HU177566B - Process for the recirculation of nitrogene oxides - Google Patents

Description

The present invention relates to a process for the recirculation of nitric oxides in nitric acid plants.

In a narrower sense, the present invention provides a coupling solution that can advantageously provide a nitric oxide content of nitric acid tail gas; by adsorption, recirculation and utilization of bound nitrogen oxides.

Nitrogen oxides are the main source of air pollutants after dust and sulfur dioxide in industrial activities. Nitrogen plants, where flue gas fluctuates around 1000-5000 ppm at present, are also a major source of unpleasant and problematic local contamination, mainly from combustion plants and engines. This nitrous oxide emission not only damages the immediate environment of l factories but also causes a significant loss in nitric acid production. In Hungary, the resulting loss of nitric acid, based on 100% acid, is currently more than 10,000 t per year.

Numerous attempts have been made to reduce the nitric oxide content of the nitrous acid waste gases. They have been experimenting with alkaline absorption and have developed methods for the catalytic reduction of nitrogen oxides to nitrogen in methane and methanol. with ammonia.

Reduction with natural gas, which is already widely used in industry 2 to enforce environmental protection considerations, is considered a temporary emergency solution as it cannot ensure the utilization of nitrogen oxides. For the same reason, the implementation of the ammonia version cannot be conclusive. In addition to the economic disadvantages, the nitrogen oxide content of the waste gases, reduced by catalytic reduction, is still 200-500 ppm.

Utilization of the nitrogen oxide content of the waste gases can be achieved by increasing the efficiency of the absorption towers or by recovering the nitrogen oxides with an adsorbent. Designers of nitric acid plants are currently making significant efforts to increase the efficiency of absorption absorption, but in parallel with the use of molecular sieve adsorbents, new adsorption technologies are emerging.

The major advantage of molecular filter adsorption tail gas purification is that it also allows nitric oxide levels below 200 ppm in the tail gas. By improving absorption efficiency, it is not possible to achieve such a low level of contamination at a tolerable cost.

There are several ideas for purifying nitric acid waste gases by molecular sieve adsorption. They often only reflect the results of laboratory experiments and as a result, the adsorption units do not fit well and are not energetically linked to nitric acid plants. In addition, it is a disadvantage of the known processes that, in addition to existing streams in nitric acid plant, a new stream (mostly air) is used to regenerate the depleted adsorber, which must be heated to the desorption temperature by an external source of energy.

The PuraSiv N procedure [Fomoff, L.L. AlChe. Symposium Ser., 68,126,111 (i972)] is energetically independent of the nitric acid plant, the regeneration is carried out by means of air heated in an oven equipped with an external energy carrier. The use of an external energy carrier is also required by the procedure of Norton (Kiovsky, J.R., Koradia, P, B., Hook, D.S., Chem Eng. Progress, 72, 8, 98 (1976)). Indirectly heated, that is to say, regeneration using an external energy carrier, is disclosed in U.S. Patent No. 158,954. The Hungarian patent and similarly energetically disclose the coupling of the molecular scavenging adsorption unit according to U.S. Patent No. 3389961. Figure 3 of the US patent.

With the solutions described, waste gas purification is feasible, but the equipment is not energetically compatible with the nitric acid plant and therefore requires an additional source of energy. Mostly air is used for regeneration and usually does not provide complete recirculation of the recovered nitrogen oxides, but is partially made up of nitric acid in a separate unit (eg, US Patent No. 3389961, Norton Process), which requires the installation of additional equipment and fittings.

In order to overcome these drawbacks, we have sought to develop an adsorption unit that integrates the nitric acid plant energetically and with respect to material streams, which is capable of recirculating the extracted nitric oxides completely in the presence of nitrous oxide, using it.

The present invention is based on the discovery that, when appropriately heat- and acid-resistant adsorbents are used, desorption can be accomplished by directly passing the hot gas mixture leaving the oxidizer through the adsorber. This discovery is surprising because the oxidant effluent contains 7 to 13% by volume of nitric oxide, yet this high nitrogen dioxide gas mixture is unexpectedly capable of removing the adsorbed nitrogen oxides there with good efficiency.

