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Water is one of the most widespread substances in the nature, especially in the liquid state – form in which it covers more than 2/3 of the surface of the globe, forming the surface waters.
One can observe the circuit of the water in the nature. The heat of the sun causes the evaporation of the surface waters, and the formed vapours rise in the atmosphere. If the atmosphere is saturated in water vapours a temperature decrease occurs, a part of the vapours condense and take the form of mist (fog) clouds, rain, snow, hailstone; during the cool nights in the warm seasons dew falls, and when the temperature of the ground is below 0°C, there is hoarfrost. All these form the meteoric waters.
The water fallen onto the ground or resulting from the melting of the snow, partly fill again the lakes, rivers, seas and oceans, and partly penetrate the ground layers to different depths, forming the phreatic waters. If in their way they reach a waterproof layer, such as a clay layer, they gather there and they form underground water..
If the underground water, circulating through different layers, approach again the surface and get out of the earth, they form the natural springs.
at 00C – solid = 0.9168
– liquid = 0.999868
at 40C = 1.000
at 200C = 0.998230
Specific heat at 150C
Vapour pressure, torr
at -100C = 2.15
00C = 4.68
200C = 17.53
400C = 55.1
800C = 355.1
1000C = 760
Dielectric constant at 250C
Dielectric moment, D
Conductivity, for pure water
H2O = H+ + OH–
Ionic product, at 250C
1.008 x 10 -14
The water is used in the industry in the engineering processes (chemical, food industry, etc.), or in the electro-energetic industry in order to produce steam and hot water (thermal plants) but it is used as cooling agent in the cooling units having outstanding advantages in comparison with other substances because of its widespread in nature (70%, of the cost price relatively low) as well as of its physical and chemical features, suitable to the proposed purpose).
The sources of natural water for the provision of the required quantity of industrial water, drinking or for other use, started to be during the last decades a serious problem for mankind; massive and costly works are being done in order to protect the water sources and at the same time to prevent pollution. Therefore, recirculation of the industrial and waste waters is more and more required in order to reduce the consumption of natural water.
From the chemical point of view, pure water corresponds to the known formula H2O. However, the natural waters used as sources for the drinking or industrial water are not clean. They contain dissolved gases or insoluble mineral and organic substances, according to the impact of the meteoric substances with the atmosphere or with the ground where these impurities are.
The natural water exists in the three states of aggregation, according to the ambient temperature:
- solid – ice or snow
- liquid – in the temperature range 0 – 100°C; in this range, the water can be also in gaseous state, forming the atmospheric humidity
- only in gaseous state (vapours), above the boiling temperature which in its turn depends on the pressure.
PFor the use of the liquid natural water as cooling agent or for the supply of the boilers, it must comply with the allowance limits in Romania specified by the I.S.C.I.R. or STAS norms, respectively in other international standards. Most of the natural sweet waters are not suitable to the above-mentioned purposes, which requires their pre-treatment before use, by physical and chemical procedures: it eliminates the impurities until their compliance to the allowance limits specified by the lawful norms.
The presence of gases and of the dissolved solids and the pressure modify the freezing point and especially the water boiling point. The boiling of a liquid is its passage under the state of vapours under the influence of the temperature, this phenomenon occurring in the whole matter of the liquid; unlike the evaporation where its passage in the state of vapours under the influence of the temperature, this phenomenon occurring in the whole matter of the liquid, unlike the evaporation where the passage of the liquids in gaseous state occurs only at its free surface.
Further on are mentioned the main impurities in the natural waters.
The gases existing in the natural waters are in general the oxygen, nitrogen, the carbon dioxide and proceed from the atmosphere, and if the latter is polluted, in the water are also dissolved ammonia, hydrogen sulphide, sulphur dioxide, chlorine etc.
The running waters are generally polluted, in the period of the meteorological phenomena, with insoluble solids (clays, silicon dioxide, organic matters, etc.) which deposit on the bottom of the lakes as mud. If they get into the boilers, they build sludge – to which add the insoluble substances built during the boiling of the water. Therefore, a first condition for waters for supplying the boilers is the degree of flushing out.
The soluble solids which pollute the natural waters are:
Substances soluble in water
5 – 20.000
Calcium and magnesium chloride
5 – 250
5 – 20
0.1 – 5
0.1 – 150
0.1 – 500
0.1 – 50
o which are added small quantities of iron salts, potassium etc, respectively organic raw materials.
It must be stated especially that all the soluble solid minerals, considering the above concentrations, are totally dissociated into anions and cations; therefore from the electrochemical point of view only the concentration of the respective ions is discussed, not also the composed body from which they resulted by dissociation.
According to the concentration of the water in the chlorides (Cl– and Na+ ions), sulphates and other salts, the natural waters can be:
- sweet waters, with content in chlorides below 250 mg/l, and the ratio Ca:Mg = 4:1.
- mineral carbonated waters used for food industry, with a high content of calcium and sodium bicarbonate, CO2 etc.
- mineral sulphurous waters
- brackish waters, with a high content of magnesium salts.
- salted and very salted waters (lakes, seas, etc.) with content of chlorides above 500 mg/l up to 18.000 – 25.000 mg/l in the sea water and up to 33% in certain lakes or closed seas.
Crust is composed of minerals, hard soluble in water, especially calcium and magnesium carbonate and calcium sulphate, which, during the vaporization of water in boilers, are deposited on their walls in the form of scaling. Crust submitted prevent transmission of heat and therefore increases fuel consumption necessary water vaporization (e.g., a layer 3 mm thick crust requires an increased fuel consumption by 20%). Also, because the heating boiler is not uniform, the walls do not expand uniformly, which can cause cracks or boiler explosion. Therefore, before entering the boiler water is removed from it substances that form crusts, called descaling operation.
