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Identifying the CO2 capture performance of CaCl2 supported amine adsorbent by the improved field synergy theory Xiao M. Wu, Yunsong Yu, Chao Y. Zhang, Geoff G. X. Wang, and Bo Feng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie500841n • Publication Date (Web): 21 May 2014 Downloaded from http://pubs.acs.org on May 27, 2014

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Industrial & Engineering Chemistry Research

Identifying the CO2 capture performance of CaCl2 supported amine adsorbent by the improved field synergy theory Xiao M. Wua, Yun S. Yua*, Chao Y. Zhangb, Geoff X. Wangc, Bo Fengd a.

School of Chemical Engineering and Technology, Xi’an Jiaotong University, No.28 Xianning West Road, Xi’an 710049, P.R. China b.

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China

c.

School of Chemical Engineering, The University of Queensland, QLD 4072, Australia

d.

School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia

*Corresponding Author: Tel.: +86-29-8266 8566, Fax: +86-29-8266 8566. Email: [email protected]

ABSTRACT: Amine solutions absorption and supported amine adsorbents adsorption are typical CO2 capture routes. However, amine solutions absorption normally consumes substantial amount of energy and supported amine adsorbents adsorption suffers from low capture capacity. Herein, a CaCl2 supported amine adsorbent was prepared to combine the high capture capacity and low energy consumption in a packed bed system. A gas and solid phase model coupled with a changing particle size model was developed to predict the adsorption/desorption behaviour. A new index of synergy angle distribution gradient (SADG) was used to characterize the synergy effects. An experiment was performed to determine the adsorption/desorption reaction kinetics and validate the model. The improved field synergy theory determined that the diffusion and reaction dominated the CO2 capture by the CaCl2 supported amine adsorbent. This process improved the capture capacity and energy consumption by 30% and 10% against the typical CO2 capture process. 1

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KEYWORDS: Calcium chloride; Amine; Carbon dioxide; Packed bed reactor; Field synergy

1. INTRODUCTION There are increasing interests in greenhouse gas reduction by effective CO2 capture methods in the world currently

1-2

. One of the recommended CO2 capture processes is amine solutions

absorption which shows great promise for industrial CO2 capture

3-4

. Typically, the amine

solutions absorption employs the monoethanolamine (MEA) solvent to absorb CO2 in an absorber and desorb the CO2 in a stripper via the reversible reaction between MEA and CO2 when heating by steam. The amine solutions absorption normally offers high CO2 capture capacity. However, the amount of steam consumption is quite large due to the water adsorbing heat in the rich solution. In order to reduce the energy consumption due to the inevitably heating water process in the CO2 desorption, the supported amine adsorbents are recently developed to capture CO2, where the gas and solid reaction replaces the gas and liquid reaction and thus the heating water process is avoided.5-7 In these researches, the supported amine adsorbents allowed the adsorption and desorption to occur at ambient temperature and around 383K and hence reduced the energy consumption.

5-7

The conventional supported amine adsorbents are composed of the primary,

secondary or tertiary amines and support material silica 5-7, which show great promise to capture CO2 from the simulated flue gas. However, the CO2 capture capacity of the conventional supported amine adsorbents is normally low. The idea of this work is to improve the capture capacity by utilizing the support material to increase the loading of the conventional supported amine adsorbents.

2


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Since CaCl2 intensifies the amine reaction with CO2 in the aqueous environment

8-11

, CaCl2 is

here utilized as the support material to improve the conventional supported amine adsorbents, which tries simultaneously to offer high CO2 capture capacity and low energy consumption. The adsorbent was prepared by monoethanolamine, H2O, CaCl2 and the adhesive attapulgite with large surface area 12. Here, the monoethanolamine, H2O, CaCl2 reaction with CO2 enhanced the adsorption reaction and increased the capacity as it has been proven in the amine solutions absorption process8-11. It is expected that the produced HCl will intensify the desorption process due to its reaction with CaCO3. Thus, the prepared adsorbent shows great potential to capture CO2 at lower adsorption and desorption temperature in the experiment in Section 2. In order to meet the requirement of CCS (CO2 Capture and Sequestration), two parallel packed bed reactors were designed to realize the continuous operation. One packed bed was designed to adsorb the CO2 while the other was used to desorb the CO2. After both adsorption and desorption finished accordingly, the adsorption and desorption altered by adjusting the inputs. The exothermic heat in adsorption was reused in CO2 desorption directly. Since the packed bed normally shows lower mass/heat transfer, the work tries to enhance them by introducing internals. The internals are anticipated to improve the fluid flow in the packed bed, as what they have performed in the CO2 aqueous desorption process 13. Since the prepared CaCl2 supported amine adsorbent and internals are used in the packed bed, the conventional gas and solid interaction model is insufficient to characterize the new system. Hence, a gas and solid phase model coupled with a changing particle size model was developed to assess the packed bed reactor performance. Additionally, since the chemical reaction, mass transfer and heat transfer produce coupling effects which significantly affect the adsorption and desorption, the synergy effect between the coupled processes is determined by the field synergy 3



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theory. Previously, the field synergy analysis

13-16

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has been used to analyse the coupling effects

of chemical reaction, mass transfer and heat transfer in the gas and liquid two phase reaction system by the parameter of synergy angle. These field synergy researches have offered the optimized velocity and gas concentration during amine solution absorption of CO2. Due to similarities, the field synergy analysis based on gas-solid reaction model

17

was employed to

perform the parametric analysis for the CO2 capture by the CaCl2 supported amine adsorbent. The improvement of the field synergy theory concentrated on the synergy effects between particle internal diffusion and reaction and phase flow by considering their great importance to gas-solid reaction, which theoretically extends the field synergy theory. After determining the proper parameters for the packed bed reactor system, the performance comparisons, such as CO2 capture capacity and energy consumption, were performed between the developed system and the conventional system. The objective is to assess the technical feasibility of the packed bed reactor system for CO2 capture by CaCl2 supported amine adsorbent. 2. EXPERIMENTAL 2.1 PACKED BED REACTOR SYSTEM Figure 1 presents the packed bed reactor system for CO2 capture by CaCl2 supported amine adsorbent, which is composed of packed bed reactors 1 and 2. In this system, the adsorption and desorption alternately occurs in the two reactors with internals. The alternate opening and closing are controlled by two sets of valves (refer to SET1 V2, V3, V6 and V8; and SET2 V1, V4, V5 and V7 in Figure 1, respectively).

