2 replies to this topic

[raj25]

Hello All

 

I am working at a client site and am looking after the gas dehydration system. We are using TEG and until now it has worked fine. However as one of the projects is to increase the sales gas capacity - a study to increase the dehydration capacity of the TEG unit has commenced.

 

During my investigations, I have found the following:

 

1. We are using tempered hot oil (177 °C supply) in our TEG reboiler instead of hot oil (260 °C supply). Upon going through some inspection records and talking to personnel I found that there were some issues of corrosion in the TEG reboiler and hence it was decided to decrease the operating temperature of TEG reboiler. This would have an effect on the TEG purity, but was acceptable till now. With an increase in forecast gas demand we want to go back to using hot oil once again and operate at the maximum allowable TEG shellside temperature of 204 °C.

 

2. The MOC for the shell is SA 515 Gr. 60 and the tubes are SA-179 with the tubesheets MOC as SA 516 Gr. 70.

 

The sales gas to the TEG unit is supplied from the amine unit which removes the CO2 & H2S. Upon talking to Operations, I believe there is a minor carryover of MDEA (about 2%) in the sales gas to TEG unit.

 

My questions are:

1. Is the MOC of the reboiler suitable for TEG service at 204 °C

2. Will the presence of amine present any corrosion problems in the TEG reboiler - I presume amine is not absorbed in TEG

3. There may be minute amount of CO2 present as well as BTEX. Is the current MOC suitable for the service or a SS 316L clad would be a better choice

 

Your experiences and opinions will be greatly appreciated.

 

[Propacket]

Are you monitoring PH of the Rich Glycol? It will tell you the reason behind corrosion.

[Art Montemayor]

Answers to your questions are:

 

1. Is the MOC of the reboiler suitable for TEG service at 204 °C?

SA 515 Gr. 60 is adequate for the reboiler.  I would prefer to use SA 515 Gr. 70, but what you have is OK.  The design temperature of the reboiler should be approx. 450 ^oF (232 ^oC)

 

2. Will the presence of amine present any corrosion problems in the TEG reboiler - I presume amine is not absorbed in TEG?

The entry moist gas into the TEG contactor should be totally devoid of any liquids or Amine droplets.  You cannot avoid the existing vapor pressure of MDEA, but I presume that the inlet gas to the contactor is at about 90 ^oF or less, and at that temperature, the vapor pressure of MDEA is miniscule.  However, the need for good upstream vapor liquid separation cannot be over emphasized.  There should be absolutely no MDEA droplets entering the TEG Contactor.

 

3. There may be minute amount of CO₂ present as well as BTEX.  Is the current MOC suitable for the service or a SS 316L clad would be a better choice?

There is nothing to fear from CO2 and BTEX when using SA 515 Gr. 60 as the material of construction.  CO2 is harmless when dry.  It is when there is liquid water present together with CO2 that the formation of carbonic acid takes place with subsequent corrosion.  You don’t have that in a TEG unit.  The BTEX is practically inert to the carbon steel.

 

My further comments:

 

Why are you running a TEG unit reboiler with only 350 ^oF of heating oil?  The desing for a TEG reboiler requires you run the reboiler at 390 – 400 ^oF for the TEG constantly.  In order for you to do that you should be applying the 500 ^oF hot oil.  With the 350 ^oF heating oil you will never meet minimum pipe line specifications (7 lb water/MMScf).