GB2139207A - Removing dissolved oxygen from water - Google Patents

Abstract

In a process for the removal of oxygen dissolved in water for corrosion prevention and based on hydrazine, the activators used comprise cationic and/or anionic complexes of trivalent cobalt with inorganic ligands or mixtures thereof. Among the preferred activators are (Co(NH3)5Cl)Cl2 and Na3Co(NO2)6.

Description

SPECIFICATION Process for removal of oxygen dissolved in water This invention relates to a process for the removal of oxygen from water, such as boiler feed water.

It is generally known that oxygen dissolved in water has a corrosive effect on steel and it is necessary, even with closed systems having tanks, pipes, steam boilers, heat exchangers and other units, to add a reducing agent in order to prevent corrosion and to maintain anti-corrosion protection even when in shut-down or starting-up periods low temperatures prevail. The application of this invention includes in particular the low-temperature ranges. It also includes the removal of oxygen from gases containing oxygen.

Various processes are used for removing oxygen dissolved in water.

Sulphites, for instance, have been employed as reducing agents but these suffer from the drawback that they are oxidized to sulphates, leading to an undersirable concentration of salts. Higher temperatures, moreover, may produce corrosive gases such as sulphur dioxide. This is the reason why hydrazine, which is converted by oxygen to harmless nitrogen, has long been used as a reduction agent. This system has been found to suffer from the drawback that a satisfactory reducing action is only obtained after higher temperatures are reached. This problem has been overcome by use of activators which in general are added to the aqueous hydrazine solution which is of about 10 to 25% strength.Among the activators particularly recommended are quinoid compounds, aromatic diamines, aminophenols, sulphonic acids and heterocyclic compounds (DE 1 521 749, DE 2601 466, DE 2 115463). These activators ave the disadvantage that they have to be used in relatively high concentrations of about 0.1 to 0.3% by mass, and more, in relation to the aqueous hydrazine solution and this greatly increases the cost. A further disadvantage is that organic compounds may find their way into the vapour phase and cause complications where the vapour is used in a proces. An additional drawback is that in order to ensure a high reaction speed a narrow pH range of 10.0 10.4 has to be maintained. The pH value has to be adjusted by continuous addition of alkali.Many of the activators proposed are so costly tht for practical purposes they cannot be considered.

The use of manganese, copper, cobalt or vanadium compounds (US 4022711, US 4 079 018, US 3 764 548) has also been suggested but these have only slight activity and even cause undesirable corrosiion effects. In addition, they lead to a breakdown of the hydrazine during storage. If these drawbacks are surmounted by the use of complex compounds of these metals with organic ligands as complex-formers, for example by using hydrazide of cobalt and maleic acid (DE 232 548) or cobalt (Il)-complexes with 3,4-diamino tuluene, 1-amino-2-naphthol-4-sulphonic acid or pyrocatechin (DE 2 636 955), they still have to be used at relatively high concentrations and with high pH values. The problem of the organic constituents has not thereby been solved.

Organic activators also suffer from the serious problem that the effect decreases as time proceeds in the course of the removal of the oxygen.

Attempts have already been made to make inorganic activators by adding small quantities of iodine or bromine or salts of the acids of these elements to the hydrazine solution (DE 1 186 305). In this case the water from which oxygen is to be removed has to be caused to pass through active carbon filters, after the activated hydrazine has been mixed into them; in order to ensure satisfactory decomposition of the oxygen and this process involves further expenditure. A further drawback is the low speed at which the hydrazine activated with iodine or bromine reacts with oxygen dissolved in water.

This invention seeks to provide a method to enable oxygen dissolved in water to be removed using hydrazine as a reducing agent and activator and without the need for any organic compounds as activators and with the use of only very small quantities of activator, a range of pH values being possible and the oxygen being removable at a high reaction speed and without any reduction in the effectiveness over a period of time during the action of the oxygen. Low or high temperatures being accommodated and no treatment of the aqueous phase with active carbon being required.

The aim of the invention is to develop, for the removal of oxygen dissolved in water with hydrazine as a reducing agent, a suitable activator which contained no organic constituents and which would prove effective in small quantities, ensure a high reaction speed at low temperatures within a wide range of pH values and not cause tile hydrazine to decompose in storage with the effectiveness not decreasing in the course of time during removal of the oxygen.

