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  2. 2. 1 GOVERNMENT OF INDIA DEPARTMENT OF ATOMIC ENERGY NUCLEAR FUEL COMPLEX BONAFIDE CERTIFICATE This is to certify that MR.RATAN MONDAL has done his Project Work under my guidance during the period from 18th May 2015 to 17th June 2015 on the topic entitled ZIRCONIUM OXIDE PRODUCTION AND DESIGNING OF DOUBLE PIPE HEAT EXCHANGERS. During this period his conduct was found to be _________________ It is ensured that the report does not contain classified or Plant operational live data in any form. HEYDERABAD SIGNATURE: DATE: APPROVED BY, JOHNSON D’SOUZA THE MANAGER OF PLANT SENIOR MANAGER ZOP, NFC
  3. 3. 2 ACKNOWLEDGEMENTS First of all I am extremely thankful to Shri N. Saibaba, Chairman ,NFC Board & Chief Executive, NFC for giving me opportunity to carry out Project Work at Nuclear Fuel Complex. I would like to express my sincere thanks to Mr. Johnson D’souza(Senior manager) for accepting to be our Guide and helping us throughout our Project Work. I would like to thank Mr. Charan (S.O) and Mr. Arun Anand (T.O) for helping us throughout our Project Work. I express my sincere thanks to Shri H. R. Ravindra, DGM (HR) & Dr. B. N. Murty, AGM (HRD, Q. Cir., & QIS) for helping me throughout our training period at NFC and also conducting Awareness Programme on DAE/NFC activities at HRD. We are indebted to our director Dr. Shailendra Kumar and the Head of the Department of Chemical Engineering Mr.Neeraj Chandrakar, for making it possible to undergo in-plant training in Nuclear Fuel Complex, Hyderabad. I am grateful to my parents who have given me constant encouragement and inspiration to pursue my graduation. Finally I would like to thank everyone who have directly or indirectly help me in the successful completion of this Project.
  4. 4. 3 CONTENTS Error! Bookmark not defined. 1. INTRODUCTION_______________________________________________________________ 4 2. PRODUCTION OF ZIRCALLOY COMPONENTS __________________________________ 5 3. ABOUT ZIRCONIUM AND HAFNIUM ____________________________________________ 6 4. FLOW SHEET OF ZIRCONIUM OXIDE PRODUCTION ___________________________ 11 5. PROCESS OF PRODUCTION ___________________________________________________ 11 5.1 DISSOLUTION_______________________________________________________________ 13 5.2 SOLVENT EXTRACTION _____________________________________________________ 15 I. SLURRY EXTRACTION________________________________________________________ 17 II. SCRUBBING _________________________________________________________________ 18 III. STRIPPING _________________________________________________________________ 20 TREATMENT WITH SODA SOLUTION: ___________________________________________ 21 5.3 PRECIPITATION_____________________________________________________________ 21 5.4 REPULPING_________________________________________________________________ 23 5.5 VACUUM FILTRATION _______________________________________________________ 23 5.6 DRYING ____________________________________________________________________ 25 5.7 CALCINATION ______________________________________________________________ 26 5.8 GRINDING __________________________________________________________________ 28 5.9 BLENDING __________________________________________________________________ 29 6. PROPERTIES AND USES OF ZIRCONIUM OXIDE ________________________________ 30 7.HEAT EXCHANGERS AND ITS CLASSIFICATION_________________________________30 8. TYPES OF HEAT EXCHANGERS ________________________________________________32 9. DESIGNING OF DOUBLE PIPE HEAT EXCHANGERS ______________________________35 10. NUMERICAL ON DESIGNING DOUBLE PIPE HEAT EXCHANGERS________________40
  5. 5. 4 1. INTRODUCTION NEED OF NUCLEAR POWER From the dawn of civilization, mankind has depended upon nature’s gifts such as solar energy and wind energy, in addition to firewood as fuel. However, the discovery of coal and petroleum as fuels sparked the industrial development of humanity, and helped it grow by leaps and bounds. As a result, man became overly dependent on these, and exploited these resources greatly, unaware of sustainability issues, all well as environmental issues. In the present world, where all these issues are persisting as a cancerous growth, it is obvious that one must look for a cleaner source of power, which does not only save the environment, but is sustainable, and is available to us at a reasonable cost. Solar, wind and hydro energy satisfy most of these criteria, but fail terribly on certain counts. Considering all of this, one may wonder, what is the true solution to mankind’s problems? The answer is NUCLEAR POWER. Nuclear power is a clean power source. With proper and sustainable implementation it is virtually an inexhaustible source of energy. The same mass of nuclear fuel is capable of producing millions of times more energy than coal. In India, the second most populous country of the world, there is an exceeding increase in the need of power. Our coal is limited. So what do we do? The answer once again, is nuclear power. India’s approach to nuclear power production is unique, owing to the lesser uranium deposits in comparison to the thorium deposits. India is pursuing a three stage nuclear power program linking the fuel cycle of pressurized Heavy water plant reactor (PHWR) &liquid metal cooled fast breeder reactor (LMFBR). In addition light water reactor (LWR) has also seen included in program. The program is currently completing the second stage of its implementation, and once fully implemented can supply up to 30% of India’s power needs by 2050. The driving force behind nuclear energy is nuclear fuel. And this is where the Nuclear Fuel Complex comes into play. THE NUCLEAR FUEL COMPLEX (NFC) Nuclear Fuel Complex (NFC) is an industrial unit under the Department of Atomic Energy (DAE). It manufactures enriched Uranium Oxide Fuels and zirconium alloy structural components for water-cooled nuclear power reactors in India. NFC is perhaps the only facility in the world where in under the same roof, both Uranium Oxide fuels and Zirconium alloy components are fabricated starting from the basic raw materials namely Magnesium diuranate and zircon sand respectively. Indian Rare Earth Ltd. (IREL) supplies zircon sand in the manufacture of reactor grade zirconium oxide, zirconium sponge and finished zirconium alloy mill products.
