The need for sustainable development has challenged the chemical process industries to seek new approaches to tackle the waste problem. This includes exploitation of popular, commercial tools such as process simulators to evaluate process retrofit options. While process simulators are useful, their application to waste minimization problem is not straightforward as considerable knowledge, skill and expertise are required in the part of the user to identify the key units and variables that control the overall waste feature of the plant. This shortcoming of the process simulator-based approaches has indeed been highlighted during a joint workshop organized by the U.S. Environmental Protection Agency, the Department of Energy, and the Center for Waste Reduction Technologies [1]. Their recommendation was to develop an integrated framework comprising of different PSE tools. The rationale behind this is that since waste minimization is a multifaceted problem, its analysis requires the application of different computational tools, each providing a different perspective.

We present one such framework that integrate two computational tools for sustainability analysis of chemical processes: ENVOPExpert and SustainPro. ENVOPExpert is an expert system for waste minimization analysis of process plants [2]. It performs a two-step analysis to derive cost-effective waste solution: heuristic waste analysis and simulation-optimization procedure. First, the process is analyzed to identify the sources of wastes and to propose qualitative alternatives for minimizing them. Next, quantitative assessment of those alternatives is performed using a simulation-optimization technique to obtain reduction in the waste flow. SustainPro is a computer aided system for identification, screening and evaluation of sustainable design alternatives of chemical processes [3]. It applies a set of mass and energy indicators to determine potential alternatives that can be applied to improve the process. Its methodology is based on reverse design approach, where target values are assigned to the indicators and the variables that are most sensitive to the indicators are evaluated for the objective of improving the process. In this work, we combine the twos to derive comprehensive waste solutions. This is done by capitalizing the strengths of each tool. The heuristic based analysis of ENVOPExpert can quickly identify top-level design alternatives and evaluate trade-offs between the environmental and economic objectives (Pareto solutions). The indicator model of SustainPro is useful for highlighting potential alternatives and screening the Pareto solutions at in-depth level through its metrics analysis. The overall procedure involves three steps. First, we capture the information about the process. Here, the flowsheet information from HYSYS simulator is used as the base design information. This information is used by ENVOPExpert to generate qualitative waste minimization alternatives to the process. In parallel, the same information is also processed by SustainPro for generating the process alternatives, all of which are combined with the solutions of ENVOPExpert. The next stage of the procedure involves identifying process variables that are candidates for optimization. For this, functional modeling algorithm of ENVOPExpert is used to highlight process variables that control the waste generation in the process. In the mean time, the minimum and maximum range of these variables are set by SustainPro through its indicator analysis. Finally, the resulting multi-objective optimization problem is solved using meta-heuristic based simulated annealing method. Detailed screening of each of the Pareto solution is performed next by SustainPro to rank it in order of preference. We illustrate our integrated framework using an ammonia production case study.

Reference:

1. Eisenhauer, J., & McQueen, S. (1993). Environmental Considerations in Process Design and Simulation. New York: American Institute of Chemical Engineers.

2. Halim, I., & Srinivasan, R. (2002). An Integrated Decision Support System for Waste Minimization Analysis in Chemical Processes. Environmental Science and Technology, 36, 1640.

3. Carvalho, A., Matos, H.A., & Gani, R. (2007). Design of Sustainable Chemical Processes: Systematic Retrofit Analysis Generation and evaluation of alternatives. Process Safety and Environmental Protection – Accepted for Publication