Date of Award

Fall 2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Rakesh Agrwal

Committee Chair

Rakesh Agrawal

Committee Member 1

Mohit Tawarmalani

Committee Member 2

Joseph Pekny

Committee Member 3

Gintaras Rekaitis

Abstract

Separations account for as much as 85% of plant operating costs in chemical production; it is therefore important that they be designed with energy efficiency in mind. This can only be achieved if two things are achieved: the complete space of design options is known, and an accurate way is developed to compare all possible design options. For both membrane separation cascades and multicomponent distillation configurations, this dissertation explores methods for designing energy efficient separations.^ The operating cost of membranes used in production of nitrogen gas from air is largely driven by the compressors required to maintain a pressure differential. Optimization of the total compressor duty can reveal an ideal cascade arrangement and set of operating conditions for a given feed and recovery. With this optimization technique in hand, it is then possible to examine the effect of introducing extra stages to form intermediate stage cascades. Furthermore, the effect of varying the recovery of the nitrogen stream can be examined to discover a U-shaped relationship between recovery and energy requirement.^ Conventional distillation configurations use n – 1 distillation columns to separate a multicomponent feed mixture into pure products. Past research has identified a way to quickly and algorithmically generate the complete ranklist of regular-column configurations using an integer programming formulation called the matrix method. Using this method, a formulation is here presented for the complete nonlinear programming problem which, for a given configuration, can ensure the globally minimum vapor duty of the configuration. Furthermore, a set of nonlinear equations designed to represent the capital and operating costs of the system are described. The need for a global optimization algorithm in the formulation of the cost product is demonstrated by comparison with a two-stage search algorithm; in addition, the cost formulation is compared to that of the vapor duty formulation and the relative effect of capital and operating cost is weighed for an example feed.^ Previous methods based on Underwood's equations have no accounting for the temperature at which utilities are required. To account for this, a thermodynamic efficiency function is developed which allows the complete search space to be ranklisted in order of the exergy loss occurring within the configuration. Examining these results shows that this objective function favors configurations which move their reboiler and condenser duties to milder temperature exchangers. ^ A graphical interface is presented which allows interpretation of any of the above results in a quick and intuitive fashion, complete with system flow and composition data and the ability to filter the complete search space based on numerical and structural criteria. This provides a unique way to compare and contrast configurations as well as allowing considerations like column retrofit and maximum controllability to be considered.^ Using all five of these screening techniques, the traditional intuition-based methods of separations process design can be augmented with analytical and algorithmic tools which enable selection of a process design with low cost and high efficiency.

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