Final Report Avishek

8/6/2019 Final Report Avishek

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A REPORT

ON

Process Study and Water Balance in Urea-I plant

BY

Pratim Palit 2007A1PS228G

Ganta Avishek 2007A1PS465G

AT

Chambal Fertilizer & Chemicals Limited

A Practice School-I station of

BIRLA INSTITUTE OF TECHNOLOGY & SCIENCE, PILANI

JUNE, 2009


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A REPORT

ON

Process Study and Water Balance in Urea-I plant

BY

Pratim Palit 2007A1PS228G B.E.CHEMICAL (Hons.)

Ganta Avishek 2007A1PS465G B.E. CHEMICAL (Hons.)

Prepared in partial fulfilment of the

Practice school I

BITS C221

AT

Chambal Fertilizer & Chemicals Limited

A Practice School-I station of


JUNE, 2009


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I

PREFACE

Industrial Training refers to work experience that is relevant to professional development prior to

graduation. Industrial Training is an essential component in the development of the practical andprofessional skills required of an Engineer and an aid to prospective employment.

Industrial Training Objectives

y To expose students to engineering experience and knowledge which is required in industry,where these are not taught in the lecture rooms.

y To apply the engineering knowledge taught in the lecture rooms in real industrial situations.y To use the experience gained from the Industrial Training in discussions held in the lecture

rooms.

y To get a feel of the work environment.y To gain experience in writing reports in engineering works/projects.y To expose students to the engineers responsibilities and ethics.y To expose the students to future employers.y With all the experience and knowledge acquired, it is hoped at the students will be able to

choose appropriate work upon graduation.

This report was prepared to present the process, functions and operations of various equipments

used and to verify the water used and utilized in Urea I plant of Chambal Fertilizers and Chemicals

Limited, Gadepan.

Last but not the least; we would like to say that by this vocational training we tried to imbibe all the

important aspects that an industry requires for befitting its work. We once again thank all those who

helped us in this endeavours.


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II

Acknowledgement

We wish to thank Mr. A.K. Saxena (Manager, HR), for providing us this valuable opportunity to

pursue practical training at Chambal Fertilizers & Chemicals Limited, Gadepan.

We would also like to thank Mr. Kaushik (Manager, HR), Mr. Kapil Mittal (Sr. Manager, Urea-I), for

their support and guidance.

We are also indebted to all those who are directly or indirectly involved in making this training a

success and who have given us the chance to develop our project in detail.

We take this chance to express our sincere gratitude to our PS-I Instructors Dr. Amit Dubey and Mr.

Irfan Basha Shaik for his valuable help and support.

This training, as a whole, is a great learning experience for us due to all these people and we hope

this valuable experience would help us to frame our career.

Date: July 16, 2009

Place: Gadepan, Kota


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III


Practice School Division

Station: Chambal Fertilizer & Chemicals Limited

Centre: Kota

Duration: From May 25th

, 2009 To: July 17th

, 2009

Date of Submission: July 16th

, 2009

Title of the project: Orientation report on basic industrial functioning at Chambal Fertilizer

& Chemicals Limited

By

2007A1PS465G Pratim Palit B.E. (Hons.)Chemical

2007A1PS465G Ganta Avishek B.E. (Hons.)Chemical

Name of Experts: Mr. Kapil MIttal ( Manager Urea I)

Name of the PS-1 Faculty: Dr. Amit Dubey and Mr. Irfan Basha Shaik

Project Area: Urea I plant


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IV

ABSTRACT

This report gives an overview of the processes involved in the Urea I Plant and verification and

calculations of the water balance in the Urea I plant. The report is based on the information given

to us by our Project Mentor Mr. Kapil Mittal and the material gathered from Documentation

Section. Most of the values (Water flow rates and weight percentages and details taken are fromthe Snamprogetti Manual which are theoretical. The report was prepared as an essential

component of the Practice-I school programme of Birla Institute of Technology & Science.

Signatures of Students Signature of PS Faculty

Date Date


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0

Table of Contents

S.NO TITLE Page No.

Preface I

Acknowledgement II

Abstract IV

Table of Contents 0

1 Plant Description 1

1.1 Description of Process 1

1.1.1 Urea synthesis and high pressure recovery 1

1.1.2 Urea purification and low pressure recovery 4

1.1.3 Urea Concentration 6

1.1.4 Urea Prilling 6

1.1.5 Waste Water Treatment 7

1.1.6 Auxiliary Installations 8

1.1.7 Steam Networks 91.2 Description of Equipment 11

2 Stabilization of Running Condition 27

2.1 H.P.Section 27

2.2 M.P.Section 27

2.3 L.P.Section 28

2.4 Evaporation and Prilling Section 28

2.5 Waste Water Treatment Section 28

3 Water Balance 29

3.1 Water Balance for each urea unit before waste water

treatment plant

29

3.2 Calculation of Water Loss 313.3 Water balance for the waste water treatment plant 32

3.4 Actual Readings 33

Conclusion 34

Bibliography 35


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1)PLANT DESCRIPTION

1.1) Description of the process

The Urea production process takes place through the following main operation:

y Urea synthesis and high pressure recoveryy Urea purification and low pressure recoveryy Urea Concentrationy Urea Prillingy Waste Water Treatment

1.1.1) Urea synthesis and high pressure recoveryUrea is produced by synthesis from liquid ammonia and gaseous CO2 . In the reactor 11-R-1

the ammonia and carbon dioxide react to form ammonium carbamate, a portion of which

dehydrates to form Urea and Water. The reactions are as follows:

Carbamate Formation

2 NH3 + CO2 NH2 COO NH4 + Heat

(Ammonia) (Carbon Di-oxide) (Ammonium Carbamate)

Dehydration

NH2 COO NH4 NH2 CO NH2 + H2O - Heat

(Ammonium Carbamate) (Urea) (Water)

In synthesis conditions ( T = 188oC, P = 156Kg/Cm

2) the first reaction occurs rapidly and is

completed, the second reaction occurs slowly and determines the reactor volume.

