HEAT RECOVERY FROM COOLING TOWERS A PROJECT REPORT ON HEAT RECOVERY This project basically focuses on the methods that can adopted for heat recovery from process plants. Submitted by- Chandra kant verma 6/21/2013
HEAT
RECOVERY
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ACKNOWLEDGEMENT
I would like to thank all the people who have helped me
in the completion of my training. Knowledge gained here
is precious which would definitely help me in my near
future. First of all i would like to thank my training-in-
charge Mr. S.K. Das for giving me an opportunity to get
training here.
My sincere thanks to Mr. Sameer soni, my training
coordinator of Alumina plant who has guided me all
through my training.
I would also like to thank Mr. Y.P. Singh (technical head,
alumina plant, HINDALCO),Mr. Vimal kumar(Assistant
professor IIT Roorkee),Ms. Soumya singh, Mr. Pawan
kumar singh, Mr. Anuj verma for their invaluable
support.
Lastly, I am thankful to all the persons associated with
my training for friendly and helpful support.
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Contents
Resources used in alumina plant
Main chemicals and mineralogical constitutes of
bauxite
Introduction to Aditya Birla group
Integrated operation of the company
Wagon unloading to desilication process
Digestion
Clarification
Precipitation
Calcinations
Evaporation
Alumina technical
Cooling systems overview
Cooling towers
project - heat recovery from cooling towers at
hindalco
Cost effectiveness and optimum production
Bibliography
Conclusion
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RESOURCES USED IN ALUMINA REFINERY
Bauxite : hindalco obtained about 65 % of its bauxite
requirement from its own mines and purchased
around 35% of the bauxite from market.
Caustic: To cater its caustic soda needs the company
has set up BCCL(Bihar caustic and chemicals Ltd) in
joint venture with the state govt. of bihar. BCCL
caters around 90% of the caustic needs of hindalco.
Steam: Used extensively in digestion process for
attaining high temp.
Furnace oil: Used in calcinations process for OIL
FIRING operation in cyclones.
Electricity: power plays a vital role in the aluminium
industry. It takes 16000 KW of power to produce one
ton of aluminium. Hindalco has its own captive power
plant of 900 MW at renusagar.
Lubricants: For lubrication of various machines and
equipments.
Water: Used for cooling, dilution etc. Purposes.
Flocculants: Used in HRD for fast settling of mud
particles.
Crystal Growth Modifier (C.G.M.): Used in the
precipitation circuit for aiding the precipitation
process.
Filter clothes: Used in the clarification circuit for
filtering mud particles from pregnant liquor.
Lime: Used in the filtration area for aiding the filter
cloth for its proper functioning.
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MAIN CHEMICAL AND MINERALOGICAL
CONSTITUENTS OF BAUXITE
TAA(Total Available alumina-42-43%)- This is very
important for cost effective production in alumina refinery.
Gibbsite (Al2O3.3H2O – 32-34%) - Gibbsite is crystalline
euhedral with variable grain sizes. It is more easily soluble
in caustic (<135°C) and most preferred bauxite. %Gibbsite
of TAA should be 80-82%. When the bauxite is
predominantly Gibbsitic, the digestion can be carried out
at 105°C and low pressure digestion at 140-145°C.Energy
consumption is less in case of A.D. or L.P.D.
Boehmite(Al2O3.H2O- 9-11%)- It is soluble in caustic Soda
at higher temperature and pressure. % boehmite of TAA
should be 18-20%. For boehmite bauxite higher
temperature digestion 240-245°C is preferred. Energy
consumption in alumina refinery is higher in case of such
bauxite. It is tough to digest and result in low extraction
efficiency.
Diaspore(0.8-1.2%)- It is soluble in caustic soda at very
high temperature (greater than 300°C) and pressure. It not
preferred.
Silica(3-4%)- Caustic loss is basically due to reaction of
silica, alumina and caustic leading to formation of
desilicated product(DSP, Sodalite or cancrinite) which is
insoluble during digestion process.
Kaolinite (Al4(SiO1o)OH) – It is reactive part of silica
causes loss of both alumina and caustic soda as well
contaminate product and forms scales.
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Quartz (SiO2 – 1.0%)- Non reactive part of silica but at
high temperature and a certain amount of quartz can also
dissolve in the liquor.
Iron (15-18%)- Iron in bauxite is essentially insoluble
during digestion process and increases mud load. Iron is
distributed in 60:40 proportions as hematite and Goethite
in most of Indian Bauxite.
Goethite (Fe2O3.H2O- 5-7%)- Goethite in iron should be
30-35%. Goethite causes settling problem and alumina
locked in Fe- minerals is difficult to extract. Unreacted
Goethite can be colloidal in nature and result in settling
trouble. It also acts as an active seed for auto precipitation
in the mud circuit. Higher Goethite/Hematite ratio will
result in higher potential for this problem.
Hematite(Fe2O3- 9-10%)- Higher proportion of hematite in
iron is good for settling.
Vanadium (V2O5- 9-10%) - It is valuable by product from
bayers alumina refinery. More vanadium dissolves at
higher temperature digestion and if not removed forms
harmful vandate compound.
Gallium(Ga)- Found in traces (15 to 150 ppm) bayer liquor
has been found world richest source of gallium.
Organic matters (0.02-0.5%)- Increases impurity level and
alumina precipitation. Organic matter as organics is highly
soluble in bayer liquor in bayer liquor and forms sodium
oxalate.
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AN INTRODUCTION TO ADITYA BIRLA
GROUP
The Aditya Birla Group is India’s second largest
business house, with a turnover of over Rs.280 billion,
assets base valued at over Rs.265 billion and nearly
72,000 employees in 18 countries the world over.
Over 16 units in India and overseas as well (in Thailand,
Indonesia, Malaysia, Philippines, Egypt and Canada)
and international trading operation spanning several
countries including Singapore, Dubai, Russia, Vietnam,
Myanmar and China make it India’s first truly
multinational conglomerate.
Committed to being a global benchmark Group, the
Aditya Birla group reaches out to the core sector in
India- in Industries integral to the nation’s growth-
Cement, Aluminium, Fertilizers, Power, Software,
Viscose Staple fiber, textiles, Petroleum Refining,
Telecommunications, Industrial Chemicals & Financial
Services.
The Aditya Birla group is the worlds:-
Largest producer of Viscose Staple Fiber with 20%
global market share.
Third largest manufacturer of Insulators.
Fifth largest producer of Carbon Black.
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In India, it has a leadership position in:-
Aluminium- India’s largest integrated Aluminium
producer and among the World’s lowest cost
producers.
Viscose Filament Yarn- No.1
White Cement:- No.1
Rayon Grade Pulp- No.1
Grey Cement- No.3
Apparel/ Garments: India’s premium branded
apparel brand include Louis phillipe, Van Heusan,
Byford, Peter England and San frisco.
The group also has a significant presence in the
financial sector, power and telecommunications sectors
in Tie-ups with giants like Sun life Insurance (Canada),
powergen pic. (U.K.) and AT & T(U.S.A.) and Tata
respectively.
On the social front: a value based, caring corporate
citizen, the Aditya Birla Group inherently believes in
trusteeship concept of management. Part of the group’s
profit is ploughed back into meaningful welfare driven
initiatives that make a qualitative difference to the lives
of a marginalised people.
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GROUP COMPANIES AT A GLANCE IN INDIA
HINDALCO Aluminium
INDAL Aluminium
GRASIM Viscose Staple Fiber, Cement, Sponge iron, textiles (fabrics and apparels), chemical,
international business
Indian Rayon Viscose Filament Yarn, Textiles, Insulators, Carbon
Black, Ready to wear Apparels, Software,
International Business
INDO GULF Fertilizers, copper
BIRLA GLOBAL FINANCE Financial services
HGI INDUSTRIES Casting, Gases
ESSEL MINING Iron and Manganese ore mining, Ferro alloys, HDPE
woven sacks
SHREE DIGVIJAY CEMENT (subsidiary to Grasim)
Cement
DHARANI CEMENT (subsidiary to Grasim)
Cement
BIRLA TECHNOLOGIES LIMITED IDEA CELLULAR
Financial Services, Telecom, Insurance and e-
learning
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ORGANIZATION AT A GLANCE
HISTORY-
‘HINDALCO’ was setup in collaboration with Kaiser
Aluminium Chemicals Corporation, U.S.A., in a record
time of 18 months. The plant started its commercial
production in the year 1962 with a capacity of 20,000
tons per annum.
The company has grown manifold and is
managed by Board of Directors, with Shri. Kumar
Mangalam Birla as the chairman of the Board of
Directors. Day to day affairs of the company are
managed by a team of Professional Executives headed
by Shri. D.Bhattacharya as the Managing Director
(whole team).
HINDALCO TODAY
Aluminium has turned out to be the wonder metal of
the industrial world.
Aluminium growth rate is the highest
amongst the major basic metal today. Hindalco ranks
as the largest Aluminium producer in India and
contributes about 40% share in total production of the
country. The company’s fully integrated aluminium
operations consist of the mining of bauxite, conversion
of bauxite into alumina, production of primary
aluminium from alumina by electrolysis.
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LOCATION
Lying in the foothills of the Vindhya Range,
Renukoot is about 160kms from Varanasi and
154kms from Mirzapur. It is well connected to both
cities by beautiful metalled roads passing through
green forests.
There is directly daily train named Tata-Hatia-
Amritsar Express also famous as, Moori Express
between Amritsar to Tatanagar and Ranchi via
Renukoot. Besides this, there is a direct train
Swarnajayanti Express between Ranchi to New
Delhi via Renukoot commuting both twice a week.
