WO1995024945A1

WO1995024945A1 - Treatment of undesirable halogenated organic compounds

Info

Publication number: WO1995024945A1
Application number: PCT/GB1995/000552
Authority: WO
Inventors: Raikko Urmas Ilari Seppa; Willem Johannes Engelbrecht; Ian Russell Hearn; Gideon François VAN STADEN; David François SCHNEIDER
Original assignee: Eskom
Priority date: 1994-03-15
Filing date: 1995-03-15
Publication date: 1995-09-21
Also published as: FI963617A0; FI963617A; AU698189B2; GB2301353B; NZ281706A; NL9520020A; ZA952128B; GB2301353A; AU1859395A; DE19581565T1; CA2184253A1

Abstract

A process for treating an undesirable halogenated organic compound comprises heating a reactor wall by means of electrical induction or resistance heating; maintaining the reactor wall at a predetermined reaction temperature of at least 1500 °C; and allowing heat to radiate from the reactor wall into a reaction zone adjacent to, and in contact with, the reactor wall. A substantially solids-free gaseous feedstock comprising an undesirable halogenated organic compound is fed into the reaction zone. A non-oxidizing substantially solids-free gaseous atmosphere is maintained in the reaction zone. The compound is heated up sufficiently by means of the heat radiated by the reactor wall in order for it to pyrolyse and thus be transformed into more desirable components. A gaseous product comprising the more desirable components is withdrawn from the reaction zone.

Description

(i)

TREATMENT OF UNDESIRABLE HALOGENATED ORGANIC COMPOUNDS

THIS INVENTION relates to the treatment of undesirable halogenated organic compounds. It relates in particular to a process and installation for treating such a compound.

According to a first aspect of the invention, there is provided a process for treating an undesirable halogenated organic compound, which process comprises heating a reactor wall by means of electrical induction or resistance heating; maintaining the reactor wall at a predetermined reaction temperature of at least 1500°C; allowing heat to radiate from the reactor wall into a reaction zone adjacent to, and in contact with, the reactor wall; feeding a substantially solids-free gaseous feedstock comprising an undesirable halogenated organic compound, into the reaction zone; maintaining a non-oxidizing substantially solids-free gaseous atmosphere in the reaction zone; directly heating up the compound by means of the heat radiated by the reactor wall, with the compound being heated sufficiently for it to pyrolyse and thus be transformed into more desirable components; and withdrawing a gaseous product comprising the more desirable components, from the reaction zone. The Applicant is aware that halogenated organic compounds are produced in industry as hazardous waste products . It is required to destroy such hazardous or toxic waste products, and the process of the present invention provides a means of effectively destroying such halogenated compounds, with the production of at least one more desirable compound. By 'effectively destroying' is meant converting the halogenated compound substantially entirely into the more desirable compound, with at most an acceptable limit of the original halogenated compound remaining and/or at most an acceptable limit of another non desirable compound being produced.

It will be appreciated that, while reference has been made to a feedstock comprising a single undesirable halogenated compound, the process can equally be applied to a process comprising a mixture of undesirable halogenated compounds.

The undesirable halogenated organic compound may typically be one of the following, or mixtures of two or more thereof: PCB (PolyChloroBiphenyl) , TCB (TriChloroBenzene) , lindane (HexaChloroCycloHexane ) . DDT, TCDD

(TetraChlσroDiphenylDioxine) , SF₆ (HexaFluoroSulphide) ,

F₅SSF₅ (DecaFluoroDisulphide) , PCP (PentaChloroPhenol) , chloroform, dieldrin, perchlorinated aliphatics and RFC1

(Chlorinated Fluoro Hydrocarbons, or Freons) . The process of the invention is -'thus suitable for destroying a range or family of undesirable halogenated compounds.

The process may include preheating the feedstock, prior to feeding it into the reaction zone, to remove contaminants therefrom and/or to preheat it. Thus, in one embodiment of the invention, the halogenated compound may be available as, or as part of, a gaseous waste stream or product. If the concentration of the halogenated compound in the waste stream is sufficiently high, the waste stream can be used directly as the feedstock. However, the gaseous waste stream can be pretreated, eg by subjecting it to suitable absorption for the halogenated compound, if its concentration in the waste stream is too low, to produce the feedstock.

In another embodiment, the waste stream, and hence the halogenated compound, may be in liquid form. The process may then include pretreating the waste stream to remove either the halogenated compound or the other liquids, depending on which is the major component. Such pretreatment may comprise leaching or the like. The liquid stream may then be heated and vaporised, to form the feedstock.

