Professional Reference Shelf 

 

1. Mass Transfer-Limited Reactions on Metallic Surfaces

In this section we develop the design equations and give the mass transfer correlations for two common types of catalytic reactors: the wire screen or catalyst gauze reactor and the monolith reactor.

 

       

   

A. Catalyst Monolith. The previous discussion in this chapter focused primarily on chemical reactions taking place in packed-bed reactors. However, when a gaseous feedstream contains significant amounts of particulate matter, dust tends to clog the catalyst bed. To process feedstreams of this type, parallel-plate reactors (monoliths) are commonly used. Figure CD11-1 shows a schematic diagram of a monolith reactor. The reacting gas mixture flows between the parallel plates, and the reaction takes place on the surface of the plates. In deriving the design equation we carry out a balance on a differential section of the reactor (Figure CD11-2).

 

 

Figure CD11-1

 

Catalyst monolith.

Figure CD11-2Top view of monolith.

Mole balance

for a monolith

catalyst

 (CD11-1)

       

   

where am is the catalytic surface area per unit volume of reactor and A cis the cross-sectional area normal to the direction of gas flow. <FONTThe rate of surface reaction is equal to mass flux to the surface. Taking the surface concentration equal to zero for mass transfer-limited reactions gives

 

       

   (CD11-2)

           Substituting Equation (11-71) into (11-70)  

and taking the limit as 0 yields       

   (CD11-3)

           In terms of volume         

   (CD11-4)

       

   The surface area per unit volume, a, for n plates is

 

       

    (CD11-5)

       

   

Typical spacings between the plates are usually between 0.005 and 0.01 m. The length ranges between 0.05 and 0.5 m and gas velocities between 5 and 20 m/s are not uncommon.

The mass transfer coefficient can be calculated from the correlation

 

       

Mass transfer

correlation for

a monolith catalyst

  (CD11-6)

           The approximate error in the correlation is 20%. Other

limitations of the correlation can be found in the article just cited by Arashi et al.1

 

For no volume change with reaction, Equation (CD11-4) can be integrated to give

       

   (CD11-7)

       

Ford and Chrysler

use monolith catalytic

afterburners

 

A variation of the monolith reactor has the gas flowing through square (or other shape) channels as shown in Figure CD11-3. This reactor is also known as a honeycomb reactor. Monolith reactors are used as catalytic afterburners on automobiles and are manufactured by Chrysler and Ford.2

 

 

(b)

 

Figure CD11-3(a) Honeycomb reactor; (b) catalytic afterburner. (Photo

courtesy of Engelhard Corporation)

       

   

B. Wire Gauzes Wire gauzes are commonly used in the oxidation of ammonia and hydrocarbons. A gauze is a series of wire screens, stacked one on top of another (Figure CD11-4). The wire is typically made out of platinum or a platinum-rhodium alloy. The wire diameter ranges between 0.004 and 0.01 cm.

Figure CD11-4Wire gauzes.

As a first approximation, one can assume plug flow through the gauze, in which case the design equation is similar to that for monolith reactors,

 

       Differential

form of the wire

gauze design

equation

 (CD11-8)

       

   where ag = total screen surface area per total volume of one screen, m2/m3 or in2/in3  

n = number of screens in seriesV = n (volume per screen)

The values of ag can be calculated from the equations3

   

where d = wire diameter, in.      N = mesh size, number of wires per linear inch

In calculating the volume of the screen, the thickness is taken as twice the wire diameter (i.e., 2d). The porosity can be calculated from the equation

 

       

     

       

   The mass transfer coefficient can be obtained from the correlation for one to three screens,

 

       

Mass transfer

correlation for wire

gauzes

 

(CD11-9)

(CD11-10)

           For one to five screens, the correlation is         

   (CD11-11)

     

   where is the minimum fractional opening of a single screen:

 

           (CD11-

12)       

   

In the commercial process for the oxidation of ammonia, typical parameter values are

conversion of ammonia.

 

   

When more than one or two screens are necessary, some backmixing takes place. Shimizu et al. 4 account for this backmixing by introducing dispersion in the axial direction:

 

       

   (CD11-13)

       

Too few screens?  

Equation (CD11-13) is then combined with Equation (CD11-8) and solved. When dispersion is significant it was shown that, depending on the flow conditions, 33 to 300% more screens were required than predicted by the plug-flow model.