Aqueous Phase Catalytic Oxidation is an effective post-treatment technology for the removal of low molecular weight polar (but non-ionic) organics which are not removed by sorption in the multifiltration (MF) train. Typical contaminants of this kind are ethanol, methanol, isopropanol, acetone, and urea. The MF effluent is saturated with oxygen and then fed into the reactor which operates at temperatures near 125C and pressures in the neighborhood of 30 psig. Inside the reactor, a packed bed containing a supported noble metal catalyst promotes the oxidation of dissolved organics by molecular oxygen. The reactor effluent is then degassed to remove excess oxygen and carbon dioxide. As an added benefit, most microorganisms do not survive the high temperature reactor environment.
The reactor operating pressure is determined primarily by the requirement to maintain water in the liquid phase. Hence minimal operating pressures are determined by the relationship between the vapor pressure of water and temperature shown below.
In the above, the curve describes the vapor pressure of water versus temperature (or the boiling point versus pressure). This is the maximum temperature which can be achieved for a liquid phase system at the given pressure. Conversely, for a given temperature, the curve also depicts the minimum operation pressure for liquid phase systems.
Aqueous phase catalytic oxidation reactors are embodied in two variations: VRA, and CATOX. The Volatile Removal Assembly (VRA) provides post-treatment for the American designed water processor for ISSA. This unit introduces oxidant into the reactor influent by direct injection of gas-phase molecular oxygen. The catalyst is proporietary (Hamilton Standard), but is believed to be composed of platinum on an alumina support. The VRA reactor is designed to oxidize organics to the corresponding carboxylic acids which, as ionic species, can be subsequently removed from the reactor effluent by ion exchange. The performance of the VRA reactor has been studied extensively during integrated water recovery testing at Marshall Space Flight Center.
The CATOX reactor differs primarily in the manner of oxidant addition and in the nature of the catalyst. In this system, molecular oxygen is introduced into the reactor influent using a hollow fiber membrane saturator in either single or dual stage configurations. A single stage system is schematically illustrated below.
Molecular oxygen equilibrates across the membrane according to Henry's Law,
where k is the Henry's Law constant, X is mole fraction of aqueous O2, and pO2 is the partial pressure of oxygen in the gas phase. Using this arrangement, the aqueous concentration is determined by the pressure applied to the gas phase side of the membrane. The carbon supported bimetallic noble metal catalyst used in the CATOX variant is also proprietary (UMPQUA Research). This system is designed to provide complete oxidation of all dissolved organics to form inorganic products, primarily CO2 and H2O.
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