JPH0464758B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for obtaining drinking water by adding hardness components to fresh water produced in a seawater desalination apparatus using an evaporation method.

[Conventional technology]

Since the fresh water obtained by the evaporative seawater freshwater apparatus is distilled water, it contains almost no various ions and dissolved gases, and almost no hardness components such as calcium or magnesium. For this reason, if the above-mentioned fresh water is directly conveyed using general-purpose water conveyance equipment, corrosion of the steel pipes used in the water conveyance equipment and elution of concrete materials may occur, which may impair the function of the water conveyance equipment. On the other hand, when used as drinking water, it has been pointed out that it is tasteless for the reasons mentioned above, and not only does it taste bad, but can also cause heart disease.

For this reason, for example, the literature ``Desalination,
39 (1981) 503-520, quicklime, slaked lime, limestone, dolomite, etc. have been used as hardness increasing agents, and a method of dissolving these in fresh water has been used. Or, when dolomite is used, fresh water into which carbon dioxide gas has been blown is introduced into a filter filled with granulated dolomite (in drinking water production equipment, a packed bed of limestone and/or dolomite is usually called a filter), and calcium or magnesium is added to the filter. A common method is to increase the hardness of water by eluting it as bicarbonate.

As a source of carbon dioxide, methods have been proposed, such as recovering carbon dioxide from exhaust gas obtained by separately burning fuel, and transporting carbon dioxide gas in carbon dioxide cylinders. Methods for effectively utilizing it are attracting attention as they are attractive from an economical perspective.

[Problem that the invention seeks to solve]

Generally, seawater desalination equipment using the evaporation method is large-scale, and the amount of fresh water produced per unit is 20,000 to 35,000.
Several to dozens of units are constructed in the same location, reaching a maximum of tons per day. Therefore, the amount of fresh water processed by the drinking water conversion equipment, which is ancillary equipment, is enormous, and the consumption of alkaline agents used to adjust the pH of the water after it passes through the filter increases, which poses a problem in terms of operating costs. . Sodium hydroxide, sodium carbonate, etc. can be used as the alkaline agent, but securing the alkaline agent becomes a problem when installing a drinking water conversion device in a place where it is difficult to obtain these agents.

[Means for solving problems]

The present invention was discovered as a result of intensive research in order to solve the problems of the above-mentioned conventional methods. This is a method for producing drinking water using carbon dioxide gas, which is characterized in that the obtained carbon dioxide gas is reused.

Specifically, fresh water produced by a seawater desalination device using an evaporation method is made into carbonated water by blowing carbonic acid-containing gas generated from the desalination device, and then passed through a filter filled with limestone and/or dolomite particles. In the method of obtaining drinking water, fresh water after passing through a filter is vacuum degassed to reduce the pH of the fresh water.
The present invention proposes a method for producing drinking water using carbon dioxide gas, which is characterized in that the carbon dioxide gas is set to a predetermined value and the obtained carbon dioxide gas is blown into the fresh water at the inlet of the filter.

[Effect]

The first advantage of the method of the present invention is that the fresh water that has passed through the filter is vacuum degassed to dissipate carbon dioxide gas and increase the pH value to reach a predetermined pH value. It may not be necessary, or even if it is needed, it will only be needed in small amounts. The second advantage is that the carbon dioxide gas obtained by vacuum degassing can be reused by being blown into the fresh water at the inlet of the filter, so the utilization rate of carbon dioxide gas can be significantly improved.

The third advantage is that the filter capacity can be reduced due to the improved carbon dioxide utilization rate and the effect of reducing the amount of alkali required, resulting in a reduction in construction costs. In other words, when reducing the filter capacity, increase the carbon dioxide concentration in the carbonated water at the filter inlet to reduce the amount of water passing through the filter and dissolve highly concentrated mineral components in the filter outlet water.
One possible method is to mix this into the fresh water produced by the desalination equipment, but in this case, due to solubility equilibrium, a large amount of unreacted carbon dioxide gas remains in the water at the filter outlet, resulting in the consumption of alkali agent for neutralization. As the carbon dioxide gas increases, the carbon dioxide gas is also neutralized and wasted, which has been a major obstacle to reducing the filter capacity in the past.

The effects of the present invention will be explained in detail below.

When carbonated water passes through a filter filled with limestone and/or dolomite, hardness components of calcium and/or magnesium are eluted according to equations (1) and/or (2) below.

