Less waste MORE HEAT

There are very few processes which don't involve, at some stage, the evaporation of water. This ubiquitous operation requires a large amount of energy, however, which has stimulated the development of systems designed to reduce energy usage.

Multiple-effect steam heated evaporators, sometimes combined with thermal vapour recompression (tvr), are widely used, and can provide very efficient systems. There are limitations to the application of this approach, however, and capital costs must be considered, along with the electricity consumption of pumps used for product, condensate, air removal and cooling water. These rise as the plant becomes more complicated, offsetting the reduction of steam and cooling water consumptions.

Latent heat cuts costs

Mechanical vapour recompression (mvr) has also been used in this field. Some or all of the evaporated vapour is compressed so that it can be used to heat the evaporator, retaining the latent heat within the system. Little or no heat is rejected to cooling water, and the steam input is largely replaced by the much lower energy consumption of the compressor, usually electric power.

Once again, there are limits to which this technique can be used, and additional technical and economic constraints. Generally speaking, the capital and maintenance costs of the plant will be higher, and a more sophisticated control system will be needed.

An alternative to the above is to use surplus energy from another process in the evaporator. Some interesting examples of this are to be found in malt whisky distilleries. The production of malt whisky includes two or three batch distillation operations carried out at atmospheric pressure. The aqueous residue from the first distillation stage, called pot ale, can be concentrated to produce a protein- and carbohydrate-rich syrup used to make animal feeds. Multiple effect and mvr evaporators have been used for this duty, but as the pot ale is initially very dilute, large quantities of water must be removed. Capital and operating costs for the evaporator are therefore of great importance.

The still condensers can be designed to produce hot water at temperatures up to 85 degrees C, and some of this hot water has been used to heat evaporators for pot ale. Until recently, the hot water was flashed to produce vacuum steam, which was used in conventional tubular evaporators in a multiple effect system. Several flash stages were required, and as the water had to be cooled to the return temperature for the condensers (typically 50 degrees C), temperatures within the evaporator were severely constrained. The pressure in the last effect of the evaporator had to be low, which led to a high cooling water requirement. Including the pumps' power consumption, the system used a similar amount of energy to an mvr system.

An existing double effect evaporator of this type has recently been modified by a major distillery company at one of its Highland sites, replacing the tubular evaporators and flash vesselswith a novel combination of plate heat exchangers. In the first effect, a circulating flow of product is heated directly by hot water; while in the second effect, the water is further cooled and vapour from the first effect is condensed in two further plate heat exchangers.

Elimination of the flash vessels and their associated pumps and pipework greatly simplifies operation of the evaporator. It reduces the power required for the product pumps considerably, and raises the pressure in the condenser. This reduces the cooling water consumption, and changes to the temperature profile within the system provide more hot water for other applications.

Careful measurements made during and after commissioning have indicated ways in which the efficiency of the evaporator can be further improved, and show that if the hot water is available at 90 degrees C, a three-effect system can be used. This would retain the simplicity of operation while reducing the specific hot water and cooling water consumptions.

This system is of particular interest in cases where solid and liquid distillery effluents are processed together to produce `dark grains'. These are usually made in large centralised facilities which incorporate a dryer for solids and pot ale syrup. The dryer exhaust has a high energy content, and this can be used to produce hot water in a scrubbing system. Such a system would recover more of the exhaust's latent heat than the direct use of the exhaust to heat an evaporator.

Mr Brotherton is with Beedes, a chemical engineering consultancy based in West Sussex

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