The Role of Organophosphates in Cooling Water Treatment

Cooling Water is used in large industries to remove unwanted process heat with the help of heat exchangers, condensers. Due to the continuous contact of water with the metallic surface corrosion, scale, deposition & fouling of the heat transfer surfaces occur. These cause equipment damage and operating losses and sometimes result in costly shutdown of the plant. Addition of chemical inhibitors in controlling these problems.

Chromate based inhibitors along with polyphosphates, have been in use for long and they have been reasonably effective against both corrosion and scaling. But they have their own limitation. Polyphosphates are hydrolyzed to orthophosphate at higher operating temperature of the cooling water circuit, which lead to orthophosphate scale. Orthophosphate formed is also a nutrient for bacterial growth chromates are toxic to aquatic life and ecological concern has been the primary reason for the search for new substances.

Among the various substance stadius organophosphates have been found to be the best. Organophosphate based formulations give comparable protection with respect to corrosion, scaling and fouling and they are hydrolytically stable. This stability of organophosphates permits greater flexibility, during operation, as they are stable over a greater range of pH and at higher temperature, thereby retaining their activity for longer periods of time. The control on various parameters is more relaxed and they are non – toxic. Phosphonate are compatible with most other chemicals used in cooling systems like chlorine, non-oxidizing biocides, silt control chemicals etc. slowly organophosphates are finding more acceptance the world over, primarily because of its freedom from environment problem.

Properties of Organophosphonates: Structurally, organophosphates have the carbon atom directly linked to the phosphorus atom. The two most widely used Organophosphonates in cooling systems are:

“Six phosphate based cooling system corrosion inhibitors were studied for their relative corrosion inhibiting capabilities by an accelerated static test. The static test was standardized by controlling pH and temperature and using an oxidizing accelerator, potassium persulphate. Results indicate that orthophosphate aminotrimethylene phosphonate and hexametaphosphate are the prospective corrosion inhibitors.

Six phosphate based inhibitors selected are Sodium Hexametaphosphate (SHMP), Sodium tripolyphosphate (STPP), tetrasodium pyrophosphate (TSPP), Sodium Orthophosphate (O-PO4), Aminotrimethylene Phosphonate (ATMP) and Hydroxyethylidene diphosphonate (HEDP), these are frequently used in non-chromate based cooling water treatment.

The key properties of Organophosphonates are:

1. Threshold effect and crystal distortion.

2. Hydrolytic Stability.

3. Sequestration characteristics.

4. deflocculation.

5. Chlorine stability

Corrosion Control: Corrosion Control of metallic surface can be obtained by inhibiting the cathodic, the anodic or both these reaction. A combination of Organophosphonates and zinc works synergistically to give very good corrosion protection by interfering with the cathodic reaction. Polyphosphates, when used as corrosion inhibitor, give rise to excessive orthophosphate sludges whereas Organophosphonates with Zinc give good corrosion protection without leading to any sludge formation.

Due to the sequestering ability of Phosphonate, the zinc ions are present in a complexed form limiting the rate of reaction of zinc with hydroxyl ions. Therefore, useful concentration of zinc hydroxide allows the formation of a thin hydroxide film at the surface giving the desired corrosion resistance.

Scale and Fouling Control: Organophosphonates are one of the best deposit control agents presently available. The threshold and crystal distortion property of these compounds interferes with the nucleation of the hardness crystals causing much higher levels of hardness to stay in solution. When scales are formed they are so distorted that they are non-adherent and form very soft sludges. Phosphonate also provide excellent

Control of hydrated ferric oxide deposits which are formed as a result of corrosion. They adsorb on the particle surfaces and reduce the attractive forces between individual iron particles. The sequestering ability of phosphonate enables it to control heavy matter (Fe, Cu & Zn) deposits and this control is far superior to other traditional chelants. Phosphonate also help to disperse suspended particles.

Choice of Phosphonate: From the two most commonly used phosphonate for cooling water treatment HEDP is preferred to ATMP for the following reasons:

  1. ATMP is more corrosive to Cu cooling systems involving Cu or Cu alloys, are therefore very sensitive to ATMP. With ATMP one would then have to use Cu corrosion inhibitors like Thiazoles and Trizoles making the treatment more expensive. The corrosivity towards Cu is due to the fact that a very strong complex with Cu is formed, the dissociation constant of the chelate being about 10 –13.

  2. HEDP has better stability to chlorine than the Nitrogen containing ATMP. Nitrogen containing compound have a tendency to form chloramines. Though, when complexed with Zinc. ATMP exhibits stability towards chlorine it should be used with caution in chlorinated cooling water systems especially when continuous chlorination is used.

    The addition of Zinc to ATMP to a certain extent inhibits the dissolution of copper. In the presence of ATMP, however, the powerful oxidizing potential of chlorine promotes the dissolution of copper, when chlorine is used as a biocide.

    The other phosphonate used to a much lesser extent are, ethylene diamine tetramethylene phosphoric acid, hexamethylene diamine tetramethylene phosphoric acid and diethylene triamine pentamethylene phosphoric acid.

Check out the complete article at http://www.altret.com/templates/images/editor/role-of-organo-phosphate-in-cwt.pdf

Ortofosfatos versus polifosfatos

La selección de un tratamiento de agua aditivo químico de fosfato puede ser una de las decisiones de tratamiento químico más difíciles que muchos de los sistemas públicos de agua harán. Esto es particularmente cierto porque la química de aditivos químicos de ortofosfato y polifosfato es complejo, y aditivos químicos de tratamiento de agua de fosfato están disponibles comercialmente en un número abrumador de mezclas químicas. Ortofosfatos y polifosfatos son sales derivadas de dos formas diferentes de ácido fosfórico. Ortofosfatos son moléculas pequeñas, formadas a partir de la forma más pequeña y la más básica de ácido fosfórico. Los polifosfatos son moléculas más grandes, formados a partir de una versión de cadena más larga de ácido fosfórico. A pesar de que las palabras de ortofosfato y polifosfato contienen la palabra "fosfato", estos dos compuestos químicos sirven radicalmente diferentes propósitos de tratamiento de agua. La falta de un sistema de servicio público a entender las diferencias significativas entre estos dos compuestos de tratamiento podría dar lugar a graves problemas de calidad del agua y las posibles violaciónes MCL. Una selección incorrecta de las mezclas químicas de fosfato mediante un sistema de servicios públicos podría incluso crear graves problemas de salud pública. En los sistemas públicos de agua, ortofosfatos se utilizan para fines de plomo y cobre la corrosión de control. Ortofosfatos reaccionan químicamente con los átomos de plomo y cobre que han lixiviado fuera de la tubería y han entrado en el agua circundante. Esta reacción química de ortofosfatos con átomos de plomo y cobre formas plomo y fosfato de cobre. El plomo y cobre fosfato es entonces electroquímicamente extrae de nuevo sobre la superficie de la tubería, donde se forma un revestimiento duro, resistente al agua en la tubería. Este revestimiento duro, resistente al agua ayuda a prevenir más lixiviación fuera de átomos de plomo y cobre en el agua circundante. La mayoría de los sistemas de servicios públicos han experimentado un éxito mucho mayor con el control de la corrosión de plomo ortofosfato de lo que han experimentado con el control de la corrosión del cobre ortofosfato. Los polifosfatos son agentes que son prácticamente ineficaces contra el plomo y la corrosión del cobre secuestrante. Cuando se secuestra un jurado en un juicio penal, que el jurado se "mantiene en reclusión." Un agente secuestrante químico es un agente químico que rodea otra molécula o átomo y sostiene que otra molécula o átomo "en reclusión." Al rodear la otra molécula o átomo y manteniéndolo en su aislamiento, el agente secuestrante química oculta la molécula o átomo de la vista y evita que entren en diversas reacciones químicas. Como un agente secuestrante, polifosfatos sólo secuestrar metales solubles "invisibles-en-agua" que no se han oxidado en sus formas insolubles. Polifosfato aplica al agua antes de que el agua se clora evitará hierro invisible y manganeso de convertirse en visible después de que el agua es clorada. Como un agente secuestrante, polifosfato de tratamiento de agua se utiliza para secuestrar átomos de hierro solubles que permanecen en agua sedimentada antes de que sea clorado o que de lixiviación fuera de la tubería de hierro en sistemas de distribución de agua. Por circundante y el secuestro de estos átomos de hierro solubles, se les impide la visualización de los colores rojizos típicos asociados con óxidos de hierro e hidróxidos de hierro. polifosfatos de tratamiento de agua también interfieren con la cristalización de y formación de escamas de calcio y carbonato de magnesio, pero no con la cystallization de y formación de escamas de hidróxido de magnesio. Si cualquiera de los átomos de manganeso solubles todavía están presentes en el agua después de que el flóculo se ha asentado fuera, polifosfatos también servirán para secuestrar estos átomos de manganeso solubles, evitando que se presentan el color típico de dióxido de manganeso oscuro. Un error bruto sobre polifosfato es la creencia de que el uso de secuestrantes de polifosfato para ocultar hierro y manganeso es una técnica de tratamiento casual, de rutina para la eliminación del exceso de hierro y manganeso que no se elimina durante los procesos de sedimentación y filtración de una planta de tratamiento de agua. En realidad, el uso de polifosfato para secuestrar hierro y manganeso que una planta no pudo eliminar durante los procesos de sedimentación y filtración es una maniobra desesperación. los quantites adversas de hierro y manganeso en el agua cruda deben estar debidamente oxidados por aireación, permanganato, o el ozono y deben ser depositados en depósitos de sedimentación como parte del floc. Polifosfatos, que secuestran hierro y manganeso por sólo un período limitado de tiempo, no son el ideal o la solución preferida para el hierro de cualquier planta de tratamiento de agua y problemas de manganeso. Los polifosfatos sólo deben utilizarse para atrapar las pocas partículas de hierro y manganeso que se perdieron durante el proceso de aireación y oxidación inicial. La mayoría de los compuestos de tratamiento de fosfato utilizados por los sistemas de tratamiento de agua públicos son en realidad mezclas de polifosfatos y ortofosfatos. Los polifosfatos se añaden habitualmente en su forma polifosfato de sodio o potasio. Ortofosfatos se añaden en forma de ortofosfato de sodio o de potasio o en una forma de ortofosfato que se mezcla con cloruro de zinc o sulfato de zinc. El zinc en la mezcla no juega ningún papel en la formación de los revestimientos que impiden plomo y cobre la corrosión. En cambio, el zinc juega un papel importante en la protección de superficies galvinized (medios galvanizado "recubierto de zinc") y en la prevención de las fibras de asbesto de la erosión fuera de las tuberías de amianto-cemento. Sin embargo, debe tenerse en cuenta, algunas mezclas de fosfato también pueden contener polifosfatos de zinc, pero las formulaciones de ortofosfato de zinc se utilizan mucho más comúnmente en las operaciones de tratamiento de agua pública. mezclas de tratamiento de agua de fosfato están disponibles en diferentes mezclas de decenas. No hay una mezcla perfecta que es de uso universal en todas las situaciones. Si un sistema de tratamiento de agua utiliza una mezcla con más polifosfato de que sus necesidades del sistema, los recubrimientos establecidos por el ortofosfato, se pueden quitar. Si un sistema de tratamiento de agua utiliza una mezcla de fosfato con más ortofosfato de que sus necesidades del sistema, el hierro puede ser despojado de distancia de las tuberías de hierro. plantas de tratamiento de agua deben evaluar regularmente los plomo, cobre, hierro, manganeso, y los niveles de asbesto en su agua y consultar con un especialista fosfato profesional siempre que un cambio en las mezclas de fosfato parece estar justificado. Sales de polifosfato COMUNES: pirofosfato de sodio ácido, pirofosfato de tetrasodio, pirofosfato de tetrapotasio, tripolifosfato de sodio, tripolifosfato de potasio, trimetafosfato de sodio, hexametafosfato de sodio (vítreo) COMUNES SALES ortofosfato: ortofosfato monosódico, monopotásico ortofosfato, ortofosfato de disodio, dipotasio ortofosfato trisódico ortofosfato, ortofosfato tripotásico, ortofosfato de zinc

