Technical -economic and environmental [628919]

U.P.B. Sci. B ull., Series C, Vol. 79, Iss. 1, 2017 ISSN 2286 -3540
EVALUATION OF THE PH YSICAL SOLVENTS USED IN CO 2
POST -COMBUSTION PROC ESSES
Adrian PASCU1, Nela SLAVU2, Adrian BADEA3, Cristian DINCA4
The aim of this article consists in comparing the physical adsorption process
using various physical solvents with the chem ical absorption process which uses the
monoethanolamine (MEA) chemical solvents. We noted that the CO 2 capture
efficiency increased with increasing the L/G ratio respectively with decreasing
temperature of the solvent (in the case of physical solvents) whe n entering the
absorption column. The L/G ratio value obtained for a 90% efficiency varied
depending on the physical solvent used. Thus, when using the methanol (MeOH) the
L/G ratio was of 61.22 mol solvent/mol flue gas (considering the input temperature of the
solvent 20 oC), when using the propylene carbonate (PC) the L/G ratio was of 6.12
mol solvent/mol flue gas (considering the input temperature of the solvent -50 oC) and
when using the N -Methyl -2-pyrrolidone (NMP) the L/G ratio was of 4
mol solvent/mol flue gas (considering the input temperature of the solvent -50 oC).
Keywords : physical absorption process, Aspen Plus, physical absorption process
integration, CO 2 capture
1. Introduction
The main method used today to capture the carbon dioxide from flue gases
resulted from fossil fuel power plants is based on chemical absorption which uses
chemical solvents [1]. The most common chemical solvent in the process of
retaining through chemical absorption is monoethanolamine due to its physico –
chemical properties an d high capacity to absorb carbon dioxide [2 – 6].
The physical absorption process takes place at high pressures and low
temperatures (in the case of methanol the process is conducted at a pressure of 28
bar and a temperature of 30°C). The carbon dioxide se paration from the physical
solvent takes place by expanding it in expanders or turbines when we want to
recover a share of the mechanical energy used by the compressors. The separation

1 PhD eng. , Power Plant Department, University POLITEHNICA of Bucharest, Romania, e -mail:
[anonimizat]
2 PhD eng. , Power Plant Department, University POLITEHNICA of Bucharest, Romania, e -mail:
[anonimizat]
3 Prof., Power Plant Department, University POLITEHNICA of Bucharest, Academy of Romanian
Scientists , Romania, e -mail: [anonimizat]
4 Assoc. Prof., Power Plant Department, University POLITEHNICA of Bucharest, Romania, e –
mail: [anonimizat]

304 Adrian Pascu, Nela Slavu, Adrian Badea, Cristian Dinca
process can take place by using expanders / turbines connected in serie s when the
pressure is high [ 7].
Solubility measures the dissolved gas in a homogeneous liquid in which
the atoms and molecules distribution is uniform. In general, in order to increase
the solubility of a gas in a liquid we have to increase the gas parti al pressure and
decrease temperature of the solvent. The physical solvents analysed in this study (its
physical properties are presented in Table 1) are non -corrosive and non -toxic. Propylene
Carbonate solvent (PC) works at lower temperatures without becomi ng too viscous, in which case
the mass transfer coefficient is better. N -methyl -2-pyrrolidone (NMP) and methanol (MeOH)
solvents show a higher degree of selectivity than the PC to remove the H 2S from gases containing
CO 2 [8]. The physical solvents capacity to absorb gases resulted from fossil fuel combustion
increases as temperature decreases.
Table 1
Properties of physical solvents [ 7, 9, 10 ]
Solvent/
Parameter PC NMP MeOH
Process Name Fluor
Solvent Purisol Rectisol
Viscosity at 25 °C (cP) 3.0 1.65 0.6
Specific Gravity at 25 °C
(kg/m3) 1195 1027 785
Molecular Weight 102 99 32
Vapor Pressure at 25 °C
(mmHg) 0.085 0.40 125
Freezing Point (°C) -48 -24 -92
Boiling Point at 760
mmHg (°C) 240 202 65
Maximum Operating
Temperature (°C) 65 – –
Specific Hea t 25°C 0.339 0.40 0.566
CO 2 solubility at 25 °C
(m3/l) 0.003402 0.003567 0.003178

