DECLARATION
I hereby declare that this thesis work entitled "A study report on COAL CHEMICALS "is my work, carried out under the guidance of my faculty guide Miss CH.NAGAMANI and company guide Sri. G.VENKATA RAO, asst
general
Manager (O) of Visakhapatnam Steel Plant this report neither
full nor in part has ever been submitted for award of any other degree of either this university or any other university.
PLACE: Visakhapatnam
DATE:
GSANJEEV SK.MOULALI ROHINI KUMAR.O
ACKNOWLEDGEMENT: This project is a result of the hard work & sincere effort put by many hands. We would like to thank SRI G.VENKATA RAO, asst general Manager (O), Visakhapatnam Steel Plant, for his valuable guidance and timely advice during the study period for the successful completion of the Project work. We wish to render my sincere gratitude to the staff & Officers of Personnel
section,
Plant
Visakhapatnam Steel Plant for their
support in providing all relevant data, help and guidance in completing this project. Finally we thank the management of Visakhapatnam
Steel Plant for permitting me to take up this
Project.
Place: Visakhapatnam Date:
G.SANJEEV SK.MOULALI ROHINI KUMAR
INDEX
Introduction of “VSP” Coke Ovens &Coal Chemical Plant Coal Preparation Plant Coke Oven Batteries Coal Chemical Plant Primary Gas Coolers Ammonium Sulphate Plant Final Gas Coolers M.B.C Tar Distillation P.C.L.A Naphthalene Fraction Crystallization Benzol Recovery Hydro Refining Extractive Distillation Unit
INTRODUCTION The role of ferrous metals in general and of steel in particular in national economy is enormous. One cannot name an economic branch where ferrous metals find no applications. The economic power of country is determined by its output of steel, since it determines the progress in the principle economic branches, be it mining, transport, manufacture engineering or agriculture implements is unthinkable without steel. An additional impetus for increasing the scope of steel manufacturers had been the vigorous progress in chemical engineering. It has turned out that steel can be very profitable combined with certain novel materials for instance plastic combined with stainless steel are excellent materials fore making furniture, decorating automobiles internal lining of houses and building purposes. As a result the manufacture of stainless steel has been appreciably increased in order to cover these new demands in recent years. The world for the steel rises continuously and is expected to reach the level of thousand million tons per year by the end of this century. The steel will obviously remains the principle structural material in for seeable. To meet the above requirements the following iron and steel companies were established: Tata iron and steel company is the first ever-integrated steel plant in India in 1908 at Jamshedpur. TISCO in Bihar IISCO in Burnapur Bhadravathi steel in Karnataka Hindustan steel plant at Bhilai, Roerkela and Durgapur Visakhapatnam steel plant at Visakhapatnam
VISAKHAPATNAM STEEL PLANT In order to increase the steel production reasonably high in the nation and remove the regional imbalances in industrial developments, the government of India took a great step in setting up the coastal-based steel plant of India is Visakhapatnam steel plant in Andhra Pradesh. This plant is located 16km south west of the city limits. A great emphasis has been made on total automation, seamless integration and efficient up gradiation at Visakhapatnam steel plant.This has resulted in a great demand for Visakhapatnam steel plant product in India and abroad which are having international standards. Visakhapatnam steel plant is considered to be the first integrated steel plant in India to become fully ISO-9002 certified company. This certificate covers quality systems of training and marketing functions over four regional marketing functions and 22 stock yards located allover the country. The decision of the government of India to set up an integrated steel plant at Visakhapatnam was announced by the Prime Minister Smt.Indira Gandhi. The plant was inaugurated formally on 20th January 1971 by the prime minister. The project was estimated to the cost of rupees 3,897.28 crores based on process on 4th quarter of 1981 but during the implementation of VSP is has been on served that the project cost as increased substantially over the sanctioned coast mainly due to this and the approved concept were studied in 1986 the rationalization has basically been from the point of view of obtaining maximum output from the equipment already installed paneled for procurement, achieving the higher level of operation efficiency and procurement over what was envisaged earlier under the rationalized concept. 3.1 million tons of liquid steel is to be produced in a year and the project is estimated to cost 5,822 crores based on 4th quarter of 1987. VSP‟S FRUITFUL ACHIVEMENTS: It has crossed many milestones in the fields of production, productivity and exports. Coke rate at an order of 543kg/tonn hot metal Average converter life of 649 heats. An average of 11.5 heats per sequence in continuous bloom caster.
Specific energy consumption of 7.51 Kcal/ton of liquid steel. Specific refractory consumption of 15.2kg. A labor productivity of 192-ton/man yr.
COKE OVENS & COAL CHEMICAL PLANT ORIGIN OF COAL: Coal originated from the arrested decay of the remains of trees, bushes, mosses, vines and other forms of plant life, which flourished in huge swamps and bogs millions of years ago, during prolonged periods of humid, tropical climate and abundant rainfall. Streams into the swamps and lake basins to form the coal beds carried an enormous amount of vegetations. Owing to pressure, the streams have generally been crushed to an elliptical section and formed coal. USE OF COAL IN VSP: Coal is used in the form of coke to serve the purpose of iron ore reduction in blast furnace. It also serves as a heat source. TYPES OF COAL: There are 2 types of coal: (1) Coking Coal. (2) Non-Coking Coal. The different coking coals used in VSP are: 1) M.C.C Medium coking coal 2) I.C.C 3) I.S.S.A.C 4) SOFT -
Imported coking coal Imported coking coal Imported coking coal
-
BENGAL, BIHAR
-
AUSTRALIA AUSTRALIA AUSTRALIA
In VSP coking coal is used for producing metallurgical coke where as noncoking coal is used for producing thermal power (in boilers). TYPES OF COAL AND PROPERTIES: S.NO.
TYPE OF COAL
%
% ASH
MEAN
MOISTURE 1. 2. 3. 4. COKE:
M.C.C I.C.C I.S.S.A.C SOFT
25-28 24-26 23-25 30-34
17-22 8-10 8-10 8-10
MAXIMUM REFLUTANCE 0.9 1.10-1.3 1.16-1.3 0.9-1.0
It is a strong porous hard mass that is obtained by heating of the coal in the absence of air at high temperature. It is a reactive fuel and satisfies the need for blast furnace. FUNCTIONS OF COKE: 1. It acts as heat producer in blast furnace 2. It acts as reducing agent by carbon reduction in blast furnace with oxygen reaction. 3. It gives a permeable bed and also as a slag carrier. CARBONIZATION OF COAL: Heating of coal in the absence of air at high temperatures to produce residue coke, coke oven gas is called “CARBONISATION OF COAL” or “DESTRUCTIVE DISTILLATION”. Its main purpose is to produce coke and the by-product known as coke oven gas from which various products are obtained and this is used as fuel of high calorific value. NEED FOR MANUFACTURE OF COKE FROM COAL: 1. Natural coal is too dense and fragile to be used as a fuel in the furnace. 2. Coal is not strong enough to withstand nearly 25 mts of burden lying on it inside the furnace. 3. Coal is nearly “VOLATILE MATTER FREE” so it does not create problems of hot shortness and coal shortness. 4. As compared to coal coke is of high quality and is highly reactive. 5. Coke is highly porous mass and it equalizes the blast coming from the bottom of the charge. 6. As coke is a rigid hard mass it does not create the problems of dust nuscence. 7. The ASH CONTENT in coke is very low i.e.) around 10%. So it does not arise problems of striking on the grates.
The coke oven and coal chemical plant is mainly divided into the following department: 1. Coal Preparation Plant (C.P.P) 2. Coke Oven Batteries 3. Coal Chemical Plant (C.C.P)
COAL PREPARATION PLANT PURPOSE OF COAL PREPARATION PLANT: The main purpose of this plant is to prepare the coal by removing the foreign matter and bringing to the size suitable for carbonization or coking process. COAL BLEND COMPOSITION: 1. 2. 3. 4.
M.C.C I.C.C I.S.S.A.C.C SOFT
-
15% 40% 35% 10%
PROPERTIES:
VOLATILE MATTER: It is the matter which is unstable at high temperature and converts itself into gaseous state like tar, benzene compounds etc. it is around 20-25%. MEAN MAXIMUM REFLUCTANCE: It is a coal ranking. It decides the properties of coal. It is around 1.12-1.14. CRUSING INDEX: % of -3mm size particles which should be around 70% - 75% FIXED CARBON: The carbon left behind after the removal of the volatile matter is fixed carbon. ASH CONTENT: The % of ash present in coal. It can known by testing in the laboratory. It is around 17-22 in Indian coking coal & 8-10 in imported coking coal. TOTAL MOISTURE: The amount of total moisture present in coal. There lies some internal moisture also.
CONTENTS IN 1 TON OF COAL: 1. 2. 3. 4. 5. 6.
