Equipment Design and Costs for Separating Homogeneous Mixtures Dr. Syeda Sultana Razia Department of Chemical Engineering, Bangladesh University of Engineering and Technology (BUET), Dhaka -1000
1. Distillation
Design Procedures for Columns with Sieve Trays Designation of design bases Composition and physical properties of feed and product Special limitations: maximum temperature and pressure drop restrictions, presence of reactive materials or toxic components etc Selection of design variables: operating pressure, reflux ratio, feed condition Establishment of physical equilibria data data for binary pairs are combined with a model (Wilson, NRTL and UNIQUAC) to predict multicomponent behavior; UNIFAC model is used for prediction based on functional group
Design Procedures for Columns with Sieve Trays (cont d) Determination of number of equilibrium stages Minimum number of stages from Fenske equation N min x ln x ln LK x x HK HK D LK LK / HK av B 1/ 2 LK / HK av LK / HK D LK / HK B Number of equilibrium stages, N as a function of and (Gilliand equation) R min N min N N N 1 min 0.751 R R R 1 min 0.566
Design Procedures for Columns with Sieve Trays (cont d) Selection of column internals
Design Procedures for Columns with Sieve Trays (cont d) Determination of column diameter V V nf C sb 20 50 90% 0.2 n V nf L v v 0.5 m An V A C " v n A n A d D 4A c 1/ 2
Design Procedures for Columns with Sieve Trays (cont d) Efficiency: Eo 0. 492 0. 245 L LK / HK av Tray Spacing: 0.46 to 0.61 m (0.3 and 0.91 m are also used) Column height H N c act N E o N 1H H act s H s
Design Procedures for Columns with Sieve Trays (cont d) Sieve Tray Geometry Hole dia: 0.005-0.025 m Fractional free area: 0.06-0.16 m2 Fractional downcomer area: 0.05-0.3 m2 Pitch/hole dia ratio: 2.5-4.0 Tray spacing: 0.305-0.915 m Weir height: 0.025-0.075 m
Design Procedures for Columns with Random Packing Determination of diameter Vapor velocity is 70 to 90 % of flooding velocity Recommended pressure drop 400 to 600 Pa/m for atmospheric and high-pressure separation 4 to 50 Pa/m for vacuum operations 200 to 400 Pa/m for absorption and stripping column
Design Procedures for Columns with Random Packing (cont d) Heights of columns HTU method Z (HTU)(NTU) HTU m K a G " v e A c NTU y 1 dy y y y 2 * HTU HTU v HTU L Vv k a v e VL k a L e mv L
Design Procedures for Columns with Random Packing (cont d) HETP method ln HETP HTU 1 Z HETPN
Design Procedures for Columns with Structured Packing
Design Procedures for Columns with Structured Packing (cont d) Diameter Height 9.29 a HETP: Rule of thumb HETP 10 p
Other Distillation Processes Batch distillation: Food, pharmaceuticals and biotechnolgy industries Rayleigh equation ln W F x x i 2 i1 dxi y x i i
Other Distillation Processes (cont d) Azeotropic distillation
Cost Estimation
Cost Estimation (cont d) Costs of distillation column 25 trays 50 trays
Cost Estimation (cont d) Costs of sieve tray
Cost Estimation (cont d)
Cost Estimation (cont d)
Cost Estimation (cont d)
Cost Estimation (cont d)
2. Absorption and Stripping
Gas Treatment with Solvent Recovery
Design Procedures Column diameter: 70 to 90% of the flooding velocity, Larger of the top or bottom diameter is used Number of equilibrium stages: Modified Kremser equation A A N1 i N1 i A 1 i Solute fraction absorbed A i L K V i S S N1 i N1 i Si 1 Solute fraction stripped S i KiV L
Design Procedures (cont d) Stage efficiency and column height Overall efficiency E o 19.2 57.8log L Column height: Tray spacing/htu/hetp
3. Membrane Separation
Selection of Membranes Fabricated from natural and synthetic polymers Membrane modules Plate and Frame ($250-400/m2) Spiral-wound ($25-100/m2) Hollow fiber ($10-20/m2) Tubular ($250-400/m2) Capillary ($25-100/m2) Ceramic ($1000-1600/m2)
Concentration Profile across Membranes
Design Parameter Permeance: porosity, solubility or partition coefficient Separation factor/selectivity Purity and yield
Flow Patterns
4. Adsorption
Selection of Adsorbent Activated Carbon Molecular Sieve Zeolites Silica gel Activated Alumina
Basic Adsorption Cycles Temperature Swing cycle Cycle time: few hours Capacity: 1 kg per 100 kg of adsorbent
Basic Adsorption Cycles (cont d) Inert Purge cycle Regeneration is done by purging inert gas and lowering the partial pressure of the adsorbate Cycle times are only a few minutes Capacity 1 to 2 kg adsorbate per 100 kg adsorbent
Basic Adsorption Cycles (cont d) Pressure Swing cycle Cycle time: few minutes Capacity: 1 to 2 kg per 100 kg adsorbent