Hydrogen Production by the Cataylitic Auto-Thermal Reforming of Synthetic Crude Glycerol in a Packed Bed Tubular Reactor

Abstract

The target of this work was to develop an efficient autothermal reforming (ATR) process for the production of renewable hydrogen from synthetic crude glycerol (CG). Hence, the work was divided into three phases: (1) development of a high performance catalyst, (2) optimization of process variables, and (3) investigating the kinetics of the involved reactions. A portfolio of ternary oxide catalysts with a nominal composition of 5Ni/CeZrM (where M= Ca, Gd, Mg) was prepared, characterized and tested in the process. A series of experiments was conducted in a Packed Bed Tubular Reactor (PBTR) using a factorial design technique to investigate the effects of the different operating parameters. A rate model expression was then developed based on the experimental kinetic data. Synthetic CG was reformed over a modified cerium-zirconium support loaded with nickel catalyst (5%Ni/CeZrM) by a combination of partial oxidation and steam reforming reactions to generate hydrogen via an overall auto-thermal process. Amongst the tested catalysts, calcium promoted one showed the highest catalytic activity due mainly to its reducibility and nickel dispersion properties. The prepared catalysts were characterized by N2 physisorption (BET), thermogravemetric analysis (TGA), temperature programmed oxidation (TPO), temperature programmed reduction (TPR), inductively coupled plasma-mass spectrometry (ICP-MS) and x-ray diffraction (XRD) techniques. Likewise, the composition of crude glycerol mixture generated at biodiesel plants, free glycerol, methanol, soap, free fatty acids and ashes (NaCl and KCl), were contained in the synthetic crude glycerol. The catalytic performance was evaluated based on conversion, hydrogen selectivity, hydrogen yield, turnover frequency and rate of coke formation. A reforming temperature of 575°C, steam-to-carbon ratio (S/C) of 2.6, oxygen-to-carbon ratio (O/C) of 0.125, reduction temperature of 600°C and calcination temperature of 550°C were experimentally found to be the best operating conditions based on hydrogen yield and process stability. Analysis of Variance (ANOVA) was performed to study the main effects and interactions among the different parameters and quantify the significance of each parameter on the overall performance. Reaction temperature and S/C ratio were found to be the most influential variables on conversion and H2 selectivity. The kinetics of synthetic CG ATR reactions were studied in a temperature range of 500-650°C, steam-to-carbon (S/C) range of 1.6-3.6, oxygen-to-carbon (O/C) range of 0.05-0.2, weight space velocity (WSV) range of 0-158.2 gcat.min/mol C and at atmospheric pressure. In preparation for collecting intrinsic kinetic data, a region free of heat and mass transfer limitations was established by varying catalyst particle size and inlet flow rates in the ranges of 0.55-1.27 mm and 0.0019-0.0033 mol C/min, respectively, and the stability of the catalyst was tested in an extended period run for 15 hours time on stream (TOS) experiment. The integral method of kinetic analysis was then applied to estimate the parameters of the proposed power law model. The activation energy was found to be 93.7 kJ/mol, alongside with reaction orders of 1, 0.5 and 2 with respect to synthetic CG, steam and oxygen, respectively. Excellent agreement between the experimental conversion results and those predicted by the model was observed with an absolute average deviation (AAD) of 5.2%.