soap is then called "fully boiled". Another consideration for removing the glycerin is the difference between soap price and glycerin price. At higher glycerin prices (as it historically has been), extra income can be derived from selling the glycerin separately. Recently, however, glycerin prices are low (as glycerin is a by-product of bio-diesel plants) and soap manufacturing might be more lucrative if the glycerin could be left in the soap.
The removal of glycerin from semi-boiled soap is done through repeated washing with salt water (brine). This makes the process plant a bit more extensive and It also produces a salty waste water loaded with glycerin - thus a water treatment plant is required.
7)
Soap plants traditionally are batch plants where the feed materials are mixed and reacted in large heated kettles (called "crutchers") under atmospheric pressure and made quasi-continuous by operating several kettles in turn. The process takes several days and the natural cooling and drying might even take several weeks. To speed up the process and get more economy of scale, most modern plants are continuous plants where the reaction takes place in a pressurized loop reactor and the drying in vacuum spray driers.
As rule of thumb, any production above 2 tons per hour of dried soap is better done on a continuous plant. The largest possible plants are determined by the capacity of the extruder and can reach around 10 tons of dried soap per hour.
8)
The batch processes with natural drying -