De-ethanizer modification
De-ethanizer modification
(OP)
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





RE: De-ethanizer modification
Recommend reading chapter 8 of D.S.J. Jones' Elements of Petroleum Processing, WILEY.
RE: De-ethanizer modification
>> 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.
RE: De-ethanizer modification
RE: De-ethanizer modification
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.
RE: De-ethanizer modification
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.
RE: De-ethanizer modification
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.