I.- Ping Chung and Steve Londerville
14. Pollutant Emissions
6.3 Process Details
6.3.1 Reactor Types
The reactor’s design and form depend on the type of catalyst, the operating temperature, the working pres- sure, and the conduction of the flow. For bulk catalysts, a cylindrical reactor cross section (Figure 6.6) is preferred for high operating temperatures. Experience has shown that the cylindrical design works best if the operation temperature is greater than 480°C (900°F) and tempera- ture difference will be greater than (ΔTc) 150 K (270°F).
The catalyst is arranged here as a packed bed layer and is normally placed on a grid with a wire screen. Under low thermal specifications (low internal stresses), reac- tors with square or rectangular cross sections can also be used. They can be designed as single-bed reactors
or, if larger cross sections are required, as multiple-bed reactors (see Figure 6.6). Horizontal arrangements can be chosen in cylindrical or, in compact facilities, rect- angular form. (This design is, however, limited by a maximum operating temperature of 480°C (900°F) and temperature difference (ΔTc) lower than 150 K = 270°F.) Catalyst beds in the form of a cylindrical ring are less common. This arrangement is susceptible to bed set- tling, which can cause undesirable marginal flows.
In facility designs that incorporate regenerative heat recovery, often the reactor is equipped with two sepa- rate chambers, each of which holds a catalyst bulk bed and a filling body packed bed as a regenerative heat transfer system.
Channeling in the perfusion flow and undesirable cavities at the reactor’s wall or within the packed bed decreases the system efficiency and can lead to prob- lems. Structural approaches to the prevention of chan- neling include a suitable sealing of the grids or of the wire cloth relative to the container’s wall. The wire cloth should be attached in packed beds such that the catalyst cannot be deformed or run off as a result of mechanical or temperature-dependent movements in the bed or in the reactor material.
Reactors constructed for monolithic (honeycomb and plate) catalysts are different from those constructed for bulk material catalysts. Monolithic catalysts are used either in cylindrical or in cuboids form. Unlike packed beds, no transverse mixing takes place inside a mono- lith. These designs require a uniform incoming flow and a successful flow distribution.
Cylindrical catalysts consist normally of a catalyti- cally coated metal matrix framed by a sheet-metal jacket.
With small waste gas volume flows in particular, such cylindrical catalysts are assembled into reactors by way of simple welded construction, by welding the sheet- metal jacket to the required pipeline part (cone, flange, etc.). This form offers the advantage of an especially low pressure drop, in any assembled position.
More expensive reactors permit the replacement of cylindrical catalyst modules without disassembling the reactor itself. The systems feature removable side- sections. One or several catalyst modules are inserted with a resilient sealing (“soft sealing”) and can be removed again at any time. The resilient sealing serves to space the elements apart and prevent bypass flows.
Cuboid catalysts are based either on metallic or ceramic monoliths (see Figure 6.7). Metal monolithic catalysts with rectangular flow cross sections are also available as modules with a sheet-metal jacket. In the event of larger waste gas volume flows, several of these modules are assembled into a rectangular flow cross section of any required size. If necessary, multiples of such a system can be arranged in sequence in the flow direction in “layers.”
Table 6.3
Examples of Minimum Inlet Temperature for Various Fresh Catalysts
Raw Gas Components
Raw Gas Components
Ethylene 300°C Dibutyl phthalate 275°C
Butane 290°C Pyridine 250°C
Butylene 230°C Dibutyl phthalate 275°C
Heptane 275°C Pyridine 250°C
Benzene 300°C Tributylamine 200°C
Toluene 270°C Dimethylformamide 230°C
Xylene 280°C Toluol diisocyanate 285°C
Naphthalene 270°C Chlorobenzene 350°C
Methane 450°C Chloroform 350°C
Methanol 190°C Thiophene 320°C
Formaldehyde 190°C Offset printing
solvent 260°C
Ethanol 210°C
Propanol-1 210°C CO 180°C
Pentanol-1 200°C HCN 250°C
Cresol 240°C Odor removal in the processing of Di-isobutyl
ketone 210°C onion/garlic 200°C
H2S and CS2 200°C Methyl ethyl
ketone 240°C Coffee 180°C
Carcasses in
rendering plants 200°C Butyric acid 200°C
Phthalic acid
anhydride 270°C NH3 270°C
Large kitchens,
restaurants From 20°C Malenic acid
anhydride 200°C
Catalytic Combustion 147
Ceramic catalyst monoliths (Figure 6.7) are available as standard with a flow cross section of 150 mm × 150 mm (5.9 in. x 5.9 in.). Normally, several such monoliths are packed with “soft sealing” in a frame or a sheet-metal jacket. The resulting module is used either as a complete catalyst layer in a reactor or as a component of one. Depending on the reactor’s con- struction, one or several catalyst layers may be used.
Both vertical and horizontal flow reactors are com- mon; in principle, tilted arrangements are also pos- sible. When assembling monolithic modules, leaks between the modules or cassettes and at the seating surfaces should be prevented structurally and by means of sealing.
Depending on the design of particular catalytic waste gas cleaning facilities, the excess heat released during the catalytic reaction should be removed by means of suitable systems. The design is determined by the type of primary heat recovery (regenerative or recupera- tive) or the installed waste heat usage, since unregu- lated operation may cause the catalyst to overheat.