post original por Bradley C. Williams en http://water-treatment-training.blogspot.com/2010/02/polyphosphates-and-orthophosphates.html

Orthophosphates versus Polyphosphates

The selection of a phosphate water treatment chemical additive can be one of the most difficult chemical treatment decisions that many public water systems will make. This is particularly true because the chemistry of orthophosphate and polyphosphate chemical additives is complex, and phosphate water treatment chemical additives are commercially available in an overwhelming number of chemical blends.

Orthophosphates and polyphosphates are salts derived from two different forms of phosphoric acid. Orthophosphates are small molecules, formed from the smallest and most basic form of phosphoric acid. Polyphosphates are larger molecules, formed from a longer chain version of phosphoric acid.

Even though the words orthophosphate and polyphosphate contain the word "phosphate," these two chemical compounds serve radically different water treatment purposes. A public utility system's failure to understand the significant differences between these two treatment compounds could result in serious water-quality problems and possible MCL violations. An incorrect selection of phosphate chemical blends by a public utility system could even create serious public health problems.

In public water systems, orthophosphates are used for lead and copper corrosion-control purposes. Orthophosphates chemically react with lead and copper atoms that have leached off of piping and have entered into the surrounding water. This chemical reaction of orthophosphates with lead and copper atoms forms lead and copper phosphate. The lead and copper phosphate is then electrochemically drawn back down onto the piping surface, where it forms a tough, water-resistant coating on the piping. This tough, water-resistant coating helps prevent further leaching off of lead and copper atoms into the surrounding water. Most public utility systems have experienced far greater success with orthophosphate lead corrosion control than they have experienced with orthophosphate copper corrosion control.

Polyphosphates are sequestering agents that are virtually ineffective against lead and copper corrosion. When a jury in a criminal trial is sequestered, that jury is "held in seclusion." A chemical sequestering agent is a chemical agent that surrounds another molecule or atom and holds that other molecule or atom "in seclusion." By surrounding the other molecule or atom and holding it in seclusion, the chemical sequestering agent hides the molecule or atom from sight and prevents it from entering into various chemical reactions. As a sequestering agent, polyphosphates will only sequester soluble "invisible-in-water" metals that have not been oxidized into their insoluble forms. Polyphosphate applied to water before the water is chlorinated will prevent invisible iron and manganese from becoming visible after the water is chlorinated.

As a sequestering agent, water treatment polyphosphate is used to sequester soluble iron atoms that remain in settled water before it is chlorinated or that leach off of iron piping in water distribution systems. By surrounding and sequestering these soluble iron atoms, they are prevented from displaying the typical reddish colors associated with iron oxides and iron hydroxides. Water treatment polyphosphates also interfere with the crystallization of and formation of calcium and magnesium carbonate scales, but not with the cystallization of and formation of magnesium hydroxide scales. If any soluble manganese atoms are still present in water after the floc has settled out, polyphosphates will also serve to sequester these soluble manganese atoms, preventing them from displaying the typical dark manganese dioxide color.

A gross misconception about polyphosphate is the belief that the use of polyphosphate sequestrants to hide iron and manganese is a casual, routine treatment technique for the removal of excess iron and manganese that was not removed during a water treatment plant's sedimentation and filtration processes. In reality, the use of polyphosphate to sequester iron and manganese that a plant failed to remove during the sedimentation and filtration processes is a desperation maneuver. Undesirable quantites of iron and manganese in raw water should be properly oxidized by aeration, permanganate, or ozone and should be deposited in sedimentation basins as part of the floc. Polyphosphates, which sequester iron and manganese for only a limited period of time, are not the ideal or the preferred solution for any water treatment plant's iron and manganese problems. Polyphosphates should only be used to catch the few particles of iron and manganese that were missed during the initial aeration and oxidation process.

Most phosphate treatment compounds used by public water treatment systems are actually blends of polyphosphates and orthophosphates. Polyphosphates are usually added in their sodium or potassium polyphosphate form. Orthophosphates are added in the sodium or potassium orthophosphate form or in an orthophosphate form that is mixed with zinc chloride or zinc sulfate. The zinc in the mixture plays no role in forming the coatings that prevent lead and copper corrosion. Instead, the zinc plays a significant role in protecting galvinized surfaces (galvanized means "zinc-coated") and in preventing asbestos fibers from eroding off of asbestos-cement piping. It should however be noted, some phosphate blends may also contain zinc polyphosphates, but zinc orthophosphate formulations are much more commonly used in public water treatment operations.

Phosphate water treatment mixtures are available in dozens of different blends. There is no perfect blend that is universally usable in all situations. If a water treatment system uses a blend with more polyphosphate than their system needs, the coatings laid down by the orthophosphate can be stripped away. If a water treatment system uses a phosphate blend with more orthophosphate than their system needs, iron can be stripped away from iron piping. Water treatment plants need to regularly assess the lead, copper, iron, manganese, and asbestos levels in their water and consult with a professional phosphate specialist whenever a change in phosphate blends seems to be warranted.

COMMON POLYPHOSPHATE SALTS: Sodium acid pyrophosphate, Tetrasodium pyrophosphate, Tetrapotassium pyrophosphate, Sodium tripolyphosphate, Potassium tripolyphosphate, Sodium trimetaphosphate, Sodium hexametaphosphate (glassy)

COMMON ORTHOPHOSPHATE SALTS: Monosodium orthophosphate, Monopotassium orthophosphate, Disodium orthophosphate, Dipotassium orthophosphate, Trisodium orthophosphate, Tripotassium orthophosphate, Zinc orthophosphate

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Cooling Product Testing and Control

With the extremely high cost of molybdenum in recent years, its use as a corrosion inhibitor or tracing agent in cooling water products, where product consumption is significant, has become essentially cost prohibitive. Other corrosion inhibitors such as phosphates, zinc, silicates, and organo- phosphorous compounds are now used largely in the absence of molybdates. Also, the use of molybdenum has been restricted in some areas because of environmental concerns, mostly centered around concentration limitations in municipally generated sludges.

