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The following query is regarding the MEG solution circulation system in Propylene Recovery Unit (PRU).
PRU unit has a vent condenser at the downstream of DeEthaniser. Vent condenser typically operates at 3 ~5 degC. 30% MEG solution in DM water is used as a refrigerant. Metallurgy followed in this MEG solution circulation system is carbon steel. During start-up, if DM water is received in the circulation system (which is having a surge drum - with nitrogen blanketed), is there a possibility of corrosion due to DM water contacting directly with carbon steel? Is it a good idea to maintain slight nitrogen atmosphere (may be around 50 mmWC of nitrogen pressure) in the surge vessel before making up DM water? How best this MEG solution system can be commissioned avoiding corrosion?
 
Answers
02/11/2011 A: G KAMALAKANNAN, Foster Wheeler India Ltd, siddhukamal@yahoo.co.in
CO2 and H2S are not present in this system. The query is regarding the corrosion of carbon steel due to DM water. Since DM water is slightly acidic (pH is less than 7), in the oxygen environment, corrosion can happen. Hence, if oxygen environment is avoided by having nitrogen blanket, will it help to avoid corrosion?
28/10/2011 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
Carbon steel is fine in this environment.
Oxygen should be avoided as much as possible. It can oxidise the MEG and create corrosive products.
Corrosion in this system will depend on the presence of CO2 and or H2S.
H2S can help to protect the system. No information was provided.