Capture

AGA AB ndustrial operators worldwide expend considerable time and money to control the release of volatile organic compounds (VOCs) to the atmosphere. Such emissions react with nitrogen oxides to form photochemical oxidants in I the troposphere. Resulting ground-level ozone or smog causes irreparable damage to crops and has been implicated in forest decline. Certain VOCs, particularly chlorinated compounds, are toxic to human health, and many VOCs are malodorous. VOC emissions from a process are controlled in two fundamental ways: Operating conditions are changed to minimize the use of organic compounds. And, control equipment is installed to to capture or destroy VOCs from the exhaust stream. Process changes may involve switching to less harmful solvents, revising operating and maintenance practices, or installing upgraded versions of process equipment. Such modifications can drastically reduce VOC level. However, today's strict regulatory thresholds often call for the installation of end-ofpipe controls. The most common VOGcontrol methods are adsorption, a b sorption, condensation and incineration. To choose the most applicable method for a particular application, the following parameters must be considered: Nature, number and concentration of VOCs in the effluent Exhaust stream flowrate and temperature Viability or desirability of VOC recovery Capital and operating costs Reliability of equipment Required operating time It is not within the scope of this article to present detailed economic comparisons of the competing VOC-control techiques mentioned above. But the results from a recent study comparing adsorption, absorption and condensation are presented in Table 1. Condensation is a well-known VOC-control technique, which is most often used for exhaust streams with relatively low flowrates or high vapor concentrations. To capture organics with relatively low volatility (such as toluene), conventional condensation systems typically use cooling water or refrigeration to attain temperatures to -40°C. However, since most VOCs need substantitally lower condensing temperatures, enhanced condensation is often required. Such systems typically rely on cryogenic coolants, or cascaded, refrigeration units based on chlorofluorocarbon chilling agents. For years, cryogenic condensation with liquid nitrogen has been overlooked due to it5 perceived high operating costs. However, in certain cases, cryogenic condensation is competitive, both technically and economically, thanks in part to its relatively simple and straightloorward operation, which requires very little operator attention. Since the entire exhaust stream must be cooled to condense the offending vapors, the operating costs for a cryogenic condensation system may be prohibitive beyond a certain flowrate, or below a certain VOC concentration. As a general rule of thumb, cryogenic condensation is best suited for flowrates below rough1 1,000 Nm3/h, or vapor concentrations above about 40