CORRELATIONS Correlation of Viscosity of Coal Liquids Raj Sharma* and Sunil Goel Department of Chemical Engineering, Malaviya Regional Engineering College, Jaipur 302017, India Applicability of viscosity correlations for predicting viscosities of coal liquids was tested against experimental viscosity data of seven coal liquids. The data considered in this work were at ambient pressure and moderate temperatures (294-360 K). The predicted viscosities were much lower than the experimental viscosities with an average absolute deviation of about 30% at best and 70% maximum. It appears that the association effects owing to hydrogen bonding in coal liquids are responsible for this deviation since the petroleum correlations for viscosity predictions were developed for nonpolar, nonassociating fluids. A simple correction is suggested which helps in predicting coal liquid viscosities more accurately to within an average absolute deviation of 12%. Introduction Coal-derived liquids have always been considered as an alternative to petroleum crude, and after proper treatment and refining can be substituted wherever petroleum finds its uses. Transport properties of coal liquids, among other property data, are required for efficient design of equipment for processing these coal liquids. Viscosity is one such property which requires immediate attention. Information on viscosity is gener- ally required for designing process equipment such as heat exchangers, pumps, and distillation columns. Viscosity data for coal liquids is scarce, and a method for predicting their viscosities is of great practical interest. However, before attempting to develop a new predictive approach, it is important to determine the applicability of existing correlations for predicting vis- cosities of coal liquids. This article presents the results of testing the applicability and accuracy of selected existing petroleum-fraction viscosity correlations against the limited experimental data of coal liquids and their model compounds and our attempts at development of a new correlation for coal-derived liquids. Data Bank For our correlational effort, viscosity data for coal liquids of Sharma (1980) were used. An average error of (5% in the experimental values was reported by Sharma (1980). Table 1 presents a compilation of the coal liquids viscosity data used in this work. In addition to coal liquids, several model compounds were chosen on the basis of the work of Holliman (1979), representative of the chemical species believed to be present in coal liquids. Viscosity data for these model compounds and three model mixtures reported by Oshmyansky, et al. (1987) were used in this work and are summarized in Table 2. The entire data considered in this work is at ambient pressure and moderate temperatures (294-360 K). Application of Petroleum Correlations To Predict Viscosities of Coal Liquids and Pure Compounds Comparison of Viscosity Predictions with Ex- perimental Data. As an initial stage in the effort to correlate the viscosity data for coal liquids, the experi- mental viscosities were compared with the predicted viscosities using correlations developed for petroleum fractions. However, it is important to note that the conditions under which the coal liquids are formed and the hydrocarbon-type distribution often differ radically from those of petroleum fractions (Sharma et al., 1982). For example, coal liquids contain a significant amount of heteroatoms (2-20 mol %) and are more aromatic than the conventional petroleum fractions (Sharma, 1980; Holliman, 1979; Sharma et al., 1982; Probstein and Hicks, 1982). Several correlations exist for estimating viscosities of “mixtures” and petroleum fractions (Hanley et al., 1975; Hanley, 1976; Ely and Hanley, 1981; Baltatu, 1982; Twu, 1985,1986) and have been discussed in detail by Goel (1987). More recently, Moharam and Fahim (1995) extended the work of Peterson et al. (1991) to predict viscosities of heavy petroleum fractions and crude oils. Most of these correlations are based on the correspond- ing states approach with variations in the choice of reference fluid(s). In this work, Ely and Hanley’s method (1981) (mainly because it set the tone for later developments in correlations for predicting viscosities of undefined mixtures using the corresponding states approach) and Twu’s correlations (1985 and 1986) (because the industry-accepted bulk properties of un- defined mixtures and petroleum fractions as reference fluids are used to predict petroleum liquid viscosities) are applied for predicting coal liquid viscosities. Table 3 presents a summary of correlations considered in this work. Also included are methods applied for estimating the various characterization parameters of coal liquids for use with the Ely and Hanley correlation. As shown in Table 3, since Ely and Hanley’s correla- tion is based on the corresponding states approach, estimation of critical properties and the acentric factor (psuedocritical constants for defined and undefined mixtures) acquires utmost importance. There is no * Author to whom all correspondence should be addressed. Tel: 91-141-521 591. Fax: 91-141-520 954. E-mail: rajsh@ recjai.ernet.in. 3999 Ind. Eng. Chem. Res. 1997, 36, 3999-4007 S0888-5885(96)00733-6 CCC: $14.00 © 1997 American Chemical Society