Tratamiento de emisiones de COVS en la industria química farmacéutica mediante oxidación térmica regenerativa

Abstract

The organic fine chemical industry, for the development of its processes, needs the organic solvents as reaction medium. These solvents are called volatile organic compounds (VOCs) which emissions must be treated suitably due to the most common of them have several risks, both environmental and health, so the emission limit values (ELV) of these are fixed by the Royal Decree 117/2003. Among the best available technology for the VOCs abatement it's found the regenerative oxidation thermal. The process is based on heating a gas mixture with air over the ignition point and maintain it at a high temperature during the enough time to complete the combustion to CO2 and H2O. The resident time (tr), the average temperature of the chamber (ATC) and the oxygen concentration in the mixture are the factors which affect the efficiency of the process. As by-products of oxidation process, in the presence of a source of chlorine, are formed polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). These aromatics tricyclic hydrocarbons are very stable, fat soluble and bioaccumulative substances with extremely toxic properties in the most of its congeners. In this study, an exhaustive bibliographic revision was performed and a detailed study of two regenerative thermal oxidizers (RTO) was done. For it, at the beginning, the strategies about sampling and analysis were fixed in order to obtain the composition of waste steam, as well as the exhaust gas and bottom ash of the post-combustion chamber. Later, several operation conditions (ATC and tr) were tested, with the objective of relate the composition of exhaust gas with the waste steam composition and the operation conditions verifying, at the same time, the ELV compliance in order to determine the most suitable operation conditions for the VOCs treatment. The VOCs destruction efficiency, for the tested conditions, raises with the ATC, linked to high tr, turbulence and oxygen concentration values in the combustion gases. The total organic carbon (TOC) and individual VOCs of exhaust gas are related with the waste steam concentration of these, the ATC and tr. Even if the environmental prescriptions and the recommendations about the RTO design for the chlorinated VOCs treatment are accomplishing, inside the ceramic media chamber, located in the post-combustion zone, the PCDD/F formation occurs. This is due to the optimum conditions to the formation of these compounds by de novo route, precursor route or homogeneous synthesis, how are the temperature conditions (from 950-1100 ºC to 200 ºC), tr (60-120 s by cycle), active catalyst presence, porous medium for fix them, incomplete combustion products, non-destroyed VOCs and a chlorine source. So that, in function of catalyst nature and the relative position inside the ceramic medium will be given the optimum conditions for the PCDD/F formation by one route or another, due to thermal gradient which is established in it. With an ATC > 950 ºC and tr > 2 s conditions is produced an increase both in the PCDD/F and CO concentration as well as mass emission flowrate of CO2 so that an increasing value of ATC and tr doesn't generate emissions improvement in test conditions. In conclusion, in the ATC = 950 ºC and tr = 2 s case are achieved the goals imposed by the integrated environmental authorization with a wide margin of safety.