1

A Mini Project Report on

COKE OVEN AND COAL CHEMICALS PLANT (CO&CCP)

Submitted in partial fulfillment of their requirements for the award of the degree

of

Bachelor of Technology in

PETROCHEMICAL ENGINEERING

Submitted by

M. YESUVARAPRASAD

(12021A2537)

PETRO CHEMICAL ENGINEERING

Department of Petroleum Engineering and Petrochemical Engineering

University College of Engineering (A), Kakinada

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ABSTRACT

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. 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 remain the principle structural material in for seeable.

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PREFACE

In this competitive world we must be well equipped with advanced knowledge so as to

stand at this stage we must have good experience of the industry. Although we have theoretical

knowledge but it is not enough as it is incomplete without industrial experience so as to balance

our knowledge in concern field, we must have knowledge about industry. In a practical training

a person deals with many technical problems, real operations and procedures. Another aim of

implant training is to learn industrial knowledge and discipline.

This type of industrial training can put us one step ahead in related field by an addition

to theoretical knowledge. Training is essential for us to get a golden future. We hope to be

successful in our attempt to learn practical aspects of industries.

Visakhapatnam steel plant produces steel from iron ore. To remove impurities in iron

ore it is heated directly with coke which is produced in CO&CCP plant. In this process

carbonization process co gas is produced. In co gas it contains many organic and inorganic

chemicals. These chemicals are separated in coal chemical plant (CCP).

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ACKNOWLEDGEMENTS

I M.YESUVARAPRASAD student of department of petrochemical engineering,

Jawaharlal Nehru Technological University, Kakinada feel exhilarated and honored while

taking this prestigious opportunity to thank one and all of the esteemed staff members of

Rastriya Ispat Nigam Limited, who guided me through my training for their impeccable

cooperation and motivation.

First, I would like to thank to our mentor of this project, Shri. P. Arjun Kumar (Asst.

General Manager, CO & CPP, VSP) for the valuable guidance and advice. He inspired us

greatly to work in this project. His willingness to motivate us contributed tremendously to our

project.

To name few of the inspirational persons who left an incredible mark on me are Shri Dr.

K. Padma Raju (Principal UCEK, JNTU Kakinada). Prof. K. V. Rao (J.N.T.U.K, Academic

Advisor, Petroleum Courses) Dr. L. Vinod Babu (Head of the Department, Petrochemical

Eng., UCEK, JNTUK), Mr. J. Prabhakararao (Training and placement officer, RINL-AP)

and Mr. P. Rajeshvarma (Deputy Manager, CO &CCP).

We are also grateful to our Institute University College of Engineering (A), Kakinada,

for providing us with a wonderful opportunity to learn and work in such a world class facility.

I also thank my parents and all others who in one or the other way extended their support

and continuous inspiration which gave me strength and motivation to complete this training.

Submitted by

M.Yesuvaraprasad

(12021A2537)

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CONTENTS PAGE NO:-

1. LIST OF FIGURES 7

2. INTRODUCTION 8

2.1 Modern technology 8

2.2 Major plant facilities 8

2.3 Major departments in VSP 9

2.4 Various departments in brief 9

2.4.1 Raw material handling plant 9

2.4.2 Coke ovens and coal chemical division 9

2.4.3 Sinter plant 10

2.4.4 Blast furnace 10

2.4.5 Steel Melt Shop 10

2.4.6 Rolling mills 10

3. COKE OVENS & COAL CHEMICAL PLANT (CO&CCP) 11

3.1 Description of raw material (coal) 11

3.2 Coal preparation plant (CPP) 12

3.3 Coke oven batteries 14

3.4 Coke dry cooling plant (CDCP) 16

3.5 Coke sorting plant (CSP) 17

4. COAL CHEMICAL PLANT (CCP) 18

4.1 Primary gas cooler (PGC) 18

4.2 Ammonium sulphate plant (ASP) 20

4.3 Mechanical biological and chemical treatment plant (MBC) 25

4.4 Tar distillation plant (TDP) 27

4.4.1 Pitch cooling and loading area (PCLA) 32

4.4.2 Naphthalene anthracene fraction crystallization (NAFC) 34

4.5 Benzol distillation column (BDC) 38

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1. LIST OF FIGURES & TABLES

1. LIST OF FIGURES Page No.

1. Figure 4.1 Flow Diagram of Benzol distillation 35

2. Figure 4.2 Flow Diagram of H.R. Process 39

3. Figure 4.3 Flow Diagram of E.D. Process 46

2. LIST OF TABLES

1. Types of coal and properties 11

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2. INTRODUCTION

Rashtriya Ispat Nigam Limited (RINL) is the corporate entity of Visakhapatnam Steel

Plant (VSP), the first coastal based Steel Plant of India is located, 26 Km south west of city of

destiny i.e. Visakhapatnam.VSP has an installed capacity of 3Million Tons per annum of liquid

steel and 2.656 Million Tons of saleable steel. VSP has become the first integrated Steel Plant

in the country to be certified to all the three international standards for quality (ISO-9001), for

Environment Management (ISO- 14001) & for Occupational Health & Safety (OHSAS-

18001). 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.

