8 replies to this topic

[engg]

I wanted to know the steam flow control configuration normally employed in steam supply to reboiler line (in kettle type-reboiler of stripper in amine plants).

what i have is a flow controller that takes, input from rich-amine(to stripper) flow transmitter, steam temperature and steam pressure before the steam flow cotrol valve. and then the controller will do "something" and give an output to the steam flow control valve....

i want to know how the controller will decide the flow of steam based on all the inputs....

is it enough to take the design rich-amine flow rate and design steam flow to calculate the design amineflow/steamflow ratio and put it in the controller which will then simply multuply rich amine flow with factor to know the steam flow needed?

thanks

[fallah]

As far as i know,rich amine enters the stripper through a LV which controls amine liquid level in upstream drum.

On the other hand,the heat input to the stripper reboiler is from LP steam which flows to the reboiler via a FV.The gas temperature of the acid gas leaving the stripper overhead is measured by a TIC which cascades onto the LP steam flow through stripper reboiler,to fine tune the stripper operation.

[pavanayi]

Engg,
Firstly, It would have been much helpful for everyone else to visualise the situation if you had reproduced the P&ID showing the location of transmitters and the control valve that you described in words. As they say, a picture is worth a thousand words. Even a sketch showing the things that matter would do.

Secondly, if you have the system installed and working, the first port of call (and the best) is the instrumentation department of your plant. They can say what signals go into the controller, and what factor(s) the controller takes as input, and what is the output going into the control valve.

Thirdly, 'Normally', in strippers, reboiling media (here steam) is usually controlled by an indirect quality control. The composition at one point in the column is measured in terms of temperature (indirect quality) and controlled via the reboiler. I have not personally seen a quality control in a stripper where feed flow rate is used as the measured entity.

[Art Montemayor]

Engg:

I have designed, built, installed, and operated amine acid gas process units. I haven’t done this in a long time, but I still remember the design basis, criteria, and methodology of process control. There is no automatic flow control on the low pressure steam fed to the kettle reboiler U-tube bundle – nor is any required. You don’t define or tell the details of what you are doing or what you are describing, so it impossible to comment on your installation – or proposed design. I can only relate on how I have done it successfully - and others – and why. If you want or need details, you must furnish the basic details and information in order to be able to comment and give helpful advise.

An amine acid gas unit is designed as a basic, constant steam flow rate supplied to an amine reboiler. The reboiler is the driving force for a gas stripper operation. Depending on the acid gas back-pressure you impose on the Low Pressure acid gas Cooler-Condenser, you will have a constant Stripper overheads temperature of approximately 220 to 225 ^oF. That is based on MEA. On the higher amines (DEA MDEA, etc.) the temperature will be different – but the process is the same: you are stripping out absorbed gases in the Rich Amine using steam generated in the reboiler. The overhead temperature will be constant if you maintain your LP Cooler Condenser at a constant pressure – which has been 100% of the applications I’ve seen and been related to over the past 51 years.

By nature, or inherently, an amine process is carried out with a constant Lean and Rich amine solution flow rate. It is not normally varied. So, why would you want to vary the steam flow rate?????

If you have something different – or you want to do something different, state it and tell us what your scope of work is.

[engg]

Thanks all for your reply;



find the sketch attached here. can you comment as to why, the steam FV can be controlled in two modes. stripper top temperture control or stripper inlet flow control mode...

another question is.

Art;
I thought that the stripper top temperature varies with reflux and inlet rich flow, so the top temeprature varies, so if i increase the steam, i can expect to rise the top temperature. the bottoms will remain constant at bubble point.

So i want to strip more to effect loading i may want to increase the top temperature by increasing the steam supply??!


or maybe i can decrease the backpressure on cooler (stripper presssure)to strip more, is it possible, any negative effects (stripper is pakced column-rashig rings)?

Attached Files

•  steam line.png   78.83KB   31 downloads

Edited by engg, 10 March 2011 - 06:11 AM.

[Art Montemayor]

Engg:

You still have not addressed our requests and questions.

If you want/need pavanayi's input, address his request. If you want/need my response, do the same.

[Dacs]

I've seen a system where the Regenerator operation can be controlled by either [A] Rich Amine flow going to the Regenerator and [B] column temperature (which is the usual method).

As I understand, [A] is provided to account for changes in amine circulation rate, whenever you have a change in sour gas feed rate on the Absorber (where the Lean Amine from the Regenerator is getting used). I was told that method [A] is usually used during start-up and whenever you expect changes in sour gas flow rate.

But normally you'd want to operate the control via temperature since as Mr Art has said, you don't expect variation in amine circulation rate (apart from what I mentioned above).

