Discuss various organic unit processes including polymerization, alkylation,. hydrolysis and their application in the chemical industry. Unit 3. Chapter 2 deals with the four C, and C, hydrocarbons-methane, ethane, ethene, and ethyne-and discusses their conformational and configurational properties and. A highly interactive comprehensive scientific resource, containing over interactive tables in over documents. BOB DYLAN NASHVILLE SKYLINE TORRENT Login credentials are Citrix Recommended Antivirus Exclusions : the specific issue. Firstly, there are has leaked a Cisco Jabber for. I had a service desk providers version of Slack, only the most. VPN : TeamViewer are incredibly professional.
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Dinesh Kumar, A. January March Never Miss. Sponsored By. Sharing is Caring. Removed sludge is sent to a digestion tower where any organic material is converted into methane gas and can be used to produce electrical energy to power the processing plant. Following the mechanical treatment stage, effluent is passed to a biological-based process for further purification. Aeration tanks are utilized to add oxygen to the water and to put the water into circulation through the use of propellers.
The oxygen stimulates the growth of bacteria and microorganisms which feed off of any organic contaminants in the water and convert those into inorganic substances. This process forms activated sludge flocks that float freely in the water. From the aeration or circulation tanks, the water is passed to a secondary wastewater treatment tank and once again, the velocity of the water is slowed allowing sedimentation to take place.
The sludge settles to the bottom of the purified water and can, therefore, be removed by mechanical means from the bottom of the tank. A portion of the sludge, known as the return sludge, is not removed, however, and is instead fed back to the circulation tank to assure that a sufficient quantity of bacteria and microorganisms is available to keep the biological treatment process viable. The sludge that is removed is usually sent to the digester for further processing and creation of methane gas to be used for power generation.
In many cases, the completion of the first two phases of wastewater treatment is sufficient to allow the water to be reentered into a river or stream. However, for some industrial and agricultural waste streams, further processing steps are needed. This is where chemical wastewater treatment processes come into play.
The balance of this article will mainly focus on these processes. Chemicals are used during wastewater treatment in an array of processes to expedite disinfection. These chemical processes, which induce chemical reactions, are called chemical unit processes and are used alongside biological and physical cleaning processes to achieve various water standards.
Specialized chemicals such as chlorine, hydrogen peroxide, sodium chlorite, and sodium hypochlorite bleach act as agents that disinfect, sanitize, and assist in the purification of wastewater at treatment facilities. There are several distinct chemical unit processes, including chemical coagulation, chemical precipitation, chemical oxidation, and advanced oxidation, ion exchange, and chemical neutralization and stabilization, which can be applied to wastewater during cleaning.
Neutralization involves the addition of chemicals for the purpose of adjusting the pH of the wastewater. This involves the addition of acids to lower pH or alkalis to raise pH depending on the initial pH of the influent. Chemical precipitation is the most common method for removing dissolved metals from wastewater solution containing toxic metals.
To convert the dissolved metals into solid particle form, a precipitation reagent is added to the mixture. A chemical reaction, triggered by the reagent, causes the dissolved metals to form solid particles. Filtration can then be used to remove the particles from the mixture. How well the process works is dependent upon the kind of metal present, the concentration of the metal, and the kind of reagent used.
In hydroxide precipitation, a commonly used chemical precipitation process, calcium or sodium hydroxide is used as the reagent to create solid metal hydroxides. However, it can be difficult to create hydroxides from dissolved metal particles in wastewater because many wastewater solutions contain mixed metals. This chemical process involves destabilizing wastewater particles so that they aggregate during chemical flocculation.
Fine solid particles dispersed in wastewater carry negative electric surface charges in their normal stable state , which prevent them from forming larger groups and settling. Once the charge is reduced, the particles freely form larger groups. Next, an anionic flocculant is introduced to the mixture.
Because the flocculant reacts against the positively charged mixture, it either neutralizes the particle groups or creates bridges between them to bind the particles into larger groups. After larger particle groups are formed, sedimentation can be used to remove the particles from the mixture. With the introduction of an oxidizing agent during chemical oxidation, electrons move from the oxidant to the pollutants in wastewater.
The pollutants then undergo structural modification, becoming less destructive compounds. Alkaline chlorination uses chlorine as an oxidant against cyanide. However, alkaline chlorination as a chemical oxidation process can lead to the creation of toxic chlorinated compounds, and additional steps may be required. Advanced oxidation can help remove any organic compounds that are produced as a byproduct of chemical oxidation, through processes such as steam stripping, air stripping, or activated carbon adsorption.
Redox reactions are used for the treatment of potable water. Chlorinated hydrocarbons and pesticides can be effectively removed from wastewater by the use of ozone and hydrogen peroxide treatments. Advanced oxidation processes are also used for the degradation of drug substances like antibiotics or cytostatic drugs that might be found in the water. Reduction processes can also be used for the transformation of heavy metal ions into sulfides.
When water is too hard, it is difficult to use to clean and often leaves a grey residue. This is why clothing washed in hard water often retains a dingy tint. An ion exchange process, similar to the reverse osmosis process, can be used to soften the water. Calcium and magnesium are common ions that lead to water hardness.
To soften the water, positively charged sodium ions are introduced in the form of dissolved sodium chloride salt or brine.
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Probability Theory and Mathematical Statistics. Machine tool reconditioning and applications of hand scraping. At ambient temperature they find maximum use in industry and agriculture. Nearly 90 per cent of the water employed in industry is for cooling purposes and the balance for steam generation. Surface waters might possess colour, odour, taste, suspended solids etc. Ground waters are expected to be free from organic odour and have a relatively less variable composition at the same source.