The fraction of ammonium carbamate that dehydrates is determined by the ratio of the

various reactants, the operating and the stay time in the reactor. The molar ratio of ammonia

to carbon dioxide is around 3.6:1.


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The molar ratio of water to carbon dioxide is around .67:1.

The liquid ammonia coming directly from the battery limits (t1-1-113 + 12oC), is collected in

the ammonia receiver tank 11-V-1.

From V-1 it is drawn and compressed at about 22.4Kg/Cm2g pressure by means of

centrifugal pump P-5 A/B.

Part of the ammonia is sent to the medium pressure absorber 11-C-1; the remaining part

enters the high pressure synthesis loop.

The ammonia feeding the synthesis loop is compressed by two low-speed, heavy duty

reciprocating pumps 11-P-1 A/B at a pressure of about 240Ata. Before entering the reactor

the ammonia feed is used as motive fluid in the carbamate ejector 11-EJ-1, where the

carbamate coming from the carbamate separator 11-MW-1 is compressed up to the synthesis

pressure.

The liquid mixture of ammonia and carbamte enters the reactor where ammonia reacts with

the compressed carbon dioxide.

The carbon dioxide drawn at the Urea plant battery limits at about 1.6 Ata pressure and about

40oC temperature enters the centrifugal compressor 11-K-1 and leaves it at a pressure of

about 160Ata.

A small quantity of air is added to the carbon dioxide at the compressor`s suction in order to

passivate the stainless steel surfaces, thus protecting them from corrosion due to both reagent

and reaction product.

The reaction products leaving the reactor flow to the steam heated falling stripper 11-E-1,

which operates at the same pressure as the reactor. The mixture is heated up as it flows down

the falling film exchanger. The carbon dioxide content of the solution is reduced by the

stripping action of the ammonia as it boils out of the solution.

The carbamate decomposition heat is supplied by 24Ata saturated steam. The overhead gases

from the stripper and the recovered solution for the medium pressure absorber 11-C-1, all

flow to the high pressure carbamate condensers 11-E-5 where the total mixture, except for

few inert, is condensed and recycled to the reactor by means of ejector 11-EJ-1.

The condensation of gases at high pressure and temperature permits the production of 4.5 Ata

steam in the high pressure carbamate condenser.

In the carbamate separator 11-MV-1 the incondensable gases, consisting of inert gases

containing a little quantity of ammonia and carbon dioxide unreacted in the condenser, are

separated from the carbamate solution and sent to the medium pressure decomposer 11-E-2.


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1.1.2) Urea purification and low pressure recoveryUrea purification takes place in the two stages at decreasing pressure as follows:

- 1st Stage at 18Ata pressure- 2nd Stage at 4.5 Ata pressure

It is pointed out the exchangers where Urea purification occurs are called decomposers

because in this apparatus the residual carbamate decomposition takes place.

1st

purification and recovery stage at 18Ata

The solution, with a low residual CO2 content, leaving the bottom of the stripper is expanded

at the pressure of 18Ata and enters the medium pressure decomposer 11-E-2 (falling film

type).

This equipment is divided into two parts:

- Top separator 11-MV-2 where the released flash gases are removed before the solutionenters the tube bundle.

- Decomposition section where the residual carbamate is decomposed and the requiredheat is supplied by means of 24Ata steam condensate flowing out of the stripper.

The ammonia and carbon dioxide rich gases leaving the top separator are sent to the medium

pressure condenser 11-E-7 where they are partially absorbed in aqueous carbonate solution

coming from the recovery section at 4.5Ata.

The absorption heat is removed by cooling water.

A tempered water circuit is provided to prevent carbamate solidification and to keep a

suitable cooling water temperature at the medium pressure condenser inlet recirculating the

cooling water by means of pump 11-P4.

In the condenser, Carbon Dioxide is almost totally absorbed.

The mixture from 11-E-7 flows to the medium pressure absorber 11-C-1 where the gaseous

phase coming from the solution enters the rectification section.

This is of bubble-cap trays type and performs C02 absorption and ammonia rectification.


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The trays are fed by pure reflux ammonia which eliminates residual C02 and water contained

in the inert gases.

Reflux ammonia is drawn from the ammonia receiver and sent to the column 11-C-1 by

means of centrifugal pump 11-P-5 A/B.

A current of inert gases saturated with ammonia with the same ppm of C02 residue comes out

from the top of the rectification section. The bottom solution is recycled by pump 11-P-2 A/B

to the synthesis recovery section.

Ammonia with inert gases leaving the column top is partially condensed in the ammonia

condenser E-9 A/B. From here the liquid and gaseous ammonia phases are sent to the

ammonia receiver 11-V-1.

