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N2 on binderless 5A zeolite
Journal of CO₂ Utilization 20 (2017) 224–233
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Separation of CO2/N2 on binderless 5A zeolite a
a,⁎
b
a,⁎
MARK
Patrícia A.P. Mendes , Ana M. Ribeiro , Kristin Gleichmann , Alexandre F.P. Ferreira , Alírio E. Rodriguesa a Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and Materials (LSRE-LCM), Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua do Dr. Roberto Frias, S/N, Portugal b Chemiewerk Bad Köstritz GmbH, Heinrichshall 2, 07586 Bad Köstritz, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Binderless 5A zeolite Flue gases CO2 capture PTSA
The study of CO2 separation from typical flue gas streams containing other gases like N2, CO, H2 and CH4 by Pressure Swing Adsorption (PSA) process using binderless 5A zeolite beads as adsorbent is the main objective of this work. Firstly, single component adsorption equilibrium isotherms of CO2, CH4, N2, CO and H2 have been determined by gravimetric technique in a magnetic suspension microbalance (Rubotherm®) at three different temperatures (305, 333, and 363 K), up to 5 bar. A comparison between the obtained adsorption equilibrium data and reported values in the literature for other 5A zeolite adsorbents was performed. Dynamic studies were also done through single and binary breakthrough curves of N2, CO, H2 and CH4 at 4 bar and 308 K. A PSA experiment was performed, at pilot scale, with the aim of validating the developed PSA model for CO2 capture with the binderless 5A zeolite adsorbent. The adsorption equilibrium isotherms were fitted with the Toth model and breakthrough curves and the PSA experiment were simulated taking into account the adsorption dynamic behaviour of a fixed bed. CO2 recovery values between 81 and 98% and energy consumption between 5.7 and 3.8 MJ/kgCO2, for purification of a stream containing 10% of CO2 by different PTSA cycles were attained. For all designed PTSA cycles the product purity was higher than 95% in CO2; and the productivity was always above 13 kg/mbed3/h. The best results are obtained with a high pressure of 3.5 bar, a low pressure of 0.3 bar and a heating temperature of 403 K. For these operating conditions the lowest value of energy consumption is obtained (3.8 MJ/kgCO2) with a high CO2 recovery (94.8%) and high CO2 purity (95.2%). The achieved productivity was of 16 kg/mbed3/h.
1. Introduction
reducing the CO2 emissions to levels below 80 percent of the 1990 emissions, during the period 2013–2020 [6]. According to the United Nations, the amount of carbon dioxide in the atmosphere increases 2 ppm per year in the world and according to the International Panel on Climate Change (IPCC) the CO2 amount will be about 570 ppm in the year 2100, therefore, ways to reduce CO2 emissions are particularly necessary. There are four significant CO2 sources that need special attention: biogas, natural-gas and syngas as pre-combustion sources and flue gas as post-combustion source; due to their high CO2 contribution. Biogas is a renewable combustible gas mixture produced by anaerobic decomposition of organic matter (biomass) in a short period of time composed predominantly by the combustible CH4 and non-combustible CO2. Domestic garbage landfills are an example of biogas production. Syngas is an abbreviation of synthesis gas, which consists mainly of hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2) [7,8]. This fuel can be used, for electricity production, or hydrogen
In a constantly changing world, where the industry creates innovative and advanced products, it’s also important to take into account the footprint that is being left behind for future generations. Indeed, the emission of greenhouse gases (GHG), such as, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFC’s), sulphur hexafluoride (SF6), and nitrogen trifluoride (NF3) is leading to the global warming. The high amount of CO2 emitted to atmosphere contributes to more than 60% of the global warming; and 80% of this CO2 production comes from anthropogenic sources [1,2]. The CO2 emissions to the atmosphere originate from fuel combustion activities, industrial processes (e.g. cement, hydrogen production, steel production, petroleum refining, gas refining), and biogas/natural/shale gas processing [3–5]. To reduce GHG emissions, a second commitment period of the Kyoto protocol was implemented on December 8th 2012, with the aim of
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Corresponding authors. E-mail addresses: [email protected] (A.M. Ribeiro), [email protected] (A.F.P. Ferreira).
http://dx.doi.org/10.1016/j.jcou.2017.05.003 Received 11 July 2016; Received in revised form 11 April 2017; Accepted 4 May 2017 Available online 07 June 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
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Nomenclature b n P q
qsat R t T Tp ΔH
Affinity parameter (bar−1) Toth heterogeneity parameter (−) Pressure (bar) Total adsorbed amount (mmol g−1)
Maximum adsorption capacity (mmol g−1) Ideal gas constant (kJ K−1 mol−1) Time (s) Temperature (K) Temperature of the particle (K) Adsorption enthalpy (kJ mol−1)
combustion processes, either using coal or other fossil fuels [29,30]. Cryogenic CO2 capture using dynamically operated packed beds [31,32], the so called CryoCell® technology, removes CO2 from natural gas, avoiding limitations of the conventional acid gas treatment processes [33]. Recently advanced cryogenic air separation process for oxy-combustion effluents, based on self-heat recuperation technology, with energy reduction of 20.2%, has been introduced as a promising technology [34]. The separation/capture of CO2 by adsorption can be by chemical adsorption if a chemical reaction occurs at the exposed surface or by physical adsorption [35,36]. Chemical or physical adsorption is simply determined by the adsorbent and operating conditions used. Calcium oxide, metal oxides, lithium zirconium oxides, lithium salts or hydrotalcites (HT) are generally considered if chemical adsorption is targeted. Activated carbons, metal-organic-frameworks (MOFs), and zeolites are usually considered for physical adsorption [37,38]. There are different adsorption based technologies, mainly differentiated by its regeneration principle: Pressure Swing Adsorption (PSA) [39–43], Vacuum Pressure Swing Adsorption (VPSA) [44,45], Temperature Swing Adsorption (TSA) [46], and Electric Swing Adsorption (ESA) [47,48]. Recent studies reported microwave assisted vacuum swing adsorption process (MVSA) [49], where a hybrid system was used in the CO2 desorption steps in order to improve the performance. ESA is an interesting methodology, where an electric current is applied on the adsorbent, after the CO2 adsorption, in order to heat the bed. This corresponds to the electrification step and afterwards an inert stream is introduced counter-currently to produce a CO2 concentrated stream [50–52]. A broad range of different adsorption processes combining different steps and different adsorbents can offer an interesting line for the development of new CO2 separation/capture solutions. That is to say, the nexus adsorbent-process cannot be disassociated, and all research and development studies in the adsorption area must be integrated, having in mind adsorbent and process. Adsorption based technologies are applicable for CO2 separation and capture, from dilute to high CO2 concentration streams, with low energy consumption [12]. To conclude, post-combustion capture processes can be used in coalfired and gas-fired plants, the available technology is more mature than
production; and as feedstock to chemical processes such as the FischerTropsch (FT) process. Flue gas is the fuels combustion off-gas stream, mainly constituted by CO2, CO, and N2 [9]. In order to capture the CO2 from the target streams, it is important to know the typical streams composition. Table 1 shows the typical compositions of general biogas, landfill gas, natural gas [10], syngas [7] and flue gas [4,9,11]. There are different available technologies for the CO2 capture and separation with different industrial applications. In a general view, we can have CO2 separation/capture by absorption, adsorption, cryogenic distillation and membrane separation as conventional methods [4,12–14]. Table 2 shows a summary of industrial applications using the conventional methods [4,15]. In the absorption separation, the CO2 is dissolved in a solvent, which can be dimethyl ether or propylene glycol (Selexol process), glycol carbonate, methanol (Rectisol process), N-methylpyrrolidone (Purisol process) morpholine (Morphysorb process), and propylene carbonate (Fluor process); they all are physical absorption examples that use organic solvents. In chemical absorption, the solvents that are usually used are alkanolamines (monoethanolamine-MEA, diethanolamine-DEA, and methyl diethanolamine-MDEA) [16,17], amino acids, ammonia, ionic liquids, and aqueous piperazine that are alkaline solvents applied in the acid-base neutralization required for the chemical absorption [2,12,18–20]. Another method is the separation of CO2 by membranes, which allow the passing of CO2 and exclude other components from the mixtures to be separated [21]. Different materials can be used resulting in membranes with different structures. The different membranes can be categorized in inorganic membranes (porous membranes: zeolite, carbon, alumina; or dense membranes: palladium and liquid-immobilized membranes), polymeric membranes (polyacetylenes, polycarbonates, polyamides, amino groups, polymethacrylates), and mixed matrix membranes [12,13,22–27]. Cryogenic distillation is also used for CO2 separation at low temperature and high pressures, where the CO2 is separated from the other components based on the vapour-liquid equilibrium of the mixture and their different boiling points [12,28]. New developments on this method have been appearing under the framework of the CANMET’s pilot-scale of CO2 cryogenic capture and compression unit (CO2CCU). It can be coupled with oxy-fuel
Table 1 Typical composition of global biogas and landfill gas, syngas, natural gas and flue gas, syngas and flue gas [4,7,9–11]. Gas
CH4 (%) Other Hydrocarbons (%) H2 (%) CO2 (%) CO (%) N2 (%) O2 (%) H2S (ppm) NH3 (ppm) H2/CO ratio NO2 (%) NO (%) SO2 (%) Ash (%)
Biogas
Syngas
H2-rich
General gas
Landfill gas
Slurry feed
Dry feed
H2 lean
60–70 – – 30–40 – ∼0.2 – 0–4000 100 – – – – –
65 – 0–3 15–50 – 5–40 0–5 0–100 5 – – – – –
– – 35 14 50 1 – – – 0.70 – – – –
– – 28 3 64 2 – – – 0.44 – – – –
– – 1 21 76 2 – – – 0.013 – – – –
225
– – 95 1 1 3 – – – 95 – – – –
Natural gas
90 9 – 1 0.3 – – 3 – – – – – –
Flue gas Fuel Oil
Natural gas
Coal
– – – 12–14 – 78–80 2–6 – – – – – – –
– – – 10–12 70–110 ppm 78–80 2–3 – – – – – – –
– – – 10.6 5.579 ppm 78–80 7 – – – 1 1 > 2 ppm 12
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Table 2 Industrial applications of different technologies for the CO2 separation/capture [4,15]. Industrial applications
Absorption
Removal of CO2 to upgrade natural gas Removal of CO2 from flue gas CO2 separation in H2 production Flue gas separations CO2 liquefaction from gas wells
Adsorption
Membranes
Cryogenic distillation
Chemical
Physical
PSA/VSA
TSA
Inorganic
Organic
✓ ✓
✓ ✓
✓
✓
✓
✓
✓ ✓
✓
✓
✓
✓
✓
