Chapter 8
Vapor Liquid Equilibrium in Polycyclic Aromatic Compound Mixtures and in Coal Tars Downloaded by UNIV OF GUELPH LIBRARY on September 8, 2012 | http://pubs.acs.org Publication Date: January 24, 2005 | doi: 10.1021/bk-2005-0895.ch008
Vahur Oja1,2 and Eric M. Suuberg1,*
Suu
1
Division of Engineering, Brown University, Providence, RI 02912 Current address: Department of Chemical Technology, Tallinn Technical University, Tallinn, Estonia 2
This paper presents the results of an experimental study on the vapor pressures of mixtures of high molecular weight aromatic compounds. Of particular interest are aromatics with heteroatomic content, especially nitrogen and oxygen. Such mixtures are of some interest in understanding the behavior of practical systems such as coal tars. The results demonstrate that significantly non-ideal behavior can be obtained in mixtures of high molecular weight (several hundred daltons), in the presence of even a single heteroatom in a molecule. On the other hand, the presence of heteroatomic species does not automatically mean that simple mixing rules will not be followed. Coal tar mixtures follow ideal-like behavior, even in the presence of significant hydroxyl content.
© 2 0 0 5 American Chemical Society In Heavy Hydrocarbon Resources; Nomura, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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Downloaded by UNIV OF GUELPH LIBRARY on September 8, 2012 | http://pubs.acs.org Publication Date: January 24, 2005 | doi: 10.1021/bk-2005-0895.ch008
114 There exist very few data in the literature concerning the vapor-liquid equilibrium (VLE) behavior of mixtures of high molecular weight organics. This is because it is difficult to measure the vapor pressures of high molecular weight organics without their undergoing thermal degradation during measurement. This has typically required use of indirect vapor pressure measurement techniques. In this laboratory, one such technique, the Knudsen effusion method, has been used to measure the vapor pressures of pure aromatics (1), pure saccharides (2% and complex mixtures such as coal tars (3) and biomass tars (4). For the latter mixtures, a non-isothermal Knudsen effusion method has proven useful (5). The present study addresses the problem of mixture behavior in complex high molecular weight organics, using the Knudsen effusion method, to measure vapor pressures. Again, this permits avoiding the problem of thermal degradation of compounds during measurement, the occurrence of which would of course invalidate the measurements. Vapor pressure data on such mixtures are of particular importance in the fields of fuels processing and combustion, in which vaporization of such species is a key step. The main focus of the present study is the question of whether high molecular weight polycyclic aromatic compounds (PAC) mixtures can be regarded as "ideal", and therefore, whether Raoult's law can describe their V L E behavior. It is of particular interest to examine how heteroatoms might affect the V L E behavior. The present results must be viewed as only a first step in addressing this complicated issue.
Experimental The experiments involved preparing mixtures of known composition, and measuring their vapor pressures. In the case of many of the compounds of interest here, keeping temperatures low enough such that no thermal degradation would occur during measurements meant that the experiments were conducted under, or near, sublimation conditions. This might have had certain consequences for the results, as will be discussed below.
Equipment and Method Vapor pressures of selected mixtures were indirectly determined, using the implementation of the Knudsen effusion method published earlier (5). Briefly, this method involves the measurement of the rate of effusion of a compound (or mixture components) through a pinhole leak in a capsule that is suspended from one arm of a continuously recording microbalance. These results are readily related to vapor pressures. The measurements were conducted in a high vacuum (