CaCO ₃ +CO ₂ +H ₂ OCa (HCO ₃ ) ₂ (1) Ca・Mg (CO ₃ ) ₂ +2CO ₂ +2H ₂ OCa (HCO ₃
) ₂ +Mg(HCO ₃ ) ₂ (2) Since the liquid phase reactions shown in equations (1) and (2) are equilibrium reactions, unreacted carbon dioxide gas remains in the water after passing through the filter. Therefore, the pH generally tends to be lower. Not only is it unsuitable for drinking if the pH value remains low (according to WHO, it is 7.0~
8.5 is considered appropriate), the Langelier Saturation Index, which is an index indicating the tendency of water to corrode, is negative,
The original objectives of making water into potable water, which are to improve drinking taste and reduce the tendency of water to corrode, become impossible to achieve. Therefore, conventional methods have been used to adjust the pH value by neutralizing the carbon dioxide gas remaining in the water at the filter outlet using an alkaline agent such as sodium hydroxide or sodium carbonate, as shown in the following equation.

CO ₂ +NaOHNaHCO ₃ (3) CO ₂ +Na ₂ CO ₃ +H ₂ O2NaHCO ₃ (4) The inventors of the present invention conducted extensive studies on a method for reducing the amount of alkaline agents mentioned above, and found that the water that had passed through the filter containing residual carbon dioxide gas was vacuum desorbed. By air treatment, residual carbon dioxide is released from the water and the PH value increases,
Furthermore, we discovered that the obtained carbon dioxide gas can be used again as a carbon dioxide source, leading to the present invention.

As a vacuum deaeration treatment device, for example, a spray tower or a packed tower type, which has been widely used as general-purpose water treatment equipment, is sufficient, and it is extremely easy to implement as it does not require any special measures. Furthermore, since fresh water produced by a seawater desalination apparatus using the evaporation method originally contains almost no dissolved gases other than carbon dioxide, only carbon dioxide gas is mainly released during vacuum degassing treatment. Therefore, the concentration of the obtained carbon dioxide gas is high, and when it is used again as a carbon dioxide gas source, it is advantageous that the capacity of the absorption device can be reduced in size. PH
The set value after adjustment is often in the range of 7.5 to 8.5, but if the set PH value is high, it may be impossible to raise it to the specified PH value with vacuum degassing alone. If necessary, an alkaline agent is further added after the deaeration treatment to obtain a predetermined pH value. Of course, in this case as well, the amount of alkaline agent required can be significantly reduced compared to when adjusting the PH value using an alkaline agent alone. Furthermore, due to the gas-liquid equilibrium relationship, as the liquid pH increases, the rate of carbon dioxide gas dissipation decreases, making it necessary to increase the capacity of the vacuum degassing equipment.To deal with this, it is also effective to add an alkali agent. There are cases.

〔implementation〕

Next, the method of the present invention will be explained in detail with reference to an example.

FIG. 1 shows the case where the present invention is applied to the drinking of fresh water obtained from a multi-stage flash evaporation method of seawater.

Fresh water produced from the multi-stage flash evaporator 1 of seawater is extracted from line a, and branched into bypass line b and line c.

Next, the fresh water branched by line c is further branched by line d and line e, and line d
The fresh water that has passed through is introduced into the CO ₂ absorption tower 2. _CO2
The absorption tower 2 contains CO ₂ -containing gas extracted by the multistage flash evaporator 1 and pressurized by the pump 7 through the line p, and CO ₂ -containing gas extracted from the vacuum degassing tower 4 and pressurized by the pump 5 . are mixed and introduced through line n.

The water that has become carbonated water by absorbing CO ₂ in the CO ₂ absorption tower 2 is extracted through line f, and the CO ₂ absorption tower 2 mixes it with fresh water that has passed through line e, which bypasses it, and then connects it to line g. and is introduced into the filter 3. Unabsorbed CO ₂ gas and gases such as nitrogen and oxygen are discharged to the outside of the system through line o. As the CO ₂ absorption tower 2, a packed tower or a line mixer is suitable.

Next, the carbonated water sent from line g dissolves limestone and/or dolomite particles filled in the filter while passing through the filter 3, increases hardness and total alkalinity, and then goes to line h It is extracted from the vacuum deaerator 4 and supplied to the vacuum degassing tower 4. A small amount remains in the water in line h due to vacuum degassing.
Dissipates CO ₂ gas and increases liquid PH value. The dissipated CO2 _- containing gas passes through lines m and n again.
The CO ₂ is introduced into the absorption tower 2. The water extracted from the vacuum degassing tower 4 through line i is mixed with fresh water flowing through line b, and after being adjusted to a predetermined hardness, is transferred to line j.
is sent to the final pH adjustment process. In the PH adjustment step, an alkali solution is injected from the alkali tank 6 into the line j through the line k and adjusted to a predetermined PH value. Line j due to low set PH value
If the water passing through has already reached a predetermined value, the final PH adjustment step described above is not necessary. The pH-adjusted drinking water is taken out of the system through line 1.

