PROCESS FOR WORKING-UP A NITROUS OXIDE COMPRISING OFF-GAS STREAM

Abstract

The invention relates on a process for working-up a nitrous oxide comprising off-gas stream from a production process of adipic acid by decomposing the nitrous oxide contained in the off-gas stream into nitrogen and oxygen in a fixed bed reactor (21) of a N.sub.2O decomposition unit (9) at a temperature in the range from 430 to 800? C. to obtain a purified gas, wherein for controlling the N.sub.2O decomposition unit (9) a nonlinear model predictive control is used which is based on a reactor model of the fixed bed reactor based on equations of energy transport and species transport for nitrogen, oxygen and N.sub.2O.

Claims

1.-11. (canceled)

12. A process for working-up a nitrous oxide comprising off-gas stream from a production process of adipic acid, the process comprising: (i) feeding the off-gas stream into a N.sub.2O decomposition unit with a pressure between 5 and 10 bar(abs) and a temperature between 100 and 300? C.; (ii) heating the off-gas stream to a temperature in the range from 430 to 650? C. in a regenerative heat exchanger and/or a heater; (iii) decomposing the nitrous oxide contained in the off-gas stream into nitrogen and oxygen in a fixed bed reactor of the N.sub.2O decomposition unit at a temperature in the range from 430 to 800? C. to obtain a purified gas; (iv) optionally recycling a part of the purified gas into the N.sub.2O decomposition unit (9); wherein for controlling the N.sub.2O decomposition unit a nonlinear model predictive control is used which is based on a reactor model of the fixed bed reactor based on equations of energy transport and species transport for nitrogen, oxygen and N.sub.2O, the nonlinear model predictive control comprises: (a) measuring a bed inlet temperature, a bed inlet volume flow, a bed outlet temperature, temperatures along the fixed bed, inlet concentrations of the off-gas stream fed into the N.sub.2O decomposition unit, and outlet concentrations of the purified gas as parameters for the model; (b) calculating bed activity using an extended Kalman-Filter; (c) calculating an optimum inlet temperature and optionally a recycle flow as function of time by a nonlinear model predictive control based on the reactor model for a given horizon in time and using calculated values for a given time step of optimum inlet temperature and optimum recycle flow as reference variable for control, wherein (c1) a degree of bypass of the regenerative heat exchanger and/or heating power of the heater and the rotational speed of a recycle blower and/or degree of internal recirculation of the recycle blower are adapted as actors of conventional PID-control of bed inlet temperature and recycle flow, and the nonlinear model predictive control based on the reactor model proposes only the optimum inlet temperature and recycle flow; or (c2) calculating an optimum degree of bypass of the regenerative heat exchanger and/or heating power of the heater and the rotational speed of a recycle blower and/or degree of internal recirculation of the recycle blower as function of time by using the reactor model for given horizon in time and adapting the degree of bypass of the regenerative heat exchanger and/or heating power and optionally the rotational speed of a recycle blower and/or degree of internal recirculation of the recycle blower as direct actors of the nonlinear model predictive control based on the reactor model; (d) shifting horizon in time with a given time step; (e) repeating steps (a) to (d).

13. The process according to claim 12, wherein the nonlinear model predictive control has the target for optimization that a time-integrated N.sub.2O-emission rate during prediction horizon is minimized.

14. The process according to claim 12, wherein the N.sub.2O-outlet concentration is as close as possible equal to a set-point of the control.

15. The process according to claim 12, wherein the value for the recycle flow (iv) is ?0 and ?a maximum capacity of the recycle blower, the gas inlet temperature into the fixed bed reactor is ?a set-point of safety circuit minus an offset and the bed temperature and the bed outlet temperature are ?the set-point of safety circuit minus an offset and the bed temperature and the bed outlet temperature ?the start temperature of catalyst sintering and deactivation, wherein the set-points of the safety circuits result from design limits of the N.sub.2O decomposition unit.

