7.5
10 12 14 16 18 24 26 20 22
7.0
6.5
6.0
5.5
5.0
4.5
0 6 4 2 8
4.0
Corrosion rate (mil/yr) pH (Y2)
3.5
3.0
Temperature (˚C)
Figure 4 Corrosiveness of the TPA stream
• Pump discharge pressure at 11 barG • Water content in TPA of 500 ppm
potential of carbon steel. As a result, the propensity for pitting due to differential current is elevated, as illustrated in Figure 5 . Finally, we highlight that all the previous surveys have been conducted assuming a constant input of HCl and NH 3 into the system. To illustrate the effect of varying amounts of HCl and NH 3, we present one last surface graph (see Figure 6 ) to demonstrate the impact of changes in both compounds at 170°C and 11 barG at the inlet of the TPA pump. These simulations underscore the necessity of employ- ing wash water in the TPA circuit in quantities sufficient to ensure that a substantial amount of water remains liquid at the injection point. This prevents solid salt deposition in the hot zone and the formation of a highly corrosive liquid phase in the cold zone. Determining the appropriate amount of wash water to be injected is crucial to ensure that downstream of the injec- tion point, at least 25 wt% of the injected quantity remains in the liquid phase. This ensures effective dispersion and protection of all piping and heat exchanger zones. This also
• Saturated water conditions during extraction: 600 ppm Cl-, 850 ppm NH 3 , 1 ppm HCN referred to the entrained water. NH 4 Cl deposition starts at around 137°C and increases to ~150 g/hr by 95°C. Below this temperature, a condensed water phase forms and dissolves the salt. This condensed liquid phase is corrosive because the dissolved NH 4 Cl is the combination of a weak base (NH3 ) and a strong acid (HCl) and produces an acidic solution. Figure 4 evaluates the corrosiveness of the TPA stream as a function of tempera- ture on carbon steel. This evaluation is conducted within the temperature range where an aqueous phase is present based on the previously simulated process conditions. The preceding graph highlights generalised corro- sion rates exceeding 2 MPY at temperatures above 74°C and reaching >20 MPY at the condensation point. Also, the presence of a highly concentrated ionic phase results in a corrosion potential exceeding the re-passivation
10.0 10.5
1778
Corrosion potential (mV (Ag/AgCl)) Repassivation potential (mV (Ag/AgCl) Maximum pit current density (micro-Amp/sq-cm) (Y2)
1.0 2.5 2.0 1.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 7.0 6.5 9.5 8.0 8.5 9.0 7.5
1278
778
278
-222
-722
-1222
-1722
-2222
Temperature (˚C)
Figure 5 Propensity for pitting
113
PTQ Q4 2023
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