Figure 1. Continuous CO2 capture by CaCl2 supported amine adsorbent in a packed bed system. 4


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Initially, the CaCl2 supported amine adsorbent is packed in reactor 1 while the product of the adsorbent reacted with CO2 is packed in reactor 2. The SET1 valves are closed while the SET2 valves are open to vent gas. Under this circumstance, the flue gas flows through the valve V1 and further into reactor 1 to carbonate the adsorbent. The remnant gas N2 ventilates via the valve V5. Meanwhile, the valve V4 is activated to access the steam for desorption of the product carbonates. The product of high concentration CO2 is transported to the sequestration process by the valve V7. When the adsorption in reactor 1 and the desorption in reactor 2 both finish, the operational states of the two sets valves are altered to accomplish continuous CO2 capture. Therefore, adsorption and desorption alternatively occurring in the packed bed reactor system achieve the continuous CO2 capture by CaCl2 supported amine adsorbent. The strong exothermic heat is employed to decompose the product carbonate in reactors 1 and 2, providing some extra benefits for the packed bed reactor system. Adsorption and desorption in the packed bed reactors are complicated and new processes. Thus, a new model is required to describe the gas-solid reaction in the two reactors, which is expected to identify the synergy effect and capture performance. In addition, the comparison analysis needs to be performed between the packed bed system and the typical one to offer the traits of the new packed bed system. In the experiment, the packed bed is a cylindrical reactor which is 0.04 m in internal diameter and 0.1 m in height. The steam is generated by an electrical heater. The flow rate of flue gas and steam are respectively given as 4L/min and 3L/min through the system. The temperature is measured by thermal couples along the reactor. The concentration of CO2 at the outlet of the

5



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reactor is monitored by IRME-S Infrared Gas Analyzer, which owns the gas concentration range of 0 to 100%. 2.2 ADSORBENT PREPARATION The 4.8 g adsorbents are respectively prepared with the 5% to 25% monoethanolamine (MEA) under the same ratio of CaCl2 and attapulgite. The MEA and CaCl2 are provided by Tianjin ZHIYUAN

Chemical

Solvent

Company,

China

(MEA

concentration>99%,

CaCl2

concentration >96%). The attapulgite is supplied by JingYuuan HaoDi Industry and Trade Co., LTD, China (SiO2 55.6-60.5%, Al2O3 9.0-10.1%, Fe2O3 5.7-6.7%, Na2O 0.03-0.11%, K2O 0.961.3%, CaO 0.42-1.95%, MgO 10.7-11.35%).The adsorbents are pelletized at the atmosphere temperature and pressure and then crushed and sieved to 0.5mm to 2mm. 2.3 KINETICS After the preparation, the 5mg adsorbents are used to determine the kinetics. The adsorption occurs at the temperature from 293 K to 313 K and the desorption occurs at the temperature from 403K to 443K, which are referenced by the typical temperature range in the literature

5-6, 12, 18

.

Based on these parameters, the uniform design is used for the experiment results analysis. The n

parameter assessment is performed by minimizing the objective function

∑( X i =1

−X) . 2

exp

Through the statistics analysis, the correlation coefficients between the experimental conversion and the modeling conversion are 0.996 for the adsorption and 0.954 for the desorption. The residual sum of squares for the adsorption and desorption are respectively 0.0325 and 0.0294. Therefore, the kinetics is obtained accordingly when the conditions are in the experiment range.

6


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In the reaction mechanism, the CaCl2 is involved in the CO2 reaction in addition to the conventional MEA reaction with CO2. The adsorption follows the mechanism below MEA+CO 2 +CaCl2 +H 2 O → CaCO3 +MEA ⋅ HCl

(1)

MEA+CO 2 +H 2 O → MEAH + ⋅ MEACOO -

(2)

According to the adsorption experiment results above, the rate constant is clearly presented as  47870.66  ka = 2.484 × 106 exp  −  RT  

(3)

with the adsorption kinetics being obtained by the regression method

ra = kaVCMEA

P YCO2 CCaCl2 1.2 xH2O Peq

(4)

The desorption is described as follows MEA ⋅ HCl → MEA+HCl

(5)

CaCO3 +HCl → CO 2 +CaCl 2 +H 2 O

(6)

MEAH + ⋅ MEACOO - → MEA+CO 2 +H 2 O

(7)

By the desorption experiment, the desorption rate constant is determined as  25000  kd = 4.848 × 108 × exp    RT 

(8)

with the desorption kinetics given by regression method 7



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(

rd = kd CMEA⋅HCl1.3VCMEAH+ ⋅MEACOO-VCCaCO3 YCO2 − Y0

)(P

d

Peq )

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(9)

During the desorption experiment, there are a small amount of amine vapour and steam, which are cooled and recycled to keep the amine and water environment for the adsorption. However, the impacts of the amine vapour and steam are ignored in the simulation by considering the small amount and calculation simplification in Section 4. The adsorption and desorption kinetics are included in the model development for the packed bed reactor system.

3. MODEL The model is comprised of gas and solid phase model coupled with a changing particle size model and an improved field synergy model, which are respectively given as follows.

