Water Technology: Hardness of water and types of hardness Domestic water treatment: Brief discussion of coagulation and ...
A Textbook on:
E
ngineering
C
hemistry
1/e 2012-13 onwards
THE MANAGEMENT CONSORTIUM First Edition: 2012-13 onwards MRP: - ` 150/Student’s Discounted Price: - ` 100/Published by: TMC Gokulpeth, Nagpur
Contact : 9422864426
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Preface
T he p urp os e o f t his te xt book is t o p res ent an int r od uct ion t o th e su bj e ct of E ng i ne er in g C he m ist ry of B a ch e lor o f E ng i ne er in g ( BE ) Se me ste r -I. Th e bo ok co nt a ins th e sy lla bus f r om bas ics o f th e su bj e cts go ing in to th e in t rica cies o f th e su bj e cts. A ll the con ce pts h av e be en e xp la in e d wit h r e le va nt examp les a nd d iag rams to m ake it int e res t ing fo r the r ea de rs. An at tem pt is ma de h e re by th e e xpe rts o f TMC t o ass is t th e stu de nts by way of p ro v id ing S tu dy t ex t as p er th e cu rr icu lum wit h no n - comm er cia l cons id er at ions. W e o w e to ma ny w eb si te s a nd t h ei r fr ee con t en t s; w e wo u ld l i ke t o spe cia l ly a ck now l edg e con t en t s of we b sit e w ww . wi k ip ed i a . com a n d v ar io u s a ut ho rs w ho se w r it i ng s fo rme d th e b asi s fo r th is boo k. W e a ckno w le dge o ur th a nk s to t h em. A t th e end we wou ld lik e t o s ay tha t th er e is a lw ays a r oom fo r im pr ov eme nt in wh at ev er w e d o. We w ou ld app re ciat e a ny sugg est ions r ega r ding th is stu dy m at er ia l fr om th e re ad e rs s o t ha t the con te nts can be m ad e m o re int er est ing and m ean in gf u l. R ea de r s can em ai l t he ir que r ie s an d do ub t s to
[email protected]. W e sh a l l b e g la d to h e lp yo u im me di a tel y.
Edi ted and Co mpil ed by : Team TMC Nagpu r
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Syllabus and TMC Contents Unit 1
Syllabus and TMC Contents
Page
Water Technology: Hardness of water and types of hardness
05
Domestic water treatment: Brief discussion of coagulation and sterilization using UV. Ozone, chlorine, Break point chlorination. Softening of water-principle, reactions, advantages, limitations and Comparison of – Lime-Soda process, Zeolite process, and de-mineralization process. Boiler Troubles-(causes, effect on boiler operation and methods of prevention) – Carry over-priming and foaming; Scales and sludges, caustic embrittlement, boiler corrosion; internal conditioning-phosphate, carbonate, calgon conditioning. Numerical based on lime-soda and Zeolite process. Desalination-using electro dialysis and reverse osmosis processes. Waste water treatment (introduction and importance) – Brief idea about tertiary treatment methods.
2
Corrosion Science: Introduction, Causes and Consequence of corrosion, brief idea
75
about electrochemical & galvanic series, Factors influencing corrosion: a) Nature of metal b) Nature of environment, Chemical and electrochemical corrosion, Mechanisms
of electrochemical
corrosion; Pilling Bed worth rule; Differential aeration theory of corrosion. Types of Corrosion – Pitting, inter granular, stress, waterline and galvanic corrosion. Corrosion Prevention – a) Design and material selection b) Cathodic and anodic protection, c) Protective surface coatings- tinning, galvanizing and powder coating, metal cladding and electroplating.
3
Construction Materials : Cement: Portland cement – Raw material, Dry and wet process of manufacture, Proportion and role of microscopic constituents, Additives of cement ,Setting and hardening of cement; heat of hydration, soundness;
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117
Types of cement ( characteristics & applications ) – White, High alumina, Low heat ,Rapid hardening cement, Ready Mix Concrete, fly ash as cementing material( properties, advantages, limitations & application)
4
Green Chemistry and Battery Technology: Green Chemistry: Introduction,
149
Principles and significance, industrial application (supercritical fluids as Solvents, Example-super critical CO2 ), Biocatalysis and concept of carbon credits. Battery Technology: Types of batteries, primary, secondary and reverse batteries, important definition-energy density, power density. a) Secondary Battery: Lithium ion, Nickel-Cadmium b) Fuel cell application, advantages and limitation (Example: Alkaline fuel Cell).
