J5. Comparative raw material and utility consumption
4.2.1 Commissioning Runs and Modifications
Before the graphite could be conditioned and the apparatus commissioned, calibrations of the following were carried out:
• Argon gas rotameter
• Thermocouples
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• Vapourisation system (water bath temperature effect on feed composition)
It was decided to use the following compounds as reactants for the experimental runs:
• Aliphatic compound:
• Aromatic compound:
• Industrial waste sample:
methylene chloride (CH2Cl
v
1.2,4wlrichlorobenzene (C6H)CI)) lindane (C6H6C16)
Hexane was selected for the test runs and commissioning of the process. Hexane has a boiling point (68.7°C) close to that of methylene chloride (40°C) and would be suitable to test the operation of the vapourisation system.
On first testing the induction unit. it was found that the unit was unstable and the power would regularly cui out. It was also found that a pressure switch on the inlet and outlet cooling water lines
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did not function. On ex.amination of the unit, a defective circuit board was found. The repair and return of the board halted further commissioning work on the reactor.
An electrician was consulted to advise and check on safe working conditions of the induction unit and reactor.
The first test was carried out to condition the graphite tube 10 high temperatures. The induction unit was turned on for 20 minutes at 10% of its rated power output (7kW). A temperature of
lOoD e
was achieved in this time. It was important to raise the temperature very slowly to prevent the graphite from fracturing under sudden temperature fluctuations. The power output was then raised 10 2QkW and the graphite reactor tube heated to 1000°C. The healing rate for this period was 13°C per minute. Argon flow into the shell and reactor tube was introduced at a reactor temperature of 200°C. This was done to flush out the oxygen present in the system and to prevent the graphite from heating up too quickly. It was found that a large amount of water had collected in the space below the cooling water jacket. This water had been trapped in the graphite prior to its heating.The conditioning of the graphite was repeated until no more water was collected.
During these trials. it was found that the thermocouple signals to the PC card were very erratic
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while the induction unit was running. The strong magnetic fields around the electrical coils interfered with the signals and thus the temperature readings. The thermocouple cables were covered with aluminium tape and this reduced the interference caused by the magnetic field.
It was also found that the functionality of the variac was affected by the magnetic field. The variac was relocated away from the electrical coils and successfully heated the inlet pipe in this position.
The required cooling water pressure drop through the inductioQ unit and the induction coil set-up is I atm and the induction unit is automatically shut down by a pressure switch if the this pressure drop is not met. It was found that the mains water pressure was insufficient for this purpose when it was used for other processes at the same time. A water-cooling tower was commissioned and used solely for the pyrolysis process. The set-up is such that water for the cooling of the induction unit and the electrical coils can be supplied either from water mains or from the cooling tower. The water ejector vacuum pump and the cooling jacket downstream of the reactor are supplied by mains only and the water runs to drain.
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two oil vacuum pumps showed that the pumps were erratic and that the vacuum attainable was insufficient. A compressed air vacuum pump was then used, and with this, a maximum vacuum of 80%
was
achieved. It became a concern that the oxygen remaining in the tube could react with the sample. Also, it was only possible to analyse samples once a week due to'the availability of the instruments. Thus storage of the gas samples posed a problem. It was decided to modify the process such that the fihered product gas is bubbled through a hexane solvent before the hydrogen chloride is neutralised in the caustic soda scrubbers. A sufficient amount of solvent would dissolve all the other gases and can be easily stored in a refrigerator.During the commissioning runs, a leak was located at the flange at the reactor inlet and merely required tightening of the bolts to solve the problem. The tape enclosing the nichrome wire on the inlet line to the reactor burnt out after the second run, causing a short circuit. The line was re~taped, this time with two layers of glass tape. There has been no need to replace this tape again.
It was also noticed that some of the carbon produced had passed through the glass wool filter and was deposited in the scrubber. On subsequent runs, a thicker layer of wool was used in the filter pot and only traces of carbon were found in the scrubber.
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Larger amounts of carbon were found deposited at the end of the reactor, at the plug at the boltom end. This is dictated by the orientation of the reactor outlet pipe and could not be changed owing to spatial and time considerations. It was thus decided to use a vacuum on the end of the carbon filter and then at the reactor plug after each run to remove carbon that had settled in the horizontal section of pipe. It was noted that this set·up would affect the carbon mass balance.
The final process flow diagram is illustrated in Figure 4~ 10.
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OOOUN) WAttI'!CAAIONFlllER