3 replies to this topic

[rxnarang]

We have a control valve which will see two-phase upstream.
Q1) Does this pose special requirements on the control valve? I am aware control valves can be designed to handle two phase, but the flow regime in this case can be slug flow. What about effects on stable control and flow?

The fluid ( amine+water) further flashes across this control valve and the flow regime downstream can be slug too. Destination of this stream is the amine regenerator.

Q2) Does this mean we have to place the control valve as close as possible to the regenrator (to avoid any upsets in the column)? Does anybody have any experience in locating this valve relatively far away from the regenrator (say at the base of the column)?

Regards

[Art Montemayor]

Rajiv:

This is an interesting query to me because I've designed, operated, and constructed a few amine solution-based processing plants - most of them for acidic (CO2) gas removal and production. As you are probably well aware, the Amine process (originally the Girbotol process) has been around for well over 60 years and is primarily used in two basic configurations:

1) the Absorber tower is operated under a pressurized atmosphere; an example of this application is a conventional Ammonia production plant where the Steam reforming process is used to generate the H2 required for the synthesis. Here, the amine solution is introduced into a pressurized absorber where the CO2 is selectively removed by the solution which is, in turn, routed to a regeneration portion of the solution's closed-circuit: an Amine Stripper connected to a reboiler. The "rich" amine solution from the absorber (which is what I believe you are referring to) is subsequently flashed into the top section of the Stripper through use of a conventional control valve. The stripper-reboiler (as in most all amine systems - especially MEA) operate at essentially atmospheric pressure (actually, around 5-10 psig) and therefore, the resultant flash of the Rich amine solution is thoroughly 2-phase. However, the feed into the control valve (while it may be saturated with dissolved gases) is not 2-phase.

2) In other applications the absorber is not pressurized at relatively high pressures and may, in fact, be operated with a few inches of water column. In this application, the Rich amine that collects at the bottom of the absorber has to be pumped out (under level control) and sent to the amine Stripper. In this case, as well, the amine solution is not 2-phase and also undergoes a flash operation upon being introduced into the Stripper under the presssure of the Rich solution pump.

At this point, allow me to introduce a caveat to both types of operation: The Rich solution is always routed (for heat recovery and economic reasons) through a heat exchanger that employs the recirculating, hot Lean amine exiting the amine reboiler as a heat source. Normal and conventional amine solution temperature entering the flash control valve at the top of the Stripper is about 200 - 205 oF. An amine reboiler usually operates in the order of around 220-225 oF.

The resultant "Lean" amine solution that exits the reboiler is sent first to the amine heat exchanger (mentioned above) and then to an amine cooler prior to it being pumped back to the top of the amine absorber to complete the solution circuit and pick up more acidic gases as Rich solution.

In all of the amine solution circuit there is no 2-phase flow that I have ever come across (with the exception of the Rich solution control valve flash product that enters the Stripper) and that is why I have gone into this long and detailed write-up in order to get a clear understanding of where your application differs from the one(s) I have explained (or tried to). Perhaps I have missed the boat on a new or unique application of the amine solution process.

With respect to your second question: I always try to locate the Rich solution control valve as close to the Stripper as I possibly can in a physical manner. What this winds up as (in an empirical plant) is a typical, Fisher, 90 degree angle valve where the Rich solution enters the top of the control plug rather the bottom and exits the CV out through the plug's bottom and into the Stripper. In other words, the CV's stem is mounted in a horizontal fashion with the valve's diaphragm cover oriented vertically. The Rich amine solution piping is installed vertically alongside the Stripper and goes up directly into the CV's inlet nozzle and makes a 90 degree turn inside the CV while undergoing the flash expansion. The nozzle that is used to mount the CV should be stricly designed to handle the expected violent, corrosive flash products. Case-hardened SS is the CV preferred choice on trim and the nozzle should also be size accordingly. I like to employ a conical expander right up against the Stripper to allow easy expansion and immediate lessening of the resultant vapor velocities. The entry section into the Stripper is, of course, also designed for this effect. Just about every experienced and serious application of the Girbotol process I've seen in the field - including the original design of the Girder Corp., who patented the process - uses this type of mechanical and process arrangement in some sort or other. The intent is to effect a useful and resulting flash - but right in the Stripper itself. The flashing is never done within the piping - nor is any 2-phase flow tolerated within the amine solution circuit piping. The potential of the Rich solution to go into 2-phase is controlled by maintaining this solution totally pressurized up to the CV that I've described above. After that, the 2-phase potential problem is history.

Has my description, explanation, and experience helped at all? I hope it has and if it hasn't, could you explain your application more specifically?

Regards

Art Montemayor
Spring, TX

[rxnarang]

Art,

Thanks for your patient reply. I was motivated by your efforts to delve further into this problem, although that was not my original mandate.

I agree that this valve should be placed close to the regenerator. I am endeavouring to convince the project to do so.

Regards

[Parmindar Singh Rana]

Hi,
I have a query which relates to subject of the thread.
Please find attached the rich amine circuit.
The rich amine flash drum is operating at pressure of 7.5 barg and 84 Deg C.
The problem is that the regenerator and the lean/ rich amine exchanger are located about 400 m from the
rich amine flash drum. When the rich amine solutions exits from Lean amine/ rich amine exchanger it is already two phase with about 3.5wt% vapor/ flash gases (as given by licensor and predicted in Promax). The control valve per present scheme is located at elevation of about 37 meter at platform near regenerator feed nozzle. The control valve inlet consists of as high as 90 vol% vapor.
Please suggest where this control valve should be located?


Thanks in advance.

Parmindar

Attached Files

•  Rich Amine circuit.xls   19KB   70 downloads