16. The process according to claim 15, wherein the offset is in a range from 3 to 100 K.

17. The process according to claim 12, wherein the off-gas stream is heated at least partly in the regenerative heat exchanger by heat transfer from the purified gas.

18. The process according to claim 12, wherein the off-gas stream fed into the N.sub.2O decomposition unit comprises 200 to 5000 Vol-ppm NO.sub.x, 0.5 to 5 Vol % CO.sub.2, 2 to 12 Vol-% 02, 0 to 0.3 Vol % CO, 3 to 25 Vol-% N.sub.2O and 0.2 to 0.8 Vol-% Ar.

19. The process according to claim 12, wherein the purified gas is passed through a DeNox unit for reducing the amount of NO.sub.x.

20. The process according to claim 12, wherein nitrous oxide is absorbed from the off-gas stream in water by pressure absorption before the off-gas stream is fed into the N.sub.2O decomposition unit.

21. The process according to claim 12, wherein nitrogen oxides contained in the off-gas stream are removed by pressure absorption in water before feeding the off-gas stream into the N.sub.2O decomposition unit.

22. The process according to claim 12, wherein the part of the purified gas is recycled into the N.sub.2O decomposition unit if the N.sub.2O concentration in the off-gas stream fed into the N.sub.2O decomposition unit is above 11 wt-%.

Description

In the Figures:

[0108] FIG. 1 shows schematically a process for working-up a nitrous oxide comprising off-gas stream,

[0109] FIG. 2 shows a N.sub.2O decomposition unit.

[0110] A process for working-up a nitrous oxide comprising off-gas stream from a production process of adipic acid is shown schematically in FIG. 1.

[0111] For working-up, a nitrous oxide comprising off-gas stream 1 is fed into a pressure absorption 3 for removing nitrogen oxides. To allow continuous operation in case of failures of a pressure absorption unit, the process comprises a first pressure absorption 3.1 and a second pressure absorption 3.2 for removing nitrogen oxides. This enables to continue the process by using one of the pressure absorptions 3.1, 3.2 for removing nitrogen oxides if the other pressure absorption 3.2, 3.1 is shut down, for example for maintenance purposes. Further, it is also possible to use both pressure absorptions 3.1, 3.2 for removing nitrogen oxides simultaneously.

[0112] In each pressure absorption 3 for removing nitrogen oxides, the nitrous oxide comprising off-gas stream 1 is compressed and cooled and passes a residence time in which nitrogen monoxide is converted into nitrogen dioxide. Subsequently, the gas stream is fed into an absorption column which comprises cooled trays, particularly trays which comprise cooling coils through which water flows. In the absorption column, a part of the nitrogen dioxide reacts with water forming nitric acid. This process preferably corresponds to a standard process for producing nitric acid.

[0113] Additionally, the off-gas of combustion of ammonia 5 also may be introduced into the pressure absorption 3. If off-gas of the combustion of ammonia 5 is fed into the pressure absorption 3, it is particularly preferred that the nitrous oxide comprising gas stream 1 from the production process of adipic acid and the off-gas of the combustion of ammonia 5 are mixed before the gas stream is compressed.

[0114] The off-gas of the pressure absorption 3 is fed into an absorption/desorption process 7 in which nitrous oxide in the gas stream is removed partially by absorption in water and subsequent desorption processes, where a first absorption and desorption process is followed by at least one more absorption and desorption process. The absorption and desorption processes thereby are carried out at different pressures. The nitrous oxide 8 separated off and concentrated in the absorption/desorption process preferably is used in the production of cyclododecanone or cylcopentanone. However, the nitrous oxide also may be used in any other process which uses nitrous oxide.

[0115] Since the gas stream which is fed into the absorption/desorption process 7 particularly contains the residual nitrous oxide besides traces of nitrogen dioxide, after leaving the absorption/desorption process 7, the gas stream is fed into a N.sub.2O decomposition unit 9. To allow continuous operation, it is preferred to provide a first N.sub.2O decomposition unit 9.1 and a second N.sub.2O decomposition unit 9.2. During normal operation, one of the N.sub.2O decomposition units 9.1, 9.2 runs and the second N.sub.2O decomposition unit 9.2, 9.1 is in stand-by mode and can be used as redundancy in case of maintenance of one N.sub.2O decomposition unit for example during catalyst change. Further, the N.sub.2O decomposition unit which is in stand-by can be used in case of a failure of the N.sub.2O decomposition unit which is in operation.

