Vapor-liquid separators Solid deposition

Condensation refers to the formation of a liquid or solid directly from a gas. (Sometimes, forming a solid directly from a gas is called deposition.) The opposite processes are evaporation or sublimation, respectively. During all of these processes, pressure and temperature cannot be varied separately. If one is fixed, so is the other. It is common experience that water boiling at 1.0 atm pressure remains at 100°C as heat is added and its volume increases, until there is no more liquid remaining in the system. During the process of vaporization, liquid and gaseous water simultaneously exist in the system. The 1.0-atm isobar of water is shown in Fig. 7. 

The major application of vapor-liquid equilibrium (VLE) is distillation. The uses of liquid-liquid equilibrium (LLE), liquid-solid equilibrium (LSE), and gas-solid equilibrium (GSE) are much more diverse. They include extraction (both solid and liquid), decantation as a phase separation, vapor-phase deposition (the heart of the semiconductor business), and a host of environmental applications. In all of these applications the equilibrium state and the rate of approaching it are both important. This book discusses only the equiUb-rium state, normally asking what are the compositions of the equilibrium phases when the system has minimized its Gibbs energy, subject to the external constraints and the starting conditions. As with VLE, the working criterion for LLE, LSE, and GSE is that the fugacity of any individual species must be the same in aU the phases at equilibrium (Eq. 7.4). 

The ablation threshold is the irradiance value at which the material starts to separate from the sample. As a rule, the material separates through vaporization to a depth where the energy supplied by the laser beam equals the energy of vaporization. The material can separate in other forms including solid or liquid fragments and products of the photochemical decomposition of the sample. Because the energy deposited over the sample must exceed its latent heat of vaporization (//, J/lcg), the threshold fluence J/m-) is given by... 

FIGURE 13 Examples of kinetic processes classified by types of phases involved, (a) Gas-gas reaction equilibrium between hydrogen gas, iodine gas, and hydrogen iodide gas. (f>) Gas-Uquid evaporation of liquid water from a glass, (c) Liquid-Liquid gradual separation of an oil-water mixture, (d) Gas-solid chemical vapor deposition of a thin Si film, (e) Liquid-solid corrosion of Cu metal in seawater, (f) Solid-solid precipitation of CuAlj particles from a copper-aluminum alloy during a heat treatment process. 

Armitt et al. investigated the influence of acid vapors (98% H2SO4,35% H2SO4, and 37% HCl) on the decomposition of a solid sample of TATP placed into a crimped vial, where the TATP was separated from the deposited 100 pL liquid acid sample by a plug of cotton wool [62]. The headspace in the sealed vials was sampled with polydimethylsiloxane/Carboxen/divinylbenzene (PDMS/Carboxen/DVB) SPME fibers of film thickness 30 and 50 pm, at 1, 3, 5, and 8 h, and at 1,3, 7, and 10 days. Analysis of the headspace from the TATP samples exposed to vapor from the two sulfuric acids produced similar results, with the sample exposed to the 98% sulfuric acid vapor decomposing at a more rapid rate. The principle decomposition products were DADP and acetone, with some minor production of acetic acid. After 7 days, acetone was the major species observed in the headspace. The analysis of the headspace of the TATP samples exposed to HCl vapors showed rapid decomposition of TATP, with no detection of TATP or DADP after 1 day. Additionally, various chlorinated acetones were also observed, indicating a different decomposition pathway than that observed for the TATP exposed to sulfuric acid vapors. 

The chemical species present in the electrolyte are actually more complex than that described. In solution, elemental bromine exists in equilibrium with bromide ions to form polybromide ions, Br, where = 3, 5, 7. Aqueous zinc bromide is ionized, and zinc ions exist as various complex ions and ion pairs. The electrolyte also contains complexing agents which associate with polybromide ions to form a low-solubility second liquid phase. The complex reduces the amount of bromine contained in the aqueous phase 10 to 100-fold, which, in addition to the separator, also reduces the amount of bromine available in the eeU for the self-discharge reaction. The complex also provides a way to store bromine at a site remote from the zinc deposits and is discussed further in the next section. Salts with organic cations such as iV-methyl-iV-ethylmorpholinium bromide (MEMBr) are commonly used as the complexing agents. One researcher has proposed a mixture of four quaternary ammonium salts for use in zinc/bromine batteries. The proposed electrolyte has favorable properties with regard to aqueous bromine concentration, resistivity, and bromine diffusion and does not form solid complexes at low temperatures (5°C and above). Complexes with quaternary ammonium ions are reversible and also have an added safety benefit due to a much reduced bromine vapor pressure (see Sec. 35.6). 

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