Carbon Capture by Physical Adsorption: Materials, Experimental Investigations and Numerical Modeling and Simulations

Applied Energy 161 (2016) 225–255

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Applied Energy

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Review Carbon capture by physical adsorption: Materials, experimental investigations and numerical modeling and simulations – A review ⇑ R. Ben-Mansour, M.A. Habib , O.E. Bamidele, M. Basha, N.A.A. Qasem, A. Peedikakkal, T. Laoui, M. Ali

Mechanical Engineering Department and KACST-TIC on CCS, KFUPM, Dhahran 31261, Saudi Arabia highlights

A review on carbon capture by physical adsorption is provided. The review covers carbon capture materials, experimental and numerical research. Challenges for the post combustion adsorption materials are presented. Gaps are found in the research of carbon dioxide adsorption of post-combustion.

Materials of high selectivity, CO2 uptake with water vapor stability are needed. article info abstract

Article history: This review focuses on the separation of carbon dioxide from typical power plant exhaust gases using the Received 4 May 2015 adsorption process. This method is believed to be one of the economic and least interfering ways for post- Received in revised form 13 September combustion carbon capture as it can accomplish the objective with small energy penalty and very few 2015 modiﬁcations to power plants. The review is divided into three main sections. These are (1) the candidate Accepted 2 October 2015 materials that can be used to adsorb carbon dioxide, (2) the experimental investigations that have been Available online 22 October 2015 carried out to study the process of separation using adsorption and (3) the numerical models developed to simulate this separation process and serve as a tool to optimize systems to be built for the purpose of Keywords: CO adsorption. The review pointed the challenges for the post combustion and the experiments utilizing Carbon capture 2 Adsorption techniques the different adsorption materials. The review indicates that many gaps are found in the research of CO2 Post-combustion adsorption of post-combustion processes. These gaps in experimental investigations need a lot of Experimental studies research work. In particular, new materials of high selectivity, uptake for carbon dioxide with stability Numerical investigations for water vapor needs signiﬁcant investigations. The major prerequisites for these potential new materi-

als are good thermal stability, distinct selectivity and high adsorption capacity for CO2 as well as sufﬁ- cient mechanical strength to endure repeated cycling. Ó 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ...... 227 2. Post-Combustion carbon capture technologies ...... 228 2.1. Adsorption ...... 229

2.1.1. CO2 capture using chemical sorbents ...... 229 2.1.2. CO2 capture using physical sorbents ...... 229 2.2. Adsorption process types ...... 230 3. Materials for adsorption carbon capture ...... 230 3.1. Introduction...... 230 3.2. Porous materials ...... 231 3.3. Carbon based adsorbents ...... 232 3.4. Solid materials...... 232

⇑ Corresponding author. Tel.: +966 138604467; fax: +966 138602949. E-mail address: [email protected] (M.A. Habib). http://dx.doi.org/10.1016/j.apenergy.2015.10.011 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved. 226 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

Nomenclature

3 CF,j feed concentration of component j (mol m ) qm;j Toth model parameter for amount of component j Cj gas phase concentration of component j (mol m3) adsorbed in activated carbon at equilibrium (mol/kg) Cv;g specific heat at constant volume for gas mixture t time of adsorption/desorption (s) 1 1 (J kg K ) tst stoichiometric time (s) Cp;g specific heat at constant pressure for gas mixture Ts temperature of solid adsorbent (K) 1 1 (J kg K ) Tw temperature of column wall (K) 1 1 Cs specific heat capacity of solid adsorbent (J kg K ) Tg gas mixture temperature (K) Cp;w specific heat capacity of adsorption column wall U superficial velocity of the gas mixture (m/s) (J kg1 K1) ux-component of the superficial velocity of the gas mix- 2 Dax axial dispersion coefficient (m /s) ture (m/s) dp adsorbent particle diameter (m) v y-component of the superficial velocity of the gas mix- dint adsorption bed diameter (m) ture (m/s) e adsorption bed void fraction wz-component of the superficial velocity of the gas mix- DHj enthalpy of component j in gas mixture (kJ/mol) ture (m/s) 3 hf film heat transfer coefficient between the gas and solid V adsorption bed volume (m ) 2 1 adsorbent (W m K ) yj mole fraction of component j in gas mixture hw internal convective heat transfer coefficient between 2 1 the gas and the column wall (W m K ) Greek Letters 1 ; KL j overall mass transfer coefficient of component j (s ) aw the ratio of the internal surface area to the volume of 1 Ko;j adsorption constant of component j at infinite dilution adsorption column wall density (m ) (Pa1) awl the ratio of the algorithm mean surface area of the l column wall thickness (m) column shell to the volume of the column wall (m1) n polytropic index e adsorption bed void fraction P total pressure of gas mixture (Pa) kL thermal conductivity of gas of the gas mixture in axial 1 1 Pj partial pressure of component j in gas mixture (Pa) direction (W m K ) 3 1 QF feed volumetric flow rate of gas mixture (m /s) l g dynamic viscosity of gas mixture (Pa s ) q average amount of adsorbed of component j (mol/kg) q 3 j g gas mixture density (kg/m ) q the amount of component j adsorbed at equilibrium q 3 j p adsorbent particle density (kg/m ) (mol/kg) q 3 w adsorption column wall density (kg/m )

3.5. Adsorption of CO2 by carbon nanotubes (CNTs) ...... 232 3.6. Metal organic frameworks (MOFs)...... 233

3.7. Comparison of different CO2 adsorbents ...... 234 4. Experimental studies on adsorption carbon capture ...... 234 4.1. Introduction...... 234 4.2. Experimental studies on adsorption by MOFs ...... 234 4.2.1. Adsorption desorption regeneration ...... 236 4.2.2. Adsorption and kinetic studies ...... 236 4.2.3. Temperature swing adsorption methods ...... 237 4.2.4. Performance in presence of water vapor ...... 237 4.3. Experimental studies on adsorption by zeolites ...... 237 4.3.1. Pressure swing adsorption process ...... 237 4.3.2. Vacuum swing adsorption ...... 238 4.3.3. Zeolite testing under humid conditions ...... 238 4.4. Experimental studies on adsorption by carbon-based materials ...... 239 4.4.1. Activated carbon...... 239 4.4.2. Carbon fibre composites...... 239 4.5. Other experimental studies on adsorption ...... 240 4.5.1. Regeneration process techniques...... 240 4.5.2. Adsorbent packing processes ...... 240 4.6. Concluding remarks ...... 241 5. Numerical investigations and mathematical models for fixed bed column adsorption...... 241 5.1. Introduction...... 241 5.2. Some existing mathematical models ...... 242

5.2.1. CO2 in a binary mixture (with CH4, N2,H2 orHe)...... 242

5.2.2. CO2 mixture (with CH4 and H2)...... 244 5.2.3. CO2 (with Air) ...... 244

5.2.4. CO2 mixture (CO2, CO, H2, and CH4) ...... 244

5.2.5. CO2 mixture (with N2 and O2)...... 244 5.3. Modeling of adsorption of CO2 for carbon capture ...... 248 5.3.1. Fixed bed adsorption model ...... 248 5.3.2. Governing equations ...... 248 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 227

5.4. Overview of results of numerical simulations of adsorptive carbon capture ...... 249 5.4.1. A comparison of breakthrough simulation results using Linear Driving Force Model (LDF) with breakthrough experimental result 249 5.4.2. Simulated results of the breakthrough behaviour of Mg-MOF-74...... 249

5.4.3. Simulated results for adsorptive storage of CO2 on MOF-5 & MOF-177 ...... 250 5.4.4. Simulated results of PSA of CO2 on Mg-MOF-74 ...... 251 6. Conclusions...... 251 Acknowledgments ...... 251 References ...... 252

1. Introduction of Carbon before combustion. This method has an advantage over Post Combustion Carbon capture because it is cheaper [4]. Post

