Process for Adsorbing and Removing Nitrogen Oxides From Waste Gases

It has generally been accepted that the nitrogen oxides (abbreviated hereafter as NO.sub.x) in combustion waste gas consist mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2). Since NO.sub.2 forms to some extent nitric acid, nitrous acid or salts thereof when absorbed in water or aqueous alkaline solution, some removal methods have been proposed based on this property.

On the other hand, there is little development in the removal of NO, because NO is not readily absorbed in water or alkaline solution. As shown in the following equation:

the oxidation efficiency of NO is proportional to the square of the NO concentration and, for this reason, the oxidation of NO contained in industrial waste gas is in such low concentration as the order of ppm, which makes its removal very difficult.

Although the use of an adsorption agent, such as activated charcoal, active alumina and zeolite has been proposed for eliminating NO.sub.x, the adsorption efficiency of these adsorption agents is low, and at the same time their regeneration by wet methods is extremely difficult. Consequently NO.sub.x is usually discharged by dry methods through heating of the adsorption agent at a high temperature. When activated charcoal is employed as an adsorption agent, it must be heated up to the temperature of 200.degree. to 300.degree.C to discharge the NO.sub.x, resulting in a red heat state. Deterioration and consumption of the charcoal are therefore inevitable and, furthermore, explosion hazards exist. For these reasons, activated charcoal has not been utilized to any extent as an adsorption agent for this purpose.

It is an object of this invention to provide a novel method for removing NO.sub.x from nitrogen oxide-containing gas. Another object of this invention is to provide a novel adsorption agent to adsorb NO.sub.x such as NO and NO.sub.2 in waste gas frequently containing together SO.sub.2 and CO.sub.2 and by regenerating the adsorption agent by elution with water or aqueous alkaline solution. This invention is characterized by the unique action of electrolytic manganese blocks and characterized, in the treatment of nitrogen oxide-containing gas, by the following steps: (a) the adsorption of nitrogen oxides through a packed layer of electrolytic manganese dioxide blocks; (b) the elution of said nitrogen oxides with water or aqueous alkaline solution; and (c) the recycle of said electrolytic manganese dioxide blocks, after drying, to the aforesaid step of adsorption of nitrogen oxides.

The advantages of this invention are:

I. Special treatment for preparing the adsorption agent, for example, in powder form is eliminated;

ii. Efficient adsorption of NO, present in low concentration is achieved, not by gas phase oxidation but with electrolytic manganese dioxide blocks.

iii. The wet solution of NO.sub.x, adsorbed in a packed layer of electrolytic manganese dioxide blocks, is accomplished with water or aqueous alkaline solution, as in the case of an ion-exchange column;

iv. The ready regeneration of the adsorption ability of electrolytic manganese dioxide blocks, merely by drying, enables the use of such blocks in a recycle of said blocks.

In a column packed with a layer of electrolytic manganese dioxide blocks of lump size (1-3cm in size) or coarse grain (0.2-1cm in size), NO.sub.x is adsorbed for a given period of time, and then, water, ammoniacal aqueous solution, or sodium hydroxide solution is passed through the above packed layer to elute and recover directly the NO.sub.x adsorbed, in the form of nitric acid, nitrous acid or salts thereof, requiring no heating to discharge the NO.sub.x. The ability of electrolytic manganese dioxide blocks to adsorb NO.sub.x can be regenerated merely by drying it at about 100.degree.C.

This invention is based on the finding that electrolytic manganese dioxide blocks are able to repeat a certain cycle, i.e. the gas phase adsorption of NO.sub.x -liquid phase removal of NO.sub.x followed by regeneration of the manganese dioxide. This is unobvious over heretofore known methods of removing NO.sub.x from waste gases.

Although the fundamental mechanism of said cycle, i.e. adsorption of NO.sub.x ; removal of NO.sub.x ; regeneration of the manganese dioxide through a packed layer of the electrolytic manganese dioxide blocks is not known, it is believed that fine micropores are held within the electrolytic manganese dioxide deposits, and thereby, partial catalytic oxidizing of the adsorbed NO.sub.x results in the more efficient adsorption of NO.sub.x.

