Process for the decomposition of nitrous oxide from a gas stream
By splitting the gas stream and heating it separately before feeding it into individual decomposition reactors, alternating catalysts, and optimizing temperature control, the problem of reduced decomposition efficiency caused by catalyst aging was solved, achieving efficient and stable nitrous oxide decomposition.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- BASF SE
- Filing Date
- 2022-01-05
- Publication Date
- 2026-07-31
AI Technical Summary
In existing technologies, the decomposition efficiency of nitrous oxide decreases due to catalyst aging, and there is a risk of catalyst sintering caused by high temperature, making it difficult to achieve efficient and long-term decomposition of nitrous oxide.
The gas stream is split into at least two streams and fed into separate decomposition reactors. The catalysts are replaced alternately, and the streams are heated and the temperature is optimized. Regenerative heat exchangers and additional heaters are used to ensure effective decomposition. The purified streams are then combined for further processing.
It improves the overall efficiency of nitrous oxide decomposition, reduces the impact of catalyst aging, avoids the risk of high-temperature sintering, and achieves high time utilization and efficient decomposition.
Smart Images
Figure CN116802311B_ABST
Abstract
Description
[0001] The present invention relates to a method for decomposing nitrous oxide from a gaseous stream, wherein the gaseous stream is preheated and fed into a decomposition reactor, in which nitrous oxide is decomposed into nitrogen and oxygen.
[0002] Typically, the decomposition of nitrous oxide into nitrogen and oxygen occurs in the presence of a catalyst, particularly a fixed-bed catalyst.
[0003] Nitrous oxide is produced, for example, in the production of adipic acid. To produce adipic acid, nitric acid is typically added to a mixture of cyclohexanone and cyclohexanol. The reaction of cyclohexanone and cyclohexanol with nitric acid forms a reaction mixture containing adipic acid, nitrous oxide, nitric oxide, and nitrogen dioxide. Adipic acid is removed from the reaction mixture by crystallization. Nitrous oxide, nitric oxide, and nitrogen dioxide are removed as exhaust gas by stripping the reaction mixture with air. Thus, at least a portion of the nitric oxide reacts with oxygen to form nitrogen dioxide. The air-containing exhaust gas, rich in nitrous oxide and nitrogen dioxide, is fed into a scrubbing unit, where nitrogen dioxide is removed from the exhaust gas by absorption with water. The nitrogen dioxide is used to produce nitric acid, which can then be used to produce adipic acid. In the scrubbing unit, the exhaust gas is optionally mixed with combustion gases from an ammonia-air combustion process, compressed, passed through a residence time reactor, cooled in at least one heat exchanger, and absorbed in an absorber (preferably in an absorption tower).
[0004] Because the crystallization process is prone to blockage during prolonged shutdowns, especially those exceeding 12 hours, long operating times are beneficial for the operation of the adipic acid unit.
[0005] Since nitrous oxide is a greenhouse gas, processes are implemented to decompose it into oxygen and nitrogen to reduce its amount in exhaust gases. Equipment for decomposing nitrous oxide typically includes a reactor, a regenerative heater, and an additional heater to control the inlet temperature of the exhaust gas entering the reactor. Furthermore, to dilute the exhaust gas supplied to the reactor, a recirculation line with a compressor is provided, through which a portion of the gas discharged from the reactor is recirculated back into the exhaust gas supplied to the reactor.
[0006] The catalytic decomposition of nitrous oxide is typically carried out at temperatures ranging from 430°C to 800°C. Depending on the state of the catalyst and the temperature window, the concentration of nitrous oxide in the inlet stream entering the decomposition unit, which can range from 6 to 20 vol%, can be reduced to a concentration in the outlet stream exiting the decomposition unit, which can range from 50 to 3000 vol ppm.
[0007] The inlet feed temperature required to achieve the desired decomposition rate is particularly dependent on the inlet concentration and the degree of catalyst aging. Therefore, the inlet temperature is typically in the range of 430°C to 650°C, increasing with the degree of catalyst aging. If the desired concentration in the outlet feed cannot be achieved at an inlet feed temperature of 650°C, the catalyst must be replaced.
[0008] The challenge in decomposing nitrous oxide from exhaust gas produced from self-generated nitrous oxide is the highly exothermic nature of the reaction. Each percentage of nitrous oxide in the exhaust stream causes an adiabatic temperature rise of 25°C. To prevent catalyst sintering, the temperature in the decomposition unit should be kept below 800°C. On the other hand, for the most efficient decomposition of nitrous oxide, the temperature should be as high as possible. When using standard chromium-nickel-based steel, a common limitation is the design temperature of the inlet section, which is typically in the range of 550°C to 650°C.
[0009] Excessive temperatures are typically avoided by diluting the exhaust stream with purge gas, particularly with a recirculated stream from the outlet stream. Optimal inlet temperature can be achieved through optimal heat recovery of the purge gas and / or additional preheating.
[0010] Methods and apparatus for removing nitrous oxide from a gaseous stream by catalytic decomposition are described, for example, in EP-B 2387 451, EP-B 1 413 349, EP-B 2 387 450, EP-B 2 165 756 or EP-B2 548 629.
[0011] In addition, to avoid prolonged downtime, the equipment can include two reactors to continue the process while maintenance work is being carried out, such as replacing the used catalyst.
[0012] However, a drawback of the known methods is that the efficiency of decomposing nitrous oxide decreases due to catalyst aging.
[0013] Therefore, one object of the present invention is to provide a method for decomposing nitrous oxide from a gaseous stream, which is unaffected by catalyst aging and can achieve high decomposition of nitrous oxide and high time utilization.