In May 2013, most world environmental organizations have Combustion capture involves the separation of CO2 from declared that a critical level of carbon dioxide concentration of nitrogen–carbon dioxide mixture as the main constituent of flue 400 ppm was reached. This event has forced all countries, includ- gas generated in power plants is nitrogen. ing those who were reluctant to take serious action about carbon Research in the Carbon Capture and Sequestration (CCS) is fast emissions, to take unprecedented measures to reduce carbon diox- growing. A broad variety of technologies is investigated and devel- ide emissions. Fossil fuels are the dominant source of the global oped by the day [8,9]. Some technologies have been developed, primary energy demand, and will likely remain so for the next sev- however most researched technology need further improvements eral decades. Carbon dioxide (CO2) is regarded as one of the main in terms of efficiency and associated cost reduction. The major promoters for climate change. Carbon capture (CC) is essential to challenges for CO2 capture methods are stated briefly as follows. enable the use of fossil fuels while reducing the emissions of CO2 In oxy-fuel combustion capture we are faced with (a) high energy into the atmosphere, and thereby mitigating global climate change. consumption for supply of pure oxygen and (b) the lack of full Research is needed to address technical challenges to CC such as readiness for this technology with very little experience on a com- improved efficiency and reduced cost of CO2 capture [1]. Among mercial scale. In pre-combustion capture, the challenges include the main sources of CO2 emissions, the road transport field (a) high cost (b) insufficient technical know-how for good operabil- accounts for about 25% of CO2 emissions, while energy electricity ity (c) absence of single concise process for overall operational generation involves 26% of the total emissions. Therefore, CO2 performance; and (d) lack of development work for industrial emissions from fixed and mobile sources should be drastically application. For post-combustion capture case, the difficulties reduced in the forthcoming decades. Reducing CO2 emissions from include: (a) additional energy requirement for compression of cap- fixed and mobile sources are equally important though the mobile tured carbon dioxide, (b) need for treatment of high gas volumes, sources may pose more difficult challenges to be addressed. Global because CO2 has low partial pressure and concentration in flue pursuit of sustainable and healthy environment has been the sub- gas and (c) large energy requirement for regeneration of sorbent ject of the day in recent years and it cannot be overemphasized. e.g. amine solution. Global warming/greenhouse effect results in increase in tempera- A wide variety of potential methods and materials for Carbon ture of the earth’s surface beyond the normal, leading to gross dis- Capture and Sequestration (CCS) applications that could be comfort for inhabitants of earth. Greenhouse effect is caused by employed in post-combustion processes are being suggested as greenhouse gases such as; carbon dioxide, nitrogen oxide, methane substitutes for the traditional chemical absorption process. The and water vapor. The most predominant of these greenhouse gases suggested processes comprise: the use of membranes, physical is carbon dioxide [2]. absorbents, adsorption of the gases on solids with the use of Tem- Due to the necessity of energy resources for man’s continual perature Swing or Pressure Swing (PSA/TSA) processes, hydrate comfortable living, development of energy efficient, fossil fuel formation, cryogenic distillation, and the use of metal oxides for operated power plants is a major task that can be used to minimize chemical-looping combustion, and adsorption. A popular technol- the level of greenhouse gases emissions [1]. In addition to this, ogy of post-combustion carbon capture involves the absorption reduction of greenhouse gas emissions due to combustion of fossil of carbon dioxide in amine solution. This method has been in use fuels to the atmosphere can be further achieved through [3,4]: (i) on industrial scale for quite a long time. At the same time, varieties reducing fossil fuels burning (ii) improving coal fired plant effi- of some other of materials are available for other similar technolo- ciency (iii) capture and storage of carbon dioxide and (iv) enhance- gies (e.g. adsorption), some of which are old while some are newly ment of CO2 partial pressure in exhaust gas. The first step might be developed. difficult because it entails reduction in electricity production and Post-Combustion Carbon Capture is advantageous because of finding a replacement for fossil fuels. The second step suggested the following reasons: may have insufficient effect when compared to the target of reducing CO2 emission to near-zero. Hence, Herzog et al. [4] suggested (a) It is easier to integrate into existing plant without needing to the third step (Carbon Capture and Storage, CCS) to be a matchless substantially change the configuration/combustion technol- method that could permit continuous use and reduction of emis- ogy of the plant. sions associated with fossil fuels combustion and it would also (b) It is more suitable for gas plants than the Oxy-Combustion buy time for the development of a new alternative to fossil fuels. or the Pre-Combustion plants. The fourth step has been suggested as a means to achieve better (c) It is flexible as its maintenance does not stop the operation electrical energy efficiency in the third step [5–7]. Carbon capture of the power plant and it can be regulated or controlled. could be executed using three methods: (i) Pre-Combustion Carbon

Capture, (ii) Oxy-Combustion Carbon Capture, and (iii) Post- The post-combustion CO2 capture technology is widely Combustion Carbon Capture. Oxy-Combustion Carbon Capture, deployed in chemical processing. However, the application of this instead of air, makes use of highly pure Oxygen (P95%) for fuel technology to CC speciﬁc applications needs further investigation combustion. Pre-Combustion Carbon Capture implies the removal especially in the area of optimizing CO2 capture systems for ﬁxed 228 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

and mobile sources. The priority activities in this task are: (1) Separated CO2 (to development of better materials for post-combustion CO2 capture; storage and (2) identifying optimal capture process designs and ways of inte- sequestration) grating the capture systems with emitting sources to reduce energy loss and environmental impact; (3) identifying advantages and limitations of precipitating systems (e.g., carbonates) and (4) Absorption carrying out a detailed assessment of the environmental impact vessel Desorption vessel of various CO2 capture technologies.

2. Post-Combustion carbon capture technologies

A few Post-Combustion separation technologies have been reported, some of which are; (a) absorption CO2 separation [10] (b) membrane CO2 separation [11,12] (c) cryogenic CO2 separation (From plant) [13] (d) Micro algal bio-fixation (e) Condensed Centrifugal Fig. 2. Schematics of absorption carbon capture process using amine. Separation [14] and (f) adsorption. Fig. 1 and the following paragraphs briefly describe these methods [1]. Absorption of carbon dioxide (Fig. 2) is a process whereby Car- from the flue gas. The membranes are made from polymer or cera- bon dioxide is taken in or embedded (absorbed) from flue gas into mic materials and their configurations are specially designed for an absorbent solution (e.g. amine) by chemical action, leaving the CO2 selectivity. Challenges are still being faced in the application remaining gas stream to pass through the absorption column freely of this technique on a large scale, and in the design of membranes [15]. The dilute absorbent is re-concentrated (regenerated) for that would operate efficiently for the desired purpose at relatively reuse in CO2 capture. CO2 absorption using amine based solvents high temperatures. presents a great deal of disadvantages. Some of these disadvan- Cryogenic CO2 separation technique, Fig. 4, uses the principle of tages are: (i) high heat/power requirement for solvent regenera- liquid state temperature and pressure difference in constituent tion, (ii) need for corrosion control measures and (iii) the gases of flue gas. In this technique, CO2 is cooled and condensed, sensitivity of the solvents to losses in chemical purity/quality thereby removed from stream of flue gases [13]. due to infiltrations from other by-products (e.g. SOx, NOx, etc.) in Micro-Algae bio fixation is a potential technique for removal of the flue gas streams, which leads to reduction in efficiencies and CO2 from flue gases. This technique entails the use of photosyn- increment in costs of power supply [16]. thetic organisms (microalgae) for anthropogenic CO2 capture in Membrane separation of carbon dioxide (Fig. 3) involves the use CCS. Aquatic microalgae have been suggested to be of greater of polymer/ceramic made membranes to sieve out the CO2 gas potential because they have higher carbon fixation rates than land

Fig. 1. Post combustion carbon capture processes. R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 229

atoms or molecules form film on the surface of the materials to which they are attached and are called adsorbate while the material on which they are attached is called the adsorbent. Adsorption is different from absorption because in absorption, the fluid (absor- bate) is dissolved by a solid or liquid (absorbent). Adsorption occurs on the surface while absorption entails the whole material volume. Sorption is related to the two processes while desorption is the counter reaction or reverse process of adsorption. In adsorption, superficial atoms of the adsorbents are not completely encompassed by the remaining adsorbent atoms. Adsorption results in surface energy due to the filling of these bonding requirements of the adsorbent by the adsorbate atoms. The particular type of bonding involved is a function of the involved species. Adsorp- tion may take place physically; this will involve weak van der Waals forces (physi-sorption). It may take place chemically, which will involve covalent bonding (chemi-sorption) and it may occur due to electrostatic attraction. Adsorption has a major advantage with regard to the ease of adsorbent regeneration by thermal or pressure modulation [18], reducing the energy of Post-Combustion Carbon Capture. Son- golzadeh et al. [18] in their review of adsorbents defined adsorp- Fig. 3. Schematics of membrane carbon capture process. tion to be; a physical process that involves attachment of fluid to solid surface. Important factors in adsorption include; (i) ease of

regeneration of adsorbed CO2, (ii) durability of adsorbent, (iii) selectivity of adsorbent for CO2, (iv) adsorption capacity and, (v) stability of adsorbent after several adsorption/desorption cycle [18]. Several challenges are being faced by scientists and engineers alike with respect to commercialization of these materials. This is so because the researched materials require further work to improve their performance and stability. Suitable materials for carbon capture must account for size of gas molecules and electronic behavior of such molecules. There is no much difference in the kinematic diameters of gas molecules; this makes it difﬁcult to

base CO2 separation solely on gas molecule size (CH4: 3.76 Å, CO2: 3.30 Å, N2: 3.64 Å) [8,9]. However, electronic properties like quadru-polar moment and polarization have been of great help, as bases of separation as they are signiﬁcantly different for each gas.