Coke Crude Tar Crude Benzol Ammonia CO Gas Losses
-
COAL PREPARATION SECTIONS:
746 kgs 32 kgs 6.9 kgs 4.1 kgs 150 kgs 61 kgs PLANT
CONSISTS
OF
FOLLOWING
Foreign material separation section Selective crushing sections. FOREIGN MATERIAL SEPERATION SECTION: The coking coal is taken from coal yard by using bucket elevators on the conveyors. Then it is sent to foreign material separation section. In foreign material separation section the following equipment exists: 1. 2 Cylindrical screens. 2. 2 Suspended iron separators. 3. 2 Self unloading suspended iron separators for separating the magnetic particles in the coal. PROCESS: First the suspended iron separators separate the iron particles in the coal. After it is demagnetized, the separated coal falls on self-unloading suspended separators. it works for 19 seconds. After that coal is separated into +150 mm & -150 mm sized particles in rotary screens. +150 mm sized particles fall in the chute and are sent back for crushing. Where as -150 mm particles are transferred to reversible conveyors. There are further connected
to shuttle conveyors for filling the bins. There are a total of 16 bins, equally divided into 2 rows. All the odds like 1, 3, 5, 7, 9, 11, 13, 15 & the evens 2, 4, 6, 8, 10, 12, 14, 16. The capacity of each bin being 800 T and a height of 20 mts. Specifications of cylindrical screen: Inclined angle Feed end length Discharge end length Mesh size -
8° 1.5 m 0.8 m 150 mm
1. SELECTIVE CRUSHING SECTION: Blended coal is send to selective crushing section for crushing of coal from -150 mm to -3mm size. In selective crushing section there are 3 primary crushers and 2 secondary crushers for crushing the coal. First coal is crushed in primary crushers. Then crushers are connected from 6.6 KV motors via variable speed coupling and gear box. Speed of motor is controlled from 1000 rpm to 400 rpm by variable speed coupling in order to avoid the breakage of crushers and sudden stoppage of crushers. In primary crushers there are 88 hammers in 12 rows of universal crushing. Total no. of crushers Primary crushers Secondary crushers Cap. of primary crusher Cap. of secondary crusher No. of hammers Wt of each hammer Wt of arm Length of arm Material of construction Rotations of hammer
-
5 3 2 400 T/hr 600 T/hr 88(12 rows) 19.7 kgs 6.0 kgs 45.0 kgs Nickel Chromium Steel 400 rpm
COKE OVEN BATTERIES
Coal is converted into coke by high temperature carbonization in the ovens of the battery. There are 3 batteries working and the Battery – 4 is being constructed for more production of hot metal. RAW MATERIAL PRODUCT BY-PRODUCT
-
Coal blend. Coke with 1-3% volatile matter. Raw coke oven gas.
CONSTRUCTION: The total length of the battery is 100 mts. It consists of rectangular chambers of length 16 mts, 7 mts high and 0.14 mts width with removal door ends. Coke oven battery is a combination of ovens and heating chambers in alternatively. There are 67 ovens and 68 heating chambers in a battery. These ovens are OTTO-HOFFOMEN‟S by-product oven type. On the roof there are metal lids for the sake of charging purpose. The width of the coke discharge side is slightly more than the pusher side for easy transport of coke outside. Heating flues are arranged in the walls between the ovens. There are regenerators underneath the battery. The oven walls are lined with silica bricks of high thermal conductivity. There is no shrinkage in the refractory walls. These bricks ensure long life. All doors are sealed with refractory clay and water mixture. Battery has 4 machines: 1) Pusher car 2) Charging car 3) Door extractor car 4) Loco car PROCESS: Coal is transported from coal preparation plant to coal tower above the battery. a charging car travels on the battery and under the coal tower. 32 T of coal is charged in each oven up to a height of 6.7 mts out of 7 mts by charging position in each oven. Since the coal drops by gravity, it should be leveled which is done by a leveling bar fixed to the pusher car. After leveling all lids and doors are replaced and coking process is continued until most of the volatile matter is removed. This process takes a coking period of 17-18 hrs by indirect heating with coke oven gas or mixed gas (CO gas + BF gas) in absence of air at 1150°c. After complete carbonization, coke oven gas is collected by hydraulic main that is connected to the ovens, which are sent to recovery plant for
cleaning of gas. Later it is sent to recovery plant for cleaning of gas. After cleaning CO gas is recycled back to the battery for heating purpose. The product is 1-3% moisture contained coke. Yield of coke by HIGH TEMPERATURE CARBONIZATION at 900°c - 1200°c is 65-75%. Calori3fi3c value of di3fferent gasses used for heating purpose: CV of Coke oven gas 4200 K Cal /Nm3 CV of Blast furnace gas 750 K Cal /Nm CV of Mixed gas 1000 K Cal /Nm3 The walls start after 5 m level. Until 5 m level it is called the regenerator. The regenerators serve the purpose of recovering heat by heat exchange between hot flue gasses and bricks. The air coming in. thus air gets preheated in the regeneration section takes this heat. In the first cycle fuel gas flows through one set of flues and flue gas goes out through other set of flues. There are 136 waste heat boxes through which air is supplied form one section and flue gas leaves through other section. As hot air is of low density and by the density difference suction is created by which flue gas rises up out of the chimney. Checker bricks made of silica are placed in the regeneration section so that the contact area and time for regeneration increases and maximum heat is recovered. SPECIFICATION: Total no of Batteries No of ovens in each Battery Heating walls Height of each oven Length of each oven Width of each oven(P/S,C/S) Heating wall thickness Coal charging in each oven Output coke from oven No of vertical flues No of waste heat boxes Heating gas used in Battery 1&2 Heating gas used in Battery 3 ratios. Coking period Coke temperature Coke fluidity temperature
-
4 67 68 7 mts 16 mts 38.5 / 43.5 mm 0.105 mts 34 ton‟s 25 ton‟s 32 136 CO gas CO + BF gas in 1:9
-
17-18 hrs 1050°c 400-500°c
Capacity of battery Coking Period Determination (C.P.D)
C.P.D
=
No of ovens x 24 ------------------------- =
97 pushing/Battery/day
67 X 24 ------------ =
1608
hrs No of pushing‟s per day
100
COKE DRY COOLING PLANT After pushing the coke from the Battery with a temperature of 1050°c, it is transferred into C.D.C.P for cooling purpose. The C.D.C.P technology is adopted only in VISAKHAPANAM STEEL PLANT only because in wet cooling the strength of the coke is reduced. The advantages of DRY COOLING are: 1) Waste heat recovery (by producing steam) 2) Pollution control (done by closed circuit) 3) Better Coke strength (there is no thermal shock as in wet cooling) PROCESS: Coke form Battery falls into loco on the coke side, which is brought into C.D.C.P. The loco is lifted up and coke is charged into the cooling chambers. Later the chambers are closed by lid and N2 gas is passed inside through a temperature of 55°c by a mill fan. The hot coke is cooled from 1050°c to 180°c-200°c. There occurs no chemical reaction, as N2 is an inert gas. The heated gas is utilized for producing steam in boilers. These boilers are water tube boilers, which are of capacity 25 T/hr. the temperature and pressure of the steam, are 440°c and 40 Kg/cm2. Cooled coke i3s convey6ed to C.S.P and the steam is used to run back pressure turbine station for producing power of 15 MW. SPECIFICATIONS OF C.D.C.P:
No of C.D.C.P No of chambers Capacity of each C.D.C.P Coke temp before cooling
-
4/4 Batteries 4 Chambers/each C.D.C.P 50-52 T/hr 1050°c
Coke temp after cooling Coolant used Dry coolant temperature
-
180°c-200°c N2 gas 55°c
COKE SORTING PLANT
Coke from C.D.C.P enters the C.S.P section. Here coke is crushed, screened and later conveyed to different consumers.
PROCESS:
In C.S.P dust will be removed from the coke with the help of dedusting fans. After that coke is sent to crushing section to crush into 80mm size particles. There B.F coke (25-80mm) is separated through 14- roll screen and sent to Blast Furnace. The remaining 0-25 mm fractions will be sent to vibrating screen. Here 0-10 mm particles called BREEZE COKE is conveyed to Sinter Plant and 10-25 mm called NUT COKE to Blast Furnace.
COAL CHEMICAL PLANT Many by products are extracted from the coke oven gas at this department. It consists of the following sections:
Exhauster house.
Ammonium sulphate plant.
M.B.C plant.
Tar distillation plant.
P.C.L.A
Naphthalene fraction crystallization.
Benzol plant. Benzol distillation plant. Hydro refining. Extractive distillation.
PRIMARY GAS COOLER The coke oven gas from the separator is fed to the PGC from the top. The cooler consists of three zones. This is a shell and tube heat exchanger in which the CO gas exchanges with service water in the top two zones and with chilled water in the bottom zone the tubes are inclined in all the zones. The inclination is maintained at 15°c with the horizontal. This provides any naphthalene condensate to drain easily. The main purpose of PGC is to cool the gas from 90°c to 30°c. During this cooling process the naphthalene and traces of tar present in the gas condense and this is collected at the bottom of the PGC. The liquid collected at the bottom is sent to the seal pot by gravity. The level in the seal pot should be maintained constant as this acts as a seal to the gas in the cooler. Tar at a temperature of 90°c is flushed from the top of the cooler to remove the condensed naphthalene on the tubes. The tar is then collected at the bottom in the seal pot. Tar and naphthalene from the seal pot is fed to the storage tank of the CPH. The CO gas from the bottom of the PGC is fed to the electrostatic precipitator. Gas temperature before PGC Gas temperature after PGC Service water inlet temperature Service water outlet temperature Gas condensate flow to each PGC Chilled water inlet temperature Chilled water outlet temperature Total number of tubes ELECTROSTATIC PRECIPITATOR:
80-95°c 27-30°c 32-33°c 43°c 10-15 Nm3/hr 11-12°c 20°c 409
The CO gas enters the ESP from the bottom. Electrostatic precipitators are cylindrical vessels with a conical bottom. Each ESP is provided with a seal pot. A round disk having electrodes in suspended state is present inside the precipitator. Electrodes are nothing but SS metal rods. Three are present at the top of precipitator, which supplies the necessary power to each ESP. high voltage of about 50,000 volts is supplied to each electrode. Due to high voltage the fine and foggy tar get sticked to the walls of the electrodes and they fall down due to gravity. The liquid thus collected at the bottom is fed to the seal pot. Each electrode is covered to prevent the connection between any two electrodes. Capacity Voltage Number of electrodes ESP insulator boxes temperature
30,000 Nm3/hr 70KV 148 80°C
EXHAUSTERS: These are centrifugal fans necessary to drive the gas from the batteries itself to various plants like ASP. Benzene recovery and finally to the main header of the coke oven gas. Exhauster sucks the CO gas from the batteries. The pressure at the suction side of the exhauster is -350mm WC and the discharge side is +2500mm WC. Due to increase in the pressure the temperature is increased to 55-60°c. The flow of gas is controlled by Askania valve, which is a butterfly valve. The function of this valve is to control the flow of gas to the exhauster. When the quantity of gas is low then the valve is closed and when the gas quantity is high valve is opened. If any condensate is collected and when the gas lines and sent to the storage tank of the condensate pump house. Capacity Power Maximum suction at inlet Maximum pressure at delivery
CONDENSATE PUMP HOUSE:
67,000-76,000Nm3/hr 1250KW 500mm WC g 2700mm WC g
Flushing liquor separated from the separators is fed to the condensate pump house. This consists of decanters and storage tanks. The tar and flushing liquor collected from the exhauster house is stored in the storage tanks.
