A study about the contribution of the α-β phase transition of quartz to thermal cycle damage of a refractory used in fluidized catalytic cracking units

Abstract

The deterioration of refractories used in fluidized catalytic cracking units (FCC-units) is responsible for high costs of maintenance for the petrochemical industry. This is commonly associated with coke deposition during the production of light hydrocarbons. However, other mechanisms responsible for causing damage may also occur, such as the generation of cracks by expansive phase transition. The aim of the work herein was to study the contribution of the a-b phase transition of quartz particles to the deterioration of a commercial aluminosilicate refractory used in a riser by the means of slow thermal cycles. Such damage may occur if the working temperature of the equipment fluctuates around the a-b transition temperature (573 °C). The current study considered the material with and without coke impregnation to evaluate the combined effect of coke presence and phase transition. To evaluate the damage, it was used the Young's modulus as a function of temperature by applying the Impulse Excitation Technique under controlled atmosphere. An equipment recently developed by the authors research group was applied. Specimens were prepared and submitted to slow thermal cycles of temperatures up to 500 °C and up to 700 °C, with a heating rate of 2 °C/min. Part of the specimens was previously impregnated with coke by a reactor using propen. To complete the evaluation, characterization by X-ray diffraction, as well as by dilatometry and scanning electron microscopy were performed. The findings of this study showed that the presence of quartz particles determine the thermo-mechanical behaviour of the material, as well as the thermocycling damage resistance. In spite of the fact that the a-b phase transition stiffens the material during the heating stage, it increases the damage by slow thermal cycling. The coke impregnation increases the resistance to slow thermal cycles, however it decreases the resistance to the damage evolution.

Analysis of a castable refractory using the wedge splitting test and cohesive zone model

“…In the present case, the FPZ length is of the order of ca. 50 mm (see Figures 12 to 16, for instance), which is one order of magnitude smaller than thus, their resistance strongly depends on the hydraulic bindings and packing of the raw material [57]. As this specimen was fired at low temperature (i.e., 500…”

“…• C [19]), it is expected to have aggregates weakly bonded to the matrix and microcracks related to the anisotropy of the phases [57]. The initiation of a crack is easy in such materials, which explains the low level of the cohesive strength σ max .…”

Laboratory Investigation of Granite Permeability after High-Temperature Exposure

“…During the heating process, physical properties present significant changes at the temperature of about 573 • C, where the α-β transition occurs. Inner cracks extend quickly and the porosity increases, leading to a reduction of the density and an increase of the conductivity [25]. It is presumed that the increase in the inter-granular thermal stress significantly induced more inter-granular and more trans-granular cracks, for the anisotropic expansion that is linked to the α/β phase transformation of quartz, which is abundant in granite, at 573 • C led to 5% volume growth in quartz [26,27].…”

Fracture energy evaluation of refractories in wedge splitting tests from notch opening displacements

“…In the following, NOD vs. CMOD histories are extracted from DIC analyses of two WSTs on an anti-erosive commercial refractory castable (grade DD40 [42][43][44]). This composition is one candidate to be applied in fluidized catalytic cracking units in petrochemical industries.…”

“…Coke impregnation may alter its microstructure [42] and affect its fracture energy [43]. If applied close to 570°C, temperature fluctuations can lead to quartz phase transformation and considerably damage the material [44]. This material contains quartz, mullite, kyanite, -cristoballite and alumina.…”