As a result of a Carbonate Stress Corrosion Cracking (CSCC) event at one refinery an investigation was made into the cause and mitigation of CSCC. At the specific refinery, the immediate cause was attributed to a change in catalyst type in the upstream cat cracker feed hydrotreater (CFHT) which reduced the feed sulfur/feed nitrogen (S/N) ratio in the feed, resulting in an increased carbonate concentration, and an increased likelihood of cracking. This paper outlines the information that was developed during the investigation and also outlines the development of tools that could be utilized by other refinery fluidized catalytic cracker units (FCCU's) to better assess risk of CSCC.

Industry guidelines (API 581) provide a method for estimating CSCC risk based on [CO3=] and pH, but no immediately available method was found to measure [CO3=] directly. Therefore ionic modeling was used to predict pH and [CO3=] based on feed S, feed N. Also developed was a method to assess risk based on the information more typically available and that could be trended and used predictively. These included pH, and feed S/N ratio. A method was developed to measure CO2 in sour water using ion specific electrodes (ISE's) that could be converted to [CO3=] using either equilibrium constants from literature and pH or using ionic modeling tools. In this way, the likelihood on the pH x [CO3=] risk space could be assessed.

Based on this review it is believed that: CSCC may be episodic in nature; understanding of the process trends (pH, and S/N) are critical in assessing the risk of CSCC; traditional alkalinity measurements (P, M alkalinity) are not useful in measuring [CO3=]⁠; mitigation of cracking can include PWHT, changing water wash variables (corrosion inhibitors, de-emulsifiers, make-up rate, etc.); establishing operating envelopes that include feed S/N and a link to the likelihood of CSCC can be used to prevent CSCC.

Subject
Fluidized catalytic cracking units, Water, Piping, Modeling, Trends, Tools, Inspection, Refineries, Acidity, Sour water, Nitrogen, Carbonate stress corrosion cracking, Stress corrosion cracking
Keywords:
FCCU, Fluid Catalytic Cracking, Carbonate, Stress Corrosion Cracking, Alkaline Stress Corrosion Cracking, SCC
© 2007 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2007
Association for Materials Protection and Performance (AMPP)
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