Chemiewerk Bad Köstritz, which the commercial name is KÖSTROLITH® 5A BFK. The main reason to choose this binderless 5A zeolite, is the fact of its enhanced capacity, making it an interesting material to study, and compare its performance with existing adsorbents, such as 13 X zeolite. The adsorbent characterization was performed by X-ray diffraction (XRD) at the Laboratório Central de Análises, Universidade de Aveiro; particle crushing strength that was performed at Delft Solids Solutions, Delft; porosimetry by mercury intrusion was performed at the Laboratório de Ensaios, Desgaste e Materiais, Instituto Pedro Nunes, Coimbra; N2 (at 77 K) and CO2 (at 273 K) adsorption measurements to determine the BET surface area and micropores size distribution was performed in Universidad de Málaga; Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS) was performed at the Centro de Materiais da Universidade do Porto (CEMUP), Porto. X-ray diffraction pattern was collected in an Empyrean diffractometer using Cu K-α1 radiation (λ ∼ 1.54056 Å) from 0 to 70° (2θ) with a step size of 0.0260 and 56.8650 s per step in continuous mode. The N2 and CO2 adsorption equilibrium isotherms were measured using about 0.21 g of adsorbent in a Micromeritics ASAP 2020 Surface Area and Porosity Analyser. The crushing strength was measured in a Dr Schleuniger type 5Y tablet hardness tester with a measuring range of 1-400 N. A high resolution (Schottky) Environmental Scanning Electron Microscope with X-Ray Microanalysis and Electron Backscattered Diffraction analysis: Quanta 400 FEG ESEM/EDAX Genesis X4 M was used to perform SEM and EDS analyses. In this analyses, the sample was previously coated with an Au/Pd thin film, by sputtering, using the SPI Module Sputter Coater equipment. A magnetic suspension microbalance (Rubotherm®, Germany) was used to assess the adsorption equilibrium. The equipment details and
other alternatives; and can easily be retrofitted into existing plants; the main disadvantage is low efficiency due to the low CO2 concentration in the streams to be treated. In Table 3 is presented a comparison of different CO2 separation methods for post-combustion treatment of flue gases. A detailed overview of CO2 capture and storage technologies can be found in Leung, et al. [21]. The main objective of this work is to design a PTSA process for CO2 capture using a commercial zeolite. Therefore, a bindereless 5A zeolite, with enhanced capacity, was studied as adsorbent for CO2 separation and capture. The capacity gain by not having binder was assessed by measuring single component adsorption equilibrium isotherms of CO2, CH4, N2, CO and H2 with a magnetic suspension microbalance. Furthermore, single and multicomponent breakthrough curves at 4 bar and 308 K were performed to study the fixed-bed adsorption dynamics. A 5-steps PSA cycle was designed and experimentally performed with the aim of validating the global model for CO2 capture with the enhanced binderless 5A zeolite material. The adsorption equilibrium was well described using the Toth model. A PTSA process for CO2 capture from a stream containing 10% CO2 was designed and optimized. The breakthrough curves and the cyclic processes were simulated with gPROMS ModelBuilder software package 2. Experimental section 2.1. Materials and equipment The sorbates used were CO2 (99.99%), CH4 (99.95%), N2 (99.95%) and CO (99.997%); the inert gas for adsorbent activation/regeneration was He (99.999%); all gases were supplied by Air Liquid. The selected adsorbent was a binderless 5A zeolite, supplied by
Table 3 Comparison of different separation technologies available for CO2 capture, based on Leung, et al. [21]. Technology Absorption
Adsorption
Chemical looping combustion
Advantage
Disadvantage
efficiency • High can be regenerated • Sorbents • Most mature technology efficiency • High can be regenerated and the process • Adsorbent operates in a continuous cyclic way is the main combustion product, which remains • CO2 unmixed with N2 thus avoiding energy intensive separation
Membrane separation
Hydrate-based separation Cryogenic Distillation
efficiency • High amount of knowledge existing from • Considerable other gas mixtures separation • Low energy requirements technology • Mature already operational in industry for many • Processes years for CO recovery. 2
Purity and Recovery
highly depends on CO concentration • Absorption amount of heat is required for the • Substantial sorbent regeneration leaching/degradation into the environment • Amine has to be better understood adsorbents that can be efficient at • Requires different temperatures, from ambient to high 2
temperatures.
energy requirements for the solid • High regenerations, i.e. CO2 desorption. mature technology, still under development • Not and no large scale processes in operation yet. Low fluxes and fouling, or high fluxes but low • selectivity.
• Still under development technology, still very new. viable for very high CO2 concentration • Only temperatures required/refrigeration loops are • Low needed. • Very energy intensive 226
Pur: 99% & Rec: 98% [53]
PSA: Pur: 50–99% & Rec: 30–90% [53] TSA: Pur: 95% & Rec: 80% [53] ESA: Pur: 20% & Rec: 93% [53] Pur: ∼100% & Rec: 95% [54]
Pur: > 95% & Rec: 90% [55]
Pur: n.a. & Rec: 87% [56] Pur: 99.9% & Rec: 90% [57]
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parameters qsat, b0, ΔH, and n were optimized by the Excel solver optimization tool.
operating procedure are described elsewhere [58]. The adsorption dynamic studies and the Pressure Swing Adsorption (PSA) cycles were performed in a home-built PSA set-up equipped with a stainless steel column. The temperature was monitored by type-K thermocouples placed in the bottom, middle and top of the column. The outlet flow rate is measured by a mass flow meter (MFM) at the exit of the system; the feed flow rate and composition are controlled by three mass flow controllers (MFC) at the entrance of the system with a maximum capacity of 1, 5 and 0.2 SLPM, respectively. The pressure is controlled by a back pressure controller (BPC). The fluid path is controlled by Solenoid valves that afterwards can be conducted to the exhaust or to the Gas Chromatography (GC −Varian CP-3800) analysis system, which allows the determination of the composition of the outlet stream. The GC unit is connected to the fixed-bed set-up by a loop collector (LP) valve and a six-ports, two position valve that can control the collecting (if the valve is in storage mode) or analysis of the sample by the GC (if the valve is in analyses mode). The loop collector has 15 loops, and each sample trapped in the loops is analysed in a packed column (Supelco 1-2390-U) located into the GC oven at 483 K. The sample molar fraction in each loop is quantified by means of a Thermal Conductivity Detector (TCD), because TCD detector responds to all target compounds. After TCD detection, a signal (mV) is sent to the computer and a relation between the signal detected and the physical concentration is obtained by means of a calibration curve specific for each loop, taking in account the possible small differences in the loop’s volumes. The equipment detailed description can be found elsewhere [59] and is presented in Figs. S1and S2.
q = qsat
bP 1
[1 + (bP )n]
(1)
n
−ΔH ⎞ b = b0 exp ⎛ ⎝ RT ⎠
(2)
−1
−1
q (mmol g ) is the total adsorbed amount, qsat (mmol g ) is the maximum adsorption capacity, b (bar−1) is the affinity parameter, P (bar) is the pressure in the system, n (−) is the Toth heterogeneity parameter, ΔH (kJ mol−1) is the adsorption enthalpy, T (K) is the system’s temperature, and R (kJ K−1 mol−1) is the ideal gas constant. 2.3. Breakthrough curves experimental protocol Single and multicomponent breakthrough curves were performed in a fixed bed column set-up packed with binderless 5A zeolite in order to study the adsorption dynamics. The column (Øint = 2.1 cm) was filled first with glass beads in the bottom, followed by a layer of glass wool, and then 79.8 g of zeolite binderless 5A (adsorbent layer length of 31.7 cm). Finally, the column was topped with a layer of glass wool (see Fig. S3). Subsequently, the adsorbent was activated at 520 K during 15 h, under He flow. After the activation, a continuously feed of the gaseous stream was introduced into the column and the adsorption dynamic behaviour is studied by means of the temperature, total flowrate, and outlet composition histories. The column outlet composition was analysed by GC analyses. When the steady-state is reached the adsorbent is regenerated. In order to regenerate the bed, helium flow was fed into it co-currently. Subsequently, the helium flow was kept overnight after the end of the experiment, in order to guarantee full regeneration. A set of single and binary breakthrough experiments was performed at 4 bar and 308 K, the experimental conditions of each run are showed in Table S1.
2.2. Adsorption equilibrium isotherm measurement procedure Single component adsorption equilibrium isotherms have been measured, in a magnetic suspension microbalance (Rubotherm, Bochum, Germany) to determine the adsorption capacity of CO2, CH4, CO, N2 and H2 in a shaped binderless 5A zeolite. A sample of 3.7 g of binderless 5A zeolite was placed in a basket that is suspended by an electromagnet. The sample was activated at 588 K during 12 h. After activation, the weight loss was of 9% and subsequently the adsorption equilibrium isotherms were measured at 305, 333, 363 K and in a pressure range from 0 to 5 bar. The system operates in batch mode, thus the equilibrium is reached when the pressure and the mass are constant (30 min at least). The adsorption equilibrium isotherms were described by the Toth model, which is shown in Eqs. (1) and (2) [35]. The
2.4. Pressure/vacuum swing adsorption pilot-scale cycle and experimental protocol A Pressure Vacuum Swing Adsorption (PVSA) cycle was designed to be tested in the pilot-scale unit with the aim of capturing CO2 from a mixture of N2/CO2 (synthetic flue gas). The cycle consists of five steps, having the following order: Pressurization (cocurrent), Adsorption,
Table 4 Steps, experimental conditions of pressure, feed composition and steps duration. STEP
Pressurization Coc.
Pressure (bar) Duration (s) Feed (SLPM) Feed composition (%)
0.35 → 4
Adsorption
Depressurization Coc.
Blowdown
Purge
4
4 → 1.1 104 – –
1.1 → 0.16 204 – –
0.35 198 0.2 100 N2
1152 0.38 80 N2 20 CO2
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zeolite beads have a pellet density of 1300 kg/m3. The complete results of textural characterization are presented in Table S2. The crushing strength test of 63 representative particles of the binderless 5A zeolite revealed an average strength of 10.1 ± 3.6 N (95% confidence interval).
Depressurization (cocurrent), Blowdown and Purge, as presented in Table 4. The light component is produced during the adsorption and cocurrent depressurization steps, while the heavy component is produced in the last two steps. To perform the designed cycle two streams of N2 and CO2, previously mixed, were fed to the column, in order to simulate a typical flue gas composition at 308 K and 4 bar. The column was initially filled with N2 also at 308 K and 4 bar. After pressurization of the column to 4 bar, the feed step begins, the two steps together last for 1152 s. Subsequently, the depressurization to 1.1 bar occurs in cocurrent mode, during 104 s. In the Blowdown (counter-current, 0.16 bar) and Purge steps, regeneration is performed. A stream of N2 (0.2 SLPM) is introduced as purge for 198 s. Samples of the outlet stream were collected at desired times of a step during one cycle and posteriorly analysed. When the temperature, pressure and composition histories of a cycle are repeated in the following cycle, the cyclic steady state (CSS) is achieved. In this experiment, a total of 17 cycles were performed.
