urea 1 report to a k saxena sir

INDIAN SCHOOL OF MINESDHANBAD [JHARKHAND]

Department Of Chemical Engineering

A TRAINING REPORT

ON

MANUFACTURING OF UREA

AND

MATERIAL BALANCE OF UREA – 1 PLANT

AT

CHAMBAL FERTILIZERS & CHEMICAL LIMITED.

DURATION: May 18, 2015 to June 30, 2015

[1]

SUBMITTED BY:- SUBMITTED TO:-

DESHRAJ MEENA MR. A K SAXENA

ISM DHANBAD H R DEPARTMENT

2012 JE 1286 CHAMBAL FERTILISERS & CHEMICALS LIMITED

GADEPAN, KOTA

[2]

CONTENTS

OVERVIEW 4

UREA 5

PHYSICAL PROPERTIES 5

CHEMICAL PROPERTIES 5

COMMERCIAL PRODUCTION 5

USES 6

[3]

UREA PLANT 6

INTRODUCTION 6

SNAMPROGETTI AMMONIA STRIPPING PROCESS 7

1. Urea synthesis and High pressure recovery 7

2. Medium pressure (MP) recovery and purification (17 Kg/Cm2g) 8

3. Low pressure (LP) recovery and purification (3.5 Kg/Cm2g) 9

4. Urea concentration 10

5. Urea prilling 10

6. Waste water treatment 11

PROCESS OPERATING VARIABLES 12

PROCESS OPERATING CONDITIONS/REQUIREMENTS 16

STREAMWISE MATERIAL BALANCE CALCULATIONS 19

CHALLENGES IN UREA PRODUCTION 35

CONCLUSIONS 38

OVERVIEW

Chambal Fertilisers and Chemicals Limited is one of the largest private sector fertilizer producers in India. It was promoted by Zuari Industries Limited in the year 1985. Its two hi-tech nitrogenous fertiliser (urea) plants are located at Gadepan in Kota district of Rajasthan. Built at a price of over Rs. 25 billion (USD 500 million), the two plants produce about 2 million tones of Urea per annum. The first plant was commissioned in 1993 and second plant in 1999. These plants use state-of-the-art technology including that from Denmark, Italy, United States and Japan.

Chambal Fertilisers caters to the need of the farmers in ten states in Northern, Central and Western regions of India and is the lead fertiliser supplier in the State of Rajasthan. The Company has a vast marketing network comprising 11 regional offices, 1,300 dealers and 20,000 village level outlets.

[4]

The company has won the Sword of Honour from the British Safety Council for two years running. It has been awarded ISO 14001 (Environment Management System Standard), ISO 9001 (Quality Management System Standard) and OHSAS 18001 (Occupational Health and Safety Management System Standard) certifications.

In addition, Chambal Fertilisers has other business interests through its subsidiary in the software sector. It also has a joint venture in Morocco for manufacturing phosphoric acid.

UREA

Urea is an organic compound with the chemical formula (NH2)2CO. It was the first organic compound to be artificially synthesized from inorganic starting materials, in 1828 by Friedrich Wöhler, NH3 + HCNO → CO(NH2)2.

PHYSICAL PROPERTIES

Molar Mass 60.06 g/mol

Appearance White (solid), colourless (liquid)Specific Gravity 1.33 (solid)Solubility in water 51.8/100 ml (20 °C)

62.3 g/100 ml (40 °C)

[5]

71.7/100 ml (60 °C) 80.2 g/100 ml (80 °C) 88.1 g/100 ml (100 °C)

Melting Point 132.7 °C

CHEMICAL PROPERTIES

Heating: Upon heating, urea decomposes primarily to ammonia and isocyanic acid. As a result, the gas phase above a urea solution contains a considerable amount of HNCO.

NH2CONH2 NH3 +HNCO

Biuret Formation: In a more concentrated solution or in a urea melt, the isocyanic acid reacts further with urea, at relatively low temperature, to form biuret.

NH2CONH2 + HNCO NH2CONHCONH2 (biuret)

Hydrolysis: NH2CONH2 + H2O 2NH3 + CO2

COMMERCIAL PRODUCTION

Urea is commercially produced from two raw materials, ammonia, and carbon dioxide. Large quantities of carbon dioxide are produced during the manufacture of ammonia. This allows direct synthesis of urea from these raw materials..

2 NH3 + CO2 ↔ H2N-COONH4 (ammonium carbamate)

H2N-COONH4 ↔ (NH2)2CO + H2O

The first reaction is exothermic: Whereas the second reaction is endothermic: Both reactions combined are exothermic.

USES

1. More than 90% of world production is destined for use as a fertilizer. Urea has the

highest nitrogen content of all solid nitrogeneous fertilizers in common use

(46.4%N.)