2.1 MODERN TECHNOLOGY:

Visakhapatnam steel plant is the most sophisticated and modern integrated steel plant

in the country.

Among these are:

Selective crushing of coal Dry quenching of coke On ground blending of sinter base mix Conveyor charging and bell less top blast furnace Cast house slag granulation for blast furnace 100% continuous casting of liquid steel Gas expansion turbine for power generation utilization blast furnace top gas pressure Computerization for process control

2.2 THE MAJOR PLANT FACILITIES:

Coke oven batteries of 67 ovens each having 41.6 cu.m volume. Sinter machines of 312 m2 area. Blast furnace of 3200m3 useful volume. Steel melt shop with three L.D converters of 150 ton capacity each and 6 nos. of 4

strand continuous bloom casters have been provided.

Light and medium merchant mill of 710,000 tons per year capacity Wire rod mill of 850,000 tons per year capacity Medium merchant and structural mill of 850,000 tons per year capacity

The plant has in-plant power generation from a power plant having 3 nos. of 60MW

sets installed. Additional requirements of operational power, around 150MVA is being met

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from the APSEB grid. Operational power supply is initially at 220 KV, which is subsequently

stepped down to 400 KV.

2.3 MAJOR DEPARTMENTS IN VSP:

1. Raw materials handling plant 2. Coke ovens and coal chemical plant 3. Sinter plant 4. Blast furnaces 5. Steel melt shop 6. Rolling mills

a. LMMM (Light and medium merchant mill) b. WRM (Wire rod mill) c. MMSM (Medium merchant and structure mill)

2.4 VARIOUS DEPARTMENTS IN BRIEF:

2.4.1 Raw material handling plant:

VSP annually requires quality raw materials like iron ore, fluxes (dolomite,

limestone) coking coal and non-coking coal. It requires 12-13 million tons of raw

materials, which produce three million tons of liquid steel.

2.4.2Coke ovens and coal chemical division:

Coke is manufactured by heating of crushed coking coal in absence of air at a

temperature of 1000c and for about 16-18 hrs. A coke oven comprises of two hallow

chambers namely coal chamber and heating chamber. In heating chamber a gaseous fuel

such as gas, coke oven gas is burnt. The heat so generated is conducted through the

common wall to heat & carbonize the coking coal placed in the adjacent coal chamber.

Various features of coke oven are:

7 mts tall coke oven batteries. Coke dry cooling or quenching using Nitrogen. Recovery of BY-Product from coke oven gas by Distillation process. Twin fuel gas regenerative system for power generation from coke oven gas. Back pressure turbine station for power generation from coke oven gas. Selective coal crushing. High productivity. High capacity of coke ovens.

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2.4.3 Sinter plant:

Sinter is hard, porous ferrous material obtained by agglomeration of iron fines, coke

breeze, lime stone fines, metallurgical waste like dust, mill scale, LD slag. Usage of sinter in

B.F increase productivity by decrease in the coke rate and imposing the quality of hot metal

produced.

2.4.4 Blast furnace:

Hot metal is produced in blast furnace, which are tall and vertical. Raw materials

are Iron Ore, Coke, Dolomite, and Limestone. It is charged from the top and hot blast at 1100c-

1300c. there are two B.Fs each with a volume of 3200m3, each with 4 tap holes and with a

daily production of 9720 tons of liquid steel. The technical parameters are:

Effective volume 3200m3

Capacity 4860TPD

Height 33.1m

Number of tap holes 4

2.4.5Steel Melt Shop:

Steel is an alloy of iron with carbon up to 18%. Hot metal produced in B.F contains

impurities such as carbon (3.5-4.25%), silicon (0.4-0.5%, manganese (0.3-0.4%), sulphur

(0.04%max) and phosphorous (0.14%max). to improve the quality of steel, the impurities have

to be removed by oxidation process which is done in converter shop having 3 LD converters.

