Sweet success

The background of Charles Edward Howard and Norbert Rillieux could have scarcely been more different – Howard the younger brother of the Duke of Norfolk, and Rillieux the son of a slave. Yet both in the early 19th century made crucial improvements to the important process of sugar refining, making important contributions to the pre-history of chemical engineering.

The sugar plantations in North and South America and the Caribbean were a major business at the time, and one that could only be operated with a significant investment in slave labour. Harvested cane had to be processed within two days in a process that was so backbreaking and dangerous that on average it cost one slave life per two tons of sugar.

Juice was extracted from the cut cane by crushing it in a three-roller mill, initially driven by mules and later by steam engines. A wooden gutter would transport the juice into the boiling house, where it entered a series of flat-bottomed copper kettles of decreasing size (the so-called Jamaica train), in which the juice was repeatedly boiled to produce a thick syrup. Milk of lime and ox blood were added to clarify the solution, and slaves would skim off the impurities, before the syrup was poured into pots to set into semi-refined sugar.

Howard’s contribution was to replace the open Jamaica train with a vacuum pan and filter units. Reducing the pressure meant that the sugar juice could be boiled at much lower temperatures, cutting the risk of burns and reducing caramelisation and allowing the evaporation step to continue till the sugar crystallised, resulting in much better, purer sugar.

Rillieux improved Howard’s vacuum pans further by using the hot steam from the first evaporator to heat the second and so on. This improved the energy of the process so drastically that the installation of Rillieux’ improved multi-effect evaporator paid for itself within a year.

Both Howard and Rillieux saw the success of their innovations during their lifetime though Rillieux, because of his colour (he was one quarter black), occasionally found it difficult to enjoy the fruits of his labour. As for slaves, the improvement in their lot was only temporary: while the improved evaporators made processing sugar safer and less arduous, the increased capacity of the processing plants meant that ever-more sugar had to be planted and harvested – equally backbreaking and dangerous work. But vacuum evaporation and multiple-effect evaporators are still very much with us today.

Full article published in: tce 842, August 2011

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