According to this invention there is provided a process for the removal of oxygen dissolved in water, using hydrazine as-a reducing agent and with the addition of activators, characterized by the use as activators of cationic andlor anionic complexes of trivalent cobalt with inorganic ligands. It is equally possible to use cationic and anionic cobalt (III)-complexes or mixtures of cationic and anionic cobalt (Ill)-complexes. Sodium hexanitritocobaltate (III, cobalt (Ill)-hexamminochloride, chloropentamminocobalt (Ill)-chloride, carbona topentamminocobalt (Ill-nitrate and potasium trioxalatocabalate (III) are particularly suitable.

These activators are either commercial products or can be economically produced by known methods of preparative inorganic chemistry. The activators are added in quantities of 0.01 - 0.3% by mass of which the hydrazine content ranges commercially from 10 to 25% by mass, preferably from 20 to 25% by mass. The hydrazine activated in this manner does not decompose in the course of storage in closed vessels.

The process to which the invention relates for the removal of oxygen from water, is effected by the addition of the hydrazine solution, containing activators, to the water to be freed of oxygen and in this process the quantity of hydrazine must amount to at least one mol per mol of dissolved oxygen. It is of advantage to adopt a surplus of hydrazine. The pH rang should be 8-11. Although high pH values increase the reaction speed, a pH range of about 9 already results in sufficient rapid decomposition of the oxygen to ensure that no alkalizing substances such as alkali hydroxides or ammonia, need be added, and this constitues a substantial advantage in comparison with the prior art. The temperature may be kept within the range 283-303OK. High tempertures are advantageous but not necessary.

Corrosion tests carried out over a period of 3 months with steel specimens immersed in oxygenous water which contained hydrazine activated in accordance with the invention or, for purposes of comparison, hydrazine without any activator or hydrazine with organic activators, and which was fed twice daily with further oxygen, by blowing air into it, showed that the use of the activators according to the invention obviated corrosion phenomena, while the comparative specimens, even after only a few days, showed signs of corrosion, which increased with time.

A hydrazine solution containing activators in accordance with the invention can also be usd with advantage, due to the high reactivity, for the removal of oxygen from oxygenous gases if the hydrazine content is adapted to the quantity of oxygen contained in them. It is thus possible for the oxygen to be almost completely removed from the air by agitation with a hydrazine solution containing activator in a closed vessel and at room temperature. The said properties are particularly desirable for compensating unforeseen ingress of air into closed systems.

Examples of the invention Example 1 0.3 - 3 g of activator per litre was dissolved in an aqueous hydrazine solution containing 200 g of hydrazine per litre. From the solution containing activator and prepared in this manner 1 ml was added to a litre of water which had come in contact with air and contained 5-6 mg of oxygen per litre, the mixture then being measured with an apparatus enabling the oxygen to be determined. The pH value fo the oxygenous water was set with alkali and once again measured after the addition of the activated hydrazine. The tests were performed at 293 K. The results are shown in Table 1.

Example 2 A hydrazine solution containing activator and prepared in accordance with Example 1 was added, as described, to oxygenous water. After the decomposition of the dissolved oxygen the water was gasified for 10 minutes, air being passed through it, and then once again supplied with oxygen. This process was repeated a number of times. The oxygen content was in each case determined immediately after the degasification and after a reaction time of 10 minutes. For purposes of comparison the same treatment was applied to hydrazine solutions to which organic activators had been added and to hydrazine solution to which iodine had been added. The results are shown in Table 2.

Example 3 500 ml of water, to which 15 ml of activated hydrazine containing 1 g of activator per litre had been added, were introduced into a closed air-filled 2000 ml flask. The solution in the flask had an initial concentration of 6 g of hydrazine per litre. After the flask had been closed it was shaken for 1 hour at room temperature. A specimen was then taken from the gas phase and examined for oxygen by known methods. The oxygen content in the residual air had been reduced to 12.5% by vol with the use of hydroquinone, to 6f5% by vol with the use of hexamminocobalt (Ill)-chloride, to 6% by vol with the use of sodium hexanitritocobaltate (III) and to 14% by vol with the use of an ordinary commercial product with organic activators.