180
Bibliography and further readings; Model Question Paper
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Unit
1
: Water Technology
Introduction Water is the most abundant, familiar and important of all compounds that occur in nature. It is found in the liquid state as water of lakes, rivers and oceans, in the solid state as thick layers of ice in Polar Regions and as water vapour in air. It is this water vapour that returns to the earth as rain, fog, snow and dew. Water is the very essence of life of almost all plants and animals. A proper balance between its consumption and excretion is vital to prevent dehydration of living organism. Water cannot be produced or added as and when required by any technological means. The major constituent of water in any case is H2O which is the internal medium for almost all organisms, and principal external medium for several organisms. Although biological and physio-chemical properties of the pure chemical water are fascinating, its use or consumption determines its importance. Water is the substance which is present in all the three states of matter, i.e. gaseous, liquid and solid within the ranges of temperature and pressure common to the earth. The availability of a water supply adequate in terms of both quantity and quality is essential for its use or consumption for different purposes like human use, crop production, industry etc. Many industrial and domestic water users are concerned about the hardness of their water. Hard water requires more soap and synthetic detergents for home laundry and washing, and contributes to scaling in boilers and industrial equipment. Hardness is caused by compounds of calcium and magnesium, and by a variety of other metals. Water is an excellent solvent and readily dissolves minerals it comes in contact with. As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution. Calcium and magnesium dissolved in water are the two most common minerals that make water "hard."
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Specification for Water Different uses of water demand different specifications: i) Textile Industry: Textile industry needs frequent dyeing of clothes and the water used by this industry should be soft and free from organic matter. Uniform dyeing is not possible with hard water as it decreases the solubility of acidic dyes. If water contains Fe, Mn, colour or turbidity, it causes uneven dyeing and leaves stains on fabrics. Hence, water should be free from these impurities. ii) Laundries: Laundries require soft water free from colour, Mn and Fe because hardness increases the consumption of soaps. Salts of Fe and Mn impart a grey or yellow shade to the fabric. iii) Boilers: Boilers require water of zero hardness otherwise; heat transfer is hindered by scale formation. Untreated water can also lead to corrosion of boiler materials. iv) Paper Industry: Paper Industry require water free from SiO2 (as it produces cracks in the paper); Turbidity (Fe and Mn as they affect the brightness and colour of the paper); Alkalinity (consumes alum and increases the cost of production); Hardness (as Ca and Mg salts increase the ash content of the paper.) v) Beverage: Beverages require which isn't alkaline as it destroys or modifies the taste as it tends to neutralize the fruit acids. vi) Dairies and Pharmaceutical Industries: Such industries require ultra pure water which should be colourless, tasteless, odourless and free form pathogenic organisms. Water needs to be treated to remove all the undesirable impurities. Water treatment is the process by which all types of undesirable impurities are removed from water and making it fit for domestic or industrial purposes.
Hardness of water and types of hardness Hard water is water that has high mineral content (in contrast with “soft water”). Hard water is generally not harmful to one's health, but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling towers, and other equipment that handles water. In domestic settings, hard water is often indicated by a lack of suds formation when soap is agitated in water. Wherever water hardness is a concern, water softening is commonly used to reduce hard water's adverse effects. People whose water comes from places with lots of
limestone or chalk have hard
water.
Remember that limestone and chalk are both made up mainly from calcium carbonate. So, perhaps hard water is produced when calcium [© 2 0 1 2 - 1 3 O n w a r d s : T M C T e x t b o o k o n E C ]
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carbonate dissolves in the water. Hard water is a very common problem, affecting water in more than 85% of the country. It is a result of the dissolved minerals calcium, magnesium and manganese. With an increase in these minerals, the following are seen:
Soap scum in sinks and bathtubs
Bathtub rings
Spots on dishes or shower doors
Reduced foaming and cleaning abilities of soaps and detergents
Dingy and yellowed clothes with soapy residues that require extra rinsing to remove
Clogged pipes from buildup of minerals
Increased water heating costs from buildup of minerals, reducing efficiency of water heaters
Possible skin infections from bacteria trapped in pores underneath soap scum
Definition Water which does not produce lather with water is known as hard water. It is due to some of the salts dissolved into the water. When we treat the water with soap, it gets precipitated in the form of insoluble salts of calcium and magnesium. CaCl2
+
2C17 H35 COONa
(From soap)
(Soap)
→
(C17 H35 COO)2 Ca +2NaCl (Insoluble precipitate)
Whereas, The soft water when treated with soap produces more lather and consumes less soap and this is due to the absence of dissolved salts of Ca & Mg in water. C17H35 COONa + H2O → NaOH + 2C17H35 COOH
Types of Hardness: 1. Temporary Hardness: Temporary hardness is caused by a combination of calcium ions and bicarbonate ions in the water. It is due to the presence of bicarbonates of calcium and magnesium. It can be easily removed by boiling. It can be removed by boiling the water or by the addition of lime (calcium hydroxide). Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling. Boil Ca (HCO3)2
→
CaCO3 ↓+ CO2 ↑+ H2 O
Boil Mg (HCO3)2
→
MgCO3 ↓+ CO2↑ + H2O
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Temporary Hardness:
Calcium bicarbonate:
Ca (HCO3)2
Mol.wt : 162gm/mole
Magnesium bicarbonate:
Mg (HCO3)2
Mol.wt : 146gm/mole
2. Permanent Hardness: Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises. This type of hardness cannot be removed by boiling. This is due to the presence of chlorides and sulphates of calcium and magnesium. The hardness can be removed by the addition of some agents. Permanent Hardness:
Calciumchloride
CaCl2
Mol.wt : 111gm/mole
Magnesiumchloride
MgCl2
Mol.wt : 95gm/mole
Calciumsulphate
CaSO4
Mol.wt : 136gm/mole
Magnesiumsulphate
MgSO4
Mol.wt : 120 gm/mole
How is hardness expressed? Water hardness is, unfortunately, expressed in several different units, and thus it is often necessary to convert from one unit to another when making calculations. The most commonly used units include grains per gallon (gpg), parts per million (ppm), and milligrams per liter (mg/L). The grain per gallon is based on the old English system of weights and measures. It is based on the average weight of a dry kernel of grain (or wheat). The part per million is a weight to weight ratio. For example, one ppm of calcium means 1 pound of calcium in 1 million pounds of water, or 1 gram of calcium in 1 million grams of water. Since pure water weighs 1000 grams per liter, the mg/L is the same as the ppm in the dilute solutions present in most raw and treated water. Units of Hardness: 1. Parts per million (ppm) 2. Milligrams per litre (mg/l) 3. Degree French (ºFr) 4. Degree Clark (ºCl) Relationship: 1ppm = 1mg/L = 0.1 ºFr = 0.07 ºCl To convert
To
Multiply by
Grains per gallon
Milligrams per liter
17.12
Milligrams per liter
Grains per gallon
0.05841
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The hardness of water is referred to by three types of measurements: grains per gallon, milligrams per liter (mg/L), or parts per million (ppm). Typically, the water produced by Fairfax Water is considered "moderately hard" to "hard." The table below is provided as a reference.
