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Abatement technologies for N2O emissions in the adipic acid industry
Chemosphere ± Global Change Science 2 (2000) 425±434
Abatement technologies for N2O emissions in the adipic acid industry A. Shimizu *, K. Tanaka, M. Fujimori Asahi Chemical Industry Co., Ltd., Leona Plant 4-3401, Nagahama-cho, Nobeoka, Miyazaki 882-0854, Japan Received 21 June 1999; accepted 11 January 2000
Importance of this paper. Plants manufacturing adipic acid (AA), the material for nylon 6, 6, are operating throughout the world and have been pointed to as one of the sources of nitrous oxide. The capacity of nitrous oxide from all adipic acid plants in the world is estimated at 576,250 t per year. The major adipic acid manufacturers have now begun to operate nitrous oxide reduction facilities and these plants now emit only a small part of the nitrous oxide that they produce. Forecasts say that more than 80% of the nitrous oxide produced by all adipic acid plants in the world will be eliminated by 1999±2000. Abstract Adipic acid (AA) is the main intermediate in nylon 6, 6 that is manufactured by polymerization condensation of AH salt (hexamethylenediammonium adipate). Adipic acid is also an intermediate in the production of polyester-polyol, a material used in polyurethane. Annual production capacity of AA for 1998 was estimated to be 2.3 million metric tons and about 80% of that AA is used to manufacture nylon 6, 6. Almost all AA is produced by nitric acid oxidation of KA oil, a mixture of cyclohexanone and cyclohexanol. The reaction of nitric acid oxidation unavoidably generates nitrous oxide. The N2 O emission coecient for Japan's AA plant is approximately 0.25 kg-N2 O/kg-AA. If the N2 O output from all adipic acid plants is calculated using the N2 O emission coecient described above and the world's AA production capacity then we obtain a ®gure of 576,250 metric tons per year, but if we calculate only that which will be clearly reduced by 1999±2000, a reduction of ca. 80% has already been achieved. This is because the N2 O abatement equipment of the major AA manufacturers is scheduled to have completed startup by 1999±2000. The main technologies used to reduce nitrous oxide in the adipic acid industry are catalytic decomposition and thermal destruction. These methods convert nitrous oxide into nitrogen and oxygen. Catalytic decomposition operates at about 500°C and thermal destruction operates at and over 1000°C. Using these reduction technologies allows the adipic acid manufacturers to reduce N2 O emissions by 90% or more. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Catalytic decomposition; Thermal destruction; Nitric acid oxidation; Nitrous oxide; Abatement technology
1. Introduction
*
Corresponding author. Fax: +81-982-22-6557. E-mail address: [email protected] (A. Shimizu).
Literature on nitrous oxide emissions point to chemical equipment in the adipic acid (AA) process as generators of nitrous oxide (Thiemens and Trogler, 1991; Reimer et al., 1995, 1994; RITE, 1996, 1997, 1998, 1999). Thiemens and Trogler (1991) have pointed out the in¯uence of nitrous oxide emission caused by AA
1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 0 0 ) 0 0 0 2 4 - 6
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plants upon the EarthÕs atmosphere. The 1992 IPCC report says that the total N2 O emitted from all natural and anthropogenic sources is 5.2±16.1 Tg-N/y and the nitrous oxide emitted by the AA industry is 0.4±0.6 Tg-N/y. In 1991, the main AA manufacturing companies, Asahi, BASF, Bayer, DuPont, ICI (cf. Appendix A (a)) and Rhone-Poulenc (cf. Appendix A (b)) convened a meeting, under their own volition, called the N2 O InterIndustry Group Meeting (cf. Appendix A (d)) and from then on these companies exchanged technology that accelerated research and development on N2 O abatement techniques. All these companies have already announced the N2 O abatement technologies they are using and are reporting their schedules for the operation of N2 O abatement processes in the world's major AA plants by 1999±2000 (Reimer et al., 1995, 1994). This paper gives the present state of N2 O abatement technologies and the future trends in development.
2. The adipic acid business Adipic acid is an important monomer that is an intermediate of nylon 6, 6, made by polymerization condensation with hexamethylene diamine (HMDA) and as an intermediate in the polyester polyol that makes up polyurethane. The world's AA production capacity for 1998 is estimated at 2.3 million tons per year. About 80% of that AA goes into the production of nylon 6, 6. Nylon 6, 6 is used to make the widely used products of tire cord, carpets, textiles, upholstery and auto parts. Table 1 shows the estimated manufacturing capacity of all AA manufacturers in the world and that the capacity in the entire world is 2,305,000 t/yr in 1998 (PCI, 1996; cf. Appendix A (e)).
3. The mechanism by which N2 O is emitted from adipic acid plants Reactions (1) and (2) indicate the nitric acid oxidation reaction.
Table 1 Production capacities of adipic acid (1998) Producer
Region
DuPont
North America UK Singapore USA USA France Brazil Korea Germany Germany Italy
Solutia Others Rhodia BASF Bayer Radici CIS, East Europe China Asahi Others
Japan Japan
Total
Capacity t/yr 695,000 205,000 110,000 320,000 15,000 240,000 55,000 50,000 235,000 50,000 50,000 65,000 92,000 120,000 3000 2,305,000
Nitrous oxide is a byproduct in the course of nitric acid oxidation as reactions (1) and (2) show. The logical emission coecient in reaction (2) is 0.30 kg-N2 O/kgAA, but the emission coecient actually measured at the Nobeoka Works of Asahi Chemical was 0.25 kg-N2 O/ kg-AA (survey by Miyazaki Prefecture in 1994), which is because in the actual reaction, part of the nitric acid becomes NOx and N2 (Thiemens and Trogler, 1991).
4. N2 O generated quantities Table 2 shows the estimates for manufacturing capacity of nitrous oxide by all AA manufacturing plants and the estimated reductions in nitrous oxide for 1999± 2000. When we use the 0.25 nitrous oxide emission coecient described above, the total nitrous oxide capacity for AA manufacturing plants is 576,250 t/yr (0.37 Tg-N/y). These ®gures are in good accordance with ®gures in the previous paper by Thiemens and Trogler (1991). In short, if there were absolutely no abatement of N2 O by AA plants this is the quantity of N2 O that would be emitted, but because the N2 O abatement of the major manufacturers has proceeded to the great extent that it has at present, as will be described later, the numbers for actual N2 O emissions are quite a bit below this. If we take the rate of decomposition for the N2 O decomposition process to be an average of 90% (cf. Appendix A (f)), then the total N2 O reduction quantities for 1999± 2000, for just the main companies, is a total 520,000 t/yr, and 81% of the entire N2 O generation capacity is reduced for 1999±2000. It probably understates the number of N2 O reduction quantities because the announced
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Table 2 Estimated amount of N2 O abatementa Capacity t/yr
N2 O t/yr
AA
N2 O
Reduced
Emitted
Major producers Others
2,080,000 225,000
520,000 56,250
>468,000 No information
<52,000
World total
2,305,000
576,250 0.37 Tg-N/y
>468,000 Reduction rate >81%
a
Major producers: Asahi, BASF, Bayer, DuPont, Rhodia, Solutia. Premises: Proportion of N2 O generation 0.25 kg-N2 O/kg-AA. Decomposition rate of N2 O 90%. Reduced N2 O N2 O Capa. ´ 0.9 Emitted N2 O N2 O Capa. ± Reduced N2 O. Announced decomposition values of the actual processes: Asahi 98%, BASF 90±95%, Dupont 98%. No information on other plants.