Edited by Dacs, 11 March 2011 - 12:05 AM.

[engg]

Art;


everyones reply is needed and welcome, and since you are well experienced in this field your reply is valuable.

well i am commissioning a new plant and want to understand this control well before startup, so the instrument guys havent yet configured the steam control.

anyways, our feed gas co2 content will vary, sometimes the
co2 slip will increase so i will want to increase or decrease the flow accordingly.

regarding the steam control, the flow of steam can be controlled in flow mode (of rich amine) or stripper top temeprature control so its our choice. when we see teh sketch we can see that the flow control input to controller taken input from steam temperature and pressure as well as flow of rich amine, which means that the controller is calculating the heat needed and deliverd by steam at that conditions and accordingly will decide and flow. is it right? now i dont know why i want to do that maybe to react better and faster during changing conditions...

[Art Montemayor]

engg:

In order to control a process, you must understand, excel, and dominate the process. You should know exactly what is the purpose and expectation of each piece of major equipment in your process. This belief and principle was taught to me by my first mentor – who started me out in the amine process for the removal of acid gas. The principle has worked for me with increasing success ever since. I’ve never walked away from a process, a startup, an upset, or a modification that I couldn’t succeed in. I owe it all to my mentor who taught me the basic, simple truths.

An amine stripper is a special piece of equipment. It releases the acid gas as overhead product and – now contemplate and understand this well – IT DOES NOT REQUIRE ANY REFLUX. This statement is going to upset a lot of process engineers – especially those who live and swear by process simulations – but if the truth be known, the stripped gas is not returned back to the stripper top plate or section as reflux. Therefore, by application of simple, basic distillation and stripping theory THERE IS NO REFLUX TAKING PLACE IN A CO2 STRIPPER. You can readily see this on an equilibrium diagram used to apply the McCabe-Thiele method of solution.

The condensed, weak amine solution (approx. 0.5% wt.) that is condensed in the low pressure overhead condenser is what some people return back to the stripper as “reflux”. THIS IS NOT REFLUX. That is nothing more than a scrubbing action applied to the top section of the stripper that “scrubs” the outgoing acid gas (and it requires additional reboiler steam). If you were to apply a veritable, true reflux, you would have to liquefy the acid gas and return it back as reflux to control the purity of the outlet overhead. That is not what takes place because you can’t liquefy the acid gas. Therefore, you can simply drain the weak condensate downstream of the LP cooler-condenser and return it back to the amine system where it belongs. I always tried to return it back to the top of the acid gas absorber where it served a useful purpose – it scrubbed the outlet process gas and captured entrained amine droplets and lowered my unit amine consumption figure significantly. When I modified and operated my amine plants that way, I always had the lowest amine consumptions reported in our corporation. And the stripper worked just fine.

I always used the ratio of kgs of LP condensate/kg of released acid gas as the measuring stick of controlling the steam added to my reboiler. Low pressure steam consumption at the reboiler the highest unit operating cost in an amine process and you want to be able to control it. Normally, the amine flow rate is kept constant and only varied by manual control – in accordance with the acid gas slip that takes place in the absorber with the effects of time, the acid gas content in the process, or the flow rate of process gas into the amine absorber. But these effects don’t usually take place instantly or dynamically. Therefore, an instrumentation network is not needed to control the amine flow rate. Operators respond by changing it manually. Your reboiler – stripper should be designed correctly for the largest amount of amine flow rate that you are going to employ. You certainly don’t want to waste excess steam in the reboiler – but the truth is that in a real-world, practical amine plant, you are always supplying a contingent amount of excess steam that ensures that all of your acid gas is stripped out in the stripper. You control and measure this effectiveness by referring to the above-mentioned lb weak condensate/lb of acid gas. You should be measuring these quantities and working out the ratio at the weak condensate separator vessel that is directly downstream of your stripper overheads cooler-condenser. That’s the way I always did it and it worked out fine – with a minimum of instrumentation and good operator control. If I had to increase the amine flow, we increased the steam to the reboiler as well. We did this manually, step-wise.

Note that the way I design and operate an amine system, there is no varying stripper overhead temperature. It is, and remains, constant. So I don't need any "controls". It is that simple.

I would insist on operating an amine process that way today because it is simple, direct, and a practical way to control the operation and results. The Amine process has been around for a longer period of time than I have been on this planet and there isn’t anything that anyone can tell us today of how an amine process works and what you can do to it or how to control it. The process operates on simple, common sense based on a closed cycle. If you keep it that way, my experience shows that you will not have any problems or upsets.

You haven't responded to all our requests for basic data and information (we haven't seen a simple P&ID), so it is not possible to comment any further. I hope the above helps out.