[0116] In the N.sub.2O decomposition unit 9.1, 9.2, the nitrous oxide is decomposed into nitrogen and oxygen.

[0117] The gas stream leaving the N.sub.2O decomposition unit 9.1, 9.2 may still contain nitrogen oxides. Therefore, it is particularly preferred, as shown in FIG. 1, to feed the gas stream after leaving the N.sub.2O decomposition unit 9.1, 9.2 into a DeNox unit 11 for reducing the amount of nitrogen oxides. After passing the DeNox unit 11, the gas stream is expanded and released into the atmosphere.

[0118] A N.sub.2O decomposition unit is shown in FIG. 2.

[0119] Decomposition of nitrous oxide into nitrogen and oxygen is carried out at elevated temperatures, particularly at a temperature in the range between 430 to 800? C. in a reactor 21 in presence of a catalyst. The reactor 21 may be for example a fixed bed reactor as shown in FIG. 2. The fixed bed 23 contains a heterogeneous catalyst which promotes the decomposition reaction of nitrous oxide into nitrogen and oxygen. The decomposition reaction of nitrous oxide is exothermal.

[0120] For heating the gas stream containing the nitrous oxide, the gas stream preferably is fed into a regenerative heat exchanger 25 in which heat is transferred from the gas stream leaving the reactor 21 to the gas stream entering the N.sub.2O decomposition unit 9. By this internal heat transfer it is possible to reduce the energy which must be supplied to the process.

[0121] As generally heating of the gas stream being fed into the N.sub.2O decomposition unit 9, after leaving the regenerative heat exchanger 25, the gas stream is fed into a heater 27. The heater may be any type of heater in which the gas can be heated to the temperature with which the gas stream is fed into the reactor 21. Particularly preferably, the heater 27 is an electrical heater.

[0122] In the fixed bed 23 in the reactor 21, the nitrous oxide is decomposed forming oxygen and nitrogen. The gas stream obtained by the reaction then is fed into the regenerative heat exchanger 25 for heating the gas stream fed into the N.sub.2O decomposition unit 9. By this heat transfer simultaneously the off-gas withdrawn from the N.sub.2O decomposition unit 9 is cooled. The off-gas withdrawn from the regenerative heat exchanger 25 may be further cooled in at least one additional heat exchanger 34. The additional heat exchanger 34 preferably is a heat exchanger for producing steam by evaporation of water or for superheating water. The off-gas withdrawn from the additional heat exchanger 34 can be fed into the DeNox unit 11 or released into the atmosphere. If multiple additional heat exchangers are used, the off-gas is first cooled in a heat exchanger, where saturated steam is superheated and is then cooled in a further heat exchanger where water is evaporated.

[0123] For a largely complete or particularly a complete decomposition of the nitrous oxide it is necessary to keep the amount of nitrous oxide in the gas stream fed into the reactor 21 below a predefined upper range, particularly below 11 wt-%. For reducing the amount of nitrous oxide in the gas stream fed into the reactor if this gas stream contains too much nitrous oxide, a recycle line 29 with a recycle blower 31 is provided, the recycle line 29 connecting a feed line 33 by which the gas stream is fed into the N.sub.2O decomposition unit 9 and an exit line 35 by which the gas is withdrawn from the N.sub.2O decomposition unit 9.