3.1 GAS AND SOLID PHASE MODEL COUPLED WITH CHANGING PARTICLE SIZE MODEL A two phase model is employed to describe CO2 capture by CaCl2 supported amine adsorbent in a packed bed reactor. Solid phase of CaCl2 supported amine adsorbent is taken as the static phase while gas phase is regarded as the continuous phase. In order to determine the effect of the particle size change on gas and solid phase reaction, the changing grain size model19-20 is revised to develop the changing particle size model. The changing grain size model, which considers the grain size as the function of the initial grain size and un-reacted grain size, provides the similar technical routes to describe the particle change by the initial particle size and un-reacted particle size. The model for adsorption and desorption is comprised of governing equations and auxiliary equations, which are developed as follows. 3.1.1 Gas phase model 8


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(1) Governing equations For characterizing the gas phase in the adsorption, the continuity equation is used to provide the amount of CO2 adsorbed in the packed bed reactor, given ∂ (ερ ) + ∇ ⋅ (ερU ) = S ∂t

(10)

For the desorption, the source term S is substituted by SR, which will be given in detail later. During the adsorption, gas phase momentum equation is described as follows ∂ ( ερU ) + ∇ ⋅ (ερUU ) = −ε∇P +ερ g + ∇ ⋅ ετ − βU ∂t

(11)

which provides the gas velocity in the packed bed reactor to determine the synergy effects. For the desorption, the momentum equation (11) is used by revising the relevant parameters. Gas phase energy equation in the adsorption is written as ∂ (ερ c pT ) + ∇ ⋅ (ερUc pT ) = ∇ (ελ∇T ) +Sc pT + ha (Ts − T ) ∂t

(12)

It indicates that the temperature variation affected by the exothermic/endothermic heat is produced in the gas-solid reaction. The temperature distribution supplies a benchmark to control the reaction at proper temperature. As for describing the desorption, the source term SR replaces S term and the endothermic effect produces the temperature difference of (T-Ts).

9



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Concentration equation during adsorption describes the CO2 concentration variation along the packed bed reactor versus time, given ∂ ( ερY ) + ∇ ⋅ ( ερUY ) = ∇ ⋅ ε J +S ∂t

(13)

Considering the desorption, the different source term SR influences the CO2 concentration. Thus, the source terms S and SR are regarded as important factors as they are directly related to the governing equations. The following will give the detail information about the two terms. (2) Auxiliary equations of the continuity equations Reaction rate is required to close the continuity equations. The effective reaction rate is determined by gas phase mass transfer coefficient and reaction rate between CO2 and the prepared adsorbent 17, 21, given

keff =

η k g ae ka a k g ae + η ka a

(14)

where the effectiveness factor η presents the pore diffusion impact on intrinsic reaction rate constant, which is given 22 as

η=

1 1 1 −   φ  tanh ( 3φ ) 3φ 

(15)

where the related Thiele modulus φ is introduced as

φ = d p / 6 ( k r / De )

0.5

(16)

10


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Since the particle size dp is an important factor for gas and solid phase reaction, the changing particle size model is developed as follows, by revising the changing grain size model19-20. The revision is to replace the grain size with the particle size. 3

3

 dp   d0   di    = Z   + (1 − Z )    2 2  2 

3

(17)

with the un-reacted particle size di being given as

d ( di / 2 )  r r  = − a − d  dt  C a aa C d ad 

(18)

In the previous research, the particle size is normally assumed as a constant in the reactor. This work provides the more realistic particle size change inside the reactor. Hence, it gives more reasonable information. The particle size influences the surface area of the adsorbent. The surface area reduction is developed by revising the literature correlation 23 d (a ρ ) 2 = k s ( a ρ ) / S a  dt

(19)

with the changing particle size to replace the conventional constant one and hence the rate constant for sintering being revised as   CCaCO ,0 ( d p 2 ) dX 3 k s = 2.45 1 + 1.26   kd CCaCO a / 60 dt  3  

   

0.59

  exp ( −29000 T )  

(20)

The mass transfer coefficient kg is affected by the particle size, which is introduced as 11



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kg =

ShDe dp

Page 12 of 54

(21)

The Sherwood number is obtained by the following correlation 24  π 1− εp  Sh = 3.66 + 0.101 1 + (ς − 1)  ×   4 ε p  εp 13  1 1− εp  ς z Xew1 8   × Xew11 48 Sc1 3  1 + 2 Sc exp  −   ς  3 εp  

(22)

For the porosity change during the adsorption and desorption, it is introduced as the following equation

ε = ε0 − ρ r ⋅

f ω ,CaCl

2

ρCaCl

⋅ ( Z − 1) ⋅ (1 − ε 0 ) ⋅ X

(23)

2

After identifying the reaction rate and particle size, the source term of continuity equation is determined as follows 25.

keff  d p  S = ε Mra   ka  d 0 

2

(24)

For the desorption, the source term SR is given  dp  S R = CCaCO3VM CaCO3 rd    d0 

2

(25)

This source term SR is used to substitute the S term for the desorption simulation. As a result, the adsorption and desorption are simulated continuously to achieve the multi-cycle performance of CO2 capture by CaCl2 supported amine adsorbent. 12


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(3) Auxiliary equations of the momentum and concentration equations Drag force between gas and solid phase influences the momentum transfer, which is given in terms of Ergun equation 2 ρ (1 − ε ) ug 1− ε ) µ ( + 1.75 β = 150

ε 2d 2

εd

(26)

In the momentum equation, the relevant parameter τ should be determined to close the equation, which is cited by literatures 17, 26-27, given  ∂U x ∂U y  +  ∂x   ∂y

τ = µ

(27)

For the concentration equation, the auxiliary equation is to identify the diffusion flux J, giving J = ρ De∇Y

(28)

The effective diffusion coefficient De is estimated as 28

1 ς  1 1  =  +  De ε s  DA DK 

(29)

where DA is molecular diffusion coefficient and DK is Knudsen diffusion coefficient. The tortuosity factor is approximately related to the particle porosity 28,

ς=

1

(30)

εs

(4) Auxiliary equations of the energy equations

13



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The heat transfer is the key to balance the energy equations. The heat transfer coefficient is determined by the following correlation to close the energy equations. Nu = 0.6 Re 0.5 Pr 0.33

(31)

with the Reynolds number defined as

Re =

ρ g d pug µg

(32)

Using the obtained Nusselt number, the heat transfer coefficient is estimated by

h=

Nuλg

(33)

dp

The effective thermal conductivity is calculated by 29

λse = λg ( λs / λg )

(

0.28− 0.757log ε b −0.057log λs / λg

)

(34)

