Question 4 of 6: Distillation Column Design Steps; Tray vs. Packed Selection

Question 4: Distillation Column Design Steps; Tray vs. Packed Selection (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — Overall design steps for a distillation column

The overall design proceeds from thermodynamics, through stage requirements, to mechanical hardware:

1. Define the separation and collect data. Fix feed flow, composition and thermal condition, the desired distillate and bottoms purities (or recoveries), and gather vapour–liquid equilibrium (VLE), enthalpy and physical-property data for the system.
2. Set the operating pressure. Choose a pressure that lets condensation occur against available cooling water (avoids refrigeration) while keeping reboiler temperature below the point of thermal degradation; check for azeotropes at the chosen pressure.
3. Determine minimum reflux and minimum stages. Use the Underwood equation for $R_{min}$ and the Fenske equation for $N_{min}$ at total reflux, establishing the thermodynamic limits.
4. Select the actual reflux ratio. Fix $R$ at roughly 1.1–1.5 × $R_{min}$ — the economic optimum balancing more stages (taller column, higher capital) against more reflux (larger reboiler/condenser and energy cost).
5. Find the number of theoretical stages and feed location. By the Gilliland correlation or a stage-by-stage (McCabe–Thiele/rigorous) method, then apply an overall tray efficiency to get actual trays; locate the feed at the stage matching its composition.
6. Size the column hydraulically. Compute vapour and liquid loads, then set the diameter from a flooding correlation (e.g. 80 % of flood) and the height from tray spacing (or packed height from HETP), adding space for the feed, reboiler return and disengagement.
7. Design the internals and auxiliaries. Specify trays (type, weirs, downcomers) or packing and its distributors, and size the reboiler, condenser, reflux drum and pumps.
8. Mechanical design and costing. Set wall thickness for pressure and corrosion, choose materials of construction, and complete the capital/operating cost estimate, iterating on pressure and reflux if the economics warrant.

Part (b) — Tray tower vs. packed tower selection criteria

The choice turns on diameter, service and duty:

• Column diameter. Packed towers are usually favoured for small diameters (< about 0.6–0.9 m), where trays are hard to install and inspect; trays dominate large-diameter columns where good liquid distribution over packing is difficult to maintain.
• Pressure drop. Packing gives a much lower pressure drop per stage, so it is preferred for vacuum service and for heat-sensitive materials where a low bottoms temperature matters.
• Liquid rate and turndown. Trays handle high liquid loads and wide turndown (flow variation) well; random packing can weep or channel at low liquid rates, though structured packing improves this.
• Fouling, solids and corrosion. Trays are easier to clean and inspect, so they suit fouling or solids-bearing service; ceramic or plastic packing is attractive for highly corrosive duties where cheap corrosion-resistant material is needed.
• Number of stages and holdup. Many stages or a need for side-draws favour trays (each tray is a clear stage); low liquid holdup favours packing where residence time must be short (e.g. reactive or degradable material).
• Cost. For small columns packing is cheaper; for large columns trays are usually the lower-cost, better-understood option.