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Thin Film Evaporator

Evaporators are used to separate the components of a process liquid based on volatility. The basic process involves applying heat to a liquid which causes it to evaporate, thereby leaving the less volatile substance behind. The more volatile – and now gaseous – substance is then condensed and collected at a separate location.

Traditionally, batch or continuous types of evaporators such as natural/forced circulation, falling and rising film, and climbing and falling plate evaporators have been used successfully with a range of process liquids. However, these evaporators have proven to be inefficient with high-boiling, viscous, fouling, and heat sensitive liquids.

The design principle of thin film evaporators allows them to successfully separate difficult-to-handle products. This method uses indirect heat transfer and mechanical agitation to evaporate a thin layer (0.1 mm to 1.0 mm) of flowing substance under controlled conditions.

HOW DO THIN FILM EVAPORATORS WORK?

The key to the efficiency of thin film evaporators is their ability to evaporate the target solution at relatively low temperatures with minimum residence time in the evaporator. This is due to operating the evaporator under vacuum conditions and the high-speed rotor creating a thin liquid film.

The main component of the thin film evaporator consists of a rotor/wiper encased in a cylindrical heating jacket. The process can be broken up in several stages:

  1. The product first enters above the heated zone where it is evenly distributed against the inner wall of the heating jacket by the rotating wipers.

  2. A rotor then forms a thin turbulent layer of liquid, which creates ideal heat flux and mass transfer conditions.

  3. The end result is rapid evaporation of the more volatile substance, which is then expelled from the heating cylinder to be condensed or subjected to further processing.

  4. The less volatile materials are collected and sent to the discharge nozzle.

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THIN FILM EVAPORATION ADVANTAGES

Thin film evaporation offers several advantages over other types of conventional evaporation processes. These include:

  • Significantly reduced residence time

  • Fast and efficient heat transfer

  • Reduced pressure drop

  • Ability to process high-viscosity and high-solids materials

  • Ability to process materials that are sensitive to thermal degradation or

    prone to fouling

  • High evaporation ratios (>95%)

  • Easy to clean/maintain equipment

Thin film evaporation is typically used for specialty applications in the chemical, food and

beverage, pharmaceutical, biotechnology, and nuclear industries, among others. Although the

application across sectors varies, thin film evaporators are commonly used to perform

specific operations such as concentration, resource recovery, devolatilization, purification,

distillation, and stripping.

Deodorization

Deodorization refers to removing odor in the atmosphere, and the word is used both for chemical and physical deodorization.

How deodorization of oil Works

  1. Deaeration. Before heating the oil, air must be removed under vacuum (deaeration) to prevent oxidation, thereby protecting product quality. ...

  2. Pre-stripping and retention. ...

  3. Post-stripping and GE stripping. ...

  4. Condensing removed impurities. ...

  5. Cooling.

Oil deaeration

Degummed, bleached oil is deaerated prior to heating to deodorizing temperature to avoid oxidation and polymerization. It is accomplished in a separate external vessel connected to the vacuum system of the bleacher (50 mbar) or, at even lower pressure, in an integrated compartment of the deodorizer (Fig. 3). Some refiners add a bit of sparge steam to improve deaeration.

Heating and cooling

Heating of the oil is usually accomplished in two or more stages. To minimize the net energy cost, bleached oil is first pre-heated in one or two stages in a heat exchange device with either hot deodorized oil or steam.

The highest energy recovery (up to 85%) can be achieved in continuous deodorizers in which bleached oil is pre-heated indirectly with hot deodorized oil. This heat recovery usually takes place in a heat recovery compartment of the deodorizer, but it can also be realized in a separate, external heat exchanger. Both options have their pros and cons. External heat exchangers result in a high heat recovery and provide easier access for cleaning. On the other hand, heat exchange in the deodorizer ensures less product intermixing and less risk of fouling and it also takes place under vacuum
The thermosiphon system is a special method of heat recovery that is used in semicontinuous deodorizers. Steam produced in the oil cooling section flows in a closed loop to the oil pre- heating section. It will condense there and the water flows back to the cooling section. In this way, a heat recovery of 45-75% can be achieved, depending on the design of the thermosiphon system (single or double loop, with or without generation of low-pressure steam)

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Final cooling of the oil can be done under vacuum or under pressure. Which practice is the best has always been a matter of discussion.

Deodorization-Deacidification

Since the concentration of most volatile components in oils is quite low, a stripping agent must be injected during deodorization. For economic reasons, steam is the most commonly used stripping agent, but the use of nitrogen has also been studied extensively. Nitrogen is an inert gas and theoretically, its use will result in lower losses (no hydrolysis) and also a higher quality of the deodorizer distillate. However, in industrial practice, nitrogen is not used primarily because it is a no condensable gas. This makes the required vacuum system much more expensive than the use of steam, which is condensable.

Most semi- and continuous deodorizers are so-called tray deodorizers which operate according to the cross-flow principle. Deodorization-deacidification is accomplished in a number of compartments (trays) where stripping steam is introduced into the oil through special sparge coils with very fine holes or by steam lift pumps. The latter give good agitation with continuous refreshing of the oil in the top layer (where deodorization effectively takes place), thereby ensuring a high overall deodorization efficiency. A minimal oil layer depth (more than 0.8 m) is required to allow good operation of the steam lift pump.

Vapour Scrubbing Systems

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The vapors leaving the deodorizer consist of steam, volatile components (fatty acids, sterols, tocopherols, contaminants, etc.), minor amounts of mechanically entrained neutral oil (mono-, di- and triacylglycerols) and some non condensable (e.g. air, etc.). Condensation of the volatile components is achieved in a scrubber and results in a by product called deodorizer distillate. 

Condensation is achieved by creating a very good contact between the hot vapour phase and the cold deodorizer distillate that is partially recirculating over the scrubber. In practice, this is done by a series of sprayers built in the duct or on a packed bed of limited height in the scrubber vessel itself. An additional demister is usually installed ahead of the vacuum unit to minimize carryover of fatty matter to the barometric condenser water

Vacuum Systems

The vacuum in the deodorizer is usually created by a combination of steam ejectors (boosters), vapor condensers and mechanical (liquid-ring) vacuum pumps. These quite robust systems typically reach pressures in the deodorizer between 2.5 and 5 mbar but motive steam consumption is high (up to 85% of the total steam consumption). Motive steam consumption can be significantly reduced (by a factor of 2.5-3) by cooling the barometric condenser water. However, the benefit of a lower motive steam consumption must be weighed against the extra chilling capacity required (higher electricity consumption). Another benefit from using a chilled water barometric vacuum system is a better condensation of the volatile matter, which also gives a lower pressure in the deodorize. 

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These classical vacuum systems are increasingly being replaced by dry (ice) condensing systems. With such systems, the stripping steam is condensed on surface condensers operating alternately at very low temperature (-30°C). The efficient solidification of steam and other volatile matter will give a very low pressure in the deodorizer (<1.5 mbar) and will also strongly reduce odor emission. As with the chilled water barometric vacuum system, dry ice condensers require extra electrical energy. Commercially available systems consist of two or more freeze condensers with horizontally or vertically orientated straight tubes, a refrigeration plant for the generation of the cold refrigerant which is evaporated in the tubes and a vessel for defrosting and cleaning of the tubes after a certain period of freezing

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