- Email: [email protected]
Development of a gas-cleaning system for a scrap-tire vacuum-pyrolysis plant
Development of a gas-cleaning system for a scrap-tire vacuum-pyrolysis plant* V. Roy, B. de Caumia Universitk
and C. Roy**
Laval, IXpartement
de genie chimique, Sainte-Foy (Quebec) Canada Gl K 7P4
Received 24 April 199 1; revised 8 November
199 1
When used tires are subjected to heat in the absence of air, large quantities of vapours and gases are produced. A large portion of the vapours can be condensed to oils with a yield of 55% by weight. However, the non-condensable gas phase (6% yield) is contaminated with oil droplets, tar, fine particles as well as nitrogen- and sulphur-compounds. The project objective is to develop a gas-cleaning system and design a pumping unit that can maintain the total reactor pressure below 10 kPa and burn the process gas without emission problems. A series of experiments was made in a 25 kg h-’ vacuumpyrolysis process-development unit, for which different gas-cleaning units such as a shell and tube heat exchanger, cold traps, cyclones, demisters and filters were tested. A gas burner with a variable orifice was used to burn the pyrolysis gas efficiently. Standard stack sampling was performed to identify the flue gas contaminants. Gas burning in general was successful, but the gas-cleaning system could not remove all the contaminants in the gaseous stream. According to preliminary experimental results, an alternative promising approach is to replace the shell and tube exchanger, cold traps and filters by a liquid-ring vacuum pump. Keywords:
gas-cleaning;
vacuum-pyrolysis;
Introduction Tire recycling has become a necessity because of the accumulation of discarded tires which are a potential environmental risk. Each year 24 million tires are disposed of in Canada and about 240 million tires in the USA. While some of these tires are recapped or ground for special uses, most are dumped in rural areas or in landfills. When buried in landfills. they eventually float to the surface. In piles, the non-biodegradable rubber can cause serious harm if ignited. Tires infested with mosquitos are the subject of increasing concern’. Tires represent a source of energy and chemicals.
By
thermal decomposition, it is possible to recover useful products’. There have been numerous attempts to pyrolyse tires. It is beyond the scope of this paper to describe the various ventures and adaptations of technology. Literature reviews have been published”“. Vacuum pyrolysis is an old concept. It enables the production of large quantities of pyrolysis oils from organic substances. Vacuum minimizes secondary reactions such as thermal cracking, repolymerization and recondensation reactions, gas-phase collision. catalytic cracking and reduction and oxidation reactions. If the vapour-phase products are quenched. the yield of organic liquids such as pyrolysis oils is increased at the expense of solid residues and gases. The physico-chemical properties of the end-products are specific.
tires
The vacuum pyrolysis process has been under development in Quebec since the early 1980s. The research programme focused on establishing background knowledge about the reaction mechanism, pyrolysisproduct characterization and design and operation of process-development units with capacities ranging from batch systems to 50-200 kg h-’ continuous units. Examples of waste materials investigated are: petroleum sludges. activated sludges, plastics, bark residues, municipal solid wastes, biomedical wastes, contaminated soils, automobile shredder residues and used tires. The specific objective of the present study was to develop and test a gas-cleaning system capable of eliminating all potential pollutants from the noncondensable gas phase generated during vacuum pyrolysis of used tires in a 25 kg h-’ process development unit. The process gas will be burned to provide a make-up heat source for the pyrolysis process and the quality of the emissions must be environmentally acceptable. One peculiarity of the process that had to be taken into account for the design of the gas-cleaning unit was the system total pressure, which should be maintained below IO kPa.
Experimental
*Paper presented at GAS Separation International, Austin, TX, USA, 22-24 April 199 1 **Author to whom correspondence should be addressed
The existing vacuum-pyrolysis process-development unit (PDU) operated on a semi-continuous feed mode using uniform cylindrical pieces of used. cross-ply tires sieved to 6.35-12.7 mm mesh sizes. The feedstock was initially poured in a hopperwhich contained all the material to be
0950-4214/92/020083-05 0 1992 Butterworth-Heinemann
Gas Separation Et Purification 1992
Ltd
Vol 6 No 2
83
Gas-cleaning system for a scrap-tire pyrolysis plant: K Roy et al.