  6. 6. 5 NFC manufactures seamless stainless steel and special alloy tubes, high purity and advanced materials for various high tech applications in Atomic Energy, Defence, Space, and other industries and zirconium alloy components for non-nucleus appliances in fertilizers and heavy chemical industries. NFC, practicing continuous in-house technology up gradation in the manufacture of strategic material meeting stringent quality requirements, has developed expertise and built a number of sophisticated equipment as special chemical process reactors, high temperature sintering furnaces and filter mills which made rapid strides in the field of mechanized material handling processes and automation. ”. A production activity in its various plants was started in early 1970’s and today it has a strong work force of about 3600 people comprising of scientists, engineers, supervisors, workmen and the administrative staff. 2. PRODUCTION OF ZIRCALLOY COMPONENTS ZIRCONIUM OXIDE PLANT (ZOP) Zirconium oxide powder is produced in ZOP. The basic raw material for producing zirconium is zircon sand. Zircon sand has silicates as major impurities and hafnium as critical impurity. Zircon is subjected to fusion process with caustic soda (at 6500 C), and frit is formed is sent to a series of three leaching tanks for removing sodium silicate. After leaching zirconate is washed again in plate and frame filter press, to remove the impurities and reduce the alkalinity. The obtained hydrated zirconia product is dried in turbo dryer and subjected to dissolution with nitric acid. The crude zirconium nitrate solution is then sent for solvent extraction process and pure zirconium nitrate is produced. The obtained zirconium nitrate is then precipitated and subjected to drying and calcinations to obtain granules, which are pulverized to get fine powder of zirconium oxide. This is packed and sent to zirconium sponge plant. ZIRCONIUM SPONGE PLANT (ZSP) Anhydrous zirconium tetrachloride is the preferred intermediate for the production of reactor grade sponge. It is produced by the chlorination of pure zirconium oxide. Thermodynamically ZrO2 is more stable than ZrCl4 as evident from the free energy values -216kcal mol-1 and -178 kcal mol-1 respectively at 1000o C. Direct chlorination of ZrO2 is not possible because the net change of free energy is positive. ZrO2 + 2Cl2 ZrCl4 For the production of zirconium sponge, ZrO2, petroleum coke and starch solution are mixed thoroughly and the mixture so formed is extruded to produce briquettes. The briquettes are subjected to coking to remove starch in a furnace with continuous supply of cooling water and N2. This briquette is chlorinated at high temperature to obtain zirconium chloride. ZrCl4 is converted to zirconium metal by Kroll’s reduction reaction. In these the ZrCl4 vapours react with molten magnesium to form zirconium metal. ZrCl4 (g) + 2Mg (l) Zr (s) + 2MgCl2 + 76 Calmole-1 at 11500 C
  7. 7. 6 The reduced mass is vacuum treated at high temperature to distil out MgCl2 and magnesium leaving out pure zirconium in the crucible. Before exposing the pure pyrophoric metal to the atmosphere, the metal is conditioned with argon, the air mixture progressively, enriched with air, to form a protective film of oxide on the metal surface thus prevent spontaneous of reactive metal. ZIRCALLOY FABRICATION PLANT (ZFP) The activities of this plant can be divided into three categories: Ingot making, Hot extrusion and finishing operations.  INGOT MAKING The alloys made in the melt shop are Zircalloy-2 for BWR fuel, zircalloy-4 for PHWR fuel and structural, Zr-2.5% Nb for pressure tubes, Zr-1% Nb for special application and Zr-Nb-Cu for a special PHWR component. The process involves mixing of alloying elements during briquetting these are welded in electro-beam welding equipment to form electrode. The electrode melted in furnace vacuum arc melting to make the primary ingots. These ingots are subjected to re-melting to make a homogeneous melt. For certain special application four times re-melting is also carried out.  HOT EXTRUSION The ingots are melted in two sizes viz., 300mm diameter and 350mm diameter, which are broken down in hot extrusion press to either rounds or slabs depending on the end products.  FINISHING OPERATIONS Hot extruded rounds are subjected to pulgering to produce fuel tubes. Slabs are rolled into sheets and further cold rolled to get the final dimensions. These sheets are hot rolled into sheets and further cold rolled to get the final dimensions. These sheets are used for making PHWR/BWR fuel component or calandria tubes by steam making. 3. ABOUT ZIRCONIUM AND HAFNIUM Zirconium occurs widely in the earth’s crust, but not in concentrated deposits. The mineral zircon, ZrSiO4 (zirconium silicate) that is alluvial deposits in streambeds, ocean beaches, old lakebeds, is the only commercial source of zirconium oxide. ZrSiO4 is the only important zirconium mineral. These zirconium minerals generally have hafnium contents that vary from a few tenths of 1 percent to several percent. For some purposes separation of these two elements is not important. Zirconium containing about 1 percent of hafnium is accepted as pure zirconium. In the case of largest single use of zirconium, however, namely, as a structural & cladding material in atomic reactors, it is essentially free of hafnium, absorption cross section of neutrons (0.18 barn). Hafnium on the other hand has an exceptionally high cross section (115barn) (1 barn = 10-24 cm2 ), & accordingly even slight hafnium contaminants nullify the intrinsic advantage of
  8. 8. 7 zirconium. Pure hafnium is in fact used in some atomic reactors as control element materials because of high neutron-capture cross-section. The atomic radii of zirconium & hafnium are 1.45 0 A & 1.44 0 A respectively. While the radii of the ions are Zr4+ ---0.74 0 A & Hf4+ ---0.74 0 A. The virtual identity of atomic & ionic sizes, resulting from the lanthanide contraction, has the effect of making the chemical behaviour of these two elements more similar than for any other pair of elements known. Although the chemistry of hafnium has been studied less than that of zirconium, the two are similar that only very small quantity differences for e.g. in solubility’s & volatilities of compounds would be expected in cases that have not been actually investigated. The most important aspect in which these two elements differ from titanium is that lower oxidation states are of minor importance, they are relatively few compounds of hafnium or zirconium in other than their tetravalent states compounds. ZIRCONIUM Zirconium, a chemical element, metal of group IV B of the periodic table, is used as structural material for nuclear reactors. Properties- Zirconium and hafnium are chemical elements of the IV B group of the periodic table. Zirconium was discovered in 1789 by German chemist Martin Heinrich Klaproth, and the metal was isolated (1824) in impure form by the Swedish chemist Johns Jacob Berzeius. The impure metal, even when 99% pure is brittle. The white, soft malleable, and ductile metal of high purity was first produced in quantity (1925) by the Dutch chemists Anton E. Van Arkel and J.H. Boer by the thermal decomposition of zirconium tetrachloride, ZrCl4. It has several valuable chemical and physical properties. It has small thermal neutron capture cross section and remarkable anticorrosion and mechanical properties, and is therefore widely used in atomic and chemical engineering and metallurgy. In the early 1940’s William Justin Kroll, of Luxemburg developed his cheaper process of making metal based on the reduction of zirconium tetrachloride, ZrCl4, by magnesium. It is relatively abundant in the earth’s crust and is characteristically observed in ‘S’ type stars. Zirconium is commercially obtained principally from the minerals, zircon and baddelyite. Occurrence The most important use of zirconium is in nuclear reactors for cladding fuel rods, for alloying with uranium and for reactor core structures because of its unique properties. Zirconium has strength at elevated temperatures, resists corrosion from the rapidly circulating coolants, does not form highly radioactive isotopes and withstands mechanical damage from neutron bombardment. Zirconium absorbs oxygen, nitrogen and hydrogen in astonishing amounts at about 8000 C. It combines chemically with oxygen to yield the oxide (ZrO2); zirconium reduces such refractory crucible materials as the oxides of magnesium, beryllium and thorium. This strong affinity for oxygen and other gases accounts for its use as a getter for removing residual gases in electron tubes. At normal temperature in air, zirconium is passive because of the formation of protective film of oxide or nitrate. Even without this film, the metal is resistant to the action of weak acids, acidic salts.
  9. 9. 8 Because of its high corrosion resistance, zirconium has found wide spread used in the fabrication of pumps, tubes, valves and heat exchangers. Zirconium is also used as an alloying agent in the production of magnesium alloys and as an additive in the manufacture of certain steels. Applications: The properties of zirconium are such as to indicate that it may find several uses in modern industry. It possesses a combination of physical, chemical and nuclear properties, which are unique. The zirconium chemicals have a wide range of applications, including automotive catalysts, electro-ceramics, structural ceramics, thermal barrier coatings, optical glass fibre/fibre optics, paints/pigments and solid oxide fuel cells. HAFNIUM Until 1922, it was not known that zirconium and its compounds always contain small amounts of chemical elements of atomic number 72. In 1922, Hevesy and Coster, while carrying out X-ray spectroscopic investigation of zirconium, discovered new X-ray lines, which coincided with the characteristic lines that had been calculated for the element of atomic number 72. It was named hafnium. The simultaneous occurrence of zirconium and hafnium is because of the effect “Lanthanide contraction” in hafnium. Hafnium is a ductile metal with brilliant silvery lustre. Hafnium is dispersed in the earth’s crust to 3ppm and is invariably found in zirconium minerals up to a few % compared with Zirconium, altered Zircons, and some other zircon compounds. Zirconium and hafnium are extremely similar in physiochemical properties. Hafnium’s large thermal neutron capture cross section, high resistance to corrosion in hot water and good chemical properties make hafnium an excellent material for the manufacture of control rods for thermal nuclear reactors. Hafnium can also be alloyed or composite with other materials, which can be used in the outlet nozzle of a rocket. Hafnium is used for fabricating nuclear control rods because it easily absorbs thermal neutrons and has excellent mechanical properties. Hafnium produces a protective film of oxides or nitride upon contact with air and thus has high corrosion resistance. It forms alloys with Iron, Tantalum and other transition metals. The alloy Tantalum Hafnium Carbide (Ta4 HfC5), with a melting point of 41250 C (76190 F) is one of the most refractory substances known. SEPARATION OF ZIRCONIUM AND HAFNIUM: Separation of Hafnium & Zirconium is generally accomplished by liquid-liquid counter current extraction process. In this procedure, crude Zirconium oxide is dissolved in a nitric acid solution & tri butyl phosphate is passed counter current to the aqueous mixture, with the result that the Zirconium nitrate is perfectly extracted. For reactor grade zirconium (<50 ppm) the solvent extraction method using tri butyl phosphate is appropriate with high purity hafnium (0.1 % Zr) can be obtained efficiently (42% recovery) by ion exchange using Dowex-50 cation exchanger. A pure grade of Hf (0.02 % Zr) may be obtained by less efficient process (20-30 % yield) using solvent extraction with trifluroacetylacetone.
