6 replies to this topic

[attya]

Hello,

I have been recently employed in one of BP gas sites in Egypt and i have some questions:

Design: First of all, the amine sweetening plant is designed to treat 55 MMSCFD gas (7%mole CO₂, 1,200 PPM H₂S) with an amine flow rate of 258,000 lb/hr (42% MDEA; 3% Hydroxy-ethyl piperazine; 55% H₂O)

Gas temperature = 115 F

Gas pressure = 158 PSI

Lean amine temperature = 130 F

Lean amine pressure = 155 psi

Currently: gas flow rate = 12 MMSCFD (4%mole CO₂, 250 PPM H₂S) and amine flow rate = 150,000 lb/hr (42%MDEA 3%HEP 55%H₂O)

Gas temperature = 90 F

Gas pressure = 160 psi

Lean amine temperature = 130 F

Lean amine pressure = ???

Questions:

• I believe that the amine circulation rate should be lower than that since the gas flow rate has dropped to around 20% from design data. However, the circulation pump is a centrifugal pump with no VFD - so the pressure will be different if I lower the circulation rate. So how can i work this out?
• Sweet gas outlet temperature is 120 F, Which is kind of weird, since it's 10 F lower than lean amine temperature, Personally (From my little experience) I haven't seen the sweet gas temperature lower than the lean amine temperature before. What could be the possible cause for that?
• What is the definition of flooding factor? And how can i know/calculate it if the flow rates/pressures for lean amine and gas rate are not adequate for the amine absorber.

Thanks in advance.

Edited by Art Montemayor, 17 April 2012 - 11:13 AM.

[attya]

bump ..,
can anyone help ?

[Dacs]

My answer is based on my experience on what I expect on a typical amine sweetening column since you have not provided any P&ID/schematic to base my answer on. I also assume that this is a running plant we're talking about:

1. I guess there's a flow control valve/FCV on your lean amine discharge line and if so, change the setpoint to match the 155,000 lb/hr requirement. Even your pump is a single speed motor, the FCV will handle the increased head due to decreased flow.

The lean amine pump, of course will have a higher discharge pressure, due to lower flow. To know exactly the value, you have to check the pump performance curve and check the head based on 115,000 lb/hr flow.

By the way, lean amine will not control the tower pressure. Basically the sour gas will dictate (through a pressure control valve/PCV located overhead your amine column) the column pressure. Since the gas pressure is roughly the same as the 1st one, you'll have no problem about this.

2. Normally, I'd expect that your sweet gas temperature to be around your lean amine temperature. Your sour gas (inlet) temperature is 90°F however, so this may cause a lower sweet gas temperature.

May I suggest that you lower your lean amine temperature to about 10°F higher than your sweet gas inlet temperature and see what happens.

Another thing is you may have bypass on your column that keeps the gas in contact with lean amine, causing a lower outlet temperature (go to point 3)

3. My concern with your scenario is your current flowrate is outside of what I expect for a typical column turndown conditions. Indeed, your column might experience flooding.

Normally, turndown depends on the internals type. What kind of internals do you have? Tray or packed internals?

Have you experienced a higher pressure drop across your column?

[attya]

thank you for your response.
I attach the following for you:

P&ID for amine absorber
P&ID for circulation pumps
DATA SHEET for amine absorber

1. Yes, the pressure will be higher - of course - for the 150,000 lb/hr discharge flow rate. I have checked the pump's performance curve and thought that it said 190 psi. Is that still ok for sour gas pressure of 160 psi? what is the effect of non equivalent pressures between lean amine and sour gas?

2. I dont think I can reduce my amine temperature low enough to be +10 F on top of my sour gas, because in winter sour gas temprature drops to 79 F. I think they are about to install insulation between the 3 phase separator (Slug catcher) for the gas stream all the way to the amine absorber and probably heat tracing to increase the sour gas temprature - especially in the winter time.

3. The pressure drop on the DPIT is around 3 psi for all trays. In the data sheet you can see my tower specifications, but I will summerize them here:

Tower type = Trays

Number of trays = 24

number of passes = 2

Tray inside diameter = 2,400 mm

tray spacing = 600 mm

weir height = 2 inch

foaming factor = 0.73

flooding factor = 0.65

One last thing is that I do not personally know how to figure out if my tower is flooding or not; or what is the exact definition of "flooding factor" and how can I use all these factors together to know what's the suitable amine circulation rate per incoming gas rate.

Thanks in advance,

Regards.

Attached Files

•  PME01A-C0104.pdf   689.39KB   355 downloads
•  PME01A-C0004.pdf   126.32KB   264 downloads
•  PME01A-C0008.pdf   145.52KB   257 downloads

Edited by Art Montemayor, 17 April 2012 - 10:51 AM.

[Art Montemayor]

Attya:

You are right. You definitely have to reduce your MDEA solution circulation rate. But the reason for doing that is to conserve energy and be more efficient – not to prevent “flooding”. Flooding in a distillation tower or an absorber is defined as a state brought about by excessive vapor flow inside the tower, causing the downward falling liquid to be entrained in the vapor up the column. The increased internal pressure caused by the excessive vapor also backs up the liquid in the downcomers, causing an increase in liquid holdup on the plates above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding in a tower is detected by sharp increases in column differential pressure across the trayed section with a significant decrease in separation efficiency and a liquid entrainment problem in the overhead vapor/gas stream exiting the tower.

