CO2 kinetics

CO2. Kinetic evidence indicates that the reaction is reversible, has a low activation energy, and proceeds in two steps. A suggested sequence of events is shown in the formulation. 

Bertucco, A. Canu, P. Devetta, L Zwahlen, A.G. Catalytic hydrogenation in supercritical CO2 kinetic measurements in a gradientless internal-recycle reactor. Ind. Eng. Chem. Res. 1997, 36 (7), 2626-2633. 

Besides hght supply, carbon dioxide uptake is most important to be considered. CO2 transfer to photobioreactors has been shown to be one of the reasons for energy dissipation. Furthermore, it influences pH value what on the other hand can be used for control purposes. CO2 kinetics is therefore an important issue for optimizing cultivation conditions. 

Although quite a few semiconductors are good photomaterials for CO2 multielectron reduction under proton transfer regime, the one-electron transfer is not easily feasible. Moreover, in this case too, as in the electrochemical reduction of CO2, kinetic issues cause an overvoltage increase which makes more energy required for the reduction processes. 

Srinivas and Mukhopadhyay (246) have investigated the selective thermal oxidation of cyclohexane in SCCO2 to produce cyclohexanone and cyclohexanol as the primary reaction products. Kinetic experiments were conducted at three temperatures (137°C, 150°C, and 160°C) and two pressures (170 and 205 bar), and the presence of a homogeneous SCF was verified experimentally for the initial reactor composition under these conditions (10 mol % cyclohexane, 10% O2, and 80% CO2). Kinetic results were interpreted assuming a free-radical reaction mechanism comparable to that observed in the conventional liquid-phase process, and the reaction was observed to be autocatalytic. Reported conversions are low relative to those in the liquid-phase process, which the authors attribute to... 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

Figure C3.1.8. Schematic diagram of a transient kinetic apparatus using iaser-induced fluorescence (LIF) as a probe and a CO2 iaser as a pump source. (From Steinfeid J I, Francisco J S and Fiase W L i989 Chemical Kinetics and. Dynamics (Engiewood Ciiffs, NJ Prentice-Fiaii).)...

Pentaeiythritol Tetranitrate. Specification-grade pentaerythritol tetranitrate (PETN) can be stored up to 18 months at 65°C without significant deterioration. However, many materials have been found to be incompatible with PETN and the presence of as Htfle as 0.01% occluded acid or alkah greatly accelerates decomposition. The decomposition of PETN is autocatalytic with reported kinetic constants oi E = 196.6 kJ/mol (47 kcal/mol) and Z = 6.31 x 10 . The decomposition products of PETN at 210°C in wt % are 47.7 NO, 21.0 CO, 11.8 NO2, 9.5 N2O, 6.3 CO2, 2.0 H2, and 1.6... 

Formic acid can decompose either by dehydration, HCOOH — H2O + CO (AG° = —30.1 kJ/mol AH° = 10.5 kJ/mol) or by dehydrogenation, HCOOH H2 + CO2 (AG° = —58.6 kJ/mol AH° = —31.0 kJ/mol). The kinetics of these reactions have been extensively studied (19). In the gas phase metallic catalysts favor dehydrogenation, whereas oxide catalysts favor dehydration. Dehydration is the predominant mode of decomposition ia the Hquid phase, and is cataly2ed by strong acids. The mechanism is beheved to be as follows (19) ... 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

Reaction measurement studies also show that the chemistry is often not a simple one-step reaction process (37). There are usually several key intermediates, and the reaction is better thought of as a network of series and parallel steps. Kinetic parameters for each of the steps can be derived from the data. The appearance of these intermediates can add to the time required to achieve a desired level of total breakdown to the simple, thermodynamically stable products, eg, CO2, H2O, or N2. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

The kinetics of the formation of the magnesium hydroxide and calcium carbonate are functions of the concentration of the bicarbonate ions, the temperature, and the rate of release of CO2 from the solution. At temperatures up to 82°C, CaCO predominates, but as the temperature exceeds 93°C, Mg(OH)2 becomes the principal scale. Thus, ia seawater, there is a coasiderable teadeacy for surfaces to scale with an iacrease ia temperature. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

For the reaction KCIO4 -t- 2C KCl -t- CO2, fine powders were compressed to 69 MPa (10,000 psi) and reacted at 350°C (662°F), well Below the 500°C (932°F) melting point. The kinetic data were fitted by the equation... 

