9 replies to this topic

[chem101]

Hi ,
We have Stripper+ re-boiler in the amine system. There are two options to control the Over head temperature.
(1)In order to control Overhead Temperature we can reduce the area inside the re-boiler by adjusting the valve on downstream of the re-boiler.in this way the area will be reduced to decrease heat transfer
(2)We can have a bypass whenever the over head temperature need to be controlled we can use by pass valve to reduce or increase the hot oil flow.
Now the question is which option is more favorable. I am in the favor of option #1 because its cost effective.but at the same time does it effect the pump outlet pressure or can it cause back flow?
(2) Option is better for the process but costly. in this option flow is split. the only issue i have its costly.
I Have attached the drawing in order to understand the pressue corrrectly.
I need suggestion from you experienced engineers.
Thank you very much for your help.

Attached Files

•  Stripper+Re-boiler.xlsx   120.69KB   41 downloads

[Zauberberg]

The way it is shown, the Option #1 does not reduce the heat transfer area - it just reduces Hot Oil flow through the reboiler.

[Art Montemayor]

Before starting what appears as a legitimate query, please furnish enough basic data to identify exactly what it is that you are trying to control.

You state you "have Stripper+ re-boiler in the amine system". Then you go on to assert that there are two options to control the Over head temperature.

I presume that you are referring to a conventional acid gas removal process - such as the conventional "amine" (MEA, DEA, MDEA, etc.) process. If I am correct, why in the world would you have any incentive or necessity to "control" the saturated acid gas overheads temperature coming out of an amine stripper? Please explain this to an old acid gas engineer who has designed, fabricated, installed, and operated these units. I never found a need to do this and I am interested in why you find a need to do so.

I hope I have interpreted your query correctly. If not, correct me.

[Propacket]

chem101,

Zauberberg & Art are right.

Please look at your PFD carefully. There is no temperature control on overhead gas. Actually, this control loop is provided to control heat input to the reboiler. Let us assume that temperature of feed to the stripper has increased. The transmitter on the overhead gas will give high temperature signal. What will you do? Naturally, you will control the heat input to the reboiler. If the reboiler is hot oil based, you can control the heat input by controlling the hot oil flow to the reboiler.

Both option-1 and option-2 control the heat input by controlling hot oil flow to the reboiler. Option-1 operates by throtlling of FCV on main hot oil line and option-2 operates by bypassing the hot oil to discharge line. In my opinion, option-1 is better. Using option-2, you will be sending extra flow (Hot oil actual flow + Bypass Flow) to the hot oil system. Therefore, your hot oil system must be designed for this extra flow.

I would also recommend to move the temperature transmitter from overhead gas line to the stripper on feed tray to get a direct reading of the feed temperature. Also, per option-2, FCV shown on the main discharge line will not be required.

Thanks

Edited by P.Engr, 21 September 2010 - 07:35 AM.

[chem101]

Dear Art:
You are absolutely correct i am referring to acid gas removal process such as amine. The reasons why we need to control the temperature are
(1)To control hot oil flow by measuring the overhead temperature
(2)Not to put too much load on reflux condenser in case of very high overhead Temperature.
(3)To prevent amine from degradation by heating it up more than 260 or 265F.
As a matter of fact we have a client who asked for the recycle line on re-boiler. This 400-600 GPM amine plant.
(In reason 3 you might say that we can measure the Temperature in the bottom of the stripper,but having temperature transmitter on overhead can also help you.)

Edited by chem101, 21 September 2010 - 07:47 AM.

[chem101]

Hi P. Engineer:

I appreciate your reply. I agree with you on some point and on some point i don't. The point where i don't agree is:

-you will be sending extra flow (Hot oil actual flow + Bypass Flow) to the hot oil system. Therefore, your hot oil system must be designed for this extra flow.

Well, we are not sending extra flow to the hot oil system. it’s a same flow rate, we are actually dividing the flow rate. Assume you have 1000 GPM hot oil is going to re-boiler. By opening the bypass valve in case when it’s required to control the heat input to the re-boiler, we will send 800 GPM to the re-boiler. The 200 GPM will be bypassed the re-boiler. I hope I made my point clear here.

Thank you

[Art Montemayor]

chem101

I thought that what you expressed in your original post was a misunderstanding of what is happening in an amine regeneration system. And I also believe that Zauberberg and P.Engr are also of the same opinion. And because of that belief, we are all trying to help you come to understand that what you are proposing is not reasonable or applicable to an amine regeneration system. Let me explain by revisiting some very basic Amine plant details surrounding the circulation and regeneration of an amine solution:

An aqueous amine solution initially picks up an acid gas (CO₂) or a sour gas (H₂S) in an amine absorber and converts itself into Rich amine with a certain amount of gas “loadings” - gas absorbed;

The Rich amine solution is regenerated into a Lean amine solution by subjecting it to heat and mass transfer by preheating it in heat exchangers and introducing it into the top of a gas stripper tower that is kept heated by a reboiler. Upon entering the stripper, the Rich amine is “stripped” of its gas content by flashing from a high pressure into the low pressure stripper (as low pressure as can be justified) and by hot aqueous vapor rising from the reboiler.

