Feature

Unsung Heroes

The Eighth Wonder of the Modern World!

By Dr John Walker

• the microprocessor
• the contraceptive pill
• the telephone network
• the jumbo jet
• the Colossus offshore oil platform
• the hydrogen bomb
• Tranquillity Base, where Apollo 11 made the first Moon landing

The selection is a salutatory reminder of how our world has been changed by the mind of Man over the last half century. The choice of Tranquillity Base on the Moon was an especially poignant choice as it could last longer than humanity itself.

On a more prosaic level one is constantly struck by the ingenuity of engineers and scientists. Like a school athlete whom one would not necessarily expect to compete successfully in the wider arena, there are a wealth of wonders which nevertheless deserve their moment of glory. It is at this more homely level I offer a candidate for the Eighth Wonder of the Modern World -- the multiple-effect evaporator.

The laws of thermodynamics tell us that there is no such thing as a free lunch, but the multiple-effect evaporator helps make some lunches go a lot further. The process has many uses but it is best exemplified in New Zealand at Kinleith and Kawerau, where it forms part of the kraft chemical recovery cycle.

In kraft pulping, wood chips are cooked in a pressurised digester at 180^oC for a couple of hours using a strong solution of sodium hydroxide and sodium sulphide. After cooking, the digester is "blown" and a softened fibre mass is discharged together with spent chemicals in the form of a black liquor. This liquor also contains fragments of lignin, the glue that held the fibres together in the tree.

For chemical pulping to be viable it is necessary to recover the pulping chemicals, which involves separating them from the debris of lignin. Lignin, being an aromatic polymer, has a high calorific value, so burning the black liquor generates an enormous amount of process steam, enough to make the entire mill energy self-sufficient. Also, by burning the liquor in the recovery furnace, the cooking chemicals can be separated as a melt and subsequently regenerated. But first the liquor must be condensed by evaporating water from it until it is sufficiently concentrated to burn efficiently in the recovery furnace. This involves increasing the solids content of the liquor from about 15% to about 70%.

Evaporators are heat transfer devices. The two major requirements are to provide the necessary heat transfer and to separate the evaporated vapour from the concentrated liquor. An evaporator consists of a packed array of pipes through which the black liquor is pumped, with the array surrounded by the heating medium -- steam.

Multiple-effect evaporators operate at different pressures (and hence temperatures) for each "effect" -- each separate evaporator. The vapour released by heating is separated from the concentrated liquor and is in turn used to heat the next cooler evaporator in the series, while the liquor moves on in the other direction to the next hotter evaporator. The energy necessary to evaporate a gram of water in one evaporator is given up when that moisture condenses out in the next evaporator, where it is used to evaporate another gram of water, and so on.

In theory one could have any number of evaporator systems, but cost and constraints such as the temperature drop between effects (7-15^oC), the corresponding areas of the heating surfaces (1000 m²) and the initial steam pressure/temperature (140^oC) limit the number of evaporators that can be connected in series.

Most mills use multiple-effect evaporators with between five and seven effects, arranged so that the vapour from one evaporator becomes the steam supply for the next in the series. On average, the multiple-effect evaporators will remove 4.5 to 6 grams of water per gram of original steam.

That's not magic, but it is wonderfully efficient!

Dr John Walker is in the University of Canterbury's School of Forestry.