Figure 1
stream of the second quench tower by other gasThe and dry ice traps were substituted Methanol flowing through the shell side was cooled in a 1250 W refrigerating equipped with a Figure2 illustrates droplets a stainless-steel installed at the top ofthe second quenching sampling trains were to the system. Monitoring of the gas emissions achieved by using the procedures described by Environment Canadax-‘I’. Samples of the stack gases were taken during run no D015(12 h)inordertodetect presence of the following pollutants: NO,. SO?. fly ash and unburnt particles. NO, was after about 3 h of the onset of the run, once the system was under steadystate conditions. Sampling particles nearly 7 h after the beginning
Original PDU flowsheet
processed during each of the two experiments performed in this study. The reactor was a six-hearth furnace, 2 m in height and 0.7 m in diameter, heated by electrical heat tracing. The average throughput reached by the system during this study was 22 kg h-‘. A schematic diagram of the PDU is shown in Figure 1. A detailed description of this unit6 its elsewhere. residues. The solid residues, comprised carbon black and tibres, found their way across the reactor towards the bottom, where they accumulated separate vessel. The vapours and gases during the thermal decomposition
(
Infrared
spectroscopy ofgaseous compounds automatic stack-sampling Infrared (FTIR) (Bomem Inc., Quebec) a sample every four minutes (Figure 3). A solid, white was found in the manifolds tertiary This was by FTIR. Elemental proximate were performed apparatus and a LECO SC-I 32 apparatus. respectively.
pressure obtained
To Vent t
Figure 2
First modification of the PDU (run no DO1 4)
Figure 3
Second modification of the PDU (run no DO1 5)
equipped with a Molecular Porapak-Q column was used to analyse the process gas. At the end, the gas was flared. When this study was. undertaken, described above was not satisfactory. example, the large across the mechanical vacuum pump caused tar deposition and vapour condensation pyrolytic gases was neither environmentally modified and tested. Run no D014, the ‘orientation experiment’, was performed equipment located down-
84
Gas Separation
Et Purification
1992
Vol 6 No 2
Gas-cleaning system for a strep-tire pyrolysis plant: V. Roy et al. Flue Table 1 procedure
Results and discussion The first pyrolysis experiment (run no DOl4) lasted 8 h and the second (run no DO15, the ‘gas analysis experiment’) 12 h. During the orientation experiment the new shell and tube heat exchanger unit installed at the suction side of the pump did not significantly reduce the amount of liquids accumulating inside the vacuum pump. Such results were not in line with what was reported by other investigators during high-pressure. pilot-plant wood and municipal-waste gasification tests”. “. However, results from run no DO15 showed that the major pyrolytic gaseous components are carbon dioxide and a few light hydrocarbons with pentane and butene leading the list (F&u~ 4). All component concentrations were relatively constant, except for CO, which slowly dropped from 25% by weight to 13% near rhe end of the 12 h run. Consequently the process-gas caiorific value did increase. whereas its average molecular weight remained constant over the experiment. The Environment Canada wet test method to characterize stack emissions is long and tedious and does not easily provide several sets of data simultaneously. Table I shows that. except for NO,, the concentrations observed were ail below the emission levels allowed for ambient air by the QuCbec provincial regulatory agency (Ministry of Environment. QuCbec). It is noticeable. however, that no specific regulations exist for flue gases o~ginating from a pyrolysis unit. The values indicated in Table I are those used for the combustion of fossil fuels. The Bomem continuous gas analysis equipment was well adapted to the harsh environment at the plant site (see Figum 5 and 6). Figure 5. for example, provides some comparison for SO? concentration as calculated by the standard procedure and that determined by FTIR.
