SU1664740A1 - Method of automatic process control for cleaning effluent gases from nitrogen oxides - Google Patents

Abstract

The invention relates to methods for the automatic control of technological processes, can be used in the industry for the production of mineral fertilizers and in the chemical industry in the production of nitric acid, reduces the consumption of ammonia and reduces the total content of ammonia and nitrogen oxides in atmospheric emissions. The content of nitrogen oxides in waste tail gases after the reactor is measured, the content of nitrogen oxides at the reactor inlet and the supply of ammonia are calculated by the temperature in the absorption column, the degree of purification and the optimum content of oxides in the reactor waste are calculated by the temperature in the reactor and the load. the tail gases and the deviation in them of the nitrogen oxides from the calculated value corrects the flow of ammonia into the reactor. 1 il.

Description

This invention relates to the automation of chemical production and can be used in the mineral fertilizer industry and in the chemical industry in the production of nitric acid.

The purpose of the invention is to reduce the specific consumption of ammonia and the total content of ammonia and nitrogen oxides in atmospheric emissions.

An analysis of the results of research on the purification of tail nitrous gases shows that in order to provide the necessary degree of purification with increasing volumetric gas velocity, it is necessary to increase the temperature of the process in the catalytic purification reactor. With a gas flow rate of 5000 hours 1, the required value is achieved by

the total content of nitrogen oxides and ammonia in the purified gas (less than 0.005% by volume) at 200 ° C in the reactor, and with a space velocity of 15000 h 1, temperatures over 300 ° C are required to achieve the same degree of reduction.

Due to the fact that the volumetric rate of tail nitrous gases in the catalytic purification reactor is determined by the operating conditions of the previous stages of the workshop (load), and the temperature in the reactor is mainly determined by the heat exchange in the flue gas preheater, it is generally not possible to achieve in the reactor an optimal fit gas velocity and temperature, while for each load and temperature there is a certain degree of purification, which is impossible

about

4 XI O

but increased by increasing the dosage of ammonia. Experimentally for loads in the field Q "(5000-15000), the dependence of the degree of purification a (%) on the space velocity Q of the gas and the temperature T1 in the catalytic purification reactor was established

 . (0.37,, 98) + 0.25. T1 + 100.2,

where a. - degree of purification of tail nitrous gases from nitrogen oxides,%;

Q is the volumetric gas velocity in the reactor, h 1;

T1 is the temperature in the reactor, ° C.

Another important factor influencing the content of nitrogen oxides in the tail gases is the efficiency of the absorption column (physicochemical and hydrodynamic absorption conditions). Since the dissolution of nitrogen oxides produces heat of reaction, the most important factor among these is process temperature, In industrial practice, the heat of reactions taking place in the absorption column is removed by water supplied to the coils located on the column plates. A decrease in temperature contributes to a decrease in the content of nitrogen oxides in the exhaust gases, since the rate of absorption of nitrogen oxides increases, the equilibrium of the reaction shifts towards the formation of nitric acid.

Under industrial conditions, obtaining cooled water with a sufficiently low and stable temperature is not always possible. The climatic daily fluctuations in the temperature of the medium significantly affect the temperature of the cooled water, and as a result, the temperature of the absorption process.

Since the gas phase load change automatically changes the column irrigation and does not significantly affect the content of nitrogen oxides in the exhaust gases, the dependence of the content of nitrogen oxides at the outlet of the column depending on the temperature T2 in the column for working range (20-401 ° C:

J 8.8 J. T2 ° 87 -1., Where p is the content of oxides of nitrogen in the gases leaving the column,% by vol .;

T2 — process temperature in the absorption column, ° C.

The drawing is a diagram of the implementation of the method.