In 2015, at UN Climate Change Conference (COP21) in Paris, was
approved the framework for action on Climate and Energy policies for period
2020 – 2030 [11]. For the Europe an Union was established a GHG emission
reduction by 20% until 2020 compared whit year 1990 [11]. For the European
Union was established a GHG emission reduction by 30% until 2020 compared
whit year 1990 [11]. The target for Romania is to reduce the GHG em issions by
19% until 2030 [12].
The end of pipe technologies that are used for capturing the CO 2 from flue
gases is mainly based on physical and chemical absorption processes. The
drawbacks of the chemical absorption processes consist of a high thermal ene rgy
consumption for the solvents regeneration and of a high rate of corrosion of

Evaluation of the physical solvents used in CO 2 post-combustion processes 305
metallic surface [13, 14]. The CO 2 capture by physical absorption processes
depends both on the partial pressure of the physical solvent and on the flue gases
temperature. As the partial pressure increases and the temperature decreases, the
CO 2 solubility in the solvent increases [15]. The physical and chemical processes
could be both integrated in the new or existing coal power plants.
The aim of this study was to establish th e physical solvents advantages
compared to the chemical ones knowing that the latter have two major drawbacks:
a) their corrosive nature greatly affect the metal surfaces of the equipment in the
CO 2 capture power plant; and b) in the regeneration of chemic al solvents we need
a significant amount of heat for their regeneration.
2. Description of the physical absorption process in Aspen PLUS
The scheme of the analyzed process was built in the ASPEN Plus work
environment and in which we defined the parameters for all the equipment used
[18]. Table 2 shows the parameters defined for each equipment used in the
physical absorption process shown in Figure 1.
Flue
gases
treatedExpandor
Absorber
unitCO2
pure
Compressor
flue gasesCoolerPhysical
solvent
Flue
gases
CO2 rich
solventCooler Compressor
Separator
ExpandorWater
CO2 lean
solventDehydratorRefrigeration
CO2 rich
solvent CO2 lean
solvent
CO2 lean
solvent CO2 lean
solvent
ECooler
QESeparatorQ
CO2 lean solvent
PumpLegend
E – Electrical energy consumption
Q – Thermal energy consumption

Fig. 1. The CO 2 physical absorption process [ 16]

The flue gases are introdu ced into the absorption unit at the bottom, but
not before being compressed and cooled so as to maintain in the absorption
column a cold temperature. The gases introduced in the absorption unit circulate

306 Adrian Pascu, Nela Slavu, Adrian Badea, Cristian Dinca
in countercurrent with the physical solvent introduc ed at the top of the absorption
column.
For a high value of CO 2 capture efficiency, the process is performed at
high pressure (between 20 bar and 30 bar) [17] and low temperatures (between –
50oC and 20oC) [18]. In this context, we have chosen the values f or each
equipment in the absorption process in order to reduce the energy consumption
and to increase the CO 2 capture efficiency. After the absorption, the CO 2-rich
solvent exits at the bottom of the absorption column and enters an expander
(depending on t he pressure of the solvent we can use more expanders / turbines
connected in series). The expander’s aim is to reduce solvent’s pressure (from 28
bar to about 6 bar) [ 19]. Subsequently, the CO 2 rich solvent is introduced into a
separator where the CO 2 is separated from the physical solvent. The CO 2 flow
with a purity of over 95% is cooled and compressed to 70 bar for shipment to the
storage area. The lean CO 2 physical solvent comes out at the bottom of the
separator and enters a moisture separator where the water will be eliminated from
the solvent. The lean CO 2 physical solvent is transported through a heat exchanger
to be cooled to the operating temperature (the parameter values are presented in
Table 2) and reintroduced into the absorption column.
Table 2
Installation parameters
No.
Crt. Equipment Parameters
1. Compressor flue gases T = 80°C / P in = 1 bar / P out = 28 bar
2. Cooler flue gases Tin = 80°C / T out = 30°C / P = 28 bar
3. Absorber unit Stage no. : 10 / P = 28 bar
4. Expander CO 2 rich
solven t T = -15.4°C / Pin = 28 bar / P out = 6 bar
5. Separator CO 2 rich
solvent T = 40 °C / Pin = 6 bar / P out = 1 bar
6. Dehydrator T = 30 °C / Pin = 1 bar / P out = 1 bar
7. Refrigeration Tin = 18°C / Tout = 0°C / Pin = 1 bar /
Pout = 1 bar
8. Separator flue gases
treated T = 7°C / Pin = 28 bar / P out = 28 bar
9. Expander flue geses
treated T = 26 °C / Pin = 28 bar / P out = 1 bar