[0014] This objective is achieved by a method for decomposing nitrous oxide from a gaseous feed stream, the method comprising:
[0015] (a) Dividing the gas stream into at least two streams and heating the streams;
[0016] (b) The feed streams are each fed into a separate decomposition reactor, wherein each reactor contains a catalyst;
[0017] (c) Nitrous oxide is decomposed into nitrogen and oxygen in a decomposition reactor to obtain a purified feed stream;
[0018] (d) Optionally, each purified feed stream may be fed into a unit for decomposing nitrogen dioxide and / or nitric oxide, or at least two purified feed streams may be combined and the combined purified feed stream fed into a unit for decomposing nitrogen dioxide and / or nitric oxide.
[0019] The catalysts in the decomposition reactors are replaced alternately, and one of the catalysts is replaced when the arithmetic mean of the lifetimes of the catalysts in the other reactors has reached 25% to 75% of the lifetime of one catalyst.
[0020] Surprisingly, it has been shown that the overall efficiency of the decomposition process can be improved by splitting the gas stream into separate streams and feeding each stream into a separate decomposition reactor, specifically by splitting the gas stream into a first stream and a second stream and feeding the first stream into a first decomposition reactor and the second stream into a second decomposition reactor. Furthermore, the total amount of residual nitrous oxide in the exhaust gas after the streams have passed through their respective decomposition reactors and then been recombined is much lower than when the entire gas stream passes through only one decomposition reactor.
[0021] The reaction of nitrous oxide decomposing into oxygen and nitrogen occurs at elevated temperatures and usually in the presence of a catalyst. To carry out the decomposition reaction at elevated temperatures, the gas stream must be heated, or if the gas stream is first split into component streams, the component streams must be heated.
[0022] According to the present invention, the gas stream is first split into component streams, and then each component stream is heated separately. This enables greater flexibility of the method and, in particular, allows the component streams to be fed into their respective decomposition reactors at temperatures optimized for the decomposition reaction.
[0023] Heating of the gas stream can be carried out by any heating method known to those skilled in the art. To improve the efficiency of the heating process and reduce the energy consumption for heating the gas stream, it is particularly preferred to preheat the gas stream via indirect heat transfer from the purified stream taken from the decomposition reactor.
[0024] To heat the feed streams, it is preferable to preheat the respective feed streams using purified feed streams taken from their respective decomposition reactors.
[0025] Alternatively, the entire gas stream can be preheated before splitting it into component streams and heating the component streams. In this case, it is particularly preferable to preheat the entire gas stream via indirect heat transfer from the combined purified stream taken from the decomposition reactor.
[0026] To optimize decomposition in each decomposition reactor based on catalyst aging, it is preferable to be able to individually set the temperature of each feed stream supplied to the decomposition reactor. This allows feed streams to be supplied to decomposition reactors containing older catalysts at higher temperatures and to decomposition reactors containing fresher catalysts at lower temperatures, thus optimizing nitrous oxide decomposition in each decomposition reactor. To set the temperature individually, the entire gas stream can be preheated before splitting into feed streams, and then the feed streams can be heated, or the gas stream can be split into feed streams first, and then the feed streams can be heated separately. Particularly preferred is to first split the gas stream into feed streams and then heat the feed streams separately.
[0027] The heat exchanger used for heating the gas stream or the diverted stream (hereinafter also referred to as a "regenerative heat exchanger") can be any heat exchanger known to those skilled in the art, which performs the heating by heat transfer from a purified stream drawn from the decomposition reactor or from a respective purified stream drawn from the decomposition reactor. Suitable heat exchangers are, for example, shell-and-tube heat exchangers, tube bundle heat exchangers with U-shaped tubes, spiral tube heat exchangers, or coils. If a shell-and-tube heat exchanger or a spiral tube heat exchanger is used, the gas stream to be heated and the gas stream drawn from the decomposition reactor can flow countercurrently or concurrently, with countercurrent flow being preferred. If a tube bundle heat exchanger with U-shaped tubes is used, the gas stream to be heated and the gas stream drawn from the decomposition reactor flow in a cross-flow manner. Shell-and-tube heat exchangers are preferred.
[0028] If the heat transfer in the regenerative heat exchanger is insufficient to heat the exhaust stream to the predetermined temperature for supplying it to the fixed-bed reactor, an additional heater is preferably provided. Alternatively, a heater may be used without the regenerative heat exchanger. However, for energy conservation reasons, it is preferable to heat the exhaust stream in the regenerative heat exchanger, and if the heating in the regenerative heat exchanger is insufficient, an additional heater is provided to allow for further heating of the exhaust stream to the predetermined temperature. The additional heater when a regenerative heat exchanger is included, or the heater when a regenerative heat exchanger is not used, is preferably an electric heater. However, in addition to electric heaters, direct or indirect gas-fired heaters can also be used as additional heaters or heaters. A direct gas-fired heater refers to adding the exhaust gas from a gas-fired heater to the exhaust stream containing nitrous oxide. An indirect gas-fired heater refers to transferring heat from the exhaust gas from a gas-fired heater to the exhaust stream containing nitrous oxide through a separate heat exchanger. Among gas-fired heaters, a direct gas-fired heater is preferred.
[0029] The temperature at which the gas stream or the distribution stream is heated depends particularly on the nitrous oxide concentration in the gas stream and the degree of catalyst aging. Typically, the gas stream or distribution stream is heated to a temperature in the range of 430 to 650°C, more preferably to a temperature in the range of 450 to 550°C, and especially to a temperature in the range of 480 to 520°C.
[0030] To set the temperature, it is preferable to measure the concentration of nitrous oxide in the purified feed stream taken from the decomposition reactor. If this concentration exceeds a given target, the temperature is increased until the target concentration is reached or the upper temperature limit is reached. The upper temperature limit is a temperature that is below the temperature defined by the safety interlock device of the decomposition reactor by a given offset. If the concentration of nitrous oxide in the purified feed stream taken from the decomposition reactor exceeds the target concentration and reaches the maximum temperature used to supply the gas feed stream into the decomposition reactor, the catalyst must be replaced.