2. A component of animal feed providing a relatively cheap source of nitrogen.

3. A raw material for the manufacture of plastics eg. urea-formaldehyde resin

4. An ingredient in some hair conditioners, skin cream.

5. A raw material for the manufacture of various glues (urea-melamine-formaldehyde);

the latter is waterproof and is used for marine plywood.

6. An alternative to rock salt in the de-icing of roadways and runways; it does not

promote

metal corrosion to the extent that salt does.

[6]

7. A raw material for melamine production More than 95% of all melamine production

is

based on urea.

UREA PLANT

INTRODUCTION:

UREA-I PLANT has two units of total 2620 MT/day. Unit 11 has been commissioned in Dec.,93 & Unit 21 in Jan., 94. This is one of the most modern plant on Snamprogetti process. The whole process is controlled by Distributed control system (DCS). The required raw materials i.e. Ammonia & Carbon dioxide are supplied by Ammonia Plant.

Snamprogetti is the process licensor for Urea and their well- proven technology of ammonia stripping has been employed in the plant design. This is based on the principle of decomposing and recondensing the unconverted Ammonia and CO2 from the reactor at synthesis pressure itself. All the condensation heat thus generated can be recovered in the form of steam. A deliberate choice of high NH3/CO2 ratio in the reactor provides sufficient ammonia to carry out this stripping and has further beneficial effects.

High conversion of CO2 to urea in the reactor Minimal biuret formation

The urea solution left after the stripping processed in low pressure recirculation stages in which balance quantities of ammonia and carbon dioxide remaining in the solution are expelled, dissolved in water and recycled to synthesis.

Other unique features of the plant are

Total recycle/recovery of process condensate to DM plant during operation. Storage of process condensate in guard pond during up-set conditions and recovery of

same once normal operations resume. Total recycle/recovery of steam condensate from heat exchangers etc to DM plant Complete isolation of urea plant drainage system frin storm drain so as to avoid any

leakage of the floor washings to the same. Replacement of oil in the seal of HP ammonia pumps with DM water so as to

eliminate any oil carry-over to process condensate recovery section..

[7]

SNAMPROGETTI AMMONIA STRIPPING PROCESS

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

1. Urea Synthesis & High pressure recovery

2. Medium pressure (MP) purification and recovery

3. Low pressure (LP) purification and recovery

4. Urea Concentration

5. Urea Prilling

6. Waste water treatment

1. Urea Synthesis & High pressure recoveryUrea is produced by synthesis from Liq. NH3 & gaseous CO2. In the reactor NH3 & CO2 react to form ammonium carbamate, a portion of which dehydrates to form Urea & water. The molar ration of NH3 to CO2 is about 3.3 to 3.5 and H2O to CO2 is about 0.60 to 1.0 The reactions are as follows: -

2NH3 + CO2 NH4COONH2

NH4COONH2 NH2CONH2 + H2O

Process Description:

The liq. NH3 coming directly from the battery limit is collected in the Ammonia receiver tank (V-1). From V-1 it is drawn & compressed at about 22 kg/cm2 (g) by means of centrifugal pump P-5A/B. The NH3 feeding the synthesis loop is compressed by the heavy duty reciprocating pump (P-01) at 240 kg/cm2(g). This NH3 is used as a motive fluid in the carbamate ejector (EJ-1), Here the carbamate coming from the carbamate separator is mixed with Ammonia & sent to the reactor.

The CO2 drawn at urea battery limit at about1.8 kg/cm2 (g) enters the centrifugal compressor & leaves at about 160 kg/cm2 (g). A small quantity of air is also added to the CO2 at the compressor suction in order to passivate the stainless steel surface from corrosion.

Reactor pressure at 156 Kg/cm2 and temperature at 188oC are maintained.

The reaction products leaving the reactor containing 34% urea, 33% NH3 and 14% CO2 (wt %) flow to the steam heated falling film stripper E-1. This mixture is heated up with 23 Kg/cm2g steam as it flows down the falling film exchanger. The CO2 content of the solution is reduced by the stripping action of the ammonia as it boils out of the solution. Since stripping is achieved thermally, relatively high temperatures (200-210 0C) are required

[8]

to obtain a reasonable stripping efficiency. Because of this high temperature, stainless steel is not suitable as a construction material for the stripper from a corrosion point of view; titanium tubes have been used.

The overhead gases from the stripper & the recovered solution from the M.P. absorber, all flow to the carbamate condenser (E-5). It is a kettle type boiler. At the tube side of this condenser the off gas is absorbed in recycled liquid carbamate from the medium pressure recovery section. The heat of absorption is removed through the tubes, which are cooled by the production of low pressure steam at the shell side.

In MV-1 the incondensable gases, consisting of inert gases containing a little quantity of NH3 & CO2 unreacted in the condenser, are separated from the solution and sent to the MP Decomposer (E-2). The stripper outlet solution is also sent to E-2.

