Distillation Column relief load for simultenous loss of feed and ovhd cooling/reflux.

[CCNY]

Appreciate learning about methodologies to use for calculation of relief load for a distillation column when both feed and overhead cooling/reflux are lost simultaneously. The standard unbalanced heat method of using the normal reboiler duty to vaporize top tray liquid seems too conservative. Without having to resort to a dynamic analysis , are there any alternative methodologies to use for this scenario.

Thanks

 

[vladrath]

Why would feed AND overhead cooling be lost when evaluating a specific relief scenario?

 

[CCNY]

vladrath,

Examples where simultaneous loss of feed and ovhd cooling/reflux can happen are:
1- Feed pump, reflux pump, and cooling water pumps are lost in a plant wide electric power scenario
2- Reflux pump/ovhd cooling are lost causing the column to over pressure and the resulting over pressure backs out the feed as the feed does not have enough pressure to flow to the column.

CCNY

 

[don1980]

For loss of cooling, I first check to see if there is a temperature pinch - will the boiling temperature of the process fluid at the relieving pressure exceed the temperature of the heating source. In some cases it does. If so, the scenario is non-credible. If it doesn't pinch, then I proceed with calculating the heat input (boil-up) as follows:

Q = (Design Q)(dT at relief)/(dT design)

In most cases the heat input during the relief event is less than the normal heat input. That's because of a lower dT at the relief conditions, as compared to the dT during normal or design conditions. As the pressure rises (approaches relief pressure) the boiling temperature also rises. This reduces the delta-T in the reboiler, thus reducing the Q which is directly proportional to dT.

 

[CCNY]

don1980

Thanks for your response. In most cases I have come across with steam reboilers, with a clean exchanger U value the reboiler duty does not get reduced when looking at the boilng point of normal bottoms liquid at relieving pressure vs steam condensing temperature at full steam supply pressure.

The issue I see is with loss of both feed and reflux, the assumption of bottoms liquid having the same composition as normal operation when the column has reached it relief pressure is too optimistic. This then puts in a situation that the normal reboiler duty stays in tact and using the unbalanced heat approach would then result in a way conservative relief load when using the normal rebolier duty to vaporize top tray liquid.

 

[CMA010]

What is optimistic about the assumption of the normal bottoms composition during relief? Doesn't the manual heat balance method result in normal reboiler duty / latent heat of top tray liquid for your case?

 

[CCNY]

CMA010

Thanks for your response.The optimism relates to the assumption of feed to the reboiler having the same composition as normal operation. With this assumption, the heat input to the reboiler for most steam reboilers would stay same as normal duty. The fact that in this scenario both feed and reflux are lost, one would expect the bottoms composition starts to heavy-up and the lighter material would be boiling off as the column is pressuring up. The question then becomes what is the reboiler duty once the column is at relieving pressure? This duty could then be used in the "manually" heat balance method as you have pointed out. The question comes down to is there a way to approximate the reboiler duty at relief conditions without having to resort to dynamic calculations? In some cases I have seen the dynamic modeling relief rates were less than half of the relief rate using normal reboiler duty.

 

[CMA010]

Make a flash curve of the bottoms product and make corresponding reboiler duty and relief rate calcs for each vapourisation step. For a narrow boiling mixture I wouldn't bother tho'.

 

[georgeverghese]

Implicit to this assumption that the top tray liquid composition is used for calculating the relief load is that the upward vapor flow through the top tray is more than adequate to prevent the tray inventory from weeping down to the reboiler sump or kettle - a tray hydraulics check can help to determine if this assumption is correct. Else all or part of the tray inventories may weep down into the reboiler section during relief, depending on the type of tray contacting element used.