JPH0358769B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an electrodialysis desalination device, and in particular an electrodialysis device suitable for a batch operation method in which the salt concentration in the desalination solution varies greatly over time. Related to type desalination equipment.

Furthermore, the present invention relates to an electrodialysis device useful as a small desalination device that can be easily used in a laboratory.

[Background of the invention and prior art]

An electrodialysis desalination device has a configuration in which a large number of anion exchange membranes and cation exchange membranes are alternately arranged between an anode provided at one end of an electrodialysis tank and a cathode provided at the other end. It is used for desalting or desalting and concentrating a solution by applying a DC voltage between the above-mentioned two electrodes, for example, underground brine, cheese, whey, sugar solution, wine, amino acids, protein chambers, etc. It is widely used for desalination, etc.

This type of electrodialysis apparatus is usually used in a batch operation method, and this method is used exclusively when desalting a solution with a particularly high salt concentration.

Conventionally, electricity is applied to an electrodialysis machine using the batch operation method by applying a constant voltage between the anode and both electrodes, but there is an upper limit value called the critical current density for the current density that can be applied. If the dialysate is operated at a higher current density, the pH of the dialysate will fluctuate widely, resulting in problems such as scale formation, alteration of the active ingredients in the dialysate, and membrane deterioration. Stable driving is impaired. Therefore, the device is normally operated by applying a constant voltage such that the current density is less than the critical current density.

By the way, it is generally known that the limiting current density is approximately proportional to the salt concentration in the membrane salt chamber.
Therefore, in a batch operation method in which the salt concentration in the desalination chamber varies greatly over time, the critical current density naturally also varies greatly. However, the electrical resistance of the entire electrodialysis device is determined by many factors, including the membrane resistance and the resistance in the electrode chamber, in addition to the salt concentration in the demineralization chamber. When applied, the operating current density is not considered to change in proportion only to the salt concentration in the desalination chamber, and in order to operate at a constant current density below the limit current density from the start to the end of batch operation, it is usually necessary to A method is used in which a constant voltage is applied such that the current density is 70 to 95% of the critical current density when the concentration in the demineralization chamber is at its lowest, that is, just before the end of operation. However, in this case, the current density in the high concentration region of the desalination chamber is much lower than the critical current density, and may even be less than 20% depending on the conditions. The processing time for driving becomes longer.

Conventionally, in order to solve the above-mentioned problems, for example, there was a method of detecting the salt concentration in the desalination chamber with a conductivity meter and controlling the current value, and a method described in Japanese Patent Application Laid-Open No. 130504/1983. As shown in the figure, a pair of detection electrodes are installed near the ion exchange membrane in both the anode and cathode electrode chambers to detect voltage, and the voltage applied to the electrodialysis machine is controlled so that the voltage between the detection electrodes is always constant. It was designed to do so.

[Problem that the invention seeks to solve]

The above-mentioned conventional techniques for completing batch desalination in as short a time as possible have complicated control methods and problems in terms of reliability. With the method of controlling the current value, there is a problem that organic substances such as proteins normally contained in the liquid to be desalted adhere to the conductivity electrode and change the measured value. This problem can also occur in the invention described in the above-mentioned Japanese Patent Application Laid-Open No. 130504/1983, in which a constant voltage is applied while measuring the voltage by inserting an electrode for the DC current. In order to
The detection electrode surface gradually deteriorates due to oxidation-reduction reactions on the electrode surface, which may cause measurement values to fluctuate and the applied voltage to become inaccurate.

In particular, electrodialysis desalination equipment with a current-carrying area of up to 200 cm ² and in which an anion exchange membrane and a cation exchange membrane are integrated via a frame-like gasket are easy to handle. Therefore, using platinum wire or conductivity cells that easily deteriorate is undesirable as handling such as replacement and cleaning becomes complicated, and the number of parts increases significantly, increasing the probability of failure. That will happen. Furthermore, with a platinum wire, it is difficult to measure accurately when the absolute value of the voltage is small, such as when the number of ion exchange membranes is small, and conductivity cells require a liquid volume of several tens of cc for measurement. There is a problem that it is difficult to detect the electric power when the liquid amount is smaller than this.