[0031] The heated exhaust stream is then fed into a fixed-bed reactor to decompose nitrous oxide into nitrogen and oxygen. The fixed bed contains a suitable catalyst for the decomposition of nitrous oxide. Therefore, any catalyst suitable for the decomposition reaction can be used. Suitable catalysts are particularly those based on copper oxide and zinc oxide as catalytic materials. Particularly preferred is a mixture of 14 to 18 wt% copper oxide and less than 20 wt% zinc oxide on alumina and / or magnesium oxide as support materials, serving as the catalytically active material. The catalyst is preferably provided as an extrusion having a length of 2 mm to 2 cm, preferably 4 mm to 1 cm, and a diameter of 2 mm to 10 mm, preferably 4 to 8 mm. The cross-sectional shape of the catalyst is disc-shaped or star-shaped, preferably star-shaped.
[0032] The reaction in the fixed-bed reactor is carried out at a temperature of 430 to 800°C, more preferably 450 to 750°C, and especially 520 to 700°C.
[0033] Due to catalyst aging and the resulting partial deactivation, the temperature used for the decomposition reaction must be increased with the degree of catalyst aging. However, to avoid catalyst sintering, the temperature in the fixed-bed reactor should not exceed 800°C.
[0034] The catalyst can be included in each decomposition reactor in any suitable form, such as as a fluidized bed or a fixed bed. Preferably, the catalyst is a fixed-bed catalyst. If the catalyst is a fixed-bed catalyst, it can be a structured, solid, or irregular packing of catalyst particles. Preferably, the fixed bed is an irregular packing of catalyst particles. As the catalyst ages during its lifespan, it must be replaced after reaching its maximum lifespan. For continuous operation of the method for decomposing nitrous oxide, the catalysts in the decomposition reactors are alternately replaced. By alternating catalyst replacement, all decomposition reactors that have not had their catalysts replaced can be operated while only a portion of the decomposition reactors (preferably only one decomposition reactor) is shut down for catalyst replacement. Whenever one reactor is shut down, all feed streams are supplied to the other decomposition reactors that are not shut down. For this purpose, if only two decomposition reactors are used, the entire gas stream is supplied to the decomposition reactors that are not shut down. If more than two decomposition reactors are used, the gas stream is divided into feed streams, the number of which corresponds to the total number of decomposition reactors minus the number of decomposition reactors that are shut down, and each feed stream is supplied to one of the decomposition reactors that are not shut down.
[0035] Catalyst replacement is preferably performed at regular intervals, where the catalyst in one of the reactors has reached 25 to 75% of its lifespan, preferably 1 / 3 to 2 / 3, of its lifespan in the other reactors. Therefore, intervals are preferably set so that the operating time of all reactors remains substantially the same until one reactor is shut down for catalyst replacement. This means, for example, if two decomposition reactors are used, the catalyst in one decomposition reactor is replaced when the catalyst in the other decomposition reactor has reached 25-75% of its lifespan, preferably 1 / 3 to 2 / 3, particularly half of its lifespan. If three decomposition reactors are used, the catalyst in one decomposition reactor is replaced when the catalysts in the other reactors have reached 25% to 75% of their lifespan, particularly when the catalysts in the other reactors have reached 1 / 3 to 2 / 3 of their lifespan. Particularly preferably, the catalyst in one decomposition reactor is replaced when the catalyst in the second decomposition reactor has reached approximately 1 / 3 of its lifespan and the catalyst in the third decomposition reactor has reached approximately 2 / 3 of its lifespan.
[0036] This replacement interval avoids the possibility that at least two catalysts may be nearing the end of their lifespan, and independently of the decomposition reactor undergoing catalyst replacement, there is no risk that the catalyst efficiency of other operating decomposition reactors may be approaching the end of their lifespan. When one of the decomposition reactors is shut down for catalyst replacement, the catalyst in the operating decomposition reactors has a substantially consistent limited efficiency because it has only reached a portion of its lifespan.
[0037] As catalysts age, their efficiency decreases with increasing catalyst lifetime. This leads to reduced nitrous oxide decomposition. To minimize the amount of nitrous oxide in the gas stream, it is preferable to reduce the feed stream to the decomposition reactor containing older catalyst and increase the feed stream to the decomposition reactor containing fresher catalyst. By reducing the feed stream to the decomposition reactor containing older catalyst, the overall process efficiency can be optimized.
[0038] As an alternative or supplement to reducing the amount of gas feed into the decomposition reactor containing older catalyst, the temperature of the feed stream into the decomposition reactor containing older catalyst can be increased to optimize the efficiency of the catalyst as described above. Particularly preferably, the temperature of the feed stream is increased while reducing the amount of gas feed into the decomposition reactor containing older catalyst.
[0039] The gaseous feed stream can originate from any process that forms or uses nitrous oxide. Such a process is, for example, the production of adipic acid. To produce adipic acid, nitric acid is added to a mixture of cyclohexanone and cyclohexanol. The reaction of cyclohexanone and cyclohexanol with nitric acid forms a reaction mixture containing adipic acid, nitrous oxide, nitric oxide, and nitrogen dioxide. Nitric oxide, nitric oxide, and nitrogen dioxide are removed from the reaction product, for example, by air stripping. Stripping yields a crude product containing adipic acid and an exhaust gas rich in nitrous oxide, nitric oxide, and nitrogen dioxide. Typically, at least a portion of the nitric oxide reacts with oxygen to form nitrogen dioxide. Nitrogen dioxide is preferably removed by washing and reused in the production of nitric acid.
[0040] If the gas stream originates from exhaust gas from a process for producing adipic acid, it is particularly preferable to post-treat the exhaust gas stream to remove nitrogen oxides before supplying the gas to a method for decomposing nitrous oxide.