  10. 10. 9 PHYSICAL CONSTANTS FOR ZIRCONIUM AND HAFNIUM Properties Zr Hf Atomic number 40 72 Atomic weight 91.22 178.49 Melting point, 0C 1830 2222 Density, g/cc 6.49 13.01-13.09 Boiling point, 0C 2900 3100 Transition temperature,0C 862 1670 CHEMICAL PROPERTIES: Natural Zr is a mixture of five stable isotopes and natural Hf is a mixture of six isotopes. The thermal neutron capture cross section of zirconium metal (0.18 barn) is very small when compared with that of other metals, iron (2.53 barn), nickel (4.60 barn) or copper (3.69 barn). In contrast to Zr, Hf has a large thermal neutron capture cross section of 115 barns. Hf in Zr plays a decisive part, which is about 1-2% in impure Zr. The presence of this amount of Hf in Zr causes a considerable rise in the value of thermal neutron capture cross section from 0.18 barn to 1 barn. The valency of Zr&Hf may be 2, 3 or 4. The stability of their compounds increases at high valencies. Compounds of bi- and trivalent Zr&Hf are known, but they are unstable & have strong reducing properties. The characteristic oxidation state of the two elements is +4. Zr&Hf are highly resistant to corrosion. Zr is practically unaffected by water, HCl, HNO3 or dil. H2SO4 and alkali solutions even on heating. However both metals are readily dissolved by H2F2 Property Zr Hf Atomic radius,0A 1.452 1.442 Ionic radius, 0A 0.74 0.75
  11. 11. 10 and by a mixture of HCl& HNO3, and at high temperature vigorously combine with O2, N2, H2, halogens S, C, Si, and B; with the last mentioned compounds, Zr&Hf form refractor compounds. The basic properties increase in the sequence Ti<Zr<Hf<Th, Hf being slightly more basic than Zr. FEATURES OF IMPORTANCE IN NUCLEAR INDUSTRY:  Ready availability  Low cross-sectional absorption of thermal neutron  Resistance to radiation damage  Excellent corrosion resistance in pressurizes hot water up to 3500 C OTHER SALIENT FEATURES:  Good guttering  Highly pyrophoric  Super conductive at low temperature with niobium  High refractive index in oxide form  Good refractive properties in oxide form ZIRCALLOY USED IN INDIAN NUCLEAR REACTORS: ZIRCALLOY -2(FUEL TUBES BWR’S) Zircalloy -4(fuel tubes and calandria tubes of PHWR’s, coolant channels of BWR’s) Zr- Nb (pressure tubes of PHWR’s) Zr-Nb-Cu (garter springs PHWR’s) Uses: a) Fuel clad and structural material in alloy form for core components of Detonators and Pyrotechnics. b) Corrosion resistance applications as components in chemical industry. c) Getter in vacuum tubes. d) Refractory material in the form of oxide in lass and ceramic industries. e) Lining of metallurgical furnaces in the form of oxide. f) Artificial gems. Deposits of zircon are high along the coastline of Needakara and Kayamkulum in Kerala. Chavara mineral division near Kollam in Kerala, Manavalakurichi mineral division near
  12. 12. 11 Kanyakumari in Tamil Nadu and OSCOM at Chatrapur in Orissa are the Indian Rate Earth Division (IREL) units exploiting the huge zircon deposits in India. 4. FLOW SHEET OF ZIRCONIUM OXIDE PRODUCTION
  14. 14. 13 5.1 DISSOLUTION Raw materials:  Dry powder  Nitric Acid (60% concentrated) Storage of nitric acid: Nitric acid is received in a horizontal SS tank. The receiving tank is placed below the ground level, so as to receive the nitric acid from the tankers by the force of gravity. There are two vertical storage tanks each of 150KL capacity. The vertical tanks are filled from the horizontal tank by the use of vertical pumps. The specialty of the vertical pumps is that they are inside the horizontal tank, so the chance of nitric acid to leak or spill is eliminated. To measure the level of nitric acid in the vertical tank, the float and weight method is used. The tanks are made of stainless steel. The nitric acid from storage tanks is pumped to the dozing tanks from where the required amount of nitric acid is fed into the reactor. Storage of dry powder: The dry powder is procured in 50 kg bags. The dry powder is mainly tested for the above required compositions of the compound mainly zirconium before being charged into the reactor. It is tested in the control lab, which plays a pivotal role in each and every part of production of zirconium. The dry powder is taken to level above that of the height of the reactor and is charged into the reactor from conical opening, because of which the chances of the powder being wasted is reduced and the handlings becomes easier. The dry powder is taken to the required height by using a 2-ton hoist. Material of construction: Dissolution is done in SS tanks. It consists of an agitator and the feed is heated by steam. Chemical reaction: ZrO2 + 4HNO3  Zr (NO3)4 + 2H2O
  15. 15. 14 PROCESS DESCRIPTION The requisite amount of nitric acid (12N) is charged into a reactor made of stainless steel. It is agitated using an impeller and moderately heated using direct contact steam at a pressure of 3 kg/cm2 . Then the calculated amount of dry powder is added to the reactor and it is agitated for 2 hrs. This DP is dissolved in 12N (60%) Nitric acid, which is already present in the dissolution tank to produce Zirconium Nitrate solution, which is the feed material for purification by solvent extraction. It is an exothermic reaction so the temperature rises .The solution is diluted with scrub raffinate and allowed to settle for 2 hrs. After 2 hrs. The insoluble solids settle down and a clear layer of zirconium nitrate is formed above the solids. The solids are drained off once a week in order to prevent choking of lines. The nitrate solution is checked for the required qualities such as free acidity, total acidity and the composition of zirconium and hafnium values. The nitrate solution is then sent to feed tanks, which is made of SS by means of centrifugal pump whose impeller is also made of stainless steel. The feed tanks have a conical bottom to ensure free draining of the silt that settles in due course time. The silt is cleaned once in 20 days. Large care is taken while handling nitric acid since it corrodes materials like Mild Steel, so, only Stainless Steel components are used while handling nitric acid. Here the material of construction of the reactor is SS-304L. Some safety precautions that must be taken while dissolution, is that the temperature must not rise abruptly to high value. If the temperature rises above this range then nitrate Fumes are released and the batch gets wasted. Usually sodium carbonate is sprinkled on the floor to neutralize the nitric acid that HN03 STEAM m DRY POWDER Baffles Acidic slit Feed slurry SCRUB RAFFINATE
  16. 16. 15 leaks from pipelines. The other main factor is the concentration of nitric acid: if a higher concentration is used then the required acidity cannot be maintained. If the free acidity of the nitrate solution is above the required value then in slurry extraction even hafnium is extracted and if free acidity is less than the required value then some amount of zirconium is lost, so it is very necessary to maintain the free acid. If free acidity is less than the required value acid is added to increase the free acidity and if the free acidity exceeds the required value then acid addition is stopped. The foremost and the important chemical unit operation of zirconium oxide is “solvent extraction”. This process takes place as slurry extraction, scrubbing and stripping. 5.2 SOLVENT EXTRACTION The process of separation of the components of a solution depends upon the unequal distribution of the components between two immiscible liquids is known as “LIQUID – LIQUID EXTRACTION” or more simply “Liquid Extraction”. Liquid extraction is sometimes called as “SOLVENT EXTRACTION”. Solvent extraction is a chemical engineering separation that has many variations and many applications in that process industries and uses many types of equipment. Solvent extraction is based on the principle that a solute can distribute itself in a ratio between two immiscible solvents, one of which is usually water and the other an organic solvent such as benzene, carbon tetrachloride, etc. in certain cases the solute can be more or less completely transferred into the organic phase. For a given metal, present in various species M1, M2, and so on up to Mi and distributed between an organic and aqueous phase, the extraction can be defined in the following terms. Distribution ratio or extraction coefficient, E E = Morg/Maq Where Morg = M1org + M2org + ……. Maq = M1aq + M2aq + ……… The above expression is valid for a simple system in which only one species is distributed and exists in the same form in both phases. Under ideal conditions, where the solute exists in the same form in both phases, interactions between solute and solvent are absent and association and dissociation reactions do not occur. When separation by distillation is ineffective or very difficult, liquid extraction is one of the main alternatives to consider. Close boiling mixtures or substances that cannot withstand the temperature of distillation, even under a vacuum, may often be separated from impurities by extraction, which utilizes chemical potential difference instead of vapour pressure difference. Separations done by solvent extractions are essentially physical in character and the various components are unchanged chemically. Nevertheless, the chemical nature of the liquids influences the extent of solutions involved. The minimum requirement for liquid extraction is the intimate contact of the two immiscible liquids for the purpose of mass transfer of the constituents from one liquid to the other, followed by physical separation of the two immiscible liquids.
