Compressing saturated vapor - HELP!

[dsengere]

Hi. I need some serious help!

I'm trying to find the energy (or work, in kj/kg) required to compress saturated water vapor BACK into liquid water.

The idea is that instead of condensing the water through loss of the enthalpy of vaporization, I instead turn the vapor back into water via compression. The purpose for this method is so that I can transfer heat back into the parent liquid (to vaporize it) instead of wasting the energy required to vaporize the water in the first place (enthalpy of vaporization).

I've seen this image:

where work is equal to the pressure of the vapor times the change in specific volume (w=P(v_2-v_1). I don't quite understand how this would look on a PV or TV diagram. Can someone help me visualize and calculate this work?

Any help would be much appreciated!

 

[gruntguru]

Can't be done without removing some heat. An adiabatic compression process proceeeds up and left on the diagram (pv or ts). You end up with a supercritical gas.

je suis charlie

 

[Compositepro]

Look-up multi-effect evaporation. Compressing the vapor causes the temperature to rise so that the heat of condensation can be used to boil more liquid at the lower temperature and pressure. This has been used for the desalination of sea water.

 

[georgeverghese]

Couldnt we pass this sat steam through a quench tower with the parent liquid flowing countercurrent instead of this far more expensive compression route ?

 

[dsengere]

Thanks for the replies (Compositepro, that's the idea I mentioned in the OP). It's actually super simple, just m(H2-H1), and the saturated steam becomes superheated. If you look up the compression cycle for refrigeration (Pv, Tv, Ts, etc.) diagrams, you can see what it would look like. I was wrong to think it would simply move left back to the left end of the curve.

 

[gruntguru]

Ahh - so you are removing some heat (to vaporise the water in the other stream). Is this a distillation process? It is already done this way - condensing vapour transfers heat to the inlet stream of liquid being vaporised. Both processes occur at constant pressure and temperature, the former being slightly higher (T and P) to drive the heat transfer. Amount of pumping work required will be equal to heat loss through apparatus insulation, plus mdot x Cp x delta T (Twater out minus Twater in).

je suis charlie

 

[georgeverghese]

Unless we are driving this compressor with a steam turbine, with steam derived from a cogen plant, net overall thermal eff for gas engine or other drivers would be poorer than if we we to heat this parent liquid / quenched steam with live injection of some HP steam.