less than 1.0 1.0 - 3.5 3.5 - 7.0
Water Hardness Scale Milligrams Per Liter (mg/L)or Parts Per Million (ppm) less than 17.1 17.1 - 60 60 - 120
7.0 - 10.5 over 10.5
120 - 180 over 180
Grains Per Gallon
Classification Soft Slightly Hard Moderately Hard Hard Very Hard
Since calcium carbonate is one of the more common causes of hardness, total hardness is usually reported in terms of calcium carbonate concentration (mg/L as CaCO 3), using either of two methods:
a) Calcium and magnesium hardness. b) Carbonate and non-carbonate hardness. (a) Hardness caused by calcium is called calcium hardness, regardless of the salts associated with it. Likewise, hardness caused by magnesium is called magnesium hardness. Since calcium and magnesium are normally the only significant minerals that cause hardness, it is generally assumed that: Total Hardness = Calcium Hardness + Magnesium Hardness (mg/L as CaCO3) (mg/L as CaCO3)
(mg/L as CaCO3)
= 2.50 X Calcium concentration + 4.12 X Magnesium concentration 2+
(mg/L as Ca )
2+
(mg/L as Mg )
(b) Carbonate hardness is primarily caused by the carbonate and bicarbonate salts of calcium and magnesium. Non-carbonate hardness is a measure of calcium and magnesium salts other than carbonate and bicarbonate salts, such as calcium sulfate, CaSO4, or magnesium chloride, MgCl2. Total hardness is expressed as the sum of the carbonate hardness and non-carbonate hardness. Total hardness = Carbonate hardness + Non-carbonate hardness (mg/L as CaCO3) (mg/L as CaCO3)
(mg/L as CaCO3)
The amount of carbonate and non-carbonate hardness depends on the alkalinity of the water. When a laboratory reports a value for total hardness of, for instance, 150 mg/l as CaCO 3, this indicates that the combined effect of the different hardness causing agents is the same as if the water contained exactly 150 mg/l of CaCO3.
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There are advantages and disadvantages for people who live in hard water areas.Some of the advantages and disadvantages of hard water are as follows:
Disadvantage of hard water 1. Domestic:
Hard water affects cleaning ability of soap.
When hard water is used for washing, large amount of soap is consumed.
2. Industrial:
Hard water can cause "Scaling" inside the pipes that transport water. Therefore if we use hard water in turbines and heat exchangers, their pipes will be corroded
3. Health:
Hard water when used for drinking for long period can lead to stomach disorders. Especially hard water contains magnesium sulphate can weaken the stomach permanently.
Advantages of hard water 1.
Some people prefer the taste.
2. Calcium ions in the water are good for children's teeth and bones. 3.
It helps to reduce heart disease.
4.
Some brewers prefer using hard water for making beer.
5. A coating of lime scale inside copper pipes, or especially old lead pipes, stops poisonous salts dissolving into water.
Softening of water Hard water is a problem for millions of households across the country. Hard water is water that has high mineral content. Due to the minerals present in hard water, the sides of pipe lines are clogged. Over a period of time, deposits and build up prevent water to flow like it should, pipes becomes too small to allow fast passage of water. Once the holes become smaller, the pressure in pipes increases so much that there is a high risk of the lines deteriorating or may burst. Softened water still contains all the natural minerals that we need. It is only deprived off its calcium and magnesium contents, and some sodium is added during the softening process. Water softening is the reduction of the concentration of calcium, magnesium, and certain other metal cations in hard water. These "hardness ions" can cause a variety of undesired effects including interfering with the action of soaps, the buildup of lime scale, which can foul plumbing, and galvanic corrosion. Conventional water-softening appliances intended for household use depend on an ion-exchange resin in which hardness ions are exchanged for sodium ions. Water softening may be desirable where the source of water is hard.
