Facilities such as processing plants, offshore drilling operations, refineries and wastewater treatment facilities often need to perform mass transfer operations. They do this to purify gas and other liquids, as well as to remove pollution and contaminants.
Typically, these facilities pump the liquid into columns, where it then undergoes the separation process. These processes can take a few different forms, including:
- Scrubbing: This is a process in which a liquid is used to assist or accomplish the collection of dust or mist.
- Distillation: This is the process of separating two substances that have different boiling points.
- Stripping: This is removing the more volatile substance from the liquid at high temperature.
- Absorption: This is using a liquid to absorb one component of a gas.
Inside these process columns is packing. Packing optimizes the separation process by providing a large, wet surface where chemical separation, also known as mass transfer, can take place. In the process of mass transfer, separation is usually achieved through the opposing forces of heat and pressure vs. gravity. Heat and pressure drive water vapor upward, whereas gravity impels liquid material downward. Packing is used to amplify these forces and facilitate faster and more efficient chemical separation processes.
Packing materials come in a variety of different forms. Plants can choose their material to fit the surface area, weight, corrosion resistance and pressure drop that they need. For example, metal packing materials are known for their strength, but plastic packing materials are typically more cost-effective. Ceramic packing materials can be brittle, but they are in high demand for corrosive substances such as chemical waste because they resist corrosion well.
Packing materials must meet a few requirements to perform effectively. They must not interact chemically with the fluids being packed. They must be strong but lightweight. They must contain enough passageways that the liquid material can flow through without obstructing the liquid or causing drops in pressure. They must also allow for the proper amount of contact between liquid and gas.
Two main types of packing in a packed column are random packing and structured packing. In this article, we outline the differences between these two types of tower packing.
What Is the Difference Between Structured Packing and Random Packing?
Random packing uses a random distribution of small packing materials to assist in the separation process, while structured packing uses larger, fixed packing structures. These more formal materials guide the liquid materials through complex structural channels into a specific, fixed shape.
Below you will find a more detailed random and structured packing comparison.
What Is Random Packing?
Random packing is used in separation columns, such as a distillation column, to increase surface area for vapor/liquid contact so that chemical separation is more efficient. The small pieces of random packing in a distillation column are designed to form a large surface area where the reactants can interact while minimizing complexity within the column. Random packing is designed to maximize the surface-to-volume ratio and minimize pressure drop.
The efficacy of random packaging depends upon a few factors — efficiency, pressure drop and capacity. Typically, when random packing is large, capacity is increased at the cost of lower efficiency. Conversely, when random packing is small, efficiency is increased at the cost of lower capacity. Low pressure drop is ideal because high pressure drop diminishes both performance and efficiency.
Initially, random packing materials were made of ceramic. The use of ceramic has declined because of its brittleness, but it still is used in some applications that require strong corrosion resistance. Today, many random packing materials are made of metal and plastic, with some other materials used in specialized applications.
How Does Random Packing Work?
To install random packing, materials are dumped into a column and allowed to settle. The materials are collected randomly in the packaging bed inside the collection container. Then the liquid to be separated flows through the materials.
Random packing makes use of a few different structures:
1. Raschig Rings
Raschig ring packing makes use of small pieces of tube to make a packing bed. These small tubes are usually about as long as they are wide. Raschig rings belong to an earlier, less sophisticated generation of packing material. They are smooth, without holes, grooves, ribbing or other textured elements. They have low capacity and low efficiency, and they tend to cost more than other types of random packing. Because Raschig rings have such high cost in comparison to their efficacy, applications for this type of packing are limited. In situations where corrosive material is being separated, however, ceramic Raschig rings are effective thanks to their high corrosion-resistance.
2. Pall Rings
Pall rings are similar to Raschig rings but come with added sophistication. Pall rings include added internal support structures and external surfacing. The texture within the ring walls allows for points of internal dripping that significantly increase the capacity and the efficiency of the packing. Pall rings work particularly well for distillation and absorption applications.
3. Saddle Rings
Saddle rings come in two primary types — Berl saddles and Intalox® saddles. As their name suggests, they’re shaped like tiny saddles. They differ from Raschig and Pall rings in that their length exceeds their diameter. Smooth Berl saddles are often shorter, with a more traditional saddle shape. Intalox® saddles are significantly longer, shaped much like pieces of macaroni halved lengthwise into long, open curved pieces. They also offer small holes and grooves for increased surface area and contact opportunity. Saddle rings, with their increased surface area for liquid and vapor contact, offer increased capacity and efficiency. They are ideal for chemical distillation, stripping and absorption.
4. Lessing Rings
Lessing rings are made of ceramic. They have internal partitions to increase surface area and enhance efficiency. Like all ceramic packing pieces, they are highly resistant to heat and corrosion. These qualities make them ideal for applications such as regenerative oxide systems.
Tri-Packs were first developed in the late 70s and are now broadly used. Tri-Packs offer many advantages over older types of packings. Thanks to their spherical shape, they are not prone to nesting and settling. The interior ribs also maximize surface area and wetting qualities. To achieve low pressure drop and high-mass transfer rates, Tri-Packs are a great choice. Tri-Packs are commonly offered in a range of plastics, providing corrosion and temperature resistance.
Benefits of Random Packing
Most separations call for random packing. The major advantage of random packing is that it is much less expensive to implement than structured packing. Other benefits of random packing, such as improved contact area, mass transfer and efficiency over older technologies such as tray technology, come without high costs.
In addition to the applications discussed above, random packing applications also include stripping, distillation, carbon dioxide scrubbing and liquid-liquid extraction.
What Is Structured Packing?
Sometimes, however, more structure is called for than random packing can provide. In this case, structured packing is necessary.
