Split ratio control for split gas flow (99% and 1% split) 2

[aegis4048]

I'm trying to design a small (1.2 MMSCFD) glycol dehydration unit for an upstream facility. My client wants me to run the worst case scenario, which is 130F gas inlet with 2% water. I ran extensive simulations, and concluded that I must have a stripping gas going into the reboiler to achieve 7 lbm/MMSCF water pipeline limit. In the attached pdf file, dry gas (str 16) comes out of the glycol contactor tower and goes to the splitter (SPLT-100). The stripping gas (str 8) goes to the reboiler to enhance water separation from the glycol. I set up the splitter so that 1% of the gas goes to str 8, and the other 99% to str 17 (sales). Without this stripping gas entering the reboiler, my water content at the end is too high (more than 10 lbm/MMSCF). I did line sizing analysis: str 16 and 17 will be 4" (main gas line), and str 8 (stripping gas line to the reboiler) will be 0.5" or less.

The gas outlet of the contactor is 1250 psig, but the gas entering the reboiler should be ~30 psig, so there must be a pressure reducing valve.

Questions:
1) How can I control the split ratio using control valves? What valves should I use?
2) I'm sure there are multiple ways to do it, please provide details
3) Would simply putting a pressure reducing regulator and putting a small pipe ID on the stripping gas line reduce the flow going into the 0.5" line instead of the 4" line?
4) Any other suggestions & thoughts are appreciated. If you have an example P&ID for this kind of application, that would be great.

 

[LittleInch]

1) Just take a side stream off the main line and send the data sheet to calve vendors and see what they say.

2) Or you could use an orifice/ restriction orifice or manually set up a needle valve and an isolation valve for when you need the stripping gas.

3 is the same as 1

4) I'm sure you can draw one.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

For this high dp service, its better to preheat this stripping gas before letdown to 30psig. What is HP gas source pressure from glycol contactor ? Install a dedicated stripping gas preheat pipe coil submerged in the reboiler and calculate the length to get desired preheat. Preheat should be sufficient to keep the gas at 30psig above hydrate temp.

You've got 2 choices on process controls;
a)a fully automated FIC and FCV for this 500scfh bleed stream going to the HP stripping gas preheat coil - preferable for remote unmanned operation. Locate the FE on the hot preheated gas side. If line size is too small for orifice plate type FE, swage up the line size to make it work.
b)a manual globe or needle valve with local FE / FI
At 30psig reboiler pressure, you must be floating this reboiler on the HP stage of the vapor recovery compressor ?

 

[LittleInch]

Good point, I'd forgotten about temperature and hydrates...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Very high pressure. Choose a good valve.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[aegis4048]

@georgeverghese
The HP source is the discharge side of a 3-stage recip. Also I mentioned 30 psig for the stripping gas, but this may be misleading. I'm just choosing to first reduce from +1000 psig HP pressure to 30 psig with one big regulator (may be two), and have an option to use that 30 psig gas for instrument gas for other equipment. But the reboiler will be operating at atmospheric or 1 psig. Injecting 30 psig gas to reboiler should be find cuz the reboiler will be rated at 125 psig MAWP.

I'm not sure what you mean by preheating the stripping gas submerged in the reboiler. Am I preheating the stripping gas (1) when its inside the reboiler or (2) preheating it before it enters the reboiler? If (2), what would be the source of the heating medium?

(1) Doesn't make sense to me, because my understanding of the stripping gas is that the gas seeps from the bottom of the reboiler through a hole (like its shown in the attached image) so there's not much room to preheat the stripping gas (my understanding maybe wrong, plz correct me if I am)

Regarding option 1 and 2, I don't think my client has advanced remote control system so we will probably have to go with option 2. The below diagram is what I'm thinking. I mentioned 1% split, but tbh 10~20 MSCFD is really what I need for the stripping gas. To reduce cost, I think its ok to put a flow meter only on the stripping gas side. I can adjust the open ratio of the globe valves until I get desired flowrate shown on the flowmeter. The below figure is how I think I can implement it, please lmk what yall think.

 

[georgeverghese]

Preheat the stripping gas stream while it is still at 1000psig. Source of heat will be hot glycol at the bottom of the reboiler. Install this coil as a length of B31.3 heavy wall pipe beneath the firetubes. But first check if the gas at 30psig will be at or below hydrate temp, starting from 1000psig and minimum operating temp for exit gas from the TEG contactor. If it is above hydrate temp, then the preheat wont be required.

If this plant has a fuel gas system for running gas engine drive power generators and other drivers, then take the stripping gas from the LP fuel gas header. What driver do you have for the recip compressors ?

No need for globe valve on the 99% side if you have no choice but to take stripping gas directly from this 1000psig dry gas exit stream from the contactor.

 

[aegis4048]

@georgeverghese
I checked the hydrate temperature, it seems to be well above the hydrate temp (-34F) at 30 psig. Two follow up questions:
(Most likely I don't need pre-heating but still would like to learn for future applications.)

1) Are you suggesting to preheat before I drop pressure due to JT effect potentially forming hydrates?
2) When you say beneath the firetubes, you mean inside the reboiler right? I don't think there's enough space inside the reboiler to install heat exchange tubes beneath the fire tube. The other heat exchange coils I've seen (I've seen only two examples tho) are installed outside the reboilers.

Also your suggestion on using fuel gas line for stripping gas, instead of directly using the 1000 psig gas, sounds much better. The dehy unit is located right next to an upstream tank battery, so there must be plenty fuel gas sources. Also the compressor has gas engine so I could pull from the LP fuel header for the recip as well. Thanks for new insight.

 

[georgeverghese]

1) Yes
2) When preheat is required, thermal / mechanical designers for the reboiler will make space beneath the fire tubes to include this preheat pipe type exchanger.

Good, its much safer to draw stripping gas from this much warmer LP gas supply.