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The process whereby we remove or reduce the hardness of water irrespective of whether it is temporary or permanent is termed as ‘softening’ of water. Water softening is very essential since hard water is unsuitable for domestic as well as industrial use. Water can be made soft by external as well internal treatment.
To remove hardness from water, three methods are used on a large scale:
(1) Lime-Soda process (2) Zeolite process or Permutit process (3) De-mineralization process or Ion-exchange process
Principle It is the process by which hard water is converted to soft water by removing inorganic impurities in it as an insoluble precipitate. Hard water can be of two types, temporary and permanent. They can soften as follows:
Temporary hard water is softened by: ● Simple boiling; calcium/magnesium bicarbonates are precipitated as insoluble carbonates
Permanent hard water is softened by adding: ● Washing soda (sodium carbonate). . ● Lime (calcium hydroxide) and washing soda mixture. ● Ammonia ● Sodium hydroxide ● Sodium tetraborate (Borax) ● Trisodium phosphate
Permanent hard water contains sulphates and chlorides of calcium or magnesium. Boiling the water does not remove them. Only addition of the substances listed above will remove them and make the water soft.
1. Lime-Soda process Lime-Soda is a process used in water treatment to remove hardness from water. This process is now obsolete but was very useful for the treatment of large volumes of hard water. Chemical precipitation is one of the more common methods used to soften water. The chemicals normally used are lime (calcium hydroxide, Ca(OH)2) and soda ash (sodium carbonate, Na2CO3). Lime is used to remove the chemicals that cause the carbonate hardness. Soda ash is used to remove the chemicals that cause the non-carbonate hardness.
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When lime and soda ash are added, the hardness-causing minerals form nearly insoluble precipitates. When calcium
hardness
is
removed in a chemical softener, it is precipitated as
calcium
carbonate
(CaCO3).
When
magnesium hardness is removed in a chemical softener, it is precipitated as magnesium hydroxide (Mg(OH)2).
These
precipitates are removed by the conventional processes of coagulation/flocculation, sedimentation, and filtration. Because the precipitates are very slightly soluble, some hardness remains in the water--usually about 50 to 85 mg/l (as CaCO3). This hardness level is desirable to prevent corrosion problems associated with water being too soft and having little or no hardness ions.
Reactions: What are the chemical reactions that happen with lime addition? Hardness
Softening
species
chemical
Precipitate
CO2.
+
Ca(OH)2
->
CaCO3 + H2O
Ca(HCO3)2
+
Ca(OH)2
->
2CaCO3 + 2H2O
Mg(HCO3)2
+
Ca(OH)2
->
CaCO3 + MgCO3 + 2H2O
MgCO3
+
Ca(OH)2
->
CaCO3 + Mg(OH)2
CO2 = carbon dioxide, Ca(OH)2 = calcium hydroxide or hydrated lime, CaCO3 = calcium carbonate, Ca(HCO3)2
=
calcium bicarbonate, Mg(HCO3)2 = magnesium bicarbonate, MgCO3 = magnesium carbonate,
Mg(OH)2 = magnesium hydroxide, MgSO4 = magnesium sulfate, CaSO4 = calcium sulfate, H2O - water.
What are the chemical reactions with soda ash? MgSO4
+
Ca(OH)2
->
Mg(OH)2 +CaSO4
CaSO4
+
Na2CO3
->
CaCO3 + Na2SO4
Na2CO3 = sodium carbonate or soda ash For each molecule of calcium bicarbonate hardness removed, one molecule of lime is used. For each molecule of magnesium bicarbonate hardness removed, two molecules of lime are used.
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For each molecule of non-carbonate calcium hardness removed, one molecule of soda ash is used. Each molecule of non-carbonate magnesium hardness requires one molecule of lime plus one molecule of soda ash.
Lime soda processes are of two types: a) Cold lime soda process b) Hot lime soda process a. Cold lime soda process: In this process, calculated quantities of lime and soda are mixed with water at room temperature. The precipitate formed at room temperature is finely divided and does not settle down easily. They cannot be easily filtered so, it is essential to add a small quantity of coagulant which hydrolysis to give flocculent and gelatinous precipitate of aluminium hydroxide, thus it entraps the fine precipitate. There are two kind of softeners used for softening water by this process. i) Intermittent type softeners, where the softening is done by batch process. ii) Continuous type softeners.
b. Hot lime soda process: In this process, water is treated with chemicals at a temperature of 94-1000C. The softeners used are of intermittent type or continuous type. Advantages of Hot lime soda process: i) They are more rapid in operation. The time taken for completion is 15 minutes and several hours for hot and cold lime soda processes respectively. ii) Elevated temperature accelerates the actual chemical reaction and reduces the viscosity of the water. This increases the rate of aggregation of the particle. Hence, both the setting rates and filtration rates are increased. Thus the softening capacity of the hot process is several times higher than the cold process. iii) The sludge and the precipitate formed settle down rapidly and hence no coagulant is needed. iv) Quantity of chemical required for softening is low. v) At the higher temperature, the dissolved gases such as CO 2 are driven out of the solution to some extent. Advantages of Lime-soda Process: i) It is very economical. ii) Treated water is alkaline and hence has less corrosion tendencies. iii) It removes not only hardness causing salt but also minerals. iv) Due to alkaline nature of treated water, amount of pathogenic bacteria in water is considerably reduced. v) Iron and manganese are also removed from the water to some extent. Disadvantages of Lime-soda process: i) It requires careful operation and skilled supervision for economical and efficient softening. [© 2 0 1 2 - 1 3 O n w a r d s : T M C T e x t b o o k o n E C ]
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ii) Sludge disposal is a problem. iii) Water softened by this process contains appreciable concentration of soluble salts, such as sodium sulphate and cannot be used in high pressure boiler.
