emulsions are less stable. Efficient desalter designs require less wash water. To achieve stringent salt content targets, such as salt <0.5 PTB in desalted crudes, a higher percent- age of wash water of up to 10% is needed. Deploying <3 vol% wash water drastically affects desalter performance. It is recommended to split the wash water feed rate at the crude battery limit, specifically before the cold preheat exchanger (to prevent solids fouling in the cold preheat and increase the contact time between crude and water to effectively remove upstream chemical-based crude impu- rities) and upstream of the mix valve. Stripped sour water, free from chloride (<10 ppm), dissolved oxygen <7 ppb, no dissolved carbon dioxide, and pH 7-8, is best for desalting operations. While higher wash water percentages improve desalting, the availability of wash water sources from plant overhead stripped sour water is also limited. An increase in desalter wash water increases effluent generation and treatment costs in the wastewater treatment plant. In crude desalting, it is important to remember that salt removal from 10 PTB to <2 PTB will be comparatively simpler than achieving a reduction of salt from 2 PTB to <0.5 PTB. At low salt levels, specifically <2 PTB with 8% wash operation conditions, the conductivity of water droplets dispersed in crude will be low. This leads to a poor force of attraction between water droplets, compromising water coalescence efficacy. Hence, maintaining a minimum con - ductivity of 1,000 µS/cm in the desalter outlet brine water is crucial to ensure a healthy force of attraction between the particles. When the desalted brine conductivity ranges from 2,000 to 5,000 µS/cm, water carryover in the desalted crude ceases, and consistent salt removal efficiency is achieved. Hence, it is important to monitor the conductivity of the desalter brine water from each stage of the desalter for a holistic desalter optimisation approach. With all the previ- ously noted best practice adoptions, deploying low chloride wash water (<10 ppm) with a wash water rate of 4.5-5% at the mix valve led to achieving the salt outlet KPI (see Figure 6 ). The desalter brine pH is equally important compared to the wash water pH for overall desalter performance. Operating the brine pH slightly towards the neutral to slightly acidic side (6-7) is recommended for effective desalting operation, as it helps minimise the corrosion envi- ronment. High pH in the wash water and desalted brine can deprotonate emulsifying agents, enhancing their ability to stabilise water droplets in oil. For crudes with chemical impurities like phosphate esters, amine chlorides, calcium naphthenates, or sodium naphthenates, it is recommended to operate the desalter brine pH slightly acidic (5-6) to wash away these impurities in the desalter brine. It is important to inform the down- stream wastewater treatment plant to monitor chemical oxygen demand (COD) control, as it can increase with low pH operations. To overcome this issue, a pH modifier with a corrosion inhibitor, such as Prochem or Predator, was deployed to control brine pH. In case service water or raw water is used as wash water
160
0.80
0.70
155
0.60
150
Temp > 150 ˚C BS & W > 0.45 vol%
145
0.50
Temp < 145˚C BS & W < 0.3 vol%
140
0.40
0.30
135
130
0.20
Post mix valve optimisation
Desalter temperature
0.10
125
Water yield from crude BS & W
120
0.00
Figure 4 Water carryover in desalted crude vs temperature
oil, effectively washing salts from the crude. Globally, globe valves are the most common mix valves deployed for desalters due to their precise control, versatility, durability, reliability, and ease of maintenance. Mix valves typically accommodate pressure drops ranging from 10 to 50 psi. For light to medium API crude, the recommended pressure drop operation is 10-35 psi (0.8-2.4 bar). Too low produces more coarse droplet sizes, leading to poor desalting, and too high produces fine droplet sizes that can make the emulsion difficult to break. The droplet size and distribution are not limited to the pressure drop in the mix valve. They depend on the elec- tric field, temperature, wash water, and crude density.2 In refinery day-to-day desalter optimisation, simple baseline monitoring of salt outlet vs mix valve DP will help establish the safe operating range for the mix valve DP. The best practices followed for operating the two-stage desalting mix valve include: u Operate the first stage of the desalter at a high-pressure drop to achieve intense mixing, maximising the removal of salts during the desalting process. v Operate the second stage at a relatively lower DP to pro- mote dehydration, ensuring that all the salt-washed water carried from the first-stage crude will be fully dehydrated. Figure 5 illustrates how to achieve the desired crude salt outlet range with a sharper interface layer at high voltage and low amperage conditions in the electric grid. This ensures there is no fine emulsion formation and water carryover in the desalted crude. Best practices such as mix valve calibration, check valve opening, and actuator response were adopted annually for reliable mix valve operations. For high solids crudes processing timeframes, the mix valve was cleaned every two years to remove any deposits around the valve positions. Wash water Wash water = f {wash water %, wash water quality, salt & BS&W, electric grid, brine pH, chemicals} A common industry practice is to deploy 3-10% wash water relative to the volume of crude oil. 3 Typically, medium to heavy crude oils require more wash water (6-10%) due to higher viscosity and stable emulsions, while lighter crudes (4-6%) need less wash water as they are less viscous and
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PTQ Q1 2025
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