Refining Heavy, High-Sulfur Opportunity Crudes: Desalter Performance and Vacuum Unit Impacts

Opportunity crudes look attractive on a price differential, but they push the problem to the front of the plant. Heavier, higher-sulfur, higher-acid crudes are viscous and carry more salt, solids and metals. They also sit closer to the edge of asphaltene stability. The desalter is usually the first unit to show it: brine creeping into the crude, a thickening interface layer, or a desalting rate that keeps falling. The consequences then travel downstream. Poor desalting feeds chlorides and solids into the crude column and overhead system, and a marginal blend can foul preheat exchangers and the vacuum heater. A vacuum unit that runs short of wash oil or coils up too early can limit the whole refinery. The economic case for a crude slate change therefore has to include desalter capability, vacuum unit limits and downstream tolerance for the residue and gas oil quality that results.

Standards and requirements context

No single standard governs crude slate changes. These are the reference points a process and integrity team would normally reach for:

• API RP 571 describes damage mechanisms affecting fixed equipment in refining, including naphthenic acid corrosion, sulfidation and ammonium chloride attack. A heavier, sourer, higher-acid slate tends to accelerate these.
• API RP 939-C gives guidelines for avoiding sulfidation (sulfidic) corrosion failures in oil refineries. Use it when crude sulfur and operating temperature change together.
• ASTM D3230 covers salts in crude oil by the electrometric method. It is a quick check on desalter inlet and outlet, but it does not replace laboratory chloride and solids work.
• ASTM D664 determines acid number by potentiometric titration, the usual basis for screening naphthenic acid risk.
• ASTM D7157 determines the intrinsic stability of asphaltene-containing residues, heavy fuel oils and crude oils by n-heptane phase separation with optical detection. It is used to compare blends and judge compatibility margins.

Management of change, a hazard review of the new slate (including H2S in sampling and drain systems) and revised corrosion monitoring plans are required regardless of the crude assay.

Why heavy sour opportunity crudes behave differently

Heavy sour crudes are highly viscous and contain high sulfur, high salt and metals. Each property hits a different part of the plant:

• Viscosity and density: Water droplets settle through oil under gravity. As the oil gets denser and more viscous, the density difference shrinks and settling slows, so a desalter designed for a lighter slate loses capacity.
• Salt and solids: More salt and more fine solids mean more material to remove. Solids stabilise emulsions and collect at the interface.
• Sulfur and acid: These drive corrosion in the furnaces, transfer lines and vacuum column internals, and they change the metallurgy you need.
• Metals and residue content: Metals and residue concentrate in the vacuum bottoms and heavy gas oil, which is where downstream catalyst units feel the change.

The same logic applies to existing units. A unit designed for a lighter slate carries implicit assumptions about settling, heater coil residence time and wash zone behaviour, and heavier feeds will test every one of them.

Desalter performance on heavy feeds

What typically goes wrong

Rarely is there one cause.

Practical levers

1. Temperature. Higher temperature lowers viscosity and improves settling, but it is capped by the vapour pressure of the crude at desalter pressure and by the electrical conditions in the vessel. Check the pressure margin before raising temperature on a lighter-ended blend.
2. Mixing energy. Heavy crudes need enough mixing to bring wash water into contact with brine droplets. Too much, though, gives a tight emulsion the electric field cannot break. Tune the mix valve against outlet salt and brine quality, not a fixed differential.
3. Wash water. Both quality and rate matter. Poor-quality wash water, especially with high hardness or amines carried in recycled streams, can undermine the chemical programme.
4. Chemical programme. Review demulsifier selection for every major slate change. A product that worked on the previous blend may fail on a crude with different solids or asphaltene character. Bottle tests on representative blends are a starting point, but confirm the results on the unit.
5. Interface management. Treat the interface profile as a primary performance signal, not an afterthought. Rag layer build-up tells you the desalter is losing capacity before outlet salt moves.

Dion and Perez specifically flag tramp amines. They can enter through slop or contaminated cargoes and then report downstream as chloride salting in the overhead. Suspect them when overhead corrosion indicators worsen while desalter outlet salt looks acceptable.

Wastewater consequences

Desalter brine from heavy crudes can carry more oil, solids and organics into the effluent treatment plant. A change in demulsifier chemistry, or heavier interface carryover, can reach the biological treatment stage. Bring the effluent treatment team into slate change reviews.

Asphaltene stability and blending

Asphaltenes stay in a crude as long as the surrounding oil keeps them peptised. Blend a paraffinic or light crude with a heavy asphaltenic one and the mixture can tip into precipitation, even when each crude is stable on its own. Precipitated asphaltenes act as fouling material and as emulsion stabilisers. They show up at the desalter interface and again in preheat exchangers, furnace tubes and vacuum residue handling.

Practical consequences:

• Blend order and ratio matter. A blend that is stable at the final ratio may pass through an unstable composition while tanks or pipelines are filling. Review the transition, not just the end state.
• Stability testing should be blend-specific. Use a method such as ASTM D7157 on actual or representative blends, and track the margin as ratios change, rather than relying on individual crude assays.
• Treat compatibility as a limit alongside salt and acid. Crude scheduling should carry a stability constraint in the same way it carries a sulfur or TAN constraint.
• Watch for downstream signs. Rising preheat fouling rates, faster furnace pass-pressure drop and desalter interface solids can all point to marginal stability, even when each unit looks tolerable by itself.

