Sulphonation – The Process
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Most electrophilic substitution reactions are irreversible but sulfonation is an exception. Treatment of benzene with “oleum” (a solution of SO3 in concentrated sulfuric acid) will give the sulfonic acid, the electrophilic species being sulfur trioxide which is Lewis acidic.
Fig – 1 Sulphonation : Benzene equation
The sulfonic acid can be converted back by treatment with hot aqueous acid. The reason for this reversibility is the fact that the Wheland intermediate is overall neutral and therefore more stable than other, positively charged intermediates. Hence, under forcing reaction conditions, the energy difference in progressing in either the forward or backward sense from the Wheland intermediate is proportionately smaller compared to the barrier to activation and hence discrimination is lost.
Fig – 2 Progress of Reaction against Energy
This makes the SO3H a useful directing group if it is desired to carry our selective ortho– substitution of a monosubstituted benzene possessing an ortho/para– activating group. Under normal circumstances, para– substitution would dominate, despite the statistical favouring of the ortho– positions due to steric hindrance of the original substituent. Initial sulfonation para– gives a Disubstituted benzene in which both substituents direct to the same position. Subsequent directed electrophilic substitution and removal of the sulfonic acid group gives theortho– disubstituted product.
Process Involved In the Manufacture of LABSA
Fig 3 – Flowchart of the process behind the production of LABSA.
The manufacturing of LABSA at Sasol gulf is a continuous process. An interval of 1 day after a period of 20-21 days for the purpose of shut-down and start-up is essential for maintenance purposes. The key reactions involved in the formation of LABSA are as follows:
•Sulphur Melting – Ignition of sulphur (S) to sulphur dioxide (SO2) •SO3 Production – Oxidation of sulphur dioxide to sulphur trioxide (SO3) using catalyst vanadium pentoxide (V2O5) under optimum temperature. •Film Sulphonation – Reaction of Linear Alkyl Benzene (LAB) with Sulphur trioxide to yield the end product LABSA. •Ageing & Hydrolysis
•Gas Separation & Gas Scrubbing – Separation of LABSA from unreacted gases.
The air taken from outside is compressed and dehumidified by means of the following units:
1)Intermediate Cooling Unit.
2)Silica gel Dehumidifying Tower.
The cooling has the purpose to remove the humidity from the air, up to a saturation humidity of 2 degree Celsius & also to convey low temperature air to the dehumidifying tower, thus favoring the water absorption in the silica gel. The silica gel air drying has the object to reduce to very low values (dew point -60 approx.) the moisture content of air intended for the sulfur combustion, & then for conversion.
This reduces to acceptable values the oleum quantity produced in the conversion unit, which depends directly to the quantity of water contained in the air.
The air filtered is sucked by the compressor that sends the process air to the refrigerating group. This unit removes the compression heat by water & moreover cools the air to 2 degree Celsius through the intermediate medium cooling unit which is kept at constant temperature. The equipment for air drying is a vertical cylindrical vessel, which is horizontally divided in two parts by a partition containing insulating material. The two silica gel layers are placed on nets; 2 spaces are left free above and below such layers for air inlet and outlet respectively. On the plates, at the level of both silica gel layer, two light glasses are located to check visually their conditions. Some silica gel indicator is placed near the sight glasses, changing its color accordingto the quantity of absorbed water; thus saturating with water, it changes blue to pink.
Fig 4 – Boiler to produce and supply steam.
Characteristics of Silica Gel
Appearance – white color, granules of 3-6 mm approx.
Bulk Density: 700kg/dm3 approx
The regeneration is carried out by heating with air at 150 degree Celsius. A checking about the effective regeneration is made by verifying the Silica gel indicator through the sight glasses, as well as by verifying the outlet temp., of the regeneration air on the recording thermometer. When the regeneration is accomplished the silica gel mass has a temperature quite near 150 degree Celsius. Therefore it is necessary to cool the silica gel thus allowing it to adsorb the humidity of the air crossing it.
Note – The four way valves are provided with a drive by pneumatic cylinders which are remote controlled energizing some solenoid valve suitably.
Air drying and cooling
Air that is utilized in the production of LABSA has to be cooled first and then dried to ensure its feasibility for further reactions. Air is cooled by passing it through a heat exchanger containing the coolant mono-ethylene glycol; at a temperature of 0 to -2 degree Celsius. The coolant ensures condensation of air to around 5 degree Celsius.
