Some advantages of high pressure technology are to reduce energy loss of expansion. Figure11.1 shows an example. As mentioned above already, water vaporization at atmospheric pressure loses a huge quantity of energy as kinetic energy (when 1 kg of water at 100C is vaporized completely, 2.6 MJ of energy is required). On the other hand, heat-up of 1 kg of liquid water up to 300C without phase change requires only 1.2 MJ.
The other advantage is to keep or control density of fluid by controlling temperatures. Density of fluid is basic property and governs physical properties such as dielectric constant, viscosity, and so on. Therefore, flexible control of fluid density achieves wide control of solubility and reactivity.
The controllable of solubility as well as density of reactants brings high flex- ibility for reaction equilibrium known as Le Chatelier’s principal. In addition, the adjustability of concentration by changing temperature and pressure (namely density) gives high operability in reaction dynamics (kinetics) as well as reaction equilibrium. Concentration (the amount of reactant in reaction environment:
density and phase behavior are closely related) is the major factor for the reaction rate and product distribution. One of the typical examples is hydrocarbon cracking.
The cracking is radical chain reaction. Simply, it consists of 4 types of reactions:
initiation, radical decomposition, H abstraction, and termination (Eqs.11.1–11.4, respectively).
S!ki 2b ð11:1ị a!kb bỵc ð11:2ị
bỵS!kH bHỵa ð11:3ị
2radicalsðaorbị !kTproducts ð11:4ị Here,S is reactant,a andbare radicals (a is relatively large radical which is produced by H abstraction, b is smaller radicals produced from initiation and radical decomposition), andcis small product formed from radical decomposition.
The rate constants of each reaction areki(initiation),kb(radical decomposition), kH (H abstraction), and kT (termination). When the concentration of S is high
enough, hydrogenated product (bH) is main product, while c is main at low S concentration. To consider the concentration dependence, the overall rate con- stant is developed with steady-state approximation for the concentration of radicals (aandb). As a result, Eq. 11.5is obtained.
ksẳ kbkH
kbỵkHẵS ffiffiffiffiffiffiffiffiffi kiẵS kT
s
ð11:5ị
Here,ksis overall rate constant of the cracking ofS. Equation 11.5tells us that ksdepends on the concentration ofS:ksincreases at low concentration region with an order of 0.5 toS concentration,ks is not sensitive to S concentration at mild concentration region, andksdecreases at highSconcentration with-0.5 order to S concentration. For hydrocarbon cracking, supercritical water (up to 500C) is inert as reactant but phase behavior of hydrocarbon is affected by the existence of supercritical water [1]. When supercritical water dissolve some amount in dense hydrocarbon phase, the concentration of hydrocarbon in the dense phase is reduced. The reduction of concentration enhances the rate of hydrocarbon cracking and thus supercritical water works as cracking promoter. One has to notice that the promotion of the reaction is not the change of reaction but only the change of concentration of reactant.
At high pressure media, pressure itself sometimes promotes reaction rate. When a reaction pass through an activated complex, the reaction rate of the reaction is promoted with increasing pressure if volume of the activated complex (activation volume) is smaller than that of the reactant.
Fig. 11.1 Enthalpy of water at saturation condition
11.2.2 Special Features of Water at High Pressure High Temperature (HHW)
Water is ecological friendly and essential solvent on the earth. It has wider application if high pressure environment is available. When reaction field is in a solvent, solubility of a solute in the solvent is the critical factor of the reaction rate;
for example, the rates of oxidation and hydrogenation are limited by the solubility of oxygen and hydrogen, respectively. One of famous examples for usefulness of supercritical water as reaction field is supercritical water oxidation. In supercritical state of water, organic compounds such as hydrocarbon and oxygen (of which solubility is restricted at ambient condition) are miscible and thus complete oxidation of organics can be achieved at short reaction time [2]. There have also been several reports that show effectiveness of supercritical water as reaction environment for hydrogenation [3,4].
Furthermore, some features of water are unique and these result in attractive media for biomass conversions. The rate of some reactions of polar molecules is affected by dielectric constant as shown in Eq.11.6 (Kirkwood equation) [5].
SupposeA+B=[AB =[C.AandBare reactants,ABis activated complex and Cis product. SometimesABhas higher polarity than reactants.
lnksolẳlnk0 NA 4pe0RT
e1 2eþ1
l2AB rAB3 l2A
rA3 l2B r3B
ð11:6ị
Here,ksolis rate constant in a solution,k0is rate constant in vacuum, andNAis Avogadro number. Dipole moment of the reactants (A and B) and the active complex are denoted asliand radius of molecules is denoted asri(i=A,B, and AB: the reactants and the activated complex). According to this equation, when the dipole moment of activated complex is high enough compared with that of the reactants, the rate of the reaction is accelerated at the solvent having high dielectric constant. Dielectric constant of water can be adjusted by density and temperature, as shown in Eq.11.7[6].
eẳ1ỵ A1
T qþ A2
T þA3þA4T
q2þ A5
T þA6TþA7T2
q3 þ A8
T2þA9
T þA10
q4 ð11:7ị
HereAi (i=1–10) is fitting parameter. Indeed, there are some examples that can be explained well based on Kirkwood analysis [7,8].
Additionally, the other feature of water is high dissociation into proton and hydroxyl ion. The dissociation of water is deeply related to control of many kinds of reactions such as acid–base reactions. Typical biomass molecules such as cel- lulose, hemicellulose, lignin, protein, and so on, have reactive chemical bonds such as ether, which can be attacked by water molecules. The reaction of the bond with water can be enhanced by proton or hydroxyl ion and thus, if these ions could
provide effectively by controlling temperature and pressure, the reaction occurs without catalyst [9]. Ion product (Kw), which is index of water dissociation, can be controlled by temperature and water and thus is also governed by density and temperature as shown in Eq.11.8.
logKW ẳAỵB TþC
T2þ D
T3þ EþF TþG
T2
logq ð11:8ị
HereA–Gare fitting parameters [10]. Some organic reactions such as hydro- lysis, dehydration, decarboxylation, and so on, are controlled by acid and base catalyst as well as temperature. For high pressure high temperature water (HHW) media, density could be controlled by temperature and pressure. Therefore the acid–base reactions could be controlled by these parameters, in particular at supercritical state because density can be changed continuously by changing the operation parameters (temperature and pressure) there.
Ion product of water affects surface acidity or basicity of solid catalysts. Of course, it is the best way of performing reactions in high pressure high temperature water (HHW) that the rate of the reaction can be controlled by only temperature and pressure. However, the concentrations of proton and hydroxyl ion in neutral water are always the same and adjustability of acidity and basicity of water must be limited. To expand the controllability of acid–base property of water, hetero- geneous catalyst would be better than homogeneous catalyst because of the existence of both acid and base sites on the surface and wide controllability of acid–base properties (of course, heterogeneous catalysts are favored from the feasibility point of view, such as recyclability, reusability, and environmentally friendliness). Indeed, the research activities of using the heterogeneous catalyst for controlling biomass reactions in high pressure high temperature water (HHW) are increasing year by year. According to the detailed studies of hydration of olefin in the presence of solid acid catalyst (acidic metal oxide such as MoO3) in sub and supercritical water, the surface acidity increased with increasing ion product with an order of 0.45 [11]. It was considered that proton concentration of bulk water contributed the amount of surface proton on the acid catalyst. Thus, to control acid–base reactions either in the presence or absence of catalyst (both homoge- neous and heterogeneous), density of water is essential key parameter.