To ensure a homogenous temperature profile across the entire catalyst bed, the reactor’s container should be thermally insulated in accordance with the specific requirements.
The choice of the reactor’s fabrication material is based on the gas composition (paying careful attention to the reaction products) and the design temperature, taking into account a possible temperature rise. When select- ing the material, consider acid formation in the process gas, especially through halogen-containing pollutants, which requires the use of special corrosion-resistant materials (e.g., austenitic steels). Table 6.4 contains some general suggestions for material selection.
The calculation of catalytic reactors proceeds on the basis of the specific conditions and problems associ- ated with any given case, the known waste gas data and the technical reaction quantities (kinetic and thermo- dynamic quantities). These can be determined experi- mentally for the particular components undergoing treatment, or found in manuals or tabulated reference books. Where, for example, a multicomponent mixture
B C
Other sizes are possible Module size
10 × 6 10 × 8 10 × 10 10 × 12 12 × 12 12 × 14 12 × 16 14 × 14 14 × 16
1.680 1.680 1.680 1.680 2.000 2.000 2.000 2.320 2.320
1.020 1.340 1.670 1.980 1.980 2.310 2.630 2.310 2.630
80–180 80–180 80–180 80–180 80–180 80–180 80–180 80–180 80–180 A in mm B in mm C in mm
Reactor with oxidation catalyst
Module for catalyst
Exhaust gas inlet Reactor
A
Figure 6.6
The arrangement is a catalyst facility consisting of ceramic monoliths.
148 The John Zink Hamworthy Combustion Handbook
of unknown composition is involved, such reactor calculations are only possible to a limited degree.
Therefore, sufficiently extensive measurements should be performed on the raw gas being cleaned. In practice, semi-empirical reactor designs are referenced. Often, the calculations are based on conversion-temperature curves obtained experimentally in testing facilities.
The conversion rate for particular substances is affected by factors such as temperature, space velocity and geometry, empty space velocity, catalyst type and active components, active catalyst surface, activity loss/
aging or catalyst poisoning, partial oxygen pressure, and partial water vapor pressure.
At the heart of the catalytic method lies the reactor that serves to hold the catalyst. The reactor’s design and dimensions should be established by the size and shape of the catalyst bed. The size and shape of the catalyst bed is determined by the specified process conditions, required reaction temperature curve, the permissible space veloc- ity, the permissible pressure drop over the catalyst bed, the selected catalyst type and reaction behavior.
Begin by selecting a suitable space velocity as a func- tion of the associated temperature. The space velocity and, thus, the catalyst’s content should be selected on the basis of factors such as catalyst type and geometry, required service life (operating hours), gas composition, possible catalyst poisons, required conversion rate, and accruing operating costs (fuel, catalyst, blower operation).
The space velocity RG is defined as the ratio of the total volume flow V˙ to catalyst volume VK:
RG V
=V
K
(6.5) The space velocity RG is, therefore, the volume flow that can be fed in 1 h over 1 m3 of catalyst, and is the param- eter that determines the facility’s size. The catalyst’s content is calculated as follows:
V V
K= RG
(6.6) V˙ in m3/h, VK in m3, and RG in m / h mN3 ( ⋅ 3) or in h−1
In the event of additional heating, the waste gas from the burner’s operation should be considered when determining the space velocity. The space velocity for pollutants that decompose with difficulty can be under 5000 h−1. Gases with normal and easily decom- posable pollutants can be cleaned with space velocities of 10,000–40,000 h−1 (even up to over 100,000 h−1 for an engine waste gas catalyst).
The catalyst bed’s design can be varied as a function of the available space and the reactor type. The catalyst’s leading surface A is obtained from the ratio of the total volume flow to the empty space velocity v:
A V
= v
(6.7)
A in m2, V˙ in m /h,N3 and v in m/s
The common values for empty space velocities are 0.7–1.5 m/s (2.3–4.9 ft/s). The bed height H is obtained from the catalyst volume VK:
H V
= AK (6.8)
H in m, A in m2, and VK in m3
The bed height H can be obtained also from the ratio of the empty space velocity v to the space velocity RG:
H v
= RG (6.9)
RG in h−1 and v in mN/h.
(c) (d)
(a) (b)
Figure 6.7
Reactor designs and flows: (a) Single-bed reactor, (b) vertical two-bed reactor (operating temperature > 480°C, ΔTc > 150 K), (c) horizontal cylinder two-bed reactor (operating temperature > 480°C, ΔTc >
150 K), and (d) multiple-bed reactor.
Table 6.4
Notes on Material Selection
Design Temperature in °C Construction Material
Up to 450 Ferritic steels
500 High-temperature ferritic steels
>550 Austenitic steels
>600 (700) High-temperatures austenitic steels
Catalytic Combustion 149
Gas velocity, catalyst form and dimensions, and bed height determine the pressure loss that, for example, a blower needs to overcome. A minimum pressure loss is necessary for adequate gas distribution. The pressure loss of the catalyst layers is usually determined experi- mentally, or stated by the catalyst’s manufacturer as a function of empty space velocity for the relevant cata- lyst type.