2.4.6 Rolling mills:

Blooms produced in SMS-CCD are shaped into Billets, Rounds, Squares, Angles,

Channels, Wire Rods, and Rein Forced Bars etc. by rolling LMMM, WRM, and MMSM.

Light and Medium Merchant Mill:

It consists of Billet Mill and Bar Mill. It is facilitated with 2 walking beam

furnaces of 200 TPH heating capacity and 2 stand roller hearth furnaces.

Wire Rod Mill:

It is a 4-stand mill. The mill has 4 zone combination type pre heating furnace

of 2100 TPH capacity. The mill products include rounds and ribbed wire in size of 5.5mm-

12.7mm dia. Wire rods are made in coil from having maximum weight of 1200kg.

Medium Merchant & Structure Mill:

This mill is installed at ground level and has the capacity of roll 8, 50,000 ton

of medium merchant products per annum. The feed materials to mill are 250mm x 250mm

blooms.

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3. COKE OVENS AND COAL CHEMICALS PLANT

3.1 Description of raw material (coal)

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 vegetation.

Owing to pressure, the streams have generally been crushed to an elliptical section and formed

coal. 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.

The different coking coals used in VSP are:

M.C.C - Medium coking coal - BENGAL,

I.C.C - Imported coking coal - AUSTRALIA

I.S.S.A.C - Imported coking coal - AUSTRALIA

SOFT - Imported coking coal - AUSTRALIA

TYPES OF COAL AND PROPERTIES:

S.NO.

TYPE OF COAL

% MOISTURE

% ASH

MEAN

MAXIMUM

REFLUTANCE

1. M.C.C 25-28 17-22 0.9

2. I.C.C 24-26 8-10 1.10-1.3

3. I.S.S.A.C 23-25 8-10 1.16-1.3

4. SOFT 30-34 8-10 0.9-1.0

Table 3.1 Types of coal and properties

COKE:

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.

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FUNCTIONS OF COKE:

It acts as heat producer in blast furnace It acts as reducing agent by carbon reduction in blast furnace with oxygen reaction. 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

NEED FOR MANUFACTURE OF COKE FROM COAL:

Natural coal is too dense and fragile to be used as a fuel in the furnace. Coal is nearly volatile matter free so it does not create problems of hot shortness and

coal shortness.

As compared to coal coke is of high quality and is highly reactive. Coke is highly porous mass. As coke is a rigid hard mass it does not create the problems of dust nascence. 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:

Coal Preparation Plant (C.P.P) Coke Oven Batteries Coke Dry Cooling Plant (C.D.C.P) Coke Sorting Plant (C.S.P) Coal Chemical Plant (C.C.P)

3.2 COAL PREPARATION PLANT (C.P.P.)

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. M.C.C - 15% 2. I.C.C - 40% 3. I.S.S.A.C.C - 35% 4. SOFT - 10%

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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:

Percentage 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 ash percentage present in coal. 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. Coke - 746 kgs 2. Crude Tar - 32 kgs 3. Crude Benzol - 6.9 kgs 4. Ammonia - 4.1 kgs 5. CO Gas - 150 kgs 6. Losses - 61 kgs

COAL PREPARATION PLANT CONSISTS OF FOLLOWING 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.

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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. -150 mm

particles are transferred to reversible conveyors.

DOGIMETERS:

These are arranged under each bin for blending the coal to produce quality of coke. In

the old dogimeters the doors at the chute were operated and coal discharge was controlled.

There occurred some jamming of coal. So vibrators were used to clear the jamming which is

removed now

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.

PNEUMATIC SEPERATION:

The separation and collection of required size materials by utilizing air pressure is

known as pneumatic separation. The crushed coal is subjected to pneumatic separator.

3.3 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 - Coal blend.

PRODUCT - Coke with 1-3% volatile matter.

BY-PRODUCT - 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-HOFFOMENS by-product oven type. On the roof

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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. 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. 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

Fig 3.1 Coke oven battery

period of 17-18 hrs by indirect heating with coke oven gas or mixed gas (CO gas + BF gas) in

absence of air at 1150c.

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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. 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 900c - 1200c is 65-75%.

Calorific value of different gasses used for heating purpose:

CV of Coke oven gas - 4200 K Cal /Nm3

CV of Blast furnace gas - 750 K Cal /Nm3

CV of Mixed gas - 1000 K Cal /Nm3

3.4 COKE DRY COOLING PLANT (CDCP)

After pushing the coke from the Battery with a temperature of 1050c, it is transferred

into CDCP for cooling purpose. The CDCP 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 CDCP. 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 55c by a mill fan. The hot coke is

cooled from 1050c to 180c-200c. 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 440c and 40

Kg/cm2. Cooled coke is conveyed to CSP and the steam is used to run back pressure turbine

station for producing power of 15 MW.

