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CPB30803 DESIGN PROJECT 2 (PLANT & PROCESS OPTIMIZATION) L01-P10 JANUARY 2016 DESIGN A UREA PLANT WITH CAPACITY OF 100, 000 METRIC TONNES / YEAR SUPERVISOR: MR SYAHIDI FADZLI ALFAN SHARIFAH ADAWIYAH SYED IDRUS 55201113513 MUHAMAD NIZAMUDIN MUSTAFA 55201113601 MOHAMAD AZHAM SHAHARUDDIN 55201113660 SITI SYAZWANI MOHD NASIR 55201113584 MUHAMMAD IZZAT HAFIZUDDIN MOHD SHAH 55201214260

Designing Urea Reactor

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Page 1: Designing Urea Reactor

CPB30803

DESIGN PROJECT 2

(PLANT & PROCESS OPTIMIZATION)

L01-P10

JANUARY 2016

DESIGN A UREA PLANT WITH CAPACITY OF 100, 000

METRIC TONNES / YEAR

SUPERVISOR: MR SYAHIDI FADZLI ALFAN

SHARIFAH ADAWIYAH SYED IDRUS 55201113513

MUHAMAD NIZAMUDIN MUSTAFA 55201113601

MOHAMAD AZHAM SHAHARUDDIN 55201113660

SITI SYAZWANI MOHD NASIR 55201113584

MUHAMMAD IZZAT HAFIZUDDIN MOHD SHAH 55201214260

Page 2: Designing Urea Reactor

Designing Urea Reactor

Consideration Features in Chosen Type of Reactor

This process implicated the reaction between gas and liquid. The liquid mixture of NH3

and carbamate (180˚C) and gaseous CO2 (140˚C) are fed to reactor. These two different phases

classified as heterogeneous reaction. They meet at 190ᵒC and 175 atm pressure inside the reactor

to form ammonia carbamate (NH2COONH4). The reaction taking place in the parameters of

reactor are as follows.

2NH3 + CO2 ↔NH2COONH4 + Heat ΔH = 136.23 kJ/mol(Exothermic reaction)

NH2COONH4 + Heat ↔ NH2CONH2 + H20 ΔH = -17.57 kJ/mol(Endothermic reaction) – dehydration process

Overall reaction: 2 NH3 + CO2 ↔ NH2CONH2 + H20 ΔH = 118.66 kJ/mol(Exothermic reaction)

From the above reaction which occurs in the reactor, it can be identified as reversible

process. Whereby, the reaction between 2 mols of liquid NH3 and 1 mol of gaseous CO2 will

produce 1 mol of NH2COONH4 and 1 mol of H2O. This reaction will react reversely forming

back 2 mols of NH3 and 1 mol CO2. This reversible reaction basically evaluated at equilibrium

condition. Following are the kinetics parameter involving in the reaction inside the reactor.

However, this reaction considered as liquid phase reaction due to the outlet flow out from the

reactor in liquid form.

Table 1 Parameter in the reactor

Parameter Value

Working temperature 190ᵒCDesign temperature 250ᵒCWorking pressure 175 atmDesign pressure 210 atm

Ammonium carbamate conversion 70%Activation Energy, Ea 60.93kJ/molFrequency factor, A 4.259 X 105 min -1

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Page 3: Designing Urea Reactor

The reaction rate constant, k was determined by using Arrhenius equation

k=Ae−E a

RT

From the existence parameter, the value of k was calculated as follows.

k=(4.259 X 105min ˗1)e−60930J /mol

(8.314 J /molK )(463.15Kk)

k=0.05719min−1

k=3.4314hr−1(First order reaction)

The value and unit obtained drive to the first order of reaction. So, the appropriate rate law is

−r A=k C A

This shows the reaction obeys a non-elementary Rate Law whereas the rate equation cannot be

determined by looking at the stoichiometric coefficient. Therefore, all the features obtained

correspond to plug flow reactor (PFR).