Phosphates

Where orthophosphate or polyphosphates are in use, testing for the phosphate is a good and accurate test. There are a number of phosphate procedures, but all tests determine orthophosphate. Other forms of phosphate such as polyphosphate or organo-phosphates must first be converted to orthophosphate to determine their concentrations with a phosphate test procedure.

Control can become more complicated when there is phosphate in the makeup water. The form of the phosphate (orthophosphate, polyphosphate, or both) and the concentration range needs to be known so that it is accounted for in the cycled cooling water.

Example

Makeup water contains 0.5 ppm of orthophosphate and 0.4 ppm of polyphosphate as PO4. The cooling tower is operated at five cycles of concentration and a cooling water product that contains 4 % of orthophosphate is being applied. The desired inhibitor product dosage is 100 ppm.

At five cycles, there will be 2.5 ppm of orthophosphate from the makeup water orthophosphate, and 2.0 ppm of polyphosphate applied from the makeup water, but some of it will have reverted to orthophosphate. You should test for polyphosphate in the tower water initially and then periodically to determine the reversion rate for your system. Typically, we assume about a 50% reversion rate. The actual reversion rate will depend upon pH and retention time, and the specific type of polyphosphate.

If when tested the polyphosphate showed to be 1 ppm in the cycled tower water, then the total orthophosphate from the makeup would be 3.5 ppm. 100 ppm of the inhibitor product would add 4 ppm orthophosphate, so a tested residual of 7.5 ppm or orthophosphate would indicate that 100 ppm of the product was in the system.

Table 1: Phosphate Summary

Phosphate Concentrations

Orthophosphate (ppm)

Polyphosphate (ppm)

Makeup Water

0.5

0.4 as PO4

Tower Water,
5 Cycles Before Reversion

2.5

2.0

Tower Water, 5 Cycles After Reversion

3.5

1.0

Orthophosphate From Product

4.0

Total in Cycled Tower Water

7.5

1.0

 

Phosphonates

Most all cooling tower products contain one or more phosphonates that are used for scale inhibition, corrosion inhibition, or both. Phosphonate testing is not as accurate as phosphate testing, but they can be used for controlling product feed. Phosphonates are subject to oxidation to orthophosphate by chlorine or bromine and are lost to precipitation with cations such as calcium. If the system is chlorinated or brominated, assume a 20 – 30% degradation to phosphate. The actual amount can be determined by testing for residual phosphonates and phosphate.

There are several phosphonates tests that can be used:

Hach UV digestion, then phosphate test.
Boiling with acid and persulfate, followed by phosphate test. Palintest drop test.
Taylor drop test.

UV Digestion

The test procedure is the most accurate and has a reproducibility of about ± 10%. A persulfate reagent is used along with a UV light to decompose the organo-phosphate (phosphonate) to orthophosphate. An orthophosphate test procedure then determines the amount of phosphate contributed by the phosphonates. Any orthophosphate already present before the digestion is subtracted from the total orthophosphate after digestion. This can be done by adding reagent to the tower water that has not had the digestion and use this as the blank, or actually determine orthophosphate in the tower water and subtract it from the total orthophosphate determined after the persulfate digestion.

The amount of phosphorus in each specific phosphonate molecule varies, so there is a specific conversion factor from orthophosphate to phosphonate. Each ppm of orthophosphate created by HEDP digestion = 1.085 ppm HEDP. The phosphorus content of PBTC is much lower. Each ppm of orthophosphate created from the digestion of PBTC = 2.84 ppm of the PBTC molecule.

Phosphonate Test Example:

The cycled tower water has 6 ppm orthophosphate and a cooling water product that contains 2.5% PBTC and 1.8% HEDP is being applied at a desired dosage of 120 ppm.

Assuming all of the phosphonates remain as phosphonates and have not been oxidized in the cooling tower by bromine or chlorine and assuming it has not been lost to precipitation, you should get 3.05 ppm of orthophosphate from the phosphonates after a persulfate / UV digestion.

Table 2: Phosphate Summary

From PBTC: 120 ppm x 2.5% = 3 ppm
3 ppm PBTC
÷ 2.84 ppm PBTC per ppm PO4 =

1.06 ppm orthophosphate

From HEDP: 120 ppm x 1.8% = 2.16 ppm

2.16 HEDP ÷ 1.085 ppm HEDP per ppm PO4 =

1.99 ppm orthophosphate

From orthophosphate in the tower water:

6 ppm

Total orthophosphate in sample after digestion:

9.05 ppm

Orthophosphate from phosphonate digestion:

3.05 ppm

 

Boiling With Acid and Persulfate

A digestion can also be accomplished by adding acid and persulfate, then boiling for about 30 minutes. If just acid were used, only polyphosphate would be hydrolyzed or reverted to orthophosphate. It persulfate is also added, the organo-phosphates and polyphosphates will be digested to orthophosphate. This test would be more applicable for samples that do not have polyphosphates, since the test will not distinguish between orthophosphate developed from phosphonates or polyphosphates.

Phosphonate Drop Counts

We recommend the Palintest procedure. This procedure is less accurate and subject to interferences. It is best to determine the number of drops on a known product concentration and relate the number of drops to that concentration. It is advisable to also compare these results initially and periodically to the digestion method.

Where PBTC is in use, the Palintest method is preferred. The procedure buffers the pH to around 3.0 and is more effective at detecting the PBTC along with the HEDP and AMP.

On the Palintest method, each 0.7 ppm of HEDP or AMP in the water should require one drop of titrant, and each 2.0 ppm of PBTC should require one drop.

Polyphosphate and some organics will interfere with the test and show up as phosphonates. To account for this, a blank is run on the makeup water. If it takes two drops for the color change on the blank, then those two drops are subtracted from the test results of the treated water. Note that the blank results are not cycled up by the tower cycles. Polyphosphates revert to orthophosphate which does not interfere and experience has shown that cycling the blank should not be done. If the product contains polyphosphate and a residual in the cycled water, it will increase the number of drops required.

If fluorides are in the cycled tested water at > 1.0 ppm, this causes a substantial interference that may disqualify the drop test procedure from being usable. It is advisable to check with the city supplier to see if they add fluorides and at what level. If high fluorides are present, an idea that may work is to first run the drop test procedure on the tower water to get a baseline number. Then take a sample of the cycled tower water and add 100 ppm of product and see how many drops are required. Subtract the number of drops used for the baseline from the drops required for the 100 ppm sample to determine how many drops represent 100 ppm of product as a basis for setting control limits.

The Palintest end point is the drop when the color change from green/gray to blue/purple first occurs.

Palintest Phosphonate Drop Count Example:

The cooling water is treated with 140 ppm of a product that contains 2.5% PBTC and 1.8% HEDP. The product has a specific gravity of 1.16. There is no fluoride in the water.

First, determine the interferences in the makeup water by running the test procedure on an untreated sample. On this example assume it took two drops.

Next, make a 100 ppm solution. To do this add 1 gram or 0.86 mL (1mL/1.16 gram/mL) of the chemical product to 99 grams (99 mL) of makeup water. Mix this up well, then add 1 gram (1 mL) of this 1% solution to 99 grams (99 mLs) of makeup water. This is now a 0.01% solution or 100 ppm of the product. This would place 1.8 ppm of HEDP and 2.5 ppm of PBTC in the solution. Run the phosphonates test on this solution, and for this example it required the theoretical number of drops of about 6.

Table 3: Theoretical Phosphonate Titrant Usage

From HEDP: 1.8 ppm ÷ 0.7 ppm HEDP / Drop

2.5 Drops

From PBTC: 2.5 ppm ÷ 2.0 ppm PBTC / Drop

1.25 Drops

From Blank:

2 Drops

Total Drops:

5.75 drops, which will require 6 drops to see the color change.

140 ppm of product would be about (140 ÷ 100) x 4 drops = 5.6 drops or 5-6 drops + 2 drops for the blank = 8 drops. This can be confirmed by making a 140 ppm solution and testing it.

Azole, Zinc, or Silica Tests

The Hach test procedures for azole, zinc, or silica can be used to check product dosage if the specific ingredient is in the applied product. Remember, as with phosphonates, the applied concentrations and actual residuals can be different. Azole residuals decrease as they film with copper. Zinc is lost as it precipitates at the cathode or in the bulk water. Silica is lost as it films metal surfaces. In establishing control ranges and dosages, take into account some of this loss. For example, we may apply azole at 2 ppm, but have a desired residual in the water of only 1 ppm.

Mass Balance

Chemical dosages should be confirmed by mass balances and compared to chemical testing. Mechanisms should be set up on each system to conveniently determine water makeup, cycles, water loss, and chemical consumption. The concentration in the recirculating water should be calculated from the actual product usage and blowdown or water loss.

Mass Balance Example:

The cooling tower is operating at five cycles of concentration. The makeup meter shows 120,000 gpd makeup. At five cycles, this is a water loss of 24,000 gpd. The product being fed contains 1.8% HEDP, 2.5% PBTC, 1.5% BZT, and 1% zinc; and the desired dosage is 100 ppm.