[Means and actions for solving problems]

The electrodialysis device of the present invention has between the anode and the cathode,
In an electrodialysis apparatus in which one or more pairs of anion exchange membranes and cation exchange membranes are arranged alternately, means for applying constant voltages of two or more different values between the anode and cathode electrodes, and between the two electrodes. means for detecting the value of the current flowing between the two electrodes, and when the value of the current flowing between the two electrodes matches one of one or more preset current values, the value of a constant voltage applied between the two electrodes is set to the current value. It is characterized by having means for automatically switching to a corresponding preset value.

The anode, cathode, cation exchange membrane, anion exchange membrane, gasket, filter press of the electrodialysis tank, etc. used in the present invention are not particularly limited, and conventional ones can be used.

As a means of applying constant voltages of two or more different values between the anode and cathode electrodes, any one of multiple preset DC voltages can be used, and the circuit must be able to conduct electricity under constant voltage conditions. Therefore, it is sufficient to set the number of DC voltages in advance in two or three types, or at most five types; increasing the number of DC voltages more than that is not preferable because the circuit becomes complicated.

An ordinary ammeter or an equivalent circuit can be used as a means for detecting the value of the current flowing between the two electrodes.

When the current value flowing between both electrodes matches one of one or more preset current values, the value of the constant voltage applied between both electrodes is automatically set to a preset value corresponding to the current value. As the switching means, current control relays or equivalent relay circuits can be used. In this case, the preset current value should be at least one type from the above preset DC voltage, but the current value should be the same as the set DC voltage, including the current setting value for detecting the end of desalination. It's good to keep it.

The order of switching is that the voltage, which was relatively uniformly distributed throughout the dialysis tank at the initial stage of desalination, shifts to apply only to the desalination chamber as desalination progresses. It is necessary to switch to lower the DC voltage as it progresses. If the voltage is not switched to lower the voltage, the limiting current density is likely to be exceeded.

Although the present invention can be applied to large-sized electrodialysis devices, it is particularly preferable to apply it to small-sized electrodialysis devices with a current-carrying area of about 200 cm ² or less. This is because, in the case of a small-sized electrodialysis apparatus, there are few demineralization chambers, so that the voltage applied to the demineralization chamber tends to become relatively large as the electricity supply progresses.

In particular, in electrodialysis equipment equipped with an ion exchange membrane integrated through a gasket to improve handling, energization conditions can be controlled using only a normal electric circuit without using voltage measurement terminals or conductivity cells. Therefore, the operation caused by contamination by organic matter in the liquid to be desalted does not occur, and the influence of contamination by organic matter can be easily changed by replacing the ion exchange membrane integrated through the gasket. It is. In addition, the minimum amount of liquid required is not controlled by the capacity of the conductivity cell, and even if the liquid to be desalted is as small as a few cc, as long as it is within the dead volume of the desalting equipment, desalination can be done in a short time. It is possible to carry out desalting procedures.

〔Example〕

Embodiments of the electrodialysis apparatus of the present invention will be described in detail based on the drawings.

FIG. 1 is a configuration explanatory diagram showing an embodiment of the electrodialysis apparatus of the present invention. In the figure, A is an electrodialysis tank, and an anode 1 is provided at one end of the electrodialysis tank A and a cathode 2 is provided at the other end. A large number of anion exchange membranes 3 and cation exchange membranes 4 are alternately arranged between the anode 1 and the cathode 2. A frame-shaped gasket (not shown) is interposed between the anion exchange membrane 3 and the cation exchange membrane 4 as usual to seal the spacer between the membranes and the inner peripheral wall of the electrodialysis cell A. It is meant to be helpful.

The desalting chamber 5 where salt is removed by applying a DC voltage between the anode 1 and the cathode 2 described above and the concentration chamber 6 where salt is concentrated are connected to the anion exchange membrane 3 and the cation exchange membrane 4. are formed alternately in between.

