Phase equilibrium studies of NFM and toluene with heavy hydrocarbons and the conceptual process design of an aromatics recovery unit.

Abstract

Distillation and extraction are commonly employed phase separation techniques, and improved efficiency and cost reduction in these large-scale processes are motivating factors behind thermodynamic equilibrium investigations. This first objective of the research undertaken was phase equilibrium studies of two ternary systems comprising of a heavy hydrocarbon and toluene, with the suitability of NFM as an extraction solvent investigated, due to its good selectivity and heat stability (Xia et al., 2008). The other objective was the development and simulation of a conceptual process design using Aspen Plus V8.4 to demonstrate the separation and recovery of aromatics using NFM, and to make a comparison to an existing process in terms of energy and cost efficiency. Ternary liquid-liquid equilibrium (LLE) phase compositions were generated for the systems n-nonane (1) + toluene (2) + NFM (3), as well as n-decane (1) + toluene (2) + NFM (3). The measurements were conducted at 303.15 K, 323.15 K, and 343.15 K for each system. The modified apparatus of Raal and Brouckaert (1992) was used, with the latest modifications to the cell incorporating an adjustable temperature sleeve and magnetic stirrer (Narasigadu et al., 2014). The uncertainty in temperature of each cell was 0.02 and 0.01 respectively. Composition uncertainty was minimized by ensuring that phase composition samples were within 1% of the repeatability error for the average absolute deviation of at least 3 samples taken. Samples were analysed using gas chromatography. The ternary systems measured in this work were modelled in terms of the NRTL model (Renon and Prausnitz, 1968) and the UNIQUAC model (Abrams and Prausnitz, 1975). Calculated RMSD values were between 0.002 and 0.02 for both models, indicating that the models represented the data satisfactorily, with the NRTL model displaying superior representation due to lower RMSD values compared to UNIQUAC. The effectiveness of using NFM an alternative solvent to extract toluene from a mixture containing n-nonane and n-decane was evaluated by determining the distribution coefficient, selectivity, and separation factor. A process design simulation was developed using Aspen Plus V8.4 for the separation of benzene, toluene, ethylbenzene and xylene (BTEX) isomers from a hydrocarbon mixture using NFM as the ABSTRACT ii solvent. Process conditions and column specifications were optimized by investigating numerous unit configurations and running sensitivity analyses on these parameters. The aim was to target a recovery of at least 99% aromatics, which was achieved. A sequence of columns was used to effect the aromatics recovery, consisting of a counter-current liquid-liquid extraction column, followed by four distillation columns in series. The simulation results indicated that the process would consume at least 11 kcal/kg extract less energy than the sulfolane process. This manifests as lower heating and steam requirements, resulting in reduced costs of at least R19 million per annum.