Pollutant
gas
analysis
Sampling
following
Environment
Concentration* (ppm by volume]
Canada
(ppm by volume)
*Calculated on a 12% excess oxygen basis
a Environment
+ FTlR
Canada
method
analysis
. i5
loo-
++
++
f+
++
N x
+++ .
++ ++
0
+t-t+
2
4
6
8
.2
10
Time (h)
Figure 5 DO1 5)
T
SO1 flue gas concentration as a function of time (run no
500---
‘10
i-
i
Cxrbon dloxldx 20%
Time (h)
Figure 6
DO15)
The white. translucent crystal found in the piping system after the mechanicai pump was determined by elemental analysis to be ammonium bicarbonate, confirmed by FTIR spectroscopic analysis.
Carbon monoxide 2% Oulrx
CO, and CO concentration in the flue gas (run no
99b
Pumping system
Standard Deviation (%) BUWIW Pentan Carbon Moxlde Yethxno Elhans Propen. ElhOIW pr0plw
Blltur. Carbon Monoxldo othor8 Figure 4
1.4 1.5 6.2 0.9 0.7 1.3 0.9 0.7 0.7 0.2 1.9
Pyrolysis gas average mass com~sition
{run no 00 15)
Despite the improvements brought to the existing gascleaning system. tar and water still found their way inside the vacuum pump, where they contaminated the lubricating oil, thus lowering the pump capacity and efficiency. It was concluded that this chemical reaction system needed a pumping unit that could handle both organic contaminants and water while maintaining a total pressure no higher than IO kPa. Two different pumping systems were considered: a rotary screw vacuum pump (oil seal) or a liquid-ring vacuum pump (water seal). The rotary screw pump was rejected since preliminary testing in the laboratory showed that the condensed liquid hydrocarbons on mixing with the sealing oil formed a
Gas Separation 8 Purification 1992
Vol 6 No 2
85
Gas-cleaning system for a scrap-tire pyrolysis plant: V. Roy et al.
the combustion efficiency. It was maintained at a level of 99.936% with a standard deviation of 0.045% throughout the experimental run. The only comparable value available is the efficiency required (99.900%) by the provincial regulation (MENVIQ) for hazardous-waste incinerators. Although the pyrolysis of used tires cannot be classified in the same category this comparison shows that combustion of the process gas from used-tire vacuumpyrolysis is a highly efficient reaction.
Conclusions
Figure 7 Process flow diagram of the PDU with the liquid-ring vacuum-pump system
stable, undesirable emulsion. Liquid-ring vacuum pumps can handle contaminants immiscible with water (like hydrocarbons). Moreover this type of pump can provide the level of vacuum needed by the addition of one or several steam-ejector stages. Finally, the cleaning effect of water used as a sealing medium can avoid the plugging problems encountered with the mechanical pump. This new pumping/cleaning system, which uses the liquid-ring vacuum-pump concept, is shown in Figure 7. Substitution of the mechanical pump by a liquid-ring vacuum pump may also improve the quality of the outlet process gas by absorbing the ammonium bicarbonate deposit that plugged the lines in the previous system. This chemical can be removed from the gas phase by either injecting water or by warming up the pipes above the compound degradation temperature (36°C). The new pumping system has recently been successfully demonstrated in our laboratories during another study for the treatment of petroleum sludges by vacuum pyrolysis”.
This study indicated that the combination of spray towers in series with steam-ejectors and a liquid-ring vacuum pump will probably produce a clean gas effluent while maintaining the pyrolysis unit below the specified pressure requirements (< 10 kPa). Other criteria such as the economics of liquid-ring vacuum pumps were not taken into consideration at this point. Further work is also needed to substantiate the data obtained for the quality of the gas emissions and to test the long-term reliability of the recommended pumping/gas-cleaning equipment.
