Many important unit operations such as distillation, stripping and scrubbing rely on the proper selection of tower internals and packings for best separation. Tower packing creates a large amount of surface area to volume where the liquid-vapor mass transfer can occur. Furthermore, it increases the production capacity and the separation efficiency, lowers the pressure drop and carries high operational flexibility.
There exist about 200 different types and sizes of random packing, with specific packing area ranging from 60-450 m2/m3. The development in tower packing began at the end of the 19th century with the aim of increasing efficiency and capacity. As early as 1820s, broken glass or glass balls were used as random packing and by 1850s, these were replaced by pumice stone or pieces of coke. A main drawback related to these early packing types, was its unpredictable efficiency.
In 1914, the thin-walled metal cylinder called Raschig Rings were introduced. Its invention took into consideration the importance of void fraction and the surface area in relation to mass transfer. Later in 1930s, Germany and US started collecting data for packed columns using metallic and ceramic Raschig rings as well as ceramic Berl saddles, which got introduced during this time as well. These random packings are known as the first generation of random packings.
Up to 1960s, the use of packed columns was limited to small plants with column diameters of not more than 0.5 m. This was due to the properties of the first generation of random packings, which suffered from decreasing efficiency when column diameters exceeded 0.5m. The low efficiency resulted from the high tendency of maldistribution. The invention of Pall Ring, introduced by the BASF in 1950s and patented in the early 1960s, reduced the limitations associated with prior packing types. These had openings in the wall which improved packing area distribution, wetting, and distribution of liquid. These had higher capacity and efficiency and lower pressure drop than the packings described so far. With their partly open area in the wall, Pall Rings were the prototype of the second generation of the random packings. The latter generation was characterized by the improvements made on the first generation of random packings and the opening of walls. The third-generation packing was based on the standard packing shapes including cylindrical ring, saddle and sphere. The goal of the third generation was to remove as much of the closed wall of the basic shape as possible. In the late 1970s and 1980s, many packing shapes with relatively large open areas were introduced like Nor-Pac, Hiflow rings, Tri-PAK, VSP, Cascade Mini Rings and more. Most of the packings were available in all three different types of material including metal, ceramic and plastic.
New development began in the plastic sector which eventually led to new plastic packing types. Many complicated shapes were constructed out of plastic which would have been impossible to make from steel. The plastic packing had a wide range of applications not only in chemical and process industries but also in air treatment, wastewater treatment, gas cooling and in other applications related to environmental protection. Among many types of plastic, Polypropylene is inexpensive and is most popular when temperature do not exceed 250 F or ranges from low to moderate. In cases of high temperature, other plastics should be considered. Generally, plastic pall rings are stiffer and resist softening more than other shapes because of their internal “arms“.
Along with polypropylene, polyethylene (PE), reinforced polypropylene (RPP), polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) and polyvinylidene fluoride (PVDF) are some of the other prominent plastic filler material in plastic packing. Plastic has good corrosion resistance, large flux, low energy consumption, and low operating cost. They are also light weight and easy to handle.