Emulsions containing small amounts of oil: the root cause of clogging in low-temperature evaporation demisters and the inability of conventional backwashing to clean them thoroughly

I. Why clogging occurs so easily

Emulsified oil is carried directly to the demister by secondary steam mist

Oils exist as ultra-fine particles in an emulsified state and do not settle naturally. The negative pressure generated by low-temperature evaporation draws micro-oil droplets, surfactants and colloids into the vapour phase, where they are captured and retained upon impact with the demister’s mesh or baffles. This is not large foam, but rather a continuous accumulation of micron-sized oil mist.

Low-temperature conditions increase the viscosity of oils and enhance their adhesive properties

At low temperatures (30–55°C), emulsified oil does not flow or drip spontaneously; instead, it becomes viscous and sticky, adhering firmly to the mesh. It does not flow back under its own weight, but accumulates to form a thick layer of oil sludge that clogs the mesh pores.

Oil + surfactants + salts form a composite sticky deposit

Emulsified oil + trace surfactants + salt powders and colloids precipitated by evaporation combine on the surface of the demister to form a dense oil-salt composite film. This is not loose scum, but a film that seals off the mesh, effectively plugging the mesh pores from the surface.

The mesh structure of demisters is prone to trapping oil contaminants

With its fine, dense pores and multi-layered interlacing, oil mist penetrates deep into the mesh layers and becomes trapped in the gaps between fibres. Whilst the surface may appear intact, the interior is already completely blocked, causing increasing resistance to gas flow.

II. Why conventional water backflushing fails to clean effectively

Emulsified oil is inherently hydrophobic, and plain water cannot wet or strip it away

Ordinary water lacks the ability to demulsify, dissolve or saponify emulsified oil; it can only wash away loose surface dust, whilst the oil film embedded within the mesh pores remains completely unaffected.

The effectiveness of water-based cleaning is even poorer at low temperatures

At low water temperatures, oils become more viscous and solidify more readily. Water lacks the ability to melt or dissolve oil; the colder the water, the less effective it is at removing oil sludge.

Composite deposits have an oil-in-salt structure that cannot be dispersed by plain water

The oil film encapsulates salt particles and colloids, forming a dense oil-in-salt layer that water cannot penetrate. Conventional backwashing only flows over the surface, leaving internal blockages completely untouched by the water flow.

Deep-seated contamination within the mesh, with insufficient penetration of backwash flow velocity

Conventional backwashing has limited pressure and flow rates, capable of cleaning only the surface layer. The washing water cannot reach the dead corners and interlayers of oil contamination within the multi-layered mesh; residual oil deposits act as nucleation sites, leading to rapid re-blockage within a few hours of operation.

III. Consequential Chain Reactions

Dewaterer clogging → Soaring vapour phase resistance → Pressure build-up and vacuum fluctuations in the evaporation chamber → Reduced evaporation efficiency → More severe mist entrainment → Increased oil content and COD in condensate, leading to a vicious cycle.