benzene condensation problem 1

[Pepijn76]

For a client I'm designing a condensor (chiller) in a vapour treatment unit (adsorption by active carbon)which removes benzene from an air stream. The active carbon is regenerated with steam.
The steam with desolved benzene is condensed in a condensor and catched up in a atmospheric vessel. The vapour that is formed in the vessel is mixed with air (dosed by an handvalve) in order to keep the vessel at atmospheric pressure. The vapour is sucked out of the vessel (by a blower) to the mentioned chiller which removes the last bits of benzene and water out of the airstream. I simulated this regenerationsystem in ASPEN HYSYS.
I discovered that the higher the amount of suppleted air (with the same amount of benzene and water -> lower concentration) the less benzene and water can be condensed in this chiller (fixed outlet temperature of air). I cannot explain this phenomenon? Can someone give me the clue?

 

[StoneCold]

That makes perfect sense. The air leaving the chiller is saturated with water and benzene. The more air that leaves the less you are able to recover.
Why don't you use Nitrogen to purge the tank instead of Air. I usually avoid the air fuel mixures on projects I want to keep.

Regards
StoneCold

 

[Pepijn76]

The air is suppleted via an air-break in the vapourline from the collection vessel. It's an existing installation which is not very sophisticated. So using N2 is not really an option. But thanks for your feedback.

 

[bchoate]

Pepij

This is explained as follows; A condensible vapour in air (or other gas) condenses when the temperature of the mixture is decreased to or below the temperature at which the gas is saturated with the vapor (dew point). By increasing the makeup air, the mole fraction of benzene in the mixture is decreased. The temperature at which that amount of benzene will saturate the air is decreased and it becomes harder to condense the benzene. In summary as the amount of air is increased for a fixed amount of benzene, the dew point of the mixture decreases and the mixture will have to be cooled to lower temperatures to condense the benzene.

 

[Homayun]

I agree with the explanation provided by "bchoate".
Another way to explain this is that since the Benzene Partial pressure is lower, the dewpoint will also be lower.

 

[Pepijn76]

To all,

Thanks for your input, it made things clear to me.
In my simulation I discovered another phenomenon that I can't explain. At a temperature of 69.64°C all the benzene begins to condensate. While the boilingpoint of benzene is 80.1°C. I think this decrease is caused by the "large" amount of water. A three phase system arises.But in my model all the benzene condenses. At temperatures below 69.64°C there is no benzene vapour present in the vessel. In my opinion there should be benzene vapour present.(because of the vapour pressure) Pressure in the vessel is atmospheric. Can someone help me to explain this phenomenon?
Is ASPEN HYSYS simulating this right? I used the peng-robinson property package.

 

[bchoate]

Pepijn76

I am lost trying to follow where you are in your process but I can give you a piece of information that may help. Benzene and water form an azeotrope that boils lower than benzene or water. The azeotrope boils at 64.9 C with 7.5% water in the azeotrope. This azeotrope has been used in the past to dry ethanol. Ethanol forms a 95/5 azeotrope approximately and cannot be made drier by distillation. Most ethanol plants use molecular sieves to dry ethanol today. The benzene water azeotrope is a heterogeneous azeotrope since water and benzene are essentially immiscible. That is, you get phase separation.

 

[Pepijn76]

bchoate,

I tried to describe the condensor that is situated after the adsorbtion vessels. The purpose of the condensor is condensate steam and benzene during a steam regeneration of the adsorbtion vessels. The condensed steam and benzene is collected in a vessel underneath the condensor. When I vary the outlet temperature of the hotside in ASPEN (condensed steam) from 90 C to 50 C at 69.64°C all the benzene begins to condensate. At an outlet temperature of 50 C the temperature in the collection vessel can be considered also at 50 C. According to the simulationmodel the liquid (water and benzene) won't evaporate at that temperature. I should expect that there will arise benzene and water vapour. Can you explain this phenomenon? Is ASPEN simulating this correctly? Thanks for your help!
I will attach a PFD and a graph.

 

[Pepijn76]

And the PFD

 

[bchoate]

Pepijn76

OK. Stream you are talking about is the 100C benzene/steam from regeneration. When you reduce condenser process outlet T you get the scenario you described. The azeotrope explains the behaviour. The water/benzene azeotrope boils at 69.2 C. As the feed moves through the condenser the temperature (and pressure) drops from the inlet conditions toward the exit conditions. Benzene/water mixtures up to 30% mole/mole have dew points of 95 C - 99 C Where mostly water begins to condense at first.

Benzene/water is a heterogenous azeotrope since water and benzene are immiscible in the liquid phase. The simulation seems to be picking up the azeotrope. At the azeotrope the vapor and liquid compositions are the same but the phase split suggests that any air moving across the surface of the benzene will saturate with benzene at the temperature in the system. See phase diagram for heterogenous azeotrope at
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http://en.wikipedia.org/wiki/Heteroazeotrope}