2.1.1. CO2 capture using chemical sorbents In order to overcome these challenges, a lot of research has been carried out on advanced materials. However, despite the extent of Fig. 4. Schematics of cryogenic carbon capture process. investigations, it has been difficult to find a single technology that is able to meet the requirements set by the Department of Energy (DOE) and National Energy Technology Laboratory (NETL): i.e. plants. Micro-algal culturing is quite expensive but the process below 35% increment in cost of electricity for 90% CO2 capture produces other compounds of high value that can be used for [8,9]. Most chemical adsorption and absorption processes, in car- revenue generation. Micro-algal photosynthesis also leads to bon capture/separation procedures involve the interaction precipitation of calcium carbonate that can serve as long lasting between chemicals which lead to the formation of molecular struc- sink for Carbon [17]. tures that are CO2-based, after which regeneration of the captured The present review is focused on carbon capture by physical CO2 is done through sufficient increase in temperature by heating. adsorption and considers materials and experimental investiga- This procedure (i.e. regeneration) consumes most of the power tions. The remaining sections provide critical reviews of carbon requirement in CCS. Hence, there is a need to develop efficient capture methods, materials for adsorption carbon capture, materials and processes for CO2 capture that can greatly decrease experimental studies on adsorption carbon capture and numerical operation cost through reduction in regeneration cost. investigations and mathematical models for fixed bed column adsorption. 2.1.2. CO2 capture using physical sorbents CO2 capture using physical sorbents and inorganic porous mate- 2.1. Adsorption rials (e.g. carbonaceous materials and zeolites respectively) consumes lesser energy when compared with CCS with chemical Adsorptive separation, Fig. 5, is a mixture separating process sorbents. This is because no new bond is formed between the sor- which works on the principle of differences in adsorption/desorp- bate and sorbent, therefore much lesser energy is required for CO2 tion properties of the constituent of the mixture [5–7]. The word regeneration. Nevertheless, some well-known materials (e.g. acti- adsorption is defined as the adhesion of ions, atoms or molecules vated carbon), have the disadvantage of poor CO2/N2 selectivity. from a liquid, gas or dissolved solid to a surface. The adhered ions, If the challenges of selectivity in physical sorbents and membranes 230 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

(a) Adsorption bed

(b)Typical experimental breakthrough curve

Fig. 5. Schematics of adsorption carbon capture process in a cylindrical bed (a) and typical breakthrough curve (b).

are successfully overcome, their use for CO2 capture could be a (b) Temperature Swing Adsorption (TSA) [23,24]. In tempera- good potential for energy saving by the dominant amine-based ture swing system, the adsorption bed heating is done using absorption systems. Zeolites show much higher selectivity, but, a feed of hot gas or steam. Following the regeneration step is they also have a disadvantage of lower CO2 loading and their efﬁ- the cooling of the adsorption bed by a feed of cold gas ciency is reduced in the presence of water [8,9]. Furthermore, stream before the next adsorption step. molecular sieve membranes have great potentials, however, traditional molecular sieves (e.g. zeolites) have restricted use in CO2/N2 Of these two processes, it has been demonstrated that PSA is a separation because of similar kinetic diameters of N2 (3.64 Å) and better option [23] because of (i) simplicity in application with wide CO2 (3.3 Å). In all, development of advanced physical adsorbents range of temperature and pressure application, (ii) low energy with high CO2 capacity and selectivity is crucial. Good stability, demand and (iii) lower investment cost. CO2 afﬁnity, scalability and additional required energy are major In adsorption carbon capture process, material selection pre- concerns in carbon capture research. This is crucial to the research cedes process design. Before an adsorption process is designed, and development of potential carbon capture materials that will selection of suitable adsorbent, with desired properties for the challenge the available technologies that have been discussed required purpose must be done. In doing this, properties such as: above. More attention should be paid to better understanding adsorbent selectivity, adsorption capacity, ease of and energy molecular level gas-sorbent synergy. required in desorption are of great importance. In view of this, a lot of research has been carried on broad species of materials such 2.2. Adsorption process types as: synthetic zeolite, metal oxides, silica’s, carbon molecular sieves, and activated carbon.

It has been reported that the incurred cost in CO2 capture and its associated procedures, with the use of liquid solvent absorption, 3. Materials for adsorption carbon capture can be cut down by a great deal if adsorption separation technique is used [19]. Numerous technological successes have been reported 3.1. Introduction recently in the research of adsorption carbon capture processes. Out of the researched technologies for adsorption carbon capture, Different classes of Carbon capture materials have been identi- two potential technologies have been considered feasible for fied over the years e.g. Songolzadeh et al. [18] discussed two industrial scale CCS: classes of CO2 adsorbents: (i) physical and (ii) chemical adsorbents. Physical adsorbents have substantial benefits for energy efficiency (a) Pressure/Vacuum Swing Adsorption (PSA/VSA) [20,21] Car- in comparison with chemical and physical absorption routes. The bon capture capacity in a PSA system is affected by two main adsorption involves either physisorption (van der Waals) or factors: Adsorption selectivity and carbon dioxide working chemisorption (covalent bonding) interaction between the gas capacity [22]. In PSA, adsorption step is done at elevated molecules and the surface of the material. An important factor in pressure than atmospheric pressure while in VSA adsorption the case of physical adsorbent is balancing a solid affinity for is performed at atmospheric pressure or lower. removing the undesired component from a gas mixture with the R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 231 energy consumption required for their regeneration. Selectivity is support with high Amine loading, by increasing the nitrogen con- another factor in addition to the adsorption capacity, which is rel- tent in amines and by improving methods of Amine introduction. evant to the adsorptive gas separation. The following mechanism Two special cases are amine impregnated adsorbents and amine- are proposed for adsorptive separation: (a) the molecular sieving grafted adsorbents. In amine impregnated adsorbents, increased effect, based on size/shape exclusion of the components in the polyethyleneimine loading would lead to improved CO2 adsorption gas mixture; (b) the thermodynamic equilibrium effect, that capacity, reduced surface area for adsorption, pore size and vol- depends on the surface–adsorbate interactions; (c) the kinetic ume. Therefore, it was suggested [37] that amine impregnated effect, due the diffusion rate differences in the gas mixture compo- adsorbents do not have thermal stability in desorption. In amine- nents [25]. grafted adsorbents and in order to overcome the limitations of

Several physical adsorbents have been studied for CO2 capture amine impregnated adsorbents it is suggested that CO2 adsorption including metal oxides, hydrotalcite-like compounds, microporous capacity for this group of materials can be improved through silyli- and mesoporous materials (including activated carbon and carbon sation. They can be grafted covalently to the intra-channel surface molecular sieves, zeolites, chemically modified mesoporous mate- of meso-porous Silica. It is indicated that improvement of Amine rials) [26–29]. Physical adsorbents (physisorbents) are barely dis- loaded adsorbent could be improved by infusing amines into turbed during adsorption. Pore sizes are of great importance in meso-porous support with the use of effective solvents. This was physical adsorption. When pores are of size 2 nm, they are termed termed supercritical fluid approach. However, this group of mate- micro-pores, pores of sizes between 2 and 50 nm are termed meso- rials has disadvantages of high toxicity, low diffusivity and high pores, and when pores are of size 50 nm, they are termed viscosity. These features can lead to lower adsorption capacity macro-pores. Materials with micro pores have better adsorption and high pressure drop. Due to large volume of flue gases are to selectivity for CO2 over CH4. Some examples of physical adsorbents be treated, and low partial pressure of CO2 in flue gas, chemical include activated carbon, zeolite, hydrotalcites, carbon nanotubes adsorption would be more feasible for CO2 capture than physical (CNTs), coal, etc. Activated carbon has high adsorption capacity adsorption. However, it has the disadvantage of being an energy for CO2, high hydrophobicity, low cost, little regeneration energy intensive process. It was indicated that that physical adsorption requirement and is insensitive to moisture. Zeolite on the is good for CO2 adsorption at high pressure and low temperature. other hand has better selectivity for CO2/N2 than carbonaceous In this light, they might not me practically applicable for post com- materials. bustion carbon capture. Some examples of metal oxides that have been studied for car- Physical adsorbents. These include activated carbon with advan- bon capture include: calcium oxide (CaO), magnesium oxide (MgO) tage of enormous availability, zeolites with advantage of highly and lithium oxides (e.g. Li2ZrO3, Li4SiO4) [30,31]. Some examples of crystalline structure, high surface area, ability to alter their compo- metal salts are lithium silicate and lithium zirconate, both of which sition structure and ratio. They also include meso-porous silica are alkali metal compounds. Magnesium oxide and calcium oxide with advantage of high volume, surface area and tunable pore size, are examples of alkali earth metal compounds. Some other exam- thermal and mechanical stability and Metal Organic Frameworks ples of chemical adsorbents are the hydrotalcites and double salts. (MOFs) with advantages of very high surface area, adjustable pore