values of the N2 O reduction processes at the actual plants are between 90% and 98% (Reimer et al., 1994, 1995; cf. Appendix A (g)). Since it is possible, in principle, to reach a rate of N2 O decomposition of close to 100%, there is also a possibility that future technological development will increase the mean rate of decomposition of these processes even further. The emissions from AA plants were calculated to be less than 5% of all N2 O generated by natural and anthropogenic sources in the entire world in the 1992 IPCC report. With the current abatement technologies this value falls to 1%, or less, by 1999±2000. The industrial source of N2 O except AA plant is dodecanedioic acid plant (Thiemens and Trogler, 1991) Dodecanedioic acid is a material for nylon 6, 12, and manufactured by nitric acid oxidation of cyclododecanol and cyclododecanone (Weissermel and Arpe, 1994). Compared with AA plant, production capacities of dodecanedioic acid are small. Therefore, the N2 O generated from dodecanedioic acid plant may be negligible. 5. Technology for reducing N2 O in the adipic acid industry 5.1. Mechanisms for decomposing N2 O Many scholars have been examining the chemistry for the decomposition of N2 O (Loirat et al., 1985; Kuratani and Tsuchiya, 1978). Scheme 1 shows the elementary reactions of N2 O decomposition. Eq. (1) shows the initial reaction. M signi®es many dierent kinds of compounds such as N2 O, N2 O or O2 . Eq. (2) shows that the major reaction generating material in the decomposition of N2 O is N2 and O2 . Eqs. (3) and (6) are for the NO generation reaction and Eq. (5) is the NO2 generation reaction. Scheme 2 shows the mutual relationship between the elementary reactions. N2 O and NO are mutually converted by Eqs. (3), (6) and (5). N2 O is converted into N2 and O2 as indicated by Eq. (2). NO generation is not
Scheme 1. Elementary reactions of N2 O decomposition.
Scheme 2. Reaction network for N2 O decomposition reaction.
important until the reaction temperature goes over 800°C. The generation of N2 and O2 is exothermic, and the heat of reaction is calculated from Eqs. (1) and (2) as ÿ8:5 kJ/mol. The NO generation reaction is endothermic and its heat of reaction is calculated from Eqs. (1) and (3) as +8.5 kJ/mol. 5.2. The processes of decomposing N2 O by the companies The N2 O abatement technology of the major AA manufacturers is chie¯y the catalytic decomposition process or the thermal destruction process (Reimer et al., 1994, 1995). Other than these processes, there are reports of a method that consumes N2 O as an oxidant for phenol synthesis (cf. Appendix A (g)). Table 3 brings together all the processes used by the companies (Reimer et al., 1994, 1995). Below we will give a general description of the technology for each company that has been made known through their patent applications.
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Table 3 N2 O abatement technologies Producer
Technology
Start up
Asahi BASF Bayer DuPont
Thermal Catalytic RFBa Catalytic RFBa
Rhone-Poulenc
Thermal Catalytic Phenol production
1999 1997 1993 1997 1958, 1976, 1994 1997 1998 2000
Solutia a
Reducing ¯ame burner.
5.2.1. Catalytic decomposition process BASF announced in 1997 that it would introduce N2 O decomposition equipment that uses the catalytic method (cf. Appendix A (f)). It then completed N2 O decomposition equipment for an AA 240,000 t/yr plant at 13 million Deutsche Marks and announced that it had achieved a decomposition rate of 95% (cf. Appendix A (g)). The catalyst used in the patent application is either a spinel type CuAl2 O4 , a catalyst that are Ag and CuO supported on Al2 O3 , or a catalyst that is Ag supported on Al2 O3 . When the catalysis is around 500°C using the o gas which contains 23% N2 O, from the actual AA process, all of these catalysts give a decomposition rate of 99% or higher. Table 4 shows the major gas compositions that have been emitted from the AA process (BASF, 1991, 1993, 1994a,b). DuPont announced that its catalytic process worked in 1997 and that the catalyst was CoO and NiO supported on ZrO2 (cf. Appendix A (d)). When N2 O of 100% concentration is processed at a reaction temperature of 402°C, the N2 O decomposition rate is 98.5%. When ZrO2 is used as the support, it has good low temperature activity compared to Al2 O3 , with degradation also decreased (DuPont, 1993). Asahi Chemical (1993) has proposed a catalyst that is CuO supported on Al2 O3 . According to our investigation, processing the AA plant emission gases that contain 34% N2 O at a reaction temperature of 620°C gives an N2 O decomposition rate of 99.5% and higher. Asahi (1999) has also proposed a further improved catalytic decomposition device. The catalytic decomposition in Table 4 O-gas analysis of ADA plant Component
Mol/%
N2 O NO2 N2 O2 H2 O
23 17 47 7.5 3.0
the ¯ow system requires rarefying the process gases to maintain the catalyst temperature at optimum level. The improved method introduces the process gas containing N2 O into several stages of a ®xed bed plug ¯ow reaction column that reduces the quantity of rare®ed gases to control the reaction temperature and maintains the N2 O decomposition reaction at the prescribed temperature to reduce energy costs. Fig. 1 shows the reactor described in the patent. Numbers 17, 18, 24 and 25 are inlets for the process gas, 19 is the preheating chamber, 22 is the catalyst layer and 26 is the outlet for generated gas. Descriptions of patents other than those that have been submitted by the AA manufacturers are given here. Air Products & Chemical and Engelhard (1993, 1991) has reported a hydrotalcite catalyst that contains metal and a metal-ion-exchanged zeolite catalyst, Grande Paroisee (1993) has reported on a moldenite/NH4 /Fe catalyst, and Agency of Industrial Science and Technology (1996) has reported on a catalyst that is Rh2 O3 supported on Al2 O3 . Table 5 gives a summary of the reaction conditions and results for using the above catalysts. Fig. 2 shows the process that was made public by BASF (1997). Exhaust gases 1 and 3 containing N2 O and NOx from the plant pass through compressor V1 with air ¯ow 2 to increase the pressure. That makes the temperature of mixed air ¯ow 4 250±350°C. Heat exchangers WT1 and WT2 cool air ¯ow 4 to 30±40°C, it then becomes air ¯ow 5 that passes through absorption column K1 to recover NOx as nitric acid. Air ¯ow 6 that exits absorption column K1 is heated to 450±500°C by heat exchangers WT1 and WT3 and is then supplied to decomposition reactor C1. WT3's heat source is the gas after N2 O decomposition. The catalyst in reactor C1 decomposes N2 O and discharges air ¯ow 9 at approximately 800°C temperature. The air ¯ow after decomposition is cooled to 260±300°C by heat exchangers WT3 and WT4 to become air ¯ow 10. Heat exchanger WT4 recovers the decomposition heat from N2 O as steam. Air ¯ow 10 is introduced into adiabatic reactor C2, that contains an NOx reduction catalyst which reduces the NOx to N2 and O2 . The temperature of air ¯ow 11 at exit C2 is 265±310°C, and turbine T1 gives adiabatic expansion of the air ¯ow and discharges it into the atmosphere at 100°C. Either the isothermal reactor or the adiabatic reactor is believed appropriate to use as the decomposition reactor in catalytic decomposition of N2 O, but it is not clear what type of reactors the AA manufacturers are actually using. The form of the adiabatic reactor is one in which the reaction device is a ¯ow system column type ®xed bed. With the isothermal reactor, a reactor device is used that has a structure made from a cooling agent and a heat exchanger and a ¯ow system multi-duct catalyst ®lled layer (BASF, 1997). The N2 O emitted from an AA plant has high concentration, thus
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Fig. 1. Reactor for the catalytic decomposition.