The present invention will be more fully understood by the following examples, which are intended to be illustrative only. In the examples, NO.sub.x was analyzed by the chemiluminescence method.

In a cylindrical column, 60 mm in diameter, 600 mm in height, and fitted at the bottom with a perforated plate (pores in 1 mm diameter), 1000 ml of electrolytic manganese dioxide blocks (4-10 meshes grain) was packed and a gas of a composition adjusted to that of a combustion gas was introduced to the bottom of this column at room temperature and at a rate of 4000 ml/min. The composition was as follows: NO: 600-800 ppm; O.sub.2 : 10%; N.sub.2 : the remainder. For comparison, each 1000ml of the hitherto used adsorption agent, such as activated alumina and zeolite, was packed in similar columns and control experiments were performed. The results obtained are shown on the following Table 1.

As shown in Table 1, when the packed layer for NO.sub.x adsorption according to the present invention was employed to remove NO from combustion waste gas, its NO adsorption ability surpassed that of the hitherto used adsorption agents such as activated alumina and activated zeolite.

A series of cyclic tests, i.e. adsorption, discharge, and regeneration were carried out with the same column as in Example 1. The electrolytic manganese dioxide blocks of this invention and activated alumina, for comparison purposes, were respectively packed, and a gas prepared similar to combustion waste gas was introduced at room temperature and at a rate of 4000 ml/min. for the NO.sub.x -absorption. The gas composition was as follows: NO.sub.x, 1200 ppm; No, 1000 ppm; NO.sub.2, 10%; N.sub.2, the remainder.

The adsorbed NO.sub.x was eluted with water by spraying at a rate of 30 ml/min. for 60 minutes as a mixed solution of nitrous acid and nitric acid.

The rate of NO.sub.x -adsorption during the first 5 hours and rate of NO.sub.x elution are compared in Tables 2 and 3, respectively.

It is clear that the rate of NO.sub.x adsorption by the method of this invention is exceedingly superior to that of alumina within a certain period of time. The figures at zero time show the rate of NO.sub.x adsorption of the initially introduced gas.

Analysis of NO.sub.2 .sup.-(nitrite ion) and NO.sub.3 .sup.-(nitrate ion) in effluents indicates that, though the ratio NO/NO.sub.x in the gas remains unchanged after elution by activated alumina, the ratio of NO.sub.3 present is larger and rate of discharge of NO.sub.x is excellent by the method of the present invention.

Table 4 shows the results of an NO.sub.x -adsorption test after drying the adsorption agents at 100.degree.C for 5 hours as the regeneration process.

Thus, according to this invention, the ability of NO.sub.x adsorption of the packed layer is restored to the original condition merely by drying. This is a great advantage, and represents a significant saving in cost, as a result of this invention.

The elution properties of the adsorption agent of this invention were examined by using activated charcoal as a comparison adsorption agent. NO.sub.x was adsorbed in both activated charcoal and the electrolytic manganese dioxide blocks of this invention under the same conditions. The quantities of the retained NO.sub.x in the respective adsorption agents were nearly equal: 2000 ml of NO.sub.x per 1000 ml of adsorption agent. In table 5 are shown the results of the elution of NO.sub.x with 1% ammoniacal aqueous solution.

It is obvious from Table 5 that the elution process can be put in practice more efficiently and simply by the method of this invention than by the use of activated charcoal. Since a considerable amount of NO.sub.x remains in the activated charcoal because of incomplete elution, a completely reversible cycle as accomplished by this invention (such as absorption, elution, regeneration) can not be performed with activated charcoal. Similar results were obtained when 1% sodium hydroxide solution was employed as an eluent.

The ability of NO adsorption was examined by using a similar column as in Example 1, packed with electrolytic manganese dioxide blocks (2-10 meshes grain), and introducing a mixed gas at 60.degree.C at a rate of 8000 ml/min. The gas composition was as follows: NO: 500 ppm; SO.sub.2 : 200 ppm; O.sub.2 : 10%; N.sub.2 : the remainder. Similar tests were conducted on 1000 ml of silica gel for comparison purposes. The results obtained are shown in Table 6.

