Visualising supercrtical phase changes 2

[athomas236]

With sub-critical boilers I find it easy to visualise the change from water in the downcomers to a mixture of saturated steam and water leaving the furnace tubes.

My problem is in visualising what happens with supercritical boilers when liquid changes to steam. In this regard the pressure/temperature phase diagram is not very helpful.

In its simplest form the pressure/temperature phase diagram is clearly a single line (saturation line) starting at the triple point and ending at the critical point with liquid on one sde and vapour on the other.

If water above the critical pressure is heated to say 700K what happens at the critical temperature, does the liquid instaneously become steam.

Any help would be appreciated.

athomas236

 

[DickRussell]

It may be useful to view a pressure-enthalpy diagram as three-dimensional, with temperature on the X-axis, pressure on the Y-axis, and enthalpy coming out at you on the Z-axis. To build this mental picture, imagine cutting the paper along the vapor pressure line, stopping at the critical point; lift the right edge of the sheet, so that higher elevation represents higher enthalpy. For a pure component that is liquid and below the critical pressure, heating at constant pressure to the vapor-liquid equilibrium line of course raises the temperature and enthalpy until the line is reached, at which point a vertical rise (large increase in enthalpy, no increase in temperature) must occur to jump up to the vapor side of the cut; this is vaporization. After vaporization is complete (top of the "cliff"), further heating produces superheat and increases temperature.

If you start over with cold liquid but at pressure above the critical pressure and add heat, you move smoothly to higher temperatures and enthalpies without ever seeing a phase transition (no vertical "cliff" to climb). Basically, the high pressure fluid just expands in volume smoothly as you add heat. Above the critical point (pure substance), the stuff is just "fluid," with no phase changes at all.

This also can be used to explain to someone how you would go from low pressure cold liquid to low pressure high temperature vapor without ever seeing a phase transition. You raise the pressure to above the critical pressure with little temperature change (pump), add heat and let the fluid expand isobarically until the temperature is well beyond the critical temperature, then reduce the pressure with little temperature drop until the fluid is hot and at low pressure, ending up with what you would conclude is "vapor." You've walked around the critical point. On that cut sheet of paper (P-T diagram), the path that avoids the "cliff" is obvious.

With a multicomponent fluid, the same thing can be done, except that the plotted lines of constant vapor fraction are separate within the two-phase region and meet at the mixture critical point. In the region of vaporization with distinct phases, there is no vertical "cliff" but a steep uphill, corresponding to large increase in enthalpy and only a small increase in temperature. The critical point usually is not at the top of the envelope, but off to one side. The lowest pressure that just touches the top of the envelope is called the cricondenbar, and the temperature that just touches the right edge of the envelope is called the cricondentherm. This plot can be used to visualize retrograde condensation and other things. Take a look at the API Technical Data Book - Petroleum Refining, Chapter 4 ("Critical Properties").

 

[athomas236]

Thanks for your explanantion DickRussell

athomas236

 

[25362]

Phase (P,T) diagrams show a final point on the liquid-vapor boundary line corresponding to the critical point, which for water is 374^oC and 218 atm.

To athomas236, the opposite process would probably be easier to imagine. Above 374^oC liquid and vapor cannot coexist in equilibrium, and any isothermic vapor compression wouldn't result in condensation. The density would gradually increase with no transition to a liquid.

Just to enlarge the picture (I hope not muddle it) let's add that phase (P,T) diagrams don't show critical points (CP's) for the solid-liquid boundary lines, no matter how high the pressure. IMHO such CP's haven't yet been found.

 

[Inthevalley]

You did say visualise...
Subcritical is your wife's reaction when you return home "late from work" smelling of beer. The phase change happens over time as she has gradually figures out that you have been out with the boys.
Supercritical phase change is your wife's reaction when you return home late smelling of perfume.

 

[TheRaptor]

Inthevalley,

That's a good one. I hope you never get caught in that supercritical state.

Cheers.

 

[davefitz]

The supercritical state is a tricky area indeed. There are peculiar physical behavior associated with the "psuedocritical point", ie, the (P,H ) point for which the heat capacity is at a maximum. Other physical properties take a radical change near this point ( solubility, conductivity, viscosity, density), and they invite unusual process behavior if you are not wise about it. The changes are most dramatic just marginally above the critical pressure. For that reason, if you have a process that is above the critical pressure, then you need to closely focus on detailed process behavior of components for which the fluid H is near the psuedo-critical point.

For water flowing thru a supercritical steam generator (ie a boiler), some issues associated with this phenomena are:

a) for ensembles of 2 to 1000+ tubes absorbing heat and operating in parrallel between to equal pressure headers ( so called parallel channels) we have both static (Ledineg) and dynamic flow instabilities to contend with - prediction of flow distribution between parallel tubes requires sophisticated analyses

b) for heated components undergoing a high heat flux, there is a severe deterioration in the physical properties across the fluid boundary layer between the metal surface ( fluid temp at wall temp with low k, visc, rho) while the bulf of the fluid in the middle of the channel is cooler and has a low velocity shear force due to hi rho. This can lead to psuedo nucleate boiling , espescially when combined with an unusually low particular cahnel flow rate due to the effects of item (a) above.

c) There is also a change in fluid solubility across the boundary layer near the wall of a heated tube, such tha there will be a net transport of impurites from the bulk of teh fluid ( at midchannel) to the heated wall, leading to a rapid buildup of crud on the heated tube's wall. this crud acts as an insulator and causes the tube metal to run very hot and fail in a short service life.


Of course these peculiar changes in physical properties can be used to advantage, and there are many supercritical extraction process that are used to process pharmacueticals etc.