Page 274 - Modeling of Chemical Kinetics and Reactor Design

244 Modeling of Chemical Kinetics and Reactor Design

reaction kinetic study from laboratory reactors, which are useful in
scale-up. The design and construction of laboratory and pilot plant
reactors together with experimental programs can be both time con-
suming and expensive, therefore it is imperative that the correct choice
of laboratory reactor be used. Weekman [4] presented an excellent
review on the choice of various laboratory reactors. The criteria
employed to determine the various types of these reactors are listed
in Table 4-3.
Sampling of a two-fluid phase system containing powdered catalyst
can be problematic and should be considered in the reactor design. In
the case of complex reacting systems with multiple reaction paths, it
is important that isothermal data are obtained. Also, different activation
energies for the various reaction paths will make it difficult to evaluate
the rate constants from non-isothermal data.
Measurements of the true reaction times are sometimes difficult to
determine due to the two-phase nature of the fluid reactants in contact
with the solid phase. Adsorption of reactants on the catalyst surface
can result in catalyst-reactant contact times that are different from the
fluid dynamic residence times. Additionally, different velocities between
the vapor, liquid, and solid phases must be considered when measuring
reaction times. Various laboratory reactors and their limitations for
industrial use are reviewed below.

DIFFERENTIAL REACTOR

The differential reactor is used to evaluate the reaction rate as a
function of concentration for a heterogeneous system. It consists of a
tube that contains a small amount of catalyst as shown schematically
in Figure 4-17. The conversion of the reactants in the bed is extremely
small due to the small amount of catalyst used, as is the change in
reactant concentration through the bed. The result is that the reactant
concentration through the reactor is constant and nearly equal to the

Table 4-3
Criteria used to determine laboratory reactors
1. Ease of sampling and product composition analysis.
2. Degree of isothermality.
3. Effectiveness of contact between catalyst and reactant.
4. Handling of catalyst decay.
5. Ease of construction and cost.