PTQ Q3 2024 Issue

Transforming packed column efficiency

Column internals, such as packings and distributors, along with application expertise and distillation know-how, are critical to achieving greater productivity and efficiency

Shwu Tyng Goh and Thomas Linder Sulzer Chemtech

D istillation is the most widely used separation tech- nology in the chemical industry. There are three main groups of distillation mass transfer compo- nents: trays, rings (random packing), and structured pack- ing. Trays were invented in the early 1800s, while the first generation of thin-walled metal random packing was intro- duced in the 1900s. Structured packing was introduced later and was initially used mainly in vacuum and low liquid load applications. Over the years, with a better understanding of the hydrau- lic performance of structured packing and innovative prod- uct development, structured packing can now be found in vacuum and pressure distillation columns, high liquid load absorption columns, and extraction columns. Although a well-established technology, there is still untapped potential for further research and development of structured packing and associated column internals. Catering to meet diverse needs The first structured packing successfully used in industrial applications was the Sulzer BX gauze packing, introduced in 1964. New generations of BXPlus and CYPlus gauze pack- ings were successively introduced to the market in 2003 and 2013. Due to the excellent liquid wettability of the spe- cial gauze material, these packings are particularly suitable for applications with low liquid loads. For distillation processes that require a high theoretical number of stages with a low allowance for total pressure drop, gauze packing is particularly suitable. It offers high separation efficiency with minimal pressure drop com - pared to other types of packing. These applications typically involve thermally sensitive products, and the low pressure drop offered by the gauze packing is essential to prevent product degradation or polymerisation. Another structured packing specifically designed for ultra-low liquid loads (less than 1 m3/m2.hr) is the AYPlus DC packing. Its hybrid gauze material provides superior fluid distribution even in aqueous operating conditions. The measured number of transfer units per meter packing height (NTUM) for this hybrid gauze packing can be three times that of a conventional sheet metal structured packing. In highly corrosive environments, special materials of construction must be considered. Plastic materials such as

polypropylene (PP) or polyvinylidene fluoride (PVDF) are used. However, they are not suitable if the operating tem- perature in the column is high and exceeds the maximum allowable temperature of these polymers. The MellaCarbon packing is tailor-made to meet this challenge. Made of car- bon material, it offers corrosion resistance against caustic solutions and non-oxidising inorganic acids, as well as ther- mal stability of more than 400°C. The pressure drop across the carbon dioxide (CO 2 ) absorber in a solvent-based carbon capture plant plays a significant role in energy consumption. A lower pres - sure drop translates to lower demand on the booster fan to deliver flue gas to the bottom of the CO2 absorber. MellapakCC packing is designed to provide equivalent absorption efficiency with remarkably lower pressure drop compared to conventional structured packing. Depending on the specific liquid load and f-factor of the CO2 absorp- tion process, the pressure drop could be reduced by up to 60% when compared to conventional type packing of similar absorption efficiency. It has been estimated that a pressure drop reduction of 10 mbar for a 500 MW natural gas combined cycle power plant could result in annual elec- tricity cost savings of approximately €450,000, based on an electricity cost of €0.05 per kWh.1 The most widely used structured packing types across various applications are the metal sheet type structured packing featuring textured, perforated, and corrugated sur- faces. Second-generation metal sheet structured packings, the MellapakPlus packing, features critical change in the packing geometry, which minimises the local pressure drop at the interface between two packing elements, thereby preventing premature flooding of a packed bed. The useful capacity of a column can be increased significantly by up to 50% with this packing. Over the last few decades, both first-generation Mellapak and MellapakPlus packings have successfully replaced and revamped numerous tray and ran- domly packed columns to improve capacity and efficiency. Next-generation structured packing As global concerns about environmental challenges and resource depletion intensify, the quest for sustainability is becoming increasingly important for the chemical sepa- ration industry. In the European Union, the chemical and

PTQ Q3 2024