PTQ Q1 2025 Issue

1.40

3.00

3.0

7.0%

1.20

6.0%

2.50

2.5

1.00

5.0%

Delta b/w rst & second stage 0.2 bar

2.00

2.0

0.80

4.0%

1.50

1.5

Consist e nt 5 to 4.5 vol% of wash water into mix is key for eective desalting, helped to achieve salt <0.5 ptb

Post adopted st ra tegy of 1st stage mix valve high DP (Desalting focus) 2nd stage mix valve low DP (Dehydration focus)

0.60

3.0%

1.00

1.0

0.40

2.0%

0.50

0.5

0.20

1.0%

0.00

0.0%

0.0

Train A - 1st stage Mix valve DP, bar

Train A - 2nd stage Mix valve DP, bar

Train A Salt out, Ptb

Train A - Salt out, Ptb

Train A - 1st stage Wash water %

Train A - 2nd stage Wash water %

Figure 5 Mix valve strategies for two-stage desalter

Figure 6 Wash water per cent vs salt outlet

during refinery CDU start-up or due to stripped sour water challenges (which is not recommended for long-term use), high levels of dissolved CO₂ and O₂ in the raw water can acidify the desalter brine. Sulphur-rich crudes produce more acidic byproducts, and higher desalter temperatures accelerate oxidation reactions, increasing the acidity of the desalter brine water. The desalter experienced a pH drop of 1.5-3 units while using raw water during high sulphur crude processing periods. The pH modifier Prochem was used to maintain the brine pH at 6-7 and sustain desalter performance. Interface level Interface level = f {try-line emulsion thickness, residence time, chemical dosage, mix valve, solids in crude, mud wash} The term ‘interface level’ in a desalter specifically refers to the boundary between the water and emulsion/oil lay- ers. Interface level and residence time are interdepend- ent parameters. A rising interface level often indicates the growth of the emulsion layer. Hence, monitoring the interface level is critical, and it helps achieve the desired residence time for both oil and water phases. Regular mon- itoring can help identify trends and patterns in emulsion formation. An increased pressure drop across the desalter can indicate the presence of a thick emulsion layer, causing resistance to flow. Also, high chemical consumption often correlates to a lack of proper interface monitoring. Combining interface level measurements with other oper- ational parameters like pressure drop and chemical usage is recommended to get a comprehensive view of emulsion behaviour. Advanced capacitance and gamma densitome- ters can provide detailed profiles of the layers within the desalter. Therefore, conducting periodic visual inspections through try-line/try-cock sampling points to validate the accuracy of interface measurement (as shared in detail in the troubleshooting section) is crucial. Typically, desalter try-line sample visual validation is the first step in desalter troubleshooting. Any deviations in measured vs observed levels in try-lines should be addressed as part of the troubleshooting approach dis- cussed in Part 1 of this article.

All desalter designers commonly provide three to five interface layer sample points, with 6in between each sam- ple point. Typically, emulsion layer growth of more than 6in in the desalter vessel indicates strong emulsion formation, which significantly compromises the crude oil and water residence time in the desalter. This affects the efficacy of the desalting process and can create uncertainty, leading to slugs of salt and water entering the desalted crude under the event of small disturbances, causing fouling in down- stream heat exchangers. The best operating guidelines for interface level adjust- ment for two-stage desalters are: u Target the first-stage desalter operation at a higher interface level (60-70%). It will increase the water phase residence, helping to control oil carryover in the brine. v Target the second-stage desalter operation at a lower interface level (40-50%). It will increase the crude resi- dence time, helping to prevent water carryover in the crude. Upon adopting these shared operating guidelines, the salt outlet was in control, as the oil in brine remained <100 ppm (within the design limit) 100% of the time. The presence of solids, such as clay particles, and heavy organic compounds, like asphaltenes, can stabilise emul- sions, creating a complex, heterogeneous layer that results in challenges in interface level measurement. From the literature and lab PED experiments, it was learned that solids-stabilised emulsions have relatively high viscosities and densities compared to typical oil-water emulsions without solids. This heterogeneity can confuse measure- ment devices. Hence, preventing solids stabilisation at the desalter interface layer is crucial, as it poses challenges in interface level measurements. Controlling interface emul- sion layer solids to <1,000 ppm is critical to prevent solids pickering stabilisation. As shown in Figure 7 , a real-time study on desalter emul- sion layer solids stabilisation helped to understand the sol- ids distribution and growth of the emulsion layer in both the first and second stages. During this solids stabilisation, the interface level measurement showed 10-15% devia- tions from the actual desalter level observations in the try- line sample points. Figure 7 is a graphical representation of where samples were taken and the solids ppm levels at

PTQ Q1 2025