TABLE 1 O2-concentration Activator after 10 min Activator concentration pH value reaction time in g/l in mgO2/l (1)(Co(NH3)6)Cl3 0.5 10.6 3.3 1.0 10.6 2.6 1.6 10.6 1.8 2.1 10.6 1.8 2.7 10.6 1.7 (2) (Co(NH3)5Cl)Cl2 1.0 10.6 1.6 1.0 9.4 3.0 (3) (Co(NH3)sCO3)NO3 1.1 10.6 0.3 1.1 9.3 2.5 (4) Na3(Co(NO2)6) 1.6 10.6 1.2 1.6 9.3 2.5 2.0 10.5 0.8 3.0 10.5 0.2 (5) Mixture (in ratio of : 1) of 1 and 4 1.5 10.5 2.7 1.5 9.4 3.0 (6) Iodine 0.02 10.8 4.3 0.2 10.8 4.3 1.0 10.8 4.3 (7) Without - 10.4 5.2 TABLE 2 1) 10.4 5.0 1.8 2.2 0.05 1.7 0.02 1.5 0.02 9.3 4.9 3.5 3.5 2.0 2.5 1.0 2.2 0.5 2) 10.5 5.0 1.5 3.1 0.3 2.2 0.05 1.9 0.02 9.5 5.0 2.5 2.9 0.5 1.8 0.1 1.0 0.02 8.6 5.1 3.3 4.2 2.9 4.0 1.5 3.1 0.3 3) 10.5 5.1 0.2 2.2 0.5 3.5 1.8 4.4 4.1 9.5 4.5 2.7 4.4 3.5 4.4 4.3 4.5 4.3 4) 10.5 4.6 0.2 1.1 0.2 2.0 0.4 2.8 3.3 9.5 4.7 1.7 4.1 3.2 4.3 4.0 4.6 4.4 5) 10.5 4.5 4.5 4.6 4.5 4.8 4.8 4.9 4.9 9.5 4.5 4.4 4.5 4.5 4.7 4.7 4.8 4.8 Acitvator pH value O2-content in mg/I before addition of hydrazine Orcontent after 10 minute reaction time 02-content after first gasification O2-content in mg/l after 10 minute reaction time O2-content after second gasification O2-content after 10 minute reaction time O2-content after third gasification content after 10 minute reaction time Activator pH value O2-content after fourth gasification 02content after 10 minute reaction time O2-content after fifth gasification O2-content after 10 minute reaction time 1) 10.4 2.1 0.02 2.0 0.02 9.3 2.2 0.2 2.2 0.05 2) 10.5 1.8 0.02 1.9 0.02 9.5 0.8 0.02 0.9 0.02 8.6 2.4 0.2 2.3 0.1 3) 10.5 4.7 4.4 4.8 4.5 9.5 4.4 4.3 4.4 4.3 4) 10.5 4.4 4.2 4.5 4.4 9.5 4.5 4.3 4.6 4.4 5) 10.5 5.1 5.1 5.1 5.1 9.5 5.0 5.0 5.1 5.1 1 = (Co(NH3)6)Cl3 2 = Na3(Co(NO2)e) 3 commercial product with organic activator system 4 hydroquinone S 5 = iodine

Claims (9)

1. Process for the removal of oxygen dissolved in water, using hydrazine as a reducing agent and with the addition of activators, characterized by the use as activators of cationic and/or anionic complexes of trivalent cobalt with inorganic ligands.

2. Process in accordance with Claim 1, wherein hexamminocobalt (III)-chloride is used as a cationic cobalt (Ill) complex.

3. Process in accordance with Claim 1, wherein hexanitrito (III)-chloride is used as an anionic cobalt (Ill) complex.

4. Process in accordance with Claim 1, wherein mixtures of hexaminnocobalt (III)-chloride and sodium hexanitritocobalt (III) are used.

5. Process in accordance with any one of Claims 1 to 4wherein activators are added in quantities of 0.01-0.3% by mass, to a hydrazine solution of 10-25% strength.

6. Process as claimed in Claim 5, werein the activators are added in quantities of 0.05% to 0.1% by mass.

7. Process as claimed in Claim 5 or 6, wherein the hydrazine solution has 20 to 25% strength.

8. Process in accordance with any one of Claims 1 to 5, wherein pH values of between 8 and 11 are used.

9. Process as described herein and exemplified.