Food drying with renewable energy

Increases in energy costs are a signal for industry to audit its energy use and survey new plant that lowers energy use.

The food processing industry in Victoria has received quotes for natural gas with prices more than doubling in just a few years.

Echuca-based food processor Kagome expects to pay $3.6 million for gas this year, up from $2.4 million last year, despite plans to use less gas. Kagome employs more than 200 people. 

Natural gas is the dominant form of energy use for the food processing plant at KAGOME Australia

Kagome Australia's processing plant receives about 4,000 tonnes of tomatoes each day during the harvest period of 70 days. Natural gas is used to evaporate water from the tomatoes for the production of tomato paste.

Evaporating 1,000 tonnes of water from 4,000 tonnes of tomatoes each day can use an enormous amount of energy. This isn't necessary but it depends on how it is done.

One way to evaporate 1,000 tonnes of water that does use an enormous amount of energy is to simply put batches into large cauldrons with gas burners beneath them. Allow the tomatoes in the cauldrons to simmer until the desired volume of water has evaporated.

This way requires 2,257 gigajoules of thermal energy that converts 1,000 tonnes of water into steam. If this heat energy is supplied by natural gas costing $9 per gigajoule, the daily energy bill would be about $20,000 and the total bill over the tomato harvest period of 70 days woul be about $1.4 million.

There are several other ways to perform the same process using much less energy.

For instance, the energy needed to convert 1 kilogram of water into water vapour is 2,257 kilojoules. The same amount of energy can be recovered when that kilogram of water vapour is condensed back into water.

Mechanical Vapour Recompression (MVR)

The mechanical vapour compressor uses a very small amount of electrical energy to transfer a very large quantity of heat energy from the condensing steam back into the cauldron of tomatoes where it boils off an identical amount of water.
The cost saving of this method is all of the natural gas used in the inefficient method of converting 1,000 tonnes of water into water vapour. This method also produces distilled water while continually recycling the latent heat of evaporation in the water vapour as it condenses back into water.

The condensed water produced may have some value too as a pure, distilled by-product.

Equipment using this method is commercially available. One type is marketed as "forced circulation evaporators". These are for concentrating fruit paste (tomato paste, peach paste, apricot paste and etc.) and some other products with high viscosity. Another type is marketed as "falling film evaporators". These are for concentrating products with low viscosity, for example: fruit juice, milk etc.

The value of the energy savings may make it worthwhile for Kagome Australia to invest in a forced circulation evaporator and eliminate the need for natural gas.

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