Page 420 - Chemical process engineering design and economics

400 Chapter 7

According to the rule given in Step 6 in Table 7.12, there is no need to round off
the reactor length.
From Equation 7.11.2, an estimate of the pressure drop is 0.11 psi/ft (0.0249
bar/m) of bed. Allowing for a pressure drop of 3 ft (0.914 m) of bed height for
internals, the pressure drop across the reactor,

Ap = 0.11 (22 + 3) = 2.75 psi (0.190 bar)

From Equation 7.11.6, the actual bed volume,

2
3
3
V B = 22.0 (3.142 / 4) (12.5) = 2700 ft (765 m )
Finally, calculate the catalyst mass from Equation 7.11.5.
5
5
W B = 90 (2700) = 2.430xl0 Ib (l.lOxlO kg)

Plug-Flow Reactor Model

First, select a reactor arrangement and catalyst configuration. The next step is to
select a reactor model for calculating the reaction volume. An exact model of
reactor performance must include mass transfer of reactants from the fluid to the
catalyst sites within the pellet, chemical reaction, and then mass transfer of
products back into the fluid. Table 7.13 lists the steps, and Figure 7.5 illustrates
the processes involved. Here, only simple models are of interest to estimate the
reaction volume for a preliminary design. The reaction volume is that volume
occupied by the catalyst pellets and the space between them. We must provide
additional volume for internals to promote uniform flow and for entrance and
exit sections. The total volume is called the reactor volume. After calculating the
reactor volume, the next step is to determine the reactor length and diameter.
A simple model is the one-dimensional, plug-flow, pseudo-homogeneous
model. In this model, we will consider the fluid and solid phases as a single
phase. For this model to apply we must fulfill the following conditions:

1. adiabatic operation
2. flat velocity profile
3. no axial dispersion
4. no radial dispersion
5. pseudo-homogeneous assumption

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