In yet another embodiment of the invention, the waste product may be in solid form. Pretreatment to extract the halogenated compound may comprise leaching or the like, or heating the solid waste to sublime or vaporize the halogenated compound. The solid waste product may also be dissolved in a suitable solvent prior to vaporisation to form the feedstock. The solvent may itself be a halogenated organic waste product or stream.

When the halogenated compound is not in gaseous form, the method thus includes vaporizing or gasifying it prior to feeding it into the reaction zone as the feedstock.

The process may, however, also include preheating the vaporized feedstock, if necessary. Thus, the feedstock may be superheated, ie heated to above the condensation temperature of all components present therein. Thus, the feedstock may typically be preheated to a temperature in the range 400°C-700°C, but ensuring that destruction of feedstock does not yet commence, ie the feedstock is preheated to below the incipient pyrolysis temperature.

The non-oxidizing atmosphere may be a neutral, ie a non-reducing, atmosphere in the reaction zone. Instead, a reducing atmosphere can be maintained in the reaction zone. This may be effected by maintaining a slight hydrogen excess in the reaction zone. Thus, the process may include adding hydrogen or a hydrogen donating compound such as methane, to the reaction zone.

The heat radiated from the reactor wall into the reaction zone is thus in the form of electromagnetic waves, covering the entire electromagnetic spectrum, but with the infrared portion thereof predominating. The transformation of the undesirable halogenated compound can comprise one or more of the following steps: exciting the halogenated compound sufficiently by means of the radiated heat, and in particular by means of infrared radiation, to dissociate it into a halogen radical as well as a further acceptable radical ('radical A'); exchanging the halogen radical in the halogenated compound with a further radical ('radical

B') to form a non hazardous or more acceptable compound; combining the halogen radical with another radical

('radical C ) to form a more acceptable compound; and breaking down radical A into more acceptable smaller radicals or components.

More particularly, the reaction mechanism is believed to involve allowing the compound to absorb sufficient infrared radiation or energy from the radiated heat to heat it up to its pyrolysis temperature, at which temperature thermal decomposition of the compound into the radicals takes place. In other words, the carbon-halogen bonds in the molecules of the compound break, and optionally react with hydrogen radicals present in the reaction zone, to form the more desirable compounds. Thus, molecules containing asymmetric components, such as carbon-halogen components, are grey or opaque to infrared radiation, and absorb radiation heat until sufficient heat has been absorbed for them to become excited, as described above; the resultant fine carbon dust and radicals which are formed, or the resultant products after reaction with the further radicals, as hereinbefore described, can be symmetrical, in which case they will be transparent to infrared radiation, and not absorb further radiant heat, with such heat thus being available to heat up other or residual asymmetric molecules until substantially all asymmetric molecules have been converted to symmetric molecules. The heat content of the asymmetric molecules, ie the heat absorbed, is utilized in the endothermic splitting reactions, with the resultant symmetric products being, as stated, transparent to radiant heat and thus not being heat absorbant, so that the product gas exits the reaction zone at low temperature, usually at less than 100°C, and typically at about 40°C-60°C.

The process may thus include adding a secondary reactant to provide radical C. The secondary reactant may contain radical C in relatively pure form, or it may be in molecular form. When in molecular form, the process may include exciting the molecular reactant to dissociate it into radical form. ' This excitation may then be effected prior to adding the secondary reactant to the reaction zone. Alternatively, the secondary reactant can be added in non excited form to the reaction zone so that it and the halogenated compound are excited simultaneously. Radical C can thus be a hydrogen radical; however, it can instead be any other suitable^" radical to provide a desired end product. The secondary reactant can thus be methane or hydrogen, as hereinbefore described, or a silicon-containing compound.

The reactor wall is thus maintained at a suitable elevated temperature of at least 1500°C, normally above 1600°C, eg about 2000-3000°C, to ensure the desired pyrolysis and transformation of the halogenated compound. The temperature is thus dictated by the temperature at which the particular compound dissociates into its component atoms or radicals . Where a mixture of such compounds is used, the pyrolysis temperature will be determined by that compound which has the highest transformation temperature.

The heating of the reactor wall by resistance or induction heating results in high thermal efficiencies. In particular, substantial heat losses associated with heating means located externally of the reactor wall, such as radiation coupling, are avoided or at least substantially reduced. For example, with externally located heating means, some heat is reflected off the outer surface of the reactor wall, and uneven temperature distributions often occur.

Heating of the feedstock in the reaction zone is thus only, or primarily, effected by means of the heat radiated from the reactor . wall, which heat heats up the feedstock directly. Thus, indirect heating of the feedstock, eg by means of contact thereof with elements, which are heated up by, for example, induction or by the heat radiated from the reactor wall, and which are located in the reaction zone, as the only or the primary heating of the feedstock in the reaction zone, is avoided. However, if desired, additional secondary heating may be provided in the reaction zone.

