Quantifying Absorption Effects during Hydrogen Peroxide Decontamination

Abstract

Understanding mass transfer effects (e.g., condensation and absorption) of hydrogen peroxide is essential for clean room decontamination technology. For example, absorption of hydrogen peroxide in polymers often causes unwanted effects in the final aeration phase of a decontamination cycle. This currently leads to significant challenges in the design and operation of clean rooms or isolators. We address here the absorption of H₂O₂ in polymers, which is a key for an understanding of mass transfer effects. Specifically, we developed a novel experimental setup to measure the desorption rate from a polymer sheet. Together with a model for the diffusion in a polymer, we are able to theoretically predict the absorption and desorption kinetics of hydrogen peroxide. We then performed measurements of still unknown properties of hydrogen peroxide in polymers, i.e., the saturation concentration and the diffusion coefficient using our experimental setup. As expected, the diffusion in the polymer is the rate-limiting step for the absorption and the release of H₂O₂. We find that considerable amounts of H₂O₂ can be absorbed in certain polymers on a time scale of less than an hour and may lead to a temporary violation of the Occupational Safety and Health Administration safety level in a typical clean room.

Appendices

Appendix A—Mass Transfer Regime Determination

In order to understand the limiting factor in the release of H₂O₂ from the polymer, here we briefly analyze the ratio of the mass transfer resistances in the fluid and in the polymer phase. This ratio indicates whether a regime with transport limitation in the polymer or with limitation in the fluid phase is present.

A sketch of the concentration profiles during the desorption process is shown in Fig. 11. Initially, the polymer is assumed to be fully saturated with H₂O₂. The concentration of H₂O₂ in the fluid phase (e.g., in the aqueous solution used for the desorption experiments or in the surrounding air in the industrial application) is assumed to remain zero far away from the interface. During a desorption experiment, the H₂O₂ concentration in the fluid phase will slightly increase. However, typically this increase is small due to the large ratio between the volume of the fluid phase (e.g., the room volume) and the polymer. Hence, this increase can be safely neglected.

Fig. 11

Schematic sketch of the concentration profiles occurring during the desorption process from a polymer

The system compensates the occurring concentration difference by mass transport of H₂O₂ from the polymer phase to the fluid phase (see Fig. 11). During this process, H₂O₂ in the fluid and the polymer phase is in equilibrium only at the polymer surface. This equilibrium between a fluid phase and the polymer is defined by the partition coefficient K _i . Thus, next to the polymer surface we write:

$$ {K_i} = \frac{{{C_{{i,s}}}}}{{{C_{{i,f}}}}} $$

(A-1)

Here “i” denotes the diffusing species (in our case hydrogen peroxide), “s” denotes the solid, and “f” denotes the fluid phase (i.e., a gas or a liquid). The partition coefficient is the ratio of the solute’s concentration (i.e., H₂O₂) between two phases under equilibrium conditions. It is a function of the solute and the properties (essentially temperature and pressure) of the two phases in contact.

Estimation of the Mass Transfer Coefficient in the Gas Phase

To describe the characteristics of the mass transfer of H₂O₂ in the gas phase, the mass transfer coefficient β [m s⁻¹] has to be estimated:

$$ \beta = \frac{{Sh \cdot {D_{{i,\,{\text{gas}}}}}}}{L} $$

(A-2)

Here, Sh is the Sherwood number, D _i,gas is the diffusion coefficient in the gas, and L is a reference length for the flow. Sh depends on the flow conditions, and the flow of air with three different velocities u _∞ (0.1, 5, and 10 m s⁻¹) over a plane sheet with L = 0.2 m was considered for our analysis. The kinematic viscosity ν _gas of air at the temperature of 293.15 K was assumed to be 1.51E–05 m² s⁻¹. The Reynolds number is defined as:

$$ {\text{Re}} = \frac{{{u_{\infty }} \cdot L}}{{{\nu_{\text{gas}}}}} $$

(A-3)