[0041] Preferably, nitrogen oxides are absorbed from the exhaust gas stream into water by pressure absorption. An absorption tower is preferably used for absorbing nitrogen oxides in water. The absorption tower can operate counter-currently or concurrently, preferably counter-currently. For this purpose, the exhaust gas stream is fed into the absorption tower at one end, preferably at the bottom, and water is fed into the absorption tower at the other end, preferably at the top. Inside the absorption tower, nitrogen dioxide is absorbed into the water by forming nitric acid and nitric oxide. Nitric oxide reacts with oxygen in the exhaust gas stream to form nitrogen dioxide, which is again absorbed into the water by forming nitric acid and nitric oxide. This process is repeated throughout the tower, such that at the tower outlet, the amount of nitrogen dioxide and nitric oxide in the exhaust gas stream is reduced. After the nitrogen oxides are absorbed by this mechanism, nitric acid containing water is removed from the bottom of the absorption tower. The absorption tower preferably contains 10 to 80 stages, more preferably 20 to 70 stages, and particularly 40 to 60 stages. The temperature at which absorption takes place is preferably in the range of 5 to 70°C, more preferably in the range of 10 to 60°C, and particularly in the range of 20 to 50°C.
[0042] The absorber preferably includes internal components. Suitable internal components include, for example, packing material, such as a packed bed or structured packing. Alternatively, the absorber may include trays as internal components. Particularly preferred are water-cooled trays as internal components in the absorber.
[0043] The exhaust gas stream is preferably fed into the absorption tower together with an additional gas mixture from ammonia combustion, the additional gas mixture mainly comprising nitric oxide and nitrogen dioxide, water, oxygen, nitrogen, argon and carbon dioxide.
[0044] Additionally or alternatively, besides optionally feeding the exhaust stream containing nitrous oxide together with an additional gas mixture from ammonia combustion into the absorber, it is preferable to oxidize the nitrous oxide in the exhaust stream to nitrogen dioxide before feeding it into the absorber. To oxidize the nitrous oxide, the exhaust stream is preferably compressed to a pressure of 1.2 to 3 bar (absolute), more preferably 1.3 to 2.7 bar (absolute), and particularly 1.5 to 2.5 bar (absolute). In a subsequent step or simultaneously, the exhaust stream is cooled to a temperature of 20 to 50°C, more preferably 25 to 45°C, and particularly 30 to 40°C. After cooling, the exhaust stream is compressed in a second step to a pressure of 3 to 12 bar (absolute), more preferably 4 to 11 bar (absolute), and particularly 5 to 10 bar (absolute).
[0045] The compressed exhaust stream and make-up air are passed through a residence time reactor, wherein the ratio of the normalized volumetric flow rates of the compressed exhaust stream and make-up air is preferably in the range of 1.5 to 3.5, more preferably in the range of 1.6 to 2.9, and particularly in the range of 1.8 to 2.5. The inlet temperature of the exhaust stream entering the residence time reactor is preferably in the range of 120 to 300°C, more preferably in the range of 150 to 275°C, and particularly in the range of 220 to 250°C. The outlet temperature of the exhaust stream leaving the residence time reactor is preferably in the range of 250 to 360°C, more preferably in the range of 270 to 350°C, and particularly in the range of 280 to 340°C. The residence time of the exhaust stream in the residence time reactor is preferably in the range of 10 to 60 seconds, more preferably in the range of 15 to 50 seconds, and particularly in the range of 20 to 30 seconds.
[0046] After passing through the residence time reactor, the exhaust stream is cooled to 10 to 50°C, more preferably 20 to 40°C, and especially 30 to 35°C.
[0047] After pressure absorption and / or oxidation of nitrous oxide to remove nitrous oxide, the exhaust stream is partially or completely fed into an N2O separator, wherein nitrous oxide from at least a portion of the exhaust stream is concentrated by using a two-stage absorption / desorption process with water.
[0048] In the first stage of the two-stage absorption / desorption process, N2O is absorbed in water in a first absorption tower. The first absorption tower operates at a pressure of 15 to 30 bar (absolute), more preferably 17 to 28 bar (absolute), particularly 19 to 26 bar (absolute) and a temperature of 10 to 45°C, more preferably 20 to 42°C, particularly 30 to 35°C. After the first absorption stage, nitrous oxide is desorbed from the water in a first desorber, which operates at a pressure of 1 to 1.5 bar (absolute), more preferably 1.05 to 1.3 bar (absolute), particularly 1.1 to 1.2 bar (absolute) and a temperature of 10 to 45°C, more preferably 20 to 42°C, particularly 30 to 35°C. After desorption from water, the resulting gas stream is fed into a second absorption tower, which operates at a pressure of 15 to 30 bar (absolute), more preferably 17 to 25 bar (absolute), particularly 19 to 23 bar (absolute) and a temperature of 10 to 45°C, more preferably 15 to 30°C, particularly 20 to 35°C. In the second desorber, operating at a pressure of 1 to 1.5 bar (absolute), more preferably 1.05 to 1.3 bar (absolute), particularly 1.1 to 1.2 bar (absolute), concentrated nitrous oxide is obtained, preferably fed into the process for producing cyclododecanone and cyclopentanone.
[0049] Only a portion of the nitrous oxide in the exhaust stream from pressure absorption and / or oxidation of nitrous oxide is absorbed during the two-stage absorption / desorption process. Preferably, the remaining portion is diverted to be recycled to an exhaust stream containing a mixture of exhaust gas from the production of diacid and optionally exhaust gas from ammonia combustion, prior to the oxidation reaction of nitrous oxide, and to the decomposition process of nitrous oxide.
[0050] Even after most nitrogen oxides are removed from the exhaust stream during the two-stage absorption / desorption process, the exhaust stream still contains 5 to 18% by volume of nitrous oxide. Since nitrous oxide has a greenhouse gas factor of approximately 300, it must be removed from the exhaust stream.
[0051] To remove nitrous oxide from the exhaust stream, the exhaust stream is heated to a temperature of 100 to 300°C, more preferably 170 to 270°C, and particularly 210 to 260°C, before being fed into the nitrous oxide decomposition method of the present invention. The pressure at which the exhaust stream is fed into the nitrous oxide decomposition method is preferably in the range of 5 to 10 bar (absolute), more preferably in the range of 6 to 9.5 bar (absolute), and particularly in the range of 7 to 9 bar (absolute).
