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Thermodynamic Analysis of Distillation Columns

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Publication Date: 2012-May-04

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Paper Number:

Thermodynamic Analysis of Distillation Columns

by Thomas P. Ognisty

The M.W. Kellogg Company 601 Jefferson Ave.
Houston, Texas 77002

     Prepared for presentation at the
1993 AIChE Spring National Meeting Recent Advances in Distillation and Absorption-IT:

Improved Energy Utilization in Distillation

March 30, 1993

UNPUBLISHED


"AIChE shall not be responsible for statements or opinions contained in papers or printed in its publications."


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Introduction.

    The word thermodynamic means heat power or power developed from heat (1). Inthe broader sense, thermodynamicsdefines the relationshipsbetween all forms of energy. The rules of the transformations are formally known as the first and second laws of thermodynamics. The f n tlaw states that energy is conserved, whereas the second law indicates that there usudly is a loss associated with the transfer of energy.

    A distillation column utilizes heat to supply the work of separating the components in a feed stream into products. By analyzing the thermodynamics of distillation columns, the thermodynamic efficiency of the process can be quantified and ways to reduce or better utilize the energy consumption can be readily identified. Distillation columns are the most common separation method and consume some 95 % of the total energy used in separations(2). This amounts to roughly 3%of the energy consumed in the U.S.(3). Distillation has been shown to be a cost effective and a reliable means of separation for large throughput and high product purity(4). Since distillation is energy intensive and requires significant capital outlays, an endless quest to improve the economics has continued since the beginning of the industry.

    The following paper is intended to reintroduce the engineer to some basic thermodynamic concepts as applied to distillation columns, and then explain how to use the information to improve a design. The minimum work required for a particular separation and the amount of heat necessary to generate the work will be used as a yardsticks to measure thermodynamic efficiency. Lost work analysis will be used to identify thermodynamic inefficiency and the most cost effectiveareas for improvement. By examining the quantity of heat flow between the vapor and liquid on each stage, opportunities for using utilities at different temperatures or heat integration with other process streams can be realized. Lost work and heat flow analysis are powerful tools that can be used to quickly identify areas for improving distillation column design.

Thermodynamic Analysis of Distillation Columns

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AIChE March 30, 1993


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Concept of Availability and Exergy.

    To characterize the amount of energy required to produce the products from the feed stream, the concept of availability has become popular in recent years. The availability function, exergy or simply availability is usually referenced to an ambient temperature of 77°F (25°C) and is defined as (5,6,7)...