Halide Salt Solubility in Supercritical Water Relevant to the Destruction of Halogenated Pollutants.

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Supercritical states exist at temperatures and pressures at which there is no distinction between the gas phase and the liquid phase. Usually it is recognized by the disappearance of a heat of vaporization between liquid and gas phases. At atmospheric pressure, when water boils at 100C, heat is added to the system without a change in temperature being recorded. The energy being added to the system has no effect on temperature, as anyone who cooks observes.

Water in this different phase, supercritical water, has very different properties. It is very acidic relative to normal phase water either as liquid or steam, and organic molecules, including many pollutants are far more soluble in it, while, conversely salts, readily soluble in liquid water are not soluble in supercritical water. One can also have fire in supercritical water, as oxygen is miscible with water in the supercritical state.

A relatively recent paper in the scientific literature discusses the problem of insoluble salts generated when the serious pollutants represented by organohalide compounds such as PCB’s, chlorocarbons and other toxic compounds are destroyed in a supercritical halide matrix, which offers many environmental advantages with respect to traditional methods of destruction such as incineration.

Here is the abstract of the paper.

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VMF-4W38RJ1-1&_user=1082852&_coverDate=08%2F31%2F2009&_rdoc=3&_fmt=high&_orig=browse&_srch=doc-info(%23toc%236149%232009%23999499998%231130268%23FLA%23display%23Volume)&_cdi=6149&_sort=d&_docanchor=&_ct=15&_acct=C000051401&_version=1&_urlVersion=0&_userid=1082852&md5=69b9c49983dfee5b8504f0fcd0c90535">J. of Supercritical Fluids 50 (2009) 1–5

Supercritical water oxidation (SCWO) technologies have been applied to decompose the toxic organics <1,2>, such as dioxin and poly-chloro-biphenyl (PCB), to CO2 and H2O. In these processes, hydrogen halides such as hydrochloric acid are produced and cause remarkable corrosion of reactor. Alkalis are added as the neutralization reagent in order to prevent the corrosion and metal chlorides are produced. These metal chlorides in supercritical water are precipitated in the reactor or lines because of the very low solubilities of the metal chlorides in supercritical water. The effective discharges of the metal chlorides from the system are required. Therefore, it is important for the development of the supercritical water oxidation processes to understand the solubilities of metal chlorides in supercritical water and the phase behavior for supercritical water +metal chloride systems. The solubilities of sodium chloride in water vapor at high temperatures and pressures have been reported by some research groups <3–6>. Bischoff et al. <3> measured the solubilities of sodium chloride in sub- and supercritical water at the vapor–liquid equilibrium regions by a static method.

One may reasonably ask about the experimental details, which are somewhat problematic since one can not use in general, glass systems to make these determination. (A device which does allow for the visual characterization of these compounds is the diamond anvil, which uses diamond as the window, because diamond is able to withstand high pressures and temperatures, which are in fact, the conditions at which diamond is formed. However diamond anvils generally are microscale and are not – yet- useful for macroscopic property determinations.) Here’s a brief description of the apparatus used from the paper:

2.2. Apparatus and procedure
A flow type apparatus was used for the measurements of the solubilities for lithium chloride and calcium chloride in water vapor at high temperatures and pressures. The experimental apparatus and procedure were quite similar to those in a previous work <9>. The detailed descriptions of the apparatus and operating procedures were given in the previous work. The apparatus and procedures are only briefly described here. An equilibrium cell was made of Hastelloy C. The inside diameter and volume were about 12mm and 10 ml, respectively. The cell was set in an air bath in which the temperatures were controlled within ±0.5 K. In the equilibrium cell, 2 g of lithium chloride or calcium chloride was introduced beforehand with the solubility measurements. This initial mass of metal chloride was larger than those in the case of NaCl and KCl in the previous work <9>. The equilibrium cell in the air bath was heated up to the desired temperatures. Pure water was supplied by a feed pump. A back pressure regulator was used for the control of the pressures in the system. The pressures in the system were maintained within ±0.05MPa. The pressurized water was heated through a preheating coil and supplied to the equilibrium cell. At the outlet of the cell, pure water was also supplied to the outlet of the cell in order to avoid the precipitations of metal chlorides. Water vapor passed through the cell was decompressed at an expansion valve. The trapped samples were analyzed by ion chromatography (JASCO Co.,Ltd.). The cation concentrations were obtained from the analysis. The solubilities of metal chlorides in water vapor were determined from the cation concentrations of metal chlorides and the flow rates of water measured at the sampling unit.

The results of the work were quite interesting. For some ions, lithium and calcium, it was possible to actually increase the solubility of ions by increasing temperature.

The conclusion of the paper is given here:

The solubilities of lithium chloride and calcium chloride in water vapor were measured from 623 to 673 K and from 6.0 to 14.0 MPa by a flow method. At constant pressure, the solubilities of lithium chloride and calcium chloride increase with temperature increase unlike the case of sodium chloride and potassium chloride. It is though that the solubility behavior is resulted from the difference of the phase states, vapor–liquid equilibria for lithium chloride and calcium chloride and vapor–solid equilibria for sodium chloride and potassium chloride.

The correlation model for the solubilities of the metal chlorides in water vapor was developed newly in consideration of the ionization and hydration of the metal chlorides. For the metal chlorides with monovalent cations, the hydration numbers in the correlation model increase with the radii of cations. Calcium chloride gives the large hydration number compared with those of metal chlorides with monovalent cations. The correlated results are in good agreement with the experimental data for the four metal chlorides.

This has relevance to catalytic systems that may have import beyond the mere destruction of problem compounds.

This is perhaps an esoteric bit of business, but may be of interest to environmental engineers and scientists.