Flooding is definitely not your concern. Just look at your present situation with regards to the design conditions for the system:

You are operating with a 78% reduction in design gas capacity (= (55-12)/55); your problem is that you have a gross TURNDOWN problem – not a turnup problem.

Instead of trying to remove 3.85 MMScfd of acid gas (= 55x0.07), you are now trying to remove only 0.48 MMScfd of acid gas (= 12x0.04). this is 12.5% of the design loading on the MDEA solution being circulated (258,000 lb/hr).

Normally, I would expect to circulate approximately 38,000 lb/hr of the 42% MDEA solution through the absorber to pick up the acid gas, but this involves careful study of the tower’s hydraulics and requirements for good control and proper acid gas loadings. This information should all be explained in the KBR operating manual for the absorber. Kellogg Brown&Root should have furnished you with a detailed explanation of how the MDEA absorber works and how it is expected to respond to both turndowns and turnups. You should have a clear and concise understanding of what acid gas loadings are and why/how they are monitored and controlled within the system.

If the operating manual does not discuss how to turndown the unit and if your lean MDEA circulation pump does not have the capability to reduce the flow rate, then the design Scope of Work for KBR did not specify a need for this and you now have a problem of designing a method and implementing a turndown operation to function at the reduced acid gas rates. I certainly hope you have a chemical engineering process background and training. From what you write and how you describe the situation I don’t believe you do and if I am right, you should obtain an experienced process engineer to do the job. The lean MDEA pump has to be designed to meet the head requirements of the absorber operating pressure(s) and the system’s resistance. This may involve a new pump and driver. The MDEA reboiler will have to be turned down as well since you will have a reduced reboiler/stripper load.

I would not waste time worrying about the MDEA and acid gas temperatures. They seem to be OK because they should be in the ambient range. The cooler the acid gas and the lean MDEA, the better the absorber will operate. The only reason for keeping the lean MDEA solution temperature above the acid gas temperature is the fear of condensing any hydrocarbons in the acid gas. If you don’t have heavy hydrocarbons in the acid gas (which you don’t mention), then keep it as cool as you can as well as the lean MDEA going into the absorber. The cooler the absorber operates, the better the absorption of CO2 and the more efficient the operation. Lean MDEA at 130 oF is much too hot. I would run it as cold as I could – maybe 80 – 90 oF. I presume your problem in Egypt is cooling and not heating.

You should be specific in describing your basic data. State exactly where the fluid temperature and pressure are taken when you are describing them. Also always state if your pressures are absolute or gauge.

In my opinion, you are in a fortunate situation because you have a trayed absorber and not a packed one. It is far easier to turndown a trayed tower, and you might escape without having to make a vessel entry and internal modifications.

I highly recommend you obtain and vigorously study the book “Gas Purification” by Arthur L. Kohl and Fred C. Riesenfeld – now published as by Arthur L. Kohl and Richard B. Nielsen (Fred Riesenfeld died). This massively updated and expanded edition is the most complete, authoritative engineering treatment of the gas purification processes used in industry today. Of great value to design and operations engineers, it gives practical process and equipment design descriptions, basic knowledge and design information. I also recommend you get the GPSA Engineering Data Book.

[attya]

Hello ..,
First of all i would like to thank you Art for spending the time to respond to my topic ,
Second , i stated already that i recently got employed in the plant "less than a month now" plus i just graduated 6 months ago so excuse my little exprience ,
I ordered today "Gas purification" book from amazon , and will definetly get a careful study on it.

About things that i didnt mention in my last respond , The pressure was psi-g , Sorry if i didnt mention that , and sour gas doesnt contain heavy hydrocarbons .

[Art Montemayor]

Attya:

Do not worry about thanking me; any young engineer that is frank, demanding, willing to work hard and learn all he can is well worth all the effort we can put out in our Forums. We all were in your same shoes at one time early in our career – so we know the feeling well.

And, as far as I am concerned, you have no reason to explain your novice situation and that you are new at this game. Your eagerness to learn and improve yourself is the most important and good thing you have going for you.

You should also try hard to get a copy of the GPSA Engineering Data Book, which you can obtain in an electronic version. You will never regret making that investment in your personal career. Everyone in your industry has a copy on their desk or in their computer and never stops using it during their working hours. Go to: http://gpsa.gpaglobal.org/databook/

For detailed and expert information on gas sweetening and gas treating, you should always look to BR&E (Bryan Research and Engineering) at: http://www.bre.com/s...s-treating.aspx

They are the world experts in using MDEA and all variations of amine solutions to remove both CO₂ and H₂S. You should go to their home webpage and click on SUPPORT and TECHNICAL ARTICLES. There, you will find 25 papers on amine processes and plant operations – especially dealing with MDEA. You should download all 25 papers and carefully study them and the information that they contain. You will learn a lot quickly about the process that you are operating.

If you are going to get involved in making a retro-fit modification to your unit in order to turndown its operation and run it efficiently and without problems, you should have or own a registered copy of BR&E’s simulation program, “PROMAX”. This simulation program was specifically developed to design and evaluate amine processes – especially MDEA. This program, in my opinion, is the state of the art in amine plants and is recognized world-wide as the program of choice for designing and simulating an amine process. Promax should be a valuable and profitable asset when you deal with a reduction in design capacity (turndown) of an amine plant. You will be able to predict what can – and will – happen in your process when you reduce the amine flow rate. This program will definitely pay back for itself in no time at all in your application.

I hope this information helps you out.