During CO2 adsorption, solid compounds react with CO2 to form spaces, pore surface properties, and exceptional adsorption capac- new compounds e.g. Metal Carbonates. These reactions can be ity for CO2. They however stated that activated carbon has disad- reversed in regenerators to harvest CO2 for storage. Metal oxides vantage of application to only high pressure gases, at high are promising capture materials with high adsorption capacities temperature they have high sensitivity and low selectivity. They at above 300 °C [32]. Lithium based oxides found recent attraction also stated that Zeolites have very low selectivity, zeolites are for their high CO2 adsorption capacities [33]. Calcium oxide is of hydrophilic and their CO2 adsorption capacity drops with the pres- special interest to researchers because it is cheap and it has high ence of moisture in gas. The authors further mentioned that the adsorption capacity for CO2 compared to lithium salts which are adsorption capacity of meso-porous silica is not sufﬁcient most more expensive especially in production. Hydrotalcites are anionic especially at atmospheric pressure. They stated that MOFs have and basic clays and their derivatives are also found suitable for CO2 the disadvantages of reduction in adsorption capacity on exposure adsorbents at temperatures as high as 400 °C [34]. Most naturally to gas mixture and insufﬁcient research on them, however, they occurring and well-studied hydrotalcite is Mg–Al–CO3. Hydrotal- are prospective materials. Generally, CO2 capture by physical pro- cites have the disadvantage of high loss in adsorption capacity after cess requires less energy when compared to typical procedure cycles of operation. During CO2 adsorption, solid compounds react using chemical sorbents. As mentioned earlier, this is because of with CO2 to form new compounds e.g. metal carbonates. Materials the absence of newly formed chemical bonds between the sorbate with at least one dimension less than 100 nm (nanomaterials) have and sorbent, which reduce the energy requirement for regenera- also been investigated [35]. These materials have improved tion [9]. stability and they maintain CO2 capturing capacity for longer adsorption/desorption cycles. However, nanomaterials have disad- 3.2. Porous materials vantage of high cost and complicated process of synthesis. Webb

[36] stated that CO2 capture efﬁciency, rate of absorption, required Zeolites are the most commonly used physical adsorbents for regeneration energy and volume of absorber are some of the major commercial hydrogen production using pressure swing adsorption challenges of CO2 absorption method. They reviewed adsorbents with most popular zeolites 13X [26,38]. They are used at high pres- and some meso-porous solid adsorbents with polyamines embed- sures (above 2 bars) and their capacity is greatly reduced by the ded in them. They stated that some factors for adsorbent selection presence of moisture in the gas; resulting in very high regeneration are rate of adsorbent, cost, and capacity of the adsorbent to adsorb temperatures [39,40]. Experimental and computational studies of

CO2 and thermal stability. They identiﬁed of the following types of CO2 removal from ﬂue gas using naturally occurring zeolites and adsorbents; other synthetic zeolites 5A and 13X indicate that synthetic zeolites

Chemical adsorbents e.g. amine based adsorbent. Amines were are most promising adsorbents for CO2 capture from ﬂue gas mix- said to have low heat of regeneration due to low heat capacity of ture [39,41]. However, they experience weak adsorbent–adsorbate solid support. They are costly and they have low CO2 adsorption interactions which are not well-suited with a high CO2/N2 selectiv- capacity, therefore, they are difﬁcult to commercialize. CO2 adsorp- ity. The low SiO2/Al2O3 ratio and presence of cations in the zeolite tion properties of amines can be improved by preparation of structure can enhance the adsorption. The presence of cations 232 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

leads to strong electrostatic interactions of the zeolites with CO2 Moreover, consistent with the lower heat of adsorption for CO2, [39]. Although these adsorbents are satisfactory for pressure swing activated carbon requires a lower temperature for regeneration adsorption, significant energy is needed for their regeneration and compared with zeolites. Activated carbon and charcoal were that possibly leads to the disadvantages of these materials. shown moderate adsorption selectivity for CO2 over N2 at low pres- In the meantime, it is possible to modify these porous solid sure (below 1 atm.) and increasing the pressure reduces the selec- materials by impregnating active alkyl amines into their internal tivity [52]. Moreover, consistent with the lower heat of adsorption surfaces leading to an enhancement in their gas adsorption proper- for CO2, activated carbons require a lower temperature for regener- ties at low pressures. Several amine modified silica have been ation compared with zeolites [52]. Activated carbon and charcoal investigated [26,42]. Carbamate species are formed through were shown moderate adsorption selectivity for CO2 over N2 at adsorption of CO2 in the surface modified silica with primary low pressure below 1 atm. and increasing the pressure reduces amines. Removal of CO2 can be performed at lower temperatures the selectivity [53]. The CO2 capture using physical adsorbents than those required for the regeneration of amine solvents including carbon based and zeolites is much more energy efficient

[43,44]. A significant enhancement in the CO2 adsorption capacity compared to the metal oxides and others. This is due to the is obtained through pressure swing adsorption using MCM-41 with absence of the formation of new chemical bonds between the sor- impregnated polyethyleneimine [45]. Amine immobilized support bate and sorbent, thereby requiring significantly less energy for such as poly(methyl methacrylate) has exhibited increased adsorp- regeneration. However, the selectivity of carbon based materials tion capacities [46]. However, after impregnation, the materials is very low, whereas zeolites exhibit significantly higher selectivity suffer from a lack of stability over repeated cycles. To increase while they suffer from lower CO2 loading and their performance is the stability of the materials in repeated cycles, alkylamines have reduced in the presence of moisture. been covalently tethered to the surface of the mesoporous support. For example, polymerization of aziridine on the surface of meso- 3.4. Solid materials porous silica generates a hyperbranched material which shows reversible CO2 binding and multi-cycle stability under simulated Organic calixarene compounds, for example non-porous self- flue gas conditions using temperature swing adsorption [42]. The assembled p-tert-butylcalix[4]arene organic solids have been con- grafted monoamino, diamino, triamino ethoxysilanes SBA-15 have sidered for CO2 capture [54,55]. Their structure involves cone- been used to study the effect of amine and the presence of mois- shaped calixarene molecules and the molecules are stabilized by ture on CO2 adsorption performance [47]. The capacity slightly intramolecular hydrogen bonds and the presence of hydrophobic decreased for primary amine, but increased for secondary and ter- nanodimensional channels [54]. The material may be suitable for tiary amines. Although amine grafting materials show significant high pressure CO2/H2 syngas separations. Other potential solids improvement over non-grafted materials, it is very important that reported for CO2 capture are covalent organic frameworks (COFs) the amount of grafted amine be optimal for the particular CO2 cap- [56]. They are microporous materials similar to MOFs but with ture process. It is also important to study the influence of the quan- frameworks with light weight organic components instead of the tity of grafting reagent added to the actual amount of amine that is metal connectors. For example, COF-102 (C25H24B4O8) is con- covalently attached to the surface. structed with tetra(4-(dihydroxy)borylphenyl)methane unit and 1 shows the highest CO2 uptake in this class (27 mmol g at 3.3. Carbon based adsorbents 55 bar and 298 K) [56]. Molecular simulation studies performed on these materials predict also their exceptional high uptake Carbon based materials such as activated carbon, charcoal and [57,58]. coal have been reported for high pressure CO2 capture applications [26,48]. The key advantages of these materials are their low cost, 3.5. Adsorption of CO2 by carbon nanotubes (CNTs) their insensitivity to moisture and the possibility of their production/synthesis from numerous carbon based naturally existing or The adsorption of CO2 on various carbonaceous materials such spent materials [49]. The activated carbon materials are amor- as activated carbon [59–63] and CNTs [64–70] attracted the atten- phous porous forms of carbon that can be prepared by pyrolysis tion of many researchers in recent years. AC, derived from different of various carbon containing resins, fly ash, or biomass [26]. These sources of carbon materials, was the first carbon adsorbent agent materials have lower capacities for CO2 compared with zeolites at used for CO2 capture [71–74]. Currently, CNTs are being considered lower pressures due to relatively uniform electric potential on the in this field due to their promising physical and chemical proper- surfaces of activated carbons leading to a lower enthalpy of ties, high thermal and electrical conductivity, along with the possi- adsorption for CO2. However, their significantly high surface area bility to modify their surfaces chemically by adding a chemical lead to greater adsorption capacities at high pressures, which has function group, using fisher esterification method, yielding high resulted in activated carbons being considered for a variety of adsorption storage capacity [75–84]. These CNTs have proven to high-pressure gas separation applications. The major target appli- have good potential as highly adsorbent materials for removing cation for these materials is the high-pressure flue gas produced in different kinds of inorganic and organic pollutants and microor- pre-combustion CO2 capture. Indeed, one study has shown that the ganisms [85–91]. The large adsorption capacity of pollutants by upper limit for the CO2 adsorption capacity within activated car- CNTs is mainly attributed to their surface charge densities, and bon materials is approximately 10–11 wt.% under post- wide spectrum of surface functional groups, achieved by chemical combustion CO2 capture conditions, while it reaches 60–70 wt.% modification or thermal treatment to make CNTs possess optimum under pre-combustion CO2 capture conditions [50]. The volumetric performance for a particular purpose. Therefore, it is believed that CO2 adsorption capacity of carbon-based adsorbents is greater than a chemical modification of CNTs would also be expected to have a some of the highest surface area MOFs at high pressure through good potential for CO2 capture from a flue gas. However, such the careful selection of the material precursors and the reaction studies are still very limited in the literature. Functionalized CNTs conditions employed [51]. with amino-functional groups [92–95] have been considered. Su One additional advantage of activated carbons over zeolites is et al. [96] investigated the effect of functionalized CNTs with that their hydrophobic nature results in a reduced effect of the 3-aminopropyltriethoxysilane (APTES) at different adsorption presence of water, and they subsequently do not suffer from temperatures. They found that by increasing the temperature of breakdown or decreased capacities under hydrated conditions. the system, the adsorption storage capacity decreased, while R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 233 increasing the water content increased the adsorption capacity, interpenetration. The interpenetration can be avoided by targeting which reflected the exothermic process of adsorption. Their exper- the topology which are not prone to interpenetrate. Since the imental CO2 adsorption capacity of 2.59 mmol/g at 293 K for emergence of MOFs as potential material for carbon capture, a APTES–CNT is the evidence for the potential of CNTs as low- lot of research has been done on MOFs. temperature adsorbents. Hsu et al. [66] combined vacuum and Since MOFs provide reversible carbon dioxide adsorption, they thermal adsorption system in order to trim down the regeneration are excellent materials for the carbon capture. Carbon dioxide time. They were able to sustain adsorption/regeneration of CNT– adsorption first reported using MOF-2 in 1998. The systematic car- APTES for twenty cycles at 493 K while maintaining the CNTs’ bon dioxide adsorption study of MOF-177 with an uptake of physiochemical properties and adsorption capacity. Dillon et al. 1470 mg/g at 35 bar which exceeded that of any known porous [97] functionalized the surfaces of single-walled CNTs with poly- material in similar conditions. Li et al. [8], worked on carbon cap- ethylene Imine (PEI) functional group and reached a maximum ture using MOFs as adsorbent. CO2 adsorption in MOFs depends on adsorption capacity of 2.1 mmol/g at 300 K. The reported good pore size or volume and nature of pore surface. MOFs have higher

