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• What is the best MOC for the NMP recovery and Dehydration portions of a Solvent Extraction System? We are finding that acid in the feed oil concentrates in the recirculating NMP and that this is degrading the 304SS process vessels. Is 316SS a good choice or will we need to go to more exotic alloys? The recirculating NMP can hit a pH of 4.0 and sometimes down to 3.7.

  Jan-2023

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Answers

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• Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

  Unfortunately, the concentration of acidic compounds in NMP (N-Methyl Pyrrolidone) is a relatively common issue in lube oil dearomatization  units.

  Before considering changing the material of construction of the process equipment, please verify the possibility to raise the frequency of purge and make up the NMP aiming to reduce the concentration of acidic compounds in the solvent. Additionally, some references describe the use of sacrificial metals like magnesium and zinc installed in the critical of solvent recovery and dehydration sections of dearomatization units as an effective way to deal with corrosion issues in these processing units, I believe that this can be most economically attractive face to change the MOC (Material of Construction) of the processing unit.

  The use of stainless steal like AISI 316 can be interesting, but it's expensive and can led to another issues once these materials are very sensitive to stress corrosion due chlorides and it's very difficult to ensure the absence of these compounds in a processing unit (a simple cooling water leakage can contaminate the system with chlorides). I believe that it can be interesting to make an economic analysis comparing the cost of replacing the construction material of the processing unit face to raise the frequency of solvent change or make up flow rate in order to reduce the acid concentration in the recirculating solvent (NMP).

   

  Jan-2023