JPS581604B2 - Denki Touseki Hohou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrodialysis apparatus that has excellent dialysis capacity per unit time, prevents scale formation, and ensures stable operation.

Conventionally, a concentration chamber, a concentration chamber, and the like are formed in an apparatus by alternately stacking and tightening a large number of cation exchange membranes and anion exchange membranes with a gasket having a liquid supply/discharge mechanism and a gasket having a central cut-off section and a mesh spacer. It is widely known that electrodialysis treatments such as electrolyte concentration and desalination can be performed using an electrodialysis tank in which a plurality of stacks each having a large number of dilution chambers are arranged in a row between a cathode and an anode.

When performing such electrodialysis treatment, when the current applied to the device is increased, there is a phenomenon in which the voltage to be applied to the device suddenly increases when it exceeds a certain value.

Moreover, not only does the voltage increase, but also a neutral disturbance phenomenon occurs in the dialysate, and when processing seawater, hydroxides such as magnesium and calcium, carbonates, etc. are deposited on the ion exchange membrane surface. This often makes it impossible to continue operating the equipment.

These phenomena are caused by concentration polarization as well as by the difference in the ring ratio of ions between the ion exchange membrane phase and the dialysate phase.
It is said that this is because when the above-mentioned constant current is exceeded, the ion concentration on the ion exchange membrane surface on the dilution chamber side becomes zero, and as a result, water electrolysis occurs.

This constant current value is called the critical current density, and it is generally considered an important requirement that dialysis treatment be performed at a current value smaller than the critical current density.

However, the magnitude of the above-mentioned critical current density is a value specific to the electrodialysis device, and is used in order to increase the dialysis capacity of the device per unit time, and to prevent various deposits on the ion exchange membrane surface. In order to ensure safe operation, it is required to have as large a value as possible, and a great deal of effort is devoted to this point in the design of dialysis machines.

By stacking a large number of cation exchange membranes and anion exchange membranes alternately between electrodes via gaskets, mesh spacers, etc., and tightening them, a large number of electrolyte concentration chambers and dilution chambers are created in the device. In a dialysis machine with multiple stacks arranged between the cathode and the anode, a clamping frame is required at both ends of each stack for operations such as disassembly and assembly, and for supplying and discharging dialysate. Usually, a liquid supply frame is installed accordingly, and this tightening frame and liquid supply frame need to be filled with electrolyte solution.

The present inventor conducted research on the limiting current density when performing dialysis using such an electrodialysis device, and found that the pressure of the liquid supplied to the gap in the current-carrying part of the clamping frame was adjusted to the concentration chamber of the stack. By increasing the hydraulic pressure higher than either the liquid pressure or the dilution retention liquid pressure, dialysis is performed while applying pressure to the concentration chamber and dilution chamber inside the stack from the clamping frame or the liquid supply frame toward the inside of the stack. We have discovered a phenomenon in which the value of the limiting current density increases dramatically.

That is, the present invention provides a stack in which a large number of anion and cation exchange membranes are alternately stacked and a large number of concentration chambers and dilution chambers are alternately provided.
In an electrodialysis device in which a plurality of electrodialysis devices are arranged between cathodes and anodes, the pressure of the liquid supplied to the current-carrying part gap of the clamping frame used for each stack is set higher than the liquid pressure in the concentration chamber and the liquid pressure in the dilution chamber of the stack. This is an electrodialysis method characterized by:

Conventionally, when performing dialysis treatment using an electrodialysis tank such as the one described above, the flow rate in the tightening frame and liquid supply frame is too high to be equal to that in the concentration chamber and dilution chamber because the liquid flow rate increases too much. , the pressure in the clamping frame and liquid supply frame was usually lower than that in the concentration chamber and dilution chamber.

However, contrary to the conventional technology, the present invention is capable of increasing the pressure of the liquid supplied to the current-carrying part gap of the clamping frame higher than the liquid pressure of the concentration chamber and the liquid pressure of the dilution chamber of the stack. The feature is that the pressure is higher than both the chamber liquid pressure and the dilution chamber liquid pressure.

Therefore, the present invention is a completely new technical idea that has not existed in the past, and it brings about a dramatic increase in the limiting current density.

Note that in addition to the clamping frame, a liquid supply frame may be provided between the stacks, but in this case, the gap is generally common or in communication, and the gap in the current-carrying part of the clamping frame is It is sufficient to control the hydraulic pressure supplied to the parts.