3.2. Adsorption equilibrium isotherms As previously referred, the adsorption equilibrium isotherms were fitted with the Toth model and the parameters obtained from the fitting are presented in Table 5. The highest adsorption enthalpy was obtained for CO2. Fig. 2 shows the experimental adsorption equilibrium isotherms on Binderless 5A zeolite at 305 K for CO2, CH4, N2, CO and H2 adsorption/ desorption (filled/open symbols) and the adsorption equilibrium isotherms fitted with the Toth model (represented by solid lines). In the case of H2, it was only performed the experimental adsorption equilibrium. The results per component for the three temperatures are presented in Fig. S5. A reversibility between the adsorption and desorption of all components is observed, so, no hysteresis phenomena is observed. As we can see in the figure, the carbon dioxide demonstrates higher adsorption capacity and the order from higher adsorption capacity to lower adsorption capacity is: CO2 ≫ CO > CH4 > N2 > H2. Furthermore, this binderless material also shows higher CO2 capacity when compared with binder containing materials (binder around 20%) as can be seen in Fig. 3. The comparison was done for adsorption equilibrium data obtained at around the same temperature (305 K) using gravimetric methods [60–63] and a volumetric method [63,64]. A maximum CO2 loading capacity of 5 mmol g−1 was obtained in the binderless beads of the 5A zeolite, which demonstrates a visible difference compared with the tendency of 4 mmol g−1 and lower of the binder containing materials. However, correcting the capacity for the binder content (∼20%) a good agreement between the different materials is obtained (see Fig. S6). Additionally, in Fig. S14 and Fig. S15, the adiabats as well as the adiabatic operation line are shown. In these figures is possible to get a general idea of how adiabatic operation can affect the working capacity.
3. Results and discussion 3.1. Binderless 5A zeolite beads characterization Fig. 1 shows SEM photographs of the binderless 5A beads, where it is possible to notice that the chosen bead has a diameter of 1.6 mm, which is in accordance with the provider’s specification of 1.0 −2.0 mm range for particle size; and crystal diameter presents values between 0.6 μm and 1.5 μm. EDS analyses showed the presence of oxygen, sodium, aluminium, silicon, and calcium, which is in agreement with the expected composition of the 5A zeolite. The observed presence of Na, can be attributed to an incomplete ion exchange during the synthesis of 5A binderless zeolite from the original 4A zeolite. The crystalline structure was confirmed by XRD, and Fig. S4 shows the comparison between the 5A zeolite reference powder pattern (blue) and our sample (red). The peak of the experimental sample corresponds to the Bragg peaks of 5A zeolite pattern. Textural analysis was performed by means of N2 adsorption at 77 K and CO2 adsorption at 273 K. The values of 526 m2 g−1 and 0.25 cm3 g−1 were obtained for the BET surface area and micropore volume, respectively. The binderless 5A
Fig. 1. SEM photographs of binderless 5A zeolite with the following magnifications: (a) 100, (b) 1000, (c) 10000 and (d) 50000×.
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Table 5 Fitting parameters of the Toth isotherm.
qsat (mmol.g−1) b0 (bar−1) −ΔHads (kJ mol−1) n (−)
CO2
CH4
N2
H2
CO
5.11 2.1 × 10−5 38.1 0.61
4.28 2.0 × 10−4 17.2 1.09
5.42 3.3 × 10−4 13.4 2.95
2.10 2.3 × 10−2 3.8 0.55
3.25 3.5 × 10−5 24.4 0.91
0.25 CO2
F (mmol/s)
0.20
0.15 0.10 0.05 0.00 0
5000
10000
15000
t (s)
400
T (K)
Fig. 2. Single adsorption equilibrium isotherms of CO2, CO, CH4, N2 and H2 adsorption and desorption equilibrium in binderless 5A zeolite at 305 K (points – experimental; lines – Toth model). 6
380
TT 2.3 cm
360
TT 13.2 cm
340
TT 24.2 cm
320
300 280
q / mmol.g-1
4
0
10000
15000
t (s)
305 K; this work; gravimetric 303 K; ref [60]; gravimetric
Fig. 4. Single breakthrough curve of 50% CO2 + 50% He in Binderless 5A zeolite at 308 K and 4 bar: top – molar flowrate at the outlet; bottom – temperature histories (points – experimental; lines – simulation).
303 K; ref [61]; gravimetric
2
5000
298 K; ref [62]; gravimetric 298 K; ref [63]; gravimetric 298 K; ref [63]; volumetric
Fig. 4 corresponds to the results of CO2 single adsorption breakthrough curve, adsorption and desorption are separated by the dashed vertical line; the solid lines correspond to the predicted breakthrough curve by the fixed-bed model. Comparing experimental and simulated results, one can say that they agree well. As we can see, the CO2 mass front begins to be detected only at 3340 s and demonstrates a high adsorbed amount of 5.2 mmol g−1, which is an important feature for industrial applications. Due to the very sharp CO2 isotherm, the desorption is rather unfavourable requiring a long period for complete regeneration of the bed, also predicted by the simulations. During adsorption a temperature increase of over 65 K is observed; this is mainly due to the high CO2 heat of adsorption. CH4, N2 and CO single component adsorption/desorption breakthrough experiments were also performed with binderless 5A zeolite in 50% of Helium (flow rate of 0.3 SLPM) at 308 K and a total pressure of 4 bar. The results obtained are presented respectively in Figs. S7, S8 and S9 of the Supporting information and it can be seen that the mass front history in the outlet stream and temperature histories are well predicted by the simulations. In order to study CO2 capture from gaseous mixtures with other components, binary studies were performed with CO2/CH4, CO2/N2 and CO2/CO, reconstituting diverse possible sources. In the case study of CO2 separation from a typical flue gas source, high percentages of N2 are present and binary breakthrough experiments with N2 and CO2 are the most relevant. Therefore, two different ratios (50%:50% and 83%:17%), in both cases at 4 bar and 308 K, were considered in our experimental work. Fig. 5 shows the N2eCO2 (50%:50%) binary
303 K; ref [64]; volumetric 0 0
1
2
P/bar
3
4
5
Fig. 3. Comparison of CO2 adsorption equilibrium data of the binderless 5A zeolite (this work) and binder containing materials.
3.3. Breakthrough curve experiments Single adsorption and desorption breakthrough experiments of CO2, N2, CH4 and CO in binderless 5A were performed with fifty percent of helium (total flow rate of 0.3 SLPM) at 308 K and a total pressure of 4 bar (experimental conditions in Table S1). A mathematical model to describe the adsorption on a fixed-bed column was used to predict the breakthrough curve results. The required mass, energy momentum balances were obtained by applying the following assumptions [65,66]: ideal gas behaviour, axial dispersed plug flow, film model for the external mass and heat transfer resistances, linear driving force model for the macropore and micropore mass transfer resistances, no temperature gradient inside the particles, axial heat conduction along the column wall and heat exchange of the column wall with the gas phase inside the bed and with the ambient air, Ergun equation valid locally. The mathematical model equations are given in Table S3. The values of the transport parameters of the model were calculated by correlations as described in Silva, et al. [67] and are presented in Table S4. The crystal diffusivity values were taken from the literature [60,61]. The adsorbent heat capacity was calculated by the following equation [68].
Cˆps (J / kgK ) = −0.0062Tp2 + 5.2124Tp − 69.73
(3) 229
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F (mmol/s)
P.A.P. Mendes et al.
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
N2
0
T (K)
model (lines). The experimental conditions such as pressure, feed composition and steps duration are presented in Table 4. During 1152 s, a stream with a constant composition of N2:CO2 (80:20) was fed to the bed which corresponds to the co-current pressurization step followed by the feed step once the high pressure is reached. Thereafter, the feed was stopped and a cocurrent depressurization (Dep. coc.) step was started with duration of 104 s to enhance the N2 recovery. The final two steps blowdown (204 s) and purge (198 s) allow the regeneration of the bed, producing a CO2 enriched stream. Cyclic steady state was achieved after 14 cycles. Regarding CO2 capture few figures of merit must be considered when a process is taken into consideration. These parameters are the purity of the N2 stream, enriched CO2 stream composition, CO2 recovery, amount of CO2 captured per mass of adsorbent and per time (productivity), and power consumption per mole of captured CO2. The energy consumption was calculated as described by Ribeiro, et al. [70]:
CO2
5000
t (s)
10000
15000
380
TT 2.3 cm
360
TT 13.2 cm
340
TT 24.2 cm
320
eel =
300 5000
t (s)
10000
nCO2MwCO2
1 η
tstep
∫
˙ gT nR
0
γ ⎡ ⎛ P1 ⎞ ⎢ γ − 1 ⎢ ⎝ P2 ⎠ ⎣ ⎜
γ−1 γ
⎟
⎤ − 1⎥ dt ⎥ ⎦
(4)
Where η is the efficiency, n˙ is the molar flow rate, Rg is the ideal gas constant, T is the inlet temperature, P1 and P2 are respectively the inlet and outlet pressure, γ is the ratio between the heat capacity of the gas mixture at constant pressure and the heat capacity of the gas mixture at constant volume, tstep is the step duration and nCO2 is the number of moles of CO2 produced during one cycle. For the pilot scale designed cycle these parameters were obtained by simulation: the purity of the N2 stream was 96.5% and the N2 recovery was of 82%. A purity of 64.2% and a recovery of 80% were achieved for the enriched CO2 stream. CO2 productivity was 45.3 kg/mbed3/h and the energy consumption was 1.4 MJ/kgCO2 considering efficiencies of 85% and 60% for the compressor (pressurization of the feed stream from 1 to 4 bar) and vacuum pump (blowdown and purge steps), respectively.
280 0
1
15000
Fig. 5. Binary breakthrough curve of N2:CO2 (50%:50%) in binderless 5A zeolite at 308 K and 4 bar: top – molar flowrate at the outlet; bottom – temperature histories (points – experimental; lines – simulation).
breakthrough, while the N2eCO2 (83:17) binary breakthrough and its temperature history is presented in Fig. S10 of the Supporting information. The breakthrough experimental results of both scenarios were well predicted by the simulations (solid lines). An overshoot of the N2 component is observed in Fig. 5. The observed behaviour was expectable taking into consideration the adsorption equilibrium isotherms of both components presented in Fig. 2. The higher affinity of CO2 at the experimental conditions leads to a displacement of a significant part of the adsorbed N2. Therefore, in the outlet stream values of molar flow rate above the feed value are observed until the CO2 rupture. In Table S1, the experimental conditions for other binary experiments are also presented: 60% CH4–40% CO2, 83% CH4–17% CO2 and 70% CH4–30% N2. The respective results are presented in the Supporting information (Figs. S11, S12 and S13). All binary breakthrough curves (mass and temperature fronts) were well predicted by the simulations (solid lines). Taking into account these results a cyclic PSA was designed and performed experimentally for a typical flue gas stream of N2:CO2 (80%:20%) to demonstrate the possibility of using the binderless 5A zeolite in a real industrial application.