Table 3 shows the parameters defined for each stream :

Evaluation of the physical solvents used in CO 2 post-combustion processes 307
a) physical solvents: temperature, pressure, weight concentration, lean /
rich loading solvent ; b) combustion gases: composition, temperature, pressure.
The physical solvents and flue gases parameters were chosen to have a
high CO 2 capture efficiency. Thus, in the absorption unit, the solvent temperature
was established at 40°C a nd the pressure of the CO 2 capture process was
maintained at 1.4 bar. The weight concentration of the physical solvent was
chosen of 100% due to the non – corrosive its properties [20].
Table 3
Stream parameters
No.
Crt. Stream Parameters
1. Physical
solvent T = – 20°C / P = 28 bar
MeOH = 100 wt. % / NMP = 100 wt. % /
PC = 100 wt. %
2. Flue gases T = 12 °C / P = 28 bar
CO 2 = 10.081 %; N 2 = 70.629 %; H 2O =
12.048 %; O 2 = 7.041 %
3. Rich loading
solvent T = – 15.4°C / P = 28 bar
γrich = 0.005 mol CO 2/ mol solvent

The elementary analysis of the flue gases analyzed in this study is
presented in Table 4.
Table 4
Analysis of the lignite used in the combustion process [ 2, 6]
Elementary composition of lignite
Ci, [%] Hi, [%] Si, [%] Oi, [%] Ni, [%] Wi, [%] Ai, [%] LHV*,
[kJ/kg]
24.27 1.4 1.3 1.8 0.86 31 39.37 8935.54
Flue gases composition
Dry condition Wet condition
CO 2, [%] 11.461 10.081
SO 2, [%] 0.23 0.202
N2, [%] 80.304 70.629
O2, [%] 8.005 7.041
H2O,
[%] – 12.048
* LHV – low heating value of lignite

308 Adrian Pascu, Nela Slavu, Adrian Badea, Cristian Dinca
3. Results and discussions

In this study we performed a comparative analysis of the CO 2 capture
efficiency from the flue gases for the (MEA) chemical solvent and, respectively, for
the physical solvents: MeOH, PC, NMP, for different values of t he L/G ratio and for
different temperatures of the solvent .
Table 5 presents the results obtained from the analysis carried out on
chemical solvents. The simulations were made for different chemical solvents in
order to see which solvent has the highest a bsorption capacity. In these
simulations, there were varied the followings parameters: the L/G ratio, the
solvent temperature, and weight concentration, in order to identify their influence
on the CO 2 capture efficiency. So, we determined the values of eac h parameter
considering the carbon dioxide capture efficiency of 90% and the lean loading
solvent of 0.21 mol CO2/mol solvent .
Table 5
The chemical absorption simultion
Nr. Chemical solvent Weight
concentration
(%) L/G ratio
(mol_liquid/
mol_flue_gas) Therm al energy
consumption
(GJ/tCO 2)
1
MEA 20 1.86 2.27
2 30 1.13 2.37
3 40 0.8 2.49
4
DEA 20 2.86 2.37
5 30 2.46 2.12
6 40 1.6 1.78
7 MDEA 20 3.25 1.38
8 30 2.33 0.91
9
TEA 20 6.25 5.72
10 30 4.79 3.61
11 40 3.91 2.25
12
MEA + MD EA 20 – 10 1.18 3.23
13 20 – 20 0.85 2.88
14 20 – 30 0.65 2.65
15
MEA + TEA 20 – 10 1.16 3.22
16 20 – 20 0,93 2.99
17 20 – 30 0.77 3.42
18
DEA + MDEA 20 – 10 2.0 5.37
19 20 – 20 1.67 4.37
20 20 – 30 1.55 3.77