Further, in the case where independent gap portions are provided in the clamping frame and the liquid supply frame, it is preferable to maintain the hydraulic pressure of the other side higher than the hydraulic pressure supplied to the stack.

FIG. 1 shows an example of the configuration of a typical electrodialysis tank to which the method of the present invention is applied.

A device is constructed in which a large number of cation exchange membranes and anion exchange membranes are alternately stacked via a gasket having a liquid supply and discharge mechanism and a central slit, and a mesh spacer inserted into the slit. This is a clamping type dialysis device in which a plurality of stacks 5 each having a large number of concentration chambers and dilution chambers arranged alternately are placed between an anode chamber 2 and a cathode chamber 6, and are clamped by a press plate 1.

At both ends of each stack 5, liquid supply frames 3 are installed as necessary for operations such as disassembly and assembly, and for supplying and discharging dialysate, as well as clamping frames 4 and as necessary for supplying and discharging dialysate. .

In the present invention, in order to make the pressure of the liquid supplied to the current-carrying part gap of the clamping frame 4 of each stack higher than the concentration chamber liquid pressure and dilution chamber liquid pressure of the stack, the clamping frame 4 or the liquid supply frame Either a method of applying pressure to the liquid in the concentration chamber or a method of decreasing the pressure of the liquid in the concentration chamber and the dilution chamber can be adopted.

At this time, of course, appropriate valves may be provided at the inlet and outlet of the tightening frame 4 or liquid supply frame 3, concentration chamber, and dilution chamber to adjust the liquid amount, pressure, etc.

The difference between the liquid pressure supplied to the current-carrying part gap of the clamping frame 4 and the liquid pressure in the concentration chamber and dilution chamber varies depending on the dialysis tank, but is generally 5 to 700 mmHg, preferably 40 to 200 mmHg.
mmHg column level is good.

If the difference in liquid pressure is made too large, the ion exchange membrane may become wedged into the spacer, causing damage to the ion exchange membrane, and the voltage may also increase.

The following may be considered as the reason why the limiting current density can be increased as an effect of the present invention.

As mentioned above, the spacer used in a dialysis tank is inserted between ion exchange membranes to regulate the ion exchange membrane spacing, that is, the thickness of each dialysis chamber (concentration chamber and dilution chamber), and to make contact with the ion exchange membrane. , it works to improve the critical current density by disturbing the liquid flow in the dilution chamber and the concentration chamber and reducing the thickness of the mass transfer film formed in the dialysate in contact with the membrane.

Such spacers are usually formed by various methods, but making the thickness uniform over the entire area is
This is practically impossible, and there is considerable thickness non-uniformity in some areas.

Therefore, although the spacer and each ion exchange membrane are in contact with each other in some parts, they are in contact with others in other parts, and a space exists between them.

When liquid is introduced into the concentration chamber and dilution chamber under such conditions, and also when it is introduced with pressure to give a constant flow rate, due to the non-uniformity of the spacer thickness and the hydraulic pressure of the dialysis chamber liquid. As a result, the stack portion bulges into the tightening frame and the liquid supply frame, so it is thought that a considerable amount of space is formed between the ion exchange membrane and the spacer.

In such a case, the dialysate should collide with the spacer and disperse into the dilution chamber and concentration chamber with appropriate turbulence, but due to the space formed above, the solution short-circuits. Not enough turbulence is provided and the uniformity of dispersion is also impaired.

However, when the method of the present invention is carried out, the space between the ion exchange membrane and the spacer as described above is not formed, and the spacer and the ion exchange membrane can be brought into contact over the entire current-carrying surface of the dialysis chamber. It is believed that the above-mentioned troubles will be resolved and the limiting current density will increase.

Although the present invention has been described above using an example of a typical electrodialyzer, the present invention is not limited thereto, and various modifications can be made as long as the principles of the present invention are followed.

Examples are shown below to concretely demonstrate the effects of the present invention.

Example 1 and Comparative Example 1 A rectangular shape (outside dimensions 125 cm x 55 cm) having a liquid supply and discharge mechanism and a central notch with a plate thickness of 1 cm was used, and a gas cut of approximately the same size as the gas cut (effective current carrying area 50 cm) was used.
dm2) cation exchange membrane Neoceptor C manufactured by Tokuyama Soda
Fifty pairs of L-25T and anion exchange membrane AVS-4T are alternately stacked between the gas holes, and a diagonal network of polyethylene dilution chamber spacers and concentration chamber spacers with a thickness of 1 mm is placed between them. were inserted, and these were tightened with a pair of tightening frames made of polyvinyl chloride to form one stack.

