7 replies to this topic

[engineer1985]

Hi all,

we have encountered a serious problem in amine stripper operations.

The differential pressure along the stripper increases gradually and causes a level hold up in the striiper (i.e bottom pressure increases and top reduces). this differentail pressure exceeds even more than 3 psig, which normally retains 0.5 psig.this situation lasts for about 10 mins until we exert back pressure from the top by closing acid gases valve.so that top pressure increases than the bottom pressure, and all the liquid level that was hold up in the trays is released to reboiler.after sometimes again this problem will happen.it seems that there is some kind of chocking in the bottom of the stripper which causes amine to retain in the trays as long as the top pressure is increased.tower bottom internals have never been inspected.but if for instance, we say that there's some chocking in the tower bottom, then due to this chocking no steam will pass through these trays , however the top temperature remains in the same in all these situations...i am unable to understand this situation. can anyone help me in this regard?

[Art Montemayor]

You haven’t given sufficient information (basic data) to help you out. Supply a schematic flow diagram and tell us specifically WHAT it is you are stripping and how.

I am very experienced in amine acid gas and sour gas applications and designs, but without you telling me all the specifics, I am forced to guess – something I’m not going to do. For example:

• What type of stripper / reboiler combination do you have?
• What is the amine? Concentration? Flow rate? Temperatures?
• What are the pressures and temperatures in the reboiler and stripper?
• Is the stripper trayed or packed?
• Etc., etc.

A detailed schematic diagram is really indispensable for this resolution – complete with heat and mass balances.

[engineer1985]

Art Montemayor

i have attached a simple schematic of the process.

we have a trayed stripper with a direct fired tube reboiler.
we are using MDEA for absorption with a flow rate of 78 USGPM and a strength of 14%...all the temperatures and pressures have been indicated in the attachment.

Attached Files

•  plant schmetic.xlsx   14.59KB   105 downloads

[Art Montemayor]

Adil:

Thank you very much for your quick and rapid response with a schematic and additional basic data. I can understand your concern and frustration. This is an important problem that requires a quick and definite solution. Additional data is needed for me, but I won’t bother you because we can resolve this as we continue on with this thread. Bear in mind the following facts about your sour gas treating stripper operation:

Your direct-fired reboiler is OK to use – but there is always a trade off with this type of reboiler – it is nearly impossible to estimate the amount of reboil heat that is being absorbed in the stripper and generating the required stripping steam.

If your reboiler is physically segregated from your stripper, then you have a rich MDEA line from the bottom of the stripper leading to the separated reboiler. The classical manner of attaching strippers to reboilers in relatively small amine units has always been to mount the stripper directly on top of the kettle reboiler. However, in your sketch, you show the stripper and reboiler as being separate, so I have to accept that as a fact.

Because of a lack of sufficient data, I have to assume you have BUBBLE CAP TRAYS instead of sieve trays (or valve trays). Is that assumption OK?

What you are describing is a classic case of insidious stripper tower flooding. Flooding in a tower is usually the result of one or more of the following:

• Excess reboil heat (and resulting reboil vapors/steam);
• Excess solution feed into the top of the tower;
• Tower tray plugging – due to trays collapsing or bubble caps corroding and/or working loose;
• Tray downcomers plugging;
• Excessive dissolved light hydrocarbons released in the reboiler;

I am assuming that the unit has operated satisfactorily in the past at the same identical process conditions and that there have been no changes or modifications in the unit or the process. Is this correct?

The way to check for correct reboil heat is to monitor the amount of overhead condensed weak MDEA solution that is formed in the overhead gas cooler. I always had my unit operators measuring this quantity and reporting the lbs of condensate formed/M Scf of product gas. This criteria should be identified in your Operation Manual. Any excess stripping steam should appear at this measurement. This check should be made as soon as you possibly can do it.

I am going to assume that you are controlling the solution flow rate and that there are no dissolved gases in the rich solution because the process hasn’t changed from past proper operation.

