Article pubs.acs.org/IECR
Onsite CO2 Capture from Flue Gas by an Adsorption Process in a Coal-Fired Power Plant Zhen Liu,† Lu Wang,† Xiangming Kong,† Ping Li,† Jianguo Yu,*,† and Alirio E. Rodrigues‡ †
State Key Laboratory of Chemical Engineering, College of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal ABSTRACT: The feasibility and efficiency of adsorption technology were evaluated experimentally and theoretically for CO2 capture from the flue gas in an existing coal-fired power plant, where a three-bed VPSA unit was set up to test 282 kg of adsorbent materials. In this work, the experimental results are reported for zeolite 5A as the adsorbent. The composition of the flue gas after dehydration was 15.0 vol % CO2, 76.5 vol % N2, and 8.5 vol % O2. With a three-bed seven-step VPSA process including rinse and pressure equalization steps, 85% CO2 was obtained with recovery of 79% from flue gas at a feed flow rate of 46.0 Nm3/h. The experimentally measured energy consumption was 2.37 MJ/(kg of CO2). The experimental work was compared with numerical simulations through the multibed VPSA modeling framework developed in a previous work. The simulated results were found to agree well with the experimental results. from simulated flue gases by various VPSA cycles have been published by Ritter et al.8 and Biegler et al.15 In most of the previous work, activated carbon and zeolites have been the commercial adsorbents used for CO2 capture. The capacity of zeolites for CO2 is higher than that of activated carbon. However, it should be noted that zeolites are highly sensitive to water in the flue gas, which is strongly adsorbed and difficult to remove. To simplify laboratory-scale experiments and simulation works, mixture streams of CO2 and N2 and/or O2 have usually been employed to mimic actual flue gas. VPSA processes for pilot-scale CO2 capture have also been reported for actual flue gases from utility power plants. Takamura et al.16 studied an eight-step four-bed VPSA process for CO2 recovery from boiler exhaust gas (13 vol % CO2, 79 vol % N2, and 8 vol % O2); CO2 with 58.8% purity and with a recovery of 91.6% was obtained at a product flow rate of 7.44 Nm3/h. Cho et al.17 investigated a two-stage VPSA with two beds at each stage using zeolite 13X to achieve 99% CO2 with 80% recovery from flue gas. The power consumption was about 1.26 kWh/(Nm3 of CO2), and the productivity of the process was 30.2 Nm3/(t h) with a CO2 feed concentration of 12%. Pilot-scale experiments are essential for examining the power consumption of VPSA for CO2 capture, which is one of the major concerns of applying CO2 capture. Still, reports in the literature on pilot-scale VPSA separations of CO2 with a suitable adsorbent are quite scarce. In this work, a pilot-scale three-bed VPSA process including rinse and pressure equalization steps was studied for CO2 capture from flue gas to obtain high CO2 recovery and purity; the power consumption of the VPSA process for CO2 capture
1. INTRODUCTION Increased concentration of carbon dioxide (CO2) in the atmosphere due to the combustion of fossil fuels has become a critical environmental problem.1 The solution of this problem could be achieved by several strategies such as energy efficiency, renewable energy production, and CO2 capture and storage (CCS).2 With the continuing increase consumption and requirements of the fossil fuels energy at present, CCS is an effective method for achieving CO2 mitigation targets while maintaining a secure energy supply. To make CCS viable, it is necessary to have economic techniques to capture CO2 from flue gases of power plants and also to store the captured CO2.3 The recovery of CO2 from industrial streams has been achieved by absorption using alkanolamines. However, this process is energy-intensive for the regeneration of the solvent and is also plagued by corrosion problems.4 Recently, vacuum pressure swing adsorption (VPSA) has become a potential tool for the removal of CO2 from flue gas because of its energy efficiency and operating flexibility.5−15 Many sophisticated VPSA processes with different cycles have been investigated to recover CO2. In the case of concentrating CO2 from flue gases, CO2 is one of the most adsorbed gases (heavy component), and usually, a heavy or dual (heavy and light) reflux cycle configuration has been included.15 Na et al.5 investigated a VPSA experimental study for CO2 capture with three columns packed with activated carbon. Their proposed PSA process can separate CO2 at a purity of 99% and a recovery of 55% from flue gas with a composition of 83 vol % N2, 13 vol % CO2, and 4 vol % O2. Chou et al.11 obtained a CO2 purity of 63% at a recovery of 67% by a three-bed VPSA process using zeolite 13X while processing a feed that contained 13% CO2. Zhang et al.14 performed an experimental study of a three-bed VSA and achieved a CO2 recovery of 90% and purity of 80% from flue gas containing 12% CO2 using zeolite 13X. Comprehensive reviews of relevant studies addressing the removal and concentration CO2 © 2012 American Chemical Society
Received: Revised: Accepted: Published: 7355
February 28, 2012 April 18, 2012 May 2, 2012 May 2, 2012 dx.doi.org/10.1021/ie3005308 | Ind. Eng. Chem. Res. 2012, 51, 7355−7363
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was also investigated through experiments. Zeolite 5A was employed as the adsorbent. In a previous work, adsorption equilibrium isotherms and adsorption kinetics of CO2 and N2 on zeolite 5A were reported.18 The flue gas from coal-fired power plants contains N2, CO2, O2, H2O, and minor impurities such as SOx and NOx. Before the VPSA process, H2O, SOx, and NOx were removed in a pretreatment processes. The experimental work was also complemented with numerical simulations through the multibed VPSA modeling framework developed in our previous work.
