Materials Selection for the Urea Production Process-Enrique Maya Teaching Project ChemRxn

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

Corrosion under Urea production process

Substantial savings can be obtained in many types of plants through the use of corrosion –resistant materials of construction (Fontana 1986). The most severe corrosion in a urea plant can occur in the reactors. However, it can be controlled through the careful selection of construction materials and initiation of proper corrosion control procedures. (Krystow 1971)

The objective of every industry is to generate money at the lower cost operation; material selection will help to avoid non schedule shutdowns, longer life equipment and a continuous operation increasing the profits of the company.

The production of Urea involves the reaction of ammonia with carbon dioxide at 20 to 25 MPa and 190 ◦C. A corrosive intermediate, ammonium carbamate, is responsible for the need for corrosion-resistant construction:

2NH3 + CO2 → NH2COONH4 → NH2CONH2 + H2O eq 1

Where

NH3: Ammonia (it is substantially noncorrosive)

NH2COONH4: Ammonium Carbamate (extremely corrosive)

NH2CONH2: Urea (it is substantially noncorrosive)

The idea of this job is to find an explanation of the corrosion process that attack reactors during the production of urea. There are multiple failures reported, some catastrophically that yields to explore more in detail of this process. (Wang 2009)

The present study makes an estimation of the kinetics parameters of the production of urea; the objective is to find an explanation why this reaction is so aggressive in terms of corrosion to the materials to the construction material of the reactor (titanium, stainless steel)

The production of ammonia into urea can be used to produced fertilizer or just as a carbon collector in the flue gas (Conway 2011).

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

The Corrosion process in any metal needs the following to occur:

1. Anode2. Cathode3. Electronic path4. Ionic path or electrolyte

The corrosion process is an electrochemical process that involves two reactions:

M → Mn+ + ne- anodic reaction

2H+ + 2e- → H2 cathodic reaction

The Pourbaix diagrams are an excellent tool to exemplify the behavior of the metals in aqueous solutions (Uhlig 2011), Figure 1 shows the Fe Pourbaix diagram at 298 K (25 C), the disadvantages of this tools is that can only be constructed from pure materials, but ii is a good approximation.

Figure 1 The Fe Pourbaix diagram at 298K (25 C) (Uhlig 2011)

The following figures show the way we should read these diagrams and help us to understand the chemical stability of the metal in a certain environment.

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

Figure 2 Maps of the 3 things that can Happen (Corrosion-Akron n.d.)

Figure 3 Iron zones of stabilities (Corrosion-Akron n.d.)

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

Urea corrosion mechanismUrea is produced on a scale of some 100,000,000 tons per year worldwide (Wikipedia 2011). However, the corrosion problems, and the corrosion mechanism are not yet understood, but with this work with its limitations have the idea to visualize what is happening inside these reactors.

NH3 + CO2 → NH2COOH (1)

NH2COOH + NH3 ↔ (NH2)2CO + H2O (2)

There are not enough information of the kinetics of urea formation from ammonia and carbon dioxide reaction, but this give me the freedom to play with this kinetics and estimate how these reactions goes forward inside the reactor.

Writing the reaction in a simplify way we can get the following:

A + B → C r1A= -k1CACB

C + A ↔ D r2A= - k2(CACC- CD/K)

Assuming that these reactions are first order and there are no any change in T and P, we can get the following result. We can see in the graph that the carbamic acid (NH2COOH) has a really slow conversion into urea, giving it the opportunity to interact with the metal surface and starting the corrosion process. The corrosion process could be explained as follows (Han 1998):

NH2COOH ↔ NH2COO- + H+ (3)

Me + xNH2COO- ↔ Mez+( NH2COO-)x + Ze- (4)

Mez+( NH2COO-)x ↔ Mez+ + xNH2COO- (5)

The corrosion process can be illustrated in Figure 5, the metal starts from point 1 in the passive behavior and after the urea production starts, the pH inside the reactor changes to more acidic values making the material more susceptible to the corrosion attack point 2. Carbamic acid reaction to form Urea is really slow, compare to the first reaction that forms the carbamic acid. This explains all the time that carbamic acid has to attack the surface of the material and start the corrosion process. As urea production continues the corrosion grows until a failure is present and brakes the material.

