Produced water, desalter effluent and oily wastewater all carry the same problem in different forms: oil droplets and fine solids stabilised by negative surface charge, refusing to separate no matter how long you give them. Cationic polyamine is one of the standard tools for breaking that stability – and one of the most commonly misapplied.
This guide covers polyamine (CAS 42751-79-1) in oil-water separation: what stabilises an emulsion, how a cationic polymer destabilises it, which grade to use, and how to test properly in brine rather than in tap water.
Why oily water does not separate
An oil-in-water emulsion is stable for three reasons, and a treatment programme has to address the one that dominates:
| Stabilising mechanism | What it looks like | What breaks it |
|---|---|---|
| Electrostatic repulsion | Small droplets with negative surface charge repel each other | Cationic polymer or inorganic coagulant |
| Interfacial film | Surfactants, asphaltenes or solids form a skin around droplets | Demulsifier chemistry (surfactant-based) |
| Steric / solids stabilisation | Fine solids (clays, corrosion products) sit at the interface | Charge neutralisation plus flocculation |
In refinery desalting and produced water, the electrostatic and solids mechanisms usually dominate – which is why a cationic polymer is the first tool, and a demulsifier the second.
How polyamine destabilises the emulsion
The polymer adsorbs on the droplet surface and on the fine solids, neutralising the negative charge. Once the electrostatic barrier drops, droplets can approach, coalesce, and rise. Fine solids that were stabilising the interface flocculate and are removed with the oil or with the water in the separator.
| Effect | Why it matters in a separator |
|---|---|
| Charge neutralisation of droplets | Coalescence starts; the oil layer forms faster |
| Charge neutralisation of solids | Interface-stabilising fines are removed from the interface |
| Mild bridging | Larger flocs, better separation in a plate pack or flotation unit |
| No added aluminium or iron | Less sludge, no pH swing, no impact on downstream catalysis |
Grade selection for oily water
| System | Grade | Reason |
|---|---|---|
| Produced water, fine droplets | B or C | Charge neutralisation dominates; low viscosity disperses quickly |
| Refinery desalter wash water | B or C | Fast neutralisation before the electric field; no viscosity penalty |
| Oily wastewater with coarse solids | D or E | Bridging helps capture solids and carry floc |
| High-salinity brine | B or C | Charge still works at high conductivity; low-viscosity grades disperse better in brine |
Grades F and G (40% solids) are worth considering where the site dilutes on location anyway, or where the dosing pump cannot handle the 50% product at the required viscosity.
Test in the real brine, at the real temperature
This is where most demulsifier programmes fail before they start. Synthetic test water made from tap water behaves nothing like produced water:
| Parameter | Why it changes the result |
|---|---|
| Salinity / conductivity | Compresses the electrical double layer, so charge effects differ |
| Temperature | Viscosity of both phases, and the solubility of the polymer, change |
| Residual chemicals | Corrosion inhibitor, scale inhibitor, biocide and antifoam all interact with cationic polymers |
| Solids content | Fines consume polymer; a clean test water flatters the product |
| Oil composition | Asphaltenic and paraffinic crudes behave differently at the interface |
Practical protocol: use a sample of the actual water, hold it at the separator temperature, run a dose series of two or three grades, and measure both oil remaining in the water phase and water remaining in the oil phase. A programme that cleans the water but leaves a wet oil stream has simply moved the problem.
Dosing and programme design
| Point | Guidance |
|---|---|
| Dilution | 0.1-1% working solution; in brine, prepare with fresh water and dose immediately to avoid precipitation |
| Injection point | Upstream of the separator with enough mixing to disperse but not enough to re-emulsify |
| Typical dose | 5-100 mg/L product depending on oil content and charge demand – set by bench test |
| Sequence with demulsifier | Test both orders; usually polyamine first (charge) then demulsifier (film), but some systems reverse it |
| Overdosing | Cationic overdose restabilises the emulsion – treat it as a real risk, not a theoretical one |
Compatibility issues that bite
- Anionic scale inhibitors and dispersants – can precipitate the cationic polymer. Dose at separate points.
- Anionic surfactants and detergents – form complexes that become the emulsion stabiliser instead of breaking it.
- High shearing pumps – shear degrades the polymer and reduces its effectiveness; use low-shear dosing pumps.
- Cold weather – the 50% product becomes very viscous below 5 deg C; store heated or use the 40% grade.
Frequently asked questions
Can polyamine replace a demulsifier? Not usually. Polyamine neutralises charge; a demulsifier disrupts the interfacial film. Most programmes need both, in a tested order.
Why does the product work in the lab and fail on site? Almost always because the lab used synthetic water. Salinity, temperature, residual chemicals and solids all shift the optimum dose and the mechanism.
Does high salinity deactivate the polymer? No, but it changes the charge environment. Higher salinity usually means a lower optimum dose, not an ineffective one.
Which grade for a desalter? Grade B or C, dosed upstream of the mixing valve; confirm with a bench test on the actual wash water.
Contact PolyBlueChem for polyamine samples and bench-test support.