Apart from above, Renukoot is also connected
with Kolkata through direct train named Shaktipunj
Express commuting between Jabalpur to Kolkata
via Renukoot.
The nearest Airport is at Babatpur, Varanasi,
Which is about 180kms by road from Renukoot.
For any further information regarding reservation
facilities etc., one can contact transport
department of hindalco.
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INTEGRATED OPERATION OF THE
COMPANY
The company has its own mine of bauxite. Bauxite is mined by
open Lohardanga & Richuguts mines of bauxite and then
transported by means of ropeways to the nearest railway head.
From there it is transported to Renukoot in railways wagons.
POWER GENERATION-
Renusagar power company is a captive power plant of
company. It is situated at the pinhead of Asia’s biggest open
mines. Renusagar power plant is the best performing power
plant in India. It operates at a plant load factor of above 92%.
All the ten turbines of the company operate to generate 765
MW electricity for plant townships.
ALUMINA PLANT-
From bauxite alumina is extracted in the alumina plant. This plant has the capacity to produce 350,000 MTPA of alumina. The capacity of this plant will shortly be increased to 370,000 MTPA. The Company has been inducting new technology from time to time and the most recent initiative in this regard is the adoption of Alusuisse Precipitation Technology for energy efficiency and capacity enhancement. The major raw materials for the Alumina Plant are Bauxite, Steam and Caustic Soda. Bauxite is procured from the
Company's Mines in Jharkhand and Chhatisgarh, as well as
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through market purchases and requirement of steam is met
thru Cogeneration plant at Renukoot.
REDUCTION PLANT-
This plant has two units – (1)Potroom- In the potrooms, aluminium is extracted by reducing the alumina. (2)Carbon Plant- The carbon anodes for the extraction of aluminium is manufactured here. The Smelter employs the Hall Heroult Electrolysis Process for the extraction of Aluminium from Alumina. Basic raw materials for the smelter are Alumina, Power, Anodes and Aluminium Fluoride. Alumina is produced by the Company's Alumina Refinery at Renukoot, Power is made available from the Company's Captive Power Plant at Renusagar and Cogeneration plant at Renukoot and Anodes are produced at the Carbon Plant located in the Renukoot . Aluminium Fluoride is sourced from the Company’s JV, amongst other sources.
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FABRICATION PLANT- The Fabrication Plant at Renukoot comprises of 4 Main Sections: � Remelt Shop � Cast House � Rolling Mills � Extrusion & Conform The Remelt Shop houses 3 Properzi Mills for the production of Wirerods and Feedstock to Conform Machine. Cast House is comprised of a state of the art Pig Ingot casting, Rolling Ingot (slab) casting and Extrusion Billet casting facilities. Product of Pig Ingot Casting is directly sold to customer and product of slab casting and billet casting are the feedstock to Rolling Mills and Extrusion presses respectively. The Company has a Hot Rolling Mill and 2 Cold Rolling Mills. The Rolling Mill facilities also include a Continuous Strip Caster, which contributes substantially to Energy efficiency since it eliminates numerous intermediate operations. The Company’s 3 Extrusion presses and one Conform Machine are well supported by a well-equipped Die Shop.
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Wagon unloading to desilication process
Bauxite is brought to site by means of railway wagons (8”-12”
size). Unloaded through automated wagon tippler. This 8”-12”
bauxite undergoes two stages of crushing. In the first stage,
bauxite is crushed up to size 3”-4” with the help of cone
crushers and stock piled. From stockpile, bauxite is reclaimed
to second stage crushing i.e.; hammer mills to bring down the
size further to -1/2” and conveyed to bauxite day bins.
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HAMMER MILLS
The purpose of hammer mill area is to reclaim and grind 3”-4”
bauxite to -1/2” size (95%) by hammer mill and transport it to
bauxite day bin.
There are three hammer mill
circuits each containing two hammer mills namely:-
Hammer mill # 1 & 2 – Sweetening bauxite
Hammer mill #3 & 4 – Normal bauxite
Hammer mill #5 & 6 – Normal bauxite
Hammer mill #7 – Normal bauxite
Equipment provided in each circuit are bauxite feeder(reclaim &
emergency) 2 No’s, hammer mill feed conveyor , magnet &
metal detector on conveyer, sump pump in conveyer pits, Dust
collection system (Bag filters), weight-o-meter on hammer mill
discharge conveyer (Old circuit only), air blasters.
Hammer Mill Description: It contains a high speed rotor turning
a cylindrical body. The shaft is horizontal. Here the bauxite is
dropped into the top of the casing and gets broken by set of
swing hammers pinned to a rotor disk. Bauxite shatters into
pieces, which fly against a liner inside the casing and gets
breaks into small fragments. This in turn is further crushed by
the hammers and pushed through cage bars, which covers the
discharge opening.
The criticality of the operation is to maintain the size of hammer
mill discharge, it should be +1/2” up to 5%. But in the raining
season it increases to 10%.
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For health and safety purpose chemicals and water are spread
in the unloading area at the wagoner end, for suppressing the
fine dust of bauxite.
Trouble shooting practices are for oversize product, abnormal
sound coming from Hammer Mill, abnormal vibration in
Hammer Mill, bauxite not coming through feeders to feed end
conveyer of Hammer mill, misalignment of conveyors and
Hammer Mill tripping frequency.
OPERATION OF HAMMER MILL DURING RAINY SEASON
Due to wet, bauxite operation becomes difficult as plugging of
feed and discharge chute, etc. Occurs.
To avoid difficulty following steps are taken:-
1/2” cage bars are replaced by 1”.
1/2” bypass screen is replaced by 1”.
Feed rate is reduced.
BALL MILL
There are eight numbers of ball mills, which operate at different
capacities. The ½” size of bauxite from the hammer mill is
stored in the day bin. Then with the help of belt conveyer it is
feed into the ball mill. The load shell of belt conveyer takes care
of feed rate of the ball mill. Out of eight ball mills, seven ball
mills always remain in line. Here wet grinding with the spent
liquor in the ball mill is not done. For wet grinding maximum
speed of ball mill should be 70-75% of its critical speed.
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Critical speed:-
The critical speed is the theoretical speed at which the
centrifugal force acting on a ball mill in contact the mill shell at
the height of its path equals the force on it due to gravity.
The expression for critical speed is
Nc = 76.6/D1/2
Where Nc = critical speed, D= diameter of ball mill in feet.
Ball changed inside a ball is 50% of its volume that is called
bed height which gives maximum capacity. The discharge of
ball mill is solid 50% with -200 mesh size (60-65%). From the
discharge point of view ball mills are of two types. In the first
type the discharge of ball mill is passed through the simple
sieve of -200 mesh size. The underflow goes to the desilication
unit where as overflow again goes into the ball mill. In the other
type the discharge of the ball mill is passed through the sump
tank to the primary cyclone. The overflow of the PC goes to
secondary cyclone feed tank(SCFT) & underflow goes to the
ball mill. From the SCFT slurry is feed to the secondary
cyclone. The overflow of the SC goes to the sum tank. The
density of the discharged slurry is maintained at 1.82kg/l.
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DESILICATION
The product of the ball mill (bauxite slurry) of all ball mills is
passed through a desilication heater where slurry is preheated
up to 98°C by means of steam of 2.4kg/cm2 pressure &
temperature of 130°C. This slurry is passed through six no of
desilication tanks to provide holding time of 10-12 hrs. Ball mill
1,2 & 3 for the grinding of Gibbsitic bauxite. The main reaction-
taking place in the tank is
5[Al2O3.2SiO2.2H2O] + 2Al(OH)3 + 12NaOH
2[3Na2O.3Al2O3.5SiO2.5H2O] + 9H2O
The temperature maintained for this reaction is nearly 98°C.
The slurry first comes in the desilication tank no. 1, and then it
is passed through two heaters.
After passing through heater it goes to desilication tank number
2,3,4,5 & 6, where temp maintained is around 98°C and holding
time is 12hrs to 14hrs.The maintained temperature and holding
time is the critical parameters for the desilication tank. The
main problem of the desilication area is the scaling in the
desilication heaters. Life of heater is 30 days. After every 30
days heater is being cleaned with chemicals.
Efficiency of heater is around 60% & level maintained in
desilication tank is 90%. We can increase the service life of
heater by using some other metal instead of mild steel, like Al
alloys, which are less corrosive than mild steel. Also the speed
of slurry which passes through heater is important parameter to
be governed. The desilication product gets stick to the tank
wall, which are removed after every 5 to 6 months. The main
problem of the desilication area is the low pick of desilication
heater temperature.
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DIGESTION
Desilicated and pre-heated bauxite slurry is pumped into the
digestion having two streams and direct heating system
through slurry heaters where alumina content of bauxite is
dissolved into caustic solution at 242°C temperature and 36kg
per sq.cm pressure, in digestion I and II. Digestion III is of
single stream and indirect steam reaction occurring in digestion
is
Al2O3.3H2O + 2NaOH 242°C & 36kg/cm2
2NaAlO2 + 4H2O
The above reaction is an endothermic reaction and requires
heat for reaction to take place. Two methods are in use for the
digestion of bauxite, namely “two stream” and “one stream”
process.
Two streams is a process in which the digesting liquor is
divided into two unequal streams. The main stream (80-85%) is
heated step-by-step in tubular heat exchangers with steam
from flash tanks. The remaining liquor is led to the wet grinding
of bauxite and fed finally to the digesters. This method is used
in digester I & II. In one stream method bauxite liquor slurry is
heated indirectly to the digestion temperature by passing
through in series connected autoclaves. This method is used in
digestion III.
To push bauxite slurry into the mixing tee Geho pump is used.
The Geho pump has a diaphragm, which sucks and creates the
adequate pressure. Geho pump is used for high pressure and
low flow. The capacity of geho pump of Digestion area II and I
is 17.75-13.5m3/hr respectively.