DECANTERS:
Decanters are used to settle the flushing liquor by gravity. These are mechanical decanters provided with scraper mechanism. This scraper is a chain like arrangement provided at the bottom. The flushing liquor is fed at the middle of the decanter. In decanters due to density difference, flushing liquor, tar & sludge form as three layers from top, and the flushing liquor is then sent to the storage tanks.
A telescopic valve provided to the decanter removes the tar settled in the middle layer. The valve works on the principle of „U‟ tube manometer. Sludge removed by scraper mechanism provided at the bottom of the decanter. The scraper then dumps the sludge into a bunker provided at the end of the scraper. A motor provided with a gearbox runs the scraper. The sludge from the bunker is removed periodically.
Tar collected from the flushing liquor decanter is stored in the tar storage tanks. Then it is pumped to tar decanters. The tar from the tar decanters contains less moisture and sludge. Flushing liquor from the decanter is stored in the storage tank and again pumped to batteries. If the flushing liquor is excess then 60% is sent to excess flushing liquor tanks.
AMMONIUM SULPHATE PLANT Coke oven gas with a pressure of 2500mm WC from exhauster is fed to the ASP. Ammonia present in the CO gas is recovered in ASP as ammonium sulphate fertilizer. PROCESS: The CO gas from the exhauster is fed to the pre-heater to preheat the gas to 60-70°c. But according to our atmospheric temperature, this temperature is obtained after the exhauster. So the gas is directly fed to the saturator. SATURATOR: Saturator is a cylindrical vessel with conical bottom. It is provided with a bubbler hood, which is duct prolonged to the middle of the saturator. The duct has a hood at the bottom provided with vanes like arrangement. Another ring like structure with small openings is provided at the conical portion, which is used for nitrogen feeding. Hot water rings are provided at the top of the saturator. Saturator is always maintained with acid bath called mother liquor, which contains 4-5% of sulphuric acid. The CO gas enters through the bubbler hood which is dipped in the bath. The gas rises through the mother liquor. During this period, the ammonia present in the gas reacts with the sulphuric acid in the liquor.
NH3
+
H2SO4
NH4 (HSO4)
+
NH3
NH4 (HSO4) (NH4)2SO4
Ammonium sulphate thus formed settles at the bottom of the saturator. Pure nitrogen is purged into the saturator through N2 rings at 4-5 kg/cm2. N2 purging increases the crystal growth. Pure sulphuric acid (98%) is fed to the saturator to maintain the acidity in the saturator. The gas collected at the top of the saturator is fed to the acid trap. As the gas rises up, some of the crystals may be carried with the gas and they get stacked to the walls of the saturator at the top. Then the hot water is sprayed to the rings provided. The crystals attached to the walls of the saturator are washed away. When hot water is sprayed the concentration of the liquor decreases. So inlet acid concentration increases to 6-7% at that period. After the reaction mother liquor is continuously drawn to the circulating tank provided at the side of the saturator. This acts as a seal for the saturator. From the circulating tank, mother liquor is fed to the mother liquor tank. The crystals collected at the bottom are fed to the crystal receiver tank by using pump. ACID TRAP: The outlet gas of the saturator carries some acid mist. In order to remove the acid mist, the gas is sent to the acid trap. It is a hollow cylindrical vessel. The coke oven gas from saturator enters tangentially to the trap. Due to the centrifugal motion, the acid mist gets separated. The acid collected a the bottom is fed to the circulating tank. The CO gas is fed to the Benzol recovery. CRYSTAL RECIEVER TANK: Ammonium slurry from the bottom of the saturator is pumped to the crystal receiver tank with conical bottom. The ammonium sulphate crystal settled at the conical portion of the tank, which is wet with liquor. The mother liquor from the top of the receiver is fed to the saturator. The slurry from the bottom is fed to the centrifuge. CENTRIFUGE: Centrifuge is a horizontal cylindrical structure having two drums inside it. One drum moves with rotary motion and the other in reciprocating
motion. The feed enters at the center of the rotation drum through pipe known as cone pipe. Crystals present in the slurry are separated by the centrifugal force of the rum. Hot water is sprayed into the centrifuge to wash of the free acidity that is the acid layer on the crystal. The cleaned crystal is discharged into the outer drum which is reciprocating. The reciprocating drum pushes the material into the discharge chute. The liquor seper4ated is then sent to the saturator. The discharge chute of crystals opens onto a conveyor.
DRIER: Drier is fluidized bed type. The principle is based on the loose materials property to acquire fluidity in the airflow under a definite air velocity. The crystal from the centrifuge contains some amount of moisture. To remove this moisture crystals are to be dried. The drier is provided with a screen at the bottom, ceramic rings are arranged at the bottom of the screen. The drier is provided with forced draught fan and air, heated in the duct. A spreader at the feed chute of the drier spreads the feed in all directions. Forced draught fan sucks the atmospheric air and feeds to the drier. The discharge chute of the fan is divided into 2 sections. One for hot air and the other for cold air. The hot air duct is provided with a steam heater. The air is heated to 120-150°c by using steam and this hot air is fed form the bottom of the screen. The ceramic rings distribute the air in all directions and allow the crystals in fluidized state. The temperature of the air is sufficient the moisture of the crystals. At the discharge end of the drier, cold air is passed which cools the crystals. When the pressure level of the fluidized bed reaches the set point (300-400 mmWc) an automatic discharge feeder discharge the dry ammonium sulphate to the bucket elevator .The elevator discharges the dry product into the bunker, which in turn feeds the product to the bagging machine. The zone above the fluidized bed is kept under 5-10 mmWc in order to avoid carryover of the ammonium sulphate particles out of the dr4ying unit to the dust catcher. CYCLONES: The air form the drier is sucked by the suction fan and fed to the cyclone separators. Cyclone separators separate fine ammonium sulphate crystals in the air and feed to the bunker. The air from the cyclones is fed to the dust collecting tank which contains flushing liquor up to certain level.
The dust – laden air is then fed to the bottom of the tank. The crystals then dissolved in the water and the air is vented into the atmosphere. MOTHER LIQUOR TANK: The excess liquor from the saturator enters the mother liquor tank. Each saturator is provided with two mother liquor tanks. One is vertical and the other is horizontal. First the liquor enters the horizontal tank. As the liquor has less density than the tar it floats. Then the clear mother liquor from the bottom is fed to the vertical tank. Form the bottom of the vertical tank mother liquor is fed to the saturator through the pumps provided. The concentration of the liquor is maintained 10-12%. If the concentration decreases, the density of the liquor decreases and it may be contaminated. Then crystals may become back. AMMONIA COLUMN: During carbonization of coal ammonia gets vaporized and flows along with flushing liquor in gas collecting mains, a little amount of ammonia gets absorbed in flushing liquor. The excess ammonium liquor after separation of tar, containing free and fixed ammonia, phenols, pyridine bases and cyanides, rodents are pumped to the fourth plate of the ammonia column for distillation of free ammonia. The column consists of 24 bubble cap trays. 40% of liquor level is maintained in the column. Steam is injected at the bottom by steam coils. The purpose of steam is to supply the necessary heat required for distillation. As the liquor is distilled, free ammonia present in the flushing liquor evolves and rises tot the top of the column. The distilled liquor from the bottom of the column is fed to the pitch tank. Certain level of the flushing liquor is maintained in the tank and the over flow from the tank is fed to the M.B.C Plant for treatment. The pH of the flushing liquor at the discharge end of the column should be 7-7.5 and the free ammonia should be less than 50 PPM.
(NH4)2S
2NH3 + H2S
NH4CN
NH3 + HCN
(NH4)2CO3
2NH3 + CO2
+ H2O
NH4HCO3 NH4HS
NH3 + H2O + CO2 NH3 + H2S
The ammonia vapors collected at the top of the column are fed before primary gas coolers. The gases are not fed to the saturators directly as the temperature of the vapors is high and secondly the vapors contains not only ammonia but also some traces of rhodonides and cyanides. These chemicals should not be present in the ammonium sulphate fertilizer. To remove some content of fixed ammonia in the liquor, dilute NaOH solution is injected at the inlet of the ammonia column. The NaOH reacts with the fixed ammonia compounds and forms as ammonia gas. Capacity Total trays Types of trays Working pressure Steam pressure Flow rate of liquor Ammonia content before column Ammonia content after column Temperature of vapors at outlet Level in the column Steam flow rate PH of liquor after column
25m3/hr 24 Bubble cap 0.5 atm 6 atm 25m3/hr 4 g/lit 50 mg/lit 105°c 40-50% 3-3.5 ton/hr 7-7.5
FINAL GAS COOLERS Coke oven gas after ammonia recovery consists of 0.8-1.12 mg/lit of naphthalene. These naphthalene particles are removed by spraying tar. Temperature at the bottom of FGC Naphthalene in CO gas after FGC FGC tar temperature Specific gravity of FGC tar moisture
50-60°c 0.4 g/Nm3 80-90°c 5%
GAS PRE-COOLER: The gas at 30-35°c is fed to the pre-cooler, which is indirect contact cooler. The gas is cooled by chilled water (150°c), which passes through the tubes. The gas is cooled to 25-27°c so that any particles condensed are received in the seal pot. SCRUBBERS: Scrubbers are long towers consisting of aluminum packing or wooden sheets provided with three layers. The main draw back of wooden sheets is that it requires large cross-section and more height, while aluminum is efficient. But the condensate should not have muggy materials, which stick to the plates and scrubbing cannot be done efficiently. The CO gas from the gas pre-coolers is sent to the first benzol scrubber at the bottom. Solar oil or de-benzolised oil is used for scrubbing the gas to recover benzol. Large amount of the benzol is absorbed in the first scrubber. At the bottom of the first scrubber a certain level of benzolised oil is maintained. CO gas from the top of the first scrubber is fed to the bottom of the second scrubber in which it comes to contact with fresh de-benzolised oil, which removes the traces of benzol remaining in the gas. The benzol free CO gas is sent to customers.