Industry employs water from all types of water resources. This is not the case with agriculture or domestic use. The water quality requirements are somewhat different for different uses. The important characteristics that signify water quality are described below. For this reason the concentrations of inorganic and organic substances dissolved i,n a body of water and their spatial and temporal variations need to be monitored. This exercise should cover not only the major dissolved constituents.
Table 2. The seasonal variations in the quality of some surface waters could be large enough to make the use of such waters more problematic. Under this category comes silt and suspended solids. The bacterial content, specially the presence of pathogens.
Whatever might be the quality of water available to a user. It is not advisable to condemn a particular body of water as unsuitable. The United States Geological Survey l has given the significant concentration. Above these levels. Physioo - Chemical Characteristics 7 2. The World Health Organisation has stipulated that drinking water should have a TDS content of less than mgll, although this can be relaxed to mgll, in case no alternative supply is available 3.
Drinking water should also be free from colour and turbidity. It should have no unpleasant odour dissolved gases or taste absence of certain dissolved solids. A case in point is the smell of chlorine that is once in a way detected in domestic water supply, as a result of excessive chlorination. With an increase in the hardness of water Ca, Mg, carbonate, sulphate , its suitability decreases with respect to cooking, cleaning and laundry jobs.
One of the well documented problems concerning drinking water, is the presence of fluoride. In India, the Technology Mission on Drinking Water laid special emphasis on fluoride, as well as iron contamination in rural water supplies 4. There is also a certain amount of avoidable confusion, since the beneficial effects of a little fluoride in dental care are also known.
What is not well publicised is the temperature effect on the fluoride limits in drinking waterS. These are as fol! The upper control limit for fluoride in the same temperature range is reduced from 1. Thus the flexibility in the range of fluoride control limits in India as well as in other tropical -:ountries is much less than say in England or Canada.
This is due to the dependence on temperature of the rate of the biological uptake of fluoride by body fluids. The WHO guidelines for the quality of drinking water as given in Table 2. In addition WHO has also issued guidelines for the "aesthetic quality" of drinking water , which are a little difficult to quantify. These are summarised in Table 2. The presence of toxic elements usually arises due to contamination by effluents discharged from nearby industries.
OpS 0. OS Chromium mgll O. OS Cyanide mgll 0. OS Barium mgll 1. JgII 0. Table 1. S Zinc mgll S Sodium and Potassium ion concentrations in natural' waters are relevant to irrigation as these cations reduce the permeability of soils. Physico - Chemical Characteristics 9 Ca and Mg ions, being divalent, are pleferentially taken up by the exchange sites in soil, thus reducing Na and K uptake and helping to restore soil permeability. Since Ca and Mg concentrations are also governed by presence of bicarbonate and carbonate ions i.
If RSC is greater than 2. Sea water with a salinity of 35 gIl has an average der. A variation in salinity of 1 gil causes the density to change by 0. In recent decades, desalination of brackish as well as sea water an industry by itselt has come into vogue in arid and desert locations, for producing drinking water. This is used for horticulture rather than for agricultural purposes.
Sea water is used for cooling power rlant condensers, when the power station is on the coast. In this context, the biofouling characteristics of sea water at that particular lOCation are of much greater relevance than the chemical parameters. This poses several problems with respect to power station water chemistry. The two , main areas of concern are as follows: a It can lead to blocking of functional groups of the ion exchange resins of water treatment plants because of irreversible absorption, leading to reduction in the ion exchange capacity as well as damage to the resins.
In addition, corrosion in the condensation zone can also result because of volatile decomposition products. Several techniques have been developed to isolate organic substances from water and to estimate them quantitatively I. Therefore, power plant laboratories usually determiIie only the potassium permanganate value. The breakdown of organics in steam generating systems is leaaing to problem situations in several power stations.
Sophisticated analysers are marketed fot this task. These are flocculation, flocculation-decarbonizathmand use. Oxidising agents such as chlorine or ozone i:an also be tried. Under certain conditions, however, it is possible to carry out the ion exchange as well as organic substance removal within the plant. Since, water qualit "'Varies with location and seasons, water quality monitoring is an essential activity for any industry thatmakes-use of a water source.
Biofouling due to surface water is also a problem that has to be tackled. In certain instances, sub- surface or groundwater from a borewell farm is also used. Thus, more care needs tq:biexerclsed. It is essential to appr. The hydrogen ion concentration is represented by the pH value. By IlhcHarge the pH of natural waters lies in the neutral range. For drinking water a pH of 6.
Some surface waters passing over areas that are rich in sodium and potassium exhibit an alkaline pH. Such examples of acidic or alkaline water, are however, not common. Clean sea water usually has a pH of 8. The electrical conductivity EC of water is related to its total dissolved solids content.
Since it is easy to measure this. Physico - Chemical Characteristics 11 Table 2. Spec:ifie Conduetlvity vs. The presence of these two divalent cations is essential for ensuring soil permeability as well as for the growth of crops. Thus, one measures what is known as Ca hardness, Mg hardness and the sum of these two viz.
The measurement of Ca and Mg is through simple volumetric procedures. On the other hand, extra hardness will mean the consumption of more soap in washing and also scale formation in cooling water circuits and boilers. It should be remembered that very soft water induces corrosion in iron pipe line.
In tenns of hardness, the water quality is designated as shown in Table 2. Hardaess VI. Itt concentrations abOve mgll, Mg is toxic. Bicarbonate alkalinity is also called methyl orange alkhlinity or M-alkalinity, while P-alkalinity Phenophalien alkalinity signifies thepresence of carbonates and hydroxide ions. Highqi alk,Hnity causes the precipitation of Ca and Mg leading to the problelll of scaling on heat transfer surfaces. If chloride is present at over mgll, it is not suitable in food processing and if it is over mgll, the water is not suitable for industrial cooling because of the corrosive effects of the chloride ion on several metallic surfaces.
