Method for vacuum deaeration

Abstract: A method for vacuum deaeration of a dissoved gas in a pure water, the pure water having a specific resistivity of not less than 0.1 MΩ·cm, in a vacuum degasifier column (1) maintained at a pressure slightly higher than a saturated steam pressure at a temperature of the pure water, the column having a region filled with packings (3), spraying (7) the pure water downwardly from above the region and meantime evacuating (5) inside the column to maintain the pressure, the method comprising the steps of: carrying out the evacuation from above the region to maintain a pressure made by adding 1 mmHg to 10 mmHg to the saturated steam pressure at the temperature of the pure water inside the vacuum degasifier column; and introducing an inert gas (11) upwardly from below the region at a volumetric flow rate in the range of from 0.001 to 1.0, based on the volumetric feed flow rate of the pure water taken as 1, simultaneously with the evacuation, to lower a dissolved oxygen concentration in the pure water below 10 ppb.

Description: This invention relates to a method for effecting vacuum deaeration with improved operational efficiency and more particularly to a method for vacuum deaeration capable of removing dissolved oxygen in pure water. In aqueous systems using pure water, the pure water is required to possess an extremely low dissolved oxygen concentration to prevent various component parts of the system such as, for example, pipes, heat exchangers, and boilers from corrosion or to protect ion-exchange resin against deterioration by oxidation. Particularly, in recent years, electronic industry specializing in the production of LSI's and super-LSI's has required or specified pure water possessing a finally attainable dissolved oxygen concentration of not more than 10 bbp, to prevent formation of a native oxide film on semiconductor wafers. Previously known methods for decreasing the dissolved oxygen content in water, have included: vacuum deaeration using a vacuum degasifier column, heat deaeration using a heat deaerator, nitrogen bubbling method for depriving water of dissolved oxygen by bubbling nitrogen gas through the water, membrane deaeration which deprives water of dissolved gas by passing the water through a hydrophobic membrane and vacuumizing the outer side of the membrane, and catalytic deaeration wherein dissolved oxygen reacts with a reducing agent thereby effecting removal of the oxygen by conversion into water, for example. The method for heat deaeration has the disadvantage that the heat deaerator is difficult to handle and is prone to external contamination; the nitrogen bubbling method has the disadvantage that since the treated water is saturated with nitrogen, the dissolved nitrogen will eventually escape in the form of bubbles from the treated water; the method for membrane deaeration has the disadvantage that a special membrane dramatically increases running costs; and catalytic deaeration has the disadvantage that the use of expensive Pd adds to the cost of operation. Vacuum deaeration effects removal of dissolved oxygen from water by keeping the pressure inside the vacuum degasifier column at a degree of vacuum slightly higher than the vapour pressure at a predetermined water temperature and causing the water meanwhile to be sprayed downwardly onto packings placed in the column. Because of good economy and freedom from the danger of contamination, this method has been finding popular acceptance. This method, however, is deficient in its capacity for deaeration because the lowest attainable dissolved oxygen concentration is rather high, only reaching the order of some tens of ppb. JP-B-2 909 844 and JP-B-6135274 disclose methods for effecting gas-liquid contact by injecting an inert gas into the interior of a column whilst simultaneously causing the water subject to treatment to fall down the column interior from above. By these methods, however, the gas/liquid contact is carried out invariably under the atmospheric pressure and the treated water is not deaerated. JP-A-59179113 discloses a method for deaerating a seal oil by causing an inert gas to leak into a vacuum degasifier column and allowing a flow of the inert gas to contact a flow of the seal oil. This method is aimed at volatilizing such low boiling inflammable gas components as hydrocarbons and H₂S in the seal oil thereby lowering the flash point of the seal oil and reducing the seal oil from the hazards of fire or explosion in storage. The present invention which accomplishes the removal of a dissolved gas present, even in a relatively minute amount, in water, is neither disclosed nor suggested in this latter reference. An object of this invention resides in providing a method for effecting vacuum deaeration with improved operational efficiency such that the lowest attainable dissolved oxygen concentration can be below 10 ppb. A second object of this invention resides in providing a method for effecting vacuum deaeration with improved operational efficiency such that the conventional vacuum degasifier column is rendered usable for the present vacuum deaeration through a minor remodeling and operable easily at a low running cost whilst the lowest attainable dissolved oxygen concentration is still below 10 ppb. A third object of this invention resides in providing a method for effecting