A New Approach to Energy Efficient Chemical Process Design
Advancing the chemical engineering fundamentals
Distillation, Absorption & Extraction - II (T2-10b)
Keywords: energy, distillation, design, reactive distillation
The rising cost of energy as well as increased demands from China and India have prompted renewed interest in energy conservation and stewardship – in both residential and commercial use. 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 remains 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 (i.e., McCabe-Thiele, Underwood’s method, the boundary value approach, and so on) for finding 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 cannot.
10) Enhances the teaching and practice of energy efficiency in process design through a simple and straightforward theory that is easily understood.
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, multi-column configurations, divided wall columns, single and multi-unit hybrid separation processes like reactive separation and extraction/distillation, reactive and extractive distillation, reaction/separation/recycle processes, and the influence of mass transfer on energy consumption. Many geometric illustrations for three and four component mixtures are used to elucidate key points and show the that the concept of shortest separation lines represents a true unification of all existing methods for finding minimum energy requirements. In addition, it is well known that methodologies such as the boundary value approach suffer from computational difficulties for mixtures involving four or more components due to sensitivity to trace component compositions in product streams. Therefore, other processing examples with mixtures of five or more components are presented to show how these sensitivity issues can be resolved and to showcase the true capabilities of our synthesis and design methodology based on the concept of shortest separation lines.
Presented Monday 17, 15:00 to 15:20, in session Distillation, Absorption & Extraction - III (T2-10b).
