**Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids**

Tirzhá L. P. Dantas1, Alírio E. Rodrigues2 and Regina F. P. M. Moreira3 *1Federal University of Paraná, Department of Chemical Engineering 2University of Porto, Faculty of Engineering 3Federal University of de Santa Catarina, Department of Chemical and Food Engineering 1,3Brazil 2Portugal* 

#### **1. Introduction**

56 Greenhouse Gases – Capturing, Utilization and Reduction

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The generation of CO2 is inherent in the combustion of fossil fuels, and the efficient capture of CO2 from industrial operations is regarded as an important strategy through which to achieve a significant reduction in atmospheric CO2 levels. There are three basic CO2 capture routes: (1) pre-combustion capture (via oxygen-blown gasification); (2) oxy-fuel combustion, i.e. removing nitrogen before combustion; and (3) post-combustion capture.

Adopting the post-combustion capture route avoids the potentially long time periods required to develop cost-effective coal-derived syngas separation technologies, hydrogen turbine technology, and fuel-cell technology, etc. It can also provide a means of CO2 capture in the near-term for new and existing stationary fossil fuel-fired power plants.

Concentrations of CO2 in power station flue gases range from around 4% by volume for natural gas combined cycle (NGCC) plants to 14% for pulverized fuel-fired plants. In the carbon capture and storage chain (capture, transport and storage) different requirements have been set for the composition of the gas stream mainly containing CO2, which can vary within the range of 95-97% CO2 with less than 4% N2.

There are several post-combustion gas separation and capture technologies currently being investigated, namely: (a) absorption, (b) cryogenic separation, (c) membrane separation, (d) micro-algal bio-fixation, and (e) adsorption.

Current absorption technologies which propose the capture of CO2 from flue gas are costly and energy intensive. Membrane technology is an attractive CO2 capture option because of advantages such as energy-efficient passive operation, no use of hazardous chemicals, and tolerance to acid gases and oxygen. However, an important challenge associated with membrane technology is how to create the driving force efficiently, because the feed flue gas is at ambient pressure and contains a relatively low CO2 content.

Solid sorbents are another promising capture technology. These sorbents can either react with the CO2 or it can be adsorbed onto the surface. Chemical sorbents that react with the

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 59

Thermogravimetric experiments were carried out with a TGA-50 thermogravimetric analyzer (Shimadzu, Japan) in the temperature range of 30**°C** –900**°C**, at a heating rate of

Fourier transform infrared (FTIR) spectroscopy was used to qualitatively identify the chemical functionality of activated carbon. To obtain the observable adsorption spectra, the solids were grounded to an average diameter of ca. 0.5 mm. The transmission spectra of the samples were recorded using KBr pellets containing 0.1% of carbon. The pellets were 12.7mm in diameter and ca. 1mm thick and were prepared in a manual hydraulic press set at 10 ton. The spectra were measured from 4000 to 400 cm\_1 and recorded on a 16PC FTIR

X-ray photoelectron spectroscopy (XPS) measurements were carried out with a VG Microtech ESCA3000 MULTILAB spectrometer using monochromatic Al Ka X-rays. The pass energy of the analyzer was 58.7 eV for high-resolution scans. Relative elemental concentrations on the surface of the sorbents were calculated by measuring peak areas in the high-resolution spectra and then converting to atomic concentrations using sensitivity

The equilibrium of CO2 and N2 adsorption on activated carbon was measured at different temperatures of 30**°C**, 50**°C**, 100**°C**, and 150**°C** using the static method in a Rubotherm

The equilibrium of CO2 adsorption on CPHCL was measured at different temperatures of 30**°C**, 50**°C**, 100**°C**, and 150**°C** by the volumetric method, in an automatic sorptometer,

Before the adsorption measurements, the solid samples were pre-treated for 12 hours at 150**°**C under vacuum. This temperature ensures that the amine is homogeneously tethered

**2.4 Breakthrough curves: Fixed-bed CO2 adsorption and CO2/N2 mixture adsorption**  All the experimental breakthrough curves were obtained by passing the appropriate gas mixture through the packed column with the adsorbent: activated carbon or CPHCL. The solid adsorbent was pre-treated by passing helium at a flow rate of 30 mL.min-1 n and at 150**°C** for 2 hours. These breakthrough curves were obtained 30**°C**, 50**°C**, 100**°C**, and 150**°C**. The dynamic of adsorption of CO2 in a fixed bed was studied using CO2 diluted in helium (CO2/He = 20%/80% v/v) in order to obtain the breakthrough curves. For the fixed-bed CO2/N2 separation dynamics, the breakthrough curves were obtained by passing the

The total gas flow rate was maintained at 30 mL.min-1 which was controlled by a mass flow unit (Matheson, USA). The column was located inside a furnace with controlled temperature. A gas chromatographic model CG35 (CG Instrumentos Científicos, Brazil) equipped with a Porapak-N packed column (Cromacon, Brazil) and with a thermal

magnetic suspension microbalance (Bochum, Germany) up to approximately 5 bar.

10**°C** /min under nitrogen flow.

spectrometer (Perkin Elmer, USA).

factors provided by the instrument manufacturer.

Autosorb 1C (Quantachome, USA), up to approximately 1 bar.

to the solid surface without devolatilize or decompose it.

standard gas mixture – 20% CO2/ 80%N2 v/v.

**2.3 Adsorption equilibrium isotherms** 

CO2 in the flue gas can be comprised of a support, usually of high surface area, with an immobilized amine or other reactant on the surface. Physical adsorbents can separate the CO2 from the other flue gas constituents, but do not react with it. Instead, they use their cage-like structure to act as molecular sieves. These sorbents can be regenerated using a pressure swing or a temperature swing, although the costs associated with a pressure swing may be prohibitively high. Physisorbents such as activated carbon and zeolites will be safe for the local environment, and are generally relatively inexpensive to manufacture. Conventionally, activated carbon materials have been widely applied in industry for gas separation, and also have been investigated for CO2 capture. Carbon dioxide emissions are frequently associated with large amounts of nitrogen gas, and thus an adsorbent selective to one of these compounds is required. These adsorbents should also be selective even at high temperatures, i.e., temperatures typical of carbon dioxide emission sources. Activated carbon is a suitable adsorbent and its CO2 adsorption characteristics are dependent on its surface area and chemical surface characteristics. The surface chemistry of activated carbon is determined by the amount and type of heteroatom, for example oxygen, nitrogen, etc. Therefore, the adsorption capacity of activated carbon for carbon dioxide is a function of its pore structure and the properties of the surface chemistry.

Strategies like PSA (pressure swing adsorption), TSA (temperature swing adsorption) and ESA (electric swing adsorption) processes have been proposed and investigated for adsorption in a cyclic process (Cavenati *et al*., 2006; Grande & Rodrigues, 2008; Zhang et al., 2008). PSA is a cyclical process of adsorption/desorption that occurs through pressure changes and can be very suitable for carbon dioxide separation from exhaust gases due to its easy application in a large temperature range. The most studies presents the CO2/N2 separation using PSA process at room temperature, but it has been reported that is possible to obtain high purity CO2 (~90%) at high temperature (Ko *et al*., 2005). Recently, Grande and Rodrigues (2008) reported that it is possible to recover around 89% of the CO2 from a CO2/N2 mixture using honeycomb monoliths of activated carbon through ESA. However, the temperature of the CO2/N2 mixture in a typical exhaust gas can exceed 100oC and at such temperatures the recovery and purity of CO2 can be significantly modified.

#### **2. Experimental section**

#### **2.1 Selection and preparation of adsorbents**

The commercial activated carbon used was Norit R2030 (Norit, Netherlands) which was selected due to its high adsorption capacity for CO2. The nitrogen-enriched activated carbon, denoted as CPHCL, was prepared in a way similar way to that as previously reported (Gray et al., 2004), mixing 10 g of activated carbon with 500mL of 10-1M 3-chloropropylamine hydrochloride solution. The mixture was kept under constant stirring, at ambient temperature for 5 hours. The CPHCL adsorbent was then left to dry for 12 hours in an oven at 105**°**C.

#### **2.2 Characterization of the adsorbents**

The content of carbon, hydrogen and nitrogen was determined by elemental analysis using CHNS EA1100 equipment (CE Instruments, Italy).

Thermogravimetric experiments were carried out with a TGA-50 thermogravimetric analyzer (Shimadzu, Japan) in the temperature range of 30**°C** –900**°C**, at a heating rate of 10**°C** /min under nitrogen flow.

Fourier transform infrared (FTIR) spectroscopy was used to qualitatively identify the chemical functionality of activated carbon. To obtain the observable adsorption spectra, the solids were grounded to an average diameter of ca. 0.5 mm. The transmission spectra of the samples were recorded using KBr pellets containing 0.1% of carbon. The pellets were 12.7mm in diameter and ca. 1mm thick and were prepared in a manual hydraulic press set at 10 ton. The spectra were measured from 4000 to 400 cm\_1 and recorded on a 16PC FTIR spectrometer (Perkin Elmer, USA).

X-ray photoelectron spectroscopy (XPS) measurements were carried out with a VG Microtech ESCA3000 MULTILAB spectrometer using monochromatic Al Ka X-rays. The pass energy of the analyzer was 58.7 eV for high-resolution scans. Relative elemental concentrations on the surface of the sorbents were calculated by measuring peak areas in the

high-resolution spectra and then converting to atomic concentrations using sensitivity factors provided by the instrument manufacturer.

#### **2.3 Adsorption equilibrium isotherms**

58 Greenhouse Gases – Capturing, Utilization and Reduction

CO2 in the flue gas can be comprised of a support, usually of high surface area, with an immobilized amine or other reactant on the surface. Physical adsorbents can separate the CO2 from the other flue gas constituents, but do not react with it. Instead, they use their cage-like structure to act as molecular sieves. These sorbents can be regenerated using a pressure swing or a temperature swing, although the costs associated with a pressure swing may be prohibitively high. Physisorbents such as activated carbon and zeolites will be safe for the local environment, and are generally relatively inexpensive to manufacture. Conventionally, activated carbon materials have been widely applied in industry for gas separation, and also have been investigated for CO2 capture. Carbon dioxide emissions are frequently associated with large amounts of nitrogen gas, and thus an adsorbent selective to one of these compounds is required. These adsorbents should also be selective even at high temperatures, i.e., temperatures typical of carbon dioxide emission sources. Activated carbon is a suitable adsorbent and its CO2 adsorption characteristics are dependent on its surface area and chemical surface characteristics. The surface chemistry of activated carbon is determined by the amount and type of heteroatom, for example oxygen, nitrogen, etc. Therefore, the adsorption capacity of activated carbon for carbon dioxide is a function of its

Strategies like PSA (pressure swing adsorption), TSA (temperature swing adsorption) and ESA (electric swing adsorption) processes have been proposed and investigated for adsorption in a cyclic process (Cavenati *et al*., 2006; Grande & Rodrigues, 2008; Zhang et al., 2008). PSA is a cyclical process of adsorption/desorption that occurs through pressure changes and can be very suitable for carbon dioxide separation from exhaust gases due to its easy application in a large temperature range. The most studies presents the CO2/N2 separation using PSA process at room temperature, but it has been reported that is possible to obtain high purity CO2 (~90%) at high temperature (Ko *et al*., 2005). Recently, Grande and Rodrigues (2008) reported that it is possible to recover around 89% of the CO2 from a CO2/N2 mixture using honeycomb monoliths of activated carbon through ESA. However, the temperature of the CO2/N2 mixture in a typical exhaust gas can exceed 100oC and at

such temperatures the recovery and purity of CO2 can be significantly modified.

