11 replies to this topic

[Dmitry]

Hello dear all,

 

On our future TEG dehydration plant we have possibility to use a low pressure gas (4-6 bar) from condensate flash tank as a fuel gas instead of dry gas, since there is no way where we can send around 1000 nm3/d of low pressure  flash gas. We think that should be ok for the heater and regenerator burners. But if we want to circulate stripping gas through the lean glycol to absorb the last traces of water -  can we use this wet flash gas as a striping gas? As I see from Hysys this gas is not 100%  saturated with water vapours. Does Hysys calculated it correct? Tell me please about this option? Do you have any experience with such flash gas as stripping gas? 

 

Many thanks, any replies would be useful.

Dmitry

[Zauberberg]

Yes, flash gas from rich TEG flash drum can be used as a stripping gas. It is undersaturated with water because its pressure is reduced down to atmospheric, when entering the stripping column.

 

Ideally, provide a spare connection of dry export gas to the stripping gas line and you will eliminate any concerns if for any reason the stripping with wet flash gas does not work.

 

Overhead gas from the regeneration column can be used as reboiler fuel gas. We are just about to commission one TEG Unit, being designed as described above.

[Art Montemayor]

Dmitry:

 

Zauberberg believes that the flash gas you are referring to is actually gas resulting from the expansion of high pressure Rich TEG in the TEG flash drum.

 

I don’t interpret your description that way at all.  (Your description is not very good; that is why you should always generate a decent, detailed sketch to describe what you have)  I understand you to say that you are expanding a high pressure, field hydrocarbon stream from a flow line at high pressure (100 barg?) down to 6 barg in a condensate flash drum where you are separating the hydrocarbon condensate formed there (C₅+).  The resulting hydrocarbon gas (C₁, C₂, C₃) that is flashed off at 6 barg is in excess of what you are using for fuel in the TEG reboiler and heaters and you want to use that excess gas as stripping gas in your TEG unit.  Am I correct in this interpretation?

 

If I am correct, the answer is NO, you can’t use that field gas as stripping gas.  You are going to really mess up your regen system.  Firstly, the field gas is probably not devoid of water content and as such, can’t act as stripping gas.  All stripping gas must be of a low molecular weight and be as dry (without water) as possible.  That is why a side stream of the product dry gas out of the TEG contactor is used as stripping gas.  Additionally, you must first pre-heat the stripping gas.  This is normally done in a coil that is installed at the bottom of the TEG reboiler.  This hot, dry gas is then injected into the small gas stripper where it removes the remaining remnants of the water in the hot Lean TEG coming out of the TEG reboiler.

 

In Zauberberg’s case, his flashed gas is indeed partially dry – although not as dry as the product gas.  Therefore, Zauberberg has to use more flash stripping gas than he normally would if he used product gas; but that is OK, because he probably has an excess of the flash gas and probably also has a problem getting rid of it.  Also, Zauberberg’s flash gas is essentially lean (essentially 100% methane), while yours is probably rich in heavies (C₂+) and not dehydrated.

 

I think what you are trying to do is “kill two birds with one stone”: get rid of the troublesome flash gas while also resolving the need for stripping gas.  Good idea, but I don’t think it will work in your specific case.

 

 

[Dmitry]

Art Montemayor:

 

Hello,

 

Thanks for your reply. Please see attached sketch of unit.

 

We can always take all fuel gas from dry export gas line, so realy we want to try use maximum flash gas from condensate flash tank as a fuel gas and do not want to send all flash gas to the flare system.  Based on Hysys model flash gas has water dew point equal to operation gas temperature, so it's saturated with water by 100 %. If we will preheat this gas before entering the fuel scrubber? it will help to prevent water separation / freezing during pressure reducing through PRV's upstream burners and so on. What do you think? Is any another option?

 

Dmitry

Attached Files

•  PDF of GPF.pdf   94.71KB   42 downloads

[Art Montemayor]

Dmitry:

 

Tell me if I was correct in my original assumptions.  I think I was.

 

Your diagram shows that you are presently taking a side stream of dry product gas and mixing it with wet, field flash gas before introducing it into a fuel scrubber and subsequently sending it to fuel and stripping gas usage.  This simply is a bad process design.  Why do you want to introduce water into a product gas that you just dried??  The normal, obvious way to strip the lean solution is to use pure, un-contaminated, dryproduct gas and send it directly to the stripping column.   If you introduce wet gas into the regen system, you will mess it up because you are introduing water into the regen system when you are supposed to be removing it.  Stripping gas should be as dry as you can get it.  That's how you are able to remove the last and hardest traces of water moisture from the Lean TEG.

 

 

[Dmitry]

Hello Art,

 

 

Yes, you are correct! If I will send flash gas saturated with water into reboiler I will not get any good results.

But...

Let's say that flash is gas saturated with water at operation pressure and temperature at 6 bara and +10 deg C. At this conditions water dew point is + 10 deg C.  Stripping gas flow is 1000 nm3/d.  Amount of water according to Hysys saturation unit op. per 1000 nm3/day is 1,6 kg/day. So we will put additional 1,6 kg/day of water into reboiler! 