Figure 1 Plug flow reactor

Mass & Heat Balance of Reactor3

Page 4: Designing Urea Reactor

Table 2 Mass Balance of each component inside reactor

6.3.1.1 Reactor

Reactor Inlet

NH3: 56668.0559/YEAR

Stream 1(i): 3509.447527T/YEAR

C02: 0.8709

NH3: 0.1291

Stream 1(ii): 270994.4333T/YEAR

Stream 1(i): 3509.447527T/YEAR

C02: 0.114

A.C: 0.675

NH3: 0.211

Reactor

NH2COONH4 + heat ↔ NH2CONH2 + H2O ΔH = -17.57 kJ/mol2NH3 + CO2 ↔ NH2COONH4 + heat ΔH = 136.23 kJ/mol

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Page 5: Designing Urea Reactor

Table 3 Heat balance for Reactor

1. Total,

=∆Ḣ1+∆Ḣ2+∆Ḣ3

=185330.5438 MJ/day

2. Ammonium Carbamate formation heat, ∆Ḣ5

∆H5= 409.34477*(−117 )

78∗103

∆H5 = -614021.5741 MJ/day

3. Urea formation heat by decomposing Ammonium Carbamate, ∆Ḣ6

∆H6 = 1060.256*15.5/60* 〖10〗^3

= 273899.4667 MJ/day

4. Energy generated in to reactor

Q=∆Ḣ=∑out

ṅḢ-∑¿ṅḢ

= 273899.5744+185330.5438-614021.5841

= -154791.5744 MJ/day

Reactor Volume5

Page 6: Designing Urea Reactor

Based on Table 2, inlet each component for the reactor are converted from T/Yr to kg/hr.

The values are as follows.

Table 4 Inlet reactor

Components Inlet Reactor, T/Yr Inlet Reactor, g/hr

NH3 102526.1973 10617.6

CO2 24839.6430 2572.39

NH2COONH4 147138.1395 16796.59

Determination of inlet volumetric flowrate, V0 for each component, density inlet must be

considered.

Table 5 Density Inlet

Components Density (kg/m3)

NH3 (liquid) 618 ( Ref: J H Perry)

CO2 (gas) at 40ᵒC 277.38 (density=PM/RT; P=162 atm,T=313 K)

Ammonium carbamate 1600 (Ref: http://www.inorganics.basf.com)

Following are the calculation and value of V0 for each components and total.

Table 6 Inlet volumetric flowrate

Components mρ

Inlet volumetric flowrate, V 0 (m3/hr

V Ao, NH 3 10617.6 kghr ˗ 1618 kgm‐ 3

17.181

V Bo, CO2 2572.39 kghr ˗ 1277.38 kgm‐ 3

9.274

V Co, NH 2COO NH 4 16796.59 kghr ˗ 11600 kg m‐3

10.498

Total, V ¿ 36.953

Reactor Volume, V PFR

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Page 7: Designing Urea Reactor

V PFR=FAo∫0

x dX−r A

−r A=k C A

Ca=FA

V=F Ao(1−X )

V 0=C Ao (1−X )

V PFR=FAo∫0

x dX−r A

V PFR=FAo∫0

x dXkC A

V PFR=FAo∫0

x dXkC Ao (1−X )

V PFR=F Ao

kC Ao∫0

x dX(1−X )

V PFR=F Ao

kC Ao[ ln 1

1−x ]V PFR=

V o

k [ ln 11−x ]

V PFR=36.953m3/hr3.4314hr−1 [ ln 1

1−0.7 ]V PFR=12.966m3

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Page 8: Designing Urea Reactor

Residence time, t

t= VV 0

t= 12.966m3

36.953m3/hr

t=0.351hr X 60min1hr

t=21.05min

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Mechanical design of the reactor

Materials for designing reactor

Ammonium Carbonate is one of chemical which is physically very corrosive. So

equipment and piping of High pressure (HP) synthesis section need to be constructed with a

corrosion resistant material. Moreover, stainless steels (SS) are the candidate materials

unfortunately conventional SS grades such as SS 304, SS 316 do not withstand with corrosion

due to carbonate. Before this, SS 316L UG (urea grade) was used with high amount of oxygen

supply. Coefficient of thermal expansion of this alloy is low (1.2 mm/m/100oC) compare to

other metals like steel (1.2 mm/m/100oC) making it perfect to use in industries (Brouwer, 2009)

Equipment used in urea process such as Urea reactor, Urea stripper, Carbonate condenser,

are constructed with Carbon steel lined or covered with a Protective layer of the above

mentioned Stainless steel materials. The thickness of the liner varies between 4 and 10 mm

(Brouwer, 2009). Welding is used as prominent method for fabricating such huge equipment.

