(26b) Process Intensification of Gas-Liquid-Solid Reactions in the Production of Fine Chemicals With Milli Packed Bed Reactors

Process intensification of gas-liquid-solid reactions in the production
fine chemicals with milli packed bed reactors

1. Introduction

Flavors and fragrances are usually produced by
selective liquid phase hydrogenations in batch stirred-tank reactors with
suspended catalyst powder. Disadvantages of these processes are an elaborate
catalyst separation, a varying product quality in each batch and an inefficient
heat transfer so that the educt must be diluted. The transformation of the
discontinuous processes into continuous ones using reactors with fixed catalysts and a high heat transfer capability could lead to a
more efficient and cheaper production, but requires novel miniaturized reactor
systems [1].

Milli packed bed reactors offer high mass and heat transfer rates
and seem to be a promising reactor design for the production of fine chemicals
by gas-liquid-solid reactions. The reactors consist of parallel flow channels with
inner diameters of a few millimeters which are filled with conventional catalyst
pellets. In dependence on the channel-to-particle-diameter ratio, dissimilar bed
geometries are obtained leading to different hydrodynamics and mass transfer
rates [2].

2. Main Objective

The aim of this work was the experimental investigation of overall
(gas-liquid-solid) mass transfer coefficients of hydrogen for milli packed bed
reactors with different bed geometries for different flow directions. Based on
this investigation, the reactor configuration with the highest mass transfer
coefficients was applied to study the selective hydrogenation of cinnamaldehyde
on Pd/Al₂O₃ catalysts in order to evaluate the
applicability of milli packed bed reactors as production unit for fine
chemicals.

3. Experimental

Four milli packed bed reactors with different bed geometries were
created by filling catalyst spheres with diameters of 0.8 mm and 1.6 mm,
respectively, in round channels with different inner diameters (d_CH:
1.0; 1.4; 2.0 mm). Schematic illustrations of the packing structures are shown
in Figure 1.

Figure 1      Schematic illustration of the investigated milli packed bed
reactors.

These reactors were experimentally investigated with regard to flow
patterns and overall mass transfer coefficients of hydrogen under reacting
conditions (liquid phase hydrogenation of α-methylstyrene; T:
343-423 K; p: 0.5-1.1 MPa) for different flow directions (downflow, upflow
and horizontal flow). The mass transfer studies were performed for a wide range
of superficial gas (0.009-1.18 m∙s^-1) and liquid (0.005-0.10
m∙s^-1) velocities.

The selective hydrogenation of cinnamaldehyde was performed in milli
packed bed reactors with a channel diameter of 1.0 mm and different reactors
with lengths between 0.25 m and 0.55 m in vertical downflow. The temperature
was set to 353 K and the pressure was varied between 1.1 and 3.0 MPa.

4. Results

It was found that mass transfer increased with raised fluid velocities
in all reactors. For the same catalyst particles (d_P= 0.8 mm), Reactor
1 offered the highest volumetric mass transfer coefficients (ka)_OV up
to 15 s^-1 compared to (ka)_OV= 4.9 s^-1 for Reactor
2 and (ka)_OV= 5.0 s^-1 for Reactor 3 at constant
superficial fluid velocities (Figure 2).

Figure 2      Influence of the bed geometry on
volumetric overall mass transfer coefficients for different superficial fluid
velocities.

The packing of Reactor 3 and Reactor 4 showed similar volumetric mass
transfer coefficients (ka)_OV, although the volumetric surface area
of Reactor 3 (a_OV= 3012 m^-1) is almost twice as large as
Reactor 4 (a_OV= 1717 m^-1). It can be concluded that the slug
flow regime, which is present in Reactor 4 and not in Reactor 3, offers higher
mass transfer coefficients k_OV compared to the film flow and bubble
flow regimes, which are dominate in Reactor 3.

Reactor 2 was used to study the influence of flow direction on
overall mass transfer coefficients under the same operation conditions. The
experiments showed that downflow offered the highest and horizontal flow mode the
lowest mass transfer coefficients for the whole range of superficial fluid
velocities. A variation of the flow direction in Reactor 1 gave no difference with
respect to the mass transfer coefficients. Such a behavior can be explained by
the strong impact of surface tension forces in small channels which make the
hydrodynamics independent on the flow regime.

Based on the results of the mass transfer studies, Reactor 1 was applied
to study the selective hydrogenation of cinnamaldehyde (CA) to hydrocinnamaldehyde
(H-CA) in downflow operation mode. Figure 3 illustrates the conversion of CA
and the selectivity of

H-CA related to CA in dependence on the superficial gas velocity for different
liquid velocities. It can be seen that conversion of CA increased with raised
superficial gas velocity for gas velocities below 0.2 m s^-1 and
decreases slowly for higher velocity. This observation suggests that the
external mass transfer is limited for hydrogen at low gas velocities and for CA
at high gas velocities. An increased liquid velocity led to smaller conversion
due to an

incline of the residence time. The selectivity of H-CA increased
with raised gas and decreased liquid velocity.

Figure 3      Influence of superficial gas velocity on the conversion of CA (open
symbols) and the selectivity of H-CA (closed symbols) for two different liquid
velocities (p= 1.1 MPa, T= 353 K, L_R= 0.25 m, 5 % w/w CA in toluene).

Many further reaction studies with different feed concentrations of
CA (5 % w/w < w_CA < 20 % w/w), hydrogen pressures
(1.1 MPa < p < 3.0 MPa) and reactor lengths (0.25 m < L_R
< 0.55 m) were performed in order to evaluate the applicability of Reactor 1
for fine chemical syntheses. The results showed that for a feed concentration
of 20 % w/w, full conversion of CA can be achieved in a reactor with a length
of 1.5 m (p= 3.0 MPa, T= 353 K, d_CH= 1.0 mm, u_G,S= 0.5 m
s^-1, u_L,S= 0.1 m s^-1). Considering a scale-up of
this reactor in order to produce 1 ton H-CA per year, only 3 parallel channels
with a diameter of 1.0 mm are required.

5. Summary/ Conclusions

In this work, milli packed bed reactors with different bed geometries
operated in 3 flow directions were analyzed with respect to overall mass
transfer coefficients under reacting conditions. The selective hydrogenation of
cinnamaldehyde was studied in the reactor with the highest mass transfer rates.
It can be concluded that milli packed bed reactors can be easily realized due
their simple geometry and already a small number of channels can produce
several tons per year which make them suitable for application in fine chemical
industry.

6. References

1.         E. H. Stitt, Chem. Eng. J. 2002, 90
(1-2), 47-60.

2.         L.
E. Kallinikos et al., Chem. Eng. Process. 2010, 49 (10), 1025-1030.