  17. 17. 16 The solvent extraction contains three units i.e., Extraction, scrubbing, stripping to produce the pure solution. During the extraction process, the organic solvent (TBP+Kerosene) is contained with the inorganic zirconium nitrate solution counter currently. Calculated amount of nitric acid is also added to maintain free acidity. The ZrO2 is then loaded into the organic solvent by contact, which is again loaded back into the pure solution (inorganic) as pre extract in the scrubbing unit. The function of a stage is to contact the liquids, allow equilibrium to be approached and to make a mechanical separation of the liquids. The contacting and separating correspond to mixing the liquids and settling the resultant dispersion, so these devices are usually called “MIXER SETTLERS”. Over the past many years, various designs of mixers settlers have been with general aim to decrease the required while maintaining high throughput and efficiency. Mixers are relatively easy to operate, reliable, flexible and fairly simple to design, are free of back mixing and the stage efficiencies are usually greater than 90%. With sufficient residence time and power in the mixer, and sufficient residence time in the settler, practically 100% stage efficiencies can be reached. Uncertainties in operation are considerably decreased by high stage efficiencies. A mixer settler transfers a solute from one liquid phase into another immiscible, or only partially miscible liquid phase. It consists essentially of a chamber where two liquids are mixed by stirring or some other means of agitation and a settler where two liquids are separated by gravity. Each stage consists of mixing and settling chambers, which alternate along the box so that mixers and settlers of adjacent stages are in juxtaposition. The liquids are brought into intimate contact in a mixing chamber and pass together, in the form of an unstable emulsion, to the settler through a port or slot placed about midway up the dividing wall. In the settler the phases disengage, the heavy phase on to the mixer of the next stage through a port plated low in the wall, while the lighter passes over a weir to the next adjacent mixer in the opposite direction. It will be seen that this flow pattern represents co current flow in each stage and counter current flow overall. These mixer settlers thus show little change in performance with moderate variations in through put and phase ratio, resulting in flexibility and ease of design. The settler size is a critical factor in mixer settler designs. The size is governed by the throughput limitations imposed by the rate of coalescence of the dispersed phase. The power input to the mixer has apparently little effect on the specific settling rate, but the rate was found to vary in a TBP/HNO3/Kerosene system with chemical composition, phase ratio and temperature. Advantages of mixer settlers:  Good contacting of phases  Handles wide range of flow ratio (with recycle)  Low head room  High efficiency  Many stages can be accommodated  Reliable scale up  Low cost and maintenance Disadvantages of mixer settlers:  Large holdup  High power costs  High solvent inventory  Large floor space
  18. 18. 17  Inter stage pumping may be necessary I. SLURRY EXTRACTION Raw materials:  Zirconium nitrate solution from the feed tanks  Lean solvent (TBP + Kerosene)  Nitric acid (to maintain free acidity) Extraction: In extraction, a solvent that preferentially dissolves one or more components in the mixture treats a mixture of two or more components. Mixer-settler is the most common type of extractor. Objective: For continuous operation a battery of mixer-settlers is used. Slurry extractor is multistage equipment. Except the last stage, all the other stages contain small settling tanks where no interphase is maintained between organic and aqueous. The last stage consists bigger settler tank (raffinate tank) where well defined inter-phase is maintained between organic and aqueous. Each stage is provided with an airlift for effective zirconium extraction and inter-stage pumping. Compressed air is used for operating airlift pumps. A long vertical disengagement section is followed by an airlift for separating compressed air from the mixed phases. A tangential entry is given from air disengagement section to settling tank for a better phase separation in settling tank. Each disengagement section is provided with a mist-eliminator where left out solvent in air is entrained and remaining air is vented out to a duct. Material of construction: A typical slurry extraction unit consists of mixers and settlers that are made up of SS-304. In this unit a mixer and a settler comprises a stage. Process description: The nitrate solution (feed) is pumped through a digital rota meter (which is used for slurries) into the first stage and the lean solvent (TBP+ Kerosene) is passed in a counter current fashion into the last stage. A float rota meter cannot be used for slurries because the solids present in the slurry accumulate on the surface of the float and tend to increase the weight of the float thereby faltering the flow rate reading of the feed solution, so a digital rota meter is used. Initially all the stages are filed up with the lean solvent before starting the extraction. The solvent TBP is selected because of its tendency to extract only zirconium at a given acidity in the nitrate medium. The principle behind this type of extraction is that the zirconium in the aqueous phase is extracted in the organic phase. Nitric acid is added accordingly to maintain the free acidity. The extraction equipment used by ZOP is the new mixer-settler (made of SS316/304L) that has been indigenously developed by NFC. It mainly consists of a mixing section and a disengaging section. The main merit of this equipment is that the mixing and propagation is done by means of compressed air. The airlift mechanism follows the principle that when air mixes with the solution the density of the solution decreases and thereby it causes
  19. 19. 18 the solution to rise and also it enhances the mixing operation. The disengaging section’s primary use is to disengage the air from the solution after it has been propagated. The primary advantage of using such airlift mechanism is that unlike the conventional mixer-settler it does not use any mechanical parts, which can develop various types of mechanical problems, which is absent in airlift since there is no use of mechanical components. The problems relating to the slurry extraction are: 1) The build-up of aqueous and organic phases. 2) This build up is mainly caused due to presence of excessive amount of either feed or organic in one stage. 3) This excessive amount of either feed or organic is mainly due to the presence of some amount of unreacted silica in the feed. 4) This unreacted silica forms a coating inside the pipeline and tends decrease the diameter of flow. 5) The reduced diameter of flow reduces the flow rate and thereby creates a build-up. 6) Thus this coating is root cause for the above said problems, so it has to be cleared. It is cleared by purging the equipment with steam, which melts the silica coating and thereby clears all the above said problems. II. SCRUBBING Raw materials:  Organic (extract) from the slurry extraction  Pure solution from stripping  Nitric acid Material of construction: The material of construction is SS316. Process description: The extract from slurry extraction that contains minimum amount of hafnium compounds is sent for scrubbing into the conventional mixer-settler where it is scrubbed with pure solution, which is the aqueous solution of pure zirconium nitrate obtained from the stripping process. The scrubbing operation is a counter current operation. Both the phases are mixed in a mixer, which essentially consists of an agitator, side baffles and two inlets for organic and aqueous phases. The side baffles are provided in order to provide turbulence, which in turn provides good mass-transfer. If there is no side baffle then the liquid moves in a circular motion without mixing with the bulk of the solution. After the solution is mixed it is allowed to settle through an opening in the side. The settling compartment is comparatively longer than that of the mixing chamber. The settled solution is again transferred to the next stages by means of the density differences and again mixed and settled. This is carried out repeatedly and finally we obtain the scrub raffinate and the extract pure (which is the feed for stripping). The salient problems encountered here are those of the mechanical parts or the moving parts, so regular maintenance has to be made in order to keep the unit efficiently working.
  20. 20. 19 The main importance of this unit is that the hafnium composition is totally eliminated (<50 ppm). The other point to be noted is that the some amount of zirconium is associated in the scrub raffinate, which is used for dilution in the reactor, and some part is again sent into another extraction chamber to get back the zirconium. Other than Lean solvent (TBP+Kerosene), the use of methyl isobutyl ketone (MIBP) in a cyanide medium can be used as solvent for extraction. The main drawbacks of using MIBK are that it has a low flash point, it is toxic and corrosive. EXTRACTION: The scrub raffinate is the feed and it counter currently extracted with lean solvent (TBP+ Kerosene). The zirconium left out in the scrub raffinate is extracted in the acidic medium (nitric acid), which is again sent to the scrubbingunit. The product obtained here is “Extract”.
  21. 21. 20 III. STRIPPING Raw materials:  Extract pure from the scrubbing section  De Mineralized water Process description: The stripping unit is the same as the scrubbing and the extraction units, the only difference lies in that of the minerals used. Here the extract pure from the scrubbing unit is counter currently mixed with demineralized water. Here the zirconium in the organic phase is transferred to the aqueous phase. The stripped organic solvent is sent for treatment with soda solution. The zirconium that is in the aqueous phase is known as pure solution. Some part of the pure solution is transferred to the storage tanks and the other part is recycled to the scrubbing section.
