**2.5 Reagent mixing strategies**

the range of 0.01–30 mL/min [6–9]. Diaphragm, syringe, and piston pumps can be employed when higher flow rates are needed or when pumping a slurry [7, 10]. Reagent heating is important to consider as water can exhibit complex heat transfer characteristics near the critical point. Enhanced or deteriorated heat transfer can occur due to a combination of rapidly changing thermophysical properties and factors such as reactor geometry and the ratio of mass flux to heat flux. Generally, deteriorated heat transfer can be avoided by installing a downwardoriented heating section, which takes advantage of buoyancy effects for more efficient heating. A coiled heating section with a small diameter for high S/V is also

Resistive heaters, electric furnaces, and immersive fluidized baths have been used to reach the desired reaction temperatures [5, 7, 10, 13–15]. Resistive cartridge heaters are attractive options for preheating, as the tubing can be tightly wound around the cartridge to minimize heat loss. Electric furnaces offer precise control, are well-insulated, and are easy to install. A fluidized bath is a great option for maintaining isothermal conditions in the reactor section but can be expensive and bulky. Some combination of these heating methods is generally sufficient to achieve (a) rapid and efficient heating past the critical point and (b) isothermal reactor

Corollary to its ability to rapidly decompose organic compounds, supercritical water is extremely corrosive to most metals and metal alloys, especially if alkali metals or halogens are present. Thus, corrosion mitigation strategies need to be considered during SCWR design. Many studies have focused on corrosion control methods in SCWRs and SCW heat exchangers [16–18]. Generally, four corrosion mitigation strategies have been proposed and are thoroughly discussed in a review by Marrone et al. [16]. These are (i) preventing corrosive species from interacting

with the reactor surface, (ii) forming a corrosion-resistant barrier, (iii)

manufacturing the reactor from materials resistant to corrosion, and (iv) tuning operating conditions to minimize severe corrosion conditions. For reactors used to study reaction chemistry of organic compounds that do not contain heteroatoms, it is generally sufficient to rely on the corrosion resistance of the reactor material.

Char has been reported as a common recalcitrated product formed during the gasification of aromatic compounds or homogeneous biomass components, such as lignin and cellulose [19–23]. Char can rapidly clog reactors, and it should be avoided or suppressed if possible. Broadly, char yields are known to decrease in the presence of certain metal catalysts (such as nickel and ruthenium), which are thought to effectively cleave C–C bonds in the aromatic rings of polycyclic aromatic hydrocarbons (PAHs). Many open questions remain surrounding the exact mechanisms responsible for char formation in SCW. Multiple studies have confirmed that char formation rates are highly dependent on temperature and the initial feedstock concentration. The literature suggests that ionic mechanisms near the critical point are responsible for charring and coking from compounds such as glucose, fructose, and cellulose [20], while free radical mechanisms form char during SCWG of aromatic compounds at higher temperatures, such as phenol, benzene, and lignin [21, 22]. While the industrial implementation of SCWG would require a method for suppressing char formation at high feedstock loadings, researchers can circumvent this issue by performing experiments with low feedstock concentrations [19]. There

desirable for improved heat transfer near the critical point [11, 12].

conditions.

**116**

**2.3 Corrosion mitigation**

*Advanced Supercritical Fluids Technologies*

**2.4 Mitigating char formation**

For studying reaction chemistry in a lab-scale SCWR, the mixing strategy used to introduce the reagent into the SCW environment should be carefully considered. Mixing can be achieved by (i) premixing water and reagent before heating to supercritical conditions or (ii) injecting reagent directly into supercritical water. If chemical kinetic rates are sought, post-critical injection is a preferred mixing strategy, as it rapidly heats the reagent to reaction temperatures and establishes a definite reaction start time [24].

Premixing is required if the feedstock is solid or viscous and must be pumped as an emulsion or when high reagent loading is considered. Premixed reagents should be rapidly heated, as char and tar formation can be significant when reagents are heated slowly [6, 16].