Structured packing is a type of organized packing used to channel liquid material into a specific shape. It uses discs composed of materials such as metal, plastic or porcelain with their internal structures arranged into different types of honeycombed shapes. These honeycombed shapes are always found within cylindrical columns.
Structured packing cylinders are precisely engineered to provide a large surface area for the liquid to contact without causing resistance that impedes the liquid’s flow.
How Structured Packing Works
While using structured packing, a liquid is forced into specific, preordained arrangements within the column.
Instead of utilizing many small pieces, structured packing consists of large pieces of material. This material contains holes, grooves, corrugation and other textured elements that allow for increased surface area and contact ability. Structured packing, especially older structured packing, is often made of sheet metal, but it can also be made of plastic, porcelain, wire gauze and other materials. Plastic is particularly useful in applications involving corrosive liquids.
Structured packing is used in processing plants to package and store liquid. It is particularly useful in offshore applications, such as offshore drilling, because the increased liquid spreading helps counterbalance the effects of the constant motion of the offshore plant.
The following are a few different types of structured packing:
1. Knitted Wire Structured Packing
The multitude of small wires of knitted packing offer a large surface area and efficient mass transfer. Knitted packing can be formed into other shapes, such as rolled into cylinders or wrapped to form a solid block.
Gauze is used for applications that require the lowest possible pressure drop. It is ideal for processing pharmaceuticals and other chemicals that require a specific temperature range.
3. Corrugated Structured Packing
Sheet metal is often used in applications such as vacuum distillation and high-pressure absorption. Because it is stronger and more durable than knitted packing and gauze, it can process much higher volumes of liquid and vapor. When necessary, strong sheet metal can be corrugated into smooth, thick grids. The advantage of corrugated grid sheet metal is that its smooth, open surface has some resistance to chemical fouling. This quality makes these types of packing ideal for wastewater and other fouling-prone environments.
Benefits of Structured Packing
In tower packing, it’s essential to create contact opportunities for liquid and vapor. One of the best ways to do this is to use packing that spreads the liquid into a thin film. This configuration leads to more contact and enhances performance. Some types of structured packing materials have additional textured designs to increase liquid spreading. Liquid spreading is particularly important in low-pressure applications where internal pressure alone cannot be relied upon to spread the liquid.
Structured packing usually has a smaller pressure drop and can handle a larger flow volume than random packing, so it is beneficial in situations involving extremely low pressure or extremely high flow rates. This lower pressure drop, in turn, leads to several other benefits. It allows for higher volatility, which is beneficial in separation processes that are more difficult. It also increases energy efficiency and reduces foaming.
Because of their tightly organized internal infrastructure, these packing materials also offer increased efficiency and the ability to pack more volume. The higher capacity possible with structured packing, in turn, leads to increased operating rates.
Structured Packing Applications
Structured packing is designed specifically for use in distillation and absorption processes. It can be used in a variety of applications, including seawater scrubbing, deaerating and more. Below are a couple of specific examples of structured packing uses in specific applications.
1. Natural Gas Dehydration
Structured packing is particularly useful in natural gas dehydration. The glycol in natural gas, for example, must be dehydrated before it can pass through pipelines, or the water vapor present in the natural gas may freeze in cold temperatures. If this happens, then ice may damage or burst the pipeline. So glycol can be used to remove the water vapor from the natural gas to prevent ice damage.
The use of structured packing for natural gas dehydration has been proven to provide up to twice the capacity and 50% more efficiency than the use of older tray technology. Structured packing can also be used to decrease the diameter of the absorption column. The many textured elements of the structured packing increase surface area without necessitating a corresponding increase in diameter to achieve the same efficiency. This makes the absorption vessel, smaller, lighter and cheaper.
2. Styrene Manufacture
Another application of structured packing is in the manufacture of styrene, a chemical used to to make latex, plastic packaging, disposable cups and a variety of other products. Styrene is also manufactured using the distillation process, typically with the help of steam as a heating agent. Structured packing is useful here because styrene polymerizes rapidly at high temperatures. Structured packing offers low bottom temperature and low pressure drop, both conditions that help prevent chemical reactions as the styrene is produced.
When metal structured packing materials were first implemented in styrene production, the pressure drop was reduced from 500 millibars under the previous tray system to only 40 millibars — a substantial and groundbreaking improvement in performance. This much lower pressure drop allows for much purer styrene that does not have quantities of ethylbenzene, the chemical precursor of styrene, mixed in.
When to Use Random vs. Structured Packing
Random packing advantages include high cost-effectiveness. The benefits of structured packing come at a price. If high capacity and efficiency are lower priorities but cost is a restricting factor, random packing offers quality performance at a much lower price. Figures from the EPA show that the cost for random packing can vary from as low as $6 per cubic foot for polypropylene material to over $100 per cubic foot for some types of stainless steel rings. Structured packing material costs vary over a much wider and higher range. Structured packing costs can range from $45 per cubic foot to over $400 per cubic foot.
Conversely, use structured packing in applications that require high capacity and efficiency. When high capacity is a necessity, the intricate internal structure of structured packing allows for a high surface area that translates into extremely high capacity. Similarly, the smooth honeycombed form of structured packing offers minimal impediment to the liquid, which leads to higher efficiency.
Also, structured packing can be used in situations where low pressure drop is required since structured packings offer lower pressure drop than random packaging.
Contact MACH Engineering for Random Packing and Structured Packing
Now that you’ve learned about structured packing vs. random packing, put your knowledge to good use.
MACH Engineering takes pride in offering quality internals, including structured and random packing, for a variety of applications. We can supply the appropriate packing for your application, or we can design an entire system if that’s what you need. Our consultants are also ready to help with technical questions.
We offer a range of features and materials, so you can find the right internals solution to fit your industry needs and budget. Contact us today.