(674a) A New Approach to Energy Efficient Process Design

The rapidly rising costs of energy and U.S. dependence on foreign oil have prompted renewed interest in energy conservation and stewardship – in both residential and commercial use. In fact, energy has been one of the major platforms of the current federal administration. Energy use in the chemical industries is dominated by the cost of separation, particularly distillation. There are an estimated 40,000 distillation columns in the U.S. that consume approximately 18% of all of the energy in the manufacturing sector. Recent estimates put this use at 2.4 quadrillion Btu/yr. This is a staggering amount. However, distillation is perhaps the most versatile means of separation and thus will continue to be used in some capacity to address a wide variety of separation needs. Other current separation techniques simply are not competitive in terms of both volume produced and purity of product. Therefore, new synthesis and design methodologies for overall energy efficiency must not, in our opinion, dismiss energy needs associated with distillation but rather extend the current knowledge base for finding minimum energy requirements in separations to processes involving multiple units (e.g., hybrid separation schemes and reaction/separation/recycle processes). This is the approach adopted in this talk.

To allow engineers to find creative and energy efficient solutions to ever changing processing challenges, new methodologies are needed to support synthesis and design efforts. The particular design and optimization approach presented in this talk is based on the novel concept of shortest separation lines. Through new global optimization formulations, the proposed methodology 1) Represents a unification of existing methodologies for finding minimum flows and minimum energy requirements in the presence of feed, saddle point or tangent pinch points and applies to all distillations. 2) Easily finds minimum energy solutions that do not correspond to separation pinch points. 3) Is unaffected by the number of components and the presence of reverse separation. 4) Is readily combined with other synthesis methods such as the attainable regions approach. 5) Uses a back-to-front philosophy to identify correct processing targets for processes with multiple units (e.g., reaction/separation/recycle, hybrid separation schemes) such that overall energy consumption is minimized. 6) Provides knowledge of other solutions that have near minimum energy consumption. 7) Can provide starting values for more detailed rating optimization calculations. 8) Can be used to establish that longest and shortest paths are unifying geometric principles for the design of energy efficient chemical processes. 9) Solves problems other synthesis methodologies can not. 10) Enhances the teaching and practice of energy efficiency in process design through a simple and straightforward theory that is easily understood.

Basic theoretical results are presented that show that the concept of shortest separation lines is a fundamental principle in the energy efficient design of chemical processes. General optimization formulations are given and detailed numerical results for several examples are presented that show that the concept of shortest separation lines provides a clear and concise way of finding feasible designs that are energy efficient. These synthesis and design examples include single distillation columns with feed, tangent, and/or saddle pinch points, columns whose minimum energy solutions do not occur at a pinch point, single and multi-unit hybrid separation processes like reactive separation and extraction/distillation, as well as reaction/separation/recycle processes. Many geometric illustrations for binary, ternary and quaternary mixtures are used to elucidate key points.