POO Topic 4 Distillation Slides

Topic 4: Distillation Tower Operations

Overview of Distillation Towers

  • Purpose: Separation process using various components and methods for optimizing yield and efficiency.

Main Components of Distillation Column

  • Trays/Packing: Enhance vapor/liquid contact.

  • Tower/Column: Contains internal mechanisms for separation.

  • Reboiler: Provides heat for vaporization.

  • Condenser: Condenses vapor from the top of the tower.

  • Accumulator: Holds condensed liquid, possibly used as reflux.

Basic Operation and Terminology

Feed Tray

  • Function: Introduces feed into the system, dividing the column into:

    • Rectification Section: Top of the column.

    • Stripping Section: Bottom of the column.

Boiling Curve

  • IBP (Initial Boiling Point): First boiling occurs.

  • FBP (Final Boiling Point): All liquid is vaporized.

Section Objectives

Stripping Section

  • Aim: Increase yield of lighter product.

  • Controls IBP of heavier product (Note: FBP of heaviest product is not controllable).

Rectification Section

  • Aim: Increase yield of heavier product.

  • Controls FBP of lighter product (IBP of lightest product is not controllable).

Internal Reflux

  • Definition: Liquid that flows down the tower. Minimum internal reflux is required for efficient operation.

  • Control Method: Achieved with pumparound heat removal, ensuring uniform load distributions and cost savings.

Column Internals - Trays

  1. Bubble Cap Trays

    • Low capacity and efficiency, maintain low liquid flow.

  2. Sieve Trays

    • Metal plates with holes; vapor passes straight through liquid.

  3. Valve Trays

    • Liftable caps allow vapor to lift and create flow area.

Tray Design Comparison

  • Capacity & Efficiency: Bubble-cap (lower) > Valve > Sieve.

  • Cost: Bubble-cap > Valve > Sieve.

  • Maintenance: Sieve trays least fouling tendency.

Column Internals - Packings

  1. Random Packings: Discrete geometric shapes randomly packed for vapor-liquid contact.

  2. Structured Packings: Ordered arrangements (e.g., crimped wire mesh).

  3. Grid Packings: Open-lattice structure; limited use primarily in heat transfer.

Packing vs. Trays

  • Packings less tolerant to fouling; installation cost usually higher.

Common Distillation Tower Problems

Flooding

  • Excessive liquid accumulation, types include:

    • Jet flooding

    • Liquid entrainment

    • Downcomer backup and choke flooding.

Foaming

  • Vapor bubbles do not coalesce, often caused by contaminants, leading to:

    • Premature flooding

    • Solvent losses.

Tower Internal Fouling

  • Caused by corrosion products; trays handle solids better.

Reboiler System Problems

  • Key Issues: Liquid level instability, fouling, condensate flooding, loss of condensate seal, vapor blanketing, gas binding.

Overhead System Problems

  • Common Problems: Fouling, flooding, inadequate coolant flow, affecting condenser performance.

Distillation Tower Optimisation

  • Adjust boiling ranges by manipulating various parameters:

    • Cut Point Adjustment: Affects liquid top product based on tower top temperature and pressure.

Product Properties & Yield Optimisation

  • Key properties to monitor: Pour Point, Freeze Point, Flash Point, Reid Vapor Pressure (RVP).

  • Changes in product composition can affect flash point and boiling range.