After cooling air to the required temperature it has to be dried to remove traces of moisture present in it. For this purpose air is passed through a cylinder filled with silica gel. The silica-gel brings the dew point of the water in the air down to stay -40 degrees Celsius. This means that the air is dry as if the air was cooled down to -40 degrees Celsius. In practice two cylinders with silica-gel are used; one for drying the air, the other is reconditioned. In general the changing of the cylinders is done automatically.
Fig 5 – Air Drying Process
Fig 6 – Glycol Tower
In order to remove the compression heat and to condense the moisture, the air is first cooled by cooling water and then by a glycol solution in the glycol tower. The air flow is then conveyed at a constant temperature (less than 5℃) to the silica gel dryers.
Sulphur is melted to allow Sulphur that has been acquired has to be melted prior to its ignition. This is done by heating it in the melter at temperatures of 135℃ to 145℃. If the temperature exceeds 145℃ it would result in vaporisation of sulphur while temperatures less than 135℃ would be insufficient to melt sulphur. Therefore a temperature of 140 is maintained to prevent wastage of sulphur due to the above mentioned causes.
Fig 7–Sulphur Melting Tank.
The solid sulphur is melted and filtered to avoid the pump valves clogging and then fed to the sulphur burner by a proportioning pump. The viscosity of molten sulphur is minimum between 135℃ to 140 ℃.
Fig 8 – Viscosity Of Sulphur
Molten sulphur is fed under mass control by means of a mass flow meter. The pump and the relevant piping are steam heated in order to to keep the
temperature constant and to minimize sulphur viscosity.
Fig 9 – Sulphur Burning
The oxidation reaction of sulphur dioxide and trioxide is exothermic and heat produced by it is quite sufficient to keep the catalyst layers at the required at the required temp. to obtain a good conversion progress. In order to get the best efficiency, the gas inlet temperature in the first catalyst layer has to be about 420 degree Celsius.
To start the reaction, the catalyst layers of the first & second stage of the conversion tower have to be brought to the necessary temperature. This is reached by preheating with hot air and the upper part of the catalyst tower is heated upto a temperature of 400-420 degree Celsius.
Fig 10 – Catalyst Bed
Sulphur Dioxide & Sulphur Trioxide Production
The sulphur combustion furnace has been designed for this special purpose. In the furnace, the sulphur is fed through a pipe and falls on a surface of refractory balls, while the combustion air is supplied in counter current, thus obtaining the complete combustion of sulphur without its spraying through a nozzle; which might often clog owing to sulphur impurities.
This system is quite simple; it does not require any maintenance and the gas composition does not change. The temperature of the gas at the burner outlet is around 700 degree Celsius (corresponding approximately to a SO2 concentration of 7% by volume). Thereafter a heat exchanger cools the gas so that it reaches the conversion tower at the required temperature. The conversion tower is composed of three layers of vanadium pentoxide (V2O5) catalyst. The gas, passing from a layer to next one, crosses a heat exchanger to take the gas temperature to optimum conversion values on every stage. In order to allow quick startup, a preheating system has been provided. The main characteristics of Ballestra pre-heating system have been provided: ➢No electric pre-heater is required because if it were used, being in the presence of SO3, it would be corroded very quickly. ➢Moreover, with Ballestra system there is no need to either cut off or regulate values in the circuit of conversion lines, which should operate a temperature of about 500 degree Celsius in the presence SO2 / SO3, thus being easily corroded. ➢The gas temperature is of course too high to be suitable for sulphonation; therefore some heat exchangers in the series are used to cool the gas down to proper sulphonation temperature. The hot air coming from SO3 coolers is utilized for silica gel regeneration. Fig 11 – SO2 / SO3 Production
This group is composed by a film reactor, multi-tube type, having dimensions and number of tubes proportional to the plant capacity. The sulphonation gas is automatically fed on the reactor top and distributed in part strictly equal on each of the pipes composing the reactor. The raw material to be sulphonated is fed in co-current with the gas. Outside the reaction tubes in the reactor jacket the cooling water circulates in co-current with the film, thus allowing a control of the reaction temperature by heat removing.