3.5. PTSA cycles simulation A pressure and temperature swing adsorption process was designed for CO2 capture from flue gas. A stream with 10% CO2 in N2 at a flow rate of 1.5 SLPM and at 303 K was assumed. The dimensions of the column employed are 1 m long and 2.1 cm in diameter. A cycle with four steps, co-current pressurization, adsorption, heating and cooling, was designed (Fig. 10). It is considered that the cooling and heating of the bed is done through the column wall by circulation of respectively a cold (at 298 K) or hot (at Theating) stream. During the pressurization step, the feed stream is introduced in the bed and the pressure inside the bed increases to the high pressure of the cycle (Phigh). During the adsorption step, purified nitrogen is obtained. Afterwards, the bed is heated and depressurized to the low pressure (Plow) and a CO2 enriched stream is produced on the other end of the bed. Finally, the bed is
3.4. PSA experiment
5.0
A PSA cycle was designed and experimentally performed with the only aim of validating the global model for CO2 capture with the enhanced binderless 5A zeolite material. Figs. 6 to 9 show experimental results of the PSA experiment with a feed of 20% CO2 and 80% N2, at 308 K and 4 bar (17 cycles). Figs. 6 and 7 show, respectively, the pressure and outlet molar flow rate histories of the first four cycles, while Fig. 8 shows the temperature histories of the first 10 cycles. An individual cycle takes 1658 s, and in these figures the vertical dashed lines mark the beginning and end of each cycle. In Fig. 9, the outlet molar flow rate history at cyclic steady state is given (vertical lines separate the steps). The same mathematical model coupled with the appropriate boundary conditions for each step [59] was employed for the simulation of the PSA experiment. As can be observed in the figures, a good prediction of the experimental results was achieved by the
P (bar)
4.0 3.0 2.0 1.0 0.0 0
2000
4000
6000
t (s) Fig. 6. Pressure history of the first 4 cycles of the PSA experiment (points – experimental; black line – simulation; vertical dashed line – cycle separation).
230
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P.A.P. Mendes et al.
1.0
F (mol/s)
Table 6 Bed characteristics and operating condition of the PTSA cycle.
N2
0.8
CO2
0.6 0.4 0.2
Bed length (m)
1
Bed diameter (m) Bed porosity Feed stream flow rate (SLPM) Feed stream temperature (K)
2.1 × 10−2 0.4 1.5 303
0.0 0
1000
2000
3000
4000
5000
6000
recovery as well as the process productivity [69] are reported. The total thermal energy consumption is also given in Table 7. For these calculations, two terms were considered, the thermal energy required for the heating step (et) and the energy (eel) required for the vacuum. The first term was calculated by:
t (s) Fig. 7. Outlet molar flow rate history of the first 4 cycles of the PSA experiment (points – experimental; lines – simulation; vertical dashed line – cycle separation). 360
TT 24.2 cm TT 13.2 cm TT 2.3 cm
350
T (K)
340 330
Vw Cpw ρw (TH − TL) + Vcol εCpg (TH − TL) et =
320 310 290 1658
3316
4974
6632
8290
9948
11606 13264 14922 16580
t (s)
Fig. 8. Temperature histories of the first 10 cycles of the PSA experiment (points – experimental; lines – simulation; vertical dashed line – cycle separation).
1.0 N2
F (mol/s)
0.8
CO2
0.6 0.4 0.2 0.0 0
500
t (s)
1000
nCO2MwCO2
(5)
Where TH is the heating temperature, TL is the cooling temperature, Vw is the column wall volume, Cpw is the column wall heat capacity, ρW is the column wall density, Vcol is the column inner volume, ε is the bed porosity, Cpg and Cps are the gas phase and adsorbent heat capacities, ρap is the pellet density, ΔH are the heats of adsorption and Δq are the differences between the amounts adsorbed at the end and beginning of the heating step. The electric energy consumption was also calculated as described by Ribeiro, et al. [70] considering efficiencies of 85% and 60% for the compressor (pressurization of the feed stream) and vacuum pump and then an equivalence for the thermal energy, eth = 3 eel. It can be seen that the CO2 recovery values are within 81 and 98% and the energy consumption between 5.7 and 3.8 MJ/kgCO2. The best results are obtained with a high pressure of 3.5 bar, a low pressure of 0.3 bar and a heating temperature of 403 K. For these operating conditions the lowest value of energy consumption is obtained (3.8 MJ/ kgCO2) with a high CO2 recovery (94.8%). Indeed this process is able to achieve high values for both the purity and recovery contrary to the majority of the PSA processes [53]. PSA processes are not very efficient for CO2 capture when zeolites; e.g. 13X and 5A, are employed as adsorbents, since the regeneration of the bed is hard to attain only by modulating the pressure. Clause et al. [53] reported a comparison for different processes (amine absorption, PSA, and TSA), furthermore Joss and co-workers [71] presented an extensive study of TSA process using 13 X zeolite. In a summarized way one can say that the reported values in this work are lower than the amine absorption process that requires 4.2–4.8 MJ/kgCO2 to obtain CO2 with a purity of 99% and recovery of 98% [53]. For TSA processes, the reported values are 3.22 MJ/kgCO2 to obtain CO2 with a purity of 95% and recovery of 81% with 5A zeolite [53], and 3.2–4.2 MJ/kgCO2 to obtain CO2 with a purity of 66–93% and recovery above 30% with 13X zeolite [71]. Therefore, the values
300 0
+ Vcol (1 − ε )(Cps (TH − TL) + ρap ∑ ΔqΔH )
1500
Fig. 9. Outlet molar flow rate history of the PSA experiment at cyclic steady state (points – experimental; lines – simulation; vertical dashed line – step separation).
closed (no inlet or outlet streams) and cooled. Equal step times for the pressurization plus adsorption, heating and cooling steps (tpre+ads=theat=tcool=tstep) was considered which implies the use of a three column process for the continuous consumption of feed stream. The bed properties and operating conditions used are summarized in Table 6. The performance of the process was assessed considering different values for the high and low pressure, the heating temperature and the steps duration. Table 7 presents results that obeyed to the CO2 purity constraint ( > 95%). The values of the N2 and CO2 purity and
Fig. 10. PTSA cycle for CO2 capture.
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Table 7 Simulation results of the PTSA process. Phigh (bar)
Plow (bar)
Theating (K)
tstep (s)
N2 Purity (%)
N2 Recovery (%)
CO2 Purity (%)
CO2 Recovery (%)
Productivity (kg/mbed3/ h)
Energyth consumption (MJ/ kgCO2)
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 3.5 3.5
1.0 0.5 0.3 0.3 0.3 1.0 0.3 0.3 0.3 0.3 0.3
423 423 423 403 383 423 423 403 383 423 403
6500 6500 7500 6500 6500 5000 6500 5500 5000 8000 7500
98.0 99.7 99.8 99.7 98.3 97.9 99.7 99.4 98.4 99.8 99.4
99.5 99.5 99.6 99.5 99.5 99.5 99.6 99.5 99.5 99.5 99.5
95.0 95.3 96.1 95.3 95.2 95.0 96.3 95.5 95.2 95.6 95.2
81.7 97.0 98.0 96.9 84.7 81.2 97.1 94.7 85.6 98.4 94.8
13.8 16.3 16.5 16.3 14.2 13.7 16.3 15.9 14.4 16.5 15.9
4.8 4.4 4.2 4.0 3.8 5.7 4.6 4.5 4.4 4.0 3.8
We thank the anonymous reviewers whose comments helped improve and clarify this manuscript.
reported in Table 7 are in general better than the reported by the two previously mentioned studies. Although PSA processes present lower energy consumption, both targets (purity and recovery) cannot be achieve at same time [53]. Regarding productivity Clause et al. [53] presents a value of 25.7 kg/mbed3/h for the best cycle, slightly higher than the values reported in Table 7, however one must draw attention to the fact that the achieved recovery on the mentioned cycle was only 81% [53]. While, Joss, et al. [71] presents productivities ranging from 42 to 92 kg/mbed3/h, but the recovery is not clearly mentioned, just reporting that it was above 30%.