Capacity Oil flow rate Benzol content in gas after benzol scrubber Specific gravity of solar oil Molecular weight of solar oil Flash point Initial boiling point
58,000Nm3/hr 150m3/hr 3 g/Nm3 0.8428 230 132°c 270°c
OIL CYCLE: The DBO from benzol distillation unit is fed to the top of the second scrubber. Benzolised oil from the bottom of the second scrubber is pumped to the top of the first scrubber through the distribution nozzles. The benzolised oil collected at the bottom of the first scrubber is sent to the benzolised oil tank from where it is pumped to the benzol distillation section. DRY PURIFICATION UNIT: 700-1000 Nm3 /Hr of coke oven gas is sent to dry purification unit when H2S of the gas is required to be removed from 0.6-0.7 gms/ Nm3 to 0.02 gms/ Nm3. The H2S free CO gas is sent to laborites and other special consumers like GETS at BF, CO gas is supplied to dry purification unit After the electrostatic precipitators. Synthetic bog is used as a purification mass. Spent purification mass containing 24 to 25 % sulpher is transferred to dump. COMPRESSOR HOUSE: Compressor house provided in the recovery plant is used to supply the plant air and instrument air. In this process air from the atmosphere is fed to the suction side of the compressor through filters for removing the dust particles. COMPRESSOR: Reciprocating compressors are used to compress the air up to 6 kg/cm . This is double acting piston type, which consists of low-pressure side and high-pressure side. A motor drives the compressor. An inter cooler is provided between the LP side and the HP side. An after cooler is provided to cool the air. 2
Air initially fed to the low-pressure side where it is compressed to 2 kg/cm . Due to compressor the temperature of the air is increases and it is cooled in the inter cooler. This avoids decrease in the compression efficiency on the high-pressure side. Air is compressed up to 6 kg/cm2 in the HP side. The compressed air is fed to the after cooler. The purpose of this cooler is to remove the moisture in the air. The air from the after cooler is fed to air receiving tanks. Air in the receiving tanks contain moisture and oil particles. So this air cannot be used for instruments. This air is used for cooling and other purposes. 2
In the compression of air the lube oil is used for compressor bearings. In this process, some oil is mixed with air. To remove this oil the air is fed to the oil filter. The air is then fed to the coke filters where coke layers are placed. This removes moisture and dust particles in the air. Then the air is fed to the dryers in which a bed of activated alumina is placed. This eliminates complete moisture and from dryers the air is fed to the candy filters in which very fine particles of moisture and dust is removed. Then the air is again fed to the buffer vessels, which is an intermediate storage. The dried air is supplied to the instruments and its pressure is in the range of 4 kg/cm2.
MECHANICAL BIOLOGICAL AND CHEMICAL TREATMENT PLANT PROCESS: Toxic effluents generated in various sections of coke oven and byproduct plant are collected and pumped to the treatment plant from two pump houses. One at ammonium sulphate section and the other at tar distillation section. Excess flushing after removal of ammonia in ammonium stripping unit is also fed to this treatment plant. The combined effluent contains large amount of tar and oils and toxicants like phenol, cyanide, thiocyanides etc. The effluent plant is designed to remove tar and oils with the help of mechanical separation methods followed by biological treatment at effluent namely two stage activated sludge process to remove other toxicants. Phenolic effluent from two phenolic water pump houses located in CO & CCP are directly fed to pre-aerators of tar settling plants. Excess flushing liquor ammonia stripping is also pumped to the same pre-aerators to
the double pipe heat exchangers where it is cooled from 90°c with the help of recalculating cooling water. Water inlet temperature is 34°c and outlet temperature is 45°c. Mixed effluent at a temperature of about 50°c is distributed in equal positions in tar settling tanks by gravity. In pre-aerators effluent mixed thoroughly with the help of air. The tar settling tanks are provided with steam heating coil and scrapper mechanism at the conical hopper bottom. Tar collected at the bottom of each tank is pumped out in the tar collecting tanks ones in 3-5 days time. Tar settling tanks are provided for removing tar and oil form the effluents. Oil floated on the surface of water and tar settles down in the conical bottom. Floated oil removed with the help of scrapping device and flows to oil collecting tanks by gravity. Over flow from the tar settling tank is collected in vertical steel tanks named phenolic water collecting tanks from where it is pumped to the oil flotation tank through pressurized head tank. Air (5% by volume of water) is injected in the suction line of the pumps before it is pumped to the pressures tank. In the flotation tank air bubbles through the water as the water is depressurized and the oil is entrained by air bubbles and floats at the water surface. The entrained oil is skimmed with oil skimmer mechanism of the flotation tanks and collected to the oil discharge to a tank called tar and oil collecting tank. After oil flotation tank, water goes to the second phenolic water collecting tank, with the help of pump collected water is send to second flotation tank to pressurized tank. Tar and oil from the collecting tank are finally pumped to tar acid utilization plant. After removal of tar and oil the effluent is collected in averaging tank. The averaging tank consists of two chambers and each chamber is having one over flow tank. In this tank adding ortho phosphoric acid and bacteria nutrient. 73% strong ortho phosphoric acid at the rate of 20 g/m3 of water to be purified. From these tanks effluent is pumped to aeration tank of first stage purification through shell and tube heat exchangers to maintain effluent temperature between 35-38°c. Cold circulating water is used for this purpose. In this first stage purification is done with the help of phenol destruction bacteria. To maintain their vital activity, compressed air from the air blower is applied to the aeration tank. Phenolic water after the first stage of purification is collected in a tank from where it is pumped to aeration tank of second stage purification. In second stage of purification rodents and cyanides are destroyed with the help of rodents destructing bacteria. With each aeration tank (both 1 & 2 stage) of purification sludge settling tank regeneration are attached. The over
flow from each tank first goes to the settling tank where sludge settles down and the super latent water overflows to the collecting of first stage purified water collecting tank. The settled sludge then flows to the attached regeneration from where it is recycled back to the aeration tanks with the help of air lifting pump. Volumetric flow rate of sludge in each tank it‟s about 80% of the volumetric flow rate of water being purified, to treatment unit for further treatment. Inlet to the treatment plant as well as excess sludge is taken from the bottom of the settling tank and taken to sludge disposal facilities having sludge-drying beds. Treatment effluent from the second stage of purification is collected on a tank from where it is pumped to fecal sewage treatment plant for further treatment and dilution. Further culture of bacteria their accumulation two separate bacteria culture tanks requisite facilities are also provided which can be utilized and when required. Two MS tanks each of capacity 800m3 are provided as balancing tanks. Water can be pumped from any of the equipment, provision is there to take this water back treated water from the collecting tank after second of purification can also be directed to a balancing tanks.
TAR
DISTILLATION PLANT
Tar is a viscous fraction obtained by the cooling of coke oven gas with ammonia liquor. During this cooling coke dust particles mixed with liquor and forms coal tar. It consists of large chain aromatic compounds, which can be distilled into light oils, phenols, naphthalene oil, wash oil and pitch. Coal tar in coke oven gas is collected from CPH and final gas a cooler of benzol recovery is transferred to tar and oil storage. From there it is pumped to mechanized decanter in tar distillation plant. COMPOSITION OF COAL TAR COMING TO TDS: Light oil Phenol fraction Naphthalene Wash oil Anthracene oil Moistur4e Pi3tch
0.5% 1-1.5% 5-6% 8-9% 18-19% 5-6% 55-60%
PROCESS: The tar from the condensate pump house is fed to the tar and oil storage of the TDP. The tar to the tar distillation section is fed from the tar and oil storage tanks. This tar contains 5% of moisture, which can be removed in the decanters. DECANTERS: The tar from the TOS is fed to the decanters. It is provided with a scraper conveyer to remove the sludge collected at the bottom and transferred to a bunker in the decanter small quantities of water and flushing liquor collects at the top of layer which are continuously removed. The tar from the decanters flows through two strainers to remove suspended matter to a common suction header of liquor plunger pumps for first stage. A plunger pump consists of three piston, suction and delivery valves, which pump with high pressure. Tar initially consists chloride salts and acid, which causes corrosion and damages the piping present in the furnace and the plant. To prevent the soda ash solution of 8% concentration is injected at the suction side of the pumps. 2NH4Cl + 2HCl +
Na2CO3 Na2CO3
---------- 2NaCl + ---------- 2NaCl +
2NH3 + H2O +
CO2 + CO2
H2O
The ammonia formed from the above reactions vaporizes along with water vapor and light oils. FURNACE: It consists of two zones one is convection zone and the other is radiation zone. Furnace is having four burners where CO gas is used as fuel. Air draught is taken from the bottom of the furnace. The tar from the filter is pumped to convection zone, which is at the top of the furnace. Here the temperature increases to 120-130°c. Controlling the airflow can control the flame length more the air longer the flame and vice versa. This arrangement helps in controlling the temperatures of tar in both the zones.
