Evaporation
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Sweetener Index: WEB PAGE ONE-PAGE PDF Sweets

A starch hydrolysate is concentrated by evaporation in a falling film evaporator - a strict thin film evaporator - or plate evaporator. In the falling film evaporator the liquid completely covers the interior surface of vertical tubes with a seething layer moving downwards by its own weight. The vapour, which will be formed, pushes the downward movement too. As the movement is gravitational it does not depend on thermal energy. A falling film evaporator may be operated with a very small temperature difference, which in turn reduces consumption of steam and cooling water.

The successful operation depends on an even distribution of liquid at the top of the calandria. The distributor design depends on product. The forming of an even and coherent film comes naturally inside a tube.

In a plate evaporator the liquid moves upward or up- and downward in partly filled chambers by the generated steam bobbles. The movement depends on much thermal energy. The liquid column adds static pressure and a higher temperature difference is required to power the evaporation and movement. Plug flow with an even exposure to heat is not possible in a plate evaporator.

The plate evaporator can easily be taken apart and put together again. This feature however is quite demanding in regard to sealing parts and their sanitary keeping. The falling film evaporator is normally built to great height, 10 - 15 m, but with a smaller footprint that required by the plate evaporator. The top and bottom cover and the distributor can be removed for inspection of the fixed tubes.

The falling film evaporator has a very small hold up of liquid. This is advantageous during operation, avoiding browning of sensitive products and during cleaning requiring a minimum of cleaning liquid. A small hold up is also advantageously when shifting products.

By putting more calandrias in series and lowering the boiling temperature in steps saves steam. According to convention the calandria with the highest boiling temperature is designated "Effect One". Because of the high temperature we like to use this calandria as our finishing step (backwards feeding) for viscous low DE syrups, where the product flows in counter current with the steam.

The number of steps goes from one in case of small capacities and upward.

For high efficiency a termo compressor may be used. The termo compressor supplies a vapour mixture to effect one heating body. That vapour is a mixture of live steam and vapour out of the effect one separator. The termo compressor requires steam pressure of not less than 5 bar in order to reach a good efficiency. Without a termo compressor three bar will do. In case of ion exchange and the fluctuations in concentration of the feed it may introduce, it may be advantageously to run without a termo compressor. A direct heated plant is more flexible concerning capacity and control. That range is smaller with a compressor. A termo compressor increases vapour condensate, which normally cannot be reused in the boiler. The vapour condensate may find use as process water and for cleaning purposes.

A properly optimised evaporator will run continuously and trouble free for extended period of time with little supervision.