Table 1. Characteristics of the Adsorbent and Bed Column bed radius bed length bed porosity bulk density wall density wall heat capacity weight of adsorbent for each column column wall thickness Adsorbent
2. EXPERIMENTAL WORK 2.1. Experimental Setup. A schematic diagram of the experimental VPSA setup is shown in Figure 1. A dryer and the
pellet radius pellet density pellet void fraction crystal radius solid heat capacity
0.2 m 1m 0.31 747 kg/m3 8238 kg/m3 500 J/kg·K−1 94 kg 5 × 10−3 m 0.0135 m 1083 kg/m3 0.30 2 μm 920 J/kg·K−1
be based on this requirement.12 Usually, two modifications to the Skarstrom cycle have been proposed to improve the heavyproduct purity or enrichment, namely, the addition of a cocurrent depressurization step or the addition of a highpressure rinse (i.e., heavy reflux) step.8 In this work, a sevenstep VSA process was employed to producing high-purity CO2 as follows: Step 1: Pressurization and Feed (FEED). The pressure is increased from low pressure to a higher pressure with feed gas. Then, the feed is kept at the high pressure, and CO2 is selectively adsorbed. Because the valve control programs for the pressurization and feed are actually the same, those two steps are combined into one. Step 2: Depressurization (D). After the feed step, the pressure of the column is reduced to atmospheric pressure (cocurrently to feed). Step 3: Rinse (RINSE). Part of the heavy product is recycled to the column before desorption. The product gas, which is already highly enriched in the heavy component, displaces the light component from the adsorbed phase near the feed end of the column and flushes it downstream toward the light-product end of the column. Step 4: Provided Pressure Equalization (EQ). This step is performed by putting two columns at different pressure levels into contact to save energy. The high pressure of the column can be reduced. Step 5: Blowdown (B). In this step, the most adsorbed components are partially removed from the adsorbent. In VPSA technology for CO2 capture, high-purity CO2 will be recovered in this step. The blowdown is carried out at lower pressure. Step 6: Purge (PURGE). To remove the heavy gas from the gas phase, a counter-current purge with inert gas exiting from the other column at the feed step is carried out. This step is also carried out at the lowest pressure of the system. Step 7: Received Pressure Equalization (EQ). The streams taken from one column from the depressurization stages are recycled to another column to reduce compression energy. To arrange the steps in continuous ordering, three columns were chosen in this work. The time schedule and cycle sequence are shown in Figure 2.
Figure 1. Schematic diagram of the VPSA process for CO2 capture from flue gas.