2. Zeolite process or Permutit process Permutit is also known as Zeolite. They are capable of exchanging ions reversibly. The chemical formula for permutit is Na2O, Al2O3SiO2 6H2O. In short it is written as Na2-P or Na2-Z. For softening of water by this method, hard water is percolated at a specified rate through a bed of zeolite kept in a cylinder. The hardness causing ions (Ca++ & Mg++) are retained by the permutit as Ca-P & Mg-P. While the outgoing water contains sodium salts.
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2. Alkaline Electrolyte Fuel Cell (AFC) Alkaline fuel cells (AFCs) were one of the first fuel cell technologies developed, and they were the first type widely used in the U.S. space program to produce electrical energy and water onboard spacecraft. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. High-temperature AFCs operate at temperatures between 100°C and 250°C (212°F and 482°F). However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F)
Reactions The electrolyte employed in AFC is potassium hydroxide and unlike the acid electrolyte fuel cell where H+ is conducted from the anode to the cathode in the AFC OH- is conducted from the cathode to the anode. The reaction at the anode and cathode is shown below:
Water is produced twice as fast as is been consumed hence the need to remove excess water to avoid dilution of the electrolyte. The main limitation of AFC for terrestrial application is the degradation the KOH electrolyte by CO2 poisoning as shown below, therefore pure H2-O2 must be used.
Advantages 1. Advantages offered by AFC include improved cathode performance, non-precious metal such as nickel can be used as catalyst and extremely inexpensive electrolyte. 2. AFCs' high performance is due to the rate at which chemical reactions take place in the cell. They have also demonstrated efficiencies near 60 percent in space applications.
Disadvantages 1.
The disadvantage of this fuel cell type is that it is easily poisoned by carbon dioxide (CO2 ). In fact, even the small amount of CO2 in the air can affect this cell's operation, making it necessary to purify both the hydrogen and oxygen used in the cell. This purification process is costly. Susceptibility to poisoning also affects the cell's lifetime (the amount of time before it must be replaced), further adding to cost.
2.
Cost is less of a factor for remote locations such as space or under the sea. However, to effectively compete in most mainstream commercial markets, these fuel cells will have to become more costeffective. AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to
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reach operating times exceeding 40,000 hours, something that has not yet been achieved due to material durability issues. This is possibly the most significant obstacle in commercializing this fuel cell technology.
3. Phosphoric Acid Fuel Cell (PAFC) Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte—the acid is contained in a Teflonbonded
silicon
carbide
matrix—and
porous
carbon
electrodes
containing a platinum catalyst. The chemical reactions that take place in the cell are shown in the diagram to the right.
Reactions Phosphorous acid fuel cell uses liquid H3PO4 as electrolyte. The electrolyte is embedded in SiC matrix between two porous graphite electrodes coated with a platinum catalyst. The half reaction is as follows:
PAFC will operate optimally in the temperature range of about 180- 210 0C
with electrical efficiency of about 40% but could be as high as 70%
when used in a combined heat and power application. The technology of PAFC is relatively matured but current research interest is how to make it cost competitive with conventional power technologies.
Advantages The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially, with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells, which are easily "poisoned" by carbon monoxide—carbon monoxide binds to the platinum catalyst at the anode, decreasing the fuel cell's efficiency. They are 85 percent efficient when used for the co-generation of electricity and heat, but less efficient at generating electricity alone (37 to 42 percent). This is only slightly more efficient than combustion-based power plants, which typically operate at 33 to 35 percent efficiency.
Disadvantages PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. Like PEM fuel cells, PAFCs require an expensive platinum catalyst, which raises the cost of the fuel cell. A typical phosphoric acid fuel cell costs between $4,000 and $4,500 per kilowatt to operate.
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4. Molten Carbonate Fuel Cell (MCFC) Molten carbonate fuel cells (MCFCs) are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications. MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO2) matrix. Since they operate at extremely high temperatures of 650°C (roughly 1,200°F) and above, non-precious metals can be used as catalysts at the anode and cathode, reducing costs.
Reactions In MCFC the electrolyte is a molten mixture of Li2CO3 and K2CO3 sustained by LiOAlO2 matrix. The mobile charge carrier is carbonate ion, CO32- and the reaction at the anode and cathode is as follows:
There is the need for recirculation of CO2 because the CO2 is produced at the anode to be consumed at the cathode. It does not suffer from CO poisoning like most other fuel cell, the CO is actually a fuel. The anode electrode is usually nickel/chromium alloy while the cathode is made of lithiated nickel oxide hence the advantages of using non-precious catalyst. It enjoys fuel flexibility as hydrogen, methane and simple alcohol could be used as fuel.