When sizing the CO 2 capture plants by chemical absorption it is preferable
that the ratio L/G to be as low as possile because of the high degree of the amines
corrosion and high thermal energy consumption necessary for the regeneration. In

Evaluation of the physical solvents used in CO 2 post-combustion processes 309
the case of 30 wt. %, MEA, the thermal energy consumption obtained was of 2.37
GJ/tCO 2 for a L/G ratio of 1.13 mol MEA/mol flue gases .

3.1. The influence of the L/G ratio on the CO 2 capture efficiency when
using the monoethanolamine (MEA)

Fig. 2 shows the influence of the L/G ratio on the CO 2 capture efficiency
and respectively on the rich loading solvent at the bottom of the absorption
column. We can see that the higher the L/G ratio, the greater the CO 2 capture
efficiency. Thus, when using the MEA with a weight concentration of 30% we
obtai ned a capture efficiency of 90% for an L/G ratio of 1.2 mol MEA/mole flue gas .
To obtain a higher CO 2 capture efficiency we increased the L/G ratio, but with
increasing the L/G ratio there also increased the energy consumption for the
solvent regeneration. H owever, given that the rich loading solvent decreases with
increasing the L/G ratio, the consumption of energy showed a maximum [5].

Fig. 2. The influence of the L/G ratio on the CO 2 capture efficiency and on the absorption capacity of
the solvent using the MEA of 30%

3.2. The influence of the L/G ratio on the CO 2 capture efficiency when
using the methanol (MeOH)

In Fig. 3 we present the results obtained when using the methanol (MeOH)
physical solvent for capturing CO 2 from the flue gases. In the case of using the
methanol, the simulations were carried out varying both the L/G ratio and the
temperature of the solvent (the temperatures used were of 20°C, -20°C, -37°C, –
50°C). When using the MeOH, we can see that the highest CO 2 capture efficiency
(70%) was obtained for the solvent temperature of -20°C and respectively for an L/G
ratio of 14.29 mol MeOH/mol flue gas . However, we do not recommend using a high flow

310 Adrian Pascu, Nela Slavu, Adrian Badea, Cristian Dinca
rate of the physical solvent which even if it is not corrosive it requires a high energy
pumpin g consumption.

Fig. 3. The influence of the L/G ratio on the CO 2 capture efficiency using the MeOH for different
temperatures

3.3. The influence of the L/G ratio on the CO 2 capture efficiency when
using the Propylene – Carbonate (PC)
When using the PC ( Propylene – Carbonate) solvent ( Fig. 4), we observed
that the CO 2 capture efficiency from the flue gases has increased with the L/G ratio.
The amount of the CO 2 capture efficiency of 90% was obtained for the L/G ratio of 6
mol PC/mol flue gas , and for the te mperature of the solvent of -50°C. From t he
simulations performed at different temperatures and different L/G ratios we observed
that with decreasing the solvent temperature, the CO 2 capture efficiency increased.
Moreover, the physical properties of the PC solvent indicate that its capture
efficiency is better in the case of reducing the temperature from 0°C to – 60°C.

Fig. 4. The influence of the L/G ratio on the CO 2 capture efficiency using the PC

Evaluation of the physical solvents used in CO 2 post-combustion processes 311
3.4. The influence of the L/G ratio on the CO 2 capture e fficiency when
using the N -Methyl -2-pyrrolidone (NMP)
In the case of using the NMP (N -Methyl -2-pyrrolidone) solvent for an L/G
ratio of 4 mol NMP/mol flue gas we had 90% efficiency at a solvent temperature of -50°C.
As with using the PC solvents we observed that the lower the temperature is, the
higher the capture efficiency is.