While nitrates are needed for increa"ing agriculture productivity, more than 50 mgll is not to be allowed in water for domestic use. The problem of fluoride has already been dealt with. In waters meant for irrigation, boron concentration should not exceed 1 mgll, as otherwise it is harmful to plant growth.
A discussion on water quality is not complete unless mention is made of the biological monitoring of surface waters ll. The fish swimming against the stream of. Their muscle potentials are of the order of 60 to 80 mV wl1ich are attenuated by the dielectric constant of water. By suitable amplication and integration, the normal activity of the fishes can be recorded. If the water quality deteriorates low dissolved oxygen, presence of toxic chemical etc.
While such systems have been used in many countries for monitoring the quality of flowing river water, the best results are obtained in less dynamic laboratory applications and in monitoring the quality of cooling tower water in industry. There are five groupings in terms of conductivity and four in tc:rms of SAR. The geochemical system of water quality classification rests on the basis of the predominant cations and anions that are present in equivalents per million.
This leads to five types, viz. Another classification makes use of the specific conductivity and Biological Oxygen Demand BOD as the defining parameters 3. The BOD ofa water body, although its practical determination is open to a number of reservations, is the most satisfactory parameter for characterising the concentration of organic matter.
If BOD is greater than this value, a part of the organic matter carrying bacteria and pathogens is likely to escape removal and pass into the water distribution system. Considering specific conductivity and BOD together, natural waters have been divided into five classes as shown in Table 2. Class 1: Suitable for public consumption as well as other uses.
Class 2: Suitable after some treatment, but not fit for irrigation if a better source is available. Class 3: Not suitable without proper treatment for any purpose, except for. Class 4: Suitable for irrigation, but treatment required for drinking and for industry. Class 5: Unsuitable for all purposes. These are only typical examples and a voluminous data is available on water quality of surface and ground waters in India. Government of Tamil Nadu 9 , it has been shown that all along its course, the water is of the calcium bicarbonate type, except at certain locations in Salem and Tiruchirapally districts where the discharge of industrial effluents into the river, turns it into sodium bicarbonate type.
Obviously water drawn from these locations, will be less suitable for irrigation. The water quality as a function of the beginning and end of flow season in the river all along its course indicated. A study was also conducted of the water quality in 14 reservoirs and an attempt was made to correlate the electrical conductivity with either bicarbonate, chloride and sulphate.
There was one reservoir which showed sulphate correlation with EC. Interestingly the C. The Maharashtra Engineering Research Institute has carried out water quality studies of Krishna, Godavari, Bhima and Tapi rivers as well as of several reservoirs In addition, heavy pollution was noticed down stream at Nasik. Several variations of water quality can be seen from, the data on Godavari and Tungabhadra river waters at relatively unpolluted locations.
S Na mgll 64 62 K mgtl 14 HC03' mgtl At Tungabhadra's sample location, however, the parameters do not vary much between the beginning and the end of the rainy season indicating scanty rainfall. These studies were extended to locations down stream of paper mill discharges into both the rivera. It was seen that the change in water quality after mixing with the effiuents was more marked for Tungabhadra than' with Godavari.
While the bicarbonate value diPPed from 44 percent to 17 per cent of the total anions, the chloride went up from 9 to 13 percent 1J. Physico - Chemical Characteristics 2. A study of wells along the coast line ofDakshina Kannada district, Karnataka is illustrative of this phenomenon Table 2.
Saline Water Intru. In Karnataka 14 pH 5. Similar studies on sea water intrusion have been reported from Thane District in Maharashtra About 50 km southwest of Delhi, in the Gurgaon District of Haryana, a number of such brine wells exist and are being used as a base for thriving salt industry.
The chemical composition of some of these well waters is given in Table 2. From the same source of raw water, they make use of a smaller amount for the production of demineralised water. As such it would be instructive to have data of the type of water quality available to such utilities. A few comments on the data in Table 2. A although it is from a canal drawn from a big river, is also a partial dumping ground for the sewage of a metropolis. The latter poses problems for the demineralisation plant.
Specific Conductivity 84 85 IlStcm pH 8. River water in East India. C: River water in Central India with other industries nearby. D: River water in South India. E: Ground water Borewell farm in South India by the side of an urban metropolis and sea. Organic load is seen in Source C, probably due to the locations of industries nearby.
Source D, although river water, has a greater content of dissolved impurities as seen by high values for specific conductivity and chloride. Source E is from a groundwater farm typical borewell waters and one can readily see the high salt content. This imposes a considerable load on the demineralisation plant of the utility. Source F is moderately "clean". The power plant condensers are cooled by the same raw water in case of A,B,C,D and F, while at location E, the condensers are cooled by sea water.
Apart from E, in all other cases, seasonal variation is seen. In general, the values of specific conductivity are lower in September, indicating the general dilution effect of the monsoon. A nuclear power station located near an artificial reservoir created by a dam on a river in India uses raw water whose typical analysis is shown in Table 2.
While all the cations belong to what is known as the class of strong cations, the major anion, the bicarbonate, belongs to the class of weak anions. Basic Groundwater Hydrology. Water Research. Water and its Impurities. Academic Press. World Health Organisation. Guidelines for Drinking Water Quality.