3.1.2 Solid phase model For solid phase, the reactant mass balance gives ∂ ( ε s ρ ) = ε s rs M s ∂t

(35)

The CaCl2 supported amine adsorbent is stationary in the packed bed reactor. Thus, there is no momentum equation for solid phase. The solid phase temperature during adsorption is identified by reaction heat and heat transfer amount between gas and solid phase. ∂ (ε s ρ s c psTs ) = ∇ (ε s λse∇Ts ) +S ( Q − c psTs ) + ha (T − Ts ) ∂t

(36) 14


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Likewise, it is feasible to replace the source term S by SR and incorporate the temperature difference (Ts -T) to obtain the solid temperature in the desorption. The solid phase can store part of the adsorption reaction heat as mentioned in Section 1, which could reduce the energy consumption and here is defined as potential recovery energy. T

QR

mc dTdV ∫ ∫ = ∫ ∫ C dYdV T0 Y0 Y

ps

(37)

0

The conversion is characterized by the time average of the removal amount integrated over time

X =

∫

t0

0

Y0 − Y dt Y0 t0

(38)

By equation (38), the capture capacity per energy consumption unit (Cap) is determined as

Cap =

mc × X msorbent × E

(39)

which is taken as a more comprehensive index against the conventional capture capacity or the energy consumption. In equation (39), the energy consumption E is calculated by summing up the reaction heat and the heat adsorbed by the adsorbent.

3.2 IMPROVED FIELD SYNERGY MODEL By referencing the previous gas and liquid two phase system analysed by field synergy theory 1316

, the adsorption and desorption of CO2 by CaCl2 supported amine adsorbent shows the similar

coupled effects of chemical reaction, mass transfer and heat transfer. It is possible to introduce the field synergy theory into the gas-solid reaction. It is anticipated that the field synergy theory 15



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Page 16 of 54

is capable to correlate various physical processes and determine the control steps of the adsorption/desorption. According to the definition in the field synergy theory, the synergy angle between gas velocity and temperature gradient is given as

 U ⋅ ∇T    U ∇T 

θT = arccos 

(40)

with another synergy angle between gas velocity and concentration gradient written as

 U ⋅ ∇Y    U ∇Y 

θY = arccos 

(41)

Different from the field synergy used in the gas and liquid two phase system, the synergy effects between the diffusion, reaction and flow should be focused on, as these parameters are key factors to the gas-solid phase interactions. Herein, the improvement of the field synergy is to develop the synergy angle between particle internal phenomenon (diffusion and reaction) and phase flow, which directly offers the index to describe how these processes affect each other. Hence, the synergy angle between phase velocity, reaction rate gradient ∇k and diffusivity gradient ∇D inside the particle is respectively developed as

 U ⋅ ∇k    U ∇k 

(42)

 U ⋅∇D    U ∇D 

(43)

θ k = arccos 

θ D = arccos 

Additionally, the synergy angle between reaction rate gradient ∇k and diffusivity gradient

∇D inside the particle is provided as 16


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 ∇D ⋅ ∇k    ∇D ∇k 

θ kD = arccos 

(44)

Since the physical parameters varies during the adsorption and desorption cycles, the synergy angle in each cycle varies. Thus, it is required to evaluate the synergy effects between cycles. In order to achieve this, the synergy angle distribution gradient (SADG) is defined by the sum of the synergy angles and the cycle number N as follows,

∂ (θT + θY + θ k + θ D + θ kD ) 5  SADG =  ∂N (θT + θY + θ k + θ D + θ kD ) 5 − (θT + θY + θ k + θ D + θ kD ) 5 n +∆N n = ∆N

(45)

Compared with the previous field synergy analysis, equation (45) simultaneously offers the synergy effects between time and space. By the conventional field synergy principle, the smaller the synergy angle is, the better performance is achieved. Here, the complementary principle is that the smaller the SADG is, the better performance is obtained. Hence, the synergy angle and SADG are both used as indices to describe the synergy effects in this work. Based on the equations above, synergy effect and capture performance in a packed bed reactor for the CO2 capture by CaCl2 supported amine adsorbent are evaluated numerically. The selfdeveloped program implemented by C++ software is used to solve the equations.

In the

numerical prediction, the finite volume method (FVM) is employed to solve the differential equations while additional source term method is applied to improve the convergence. The

17



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Page 18 of 54

procedure to solve the model is presented in Figure 2, which also summarizes how the model is structured logically.

Figure 2. Schematic of the model structure and the solution procedure. 4. MODEL VALIDATION

In order to test the model for a packed bed reactor, an experiment system is set up as that in Section 2. The characteristic of CaCl2 supported amine adsorbent and operating conditions are given in Tables 1 and 2, respectively. The adsorbent particle size for comparison is 0.5mm. The reactor is working at two conditions of 303K and 313K. Meanwhile, the adsorbent with 15% and 25% MEA are used in the experiment. These experiment results are used to validate the performance at varying conditions. The simulated curves have fitted the distributions obtained at 303K and 313K (see Figure 3(a)). As can be seen, the gas and solid phase model predicts well the CO2 conversion during the adsorption period for the prepared adsorbent (i.e. from 1 minute to 13 minutes in Figure 3(a)), demonstrating its effectiveness at varying conditions.

Table 1. Physical property of the prepared adsorbent. Table 2. Conditions for CO2 capture by the prepared adsorbent in packed bed reactors. Figure 3. CO2 adsorption curves (a) and continuous adsorption and desorption (b) in a packed bed reactor system.

18


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In order to validate the multi-cycle behaviour, the same size reactors are alternatively taken as the adsorption reactor and desorption reactor. The continuous performance of adsorption at 303K and desorption at 423K is explicitly given in Figure 3(b), which shows good agreement with the experimental data. This offers the multi-cycle modelling precisions for describing continuous CO2 capture. In order to further validate the model, the temperature distributions are compared against the experimental data in Figure 4. The initial adsorption and desorption temperature are 293K and 403K. As shown in Figure 4, the continuous adsorption and desorption modelling predicts well the temperature along the reactor during adsorption and desorption, which verifies its accuracy.