The inert gases, saturated with ammonia, leaving the receiver, enter the ammonia recovery

tower 11-C-5 where an additional amount of ammonia is condensed by the cold ammonia

coming from the Urea plant battery limits.

The condensed ammonia is recovered in the 11-V-1.

The inert gases, containing residue ammonia, are sent to the medium pressure falling film

absorber 11-E-11, where they meet a countercurrent water flow which absorbs gaseous

ammonia.

The absorption heat is removed by cooling water. From the bottom of 11-E-11 the water

ammonia solution is recycled back to the medium pressure absorber 11-C-1 by means of the

centrifugal pump 11-P-7 A/B.

The upper part of the medium pressure absorber consists of three valve trays (11-C-3) where

the inert gases are submitted to a final washing by means of the same absorption water.

In this way the inerts are collected to blow-down practically free from ammonia.

2nd

purification and recovery stage at 4.5 Ata

The solution leaving the bottom of medium pressure decomposer is expanded at 4.5Ata

pressure and enters the low pressure decomposer 11-E-3 (falling film type).

This is divided into two parts:

- Top separator (11-MV-3) where the released flash gases are removed before the solutionenters the tube bundle.

- Decomposition section where the last residual carbamate is decomposed and the requiredheat is supplied by means of 4.5 Ata saturated steam.


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The gases leaving the top separator are sent to the low pressure condenser 11-E-8 where they

are absorbed in an aqueous carbonate solution coming from the waste water treatment

section.

The absorption and condensation heat is removed by cooling water.

From the condenser bottom, the liquid phase, with the remaining inert gases, is sent to the

carbonate solution tank 11-V-3. From here the carbonate solution is recycled back to the

medium pressure condenser 11-E-7 by means of the centrifugal pump 11- P-3 A/B.

The inert gases, which essentially contain ammonia vapor, washed in the low pressure inert

washing tower 11-C-4, are collected to blow down practically free from ammonia.

1.1.3)Urea Concentration Section

As it is necessary, in order to prill Urea, to concentrate the Urea solution up to 99.8% wt, a

vacuum concentration section in two stages is provided.

The solution leaving the low pressure decomposer bottom with about 71% with Urea is sent

to the first vacuum concentrator 11-E-14 operating at a pressure of 0.3Ata.

The mixed phase coming out of 11-E-14 enters the gas liquid separator 11-MV-6, from where

vapors are extracted by the first vacuum system 11-ME-4, while the solution enters the

second vacuum concentrator 11-E-15 operating at a pressure of 0.03Ata.

The concentrations are fed by the saturated steam at 4.5Ata.

The mixed phase coming out of 11-E-15 enters the gas liquid separator 11-MV-7 from where

vapors are extracted by the second vacuum system 11-ME-5.

1.1.4)Urea Prilling

The melted Urea leaving the second vacuum separator is sent to the prilling bucket 11-ME-8

by means of centrifugal pump 11-P-8 A/B.

The Urea coming out of the bucket in the form of drops falls along the prilling tower 11-ME-

6 and encounters a cold air flow which causes its solidification.

The solid prills falling to the bottom of the prilling tower are sent into the belt conveyor 11-

MT-1 by the rotary scraper 11-ME-10. From here they are sent through screeners 11-ME-11


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to retain lumps only, and then to belt conveyor 11-MT-3 which carries the product to the

automatic weighing machine 11-WD-1 and to the Urea storage section.

Urea lumps by means of belt conveyor 11-MT-2 are recycled back to the underground tank

11-V-4 where they are dissolved.

1.1.5) Waste Water Treatment

The condensed vapors from the overhead vacuum systems, containing ammonia and carbon

dioxide, are collected in the waste water tank 11-V-9. In this tank the drain waters collected

in the tank 11-V-7 are fed by means of pump 11-P-12 A/B.

From here they are pumped by 11-P-21 into the buffer waste water tank V-6 and then, by

means of P-16 they are pumped into the waste water distillation tower 11-C-2 operating at a

pressure of about 2.5Ata. Before entering the distillation tower 11-C-2 the solution is

preheated in the exchanger 11-E-18 A/B by means of the purified water flowing out of the

tower bottom.

Since the solution is contaminated by Urea, after a first stripping in the upper part of the

tower it is pumped by 11-P-14 A/B into the hydrolyzer 11-R-2 where the Urea is decomposed

by means of steam at 38Ata. Before entering the hydrolyser, the solution is preheated in the

exchanger 11-E-19 with the solution coming out of the hydrolyzer.

The vapors produced in the hydrolyzer are sent to the bottom of the upper part of distillationtower 11-C-2, while the solution returns to the top of the lower part of 11-C-2 tower, in

which the remaining ammonia is stripped out by means of 4.5 Ata saturated steam fed to the

column bottom.

The vapor leaving the top of the tower are condensed in the overhead condenser 11-E-17

from where the carbonate solution flows to the reflux accumulator 11-V-8. Part of this

solution is sent by P-15 A/B to the top of distillation tower 11-C-2 as reflux, the balance is

recycled back to L.P. condenser E-8.

The purified waste water from the bottom of 11-C-2 is sent by P-18 A/B to B.L. after being

cooled in 11-E-20 A/B exchanger.

During start up and transistors, the water is recycled to 11-V-6 until it contains only traces of

ammonia.