vacuum deaeration with improved operational efficiency such that the water under treatment suffers no evolution of bubbles, whilst the lowest attainable dissolved oxygen concentration is still below 10 ppb. The objects of this invention described above are accomplished by the method as delineated in claim 1. Some preferred features of the method are delineated in dependent claims 2 to 6 To the design of the vacuum degasifier column constructed as described above, the technique available for designing gas dispersion can be applied, in accordance with Henry's law governing the relation of equilibrium between a gas and a liquid. The volumetric feed flow rate of the liquid subjected to treatment, the concentration of dissolved gas in the liquid, the height of the column, the height of the bed of packings, the kind of packings, and the surface area of the packings available for the treatment, therefore, can be found by calculations. These calculations are described in detail in Perry's "Chemical Engineer's Handbook," Sixth Edition, Section 14, McGraw Hill. The features of the method and of the apparatus suitable for carrying it into effect are now described. The linear velocity (LV: m³/m²/h) inside the column in its empty state can preferably be selected in the range of from 50 to 100 in operations likely to be of interest. The degree of vacuum in the vacuum degasifier column itself is the range of from A + 1 to A + 10 mmHg, wherein A (mmHg) stands for the pressure of the saturated steam at the temperature of the water. For operation according to the present invention, the volumetric flow rate of the inert gas is required to be in the range of from 0.001 to 1.0, based on the volumetric feed flow rate of the water taken as 1. Amongst inert gases suitable for use in this invention there is nitrogen gas and argon gas, for example. The method of this invention prefers nitrogen gas over argon gas for economy of operation. The method of this invention is used for removal of dissolved oxygen from pure water having a specific resistivity of not less than 0.1 MΩ.cm. Besides the removal of dissolved oxygen from water, this invention can also or simultaneously be used in the removal of dissolved carbon dioxide gas and gases other than the inert gas being used for the treatment, from liquids containing such dissolved gases, even in minute amounts. In order that the invention may be illustrated and readily carried into effect, an embodiment thereof will now be described, purely by way of non-limiting example only, and with reference to the accompanying drawing, wherein: Figure 1 shows a diagram schematically illustrating a suitable vacuum degasifier column useful in the performance of the present invention. Referring to the drawing, the vacuum degasifier column (0.25 m in diameter and 8 m in height), though illustrated with omission of some detail , is designed in accordance with the conventional theory of gas absorption. It has a main part composed of a vacuum degasifier column proper 1 and a storage part 2 for treated liquid. The vacuum degasifier column proper 1 is packed with packings ("Terallet Type S, "50 mm in diameter, 20 t, 180 m²/m³ in surface area) (packed bed 4.1 m in height and 1.05 m² in cross-sectional area). Evacuation means such as, for example, a degasifying pipe 5 connected to a vacuum pump 4, adapted to evacuate the interior of the vacuum degasifier column proper 1 communicates above the region filled with the packings, i.e. into the top part of the vacuum degasifier column proper 1. Near this opening of the evacuation means, a liquid dispersing device 7 such as a spray nozzle connected to a liquid supply pipe 6 and adapted to supply a raw liquid under treatment is disposed in such a manner as to spray the raw liquid downwardly, namely in the direction of the packings 3. Below the region filled with the packings 3, a partition plate 6a permeable to gas and liquid is disposed. An empty part 8 is provided below the partition plate. The raw liquid which is sprayed from the liquid dispersing device 7 and treated by the downward flow of itself through the bed of packings 3 drops through the empty part 8 into the treated liquid storage part 2. The treated raw liquid is supplied for subsequent use through a treated liquid transfer pipe 10 which communicates with the bottom part of the treated liquid storage part 2 and is provided with a transfer pump 9. In the vacuum degasifier column, an inert gas supply pipe 11 adapted to supply a flow of an inert gas such as nitrogen gas to the interior of the vacuum degasifier column proper 1, communicates with the said empty part 8. In the diagram, reference 12 identifies a ring for re-distribution disposed on the inner wall of the region filled with the packings inside the vacuum degasifier column proper 1, 13 identifies a feedback control system for the vacuum pump 4, 14, identifies a feedback control system for the pump 9, F1 identifies flow meters, P1 identifies pressure gauges, and LC identifies a level controller. In the present invention, first the vacuum pump 4 is set into operation to evacuate the interior of the vacuum degasifier column proper 1 to a level of vacuum of the order of 10 to 40 torrs, namely a pressure 1 to 10 mmHg plus the saturated steam pressure at the temperature of the water under treatment. 