Acknowledgments The comments and information provided by Mr G. Drouin, Biothermica International Inc.. and Dr H. Pakdel at U. Lava1 are gratefully acknowledged by the authors. MS V. Roy wishes to thank the Natural Sciences and Engineering Research Council of Canada, the Fonds pour la Formation de Chercheurs et I’Aide ti la Recherche. Environment Canada, Gaz Metropolitain and Roche et AssociCs for the scholarships and fellowships awarded during the length of this research project. The collaboration of MS G. Couture has been appreciated by the authors. This study has been partly supported by Energy Mines and Resources Canada, Energie Ressources Quebec and Ultramar
Canada
Inc.
Emissions
According to this study, the concentrations of SO2 and particles in the flue gas emissions are both below the concentration levels allowed by the regulating environmental agency. Limited data also revealed that nitrogen oxides slightly exceeded the acceptable emission level. Usually NO, are generated at high temperature in the combustion chamber by the oxidation of atmospheric nitrogen. During the experiment, the flame was always yellow and temperature of the water in the boiler was maintained below 100°C. Consequently, it is assumed that oxidation of atmospheric nitrogen was not the major reaction producing NO,. The presence of ammonium bicarbonate in the vacuum pump and the high NO, concentration level in the flue gas suggest that the pyrolytic gas phase contained at least one unidentified, major nitrogenous component. The combustion efficiency can be calculated by: ICO21 tl
bt’ com US’o” =
[CO]
+
(CO2
J
(1)
where [CO2J is the carbon dioxide volume fraction, [CO] is the carbon monoxide volume fraction and ocombustion is
86
Gas Separation 8 Purification 1992
Vol 6 No 2
References Craven, R.B., Eliason, D.A., Francy, D.B., Reiter, P., Campos, E.G., Jakob, W.L., Smith, G.C., Bozzi, C.J., Moore, C-G., Maupin, G.O. and Monath, T.P. Importation of Aedes albopictus and other exotic mosquito species into the United States in used tires from Asia J Am Mosquiro Control Assoc (1988) 4 138-142 Pakdel, H., ROY,C.. Aubin. H.. Jean. G. and Coulombe. S. Formation of di-limonene in used tire vacuum pyrolysis &Is Env Sci Technol(l991) 25 1646-1649 Dufton, P.W. ‘The ‘Value and ike qf Scrap Qwes Rapra Technology Ltd. UK (1987) Dodds, J., Domenico, W.F. and Evans, D.R. Scrap tires: A resource and technology evaluation of tire pyrolysis and other selected alternate technologies NTIS Rep No EGG-224I US Dept of Energy (1983) Rivard, P., Roy, C. and Vendette, A. Les possibilitks de rkutilisation commerciale des pneumatiques usagCs L’/ng&~ieur (I 985) 366 25-30 Labrecque, B. Etude du transfert de chaleur par radiation thermique dans un rkacteur de pyrolyse sous vide des vieux pneumatiques K9c.A. Thesk UniversitC de Sherbrooke. Sherbrooke, QuCbec (1987) Roy, C., Labrecque, B. and de Caumia, B. Recycling of scrap tires to oil and carbon black by vacuum pyrolysis Rev Conserv Recyc ( 1990) 4 203-2 I3 M6thode.s nomali.&s
de r@&ence pour le conttile
ci la source:
Gas-cleaning dosage du dioxyde de soyfre Pmis par 1e.ssource.T,fi.ws Service de la protection de I’environnement Rapport EPS I-AP-74-3
Environnement 9
IO
II
Canada (1975)
MPth0de.s de rkfkence normali.sPe.s en vue d’essais aux source.v: me.sure de.s Pmksions dbxyde.s d ‘azote provenant de s0urce.s fixe.y Service de la protection de I’environnement Rapport EPS I-AP77-3 Environnement Canada (1979) MPth0de.s de r
Environnement
system
Canada (1974)
I2
for a scrap-tire
pyrolysis
plant:
K Roy et al.
Assessment of the fiasibility of jluidized refuse derived fuel (RDF) gasification Internal Report. Biothermica International Inc..