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1.1.6) Auxiliary installations

To make the plant operation easier, the following auxiliary installations have been provided:

- Urea lumps dissolving tank 11-V-4.The buried tank 11-V-4, with a capacity of approx. 12 cu.m. is used to collect Urea

solution drains and to dissolve lumps.

This tank is equipped with stripper 11-ME-13 and immerged pumps 11-P-19 A/B to

recover the solution.

The possibility of heating the solution with 4.5 Ata saturated steam has also been

envisaged.

- Urea solution tank 11-V-5The tank 11-V-5 (580cu.m. capacity) is used to collect both the 71% Urea solution in

case of concentration section failure and the melted Urea in case of prilling section

failure. In the 11-V-5 it has also been envisaged to recover the Urea solution coming

from tank 11-V-4 after being filtered though filters 11-FL-1 A/B. The solution contained

in the tank 11-V-5 can be heated by means of 4.5Ata saturated steam.

- Urea solution pumps 11-P-9 A/BThese pumps suck the solution from 11-V-5 and recycle it to the first vacuum

concentration 11-E-14.

- Drain collecting tank 11-V-7The buried tank 11-V-7 having a capacity of approx. 12 cu.m. is used to collect the

different drains of the plant containing ammonia and carbon dioxide.

The tank is equipped with immerged pumps 11-P-12 A/B to send the solution back to the

waste water tank 11-V-9.


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1.1.7) Steam Networks

Three stream networks are provided in the Urea Plant area:

- Superheated steam network at P = 106Ata T= 510 oC- Superheated steam network at P =38Ata T= 381oC- Superheated steam network at P = 25Ata T= 340oC- Saturated steam network at P = 24Ata T= 220oC- Saturated steam network at P = 4.5Ata T= 147oC

1) Superheated steam network at P= 106 Ata T = 510oCThis steam is available at Urea Plant battery limits and feeds the carbon dioxide compressor

turbine TK-1.

Furthermore, it feeds the 25Ata steam network in case of failure of the turbine 11-TK-1

extraction steam.

2) Superheated steam network at P =38Ata T= 381 oCThis steam is available at Urea Plant battery limits and feeds the hydrolyzer 11-R-2.

3) Superheated steam network at P = 25Ata T= 340oCThis steam is obtained by the extraction of the carbon dioxide compressor turbine or fed by

the 106Ata steam in case of turbine failure.

4)Saturated steam network at P = 24Ata T= 220

o

C

This steam is obtained by desuperheating the 25Ata steam it is used in the stripper 11-E-1.

In particular cases it can feed the 4.5 Ata network after depressurization.

The condensate coming from the 11-E-1 is collected in vessel 11-MV-4 and utilized in the

M.P. decomposer 11-E-2.


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The condensate coming from the 11-E-2 is used to feed carbamate condenser 11-E-5 shells.

5) Saturated steam network at P = 4.5Ata T= 147oCThe steam of this network is produced in boiler 11-E-5 and is utilized in the following

equipment:

y L.P. decomposer 11-E-3y First Vacuum Concentrator 11-E-14y Second Vacuum Concentrator 11-E-15y Vacuum system ejectorsy Bottom of distillation tower C-2

The condensate coming from 11-E-3, 11-E-14, 11-E-15 are collected in tank 11-V-2 at

atmospheric pressure; Since, during normal operating conditions, the 4.5Ata steam

production is higher than the relevant steam consumption, the excess steam is returned to

Urea plant battery limits.


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1.2) Description of equipment

A list is given here below of the main equipment of Urea plant as well as the operation of

each of them.

y Ammonia Receiver 11-V-1Function

In this tank the ammonia recovered during plant shut down is stored when emptying of high

pressure equipment is requested. Ammonia condensed in the recovery system is also stored.

Ammonia coming from the battery limits is measured and sent to the tank before being

delivered to the reactor.

Operation

In order to recover all ammonia in case of plant shut down the tank is to be normally kept at a

minimum level.

The rate of ammonia from the battery limits is recorded by 11-FR-104 and should be equal to

the amount of ammonia consumed by the plant in normal operation.

y Ammonia Booster Pump 11-P-5 A/BFunction

The booster pump sends ammonia both to the suction of the reciprocating pump which feeds

the reactor and to the M.P. absorber 1 0C-1 as reflux.

Operation

The booster pump is equipped with mechanical seal flushed with ammonia.

The pump is of the single stage type and is electric motor operated.


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It has been designed for a differential pressure of 4.9Kg/Cm2

.

It has not been designed to pump water at full charge.

y Ammonia reactor feed pump 11-P-1 A/BFunction

The pump is designed to pump liquid ammonia into reactor through carbamate ejector.

Operation

The pump is operated by electrical motor. IT has been designed for a differential pressure of

219 Kg/Cm2. A torque convertor is used to control the rate.

y Carbamate Ejector 11-EJ-1Function

It recycles carbamate in the reactor. Liquid ammonia is the motive fluid. In the ejector the

high pressure of liquid ammonia is transformed into high speed to let carbamate enter the

reactor.

Operation

The pressure controller 11-PTC-12 sets the position of the ejector needle. The rate of

carbamate sucked by ejector depends on the flow rate of motive fluid and from its pressure at

ejector inlet. If the level in separator 11-MV-1 increases, with reactor outlet valve completely

open it means that the ammonia recycle more solution to the reactor. However ammonia must

not be used at a pressure higher than required, to keep the normal level in MV-1 for saving

energy.


