LIQUID PHASE HYDROGENATION IN A STRUCTURED
MULTICHANNEL REACTOR
Xiaolei Fan, A.A. Lapkin and P.K. Plucinski
x.fan@bath.ac.uk; a.lapkin@bath.ac.uk; p.plucinski@bath.ac.uk
Catalysis and Reaction Engineering Group, Department of Chemical Engineering,
University of Bath, Claverton Down, Bath BA2 7AK, UK
Topic 6: Structured catalysts and reactors in intensification of industrial processes.
Abstract
Advances in the field of process intensification have led to the development of compact heat
exchangers and reactors. These consist of multichannel structures that may have individual
channels with hydraulic diameters in the range of 0.5 to 5 mm. These compact structures
provide high rates of heat transfer even when two-phase flow conditions prevail in the
channels.
In our group, a structured, multifunctional packed bed catalytic reactor for the applications in
the pharmaceutical and speciality chemicals industries has been developed. The reactor
consists of an integrated micro-heat exchanger, gas/liquid mixing zone, and a reactant
injection system. The reactor was successfully used in our group for partial oxidation of
organic feedstocks.
In this study, we expanded the application scope of the structured reactor to the three phase
catalytic hydrogenation reactions, using a model substance benzyl aldehyde and Pt/C (3 wt %)
catalyst. Up to 50% yield was attained in a single pass channel (10 cm length of single
channel) at a liquid phase residence time of ca. 10 s, proving the effectiveness of the designed
reactor. Integrated heat exchange function ensured isothermal mode of operation, in spite of
exothermic reaction (Hr = - 67 kJ mol-1). By controlling the point of injection of hydrogen
into the reactor an increase in the yield of hydrogenation was achieved using two channels in
consecutive mode (Figure 1).
70
0.14
0.12
60
0.10
50
0.08
40
0.06
30
0.04
Channel arrangement C
Channel arrangement B
Channel arrangement A
0.02
0.00
20
Channels arrangement:
Yield / %
BALC concentration / mol L
-1
0.16
C
B
A
10
0
0
2
4
6
8
10
Gas flow rate / mL(STP) min
12
-1
Figure 1. The yield of benzyl alcohol as a function of hydrogen flow rate for different
hydrogen injection modes (p = 8 bar, T = 318 K, FL = 1 mL min-1, cbald,0 = 0.2 mol L-1.
Designed compact reactor proved to be an excellent kinetic tool: the kinetics of three-phase
hydrogenation was evaluated showing: (i) reaction limitation for applied reaction conditions, (ii)
Langmuir-Hinshelwood mechanism of hydrogenation, and finally (iii) the dominating role of
adsorption of reactant in the mechanism of hydrogenation by higher temperatures (Figure 2).
50
increase temperature
-3
1.6x10
-3
1.4x10
-3
1.2x10
-3
1.0x10
-3
8.0x10
-4
6.0x10
-4
4.0x10
-4
40
30
20
decrease temperature
310
320
330
340
350
-1
10
300
-1
1.8x10
Average reaction rate / kmol kgcat s
Yield of benzyl alcohol / %
60
Reaction temperature / K
Figure 2. The influence of temperature on the yield of benzyl alcohol (p = 8 bar, FL = 1 mL
min-1, FG = 16 mL(STP) min-1)
Finally, the designed structured reactor has been successfully used for a more complex
continuous chemical synthesis comprising a Heck cross-coupling reaction followed by the
hydrogenation of the formed unsaturated bond (see reaction scheme below). In this last
process Pd/C catalyst was used (5 wt %). The exemplary results are shown in Table 1.
Table 1. The results of the flow chemistrya in the compact reactor for a multi-step organic
synthesis.
Pressure
[bar]
4
8
Solvent
EtOH
EtOH
PhI conversion
[%]
100
100
Selectivity [%]
trans/cis-stilbene 3/4 Dibenzyl 5
intermediate
product
0
0
83
83
benzene
byproduct
13
12
a
reaction conditions: initial concentration of 1 = 0.4 mol dm-1, reaction temperature 120 ºC,
FL = 0.25 ml min-1, FG = 8 ml min-1.
In conclusion, it was shown that the developed structured multichannel reactor shows
considerable promise for the catalytic hydrogenation of organic feedstocks as well as for
continuous multi-step organic synthesis. This, together with the earlier results for catalytic
oxidation shows unequivocally the great potential of structured, compact reactors for
continuous synthesis of pharmaceutical intermediates and specialty chemicals.