Due to the previously mentioned advantages and benefits, we applied microwave irradia­tion technique to the synthesis of ETBE from EtOH and TBA. Microwave-assisted experi­ments were performed using various microwave apparatuses working at 2.45 GHz frequency, with a power programmable from 0 to 1000W.

1.2. Batch experiments under atmospheric conditions

At first, experiments were carried out in batch mode under atmospheric pressure using the apparatus, shown as an actual image in Figure 5.

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Figure 5. Microwave apparatus for batch synthesis of ETBE under atmospheric conditions (Shikoku Instrumentation Co., Ltd.)

In a typical run, about 0.25mol of EtOH and TBA, and 20 g of catalyst were placed in a reac­tor vessel, and heated using a microwave apparatus described above. Amberlyst 15, an ion exchange resin in H+ form was used as catalyst, unless otherwise specified. GC-FID appara­tus equipped with a CP-Sil 8CB-MS (60mx0.25mm, df=0.25) column for component separa­tion was used for the analyses of the products. Isopropanol was used as an internal standard.

Figure 6 shows typical experimental results obtained under atmospheric conditions. Experi­ments at atmospheric pressures using a batch reactor showed that the yield hardly increased above the 20% level. Similar results were obtained by other researchers [20,21], and the low­
er yield was likely due to the selective dehydration of TBA to IB, a highly volatile com­pound that easily escaped from the reaction zone. The maximum attained temperature was 80 oC, corresponding to the boiling point of the mixture.

Experiments under atmospheric conditions were extended to a continuous flow system us­ing the apparatus shown in Figure 7. A Masterflex digital pump was used to deliver the re­actants into the glass reactor filled with about 50g Amberlyst 15 catalyst. The temperature was set at 70 oC. Flowrates were varied to study the effect of residence times. The residence time of the reactants inside the reactor was calculated based on the reactor void space vol­ume and the flowrates. Products were collected continuously after certain time has elapsed, and until the system reached equilibrium.

Results in Figure 8 show that the yield increases with increasing residence time, getting maximum yield of about 30% at a microwave duty of 13%. Increasing the duty to 20% did not have any significant effect on the yield. The maximum yield of 30% obtained using this method was in agreement with our previous studies on reactive distillation [14] and the re­sults obtained by other researchers [20]. This low yield of ETBE was likely due to the selec­tive dehydration of TBA to IB, a highly volatile compound that easily escaped from the reaction zone. If IB could be allowed to further react with EtOH to produce ETBE, better yield could be obtained. Thus performing the experiments in a sealed reactor vessel was thought to be effective in overcoming this limitation under atmospheric conditions.