[0052] Preferably, the exhaust gas stream is heated in a tube bundle heat exchanger, a U-tube bundle heat exchanger, or a spiral heat exchanger. Preferably, a tube bundle heat exchanger is used for heating. Hot exhaust gas taken from a residence time reactor is preferably used as the heating medium. The thus heated gas stream is then fed into step (a) of the method of the invention, wherein the stream is split into component streams and heated to the temperature at which the component streams are fed into the decomposition reactor. Particularly preferably, the entire gas stream is first heated to 100 to 300°C, more preferably 170 to 270°C, particularly 170 to 270°C, before splitting into component streams, and then the split component streams are individually heated to the temperature at which they are fed into the decomposition reactor.
[0053] To wash the exhaust gas from adipic acid production, two pressure absorption units are typically provided. Of these two pressure absorption units, only one operates with the adipic acid exhaust gas. According to the invention, if only one pressure absorption unit is operating, the gas stream leaving this pressure absorption unit is the gas stream that has been treated by the method of the invention and heated in the diversion step (a) of the method of the invention.
[0054] The residual exhaust gas containing nitrous oxide and optionally remaining nitrous oxide and nitrogen dioxide is the gas stream supplied to the method of the present invention. Preferably, the gas stream supplied to the method contains 3 to 25 vol% nitrous oxide, more preferably 4 to 18 vol% nitrous oxide, and particularly 6 to 13 vol% nitrous oxide. Furthermore, the exhaust stream supplied to the N2O decomposition unit may contain 1 to 12 vol% oxygen, 0.5 to 5 vol% carbon dioxide, 0 to 0.3 vol% carbon monoxide, 0.2 to 0.8 vol% rare gases (especially argon), 0.1 to 2 vol% water, and 200 to 5000 vol ppm nitrogen oxides, more preferably 1.5 to 6 vol% oxygen, 0.7 to 3.5 vol% carbon dioxide, 0 to 0.25 vol% carbon monoxide, 0.3 to 0.6 vol% rare gases (especially argon), more preferably 0.2 to 1.3 vol% water, and 300 to 3000 vol ppm nitrogen oxides, particularly 2 to 4 vol% oxygen, 1 to 2 vol% carbon dioxide, 0 to 0.2 vol% carbon monoxide, 0.2 to 0.5 vol% rare gases (especially argon), 0.3 to 1 vol% water, and 500 to 1000 vol ppm nitrogen oxides.
[0055] The main component of the exhaust stream is nitrogen, which is contained in an amount of 55 to 95% by volume, more preferably 70 to 92% by volume, and particularly 80 to 90% by volume. In addition to these components, the exhaust stream may also contain other impurities, typically in amounts less than 2% by volume, more preferably less than 1% by volume, and particularly less than 0.5% by volume.
[0056] To achieve a sufficiently low nitrous oxide concentration in the purified gas extracted from the decomposition reactor, the nitrous oxide concentration in the gas stream fed into the decomposition reactor should be below 13% by volume, more preferably below 12% by volume, and particularly below 11% by volume. If the nitrous oxide concentration in the gas stream exceeds these values, it is preferable to recycle a portion of the purified gas extracted from the decomposition reactor as a recirculation stream back to the decomposition reactor. The amount of recirculation stream is set such that the amount of N2O in the gas stream fed into the decomposition reactor remains below these concentration values.
[0057] A recirculation flow is established using a recirculation blower. The amount of recirculated purified gas depends on the rotational speed of the recirculation blower and / or the internal recirculation rate of the recirculation blower. The effective recirculation flow can be varied without changing the rotational speed of the recirculation blower by changing the pressure of the recirculation blower and the degree of internal recirculation between the inlet and outlet sides.
[0058] Preferably, the recirculation flow of each decomposition reactor is set individually. By setting the recirculation flow of each decomposition reactor individually, each decomposition reactor can be operated under optimal conditions.
[0059] The method for decomposing nitrous oxide according to the present invention is particularly suitable for industrial-scale processes. Therefore, the volumetric flow rate supplied to an N₂O decomposition unit comprising at least two decomposition reactors can be between 5000 and 70000 Nm³. 3 More preferably, it is within the range of 7000 to 60000 Nm / h. 3 In the range of / h, especially in the 10,000 to 50,000 Nm 3 Within the range of / h. However, in addition to industrial-scale post-treatment of nitrous oxide-containing exhaust gas, this method can also be used in processes that generate small amounts of nitrous oxide exhaust gas, such as in pilot plants or laboratories. However, it is particularly preferred that this method be used on an industrial scale.
[0060] The catalyst loading, measured as normalized cubic meters per kilogram of nitrous oxide per hour and per kilogram of catalyst in a fixed bed, ranges from 0.5 to 10 Nm³. 3 More preferably, it is in the range of / h / kg catalyst, from 1.5 to 6.5 Nm. 3 In the range of / h / kg catalyst, especially in the range of 2 to 4 Nm 3 Within the range of / h / kg catalyst.
[0061] The decomposition of nitrous oxide into nitrogen and oxygen can occur in an ideal isothermal or ideal adiabatic mode, or in a mode between ideal isothermal and ideal adiabatic. The decomposition of nitrous oxide into nitrogen and oxygen is a strongly exothermic reaction. In an ideal adiabatic mode, each percentage of nitrous oxide in the exhaust gas fed into the reactor results in a 25 Kelvin increase in exhaust gas temperature. The reactor mode between ideal isothermal and ideal adiabatic is defined by the degree of adiabaticity, which is 100% minus the heat flux transferred from the catalyst bed to its environment, such as the walls and catalyst support material, via conduction and radiation, divided by the heat flux of the reaction. The decomposition reaction is carried out adiabatically with a degree greater than 55%, preferably greater than 75%, and more preferably greater than 85%.