VPSA apparatus are the two main parts of the setup. The dryer removes the water from the flue gas before it enters the main VPSA process. Alumina is used as the desiccant. The dryer can also capture small amounts of SOx and NOx present in the flue gas, which are easily adsorbed on and difficult to desorb from zeolites.9 In this work, we focused on the VPSA process for CO2 capture. Dried flue gas fed to the VPSA process was composed of 15.0 vol % CO2, 76.5 vol % N2, and 8.5 vol % O2. The flow rate of the feed stream was adjusted by a blower with a variable-frequency drive (VFD). The VPSA apparatus contains three columns of 400-mm i.d. and 1000-mm height packed with zeolite 5A, where the adsorption equilibrium and kinetics data were already measured and reported previously.18 All columns were made of stainless steel 316 L. The physical properties of the adsorbent and columns used in the VPSA process are listed in Table 1. Two flowmeters were used to monitor the flow rates of the feed stream and product. The concentrations of CO2 in the gases were analyzed with an infrared analyzer. The equipment allows different cycle configurations and temperatures inside the column to be measured at three different points: 0.25, 0.55, and 0.90 m from the feed inlet. A back-pressure regulator and an oil-free vacuum pump were used to regulate the pressure in the columns. A programmable logic controller (Siemens) and software were used for data acquisition and control of the pneumatic solenoid valves. Two wattmeters were used to monitor the power consumption of the blower and the vacuum pump. 2.2. Process Description. For CO2 capture from flue gases, CO2 is one of the most adsorbed gases that needs to be concentrated, and the scheduling of the VPSA process should 7356
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The most essential part of modeling a VPSA process is correctly describing the adsorption and desorption of gases in a fixed bed. The model used to describe the fixed-bed dynamics was derived from mass, energy, and momentum balances, including the following assumptions:20 (1) Fluid flow is governed by an axially dispersed plug-flow model. (2) One-dimensional macroscopic balance equations following the axial coordinate of the column are considered. (3) The cross-sectional area is constant, and the void fraction is uniform in the column. (4) The momentum balance can be simplified to the Ergun equation. (5) The gas phase follows the ideal gas law. (6) Film mass- and heat-transfer resistance in the boundary layer surrounding the pellets is considered. (7) According to Siriwardane et al.21 in their study of suitable adsorbents for CO2 separation from power-plant flue gas using VPSA, the amount of CO2 adsorbed on zeolite was similar for a multicomponent mixture of CO2, N2, and O2 and for pure CO2. Thus, for simplicity, a binary gas mixture of 15% CO2 and 85% N2 is assumed as the feed composition of the dried flue gas for the simulation. The multisite Langmuir model describes the adsorption equilibrium behavior of the mixture with parameters (see Table 2) taken from previously reported pure-component data. (8) Diffusion of gas in the macropores and micropores of the adsorbent pellets is described by a bilinear driving force (bi-LDF) model. The macropore diffusivity was calculated with the Bosanquet equation22 taking into account the Knudsen diffusivity, which was calculated with the Kauzmann correlation23 and molecular diffusivities represented by the Chapman−Enskog equation.23 The micropore diffusivities of CO2/N2 on zeolite 5A have a temperature-dependent form with the parameters listed in Table 2. The complete model equations used for the description of the adsorption bed in the VPSA process are detailed in Table 3. The boundary and initial conditions employed in the multiplebed VPSA simulations can be found elsewhere.19 The values of mass- and heat-transport parameters were calculated according to frequently used correlations: axial mass- and heat-dispersion coefficients and the mass-transfer and heat convective coefficients were estimated using the Wakao and Funazkri correlations.22 General properties of the gases, such as density, viscosity, and molar specific heat, were calculated according to Poling et al.23 The specific heat and viscosity of the gas were estimated at the inlet conditions and taken as constant throughout the bed. In this work, we calculated the power consumption of the VPSA processes through both simulation and experimental work. The experimental power consumption was calculated directly from the sum of the power for the blower and vacuum pump divided by the CO2 captured after cyclic steady state
Figure 2. Three-column schemes and cyclic configurations employed for VPSA for CO2 capture from flue gas using zeolite 5A.
3. THEORETICAL WORK The modeling framework for multiple-bed VPSA processes was developed in the gPROMS modeling environment (PSE, London) and reported in a previous work.19 The flow sheet of the VPSA process was established by connecting individual models for the following equipment: gas source, valve, mass flow controller (MFC), sink, and adsorption bed. The MFC delivers a constant specified amount of gas for feed, purge, and rinse steps; valves are considered as delay elements and can result in a significant disturbance to the flow regime or severe pressure drop; headers are used to mimic the empty space in the top and bottom of each column for flow distribution; sources and sinks are destination devices that supply initial and final operating conditions. The model equations for each of these individual units were detailed in a previous work.19 The three-bed VPSA modeling flow sheet is shown in Figure 3.
