Page 286 - Petroleum and Gas Field Processing

increasing the temperature in the absorber. Both of these conditions favor
H 2 S absorption with CO 2 rejection.

Mixed Amines
Mixtures of amines are generally mixtures of MDEA and DEA or MEA
and are used to enhance CO 2 removal by MDEA. Such mixtures are
referred to as MDEA-based amines. The secondary amine generally
comprises less than 20% (mole) of the total amine. At lower MEA and
DEA concentrations, the overall amine strength can be as high as 55% (wt),
without invoking corrosion problems.
Amine mixtures are particularly useful for low-pressure applications
because MDEA becomes less capable of CO 2 pickup sufficient enough to
meet pipeline specifications. At higher pressures, amine mixtures appear to
have little or no advantage over MDEA. Mixed amines are also useful for
cases where the CO 2 content of the feed gas is increasing over time due to
field aging.

Design of Amine Units
The design of amine units are similar for different solvents; however, a
reclaimer should be added in the case of more reactive amines such as MEA
to recover these amines. The main equipment in the amine processes is the
absorber, and emphasis here will be on the design of the absorber. The main
operating conditions such as circulation rate, solvent concentration, acid gas
loading, steam rate for regeneration are given in Table 3.
Absorber Design

For 35 wt% DEA and using 0.5 mol acid gas/mol DEA, the amine
circulation rate q Am is

q Am ðgpmÞ¼ 126 Q g X AG ð10Þ
where Q g is the gas flow rate (MMSCFD) and X AG the mole percent of
acid gas in the feed. Assuming an amine drop size of 100 mm, the diameter
d (in.) of the absorber can be calculated as (Arnod and Stewart)
1=2

2 TZQ g g
d ¼ 504 C d ð11Þ
P l g
where T is the temperature ( R), Z is the compressibility factor, P is the

pressure (psia), and C d is the drag coefficient. The density of gas and
3
liquid are g and l ðlb=ft Þ, respectively. Usually 20 valve-type trays are
used with a spacing of 24 in.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.