[0062] For the reaction to initiate in the catalyst bed, a certain inlet temperature of the exhaust gas flowing into the catalyst bed and / or the catalyst bed itself is necessary. In reaction modes involving high concentrations of nitrous oxide, near-complete decomposition of nitrous oxide, and high adiabatic conditions in the reaction bed, this can lead to gas outlet temperatures exceeding the maximum permissible temperature for safety reasons. Typically, safety interlocks prevent exceeding such temperature limits, and these interlocks have a given setpoint for the maximum temperature. To account for temperature fluctuations and uncertainties in temperature measurement, a maximum operating temperature is selected that is a given offset below the setpoint of the safety interlock device. The selected offset is preferably between 100 and 3 K, more preferably between 50 and 3 K, more preferably between 30 and 3 K, and particularly between 20 and 3 K.
[0063] To prevent high temperatures in the catalyst bed or gas phase, it is preferable to add a gas that contains no nitrous oxide or contains a low amount of nitrous oxide to achieve a preferred nitrous oxide content. By optimizing the nitrous oxide content within the aforementioned range, the decomposition reaction is optimized, thus minimizing the amount of residual nitrous oxide in the purified feed stream taken from the decomposition reactor. To reduce the nitrous oxide content in the feed stream, it is particularly preferable to recycle a portion of the purified feed stream taken from the decomposition reactor back into the feed stream fed into the decomposition reactor before heating.
[0064] If a regenerative heat exchanger is used, the purified gas stream exiting the decomposition reactor is fed into the regenerative heat exchanger to heat the gas stream subsequently fed into the decomposition reactor. If only one regenerative heat exchanger is used and the gas stream is split after passing through the regenerative heat exchanger, the purified gas streams are combined before being fed into the regenerative heat exchanger. On the other hand, if the gas stream is split before being heated in the regenerative heat exchanger, the respective split streams exiting the decomposition reactor are fed into their respective regenerative heat exchangers, where the gas streams are heated before being fed into the decomposition reactor. In each regenerative heat exchanger, the purified gas stream is cooled by heat transfer to the gas streams fed into the method.
[0065] Regardless of whether a regenerative heat exchanger is included, it is preferable to further cool the purified gas extracted from the fixed-bed reactor. Cooling is preferably carried out via heat transfer in at least one heat exchanger, where heat is transferred to a cooling medium, particularly water. Particularly preferably, in at least one heat exchanger, water is evaporated and / or superheated via indirect heat transfer from the purified gas extracted from the fixed-bed reactor. Suitable heat exchangers for cooling the purified gas are, for example, tube bundle heat exchangers, U-tube bundle heat exchangers, or spiral heat exchangers, or coils in the exhaust path.
[0066] Since the gas stream may contain nitric oxide and / or nitrogen dioxide that were not decomposed in the decomposition reactor, it is preferable to supply the purified stream to a unit for decomposing nitrogen dioxide and / or nitric oxide. Therefore, a unit for decomposing nitrogen dioxide and / or nitric oxide can be provided for each purified stream, or alternatively and preferably, the purified streams can be combined and the combined purified stream can be supplied to a single unit for decomposing nitrogen dioxide and / or nitric oxide. Furthermore, if more than one unit for decomposing nitrogen dioxide and / or nitric oxide is included, the combined purified gas stream can be supplied to only one unit for decomposing nitrogen dioxide and / or nitric oxide. In this case, the second unit for decomposing nitrogen dioxide and / or nitric oxide is, for example, a spare unit that can be used during maintenance operations of another unit for decomposing nitrogen dioxide and / or nitric oxide.
[0067] The unit used for decomposing nitrogen dioxide and / or nitric oxide can be any unit known to those skilled in the art for decomposing nitrogen dioxide and / or nitric oxide. Typically, in such a unit for decomposing nitrogen dioxide and / or nitric oxide, nitrogen dioxide and / or nitric oxide are decomposed by selective non-catalytic reduction or selective catalytic reduction. In this reaction, ammonia or urea is used as a reducing agent.
[0068] Due to the high pressure of the gas stream after passing through the decomposition reactor and optionally through a unit for decomposing nitrogen dioxide and / or nitric oxide, it is preferable to pass the gas stream through an expansion turbine before releasing the gas into the atmosphere. By passing the gas stream through the expansion turbine, the pressure can be used to generate an electric current or, for example, to drive a compressor or any other device with rotating components that should be driven.
[0069] An exemplary embodiment of the present invention is shown in the accompanying drawings and explained in more detail in the following description.
[0070] In the attached diagram:
[0071] Figure 1 A schematic flowchart showing the method for decomposing nitrous oxide is displayed.
[0072] Figure 2 This shows the unit used to decompose nitrous oxide.
[0073] Figure 1 A schematic flowchart showing the method for decomposing nitrous oxide.
[0074] In the method for decomposing nitrous oxide according to the present invention, a gas stream 1 containing nitrous oxide and optionally nitric oxide and nitrogen dioxide is split into a first stream 3 and a second stream 5. The first stream 3 is fed into a first decomposition unit 7, and the second stream 5 is fed into a second decomposition unit 9.
[0075] In the first decomposition unit 7 and the second decomposition unit 9, nitrous oxide in the first feed stream 3 and the second feed stream 5 is decomposed into nitrogen and oxygen, resulting in a first purified feed stream 13 and a second purified feed stream 15. Figure 1 In the embodiment shown, after passing through the first decomposition unit 7 and the second decomposition unit 9, the resulting first purified feed stream 13 and the second purified feed stream 15 are recombined and fed into unit 11 for decomposing nitric oxide and / or nitrogen dioxide. Unit 11 for decomposing nitric oxide and / or nitrogen dioxide can therefore be any nitrogen oxide decomposition unit known to those skilled in the art.