Figure 3. Modeling framework of the three-column VPSA unit employed for process simulations of CO2 removal from flue gases.
Table 2. Adsorption Equilibrium and Micropore Diffusion Parameters of Pure CO2 and N2 in Zeolite 5A gas CO2 N2
K0i (kPa−1) −7
1.4680 × 10 3.7885 × 10−7
−ΔHi (kJ/mol)
qm,i (mol/kg)
ai
D0μ,i (m2/s)
−Ea (kJ/mol)
37.8530 19.4346
3.9188 3.2793
2.0589 2.4604
5.9 × 10−11 5.2 × 10−13
26.34 6.28
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Table 3. Mathematical Adsorbent Bed Model Used for Multiple VPSA Processes for CO2 Capture from Flue Gas with Zeolite 5A Adsorption Isotherm for CO2−N2 Mixture multisite Langmuir isotherm
qi* qmax , i
⎡ = K iPyi ⎢1 − ⎢ ⎣
a ⎛ q * ⎞⎤ i i ⎥ ⎟ ⎜ ∑⎜ ⎟⎥ q i ⎝ max , i ⎠⎦
van’t Hoff equation
⎛ ΔH ⎞ i⎟ K i = K i0 exp⎜⎜− ⎟ ⎝ R gTs ⎠ Equations for Mass, Energy, and Momentum Balance component mass balance
∂Ci ∂(uCi) ∂ ⎛ ∂C ⎞ 1 = εc ⎜Dax i ⎟ − − (1 − εc)a′k fi (Ci − Cip) ∂t ∂z ⎝ ∂z ⎠ ∂z 1 + Bii
εc
Ergun equation
150μg (1 − εc)2 1.75(1 − εc)ρg ∂P =− u+ |u|u 3 2 ∂z εc d p εc 3d p LDF equation for the macropores
εp
∂⟨qi⟩ 15Dp, i Bii ∂ Ci p + ρp = εp (Ci − Ci p) ∂t ∂t R p2 1 + Bii
LDF equation for the micropores
∂⟨qi⟩
15Dμ , i
=
∂t
rc 2
(qi* − ⟨qi⟩)
gas-phase energy balance
εcC tCv
∂Tg ∂t
∂Tg ∂C ∂ ⎛ ∂Tg ⎞ + εcR gTg t − (1 − εc)a′hf (Tg − Ts) ⎜λ ⎟ − uC tCp ∂z ⎝ ∂z ⎠ ∂z ∂t
=
−
2hw (Tg − Tw) Rw
solid-phase energy balance n ⎡ n ⎤ ∂T (1 − εc)⎢εp ∑ Ci pCvi + ρp ∑ ⟨qi⟩Cv,ads, i + ρp Cps ⎥ s ⎢⎣ i = 1 ⎥⎦ ∂t i=1 n
= (1 − εc)εpR gTs
∂⟨qi⟩ ∂Ci + ρb ∑ (−ΔHi) + (1 − εc)a′hf (Tg − Ts) ∂t ∂t i=1
wall energy balance
ρw Cpw
∂Tw = αwhw (Tg − Tw) − αwlU (Tw − T∞) ∂t
with
αw =
Dw e(Dw + e)
and
1
αwl =
(
(Dw + e)ln
Dw + e Dw
)
ideal-gas behavior n
P = C tR gTg
Ct =
∑ Ci i=1
(CSS) was achieved. The theoretical definition of the power consumption is
The power consumption at the blower during the pressurization and high-pressure feed step can be represented by the equation
Specific power ave t
=
∫0 feed
+ t rinse
powerblowerdt + ∫ 0
tblow + t purge
power vacuumdt
t purge ⎡ tblow ⎤ ⎣∫0 CCO2u(0)dt + ∫0 CCO2u(0)dt ⎦A
powerblower = (1) 7358
γ − 1/ γ ⎡⎛ ⎤ P ⎞ P γ − 1⎥u(0)πR c 2 feed R gTfeed⎢⎜ feed ⎟ ⎢ ⎥ γ−1 R gTfeed ⎣⎝ Patm ⎠ ⎦ (2)
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The power consumption in the blowdown and purge steps (steps using vacuum) can be estimated by the equation power vacuum =
γ − 1/ γ ⎡⎛ ⎤ P ⎞ P γ − 1⎥u(0)πR c 2 blow R gTblow ⎢⎜ atm ⎟ ⎢ ⎥ γ−1 R gTblow ⎣⎝ Pblow ⎠ ⎦
(3)
In all of the simulations, the mathematical model was solved using gPROMS software (PSE, London). The discretization method for the spatial domain in the column was orthogonal collocation with the finite-element method (OCFEM) with 200 intervals in the whole column.