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CAUSTIC ADDITION (290gpl)
67-70°C
Bauxite slurry
Live steam
Fig: Circuit diagram of digestion
The spent liquor comes in the test tank where the concentration
of the caustic is maintained at 290 gpl by the addition of fresh
caustic 900gpl. Then with the help of pumps it is sent to the
low-pressure heater and medium pressure heater, before it
comes to the mixing tee.
From the mixing tee it (bauxite slurry) goes to the process
slurry heater where temperature is increased to 193°C.
TEST TANK Primary Booster Charge
pump 20-
22kg/m3
Low Pressure
heater
Booster
pump
Medium
pressure Injection
Pump Mixing Tee
Process slurry Indirect steam
heater
Digesters
35kg/cm2
Tail Valve
35kg/cm2
Flash Tank 1 to 7 Blow off
Blow Off
Pump Clarification
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Then it goes to ISH, where live steam is used to raise the
temperature up to 230°C. And finally the mixture is passed
through the digesters at 242°C and 36kg/cm2 pressure. Holding
time in digesters is 2hrs. The digested boehmite bauxite slurry
is flashed in successive stages of flash tanks and finally the
pressure is brought down near to atmosphere.
Sweetening is done in flash tank 2, 3 or 4 at temperature of
150°C. In sweetening process slurry is heated from 65°C to
100°C by means of 2.4kg/cm2 steam. And this slurry is fed to
desired flash tank inlet line in digestion unit I, II, III through
centrifugal pumps where it mixes with the digester normal
bauxite slurry. The Gibbsitic bauxite has higher solubility at
lower temperature (140°C) resulting in increasing the finished
A/C ratio and enhanced rate of alumina dissolution. The main
advantages of this process are increased rate of alumina
dissolution only through marginal addition steam requirement
and higher liquor productivity.
The main operating parameters are steam temp., slurry flow,
digestion pressure, steam pressure, back pressure(pressure of
control valve), heater temperature. The main criticality of the
operation is scaling in the heater pipe, vessel and pepelines,
leakage through joint, vent, pipeline, gasket and welding joints.
Scaling in the pipe is removed by caustic cleaning. And critical
equipments are charging pump, booster pump, injection pump,
geho pump, heater and agitator.
The heat recovery system of the digestion area is temp. Pick-
up, line and vessel insulation, steam line safety valve; if
pressure will increase it will open to release the pressure. Other
arrangements are facilities for tripping of booster pump, Geho
pump, digestion slurry control valve and injection pump.
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Condensate generated from the process slurry flashing in
series of flash tanks is pumped to boiler house for its heat
recovery and returned back to clear water header, used in
refinery in pump gland sealing and cooling tower make up.
The condensate management includes flash tank level to
control. Some of the preventions require during the operation
are restricting moisture to enter into the air, no choking of valve,
smooth air supply, restricting flow of wrong signals from control
room, condensate contamination, slurry leakage and slurry
coming out from relief tank bottom. The whole process of
digestion area is DCS (Distribution Control System) controlled.
We can see and control all parameters through DCS.
CLARIFICATION
PURPOSE: To efficiently separate sand contents and red mud
from pregnant liquor, to remove maximum fine suspended mud
solid particles from pregnant, to recover physical soda of red
mud by counter current washing and maximum possible
economic recovery of bound soda of mud of causticization
process, Filtration of red mud slurry Techno-economically and
environmentally viable disposable of red mud.
HRD (High Rate Decanter)
Inside HRD sedimentation process takes place where
separation of mud and pregnant liquor take place.
In HRD, the feed slurry is admitted tangentially into the unit
(feed well) at a depth of 2-3 feet below the surface of liquid. On
entrance, the slurry spread rapidly through the cross section of
the settler. Liquor than flows upward to be withdrawn at the
overflow launder, and the solid settles down to the bottom. The
flocculent used for the settling of solid particles are polymers
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(Flomina AI99F). The upper zone is free from particles and
increases slightly in solid below the entrance of the feed.
Particles settle in this zone by free settling. Below the dilute
zone is a compression in which the concentration of solid
increases rapidly with distance from the boundary between the
zones. The rakes, which operate in the bottom of compression
zone, tend to break the flock’s structure and compact the
underflow to a solid contact. The main purpose of the rake is to
keep mud pump able to bring mud towards pumping end and to
remove trapped liquor between mud portable (underflow). In
practice a clear overflow can be obtained if the upward velocity
of liquid in the dilute zone is less than the minimum terminal
velocity of the solid at all point in the zone. Upward velocity of
the liquid is directly proportional to the overflow rate. If the solid
will be more then load on the rake will increase and in that case
dosing of flocculent is to be reduced.
Components of HRD are drive assembly, feed as well, cable
torque mechanism and overflow launder.
Flocculent: it consolidates the underflow mud slurries. It
neutralizes the charges on the slurry, which in turn
agglomerates to form flocs each containing many particles.
SAND CLASSIFIER
The sand from the pregnant liquor in cyclone by cyclonic effect
and comes in screw conveyer where the leaching of soda from
sand takes place and the sand collected in hopper is disposed
off through dumpers. The overflow of first screw conveyer is
collected in 1st wash circuit from where it is pumped in one of
the settler in line. The overflow of second screw goes either in
settler or washer.
WASHER
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This system leaches out caustic from mud slurry and disposes
liquor free mud. There are 7 washers in one series. HRD
underflow is introduced into the 1st washer feed tank. Polymer
is added with the feed. A counter current washing by hot water
takes place here. The overflow of washer #1 is the final product
of washing operation and contains recovered caustic. Dilution is
pumped to digestion area in order to maintain L to P
concentration. Wash water temperature is maintained around
90°C into last washer for effective leaching of soda. Mud from
last washer is pumped to drum filter directly. The other streams
received in washer circuit are drum filtrate liquor, fine seed
wash caustic zed liquor, sand classifier drained liquor & filter
press washed cake.
The diameter of the washer is 125’ and they are 7 in number.
Other equipment associated with the washer is overflow pump,
wash water tank & underflow pump, dilution tank & dilution
pumps and drum filter heater (3 in nos.).
DRUM FILTER
Filtration is the removal of solid particles from a fluid by passing
the fluid through a filtering medium or septum on which the
solids are deposited.
Drum filter is a continuous vacuum filter i.e. filtration is done
under vacuum. Drum filter is a drum rotating about a horizontal
axis with a portion of drum submerged in the slurry to be filtered
in a vat. Agitators in the vat keep the slurry thoroughly mixed.
The drum surface is divided into number of longitudinal
sections comparatively shallow in depth, each of which is
connected to a vacuum system. The filter valve is connected to
a vacuum system. The drum suction is equipped with a
perforated plate, which forms the outer cylindrical surface of the
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drum and supports the filter medium. The drum is then covered
with a suitable filter cloth by lateral caulking.
As the drum rotates through the slurry in the tank, the liquid is
sucked through the cloth into the drum piping and out through
the filter valve. The solids are trapped on the surface of the
filter cloth, forming what is known as filter cake and deposit on
the outer surface of the rotating drum, where it is subjected to
water spray (Temp. is 90°C). Air is then passed through the
cake by suction as the rotation cycle progresses, to remove
residual cake moisture as much as possible.
As the filter section rotates towards the cake discharge point
the filter valve shuts off the vacuum on that particular section
ready to discharge. Mud cake is finally discharged to hopper
after being led off the drum over a small roller and scraped by a
string scrapper. When the mud hopper gets full, the mud is
emptied out in dumper stationed below the hopper. The mud is
then disposed off in Mud Yard. The product of the drum filter
should be of below specifications:
Solid after filtration – 70%
Leachable Soda – 1.0%
SECURITY FILTER:
Security filter is the last check point to trace and remove red
mud. We use Kelly filters for this purpose. A new filter known as
disaster filter is proposed to be used as it does not requires
maintenance since it will have no moving parts. However, it has
filter cloth, etc present in Kelly filters.
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RED MUD YARD
Mud Yard has been divided into 8 different dumping zones.
Mud is unloaded by Dumpers from to these dumping stations,
which have a earthen Dam, at the outer periphery of valley to
prevent mud from going to nearby forest land. It also ensures
pollution norms.
ADDITIVE AREA
The main purpose of this unit is to prepare two additive namely
TCA and Starch solution.
1. Tricalcium Aluminate (TCA): It activates the filtration rate
and porosity of filter cloth. It is used in filter presses. It is more
effective than filter Aid by reducing solid of filtration and
increasing flow rate of presses.
Reaction- 6NaAlO2 + 3Ca(OH)2 Ca3(AlO2)6 + 6NaOH
Preparation: Measured amount of press filter liquor (sodium
aluminate liquor) is taken in TCA preparation tank (12m3
capacity) then measured amount of lime slurry is taken in the
same tank. The temperature is maintained at 102°C for a
duration of around 2 Hrs. When the preparation of TCA is
completed the colours become Ivory.
2. Starch- It activates the settling of Red Mud. It is most
effective on fine particles since it acts as a surface-active
agent.
28
PRECIPITATION AND CALCINATION
Precipitation of alumina tri-hydrate fro seeded caustic aluminate
liquor is an important step of bayer’s process for production of
alumina from bauxite. The importance of precipitation step arise
from the fact that by and large it determines the ultimate
characteristics of the product (calcined alumina) and much of
the success of the plant’s performance depends on this aspect.
During precipitation precipitate of alumina tri hydrate gets
deposited on seed crystals as well as on nuclei that get formed
during the precipitation cycle. The new nuclei also grow by
fresh deposition of precipitate. There are four main
mechanisms taking place during a precipitation process. They
are nucleation, crystal growth, agglomeration and crystal
breakage.