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4. Conclusions A cyclic pressure swing adsorption (PSA) test was performed for a N2eCO2 (80:20) binary feed at 308 K and 4 bar using the adsorbent binderless 5A zeolite with the aim of capturing the CO2 from a typical synthetic flue gas stream. Single adsorption equilibrium isotherms were measured and described by the Toth model. Breakthrough curves for single and binary feeds have been measured and a mathematical model which describes the dynamic behaviour of multicomponent adsorption in a fixed bed was implemented in gPROMS (v4.0) environment to simulate the breakthrough experiments. The studied components are CO2 and the components that are normally presents in biogas, natural-gas, syngas and flue gas, which are CH4, CO, N2 and H2. The single component adsorption equilibrium isotherms demonstrated the following adsorption capacities order from high to lower adsorption capacity: CO2 > CH4 > N2 > CO > H2, and were well fitted by Toth model. CO2 single breakthrough demonstrates an adsorption capacity of 5.2 mmol/g and high retention times, which is conveniently to the industrial applications. The PSA experiment showed that cyclic steady state was reached after 14 cycles with purity and recovery values of 64.2 and 80% for CO2 and adsorbent productivity of 45.3 kg/mbed3/h, while the energy consumption was 1.4 MJ/kgCO2. The nitrogen stream was obtained with a purity of 96.5% and a recovery of 82%. A PTSA cycle with four steps was designed and simulation results showed that it is possible to increase the concentration of a flue gas stream with 10% CO2 to 95.2% with a CO2 recovery of 94.8%, a productivity of 15.9 kg/ mbed3/h and an energy consumption of 3.8 MJ/kgCO2. These results show that binderless 5A zeolite exhibits the capacity to separate the CO2 from N2 by a PTSA process; with fairly good performance parameters (purity, recovery, and productivity) when compared with other materials and/or processes. Acknowledgements This work was financially supported by: Project POCI-01-0145FEDER-006984 – Associate Laboratory LSRE-LCM funded by FEDER funds through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT – Fundação para a Ciência e a Tecnologia. 232
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Separation of CO2/N2 on binderless 5A zeolite a
a,⁎
b
a,⁎
MARK
Patrícia A.P. Mendes , Ana M. Ribeiro , Kristin Gleichmann , Alexandre F.P. Ferreira , Alírio E. Rodriguesa a Laboratory of Separation and Reaction Engineering – Laboratory of Catalysis and Materials (LSRE-LCM), Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua do Dr. Roberto Frias, S/N, Portugal b Chemiewerk Bad Köstritz GmbH, Heinrichshall 2, 07586 Bad Köstritz, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Binderless 5A zeolite Flue gases CO2 capture PTSA
The study of CO2 separation from typical flue gas streams containing other gases like N2, CO, H2 and CH4 by Pressure Swing Adsorption (PSA) process using binderless 5A zeolite beads as adsorbent is the main objective of this work. Firstly, single component adsorption equilibrium isotherms of CO2, CH4, N2, CO and H2 have been determined by gravimetric technique in a magnetic suspension microbalance (Rubotherm®) at three different temperatures (305, 333, and 363 K), up to 5 bar. A comparison between the obtained adsorption equilibrium data and reported values in the literature for other 5A zeolite adsorbents was performed. Dynamic studies were also done through single and binary breakthrough curves of N2, CO, H2 and CH4 at 4 bar and 308 K. A PSA experiment was performed, at pilot scale, with the aim of validating the developed PSA model for CO2 capture with the binderless 5A zeolite adsorbent. The adsorption equilibrium isotherms were fitted with the Toth model and breakthrough curves and the PSA experiment were simulated taking into account the adsorption dynamic behaviour of a fixed bed. CO2 recovery values between 81 and 98% and energy consumption between 5.7 and 3.8 MJ/kgCO2, for purification of a stream containing 10% of CO2 by different PTSA cycles were attained. For all designed PTSA cycles the product purity was higher than 95% in CO2; and the productivity was always above 13 kg/mbed3/h. The best results are obtained with a high pressure of 3.5 bar, a low pressure of 0.3 bar and a heating temperature of 403 K. For these operating conditions the lowest value of energy consumption is obtained (3.8 MJ/kgCO2) with a high CO2 recovery (94.8%) and high CO2 purity (95.2%). The achieved productivity was of 16 kg/mbed3/h.
1. Introduction
reducing the CO2 emissions to levels below 80 percent of the 1990 emissions, during the period 2013–2020 [6]. According to the United Nations, the amount of carbon dioxide in the atmosphere increases 2 ppm per year in the world and according to the International Panel on Climate Change (IPCC) the CO2 amount will be about 570 ppm in the year 2100, therefore, ways to reduce CO2 emissions are particularly necessary. There are four significant CO2 sources that need special attention: biogas, natural-gas and syngas as pre-combustion sources and flue gas as post-combustion source; due to their high CO2 contribution. Biogas is a renewable combustible gas mixture produced by anaerobic decomposition of organic matter (biomass) in a short period of time composed predominantly by the combustible CH4 and non-combustible CO2. Domestic garbage landfills are an example of biogas production. Syngas is an abbreviation of synthesis gas, which consists mainly of hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2) [7,8]. This fuel can be used, for electricity production, or hydrogen
In a constantly changing world, where the industry creates innovative and advanced products, it’s also important to take into account the footprint that is being left behind for future generations. Indeed, the emission of greenhouse gases (GHG), such as, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFC’s), sulphur hexafluoride (SF6), and nitrogen trifluoride (NF3) is leading to the global warming. The high amount of CO2 emitted to atmosphere contributes to more than 60% of the global warming; and 80% of this CO2 production comes from anthropogenic sources [1,2]. The CO2 emissions to the atmosphere originate from fuel combustion activities, industrial processes (e.g. cement, hydrogen production, steel production, petroleum refining, gas refining), and biogas/natural/shale gas processing [3–5]. To reduce GHG emissions, a second commitment period of the Kyoto protocol was implemented on December 8th 2012, with the aim of
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Corresponding authors. E-mail addresses: [email protected] (A.M. Ribeiro), [email protected] (A.F.P. Ferreira).
http://dx.doi.org/10.1016/j.jcou.2017.05.003 Received 11 July 2016; Received in revised form 11 April 2017; Accepted 4 May 2017 Available online 07 June 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.
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Nomenclature b n P q
qsat R t T Tp ΔH
Affinity parameter (bar−1) Toth heterogeneity parameter (−) Pressure (bar) Total adsorbed amount (mmol g−1)
Maximum adsorption capacity (mmol g−1) Ideal gas constant (kJ K−1 mol−1) Time (s) Temperature (K) Temperature of the particle (K) Adsorption enthalpy (kJ mol−1)
combustion processes, either using coal or other fossil fuels [29,30]. Cryogenic CO2 capture using dynamically operated packed beds [31,32], the so called CryoCell® technology, removes CO2 from natural gas, avoiding limitations of the conventional acid gas treatment processes [33]. Recently advanced cryogenic air separation process for oxy-combustion effluents, based on self-heat recuperation technology, with energy reduction of 20.2%, has been introduced as a promising technology [34]. The separation/capture of CO2 by adsorption can be by chemical adsorption if a chemical reaction occurs at the exposed surface or by physical adsorption [35,36]. Chemical or physical adsorption is simply determined by the adsorbent and operating conditions used. Calcium oxide, metal oxides, lithium zirconium oxides, lithium salts or hydrotalcites (HT) are generally considered if chemical adsorption is targeted. Activated carbons, metal-organic-frameworks (MOFs), and zeolites are usually considered for physical adsorption [37,38]. There are different adsorption based technologies, mainly differentiated by its regeneration principle: Pressure Swing Adsorption (PSA) [39–43], Vacuum Pressure Swing Adsorption (VPSA) [44,45], Temperature Swing Adsorption (TSA) [46], and Electric Swing Adsorption (ESA) [47,48]. Recent studies reported microwave assisted vacuum swing adsorption process (MVSA) [49], where a hybrid system was used in the CO2 desorption steps in order to improve the performance. ESA is an interesting methodology, where an electric current is applied on the adsorbent, after the CO2 adsorption, in order to heat the bed. This corresponds to the electrification step and afterwards an inert stream is introduced counter-currently to produce a CO2 concentrated stream [50–52]. A broad range of different adsorption processes combining different steps and different adsorbents can offer an interesting line for the development of new CO2 separation/capture solutions. That is to say, the nexus adsorbent-process cannot be disassociated, and all research and development studies in the adsorption area must be integrated, having in mind adsorbent and process. Adsorption based technologies are applicable for CO2 separation and capture, from dilute to high CO2 concentration streams, with low energy consumption [12]. To conclude, post-combustion capture processes can be used in coalfired and gas-fired plants, the available technology is more mature than
production; and as feedstock to chemical processes such as the FischerTropsch (FT) process. Flue gas is the fuels combustion off-gas stream, mainly constituted by CO2, CO, and N2 [9]. In order to capture the CO2 from the target streams, it is important to know the typical streams composition. Table 1 shows the typical compositions of general biogas, landfill gas, natural gas [10], syngas [7] and flue gas [4,9,11]. There are different available technologies for the CO2 capture and separation with different industrial applications. In a general view, we can have CO2 separation/capture by absorption, adsorption, cryogenic distillation and membrane separation as conventional methods [4,12–14]. Table 2 shows a summary of industrial applications using the conventional methods [4,15]. In the absorption separation, the CO2 is dissolved in a solvent, which can be dimethyl ether or propylene glycol (Selexol process), glycol carbonate, methanol (Rectisol process), N-methylpyrrolidone (Purisol process) morpholine (Morphysorb process), and propylene carbonate (Fluor process); they all are physical absorption examples that use organic solvents. In chemical absorption, the solvents that are usually used are alkanolamines (monoethanolamine-MEA, diethanolamine-DEA, and methyl diethanolamine-MDEA) [16,17], amino acids, ammonia, ionic liquids, and aqueous piperazine that are alkaline solvents applied in the acid-base neutralization required for the chemical absorption [2,12,18–20]. Another method is the separation of CO2 by membranes, which allow the passing of CO2 and exclude other components from the mixtures to be separated [21]. Different materials can be used resulting in membranes with different structures. The different membranes can be categorized in inorganic membranes (porous membranes: zeolite, carbon, alumina; or dense membranes: palladium and liquid-immobilized membranes), polymeric membranes (polyacetylenes, polycarbonates, polyamides, amino groups, polymethacrylates), and mixed matrix membranes [12,13,22–27]. Cryogenic distillation is also used for CO2 separation at low temperature and high pressures, where the CO2 is separated from the other components based on the vapour-liquid equilibrium of the mixture and their different boiling points [12,28]. New developments on this method have been appearing under the framework of the CANMET’s pilot-scale of CO2 cryogenic capture and compression unit (CO2CCU). It can be coupled with oxy-fuel
Table 1 Typical composition of global biogas and landfill gas, syngas, natural gas and flue gas, syngas and flue gas [4,7,9–11]. Gas
CH4 (%) Other Hydrocarbons (%) H2 (%) CO2 (%) CO (%) N2 (%) O2 (%) H2S (ppm) NH3 (ppm) H2/CO ratio NO2 (%) NO (%) SO2 (%) Ash (%)
Biogas
Syngas
H2-rich
General gas
Landfill gas
Slurry feed
Dry feed
H2 lean
60–70 – – 30–40 – ∼0.2 – 0–4000 100 – – – – –
65 – 0–3 15–50 – 5–40 0–5 0–100 5 – – – – –