  22. 22. 21 TREATMENT WITH SODA SOLUTION: Raw materials:  Lean solvent from the stripping unit  Soda solution  DM water Process Description: The Lean solvent from the stripping section is sent to mixer-settler unit in which soda solution is passed in a counter current flow. The lean solution i.e., tri butyl phosphate degrades into mono butyl phosphate and di butyl phosphate. The main use of this treatment is to remove the mono butyl phosphate and di butyl phosphate, which dissolve in soda solution thereby producing free tri butyl phosphate. MBP and DBP have higher solubility in aqueous medium than that of TBP. So, it always betters to treat the degraded TBP before sending it for extraction. The most important point to be noted here is that the plant does not consume excess TBP as it can be treated and recycled back to the extraction process. There is certainly some wastage but it is minor. TBP has 0.39g/l solubility in water DBP has 0.64g/l solubility in water MBP has complete solubility in water Safety precautions: o Use personal protection accessories like gloves, masks etc. o Avoid chemical spillage. o TBP and kerosene are flammable. Hence all fire prevention measures must be taken. o Wall and local exhaust must be on. o Emulsion formation should be avoided. o Ensure clear phase separation such that entrainment is avoided in organic aqueous streams. o Maintain correct inter phase levels. o Check airlift pump also for sufficient pressure. o In this we should check the pulleys, belts and agitators before starting the operation. o The silica coming from the slurry extraction may cause problems. So we have to check for scaling. 5.3 PRECIPITATION Raw materials:  Pure solution from the stripping unit  Ammonium hydroxide  Sulphuric acid
  23. 23. 22 Material of construction: Precipitation tank is made up of SS-304. An agitator is provided for mixing. Chemical reaction: The required amount of pure solution, ammonium hydroxide and sulphuric acid are taken into the precipitation tank and are agitated. The temperature is maintained around 600 C. The following reaction takes place: - Zr (NO3)4 + 6NH4OH + H2SO4Zr (OH) 4 + 4NH4NO3 + (NH4)2SO4 Sulphuric acid is added to the solution in order to make the cake fluffier and to reduce the density of the zirconium oxide. Ammonium hydroxide is added to precipitate zirconium hydroxide from zirconium nitrate.
  24. 24. 23 Process description: The solution coming from the precipitation tank is sent to a vacuum rotary drum filter. We have to maintain vacuum of around 450 mm Hg. The filter cloth we are using here is polypropylene. The drum rotates with a speed of 1.33 revolutions per minute. The cake coming out contains 80-85% moisture which consists of water, ammonium nitrate and ammonium sulphate. This wet cake is sent for repulping. In repulping the wet cake is mixed with water where ammonium nitrate will dissolve in water and again sent it for filtration. The slurry free of solids transferred to scrubbing unit where alkalinity changes to neutralization. We have to agitate the slurry during the filtration to avoid settling of any solids in the equipment and also we should maintain 600 C with manual controlled steam valve. Safety precautions: 1. Use personal protective appliances. 2. Check for any damage to the polypropylene cloth. 3. Avoid spillage of slurry by overflow. Note: pH of the slurry is maintained at 7 before filtration. 5.4 REPULPING Raw materials:  Filtered cake  Demineralized water Process Description: The slurry from the precipitation tank is passed through a rotary vacuum drum filter. The cake thus obtained is mixed with demineralized water in a tank. This process is called repulping. The importance of repulping is that is ammonium compounds dissolve in demineralized water thereby reducing the possibility of explosion in the drying chamber. 5.5 VACUUM FILTRATION Raw materials: Filtered cake from repulping
  25. 25. 24 Filtration: Filtration is the removal of solid particles from a fluid passing the fluid through a filtering medium, or septum, on which solids are deposited. Objective: It consists of a cylindrical drum mounted horizontally. Their outer surface of the drum is formed of perforated plate. A filter medium such as polypropylene cloth covers the outer surface of the drum, which turns at 0.1-2 rpm in an agitated slurry trough. The annular surface between the two drums is dividing into number of compartments/sectors (12) by radial partition through a rotary valve (12 holes). Here in ZOP a continuous vacuum drum filter is used, in which filtration & discharge of cake takes place continuously. Material of construction: Apart from cast iron, other materials of construction include stainless steel, titanium and plastics such as poly vinyl chloride etc. These materials give much improved corrosion resistance for many types of slurry. Process Description: The slurry from the repulping is sent for vacuum filter. As the drum rotates, vacuum is applied to all compartments except the one at which the cake has to be released, air is applied. The solution coming from the precipitation tank is sent to a vacuum rotary drum filter we have to maintain the vacuum of around 450mmHg. The filter cloth here we are using is polypropylene. The drum rotates with a speed of 1.33 revolutions per minute. The cake coming out contains 80-85% moisture which consists of water, ammonium nitrate and ammonium sulphate. We have to agitate the slurry during the filtration to avoid settling of any solids in the equipment and also we should maintain 60c with manual controlled steam valve. When the drum dips into the slurry vacuum is applied because of which the slurry is sucked into the drum. The vacuum drum filter essentially consists of a boot in which the slurry is allowed. The drum is placed inside the boot. There is a factor known as submergence, which affects the rate of formation of cake. The submergence is about 30% in this case. When the drum comes out of the boot then air is applied to blow out the filter cloth thereby helping the easy scraping of the cake. The cake is scraped using a doctor blade. The drum has a filter cloth, which is used to retain the solid, and the liquid passes through the cloth and it goes into a receiver. The liquid in the receiver is then drained into a waste storage tank, from where it is taken away by other companies for the production of ammonium nitrate.
  26. 26. 25 ADVANTAGES  The filter is continuous in operation, as the rotary drum is rotated by electric motor. So the manpower requirement is very low.  With cake consisting of coarse solids, it is possible to remove most of the liquids from the cake before discharging. DISADVANTAGES  The maximum available pressure difference is limited as it being a vacuum filter. 5.6 DRYING To bring the moisture content of wet cake from 85% to 30%, to make the material free flowing and to remove ammonium nitrate from the cake, high temperature drying is done.
  27. 27. 26 Material of construction: Process description: The cake obtained from the filtration section is collected in bunkers and is collected in bunkers and is charged in the static bed dryers. The bunkers are lifted by means of a hoist. The material is dried for about 12-16 hours at high temperature with intermittent baking of the material. Raking is done to distribute the heat uniformly in the material. Later the oven is checked for every 4 hours to level the surface for faster drying. The dryer has shutters through which the material is charged. After completion of drying, the shutters are opened by means of rotating wheels provided. The dried product is collected in containers and is taken to the charging section and transported into calcinations hopper. Then the oven is available for next cycle. 5.7 CALCINATION It is the process of removing volatile impurities to specified limits, present in the substance. This type of furnace that is indirect heated and is adopted for drying of free granular material on a large scale. Exhaust Discharge BLOWER Heating elements FEED POINT HOT TEMPERATURE OVEN
  28. 28. 27 Objective: It consists of hollow cylindrical shell of diameter 350mm-500mm and a length of 5m-8m with it axis at a slight angle of horizontal. So that the material is consequently advances through the dryer from one end to another end. It is supported on rollers so that it can be rotated. To avoid slipping over rollers, it is fitted with thrust wheels. It is fitted inside with flights, which lefts the material upwards and showered if down from the top. Few spiral flights are fitted near the feed end, which helps initial forward motion of the material before the principal flights are reached. The material, which is to be dried, is fed at higher end of the drier a by the hopper the product is to be taken from the lower end. Materials move through dryer by virtue of its motion, heat effects and inclination of the cylindrical shell. The cylindrical shell is rotated by a gear mechanism at a speed of 2-2.5rpm. This is fixed in a furnace refractory lined of sellamanite bricks. The heating coils are arranged in the refractory bricks. The heating coils are made up of Nichrome. PPrroocceessss ddeessccrriippttiioonn:: The dried material is charged through an opening at the top and is fed into the rotary furnace. The chamber has a cylindrical tube rotating inside a rectangular box. The calciner is a 150 KW capacity heater. The cylindrical tube is made up of SS310 and the heating elements are made of nichrome. The temperature reached in a calcinations chamber is about 800O C. the main use of this furnace is to drive away the moisture and the other volatile impurities to the specified limits. The calcined material is Zirconium oxide, which is collected in drums at the other end of the furnace. The collected oxide is sent for grinding. CChheemmiiccaall RReeaaccttiioonn:: ZZrr ((OOHH))44 -------------------------------------------------- ZZrrOO22 ++ 22HH22OO
  29. 29. 28 AAddvvaannttaaggeess::  Moderate drying time.  Low capital cost  High thermal efficiency  Continuous operation DDiissaaddvvaannttaaggeess  Difficulty of sealing  High structural load Safety precautions: 1. Clean exhaust duct for every 8 hours 2. Avoid high feed rate 3. Ensure emergency power supply 4. Collect the exhaust duct material 5.8 GRINDING PPrriinncciippllee:: Size reduction is achieved by impact and attrition. CCoonnssttrruuccttiioonn:: The hammer mill consists of essentially of high-speed rotor turning inside a cylindrical casing. The rotor is mounted on a shaft, which is usually horizontal. In this mill, the particles are broken by sets of swing hammers. They may be straight bars of metal with plane or enlarged ends. The products fall through a gate or screen, which forms the lower portion of the casing. Several rotors discs each carrying 4 to 8 swing hammers is often mounted on a single shaft. The rotor disc diameter ranges from 150mm to 250mm as the hammers hinged. The hammers are readily replaced when they are worn out. PPrroocceessss DDeessccrriippttiioonn:: The calcined material is then sent to the grinding section. The grinding section essentially consists of a feed charger, a feed rate adjusted hammer mill, a blower and a big filter. The feed rate is adjusted using a mechanical device. If the machine reaches the overloading limit then using the feed rate adjuster the overloading can be stopped. If the machine gets overloaded then it comes halt thereby disrupting further production.
