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

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 N2O 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 N2O 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 N2O decomposition unit (9);
• wherein
• controlling the N2O 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 N2O, 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 N2O 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).