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y Urea Reactor 11-R-1Function

The reactor has a volume designed to allow the holding time necessary for carbamate

decomposition into Urea and water. Reactor is equipped with trays. Their function is to

prevent escape of gaseous CO2 which must react in the lower part of the reactor with

ammonia and prevent internal recycle of reaction products with higher specific gravity former

in the higher part of the reactor.

Operation

For a correct operation, the reagent rates to the reactor must be as constant as possible; this

will ensure the evenness of the reactor temperature, molar ratio, conversion ratio and

production. During heating, quick changes in reactor temperature must be avoided to prevent

possible ruptures in lining welds. Reactor must be heated up to 1500c with steam prior to send

liquid ammonia. Inspection holes are provided on the reactor shell to detect possible leakages

be noticed through these holes the plant must be immediately shut-down. To protect the

reactor, in case a sudden pressure increases an automatic system has been provided to stop

fluids inlet. Prior to any restart of the unit after repairing, the hydraulic test should be carried

out again.

y Stripper 11-E-1Function

Reaction products leaving the reactor are sent in to stripper for carbamate decomposition.


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Operation

The temperature of the solution leaving the stripper is kept between 205 and 2100C by

varying the steam pressure values at the stripper shell. At this temperature approx. 80% of

carbamate present in the solution is decomposed.

y Carbamate Condenser Kettle type 11-E-5Function

Gases leaving the top of the stripper 11-E-1 are mixed with carbonate coming from the

bottom of column 11-C-1 where they are condensed. L.P. steam is produced on the shell side

of condenser 11-E-5 and it is fed to the 4.5 Ata steam network.

Operation

Temeparture of carbamate condensation in E-5 is determined by shell side steam pressure

which is kept constant by pressure controller on L.P. steam network. Any variation of

condensation is indicated by variations in the L.P. steam system.

y Carbamate separator 11-MV-1Function

Its function is to separate the liquid from inert gases and maintain in a liquid head on the

ejector. When level in 11-MV-1 increases, HIC/3, positioned between reactor outlet and

stripper, will be opened, causing a small pressure drop and increasing carbamate circulation.


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y Medium Pressure decomposer 11-E-2Function

Its function is to decompose the carbamate remaining in the outlet stripper solution in order

to increase Urea conc. Up to 60.65% wt. This reaction requires heat which is supplied by the

condensate coming from the stripper and by additional steam. During start-up and shut-down

it is possible to use 24Ata steam as heating medium.

Operation

The bottom operating temperature of process solution is determined by the conc. Of the Urea

needed at the outlet. Ammonia percentage at bottom outlet is 67% wt. These conc. Are

obtained with a temp. of approx. 1580C and a pressure of 18Ata in 11-ME-2. If the operating

pressure is higher, also the temperature must be higher. The level of the solution at the outlet

is controlled by 11-LIC/101. Overhead gases are partially condensed in condenser 11-E-7 and

recovered in column 11-C-1.

y Low pressure decomposer 11-E-3Function

Its return is to decompose residual carbamate from 11-E-2 and separate it from the Urea

solution.

Operation

The final operating temperature is determined by the conc. of Urea solution needed at the

outlet. Ammonia percentage at the outlet is 1:2% wt. CO2 is 0.3:1.1%wt. These

concentrations are obtained with a temp. of approx. 1380C and an operating pressure of 4.5

Ata. If pressure is higher, the operating temperature must be increased accordingly.


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y M.P. Condenser 11-E-7Function

Its function is to condense part of the gases coming from the 11-E-2.

Operation

Carbonate solution coming from 11-P-3 A/B enters the condenser where it is mixed with the

gases coming from 11-MV-2. Reaction heat is removed by cooling water. The inlet temp. of

cooling water is maintained above 380C by recirculating a certain amount of the C.W.

leaving the condenser. 11-TIC/105 controls inlet temperature by acting on the valve installed

on recirculating pump delivery line. The purposes of this control system is to prevent

ammonium carbonate crystallization in 11-E-7.

y M.P. Absorber 11-C-1Function

CO2 absorption.

Operation

NH3-CO2-H2O mixture partially condensed from 11-E-7 enters the bottom of the column.

Gases consisting of NH3, CO2, H2O and inerts, rising from the bottom, are absorbed by cold

liquid ammonia reflux in the upper section of the column. Carbon dioxide and water

condenses as ammonium carbonate and fall back to the bottom. Condensation heat is

removed by the evaporation of NH3 coming from the 11-P5 A/B. Thus a current of inert

gases saturated with ammonia and containing just a few ppm of CO2 leaves the top of the

rectification section. The bottom level and temperature must be kept constant. In the event

that the trays are clogged with carbonate, they can be detected by means of thermocouples


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mounted in the tray. The level of carbonate in the column can be observed through the sight

glasses mounted in the lower part and it is controlled by LIC-102.

y O.H. Ammonia Condenser 11-E-9 A/BFunction

Its function is to condense NH 3 vapors coming from column 11-C-1.

Operation

Gaseous ammonia coming from the head of column 11-C-1 is condensed in this water cooler

and subsequently recovered in the receiver 11-V-1. Particular attention must be paid in the

operation of column 11-C-1 to avoid carbonate carry over which could clog the condenser.

y M.P. Ammonia Absorber 11-E-11 and M.P. Inerts GasWashing Tower 11-C-3

Function

Its function is to absorb in water, the vapors coming from 11-V-1.