This secondary heating may be provided by locating direct heated graphite heating elements in the reaction zone, or by DC/AC or RF plasma, a heating arc, additional infrared radiation,- micro waves or eximer laser energy, or laser radiation in,- or directed into the reaction zone. The secondary heating, when present, only constitutes a minor proportion of the heat supplied to the reaction zone, with the major proportion being provided by the heat radiated from the wall.

The reactor wall may be of graphite, or may be graphite lined. This will ensure that the high reaction temperatures required can be handled by the wall. In addition, graphite provides the required combination of temperature resistance and chemical resistance; furthermore, its electrical conductivity can be utilized for resistance heating of the wall.

The feedstock must then, however, contain substantially no chemical component capable of releasing reactive oxygen which can react with the graphite wall or lining to an appreciable extent. By 'reactive oxygen' is meant oxygen which is released, at the pyrolysis temperature, in the form of a radical, such as the hydroxyl radical, which can react with^' graphite. The pretreatment can thus comprise removing such components from the feedstock, if necessary. The pretreating may also include removing other substances, such as sulphur and phosphorus, which are capable of reacting to form substances which are harmful to the graphite, from the feedstock.

If desired, an inert carrier gas, such as argon, for the halogenated compound, can be used, with the feedstock then comprising the halogenated compound and the carrier gas. The velocity of the feedstock through the reaction zone may be such that there is laminar flow in the reaction zone, at the pyrolysis temperature.

The reactor wall may be of vertical cylindrical form, with the reaction zone being provided inside the cylindrical wall. The vertical location of the wall ensures that settling of carbon dust on the wall is minimized. The reaction zone may comprise a preheating section and a pyrolysis section located adjacent the preheating section, with the feedstock entering the preheating section and the product being withdrawn from the pyrolysis section. The feedstock will thus be preheated up to the required pyrolysis temperature in the preheating section, with pyrolysis being effected in the pyrolysis section. The reaction wall in at least the pyrolysis section, and optionally also the preheating section, may be substantially non-porous. Since the reactor wall in the preheating section must be clean for good heat radiation and since radicals already start forming as the feedstock is superheated in the preheating section with such radicals tending to polymerise, forming tars, soot, and the like, due to the temperature being below the pyrolysis temperature, and which tend to deposit on the wall as a layer which is opaque to radiation, the process may include periodically cleaning the reactor wall in the preheating section.

According to a second aspect of the invention, there is provided an installation for treating an undesirable halogenated organic compound, which installation comprises a reactor comprising reactor wall defining a reaction zone adjacent thereto and in contact therewith; electrical induction or resistance heating means for heating the reactor wall to a predetermined reaction temperature; feed means for feeding a gaseous feedstock comprising an undesirable halogenated organic compound into the reaction zone so that the feedstock is in contact with, and passes along the reaction wall; means for maintaining a non-oxidizing atmosphere in the reaction zone, with the reactor being adapted to heat up the feedstock in the reaction zone sufficiently by means of the heat radiated by the reactor wall for it to pyrolyse into more desirable components; and ^• withdrawal means for withdrawing a gaseous product comprising the more desirable components from the reaction zone.

The reactor wall may, as hereinbefore described, be of vertical cylindrical form with the reaction zone being provided on the inside of the tube, and comprising a preheating section in which the feed can be further preheated, and a pyrolysis section adjacent the preheating section, and with the reactor wall in at least the pyrolysis section being substantially non-porous at the reaction temperature.

The installation may include cleaning means for cleaning the reactor wall in 'the preheating section, or for ensuring that the reactor wall in the preheating section remains clean.

The cleaning means may comprise means for applying a film, blanket or envelope of inert gas, hydrogen or recycled product gas against the reactor wall, in order to preclude the feedstock from contacting the reactor wall. Instead, the cleaning means may comprise means for periodically 5/24945

(10) blowing graphite granules or the like against the reactor wall to remove the deposits therefrom. Instead, the cleaning means may comprise a mechanically operable cleaning tool, eg a drill, which may be of carbide, alumina, zirconia or other suitable ceramic material. In yet another embodiment of the invention, the cleaning means may comprise means for imparting pressure pulses or shocks to the reactor wall, thereby to remove any deposits.

The invention will now be described by way of example with reference to the accompanying drawings.

In the drawings

FIGURE 1 shows a simplified flow diagram of a process according to the invention for treating an undesirable halogenated compound; and FIGURE 2 shows a flow diagram of a pilot laboratory scale simulation of the process of Figure 1.

Referring to Figure 1, reference numeral 10 generally indicates a process according to the invention for treating an undesirable halogenated organic compound.