Nucleation is a special condition characteristic by the
spontaneous generation of very fine crystals, of submicron
sizes and as a consequence there is a rapid increase in the
production of sub sieve particles. Controlled nucleation is very
necessary to produce just the required quantity of new seed
crystals for maintaining the precipitation process in a balanced
state. Uncontrolled nucleation along with in sufficient
agglomeration and/or crystal growth would lead to seed
balance getting disturbed and this can’t be tolerated especially
in a plan that produces coarse sandy alumina.
Growth is the mechanism by which particles get enlarge by the
addition or deposition of newly precipitated alumina tri hydrate
into the original crystals. The conditions required here are more
or less similar to what are required to favour agglomeration,
which is also a process of enlargement of particles. Large
29
particles are produced predominantly by growth are dense and
strong and distinct from those produced by agglomeration,
which are generally weak.
Agglomeration is also a mechanism whereby larger particles
are produced. But as distinct from the mechanism of straight
growth, in this case the enlargement is achieved by smaller
particles colliding and then getting cemented together by
precipitation of hydrate between the colliding particles that is by
interstitial deposition of hydrate.
In addition to the above mentioned mechanisms there is yet a
fourth one, crystal breakage which could occur at any stage in
a precipitation process. It is a mechanism not directly caused
by the precipitation process but all the same influences the end
results of the process. Due to the shear forces created by
agitation and turbulence, either the newly deposited particles
are sheared-off or the particles are continuously created. Thus
during a precipitation process two opposing mechanisms must
proceed in such a way as to permit the circuit to remain in
balanced state, that is, just the amount of coarse particles to
yield a final product of acceptable granulometry and just the
amount of fine particles to provide adequate surface area for
further precipitation, while achieving maximum possible yield
from the liquor. And also it equally important for the operator to
ensure that as the particles are enlarged the final material
attains adequate strength to withstand breakdown during
subsequent operations.
One aim in alumina plant precipitation area is to achieve
maximum yield with coarse aluminium tri-hydrate particles and
lesser finishing A/C ratio. Amount of alumina tri-hydrate
extracted per litre is nothing but yield.
30
AGGLOMERATION PRECIPITATION (AGG)
In the agglomeration phase the washed fine seed slurry is
added to pregnant liquor at a strictly controlled ratio and
controlled temperature conditions. In this phase two or more
fine seed hydrate particles join to form larger particles. The
cementing action is achieved due to the highly over saturated
state of alumina in the pregnant liquor. The newly precipitated
alumina particles cement the existing particles.
NEW GROWTH PRECIPITATION AND
HYDRATE STORAGE (NG)
The precipitation process takes place by addition of seed
hydrate slurry and pregnant liquor together with natural and
forced cooling. The precipitation process takes place in
Agglomerated precipitated slurry drawn from last but one
Agglomerator is added to the last agglomerator inline.
The slurry from Last Agglomerator is pumped into first inline
3000m3 Growth precipitator. Eight Growth precipitators are
connected with each other means of launders and there is
provision of bypassing any one of them. About 70% of slurry
from third or fourth growth precipitator are gravity fed to the
inter stage cooler. The cooled slurry precipitator are gravity fed
to the interstage cooler. The cooled slurry from inter stage
cooler is returned to fourth of fifth growth precipitator and mixes
with the uncooled slurry. The total residence time available is
about 18hrs and due to natural cooling 3°C temperature drop
occurs while slurry is flowing through the precipitator and
additional temp drop of 8°C is obtained from inter stage cooling.
The agitation in these tanks is by external airlift draft tube
31
agitation. The slurry from the test tank is pumped to the feed
manifold the existing 780m3 precipitators.
INTER STAGE COOLING (ISC)
From the third fourth growth precipitator slurry is gravity feed to
the inter stage cooling, which is approx 70-75% of total flow in
new growth. The inter stage cooling comprises of flash vessel,
barometric condenser, cooling tower, hot well and associated
pumps. The slurry temp is dropped by approx 11-12 deg cent.
The cooled slurry is pumped back to fourth or fifth new growth
tank and the hot water goes to cooling tower for cooling.
EXISTING GROWTH PRECIPITATION (EG)
52 Nos. Batch ppt tanks are converted into four trains and are
numbered as A,B,C & D. The train ‘A’ consists of 13 no’s of
tanks connected in series and there is provision of by passing
single and in some cases two tanks. The train ‘B’ consists of 12
no’s of tanks connected in series. The train ‘C’ consists of 14
no’s of tanks connected in series. The train ‘D consists of 13
no’s of tanks connected in series.
The Mid precipitated slurry from the new growth tanks is
directed to manifold and distributed to first tank in operation of
four trains the slurry from last tanks in operation of all the four
trains is pumped by means of Pump off Pump (4Nos) to new
hydro cyclone classification area. The agitation in these tanks is
by external airlift draft tube agitation. The approx residence in
existing growth is 27Hrs. The over flow from both the cyclones
is collected in cyclone over flow tanks and then pumped to
primary thickener for further classification.
PRIMARY THICKNER
32
There are four primary thickener installed in plant. Three are
normally in line and one in standby. The tank Diameter is 9.1m.
CLASSIFICATION
The classification area is designed to classify hydrate from the
pump of slurry into three different size fractions. The coarsest
fraction becomes product, the finest fraction fine seed and the
fraction between is the coarse seed. In product cyclones,
coarse seed is separated from a part stream of the Pump off
slurry. After suitable dilution, it is sent to the product de-
liquoring disc filters in filtration. In the seed cyclones, coarse
hydrate is separated from the remaining pump off slurry and
becomes part of coarse seed supply.
Over flow from both sets of cyclones is collected in overflow
tanks and pumped to the PT’s. Underflow from the PT’s is
pumped to the seed cyclone underflow tanks to make up
remainder of coarse seed. The slurry in the cyclone underflow
tanks is diluted to a suitable density and pumped to the coarse
seed de-liquoring disc filters.
Overflow from the PT’s by gravity goes to ST’s. Underflow from
the ST’s is collected in the fine seed slurry tank, while the
overflow goes to the online tank. The mixture of ST & TT
underflow constitutes the fine seed slurry, which is pumped to
the fine seed deliquoring disc filter.
Hydro cyclones
Filtration
Primary thickeners (PT)
Secondary thickeners (ST)
Terminal thickeners(TT)
HYDROCYCLONES
33
There are batteries of cyclones: Product cyclone, Coarse
cyclone and Swing cyclones. The Swing cyclone is the stand by
for both product and coarse. One battery consists of 24
cyclones with feeding arrangement, underflow and overflow
piping. The product cyclone underflow is collected in product
deliquoring filter. The coarse cyclone underflow is collected in
coarse cyclone underflow tank and PT underflow also pumped
into the tank. The slurry from this tank is pumped to coarse
deliquoring filters Mts and a volume of 1440m3. The feed to all
PT’s are fed from cyclone overflow tanks and is controlled and
adjusted by online flow meters and C/V. The overflow goes to
associated ST’s.
PT underflow is pumped by PT underflow pumps to coarse
seed cyclone underflow tank. There are three sets, each
contain two pumps.
SECONDARY THICKENERS
There are four ST’s in plant. Normally three are inline and one
is in standby mode. The tank diameter of ST’s is 12.8m and
volume of 1560m3. The overflow of all ST’s goes to TT. ST
underflow is pumped by ST underflow pumps too fine seed
slurry tanks. There are four sets, each set contains two pumps.
TERMINAL THICKNERS
There are two TT’s installed in plant normally one inline and
one in standby mode. The thickeners have diameter of 38m
and operating volume of 6700m3. The TT feed is combined
overflow from three ST’s inline and spent liquor from filtration
area. The overflow is collected in a collection box, where
flocculent is added to achieve good settling and low overflow
solids. The TT’s are equipped with a rake mechanism. The
34
torque and motor current are monitored on a regular basis. The
overflow of TT goes to spent liquor tank.
CALCINATION
Calcination is another important step in Bayer’s process, where
alumina tri-hydrate is calcined to final form, alumina possessing
certain desired characteristics. This step is essential because
for the conduct of electrolysis all material added to the cells
must be completely free from water whether chemically bound
or absorbed on the surface.
Thermal dehydration of alumina tri-hydrate starts at 180-290°C
and is practically over at about 600°C. However, alumina
produced at these temperatures has a highly activated surface
and as such tends to absorb considerable amounts of moisture
when in contact with the atmosphere. Hence the dehydration
step has to be followed by energetic heating to temperature of
more than 1100°C at which its property of absorbing moisture
from the atmosphere is reduced to a level considered safe
enough for the operation of electrolytic ceels.
In HINDALCO there are two types of calciner. These are as
follows:
1. Gas Suspension Calciner (GSC) 2. Flash Calciner
The main components in the G.S.C. System comprises of:
1. Hydrate Feed System
2. Ventury Type Flash Drier
3. 2-Stage Cyclone Pre-heater(P01-P10&P02)
4. Gas Suspension Calciners PO4.
5. Disengaging Cyclone or Separating Cyclone (PO3).
6. $-stage Cyclone Cooler (CO1,CO2,CO3 & CO4).
7. Secondary Fluid Bed Cooler (KO1 & KO2)
35
8. Oil heating system
9. Dedusting and dust recycling system.
GAS SUSPENSION CALCINER FLOW SHEET (CALCINER I)
160°C
1150°C
380°C OIL
840°C 620°c
AO1
P10
PO1
PO3
PO4
CO1
CO2
CO3
ESP
A
O
36
420°c
ALUMINA 80°C AIR 250°C
AIR 30°C
CO4
COOLING
WATER
KO1 KO2
37
PROCESS
Hydrated slurry from precipitation area, is fed on 24-ft dia
hydrate Pan filter. The pan filter consists of 20 nos. metallic
perforated panels; each covered by polypropylene mono
filament clothes.