Daily product use determined by drum level and confirmed with drawdown cylinder testing is 28 lbs per day. This is a calculated applied dosage of 140 ppm of product in the cycled cooling tower water (140/120 x 24,000/1000 = 28 lbs).

Chemical testing showed 4 drops of phosphonates (6 drops from the test – 2 drops for the blank), which was previously determined to represent 100 ppm product. Testing also revealed 1.5 ppm BZT and 0.8 ppm of zinc residuals in the water. All of the chemical tests show that some portion of the active component has been consumed or residuals would have been higher at 140 ppm of applied product.

Product Component

Expected Residuals with No Loss When Applied at 140 ppm

Calculated Dosage Based on Actual Residual

Product Loss to System Reactions

Phosphonate

8 drops

6 drops

= 100 ppm Product

40 ppm Product

BZT

2.1 ppm

1.5 ppm BZT

= 100 ppm Product

40 ppm product

Zinc

1.4 ppm

0.8 ppm Zinc

= 80 ppm Product

60 ppm Product

 

Conclusion

Mass balance is the most accurate way to determine applied dosage. If the product dosage was projected to be effective at 100 ppm, it is likely that this product is being overfed by 40%. Chemical testing suggests that there is more than sufficient residual of active components even after some loss to the system, so product dosage can be lowered and results monitored to confirm that desired results are maintained. There is expected to be some loss of active components as they react with the materials in the system and the impurities in the water.

Where molybdate is used or has been used as a monitoring method for product control and consumption, generally its loss to the system is minimal. That means that if the product shown above contained 1% molybdate as Mo,

it is likely that the test results would have been very close to 1.4 ppm Mo and the product dosage would have been decreased to 100 ppm to lower Mo to 1.0 ppm. Molybdate used as a tracer, then, would commonly yield a lower product usage rate because the other active components would not ordinarily be used to control the dosage.

Reduccion del uso de agua en torres de enfriamiento con automatización

Con la iniciativa del Estado para reducir el consumo de agua en un 20 por ciento para el año 2020, muchas plantas en California están tratando de ser más respetuosos con el medio ambiente. Una de esas instalaciones incluye un hospital líder en California, que trató de reducir los costos de tratamiento de agua para su sistema de climatización. El hospital cuenta con tres sistemas de torres de enfriamiento individuales que dan servicio a tres enfriadores centrífugos, con un total combinado de 2.800 toneladas de capacidad.

El programa de tratamiento de agua actualmente en uso en la instalación estaba operando a 2,8 ciclos de concentración, resultando en 35,7 por ciento de la composición agua de la torre se sangró a la alcantarilla por el proveedor de tratamiento actual. Teniendo en cuenta la calidad del agua en la zona, estos eran los ciclos máximos de concentración que podrían lograrse sin emplear el uso de reblandecimiento ácido o agua. El ahorro que el hospital solicitó se realizaron mediante la revisión de varias formas de optimizar el programa de tratamiento de agua. Trabajando en estrecha colaboración con el Departamento de Agua y Energía (LADWP) Los Ángeles, se reveló que mediante la introducción de un programa de conservación de agua para reducir el uso del agua a través de mayores ciclos de concentración, la instalación realidad ahorraría más dinero que se gastaría para alterar el programa , por lo que el proyecto propuesto sostenible.

A través de pruebas y análisis de laboratorio, el equipo fue capaz de concluir que seis ciclos de concentración podrían alcanzarse, resultando en sólo el 16,7 por ciento del agua de maquillaje torre siendo desangrado en el sistema de tratamiento de alcantarillado. Esto se podría lograr mediante la introducción de un sistema de alimentación de ácido seguro que minimizaría escala, la corrosión y el ensuciamiento microbiológico para permitir el aumento de ciclos de concentración al mismo tiempo proteger personal de la instalación entre en contacto con los productos químicos.

La evaporación de la torre de refrigeración sigue siendo el mismo, pero el agua de Estados Unidos fue capaz de reducir la purga, cortando el consumo de agua en un estimado de 3.6 millones de galones por año y la disminución de los costos de agua y alcantarillado. La planta fue capaz de ahorrar más de $ 76.000 (ver Fig. 1).

química en cualquier momento en la torre de refrigeración está estresado por la adición de más ciclos, se requiere un control estricto de la química para evitar la formación de incrustaciones. Esto llevó a la introducción de controles de automatización avanzada de agua de Estados Unidos. El programa de automatización avanzada incluye notificaciones de vigilancia y alarmas inalámbricas para gestionar el rendimiento general del programa, y ​​el equipo de conductividad, pH, los niveles de inhibidor de incrustaciones, el uso de la torre de maquillaje, y la utilización de la torre de purga monitoreado.

En un momento dado, el personal del hospital y los representantes designados de agua de Estados Unidos, utilizando varios niveles de seguridad de la contraseña-protegida indicado por la instalación, se puede acceder de forma segura los datos para la revisión y ajuste en línea. Si los parámetros designados cayeron encima o por debajo del intervalo especificado, un representante de aguas US fue alertado para una respuesta rápida (véase Fig. 3).

Segundo para riego, torres de enfriamiento ofrecen el mayor potencial de ahorro de agua en California. Como un incentivo adicional, el estado de California ha puesto en marcha programas para rebajar la instalaciones para el coste de la automatización de sus sistemas. LADWP y el Distrito Metropolitano de Agua (MWD), por ejemplo, ofrecen tres programas que financian la automatización de las torres de enfriamiento debido a su capacidad para aumentar ciclos de concentración, lo que reduce el consumo de agua.

 

Esta permitido la financiación de agua de Estados Unidos para implementar el programa de automatización avanzada $ 34.000 a monitorear y controlar el programa de tratamiento de agua para este hospital sin costo alguno para el hospital.

Los resultados hasta la fecha para la instalación incluyen la reducción significativa en el consumo de agua, el agua baja y las facturas de aguas residuales y un control más eficiente debido a la automatización de software instalado para proteger los bienes de equipo.

You will find this article here: http://www.waterworld.com/articles/iww/print/volume-14/issue-5/columns/case-study/hospital-reduces-water-usage-in-cooling-towers-with-automation.html

Boiler Carryover – Cause, Effect and Prevention

Mechanisms

carryover or primingCarryover also known as priming is any solid, liquid or vaporous contaminant that leaves a boiler with the steam. In low/medium pressure boilers (<100 bar) entrained boiler water is the most common cause of steam contamination.

Both mechanical factors such as boiler design, high water levels, load characteristics and chemical factors such as high solids concentration, excessive alkalinity, presence of contaminants contribute to the creation of carryover.

Two of the most common mechanical causes of carryover are operation in excess of design load and sudden increases in load.

Foaming is one of the mechanisms of chemical carryover. Foaming tendencies are increased with increases in alkalinity and solids content. Stable foam bubbles contain boiler solids and are carried forward with the steam giving rise to carryover.

Oil and other organic contaminants can react with boiler water alkalinity to give crude surface active materials which cause foaming and carryover.

Effects

Boiler water solids carried over with steam will form deposits in non-return and other control valves. Process streams can be contaminated by carryover affecting product quality.

Deposition in superheaters can lead to failure due to overheating and corrosion.

Steam turbines are potentially prone to damage by carryover as deposits on turbine blades creates imbalance reducing efficiency and capacity. Solid particles in steam can lead to erosion and corrosion in both turbines and other equipment.

 

Prevention of Carryover

The prime means of preventing carryover is to have good mechanical steam separation devices. For low/medium pressure fire tube boilers where steam purity is not stringent, gravity separation is normally satisfactory. (At least 14 bar and saturation conditions the density of water is 115 times greater than that of steam). As steam pressure rises the difference in density reduces (at 69 bar water is only 20 times more dense than steam) making gravity separation less effective. Steam separators are then used to improve purity and are usually installed in the steam drum of water tube boilers.

Primary separators utilise the difference in density as the means of separation bypassing steam through a series of baffles which reduces turbulence or centrifugal (cyclone) separators.

Secondary separators, where steam is directed in a frequently reversing pattern through a large contact surface. A mist of boiler water collects on the surface and is drained from the unit.

Control of boiler water chemistry is essential to minimise carryover and allow mechanical separation to work effectively. The parameters that must be controlled are:

  • Total dissolved solids
  • Alkalinity
  • Silica
  • Organic contamination.

These should be maintained within the boiler manufacturer guidelines or those of BS 2486.

Whenever carryover is being caused by excessive boiler water concentrations an increase in boiler blowdown rate is normally the simplest and most expedient solution. If carryover is still occurring and increasing blowdown is uneconomic then the addition of antifoam agents can economically reduce carryover. Use of an antifoam may allow the boiler to operate at higher water concentrations, Feedwater offer a product called Defoamer C which is suitable for this job, for more information visit the product page for product usage guidance.

Read more at https://feedwater.co.uk/boiler-carryover-cause-effect-prevention/

 

Fundamentos de corrosion en sistemas con agua

Sistemas de análisis de líquidos y sensores están cuestan herramientas eficaces contra la corrosión.

 

Agua plus de metal es igual a la corrosión. Esta realidad ataca la línea inferior de cada planta de generación de energía de vapor impulsado en el mundo.

