Process for working-up a nitrous oxide comprising off-gas stream

1 . 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 2 O decomposition unit with a pressure between 5 and 10 bar(abs) and a temperature between 10° 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 2 O 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 2 O decomposition unit ( 9 );

wherein

controlling the N 2 O decomposition unit using a nonlinear model predictive control 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 2 O, 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 2 O 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).

2 . The process according to claim 1 , wherein the nonlinear model predictive control has the target for optimization that a time-integrated N 2 O-emission rate during prediction horizon is minimized.

3 . The process according to claim 1 , wherein the N 2 O-outlet concentration is as close as possible equal to a set-point of the control.

4 . The process according to claim 1 , 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 2 O decomposition unit.

5 . The process according to claim 4 , wherein the offset is in a range from 3 to 100 K.

6 . The process according to claim 1 , wherein the off-gas stream is heated at least partly in the regenerative heat exchanger by heat transfer from the purified gas.

7 . The process according to claim 1 , wherein the off-gas stream fed into the N 2 O decomposition unit comprises 200 to 5000 Vol-ppm NO x , 0.5 to 5 Vol-% CO 2 , 2 to 12 Vol-% O 2 , 0 to 0.3 Vol-% CO, 3 to 25 Vol-% N 2 O and 0.2 to 0.8 Vol-% Ar.

8 . The process according to claim 1 , wherein the purified gas is passed through a DeNox unit for reducing the amount of NO x .

9 . The process according to claim 1 , 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 2 O decomposition unit.

10 . The process according to claim 1 , 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 2 O decomposition unit.

11 . The process according to claim 1 , wherein the part of the purified gas is recycled into the N 2 O decomposition unit if the N 2 O concentration in the off-gas stream fed into the N 2 O decomposition unit is above 11 wt-%.