Hydrate slurry is de-liquored on the hydrate filter, then hydrate
is washed, first by dilute liquor of precious recycled washing
then by fresh water/condensate at 90-95°C and finally by
15psig steam to recover soda.
The wet hydrate containing 4-5% free moisture on an average
enters the venture drier at a temperature of about 60°C through
Roto Bin( Hydrate Surge Bin with a table feeder) at a constant
feed rate. The material dries in the venture drier is carried to
the first pre-heater (PO1,PO2) cyclones by the gas flow. The
dry Alumina Hydroxide from the upper pre-heater cyclone is
preheated and partly calcined in cyclone (PO2) and is
discharged in to the calciner (PO4) at temperature of 300-
400°C. The retention time of calcined alumina is 8-10 second in
the main Calciner. The Calciner temperature is chosen in
accordance with the desired product specification.
Calcined alumina is separated from the hot gases in the
separating cyclone (PO3) and discharged into the 4-stage
cyclone coolers. The alumina is cooled to around 250°C by
overall counter current heat exchanger with atmospheric air
used for combustion of fuel in calciner. The temperature of the
calcined Alumina is reduced to 80°C in the secondary fluid bed
coolers (KO1 & KO2), where the calcined Alumina is cooled
indirectly by water flowing counter current to the alumina.
The dust carried by flue gases from the upper pre-heater(PO1,
PO10), is de-dusted in the electrostatic precipitator.
38
The total pressure drop across the whole plant is 500-550mm
WG. The vertical arrangement of cyclones utilizes gravity flow
of solids. As a result of low overall pressure drop and low
specific air consumption, low specific power consumption per
ton Alumina is achieved.
FUEL OIL FIRING SYSTEM
Fuel oil for combustion is added through 4-burner by low-
pressure atomization in PO4. Air for automation is provided
from a compressor unit. Burners are suitable for firing furnace
oil as well as L.S.H.S. oil.
ADVANTAGE OF THE PROCESS
1. Low maintenance cost owing to simple design
2. Easy adjustment of Product Specification by control of the
calciner temperature.
3. Low power consumption due to small overall pressure
drop and low specific air consumption.
4. Low in plant solid inventory minimizing production loss
during start up and shut down operation caused by power
failure.
39
EVAPORATION AREA
The objective of evaporation is to concentrate a solution
consisting of non-volatile solute and a volatile solvent.
Evaporation is conducted by vaporizing a portion of solvent to
produce concentrated of liquor.
Evaporation is process in which the change of state from liquor
to gas can occur at any temperature up to boiling point. At any
time a variable population of molecules in liquid will have
sufficient to escape in to the atmosphere. The rate of
evaporation arises with increased temperature because as a
mean kinetic energy of liquid molecules rises and so will be the
no. of molecules having enough energy to escape. So lower the
boiling point faster is the evaporation though this also depends
on the latent heat. There are various types of evaporators
being used in various industries. Here in Hindalco there are
multiple effect flash type evaporators.
The basic principle of flash evaporation is to evaporate water
from caustic liquor in a series of stages. Flash vapours
generated during various stages are used to heat caustic liquor.
Preheated caustic liquor is passed progressively at lower
pressure through various flash stages wherein flash
evaporation leads to cooling. The vapours from each stage are
used to preheat caustic liquor by use of multi-pass heat
exchangers connected in series.
There are three evaporation units in Alumina plant they differ
only in the facility and the equipment installed and hence
capacity and steam economy but the basic principle remain
Evaporation rate= mass flow x specific heat x ∆T x 60
Latent heat x 1000
40
same. Here in Hindalco there are 3 multiple effect Flash
Evaporators. Total Evaporative capacity is 205 TPH.
Individual capacity is shown below:
UNIT CAPACITY ECONOMY
I 35 TPH 2.4
II 70 TPH 2.0
III 100 TPH 2.6
IV 60 TPH 2.0
WORKING OF EVAPORATION UNIT:
In the plant during process, water is directly added to caustic or
indirectly as steam. So the caustic concentration goes down.
To increase the concentration of caustic, the water content
inside the caustic should be removed, so three Evaporation
Units are set. In all evaporation units, process is almost similar.
Diluted caustic comes to unit, and goes to Barometric
condenser in which it creates vacuum, then goes to pre-heater
in which caustic is heated. This heated caustic goes in first
evaporator. In evaporators barometric condenser already
creates vacuum. From first to last evaporator caustic goes in
the same fashion and in last evaporator stream is used to heat
the caustic, this caustic then travels in reverse manner means
from last to first evaporator and vacuum also goes on
increasing from last to first evaporator. Due to these effect
caustic flashes inside evaporator and produces hot condensate
and it is then cooled down. The hot condensate, which is
formed inside evaporator, is used to heat incoming caustic from
first to last evaporator. So in this manner the consumption of
steam is less as compared to the evaporation rate achieved.
41
ALUMINA TECHNICAL
The main job of the alumina technical is to monitor the various
process control parameters and prepare the daily, monthly and
yearly report. These parameters are size of hammer mill and
ball mill discharge, slurry density, A/C ratio, concentration of
caustic in the pregnant liquor and in spent liquor, percentage of
solid in the pregnant liquor, addition of flocculants in the HRD
and washers, addition of seed in the agglomerates and size of
the calcined alumina. The technical also does monitoring of raw
materials used in alumina. Among the various raw materials
used, the main raw materials are bauxite and caustic. So the
monitoring of these two is important. In fact the main value
drivers and cost drivers of alumina plant are these two only.
On daily basis digestion extraction efficiency of bauxite slurry
charging is found by lab analysis. It can be controlled by
avoiding over charging of bauxite slurry, due to which the
extraction efficiency of digestion process decreases. In case
under charging there is loss in the production of alumina, So to
optimize under charging and over charging by feeding accurate
charging of bauxite slurry with a certain proportion of spent
liquor charging to Mix tee.
Extraction efficiency= TAA in bauxite- TAA in flash effluent x (%fe2o3 in bauxite %
fe2o3 in flash effluent mud)
TAA in bauxite
42
COOLING SYSTEMS OVERVIEW
COMMON TYPES:
Open re-circulating systems.
Once through systems.
Closed re-circulating systems.
Differences-
1. Open re-circulating systems: This is most widely used
industrial cooling design. It consists of pumps, heat
exchangers and cooling tower. The pumps keep the water
re-circulating through heat exchangers where it picks heat
return to the cooling tower where heat is released from the
water through evaporation. Because of evaporation the
water in open re-circulating systems undergoes changes
in its basic chemistry.
2. Once through systems: Cooling water passes through
heat exchanger equipment only once because large
volumes of cooling water are used. The mineral content at
cooling water remains practically unchanged as it passes
through the system.
3. Closed re-circulated system: Use the same cooling water
repeatedly in a continuous cycle, first, the water absorbs
the heat from process fluids, then releases it in another
heat exchanger, in this system an evaporation cooling
tower is not induced.
43
WHAT IS INVOLVED IN COOLING PROCESS?
Cooling involves the transfer of heat from one substance to
another. The substance that loses heat is said to be cooled
and the one that receives the heat is referred to as coolant.
WHY WATER IS USED FOR COOLING?
Water as coolant, yet another significant benefit the industry
gets from water other than its normal indispensable
relationship with human lives and industries. Several factors
make water as coolant-
It is normally plentiful, readily available and inexpensive.
It is easily handled.
It can carry large amount of heat per unit volume.
It doesn’t expand or compress significantly within
normally encountered temperature range.
It doesn’t decompose.
What are some important properties in cooling water
chemistry?
CONDUCTIVITY- A measure of water’s ability to conduct
electricity. In cooling water it indicates the amount of
dissolved minerals and gases.
PH- It gives an indication of the relative acidity or basicity of
water. The pH scale ranges from 0 to 14, where 0 represents
maximum acidity and 14 maximum basicity.
ALKALINITY- In cooling water two forms of alkalinity play a
key role. These are carbonate(CO32-) alkalinity and
bicarbonates (HCO3-) alkalinity.
HARDNESS- Refers to the amount of calcium and
magnesium minerals present in the water. The hardness in
natural water can vary from a few ppm to over 200 ppm.
44
COOLING TOWERS
INTRODUCTION-
A great majority of cooling towers are operating today, even
though some are newly installed, have been engineered with
techniques over 30 years old. Today’s state of art can be utilized to
Retrofit practically all towers and upgrade their capability for
producing colder water or cooling greater volume of circulating
water in the same tower.
A recent government agency survey stated, “Inadequate thermal
performance of cooling towers surveyed is costing electric utilities
more than $25 million per year in lost revenue and higher fuel cost.
In process plants, power generating stations, chemical plants and
regeneration system that depend upon the dissipation of waste
heat, it behoves the owner and operator to investigate the
possibility of upgrading the existing cooling tower rather than
installing another cooling unit which may or may not produce the
necessary colder water. It should be understood that colder water
can save energy and can create an operating profit.
Cooling towers are a very important part of many chemical plants.
The primary task of cooling tower is to reject heat into the
atmosphere. They represent a relatively inexpensive and
dependable means of removing low-grade heat from cooling water.