The commercial activated carbon used was Norit R2030 (Norit, Netherlands) which was selected due to its high adsorption capacity for CO2. The nitrogen-enriched activated carbon, denoted as CPHCL, was prepared in a way similar way to that as previously reported (Gray et al., 2004), mixing 10 g of activated carbon with 500mL of 10-1M 3-chloropropylamine hydrochloride solution. The mixture was kept under constant stirring, at ambient temperature for 5 hours. The CPHCL adsorbent was then left to dry for 12 hours in an oven

The content of carbon, hydrogen and nitrogen was determined by elemental analysis using

pore structure and the properties of the surface chemistry.

**2. Experimental section** 

at 105**°**C.

**2.1 Selection and preparation of adsorbents** 

**2.2 Characterization of the adsorbents** 

CHNS EA1100 equipment (CE Instruments, Italy).

The equilibrium of CO2 and N2 adsorption on activated carbon was measured at different temperatures of 30**°C**, 50**°C**, 100**°C**, and 150**°C** using the static method in a Rubotherm magnetic suspension microbalance (Bochum, Germany) up to approximately 5 bar.

The equilibrium of CO2 adsorption on CPHCL was measured at different temperatures of 30**°C**, 50**°C**, 100**°C**, and 150**°C** by the volumetric method, in an automatic sorptometer, Autosorb 1C (Quantachome, USA), up to approximately 1 bar.

Before the adsorption measurements, the solid samples were pre-treated for 12 hours at 150**°**C under vacuum. This temperature ensures that the amine is homogeneously tethered to the solid surface without devolatilize or decompose it.

#### **2.4 Breakthrough curves: Fixed-bed CO2 adsorption and CO2/N2 mixture adsorption**

All the experimental breakthrough curves were obtained by passing the appropriate gas mixture through the packed column with the adsorbent: activated carbon or CPHCL. The solid adsorbent was pre-treated by passing helium at a flow rate of 30 mL.min-1 n and at 150**°C** for 2 hours. These breakthrough curves were obtained 30**°C**, 50**°C**, 100**°C**, and 150**°C**.

The dynamic of adsorption of CO2 in a fixed bed was studied using CO2 diluted in helium (CO2/He = 20%/80% v/v) in order to obtain the breakthrough curves. For the fixed-bed CO2/N2 separation dynamics, the breakthrough curves were obtained by passing the standard gas mixture – 20% CO2/ 80%N2 v/v.

The total gas flow rate was maintained at 30 mL.min-1 which was controlled by a mass flow unit (Matheson, USA). The column was located inside a furnace with controlled temperature. A gas chromatographic model CG35 (CG Instrumentos Científicos, Brazil) equipped with a Porapak-N packed column (Cromacon, Brazil) and with a thermal

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 61

countercurrent purge with pure nitrogen at constant pressure and a flow rate of 0.5 L.min-1. All experiments were performed with 20 seconds of pressurization and 70 seconds of depressurization. However, different feed and purge times were used. Table 2 summarizes

> **Run T, °C Feed Time, s Purge time, s** 50 100 70 50 120 50 100 120 50 100 200 50

The definition of the performance criteria provides a common basis for comparing the

2

2

Purity of CO

Purity of N

Recovery of CO

where *Fi* is the molar flow rate of component i – carbon dioxide or nitrogen.

1

*purge*

*blowdown purge comp*

*i t*

*t*

*t n t*

1

*i t*

2

The model used to describe the fixed-bed experiments is derived from the mass, energy and momentum balances. The flow pattern is described with the axially dispersed plug flow model and the mass transfer rate is represented by a Linear Driving Force model – LDF. It was assumed that the gas phase behaves as an ideal gas and the radial concentration and

*blowdown*

*purge*

*t*

*t t*

*blowdown purge*

0

*blowdown*

2

*F dt*

*i*

*F dt*

2

*F dt*

*CO*

2

*F dt*

*CO*

*N*

*purge*

*t*

*t n t*

*blowdown purge comp*

2

*F dt*

*i*

*F dt*

(1)

(2)

(3)

*CO*

the PSA experimental conditions used in this study.

Table 2. PSA experimental conditions.

**3. Mathematical modelling** 

**3.1 Model description** 

**2.5.1 Performance criteria of the PSA process** 

different experiments. These are; Eq (1) to (3):

conductivity detector (TCD) was used to monitor the carbon dioxide or nitrogen concentration at the bed exit, using helium as the reference gas. The experimental system – column and furnace – was considered adiabatic because it was isolated with a layer of 0.10m of fiber glass and with a refractory material. The characteristics of the fixed bed and the column are presented in Table 1


Table 1. Characteristics of the fixed bed and the column used in the experiments [a Dantas *et al*., 2010; b Dantas *et al*., 2011].

#### **2.5 Pressure swing adsorption**

The PSA experimental setup consisted of one fixed-bed adsorption that simulated the operation of a unit with several fixed-beds, for which more details are given elsewhere (Da Silva & Rodrigues, 2001). The solid adsorbent used was the commercial activated carbon which was pre-treated by passing helium at a flow rate of 1.0 L.min-1 and at 150**°C** for 12 hours. The PSA experiments were performed premixed CO2 to N2 forming a mixture - 0.15 v/v. The flow rate of each gas was controlled by mass controllers (Teledyne Brown Engineering, USA).

A gas chromatographic model CP9001 (Chrompack 9001, Netherlands) equipped with a Poraplot Q capillary column (Varian, Netherlands), with a thermal conductivity detector (TCD), and with a flame ionization detector (FID) was used to monitor the carbon dioxide or nitrogen concentration at the bed exit, using helium as the reference gas. The experimental system – column and furnace – was considered adiabatic because it was isolated with a layer of 0.10m of fiber glass and with a refractory material. The temperature inside the column was continuously monitored using a K-thermocouple placed at 0.17 m and 0.43 m from the bottom of the column. The column was located inside a convective furnace and thus the system was considered to be non-adiabatic. The characteristics of the fixed bed and the column are presented in Table 1.

The cycles were of the Sharstrom-cycle type and divided by pressurization with pure nitrogen at a flow rate of 3.0 L.min-1, feeding at constant pressure of 1.3 bar and total flow rate of 3.0 L.min-1, countercurrent blowdown decreasing the pressure to 0.1bar and countercurrent purge with pure nitrogen at constant pressure and a flow rate of 0.5 L.min-1. All experiments were performed with 20 seconds of pressurization and 70 seconds of depressurization. However, different feed and purge times were used. Table 2 summarizes the PSA experimental conditions used in this study.


Table 2. PSA experimental conditions.

60 Greenhouse Gases – Capturing, Utilization and Reduction

conductivity detector (TCD) was used to monitor the carbon dioxide or nitrogen concentration at the bed exit, using helium as the reference gas. The experimental system – column and furnace – was considered adiabatic because it was isolated with a layer of 0.10m of fiber glass and with a refractory material. The characteristics of the fixed bed and the

**Bed length,** *L* 0.171 m 0.83 m **Bed diameter,** int *d* 0.022 m 0.021 m **Bed weight,** *W* 0.0352 kg 0.158 kg

**Column wall thickness,** *l* 0.0015m 0.0041m **Column wall specific heat,** *Cp*,*<sup>w</sup>* 440 J kg-1K-1 500 J kg-1K-1

**Column wall conductivity,** *wk* 1.4 W m-1K-1

Table 1. Characteristics of the fixed bed and the column used in the experiments [a Dantas *et* 

The PSA experimental setup consisted of one fixed-bed adsorption that simulated the operation of a unit with several fixed-beds, for which more details are given elsewhere (Da Silva & Rodrigues, 2001). The solid adsorbent used was the commercial activated carbon which was pre-treated by passing helium at a flow rate of 1.0 L.min-1 and at 150**°C** for 12 hours. The PSA experiments were performed premixed CO2 to N2 forming a mixture - 0.15 v/v. The flow rate of each gas was controlled by mass controllers (Teledyne Brown

A gas chromatographic model CP9001 (Chrompack 9001, Netherlands) equipped with a Poraplot Q capillary column (Varian, Netherlands), with a thermal conductivity detector (TCD), and with a flame ionization detector (FID) was used to monitor the carbon dioxide or nitrogen concentration at the bed exit, using helium as the reference gas. The experimental system – column and furnace – was considered adiabatic because it was isolated with a layer of 0.10m of fiber glass and with a refractory material. The temperature inside the column was continuously monitored using a K-thermocouple placed at 0.17 m and 0.43 m from the bottom of the column. The column was located inside a convective furnace and thus the system was considered to be non-adiabatic. The characteristics of the fixed bed and the

The cycles were of the Sharstrom-cycle type and divided by pressurization with pure nitrogen at a flow rate of 3.0 L.min-1, feeding at constant pressure of 1.3 bar and total flow rate of 3.0 L.min-1, countercurrent blowdown decreasing the pressure to 0.1bar and

CO2 and CO2/N2 adsorption in a fixed bed a

0.52 0.52

*<sup>w</sup>* 7280 kg m-3 8238 kg m-3

PSA experiments b

column are presented in Table 1

**Bed voidage fraction,**

**Wall density,** 

*al*., 2010; b Dantas *et al*., 2011].

Engineering, USA).

**2.5 Pressure swing adsorption** 

column are presented in Table 1.

#### **2.5.1 Performance criteria of the PSA process**

The definition of the performance criteria provides a common basis for comparing the different experiments. These are; Eq (1) to (3):

$$\begin{aligned} \stackrel{t\_{\text{pury}}}{\int\limits\_{t\_{\text{Munder}}}} F\_{\text{CO}\_2} dt\\ \text{Purity of CO}\_2 = \frac{t\_{\text{Munder}}}{n\_{\text{comp}}} \frac{t\_{\text{pray}}}{t\_{\text{pray}}} \end{aligned} \tag{1}$$

$$\begin{aligned} \stackrel{t\_{\text{pray}}}{\int\limits\_{t\_{\text{Munder}}}} F\_{\text{N}\_2} dt\\ \text{Purity of N}\_2 = \frac{t\_{\text{Munder}}}{n\_{\text{Munder}}} \end{aligned} \tag{2}$$

$$\begin{aligned} \text{Purity of N}\_2 &= \frac{t\_{blaudown}}{\sum\_{i=1}^{t\_{blaudown}} \int\_{t\_{blaudown}} F\_i dt} \\ &\quad \sum\_{i=1}^{t\_{blaudmon}} \int\_{t\_{blaudman}} F\_i dt} \end{aligned} \tag{2}$$

$$\begin{aligned} \stackrel{t\_{\text{pwy}}}{\int} F\_{\text{CO}\_2} dt\\ \text{Recovery of CO}\_2 = \frac{t\_{\text{blow}w}}{t\_{\text{pwy}}}\\ \stackrel{t}{\int} F\_{\text{CO}\_2} dt \end{aligned} \tag{3}$$

where *Fi* is the molar flow rate of component i – carbon dioxide or nitrogen.

