Distillation Column Internals

The Engineer's perspective
The Engineer's perspective

Table of Contents

Distillation is the process of separating the components of liquid-liquid and vapor-liquid mixtures by selective evaporation and condensation. Either a batch or continuous approach may result in complete separation of nearly pure substances or partial separation, in which a higher concentration of specific substances is achieved.

Though knowledge of the liquid mixture component’s volatility is key to the efficiency of the separation process, the distillation equipment in which the separation process is performed (called a distillation column) plays a major role.

Distillation Column Components

There are various components in a Tray distillation column:

(a) A vertical shell in which the separation process is performed;

(b) Distillation column internals that enhance the separation process of each component (these could be in the form of trays, plates, or packings);

(c) A reboiler which provides the necessary vaporization for the distillation process;

(d) A condenser to cool down the vapors leaving the top of the column, converting them back into liquid form (called condensate);

(e) A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column.

Diagram of a Distillation Process
Schematic Diagram of a Distillation Process

Types of Distillation Column Internals

 Tray Distillation column internals promote vapor-liquid contact so that every theoretical stage achieves equilibrium efficiently — enhancing the separation process of each component of the liquid mixture. Factors such as the geometry of distillation column internals affect the extent and type of contact between the vapor and liquid streams.

There are 2 general types of distillation column internals:

  • Plates/trays – usually used interchangeably, plates or trays are the most widely used in distillation columns.
  • Packings — provide a large surface area for vapor-liquid contact, which increases the column’s efficiency. As such, packings are used in the separation of vapor-liquid mixtures (often used in solvents).

Distillation Trays vs. Distillation Packings

In general, distillation trays are used for applications with liquid rates of 30 m3/m2-h and above, and/or those where solids are present or fouling is a concern. Trays are designed to provide liquid holdup (stepwise contact) to achieve the proper vapor-liquid mass transfer that the distillation process requires for separation. 

Tray Distillation ColumnTrays and Packings for Distillation

The holdup is achieved by a weir on each tray and stacks of horizontal trays that allow liquid to run across a tray before flowing over a downcomer and moving to the next stage of the distillation process. Flow across the trays allows upward-moving vapors and downward-moving liquids to have intimate contact in strategically placed passages in the tray (e.g., valve trays, bubble cap trays, sieve trays). 

On the other hand, packings are used in lower liquid rate applications (less than 50 m3/m2-h), especially when minimizing pressure drop within the column is desired. They provide a large amount of surface area per unit volume to facilitate the vapor-liquid mass transfer. Unlike the stepwise contact in distillation trays, a packed bed allows for continuous contact between the vapor and the liquid on the surfaces of the packing. 

Common Types of Distillation Trays

Sieve Trays

Tray Distillation ColumnSieve Trays
Source: Ultimo Engineers, Sieve Tray

The sieve tray is the simplest type of cross-flow plate. The tray contains small round holes, perforations ranging from 1/8 to 1 inch which are punched through the tray deck. Vapor rises through the perforations on the tray floor and bubbles through the liquid in a fairly uniform manner. The liquid then flows across the tray floor over the weir, through the downcomer to the tray below.

The holes on the tray give a multi-orifice effect. The vapor velocity keeps the liquid from flowing down through the holes. Liquid flow is transported down the tower by downcomers, and weir to overflow device to the side of the tray. In sieve plates, the liquid is prevented from flowing through the sieve tray perforations by the upward flowing action of the gas. Thus, when the gas flow is low, it is possible for some or all of the liquid to drain through the perforations, and in effect, bypass portions of the contacting zone. This phenomenon is called weeping.

The function of a tray is to mix the vapor and liquid together to form a foam. Vapors bubble up through the tray sieve holes, where the vapor comes into intimate contact with the liquid. The fluid on the tray is a mixture of vapor and liquid in the form of froth or foam. This foam should separate back into a vapor and a liquid on the tray and in the downcomer. If the foam cannot drain quickly from a downcomer onto the tray below, then the foamy liquid will back up onto the tray above. This is called flooding.

Tray Distillation ColumnUnit operations seperation process
Source: Unit Operations - Separation Process - Distillation, National University of Singapore

Bubble Cap Trays

Tray Distillation ColumnBubble Cap Trays
Source: Real Ions Technologies, Bubble Cap Tray

In bubble cap trays, vapors rise up through risers or chimneys into a cap, then come out from slots or holes in a cap and bubble through the surrounding liquid on the tray. Vapors and liquid are in total contact for their mixing and mass transfer. Liquid flows over the caps on the tray from the upper downcomer to the outlet weir and downcomer to the tray below. A liquid head is maintained on the tray by an outlet weir near the downcomer.

Unit Operations - Separation Process
Source: Unit Operations - Separation Process - Distillation, National University of Singapore

Valve Trays

Valve Trays

The openings in valve trays are covered with liftable caps that adjust themselves according to the vapor flow. Tray valves or caps close off at a low vapor rate, stopping the liquid from moving down from the tray. As the vapor rate increases, the valve lifts, and the open area for vapor flow also increases.

The valves on the trays can be fixed or moving. The fixed valves are permanently open while the moving valves adjust themselves according to the vapors flowing up through the tray. Hence, valve trays can operate over a wide range of flow rates with high separation efficiency and large flexibility. 

The valve plate is designed to minimize “weeping”. Since the valve tends to close as the gas flow becomes lower, the total orifice area varies to maintain a dynamic-pressure balance across the plate. Because of their flexibility and reasonable price, valve trays tend to replace the bubble-cap trays.