SPECIFICATIONS OF CDCP:

No of C.D.C.P - 3/3 Batteries No of chambers - 4 Chambers/each C.D.C.P Capacity of each C.D.C.P - 50-52 T/hr Coke temp before cooling - 1050C Coke temp after cooling - 180C-200C Coolant used - N2 gas Dry coolant temperature - 55C

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3.5 COKE SORTING PLANT (CSP)

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.

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4. COAL CHEMICAL PLANT (CCP)

Many by products are extracted from the coke oven gas at this department. It

consists of the following sections:

1. Exhauster house. 2. Ammonium sulphate plant. 3. M.B.C plant. 4. Tar distillation plant. 5. P.C.L.A 6. Naphthalene fraction crystallization. 7. Benzol plant.

Benzol distillation plant. Hydro refining. Extractive distillation.

4.1 PRIMARY GAS COOLER (PGC)

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. This provides any naphthalene condensate to drain easily. The main

purpose of PGC is to cool the gas from 90c to 30c.

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 90c 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 80-95c

Gas temperature after PGC 27-30c

Service water inlet temperature 32-33c

Service water outlet temperature 43c

Gas condensate flow to each PGC 10-15 Nm3/hr

Chilled water inlet temperature 11-12c

Chilled water outlet temperature 20c

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ELECTROSTATIC PRECIPITATOR:

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. These 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 getsticked 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 30,000 Nm3/hr

Voltage 70KV

Number of electrodes 148

ESP insulator boxes temperature 80C

EXHAUSTERS:

These are centrifugal fans necessary to drive the gas from the P.G.C 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 PGC. 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 55-60c.

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 67,000-76,000Nm3/hr

Power 1250KW

Maximum suction at inlet 500mm WC g

Maximum pressure at delivery 2700mm WC g

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CONDENSATE PUMP HOUSE:

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.

4.2 AMMONIUM SULPHATE PLANT (ASP)

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 60-70c. 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

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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)

NH4 (HSO4) + NH3 (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 at 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.

The ammonium sulphate crystal settled at the conical portion of the tank, which is wet with

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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. The air is heated to 120-150c by using

steam and this hot air is fed form the bottom of the screen. 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.

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.

23

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.

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 to 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.

(NH4)2S 2NH3 + H2S

NH4CN NH3 + HCN

(NH4)2CO3 2NH3 + CO2 + H2O

NH4HCO3 NH3 + H2O + CO2

NH4HS 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 rhodonidesand

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.

24

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.

GAS PRE-COOLER:

The gas at 30-35c is fed to the pre-cooler, which is indirect contact cooler. The

gas is cooled by chilled water (150c), which passes through the tubes. The gas is cooled to 25-

27c 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 drawback 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.

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.

COMPRESSOR HOUSE:

Compressor house provided in the recovery plant is used to supply the plant air and

instrument air.

25

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/cm2. 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. Air initially fed to the low-pressure side where it is

compressed to 2 kg/cm2. 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.

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.

4.3 MECHANICAL BIOLOGICAL AND CHEMICAL TREATMENT

PLANT (MBC)

PROCESS:

Toxic effluents generated in various sections of coke oven and by-product 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

26

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 90c with the help of recalculating cooling water. Water inlet temperature is 34c and

outlet temperature is 45c. Mixed effluent at a temperature of about 50c 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 35-38c. Cold circulating water is used for this purpose.

27

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 its 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.

4.4 TAR DISTILLATION PLANT (TDP)

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 cooler of

benzol recovery is transferred to tar and oil storage. From there it is pumped to mechanized

decanter in tar distillation plant.

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COMPOSITION OF COAL TAR COMING TO TDS:

Light oil 0.5%

Phenol fraction 1-1.5%

Naphthalene 5-6%

Wash oil 8-9%

Anthracene oil 18-19%

Moisture 5-6%

Pitch 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 this, the soda ash solution of 8% concentration is injected at

the suction side of the pumps.

2NH4Cl + Na2CO3 ---------- > 2NaCl + 2NH3 + CO2 + H2O

2HCl + Na2CO3 ---------- > 2NaCl + H2O + CO2

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 120-130c. Controlling the airflow can control

29

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 400c. The temperature of the tar

should not increase beyond 400c 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

120-130c. 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.