Operation

Non condensed NH3 vapours from 11-E-9 A/B are absorbed with cooled steam condensate in

11-E-11 and 11-C3 where an ammonia solution will form and subsequently it will be sent

between the third and fourth tray 11-C-1 column. Level in 11-E-11 is controlled by 11-LIC-

103. Inerts from 11-C-3 overhead are sent to blow-down under control of PRC-108.


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y L.P. Condenser 11-E-8Function

Its function is to condense NH3-CO2-H2O vapors coming from low pressure separator 11-

MV-3.

Operation

Gases are absorbed by the solution coming from 11-P-15 A/B. The solution which has

formed in 11-E-8 is sent into tank 11-V-3. The heat of solution is removed with cooling

water.

y Carbonate solution tank11-V-3Function

It is used for the recovery of NH3 and CO2 in case of shut-down accompanied by emptying of

the high pressure equipment. Normally it collects the diluted carbonate solution coming from

11-E-8 and possible solution discharge from column 11-C-1.

Operation

The diluted carbonate solution, collected in the tank, is recycled in 11-E-7. The tank level 11-

LI-133 must be kept low in order to recover all carbonates in case of shut-down. Tank

pressure is controlled by 11-PIC-133.


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y L.P. Inert Gas Washing Tower 11-C-4Function

Its function is to absorb with water NH3 vapours from 11-V-3.

Operation

Non condensed vapours from 11-E-8 are absorbed with water in 11-C-4 where an ammonia

solution will form. The ammonia solution is collected in 11-V-3.

y High pressure carbonate solution pump 11-P-2 A/BFunction

Its function is to pump carbonate from column 11-C-1 to carbamate condenser 11-E-5.

Operation

The liquid is sucked from the bottom of column 11-C-1 and sent to 11-E-5.

The pump is centrifugal high speed type. The minimum flow through the pump is controlled

by 11-FIC-34. The materials contacting the pumped liquid are in 316 L. The materials

contacting the pumped liquid must be controlled to prevent crystallization. The differential

pressure in the normal conditions is about 140Kg/Cm2.

y Medium Pressure Carbonate Solution Pump 11-P3 A/BFunction

Its function is to pump carbonate form 11-V-3 to 11-E-7.


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Operation

The liquid is sucked from bottom 11-V-3 at a temperature of 420C. The pump, electrical

motor driven, has been designed for a suction temperature of pumped liquid up to 1000C, for

a differential pressure of 20Kg/cm2.

y NH 3 Solution Pump 11-P-7 A/BFunction

Its function is to pump ammonia solution from 11-E-11 to 11-C-1 to perform the trays

cleaning.

Operation

The pump is electrical motor driven and it is designed for a differential pressure of 5.5Kg/cm2

and a suction temperature from 300C to 60

0C. It is fitted with a mechanical seal flushed with

process fluid.

y Urea Melt Pump 11-P-8 A/BFunction

Its function is to pump melted uea from 11-ME-7 to the prilling bucket.

Operation

The pump is driven by an electric motor; it is made of AISI 304 L and designed for a suction

temperature between 134 and 1700C at a differential pressure of 14Kg/cm

2. It is equipped

with mechanical seal heated with steam and flushed with melted Urea. It must be washed

with water after each shut-down. The level of melted Urea in 11-ME-7 is controlled by 11-

LIC-138.


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y 1stVacuum Concentrator 11-E-14Function

Partial evaporation of water present in Urea solution coming from the last recovery section of

the unit.

Operation

Heat for water evaporation is supplied by steam. The evaporator is fed with Urea solution at a

temperature of about 820

C.

y 2ndVacuum Concentrator 11-E-15Function

Total evaporation of water remaining in Urea production, coming from 11-MV-6.

Operation

Heat for water evaporation is supplied by steam. It is fed with Urea solution at a temperature

of about 1280C.

y 1stVacuum Separator 11-MV-6Function

Its function is to separate water, NH3 and CO2 vapours present in Urea solution coming from

exchanger 11-E-15.


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Operation

The separated overhead vapours are condensed in the second vacuum system 11-ME-5, while

melted Urea leaving the bottom and collected in 11-ME-7 is sucked by a pump P-8 A/B and

sent to the prilling bucket.

y Vacuum system 11-ME-4 , 11_ME-5Function

Its function is to make vacuum in the two evaporation system in order to concentrate Urea

solution coming from the last recovery section of the unit.

Operation

A different vacuum degree is maintained between the evaporator combination 11-E-14, 11-

MV-6 and that of 11-E-15, 11-MV-7 to allow the maximum water evaporation. Vacuum

evaporation avoids the use of high temperatures which would increase biuret content in the

Urea. With evaporators running at different pressures, the solution passes from one to the

other without the use of pumps. The separator must work empty to reduce the holding time of

the solution. Outlet vapours from vacuum separators are condensed and recovered in waste

water tank 11-V-9.

y Prilling Buckets 11-ME-8 A/DFunction

Its function is to prill melted Urea coming from the separator holder 11-ME-7.


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Operation

Melted Urea enters the rotating prilling bucket which distributes it in the prilling tower where

the air, flowing in countercurrent, causes its crystallization. Prilled Urea is collected at tower

bottom and sent to storage by means of a belt conveyor system.

y Prilling Tower 11-ME-6Function

Its function is to cool the melted Urea.

