De-ethanizer modification

[askme]

Hi there;

We have a de-ethanzier that takes its feed from a feed drum at a rate of 50 Mbbl/day liquid NGL. It is equipped with an overhead condenser (partial) and reflux gets back to the column. Our target is to recover more NGL from this column (C3+). At the column bottom we have two reboilers(Kittle Type).
The column is 60 trays and the feed located at tray # 40.


My questions are:
1-What is the advantage of having an economizer at the column feed to preheat the feed utilizing the column bottom stream as heating media?

2- This will result in energy saving by taking some load off the bottom reboilers. Will that have negative impact on the recovery?

3- Do we need to change the design or location of the feed stream to the column? In other word, does it require column design change?

Thank you

 

[25362]

Recommend reading chapter 8 of D.S.J. Jones' Elements of Petroleum Processing, WILEY.

 

[epoisses]

1-What is the advantage of having an economizer at the column feed to preheat the feed utilizing the column bottom stream as heating media?
>> You typically save around 50% of the energy on the reboiler consumption, provided (obivously) you don't have to heat up the bottom stream again downstream.

2- This will result in energy saving by taking some load off the bottom reboilers. Will that have negative impact on the recovery?
>> That depends among others on how the L/V ratio of the feed changes, you should run a simulator for a detailed calc.

3- Do we need to change the design or location of the feed stream to the column? In other word, does it require column design change?
>> My uneducated guess would be that a change of location would not be cost-effective. You may have to adapt the distributor if feed L/V changes too much. Use the simulator to find out if e.g. you won't exceed jet flooding limits.

 

[epoisses]

50% of the preheater duty, that is (of course)

 

[25362]

In addition, the reboiler working near the critical point, may not be so efficient as a fractionating tray. The partly vaporized feed should better be introduced at the tray having about equal temperatures.
Because of those facts, P and T along the column should be kept as constant as possible for better fractionating efficiency.

 

[25362]

No doubt preheating feed saves heat load on the reboilers. As for the economics of prevaporizing feed to distillation columns published studies say that heat savings from vapor feed are greatest when the split is 80% overhead, 20% bottoms. For 20% overheads and 80% bottoms, vaporizing the feed results in negligible savings.

 

[DickRussell]

Expanding on 25362's last response, consider a column in which only a tiny fraction of the feed is removed overhead, and nearly all of the feed out the bottom. To achieve a certain degree of stripping of light stuff from the bottoms, a certain boilup is required, depending on the relative volatility of the light stuff w.r.t. the heavy key(s). Except for any vapor knocked down at the feed tray by a cold feed, the vapor will run up the top section to the condenser. If only a tiny bit of material is withdrawn as distillate, then the top section L/V will be very close to 1.0 (total reflux), and increasing the vapor coming up (by means of hotter/vaporized feed) will have almost no effect on the rectification. Meanwhile, the stripping section still needs the same L/V (or V/B) to do its job, and adding heat to the feed has no effect there either; extra feed turned into vapor just gets condensed and returned to the top of the stripping section as liquid again, so the liquid running down the rectification trays and the boilup stay about the same.

For the reverse situation, with a tiny bottoms stream (say, a residue of some sort), the reasoning is similar, except that the stripping section is near total reflux, and adding heat to the feed means that much less heat needed at the reboiler to provide vapor entering the rectification section. As you move from the situation of tiny distillate to tiny bottoms, adding heat to the feed does make increasingly more difference, which may be beneficial or not, depending on the situation.

A quick series of runs on any decent process simulator will quantify the changes to duties and traffic in the column as you add heat to the feed, but understanding the extreme situations helps make sense of what the simulation is telling you.