During operation coke migrates and deposits in all cavities, which appear as the result in different (reversible) expansion between the refractory and the stainless and/or low-alloy steels. During cooling or interruptions the reversibility of the refractory expansion is restricted, contrary to the steel. Very high stresses can thus develop and the linings as a non-uniform system, i.e. refractory in (hexagonal) armour, will finally collapse by spalling.
Another phenomenon is the fact that failures and damages have increased after FCCUs either have been revamped or began processing heavier long-residue feed stocks. As a consequence a much higher circulation of improved catalyst and a higher carbon load with an increased abrasiveness caused increased erosion, especially in those areas where the original design dimensions caused further constraints, which in itself aggravated the likelihood of failures more.
In an erosion-resistant refractory lining failures can generally be related to its (traditional) design, i.e. Hex/Flex-mesh based, first on spacer bars, later as so-called off-set panels.
The differences in types of steel between armour and substrate, i.e. stainless steel versus low-alloy carbon steels, etc. and the higher thermo-mechanical stresses are more and more becoming the ‘bottleneck’ in existing and vintage FCCUs.
In the past decades, significant new installation technologies have been introduced for (erosion-resistant) refractory materials. And although their innovation has resulted in fundamental improvements in installation man- hours, physical properties and refractory quality control, there is still limited but quickly growing practical experience overall.
With increasing throughputs, heavier feed-stocks (i.e. more residual coke) and higher operating temperatures in FCCUs very strong and highly abrasion-resistant refractory materials were required, with chemical binding and highly abrasion-resistant aggregates.
As a result these materials outrank the steel in abrasion-resistance, but they are also more critical in view of uniformity, method of application, etc. Furthermore they have become less ductile and pliable when hardened and thus less stress absorbing, which means a higher risk for spalling (e.g. biscuiting), albeit at a much higher temperature and stress limit.
2 Introduction to wear
During blast wear, which is one of the known kinds of wear, grainy, mostly abrasive particles impinge on the solid surface at a relative high velocity.
The resulting wear is predominantly influenced by those factors which govern the energy concentration; in addition, the interaction between the particles and structural components will have to be duly considered.
With decreasing intensity, the elastic-plastic deformation - being decisive for wear - is gradually shifted to elastic deformation which range is finally reached upon very soft particles, low velocity or small particle size. The process is then transferred to a wear-less state.
Moreover, the tough/brittle characteristics of the material are shifted towards tough which again results in reduced wear. Therefore, the law of low level/high level wear regime enables displacing the wear process to the low level by selecting suitable materials, provided that other requirements permit to do so.
Because of hard (or unknown) abrasive particles or due to particles with inclusions of varying hardness, these targets can often not be reached with conventional materials.
The slow erosion process is transformed into high-speed-cutting action as the force of the water is increased and the impact area is reduced simultaneously, thus concentrating in a small path, say flooding. They called
March 2019 Issue ENGINEER THE REFRACTORIES 3 Catalyst
FCCU catalyst is a zeolite, a three-dimensional tetrahedral alumina-silicate material, containing SiO2
, Al2 O3 , Fe2 O3 , Na2 O, TiO2 and rare earth elements.
Catalyst is used as a fine powder (<90mu) in the petro-chemical industry to increase the yield from crude oil products in various processes.
Typical chemical composition of a catalyst for L/R-FCCU Loss on Ignition SiO2
Cl- Cd, Hg, Tl Trace elements As, Co, Ni, Se, Te Cr, Cu, CN, Pd, Pb, Pt, Rh, Sb, Zn, V
Zeolite, any member of a family of hydrated alumina-silicate minerals that contain alkali and alkaline-earth metals.
The zeolites are noted for their lability toward ion-exchange and reversible dehydration. They have a framework structure that encloses interconnected cavities occupied by large metal cations (positively charged ions) and water molecules.
4 Principles of wear
It is important to first understand the basic principles and contributing factors associated with wear; e.g. erosion, abrasion, cavitation and all those phenomena, which cause wear of materials.
Everybody knows that continuous flowing water gradually erodes the surfaces it contacts. This is how rivers and streams form paths cutting through mountains and valleys.
50,6 41,0 0,89 0,57 0,29 0,35 0,10 0,60 0,85 0,01 0,45 0,06
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