CO2 capture capacities suggest that the amine-functionalized CNTs adsorption capacity than Zeolite and activated Carbon because are promising CO2 adsorbents, given that the adsorption mainly they have more surface area and larger pore size in contrast to depends on physical effects, thus relatively low energy is required them. The volume and nature of pore to a great extent determine for the regeneration. Very few works are reported on the use of the shape of adsorption isotherms; due to interaction between

CNTs as membrane for CO2 capture. Mixed matrix membranes of molecules of CO2 leading to large condensation. Typically, MOFs polyvinylalcohol containing amines with MWCNTs dispersed as are synthesized in a hydro/solvothermal reaction which involves mechanical reinforcing fillers demonstrated high stability for gas combination of organic ligands and metal salts in dilute solution separation at high pressures up to 1.5 MPa. Selectivity and perme- of polar solvents such as water, alcohol, alkyl formamides (such ability of 43 and 836 Barrers have been achieved even at such high as DMF, DEF) or DMSO and heated at comparatively low tempera- pressures [98]. tures usually below 50–300 °C. The solvent utilized in the synthesis itself act as a template and the solvent can provide the 3.6. Metal organic frameworks (MOFs) framework intact and accessible porosity. It is important to get high quality single crystals to characterize the MOF crystals. About two decades ago, a new class of materials was discov- Although solvothermal technique used extensively other tech- ered; they are made of MOFs and are simply called MOFs [5–7]. niques also known for example slow evaporation of the solution They are organic–inorganic hybrid, porous, solid materials. Out of precursors, layering or slow diffusion. Hydro/solvothermal tech- all known materials to date, MOFs have the highest adsorption sur- niques have advantage over other former techniques since they face area per gram. They have great potentials for CO2 capture, reduce the synthesis time. The ligand properties such as ligand flexible design-ability in terms of structure and function. This has length, bulkiness, bond angles, and chirality act as major factors made these materials highly used in research works of Carbon Cap- to determine the frame work topology of the resultant compound ture and Sequestration. MOFs has emerged and first synthesized by [101]. The synthesis of MOF also depends on the concentration, Hoskins and Robson in 1989. MOFs, also known as coordination solvent polarity, pH and temperature. A minor change in the for- polymers [99] have been described as porous hybrid nano-cubes mer parameters can leads to poor quality crystals, lower yields or that harness bi-properties; they establish properties of organic even the formation of new structures. To improve the crystal and inorganic porous materials. The descriptive term MOF was first growth mixed solvent are often used which also provide to tune introduced by Yaghi and co-workers in 1995. MOFs are a class of the polarity of the solution. Besides this standard method, some porous crystalline materials constructed from metal-containing other methods have been described by researchers. These methods nodes that bonded or linked through organic ligands [9,7]. The include: The mixture of non-miscible solvents [102], spray drying linked metal and organic ligands bridges and assembled to form technic [103], an electrochemical approach [104,105], and a high- 1D, 2D and 3D coordination network., The metal containing unit throughput approach [106] and microwave irradiation. Micro wave which is referred as secondary building units (SBUs) linked with irradiation enables access to increased range of temperatures, it organic ligands using strong bonds [7]. MOFs have shown extraor- can be used to reduce crystallization time and for controlling dis- dinary porosity and can be used for wide application such as gas tribution of particle size and face morphology [107,108]. Micro- storage, gas separation and catalysis. One of the most advantages wave irradiation however has a disadvantage of small crystal size of MOFs shows its possibility of tuning the pore size from several formation, therefore difficult to get enough size crystal for single angstroms to nanometres by controlling the length and functional- crystal X-ray diffraction. ity of the ligands. These properties are not achievable in the case of Over time, several MOFs have been prepared by different group zeolites and porous carbon materials. The most prominent and dis- of researchers with the aim of arriving at a suitable formulation for tinctive property of MOFs are its large surface area. The surface efficient capture of CO2. As at August 2012, a total of about 37,241 area, pore size and framework topology can be tuned by using dif- MOF structures were available in the Cambridge Structure Data ferent organic building blocks and metal ions. base [109]. A typical example is MOF-177 [110] synthesized using 0 00 The metals ions can vary from transition metals to lanthanides Zn(NO3)26H2O and of 4,4 ,4 -benzene-1,3,5-triyl-tri-benzoic acid and even some p-block metals to form wide range of network (H3BTB) were dissolved in 10 mL of DEF inside a 20 mL vial. It topologies. There are wide range of network topologies are known was subjected to heat at temperature of 100 °C for 20 h. The solu- and they are constructed with different combination of metal ion tion drained; the resulting clear crystals were washed in DMF and and the ligands. The organic linkers and metal SBUs can be varied replaced with CHCl3 three times in three days. Evacuated of the and that leads to variety of thousands of MOFs and that number material was carried out at 125 °C for 6 h prior to further analysis. increasing year and year [100]. The layered zinc terephthalate For proper selection of appropriate building blocks for any desired was the first proof of permanent porosity of MOF observed by mea- application, a proper understanding of the influence of characteris- suring nitrogen and carbon dioxide isotherms. Later the thrust was tics of the building blocks and resulting material on the adsorption looking for ultrahigh porosity MOFs that can be achieved by using behavior is important. Hydrothermal stability of MOFs could be longer linkers which eventually increase the storage space and estimated by exposing MOFs to steam at concentration and tem- umber of adsorption sites. The longer hurdles were using the perature more than anticipated in practical operating condition longer linkers that always prone to form the network to undergo of flue gas. A throughput apparatus could be employed for the 234 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 steaming. After which, sample materials are exposed to X-ray dimensional pore is as a result of presence of sharp corners which diffraction (XRD) examination to ascertain their structural stability brings about more framework atoms in the sharp corners.