In radiation zone dehydrated tar is circulated which is pumped from second stage plunger pumps. The temperature of the tar is increased to 400°c. The temperature of the tar should not increase beyond 400°c as this cause the formation of B.F grade pitch. The flue gas from the furnace is sent to the atmosphere through the chimney. It is provided with a baffle plate in order to assure complete combustion of the gas. I STAGE EVAPORATOR: Evaporator is a cylindrical vessel having baffle plates, which change the direction of gases. Evaporator is designed for the evaporation of water and light oils from the crude tar, which are fed at the middle of the evaporator. The temperature of the tar at the inlet is about 120-130°c. Due to this temperature, the moisture in the tar is evaporated. These vapors from the top of the evaporator are fed to the twin condensed coolers. From the bottom dehydrated tar having less than 2% moisture is fed to the dehydrator tar tank and pumped to the radiation zone of the furnace. Pressure in the evaporator Temperatures of vapors Temperature of DTT Baffle plates Spilling plates
0.3 kg/cm2 102-105°c <110°c 3 2
DEHYDRATED TAR TANK: Tar from the evaporator is fed to the DTT tank, which is used as a intermediate storage. The tank is always maintained 100% and the over flow tar from the DTT is fed to the decanter4. Tar is fed to the II stage tar pump from the bottom of the DTT. The pump discharges this tar to the radiation zone of the furnace where it is heated up to 380-400°c. This tar is fed to the second stage evaporator at the rate of 10 m3/hr. IISTAGE EVAPORATOR: In second stage evaporator oil vapors and pitch are separated. Dehydrated tar from the radiation zone is fed to the evaporator. Two baffle plates are arranged below the vapor inlet and five above the vapor inlet. Four bubble cap trays are also arranged at the bottom of the evaporator. This arrangement completely separates tar fraction form pitch. A protective plate is also installed as in first stage evaporator. The oil vapors from the top of
the evaporator are fed to the distillation column. The tar at the bottom of the evaporator is called as pitch and is supplied to the pitch cooling section. Pressure in the evaporator Number of bubble cap trays No. of baffle plates Temperature of the fractions
0.5 kg/cm2 4 7 400°c
DISTILLATION COLUMN: Fractionating column is designed to separate different fractions from tar. The column is provided with 59 trays out of which 56 are bubble cap trays and the remaining three baffle plates. Each tray has 42 bubble caps. Mixed oil vapors from the top the second stage evaporator are fed to the third tray of the fractionating column at a temperature of 360-380°c. As the vapors rises to the top of the column different fractions are obtained from different trays. Initially vapors enter at about 360-400°c. Pure light oil is supplied as reflux to the top plate rectification column to maintain top temperature of the column. Reflux is fed as the top temperature increases. All the fractions are continuously drawn from the column. The vapors, which are not condensed are collected from the top of the column and fed to the twin condenser cooler of the second stage.
LIGHT OIL: Light oil is obtained from the column, which is lighter than all of the fractions the top temperatures of the column are maintained at 100°c. The light oil consists of phenol and phenol containing water (PWC). The light vapors are cooled in twin condensed coolers and fed to the separator II. PHENOL: Phenol fraction obtained is not in pure state at 160-180°c. It contains 22% phenol and 10-15% naphthalene. Three tapings are provided on 44.46 and 48 trays. NAPHTHALENE: Naphthalene fraction can be trapped from 24, 26, 28 and 30 trays. It is obtained at a temperature of 190-200°c. Steam jacketed lines are provided because decrease in temperature results in crystallization of naphthalene.
WASH OIL:
There 5 tapping for wash oil on 12, 14, 16, 18, and 20. Normally wash oil will be tapped at a temperature of 280-290°c. It is tapped from 6 & 8 trays. Anthracene II is heavier than all fractions. So it is obtained from 1 st or 2 nd trays at 320-330°c.
TWIN CONDENSED COOLERS I & II STAGES: These are shell and tube heat exchangers to condense the vapors. First stage coolers condenses the gas coming from I stage evaporator and the II stage cooler condenses the gas leaving the column. The vapors pass through the shell and service water is used to cool the vapor passing through the tube side. The vapors of I stage cooler are cooled from 102-50°c. The vapors of second stage coolers from 100-40°c. After condensation the vapors from both I & II stage condensers enter into separators I & II respectively. SEPERATORS I &II: Separators are used to separate light oils from other fractions (NH3 water and phenolic water) obtained form the top of the first stage evaporator and distillation column in separator I & II respectively. Light oil obtained from the top the two separators flow to light oil tanks by gravity. At the bottom of the separator I ammonia water will be settled and a the bottom of the separator II phenolic water will be settled. Both of them are removed through dip tubes to prevent light oil mixing with them.
SUBMERGED COOLERS: Submerged coolers contain a coil inside the coolers through which the fractions from the column are passed. This coil is submerged in the water. The vapors are cooled from 80-50°c are sent to the collected tanks.
START UP TANK: During initial starting of the plant, the temperature of the tar does not reach up 400°c in second stage evaporator will be diverted to start up tank until it reaches the temperature of 290°c similarly column bottom liquid will also be diverted to start up tank until the temperature of the column reaches to 330°c. Tar then overflows to the decanters.
USES OF DIFFERENT TAR FRACTIONS: LIGHT OIL: It can be further processed to produce crude tar basis can be used to prepar4e pyridine and heavy basis. Light oil can be fractionized to produce small amounts of benzene, toluene, xylene and heavy solvent naphtha. PHENOL: It is mainly used in plastic industry. In thermosetting resins made with formaldehyde. In pesticides preparation. Insecticides and pharmaceuticals. Dye stuffs and carpolactum for fibers. NAPHTHALENE: Used in manufacturing of pthallic anhydride. ANTHRACENE: It is purified to produce pure anthracene. It is used to produce carbozole and phenanthracene. PITCH: Pitch cake. Industrial pitches. Road tar. Refined tar.
PARAMETERS: TEMPERATURES: Tar in decanters Tar after first stage furnace Tar after DTT Vapors from first stage evaporator Tar after II stage furnace
75-80°c 120-130°c 100°c 105-110°c 380-400°c
Vapors after II stage evaporator‟s Pitch after II stage evaporator Distillation column top temperature
300-330°c 330-350°c 100°c
Tar in first stage Tar in second stage
9-10 Nm3/hr 10 Nm3/hr
FLOW:
ANALYSIS: Water content in I stage tar Water content in II stage tar
5-6% <1%
PRESSURES: Tar after I stage pump Tars after II stage pump CO gas before furnace I stage evaporator top I stage evaporator bottom II stage evaporator top II stage evaporator bottom Distillation column top Distillation column bottom
2.3 kg/cm2 4.5 kg/cm2 >100 mm WC 0.14-0.5 kg/cm2 0.24-0.25 kg/cm2 0.35 kg/cm2 0.4 kg/cm2 0.04-0.06 kg/cm2 0.28-0.3 kg/cm2
PITCH COOLING AND LOADING AREA Pitch is a mixture of resins and straight chain aromatic compounds and oils. Pitch from the bottom of II stage evaporator is fed to the storage a tank of PCLA by gravity mainly produces. 1. Pitch creosote mixture 2. Medium hard pitch 3. BF grade pitch The main consumers are BALCO, NALCO, and INDALCO & HINDALCO. This pitch is mainly used in manufacturing electrodes in aluminum industries.
PITCH CRESOTE MIXTURE: The pitch from TDS is stored in the storage tanks. The softening point of this pitch is about 65-70°c. Anthracene oil, wash oil and pitch are mixed in respective proportions in PCM tanks. Initially, anthracene and wash oil are mixed in 1:1 ratio. The tank is filled up to 1.8m with this mixture and the soft pitch is filled up to 4m heights. Then the mixture is allowed to mix properly by keeping in continuous circulation through a pump from bottom. This PCM is used as fuel in CRMP section, which produces refraction bricks in VSP. MEDIUM HARD PITCH: The softening point of the soft pitch is 65-70°c. To increase the softening point of the pitch up to 95-110°c the pitch is fed to a series of reactors.
REACTORS: Reactors are cylindrical vessels with conical bottom. A bubbler is provided inside, which is fed with air at a pressure of 5 kg/cm2. An outlet provided at the top removes the air injected. A cylindrical small vessel on the top of the reactor is provided with baffles to remove any heavier particles present in the vapors. The vapors mainly contain anthracene and wash oil fractions.
PROCESS: Initially the pitch at 360°c from TDS is fed to the middle of the first reactor. Air is injected through the bubbler, which is provided with a ring having small holes. As the air passes through the pitch, it entrains any oil present in the pitch. The overflow from the first reactor is fed to the second and similarly to the third. Aeration is done in all the three reactors to increase the softening point up to 110 °c depending on the requirement. Airflow to all the reactors increases up to 400°c. The MH pitch after obtaining the required softening point is fed from
the third reactor to electrode pitch tank. From ETP the tar is pumped to the overhead tanks. All the ETP and overhead tanks are provided with steam coils to keep the pitch in liquid state. The pitch from overhead tanks is pumped to the bay, which is a large rectangular area where the pitch is allowed for atmospheric cooling. After complete cooling the pitch in small pieces is packed in bags. The vapors from all the reactors and storage tanks contain some entrained oil particles. The vapors are first condensed in a shell and tube heat exchangers (water in tubes and vapors on shell side). The condensed vapors are sent to the washer. In the washer, the vapors are scrubbed with the light oils. The condensate collected called pitch distillates are fed to the pitch distillate tank. The washer is provided with steam jet ejector at the top to draw the oil free vapors and is let out to the atmosphere. CHEMICAL PROCESS DURING ARERATION: Pitch is a mixture of high polymers, straight chain organic compounds and oils. Due to aeration and effective mixing in the reactor, the polymer oil bond breaks and resin bonds progressively increase causing softening point elevation. Due to effective agitation, new polymeric bonds formation and bond breakage large amount of heat is evolved. This increase in temperature leads to the formation of optically sensitive isomers i.e. optical isomers; which causes the pitch increase in optical activity. The increase in aeration in the reactor increases the softening point of pitch to required levels and increases the porosity, which will effective the strength and binding properties of the pitch. The aluminum industries require various concentration of resin in the pitch, which plays main part in the bonding property of pitch. Quinilene insoluble and benzene insoluble determine this. BF GRADE PITCH: BF grade pitch is used for preparation of BF mass for Blast Furnace tapping. This requires high softening point of 170-180°c. So it is prepared separately in fourth reactor. Initially, pitch from TDS is charged for three and a half hours and air is injected through bubbler at the rate of 150 Nm3/hr for 10-12 hours. After softening point is attained to the bay by gravity for cooling purpose. PARAMETERS: Pressure (mm HG) Reactor 1
300
Temperature (°C) 325
Air flow (Nm3/hr) 50
Reactor 2 Reactor 3 Reactor 4
310 320 ----
330 340 280-330
80 120 150
NAPHTHALENE ANTHRACENE FRACTION CRYSTALLIZATION Naphthalene fraction obtai3ned from the distillation column is 7580% pure. This is purified up to 99% in NAFC section. Crystallizing out naphthalene from the melt obtained from TDP does this purification. PROCESS: Naphthalene oil from tar distillation plant at 70-80°C is pumped to the overhead tanks of the NAFC section. The tanks are provided with steam coils. The temperature of the naphthalene oil is maintained at 85-90°C. One tank receives oil from TDS and the other from melting pot. The naphthalene oil is then fed to the drum crystallizer. DRUM CRYSTALLIZER: Drum crystallizer is a hallow cylinder with ribbed surface over its entire perimeter. The drum is rotated at a rpm of 0.3 by an electric motor and reducer. The rotating drum is partially submerged in oil bath. Two jackets are provided at the bottom of the drum one for naphthalene oil and the other for hot water. The oil form the over head tank is fed to the oil jacket. Hot water is circulated under the oil jacket to prevent the solidification of naphthalene. Inlet and outlet to the service water is provided to the side of the drum. Service water is circulated inside the drum to cool the naphthalene layer formed on the surface of the drum. Knifes are provided on the surface of the drum to cut the layer formed on the drum. The flakes are then collected in a screw conveyor and fed to the press mixer.
