9 replies to this topic

[ravi.s]

Dear All,

I'll be glad if you highlight on the following:

I'm designing the Reboiler for the Amine Regenerator (MDEA solvent). As we have to utilize the MP Steam for the Reboiler, I have following doubts in mind.

Generally LP Steam (~3.5 kg/cm2g ) is used for the application & the maximum temperature of heating media should be below 175 deg C to avoid thermal degradation of Amine.
However we have supply headers for MP Steam @240 deg C & 12 kg/cm2g and Boiler Feed water @105 degC & 52 kg/cm2g. Can we use MP steam for the application after desuperheating?
if yes,
1) How do I select type of Desuperheater?
2) what should be the process conditions (temp,press, flow) for the desuperheated steam.

Does any references are available for the similar application or any pertinent information or background will be useful.

Await your reply with more meaningful information.

[Art Montemayor]

Ravi:

You don’t mention the capacity (flow rate) of the steam load on the reboiler you are sizing. That is probably the first most important basic data you need to consider before going any further. We need to know that in order to make meaningful comments.

You say you have to use MP (I have to presume you mean Medium Pressure) steam as the heat source. Is that a mandate? Have you no other steam source besides the MP steam header?

MP Steam @240 deg C & 12 kg/cm2g is a total overkill. That temperature will ruin your MDEA solution. I can’t imagine why anyone would produce a steam source with 49 ^oC of superheat except to use it in a special application – such as a steam turbine. What you need is process steam – not power steam. But if you are stuck with power steam because you’ve been ordered to do it, then you have no option but to de-superheat it – which will cost a goodly amount of money, depending on the site and the quantity involved.

You select the type of de-superheater in accordance with the capacity and the sources of heat sink available. 52 kg/cm2g pressure on the feed water doesn't correspond to the 12 kg/cm2g steam available. I can't imagine why the feed water pressire is so high. This will make problems for flashing it into superheated steam - if that is what you are inferring.

The process conditions (temp,, press, flow) for the desuperheated steam should be those that comply with your application – the MDEA reboiler. What temperature are you planning to reboil the MDEA solution at? Surely, you know that. You said you are designing it and you show that you are aware that the maximum temperature of heating media should be below 175 deg C to avoid thermal degradation of Amine.

If you are truly designing the reboiler, then you already know the heat load and the amount of steam needed (depending on the steam condition). You should already have knowledge of the process conditions required by the MDEA reboiler. You either have an existing MDEA unit that was designed by others or you are now embarking on designing your own for the first time. You haven’t said which is the case, so I don’t know the degree of your situation or problem. But either way, if you are encharged with designing the reboiler, you have to set and obtain all the required and necessary basic data – most of which you haven’t furnished us.

I recommend you use your basic data to discuss the steam desuperheating with a local supplier or a representative of a major, recognized fabricator of desuperheaters. Obtain their advice and quotes on the job required and evaluate the answer.

Good Luck.

[ravi.s]

Thanks for the prompt reply.

For the better & meaningful comments of the experts, please find below the basic information

Initially, the reboiler is supposed to be designed for Low Pressure (LP) Steam having flowrate @ 130MT/h. As the there is constraint of LP steam, we are forced to go with Medium Pressure (MP) steam for the application (Although I'm agree with you on the economics of the use of MP steam).
The flowrate rich amine is 930 MT/hr (containing mainly H2S as acid gas) is to be stripped off in the regenerator. The lean amine leaving from the regenerator is @132 degC & 1.55 kg/cm2 g.

If we go ahead with MP Steam, what pressure of steam should be maintained at the entrance of the reboiler?

Will there be any drastic effects on the designing of the reboiler? Are there any chances of reducing the number of tubes/shell of the reboiler?

Any other releavent information will be useful.
Thanks in advance.
Ravi

[Art Montemayor]

Ravi:

I’m assuming you are a practicing, professional engineer. Therefore, we know that superheated steam is of little, or no value as a heating fluid. It is a relative useless attempt to try to use superheated steam as a heating agent, thinking that because it has a higher temperature it will heat “better”, faster, and using less heat transfer area. We both know that is totally false. One cannot effectively use superheated steam as a heating agent – for various reasons.

Therefore, when you ask, “If we go ahead with MP Steam, what pressure of steam should be maintained at the entrance of the reboiler?” I have to assume you mean SATURATED steam – the only process steam you should be employing to heat the reboiler tube bundle. That is why you have to de-superheat the Medium Pressure, superheated steam before being able to use it as a heating medium. And you have to invest money to be able to do that. I thought I wrote that very clearly and in detail in the prior post.

“Will there be any drastic effects on the designing of the reboiler?”
Again, if you try to use superheated steam, you bet! That is why (I keep repeating myself!) you must employ saturated steam – only. If you design based on saturated steam, the method and effects will be no different than the existing reboiler bundle. You still haven’t defined whether this is a new unit or the design is a retro-fit to an existing one.

