Masters Theses

Date of Award

6-1988

Degree Type

Thesis

Degree Name

Master of Science

Major

Chemical Engineering

Major Professor

Charles H. Byers

Abstract

The Oak Ridge National Laboratory (ORNL) has proposed a process for purifying substitute natural gas streams by absorption-fractionation in liquid CO2. In order to further the development of this process, additional vapor-liquid equilibrium data was needed to supplement the literature data. The primary objective of the study presented here was to design, construct, and test an apparatus at the Oak Ridge National Laboratory to measure vapor-liquid equilibrium properties of binary and multicomponent systems which are of interest to this process.

The CH4-CO2-H2S system and the associated binary systems have been studied in this work between 270.0 K and 288.2 K in order to test the accuracy of the experimental apparatus, supplement the literature data, and test the ability of the Peng-Robinson (PR) and Redlich-Kwong-Soave (RKS) equations of state to predict the equilibrium properties of these systems. Binary interaction parameters have been determined for each of the binary systems and used to calculate the equilibrium phase properties of the binary and ternary systems. A comparison of these calculation with the experimental results is presented. Experimental CO2 vapor pressure measurements deviate less than 0.5 percent from the literature. Also, calculated phase equilibrium measurements of the the CH4-CO2 system using both the PR and RKS equations disagree with the experimental results by less than 3.5 percent and measured liquid phase compositions differ from the calculated by less than 15 percent. Similar results, less than 2 percent deviation on pressure and less than 13 percent for liquid phase composition, were obtained when comparing literature data for the CO2-H2S system to the calculations of the PR and RKS equations.

Questionable phase compositions were obtained when measuring systems containing H2S, especially when the H2S composition was greater than 90 percent. Large deviations resulted when the calculated equilibrium properties of both equations were compared to the experimental results for the CH4-H2S system. The calculated pressures, for this system, were up to 12 percent below the experimental values and the experimental liquid phase compositions differed from the calculated values by greater than 50 percent at pressures up to 700 psia. This difference was smaller, less than 15 percent for liquid phase compositions, at pressures of 900 to 1000 psia. The PR equation has been found to provide more accurate pressure results (12 percent deviation versus 29 percent) for the ternary mixture than the RKS equation in the temperature and pressure range of this study.

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