  30. 30. 29 The material is ground in the hammer mill. The grinding action is because of both impact and attrition. The ground power is pneumatically carried using a centrifugal blower. It is collected in a drum and very fine particles are collected in the bag filter. Coarser particles are sent back to the hammer mill and are ground to the required size. The product is of 325 mesh. Another important aspect to be considered in grinding is that of the amount of sulphuric acid is added. If less amount of sulphuric acid is added then the obtained material is hard and the grinding load on the machine increased thereby increasing the grinding time which in turn increases the current consumption. SSaaffeettyy PPrreeccaauuttiioonn:: 1. Check for foreign items like nuts, bolts, etc. 2. Clean the dust collecting system. 3. Do not keep a high feed rate. 4. Keep belt guard in position. 5.9BLENDING Blending is a process of mixing the ground solids in required proportion to get the required percentage purity of zirconium oxide. The zirconium oxide thus obtained is then sent to
  31. 31. 30 Zirconium Sponge Plant (ZSP) for production of zirconium metal and then sent to Zirconium Fabrication Plant (ZFP) for the production of Zircaloy, which is used as cladding material in nuclear reactors. 6. PROPERTIES AND USES OF ZIRCONIUM OXIDE PPrrooppeerrttiieess ooff zziirrccoonniiuumm ooxxiiddee::  High density  Thermal conductivity (20% that of alumina)  Chemical inertness  Ionic electrical conduction  Resistance to molten mass  High fracture toughness  High hardness Zirconium oxide (zircon) also has a high index of refraction TTyyppiiccaall uusseess ooff zziirrccoonniiuumm ooxxiiddee::  Precision ball valve balls and seats  High density ball and pebble mill grinding media  Rollers and guides for metal tube forming  Thread and wire guides  Hot metal extrusion  Deep well down-hole valves and seats dies  Power compacting dies  Marine pump seals and shaft guides  Oxygen sensors  High temperature induction furnace susceptors  Fuel cell membranes  Electric furnace heaters over 2000O C in oxidizing atmospheres 7. HEAT EXCHANGER & IT’S CLASSIFICATION Heat exchangers is a piece of equipment build for efficient heat transfer from one medium to another. The media may be separated by solid wall to prevent the mixing or they may be in direct contact. They are widely used in space heating, refrigeration, power plant, chemical plant petrochemical plant, petroleum refinery etc.
  32. 32. 31 It is still difficult to have an overview, and a classification needs to be made. It is possible to classify heat exchangers in a number of ways.
  33. 33. 32 8. TYPES OF HEAT EXCHANGERS There are many types of heat exchangers used in industries. The heat exchangers that are commonly used in industries are: 1. Double pipe heat exchanger: Double pipe heat exchangers are simplest heat exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance making them good choice for small scale industries. On other hand, their low efficiency coupled with the high space occupied in large scale has led modern industries to use more efficient heat exchangers like shell and tube or plate heat exchangers. However double pipe heat exchangers are used to teach heat exchanger design basics to students as the fundamental rule for all the heat exchangers are same. It is used where flow rate of fluid and heat duty is small. It is suitable for high pressure service. It is used when heat transfer area requirement is small. Advantages • Inexpensive • True countercurrent or co-current flow • Easily designed for high pressure service Disadvantages • Difficult to clean on shell side. • Only suitable for small sizes. They are generally not economical if UA > 50,000 Btu/hr-o F. • Thermal expansion can be an issue. Typical Applications 1. Single phase heating and cooling when the required heat transfer area is small. 2. Can be used for heating using condensing steam if fabricated with elbows to allow expansion U-type or hairpin construction for a double pipe heat exchanger.
  34. 34. 33 2. Shell and tube heat exchanger: It is usually a cylindrical casing through which one of the fluid flows in one or Shell is commonly made of carbon steel. The minimum thickness of shell made of carbon steel varies from 5mm to 11mm depending upon the diameter. It may be cut to the required length from a standard pipe up to 60 cm diameter or Fabricated by rolling a metal plate suitable dimension into a cylinder and welding along the length. Tubes are providing the heat transfer surface. Variety of materials including low carbon steel, stainless steel, copper, brass, aluminum, etc. are used as tube material. Outside diameter of tubes vary from 6 mm to 40 mm. The tubes with outside diameter 19 mm to 25 mm are very common. The tube lengths used are 0.5, 2.5, 3, 4, 5 and 6 m. It depends upon the material of construction and diameter. The tubes that are placed in a tube bundle inside the shell are either rolled or welded to the tube sheet. Tube side fluid first enters a channel through and then through the tubes in one or multi pass fashion. The shortest center-to-center distance between the adjacent tubes is called the tube pitch. Tubes are generally arranged in square or triangular pitch manner. The shortest distance between the two tubes is called the clearance. The minimum pitch is 1.25 times the outside diameter of tube. Baffles are commonly employed within the shell to increase the rate of heat transfer by increasing the turbulence of shell side fluid and also provides supports for the tubes and act as dampers against vibration. Fig.: Shell and tube heat exchanger.