The water used in other industries must meet certain conditions of purity, depending on the specific industry it is used. Thus, for example, water needed for the preparation of ammonia or hydrochloric acid should be pure, the drug industry to use very pure water, water used in the sugar industry must meet the requirements of drinking water and its hardness is as low because the salt content damage the sugar crystallization process, in brewing, the quality of water used is of particular importance because it determines the taste of beer, the textile wash water should be used with very low hardness because calcium and magnesium salts forms insoluble soaps, which soiled fabrics etc.
Industrial wastewater can be done through different processes: distillation, chemical treatment or ion exchange.
From the thermodynamic point of view, iron is unstable in water, in which it has always the tendency to dissolve. This process stops when a protective layer forms on the surface of the metal in contact with water, in a natural or deliberate way. The layer can be, more or less: thick, dense, homogenous, adherent and complete.
The energy of Fe2+ ions is bigger in metal than in the water, they have the tendency to pass into the liquid stage according to ec. (1).
This corrosion is an electrochemical phenomenon by which the metals, in contact with water, ‘’dissolves’’, forming ions:
Me 0 → Me z+ + z e – (1) (z = metal valence).
The equation above expresses a dynamic equilibrium which occurs at the interface metal-water by which simultaneously the ions pass, from one side to another (material transfer) and the electrons (charge transfer), therefore an electric current, is an anodic reaction and the initial stage of corrosion; the attacked metal is the supplier of electrons and forms the anode.
When the direction of the reaction is towards the right an anodic oxidation occurs, and therefore an anodic current, and conversely, a reduction, respectively a catholic current.
At the interface metal-water a double electric layer is formed, due to the metallic cations (positively charged) attracted to the metallic surface (negatively charged due to the released electrodes). This layer acts as a condenser which has a certain potential designated in electrochemistry as ‘’electrode potential‘’ and which is characteristic to each metal. Its value is according to the activity of the electrochemical sorts in the water (ions), of the temperature and of other factors.
In equilibrium conditions, the potential of the electric field is by definition – the equilibrium potential of the iron electrode in relation with water. This equilibrium is, however, very unstable; the electric field is destroyed, as the daily practice confirms it. Therefore one must admit the existence of an electrode which has a different potential than the one of the iron. In the case of the cooling circuits it is the oxygen electrode. The free electrons are “captured” by the oxygen dissolved in water, which plays the role of cathode:
O2 – 4e– → 2O2- (1)
2O2- + 2HOH → 4OH– (2)
or O2 + 4e– + 2H2O → 4OH– (3)
The reaction (1) supplies positive ions, the following reaction allows the retention of these ions:
2 Fe2+ + 4 OH – → 2 Fe(OH)2. (4)
The electric equilibrium field has disappeared and the corrosion can continue. The cathodic reaction with hydrogen electrode is not practically taken into consideration in this case, because it has weight only in completely degassed water.
The quantity of metal which enters in the solution is proportional with the current which appears during corrosion. The sum of the cathodic currents is equal as absolute value with the sum of the anodic currents. The areas of metal remaining under tension after the lamination, welding, cold-hardening, etc. are not in equilibrium, and the iron ions pass into the solution causing anodic areas. The areas covered of mud, dirt, corrosion products, have also the tendency to become anodic, which is to be preferably corroded (Evans corrosion elements). In these areas, the presence of the deposits prevents the oxygen from entering in a sufficient quantity to form cathodic areas.
The heterogeneous surfaces of the metal in the ducts, tanks, heat exchangers, etc influence the location of the cathodes and anodes. The cathodes and the anodes are as many local elements which are practically in short-circuit; when these elements discharge current, following the corrosion process, overvoltages occur, due to the passage of the electric charges through the double electric field: this fact reduces the cathodic potential and increase the anodic potential, causing their tendency towards a mixed potential. This can be measured in practice and it is the potential which takes the metal subjected to corrosion related to water.
When in the water there are ‘’consumers‘’ of electrons, they can cross the interface metal-water and in this case the potential is modified and a polarization occurs. Coming back to the general dynamic equilibrium equation (1), we will mention further on the factors which can spoil this equilibrium, respectively the movement of the reaction in only one direction.
In a cooling circuit involved several factors that contribute to the diversification of the corrosion processes, which must be taken into consideration during the water treatment.
- WATER FEATURES
The increase of the salinity of the water intensifies the corrosion: the conductivity of the water increases, which causes the increase also the corrosion current, and the cathodic areas that are farther than a certain anodic area can take part to the corrosion reactions. The available cathodic area increases and therefore intensified corrosion. The Cl – ions have a specific action on the process: with the iron it forms complex ions, avoiding or even preventing the build-up of the protection layer of iron oxides.
The organic substances, from the contamination of the circuit of coming inside together with the addition water, can cause problems during exploitation. The low values of the pH can dissolve the formed protection layer or film. The pH values of 8-9 are recommended for the protection layer.
- CIRCULATION SPEED
A high speed is favourable in case when it introduces into the system a quantity of oxygen large enough; in this way the build-up of the anodic areas is prevented, which can appear in the areas with low speed. In the latter, the available oxygen is consumed by the cathodic reaction (3) and as it is replaced only very slowly, these areas become anodic in comparison with the balance of the metal surface. The consequence is an anodic dissolution of the metal.