Advantages One of its advantage is the corrosive nature of the molten electrolyte. MCFC is most suitable for stationary, continuous power application. Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells (PAFCs). Molten carbonate fuel cells can reach efficiencies approaching 60 percent, considerably higher than the 37-42 percent efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be as high as 85 percent. Unlike alkaline, phosphoric acid, and polymer electrolyte membrane fuel cells, MCFCs don't require an external reformer to convert more energy-dense fuels to hydrogen. Due to the high temperatures at which MCFCs operate, these fuels are converted to hydrogen within the fuel cell itself by a process called internal reforming, which also reduces cost.
Disadvantages Molten carbonate fuel cells are not prone to carbon monoxide or carbon dioxide "poisoning" —they can even use carbon oxides as fuel—making them more attractive for fueling with gases made from coal. Because they are more resistant to impurities than other fuel cell types, scientists believe that they could even be capable [© 2 0 1 2 - 1 3 O n w a r d s : T M C T e x t b o o k o n E C ]
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of internal reforming of coal, assuming they can be made resistant to impurities such as sulfur and particulates that result from converting coal, a dirtier fossil fuel source than many others, into hydrogen. The primary disadvantage of current MCFC technology is durability. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life. Scientists are currently exploring corrosion-resistant materials for components as well as fuel cell designs that increase cell life without decreasing performance.
5. Solid Oxide Fuel Cell (SOFC) Solid oxide fuel cells (SOFCs) use a hard, non-porous ceramic compound as the electrolyte. Since the electrolyte is a solid, the cells do not have to be constructed in the plate-like configuration typical of other fuel cell types. SOFCs are expected to be around 50-60 percent efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 80-85 percent.
Reactions SOFC is made of solid oxide ceramic electrolyte sandwiched between two porous electrodes. The common electrolyte material is yttria-stabilised Zirconia (YSZ). The anode is usually made of Ni/8YSZ material while typical cathode material is strontium doped LaMnO3 (LSM). In SOFC O2- is the mobile ion conductor and the reactions at the anode and cathode are
Water is produced at the anode unlike PEMFC, AFC and PAFC in which water is produced at the cathode. The operating temperature is between 600 and 10000C. Although the high operating temperature could lead to low open circuit voltage but at the same time it increases its performance with the possibility of inwardly processing hydrocarbon. Solid oxide fuel cells operate at very high temperatures—around 1,000°C (1,830°F). High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels and reduces the cost associated with adding a reformer to the system.
Advantages SOFCs are also the most sulfur-resistant fuel cell type; they can tolerate several orders of magnitude more sulfur than other cell types. In addition, they are not poisoned by carbon monoxide (CO), which can even be used as fuel. This allows SOFCs to use gases made from coal.
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Disadvantages 1.
High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation and small portable applications. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.
2.
Scientists are currently exploring the potential for developing lower-temperature SOFCs operating at or below 800°C that have fewer durability problems and cost less. Lower-temperature SOFCs produce less electrical power, however, and stack materials that will function in this lower temperature range have not been identified.
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Fuel Cell Comparison Chart Fuel Cell
Common Electrolyte
Type
Operating
Applications
Advantages
Disadvantages
Temperature
Polymer
Solid organic polymer
50 - 100°C
Backup power
Solid electrolyte
Requires expensive
Electrolyte
poly-perfluorosulfonic
122 - 212°F
Portable power
reduces corrosion
catalysts
Membrane
acid
Small
& electrolyte
High sensitivity to
distributed
management
fuel impurities
generation
problems
Low temperature
Transportation
Low temperature
waste heat
Quick start-up
Waste heat
(PEM)
temperature not suitable for combined heat and power (CHP) Alkaline
Aqueous solution of
90 - 100°C
Military
Cathode reaction
Expensive removal of
(AFC)
potassium hydroxide
194 - 212°F
Space
faster in alkaline
CO2 from fuel and air
electrolyte, higher
streams required
performance
(CO2 degrades the
soaked in a matrix
electrolyte) Phosphoric
Liquid phosphoric acid
150 - 200°C
Distributed
Higher overall
Requires expensive
Acid (PAFC)
soaked in a matrix
302 - 392°F
generation
efficiency with
platinum catalysts
CHP
Low current and
Increased
power
tolerance to
Large size/weight
impurities in hydrogen Molten
Liquid solution of
600 - 700°C
Electric utility
High efficiency
High temperature
Carbonate
lithium, sodium,
1112 - 1292°F
Large
Fuel flexibility
speeds corrosion and
(MCFC)
and/or potassium
distributed
Can use a variety
breakdown of cell
carbonates soaked in a
generation
of catalysts
components
Suitable for CHP
Complex electrolyte
matrix
management Slow start-up Solid Oxide
Solid zirconium oxide
650 - 1000°C
Auxiliary
High efficiency
High temperature
(SOFC)
to which a small
1202 - 1832°F
power
Fuel flexibility
enhances corrosion
amount of Yttria is
Electric utility
Can use a variety
and breakdown of
added
Large
of catalysts
cell components
distributed
Solid electrolyte
Slow start-up
generation
reduces
Brittleness of ceramic
electrolyte
electrolyte with
management
thermal cycling
problems Suitable for CHP
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4) Reserve Batteries A fourth battery category is commonly referred to as the reserve cell. Also called deferred-action batteries, reserve batteries are special purpose primary batteries usually designed for emergency use. The electrolyte is usually stored separately from the electrodes which remain in a dry inactive state. The battery is only activated when it is actually needed by introducing the electrolyte into the active cell area. This has the double benefit of avoiding deterioration of the active materials during storage and at the same time it eliminates the loss of capacity due to self discharge until the battery is called into use. They can thus be stored for 10 years or more yet provide full power in an instant when it is required.
