because of its known efficacy in room-temperature conditions, broad material compatibility, and lack of residue, vH2O2 bio-decontamination is also often used in hospitals, metros, airplanes, and applications that require reliable decontamination.

Controlling condensation is the most critical component of any biodecontamination process. Dripping condensation should be avoided because an accumulation of concentrated liquid hydrogen peroxide can have negative effects on materials, aeration time, and uniform decontamination efficacy.

Understanding how process parameters affect condensation also provides insight to the maximum hydrogen peroxide concentration that can be maintained in a vapor state. Humidity level, temperature, and the parts per million (ppm) of vaporized hydrogen peroxide have a combined influence on the condensation point; the point at which the air is saturated. Once condensation is reached, the ppm vH2O2 cannot be increased.

Relative Humidity & Relative Saturation

Ideally, each step in the bio-decontamination cycle is controlled and monitored. Monitored parameters typically include temperature, vH2O2 ppm, and humidity (measured as both 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.

As stated, once condensation occurs and relative saturation has reached 100% RS, the H2O2 vapor concentration can no longer increase. In fact, H2O2 vapor concentration will often decrease as condensation pulls vH2O2 out of the air. 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.

Phases of vH2O2 Bio-decontamination

Vaporized hydrogen peroxide biodecontamination cycles typically comprise four separate steps:

Step 1: Dehumidification:
The initial physical conditions of temperature, humidity, and air circulation are established. The area under decontamination may need to be dehumidified prior to conditioning to mitigate condensation. Lowering humidity to a predefined percentage before conditioning will generate more repeatable results. No H2O2 vapor is yet introduced during this phase.

Step 2: Conditioning:
Hydrogen peroxide vapor is produced from an aqueous solution of H2O2 and H2O and injected into the area until the desired ppm vH2O2 concentration is reached. Process concentrations can range from 140 to 1400 ppm, depending on the microbial load.

Step 3: Bio-decontamination or Dwell:
Surfaces and microorganisms are exposed to lethal concentrations of hydrogen peroxide vapor for a sufficient duration. H2O2 vapor decomposes into water and oxygen. This can be compensated for by injecting vaporized H2O2 throughout this phase. The ppm vH2O2 measurements can be used to control injection rates, thereby maintaining a constant level of ppm vH2O2.

Step 4: Aeration:
This final step lowers the residual H2O2 vapor to a safe level. The vH2O2 can be catalyzed into water vapor and oxygen with the help of a catalytic converter

Process Parameters that Affect Condensation

In the upcoming graphs, we vary humidity level, H2O2 solution concentration, temperature, and ppm vH2O2 to demonstrate how these factors influence both the condensation point (relative saturation = 100 %RS) and the maximum achievable concentration of hydrogen peroxide vapor during the bio-decontamination phase. These graphs are simplified representations of real bio-decontamination processes to show how critical process parameters influence outcomes.

Humidity, Condensation Point, & Maximum Achievable vH2O2

Lowering the initial level of humidity increases the amount of H2O2 vapor that can be used before condensation.

The following graphs (Figure 1) show how dehumidification influences the maximum achievable ppm vH2O2. In figures 1a and 1b, the hydrogen peroxide solution used is 12%-m; figures 1c and 1d use a concentration solution of 59%-m. Figures 1a and 1b show two otherwise similar bio-decontamination cycles; orange lines indicate processes without dehumidification and a conditioning phase that begins with relative humidity at 50 %RH. The blue lines show processes where dehumidification was completed to 10 %RH prior to the conditioning phase. Figure 1a and 1c show the effect of dehumidification on humidity percentage - indicated by relative humidity and relative saturation - during conditioning and dwell phases. Figure 1b and 1d show the effect of dehumidification on the maximum achievable hydrogen peroxide vapor during conditioning and dwell phases.

Vaporized H2O2 is injected into the chamber until condensation is achieved. We see that when the relative humidity is higher at the onset of the conditioning phase (because de-humidification was not performed), condensation occurs sooner. Therefore, the lower the RH% 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.

H2O2 Solution Concentration, Condensation Point & Maximum Achievable vH2O2

A higher concentration H2O2 solution increases the H2O2 vapor that can be used before condensation.

The following graphs show two otherwise similar bio-decontamination cycles using different solutions of liquid H2O2. The black line represents a bio-decontamination cycle with a solution ratio of 59%-m H2O2 to 41% H2O. The blue line represents a cycle using a solution ratio of 12%-m H2O2 to 88% H2O. Both conditions have initial humidity levels at 50% RH and are dehumidified to 10% RH prior to conditioning.

