Water thermochemical separation

Ionic solids tend to dissolve (to varying degrees) in polar solvents such as water. Such solutions contain the separated ions, surrounded by solvent molecules. Ion-dipole attraction (in some cases with appreciable covalency as well) provides the energic compensation for the loss of electrostatic attraction which must accompany the dissolution of an ionic solid. Consider, as an example, the following thermochemical data ... 

Thermochemical splitting of water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Unfortunately the decomposition of water does not proceed until temperatures around 2500 K are reached. This and other thermal routes are discussed in Chapter 5. Solar thermal processes are handicapped by the Carnot efficiency limits. On the other hand, solar photonic processes are limited by fundamental considerations associated with band-gap excitation these have been reviewed in Refs.32 and 33. 

Fig. 2.13. Separation factor (a) versus column temperature in the chromatographic resolution of D,L-PA on L-PA imprinted polymers prepared by thermochemical initiation at 60/ 90/120°C (24 h at each temperature) using acetonitrile as porogen and photochemical initiation at 15°C for 24 h using dichloromethane as porogen. For the thermochemically polymerised material the mobile phase was 5% acetic acid in acetonitrile and for the photochemically polymerised material the mobile phase was acetonitrile/water/acetic acid 92.5/2.5/5 (v/v/v). From Sellergren et al. [27] and Sellergren and Shea [13].

Fig. 5.18. Effect of polymerisation and elution temperature on the enantiomer separation factor (a) in the separation of D- and L-PA on L-PA imprinted polymers. Polymers were prepared by thermochemical initiation at either 60 or 40°C using AIBN or ABDV respectively as initiators. The samples consisted of ca. 20 nmol of each of D- and L-PA and BOC-L-PA as void marker. Flow rate 0.5 mL/min. Mobile phase MeCN/acetic acid 95/5 (v/v). The columns were thermostatted by immersing them in a circulating water bath at the indicated temperature. From O Shannessy et al. [8].

Thermochemical production of hydrogen involves the separation of water into hydrogen and oxygen through chemical reactions at high temperatures. Ideally, water can be separated directly (thermolysis) however this process requires temperatures in excess of 2 500°C. 

Because these temperatures are impractical, the thermochemical water-splitting cycles achieve the same result (i.e., separation of water into hydrogen and oxygen) at lower temperatures. A thermochemical water-splitting cycle is a series of chemical reactions tliat sum to the decomposition of water. To be useful, each reaction must be spontaneous and clean. Chemicals are chosen to create a closed loop where water can be fed to the process, oxygen and hydrogen gas are collected, and all other reactants are regenerated and recycled [2]. 

Recent studies conducted through the Nuclear Energy Research Initiative (NERI) have identified more than 100 thermochemical water-splitting cycles. A few of the most promising cycles have been selected for further research and development, based on the simplicity- of the cycle, the efficiency of the process, and the ability to separate a pure hydrogen product. Tlie Cu-Cl cycle is one of the promising cycles which can produce hydrogen at a lower temperature (550°C) compare to that of direct themiolysis. 

Hydrogen production by a 2-step water splitting thermochemical cycle can be based on metal oxides redox pairs. A two-step, water-split-ting cycle, based on metal oxides redox pairs bypasses the separation hurdle. Multi-step thermochemical cycles can allow the use of more moderate operating temperatures, but their efficiency is still limited by the irreversibility associated with heat transfer and product separation. 

This endothermic plasma-chemical process was considered, in particular, to be an important step in the thermochemical calcium-bromine-water splitting cycle for hydrogen production (Doctor, 2000). The plasma-chemical HBr decomposition (5-182) assumes in this case effective quenching and separation of products by fast rotation of quasi-thermal plasma... 

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