Siloxane treatment by adsorption into porous materials

• Published in: Environmental technology 2009-09, Vol.30 (10), p.1073-1083
• Main Authors: Ortega, D. Ricaurte, Subrenat, A.
• Format: Article
• Language: English
• Subjects: activated carbon fibre cloth; Adsorption; Applied sciences; Biofuels; biogas; Biological and medical sciences; Biotechnology; Carbon - chemistry; Charcoal; Chemical engineering; Environmental Restoration and Remediation - methods; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General treatment and storage processes; hexamethyldisiloxane; Humidity; Kinetics; Methods. Procedures. Technologies; octamethylcyclotetrasiloxane; Others; Pollution; Porosity; Reactors; Silica Gel; Silicon Dioxide; siloxanes; Siloxanes - chemistry; Temperature; Thermodynamics; Various methods and equipments; Wastes; Water Pollutants, Chemical - chemistry; Zeolites - chemistry
• ISSN: 0959-3330; 1479-487X
• DOI: 10.1080/09593330903057540

Siloxanes are widely used in different applications: health care, dry cleaning, household products, paints and coatings, paper, personal care, for example. This explains their prevalence in the environment. Because of their volatile nature, most of the time they are dispersed in the atmosphere, but they can also be present in the slurry from landfills. During anaerobic digestion, when the temperature goes up to 60 °C, siloxanes are volatilized, forming part of the biogas. Operational problems using biogas to produce energy, heat and hydrogen have been identified. At high temperatures the siloxanes are transformed into silicate dioxide (commonly called sand transmission). These white deposits may adhere to metal or catalytic substrate surfaces, seriously reducing equipment efficiency, and this can be a reason for changing equipment warranties. Consequently, elimination of siloxanes has become very important. Unfortunately, relatively little information can be found on this subject. Nevertheless some authors have described different analytical methods for siloxane quantification, and recent studies have looked at the presence of siloxanes in landfills and the restriction on the energy recovery equipment using the biogas produced. The growing consumption of siloxanes and silicones in industrial processes consequently increase their prevalence in the environment, hampering the use of biogas as a source of 'green energy'. Therefore, the principal focus of this study is the treatment of siloxanes. Their elimination was carried out using an adsorption process with four different porous materials: activated carbon cloths (ACC), granular activated carbon (GAC), zeolite and silica gel. Two representative siloxane compounds were used in this study, hexamethyldisiloxane (L2) and octamethylcyclotetrasiloxane (D4). Adsorption kinetics and isotherms in batch reactors were performed. It was observed that the mass transfer into the porous material was more rapid for the activated carbon than for the zeolite and silica gel, congruent with the porous structure of the material. Moreover, it was found that D4 is more adsorbable than L2, due to possible interactions between the solid surface and the physical structure of the cyclic compound (D4). The influence of humidity and temperature were also studied. The increase in the temperature reduced the adsorption capacities. The influence of humidity on the adsorption was investigated under dry air and humid air at 70%. No signifi