Location of Amine Stripper Feed Control Valve

[ab84]

This is regarding DGA system to strip out acid gases from Refinery off gases. In all projects we have seen that the Amine Stripper Feed Control valve is located near the Amine Stripper Feed Nozzle at minimum distance from column.

The configuration is typical, whereby the rich amine from the Rich Amine Flash Drum (downstream of the Amine Absorber to strip out lighter hydrocarbons) passes through the Rich DGA/Lean DGA exchanger where it is heated (Temperature controlled to prevent vaporization) and then flashed across the Feed Control valve and enters the Amine stripper. The Rich Amine Flash Drum operates at around 5.5 kcsg while the Amine Stripper operates at around 1.3 kcsg and the Rich Amine flows based on pressure difference to the Stripper.

My question is how do we decide the elevation of Rich Amine Flash Drum?

The heated rich amine at inlet of Control valve must be liquid and it flashes across the valve. Since the rich amine liquid in flash drum must be considered at bubble point , the only parameter that can achieve a bubble point or slightly subcooled liquid at inlet of this control valve is the rich amine flash drum skirt height. With control valve located at height of tower feed inlet nozzle and considering increased vapor pressure due to heated rich amine, rich amine flash drum must be elevated above the tower feed nozzle to offset increased vapor pressure plus line and exchanger pressure drop.

 

[georgeverghese]

Vapor mole fraction in this 2 phase stream at the LCV inlet is usually low. Ask the control valve vendor to size the CV for 2 phase conditions at the inlet and further flashing in the valve. Extract the 2phase flow parameters necessary for valve sizing at the vena contracta in the valve, for which the process simulator will be required. There is no need to elevate this drum to the extent that the rich DGA will remain subcooled at LCV inlet. Request for hardened valve trim to withstand cavitation (anti cavitation trim) for the LCV. Standard practice is to keep the LCV at min distance to the stripper column inlet to avoid slugging / uneven 2 phase flow onto the feed tray in the stripper.

 

[ab84]

Thanks for your responae. I waa looking at having liquid phase at control valve inlet as it is difficult to control when the flow is two phase at control valve inlet

 

[georgeverghese]

There may be many reasons why the level control at the flash drum is poor. Some of these may be:
a)Incorrect LCV sizing
b)Slug flow in the vertical riser leading up to the LCV - see if you can simplify the piping route to minimise low points where liquid could pool; reduce line size to enable annular flow if pressure drop is still acceptable
c)Unstable pressure control at the flash drum
d)Incorrect orientation of the rich feed and exit nozzles at the HX which results in vapor trapping in the HX, which then leads to unstable exit temps / erratic flow to the LCV. Nozzles should be oriented such that vapor can free flow out to the exit and not get trapped at internal high points within the rich DGA side channels.

 

[ab84]

Thank you for the response.