By Paul E. Schlett
Industry has come a long way over the years with respect to refractory lining systems in FCCU Flue Gas and Transfer Lines. This article summarizes where we started, where we are now, and how we got here.
The first generation refractory linings in FCCU flue gas and transfer lines were dual layer type. The hot face layer was a “T” stud-supported, hexmesh-anchored, erosion resistant, hot face lining. The backup layer was an insulating castable. Problems that often occurred with this lining system were weld failures between the hexmesh and the “T” stud anchor supports on the hot face. This caused a breach in the hot face lining which permitted catalyst bypassing, metal casing hot spots and, in some cases, holes in the lines. As weld failure progressed, sheets of hexmesh would be detached from the “T” studs and plug the lines.
These dual layer lining systems were replaced with a second generation lining system: a single layer, erosion resistant, heat-insulating lining. With this more dense single layer lining, the assumption was made that there would be no direct flow paths back to the metal shell. In addition, the refractory linings would be erosion resistant enough to resist catalyst flow, and would have adequate thermal insulating capability to keep the metal shell cool enough from a differential thermal expansion standpoint.
In the early days of the single layer lining system, erosion resistance was adequate, but the coefficient of thermal conductivity (K-factor) of the refractory products selected for this service was too high. The high thermal conductivity resulted in higher than expected metal casing temperatures leading to greater than expected line thermal growth. Thermal conductivity values published by the manufacturers of these products were typically much lower than the actual, as-installed values.
Except for when the lines were dried improperly (i.e., in a reheat furnace from the outside) or when too much water was added during installation, permanent linear change (i.e., shrinkage) of this second generation of refractories was adequate to minimize gas and catalyst bypassing, but metal shell casing temperatures were higher than desirable.
Because of the higher than expected or desired K-factors, a third generation of refractories was developed that would have lower densities (leading to better thermal insulating properties), less erosion resistance, and the same permanent linear change. The second generation of refractories would continue to be used in transfer lines because of the need for erosion resistance, but the third generation could be used in vapor and flue gas lines where erosion is not as severe. Some third generation refractories were Resco Products RS 17 E MW and Harbison-Walker (now ANH Refractories) Thermax.
Although several suppliers were successful in providing third generation refractories in the 1980’s, the desire to have a fourth generation that would satisfy both the erosion resistance and heat insulating requirements brought about continued refractory product development. Four suppliers, and now a fifth, have been working on fourth generation-type products. North American Refractories (now ANH Refractories) made “HPV Castable”; RHI Refractories made “LEGRIT 135-1,9 COR 0-3 (D171)”; Resco Products made “Rescocast 110C”; and Thermal Ceramics made “Kaotuff 110C”. “Kaotuff 110C” was the first of the fourth generation products and was developed in 1989. Vesuvius is now offering “ACTCHEM MW VC”. There may be others available or under development that are not noted here.
Although these products have about the same erosion resistances (ACTCHEM MW-VC appears to be the most erosion resistant), and K-values are within the range originally required for second generation products, they do not represent a generic refractory group. The suppliers appear to be approaching the physical properties requirements by using differing raw materials combinations depending on the proprietary technology they have available. Note that second generation refractories were nearly all the same, but with different brand names.
One physical property that separates them is permanent linear change from room temperature cured-to-dried at 230°F (110°C). HPV Castable and ACTCHEM MW-VC shrink more than the others at lower temperatures, while the other two have only minor shrinkage in the cured-to-dried range. Those products with significant shrinkage at such low temperatures allow cracks to form during the initial dryout which may not close up back to the metal shell during operation. This means that a potential catalyst bypass situation and subsequent hot spot could develop during operation.
When a product has minimal shrinkage at the low temperatures, the cracks seen on the refractory hot face are not through-thickness cracks and will grow together in service. This minimizes the possibility of catalyst and hot gases getting back to the metal shell.
Of the four products presented above, Resco Products Rescocast 110C and Thermal Ceramics Kaotuff 110C have fairly low shrinkages at low temperatures. These two products should perform acceptably in transfer lines. Kaotuff 110C has lower shrinkage, lower K-value, and better erosion resistance than Rescocast 110C. RHI Refractories advertises physical properties for LEGRIT 135-1,9 COR 0-3 (D171) that, if correct, are better than both of these. This product would need to be evaluated carefully before it could be recommended.