The make-up water source is used to replenish water lost to
evaporation. Hot water from heat exchangers is sent to the cooling
tower. The water exits the cooling tower and is sent back to the
heat exchangers or to other units for further cooling. Typical closed
loop cooling tower system is shown in figure given below-
HOT
AIR
AIR
Plant heat
exchanger Cooling tower
Make-up
water source
45
TYPES OF COOLING TOWERS
Cooling towers fall into two main categories: Natural draft and
mechanical draft.Natural draft towers use very large concrete chimneys
to introduce air through the media. Due to the large size of these towers,
they are generally used for water flow rates above 45,000m3/hr. These
types of towers are used only by utility power stations.Mechanical draft
towers utilize large fans to force suck air through circulated water. The
water falls downward over fill surfaces, which help increase the contact
time between the water and the air- this helps maximise heat transfer
between the two. Cooling rates of mechanical draft tower depend upon
their fan diameter and speed of operation. Since, the mechanical draft
towers depend upon their fan diameter and speed of operation. Since,
the mechanical draft cooling towers are much more widely used; the
focus is on them in this project.
MECHANICAL DRAFT TOWERS
Mechanical draft towers are available in the following airflow
arrangements:
1. Counter flows induced draft.
2. Counter flow forced draft.
3. Cross flow induced draft.
In the counter flow induced draft design, hot water enters at the top,
while the air is introduced at the bottom and exits at the top. Both forced
and induced draft fans are used.
In cross flow induced draft towers, the water enters at the top and
passes over the fill. The air, however, is introduced at the side either on
one side (single-flow tower) or opposite sides (double-flow tower). An
induced draft fan draws the air across the wetted fill and expels it
through the top of the structure.
The figure shown below illustrates various cooling tower types.
Mechanical draft towers are available in a large range of capacities.
Normal capacities range from approximately 10tons, 2.5 m3/hr flow to
several thousand tons and m3/hr. towers can either factory built or field
erected – for example concrete towers are only field erected.
46
Many towers are grouped together to achieve the desired capacity.
Thus, many cooling towers are assemblies of two or more individual
cooling towers or “cells”. Multiple-cell towers can be lineal, square, or
round depending upon the shape of the individual cells and whether the
air inlets are located on the sides or bottoms of the cells.
47
COMPONENTS OF COOLING TOWER
The basic components of an evaporative tower are: frame and casing,
fill, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans.
Frame and casing- Most towers have structural frames that
support the exterior enclosures (casing), motors, fans, and other
components. With some smaller design, such as some glass fiber
units, the casing may essentially be the frame.
Fill- Most towers employs fills(made of plastic or wood) to facilitate
heat transfer by maximising water and air contact. Fill can either
be splash or film type.
With splash fill, water falls over successive layers of horizontal
splash bars, continuously breaking into smaller droplets, while also
wetting the fill surface. Plastic fill promotes better heat transfer
then the wood splash fill.
Cold water basin: The cold water basin, located at or near the
bottom of the tower, receives the cooled water that flows down
through the tower and fill. The basin usually has a sump or low
point for the cold water discharge connection. In many tower
designs, the cold water basin is beneath the entire fill. In some
forced draft counter flow design, however, the water at the bottom
of the fill is channelled to a perimeter trough that functions as the
cold water basin. Propeller fans are mounted beneath the fill to
blow the air up through the tower. With this design, the tower is
mounted on legs, providing easy access to the fans and their
motors.
Drift eliminators: These capture water droplets entrapped in the
air stream that otherwise would be lost to atmosphere.
Air inlet: This is the point of entry for the air entering the a tower.
The inlet may take up an entire side of a tower-cross flow design-
or be located low on the side or the bottom of counter flow
designs.
Louvers: Generally, cross-flow towers have inlet louvers. The
purpose of louvers is to equalize air flow into the fill and retain the
water within the tower. Many counter flow tower designs do not
require louvers.
48
Nozzles: These provide the water sprays to wet the fill. Uniform
water distribution at the top of the fill is essential to achieve proper
wetting of the entire fill surface. Nozzles can either be fixed in
place and have round or square spray patterns or can be part of a
rotating assembly as found in some circular cross-section towers.
Fans: Both axial (propeller type) and centrifugal fans are used in
towers. Generally, propeller fans are used in induced draft towers
and both propeller and centrifugal fans are found in forced draft
towers. Depending upon their size, propeller fans can either be
fixed or variable pitch.
A fan having non-automatic adjustable pitch blades permits the
same fan to be used over a wide range of kW with the fan adjusted to
deliver the desired air flow at the lowest power consumption.
Automatic variable pitch blades can vary air flow in response to
changing load conditions.
TOWER MATERIALS
In the early days of cooling tower manufacture, towers were constructed
primarily of wood. Wooden components include the frame, casing,
louvers, fill, and often the cold water basin. If the basin was not of wood,
it likely was of concrete.
Today, tower manufacturers fabricate towers and tower components
from a variety of materials. Often several materials are used to enhance
corrosion resistance, reduce maintenance, and promote reliability and
long service life. Galvanized steel, various grades of stainless steel,
glass fiber, and concrete are widely used in tower construction as well
as aluminium and various types of plastics for some components.
Wooden towers are still available, but they have glass fiber rather than
wood panels (casing) over the wood framework. The inlet air louvers
may be glass fiber, the fill may be plastic, and the cold water basin may
be steel. Larger towers sometimes are made of concrete. Many tower-
casings and basins-are constructed of galvanized steel or, where a
corrosive atmosphere is a problem, stainless steel. Sometimes a
galvanized tower has a stainless steel basin. Glass fiber is also widely
used for cooling tower casings and basins, giving long life and protection
from the harmful effects of many chemicals.
49
COOLING TOWER PERFORMANCE
The important parameters, from the point of determining the performance of cooling towers, are: i) "Range" is the difference between the cooling tower water inlet and outlet temperature. ii) "Approach" is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and approach should be monitored, the 'Approach' is a better indicator of cooling tower performance. iii) Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). iv) Cooling capacity is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference. v) Evaporation loss is the water quantity evaporated for cooling duty and, theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity works out to 1.8 m3. An empirical relation used often is:
50
*Evaporation Loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2) T1-T2 = Temp. difference between inlet and outlet water. *Source: Perry’s Chemical Engineers Handbook (Page: 12-17)
vi) Cycles of concentration (C.O.C) is the ratio of dissolved solids in
circulating water to the dissolved solids in make up water.vii) Blow down
losses depend upon cycles of concentration and the evaporation losses
and is given by relation: Blow Down = Evaporation Loss / (C.O.C. – 1)
viii) Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the
water and the air mass flow rates. Against design values, seasonal
variations require adjustment and tuning of water and air flow rates to
get the best cooling tower effectiveness through measures like water
box loading changes, blade angle adjustments. Thermodynamics also
dictate that the heat removed from the water must be equal to the heat
absorbed by the surrounding air:
L(T1 –T2) = G(h2 – h1) L/G = (h2-h1)/T1-T2 Where :
L/G = liquid to gas mass flow ratio (kg/kg) T1 = hot water temperature (°C) T2 = cold water temperature (°C) h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature (same units as above) h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature (same units as above)
FACTORS AFFECTING COOLING TOWER PERFORMANCE
Capacity: Heat dissipation and circulated flow rate are not sufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow rate m3/hr. For example, a cooling tower sized to cool 4540 m3/hr through a 13.9°C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5°C range. Range °C = Heat Load in kcals/hour / Water Circulation Rate in LPH
Thus, Range is a function of the heat load and the flow circulated through the system.
51
HEAT LOAD: The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature level of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. In some applications (e.g. internal
combustion engines), however, high operating temperatures are desirable. The size and cost of the cooling tower is proportional to the heat load. If heat load calculations are low undersized equipment will be purchased. If the calculated load is high, oversize and more costly, equipment will result. WET BULB TEMPERATURE
Wet bulb temperature is an important factor in performance of evaporative water cooling equipment. It is a controlling factor from the aspect of minimum cold water temperature to which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the air entering the cooling tower determines operating temperature levels throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering air wet bulb temperature, when a heat load is applied. The approach obtained is a function of thermal conditions and tower capability.
The Table 7.1 illustrates the effect of approach on the size and cost of a cooling tower. The towers included were sized to cool 4540 m3/hr through a 16.67°C range at a 26.7°C design wet bulb. The overall width of all towers is 21.65 meters; the overall height, 15.25 meters, and the pump head, 10.6 m approximately. Approach °C 2.77 3.33 3.88 4.44 5.0 5.55 Hot Water °C 46.11 46.66 47.22 47.77 48.3 48.88 Cold Water °C 29.44 30 30.55 31.11 31.66 32.22 No. of Cells 4 4 3 3 3 3 Length of Cells Mts. 10.98 8.54 10.98 9.76 8.54 8.54 Overall Length Mts. 43.9 34.15 32.93 29.27 25.61 25.61 No. of Fans 4 4 3 3 3 3
TABLE 7.1 APPROACH VS. COOLING TOWER SIZE (4540 m3/hr; 16.67°C
Range 26.7°C Wet Bulb; 10.7 m Pump Head)
52
Fan Diameter Mts. 7.32 7.32 7.32 7.32 7.32 6.71 Total Fan kW 270 255 240 202.5 183.8 183.8
Range, flow and heat Load
Range is a direct function of the quantity of water circulated and the heat load. Increasing the range as a result of added heat load does require an increase in the tower size. If the cold water temperature is not changed and the range is increased with higher hot water temperature, the driving force between the wet bulb temperature of the air entering the tower and the hot water temperature is increased, the higher level heat is economical to dissipate. If the hot water temperature is left constant and the range is increased by specifying a lower cold water temperature, the tower size would have to be increased considerably. Not only would the range be increased, but the lower cold water temperature would lower the approach. The resulting change in both range and approach would require a much larger cooling tower.
