Methane thermal dissociation

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Dahl, J. et al., Solar thermal dissociation of methane in a fluid-wall aerosol flow reactor, Int. J. Hydrogen Energ., 29, 725, 2004. 

One example would be ice melting or methane hydrate dissociation when rising in seawater. Convective melting rate may be obtained by analogy to convective dissolution rate. Heat diffusivity k would play the role of mass diffusivity. The thermal Peclet number (defined as Pet = 2aw/K) would play the role of the compositional Peclet number. The Nusselt number (defined as Nu = 2u/5t, where 8t is the thermal boundary layer thickness) would play the role of Sherwood number. The thermal boundary layer (thickness 8t) would play the role of compositional boundary layer. The melting equation may be written as... 

In the US, two programs were reported High Temperature Solar Splitting of Methane to Hydrogen and Carbon, and Rapid Solar-thermal Dissociation of Natural Gas in an Aerosol Flow Reactor. 

Figure 4.18. Experimental data showing how the thermal dissociation rate for methane (as measured by the C uptake) decreases rapidly as S atoms cover the 4% of steps on a Ni(14 13 13) surface. Adapted from Ref. [60].

Thermal Dissociation of Methane using a Solar Coupled Aerosol Flow Reactor University of Colorado/NREL... 

Thomas D.J., ZachosJ.C., BralowerT.J., Thomas E., Bohaty S. (2002) Warming the fuel for the fire evidence for the thermal dissociation of methane hydrate during the Paleocene—Eocene thermal maximum. Geology 30, 1067— 70. 

Demonstrate a pilot scale solar-thermal transport tube reactor process to thermally dissociate methane to hydrogen (H2) and carbon black. 

Dahl, JK, "Solar-thermal Dissociation of Methane in an Aerosol Flow Reactor," paper presented at the Fourth World Congress on Particle Technology," Sydney, Australia, July 25, 2002 (Boulder). 

Weimer, AW, "Distributed Hydrogen by Rapid Solar-thermal Dissociation of Methane," paper to be presented at the 224th American Chemical Society Meeting, Boston, MA (August 21, 2002). 

Weimer, AW and Eewandowski, S, "Thermal Dissociation of Methane Using a Solar-coupled Aerosol Flow Reactor," U.S. Department of Energy Hydrogen Program Review Presentation, Golden, CO (May 7, 2002). 

Figure 4.4 shows some typical simulation results obtained for the thermal dissociation of molecular clusters of nitrogen or methane. These molecules are simple enough for the rotational densities of states to be computed exactly in the approximations where the dissociation products are both spherical (methane), or spherical and linear (nitrogen). The details of these simulations, including the intermolecular potentials, are given in ref. 33, but it is important to... 

Primary dissociation is followed by thermal dissociation of the acetyl radicals (secondary dissociation), so that ethane and carbon monoxide are the main products of the vapor phase photolyses. When the reaction is carried out at low temperature, with higher energy light filtered out, the acetyl radicals which are produced are sufficiently long lived so that their participation in coupling and disproportionation reaction competes with decarbonylation. Under such conditions formation of biacetyl, acetone, methane, and formaldehyde follow primary dissociation and the quantum yield of carbon monoxide decreases ( co = 1 at 2537 A and about 0.75 at 3130 A Noyes et al., 1956). 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

The occurrence of proton transfer reactions between Z)3+ ions and CHa, C2H, and NDZ, between methanium ions and NH, C2HG, CzD , and partially deuterated methanes, and between ammonium ions and ND has been demonstrated in irradiated mixtures of D2 and various reactants near 1 atm. pressure. The methanium ion-methane sequence proceeds without thermal activation between —78° and 25°C. The rate constants for the methanium ion-methane and ammonium ion-ammonia proton transfer reactions are 3.3 X 10 11 cc./molecule-sec. and 1.8 X 70 10 cc./molecule-sec., respectively, assuming equal neutralization rate constants for methanium and ammonium ions (7.6 X 10 4 cc./molecule-sec.). The methanium ion-methane and ammonium ion-ammonia sequences exhibit chain character. Ethanium ions do not undergo proton transfer with ethane. Propanium ions appear to dissociate even at total pressures near 1 atm. 

The dissociation energy for C-H bond in methane (E = 436 kj/mol) is one of the highest among all organic compounds. Its electronic structure (i.e., the lack of n- and n-electrons), lack of polarity, and any functional group makes it extremely difficult to thermally decompose the methane molecule into its constituent elements. 

When the temperature is too low for methoxy to dissociate thermally, it may react through a number of H abstraction reactions to form methanol. These include reaction with methane (R19) and with formaldehyde... 

Hydrate dissociation is of key importance in gas production from natural hydrate reservoirs and in pipeline plug remediation. Hydrate dissociation is an endothermic process in which heat must be supplied externally to break the hydrogen bonds between water molecules and the van der Waals interaction forces between the guest and water molecules of the hydrate lattice to decompose the hydrate to water and gas (e.g., the methane hydrate heat of dissociation is 500 J/gm-water). The different methods that can be used to dissociate a hydrate plug (in the pipeline) or hydrate core (in oceanic or permafrost deposits) are depressurization, thermal stimulation, thermodynamic inhibitor injection, or a combination of these methods. Thermal stimulation and depressurization have been well quantified using laboratory measurements and state-of-the-art models. Chapter 7 describes the application of hydrate dissociation to gas evolution from a hydrate reservoir, while Chapter 8 describes the industrial application of hydrate dissociation. Therefore in this section, discussion is limited to a brief review of the conceptual picture, correlations, and laboratory-scale phenomena of hydrate dissociation. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

The principal methods of gas activation are thermal and electrical much less common are chemical and photochemical activation. In the most commonly used thermal activation technique - the hot filament technique - a W or Ta wire is arranged in the immediate vicinity of the substrate to be coated by diamond (Fig. 1). The wire is heated until it reaches the temperature when H2 molecules dissociate readily. The gas phase is a mixture of a carbon-containing gas (e.g. methane, acetone or methanol vapor), at a concentration of a few per cent, and hydrogen. Upon the contact of the gas with the activator surface, excited carbon-containing molecules and radicals are produced, in addition to the hydrogen atoms. They are transferred to the substrate surface, where deposition occurs. Table 2 gives an indication of the hot-filament deposition process parameters. 

A tetrakis(trimethylphosphine)ruthenium complex of benzyne has been prepared6 26 by a reaction similar to that used for the Group 4 and 5 metals thermally induced /3-hydride elimination of methane or benzene from 1 or 2, respectively [Eq. (3)]. A careful study of the kinetics of the elimination of methane from 1 revealed that dissociation of trimethylphosphine pre-... 

The mechanism of thermal decomposition (pyrolysis) of methane has been extensively studied.11 Since C- H bonds in methane molecule are significantly stronger than C-H and C-C bonds of the products, secondary and tertiary reactions contribute at the very early stages of the reaction, obscuring the initial processes. It has been shown11 that the homogeneous dissociation of methane is the only primary source of free radicals and controls the rate of the overall process ... 

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