Table 5 Reaction conditions and results of catalytic decomposition Cat ZnO, CuAl2 O4 /Al2 O3 MgO, CuAl2 O4 /Al2 O3 CaO, CuAl2 O4 /Al2 O3 Ag, CuO/Al2 O3 Ag/Al2 O3 CoO, NiO/ZrO2
Metal (wt%)
Temperature (°C)
Conv. (%)
SV (1/h)
480 480 480 490 550 402
> 99:9 > 99:9 > 99:9 > 99:9 > 99 98.5
4000 4000 4000 4000 4000 600
N2 O conc. of inlet (%) 23 23 23 23 23 100
620 450
> 99:5 99
3350 30,000
34 0.1
Cu±ZSM-5
Ag:14.9 Ag:14.2 Co:0.916 Ni:0.908 Cu:2.4 Co/ Al:2.2 Cu:4.0
400
95
30,000
0.1
Mordenite/NH4/Fe Rh2 O3 /ZnO
Rh:0.5
540 500
99.7 100
13,300 40,000
50 0.1
CuO/Al2 O3 Co±Al-hydrotalcite
presenting the possibility of temperatures rising higher than the temperature tolerances of the reactor and the catalyst and this requires special caution when an adiabatic reactor is used. It is important that the temperature at the reactor outlet will have to be held to about 800°C. The temperature of the adiabatic reactor is controlled by the quantity of rare®ed gases that are supplied with the process gas and the temperature of the isothermal reactor is controlled by the reactor coolant (molten salt). A method has been proposed for controlling reactor temperature by returning UOP to the reactor after decomposition and after the gas has cooled (UOP, 1993). Fig. 3 shows the process ¯ow in that method. In the diagram, 1 is the ¯ow of gas containing N2 O, 2 is the NOx removal process, 14 is the N2 O decomposition section, 30 is the heat exchanger, 55, 56 and 57 are the recycle lines, and 25 and 50 are the discharge lines. There are also methods for controlling reactor temperature by inserting external air, but this method is thought to be advantageous in heat terms because the gas is recycled after the reaction.
Experimental term (h) 1036 1025 1013 242 550
BASF BASF BASF BASF BASF DuPont
2400
Asahi Air products, Engelhard Air products, Engelhard Grande Paroisse MITI
The heat of the reaction of N2 O decomposition is comparatively high at ÿ81:5 J/mol, and the generated heat can be used in steam generation by process gas preheaters and boilers. The ability to recover heat is an advantage of the decomposition processing of N2 O generated from the AA process. There is a possibility of the life of the catalyst becoming a problem with catalytic decomposition. The deterioration of catalysts, for example Pd/Al2 O3 and CuO/ZnO, have been reported in the patent application (Asahi, 1993). The detailed data of deterioration for the catalyst in Table 5 have not been disclosed. We are awaiting further data in order to obtain knowledge on this point. Fig. 4 is a conceptual diagram of the catalytic process. 5.2.2. Thermal destruction process The thermodynamics and kinetics on the elementary reactions of N2 O decomposition have been studied in detail (Loirat et al., 1985; Kuratani and Tsuchiya, 1978)
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Fig. 2. Catalytic decomposition process. Table 6 Thermal destruction of N2 Oa Component
Fig. 3. Temperature control of the catalytic decomposition process.
Concentration Feed (mol/%)
Product (mol/%)
N2 O NO NO2 CO2 O2 N2
51.0 0.02 0.20 5.52 5.57 37.7
0 2.63 8.28 4.57 16.21 68.31
Total ¯ow rate
2.87 Nm3 /h
3.46 Nm3 /h
a
Reaction conditions: Temperature: 1054°C. Pressure: 1.5 kg/cm2 . LV:44.7 cm/s. Residence time: 1.2 s.
Fig. 4. Conceptual diagram of catalytic process.
and a reaction simulation is feasible. Because the decomposition reaction of N2 O into N2 and O2 is an exothermic reaction, if the decomposition is done on adiabatic condition, the heat of the reaction will continue the decomposition. Asahi Chemical (1986) has proposed a method of decomposing N2 O into nitrogen and oxygen and one that recovers NO generated as nitric acid. Table 6 shows that allowing the N2 O in the gas emitted from an AA plant to remain in a reaction furnace for 1.2 s at 1054°C decomposes the N2 O 100% and converts 21% of the N2 O supplied into NOx . Fig. 5 shows the process ¯ow described in the published patent. The exhaust gas generated from AA reaction process 8 is ®rst pressurized to 1±2 kg/cm2 by water-sealed pump 16 and after the NO contained is oxidized into NO2 in oxidation column 17, it is fed into absorption column 18 and the NO2 is
recovered as nitric acid. The air ¯ow from which NO2 has been removed is fed into the plug ¯ow reactor 19 where thermal destruction reaction is performed on the N2 O. After thermal destruction, air ¯ow feeds into a plug ¯ow reactor tube so that it can be heated externally and used to heat process gas. The air ¯ow then feeds into boiler 20 to be heat-recovered, it is then cooled to room temperature by heat exchanger 24, and then feeds to absorption column 27 where the NOx is recovered as nitric acid. Asahi Chemical (1999) has also applied for a patent on an improved N2 O processing equipment. This method heats part of the gas, which contains N2 O, supplied to the reactor to start a decomposition reaction and the remaining gas that contains N2 O is supplied in several stages to the decomposition reactor to be decomposed. Fig. 6 shows the reactor that is described in the patent description. Numbers 1 and 4 in the diagram are the entrances for the process gas 2 is the preheating chamber, 5 is the thermal destruction chamber, 6 is the exit for the generated gas. In this method, the quantity of energy that is supplied to the reactor to sustain the N2 O decomposition reaction is kept extremely low. For energy input a combustible gas such as hydrogen or methane is used, and the amount of heat input required for 1 Nm3 decomposition of the 550°C process gas that
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Fig. 5. Thermal decomposition process.