Definitions A reserve battery, also called stand-by battery, is a primary battery where part is isolated until the battery needs to be used. When long storage is required, reserve batteries are often used, since the active chemicals of the cell are segregated until needed, thus reducing self-discharge. A reserve battery is distinguished from a backup battery, in that a reserve battery is inert until it is activated, while a backup battery is already functional, even if it is not delivering current. Reserve batteries have been designed using a number of different electrochemical systems to take advantage of the long unactivated shelf life achieved by this type of battery design. Relatively few of these have achieved wide usage because of the lower capacity of the reserve structure (compared with a standard battery of the same system), poorer shelf life after activation, higher cost, and generally acceptable shelf life of active primary batteries for most applications. For the special applications that prompted their development, nevertheless, the reserve structure offers the needed advantageous characteristics. In recent years, however, the use of reserve batteries has declined because of the improved storability of active primary batteries and the limited number of applications requiring extended storage. Most of these applications are for special military weapon systems. The reserve batteries are usually designed for specific applications, each design optimized to meet the requirements of the application. What differentiates the reserve cell from primary and secondary cells in the fact that a key component of the cell is separated from the remaining components, until just prior to activation. The component most often isolated is the electrolyte. This battery structure is commonly observed in thermal batteries, whereby the electrolyte remains inactive in a solid state until the melting point of the electrolyte is reached, allowing for ionic conduction, thus activating the battery. Reserve batteries effectively eliminate the possibility of selfdischarge and minimize chemical deterioration. Most reserve batteries are used only once and then discarded. Reserve batteries are used in timing, temperature and pressure sensitive detonation devices in missiles, torpedoes, and other weapon systems.
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Types and Activation The reserve batteries can be classified by the type of activating medium or mechanism that is involved in the activation:
1. Water-activated batteries: Activation by fresh- or seawater. Activation of the reserve battery is accomplished by adding the missing component just prior to use. In the simplest designs, this is done by manually pouring or adding the electrolyte into the cell or placing the battery in the electrolyte (as in the case of sea water activated batteries).
2. Electrolyte-activated batteries: Activation by the complete electrolyte or with the electrolyte solvent. The electrolyte solute is contained in or formed in the cell. In more sophisticated applications the electrolyte storage and the activation mechanism are contained within the overall battery structure, and the electrolyte is brought automatically to the active electrochemical components by remotely activating the activation mechanism. The trigger for activation can be a mechanical or electrical impulse, the shock and spin accompanying the firing of a shell or missile, and so on. Activation can be completed very rapidly if required, usually in less than one second. The penalty for automatic activation is a substantial reduction in the specific energy and/or energy density of the battery due to the volume and weight of the activating mechanism. It is therefore not general practice to rate these batteries in terms of specific energy or energy density.
3. Gas-activated batteries: Activation by introducing a gas into the cell. The gas can be either the active cathode material or part of the electrolyte. The gas-activated batteries are a class of reserve batteries which are activated by introduction of a gas into the battery system. There are two types of gas-activated batteries: those in which the gas serves as the cathodic active material and those in which the gas serves to form the electrolyte. The gas-activated batteries were attractive because they offered the potential of a simple and positive means of activation. In addition, because the gas is nonconductive, it can be distributed through a multicell assembly without the danger of short-circuiting the battery through the distribution system. Gas-activated batteries are no longer in production, however, because of the more advantageous characteristics of other systems.
4. Heat-activated batteries: A solid salt electrolyte is heated to the molten condition and becomes ionically conductive, thus activating the cell. These are known as thermal batteries. The thermal or heat-activated battery is another class of reserve battery. It employs a salt electrolyte, which is solid and, hence, nonconductive at the normal storage temperatures when the battery must be inactive. The battery is activated by heating it to a temperature sufficiently high to melt the electrolyte, thus making it ionically conductive and permitting the flow of current. The heat source and activating mechanism, which can be set off by electrical or mechanical means, can be built into the battery in a compact configuration to give very rapid activation. In the inactive stage the thermal battery can be stored for periods of 10 years or more.
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Review Questions 1.
What is a battery? How does it differ from a cell?
2.
What are the important requirement of a battery?
3.
What is a primary battery? Give an example. Or What are primary cells?
4.
What are secondary cells? What are the advantages of alkaline battery over dry battery?
