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2

# Relief Load Of Fractionating Column

4 replies to this topic
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### #1 Poet

Poet

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Posted 22 November 2019 - 01:56 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.

### #2 Bobby Strain

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Posted 22 November 2019 - 10:16 AM

You should have a company approved method. It's rather risky to solicit such specific advice here. What you will probably get here is some reference material. What is the source of the information from which you outlined the steps above? Of course, the best solution is a dynamic simulation. Did you ask your client if they require a specific method?

Bobby

Edited by Bobby Strain, 22 November 2019 - 05:19 PM.

### #3 breizh

breizh

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Posted 24 November 2019 - 04:14 AM

Hi,

You will find some information using your favorite search engine ,Key words : relief load calculation for distillation column

Good luck

Breizh

Edited by breizh, 24 November 2019 - 04:16 AM.

### #4 Poet

Poet

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Posted 24 November 2019 - 07:42 PM

Bobby, Breizh

Bobby,
I'm not handling this topic with company now. This is my private study.
Such calculation steps I wrote here are from internet journal. There is a good example in Breizh's reply.
According to the journal linked by Breizh, Bechtel engineer may use equation 2 (Qunbalanced= F*hF - B*hB - D*hD + QR - Qc - (F - B - D)*hL).
However, some journals by other engineers specify equation 1 (Qunbalanced= F*hF - B*hB - D*hD + QR - Qc).
So I would like to discuss it and know other engineer's opinion.
I presume this site is made for communication and discussion. Am I right?
The reason that I wrote this topic is to discuss it, not to receive nitpicking.

### #5 Bobby Strain

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Posted 28 November 2019 - 05:55 PM

I have Bechtel's procedure and I don't see your equation 2. But maybe I corrected it and used my own calculation. And I have a spreadsheet for the calculation. You can use your favorite search engine and find the original development. Search for unbalanced heat method. See a paper by Ascent Engineering about 2016. You should be able to do the heat and material balance based on your sketch.

Bobby

Edited by Bobby Strain, 28 November 2019 - 05:56 PM.