Corrosions due to the low speed of the water can be noticed at the end of the ducts, on the valve seats, in connection parts, at the tube flanging, at the welding seams, in the areas with deposits of dust, organic slime, calcium carbonate or phosphate. Under the deposits of any kind occur the so-called airing elements, leading to punctiform corrosions. Also, a higher speed of the water does not allow the agglomeration of the bulky corrosion products, which contribute as well to the creation of the areas with anodic character. However, higher speeds of the water can prevent the formation of a protective layer, especially in the copper tubes, which results in the intensification of corrosion.
In the cooling circuit are used several metals and alloys: steel, copper, brass, aluminium, alloyed steels. Therefore, one has to take into account the possibility of occurrence of the galvanic effects between different metals, and the treatment, respectively the conditioning of the water, must keep into consideration the presence of these different metals, especially in the construction of the heat exchangers.
The copper can be dissolved, in small quantities, by the completing ions in the water, such as: sulphides, cyanides, ammonium. The ions of iron, as the most active metal, drive out a part of the copper ions from the solution, which deposit as metallic copper on the surface of the steel, forming a cathode which intervenes in the process of destruction of the metal.
The dry breakdown layer of the steel acts cathodically to the clean steel; at the contact with the water, the layer becomes anodic and corrodes.
Before putting in operation the cooling circuit, it must be cleaned from the concentrations of sludge, dirt, etc under which anodic areas usually appear; the impurities can absorb the conditioning reactants added to the water, annihilating their protective effect on the metal surface.
The corrosive action is intensified by the increase of the temperature of the water, up to 70 0 C. Above this temperature, the solubility of oxygen decreases quickly; the corrosion speed being determined, partly, by the diffusion of oxygen to the cathodic surface, the intensity of the corrosion decreases as well. The decrease is approximatively proportional with the content of oxygen of the water, and at the water boiling point, the corrosion speed is practically null.
They are the most unpleasant phenomenon in the exploitation of the cooling circuit. The deposits can be of several types: consisting of substances dissolved naturally in the water, for instance the calcium carbonate, from the solid suspensions, such as clay or coal dust, vegetal or animal microorganisms.
The calcium carbonate deposits in the warmest areas of the cooling circuit, therefore also in the heat exchangers. The deposits of calcium carbonate worsens the heat transfer, and the unturbulent water layer near the metallic wall gets warmer more, which intensifies the precipitation of the calcium carbonate. Finally, for this reason serious problems during exploitation can occur.
Solid suspensions, especially the sand, light ashes enter into the cooling circuit by the atmospheric air at the cooling towers, open channels, basins, etc. Settling on the metallic surface, these particles generate airing elements, causing punctiform corrosions. The transmission of the heat is also impeded by the impurities deposited on the surfaces of heat exchange, an undesirable phenomenon in any exploitation.
The impurities in the water, of colloidal kind, show a strong tendency to absorb the inhibitors added into the circuit, thus annihilating their useful action.
At the alloy steels there can occur, under the deposits, transcrystalline corrosions under tension, favoured by a local concentration of Cl – ions in the water and by a local increase of the temperature.
In the circuit of the cooling water many species of micro-organisms can develop, favoured by the presence of the impurities, and, apart from other drawbacks, can also cause corrosions.
All these factors do not act separately, they are interconnected, and their effects are linked. Together, they form a whole which only for the purpose to be easier understood is presented systematically: corrosions, deposits, settlings, biological effects. The control of these drawbacks is achieved, in general, by a number of interdependent actions.
- GALVANIC COUPLES(primary cells) represented by the contact in short circuit of two different metals is the ideal way to transport the electrons from the metal which corrodes and which is negative from the electric point of view, towards the electropositive metal which acts as a cathode, that is electron-seeking. The conventional signs of the anodes and cathodes in case of the primary cells are reverse to the ones known from the electrolysis of the water where the anode is (+) and the cathode is (-).
- 7. TECHNICAL METALS HAVE IMPURITIESin their composition, namely carbon, other metals or oxides, which in general have the role of a cathode, forming micro- primary cells in the mass of the basic metal (anode). This is the way how is explained the increased resistance to corrosion in comparison with the pure chemical iron in the water without salts and gases, when the corrosion speed is extremely low.
Formation OF SOFT SETTLINGS
The reasons which contribute to the building-up of the deposits are multiple and they differ according to the specific features of the cooling circuit:
- impurities (dust, smoke) from the atmosphere
- sand and sludge from the raw water
- corrosion products from the circuit
- organic, natural substances from the air or from the raw water
- reactants from the penetration of the addition water
- impurities from the lack of tightness of the heat exchangers
- reactants from the conditioning of the water in the circuit against corrosion and hard settlings
Often, several factors intervene simultaneously, intensifying the spooling process. These deposits have usually a gelatinous aspect and mellow state; they can embarrass the exploitation or they lead even to the damage of the equipment. The increase of the temperature aggravates the process of soiling of the surfaces, because under the influence of the temperature the initial settlings become more adherent and more compact, embarrassing the heat transfer. The slow speeds of the water (below 0.6 m/s) favour the settling of the impurities in the suspension: according to the specific features of these impurities, settlings can occur even to speeds of 1.5 m/s.
The micro-organisms (algae, ferruginous bacteria, fungi etc) form settlings by adhesion, which act as a filter to the mineral and organics suspensions in the water, enclosing them.
