Introduction to Urea

Urea

Urea (carbamide), readily produced from ammonia and carbon dioxide, is a very
important chemical in the agricultural and the polymer industries.

Uses of urea

Urea is the world's most commonly used nitrogen fertilizer and indeed more urea is
manufactured by mass than any other organic chemical. Containing 46% N, it is
the most concentrated nitrogen fertilizer, and is readily available as free
flowing prills (granules). It is the cheapest form of nitrogen fertilizer to transport
and it is also the least likely to 'cake'. It is therefore favored in developing
countries. While over 90% of urea produced is used as a fertilizer, it has other uses,
which include the manufacture of the melamine, used in melamine-methanal
resins. Urea itself also forms important resins. |
An increasingly important use of urea is in reducing air pollution from diesel
engines in cars, buses and Lorries. Diesel engines run at high temperatures and
nitrogen and oxygen, from the air, are able to react together under these conditions
to produce high concentrations of nitric oxide. One way to remove this pollutant is
to allow it to react with ammonia to form nitrogen.
However it is not possible to use ammonia directly as it is too volatile and is
poisonous. Instead a solution of urea in water is injected into the hot gases
emerging from the engine in the exhaust. Urea is thermally decomposed to
ammonia and carbon dioxide. This is the reverse of the process used to make urea:

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Manufacture of urea

Ammonia reacts with carbon dioxide to produce urea. Urea is always

manufactured close to an ammonia plant.

Ammonia and carbon dioxide are heated together at 375 F and 3200 Psig pressure.
First ammonium-carbamate is formed, which rapidly decomposes to form urea:

Reaction Parameters

Reaction is promoted by following parameters

 High Pressure (3200 Psig)

 High Temperature (375~383 F)

 Excess Ammonia (4.5:1, NH3:CO2)

 Residence Time (25 to 30 min)

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Reaction is retarded by the following parameters.

 Excess of Water

 High temperature at insufficient pressure

 Excess retention time in reactor

Synthesis Section

Urea unit synthesis section is where urea synthesis reaction takes place to
synthesize urea. In synthesis section raw materials ammonia, carbon dioxide
and recycle carbamate solution are pumped to reactors at sufficient pressure and
temperature to promote the reaction. Various equipment and machinery is
employed for this purpose.

Equipment and Machinery involved in synthesis section

No Name Quantity Tag No.

1 High pressure ammonia feed 3 P-201 A/B/C
pumps
2 High pressure carbamate feed 4 P-202 A/B/C
pumps P-2202 D

3 High Pressure CO2 2 C-201 A/B

compressors

4 Medium pressure CO2 1 C-2111

compressor

5 Ammonia preheaters 2 E-2153 A/B

6 Ammonia heaters 2 E-201A/B

7 Carbamate preheaters 2 E-2151 A/B

8 Carbamate heaters 2 E-2152 A/B

8 Reactors 2 V-2201 A/B

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Synthesis Section Equipment Description

Ammonia Pumps

The Purpose of ammonia pumps is to make desired pressure of 3200 psig.

The ammonia pump is a multiplex plunger pump having 5 Plungers. The
lubrication of the pump is done by lubrication pumps.
Several Alarms and Tripping securities have been allocated to the pump for
security purposes. At 3580 psig, the Pump trips.

Carbamate Pumps

The Purpose of Carbamate pumps is to make desired pressure of 3200 psig. The
carbamate pump is a multiplex plunger pump having 5 Plungers. The
lubrication of the pump is done by lubrication pumps.
Several Alarms and Tripping securities have been allocated to the pump for
security purposes. At 3580 psig, the Pump trips.

Hp CO2 Compressors

The Purpose of CO2 compressor is to make desired pressure of 3200 psig. The
CO2 compressor is a 5 stage Reciprocating Piston Pump. The intercooling of the
gas and cylinders of each stage is done by Tempered water.
Several Alarms and Tripping securities have been allocated to the Compressor’s
each stage for security purposes.

Reactor Feed Description

Ammonia Feed

Ammonia is obtained from 2 sources

i) From Ammonia Storage Tank –LiC-2701-1
ii) Direct from ammonia plant –LiC-2701-2

Ammonia is received in TK-200 and from here it is transferred to TK-208.

