JPS59230687A - Scale suppression of multistage flash type water making apparatus - Google Patents

Abstract

PURPOSE:To reduce the scale adhesion speed of a heat transfer pipe, by adding an acid to replenishing seawater or replenishing seawater mixed recirculation brine while sending the resulting liquid into the heat transfer pipe of an evaporation apparatus in such a state that formed CO2 is dissolved in the liquid. CONSTITUTION:The adding position of an acid is arranged to the replenishing seawater line 18 connected to the outlet of a degassing tower 6 or to the recirculation brine line 19 connected to the inlet of a recirculation pump where concn. brine after evaporation and replenishing seawater are mixed and the acid 17 is added in an amount required in decomposing at least 30% or more, pref., about 50% of alkali carbonate component in the replenishing seawater. For example, a method, wherein the acid adding amount is controlled to a predetermined value by a pH controller 16 provided to the outlet of the recirculation brine pump, can be adapted to acid addition. By this method, the pH of the recirculation brine at the inlet of the heat transfer pipe of a heat recovery part is lowered and the scale adhesion speed of the heat transfer pipe can be reduced and operation is performed in an extremely advantageous manner.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for suppressing scale within heat exchanger tubes of a multi-stage flash freshwater generator.

Figure 1 shows a schematic system diagram of a tank for producing fresh water from seawater. The raw seawater enters the heat transfer tube 3 of the heat release section 2 via the seawater supply pump 1, and exchanges heat with the steam generated in the evaporation chamber of the heat release section, so that the water vapor is cooled and condensed, and the seawater is heated.

A portion of this heated seawater is sent as make-up sea water via a make-up sea water pump 4 to a degassing tower 6 after adding an antifoaming agent 5r to degas gas such as dissolved oxygen. A scale inhibitor 7 is added to this deaerated seawater, and the mixture is mixed with concentrated seawater (referred to as concentrated brine, hereinafter referred to as concentrated brine) in the final stage evaporation chamber of the heat release section, and is transferred to the heat recovery section 9 via the circulation pump 8 as circulation brine. It is sent to the heat tube 10. The circulating brine in the heat exchanger tubes of the heat recovery section is heated while cooling and condensing the water vapor evaporated in the evaporation chamber of the heat recovery section, and is sent to the brine heater 11 through the evaporation chambers of each stage while increasing its temperature. The brine heater is heated to a predetermined temperature with steam 12 sent from a separately installed steam generator, and then enters the first stage evaporation chamber and is sequentially sent to the next stage evaporation chamber. Each evaporation chamber is connected to an ejector 16 and is sequentially depressurized.The water vapor generated in each evaporation chamber contacts a heat transfer tube to exchange heat, and the water vapor is condensed into a condensed water receiver provided in each evaporation chamber. It will be collected on 14th. The condensed water receivers 14 installed in each evaporation chamber of the heat recovery section and the heat release section are communicated and sent to a separately provided fresh water tank by a produced water pump 15. The brine in the evaporation chamber is transferred from the 1st stage to the 2nd stage to the 6th stage, where it is concentrated while passing through the final stage evaporation chamber of the heat recovery section, and then transferred to the heat release section evaporation chamber. A portion of the stage evaporation chamber is discharged as blow water.

The IAp is then mixed with degassed make-up seawater to which a scale inhibitor has been added, and sent through the circulation pump 8 to the heat exchanger tubes 10 in the heat recovery section for circulation.

In this multi-stage flash water generation device, if scale adheres to the inside of the heat transfer tubes, it will lead to a decrease in heat transfer performance, so various scale suppression measures are applied. The scale that precipitates and adheres to the heat transfer tubes is the so-called alkaline scale (also called soft scale) mainly composed of CaCO3゜Mg(OH)t, which precipitates when seawater is heated and concentrated, and CaS04, which precipitates in high temperature regions. It is broadly divided into hard scales, which are mainly based on systems.

Usually, CaSO4-based hard scale restricts the heating temperature and concentration and evaporation rate of brine to below its saturated solubility, thereby suppressing precipitation. On the other hand, alkaline scale control mainly using CaCO3゜Mg(OH)2 can be achieved by the so-called pH control method, which decomposes and removes the carbonic acid in the supplementary seawater by adding acid, or by using scales such as polymerized phosphoric acid or polymerized maleic acid. A method of suppressing precipitation by adding an inhibitor has been put into practice. However, it is difficult to completely suppress scale precipitation, so if scale adheres to the inner wall of the heat transfer tube and the fouling coefficient (increase in heat transfer resistance due to scale adhesion) exceeds a certain limit, the sponge ball is circulated through the brine. A so-called ball cleaning method is used in which scale is removed from the inner surface of the tube by pressure feeding, or pickling is used in combination.

