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Relief Load Of Fractionating Column


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#1 Poet

Poet

    Junior Member

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  • 10 posts

Posted 22 November 2019 - 01:54 AM

Hello,

I'm process engineer working on EPC.

Regarding conventional relief load calculation for fractionating column, what I found is,

 

Step 1. Set system boundary.

 

Step 2. Set heat & material balance for normal operation to check whether system boundary is correctly determined.

 

Step 3. Set unbalanced energy balance for each relief load scenario such as reflux failure, general power failure, abnormal heat input and so on.

 

Step 4. Assume there is a pseudo tray having same compositions & conditions to 1st tray.

(Compositions and conditions in pseudo tray may have different references. 2nd tray, 3rd tray...However, for now, I would like to focus on energy balance stated below.)

 

Step 5. Assume that unbalanced heat calculated by Step 3 will vaporize liquid in the pseudo tray and vapor rate will be relief load.

 

What I would like to clarify is Step 3.

I attached drawing. F, D, B, Qc, Qr denote feed, top product, bottom product flowrate, condenser, reboiler duty respectively. h is enthalpy of subscript. (e.g., hF is feed enthalpy)

 

Relief load is "(Qunbalanced / latent heat of liquid in the pseudo tray)".

Regarding Qunbalanced, many companies use different two equations.

 

<Equation 1>

Qunbalanced= F*hF - B*hB - D*hD + QR - Qc

 

 

<Equation 2>

Qunbalanced= F*hF - B*hB - D*hD + QR - Qc - (F - B - D)*hL

Where, L is enthalpy of the liquid in pseudo tray.

 

 

I think equation 2 is more exact because it shows enthalpy difference between pseudo tray liquid and each flow stream. If feed stream enthalpy is higher than pseudo tray liquid enthalpy, heat from feed is accumulated to pseudo tray liquid.

Could you explain which equation is more reasonable?

 

Thanks.

 

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