Hydrocarbon Processing - October 2021 - 49

Sustainability
librium. Reaction 2 models the absorption reaction of CO2
absorp,
which
is slow and, therefore, considered to be a kinetically limiting
reaction. Reaction 2 is the controlling step for CO2
tion with aqueous K2
CO3
r = k
⎛
⎜⎝
T
T0
⎞
⎟⎠
n
⎡
⎣
.
The kinetic reaction (Eq. 2) is typically expressed by a power
law-based kinetic model, as shown in a technical paper (Eq. 4):2
(4)
exp⎢
⎛
⎝
−E
R
⎞
⎠
⎛
⎜⎝
1
T
Mathias and Gilmartin6
1
−
T0
⎞
⎟⎠
⎤
⎦
⎥∏ ai
i=1
N
Ci
found that the simple Arrhenius
temperature-dependent expression in Eq. 4 is inadequate to represent
the reaction kinetics because it does not correctly model
the equilibrium limit. They proposed a rigorous backward kinetic
model (RBKM), which includes the activity coefficients
of each components (Eq. 5):
⎡
⎣
r = k0 × exp⎢
⎛
⎝
−E
R
⎞
⎠
⎛
⎜⎝
1
T
1
−
T0
⎞
⎟⎠
⎤
⎦
⎥CCO2
COH- ⎜1 −
⎛
⎝
where:
r = Rate of reaction, kmol/m3
k0
.sec
= Rate constant for forward reaction (Eq. 3)
at reference temperature T0
, m3
/kmol.sec
E = Reaction activation energy, kJ/kmol
C = Species concentration, kmol/m3
a = Species activity, which is the product of mole fraction
and activity coefficient
aHCO3-
aCO2aOH-Keq
⎞
⎠
⎟ (5)
Keq = Chemical equilibrium constant
R = Gas constant, 8.314462 kJ/kmol.K.
When a very large residence time is available for Reaction 2
(simulated with a large liquid holdup volume for kinetic modeling),
both kinetic models (Eqs. 4 and 5) should reach the chemical-reaction
equilibrium. Mathias and Gilmartin6
indicated that
only the RBKM reaches the true reaction equilibrium, while the
simple power law expression (Eq. 4) reaches a false equilibrium.
Therefore, the RBKM is used in this study when applying ratebased
column modeling in the proprietary software.a
A major challenge for removing CO2
with aqueous K2
CO3
is that the absorption reaction rate is slow, which renders it uneconomical
for commercial applications. Promoters have been
used to increase the kinetic reaction rate.7-10
In this study, the
effect of promoters is quantified by the bicarbonate reaction factor
(BRF) in rate-based column modeling. The BRF is defined
as the ratio of promoted reaction rate over the unpromoted rate.
To explore the effect of BRF on the overall CO2
absorption in an
absorber, a case study was conducted using a rate-based column
model. The CO2
solvent loads (α = mol-CO2
absorption was simulated with different lean
/mol-solvent) over the range of BRF
= 1-20. The result is illustrated in FIG. 4.
The results indicate that the most economically promoted
reaction rate is to have BRF = 4-6, where the promoted kinetic
reaction rate is close to the equilibrium rate. The additional promoted
reaction rate (BRF > 6), with the additional cost of more
promoters added, does not contribute to significant CO2
abON
DEMAND WEBCAST
Umesh Mathur
Principal Process Engineer
Aggreko
Mike Rhodes
Managing Editor
Hydrocarbon Processing
Overcoming Seasonal Cooling Problems in
Distillation Column Overheads
Some of the most ubiquitous and, in our opinion, most serious problems confronting
operators are the results of seasonal or diurnal variations in ambient air and cooling
water temperatures. Generally, distillation column overhead cooling systems are designed
for some sort of average summertime cooling water or air temperature. These are
routinely exceeded, according to seasonal weather data, for extended periods during the
peak summer months. Also, over time, column throughputs have been increased as units
were debottlenecked or when other process changes were made. On occasion, purity
specifi cations may change, inexorably becoming more stringent.
We will show examples of how it is feasible to restore column overheads cooling
capacities to winter-like conditions by installing temporary, or seasonal, supplemental
cooling solutions. Also, we discuss some of these techniques that have been proven
successful in a large number of past applications in the petroleum refi ning, petrochemical,
and complex chemicals industries.
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Hydrocarbon Processing - October 2021

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