11 replies to this topic

[narendrasony]

Dear forum members,
I've few queries regarding TEG (Tri-ethylene glycol) based Gas dehydration units:

1) Why direct gas fired tubes are used for glycol regeneration in these units. Can we not use any other heating media like steam ? I suspect faster glycol decomposition with direct fired tubes since glycol flow rates are very low in these units and liquid film temperature adjacent to the tube will be much higher than TEG bulk temperature.

2) Typically bubble cap trays are used in Glycol contactors. But I've heard about packed bed columns also for this application. There should be sufficient liquid loading for wetting of the packing surface. How is it possible to use packed bed columns for very small TEG flow rates (e.g. 20-40 gpm for 100 MMSCFD wet gas depending on moisture content) ?

Regards
Narendra

Edited by narendrasony, 20 June 2012 - 02:11 PM.

[Art Montemayor]

Narendra:

I don’t know why you make your query based on statements that infer that they are facts. For example, you infer that most (or all) glycol reboilers are direct-fired and that most (or all) glycol contactors use bubble caps. In my experience that isn’t necessarily true. In fact, I seriously doubt if there are present contactors being designed with bubble caps any more. I also don’t know what the situation is in Kuwait; perhaps things are different there.

I have dealt with TEG natural gas dehydration since 1976 when I worked for Black, Sivalls & Bryson. We did a lot of TEG units for both land and offshore applications. The usual TEG reboiler in those days was direct fired because most of the applications were “oil patch” locations – remote, oil field-related sites where other heat sources were not available. However, even in those days, electrical resistance, hot oil, or Dowtherm were also being used – very much as today.

I abhor direct-fired reboilers and I try to avoid them wherever I can. However, depending on the scope of work and the specific site and application, they sometimes cannot be avoided. When you have a dehydration application in a gas field environment (such as in Kuwait) it is only natural to employ a direct-fired apparatus. It is available energy on tap, cheap, and relatively very efficient if one considers the alternatives. Additionally, as in the very large units in the project I was process advisor on last year, there is an environmental incentive when you can use the combustion device to eradicate all BTX effluents via incineration in the same combustion process used to generate the reboiler flue gas. This is a more expensive combustion design, but it has merit in that it dispels the threat of BTX pollution.

Using steam to obtain a 400 ^oF reboiler temperature is not usually justifiable. This would require a 400 psig boiler which isn’t normally available at the consumption site – especially for just use on one reboiler.

I used bubble caps in TEG service (and in other services as well) in the 1970’s. But after the 1980’s, I have not seen another bubble cap. Labor and maintenance costs – as well as the hazards of confined space entry – have dealt a heavy blow to the application of bubble caps. Just about every designer is applying structured packing and sieve trays. Valve trays are employed for turndown necessities and personal choice. But structured packing has taken over a large segment of the applications due to the low pressure drop and little maintenance. This type of packing, together with good re-distribution methods, can easily handle lean TEG flow rates of 20 to 60 gpm.

I hope this responds to your query.

[ihg]

Direct firing is not the only technique - a kettle reboiler with ~260C hotoil has been used (hot oil from gasturbine heat recovery) in a couple of plants of my acquaintance. TEG boiling at ~210C, with a stripping gas, or DRIZO gives good results. See GPSA Handbook Section 20 for background.

[jrtailor09]

<sub>Dear Narendra,

Find attached Article related to DRIZO PRocess Simulation for information.

Regards,

Jatin Tailor</sub>

Attached Files

•  TEG.pdf   244.51KB   302 downloads

[narendrasony]

Thanks Jatin and ihg for sharing valuable information.
Thanks once again Art for sharing your wonderful experience. In another post recently, I liked your idea of separating the packed bed for glycol stripping.

Art , why do you dislike direct fired re-boilers ? Here two units are just 10-11 years old, they could have employed hot oil instead, it is very well feasible here.
One of the units I'm involved with, Gas sweetening (with MDEA for H2s & CO2 removal) is followed by TEG unit, within one year TEG color had changed to dark yellow, no analysis was done since local labs don't perform TEG tests. Gas is clean and Amine colour is also clear. TEG reboiler temperature never crossed 400⁰F. Glycol has been replaced with fresh batch after water flushing the entire circuit.

I still wonder, how packed column can handle such small TEG flows, are there spacial redistribution methods?
I did not understand fully BTEX incineration - is BTEX being separated from moisture (as in DRIZO process) and then mixed with the fuel gas to the TEG re-boiler ? Can you please explain in more details ?

ihg, Jatin : Are DRIZO and Cold finger processes being used commercially ?