Environmental Protection Agency. Encyclopedia of Physical Science and Technology. Seminar on. Water Quality and Its Management. Central Board of Irrigation Powe'r. New Delhi. US Salinity Laboratory Staff Diagnosis and Improvement of Saline and Alkaline Soils. US Dept. Reiff, B. Seminar on Water Quality and its Management. Central Board of Irrigation and Power. Rameshwar Rao C.
Narisimha Rao Gajendragad and C. Nliganna III - t Padhye - Gogate. Sila Indian Geographers. Biswas, A. Ramshesh and K. Corrosion and Maintenance. The two hydrogen atoms are bonded to the oxygen at an angle of The O-H bond length is 0.
The two lone pairs of electrons on the oxygen are positioned to give a psuedotetrahedral arrangement. The bent shape of the water molecule bestows it with a high dipole moment and bigh dielectric constant. These two properties make water an excellent solvent in which elctrolytes such as NaCI dissociate into ions, which in turn are hydrated specially the cations due to the co-ordinating ab.
The hydrogen bonding often leads to he formation of a secondary shell of hydration, around the first hydration shell. An important example of anion solvation 'is the formation of a hydrated electron commonly referred to as the solvated electron wherein the positive ends of the water dipoles surround the electron.
These properties are unique to water. The only other substances that have such proclivity to a limited extent are liquid ammonia and liquid hydrogen fluoride. When water is employed in industry for cooling, through the use of cooling ponds, cooling towers etc. On the other hand, when water is used to make steam, the temperature in the boilers and the associated steam turbines is in the range of oC. Thus when one considers steam generation either for production of electrical or motive power, the properties of water and steam at high temperatures and pressures become relevant for understanding the chemistry involved.
Table 3. This is due to the collapse of some of the l,drogen bonds and the release of water molecules trapped in the cavities. The decrease in density results in a decrease in the surface tension and viscosity, as shown in Table 3. Properties of Water at High Temperatures and Pressures 21 increase in the temperature of water. Consequently, the pH of pure neutral water defined to be 7. The very fact that the hydrogen ion concentration increases with temperature in pure water, makes it much more aggressive to metallic surfaces at higher temperatures than at room temperature.
As is well known, electrolytes such as sodium chloride dissociate into their component ions on dissolution in water, due to its dielectric constant. This important property of water also undergoes a change with temperature. At OOC,the value of the dielectric constant of water is This means that water loses its ability to effect the dissociation of electrolytes and a fraction of the dissolved substances remains as an undissociated or neutral chemical species.
While in the temperature range under consideration this effect might not be of that much importance for strong electrolytes such as NaCl, for weaker electrolytes such as the hydroxides of corrosion products, it has some relevance. This will be dealt with in greater detail in a later chapter. Pressure in itself has only marginal effect on the water chemistry except with respect to the elevation of its boiling point , but has a profound effect on what has come to be known as steam chemistry.
In the two phase system of water and steam, the distribution of solutes is a fucntion of temperature and pressure, rather than temperature alone. The thermophysical parameters and other properties of water as a function of temperature and pressure are given in. Tables 3. Bar 0. The decrease in density and viscosity coupled with the increase in the dissociation constant of water with increase in temperature results in increased conductivity of pure water as given in Table 3.
Compared to a value of 4. This in turn makes water very aggressive to the metallic surfaces with which it comes into contact and thus promotes corrosion. Conductivity J. Since the number of ions of a given electrolyte are proportional to the dielectric constant due to increase in temperature , the effect ofincreased ionic mobility due to the reductions in density and viscosity gets nullified at a certain temperature.
In other words, the conductivity ofan elctrolyte, like NaCI in water, goes through a maximum when measured as a function of temperature. Figure 3. Mesmer, Baes and Sweeton 5 b. Cobble 4 c. Although there is some difference in the pKw values as determined by different workers, the trend is the same, viz. As seen earlier, since the pH scale and the point of neutrality of pure water are defined in terms of pKw and the minimum of pKw value as seen in Figure 3.
Thereafter the neutral point shifts to higher values. An ammonical solution of water that exhibits a pH of 9. Similarly water spiked with LiOH that shows a pH of The consequences of such changes are discussed later. At the critical temperature In other words, under such a condition, a given solute will be distributed equally between the two phases. However, this is a limiting condition. What is of greater interest is the range of oC, in which the ratio is a function of temperature and pressure.
The vapour transport of solutes like silica are attributed to this dependence and the higher the ratio, the greater is the solute content of the steam phase, which deposits the solute at a different part of the steam water circuIt where temperature and presssure are lower. It is obvious from the above discusssion that the properties of water undergo significant changes as a function of temperature a'ld presssure. A certain amount of individuality characterises each system that arises from the interaction of specific materials of construction of the steam-water circuit.
However, there are enough common points to broadly discuss and understand the application of the basic concepts of physical chemistry of water and its dilute electrolyte and non-electrolyte solutions to industrial practice, so as to achieve maximum efficiency and economy in energy generation..
Sudarsanan M. K lyer, Water in the Environment. Analytical Scientists, pp. Todheide K. I, ed. Plenum Press. Rice M. Cobble J. Baes and F. Chem, 74, Marshall W. Franck , J. Reference Data, 10, Helgeson H. Chem, 71, Thornton E.
Central Electricity Generating Board, U. On the other hand, corrosion signifies essentially the same to all professional metallurgists and chemists. To an environmental chemist, water chemistry would mean the analytical determination of impurities especially the trace toxic inorganic and organic contaminants in water and the detoxification proC'esses thereof. On the other hand to a chemist in a power plant or other industries such as fertilisers, water chemistry would immediately signify the chemical regime that is reqUired to be maintained in the steam-water circuits so as to minimise corrosion and material transport.
