Modeling of a para-toluenesulfonic acid synthesis unit

The paper presents a simulation of a para-toluenesulfonic acid (p-toluenesulfonic acid) production unit based on the sulfonation of toluene with concentrated sulfuric acid. A process flow diagram for a batch process with a capacity of 200 t/y (as p-toluenesulfonic acid monohydrate) was developed, featuring closed recycle loops for toluene and sulfuric acid and aimed at pilot-scale and small-tonnage implementation with reduced consumption of fresh reagents. The model was implemented in Aspen Plus using the electrolyte thermodynamic property package ELECNRTL, which enables an adequate description of the thermodynamic properties and phase equilibria of the highly non-ideal “water – sulfuric acid – organic phase” system at high electrolyte concentrations. The component list includes toluene, water, and sulfuric acid as base components, while o- and p-toluenesulfonic acids were defined as user-added compounds. The synthesis stage was represented by a stoichiometric reactor accounting for the formation of an isomeric mixture; this was followed by sequential cooling and phase separation with toluene recycle to the loop, adjustment of sulfuric acid concentration to a range ensuring hydrate precipitation, and crystallization of p-toluenesulfonic acid monohydrate with subsequent filtration and washing of the crystals with a sulfuric acid solution to remove dissolved isomeric impurities from the product. After filtration, the mother liquors are sent to a regeneration section that includes hydrolysis of residual sulfonic acids to toluene and sulfuric acid, phase separation, and concentration of sulfuric acid with recycle of the regenerated streams back to the synthesis stage. A material balance was calculated for one production cycle, and production of the target product with a mass fraction of 98.9 wt% p-toluenesulfonic acid monohydrate was achieved.