Distillation is one of the most important unit operations in the Chemical Process Industry (CPI). The distillation process utilizes towers with internals. Examining the different tower types is a first step when learning about the distillation process. As one of the most essential pieces of a distillation system, towers need to fit the distillation process properly and must also be able to handle any unforeseen problems as tower internals are not easily accessible or fixed after assembly. This includes turndown ratio and changes in feed flowrate and/ or composition..
Below are some things to know about the differences between packed towers and tray towers to help you decide which is best for your operation.
Basic Operation of Tray Towers
Tray towers are designed to provide liquid holdup in order to achieve the proper vapor-liquid mass transfer that the distillation process requires for separation. Holdup is accomplished by a weir on each tray and having stacks of horizontal trays that allow liquid to run across a tray before flowing over to a downcomer and moving across to the next stage of the distillation process.
In addition, flow across the trays allows upward moving vapors and downward moving liquids to have intimate contact in strategically placed passages in the tray. These include valve trays, bubble cap trays, and sieve trays.
Basic Operation of Packed Towers
Packed towers work by providing a large amount of surface area per unit volume to facilitate the liquid-vapor mass transfer. As the liquid phase flows through the tower, the vapor and the liquid are in continuous contact on the surfaces of the packed bed, differing from the stepwise contact of tray towers. Packed towers generally come in two designs: random and structured packing.
- Random Packed towers are constructed by using a variety of metal or nonmetal materials, including ceramics or plastics. These materials provide the surface area for the distillation process. The Raschig ring was the first packing material but newer shapes provide low pressure drop and high surface area per unit volume.
- Structured packing towers offer more surface area and have a lower pressure drop than packed towers do. Structured packing can be manufactured from corrugated sheets of perforated embossed metal, plastic (including PTFE), or wire gauze. The result is a very open honeycomb structure with vertical flow channels giving a relatively high surface area to volume but with very low resistance to flow. The surfaces have been chosen to maximize liquid spreading. These characteristics tend to show significant performance benefits in low pressure and low flow rates per cross sectional area of the column.
Random packed towers and structured packed towers require liquid distributors at the top of the column and packing support plates at the bottom of the column.
Advantages and Disadvantages
So which process is better? The answer, of course, depends solely on what type of distillation process you are using. If you are unsure what type would be better, COSTELLO can use ChemCad to create a process simulation, calculate the column diameter and can help you choose the type of column needed. Some of the advantages or disadvantages of these different tower types include:
- Packed towers offer a lower pressure drop
- Tray towers are better at handling solids or other sources of fouling
- Tray towers are better at handling lower liquid rates
- Packed towers are better for corrosive liquids
- Packed towers are better at handling foaming systems
- Tray towers offer better predictability than packed towers
Let COSTELLO Design the Right Distillation Tower for You
COSTELLO’s team of experts have years of experience simulating and designing distillation towers. We can help you to pick the right tower for your chemical process. Contact us today to learn more about what services COSTELLO offers and how we would be a good fit for you.