Method for controlling pH by electrical conductivity

Abstract

Provided is a method for controlling a pH to a neutral range (pH 6 to 8) by adding an alkali or an acid to decarbonated water, including controlling the amount of the alkali or the acid added using an electrical conductivity meter. The decarbonated water is, for example, water obtained by decarbonating industrial water or groundwater so that the concentration of carbonic acid is 10 ppm or less. The method may further have a step of producing pure water by RO treatment of the decarbonated water with a pH adjusted to 6 to 8 by the pH control.

Claims

1. A method for controlling a pH to a neutral range by adding an alkali or an acid to decarbonated water, comprising: controlling an amount of the alkali or the acid added using an electrical conductivity meter, a step of measuring the pH and electrical conductivity EC.sub.1 [mS/m] of the decarbonated water; a step of calculating electrical conductivity EC.sub.2 [mS/m] of the water after the addition of the alkali or the acid according to the following formula; and a step of adding the alkali or the acid so that the electrical conductivity after the addition of the alkali or the acid is the calculated EC.sub.2 [mS/m]:
Electrical conductivity EC.sub.2 [mS/m]=Electrical conductivity EC.sub.1 [mS/m]+(ΔEC/ΔpH) [mS/m]×(Target pH−pH of decarbonated water), wherein (ΔEC/ΔpH) is a ratio of an electrical conductivity relative to pH.

2. The method for controlling a pH according to claim 1, wherein the decarbonated water is water obtained by decarbonating industrial water or groundwater so that the concentration of carbonic acid is 10 ppm or less.

3. The method for controlling a pH according to claim 1, further comprising a step of producing pure water by RO treatment of the decarbonated water with a pH adjusted to 6 to 8 by the pH control.

4. The method for controlling a pH according to claim 1, wherein ΔEC/ΔpH of the decarbonated water is experimentally determined in advance.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a block diagram illustrating the present invention.

(2) FIG. 2 is a graph showing changes in pH.

(3) FIG. 3 is a graph showing a neutralization curve.

(4) FIGS. 4(A) and 4(B) are graphs illustrating Examples.

DESCRIPTION OF EMBODIMENTS

(5) FIG. 1 is a block diagram illustrating an example of a method for controlling a pH of the present invention. A pipe 1 configured to introduce decarbonated water to a static mixer 5 is provided with a pH meter 2, a first electrical conductivity meter 3, and an alkali (NaOH aqueous solution in this embodiment) adding device 4. The alkali adding device 4 includes a tank, a chemical injection pump, an addition amount control valve 4a, and the like. The concentration of carbonic acid in the decarbonated water is preferably 10 ppm or less, more preferably 8 ppm or less, particularly preferably 5 ppm or less, in view of the accuracy of pH control by the buffering action and the electrical conductivity. As a decarbonation device, a membrane degasifier, a decarbonator, an aerator, or the like can be used, but a membrane degasifier is preferable.

(6) Water after being mixed with an alkali in the static mixer 5 is supplied to a first RO (Reverse Osmosis) device 8 via a pipe 6 and water permeated through the first RO device is passed through a second RO device 9. The pipe 6 is provided with a second electrical conductivity meter 7.

(7) The values detected by the pH meter 2 and the first and second electrical conductivity meters 3 and 7 are input into a controller 10, and the degree of opening of the valve 4a is controlled based on the calculation results, to control the amount of alkali added.

(8) The static mixer 5 is used in FIG. 1, but various mixers such as a pH adjustment tank using a stirrer can be used.

(9) In the controller 10, an electrical conductivity EC.sub.2 as a target is calculated by substitution of an electrical conductivity EC.sub.1 detected by the first electrical conductivity meter 3, the pH of decarbonated water, and a pH (an appropriate time average) as a target into the following formula, to control the amount of NaOH added using the valve 4a so that the value detected by the second electrical conductivity meter 7 is equal to the target electrical conductivity EC.sub.2.
Electrical conductivity EC.sub.2[mS/m]=Electrical conductivity EC.sub.1[mS/m]+(ΔEC/ΔpH) [mS/m]×(Target pH−pH of decarbonated water)  (2)

(10) wherein (ΔEC/ΔpH) is a ratio of an electrical conductivity relative to pH.

(11) The method for calculating ΔEC/ΔpH [mS/m] in formula (2) will be described below. Specifically, an appropriate value is to be updated based on the processing results on site.

(12) The method for controlling a pH of the present invention that uses an electrical conductivity meter with high time response can enhance the control responsiveness in adjusting the pH of decarbonated water (to a pH of preferably 6 to 8, particularly preferably 6 to 7), which is difficult to control because the pH buffering action is considerably reduced due to decarbonation, and thus the pH sharply changes due to addition of alkali. In the range of pH 6 to 8, the (proportional) correlation between the pH and the electrical conductivity is high, and the correlation is further higher in the range of pH 6 to 7. The present invention is suitable for neutralization of acidic decarbonated water to a pH of 6 to 8, particularly, a pH of 6 to 7, by adding an alkali.