The latter, although on a very small -scale is of serious concern to the chemists associated with the nuclear power industry. Here it would lead to the generation of radioactive nuctides and their transport to unshielded locations where maintenance and repair are needed l. The stress could be chemical in the sense that the aqueous environment may be acidic or alkaline. The chemical stress can be viewed or understood in terms of thermodynamic and electrochemical concepts.
The stress could be metallurgical in the sense that the material surface has defects, either inherent or as a result of the manufacturing process. It might be thermal stress as in a steam generating system. In reality, corrosion would be the consequence of a combination of all the above stress factors. To avoid or minimise corrosion, great care has to be taken in selecting the construction materials as well as in controlling the chemistry of the aqueous environment.
Basically, corrosion is a process where the metal atoms leave their location on the surface and stabilise in the form of ions in solution. In high purity water, where no other electrolyte is present to any significant extent, it is the solubilising action of water on a metal surface like iron, which is the first step in the corrosion process. The polarisability of the water molecules on contact with the iron surface leads to the weakening of the O-H bond and gives rise to reactions below, which show the combined effect of solubilisation and hydrolysi3 l.
These two chemical species appear later, in many forms due to secondary reactions as shown in Fig. StopS I 1. O ,nolec. These secondary reactions being pH dependent, the percentage of iron present in each of the ab0ve forms also shows a pH dependence. Reaction 4. The lower the dissolved oxygen content of water, the smaller will be the cathodic reaction 4.
In general, corrosion is less, if the water is deoxygenated. In high temperature, slightly alkaline and deoxygenated aqueous environment, the principal corrosion product formed on an iron surface carbon steel, stainless steel is magnetite, Fe 30 4 Fig. Under the ccnditions specified, there is a possibility for Fe OH 2 to exist as neutral molecules in solution, which get converted to magnetite in a short time, 4.
The hydrogen generated in. S is in addition to that from 4. In actual practice, the determination of the hydrogen content of steam in view of the very limited solubility ofH2 in water serves to monitor the overall corrosion process in the steam water cycle. Hence the direct formation ofFe 30 4 on ,an iron surface leads to a sudden volume expansion. On stainless steel surface, Fe unC:ergoes a series of substitution reactions with the alloying elements Cr and Ni and complex ferrites such as CrFe 20 4 and NiFe 20 4 have been identified in the corrosion products.
Before dealing with the behavior of magnetite and other oxides in high temperature aqueous environment, it would be advantageous to complete the discussion on corrosion, which as noted earlier could have its origin in chemical, electrochemical and metallurgical parameters. There are other specialised forms of corrosion 5. Pitting is one such, extremely localised attack resulting in pits and even pin holes such as in chloride induced pitting of stainless steel under stagnation.
Another form is known as stress corrosion caused by a synergistic effect of tensile stress and conosive environment. Examples are, caustic cracking of boiler, tubes, cracking of stainless steel in a chloride environment etc. The use of sacrificial electrodes is a direct application resulting from this form of corrosion.
From a metallurgical view point, intergranular corrosion and selective leaching need 0 be mentioned. The former is a localised phenomenon occurring along the grain boundaries such as the corrosion of stainless steel in heat affected zones of a weld. The de-zincification of brass is an example of selective leaching from an alloy, as a result of which porosity is developed.
A purely mechanical form of attack is the erosion - corrosion, as experienced at inlets of condenser tubes of a sea water cooled power station. Since pH scale as defined undergoes a change with temperature, one must be very clear about what is specified by pH, e. Thus changes in solubility of magnetite would mean that in a closed, but circulating heat transport system, magnetite gets transported solubilised and redeposited from relatively hot to cold sections of the system.
In fossil fuelled power stations, this phenomenon is of no serious consequence, except that unde: deposit attack is promoted by the formation of deposits allover the place. However, in a nuclear power station, this would mean the transport of corrosion protlucts through the reactor core and their activation »Y neutrons, leading to radioactive nuclides. These are transported to ollt-of-core surfaces and get deposited on them, thereby contributing to a radiation field which prevents accessibility to the system for maintenance.
The latter are as a result o normal chemical, electrochemical and metallurgical faclors operating at the interface of a metal and high temperature waterS. Two major forces are responsible for the deposition of such particles, viz.
The mass force is directly proportional to the mass of the particles i. As the size of the particle decreases, the surface forces prevail over the mass forces. When any solid is in contact with an electrolyte solution, it acquires a surface charge which is balanced by in equal and opposite charge in the liquid layer near the surface, but arranged in two layers which is known as the electrical double layer.
The first one is the adherent monoionic layer of counter ions held by chemical forces and immovable with respect to the solid. The second one is a diffuse outer layer of counter ions mobile with respect to the surface as shown Fig. From equation 4. Table 4. By implication, a steam generating system promotes concentrations of such impurities in the water phase. This is so since the pressure and temperature decrease in the various stages of the turbine leading to the deposition of substances like NaCl and Si0 2 on the turbine blades.
If allowed to go unchecked, such deposition will lead to the failure of the turbine blades. Hence the basic information on solubilities of salts and metallic oxides in steam and their distrIbution coefficIents, needs to be discussed for a proper understanding.
The distribution coefficient K IS defined as the weight ratio of the concentration in the steam phase to that in the water phase. As seen earlier decrease in the dielectric constant of water with temperature will effect this equilibrium in favour of the neutral species. Martinova 9 has described what are known as the carryover coefficients into steam from water as a function of pressure. This is generally known as a 'ray diagram' and is shown in Fig. Na2 SO. This is to be expected since in the water phase also, the oxides exist as neutral molecules.
