Detailed study notes on column internals: liquid-gas contacting systems, tray types, sizing methods, and key formulas used in distillation and absorption.
Table of Contents – Column Internals
Chapter 1: Column Internals
1.1 Introduction
1.2 Liquid-Gas Contacting System
1.3 Column Internals
1.4 Packed Column
1.5 Plate Column
1.6 Packed Columns vs Plate Columns
1.7 Comparison of Tray Types
- 1.7.1 Bubble Cap Tray
- 1.7.2 Sieve Tray with Downcomers
- 1.7.3 Perforated Plate without Downcomers
- 1.7.4 Valve Trays
- 1.7.5 Other Trays
1.8 Tray Performance - 1.8.1 Flooding
- 1.8.2 Blowing & Entrainment
- 1.8.3 Weeping
- 1.8.4 Tray Flexibility
- 1.8.5 Tray Stability
1.9 Tray Data Sheet
1.10 Preliminary Sizing of Trays/Packings - 1.10.1 Valve Tray Sizing
1.1 Introduction
Columns are the backbone of most separation processes in chemical and process industries. They are used in distillation, absorption, stripping, and extraction to separate mixtures into their pure components. The basic purpose of a column is to provide intimate contact between the liquid and vapor phases, allowing efficient mass transfer of components between them. For example, in a distillation column, the more volatile components transfer from liquid to vapor phase, while less volatile components condense back into liquid, leading to separation.
👉 Quick Points:
- Used in distillation, absorption, stripping, extraction.
- Function: maximize liquid–vapor contact.
- Aim: efficient mass transfer & separation.
1.2 Liquid-Gas Contacting System
For effective separation, the liquid-gas contact system must be carefully designed. The greater the interfacial surface area, the higher the mass transfer efficiency. Contact can be achieved by dispersing one phase into another (e.g., gas bubbles in liquid), or by spreading liquid as a thin film over surfaces. Proper mixing and turbulence also help renew the interface between phases.
👉 Quick Points:
- Efficiency depends on interfacial area.
- Contact types: bubble, film, spray.
- Turbulence → better renewal of liquid-vapor interface.
1.3 Column Internals
Column internals are devices inside a column that regulate the flow of vapor and liquid, and improve the distribution and contact between phases. They prevent short-circuiting, channeling, and flooding. Broadly, internals are divided into two groups: trays (plates) and packings (random or structured).
👉 Quick Points:
- Enhance mass transfer.
- Provide uniform liquid/vapor distribution.
- Two main types: Trays and Packings.
1.4 Packed Column
A packed column is filled with packing material such as Raschig rings, Berl saddles, Pall rings, or structured packings (corrugated sheets). The liquid flows downward over the packing surface, while vapor rises upward, promoting contact. Packed columns are favored for low-pressure operations, vacuum distillation, and corrosive liquids, because of their low pressure drop and high surface area. However, they require well-designed distributors to avoid maldistribution of liquid.
👉 Quick Points:
- Packing types: Raschig rings, Pall rings, Berl saddles, structured packings.
- Advantages: Low pressure drop, good for vacuum/corrosive systems.
- Limitations: Cleaning difficulties, liquid maldistribution.
1.5 Plate Column
Plate (or tray) columns consist of a vertical shell with trays installed at intervals. The vapor passes through openings in each tray, contacting the liquid that flows across it. The liquid moves downward via downcomers from tray to tray, while vapor rises upward. Plate columns are robust, handle wide ranges of liquid/vapor flows, and are easier to clean compared to packed columns, but they have higher pressure drops.
👉 Quick Points:
- Uses trays/plates + downcomers.
- Advantages: Good flexibility, easy maintenance.
- Disadvantages: High pressure drop, entrainment possible.
1.6 Packed Columns vs Plate Columns
Packed and plate columns are chosen depending on operating conditions. Packed columns offer lower pressure drop and are preferred for vacuum systems, small diameter columns, or corrosive services. Plate columns are better suited for large throughput, varying operating conditions, and are easier to maintain.
👉 Quick Points:
- Packed: Low pressure drop, vacuum service, corrosive fluids.
- Plate: Handles wide flow variations, easier to clean, suitable for large scale.
1.7 Comparison of Tray Types
1.7.1 Bubble Cap Tray
Vapor rises through risers and is forced under small caps with slots, bubbling into the liquid. They work well at low vapor rates but are costly and cause high pressure drop.
1.7.2 Sieve Tray with Downcomers
Simple perforated plates; vapor passes through holes to contact liquid. Cheaper than bubble caps but may suffer from weeping if vapor flow is low.
1.7.3 Perforated Plate without Downcomers
Liquid is held on the tray by vapor flow, but operation is unstable. Rarely used in practice.
1.7.4 Valve Tray
Contain perforations covered by liftable valves. Vapor flow lifts valves, adjusting the opening area to suit flow. Highly flexible and widely used.
1.7.5 Other Trays
Include dual-flow trays, jet trays, and tunnel trays used for specific applications.
👉 Quick Points:
- Bubble Cap: Flexible, costly, high pressure drop.
- Sieve Tray: Cheap, simple, risk of weeping.
- Valve Tray: Adjustable, flexible, efficient.
- Others: Special purpose designs.
1.8 Tray Performance
- Flooding: Excess vapor prevents liquid from flowing down, causing liquid accumulation.
- Blowing & Entrainment: Vapor bypasses liquid (blowing) or carries liquid droplets upward (entrainment).
- Weeping: Low vapor → liquid leaks through tray perforations.
- Tray Flexibility: Range of operating flow rates over which tray works efficiently.
- Tray Stability: Resistance against hydraulic problems like foaming or weeping.
👉 Quick Points:
- Flooding = too much vapor.
- Weeping = too little vapor.
- Entrainment = liquid carried upward.
- Valve trays = highest flexibility.
1.9 Tray Data Sheet
A tray data sheet is a technical record of tray design, including hole diameter, tray spacing, pressure drop, weeping/flooding limits, and efficiency. It is used for communication between process engineers and equipment vendors.
👉 Quick Points:
- Records design parameters (hole size, spacing, pressure drop, efficiency).
- Used for design and troubleshooting.
1.10 Preliminary Sizing of Trays/Packings
1.10.1 Valve Tray Sizing
Tray sizing determines column diameter, tray spacing, hole/valve size, and active area. The Souders–Brown equation is commonly used to estimate maximum vapor velocity: Vmax=K⋅ρL−ρVρVV_{max} = K \cdot \sqrt{\frac{\rho_L – \rho_V}{\rho_V}}Vmax=K⋅ρVρL−ρV
where:
- ρL\rho_LρL = liquid density
- ρV\rho_VρV = vapor density
- KKK = empirical constant
👉 Quick Points:
- Determines column diameter & tray spacing.
- Uses Souders–Brown equation for vapor capacity.