The process 10 includes an optional pretreatment or concentration stage 12 for pretreating a waste product stream which contains an undesirable halogenated organic compound to be destroyed. The stage 12 is linked, by means of a flow line 16, to a vaporization/gasification stage 14. The stage 14 is linked, by means of a flow line 18, to a reactor or pyrolysis stage 20. An additional or secondary reactant feed line 22 leads into the stage 20. The stage 20 is in turn linked, by means of a flow line 26, to a by-product collection stage 24. The stage 12 will be dispensed with if the waste product stream contains a sufficiently high concentration of the undesirable halogenated compound, eg is a liquid stream consisting of the undesirable halogenated compound only. Furthermore, if the waste product stream is in gaseous form and contains a sufficiently high concentration of halogenated compound, then the stage 12, and indeed the gasification stage 14, can be dispensed with. Such would typically be the case if the waste product stream emanates from a vinyl chloride manufacturing process.

If the waste product stream contains a relatively low concentration of the undesirable halogenated compound, eg if it is a ventilation air stream or liquid effluent stream, then concentration and extraction of the halogenated compound will be effected in the stage 12.

If the waste product stream is aqueous based, then the halogenated compound must be removed therefrom, eg leached therefrom using a suitable solvent such as a halogenated solvent, in the stage 12. If the liquid stream is organic based but is contaminated with water, then such water should be substantially removed, eg by suitable leaching, since the remainder of the process 10 is not tolerant to water. Any other non hazardous components present in the waste stream should then also be removed in the stage 12, eg by suitable leaching.

If the liquid waste product stream contains a sufficiently high concentration of the undesirable halogenated compound and contains substantially no compounds undesirable to the pyrolysis stage 20, it is routed through the stage 14 to the stage 20. Thus, the liquid stream passes through the stage 14 only to gasify or vaporize it. The flow line 16 may be a conduit, conveyor, or the like depending on the physical form of the stream or product being transferred from the stage 12 to the stage 14.

In the stage 14, the halogenated compound is vaporized or gasified, and superheated, while keeping it at as high a concentration as possible. The heating can be effected in an oven which can be heated by any convenient method, for example, in an induction or ohmic heated electric oven.

The gaseous stream or feedstock from the stage 14 then passes, by means of the flow line 18, which is typically a conduit, to the reaction stage 20. The pyrolysis stage 20 can be in the form of a pyrolysis furnace. The conduit 18 transfers the product from stage 14 directly to the stage 20.

The nature, type and construction of the stage 20 normally depends on the specific halogenated compound to be treated therein. Where a mixture of halogenated compounds is to be treated, then the reaction or wall temperature, and hence the construction of the stage 20, will be dictated by that component which requires the highest pyrolysing or transformation temperature. However, the temperature is also related to the residence time. The structure will thus be optimized according to the reaction temperature required, and the specific reaction volume required at a specific temperature to give a desired residence time. Furthermore, the construction material also depends on the halogenated compound(s) and the temperature to be applied.

Halogenated compounds generally dictate that a pyrolysing temperature in the region of 1500°C to 3000°C is required. The stage 20 therefore has a substantially non-porous graphite reactor tube or a substantially non-porous reactor tube lined with graphite. The furnace is heated by direct ohmic or resistance heating. Utilizing a graphite lined furnace for the high pyrolysis temperatures which are required gives, as has been demonstrated on laboratory scale, good results in view of the properties of graphite, such as excellent thermal shock resistance, large thermal gradients obtainable across the tube or lining, good electrical conductance, and increasing mechanical strength as temperature increases. An essentially similar pyrolysis furnace design based on induction heating would also provide the same functionality and can hence in principal be used. The reactor tube has a preheating section and a pyrolysing section as hereinbefore described.

In the preheating section, the feedstock is heated to the pyrolysis temperature of 1500°C-3000°C, while in the pyrolysis section, the halogenated organic molecules are exposed to a specific infrared radiation in the high temperature environment, and in an atmosphere essentially devoid of oxygen or an oxygen-donating compound such as H₂0 or C0₂, ie in a non-oxidizing atmosphere. Preferably, a reducing or hydrogen atmosphere is used. The absorbed energy is converted to chemical energy, resulting in the formation of radicals, especially hydrocarbon and halogen radicals. Sufficient energy must be available to complete the splitting reaction; if insufficient energy is available then partial splitting occurs, and the formed radicals can back react, thus rejoining the carbon halogen bonds. If sufficient energy is available, further splitting of the carbon hydrogen bonds will occur. The hydrogen and halogen radicals join to form a lower energy species, ie a hydrogen halide. Depending on the available energy, the pyrolysis may be continued until only elemental carbon radicals remain. These carbon radicals may react with others, forming hexagonal radicals, which may further join to form graphite nuclei, or amorphic carbon dust. With the process of this invention, complete destruction of halogenated organic compounds to elemental carbon radicals is achieved, and therefore the temperatures utilised are significantly higher than in known processes employing pyrolysis of chlorinated organics .