2. Medium pressure (MP) purification and recovery (17 Kg/Cm2g)

The purpose of this section is to partially strip out the reactants, ammonia and carbon dioxide from the urea solution and, after their condensation in water, to recycle solution to the reactor, together with the ammonia and carbon dioxide aqueous solution resulting from the downstream sections of the plant.


The solution leaving stripper bottom containing 48% urea, 23% ammonia and 5% carbon dioxide enters the MP pre decomposer where decomposition of carbamate occurs by heat supplied by steam generated in steam booster ejector .The mixed phase leaving the pre decomposer enters the MP separator (MV-2). Gases released hereon flashing are removed before the solution enters MP Decomposer (E-2) where the residual carbamate is decomposed to a large extent. The ammonia and carbon dioxide rich gases leaving the M.P. Separator are sent to the shell side of the falling film vacuum preconcentrator where they are partially absorbed. From the vacuum preconcentrator shell side, the mixed phase is sent to the MP condenser. The mixture from MP condenser flows to MP Absorber. Gases coming along with the solution go up to the rectification section of MP Absorber (C-1) having bubble-cap, this performs CO2 absorption and NH3 rectification. Pure NH3 reflux eliminates any residual CO2

contained in the inert gases. In this way a top product consisting of pure gaseous ammonia and a bottom product of liquid ammonium carbamate are obtained.

To prevent solidification of ammonium carbamate in the rectifier, some water is added to the bottom section of the column to dilute the ammonium carbamate below its crystallization point. The liquid ammonium carbamate-water mixture obtained in this way is also recycled to the synthesis section.

Ammonia with inert gases leaving the top of MP Absorber is partially condensed in the Ammonia condenser (E-9). From here the liquid and gaseous ammonia phases are sent to the Ammonia receiver. The inert gases, saturated with ammonia, leaving the receiver, enter the ammonia recovery tower(C-5) where ammonia is further condensed by the cold liquid

[9]

ammonia coming from the plant’s battery limit. The inert gases along with residue ammonia enter MP Ammonia Absorber (E-11), where they meet a counter current flow of cold condensate and gaseous ammonia is absorbed. Ammonia water solution is recycled back to MP Absorber.

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

3. Low pressure (LP) purification and recovery (3.5 Kg/Cm2g)

The urea solution from the medium pressure decomposer is subjected to a second low pressure decomposition step. Here further decomposition of ammonium carbamate is achieved, so that a substantially carbamate –free aqueous urea solution is obtained.


The solution leaving the bottom of MP Decomposer containing 64% urea, 7% ammonia & 1.4 % carbon dioxide enters the LP Separator (MV-3). Gases released here on flashing are removed before the solution enters the LP Decomposer (E-3) where the last residual carbamate is decomposed.

The gases leaving the LP Separator are sent to LP condenser (E-8) where they are absorbed in carbonate solution coming from Waste water treatment section.

Liquid from LP condenser bottom, with the remaining inerts, are sent to the carbonate solution tank (V-3). From here the solution is recycled back to the MP condenser. The inert gases washed in the LP inert washing tower (C-4) with counter current cold condensate flow, are vent to blow down practically free from ammonia. Normally vent remains closed.

4. Urea concentrationThe urea solution leaving the LP section is about 70% by weight and contains small quantities of ammonia and carbon dioxide. The final concentration of the urea solution (99.8% b.w.) is made under vacuum in two steps at 0.3 and 0.03 kg/cm2 abs. An important feature of this section is the pre-concentration of the urea solution to about 86% b.w. The necessary heat is provided by partial condensation of the vapours (ammonia and carbon dioxide essentially) from the MP section evaporator.


The 70% urea solution leaving LP solution holder (ME-3) is concentrated to 86% in a pre-concentrator. Pre-concentrator is an additional vacuum stage utilizing heat of condensation of Carbamate vapours coming from Medium Pressure.

The urea solution is then sent to the 1st vacuum concentrator E-14 at a temperature of 122 C. At E-14, urea solution is heated by LS to a temperature of 130 C. The mixed phase coming out of E-14 enters 1st vacuum separator (MV-6) where the vapors are extracted by 1st

[10]

vacuum system (ME-4) operating at 0.3 Kg/cm2(abs). Vapors so extracted are condensed in condensers with the help of circulating cooling water and are collected in waste water tank V-9. The liquid phase urea having about 96% concentration enters 2nd vacuum concentrator E-15.

At second vacuum concentrator E-15 urea is heated further to 140 C by LS. The solution then is delivered to second vacuum separator MV-7 where the gas phase is extracted by second vacuum system ME-5 which operates at 0.03 Kg/cm2a pressure. Extracted gases are condensed in condensers with the help of circulating cooling water and are collected in waste water tank V-9 where it maintains a hydraulic seal.

Liquid phase 99.8% wt urea at a temperature of 137 C is collected at urea solution holder ME-7 from where it is sent to rotating prill bucket at top of prilling tower with the help of urea melt pump P-8 A/B.

