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Pumparound flows through a preheat exchanger and after that passes through a control valve and back into the column. While deciding how much pump should be used, what should we consider?
Condition #1: Reduce the control valve opening, provide the pumparound more residence time inside the exchanger which will cool it down more and then it will return at a lower temperature but with a lower flow rate.
Condition #2: Increase the control valve opening which will allow more flow to be fed back into the column but with a relatively higher temperature.
Which one should we choose?
 
Answers
23/07/2020 A: Carlos Mendiondo, CHM Consult, Carloshugomendiondo@gamil.com
The variable "Residence Time" does not apply to heat exchangers. If the heat exchanger is able to transfer more heat, it will do so regardless of the residence time. The RT is used for the reactors, not in heat exchangers, with exceptions if coking reactions can take place in certain high temperatura conditions , where the kinetics of the reaction/ resident time plays a role, also the time to which a certain compound could be altered inside HE . What you say could be true, if the area is not enough or excessively tight for the heat quantity that you need to exchange. But if the heat exchanger has enough area and you reduce the flow rate , it will possibly decrease the heat transfer coefficients and instead of the outlets becoming colder, the hot fluid becomes hotter than i normal condition. If the flow rate is increased it could happen that the hot fluid comes out colder . It all depends on what is controlling the heat transfer.
21/07/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
The most important variable is the pumparound duty (i.e. MW, kcal/hr or MMBTU/hr) rather than temperature or flowrate. In practice, maximum flowrate is likely to give maximum duty, but it would be simple enough to include a duty calculation in your DCS or process monitoring spreadsheets and do some data regression to confirm.

Other factors to consider:
1. The tower needs to be able to handle the pumparound flow. Excessive pumparound flow could lead to flooding in the pumparound section.
2. Depending on suction pipework hydraulics, excessive pumparound flow could cause cavitation of the pumparound pumps, particularly if we are talking about a vacuum tower.
3. If the pumparound is a fouling service, reducing the pumparound flow will make the fouling worse.
4. Taking out too much heat from the lower pumparounds can reduce fractionation performance higher up the tower. If there is surplus cooling, pumparound duty should be optimised rather than maximised but, for many units, cooling is a constraint and maximising pumparound duty is essential to maximise throughput.

I hope that helps.
20/07/2020 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The purpose of the pumparound is to generate internal reflux in the column. The pumparound coolers generate a subcooled liquid back to the tower. The subcooled liquid is heated back up to saturated conditions by condensing some of the vapor coming up the column. The latent heat of condensing is what heats the subcooled liquid back up. The net condensed liquid flows back down the tower as reflux (either from overflowing the pumparound draw tray or by being pumped back to the column below the PA draw. The pumparound duty can by approximated by the mass flow rate times the heat capacity times the delta T. The reflux generated is approximated as the duty divided by the latent heat of the vapor coming into the PA section of the tower. The trays or packing in the pumparound section of the column serve as a direct contact heat exchanger.

The way you operate the PA depends on what you are wanting to do from a product yield and quality standpoint. Increasing the PA duty will increase the internal reflux in the column, which will improve the separation between the cuts below and above the PA and will tend to increase the product draw rate below the PA. If this were a kerosine PA, then higher PA rate would allow you to make more kerosine and diesel and less naphtha. If economics favored jet/diesel over gasoline, then you would want a high PA rate. If gasoline economics were favored you would probably want a lower PA rate in this example.

Because the heat recovered in PA's typically goes to preheat crude, the way the PA's are operated can also influence the total amount of crude preheat, which can impact the crude charge rate and/or the amount of feed that the vacuum unit must handle. So that is another factor which sometimes goes into setting the PA duty and rates. The pumparound sections will have some finite amount of liquid that they can process, so the pumparound rate needs to stay below that which will flood the trays.