The International Association for the Properties of Steam has compiled extensive data on the distribution coefficients and other relev. The data are basic in nature and valuable in understanding the vapour carry over of oxides and salts. As an example Fig. OF Fig. If one considers only those area!! Calculated data on the vapour pressures of concentrated caustic solutions are also shown in Fig.
The same data and extrapolations are shown in Fig. The overall conclusions are the same through either method of data representation. If one proceeds from the assumption that the concentration in the liquid phase can attain very high values locally, considerable quantities of the substances can be camed over into steam. Thus from Fig.
Water Chemistry. IF FII. The steam carry over ofSi02 is essentially due to the distribution of this compound between high temperature water and steam phases. Suffice to say here tha[ increasing concern for low pressure turbine blade and rotor damage by stress corrosion and corrosion fatigue has caused new limits on allowable steam contamination by NaCI, NaOH and Si02 to be recommended. Such limits have an impact on the required purity of steam generator feed water. Thus it has been a practice to make use of volatile alkalising agents such as ammonia, morpho line end hydrazine, ll in the cycle.
The volatile compounds, as compared to the non-volatile alkalies such as NaOH have the advantage of being easily carrieo over into the steam phase. They thus give corrosion protection to that part of the cycle where steam comes into contact with the surface of the construction materials.
Because of their differing dissociation constants, different quantities of each of these reagents ate required to attain the sam! Somehow, its role in increasing the pH of the water - steam circuit has not been well recognised. This means that sufficient alkalisation in the condensing phase is not achieved with ammonia.
On the other hand, the value of this ratio for morpholine increases with temperature. However, the magnitude of the ratio is smaller than that for ammonia. Hydrazine, on the other hand is a good compromise candidate. Its distribution ratio increases with temperature, while a concentration lower than morpho line, but higher than ammonia is required to attain the same pH value in the condensate. The main decomposition reaction is, 3 N2 H To minimise this problem, deoxygenation is an established practice.
The reaction has a period of induction and proceeds very slowly at room temperature. Two additional mechanisms appear to operate during the process of deoxygenation in addition to the one given in equation 4. A heterogeneous surface absorption on metal or metallic oxides reaction has been postulated.
Coming to the parameters that govern the deoxygenation the effect of temperature is appreciable. The deoxygenation is most effective in the pH range of9 - As the purity of water increases, the rate falls showing that impurities present in water catalyse the reaction. The effect ofhydrazine concentration is very pronounced.
Thus, while 10 percent excess hydrazine requires 40 hours for scavencing dissolved oxygen at a particular temperature , the time is reduced to less than 10 hours with percent excess. In general, metal ions are found to catalyse the reaction. Catalysed hydrazine is available in the market. The discussion in this chapter clearly indicates that water chemistry and corrosion are closely interlinked, leading to a favourable material compatibility or the lack of it in high temperature and high pressure water-steam circuits.
A large amount of basic physico-chemical information has been generated and collateci. The proceedings of several conferences organised by the International Association for the Properties of Steam, provide, valuable data in this regard.
Venkateswarlu K. Schikkor G. Electrochem, 35,25 and , Z. Sundaram C. Bohnsack, G. Venkateswaran G. Martinova O. Gould G. Potter and F. Heitmann H. Ramshesh V. Venkatcswarlu K. In addition, after special purification procedures, it is also made use for generating high pressure steam in thermal and nuclear power stations.
It has been roughly estimated that 90 percent of all the water used by industry is for heat transfer and cooling while about 8 per cent is used for process requirements and about 2 percent for steam generation. Tile water quality criteria and hence the treatment procedures are different for these three end uses. Stringent control of water chemistry. In this chapter, attention will be focussed on the chemical treatment and quality of water needed for industrial cooling. Although water from a particular source may be acceptable for drinking or for agriculture, certain suspended and dissolved impurities present in it, will have to be removed by suitable pretreatment procedures, so that it is acceptable for industrial cooling with or without further chemical conditioning.
The type of treatment to be adopted depends upon the quality of the available source of natural water, as well as on tbe capital and operational costs and environmental requirement of the industrial location. Thus, some site-specific variations of the general water treatment procedures are ullavoidable. Efforts are constantly being made to envolve improved water conditioning programmes that are environmentally friendly, efficient and cost effective.
Water from rivers, lakes and even underground borewells, is employed for all the three purposes mentioned above, after proper chemical treatment 1. Only at a few locations, depending upon the material of condenser tubes, a chemical treatment of sea water is practised. On the other hand, biofouling prob'lems are severe with sea water applications and antifouling - practices are all pervasive l.
Surface and groundwaters also experience biofouling, though of a different nature and magnitude. We shall first deal with the problem of biofouling encountered in the use of natural waters for coolin, condensers. Such raw waters contain their natural flora and fauna. The flora mainly comprises phytoplankton in association with bacteria and fungi fed' '. Jy nutrients like nitrates, phosphates, iron, silica and carbonic acid present in these waters.
Sunlight penetrates the low depth turbid water systems like lakes and river banks and supports the photosynthetic activity of the phytoplankton consisting of different types of algae such as the blue greens, greens and the diatoms. In India, power stations located at Dhuvaran, Tarapur and Trombay on the west coast and Ennore, Kalpakkam and Tuticorin on the east coast make use of sea water for cooling the power plant condensers and process water heat exchangers 3.
The conventional and the widely practised method for controlling biofouling is through the chlorination of the coolant water 4,s. Since large volumes are involved in once through systems, no other method appears to be economically viable. At the same time this imposes the need for handling bulk quantities of hazardous chlorine.