4. RESULTS AND DISCUSSION 4.1. Initial VPSA Process. Four different VPSA experiments were performed with different feed flow rates of the dried flue gas ranging from 32 to 46 Nm3/h. The rinse flow rate was fixed at 25 SLPM. The feed pressure was 1.2 bar controlled by a back-pressure regulator, and the evacuation pressure was 0.05 bar. The ambient temperature was 283 K. The temperature of the feed flue gas of the VPSA processes was about 298 K because of the temperature increase of the flue gas during compression in the blower. The cycle sequence and step times of the VPSA processes are shown in Figure 2. For the first VPSA run, adsorbents were degassed in an oven at 473 K overnight, whereas for the other runs, the experiments were performed under different operating conditions without initial regeneration. Figure 4 shows the temperature and pressure of the first VPSA experiment on a degassed adsorbent bed. Because the variation of the pressure evolution during each cycle was quite small, the pressure history is shown only in part to be more easily identified. In this process, the feed flow rate was 39.8 Nm3/h, and the purge flow rate was 20 SLPM. In this work, cyclic steady state (CSS) was considered to be achieved when the difference between the temperature profiles of the last two cycles was 60 K temperature differences can be seen in previous bench-scale adsorption and desorption breakthrough experiments19), which indicates that regeneration was not highly efficient during the VPSA cycles. 4.2. CO2 Capture Performance of the VPSA Process. The experimental operating conditions (feed flow rate and purge flow rate) and the performance of the VPSA processes for CO2 capture are reported in Table 5. With the seven-step three-column VPSA process, 86−91% CO2 was recovered with purity ranging from 71% to 81%. Figure 6 shows a comparison between the VPSA simulation and the experimental data for the CO2 concentration in the recovery gas and the recovery efficiencies for some feed flow rates. The results of the mathematical model have a trend similar to that of the experimental data and can explain that the CO2 concentration in the recovery gas increases and the CO2 recovery efficiency decreases with increasing flow rate. These results agree well with the values
Figure 4. Temperature and pressure profiles of the VPSA process for CO2 capture from flue gas (run 1).
Table 4. Transport Parameter Values Used in the Simulation of VPSA Processes for CO2 Capture from Flue Gas parameter axial dispersion coefficienta (m2/s) pore diffusivitya (m2/s) film mass-transfer coefficienta (m/s) overall heat-transfer coefficient (W/m2·K−1) film heat-transfer coefficient between the gas phase and the column walla (W/m2·K−1) axial heat-dispersion coefficienta (W/m2·K−1) film heat-transfer coefficient between the gas and the solid phasea (W/m2·K−1) a
value 9.1 × 10−4 CO2: 5.6 × 10−6 N2: 5.7 × 10−6 2.8 × 10−2 0 10.0 0.245 50.7
Calculated with the feed value.
predicted by the model simulation. Higher feed flow rates give higher purities but lower recoveries and vice versa. This is because higher feed flow rates result in partial breakthrough of the CO2 front during the feed step, “losing” CO2 to the waste stream and reducing CO2 recovery. However, allowing partial breakthrough displaces remaining N2/O2, giving a slightly higher CO2 product purity during the evacuation step because coadsorbed N2/O2 has been displaced. This behavior suggests that the duration of the feed step might be a useful operating parameter for controlling product purity and recovery.14 The energy consumption of the VPSA processes is also reported in Table 5. As detailed in this table, with an increase in the feed flow rate, the experimental energy consumption decreased. A power consumption of 2.37 MJ/(kg of CO2) was 7359
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Figure 6. Relationship between flow rate of dried flue gas feed, CO2 concentration, and CO2 recovery efficiency for the VPSA process. Symbols are the experimental results; the solid lines correspond to the simulation results.