Montreal. Canada (1989) Rensfelt, E. and Ekstrom, C. Fuelgasjiwm municipal waste in an integrated circulatingfluid-bedgasiJication/gas-cleaningprocess
I3
Internal Report. Studsvik Energy. Sweden (1989) Roy, C., de Caumia, B., Pakdel, H.. Plante, P., Blanchette. D. and Roy, V. Recyclage des residus huileux par pyrolyse sous vide. Phase II: Unite de develoDDement de Drocedt. Rewrt to Ultramar Canada Inc. and energy Mines and Resburces Canada DSS No 23440-B-9093 (1990)
Gas Separation 8 Purification 1992
Vol 6 No 2
87
and C. Roy**
Laval, IXpartement
de genie chimique, Sainte-Foy (Quebec) Canada Gl K 7P4
Received 24 April 199 1; revised 8 November
199 1
When used tires are subjected to heat in the absence of air, large quantities of vapours and gases are produced. A large portion of the vapours can be condensed to oils with a yield of 55% by weight. However, the non-condensable gas phase (6% yield) is contaminated with oil droplets, tar, fine particles as well as nitrogen- and sulphur-compounds. The project objective is to develop a gas-cleaning system and design a pumping unit that can maintain the total reactor pressure below 10 kPa and burn the process gas without emission problems. A series of experiments was made in a 25 kg h-’ vacuumpyrolysis process-development unit, for which different gas-cleaning units such as a shell and tube heat exchanger, cold traps, cyclones, demisters and filters were tested. A gas burner with a variable orifice was used to burn the pyrolysis gas efficiently. Standard stack sampling was performed to identify the flue gas contaminants. Gas burning in general was successful, but the gas-cleaning system could not remove all the contaminants in the gaseous stream. According to preliminary experimental results, an alternative promising approach is to replace the shell and tube exchanger, cold traps and filters by a liquid-ring vacuum pump. Keywords:
gas-cleaning;
vacuum-pyrolysis;
Introduction Tire recycling has become a necessity because of the accumulation of discarded tires which are a potential environmental risk. Each year 24 million tires are disposed of in Canada and about 240 million tires in the USA. While some of these tires are recapped or ground for special uses, most are dumped in rural areas or in landfills. When buried in landfills. they eventually float to the surface. In piles, the non-biodegradable rubber can cause serious harm if ignited. Tires infested with mosquitos are the subject of increasing concern’. Tires represent a source of energy and chemicals.
By
thermal decomposition, it is possible to recover useful products’. There have been numerous attempts to pyrolyse tires. It is beyond the scope of this paper to describe the various ventures and adaptations of technology. Literature reviews have been published”“. Vacuum pyrolysis is an old concept. It enables the production of large quantities of pyrolysis oils from organic substances. Vacuum minimizes secondary reactions such as thermal cracking, repolymerization and recondensation reactions, gas-phase collision. catalytic cracking and reduction and oxidation reactions. If the vapour-phase products are quenched. the yield of organic liquids such as pyrolysis oils is increased at the expense of solid residues and gases. The physico-chemical properties of the end-products are specific.
tires
The vacuum pyrolysis process has been under development in Quebec since the early 1980s. The research programme focused on establishing background knowledge about the reaction mechanism, pyrolysisproduct characterization and design and operation of process-development units with capacities ranging from batch systems to 50-200 kg h-’ continuous units. Examples of waste materials investigated are: petroleum sludges. activated sludges, plastics, bark residues, municipal solid wastes, biomedical wastes, contaminated soils, automobile shredder residues and used tires. The specific objective of the present study was to develop and test a gas-cleaning system capable of eliminating all potential pollutants from the noncondensable gas phase generated during vacuum pyrolysis of used tires in a 25 kg h-’ process development unit. The process gas will be burned to provide a make-up heat source for the pyrolysis process and the quality of the emissions must be environmentally acceptable. One peculiarity of the process that had to be taken into account for the design of the gas-cleaning unit was the system total pressure, which should be maintained below IO kPa.
