15 replies to this topic

[purvamehta_5]

Hello All,

 

I am trying to optimize a design of a debutanizer to recover LPG as the ovhd and condensate as the bottoms. I have currently simulated as 30 theoretical stages with feed location on the 11th stage. Currently the cleint standard is to operate at 210 psig, which I think is high.

 

the product specs are: LPG: C5+ mole frac no more than 3%

                                     condensate: RVP 12 psia

 

When I reduce the operating pressure in steps from 210 psig to 70 psig, obviously the reflux ratio and the reboiler duty is dropping. This is an indication of separation becoming easier at lower operating pressure.

 

My question is why do I not see the change in condenser duty. The temp. profile in the column is dropping from 209 F @ 210 psig to 120 F @ 70 psig. Should this reduce my condenser duty as well. can someone explain. 

 

i am attaching a simple excel table indicating the results from HYSYS.

 

Thanks

Attached Files

•  Debutanizer Profile.xlsx   12.57KB   59 downloads

[Bobby Strain]

We need to se the column specs to give you any help. And the feed conditions, too. Also, you can find an Excel sample on the AspenTech website that optimizes the feed stage for a depropanizer. You can modify this to optimize the feed stage of your debutanizer. You need to pay attention to medium for the condenser cooling. If this is a new project, you should be using some heat integration, too.

 

Bobby

[purvamehta_5]

Bobby-

Thanks for replying. the specs are:

 

Ovhd: LPG: C5+ mole frac no more than 3%

Bottoms: condensate: RVP 12 psia

[eggzit1989]

Provide the feed composition to help you in this issue.

 

Kr,

Andrei

[purvamehta_5]

Composition: mole fraction

 

Propane: 2.00137388559415e-002

i-Butane: 0.220874647240784

n-Butane: 0.233404386846283

i-Pentane: 8.86153384966068e-002

n-Pentane: 6.66272982081941e-002

n-Hexane: 0.214695737290077

Methanol: 0.155768847390503

 

Pressure 210 psig, Temp 245 F, Vapor fraction: 0.06

[Bobby Strain]

What is the source of this material that has so much methanol? I know you can't enter a spec such as "no more than". So what specifications are entered for the column operation?

 

Bobby

[Zauberberg]

Condenser duty is based on condensation of the overhead product. As long as you have the same yield and composition, duty will not change. And why it should change?

 

Don't mix the required surface area with duty, when changing the temperature (difference) across the condenser.

[purvamehta_5]

Zauberberg-

 

If the operating pressure of the column is changed from from 210 psig to 70 psig,, obviously the temp. profile of the column is going to drop. As you can see in the attached table the top tray temp. drops from 210 to 120 F. If the ovhd spec is maintained, which implies that the dew point of the condensed product is maintained, then shouldnt the duty on the condenser change. Sorry, but plz can someone explain what am i missing here?

 

Bobby-

The spec was entered as max mole frac C5 + C6 as 3 mole%. I hope this answers your quesrion. 

[Bobby Strain]

You are obviously feeding the column at the wrong point. But you are witholding all the information. You probably won't get any good response if you continue to withold.

 

Bobby

[purvamehta_5]

Sorry Bobby, I am not too sure what additional important information am I holding back. Is providing the simulation file a better option? Please provide an email address and I can send in the simulation file.

[shan]

One of restrictions for lowering column pressure is the condenser cooling medium temperature.  What is your condenser temperature at 70 psig?  What is your available cooling medium temperature?

[Bobby Strain]

No, just post a snapshot of your column setup and specifications. I don't have HYSYS. Your condenser doesn't necessarily change because the latent heat increases with decreasing pressure. And, you haven't answered the question about methanol. Optimization also requires that you find the proper feed tray, too.

 

Bobby

Edited by Bobby Strain, 23 November 2015 - 03:03 PM.

[purvamehta_5]

Shan-

The condenser temperature is 120F @ 70psig. With the max ambient of 90F, that should provide a reasonable 30 deg F approach to size the fin-fan cooler.

 

Bobby-

I am not in the office, but will send you the snap shot of the column setup and specs once I get in there. Question about methanol. In this project we are shipping CGL from US to an overseas facility on barge. Methanol is used as a pushing medium to offload CGL off through the barge and get into an onsite facility before recovering NG and NGL's. We have a methanol recovery system upstream of the DeC3, but see some amount channeling through. I have tried to optimize the feed tray location to minimize the reboiler duty.  I think you answered the question I was asking about cond duty not affecting with reduced operating pressure, as the latent heat is increasing.

[shan]

Then, you may look into the cost impact.  210 psig and 70 psig may end up the same column wall thickness but probably 70 psig may need larger column diameter and more condenser surface area.

[Padmakar Katre]

Hi,

Please generate the HMB across the column for both the cases. Note these parameters,

1. Overhead flow, overhead temperature

2. Reflux flow and temperature

 

At lower pressure, since the separation is better the reflux quantity required will come down and hence the overhead vapor flow to condenser since the distillate/LPG rate is same. In addition the latent heat is higher at lower pressure, hence m2*lambda2 will be approximately same as m1*lambda1.

 

If the column pressure is lowered and feed temperature is kept constant, condenser duty may go up since the feed flashing would be more resulting into higher vapor to condenser and it may significantly off-load reboiler. This needs verification with simulator.

 

Hope it helps.

[Alexsandres]

Purvametha,

 

Perhaps it could be better to see amount of generated vapor in the top of debutanizer column entering condenser. This is also critical to check latent heat of condensing vapor. If pressure is decreasing, latent heat is increasing.

 

I also agree with Mr Padmakar, since operating pressure is lower, more flashing will be generated. This will impact also vapor flowrate to condenser. But, it could be better to see this in simulation software.

 

From your table I could not guess how much reflux flowrate you're maintaining. But I suppose it is lower than before (operating at higher pressure). With lower reflux flowrate (or maybe the same reflux flowrate), if you compare latent heat of lower pressure and higher pressure, you can see the difference.

 

CMIIW.

 

Alex.