The energy consumption of the VPSA process can be further reduced in the larger-scale case. Figure 7 shows the relationship between the specific power (per kilowatt per amount of gas) and gas flow rate for the same type of blower and vacuum pump as used in this study. These data are the mechanical properties of the blower and vacuum pump obtained from the suppliers of the equipment. The pressure increase of the blower is 58.8 kPa, and the limited vacuum pressure of the pump is 2 kPa. The conditions for the intake flow rate are the same for the blower and vacuum pump (101.3 kPa, 20 °C, air) in Figure 7. As shown in this figure, the specific power of the blower decreases significantly as the amount of gas processed increases, whereas the value for the vacuum pump stays almost constant. The total power of the blower and pump can be further reduced when the flow rate of the flue gas increases. The performance parameters of the VPSA process for capturing CO2 using zeolite 5A, such as CO2 purity, CO2 recovery, and specific power consumption, were compared with those of other adsorption processes for CO2 capture reported previously in the literature, as summarized in Table 6. It can be observed that most of the studies used as the adsorbent zeolite 13X or 5A, which have the highest CO2 capacities and selectivities among the commercial adsorbents. The energy consumption varied because of the different processes employed and different amounts of flue gas processed. The specific power consumption required in this study is comparable to other reports. It can be observed that the capture power consumption for the VPSA process was lower than that for the amine scrubbing process [3−4.6 MJ/(kg of CO2)].26 It should be noted that the total power consumption did not include the power consumption for the regeneration of the dryer. The purity of the CO2 obtained as a product was still low compared to the requirements for CO2 storage (purity higher than 95%). One proposed solution to increase the purity of the CO2 recovered is to use a second VPSA process in series. In the
Figure 5. Temperature and pressure profiles of the cycle when CCS was achieved in VPSA processes for CO2 capture from flue gas (run 1). Symbols are the experimental results; the solid lines correspond to the simulation results.
obtained with a feed flow rate of 45.9 Nm3/h (run 2). It also can be observed that the experimental data were much higher than the theoretical results [535 kJ/(kg of CO2) for run 2]. This is because the theoretical data correspond to the minimum value of the power required to make a vacuum in the blowdown and purge steps and also a blower to increase the pressure of the feed gas. It should be multiplied by the efficiency factors of the blower and vacuum pump to get the real power consumption. Also, the pressure and heat losses throughout the setup cost a great deal of energy and should be included for the real power consumption. For the blower, the pressure drop of the flow from the outlet of the blower to the feed end of the column was about 20 kPa (run 2), and the experimental power consumption for the blower, which is significantly affected by the outlet pressure, was even slightly higher than that for the vacuum pump. It should be noted that the power consumption in the blower could be practically zero if the pressure drop could be neglected and the feed pressure in the cycle were Patm. These results indicate that there is more room for the reduction of the energy consumption by the blower than the vacuum pump. To further reduce the power consumption, the quite large pressure drop in the experimental setup should be avoided.
Table 5. Experimental Operating Conditions (Feed Flow Rate and Purge Flow Rate) and Process Performance of VPSA Processes for CO2 Capture from Flue Gas with Zeolite 5A run
Qfeed (Nm3/h)
Qpurge (SLPM)
CO2 purity (%)
CO2 recovery (%)
experimental power consumption [MJ/(kg of CO2)]
theoretical power consumption [kJ/(kg of CO2)]
1 2 3 4
39.8 45.9 32.1 32.1
20 20 20 35
81 85 76 71
86 79 88 91
2.64 2.37 3.12 3.12
552 535 585 562
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Figure 7. Specific power of the blower and vacuum pump: (A) operating range in this study, (B) larger-scale case.
