JPS608265B2 - Method for suppressing alkaline scale precipitation and its composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an evaporator for saline water in use, e.g.
This invention relates to suppressing the formation of scale in obtaining fresh water from fertilized water.

Saline water means water that contains one or more inorganic salts dissolved in the water, such as those that make it unsuitable for industrial or domestic use, including sea water, brackish water, and estuary water. Alkaline scale (e.g. in the form of CaC03 or MOH2) formed on the heat exchanger surface of brine evaporators is an important problem caused by the presence of calcium or magnesium bicarbonates in the brine. It is. Scale can be treated, for example, by adding sufficient acid, usually sulfuric acid or hydrochloric acid, to the saline water in order to neutralize all or a substantial part of the alkaline content of the water, so-called acid dosing (acid, dosjng). be done. While this method can effectively inhibit the formation of all scales, the use of acids is difficult to ensure continuous and accurate dosing under the operating conditions of saline water evaporators. Difference in difficulty (because of this,
The drawback is that the relatively low feed produced in the brine promotes corrosion of the equipment (often resulting in a lower boat than desired over a period of time). Another method of treating alkaline scale is to add inhibitors, and in some cases softer types, to the saline water that result in less scale formation than without the addition. Such additives are known and are usually limited to limit additives.
It is called es).

Examples are polyphosphates and polycarboxylic acid additives such as hydrolyzed polymaleic anhydride. The inventors have observed and demonstrated that unexpected benefits can be obtained by using a combination of acid dosing and limiting additive addition.

As described above, the present invention provides a method for suppressing the precipitation of scale from saline water having an alkalinity of bicarbonate, which is formed on the surface of the heat exchange part of an evaporator for saline water. It neutralizes some, but not all, of the alkalinity of the salt. For this neutralization, mineral acid is added to the saline water in an amount such that the pH of the saline water flowing through the device does not become less than 8.0, and in this way, the scale of the partially neutralized saline water is suppressed. It consists of adding additives.

The pH is measured on a saline water sample at 25q0. The scale control effect obtained by the method of the invention is even greater than would be expected from knowledge of the individual scale control properties of the acid and additive, and even if scale is formed, the additive or acid It is not as hard as the scale produced when used alone in less than stoichiometric amounts.

Since the acid neutralization is only partial, the corrosion problem that occurs when using bushes alone as described above is largely solved. The present inventors believe that "in the saline water flowing inside the device"
We paid attention to the pH of the CO2 produced during the neutralization process.
2 will essentially come out of such water as a gas, so the wind of the water will not be suppressed by the presence of C02.

Generally, it is desirable that the pH of the flowing water be at least 8.

The proportion of acid and scale-inhibiting additives added will depend on many factors, such as the characteristics of the seawater and the characteristics of the evaporator.

However, the total amount of acid added is often less than to neutralize 80% of the bicarbonate alkalinity of the brine, preferably between 30 and 80%, more preferably between 50 and 75%. It is between. However, the constant rate that has been very satisfactory in our tests is the amount of acid such that 50% of the bicarbonate alkalinity of the brine is neutralized.

The amount of scale-inhibiting additive used can vary between 0.5 and 20 traces, preferably 0.5 to 20 traces, preferably 0.5 to 20 traces, and particularly preferably 0.5 to 5
It's on the side. These specific proportions can, of course, be varied to satisfy specific conditions.

The acid used in this process is preferably an inorganic acid, such as sulfuric acid or hydrochloric acid, from the standpoint of cost and effectiveness. Examples of additives that can be used in this method include scale-inhibiting additives known in the art, such as polyphosphates, polyacrylates, polymethacrylates, phosphonates, aminophosphonates, carboxylic acid polymers, such as those described in West German Open No. 215917
There are hydrolysed polymaleic anhydrides having a molecular weight in the range 300 to 500 which are disclosed in US Pat.
There are polymers or turbolimers of unsaturated acids with one or more other monomers, such as those described in US Pat. No. 1/73.

The method can be applied to various types of saline water evaporators, such as multi-stage flash evaporators for the production of fresh water, multi-effect evaporators, such as vertical tube (light, film) evaporators, horizontal falling film evaporators (or Applicable to evaporation cans (sprayed film type) and underwater tube cans. The acid and additives may be added separately to the saline water, or
Alternatively, they may be premixed and administered together. According to the invention, it is also possible, as mentioned above, to provide a composition comprising an inorganic acid, in particular hydrochloric acid or sulfuric acid, and 0.5 to 50% by weight, based on the weight of the acid, of a scale-inhibiting additive. A preferred composition is 0.5 based on the weight of acid.
~25% by weight. The invention is further illustrated in the following examples. The general method was as described in the report by Elliott et al. at the Third International Symposium on Obtaining Fresh Water from Seawater, Volume 1, page 46. It is roughly as follows. Seawater is treated with acids and/or additives in the amounts mentioned in the examples below and continuously fed into a test cell where a corresponding release of brine takes place.

The contents of the cell are mixed by sparging with air to remove the carbon dioxide released from the solution.
Then, for two weeks, a certain concentrated steel in the U-tube (located in the middle of the cell) is used to maintain the required value of temperature.
heated by the heat. The scale formed on the U-tube and the cell wall is then removed, dried, and weighed. The rate of scaling for the tubes is calculated as the weight of scale formed on the U-tube per night of seawater supplied to the cell, and the total scaling rate is calculated as the weight of scale formed on the U-tube per night of seawater supplied to the cell. It was calculated as the weight of the scale formed on the wall of the cell. The percentage of scale suppression is calculated using the following formula: Scale suppression rate (%) = Main tube X,oo, where B is the scaling speed on the tube relative to the blank and T is Scaling rate on the tube for the test.

In all the experiments and examples that follow, the acid used was sulfuric acid and the pH of the treated water was greater than 8.

Experiments used various additives and 10400 and 12
It was carried out at a temperature of 0°C.

Experiments were performed first with no acid or additives added, then with 50% and 75% of the theoretical amount of acid, but without any additives.

The following results were obtained. Experiments were then carried out using the various additives described below in different amounts and alone or with acids.

Use of hydrolyzed polymaleic anhydride Polyacrylic acid at 104°C Aminotrismethylene at 104°C of turboma such as maleic anhydride, vinyl acetate and ethyl acrylate in a molar ratio of 9:2:1 Use of phosphonic acid at 104 °C Use of the following compound formula: P03 day 2 OH30 day 2 -○-NH2 P03 day 2 Use of polymethacrylic acid at 104 °C The above results demonstrate the scale inhibition effect of the method of the present invention. It shows.

Moreover, they clearly show that the inhibition rates obtained are greater than would be expected from the scale inhibition results when using additives alone or acids alone at the amounts used. This is because when 50 or 75% of the bicarbonate alkalinity present is neutralized by acid, a reduced amount of additive is used depending on the degree of neutralization, and in the absence of acid, by a sufficient amount of additive. This is because an even better scale suppression rate has been achieved.

Claims (1)

Claims: 1. An inorganic acid is added to the saline water flowing through the saline water evaporator to neutralize some, but not all, of the alkalinity of the bicarbonate, and the total amount of the acid is pH of salt water is 8.0
alkaline scale from the brine with bicarbonate alkalinity on the heat exchange part surface of the evaporator, characterized in that the scale inhibiting additive is also added to the brine. Precipitation suppression method. 2. A composition for inhibiting alkaline scale precipitation from saline water, comprising an inorganic acid and 0.5 to 50% by weight of a scale inhibiting additive based on the weight of the acid.