From TK-208 it is transferred to Ammonia pumps (P-201 A/B/C) and then to
reactor V-2201 A/B.

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CO2 Feed

CO2 is directly obtained from Ammonia plant (FRe-18).

It is then transferred to CO2 compressors for achieving the desired pressure.

Carbamate Feed

Carbamate Feed is basically the recycled/decomposed Urea into its constituents.

The Carbamate feed from E-209 (High Pressure Absorber Cooler) is received
and transferred to the reactor via Carbamate pumps at the desired pressure.

Urea Reactors V-2201 A/B

Urea Reactor is basically is type of Plug Flow reactor (Tubular Reactor). All the
input feeds at desired Temperature and Pressure fed into the reactor tube where
exothermic reaction takes place (Carbamate formation) and then this heat is
utilized to dehydrate carbamate into urea in the shell side of reactor.

Instrumentation

Major instrumentation for synthesis section are as follows.

i) Motoyama Valves
ii) Block Valves
iii) Reactor’s Instrumentation
iv) Angles Valve
v) Let Down Valves

Motoyama Valves

Motoyama valves are quick/snap shut off valves. These valves are provided at
ammonia, carbamate and carbon dioxide feed lines of reactors. These valves
close within 2 seconds of tripping of corresponding pump or compressor to
avoid the back flow of reactor solution to the respective lines.

Block Valves

These valves are provided at ammonia, carbamate and carbon dioxide feed lines
before Motoyama valves. These valves help in the protection and isolation of
Motoyama valves.

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Reactor’s Angle Valve

This valve is provided at the downstream of reactor and its purpose is for
isolation and pressure control of reactor.

Let Down Valves

There are 2 letdown valves

i) Main letdown Valve
ii) Mini letdown Valve
The purpose of both letdown valves is to reduce the reactor pressure to 320-330
psig so that effective flashing takes place in liquid distributor.

Reactor’s Instrumentation

Reactor is provided with pressure, temperature, flow and mole ratio control
system. Following instruments are installed.
 Reactor pressure indicator controller
 Top temperature indication
 Bottom temperature indication
 Coil inlet temperature indication
 Flash temperature indication
 Temperature differential indicator controller

Tempered Water System

Tempered water is basically Demineralised water treated with Sodium

Dichromate and Caustic Soda. Sodium Dichromate creates a passivation layer
against corrosion and caustic soda helps in maintain the pH of this water.
Importance:
The purpose of Tempered water is to control the temperature of CO2 gas in
compressions. As at elevated pressure CO2 may freezes which are a huge
emergency resulting in the destruction of piping and vessels.

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Activities performed

 Line Tracing of synthesis section

 Material Balance Around UREA Reactors
 Daily Urea Log sheet and Lab analysis Study
 Average calculations for DPPR
 Performance Evaluation of Urea Reactors
 Passivation air survey for Urea plant
 Critical parameters of synthesis section study
 Comparison of Design and operating parameters of C-201A/B C-2111
 Simulation of urea reactor on Aspen plus
 Simulation of Synthesis Section on Aspen plus

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Material Balance

Material Balance is performed by 2 methods

i) Conventional with Stoichiometry

ii) Elemental Balance on Carbon Basis

Conventional Balance with stoichiometry across V-2201 A

Carbamate
Composition
(E-209 Total Urea 1501 Ton/day
analysis) Production
NH3 42.1 % Urea 750.5 Ton/day
CO2 32 % production
Urea 7.54 % of Reactor
H2O 18.3 % A
68952.2 Lbs/hr
1149.2 Lbmol/hr
HP NH3 283 Gal/min
Flow rate
86262 Lbs/hr
5074.24 Lbmol/hr Carbamate 222.5 Gal/min
Flow rate
From Stoichiometry 125907 Lbs/hr
2NH3 + CO2 = Urea + H2O
CO2 Required for 750.5 1149.2 Lbmol/hr
Carbamate Flow 125907 Lbs/hr
T Urea Rate
50564.9 Lbs/hr NH3 in Carbamate 53007 Lbs/hr
NH3 Required for 750.5 2298.41 Lbmol/hr CO2 in Carbamate 40290.4 Lbs/hr
T Urea Urea in Carbamate 9493.42 Lbs/hr
39072.9 Lbs/hr H2O in Carbamate 23041.1 Lbs/hr
Water Produced 750.5 T 1149.2 Lbmol/hr
Urea
20685.7 Lbs/hr