– – 35 14 50 1 – – – 0.70 – – – –
– – 28 3 64 2 – – – 0.44 – – – –
– – 1 21 76 2 – – – 0.013 – – – –
225
– – 95 1 1 3 – – – 95 – – – –
Natural gas
90 9 – 1 0.3 – – 3 – – – – – –
Flue gas Fuel Oil
Natural gas
Coal
– – – 12–14 – 78–80 2–6 – – – – – – –
– – – 10–12 70–110 ppm 78–80 2–3 – – – – – – –
– – – 10.6 5.579 ppm 78–80 7 – – – 1 1 > 2 ppm 12
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Table 2 Industrial applications of different technologies for the CO2 separation/capture [4,15]. Industrial applications
Absorption
Removal of CO2 to upgrade natural gas Removal of CO2 from flue gas CO2 separation in H2 production Flue gas separations CO2 liquefaction from gas wells
Adsorption
Membranes
Cryogenic distillation
Chemical
Physical
PSA/VSA
TSA
Inorganic
Organic
✓ ✓
✓ ✓
✓
✓
✓
✓
✓ ✓
✓
✓
✓
✓
✓
Chemiewerk Bad Köstritz, which the commercial name is KÖSTROLITH® 5A BFK. The main reason to choose this binderless 5A zeolite, is the fact of its enhanced capacity, making it an interesting material to study, and compare its performance with existing adsorbents, such as 13 X zeolite. The adsorbent characterization was performed by X-ray diffraction (XRD) at the Laboratório Central de Análises, Universidade de Aveiro; particle crushing strength that was performed at Delft Solids Solutions, Delft; porosimetry by mercury intrusion was performed at the Laboratório de Ensaios, Desgaste e Materiais, Instituto Pedro Nunes, Coimbra; N2 (at 77 K) and CO2 (at 273 K) adsorption measurements to determine the BET surface area and micropores size distribution was performed in Universidad de Málaga; Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS) was performed at the Centro de Materiais da Universidade do Porto (CEMUP), Porto. X-ray diffraction pattern was collected in an Empyrean diffractometer using Cu K-α1 radiation (λ ∼ 1.54056 Å) from 0 to 70° (2θ) with a step size of 0.0260 and 56.8650 s per step in continuous mode. The N2 and CO2 adsorption equilibrium isotherms were measured using about 0.21 g of adsorbent in a Micromeritics ASAP 2020 Surface Area and Porosity Analyser. The crushing strength was measured in a Dr Schleuniger type 5Y tablet hardness tester with a measuring range of 1-400 N. A high resolution (Schottky) Environmental Scanning Electron Microscope with X-Ray Microanalysis and Electron Backscattered Diffraction analysis: Quanta 400 FEG ESEM/EDAX Genesis X4 M was used to perform SEM and EDS analyses. In this analyses, the sample was previously coated with an Au/Pd thin film, by sputtering, using the SPI Module Sputter Coater equipment. A magnetic suspension microbalance (Rubotherm®, Germany) was used to assess the adsorption equilibrium. The equipment details and
other alternatives; and can easily be retrofitted into existing plants; the main disadvantage is low efficiency due to the low CO2 concentration in the streams to be treated. In Table 3 is presented a comparison of different CO2 separation methods for post-combustion treatment of flue gases. A detailed overview of CO2 capture and storage technologies can be found in Leung, et al. [21]. The main objective of this work is to design a PTSA process for CO2 capture using a commercial zeolite. Therefore, a bindereless 5A zeolite, with enhanced capacity, was studied as adsorbent for CO2 separation and capture. The capacity gain by not having binder was assessed by measuring single component adsorption equilibrium isotherms of CO2, CH4, N2, CO and H2 with a magnetic suspension microbalance. Furthermore, single and multicomponent breakthrough curves at 4 bar and 308 K were performed to study the fixed-bed adsorption dynamics. A 5-steps PSA cycle was designed and experimentally performed with the aim of validating the global model for CO2 capture with the enhanced binderless 5A zeolite material. The adsorption equilibrium was well described using the Toth model. A PTSA process for CO2 capture from a stream containing 10% CO2 was designed and optimized. The breakthrough curves and the cyclic processes were simulated with gPROMS ModelBuilder software package 2. Experimental section 2.1. Materials and equipment The sorbates used were CO2 (99.99%), CH4 (99.95%), N2 (99.95%) and CO (99.997%); the inert gas for adsorbent activation/regeneration was He (99.999%); all gases were supplied by Air Liquid. The selected adsorbent was a binderless 5A zeolite, supplied by
Table 3 Comparison of different separation technologies available for CO2 capture, based on Leung, et al. [21]. Technology Absorption
Adsorption
Chemical looping combustion
Advantage
Disadvantage
efficiency • High can be regenerated • Sorbents • Most mature technology efficiency • High can be regenerated and the process • Adsorbent operates in a continuous cyclic way is the main combustion product, which remains • CO2 unmixed with N2 thus avoiding energy intensive separation
Membrane separation
Hydrate-based separation Cryogenic Distillation
efficiency • High amount of knowledge existing from • Considerable other gas mixtures separation • Low energy requirements technology • Mature already operational in industry for many • Processes years for CO recovery. 2
Purity and Recovery
highly depends on CO concentration • Absorption amount of heat is required for the • Substantial sorbent regeneration leaching/degradation into the environment • Amine has to be better understood adsorbents that can be efficient at • Requires different temperatures, from ambient to high 2
temperatures.
energy requirements for the solid • High regenerations, i.e. CO2 desorption. mature technology, still under development • Not and no large scale processes in operation yet. Low fluxes and fouling, or high fluxes but low • selectivity.
• Still under development technology, still very new. viable for very high CO2 concentration • Only temperatures required/refrigeration loops are • Low needed. • Very energy intensive 226
Pur: 99% & Rec: 98% [53]
PSA: Pur: 50–99% & Rec: 30–90% [53] TSA: Pur: 95% & Rec: 80% [53] ESA: Pur: 20% & Rec: 93% [53] Pur: ∼100% & Rec: 95% [54]
Pur: > 95% & Rec: 90% [55]
Pur: n.a. & Rec: 87% [56] Pur: 99.9% & Rec: 90% [57]
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parameters qsat, b0, ΔH, and n were optimized by the Excel solver optimization tool.
operating procedure are described elsewhere [58]. The adsorption dynamic studies and the Pressure Swing Adsorption (PSA) cycles were performed in a home-built PSA set-up equipped with a stainless steel column. The temperature was monitored by type-K thermocouples placed in the bottom, middle and top of the column. The outlet flow rate is measured by a mass flow meter (MFM) at the exit of the system; the feed flow rate and composition are controlled by three mass flow controllers (MFC) at the entrance of the system with a maximum capacity of 1, 5 and 0.2 SLPM, respectively. The pressure is controlled by a back pressure controller (BPC). The fluid path is controlled by Solenoid valves that afterwards can be conducted to the exhaust or to the Gas Chromatography (GC −Varian CP-3800) analysis system, which allows the determination of the composition of the outlet stream. The GC unit is connected to the fixed-bed set-up by a loop collector (LP) valve and a six-ports, two position valve that can control the collecting (if the valve is in storage mode) or analysis of the sample by the GC (if the valve is in analyses mode). The loop collector has 15 loops, and each sample trapped in the loops is analysed in a packed column (Supelco 1-2390-U) located into the GC oven at 483 K. The sample molar fraction in each loop is quantified by means of a Thermal Conductivity Detector (TCD), because TCD detector responds to all target compounds. After TCD detection, a signal (mV) is sent to the computer and a relation between the signal detected and the physical concentration is obtained by means of a calibration curve specific for each loop, taking in account the possible small differences in the loop’s volumes. The equipment detailed description can be found elsewhere [59] and is presented in Figs. S1and S2.
q = qsat
bP 1
[1 + (bP )n]
(1)
n
−ΔH ⎞ b = b0 exp ⎛ ⎝ RT ⎠
(2)
−1
−1
q (mmol g ) is the total adsorbed amount, qsat (mmol g ) is the maximum adsorption capacity, b (bar−1) is the affinity parameter, P (bar) is the pressure in the system, n (−) is the Toth heterogeneity parameter, ΔH (kJ mol−1) is the adsorption enthalpy, T (K) is the system’s temperature, and R (kJ K−1 mol−1) is the ideal gas constant. 2.3. Breakthrough curves experimental protocol Single and multicomponent breakthrough curves were performed in a fixed bed column set-up packed with binderless 5A zeolite in order to study the adsorption dynamics. The column (Øint = 2.1 cm) was filled first with glass beads in the bottom, followed by a layer of glass wool, and then 79.8 g of zeolite binderless 5A (adsorbent layer length of 31.7 cm). Finally, the column was topped with a layer of glass wool (see Fig. S3). Subsequently, the adsorbent was activated at 520 K during 15 h, under He flow. After the activation, a continuously feed of the gaseous stream was introduced into the column and the adsorption dynamic behaviour is studied by means of the temperature, total flowrate, and outlet composition histories. The column outlet composition was analysed by GC analyses. When the steady-state is reached the adsorbent is regenerated. In order to regenerate the bed, helium flow was fed into it co-currently. Subsequently, the helium flow was kept overnight after the end of the experiment, in order to guarantee full regeneration. A set of single and binary breakthrough experiments was performed at 4 bar and 308 K, the experimental conditions of each run are showed in Table S1.
2.2. Adsorption equilibrium isotherm measurement procedure Single component adsorption equilibrium isotherms have been measured, in a magnetic suspension microbalance (Rubotherm, Bochum, Germany) to determine the adsorption capacity of CO2, CH4, CO, N2 and H2 in a shaped binderless 5A zeolite. A sample of 3.7 g of binderless 5A zeolite was placed in a basket that is suspended by an electromagnet. The sample was activated at 588 K during 12 h. After activation, the weight loss was of 9% and subsequently the adsorption equilibrium isotherms were measured at 305, 333, 363 K and in a pressure range from 0 to 5 bar. The system operates in batch mode, thus the equilibrium is reached when the pressure and the mass are constant (30 min at least). The adsorption equilibrium isotherms were described by the Toth model, which is shown in Eqs. (1) and (2) [35]. The
2.4. Pressure/vacuum swing adsorption pilot-scale cycle and experimental protocol A Pressure Vacuum Swing Adsorption (PVSA) cycle was designed to be tested in the pilot-scale unit with the aim of capturing CO2 from a mixture of N2/CO2 (synthetic flue gas). The cycle consists of five steps, having the following order: Pressurization (cocurrent), Adsorption,
Table 4 Steps, experimental conditions of pressure, feed composition and steps duration. STEP
Pressurization Coc.
Pressure (bar) Duration (s) Feed (SLPM) Feed composition (%)
0.35 → 4
Adsorption
Depressurization Coc.
Blowdown
Purge
4
4 → 1.1 104 – –
1.1 → 0.16 204 – –
0.35 198 0.2 100 N2
1152 0.38 80 N2 20 CO2
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zeolite beads have a pellet density of 1300 kg/m3. The complete results of textural characterization are presented in Table S2. The crushing strength test of 63 representative particles of the binderless 5A zeolite revealed an average strength of 10.1 ± 3.6 N (95% confidence interval).
Depressurization (cocurrent), Blowdown and Purge, as presented in Table 4. The light component is produced during the adsorption and cocurrent depressurization steps, while the heavy component is produced in the last two steps. To perform the designed cycle two streams of N2 and CO2, previously mixed, were fed to the column, in order to simulate a typical flue gas composition at 308 K and 4 bar. The column was initially filled with N2 also at 308 K and 4 bar. After pressurization of the column to 4 bar, the feed step begins, the two steps together last for 1152 s. Subsequently, the depressurization to 1.1 bar occurs in cocurrent mode, during 104 s. In the Blowdown (counter-current, 0.16 bar) and Purge steps, regeneration is performed. A stream of N2 (0.2 SLPM) is introduced as purge for 198 s. Samples of the outlet stream were collected at desired times of a step during one cycle and posteriorly analysed. When the temperature, pressure and composition histories of a cycle are repeated in the following cycle, the cyclic steady state (CSS) is achieved. In this experiment, a total of 17 cycles were performed.