#### **3. Mathematical modelling**

#### **3.1 Model description**

The model used to describe the fixed-bed experiments is derived from the mass, energy and momentum balances. The flow pattern is described with the axially dispersed plug flow model and the mass transfer rate is represented by a Linear Driving Force model – LDF. It was assumed that the gas phase behaves as an ideal gas and the radial concentration and

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 63

*p s gs p i p i <sup>T</sup> <sup>h</sup> <sup>q</sup> C T T H t d t*

where *<sup>f</sup> h* is the coefficient for film heat transfer between the gas and the adsorbent.

*w*

For the column wall, the energy balance can be expressed by Eq. (10) to (12):

*t* 

*wl*

of the internal surface area to the volume of the column wall,

 <sup>6</sup> ( ) *<sup>f</sup> <sup>s</sup> <sup>i</sup>*

, ()( ) *<sup>w</sup> w pw w w g w wl w <sup>T</sup> <sup>C</sup> h T T UT T*

> int int

> > int

ln

*d d l d l*

logarithmic mean surface area of the column shell to the volume of the column (Cavenati *et al.*, 2006), U is the external overall heat transfer coefficient, and *T* is the furnace external air temperature. For an adiabatic system, the last term of this equation must not been

The mathematical model was solved using the commercial software gPROMS (Process System Enterprise Limited, UK) which uses the method of orthogonal collocation on finite

> .() *<sup>i</sup> L ii z z <sup>z</sup> <sup>C</sup> D uC C*

 

elements for resolution. The boundary and initial conditions were the show bellow.

The boundary conditions are described by the equations given below (Eq.14-18).

*z*

int

*d*

int

*T T TT <sup>w</sup> <sup>g</sup> s i* ; *P P* 0 and ( ,0) ( ,0) 0 *Cz qz i i* (13)

(14)

*d ld l*

int

*<sup>w</sup>* is the column wall density, *Cp*,*<sup>w</sup>* is the column wall specific heat,

 

 

(9)

 (10)

(11)

(12)

*wl* is the ratio of the

*<sup>w</sup>* is the ratio

The solid phase energy balance is expressed by Eq. (9):

with

and

where

considered.

**3.2 Boundary and initial conditions** 

**3.2.1 For the breakthrough curves** 

1. Bed inlet: (z=0)

The initial conditions for the adiabatic system are:

temperature gradients are negligible. The fixed-bed model is described by the equations given below.

The mass balance for each component is given by Eq. (4); (Ruthven, 1984):

$$
\varepsilon \frac{\partial \mathbb{C}\_i}{\partial t} + \frac{\partial (\mu \mathbb{C}\_i)}{\partial z} = \varepsilon D\_L \frac{\partial^2 \mathbb{C}\_i}{\partial z^2} - \begin{pmatrix} 1 - \varepsilon & \rangle \rho\_p \frac{\partial \overline{q}\_i}{\partial t} \end{pmatrix} \tag{4}$$

where ε is the bed void fraction, *Ci* is the gas phase concentration of component *i*, *<sup>i</sup> q* is the average amount of component i adsorbed, *DL* is the axial mass dispersion coefficient, *u* is the superficial velocity, and *<sup>p</sup>* is the particle density.

The rate of mass transfer to the particle for each component is given by Eq. (5):

$$\frac{\partial \overline{q\_i}}{\partial t} = \mathcal{K}\_{L,i} (q\_i^\* - \overline{q\_i}^\*) \tag{5}$$

where *KL* is the overall mass transfer coefficient of component i and \* *<sup>i</sup> q* is the amount adsorbed at equilibrium, i.e., \* (,) *i i <sup>g</sup> q fC T* given by the adsorption isotherm, and *<sup>i</sup> q* is the average amount adsorbed.

The concentration Ci is given by Eq (6):

$$C\_i = \frac{y\_i P}{RT\_\%} \tag{6}$$

where yi is the molar fraction of each gas in the gas phase, P is the total pressure, *Tg* is the gas temperature and R is the universal gas constant.

The Ergun equation considers the terms for the pressure drop and velocity changes; Eq. (7):

$$-\frac{\partial P}{\partial z} = 150 \frac{\mu\_{\text{g}} \left(1 - \varepsilon\right)^{2}}{\varepsilon^{3} d\_{p}^{\,^{2}}} u + \ 1.75 \frac{\left(1 - \varepsilon\right)}{\varepsilon^{3} d\_{p}} \rho\_{\text{g}} u^{2} \tag{7}$$

where *<sup>g</sup>* is the gas viscosity, *<sup>g</sup>* is the gas density, and *<sup>p</sup> d* is the particle diameter. The energy balance is; Eq. (8):

$$\begin{split} \varepsilon \mathbf{C} \mathbf{C}\_{\upsilon,g} \frac{\partial \mathbf{T}\_{g}}{\partial t} + \mathbf{C} \mathbf{C}\_{p,g} \frac{\partial (\mu \mathbf{T}\_{g})}{\partial z} &= \varepsilon \mathbf{A}\_{\mathsf{L}} \frac{\partial^{2} \mathbf{T}\_{g}}{\partial z^{2}} - \\ - (1 - \varepsilon) \rho\_{p} \mathbf{C}\_{s} \frac{\partial \mathbf{T}\_{s}}{\partial t} + (1 - \varepsilon) \rho\_{p} \sum\_{i} (-\Delta \mathbf{H}\_{i}) \frac{\partial \overline{q\_{i}}}{\partial t} - \frac{4h\_{w}}{d\_{\mathrm{int}}} (\mathbf{T}\_{g} - \mathbf{T}\_{w}) \end{split} \tag{8}$$

where *Cv*,*g* is the molar specific heat at constant volume for the gas phase, *Cp g*, is the molar specific heat at constant pressure for the gas phase, *<sup>L</sup>* is the axial heat dispersion coefficient, *Cs* is the solid specific heat, *Hi* is the heat of adsorption for component i at zero coverage, *wh* is coefficient for the internal convective heat transfer between the gas and the column wall, int *d* is the bed diameter, and *Tw* is the wall temperature.

The solid phase energy balance is expressed by Eq. (9):

$$\rho\_p \mathbf{C}\_s \frac{\partial \mathbf{T}\_s}{\partial t} = \frac{6h\_f}{d\_p} (\mathbf{T}\_g - \mathbf{T}\_s) + \rho\_p \sum\_i (-\Delta H\_i) \frac{\partial q\_i}{\partial t} \tag{9}$$

where *<sup>f</sup> h* is the coefficient for film heat transfer between the gas and the adsorbent. For the column wall, the energy balance can be expressed by Eq. (10) to (12):

$$
\rho\_w \alpha\_{p,w} \frac{\partial T\_w}{\partial t} = \alpha\_w h\_w (T\_g - T\_w) - \alpha\_{wl} \text{LI}(T\_w - T\_w) \tag{10}
$$

with

62 Greenhouse Gases – Capturing, Utilization and Reduction

temperature gradients are negligible. The fixed-bed model is described by the equations

2 2 ( ) (1 ) *ii i <sup>i</sup>*

*C uC C <sup>q</sup> <sup>D</sup> t z z t*

where ε is the bed void fraction, *Ci* is the gas phase concentration of component *i*, *<sup>i</sup> q* is the average amount of component i adsorbed, *DL* is the axial mass dispersion coefficient, *u* is

> \* , ( ) *<sup>i</sup> Li i i <sup>q</sup> K q q t*

adsorbed at equilibrium, i.e., \* (,) *i i <sup>g</sup> q fC T* given by the adsorption isotherm, and *<sup>i</sup> q* is the

*i*

*<sup>y</sup> <sup>P</sup> <sup>C</sup>*

where yi is the molar fraction of each gas in the gas phase, P is the total pressure, *Tg* is the

The Ergun equation considers the terms for the pressure drop and velocity changes; Eq. (7):

3 2 3 (1 ) (1 ) <sup>150</sup> 1.75 *<sup>g</sup>*

*p p*

<sup>4</sup> (1 ) (1 ) ( )

where *Cv*,*g* is the molar specific heat at constant volume for the gas phase, *Cp g*, is the molar

coefficient, *Cs* is the solid specific heat, *Hi* is the heat of adsorption for component i at zero coverage, *wh* is coefficient for the internal convective heat transfer between the gas and

 

*s w i p s p i g w i*

*T h <sup>q</sup> C H T T t t d*

2

*u u* 

*<sup>g</sup>* is the gas density, and *<sup>p</sup> d* is the particle diameter.

  2

*z d d*

 

, , 2

*v g p g L*

the column wall, int *d* is the bed diameter, and *Tw* is the wall temperature.

( )

*T uT T*

*t z z*

*g gg*

 

> 

*g*

*i*

 

 *<sup>p</sup>* is the particle density. The rate of mass transfer to the particle for each component is given by Eq. (5):

where *KL* is the overall mass transfer coefficient of component i and \*

*L p*

 

(4)

(5)

*RT* (6)

2

*g*

int

(7)

*<sup>i</sup> q* is the amount

(8)

*<sup>L</sup>* is the axial heat dispersion

The mass balance for each component is given by Eq. (4); (Ruthven, 1984):

given below.

the superficial velocity, and

average amount adsorbed.

where 

The concentration Ci is given by Eq (6):

*<sup>g</sup>* is the gas viscosity,

The energy balance is; Eq. (8):

gas temperature and R is the universal gas constant.

*P*

*CC CC*

specific heat at constant pressure for the gas phase,

  
$$\alpha\_w = \frac{d\_{\rm int}}{l \left(d\_{\rm int} + l\right)}\tag{11}$$

and

$$\alpha\_{wl} = \frac{d\_{\rm int}}{\left(d\_{\rm int} + l\right) \ln\left(\frac{d\_{\rm int} + l}{d\_{\rm int}}\right)}\tag{12}$$

where *<sup>w</sup>* is the column wall density, *Cp*,*<sup>w</sup>* is the column wall specific heat, *<sup>w</sup>* is the ratio of the internal surface area to the volume of the column wall, *wl* is the ratio of the logarithmic mean surface area of the column shell to the volume of the column (Cavenati *et al.*, 2006), U is the external overall heat transfer coefficient, and *T* is the furnace external air temperature. For an adiabatic system, the last term of this equation must not been considered.

#### **3.2 Boundary and initial conditions**

The mathematical model was solved using the commercial software gPROMS (Process System Enterprise Limited, UK) which uses the method of orthogonal collocation on finite elements for resolution. The boundary and initial conditions were the show bellow.

#### **3.2.1 For the breakthrough curves**

The initial conditions for the adiabatic system are:

$$T\_w = T\_g = T\_s = T\_i \; \text{: } P = P\_0 \quad \text{and} \; C\_i(z, 0) = \overline{q}\_i(z, 0) = 0 \tag{13}$$

The boundary conditions are described by the equations given below (Eq.14-18).