Tray Distillation Column
Source: Unit Operations - Separation Process - Distillation, National University of Singapore

In summary of the comparison among the distillation trays, here are their advantages and disadvantages:

advantages and disadvantages among distillation trays

Examples of Distillation Packings

Random Packed Towers

Random packed towers are constructed using a variety of metal or nonmetal materials, such as plastics and ceramics. These materials provide the surface area needed for the distillation process. When it comes to packings, a good wettability is desired since a higher wetted area increases the interfacial area where vapor and liquid come in contact for mass and heat transfer.

The packing materials can be made from metal, plastic, or ceramic. 

  • Metal packings have high strength and good wettability. 
  • Plastic packings have sufficient strength but experience poor wettability at low liquid flow rates. 
  • Ceramic packings have a higher wettability than metal packings, but are not as strong and durable. They are used at elevated temperatures where plastic packing would fail due to its resistance to corrosion.

Types of Random Packings

Tray Distillation ColumnTypes of random packings
Types of Random Packings (a) Raschig Rings, (b) Pall Rings, (c) Ceramic Berl Saddle, (d) Ceramic Intalox Saddle, (e) Metal Hypac, and (f) Ceramic Super Intalox Source: (Sinott, 2005)
  • Raschig Rings — consist of a hollow cylindrical tube with a height to diameter ratio of 1:1. They display an exceptional strength-to-weight ratio and are more resistant to fouling than other types of packing. Raschig rings were the first kind of packing used in mass transfer processes. 
  • Pall Rings —  are a progression from the Raschig rings and have the same cylindrical shape and ratio. However, they have two rows of punched-out holes with webs from the center which drastically enhances the execution of the packing with regards to the pressure drop, throughput, and efficiency.
  • Berl Saddles — are an improvement from both the Pall rings and Raschig rings with regards to an enhanced flow distribution. The shape of the saddles has both the external and internal surfaces exposed so the distribution of vapor-liquid is the same on both sides. When comparing Berl saddles to the previous two types of packing, they produce a lower pressure on the surrounding column walls.
  • Other Proprietary Shapes — the Hypac and Intalox rings are improvements from the Pall ring and Berl saddles. Furthermore, Intalox saddles, Hypac, and super Intalox are all trademarked designs by the company Norton Chemical Process Products Ltd.
 

Structured Packed Towers

Structured Packed Towers
Left: Ceramic Structured Packing by ShenZen Chem Enviro Technology Co., Ltd. Right: Metallic Structured Packing by ETW International

Compared to random packings, structured packed towers offer more surface area and have a lower pressure drop. 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 a 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.

Which is Better – Tray Distillation Columns or Packed Columns?

Distillation Trays And Distillation Packings

Understanding the pros and cons of different column internal options types is key to efficient design practice. In general, the application suitability depends on the nature of feed and flow rates.

 Trays are used in applications with liquid rates of 30 m3/m2-h and above, and/or those mixtures where fouling is a concern whilst packings are used in lower liquid rates (less than 50 m3/m2-h) and minimizing pressure drop within the distillation column is desired. 

However, these liquid rate guidelines are not absolutes as there is often significant overlap in the liquid flow rate ranges for various types of column internals. In some cases, a combination of trays and packings are used in distillation systems depending on the feed nature and composition, and the desired distillate. 

Thus, it is always recommended to evaluate all practical solutions and approaches to come up with the most suited design. As a rule of thumb, tray columns are more efficient for columns with diameters above 0.6 meters. Otherwise, packed columns are more suitable. 

References

  1. Costello, R.C. “Distillation Part 1 – Packed Towers Vs. Tray (Plate) Towers”, 2016. www.rccostello.com, https://rccostello.com/wordpress/distillation/distillation-part-1-packed-towers-vs-tray-plate-towers/. Accessed 14 April 2022
  2. Coulson, J.M. Coulson and Richardson’s Chemical Engineering Vol 2 5th Ed. Butterworth-Heinemann, 2002.
  3. Hussein, N “Types of Trays in Distillation Columns”, 2021. www.thepetrosolutions.com, https://thepetrosolutions.com/types-of-trays-in-distillation-columns/. Accessed 12 April 2022.
  4. Murtaza,G. et al., “Distillation Column Internals” vol. 0, no. 0, 2015, p. 16.  www.academia.edu, Distillation Column Internals | Sahir Chaudhary – Academia.edu. Accessed 14 April 2022.
  5. National University of Singapore. “Column Distillation — Sieve Trays, Bubble Cap Trays, Valve Trays.” Unit Operations – Separation Process, https://courses.nus.edu.sg/course/chesf/cn2113/UO/UO_SeparationProcess/. Accessed 14 April 2022.
  6. Perry, R.H. Perry’s Chemical Engineer’s Handbook 7th Ed. McGraw-Hill, 1997
  7. Pilling, Mark. “Choosing Trays and Packings for Distillation.” vol. 0, no. 0, 2009, p. 7. www.cchem.berkeley.ed, www.cchem.berkeley.edu/cbe150b/docs/dist/Pilling%20Article.pdf. Accessed 10 April 2022.
  8. Towler, Gavin, and Ray Sinnott. Chemical Engineering Design. Elsevier Science & Technology, 2019. Accessed 14 April 2022.
  9. Visual Encyclopedia of Chemical Engineering: Distillation Columns(n.d.), https://encyclopedia.che.engin.umich.edu/Pages/SeparationsChemical/DistillationColumnsAccessed 14 April 2022.

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