Fill Media Effects
In a cooling tower, hot water is distributed above fill media which flows down and is cooled due to evaporation with the intermixing air. Air draft is achieved with use of fans. Thus some power is consumed in pumping the water to a height above the fill and also by fans creating the draft. An energy efficient or low power consuming cooling tower is to have efficient designs of fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor. Power savings in a cooling tower, with use of efficient fill design, is directly reflected as savings in fan power consumption and pumping head requirement. Function of Fill media in a Cooling Tower
Heat exchange between air and water is influenced by surface area of heat
exchange, time of heat exchange (interaction) and turbulence in water effecting
thoroughness of intermixing. Fill media in a cooling tower is responsible to
achieve all of above. Splash and Film Fill Media: As the name indicates, splash fill media generates the required
heat exchange area by splashing action of water over fill media and hence breaking into
smaller water droplets. Thus, surface of heat exchange is the surface area of the water
droplets, which is in contact with air
53
Splash fill Film fill Long clog film fill
Possible L/G ratio 1.1 - 1.5 1.5 - 2.0 1.4 – 1.8
Effective heat
exchange area
30 - 45m2/m
3 150 m
2/m
3 85 – 100 m
2/m
3
Fill height required 5 - 10 m 1.2- 1.5 m 1.5- 1.8 m
Pumping head
requirement
9 - 12 m 5 – 8 m 6 – 9 m
Quantity of air
required
High Much low Low
Performance Assessment of Cooling Towers
In operational performance assessment, the typical measurements and observations involved are: • Cooling tower design data and curves to be referred to as the basis. • Intake air WBT and DBT at each cell at ground level using a whirling pyschrometer. • Exhaust air WBT and DBT at each cell using a whirling psychrometer. • CW inlet temperature at risers or top of tower, using accurate mercury in glass or a digital thermometer. • CW outlet temperature at full bottom, using accurate mercury in glass or a digital thermometer. • Process data on heat exchangers, loads on line or power plant control room readings, as relevant. • CW flow measurements, either direct or inferred from pump motor kW and pump head and flow characteristics. • CT fan motor amps, volts, kW and blade angle settings • TDS of cooling water. • Rated cycles of concentration at the site conditions. • Observations on nozzle flows, drift eliminators, condition of fills, splash bars, etc.
TYPICAL COMPARISONS BETWEEN VARIOUS FILL MEDIA
54
PROJECT - HEAT RECOVERY FROM COOLING TOWERS AT HINDALCO
Here, at hindalco we have cooling tower in evaporation unit III with a capacity of 1000TPH.
Also, we have a DM plant (for demineralising water to be used by boilers in boiler & co-
generation plant) with a capacity of 400TPH.
Cooling tower- fluid- water, capacity 1000TPH , inlet temperature= 55.5°C
DM pant- fluid- water, capacity 400TPH, inlet temperature= 26°C
So, in my view we can utilize this water from cooling tower at 55°C to heat DM water at
26°C. As a result DM water will be heated to some extent and it may reduce a significant
amount of coal consumption in boilers.
Cooling tower
Barometric
condenser
1
1
#
1
#
2
#
3
#
4
#
5
#
6
#
7
#
8
#
9
#
1
0
#
1
1
#
1
2
#
1
3
#
1
4
#
1
5
#
1
6
#
1
7
#
1
8
#
1
9
#
2
0
3
DM PLANT
2
4
Boiler house & process
55
FINDING AMOUNT OF HEAT RECOVERY
Temperature Cp density viscosity conductivity Prandtl no.
55.5°C 4.179 985.7 5.13 0.649 3.30
26°C 4.179 995.7 8.6 0.614 5.85
Applying, effectiveness NTU method,
Let us consider the outlet temperature of DM water be 45°C.
Mass flow rate of hot water (from cooling tower) = 1000/3600 x 985.7 kg/sec=273.80kg/s
Mass flow rate of COLD WATER (from DM plant ) = 400/3600 x 995.7 kg/s=110.63kg/s
Let us use 20 shell and tube heat exchangers;
Flow rate MH= 13.69kg/sec per heat exchanger
Flow rate MC= 5.53kg/sec per heat exchanger
Heat capacity rate of hot water= CpH x MH = 4.179 x 13.69= 57.21 KW/°C= CH
Heat capacity rate of cold water= CpC x MC= 4.179 x 5.53= 23.10 KW/°C= CC
Heat capacity rate ratio= Cmin/Cmax = CpC/CpH = 23.10/57.21= 0.40=C
Qmax= Cmin(55.5-26)= 23.10 x 29.5= 681.45 kW
Qactual= CC(45-26)= 438.9 kW
Outlet temp. of hot water= Thin- Qactual/ CH = 47.8°C
Efficiency=ɛ= Qactual/Qmax= 0.644
Assuming counter flow shell and tube 1 shell pass 2,4,6... tube passes,
We get NTU= 1.39
NTU= UA/Cmin
Considering U=1500 for water to water heat exchanger.
1.39= 1500 x A/ 23100
A=21.406 m2
Assuming the diameter of tubes to be 30mm,
56
We get, Length of tube = 227 m
Let, the no. of tubes be 30.
Length/tube= 227/30 = 7.566 m
Now, let us calculate the head loss in tubes-
Head loss per tube can be calculated,
Velocity of liquid per tube= flow rate per tube / cross section area per tube
= 1.85 x 10-4
m3/sec / 7.065 x 10
-4m
2 = 0.261m/s
Now, considering the pipe to be made of galvanized iron of roughness=ɛ= 0.15mm
Determining the friction factor considering completely turbulent flow,
f = [1.14 + 2 log10(D/ɛ)]-2
We get, f= 0.083
Thus, head loss due to friction per tube =
we get head loss per tube = 7.27 x 10-2
m
Thus, head loss per heat exchanger along tubes = 30 x 0.0727=2.181m
Along 20 heat exchangers tube side loss = 20 x 2.181 = 43.62m
Taking allowance of 30% we take head loss of 56.706 m along tubes of heat exchanger.
Calculating shell side pressure drop,
Inlet temp = 50°C, outlet temp.=48.7°C , tubeside flow rate= 19910 kg/hr, shell dia.= 22 inch
no. of baffles= 32 , baffle spacing= 6 inches , tube dia= 30mm , no. of tubes= 30 ,
pitch=1.25inches,triangular pitch.
Effective area of cross flow across tubes between baffles is calculated using following
equation, Ae= Ds x Bs x (P- Dt/P)
P=pitch, Dt= tube dia. , Ds= shell dia. , Bs= baffle spacing
For our case, Ae= 0.004693 m2
Velocity of cross flow then becomes, v= mass flow rate/
Thus we get v= 1.183 m/s
57
Next, the effective dia. Of fluid path is determined using following approximation-
De= 4 x {P2- )/
We get De= 0.01280m
And the factor fk is calculated as a function of Reynolds no.
-1.9
We get fk = 1.546 x 10-7
The shell side pressure drop is finally calculated using the following equation,
Where N= no. of baffles , we get ∆P= 0.15284 bar
Thus, for 20 heat exchangers ∆Ptotal= 0.15284 x 20 = 0.30 bar= 300000 Pa.
Or head loss= HL= 31.05 m
Taking allowance of 30% net shell side head loss= 40.3m
Calculating the head loss along pipe length,
Consider length=40m (approximate distance between barometric condenser to DM plant)
Also, taking 5 regular flanged elbow at 90°.
Flow rate of water from cooling tower= 1000m3/hr= 0.2778m
3/sec
Taking critical velocity of water to be 1.8m/s. The required cross section of pipes is given by,
We get diameter of pipe= 0.443m.
From darcy-weighbach equation,
Hl
Considering the pipe to be made of galvanized iron of relative roughness=ɛ= 0.15mm
Now, determining the friction factor considering flow to be completely turbulent,
-2
58
We get f= 0.015314
Now, h= f x L/D x V2/2g
We get head loss= 0.17325m
Minor loss= KL x V2/2g , KL= 0.3
We get minor losses = 0.25 m
Total loss= major loss + minor loss = 0.42325m
Taking allowance of 0.5m, total loss= 0.92325m
Now, as we have divided the flow equally into 20 separate heat exchangers in parallel mode.
Thus, new flow rate per pipe= 1000/20 m3=50m
3/hr
Considering the pipe of length 30m with 25 elbows at 90°, we calculate the head loss.
Assuming the critical velocity to be 1.8m/s. We calculate the pipe diameter to be 0.099m.
d=99mm; f= [1.14+2 log10(d/ɛ)], we get f= 0.02175
head loss= HL= f x L/D x V2/2g ; we get HL= 1.0900m
Minor loss hL= KL x v2/2g ; we get hL= 1.239m.
Total loss= 1.09m + 1.239m = 2.3297m ,
Thus, total head loss = 2.3297m + 0.92325m= 3.25295m.
Calculating the HP(Horse power) of pump required for this system
HP of pump is given by the formula,
Now calculating the HP of pump #1 & #3,
HP for pump #1, flow rate= 0.2778m3/s, density= 985.7 kg/m
3, g= 9.8m/s
2,
Head required = shell side pressure drop + major loss + minor loss + discharge head at heat
exchanger outlet + other losses = 40.3m + 2.33m + 4m + 15m= 62.553m.
Thus, horse power of pump #1 = 0.2778 x 985.7 x 9.8 x 62.55/746 = 225 HP.
Assuming the efficiency of pump to be 65%, actual HP of pump #1= 304 HP.
HP of pump #3, flow rate = 0.2778m3/s, density=985.7 kg/m
3, g=9.8m/s
2,
Suppose, head required by cooling tower including looses = 20m.
59
Thus, HP of pump #3 = 0.2778 x 985.87 x 9.8 x 20 / 746 = 72 HP.
Assuming the efficiency of pump to be 65%, actual HP of pump #3= 98 HP.