The corrosion products can form, themselves, agglomerations of settlings or they can be enclosed into the settled organic mass. In the chemical industry, due to the lack of tightness of the heat exchangers, there can appear, in the cooling circuit, according to the manufacturing processes, hydrocarbons of all kinds, ketones, aldehydes, inorganic reactants, etc. These ones settle, or, remaining in the water, can be the food of the micro-organisms, favouring their growth; they can react with the solid settlings, with the corrosion products or with the inhibitors, generating soft settlings.
CONDITIONING OF THE WATER AGAINST SOFT SETTLINGS
The combating of the soiling is performed by different means, according to the impurities in the water and to the features of the cooling circuit. Two kinds of actions can be taken: mechanical and chemical means. According to the case, the two procedures can be combined.
The following are used: oil separators, mineral suspensions catchers, filters.
The water filtering is the main operation also for the cooling circuits with high flow, it requires special equipment, so that the dimensions of the filters are not too large, however their efficiency be high.
Their purpose is the dispersing of impurities, their solubilisation or passage in the state of suspensions; then, by purging, their elimination from the circuit is achieved.
The washing with water, even in a new circuit, is less efficient in this respect and active solutions must be applied.
The washing with acid solution removes the corrosion products and some mineral impurities;
It has no effect on the organic impurities. After the rinsing of the circuit, after treatment, it is not well performed, and intense corrosions can occur. The metals cleaned with acid become very sensitive to corrosion; the operation must be performed by adding inhibitor, specific for the metals present in the circuit. In certain cases the chemical washing must be made on parts of the circuit. The parts made of concrete, cement, sandstone, cannot be washed with acid.
Immediately after the hydraulic test of the circuit will start the conditioning by the retention of the mineral suspensions (for instance by filters). In the frame of the complex conditioning will be added corrosion inhibitors and a dispersing agent, in order to ease the elimination of the soft deposits which build up in the circuit.
BUILDING UP OF HARD DEPOSITS
The hard deposits build up by precipitation on the walls of the circuit and of the cooled equipment, of some chemical compounds contained naturally by the water. The precipitation occurs due to the outrunning of the solubility, caused either by the increase of the concentration of the similar or “Foreign” ions, either by the increase of the temperature.
Usually, calcium and magnesium salts precipitate; they are substances very wide-spread in the nature and most of them have a relatively reduced solubility, which decreases with the temperature and in the presence of certain ions in the solution. Sometimes also the silica in the water has conditions to precipitate or to be included into the precipitates which form.
The calcium and magnesium bicarbonates can be eliminated from the water by simple and economic means, unlike the calcium sulphate which, having a relatively low solubility, contributes to the building up of hard deposits. Its solubility in the water in the circuit must not be exceeded.
The magnesium compounds have a higher solubility than the calcium compounds and their contribution to the building up of the hard deposits is more reduced, especially because in the natural waters in our country (and generally also in Europe) their relative concentration related to the calcium salts is low.
The polyphosphates used for the conditioning of the circuit form orthophosphates, by hydrolysis, which can build up hard deposits of calcium or magnesium phosphates.
The solid suspensions in the water, from the process of coagulation and the products of the corrosion in the circuit contribute to the building up of hard deposits. The dissolved iron, combined with some of the substances present in the water produce sparingly soluble iron salts, which will settle.
In the conditions of alternative steeping and drying, of high concentration and in the presence of solid substances, even the sodium salts can form hard deposits.
The main factors which contribute to the building up of the hard deposits are:
- the temperature, which modify positively or negatively the solubility of the substances dissolved in the water;
- the pH; a high value intensifies usually the building up of hard settlings, the solubility of several compounds decreasing in the alkaline field. The decrease of the pH causes the tendency to build up settlings of those substances which dissolve in acids.
From the shortcomings caused by the hard deposits in the cooling circuits, the following are worth mentioning:
- occurrence of corrosions under the settlings, by the effect of differential aeration (Evans), and therefore the intense clogging of the surfaces of the heat exchangers, and finally, their breakage;
- need of periodical cleaning of the equipment, related with lack of availability and expenses.
The applied methods depend on the kind of the substances in the water which forms settlings and on the features of the equipment. In principle are applied those actions which allow the concentration of the salts in the recirculated water, in order to spare the water: either eliminating certain sparingly soluble salts, or transforming them in other ones, more soluble, either keeping them artificially in the solution.
The following conditioning processes will be taken into consideration:
- decarbonisation of the water, by means of lime.
- decarbonisation of the water by cationic carboxylic exchangers
- softening of the water by cationic exchangers in the Na cycle.
- softening by cationic exchangers in the cycle Na – H.
- transformation of the temporary hardness into permanent hardness.
- metering of polyphosphates.
- stabilization with special products.
The method of the “recarbonization” is too complicated and it can cause corrosions in certain areas of the circuit, due to the free carbon dioxide.
Therefore it is necessary to combine some of these methods to obtain a satisfactory result.
Neither the economic factors should be ignored while choosing the procedure that needs to be applied, on the condition that the technical people specify previously the most suitable solution.
DecarbonIZATION OF THE WATER BY LIME
It is used usually for large flow rates of water, the reactant being cheap, or when the raw water contains materials in suspension. A solution of calcium hydroxide is used, as hydrated lime (with the CaO content of 0.8 – 13%) or powder of calcium hydroxide. The following are eliminated: the calcium and magnesium bicarbonates, the magnesium carbonate and the free CO2.The efficiency of the elimination of the bicarbonates depends finally on the solubility in the water, in the conditions given for the compounds which appeared.
The decarbonised water, even without an excess of reactant, keeps a certain capacity to neutralize a weak acidity. The sodium bicarbonate cannot be removed by this procedure.