Fig. 5. The influence of the L/G ratio on the CO 2 capture efficiency using the NMP

The results of simulations using the physical solvents are summarized in
Table 6. We considered that the CO 2 capture efficiency is of 90%; the only
exception was noticed in the case of the MeOH solvent where the CO 2 capture
maximum efficiency from the flue gases was of 70%.
Table 6
Comparative assessment of the physical – chemical solvents used accor ding to the CO 2
capture efficiency
Nr.
Crt. Solvent Concentration
[%] T_solvent
[°C] L/G ratio
[mol solvent/mol flue gas] CO 2 capture
efficiency [%]
1. MEA 30 50 1.2 90
2.
MeOH 100 20 61.22 90
3. 100 -20 91.84 90
4. 100 -37 122.45 90
5. 100 -50 163.27 90
6.
PC 100 20 28.57 90
7. 100 -20 13.27 90
8. 100 -37 8.16 90
9. 100 -50 6.12 90
10.
NMP 100 20 17.35 90
11. 100 -20 8.16 90
12. 100 -37 5.10 90
13. 100 -50 4 90

312 Adrian Pascu, Nela Slavu, Adrian Badea, Cristian Dinca
Fig. 6 presents a comparison between the chemical solvent (MEA 30%) and
the physical solvents (MeOH, PC, NMP), in the case of a CO 2 capture efficiency of
90% (except for the MeOH where the maximum CO 2 capture efficiency was of 70
%) for different L/G ratios. If we maintain a CO 2 capture efficiency of 90%, the L/G
ratio varied w ith the inlet temperature of the solvent in the absorption column. It is
interesting the fact that in the case of the MeOH solvent, decreasing the L/G ratio
was possible by raising the temperature at the entrance into the absorption column
unlike the PC a nd NMP solvents in whose case the reduction of ratio L/G the was
obtained by lowering the temperature.

Fig. 6. Comparative analysis between chemical and physical solvents
4. Conclusions
The simulation of CO 2 post-combustion capture by chemical and phys ical
absorption processes have been developed based in the simulation program
ASPEN Plus.
Following the simulations carried out we observed that with decreasing
temperature of the solvent, there increased the CO 2 absorption capacity of
physical solvents. In the case of the MeOH solvent, the CO 2 capture efficiency of
70% was obtained for an L/G ratio of 14.29 mol MeOH/mol flue gas . For the NMP
solvent, the capture efficiency of 90% was obtained for an L/G ratio of 4
mol NMP/mol flue gas . In the latter case anal ysed (the PC solvent) we saw that the CO 2
capture efficiency of 90% was obtained for an L/G ratio of 6 mol PC/mol flue gas .
From the point of view of the solvent amount used, considering the same
CO 2 capture efficiency, it is preferred to use the NMP solvent for which the L/G
ratio was 4 at the inlet temperature of – 50°C.

Evaluation of the physical solvents used in CO 2 post-combustion processes 313
Acknowledgement