[0076] In addition to Figure 1In addition to splitting the gas stream 1 into two streams as shown, the gas stream can also be split into more than two streams. In this case, each stream is fed into a separate decomposition unit. Furthermore, besides recombinating the purified streams before feeding them into unit 11 for decomposing nitric oxide and / or nitrogen dioxide, each purified stream can also be fed into a separate unit for decomposing nitric oxide and / or nitrogen dioxide. After passing through at least one unit 11 for decomposing nitric oxide and / or nitrogen dioxide, the purified gas can be released into the atmosphere. If more than one unit for decomposing nitric oxide and / or nitrogen dioxide is used, each purified stream can be released into the atmosphere, or at least two purified streams can be combined and then released into the atmosphere.
[0077] Furthermore, if the gas stream 1 is split into more than two streams, at least two purified streams can be recombined into a group, and each combined stream can be fed into a separate unit for decomposing nitric oxide and / or nitrogen dioxide.
[0078] Operating gas stream 1 using first and second decomposition units 7, 9, or optionally more than two decomposition units, allows for diversion in such a manner that the amount of gas supplied to each decomposition unit minimizes undecomposed nitrous oxide. Furthermore, the temperature in each decomposition unit can be optimized in relation to the activity of the catalyst within that unit. For example, temperature optimization can be achieved by setting the degree of bypass 43 and / or by setting the power of heater 29 and / or by setting the recirculated flow from the outlet of decomposition unit 7, 9 back to the inlet of that unit. Moreover, using at least first and second decomposition units 7, 9 enables continuous operation even if one decomposition unit is shut down (e.g., for maintenance, such as catalyst replacement). In this case, if two decomposition units are included, gas stream 1 is supplied to the one of the decomposition units 7, 9 that is not shut down; if more than two decomposition units are included, the gas stream is diverted into multiple sub-streams corresponding to the total number of decomposition units minus the number of decomposition units that are shut down, and these sub-streams are supplied to the decomposition units that are still operating.
[0079] If the device includes two decomposition units 7 and 9, the first decomposition unit 7 and the second decomposition unit 9 preferably have the same design, corresponding to, for example: Figure 2 The design of the decomposition unit 21 is shown. If there are more than two decomposition units, all decomposition units preferably have the same design, which preferably corresponds to... Figure 2 The design of the decomposition unit 21 shown.
[0080] A feed stream 23 (which can be either a first feed stream 3 or a second feed stream 5, depending on whether the decomposition unit 21 corresponds to the first decomposition unit 7 or the second decomposition unit 9) is supplied to a heat exchanger 25, where it is preheated by heat transfer from the purified feed stream 27 taken from the decomposition unit 21. If the heat transferred from the purified feed stream 27 is insufficient to heat the feed stream 23 to the temperature required for nitrous oxide decomposition, the feed stream 23 after leaving the heat exchanger 25 is supplied to a heater 29, where it is heated to the temperature at which it was supplied to the decomposition reactor 31, where nitrous oxide is decomposed into nitrogen and oxygen. The heater 29 is preferably an electric heater.
[0081] By further adjusting the heat transferred to the feed stream 23 in the heater 29, the temperature of the feed stream 23 can be set to a predetermined temperature at which the feed stream 23 is fed into the decomposition reactor 31. The temperature at which the feed stream 23 is fed into the decomposition reactor 31 is preferably in the range of 430 to 650°C.
[0082] The decomposition reaction of nitrous oxide typically takes place in the presence of a catalyst. Therefore, the decomposition reactor 31 contains a catalyst, preferably a fixed-bed catalyst 35. In the fixed-bed catalyst 35, nitrous oxide is decomposed to form oxygen and nitrogen. The gaseous stream obtained from the reaction is then fed into a heat exchanger 25 to heat the feed stream 23 subsequently fed into the decomposition reactor 31. This heat transfer simultaneously cools the purified feed stream 27 exiting the decomposition reactor 31. The purified feed stream 27 can be further cooled in an auxiliary heat exchanger 37. The auxiliary heat exchanger 37 is preferably a heat exchanger for generating steam by evaporation of water or for superheating water. The purified feed stream 27 exiting the auxiliary heat exchanger 37 can be fed into unit 11 for decomposing nitrogen dioxide and / or nitrous oxide or released into the atmosphere.
[0083] To ensure that nitrous oxide is substantially completely decomposed, or particularly completely decomposed, the amount of nitrous oxide in the gas stream supplied to the decomposition reactor 31 must be kept below a predetermined upper limit, particularly below 11% by weight. If this gas stream contains too much nitrous oxide, a recirculation line 39 with a recirculation blower 41 is provided to reduce the amount of nitrous oxide in the gas stream supplied to the decomposition reactor 31. The recirculation line 39 is connected to the feed line of the distribution stream 23 and the outlet line of the purified stream 27.
[0084] In addition, to set the temperature of the feed stream 23 supplied to the decomposition reactor 31, a bypass 43 can be provided to bypass the regenerative heat exchanger 25. The bypass 43 can be closed to allow the entire gas stream to flow through the regenerative heat exchanger 25, or the bypass 43 can be opened and the feed line into the regenerative heat exchanger 25 can be closed to allow the entire gas stream to flow through the bypass 43, or in a third alternative, the gas stream can be split, with a portion flowing through the regenerative heat exchanger 25 and a portion flowing through the bypass 43.
[0085] Each decomposition unit 21 is provided with a heat exchanger 25 and a heater 29 so that the temperature of the feed stream 23 entering the decomposition reactor 31 can be set individually. However, in addition to... Figure 2 In addition to the design shown, the entire gas stream 1 can also be at least partially heated before being split into component streams 3, 5, and 23. If the gas stream 1 is partially heated before being split into component streams 3, 5, and 23, then final heating of the component streams is performed, and this final heating allows for individual temperature setting of each component stream. If the gas stream is fully heated before being split into component streams, it is impossible to further set the temperature of the component streams, and therefore it is impossible to individually set the temperature for optimizing nitrous oxide decomposition in each decomposition unit 21. Therefore, it is preferable that each component stream 23 is at least partially heated in a separate decomposition unit 21.