  35. 35. 34 Passes are generally used to obtain higher velocities and long paths for a fluid to travel without increasing the length of the exchanger that leads to high heat transfer area. Single or two pass is used in shell side. One, two, four, six up to twelve passes used in tube side. Passes in tube side are formed by partitions placed in the shell cover and channel 3. Plate heat exchanger: It consists of a series of rectangular, parallel and corrugated plates held firmly together between substantial head frames. The plates have corner ports and are sealed and spaced by rubber gaskets around the ports and along the plate edges. These plates serve as the HT surfaces and are of stainless steel. Corrugated plates provide a high degree of turbulence even at low flow rates. Gap between plates is 1.3 to 1.5 mm. It is provided with inlet and outlet nozzles for fluid at the ends. Hot fluid passes between alternate pairs of plates, transferring heat to a cold fluid in the adjacent spaces. The plates can be readily separated for cleaning and the HT area can be increased by simply adding more plates. As it is very compact, requires very small floor space. High heat transfer coefficient, easy to clean. Plate heat exchanger are competitive with Shell and tube heat exchanger where the corrosive fluid is to be handled. Heat sensitive material, where the temperature control is required, these units are used. Fig.: Plate heat exchanger. 4. Waste heat recovery unit: A waste heat recovery unit (WHRU) is a heat exchanger that recovers heat from a hot stream while transferring it to a working medium, typically water or oil. The hot gas stream can be the exhaust gas from a gas turbine or a diesel engine or waste gas from industry or refinery. Big system with high volume and temperature and gas stream, typical in industries can be benefit from steam Rankine Cycle in WHRU, but these cycle are too expensive for small system. The recovery of heat from low temperature system requires different working fluids than steam. An organic Rankine Cycle WHRU can be more efficient at low temperature range using Refrigerant that boil at lower temperature than water. Typical organic refrigerant are Ammonia Pentafluoropropane and Toluene. The refrigerant is boiled by the heat source in the evaporator to produce super-heated vapor. This fluid is expanded in the turbine to
  36. 36. 35 convert thermal energy to kinetic energy, that is converted in to electricity in the electrical generator. This energy transfer process decrease the temperature of the refrigerant that, condenses. The cycle is close and complete using a pump to send the fluid back to the evaporator. 9. DESIGNING OF DOUBLE PIPE HEAT EXCHANGER The double-pipe heat exchanger is one of the simplest types of heat exchangers. It is called a double-pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first. This is a concentric tube construction. Flow in a double-pipe heat exchanger can be co-current or counter-current. There are two flow configurations: co-current is when the flow of the two streams is in the same direction, counter current is when the flow of the streams is in opposite directions. As conditions in the pipes change: inlet temperatures, flow rates, fluid properties, fluid composition, etc., the amount of heat transferred also changes. This transient behavior leads to change in process temperatures, which will lead to a point where the temperature distribution becomes steady. When heat is beginning to be transferred, this changes the temperature of the fluids. Until these temperatures reach a steady state their behavior is dependent on time. In this double-pipe heat exchanger a hot process fluid flowing through the inner pipe transfers its heat to cooling water flowing in the outer pipe. The system is in steady state until conditions change, such as flow rate or inlet temperature. These changes in conditions cause the temperature distribution to change with time until a new steady state is reached. The new steady state will be observed once the inlet and outlet temperatures for the process and coolant fluid become stable. In reality, the temperatures will never be completely stable, but with large enough changes in inlet temperatures or flow rates a relative steady state can be experimentally observed. . The outer tube is called the annulus. In one of the pipes a warmer fluid flows and in the other a colder one. Co-current Flow To understand what factors influence the dimensions of this heat exchanger when a certain heat rate is expected some simple equations will be examined. Countercurrent Flow
  37. 37. 36 First a simple heat balance: q = m& h ⋅ch ⋅(ThI −ThII ) = m& c ⋅cc ⋅(TcII −TcI ) (1.1) With: qh = heat transferred from the hot to the cold fluid (kW) m& h = mass flow of the hot fluid (kg/s ) ch = specific heat of the hot fluid (kJ/kg/°C) ThI = hot fluid at position I (°C) ThII = hot fluid at position II (°C) The subscript c stands for cold. But also the next equation is valid: q =U ⋅ A⋅LMTD (1.2) With: q = the heat transferred between the hot and the cold fluid (kW) U = the overall heat transfer coefficient (kW/m2 /°C) A = the heat transferring surface (m2 ) LMTD = the log mean temperature difference LMTD for counter flow the following can be written down: LMTD = (𝑇1−𝑡2)−(𝑇2−𝑡1) 𝑙𝑛 (𝑇1−𝑡2) (𝑇2−𝑡1) (1.3) Where: T1= Hot fluid inlet temperature. T2= Hot fluid outlet temperature. t1 = Cold fluid inlet temperature. t2= Cold fluid outlet temperature.
  38. 38. 37 Another big factor in heat exchanger design is of course costs. The three main relevant factors that have the greatest effect on size and therefore on costs are: - Pressure drops - Log Mean Temperature Difference - Fouling factors They will be discussed one by one. Pressure drops – If unrealistically low allowable pressure drops are imposed, the designer is forced to use lower fluid velocities to maintain the pressure drops limitations. Lower velocities can result in a large heat exchanger. Higher pressure drops result in a smaller heat exchanger, but a pumping device is needed to maintain this high pressure drop. This pumping device needs energy and so operating costs must be calculated in the overall cost for the heat exchanger. Only by considering the relationship between operating costs and investments can the economical pressure drop be determined. Log Mean Temperature Difference – The size, or surface, of a heat exchanger is inversely proportional to the overall heat-transfer coefficient and the corrected LMTD. When looking at a shell-and-tube heat exchanger a so-called ‘corrected LMTD’ must be used instead of the LMTD presented earlier when the double-pipe heat exchanger was discussed. Assuming that reasonable temperatures have been specified, a designer should try to maximize the product of the heat- transfer rate and the LMTD. Fouling factors – According to Garrett-Price (1985) “fouling is generally defined as the forming of deposits on heat transfer surfaces, which interferes with heat transfer and/or fluid flow”. In other words, by using a heat exchanger small layers of insulating material will be formed on the heat transferring surfaces of that heat exchanger. The influence of this layer is two-sided: 1) The layer has a high thermal resistance, higher then any other part of the heat exchanger, thereby increasing the total thermal resistance. This will decrease the amount of heat transferred through the surfaces and reduces the efficiency of the heat exchanger. 2) The presence of a layer will decrease cross-sectional flow area of the medium. To achieve the same throughput through this smaller area, there’s a bigger pressure drop needed. Additional pumping is needed, increasing to total amount of energy added to the system, decreasing the efficiency. So fouling is a absolutely not-wanted phenomenon. The problem is that the heat exchanger that doesn’t suffer from fouling still has to be invented. Furthermore fouling is extremely difficult to describe. That’s why recent years there’s a lot of emphasis on the analysis of this problem.
  39. 39. 38 A convenient order of calculations follow: 1. From T1, T2, t1 and t2 check the heat balance Q, using c at Tmean and tmean Q = MC(T1 –T2 ) = mc(t2 - t1). 2. Calculate LMTD assuming counter flow. 3. Tc and tc : if liquid is neither a petroleum fraction nor a hydrocarbon then the calorific temperature need not to be determine. If neither of liquid is very viscous at cold terminal say not more than 1 centipoise, if the temperature difference is less than 50o F, then the arithmetic mean temperature T1 and T2 and t1 and t2 can be used in place of Tc and tc for evaluating the physical properties of liquids. For non-viscous liquid, 𝝋= (𝝁/𝝁w)0.14 may be taken as 1. Inner pipe. 4. Flow area, ap = 𝛑𝐃 𝟐 /4, ft2 . 5. Mass velocity, GP = (mass flow rate of liquid in inner pipe ./ ap) , lb/(hr.)(ft2). 6. Obtain 𝝁 at Tc or tc depending upon which flow through the inner pipe. 𝝁 , lb/(ft)(hr) = centipoise x 2.42 . From D, ft (inner diameter of inner pipe.) and Gp , obtain Reynolds number Rep =DGp / 𝝁 . 7. From fig. 24, in which JH = (hiD/k)(c𝝁/k)-1/3(𝝁/𝝁w)0.14 vs. DGp / 𝝁 , obtain JH. 8. From c Btu /(lb)(oF) , 𝝁 lb/(ft)(hr) , k Btu / (hr)(ft2)(oF/ft) , all obtained at Tc or tc , compute (c𝝁/k)1/3. 9. To obtain hi multiply JH by (k/D) (c𝝁/k)1/3 (𝝋 = 1) gives hi Btu/(hr)(ft2)(oF). 10. Convert hi to hio by using , hio = hi x ID/OD of inner pipe. Annulus: 11. Flow area, aa = 𝛑(D2 2-D1 1) / 4, ft2.
  40. 40. 39 Equivalent diameter De = 𝟒 𝒙 𝒇𝒍𝒐𝒘 𝒂𝒓𝒆𝒂 𝒘𝒆𝒕𝒕𝒆𝒅 𝒑𝒆𝒓𝒊𝒎𝒆𝒕𝒆𝒓 = (D2 2 – D1 2) / D1 , ft. 12. Mass velocity, Ga = (mass flow rate of liquid in annlus. / aa ) , lb/(hr.)(ft2). 13. Obtain 𝝁 at Tc or tc depending upon which flow through the inner pipe. 𝝁 , lb/(ft)(hr) = centipoise x 2.42 . From De ft (inner diameter of inner pipe.) and Ga , obtain Reynolds number Rep =DeGa / 𝝁 . 14. From fig. 24, in which JH = (hoD/k)(c𝝁/k)-1/3(𝝁/𝝁w)0.14 vs. DeGa / 𝝁, obtain JH. 15. From c Btu /(lb)(oF) , 𝝁 lb/(ft)(hr) , k Btu / (hr)(ft2)(oF/ft) , all obtained at Tc or tc , compute (c𝝁/k)1/3. 16. To obtain ho multiply JH by (k/De) (c𝝁/k)1/3 (𝝋 = 1) gives ho Btu/(hr)(ft2)(oF). Overall coefficient 17. Compute Uc = hohio / (hio+ ho ) , Btu / (hr)(ft2)(oF). 18. Compute Ud , from 1/Ud =1/Uc + Rd . 19. Compute Area from Q = Ud A (LMTD), which may be translated into length. If the length should not corresponds to integral no. of hairpins required a change in dirt factor will result. The recalculated dirt factor should equal or exceed the required dirt factor by using the next larger integral no. of hairpins . Calculation of pressure drop requires a knowledge of total length of path satisfying the heat transfer requirement. Pressure drop (Inner pipe) 20. From Rep in (6) above , obtain f =0.0035 + .264/(Rep)0.42. 21. ∆𝑭p = 4fG2L /2g𝝆2D, ft. 22. ∆𝑷p psi. = ∆𝑭p 𝝆 / 144.
