Supercritical flow reactor temperature

Figure 1 is a schematic of one of the two supercritical flow reactors used in this work. The system is first brought up to the operating pressure by an air compressor. An HPLC pump forces the reactant solution through the reactor, the ten-port valve and dual-loop sampling system, and into the product accumulator, where the flow of products displaces air through a back-pressure regulator. The reactant inflow is rapidly heated to reaction temperature by an electric entry heater/water jacket combination, and maintained at isothermal conditions by a Transtemp Infrared furnace and an exit electric heater/water jacket combination. 

MODAR Inc. (Massachusetts. USA) developed the first reactor vessel [13]. It comprised an elongated, hollow cylindrical pressure-vessel, capped at both ends so as to define an interior reaction chamber. Defined within the reaction chamber are a supercritical temperature zone, in the upper region of the reactor vessel, and a subcritical temperature zone in the lower region of the reactor vessel. Oxidation of organics and oxidizable inorganics takes place in the supercritical temperature zone. Dense matter, such as inorganic material initially present and formed by reactions, if insoluble in the supercritical-temperature fluid, falls into the liquid phase provided in the lower-temperature, subcritical zone of the vessel. A perimeter curtain of downward-flowing subcritical-temperature fluid is established about a portion of the interior of the cylindrical wall of the vessel to avoid salt-deposits on the walls of the reactor vessel. 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

In nearly all cases, these complexes decompose rapidly and irreversibly at or near room temperature because of the weak H2 binding on such CO-rich metals, where less backbonding is present. Their instability is exacerbated because the 16e product of H2 dissociation is extremely reactive since it is not stabilized by internal agostic C-H interactions or solvent binding (hydrocarbon solvents are even more weakly bound than H2). The rate of room-temperature dissociation of H2 from Cr(CO)5(H2) in hexane is actually slower than that for many stable species. Thus, this complex and others like it might otherwise be stable under H2. One such complex initially presumed to be unstable, CpMn(H2)(CO)2, was in fact isolated as a solid from supercritical C02 (scC02) at room temperature in a flow reactor by rapid expansion of the scC02 (Scheme 5.21).163 164... 

In a very early study Patat (1945) investigated the hydrolysis of aniline to phenol in a water-based acidic solution in near-critical and supercritical water (Tc = 374.2°C, Pc = 220.5 bar). Phosphoric acid and its salts are used as the catalyst for this reaction. The reaction proceeds extremely slowly under normal conditions and reaches equilibrium at low conversion levels. For these reasons, Patat chooses to study the reaction in supercritical water to temperatures of 450°C and to pressures of 700 bar in a flow reactor. He finds that the reaction follows known, regular kinetics in the entire temperature and pressure space studied and the activation energy of the hydrolysis (approximately 40 kcal/mol) is the same in the supercritical as well as in the subcritical water. He suggests that the reaction is catalyzed by hydrogen ions formed from dissolution of phosphoric acid in supercritical steam. Very small amounts of phosphoric acid and the salts of the phosphoric acid are dissolved in the supercritical steam and are split into ions. Patat lists several dissolution constants for primary ammonium phosphates in supercritical steam. In this instance, the reaction performance is improved when the reaction is operated homogeneously in the mixture critical region and, thus, in intimate contact between the reactants and the catalyst. 

The discussion above suggests an approximate minimum value of 5 = 7 for most reactions. For neat supercritical CO2,5 falls below 7 at a temperature of 83 C at 6000 psi and at 130 C at 10,000 psi using equation 6. The lower temperature limit is set by the critical temperature of CO2 31. TC. Thus, for most workers, the useful range for synthesis in supercritical CO2 is likely to be 31 < T < 83 C, P < 6000 psi. The outer Imiit, without die use of fairly specialized and expensive equipment is 31 < T < 130 C, P < 10,000 psi. If higher pressures are required, they may be most easily achieved using flow reactors, (which also scale more easily). How reactor methods for synthesis in supercritical carbon dioxide have been pioneered by Poliakoff and coworkers. 74) In practice, such reactors closely resemble those ady used for hydrothermal processing. 

Koll, P., Bronstrup, P. and Metzger, J. O. (1983) Liquefaction of biomass witli supercritical fluids in high pressure/liigli temperature flow reactor in Chemical Engineering at Supercritical Conditions, Paulaitis, M.E., Penninger, J.M.L., Gray, R.D. Jr. and Davidson, P., Ed. Ann Arbor Science Ann Arbor, MI.. 

King also pointed out opportunities for selected reactions to be conducted in water at conditions substantially below the critical temperature. He felt that reactions should be examined in supercritical light hydrocarbon media (ethylene, propylene, propane, and pentane), for potential rate and selectivity enhancement. Furthermore, using an SCF in conjunction with a condensed phase, a two-phase system, may be beneficial, as shown by Tumas et al The SCF can considerably lower the viscosity of the reaction medium. Since so many studies of reactions in SCF media have been carried out in micro flow reactors, the need for scale-up studies is urgent. Modeling of reaction kinetics is lagging far behind the modeling of supercritical fluid extraction. 

For the sake of completeness, it should be mentioned that the use of microreactors and miniaturized flow reactors for the Friedel-Crafts alkylation of aromatic compounds has also been documented by other authors. For example, the Friedel-Crafts alkylation ofbenzenewithcydohexene using H2SO4 as a catalyst has been described [7]. The reaction was conducted in a static micromixer giving 58% cyclohexylbenzene. PoUakofi and coworkers have carried out the Friedel-Crafts alkylation of anisole with n-propanol in supercritical CO2, testing five different Bronsted solid acid catalysts under systematic variation of process conditions such as temperature and pressure [8]. 

Cr, Mo and W - The chromium complex [CMt -C2H4)(CO)s] has been isolated as a stable solid for die first time by the photolysis of [Cr(CO)6] in supercritical ethene at room temperature using a miniature flow reactor. Bis-ethene compounds of the type trans [M(C2H4)2(P-P)(PMe3)2] (M = Mo, P-P = dmpe, depe, dmpm M = W, P-P = dmpe, depe) have been reported and the interdependence between the electron density at the metal and the C-chemical shift of the coordinated ethene established. The reaction of carbon dioxide with some of... 

The presence of copper ions seemed to help synthesize tetrj lycine [17]. Using a supercritical water flow reactor with temperature control inside the fluids, it is suggested that condensates of glycine, which yielded amino acids after hydrolysis, formed even in supercritical water at 400 °C, under 25 MPa pressure [18]. When an aqueous mixture of ten amino acids was heated at 200-400 °C, the acid hydrolysis of the products led to a higher content in glutamic acid and a-amino acids, such as a-aminobutyric acid, 5-aminovaleric acid and 6-aminohexanoic acid than in a-amino acids even over supercritical conditions of water su esting that a-amino acids could be chemical markers of abiotic hydrothermal systems [18]. Reviews report the various conditions of amino acid syntheses [19-21 and Ref therein]. 

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