The distribution of the gas and the product to be sulphonated is designed in order to ensure a constant ratio between the two phases, inside each reaction tube. The sulphonated or sulphated product, coming out of the reactor is suitably degassed, aged and stabilized according to the fed raw material; and fed to the neutralization unit.
In this connection the Ballestra sulphonation / sulphation system by film reactor has great advantages towards the other existing systems on the market since in the case of power failure an emergency system, included in the supply, avoids any damage to the product and the necessity of cleaning the reactor before resuming operation. This system can be also used during plant startup: the material to be sulphonated is fed and recycled to the reactor until optimum SO2-SO3 conversion is reached.
Fig 12 – Top view of the Reactor Fig 13 – Bottom Nozzles Of the Reactor
This SO3 is sent to the reactor where it reacts with Linear Alkyl Benzene. Due to presence of some water vapours in air some oleum is also formed. This should be avoided as it can cause blockage. The reactor has small tubes in which the SO3 passes and the LAB passes through its sides. The main reaction takes place at bottom of these tubes and during maintenance these tubes are thoroughly cleaned because if the LAB leaks to the centre part then the reaction will take place there only and no SO3 will pass through.
Then this mixture of LABSA (desired product), LAB, SO3 and a mixture of other waste materials including oleum is sent to a separator. The liquid product is sent to the aging vessel and the gases are sent to cyclone.
Fig 14 – Sulphonation Plant Arrangement
Ageing & Hydrolysis
This is used to stabilize the sulphonated DDB. It is composed by an ageing unit and stabilizer. The product after being sulphonated overflows into the bottom of the ageing unit. It is then conveyed into the stabilizer together with water. Afterwards it is transferred with the help of a pump to the neutralization unit.
Fig 15 – Ageing Vessel
Cyclonic separation is a method of removing particulates from an air (or gas) stream, without the use of filters, through vortex separation. Rotational effects and gravity are used to separate mixtures of solids and fluids.
Here a high speed rotating air-flow is established within a cylindrical or conical container called a cyclone. Air flows in a spiral pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top.
Larger (denser) particles in the rotating air stream have too much inertia to follow the tight curve of the air stream and strike the outside wall, falling then to the bottom of the cyclone where they can be removed. In a conical system, as the rotating air-flow moves towards the narrow end of the cyclone the rotational radius of the air stream is reduced, separating smaller and smaller particles from the stream. The cyclone geometry, together with air flow rate, defines the cut point of the cyclone.
This is the size of particle that will be removed from the air stream with a 50% efficiency. Particles larger than the cut point will be removed with a greater efficiency and smaller particles with a lower efficiency. The liquid product and reactants which still have to react are sent to the aging vessel but the product in the form of vapour and gases are sent to Electrostatic Precipitator.
Fig 16 – Gas Splitting
Gas Scrubbing / Exhaust Gas Treatment
The unit is designed to treat exhaust gas stream coming from the sulphonation reactor before being sent to the atmosphere in order to remove any possible organic, unreacted SO₃ traces and unconverted SO₂. The exhausted gas coming from the reactor pass through a cyclone which provides to separate the acid mist before getting into the electrostatic precipitator and SO2 scrubber column.
Fig 17 – Exhaust Gas Treatment
In the electrostatic precipitator the organic substances and unreacted SO3 are separated and eliminated. The residual unconverted SO2 is absorbed in the scrubbing column in which a water and caustic soda solution is continuously recycled. The gas stream is contacted with a controlled stream of fresh organic raw material.
Fig 18– Electrostatic Precipitator Functionality Details
Fig 19 – Electro Static Precipitator Construction Details
An electrostatic precipitator (ESP) or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. The LABSA is separated and sent to the ageing vessel.
Types of Heat Exchangers
Shell and Tube heat exchanger
Shell and tube heat exchangers consist of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned etc. Shell and Tube heat exchangers are typically used for high pressure applications (with pressures greater than 30 bar and temperatures greater than 260°C. This is because the shell and tube heat exchangers are robust due to their shape. There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers. These include:
•Tube diameter: Using a small tube diameter makes the heat exchanger both economical and compact. However, it is more likely for the heat exchanger to foul up faster and the small size makes mechanical cleaning of the fouling difficult. To prevail over the fouling and cleaning problems, larger tube
diameters can be used. Thus to determine the tube diameter, the available space, cost and the fouling nature of the fluids must be considered.
