8 replies to this topic

[daraj]

Hi, when dealing with vacuum distillation columns(50-500mbar) that just vent their overhead strem's heat to atmosphere and when thinking of possible ways to recover that heat and re-use it to improve energy efficiency, there are a couple of ways to do it. One of them is to use that heat of the top vapor stream and exchange it with  say  a utility stream like condensate/cooling water and generate low quality steam. This steam can be further used in the plant as such or after improving its quality. Now the problem with the idea here is, the heat exchanger or condenser at the top of the vacuum column cannot induce a large pressure drop. The heat exchange between the overheads and water must take place using a suitable exchanger/condenser design that causes low pressure drop.

Do you hve any recommendation for present day improved condenser designs for such a purpose? what type of condenser can achieve this? Something like plate condenser?the column top is around say 100-400mbar and I do not want more than 10-20% pressure drop due to the introduction of the condenser.

 

Thanks

[Padmakar Katre]

 

Hi,

Just check the pre-condenser designs for vacuum ejector systems. Special design features with low pressure drop. Article is attached for ready reference.

Attached Files

•  Optimizing Process Vacuum Condensers.PDF   821.73KB   2181 downloads

[xavio]

daraj,

 

"...when dealing with vacuum distillation columns(50-500mbar) that just vent their overhead strem's heat to atmosphere..."

Are you referring to an air fin cooler?

 

You intend to do heat recovery, but you don't tell us the temperature of you overhead stream.

People use air fin cooler for two main reasons: unavailability of low cost cooling water, high temperature unsuitable for cooling water.

In case that temperature is high enough to generate steam (either very low pressure or vacuum steam), we still need to assess whether there are recipients for this 'low quality' heat.

 

If you have determined that one method of heat recovery is economically viable, then you can proceed with condenser design.

Plate condenser is one good option, but you may need to discuss the detail with vendor because many design features are proprietary.

Conventional shell and tube, when properly designed, can also satisfy your pressure drop requirement.

After all, your 100-400mbar is actually not too critical, it is just a rough vacuum level.

 

By the way, you cannot use general cooling water (i.e., CW from cooling tower) for your steam-generating condenser.

 

Good luck!

 

xavio

[daraj]

Thanks for your inputs!

Xavio, can you elaborate on the limitations of using cooling tower water or cooling water for the heat recovery?what problem do you foresee?

the top of column is  >130C

[PingPong]

One of them is to use that heat of the top vapor stream and exchange it with  say  a utility stream like condensate/cooling water and generate low quality steam.

If you want to generate steam you need to use boiler feed water (treated condensate), but definitely not cooling water as that is too dirty for steam generation.

Edited by PingPong, 14 October 2014 - 06:28 AM.

[daraj]

Thanks Pingpong

[xavio]

daraj,

 

I just want to add some points.

For 130oC condenser temperature, I think you will only get near atmospheric steam, which I doubt you can use it efficiently.

You said about improving steam quality, how do you plan to do it?

 

If you want to use water-cooled condenser, please be careful about high temperature difference which may cause excessive fouling on CW side.

Tempered water is what you need in this case.

 

Good luck.

 

xavio

[daraj]

Thanks xavio. Steam quality can be improved by compressing it to a higher pressure. Energy needed for this will be less as compared to

what you recover out of the overheads

[xavio]

daraj,

 

Are you talking about mechanical recompression or thermal recompression?

I don't have experience with mechanical compressor for steam, but I do know about thermocompressor for steam.

Usually compression ratio is limited to less than two for thermocompressor.

Anyway, you should assess the economic feasibility (capex and opex) for this option, I am inclined to say that this won't be feasible.

 

Good luck!

 

xavio