As indicated in Table 6, mixing 200 ppm of SO.sub.2 in the test gas did not affect the ability of NO adsorption at all, according to the present invention. The adsorption ability surpassed hitherto used silica gel.

The ability of NO adsorption of the electrolytic manganese dioxide blocks was tested by using a similar column as in Example 1 and introducing at 60.degree.C and at a rate of 12000 ml/min. a gas having the following composition:

No: 500-600 ppm; SO.sub.2 : 100 ppm; O.sub.2 : 5%; CO.sub.2 : 10%; N.sub.2 : the remainder. Similar tests were carried out on zeolite for comparison purposes. The results of the 5 hour adsorption is shown in Table 7.

While the rate of adsorption of zeolite decreased to zero after 3 hours, the adsorption agent according to this invention maintained the rate up to 55%.

After elution with water, the adsorption agent was dried at 100.degree.C for 5 hours. The ability of NO adsorption after this regeneration was examined by means of gas of the same composition as above and the results are shown in Table 8.

It was thus confirmed that the NO adsorption ability of the adsorption agent according to this invention was exceedingly superior to that of zeolite and was easily regenerated by elution and drying, resulting in ready use for the next cycle. The ability of the NO adsorption was maintained in fairly high space velocity even in the case of the most ordinary composition of combustion waste gas containing SO.sub.2 and CO.sub.2.

Electrolytic manganese dioxide blocks were tested as an adsorption agent for gas analysis. A gas composed of NO.sub.x (590 ppm); O.sub.2 (5%); CO.sub.2 (10%); and N.sub.2 (the remainder) was introduced continuously at a rate of 4000 ml/min for 5 minutes into the same column packed with electrolytic manganese dioxide blocks as in Example 1. Sampling of the waste gas was carried out by adsorbing it in this column. The adsorbed NO.sub.x was discharged by eluting with water at a rate of 20 ml/min for 60 minutes by spraying water from the top of the column, in the form of a mixed solution of nitrite ion (NO.sub.2 .sup.-) and nitrate ion (NO.sub.3 .sup.-). Both ions were determined respectively by the methods of quantitative analysis for nitrite ion (NEDA method) and nitrate ion (brucine method). The sum of the concentration of both ions was regarded as the amount of NO.sub.x adsorbed initially in the column. For comparison, a similar experiment was conducted on 1000 ml of activated charcoal conventionally used as a gas adsorption agent. Table 9 shows the values determined of nitrite and nitrate ions and equivalent values to NO.sub.x.

As indicated in Table 9, the method by this invention is far superior to the prior art, as illustrated by the adsorption value of NO.sub.x from nitrogen oxide-containing gas, in comparison to the standard value (NO.sub.x -value by the chemiluminescence method), and compared with the case of activated charcoal. Furthermore, the ratio of nitrate ion in the effluent with respect to the adsorption agent of this invention is by far higher than that obtained from activated charcoal. The reason is that the adsorption agent according to this invention adsorbs more NO.sub.x in the form of reactive NO.sub.2 than activated charcoal does, and NO.sub.2 thus adsorbed is more easily soluble in water than NO is.

Gas composed of NO.sub.x (86 ppm, by the chemiluminescence method), CO.sub.2 (12%), SO.sub.2 (500ppm), O.sub.2 (5%), H.sub.2 O (5%) and N.sub.2 (the remainder) was introduced at a rate of 5000 ml/min for 20 minutes into the same column packed with electrolytic manganese dioxide blocks as in Example 1. Then water was passed through the column at a rate of 20 ml/min for 60 minutes. The effluent recovered was submitted to determination of nitrite and nitrate ions as in Example 6. The amount of NO.sub.x adsorbed was calculated as the sum of the concentration of both ions. Similar experiments were carried out by using activated charcoal for comparison purposes. Table 10 shows the results obtained.

As is clear from Table 10, NO.sub.x is more efficiently adsorbed and eluted by the method of this invention than by activated charcoal, even when the gas contains low concentrations of NO.sub.x in the presence of about 500 ppm of SO.sub.2 and a considerable quantity of CO.sub.2 (generally, combustion waste gas contains 10-15% of CO.sub.2).

Resort may be had to such modifications and equivalents as fall within the spirit of the invention and the appended claims.