Drying Soaps

Nowadays, the economic manufacture of high-quality soap supposes a fully-boiled process. The following procedures proved successful for continuous steam drying.

1. Vacuum Drying
Drying and expansion under vacuum with simultaneous cooling of the soap.

2. Atmospheric Drying
Drying with atmospheric expansion and subsequent cooling of the soap on a chill roll.

3. Combined Drying
Drying in an atmospheric stage and connected in series cooling by expansion under vacuum.

When selecting the drying process, besides aspects of soap quality partly depending on speed of cooling down during or after steam drying, questions of energy consumption and availability of low temperature cooling water become important to an increasing extent.

At atmospheric drying, the evaporated water is expanded to ambient pressure and drained off vaporously. A condensation of vapor and the requirement of cooling water connected therewith are not necessary. Then the dried soap has the boiling temperature of water of ab. 100°C and has to be cooled to a suitable processing temperature of 20 to 30°C by means of a chill roll.

At vacuum drying, the expansion of evaporated water and soap is effected to a pressure of 20 to 50 mbar at simultaneous cooling down of the soap to 20 to 40°C. The vapor has to be led off the vacuum system by condensation. When drying toilet soap the yield of heat for vapor condensation in the vacuum is nearly three times bigger than heat emission necessary for cooling the soap in the chill roll after atmospheric expansion. For this reason, single-stage vacuum plants need more cooling water than atmospheric driers. For drying toilet soap, vacuum plants need 15 to 30 m3 cooling water/1000 kg soap according to the temperature of cooling water.

For atmospheric plants, which should be operated without permanent supply of cooling water, SELA developed special cold water circuits for the chill roll, with an electric power requirement of the order of ab. 30 kW per ton of dried soap.

At vacuum plants with high evaporation capacity corresponding to high fatty acid contents of the final product the requirement of cooling water can be reduced considerably by connecting it in series to an atmospheric stage. Then cold water circuits without permanent supply of cooling water are possible also for the vacuum stage by surface condensation via heat exchanger and refrigerating machine with electric power requirements of the order of ab. 30 to 40 kW per ton of dried soap.

For a further reduction of the energy and cooling water consumption when using vacuum driers, in 1984 a system for vapor extraction was developed and patented. By permanent development of this process a maximum reduction of the use of fresh vapor was achieved. At this vapor extraction process the water being evaporated already in the heat exchanger is separated from the soap and is used as motive steam for the steam ejector for vapor compression. By this means there are considerable savings not only at the cooling water required but also at the quantity of steam.

The diagram shows by means of an example in which way the costs of cooling water according to different drying systems can influence the operating costs of a drying plant. As a rule, the use of cooling circuits is profitable if the costs of cooling water are higher than the net costs of conveying.