It has been observed that residual chlorine present in the combined form and as chlorinated organics is detrimental to fish, invertebrates and algae in natural water. The effect is a function of the size of the organisms, the period of exposure, the water quality, e. Hence there is a need to regulate the release of chlorine content in the effluents 6,6. Environmental concerns have led to the lowering of the allowable discharge limits to less than 0.
Such discharges are permitted for not more than 2 to 3 hrs in a day. For continuous chlorination, the discharge limits are set much lower at 0. Chlorine also reacts with ammonia present in water producing chloramines, which possess a disinfecting property, times less than that of HOCI.
Chlorine reacts with nitrogenous compounds like proteins, and other organic matter forming chlorinated compounds or oxidised products. The difference between various disinfectants is attributed to their ability to penetrate the cell. The effectiveness of HOC I as compared to other forms of chlorine species could be due to its small molecular size and electrical neutrality which allows it to penetrate the cell wall.
The lesser bactericidal effect of OCI- could be due to its negative charge, which may impede its penetration into the cell. Chloramines have comparatively slow diffusion through tne cell wall, however, these are of importance in water chlorination due to their persistence in water for a longer period of time as compared to HOCI. From this, it is obvious that chlorination is quit,. The question that needs an answer is the universal observation that chlorination of seawater, which has a pH of 8.
Herein comes the role of dissolved bromide in sea water. The presence of 68 mgll of bromide in seawater, results in a complex chemistry of chlorinated seawater 6. Hence at the pH of seawater, chlorination leads to the formation of hypochlorous acid, OCI' ion, HOBr and OBr' ion, plus the bromamines, which co-exist with chloramines. Upto 8. Consenquently, the chlorination of seawater which leads to HOBr, is a successful method of controling marine biofouling 7.
At a coastal power station this practice should be optimised depending upon the nature of the fouling community and the chlorine demand of the seawater in the intake area. The chlorine demand varies somewhat with the season and the sea currents and needs to be analytically determined at regular intervals so that the total chlorine dose needed can be evaluated. As briefly mentioned aboye, HOCI reacts with both inorganic and organic nitrogenous compounds present in natural waters a major fraction of the chlorine demand.
Examples of such compounds are ammonia, nitrates, amino acids, proteins and humic acid. While the reaction of HOCI with ammonia is comparatively fast, that with organics is slower and depends upon the contract time. The formation of chloramines, stepwise is a follows. The reaction between chlorine and organic nitrogen is relatively slow. S, D is the final chlorine demand initial chlorine dose - final residual chlorine in mgll, K is the chlorine demand in mgll at the end of one hour initial chlorine dose - minus chlorine residual at the end of one hour , t is the contact time in hours and n is the slope of the curve that is obtained on plotting the experimentally determined chlorine demand at different time intervals vs.
A significant development following chlorination of natural waters for industrial use, is its reactions with the ever increasing' number of man made chemicals of undetermined toxicity discharged into the aquatic environment. The cumulative effect of the potentially carcinogenic chloro- organics is a matter of concern. Thus, in once-through systems, lot of care is to be exercised in regulating the chlorination practice so that the discharged water contains as little total residual chlorine as possible.
Work on algae indicated that while the effect of chlorine is algistatic, that of bromine is algicidal. Similarly reduced carbon uptake using C and depression of photosynthesis were noted. Branacles are one of the common macrofouiants and their response to chlorine has been well studied. Eighty percent average mortality of Balanus larve was recorded after five minutes exposure to seawater at a chlorine level of 2. Marine mussels, typified by Mytilus edulis, Mytilus virdi and Mytilus californicus, are the most important of the fouling organisms that restrict the cooling water flow.
Macrofouling by branacles and mussels have been the focus of many investigations, including studies at Bombay and Kalpakkam, India! The time to achieve a percent kill has been related to the temperature and residual chlorine level through a generalised regression equation. It has been realised, of late, that instead of intermittent moderate chlorination 2. Chlorine interferes with the process or thread of filament formation by the mussel larve, that is vital to the growth of these organisms.
They found that chlorination is the most economical method for combating all biogrowth in power station circulating water systems According to the CEGB, c. Chlorine injection must be such that residual chlorine is distributed uniformly throughout the intake water.
The Tarapur Atomic Power Plant consumes - kg. There are four bays at the intake. While all the seawater cooling systems are once-through, a number of fresh water cooling systems are not. In fact it is becoming the norm to use cooling towers to dissipate heat into the atmospheric environment.
Fresh make up water is added only to compensate for the evaportion losses from the cooling tower and the blow down is discharged into the aquatic environment, so that a balanced chemical conditioning is maintained in the recirculating cooling water. Since, it is a recirculating system, once in a while additional biocides can also be used without economic penalty.
Micro organisms, dead or alive, form slime, which acts as material for cementing particles together. It is, therefore, essential that tt. Dispersed biological matter also makes the chlorination programme effective, by allowing chlorine to interact easily with the binding material. It has been found, for example, that algae and bacteria can be flocculated using cationic polymer.
Biocides used in water treatment programmes are mostly cationic surface active agents. Recently it has been established that chlorination and the use of low molecular weight cationic polymers alone can control biofouling of cooling water systems, and the use of biocides can be eliminated. In fresh water cooling systems,the trend is towards a programme of chemical conditionirrg of the circulating water, whose pH is maintained towards the alkaline side. Under such conditions, as seen earlier, chlorination loses its efficacy and research was focussed on the use of alternative biocides.
Keeping the experience with seawater in mind, the application of bromine in the form of bromide or C? Bromine chemistry offers reduced corrosion and environmental hazard. Rapid decay of bromine and its compounds sllch as bromamines, minimises the environmental impact ofthe biocide on the receiving aquatic eco-system.
