JPS6363272B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for removing dissolved oxygen from boiler water. Efficient operation of boilers and other steam-operated equipment requires chemical treatment of feed water to inhibit corrosion. Corrosion within such equipment generally occurs in water supply equipment, pre-boiler systems,
Occurs as a result of oxygen invasion of the steel in the boiler and condensate return pipes. Unfortunately, oxygen invasion of steel is accelerated by the unavoidably high temperatures found within boiler equipment. Acidic PH also promotes corrosion, so most boiler systems operate at alkaline PH. In most modern boiler systems, most of the dissolved oxygen is treated by first removing it mechanically and then removing the remainder chemically. Mechanical degassing is typically 0.5-1.0 mg/
It is carried out in a vacuum degasser which reduces the oxygen to lower levels or in a degassing heater which reduces the oxygen concentration to a range of 0.005 to 0.01 mg/. Chemical removal of residual dissolved oxygen is extensively accomplished by treating water with hydrazine. For example, see the Encyclopedia by Kirk Osmer published by Interscience Publishers.
of Chemical Technology” 2nd Edition Volume 2
Please see page 187. As explained by Kirk Osmer, hydrazine efficiently removes residual oxygen by reacting with oxygen to produce water and gaseous nitrogen. In addition, hydrazine is a good metal passivator because it forms and maintains an adherent protective layer of magnetite on the iron surface. Unfortunately, however, it has become widely recognized that hydrazine is a highly toxic chemical. As a result, exposure of workers to this compound will likely be subject to extremely strict tolerance limits by government occupational health authorities. Therefore,
It would be desirable to provide an alternative boiler water treatment chemical that generally does not have the inherent risks of using hydrazine, yet which removes oxygen and passivates steel surfaces under typical boiler conditions. The purpose of the present invention is to inhibit corrosion in boiler systems, and more particularly to treat boiler water to remove dissolved oxygen and passivate metal surfaces. Therefore, the present invention provides boiler water characterized in that an oxygen-removed amount of carbohydrazide is added to boiler water having an alkaline pH, and that this oxygen-removed amount is at a level of at least 0.5 mol per mol of dissolved oxygen. Provided is a method for removing dissolved oxygen from. Carbohydrazide, also known as NADU or N,N' diaminourea, is a high-melting solid (melting point 157°-158°C) that freely dissolves in water, so carbohydrazide can be used in dry powder form or in solution. I can do it. Although the carbohydrazide can be charged at any point in the boiler system, it is preferred to treat the boiler feed water, preferably as it comes from the degassed gas. Residence time before water vapor generation should be maximized to maximize corrosion protection. Carbohydrazides inhibit corrosion even with residence times as short as 2 to 3 minutes, although residence times of 15 to 20 minutes or more are preferred. The amount of carbohydrazide required to efficiently remove oxygen from boiler water depends on the amount of oxygen actually present in the water. It is generally preferred to use at least 0.5 moles of carbohydrazide per mole of oxygen. This minimum level of carbohydrazide has the added advantage of effectively passivating the metal surface. Of course, especially for treating boiler water under static storage conditions, per mole of oxygen
Carbohydrazide levels well above 0.5 molar may be required. Under such static conditions, treatment levels of, for example, 160 moles of carbohydrazide per mole of oxygen or greater have been found to be effective in inhibiting corrosion. Carbohydrazides are efficient oxygen scavengers as well as metal passivators over the entire range of temperatures at which boiler water is commonly subjected. Typically this temperature will be in the range of 190° to 350°. Although it is well known that one molecule of carbohydrazide can be hydrolyzed into two molecules of hydrazine, the degree of hydrolysis under typical boiler conditions is very low. This fact illustrates that carbohydrazides are efficient oxygen scavengers and metal passivators themselves. Although the carbohydrazide can be used alone in the process, it is preferred to catalyze it. For this purpose, it is preferable to use a catalyst that carries out an oxidation-reduction reaction. For example, hydroquinone and other quinones can undergo oxidation-reduction reactions and can be used to contact carbohydrazides. When using a quinone catalyst, the amount of quinone added to the carbohydrazide is 0.2 of the carbohydrazide.
and about 20% by weight. Another oxidation-reduction catalyst useful in combination with the carbohydrazide in the present process is cobalt, preferably in a stabilized form. With respect to the carbohydrazide, the amount of cobalt used should be in the range of 0.2 to about 20% by weight. Representative useful stabilized cobalt complexes are described in the following U.S. patents, which are hereby incorporated by reference: U.S. Pat.
No. 4012195; No. 4022711; No. 4022712; 4026664
No.; and No. 4096090. The following examples are provided to illustratively explain the invention. Example 1 A small laboratory boiler was used to screen a large number of potential oxygen scavengers. Oxygen saturated feed water containing sodium hydroxide was used as the feed. Residence time for processing chemicals is approximately 11
It was hot in minutes. The temperature of boiler water was maintained between 200° and 220°. 2% hydroquinone was used as the catalyst in these tests. The results are shown in Table 1 below.