En una planta de energía de vapor, agua de alta pureza se calienta y se hierve para producir vapor, que energiza y acciona una turbina para producir electricidad.

El agua y el vapor están en constante contacto con las superficies metálicas que amenazan la integridad de equipos de la planta como condensadores, calentadores, bombas, tuberías, calderas y turbinas.

Afortunadamente, purificación de agua y producto químico de tratamiento a reducir y controlar la corrosión en la planta en gran medida. Asegurar buena química ciclo para evitar la corrosión, sin embargo, requiere mediciones precisas y continuas de análisis en el tren de desmineralización, agua de enfriamiento, el condensado, y la caldera de agua de alimentación y los sistemas de vapor.

Si bien las directrices dadas a continuación abordan las necesidades de una planta de generación de energía de vapor impulsado, también pueden ser útiles en otras instalaciones de fabricación en donde el agua juega un papel importante.

La corrosión se produce cuando los iones metálicos transfieren de un metal de base al agua y se combinan con el oxígeno para convertirse en hidróxidos e hidróxidos de metal sólido. partículas resultantes a menudo viajan a otras partes del sistema y se depositan.

Reaccion de corrosion

 

El depósito es un mal conductor

Una vez que se forma un depósito, que atrae más sólidos en suspensión y el depósito crece. Depósitos con frecuencia se acumulan en las superficies de intercambio de calor, tubos de calderas, y calentadores.

El depósito es un conductor pobre de calor que el metal y, por lo tanto interfiere con la transferencia de calor a través del tubo. Esto reduce la eficiencia global del ciclo y puede causar fallos de sobrecalentamiento del tubo locales. Los depósitos también pueden reducir significativamente la eficiencia de las turbinas y, a su vez, se convierten en sitios de corrosión cuando se disuelven los sólidos atrapados en el concentrado de depósito como el líquido hierve lejos. Eventualmente, la concentración alcanza niveles muy corrosivos y graves deficiencias de depósito se produce la corrosión.

Una película de óxido resistente que protege el metal de base es la mejor manera de defender hierro y cobre de la corrosión. Para el hierro y el acero al carbono, la película protectora es magnetita.

Para las aleaciones de cobre y de cobre, la película protectora es óxido cuproso. Esta película funciona sólo en la presencia de la química del agua adecuadamente controlado.

la química del agua adecuada también asegura que la película no se desgasta y, si se produce una ruptura, la película se repara rápidamente.

El control de la química del agua requiere el mantenimiento de agua de alta pureza, el control de pH, el seguimiento de las cantidades de trazas de oxígeno disuelto, y, si es necesario, controlar la alimentación de un agente de eliminación como hidrazina.

Tren de desmineralización

La primera línea de defensa contra la corrosión en una planta de energía de vapor es el uso de agua de alta pureza. La producción de que el agua es la función del tren de desmineralización, que convierte el agua en bruto que contiene entre 100 y 1.500 ppm de sólidos disueltos en el agua que contiene no más de 10 a 20 ppb sólidos disueltos. Los pasos de tratamiento pueden incluir filtración, ablandamiento, la eliminación de cloro, ósmosis inversa, desgasificación, y el intercambio de iones.

ósmosis inversa eficaz (RO), en la que las fuerzas de agua a través de una membrana semi-permeable, puede eliminar aproximadamente el 98% de las sales disueltas y de sílice en el agua en bruto y moléculas orgánicas casi todas las grandes. Ponerse en contacto con sensores de conductividad colocados en el agua de alimentación y del permeado de la RO permiten operadores de plantas de supervisar la calidad del agua y la eficiencia general del sistema de RO.

Las mediciones de conductividad en RO impregnan y agua de alta pureza no son simples, sin embargo. La calibración de sensores es complejo y debe tener lugar mediante la comparación del sensor contra un Instituto Nacional de Estándares y celular calibrada trazable Tecnología (NIST) de una constante de celda conocidos o mediante la calibración del sensor en una solución certificada. Sin embargo, tras la exposición a la atmósfera, patrones de conductividad de alta pureza y falta de agua a través de la absorción de dióxido de carbono del aire circundante y cualquier residuo en el recipiente de muestra. Para evitar la contaminación, puede ser deseable usar sensores de pre-calibrados a los estándares NIST. instrumentos de validación de conductividad están disponibles que se conectan al proceso a través de la tubería, eliminando los efectos de la atmósfera en la medición.

Típicamente, agua de alimentación a un sistema de RO se someterá a tratamiento y ya contendrá los productos químicos para asegurar un funcionamiento óptimo. Estos productos químicos, sin embargo, requieren una vigilancia cuidadosa, o pueden atacar a las membranas de OI. Esto es particularmente cierto si el agua de alimentación se encuentra fuera del intervalo ácido deseado. Los operadores de planta requieren sensores de pH de uso general para mantener la acidez leve en el agua de alimentación. El cloro puede estar en el agua de alimentación en algunas plantas como un biocida o necesita la eliminación en otros por medio de un lecho de carbón porque ataca las membranas de OI. Sin embargo, lechos de carbón alcanzan la saturación con el tiempo, por lo tanto, los monitores de cloro detectan avance de cloro.

La ósmosis inversa sola rara vez puede producir agua de pureza suficiente para el maquillaje. El permeado de RO se suele pulido usando un intercambiador de iones (IX). Estos sistemas consisten en tanques que contienen perlas de resina tratados selectivamente para adsorber o bien cationes o aniones. A los intercambios de cama de cationes cargados positivamente iones (tales como calcio, magnesio y sodio) para el hidrógeno, y los intercambios de cama anión cargado negativamente iones (tales como cloruro, sulfato y bicarbonato) para hidroxilo. El hidrógeno desplazados e hidroxilo se combinan para formar agua pura. Después de una cierta cantidad de uso, estos sistemas se agotan y deben ser regeneradas usando ácido sulfúrico o clorhídrico para la resina de catión y el hidróxido de sodio para aniones. El seguimiento de la concentración de ambas de estas sustancias debe ocurrir continuamente con sensores de conductividad medir el regenerante medida que entra en el tanque. Durante enjuague, las mediciones de conductividad toroidales realizadas en el efluente del lecho determinar qué tan bien enjuagados los regenerantes son.

 

Las variaciones en el diseño de la torre de enfriamiento

En el condensador, la recirculación de agua de refrigeración convierte turbina de vapor de escape en el condensado. El agua de enfriamiento generalmente contiene altos niveles de sólidos disueltos, y las fugas de agua de refrigeración en el ciclo de vapor es una fuente importante de contaminación.

Las fugas introducen iones que aumentan la conductividad y aumentar la corrosividad del agua de alimentación, caldera de agua y vapor. Para dar indicación temprana de fugas y para supervisar el rendimiento del condensador en general, la conductividad de cationes de los registros de descarga de la bomba de condensado en un sensor de conductividad de flujo a través.

Además, el seguimiento de condensado y de agua de alimentación pureza requiere medir la conductividad de cationes. Después de que el condensado pasa a través de la columna de cationes, la conductancia de los aumentos de sal contaminantes, ya que convierte a un ácido significativamente más conductor.

Hay un mayor énfasis en la industria de la reutilización de agua de refrigeración mediante torres de refrigeración. El efecto de enfriamiento viene por la evaporación de una pequeña fracción de intercambio de agua y el calor con el aire que pasa a través de la torre de refrigeración. Como el agua se evapora, sin embargo, los sólidos disueltos se concentran, provocando finalmente que la escala y la corrosión en el equipo de intercambio de calor. Aunque hay muchas variaciones en el enfriamiento de diseño de la torre, una característica común es el control de la calidad del agua con el uso de mediciones de conductividad y pH continuos para mantener un conjunto dado de condiciones. Un sensor de conductividad en contacto mide la concentración relativa de las impurezas en el agua. El analizador de ese sensor inicia la apertura de una válvula de purga cuando la conductividad es demasiado alta. A continuación se introduce la pureza del agua más alta de maquillaje que reduce la conductividad.

Como la mayoría de impurezas en el agua de refrigeración son alcalinas, una pequeña cantidad de ácido sulfúrico se agrega en al agua en circulación para bajar el pH y por lo tanto prevenir la formación de incrustaciones. La medición de este concentración de ácido sulfúrico y manteniendo el pH por debajo de siete, donde es menos probable que ocurra de escala (como se indica por el índice de Langelier), se logra mejor por un sensor de pH de propósito general. agua que contiene un alto nivel de sólidos en suspensión de refrigeración, sin embargo, requiere el uso de sensores de pH más especializados más resistente al ensuciamiento.

El condensado de agua de alimentación

La torre de refrigeración se convierte en vapor de agua en el agua después de salir de la turbina. El agua de reposición del tren desmineralización se suma a esta agua para convertirse en agua de alimentación, que bombea a través de una serie de calentadores a la caldera. El control de la corrosión en el condensado y el sistema de alimentación de agua se logra generalmente en una de dos maneras, todo tratamiento volátil (AVT) y el tratamiento oxigenada (OT). AVT utiliza amoníaco para controlar el pH y la hidrazina para proporcionar un ambiente reductor para la protección de aleaciones de cobre. AVT requiere la medición de amoniaco, oxígeno disuelto, y la hidrazina. medición de amoníaco puede ocurrir ya sea directamente o indirectamente de pH y conductividad. El método indirecto es útil porque el amoníaco reacciona en agua para producir iones hidróxido. Tanto la conductividad, que es una medida de los iones en soluciones, y pH, que es una medida indirecta de iones hidróxido, puede combinar para producir la concentración de amoníaco.

