Coenen, Kai (PhD)

Sorbent Development on the basis of Kinetics, and Mass- and Heat-transport Phenomena in Sorption-Enhance Processes at Elevated Temperatures

Introduction

The worldwide demand for hydrogen has steadily increased in the recent years. More than 50 million tons of hydrogen is currently produced annually in the world, of which more than 80% is produced by conventional steam methane reforming (SMR). The water gas shift reaction (WGS) is one important part of hydrogen production via SMR.

WGS Reaction:                                  H2O + CO            CO2 + H2

The sorption enhanced water-gas shift (SEWGS) process is a promising technology for pre-combustion decarbonization and hydrogen production. The SEWGS process can yield higher capture ratios at lower efficiency penalties and at lower costs than other mature technologies using solvents.  The SEWGS is a kind of pressure swing adsorption (PSA) process based on reversible CO2 adsorption on solid materials at temperatures between 350 and 550°C. The overall process directly converts syngas into separate streams of H2 and CO2, which makes the SEWGS process exceptionally suitable for pre-combustion CO2 capture, mitigating greenhouse gas emissions.

Sorbents based on hydrotalcites are capable of capturing CO2 at high temperatures (300-500°C) and are therefore ideal candidates for use in high temperature pressure swing adsorption (PSA) systems in which reaction and adsorption occur simultaneously at high pressure (up to 40bar) and temperature, followed by regeneration of the sorbent by release of the CO2 at the same temperature but lower pressure (<5bar). Hydrotalcites have a layered structure at low temperature and are available in a wide range of compositions. At higher temperatures the structure of the hydrotalcites changes and can be best described by mixed-metal oxides.

Potassium promoted hydrotalcites, which have been widely studied in literature for SEWGS can be described as mixed magnesium –aluminum oxides with basic character. Although applications of these materials have been studied and shown to be feasible at a larger scale, there is still a lack of understanding of the fundamental interactions occurring with small gas molecules (H2O, H2S, CO2 and other possible acidic species). In order to develop new materials for these applications, a deeper understanding of physical and chemical properties including composition, morphology, microstructure and porosity, with aim to better describe the kinetics, mass – and heat transport phenomena, is necessary.

Objective

An intensive experimental and theoretical study will be carried out to understand adsorption phenomena in hydrotalcite based adsorbents. Therefore a TGA and PBR system will be built, which will make it possible to investigate the adsorption of different species (H2O, CO2, H2S). Additionally ex-situ and in-situ characterization methods as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Infrared (IR) Spectroscopy , BET, chemisorption will be used to characterize the adsorbents at different adsorption states.

A phenomenological model will be used to describe the adsorption behaviour of different components and will be implemented in a packed bed reactor model. The model will be validated with experiments in  a packed bed reactor system and tested at various operating conditions.