Vaporized H2O2 is injected into the chamber until condensation is achieved. Because the lower concentration solution (12%-m) contains 88% water, the chamber reaches 100% relative saturation faster than the chamber injected with the higher concentration (59%-m) solution. Once condensation is achieved, the level of vH2O2 can no longer increase. Therefore, the maximum achievable vH2O2 concentration with the 59%-m solution is 1400 ppm (black line) prior to condensation. The maximum achievable vH2O2 concentration with the 12%-m solution is 700 ppm.

Temperature, Condensation Point & Maximum Achievable vH2O2

Increasing the temperature increases the amount of water and hydrogen peroxide vapor the air can hold, thereby increasing the maximum achievable vH2O2.

In the previous conditions, the bio-decontamination cycle temperature was maintained at 23 °C. At a given temperature, air can only hold a certain amount of vapor, whether H2O or H2O2. By changing the temperature, we change both the condensation point and the maximum achievable hydrogen peroxide vapor concentration.

Figures 3a and 3b show two otherwise similar bio-decontamination cycles. The black lines represent temperature at 40 °C and the blue lines represent temperature at 23 °C. In both cases, dehumidification is performed to lower humidity to 10% prior to conditioning and both use a 59 m-% H2O2 solution concentration. The black line in Figure 3b shows that temperature at 40 °C allows a higher ppm vH2O2 than temperature at 23 °C, represented by the blue line

We can further examine the effect of temperature by looking at humidity, both relative humidity and relative saturation. Figure 4, shows ppm vH2O2 with temperature at 5°C. Relative saturation is on the x axis and relative humidity is on the y axis. The coordinate lines within the x and y axes represent H2O2 vapor concentration from 0 ppm to around 500 ppm. The 0 ppm line represents a vapor that has been created using pure water only. When the H2O2 solution concentration is increased, the vH2O2 also increases. Theoretically, the line along the X-axis represents a vapor that has been created using 100% H2O2 liquid. Condensation occurs once relative saturation is 100% RS and ppm vH2O2 cannot be increased thereafter. Thus, with temperature at 5 °C, initial relative humidity at 0% RH, and vaporizing an H2O2 solution of 100 m-%, the theoretical maximum achievable ppm vH2O2 is 548 ppm.

All conditions in Figure 5 are the same as Figure 4 (0% RH, 100 m-% solution), and only temperature is changed to 50 °C. The theoretical maximum achievable ppm vH2O2 is now 13,019.

Figure 6 shows the maximum ppm vH2O2 at various temperatures and using different H2O2 solution concentrations. We compare two frequently used H2O2 solution concentrations: 35% and 59%. The blue trend line represents a 35% H2O2 solution. With temperature at 40 °C, the maximum achievable ppm vH2O2 is 4,210. At the same temperature (40 °C), a 59% H2O2 solution gives a maximum achievable ppm vH2O2 of 5,461.

H2O2 Vapor Concentration & Condensation Point

Increasing the H2O2 vapor concentration decreases the amount of water vapor the air can hold and condensation occurs sooner.

Each point in Figure 7 is a condensation point, meaning that RS = 100 %. Temperature is on the x axis and ppm vH2O2 is on the y axis. The curve in the graphs shows the maximum relative humidity at a given temperature and concentration of ppm vH2O2.

As shown, at 20 °C and 300 ppm hydrogen peroxide, relative humidity will be 60% and relative saturation will be 100%. If we increase vH2O2 to 600 ppm at 20 °C, relative humidity is then 39 % and relative saturation is 100 %. By increasing the air temperature to 40 °C with vH2O2 concentration at 300 ppm, relative humidity will be 87% and relative saturation will be 100%. The higher the temperature, the more water vapor the air can hold; relative humidity is increased.

Four vH2O2 Process Parameter Rules

1. Lowering the initial levelof humidity increases the amount of H2O2 vapor that can be used before condensation.

2. A higher concentration H2O2 solution increases the H2O2 vapor that can be used before condensation.

3. Increasing the temperature increases the amount of water and hydrogen peroxide vapor the air can hold, thereby increasing the maximum achievable ppm vH2O2.

4. Increasing the H2O2 vapor concentration decreases the amount of water vapor the air can hold, and therefore, condensation occurs sooner.

Conclusion

We have demonstrated how an in-depth understanding of the relation between critical process parameters allows for the development of effective, repeatable vH2O2 bio-decontamination cycles. Single-parameter measurement is often insufficient for monitoring and ineffective for process control. We have also shown why relative saturation is an important value in accurately predicting condensation. This is why Vaisala’s unique PEROXCAP® technology measures multiple parameters in a single sensing unit, including: hydrogen peroxide vapor ppm, temperature, dew point, vapor pressure, and humidity as both relative humidity and relative saturation.