Fig. 6. Reactor for the thermal destruction.
contains 33.9 mol% N2 O is 46.0 kJ and the decomposition rate is given as 99% or higher. The thermal destruction method has the advantage of being able to recover NO as nitric acid. It is also superior on the point of given o with almost no industrial waste discharge because there is no replacement of catalyst. Asahi Chemical began the operation of its thermal destruction method in 1999 at an investment cost of 600 million yen to achieve a 98-plus-percentage rate of decomposition. DuPont (1995) has proposed a method of spontaneously maintaining the thermal destruction of N2 O, by starting the destruction by placing methane combustion of process gas containing N2 O in contact with
a ¯ame. This method mixes methane gas with a gas containing 660°C N2 O in a mixing tee and heats it to 850°C or higher to create a decomposition reaction. Using this method to process the exhaust gas from an AA plant that contains 57% N2 O gives an N2 O concentration at the reactor exit of 200 ppm or less at 1050±1254°C of decomposition temperature. It also generates about 10% NOx from the N2 O which can be recovered as nitric acid. The amount of methane for processing gas 20 SCFM (ft3 /min) containing 57 mol% N2 O is 30,000 BTU/h (1 BTU 1.055 J) by conversion of heat volume. Hoechst (1992) has proposed a method of decomposition that sprays the process gas containing N2 O into
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a ¯ame created by combustion of natural gas. This method uses ¯ame temperature to decompose the N2 O, but it uses more fuel than the DuPont method described above. Processing 240 m3 /h cubic meters per hour of N2 O, for example, takes 15.2 m3 /h of natural gas and 113 m3 /h of air. The concentration of N2 O in the gas after it has passed the reactor is 80 ppm. Bayer does its processing using a reducing ¯ame burner (RFB) (Reimer et al., 1994, 1995). A patent has been applied for the combustion device (Bayer, 1991) for achieving the reducing atmosphere, but it is not clear whether this method will be used in the processing of N2 O. Rhone-Poulenc (1995) has proposed a method that uses plasma to convert N2 O into NOx . This method discharges an arc into the air ¯ow containing N2 O to generate plasma and convert that N2 O into NOx . Fig. 7 shows the device. The air ¯ow containing N2 O is preheated in gas preheater 7 to pass out of nozzle 3, becomes plasma by the discharge of an arc across electrodes 10, converts to NOx in area 6, and then exits from 17. In radiation form, the six electrodes are set in opposition so that there are three pairs each. The amount of electricity required to process gas containing 31.6±50.9% N2 O at 22.0±81.8 Ndm3 /min is 0.8 to 2.1 kW. The reaction rate when process gas containing 50.9% N2 O is supplied at 25°C and an equivalent of 37.4 Ndm3 /min is 26.1%, and the NOx selection rate is 55.7%. Fig. 8 shows a conceptual diagram of the thermal destruction process. It has the major feature of being
Fig. 8. Conceptual diagram of thermal process.
able to recover NOx because its decomposition temperature is higher than the catalytic process. 5.2.3. Phenol synthesis Monsanto (cf. Appendix A (c)) has announced its schedule of placing in operation by a target date of 1999±2000 a process for processing N2 O as the result of using zeolite as the catalyst and causing benzene to react with N2 O and producing phenol (cf. Appendix A (h)). This method takes as its base the technology from the Boreskov Institute of Catalysis (1995). When a reaction is made at SV 600 hÿ1 and a reaction temperature of 430°C using a ZSM-5 zeolite catalyst that contains 0.45% Fe2 O3 , they have achieved results of a 98% phenol selection rate and a 100% N2 O reaction rate. The selectivity with the cumene method is reportedly about 93%, so that the selection rate with the N2 O oxidation method is much better. This method might be dependent on location and production quantity of the AA plant so that it places restrictions on the freedom of plant design but a lot of attention is being given to it as a method of N2 O decomposition and phenol production.
6. Future trends
Fig. 7. Plasma chemical reactor.
Because of the high 20±60% N2 O concentration in exhaust gas from the AA process, N2 O decomposition heat can be recovered and reused in N2 O decomposition or it can be used as steam for other processes in the factory. Taking into consideration the large eect that these processes have on reducing the environmental burden, the capital costs are relatively low. However, there are problems with the heat source in the abatement of exhaust gases with low N2 O concentration in processes other than AA, because of the low quantity of heat generated along with the decomposition and the need to heat the large quantity of gases that coexist with N2 O to a temperature sucient for decomposition. If petroleum products are used for the heating source, then
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carbon dioxide will be produced. This is why it is thought that highly active low temperature catalysts will have to be developed. Caution must be given to carbon dioxide generated from the heating source and the quantity of N2 O that is decomposed in the thermal destruction processes. The technologies for N2 O abatement in AA manufacturing seem generally to be complete but it is believed that there is still room for improvement. It is hard to discuss the question of catalyst deterioration because few data on this subject have been released, but if we assume that catalyst deterioration is a general phenomenon, there will be costs for new purchases of replacement catalyst. The thermal destruction process generates relatively large amount of NO in high temperature decomposition. Therefore, it is hoped that equipment materials with high temperature resistance will be developed because if the furnace's ability to withstand heat increases, the amount of recovered nitric acid increases. Since new AA plants are scheduled to be built in various parts of the world (cf. Appendix A (i)) and it can be predicted that some of the plants do not have N2 O decomposition processing, there should be continued improvement in the technology to introduce it into those plants. N2 O is a gas with the properties of an oxidizing agent so that one of the future means of N2 O abatement will be the development of technologies for separating and recovering N2 O cheaply from exhaust gases and the development of technologies for using N2 O in a way that will take advantage of these features (BASF, 1982).
Acknowledgements Contained in this paper are results from surveys made by the Research Institute of Innovative Technology for the Earth (RITE) and the New Energy and Industrial Technology Development Organization (NEDO). The authors wish to thank both organizations for giving the opportunity to participate in their committees.
Appendix A Adipic acid (AA) business was restructured as follows: (a) The AA business of ICI was succeeded by DuPont, (b) the AA business of Rhone-Poulenc was succeeded by Rhodia, and (c) the AA business of Monsanto was succeeded by Solutia. The authors have referred to the following industrial information: (d) 1992. European Chemical News May 25, p. 35, (e) 1997. Asian
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Chemical News Decmber 1, 4, p. 38, (f) 1997. Financial Times February 3, p. 20, (g) 1998. European Chemical News 12±18 January, p. 43, (h) 1996. Chemical Market Reporter December 30, p. 1, (i) 1997. Chemical Week November 5, 159, p. 22.
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Thiemens, M.H., Trogler, W.C., 1991. Nylon production: an unknown source of atmospheric nitrous oxide. Science 251, 932. UOP, 1993. US 5, 200, 162. Weissermel, K., Arpe, H.-J., 1994. Industrielle Organishe Chemie. VCH Verlagsgesellschaft, Germany.
