2002 Volume 35 Issue 8 Pages 729-736
Implementation of polychlorinated biphenyls detoxification is being accelerated globally. We have developed an environmentally sound chemical polychlorinated biphenyls detoxification plant (codenamed: HM1), using the Ultraviolet ray (UV)/Catalyst Method. Safety analysis greatly facilitates communication on the subject of risk between the entity wishing to construct and operate the plant and the local residents.
In the UV/Catalyst Method, PCB is mixed with sodium hydroxide (NaOH) and isopropyl alcohol (IPA: solvent) to realize PCB concentration in IPA of 1 wt% and subsequently PCB is dechlorinated by two independent process steps. The first step is the UV irradiating process (UV process), and the second step is the catalyst reaction process. As a result, biphenyl, NaOH, acetone, and water are generated after PCB is dechlorinated. A distilling column is designed in order to separate IPA from the solution, and IPA can be recycled many times as a solvent of PCB. IPA and acetone may form an inflammable mixture after leakage.
Concerning HM1, first, hazardous events which produce severe accidents were identified by Failure Mode and Effect Analysis (FMEA); and secondly, leak of isopropyl alcohol (IPA), that causes fire/explosion, is defined as the most serious event. Based on this result, numerical safety analyses were carried out and the following findings were obtained.
• Thermal runaway experiment of residual liquid in the bottom of a distilling column: It was confirmed experimentally that no possibility of thermal runaway exists.
• Stress analysis of the distilling column after fire/explosion: The strength of the distilling column was proved.
• Impact evaluation concerning the surrounding facilities and residents after fire/explosion: It was confirmed that impact would be slight.
• Impact evaluation concerning exposure of residents to PCB: Exposure of residents to PCB would be negligibly small.
In conclusion, the standardized methodologies for both safety evaluations and procedures concerning PCB plants were proposed and their validity verified.