The invention is further based on the recognition that the waste gases do not have to be dried prior to adsorption, and that the waste gas leaving the absorption tower (and saturated with water vapor at a given temperature) can be introduced directly into the adsorber. This discovery is surprising because the adsorbents used generally have good water-binding properties, so that known processes (e.g. PuraSiv N) have previously dried the waste gases to be purified. Not only is this not necessary in the process of the invention, but during regeneration, after desorption, the adsorbers are rinsed and cooled: the adsorbent subsequently has an unexpected high nitrogen oxide binding capacity.

The present invention relates to a process for reducing the nitric oxide content of nitric acid waste gases by adsorption and to recycle bound nitrogen oxides using adsorbers operated in a temperature cycle. SUMMARY OF THE INVENTION It is an object of the present invention to cover the energy required to desorb adsorbed nitrogen oxides by absorbing the heat content of the gasses obtained by oxidation of the nitric oxide by passing hot gas from the oxidant or a partial stream thereof through the absorber and the gas stream leaving the adsorber is recirculated to the main stream of the nitric acid plant, after desorption, the adsorber is flushed with waste gas or air while the flushing gas is introduced into the main stream of the plant and finally cooled with the adsorber waste stream.

A simplified flowchart of nitric acid plants as shown in Figure 1 illustrates the main material streams and how they are coupled to the tail gas purification adsorption unit according to the present invention.

Ammonia is burned with mixed air in the converter 1 to nitric oxide, and the gases at 800-850 ° C are cooled in the cooler 2 to 100-150 ° C and supplied with auxiliary air to the turbo-compressor 3 into the oxidizer 4 where the nitric oxide is converted to nitrogen dioxide. The 300-350 ° C hot gases exiting the oxidizer 4 are led through the heat exchanger 5 to the absorption tower 6, where water forms nitric acid, and the 20-40 ° C tail gas is discharged through the energy recovery unit 7 and the chimney 8. In the process of the invention, the final purification unit 9 is connected to the gas stream of the nitric acid plant as shown in Figure 1.

The waste gas purification unit 9 comprises at least two adsorption columns (9a, 9b). A prerequisite for using two columns is that the regeneration time is shorter than the time required for the adsorber to saturate during the adsorption phase. At the time shown in Figure 1, the waste gas to be purified is passed through adsorber 9a and hot nitrosate gas is passed through adsorber 9b: adsorption in the first column and desorption in the second. After desorption is complete, the nitrosatable gas from column 9b must be removed by flushing. The flushing can be accomplished by using a partial flow of the waste gas available in the plant or any additional air required, the flushing gas being introduced into the main gas stream to the plant through the suction side of the turbocharger. After rinsing, the column should be cooled to the adsorption temperature, this can be done by suctioning cold gas on a commercial scale. A preferred solution for cooling is the available purified waste gas stream. When cooling is complete, regenerated adsorber 9b will not run until adsorber 9a is exhausted.

Thus, if two columns are used, the present invention preferably proceeds by passing one of the adsorbents 9a through the exhaust gases leaving the absorber 6, while the other adsorbent 9b is regenerated by passing hot gas from the oxidizer 4 and containing the desorbed nitrogen oxides. the gas is introduced into the absorber 6, after desorption, the adsorber 9b to be regenerated is flushed with the purified tail gas while the flushing gas is introduced into the main stream of the nitric acid plant, then the adsorber 9b When the adsorber 9a is exhausted, the waste gas stream to be purified is switched to the regenerated adsorber 9b and regeneration of the adsorber 9a depleted in the previous cycle is started. This coupling mode is illustrated in Fig. 2, which is a graphical representation of the elementary steps of the temperature change adsorption cycle; adsorber 9a is in the adsorption phase of the cycle, while adsorber 9b is in its regeneration phase:

Figure 2 is a view of adsorber 9a in adsorber 9b

Adsorption is desorption

B adsorption rinse

C adsorption cooling

Adsorption D - (Desorption includes heating and keeping heat.)

Nitrogen oxides can be recirculated efficiently with three or more columns, and the number of adsorbers must be determined from economic and operational safety considerations. By increasing the number of columns, the size of each column can be reduced, but the number of fittings and pipelines required increases.

