Page 416 - Chemical process engineering design and economics

396 Chapter 7

6. Calculate the reactor length, L R, from Equation 7.11.7. Round off L R in 3 in (0.25 ft;
0.0762 m) increments (for example, 5.0, 5.25, 5.5, 5.75 etc.).
7. Calculate the reactor pressure drop, Ap, from Equation 7.11.2.

8. Calculate the actual reaction volume from Equation 7.11.6, using the corrected bed di-
ameter and length.

9. Calculate the catalyst weight, W B, from Equation 7.11.5.

Finally, estimate the pressure drop across the bed to complete the design of
the reactor system. To promote uniform flow distribution across the bed, Tram-
bouze et al. [8] recommend a pressure drop per unit length of bed of at least
2500 Pa/m (0.11 psi/ft). To the pressure drop across the bed, add an additional
pressure drop equivalent to about 3 ft (0.914 m) of bed height [21] to account for
pressure losses caused by the vessel nozzles, distributor (balls or other devices),
and bed supports, if needed.
Example 7.3 Packed-Bed, Catalytic, Reactor Sizing Using Space Velocity

In 1973, because of a natural gas shortage, the US evaluated two methods of
transporting natural gas from overseas producers. One method was to liquefy the
natural gas (LNG). LNG is produced by well established processes and then
shipped in cryogenic tankers at -161 °C (-258 °F). The other method was to con-
vert the natural gas to methanol, as discussed by Winter and Kohle [26], by a
process similar to the one described in Chapter 3. Then, the methanol would be
shipped to the US and converted back to methane in two catalytic reactors in se-
ries. The first reactor converts methanol to a mixture of gases, which contains
methane. The composition of the gases leaving this reactor, which is given in Ta-
ble 7.3.1, becomes the input to the second reactor. In the second reactor, some of
the carbon monoxide and dioxide in the mixture is converted to additional meth-
ane. Table 7.3.1 gives the gas analysis out of the second reactor.
After the second reactor, the methane is separated from the mixture before
entering the natural-gas pipeline. Estimate the reactor size using the space veloc-
ity given below.

Data (Source: Ref. 27).
Catalyst nickel deposited on kieselguhr
Catalyst size 1 /8 in tablets (3.18 mm)
Bed void fraction 0.38
3
Bulk density 901b/ft 3 (1440 kg/m )
Space velocity 3000 h" 1 (at 60 °F, 1 arm) (289 K, 1.01 bar)
Molecular weight in 20.4

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