The authors as a group investigate a new production process of nylon 6, 6 intermediates. The backgrounds of authors, Shimizu, Tanaka and Fujimori, are physical organic chemistry, chemical engineering and organic chemistry respectively.
Abatement technologies for N2O emissions in the adipic acid industry A. Shimizu *, K. Tanaka, M. Fujimori Asahi Chemical Industry Co., Ltd., Leona Plant 4-3401, Nagahama-cho, Nobeoka, Miyazaki 882-0854, Japan Received 21 June 1999; accepted 11 January 2000
Importance of this paper. Plants manufacturing adipic acid (AA), the material for nylon 6, 6, are operating throughout the world and have been pointed to as one of the sources of nitrous oxide. The capacity of nitrous oxide from all adipic acid plants in the world is estimated at 576,250 t per year. The major adipic acid manufacturers have now begun to operate nitrous oxide reduction facilities and these plants now emit only a small part of the nitrous oxide that they produce. Forecasts say that more than 80% of the nitrous oxide produced by all adipic acid plants in the world will be eliminated by 1999±2000. Abstract Adipic acid (AA) is the main intermediate in nylon 6, 6 that is manufactured by polymerization condensation of AH salt (hexamethylenediammonium adipate). Adipic acid is also an intermediate in the production of polyester-polyol, a material used in polyurethane. Annual production capacity of AA for 1998 was estimated to be 2.3 million metric tons and about 80% of that AA is used to manufacture nylon 6, 6. Almost all AA is produced by nitric acid oxidation of KA oil, a mixture of cyclohexanone and cyclohexanol. The reaction of nitric acid oxidation unavoidably generates nitrous oxide. The N2 O emission coecient for Japan's AA plant is approximately 0.25 kg-N2 O/kg-AA. If the N2 O output from all adipic acid plants is calculated using the N2 O emission coecient described above and the world's AA production capacity then we obtain a ®gure of 576,250 metric tons per year, but if we calculate only that which will be clearly reduced by 1999±2000, a reduction of ca. 80% has already been achieved. This is because the N2 O abatement equipment of the major AA manufacturers is scheduled to have completed startup by 1999±2000. The main technologies used to reduce nitrous oxide in the adipic acid industry are catalytic decomposition and thermal destruction. These methods convert nitrous oxide into nitrogen and oxygen. Catalytic decomposition operates at about 500°C and thermal destruction operates at and over 1000°C. Using these reduction technologies allows the adipic acid manufacturers to reduce N2 O emissions by 90% or more. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Catalytic decomposition; Thermal destruction; Nitric acid oxidation; Nitrous oxide; Abatement technology
1. Introduction
*
Corresponding author. Fax: +81-982-22-6557. E-mail address: [email protected] (A. Shimizu).
Literature on nitrous oxide emissions point to chemical equipment in the adipic acid (AA) process as generators of nitrous oxide (Thiemens and Trogler, 1991; Reimer et al., 1995, 1994; RITE, 1996, 1997, 1998, 1999). Thiemens and Trogler (1991) have pointed out the in¯uence of nitrous oxide emission caused by AA
1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 0 0 ) 0 0 0 2 4 - 6
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plants upon the EarthÕs atmosphere. The 1992 IPCC report says that the total N2 O emitted from all natural and anthropogenic sources is 5.2±16.1 Tg-N/y and the nitrous oxide emitted by the AA industry is 0.4±0.6 Tg-N/y. In 1991, the main AA manufacturing companies, Asahi, BASF, Bayer, DuPont, ICI (cf. Appendix A (a)) and Rhone-Poulenc (cf. Appendix A (b)) convened a meeting, under their own volition, called the N2 O InterIndustry Group Meeting (cf. Appendix A (d)) and from then on these companies exchanged technology that accelerated research and development on N2 O abatement techniques. All these companies have already announced the N2 O abatement technologies they are using and are reporting their schedules for the operation of N2 O abatement processes in the world's major AA plants by 1999±2000 (Reimer et al., 1995, 1994). This paper gives the present state of N2 O abatement technologies and the future trends in development.
2. The adipic acid business Adipic acid is an important monomer that is an intermediate of nylon 6, 6, made by polymerization condensation with hexamethylene diamine (HMDA) and as an intermediate in the polyester polyol that makes up polyurethane. The world's AA production capacity for 1998 is estimated at 2.3 million tons per year. About 80% of that AA goes into the production of nylon 6, 6. Nylon 6, 6 is used to make the widely used products of tire cord, carpets, textiles, upholstery and auto parts. Table 1 shows the estimated manufacturing capacity of all AA manufacturers in the world and that the capacity in the entire world is 2,305,000 t/yr in 1998 (PCI, 1996; cf. Appendix A (e)).
3. The mechanism by which N2 O is emitted from adipic acid plants Reactions (1) and (2) indicate the nitric acid oxidation reaction.
Table 1 Production capacities of adipic acid (1998) Producer
Region
DuPont
North America UK Singapore USA USA France Brazil Korea Germany Germany Italy
Solutia Others Rhodia BASF Bayer Radici CIS, East Europe China Asahi Others
Japan Japan
Total
Capacity t/yr 695,000 205,000 110,000 320,000 15,000 240,000 55,000 50,000 235,000 50,000 50,000 65,000 92,000 120,000 3000 2,305,000
Nitrous oxide is a byproduct in the course of nitric acid oxidation as reactions (1) and (2) show. The logical emission coecient in reaction (2) is 0.30 kg-N2 O/kgAA, but the emission coecient actually measured at the Nobeoka Works of Asahi Chemical was 0.25 kg-N2 O/ kg-AA (survey by Miyazaki Prefecture in 1994), which is because in the actual reaction, part of the nitric acid becomes NOx and N2 (Thiemens and Trogler, 1991).
4. N2 O generated quantities Table 2 shows the estimates for manufacturing capacity of nitrous oxide by all AA manufacturing plants and the estimated reductions in nitrous oxide for 1999± 2000. When we use the 0.25 nitrous oxide emission coecient described above, the total nitrous oxide capacity for AA manufacturing plants is 576,250 t/yr (0.37 Tg-N/y). These ®gures are in good accordance with ®gures in the previous paper by Thiemens and Trogler (1991). In short, if there were absolutely no abatement of N2 O by AA plants this is the quantity of N2 O that would be emitted, but because the N2 O abatement of the major manufacturers has proceeded to the great extent that it has at present, as will be described later, the numbers for actual N2 O emissions are quite a bit below this. If we take the rate of decomposition for the N2 O decomposition process to be an average of 90% (cf. Appendix A (f)), then the total N2 O reduction quantities for 1999± 2000, for just the main companies, is a total 520,000 t/yr, and 81% of the entire N2 O generation capacity is reduced for 1999±2000. It probably understates the number of N2 O reduction quantities because the announced
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Table 2 Estimated amount of N2 O abatementa Capacity t/yr
N2 O t/yr
AA
N2 O
Reduced
Emitted
Major producers Others
2,080,000 225,000
520,000 56,250
>468,000 No information
<52,000
World total
2,305,000
576,250 0.37 Tg-N/y
>468,000 Reduction rate >81%
a
Major producers: Asahi, BASF, Bayer, DuPont, Rhodia, Solutia. Premises: Proportion of N2 O generation 0.25 kg-N2 O/kg-AA. Decomposition rate of N2 O 90%. Reduced N2 O N2 O Capa. ´ 0.9 Emitted N2 O N2 O Capa. ± Reduced N2 O. Announced decomposition values of the actual processes: Asahi 98%, BASF 90±95%, Dupont 98%. No information on other plants.
