5.0e-05
7.0
0.008
Carbonate Bicarbonate
4.0e-05
6.5
6.0
0.006
3.0e-05
5.5
0.004
5.0
2.0e-05
4.5
0.002
1.0e-05
4.0
3.5
0.000
0.0e-01
50
100
150
200
250
3.0
Temperature (˚C)
50.0
100
150
200
250
Temperature (˚C)
Figure 5 Bicarbonate and carbonate concentration as function of temperature in the REAC
Figure 4 pH dependency with temperature in the REAC
is applied with 5 ppm of H₂S and 40 ppm of NH₃, with 25 wt% of water phases. The specific outlet temperature of the REAC will significantly affect the physical properties of the sour water. Another important parameter to estimate is the potential for salts to form in the feed/effluent exchangers that absorb water from the hot effluent vapours. If NH₄Cl salts get wet, they form pits, known as pitting corrosion. In addition to causing metal loss that can develop leaks, pitting corrosion creates weak points in the metal structure that may cause cracks. Therefore, operators need to know under what con - ditions the NH₄Cl salts can get wet to prevent this issue. The relative humidity (RH) parameter is used by corrosion specialists to calculate if NH₄Cl salts will absorb water from the hot vapour stream. RH is calculated by obtaining the ratio of the partial pressure of water in the vapour stream and the saturated pressure of water at the process temper - ature, then multiplying it by 100%. NH₄Cl salts will start to absorb water from the vapour stream when the RH reaches at least 10%, with values between 30% and 50% showing the highest degree of absorption. In this case study, RH has been plotted across the tem - perature difference on the process side for the feed efflu - ent exchangers. The 10% RH threshold is surpassed once
temperatures get below 225ºC. From the graphs in Figures 2 and 3 , we can infer that the potential for salts to form and start to get wet happens at the NH₄Cl formation temperature. Therefore, the risk of corrosion and fouling significantly increases at temperatures below 190ºC. We conclude that when the outlet temperature of the feed/effluent exchangers stays at the current temperature of 250ºC, then the NH₄Cl salts will only form at the REAC, and the potential for the salts to get wet is mitigated with a proper water wash that will completely dissolve the salts. According to the Alkaline Carbonate Stress Corrosion Cracking (ACSCC) guidelines,5 , 6 the four conditions pro - moting corrosion in sour water systems are: a) presence of an aqueous phase, b) pH above 8, c) carbonates content ( CO 2₃ – ) above 100 ppm-wt, and d) piping not post-welding heat treated. Figure 4 shows the dependence of the pH with respect to temperature. In this case, the pH falls below 8. Additionally, the computer model can estimate the formation of bicarbo - nate ( HCO ₃ – ) and CO 2₃ – , shown in Figure 5 , and the concen - tration of CO 2₃ – does not exceed the ACSCC guidelines. The analysis is extended to the overhead system of the stripper, which includes the overhead condenser and the stripper reflux drum. The evolution of the pH in the
0.008
5.0e-05
8.0
0.0e-01 1.0e-05 1.5e-05 5.0e-06 2.0e-05 2.5e-05 3.0e-05 3.5e-05 4.0e-05 4.5e-05
0.007
Carbonate Bicarbonate
7.0
0.006
6.0
0.005
5.0
0.004
4.0
0.003
3.0
0.002
2.0
1.0
0.001
0.000
0.00
40.0 60.0
80.0
100 120
140 160
40.0
60.0
80.0
100
120
140
160
Temperature (˚C)
Temperature (˚C)
Figure 6 pH dependency with temperature in the overhead condenser
Figure 7 Bicarbonate and carbonate concentrations as function of temperature in the overhead condenser
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