I am having problems similar to the second post by vinya in this old thread:
Basically, our lean TEG surge tank is collecting a layer of hydrocarbon on top of it, filling up to the skimming level, and continuously dumping (why it doesn't go back to our NGL product tanks is a different matter...)
I am wondering how this hydrocarbon (drizo solvent) is getting in to the lean TEG surge tank - the drizo solvent is used to strip the lean TEG coming from the TEG reboiler, and it should exit the TEG still column with the water vapor. The still overhead stream is condensed to a 3 phase separator (drizo/water separator), and the condensed drizo solvent is sent through a superheater back to the stripping column.
So one possibility I guess is that our superheater is not working right, and letting only partially vaporized drizo solvent into the stripping column, and the drizo liquids fall with the TEG into the surge tank.
Another thought I had: In other plants I've looked at, the amount of drizo solvent in the loop is maintained by the TEG stripping BTEX out of the gas in the TEG contactor, but our gas stream is 88% CO2, so I'm thinking the TEG is not picking up enough drizo solvent in the contactor. This results in a low level in the drizo bucket in the drizo/water separator, which opens a drizo makeup valve. The makeup drizo solvent comes from the bottom of a fractionation column - a single column ryan holmes process, where the above mentioned 88% CO2 stream is sent to a refluxed column, at about 300 pounds, top temperature -5F, bottom temperature 370F. This bottoms product is the source of the drizo solvent makeup. The makeup goes into the Rich TEG into the TEG still column.
So my thought is that this bottom product may not be the right composition for drizo solvent. I was under the impression that drizo solvent was supposed to be mostly BTEX, but the bottoms from the column are a spread of C5+ NGL, mostly iC5 and nC5, with a long tail. I guess the lighter components go out with the non condensibles in the solvent/water separator, and whatever BTEX range there is goes into the solvent loop, but the heavies never even make it into the overhead, so end up in the TEG reboiler, eventually down the still column and into the surge tank. So makeup is continuously added until the solvent loop is full again, but by that point so many heavies have accumulated that we get a high level of hydrocarbon on top of the lean TEG in the surge tank.
If there are gaps in my reasoning or other possible explanations, please point them out!
And finally, what are the solutions to the problem? I will check the superheater to see if we are getting adequate heating there, but if the makeup stream we are using is not a good composition for drizo solvent makeup, what to do about it? Does anyone add drizo solvent makeup from an outside source eg. trucking in benzene? I don't think trying to separate the makeup stream to provide a stream with better drizo solvent composition will be economical, but is that something I should look at? (maybe extraction with a TEG stream? afterall, TEG has a high affinity for the ideal drizo solvent components).
Or maybe this is the way it is supposed to work, and I just need to route the NGL that ends up in the TEG surge tank back to the process rather than disposal.
Sorry for such a long first post, just wanted to provide as much detail as possible.
Basically, our lean TEG surge tank is collecting a layer of hydrocarbon on top of it, filling up to the skimming level, and continuously dumping (why it doesn't go back to our NGL product tanks is a different matter...)
I am wondering how this hydrocarbon (drizo solvent) is getting in to the lean TEG surge tank - the drizo solvent is used to strip the lean TEG coming from the TEG reboiler, and it should exit the TEG still column with the water vapor. The still overhead stream is condensed to a 3 phase separator (drizo/water separator), and the condensed drizo solvent is sent through a superheater back to the stripping column.
So one possibility I guess is that our superheater is not working right, and letting only partially vaporized drizo solvent into the stripping column, and the drizo liquids fall with the TEG into the surge tank.
Another thought I had: In other plants I've looked at, the amount of drizo solvent in the loop is maintained by the TEG stripping BTEX out of the gas in the TEG contactor, but our gas stream is 88% CO2, so I'm thinking the TEG is not picking up enough drizo solvent in the contactor. This results in a low level in the drizo bucket in the drizo/water separator, which opens a drizo makeup valve. The makeup drizo solvent comes from the bottom of a fractionation column - a single column ryan holmes process, where the above mentioned 88% CO2 stream is sent to a refluxed column, at about 300 pounds, top temperature -5F, bottom temperature 370F. This bottoms product is the source of the drizo solvent makeup. The makeup goes into the Rich TEG into the TEG still column.
So my thought is that this bottom product may not be the right composition for drizo solvent. I was under the impression that drizo solvent was supposed to be mostly BTEX, but the bottoms from the column are a spread of C5+ NGL, mostly iC5 and nC5, with a long tail. I guess the lighter components go out with the non condensibles in the solvent/water separator, and whatever BTEX range there is goes into the solvent loop, but the heavies never even make it into the overhead, so end up in the TEG reboiler, eventually down the still column and into the surge tank. So makeup is continuously added until the solvent loop is full again, but by that point so many heavies have accumulated that we get a high level of hydrocarbon on top of the lean TEG in the surge tank.
If there are gaps in my reasoning or other possible explanations, please point them out!
And finally, what are the solutions to the problem? I will check the superheater to see if we are getting adequate heating there, but if the makeup stream we are using is not a good composition for drizo solvent makeup, what to do about it? Does anyone add drizo solvent makeup from an outside source eg. trucking in benzene? I don't think trying to separate the makeup stream to provide a stream with better drizo solvent composition will be economical, but is that something I should look at? (maybe extraction with a TEG stream? afterall, TEG has a high affinity for the ideal drizo solvent components).
Or maybe this is the way it is supposed to work, and I just need to route the NGL that ends up in the TEG surge tank back to the process rather than disposal.
Sorry for such a long first post, just wanted to provide as much detail as possible.
