6 replies to this topic

[Dizzy]

Hi,

I have a few questions about a condenser which I have to design for my undergrad final project so this may be a bit long. x.x

 

1. I have to design a heat exchanger to condense a mixture of light hydrocarbons from 80 C to -15 C but I am unsure of whether I should put the mixture on the tube or shell side. Many sources say that condensing is usually done on the shell side but from what I have read this can cause to vapour and liquid to separate resulting in differential condensation which, according to the text I'm using, should be avoided.

Therefore, I think I should put the mixture to condense on the tube side so that the liquid and vapour won't separate and the mixture is also at 2600 kPa - another reason why it should go tube-side imo. It's a mixture of ethylene, ethane and a  little bit of propylene. Am I right to put it tube-side? 

 

2. If I'm correct in choosing tube-side condensing, should I use U-tubes so that there won't be vapour separation at the head? Or would the amount of separation be negligible? I'm hesitant to use them because of how if one tube breaks you'd have to replace the whole bundle and they're harder to clean.

 

3.For tube-side condensing, a lot of the correlations for the tube-side heat transfer coefficient (h_t) include the vapour quality in the equations. Should I use a quality of 0.5 or 0 when calculating this? Or should I find h_t for various qualities and use the average value? Or should I be doing some kind of integration over the length of the exchanger?

 

4. This is just a by-the-way question...the coolant fluid I chose was isobutane (refrigerant R600a) but are there any others that may be more efficient? (preferably single-phase and available on HYSYS). 

 

I don't know if this will be useful but according to HYSYS the condensing range is -10.05 to -11.88 C.

 

Ok that's all the questions. Sorry for asking so many...I did a lot of reading but the more I read, the more questions I have.    

 

 

[Pilesar]

Does your heat transfer need to be accomplished in one exchanger? That is a large temperature range for cooling. Consider a brazed aluminum heat exchanger instead of shell and tube. What pressure is required for your condensed light hydrocarbon product? If you are going to store the liquid stream near atmospheric pressure, then you must consider what happens to the temperature and phase of the stream as you reduce pressure. At the temperatures you stated, will the stream be in liquid phase at storage conditions? Can you use the light hydrocarbon stream itself as the refrigerant? This would eliminate the need for a separate storage system for refrigerant and may make your process more efficient.

[Pilesar]

Your stream suggestive of the top of a deethanizer column in an olefins plant which is not nearly as hot. There the condenser might be configured as a kettle exchanger with boiling C3 refrigerant on the shell side. You normally do not want to use refrigerant to cool a stream that might be cooled with a cheaper utility. Using refrigerant against an 80 C stream is not very efficient when you can knock some of that heat out with a cooling water exchanger first.

[Padmakar Katre]

Hi,

I have a few questions about a condenser which I have to design for my undergrad final project so this may be a bit long. x.x

 

1. I have to design a heat exchanger to condense a mixture of light hydrocarbons from 80 C to -15 C but I am unsure of whether I should put the mixture on the tube or shell side. Many sources say that condensing is usually done on the shell side but from what I have read this can cause to vapour and liquid to separate resulting in differential condensation which, according to the text I'm using, should be avoided.

Therefore, I think I should put the mixture to condense on the tube side so that the liquid and vapour won't separate and the mixture is also at 2600 kPa - another reason why it should go tube-side imo. It's a mixture of ethylene, ethane and a  little bit of propylene. Am I right to put it tube-side? 

 

2. If I'm correct in choosing tube-side condensing, should I use U-tubes so that there won't be vapour separation at the head? Or would the amount of separation be negligible? I'm hesitant to use them because of how if one tube breaks you'd have to replace the whole bundle and they're harder to clean.

 

3.For tube-side condensing, a lot of the correlations for the tube-side heat transfer coefficient (h_t) include the vapour quality in the equations. Should I use a quality of 0.5 or 0 when calculating this? Or should I find h_t for various qualities and use the average value? Or should I be doing some kind of integration over the length of the exchanger?

 

4. This is just a by-the-way question...the coolant fluid I chose was isobutane (refrigerant R600a) but are there any others that may be more efficient? (preferably single-phase and available on HYSYS). 

 

I don't know if this will be useful but according to HYSYS the condensing range is -10.05 to -11.88 C.

 

Ok that's all the questions. Sorry for asking so many...I did a lot of reading but the more I read, the more questions I have.    

 

Hi,

You are absolutely correct to choose the mixture condensation inside U tubes to avoid phase stratification at rear head and also to avoid shell side condensation. If the coolent is refrigerant and undergoes phase change, use K type shell or kettle.

[Dizzy]

Thanks for replying

 

 

Does your heat transfer need to be accomplished in one exchanger? That is a large temperature range for cooling. Consider a brazed aluminum heat exchanger instead of shell and tube. What pressure is required for your condensed light hydrocarbon product? If you are going to store the liquid stream near atmospheric pressure, then you must consider what happens to the temperature and phase of the stream as you reduce pressure. At the temperatures you stated, will the stream be in liquid phase at storage conditions? Can you use the light hydrocarbon stream itself as the refrigerant? This would eliminate the need for a separate storage system for refrigerant and may make your process more efficient

I was going to split it into heat exchangers - one with cooling water for desuperheating, but I was told that if I did that I would have to design the two exchangers. (I also have a tank, a pump and a reactor to design so I didn't want to add another piece of equipment to the list). 

The condensed product is going to a distillation column which operates at 1900 kPa.

I don't think I can use the stream as the refrigerant since it's needed downstream.

 

Your stream suggestive of the top of a deethanizer column in an olefins plant which is not nearly as hot. 

Sort of. It came from the top of a deethanizer, then was vaporized to pass through an acetylene converter, and now has to be re-condensed to go to a c2 fractionator.

[Pilesar]

The stream from your deethanizer was probably already very cold because it was condensed with refrigerant. You should use that cold stream to help cool your acetylene converter effluent. The order will be something like: deethanizer -> feed-effluent exchanger (cold side) -> steam exchanger -> acetylene converter -> cooling water exchanger -> feed-effluent exchanger (hot side). This will save utilities for both the hot and cold duties. Your C2 fractionator will have a condenser at the top of the column with a refrigerant utility. This means that you do not have to completely condense the feed to the C2 fractionator! You need both vapor and liquid in the C2 fractionator anyway for the separation. The vapor in the feed will travel up to the C2 fractionator condenser where it will be condensed.

 

I understand your not wanting to design more exchangers, but sometimes it is advisable. Trying to put every exchanger service into a single exchanger really complicates the design and makes it likely that your final design will not work at all. If I were your professor and compared a project with a single exchanger that did not work to a project with three exchangers that at least two of them worked, then I would give a better grade to the project with three exchangers.

[Pilesar]

If you are designing an olefins plant, you will have refrigeration systems needed for other exchangers. There is usually an ethylene refrigeration and a propylene refrigeration system in the plant. Those components are purified products from the plant and cover most of the refrigerant needs. You would not choose a different special refrigerant component for one exchanger when the site has refrigeration systems available that can work.