  41. 41. 40 Pressure drop (Annulus side) 23. Obtain D’e = ( D2 – D1 ) 24. Compute the frictional Reynolds number using the above D’e , and then calculate f by using no. 20 and frictional Reynolds number. 25. ∆Fa = 4FG2L / 2g𝝆2D’e , ft. 26. Entrance and exit losses , one velocity head per hairpin, ∆Fl =V2 /2g’ ft. /hairpin 27. ∆𝑷a psi = (∆Fl + ∆Fa )𝝆 / 144 There is an advantage if both fluid calculation is computed side by side. 10. NUMERICAL ON DESIGING OF DOUBLE PIPE HEAT EXCHANGER. Q.) Ammonia hydroxide is formed by reaction of ammonia and water according to following reaction, NH3 + H2O ----------- NH4OH. Reaction is an exothermic reaction in which heat is liberated. As the reaction proceeds temperature rises. According to Le- chatelier’s principal, backward reaction occurs if exothermic reaction occurs at higher temperature. So, heat is removed from reaction by use of cold water in double pipe heat exchanger. A 2 𝟏 𝟐 inch by 1 𝟏 𝟒 inch I.P.S pipe( shec. No. = 40) double pipe heat exchanger is used. The temperature of hot fluid at inlet and outlet are 96.8 oF and 78.8oF respectively. The temperature of cold water at inlet and outlet are 77oF and 93.2oF respectively. Design a double pipe heat exchanger considering fouling factor to be 0.001. Consider pressure
  42. 42. 41 drop should not exceed more than 10 psi. Calculate the required no. of hairpins used if the length of pipe is 20 ft. The flow rate of ammonia is 500 kg/hr .The required normality of ammonia hydroxide solution is 10N. Ans.) NH3 + H2O -------- NH4OH 17kg 18kg 35kg For 10N ammonia hydroxide solution 350kg of ammonia hydroxide must be dissolved in 1000kg water which required 170 kg ammonia. As 10N solution is prepared, it have density same as that of density of water. So , 1000 liters is same as 1000 kg. 170 kg ammonia ------------- 350 kg ammonia hydroxide-------------- 1000kg solution For, 500 kg ammonia ----------1029.41kg ammonia hydroxide------------2941 kg of solution Therefore the flow rate of hot fluid = 2941 kg /hr = 6483.8 lb/hr. As fluid is not petroleum or hydrocarbon so we can take mean temperatures as calorific temperature. Both the fluid is non-viscous so, 𝝋= (𝝁/𝝁w)0.14 = 1 Tmean = (96.8+78.8) / 2 = 87.8oF tmean = (77+93.2) /2 = 85.1oF Physical properties of fluid at this mean temperature. For hot fluid. (mean temp.=87.8oF) For cold fluid (mean temp. =85.2oF) C (Btu/hr. oF ) =1.145 c (Btu/hr. oF ) = 1 𝝁 lb/(ft)(hr) = 2.444 𝝁 lb/(ft)(hr) = 2.187 k Btu / (hr)(ft2)(oF/ft) = 0.269 k Btu / (hr)(ft2)(oF/ft) = 0.355 𝝆 = 56.875 Btu /ft 3 𝝆 = 62.5 Btu /ft 3
  43. 43. 42 All above data from appendix of D.KERN HEAT BALANCE Q = MC(T1 –T2 ) = mc(t2 - t1). Putting the values, we get flow rate of cold fluid m = 7423.951 lb/hr. LMTD CALCULATION LMTD = (𝑇1−𝑡2)−(𝑇2−𝑡1) 𝑙𝑛 (𝑇1−𝑡2) (𝑇2−𝑡1) = 2.59 OUTER PIPE Outer diameter = .24 ft. Inner diameter = .20575 ft. INNER PIPE Outer diameter = .138ft. Inner diameter = .115ft. INNER PIPE (HOT FLUID) (AMMONIA SOLUTION)  Flow area , ap = 𝛑𝐃 𝟐 /4, ft2 =0.0138 ft2  Equivalent dia . = .115 ft2  Mass velocity GP = (mass flow rate of liquid in inner pipe ./ ap) , lb/(hr.)(ft2). = 624643.54 lb/(hr.)(ft2)  Rep =DeGa / 𝝁 = 624643.54 X .115 / 2.444 = 29391.95  JH = 109 ( referring fig. 24 , Appendix ,D. Kern)  hi Btu/(hr)(ft2)(oF).= JH X (k/De) (c𝝁/k)1/3 (𝝋 = 1) = 556.58 Btu/(hr)(ft2)(oF).  hio = hi x ID/OD of inner pipe = 556.58 X .115 / .138 = 463.81 Btu/(hr)(ft2)(oF).
  44. 44. 43 ANNULUS REGION (COLD FLUID)(WATER)  Flow area, , aa = 𝛑(D2 2-D1 1) / 4, ft2 =0.01829 ft2.  Equivalent diameter De = 𝟒 𝒙 𝒇𝒍𝒐𝒘 𝒂𝒓𝒆𝒂 𝒘𝒆𝒕𝒕𝒆𝒅 𝒑𝒆𝒓𝒊𝒎𝒆𝒕𝒆𝒓 = (D2 2 – D1 2) /D1 , ft. = (.𝟐𝟎𝟓𝟕𝟓)𝟐−(.𝟏𝟑𝟖)𝟐 .𝟏𝟑𝟖 = 0.168 ft2.  Ga = (mass flow rate of liquid in annlus. / aa ) , lb/(hr.)(ft2). = 7423.951/ .01829 = 405902.187 lb/(hr.)(ft2).  Rep =DeGa / 𝝁 = .168 X 405902.2 / 2.180 = 32472.176  JH. =122 (referring fig.24 , Appendix ,D. Kern).  For ho multiply JH by (k/De) (c𝝁/k)1/3 (𝝋 = 1) gives ho = 466 Btu/(hr)(ft2)(oF). Overall coefficient.  Clean over all heat transfer coefficient Uc = hohio / (hio+ ho ) , = 232.45 Btu / (hr)(ft2)(oF).  Ud , from 1/Ud =1/Uc + Rd . Ud = 188 Btu / (hr)(ft2)(oF). (taking Rd = 0.001).  Computing Area by using ,
  45. 45. 44 Q = Ud A (LMTD) A= Q/Ud X LMTD = 274 ft2.  Computing length of pipe For 1 𝟏 𝟒 inch I.P.S pipe 0.435ft2 of external surface / ft length Required length = 𝑨 𝟎.𝟒𝟑𝟓 = 274/0.435 =629.88 ft. As 20 ft pipe is used , so for one hairpin the length will be 40 ft. Therefore, no. of hairpins required = required length/ 40 = 629.88/40 = 15.72 =16 Number of hairpins required = 16. PRESSURE DROP INNER PIPE  Rep in (6) above , obtain f =0.0035 + .264/(Rep)0.42 = 0.0035 + .264/(29391.95)0.42 = 4.83 X 10-3 .  ∆𝑭p = 4fG2L /2g𝝆2D, ft. By putting the values from above we get, ∆𝑭p=22.50 ft.  ∆𝑷p psi. = ∆𝑭p 𝝆 / 144. = 22.50 X 56.87 /144 = 8.88 psi
  46. 46. 45 Annulus region  D’e = ( D2 – D1)= .20575-.138 = .06775ft.  Nre’= .06775 X405902.187 /2.30 = 11956.466.  f = .0035 + .264/(11956.46).42 = 8.61 X 10-3.  ∆Fa = 4FG2L / 2g𝝆2D’e , ft. By putting the values from above, we get ∆Fa= 15.270 ft.  Entrance and exit losses , one velocity head per hairpin, ∆Fl =V2 /2g’ ft. /hairpin. =0.804 ft. /hairpin.  ∆𝑷a psi = (∆Fl + ∆Fa )𝝆 / 144 = ( .804 + 15.270) X 62.5 / 144 = 6.97 psi.