Operation

Urea coming out from the prilling bucket in the form of drops falls into the tower and by

means of an air current is cooled and solidified. Air rises through the tower by natural draft. It

enters through a series of windows located at the tower bottom and, after cooling the product,

it is vented to the atmosphere. The prilling buckets with relevant supporting and control

system are installed in a room located at the prilling tower top. Prilled Urea is collected on

the conveyor belt 11-MT-1 and discharged into the hopper provided with a screen, ME-11, to

separate possible limps that may have formed on the walls of the tower 11-ME-6. The

product coming out from the hopper is weighed and then sent to the warehouse. The rough

product separated on the hopper screen is sent to the dissolving tank 11-V-4 by means of a

belt.

y Waste Water Tanks 11-V-9, Buffer waste water tank11-V-6Function

The function of 11-V-9 is to collect the waste water and to act as hydraulic seal. From here

the waste water is sent to V-6 by means feed pumps 11-P-21 A/B.


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Operation

The waste water, containing NH3 and CO2 from 11-V-9 is collected in the buffer tank 11-V-

6. Also the ammonia water from close drains collected in 11-V-7 is sent to this tank. The tank

is equipped with a temperature indicator 11-TI-1-173 and with a level indicator 11-LI-173.

y Distillation tower preheater 11-E-18Function

Its function is to heat the waste water before entering into the upper part of distillation tower

11-C-2.

Operation

Waste water from 11-V-6 is sent by centrifugal pumps 11-P-16 A/B to the exchanger 11-E-18

where it is heated by means of the treated water leaving the bottom of column 11-C-2. Hot

waste water at a temperature of about 1070C feeds the column 11-C-2.

y Hydrolyzer preheater 11-E-19 A/BFunction

Its function is to preheat the waste water leaving the bottom of the upper of distillation tower

11-C-2, before the entering hydrolyzer 11-R-2.

Operation

The waste water from bottom of 11-C-2 upper part is sent by centrifugal pumps 11-P-14 A/B

to the horizontal shell and tube exchangers 11-E-19 A/B where it is heated up in

countercurrent with the water leaving the hydrolyzer. The waste water at a temperature of

about 2200C feeds the hydrolyzer itself.


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y Hydrolyzer 11-R-2Function

Its function is to transform the Urea contained in the waste water, into NH3 and CO2 by

hydrolization.

Operation

The waste water coming from bottom of the upper part of tower 11-C-2 is preheated in 11-E-

19 A/B and sent to the hydrolyzer 11-R-2 where the Urea is decomposed by means of 38Ata

steam injected directly into the hydrolyzer. Vapors produced in the hydrolyzer are sent to the

distillation tower overhead condenser 11-E-17 while the solution returns to the top of the

lower part of tower 11-C-2.

y Distillation tower 11-C-2 Distillation tower O.H. condenser11-E-17 Reflux accumulator 11-V-8

Function

Their function is to recover the ammonia and carbon dioxide contained in the waste

water.

Operation

The distillation tower is divided into two parts; the; lower part consists of 35 valve trays

and the upper part of 20 valve trays. The waste water feed is placed on the 50th

or 44th

tray. The waste water containing ammonia , carbon dioxide and Urea after a first stripping

of the ammonia in the upper part of the tower is sent to the hydrolyzer for the

decomposition of Urea. The treated water from the hydrolyer comes back to the tower


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feeding the top of lower part where the remaining ammonia is stripped out by means of

4.5 Ata saturated stream fed directly into the C-2 bottom.

y H.P. washing pump 11-P-11 A/BFunction

Its function is to flush sample connections, before and after taking samples for the

analysis, to carry out hydraulic tests of the synthesis loop, to flush H.P. process lines after

machine shut-down and in case of plugging due to crystallization of product in the lines.

Operation

The pump fed by steam condensate is electric motor driven. It is desgined for a

differential pressure of 178.5Kg/cm2

and a suction temperature from 70 to 1200C.


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2) Stabilization of Running Condition

2.1) H.P.Section

y If the reactor top temperature increases very slowly : increase CO2.y If the reactor top temperature increases rapidly : increase CO2.y If NH3: CO2 ratio is correct i.e. 3.0, the reactor bottom temperature will increase

steadily.

y Maintain ammonia plus carbamate solution temperature to Reactor (at ejector) around118

0C.

y If the temperature at Ejector outlet decreases, this can be due to:A. Drop in flow rate of carbamate to Ejector : Increase ammonia pressure to

Ejector.

B. Ammonia to ejector is more than required : Reduce ammonia flow from P-1 toEjector.

y Maintain liquid outlet temperature of Stripper 205 to 2100C.y The operating pressure in synthesis loop must increase very gradually.y If MV-1 level increases, increase ammonia pressure to Ejector.

2.2) M.P.Section

y Maintain E-7 (M.P.Condenser) outlet temperature to about 800C.y C-1 (M.P.Absorber) bottom temperature to be maintained around 720C with ammonia

reflux to C-1 and top temperature about 400C.

y Maintain level in C-1 and with P-2.y Maintain ME-2 liquid outlet temperature at about 1500C.y Control M.P.Section pressure at about 17kg/cm2g by PIC-108.


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2.3) L.P.Section

yMaintain ME-2 liquid outlet temperature at about 140

0

Cy Maintain level in V-3 with LIC-102 at minimum.y Maintain section pressure at 3.5kg/cm2g by PIC-133.y Feed minimum cold condensate to C-3 to avoid excess water in the systems.