3.2. Adsorption equilibrium isotherms As previously referred, the adsorption equilibrium isotherms were fitted with the Toth model and the parameters obtained from the fitting are presented in Table 5. The highest adsorption enthalpy was obtained for CO2. Fig. 2 shows the experimental adsorption equilibrium isotherms on Binderless 5A zeolite at 305 K for CO2, CH4, N2, CO and H2 adsorption/ desorption (filled/open symbols) and the adsorption equilibrium isotherms fitted with the Toth model (represented by solid lines). In the case of H2, it was only performed the experimental adsorption equilibrium. The results per component for the three temperatures are presented in Fig. S5. A reversibility between the adsorption and desorption of all components is observed, so, no hysteresis phenomena is observed. As we can see in the figure, the carbon dioxide demonstrates higher adsorption capacity and the order from higher adsorption capacity to lower adsorption capacity is: CO2 ≫ CO > CH4 > N2 > H2. Furthermore, this binderless material also shows higher CO2 capacity when compared with binder containing materials (binder around 20%) as can be seen in Fig. 3. The comparison was done for adsorption equilibrium data obtained at around the same temperature (305 K) using gravimetric methods [60–63] and a volumetric method [63,64]. A maximum CO2 loading capacity of 5 mmol g−1 was obtained in the binderless beads of the 5A zeolite, which demonstrates a visible difference compared with the tendency of 4 mmol g−1 and lower of the binder containing materials. However, correcting the capacity for the binder content (∼20%) a good agreement between the different materials is obtained (see Fig. S6). Additionally, in Fig. S14 and Fig. S15, the adiabats as well as the adiabatic operation line are shown. In these figures is possible to get a general idea of how adiabatic operation can affect the working capacity.
3. Results and discussion 3.1. Binderless 5A zeolite beads characterization Fig. 1 shows SEM photographs of the binderless 5A beads, where it is possible to notice that the chosen bead has a diameter of 1.6 mm, which is in accordance with the provider’s specification of 1.0 −2.0 mm range for particle size; and crystal diameter presents values between 0.6 μm and 1.5 μm. EDS analyses showed the presence of oxygen, sodium, aluminium, silicon, and calcium, which is in agreement with the expected composition of the 5A zeolite. The observed presence of Na, can be attributed to an incomplete ion exchange during the synthesis of 5A binderless zeolite from the original 4A zeolite. The crystalline structure was confirmed by XRD, and Fig. S4 shows the comparison between the 5A zeolite reference powder pattern (blue) and our sample (red). The peak of the experimental sample corresponds to the Bragg peaks of 5A zeolite pattern. Textural analysis was performed by means of N2 adsorption at 77 K and CO2 adsorption at 273 K. The values of 526 m2 g−1 and 0.25 cm3 g−1 were obtained for the BET surface area and micropore volume, respectively. The binderless 5A
Fig. 1. SEM photographs of binderless 5A zeolite with the following magnifications: (a) 100, (b) 1000, (c) 10000 and (d) 50000×.
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Table 5 Fitting parameters of the Toth isotherm.
qsat (mmol.g−1) b0 (bar−1) −ΔHads (kJ mol−1) n (−)
CO2
CH4
N2
H2
CO
5.11 2.1 × 10−5 38.1 0.61
4.28 2.0 × 10−4 17.2 1.09
5.42 3.3 × 10−4 13.4 2.95
2.10 2.3 × 10−2 3.8 0.55
3.25 3.5 × 10−5 24.4 0.91
0.25 CO2
F (mmol/s)
0.20
0.15 0.10 0.05 0.00 0
5000
10000
15000
t (s)
400
T (K)
Fig. 2. Single adsorption equilibrium isotherms of CO2, CO, CH4, N2 and H2 adsorption and desorption equilibrium in binderless 5A zeolite at 305 K (points – experimental; lines – Toth model). 6
380
TT 2.3 cm
360
TT 13.2 cm
340
TT 24.2 cm
320
300 280
q / mmol.g-1
4
0
10000
15000
t (s)
305 K; this work; gravimetric 303 K; ref [60]; gravimetric
Fig. 4. Single breakthrough curve of 50% CO2 + 50% He in Binderless 5A zeolite at 308 K and 4 bar: top – molar flowrate at the outlet; bottom – temperature histories (points – experimental; lines – simulation).
303 K; ref [61]; gravimetric
2
5000
298 K; ref [62]; gravimetric 298 K; ref [63]; gravimetric 298 K; ref [63]; volumetric
Fig. 4 corresponds to the results of CO2 single adsorption breakthrough curve, adsorption and desorption are separated by the dashed vertical line; the solid lines correspond to the predicted breakthrough curve by the fixed-bed model. Comparing experimental and simulated results, one can say that they agree well. As we can see, the CO2 mass front begins to be detected only at 3340 s and demonstrates a high adsorbed amount of 5.2 mmol g−1, which is an important feature for industrial applications. Due to the very sharp CO2 isotherm, the desorption is rather unfavourable requiring a long period for complete regeneration of the bed, also predicted by the simulations. During adsorption a temperature increase of over 65 K is observed; this is mainly due to the high CO2 heat of adsorption. CH4, N2 and CO single component adsorption/desorption breakthrough experiments were also performed with binderless 5A zeolite in 50% of Helium (flow rate of 0.3 SLPM) at 308 K and a total pressure of 4 bar. The results obtained are presented respectively in Figs. S7, S8 and S9 of the Supporting information and it can be seen that the mass front history in the outlet stream and temperature histories are well predicted by the simulations. In order to study CO2 capture from gaseous mixtures with other components, binary studies were performed with CO2/CH4, CO2/N2 and CO2/CO, reconstituting diverse possible sources. In the case study of CO2 separation from a typical flue gas source, high percentages of N2 are present and binary breakthrough experiments with N2 and CO2 are the most relevant. Therefore, two different ratios (50%:50% and 83%:17%), in both cases at 4 bar and 308 K, were considered in our experimental work. Fig. 5 shows the N2eCO2 (50%:50%) binary
303 K; ref [64]; volumetric 0 0
1
2
P/bar
3
4
5
Fig. 3. Comparison of CO2 adsorption equilibrium data of the binderless 5A zeolite (this work) and binder containing materials.
3.3. Breakthrough curve experiments Single adsorption and desorption breakthrough experiments of CO2, N2, CH4 and CO in binderless 5A were performed with fifty percent of helium (total flow rate of 0.3 SLPM) at 308 K and a total pressure of 4 bar (experimental conditions in Table S1). A mathematical model to describe the adsorption on a fixed-bed column was used to predict the breakthrough curve results. The required mass, energy momentum balances were obtained by applying the following assumptions [65,66]: ideal gas behaviour, axial dispersed plug flow, film model for the external mass and heat transfer resistances, linear driving force model for the macropore and micropore mass transfer resistances, no temperature gradient inside the particles, axial heat conduction along the column wall and heat exchange of the column wall with the gas phase inside the bed and with the ambient air, Ergun equation valid locally. The mathematical model equations are given in Table S3. The values of the transport parameters of the model were calculated by correlations as described in Silva, et al. [67] and are presented in Table S4. The crystal diffusivity values were taken from the literature [60,61]. The adsorbent heat capacity was calculated by the following equation [68].
Cˆps (J / kgK ) = −0.0062Tp2 + 5.2124Tp − 69.73
(3) 229
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F (mmol/s)
P.A.P. Mendes et al.
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
N2
0
T (K)
model (lines). The experimental conditions such as pressure, feed composition and steps duration are presented in Table 4. During 1152 s, a stream with a constant composition of N2:CO2 (80:20) was fed to the bed which corresponds to the co-current pressurization step followed by the feed step once the high pressure is reached. Thereafter, the feed was stopped and a cocurrent depressurization (Dep. coc.) step was started with duration of 104 s to enhance the N2 recovery. The final two steps blowdown (204 s) and purge (198 s) allow the regeneration of the bed, producing a CO2 enriched stream. Cyclic steady state was achieved after 14 cycles. Regarding CO2 capture few figures of merit must be considered when a process is taken into consideration. These parameters are the purity of the N2 stream, enriched CO2 stream composition, CO2 recovery, amount of CO2 captured per mass of adsorbent and per time (productivity), and power consumption per mole of captured CO2. The energy consumption was calculated as described by Ribeiro, et al. [70]:
CO2
5000
t (s)
10000
15000
380
TT 2.3 cm
360
TT 13.2 cm
340
TT 24.2 cm
320
eel =
300 5000
t (s)
10000
nCO2MwCO2
1 η
tstep
∫
˙ gT nR
0
γ ⎡ ⎛ P1 ⎞ ⎢ γ − 1 ⎢ ⎝ P2 ⎠ ⎣ ⎜
γ−1 γ
⎟
⎤ − 1⎥ dt ⎥ ⎦
(4)
Where η is the efficiency, n˙ is the molar flow rate, Rg is the ideal gas constant, T is the inlet temperature, P1 and P2 are respectively the inlet and outlet pressure, γ is the ratio between the heat capacity of the gas mixture at constant pressure and the heat capacity of the gas mixture at constant volume, tstep is the step duration and nCO2 is the number of moles of CO2 produced during one cycle. For the pilot scale designed cycle these parameters were obtained by simulation: the purity of the N2 stream was 96.5% and the N2 recovery was of 82%. A purity of 64.2% and a recovery of 80% were achieved for the enriched CO2 stream. CO2 productivity was 45.3 kg/mbed3/h and the energy consumption was 1.4 MJ/kgCO2 considering efficiencies of 85% and 60% for the compressor (pressurization of the feed stream from 1 to 4 bar) and vacuum pump (blowdown and purge steps), respectively.
280 0
1
15000
Fig. 5. Binary breakthrough curve of N2:CO2 (50%:50%) in binderless 5A zeolite at 308 K and 4 bar: top – molar flowrate at the outlet; bottom – temperature histories (points – experimental; lines – simulation).
breakthrough, while the N2eCO2 (83:17) binary breakthrough and its temperature history is presented in Fig. S10 of the Supporting information. The breakthrough experimental results of both scenarios were well predicted by the simulations (solid lines). An overshoot of the N2 component is observed in Fig. 5. The observed behaviour was expectable taking into consideration the adsorption equilibrium isotherms of both components presented in Fig. 2. The higher affinity of CO2 at the experimental conditions leads to a displacement of a significant part of the adsorbed N2. Therefore, in the outlet stream values of molar flow rate above the feed value are observed until the CO2 rupture. In Table S1, the experimental conditions for other binary experiments are also presented: 60% CH4–40% CO2, 83% CH4–17% CO2 and 70% CH4–30% N2. The respective results are presented in the Supporting information (Figs. S11, S12 and S13). All binary breakthrough curves (mass and temperature fronts) were well predicted by the simulations (solid lines). Taking into account these results a cyclic PSA was designed and performed experimentally for a typical flue gas stream of N2:CO2 (80%:20%) to demonstrate the possibility of using the binderless 5A zeolite in a real industrial application.