•Tube thickness: The thickness of the wall of the tubes is usually determined to ensure: oThere is enough room for corrosion
oThat flow-induced vibration has resistance
oAbility to easily stock spare parts cost Sometimes the wall thickness is determined by the maximum pressure differential across the wall.
•Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as possible. However, there are many limitations for this, including the space available at the site where it is going to be used and the need to ensure that there are tubes available in lengths that are twice the required length (so that the tubes can be withdrawn and replaced). Also, it has to be remembered that lone, thin tubes are difficult to take out and replace. •Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e. the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes’ outside diameter
LAS/LABSA ( Linear Alkyl Benzene Suplhonic Acid ) – The Product
Description of LAS
Linear alkylbenzene sulfonic acid is the largest-volume synthetic surfactant because of its relatively low cost, good performance, the fact that it can be dried to a stable powder and the biodegradable environmental friendliness as it has straight chain. LAS is an anionic surfactants with molecules characterized by a hydrophobic and a hydrophilic group. Alpha-olefin sulfonates (AOS) alkyl sulfates (AS) are also examples of commercial anionic surfactants. They are nonvolatile compounds produced by sulfonation. LAS are complex mixtures of homologues of different alkyl chain lengths (C10 to C13 or C14) and phenyl positional isomers of 2 to 5-phenyl in proportions dictated by the starting materials and reaction conditions, each containing an aromatic ring sulfonated at the para position and attached to a linear alkyl chain at any position with the exception of terminal one (1-phenyl). The properties of LAS differ in physical and chemical properties according to the alkyl chain length, resulting in formulations for various applications. The starting material LAB (linear alkylbenzene) is produced by the alkylation of benzene with n-paraffins in the presence of hydrogen fluoride (HF) or aluminium chloride (AlCl3) as a catalyst. LAS is produced by the sulfonation of LAB with oleum in batch reactors. Other sulfonation alternative reagents are sulfuric acid, diluted sulfur trioxide, chlorosulfonic acid and sulfamic acid on falling film reactors. LAS are then neutralized to the desired salt (sodium, ammonium, calcium, potassium, and triethanolamine salts). Surfactants are widely used in the industry needed to improve contact between polar and non-polar media such as between oil and water or between water and minerals.
MASS DENSITY AT 20 DEGREES C : ~ 1.070 g/cm3 VISCOSITY AT 20 DEGREES C : ~ 1500 – 2000 mPa.s.
MELTING RANGE : ~ – 10 DEGREES C.
BOILING POINT : ~ 315 DEGREES C.
VAPOUR PRESSURE at 20 DEGREES c : Below 0.15 (0.001 mm Hg). FLASH POINT (PMcc) : >200 DEGREES C.
DECOMPOSITION TEMPERATURE : > 100 DEGREES C.
Ph : ~ 2.
Applications of LAS
Alkylbenzene sulfonic acid, as the raw material of detergent, is used to produce alkylbenzene sulfonic acid sodium (LAS), which has the performances of cleaning, wetting, foaming, emulsifying and dispersing, etc. The rate of biodegradation is more than 90%. The product is widely used for producing various detergents and emulsifiers for agricultural herbicides and in emulsion polymerization. It is mainly used to produce household detergents such as washing powder, detergent of dishware, detergent of light or hard dirt, cleaner of textile industry, dyeing assistant, degreaser of plating
and leather making industry, and the deinking agent of paper-making industry, etc.
•Good Surface active properties
•Low cost surfactant for detergents
•Easy processing into dried powders
•Desirable solubility in both liquid and powder formulation •Biodegradable
•Compatible with other surface active agents
APPEARANCEViscous Light Brown liquid
ACTIVE MATTER96.0% min
ACID VALUE180 – 190
FREE OIL1.5% max
COLOR, KLETTE50 max (5% Sol. pH=7, 40mm cell)
FREE SULFURIC ACID1.5% max
•Cost effective, anionic surfactant.
•Due to its stable foam, suitable for detergent applications in combination with other surfactants. •Compatible with enzymes and builders.
•Outstanding performance with other anionic surfactants due to its synergistic effect. •Consumes less alkali for neutralisation.