Welding procedures are qualified by optimizing weld parameters in order to meet stringent. The

qualification is adopted for job, with appropriate quality checks in production welds also. Recent

developments in construction materials to curb corrosion mechanism operative in HP section is

also integrated.

Urea plant operates at high temperature / high pressure and carbonate solution, the

intermediate product is extremely corrosive. Ammonium carbonate at approximately 180-250°C

and 180bar is extremely aggressive to materials. Candidate MOC for such aggressive

environments is Zirconium, Titanium, Duplex Stainless steel (SAFUREX & DP-28W) and

stainless steel (25Cr-22Ni-2Mo & 316L UG). Titanium had been used widely in the synthesis

reactor of the total recycle plants till early 1970s and the high pressure (HP) stripper of ammonia

stripping process till early 1990s. It has good passivation property with less passivation air.

However titanium is susceptible to erosion and it is difficult to weld. Other than that, The life

time of titanium is limited (Juneja, 2013).Due to this disadvantages, titanium has been gradually

taken over by stainless steel. Stainless steel has been widely used for this equipment in urea

plants. Stainless steel is almost immune to erosion and has good weldability but requires large

amount of passivation air for urea synthesis equipment compared to titanium. Type 316L UG has

been used for a long time in urea plants mainly because of its excellent weldability, fair

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Page 11: Designing Urea Reactor

corrosion resistance and relatively low cost. Requirement of huge amount of passivation air by

316L UG in synthesis and recycle sections restricts its operability.

Table 7 Advantages and disadvantages of material types

Type 25Cr-22Ni-2Mo SS is being used due to its better corrosion resistance than 316L

UG and excellent weldability. This type of metal has been used in reactor and strippers but it is

susceptible by chloride to SCC and costly. Duplex SS shows excellent corrosion resistance in

both Weld metal and HAZ. They possess better resistance to Stress Corrosion Cracking, hence

used in chloride environments as well. In oxygen free carbonate solutions, duplex has proved to

be more corrosion resistant than much more costly materials such as Titanium and high nickel

alloys. Various MOC used in urea plants worldwide, their advantages and disadvantages are

depicted in Table 1.

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Page 12: Designing Urea Reactor

Accessories of the reactors

1. Cooling jacket.

There are different cooling jackets are available in the market depending the operating

conditions. However in order to find the right jacket, several factors need to be considered, for

example, cost, heat transfer rate required and pressure as a rough guide. Half pipe cooling jacket

can withstand pressure up to 70 bar (Sinnot & Towler, 2009, pp. 956-957) and less pressure

drop than other jackets. In order to fulfil the design operating conditions (140bar and 453K), half

pipe cooling jacket can be used as it can withstand high pressure. Standard sizes of the half pipe

jacket are 2 3/8, 3 ½ and 4 ½-in.o.d. Thickness can be used as 3/16-in. for 2 3/8-in.od. and ¼ in

for both 3 ½ and 4 1/2 –in.od (McKetta, 1992, pp. 423-424) however cooling jacket diameter

and thickness can be slightly varied in order to accommodate the higher pressure and

temperature conditions and will be shown in the below calculations. Half pipe cooling jacket is

normally fabricated by 304 stainless steel to avoid problem like differential thermal expansion.

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Page 13: Designing Urea Reactor

Heat transfer device

DESIGN OF AMMONIA PREHEATER

Ammonia inlet flow rate = 13.82503 kg/hr

Specific heat of ammonia = 5.14 KJ/kgᵒC

Inlet ammonia temp.= 89.6F = 34 ᵒC

Outlet ammonia temp.= 338 F = 170 ᵒC

Heat required by ammonia, mcѲ

= (13.82503 kg/ 3600 sec) * 5.14 KJ/kgᵒC * (170-34) ᵒC

= 2.6845 KJ/sec

= 2.6845 KW

Latent heat of steam = 503.7 cal/mol= 2105.466 J/mol

Therefore, m*2105.466 = = 2.6845

m = (2.6845*1000)/2105.466

m = 1.27501 mol/sec =0.07082 kg/sec

LMTD = (ΔT1 - ΔT2) / ln(ΔT1/ΔT2)

= {(374 – 89.6) – (374 – 338 )}/ ln (284.4/36) = 120.182

Area A = Q/(U*LMTD)

