Many industries use random packed columns, and they are crucial to many processes. If you require a random packed column, it’s important to know what goes into designing the right one for your needs. You should also understand what packed columns are, why they’re used and the basics of how they work before you order or build your own random packed column.
Besides knowing how a random packed column works and its advantages, you should also be aware of the primary design procedures a company needs to follow to get the best performance possible.
What Is a Packed Column?
A packed column is a pipe or hollow tube filled with fine particles and packing materials. You can think of them as a pressure vessel with a packed section included in its final design. They’re usually cylindrical and shorter than capillary columns. Since they’re packed with materials, the column has a higher pressure drop.
These columns are used primarily for liquid-liquid extraction, gas absorption and distillation. Unlike a plate column where the gas-liquid contact is stagewise, a packed column’s gas-liquid contact is continuous. In this design, the liquid will flow down the column and pass through the packaging material. While the liquid flows down, the vapor or gas will go up the column in a counter-current manner. Occasionally, gas-absorption columns will utilize a co-current flow.
A packed column’s performance often relies on the proper distribution of gas and liquid in the packed bed. As such, a packed bed column’s design needs to take this proper distribution into account.
The Uses of Packed Columns
Packed columns are used in the chemical and food industry as well as in environmental protection. They also are commonly utilized in thermal power stations to assist with flue gas heat utilization, SO2 removal and water purification. Their role in the process industry is quite large, as the separation processes of packed columns account for somewhere between 40% to 70% of the industry’s capital and operating costs.
In addition to their separation abilities, packed columns can also handle the direction of heat transfer between liquid and gas. This ability to transfer heat makes them valuable to many types of companies. From a thermodynamical advantage point, they are the top apparatuses for transfer processes involving mass and heat during the gas and liquid phase.
Advantages of Packed Columns
Packed columns are still widely used for various applications. From their affordability and lower pressure to their ability to fit in smaller spaces and their suitability for foaming systems, they’re a great choice for many companies across a wide swath of industries.
Below you can find some of the top benefits that packed columns provide:
- Cost-effective: When compared to an equivalent plate column, a packed column will usually be less expensive when handling corrosive liquids. This affordability makes packed columns attractive to many companies.
- Ideal for foaming systems: Due to their construction, packed columns are best for handling foaming systems when compared to other options.
- Fit small diameter columns: Plates can often be expensive and hard to install in columns with a smaller diameter. If you have a column smaller than 0.6 meters, packed columns are an excellent option since they can fit in areas with less space.
- Lower liquid hold-up: Packed columns will have a lower liquid hold-up than plate columns. This lower placement is especially useful if you’re dealing with flammable or toxic liquids. Since you need to keep potentially dangerous liquids in small quantities for safety, the lower liquid hold-up of a packed column is ideal.
- Reduced pressure drop: Typically, plate columns have higher pressure drops than packed columns. If you need a more consistent pressure in your columns, a packed column is a great choice.
Components of a Packed Column
A packed column typically consists of tubing and packings, with other materials to keep the gas and liquid flowing smoothly. The tubing will contain most of the materials, as the gas and liquid will be passing each other while inside the vessel. Below you can find a list of the main components of a packed column:
- Vessel: The vessel or tower is the part of the packed column that holds the rest of the components. They’re usually cylindrical and made of steel. Depending on their application, the vessel will be coated to stop corrosion and other potentially harmful effects.
- Packing: In a packed column, the packing assists with intimate contact between the liquid absorbent and target pollutant molecules.
- Spray distribution system: The spray distribution system for the packed column is used to spread your liquid absorbent evenly across the top of the packing.
- Reservoir: You’ll usually find the reservoir at the tower’s bottom. It’s used as the pump’s wet well.
- Pump: The vessel’s pump is designed to take the liquid absorbent and transfer it to the spray system.
- Blower: The blower forces a gas stream up from its source and through the packing.
- Support floor: A support floor is crucial to several functions of the packed column. It allows the liquid absorbent to drain away from the column’s packing and acts as an inlet device that assists with influent gases being evenly applied to the bottom of the packing column. It is also regularly used to create a space to hold the packing above the reservoir, which helps influent incoming gas to spread across the tower’s cross-section.
- Demister: A demister prevents moisture droplets from leaving the tower alongside the gas stream.
In a packed tower, the liquid absorbent gets pumped to the spray distribution system from the tower’s reservoir. This absorbent is then sprayed evenly over the packing material’s top surface. After being sprayed, the liquid absorbent then trickles through the packing.
Simultaneously, the gas stream is pumped from below the packing and up through it. When the gas and the liquid meet, the liquid dissolves substances in the gas, getting rid of them before the gas continues its upward flow to the exit point at the top of the tower. The demister will also catch any excess moisture, trapping molecules before they can exit the column.
What Packing Materials Are Used for Packed Columns?
One of the most important parts of a packed column is the material you choose to fill it with. To give you a clearer idea of what comes in packed columns, consider the most common materials used for packing:
- Ceramic: Ceramic offers exceptional corrosion resistance when placed in higher temperatures. It also offers superior wettability to ensure that your liquid can make consistent contact with the ceramic.