values of the N2 O reduction processes at the actual plants are between 90% and 98% (Reimer et al., 1994, 1995; cf. Appendix A (g)). Since it is possible, in principle, to reach a rate of N2 O decomposition of close to 100%, there is also a possibility that future technological development will increase the mean rate of decomposition of these processes even further. The emissions from AA plants were calculated to be less than 5% of all N2 O generated by natural and anthropogenic sources in the entire world in the 1992 IPCC report. With the current abatement technologies this value falls to 1%, or less, by 1999±2000. The industrial source of N2 O except AA plant is dodecanedioic acid plant (Thiemens and Trogler, 1991) Dodecanedioic acid is a material for nylon 6, 12, and manufactured by nitric acid oxidation of cyclododecanol and cyclododecanone (Weissermel and Arpe, 1994). Compared with AA plant, production capacities of dodecanedioic acid are small. Therefore, the N2 O generated from dodecanedioic acid plant may be negligible. 5. Technology for reducing N2 O in the adipic acid industry 5.1. Mechanisms for decomposing N2 O Many scholars have been examining the chemistry for the decomposition of N2 O (Loirat et al., 1985; Kuratani and Tsuchiya, 1978). Scheme 1 shows the elementary reactions of N2 O decomposition. Eq. (1) shows the initial reaction. M signi®es many dierent kinds of compounds such as N2 O, N2 O or O2 . Eq. (2) shows that the major reaction generating material in the decomposition of N2 O is N2 and O2 . Eqs. (3) and (6) are for the NO generation reaction and Eq. (5) is the NO2 generation reaction. Scheme 2 shows the mutual relationship between the elementary reactions. N2 O and NO are mutually converted by Eqs. (3), (6) and (5). N2 O is converted into N2 and O2 as indicated by Eq. (2). NO generation is not
Scheme 1. Elementary reactions of N2 O decomposition.
Scheme 2. Reaction network for N2 O decomposition reaction.
important until the reaction temperature goes over 800°C. The generation of N2 and O2 is exothermic, and the heat of reaction is calculated from Eqs. (1) and (2) as ÿ8:5 kJ/mol. The NO generation reaction is endothermic and its heat of reaction is calculated from Eqs. (1) and (3) as +8.5 kJ/mol. 5.2. The processes of decomposing N2 O by the companies The N2 O abatement technology of the major AA manufacturers is chie¯y the catalytic decomposition process or the thermal destruction process (Reimer et al., 1994, 1995). Other than these processes, there are reports of a method that consumes N2 O as an oxidant for phenol synthesis (cf. Appendix A (g)). Table 3 brings together all the processes used by the companies (Reimer et al., 1994, 1995). Below we will give a general description of the technology for each company that has been made known through their patent applications.
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Table 3 N2 O abatement technologies Producer
Technology
Start up
Asahi BASF Bayer DuPont
Thermal Catalytic RFBa Catalytic RFBa
Rhone-Poulenc
Thermal Catalytic Phenol production
1999 1997 1993 1997 1958, 1976, 1994 1997 1998 2000
Solutia a
Reducing ¯ame burner.
5.2.1. Catalytic decomposition process BASF announced in 1997 that it would introduce N2 O decomposition equipment that uses the catalytic method (cf. Appendix A (f)). It then completed N2 O decomposition equipment for an AA 240,000 t/yr plant at 13 million Deutsche Marks and announced that it had achieved a decomposition rate of 95% (cf. Appendix A (g)). The catalyst used in the patent application is either a spinel type CuAl2 O4 , a catalyst that are Ag and CuO supported on Al2 O3 , or a catalyst that is Ag supported on Al2 O3 . When the catalysis is around 500°C using the o gas which contains 23% N2 O, from the actual AA process, all of these catalysts give a decomposition rate of 99% or higher. Table 4 shows the major gas compositions that have been emitted from the AA process (BASF, 1991, 1993, 1994a,b). DuPont announced that its catalytic process worked in 1997 and that the catalyst was CoO and NiO supported on ZrO2 (cf. Appendix A (d)). When N2 O of 100% concentration is processed at a reaction temperature of 402°C, the N2 O decomposition rate is 98.5%. When ZrO2 is used as the support, it has good low temperature activity compared to Al2 O3 , with degradation also decreased (DuPont, 1993). Asahi Chemical (1993) has proposed a catalyst that is CuO supported on Al2 O3 . According to our investigation, processing the AA plant emission gases that contain 34% N2 O at a reaction temperature of 620°C gives an N2 O decomposition rate of 99.5% and higher. Asahi (1999) has also proposed a further improved catalytic decomposition device. The catalytic decomposition in Table 4 O-gas analysis of ADA plant Component
Mol/%
N2 O NO2 N2 O2 H2 O
23 17 47 7.5 3.0
the ¯ow system requires rarefying the process gases to maintain the catalyst temperature at optimum level. The improved method introduces the process gas containing N2 O into several stages of a ®xed bed plug ¯ow reaction column that reduces the quantity of rare®ed gases to control the reaction temperature and maintains the N2 O decomposition reaction at the prescribed temperature to reduce energy costs. Fig. 1 shows the reactor described in the patent. Numbers 17, 18, 24 and 25 are inlets for the process gas, 19 is the preheating chamber, 22 is the catalyst layer and 26 is the outlet for generated gas. Descriptions of patents other than those that have been submitted by the AA manufacturers are given here. Air Products & Chemical and Engelhard (1993, 1991) has reported a hydrotalcite catalyst that contains metal and a metal-ion-exchanged zeolite catalyst, Grande Paroisee (1993) has reported on a moldenite/NH4 /Fe catalyst, and Agency of Industrial Science and Technology (1996) has reported on a catalyst that is Rh2 O3 supported on Al2 O3 . Table 5 gives a summary of the reaction conditions and results for using the above catalysts. Fig. 2 shows the process that was made public by BASF (1997). Exhaust gases 1 and 3 containing N2 O and NOx from the plant pass through compressor V1 with air ¯ow 2 to increase the pressure. That makes the temperature of mixed air ¯ow 4 250±350°C. Heat exchangers WT1 and WT2 cool air ¯ow 4 to 30±40°C, it then becomes air ¯ow 5 that passes through absorption column K1 to recover NOx as nitric acid. Air ¯ow 6 that exits absorption column K1 is heated to 450±500°C by heat exchangers WT1 and WT3 and is then supplied to decomposition reactor C1. WT3's heat source is the gas after N2 O decomposition. The catalyst in reactor C1 decomposes N2 O and discharges air ¯ow 9 at approximately 800°C temperature. The air ¯ow after decomposition is cooled to 260±300°C by heat exchangers WT3 and WT4 to become air ¯ow 10. Heat exchanger WT4 recovers the decomposition heat from N2 O as steam. Air ¯ow 10 is introduced into adiabatic reactor C2, that contains an NOx reduction catalyst which reduces the NOx to N2 and O2 . The temperature of air ¯ow 11 at exit C2 is 265±310°C, and turbine T1 gives adiabatic expansion of the air ¯ow and discharges it into the atmosphere at 100°C. Either the isothermal reactor or the adiabatic reactor is believed appropriate to use as the decomposition reactor in catalytic decomposition of N2 O, but it is not clear what type of reactors the AA manufacturers are actually using. The form of the adiabatic reactor is one in which the reaction device is a ¯ow system column type ®xed bed. With the isothermal reactor, a reactor device is used that has a structure made from a cooling agent and a heat exchanger and a ¯ow system multi-duct catalyst ®lled layer (BASF, 1997). The N2 O emitted from an AA plant has high concentration, thus
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Fig. 1. Reactor for the catalytic decomposition.