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 380-400c. This tar is fed

to the second stage evaporator at the rate of 10 m3/hr.

II STAGE 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.

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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 360-380c.

As the vapors rises to the top of the column different fractions are obtained from

different trays. Initially vapors enter at about 360-400c. 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 100c. 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 160-180c. 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 190-200c. 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 280-290c. It is tapped from 6 & 8 trays. Anthracene II is heavier

than all fractions. So it is obtained from 1 stor 2 ndtrays at 320-330c.

31

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 102-50c.

The vapors of second stage coolers from 100-40c. 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 from 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 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

80-50c are sent to the collected tanks.

START UP TANK:

During initial starting of the plant, the temperature of the tar does not reach up

400c in second stage evaporator will be diverted to start up tank until it reaches the temperature

of 290c similarly column bottom liquid will also be diverted to start up tank until the

temperature of the column reaches to 330c. 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.

32

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.

4.4.1 PITCH COOLING AND LOADING AREA (PCLA)

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 65-70c. 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

33

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 65-70c. To increase the softening point of

the pitch up to 95-110c 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 360c 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 400c. 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.

34

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 170-180c. 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.

4.4.2 NAPHTHALENE FRACTION CRYSTALLIZATION

Naphthalene fraction obtai3ned from the distillation column is 75-80% 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 70-80C 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 85-90C. 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 an 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

35

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

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 60-65c 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.

36

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 kg and 98% pure. 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 godown.

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.

37

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.

38

4.5 BENZOL DISTILLATION COLUMN

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.

Figure 4.1 Flow Diagram of Benzol 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

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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 130-135c.

Tube side Shell side

Oil dephlegmator vapors of stripping column BO

Oil-oil exchangers DBO from stripping column BO

Steam pre-heaters medium pressure steam 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

180c 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 92-95c 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

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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 110-115c

Bottom temperature 120-130c

No. of trays 23

Feed tray 17th tray

Pressure in the column 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 here 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 80-85c is fed to the 6th 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 70-80c

Bottom temperature 115-120c

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 120C

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Bottom temperature 140C

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 45-50C. 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 45-50C. 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 80-90C.

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, non-aromatics and residue. Initially, the LCB is purified from

sulphur, non-aromatics and other compounds. This consists the following sections. They are,

De-fronting section

Reaction section

Purification section

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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 70c.

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.

Figure 4.2 Flow Diagram of H.R. Process

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 de-fronted crude benzol is fed to the reaction section through

feed pre-heater. 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

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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 70c and this is fed to the

intermediate storage.

Feed rate to the column 3T/hr

Pressure in the column 0.5 kg/cm2

Sulphur content in the feed 2000-1800 ppm

Sulphur content in DCB 1200 ppm

No. of bubble cap trays 30

Boiling point of CS2 45

Temperature at the top of the column 55-65c

Column bottom temperature 105c.

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 160-165C by means of the main reactor effluent

passing through shell side. The feed at a temperature of 160-165C is fed to the third mixing

nozzle of stage evaporator.

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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 210C by the main reactor effluent. E-402 and E-403 are

heated by hot oil through tubes at a temperature of 250C. 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 210C. 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.

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PRE-REACTOR:

The vapors from the top of the evaporator at 180C are heated in a heat exchanger E-

406 to 190-225C 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 190-225C and this depends on the life

cycle of the catalyst. Due to the exothermic reaction the outlet temperatures increases to 200-

235C. 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 10C. Catalyst can be

regenerated by heating the bed with steam and air. The reactions in the pre-reactor

Diolefins + H2 mono olefins

CnH2n-2 CnH2n

Cyclopentadiene + H2 cyclopentane

C5H6 C5H8

Styrene + H2 ethyl benzene

C8H8 C9H10

Carbon disulphide + H2 methane + H2S

CS2 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 270C and the outlet

temperature is 330C 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

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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 + H2 Paraffin

Ethyl mercaptans + H2 Ethane + H2S

Thyopene + H2 Butane + H2S

Coumarone + H2 Ethyl benzene + H2

Pyridine + H2 Pentene + H2

Pyridine + H2 Butane + H2

Benzene + H2 Cyclohexane

Toluene + H2 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 is at 330c. 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 50c 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. unreacted 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 340-350c. The hot oil is pumped by P-404 pump. The oil at

temperature of 340c is fed to the HR unit by using another pump. This is again recycled to the

suction side of P-404.

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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