Since the halogen atoms are present as radicals in the furnace, they are highly reactive, and these 'radicalized' halogen nuclei may react with other radicals present, resulting in formation of the lowest chemical energy species in equilibrium. These species become lower in energy with hydrogen radicals than with carbon or oxygen radicals, thereby promoting destruction of the halogenated compounds. Where there is sufficient excess, on a molar basis, of free hydrogen radicals available compared to halogen radicals, the graphite nuclei can be expected to be free of halogens. It is essential, therefore, in the process of the invention, that sufficient hydrogen is available for the complete reaction of all halogen nuclei with hydrogen.

If the feed to the pyrolysis reactor 20 contains non-reactive oxygen containing molecules, the halogen radicals will be split off first . The oxygen atoms will initially remain with the mother carbon atoms, which will finally be split off as carbon monoxide which is very inert in the reducing atmosphere in the furnace. Minor amounts of water and carbon dioxide will form carbon monoxide and hydrogen where applicable, in the furnace, thus consuming some carbon radicals. The thermodynamics prevalent under the pyrolysis conditions indicate that formation of dioxines, furanes and phosgene are energetically unfavourable; if such substances are present in the feedstream, they will also be destroyed. Heat transfer into the organic media in the pyrolysis reactor is^•by radiation heat transfer. At higher applied temperatures, the non symmetric organic molecules, especially halogenated organics, effectively absorb radiation heat. The molecules are transformed into radicals with hydrogen and chlorine being split off as radicals. Components which do not contain halogen have as a rule high negative Gibbs Helmholz free energy. Once a halogen radical has been split off, the remaining radical becomes less 'grey' for absorption of thermal radiation. Finally, the remaining free elemental carbon, hydrogen, and halogen radicals are transparent to thermal radiation, and the radiation heat can thus pass through the gas layers to reach all halogenated molecules and radicals. The result of this is that energy will be absorbed by the media, until all the molecules and radicals have been destroyed or transformed to single radicals. The heat energy absorbed by the compounds is consumed in the highly endothermic splitting reactions and, as the materials produced by this reaction are transparent to the infrared radiation in the reaction zone, these products only gain heat by convection and therefore remain at relatively low temperatures. Argon carrier gas utilized in the reactor and secondary reactants such as hydrogen are transparent to infrared radiation and therefore are heated only slightly by convection in the reaction tube. These phenomena ensure that the exit temperature of the product stream remains relatively low, eg typically at 40°C-60°C.

In the pyrolysis section, any non-symmetric agglomerated carbon radicals, forming fine carbon dust, will effectively absorb and further radiate heat, completing the reaction. Since the heating energy enters through the walls of the furnace, the formation of tar and fouling of the heating surfaces at cold spots will not readily occur. The carbon dust which is formed has an extremely fine particle size, and does not settle readily. Furthermore, the carbon particles, being conductive, do not readily attach to the walls by means of electrostatic forces.

From the stage 20, the secondary products produced therein pass along the flow line 26 to the post treatment or by product collection stage 24. The post treatment applied will depend on the secondary products in question.

Thus, for example, if the feedstock to the process comprises chlorinated hydrocarbons, and methane or hydrogen, preheated to an adequate temperature to preclude condensation of the feedstock to any appreciable extent, is introduced along the flow line 22, then the secondary products obtained from the stage 20 will comprise essentially carbon dust, hydrochloric acid, and hydrogen. The stage 24 will then comprise HC1 scrubbers, carbon dust filters and waste gas recycling.

In the event that PCB or TriChlorBenzene is the halogenated compound, a suitable preheated silicon bearing compound may be introduced into the stage 20 along the flow line 22. The secondary products from the stage 20 will then include silicon carbide. In addition, when the secondary products include silicon chloride (SiCl₄) , this can be rerouted back to the stage 20 for excitation therein, as hereinbefore described, together with additional HC1, H₂, Si or C, depending on the stoichio etric requirements. The fine silicon carbide powder thus obtained can be used to make highly ■ homogeneous silicon carbide elements utilizing sintering techniques.

Similarly, when the secondary products which pass from the stage 20 along the flow line 26 include boron chloride (BC1₃) or boron hydride (BH₃) , these can be rerouted back from the stage 24 to the stage 20 where they are vaporized and excited, and the resultant radicals allowed to react with carbon radicals, to form boron carbide (C₃B₄) dust, which can also be used in hard metal applications.

