PTQ Q2 2024 Issue

NH 4 HS corrosion risk table

Stream

Temperature (⁰C)

Corrosion rate (mm/y) *

pH 7.0 7.6 7.8 8.0 8.0

Metallurgy

[NH4 HS] (% wt )

HEX 10 out AC 01 out

135

0.35 0.50 0.45 0.75 1.25

CS CS CS CS CS

0.6 3.1 4.1 5.5 8.0

70 50 40 35

HP sep. water out LP sep. water out

Diesel stripper condensate

* For long-term service to be deemed acceptable, the corrosion rate must remain below 0.125 mm per year. The corrosion rates were determined based on the operational fluid velocity, which consistently remained below 6 m/s.

Table 3

balance was carried out on OLI Systems’ proprietary leg- acy process simulator, Flowsheet ESP, by reproducing the DHT process scheme and simulating its behaviour at aver- age process and contaminant level conditions (see Table 1 ). Each stream was then analysed in the proprietary OLI Studio: Corrosion & Stream Analyzer to gain insights into the electrolyte chemistry and assess the corrosion and foul- ing risk of each of the streams. It is worth noting that the unit is equipped with both inter- mittent and continuous wash water injection facilities. These are situated upstream and downstream of HEX 10, respec- tively, with an overall injection rate of 15-20 t/h of wash water. The wash water mainly originates from the vacuum distillation unit (VDU), which explains the elevated concen- trations of ammonia, sulphides, and observed oxygen. The result of the ionic survey evidenced the following issues: • Fouling issue: Dry NH 4 Cl β -solid formation in HEX 08-09 tube bundles with a RH of 0.7%. • Fouling issue: NH 4 Cl α -solid formation in HEX 09-10 tube bundles (the NH 4 Cl β -crystalline form transition to the α -form at 185⁰C) with a RH of 1.0%. • NH 4 HS corrosion: AC 01 outlet – HP and LP separator outlet (NH 4 HS concentration 3.0-5.5% wt ). NH 4 Cl deposition in HEX 08-09 is a commonly over- looked occurrence, with no proactive measures taken to address it (for example, feedstock quality control, feed pretreatment, wash water or salt dispersant injection, or operational parameter change). However, it is widely rec- ognised in both literature and practical plant experience that there is a significant risk of pitting corrosion when rel - ative humidity exceeds 10-15% for carbon steel material. While NH 4 Cl is not corrosive and primarily causes fouling when dry, it becomes corrosive when exposed to moisture due to its hygroscopic nature. The NH 4 HS corrosion issue is relatively mild, extending from the AC01 outlet to the LP separator cold and diesel stripper overhead areas (see Table 3 ). This corrosion is sen- sitive to both concentration and velocity. While engineering best practices suggest that a concentration of 2% wt and a velocity of 6.0 m/sec are the safe operation limits for carbon steel, rather than solely relying on these practices, we con- ducted an analysis of corrosion rates and the risk of pitting corrosion to which the unit is exposed by surveying the streams highlighted in purple in the proprietary OLI Studio: Corrosion Analyzer (see Figure 3 ).

Another factor contributing to NH 4 HS corrosion is the presence of oxygen, which can accelerate corrosion. It is recommended that wash water be oxygen-free, with a suggested oxygen limit of <15 ppb wt . From the first phase evaluation, it is evident that the unit is already experiencing some issues, with corrosion and fouling, particularly chloride salt deposition, being the most problematic. Therefore, any increase in chloride intake does not seem feasible. Additionally, the chloride content in the fossil feed should be limited. Alternatively, a new intermit- tent wash water injection facility with a capacity of 15-20 Another factor contributing to NH 4 HS corrosion is the presence of oxygen, which can accelerate corrosion t/h should be installed upstream of HEX 08 to dissolve the dry chloride salts formed. Additionally, an upgrade in the material of construction (MoC) for the carbon steel HEX tube bundles and piping should be implemented. This upgrade is necessary to protect HEX 08-09 from chloride stress cor- rosion cracking due to intermittent water washing and to resist NH 4 HS corrosion in REAC downstream cold areas. Going forward The second phase of the project to be discussed in PTQ Q3 2024 assesses the behaviour of the unit when incorporat- ing different biogenic feedstocks and different rates, with a minimum target of 10 wt%. The feasibility study results revealed that achieving the targeted minimum biofeedstock incorporation rate of 10% was not feasible with the cur- rent DHT configuration, necessitating significant capital expenditure (Capex) for implementation. Cristian Spica is a Senior Application Engineer with OLI Systems, Inc. He has 12 years of experience in chemical engineering activities including process engineering, modelling and simulation, engineering, design and construction, project management, technical support and training. He holds a chemical engineering degree from the University of L’ Aquila, Italy (MSc equivalent) and is an Executive MBA candidate

at Luiss Business School in Rome. Email:cristian.spica@olisystems.com

PTQ Q2 2024