For 10 ≤ Re ≤ 10⁷ and 0.7 ≤ Sc ≤ 70.000, the mean Sherwood number (i.e., the Sherwood number averaged over the length L) for the flow over a flat plate was estimated by using the Krischer and Kast correlation:

$$ Sh = \sqrt {{Sh_{\text{lam}}^{{2}} + Sh_{\text{turb}}^{{2}}}} $$

(A-4)

Sh _lam was calculated by using the Pohlhausen correlation:

$$ {\text{S}}{{\text{h}}_{\text{lam}}} = 0.664 \times \sqrt {\text{Re}} \times {\text{S}}{{\text{c}}^{{{1/3}}}} $$

(A-5)

while Sh _turb was estimated by using the Petukhov and Popov correlation:

$$ S{h_{\text{turb}}} = \frac{{0.037 \cdot {\text{R}}{{\text{e}}^{{0.8}}} \cdot {\text{Sc}}}}{{1 + 2.443 \cdot {\text{R}}{{\text{e}}^{{ - 0.1}}} \cdot {\text{(S}}{{\text{c}}^{{2/3}}} - 1{)}}} $$

(A-6)

For the calculation of the diffusion coefficient of H₂O₂ in air, the correlation of Fuller/Schettler/Giddings was used:

$$ {D_{{i{\text{,gas}}}}} = \frac{{1.43 \cdot {{10}^{{ - 7}}} \cdot {T^{{1.75}}}}}{{p \cdot M_{{G,12}}^{{0.5}} \cdot {{\left[ {\left( {\sum \upsilon } \right)_1^{{1/3}} + \left( {\sum \upsilon } \right)_2^{{1/3}}} \right]}^2}}} $$

(A-7)

Here p, T, ν, and M _G,12 refer to the pressure (1 bar), temperature (293.15 K), atomic diffusion volume and a mean molecular weight, defined as:

$$ {\text{M}}{{\text{W}}_{{12}}} = \frac{{2}}{{\frac{{1}}{{{\text{M}}{{\text{W}}_{{1}}}}} + \frac{{1}}{{{\text{M}}{{\text{W}}_{{2}}}}}}} $$

(A-8)

In this equation, MW₁ and MW₂ is the molecular weight of H₂O₂ and air, respectively.

The results of the calculation of the mass transfer coefficient for the flow over a flat plate are summarized in Table 5.

Table 5 Results of the mass transfer coefficient for three different velocities (L = 0.2 m, Sc = 0.79, D _i,gas = 1.92E–05 m² s⁻¹)

Now, the mass transport resistance in the gas phase with respect to the solid phase (i.e., the polymer; to compare mass transport resistances, they must be defined with respect to the same phase) can be defined as:

$$ {r_{{{\text{solid}}\;{\text{phase,}}\;{\text{gas}}}}} = \frac{{{K_i}}}{\beta } = \frac{{{K_i} L}}{{Sh\,{D_{{i,{\text{gas}}}}}}} $$

(A-9)

Here, K _i denotes the partition coefficient, and we used the value for water in LDPE as a typical example (i.e., K _i  = 5^.10⁻⁵, based on [1]). This value is thought to be also representative for the partition coefficient of H₂O₂ in polymers.

The mass transport resistance in the polymer can be defined as:

$$ {r_{{solid\,phase,\;polymer}}} = \frac{1}{{{\beta_{{polymer}}}}} = \frac{{{l_c}}}{{{D_{{i,\;polymer}}}}} $$

(A-10)

Here, β _polymer and D _i,polymer refer to the mass transfer coefficient in the polymer, and the diffusion coefficient of H₂O₂ in the polymer. l _c describes a characteristic length of the polymer sheet. Taking the ratio of the mass transport resistance in the polymer and in the gas, one obtains the expression for the Biot number shown in Eq. 1.

For our example, mass transfer of hydrogen peroxide to LDPE, D _i,polymer is approximately 1E-11 [m² s⁻¹]. Assuming a 1 mm thick sheet of this polymer, and a mass transfer coefficient in the gas phase of β = 0.027 m/s (see the value for u _∞ = 5 m s⁻¹ in Table 5), one obtains for the resistances in the gas phase and in the polymer 1.9E–03 and 1E08 m⁻¹ s, respectively.