Sustainable Design of Chemical and Biochemical Processes: The Role of Models and Modelling

Models play a very important role in the sustainable design of chemical and biochemical procsses. The design related calculations use models of different scales, from different sources and of different types. The reliability, consistency and applicability of the models are issues that need to be addressed by methods specially developed for sustainable design of processes.

A systematic methodology for sustainable process design and a corresponding software (SustainPro) has been developed by the authors. This methodology can be applied for sustainable design and analysis of chemical processes operating on batch as well as continous operation modes. A computer-aided framework has been designed and implemented to allow the integration of models and data from various sources with the work-flow and data-flow corresponding to sustainable design of processes. At each step of the work-flow, model-based methods and tools are applied with their corresponding data-flow requirements.

The methodology (and the software, SustainPro) first performs a flowsheet decomposition based on the identified component mass and energy ?paths?. Then it calculates a set of mass and energy indicators that trace the paths of the components ?mass-flows? and ?energy-flows? as they enter (or generated) and leave (or consumed) the process. With this information, the critical points of the process are identified, followed by a sensitivity analysis to locate those critical points which have the potential for effecting the most sustainable improvements in the process. Through this sensitivity analysis the target indicators are selected and once the target values are established, another sensitivity analysis is performed to determine the design and the operational process variables that most influence the targets. The next step involves the matching of the defined targets with process designs in terms of changes in the sensitive design and operational process variables. This last step is performed using a set of algorithms for process synthesis that allow the generation of feasible flowsheet or operational alternatives. In order to determine if the generated alternatives are better than the reference design, a set of sustainable metrics and safety indices are used as performance criteria.

from the above description, it is clear that models and/or data are used in almost all steps of the methodology. For example, process simulation models (and corresponding data) are employed to establish the reference design and for analysis of design alternatives. Indicator-models are needed to identify process points that can be targeted for improvements. These models use process simulation data, cost data as well as physical properties data. For evaluation of sustainable design alternatives, models for metrics (sustainability and safety) are needed. Also, models and associated data for LCA would be needed if this option is to be used. Finally, for generation of design alternatives, model-based process synthesis methods are needed. Note that almost all the models employed at various steps of the design work-flow, employ parameters that are usually obtained from different sources. The models need to be qualitatively correct during the synthesis steps while they need to be quantitatively correct during the final design calculations.

The presentation will highlight the work-flow and data-flow for the developed systematic methodology for sustainable process design together with the models used at each step. The need and use for a computer aided modelling tool to quickly generate, analyze and adopt models for the various steps of the design work-flow will be illustrated. The uncertainty of model parameters and their effect on design decisions will be highlighted through two case studies operating in batch and continous modes of operation.