Table 6. Comparison of Performance Employing Different Processes for CO2 Capture from Flue Gas step sequencea
process
adsorbent
yCO2,feed (%)
PTSA9
Ca-X
11.5
VPSA12
FP, FD, RIN, BD, PUR
13X
10
2-stage VPSA17
first stage: EQ, FP, FD, EQ, BD, PUR second stage: EQ, FP, FD, EQ, BD
13X
10.5
5A
2-stage VPSA19 TSA25
first stage: FP, FD, RIN, BD, PUR second stage: FP, FD, EQ, BD, PUR, EQ
FVPSA7 VPSA16
VPSA19
VPSA15 VPSA28
regeneration method
PCO2 (%)
RCO2 (%)
flow rate of feed flue gas
power consumption
result typeb
(0.05−0.15) atm (323−373) K 6.67 kPa
99
90
1000 Nm3/h
2016 kJ/(kg of CO2)
exp
50−70
30−90
8.1 Nm3/h
sim
99
80
110 Nm3/h
99.7
78.8
15
first stage: 6.67 kPa second stage:13.34 kPa 10 kPa
(90−1100)kJ/ (kg of CO2) 2.3−2.8 MJ/(kg of CO2) 513 kJ/(kg of CO2)
69.2
98.9
0.048 Nm3/h
sim
5A
15
10 kPa
96
91
0.048 Nm3/h
5A
10
423 K
≥94
75−85
1.2 Nm3/h
FP, FD, DP, BD
13X
15
(0.1−0.7) atm
88.9
96.9
∼32 Nm3/h
FD, RIN, EQ, BD, PUR, EQ, N
NaX
13
58.8
91.6
35.6 Nm3/h
exp
NaX/ NaA 13X
13
56.9
88.0
38.0 Nm3/h
exp
15
(0.01−0.12) atm (0.01−0.12) atm 600/50 kPa
452.8KJ/(kg of CO2) 645.7KJ/(kg of CO2) (6120−6460) kJ/ (kg of CO2) 150.4 KJ/(kg of CO2)
95
80
2.29 MJ/(kg of CO2)
sim
HTlc
15
11.6 kPa
98.7
98.7
0.012 Nm3/h
RIN, FP + FD, EQ, DP, BD, EQ FD, RIN, BD, PUR, LP
exp sim
sim exp sim
sim
a
BD, blowdown; DP, depressurization; EQ, pressure equalization; FD, feed; FP, feed with pressurization; LP, light product pressurization; N, null operation; PUR; low-pressure purge; RIN, rinse. bexp, experimental results; sim, simulation results.
first VPSA unit, the CO2 concentration is increased to 40−60%, and then the stream is further concentrated to >95% in the second unit. This study will be performed in a future work. One improvement of the VPSA process is to increase the temperature of the rinse stream by using waste steam in the power plant. The heating can lead to a easier desorption during the blowdown and purge step. This method is called combined pressure and temperature swing adsorption (PTSA) and could reduce the power consumption by about 11%.15 The area of the capture plant should be small enough to be retrofitted into existing power plants. A reference for an amine scrubbing process in a 500 MW coal-fired power station (670 Nm3/s flue gas flow rate27) is a process with two columns of 35-m length and 12-m diameter (total volume of 7913 m3) plus two regenerators and heat exchangers.28 As a simple analysis, unit productivity, defined as the amount of flue gas per unit volume of the column, was used to evaluate the size of the capture plant. The unit productivity of the column packed with zeolite 5A in this work was 0.034 (Nm3/s)/m3, whereas the
unit productivity of the reference was 0.085 (Nm3/s)/m3. The area of the VPSA equipment is larger than that of the absorption process because of the much lower unit productivity. The practical CO2 capture costs include the capital and operating costs, which account for all process equipment and fixed general maintenance costs and labor costs.27 The power consumption of adsorption [2.02−2.79 MJ/(kg of CO2)]16,17 is much lower than that of absorption with monoethanolamine (MEA). However, the CO2 capture cost of adsorption might be higher than the cost of the MEA absorption process because of the investment in the equipment. With further improvements in the capacity of the adsorbent, both the operating and capital costs can be reduced, and the VPSA process can be a promising and economical CO2 capture technology.