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 Total Feed Effluent
to Reactor Composition
A NH3 100196 Lbs/hr 40%
Total NH3 139269 Lbs/hr 56% CO2 26834.4 Lbs/hr 11%
Total CO2 77399.3 Lbs/hr 31% Urea 78445.6 Lbs/hr 31%
Total H2O 23041.1 Lbs/hr 9% H2O 43726.7 Lbs/hr 18%
Total Urea 9493.42 Lbs/hr 4% Total 249203 Lbs/hr
Total 249203 Lbs/hr 100% Product

Elemental Balance on Carbon Basis

HP CO2 37108.91
(Lbs/hr)
Carbamate 125907.45 Lbs/hr
NH3 53007.03 Lbs/hr
CO2 40290.38 Lbs/hr
Urea 9493.42 Lbs/hr
H2O 23041.0 Lbs/hr

UREA REACTOR

NH3
86262.03 Lbs/hr

Product 237820.1 (Lbs/hr)

NH3 87256.2 37% (Lbs/hr)
CO2 23425.28 10% (Lbs/hr)
Urea 83094.35 35% (Lbs/hr)
H2O 44044.29 19% (Lbs/hr)

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Carbon Balance

𝐶 𝑖𝑛 𝑐𝑎𝑟𝑏 𝑎𝑠 𝐶𝑂2 𝐶 𝑖𝑛 𝑐𝑎𝑟𝑏 𝑎𝑠 𝑢𝑟𝑒𝑎 𝐶 𝑎𝑠 𝑝𝑢𝑟𝑒 𝐶𝑂2

( )+( )+( )
44 60 44
𝐶𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛/100 𝐶𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛/100
=( )𝐹 + ( )𝐹
44 60
All Composition and input flows are known and Outlet flow F is calculated on
basis of carbon balance.

Reactor Performance

Reactor Performance tells about the NH3/CO2 ratio Water/CO2 ratio with reactor
efficiency as follows.

So on the basis of Material balances performed Reactor performances are as

follows

Elemental Balance Stoichiometrically

NH3/CO2 4.657163 NH3/CO2 4.657163
X CO2 0.697345 X CO2 0.6533
H2O/CO2 0.727689 H2O/CO2 0.727689
K K

Comparison with Daily performance Report

Parameters DPPR Elemental Balance Stoichiometric

NH3/CO2 4.69 4.657163 4.657163
X CO2 0.6837 0.697345 0.6533
H2O/CO2 0.72 0.727689 0.727689
K 1.57 1.75 1.7

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PSV’s and Vents Surveys

This survey is done after the startup of Urea Plant to ensure any leakage of PSV or
Vent for safe operation. If the upstream and downstream temperatures are equal
then it shows there is leakage.
If the downstream temperature is less than the upstream temperature then the PSV
or Vent is fine and need no maintenance. Sometimes the downstream temperature
is higher than the upstream temperature this is in case of critical equipment’s
where steaming is done such as reactor or carbamate lines.
Upstream Downstream
Sr. No Equipment
temperature (oC) temperature (oC)
1 C-201 A 2nd stage 42 31
2 C-201 A 3rd stage 40 33
3 C-201 A 4th stage
4 C-201 A 5th stage 58 30
5 C-201B 1st stage 39 29
6 C-201B 2nd stage 38 30
7 C-201B 3rd stage 38 32
8 C-201B 4th stage
9 C-201B 5th stage 62 29
10 PICe-24a vent 24 23
st
11 C-2111 1 stage 81 31
nd
12 C-2111 2 stage 108 30
rd
13 C-2111 3 stage 85 45
14 Reactor A-1 52 87
15 Reactor A-2 61 67
16 Reactor B-1 43 84
17 Reactor B-2 45 65
18 E-2161 A 65 68
19 E-2161 B 70 50
20 V-2161 (from Rx A) 85 32
21 V-2161 (from Rx B) 81 29
22 V-2162 96 83
23 V-2163 41 83
24 V-2171 73 42

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Simulations

2 simulations are performed on aspen plus

i) Urea Reactor Simulation with conventional elementary kinetics.

ii) Simulation of Synthesis section.

Reactor Simulation

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