Figure 4. Temperature distributions in the packed bed. The results above suggest that the gas and solid phase model coupled with changing particle size model predicts the adsorption/desorption quite well. In the packed bed reactor for CO2 capture by CaCl2 supported amine adsorbent, there are substantial parameters, including temperature, particle diameter, porosity, internals and CaCl2 fraction, which significantly affect the adsorption/desorption process. The following section discusses the impacts of these parameters on adsorption/desorption, which is mainly characterized by a new index of Cap given in Section 3. The analysis is performed by the improved field synergy theory to demonstrate the synergy effects by synergy angle and SADG.

5. PARAMETER STUDIES

The parametric analysis generally focuses on the synergy effects (synergy angle θT , θY , θ k , θ D and θ kD ), synergy angle distribution gradient (SADG) and capture capacity per energy 19



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consumption unit (Cap) at varying particle properties (size, porosity and CaCl2 fraction) and operating conditions (temperature and internals). The pie charts to offer the ratio of the different synergy angles, are used to quantitatively demonstrate how the different process (diffusion, reaction and gas flow) affects the CO2 capture process. The larger the ratio, the corresponding process is identified to dominate the CO2 capture process. After determining the dominant process, the parameter distributions (CO2 concentration and CO2 diffusivity) are used to help interpret the results.

5.1. EFFECT OF ADSORBENT PROPERTIES Figure 5. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under various particle sizes.

The particle size is of great importance for the gas adsorption

19

, as it significantly affects the

fluid flow and reaction. This is proved by the results shown in Figure 5. It can be discerned that there is respective 28% and 17% increase of Cap when

0.5mm and 1mm particle are

respectively used to compare with 2mm particle in Figure 5(a). The SADG is reduced as smaller particle is used, which offers that the 0.5mm, 1mm and 2mm CaCl2 supported amine adsorbents respectively achieve the maximum Cap at recycle number of 4, 5 and 7(determined by the corresponding SADG) . The reason is that smaller particle size improves the synergy effects between diffusion, reaction and gas phase flow and then makes the CO2 capture proceed faster. Moreover, the detail synergy angles are required to determine the control step. Hence, the synergy angle at the centre of the reactor (r=0.02m) packed with 0.5mm particle is accordingly taken as the typical one in Figure 5(b). This provides that the synergy effects becomes better 20


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from the bottom to the top of the reactor. The synergy angles θ k , θ D and θ kD show larger than θT and θY , especially θ kD showing 12% greater than θT , which suggests that diffusion and reaction dominate the CaCl2 supported amine adsorbent adsorption process. This is proven by the one that 0.5mm particle produces higher CO2 diffusivity and hence produces the higher CO2 concentration (Figure 5(c)). This indicates that the smaller particle size improves the synergy effects by increasing the contact area and reaction time. It should be clear that the particle size here is relatively small for a fixed bed reactor type of design. However, the results will guide the practical design in the future.

Figure 6. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under various adsorbent porosities.

Porosity reflects the particle distribution in the packed bed reactor, which is a key parameter for gas-solid reaction. Under the porosity variation, the Cap, SADG and synergy angles are presented in Figure 6. During the adsorption and desorption process, the porosity at 0.5 and 0.6 respectively produces an increase Cap by about 30% and 24% against that porosity at 0.4 (in Figure 6(a)). The SADG shows a reduction tendency as the adsorbent porosity increases, which clearly demonstrates that the maximum Cap is respectively achieved at recycle number 3, 4 and 6(determined by the corresponding SADG) corresponding to porosity 0.6, 0.5 and 0.4. It is found that the smaller porosity improves the synergy effects and thus achieves the maximum Cap with less cycle number. In order to determine the control step, the synergy angle at centre of reactor (r=0.02m) packed with porosity of 0.6 is analysed in detail (Figure 6(b)). As can be seen, the 21



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synergy angle is increased from the top to the bottom, which suggests better synergy effects at the top. The synergy angles θ k , θ D and θ kD respectively account for 22.09%, 22.93% and 25.15%, which are larger than θT and θY along the reactor, showing that diffusion and reaction still dominate the CO2 capture process with the expected lower synergy angle around reactor position of 40%. This is validated by the expected one that the 0.6 porosity produces the higher CO2 diffusivity and thus offers the higher CO2 concentration along the reactor (Figure 6(c)). Thus, it is possible to improve the synergy effects by increasing the porosity properly due to greater porosity enhancing the diffusion and reaction.

Figure 7. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under various CaCl2 amounts.

Usually, CaCl2 influences the capture capacity greatly since it is one of the main reactants and accelerates CO2 adsorption. On account of this, 20%, 25% and 30% CaCl2 are considered in this study. Under the circumstance, the Cap, SADG and synergy angles are presented in Figure 7. During the CaCl2 supported amine adsorbent adsorption process, the 25% and 30% CaCl2 respectively produces 13.5% and 33% larger Cap against 20% CaCl2 (Figure 7(a)). The SADG clearly decreases as CaCl2 fraction increases from 20% to 30%, which respectively produces that the maximum Cap for CaCl2 of 30%, 25% and 20% are obtained at recycle number 4, 5 and 7 (determined by the corresponding SADG). This improvement is mainly due to the fact that CaCl2 chloride stabilises the MEAH+ and thus increases the capture capacity30. For purpose of identifying the control step, the representative synergy angle distribution at 30% CaCl2 is 22


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provided in Figure 7(b). The synergy angles θ k , θ D and θ kD accounting for 22.47%, 22.43% and 23.08%, are larger than θT and θY along the reactor (Figure 7(b)). Hence, the synergy between the diffusion, reaction and gas flow dominates the CO2 capture process, with an exception that around the 40% position of the reactor due to strong temperature effect. This is strongly supported by the results (shown in Figure 7(c)) that the 30% CaCl2 produces higher CO2 diffusivity and thus produces the higher CO2 concentration compared against the 20% CaCl2. Figure 7(c) also indicates that increasing the CaCl2 weight fraction will improve the synergy effects.

