Pyrolysis of chlorinated organic chemicals.

South Africa currently lacks the appropriate technology to dispose of its hazardous waste, with approximately 18,000 liters of chlorinated hazardous waste stored in the country. Due to the toxicity of chlorinated waste and its long-term impact on the environment, a sustainable way of handling this type of waste is necessary.

INTRODUCTION

Chemis try of PCBs
Chemistry of Polychlorinated dibenzo -p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) & polycyclic aromatic

PCBs
PCDDlFs
PAHs

International requirements for hazardous waste destruction
Motivation & Objective

Homologs exist and differ from each other in the number of chlorine atoms contained in the molecule. The Department of Health and Human Services (DHHS) in the United States has determined this.

PYROLYSIS EXPERIMENTS, RESULTS AND PROPOSED REACTION MECHANISMS

Introduction
Pyrolysis of Chlorinated Aliphatic Compounds
Pyrolysis of C hlorinated Aromatic Compounds
Mechanisms of Formation of Polycyclic Aromatic and High Molecular Weight Hydrocarbons
Mechani sm s of Forma ti on of Polychlorina ted Dibe nzofura ns a nd Polychlorin a ted Dibenzodi oxins
Conclusions

H;h has been proposed as a key intermediate in the formation of aromatic hydrocarbons by addition to C2H2 followed by cyclization to the phenyl radical. The conversion of chlorobenzenes to PCDD/F does not involve the formation of the intermediate chlorophenol.

Figure 2-1: Main reaction families for hydrocarbon pyrolysis

EXISTING TECHNOLOGIES FOR HAZARDOUS WASTE DESTRUCTION

Thermal m ethods of waste destruction

Rotary kiln incineration
Rotary barrel combustor
Circulating bed combustor
Infrared systems
Plasma arc technology
Electric pyrolyser

Also, the movement of the furnace makes the refractory material susceptible to damage from thermal shock. The perforations regulate the distribution of the combustion air, while the heat is removed from the barrel by the cooling water.

Table 3-2: Infrared System - Results of Product Analys is

Non-thermal methods of waste destruction

Adsorption processes
Catalytic dehydrochlorination
Catalytic extraction process (CEP)
Sun ohio process
Catalysed wet air oxidation
Electrochemical process
Supported liquid phase reagent process
Supercritical water oxidation

The pes is oxidized at high temperature in air or oxygen in an acidic aqueous medium. Non-precious metal electrodes were used in an electrochemical solution consisting of the pes-contaminated liquid and a proprietary reagent, at 5.15 vol.

Conclusion

An Overview of Pyrolysis Reactors and Processes

One embodiment of the reactor was a block of graphite, in which a number of holes (tubes into which the reactants are introduced) are drilled. The reactor used for this project is derived from a combination of the technology described above and is discussed in detail in the next section.

Figure 4~ I: Inlet e nd elevati on of one embodiment Matovich's [1977J reactor

Equipment and Process for the Pyrolysis of Chlorinated Hydrocarbon Waste

The chlorinated feed had to be vaporized and then mixed with an inert gas before being fed into the reactor. Part of the hydrogen chloride in the product had to be cleaned with caustic soda, and all the remaining gases were dissolved in water and drained down the drain. The flow of the chlorinated organic liquid had to be regulated by a metering pump and then allowed to evaporate in a heated section of pipe through which the feed enters the reactor.

Materials for the reactor were sponsored by Quadro Engineering, and the reactor was manufactured at the Faculty of Chemical Engineering. Graphite is also porous and will prevent the flow of inert gas through the tube walls to provide a protective blanket against reaction occurring on the tube walls. Graphite exhibits all the mechanical, chemical and thermal properties required for this process, while other chemically stable materials such as high alumina and zirconia are too brittle.

J5. Comparative raw material and utility consumption

Commissioning Runs and Modifications

An electrician was consulted to advise and check the safe working conditions of the induction unit and reactor. It was also found that the functionality of the variac was affected by the magnetic field. The configuration is such that the water for cooling the induction unit and electric coils can be supplied either from the water mains or from the cooling tower.

Furthermore, due to the availability of the instruments, it was only possible to analyze samples once a week. It was also noted that some of the carbon produced had passed through the glass wool filter and was deposited in the scrubber. So it was decided to use a vacuum at the end of the carbon filter and then at the reactor plug after each run to remove carbon that had settled in the horizontal portion of the pipe.

E xperimental P ro cedure

Argon was thus allowed to bubble through the methylene chloride and lead it into the reactor. It was observed that the temperature in the reactor tube increased by approx. 50·80°C from the temperature at which the induction unit is switched off. The argon flow rate was then increased to flush all the products in the reactor into the solvent and scrubber, and the shell pressure was increased accordingly to be maintained at 200 Pa higher than the tube pressure.

This was to prevent oxygen entering the reactor when the filter container was opened. An inlet for argon gas and an outlet for the reactor were made in the lid. This method was crude and offered no control over the flow rate into the reactor.

