Are the declining costs of metallurgy providing an incentive for construction of 2000+ ton heavy-walled hydrocracking reactors? Is the application of advanced manufacturing techniques, such as Cr-Mo vanadium welding, becoming the 'norm' for fabrication of heavy walled hydrocracking reactors? What other developments coincide with new hydrocrackers designed to operate in a highly corrosive environment?
Answers
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18/03/2009
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A:
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keith bowers, B and B Consulting, kebowers47@gmail.com
Metallurgy costs are not a significant driver of ever larger hydrocracking reactors. The significant economy of a single process train over smaller parallel trains is the economic driver.
Parallel reactors in a 'single train' process are not likely. The high heat release of the hydrocracking reaction requires high precision in apportioning feed, recycle gas and purge gas to parallel reactors. It is extremely difficult, if not fundamentally impossible to precisely and continuously divide a two phase stream between parallel reactors. Short and long term flow oscillations would cause wide divergence in reaction conditions and resultant yields. Hence the drive to very large single trains.
"Corrosion' is not the most problematic issue in the design (metallurgy, fabrication, assembly) of high pressure hydrogenation reactors. Hydrogen dissolving into the metal causes it to grow increasingly brittle and subject to massive and sudden rupture. Metallurgy and fabrication technologies are selected to lower the risk is this.
Very, very few facilities can manufacture these thick wall large diameter vessels. Extrusion of full thickness 'rings' and subsequent welding of the rings together to attain desired length is the dominant method.
A high chrome+nickle inner layer is essential to control hydrogen embrittlement and corrosion. Usually a weld overlay is applied in one or more weld passes to a depth of 1/4" or more. Methods of providing support point for catalyst bed supports vary from weldments attached after extrusion to extruding thicker than needed and machining away excess everywhere except at support points.
Numerous penetrations of the shell also have to be provided for thermowells for many thermocouple arrays and quench gas injection. Different process licensors have different designs for these critical features. Some call for 'blow-out proof' penetrations while others incorporate conventional externally weld attached nozzles.
Each design must include fabrication details to ensure complete, crack free coverage with the protective weld overlay as well as avoiding any stress concentration effects.
The overall manufacturing process for these very costly reactors is critically dependent on absolute quality control and verification at ever single step of the entire process--from composition and time/temperature control of the steel 'heats to final exterior painting. Errors in any step of the process may lead to catastrophic failure in service.
Larger single train processes are more economical, but overall risk may increase significantly if ANY component in the 'train' requires a 'step-out' from well proven designs. History is richly populated with instances where 'step-outs' did not perform as expected.
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