Thanks and regards
Narendra

[Art Montemayor]

Narendra:

I dislike direct-fired reboilers for the same reasons any other operations engineer does: they are firing mechanisms that require complex controls and still put flue gas into the atmosphere. They also have the ability to decompose TEG in hot spots around the fire tube (create film temperatures in excess of 400 oF) and they are a potential fire and explosion hazard that must be carefully supervised and operated. However, as I stated, one has to confront reality and employ them because often times there is no alternative on a practical or energy basis. Using hot oil, electrical resistances, and Dowtherm is nothing new. I have used hot oil and electrical resistance. But the energy consumption is much higher than a direct-fired unit. It’s a matter of practical economics. I don’t know why you keep asking and making the same point.

Unless you are using a water scrubbing operation between the outlet of your amine absorber and the inlet to the TEG contactor, I would highly suspect that you are contaminating your TEG with amine. That would explain the yellow color in the TEG. That is why I always tried to stop all reflux to the acid gas stripper and used the LP condensate from the acid gas overheads cooler-condenser as a weak solution wash on the product gas out of the amine absorber. I succeeded in reducing the amine consumption in one unit down to 1.0 kg amine / tonne of acid gas produced.

The BTX is incinerated in a firing plenum chamber of the reboiler and the product gas is used as the heating medium in the reboiler tube bundle. This is not a new invention; the project I was process lead advisor on during 2009 - 2010, used this system.

The Drizo (Dow Chemical) and ColdFinger processes have been around for sometime also. I knew Drizo back in the late 1970’s. Drizo is nothing more than a conventional TEG design with more equipment added to it using iso-octane as a vapor pressure fluid. Accumulation of BTX in Drizo fluid can play havoc with the operation. ColdFinger also is additional equipment added to the conventional TEG design. Both are proprietary and patented processes and so carry a “royalty” fee.

[narendrasony]

Dear Art,
Thanks for your reply. I got most of my doubts cleared now.

I was suspecting glycol contamination due to degradation in the reboiler, you have given one more direction . Basic amine analysis is done here, we'll send TEG sample for amine (degraded) content, I hope that should give a clue.

Thanks once again.

Regards
Narendra

[hirengdesai]

Dear Narenji,

In addition to Art's most valuable and expert comments, would like to share few personal learning regarding use of structured packing :

Now a days, structured packings is more common in glycol absorbers as they have the following advantages :

- the gas handling capacity of the structured packing is more than 1.5 times greater than that of bubble cap trays which allows smaller column diameter.
- also preferred from safety point as lower hydrocarbon inventories are involved.

Valve trays are prone to weeping especially at low liquid rates and hence they are not so popular for glycol absorbers.

Regards,
Hiren

[Alexsandres]

Dear Mr Art and Mr Narendra,

What about iron? If excessive corrosion does exist, it will lead excessive iron content on TEG, resulting yellowish TEG. Isn't it?

Commonly, contactor and DHU vessel is design using carbon steel. Carbon steel is also used in back end system of glycol plant. Ussually, we checked acid content and iron content on glycol. Sometimes we get yellowish glycol too, esspecialy when acid content is high, so does iron.

[SD0775]

Excessive corrosion could also actually indicate high degradation rate of TEG.

[Art Montemayor]

Alexsandres / SD0775:

Corrosion can take place with degradation products formed in the TEG. Organic acids built up within the system due to TEG degradation can "eat" up your carbon steel components. This is usually monitored and controlled by taking pH readings of the TEG on a regular schedule.

Any excessive reboiler temperature (greater than 404 ^oF) will be conducive to the formation of degraded TEG - and resulting in circulating corrosive by-products with the TEG. That is one of the main reasons I dislike the use of direct-fired reboilers - as I keep repeating on these forums. The type of reboiler doesn't directly cause degradation; it is the quality of the design and the eventual good firing control that is applied that are the ultimate features that can lead to a bad, corrosive TEG.

[SD0775]

Just wanted to add one thing about pH monitoring here especially for those who are not familiar with TEG system. The TEG sample needs to be in aquous solution, in other word needs to be diluted with water before measuring pH.

As it is mentioned in earlier posts, high tendency of TEG degradation due to use of direct fired reboiler has to be traded off with something else especially capital cost at the first place. In offshore processing platform TEG with direct fired reboiler is still preferred as it comes with simpler and lighter tonnage of equipment.

SD