2 replies to this topic

[law4msc]

Hi,

I am an energy and environmental engineering student undertaking a carbon capture project in my 3rd year at university. I have been asked to design the regenerator column for our process to recover lean 20% MEA and strip CO2.

Not coming from a chemical engineering background has caused me to struggle a bit. Anyway, I am trying to obtain my vapor equilibrium data.... looking in Gas Purification by Kohl and Nielsen I came across a graph showing the partial pressure of CO2 against the mole ratio of CO2 to MEA. Is this my VLE curve? Because in the worked examples in Coulson & Richardson the equilibrium curves used for the calculations to determine the number of stages look different.

Also, we are working on a 800MWe power plant, and the flue gas flow rate entering the CO2 absorber is nearly half a tonne per second! The rich MEA solution entering the regenerator is nearly a tonne a second.

Are these figures realistic? Im guessing there will have to be more than one absorber and regenerator? If so, does anybody know what flow rates are typically used in industry?

I am also attempting to start my calculations to determine the number of stages required in the column. I know the liquid flow rate, but what is the gas flow rate (V)? Is that the flow rate of the vapors leaving the reboiler? Or is it the flow rate of CO2 leaving the column? Therefore at stage 1 the gas flow rate will be very small and at the top of the column the gas flow rate will be high (near enough the theoretical CO2 exit flow)?

Thanking anyone who can guide me.


Regards


Ramzi

Attached Files

•  Solubility_of_CO2_in_30_wt_MEA_solution.jpg   73.37KB   123 downloads

[Art Montemayor]

Ramzi:

If you have no formal Chem Engineering university training in such courses as Unit Operations, Heat Transfer, mass transfer, and Fluid Mechanics you are going to be swimming upstream through some deep waters. I don't know the curriculum for the degree program you cite, but it can't be as versatile and thorough as a Chem Engr degree. Nevertheless, if you have to undertake it then you have no choice.

I've designed, built, and operated CO2 plants using MEA long before it was popular to call it "sequestering". I've designed most of my plants using a slide rule, but I've also resorted to simulators - such as Aspen and HySys. The simulators always have errors or flawed results in the design. But since I have 49 years of experience, I know where to make the corrections on the computer's output. The best and most reliable simulator used today, in my opinion, is ProMax. If you have never designed a process plant before, you probably have to rely on a simulator. To do that, you must have the Chem Engineering training to do so.

You are asking a lot of questions and probing on how to start your project. You have to organize your methodology and attack your challenge in an orderly way. If you don't have any knowledge of how the Amine system works, then you already have a major problem. Kohl & Nielson probably have the best textbook on the subject, but you need additional information - such as that found in the GPSA Handbook and in John Campbell's "Gas Conditioning".

You ask if your information is realistic. My professional opinion is that depending on a 20% MEA wt. solution to operate continuously is NOT realistic. The corrosion rate, in my opinion, will overcome the process. The solution loadings are just too high. You give a flue gas rate of 1/2 tonne per second, but you don't even mention the amount of CO2 you are absorbing and the solution loadings you are designing for. These are all critical data that you must establish before anything else.

My recommendation is to create a Process Flow Diagram first - complete with expected or anticipated temperatures and pressures. This is a basic requirement in order to base your questions. Once the basic process is accepted then, you can enter into your basic design using a simulator. If you want to do it manually, this is also OK. In fact, I believe you will learn more and profit more doing all the calculations by hand - but that is your call.

I recommend you employ a solution strength of no more than 15% wt. MEA at the design level. In actual practice you may operate it a little less than that. The underpinnings of the MEA process are going to be the MEA solution flow rate and the reboiler heat load as expressed in low pressure steam consumed there. The MEA process is the highest steam consumer (at the reboiler) of all the Amine processes. That is one of its weaknesses.

If you prepare a formal PFD as I recommend, submit it to our Forum and we will then make comments and advise you on what we review.

I look forward to your submittal.

[law4msc]

Art,

Thank you for your quick reply and offer for guidance.

I've attached a pdf with my process flow and a few tables indicating the stream temperatures, pressures and compositions.

I have changed the MEA concentration to 15%. I'm assuming the only change is in the amount of water and there will be no effect on the mole ratio (i.e. 0.5 mol CO2 / mol MEA and therefore 7.95 kmol of MEA will still be req'd). In my interim report last term, I set the absorber efficiency to be 85%. The input stream contains 3.97 kmol/s CO2 (175 kg/s) and 85% of that will be absorbed by MEA.

I also set the stripper efficiency at 99% which I realise now is very optimistic. Settling for 80% efficiency would be better, right?

As you stated, I have to organise my methodology and attack my challenge in an orderly way. I do plan on doing the calculations manually (by hand). I would appreciate a dummies step by step guide on what to do. I plan on doing all the necessary work and I am not asking for anyone to do it for me. Just a guide on where to get started and what needs to be done first!

Many Thanks again

Ramzi

Attached Files

•  CCPlant_1_.pdf   82.39KB   249 downloads