Experimental
*Paper presented at GAS Separation International, Austin, TX, USA, 22-24 April 199 1 **Author to whom correspondence should be addressed
The existing vacuum-pyrolysis process-development unit (PDU) operated on a semi-continuous feed mode using uniform cylindrical pieces of used. cross-ply tires sieved to 6.35-12.7 mm mesh sizes. The feedstock was initially poured in a hopperwhich contained all the material to be
0950-4214/92/020083-05 0 1992 Butterworth-Heinemann
Gas Separation Et Purification 1992
Ltd
Vol 6 No 2
83
Gas-cleaning system for a scrap-tire pyrolysis plant: K Roy et al.
Figure 1
stream of the second quench tower by other gasThe and dry ice traps were substituted Methanol flowing through the shell side was cooled in a 1250 W refrigerating equipped with a Figure2 illustrates droplets a stainless-steel installed at the top ofthe second quenching sampling trains were to the system. Monitoring of the gas emissions achieved by using the procedures described by Environment Canadax-‘I’. Samples of the stack gases were taken during run no D015(12 h)inordertodetect presence of the following pollutants: NO,. SO?. fly ash and unburnt particles. NO, was after about 3 h of the onset of the run, once the system was under steadystate conditions. Sampling particles nearly 7 h after the beginning
Original PDU flowsheet
processed during each of the two experiments performed in this study. The reactor was a six-hearth furnace, 2 m in height and 0.7 m in diameter, heated by electrical heat tracing. The average throughput reached by the system during this study was 22 kg h-‘. A schematic diagram of the PDU is shown in Figure 1. A detailed description of this unit6 its elsewhere. residues. The solid residues, comprised carbon black and tibres, found their way across the reactor towards the bottom, where they accumulated separate vessel. The vapours and gases during the thermal decomposition
(
Infrared
spectroscopy ofgaseous compounds automatic stack-sampling Infrared (FTIR) (Bomem Inc., Quebec) a sample every four minutes (Figure 3). A solid, white was found in the manifolds tertiary This was by FTIR. Elemental proximate were performed apparatus and a LECO SC-I 32 apparatus. respectively.
pressure obtained
To Vent t
Figure 2
First modification of the PDU (run no DO1 4)
Figure 3
Second modification of the PDU (run no DO1 5)
equipped with a Molecular Porapak-Q column was used to analyse the process gas. At the end, the gas was flared. When this study was. undertaken, described above was not satisfactory. example, the large across the mechanical vacuum pump caused tar deposition and vapour condensation pyrolytic gases was neither environmentally modified and tested. Run no D014, the ‘orientation experiment’, was performed equipment located down-
84
Gas Separation
Et Purification
1992
Vol 6 No 2
Gas-cleaning system for a strep-tire pyrolysis plant: V. Roy et al. Flue Table 1 procedure
Results and discussion The first pyrolysis experiment (run no DOl4) lasted 8 h and the second (run no DO15, the ‘gas analysis experiment’) 12 h. During the orientation experiment the new shell and tube heat exchanger unit installed at the suction side of the pump did not significantly reduce the amount of liquids accumulating inside the vacuum pump. Such results were not in line with what was reported by other investigators during high-pressure. pilot-plant wood and municipal-waste gasification tests”. “. However, results from run no DO15 showed that the major pyrolytic gaseous components are carbon dioxide and a few light hydrocarbons with pentane and butene leading the list (F&u~ 4). All component concentrations were relatively constant, except for CO, which slowly dropped from 25% by weight to 13% near rhe end of the 12 h run. Consequently the process-gas caiorific value did increase. whereas its average molecular weight remained constant over the experiment. The Environment Canada wet test method to characterize stack emissions is long and tedious and does not easily provide several sets of data simultaneously. Table I shows that. except for NO,, the concentrations observed were ail below the emission levels allowed for ambient air by the QuCbec provincial regulatory agency (Ministry of Environment. QuCbec). It is noticeable. however, that no specific regulations exist for flue gases o~ginating from a pyrolysis unit. The values indicated in Table I are those used for the combustion of fossil fuels. The Bomem continuous gas analysis equipment was well adapted to the harsh environment at the plant site (see Figum 5 and 6). Figure 5. for example, provides some comparison for SO? concentration as calculated by the standard procedure and that determined by FTIR.
