7 replies to this topic

[GASCO]

hi all

our NGL plant have two operating modes
1-Ethane rejection
2-Ethane Recovery

when we oeprate @ the ethane rejection mode there is more vapor flow to the turbo expander than in the ethane recovery mode

when we are operating in ethane rejection mode, the pressure in the demethaniser drop from 30 bar to 26.6 bar ? why this phonemena is happening ? shall we suspect flooding of the column ?

[Zauberberg]

You need to post a P&ID or a sketch, as NGL Recovery units are always specific. Is this a design by Ortloff?

Why the flow rate to turboexpander should change, depending on the operating mode? Is there any internal recycle stream? Normally De-C1 towers operate at lower pressures than De-C2 towers.

Provide some more data and tell us what makes you think that there is flooding in the column, or anything that you consider unusual.

[GASCO]

hi Zauberberg
yes its by Ortloff
we do not change the pressure of demethaniser to control the mode
we only change the bottom temeperature of the reboiler and we change the path of the liquid going to the column "see attached doucments"


what do u mean by "Is there any internal recycle stream?"

Attached Files

•  recovery conditions.pdf   99.16KB   194 downloads
•  rejection conditions.pdf   103.51KB   139 downloads

[Zauberberg]

I'm not sure if I understood you well. I can't see how the pressure can simply "drop" - it should be controlled somewhere in the Expander Feed Separator/Column/Residue Gas circuit, but there is no pressure controller shown in the PFD. Is it located at the suction of Residue gas compressor?

There must be a logical explanation of what causes operating pressure to change so drastically (3.4 bar) when switched from De-C1 to De-C2 mode.

As for the first question - expander feed separator temperature is higher in the Rejection mode (-34 degC) than in the Recovery mode (-42 degC), that is why more gas flows to the expander section.

[Zauberberg]

A thing worth checking, is to see if there's any of the key control loops (pressure, flow, turbine/compressor speed) operating in MAN mode, instead of AUTO/CAS.

In many of the Ortloff's plants, the Master/Performance controller which takes the pressure signal upstream of the Residue Gas compressor (not shown in your PFDs), sets the position of expander IGV and the speed of Residue Gas compressor, so the pressure upstream of the RGC basically "floats" on the RGC suction pressure. If this controller is in AUTO, it is impossible to have such a huge difference in the tower operating pressure, unless some unreasonably high pressure drop takes place in between the two systems, due to flow variations.

If you can't get the clue what is going on, contacting Ortloff is probably the best path forward. During commissioning and startup of LNG Trains (including two SCORE units for NGL recovery) there are two guys from Ortloff here with us on a continuous day/night basis, and they have been a tremendous help.

[GASCO]

Dear Zauberberg

we found the problem it was the IGV of the turboexpander, the IGV opening showing in the DSC in the control room was showing 100% but actually its not really 100% in the field, after adjusting the IGV

and increasing the sour header pressure from 72 to 74 bar "this is upstream in the amine unit" we were able to obtain enough flow and the demethanizer perssure was holding

regards

[NGL Licensor]

Normally in the ethane recovery mode you need to recover ethane from the gas part of the expander outlet gas as well as from the gas leaving the stripping section of the column.

Hence you need more liquid reflux on the absorber trays. To achieve this you split more gas to the gas sub-cooler from the expander feed gas (in your case it is around 96 mmscfd). Also all liquid from OVHD reflux drum 141-V-111 is routed to top tray of the absorber section

In ethane rejection mode (Propane recovery mode) no gas split is made and that is why you have basically more gas to the turbo-expander (in your case the gas to GSP cooler 141-E-111 is zero) and some of liquid reflux from 141-V-111 is routed to top tray of the absorber section of the column and 42 % of the liquid reflux from 141-V-111 is routed to the top tray of the de-ethanizer section of the column.

Do you switch from ethane recovery to ethane rejection quite often? I believe this depends on downstream GASCO fractionation plant at Rewais area and feed stock requirement of the ethylene complex of Brouge


Regards,

A.King
ngltechnology@yahoo.com
nglprocess@gmail.com

[thaihy]

GASCO

Steam seems to be heating medium in your plant. in case of column blowdown, steam may be leaked into the column, any problems to column freezed up after resuming normal operation?

It may be ok to your process because the botom temp. is below water condesation temp. and it may be dumped off the system.

Some of Ortloff processes such as SCORE , the reboiler temp. is around 100 degree C, there may be some incident that steam can be accumulated and freezed up in higher section of the column. Should Hot Oil be preferred?

Thanks.

Edited by thaihy, 16 June 2012 - 05:46 AM.