Scanning electron microscopy of human heart valves

Abstract

The Claus Sulfur recovery process is one of the most widely used methods of converting large quantities of highly toxic Hydrogen Sulfide gas (H2S) to a more harmless form, in this elemental case, Sulfur. In this process, a portion of the acid gas is reacted with Air or Oxygen in a combustion chamber in order to partially oxidize the H2S, and begin the process of converting it to an useful harmless product [1]. On the far side of the combustion chamber is located the inlet end of a heat exchanger, through which the gas is cooled and further downstream, condensed. The face of the tubesheet and the inlet ends of the boiler tubes must be protected from the detrimental effects of temperature and corrosion. Monolithic refractory and ceramic inserts called ferrules are used to protect both, respectively [2,3]. This research presents the characterization and the determination of failure mode of a Zirconium Silicate Ferrule failed in services. These ferrules were visually inspected and cut in sections to obtain samples for analysis. A cross sections from the ferrules collapsed zone (polished to 3 μm) were characterized by Scanning Electron Microscopy (SEM), incorporating Energy Dispersive X-ray analysis (EDS) using a CAMBRIDGE S240 Scanning Electron Microscope and a EDS LINK System. The phases inside the refractory ferrule were determined using a PHILLIPS X-ray Diffractometer (XRD) operated at a tube voltage and current of 40 kV and 30 mA, respectively, with Cu Kα radiation. SEM and EDS results show that the ferrules are constituted of two phases, grains of ZrSiO4 into a Silico-Aluminate matrix (SiO2,Al2O3 and Fe2O3), that is characteristic of low pure Zircon refractories [4,5] (Fig.1 a and b). The analyses in the affected and deformed zone (Fig.1 c and e) show evidence of thermal degradation of Zircon grain into tetragonal Zirconia (ZrO2-t) and Cristobalite (SiO2). This reaction takes place at temperatures between 1285°C and 1720°C, and are dependent on the refractory chemical composition, grain size, chemical environment, and exposure time [4]. Also, a deposit of particles found inside the failed ferrule was analyzed by SEM, EDX and XRD. This deposit is rich in Aluminum (Al), Silicon (Si), Iron (Fe), Calcium (Ca) and Sulfur (S) and the XRD showed that is composed of Corundum (Al2O3), Quartz (SiO2), Cristobalite (SiO2) and minor quantities of Hematite (F2O3), Pirite (FeS), Makinawite (Fe2S) and Lime (CaO). This deposits are derived from the reaction between the ferrule, the process gases (H2S) and particles of the high Alumina refractory, that fixes these ceramics insert in front of the combustion tubesheet chamber. Operational data and the evidence found in this research suggested that the principal failure reason was mechanical blockage (deposits) inside the ferrule that produces an increase in the local temperature, inducing the thermal degradation of ZrSiO4 into ZrO2-t and SiO2. This chemical changes in microstructure, the reaction of the gas processes with the ceramic materials, and the decrease in the thermal resistance of the ferrule, probably induce a thermofluence process, that finally, produces the deformation and collapsing of this sections. This research was conducted to evaluate the Zircon ferrules behavior in this kind of static equipment, and advice the replacement with high Alumina ceramic ferrules that are less susceptible to thermal cycles and changes, producing money saving and less non-scheduled maintenance shutdowns. Acta Microscopica Vol 16 No1-2,(Supp.2)2007