The scheme includes an absorption column 1, a high-speed gas heat exchanger 2, a catalytic purification reactor 3, sensors

-

0

5 0 5 0

50

five -

0

five

4 and 5 of the temperature of the media, respectively, in column 1 and reactor 3, sensor (flow meter) 6 of flow rate Q of the gas phase in column 1, sensor 7 of nitrogen oxides in the tail gas 3 leaving the reactor, unit 8 for calculating the composition of gases at the outlet of column 1 , unit 9 for calculating the degree of purification in reactor 3, unit 10 for calculating the content of nitrogen oxides in the tail gas leaving the reactor 3, comparison circuit 11, ratio unit 12 and valve 13 for supplying ammonia to the catalytic purification reactor 3.

The scheme works as follows.

After oxidation, nitrous gases at the previous stage enter at a temperature of 300–340 ° C to heat exchanger 2, where they are cooled to a temperature of 20–40 ° C, and then go to the bottom of the absorption column 1. The volume of incoming nitrose gases is measured by a flow meter 6. A weak solution of nitric acid is fed to the top of column 1 as reflux. The coils located on the plates of column 1 are fed with cooled water to remove the heat of the reactions occurring in column 1. The process temperature in column 1 is measured by sensor 4 and the signal T2 is fed to block 8 for calculating the composition of the gas phase at the outlet of column 1. Block 8 forms a signal / 3 of nitrogen oxides in the gas phase according to

,eight" . T2a87-1.9. ,

The signal / 3, entering into the ratio block 12, acts on the valve 13 for supplying ammonia to the reactor 3, thus determining a ratio of the order of 1.1.

The process temperature T1 in the reactor 3 is measured by the sensor 5 and fed to the purification degree calculation unit 9 in the reactor 3. In block 9, the gas phase flow rate signal Q from the sensor 6 is also input. Based on the T1 and Q signals, unit 9 generates a purification degree signal according to dependence

a (0.37 ". T1-1.98) + 0.25 T1 +100.2.

Based on the signal / content of nitrogen oxides in the gas phase supplied to the catalytic purification reactor 3 and the degree of purification signal in this reactor, unit 10 generates a signal of the calculated content of nitrogen oxides in the 3 gases leaving the reactor according to the formula

(lg -DOOO-D) (NQ) pac4. I'm 100

and then the calculated value is fed to the first input of the comparison circuit 11, to the second input of which a signal of the actual content of nitrogen oxides in the exhaust gases is measured, measured by the sensor 7. When exceeding

The actual signal above the calculated comparison circuit 11 generates a corrective signal in block 12, under the action of which, in block 12, the feed ratio of the IV: {MOh changes, increasing up to 1.15 depending on the degree of mismatch. With a smaller value of the actual signal compared to the calculated ratio, the NOx ratio is adjusted accordingly downwards.

Thus, the proposed method, in comparison with the known method, allows to increase the accuracy of controlling the supply of ammonia and thereby reduce the consumption of ammonia by 15-16%, as well as minimize the total amount of ammonia and nitrogen oxides in atmospheric emissions.

Claims (1)

The invention The method of automatically controlling the process of cleaning tail gas from nitrogen oxides in the production of weak nitric acid in a plant that includes series-connected absorption column and catalytic cleaning reactor, by adjusting the supply of reflux to the absorption column, depending on the load of the column in the gas phase, adjusting the flow ammonia into the reactor and measurement of the consumption of nitrous gases in the absorption column and the content of nitrogen oxides in the tail hectares leaving the reactor ah, wherein. that, in order to reduce the specific consumption of ammonia, the total content of ammonia and nitrogen oxides in atmospheric emissions, the temperatures of the media in the reactor and the absorption column are additionally measured, the content of nitrogen oxides in the gas at the outlet of the absorption column is calculated from the measured temperature of the medium in the absorption column the measured values of temperature in the reactor and the flow rate of nitrous gases in the absorption column are used to calculate the degree of purification of gases in the reactor, based on the calculated values of the content of nitrogen oxides in the gas at the outlet of the absorption column and the degree of gas purification in the reactor calculates the optimal content of nitrogen oxides in the tail gas leaving the reactor, compares it with the measured value and, depending on the deviation, corrects the flow of ammonia into the reactor. In vol. L, Irrigation loose Soda INHO