(287f) PDMS Membrane for O2/O3 Gas Separation Using a Spiral Wound Module

Ozone (O₃), a triatomic oxygen molecule, is widely used as an oxidant and disinfectant in water and wastewater treatment. Given its rapid decomposition into oxygen (O₂), on-site generation is required, usually using electrical discharge ozone generators fed with oxygen-rich streams. The problem with this process is that only a small fraction of the initial stream is converted to ozone (6 to 13 wt.% of O₃), resulting in high energy consumption.

To address this, ozone-selective membranes can be used to separate oxygen and ozone, purifying the generated ozone stream, and yielding an O₃-enriched gas stream. Therefore, the main goal of the present work is to develop a technology for ozone stream purification based on membrane separation, enabling: i) O₃ generation at lower concentration utilizing a lower specific power; ii) the recovery of O₂ from the mixture of O₂/O₃ and its recycle to the O₃ generator, thereby reducing its consumption; iii) production of ozone streams at higher and safer concentrations, increasing the effectiveness of ozone-based treatment processes because the higher partial pressure provides a larger driving force for gas-liquid mass transfer, and consequent reaction with pollutants in the liquid phase.

A PDMS membrane with an effective area of 0.14 m² was employed in a spiral wound module to study O₂/O₃ separation. Single-component (with oxygen) and multi-component (oxygen and ozone) experiments were carried out at ambient temperature (T=20-25 ºC) and at a feed and permeate pressure of 1-2 bar and 1 bar, respectively. The laboratory gas separation set-up, shown in Figure 1, consists of the following components: i) oxygen cylinder; ii) mass flow controller to regulate the flow of oxygen that is fed into the system; iii) electrical discharge ozone generator ; iv) spiral wound membrane module; v) nitrogen line, where nitrogen can be used as a carrier gas to increase the partial pressure difference (driving force in a gas separation process); vi) pressure gauge to measure the pressure in the retentate stream up to 10 bar; vii) mass flow meter to measure the permeate stream flow; viii) ozone analyzer capable of measuring ozone concentration in the feed, permeate, and retentate streams; iv) catalytic O₃ destruction unit and O₃ destroyer flask (containing a 2 wt.% KI solution) to ensure total ozone destruction.

The best result was obtained for a nitrogen flow rate of 0.265 SLPM (carrier gas), with an oxygen and ozone permeance of 9.05×10^-8 mol·m^-2·s^-1·Pa^-1 and 3.45×10^-8 mol·m^-2·s^-1·Pa^-1, respectively, and a real O₂/O₃ selectivity of 1.43, which is equivalent to an increase of 6 to 8.3 wt.% of O₃ in the generator output stream. It can therefore be concluded that the PDMS membrane was able to achieve O₂/O₃ separation. Additionally, membrane stability tests will be carried out to assess the feasibility of the membrane as a long-term O₂/O₃ gas separation process. For this purpose, stability tests will be performed on a flat sheet module (effective are of 23 cm²), since the components inside the spiral wound module are unknown, as well as its resistance to ozone. The PDMS membrane was exposed to ozone and the O₂ permeance parameter was determined before ozone exposure and after every 24 hours of exposure to understand whether ozone affects its properties. SEM analysis were performed on the membrane at the end of the test (total of hours of ozone exposure) and on a membrane without exposure to ozone.