The pH control method involves adding acid to make-up seawater to decompose almost all of the carbonate alkaline content in the seawater.
The water is removed in a decarboxylation tower (not shown) and then passed through a deaeration tower to remove dissolved oxygen and then mixed with circulating brine.Since most of the alkaline scale components in seawater are removed, transmission The heating temperature and concentration ratio of the circulating brine flowing in the heat tube should be below the saturation solubility of the hard scale, the normal heating temperature should be about 120°C or below, and the total dissolved solid content in the brine should be about 70,000 ppm or below (the concentration ratio should be about 2.0 This is the most effective method for controlling scale. However, the pH of circulating brine is generally in the range of approximately Z5 to 18, and in order to adjust this pH, it is necessary to decompose almost all of the carbonate fraction in seawater, which requires a large amount of acid consumption. It is difficult to handle and adjust the A at the start of operation or during load fluctuations, and the low pH causes problems such as corrosion of the equipment's constituent materials.

On the other hand, the scale inhibitor addition method is easy to operate and has a high brine pH (usually 9 or less), so it is very advantageous in terms of preventing corrosion of equipment constituent materials, but it is less effective than the pH control method in terms of scale inhibition effect. Since the pH control method is also inferior, it is necessary to perform ball cleaning more frequently than with the pH control method. Furthermore, the pH of the circulating brine is approximately 91.
When the temperature increases, OH- is generated in the brine, and the precipitation of Mg(OH)2 increases in the heat exchanger tubes, which increases the contamination coefficient of the heat exchanger tubes and acts as a scale spool, and is removed by peeling during hole cleaning. Ball cleaning effectiveness is reduced because the scale becomes difficult to clean.

FIG. 2 shows an example of the effect of ball cleaning when the scale inhibitor is added alone. In Figure 2, the solid line indicates a maximum brine temperature of 112°C, a pH of 9, 1 to 96 or higher, and a concentration ratio of 1.
．． In the case of 95-2, the dotted line is the maximum brine temperature of 112℃
, shows the effect of ball cleaning when the pH is 9 or less and the concentration ratio is 17 or less, and the lower end of each broken line shows the state after ball cleaning. As shown in Fig. 2, in the case where the pH of the circulating brine at the inlet of the heat pipe of the heat recovery section is 9 or higher, the recovery rate of the dirt coefficient before and after ball cleaning is poor, and the recovery rate decreases as time passes. showed a trend. When the pH is set to 9 or less, the recovery rate becomes good, and although the recovery rate decreases somewhat with the passage of time, it shows an almost stable recovery rate. The pI-I of circulating brine is influenced by the concentration ratio as described below. In the evaporation chamber, the brine is evaporated and concentrated, and CO and gas are released by thermal decomposition of HCO,- to produce CO2 gas. The pH of the concentrated brine after evaporation will be higher than that before condensation.The rate of CO2 dissipation is affected by the temperature and residence time of the brine in the evaporation chamber. ,
The higher the concentration ratio of the circulating brine, the longer the residence time of the brine in the evaporation chamber, and the COζ = concentration becomes 9,C.
The dissipation of O2 gas and the production of OH- increase, increasing the pH of the circulating brine.

FIG. 3 shows the relationship between the concentration ratio of circulating brine and the pH of circulating brine. In Figure 6, 1 lotus/7--, L
/In the case of the supine stimulant addition method alone, 2 is the combination method of combining the pH control method and the scale inhibitor addition method (described later) (however, when CO2 is discharged), and 6 is the above in the case of the present invention method described later. Show relationships. In the figure, F indicates supplementary seawater.

Therefore, in the scale inhibitor addition method, it is necessary to adjust the amount of make-up seawater and the amount of blowdown so that the pH of the circulating brine at the inlet of the heat exchanger tube in the heat recovery section is approximately 9 or less, so the concentration ratio of the circulating brine is usually The limit is around 1.7 or less (when using seawater similar to standard seawater).

On the other hand, as an intermediate method, a method is known in which the drawbacks of both methods are compensated for by a combination method that combines the pH control method and the scale inhibitor addition method (the combination of both is hereinafter referred to as the combination method). In other words, by adding a small amount of acid to make-up seawater using a pH control method, a portion of the carbonate alkaline component (pH
In the control method, almost all (ht) is decomposed and generated Co2
After removing the gas through a decarboxylation tower and removing dissolved oxygen through the entire deaeration tower, a scale inhibitor is added.
It is mixed with concentrated brine to form circulating brine at the inlet of the heat exchanger tube in the heat recovery section. As a result of detailed investigation into this combined method, we found that, for example,
Acid (e.g. sulfuric acid) required to decompose about 0% is added, the generated CO2 gas is removed through a coal finishing tower, and dissolved oxygen is removed through a degassing tower, after which a specified scale inhibitor is added. The pH is in the range of Z9 to 8.0. If this make-up seawater is mixed with concentrated brine and used as circulating brine at the inlet of the heat exchanger tube in the heat recovery section, the pH of this circulating brine will be around 8.8 when the concentration ratio is 17, but when operated at around 20, the pH will be around 9. The change in fouling coefficient over time and the ball cleaning effect are almost the same as in the case where the circulating brine pH is around 9 when the scale inhibitor addition method is used alone (however, when the concentration ratio is about t7), and the scale Although it has the advantage of increasing the concentration ratio compared to the inhibitor addition method alone, it is less effective in terms of ball cleaning frequency and recovery rate.