Another possibility tliat would reduce the overall process efficiency is the kinetics of CO2 formation at the anode. There is some evidence that die first step in tire sequence... 

Ethylene oxide secondary oxidation with C-tagged ethylene oxide, to clarify the source of CO2, was done at Union Carbide but not published. This was about 10 years before the publication of Happel (1977). With very limited radioactive supply only a semi-quantitative result could be gained but it helped the kinetic modeling work. It became clear that most CO2 comes from ethylene directly and only about 20% from the secondary oxidation of ethylene oxide. 

Remarks The aim here was not the description of the mechanism of the real methanol synthesis, where CO2 may have a significant role. Here we created the simplest mechanistic scheme requiring only that it should represent the known laws of thermodynamics, kinetics in general, and mathematics in exact form without approximations. This was done for the purpose of testing our own skills in kinetic modeling and reactor design on an exact mathematical description of a reaction rate that does not even invoke the rate-limiting step assumption. 

Step 4 Define the System Boundaries. This depends on the nature of the unit process and individual unit operations. For example, some processes involve only mass flowthrough. An example is filtration. This unit operation involves only the physical separation of materials (e.g., particulates from air). Hence, we view the filtration equipment as a simple box on the process flow sheet, with one flow input (contaminated air) and two flow outputs (clean air and captured dust). This is an example of a system where no chemical reaction is involved. In contrast, if a chemical reaction is involved, then we must take into consideration the kinetics of the reaction, the stoichiometry of the reaction, and the by-products produced. An example is the combustion of coal in a boiler. On a process flow sheet, coal, water, and energy are the inputs to the box (the furnace), and the outputs are steam, ash, NOj, SOj, and CO2. 

Figure 8.46. Effect of Pt catalyst dimensionless potential ]1=FUwr/RT on the kinetic constants of formation of formaldehyde (a) and CO2 (b) during CH3OH oxidation on Pt/YSZ Conditions as in Fig. 8.45.50 Reprinted with permission from Academic Press.

Since the pressure build up is primarily due to the evolution of CO as MDI is being decomposed to carbodiimide, the thermodynamic relationship PV = nRT may be applied to convert the pressure profiles to plots of moles of CO2 generated vs. time. This is shown for the 225 °C isotherm in Figure 3. The theoretical curve obtained through the application of zero-order kinetics is also shown in this plot and the data seem to be well accommodated by this rate law throughout the majority of the run. 

One of the advanced concepts for capturing CO2 is an absorption process that utilizes dry regenerable sorbents. Pure sodium bicarbonate from Dongyang Chemical Company and spray-dried sorbents were used to examine the characteristics of CO2 reaction in a flue gas environment. The chemical characteristics were investigated in a fast fluidized reactor of 0.025 m i.d., and the effects of several variables on sorbent activity, including gas velocity (1.5 to 3.5 m/s), temperature (40 to 70 °C), and solid concentration (15 to 25 kg/m /s)], were examined in a fast fluidized-bed. Spray-dried Sorb NX30 showed fast kinetics in the fluidized reactor. 

The reactivities of pure NaHCOa solid. Sorb NHR, NHR5, and NX30 sorbents were examined in a fast fluidized bed reactor. The CO2 removal of the pure NaHCOa solid increased from 3 % to 25 % when the variables were altered. Removal increased as gas velocity was decreased, as the carbonation temperature was decreased, or as the solid circulation rate was increased. The CO2 removal of Sorb NHR and NHR5 was initially maintained at 100 % for a short period of time but quickly dropped to a 10 to 20 % removal. However, the Sorb NX30 sorbent showed fast kinetics in the fast fluidized reactor, capturing all of the 10 % of the CO2 in the flue gas within 3 seconds in the fast fluidized reactor. 

A Kinetic Study on the Solid State Grafting of Maleic Anhydride onto Isotactic Polypropylene in Supercritical CO2... 

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