Table: Drazide Example 2 A second, more extensive group of potential oxygen scavengers was compared, again using a small laboratory boiler and oxygen saturated feed water containing sodium hydroxide.
The results obtained are shown in Table 2. Table 2 shows that the following compounds exhibited comparable activity to hydrazine: hydrazine sulfate, N,N'diaminourea, disodium dihydroazodicarboxylate, and hydrazine sulfite. Of this group, hydrazine salts are toxic like hydrazine and were therefore omitted from further studies. Of the two remaining compounds, carbohydrazide was the best remover. Additionally, carbohydrazides are preferred because they are solids at room temperature and therefore could be used in dry matter formulations.

TABLE Example 3 The oxygen removal efficiency of carbohydrazide was compared to hydrazine in the following series of tests and the results are shown in Table 3 below. In this example, a single pass experimental setup was utilized to simulate boiler water transport.
In this example, the water temperature is approximately 227° to 239°, and the pH is approximately
I kept it at 10.5. The data obtained in this example show that the level of oxygen removal achieved with a given concentration of hydrazine is generally achieved with half the amount of carbohydrazide. Additionally, this data shows that there is little difference between the contacted remover and the uncontacted one.

EXAMPLE 4 Another series of tests comparing hydrazine and carbohydrazide was conducted and is shown in the form of a graph in FIG. These tests were conducted at 135 〓 and PH 9.0 to prevent hydrolysis of the carbohydrazide. The equipment used included a reciprocating pump that drew water from a water reservoir and passed it through a heated steel coil at a rate of approximately 37 ml per minute. The coil was placed in a cooling tower and dissolved oxygen was measured at the exit of the tower. The initial concentration of oxygen obtained at this location for these tests was 7.7 ppm. After measuring the initial concentration of oxygen, the scavenger to be tested was injected into the feed line upstream of the reciprocating pump. Figure 1 shows the dissolved oxygen concentration measured at the coil outlet. This figure shows that carbohydrazide is superior to hydrazine at the corresponding scavenger to oxygen level. Example 5 A commercial scale comparison was made between carbohydrazide and hydrazine. This test tested the feed water and condensate water mixture before feeding it into a 160 p.si, 360° to 366〓 first stage heater and a 400 p.si, 446゜ to 450〓 second stage heater. was carried out in an apparatus containing a deaerator operating at 60 p.si and 300° to 310°. Then the second
Three boilers from stage heaters, i.e. 1275p.si
combination boiler, a 1275 p.si power boiler and a 400 p.si second power boiler. Processing chemicals were fed into the deaerator and its outlet, respectively. Samples at various locations on this device showed that carbohydrazide was comparable to or superior to hydrazine at levels of carbohydrazide half that of hydrazine. In particular, the sample at 300〓 (just after the deaerator) showed comparable results to hydrazine, and the sample at 360〓 (just after the first stage heater) and the sample at 446〓 (just after the second stage heater) showed results comparable to those of hydrazine. Carbohydrazide was shown to remove oxygen more efficiently than hydrazine (carbohydrazide is still at a level 50% that of hydrazine). Example 6 A metal passivation study was conducted using an experimental firerod boiler under the following test conditions: 600 p.si 110,000 BTU/ft ² /hour Boiler water concentration 10 Test time 48 hour supply _Water : Ca4ppm (as _CaCO3 ) Mg2ppm (as _CaCO3 ) _SiO2 1ppm (as _SiO2 ) M alkaline 40ppm (as _CaCO3 ) Na2SO4 _42.6ppm ( _Na2SO4 ) _NaCl10ppm (as _NaC1 ) Hydrazine, The tests with carbohydrazide and sodium sulfite were followed by a blank test without oxygen scavenger. The state of the fire rods after the test was compared and shown in Table 4. This study shows that carbohydrazide is the most effective metal passivating agent under the conditions tested.

