Tube-wall reactor product yield

A numerical example was used to examine the desired product yield versus reactant conversion behavior in a tube-wall reactor described above. Values assumed for the physical and chemical properties and the operating conditions are ... 

Fickert et al. (1999) examined the production of Br2 and BrCl from the uptake of HOBr onto aqueous salt solutions in a wetted-wall flow tube reactor. The yield of Br2 and BrCl was found to depend on the Cl to Br ratio, with more than 90% yield of Br2 when [Cl ]/[Br ] (in molL ) was less than 1,000. With increasing [Cl ]/[Br ] BrCl was the main product (see Figure 2). They also found a pH dependence of the outgassing of Br2 and BrCl with greater release rates at lower pH. 

All of this is, of course, speculative at present and will remain so until more details become available about the wall active species, the elementary reaction steps, and the physical transformations occurring within a pyrolysis reactor. However, precise control of reactor treatment times and conditions still provides excellent conditions for investigating the effect of such treatments during pyrolysis reactions. As research workers continue to identify the effects and reactions of both the homogeneous and heterogeneous pyrolysis of hydrocarbons, and as these reactions mechanisms become quantitative then such efforts will lead to better control of conversion, product yields and in carbon laydown in commercial tubes. 

Coke and carbon oxides, both undesirable by-products, are always formed to some extent In commercial pyrolysis units. The carbon oxides, are produced when part of the coke reacts with steam that Is used as a diluent with the hydrocarbon feedstock. Most, If not all, of these undesired products are formed by surface reactions that reduce the yields of olefins and other desired products. Coke also acts to Increase heat transfer resistances through the tube walls, and most pyrolysis units must be periodically shut down for decoking of the tubes. During decoking, pure steam or steam to which a small amount of oxygen (or air) Is added Is fed to the reactor, and the coke Is oxidized to produce carbon oxides. 

In a typical gas-phase carbon monoxide-UF6 reduction, 1.7 g (4.9 mmole) of UF6 is condensed at -196° into an evacuated 30-ml Kel-F tube. Carbon monoxide (2.3 mmole) is expanded into the tube, and the valve of the reactor is closed. A Hanovia, 6515-34, 450-W Hg lamp is placed about 30 cm from the Kel-F tube and the gaseous sample is irradiated for 2-4 hours. ( Caution. The lamp should be shielded by a heavy black screen from laboratory personnel.) The pentafluoride product collects on the walls and the remaining volatile products can be vacuum evaporated. The yield is in excess of 90%. Anal. Calcd. for UFS U, 71.47 F, 28.52. Found U, 71.33 F, 28.45. 

A mixture of 1//-decafluoroheptane (24.7g, 67mmol), NOz (6.6g), and Cl2 (5.2g) (molar ratio 1 2 1) was passed through the reactor (sloping 52 x 2.5 cm empty Ni pipe in a 33-cm electric tube furnace) at a wall temperature of 600 C in 15 min with a contact time of 11 s. The product in the ice trap (17.8 g) was hydrolyzed in cold H20 (100 mL), and coned H2S04 (25 mL) was added which caused the separation of a lower phase containing theperfluoro acid. The aqueous phase was extracted with perfluoro inert c-C6F 20 (a cyclic ether), in which fluorocarbon acids are soluble. The lower layer was separated and the product was recovered from the perfluorinated solvent by distillation yield 11.1 g(46%) bp 170-175 C/740Torr. 

These experiments show that addition of water vapor to the reactant mixture has a strong inhibitory effect on the production of hydrocarbons. The C02 yield is increased owing to the reaction H20 + CO — C02 + H2. When the water vapor partial pressure is approximately equal to the CO partial pressure in the reactant gas mixture, no hydrocarbons are produced at all. The pronounced inhibitory effect of H20 vapor on hydrocarbon formation in this reaction may explain some of the scatter of the data shown in Figures 1, 2, and 3. If small (and variable) amounts of H20 vapor were adsorbed on the walls of the reactor tubes, this would decrease the hydrocarbon yield in (some of) the runs by a varying amount which would show up as scatter in the data. However, there are other, as yet unknown, sources of variability in the experimental conditions which also undoubtedly contribute to the scatter in the data. 

The system actually is comprised of two reactors in series I. the plasma reactor, wherein high-temperature transient species are generated, followed by II. the quench reactor, wherein the plasma precursors are rapidly cooled within the cold-walled sampling tube to yield room-temperature stable products. 

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