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A schematic o f the experimental set-up is shown in Figure 4.6. A HPLC pump (Knauer K-120, 50 ml pump head) was used to feed the reaction mixture into the reactor. Prior to entering the reactor, the feed mixture was passed through a water bath (Grant W14) to bring it closer to the reaction temperature. A syringe pump (Razel A-99) was connected to the other end o f the 3-way valve. It was used to clean the system with ethanol before shutdown. The HPLC pump was not used for cleaning purposes as there would be a volume o f ethanol left inside it, which would need to be

p u r g e d t h r o u g h t h e s y s t e m b e f o r e r u n n in g a n y e x p e r im e n t s t h u s le n g t h e n i n g sta r t-u p time. H ydrogen - O k } P R otam eter Reactor

a

Figure 4.6: Schem atic o f the exp erim en tal set-up

The hydrogen flowrate through the system was controlled using a rotameter (Cole Palmer). The gas line was connected to the top gas port o f the reactor. The bottom gas port was blocked off. This was done to maintain pressure in the reactor, and to use the gas pressure within the reactor to push out the product fluid from the reactor. A nitrogen line was also connected later on for use as purge gas. A needle valve was used to control the outlet flowrate. This was necessary to maintain pressure within the reactor as well as to prevent flooding. Product was collected in a sample bottle for analysis using a gas chromatograph (Agilent 6890).

Stainless steel tubing was used throughout the system, except for the HPLC pump inlet (PTFE tubing), the product outlet (glass), the gas lines (copper) and the water bath inlet and outlet (silicone).

4.4.1 Heating

Because temperatures no greater than 80 °C would be used, water was used as the heating fluid. First it had to be determined whether the temperature o f the water bath would match the temperature o f the reactor. This was done by inserting a themocouple (industrial mineral insulated probe, type K) into the reactor from the product outlet. It was found that there was a discrepancy 3-6 °C over a temperature range o f 50-70 °C. Therefore, the water bath temperature was kept higher to maintain the reactor at the desired temperature. Distilled water was used to avoid scale build­ up, both in the water bath and in the heating channels o f the reactor.

4.4.2 Liquid Flow Testing

Tests were run using both water and ethanol. Liquid flowing down the channels could be clearly seen in both cases. It was observed that the side channels would be wetted before the middle channels, but the flow would quickly become evenly distributed. As predicted by the film thickness calculations presented earlier, a flowrate o f 3 ml/min resulted in flooding o f the channels. Note however that the flooding occurred only in the middle channels.

Further testing was done using ethanol, as that was the solvent to be used in the reaction. Tests were made with hot water passing through the system (50-80 °C) and a system pressure o f 4 bar. While ethanol has a boiling point o f 78.4 °C at atmospheric pressure, there was a high chance that it would evaporate at temperatures lower than that due to the thin films involved even though the pressure was higher than atmospheric. This was indeed found to be true as shown in the Table 4.5.

Tem perature (°C ) M inim um flow rate b efore dryout (m l/m in)

50 0.03

60 0 .0 7

70 0 . 2

80 0 . 2

T able 4.5: M inim um flow rates b efore dryout at various tem peratures at 4 bar

An unforeseen phenomenon also occurred: at higher flowrates (above 2 ml/min), the liquid flow was not smooth, i.e. liquid would periodically spurt out from the entrance o f the falling film section thus resulting in a thicker liquid film in the centre o f the reactor. This spurting was caused by the fact that both gas and liquid phases shared the same outlet, and that the gas was used to push the liquid out o f the reactor. If the outlet valve was not opened wide enough, liquid would build up faster than the gas would be able to push it out, leading to flooding. However, if the valve was opened too wide, the gas would flow too rapidly through the reactor and cause problems for liquid delivery. Therefore, it was necessary to balance the opening o f the outlet valve. But, this was more difficult at higher pressures. The spurting essentially resulted from the following series o f events:

a) liquid would build up at the outlet and trap the gas in the reactor, thus building up pressure

b) when pressure build-up exceeded the head o f liquid trapped at the outlet, the gas would push the liquid out

c) the sudden drop in pressure would cause the liquid feed to spurt into the reactor, and restart the cycle

This occurrence was likely to cause some experimental error, especially at higher flowrates and pressures.

4.4.3 Estimation o f Reactant Concentration

A simple model was used to estimate the nitrobenzene concentration that would be suitable for this system, based on kinetic and experimental parameters found in Bartholomew, et al (1997). Each channel o f the falling film reactor can be described essentially as a plate reactor. Therefore, the falling film reactor can be assumed to be made up o f several plate reactors operating in parallel. The liquid reactant flows in one direction. At the same time, the hydrogen pressure is kept constant. As dissolved hydrogen reacts with the liquid reactant on the catalyst, more hydrogen dissolves thereby maintaining the equilibrium concentration o f hydrogen at the liquid surface. As such, there is constant renewal o f the hydrogen supply at the gas-liquid interface. Assuming the reaction is first order with respect to hydrogen, a model which satisfies this condition was proposed by Gobby, et al (2001) (see Appendix 4-1 for the method used). The parameters used for the calculation are shown in Table 4.6.

C hannel w idth (m ) 0.0003

C hannel length (m ) 0.065

N um ber o f channels 64

V olu m etric flow rate (m^/s) 6.67 X 10'^

L iquid v is c o sity (Pa s) 8.57 X 10"^

L iquid den sity (kg/m^) 815

D iffiisiv ity o f hydrogen in ethanol (m^/s) 1.05 X 10'^

R eaction rate constant (s ')^ 4.158

G ravitational acceleration (m/s^) 9.81

Initial concentration o f nitrobenzene (m ol/m ^)

400

S y stem pressure (atm) 7

Henry's constant (m ol m^/atm) 1.46

R esid en ce tim e (s) 9.3

T able 4.6: F allin g film calcu lation parameters (p h ysical constants, d iffiisivity, reaction rate constant

obtained from Farrauto, et al, 1997)

Based on the parameters above, the amount o f aniline produced would be 16.1 mol/m^ or a conversion o f 4 %. Hydrogen was the limiting reactant. Even though this result was rather low, it was decided to run the experiments with an initial nitrobenzene

concentration o f 0.4 mol/1 ju st to confirm the results. If the experimental conversion was as low as predicted, then further experiments would be performed using nitrobenzene coneentration o f 0.04 mol/1.

4.4.4 Analysis Method

Product analysis was perfomed using an Agilent 6890 GC with an auto-sampler. A fused silica capillary column (Rtx-5, crossbond 5% diphenyl - 95% dimethyl polysiloxane, 7 m, 0.32 mm ID, 0.25 pm df, Thames Restek) was used. Analysis was earried out using a TCD detector. The analysis recipe and calibration method can be found in Appendix 4-2 and 4-3, respectively.

4.4.5 Experimental Procedure

Experiments were conducted at 1-8 bar pressure, 60-70 °C, 0.04-1 mol nitrobenzene/1 concentration and flowrates o f 0.2-3 ml/min nitrobenzene solution (99.5%, Fluka, diluted in ethanol). Blank experiments carried out to test the catalytic activity o f stainless steel concluded that no discernible reactions took place. In total, seven catalyst plates were examined: two each for the sputtered, impregnated and incipient wetness catalysts and one for the UV-decomposed catalyst. Nitrobenzene conversion, X, and aniline selectivity, S, were calculated by:

Moles nitrobenzene reacted

X =

s =

Moles nitrobenzene entering Moles aniline produced Moles nitrobenzene reacted