Absorption & Stripping Design •  Common assumptions:

–  Dilute and immiscible gas-liquid phases – L and V streams are constant from stage to stage

–  Concentrated or miscible – L and V streams can increase or decrease from stage to stage

–  How will this affect the number of theoretical stages? –  Are the analytical equations still valid?

•  What is the goal of absorption or stripping operations? –  Maximum extraction with 100% efficiency (i.e. actual # of stages =

theoretical)

–  High throughput (low residence time)

–  Smallest and simplest possible system (low capital)

•  How do we accomplish this? –  Choose the right absorbing (liquid) or stripping (gas) phase

(equilibrium and immiscibility)

–  Optimize column diameter and height

–  Maximize contact area/volume within column WITHOUT flooding the column or making it unreasonably “slow”

1. Tray designs

Perforated

Valve cap

Bubble cap

2. Random packing materials (p. 659 in text gives properties)

2. Pressure drop across random packing (Fig. 10.6-5)

2. Structured packing (pressure drop correlation in Fig. 10.6-6)

•  Flooding pressure drop given by:

•  How to calculate limiting flow rates and column diameter 1. Select a suitable packing material with Fp (random or structured) 2. Select a suitable GL/GG ratio and a total gas flowrate 3. Calculate ΔPflood

4. Calculate capacity (i.e. flow) parameter at ΔPflood from correlation (Fig. 10.6-5 or 10.6-6)

5. Calculate GG from capacity parameter (this is at flooding) 6. Using suitable % of GG at flooding for actual GG (typically 50-70%,

but varies). Determie GL. 7. Calculate cross sectional area of tower from GG and total gas

flowrate

Putting these pieces together…

Diagram of plate absorption towers

L0, x0 V1, y1

LN, xN VN+1, yN+1

L0, x0 V1, y1

LN, xN VN+1, yN+1

Diagram of packed absorption towers

V2

y2

L2

x2

V1

y1

L1

x1

z dz

y x