Although this figure shows an example of turning fresh water obtained by the multi-stage flash evaporation method into drinking water, the application of the present invention is not limited to this. is also applicable.

Example 1 Next, an example will be shown to clarify the effects of the present invention.

Fresh water and carbon dioxide-containing gas produced by a multi-stage seawater flash evaporator were separated and treated according to the drinking water production method shown in FIG. The amount of fresh water extracted is 400/h, of which 200/h is branched off and sent to the carbon dioxide absorption process via line c.
The remainder was passed through bypass line b.

A Raschig ring packed tower was used as the absorption tower 2, and the filter 3 was filled with limestone having a sieve diameter of 1 mm to 4 mm. The filling volume was 20. The deaeration tower 4 was filled with interlock saddles up to a height of 2 m, and decarboxylation was carried out by suction with the pump 5 so that the internal pressure was approximately 50 mmHg.

The amount of carbon dioxide-containing gas injected from line n was set so that the total hardness of the final treated drinking water taken out from line 1 was 60 mg/based on calcium carbonate. In addition, the amount of alkaline agent (caustic soda) injected was adjusted so that the pH of the drinking water after final treatment was 8.4.

Continuous operation was carried out for two days under the above conditions, and 1% of the caustic soda and carbon dioxide gas injected from line p were
When we measured the amount consumed per day, we obtained the following results.

Caustic soda consumption = 19g Carbon dioxide consumption = 311g Example 2 Fresh water was treated by the same method as in Example 1 under the following conditions.

The amount of fresh water collected was 400/h, the same as in Example 1,
Of this, 100/h is branched off and sent to the carbon dioxide absorption process via line c, and the rest is bypass line b.
flowed. The filter was filled with 10 pieces of the same limestone as in Example 1. Other operating conditions are Example 1
When continuous operation was carried out for two days and the daily consumption of caustic soda and carbon dioxide gas injected from line p was measured, the following results were obtained.

Caustic soda consumption = 31 g Carbon dioxide consumption = 327 g Comparative Example 1 The gas extraction pump 5 from the vacuum degassing tower 4 was stopped, and the other conditions were exactly the same as in Example 1, and continuous operation was carried out for two days. When the daily consumption of caustic soda and carbon dioxide gas injected from line p was measured, the following results were obtained.

Caustic soda consumption = 58 g Carbon dioxide consumption = 352 g Comparative Example 2 Continuous operation for two days was carried out under the same conditions as in Example 2, except that the pump 5 for removing gas from the vacuum degassing tower 4 was stopped. When the daily consumption of caustic soda and carbon dioxide gas injected from line p was measured, the following results were obtained.

Caustic soda consumption = 203g Carbon dioxide consumption = 522g Comparing the examples and comparative examples, it is found that the method of the present invention, which vacuum degasses fresh water after passing through a filter and reuses the obtained carbon dioxide, It is clear that this method has the effect of reducing the consumption of alkaline agents for pH adjustment and the consumption of carbon dioxide gas.

〔Effect of the invention〕

According to the method of the present invention, the alkaline agent conventionally required for adjusting the pH of fresh water after passing through a filter is not required at all or can be significantly reduced. At the same time, the amount of carbon dioxide gas supplied can be reduced. Since the amount of fresh water to be treated for drinking water is enormous, the consumption of alkaline agents and the amount of carbon dioxide gas are also large. Therefore, its reduction is very effective in reducing operating costs.

[Brief explanation of drawings]

FIG. 1 is a flow sheet for explaining the method for producing drinking water according to the present invention. 1...Multi-stage flash evaporator, 2... _CO2 absorption tower, 3...Filter, 4...Vacuum degassing tower, 5
...Pump, 6...Alkaline agent tank, 7...Pump.

Claims (1)

[Claims] 1 Freshwater produced by a seawater desalination device using the evaporation method is injected with carbonic acid-containing gas generated by the desalination device to make carbonated water, and then limestone and/or
Alternatively, in a method of obtaining drinking water by passing it through a filter filled with dolomite particles, the fresh water after passing through the filter is vacuum degassed to bring the PH value of the fresh water to a predetermined value, and the obtained carbonate is A method for producing drinking water using carbon dioxide gas, which comprises blowing gas into the fresh water at the inlet of the filter.