[110]. Some of the ways by which CO2 uptake of MOFs have to be MOFs could be rigid or flexible, depending on whether there is improved include the following. (1) Capacity of MOFs at pressure relative movement within their frameworks or not [9]. Several can be improved by introduction of metal ions like Magnesium, researches have been carried out on this topic: [111–114]. Usually, Cobalt, Vanadium, Titanium etc. [110,119]. (2) After-synthesis- rigid MOFs; MOFs that do not display movement within frame- exchange of extra framework cations inside anionic MOFs. (3) works show adsorption isotherms that are I-shaped. However, Introduction CNTs into MOFs, which could be ameliorated by addi- some MOFs have bi-porous structures that have channels and tion of lithium and (4) Functionalizing the pores with alkyl amino cages existing together within them. This makes them having group. stepwise adsorption isotherms [115] e.g. at low temperature, II III 3.7. Comparison of different CO adsorbents Ni2Ni (_3-OH)(pba)3(2,6-ndc)1.5 (MCF-19; pba = 4-(pyridin-4-yl) 2 benzoate, 2,6-ndc = 2,6-naphthalenedicarboxylate). Some other MOFs with ultrahigh pores have sigmoidal isotherms at low temper- The data of the different materials are summarized in Table 1. The table provides the different properties of CO uptake, surface ature (close to room temperature) and high pressure e.g. Zn4O(btb)2 2 area, CO /N selectivity and stability in humid conditions. The data (MOF-177, btb = benzene-1,3,5-tribenzoate), Zn4O(bdc)3 (MOF-5 or 2 2 are provided for materials of the different groups including carbon- IRMOF-1, bdc = 1,4-benzenedicarboxylate), and Zn4O(bte)14/9(- 0 00 based adsorbents, Zeolites and MOFs. The table indicates the bpdc)6/9 (MOF-210, bte = 4,4 ,4 -(benzene-1,3,5-triyltris(ethyne-2,1 -diyl))tribenzoate, bpdc = biphenyl-4,40-dicarboxylate). On the other dependence of the properties on the application pressure. It also hand, flexible MOFs; MOFs that show flexible behavior due to move- indicates that some new materials are well stable in humid condi- ment within frameworks; display stepwise or hysteretic desorption tions. However, many materials require more development for consideration for carbon capture of flue gases of the industrial for CO2 and other gases [8]. Such MOFs are said to ‘breath’ during applications. As well, the CO uptake in some materials needs adsorption/desorption e.g. M(OH)(bdc) (MIL-53) series, Sc2(bdc)3 2 etc. Flexible MOFs show great potential for selectivity and they have improvement. Another table (Table 2) provides a comparison of the different the advantage of smooth increment in volume with increase in CO2 loading. The flexibility of such MOFs can be improved as it is related materials group of zeolites, MOFs and activated carbon based to the post added group alkyl chain length. Gate phenomenon in materials. It is shown that MOFS have much priority on other MOFs has been given quite attention over the years [116,117]. materials regarding the capacity but it is very expensive. As well Kitagawa et al. observed a phenomenon which was termed ‘‘gate” MOFs in general are not stable in humid conditions. The three effect in some flexible MOFs e.g. Cu(pyrdc)(bpp) (pyrdc = pyridine- groups discussed in the table differ in terms of conductivity, ther- 2,3 dicarboxylate, bpp = 1,3-bis(4-pyridyl)propane). This was mal and chemical stability and possibility of tuning. The selectivity described as an abrupt rise in adsorption isotherm at relatively of CO2/N2 changes form low in zeolites to moderate in carbon- low pressure. This pressure was termed ‘‘gate” opening pressure. based absorbents and becomes high in MOFs. Saturation of the materials occurred at a different pressure. How- ever, the isotherms for desorption, did not follow reverse trace of 4. Experimental studies on adsorption carbon capture the adsorption isotherm, rather, it showed a sudden drop at another pressure (third pressure). Gate phenomenon also noticed in 4.1. Introduction 0 0 0 0 [Cu(4,4 -bipy)(H2O)2(BF4)2](4,4 -bipy) (4,4 -bipy = 4,4 -bipyridine), when bared to water. Similarly, Rosseinsky et al. reported that Zn Generally speaking, post-combustion carbon capture is a costly

(Gly-Ala)2; a peptide base MOF; exhibited ‘‘gate” behavior at process due to process challenges including many parameters. pressure of about 2 bar. These include design of capture CO2 process and materials, struc- Another property, for gas adsorption, which can affect CO2 turing of carbon capture materials, dealing with impurities with uptake capacity of MOFs, is heat of adsorption [9]. Heat of adsorp- CO2 that can cause adverse effect on capture materials. They also tion can be estimated with the use of adsorption isotherms of a CCS include CO2 storage and thermodynamics of power plants, integra- process at various temperatures. This property is an important fac- tion of heat dissipation during carbon capture with heat dissipated tor in desorption. High of heat of adsorption brings about high in power plants, optimization of carbon capture materials with energy requirement for regeneration/desorption. Heat of adsorp- respect to ease of recycling, rate of carbon capture, CO2 selectivity tion reduces with increase in loading. The tenability of pores in and capacity etc. [140]. Many types of MOFs and zeolites as adsor- MOFs is one of the important properties that distinguish them bents for carbon capture by adsorption in post combustion were from other porous materials. Often, the length of organic linkers studied in terms of CO2/N2 selectivity, adsorption capacity and is the major determinant of the pores size in MOFs [118]. An anal- breakthrough time [22]. Furthermore, many types of MOFs studied ysis of the sorbate/framework interactions by Düren [99] showed in literature for post combustion CO2 capture were tabulated [141] that one dimensional pores with sharp edges are good for gas sep- regarding to CO2 and N2 uptake and selectivity for conditions aration and gas storage at low pressure. However, this is less feasi- closed to the ambient conditions which generally mimicked the ble at higher pressure because of the small volume of these post combustion exhaust conditions. This section presents the preferred energetic corner regions. This was illustrated with the experimental studies that are available for CO2 adsorption. These investigation of the adsorption of pure methane and ethane in Zn are provided in two sub-sections including adsorption by MOFs MOFs of different pore morphologies (e.g. 3D cubic, 1D Rhombic, and adsorption by zeolites and other materials. 1D triangular). It was shown that at lower pressure, as the pore volume is designed smaller, the selectivity becomes better while 4.2. Experimental studies on adsorption by MOFs the adsorption rate per unit volume becomes higher. However, saturation is quicker due to smaller pore volume. However, at higher A large number of literature investigations related to carbon pressure, there is much lower uptake because of the small pore capture is focused on methods and procedures for synthesis and volumes. It was concluded that adsorption in MOFs with one testing of materials for post combustion capture. MOF type Table 1 Adsorbent materials utilized for CO2 capture.

2 Sorbent Temp. Pressure CO 2 molar fraction (%) Uptake CO 2 (mol/ Surface area (m /g) BET Selectivity CO 2/N2 Stability in humid conditions Reference (°C) (kPa) kg) Activated carbon based NCLK3 25 120 – 3.5 – 30 (at 130 kPa, 323 K) – González et al. [120] NCHA29 25 120 – 2.3 – 20 (at 130 kPa, 323 K) – González et al. [120] NaSB31 25 4000 100 27 3024 – – Marco-Lozar et al. [121] KL31 25 4000 100 22 2540 – – Marco-Lozar et al. [121] KA21 25 4000 100 17.5 2156 – – Marco-Lozar et al. [121] NORIT R2030CO2 30 120 17 2.4 942 7 Plaza et al. [122] Carbon ﬁber 25 101.3 13 3.1 490.6 – – Thiruvenkatachari et al. composites [123] Olive stones 50 120 14 0.61 1113 18 Hydrophobic and high González et al. [124] stability Almond shells 50 120 14 0.58 822 20 Hydrophobic and high González et al. [124] stability

– Gil et al. [125] 225–255 (2016) 161 Energy Applied / al. et Ben-Mansour R. No1KCla-600 25 120 50 2.03 1091 2.54 over CH4 No1KClb-1000 25 120 50 1.91 804 2.69 over CH4 – Gil et al. [125] No2OS-1000 25 120 50 1.83 1233 2.26 over CH4 – Gil et al. [125] Cu/Zn–16% AC 30 100 15 1.98 730.53 – – Hosseini et al. [126] Cu/Zn–20% AC 30 100 15 2.26 599.41 – – Hosseini et al. [126] Cu–20% AC 30 100 15 1.99 645.21 – – Hosseini et al. [126] Zeolite Zeolite 13X 50 100 15 3 585.5 – – Dantas et al. [127–129] Zeolite 13X-APG 30 100 15.9 4.3 – – – Wang et al. [130] Zeolite A5 30 100 16 3 499 – – Wang et al. [130] LEZ -13X 50 101.3 – 4.6 12.7 – Stable Cho et al. [131] LEZ -A5 50 101.3 – 5.2 16.8 Stable Cho et al. [131] ZSM5 25 120 25 0.7 – 4.6 – Hefti et al. [132] Zeolite 13X 25 120 25 4.5 – 28 – Hefti et al. [132] MOFS HKUST-1 30 1000 20 8.07 1326 – Stable Ye et al. [133] MIL-101(Cr) 30 1000 20 7.19 2549 – Stable Ye et al. [133] (hﬁpbb) (ted) 25 101.3 – 0.4545 – 40 – Xu et al. [134] 2 Zn 2 CPM-5 0–25–40 105 15 3–2.3–1 – 14.2 (273 K)–16.1 Stable for few weeks Sabouni et al. [135] (298 K) MOF-177 40 100 15 0.65 4690 3 – Mason et al. [24] Mg2-MOF-74 40 100 15 7.5 1800 63 – Mason et al. [24] IRMOF-1 25 3500 100 11.1 2833 – – Millward and Yaghi [136] IRMOF-3 25 3500 100 10.3 2160 – – Millward and Yaghi [136] IRMOF-6 25 3500 100 10.5 2516 – – Millward and Yaghi [136] IRMOF-11 25 3500 100 8.9 2096 – – Millward and Yaghi [136] HKUST-1 25 3500 100 7.3 1781 – – Millward and Yaghi [136] Zn-MOF-74 25 3500 100 7.1 816 – – Millward and Yaghi [136] MOF-505 25 3500 100 0.70 1547 – – Millward and Yaghi [136] Cu-TDPAT 25 100 10 0.59 1938 79 – Li et al. [5–7] Na-rhoZMOF 25 100 20 6.2 – 440 – Nalaparaju et al. [137] Mg-rhoZMOF 25 100 20 8 – 680 – Nalaparaju et al. [137] Al-rhoZMOF 25 100 20 8 – 590 – Nalaparaju et al. [137] MIL-53(Al) 30 1000 100 5 – 5.5 – Camacho et al. [138] MIL-100(Fe) 30 101.3 15 0.67 1894 4.6 Stable Xian et al. [139] MIL-101(Cr) 30 101.3 15 1.05 3360 5.5 Stable Xian et al. [139] 235 236 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

Table 2 Comparison of different adsorbents.

