A thermodynamic study of high temperature equilibria involving zinc chloride

Abstract

Molten zinc chloride has been used as a solvent and catalyst for the hydrocracking of heavy oils and coal slurries. Although there has been some systematic study of the mechanism of these catalytic reactions, little attention has been directed to the fate of the inorganic species which accumulate in the melt. These species derive from heteroatoms in the feedstock, and comprise mainly of zinc sulphide, zinc oxide, zinc-ammine chlorides and other minor chloride complexes. Their steady accumulation poisons the catalyst and renders it cost-inefficient.This research aims (i) to appraise the available thermodynamic data and (ii) to determine such thermodynamic parameters as are necessary to define equilibria involving zinc-containing species in a zinc chloride melt.The experimental technique used in this work is the Modified Entrainment Method (MEM). The principal modification of our method from conventional entrainment (or transpiration) is to isolate the sample from the flowing gas stream by a capillary of well-defined geometry so permitting equilibrium partial pressures to obtain above the sample. In this manner equilibrium thermodynamic results may be derived that are independent of the flow rate. A new design of silica capsule involving a re-entrant capillary has been developed for the MEM which replaces the earlier flared capillaries and eliminates the correction for their variable geometry. The following systems have been studied: 1) the vaporisation of ZnCl2 involving monomer and dimer species (360 to 710°C), 2) the reductive transport of ZnS in H2 (740 to 1180° C) (including the phase transition ZnS (1020°C)) 3) the transport of ZnS by HCl (750 to 990°C), 4) the thermal dissociation of ZnCl2(NH3)2 (170 to 400°C), and 5) the diffusion of Zn(g) in H2, He, N2 and Ar (420to 87 0°C) and of Hg(g) in the same gases (150 to 320°C).