In addition to HOBr, bromine is available in different chemical forms which can be added as a biocide. Brominated propionamides like, 2,2-dibromo - :3 - nitrilo propionamide are extremely potent broad spectrum microbiocides. The compound is an oxidising type microbiocide and is unstable with increasing pH and temperature. Thus, the cooling water dosed with this chemical can easily be detoxified before being.
Equally effective are the bromamines. In addition, the reaction leading to the formation ofbromamines can be made reversible by lowering the pH. Thus they are less persistant than the corresponding chloramines. It is shown that manganese fouling of stainless steel SS condenser tubing is arrested on switching over to bromine based biocides IS.
As in seawater chlorination, the chlorine will release bromine in the form of HOBr as an effective biocide. Manufacturers of cooling water chemicals offer a number of proprietory pjpcides, These are recommended to be used once a week, while chlorination is suspended for that day. Their effectiveness depends upon site specific studies and no generalisation can be made.
Tin - organic biocides made a major entry into the markel, but of late, environmental considerations have imposed servre constraints on their large scale application. The compound is made use of in combination with a dispersant to enhance its penetration of algal and bacterial slime layers.
From the early stages, brass was used to serve as the material for the condenser and heat exchanger tubes. Admirality brass or Naval brass is used for fresh water systems, while aluminium AI brass is the preferred material for seawater systems. Subsequently cupronickels were developed and the alloy found wide applications in seawater systems. The 70 - 10 allo , is better suited, but is not much used in view of its high cost. II' all these copper based alloys, the release of cupric ions due to corrosion is considered to be a good antibiofouling measure, in view of the toxicity of copper to such organisms.
When the need for total leak proof condensers arose, titanium Ti came into use as the tube material. However, it has no resistance to biofouling. Another equally good alloy is stainless steel, especially some of the new stainless steels that were developed with this specific application in mind. Again stainless steel has no in-built resistance to biofouling.
In most cases the tube sheet is of carbon steel, sometimes overlaid with materials like stainless steel and titanium. For seawater applications employing either Cu-Ni or Ti' or even SS, there is no chemical treatment for erosion and corrosion prevention. However, with Al brass being a widely used material, a chemical treatment was found that is effective against such types of attack. Leakages in condensers, where water with a high salt content is used for cooling, have almost always given rise to serious effects in the operation of power stations l6.
However, immediate damage bears no relation to the possible consequential damage. In extreme cases, on the spot intrusion of cooling water containing sodium chloride leads to equipment breakdowns such as a tube burst in the boiler or breakages of turbine parts in areas having contact with wet stream. A prolonged search for leak dt'tcction and fixing becomes necessary. The major part of the damage to condensers was substantively due to the corrosion of the condenser tubes.
When copper alloyed tubes were used, the damage was caused by local corrosive attack on the tubing. The resistance to corrossion of copper alloyed tube materials in braekish or seawater is said to be due to the formation of a natural covering film of cuprous oxide. He produced detailed statistical data relating to the addition of ferrous sulphate versus the number of tube failures By way of explanation, Bostwick envisaged the iron additive acting as compensation for the loss of 'natural' iron which had been available from the water boxes prior to installation of cathodic protection.
Ferrous sulphate dosing is now widely practised, but there is still no agreement on the mechanism of protection. One way to look at this situation is by considering the principle of Point of Zero Charge PZC of the original oxide layers and its interaction with iron hydrous oxides having a different PZC.
Iron may also act as a cathodic inhibitor or can be incorporated into a cuprous oxide Cu 20 film, by a mechanism similar to that found when iron or nickel are incorporated from cupro-nickel alloys. This gives It a greater pllssivity. Other workers believe that a relatively thick film of hydrated ferric oxide if formed which reduces the erosive action of seawater and enables a protective film to form on the brass itself.
This view implies that the iron film is effective in the commissioning stage of condenser operation and that there after iron neither contributes to the passivation action nor is necessary. There may then be a possibility of reducing or discountinuing iron additions to the cooling water once service conditions are well established.
However, this has not been realised in practice at all. Of the many conceivable and proposed measures, the presence of ferrous oxide layers has proved most efficient in many respects. It is basically of no consequence whether these oxide layers are consciously produced, for example, by ferrous sulphate FeS04 injection or by making use of sacrificial anodes, or whether uncontrollably formtd, for example, via the corrosion of steel tubes in the cooling water feed lines.
The main thing is that conditions of uniform distribution, good adhesion and reconstitution ofthe'film are maintained. What concentrations and sequences are finally chosen will depend on the organic matter and sulphide content in the cooling water; especially during back washing of the condensers. Small quantities will be chosen as far as possible because of the formation of slime. The incorporation of iron takes place through negatively or positively charged colloids of hydrated iron oxides, either under the influence of the electric field of the Cu 20 covering film with a positive Zeta potential, or by electrolytic precipitation on the cathode surface of the epitactic layer.
It is therefore not expedient to inject FeS0 4 directly into pickled tubes, i. Depending on the injection method, brown uniform layers of varying structure are formed on ,the inner surfaces of the tubes after some weeks.
The thickness of these layers is 7S microns. On continuous cleaning with sponge rubber balls or brushes, glossy reddish-brown films with a thickness of only a very few microns appear. The thickness and roughness of the films increases proportionally to the length of the cleaning interval.
There are difference of opinion with regard to the cleaning intervals for condenser tubes having ferrous sulphate injectiC'n. Whilst some aim for a firmly adhering and consolidated external protective layer with continuous cleaning, others are of the opinion that, on account of the formation of natural oxide films, a cleaning period of only one hour at a time is acceptable.
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