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.