Capacity Rotation per minute Temperature of hot water in jacket Cooling water temperature Temperature of fraction in bath Naphthalene temperature
25T/hr4of fraction 0.3 90-100°c 18-20°c 80-90°c 60°c
HYDRAULIC PRESS: It consists of a mixer and a press. Flakes from the conveyor are fed to the mixer in which agitator mixes the crystals homogeneously. The temperature in the mixer is maintained at 60-65°c by circulating hot water in the jacket. PRESS: It consists of three filters mounted in a rotary table. This system is operated by hydraulic pressure at a pressure of 160 kg/cm2. Oil is compressed and fed to different parts of the press. The press consists of squeezing device, pushing device and table stopper. Squeezing device is provided with shift to compress the naphthalene into a cake. The filters are at an angle of 120 ° to each other. The naphthalene mass from the mixer is fed to the filter through the charging neck of the mixer the press is operated by six operations. CLAMPING: The filter coincides with the axis of charging neck of the mixer. The mixer discharges the feed into the filter. Then the table rotates. SQUEEZING OR PRESSING: After clamping the filter moves to the squeezing device. The squeezing device consists of two cylinders, which compresses from top and bottom with 160-180 kg/cm2 pressure. The oil present in the cake will be separated from the holes provided in the filter. This is collected at the bottom. Then the cake is formed.
PUSHOUT: This operation is done after squeezing. a cylinder from the bottom of the filter lifts the cake forward up. PUSH OFF: An arm like device pushes the cake into a chute. At this position, the lifted cylinder is in the same position. Push out and push off operations is done in the third position of the filter.
RETURN STROKE: In this operation all the cylinders return to their original position. TABLE ROTATION: After all the cylinders reach to their original positions, the table rotates so that the filter again comes to the first position i.e. charging position. All these operations are done automatically by gear mechanism. The temperature inside the filter is maintained by supplying steam. All these six operations are completed in three & a half minutes. The naphthalene cakes formed are conveyed to the jaw crushers. The weight of the cake form is about 40 kgs and 98% pur4e. The jaw crusher crushes the cakes and the crusher pieces are conveyed to the bagging machine through a chain conveyor. Naphthalene bags weighting 50kg each are stored in the godow. PRESSED OIL: The oil separated from the filter is further processed to recover naphthalene present in the oil as the oil contains 60-65% naphthalene. This pressed oil collected in the ground floor is pumped to the overhead tank. This oil is processed in mechanical crystallizes. MECHANICAL CRYSTALLIZERS: These are horizontal cylindrical vessels with the shaft running through the crystallizer. The shaft is a propeller type agitator, which is fixed with a number of vanes. The shaft is run by a motor, which connected by a gearbox. There is a provision for spraying of water on the drum. The water is collected in the water jacket under the drum.
The de-naphthalene oil from the overhead tank is fed to the mechanical crystallizer. Half of the volume of the crystallizer is fed with the DN oil. Due to slow and continuous agitation of the shaft, high retention time and slow cooling the oil in the crystallizer thickness. Small nucleuses of the crystals are formed and the oil forms as the slurry. Constant agitation of the melt results in the breakage of weak small crystals originally formed which then act as nucleus to new crystals. Cooling is done by natural convention by atmosphere for 16 hours. After 16 hours of atmospheric cooling, 8 hours water-cooling is done by spraying water on the drum. After the completion of crystallization, the valve open and the slurry is transferred to mixer through screw conveyor. MIXER: The slurry enters into the mixer through a filter, which filters any suspended particles in the slurry. Mixer is a horizontal cylindrical drum provided with agitator. Mixer acts as a storage drum as well as maintains the uniform mixture by agitating the slurry. Then the slurry flows by gravity to the centrifuge by controlling the valve. CENTRIFUGE: The centrifuge used in the NAFC is of basket type. It is a batch wise centrifuge operating at a rate of 400 liters of naphthalene slurry per batch. The retention time of each is 2 minutes. The centrifuge consists of a basket rotated by a motor. Feed enters through a feed chute, which aids provided with an automatic valve. There is a knife arrangement, which collects the naphthalene powder into a basket type chute. The slurry is discharged from the rotating basket tangentially through the holes arranged. The centrifuge does three operations. CHARGING: During this period (20seconds) the material is fed into the basket after a flow rate of 400 liters, the valve is closed. DRYING: The power is separated from the slurry and the basket is rotated without slurry for about 1 minute. The naphthalene powder is allowed to dry during this period.
DISCHARGE: After the material is dried, the knife collects all the powder into a chute that is fixed to the door. The oil separated contains 40-60% of naphthalene. The soil is again recycled till its concentration reaches below 30%. After this DB oil is pumped to the tar and oil storage section. The naphthalene powder so obtained is about 98-99% pure and this fed to the melting pot through a belt conveyor. MELTING POT: This is a horizontal cylindrical drum, which consist of steam coils inside it. LP steam is circulated in these coils. The naphthalene powder gets melted due to high temperature and the liquid thus obtained is fed to the main naphthalene friction overhead tank to increase the purity of the naphthalene oil coming from the distillation section.
BENZOL PLANT Benzol plant is provided in order to produce pure benzene, toluene, and solvent naphtha. Benzol plant consists of three sections: Benzol distillation plant. Hydro refining unit. Extractive distillation unit. Crude benzol recovered from the coke gas is fed to the benzol distillation plant. Various chemicals in the benzol are recovered by distillation. In the benzol distillation plant, the benzolised oil from final absorption is pumped to the storage tanks of the benzol distillation. The BO is then stripped of with steam to get the crude benzol and debenzolised oil. This DBO is again pumped to the benzol recovery section. Makeup solar oil is added continuously to compensate for the losses in the equipment. The benzolised oil is initially pre-heated in three pre-heaters, which are shell and tube heat exchangers. Pre heating is first done in oil dephelegmators, then oil-oil heat exchanger and finally in steam pre heaters. The temperature is slowly in order to prevent chemical decomposition of benziolised oil. The temperature of the BO fed to the stripping column is about 130-135˚c.
Tube side Oil dephlegmator Oil-oil exchangers Steam pre-heaters
Shell side vapors of stripping column DBO from stripping column medium pressure steam
BO BO BO
STRIPPING COLUMN: Pre-heated BO from the exchangers is fed to the 17th tray of the stripping column. The column consists of bubble cap trays. Low pressure of steam at a temperature of 180°c and 3.8kg/cm2 is injected through DBO at the bottom of the column. Crude benzol in the BO is recovered by steam distillation. Steam distillation is done so that the partial pressure of the benzol decreases and easily get vaporized. LP steam injected at the bottom not only maintains temperature of the column but also decrease the partial pressure of the crude benzol. The crude benzol vapors along with steam from the top of the column are fed to the oil dephlegrmators. The DBO from the bottom in which crude benzol is recovered is pumped to the decanter through oil-oil heat exchangers. The crude benzol vapors are partially condensed in the oil dephlegmator. The partial condensation removes any higher fractions present in the vapors which further increases the purity of the vapors. Three sets of dephlegmator are provided, two sets for oil and one for water. The vapors are cooled to 92-95°c in oil dephlegmator by pre-heating the
feed to the stripping column and further cooled to 84 °c in water dephlegmator. The condensate collected in the heat exchanger is called PHLEGMA. The phlegma from the exchangers, which contains water, is collected in a separator. Water is separated and phlegma over flows to the phlegma collecting tank from this tank phlegma is sent to stripping column as reflux. REGENERATOR: The continuous circulation of DBO forms some polymer due to heating and cooling. This polymer must be removed from the DBO by regeneration. Regeneration is a hollow tank in which steam coils are arranged MP steam is circulated through these coils. Part of the stripping column is fed to the regenerator. Due to the pressure CB vapors are collected at the top, which are in turn to the stripping column. The bottom liquid from
the regenerator is pumped out and stored in Crude and Finished Product Storage. Top temperature Bottom temperature No. of trays Feed tray Pressure in the column
110-115°c 120-130°c 23 17 th tray 0.3-0.35
CRUDE BENZOL COLUMN I: The vapors containing crude benzol from the top of the stripping column is fed to the crude benzol column I through water dephlegmator where the crude benzol is separated to heavy crude benzol and light crude benzol. Crude benzol mainly consists of LCB, HCB and polymer. Lighter fractions like benzene, toluene, and xylene are present in LCB and HCB is similar to that of heavy polymer, which is used as furnace oil. The column consists of 16 bubble cap trays. Simple distillation is carried out in this column. A reboiler provided at the bottom of the column supplies the necessary heat. MP steam is used as heating media. A CB vapor at a temperature of 80-85°c is fed to the 6 th tray of the column. The lighter components are vaporized and these are collected at top of the column, which are then condensed in a condenser by water. The condensed vapors are then fed to the separator where the moisture present in the vapors is separated and the LCB obtained is stored in CB1 tanks. Part of the LCB is fed as reflux to the CB1 column. The bottom product obtained from CB1 column is fed to the CB II. Top temperature Bottom temperature
70-80°c 115-120°c
CRUDE BENZOL COLUMN II: CB II consists of 6 bubble cap trays. The bottom product of the CB I which mainly contains HCB with small amount of LCB is fed to the CB II column. To recover the LCB the liquid is to be distilled. The LCB vapors obtained from the top of CB II is fed to the CB I column as reflux. The bottom product obtained from the CB II is called as Heavy Crude Benzol (HCB).
