The Phase Diagram of Supercritical Water + Sodium Chloride
The paper I'll discuss briefly in this post is this one, which is an oldie but goody, as they say: The system H2O–NaCl. Part I: Correlation formulae for phase relations in temperature–pressure–composition space from 0 to 1000 °C, 0 to 5000 bar, and 0 to 1 XNaCl (Thomas Driesner *, Christoph A. Heinrich, Geochimica et Cosmochimica Acta 71 (2007) 4880–4901).
Lately, given the problem of microplastic pollution of seawater, the changes and volatility with respect to the availability of fresh water owing to climate change, and climate change itself, I have been thinking quite a bit about supercritical seawater as a desalination, water cleaning and carbon dioxide recovery tool, the latter recovering carbon dioxide from air, an extremely challenging engineering task that will become necessary as a result of our contempt for all future generations.
The supercritical state of pure water is that at temperatures higher than 373 °C and 25 MPa of pressure, a point at which the distinction between steam and liquid water disappears. It has special properties, notably the ability to dissolve organic compounds, and the insolubility of many salts, in other words, the inverse of what we experience with liquid water at ambient conditions.
As of now, this technology, bringing seawater to the supercritical state, is not economically feasible because of the extremely corrosive nature of supercritical water, particularly in the presence of salts. However, recent advances in materials science, particularly involved with developments in nanostructural semimetallic ceramics would seem to make this idea worthy of consideration. There have been a number of advances in supercritical water separation reactors in recent years, with the separation of salts being a critical goal.
Thinking about this problem led me to dig around in my files whereupon I came upon the paper referenced above, which was in my files in connection with considerations, a number of years ago, of the supercritical water oxidation (SCWO) of biomass.
The paper is connected with the now widely held view that some geological salt deposits were not formed by the evaporation of ancient seas, but rather formed as the result of the precipitation of salts from supercritical seawater interacting with magma in volcanic vents.
So here is the phase diagram of the NaCl Water system up to 1000 °C:
It would seem that the optimum separation temperature would be around 550 °C, which is lower than the melting point of sodium chloride, 801 °C.
Some text from the paper:
At temperatures below the critical temperature of pure water (e.g., 300 _C, Fig. 2a and b), the V + L coexistence has its upper pressure limit at the boiling curve of pure water. The latter appears as a point in an isothermal section, located on the left vertical axis. The liquid branch of the V + L coexistence surface monotonously descends from this point to the halite-saturated liquid point on the V + L + H surface. At pressures above this curve, only a single-phase liquid is stable. The vapor branch of the V + L coexistence surface always has a much smaller NaCl content than the coexisting liquid. Upon pressure decrease from the boiling curve, the vapor’s NaCl content first increases, passes through a maximum and then decreases again towards the halite-saturated vapor point on the V + L + H surface. The V + L + H surface appears as a horizontal tie-line in this projection and has one end-point at the pure NaCl side (halite), runs through the point of halite-saturated liquid and ends at the halite-saturated vapor, which has a very low but finite NaCl-concentration, i.e., the V + L + H surface does not touch the pure water side…
…2.2.2. Phase diagram topology above the critical temperature of water.
The major change in the phase diagram topology at temperatures above the critical temperature of water is that the upper pressure limit of V + L coexistence is now the critical curve with a finite NaCl concentration such that the V + L coexistence field does not touch the pure water side anymore (e.g., 375 _C, Fig. 2e and f). On the low concentration side of the halite-saturated vapor curve and the vapor branch of the V + L surface, a single phase with vapor-like properties exists that contains no or only very little NaCl. This has the important consequence that upon pressure increase, a fluid with vapor-like properties from the waterrich side of the phase diagram can continuously change its properties to become a liquid-like fluid (and vice versa) without any heterogeneous phase change. A variety of other paths in P–T–XNaCl space can cause similar homogeneous density changes, including isothermal–isobaric (by changing the salt concentration at P > Pcrit, T) and isobaric– isoplethic (isobaric cooling at P > Pcrit,X) paths. This phenomenon has recently been proposed as a key to the formation of gold-rich epithermal ore fluids from magmatic vapor (Heinrich et al., 2004). Because no clear distinction between liquid and vapor is possible in this region, the terms ‘‘fluid’’ or ‘‘single phase fluid’’ are preferred by many authors, although the transition from the ‘‘supercritical’’ to the ‘‘subcritical’’ (with respect to the critical temperature of water) regime may transform ‘‘fluid’’ gradually to a liquid or vapor. Obviously, no universally meaningful definition of ‘‘fluid’’, ‘‘liquid’’, and ‘‘vapor’’ is possible. Nevertheless, in the literature the terms ‘‘liquid’’ and ‘‘vapor’’ are frequently used for fluids above the critical temperature of water. In such a case, the terms ‘‘liquid-like fluid’’ or ‘‘vapor-like fluid’’ seem appropriate, and specific conventions based on the fluid’s density may be introduced (e.g., Heinrich, 2005) to facilitate discussion. In this study, we will make use of ‘‘liquid’’ mainly to denote a fluid phase at temperatures below the critical temperature of water that cannot isothermally or isobarically be transformed into a vapor or vapor-like fluid without a heterogeneous phase transition or without heating above the critical temperature of water. Similarly, ‘‘vapor’’ will be used for a fluid phase at temperatures below the critical temperature of water that cannot isothermally or isobarically be transformed into a liquid or liquid-like fluid without a heterogeneous phase transition without a heterogeneous phase transition or without heating above the critical temperature of water…
These seem to me to be good ideas, although it appears that not much has been written about it, although a great deal has been written about the wonders of supercritical water itself as an environmental and synthesis tool.
Interesting I think.
I trust you are enjoying the New Year.