•Ideal for liquid detergent application due to high solubility and low salt content.
Packaging & Transportation
First the truck is parked in the heavy loading station directly under a valve. Laborers enter the truck and fix metal rods in grooves near the door. Then a cardboard perimeter is set up in order to provide support. A flexi-bag is spread out on the bottom of the container, above and within the cardboard perimeter. A hose is connected to the valve and to the flexi-bag. The initial reading is taken from the main LAB tank. The flexi-bag can withstand 20-25 tons of LAB. Calculations are carried out to transfer an approx. 20 tons. The meter reading must reduce by 52cm.
Fig 20 – Flexi Bags within cardboard perimeter
The sulphonic acid is corrosive in nature and therefore requires a vehicle with a pre-fitted tanker. The tanker is made of a special material (commonly stainless steel) Also the tank must be able to keep the sulphonic acid at a desirable temperature.
Fig 21 – Fitted Tank
LABSA is packaged into plastic drums and then transported. Each drum contains 210kg of the product. Sasol buys second hand drums in order to cut costs. Once the drums are loaded with LABSA on a wooden platform a forklift will move them to the storage shed. At the time of loading the forklift will carry these drums to a loading station with an adjustable ramp. The forklift will carry the containers into the truck and load them there.
Fig 22 – Plastic Drums for packaging
Every two hours, regular analysis of the product is carried out to make sure the quality of the product is maintained. A sample of the product is taken
in a beaker and taken to the analysis room.
Color Klett Determination
A Klett colorimeter allows light to pass through and determines the colour Klett of the substance. The beaker is put on the colorimeter and the value of the color Klett is obtained. Lesser the color Klett, better the quality of the product. The standard value for Klett is around 50%(maximum). Here at Sasol, it ranges from 5-10% and is therefore great in quality.
•Blue Filter No. 42 with 400-465mm wavelength range
•Pair of cells with 40mm path length
•Ethanol 99.9% GPR
•Propan-2-ol ( Isopropyl alcohol ) GPR
•Prepare solvent of ethanol 99.99%, methanol, propan-2-ol or distilled water or a mix of all.
•Weigh 5g of active substance and dilute with a weighed amount of solvent so that a solution of 5% m/m is prepared for color measurement.
•In case of 5% m/n color measurement , weigh 5g of active substance, then times the volume of solvent required by relevant solvent density and weigh solvent.
•Mix upto complete dissolution.
•Fill 40mm path length cell (clean and dry) with mixture and other cell as reference cell. •Colorimeter must be switched on 15 mins prior to the
Acid value determination
After finding the color Klett, the sample of the product is then titrated with ethanol, drop by drop after adding the indicator. Through this, the amount of ethanol required to reach the n point is noted and the acid level of the product is calculated. It should be approx. 180. If the acid value is higher or lower than the required value, the air flow rate is adjusted and analysis is carried out till the desired acid value is obtained.
•250ml conical flask
•Lab analytic balance reading upto 3 decimal places
•Sodium Hydroxide Volumetric Solution
This method covers determination of acid value for sulphonic acid, however it can also be used for pure fatty acids.
•Weigh accurately about 2g of sulphonic acid into the conical flask and note the weight. •Add 25ml of ethanol and mix well to ensure the sample is dissolved completely. •Titrate with NaOH solution using phenolphthalein until the solution retains a faint pink colour. Note T1.
•Acid Value = ( T1 x Molarity of NaOH x 56.1 ) / Wt
•Free Acid % = ( T1 x Molarity of NaOH x titrated acid molecular weight ) 
In a nutshell, the previous month at Sasol has been very productive in terms of the knowledge gained regarding the manufacturing operations of the LABSA plant in Dubai, UAE.
This is a detailed report on: Production Operations and Manufacturing Processes of LABSA. It is highly informative on processes such as Air Drying, SO₃ Production, SO₃ Treatment, Linear Alkyl Benzene Sulphonation etc. The report can be used to bring about the following functionality:
•Set up companies
•Set up detergent raw material supply
•Improve plant production quality
•Reduce production costs
 Sasol Gulf – Operation Manual
 Sulphonation Technology in the Detergent Industry by W. Herman de Groot  www.lasinfo.org
 Test Method Control Room File