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.