The study has been funded by the UEFISCDI within the National Project
number 38/2012 with the title: “Technical -economic and environmental
optimization of CCS technologi es integration in power plants based on solid fossil
fuel and renewable energy sources (biomass)” – CARBOTECH. The work has
also been funded by the Sectorial Operational Program Human Resources
Development 2007 -2013 of the Ministry of European Funds throug h the Financial
Agreement POSDRU/159/1.5/S/134398 and of the national project P 95/2014 .
R E F E R E N C E S
[1]. http://www.iea.org/topics/climatechange/ , accessed 07 December 2015 ;
[2]. A. Pascu, A . Badea, C. Dincǎ, and L. Stoica, “Simulation of polymeric membrane in Aspen
Plus for CO 2 post-combustion capture”, in Engineering Optimization, Vol. IV, 2015, pp.
303-307;
[3]. M. Norisor, A. Badea, and C. Dincǎ, “Economical and technical analysis of CO 2 transort
ways”, in U.P.B Sci. Bull., Series C, Vol. 74, Iss. 1, 2012, pp. 127 -138;
[4]. A.M. Cormos, C. Dincǎ, and C.C. Cormos, “Multi -fuel multi -product operation of IGCC
powerplants with carbon capture and storage (CCS)”, in Applied Thermal Engineering , Vol.
74, 2015, pp. 20 -27;
[5]. A. Badea, and C. Dincǎ, “CO 2 capture from post -combustion gas by employing MEA
absorption process – experimental investigations for pilot studies”, in U.P.B Sci. Bull.,
Series D, Vol. 74, Iss. 1, 2012, pp. 21 -32;
[6]. C. Di ncǎ, A. Badea, L. Stoica, and A. Pascu, “Absorber design for the improvement of the
efficiency of post -combustion CO 2 capture”, in Journal of the Energy Institute, Vol. 88,
2015, pp. 304 -313;
[7]. R.W. Bucklin, and R.L. Schendel, “Comparison of physical so lvent used for gas processing”, in
Energy Progress, Octomber, 1948;
[8]. R.W. Rousseau, J.N. Matange, and J.K. Ferrell, “Solubilities of carbon dioxide, hydrogen
sulfide, and nitrogen mixtures in methanol”, in AIChE Journal, Vol. 27, No. 4, July, 1981,
pp 605-613;
[9]. S.H. Park, S.J. Lee, J.W. Lee, J.Wook. Lee, S.N. Chun, and J.B. Lee , “The quantitative
evaluation of two -stage pre -combustion CO 2 capture processes using the physical solvents
with various design parameters”, in Energy, Vol. 81, 2015, pp. 47 – 55;
[10]. M. Sharifzadeh, and N. Shah, “Comparative studies of CO 2 capture solvents for gas – fired
power plants: Integrated modelling and pilot plant assessments”, in International Journal of
Greenhouse Gas Control, Vol. 43, 2015, pp. 124 – 132;
[11]. Institute for Climate Economics „Key figures on climate – France and worldwide” COP21 –
CMP11, PARIS 2015, 2016 Edition;
[12]. European Environmental Agency „Greenhouse gase emission trends and projections in
Europe 2011 – Tracking progress towards Kyot o and 2020 targets ” EEA Report, No 4,
2011;
[13]. S. Zhao, P.H.M. Feron, L. Deng, E. Favre, E. Chabanon, S. Yan, J. Hou, V. Chen, and H. Qi,
“Status and progress of membrane contactors in post – combustion carbon capture: Astate –
of-the-art review of new development”, in Journal of Membrane Science, Vol. 511, 2016,
pp. 180 – 206;

314 Adrian Pascu, Nela Slavu, Adrian Badea, Cristian Dinca
[14]. X. Wu, Y. Yu, Z. Qin, Z. Zhang, “The advances of post – combustion CO 2 capture with
chemical solvents: review and quidelines”, in Energy Procedia, Vol. 63, 2014, pp. 1339 –
1346;
[15]. E. Ali, M.K. Hadj -Kali, S. Mulyono, and I. Alnashef, “Analysis of operating conditions for
CO 2 capturing process using deep eutectic solvents”, in International Journal of Greenhouse
Gas Control, Vol. 47, 2016, pp 342 – 350;
[16]. W. Guo, F. F eng, G. Song, J. Xion, L. Shen, “Simulation and energy performance assessment
of CO2 removal from crude synthetic natural gas via physical absorption process”, in
Journal of Natural Gas Chemistry, Vol. 21, 2012, pp. 633 – 638;
[17]. W. Guo, F. Feng, G. Son g, J. Xiao and L. Shen, “Simulation and energy performance
assessment of CO 2 removal from crude synthetic natural gas via physical absorption
process”, in Journal of Natural Gas Chemistry, Vol. 21, 2012, pp. 633 -638;
[18]. Y. J. Heintz, L. Sehabiague, B.I. Morsi, K. L. Jones, and H.W. Pennline, “Novel physical
solvents for selective CO2 capture from fuel gas streams at elevated pressures and
temperatures”, in Energy Fuels, Vol. 22, 2008, pp. 3824 – 3837;
[19]. C. A. Scholes, C. J. Andreson, G. W. Stevens, S . E. Kentish, “Membrane gas separation –
physical solvent absorption combined plant simulation for pre – combustion capture”, in
Energy Procedia, Vol. 37, 2013, pp. 1039 – 1049;
[20]. D. Berstad, R. Anantharaman, P. Neksa, “Low – temperature CCs from an IG CC power
plant and comparison with physical solvents”, in Energy Procedia, Vol. 37, 2013, pp. 2204 –
2211;