Jul-2024

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

Article Summary

Distillation is the most widely used separation technology in the chemical industry. There are three main groups of distillation mass transfer components: trays, rings (random packing), and structured packing. Trays were invented in the early 1800s, while the first generation of thin-walled metal random packing was introduced 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 hydraulic performance of structured packing and innovative product 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 packings were successively introduced to the market in 2003 and 2013. Due to the excellent liquid wettability of the special 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 compared 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 temperature 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 carbon material, it offers corrosion resistance against caustic solutions and non-oxidising inorganic acids, as well as thermal stability of more than 400°C.

The pressure drop across the carbon dioxide (CO2) absorber in a solvent-based carbon capture plant plays a significant role in energy consumption. A lower pressure 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 absorption 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 electricity 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 surfaces. 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 randomly 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 separation industry. In the European Union, the chemical and petrochemical industry was the largest industrial energy consumer in 2021. In the US, about half of the energy consumed in the industrial sector comes from both thermal and non-thermal separation processes. Distillation alone accounts for 49% of the energy consumed by separation processes (see Figure 1).²

It becomes evident that crucial effort needs to be made to improve the eco-efficiency of distillation processes. Energy-efficient design can be achieved through process design and/or equipment design. The most straightforward approach is to increase the separation efficiency in the distillation column by introducing a high-performance mass transfer component without compromising the hydraulic performance (capacity).

An advanced structured packing is characterised by a highly effective interfacial or wetted area promoting the mass transfer between the vapour and liquid phase to separate components in a distillation column. At the same time, the pressure drop is kept to a minimum due to the packing’s low resistance to the gas flow, thereby extending the useful capacity of the packing. Wettability on the packing surface has a remarkable effect on efficiency. Superior surface texture ensures that the liquid film on the wetted surface is created, maintained, and continually renewed to maximise utilisation of the packing surface. Performance of the distillation column is enhanced as the useful capacity is extended.

The latest generation structured packing MellapakEvo embodies these features and outperforms the current benchmark MellapakPlus packing in both efficiency and capacity (see Figure 2). Tested and proven in Sulzer’s in-house R&D facility in Oberwinterthur, Switzerland, and in an independent testing facility located in the US, the new packing offers up to 40% higher separation efficiency at the same useful capacity and pressure drop than MellapakPlus 252.Y. Compared to MellapakPlus 452.Y, it offers similar separation efficiency with approximately 20% higher capacity (see Figure 3).

The superior mass transfer and hydraulics performance of the latest generation of structured packing benefits distillation columns in several ways.

Typically, the diameter and height of a separation column are determined by the required separation stages (efficiency) and energy (reboiler duty). The reflux ratio of the column can be reduced with a higher number of theoretical stages supplied in a column. Provided that the overall pressure drop is maintained, the same product quality can be achieved with lower reboiler duty.3 An optimised column design will balance the trade-off between the capital cost associated with a taller column due to greater packed bed height and the operating cost of energy to supply the required heat to the reboiler. MellapakEvo benefits the column design by increasing the number of separation stages without the penalty of introducing taller columns, which incur higher capital costs. With increasing energy costs, the economic benefits of installing higher efficiency in the column would be substantial.