Natural and forced circulation evaporation

Natural circulation evaporator (NCE)

The natural circulation evaporator is a vertical or horizontal shell and tube heat exchanger where the liquid product to be partially vaporized naturally flows upwards through the tubes, driven by the difference in density between the generated two-phase mixture and the liquid at the evaporator inlet.
The heat duty is transferred by the heating medium (e.g., hot water, live steam or heating oil) on the shell side.

The generated liquid and/or vapour mixture flows to a downstream separator to disengage the vapour phase from the liquid concentrated phase.
A shell and tube exchanger type allows for easy cleanability.

Natural circulation evaporator (NCE) plants

A natural circulation evaporation plant typically consists of:

• The evaporator
• A separator to separate the produced vapour/liquid mixture
• A condenser to condense the vaporized fraction
• A condensate collection vessel and pump
• A pump to transfer the separated concentrated product

When vacuum pressure is needed due to product and/or process requirements, a vacuum group is required.
To recover energy, the feed can be pre-heated by the produced process condensate and/or the heating medium return stream.

Forced circulation evaporator (FCE) and crystallizer

A forced circulation evaporator is a vertical or horizontal shell and tube heat exchanger where the liquid product to be evaporated flows through the tubes under the driving force of a circulation pump loop.
The heat duty is transferred by the heating medium (e.g., hot water, live steam or diathermic oil) on the shell side.

The pumped circulating liquid product is heated when it flows through the heat exchanger. As a consequence, temperature increases and the liquid partially vaporizes when the pressure is reduced in the downstream separator.

Forced circulation evaporators can also be used as crystallizers. Crystals grow inside the separator and special separator designs are used to handle crystals in the recirculated crystal slurry.
A shell and tube exchanger type allows for easy cleanability.

Forced circulation evaporator (FCE) plants

A forced circulation evaporation plant is typically composed of:

• The evaporator
• The circulation pump to enable the process
• A condenser to condense the vaporized fraction
• A condensate collection vessel and pump
• A concentrate/slurry pump to transfer the separated concentrated product

When a vacuum is needed due to product and/or process requirements, a vacuum group is used.
To recover energy, the feed can be pre-heated by the produced process condensate and/or the heating medium return stream.

Forced circulation evaporator plants are also an optimal solution to work as crystallizing units for saline wastewater: crystals are separated from the recirculated slurry thanks to special separators.
A decanter/centrifuge is required to separate the solid crystals from the mother liquor to obtain a dry or wet salt cake.

Multiple effect (ME) natural or forced circulation evaporation plants

Multiple effect natural or forced evaporation plants usually consist in a series of evaporators operating at progressively lower pressure, to allow using the vapour generated in the upstream effect as the heating medium of the next effect.

The type of evaporator is selected according to the specific application and the solid content in the effluent.
Live steam or another heating medium (e.g., hot water, heating oil) is required for the first evaporation step only.
Vapour from the final effect is condensed by cooling water.

The number of effects is limited by material corrosion and the boiling point elevation (BPE) of the concentrate stream. For condensate or concentrate recovery, also thermal degradation can be a relevant aspect to be considered.

More about equipment

Natural or forced circulation evaporation plants are customised according to the customer’s needs and expectations. Typical features are shown below.

Also supplied as skid mounted units to minimise installation operations at the customer’s site.

The most suitable material is selected based on the nature of the process stream, wastewater composition and operating conditions.
The main materials usually selected are the following:

• Stainless steel
• High performance alloys
• Titanium

A wide range of services from technical feasibility studies to revamping existing plants is also available.

More about DBO

Depending on the wastewater composition and the treated water’s (the condensate) end use, the evaporation plant is coupled with other process technologies to achieve the desired water quality.

More about wastewater tratment processes

• Minimum up to zero liquid discharge
• Water reuse or recycling
• Less wastewater off-site disposal volumes and costs
• Potential recovery of valuable substances
• Customised design
• Combination with other process units
• Bespoke thermal configuration
• High automation level
• Remote assistance
• Easy cleanability

Specific benefits

• Natural circulation evaporators: power consumption reduction since no pump is required.
• Forced circulation evaporators: reduced maintenance thanks to reduced fouling.

The above benefits are further increased in our unique “multiple effect natural + forced circulation evaporation” arrangement, providing maximum energy savings, thanks to:

• Using the vapour generated in the upstream effect as the heating medium of the next effect
• Condensate heat recovery

A wide range of applications is possible for both municipal and industrial wastewater, having a medium-high content of dissolved solids such as salts, metals or other organic contaminants, and for different wastewater end use:

• Discharge to surface, sewer or other treatment plant
• Reuse/recycling
• Concentration
• Solvent recovery

Natural and forced circulation evaporation plants address the needs of the following different industries:

• Chemicals
• Food & beverage
• Landfill
• Mining
• Oil & Gas
• Pharmaceutical
• Power

A broad range of industrial processes generates extensive wastewater compositions to be treated and, whether easy or difficult to treat, each wastewater requires a customised process and a bespoke plant design.
We provide support to our customers conducting tests at the laboratory and pilot plant scale, as well feasibility studies leveraging our R&D centre and multi-disciplinary teams.

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