The decarbonisation equipment must operate continuously and, in case of a short duration, it must be cleaned before re-starting it. One will work at a temperature as low as possible, in quick reactive vessels (with turbulence): hexagonal calcite crystals are produced, which form the crystallization “germs” and contribute to the efficiency of the decarbonisation. At higher temperatures the rhombic aragonite appears, which worsens the elimination of the temporary hardness. The control of the decarbonisation is performed by the titration of the p and m alkalinities in samples of filtered, titrated water.
The relation 2p-m, represents the alkalinity given by the OH- ions; the relation 2p = m, corresponds to a minimum of the concentration in Ca2+ ions and to a maximum of the concentration in CO32- ions, representing the best case of the decarbonisation, namely to a correct metering of lime.
The process of decarbonisation cannot be controlled on the basis of the measurement of the pH index, because there is no pure system H2O – CaO – CO2, where the relations between p, m , 2p = m and pH were univocal, but they are natural waters which contain magnesium salts, salts of strong acids, etc, which introduces sources of errors.
WATER DECARBONIZATION BY CARBOXYLIC CATHIONIC EXCHANGERS
It will be worked in cycle H+, in a similar way to the softening of the water rich in bicarbonates. In the case of the waters rich in sodium bicarbonate, the efficiency of the carboxylic exchanger is reduced, which could modify essentially the economic aspect of the process.
The salinity of the treated water is lower than the one of the raw water with the value of the eliminated bicarbonates: therefore, the cooling circuit can operate with small purgings. The procedure requires that the water to be treated should be clear, which sometimes requires, in most cases, a previous mechanical filtering and sometimes a flocculation – coagulation followed by a filtering. The elimination of CO2 which results is made either in a degasifier, either in a cooling tower. The economical way is to use for regeneration a “scrap” acid, recovered from the regeneration of the cationic filters of the water softening equipment for engineering purposes or for boilers.
WATER SOFTENING WITH CATHIONIC EXCHANGERS IN THE Na CYCLE
The procedure is known in detail. The salinity of the treated water varies compared with the salinity of the raw water in the ratio 100 (Na) / [ a(Ca) + b(Mg) + C(Na) ], where a, b, c are the relative concentrations, expressed in percentages, of the cathions of Ca, Mg and Na in the raw water.
The sodium salts have a high solubility, and their concentration in the circuit can be quite high, which leads to an economy of water.
By this procedure are avoided the hard deposits in the circuit; the presence of sulphates, chlorides and carbon dioxide (from the thermal decomposition of the bicarbonate) require, however, a careful conditioning of the circuit.
WATER SOFTENING WITH CATHIONIC EXCHANGERS IN THE Na – H CYCLE
Several schemes can be used. The easiest and safest is the scheme with the cathionic filters connected in parallel; the adjustment of the softening is done in the point of mixing of the partial water currents by their reciprocal volumetric ratio. A degasifier by blast eliminates the excess of carbon dioxide from the water. The Na – cathionic filter warranties safety against the presence of the free acids in the softened water.
TRANSFORMATION OF THE TEMPORARY HARDNESS INTO PERMANENT HARDNESS
This method is also designated in an improper way, “vaccination”. Mineral acids are used, especially sulphuric acid and chlorine hydride; the use of each of them is usually done according to economic or opportunity reasons.
However, in case of certain waters, the choice of the acid is done according to technical reasons:
- the waters rich in chlorides and bicarbonates will be treated with sulphuric acid
- the waters rich in sulphates and bicarbonates will be treated with chlorine hydride
A special attention must be given to the concentration in the circuit of the calcium sulphate, in order not to exceed its solubility at the given temperature. The configuration of the circuit, predominance of the metal parts or of those of construction materials, their technical features can influence the choice of the acid with the purpose to eliminate any source of drawbacks during exploitation. The addition of acid is done in quantities equivalent to the temporary hardness which must be transformed into permanent hardness. The specific consumption of acid is lower than in the scheme of decarbonisation with carboxylic cathionic exchanger. As a precaution action, the temporary hardness is not transformed completely into permanent hardness. The elimination of CO2 resulting from the reaction can be made in the cooling tower, and for this reason the addition of water is done either in the warm water tank, either directly in the tower.
It is applied for the conditioning of the cooling circuit, namely for the stabilization of the calcium bicarbonate in the solution, which results from the decomposition of the bicarbonate.
The metaphosphates, which represent another group on condensed phosphates are not proper to this purpose, as they do not have a stabilization effect.
The polyphosphates have the general composition formula (Me I PO3)n . H2O, respectively (MeI PO3)n . Me2O are catenaries, without ramification; for the conditioning of the water are used polyphosphates of high polymerization, the hexametaphosphate being one of the most well-known (NaPO3) . Na2O.
In the process of formation of hard settlings solid germs appear in the water, around which develops the crystalline construction of the substance which precipitates, by the juxtaposition of other germs which have the solid forms characteristic to the respective substance (especially calcium carbonate). The initiation of the first centre of crystalline settling lasts a certain time, according to the conditions in the circuit (salinity of the water, temperature, kind of dissolved substances etc.); the other crystallization centres form afterwards quicker and quicker, until a crystalline network occurs, which forms the settling. The high polymerization polyphosphates have the property to delay the precipitation of the carbonate, complexing the germs in their chain as long as they form, embezzling them from the environment. As a result, the time for forming new germs returns to the initial time of forming of the first germ, so that the precipitation is delayed for a quite long period. Low concentrations of polyphosphate can thus prevent the forming of crystalline settlings.