“Are there any chances of reducing the number of tubes/shell of the reboiler?”
What do you mean by chances? Saturated steam is saturated steam. If the existing unit is working properly, then the replacement will behave the same way. I don’t understand why you or any one else should expect to save a bunch of money on reboiler tubes in this application – unless you (or your bosses) are laboring under the false idea that because you can use superheated steam, you should get by with less tubes due to the superheated steam higher temperature. As I stated above, this is totally wrong thinking and I don’t think you would be doing that. That’s why I don’t understand this question.

[fallah]

One cannot effectively use superheated steam as a heating agent – for various reasons.




Dear Art

What are the main reasons for : "superheated steam not to be used as a heating agent "?

Thanks
Fallah

[Padmakar Katre]

Dear Ravi,
According to your problem stated in the forum and the data provided I want to give the solution for your query. As stated by Mr. Arthur Montemayer that superheated steam is not a good option for heating I am 100 % agree with him as well as per your reply to his solution is ,you have constraint to have LP steam now if you want to use the same 12 Kg/cm2g pressure steam (Saturation temperature is 191 C ) at 240 C so you have a degree of superheat is around 50 C(Normally Reboilers are designed to 20-30 C plus temperature than the saturation temperature of the inlet steam) so first you have to suppress this degree of superheat. I don't agreee to Mr. Arthur that there will be flashing of the Boiler Feed water as it is at higher pressure but if we see the temperature which is quite low even if we inject this water in either PRDS i.e. of 12 Kg/cm2g or 3.5 Kg/cm2g header there is not at all chance of flashing.
My suggestion to you is to go for a let down control valve from 12 Kg/cm2g header to 3.5 kg/cm2g pressure followed by a desuperheater which you have to have in both these cases. In the let down case you will save money in the rebolier as well the piping with an extra cost of pressure let down control valve. I hope it will help you.

If you will share your mail id I will will send you an excel spreadsheet to calculate the Boiler feed requirement in the desuperheaters or just click on the link below......
http://www.cheresour...l=desuperheater

Regards,
Padmakar

[Art Montemayor]

Fallah:

It is a well-known fact that it is very difficult to extract the sensible heat from superheated steam. The film transfer coefficient is terrible and the equipment required to effect the sensible heat transfer is not economically effective. That is why superheat heat is never employed as a process heating fluid. It is strictly used as a power fluid. Process steam boilers are designed to produce up to 10 degrees of superheat in order to maintain the steam “live” in the main distribution headers. But by the time the steam gets to the process, it is essentially saturated. The superheat employed in process headers is to try to avoid condensation in the distribution system. The effect of trying to heat with superheated steam is well described in Donald Kern’s book, “Process Heat Transfer”.


Padmakar:

Introducing Boiler Feed water @105 ^oC & 52 kg/cm²g into a 12 kg/cm²g superheated steam system is going to produce the instantaneous evaporation of the feed water. This is conventionally called “flashing” when it takes place across the orifice of a control valve. This is the effect that I am describing.

Additionally, I have designed for - and obtained – a 15 ^oC temperature difference between the Lean Amine exiting the reboiler and the inlet temperature of the saturated steam into the reboiler tube bundle. I used 316 ss reboiler tubes in a U-tube configuration. I never had any trouble with this design and have never had to resort to such high temperatures as you recommend (20-30 ^oC). As I said, I always try to keep the Amine temperature as low as possible.

[Padmakar Katre]

Dear Art Montemayer,
I don't have an experience of amine regeneration.As you are telling about keeping the 15 C plus temperature of steam at reboiler inlet than the exit temperature of the amines from regenerators here you are concerned about the degradtion of the amines.As I mentioned above that I don't have an experience of amine regeneration so I told the reboiler design temperature as 20-30 C plus than the steam saturaion temperature as I was concerned about the MOC of the Reboiler in the event of low flow of Boiler flow water or no flow of BFW to desuperheater.
Anyways thanks.

Regards,
Padmakar Katre

[Pronab]

Ravi

You mention amine degradiation temperature 175 Deg. C which is not accaptable to me.
It is quite higher side, we are maintaing reboiler haeting medium within 150 Deg. C to avoid any
MDEA degradiation. A lot of litteratures in (www.bre .com ), you will find. Think about if your desuperheating system fail, what will happen . You will destroy the amine solvent.We are using LP steam ( for the same system you describe) in reboiler and off-course it is saturated ( Tempe: ~ 140 deg. C). Also if you use 12 Kg /cm2 steam in tube side and 1.55 Kg.cm2 amine solvent in shell side , there is a big pressure difference in tube and shell side. Did you do a HAZOP study to use MP steam ?

Regards.
Pronab.

Dear Mr. Art/Pronab,

You have provided most useful infortmation regarding the operation/design of Amine Regenerator Reboiler.

Can you please upload the Excel spreadsheet for Design calculations of Amine Regenerator Reboiler.

Thanks in advance.

Regards,
Abhijeet