The present invention provides a means to further enhance the effect of acid addition, and the acid required to decompose at least 30 t, preferably about 50 t, of the carbonate alkali component in the make-up seawater is transferred to the outlet of the degassing tower. By injecting make-up seawater or a mixture of make-up seawater and concentrated brine into the circulating brine pump inlet, the make-up seawater can be directly removed without removing the CO2 produced by acid addition to the outside through the decarboxylation tower or deaeration tower. Alternatively, it can be left soluble in the circulating brine and reacted with CO1- and OH- in the circulating brine. C generated by adding an acid that is easy to decompose about %
If O2 is mixed with the concentrated brine without releasing it to the outside and operated, the pH of the circulating brine will be lower than that of the conventional method even if the pH is raised to around 20, which is about the same as the sardine ratio t pH control method. The value is around 8.4, which is between the scale inhibitor addition method and the scale inhibitor addition method.
As the pH decreases, scale precipitation on the heat exchanger tubes decreases, and the rate of recovery of the fouling coefficient by ball cleaning also improves.

FIG. 4 compares the change in fouling coefficient over time and the ball cleaning effect when CO2 is diffused to the outside after addition of an acid and when CO2 produced is dissolved in the circulating brine to -2 without removing it. The former is shown as a fruit, and the latter is shown as a dotted line. The former is a braai 7ft high'Qt W 112℃,
pH approx. 9, a compression ratio 2.0, brine maximum temperature 1
The temperature was 12°C, the pH was around 8.4, and the concentration ratio was 20. Note that the lower end of each broken line indicates the state after ball cleaning. As shown in this figure, in the latter case, the scale removal effect by ball cleaning was greater, and the recovery effect was almost the same as in pickling.

As described above, the method of the present invention is a scale j1111tilJ method that is able to lower the pH of the circulating brine at the inlet of the heat exchanger tube in the heat recovery section than the conventional method having a decarboxylation tower.

Hereinafter, specific embodiments of the present invention will be described below based on the system diagram shown in FIG. In FIG. 5, the same reference numerals as in FIG. 1 indicate the same functional parts as in FIG. 1.

The addition position of f-17 is placed in the make-up seawater 18 at the outlet of the degassing tower 6 or in the circulation brine 19 at the inlet of the circulation brine pump where concentrated brine after evaporation and make-up seawater are mixed, and the carbonate alkali component in the make-up seawater is added. Acid equivalent to decomposing at least 30 or more, preferably about 50, is added. The amount of acid added can be determined, for example, by using a circulating brine pump installed at the outlet.
E (a method of adjusting to a predetermined value by the controller 16 can be applied).

As described above, according to the method of the present invention, the pH of the circulating brine at the inlet of the heat tube in the heat recovery section is reduced by the combined method of combining the pH control method and the scale inhibitor addition method. Not only can the adhesion speed be reduced, but the ball cleaning effect is the same as that of washing, so it is extremely advantageous in terms of operation and has an extremely remarkable effect industrially.

[Brief explanation of the drawing]

Figure 1 is a system diagram of a conventional multi-stage Fra-Sauche type water generator.
Figure 2 is a chart showing an example of the effect of ball cleaning in the case of Sk-1 and inhibitor additive alone; Figure 6 is a chart showing the relationship between the concentration ratio of circulating brine and the pH of circulating brine;
The figure shows the conventional method of releasing CO, to the outside after adding acid, and CO□
Figure 5 is a graph comparing the circulation brine pH at the inlet of the heat exchanger tube in the heat recovery section and the recovery rate of the fouling coefficient due to ball cleaning with the method of the present invention in which the method of the present invention remains dissolved. It is a system diagram. Sub-agents 1) Mei-Fu agent Ryo Hagiwara - Concentration ratio of circulating brine during the morning of operation 4th garden operation period

Claims (1)

[Claims] As a method for suppressing scale in the heat transfer tubes of a multi-stage flash water generation system, the acid required to decompose part of the carbonate alkaline components contained in the make-up seawater is added to the make-up seawater or circulating brine mixed with the make-up seawater. C02 is dissolved in the liquid,
A scale suppression method for a multi-stage flash water supply device, which is characterized by feeding the mixture as it is into a heat transfer tube of an evaporator without dissipating it to the outside.