5. Urea prilling

Urea prills are formed by spraying molten urea in droplets and cooling them to solidification.

This is achieved in a prilling tower which is a hollow cylindrical R.C.C. construction in vertical configuration. The most widely used method of making prills is spraying molten urea through a large number of holes on a revolving container called prill bucket installed at top of prilling tower and cooling these droplets to solidification while falling down by counter current flow of cool air.

The whole prilling tower system comprises of :

A prill bucket assembly at top of prill tower inlet and outlet openings for air at bottom and top of tower; a rotary scrapper unit to collect urea prills and conveying belt system to receive prill discharged by scrapper and these to transport it

to bagging facilities after separating of any possible lumps or oversize which may develop on prill tower wall or on scrapper body.

The air after solidifying molten urea to prills is discharged to atmosphere at prill tower top.


The molten urea at 137 C sprayed by rotating bucket is cooled down and solidified by counter current flow of cold air by natural draft through louvers at PT bottom.

[11]

The cooled and solidified urea prills are collected at PT bottom floor (Called as scrapper floor) and discharged by a motor drive double arm scrapper ME-10 to a running belt conveyer MT-1 through a slit on scrapper floor.

This belt MT-1 delivers the urea prills to product belt conveyer MT-3 through a hopper and lump separator screen ME-11. Product via MT-3 is sent to bagging plant for bagging and dispatch while the oversize/lumps are recovered on recycle belt conveyer MT-2 and dissolved in lump dissolving tank V-4 from where this is transferred to urea solution tank V-5 with the help of urea solution recovery pump P-19 .

6. Waste Water Treatment

Wastewater section (WWS) or process condensate treatment section and effluent treatment section constitute a very important part of urea plant. This section serves following purpose:

1. Solves the pollution problem of liquid discharge from urea Plant; Otherwise it would have been difficult to dispose of 70 M3/hr of contaminated water.

2. The treated water coming out of the waste water section has NH3 less than 4 ppm and urea around 1 ppm.

3. It recovers the NH3 and CO2 which would otherwise be lost, by recycling them into the process, thus improving the production yield.

Process description:

The condensed vapour from the overhead vacuum system, containing ammonia and carbon dioxide, are collected in waste water tank V 9. From V-9 they are pumped to buffer waste water tank V-6 through the pumps P-21.

Then by means of P-16 they are pumped into waste water distillation tower 11-C-2. Before entering the distillation tower the solution is preheated in the plate and frame type heat exchanger E-18 by means of the purified water flowing out of the tower bottom.

The tower is divided into two parts – Top section (above chimney tray) as enriching section and lower section (below chimney tray) as stripping section. The tower is operated at a pressure of 2.5 Kg/Cm2g.

The feed is given in the top section of the tower. Since the solution is contaminated by urea, after first stripping in the upper section of the column it is pumped by P-14 into the hydrolyser R-2 where the urea is decomposed by the means of steam at 38 Kg/Cm2g and 230 C temperature. The solution is preheated, before entering the hydrolyser, in the exchanger E-19 with the solution coming out of the hydrolyser.

[12]

The solution from the hydrolyser after getting hydrolysed returns to the top of the lower part of the distillation tower, here the remaining ammonia is stripped out by means of 4.5 Kg/Cm2g saturated steam which is fed to the column bottom.

The vapors produced in the hydrolyser are sent to overhead condenser E-17 along with the vapor leaving the top of the distillation tower. From E-17 the carbonate solution flows to the reflux accumulator V-8. Part of this solution is sent by P-15 to the top of distillation tower C-2 as reflux, the balance is recycled back to LP condensers E-8.

The purified waste water from the bottom of C-2 is sent by P-18 to B/L after being cooled in E-18 and E-20 A/B exchangers.

PROCESS OPERATING VARIABLES

There are two kinds of chemicals reaction occur at the same time in the urea reactor: -

1. 2NH3 + CO2 NH4COONH2 + 32560 Kcal/Kmole 2. NH4COONH2 NH2CONH2 + H2O – 4200 Kcal/Kmole

Reaction 1 is a very hard exothermic reaction & is almost complete & reaction 2 is slow & weak endothermic. As from the above 2 equation, the equilibrium conversion to urea will become higher under the following condition:

NH3 Excess H2O deficiency High temperature & pressure

Effect of NH3 /CO2 ratio: -

Figure 1 shows that the ureas yield as a function of NH3: CO2 ratio reaches a maximum somewhat above the stoichiometric ratio (2: 1).This is one of the reasons that all commercial processes operate at NH3:CO2 ratios above the stoichiometric ratio.

[13]

Figure 2 "

Figure 3 "

Effect of H2O/CO2 ratio: -

From eqn-2, 1 mole of water is the by-product when 1 mole of urea is produced from 1 mole of ammonium carbamate. Thus excess water in the solution disturbs the formation of urea from carbamate. But if the water content is very low, the carbamate conc. becomes high, which may be the cause of clogging. Therefore, from, the H2O/CO2 ratio is normally 0.5 ~ 0.6 in use.