OT utiliza amoníaco para controlar el pH y rastrear de oxígeno para proporcionar un ambiente ligeramente oxidante que promueve la formación de una película de óxido modificado resistente. La calidad del agua para OT es más estricta que para AVT, lo que requiere la conductividad de cationes de menos de 0.15 micro Siemens / centímetro. Es necesario para medir el oxígeno disuelto, pH, y la conductividad de cationes en sistemas de agua de alimentación utilizando el método de OT. medición del pH puede ser difícil en agua de baja conductividad y requiere el uso de tecnología que fluye referencia. Una medición de pH requiere continuidad eléctrica entre la referencia y electrodos de vidrio y un camino a la tierra solución. agua de alta pureza no proporciona suficiente conductividad para completar de forma fiable estos caminos y causa potencial de unión que registra la deriva como errático y compensado en la medición de pH. Una referencia que fluye elimina este efecto mediante la estabilización de la potencial de unión. Esta medición se lleva a cabo en una línea de derivación con el fin de preservar la calidad de la alimentación de agua y preferiblemente en una cámara de medición de acero inoxidable para disipar la corriente electrostática generada por el agua de alta pureza. Desde alta pureza pH es de flujo sensible, las tasas de flujo debe ser muy bajo y constante.

tratamiento con vapor de agua de la caldera

La caldera es el punto de recogida final para todos los contaminantes corrosivos y escala productoras generados aguas arriba. corrosión sólido aterriza en las superficies de los tubos de la caldera y crece mediante la recopilación de más materia suspendida. Eventualmente, el sobrecalentamiento y producir fallo de los tubos. El mantenimiento de una película de óxido protectora es la forma óptima para limitar la corrosión del agua, y esto ocurre más fácilmente cuando el mantenimiento de una baja concentración de sólidos disueltos en un entorno de pH ligeramente alcalino. Para lograr esto, la medición continua tanto de pH y conductividad tiene que ocurrir. se requiere medidas de conductividad, la concentración de sólidos disueltos y un sensor de conductividad de larga duración. Para mantener el ambiente alcalino necesario, las plantas de energía comúnmente tamponar el agua de la caldera con sales de hidróxido de sodio y fosfato de sodio. La sobrealimentación o subalimentación de estos productos químicos pueden ser perjudiciales, sin embargo, y las mediciones de pH y fosfato, por lo tanto precisas son críticas.

agua de la caldera también se somete a tratamiento con el fin de producir vapor de agua de alta pureza. Impurezas en el agua de la caldera de la caldera y de tambor de arrastre en forma de vapor, que se deposita sobre la turbina y causa daños por erosión. La sílice es el contaminante más notoria, y es necesario medirlo en el agua de la caldera y el vapor. Las sales tales como sales de hidróxido de sodio y amoníaco también se vaporizan en el vapor y el flujo en la turbina, donde se precipitan, se concentran, y se convierten en altamente corrosivo. Para controlar la contaminación en el vapor, la medición de la conductividad del agua de la caldera debe suceder, que mide indirectamente los sólidos disueltos. A continuación, purga controla la cantidad de contaminación.

Por lo tanto, para evitar la corrosión incontrolada que cuesta los mil millones de la industria eléctrica de dólares cada año, monitorear la calidad del agua y controlar rigurosamente que la calidad de forma continua.

sistemas de análisis de líquidos y sensores son de trabajo duro, fácil de usar, cuesta herramientas eficaces cuando se mide contra el impacto de la corrosión en los costes y operaciones de la planta.

Mientras que cada planta es diferente, se requiere generalmente una gran variedad de instrumentos de detección de pH y conductividad para prácticamente cada paso del proceso de generación de fuerza de vapor.

Más allá de eso, las plantas individuales requerirán oxígeno disuelto, el ozono, cloro, y otras mediciones más especializados.

Muchas plantas están optando por sistemas de control digital centralizada para controlar continuamente la salida de los analizadores y automatizar muchas funciones de control. Esto reduce el impacto sobre el personal y permite la gestión de control de la corrosión para funcionar como una máquina bien engrasada.

Lo más importante, la clave para el control de la corrosión éxito es la continuidad de la medición.

Las muestras individuales y otras técnicas de medición periódicas son inadecuados para la tarea. Sólo continua, análisis en tiempo real ofrece la garantía de la calidad del agua que requiere control de la corrosión.

Detrás del carril

pH detectar una persecución venerado

En el siglo decimosexto, alquimista Leonard Thurneysser descubrió que el matiz de la savia violeta cambió con la adición de ácidos sulfurosos o sulfúrico. Este indicador temprano fue ampliamente utilizado a través de los siglos posteriores para detectar ácidos.

Con introducción de la teoría iónica en la década de 1880 de Svante Arrhenius, se desarrollaron las primeras teorías referentes a la disociación de ácidos y bases. Johannes Bronsted, que postularon que los ácidos y bases son sustancias capaces de cualquiera de donar o aceptar iones de hidrógeno, refinó aún más estas teorías iniciales.

Por 1904, Hans Friedenthal había establecido con éxito la primera escala de clasificación de ácidos mediante la determinación de las constantes de disociación para los ácidos débiles, de acuerdo con la conductividad y la correlación de los cambios de color que corresponden a diferentes concentraciones de iones hidrógeno utilizando 14 colorantes que indican.

Los números de la concentración de iones de hidrógeno a partir de los cálculos de Friedenthal eran pequeños y difíciles de manipular. Por lo tanto, Lauritz Sorensen sugirió utilizar el logaritmo negativo de estos números, que él dobló el "exponente de hidrógeno" o "pondus Hydrogennii."

Esto llevó al desarrollo de la expresión del pH y la creación de la escala de pH moderna.

 

 

 

 

Originó publicada en: https://www.isa.org/standards-and-publications/isa-publications/intech-magazine/2005/may/sensing-ph-controlling-ph/

Fundamentals of corrosion control in water systems

Liquid analysis systems and sensors are cost effective tools against corrosion.

Water plus metal equals corrosion. This reality attacks the bottom line of every steam driven power generation plant in the world.

In a steam power plant, high purity water is heated and boiled to make steam, which energizes and powers a turbine to produce electricity.

Water and steam are in constant contact with metal surfaces threatening the integrity of plant equipment like condensers, heaters, pumps, piping, boilers, and turbines.

Fortunately, water purification and chemical treatment greatly reduce and control the corrosion in the plant. Ensuring good cycle chemistry to prevent corrosion, however, requires accurate and continuous analytical measurements in the demineralization train, cooling water, condensate, and boiler feed-water and steam systems.

While the guidelines given below address the needs of a steam driven power generation facility, they can also be useful in other manufacturing facilities where water plays an important role.

Corrosion occurs when metal ions transfer from a base metal to water and combine with oxygen to become hydroxides and solid metal hydroxides. Resultant particles often travel to other parts of the system and are deposited.

Rust reaction
Rust reaction

Deposit is a poor conductor

Once a deposit forms, it attracts more suspended solids and the deposit grows. Deposits frequently accumulate on heat exchange surfaces, boiler tubes, and heaters.

The deposit is a poorer conductor of heat than metal and therefore interferes with heat transfer across the tube. This lowers the overall cycle efficiency and can cause local tube overheating failures. Deposits can also significantly lower the efficiency of the turbines and, in turn, become corrosion sites when dissolved solids trapped in the deposit concentrate as the liquid boils away. Eventually, the concentration reaches highly corrosive levels and severe under-deposit corrosion occurs.

A tough oxide film that protects the base metal is the best way to defend iron and copper from corrosion. For iron and carbon steel, the protective film is magnetite.

For copper and copper alloys, the protective film is cuprous oxide. This film works only in the presence of properly controlled water chemistry.

Proper water chemistry also ensures that the film won't wear away and, if a break occurs, the film quickly repairs itself.

Controlling water chemistry requires maintaining high purity water, controlling pH, monitoring for trace quantities of dissolved oxygen, and, if necessary, controlling the feed of a scavenging agent like hydrazine.

Demineralization train

The first line of defense against corrosion in a steam power plant is the use of high purity water. Producing that water is the function of the demineralization train, which converts raw water containing between 100 and 1,500 ppm dissolved solids into water that contains no more than 10 to 20 ppb dissolved solids. Treatment steps may include filtration, softening, chlorine removal, reverse osmosis, degasification, and ion exchange.

Efficient reverse osmosis (RO), in which water forces through a semi-permeable membrane, can remove approximately 98% of the dissolved salts and silica in raw water and nearly all large organic molecules. Contacting conductivity sensors placed in the feed water and the permeate of the RO let plant operators monitor the water quality and overall efficiency of the RO system.

