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Simulation of CO2 removal from pressurized natural gas stream contains high CO2 concentration by absorption process using membrane contactors

Hamza A. Ahmed, Harith N. Mohammed ORCID logo, Omar S. Lateef and Ghassan H. Abdullah


Despite the importance of natural gas (NG) as an energy source, there is a lot of pressurized landfill gas not exploited so far because it contains high CO2 concentration. Therefore, this study aimed to develop a 2-D mathematical model to simulate CO2 removal from NG stream contains high CO2 concentration up to 70% at high-pressure up to 60 bar using three different dimensions of polyvinylidene fluoride (PVDF) hollow fiber membrane contactors. Aqueous solutions of activated methyldiethanolamine (MDEA) with piperazine (PZ) were adopted. The performance of considered absorbent at high-pressure was evaluated at the non-wetting mode condition of membrane contactor. Moreover, the effect of pressure, contact area, gas flow rate, MDEA concentration into the amine mixture, PZ concentration, temperature and membrane properties were theoretically investigated. The findings stated that activated MDEA had different performance in terms of membrane wetting compared with other amines, which used at high pressure in previous studies. In addition, the simulation results showed that CO2 removal efficiency was significantly enhanced, when the operating pressure, contact area, PZ concentration and temperature were increased. However, increasing gas flow rate leads to reduce CO2 removal efficiency. Furthermore, the CO2 absorption was significantly improved by adding a small amount of PZ to MDEA. The predicted model results showed a good agreement with experimental data obtained from the literature.

Corresponding author: Harith N. Mohammed, Department of Chemical Engineering, Tikrit University, Saladin, Iraq, E-mail:


A.1 Reaction rate constant

The reaction of CO2 with respect to MDEA and PZ is second order reaction. The reaction rate constants for MDEA (k2.MDEA, m3 Kmol−1 s−1) and PZ (k2.PZ, m3, Kmol−1 s−1) can be estimated from the following [32]:


A.2 Liquid phase density

The density of the liquid phase (ρsol) in g cm−3 may calculate according to the following [33]:


where Mi and xi are the molar mass and mole fraction, respectively, of pure component i in the amine aqueous solution. Vm is the molar volume of the liquid mixtures (cm3 mol−1) which correlated as the following expression




where Ai are pair parameters and are assumed to be temperature dependent


where a, b, c and c are the constant which its values were listed in tables presented by Paul and Mandal [33].

A.3 Liquid phase viscosity

The viscosity of the mixture of PZ, MDEA and water (µsol) in mPa s−1 was correlated Paul and Mandal [33] as follows:


Gij are temperature-dependent and written as


Where a, b and c are constant can be obtained from the study performed by Paul and Mandal [33].

A.4 CO2 diffusion coefficient in amine

The diffusion coefficient of CO2 in the amine solution (DCO2,m2s1) can be estimated from the following expression [34]:

(A.9)DCO2, sol=DN2O,sol(DCO2,wDN2O,w)

where DCO2,w and DN2O,w (m2 s−1) are the diffusion coefficients of CO2 and N2O in the water which expressed as follows [34]:

(A.10)DCO2,w=(2.35106) exp(2119T)

The diffusion coefficients of N2O (m2 s−1) in the aqueous amine solution are determined according to the modified Stokes−Einstein equation as:

(A.12)DN2O,solμsol0.6=DN2O,w μw0.6

where µw (mPa s−1) is the viscosity of water which are estimated from the following relation [35]:


A.5 Diffusion coefficients of amine

The modified correlation of Stokes−Einstein relation was adopted to estimate the diffusion coefficient of activated MDEA (Dsol, m2 s−1) in the shell side of HFMC which written as [36]:

(A.14)Damine,L=Damine,wT273 (μwμsol)0.6

where Damine,w (m2 s−1) is the diffusion coefficients of mixed amines in water which can be calculated from the following correlation [37]


where M and ρ are the molar mass and density of the amine, respectively.

A.6 Solubility of CO2 in amine liquid.

The solubility of CO2 in mixed amines solution can be estimated from the following relation [34]:


where HCO2amine, HN2Oamine are the physical solubility of CO2 and N2O in amine solution (kPa m3 kmol−1), respectively. HCO2water, HN2Owater are the physical solubility of CO2 and N2O in water (kPa m3 kmol−1), respectively, which can be calculated as follows [34]:


The solubility of N2O in binary and ternary mixtures was correlated by Arunkumar et al. [38] as function of temperature and amine concentration according the following relation:


where the parameters, Ki’s, are expressed as


where ai, bi, ci, and α are the correlation parameters and w1and w2 are the mass percent of individual amine. The distribution coefficient of CO2 in amine solution mCO2 can be determined as follows [30]:


where R is the ideal gas constant, 8.314 kJ/(kmol K).

A.7 Compressibility factor (Z)

The value of Z can be calculated from Pitzer correlation as [39],


where Zo and Z1 are functions of both reduced temperature (Tr) and reduced pressure (Pr) and ω is the Pitzer acentric factor. The reduced temperature is calculated from the following


where T and Tc are the absolute temperature and critical temperature of the gas respectively. The reduced pressure is determined as


where P and Pc are the absolute pressure and critical pressure of the gas, respectively.

  1. Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: None declared.

  3. Employment or leadership: None declared.

  4. Honorarium: None declared.

  5. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.


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Received: 2019-12-15
Accepted: 2020-03-05
Published Online: 2020-06-01

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