2.4) Evaporation andPrillingSection

y Maintain E-14 outlet temperature at 1300C and vacuum in MV-6 at 0.3kg/cm2 abs.y Maintain E-15 outlet temperature at 1400C and vacuum in MV-7 at 0.03kg.cm2 abs.y Keep stream ON to Urea Melt recycle line and its jacket.y Control Bucket Speed to maintain Prills size (without getting overflow).

2.5) Waste Water TreatmentSection

y Maintain distillation tower and Hydrolizer temperatures and pressure at 1330C and235

0C and 1.5 kg/cm

2g and 35kg.cm

2g respectively.


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5) Vacuum concentration section:

L.P steam to 1st

, 2nd

vacuum concentrators and pre-concentrator

Water Content = 8500 kg/h.

Section Water added or produced (kg/h.)

Reactor 22458

M.P Absorber 1500

M.P Ammonia absorber 492

L.P decomposer Separator 4917

Vacuum concentration section 8500

Total 37867

So,

Water going into or produced by each urea unit = 37867 kg/h.

But,

There are 2 units.

Therefore, total water in = 75734 kg/h.

Note- This water should be equal to the water going to the waste water treatment

section.

Waste water coming into the waste water treatment section from both the urea units

(stream 81)-

Water content = 74378 kg/h.

Note This value is not consistent with the water coming into the plant.

This loss of water is accounted for in the urea stripper (E-1). In this section, due to the

conditions in the stripper, some of the urea coming from the reactor absorbs water and

hydrolyzes back to ammonia and carbon dioxide.


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This is evident as the urea in the entry stream and the sum of the urea in the exit streams are

not consistent with each other.

3.2) Calculation of water loss :-

a) Reactor outlet ( stream entering stripper)-Water content = 46486 kg/h.

b) Stripper soln. outlet-Water content = 41448 kg/h.

c) Stripper vapours-Water content = 4342 kg/h.

Water lost in urea hydrolysis (in single urea unit) = 46468- (41448+4342)

= 678 kg/h.

Total water lost = 678 * 2

= 1356 kg/h

Therefore,

Total water in = 75734 1356

= 74378 kg/h.

Total water out = 74378 kg/h.

Hence, the values are consistent.


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3.3) Water balance for the waste water treatment plant:

1) Waste water coming from the urea plant ( stream 81)-Water content = 74378 kg/h.2) L.P steam coming into the distillation tower (stream 92)-

Water content = 15200 kg/h.

3) H.P steam coming into the hydrolyzer (stream 93)-Water content = 5200 kg/h.

Section Water added (kg/h.)

Buffer Waste water tank 74378

M.P Absorber 15200

M.P Ammonia absorber 5200

Total water entering 94778

4) Process Water leaving the Waste water treatment section (stream 86)-Water content = 84746 kg/h.

5) Carbonate soln. recycled back to E-6 (stream 96)-Water content = 4917 * 2 (2 units)

= 9834 kg/h.

Section Water added (kg/h.)

Process water 84746

Recycled carbonate soln. 9834

Total water leaving plant 94580

Again , water entering is not equal to the water leaving. This, too, can attributed to

urea hydrolysis in the hydrolyzer.


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Water loss in hydrolyzer :

Streams entering: stream 93 + stream 88.

Streams leaving: stream 89 + stream 90.

Water content:Stream 93 = 5200 kg/h.

Stream 88 = 83718 kg/h.

Stream 89 = 1876 kg/h.

Stream 90 = 86844 kg/h.

Water loss = 5200+83718-(1876+86844)

= 198 kg/h.

So,

Water in = Water out + water loss.

Water in = 198 + 94580

= 94778 kg/h. which is equal to water (in) tabulated.

3.4) Actual Readings

Actual Readings noted on a particular day :-

Stream to 11-E-8 7.320 m3/hr

Stream to 11-P-2 (HP Carbonate Solution Pumps) 0.7 m3/hr

Stream to 11-EJ-1 (Carbamate Ejector) 1.4 m3/hr

Stream to 11-C-4 (LT purging) 0.92 2 = 1.84 m3/hr

Stream to Urea Scrubber 1 m3/hr

Stream to 11-C-3 (MP Inerts Washing Tower) 1 m3/hr


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Conclusion

Water, as we know, is one of the most important chemical in any industry. In the Urea plant

also water is an invaluable substance. It is used in flushing of inerts, ammonia, carbonate,

absorption as well as in the vacuum concentration unit. Water is also produced as an

additional product in Urea production which has to be utilized properly. Thus, a proper waste

water management section is necessary.

As we have seen in CFCL has quite an efficient waste water treatment plant. Water from both

the Urea plants is collected in buffer tanks, purified and utilized in many ways. Some of the

water recycled back to the Urea plant but the majority of the water is left out. To utilize this

properly, this water is sent to the D.M. plant for treatment and then most of it is converted

into steam which is used in various utilities of the plant.

Thus, it is evident that a proper water balance and water management is essential for the

flawless performance of both the plants.


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Bibliopraphy

y Snamprogetti Milan-Italy, `` Chambal Fertilizers andChemicals LtdGadepan India Operating Manual `, Volume I, May 1991.

y Operation Manual Urea I

Related Web Sites

1.www.google.com2.www.sciencedirect.com

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