The sorbitan ester production uses an esterification process where fatty acid and sorbitol are heated to a temperature of 235°C where the reaction occurs, forming sorbitan. Carbon black is added to reduce color formation. Since the reaction is reversible, the water is vaporized and drawn off as soon as it is formed, leaving the sorbitan ester in the reaction vessel. After the water has been removed, the ester is refined by removing carbon black with a filter press. The product can be further refined to produce different grades of sorbitan ester by “bleaching”. Trace catalyst is removed and the color improved by bleaching with hydrogen peroxide. The finished product is then cooled and filtered. This sorbitan ester manufacturing plant can produce both bleached and unbleached sorbitan mono-oleate (SMO), sorbitan sesqui-oleate (SSO), sorbitan tri-oleate (STO), and sorbitan mono-laurate (SML).
The sorbitan ester production is a highly automated and flexible batch ester process. Sorbitol and fatty acids are prepared and heated in a pre-reactor before being transferred to the main reactor vessel. Carbon black is also added to the batch to control color. Both reactors are agitated and steam heated with jackets. Recirculation is used during the reaction step to achieve additional mixing. When the reaction is completed the batch is cooled though an external cooler before being transferred to filter tank. The batch is then filtered through two Boulton plate and frame filter presses prior to storage.
Three types of fatty acid can be used in this esterification plant for sorbitan ester manufacturing: captive tallow-based oleine, vegetable-based oleine, and coconut fatty acid. The poly-functional alcohol sorbitol is used to produce bleached and non-bleached sorbitan esters. The sorbitol is received in the form of sorbidex, a 70% solution of sorbitol in water. To assist in the reaction process, a homogeneous catalyst mixture is used. There are two types of catalysts: Standard Catalyst is 50% phosphoric acid and 28% sodium hydroxide and Lite Catalyst is 70% phosphorous acid and 28% sodium hydroxide. Carbon black is used to prevent color formation and hydrogen peroxide is used for bleaching.
The production of bleached sorbitan esters can be separated into three main processing steps: reaction, filtration, and bleaching. The first stage of the esterification reaction is to mix known weights of oleic acid and sorbitol solution. Carbon black is added with the oleic acid to protect the color of the batch during reaction. A catalyst mixture of 28% caustic and either 50% or 70% phosphoric acid is added to the reactor contents to promote the reaction process. The reactor contents are subjected to heat and a reaction takes place between the fatty acid and alcohol, producing an ester and water.
This reaction is reversible, and to obtain a completely one-way reaction, the generated water (reaction water) must be drawn off as it is formed. Water of reaction is produced in three different phases of the reaction. The first is at temperatures up to 120°C where the water associated with the sorbitol solution is liberated. The second phase occurs between 180 and 235°C where the water of esterification is liberated. The final phase occurs between 230 and 235°C where water from the cyclization reaction is liberated.
The reaction water is boiled off and condensed in the two overhead condensers combined with a packed column. Non-condensables that pass through the condensers are run through a water scrubber to remove any VOCs prior to releasing to atmosphere. When the end of the reaction is reached, the batch is crash cooled to 85°C using an external cooler. The cool ester is filtered using a plate and frame filter press until clear and bright. The ester can then be bleached in batches of 17 metric tons in the bleaching vessel. The ester is heated to 95°C and hydrogen peroxide is added to refine the ester. The refined ester is then filtered through a second plate and frame press to remove water and traces of peroxide.
Process control systems and programming are completely up-to-date and are for sale with the facility. They are Siemens PCS7 and Fisher-Provox systems. Fatty acids and glycerine production units use the Siemens system, which was installed in the 1980s but has been periodically updated. Promace is used as the process information management system (PIMS).
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