[Table] * Pipes are graded based on the appearance of the metal surface.
Example 7 The effectiveness of the catalyst was tested in this series of tests using the equipment described in Example 4. The results obtained, illustrated graphically in FIG. 2, showed that the contacted carbohydrazide (catalyst active ingredient level was 1%) outperformed the carbohydrazide or hydrazine alone. In addition, cobalt-contacted carbohydrazides generally performed better than hydroquinone-contacted carbohydrazides. Example 8 Low temperature hydrolysis studies were conducted using reagent grade carbohydrazide containing approximately 2.7±0.1% hydrazine as an impurity. With degassed and deionized water
A 1000 ppm carbohydrazide solution and a 100 ppm hydrazine solution were prepared. Half of the carbohydrazide solution was adjusted to pH 10.2 with sodium hydroxide, while the other half remained at its original pH of 6.9. The solution was injected into the Nessler tube until it overflowed and sealed with a stopper equipped with a hypodermic needle to minimize air contact and release pressure. Four samples of both carbohydrazide solutions and two samples of the hydrazine control were stored at various temperatures for various periods of time as shown in Table 5 below. The hydrazine comparator was
After days it showed a decrease of about 10% of the initial concentration. Results for carbohydrazides at room temperature and 120°C show that no hydrolysis occurred at these temperatures. At 180㎓, hydrolysis occurred only 3% above the original free hydrazine level. Values for the PH range of 7 to 10 remained virtually unchanged.

【table】

Table: Example 9 Again, a reagent grade carbohydrazide containing 2% hydrazine as an impurity was then used for high temperature hydrolysis studies. The equipment used in this study is a single-pass experimental equipment to simulate the transport of boiler water described in Example 2, and the pH of the feed water is
At 8.0, the device pressure was set at 600 p.si. The results for high temperature hydrolysis of carbohydrazides are shown in Table 6. These results show that 220〓 and 282〓 have the lowest hydrolysis, and 340〓
400〓 indicates that hydrolysis is still incomplete. Referring to the results of Example 2 shown in Table 2, the carbohydrazide was shown to efficiently remove oxygen at about 220㎓. Since less than 7% hydrazine was found at this temperature in this example, the oxygen removal achieved by the practice of the present invention must be due to the action of species other than hydrazine.

[Table] Theoretical total hydrogen
Radin 71.1ppm
Example 10 The result of Example 9 shows that the hydrolysis of carbohydrazide at 400㎓ is incomplete, so the residual hydrazine level of carbohydrazide is reduced by treatment with hydrazine alone (ratio of carbohydrazide to hydrazine 1:2).
This suggests that the level is likely to be lower than that of This expected result was explored in a residual hydrazine study conducted in a commercial boiler described in Example 5. The results in each of the three samples shown in this example (300〓, 360〓 and 450〓) confirmed the results predicted above.
The data obtained with the 360〓 sample is representative of the data obtained with other samples and is shown graphically in Figure 3. In connection with this figure, it can be seen that the carbohydrazide used in this study was actually 79.9% pure. When this lower concentration is taken into account, the residual hydrazine level observed for carbohydrazide is approximately half that observed for hydrazine, regardless of temperature. Therefore, it again appears that the carbohydrazide is far from completely hydrolyzed in the system and that hydrazine is probably not the scavenger species involved in removing dissolved oxygen from the boiler water system. .

[Brief explanation of drawings]

FIG. 1 is a graph showing the amount of oxygen removed in each case for comparison of the effects of hydrazine and carbohydrazide. FIG. 2 is a graph to show a comparison of the effectiveness of using a contacted carbohydrazide catalyst versus the case of carbohydrazide or hydrazine alone. FIG. 3 is a graph showing the residual hydrazine level due to the respective hydrolysis of hydrazine and carbohydrazide.

Claims (1)

[Scope of Claims] 1. Adding an oxygen-removing amount of carbohydrazide to boiler water having an alkaline pH, and the oxygen removing amount is at least 0.5 per mole of dissolved oxygen.
A method for removing dissolved oxygen from boiler water characterized by molar carbohydrazide levels. 2. The method of item 1 above, in which boiler water is deaerated to reduce the level of dissolved oxygen, and then carbohydrazide is added to the degassed boiler water to remove remaining dissolved oxygen. 3. The method of item 1 above, using a carbohydrazide together with a catalyst capable of undergoing an oxidation-reduction reaction. 4 0.2 to about 20% by weight based on carbohydrazide
The method of paragraph 3 above, wherein the catalyst is present in an amount of . 5. The method of item 3 or 4 above, wherein the catalyst is hydroquinone. 6. The method of item 3 or 4 above, wherein the catalyst is cobalt.