The greatest amount of gas is liberated at the top entry of the stripper tower; the remaining lean amine that trickles down the stripper, through trays or packing, subsequently loses more gas as it comes in contact with hot aqueous vapor rising from the reboiler. By the time the lean solution reaches the reboiler, it is essentially stripped of most of the gas it absorbed in the absorber and it is used to generate more hot vapors in the reboiler before exiting out as hot, lean amine.

We know (and you have acknowledged) that amine solutions are subject to chemical degradation when subjected to excessive heat. Therefore, we want to keep the temperature in the stripper and reboiler as low as we can – and yet regenerate the Rich amine as much as possible. Experience tells us that an amine solution (such as MEA) will work best at 15% wt. and a temperature at the top of the stripper of 210 – 220 ^oF, and 240 – 250 ^oF at the reboiler while maintaining a reboiler pressure of 3 – 5 psig.

Note that the above temperatures are taken as practically saturated conditions. In fact, each ideal stage at each stripper tray or HETP in a packed bed will have equilibrium and saturated conditions related to its conditions. The vapors ascending the tower are not superheated – except for the non-condensable gas.

Normally, the Rich solution flow rate into the stripper is a fixed, steady-state flow rate and so is the reboiler heat rate. If the Rich solution flowrate varies or if its temperature and pressure also vary, then you must vary the reboiler heat rate. But you have not stated that – although I suspect that is really the crux of the problem you are facing. If you have a requirement to vary the reboiler heat load because of a varying Rich solution heat load, then please state so. We can’t start guessing as to why you need to “control” the stripper top temperature. The stripper top temperature is fixed by the reboiler heat rate – which is normally fixed. The only reason you would want/need to vary the reboiler heat rate would be because you are varying the Rich solution flow rate, the rich solution inlet temperature, or the gas loadings (least effect of all). But you haven’t told us that.

If you control the temperature of the reboiler by heating the Lean solution in the reboiler at a fixed temperature (and varying the reboiler heat rate) you will also be “controlling the stripper overhead vapor temperature. And that is the way I would vary the reboiler heat rate. The basic parameter that you want to control is the reboiler heat rate – not the overhead vapor temperature – in a stripping operation that involves non-condensables. This is a very easy application.

You say you “need to control the overheads temperature” in order to:

(1) Control hot oil flow by measuring the overhead temperature
(2) Not put too much load on reflux condenser in case of very high overhead Temperature.
(3) Prevent amine from degradation by heating it up more than 260 or 265 F.

I claim that to control the hot oil flow into the reboiler you should control the main parameter: the reboiler Lean amine temperature - by bypassing excess hot oil around the reboiler. You don’t have to worry about putting too much load on the overheads condenser. The overheads temperature will be as constant as the reboiler controlled temperature. You will always have a certain percentage of excess reboiler vapors that have to be handled by the overheads condenser – this is used as reflux and that is a measure of how you operate the reboiler.

A temperature transmitter on the overheads vapor line will not help you at all. All you need there is a temperature readout.

I hope this helps you out.

[chem101]

Thank you very much Mr. Art and all other Engineers.
Mr. Art I appreciate taking your important time to explain me the amine process in very short but understandable way.
I take very much interest in all your answers posted on this website. I find you topic very interesting and very meaningful.

[Zauberberg]

Controlling amine regenerator overhead temperature is just another way of controlling heat input in the regenerator reboiler. Since it is a total reflux system, increasing reboiler duty leads to increase of tower overhead temperature. That is why we employ such (or similar) control system, because it is very difficult to control reboiler heat duty by controlling reboiler outlet temperature: since it corresponds to the boiling point of water (at given pressure), increasing or decreasing reboiler duty does not change the outlet temperature so much. And that is why we opt for controlling overhead temperature.

This arrangement works well, although a bit slow and with a certain dead time as in all temperature-controlled processes. The good thing is that it reflects actual reflux ratio (= moles of water vapor divided by moles of acid gas, in the overhead stream), which is calculated by dividing water partial pressure by acid gas partial pressure, at regenerator overhead conditions. By knowing the temperature, pressure, and feed acid gas content, the calculation is straightforward. Keeping the reflux ratio constant, we ensure that the regenerated solvent quality (acid gas loading) is also maintained constant. Read page #6 of the attached DOW article.

As Art has correctly pointed out, bypassing a portion of hot oil flow is an easy way to achieve heat duty control in the reboiler. It is easily done by employing a bypass line and a three-way valve, nothing more than that.

Attached Files

•  DOW Gas Sweetening.pdf   1.22MB   39 downloads

[Zauberberg]

Also read these materials, it's a nice overview of Gas Sweetening design and operational issues:

http://training.nigc...gf_June2008.pdf