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Heat Recovery Options


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#1 fangs569

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Posted 21 February 2018 - 05:53 AM

I would like to seek your opinions on the way ahead for my final year process design

 

My team and I performed heat integration our process plant and turns out that:

 

for Hot Utility: we require a boiler to top up additional heat input (14364.34kW)

for Cold Utility: we have a lot of heat to remove from the system (232608.74 kW of low grade heat)

 

*I know the amount of energy calculated are too high, its due to the specifications of the process plant*

 

Suggestions:

 

- Design an Organic Rankine Cycle to recover energy wherever possible (between 90degC to 220degC)

 

- Design Heat removal system (i.e. cooling tower to cool temperatures from 90 degC to 20 degC)

 

Are there any other methods to recover heat energy that's not even steam?

 

Kindly enlighten me

Much appreciated



#2 Pilesar

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Posted 21 February 2018 - 09:38 AM

Did you use Pinch Analysis techniques to find feasible process-to-process heat exchange? If you have a lot of heat available at 90 C then I think you probably should look again at your heat integration. Can you use these hot streams to preheat plant feed? To preheat column feed? To preheat reactor feed? To preheat boiler feed water? Heat integration is one of the most difficult parts of scholastic design projects because it may mean redesigning plant equipment you thought you had completed.



#3 Saml

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Posted 21 February 2018 - 01:32 PM

I wonder what your process is about.

 

You need 14 MW of hot utility and you reject 232 MW of heat.

This implies that it is an overall exothermic process or you are using fired heaters or other heating media for parts of the process that you are not counting as "hot utility"

 

If you have already done heat integration, the next thing to do is to try to move some of the hot stream to higher temperature (ie. doing the reaction at a higher temperature/pressure)

 

This, however, goes up to the point where you don't need the 14 MW hot utility.

 

If you want to reduce cold utility, the next thing you should think about is exporting energy, either as steam, power or electricity.

 

Yes, from 90°C (unless energy is extremely expensive) economics will dictate that you reject that heat to atmosphere thru cooling tower (if you have water available) or air cooled exchangers. 234 MW is more than 300 t/h of water evaporation and may imply a cooling tower circulation of about 20 thousands cubic meter per hour with a 4 to 5 MW pump(s).  It is not a modular cooling tower what you need.



#4 fangs569

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Posted 22 February 2018 - 02:12 AM

Pilesar:

 

Thank you for your replies, yes we used the pinch technology to create the heat exchanger network

 

Saml:

 

 

Our problem statement is quite over-specified; we are tasked to design a plant that converts carbon dioxide into methanol utilising only renewable energy, furthermore the product flow is 2500 metrics ton per day 

 

It is as you said, we managed to identify a stream that we had overlooked, and now there's no more pinch point (i.e. only cooling utility required process)

 

PS: The boundary of my design is within the methanol synthesis part of the process plant (not including amine scrubbing and electrolysers), therefore I can say that only cooling utility is required



#5 Dazzler

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Posted 24 February 2018 - 03:26 AM

Hello Fangs,

 

I don't know your process for CO2 to CH3OH, but if there is that huge cooling need it makes me think that you are converting (reactions and or catalysis) too much in the first pass and trying to get the product right first time, when in fact the conversion in first pass could be less % and then having a second pass or a recycle stream.  There is a chance that modelling that my indicate that the final product after first pass is not so hot and therefore not so much cooling needed later?  By recycling to achieve a higher final conversion the feed is effectively hotter so there are efficiencies

 

Let us know if this turns out to be useful.

 

Dazzler



#6 Saml

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Posted 25 February 2018 - 06:51 PM

There is not too much we can guess from your description.

- Are you using H2 from water electrolysis?

- Is the CO2 to methanol reaction done with one of the newer catalyst/processes that don't need CO?

- Why do you need "amine scrubbing"? Where are you obtaining the CO2 from?



#7 fangs569

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Posted 26 February 2018 - 04:04 AM

Hello Fangs,

 

I don't know your process for CO2 to CH3OH, but if there is that huge cooling need it makes me think that you are converting (reactions and or catalysis) too much in the first pass and trying to get the product right first time, when in fact the conversion in first pass could be less % and then having a second pass or a recycle stream.  There is a chance that modelling that my indicate that the final product after first pass is not so hot and therefore not so much cooling needed later?  By recycling to achieve a higher final conversion the feed is effectively hotter so there are efficiencies

 

Let us know if this turns out to be useful.

 

Dazzler

 

 

Hello Fangs,

 

I don't know your process for CO2 to CH3OH, but if there is that huge cooling need it makes me think that you are converting (reactions and or catalysis) too much in the first pass and trying to get the product right first time, when in fact the conversion in first pass could be less % and then having a second pass or a recycle stream.  There is a chance that modelling that my indicate that the final product after first pass is not so hot and therefore not so much cooling needed later?  By recycling to achieve a higher final conversion the feed is effectively hotter so there are efficiencies

 

Let us know if this turns out to be useful.

 

Dazzler

 

Hi Dazzler, 

 

Thank you for your reply, I suspect the most energy intensive cooling duty is around the condensation of the process fluids before sending to knockout drum, as we want to separate our product from the recyclable gases. Still, we will look into the design of the reactor in detail 

 

There is not too much we can guess from your description.

- Are you using H2 from water electrolysis?

- Is the CO2 to methanol reaction done with one of the newer catalyst/processes that don't need CO?

- Why do you need "amine scrubbing"? Where are you obtaining the CO2 from?

 

Hi Saml

 

Thank you for your reply,

 

1. Yes indeed, we are using H2 from water electrolysis, it is an energy intensive process...

2. We are still using the conventional Cu/ZnO/Al2Ocatalyst so that we can react CO to form methanol as well

3. We are treating flue gas using MEA to extract CO2 for our process

 

 


Edited by fangs569, 26 February 2018 - 04:04 AM.





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