Top temperature Bottom temperature
120°C 140°C
DEBENZOLISED OIL: The DBO from the bottom of the stripping column is pumped through oil-oil heat exchanger to DBO cooler. In DBO cooler it is cooled to 45-50°C. Due to high temperature exposure, part of the solar oil may get decomposed. This decreases the absorption efficiency of the solar oil. To remove this decomposed matter DBO is fed to the decanter. DECANTER: It is a horizontal cylindrical tank unlike mechanical decanters DBO is fed to the decanter at a temperature of 45-50°C. Small amount of water is fed to the decanter which provides better removal of sludge or muck form of oil. Water settles at the bottom carrying sludge with it. Muck or sludge layer is formed the water layer. Oil layer is formed above the muck layer. The residence time in the decanter is three to four hours. Water is continuously drained from the decanter. Oil after 3-4 hours is fed to the DBO tank. Muck from the decanter is drained and sent to the emulsion beaker. Due to contact of oil, water and muck oil-water emulsions and muck-water emulsions are formed. These emulsions float on the surface of the water, which is fed to the emulsion beaker along with muck. EMULSION BEAKER: This is a horizontal cylindrical vessel provided with insulation. Medium pressure steam is fed through a coil into the beaker. Residence time for setting the oil, muck and water in the beaker is 2 hours. Due to heating of emulsion, oil and water get separated which is called as De-emulsification. Emulsion thus formed is broken and muck will float on water. This muck is fed to the muck tank and the water is drained. The temperature inside the beaker is 80-90°C.
HYDRO REFINING
In this unit using hydrogen gas purifies the light crude benzol. Hydrogen is recovered from coke oven gas and LCB from benzol distillation plant. LCB consists of benzene, toluene, xylene, solvent naphtha, nonaromatics and residue. Initially, the LCB is purified from sulphur, nonaromatics and other compounds. This consists the following sections. They are,
De-fronting section Reaction section Purification section
DE FRONTING SECTION: In this section, carbon disulphide is removed from the crude benzol and this is called as de-fronted crude benzol. LCB from the storage tank is pumped to a surge tank, which is meant for intermediate storage. The LCB from surge tank is pumped to the distillation column through feed pre-heater. The feed enters the column at a rate of 3 T/hr and at 70°c. Pressure in the column will be 0.5 kg/cm2. Sulphur content in the feed is 2000-1800 ppm. This is decreased to about 1200 ppm in the column. Distillation column consists of 30 bubble cap trays of which 17th tray is the feed tray. Steam is fed into the reboiler, which heats the bottom product recycled to the column. The remaining bottom called defronted crude benzol is fed to the reaction section through feed preheater. The sulphur is removed in the form of CS2. Simple distillation is carried out and due to heating CS2 vapors rise in the top and these are condensed in a water condenser. Condensed CS2 is collected in CS2 vaporizer. Part of it is fed to the column as reflux and the other part is stored. The DCB obtained is at 70°c and this is fed to the intermediate storage. Feed rate to the column Pressure in the column Sulphur content in the feed Sulphur content in DCB No. of bubble cap trays
3T/hr 0.5 kg/cm2 2000-1800 ppm 1200 ppm 30
Boiling point of CS2 Temperature at the top of the column Column bottom temperature
45 55-65°c 105°c.
REACTION SECTION: This section consists of reactors and evaporators. Here the hydro refining takes place in the reactors provided which removes the oxygen, nitrogen and sulphur content in DCB. PROCESS: The de-fronted crude benzol is pumped to the de-fronted storage tank (V-401) through a filter. The filter is provided to remove the solid particles and polymers, which may be present in the crude benzol. The benzol filter is an edge type filter and consists of a slotted tube inside a shell with a specified filter fineness, which is determined by the slots and scrappers. This is agitated by a hard crank. The particles are retained at the edges of the slots and must be scrapped off. If the pressure difference between the inlet and the outlet streams is too high the concerned filter must be opened and cleaned. The filtered DCB is stored in the surge drum (V-401). The drum is set to approximately two bars split range controlled by feeding N2and venting gases. From surge drum, the DCB is fed to pre-vaporizer at a pressure of 30 bars using 32 stage centrifugal pumps. PRE-VAPORIZER: It is nothing but a vertically mounted shell and tube heat exchanger. The feed is mixed with a part of cycle gas (containing H2 approximately 15% of the total gas) before it is fed to the vaporizer. This feed is pre-vaporized to about 160-165°C by means of the main reactor effluent passing through shell side. The feed at a temperature of 160165°C is fed to the third mixing nozzle of stage evaporator. This vertical heat exchanger is provided with turbulence promoters in the tube side to achieve high turbulence so that more heat exchange will occur and no scale formation is attained. This arrangement is provided as the feedstock is in partial vapor stage (gas-liquid stage) and so fouling of the tubes will occur rapidly. This arrangement also provides easy cleaning of tubes by simply pulling the turbulence promoters.
STAGE EVAPORATOR: The stage evaporator is a long cylindrical vessel provided with three stages, which are separated by two plates. Demister pads are provided at the top of the evaporator. Each stage is provided with a mixing nozzle. Two reboiler E-402 and E-403 are provided for second and first stage respectively. A gas pre-heater E-404 is also provided in which the rectangle gas (85% of the total gas) is pre-heated to 210°C by the main reactor effluent. E-402 and E-403 are heated by hot oil through tubes at a temperature of 250°C. Rectangle gas mixed with feed is passed through the shell side. Down comers are placed so that the liquid in the third stage will enter the second and from second to first. Pressure inside is about 20kg/cmm. The DCB mixed with 15% of rectangle gas is fed at the third mixing nozzle of the evaporator. The vapors coming from the second stage and the feed are mixed thoroughly and fed to the third stage. Lighter vapors are passed through the demister pads and to the pre-reactor. The liquid containing lighter and heavier substance is passed through down comers to the second stage. Here the fed is mixed with the vapors from first stage in the mixing nozzle II and heated in reboiler E-402. This is fed to the top of the second stage. Similarly liquid from second stage flows to first stage. This liquid is pre-heated in E-403 and mixed with 85% of the rectangle gas in first mixing nozzle and again fed to the first stage. The temperature at the bottom of the evaporator is 210°C. Due to heating of the feed the vapors are sent to the top and any residue or polymers in the feed are collected at the bottom. Part of the liquid from the first stage is fed to the residue flash drum (V-406) from where they are recycled to benzol distillation plant. The lighter vapors from the flash drum are fed to the surge drum (V-401) nearly this residue would be 3-4% if total feed. The vaporization of feed (DCB) in the evaporator is done by reduction of partial pressure of DCB, which is manipulated by addition of the rectangle gas. This results in lower operating temperature even at higher pressures. Vaporization of feed in heat exchanger should be avoided to reduce fouling of surfaces.
PRE-REACTOR:
The vapors from the top of the evaporator at 180°C are heated in a heat exchanger E-406 to 190-225°C by passing main reactor effluent through shell side. The reactor is provided with a bed of catalyst i.e. NICKEL MOLYBDEBUM. In this pre-reactor such as diolefins, styrene and CS2 are removed by hydrogenation. Feed enters from the bottom of the reactors through catalyst bed. Hydrogenation of diolefins, styrene takes place in the presence of catalyst. The temperature at the inlet of the reactor is 190-225°C and this depends on the life cycle of the catalyst. Due to the exothermic reaction the outlet temperatures increases to 200-235°C. Due to continuous operation of the catalyst bed coke like polymerization products deposit on the catalyst bed resulting in the lower efficiency. This can be overcome by increasing the inlet temperature of the reactor. Catalyst activity can be determined by the temperature difference between inlet and outlet, which should be more than 10°C. Catalyst can be regenerated by heating the bed with steam and air. The reactions in the pre-reactor are
Diolefins CnH2n-2
+
H2
mono olefins CnH2n
Cyclopentadiene C5H6
+
H2
cyclopentane C5H8
Styrene C8H8
+
H2
ethyl benzene C9H10
Carbon disulphide + CS2
H2
methane + H2S CH4
MAIN REACTOR: In main reactor treated pre-reactor effluent is hydrogenated on special sulphide molybdenum catalyst. The main reactor consists of two beds of catalyst makeup gas i.e. pure H2 gas from the compressor at pressure of 18 bars provided more hydrogenation and hence complete saturation of olefin hydrocarbons. The inlet temperature is about 270°C and the outlet temperature is 330°C due to exothermic reaction. Mainly desulphurization, densification and olefin saturation feed stock occurs in main reactor. The hydrogen is fed through a distributor below first bed of catalyst oxygen
content in H2 gas should be very low so that no polymerization occurs in the reactor. Hydrogenation of aromatics should be prevented. Catalyst deactivation can be determined by the amount of thyophene content at the outlet of the reactor. If this increases hydrogenation of aromatics, coke formation increases. So the temperature of the reactor should be increased or other regenerations should be done. Main reactions are: Mono olefins Ethyl mercaptans Thyopene Coumarone Pyridine Pyridine Benzene Toluene
+ + + + + + +
H2 + H2 H2 H2 H2 H2 H2
H2
Paraffin Ethane + H2S Butane + H2S Ethyl benzene + H2 Pentene + H2 Butane + H2 Cyclohexane Methyl cyclo hexane
Hence required to maintain a heater to which part of the effluent is passed, Heated and fed to the main reactor supplies the temperature. Coke oven gas is used as fuel in the heater. The effluent from the main reactor collected at the bottom, which о is at 330 c. This effluent is passed through E-407, E-406, E-404, E-401 and finally cooled in water cooler E-408. This condenser effluent is fed to the separator. Before water cooler hot water is dosed into the effluent. This dissolves the deposits of salts such as NH4HS2 and NH4Cl. The cooled effluent at 50 оc is fed to the separator. A water leg provided separates the dosed water. The water free effluent is fed to the stripping column. The gases i.e. un reacted hydrogen gas and other gasses are sucked by recycle gas compressor and are recycled part of the gas is purged out through vent provided. HOT OIL SYSTEM: The heat demand of the process is supplied by a separate hot oil system. The hot oil is used as a heating medium for several heat exchangers in hydro refining unit and extractive distillation unit. A horizontal furnace is used to heat the oil; the furnace is fired using coke oven gas. Hot oil is pumped in to the coils into the furnace. The temperature of the oil increases to about 340-350 оc. The hot oil is pumped by P-404 pump. The oil at temperature of 340 оc is fed to the HR unit by using another pump. This is again recycled to the suction side of P-404.