1 mmHg = 1 torr = 0.133 kPa. Then, the evacuation is continued to maintain that level of vacuum, and inert gas (such as nitrogen gas) is introduced through the inert gas supply pipe 11 and the water is sprayed from the liquid dispersing device 7. The flow rate of the inert gas in this case is adjusted so that the volumetric flow rate thereof falls in the range of from 0.001 to 1.0, based on the volumetric feed flow rate of the water taken as 1. The reason for this particular range is that sufficient effect is not obtained in capacity for deaeration if the flow rate is less than 0.001 and the amount of the inert gas remaining dissolved in the treated water unduly increases if the flow rate exceeds 1.0. Desirably, the flow rate of the inert gas is such that the volumetric flow rate thereof falls in the range of from 0.01 to 0.5, preferably from 0.03 to 0.08, based on the volumetric feed flow rate of the water taken as 1. While the raw liquid sprayed under the conditions described above is falling inside the vacuum degasifier column proper 1, it is deprived of the dissolved gas and allowed to fall into the treated liquid storage part 2 and subsequently forwarded to the next step through the treated liquid transfer pipe 10. By this method, the dissolved gas and not the inert gas present in the raw is removed in a high ratio because the inert gas is introduced at the prescribed flow rate upwardly into the vacuum degasifier through the lower part thereof and the interior of the vacuum degasifier column proper 1 is continuously evacuated through the top part thereof. For the present invention, the temperature of the raw to be supplied is desired to be as high as permissible in the light of the fact that the solubility of the gas decreases in proportion as the temperature of the raw liquid increases. The effect of this invention is fully attained when this temperature falls in the approximate range of from 5°C to 30°C at which the water is generally used in a pure water or super-pure water plant. Now, the present invention will be described specifically below with reference to working examples. Example 1: In the vacuum degasifier column proper 1 illustrated in Fig. 1, with the interior of this column kept at a degree of vacuum of about 25 Torrs, nitrogen gas was continuously introduced upwardly at a flow rate of 0.1m³/hr through the lower part thereof and, at the same time, a raw water (pure water having a dissolved oxygen concentration of 8 ppm and a dissolved carbon dioxide gas concentration of 8 ppm) at 25°C was continuously sprayed downwardly from the liquid dispersing device 7 disposed in the upper part to effect deaeration of the raw water. In the treated water, the dissolved oxygen concentration was 5 ppb and the dissolved carbon dioxide gas concentration was 4.0 ppm. Example 2: The same raw water at 25°C was subjected to a deaeration treatment by following the procedure of example A, except argon gas was continuously introduced at a flow rate of 0.2 m³/hr in the place of the nitrogen gas. Comparative Experiment: When the same raw water was treated by following the same procedure as used in the working examples except for omission of the continuous supply of nitrogen gas or argon gas, the treated water had a dissolved oxygen concentration of 20 ppb and a dissolved carbon dioxide gas concentration of 6.5 ppm. The results of the working examples and the comparative experiment and the conditions of treatment used therein are collectively shown in the following table.

Claims: A method for vacuum deaeration of dissolved gas in substantially pure water, in a vacuum degasifier column maintained at a pressure slightly higher than saturated steam pressure at the temperature of the water which is 5°C to 30°C, the column having a region filled with packings, spraying the water downwardly from above said region, evacuating the interior of said column from above said region to maintain said pressure, characterised in that the water has a specific resistivity of not less than 0.1MΩ.cm, nitrogen gas or argon gas is introduced upwardly from below said region simultaneously with said evacuation and at a volumetric flow rate of 0.001 to 1.0, based on the volumetric feed flow rate of said water taken as 1, the level of vacuum within said column is (A + 1)mm Hg to (A + 10)mm Hg wherein A mm Hg represents saturated steam pressure at the temperature of the water, whereby the dissolved gas concentration in the water is reduced to 10 ppb or less. A method according to claim 1, wherein the volumetric flow rate of the nitrogen or argon gas is from 0.01 to 0.5, based on the volumetric feed flow rate of the water taken as 1. A method according to claim 2, wherein the volumetric flow rate of the nitrogen or argon gas is from 0.03 to 0.08, based on the volumetric feed flow rate of the water taken as 1. A method as claimed in claim 4 wherein the temperature of the water is 25°C. A method as claimed in any preceding claim wherein the gas dissolved in the water includes oxygen. A method as claimed in any preceding claim, wherein the gas dissolved in the water includes carbon dioxide.

Inventors: KOGURE MASAHIKO C O RESEARCH D, KOGURE, MASAHIKO, C/O RESEARCH & DEVELOPMENT LAB., Kogure, Masahiko, c/o Research &amp; Development Lab.
Applicants: NOMURA MICRO SCIENCE KK, NOMURA MICRO SCIENCE CO., LTD.
Application year: 1991
Main classification: B01D19
IPC codes: B01D19, C02F1