Evaluation of zinc oxide sorbents in a pilot-scale transport reactor: Sulfidation kinetics and reactor modeling

Under conditions of negligible external mass-transfer resistance, kinetic studies on the sulfidation of zinc oxide were performed using thermogravimetric analysis (TGA) for three sorbent samples comprising approximately 60, 80, and 100 wt % zinc oxide with the balance being inert binder. The size range of particles for each sorbent was quite broad, with average diameters in the range of 60-80 mum. Conversion-time profiles were obtained for each sample of sorbent over the temperature range of 482-593 degreesC at 101.3 kPa pressure using between 0.5 and 2 vol % of hydrogen sulfide in nitrogen. Initial reaction rate data for the three sorbent samples were correlated on the basis of the surface area of zinc oxide available for reaction. For the 60 wt % zinc oxide sorbent, an additional series of experiments was performed on different size fractions. The conversion-time profiles for all size fractions were found to be essentially identical at 593 degreesC, confirming that intrapellet (between-grain) diffusion was not significant. The reaction of hydrogen sulfide with the zinc oxide sorbent and the diffusion of gas through the zinc sulfide product layer (within-grain diffusion) were modeled successfully using a grainy-pellet model with a bimodal size distribution of grains. Results were also obtained for repeated sulfidation runs in a 11.54-m-long, 0.84-cm-diameter transport reactor at a temperature of 538 degreesC and a pressure of 2.05 MPa, using the 60 and 80 wt % sorbents. A simple plug-flow model was developed to describe the sulfidation of the zinc oxide sorbents, and predictions from this model were found to be in close agreement with experimental results.