(13) When the decarbonation membrane has low decarbonation rate so that the electrical conductivity of treated water in the second RO device 9 cannot satisfy the target electrical conductivity, the same pH adjustment device (including the first and second electrical conductivity meters 3 and 7, the alkali adding device 4, a mixer such as the static mixer 5, and the controller 10) may be provided between the first RO device 8 and the second RO device 9.

EXAMPLES

Experimental Example 1

(14) Table 3 shows the properties of water decarbonated (residual IC=2.5 mg-C/L) using a membrane degassing device after sulfuric acid was added to each of industrial waters 1 to 4 having the properties shown in Table 2 to adjust the pH to 5.

(15) TABLE-US-00002 TABLE 2 Industrial Industrial Industrial Industrial Item Unit water (1) water (2) water (3) water (4) pH — 7.4 7.01 7.6 7.42 Electrical mS/m 34.9 30.9 30.5 37.1 conductivity Na mg/L 26.2 8.69 11.83 22.9 K mg/L 5.65 4.09 4 3.8 Ca mg/L 17.9 28.6 27.2 30.5 Mg mg/L 8.77 6.98 7.5 5.6 Cl mg/L 40.3 20.31 16.16 18.4 SO.sub.4 mg/L 22.3 42.1 25.3 48.7 NO.sub.3 mg/L 2.8 4.6 1.3 2.1 IC mg- 16.38 14.05 20.08 19.63 C/L

(16) TABLE-US-00003 TABLE 3 Decarbonated Decarbonated Decarbonated Decarbonated Item Unit water (1) water (2) water (3) water (4) pH — 5.85 5.75 5.98 5.97 Electrical mS/m 28.6 24.8 25.1 29.6 conductivity Na mg/L 26.2 8.69 11.8 22.9 K mg/L 5.65 4.09 4.00 3.80 Ca mg/L 17.9 28.6 27.2 30.5 Mg mg/L 8.77 6.98 7.5 5.6 Cl mg/L 40.3 20.3 16.2 18.4 SO.sub.4 mg/L 80.2 86.3 98.6 118.3 NO.sub.3 mg/L 2.8 4.6 1.3 2.1 IC mg-C/L 2.50 2.50 2.50 2.50

(17) Table 4 shows the water quality after NaOH was added to the decarbonated waters 1 to 4 to adjust the pH to 7.

(18) TABLE-US-00004 TABLE 4 Adjustment Adjustment Adjustment Adjustment to pH 7 after to pH 7 after to pH 7 after to pH 7 after decarbonation decarbonation decarbonation decarbonation Item Unit (1) (2) (3) (4) pH — 7.00 7.00 7.00 7.00 Electrical mS/m 29.77 26.07 26.16 30.66 conductivity Na mg/L 29.0 11.7 14.3 25.5 K mg/L 5.65 4.09 4 3.8 Ca mg/L 17.9 28.6 27.2 30.46 Mg mg/L 8.77 6.98 7.5 5.6 Cl mg/L 40.3 20.3 16.2 18.4 SO.sub.4 mg/L 80.2 86.3 98.6 118.3 NO.sub.3 mg/L 2.8 4.6 1.3 2.1 IC mg- 2.50 2.50 2.50 2.50 C/L NaOH mg-L 4.882 5.213 4.356 4.420

(19) As shown in Table 4, the electrical conductivity after decarbonation and adjustment to pH 7 varied depending on the quality of raw water. FIG. 4 shows changes in electrical conductivity corresponding to the amount of NaOH added, in the case of adding NaOH to water after decarbonation (Table 3). Sample waters 1 to 4 in FIG. 4 are respectively the decarbonated waters 1 to 4 in Table 3.

(20) As shown in FIG. 4, in the case of adding NaOH to adjust the pH of sample water (decarbonated water) to 7, the pH and the electrical conductivity increased in the same manner. Further, it is understood that the electrical conductivity increased linearly at a constant rate in each case.

(21) From the slopes of the graphs of FIG. 4(A) by plotting the values of pH on the horizontal axis and FIG. 4(B) by plotting the values of electrical conductivity on the vertical axis, ΔEC/ΔpH is determined.

(22) The rate of change in electrical conductivity (ΔEC/ΔpH) with respect to changes in pH is as shown in Table 5.

(23) TABLE-US-00005 TABLE 5 Sample Sample Sample water Sample water Target water water (1) (2) water (3) (4) Rate of change in electrical 0.894 0.881 0.893 0.897 conductivity due to addition of NaOH (ΔEC/ΔpH) [mS/m]

(24) The coefficient, 0.89, in calculation formula (2) of the target electrical conductivity for adjusting the pH of decarbonated water to 7 (target pH) was determined as an average value of ΔEC/ΔpH in Table 5.

(25) Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible without departing from the spirit and scope of the invention.

(26) This application is based on Japanese Patent Application No. 2017-014354 filed on Jan. 30, 2017, which is incorporated by reference in its entirety.

REFERENCE SIGNS LIST

(27) 3, 7: Electrical conductivity meter 4: Alkali adding device 5: Static mixer 8, 9: RO device