Carbon dioxide reaction, rate

Usually, the reaction between C02and water is very slow and hardly contributes to the total carbon dioxide reaction rate. Nevertheless, for the sake of completeness, it has been considered as a reaction of the first order with respect to the CO2, since the reaction kinetics depends on the carbonation ratio (see Ref. [87]). 

Carbon-Carbon Dioxide Reaction. There is general agreement 6, 37-43) that experimental data on the rate of gasification of carbon by carbon dioxide fit an equation of the form... 

Key (46) and Strickland-Constable (47) also support mechanism B for the carbon-carbon dioxide reaction. Strickland-Constable concludes from earlier measurements (46) that the rate of adsorption of carbon monoxide on carbon is too low to account for the retardation. 

As in the carbon-carbon dioxide reaction, mechanisms A and B can be treated for the cases where either the surface rearrangement or desorption of the carbon-oxygen complex is the slow step. This has no effect on the discussion except that the significance of the rate constant js in Equation (10) is altered, as previously discussed. 

Other workers 67, 85-89) have determined over-all activation energies for the carbon-carbon dioxide reaction, but the values have been affected to some extent by mass-transport control. Workers 6, 39, 40, 41) have also determined activation energies for the individual rate constants in Equation (5) but do not agree on their magnitude. The values of activation energy reported for rate constant i vary from 26.5 41) to 61.5 kcal./ mole 40). 

Gadsby and co-workers (63) report that for a coal charcoal, the rate of the carbon-steam reaction is greater by a factor of about three than the carbon-carbon dioxide reaction at 800° and a pressure range of 50 to 500 mm. Hg. The results of Pilcher et al. (68) and Walker et al. (85), using the same graphitized carbon rods and apparatus, essentially agree with this finding. At 1100°, the former workers report a reaction rate of 1.6 g./hr. at a steam partial pressure of 142 mm. Hg, which can be extrapolated to 4.8... 

Wicke (31), reacting spectroscopic electrode carbon at 0.1 atm. reactant pressure, gives the following equations for the rates of the carbon-oxygen and carbon-carbon dioxide reactions... 

As was discussed in Sec. Ill, the rate of the carbon-carbon dioxide reaction can be expressed as... 

The capillary plasma reactor consists of a Pyrex glass body and mounted electrodes which are not in direct contact with the gas flow in order to eliminate the influence of the cathode and anode region on CO2 decomposition. Analysis of downscaling effects on the plasma chemistry and discharge characteristics showed that the carbon dioxide conversion rate is mainly determined by electron impact dissociation and gas-phase reverse reactions in the capillary microreactor. The extremely high CO2 conversion rate was attributed to an increased current density rather than to surface reactions or an increased electric field. 

Carbonic anhydrases catalyze the reaction of water with carbon dioxide to generate carbonic acid. The catalysis can be extremely fast molecules of some carbonic anhydrases hydrate carbon dioxide at rates as high as 1 million times per second. A tightly bound zinc ion is a crucial component of the active sites of these enzymes. Each zinc ion binds a water molecule and promotes its deprotonation to generate a hydroxide ion at neutral pH. This hydroxide attacks carbon dioxide to form bicarbonate ion, HCO3 ". Because of the physiological roles of carbon dioxide and bicarbonate ions, speed is of the essence for this enzyme. To overcome limitations imposed by the rate of proton transfer from the zinc-bound water molecule, the most active carbonic anhydrases have evolved a proton shuttle to transfer protons to a buffer. 

As noted earlier, some carbonic anhydrases can hydrate carbon dioxide at rates as high as a million times a second (10 s ). The magnitude of this can be understood from the following observations. In the first step of a carbon dioxide hydration reaction, the zinc-bound water molecule must lose a proton to regenerate the active form of the enzyme (Figure 9.27). The rate of the reverse reaction, the protonation of the zinc-bound hydroxide ion. is limited by the rate of proton diffusion. Protons diffuse very rapidly with second-order rate constants near 10 M. Thus, the backward rale constant i must be less than 10 s F Because the equilibrium... 

Acidic gases. Powdered hydrated lime, whether in the form of Ca(OH)2, Ca(OH)2 MgO, or Ca(OH)2 Mg(OH)2, reacts readily with acidic gases such as the oxides of sulfur and nitrogen, and carbon dioxide. The rate of reaction depends largely on the particle size and the degree of dispersion of the hydrate (see sections 29.4 and 29.5). 

They also noted that hydrogen sulfide selectivity over carbon dioxide can be Improved by introducing gas gaps in a multilayer membrane because of the higher reaction rates of hydrogen sulfide. They found that catalysts increased the carbon dioxide hydration rate by a factor of 2 at low carbon dioxide partial pressure, this effect was observed to increase as carbon dioxide partial pressure decreased. Economic studies based on experimental data Indicate that cost savings on the order of 30-50% over conventional acid gas scrubbing were possible with immobilized liquid membranes. 

This equation represents accumulation, net transport as influx minus out-flux, and uptake/reaction by the biomass. For batch cultivations, the dilution rate < feed/ Rr = 0. For dissolved gases a transport term via the gas phase has to be amended. For the carbon dioxide transfer rate (CTR) this reads... 

Fig.4. Second-order rate constant of DEA-PZ and carbon dioxide reaction.

Laser flash photolysis techniques have been employed to evaluate the dynamics of decarboxylation reactions of cation radicals derived from a-aminocarboxylates. > In one report, variously substituted aminium radicals 47 were generated by laser flash excitation of anilinocarboxylates 46 (Scheme 21) in MeCN solutions containing the acceptor, 1,4-dicyanobenzene. These transients undergo fast, first-order decay by a pathway involving loss of carbon dioxide. The rate constants for decarboxylation were found to be in the range of 8 x 10 to 4 x 10 s . In addition, the rates show the same dependence on nitrogen, a-alkyl and a-phenyl substituents, as do the related a-CH deprotonation, a-desilylation, and retro-Aldol cleavage reactions. 

The complete assembly for carrying out the catalytic decomposition of acids into ketones is shown in Fig. Ill, 72, 1. The main part of the apparatus consists of a device for dropping the acid at constant rate into a combustion tube containing the catalyst (manganous oxide deposited upon pumice) and heated electrically to about 350° the reaction products are condensed by a double surface condenser and coUected in a flask (which may be cooled in ice, if necessary) a glass bubbler at the end of the apparatus indicates the rate of decomposition (evolution of carbon dioxide). The furnace may be a commercial cylindrical furnace, about 70 cm. in length, but it is excellent practice, and certainly very much cheaper, to construct it from simple materials. 

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