1. Bed inlet: (z=0)

$$
\left.\varepsilon D\_L \frac{\partial \mathbf{C}\_i}{\partial z}\right|\_{z^+} = -\mu (\mathbf{C}\_i \big|\_{z^-} - \mathbf{C}\_i \big|\_{z^+}) \tag{14}
$$

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 65

2. Bed outlet for pressurization step (z=L), and bed inlet for countercurrent blowdown

4. Bed outlet: countercurrent blowdown step (z=0), and countercurrent purge step (z=0).

**3.3 LDF global mass transfer coefficient and correlations used to estimation of model** 

Table 3 summarizes the experimental conditions of temperatures and Reynolds numbers in

Table 4 shows the LDF global mass transfer coefficient, the axial dispersion coefficient, and the film mass transfer coefficient, should be evaluated using different correlations due

For the all experiments performed at adiabatic system – low Reynols number, the value of the LDF global mass transfer coefficient was estimated using the expression proposed by Farooq and Ruthven (1990) which considers all of the resistances to the mass transfer, i.e.,

> <sup>2</sup> <sup>2</sup> 1 3 15 15 *po p o c L f o p eo c rq r q r K k C DC D*

where *pr* is the particle radius, *kf* the external mass transfer coefficient, *qo* the value of q at equilibrium with *Co* (adsorbate concentration in the feed at feed temperature *To* and expressed in suitable units), P the particle porosity, *rc* the radius of activated carbon crystal and *DC* is the micropore diffusivity. The micropore diffusivity values were those reported by Cavenati and coworkers (2006) since the micropore distribution of the adsorbents are similar

For the all experiments performed at non-adiabatic system – high Reynols number, the value of the LDF global mass transfer coefficient was estimated that intraparticke resistance

All others correlations used to evaluate the mass and heat transport parameters are summarized in Table 4. The gas phase viscosity was estimated using Wilkes's equation (Bird *et al*., 2007). The axial mass dispersion coefficient (*DL*), for the adiabatic system, was evaluated by Leitão & Rodrigues (1995); for the non-adiabatic system, according to Wakao and coworkers (1978). The film mass transfer coefficient (*kf*), for the adiabatic system, was evaluated by Seguin et al. (1995); for the non-adiabatic system, according to Wakao and Funazkri (1978). The axial

to those of carbon molecular sieves (Cavenati *et al.,* 2006; Vinu & Hartmann, 2005).

different Reynolds numbers in Runs 1 -4 under adiabatic or non-adiabatic systems.

3. Bed inlet: feed step (z=0), and countercurrent purge step (z=L).

0 *<sup>z</sup> u*  (25)

*z z u u* (26)

*purge <sup>z</sup> P P* (27)

(28)

*L*) and the film heat transfer coefficient (*hf*) were evaluated by

step (z=L).

**parameters** 

Runs 1 - 4.

intra- and extraparticle resistances; Eq. (28):

is only controlled by molecular diffusion.

heat dispersion coefficient (

$$\left. \varepsilon \mathcal{A}\_{\rm L} \frac{\partial T\_{\rm g}}{\partial z} \right|\_{z^+} = -\mu \text{CC}\_{p,\rm g} \left( T\_{\rm g} \Big|\_{z^-} - T\_{\rm g} \Big|\_{z^+} \right) \tag{15}$$

$$\left. \mu \mathbf{C} \right|\_{z^-} = \mu \mathbf{C} \Big|\_{z^+} \tag{16}$$

2. Bed outlet: (z=L)

$$\left.\frac{\partial \mathbf{C}\_i}{\partial z}\right|\_{z^-} = \mathbf{0} \tag{17}$$

$$
\left.\frac{\partial T\_{\text{g}}}{\partial z}\right|\_{z^-} = 0\tag{18}
$$

#### **3.2.2 For PSA experiments**

The initial conditions, only considered for the unused bed, are:

$$T\_w = T\_g = T\_s = T\_0 \; \text{;} \; P = P\_0 \; \text{and} \; C\_i(z, 0) = \overline{q}\_i(z, 0) = 0 \tag{19}$$

The initial condition of each new cycle corresponds to the final condition of the previous cycle. The boundary conditions for the mass and energy balances are described by the equations given below (Eq.20-27).

1. Bed inlet: pressurization step (z=0), feed step (z=0) and countercurrent purge step (z=L).

$$\left. \varepsilon D\_{\rm L} \frac{\partial \mathcal{C}\_{i}}{\partial \boldsymbol{z}} \right|\_{\boldsymbol{z}^{+}} = -\mu (\mathcal{C}\_{i} \big|\_{\boldsymbol{z}^{-}} - \mathcal{C}\_{i} \big|\_{\boldsymbol{z}^{+}}) \tag{20}$$

$$
\left.\varepsilon\omega\mathbf{j}\_{\rm L}\frac{\partial T\_{\rm g}}{\partial z}\right|\_{z^{+}} = -\mu\mathbf{C}\_{p\star\mathcal{g}}(T\_{\rm g}\Big|\_{z^{-}} - T\_{\rm g}\Big|\_{z^{+}})\tag{21}
$$

2. Bed outlet: pressurization step (z=L), feed step (z=L), countercurrent purge step (z=0), and bed inlet and outlet for countercurrent blowdown step (z=L and z=0, respectively).

$$\left.\frac{\partial \mathbf{C}\_i}{\partial z}\right|\_{z^-} = 0\tag{22}$$

$$
\left.\frac{\partial T\_{\mathcal{S}}}{\partial z}\right|\_{z^{-}} = 0\tag{23}
$$

The boundary conditions for the momentum balance are the following:

1. Bed inlet: pressurization step (z=0).

$$P\_{z^{+}} = P\_{feed} \tag{24}$$

*T*

The initial conditions, only considered for the unused bed, are:

2. Bed outlet: (z=L)

**3.2.2 For PSA experiments** 

equations given below (Eq.20-27).

1. Bed inlet: pressurization step (z=0).

(z=L).

*z*

, . () *<sup>g</sup> <sup>L</sup> pg g g z z <sup>z</sup>*

> 0 *<sup>i</sup> z C z*

> 0 *<sup>g</sup> z T z*

The initial condition of each new cycle corresponds to the final condition of the previous cycle. The boundary conditions for the mass and energy balances are described by the

1. Bed inlet: pressurization step (z=0), feed step (z=0) and countercurrent purge step

 

*z*

*T*

*z*

The boundary conditions for the momentum balance are the following:

( ) *<sup>i</sup> L ii z z <sup>z</sup> <sup>C</sup> D uC C*

, . () *<sup>g</sup> <sup>L</sup> pg g g z z <sup>z</sup>*

2. Bed outlet: pressurization step (z=L), feed step (z=L), countercurrent purge step (z=0), and bed inlet and outlet for countercurrent blowdown step (z=L and z=0, respectively).

> 0 *<sup>i</sup> z C z*

0 *<sup>g</sup> z T z*

*uC T T*

*uCC T T*

(15)

*z z uC uC* (16)

(17)

(18)

(20)

(21)

(22)

(23)

*P P <sup>z</sup> feed* (24)

*T T TT wgs* <sup>0</sup> ; *P P* 0 and ( ,0) ( ,0) 0 *Cz qz i i* (19)

2. Bed outlet for pressurization step (z=L), and bed inlet for countercurrent blowdown step (z=L).

$$\left.u\right|\_{z^{-}} = 0\tag{25}$$

3. Bed inlet: feed step (z=0), and countercurrent purge step (z=L).

$$\left.u\right|\_{z^-} = \left.u\right|\_{z^+} \tag{26}$$

4. Bed outlet: countercurrent blowdown step (z=0), and countercurrent purge step (z=0).

$$P\big|\_{x^{-}} = P\_{pure} \tag{27}$$

#### **3.3 LDF global mass transfer coefficient and correlations used to estimation of model parameters**

Table 3 summarizes the experimental conditions of temperatures and Reynolds numbers in Runs 1 - 4.

Table 4 shows the LDF global mass transfer coefficient, the axial dispersion coefficient, and the film mass transfer coefficient, should be evaluated using different correlations due different Reynolds numbers in Runs 1 -4 under adiabatic or non-adiabatic systems.

For the all experiments performed at adiabatic system – low Reynols number, the value of the LDF global mass transfer coefficient was estimated using the expression proposed by Farooq and Ruthven (1990) which considers all of the resistances to the mass transfer, i.e., intra- and extraparticle resistances; Eq. (28):

$$\frac{1}{K\_L} = \frac{r\_p q\_o}{3k\_f C\_o} + \frac{r\_p^2 q\_o}{15\varepsilon\_p D\_e C\_o} + \frac{r\_c^2}{15D\_c} \tag{28}$$

where *pr* is the particle radius, *kf* the external mass transfer coefficient, *qo* the value of q at equilibrium with *Co* (adsorbate concentration in the feed at feed temperature *To* and expressed in suitable units), P the particle porosity, *rc* the radius of activated carbon crystal and *DC* is the micropore diffusivity. The micropore diffusivity values were those reported by Cavenati and coworkers (2006) since the micropore distribution of the adsorbents are similar to those of carbon molecular sieves (Cavenati *et al.,* 2006; Vinu & Hartmann, 2005).

For the all experiments performed at non-adiabatic system – high Reynols number, the value of the LDF global mass transfer coefficient was estimated that intraparticke resistance is only controlled by molecular diffusion.

All others correlations used to evaluate the mass and heat transport parameters are summarized in Table 4. The gas phase viscosity was estimated using Wilkes's equation (Bird *et al*., 2007). The axial mass dispersion coefficient (*DL*), for the adiabatic system, was evaluated by Leitão & Rodrigues (1995); for the non-adiabatic system, according to Wakao and coworkers (1978). The film mass transfer coefficient (*kf*), for the adiabatic system, was evaluated by Seguin et al. (1995); for the non-adiabatic system, according to Wakao and Funazkri (1978). The axial heat dispersion coefficient (*L*) and the film heat transfer coefficient (*hf*) were evaluated by

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 67

The effective diffusivities were calculated by Bosanquet equation and the molecular diffusivities were calculated with the Chapman-Enskong equation (Bird *et al*., 2007). A tortuosity of 2.2 and 1.8 was admitted to the activated carbon particle and CPHCL,

Textural properties of activated carbon and CPHCL were previously described (Dantas *et al*., 2010). The adsorbents are microporous and the BET surface areas are shown in Table 5. Modifications with nitrogen-containing species may also result in changes in the porous structure (Arenilas *et al*., 2005). The CPHCL had a lower BET area when compared to the commercial activated carbon. The micropores volume of the CPHCL decreases considerably compared with the commercial activated carbon, suggesting that the nitrogen incorporation

The chemical characteristics of the adsorbents are given in Table 6. As expected, the adsorbent CPHCL has the greater nitrogen content and an N/C atomic ratio which is twice

The FTIR spectra of commercial activated carbon and CPHCL are shown in Figure 1. All spectra show the contribution from ambient water (at about 3600 cm–1) and carbon dioxide (doublet at 2360 cm–1 and sharp spike at 667 cm–1) present in the optical bench. The band of O-H stretching vibrations (3600 - 3100 cm−1) was due to surface hydroxyl groups and chemisorbed water. The band at 2844 and 2925 cm−1 is frequently ascribed to the C–H stretching. The asymmetry of the band at 3600 – 3100 cm-1 indicates the presence of strong

**Commercial activated** 

0.47 0.37

**SBET, m2/g** 1053**.**0 664.6 **Smicro, m2/g** 1343.0 753.0 **Vmicro, cm3/g** 0.0972 0.0388

**Mean pore radius** *or* **, nm** 1.23 1.54

**N2 Micropore Capacity, kg/kg** 300 155

**Particle diameter** *<sup>p</sup> d* **103, m** 3.8

Table 5. Textural properties of the adsorbents studied [Dantas *et al*., 2010].