Now, calculating HP of pump #2 & #4,
For, pump #2, flow rate = 400/3600m3/s= 0.112m
3/s, density= 995.7kg/m
3, g=9.8m/s
2
Head required = tube side pressure drop in heat exchangers + major losses + minor losses +
required discharge head from heat exchangers+ other losses
Assuming, major + minor losses to be 4m, other losses= 15m.
Given, total tube side head loss= 66.706m
Thus, head required = 4m + 56.706m + 15m = 66.706m.
Thus HP of pump #2 = 0.112 x 995.7 x 9.8 x 66.706 / 746 = 97.25HP
Assuming the efficiency of pump to be 65%, actual HP for pump #2= 132 HP.
For, pump #4, flow rate = 0.112m3/s, density= 995.7 kg/m
3, g= 9.8m/s
2
Let the required head to boiler house and process(including losses) be 20m.
Thus HP #4= 0.112 x 995.7 x 9.8 x20 / 746 =29.28 HP
Taking efficiency of pump to be 30 %, actual HP of pump #4 = 42 HP.
Total HP of all pumps = 576 HP.
Again assuming 40% allowance HPtotal= 806HP.
Thus, amount of heat recovered from heat exchangers= 8784.1326KW.
And power consumed by all pumps= 806 x 746 /1000 = 601.27KW.
But, in actual practice it is very difficult to install such large heat exchangers, but as we can
see if we will be able to set up such a system we can reduce a significant amount of coal
consumption in boilers and also reducing our CO2 emissions. Thus, making a big change for
industry in particular and environment as a whole.
Amount of coal saved , calorific value of coal = 2600kcal/kg
Amount of coal saved per second = 0.8kg.
Amount of coal saved per day = 69120kg
60
Cost effectiveness & optimum Production
Every plant’s goal is higher profitability, controlled production costs, competitiveness, higher
asset utilization, better productivity and lower rejections.
The various raw materials consumed in the plant are:
RAW MATERIAL UNIT TYPICAL SP. CONSUMPTION
BAUXITE T/T 2.756
CAUSTIC T/T 0.124
LIME T/T 0.063
HRD FLOCCULANT T/T 0.000153
WASHER FLOCCULANT T/T 0.000245
STARCH/ SETFAST T/T 0.000400
FUEL OIL L/T 77.5
FILTER CLOTH MET/T 0.0046
CGM/NALCO-85700 KG/T 0.165
STEAM T/T 3.05
POWER KWH/T 413
ALUMINA COST DRIVERS
OPTIMIZING STEAM UTILIZATION
Steam is one of the raw materials for the alumina plant. It is used in the digestion unit of the
bayers process. The steam used in the alumina plant is generated in the boilers. Process
equipments actually consume much more steam than the theoretical load on the equipment.
This excess steam consumption (generally) in equipments occurs due to the following:
42%
29%
17%
9%
3%
ALUMINA COST DRIVERS
ENERGY(27%) CAUSTIC(19%)
STORES,MAINT,OVERHEAD(11%) WAGES(6%)
RAW MATERIALS(2%)
61
1. Improper trap selection and by-pass kept open
2. Supply steam pressure variations
3. Temperature overshoot
4. Manual provision for air-venting
5. Manual control for steam flow
Steam Trapping
Improper steam trapping is one of the key factors resulting in excess sream consumption in
equipments. As steam loses heat, it turns back into water. Steam inevitably begins to do this
as soon as it leaves the boiler. The water formed, known as condensate, does not transmit
heat effectively and hence needs to be removed from the steam system.
A steam trap is used to release condensate from the equipment preventing condensate logging
in heating equipment while simultaneously preventing steam from escaping from the system.
This results higher heat transfer rates and thus less steam consumption, resulting in fuel
savings.
Correct selection of steam trap results in lower steam consumption in the equipment through
more effective condensate evacuation.
Condensate and Flash recovery
Condensate contains almost 20% of the energy supplied to the boiler. When steam condenses
by giving away its latent heat, the resulting condensate is still at the same temperature as
steam. This heat, termed sensible heat, can be recovered. Thus recovering condensate is
absolutely essential and the plant can significantly reduce its fuel consumption by completely
and effectively recovering and utilizing condensate.
Energy consumption in bayer process
The processes to produce alumina from bauxite are energy consumption intensive processes.
There is about 20-50% of operation cost for the energy expenses in alumina production
depending on various factors such as bauxite types and grade, process flow and its parameters
and enegy efficiency of the equipment applied in the refineries etc. It is essentially important
to cut down the energy consumption for operation cost saving in alumina production,
especially for the highly energy consumption processes.
The actual energy consumption for alumina production includes not only the reaction heat in
digestion, precipitation and hydrate calcinations but also the heat consumed in physical
processes during all stages in the whole bayer cycle and calcinations. e.g. preheating for
temperature raising, evaporation for concentration increase and heat loss of vessels etc.
Optimization of concentration system
The caustic concentration of pregnant liquor can be properly increased by the following
means:
62
Application of new, high efficient solid- liquid separation processes, additives and
equipment for red mud settling and washing to enhance the separation efficiency and
to reduce the amount of water.
Application of high temperature settling processes for red mud separation from high
concentration liquor.
Spent liquor caustic concentration for digestion can be properly reduced as follows:
Increasing digestion temperatures.
Longer digestion time.
Increasing of pregnant liquor concentration and reducing spent liquor concentration are
effective approaches for optimization of the liquor concentration system and will reach in
energy consumption reduction by decreasing water evaporation.
Increase of precipitation efficiency and productivity
The maximum precipitation productivity could be obtained by delicate optimization of
precipitation parameters according to the existing precipitation and equipment conditions as
follows:
Reducing molar ratio of pregnant liquor
Controlling precipitation temperature system and properly reducing temperature in the
final precipitator
Relatively increasing seed addition and application of activated seed
Prolonging precipitation type
CGM addition
Utilization of energy saving equipment and control system
Application of high efficient and energy saving equipment and automation system in Bayer
process is essential to reducing energy consumption since most of the energy such as steam,
electricity and oil etc is consumed for the equipments.
Energy efficiency of the equipments including the specific energy consumption and electrical
efficiency of the equipments etc plays an important role for energy and electricity
consumption of the equipments.
Energy efficiency of the equipments including the specific energy consumption and electrical
efficiency of the equipments etc plays an important role for energy and electricity
consumption of the equipments.
The energy utilization efficiency in the highly energy consuming equipments such as
calciners, evaporators and preheaters etc. is closely related to total energy consumption in
bayer process.
The solutions for the equipments to save energy are as follows:
63
Developing and application of energy saving pumps, agitators and fans etc. in
optimized and energy saving operation status to reduce electricity consumption
Heat can be saved by using high efficient fludized calciners with high fuel
combustion and heat efficiency, falling film evaporators are heat exchangers with
excellent heat transfer coefficient.
The online inspection and automatic control are the important measures to keep
steady operation of process, optimize process technological parameters and improve
process productivity as well, especially for accuracy of preparation of bauxite slurry
composition and just in time adjustment of some important parameters in the process.
Better energy recovery and less heat loss
Heat recovery efficiency and heat loss impacted by the heat utilization during calcination,
evaporation and temperature changes of bauxite slurry or liquor etc are the great factors to
influence energy consumption.
The measures for energy saving are proposed as follows:
Developing the processes or equipments for waste heat recovery and reducing heat
loss, such as setting up more efficient flash tanks and heat exchangers for better heat
recovery and reducing scale on heat transfer surfaces to raise heat exchanger
efficiency.
Applying new kinds of materials with low cost and high performance including
various lining materials, wearable materials, seal materials, heat transfer materials and
insulating materials etc. to increase operation cycle time and yield, to raise heat
efficiency and to reduce heat dissipating in operaon.
Energy saving technology by combination of heat and electricity generation with
alumina production.
64
BIBLIOGRAPHY
Kern process heat transfer
Heat_Transfer__Yunus_A._Cengel__2nd_Edition
Introduction of Heat Recovery Chiller Control and Water
System Design Jing JiaMarket Development ManagerTrane
Air Conditioning
Data Center Heat Recovery in intel company using chillers.
HEAT EXCHANGER NETWORKS Submitted
by
Michelle Villasin
Keith Obenza
Mary Lanuza
Toan Nguyen
65
CONCLUSIONS
The above study explains the process flow of the alumina plant in HINDALCO
Renukoot. It covers all the various units and sub-units of the plant. It explains
the various stages in Bayer Process: Digestion, Clarification, Precipitation,
Calcination and Evaporation. Bauxite is crushed and liquor is added to form
slurry. It is heated to 248°C and then cooled in flash tanks. Sand and red mud is
separated from pregnant liquor. Super saturated solution of alumina in caustic
soda is allowed to crystallize. The saturated solution of Alumina in caustic
decomposes in to Alumina tri-hydrate (ATH) and caustic soda. Anhydrous
alumina is then produced by removing water of crystallization at high
temperature.
The report also focuses on the enhancement of production at lowest cost. Also
we discussed the concept of heat recovery from cooling towers by employing
heat exchangers to transfer this heat to other fluids in the process plant. It shows
the success of Hindalco over the years in the field of cost effectiveness and also
focuses on measures to further make the processes more cost effective. The
measures suggested are steam trapping, condensate and flash recovery,
optimization of concentration systems, increase of precipitation efficiency and
productivity, utilization of energy saving equipment and control system, better
energy recovery and less heat loss.
It has been a learning exercise to complete this training. Working on this project
entitled ‘HEAT RECOVERY FROM COOLING TOWERS’ has been an
enjoyable and fruitful experience to us. I certainly found a more practical
student in myself after 2 months of long interaction with this organization.