Conductivity measurements in RO permeate and high purity water are not simple, however. Calibration of sensors is complex and must take place by comparing the sensor against a National Institute of Standards and Technology (NIST) traceable calibrated cell of a known cell constant or by calibrating the sensor in a certified solution. However, upon exposure to the atmosphere, high purity conductivity standards and water foul through the absorption of carbon dioxide from the surrounding air and any residue in the sample container. To prevent contamination, it may be desirable to use sensors pre-calibrated to NIST standards. Conductivity validation instruments are available that connect to the process via tubing, eliminating the effects of the atmosphere on the measurement.

Typically, feed-water to an RO system will undergo treatment and will already contain chemicals to ensure optimum operation. These chemicals, however, require careful monitoring, or they may attack the RO membranes. This is particularly true if the feed-water is outside the desired acidic range. Plant operators require general-purpose pH sensors to maintain mild acidity in the feed-water. Chlorine may be in the feed water in some plants as a biocide or need removal in others by means of a carbon bed because it attacks the RO membranes. However, carbon beds reach saturation over time, therefore, chlorine monitors detect breakthrough of chlorine.

Reverse osmosis alone can rarely produce water of sufficient purity for make-up. The RO permeate is usually polished using an Ion Exchanger (IX). These systems consist of tanks containing resin beads selectively treated to adsorb either cations or anions. A cation bed exchanges positively charged ions (such as calcium, magnesium, and sodium) for hydrogen, and the anion bed exchanges negatively charged ions (such as chloride, sulfate, and bicarbonate) for hydroxyl. The displaced hydrogen and hydroxyl combine to form pure water. After a certain amount of use, these systems become exhausted and must be regenerated using sulfuric or hydrochloric acid for cation resin and sodium hydroxide for anion. The monitoring of the concentration of both of these substances must happen continuously with conductivity sensors measuring the regenerant as it enters the tank. During rinse, toroidal conductivity measurements made on the bed effluent determine how well rinsed the regenerants are.

Ammonia, Conductivity, and pH

Variations in cooling tower design

In the condenser, recirculating cooling water converts turbine exhaust steam into condensate. Cooling water usually contains high levels of dissolved solids, and leakage of cooling water into the steam cycle is a major source of contamination.

Leaks introduce ions that raise the conductivity and increase the corrosiveness of the feed-water, boiler-water, and steam. To give early indication of leakage and to monitor the overall condenser performance, the cation conductivity of the condensate pump discharge registers on a flow-through conductivity sensor.

In addition, monitoring condensate and feed-water purity requires measuring cation conductivity. After the condensate passes through the cation column, the conductance of the contaminating salt increases as it converts to a significantly more conductive acid.

There is an increased emphasis in the industry on the re-use of cooling water using cooling towers. The cooling effect comes by the evaporation of a small fraction of water and heat exchange with the air passing through the cooling tower. As the water evaporates, however, the dissolved solids concentrate, ultimately causing scale and corrosion in the heat exchange equipment. While there are many variations in cooling tower design, a common feature is the control of water quality with the use of continuous pH and conductivity measurements to maintain a given set of conditions. A contacting conductivity sensor measures the relative concentration of the impurities in the water. The analyzer for that sensor initiates the opening of a blowdown valve when the conductivity becomes too high. Higher purity make-up water is then introduced which lowers the conductivity.

Since most impurities in cooling water are alkaline, a small quantity of sulfuric acid adds in to the circulating water to lower the pH and thus prevent the formation of scale. Measuring this sulfuric acid concentration and keeping the pH below seven, where scaling is less likely to occur (as indicated by the Langelier Index), is best accomplished by a general-purpose pH sensor. Cooling water that contains a high level of suspended solids, however, requires the use of more specialized pH sensors more resistant to fouling.

Liquid analysis in steam power generation

Condensate feed-water

The cooling tower turns steam into water after leaving the turbine. Make-up water from the demineralization train adds to this water to become feed-water, which pumps through a series of heaters to the boiler. Controlling corrosion in the condensate and feed-water system is usually accomplished in one of two ways-all volatile treatment (AVT) and oxygenated treatment (OT). AVT uses ammonia to control pH and hydrazine to provide a reducing environment for protection of copper alloys. AVT requires measurement of ammonia, dissolved oxygen, and hydrazine. Ammonia measurement can happen either directly or indirectly from pH and conductivity. The indirect method is useful because ammonia reacts in water to produce hydroxide ion. Both conductivity, which is a measurement of ions in solutions, and pH, which is an indirect measurement of hydroxide ion, can combine to yield the ammonia concentration.

OT uses ammonia to control pH and trace oxygen to provide a slightly oxidizing environment that promotes formation of a tough modified oxide film. Water quality for OT is more stringent than for AVT, requiring cation conductivity of less than 0.15 micro Siemens/centimeter. It is necessary to measure dissolved oxygen, pH, and cation conductivity in feed-water systems using the OT method. pH measurement can be difficult in low conductivity water and requires the use of flowing reference technology. A pH measurement requires electrical continuity between the reference and glass electrodes and a path to the solution ground. High purity water does not provide enough conductivity to reliably complete these paths and causes junction potential that registers as erratic drift and offset in the pH measurement. A flowing reference eliminates this effect by stabilizing the junction potential. This measurement takes place in a bypass line in order to preserve the quality of the feed-water and preferably in a stainless steel measurement chamber to dissipate the electrostatic current generated by the high purity water. Since high purity pH is flow sensitive, flow rates should be very low and constant.

Boiler water steam treatment

The boiler is the final collection point for all the corrosive and scale-producing contaminants generated upstream. Solid corrosion lands on the boiler tube surfaces and grows by collecting more suspended matter. Eventually, overheating and tube failure occur. Maintenance of a protective oxide film is the optimum way to limit water corrosion, and this more readily happens when maintaining a low concentration of dissolved solids in a slightly alkaline pH environment. To accomplish this, continuous measurement of both pH and conductivity needs to happen. Conductivity measures the concentration of dissolved solids and a long-life conductivity sensor is required. To maintain the alkaline environment required, power plants commonly buffer the boiler water with sodium hydroxide and sodium phosphate salts. Overfeeding or underfeeding of these chemicals can be damaging, however, and therefore accurate pH and phosphate measurements are critical.

Boiler water also undergoes treatment in order to produce high purity steam. Impurities enter this boiler water from the boiler drum and from vaporous carryover, which deposits on the turbine and causes erosion damage. Silica is the most notorious contaminant, and it is necessary to measure it in the boiler water and steam. Salts such as sodium hydroxide and ammonia salts also vaporize in the steam and flow into the turbine where they precipitate, concentrate, and become highly corrosive. To control contamination in the steam, the conductivity measurement of the boiler water must happen, which indirectly measures the dissolved solids. Then, blowdown controls the amount of contamination.

So, to avoid the uncontrolled corrosion that costs the power industry billions of dollars every year, monitor water quality rigorously and control that quality continuously.

Liquid analysis systems and sensors are hard working, easy-to-use, cost effective tools when measured against the impact of corrosion on plant costs and operations.

While every plant is different, generally an array of pH and conductivity sensing instruments is required for virtually every step of the steam-power generation process.

Beyond that, individual plants will require dissolved oxygen, ozone, chlorine, and other more specialized measurements.

Many plants are opting for centralized digital control systems to continuously monitor the output of analyzers and automate many control functions. This reduces impact on staff and allows corrosion control management to run like a well-oiled machine.

Most important, the key to successful corrosion control is the continuity of measurement.

Grab samples and other periodic measurement techniques are inadequate to the task. Only continuous, real-time analysis offers the assurance of water quality that corrosion control requires.

Sensing pH a venerated pursuit

In the sixteenth century, alchemist Leonard Thurneysser discovered that the hue of violet sap changed with the addition of either sulfurous or sulfuric acids. This early indicator was widely used through the subsequent centuries to detect acids.

With Svante Arrhenius's introduction of ionic theory in the 1880s, the first theories concerning disassociation of acids and bases were developed. Johannes Bronsted, who postulated that acids and bases are substances capable of either donating or accepting hydrogen ions, further refined these initial theories.

By 1904, Hans Friedenthal had successfully established the first scale for classifying acids by determining the dissociation constants for weak acids, according to conductivity and correlating color changes corresponding to different hydrogen ion concentrations using 14 indicating dyes.

The hydrogen ion concentration numbers from Friedenthal's calculations were small and awkward to manipulate. Thus, Lauritz Sorensen suggested using the negative logarithm of these numbers, which he dubbed the "hydrogen exponent" or "pondus Hydrogennii."

This led to the development of the term pH and the creation of the modern pH scale.

Modern pH Scale
The modern pH scale

 

 

Originated published at: https://www.isa.org/standards-and-publications/isa-publications/intech-magazine/2005/may/sensing-ph-controlling-ph/

Ozonation of Cooling Towers

The objective of ozone use with cooling towers is to maintain the highest purity of water with the least amount of water waste and chemical use. Chemical use in cooling towers leads to ever-increasing total dissolved solids (TDS), which must be reduced by eliminating water (blow down/bleed off) and then refilling with raw/lower TDS water. This is a vicious circle that will never end unless one of the TDS-increasing culprits (a.k.a. chemicals) is eliminated or reduced.

