GB2223960A

GB2223960A - De-oxygenating water

Info

Publication number: GB2223960A
Application number: GB8821895A
Authority: GB
Inventors: David Maldwyn James; Wallace John Elliot
Original assignee: British Steel PLC
Current assignee: British Steel PLC
Priority date: 1988-09-06
Filing date: 1988-09-06
Publication date: 1990-04-25
Also published as: GB8821895D0

Abstract

The present invention provides a method of de-oxygenating water by bubbling nitrogen gas via pipes 9 through the water to scavenge the dissolved oxygen therein. Further oxygen is removed by bubbling steam via port 4 through the liquid, thus raising its temperature sodium sulphite is introduced at 10 to scavenge the last residues of oxygen and the resulting acidity is neutralised by sodium hydroxide. The nitrogen is preferably pressed through the water whilst this is at ambient temperature. <IMAGE>

Description

-AERATION OF WATER This invention relates to de-oxygenating (de-aeration) of water.

Boiler feed water requires de-oxygenating to remove the dissolved oxygen from e.g. saturation levels of 9 to 10 ppm to practically zero before use in the boiler. This is necessary because of the corrosive effect of oxygen on the iron-and steel-based structure and pipework of the boiler system, especially at elevated temperatures.

Hitherto de-oxygenation has involved the use of steam injection plus chemical additions to the feedwater; more particularly the steam is used to raise the temperature of the feedwater in a holding tank (thus reducing the oxygen to a low level of. say, 2/3 ppmj and e.g. catalysed sodium sulphite is used as a scavenging medium to remove the last traces of oxygen before passage to the boiler economiser, and thence to the boiler(s).

It is an object of this invention to provide an improved method of de-oxygenating water.

From one aspect the present invention provides method of de-oxygenating water by bubbling nitrogen gas through the water to scavenge the dissolved oxygen therein.

Preferably the method is performed at ambient temperature thus readily permitting the use of condensing economisers upstream of the boiler(s).

Basically, this invention is based on the recognition that when a gas is bubbled through water in which another gas is dissolved the bubbled gas gradually removes and replaces the dissolved gas provided that their solubilities are similar. In particular, we have found that in one scheme adopted approximately one volume of oxygen-saturated water was readily de-oxygenated to a level of 0.05 ppm when two volumes of nitrogen were bubbled through that water for a specific period, accompanied by a small degree of chemical scavenging/polishing.

In order that the invention may be fully understood one embodiment thereof will now be oescribed with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the method applied to a boiler; Figures 2(a) and 2(b) are plan and side elevations, respectively, of a de-oxygenating unit according to this invention; and Figure 3 is a graph illustrating the dissolved oxygen content with, and without, de-oxygenation according to this invention, and the effect of the feed water temperature.

Referring now to Figure 1 cold feedwater which may be partially recycled from the boiler, is fed through port 1 into one section of a large water tank 2 into which section nitrogen gas is bubbled through port 3; the de-oxygenation is effected in this section and the water thus treated enters into another section of the tank through which steam is bubbled (via port 4) to raise the temperature of the water and effect a further degree of de-oxygenation. From here the water is fed to an economiser 5, which may conveniently be of the condensing type rather than the non-condensing type - see discussion below - and thence to the boiler(s) 6.

The design of water tank is shown in more detail in Figures 2(a) and 2(b). Referring to these Figures the tank 2 is divided into two sections 7, 8 identified as a cold well and a hot well, respectively. The nitrogen is fed into a series of pipes 9 set into the cold well, these pipes having a multiplicity of outlets by which the gas bubbles upwardly through the water in a horizontal #-nc vertical purge pattern effecting de-aeration. Chemical dosing vwith sodium sulphite introduced at 10 into the feedwater stream effects a scavenging and polishing action such that in the hot weil the last traces of oxygen are removed.

Typically, the temperature of the feedwater may be about 8r with the temperature at the output from the hot well about 5#0C.

Whereas a reduction in the temperature of the hot well can be accomplished without adversely affecting the degree of de-aeration it has been found that with a condensing economiser external condensation may occur on the stack as the dew point temperature of the stack gases is reached with lower temperatures.

Thus, in the scheme described 550C is regarded as the minimum temperature.

The chemical reaction within the tank is such that the sodium sulphite is converted to sodium sulphate by taking up the oxygen.

This tuns the water slightly acidic which thus has to be neutralised before passage to the economise by e.g. sodium hydroxide. This is conveniently introduced in the pipework circuit between the hot well outlet of the tank and the economiser.

Figure 3 graphically illustrates the dissolved oxygen content in the input feedwater to the boiler, as a function of temperature, without de-oxygenation (trace A) on the one hand, and with de-oxygenation in accordance with this invention (trace B), on the other.

The efficiency of this 'low temperature' proposal for nitrogen purging the tank is principally characterised by a reduction in the steam consumption in the hot well together with a reduction in the chemical scavenging cost (sodium sulphite) but for the scheme as a whole the facility to utilise condensing economisers is of paramount benefit. Condensing economisers are over 90% efficient much higher than non-condensing economisers - the latent heat of the water vapour formed in the combustion reaction being recovered in the form of extra preheat in the boiler feedwater.

Further, a small saving is effected in the reduced level of neutraliser (sodium hydroxide) now necessary by reason of the reduction in the scavenging chemical treatment.

Nitrogen costs must be discounted from the above calculations but in many instances of bulk usage where such a boiler scheme is in operation, e.g. in steelmaking or steel processing plants, contract conditions are such that excess nitrogen is available at no or only nominal cost.

Potential overall boiler thermal efficiency savings of the order of 10% may be achieved with this scheme, with much reduced corrosion level 5.

Although this invention has been described with reference to the specific embodiment illustrated, it is to be understood that various modifications may readily be made without departing from the scope of this invention. Furtner, the principle of nitrooen de-oxygenation is not necessarily linked to boiler usage; for example, cooling water for bearings etc. may readily be treated in this fashion to reduce the incidence of corrosion.

Claims

We claim:

1. A method of de-oxygenating water, by bubbling nitrogen gas through the water to scavenge the dissolved oxygen therein.

2. A method according to Claim 1, in which the water is at ambient temperature.

3. A method according to Claim 1 or Claim 2, in which the water is dosed with chemicals operative to reduce still further the oxygen content.

4. A method according to Claim 3, in which the chemical introduced is sodium sulphite.

5. A method according to any one of Claims 1 to 4, in which, subsequent to nitrogen treatment the temperature of the treated water is raised to a controlled level.

6. A method according to claim 5, in which steam is used to raise the temperature of the water.

7. A method according to claim 6, in which the nitrogen bubbling is effected in one section of a two-part water tank, the steam heating being effected in the other part.

8. A method according to any one of claims 1 to 7, in which the de-oxygenated water is fed to a boiler.

9. A method according to Claim 8, in which the de-oxygenated water is fed to the boiler via a condensing economiser.

10. A method of de-oxygenating water subtantially as herein described with reference to the accompanying drawings.

11. Apparatus for performing a method according to any one of claims 1 to 10, substantially as herein described with reference to the accompanying drawings.