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UNS S31400 Stainless Steel as Reactor Material in a Thermal Cracking Process

UNS S31400 stainless steel was tested as a reactor material for a thermal cracking process of agricultural, silvicultural, industrial and anthropogenic resources containing HCl. This material was chosen as it offers proper corrosion resistance in hot gases besides a good price and a high availability.

Product Number: 51317--9320-SG
ISBN: 9320 2017 CP
Author: Alexander Schmid
Publication Date: 2017
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UNS S31400 stainless steel was tested as a reactor material for a thermal cracking process of post-consumer plastics containing PVC. In a first step a bench scale pilot plant has been established and subsequently test runs have been performed. Cross sections of reactor pipes of the bench scale pilot plant were investigated after 30 h test runs by metallography SEM / EDX and STEM / EDX. In addition an independent experimental laboratory setup for corrosion tests was designed consisting of an inert quartz glass tube which was heated by use of a tube furnace.Sheet material of S31400 was tested between 450 – 650°C for 24 to 240 h in most aggressive conditions including 5 wt.% HCl 3 wt.% CO2 0.3 wt.% CO 0.2 wt.% H2 0.02 wt.% H2S bal. N2. The oxygen and chlorine partial pressures of this artificial gas mixture corresponded to the gas atmosphere in the reactor zone of the pilot plant. While the gas composition was evaluated with the software PetroSIM besides the gas chromatographic measurements the chlorine and oxygen partial pressures were calculated with FactSage© 7.0.In contrast to the pilot plant test runs the tests in the independent glass equipment did not cause coking on the metal samples. Hence sheet metals with a thin coke layer were prepared and tested analogously. Moreover to simulate long term operations immersion tests of reactor pipes with a coke layer were performed at 650°C and up to 1600h. Metallographic investigations of these immersed coked samples showed numerous carbide precipitations. Their quantity increased with immersion time. To determine also the influence of the precipitations on the corrosion behaviour of the steel in the gas atmosphere these immersed samples were tested too.The thin coke layer acted as a diffusion barrier and therefore inhibited the corrosion process. However this is only valid for short term operations. In case of long term operations the increasing amount of carbide precipitations enhances internal corrosion along the grain boundaries.

Key words: Thermal cracking, stainless steel, hydrogen chloride, high temperature corrosion, metal chlorides, sublimation

UNS S31400 stainless steel was tested as a reactor material for a thermal cracking process of post-consumer plastics containing PVC. In a first step a bench scale pilot plant has been established and subsequently test runs have been performed. Cross sections of reactor pipes of the bench scale pilot plant were investigated after 30 h test runs by metallography SEM / EDX and STEM / EDX. In addition an independent experimental laboratory setup for corrosion tests was designed consisting of an inert quartz glass tube which was heated by use of a tube furnace.Sheet material of S31400 was tested between 450 – 650°C for 24 to 240 h in most aggressive conditions including 5 wt.% HCl 3 wt.% CO2 0.3 wt.% CO 0.2 wt.% H2 0.02 wt.% H2S bal. N2. The oxygen and chlorine partial pressures of this artificial gas mixture corresponded to the gas atmosphere in the reactor zone of the pilot plant. While the gas composition was evaluated with the software PetroSIM besides the gas chromatographic measurements the chlorine and oxygen partial pressures were calculated with FactSage© 7.0.In contrast to the pilot plant test runs the tests in the independent glass equipment did not cause coking on the metal samples. Hence sheet metals with a thin coke layer were prepared and tested analogously. Moreover to simulate long term operations immersion tests of reactor pipes with a coke layer were performed at 650°C and up to 1600h. Metallographic investigations of these immersed coked samples showed numerous carbide precipitations. Their quantity increased with immersion time. To determine also the influence of the precipitations on the corrosion behaviour of the steel in the gas atmosphere these immersed samples were tested too.The thin coke layer acted as a diffusion barrier and therefore inhibited the corrosion process. However this is only valid for short term operations. In case of long term operations the increasing amount of carbide precipitations enhances internal corrosion along the grain boundaries.

Key words: Thermal cracking, stainless steel, hydrogen chloride, high temperature corrosion, metal chlorides, sublimation

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