5.2. EFFECT OF OPERATING CONDITIONS Figure 8. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) at different temperature.

Temperature is normally regarded as an important parameter as it affects the adsorption and desorption in the reactor. The influence of temperature on the Cap, SADG and synergy effects are shown in Figure 8. It is clearly discerned that the Cap is respectively increased by 28% and 9%, as the temperature decreases from Ta=313K and Td=443K (Figure 7(a)). In this sense, a decrease of temperature synergizes the heat transfer and mass transfer between gas and adsorbent, which is also validated by the reduced SADG when the temperature decreases (Figure 8(a)). Hence, at the maximum Cap position, the recycle number is determined as 4, 6 and 8 (determined by the corresponding SADG) corresponding to the different temperature in Figure 23



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8(a). This is due to the fact that low temperature favours exothermic adsorption and low temperature desorption consumes less energy. After the SADG is determined, the synergy angles are given at the typical conditions of adsorption at 293K and desorption at 403K, which are used to identify the control step. It is apparently found that the synergy angles θ k , θ D and θ kD account for 22.67%, 22.84% and 23.71%, which are larger than 16.91% θT and 13.87% θY . This indicates that the diffusion and reaction dominate the CO2 capture process. This is proven by the results that the higher CO2 diffusivity contributes to the higher CO2 concentration (Figure 8(c)). By carefully analysing the results in Figure 8(c), it is found that decreasing the adsorption and desorption temperature properly will improve the CO2 capture performance.

5.3. EFFECT OF INTERNALS

Figure 9. Schematic of the ring and wedge internals. Table 3. Structural parameters of the ring and wedge internals. Figure 10. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under different internals.

Internals have been used previously 13 to enhance the heat transfer in the packed bed for gas and liquid absorption. Here, the internals are employed to intensify the heat transfer in the gas-solid reactor by changing the gas flow boundary. Figure 9 shows the structures of the wedge and ring internals with their parameters presented in Table 3. Figure 10 shows the impacts of internals on 24


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the reactor performance. It is apparently observed that assembling wedge and ring internals improves the Cap by 24% and 5% against that without internals (Figure 10(a)). At the maximum Cap position, the recycle number (determined by the corresponding SADG) is reduced from 7 to 6 and 3 as SADG is reduced due to internals improving the gas phase flow. On account of determining the control step, the synergy angles are provided in detail in Figure 10(b). As shown in Figure 10(b), the synergy angles θ k , θ D and θ kD account for 22.09%, 22.92% and 26.12%, which are larger than 15.92% θT and 12.96% θY . This strongly proves that the diffusion and reaction dominate the CO2 capture process. This is supported by the fact that the wedge internals increases the CO2 diffusivity and hence increase the CO2 concentration against that without internals (Figure 10(c)). The results in Figure 10(c) suggest that adding wedge internals will improve more the CO2 capture performance against the ring internals.

6. DISCUSSION

As discussed in Section 5, a CaCl2 supported amine adsorbent is successfully developed to achieve high CO2 capture performance. The following section will provide a detailed comparison between typical amine solutions absorption and conventional supported amine adsorbent and CaCl2 supported amine adsorbent adsorption. The objective is to demonstrate the improved performance of the CaCl2 supported amine adsorbent for CO2 capture in the future application. These discussions cover the capture capacity and energy consumption as follows.

Figure 11. Capture capacity comparison between amine solutions absorption and CaCl2 supported amine adsorbent and conventional supported amine adsorbent adsorption.

25



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Table 4. Energy consumption in amine solutions absorption and conventional supported amine adsorbent adsorption and CaCl2 supported MEA adsorbent adsorption.

The capture capacity in the amine solutions absorption is found in the literatures

3, 31

, with the

maximum capture capacity of 0.4 gCO2/gMEA. The operating condition is at typical temperature of 298K. The MEA concentration is 7% and 35%, which agrees well with the range in the CaCl2 supported amine adsorbent. The capture capacity during the conventional supported amine adsorbent adsorption is given clearly 32-34. Hence, the detailed comparison between the research results with this work is offered in Figure 11. Based on the field synergy analysis in Section 5, the optimized one is that the 0.5mm CaCl2 supported 30% MEA adsorbent adsorbs CO2 at 293K and desorbs CO2 at 403K and thus it is used for the comparison. It is found that the CaCl2 supported amine adsorbent affords to capture the CO2 effectively, i.e., 30% increase of capture capacity against the amine solutions absorption. Compared with the conventional supported amine adsorbent adsorption, the CaCl2 supported amine adsorbent shows almost 46% higher of capture capacity. This improvement is mainly due to the CaCl2 enhancing the adsorption reaction. This larger capture capacity allows for the high availability to capture CO2 by CaCl2 supported amine adsorbent. Theoretically, the storage energy in the adsorbent particles can save energy for desorption as high as 15%20. As there are some different operating conditions, the energy consumption under the optimized parameters by synergy optimization (in Section 5) is presented in Table 4. The results are compared against the energy consumption given in the open literatures

3-4, 35-36

. It is

clearly found that the energy consumption during CO2 capture by CaCl2 supported amine adsorbent is respectively 30% and 10% below the typical amine solutions absorption and 26


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conventional supported amine adsorbent adsorption. The reason is that the energy consumption during the CO2 capture by CaCl2 supported amine adsorbent depends much on the MEA·HCl decomposition reaction and CaCO3 reaction with HCl. Compared with the typical decomposition temperature 473K of MEA·HCl

37-38

, the experiment desorption performs well at relatively low

temperature due to MEA·HCl being close to liquid state rather than solid state during cycles. As for the previous CaCO3 reaction with HCl, it has been suggested that the reaction rate is relatively slow

39-41

under the 0.1% to 0.49% HCl concentration. However, the CaCO3 reaction

with HCl here is fast mainly due to almost 50% HCl concentration and amine product producing a larger partial pressure. The results above show great potential of CaCl2 supported amine adsorbent for CO2 capture during long cycle adsorption and desorption. However, in order to achieve an alternative operation, the system control for the packed bed reactors becomes relatively complex compared with the typical system. Moreover, if the potential emission of HCl is considered in the future, the additional HCl traps will make the system more complicated. These factors may inevitably increase some costs, which need to be further assessed.