Speciﬁcations Zeolites Carbon-based adsorbents MOFs

Major application H2 production High pressure CO2 adsorption ﬂue gas CO2 separation

CO2/N2 selectivity Low Moderate selectivity for CO2 over N2 High Energy for regeneration Significant Lower temperature for regeneration compared to Limited by low temperatures for zeolites. Better energy efficiency compared to metal generation, but still low economic oxides efficiency Capacity Moderate Lower than zeolites at low pressures and gets high at High high pressures Stability under moisture Reduced capacity Do not suffer from breakthrough or decreased Mainly unstable: improvement under conditions capacity under moist conditions research Cost Low production cost Reasonable cost Expensive Advantages Large micropores/mesopores High conductivity Possibility of tuning the pore size

Medium CO2 adsorption at High thermal and chemical stabilities Large surface area ambient conditions Light weight with high surface areas as well as large pore volumes Energy consumption is low

Disadvantages Adsorb moisture, so CO2 Low adsorption and desorption temperatures Has low performance at partial pres-

adsorption is poor with mois- Low CO2 uptake compared to some types of Zeo- sure of CO2 ture existence lites and MOFs Low economic efficiency High energy consumption Synthesis is tedious and complicated Difficult readiness So sensitive to moisture It is difficult to use at high temperatures due to destroying the MOF construction

UiO-66 was synthesized and evaluated by Andersen et al. [142] as (Cr), respectively, at 30 °C and 10 bar. This is attributed to the fact 3 adsorbent for post combustion CO2 capture using vacuum swing that the pore volume of HKUST-1 (0.58 cm /g) is smaller than that adsorption (VSA) process. The study focused on equilibrium iso- in MIL-101(Cr) (1.3 cm3/g), even though, the surface area of MIL- 2 2 therm, breakthrough curves, purity, and recovery of CO2 (for 15% 101(Cr) (2549 m /g) was over that of HKUST-1 (1326 m /g). The dry CO2 and for 15% of CO2 associated with 9% of water vapor; comparison between both MOFs was done by TSA at 25 °C for the remaining fraction was N2). Single adsorber column of 1.1 cm adsorption and 100 °C for desorption (with purging N2). It was diameter and 10.5 cm of length was used in experimental work. noticed that HKUST-1 had a higher CO2 adsorption capacity The gases were directed by solenoid valves while the mass flow (1.82 mmol/g) than MIL-101(Cr) (1.17 mmol/g) at this condition. controllers determined the need amounts of CO2 and N2 to mix Furthermore, HKUST-1 was exploited to compare the sorption and to purge into the adsorbent. Six steps represented the VSA capacity for TSA and VSA processes. The CO2 regeneration showed cycle. These are feed pressurization, counter-current blow-down obviously that the TSA is better than VSA. The amount of CO2 des- (adsorption), concurrent rinse with CO2, counter-current evacua- orbed by VSA was about 1.05 mmol/g for 16 min while the desorption (desorption), and counter-current evacuation with nitrogen tion of CO2 by TSA process was up to 1.85 mmol/g for 100 °C after purge (completing desorption). Equilibrium isotherms of CO2 and 6 min only. These behaviors were interpreted by the MOFs con- N2 were obtained at 303 K and 328 K for pressure increased up taining co-ordinately unsaturated metal sites (CUMs) that might to 100 kPa. The results showed that the best CO2 adsorbed not be efficient desorption by VSA. Xu et al. [134] synthesized amounts were obtained at high pressures and low temperatures. two types of MOFs (Zn2(hfipbb)2(ted) and Co2(hfipbb)2(ted)) Breakthrough curves were evaluated for three different conditions and only investigated the CO2 adsorption in one of them of pressure (2 bar, 3 bar and 4 bar) and the obtained values showed (Zn2(hfipbb)2(ted)). The study reported microporous MOFs synthe- the longer time was for the higher pressure which exhibited the sis, crystal structure analysis, porosity characterization and CO2 better adsorption process. Increasing the times for adsorption adsorption selectivity and capacity as well. For 298 K and 1 atm and rinse processes (up to 61% and 13% of CO2 breakthrough time condition, the equilibrium isotherms showed the maximum CO2 for adsorption and rinse time, respectively) enhanced the recovery adsorption was about 2% (by wt.) and the selectivity ranged and purity of CO2 up to 70% and 60%, respectively. The effect of between 208 and 40 for low vacuum pressure and up to 1 atm. water vapor was also studied through 50 consecutive cycles; it These values of selectivity were claimed to be higher than zeolite showed that the CO2 capacity of adsorbent is reduced 25% without materials and some MOFs as Cu-TPBTM, CuBTTri and PCN-61. It any deterioration of MOF compared to dry cases. was observed that the adsorption heat was close to be constant

(27 kJ/mol). The other results concerned with H2 adsorption and 4.2.1. Adsorption desorption regeneration pure CO2 adsorption.

Adsorption, desorption and regeneration of CO2 in two types of MOFs (HKUST-1 and MIL-101(Cr)) were experimentally investi- 4.2.2. Adsorption and kinetic studies gated by Ye et al. [133]. The experimental set-up was built from Another MOF called CPM-5 was synthesized and undergone to one adsorbent bed connected to two cylinders; one had mixture CO2 adsorption equilibrium and kinetic study by Sabouni et al. of CO2 (20% by volume) and N2 and the other was filled by pure [135]. Adsorption studies of carbon dioxide started by investigat- N2 (for supporting desorption process). The concentrations of efflu- ing the adsorption equilibriums of CO2 and N2 for pressure up to ent gases from adsorbent bed were measured by a dual channel gas 105 kPa and for three different temperatures (0, 25 and 40 °C). chromatograph fitted with a thermal conducted detector using H2 BET instruments were used for measuring the adsorption equilibri- as the carrier gas. The study started focusing on the CO2 adsorption ums volumetrically and ASAP 2010 system equipped with software capacity of both HKUST-1 and MIL-101(Cr) at temperature (Rate of Adsorption program) to measure CO2 adsorption rates. The varied between 30 and 200 °C and pressure up to 10 bar. The experiments commenced with degassed process at 423 K and corresponding results showed that the maximum CO2 adsorption vacuum pressure (10–6 kPa) previous to adsorption process. capacities were 8.07 and 7.19 mmol/g for HKUST-1 and MIL-101 Unlike many of MOFs, CPM-5 showed stable structure under Lab R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 237

conditions with relevant humidity of 62% for several weeks. and Cu–HF) under exposure to moist air, liquid water, SO2 and Regarding to experimental isotherms at several conditions, CO2 NO2. They signiﬁcantly emphasized on three types: Cu–HF, adsorption rate was about 3 mmol/g (13.2 wt.%), 2.3 mmol/g Zn–NDC and Ni–Nic as they had larger adsorption capacity and (10.1 wt.%) and 1 mmol/g (4.3 wt.%) at 105 kPa for 273 K, 298 K selectivity than the other four types. Exposing Cu–HF, Zn–NDC and 318 K, respectively. Moreover, the selectivity factor of CPM-5 and Ni–Nic to liquid water and NO2 during 5 days decreased the was evaluated as 14.2 for 273 K and 16.1 for 298 K. CO2 diffusivity CO2 adsorption capacity of Zn–NDC by about 30% due to partial in CPM-5 at 273 K, 289 K and 318 K for the same pressure decomposition of organic structure, whereas, Cu–HF and Ni–Nic (105 kPa) was estimated as 1.86 ⁄ 1012 m2/s, 7.04 ⁄ 1012 m2/s did not suffer from decompositions. Oppositely, Cu–HF and 12 2 and 7.87 ⁄ 10 m /s, respectively, while the maximum Ni–Nic showed decreases in CO2 adsorption capacity under expos- adsorption heat was about 36 kJ/mol. Comparison to other MOFs ing to humidity (3 days) and SO2 (2 days) while Zn–NDC expressed in the literature in terms of adsorption capacity performance, the some increasing in adsorption in the same exposed gases.

