Page 179 - Gas Purification 5E

166 Gas Purijkation

cal recommended rich solution loading values for MEA, DEA, and DGA are given in Table
2-32. These rich amine loading recommendations should be used with caution as the mini-
mum amine circulation requirement is usually set by the approach to equilibrium at the bot-
tom of the contactor. The calculation methods outlined by Honerkamp (1975) and Holmes et
al. (1984) can be used to set the approach to equilibrium. Setting the approach to eqilibriw
at the bottom of the contactor at 60 to 70% usually provides a conservative design margin.
Table 2-32 also lists the amine concentrations recommended by several investigators.

Table 2-32
Recommended Solution Conditions for LPG Treating with Alkanolamines
I Reference MEA DEA DGA I

Amine Concentration wt% A 5-20 25-35 50-70
B 48 - -
C 10-18 20-30 40
Rich Solution, Moles Acid Gas/
Mole Amine A 0.3-0.4 0.35-0.65 -
B c0.4
D COS (CO?)
References:
A. Holmes et al. (1984); B. Bacon (1972): C. Dupart and Marchant (1989); D. Honerkamp (1975)

Once the lean and rich amine solution loadings have been selected, the required solution
flow rate can be calculated by a simple material balance around the contactor. The required
number of theoretical stages can be estimated using graphical or analytical techniques
described in many standard texts (e.g., Treybal, 1963; Perry. 1963).
If a graphical method is used, the material balance establishes the operating line, and the
required number of theoretical stages can be stepped off between the operating line and the
equilibrium curve. If too many theoretical stages are indicated (i.e., more than about 3), a
lower acid gas loading in the rich solution should be assumed and the design procedure
repeated.
In addition to the restrictions on amine concentration and rich solution loading, the volu-
metric flow rate ratio of the continuous phase (amine) to the dispersed phase (LPG) should
be more than about 1:30 to ensure good mass transfer. Conservative design would call for a
volumetric ratio of amine to LPG of 1:15 or greater.
Absorber Designs

Random Packed, Countercurrent Columns

Packing Selection and Bed Height The use of packing increases mass transfer in the LPG
contactor by increasing the residence time of the LPG droplets, reducing the possibility of
back-mixing, and renewing the droplet surface film (Tse and Santos, 1993). Either metal or
ceramic packings may be used. If amine is the continuous phase and if metal packing is used,