PRESSURE SWING ADSORPTION UNIT: The required hydrogen gas to HR units is supplied from this section. The clean coke oven gas after benzol recovery is fed to a filter at a pressure of 800mm WC. Moisture and carbon particles present in the gas are filtered and the filtered coke oven gas is fed to a reciprocating compressor, which compresses the gas to about 2.5 kg/cm2. The compressed gas is again fed to the other compressor where the pressure of the gas increases to 6.5 kg/cm2. The gas is then fed to another filter, which removes the moisture in the gas. From the filter the gas is fed to the pressure swing adsorption unit. It consists of 4 cylindrical vessels in a bed of molecular sieves is placed. The coke oven gas is passed form the bottom of the bed and the molecular sieves absorb the hydrogen present in the gas. The hydrogen thus collected is fed to the make up gas compressor. The gas is passed through one catalyst bed only. At this time, the remaining beds are in regeneration. This is because catalyst for 180 seconds only. Then it has to be regenerated. This is done by using pure H2 gas. The regeneration of then bed is done automatically. The H2 gas is collected form the top of the bed and is fed to the make up gas compressor. This is a vertical reciprocating compressor of double stage. The H2 gas is compressed to about 30 bar. The recycle gas from the gas separator is fed to the recycle gas compressor, which is a horizontal single stage compressor. PURIFICATION: This section consists of a stripping column in which the sulphur content as H2S and any dissolved gases in the DCB are removed. PROCESS: The liquid part from the separator is fed to the stripping column through a pre-heater, which is heated by BTX solvent from the stripping column. The fed at a temperature of 135 оc is fed to the column. The column consists of sieve trays. Top temperature is 125-135 оc and bottom temperature is 150 оc. Pressure is about 4.3 kg/cm2. Re boiler is provided which supplies the required heat to the column. MP steam is fed to the shell side of the re boiler. The gas from the column contains H2S. These are condensed in the condenser where water issued. This condensate (70 оc) is fed to the reflux drum. Part of the condensate is refluxed to the column. Moisture present in the gas is removed from the water leg and the off gasses are fed to the off gas mains.
The bottom product called BTX solvent raffinate is passed through the pre heater where it is cooled and finally raffinate is cooled in the raffinate cooler which is cooled by water. This is stored in intermediate storage.
EXTRACTIVE DISTILLATION UNIT In this unit, the BTX raffinate is processed to separate benzene, toluene and xylene solvent. Further benzene and toluene are also separated. Using „Extractive Distillation‟ in which N-formylmoropholine (NFM) is used as solvent does separation of BTX into BT and X. Non aromatic compounds present in BTX are removed by pressure distillation solvent is recovered in solvent recovery column. Benzene and toluene are separated in BT separation column. The total heat required for the unit is supplied from various means pressure distillation receives heat from hot oil. Aromatic separation column and solvent recovery column receives heat from the vapors of the pressure distillation column. The BT column receives heat from the LP steam. The unit consists of the following sections:
Pressure distillation section Extractive distillation section Solvent recovery section Aromatic stripper BT separation section Xylene solvent section
PRESSURE DISTILLATION SECTION: This section consists of a distillation column in which the BT & X solvent are separated by simple distillation from reffinate. PROCESS: The BTX solvents raffinate from the IPS is pumped to the feed surge drum (V-513). The drum is a horizontal tank provided with a vane and line from reflux drum (V-501) that carries vapors to this drum. The BTX solvents raffinate from the surge drum is pumped to pressure distillation column through four heat exchangers I series E-502, E-503, E-504 & E505respectively. E-502 and E-505 are heated by bottom product i.e. Xs fraction. E-503 and E-504 are heated by BT fraction. The column consists of
50 bubble cap trays of which 25th trays is the feed tray. Column pressure is about 15 kg/cm2. A re boiler is provided to the column through which hot oil passes through shell side. These supplies the heat required for the column. The BT vapors from the top of the column a collected in reflux drum before which they are condensed in E-504 and E-509. This condensed BT fraction is collected in reflux drum. Some of it is reflux to the column and the remaining is passed to E-503 and cooled in BT condensed in E-501 by using water.
EXTRACTIVE DISTILLATION COLUMN (C502): The BT surplus is conveyed by steam pressure from V-501 via heat exchangerE-503 and cooler E-501 as feed to extractive distillation column C-502. The feed is introduced on the 31st tray at the middle of the column. The N-formylropholine (NFM) solvent is introduced on the top tray of the ED Column at the physically required conditions at the flow ratio of 56 kg NFM per kg of feed at 92оc. The NFM temperature is regulated for the achievement of the low level of aromatics in the non-aromatics. ED column serves for the separation of non-aromatics contained in the feed, which is not possible under normal distillation conditions. This means that non-aromatics originally with boiling points higher than aromatics, becomes low boiling non-aromatics which can be with drawn at the top of the ED column while the aromatic substances dissolve in the NFM is yielded at the bottom of the ED column. ED column is supplied by the reboiler E-507 (LP Steam), E-508 (Hot NFM) and partially via vapor heated reboiler E-509. The NFM at the top of the column promotes the scrubbing of aromatics out of ascending vapors; where as non –aromatic vapors are dissolved only to a slight extent. ED column trays Feed plate Top temperature Bottom temperature Top pressure Bottom pressure
60-bubble cap 31 tray 110оc 150 оc 0.8kg/cm2 0.4kg/cm2
SOLVENT RECOVERY COLUMN: The column is used for separation of non- aromatics yielded at the top of ED column from the residual carried over solvent contents. For this purpose the top vapor of the ED column are fed at the point below at the pall rings packing in the column. Bottom heating to the column is affected using
reboilerE-510, also by means of hot NFM from the solvent circulation. The top phase in the column, consisting principally of non-aromatics is condensed in condenser E-511 and the liquid phase yielded is routed to the reflux vessel V-502. A portion of the non-aromatics is routed as reflux to the column. While bottom product is discharged through a level controller to CFPS. NFM recovered at the bottom of the column is returned to the ED column. The solvent recovery column minimizes the NFM losses by means of extensive recalculation of NFM flow inevitably leaving the top of the extraction column. The basic difference as compared to the normal hydrocarbon distillation and ED column is that its bottom section must be operated in the phase occurs in V-509. The bottom contains large quantities of non-aromatics due to the reflux required for scrubbing. In contrast to an ED column, the recovery column is operated under normal circumstances with a two-phase bottom product. Packing Bottom pressure Top pressure Top temperature Bottom temperature
Pall rings 0.25 kg/cm2 0.2 kg/cm2 100оc 125 оc
STRIPPING COLUMN: The product yielded at the bottom of the ED column consists of NFM in which the extracted aromatic substances are dissolved. The non-aromatic content will be in low PPM due to the existing pressure drop. This flow is conveyed into the aromatics column, which is operated under vacuum. In this column the pure aromatics are separated from the NFM, which is yielded as the bottom product and cooled in the heat exchanger system of the equipment prior to be being returned to the ED column. Before feeding NFM to the ED column, it is passed through the following equipment: 1. Center re boiler E-508 on ED column C-502. 2. Re boiler E-512 on solvent recovery column. 3. NFM re boiler E-514 on the stripper column and then fed to the ED column. Solvent cooler E-522 serves as a trim cooler for NFM. The bottom of the stripper column is heated by means of the two continuous re boilers E-
512 and E-513, which are heated by BT vapors and E-514 heated by means of NFM. The reflux to the stripping column serves to remove the solvent in the lower section of the column. The vapor liquid mixture discharges from the re boiler E-514 is fed below the chimney tray in the stripping column. Traces of the solvent flash are washed back by the aromatics reflux and directed into the lower part of the column. Total trays Feed tray Top temperature Bottom temperature Pressure
30 5 th 56 оc 119 оc 0.36-kg/cm3 vacuums
BENZENE TOLUENE SEPERATION COLUMN: BT separator is a normal two-phase distillation for pure aromatics. The BT fraction is routed using a reflux pump from reflux drum via exchanger E-517 to separation column C-505. The heat required for distillation is supplied to the system via re boilers E-518 by means of LP steam. Overheads are pure benzene, bottom are pure toluene as specified. Total trays Feed tray Pressure Benzene purity Toluene purity
65 Bubble cap 30 th 1.2 bars 99.97% 99.95%