7. CONCLUSIONS

A feasible packed bed reactor system for CO2 capture by CaCl2 supported amine adsorbent was successfully proposed to mitigate the greenhouse gas emission. This alternative adsorption and desorption system

simultaneously showed the high capture capacity and low energy

consumption against the conventional amine solutions absorption and supported amine adsorbent 27



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adsorption. The gas and solid phase model, combined with the improved field synergy model and the changing particle size model, was successfully developed to characterize the continuous adsorption and desorption behaviour. The new index SADG and synergy angle were employed to identify the proper parameters for the packed bed system. The results demonstrated that the diffusion and reaction dominated the process of CO2 capture by CaCl2 supported amine adsorbent. The packed bed system improved capture capacity by 30% against the typical amine solutions absorption system. Finally, it was found that there was 10% energy conservation potential for CO2 capture by CaCl2 supported amine adsorbent compared with the conventional supported amine adsorbent adsorption.

ACKNOWLEDGEMENTS

Financial supports of the National Natural Science Foundation of China (nos. 51276141 and 20936004) are gratefully acknowledged. This work is also supported by the China Postdoctoral Science Foundation funded project (no. 2013M530422) and “Fundamental Research Funds for the Central Universities”.

ASSOCIATED CONTENT Supporting Information Photograph of CaCl2 supported MEA adsorbent used in the experiment. This material is available free

of charge via the Internet at http://pubs.acs.org.

