Scheme

\ products rr

As these schemes show, for metal or nonreducible oxide catalysts, excess oxygen in the gas phase means that the catalyst surface is well-covered with oxygen and that little if any VOC is adsorbed. Thus, the Eley-Rideal mechanism is expected to be important. For metal oxide catalysts containing readily reducible metals, the Mars-van Krevelen mechanism is important.

Metal oxides that are n-type semiconductors are rich in electrons and are generally not highly active as oxidation catalysts. Vanadium pentoxide is the notable exception. In contrast, p-type semiconductors are conductive because of the electron flow into positive holes. The electron-deficient surfaces of such metal oxides readily adsorb oxygen, and if the adsorption is not too strong, they are active catalysts. Insulators, which are inexpensive and not friable or thermally unsuitable, have values as supports for more expensive, catalytically active metal oxides or noble metals.

For all mechanisms, a key factor is the strength of the interaction between the surface and the oxygen (atom, molecule, or ion) required for oxidation of the VOC. If the oxygen is too tightly bound to a surface, that surface is not highly active as a catalyst. Similarly, if the interaction is too weak, the surface coverage with oxygen is low, and the catalytic activity is consequently diminished. Various thermodynamic properties are considered as the parameter that best represents the strength of adsorption. For metals, the initial heat of oxygen adsorption is a reasonable choice (Bond 1987). For metal oxides, the reaction enthalpy for reoxidation of the used catalyst (Mars-van Krevelen mechanism) is considered the most representative (Satterfield 1991). The maximum rate for VOC oxidation over an oxide catalyst is estimated to occur when the reaction enthalpy for reoxidation of the catalyst is one-half of the reaction enthalpy for total oxidation of the VOC. Thus, the selection of a catalyst for an application depends strongly on the nature of the VOC to be destroyed, and the conditions depend on the destruc-tabilities of the VOC (see Table 5.20.9).

Complete oxidation of each VOC in a multicomponent stream is ensured with high temperatures and excess oxygen, and mixed-oxide catalysts are frequently used. The mixed oxides, especially when promoted with alkali or alkaline earth metal oxides, frequently have activities that are different from the combination of properties of the components, and in general the activities are higher. This phenomenon probably arises from two factors: the availability and mobility of the different forms of available oxygen and the accessibility of different binding sites with various energy levels.

More than one type of surface oxygen species can be involved: adsorbed dioxygen (O2), ions (O2~; Of"), or radical ions (O_; O2) on the surface or incorporated into the lattice of the catalyst. Sachtler (1970) and Sokolovskii (1990) review the roles of various forms of adsorped oxygen.

Kinetics

The following equations summarize the steps required for destruction of a VOC by an Eley-Rideal mechanism:

where [ ] represents a surface site and [Sa]i represents the ith in a series of partially oxidized species Sa at the surface of the catalyst. The following rate expression is derived from the preceding mechanism:

Region C:

catalytically initiated homogeneous combustion

Region B: mass-transfer control

Region C:

catalytically initiated homogeneous combustion

Region B: mass-transfer control

Combustor Temperature

FIG. 5.20.16 Overall reaction rate. (Reprinted, with permission, from D.L. Trimm, 1991, Catalytic combustion, Chap. 3 in Studies in inorganic chemistry 1991 11:60.)

Combustor Temperature

FIG. 5.20.16 Overall reaction rate. (Reprinted, with permission, from D.L. Trimm, 1991, Catalytic combustion, Chap. 3 in Studies in inorganic chemistry 1991 11:60.)

kaPoP

kbPo2 + vkcPvoC

where ka, kb, and kc are constants and v is the stoichio-metric coefficient of oxygen in the overall oxidation reaction of the VOC. Under conditions of excess oxygen where kbPo2 vkcPVOC, this equation reduces to the following approximate form:

For most applications, the kinetics are described by the following fractional power expression:

b O2 VOC

in which a and b are fractional coefficients with values close to zero and unity, respectively.

The oxidation of a VOC can occur at the catalyst surface and in the gas phase. The overall reaction rate is the sum of these two components and is a strong function of temperature (see Figure 5.20.16) (Prasad 1984, Trimm 1991). The catalytic oxidation of hydrocarbons over supported metal catalysts is thought to occur via dissociative chemisorption of the VOC, followed by reaction with coadsorbed oxygen at the surface and then desorption of the combustion products (Chu and Windawi 1996). The rate determining step in this Langmuir-Hinshelwood type of mechanism is hydrogen abstraction from the VOC. Thus the ease of oxidation of the VOC is directly related to the strength of the C-H bond. Methane is more difficult to oxidize than other paraffins, aromatics, or olefins, and oxygenates are relatively easier to oxidize.

Reactors

The reactor's heat requirement heat arises mainly from the need to preheat the inlet gases or to heat the catalyst bed. For efficient operation of a catalytic oxidation system, the exhaust heat must be recovered and used to preheat the feed, as shown in Figure 5.20.17 for a system manufac r =

permission, from Salem Engelhard.)

tured by Salem Engelhard (South Lyon, Mich.). The residual heat is recovered by a secondary heat exchanger and used for area heating or other purposes requiring low-grade energy.

In some cases, destruction of all VOCs and intermediates requires a higher operating temperature than that required to combust a single VOC (see Figure 5.20.18).

The waste gases from several chemical, printing, or related industries contain mixtures of halogenated and non-halogenated VOCs. Converting each VOC requires a combination of catalysts. Further, scrubbing the effluent from the reactor is necessary to remove the acidic components generated. As an example of such a system, the catalytic solvent abatement (CSA) process designed and marketed by Tebodin V.B. is shown in Figure 5.20.19.

82 so lu

82 so lu

0 0

Post a comment