Reactive Distillation Technology - Process Intensification for Green Chemistry - page 257

Da and low K EQ ; this represents a slow forward reaction and fast reverse reaction, which
essentially leads to no product formation. This class of process requires an optimally
designed reactor with a large holdup.
This technical feasibility analysis of RD processes and process limitations based on Da
and the chemical equilibrium constant is represented in Figure 9.6 [50].
The working regime of the process must be identified in order to confirm whether the
process is mass transfer or kinetically controlled and whether the reaction takes place only
in the bulk or also in the film. This allows both the requirements of internals for the RDC
and the modelling approach that must be applied in order to design the RD process to be
established. Identification of the working regime can be performed by using the Hatta
number (Ha), pseudo first-order rate constant (k f ) and product of mass-transfer coefficient
and interfacial area (k L a). The Hatta number is the ratio of the maximum possible
conversion in the film to the maximum diffusion transport through the film. For
higher-order reactions of two components, the Hatta number is defined as:

s

2

þ 1 k f C n 1

C B D A

n A

Ha ¼ (9.2)

k L

where n is the order of reaction ( ), k f is the forward reaction rate constant (1/s), C is the

concentration (mol/m 3 ), D is the diffusivity (m 2 /s) and k L is the mass-transfer coefficient
(m/s). A Hatta number less than one represents a slow RD process where the reactions
take place in the bulk, while a Hatta number greater than one represents a fast RD
process where the reaction taking place in both the bulk and the film. This identification
of the working regime based on the Hatta number is illustrated in Figure 9.6. The pseudo
first-order rate constant (k f ) and the product of the mass-transfer coefficient and
interfacial area (k L a) particularly determine whether the process operates under the
slow kinetic regime (k f < k L a), slow diffusion regime (k f > k L a) or slow mixed regime

(k f ¼ k L a). Depending on the working regime, a proper selection of the internals can be

conveniently performed [50]. Figure 9.6 also summarizes the internal and model
requirements for different working regimes. As clearly shown in this section, the
technical evaluation of any RD process can be quickly but systematically performed
based only on the Damkohler number, the chemical equilibrium constant, the Hatta
number, the pseudo first-order rate constant and the product of mass-transfer coefficient
and interfacial area [50].

9.6 Case Studies: RD

9.6.1 Biodiesel Production by Heat-Integrated RD

Biodiesel is an alternative, renewable and biodegradable fuel produced mainly from green
sources such as vegetable oils, animal fat or even waste cooking oils from the food industry.
However, waste raw materials can contain a substantial amount of free fatty acid (FFA), up
to 100%. Accordingly, the development of an efficient continuous biodiesel process is
required, in which the use of a solid catalyst is especially desirable in order to suppress the
costly processing and waste treatment steps. Note that the common problem of all