= (2.6845 *1000)/( 0.1435*120.182)= 155.658 m2

Choose 20mm O.D., 16mm I.D., 4.88m long tubes,

L= 4.83m

Area of one tube = 3.14*d*l= 3.14*4.83*(20/1000) = 0.303 m2

No.of tubes= 155.658/0.303 = 513

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Page 14: Designing Urea Reactor

COLD FLUID TUBE SIDE

Mean ammonia temp.= (338+89.6)/2 =213.8 F= 101 o c

Tube cross-sectional area =(3.14/4)*162= 201mm2

Tubes per pass =no.of tubes/2 = 513/2 = 256.5 = 256

Tube flow area = (256*201)/1000000= 0.051m2

Ammonia mass velocity= 13.82503 /(60*60*0.051)= 0.075 kg/sec m2

Density of ammonia = 0.618 g/ml= 618 kg/m3

Ammonia linear velocity, ut= 0.075/618 = 1.21 x 10-4 m/sec

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Page 15: Designing Urea Reactor

Instrumentation and Process Control

Level Control inside the Reactor ( Cascade Control )

Output Flowrate

Liquid Level

Reactor Level is affected by changes in output flow rate

Control Strategy: handle reactor level by adjusting the flow rate of the product output.

If a disturbance in output flow rate occurs, FC will act quickly to hold the output flow rate at its Set Point.

Control system measures Reactor level and compare it to set point level of the reactor. Then uses the resulting error signal as the input to

a controller for output flow rate.

Liquid level

Flow Controller Output Valve Output Flow Process

Reactor LevelLevel Controller

Page 16: Designing Urea Reactor

Temperature Control inside the Reactor ( Cascade control )

Steam Flowrate

Temperature inside reactor

Reactor Temperature is affected by changes in reactant feed Temperature

Control Strategy: handle reactor temperature by adjusting the flow rate of the steam on the steam jacket.

Primary control loop (TT & TC)

Secondary (FT & FC)

The hot steam is used by MASTER controller (TC) to establish the set point for SLAVE loop controller.

Secondary measurement is fuel steam flow rate.

If a disturbance in steam supply occurs, FC will act quickly to hold the Steam flow rate at its Set Point.

Temperature

Level

Flow Controller Steam Valve Steam Flow Process

Reactor Temperature

Temperature Controller

Page 17: Designing Urea Reactor

Control system measures Jacket Temperature and compare it to set point temperature of the reactor. Then uses the resulting error signal as

the input to a controller for steam makeup.

PRINCIPAL: the 2ndmanipulated variable is located closed to potential disturbance & react quickly

Page 18: Designing Urea Reactor

Pressure control inside the Ractor ( Cascade control )

Gas Flowrate

Reactor Pressure

Reactor Pressure is affected by changes in gas flow rate

Control Strategy: handle reactor pressure by adjusting the flow rate of the gas.

If a disturbance in gas flow rate occurs, FC will act quickly to hold the gas flow rate at its Set Point.

Control system measures Reactor pressure and compare it to set point level of the reactor. Then uses the resulting error signal as the input

to a controller for gas flow rate.

Pressure Level

Gas Flow Controller Gas Valve Gas Flow

ProcessReactor Pressure

Pressure Controller

Page 19: Designing Urea Reactor

Overall Process Control at Reactor

Heat Exchanger

Gas

Ammonia CarbamateNH3

steam

TTTC

TC

FC

FC

LC

PT

PCFC

Page 20: Designing Urea Reactor
Page 21: Designing Urea Reactor

Cascade is desired when the single loop performance is unacceptable and a measured

variable is available. Besides that, the secondary variable must indicate the occurrence of an

important disturbancein the system. Furthermore, the secondary variable also must have a

faster response than the primary which is 4 times faster than the primary in order to get a

better control.

ADVANTAGES OF CASCADE CONTROL

The cascade control is an improvement of the feedback and feed forward control

system because the conventional feedback usually take the corrective action for disturbance

after the control variable deviates from set point. Besides that, the feed forward requires to

calculate the disturbance explicitly and hence available to calculate the control variable.

Furthermore, employment of secondary measurement point and secondary feedback controller

are required for recognizes the upset condition sooner.

In conclusion, cascade control system are much more applicable in the reactor control

system because it have large improvement in performance when the

secondary is much faster than primary, simple technology with PID algorithms, use of

feedback at all levels since primary has zero offset for “step-like” disturbances.

Furthermore, plant operating personnel find cascades easy to operate because opening

cascade at one level cause all controllers above to become inactive.