In the event that it is not desired to react any of the secondary products obtained from the stage 20 to produce further products, the off-gas from the stage 20 must be treated. The gas has a high calorific value, and can be used as a fuel. The gas will contain HCl which can be collected in a series of wet scrubbers or by a dry recovery process .

The process 10 was simulated on pilot laboratory scale, utilizing the layout of Figure 2.

In Figure 2, reference numeral 100 generally indicates the pilot laboratory scale apparatus used. The apparatus 100 has a section 102 corresponding to stage 14 of Figure 1; a section 104 corresponding to stage 20 of Figure 1; and a section 106 corresponding to stage 24 of Figure 1.

The section 102 comprises a batch solids feeder 108, with a conduit 110 leading from the feeder 108 to an electrically operable evaporator or vaporizer 112. The evaporator 112 includes a Variac (trade name) 2kW heating arrangement 11 .

The section 102 also includes a continuous liquid stream feeder, generally indicated by^' reference numeral 116.

The continuous liquid stream feeder 116 includes . a container 118 containing a supply of the unwanted liquid hydrocarbon product to be destroyed, with a conduit 120 leading from the container 118 to a peristaltic pump 122. The conduit 120 leads from the peristaltic pump to a hot oil bath and stirrer arrangement 124, and then ties into the conduit 110.

The evaporator 112 is integral with a superheater 126 fitted with a Variac (trade name) 2kW heating arrangement 128. A conduit 130, through which argon carrier gas, and methane or hydrogen can be introduced, leads into the superheater 126.

The section 104 comprises an upright cylindrical or tubular reactor, generally indicated by reference numeral 132. The reactor 132 comprises an outer cylindrical shell 134 to which is connected a tubular component 136 forming part of the superheater 126. Inside the outer shell 134 is mounted an outer graphite tube 138 along the inside of which extends an inner graphite reaction tube 140, which is operatively connected to the superheater 126. The upper and lower ends of the outer graphite tube are mounted in graphite limpets 142, while the limpets are mounted to oil cooled aluminium contact rings 142 located around the outside of the outer casing 134. The tube 140 was isolated by means of graphite and alumina wool.

A pyrometer 146 is also mounted to the outer casing, and is operatively connected to the inner tube 140. A Variac (trade name) 44kW heating arrangement 148 is connected, by means of seven cables, to each of the aluminium contact rings 144 so that the graphite limpets, and hence the inner graphite tube 140, can thereby be heated up to the required pyrolysis temperature.

The inner or central graphite tube had a 22mm ID and an effective length of 2m. During the pilot scale test it was heated up to 2660°C, by using it as the resistance element in the high current AC heating arrangement or circuit 148.

The section 106 comprises a dry filter or trap 150 for carbon, with the lower end of the inner tube 140 leading into the dry filter 150. A conduit 152 connects the dry filter 150 to a wet filter 154. A conduit 156 connects the wet filter 154 to a first hydrochloric acid scrubber 158, with a conduit 160 connecting the scrubber 158 to a second hydrochloric acid scrubber 162. A conduit 164 leads from the scrubber 162 to a vacuum pump (not shown) .

Thermal insulation materials used throughout, eg. inside the shell 134 were carbon fibre felt and high alumina felt. Temperatures were measured by means of an optical pyrometer, and the measuring gate was mounted in an argon atmosphere containing steel case.

Tests were conducted on the pilot laboratory scale apparatus of Figure 2, as follows:

EXAMPLE 1

The product gas from the reactor 132 was analyzed and found to consist essentially of hydrogen and HC1. The solid carbon product from the reactor 132 was analyzed by leaching it with n-hexane according to the standard ASTM method to isolate any residual chlorinated organic material. In the test program chloroform, TCB and PCB were used as the undesired halogenated compound, with methane as a hydrogen donor (secondary reactant) and argon as an inert carrier. For analysis, a lOg sample of the carbon produced was leached twice for 20 hours and the leachate concentrated to lmf . This sample was analyzed in a gas chro atograph mass spectrometer (GC-MS) . GC-MS analysis sensitivity allows a detection of lOpg (pico grams) from 1+1 feed, ie. five 9's. In the analyses 5 - 6 significant figures were obtained. Traces of the original feedstock were detected at between 100 and 1 ppm - equivalent to between 99.99% and 99.9999% respectively destruction of the feedstock. The efficiency of destruction was found to be directly related to the feed rate and therefore more complete destruction can be anticipated with longer residence times at the pyrolysis temperature. A longer graphite reaction tube 140 will thus lead to increase destruction and enhanced energy efficiency.

