3 replies to this topic

[campiloo]

Dear Gents
I will be very appreciated if you help me to find...
1- during propane treatment (mercaptan removal) we use caustic 15% ,what will happen if we use more concentrated caustic (20% , 40%,...) ?
2-after washing propane with caustic it goes to another drum which contact with fresh caustic 7% , why 7%? (Before contacting, it passes a heat exchanger to increase its temperature up to 70C and 10% of COS would be extracted in this section.
I look forward for your kind reply
Thanks and Regards

[Andrei]

campiloo,

It seams to me you are using two stage caustic wash. Here are my answers:
Question 1: Using more concentrated caustic solution is only affecting the quantity of caustic entrained from that stage and supposed lost in the subsequent water wash phases.
Question 2: It is a little strange to me to see a lower caustic concentration in the second stage, I would expect to be higher than the first stage. What is done usually is ensuring a certain constant driving force for the wash process by contacting high mercaptane concentration gas with low caustic solution and low mercaptane gas with high caustic composition, a kind of countercurrent process.
Heating at 70ºC is related to hydrates formation, I think one of the hydrates formes at 63ºC, and that's why heating is required.
I hope these helped. You can find more in GPSA Engineering Data Book, Section 21-Hydrocarbon Treating.


Regards

[campiloo]

Hi
Thanks for your Reply
in 2nd stage which is called finishing drum , in order to get rid of all cos left(10% of initial content in the feed), the propan is heated at 70 C before entering the caustic finishing drum , actually the reaction with caustic solution must be complete and it can be done at a higher temperature (about 70 C)
this caustic finishing drum is operated on liquid counter current mode : sour fresh feed is introduced at its bottom level , while fresh caustic solution (7%wt NAOH average ) is continuosly injected at its top level under contrl flow
so the reason of 70C is Clear but why caustic 7%?

[Merox Boy]

campiloo,

7% isn't a magic number. It's a balance between the availability of NaOH to neutralize acid gases, specific gravity difference to separate bulk hydrocarbon/aqueous phases, what is the maximum concentration of salts in solution before you stabilize emulsions and cause aqueous phase carryover, etc. If you want to use a higher concentration of caustic, go ahead and try. Just keep an eye out for aqueous phase carryover and ensure that you are avoiding caustic embrittlement concerns.