7 replies to this topic

[abdouabderrazak]

hi engineers 

i have a question about heat  exchanges and its turn-down

what happen if we operate with an heat exchanger, whith the bundle is not fully submerged (86% of bundle is submerged)

 

for shell side:hot oil

T(in)= 315°C (600°F) 

T(out) = 189°C (372°F)

 

tube side: crude oil 

T(in)= 173°C (344°F) 

T(out) = 238°C (462°F)

 

 -I search the trundown of this exchanger, 

haw and when i should stop using this exchanger

thank you

[srfish]

Hasn't this been posted before?

 

I can't imagine operating with 86% submergence with single phase hot oil.

[abdouabderrazak]

thank you Mr Srfish

yes, this question has been posted before. but i need somme arguments to justify why we can't use this exchanger, and what are the problems that can be occurs in this condition

thank you

Edited by abdouabderrazak, 03 April 2017 - 02:36 AM.

[Saml]

Can you please post an schematic of your exchanger and controls. It is difficult to imagine what you mean with "86% of bundle is submerged"

[abdouabderrazak]

thanks Mr SamI

I'm looking for the turndown by simulation (Hysys), I use the hysys EDR option for analysis, when I reduce the flow (tube side (crude oil)), I decrease the flow of hot oil, to have A crude temperature at the outlet of 238 ° C.

for 14.000 Bpd of crude oil, I need 140m^{3 of} of hot oil, with this flow i have my desired temperature, but a message appear in HYSYS, tell me:

( the fraction of the surface area for cooling the liquid phase is estimated to be 86%, the number of naturally submerged tubes is estimated to be 1275. thus only 100% of the surface is considered submerged by the liquid phase).

Attached Files

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[Saml]

It is not totally clear, but the EDR message seems to indicate that you are using a vapor heat transfer agent, and that the area specified for subcooling is 86% of the total area.

 

Apart from the fact that I don't quite understand why you need to heat crude oil with hot-oil unless it is a problem that your professor gave to you,  you are doing the control in an unusual way. It seems that you are using varying the parameter "Fraction of the tube area submerged for shell side condenser". It is what you are doing? What for?

 

If it is vapor, probably (I am guessing here) you are using Dowtherm A or Therminol VP-1 (are almost the same).

 

For a design guide  I recommend that you go to Therminol site and check the document "TF-33_VaporPhaseDesign".  On the Dow site you may find something similar for Dowhterm.

Edited by Saml, 03 April 2017 - 11:25 AM.

[abdouabderrazak]

Thank you Mr SamI

I will explain otherwise

This unit exists, it is desiged for 70.000 BPD, it is a crude oil stabilization unit, to have an RVP = 10 psi of the product.

This unit has two feed, cold and hot, the cold feed is connected to the first stage, the hot feed is préheated by the stabilised crude oil and entre to the stage13. the column have 22 trays.

The bottom of the column is heated by the hot oil (dowtherm Q) to have a temperature of 238 ° C.
I checked the design by hysys ok.
The objective of this study is to:
Determine the turndown of the entire unit.
By simulation, I decrease the flow of the feed gradually , and I verify the RVP of the product, the state of heat exchangers, the refrigerant, the hydrodynamic behavior of the column.
For a flow of 20.000 BPD I found the following problem:
-weeping in column
- temperature cross (but I have the desired temperature)
- the previous message concerning the exchangers

The questions are:
- when i should stop using this unit

- when i should stop using the exchangers

- what are the limites.

thank you

 

Attached Files

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[Saml]

Sorry, I am confused now. Dowtherm Q is for liquid service, and the control may be fine. However I do not understand the message. That message in EDR indicates a two phase transition in the shell.

 

About the turndown. You should stop when:

 - It not longer produces the required spec

 - You cannot control it 

 - There is a predicted safety or reliability problem (i.e. condensation in boiler economizer that leads to corrosion)

 

That said. Simulation will tell you about where you may find problems, and to understand what you are seeing. It may also bee good to set conservative limits. But to know the actual limits, you have to carry a planned test run. A weeping column and a exchanger with temperature cross may still produce on spec product.