Distillation Column Feeds Mixing

[EmmanuelTop]

Problem description:

Regular Feed #1 to deisobutanizer column, consisting of 6m3/h n-C4 and i-C4, is coming from the upstream depropanizer column bottoms (105C, 17barg, 100% liquid). By flowing across the flow control valve (FCV), its pressure drops to 6.7barg, causing partial feed vaporization and cooling (63C, 6.7barg, 23% vapor); as such, feed enters deisobutanizer column. This is regular way of operation.

Feed #2, coming from the FCC gas plant depropanizer (6m3/h unsaturated & saturated C4s), should be routed to deisobutanizer column I mentioned, downstream of the existing FCV where flashing of regular feed occurs. There are 2 options we have to choose between:

1) To take hot C4 stream (95C, 19barg) and mix it with regular (flashed) feed. By choosing this option, combined feed has 26% vapor fraction.
2) To take the same cold C4 stream (35C, 12barg) and mix it with regular (flashed) feed. By choosing this option, combined feed is 100% liquid.

In both cases, flow of unsaturated C4s is controlled by new FCV, with minimum allowable downstream pressure of 8barg.
Column calculations showed insignificant changes in reboiler heat duty demand, overhead condensers duty and column hydraulics - almost the same in option #1 and option #2. However, option #2 is far more attractive because it eliminates column feed distributor hydraulic concerns (100% liquid feed).

I need help in following: does introducing cold C4s and having 100% liquid (combined) feed influence upstream FCV operation (line marked with red colour on the attached sketch), where flashing of regular feed takes place? In other words, will something change in this FCV operation and degree/percent of regular feed vaporization, when cold C4 stream from FCC is introduced into the common feed line - since the combined feed would consist of 100% liquid?
What other issues we need to consider when choosing between these two options?

Thank you in advance, gentlemen.

 

[UmeshMathur]

I don't see any sketch, so it's not clear what the "Hot C4" and "Cold C4" streams mean.

 

[EmmanuelTop]

Hello Umesh,

You can see the sketch at:

http://img148.imageshack.us/my.php?image=sketchrj4.jpg

Or visit ChE Resources forum website, but you must be a registered user if you want to download message attachments:

http://www.cheresources.com/invision/index.php?showtopic=3431

Thank you

 

[reena1957]

Dear Emmanueltop,
Yes. The FCV will see a different(higher) downstream pressure than at present. You have to do a pressure drop calculation starting from deisobutanizerfeed nozzle backwards towards FCV for the three cases:
1. Presentcase
2. Option1 total flow in combined line + partial flow in the line from the junction point to FCV
3. Option2 total flow in combined line + partial flow in the line from the junction point to FCV
Remember it is a two flow case with varying vaporiZation along the path.
Now you will have the d/s pressure of FCV in all the cases and answers to all your questions.
Best wishes

 

[UmeshMathur]

Hi, EmmanuelTop:

Since the column pressure is 6.2 barg, the pressure downstream of both FCVs will be only slightly higher than this value, and could be calculated by any competent software package. Also, you need to be careful about static head effects in doing such calculations. You seem to have assumed that the line pressure (after the FCV on the Unsat Gas Plant feed line) is constant, known, and is 8 barg. In fact, it will vary throughout from the FCV until the connection to the main feed line. By the way, how did you arrive at the number of 8 barg, as it seems quite high.

Regarding your question about the effect of Option 1 v/s Option2, the simple answer is that the pressure drop from the end of the red line to the column will depend on the fractional vaporization in that segment, if any. If using Option 1, it seems clear that the pressure drop after mixing the two streams will be higher than for Option 2, since there is a lot more flashing of the feed from the Unsat Gas Plant in Option 1. Therefore, the pressure in the red line segment will definitely be higher for Option 1. How much higher depends on the flows and how the vaporization progresses until you get to the column.

 

[EmmanuelTop]

Umesh and Reena, thanks for your replies.

If combined feed is 100% liquid, is it reasonable to perform back-pressure drop calculation?
I am asking this by having on my mind the following example: if pump discharge pressure is, let's say 10barg, and there is only one (single) pipeline carrying 100% liquid far below flashing conditions, what is the pressure in pipeline 2cm before fluid is discharged to atmosphere (or to a enclosed vapor space with defined pressure)? I wouldn't say it is 1bar + nozzle exit loss.

I am not so familiar with fluid flow, especially when there are two phase involved (like in C4 case) - but it looks to me like special approach have to be taken.

 

[dcasto]

My simple reasoning says, whether the FCV has a 6.7 barg outlet or 6.8 because of pressure drop due to friction, it doesn't matter. Energy loss across a valve is zero. What i keyed in on is if you can have the optional feed come in at a higher temperature, the column energy balance will show a lower net energy input to the column. As for the percent vaporization, most columns like yours should have multiple feed points to optimize trays around the feed, make adjustments to feed location to assure best tray effiencies and column performance.

 

[mrobles1942]

I think there are serious mistake in your sketch.
One of them is:
-At the inlet of the Deisobutanizer with the same throughput (¿?), composition ¿?, pressure(6.2barg) and temperature(62ºC):
- Option 1: 27% V
- Option 2: 0% V

- Another mistake is:
Downstream of the both FCV the pressure must be closed to the operating pressure of the column (only line dP must be consider).However you show 6.7 barg (Saturated C4s stream) and 8 barg (Option 1 and 2 of Unsaturated C4s stream)
-Finally introducing most heat in the Deisobutanizer with the feed (Option 1) less reboiling duty is necessary