This means that you are left with only an over-firing issue at the reboiler (that can be confirmed as noted above) and potential plugging within the stripper tower.

The confirmation of internal plugging in the tower will have to be taken care of when you are prepared to suffer a shutdown and undergo an internal inspection of the unit.

One thing that you can do for now – that may alleviate the situation or help you out –is the elimination or reducing of so-called “reflux” back to the top of the stripper. The return of weak, condensate back to the stripper top is not a stripping process requirement. This stream is not a “reflux”. It is called that erroneously and is only a use of the condensate to “scrub” the stripper outlet vapors and avoid any heavy overheads entrainment of MDEA. The elimination of this so-called reflux will not change or alter the stripping of the acid gas nor its “purity”. If you eliminate or reduce the “reflux”, you will reduce the amount of liquid going back down the stripper and into the reboiler. Therefore, it might reduce the degree of flooding now taking place. Of course, you have to return the captured weak condensate back into the MDEA re-circulated system – preferably in the lean side of the process. Depending on your unit, you might be able to do this easily – as for example, in your lean surge tank.

If your stripper and reboiler are separated, then one possible plugged point is the lean solution line that exits the bottom of the stripper and goes into the reboiler. By the size of your MDEA circulation, I would guess this is a 3" of possibly a 4" line and it has a low point in it because it enters the reboiler at its (the reboiler's) bottom. I would also guess that your stripper diameter is approx. about a meter or 1.5 meters in ID.

Please keep us informed of any inspection results after you shutdown the unit.

[engineer1985]

Art Montemayor

thanks for your detailed response

i want to answer your queries step by step

1. our stripper contains valve type trays not the bubble caps

2.Yes,you are right the unit has operated satisfactorily in the past at the same identical process conditions and that there have been no changes or modifications in the unit or the process, this situation appears some days back

3. can you please tell me how to calculate the quantity of lbs of condensate formed/M Scf of product gas. because no criteria is provided in our Operation Manual.

4. there may be some kind of internal chocking in the lean MDEA line or within the tower as mentioned by you.

5. One another thing i would like to ask from you is the measurement of amount steam generated, is there any way to calculate and secondly how can i calculate the reboiler duty.


Regards

[Art Montemayor]

Engineer1985:

The following are my responses to your recent queries:

Can you please tell me how to calculate the quantity of lbs of condensate formed/M Scf of product gas. because no criteria is provided in our Operation Manual.
It is a bad situation if your unit’s Operating Manual does not draw attention to the so-called “reflux ratio” in your MDEA operation. I recommend you own, study, and apply the information and know-how available to you in such classic text books as:

“Gas Purification”; Kohl and Riesenfeld; McGraw-Hill; 1960 and following years;

GPSA Engineering DataBook;

“Gas Conditioning and Processing”; John M. Campbell.

The information found in these 3 books is sufficient to do a thorough analysis and understanding what you are dealing with. You can easily generate a heat and material balance of your unit using these texts as reference. For example, Kohl and Riesenfeld state:

“The quantity of stripping vapor required depends upon the solution-purity needed to produce the required product gas, the stripping column height, and the nature of the solution. The stripping vapor that passes out of the column with the product gas is normally condensed and returned to the column as reflux, resulting in a heat load additonal to the sensible and latent heat requirements of the operation. The ratio, moles of water in the acid gas from the stripping column to the moles of acid gas stripped, is commonly referred to as the “reflux ratio” and is used in design as a convenient measure of the quantity of stripping vapor provided. Typical reflux ratios used in designing commercial operations are:

(1) 3 to 1.0 for aqueous monoethanolamine plants in high pressure natural gas service where gas purity requirements are severe;

(2) 1.0 to 1.0 for glycol-amine units in similar service; and,

(3) 2 – 2.5 to 1.0 for aqueous monoethanolamine plants in less exacting service and for most diethanol amine plants.”

