Crude Tower Pumparound Tray Efficiency

[lgmarti]

I have been told by many experts that the pumparound trays in a crude tower have a 30% efficiency at the most. However, my crude tower simulation shows that the pumparound trays efficiency should be between 45-55% to match the pumparound duty in the middle and the bottom pumparound. In the top pumparound the efficiency is 90%! Anything less will increase the top pumparound duty to "incredible" proportions. Does anyone has experience in determining pumparound tray efficiency? What is the expected efficiency for the flash zone tray? To match distillation data, my simulation shows a 60% efficiency.

 

[TLKemp]

Pumparound trays remove heat. Model the pumpround section as one theoretical tray and install 3 to 5 real trays in the tower. The "efficiency" of P/A trays doesn't matter - after all you are moving heavy material up the tower which is exactly opposite of what the tower is supposed to accomplish!

Make your simulation life easy and model all trays as theoretical stages. Apply your efficiency corrections external to the model.

Good luck!

 

[lgmarti]

TLKemp, thanks for your reply. Initially I wanted to model only the theoretical trays. But I was giving the ASPEN theoretical tray version a chance; plus I was intending to use this simulation in their new EO configuration model.

Most people would agree that the pumparound's only purpose is to remove heat. However, I was being influenced by Lieberman's opinion that the pumparound trays do fractionate up to a point. He suggests that increasing pumparound rate increases fractionation (and efficiency) up to the incipient flood point. Thereafter, efficiency drops and fractionation deteriorates. I thought that the fact that I can't match the pumparound duties following the rules of thumb proved this point.

In my simulation, I even tied up the gap between the naphtha-kero and kero-diesel to the middle and lower pumparound rate. It worked for my base case, but even though I have a mass and energy balance within 4% of the actual values I am not sure if the simulation will work under other conditions.

I will keep trying.

 

[gregq8]

The high tray efficiency you need for the top p/a tray efficiency to match the duty is probably symptomatic of too few stages below this point. Use 1 theo. stage for the p/a trays (as suggested by TLkemp) and adjust stage in section below to see if this helps. I would start off by assuming a tray efficiency of 60% for the non-p/a trays. The 4% match in M&EB is quiet good - another thing to check is the column temperature profile.
Greg