Date of Award

2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering

Department

Chemical Engineering

First Advisor

Angelo Lucia

Abstract

Energy use in many process industries is dominated by separation processes. As energy costs are rising rapidly, there is a renewed interest in better methodologies for the synthesis, design and/or retrofitting of separation processes. In this thesis, a novel method for determining energy efficient process designs based on finding the separation with the shortest stripping line distance is proposed. A problem formulation based on mixed integer nonlinear programming (MINLP) is given and a global optimization algorithm is presented for determining energy efficient process designs. A variety of examples of separations involving ideal, non-ideal, azeotropic and reactive mixtures are used to demonstrate the versatility and advantages of the shortest stripping line distance approach over available methods in literature. One of the major advantages of the proposed methodology is that it can be used to identify minimum energy requirement for multi-unit processes such as hybrid separations involving extraction followed by distillation and reaction/separation/recycle processes.

The proposed shortest stripping line distance method is extended and a two-level distillation design procedure is developed for finding portfolios of minimum energy designs when specifications are given in terms of key component recoveries. It is shown that the proposed two-level design procedure is flexible and can find minimum energy designs for both zeotropic and azeotropic distillations. It is also shown that the two-level design method encompasses Underwood's solution, when it exists, and can find minimum energy designs when Underwood's method is not applicable. This two-level design approach also overcomes the well-know limitation of distillation line methods of sensitivity of column profiles to the product compositions.

Non-pinched, minimum energy distillation designs are an important and often overlooked class of distillation designs that provided added economic advantages in practice. All current methods for designing distillation columns available in literature are based on the concept of pinch points and are incapable of finding non-pinched, minimum energy solutions. In contrast, it is demonstrated that shortest stripping line distance approach is capable of systematically and reliably finding non-pinched, minimum energy distillation designs as well as providing insights into the reasons for the existence of non-pinched, minimum energy design. These reasons include · trajectories that follow unstable branches of a pinch point curve in azeotropic systems, the inherent looping structure of trajectories in hydrocarbon separations, and the presence of ancillary constraints in multi-unit processes like extraction/distillation.

Several examples are studied and many numerical results and geometric illustrations are presented in each section show that the shortest stripping line distance methodology is indeed a powerful and versatile tool for designing energy efficient processes and can be considered as a next generation method for conceptual design of energy efficient chemical processes.

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