An electrodialysis apparatus was assembled by arranging two stacks of the above stacks between negative and positive electrodes, and supplying the dilution chamber liquid and the concentration chamber liquid using the clamping frame of each stack as a header.

Seawater with a temperature of 25° C. and a turbidity of 0.1 ppm was supplied to the dialysis apparatus at a membrane surface linear velocity of 5 cm/sec to the dilution chamber and the concentration chamber, respectively, and was branched from the concentrated liquid as the liquid to be supplied to the tightening frame. The fluid was supplied to the clamping frame, and the fluid pressure in the clamping frame was adjusted by the inflow and outflow valves of the clamping frame so that the fluid pressure within the clamping frame was lower than either the dilution chamber hydraulic pressure or the concentration chamber hydraulic pressure over the entire energized surface.

Electrodialysis was carried out under such conditions, and the limiting current density was measured by the current-voltage curve method and was found to be 14.7 A/dm2.
Met.

Next, keeping other conditions the same, only the hydraulic pressure inside the clamping frame was adjusted to 20 mmHg higher than both the dilution chamber hydraulic pressure and the concentration chamber hydraulic pressure. Electrodialysis was performed in this state, and the limiting current density was measured in the same manner as before and was found to be 33.5 A/dm2.

From these results, it can be seen that by carrying out the present invention, the limiting current density increases by about twice as much.

Example 2 and Comparative Example 2 Using the same two-stack (one stack incorporates 50 pairs of ion exchange membranes) electrodialysis equipment used in Example 1, a seawater concentration experiment was carried out for one month.

In one stack (A stack), the hydraulic pressure within the clamping frame is maintained to be lower than both the concentration chamber hydraulic pressure and the dilution chamber hydraulic pressure in the stack, and the other stack (B Stack), the hydraulic pressure within the clamping frame is 4% higher than either the concentration chamber hydraulic pressure or the dilution chamber hydraulic pressure within the stack.
Seawater concentration was performed at an operating current density of 4 A/dm2 using sand-filtered seawater as a raw material while maintaining the column at a height of 5 mmHg.

As a result of a long-term continuous experiment for one month, the average voltage of the stack was 23.1V for the A stack and 21.7V for the B stack.
V, average chloride ion current efficiency as A stack 84.3
%, B stack 84.8%, average chlorine ion concentration of the obtained concentrated water is A stack 3.52N, B stack 3.5
It was 9N.

After one month of operation, we inspected the inside of the A and B stacks and found that in the A stack, there was a neutral disturbance phenomenon on the cation exchange membrane in the two dilution chambers adjacent to the clamping frame on the cathode side. Scaling of a mixture of Mg(OH)2 and CaC03 was occurring, which was thought to be caused by.

Note that no abnormality was observed inside the B stack.

Next, in the A stack, the scale precipitation membrane was replaced with a normal ion exchange membrane to create 50 new pairs of anion and cation exchange membranes, and the liquid pressure in the tightening frame was changed to the stack internal pressure in the same way as in the B stack. The same concentration experiment was conducted for another month with the column higher by 45 mmHg.

As a result, the A-stack custom-made concentrate had almost the same values as the B-stack in terms of voltage, current efficiency, and chloride ion concentration, and it was confirmed that good results could be obtained by increasing the clamping frame hydraulic pressure.

Furthermore, when the stacks were dismantled and inspected internally one month later, no scaling was found in either the A or B stacks.

[Brief explanation of the drawing]

FIG. 1 shows an example of the configuration of a typical electrodialysis tank to which the method of the present invention is applied. 1 is a press plate, 2 is an anode chamber, 3 is a liquid supply frame, 4 is a tightening frame,
5 is a stack and 6 is a cathode chamber.

Claims (1)

[Claims] 1. A clamping frame used for each stack in an electrodialysis apparatus consisting of a plurality of stacks in which a large number of anion and cation exchange membranes are alternately stacked and a large number of concentration chambers and dilution chambers are arranged between cathodes and anodes. An electrodialysis method characterized in that the pressure of the liquid supplied to the void portion of the current-carrying part is made higher than the liquid pressure in the concentration chamber and the liquid pressure in the dilution chamber of the stack.