- Plastic: Plastic is a common packing material since it’s one of the most affordable and has good strength. At a low liquid rate, plastic does have a downside since it can have poor wettability.
- Metal: If you’re looking for greater strength and wettability, metal is an excellent option for packing materials.
After you’ve selected a packed column for your use, you still need to make several decisions to design the best column for your needs. Consider the type, column height and column width when creating a random packed column for your company’s unique requirements:
Selecting the Size and Type of Packing
Packed columns can be made with either random dumped packing or sections of stacked or arranged packing materials. The two primary types of packing include random packing and structured packing.
Random packing refers to the packing of various geometrical shapes into the column. Once in the tower, the randomly packed shapes orient themselves on their own without a specific direction. Structured packing refers to the placement of corrugated sheets and crimpled layers that are stacked inside the column. Each layer of these materials will be placed at a 70-degree to 90-degree angle to the layers below it. While structured packing has its uses, we’ll stick to random packing for this article.
If you’re interested in designing a random packed column, you’ll need to choose random packing. This method has a few advantages that make it the choice of companies nationwide. It’s very cost-effective while still offering high-quality performance. It also has greater efficiency than other technologies, higher mass transfer and an improved contact area. If you’re conducting separation tasks, random packing is often the go-to choice.
You’ll select your packing sizes based on the size of the vessel. Overstacking the column can cause poor liquid distribution. Some of the recommended size ranges for a random packed column include:
- Less than 1 foot of column diameter should have a packing size of 1 inch or less.
- Between 1 to 3 feet of column diameter will have a packing size of 1 to 1.5 inches.
- A column diameter of greater than 3 feet will need a packing size between 2 and 3 inches.
Determining the Column Height
The packing height equals the height of transfer units multiplied by the number of transfer units. You can determine the packing height through either a mass transfer analysis or an equilibrium stage analysis. Find out more about how to use either of these analyses below:
1. Equilibrium Stage Analysis
When random packing is used instead of trays, the same enrichment of the vapor will occur over a certain height of the packing materials, and this height is termed the height equivalent to a theoretical plate (HETP).
Determining the HETP helps you figure out the proper packing height needed for an absorption column and a distillation column to ensure there is the same change in composition in the vapor or liquid phase as one theoretical plate. You’ll find these HETP values for every type of packing.
2. Mass Transfer Analysis
In a mass transfer analysis, the packed bed height is equal to the height of a transfer unit multiplied by the number of transfer units, which you obtain by numerical integration. With this method, the equation is often referred to as Z = HTU x NTU.
Determining the Column Diameter
The column diameter refers to a packed column’s capacity. It’s determined by identifying the cross-sectional area of the packed column. To figure out the diameter and column cross-section area for the required pressure drop, use a generalized pressure-drop correlation.
It’s important to understand your column diameter, as you want your random packed column to run at its highest economical pressure drop. This pressure drop ensures there is a proper distribution of gas and liquid. In a random packed column, it’s generally best for the pressure drop never to pass 80 millimeters of water per meter of packing height. This distribution will keep the gas velocity at about 80% of the flooding velocity.
Selecting and Designing the Column’s Internal Features
If you’re wondering how to design random packed columns, you should also know how to choose and design the internal features of them. Depending on the needs of your column, your random packing column design will incorporate different features to ensure that you get the performance you require out of them. Some of the internal features for columns you should pay attention to in the design of your packing column include:
- Packing support: Usually coming in the form of a support plate, packing support is crucial to holding the column’s wet packing weight, while still letting the liquid and gas flow freely. There are a few different types of support plates, but one of the best designs comes when there are gas inlets placed above the levels in the bed where liquid flows out of it.
- Liquid distributors: Another crucial internal feature is the presence of liquid distributors. These pieces will ensure the liquid is distributed equally throughout the column. In smaller diameter columns, you can often get by with a central open feed pipe or a pipe with a spray nozzle. However, for larger columns, you’ll need more complex liquid distributors.
- Liquid redistributors: You also need to choose the liquid redistributors. These redistributors collect excess liquid from the column walls and spray it back evenly across the packing. They also make sure any maldistribution of liquid is evened out within the packing. You can often find liquid redistributors that combine their base function with packing support. As you search for one, you need to make sure it doesn’t restrict gas flow, as this can produce local flooding.
- Hold-down plates: During a surge or when the column reaches high gas rates, the packing’s top layer can be fluidized. If this occurs, ceramic packing can break up and plug up the packing, while plastic and metal can end up getting blown out of the random packed column. A hold-down plate will prevent any fluidization by weighing down the packing. Ensure the one you choose doesn’t restrict liquid and gas flow but is still small enough to contain packing.
Choose MACH Engineering for Your Random Packing Needs
If you need to utilize random packed columns for your business, select an engineering company to supply you with tower packing products that can help you create the perfect packed bed column design. As a company, we pride ourselves on providing our partners with a large selection of custom and semi-custom products.
Browse our tower packing inventory to help you increase capacity without paying exorbitant prices or sacrificing efficiency. Contact us today for help with your current packed column project to learn how we can assist you.