Reactors, supercritical fluids

In work by Hanrath and Korgel, H-terminated Ge nanowires were exposed to hexene in a supercritical fluid reactor at 220 °C, and the resulting hexyl-terminated nanowires appear resistant to oxidation in either air or water. The image reveals an abrupt interface for the alkyl-terminated Ge nanowire (Figure 5.12(b)) compared to the nanowires removed from the reactor without termination (Figure 5.12(a)) [103]. 

Figure 1. Schematic diagram of supercritical fluid reactor.

Clean a silicon wafer by rinsing with distilled water, and dry with KimWipes. Place the clean silicon substrate in a tall beaker, and place in the supercritical fluid reactor using long tweezers. With assistance from the instructor, fasten the reactor to the supercritical fluid system. 

Prepare a 0.05 M copper solution using CuCl2-2H20 in absolute ethanol (200 proof), and transfer this to a clean 8-mL vial. Also prepare a 0.04 M aqueous solution of 1,4-phenylenediamine, and transfer to a second clean 8-mL vial. Position the vials within the supercritical fluid reactor, avoiding any contact between the solutions. With consultation with your instructor, allow the system to reach the desired pressure and temperature. Maintain these conditions for 45 min. 

Most of the data available in the literature are for subcritical conditions. Corrosion studies of iron alloys in supercritical water have not been reported. For supercritical fluid extraction and corrosion studies, a supercritical fluid reactor system for temperatures up to 530 C and pressures up to 300 atm was constructed. This system was used to determine the electrochemical behavior of type 304 stainless steel (304 S.S.), 316 S.S., 1080 carbon steel (1080 C.S.), and pure iron in supercritical water. 

A simplified continuous supercritical fluid reactor is shown in a clear schematic in Fig. 12.5, such reactors are the basis for all the apparatus used for the heterogeneously catalyzed reactions of organic substrates described in this chapter. 

Position the vials within the supercritical fluid reactor, avoiding any contact between the solutions. With ctmsultation with your instructor, allow the system to reach the desired pressure and temperature. Maintain these conditions for 45 min. 

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

Spinning disc reactor Supercritical fluids Static mixer reactor Reactor... 

For the elucidation of chemical reaction mechanisms, in-situ NMR spectroscopy is an established technique. For investigations at high pressure either sample tubes from sapphire [3] or metallic reactors [4] permitting high pressures and elevated temperatures are used. The latter represent autoclaves, typically machined from copper-beryllium or titanium-aluminum alloys. An earlier version thereof employs separate torus-shaped coils that are imbedded into these reactors permitting in-situ probing of the reactions within their interior. However, in this case certain drawbacks of this concept limit the filling factor of such NMR probes consequently, their sensitivity is relatively low, and so is their resolution. As a superior alternative, the metallic reactor itself may function as the resonator of the NMR probe, in which case no additional coils are required. In this way gas/liquid reactions or reactions within supercritical fluids can be studied... 

Supercritical fluids allow the formation of species that cannot be made in conventional solvents. For example, rj2-H2 complexes have been generated by direct reaction of hydrogen with a transition metal carbonyl complex [10]. In order to isolate these compounds, a continuous flow reactor was used and such compounds could be isolated with surprising ease. 

Figure 1. Experimental system for supercritical fluid extraction. LF = line filter PG = pressure gauge SV = shut-off valve CV = check valve PHC = preheating coil TC = thermocouple MV = micrometering valve TP = separator trap subscripts r = reactor 1,2 = trap 1,2.

In high pressure work, slurry reactors are used when a solid catalyst is suspended in a liquid or supercritical fluid (either reactant or inert) and the second reactant is a high pressure gas or also a supercritical fluid. The slurry catalytic reactor will be used in the laboratory to try different catalyst batches or alternatives. Or to measure the reaction rate under high rotational speeds for assessing intrinsic kinetics. Or even it can be used at different catalyst loadings to assess mass transfer resistances. It can also be used in the laboratory to check the deactivating behaviour. 

We refer to Fig. 6.7-1. Reaching once equilibrium between the supercritical fluid SCF1 and the feed in the extractor El is enough for separation. By changing pressure and temperature the produced extract EX1 and raffinate R1 concentrations can be varied following the ternary phase equilibrium. The supercritical solvent-to-feed flow rate ratio affects the amounts of products obtained from a given feed. The apparatus required to apply this method are a normal stirred reactor, where contact of the two phases takes place, followed by a separator eliminating the extract from the extraction gas, which is recycled back to the extractor. 

The isobutane-1 -butene alkylation was studied in dense CO2 in both fixed-bed and slurry reactors.165-167 Both Nafion SAC-13 and Nation SAC-25 exhibited steady-state conversions and selectivities for 50 h. Enhanced Cg alkylate selectivity could be achieved at near total butene conversion. The maximum value attained, however, was only about 40%. The higher effective alkylation rate constant for SAC-25 compared to SAC-13 indicates improved accessibility of the acid sites. Nafion SAC-13 and SAC-25 applied in a study to test the effect of supercritical fluids on alkylation exhibited only modest activities.168... 

Figure 8. Schematic representation of our technique for recovering solid products from reactions in supercritical fluids (a) shows the supercritical reactor connected to a computer controlled syringe pump (Lee Scientific Model 501) filled with scC02. (b) shows how the scC02 is used to drive the supercritical reaction mixture (colored) through an expansion valve (Jasco 880/81 Back-pressure Regulator) where the products dissolved in the fluid are precipitated.

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