A milestone in process safety was the 1974 Flixborough explosion in the United Kingdom the caused twenty-eight deaths. Following, the accident interest in chemical process industry safety increasd, as well as from government regulatory organizations and the general public. Since 1978, interest in inherent safer chemical processes and plants has grown, and that growth was particularly rapid in the 1990s . Several studies has proposed tools for evaluation and estimation of chemical process and plants such as Dow Fire & Explosion Index and the Dow Chemical Explosure Index . Also, Bollinger et al. (1996), Gupta and Edwards (2002), Khan and Amyotte (2003) developed methods for inherent safer design. Most of indices adopted relative scoring method to measuring inherent safety design. The major two indices are Prototype Inherent Safety Index (PISI) by Lawrence in 1996  and Inherent Safety Index (ISI) by Heikkila in 1999 . Actually, ISI is focused on process units and structure in basic design step. Especially, it is incorporated into process synthesis using computer aided process design tool. Also, several attempts are reported incorporation into process life cycle; chemical product design (research), basic process design, detail process design and construction, operation and management and decommissioning. However, there have some considerations to improve applicability of inherent safety design index. One of the considerations is effect of mixture which is flash point, lower and upper explosive limits and threshold limit value (TLVs). Additional, the dependence of temperature and pressure of process are considered. The second of consideration is the lack of model and knowledge of process, so, in this paper, we will suggest fuzzy logic based model to express uncertainties.
As mentioned above, existed inherent safety index has several modifications to improve applicability. The proposed inherent safety performance index (ISPI) consists of chemical characteristic index (flammability, explosiveness, toxicity and reactivity) and process unit index (temperature, pressure, inventory and equipment). Also, the complexity of process, process safety level index and stability of process are multiplied in final process inherent index. In addition, the method is integrated with process simulator to get process operation date. The proposed methods are incorporated to computer aided tool using Matlab interface and process simulators.
The evaluation was done by using the six process routes for manufacturing methyl methacrylate (MMA) as a case study. To validate our proposed method, the experts ranking and several existed index are compared.
The proposed methods can be guideline to decision makers of chemical process design when the safety and economic have conflicts because of government aspects and for practical engineering reasons. It can make easy to affect the process configuration and inherent safety in the conceptual design phase than in the later phase of process design.
 CCPS, Inherent safer chemical process: A life cycle approach, 2nd edition, New York: American Institute of Chemical Engineers, 2009.
 Dow Chemical Company, DOW’s Fire & Explosion Index Hazard Classification Guide. 6th edition, New York: American Institute of Chemical Engineers, 1987.
 D. Lawrence, Quantification inherent safety of chemical process routes, a doctoral thesis, Loughborough University, 1996.
 A. M. Heikkila, Inherent safety in process plant design: an index-based approach, a doctoral thesis, Technical research center of finland, 1999.
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