Multiple Effect Evaporator (MEE)
Steam-driven multi-stage evaporation that concentrates high-TDS effluents and minimises liquid discharge — the workhorse thermal stage in most zero liquid discharge (ZLD) plants, engineered to global standards for reliability and energy efficiency
Overview
About Multiple Effect Evaporator (MEE)
A Multiple Effect Evaporator (MEE) concentrates a dilute effluent by boiling off water in a series of evaporator bodies called effects, each operating at a progressively lower pressure and temperature than the one before it. Live steam is supplied only to the first effect; the vapour it generates becomes the heating medium for the second effect, whose vapour heats the third, and so on. Because the latent heat of the steam is reused across several effects, an MEE evaporates several kilograms of water per kilogram of live steam, making it far more economical than a single-effect evaporator.
MEE is the backbone thermal concentration stage in most Zero Liquid Discharge (ZLD) plants. It takes high-TDS streams such as RO reject, spent process liquors, and washings, and concentrates them close to saturation before a downstream crystalliser or Agitated Thin Film Dryer (ATFD) converts the residue to a dry, disposable or recoverable solid. By recovering a large fraction of the feed as clean distillate, MEE both minimises the volume of waste requiring final treatment and returns reusable water to the plant.
The number of effects is an economic decision: more effects mean higher steam economy and lower operating cost, but higher capital cost and floor area. Typical industrial installations use 2 to 6 effects. Where energy cost dominates, a Mechanical Vapour Recompression (MVR) stage is often placed ahead of or in place of the first effects to slash steam consumption, giving a hybrid MEE-MVR train that combines low energy cost with the robustness of thermal evaporation.
Configuration is matched to the liquor. Falling film evaporators suit clean, low-fouling streams and offer gentle, low-residence-time evaporation. Forced circulation evaporators handle scaling, viscous, and crystallising liquors by maintaining high tube velocities that suppress deposition. Spans engineers MEE systems to global fabrication and safety standards, selecting metallurgy, tube design, and anti-scale strategy to suit each effluent for a long, reliable service life.
Schematic
Typical System Schematic
Specifications
Technical Specifications
| Feed capacity | 0.5–100+ m³/hr (multiple trains for higher duty) |
| Number of effects | 2 to 6 (forward, backward or mixed feed) |
| Steam economy | ~1.8 (2-effect) up to ~5–6 (6-effect); higher with MVR pre-stage |
| Evaporator type | Falling film (clean liquors) / forced circulation (scaling, viscous liquors) |
| Operating vacuum | Last effect under vacuum to lower boiling temperature |
| Feed TDS handling | Dilute streams up to near-saturation; crystalliser / ATFD for final drying |
| Distillate quality | Condensate reusable as boiler feed / process water |
| Materials of construction | SS 316L / SS 904L / duplex / titanium per liquor chemistry |
| Automation | PLC / SCADA control of steam, levels, density and CIP cycles |
Process
How a Multiple Effect Evaporator Works
Feed Preheating
Raw effluent is preheated against hot condensate and product concentrate, recovering waste heat before the feed enters the first effect and reducing overall steam demand.
First Effect Evaporation
Live steam condenses on the heating surface of the first effect, boiling the feed on the other side. This is the only external steam the train consumes; the vapour it produces drives every subsequent effect.
Vapour Cascade Through the Effects
Vapour from each effect flows to the calandria of the next, which operates at lower pressure so the vapour is hot enough to boil that effect's liquor. This reuse of latent heat across effects is what gives MEE its steam economy.
Progressive Concentration
Liquor moves from effect to effect (forward, backward, or mixed feed), becoming more concentrated at each stage as water is driven off. The last effect operates under vacuum to keep boiling temperatures low.
Condensate Recovery
Vapour from the final effect is condensed in a surface condenser. The condensate from all effects is high-quality distillate, collected for reuse as boiler feed or process water.
Concentrate Discharge
The concentrated liquor leaving the last effect is sent to a forced circulation crystalliser or an Agitated Thin Film Dryer for final drying, completing the ZLD train with a dry salt or cake for disposal or recovery.
Benefits
Key Advantages
Proven, robust thermal concentration
Decades of industrial track record on the widest range of effluents, including aggressive and scaling liquors that membranes cannot handle.
High steam economy
Reusing latent heat across 2–6 effects evaporates several kilograms of water per kilogram of steam, cutting energy cost versus single-effect evaporation.
Handles high and variable TDS
Concentrates RO reject and spent liquors close to saturation regardless of feed salinity, where membrane systems reach their osmotic limit.
High-quality distillate for reuse
Recovers a large fraction of the feed as clean condensate suitable for boiler feed or process water, improving overall plant water balance.
Flexible configuration
Falling film for clean liquors, forced circulation for scaling and viscous streams, and hybrid MEE-MVR trains where energy cost dominates.
Seamless ZLD integration
Feeds directly into crystallisers and ATFDs to reach true zero liquid discharge with a dry, disposable or recoverable residue.
Engineered metallurgy
SS 316L, 904L, duplex or titanium selected per liquor chemistry for long service life against corrosion and scaling.
Automated, low-attention operation
PLC/SCADA control of steam, levels, density and CIP keeps the train running reliably with minimal operator intervention.
Applications
Industries & Use Cases
FAQ
Frequently Asked Questions
- What steam economy can I expect from an MEE?
- Steam economy rises with the number of effects: roughly 1.8 for a 2-effect train, around 3–4 for 4 effects, and up to about 5–6 for a 6-effect train. Adding a Mechanical Vapour Recompression stage lifts effective economy far higher by recycling vapour electrically. The optimum number of effects is a trade-off between steam savings and capital cost.
- MEE or MVR — which should I choose?
- MEE is preferred where waste or low-cost steam is available and for aggressive, scaling liquors; MVR is preferred where electricity is reasonably priced and evaporation volumes are large enough for the compressor to pay back through steam savings. Many ZLD plants use a hybrid MEE-MVR train to get the best of both. We size the split against your energy tariffs and effluent chemistry.
- What is the difference between falling film and forced circulation MEE?
- Falling film evaporators spread the liquor as a thin film down heated tubes for gentle, low-residence-time evaporation — ideal for clean, low-fouling and heat-sensitive streams. Forced circulation evaporators pump liquor at high velocity through the tubes to suppress scaling and handle viscous or crystallising liquors. Many trains combine falling film early effects with a forced circulation final effect.
- What happens to the concentrate from an MEE?
- Once the liquor is concentrated close to saturation, it is sent to a forced circulation crystalliser or an Agitated Thin Film Dryer (ATFD), which converts it to a dry salt or cake for disposal or recovery. This final drying step is what makes the overall plant a true Zero Liquid Discharge system.
- Can MEE handle scaling effluents like those high in calcium or silica?
- Yes, with the right configuration. Forced circulation bodies, appropriate tube velocities, anti-scale dosing, and scheduled CIP cleaning are used for liquors high in CaSO₄, CaCO₃, or silica. Metallurgy is also selected to resist the specific corrosion and scaling tendency of the stream.
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