Pollutant
gas
analysis
Sampling
following
Environment
Concentration* (ppm by volume]
Canada
(ppm by volume)
*Calculated on a 12% excess oxygen basis
a Environment
+ FTlR
Canada
method
analysis
. i5
loo-
++
++
f+
++
N x
+++ .
++ ++
0
+t-t+
2
4
6
8
.2
10
Time (h)
Figure 5 DO1 5)
T
SO1 flue gas concentration as a function of time (run no
500---
‘10
i-
i
Cxrbon dloxldx 20%
Time (h)
Figure 6
DO15)
The white. translucent crystal found in the piping system after the mechanicai pump was determined by elemental analysis to be ammonium bicarbonate, confirmed by FTIR spectroscopic analysis.
Carbon monoxide 2% Oulrx
CO, and CO concentration in the flue gas (run no
99b
Pumping system
Standard Deviation (%) BUWIW Pentan Carbon Moxlde Yethxno Elhans Propen. ElhOIW pr0plw
Blltur. Carbon Monoxldo othor8 Figure 4
1.4 1.5 6.2 0.9 0.7 1.3 0.9 0.7 0.7 0.2 1.9
Pyrolysis gas average mass com~sition
{run no 00 15)
Despite the improvements brought to the existing gascleaning system. tar and water still found their way inside the vacuum pump, where they contaminated the lubricating oil, thus lowering the pump capacity and efficiency. It was concluded that this chemical reaction system needed a pumping unit that could handle both organic contaminants and water while maintaining a total pressure no higher than IO kPa. Two different pumping systems were considered: a rotary screw vacuum pump (oil seal) or a liquid-ring vacuum pump (water seal). The rotary screw pump was rejected since preliminary testing in the laboratory showed that the condensed liquid hydrocarbons on mixing with the sealing oil formed a
Gas Separation 8 Purification 1992
Vol 6 No 2
85
Gas-cleaning system for a scrap-tire pyrolysis plant: V. Roy et al.
the combustion efficiency. It was maintained at a level of 99.936% with a standard deviation of 0.045% throughout the experimental run. The only comparable value available is the efficiency required (99.900%) by the provincial regulation (MENVIQ) for hazardous-waste incinerators. Although the pyrolysis of used tires cannot be classified in the same category this comparison shows that combustion of the process gas from used-tire vacuumpyrolysis is a highly efficient reaction.
Conclusions
Figure 7 Process flow diagram of the PDU with the liquid-ring vacuum-pump system
stable, undesirable emulsion. Liquid-ring vacuum pumps can handle contaminants immiscible with water (like hydrocarbons). Moreover this type of pump can provide the level of vacuum needed by the addition of one or several steam-ejector stages. Finally, the cleaning effect of water used as a sealing medium can avoid the plugging problems encountered with the mechanical pump. This new pumping/cleaning system, which uses the liquid-ring vacuum-pump concept, is shown in Figure 7. Substitution of the mechanical pump by a liquid-ring vacuum pump may also improve the quality of the outlet process gas by absorbing the ammonium bicarbonate deposit that plugged the lines in the previous system. This chemical can be removed from the gas phase by either injecting water or by warming up the pipes above the compound degradation temperature (36°C). The new pumping system has recently been successfully demonstrated in our laboratories during another study for the treatment of petroleum sludges by vacuum pyrolysis”.
This study indicated that the combination of spray towers in series with steam-ejectors and a liquid-ring vacuum pump will probably produce a clean gas effluent while maintaining the pyrolysis unit below the specified pressure requirements (< 10 kPa). Other criteria such as the economics of liquid-ring vacuum pumps were not taken into consideration at this point. Further work is also needed to substantiate the data obtained for the quality of the gas emissions and to test the long-term reliability of the recommended pumping/gas-cleaning equipment.