3.5. PTSA cycles simulation A pressure and temperature swing adsorption process was designed for CO2 capture from flue gas. A stream with 10% CO2 in N2 at a flow rate of 1.5 SLPM and at 303 K was assumed. The dimensions of the column employed are 1 m long and 2.1 cm in diameter. A cycle with four steps, co-current pressurization, adsorption, heating and cooling, was designed (Fig. 10). It is considered that the cooling and heating of the bed is done through the column wall by circulation of respectively a cold (at 298 K) or hot (at Theating) stream. During the pressurization step, the feed stream is introduced in the bed and the pressure inside the bed increases to the high pressure of the cycle (Phigh). During the adsorption step, purified nitrogen is obtained. Afterwards, the bed is heated and depressurized to the low pressure (Plow) and a CO2 enriched stream is produced on the other end of the bed. Finally, the bed is
3.4. PSA experiment
5.0
A PSA cycle was designed and experimentally performed with the only aim of validating the global model for CO2 capture with the enhanced binderless 5A zeolite material. Figs. 6 to 9 show experimental results of the PSA experiment with a feed of 20% CO2 and 80% N2, at 308 K and 4 bar (17 cycles). Figs. 6 and 7 show, respectively, the pressure and outlet molar flow rate histories of the first four cycles, while Fig. 8 shows the temperature histories of the first 10 cycles. An individual cycle takes 1658 s, and in these figures the vertical dashed lines mark the beginning and end of each cycle. In Fig. 9, the outlet molar flow rate history at cyclic steady state is given (vertical lines separate the steps). The same mathematical model coupled with the appropriate boundary conditions for each step [59] was employed for the simulation of the PSA experiment. As can be observed in the figures, a good prediction of the experimental results was achieved by the
P (bar)
4.0 3.0 2.0 1.0 0.0 0
2000
4000
6000
t (s) Fig. 6. Pressure history of the first 4 cycles of the PSA experiment (points – experimental; black line – simulation; vertical dashed line – cycle separation).
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1.0
F (mol/s)
Table 6 Bed characteristics and operating condition of the PTSA cycle.
N2
0.8
CO2
0.6 0.4 0.2
Bed length (m)
1
Bed diameter (m) Bed porosity Feed stream flow rate (SLPM) Feed stream temperature (K)
2.1 × 10−2 0.4 1.5 303
0.0 0
1000
2000
3000
4000
5000
6000
recovery as well as the process productivity [69] are reported. The total thermal energy consumption is also given in Table 7. For these calculations, two terms were considered, the thermal energy required for the heating step (et) and the energy (eel) required for the vacuum. The first term was calculated by:
t (s) Fig. 7. Outlet molar flow rate history of the first 4 cycles of the PSA experiment (points – experimental; lines – simulation; vertical dashed line – cycle separation). 360
TT 24.2 cm TT 13.2 cm TT 2.3 cm
350
T (K)
340 330
Vw Cpw ρw (TH − TL) + Vcol εCpg (TH − TL) et =
320 310 290 1658
3316
4974
6632
8290
9948
11606 13264 14922 16580
t (s)
Fig. 8. Temperature histories of the first 10 cycles of the PSA experiment (points – experimental; lines – simulation; vertical dashed line – cycle separation).
1.0 N2
F (mol/s)
0.8
CO2
0.6 0.4 0.2 0.0 0
500
t (s)
1000
nCO2MwCO2
(5)
Where TH is the heating temperature, TL is the cooling temperature, Vw is the column wall volume, Cpw is the column wall heat capacity, ρW is the column wall density, Vcol is the column inner volume, ε is the bed porosity, Cpg and Cps are the gas phase and adsorbent heat capacities, ρap is the pellet density, ΔH are the heats of adsorption and Δq are the differences between the amounts adsorbed at the end and beginning of the heating step. The electric energy consumption was also calculated as described by Ribeiro, et al. [70] considering efficiencies of 85% and 60% for the compressor (pressurization of the feed stream) and vacuum pump and then an equivalence for the thermal energy, eth = 3 eel. It can be seen that the CO2 recovery values are within 81 and 98% and the energy consumption between 5.7 and 3.8 MJ/kgCO2. The best results are obtained with a high pressure of 3.5 bar, a low pressure of 0.3 bar and a heating temperature of 403 K. For these operating conditions the lowest value of energy consumption is obtained (3.8 MJ/ kgCO2) with a high CO2 recovery (94.8%). Indeed this process is able to achieve high values for both the purity and recovery contrary to the majority of the PSA processes [53]. PSA processes are not very efficient for CO2 capture when zeolites; e.g. 13X and 5A, are employed as adsorbents, since the regeneration of the bed is hard to attain only by modulating the pressure. Clause et al. [53] reported a comparison for different processes (amine absorption, PSA, and TSA), furthermore Joss and co-workers [71] presented an extensive study of TSA process using 13 X zeolite. In a summarized way one can say that the reported values in this work are lower than the amine absorption process that requires 4.2–4.8 MJ/kgCO2 to obtain CO2 with a purity of 99% and recovery of 98% [53]. For TSA processes, the reported values are 3.22 MJ/kgCO2 to obtain CO2 with a purity of 95% and recovery of 81% with 5A zeolite [53], and 3.2–4.2 MJ/kgCO2 to obtain CO2 with a purity of 66–93% and recovery above 30% with 13X zeolite [71]. Therefore, the values
300 0
+ Vcol (1 − ε )(Cps (TH − TL) + ρap ∑ ΔqΔH )
1500
Fig. 9. Outlet molar flow rate history of the PSA experiment at cyclic steady state (points – experimental; lines – simulation; vertical dashed line – step separation).
closed (no inlet or outlet streams) and cooled. Equal step times for the pressurization plus adsorption, heating and cooling steps (tpre+ads=theat=tcool=tstep) was considered which implies the use of a three column process for the continuous consumption of feed stream. The bed properties and operating conditions used are summarized in Table 6. The performance of the process was assessed considering different values for the high and low pressure, the heating temperature and the steps duration. Table 7 presents results that obeyed to the CO2 purity constraint ( > 95%). The values of the N2 and CO2 purity and
Fig. 10. PTSA cycle for CO2 capture.
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Table 7 Simulation results of the PTSA process. Phigh (bar)
Plow (bar)
Theating (K)
tstep (s)
N2 Purity (%)
N2 Recovery (%)
CO2 Purity (%)
CO2 Recovery (%)
Productivity (kg/mbed3/ h)
Energyth consumption (MJ/ kgCO2)
2.5 2.5 2.5 2.5 2.5 1.5 1.5 1.5 1.5 3.5 3.5
1.0 0.5 0.3 0.3 0.3 1.0 0.3 0.3 0.3 0.3 0.3
423 423 423 403 383 423 423 403 383 423 403
6500 6500 7500 6500 6500 5000 6500 5500 5000 8000 7500
98.0 99.7 99.8 99.7 98.3 97.9 99.7 99.4 98.4 99.8 99.4
99.5 99.5 99.6 99.5 99.5 99.5 99.6 99.5 99.5 99.5 99.5
95.0 95.3 96.1 95.3 95.2 95.0 96.3 95.5 95.2 95.6 95.2
81.7 97.0 98.0 96.9 84.7 81.2 97.1 94.7 85.6 98.4 94.8
13.8 16.3 16.5 16.3 14.2 13.7 16.3 15.9 14.4 16.5 15.9
4.8 4.4 4.2 4.0 3.8 5.7 4.6 4.5 4.4 4.0 3.8
We thank the anonymous reviewers whose comments helped improve and clarify this manuscript.
reported in Table 7 are in general better than the reported by the two previously mentioned studies. Although PSA processes present lower energy consumption, both targets (purity and recovery) cannot be achieve at same time [53]. Regarding productivity Clause et al. [53] presents a value of 25.7 kg/mbed3/h for the best cycle, slightly higher than the values reported in Table 7, however one must draw attention to the fact that the achieved recovery on the mentioned cycle was only 81% [53]. While, Joss, et al. [71] presents productivities ranging from 42 to 92 kg/mbed3/h, but the recovery is not clearly mentioned, just reporting that it was above 30%.
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4. Conclusions A cyclic pressure swing adsorption (PSA) test was performed for a N2eCO2 (80:20) binary feed at 308 K and 4 bar using the adsorbent binderless 5A zeolite with the aim of capturing the CO2 from a typical synthetic flue gas stream. Single adsorption equilibrium isotherms were measured and described by the Toth model. Breakthrough curves for single and binary feeds have been measured and a mathematical model which describes the dynamic behaviour of multicomponent adsorption in a fixed bed was implemented in gPROMS (v4.0) environment to simulate the breakthrough experiments. The studied components are CO2 and the components that are normally presents in biogas, natural-gas, syngas and flue gas, which are CH4, CO, N2 and H2. The single component adsorption equilibrium isotherms demonstrated the following adsorption capacities order from high to lower adsorption capacity: CO2 > CH4 > N2 > CO > H2, and were well fitted by Toth model. CO2 single breakthrough demonstrates an adsorption capacity of 5.2 mmol/g and high retention times, which is conveniently to the industrial applications. The PSA experiment showed that cyclic steady state was reached after 14 cycles with purity and recovery values of 64.2 and 80% for CO2 and adsorbent productivity of 45.3 kg/mbed3/h, while the energy consumption was 1.4 MJ/kgCO2. The nitrogen stream was obtained with a purity of 96.5% and a recovery of 82%. A PTSA cycle with four steps was designed and simulation results showed that it is possible to increase the concentration of a flue gas stream with 10% CO2 to 95.2% with a CO2 recovery of 94.8%, a productivity of 15.9 kg/ mbed3/h and an energy consumption of 3.8 MJ/kgCO2. These results show that binderless 5A zeolite exhibits the capacity to separate the CO2 from N2 by a PTSA process; with fairly good performance parameters (purity, recovery, and productivity) when compared with other materials and/or processes. Acknowledgements This work was financially supported by: Project POCI-01-0145FEDER-006984 – Associate Laboratory LSRE-LCM funded by FEDER funds through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) – and by national funds through FCT – Fundação para a Ciência e a Tecnologia. 232
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