Dissolved oxygen removal method using activated carbon fiber...

Chemical apparatus and process disinfecting – deodorizing – preser – Process disinfecting – preserving – deodorizing – or sterilizing – Maintaining environment nondestructive to metal

Reexamination Certificate

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C422S011000, C422S017000, C210S750000, C210S757000, C252S178000

Reexamination Certificate

active

06391256

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof for preventing corrosion of metal due to dissolved oxygen contained in water used in a steam generator and a cooling water system, and in particular, to an improved dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof which are capable of significantly enhancing removal performance of dissolved oxygen contained in water when impregnating metals on an activated carbon fiber based on the characteristics that an activated carbon fiber (ACF) has an excellent adsorption force for dissolved oxygen and a high surface area, so that the method and apparatus according to the present invention are effectively applicable for a water treatment system of a steam generator and a cooling water system.
2. Description of the Conventional Art
The following table 1 illustrates dissolved oxygen concentration contained in water at atmospheric pressure. As shown therein, a large amount of dissolved oxygen (8~10 ppm) is contained in water at room temperature when water is exposed to air.
TABLE 11
Concentration of dissolved oxygen in water based on temperature
T
10
21
32
43
54
66
77
88
99
D
11.43
8.87
7.45
6.15
5.44
4.43
3.43
21.15
1.43
In the table, T represents temperature (° C.), and D represents dissolved oxygen (ppm).
When the water in which dissolved oxygen is saturated is directly used for a steam generator, a corrosion may occur in a metal for thereby decreasing the life time of the system and causing operational problems. The corrosion of the metals due to the dissolved oxygen is composed of a cathodic reaction in which a reduction reaction of dissolved oxygen occurs and an anodic reaction in which an oxidation reaction occurs.
Corrosion reaction of Fe due to dissolved oxygen
i) Cathodic reaction: O
2
+4H
+
+2H
2
O (acid solution)
 O
2
+2H
2
O+4e→4 OH
+
(neutral, alkali solution)
ii) Anodic reaction: Fe→F
+2
+2e
Fe′
2
+2 OH

→Fe(OH)
2
(or FeOnHO
2
)
4 Fe(OH)
2
+O
2
+2H
2
O→4 Fe(OH)
3
(or Fe
2
O
3
nH
2
O)
2 Fe(OH
3
)→Fe
2
O
3
+H
2
O(high temperature)
The corrosion product of Fe is classified into three products. Among which, Fe(OH)
2
is green in color and produced at a neighboring surface of the metal, Fe(OH)
3
is grey in color and produced at an outer most portion, and Fe
3
O
4
H
2
O are black color in and produced between the above-described two layers.
The metal corrosion in water due to the dissolved oxygen is proportional to the concentration of the dissolved oxygen and linearly increases. In the case that there is no dissolved oxygen, the corrosion rate of Fe is low (below 0.2 mpy), and when the water is saturated by the dissolved oxygen, the corrosion rate is increased by more than 100 times. When corrosion occurs, the process heat efficiency ratio is significantly decreased because the corrosion products are formed on the heat transfer surfaces, thereby decreasing the life time of the system.
Therefore, in order to maintain a corrosion prevention effect and a heat efficiency of a metal based on the dissolved oxygen, the concentration of the dissolved oxygen contained in water should be strictly limited. The following table 2 illustrates an example that the limits of the dissolved oxygen suggested by steam generator manufacturers. As shown therein, it is recommended to maintain the concentration of the dissolved oxygen below 10 ppb in the normal operation mode.
TABLE 2
Limits of Dissolved Oxygen Suggested by Steam Generator Manufacturers (ppb)
Hot
Cold
Source
System
Operation
stand-by
shut-down
Westing house
Boiler
<5
<100
<100
Combustion Engineering
Boiler
<10 
<100
<100
Bobcock & Wilcox
Boiler
<7
<100
<100
EPRI
Boiler
<3
<100
<100
Condenser
<10 
In the conventional dissolved oxygen removal method, the dissolved oxygen in water used for the steam generator is removed based on a mechanical degasifier and reducing agent (for example, hydrazine). However, the above-described conventional methods have the following disadvantages in view of the processing performance and cost.
1. Mechanical Deaerating Method
i) Vacuum Deaerating Method
This method has been most widely used for removing the dissolved oxygen of make-up water in a steam generator of a nuclear power plant. The operational principle is to spray water into a vacuum tower and to decrease the pressure of the tower gas phase for thereby removing a non-condensable gas. The packed towers are preferably designed in more than two stages for thereby increasing the oxygen removing efficiency.
The dissolved oxygen removing efficiency is affected by the vacuum degree, the size of the packed tower, the water temperature at an entrance, etc. The packing material preferably has a large surface area per unit volume and then the size of the packed tower is determined. There should not be a by-product of an impurity from the packing material, and the vacuum degree should be maintained at a predetermined degree using a vacuum pump and a vapor ejector for thereby fully removing oxygen, nitrogen and CO
2
from the packed tower. The water processed by the vacuum deaerating method contains a dissolved oxygen in a range of 30~40 ppb. Namely, it is impossible to perfectly remove the dissolved oxygen. The dissolved oxygen concentration is increased by air leakage at the sealed portions. In addition, a special sealing apparatus is required for maintaining the system at a predetermined vacuum degree. In addition, in order to maintain a vacuum state in the interior of the packed tower, an expensive apparatus is and needed high maintenance costs are incurred.
ii) Thermal Deaerating Method
The gas solubility in water is proportional to the partial pressure in a gas phase according to the Henry's rule. Therefore, it is possible to remove the dissolved oxygen in water by decreasing the partial pressure in a gas phase. In addition, the gas solubility is decreased as the temperature of water is increased. The thermal deaerating method heats the water using the heating steam and to removes the dissolved gas by decreasing the partial pressure of a gas.
In this thermal deaerating method, the dissolved oxygen may be decreased below 7 ppb under the optimum operation of the thermal deaerator; however, it is impossible to apply the above-described thermal deaerating method to a system in which there is not a heating source such as steam and heater.
2. Reducing Agent (Hydrazine) Processing Method
As one of the effective methods for removing the dissolved oxygen in water, there is a method using a reducing agent such as hydrazine. The oxygen removing chemical reaction of hydrazine in water is as follows. Since nitrogen gas and water molecular are produced as by-product of the reaction and do not effect the corrosion of the metal, the above-described method is widely used for removing the dissolved oxygen.
N
2
H
4
+O
2
→N
2
+2H
2
O
In this method, hydrazine 1 ppm per oxygen 1 ppm is consumed as a chemical agent. However, since the reaction is implemented at a relatively high temperature (above 80° C.), it is very difficult to remove the dissolved oxygen at room temperature.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof which overcomes the aforementioned problems encountered in the conventional art.
It is another object of the present invention to provide a dissolved oxygen removal method using an activated carbon fiber and an apparatus thereof which are capable of extending the life time of the facility and decreasing the operational and maintenance costs by fully removing dissolved oxygen from water used for an steam generator or a cooling system for thereby minimizing corrosion of metal and effectively removing the dissolved oxygen in water by a metal supp

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