However, the polyphosphates hydrolyse quickly, passing to orthophosphates, which precipitate the calcium and magnesium salts, causing settlings as sludge or even hard ones. This transformation is practically instantaneous at 70 0 C, when the effect of the polyphosphates becomes null.
The hydrolysis is also influenced by other factors: the pH, presence of neutral salts, the accelerator effects of the enzymes (organic substances soluble in water), interaction of complexes formed by the polyphosphate and by cations, catalytic action of the precipitates of oxides of the metals, microorganisms. Even at temperatures normally existing in the industrial cooling circuits, the hydrolysis of the polyphosphates can reach values of above 95%, according to the weight of the specified factors.
The polyphosphates of Zn-Na show, in similar conditions, a weaker tendency to hydrolyse (hydrolysis degree 40 – 45%), having therefore a better stabilizer effect.
A correct indication of the correct metering of polyphosphate in the water of the circuit, concerning the effect or the stabilization of the calcium and magnesium compounds is given by the soft settlings formed in the heat exchangers which should not contain more than 10% CaCO3. The dosages lower than 0.5 mg polyphosphate per litter produce hard settlings, and the dosages higher than 2 mg/l increase their quantity of soft deposits on the surfaces of the heat exchangers. At the same time the temperature of the water in the circuit must not be exceeded, neither the concentration of the calcium and magnesium bicarbonates, in this way the effect of the polyphosphates decreases or becomes even null.
STABILIZATION WITH SPECIAL PRODUCTS
During exploitation, in order to avoid hard settlings in the circuit, in most cases is applied one of the above-mentioned methods of elimination of the bicarbonates from the water. To complete the effect, as well as to avoid the soft deposits, the water is conditioned with certain special products, among which the polyphosphates.
Sometimes a complex conditioning is applied in order to prevent hard settlings, and the dispersion of soft settlings and to inhibit corrosion, because all these unfavourable effects appears, often, at the same time in the cooling circuits, even when a previous treatment is applied to the addition water.
In case when the problem of the hard deposits is the main drawback of a cooling circuit, the following stabilization procedures can be applied:
- chromium salts in synergic mixture with zinc salts.
The inhibitory effect occurs at pH = 6.5 – 7, which requires the previous reduction of the temporary hardness up to 0.05 mval/l. For this purpose, the water is decarbonised with lime and then treated with sulphuric acid up to a pH of 6.5 – 7. The effect is complex, as far as the simultaneous prevention of corrosions and of microorganisms is concerned (algae, bacteria). The consumption of inhibitors is of grams to 1 m3 water.
The purgings of the circuit must have the chromium removed so that the effluent corresponds to the sanitary prescriptions in force. For this purpose will be used an anionic exchanger working in the cycle Cl–, by which the content in CrO4 of the water can be reduced below 1 mg/l. The purgings thus treated are mixed with the industrial drainage waters in order to dilute them. The washing waters of the anionic filters must be discharged separately, which is a difficult problem; a solution might be the use of these waters in the circuit of hydraulic transport of ashes, ores, etc. to washings and others.
The control consists in the checking of the content in CrO4 of the water in the circuit (by the calorimetric method) and the checking of the parts or test specimens introduced in the tank of cold water. This procedure is recommended for the circuits with tower and “wet” cooling, with low flow rates.
- mixtures of organic substances, without chromates and phosphates.
The water in the circuit must have a pH of 7 – 8, which is achieved by metering sulphuric acid, in case of need, in the addition water.
The products are solid and stable in conditions of dry storage. The dosage is of tens of grams to 1 m3 water. The treated water cannot be used for drinking.
- mixtures of polyphosphates and stabilizing organic substances, such as lignines and tannins materials.
They have a synergic action, activating the polyphosphates.
The organic substances slow down the increase of the crystals of the salts which precipitate, forming a film around the particles: as a result, they form sludge, not hard settlings.
The dosage is of the range of grams to 1 m3 water.
It is considered that a natural protection is the formation of a layer on the surface of the metal, made of calcium carbonate and rust. It consists in waters which contain oxygen and where there is an equilibrium CaO:CO2.
During the incipient corrosion, the oxygen causes the formation of OH – ions in the limiting layer water-metal, by this the pH increases, and the balance CaO:CO2 in the water is disturbed.
A precipitation of crystalline calcium carbonate occurs, which includes also small quantities of iron oxides (rust), forming a protective layer. This layer cannot form if the oxygen is missing, neither in the presence in the water of reducer substances (hydrogen sulphide, traces of oil, organic substances). The formed sludge causes corrosions in its turn. In conditions of dead water no protective layer is formed, because the dissolved oxygen is consumed microbiologically or chemically, before reaching at the surface of the metal.
The neutral salts, especially the chlorides, have the property to keep the iron compounds in the water in colloidal stage; they increase also the solubility of the calcium carbonate and form porous, inefficient layers.
The content in Cl– ions of the water in the circuit must be low, a condition which is rarely achieved in practice. All these basic conditions must be fulfilled, the precipitation of the crystalline calcium carbonate, on the surface of the metal, which forms a protective layer without pores is conditioned by a series of factors. The precipitation must be slow, namely the balance Ca:CO2 in the water near the surface of the metal must change slowly, gradually.