EXAMPLE 2

A number of test runs were conducted, utilizing solid, liquid and gaseous feeds. Liquid feeds were passed through a falling film evaporator, and the vapours then superheated in a graphite heater to 400°C-700°C, depending on the feedstock. Solid feeds were evaporated or sublimed in a batch pot, and the vapours conducted through heated conductors into the reactor. Gaseous feeds, such as freons and halothanes were superheated, as was done for the vapours from the liquid feeds.

The product gas from the reactor was in all cases merely warm to touch, and tolerable by hand, indicating a temperature only slightly above ambient, eg about 40°C. Fine carbon dust was filtered from the product gas using a glass-wool filter, 'and the hydro-halogen products were absorbed in two large alkaline absorbers. The carbon yield removed from the wool was 80%-95%. Some of the dust was thus caught in the wool, while the rest passed through the wool into the absorbers.

Product gas was in each case sampled in a liquid nitrogen trap, as well as in a quartz cell for IR spectrophotometric analysis. Carbon was leached twice, for 24h periods in each case, using cyclohexane, in a soxhlet apparatus, to a leached concentrate of less than lmf . The leachate was analysed by injecting Iμi into GC-MS. 50m fused silica column was used. The sensitivity of the MS was claimed to be better than 0,4pg.

No halogenated feedstock or halogenated daughter compounds could be detected in the product gases . This gave a destruction efficiency better than seven 9's.

Standard Test conditions: Feedstock lg/min

H donor CH₄ or H₂ in stoichiometric ratio:

Cl:H = 1:2

Main reactor, ie wall temperature 2100°C Evaporator 420°C

Preheater 550°C

Summary of Results

Reactor Wall

Test Feed Feed Temp. Dest. Run Feed Stock H-Donor Method Rate C. Eff. %

1 PCB CH₄ Liquid 1 g/m 2100 99,9999 +

2 PCB CH₄ Liquid 1 g/m 2100 99,9999 +

3 PCB CH₄ Liquid 2 g/m 2100 99,9999+

4 PCB CH₄ Liquid 4 g/m 2100 99,9999+

5 PCB CH₄ Liquid 1 g/m 2500 99,9999+

6 PCB H₂ Liquid 1 g/m 2100 99,999 +

7 Chloroform H₂ Liquid 1 g/m 2100 99,99+

8 Tetra H₂ Liquid 1 g/m 2100 99,99+ chloride

9 Lindane H₂ Evaporate - 2100 99,9999+

10 Hexes H₂ Evaporate - 2100 -

11 Tar H₂ Evaporate - 2100 99,99+

12 DDT H₂ Liquid 1 g/m 2100 99,999+

13 Dieldrin H₂ Liquid 1 g/m 2100 99,99+

14 CFC H₂ Gas 1 g/m 2100 -

The carbon products were obtained as a loose powder and as dense agglomerated pieces.

It is often difficult to obtain desired maximum limits of super poisons, such as TCDD, in the flue gas emanating from incineration. Typically, the upper limit of such poisons can be as high as lppb (ie 0,001ppm) , but it can be as low as 0,lppb. These limits are difficult to detect due to the fact that, in fossil fuel incinerators generally used, the hazardous waste stream is diluted with a support fuel, typically by a factor of about 20. Furthermore, the support fuel requires a substantial volume of air for combustion and, since the upper limits of super poisons are based on the total volume of flue gas produced, substantial masses of such super poisons will still be emitted to the atmosphere during such incineration.

The Applicant thus believes that in the process of the present invention, wherein super poisons such as TCDD are not produced in view of the complete absence of oxygen which prevails, and wherein there is no dilution of the feedstock by means of a support fuel and/or air, these problems are to a large extent overcome.

The Applicant also believes that the process of the present invention provides a convenient means for effectively handling hazardous chemical wastes containing halogenated compounds, and in particular halogenated hydrocarbons, which are difficult to destroy with conventional methods or where there is a danger of producing toxic secondary waste materials which may then possibly be even more hazardous than the halogenated compounds.

The Applicant more particularly believes that with the process 10, problems associated with known methods of destruction of hazardous wastes containing chlorinated hydrocarbons, are at least alleviated. Presently, such hazardous wastes are destroyed by means of incineration, at applied temperatures of 1100 to 1200°C. The incineration can be effected in two stages, with hazardous components being gasified in the first stage by applying temperatures of around 700°C in an oxidizing atmosphere. In the second stage a high oxygen content is required to prevent formation of phosgene, and this stage utilizes a temperature of 1100 to 1200°C. The high oxygen atmosphere is effected by utilizing burners combusting oxygen or oxygen enriched air, and different forms of nitrogen oxides are generated. If desired, the combustion air to the burners can be heated by applying a DC plasma thereto.

Another known halogenated organic destruction system utilises a plasma furnace with controlled oxygen levels to achieve destruction. This system operates at temperatures above 5000°C, is capital and energy intensive, and is substantially inflexible for by product production.