Table 5 Reaction conditions and results of catalytic decomposition Cat ZnO, CuAl2 O4 /Al2 O3 MgO, CuAl2 O4 /Al2 O3 CaO, CuAl2 O4 /Al2 O3 Ag, CuO/Al2 O3 Ag/Al2 O3 CoO, NiO/ZrO2
Metal (wt%)
Temperature (°C)
Conv. (%)
SV (1/h)
480 480 480 490 550 402
> 99:9 > 99:9 > 99:9 > 99:9 > 99 98.5
4000 4000 4000 4000 4000 600
N2 O conc. of inlet (%) 23 23 23 23 23 100
620 450
> 99:5 99
3350 30,000
34 0.1
Cu±ZSM-5
Ag:14.9 Ag:14.2 Co:0.916 Ni:0.908 Cu:2.4 Co/ Al:2.2 Cu:4.0
400
95
30,000
0.1
Mordenite/NH4/Fe Rh2 O3 /ZnO
Rh:0.5
540 500
99.7 100
13,300 40,000
50 0.1
CuO/Al2 O3 Co±Al-hydrotalcite
presenting the possibility of temperatures rising higher than the temperature tolerances of the reactor and the catalyst and this requires special caution when an adiabatic reactor is used. It is important that the temperature at the reactor outlet will have to be held to about 800°C. The temperature of the adiabatic reactor is controlled by the quantity of rare®ed gases that are supplied with the process gas and the temperature of the isothermal reactor is controlled by the reactor coolant (molten salt). A method has been proposed for controlling reactor temperature by returning UOP to the reactor after decomposition and after the gas has cooled (UOP, 1993). Fig. 3 shows the process ¯ow in that method. In the diagram, 1 is the ¯ow of gas containing N2 O, 2 is the NOx removal process, 14 is the N2 O decomposition section, 30 is the heat exchanger, 55, 56 and 57 are the recycle lines, and 25 and 50 are the discharge lines. There are also methods for controlling reactor temperature by inserting external air, but this method is thought to be advantageous in heat terms because the gas is recycled after the reaction.
Experimental term (h) 1036 1025 1013 242 550
BASF BASF BASF BASF BASF DuPont
2400
Asahi Air products, Engelhard Air products, Engelhard Grande Paroisse MITI
The heat of the reaction of N2 O decomposition is comparatively high at ÿ81:5 J/mol, and the generated heat can be used in steam generation by process gas preheaters and boilers. The ability to recover heat is an advantage of the decomposition processing of N2 O generated from the AA process. There is a possibility of the life of the catalyst becoming a problem with catalytic decomposition. The deterioration of catalysts, for example Pd/Al2 O3 and CuO/ZnO, have been reported in the patent application (Asahi, 1993). The detailed data of deterioration for the catalyst in Table 5 have not been disclosed. We are awaiting further data in order to obtain knowledge on this point. Fig. 4 is a conceptual diagram of the catalytic process. 5.2.2. Thermal destruction process The thermodynamics and kinetics on the elementary reactions of N2 O decomposition have been studied in detail (Loirat et al., 1985; Kuratani and Tsuchiya, 1978)
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Fig. 2. Catalytic decomposition process. Table 6 Thermal destruction of N2 Oa Component
Fig. 3. Temperature control of the catalytic decomposition process.
Concentration Feed (mol/%)
Product (mol/%)
N2 O NO NO2 CO2 O2 N2
51.0 0.02 0.20 5.52 5.57 37.7
0 2.63 8.28 4.57 16.21 68.31
Total ¯ow rate
2.87 Nm3 /h
3.46 Nm3 /h
a
Reaction conditions: Temperature: 1054°C. Pressure: 1.5 kg/cm2 . LV:44.7 cm/s. Residence time: 1.2 s.
Fig. 4. Conceptual diagram of catalytic process.
and a reaction simulation is feasible. Because the decomposition reaction of N2 O into N2 and O2 is an exothermic reaction, if the decomposition is done on adiabatic condition, the heat of the reaction will continue the decomposition. Asahi Chemical (1986) has proposed a method of decomposing N2 O into nitrogen and oxygen and one that recovers NO generated as nitric acid. Table 6 shows that allowing the N2 O in the gas emitted from an AA plant to remain in a reaction furnace for 1.2 s at 1054°C decomposes the N2 O 100% and converts 21% of the N2 O supplied into NOx . Fig. 5 shows the process ¯ow described in the published patent. The exhaust gas generated from AA reaction process 8 is ®rst pressurized to 1±2 kg/cm2 by water-sealed pump 16 and after the NO contained is oxidized into NO2 in oxidation column 17, it is fed into absorption column 18 and the NO2 is
recovered as nitric acid. The air ¯ow from which NO2 has been removed is fed into the plug ¯ow reactor 19 where thermal destruction reaction is performed on the N2 O. After thermal destruction, air ¯ow feeds into a plug ¯ow reactor tube so that it can be heated externally and used to heat process gas. The air ¯ow then feeds into boiler 20 to be heat-recovered, it is then cooled to room temperature by heat exchanger 24, and then feeds to absorption column 27 where the NOx is recovered as nitric acid. Asahi Chemical (1999) has also applied for a patent on an improved N2 O processing equipment. This method heats part of the gas, which contains N2 O, supplied to the reactor to start a decomposition reaction and the remaining gas that contains N2 O is supplied in several stages to the decomposition reactor to be decomposed. Fig. 6 shows the reactor that is described in the patent description. Numbers 1 and 4 in the diagram are the entrances for the process gas 2 is the preheating chamber, 5 is the thermal destruction chamber, 6 is the exit for the generated gas. In this method, the quantity of energy that is supplied to the reactor to sustain the N2 O decomposition reaction is kept extremely low. For energy input a combustible gas such as hydrogen or methane is used, and the amount of heat input required for 1 Nm3 decomposition of the 550°C process gas that
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Fig. 5. Thermal decomposition process.