The pressure used in the scrubbing system is the same as the pressure in the main stream of the nitric acid plant. Suitable heat- and acid-resistant adsorbents include suitably pretreated natural zeolites such as mordem't or clinoptilolite type molecular sieves.

The process of the present invention involves heating and maintaining the column under a stream of gas during adsorption. is preferably countercurrent, while during flushing and cooling the gas flow direction is the same as the adsorption current.

The process of the present invention is advantageously used to purify the waste gases of any type of nitric acid plant. The energy required for the regeneration of the adsorbers is provided by the oxidative heat of nitric oxide, thus leaving behind the air-heating furnace, the heat exchangers, and the associated instruments and fittings. Because flushing can be done with a partial stream of raw or adsorption-purified tail gas or inlet air, and cooling can be accomplished by passing the purified tail gas through the column to be regenerated, not only setting up a new material stream but also setting up the machinery to move it. This cooling solution has further advantages: on the one hand, the heat energy accumulated in the adsorber can be used to heat the tail gas, and on the other hand, if desired, the capacity of the currently operating adsorption column can be used not only for breakthrough but also for full saturation.

The process of the invention is illustrated by the following embodiments.

Example 1

A cylindrical adsorbent made of 1.2 m long, 216 mm diameter stainless steel was thermally insulated and made with a clinoptilolite active substance, which is also suitable for adsorption of nitric oxide, at a height of m of 30.6kgERSORB-S away. The column was coupled to the position 9b in Figure 1 in a thermally insulated nitric acid plant and heated to 320-330 ° C over a period of 1 hour with a 340 ° C, 2.5 att pressure product. The column temperature was controlled by Fe-Ko thermocouples placed in appropriate sleeves. The column was held at 320-330 ° C for 1 hour, then rinsed with the tail gas countercurrent and cooled.

The column was then passed through the column at a flow rate of 30 m '/ h from top to bottom containing 0.3% by volume of nitric oxide. The exhaust gas is HARTMAN-BRAUN (German) URAS and LIMAS brand NO, respectively. NO was analyzed. It was found that the column so prepared kept the nitrogen oxide content of the waste gas passed through it at 200 ppm for 2.5 hours. The experiment was repeated several times. For subsequent measurements, the column showed constant performance.

Example 2

In the adsorption column of Example I, 25.8 kg of a 1 m high layer of mordenite-based acid-resistant adsorbent was treated twice with 0.7 N acid for 5 hours. This charge was also heat treated with gas from the oxidizer product line as described in Example 1, then rinsed with waste gas in a counter current and cooled. A flow of 40 m ³ / h of 0.2 vol% nitric oxide was passed through the column from top to bottom. Analyzer data show that the nitrogen oxide content of the effluent gas passed the 200 ppm limit only after 5.3 hours.

Multiple repetitions showed constant adsorption performance of the charge.

Claims (2)

Patent claims 1. A process for reducing the nitrogen oxide content of nitric acid waste gases by adsorption and for recirculating bound nitrogen oxides using temperature-controlled cycle adsorbers, Characterized in that the energy required for the desorption of the oxides bound on the adsorbent is covered by the heat content of the gases obtained from the oxidation of the nitric oxide by passing the hot gas or a partial stream thereof leaving the oxidant and passing through the adsorber the adsorber gas stream is recirculated to the nitric acid master stream, after desorption, the adsorber is flushed with tail gas or air while the flushing gas is led to the main plant stream, and finally the adsorber is cooled down with tail gas. 2. A method according to claim 1, wherein two adsorbers are used, while one of the adsorbers (9a) is passed through the exhaust gases leaving the absorber (6) while the other adsorber 45 (9b) is regenerated by passing hot gas from the oxidizer (4) through it and feeding the gas containing the desorbed nitric oxides into the sink (6), after desorption the adsorber (9b) to be regenerated is flushed with a partial stream of purified waste gas, the adsorbent (9b) is cooled to the adsorption temperature with the purified waste gas exiting another adsorbent (9a) operating in the adsorption phase, and upon exhaustion of the adsorber (9a), the waste gas stream to be purified is recycled (9b) cycle regeneration of the depleted adsorber (9a).