Effect of temperature and pressure: -

[14]

Figure 4 "

Figure 5 "

According to Fejacques etal, increasing the temperature proportionally increases equilibrium urea conversion. And the equilibrium pressure becomes higher and higher when the temperature increases. Figure below illustrates that urea yield as a function of temperature also goes through a maximum; the location of this maximum is of course composition dependent.

Decomposition of Carbamate: -

The decomposition reaction is a reverse reaction of eqn –1 NH4COONH2 2NH3 + CO2 - Heat

[15]

As from equation , it will promote by reducing pressure , or by adding heat.So in the stripper a higher temperature will cause more carbamate decomposition & subsequently residual carbamate will decompose in the lower section due to low pressure.

PROCESS OPERATING CONDITIONS /REQUIREMENTS

[16]

CO2

NH3

REACTORCARBAMATE SEPARATOR

STRIPPER

MEDIUM PRESSURE DECOMPOSER

CARBAMATECONDENSERMEDIUM PRESSURE CONDENSER

MEDIUM PRESSURE ABSORBER

AMMONIA CONDENSER

AMMONIA RECEIVER

LOW PRESSURE DECOMPOSER

LOW PRESSURE CONDENSER

CARBONATE SOLUTION TANK

1ST STAGE CONCENTRATOR AND SEPARATOR

PRILLING TOWER

2ND STAGE CONCENTRATOR AND SEPARATOR

PRODUCT UREA TO BAGGING PLANT

CO2 COMPRESSOR

AMMONIA PUMP

UREA-I PLANT

Pr-157 ata Temp-188°C Urea-33.77%



Pr-4.5 ata Temp-138°C Urea-71.20%


[17]

Material Balance Calculations Stream wise compared to

2620MTPD Urea design:-

Calculation of stable CO2 flow on the basis of CO2 Specific 0.750

CO2 sp 0.75

Urea 1525 MTPD

CO2 1143.75 MTPD 47656.25 Kg/hr 1083.097 moles/hr

Conv 64 % 24.26136 Knm3/hr

N/C 3.6

1 UREA PROD 693.18 MOLES/HR

41590.91 Kg/hr

Remaining CO2 17156.25 Kg/hr

NH3 consumed 1386.36 MOLES/HR

NH3 Feed 3899.15 MOLES/HR

Remaining NH3 2512.78 MOLES/HR

42717.33 Kg/hr

2 CO2 Feed 64812.50 Kg/hr

UREA PROD 942.73 MOLES/HR

56563.64 Kg/hr





58095.57 Kg/hr



61953.82 Kg/hr





[19]

63631.73 Kg/hr



63894.28 Kg/hr





65624.75 Kg/hr



64592.85 Kg/hr





Kg/hr



64844.34 Kg/hr





66600.54 Kg/hr

Water formed 1080.74 MOLES/HR



64934.87 Kg/hr


[20]




66693.52 Kg/hr



64967.46 Kg/hr





66727.00 Kg/hr




64979.20 Kg/hr





66739.05 Kg/hr

10 CO2 Feed 74460.17 Kg/hr


64983.42 Kg/hr





66743.39 Kg/hr

11 CO2 Feed 74461.91 Kg/hr


[21]

64984.94 Kg/hr





66744.95 Kg/hr

12 CO2 Feed 74462.54 Kg/hr


64985.49 Kg/hr





66745.51 Kg/hr

13 CO2 Feed 74462.76 Kg/hr


64985.68 Kg/hr 1559.65643 MTPD





66745.71 Kg/hr

Back Calculation of stable CO2 flow for 1525 MTPD

CO2 sp 0.75

Urea 1491.114 MTPD

CO2 1118.335 MTPD 46597.3017 Kg/hr 1059.03 moles/hr

Conv 64 % 23.72226 Knm3/hr

N/C 3.6 Actual flow 23.5 Knm3/hr

1 UREA PROD 677.78 MOLES/HRDuring HLD operation on 15/06/2013

[22]

40666.74 Kg/hr





41768.13 Kg/hr


12200.02 Kg/hr



55306.76 Kg/hr





56804.65 Kg/hr



60577.17 Kg/hr





62217.80 Kg/hr



62474.52 Kg/hr





[23]

64166.54 Kg/hr



63157.56 Kg/hr





Kg/hr



63403.46 Kg/hr





65120.64 Kg/hr




63491.98 Kg/hr





65211.56 Kg/hr



63523.85 Kg/hr


[24]




65244.29 Kg/hr




63535.32 Kg/hr





65256.07 Kg/hr

10 CO2 Feed 72805.62 Kg/hr


63539.45 Kg/hr





65260.31 Kg/hr

11 CO2 Feed 72807.33 Kg/hr


63540.94 Kg/hr





65261.84 Kg/hr

12 CO2 Feed 72807.94 Kg/hr

[25]


