4. EXPERIMENTAL WORK
4.1. E QUIPMENT
4.1.3. Heating options
A decision was taken to retain the QVF condenser which had been used in the previous plant at higher velocities to vaporise the gas, since its large available surface area was in excess of that required to heat the new plant‟s lower velocity stream. The disadvantage of using the QVF condenser as a vaporiser was that the glassware‟s thermal gradient tolerance restricted the oil
temperature to 150°C. Due to the large heat transfer area, this unit acted as a total vapouriser completely vaporising the HCl solution so that the concentration of HCl in the gas exiting the vapouriser was the same as that of the liquid solution that was pumped into the vapouriser.
The superheater used in previous testwork was replaced by a 5 mm (OD) glass tube which was wrapped in nichrome wire with 10 mm spacings. Unfortunately there was no automatic control on heat supplied to the nichrome wire and it was therefore controlled manually using a pt 100 temperature probe to measure the external temperature and a Variac to regulate the voltage.
The optimum spacing for the nichrome wire was determined through calculation; if the spacing was too wide the heat loss between the coils would be high and the vapour would lose heat more rapidly than it would gain it. If the windings were too close, the temperature would rise too high. Simulations were performed in MATLAB to determine the coil spacing which would provide sufficient heating surface while ensuring that the superheater was not overly sensitive to the change in variac voltage. Figure 8 shows the vapour exit temperature predicted at different coil spacing and different voltages. The equations used to determine the plots for this graph and the MATLAB code for this simulation can be found in Appendix A. It must be noted that the decrease in temperature is a result of the lack of heating in the final section of the tube and that the calculations are based on 2 m of wire which covers different tube lengths depending on the coil spacing.
Figure 8: Figure showing vapour exit temperature at different nichrome wire coil spacing
0 0.5 1 1.5
50 100 150 200 250 300 350 400 450 500 550
length (m)
Temperature (degC
0.005m, 40V
0.005m, 30V
0.005m, 20V
0.010m, 40V
0.010m, 20V 0.010m, 30V
0.015m, 40V
0.015m, 30V 0.015m, 20V
Length (m)
Temperature (ºC)
From Figure 8 it was apparent that coil spacings of 5 mm lead to gas temperatures which were strongly dependent on the voltage, reaching temperatures of 150, 300 and 500ºC as the voltage increased. While the 15 mm spacing did not show the large changes obtained when a coil spacing of 5 mm was used, the temperatures achieved would not have been high enough to prevent condensation of the gas phase. A spacing of 10 mm was selected as the temperature change was not as sensitive to voltage as the 5 mm spacing but it also achieved high enough temperatures to maintain a temperature above the saturated temperature of steam when voltages of 30V and higher were used.
It was concluded that the voltage provided to the superheater wire could successfully be used to control the temperature of the superheater glass and therefore the gas outlet temperature. This was demonstrated experimentally as shown in Figure 9 which shows the variation in the superheater glass temperature with time. When the Variac was set to 20V, a stable temperature approaching 120˚C could be achieved and when the Variac supply was changed to 30V the temperature increased steadily. The voltage could therefore be used to control the superheater temperature, which in turn set the reactor gas outlet temperature.
0 20 40 60 80 100 120 140 160 180 200
0 10 20 30 40 50 60 70 80 90
Time (minutes)
Temperature (degC)
20V 30V
Figure 9: Superheater temperature profiles at different voltages
The entire heating system was verified experimentally as follows: The QVF® condenser was put in place and connected so that the oil flowed into the bottom of the shell and out the top.
The water / acid solution was pumped into the condenser‟s coil inlet using a Teflon and glass connector, where it vaporised completely during its passage up the condenser. Thereafter the vapour exited the condenser via another Teflon plug which connected onto the superheater. The
vapour then passed through the superheater‟s glass tube which was surrounded by the nichrome wire coils and was well insulated. By regulating the voltage across the nichrome wire, the gas exit temperature could be controlled. Finally, the exit vapour was directed to the reactor and then through the condenser, where it was collected. The temperatures of the superheater, steam exit and oil exit were measured over a 90 minute period. The profiles are plotted in Figure 10.
0 20 40 60 80 100 120 140 160
0 10 20 30 40 50 60 70 80 90 100
Time (min)
Temperature (degC)
Superheater temperature Steam Temperature Oil Temperature
Figure 10: A temperature/time profile for the QVF® condenser and superheater over 90 minutes Figure 10 indicated that the temperature reached in the reactor (steam temperature) was at the desired level, i.e. in excess of 100°C, which is the saturated temperature of steam at atmospheric pressure.
This experiment showed that this equipment set-up, using the QVF® condenser from the previous plant as a vapouriser and a thin glass tube wrapped with nichrome wire as a superheater, was a feasible option for producing steam at the desired reaction conditions.
Difficulty with this set-up arose as there was no fine control on the Variac. The inability to source an acid resistant pt100 temperature probe meant that control was manual and based on the reading of a thermometer.