**N2 Total Capacity, kg/kg** 370 260

*<sup>p</sup>* **.103, kg/m3** 1.14

**carbon CPHCL** 

respectively.

hydrogen bonds.

**4. Results and discussion** 

**4.1 Characterization of adsorbents** 

partially blocks the access of N2 to the small pores.

that of the commercial activated carbon.

**Particle porosity** *P*

**Particle density** 


Wakao and coworkers (1978); the convective heat transfer coefficient between the gas phase and the column wall (*hw*) was evaluated according to De Wash & Froment (1972).

Table 3. Experimental conditions of temperature and Reynolds number.


Table 4. Correlations used for estimation of mass and heat parameters [Dantas *et al*., 2011].

The effective diffusivities were calculated by Bosanquet equation and the molecular diffusivities were calculated with the Chapman-Enskong equation (Bird *et al*., 2007). A tortuosity of 2.2 and 1.8 was admitted to the activated carbon particle and CPHCL, respectively.

#### **4. Results and discussion**

66 Greenhouse Gases – Capturing, Utilization and Reduction

Wakao and coworkers (1978); the convective heat transfer coefficient between the gas phase

Run T, °C Re

1 28 0.12 2 50 0.10 3 100 0.08 4 150 0.06

1 28 0.36 2 50 0.32 3 100 0.25 4 150 0.20

1 50 41.64 2 50 41.64 3 100 32.31 4 100 32.31

*p*

 

*h L Ra*

0.67 0.68

*C k* ; *f p*

int

0.492 <sup>1</sup> Pr

*g*

1/4 4/9 9/12

*Nu*

*g h d*

*<sup>k</sup>* ;

1 1 1 ln *ext w w ext ext*

*ddd h k d dh*

*g k* 

*g h d k*

*<sup>D</sup>* ; , Pr *pg g*

*<sup>d</sup>* 20 0.5 Re *<sup>L</sup>*

*m*

*D* 

*<sup>D</sup> Sc*

**Coefficient Adiabatic system Non-adiabatic system** 

*<sup>L</sup> Pe*

**Film mass transfer** 0.27 1/3 *Sh* 1.09Re *Sc* 0.27 1/3 *Sh* 2 1.1Re *Sc*

*U*

*ext ext*

> *m k d*

*k*

and the column wall (*hw*) was evaluated according to De Wash & Froment (1972).

Adiabatic

Adiabatic

Nonadiabatic

**External convective heat transfer** 

*D* ; Re *g p*

*g*

; *<sup>g</sup>*

*Sc*

 *ud* 

*L uL Pe* System

System

System

CO2 /He

CO2 /N2

PSA

Table 3. Experimental conditions of temperature and Reynolds number.

**Axial Heat Dispersion** 10 0.5PrRe *<sup>L</sup>*

**Internal convective heat transfer** int 12.5 0.048Re *<sup>w</sup>*

**Film heat transfer** 0.6 1/3 *Nu* 2.0 1.1Re Pr

**Global heat transfer** int int

*g m*

*D* 

; *f p*

 *T T <sup>w</sup>* <sup>3</sup> *Ra g L* 

Table 4. Correlations used for estimation of mass and heat parameters [Dantas *et al*., 2011].

*Sh*

**Axial Mass Dispersion** 0.020 0.508Re

#### **4.1 Characterization of adsorbents**

Textural properties of activated carbon and CPHCL were previously described (Dantas *et al*., 2010). The adsorbents are microporous and the BET surface areas are shown in Table 5.

Modifications with nitrogen-containing species may also result in changes in the porous structure (Arenilas *et al*., 2005). The CPHCL had a lower BET area when compared to the commercial activated carbon. The micropores volume of the CPHCL decreases considerably compared with the commercial activated carbon, suggesting that the nitrogen incorporation partially blocks the access of N2 to the small pores.

The chemical characteristics of the adsorbents are given in Table 6. As expected, the adsorbent CPHCL has the greater nitrogen content and an N/C atomic ratio which is twice that of the commercial activated carbon.

The FTIR spectra of commercial activated carbon and CPHCL are shown in Figure 1. All spectra show the contribution from ambient water (at about 3600 cm–1) and carbon dioxide (doublet at 2360 cm–1 and sharp spike at 667 cm–1) present in the optical bench. The band of O-H stretching vibrations (3600 - 3100 cm−1) was due to surface hydroxyl groups and chemisorbed water. The band at 2844 and 2925 cm−1 is frequently ascribed to the C–H stretching. The asymmetry of the band at 3600 – 3100 cm-1 indicates the presence of strong hydrogen bonds.


Table 5. Textural properties of the adsorbents studied [Dantas *et al*., 2010].

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 69

*qK P*

*KP*

1( ) *m eq*

where *qm* is the maximum adsorbed concentration, i.e., the monolayer capacity; *Keq* is the

The temperature dependence of the equilibrium was described according to the Van't Hoff

*R T K Ke eq o*

Table 7 gives the parameters used for Toth model isotherms of each gas. It should be noted that activated carbon has a high CO2 adsorption capacity in comparison with the N2 adsorption capacity. It is worth mentioning that the commercial activated carbon used in this studied has a high CO2 adsorption capacity in comparison with other adsorbents

The CO2 adsorption equilibrium isotherms for CPHCL, at low partial pressure, were

where *Kp* is the Henry's Law constant for the adsorption equilibrium which the temperature

**Gas qm, 10-3mol/g** *n* **Ko,bar-1 -∆Hi, kJ/mol** 

*CO2 10.05 0.68 7.62 × 10-5 21.84 N2 9.74 0.52 6.91 × 10-5 16.31* 

Table 8 gives the Henry's Law constants for the adsorption equilibrium on CPHCL, at the different temperatures studied, the pre-exponencial factor and heat of adsorption. Table 8 also shows the Henry's constants for the adsorption equilibrium on commercial activated

It should be noted, however, that the commercial activated carbon has higher Henry's Law

The nature of the N functionality is very important because it can affect the basicity of the solid surface (Vlasov & Os'kina, 2002); comparing a primary amine with a secondary amine of the same carbon number, the basic character increases due to the increase in the inductive

constant indicating that this solid has a greater carbon dioxide adsorption capacity.

Table 7. Parameters used for fitting of the Toth model for carbon dioxide and nitrogen

. . *Hi*

*q*

equilibrium adsorption constant and *n* is the heterogeneity parameter.

reported in literature (Grande & Rodrigues, 2008; Glover *et al*., 2008).

described according a linear isotherm (Dantas *et al*., 2010); (Eq.31):

dependence was also described according to the Van't Hoff equation.

where *Ko* is the adsorption constant at infinite dilution.

equation; Eq. (30):

adsorption o activated carbon.

carbon that was fitted at low pressure.

effect caused by the alkyl groups.

1/

(29)

(30)

*<sup>p</sup> q KP* (31)

*<sup>n</sup> <sup>n</sup>*


Table 6. Chemical characterization of the adsorbents studied. [Dantas *et al*., 2010].

It has been suggested that primary amine can react with the activated carbon surface, forming surface complexes with the presence of NH2 surface groups (Gray *et al*., 2004). Bands were presence at 3365 and 1607 cm-1, ascribed to asymmetric stretching (NH2) and NH2 deformation, respectively, and at 3303 cm-1. However, the CPHCL spectrum shows that these bands may be overlapped by the OH stretching band (3600-3100 cm-1) and by the aromatic ring bands and double bond (C=C) vibrations (1650-1500 cm-1) (Fanning & Vannice, 1993). The same pattern is observed for CPHCL after CO2 adsorption at 28**°C** and 150**°C**, indicating that there is no difference in the adsorption behavior.

Fig. 1. FTIR spectra of (a) commercial activated carbon; (b) CPHCL; (c) CPHCL after pretreatment and CO2 adsorption at 28**°C** and (d) CPHCL after pre-treatment and CO2 adsorption at 150**°C** [Dantas *et al*., 2010].

#### **4.2 Adsorption equilibrium isotherms**

The adsorption equilibrium of CO2 and N2 adsorption on activated carbon was previously (Dantas *et al*., 2010) described using the Toth model (Toth, 1971); Eq. (29):

$$q = \frac{q\_m K\_{eq} P}{\left[1 + \left(KP\right)^n\right]^{1/n}}\tag{29}$$

where *qm* is the maximum adsorbed concentration, i.e., the monolayer capacity; *Keq* is the equilibrium adsorption constant and *n* is the heterogeneity parameter.

The temperature dependence of the equilibrium was described according to the Van't Hoff equation; Eq. (30):

$$K\_{eq} = K\_o.e^{\left(\frac{-\Delta H\_i}{R.T}\right)}\tag{30}$$

where *Ko* is the adsorption constant at infinite dilution.

68 Greenhouse Gases – Capturing, Utilization and Reduction

 **Activated carbon CPHCL** 

**C** 86.2 70.2 **H** 1.3 2.0 **N** 0.9 1.4

**N/C. 102** 1 2

It has been suggested that primary amine can react with the activated carbon surface, forming surface complexes with the presence of NH2 surface groups (Gray *et al*., 2004). Bands were presence at 3365 and 1607 cm-1, ascribed to asymmetric stretching (NH2) and NH2 deformation, respectively, and at 3303 cm-1. However, the CPHCL spectrum shows that these bands may be overlapped by the OH stretching band (3600-3100 cm-1) and by the aromatic ring bands and double bond (C=C) vibrations (1650-1500 cm-1) (Fanning & Vannice, 1993). The same pattern is observed for CPHCL after CO2 adsorption at 28**°C** and

Fig. 1. FTIR spectra of (a) commercial activated carbon; (b) CPHCL; (c) CPHCL after pretreatment and CO2 adsorption at 28**°C** and (d) CPHCL after pre-treatment and CO2

The adsorption equilibrium of CO2 and N2 adsorption on activated carbon was previously

(Dantas *et al*., 2010) described using the Toth model (Toth, 1971); Eq. (29):

adsorption at 150**°C** [Dantas *et al*., 2010].

**4.2 Adsorption equilibrium isotherms** 

Table 6. Chemical characterization of the adsorbents studied. [Dantas *et al*., 2010].

150**°C**, indicating that there is no difference in the adsorption behavior.

Table 7 gives the parameters used for Toth model isotherms of each gas. It should be noted that activated carbon has a high CO2 adsorption capacity in comparison with the N2 adsorption capacity. It is worth mentioning that the commercial activated carbon used in this studied has a high CO2 adsorption capacity in comparison with other adsorbents reported in literature (Grande & Rodrigues, 2008; Glover *et al*., 2008).

The CO2 adsorption equilibrium isotherms for CPHCL, at low partial pressure, were described according a linear isotherm (Dantas *et al*., 2010); (Eq.31):

$$q = \mathbb{K}\_p P \tag{31}$$

where *Kp* is the Henry's Law constant for the adsorption equilibrium which the temperature dependence was also described according to the Van't Hoff equation.