Acknowledgments The comments and information provided by Mr G. Drouin, Biothermica International Inc.. and Dr H. Pakdel at U. Lava1 are gratefully acknowledged by the authors. MS V. Roy wishes to thank the Natural Sciences and Engineering Research Council of Canada, the Fonds pour la Formation de Chercheurs et I’Aide ti la Recherche. Environment Canada, Gaz Metropolitain and Roche et AssociCs for the scholarships and fellowships awarded during the length of this research project. The collaboration of MS G. Couture has been appreciated by the authors. This study has been partly supported by Energy Mines and Resources Canada, Energie Ressources Quebec and Ultramar
Canada
Inc.
Emissions
According to this study, the concentrations of SO2 and particles in the flue gas emissions are both below the concentration levels allowed by the regulating environmental agency. Limited data also revealed that nitrogen oxides slightly exceeded the acceptable emission level. Usually NO, are generated at high temperature in the combustion chamber by the oxidation of atmospheric nitrogen. During the experiment, the flame was always yellow and temperature of the water in the boiler was maintained below 100°C. Consequently, it is assumed that oxidation of atmospheric nitrogen was not the major reaction producing NO,. The presence of ammonium bicarbonate in the vacuum pump and the high NO, concentration level in the flue gas suggest that the pyrolytic gas phase contained at least one unidentified, major nitrogenous component. The combustion efficiency can be calculated by: ICO21 tl
bt’ com US’o” =
[CO]
+
(CO2
J
(1)
where [CO2J is the carbon dioxide volume fraction, [CO] is the carbon monoxide volume fraction and ocombustion is
86
Gas Separation 8 Purification 1992
Vol 6 No 2
References Craven, R.B., Eliason, D.A., Francy, D.B., Reiter, P., Campos, E.G., Jakob, W.L., Smith, G.C., Bozzi, C.J., Moore, C-G., Maupin, G.O. and Monath, T.P. Importation of Aedes albopictus and other exotic mosquito species into the United States in used tires from Asia J Am Mosquiro Control Assoc (1988) 4 138-142 Pakdel, H., ROY,C.. Aubin. H.. Jean. G. and Coulombe. S. Formation of di-limonene in used tire vacuum pyrolysis &Is Env Sci Technol(l991) 25 1646-1649 Dufton, P.W. ‘The ‘Value and ike qf Scrap Qwes Rapra Technology Ltd. UK (1987) Dodds, J., Domenico, W.F. and Evans, D.R. Scrap tires: A resource and technology evaluation of tire pyrolysis and other selected alternate technologies NTIS Rep No EGG-224I US Dept of Energy (1983) Rivard, P., Roy, C. and Vendette, A. Les possibilitks de rkutilisation commerciale des pneumatiques usagCs L’/ng&~ieur (I 985) 366 25-30 Labrecque, B. Etude du transfert de chaleur par radiation thermique dans un rkacteur de pyrolyse sous vide des vieux pneumatiques K9c.A. Thesk UniversitC de Sherbrooke. Sherbrooke, QuCbec (1987) Roy, C., Labrecque, B. and de Caumia, B. Recycling of scrap tires to oil and carbon black by vacuum pyrolysis Rev Conserv Recyc ( 1990) 4 203-2 I3 M6thode.s nomali.&s
de r@&ence pour le conttile
ci la source:
Gas-cleaning dosage du dioxyde de soyfre Pmis par 1e.ssource.T,fi.ws Service de la protection de I’environnement Rapport EPS I-AP-74-3
Environnement 9
IO
II
Canada (1975)
MPth0de.s de rkfkence normali.sPe.s en vue d’essais aux source.v: me.sure de.s Pmksions dbxyde.s d ‘azote provenant de s0urce.s fixe.y Service de la protection de I’environnement Rapport EPS I-AP77-3 Environnement Canada (1979) MPth0de.s de r
Environnement
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I2
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