63541.47 Kg/hr





65262.39 Kg/hr

13 CO2 Feed 72808.16 Kg/hr


63541.67 Kg/hr 1525 MTPD





65262.59 Kg/hr

water feed 17871.09 Kg/hr

water formed 19062.5 Kg/hr

water at reactor O/L 36933.59 Kg/hr

Stream 11 % 12 %

N 0 0 65262.59 34.0

C 46597.3 100 26210.94 13.7

U 0 0 63541.67 33.1

H 0 0 36933.59 19.2

Total 46597.3 100.0 191948.78 100.0

Assuming negligible hydrolysis:

Urea at Stripp I/l equals (Flow of soln O/L x % of Urea)

63541.67 equals F*0.4763

[26]

F 133406.8

V 58541.96

Assuming % of N/C/U/H same as in mat bal for 2620 MTPD

Stream 13 % 14 %

N 30843.66 23.12 33573.81 57.35

C 6243.44 4.68 21420.50 36.59

U 63541.67 47.63 0.00 0.0

H 32778.06 24.57 3547.64 6.06

Total 133406.8 100.0 58541.96 100.0

Stream 11 % 12 %

N 0 0 65262.6 34.00

C 46597.3 100 26210.9 13.66

U 0 0 63541.7 33.10

H 0 0 36933.6 19.24

Total 46597.3 100 191948.79 100

Urea in sol. F x 0.4763 = 63541.7

Flow F = 133406.8

V V = 58542.0

Stream 13 % 14 %

N 30843.7 23.12 33573.8 57.35

[27]

C 6243.4 4.68 21420.5 36.59

U 63541.7 47.63 0 0

H 32778.1 24.57 3547.6 6.06

Total 133406.8 100 58542.0 100

Urea in sol. F x 0.6367 = 63541.7

Flow F = 99798.4

V V = 33608.4

Stream 31 % 32 %

N 6546.8 6.56 25455.0 75.74

C 1397.2 1.40 4241.4 12.62

U 63541.7 63.67 0 0

H 28312.8 28.37 3912.0 11.64

Total 99798.4 100 33608.4 100

Urea in sol. F x 0.7120 = 63541.7

Flow F = 89243.9

V V = 10554.5

Stream 51 % 53 %

N 1588.5 1.78 4952.2 46.92

C 696.1 0.78 698.7 6.62

U 63541.7 71.20 0 0

[28]

H 23417.6 26.24 4903.6 46.46

Total 89243.9 100 10554.5 100

urea in vapors =0.15

% = 96.5

rest urea = 63445.2

Urea in sol. F x 0.9497 = 63445.2

Flow F = 66805.5

V V = 22438.4

Stream 71 % 72 %

N 13.4 0.02 1575.2 7.02

C 6.7 0.01 693.3 3.09

U 63445.2 94.97 96.5 0.43

H 3340.3 5.00 20073.4 89.46

Total 66805.5 100 22438.4 100

urea in vapors = 0.30% = 193.1

rest urea = 63252.1

Urea in sol. F x 0.9970 = 63252.1

Flow F = 63442.4

V V = 3363.1

Stream 73 % 76 %

N 0 0 12.8 0.38

[29]

C 0 0 7.1 0.21

U 63252.1 99.70 193.4 5.75

H 190.3 0.30 3149.9 93.66

Total 63442.4 100 3363.1 100

Stream 75 % 74 %

N 0 0 1588.0 4.77

C 0 0 700.4 2.10

U 0 0 289.9 0.87

H 7501 100 30724.3 92.26

Total 7501 100 33302.5 100

Stream 81 % 83 %

N 3175.9 4.77 3175.9 4.77

C 1400.8 2.10 1400.8 2.10

U 579.7 0.87 579.7 0.87

H 61448.6 92.26 61448.6 92.26

Total 66605.1 100 66605.1 100

579.7344 = 0.76 %urea in sol.

F = 76280.8

[30]

Stream 87 % 88 %

N 381.4 0.50 381.4 0.50

C 76.3 0.10 76.3 0.10

U 579.7 0.76 579.7 0.76

H 75243.4 98.64 75243.4 98.64

Total 76280.8 100 76280.8 100

urea will break in NH3 and CO2

NH3 = 19.3244802 Moles 328.5 Kg/h

CO2 = 9.66224009 Moles 425.1 Kg/h

Total NH3 = 709.9 Kg/h

TotalCO2 = 501.4 Kg/h

51.06 % of Total NH3 will in 89 stream = 362.5

11.9 % of Total flow is NH3

For 89 F = 3046.1

For 90

0.44 % of Total flow is NH3 = 347.4

F = 78962.5

Stream 89 % 90 %

N 362.5 11.9 347.4 0.44

C 446.3 14.65 55.3 0.07

U 0 0 0 0

H 2237.4 73.45 78559.799 99.49

Total 3046.1 100 78962.507 100

[31]