Table 7. Parameters used for fitting of the Toth model for carbon dioxide and nitrogen adsorption o activated carbon.

Table 8 gives the Henry's Law constants for the adsorption equilibrium on CPHCL, at the different temperatures studied, the pre-exponencial factor and heat of adsorption. Table 8 also shows the Henry's constants for the adsorption equilibrium on commercial activated carbon that was fitted at low pressure.

It should be noted, however, that the commercial activated carbon has higher Henry's Law constant indicating that this solid has a greater carbon dioxide adsorption capacity.

The nature of the N functionality is very important because it can affect the basicity of the solid surface (Vlasov & Os'kina, 2002); comparing a primary amine with a secondary amine of the same carbon number, the basic character increases due to the increase in the inductive effect caused by the alkyl groups.

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 71

It is observed that, in the case of the mass balance, the model reproduces the experimental

(a)

Fig. 2. Breakthrough curves for the CO2 adsorption (a) on activated carbon and (b) on

The global mass transfer coefficient for CO2 adsorption on CPHCL in the fixed bed is higher than that for the adsorption on activated carbon (Table 9) which was to be expected because the CPHCL is an adsorbent with less micropores and smaller CO2 adsorption capacity than the commercial activated carbon. This makes the importance of the external to the internal mass transfer resistance (Eq. (28)) greater in the case of CPHCL than commercial activated

Figures 3(a) e 3(b) shows the gas simulated temperature profile, at the end of the bed, for the

carbon dioxide adsorption at 28**°C** on activated carbon and CPHCL, respectively**.** 

(a)

Fig. 3. Gas simulated temperature profile, at the end of the bed, for the carbon dioxide

The temperature peaks is about 8°C and 4°C, for activated carbon and CPHCL, respectively. Although as the commercial activated carbon and the CPHCL have about the same heat of

28

30

32

**Temperature, °C**

34

CPHCL. Symbols: experimental data; Lines: LDF model (Dantas *et al.*, 2010).

0.0

0.2

0.4

**C/Co**

0.6

0.8

1.0

0 40 80 120 160 200

**time, min**

0 50 100 150 200

**time, minutes**

(b)

(b)

 Run 1 Run 2 Run 3 Run 4 LDF Model

data for the different feed concentration and temperatures reasonably well.

 Run 1 Run 2 Run 3 Run 4

LDF Model

0 50 100 150 200 250 300

time, min

0 50 100 150 200 250 300

adsorption at 28°C (a) on activated carbon and (b) on CPHCL.

**time, minutes**

0.0

carbon.

28

30

32

**Temperature, °C**

34

36

0.2

0.4

C/Co

0.6

0.8

1.0

Some authors have reported that although there is a reduction in the BET superficial area which is caused for the partial blockage of the lesser pores, as also observed in this paper, the enrichment of the carbonaceous materials with nitrogen tends to increase the adsorption capacity for CO2 (Arenillas *et al*., 2005). However, there is no consensus about this issue because sorbents with the high amounts of nitrogen do not have the high CO2 adsorption capacity reported in recent publications by Arenillas and coworkers (2005) and Pevida *et al*. (2008). In the present study, we show a decrease in the CO2 adsorption capacity of CPHCL in comparison with non-functionalized activated carbon. The decrease in the CO2 adsorption capacity is not related to the destruction of basic sites in the CPHCL, as shown in the FTIR studies (Figure 2). In fact, Drage *et al*. (2007) have reported that only an activation temperature higher than 600**°C** can destroy basic sites in the adsorbents.


Table 8. Henry's Law constants for the adsorption equilibrium on commercial activated carbon and CPHCL at different temperatures (Dantas *et al*., 2010).

#### **4.3 Fixed-Bed CO2 adsorption: experimental data and modeling**

As previously mentioned, a set of experiments was performed changing the temperature of the carbon dioxide to determine the breakthrough curves of carbon dioxide adsorption on activated carbon and CPHCL.

The Peclet number and the LDF global mass transfer coefficient for the adsorption of carbon dioxide on activated carbon and CPHCL are shown in Table 9.


Table 9. Experimental Conditions and LDF global mass transfer coefficient for CO2 adsorption on the commercial activated carbon and CPHCL (Adapted from Dantas *et al*., 2010).

Figures 2(a) e 2(b) shows a comparison between the experimental and theoretical curves obtained for the CO2 adsorption on commercial activated carbon and CPHCL, respectively.

Some authors have reported that although there is a reduction in the BET superficial area which is caused for the partial blockage of the lesser pores, as also observed in this paper, the enrichment of the carbonaceous materials with nitrogen tends to increase the adsorption capacity for CO2 (Arenillas *et al*., 2005). However, there is no consensus about this issue because sorbents with the high amounts of nitrogen do not have the high CO2 adsorption capacity reported in recent publications by Arenillas and coworkers (2005) and Pevida *et al*. (2008). In the present study, we show a decrease in the CO2 adsorption capacity of CPHCL in comparison with non-functionalized activated carbon. The decrease in the CO2 adsorption capacity is not related to the destruction of basic sites in the CPHCL, as shown in the FTIR studies (Figure 2). In fact, Drage *et al*. (2007) have reported that only an activation

temperature higher than 600**°C** can destroy basic sites in the adsorbents.

*28-25* 2.89 2.16

**50** 1.86 1.55 **100** 0.62 0.32 **150** 0.29 0.11

carbon and CPHCL at different temperatures (Dantas *et al*., 2010).

**4.3 Fixed-Bed CO2 adsorption: experimental data and modeling** 

dioxide on activated carbon and CPHCL are shown in Table 9.

activated carbon and CPHCL.

**Run T, °C** *Pe*

 **Activated carbon CPHCL** 

**T, °C Kp, moles/kg. bar-1 Kp, moles/kg. bar -1 -∆Hi, kJ/mol**

Table 8. Henry's Law constants for the adsorption equilibrium on commercial activated

As previously mentioned, a set of experiments was performed changing the temperature of the carbon dioxide to determine the breakthrough curves of carbon dioxide adsorption on

The Peclet number and the LDF global mass transfer coefficient for the adsorption of carbon

*28* 21.91 0.0027 0.0041 *50* 21.85 0.0043 0.0063 *100* 21.75 0.0125 0.0259 *150* 21.65 0.0259 0.0719

Table 9. Experimental Conditions and LDF global mass transfer coefficient for CO2 adsorption

Figures 2(a) e 2(b) shows a comparison between the experimental and theoretical curves obtained for the CO2 adsorption on commercial activated carbon and CPHCL, respectively.

on the commercial activated carbon and CPHCL (Adapted from Dantas *et al*., 2010).

**Activated Carbon**  *KL* **, s-1**

20.25

**CPHCL**  *KL* **, s-1**

It is observed that, in the case of the mass balance, the model reproduces the experimental data for the different feed concentration and temperatures reasonably well.

Fig. 2. Breakthrough curves for the CO2 adsorption (a) on activated carbon and (b) on CPHCL. Symbols: experimental data; Lines: LDF model (Dantas *et al.*, 2010).

The global mass transfer coefficient for CO2 adsorption on CPHCL in the fixed bed is higher than that for the adsorption on activated carbon (Table 9) which was to be expected because the CPHCL is an adsorbent with less micropores and smaller CO2 adsorption capacity than the commercial activated carbon. This makes the importance of the external to the internal mass transfer resistance (Eq. (28)) greater in the case of CPHCL than commercial activated carbon.

Figures 3(a) e 3(b) shows the gas simulated temperature profile, at the end of the bed, for the carbon dioxide adsorption at 28**°C** on activated carbon and CPHCL, respectively**.** 

Fig. 3. Gas simulated temperature profile, at the end of the bed, for the carbon dioxide adsorption at 28°C (a) on activated carbon and (b) on CPHCL.

The temperature peaks is about 8°C and 4°C, for activated carbon and CPHCL, respectively. Although as the commercial activated carbon and the CPHCL have about the same heat of

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 73

As previously mentioned, a set of experiments was performed changing the temperature of the carbon dioxide/nitrogen mixture to determine the breakthrough curves of carbon dioxide adsorption and nitrogen adsorption on activated carbon.The axial mass dispersion coefficient and the LDF global mass transfer coefficient for CO2 and N2 adsorption on

*KL* **, s-1**

**Run T, °C** *DL* **, cm2 s-1 CO2 N2**

CO2 and N2 adsorption on activated carbon (Adapted from Dantas *et al*., 2011).

breakthrough times are shorter due the exothermic character of adsorption.

*28* 0.10 0.0025 0.004 *50* 0.10 0.0042 0.011 *100* 0.10 0.0138 0.042 *150* 0.10 0.0323 0.128 Table 11. Axial mass dispersion coefficient and the LDF global mass transfer coefficient for

Figure 4 shows a comparison between the experimental and theoretical curves obtained for the N2 and CO2 adsorption on activated carbon. The model describes quite well the roll-up effect that is caused by the displacement of N2 by CO2 (Dabrowski, 1999). The roll-up is a common phenomenon happening in multicomponent adsorption processes when the concentration of one component at outlet of the adsorber exceed it inlet level (Li et al., 2011). It can be observed that, when the temperature is increased, the carbon dioxide and nitrogen

The nitrogen breakthrough times are very similar at 28**°C** and 150**°C**. This finding can be explained by the decrease in the amount of nitrogen adsorbed on activated carbon which can be compensated by its faster diffusion at high temperature as observed by Cavenati and coworkers (2006) for nitrogen adsorption on CMS 3K (as mentioned above, this molecular sieve has a similar pore size distribution to that used for the activated carbon in this study). The model reproduces very well the breakthrough curves for the different feed concentrations, including the experimental breakthrough curves obtained for nitrogen. Also, from the breakthrough curves we can note that the adsorbent is very selective towards

Figure 5 shows the pressure change as a function of the process time for the experimental conditions of Run 1 (see Table 2). A function was not found to describe with more accuracy the pressure drop during the blowdown step and therefore the model cannot predict very

Figure 6 shows the experimental and simulated changes for CO2 molar flow rate during the PSA separation. The solid line represents the first cycle and the dashed line the steady-state

Figure 7 shows the experimental and simulated N2 molar flow rate as a function of process time, under the experimental conditions of run 2. The assumption that nitrogen adsorption does not affect the CO2 adsorption is well represented by the simulation, since the flow rate

activated carbon are shown in Table 11.

carbon dioxide.

cycle (CCS).

well this process variable.

**4.5 PSA: Experimental data and modeling** 

of nitrogen is not modified during each step.

adsorption, but distinct adsorptive capacities, it is also possible to conclude that a higher adsorption capacity leads to a higher temperature peak, since the adsorption is an exothermic phenomenon.