The process of the invention is thus characterized thereby that the reaction zone contains substantially no solid material, elements or particles either introduced with the feedstock of inherently in the reaction zone, eg to heat or assist in heating the feedstock, with the only solid material being any solid product which is formed. Such exclusive gaseous phase operation promotes simplicity of construction and generation of the installation. For example, fouling of the reactor wall is minimized.

The process is further characterized thereby that the feedstock occupies the entire reaction zone, and is even in contact with the reactor wall. Since the feedstock is in gaseous form containing little or no solids, other than possibly some solid reaction products, and bearing in mind the reaction mechanism as hereinbefore described, little or no fouling of the reactor wall in the pyrolysis region occurs. Thus, the use of wall cleaning means in the pyrolysis region, such as providing an envelope of inert gas against the wall or blanket can largely, if not entirely, be 'avoided. This results in a simpler construction, and lower capital and operating costs. Such inert gas envelopes or blankets are also used to protect reactor walls against high reactor temperatures in cases where heating means other than resistance or induction heating of the wall are used, and are clearly not required for this purpose in the present instance, thereby also avoiding potential problems associated therewith, such as reduction in heating efficiency of the feedstock if the inert blanket admixes turbulence with the feedstock adjacent the wall causing carbon dust clouds which shield radiation heat transfer.

Claims

CLAIMS :'

1. A process for treating an undesirable halogenated organic compound, characterized in that the process comprises heating a reactor wall by means of electrical induction or resistance heating; maintaining the reactor wall at a predetermined reaction temperature of at least 1500°C; allowing heat to radiate from the reactor wall into a reaction zone adjacent to, and in contact with, the reactor wall; feeding a substantially solids-free gaseous feedstock comprising an undesirable halogenated organic compound, into the reaction zone; maintaining a non-oxidizing substantially solids-free gaseous atmosphere in the reaction zone; heating up the compound sufficiently by means of the heat radiated by the reactor wall in order for it to pyrolyse and thus be transformed into more desirable components; and withdrawing a gaseous product comprising the more desirable components from the reaction zone.

2. A process according to Claim 1, characterized in that it includes pretreating the feedstock, prior to feeding it into the reaction zone, to remove contaminants therefrom and/or to preheat it.

3. A process according to Claim 1 or Claim 2, characterized in that it includes maintaining a reductive

^■atmosphere in the reaction zone by feeding hydrogen or a hydrogen donating compound into the reaction zone, as a secondary reactant.

4. A process according to any one of Claim 1 to 3 inclusive, characterized in that the reactor wall is of graphite or is graphite lined in the region where it is in contact with the reaction zone, with the feedstock containing substantially no chemical component capable of releasing reactive oxygen, which can react with the graphite wall or lining, to an appreciable extent.

5. A process according to any one of Claims 1 to 4 inclusive, characterized in that the velocity of the feedstock through the reaction zone is such that there is laminar flow in the reaction zone, with the temperature of the product gas, on exiting the reaction zone, being less than 100°C.

6. A process according to any one of Claims 1 to 5 inclusive, characterized in that the reactor wall is of vertical cylindrical form, with the reaction zone being provided inside the tubular wall and comprising a preheating section and a pyrolysis section located adjacent the preheating section, with the feedstock entering the preheating section and the product gas being withdrawn from the pyrolysis section; and with the reaction wall in at least the pyrolysis section being substantially non-porous.

7. An installation for treating an undesirable halogenated organic compound, characterized in that the installation comprises a reactor comprising a reactor wall defining a reaction zone adjacent thereto and in contact therewith; electrical induction or resistance heating means for heating the reactor wall to a predetermined reaction temperature; feed means for feeding a gaseous feedstock comprising an undesirable halogenated organic compound into the reaction zone so that the feedstock is in contact with, and passes along the reaction wall; means for maintaining a non-oxidizing atmosphere in the reaction zone, with the reactor being adapted to heat up the feedstock in the reaction zone sufficiently by means of the heat radiated by the reactor wall for it to pyrolyse into more desirable components; and withdrawal means for withdrawing a gaseous product comprising the more desirable components from the reaction zone.

8. An installation according to Claim 7, characterized in that the reactor wall is of vertical cylindrical form with the reaction zone being provided on the inside of the tube, and comprising a preheating section in which the feed can be further preheated, and a pyrolysis section adjacent the preheating section, and with the reactor wall in at least the pyrolysis section being substantially non-porous at the reaction temperature.

9. An installation according to Claim 8, characterized in that it includes cleaning means for cleaning the reactor wall in the preheating section, or for ensuring that the reactor wall in the preheating section remains clean.