Supercritical flow reactor

Figure 1. Supercritical flow reactor. Key (I) Mettler balance (2) flask with 1 0 (filtered and deaerated) (3) HPLC pump (4) bypass (three-way) valve (5) feed cylinder (6) weather balloon with feed solution (7) probe thermocouple (type K) (8) ceramic annulus (9) Hastelloy C-276 tube (10) entrance cooling jacket (11) entrance heater (12) furnace coils (13) quartz gold-plated IR mirror (14) window (no coils) (15) guard heater (16) outlet cooling jacket (17) ten-port dualloop sampling value (18) product accumulator (19) air compressor (20) back-pressure regulator and (21) outflow measuring assembly.

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. 

Continuous flow reactions are considered to be more sustainable in terms of quality, safety and economic perspective [97]. Continuous reactions in ScCOj have been critically reviewed by Hans and Poliakoff [55]. The major energy burden of a packed bed supercritical flow reactor comes from the CO compression to supercritical conditions. This can be reduced if a CO stream of similar pressure is available elsewhere in the plant. Although this is not realized at present, this m become possible as the CO Capture and Storage (CCS) technology matures. 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

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. 

Another way of getting around the problem of the separation of the catalyst from the substrate is via use of a flow reactor [38], Supercritical carbon dioxide has been used successfully as a medium for the hydroformylation of 1-octene using an immobilized rhodium catalyst. The catalyst is covalently fixed to silica through the modifying ligand A-(3-trimethoxysilyl-n-propyl)-4,5-bis(diphenylphosphino)phenoxazine (Figure 8.13). Selectivity was found to be... 

Mesitylene was alkylated with propylene or 2-propanol in supercritical CO2 using Deloxane, a polysiloxane-supported solid acid catalyst in a continuous flow reactor.406 Monoisopropylation with 100% selectivity occurred with 2-propanol. 

Initial studies of phenol SCWO Involved in extensive SCWO study Investigated the unique features of supercritical water in terms of density, dielectric constant, viscosity, diffusivity, electric conductance, and solvating ability Treatment of hazardous organic compounds Application of SCWO to the decomposition of sludges Found that sludge readily decomposes at near-critical water conditions with 02 or H202 as an oxidant in a batch or continuous flow reactor Treatment of sludges... 

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 ... 

Figure 8 A continuous flow reactor system for subcritical and supercritical studies. (From SRI International.)...

Selective hardening of edible oils in discontinuous stirred tank reactors (STR), continuous trickle bed reactors and in continuous flow reactors operating with supercritical CO2... 

Supercritical fluids, particularly supercritical C02, scC02, are attractive solvents for cleaner chemical synthesis. However, optimisation of chemical reactions in supercritical fluids is more complicated than in conventional solvents because the high compressibility of the fluids means that solvent density is an additional degree of freedom in the optimisation process. Our overall aim is to combine spectroscopy with chemistry so that processes as varied as analytical separations and chemical reactions can be monitored and optimised in real time. The approach is illustrated by a brief discussion of three examples (i) polymerisation in scC02 (ii) hydrogen and hydrogenation and (iii) miniature flow reactors for synthetic chemistry. 

We have recently reviewed the use of vibrational spectroscopy in supercritical fluids [2] and the theme common to most of our projects is the use of spectroscopy for real-time optimisation of processes in supercritical solution. Such optimisation is considerably more important in supercritical fluids than in conventional solvents because the tunability of the fluids results in a greater number of parameters which can affect the outcome of a reaction. Thus, the chances of hitting the optimal conditions purely by trial and error are much less in supercritical solution than in conventional reactions. Below, we give three examples of our approach, synthesis of polymers, transition metal hydrogen compounds, and the use of flow-reactors. 

Flow reactors offer considerable advantages over sealed autoclaves for supercritical reactions. Not only do flow-reactors require a much lower volume than a batch reactor for a given throughput of material (with obvious safety advantages) but also it is much easier to optimise reaction conditions in a flow reactor. We have already reported [4,5] the use of a miniature flow-reactor for the photochemical preparation of unstable metal complexes. We are now extending these techniques to the study of thermal and catalytic reactions. As an initial stage we... 

A photochemical flow reactor (Figure 8) utilizing supercritical fluids or hydrogen saturated solvents has been developed for the preparation of gram scale quantities of labile dihydrogen and aUcene complexes. Thus, Cr(CO)s(aIkene), CpMn(CO)2(propene)," and CpMn(CO)2(H2) have been... 

Figure 8 Schematic diagram of flow reactor for photochemical synthesis in supercritical fluids. (Reproduced by permission of Ref 108)...

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