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Chemical Process Assessment and Design Based on Green Degree Method

Xiangping Zhang, Chunshan Li, and Suojiang Zhang. Research Laboratory of Green Chemistry and Technology, Institute of Process Engineering, Chinese Academy of Sciences, P.O.Box 353, Beijing, China

Sustainable development and serious environmental problems provide great challenge for chemical industry and chemical engineers. How to develop and design a greener process has become an urgent issue at present. In this work, a systematic framework has been established to optimize and design a greener chemical process to improve the environmental performance of product and process while also offer economic incentives by considering two objectives, green degree value and economic benefit.

Green degree is defined as an integrated index for assessment of the degree of environmental benignancy of material, stream, unit and flowsheet of a chemical process. In this work, the green degree value was quantatively calculated by considering eight environmental impact categories, Global Warming Potentials (GWP), Ozone Depleting Potentials(ODP), Photochemical Ozone Creation Potentials(POCP), Acidification Potentials(AP), Eutrophication Potentials(NEP), Aquatic Toxicity Potential(ATP), Human Toxicity Potential by Ingestion(HTPI), Human Toxicity Potential by either Inhalation or Dermal Exposure (HTPE). A environmental impact category database containing 2665 kinds of chemcials was established for comprehensive asseeesment of the environmental impacts of a chemical process.

By incorporating the green degree into the objective functions, a systematic framework for optimization and design of chemical process is presented, the key steps are described as follows: 1) defining the scope and boundaries of a studied system; 2) performing the process simulation to obtain material and energy flow including composition and quantity; 3) carrying out an economic analysis of a whole process; 4) implementing the green degree analysis to determine the degree of environmental benignancy of raw materials, solvents, units, reaction and separation processes and etc; 5) integrating the results of green degree assessment and economic analysis into the process modelling and optimization to discover improvement opportunities and optimal strategy for process design.

The proposed framework was applied to a typical process for producing methacrylic esters (MMA), which is an important intermediate for synthesis of organic glass and resin. Using the rigorous process simulation technology with economic and green degree objectives, a number of the processes with different raw materials, solvents and reaction routes were evaluated and compared. A multi-objective model was established and the whole process was optimized with non-dominated sorting genetic algorithm. The optimal Pareto results provide sustainable guidance for design of a greener process of MMA. Key words: green degree, multi-objective optimization, process simulation, environmental assessment