I am trying to model a vacuum ice generator for the generation of ice slurry in ASPEN Plus. In my work, seawater (water+NaCl) enters an evaporator at a temperature close to 0 deg Celsius and the evaporator is maintained at the triple point of water. and so, ice slurry is generated which is pumped out, while the vapour generated is passed through a compressor and subsequent process ensures. I tried to use a flash evaporator, but it does not give any vapour, while it is the same with crystalliser. I would be grateful if someone could guide me as to how to model it. Thank you.
Rakesh,
A couple of observations / suggestions, which might be of some help ... I'm not 100% sure of the system:
1. The tripple point of pure water may be 0°C ... I'm quite sure that for salt water it is lower, something like -5°C. the presence of salt will change the thermodynamics of the system
2. relative to the above, model the energy flow. Pressure only generates the conditions ... energy flow then drives the system - head in, and heat out ... if you can't take the heat out, you can't maintain the pressure / vaccum.
3. at 6mbar then pure water boils at ~0°C ... go lower! Suggest 2mbar or below. At 2mbar water boils at ~ -15°C which should be enough to freeze salt water.
4. if using real sea water, then it will behave a little differently ... that's a more complex mixture than salt and water! The solutes often affect the thermodynamics of boiling / sublimation. That said, in many cases, when you freeze the solutes do come out of solution, so you are back to a mixture of ice and salt crystals. The question is - how / how well do they freeze? There are also surface layer questions to answer.
5. submlimation takes more energy than boiling (2 phase changes!) at about 2.85Kj/ml whereas evaporation / boiling is "only" 2.4Kj/ml. All that energy needs be removed in the condenser, where you should get ice fomed. This condenser needs be COLDER than the boiling / sublimation point of the ice to catch it. So if pressure is 2mbar, sublimation point of pure water is ~ -15°C then the condensor MUST be colder than -15°C to collect the vapour, and make ice.
I hope this is of some assistance.
Kind regards
Rob
Dear Rob Darrington,
Thanks a lot for the reply.
1. Yes, I calculated the triple point for the salt concentration of water.
2. The vapour extracted from the saltwater converted to ice slurry is sent to compressor -> heat exchanger - similar to the heat pump and when it condenses to liquid in the heat exchanger it is fed to the slurry generator to improve the performance. Whereas the ice slurry prepared (for an ice fraction of 20%) is pumped back to the sea.
3. Since seawater might contain some gases, post the condenser, I am adding a vacuum pump to suck out the dissolved gases
4. ATtached below is the image of the schematic of what I am modelling. I want to know how would I model the ice generator component-the evaporator, while simultaneously it releasing heat because of the condition that it is maintained at triple point
Best Regards,
Rakesh
With sea water you should have a close look at the thermodynamics. Pure water is usually modeled using STEAMNBS but with salt you need to apply ELEC-NRTL for modeling the diluted salt. Try to experiment with treu component approach and apparent component approach which of the two gives the best results. Maybe you need to reinvestigate the underlying parameters in order to get suitable results. nist.gov might be a source for data.
good luck!
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