(286d) Risk-Based Design of Metal Degreasing Process and Operation Considering Local Chemical Risks and Environmental Impacts

In ordinary metal processing, metal parts are greased to avoid possible friction and confrontation by pressing or cutting. Although the process oil for greasing is regarded as an impurity in the following processes and cleaning process for metal degreasing is inevitable, various chemicals, such as chloride compound, hydrocarbon and aqueous surface-active agent, are used as cleansing agents and a significant amount of such agents is released to the environment. While chloride compounds are widely used owing to their high capability for precision cleaning, inexpensiveness and nonflammability, they are volatile and human toxic. In order to reduce the emission volume and the chemical risks, many alternative agents and improved processes have been developed. However, the improvement of cleaning operation can reduce the emission volume of chloride compounds as well. Therefore, when we design or improve a metal degreasing process, process apparatuses and operations should be taken into account simultaneously. For that purpose, an appropriate and standard measure to assess the risks originated in the alternative processes considering operational conditions should be developed.

Most cleansing agents are produced by chemical companies and mainly used in small and medium-sized enterprises (SMEs). In the use, most SMEs carry out the cleaning operation by hand. After use, they are transferred to and treated by small waste treatment sites. Because of the lack of inclusive viewpoints, individual decisions on the process design by each enterprise may cause unforeseen effects elsewhere in the life cycle. Thus, in a standard measure, environmental impacts throughout the life cycle of cleansing agents as well as local risks around an individual process should be included simultaneously. This is because cleansing agents have intrinsic hazardous properties causing the serious damage on cleaning site and surrounding area, and have potentials of global warming and ozone depletion.

In this study, a method for risk-based process design considering local risks and environmental impacts is proposed. In the proposed method, the design of process and operation are taken into account simultaneously. A case study demonstrating risk-based design of a metal degreasing process is presented.

In the proposed method, the local risks are evaluated by source-specific risk assessment as actual, not potential, risks. For evaluation of actual risks, a local dispersion model of chemicals is employed to obtain the distribution of chemicals around the site and detailed process data are applied to calculate the changes caused by modifying operation. Using this source-specific information the actual situation around the site and the effects of the change of manual operation or settings of machines can be indicated. Environmental impacts throughout the life-cycle of cleansing agents are evaluated by life cycle assessment (LCA) as potential impacts.

A case study on an existing metal degreasing process design was performed. The existing cleaning process is to degrease connector terminal precisely using dichloromethane and an open-top washing machine with manual operations. The ambient surrounding of the site is residential area. At first, the existing process is evaluated using source-specific risk assessment on workers' and inhabitants' health risks and LCA on global warming. Based on the evaluation results, process alternatives are generated. In this step, the alternatives include the improvements of cleaning operation, and substitution of cleansing agent, washing machine and local exhausting machine simultaneously. Finally, the process alternatives are evaluated, and then the results are interpreted. In this evaluation step, alternatives substituting cleansing agent are evaluated on different types of risk such as ecological risk caused by surfactants released to the aquatic environment. This case study demonstrates that the trade-off problem between the local risks and the environmental impacts occurs among alternatives including the modification of process and operation, and shows that there is possibility of failure in the decision-making without both assessments on the local risks and the environmental impacts.