10 replies to this topic

[lrfrancisco]

Hello!
I’m working on the project of a new sour gas unit, but I’ve never worked in the operations of this kind of unit. So, I’ve been studying about amine treating units and I read something that I didn’t understand. I read that the bottom temperature of the stripper tower doesn’t change even changing the reboiler duty significantly. Is that true? Why does it happen? If the reboiler duty is increased, more water is vaporized and more the reflux, right? The quantity of water vaporized isn’t high enough to chance the amine solution concentration and this way the boiling point doesn’t change significantly? Could you help me?
Thanks a lot!

[Art Montemayor]

IrFrancisco:

Yes, the reboiler temperature does not change on a sour gas unit, a distillation tower, or any other device that is held at a constant pressure – regardless of the amount of heat being transferred from the reboiler bundle to the sump of the tower (called “the reboiler duty”). What I define as the reboiler temperature is the temperature of the liquid in the reboiler (or the sump) of the tower.

Think about it. The function of the reboiler is to furnish saturated vapors to the stripper or distillaltion column that it is attached to. If you maintain the pressure in the reboiler constant – as is the normal, design, and operating custom – the temperature of the saturated vapors generated there MUST REMAIN CONSTANT.

Whether you furnish reflux to the top of the sour gas unit or not has nothing to do with the temperature of the vapors ascending from the reboiler. I personally never design for water reflux on the Amine (or sour gas) strippers I have designed and operated. But it makes no difference; the operation of the reboiler remains the same – regardless whether the duty is 10% or 100% of the maximum design rate. How much heat is transferred in the tube bundle has no effect on the reboiler liquid’s temperature. It is just like when your mama boils potatoes in her kitchen. How many potatoes, how big the pot is, or how fast she boils them has no effect on the temperature of the water in the pot. The temperature remains the same – 100 oC – simply because your mama is boiling the potatoes at a constant pressure: 1 absolute atmosphere.

[Zauberberg]

The purpose of amine regenerator reboiler - whether the heat is directly or indirectly applied - is to generate steam vapors which will act as a stripping agent: the more heat is introduced into the reboiler, the better the stripping. The reboiler temperature is dependent solely on system operating pressure, as Art has said - full stop. By increasing reboiler duty, you only generate more steam but fluid temperature will remain constant.

For each type of generic amine, there is an optimum/minimum lean amine loading and corresponding reflux ratio in regenerator tower. This parameter defines the amount of heat you need to provide in the reboiler.

[sheiko]

I take the opportunity of this post to ask the following question:

If an increase in reboiler duty directly increases the vapor weight flow from the reboiler (more stripping steam) and the tower-top temperature (more "heavy" water), then the volume (and velocity) of gas at the top of the stripper will probably go up (unless the increase in molecular weight due to more "heavy" at top is high enough to compensate and reduce the gas volume ...). So will do the tower-top pressure drop and, thus the overall pressure drop of the stripper. As a result, how can the bottom-tower pressure (which is the sum of the tower-top pressure and the tower overall pressure drop) remain constant?

[lrfrancisco]

Thank you for the answers!
Yesterday I was able to access some operational data from a similar unit to the one I'm studying and I could see everything you said. )
But I confess that I'm a little bit confused yet. On the lecture notes I read the bottom temperature of amine stripper doesn't change with reboiler duty, it's written that for petroleum distillation tower it does. In this case, is it because of bubble point of the liquid?
Kind regards!

PS: sheiko... What happened to your post about stripper top temperature and reflux?

[Zauberberg]

In amine regenerator towers, we are simply boiling water (single component); while in petroleum distillation towers reboiling of hydrocarbon mixtures takes place.

Now, if you recall the lecture related to boiling and condensation of multicomponent mixtures, you will agree that these phenomena occur accross temperature range - rather than at a fixed temperature. Components with a lower vapor pressure (heavies) need to be heated more to reach the operating pressure in the system (= beginning of vaporization). So as you heat the mixture entering the reboiler, lighter fractions evaporate predominantly, leaving the liquid phase richer with heavies - which require higher temperature in order to start to evaporate. That's why higher heat input in boiling multicomponent systems results in higher boiling temperature. It is explained quite simple with Raoult's and Dalton laws.

Sheiko: theoretically, you are correct. However, you wil notice that for basically all towers pressure drop is almost marginal (in the sense of influence on boiling temperature). Therefore by increasing the mass flow from reboiler, you'll get a higher back-pressure on the reboiler - assuming that PCV is located in the tower overhead line - and a slight increase of boiling temperature. You can check in water/steam tables and see that dP increase in order of 0.1bar will not cause significant change of water boiling temperature.

[sheiko]

lrfrancisco,

When the top reflux rate is increased, the tower-top temperature goes up, not down. This odd behaviour is easily understood if we note that there is no liquid product made from the reflux drum. Therefore the only way to increase the reflux rate, without losing the level in the reflux drum, is to increase the steam rate to the bottom of the stripper. The extra stripping steam drives up the tower-top temperature.


Zauberberg thanks. However,

Components with a lower vapor pressure (heavies) need to be heated more to reach the operating pressure in the system (= beginning of vaporization). So as you heat the mixture entering the reboiler, lighter fractions evaporate predominantly, leaving the liquid phase richer with heavies - which require higher temperature in order to start to evaporate.

in general, for hydrocarbon streams, light components have lower boiling points (and thus a higher vapor pressure as you wrote) but higher mass-based latent heats. So even though the light components in a mixture of hydrocarbons vaporize first, they require more heat to vaporize per unit of mass.

[lrfrancisco]

Hello again!
I didn’t know that amine solutions could be considered as pure substances.

[Art Montemayor]

Francisco:

Amine solutions are generally NOT USED AS 100% pure liquids. They are usually used as water-based solutions. I normally design for 10 - 15% (wt.) MEA, for example.

That is why I said previously that I prefer not to reflux the water condensed overhead from the removed acid gases leaving the Amine Stripper. It is essentially pure water (very weak in amine). I have used this condensate - after subcooling it - as a water wash to recover amine vapors exiting the top of the amine absorber. This technique has saved me a lot of amine that normally is lost and has to be made up.

[Zauberberg]

LRFrancisco,

Amine solutions are never pure substances. Depending on the type of amine used in the plant, concentrations may vary from 15% to 50%.

Refer to the attached article, it gives a very clear picture of amine unit design and operation fundamentals.
I hope you're enjoying in Brasil, is it a carnival time?

Best regards,

 Gas_Treating_Fundamentals.pdf   126.77KB   158 downloads

[lrfrancisco]

Hi!

I know that amines used in sour gas units are mixtures of water and amine and that the normal concentration of amine in solution depends on amine type (MEA, DEA, MDEA). I didn't know that it's possible to consider amine solution (amine + water) as a pure substance or simply as pure water when analyzing the behavior of stripper bottom and reboiler temperatures.

Art Montemayor: All amine units I've been studying have stripper reflux. This process configuration you usually apply is very interesting! In this case is there a knockout drum after absorber?

Zauberberg: Thanks a lot for the article! And talking about Carnival... I still have to wait until the end of February for that!

Best regards!