(277e) Optimizing Work and Heat Flows in Sustainable Chemical Processes Using Attainable Regions

A chemical process aims to transform input materials into desired products. This transformation requires the supply or rejection of heat and work between the process and the environment. Careful selection of chemical pathways, process parameters, and heat/workflows is essential to optimize process performance. In pursuit of environmental sustainability, the upper limit for process performance is a reversible process, setting the targets for process design.

The GH diagram is a valuable tool for process synthesis and assessing process reversibility. This diagram plots enthalpy (H), which corresponds to the process heat flows, against Gibbs Free Energy (G), which corresponds to the process workflows. We can determine the heat and workflows required for any chemical process or pathway through the GH plot. The GH diagram offers the advantage of presenting and analyzing any number of processes or process steps with varying complexities on a single set of two-dimensional axes.

In order to represent all processes on the GH diagram, we first determine The Material Balance Attainable Region (AR^MB), which represents all possible product flow rates achievable for a given feed(s) for all possible processes. The dimension of the AR^MB corresponds to the number of independent material balances describing the relationship between the feed and product species. However, the AR^MB can be transformed into a two-dimensional convex projection in GH space, called the Thermodynamic Attainable Region (AR^T), which represents the set of heat and workflows for all possible processes converting the given feed(s) into any combination of defined products.

Leveraging the visual representation of the AR^T, we can systematically analyze and compare the inherent heat and workflows for different processes, feeds and products. To illustrate the impact of feed selection on heat and workflows, we will consider some simple examples. Additionally, we will compare the addition of work and heat using solar energy, either captured as heat or transformed into electricity. This analysis enables us to specify renewable energy mixes tailored to the chosen chemical pathway, minimizing cost and land use and facilitating quick comparison of the suitability of different processes for utilizing a known renewable energy mix.