[0086] Furthermore, the recirculation line 39 allows the concentration of nitrous oxide supplied to each feed stream 23 of the respective decomposition reactor 31 to be set by adjusting the amount of gas recirculated from the purified gas 27 into the feed stream 23. Therefore, it is preferable that each decomposition reactor 23 is equipped with the recirculation line 39 as shown herein. Example
[0087] Example
[0088] 32000 Nm³ of nitrous oxide containing 8 vol% was extracted from the adipic acid production process at a temperature of 40.5°C and a pressure of 6.8 bar (absolute). 3 The exhaust gas stream is heated to a temperature of 273°C in the first heat exchanger, and then the exhaust gas stream is split into two separate streams.
[0089] The first material flow has a flow rate of 15600m. 3A first feed stream, with a volumetric flow rate of / h, enters the regenerative heat exchanger of the first nitrous oxide decomposition unit. In this regenerative heat exchanger, the first feed stream is heated to 490°C by electric heating, and then fed into a reactor having a fixed bed containing 8 tons of nitrous oxide decomposition catalyst. Commercially available BASF-Catalyst O3-81 is used as the catalyst. A first purified gas stream, with a temperature of 680°C and an N2O concentration of 90 ppm by volume, emanating from the fixed bed catalyst, is fed into the regenerative heat exchanger of the first nitrous oxide decomposition unit. The first purified gas stream exits the regenerative first heat exchanger of the first nitrous oxide decomposition unit and passes through a first steam generator.
[0090] With 16400m 3 A second feed stream with a volumetric flow rate of / h enters the regenerative heat exchanger of the second nitrous oxide decomposition unit, where it is heated to 490°C by electric heating, and then fed into a reactor with a fixed bed containing 8 tons of nitrous oxide decomposition catalyst. Commercially available BASF-Catalyst O3-81 is used as the catalyst. A second purified gas stream, with a temperature of 675°C and an N2O concentration of 70 ppm by volume, exits the fixed catalyst bed and is fed into the second regenerative heat exchanger of the second nitrous oxide decomposition unit. The second purified gas stream exits the regenerative heat exchanger and passes through a second steam generator.
[0091] The first and second purified gas streams are mixed. This mixed gas stream is fed into the Denox unit and released into the atmosphere after passing through an expander turbine. The concentration of nitrous oxide in the mixed exhaust gas stream is 80 ppm by volume.
[0092] Comparative Example
[0093] The 30400 Nm³ of nitrous oxide containing 8 vol% was extracted from the adipic acid production process at a temperature of 34°C and a pressure of 7.2 bar (absolute). 3 The exhaust gas stream, at a rate of / h, is heated to 254°C in a heat exchanger in the first step and to 512°C in the second step via a regenerative heat exchanger and electric heating, before being fed into a reactor with a fixed bed containing 8 tons of nitrous oxide decomposition catalyst. Commercially available BASF-Catalyst O3-81 is used as the catalyst. The purified gas stream from the fixed catalyst bed, at a temperature of 710°C, is fed into a regenerative heat exchanger and then into a steam generator. The exhaust gas from the steam generator, with a nitrous oxide concentration of 660 ppm by volume, is fed into the Denox unit and, after passing through an expander turbine, is released into the atmosphere.
Claims
1. A method for decomposing nitrous oxide from a gas stream (1), comprising: (a) Divide the gas stream (1) into at least two sub-streams (3, 5); 23) and heating the feed stream (3, 5; 23); (b) The feed streams (3, 5; 23) are each fed into a separate decomposition reactor (31), wherein each reactor (31) contains a catalyst; (c) Nitrous oxide is decomposed into nitrogen and oxygen in a decomposition reactor (31) to obtain a purified feed stream (13, 15; 27); (d) Optionally, each purified feed stream (13, 15; 27) may be fed into a unit (11) for the decomposition of nitrogen dioxide and / or nitric oxide, or at least two purified feed streams (13, 15; 27) may be combined and the combined purified feed stream fed into a unit (11) for the decomposition of nitrogen dioxide and / or nitric oxide. The catalyst in the decomposition reactor (31) is replaced alternately, and the catalyst is replaced when the arithmetic mean of the lifetimes of the catalysts in the other reactors has reached 25% to 75% of the lifetime of one catalyst.
2. The method according to claim 1, wherein the catalyst is a fixed-bed catalyst (35).
3. The method according to claim 1 or 2, wherein the gas stream (1) is split into a first stream and a second stream (3, 5) and the first stream (3) is fed into a first decomposition reactor and the second stream (5) is fed into a second decomposition reactor.
4. The method according to any one of claims 1 to 3, wherein the feed stream (23) supplied to the decomposition reactor (31) containing older catalyst is reduced, and the feed stream (23) supplied to the decomposition reactor (31) containing fresher catalyst is increased.
5. The method according to any one of claims 1 to 4, wherein during the catalyst replacement process in one decomposition reactor (31), all feed streams (3, 5; 23) are fed into other decomposition reactors.
6. The method according to any one of claims 1 to 5, wherein the temperature of each feed stream (3, 5; 23) fed into the decomposition reactor (31) can be set individually.
7. The method according to any one of claims 1 to 6, wherein the entire gas stream (1) is preheated before the gas stream (1) is divided into streams (3, 5; 23) and the streams (3, 5; 23) are heated.
8. The method according to any one of claims 1 to 7, wherein the feed streams (3, 5; 23) are preheated by heat transfer from respective purified feed streams (27) taken from the decomposition reactor (23).
9. The method according to any one of claims 1 to 8, wherein the feed stream (3, 5; 23) is further heated in the heater (29).
10. The method according to any one of claims 1 to 9, wherein a portion of the purified feed stream (27) taken from the decomposition reactor (31) is recycled to the feed stream (23) fed into the decomposition reactor (31) before heating.
11. The method of claim 10, wherein a portion of the purified feed stream (27) recirculated into the feed stream (23) can be separately set for each decomposition reactor (31).
12. The process according to any one of claims 1 to 11, wherein the gaseous stream comprising nitrous oxide is obtained in a process for the production of adipic acid.