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NOMENCLATURE ae , a

external and internal specific surface, m2 m-3

C

molar concentration, mol m-3

Cap

capture capacity per energy consumption unit, t-1GJ-1

C0

initial amount of CO2, mol

cp

thermal capacity, kJ kg-1 K-1

d

diameter, m

di

un-reacted particle diameter, m

dp

particle diameter, m

d0

initial particle diameter, m

De

diffusivity, m2 s-1

DK

Knudsen diffusion coefficient, m2 s-1

DA

molecular diffusion coefficient, m2 s-1

E

energy consumption, GJ t-1

f

weight fraction

g

acceleration of gravity, m s-2

h

heat transfer coefficient, W m-2 K-1

J

diffusion flux, kg m-2 s-1 29



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ka

adsorption reaction kinetic constant, s-1

kd

desorption reaction kinetic constant, s-1

keff

effective reaction kinetic constant, s-1

kg

mass transfer coefficient, m s-1

kr

reaction kinetic constant, s-1

ks

reaction constant for sintering, g s-1 m-2

M

molecular weight, kg kmol-1

m

phase weight, kg

MEA

monoethanolamine

N

recycle number

Nu

Nusselt number

Re

Reynolds number

Pr

Prandtl number

P

pressure tensor, Pa

Peq

equilibrium pressure, kPa

Q

heat exchange intensity, kJ kg-1

QR

storage energy, kJ mol-1

r

reaction rate, mol m-3 s-1

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30


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R

gas constant, J mol-1 K-1

S

source term, kg m-3 s-1

Sa

asymptotic specific surface area, m-2 kg-1

SADG

synergy angle distribution gradient, degree

Sh

Sherwood number

Sc

Schmidt number

T

temperature, K

t

time, s

t0

lasting time, s

T0

initial temperature, K

ug

gas velocity, m s-1

U

velocity, m s-1

V

volume, m3

X

conversion

Xew

wall energetic criterion

x

fraction

Y

concentration

Y0

initial concentration of CO2

31



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Z

aspect ratio of a particle

ρ

density, kg m-3

ε

void fraction

εp

porosity

ε0

initial porosity

τ

stress tensor, Pa

ς

tortuosity factor

β

drag coefficient, kg m-3 s-1

λ

thermal conductivity, W m-1k-1

λse

effective thermal conductivity, W m-1k-1

µ

viscosity, Pa s-1

η

effectiveness factor

φ

Thiele modulus

θ

synergy angle, degree

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Subscript a

adsorption

c

carbon dioxide

32


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d

desorption

D

synergy between diffusivity gradient and flow

eff

effective

g

gas phase

k

synergy between reaction rate gradient and phase flow

kD

synergy between reaction rate gradient and diffusivity gradient

r

reaction

R

desorption

s

solid phase

T

synergy between temperature gradient and phase flow

x

radial direction

Y

synergy between concentration gradient and phase flow

y

axial direction

CaCl2

calcium chloride

33



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(21) Kiel, J. H. A.; Prins, W.; Van Swaaij, W. P. M., Modelling of Non-Catalytic Reactions in a Gas-Solid Trickle Flow Reactor: Dry, Regenerative Flue Gas Desulphurisation Using a Silica-Supported Copper Oxide Sorbent. Chem. Eng. Sci. 1992, 47,4271. (22) Froment, G. F.; Bischoff, K. B., Chemical Reactor Analysis and Design. In Wiley, New York, 1979. (23) Sun, P.; Grace, J.; Lim, C.; Anthony, E., The Effect of Cao Sintering on Cyclic CO2 Capture in Energy Systems. AlChE J. 2007, 53,2432. (24) Comiti, J.; Mauret, E.; Renaud, M., Mass Transfer in Fixed Beds: Proposition of a Generalized Correlation Based on an Energetic Criterion. Chem. Eng. Sci. 2000, 55,5545. (25) Jung, J.; Gamwo, I. K., Multiphase CFD-Based Models for Chemical Looping Combustion Process: Fuel Reactor Modeling. Powder Technol. 2008, 183,401. (26) Deng, Z.; Xiao, R.; Jin, B.; Song, Q., Numerical Simulation of Chemical Looping Combustion Process with CaSO4 Oxygen Carrier. Int. J. Greenh. Gas Con. 2009, 3,368. (27) Shuai, W.; Guodong, L.; Huilin, L.; Juhui, C.; Yurong, H.; Jiaxing, W., Fluid Dynamic Simulation in a Chemical Looping Combustion with Two Interconnected Fluidized Beds. Fuel Process. Technol. 2011, 92,385. (28) Satterfield, C. N., Mass Transfer in Heterogeneous Catalysis. In Mit Press, Cambridge, Ma, 1970. (29) Wen, D.; Ding, Y., Heat Transfer of Gas Flow through a Packed Bed. Chem. Eng. Sci. 2006, 61,3532. (30) Huang, Q.; Li, Y.; Jin, X.; Zhao, D.; Chen, G. Z., Chloride Ion Enhanced Thermal Stability of Carbon Dioxide Captured by Monoethanolamine in Hydroxyl Imidazolium Based Ionic Liquids. Energy Environ. Sci. 2011, 4,2125. (31) Yeh, A. C.; Bai, H., Comparison of Ammonia and Monoethanolamine Solvents to Reduce CO2 Greenhouse Gas Emissions. Sci. Total Environ. 1999, 228,121. (32) Bollini, P.; Brunelli, N. A.; Didas, S. A.; Jones, C. W., Dynamics of CO2 Adsorption on Amine Adsorbents. 1. Impact of Heat Effects. Ind. Eng. Chem. Res. 2012, 51,15145. (33) Choi, S.; Gray, M. L.; Jones, C. W., Amine‐Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture from Ambient Air. ChemSusChem. 2011, 4,628. (34) Gebald, C.; Wurzbacher, J. A.; Tingaut, P.; Zimmermann, T.; Steinfeld, A., Amine-Based Nanofibrillated Cellulose as Adsorbent for CO2 Capture from Air. Environ. Sci. Technol. 2011, 45,9101. (35) Krutka, H.; Sjostrom, S. Topical Report 5: Sorbent Performance Report; 2011. (36) Sjostrom, S.; Krutka, H., Evaluation of Solid Sorbents as a Retrofit Technology for CO2 Capture. Fuel. 2010, 89,1298. (37) Amendola, S., Thermochemical Hydrogen Produced from a Vanadium Decomposition Cycle. US Patents: 2010. (38) Fearnside, P.; Murphy, C. J., Process Using Amine Blends to Inhibit Chloride Corrosion in Wet Hydrocarbon Condensing Systems. EP Patent,645, 2003. (39) Duo, W.; Kirkby, N.; Seville, J.; Clift, R., Alteration with Reaction Progress of the Rate Limiting Step for Solid-Gas Reactions of Ca-Compounds with Hcl. Chem. Eng. Sci. 1995, 50,2017. (40) Shemwell, B.; Levendis, Y. A.; Simons, G. A., Laboratory Study on the High-Temperature Capture of Hcl Gas by Dry-Injection of Calcium-Based Sorbents. Chemosphere. 2001, 42,785. (41) Weinell, C. E.; Jensen, P. I.; Dam-Johansen, K.; Livbjerg, H., Hydrogen Chloride Reaction with Lime and Limestone: Kinetics and Sorption Capacity. Ind. Eng. Chem. Res. 1992, 31,164.

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Figures and tables Figure 1. Continuous CO2 capture by CaCl2 supported amine adsorbent in a packed bed system. Figure 2. Schematic of the model structure and the solution procedure. Figure 3. CO2 adsorption curves (a) and continuous adsorption and desorption (b) in a packed bed reactor system.

Figure 4. Temperature distributions in the packed bed. Figure 5. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under various particle sizes



Figure 8. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) at different temperature.

Figure 9. Schematic of the ring and wedge internals. Figure 10. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under different internals.

Figure 11. Capture capacity comparison between amine solutions absorption and CaCl2 supported amine adsorbent and supported amine adsorbent adsorption. 36


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Table 1. Physical property of the prepared sorbent. Table 2. Conditions for CO2 capture by the prepared sorbent in packed bed reactors. Table 3. Structural parameters of the ring and wedge internals. Table 4. Energy consumption in amine solutions absorption and conventional supported amine adsorbent adsorption and CaCl2 supported MEA adsorbent adsorption.

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Figure 1. Continuous CO2 capture by CaCl2 supported amine adsorbent in a packed bed system.

Figure 2. Schematic of the model structure and the solution procedure. 38


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Figure 3. CO2 adsorption curves (a) and continuous adsorption and desorption (b) in a packed bed reactor system. 39



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Figure 4. Temperature distributions in the packed bed.

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Figure 5. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under various particle sizes

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Figure 8. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) at different temperature.

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Figure 9. Schematic of the ring and wedge internals.

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Figure 10. Cap, SADG (a), synergy angle (b) and CO2 concentration and diffusivity (c) under different internals.

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Figure 11. Capture capacity comparison between amine solutions absorption, CaCl2 supported amine adsorbent and conventional supported amine adsorbent adsorption.

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Table 1. Physical property of the prepared sorbent.

Sorbent composition

Specific surface area/m2 g-1 26.2

Calcium chloride+

Pore volume/ ml g-1 0.027

Size/ mm 0.5-2

attapulgite+MEA

Table 2. Conditions for CO2 capture by the prepared sorbent in packed bed reactors.

CO2 fraction/ % 10.0

Initial temperature/K

Porosity

Pressure /MPa

293-443

0.5

0.1

Table 3. Structural parameters of the ring and wedge internals.

Dimensions

H1

H2

Value/m

0.02

0.03

d(ring d(wedge internal) internal) 0.002

0.0251

L

0.05

Table 4. Energy consumption in amine solutions absorption and conventional supported amine adsorbent adsorption and CaCl2 supported MEA adsorbent adsorption. Sorbents

Amine solutions absorption

Adsorption temperature /K 293-313

Desorption Temperature /K 383-393

Energy consumption /GJ/t 3.1-4.03,4

Conventional supported amine adsorbent adsorption

~323

353-393

1.9-3.535,36

CaCl2 supported MEA adsorbent adsorption

293-313

403-443

1.8-3.1 53



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Identifying the CO2 capture performance of CaCl2 supported amine adsorbent by the improved field synergy theory 45x20mm (300 x 300 DPI)


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Identifying the CO2 Capture Performance of CaCl2 ... - ACS Publications

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