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I am working on CDU as a field operator.
I want to know why NH3 or NH3H2O is injected in overhead line of distillation column?
Why dont we use NAOH for nutralization there? Even NAOH is cheaper then Ammonical water.
 
Answers
27/05/2010 A: SUNDEEP GUPTA, BECHTEL, sundeepgupta1@rediffmail.com
Injecting NaOH solution will not protect the overhead vapor lines (NaOH will remain in the liquid phase). Ammonia does a better job in this aspect. Further, introducing Caustic can create problem in the downstream Reforming Unit.
21/05/2010 A: Sam Lordo, Nalco Company, salordo@comcast.net
NH3 and NH3H2Or can be used successfully in some overhead corrosion control programs, however there is a risk of ammonium chloride salt deposition and under-deposit corrosion if the program is not designed properly... using NaOH though whilst it would be a good cheap neutraliser, the salting that could result is worse that other neutralisers so you would have something similar to the ammonium chloride salt issue mentioned above...
In addition sodium can be/is a catalyst poison so hydrotreaters downstream of the crude unit could be impacted.
21/05/2010 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
The use of Ammonia solution in the overhead has two issues:
1.- it can attack Monel and Admiralty brass, the high pH will attach the copper of this metals normally used either in the overhead line or condenser exchanger tubes.
2.- pH control is difficult to produce because it does not produce a buffer solution compare with neutralizing amines
Cautic will have the same issue regarding pH control (4.5 to 6.5) because it is a strong alkaline solution so there is not buffer.
21/05/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Many plants do use caustic. Here is an excerpt from my course manual:
"Corrosion in the overhead system of the atmospheric column is controlled in some plants by
the injection of filming amines and wash water circulation to force water condensation to occur in the piping upstream of the condenser/exchanger. Some plants have upgraded the metallurgy in this section as well. Sometimes ammonia is injected to neutralize HCl and CO2. However, if the condenser is monel, the ammonia will attack it. Sometimes caustic is injected downstream of the desalter to reduce the chlorides that would hydrolyze and go overhead, and to neutralize CO2. A global survey indicated that approximately one-half of refiners inject caustic ahead of the hot train or crude furnace inlet for overhead chloride control."
21/05/2010 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
If my understanding is correct, 'filming amines' should be injected into the overhead line of the Crude Atmospheric Tower. Ammonia is sometimes substituted, but may not be as effective.
Good practices of course call for the overhead condensers and accumulator to be 'gravity flow' from after the U-bend at the tower top.
Best practices in my opinion favor 'artificial dew pointing' via two tee quill injectors upstream of the first overhead condenser [high performance air cooler?], one recycling slipstream of sour water decanted from the accumulator and one recycling slipstream wild / sour / wet naphtha from the accumulator.
Preferably, there is a recommended pH value for the sour water being decanted, and this may need to be 'trim adjusted' using metering pump drawing dilute caustic from a day tank and injecting this into the slipstream being recycled. Under no circumstances should a flow control valve be used for pH control using caustic since this typically results in wild swings in injection and pH values.