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Unit Heat Balance

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#1 Nikolai T

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Posted 26 October 2021 - 08:21 PM

Dear experts,


I’m going to retrofit an existing design of crude pre-heat train. I’ve modeled it and compose CDU+VDU unit heat balance, but there are points that confuse me. I found that to agree inlet temperature to first column of CDU I should keep temperature outcoming product streams (before air coolers) much more than it happens to be at existing plant. For example, calculated temperature of vacuum residue is 180 C degrees, but in reality, it’s about 148 C. The same thing happens with VGO stream. Measurements were performed in winter; ambient temperature was minus 12 C degrees. Calculated heat loss is about 6 MW. May it happen to be? 


Thank you.

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#2 breizh


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Posted 26 October 2021 - 09:22 PM


Make sure your sensors are well calibrated before you try to perform mass and heat balances.

Same for their location (Field vs Simulation) .

My 2 cents


#3 Nikolai T

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Posted 27 October 2021 - 01:52 AM

Agree with you. Possibly accuracy of measurement was inadequate. In field thermal imaging was applied.

#4 SilverShaded


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Posted 27 October 2021 - 04:20 AM

Typically its quite hard to balance a preheat train to due small errors having large impact.

The most usefull method i've found is to take all the data and reconcile it in excel using heat capacity data from the simulation (or estimated using the Lee-Kesler liquid enthalpy correlation).  Small errors on the crude side temperatures of only a degree or two (ie within typical thermcouple accuracy) have a much larger impact on predicted temperatures on the hot side stream due to the often large differences in MCp.  Allowing crude side temperatures to vary by a degree or two often helps bring the prehehat train into balance.


Another issue is pumparound flows can often be incorrect, especially if flow is measured at high temperature and is not compensated correctly.  In those cases, it can make sense to allow pumapround flow to vary a little as part of the reconciliation.


Another issue is some exchangers may in reality be two phase, e.g. overhead condensing or exchangers just prior to pre-flash drums or the furnace.

Done correctly all temperatures can often be matched to a within a few degrees (with an occasional outlier which may just be a faulty thermocouple).

Edited by SilverShaded, 27 October 2021 - 05:22 AM.

#5 Nikolai T

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Posted 27 October 2021 - 06:55 AM

Thank you. 

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