Water used in hydrolyser = 173.9 Kg/h

Total water = 80971.1

Stream 93 % 91 %

N 0 0 347.4 0.44

C 0 0 55.3 0.07

U 0 0 0 0

H 5727.7 100 78559.799 99.49

Total 5727.7 100 78962.5 100

water through L.S. = 19.31 %of sol.

water = 15247.7

Tota water in C-2 = 93807.5

pure water is = 81.72 % of Total water

Stream 92 % 85 %

N 0 0 0 0

C 0 0 0 0

U 0 0 0 0

H 15247.7 100 76659.5 100

Total 15247.7 100 76659.5 100

Water in upside of C2 17148.0

Water in 87 stream 13794.8

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Water in 84 stream 3353.2

water from 95 stream = 73.13 %

= 2452.2

total water in 84 = 44.67% = 5805.4

F = 12996.1

Stream 86 % 84 %

N 0 0 4909.9 37.78

C 0 0 2280.8 17.55

U 0 0 0 0

H 76659.5 100 5805.4 44.67

Total 76659.5 100 12996.1 100

Stream 95 % 82 %

N 1768.0 34.52 5272.4 32.87

C 901.0 17.59 2727.1 17.00

U 0 0 0 0

H 2452.2 47.88 8042.7 50.13

Total 5121.2 100.00 16042.2 100.00

Stream 94 % 96 %

N 5272.4 32.87 1752.1998 32.09

C 2727.1 17.00 913.0 16.72

U 0 0 0 0

H 8042.7 50.13 2795.3 51.19

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Total 16042.2 100.00 5460.4996 100.00

Stream 55 % 56 %

N 6704.4 41.86 6704.4 41.86

C 1611.8 10.06 1611.8 10.06

U 0 0 0 0

H 7698.9 48.07 7698.9 48.07

Total 16015.0 100.00 16015.0 100.00

water from 57 stream = 1.98%

= 317.1

Stream 61 % 57 %

N 0 0 0 0

C 0 0 0 0

U 0 0 0 0

H 0 0 317.1 100

Total 0 0 317.1 100

Stream 58 %

N 6704.4 41.05

C 1611.8 9.87

U 0 0

H 8016.0 49.08

Total 16332.1 100

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Stream 33 %

N 32159.4 64.40

C 5853.1 11.72

U 0 0

H 11928.0 23.88

Total 49940.5 100.00

NH3 in stream 45 = 1.2 % of NH3 in 33 stream= 385.9

F = 671.5

NH3 in stream 42 = 13.7 % of NH3 in 33 stream= 4405.8

F = 4419.1

Stream 45 % 42 %

N 385.9 57.47 4405.8 99.7

C 0 0 0 0

U 0 0 0 0

H 285.6 42.53 13.3 0.3

Total 671.5 100 4419.1 100

Total NH3 in C-1 = 36951.1

NH3 in 36 stream = 56.5 % of total NH3 = 20877.4

Stream 36 % 34 %

N 20877.4 100 16073.7 47.06

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C 0 0 5853.1 17.14

U 0 0 0 0

H 0 0 12226.8 35.80

Total 20877.4 100 34153.7 100.00

NH3 in 37 stream = 84.6 % of total NH3 in 36 stream = 17662.26

Stream 37 % 38 %

N 17662.3 100 3215.1 100

C 0 0 0 0

U 0 0 0 0

H 0 0 0 0

Total 17662.3 100

CHALLENGES IN UREA PRODUCTION

Like any process design, a urea plant design has to fulfil a number of criteria. Most importantitems are product quality, feedstocks and utilities consumptions, environmental aspects, safety, reliability of operation and a low initial investment.

1. Product Quality Control

Urea prills should have free flowing and non caking quality with uniform size pattern for storage and field application point of view. They should also be free from urea dust. Caking of urea prills create difficulty in field application/distribution and proper absorption by soil/plants.

Caking of urea dust formation is the result of high product temperature and high moisture content in it. High temperature causes breakage as the prills hit on scraper floor, while high

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moisture reduces physical strength of prill. These conditions also result in loss of prill shine and cause them to be porous and reduction in physical strength.

Moisture in product is controlled by conditions of vacuum and temperature in second stage cocentrator and is affected by a number of factors such as ejector performance, cooling water temperature and fouling of condensers etc.

It is very important to keep in mind that the right vacuum figure is by far more sensitive than temperature. The temperature is a dependent of ambient temperature, quantity of air flow available for cooling the prills and prill tower top louver condition. Besides high temperature, high prill bucket speed also tends to more urea dust formation.

Specifications urea prills:

Urea prills should have the following specification1) Prill Size : Between 1.0mm to 2.4 mm = 95 % wt. Min.