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

0 1000 2000 3000 4000 5000 60000

50

100

150

200

250

C NH2COOH, k1=1e-4,k2=1e-5C urea, k1=1e-4,k2=1e-5

t

Conc

entr

ation

Figure 4 Urea formation from carbamic acid

Figure 5 Corrosion process (Corrosion-Akron n.d.)

saas 12

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

Figure 6 Reactor explosion (Wang 2009)

Figure 7 SCC possible cause (Wang 2009)

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

SCC (Stress Corrosion Cracking)

Localized corrosion process, the conjoint of a tensile stress and a corrodent will in some instances result in the cracking of a metal alloy. Stresses that cause cracking arise from residual cold work, welding, thermal treatment or may be externally applied during service. (Carpenter 1971)

Figure 8 SCC intergranular crack (Wang 2009)

To finish this work I want to point that the corrosion process in these urea reactors are not well understood, but we can prevent the corrosion problems with the proper materials selection, for these case, stainless steel and titanium, but buy these material with and certified provider in order to avoid any mechanical problem in the material, and to have an adequate maintenance program to avoid any catastrophic consequence like the explosion on the reactor.

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

Polymath program

Calculated values of DEQ variables Variable

Initial value

Minimal value

Maximal value

Final value

1 CA 500. 100.4004 500. 100.4004

2 CB 200. 3.639E-47 200. 3.639E-47

3 CC 0 0 123.0523 0.4004307

4 CD 0 0 199.5996 199.5996

5 K 5. 5. 5. 5.

6 k1 0.0001 0.0001 0.0001 0.0001

7 k2 1.0E-05 1.0E-05 1.0E-05 1.0E-05

8 r1A -10. -10. -3.653E-49 -3.653E-49

9 r1B -10. -10. -3.653E-49 -3.653E-49

10

r1C 10. 3.653E-49 10. 3.653E-49

11

r2A 0 -0.2757209 0 -2.835E-06

12

r2C 0 -0.2757209 0 -2.835E-06

13

r2D 0 0 0.2757209 2.835E-06

14

rA -10. -10. -2.835E-06 -2.835E-06

15

rB -10. -10. -3.653E-49 -3.653E-49

16

rC 10. -0.2629426 10. -2.835E-06

17

rD 0 0 0.2757209 2.835E-06

18

t 0 0 10000. 10000.

Differential equations

1 d(CA)/d(t) = rA

2 d(CB)/d(t) = rB

3 d(CC)/d(t) = rC

4 d(CD)/d(t) =

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet

rD

Explicit equations 1 k2 = 0.00001

2 k1 = 0.0001

3 K = 5

4 r2A = -k2*(CA*CC-(CD/K))

5 r1A = -k1*CA*CB

6 r1B = r1A

7 rA = r1A+r2A

8 r2C = r2A

9 rB = r1B

10 r2D = -r2A

11 r1C = -r1A

12 rC = r1C+r2C

13 rD = r2D

References1. Carpenter. Corrosion Causes and Control. NY: McGraw-Hill, 1971.

2. Conway, William. "Kinetics of the Reversible Reaction of CO2(aq) with Ammonia in Aqueous Solution." The Journal of Physical Chemistry, 2011: 6405-6412.

3. Corrosion-Akron. n.d.

4. Fontana, Mars G. Corrosion Engineering. McGraw-Hill Book Company, 1986.

5. Han, Wenan. "Approach to the Cause of Corrosion in Urea Medium." J. Mater. Sci. Technol., 1998: 92-94.

6. Krystow, P. E. "Materials and Corrosion Problems in Urea Plants." Chemical Engineering Progress 67 (1971): 59-64.

7. Uhlig, Herbert. Corrosion Handbook, 3rd edition. The Electrochemical Society, 2011.

8. Wang, Weiqiang. "The explosion reason analysis of urea reactor of Pingyin." Engineering Failure Analysis, 2009: 972–986.

9. Wikipedia. Wikipedia. 11 19, 2011. http://en.wikipedia.org/wiki/Urea#cite_note-Ullmann-14 (accessed 11 19, 2011).

ChE 4200:605 Grad Reaction Kinetics Teaching Project Fall 2001Enrique Maya Visuet