The continuous linear alkylbenzene (LAB) sulphonation process is widely applied for the production of the corresponding sulphonic acid (LABSA), a commodity used in the manufacture of domestic and industrial detergents.
Sulphonation is used for processing in the same reactor other organic bases such as fatty alcohols, pure or more often ethoxylated, or C12 – C18 alpha olefins (AO).
A valuable product obtained from sulphonation and following neutralization is sodium salt of the sulphonated ethoxylated lauryl alcohol, commonly named SLES, a toiletry and cosmetic base commodity. Long-chained alcohols, such as lauryl alcohol, in the 2-3 moles ethoxylated form, may be treated with sulphur trioxide in the same multi-tube film reactor for production of the relevant acid (LESA).
Sodium alpha olefin sulfonate (AOS) is an anionic surfactant widely used for laundry and personal-care products. Various chain lengths alpha olefins, typically C12 – C18, may be sulphonated in the multi-tube film reactor and afterwards neutralized.
A final treatment of hydrolysis is required to obtain a high quality product.
MAIN PLANT SECTIONS
The typical Sulphonation Plant is composed of the following main sections:
- SO3 production
- Exhaust gas treatment
Complementary sections of typical sulphonation plant available are the following:
- Neutralization section for SLES or alpha olefin sulphonate (AOS) production
- Hydrolysis section for alpha olefin sulphonate (AOS)
- Sulphuric acid production section
Sulphonation is based on reaction between a sulphur trioxide stream diluted in air and an organic base. The reaction is carried out in a stainless steel multi-tube film reactor where the sulphur trioxide stream and the organic base are conveyed in co-current, for a reaction time necessary for a sulphuric group to react with the base and to form an additioned molecule. This molecule has a hydrophilic polar end (the sulphuric group) and an electrically neutral end (the hydrocarbon chain) which is hydrophobic. Such double property is the factor providing the detergent behaviour of the LABSA and its ability to modify the water surface tension.
Product quality control
Colour is a key property for the commercial value of the sulphonated organic base.
Colour may be hindered by several factors, including reaction temperature.
Since the reaction is fairly exothermal, in order to hold constant temperature, the reactor tube bundle is submerged in circulating cooling water under temperature control. Density, sulphuric acid content and free oil content are other measurable parameters of the product quality.
Common section of the production process
The sulphuric stream production is a common feature of all sulphonation processes. The sulphuric stream is manufactured by a process similar to the sulphuric acid production. Raw material is solid sulphur, normally in pellet form at high purity as produced by partial oxidation of sour gas by the Claus process. Sulphur is molten by steam in a jacketed tank, then is pumped to a combustion furnace where is burnt in a large excess of air with formation of sulphur dioxide. The sulphur dioxide stream must be cooled down for the next processing step.
Conversion and sulphonation
Sulphur dioxide duly cooled down is conveyed to a multi-layer catalytic reactor where sulphur dioxide is converted almost completely to sulphur trioxide. This stream requires cooling and filtration before being reacted with the organic base in the multi-tube film reactor.
Reaction occurs under control of temperature and control of flow rate of organic base fed to the sulphonation reactor.
Software and hardware enable the operator to carry out the control tasks automatically or to bring such key parameters under manual control, according to process management style.
The acidic LESA stream is neutralized in a loop where the reactor output is treated with a caustic soda solution and is subject to homogenizing process in a dedicated piece of equipment enabling the product to become lump-free and to meet the most stringent homogenization specifications as necessary for its use in the cosmetic industry. The significant amount of heat developing out of the neutralization reaction is removed in a special cooler, then additional on-stream equipment and injections allow for colour adjustment, deareation, pH adjustment and product ageing. Deareation consists in the removal of small air bubbles forming during the various atmospheric processes, in a degasifier working under vacuum, so providing crystal appearance to the SLES paste.
A further optional treatment aimed to the removal of dioxane can be foreseen.
1,4 – Dioxane, eventually produced as undesired by-product by the sulphonation reaction, is stripped out of the paste in a vacuum film evaporator where also deareation occurs simoultaneously.
In addition to the neutralization whit caustic soda of the acidic reactor output, a supplementary treatment is involved in the production of Sodium Alpha Olefin Sulfonate (AOS), named “hydrolysis”.
The neutralized AOS stream is heated up to high temperature, homogenized and aged for a suitable time in order to ensure the complete conversion of Sultones to surfactant compounds.
After a final cooling, the product is ready for storage or packing.