Between 1.0 mm to 2.0 mm = 85 % wt. Min. Below 1 mm = 05 % wt. Max.

2) Biuret Content = 1.5 % wt. Max.3) Nitrogen Content = 46 % wt. Min.4) Moisture Content = 1 % Wt. Max.5) Dust Content = 0.2 % wt. Max.6) Free Ammonia = Less than 200 ppm.7) Bulk Density = 700 Kg/M3

8) Colour : White crystall shine.9) Prills : Should have freeflow

2. Environmental Aspects – Waste Water treatment

The quality of purified/treated water is maintained by monitoring parameters of the process. The R-2 temperature is maintained at 230oC and pressure 33-34 Kg/cm2. Chimney tray temperature of C-2 is maintained at 126oC. C-2 top pressure is maintained at 2.5 ata and the bottom C-2 temperature is maintained at 135oC. These parameters ensure total ammonia in purified water to be less than 4 ppm and urea around 1 ppm.

3. Corrosion

Urea synthesis solutions are very corrosive. Generally, ammonium carbamate is considered the aggressive component. This follows from the observation that carbamate-containing product streams are corrosive whereas pure urea solutions are not.

Role of Oxygen Content

Since the liquid phase in urea synthesis behaves as an electrolyte, the corrosion it causes is of an electrochemical nature. Stainless steel in a corrosive medium owes its corrosion resistance to the presence of a protective oxide layer on the metal. As long as this layer is intact, the metal corrodes at a very low rate. Upon removal of the oxide layer, activation and,

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consequently, corrosion set in unless the medium contains sufficient oxygen or oxidation agent to build a new layer. Stainless steel exposed to carbamate containing solutions involved in urea synthesiscan be kept in a passivated (noncorroding) state by a given quantity of oxygen. If the oxygen content drops below this limit, corrosion starts after some time – its onset depending on process conditions and the quality of the passive layer. Hence, introduction of oxygen and maintenance of sufficiently high oxygen content in the various process streams are prerequisites to preventing corrosion of the equipment.

Role of Temperature and Other Process Parameters in Corrosion

Temperature is the most important technological factor in the behavior of the steels employed in urea synthesis. An increase in temperature increases active corrosion, but more important, above a critical temperature it causes spontaneous activation of passive steel.

Sometimes, the NH3: CO2 ratio in synthesis solutions is also claimed to have an influence onthe corrosion rate of steels under urea synthesis conditions. Experiments have showed that underpractical conditions this influence is not measurable because the steel retains passivity.

Material Selection

Stainless steels used in the snamprogetti process are 2RE-69 grades(containing 25 wt% chromium,22 wt% nickel, and 2 wt% molybdenum).

The process conditions in the high-pressure stripper are most severe with respect to corrosion. In the Snamprogetti stripping processes, titanium usually is chosen for this critical application.

4. Biuret formation & its control

During the formation of urea, biuret is formed as a byproduct which can be represented By following equation, 2 NH2 CONH 2 NH2CONHCONH2 + NH3 + Heat

This is a slow and endothermic reaction and shows that if one mole of ammonia is expelled out from two moles of urea, biuret is formed. The biuret formation is favored by following conditions:-

1) High urea concentration.2) Low ammonia concentration / Low ammonia vapour pressure.3) High Temperature.4) Longer residence/retension time at high temperature.5) particularly in places where there is sufficient time for biuret formation.

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Since the biuret is toxic/ harmful to plants/green plants, its content in fertiliser grade urea has to be kept as low as possible. This means that the combinations of above conditions are to be avoided, The most critical location in this respect are the Reactor, Stripper and Second Stage Evaporation equipments. The liquid leaving the reactor contains about 0.30 to 0.40 % wt. Biuret in relation to the total of urea plus biuret. Owing to a very short residence time in the Stripper ,biuret formation between reactor and the first evaporation section is limited to 0.20 % wt. The evaporators are designed as one pass evaporators to minimize the residence time. In order to keep the temperature low , they are operated at reduced pressure. This keeps the biuret formation down to about 0.30 % wt., which means that over all biuret formation will be about 0.80 to 0.90 % wt.

If the plants are operated below design capacity/low load, biuret formation will increase as a result of longer residence time. The biuret formation can be minimised by keeping residence time and temperature as low as possible.

PROPERTIES OF BIURET ARE AS FOLLOW.

1) Chemical Formula = NH2CONHCONH2

2) Molecular Weight = 103 3) Specific Gravity = 1.467 4) Crystallisation Point = 180 0 C - 190 0 C

Permissible maximum biuret content in fertiliser grade urea is 1.5 % wt.

CONCLUSION

The Urea-I plant at CFCL, Gadepan provided me insight knowledge in the field of chemical engineering. I got a firsthand experience of the various unit operations involved in the whole manufacturing process. I was impressed by the company’s practice of following highest safety standards as well as its commitment to maintain a pollution free environment.

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urea 1 report to a k saxena sir

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