Vacuum unit impacts

This is where heavy crude challenges concentrate. The feed is a larger fraction of the crude, heavier, higher in metals and sulfur, and more prone to thermal cracking.

Heater and transfer line

Heavier atmospheric residue needs a higher temperature for a given vaporisation, and the film temperature at the tube wall reaches the coking region sooner. Coke shortens run length and creates hot spots. Steam injection and coil velocity are the main tools for managing residence time, and both are limited by hydraulic and column vapour-load constraints. Monitor skin temperatures, pass balance and coil pressure drop trends against the clean baseline.

Wash zone and heavy vacuum gas oil quality

If the wash zone is under-wetted or poorly distributed, entrainment carries metals and carbon-forming material into the heavy vacuum gas oil. That gas oil is usually the feed to a hydrocracker or FCC unit, so the contaminants hit catalyst life directly. Heavy crudes make wash-oil management and packing or grid design more critical. The point made in the CDU/VDU design article, that conventional design practice leads to short run length and poor yield and quality, applies strongly here.

Corrosion and metallurgy

Higher sulfur and acid content push sulfidation and naphthenic acid corrosion up in the vacuum column, transfer line and bottoms circuit. Use API RP 939-C and API RP 571 as the basis for reviewing materials and inspection scope. Then confirm that existing alloy selections cover the new sulfur and acid combination at the operating temperatures you actually see.

Ejectors and overhead

A heavier feed that cracks more in the heater makes more light ends and non-condensables, and that loads the vacuum system. Check ejector and condenser capacity against the new cracked-gas load, not only the design case.

Downstream knock-ons

• Hydrotreaters and hydrocrackers: More sulfur, nitrogen and metals in the vacuum gas oil raise hydrogen consumption, cut catalyst life and tighten the margin on reactor effluent corrosion control.
• FCC: Higher metals and carbon-forming material in the feed change catalyst makeup and regenerator duty.
• Coker and residue handling: More vacuum residue means more demand on coking or other bottoms outlets, and asphaltene character affects coke behaviour and handling.
• Overhead systems: Poor desalting shows up as chloride and ammonium salt deposition and acid water corrosion in the crude column overhead.

Illustrative scenario

This scenario is illustrative and is not drawn from a specific refinery.

A refinery running a medium sour slate is considering a heavy, high-sulfur cargo for its blend. The assay shows the cargo is stable alone and has moderate salt. Planning proposes raising the heavy fraction in steps over successive tank turnovers.

A sound review would go like this:

1. Run stability tests on the intermediate blend ratios, not only the final one, and find the point of least margin.
2. Commission bottle tests of the existing demulsifier on the heavier blend, and ask the chemical supplier to check for calcium, amines and solids carried by the cargo.
3. Raise desalter interface sampling frequency through the first tank turnovers, track the rag layer and brine oil content, and tell the effluent plant in advance.
4. Review vacuum heater skin temperatures, pass balance and wash zone performance against the prior baseline. Agree a trigger for backing out the heavy fraction before coking accelerates.
5. Ask downstream unit owners to confirm their tolerance for the heavier vacuum gas oil and residue, including hydrogen balance and catalyst monitoring.

The decision then rests on observed trends at each step, with a pre-agreed rollback route. It should not rest on the cargo's price differential alone. Any sampling of hydrocarbon or brine should use closed-loop systems with H2S precautions. Any internal inspection of desalters or vacuum columns needs isolation, depressurisation, verified zero energy, lockout/tagout, gas detection, hazardous-area controls and safe venting before anyone enters or opens the equipment.

Checklist for evaluating a heavy, high-sulfur slate change

• Confirm crude assays include salt, solids, calcium, acid number, sulfur, metals and asphaltene content, and check them against receipts.
• Run blend stability tests across the intermediate ratios, not only the target blend.
• Check desalter temperature, pressure margin, mix valve range and wash water capacity against the heavier feed viscosity.
• Test the demulsifier on the actual blend and ask the supplier about calcium, amines and compatibility.
• Define interface profile and brine oil content as monitored performance indicators.
• Notify the effluent treatment team before the first heavy cargo is processed.
• Review vacuum heater coil velocity, steam injection, skin temperature margin and coil pressure drop trends.
• Check vacuum wash zone liquid rate and distribution against the heavier feed.
• Review metallurgy and inspection scope against sulfidation and naphthenic acid risk.
• Verify ejector and condenser capacity for the cracked-gas load.
• Get downstream unit confirmation for vacuum gas oil and residue quality.
• Set a rollback trigger and a communication path before the trial begins.

Conclusion

Heavy, high-sulfur opportunity crudes are manageable when the desalter, vacuum unit and downstream units are assessed together. The usual failure is optimising one unit while the next one absorbs the penalty. Start with blend-specific stability testing and a desalter chemical and mixing review. Then check vacuum heater and wash zone margins, and the downstream tolerance for heavier gas oil and residue. Trial the slate in steps with defined monitoring and rollback criteria, and bring in integrity, effluent treatment and chemical supplier specialists before the first cargo arrives.