Fig. 6. Reactor for the thermal destruction.
contains 33.9 mol% N2 O is 46.0 kJ and the decomposition rate is given as 99% or higher. The thermal destruction method has the advantage of being able to recover NO as nitric acid. It is also superior on the point of given o with almost no industrial waste discharge because there is no replacement of catalyst. Asahi Chemical began the operation of its thermal destruction method in 1999 at an investment cost of 600 million yen to achieve a 98-plus-percentage rate of decomposition. DuPont (1995) has proposed a method of spontaneously maintaining the thermal destruction of N2 O, by starting the destruction by placing methane combustion of process gas containing N2 O in contact with
a ¯ame. This method mixes methane gas with a gas containing 660°C N2 O in a mixing tee and heats it to 850°C or higher to create a decomposition reaction. Using this method to process the exhaust gas from an AA plant that contains 57% N2 O gives an N2 O concentration at the reactor exit of 200 ppm or less at 1050±1254°C of decomposition temperature. It also generates about 10% NOx from the N2 O which can be recovered as nitric acid. The amount of methane for processing gas 20 SCFM (ft3 /min) containing 57 mol% N2 O is 30,000 BTU/h (1 BTU 1.055 J) by conversion of heat volume. Hoechst (1992) has proposed a method of decomposition that sprays the process gas containing N2 O into
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a ¯ame created by combustion of natural gas. This method uses ¯ame temperature to decompose the N2 O, but it uses more fuel than the DuPont method described above. Processing 240 m3 /h cubic meters per hour of N2 O, for example, takes 15.2 m3 /h of natural gas and 113 m3 /h of air. The concentration of N2 O in the gas after it has passed the reactor is 80 ppm. Bayer does its processing using a reducing ¯ame burner (RFB) (Reimer et al., 1994, 1995). A patent has been applied for the combustion device (Bayer, 1991) for achieving the reducing atmosphere, but it is not clear whether this method will be used in the processing of N2 O. Rhone-Poulenc (1995) has proposed a method that uses plasma to convert N2 O into NOx . This method discharges an arc into the air ¯ow containing N2 O to generate plasma and convert that N2 O into NOx . Fig. 7 shows the device. The air ¯ow containing N2 O is preheated in gas preheater 7 to pass out of nozzle 3, becomes plasma by the discharge of an arc across electrodes 10, converts to NOx in area 6, and then exits from 17. In radiation form, the six electrodes are set in opposition so that there are three pairs each. The amount of electricity required to process gas containing 31.6±50.9% N2 O at 22.0±81.8 Ndm3 /min is 0.8 to 2.1 kW. The reaction rate when process gas containing 50.9% N2 O is supplied at 25°C and an equivalent of 37.4 Ndm3 /min is 26.1%, and the NOx selection rate is 55.7%. Fig. 8 shows a conceptual diagram of the thermal destruction process. It has the major feature of being
Fig. 8. Conceptual diagram of thermal process.
able to recover NOx because its decomposition temperature is higher than the catalytic process. 5.2.3. Phenol synthesis Monsanto (cf. Appendix A (c)) has announced its schedule of placing in operation by a target date of 1999±2000 a process for processing N2 O as the result of using zeolite as the catalyst and causing benzene to react with N2 O and producing phenol (cf. Appendix A (h)). This method takes as its base the technology from the Boreskov Institute of Catalysis (1995). When a reaction is made at SV 600 hÿ1 and a reaction temperature of 430°C using a ZSM-5 zeolite catalyst that contains 0.45% Fe2 O3 , they have achieved results of a 98% phenol selection rate and a 100% N2 O reaction rate. The selectivity with the cumene method is reportedly about 93%, so that the selection rate with the N2 O oxidation method is much better. This method might be dependent on location and production quantity of the AA plant so that it places restrictions on the freedom of plant design but a lot of attention is being given to it as a method of N2 O decomposition and phenol production.
6. Future trends
Fig. 7. Plasma chemical reactor.
Because of the high 20±60% N2 O concentration in exhaust gas from the AA process, N2 O decomposition heat can be recovered and reused in N2 O decomposition or it can be used as steam for other processes in the factory. Taking into consideration the large eect that these processes have on reducing the environmental burden, the capital costs are relatively low. However, there are problems with the heat source in the abatement of exhaust gases with low N2 O concentration in processes other than AA, because of the low quantity of heat generated along with the decomposition and the need to heat the large quantity of gases that coexist with N2 O to a temperature sucient for decomposition. If petroleum products are used for the heating source, then
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carbon dioxide will be produced. This is why it is thought that highly active low temperature catalysts will have to be developed. Caution must be given to carbon dioxide generated from the heating source and the quantity of N2 O that is decomposed in the thermal destruction processes. The technologies for N2 O abatement in AA manufacturing seem generally to be complete but it is believed that there is still room for improvement. It is hard to discuss the question of catalyst deterioration because few data on this subject have been released, but if we assume that catalyst deterioration is a general phenomenon, there will be costs for new purchases of replacement catalyst. The thermal destruction process generates relatively large amount of NO in high temperature decomposition. Therefore, it is hoped that equipment materials with high temperature resistance will be developed because if the furnace's ability to withstand heat increases, the amount of recovered nitric acid increases. Since new AA plants are scheduled to be built in various parts of the world (cf. Appendix A (i)) and it can be predicted that some of the plants do not have N2 O decomposition processing, there should be continued improvement in the technology to introduce it into those plants. N2 O is a gas with the properties of an oxidizing agent so that one of the future means of N2 O abatement will be the development of technologies for separating and recovering N2 O cheaply from exhaust gases and the development of technologies for using N2 O in a way that will take advantage of these features (BASF, 1982).
Acknowledgements Contained in this paper are results from surveys made by the Research Institute of Innovative Technology for the Earth (RITE) and the New Energy and Industrial Technology Development Organization (NEDO). The authors wish to thank both organizations for giving the opportunity to participate in their committees.
Appendix A Adipic acid (AA) business was restructured as follows: (a) The AA business of ICI was succeeded by DuPont, (b) the AA business of Rhone-Poulenc was succeeded by Rhodia, and (c) the AA business of Monsanto was succeeded by Solutia. The authors have referred to the following industrial information: (d) 1992. European Chemical News May 25, p. 35, (e) 1997. Asian
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Chemical News Decmber 1, 4, p. 38, (f) 1997. Financial Times February 3, p. 20, (g) 1998. European Chemical News 12±18 January, p. 43, (h) 1996. Chemical Market Reporter December 30, p. 1, (i) 1997. Chemical Week November 5, 159, p. 22.
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The authors as a group investigate a new production process of nylon 6, 6 intermediates. The backgrounds of authors, Shimizu, Tanaka and Fujimori, are physical organic chemistry, chemical engineering and organic chemistry respectively.