2 replies to this topic

[Process Engineer 1982]

Hello Cheresources Community,

 

I have a question regarding the handling calculation of the absorbed duty in a Fired Heater. I just received a data sheet from the client and I will like to check the absorbed duty reported on it. I perform some calculations but I get different results. I think, I have some trouble with the calculation of the Heat of Vaporization.

 

This is the data I have available:

 

Absorbed Duty: 67.53 MMBTU/hr

Fluid Name: Naphtha+H2

Total Flow Rate: 1092200 Lb/h 

 

Inlet Conditions

Temperature, ºF=578

Pressure, psig=259

Liquid Flow Rate, lb/h= 943400

Vapor Flow Rate, lb/h= 148800

Liquid Gravity= 0.533

Vapor Molecular Weight= 78.3

Viscosity Liq / Vap, cP= 0.088 / 0.016

Specific Heat Liq / Vap, BTU/lb-ºF= 0.788 / 0.678

Thermal Conductivity Liq / Vap, BTU/hr-ft-ºF=  0.0357 / 0.0486

 

Outlet Conditions

Temperature, ºF= 625

Pressure, psig= 232

Liquid Flow Rate, lb/h= 543400

Vapor Flow Rate, lb/h= 548800

Liquid Gravity= 0.505

Vapor Molecular Weight= 120.5

Viscosity Liq / Vap, cP= 0.074 / 0.014

Specific Heat Liq / Vap, BTU/lb-ºF= 0.847 / 0.694

Thermal Conductivity Liq / Vap, BTU/hr-ft-ºF= 0.034 / 0.035

 

I highly appreciate your help.

 

[PingPong]

Naphtha is not a single component, but a mixture of many. As the temperature of the H₂/Naphtha mixture increases more molecules of the naphtha will vaporize, but one cannot calculate by hand what fraction of the various naphtha components will have vaporized.

 

You can only do this kind of calculations accurately using a simulator like Hysys, PRO/II, Aspen Plus, et cetera, and moreover you would need the whole composition of the naphtha, as a minimum ASTM D86, but preferably TBP distillation data.

I am sure that the data you received came from one such simulator.

 

By hand you can only do a very rough order-of-magnitude estimate of the duty, that could easily be 30 % different from a simulator.

[Process Engineer 1982]

Thanks so much for your reply. Actually, I ran the heater using FRNC5 using the property grid provided by the client and the fluid is absorbing the expected duty. The simulation results are consistent with the original design. I just wanted to double check if it's a way to make some handling calculation to obtain a rough estimation of the absorbed duty. But, as you said the deviation is pretty big.