CEE 452/652 Week 9, Lecture 2 Absorption Dr. Dave DuBois Division of Atmospheric Sciences, Desert Research Institute
Today s topics Today s topic: chapter 13 on absorption Cover odor control on Tuesday, Oct 30 Also have review session next class Oct 30 Midterm is Nov 1 (next Thursday) Cumulative, information up to Oct 30 lecture is fair game 2
Major Techniques for Capture/Elimination of Gas Pollutants Oxidation to form nontoxic compounds Chemical reduction to form nontoxic compounds Adsorption onto solid surfaces Absorption into liquids Biological oxidation to form nontoxic compounds Condensation of vapors to form liquids 3
The Absorption Process The transfer of material from a gas (absorbate) to a liquid (absorbent) Transfer is based on the preferential solubility of a gaseous component in the liquid Also known as scrubbing or washing Examples include removal and recovery of NH 3 in fertilizer manufacturing Control of SO 2 from combustion source Control of odorous gases from rendering plants 4
Mass Transfer in Absorption diffusion 5
Types of Absorber Control Equipment Packed bed tower absorbers Spray tower absorbers Tray tower absorbers Venturi Absorbers Ejector Absorbers Biofiltration Bed Absorbers 6
Gas out Gas Absorption Equipment Gas in Packed bed absorbers most common Counter-current flow tower configuration Gas flow enters bottom of tower and flows upward 7
Gas exit Gas Absorption Equipment Another countercurrent flow tower configuration Gas in 8
Cross-Flow Scrubber Gas Flow in Gas Flow out Concentration gradients exist in two directions in the liquid - from top to bottom and front to rear 9
Packed Bed Abs. Applications Suited to applications where high gas removal efficiency is required Exhaust gas is relatively free from particulate matter Control of SO 2 and HCl in sulfuric acid and hydrochloric acid production 10
Packing Elements Lessing ring 11
Packing Elements Packing material provides a large surface area for mass transfer Packing elements made of plastic (polyethylene, polypropylene, polyvinylchloride), ceramic or metal Sizes range from 1 to 4 inches each Design depends on corrosiveness of gas, scrubbing liquid, size of absorber, static pressure drop and cost 12
Packing Elements 13
Spray Tower Absorber Simplest device used for absorption Consists of open vessel and a set of liquid spray nozzles to distribute scrubbing liquid (absorbent) Limited efficiency because of limited contact between gas and spray droplets Used when gases are extremely soluble in absorbent Chemical reaction in liquid could cause clogging 14
Gas Absorption Equipment Co-current Spray Tower Scrubber Spray Tower Full Cone Nozzle 15
Tray Tower Absorber 16
Packed Tower Design Diameter and height of the bed can be estimated for this design Use generalized flooding and pressure drop correlation graph 17
Simplified Design of Packed 1. calculate value of Absorber abcissa = L ρ g G ρ L In the Generalized Sherwood flooding and pressure drop correlation graph 0.5 L = mass flow rate of liquid G = mass flow rate of gas ρ g = gas density ρ L = liquid density 18
( G') ρ 2 G F ρ Φ 0. 1 L L μ g L = mass flow rate of liquid G = mass flow rate of gas G = mass flux of gas per cross sectional area of column F = Packing factor Φ = specific gravity of the scrubbing liquid μ L = liquid viscosity (in cp; 0.8 for water) L G ρg ρ L (dimensionless) 19
Simplified Design of Packed Absorber 2. calculate flooding pressure drop ΔP = flood 0.115F 0.7 p F p = packing factor (dimensionless) 20
Simplified Design of Packed Absorber 3. use graph to find ordinate at the flooding pressure drop, ΔP ( G') ρ 2 G F ρ Φ 0. 1 L L μ g G = mass flux of gas per cross sectional area of column (lb/ft 2 -s) F = Packing factor Φ = specific gravity of the scrubbing liquid μ L = liquid viscosity (lb/ft-s) g = gravitational acceleration ρ g = gas density ρ L = liquid density 4. And find gas flow rate, G 21
Simplified Design of Packed Absorber 5. Calculate actual gas flow rate per unit area as a fraction of the gas flow rate at flooding G operating = G f Where Goperating = actual flow rate per unit area (lb/ft 3 -s) f = coefficient (0.75) 22
Simplified Design of Packed Absorber 6. Calculate packed bed diameter on the actual gas flow rate per unit area in the system Tower Area = Gas Total gas flow rate flow rate per unit area Tower Diameter Tower = 4 π Area Note correction Tower Diameter = 1. 13 Tower Area 23
Problems with high gas flow Channeling: the gas or liquid flow is much greater at some points than at others Loading: the liquid flow is reduced due to the increased gas flow; liquid is held in the void space between packing Flooding: the liquid stops flowing altogether and collects in the top of the column due to very high gas flow 24