Aspen EDR studies

[ReemaK]

Hello...
I am running EDR for the first time...I need to heat a heavy stream of crude from 150 to 240 C....Different heating mediums are being considered and one among them is hot oil...I know the hot oil inlet and outlet temperatures...I am estimating the hot oil flowrate required using a simple heat exchanger...I am attaching a picture below..

when I do EDR with that the previously estimated hot oil flowrate...the heavy stream is getting heated to around 256 instead of 240 and the hot oil outlet stream is lesser than the sepcified outlet temperature of 270 C...

What can i do in this case ? Should I reduce the hot oil flowrate estimated and use lesser quantity or reduce the number of tubes ??

I am attaching the screenshots below..

Thanks in advance !!

 

[1503-44]

Assuming that flow rate changes do not substantially change heat transfer coefficients disproportionately, which could happen if flow character changes from turbulent to laminar flow, etc.

Reducing the hot oil flowrate will lower the outlet temp of the heavy as less heat is carried into the HX to give up to the heavy. Less heat is also given up by the hot oil to the heavy, so hot oil outlet temp will tend to increase.

On the other hand, increasing number of tubes increases heat transfer from hot oil to heavy, which will tend to increase heavy outlet temp and reduce hot oil outlet temp

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[ReemaK]

Thank you for your prompt response @1503-44

Actually I wanted to decrease the number of tubes...not increase them...so which would be a better option ? reducing the hot oil flowrate ?

And I have done the simulation using 1-shell, 2-tube pass...it is showing temperature cross because the outlet temperatures are different from requirement as mentioned in the previous post...whereas for 1-shell, 1-tube pass, there is no temperature cross...is 1-shell, 1-tube pass common in industry ?Can i go for 1-shell, 1-tube pass ?? This is just a preliminary design stage..

Thanks in advance !!

 

[1503-44]

Yes, of course reducing the number of tubes decreases heat transfer, heavy oil outlet temperature will be cooler, hot oil discharge temperature would tend to go higher. In fact you could try decreasing both the hot oil flow rate and the number of tubes. Decreasing the number of tubes first is probably a better way to arrive at the minimum cost solution. Decreasing length of tubes is also an option.

One tube is not common. Usually there are quite a few tubes to maximize heat transfer surface for a more efficient heat exchange. One or two tubes will not have much surface area and a lot of both fluids will not come in thermal contact, simply running through the equipment for nothing. One or 2 tubes also suggest that a shell and tube exchanger is the wrong type of exchanger to use.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[ReemaK]

Thank you !!

I will try decreasing both and check !!

 

[1503-44]

Varying flow rate is something I use to see what temperature ranges are possible after your basic equipment and piping has been sized for the design flow rates, as that depends on many other factors, pump sizing, pipe cost, distances, and pressure drop is not linear.

Please let me know how that goes.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[ReemaK]

Yes

Coming to pump and piping...actually hot oil is one heating medium we are considering...there are 3 more (steam, electric and feed/product heat integration)...so i think once the heating medium is finalized we have to look pump requirement !! This project is being done by other department , so i dont have many details regarding the whole scheme yet...

Thanks

 

[Christine74]

I would advise against using a 1-pass arrangement as they tend to be problematic designs. A 1 tube pass doesn't allow you to use "U" tubes, and based on the temperatures you show above a 1-pass fixed-tubesheet exchanger would almost certainly require an expansion joint. A 1-pass floating head heat exchanger would require an internal expansion joint which is also a mess from a maintenance perspective.


-Christine

 

[ReemaK]

yes...For U-tube we need to have a minimum of 2 passes...the 1-shell, 1-tube pass I did is also a fixed tube exchanger...
that is why I had a doubt if 1-shell, 1-tube pass can be used or not

thank you @christine