Apart of the time, the temperature is an important factor, when it is high, the thermal decomposition of bicarbonates is quick, and the deposited calcium carbonate is non-crystalline and porous. The sensitivity to corrosion of the metal creates a higher alkalinity in the limit-layer; for iron, it is more increased up to 700C and therefore, it precipitates non-crystalline calcium carbonate. Finally, in the cooling system there are different temperatures and the reaching of a stage of stable, ideal, equilibrium CaO:CO2 is not possible for the whole circuit.
In these conditions one cannot be sure, practically, on the creation of a natural protective layer, on the surface of the metal, and the anticorrosive protection, in this case, is inoperative.
The provision of the anticorrosive protection in the cooling circuits is achieved artificially.
The applied methods, based on the theoretical reasons specified above are divided as follows:
- cathodic protection, consisting in the reduction of the cathodic potential, according to the corrosion current
- anodic protection, achieved by the stressed increase of the anodic potential
- simultaneous anodic and cathodic protection
It consists either in the reduction of the H+ ions, either in the elimination of the oxygen; the first condition is achieved by the increase of the pH (metering of alkaline substances) and it can be applied only to degassed waters: the second condition cannot be applied to the cooling circuits.
As a consequence, the protection can only be achieved by the formation of a protecting film, obtained using the generated alkalinity, near the wall of the metal, by the corrosion itself, by adding certain substances to the water.
The products which are added to the water, designated as cathodic inhibitors, must lead to the forming of a homogenous, dense and sparingly conductor film. It is formed by the adsorption of the substances added to the water, at the surface of the metal or by their combination with corrosion products and it must produce a high supratension.
It is recommended as a dose, at the same time with the inhibitor and a dispersing agent, which has the role to clean the surfaces of the equipment and to create conditions for a good effect of the inhibitor.
The corrosion inhibitors have sometimes the features of a polymer. The phenomenon of adsorption by the surface of the metal, leads to the concentration of the inhibitor in the limit-layer between metal and water. The association of the polymer inhibitor can occur with the metal or with its oxide and it can be of physical kind (Van der Waals connections), or chemical (coordinative connections), by pairs of free electrons. The inhibitor must have a good adherence on the surface of the metal at the turbulent flow of the cooling water; the monomolecular films are usually insensible to thermal stress. The products used for this purpose are: mixtures of polyphosphates, salts of the metals, certain organic products.
Anodic protection is achieved by the union of the Fe2+ cathions with the OH- anions of the water, forming Fe(OH)2; its solubility decreases when the pH increases, and it precipitates on the metal surface. In the presence of the oxygen in the water the ferric hydroxide forms, which precipitates as well; the settlings of hydroxides provide a certain anodic protection, on the condition that the water speed does not entrain these hydroxides. The anodic inhibitors act as well by forming a protective film, either by their adsorption at the surface of the metal, either by their combination with the corrosion products, as in the case of cathodic protection. Such products are: chromates, orthophosphates, ferrocyanides and some organic substances. The anodic protection used alone must be considered as not recommended, because if it is not absolutely complete, all the corrosion currents are concentrated in certain points of the metal surface, which is corroded more quickly. For the prevention of this phenomenon large doses of inhibitors are required, which can be possibly done at small cooling circuits, but it cannot be applied from the economical point of view, in the case of the circuits with large flow rate.
ANODIC AND CATHODIC SIMULTANEOUS PROTECTION
It is achieved by metering some mixtures of substances into the water (inhibitors), studied especially for their efficiency; the dose is of the range of grams per cubic meter of water.
The mixtures of inhibitors can modify the properties of inhibition of the corrosion of their components.
Thus, only the use of the polyphosphates even in doses of 30 g/m3 does not provide a sufficient protection against corrosion; a mixture of polyphosphates, some soluble zinc salts, represent a good corrosion inhibitor, even in doses of 12 g/m3. However, if the concentration in zinc ions in the water in the circuit exceeds 2 g/m3, the protective effect is cancelled.
The content in Cl– ions of the water does not have an unfavourable effect on the inhibiting action of the polyphosphates.
In the case of the presence in the circuit of the copper alloys (for instance the steam condensers) the efficiency of the mixtures of inhibitors can be increased by adding organic products; if to the mixture of zinc-sodium polyphosphates one adds 2 g mercapto-2-benztiazol to 1 m3 water, the result is a considerable reduction of the copper ions in the water.
Other cations, such as Al, Cu, Sb, Fe, show an antagonistic action towards the polyphosphates.
An unfavourable effect can also have the metering, in insufficient quantity, of a synergic mixture; in this case the whole insufficient quantity of a synergic mixture is not protected, in this case not the whole metal surface is protected and corrosions can occur. Some mixtures of inhibitors do not contain polyphosphates; for instance, the chromed mixture or the sodium dichromate and a soluble zinc salt, recommended for pH £ 7, in the presence of the polyphosphates precipitate the zinc compounds. If the composition of the mixture drifts from the best one, the corrosions can intensify in the circuit. The sodium nitrite is recommended for waters containing few Cl– ions, otherwise the metering must be much increased, which is not economical. The consumption of nitrite can be reduced by using a mixture of polyphosphates, zinc sulphate and sodium nitrite. The cooling circuit must not be without microorganisms, which consume the nitrite (bacteriological oxidation).
It is therefore required, sometimes, to treat the water in the circuit with a bactericide product (for instance pentachlorophenol and a quaternary ammonium alkali), in order to get protection against corrosion; apart from the case itself of fighting against these microorganisms.
