The Carbon Capture model aims to:

• Develop a correlation between feed gas flowrate, feed gas composition, CO2 capture efficiency, length and diameter of the absorption and stripping columns, number of columns, amount of make-up solvent and power consumption.
• Estimate the cost of a carbon capture process using amines.

CO2 can be captured either from flue gas or another process stream. Before the carbon capture process, CO2 – containing stream is cleaned to eliminate particles such as dust or tar or other types of particles. The clean gas and the amine solution are then injected in the absorption column in counter current. The absorber is filled with packings to optimize the gas/liquid contact.

The pure gas is recovered at the top of the column while the amines rich with CO2 are recovered at the bottom of the column and then preheated to 100°C in a cross-heat exchanger before injection into the stripper. The heat exchange occurs thanks to the hot stream of regenerated amine leaving the stripper. After the stripping step, the CO2 is dried and compressed to meet the required pressure for its transportation and storage.

Process Chemistry

Carbon capture with amine is indeed a chemical absorption process with a complex chemistry behind. The amine reacts with the CO2 to produce an intermediate compound called carbamate. The formed compound dissociates to give back CO2 and the amine solvent by the application of heat. However, the carbamate needs to be stable, and it is granted by using unhindered amines such as MEA amine. A representation of the main reactions involved in the process are shown below.

CO2 Absorption

MEA Regeneration

Sizing of the columns :

The main equipment to be sized in a carbon capture process with amine are the absorption and stripping columns.

ABSORBER

Sizing of the absorber columns is defined by the diameter and length of the vessel. To define the calculations, a gas inlet temperature of 55 ◦C and 1.2 bar as pressure are settled. Before the sizing, some material balances and compound properties calculations are done.
The molecular weight of the gas mixture (MWgas) is determined by applying the equation below.

Where:
y_CO2in is the CO2 composition at the inlet gas (mol/mol)
y_H2O is the H2O composition at the inlet gas (mol/mol)
y_O2 is the O2 composition at the inlet gas (mol/mol)
y_N2 is the N2 composition at the inlet gas (mol/mol)
MW_CO2 is the CO2 molecular weight (44 kg/kmol)
MW_H2O is the H2O molecular weight (18 kg/kmol)
MW_O2 is the O2 molecular weight (32 kg/kmol)
MW_N2 is the N2 molecular weight (28 kg/kmol)

The gas flow rate in kmol is defined as:

Where:
P₀ is the pressure at normal conditions (1 bar)
Q_gas is the total gas flowrate at the inlet of the absorber (Nm3/h)
R is the ideal gas constant (0.08314 m3 bar / K kmol)
T₀ is the temperature at normal conditions (273.15 K)

The gas flow rate in kg is defined as:

The L/G ratio (mol/mol) in the absorption column is estimated as:

Where:
α is the CO2 loading in lean amine, established as 0.2 mol CO2 / mol MEA
n_CO2 is the CO2 capture efficiency
C is the amine concentration in % w/w (compositions between 15 – 40 % w/w)
T_gas in is the inlet gas temperature in K
L is the liquid flowrate in kmol/h

The liquid flowrate in kg (mliq) is calculated as:

With:

Where:
X_MEA is the MEA composition in the lean amine (mol/mol)
X_H2O is the H2O composition in the lean amine (mol/mol)
X_CO2 is the CO2 composition in the lean amine (mol/mol)
MW_MEA is the MEA molecular weight (61 kg/kmol)
MW_{lean amine} is the average molecular weight for lean amine

The viscosity of the liquid in Pa∙s at 40 ◦C is calculated as:

The gas mixture density (gas) is determined using an approximation to an ideal gas behaviour due to the low operating pressure.

Where:
P_gas in is the inlet gas pressure in bar

The CO2 captured in kmol / h is equal to:

CO2 efficiency is the percentage of CO2 recovered.

The outlet gas from the absorber (G') is defined as:

Absorber diameter

To estimate the absorber diameter, the following correlations is applied:

Where:
v_G is the velocity of the gas in m/s
Q_G is the volumetric flowrate of the gas in m3/h

To estimate the velocity of the gas (vG), the subsequent calculations were considered:

The flow parameter (X) is defined as:

Where:
m_gas is equal to the total gas (mol flow) per MW_gas
m_liq is the liquid flowrate in kg/h
ρ_liq is the density of the liquid (1000 kg/m3)

The pressure drop parameter and the coefficient under flooding conditions are estimated by two correlations.