5.
Write the cell representation of lead storage cell?
6.
Describe lithium battery. What are the advantages of Li-S battery?
7.
Lithium battery is the cell of future, why?
8.
What is a primary battery? Give an example.
9.
Write a note on Ni – Cd battery.
10. Explain the construction and working of lead – acid storage battery. 11. What is reversible battery? Describe the construction and working of a lead storage battery with the reaction occurring during charging and discharging. 12. What is lithium battery? Give the reactions involved. 13. How NICAD battery constructed. Explain with cell reaction . 14. Give the description of lead storage battery and explain its functioning during discharging and recharging. 15. Define Fuel cell. Explain alkaline fuel Cell. Give its merits and demerits. 16. Define green chemistry. 17. Which of the Twelve Principles of Green Chemistry deals with atom economy?
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Bibliography and Further Readings 1.
A Text book of Engineering Chemistry: Shashi Chawla; Dhanpat Rai & Sons, New Delhi.
2.
Applied Chemistry by N. Krishnamurthy:P. Vallinavagam. And K. Jeysubramanian TMH
3.
Applied Chemistry for Engineers: T.S. Gyngell.
4.
Applied Chemistry: A.V. Bharati and Walekar, Tech Max Publications, Pune.
5.
Bateman, John H., Materials of Construction Publishing Corporation, London.
6.
Chemistry of Advanced Materials: CNR Rao, Rsc Publication.
7.
Chemistry of Engineering Materials: Robert B Leighou Mc Graw – Hill Book Company, Inc New York
8.
Engineering Chemistry: Arty Dixit Dr. Kirtiwardhan Dixit, Harivansh Prakashan, Chandrapur.
9.
Engineering Materials: Kenneth G Budinski (Prentice – Hall of India)
10. Fundamentals of Corrosion: Michael Henthorne, Chemical Engineering. 11. Fundamentals of Engineering Chemistry (Theory and Practice) :S. K. Singh (New Age Materials 12. G Nagendrappa, Resonance, Vol.7, No.1, pp.64–76; No.10, pp.59–68; 13. http://en.wikipedia. org/wiki/Green, chemistry 14. http://www.epa.gov/greenchemistry/pubs/whats_gc.html 15. IS : 2547 (Part 1) 1967, Specification for Gypsum Building Plasters, 16. IS : 7i2-1973, Specification For Building Limes. 17. Larminie, J., Dicks, A. and Knovel, ( 2003), Fuel cell systems explained, 2nd ed., J. Wiley, Chichester, West Sussex. 18. Leena Rao, Resonance, Vol.12, No.8, pp.65–75; No.10, pp.30–36, 2007. 19. Makuch G. (2004), Micro Fuel Cells Strive for Commercialization 20. Materials Science and Engineering an Introduction, William D. Callister, (Jr. Wiley publisher). 21. Neville, A.M. Properties of Concrete, Pitnian Publishing, London. 22. Sammes, N.M. and SpringerLink, ( 2006), Fuel cell technology, Springer, London 23. Shetty, M.S., Concrete Technology, Chand &L Company Ltd., New Delhi. 24. Text Book of Engineering Chemistry: S.S. Dara, S. Chand and Company Ltd. New Delhi. 25. Textbook of Engineering Chemistry: P.C. Jain and Monica Jain, Dhanpat Rai and Sons, New Delhi. 26. Textbook of Engineering Chemistry: S.N. Narkhede, R.T. Jadhav, AB. Bhake, A.U. Zadgaonkar, Das Ganu Prakashan, Nagpur. 27. VKAhluwalia andMKidwai, NewTrends inGreenChemistry, Anamaya Publishers, New Delhi, 2004. 28. Water Treatment : F. I. Bilane, Mir publisher 29. Winterbone, D.E., (1997), Advanced Thermodynamics for Engineers, Elsevier. 30. Zhang J. (2008), PEM Fuel Cell Electrocatalysis, Institute for Fuel Cell Innovation, National Research Council Canada.
`
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Modal Question Paper for Winter 2012 First Semester of Bachelor of Engineering (BE) Examination
Engineering Chemistry
Time: Two Hours]
[Max. Marks: 40
NB:1) All the Five Questions are compulsory. 2) All questions carry equal marks.
Q1. (a) Compare Lime-Soda process and Zeolite process for Softening of water.
Q4. (a) What is Green Chemistry? Explain twelve principles of green chemistry
or
or
(b) What is the importance of Waste water treatment? Give a brief idea about tertiary treatment methods.
(b) What are three major types of chemical batteries?
Mention
their
advantages
disadvantages?
Q2. What is the significance of Corrosion Science? Enlist causes and Consequence of corrosion.
Q5. Write short notes on (Any Two): (a) Reverse osmosis processes
or
(b) Pilling Bed worth rule
(b) Explain any three types of corrosions based on the reactions and physical states.
(c) Ready Mix Concrete (d) Concept of carbon credits
Q3. (a) Enlist the Proportion and role of microscopic constituents of cement?
How is
cement manufactured by Dry process? or (b) Can Fly ash be used as cementing material? Elaborate
its
advantages,
limitations
&
applications.
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and