The efficient treatment and a quick effect of corrosion inhibition in the case of a given cooling system, needs a previous cleaning of the latter of settlings, sludge and corrosion products. At the beginning of the treatment a large metering of inhibitor is needed in order to ensure the formation of the complete protecting film in all the areas of the cooling system. This increased metering is applied as long as the concentration of the inhibitor in the circuit shows the tendency to decrease; it is absorbed by the surface of the metal, respectively it is consumed by the impurities in the equipment.
When the protective film was formed, the normal metering of corrosion inhibitors is applied, so that the keeping of the film is ensured.
The temporary larger meterings are necessary in order to restore the film damaged by the modification of the water pH in the circuit.
The control of the circuit is performed by means of the test specimens and of the corrosimeter. A satisfactory corrosion inhibition is characterised by:
- loss of materials, max. 0.033 mm/year.
- absence of punctiform corrosions larger than 0.025 mm.
- minimum settlings and low corrosion rate, after the test of 28 days with the corrosimeter.
Three possibilities of cooling are known for the industrial equipment with the water recirculation in closed and open circuit. Irrespective of the adopted system, at the metal/water contact surface many corrosion or settling processes occur. The initiation and development of these processes occur by means of some chemical and physical factors, that exist permanently in the cooling circuits. Because of the deposits on the surfaces of thermal transfer and of the corrosion caused by the cooling water, the heat exchange is rendered difficult, which disturbs the normal operation of the industrial equipment. The use of the water as cooling agent involved two major problems: purification and the lowering of its aggressiveness. In most cases is used the recycling of the cooling water in open system In this case the water which heated up in the heat exchangers is pumped into the cooling towers, where, in contact with the air and following the partial evaporation it is cooled and then it is reintroduced into the circuit. The evaporated water is replaced continuously with fresh water.
In an open recirculation system where the main cooling source is the evaporation, the content of salts in the water increases, until solubility reasons cause its limitation. The contact of the water with the atmosphere maintains the quantity of oxygen dissolved in the circulation water near the saturation limit. The waters with high content of bicarbonate loose the CO2 in the cooling tower and become very unstable. The gaseous contaminants present in the atmosphere, especially in the industrial atmosphere (SO2, NH3, SO3, H2S etc.) become soluble in the water, increasing its aggressiveness. The spores of algae inoculate easily in the recirculation waters, the cooling towers ensuring ideal conditions for their growth; the algae driven by the waters cause the clogging of the pipes of the pumps, etc. Apart of this, microbiological settlings build up, especially in those parts of the circuit where there is the best temperature for their growth.
Thus, the result is that the surface of the equipment, which is in contact with the cooling water, will cover of settlings formed of mixtures of bacteria and microorganisms or settlings formed by the accumulation of the corrosion products and of the substances resulting following the interaction between the metal and the components of the water.
The cooling water can be also contaminated with oil, detergents, forming in this case emulsions, or with different substances, whose source is the discharges. Usually, agents are added for the control and adjustment of the pH by the neutralization of the acids or of the alkaline substances collected by the waters.
The cooling waters with closed circuit (such as in the case of the cooling of Diesel motors) do not come systematically into contact with the oxygen in the water, but they become corrosive quickly, by the accumulation of salts, especially chlorides and even oxygen. In this case one can use a water subjected to a certain previous purification (distillation, softening) where the so corrosive oxygen was removed, and the access of new quantities of oxygen is stopped by the tight construction of the equipment and by its careful maintenance.
The satisfactory protection of the cooling systems requires more than the simple introduction of the corrosion inhibitors. For the efficient application of the protection, by the inhibition of the aggressive action of the medium, the metal surface must be free from settlings, grease, corrosion products, residues, which prevent the access of the inhibitors to the metal surface. For this reason, the surface will be subjected to a pre-treatment, which must be compatible with the treatment which follows it. The settlings can be removed by treatments which detach and spread them and then they can be removed by water current. The process involves a low activity of the metal surface for the water and a tendency of the particles to form unstable agglomerations. For this purpose one can use substances such as the polyacrylamides, polyacrylic acid and polyphosphates. The pH of the recirculation water must be brought to a favourable value, in order to form a protective film and it must be kept to this value.
As a result, the objectives of the treatment in the cooling system are the following:
- prevention of crust settling on the cooled surfaces.
- reduction of the corrosion of the metal in contact with the cooling water.
In most of the cases, the inhibitors are the most economical procedure for the control of the corrosion of the cooling water. Taking into account the large flow rates of the water in the cooling circuits, to achieve an efficient protection it is necessary to use inhibitors with high activity. In the cooling systems, it is required that the film formed by the inhibitor be thin enough, so that it does not affect the heat transfer, but at the same time to have a good mechanical strength and thermal stability, to resist to the mechanical and thermal stress and in case of destruction to be easily recovered by the excess of the inhibitor in the water.
The fighting against the corrosion of the waters in the open circuit involved difficult problems; the removal of the oxygen from these waters is not economical. On the other hand, the organic inhibitors which form films, such as the amines do not give satisfactory results, as they lose their effectiveness in the presence of the oxygen. The suspensions existing in the water (clay, etc.) can inactivate the inhibitor by adsorption.
When the ratio between the recirculation water and the water discharged from the system is too small, a too big part of the inhibitor is eliminated, and this protection method becomes unprofitable. The cooling waters in the closed circuit can be treated with organic inhibitors, which maintain clean the metallic surfaces, providing permanently a good heat transfer.
The most frequently used corrosion inhibitors for the cooling water systems are the chromates, alkaline polyphosphates, zinc polyphosphates, silicates and the organic compounds: amines, amides, pyridine, carboxylic acids, esters, emulsifying oils, especially in the closed recirculation systems.