Carbon dioxide removal in indirect gasification - SGC

SGC Rapport 2013:277
then saturated with water in a flash tank before being reintroduced in the first flash
tank after the fan. Also the air flow going into the stripper was saturated with water
in a flash tank before entering the stripper.
The impact of electrolytes and their reactions where assumed to be negligible.
Since low to moderate pressures are used in the model, the choice of physical
property method became the NRTL method. The NRTL property method uses the
ideal gas equation of state for the vapor phase and the NRTL activity coefficient
model for the liquid phase. The NRTL method is good for describing strongly nonideal
solutions when dealing with moderate temperatures and pressures. The correlations
used for random packing mass transfer and interfacial area calculations
have been described by Onda et al [53]. The model was calibrated against a set of
data obtained from Malmberg Water. To better adjust the model for the case of
SNG-production, the model has been expanded to include H2, CO, ethylene and
benzene.
5.3.2 Amine absorption model
For the purpose of purifying the gas from the SNG-process from CO2 a simulation
model of the CO2-capture process was used. This model was developed earlier at
the Department of Chemical Engineering for modeling of the CO2-removal from a
biohythane process [54]. The model uses an industry standard amine solution for
the carbon dioxide removal consisting of 50% water, 40% MDEA and 10% PZ.
The exact composition can of course vary between suppliers and other components,
such as e.g. anti-foaming agents, are also present in the solution. The
model calculates the required dimensions of the absorber, stripper and heat exchangers
in order to purify the gas. For an overview of the process layout for the
gas separation process, see Figure 19.
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Absorber
HX 1
Stripper
Figure 19. Process flow diagram for the gas separation process.
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2
HX 2
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Condensor
The amine solution used for the absorption of CO2 consists of a mixture of 40 %
N-methyldiethanolamine (MDEA), 10 % piperazine (PZ) and 50 % water, by
weight. This is a very common solvent mixture used in industrial applications for
the separation of carbon dioxide from various gases, e.g. for biogas upgrading.
Reboiler