[0124] For controlling the N.sub.2O decomposition unit 9 using the nonlinear model predictive control, several process data must be measured. Further it is necessary to control the compressor 31 to set the amount of gas recycled into the gas stream fed into the N.sub.2O decomposition unit 9 and to control the heater 27 to heat the gas stream fed into the reactor to a predefined temperature. Further, for setting the temperature of the gas stream fed into the reactor 21 a bypass 37 may be provided, bypassing the regenerative heat exchanger 25. The bypass 37 may be closed so that the whole gas stream flows through the regenerative heat exchanger 25 or the bypass 37 is open and the feed line into the regenerative heat exchanger 25 is closed so that the whole gas stream flows through the bypass 37 or in a third alternative, the gas stream is split and a part flows through the regenerative heat exchanger 25 and a part through the bypass 37.

[0125] For providing the data used for the nonlinear model predictive control, a first temperature indicator 39 is provided for measuring the temperature of the gas stream after being heated in the heater 27 and a second temperature indicator 41 for measuring the temperature of the gas stream being withdrawn from the reactor 21 after having passed the regenerative heat exchanger 25 and the at least one additional heat exchanger 34. By using a third temperature indicator 43 the temperature in the reactor 21 after the fixed bed 23 is measured. Further, the concentration of nitrous oxide in the gas stream is measured by a suitable quality indicator 45 and the flow rate of the gas stream fed into the reactor 21 by a flowmeter 47. A fourth temperature indicator 48 is used for measuring the temperature in the fixed bed 23 and by using a second quality indicator 50, the concentration of the nitrous oxide in the off-gas which flows through the exit line 35 is measured.

[0126] Temperature measuring in the fixed bed is done with at least one temperature indicator 48. As usually the temperature along the flow direction of the gas is not constant, it is preferred that 2 to 12 temperature indicators, and more preferably 3 to 10 temperature indicators which are arranged in succession in flow direction of the gas are used for measuring the temperature in the fixed bed 23.

[0127] By the nonlinear model predictive control, an optimum inlet temperature is calculated which is submitted to a temperature controller 49, which sets the amount of the gas stream flowing through the bypass 37 and/or for the power of the heater 27 to control the temperature of the gas fed into the reactor 21. Further, the nonlinear model predictive control may submit an optimum value of the amount of the gas stream which is recycled through the recycle line 29 to the flow controller 53, which sets the rotational speed and/or degree of internal recirculation of the recycle blower 31 to control the gas stream which is recycled through the recycle line 29.

[0128] The data which are used for the nonlinear model predictive control are submitted to a data processing unit 51.

[0129] In the data processing unit which can be any data processing unit known to a skilled person, particularly a computer the activity of the fixed bed 23 is calculated using an extended Kalman-Filter. Further, an optimum inlet temperature and a recycle flow are calculated by nonlinear model predictive control as a function of time by using a reactor model of the fixed bed reactor which is based on equations of energy transport and species transport for nitrogen, oxygen and nitrous oxide for a given horizon in time. The calculated values for a given time step of optimum inlet temperature and optimum recycle flow are used as reference variables for controlling the temperature by temperature controller 49 and the recycle stream which is recycled into the stream being fed into the N.sub.2O decomposition unit 9 via bypass 29 using a flow controller 53.

[0130] The nonlinear model predictive control based on the reactor model is adapted such that either a degree of bypass of the regenerative heat exchanger and/or heating power of the heater and optionally the rotational speed of the recycle blower and/or degree of internal recirculation of the recycle blower are adapted as actors of conventional PID-control of inlet temperature of the fixed bed and optionally of the amount of the recycle flow, and the nonlinear model predictive control proposes only the optimum inlet temperature and recycle flow; or an optimum degree of bypass of the regenerative heat exchanger and/or heating power of the heater and optionally the rotational speed of the recycle blower and/or degree of internal recirculation of the recycle blower as function of time is calculated by using the nonlinear model predictive control algorithm which is based on the reactor model for given horizon in time and the degree of bypass of the regenerative heat exchanger and/or heating power and optionally the rotational speed of the recycle blower and/or degree of internal recirculation of the recycle blower is adapted as direct actors of the nonlinear model predictive control.

[0131] After having calculated the optimum inlet temperature and optimum recycle flow for a given horizon in time, the horizon in time is shifted with a given time step and subsequently, all measurements and calculations are repeated for the new horizon in time.