#### **4.4 Fixed-Bed CO2/N2 mixture adsorption: Experimental data and modeling**

The basic information required to describe the fixed-bed dynamics of the adsorption of carbon dioxide-nitrogen mixtures on activated carbon is the adsorption equilibrium behavior of the single components. The adsorbed equilibrium concentration of carbon dioxide and nitrogen on activated carbon was estimated as a function of the feed concentration from a mass balance in the fixed bed. For each experimental breakthrough curve, the adsorbed concentration is given by:

$$q\_i = \frac{1}{\rho\_p} \left| \frac{\mathcal{C}\_{Fi} Q\_F t\_{st}}{\left(V - \varepsilon V\right)} - \frac{\mathcal{C}\_{Fi} \varepsilon}{\left(1 - \varepsilon\right)} \right| \tag{32}$$

where *CFi* is the feed concentration of component i, *V* is the bed volume, *QF* is the feed volumetric flow rate and *tst* is the stoichiometric time (Ruthven, 1984).

The resulting adsorbed concentrations are given in Table 10. It can be observed that the activated carbon adsorption capacity for CO2 and N2 in the CO2/N2 mixtures is the same as that predicted by the single component Toth isotherm using the previously reported adjusted. This is to be expected if the active sites for N2 and CO2 are independent, since the amount of CO2 and/or N2 adsorbed on the solid at each partial pressure from a CO2/N2 mixture is the same as that measured for the pure gases at the same partial pressure, as shown in Table 10. This assumption is in agreement with Siriwardane and coworkers (2001) who observed the same behavior for the adsorption of CO2/N2 mixtures on 13X zeolite, although Delgado and coworkers (2006) observed that the nitrogen adsorption can be neglected when it is mixed with carbon dioxide. As the presence of nitrogen in the mixture does not interfere at the CO2 adsorption on activated carbon, thus the pure component equilibrium isotherms predict very well the equilibrium of each component in the CO2/N2 mixture.


Table 10. Experimental conditions and adsorbed concentrations predicted by the Toth model for pure components and from the mass balance of breakthrough experiments on activated carbon. (a) Values calculated from the experimental data; (b) Values calculated using Toth model for single component adsorption.

adsorption, but distinct adsorptive capacities, it is also possible to conclude that a higher adsorption capacity leads to a higher temperature peak, since the adsorption is an

The basic information required to describe the fixed-bed dynamics of the adsorption of carbon dioxide-nitrogen mixtures on activated carbon is the adsorption equilibrium behavior of the single components. The adsorbed equilibrium concentration of carbon dioxide and nitrogen on activated carbon was estimated as a function of the feed concentration from a mass balance in the fixed bed. For each experimental breakthrough

*Fi F st Fi*

1

(32)

 

CO2 0.734 0.743

CO2 0.450 0.466

CO2 0.163 0.170

CO2 0.072 0.071

**4.4 Fixed-Bed CO2/N2 mixture adsorption: Experimental data and modeling** 

1

*p*

*C Qt C <sup>q</sup> V V*

where *CFi* is the feed concentration of component i, *V* is the bed volume, *QF* is the feed

The resulting adsorbed concentrations are given in Table 10. It can be observed that the activated carbon adsorption capacity for CO2 and N2 in the CO2/N2 mixtures is the same as that predicted by the single component Toth isotherm using the previously reported adjusted. This is to be expected if the active sites for N2 and CO2 are independent, since the amount of CO2 and/or N2 adsorbed on the solid at each partial pressure from a CO2/N2 mixture is the same as that measured for the pure gases at the same partial pressure, as shown in Table 10. This assumption is in agreement with Siriwardane and coworkers (2001) who observed the same behavior for the adsorption of CO2/N2 mixtures on 13X zeolite, although Delgado and coworkers (2006) observed that the nitrogen adsorption can be neglected when it is mixed with carbon dioxide. As the presence of nitrogen in the mixture does not interfere at the CO2 adsorption on activated carbon, thus the pure component equilibrium isotherms predict very well the equilibrium of each component in the CO2/N2

Run **T, °C Adsorbate** *q***, mol kg-1(a)** *q***, mol kg-1(b)**  *1 28* N2 0.272 0.294

**<sup>2</sup>***50* N2 0.178 0.173

**<sup>3</sup>***100* N2 0.097 0.096

**<sup>4</sup>***150* N2 0.054 0.059

Table 10. Experimental conditions and adsorbed concentrations predicted by the Toth model for pure components and from the mass balance of breakthrough experiments on activated carbon. (a) Values calculated from the experimental data; (b) Values calculated using Toth

*i*

volumetric flow rate and *tst* is the stoichiometric time (Ruthven, 1984).

exothermic phenomenon.

mixture.

curve, the adsorbed concentration is given by:

model for single component adsorption.

As previously mentioned, a set of experiments was performed changing the temperature of the carbon dioxide/nitrogen mixture to determine the breakthrough curves of carbon dioxide adsorption and nitrogen adsorption on activated carbon.The axial mass dispersion coefficient and the LDF global mass transfer coefficient for CO2 and N2 adsorption on activated carbon are shown in Table 11.


Table 11. Axial mass dispersion coefficient and the LDF global mass transfer coefficient for CO2 and N2 adsorption on activated carbon (Adapted from Dantas *et al*., 2011).

Figure 4 shows a comparison between the experimental and theoretical curves obtained for the N2 and CO2 adsorption on activated carbon. The model describes quite well the roll-up effect that is caused by the displacement of N2 by CO2 (Dabrowski, 1999). The roll-up is a common phenomenon happening in multicomponent adsorption processes when the concentration of one component at outlet of the adsorber exceed it inlet level (Li et al., 2011). It can be observed that, when the temperature is increased, the carbon dioxide and nitrogen breakthrough times are shorter due the exothermic character of adsorption.

The nitrogen breakthrough times are very similar at 28**°C** and 150**°C**. This finding can be explained by the decrease in the amount of nitrogen adsorbed on activated carbon which can be compensated by its faster diffusion at high temperature as observed by Cavenati and coworkers (2006) for nitrogen adsorption on CMS 3K (as mentioned above, this molecular sieve has a similar pore size distribution to that used for the activated carbon in this study). The model reproduces very well the breakthrough curves for the different feed concentrations, including the experimental breakthrough curves obtained for nitrogen. Also, from the breakthrough curves we can note that the adsorbent is very selective towards carbon dioxide.

#### **4.5 PSA: Experimental data and modeling**

Figure 5 shows the pressure change as a function of the process time for the experimental conditions of Run 1 (see Table 2). A function was not found to describe with more accuracy the pressure drop during the blowdown step and therefore the model cannot predict very well this process variable.

Figure 6 shows the experimental and simulated changes for CO2 molar flow rate during the PSA separation. The solid line represents the first cycle and the dashed line the steady-state cycle (CCS).

Figure 7 shows the experimental and simulated N2 molar flow rate as a function of process time, under the experimental conditions of run 2. The assumption that nitrogen adsorption does not affect the CO2 adsorption is well represented by the simulation, since the flow rate of nitrogen is not modified during each step.

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 75

0.0

0.0 0.2 0.4 0.6 0.8 1.0

**FCO2, 10-3mol/s**

Fig. 6. Carbon dioxide molar flow rate as a function of cycle time. Experimental conditions: (a) run 1; (b) run 2; (c) run 3; and (d) run 4. Solid line: first cycle simulation. Dashed line:

0 40 80 120 160 200 240 280

**cycle time, minutes**

Fig. 7. Nitrogen molar flow rate as a function of cycle time. Experimental conditions: run 2.

Figures 8(a) and 8(b) shows the experimental and simulated temperature profile inside the column, at 0.17 m and 0.43 m from the bottom of the column, under the experimental

**Feed**

**Purge**

**Blowdown**

**Pressurization**

0.2

**Feed**

0.4

**FCO2, 10-3mol/s**

**Pressurization**

0.6

0.8

0 40 80 120 160 200 240 280

**cycle time, minutes**

1.2 (d)

 1o Cycle 9o Cycle 18<sup>o</sup> Cycle 26<sup>o</sup> Cycle 38<sup>o</sup> Cycle 47<sup>o</sup> Cycle

0 60 120 180 240 300 360

**cycle time, minutes**

**Purge**

**Purge Blowdown**

**Blowdown**

1.0 (b)

 1o Cycle 9o Cycle 21<sup>o</sup> Cycle 36<sup>o</sup> Cycle 46<sup>o</sup> Cycle 60<sup>o</sup> Cycle

0 40 80 120 160 200 240 280

0 40 80 120 160 200 240 280

0.0

Solid line: first cycle simulation. Dashed line: CCS simulation.

0.4

0.8

**FN2, 10-3mol/s**

**Pressurization**

1.2

1.6

**cycle time, minutes**

**Purge Blowdown**

**Feed**

 1o Ciclo 9o Ciclo 21<sup>o</sup> Ciclo 36<sup>o</sup> Ciclo 46<sup>o</sup> Ciclo 60<sup>o</sup> Ciclo

**cycle time, minutes**

1.2 (c)

 1o Cycle 10<sup>o</sup> Cycle 24<sup>o</sup> Cycle 39<sup>o</sup> Cycle 51<sup>o</sup> Cycle **Purge**

**Blowdown**

1.0 (a)

 1o Cycle 10<sup>o</sup> Cycle 18<sup>o</sup> Cycle 32<sup>o</sup> Cycle 46 <sup>o</sup> Cycle

0.0

0.0 0.2 0.4 0.6 0.8 1.0

CCS simulation.

**FCO2, 10-3mol/s**

**Feed**

**Pressurization**

0.2

**Feed**

0.4

**Pressurization**

0.6

**FCO2, 10-3mol/s**

0.8

Fig. 4. Breakthrough curves for the N2 and CO2 adsorption on commercial activated carbon. Symbols: experimental data; ∆ N2 and □ CO2. Lines: LDF model. Conditions: (a) run 1; (b) run 2; (c) run 3; and (d) run 4.

Fig. 5. Pressure change as a function of process time. Experimental conditions: run 1.

(a)

(c)

Fig. 4. Breakthrough curves for the N2 and CO2 adsorption on commercial activated carbon. Symbols: experimental data; ∆ N2 and □ CO2. Lines: LDF model. Conditions: (a) run 1; (b)

 **Cycle**

0 80 160 240 320 400 480 560

**time, minutes**

Fig. 5. Pressure change as a function of process time. Experimental conditions: run 1.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

**<sup>0</sup> 1 Cycle <sup>0</sup>**

**2**

**C/Co**

**C/Co**

0 50 100 150 200 250

**time, minutes**

0 20 40 60 80 100

**time, minutes**

 N2 CO2 LDF model

 N2 CO2 LDF model

(b)

(d)

 N2 CO2 LDF model

 N2 CO2

LDF model

0 50 100 150 200 250

**time, minutes**

0 20 40 60 80 100

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

**P, bar**

**time, minutes**

run 2; (c) run 3; and (d) run 4.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

**C/C**

**o**

**C/Co**

Fig. 6. Carbon dioxide molar flow rate as a function of cycle time. Experimental conditions: (a) run 1; (b) run 2; (c) run 3; and (d) run 4. Solid line: first cycle simulation. Dashed line: CCS simulation.

Fig. 7. Nitrogen molar flow rate as a function of cycle time. Experimental conditions: run 2. Solid line: first cycle simulation. Dashed line: CCS simulation.