The pressure drop:

The coefficient under flooding conditions

Where:
Y_flood is the pressure drop parameter under flooding conditions
Cs_flood is the coefficient at flooding conditions
F_P is the pressure factor of the packing. For metallic rasching rings - 1-inch packing, Fp was considered equal to 85 ft-1
μ_L is the viscosity of the liquid in Pa∙s

Finally, the gas velocity at the flooding conditions and the gas velocity are calculated as follow:

and

Where:
vG_flood is the gas velocity at the flooding conditions
f_flood is the flooding factor, defined as 70 %

Absorber length

The height of the packing in the column can be estimated as follows:

Where:
A_c is the cross-section area of the column in m2
Z is the packing height of the column in m

and

K_Ga_v is the volumetric mass transfer coefficient in kmol/ (m3 kPa h)

Where:
P_CO2 is the CO2 partial pressure in KPa, defined as:

α_e is the CO2 loading in lean amine at equilibrium conditions, it is 0.5 mol CO2 / mol MEA
M is the lean MEA concentration in kmol / m3
L_spec is the specific flow rate in m3 / m2 h, defined as : Where QL is the liquid flowrate in m3 / h

The total height of the absorption column (Habs) is found by adding 2 m to the packing height.

STRIPPER

For the stripper, the column works at 1.5 bar and a temperature at the top and bottom of 100 and 120 ◦C respectively.
The total flowrate at the feed of the stripper is estimated by a material balance in the absorber.

The distillate containing the CO2 recovered (Dist) is calculated by a material balance in the stripper.

Where:
y_CO2A is the CO2 composition at the feed of the stripper
y_CO2D is the desired distillate CO2 composition in the stripper
y_CO2R is the CO2 composition in the residue of the stripper (CO2 composition of the lean amine)

Stripper diameter

The diameter of the stripper is defined by the tray with the greatest vapor load.
The maximum vapor load (VL) is estimated by considering the wet CO2 vapor at the top temperature of the stripper.

With

And

And

Where:
V_L is the vapor load in m3 / h
Q_v is the vapor flowrate in m3 / h
y_H2O-CO2 is the H2O composition in CO2-rich phase at 100 ◦ C
ρ_G-reg is the gas density in the stripper
MW_wet CO2 is the average molecular weight of the wet CO2
P_str is the operating pressure of the stripper
T_str-top is the temperature at the top of the stripper

Approximation of the internal diameter Di at the level of the design tray

With

And

And

Where:
Q_L str is the liquid flowrate in the stripper
Residue is the residue flowrate in the stripper in kmol /h
T_s is the spacing between trays. Since internal diameters are established between 1.2 and 3 m, spacing of 0.6 is fixed.

Stripper height

For cost estimation purposes, the height of the stripping column was considered similar to the absorber height (5 % smaller). Note that the number of absorption (Nabs) and stripping columns (Nstr) are defined by the calculated diameter of each process in meters divided by 3 m.

Utilities consumption
Regeneration heat
The regeneration of the amine consumes a considerable amount of energy. It is obtained by the following correlation:

Where:
Q is expressed in GJ/h

MEA make-up
During the CO2 process, there are some MEA losses due to acid gases, oxidation, HSS formation or with gas exhausted. It is estimated a make-up of 1.5 kg of MEA / ton CO2 captured.

Electrical Power
Electrical Power associated to the pumps needed in the process are estimated as follow:

Where:
Q_solvent is the solvent flowrate in gal/min
∆P_solvent is 30 psi

Accuracy Level

An accuracy level is added to reflect the accuracy and precision of the results and cost equations.

An accuracy level of “Fair” (Accuracy Index of 3/3) is to reflect that for this technology and sizing, the available data is sufficient to consider the costing used.

An accuracy level of “Projected” (Accuracy Index of 2/3) is given to reflect that the costing computed might rely on insufficient sources for this technology and sizing. The lack of data may reflect either that the technology is still under development and thus the results are projections of reachable costs in the future, or the sizing computed doesn't correspond to a configuration where this technology is usually deployed.

An accuracy level of “Improvable” (Accuracy Index of 1/3) is given to reflect that the costing computing might rely on an insufficient number of sources for this technology and sizing. To enhance the costing for this configuration, your feedback is welcome.