Sulfonation and sulfation are major industrial chemical processes used to make a diverse range of products, including dyes and colour intensifiers, pigments, pesticides and organic intermediates. Although there are different kinds of sulfonation processes available which differ primarily in the sulfonating agent used such as oleum, chlorosulfonic acid or SO3, SO3-based sulfonation is the dominant process for modern plants. The SO3/air process has been shown versatile, able to produce abroad range of high quality products and cost-efficient. Among such processes sulfur burning systems where sulfur is used as SO3 source gained substantial importance on the market due to distinct safety advantages and wide solid sulfur availability. The process flow scheme normally involves solid sulfur to SO2 oxidation, SO2 to SO3 conversion, sulfonation of organics and effluent gas clean-up systems. As mentioned before very dry air has to be supplied to the sulfonation process. Dew points of at least -60 °C and often up to -80 °C are normally achieved in such plants.
Air dryer in a sulfonation plant
The typical air supply system in a sulfonation process uses ambient air which is first compressed to 15 to18 psig (0.5 to 1.2 bar) pressures and cooled in a chiller to ~5 to 7 °C (45 °F). Air precooling is necessary to reach very low moisture content in the compressed air at low pressures. The chilled air is dried in two parallel desiccant-adsorbers to a dew point of -60 to -80 °C (-112 °F). Another important aspect of dry air supply systems in a sulfonation plant is a necessity to exclude dew point peaks and air flow variations. Pulsing will immediately affect the SO3/organics mole ratio (i.e. SO3 mole ratio) which is the major parameter in a production process. A stable SO3 mole ratio ensures designed product yield, optimal sulfonation efficiency and always needs to be sustained when the plant is running. This is also the reason why deep bed cooling with ambient air is employed on regenerating beds. Reducing the bed temperature to an acceptable level prior to bringing the regenerated air dryer back on-line is important to avoid dew point spikes.
Sulfonation plants that use SO3 from sulfur burners do not require external heat sources to heat air for beds regeneration. Hot air is supplied from SO3 coolers where SO3 produced in SO2 converters cooled to almost ambient temperature to be further processed. The abundance of heat energy generated upon sulfur and SO2 oxidation is an important advantage of the sulfur burning sulfonation process. Other process variations where oleum-derived SO3 or liquid SO3 is used as sulfonation agent require external heating to produce high temperature air for regeneration.
Adsorbent used in compressed air dryers installed in sulfonation plants plays an important role in ensuring cost-efficient and robust operation of the air supply system which also affects efficiency of the production process. Conventional adsorbents for compressed air dryers include activated alumina, silica gel and molecular sieves. Since an air supply system is offered as part of the sulfonation plant, process licensors are normally responsible for selecting and qualifying certain adsorbent types for their plant dryers. BASF offers high-performance Sorbead R and Sorbead WS adsorbents which are uniquely suited for demanding conditions in sulfonation plants.
Stability towards acidic components
SO2 and SO3 (i.e. SOx) are highly acidic and highly reactive gases. Even at ppm concentrations SOx reacts with a variety of inorganic oxides to produce sulfates or sulfites. Aluminum oxide is susceptible to such reactions which are strongly promoted by moisture loaded on the adsorbent bed during the air drying process. The formation of surface sulfates/sulfites contributes to adsorbent deactivation and degradation and might result in more frequent adsorbent replacement or operational issues in a plant. This effect could also be pronounced for silica gel materials with high aluminum content. While aluminum is added to improve hydrothermal stability of certain silica gels, its excessive amounts negatively impact material resistance towards acidic components. Sorbead is an acid-stable material maintaining its properties and activity in a presence of high levels of acidic impurities in air. It has been designed to optimise its dehydration efficiency and mechanical stability yielding very stable and performing adsorbent.
Robustness and mechanical stability
As the adsorbent is subject to multiple adsorption/regeneration cycles – it ages and has the tendency to lose efficiency but also mechanical stability. Low-grade silica gel materials often exhibit higher attrition and poor mechanical integrity when subject to multiple adsorption/desorption cycles. This could lead to downstream filters plugging with adsorbent fines, higher pressure drop across the adsorbent layer and non-uniform temperature profiles throughout the bed. Sorbead is designed to maintain exceptional mechanical integrity under aggressive operating conditions and was shown to be highly resistant to hydrothermal aging.
Sorbead WS is a liquid water stable adsorbent which is often used as a protective layer on top of Sorbead R to ensure the
highest possible efficiency of the dehydrogenation process. The presence of liquid water in desiccant beds negatively impacts the performance but also substantially contributes to hydrothermal aging of adsorbents. Significant amounts of liquid water in a bed could result in a complete shutdown of dryers.
Performance and energy efficiency
Desiccants are characterised by dynamic moisture uptakes. Sorbead R/WS are high capacity desiccants that overperform most of the commercially available market adsorbents. Higher dynamic moisture uptakes allow to utilise adsorbent volume more efficiently but also ensure less frequent regeneration limiting adsorbent aging and wearing. It is important to emphasise that Sorbead WS is not only water-stable but also exhibits similar dynamic water uptakes as Sorbead. It is high performance and high reliability that make Sorbead adsorbents the perfect choice for compressed air dryers.
Because of high adsorption dynamic water uptakes and the energetically favorable regeneration conditions to achieve low pressure dew points Sorbead is the first choice for low-energy consumption heat-regenerated adsorption dryers. Sorbead adsorbents need lower regeneration temperatures than most conventional adsorbents but are still able to deliver very low moisture dewpoints (down to -80 °C) under conditions normally used in sulfonation plant dryers. While in certain designs of sulfonation processes there is an abundance of heat energy available for regeneration of dryers, the efficiency of the bed cooling is mostly defined by the highest temperature achieved on regeneration of the adsorbent – regeneration temperature implies much faster and more economic cooling. Higher dynamic capacity allows sulfonation plant users to extend the cycle length and, thus, limit the number of regeneration cycles which contribute to adsorbent deactivation/degradation.
BASF SE, Ludwigshafen
Global Market Manager for Alumina, Catalyst
Intermediates and Compressed Air,
AirConsultant, Consultant for Compressed Air