What is Random Packing?
Random packing is the process of dumping filtration material randomly in a distraction column to achieve optimum surface area on which reactants interact to reduce the requirements of constructing columns. It's used in distillation, stripping, carbon dioxide scrubbing and Liquid-liquid extraction.
They are used to allow contact between 2 fluids such as liquid with gas or with liquid. The packing selected should offer large surface area to achieve optimum mass transfer.
Types of Random Packing
Large random packing has a higher capacity, but efficiency is low while small packing has high efficiency with lower capacity. You should select the most economical packing that balances efficiency and capacity.
a) Raschig Ring Packing
Raschig Ring is the first generation of random packing which was designed with a height equal to the diameter. It is made of metal and it less common due to its low efficiency, low capacity, and high costs. Its only used in corrosive applications such as ceramic or carbon/graphite construction materials. It lacks internal support known as "fingers."
b) Pall Ring Packing
It is similar to Raschig ring but with "fingers." Pall rings are punched on the side to create internal drips which increase the capacity efficiency by 50% to 80% compared to the Raschig Rings. They have the same height as diameter and are commonly used in distillation and absorption services.
c) High-Performance Saddle Ring
It was designed to create a low aspect ratio or height less than the diameter. When installed, it settles in a way that it orients more flat surfaces parallel to liquid and vapor traffic. The surfaces increase capacity while maintaining an effective contact surface area. High-Performance Saddle Rings offer higher efficiency and capacity than the Pall Ring Packing.
They are commonly used in sour water stripping, atmospheric and high-pressure distillation, demethanizers, deethanizers, quench towers, acid gas removal, and main fractionators.
d) SuperBlendTM 2-Pac Packing
It uses two distinct Saddle Ring high-Performance Packing, large and small packing. Large packing is for high capacity, and pressure drop and small packing is for higher efficiency. The small packing fits in the interstitial voids of the larger packing to create a more effective surface area without decreasing the capacity. The combination increases the efficiency of large packing by 25% or the capacity of small packing by 15%.
This packing is commonly used in fine chemical distillation, absorption, and stripping, refinery fractionators and retrofit opportunities.
e) Lessing Ring
They are designed from the Raschig Ring and are made out of ceramic. They have internal drips which enhance mass transfer and increase the surface area. They have a high density. Thus they have excellent acid and heat resistance. They are used in regenerative oxide systems and transfer systems because they withstand corrosion.
f) Dixon Rings
Dixon rings are similar to Lessing in design, but they are made of stainless steel mesh. The mesh gives Dixon rings a low-pressure drop. They have a high rate of mass transfer due to the large surface area. They still keep a high mass transfer rate despite having a high liquid hold up, large surface area and low-pressure drop. They are mainly used in scrubbing applications and laboratory distillation.
Where is Random packing used?
• When tower has a small diameter
It is impossible to install trays in towers with a diameter less than 0.9m (3feet). Packing is the most reliable and efficient choice for small diameter towers.
• Processes with Low-pressure drop
Used when pressure drop in packing towers is way too lower than in tray towers. Trays have a free area of about 5% to 10% of the tower cross space while that of packing is more than 50%.
• Corrosive systems
Random packing comes in a range of materials such as plastic, ceramic and metal which are resistant to corrosive fluids.
• Increasing capacity of operation towers
If the tower with trays has low capacity, it can be upgraded with random packing to upgrade the capacity.
Factors to consider when choosing a random packing
The general characteristics you should consider before choosing a random packing are
• Check it meets the process requirements,
• Is it the direct replacement of an already existing packing
• Your familiarity with the type of packing and its performance
• Experience with a certain packing with a specific application.
• Can it be used in a licensed process
All these are the general factors that you should check before choosing a random packing. To get the most suitable Random packing for your unique process, you need to be more specific by checking characteristics such as density, measurement, wetting rate, surface area, among others.
Here a list of the specific characteristics you should check that is associated with your process.
1. The Surface Area
You should consider surface area of the packing because the larger the surface area, the higher the efficiency due to the increased vapor contact area. Therefore the efficiency increases with lower packing size.
2. Uniform spreading surface
The spreading surface should be uniform to improve liquid-vapor contact which enhances efficiency. A Raschig ring and pall of equal size are identical in terms of surface area, but latter has a better spread surface as compared to the former, and hence latter is more efficient.
3. An uniform distribution
Random packing rings that can be fixed together with their counterpart can lower the efficiency when they form channels. Packing should also drain fluids freely to prevent liquid settling pockets as it was common with the older saddle type of packing.
4. The rate wetting
The lowest wetting rate determines the stability minimum level of the packing. Dewetting of the packing surface occurs when the liquid on the surface of packing is interrupted by falling liquid. When packing surface Dewets it causes a decrease in efficiency.
5. Void created by the packing
A high random packing voidage lowers the resistance to vapor up flow hence improving the capacity by decreasing the pressure drop. Capacity increases with the particle size of the packing. The suitable size of packing should balance between efficiency and capacity.
6. The material used to make random packing
The table above shows some of the materials used to make random packing. Metal packings are unbreakable, offer higher capacity, turndown and efficiency, comes in different shapes and they have a higher resistance to compression.
Ceramic packings work best in applications where high chemical inactivatity and high-temperature resistance are required. They offer low capacity, are breakable and are not available in popular geometries.
The most affordable packings are those made of plastics (polypropylene), but they can’t withstand temperature above 1220C (2500F) depending on the grade. They should never be used in oxidizing enviroment and solvents application because they tend to degrade. They have poor wettability hence lower efficiency.
7. Friction
Capacity is higher in random packing when friction is minimized by using open shape which has aerodynamic characteristics.
8. Strength
Effective random packing should be resistance to any mechanical damage that may occur due to weight so that it can last longer.
Conclusion
Overall the best random packing for your process should be affordable as per your budget and resistance against fouling and corrosion so that they can last for a long time.
We conclude the guide on how to choose random packing for your industry here.
Hope you now know what to look for when choosing random packing.
Whether in new installations or replacing old installation, follow this guide to ensure you get maximum efficiency of your random packing.
If there is anything you didn't understand and you need further clarification, or you have a question regarding random packing or tower packing, feel free to leave a comment below, and we will get back to you with the information. Share this post on Facebook or twitter to help someone else.
