Selecting a Suitable Structured Packing
Facilities like processing plants, offshore drilling locations and refineries often perform chemical separation functions, also called mass transfer functions. In these functions, liquids such as gasoline and water are purified of contaminants. This process is accomplished by pumping liquids into a scrubber column, where the liquid is separated from pollutants and particulates.
These facilities use packing inside the scrubber column, which provides a large surface where mass transfer can take place. The packing captures moisture and particulates, while the heat and pressure inside the column drive water vapor up and away from the liquid running down the column. As a whole, this system provides a fast and efficient chemical separation process.
Packing materials inside the tower come in a range of types and forms. One type of packing is called structured packing. Here, we’ll discuss structured packing column design, what it means, the different types available and how it compares to other packing structures.
What Is Structured Packing?
Structured packing is a type of organized column packing where materials are specifically designed and arranged into geometric patterns. This differs from random packing, which fills the column with random structures.
In structured packing, discs of honeycombed materials are placed inside the column, forcing fluids to take complicated paths down the length of the column. These long, complex paths create a large surface area for contact between the liquid and the packing material without impeding gas flow.
The History of Structured Packing
Structured packing has been an option for scrubber columns for decades. The concept was first established in the early 1940s and was patented in the 1950s. These early iterations primarily used wavy-form sheet metal. New iterations using wire mesh were developed later in the 1950s and were widely used until the 1970s. This second generation of mesh saw limited use due to high cost and low capacity.
The third generation of structured packed columns came in the late 1970s with corrugated structured patterns, which provided higher capacity at a lower cost and better performance overall. This new structure was tweaked over the following decades, changing the shape and design of the materials to improve surface area, wettability and other factors.
Today, several structured packing options are available, primarily consisting of thin metal sheet or wire gauze. Plastic and porcelain options are available as well depending on the type of liquid being processed.
How Is Structured Packing Used?
Structured packing is used in distillation and absorption processes and is best applied in low pressure and low irrigation applications. Some common applications of structured packing include the following:
1. Offshore Drilling
Structured packing is often used in offshore applications, like offshore drilling. This is because the increased liquid spread from structured packing helps counterbalance the effects of constant tilt and motion.
2. Natural Gas Dehydration
Structured packing is also used in natural gas dehydration. Natural gas must be dehydrated before it can pass through pipelines, or else the water vapor will freeze in cold temperatures and damage the pipeline. Structured packing is 50% more efficient than tray technology and can be used to decrease the diameter of the absorption column, which significantly reduces costs.
3. Styrene Manufacturing
Styrene is a chemical used to make latex, disposable cups, plastic packaging and many other commonly used products. Styrene is commonly manufactured using a distillation column with structured packing. This is because styrene polymerizes quickly at high temperatures, and structured packing offers low bottom temperature and low pressure drop, which helps prevent polymerization during manufacturing and produces purer styrene.
Types of Structured Packing
Structured packing comes in a range of design types and materials. These include the following:
- Gauze: Gauze is a woven structured packing that consists of a wire gauze material. The closely woven wires create very small openings that work to maximize surface area. This packing option is preferred for low liquid rate applications and areas that require the lowest possible pressure drop per theoretical stage. This means it is an ideal choice for processing specialty chemicals, pharmaceuticals and temperature-sensitive materials.
- Knitted: Knitted wire packing essentially consists of multi-strand wires that are knitted into a high-density mesh. This mesh can be used as one-piece layers or stacked into near-solid bricks, depending on the needs of the column and application. This type of packing offers a large surface area and efficient mass transfer, but it has a higher pressure drop than gauze packing.
- Sheet metal: Sheet metal consists of sheets of textured metal material arranged into bundles. This packing is significantly less expensive than wire materials, it is durable, it can handle a wide range of liquid and vapor rates and process services, and it can withstand some amount of fouling. Some applications include vacuum and atmospheric distillation services and wastewater treatment functions.
- Grid: Grid packing comes in two types — corrugated sheet packing and stamped blades connected to panels. These packing types are highly durable and resistant to fouling due to their thickness and smooth surfaces. Their resistance to fouling makes grid packing types the best choice for extended operation in fouling services, such as fractionation and wastewater treatment.
The above types of packing are often made with metals such as carbon steel, copper, stainless steel or titanium. Structured packing options are also available in other materials like plastics, ceramics and glass. Plastics like polypropylene, PTFE and PVC are especially useful in crude oil processing applications.
Advantages of Structured Packing
Structured packing offers a range of benefits compared to other packing options, including the following:
- Low pressure drop: Structured packing, especially gauze packing, offers a very low pressure drop compared to other packing options, making it a better choice for more volatile chemicals.
- Large surface area: In tower packing, there needs to be plenty of opportunities for liquid and vapor to make contact. The best way to do this is to spread the liquid into a thin film, which structured packing does better than any other packing method.
- High efficiency: Compared to both tray and random packing, structured packing offers excellent efficiency for separation and filtering. This means more filtering can be achieved with a shorter column.
- High capacity: Structured packing’s tightly organized internal structure allows columns to be more efficient and pack more volume than other packing types. This increased capacity also improves operational costs and rates.
- Maximized liquid and gas flow: Structured packing is designed to handle a larger flow volume than random packing, making it better for high flow rates.
In addition to these benefits, structured packing also tends to be more resistant to corrosion and temperature than other packing techniques, making it useful for a wider range of applications.
However, structured packing isn’t the best choice for every application. Structured packing tends to be more costly than other packing methods due to the quality and manufacturing of materials. This makes it a less cost-efficient choice for some applications. Additionally, structured packing techniques are more vulnerable to issues related to misdistribution.
Structured packing is a valuable packing method for a range of applications in processing facilities. If you want to learn more about structured packing, its benefits and how to choose the right packing material for your application, MACH Engineering is here to help. We offer quality internals, including a range of structured packing options, and experienced consultants to help you with everything from design to supply.
Learn more about our products and services today by contacting us.