The Problem

Cooling tower water quality tends to be extremely poor. Cooling tower traditional treatment is based on extreme chemical use only. This means that you, the water treatment professional, have a chance to create an entirely new income base and aid in environmental integrity and responsibility. There are three main problems surrounding cooling towers.

* Water quality control is difficult due to

- Evaporation rate,

- Environmental contaminants, and

- Extreme chemical use.

* Chemical dependence is promoted by an industry that serves and maintains cooling towers. Most cooling tower manufacturers do nothing about recommending or selling treatment equipment along with the towers. In most cases, it is left up to the end users to set up the treatment method. The cost of chemicals is lower on the front end than water treatment equipment, but far higher based upon the ongoing nature of the use.

* The water waste issue. For example, it is not uncommon to see a 3,000-gallon cooling tower constantly draining water, then constantly replenishing raw water just to lower TDS. This ever-increasing TDS is contributed to a great degree by the chemicals that are used for treatment.

Not only is there an extreme amount of water being wasted on a daily basis, but the environmental impact from the chemical-laden wastewater is deplorable. This chemical-laden wastewater eventually will make its way into our lakes, streams, rivers and groundwater. That is why this wastewater is becoming the subject of more stringent U.S. Environmental Protection Agency regulations.

Cooling Towers and Ozone Primary Uses

Ozone is used in cooling tower treatment for

* Bacteria/virus elimination/prevention

* Organic build-up elimination/prevention

* Blow-down reduction/elimination

* Bleed-off reduction/elimination

* Improved clarity

* Scale reduction

* Cooler running temperatures where scale is inhibited or reduced

* Reduction or elimination chemicals needed for algae control

Principle of Operation

Ozone is injected into the water flow created by a separate circulation pump. This pump pulls the water from the tower's sump or basin and sends it to the ozone injector, contact tank and scale removal/filtration system. Lastly, the treated water returns back to the sump or basin. The principle is to treat the water and eliminate/reduce the following contaminants.

* Scale-forming minerals

* Organics

* Algae

* Harmful microbes

The clean water then is used to clean the entire sump, basin, pipes and peripheral equipment.

The ozone treatment system is simple and can be broken down into three easy steps.

* Ozone injection. Ozone is injected into the side stream flow. Oxidation starts to take place immediately on microbes, organics, bacteria and viruses.

* Contact/mixing. A contact tank helps to further the ozone's ability to oxidize particles allowing them time to react prior to returning to the system. As water flows down the off-gas tank, ozonated water rises and strips any gas in the incoming water. (The off gas tank is the same design as what was discussed in my column "Ozone Installation," February 2003, and "Well-Ozone Again," December 2002, Water Quality Products.)

* Filtration, scale control, particle removal. Possibly the most important aspect of any water treatment is the removal of the particles that have been oxidized. Without this step, all you have done with the ozone is change the structure of the particles by making them larger, insoluble and/or heavier. This step is necessary for systems that require scale control and particulate removal.

It is very important not to construct an ozone unit too large to handle the bacteria, scale and algae. The problem encountered at this point could be corrosion. If you carry an ozone residual too high to de-scale downline you stand a chance of creating a corrosive situation in the sump and its adjacent equipment. For this reason, it's important to utilize existing water treatment technology and equipment in conjunction with ozonation. The result is a system that works without high maintenance, dangerous chemicals, extreme water waste and costly corrosion.

Read this article at https://www.wqpmag.com/o-zone-todays-lesson-ozonation-cooling-towers

Cooling Tower Fundamentals, Heat Transfer

The function of a cooling system is to remove heat from processes or equipment, Heat removed from one medium is transferred to another medium, or process fluid. Most often, the cooling medium is water. However, the heat transfer concepts and calculations discussed in this chapter can also be applied to other fluids.

Efficient removal of heat is an economic requirement in the design and operation of a cooling system. The driving force for the transfer of heat is the difference in temperature between the two media. In most cooling systems, this is in the range of 10—200 °F. The heat flux is generally

low and in the range of 5,000 to 15,000 Btu/ft2/hr. For exceptional cases such as the indirect cooling of molten metal, the heat flux can be as high as 3,000,000 Btu/ft2/hr.

The transfer of heat from process fluids or equipment results in a rise in temperature, or even a change of state, in the cooling water. Many of the properties of water, along with the behavior of the contaminants it contains, are affected by temperature. The tendency of a system to corrode, scale, or support microbiological growth is also affected by water temperature. These effects, and the control of Conditions that foster them, are addressed in subsequent chapters.

TYPES OF SYSTEMS

Water heated in the heat exchange process can be handled in one of two ways. The water can be discharged at the increased temperature into a receiving body (once-through cooling system), or it can be cooled and reused (recirculating cooling system).

There are two distinct types of systems for water cooling and reuse: open and closed recirculating systems. In an open recirculating system, cooling is achieved through evaporation of a fraction of the water. Evaporation results in a loss of pure water from the system and a concentration of the remaining dissolved solids. Water must be removed, or blown down, in order to control this concentration, and fresh water must then be added to replenish the system.

A closed recirculating system is actually a cooling system within a~ cooling system. The water containing the heat transferred from the process is cooled for reuse by means of an exchange with another fluid. Water losses from this type of system are usually small.

Each of the three types of cooling systems once—through, open recirculating, and closed recirculating—is described in detail in later chapters. The specific approach to designing an appropriate treatment program for each system is also contained in those chapters.

HEAT TRANSFER ECONOMICS

 

In the design of a heat transfer system, the capital cost of building the system must be weighed against the ongoing cost of operation and maintenance. Frequently, higher capital costs (more exchange surface, exotic metallurgy, more efficient tower fill, etc.) result in lower operating and maintenance costs, while lower capital costs may result in higher operating costs (pump and fan horsepower, required maintenance, etc). One important operating cost that must be considered is the chemical treatment required to prevent process or waterside corrosion, deposits and scale, and microbiological fouling. These problems can adversely affect heat transfer and can lead to equipment failure.

 

Heat Transfer

 

The following is an overview of the complex considerations involved in the design of a heat exchanger. Many texts are available to provide more detail.

In a heat transfer system, heat is exchanged as two fluids of unequal temperature approach equilibrium A higher temperature differential - results in a more rapid heat transfer.

However, temperature is only one of many factors involved in exchanger design for a dynamic system. Other considerations include the area over which heat transfer occurs, the characteristics of the fluids involved, fluid velocities, and the characteristics of the exchanger metallurgy.

Process heat duty, process temperatures, and available cooling water supply temperature are usually specified in the initial stages of design. The size of the heat exchanger(s) is calculated accordingly with important parameters such as process and water flow velocity, type of shell, layout of tubes, baffles, metallurgy, and fouling tendency of the fluids.

Factors in the design of a heat exchanger are related by the heat transfer equation:

                        Q = UAΔTm

Where

        Q = rate of heat transfer (Btu/hr)

U = heat transfer coefficient (Btu/hr/ft2/°F)

A = heat transfer surface area (ft2)

ΔTm = log mean temperature difference between fluids (ft2)

The heat transfer coefficient, U, represents the thermal conductance of the heat exchanger. The higher the value of U, the more easily heat is transferred from one fluid to the other. Thermal conductance is the reciprocal of resistance, R, to heat flow.

                        U = 1/R

The total resistance to heat flow is the sum of several individual resistances. This is shown in the following figure and mathematically expressed below.

                R1 = r1 + r2 + r3 + r4 + r5

Where

        R1 = total heat flow resistance

        r1 = heat flow resistance of the process-side film

            r2 = heat flow resistance of the process-side fouling (if any)

        r3 = heat flow resistance of the exchanger tube wall

        r4 = heat flow resistance of the water-side fouling (if any)

        r5 = heat flow resistance of the water-side film

 

The heat flow resistance of the process film and the cooling water film depends on equipment geometry, flow velocity, viscosity, specific heat, and thermal conductivity. The effect of velocity on heat transfer for water in a tube is shown in following figure.

 

Heat flow resistance due to fouling varies tremendously depending on the characteristics of [he fouling layer, the fluid, and the contaminants in the fluid that created the fouling layer. A minor amount of fouling is generally accommodated in the exchanger design. However, if fouling is not kept to a minimum, the resistance to heat transfer will increase, and the U coefficient will decrease to the point at which the exchanger cannot adequately control the process temperatures. Even if this point is not reached, the transfer process is less efficient and potentially wasteful. The resistance of the tube to heat transfer depends on the material of construction only and does not change with time. Tube walls thinned by erosion or corrosion may have less resistance, but this is not a significant change.

The log mean temperature difference (ΔTm) is a mathematical expression addressing the temperature differential between the two fluids at each point along the heat exchanger.

When there is no change in state of the fluids, a countercurrent flow exchanger is more efficient for heat transfer than a concurrent flow exchanger. Therefore, most coolers operate with a countercurrent or a variation of countercurrent flow. Calculated ΔTm formulas may be corrected for exchanger configurations that are not truly countercurrent.