J5. Comparative raw material and utility consumption
5. RESULTS AND DISCUSSION
5.3 Pyrolysis of Methylene Chloride
5.3.2 Phase Two: Mass balance and collection system
5.3.2.2 Chlorine recovery
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porosity of the tube. This in turn allows more oxygen to be leaked into the reaction zone. As mentioned earlier, the seal around the thermocouple inserted in the shell had become loose. As a result. oxygen leakage into the reactor could have occurred before the broken seal was detected. At the ~nd of the experimentation, small pieces of ceramic were found collected in the bottom flange.
The ceramic pipe insulation on which the graphite tube rested had begun to fail. This pipe was intact on the last inspection carried out bt::fore experiments 9b-l were performed, and must have failed during the course of these runs. During carbon removal downstream of the reactor, the reactor is exposed to air that could have been sucked into the spaces where the ceramic had failed.
Similar oxygenated compound formation is reported by Van der Westhuisen [1994} for lindane pyrolysis.
Only three chlorinaled oxygen-containing compounds were indentified: tri- and tetra-chlorophenol and butylated hydroxy toluene. Both were identified as trace quantities. This suggests that cleavage of all C-CI bonds took place before any radical recombination with oxygen occurred. It would thus be suitable to have an excess of hydrogen atoms available to combine with chloride radicals to form hydrogen chloride rather than for chloride radicals to combine with oxygen radicals to produce peroxides, ethers, a1cohols and carboxylic acids.
Chlorinated naphthalenes and biphenyls were also detected. It is likely that the fOOnation of these compounds could be prevented by introducing hydrogen to increase the H:CI ratio in the reactor.
This should prevent chlorination of naphthalenes and biphenyls as well as inhibit soot formation.
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Run Temperature % Clin % Clin Scrubber set-up
(d.g C) product scrubber
9b 1222 0.02 10.2 I scrubber, vacuum
9d 1032 1.21 37.1 I scrubber, vacuum, increased liquid level 9. 1385 0.02 49.5 I scrubber, vacuum, increased liquid level 9f 854 1.42 12.0 I scrubber, vacuum, increased liquid level 9g 1016 0.3IxI0·' 0.36 1 scrubber, sinlcred disc on inlet, vacuum 9h 1033 0.02 0.15 1 scrubber, nozzle on inlet. stirrer bar, vacuum 9i 1042 0.48xlO·' 35.7 2 scrubbers, sintered inlet in first scrubber, vacuum 9j 1021 O.23xlO- 40.7 2 scrubbers, recirculating NuOH
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first scrubber,smaller vacuum
9k 1031 0.66 57.7 2 scrubbers, recirculating NaOH in first scrubber, vacuum, increased liquid level
91 1020 1.88x10· 86.2 2 scrubbers, recirculating NaOH
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first scrubber,vacuum, increased liquid level
The chlorine recovery of 10.2% for run 9b was unacceptably low. Rather than being a case of low reactant conversion to hydrogen chloride, it was suspected that there was a mass thmsfer problem between the hydrogen chloride gas and the caustic soda liquid. Thus most of the hydrogen chloride gas produced was being discharged to drain. For the next run, it was decided to increase the liquid level in the scrubber, thus allowing the gas bubble to have a longer residence time in the liquid phase. This change saw immediate results, with the chlorine recovery increasing to 37.1%. The new scrubber set-up was used again, this time with the liquid level slightly increased. The chlorine recovery increased once again to 49.5% for run ge.
The liquid-level was once again increased for run 9f, however chlorine recovery from the scrubber was only 12%. Referring to Table 5-4, large amounts of hydrochloric acid were detected in both the hexane solvent and the carbon residue by GC/MS. The pressure drop through {he increased scrubber liquid level was too high, and the water ejector pump was not strong enough to provide enough driving force for the gas bubble to move through the scrubber.
During the previous runs, it was observed that the gas bubble travelling through the liquid phase was very large. Aside from increasing the bubble residence time to improve mass transfer, the
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distribution of the gas phase could be improved. This was attempted by placing a sintered glass disc on the gas inlet into the scrubber liquid. This method also provided smaller bubbles that were better distributed in the caustic soda. However, the 0.15% chlorine recovery (run 9g) was disappointing. Similar to the previous run, the glass sinter caused the pressure drop to increase such that the driving force for the gas flow through the scrubber was too Jow. Again, hydrochloric acid was detected in samples analysed by GC/MS.
Run 9h was attempted with a different set*up. A magnetic stirrer bar was introduced into the scrubber to aid mixing and 10 distribute the gas phase more evenly in the liquid. The sintered disc was replaced with a plastic nozzle. The chlorine recovery was again disappointing at 0.36%. It was observed that although the nozzle decreased the gas bubble size, the sudden contraction caused the bubble velocity to increase, thus its residence time in the liquid decreased. Hydrochloric acid was again detected by GC/MS analysis of the samples.
An improved, but not yet acceptable 35.7% recovery was obtained for run 91, using two scrubbers in series, both with high liquid levels and with no changes or adjustments to the 6 mm polyflow tubing inlet.
Two changes were made for run 9j. The first scrubber was replaced by a plastic bucket with a lid which was modified to recirculate caustic soda. This was effected by using a small fish tank pump submerged in the liquid. The pump outlet was modified to spray the caustic soda in from the top of the bucket. This was achieved by making a plastic device similar to a sh"owerhead. Unreacted hydrogen chloride then passed through the second scrubber, and then to drain. To decrease the speed of the gas bubble through the liquid, a lower vacuum was used. The result was a 40.7%
chlorine recovery. No hydrogen chloride was found in the samples. and had thus escaped to drain.
The same set-up was used for run 9k. with the liquid level in both scrubbers increased and the vacuum higher. The chlorine recovery improved to 57.7%, despite a leak found at the thermocouple insertion in the shell. The leak was repaired, and run 91 carried out using the same scrubber set-up. Chlorine recovery from the scrubber was a satisfactory 86.2%.
Reproducible chlorine recoveries were found for runs lOa (71.8%) and lOb (84.1%) where trichlorobenzene was pyrolysed and the scrubber assembly in run 91 was used. Detailed results can be found in section 5.4.
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A scanning electron microscope was used to examine the carbon black produced in runs t -3a. The carbon used for this part of the analysis had already been washed with acetone and hexane, and then dried in an oven. Micrographs can be found in Appendix 1. It became clear from the magnification and resolution of the micrographs that the carbon particles were in the order of nanometers, if not smaller. Agglomeration of carbon was observed. It was noticed that a large amount of glass fibres from the wool used in the filter pot were present in the carbon sample. Agglomeration tended to occur around these strands of fibre. Due to the small range of temperatures (1018 • 1414 QC) at which the analysed carbon was produced, no structural or other differences with reaction temperature could be observed.
The carbon analysed from run la exhibits a fairly porous surface, suggesting the possibility of conversion to activated carbon. However, more thorough investigations into this avenue are necessary.
This first set of SEMs did not provide any infonnation pertinent to the project and was thus discontinued.