Figures 8(a) and 8(b) shows the experimental and simulated temperature profile inside the column, at 0.17 m and 0.43 m from the bottom of the column, under the experimental

Separation of Carbon Dioxide from Flue Gas Using Adsorption on Porous Solids 77

There are many factors that influence CO2 capture, some of them are physical and some chemical. Textural properties are important for any adsorption processes but, in the case of CO2 capture, the surface chemistry is a particularly important factor. The enrichment of activated carbon with nitrogen using amine 3-chloropropylamine hydrochloride blocked some pores of the activated carbon. The increase in the surface basicity was not sufficient to counteract the decrease in the BET superficial area since a reduction in the CO2 adsorption

Carbon dioxide adsorption on commercial activated carbon and on a nitrogen-enriched activated carbon, named CPHCL, packed in a fixed bed was studied. The adsorption equilibrium data for carbon dioxide on the commercial activated carbon were fitted well using the Toth model equation, whereas for carbon dioxide adsorption on the CPHCL a linear isotherm was considered. A model using the LDF approximation for the mass transfer, taking into account the energy balance, described the breakthrough curves of carbon dioxide adequately. The LDF global mass transfer coefficient for the adsorption of CO2 on activated carbon is smaller than that for the CPHCL. Since part of the micropores of the activated carbon are blocked by the incorporation of the amine, probably only the largest pores would be filled by the CO2, causing a decrease on the capacity of the adsorption and

The fixed-bed adsorption of CO2/N2 mixtures on activated carbon was also studied. The adsorption dynamics was investigated at several temperatures and considering the effects caused by N2 adsorption. It was demonstrated that the solid sorbent adsorbed carbon dioxide and nitrogen to its total capacity, leading to the conclusion that the equilibrium of CO2 and N2 adsorption from CO2/N2 mixtures could be very well described through the adsorption equilibrium behavior of the single components. The activated carbon used in this study has high selectivity for CO2 and is suitable for CO2/N2 separation processes. The model proposed herein can be used to design a PSA cycle to separate the components of

The carbon dioxide–nitrogen separation applying PSA process showed that the increase in the inlet temperature of the mixture CO2/N2 increases the CO2 purity due to the great

The authors are grateful to CAPES – Comissão de Aperfeiçoamento de Pessoal de Nível Superior (Brazil) – and to CAPES/GRICES for the International Cooperation Project

Arenillas, A.; Rubiera, F; Parra, J.B.; Ania, C.O. & Pis, J.J. (2005). Surface modification of low

*Science*, 252, Vol.252, No.3, pp. 619-624, ISSN 0169-4332.

costs carbons for their application in the environmental protection. *Applied Surface* 

CO2/N2 mixtures, where the pressure drop and thermal effects are very important.

difference between the adsorption capacities of N2 and CO2.

**5. Conclusion** 

was observed.

an increase on the adsorption rate.

**6. Acknowledgments** 

(Brazil/Portugal)

**7. References** 

conditions of run 3. The temperature peak is high due to the exothermic adsorption of CO2 on activated carbon in a high amount. Therefore heat effects cannot be neglected during adsorption, especially when there is a strong adsorbent-adsorbate interaction.

Fig. 8. Temperature profile of the gas phase: (a) at 0.17 m and (b) at 0.43 m from the bottom of the column. Experimental conditions: run 3.

Table 12 shows the performance of the PSA process: carbon dioxide recovery, nitrogen recovery, and carbon dioxide purity obtained for all experimental conditions studied. It is possible note that there is an increase in the carbon dioxide purity with increasing feed time (runs 1 and 2). This indicates that the separation is strongly controlled by the equilibrium. It can be also noted that there is an increase in the carbon dioxide purity with increasing temperature and this is due to the high selectivity of activated carbon. We observed that when the temperature of the CO2/N2 mixture was 100oC, a superior CO2 purity is obtained due to the high selective toward CO2. This is a good result since it indicates that the cooling of the exhaustion gas before CO2 separation is not necessary.


Table 12. PSA Performance.

As proposed by Grande and Rodrigues (2008) for CO2 adsorption, adsorbents with a greater adsorption capacity and higher heat of adsorption than the activated carbon honeycomb monolith should be used to achieve a product purity of greater than 16%. Although the CO2 purity is lower than that is required to transport, the PSA cycle could be optimized in order to increase the CO2 purity and recovery (Ko *et al*., 2005). Transport considerations limit the CO2 purity > 95.5% to ensure a reasonable input of CO2 compression power (Vinay & Handal, 2010).

#### **5. Conclusion**

76 Greenhouse Gases – Capturing, Utilization and Reduction

conditions of run 3. The temperature peak is high due to the exothermic adsorption of CO2 on activated carbon in a high amount. Therefore heat effects cannot be neglected during

95

100

**Temperature, o**

Fig. 8. Temperature profile of the gas phase: (a) at 0.17 m and (b) at 0.43 m from the bottom

Table 12 shows the performance of the PSA process: carbon dioxide recovery, nitrogen recovery, and carbon dioxide purity obtained for all experimental conditions studied. It is possible note that there is an increase in the carbon dioxide purity with increasing feed time (runs 1 and 2). This indicates that the separation is strongly controlled by the equilibrium. It can be also noted that there is an increase in the carbon dioxide purity with increasing temperature and this is due to the high selectivity of activated carbon. We observed that when the temperature of the CO2/N2 mixture was 100oC, a superior CO2 purity is obtained due to the high selective toward CO2. This is a good result since it indicates that the cooling

**Run Feed time, s CO2 purity, % N2 purity, % CO2 recovery, %**  100 25.7 87.1 93.1 120 31.3 87.5 85.4 120 32.3 87.1 73.7 200 49.7 78.9 66.8

As proposed by Grande and Rodrigues (2008) for CO2 adsorption, adsorbents with a greater adsorption capacity and higher heat of adsorption than the activated carbon honeycomb monolith should be used to achieve a product purity of greater than 16%. Although the CO2 purity is lower than that is required to transport, the PSA cycle could be optimized in order to increase the CO2 purity and recovery (Ko *et al*., 2005). Transport considerations limit the CO2 purity > 95.5% to ensure a reasonable input of CO2 compression power (Vinay &

**C**

105

0 600 1200 1800 2400

**time, minutes**

110 (b)

adsorption, especially when there is a strong adsorbent-adsorbate interaction.

(a)

0 600 1200 1800 2400

**time, minutes**

of the exhaustion gas before CO2 separation is not necessary.

of the column. Experimental conditions: run 3.

95

Table 12. PSA Performance.

Handal, 2010).

100

**Temperature, o**

**C**

105

110

There are many factors that influence CO2 capture, some of them are physical and some chemical. Textural properties are important for any adsorption processes but, in the case of CO2 capture, the surface chemistry is a particularly important factor. The enrichment of activated carbon with nitrogen using amine 3-chloropropylamine hydrochloride blocked some pores of the activated carbon. The increase in the surface basicity was not sufficient to counteract the decrease in the BET superficial area since a reduction in the CO2 adsorption was observed.

Carbon dioxide adsorption on commercial activated carbon and on a nitrogen-enriched activated carbon, named CPHCL, packed in a fixed bed was studied. The adsorption equilibrium data for carbon dioxide on the commercial activated carbon were fitted well using the Toth model equation, whereas for carbon dioxide adsorption on the CPHCL a linear isotherm was considered. A model using the LDF approximation for the mass transfer, taking into account the energy balance, described the breakthrough curves of carbon dioxide adequately. The LDF global mass transfer coefficient for the adsorption of CO2 on activated carbon is smaller than that for the CPHCL. Since part of the micropores of the activated carbon are blocked by the incorporation of the amine, probably only the largest pores would be filled by the CO2, causing a decrease on the capacity of the adsorption and an increase on the adsorption rate.

The fixed-bed adsorption of CO2/N2 mixtures on activated carbon was also studied. The adsorption dynamics was investigated at several temperatures and considering the effects caused by N2 adsorption. It was demonstrated that the solid sorbent adsorbed carbon dioxide and nitrogen to its total capacity, leading to the conclusion that the equilibrium of CO2 and N2 adsorption from CO2/N2 mixtures could be very well described through the adsorption equilibrium behavior of the single components. The activated carbon used in this study has high selectivity for CO2 and is suitable for CO2/N2 separation processes. The model proposed herein can be used to design a PSA cycle to separate the components of CO2/N2 mixtures, where the pressure drop and thermal effects are very important.

The carbon dioxide–nitrogen separation applying PSA process showed that the increase in the inlet temperature of the mixture CO2/N2 increases the CO2 purity due to the great difference between the adsorption capacities of N2 and CO2.

#### **6. Acknowledgments**

The authors are grateful to CAPES – Comissão de Aperfeiçoamento de Pessoal de Nível Superior (Brazil) – and to CAPES/GRICES for the International Cooperation Project (Brazil/Portugal)

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**4** 

*1Malaysia 2Iran* 

**The Needs for Carbon Dioxide Capture from** 

*1University Teknologi Malaysia, Faculty of Petroleum & Renewable Energy Engineering* 

The greenhouse effect is the heating of the earth due to the presence of greenhouse gases. According to the Intergovernmental Panel on Climate Change (IPCC), the ongoing emissions of greenhouse gases from human activities are leading to an enhanced greenhouse effect. This may result, on average, in additional warming of the earth's surface (Houghton and Jenkins, 1990). During 2007, global emission of carbon was 7 billion metric tons (Bt), which are expected to increase to 14 Bt per annum by the year 2050 assuming the demand for fossil fuel keeps increasing because of the growing economies around the world (Bryant, 2007). Carbon dioxide (CO2), is considered as a raw material in chemical industry. So, its recovery from flue gas meets a great prosperity not only for economic point of view but also for its negative effects to the environment. CO2 emission control by its capturing from fossil-fuel combustion sources is applied widespread in power plants and industrial sectors. By utilization of this approach, fossil fuel could be continually allowed to be used with a lesser degree and/or without contributing significantly to greenhouse-gas warming. This chapter clearly shows the need for CO2 capture in downstream petroleum industry by demonstrating its health and environmental effects. These effects are briefly discussed the negative impacts of the increasing trend of CO2 emission in Iran. Afterward, a comparative study for capturing carbon dioxide in a petrochemical plant in Iran will be presented.

At 5% concentration in air (500,000 parts per million (ppm)), CO2 can produce shortness of breath, dizziness, mental confusion, headache and possible loss of consciousness. At 10 % concentrations, the patient normally loses consciousness and will die unless it is removed. With little or no warning from taste or odour, it is possible to enter a tank or a pit full of CO2 and be asphyxiated in a very short time. Long-term exposure at concentrations of 1-2 % can cause increased calcium deposition in body tissue, and may cause mild stress and

**1. Introduction** 

**1.1 CO2 health effects** 

*2Petroleum University of Technology, Abadan Faculty of Petroleum Engineering* 

**Petroleum Industry: A Comparative Study** 

**in an Iranian Petrochemical Plant by** 

**Using Simulated Process Data** 

Mansoor Zoveidavianpoor1, Ariffin Samsuri1, Seyed Reza Shadizadeh2 and Samir Purtjazyeri2

Zhang, J.; Webley, P.A. & Xiao, P. (2008). Effect of process parameters on power requirements of vacuum swing adsorption technology for CO2 capture from flue gas. *Energy Conversion and Management*, Vol.49, No.2, pp. 346-356, ISSN 0196-8904.