Reference numeral 7 denotes a desalination liquid flow path, and numeral 8 represents a concentrate flow path, and the desalination liquid flow path 7 and concentrate flow path 8 circulate the respective liquids to the desalination chamber 5 and the concentration chamber 6 during operation. Moreover, electrode electrolytes 9, 9 are circulated between the anode 1 and the ion exchange membrane 3 attached to the anode 1 and the cation exchange membrane 4 attached to the cathode 2. Further, a rectifier 10 is provided as a current supply source for supplying direct current to both electrodes 1 and 2.

Incidentally, in this embodiment, the rectifier 10 described above is configured to obtain two or more constant voltages of different values. The rectifier 10 shown in FIG. 1 has a negative output terminal 13 and three positive output terminals 14.
a, 14b, and 14c are provided, the voltage between the negative side output terminal 13 and the positive side output 14a is V1, and the voltage between the negative side output terminal 13 and the positive side output terminal 14b is V2, the negative side The voltage between the output terminal 13 and the positive output terminal 14c is configured so that three different constant voltages such as V3 can be obtained. Reference numeral 11 denotes an output switching device for switching between the three different constant voltages, and the output switching device 11 applies any one of the three different constant voltages to the electrodialysis tank A when electricity is applied. Then, the output switching device 11 uses the current detector 12 to select the electrodialysis tank A.
detects the current value flowing through the sensor, and is automatically activated based on the detected current value.

Next, the operation of the electrodialysis apparatus in this embodiment from the start to the end of operation in the batch operation method will be described.
V3 and the respective end current values I1, I2, I3 are determined in advance, but each value is V1>V2>V3, I1
＞I2＞I3 and its settings.The feature of the present invention is that in an electrodialysis machine that performs a batch operation method in which the concentration in the demineralization chamber decreases significantly over time, the concentration in the demineralization chamber The concentration range is divided into multiple regions over time, and operation is performed by applying a required constant voltage to the positive and negative electrodes in each concentration range.

For example, if the concentration in the desalination chamber is divided over time into three areas: the first high concentration area, the second medium concentration area, and the third low concentration area, the current density is below the critical current density in each concentration area, and By applying a constant voltage to the electrodialysis tank A that provides the highest possible current density, batch operation can be completed in a short time.
It is preferable to set VI, V2, V3 and end current values I1, I2, and I3 as follows.

First, the values of 70 to 95% of the critical current density at the lower limit concentration are set as I1, I2, and I3 at the end of each of the above concentration ranges, and the above current values are obtained at the end of each of the above concentration ranges. The voltages applied to the electrodialyzer are set as VI, V2, and V3, respectively.

VI, V2, V3 and I1, respectively, as above
When I2 and I3 are set and operation is started, first, at the start of operation, the output switching device 11 relays the output voltage V1 of the positive side output terminal 14a of the rectifier 10, and a constant voltage V1 is applied to the electrodialysis tank A. , as the desalination progresses in the first high concentration region, the electrodialysis tank A
The current flowing through the current value V1 decreases to the end current value I1,
When the second medium density region is reached, the output switching device 11
is activated, and the output voltage of the positive side output terminal 14b of the rectifier 10 is switched to V2. Furthermore, desalination progresses in the medium concentration region and the current value decreases to the end current value I2 at V2, and when the third low concentration region is reached, the output switching device 11 is activated and the positive side output terminal 14c of the rectifier 10 is switched on. When the output voltage V3 is switched to V3, and finally the current decreases to the end current value I3 at V3, the output switching device 11 does not relay any output of the rectifier,
At this point, the batch operation ends. At this time, it is possible to terminate the process at the same time as the batch operation ends, for example by stopping the circulation pump.

As described above, by setting I1, I2, I3, V1, and V2, stable operation was possible from the start to the end of the batch operation described above, and the effect of processing in a short time was obtained.

Specific examples and comparative examples of the present invention will be explained with reference to FIGS. 2 and 3.

