Abstract
The release of volatile organic compounds to the atmosphere is harmful to human health and the environment. The printing industry is one of the highest contributors to VOC emissions in the UK. According to the Solvent Emissions Directive (1999), only 30 % (by weight) or less of solvents used in the printing industry can be released into the atmosphere as emissions; this is proving to be a challenge to the industry. Thus, the aim of this project is to develop and test a complete demonstration scale capture and regeneration system capable of reusing both, the adsorbent and the adsorbed VOC (in this case IPA). A prototype adsorber was built and tested at a lithographic printing company for the purpose of capturing the isopropanol (IPA) emissions, under industrial conditions. The prototype itself consisted of an extractor pipe and an adsorbent cartridge placed inside a vacuum cleaner. Adsorption was carried out by drawing in air from the printing machines where vapour emitted from the dampening solution (which comprised of about 10% by vol IPA and 90% by vol water) was concentrated. Three trials of varying inlet concentrations with an adsorbent Dowex Optipore V503 (Dow) and a one trial with activated carbon (AC), was carried out at the printing facility. The time taken until the start of breakthrough was approximately 86 minutes and 250 minutes (of printing time) for Dow and AC respectively. Results showed that, until the start of breakthrough, all of the IPA entering the bed had been captured by both adsorbents. Of the material captured on the adsorbent, the percentage that was IPA for Dow was 66 wt% to 80 wt% and the IPA percentage that was captured on AC was 54 wt%. The rest of the captured material on the adsorbents was found to be water. A higher IPA loading, however, was evident for AC as compared to Dow. Results on microwave (MW) regenerating the two adsorbents showed that maximum regeneration of 88% for Dow and 97% for AC occurred after 12 and 13.5 minutes of microwave irradiation respectively. Tiny flashes of light across and within the whole AC bed were evident frequently during the initial stages of MW regeneration. Thus, in terms of safety, the existence of tiny sparks during AC regeneration indicates that Dow is the safer of the two adsorbents. Fractional regeneration of Dow showed that maximum IPA content was found in the regenerate collected between the 6th and 9th minute while the lowest IPA content was found between 0 and 3 minutes. For AC, the percentage of IPA in the regenerate was also found to increase with irradiation time. An attempt was made to model the process. The first step was to obtain the pure adsorption isotherms at 298 K using an Intelligent Gravimetric Analyser (IGA). For the AC and Dow adsorbents, the Toth and the CIMF model fitted extremely well to the pure IPA and water isotherms respectively. The mixture isotherms were described by the virial equation. The results from the mixture experiments involving Dow showed ideal adsorption of the mixture; AC showed highly non-ideal behaviour. A mathematical model (compiled on Matlab), which incorporated the co-adsorption isotherms, was used to predict breakthrough times during fixed-bed adsorption. This model was able to predict breakthrough data for Dow fairly accurately. An economic analysis was conducted which shows that AC is the cheaper of the two adsorbents to use, subject to safety considerations. Overall, a system that captures and regenerates the IPA from the print works had been successfully developed and tested. Microwave regeneration was found to be favourable for both adsorbents since, no loss in adsorbent capacity was found after exposure to microwave radiation. On comparing the two adsorbents, Dow was found to capture a higher percentage of IPA than water as compared to AC. However, with regards to economic viability, AC was found to be the more economic adsorbent.