Sample Preparation & Analysis

The temperature in the container was logged to a file, and a simple plot of temperature versus time indicated the time it took for the compound to evaporate completely. A beuer system with accurate control can be implemented once optimization of the process has been investigated.

RESULTS AND DISCUSSION

Characterization of reactants and solvents
GC calibration
Pyrolysis of Methylene Chloride

Phase One: Qualitative results
Phase Two: Mass balance and collection system

Pyrolysis of 1,2,4-trichlorobenzene

P r odu cts
C hlorine recovery

Pyrolysis of lindane
E n er gy Balance a nd Effici ency

Power output of three-phase supply to induction unit
Efficiency of induction heating
Heat losses from reactor
Heat losses under reaction conditions

This error can be attributed to the inaccuracy of the thermocouple measuring the tube temperature. In order to comment on the effectiveness of the reactor insulation, a value had to be found for the heat losses from the reactor tube to the atmosphere. However, the pair is then positioned to measure the temperature on the outer surface of the reactor tube.

Another source of error exists in the measurement of the mass of organic reactant fed to the reactor. This could cause the measured reactant mass to be greater than the actual mass fed to the reactor. A change in the speed of f10 could affect the heat distribution in the reaction zone.

Table 5-2: Products and DE of methylene chloride pyrolysis at different tempe ratures and residence times

COST ANALYSIS

Due to the large capacity difference, CQ could not be used to generate a fixed cost comparison between this process and the 4.73 Ipa pyrolysis process. The quality and properties of the carbon black still need to be investigated and the market for hydrochloric acid is currently saturated. The intention is for the pyrolysis process to be attached to the end of an already existing process as a waste destruction unit, rather than as a standalone installation.

Based on run data, it was calculated that the induction unit needed to run for 1 minute every 9 minutes to maintain the reactor temperature within lQoe of the desired operating temperature. It is clear from Table 6-2 that cooling water and inert gas consumption for pyrolysis is much lower than that for plasma arc destruction. However, pyrolysis uses about ten times more electrical power than the plasma arc process.

CONCLUSIONS AND RECOMMENDATIONS

Carbon buildup proved difficult and it is recommended that the reactor assembly be either moved or raised to allow the outlet pipe to be oriented vertically so that the product stream enters the filter system from the top rather than the side. This change will require a minor modification to the reactor outlet flange and will facilitate a more efficient recovery system. Residence times below 1.3 seconds could not be achieved due to the nature of the methylene chloride dreschel bottle feed system.

Facilities for monitoring the off-gas stream for volatile organic compounds would be advantageous. The reactor assembly and method of induction heating used have been shown to be both efficient and successful, but the requirements for the success of the process on a wide scale are limited by the fact that only two compounds were destroyed. However, the presented results from this project show that there is certainly room for more research to be done in the pyrolysis process, i.a.

References
Bibliography

Pyrolysis formation of chlorinated phenols from a phenolic antioxidant used as a plastic additive.' Journal of AnalyticaL a Applied Pyrolysis. Study of combustion of hexachlorobenzene in gas phase, influence of oxygen concentration, attempt at global kinetic formulation.' Journal of Analytical and Applied Pyrolysis 44: 1-11. Mechanism of free radical hydrogen abstraction reactions: simple metathesis or involvement of an intennediate complex?' Journal of Physical Chemistry.

Rate constants for hydrogen atom attacks on some chlorinated benzenes at high temperatures.' Journal of Physical Chemistry. Journal of Analytical and Applied Pno/ysis Laboratory research into thermal degradation of a mixture of hazardous organic compounds.' Environment. EYE CONTAMINATION SHOULD BE TREATED BY IMMEDIATE AND PROLONGED IRRIGATION WITH EXCESSIVE AMOUNTS OF WATER.

GISAAA 53(6),18,88

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Figure Cl: GC trace - dissolved product gases in hexane

Appendix D Oscilloscope Sp ecifi ca ti ons and l\l easllrements for Three-phase Power Output

2: Peak· t a-peak current

l Methylene chloride in n-hexane

Chlorobenz cnc in n-h exane
References
MarshaII and Swift cost indices & exchange rate
Estimation of purchased equipment cost
Estimation of fixed capital investment

The term UA in (3) can be reduced to a single variable, a, which accounts for the heat transferred from the graphite tube through the carbon black, aluminum wool, and ceramic shell to the atmosphere, and accounts for the different thermal conductivities and heat transfer. surfaces. External conditions such as cooling water flow and exhaust fans are the same in both cases, but in one case argon flow through the reactor was allowed. The above expressions take into account heat losses from the reactor in 1) losses to the atmosphere.

The change in temperature of the reactor tube with time was recorded for each run. The recorded data for the methylene chloride and trichlorobenzene runs can be seen in Figures F~l and F-2, respectively. same as used in Appendix E. Reactor temperature profiles for methylene chloride. Symbols used without captions indicate peak areas and masses of the compounds being calibrated.