Specific example: In the explanatory diagram of the configuration of the electrodialysis apparatus shown in FIG. 1, 20 Aciplex A-201 membranes manufactured by Asahi Kasei Industries, Ltd. are used as the anion exchange membrane 3, and the cation exchange membrane 4 is used as the anion exchange membrane 3.
20 Asiplex K-101 membranes manufactured by Asahi Kasei Industries, Ltd. were used, the membrane spacing was 0.5 mm, and the number of demineralization chambers 5 was 20.
A filter press type electrodialysis tank with 19 chambers and concentration chambers 6 and an energized area of 0.4 dm ³ was prepared. Using this device, 0.5M-NaCl aqueous solution 1 is used as a desalting solution.
We will explain the results when desalting to about 0.002M. However, as the concentration chamber circulating fluid, 0.1M−
50g/NaCl aqueous solution 1 as electrode electrolyte
- Na ₂ SO ₄ aqueous solution 0.5 was used to circulate each pump electrodialyzer. Further, the desalting solution was circulated by a pump at a flow rate of 5 cm/sec ² in the desalting chamber 5, and the temperature was adjusted to 25±1°C.

Figure 2 shows that the electrodialysis apparatus of the present invention is used, the applied voltage V1 at the start of operation is 14 V, the final current density I1 at V1 is 2.5 A/dm ² , and the applied voltage V2 in the second stage is
11V, ending current density at V2 I2 0.25A/dm ²
Then, the applied voltage V3 in the third stage is 9V, and the end current density at V3, that is, the end current density I3 of the desalination operation, is
This diagram shows a diagram when the setting is 0.08A/ ^dm2 . In Figure 2, the operating current density from the start of operation to the end of operation is always 50% to 50% of the limit current density.
It was between 80%. As a result, the desalination solution concentration
The time required to desalinate from 0.5M to 0.002M is
It took only 50 minutes and the results were good.

FIG. 3 shows temporal changes in the concentration in the demineralization chamber and the operating current density when the applied voltage was a constant voltage of 9 V from the start of operation to the end of operation in the apparatus of Example 1. In Figure 3, A is the desalination solution concentration, B is the current density, and dashed line C is the critical current density of approximately 80
%. In this case, the current density immediately after the start of operation was about 20% of the critical current density. It took a considerable amount of time, 87 minutes, to desalinate the desalination solution concentration from 0.5M to 0.002M.

〔Effect of the invention〕

The electrodialysis apparatus according to the present invention can be operated at a current density that is relatively close to the critical current density even if the salt concentration in the desalination chamber changes greatly, and therefore has the advantage that the operating time of batch-type operation can be significantly shortened. There is.

Furthermore, the control method for switching the constant voltage applied between both electrodes using a predetermined current value does not require any special sensors, and can be done using only voltage and current values, making it extremely simple. It has a special effect in that it can be easily configured and there is no risk of malfunction or trouble occurring.

[Brief explanation of drawings]

FIG. 1 is a configuration explanatory diagram showing one embodiment of the electrodialysis apparatus of the present invention, FIGS. 2 and 3 are diagrams showing temporal changes in desalination solution concentration and current density, and FIG.
The figure shows the results when the electrodialysis device of the present invention is used and the applied voltage and final current density are set in three stages. Figure 3 shows the results when the electrodialysis device of Example 1 is used and the applied voltage is kept constant. It is a line diagram. 1: anode, 2: cathode, 3: anion exchange membrane,
4: Cation exchange membrane, 5: Demineralization chamber, 6: Concentration chamber,
7: Desalinated liquid, 8: Concentrated liquid, 10: Rectifier, 11:
Output switching device, 12: Ammeter.

Claims (1)

[Scope of Claims] 1. In an electrodialysis device in which one or more pairs of anion exchange membranes and cation exchange membranes are alternately arranged between an anode and a cathode, two
The device includes means for applying constant voltages of different values as described above and means for detecting a current value flowing between the two electrodes, and the current value flowing between the two electrodes matches one of one or more preset current values. An electrodialysis apparatus comprising means for automatically switching the value of a constant voltage applied between both electrodes to a preset value corresponding to the current value when the current value is applied. 2 The current-carrying area is approximately 200cm2 ^or less,
2. The electrodialysis apparatus according to claim 1, wherein the anion exchange membrane and the cation exchange membrane are integrated through a frame-shaped gasket.