 

Required pressure for stripping gas is 1,35 bara. So, due to pressure drop from 6 bara to 1, 35 bara water dew point of stripping gas is goes down and now it's minus 25 deg C. at pressure 1,35 bara and temperature 6 deg C.

Amount of water according to Hysys saturation unit op. per 1000 nm3/day at 1, 35 bara and 6 deg C is 18,3 kg/day.

Сonsidering the fact that pressure will be constant and temperature of stripping gas will raise from 6 to 210 deg C inside reboiler - stripping gas will absorbe more water vapours from rich TEG solution than he contains before entering regenerator.

Tell me if I am wrong with this thoughts. Many thanks for you replies....

 

Regards, 

Dmitry

[RockDock]

Why not use the 500 sm3/d of dry gas you have diverted to mix with the flash gas as the stripping gas?

[Propacket]

Using dry product gas as stripping gas is indeed a good practice. Heavy, wet gases as Dmitry intends to use are associated with following implications:

 

1)It is water saturated at condensate flash durum outlet. Therefore, has high water content.

2)Heavy gases naturally have lower water holding capacity. When you bring this gas in contact with rich TEG, it will absorb less water from TEG as compared to a lean gas. 

So high water is one of the limitation for such a service. But when you flash this gas to a low pressure, it becomes under-saturated. Its water absorption capacity increases. Subsequently, when you preheat the gas to a suitable temperature (say 250 degF), it is able to take more water from the TEG and now it is a decent stripping gas. Off course, compared to a dry lean gas, required flow rate for this type of gas may be significantly high.

However, i would use this gas only if it has no other utility (.e.g it was being flared) and It provides a decent reduction in operating cost. Such gas should not be a favorite one as people like Art dont like it. 

 

By the way, you must have heard abut Drizzo Process for TEG regeneration. It uses a heavy solvent which is vaporized and used as stripping gas. The overhead gas is cooled and the solvent is recovered and re-used. Nobody knows about the exact composition but it is believed to be C5+. One of the logics to use a heavy solvent is to make the solvent recovery easy as it will be condensed easily out of the overheads.

 

Mr. Art,

 

If heavy gases are troublesome for TEG regenerator, how does C5+ gas works good.

 

Dmitry,

 

Just flash the gas to low pressure, use a fuel gas scrubber as pressure reduction may lead to low temperature which may cause condensation and then pre-heat the gas. Your stripping gas is ready to use.     

[Art Montemayor]

Propacket:

 

Engineers should not do things because others don’t like it; they should not do things that don’t make engineering, common sense.  Adding water to a TEG regeneration operation with wet stripping gas does not make sense.

 

Heavy gases are not necessarily “troublesome” for the TEG regenerator.  BTX compounds go through the TEG stripper column on a normal, design basis.  Who has stated that heavy gases are troublesome?  Who has said that C5+gas works “good”?

 

I don’t understand your claims or what you are asking.

[Propacket]

 

1) At first sight, it does not make sense. However, I dont think a pre-heated wet gas (more precisely a gas super-heated above its water dew point) at TEG reboiler operating conditions (atmospheric pressure, 400 degF) is any problem. At these operating conditions, water in the stripping gas will not leave it or go into TEG. It will only reduce water absorbing capacity of the gas and more stripping gas will be required.   

 

2) In your first post, you said that the stripping gas must be "lean".

 

3) I didn't mention anything regarding BTX. Could you elaborate. 

 

4) As i mentioned above, DRIZZO stripping gas is generated from a C5+ solvent. Isnt DRIZZO using C5+ based stripping gas a good process.

 

[Art Montemayor]

Propacket:

 

To address your points:

1. In my opinion, the net result is that your idea is literally “all wet” and it won’t work.  You are proposing putting more water load into the reboiler-stripper.  Where is the additional water supposed to go?  -- Out with the “lean” TEG (that wouldn’t be lean anymore)?  -- Or out with the overhead steam?  In order to do that you need more stripping gas and reboil heat – and how much more?  Regardless of the extra load you put on the stripper-reboiler, you now have negated your right to complain to the designer/fabricator about the system not working as designed.  You now have assumed the responsibility for messing up what was designed to work in another completely different manner.
2. Again, you are wrong.  I have not said that the stripping gas must be lean in my first post.  Read it carefully and you will discover that I said “Zauberberg’s flash gas is essentially lean (essentially 100% methane), while yours is probably rich in heavies (C₂+) and not dehydrated”.
3. No one has said you mentioned anything regarding BTX.  It was I that said “BTX compounds go through the TEG stripper column on a normal, design basis” – which they do in normal situations where there is BTX in the feed.  That is why it is normal here in the USA to have BTX units to remove the BTX and not contaminate.  (are we having difficulty communicating in the English language?)
4. The Drizzo process has been around for many years.  It is just that – another process.  The major gas producers in the USA certainly don’t believe it’s better than pure TEG and dry stripping gas because the greatest majority of units still being designed and installed in the USA are the latter.

 

 

 

 

[Dmitry]

Gentleman, Many thanks for replies! It very useful to get so much information before doing any progress on it. For start we will use flash gas as a fuel gas and if any problems with TEG regeneration and/or water dew point of sales gas will appear we always can to use dry gas after dehydration as a fuel gas. Regards, Dmitry