5. CONCLUSIONS Experimental and theoretical work on a pilot-scale VPSA separation of CO2 with a suitable adsorbent is essential for the evaluation of CO2 capture from power plant. In this work, a 7361
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Ea,i = activation energy of microspore diffusion for component i (kJ/mol) hf = film heat-transfer coefficient between the gas and solid phases (W/m2·K) hw = film heat-transfer coefficient between the gas phase and the column wall (W/m2·K) kft = film mass-transfer coefficient (m/s) kg = thermal conductivity of the gas mixture (W/m2·K) Ki = adsorption equilibrium constant of component i (1/kPa) K0i = adsorption equilibrium constant of component i at the limit T → ∞ (1/kPa) P = total pressure (Pa) Pblow = blowdown pressure (Pa) Pfeed = feed pressure (Pa) powerblower = power for the blower (W) powervacccum = power for the vacuum pump (W) qi = adsorbed-phase concentration of component i (mol/kg) qi* = adsorbed gas-phase concentration of component i in the equilibrium state (mol/kg) ⟨qi⟩ = adsorbed-phase concentration of crystals averaged over the entire pellet (mol/kg) qmax,i = saturation capacity of component i (mol/kg) rc = radius of the crystal (m) Rc = radius of the column (m) Rg = universal gas constant (J/mol·K) rp = radius of the pore (cm) Rp = radius of the pellet (m) Rw = radius of the wall (m) specific powerave = specific power consumption (kJ/mol of CO2) t = time (s) tblow = blowdown step time (s) tfeed = feed step time (s) tpress = pressurization step time (s) tpurge = purge step time (s) trinse = rinse step time (s) T = temperature (K) Tblow = temperature of the gas during blowdown (K) Tfeed = temperature of the feed gas (K) Tg = temperature of the gas phase (K) Ts = temperature of the solid phase (K) Tw = wall temperature (K) T∞ = environment temperature (K) u = superficial velocity of component i (m/s) U = global external heat-transfer coefficient (W/m2·K) yi = molar fraction of component i z = axial distance along the column (m)
VPSA process including rinse and pressure equalization steps was studied for CO2 capture from flue gas to obtain high CO2 recovery and purity; also the power consumption of the VPSA process for CO2 capture was investigated through experiments. The CO2 adsorbent selected was zeolite 5A, and the adsorption equilibrium isotherms and adsorption kinetics of CO2 and N2 on the adsorbent were reported previously. The composition of the feed flue gas entering the VPSA process was 15.0 vol % CO2, 76.5 vol % N2, and 8.5 vol % O2, because the H2O, SOx, and NOx were removed in pretreatment processes. Four VPSA runs were performed with different feed and purge flow rates. With a three-bed seven-step VPSA process, 85% CO2 was obtained with a recovery of 79% from flue gas with a feed flow rate of 46.0 Nm3/h. The experimentally measured energy consumption was 2.37 MJ/(kg of CO2). The process was also studied by numerical simulations through the multibed VPSA modeling framework developed in a previous work. The simulated results were found to agree well with the experimental results, so the model can be used for further scaleup of the VPSA process.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 21 64252826. Fax: +86 21 64252826. E-mail: jgyu@ ecust.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support of the China 863 program (Grant 2008AA062302). NOTATION A = cross-sectional area of the column (m2) a′ = area-to-volume ratio (1/m) ai = number of neighboring sites occupied by a molecule of component i Bii = Biot number of component i, calculated as Bii=(Rpkfi)/ (5εpDp,i) Cip = average concentration of component i in the macropores (mol/m3) Ci = concentration of component i in the gas phase (mol/m3) Cp = molar constant-pressure specific heat of the gas mixture (J/mol·K) Cps = constant-pressure specific heat of the adsorbent (J/ kg·K) Cpw = specific heat of the column wall (J/kg·K) Ct = total gas concentration (mol/m3) Cv = molar constant-volume specific heat of the gas mixture (J/mol·K) Cvi = molar constant-volume specific heat of component i (J/ mol·K) Cv,ads,i = molar constant-volume specific heat of component i adsorbed (J/mol·K) dp = pellet diameter (m) Dax = axial dispersion coefficient (m2/s) Dp,i = pore diffusivity of component i (m2/s) Dw = internal diameter of the column (m) Dμ,i = crystal diffusivity of component i (m2/s) 0 Dμ,i = limiting diffusivity at infinite temperature for component i (m2/s) e = wall thickness (m)
Greek Letters
αw
ratio of the internal surface area to the volume of the column wall (1/m) αwl ratio of the logarithmic mean surface area of the column shell to the volume of the column wall (1/m) γ heat capacity ratio, represented by γ = Cp/Cv (−ΔHi) isosteric heat of adsorption of component i (kJ/mol) εc porosity of the column εp porosity of the pellet λ axial heat dispersion (W/m2·K) μg gas viscosity (Pa.s) ρb gas density in the bulk (kg/m3) ρc column density (kg/m3) ρg gas density (kg/m3) 7362
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pellet density (kg/m3) column wall density (kg/m3)
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