I have on past opportunities always calculated my required (or preferred) “Reflux ratio” – or quantity of steam vapor exiting the top of my strippers with the product acid gas. The results have always given me a good guide by which to judge my operations and energy consumptions in the unit. The way to calculate the amount of steam vapor generated is clearly explained in the referenced texts and easily applied. If you are an engineer in charge of operations in an acid gas removal unit, you should own and master these texts – at the very least.

One another thing I would like to ask from you is the measurement of amount steam generated, is there any way to calculate and secondly how can I calculate the reboiler duty.
The amine unit reboiler duty is closely tied and linked to the selected design “reflux ratio” assigned or taken for your unit. That is why I emphasize above that you should already have that figure – or calculate it yourself, giving a reasonable engineering contingency to the calculation results. You can always design for or handle an excess quantity of overheads steam vapor that you have to condense in the overheads cooler-condenser. This ensures that you will strip out as much acid gas as you need to. However, you should bear in mind that any excess steam generated in the reboiler for stripping means that much more reboiler energy that you must charge to the unit’s operation – and costs. This is one important reason why I always have emphasized the use of “waste”, excess steam (35 – 50 psig) in an acid gas stripper reboiler tube bundle instead of a direct-fired reboiler. The use of excess steam means that you use a conservative, high reflux ratio (assuming you have designed for a suitable condenser capacity) and assure the removal of all acid gas in the stripper with a subsequent low acid gas loading in the produced lean solution exiting the reboiler.

I hope this experience and advice is of help to you.

[anjumaziz]

Hi, well same type of problem i had faced during operation of amine plant. This is due to excessive foaming in stripper column and foam layer stablized so much that immediate put effects on amine circulation rate, stripper top temperature, poor reflux, reboiler duty, steam temperature, badlly loading of stripper column, off spec gas etc.
First step in case of amine stripper erratic behaviour:

1. Add fresh (0 TDS water into the pool incase of excessive low level of amine surgedrum, shut off antifoam injection if in operation, corrosion inhibitor injection etc. By addition of fresh water, it will suddenly break the foam layer and unloaded the stripper column.
2. Apply back pressure to amine stripper column. (Within range of stripper section ESD parameters)

What is the frequency of erratic stripper behaviour / day?

Your plant needs ATA, and possiblity is in stripper top tray choking, clean up your system during ATA, intrernal cleanning of amine reboiler / fire tube, cleanning of plate type heat excahnger if insatlled. If you would go arround these important equipments then i am dead sure you would not be faced same problem after ATA.
Infact these are the signs for amine plant that demands ATA. You would definatly after ATA would be get rid of this problem.

Secondly always pay keen attention on DI water, it should be 0TDS, intime repalcement of carbon / particulate filter elements, avoid excessive injection of antifoam / corrosion inhibitors chemicals. Amine plant operats well , troublefree if you do not compromise on filteration system of amine plants.

Regards,
Anjum Aziz
Process Engineer
MGCL
+923335109501

[sheraz shaikh]

the one of the most reason for this problem is Excess reboiler heat duty, it causes to increase bottom level of tower as the level increase the steam from reboiler is condense quickly as it is easier to condense steam rather then to cool water. now the actual thing in this type of problem are

1. steam will condense so that it looks like back pressure to reboiler but at certain the pressure remain constant like we say at 14 PSIG a part of steam get condense and a part of steam moves ahead which show stripper top overhead pressure as like we say 8 PSIG and this pressure will also become constant not what is the net result Is high DP of 6 PSIG. during this stripping is poor 

2. Now what to do to overcome this issue is to stop or decreasing reflux and may be drain out from reflux accumulator but before draining check amine strength and PH . By this degree of flooding will decrease and before decrassing or stoping reflux it is highly needed to check lean amine TDS

3. After this you will see that there will be no effect on reboiler level how ever you had drain reflux 

4. check amine TDS, will increase as stripper bottom level ( Excess level, flooded ) will flow to reboiler

You will see that DP reducing to its normal range by this it may needed to change filters and then have to control TDS as well too. after all take reflux in service and maintain stripper overhead condenser inlet temp 210 F and Outlet 110 F

as low temp also reduce pressure