CPM-5 showed a better CO2 adsorption performance than some The best MOF type (Mg-MOF-74) also has some CO2 adsorption kinds of MOFs as MOF-5 and MOF-177 and in the same adsorption deﬁciency with existing of moister, unlike HKUST-1 type. The study capacity performance of MIL-53(Al), UMCM-150 and Ni-STA-12. investigated by Yu and Balbuena [145] showed the decreasing of

However, the adsorption capacity of CPM-5 is lower than function- CO2 adsorption at several conditions. For 1 bar and 298 K, the dry alized and open metal sites MOFs such as HKUST-1, Mg-MOF-74 Mg-MOF-74 could adsorb about 8.4 mmol/g of CO2 while with and NH2MIL-53(Al). hydration 6.5% and 13% the CO2 adsorbed amounts were Fourteen different types of MOFs were investigated for captur- 6.7 mmol/g and 5.4 mmol/g, respectively. Meanwhile, the CO2/N2 ing CO2 from the flue gas by Yazaydın et al. [143]. Seven types of selectivity increased significantly due to drop in N2 adsorption in MOFS were synthesized, characterized and measured regarding hydrated gas. The interpretation of CO2 decreases with existing to the adsorption properties while the other 7 types were taken humidity was the strong binding energy between CO2 and co- from the literature to study their CO2 capture capability. Some ordinately unsaturated metal sites in MOF more than the binding experimental and simulation work was done for this purpose; energy between CO2 and coordination water interacting. The the simulation study was performed by use Grand Canonical reverse action (the binding energy between CO2 and coordination Monte Carlo (GCMC) at the ambient conditions (room temperature water interacting is stronger) made the HKUST-1 adsorbing more and 0.1 bar, the normal partial pressure of CO2 in flue gas). The CO2 under increasing of hydration level. IRMOF-74-III as a MOF experimental work demonstrated that the best types could be used was covalently functionalized by anime [146] to study impact of for CO2 adsorption were Mg/DOBDC (above 250 mg/g) followed by humidity on the MOF construction and CO2 adsorption capacity. Ni/DONDC (180 mg/g) and CO/DOBDC (140 mg/g). On the other The anime compounds added to IRMOF-74-III were –CH3, –NH2, hand, the worst types were ZIF-8, IRMOF-3, IRMOF-1, UMCM-1 –CH2NHBoc, –CH2NMeBoc, –CH2NH2, and –CH2NHMe. IRMOF-74- and MOF-177 (all of them less than 10 mg/g). Another point was III-CH2NH2 showed high adsorption capacity of CO2 (3.2 mmol/g the reversal effect of the metal–organic (M–O) bond length, it at 106 kPa and 298 K) and was not affected by water vapor. Com- showed that the good captured CO2 was for lower M–O bond paring dry and wet (RH = 65%) cases of flue gas (16% CO2, and the length (Mg–O (1.069 Å) is better than Ni–O (2.003 Å)). The simula- balance was N2), the breakthrough curves ware identical for both tion study proved only some agreements with experimental data cases (dry and wet by using IRMOF-74-III-CH2NH2). in the cases of the best MOFs types for CO2 pressure about 0.5 and 1 bar. 4.3. Experimental studies on adsorption by zeolites

4.2.3. Temperature swing adsorption methods 4.3.1. Pressure swing adsorption process

Two types of MOFs (MOF-177 and Mg2-dobdc (Mg/DOBDC)) Flue gas separation by zeolite 13X through pressure swing were compared to capture CO2 for post-combustion by using tem- adsorption process (PSA) was investigated by experimental and perature swing adsorption method (TSA) [24]. Effect of tempera- mathematical model at two different temperatures (50, 100 °C), ture range between 20 °C and 200 °ConCO2 caption was Dantas et al. [129]. The experimental set-up relayed on fixed bed investigated at low pressure (0.15 bar for CO2 in flue gas) to study filled with zeolite 13X which was undergone to four steps to rep- the equilibrium isotherms of both MOFs as well as of zeolite NaX resent separation process namely: pressurization, flue gas feed

(well known in the literature). The results showed that (15% CO2, 85% N2 by volume), blowdown (depressurization), purg- Mg2-dobdc exhibited the best capture performance: in term of ing. The gas chromatograph unit was used to measure the outlet amount of adsorbed CO2,Mg2-dobdc adsorbed 189 mg/g at 40 °C concentrations of CO2 and N2 and mass ﬂow controllers were used whereas Zeolite NaX and MOF-177 captured about 81 and to control the ﬂow amount of gases during working. Pressurization

4.3 mg/g, respectively. Furthermore, the selectivity of Mg2-dobdc process was used to rise the pressure of the bed up to 1.3 bar with is the highest (148.1 at 50 °C, while 87.4 and unity for zeolite purging nitrogen, and then, the mixture of CO2 and N2 was fed to NaX and MOF-177, respectively). In addition, the working capacity the bed at constant pressure (1.3 bar) to represent the adsorption by means of desorbing amount of CO2 at higher temperatures process. After CO2 saturation observed, the inlet gases was closed indicated a superior amount for Mg2-dobdc over the others. Thus, with depressurization the bed down to 0.1 bar for remove adsor- 0–176 mg/g could be desorbed by Mg2-dobdc for temperature bent amount of CO2. For enhancing the desorption process, some between 90–120 °C and about 0–75 mg/g could be desorbed by amount of nitrogen was purging to the bed under low pressure zeolite NaX while MOF-177 did not express any positive values (0.1 bar), this process called purging process. The experimental of desorbed CO2 at the same range of temperature. and theoretical equilibrium isotherms showed that zeolite 13X could adsorb 3 mmol/g of CO2 at 1 bar and 50 °C and about 4.2.4. Performance in presence of water vapor 1 mmol/g of CO2 for 100 °C at the same pressure while the notice- The most issue faces the use of MOFs as the adsorbents in sep- able adsorbed amount of N2 was less than 0.25 mmol/g for the aration processes is the decomposition under exposure to humid same conditions. The results also showed good percentages of air. A few researches deal with this issue because the majority CO2 recovery reached about 91.8% and 90% for temperatures 50 dealt with ﬂue gas as a dry mixture gas only consists of CO2 and and 100 °C(P = 1.3 bar), respectively, while the CO2 purity exhib- N2. Han and his co-workers [144] studied the stability of seven ited low percentages about 33.3% for 50 °C and about 36.8% for types of MOFs (CdZrSr, Ni–Nic, La–Cu, Eu–Cu, Zn–NDC, ZnPO3 100 °C. The decrease in purity of CO2 can be solved by adding rinse 238 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

process after adsorption by purging pure amount of CO2 into the gas). The study also claimed that the fully filled pores adsorbed adsorbent to remove N2 and replaced by CO2. This process by N-contains had lower CO2 caption at low temperature (room increases the cycle cost, but it is a solution when the pure CO2 temperature) while significant amount of CO2 was adsorbed for (above 90%) is needed. Fig. 6 and Table 3 show a schematic of higher temperature (as 70 °C), and the vice versa for partially filled PSA and valve sequencing for different steps in the cycle pores (open pores) by N-containing. Generally, for open pores respectively. adsorbent, increasing the gas feed temperature decreased the

In the PSA set up (Fig. 6), the first column (M1) is fed with flue amount of adsorbed CO2 while increasing the feed pressure gas at a pressure above atmospheric pressure, the packed bed improved the captured CO2; the optimum vacuum pressure to selectively remove CO2 from the gas stream leaving nitrogen rich minimize the power used for adsorption process was 0.04 bar. effluent to flow out from valve 7(V7). After a set time e.g. breakthrough, the adsorbent packed in M1 is saturated hence, it no 4.3.3. Zeolite testing under humid conditions longer adsorbs CO2. The feed is then directed to the second column Experimental investigation of CO2 capture from wet (humid) (M2). In order to regenerate the saturated bed (M1), valve 3(V3) is flue gas was studied by Li et al. [148]. Zeolite X13 was used and opened to initiate pressure drop within the bed. The induced pres- the vacuum swing adsorption method was applied to study the sure causes desorption of the adsorbed CO2 making the gas exiting impact of moist flue gas (PH = 95%) on the adsorption and desorp- V3 rich in CO2. A purge step is then initiated to facilitate additional tion processes at 30 °C. The investigation demonstrated that the removal of CO2 from the column. After purging, the bed pressure is CO2 recovery reduced by 22% with existence of H2O. Furthermore, restored by pressurizing with the less adsorbed gas. These are the high concentration of H2O appeared during vacuum process and four steps that make up a typical PSA cycle. At the end of a com- about 27% of the condensed H2O was accumulated in the vacuum plete cycle additional cycles can be conducted to ensure further pump itself. A comparative experimental study between two purity of the desorbed stream. adsorbents (13X and A5 Zeolites) for CO2 capture by indirect thermal swing adsorption (indirect heating/cooling by internal heat

exchanger) was studied by Mérel et al. [149]. 90% of N2 and 10% 4.3.2. Vacuum swing adsorption of CO2 were modeled the flue gas to pursue CO2 capturing. The Zeo- The problems associated with use vacuum swing adsorption lite A5 showed the better performance than Zeolite 13X for captur- were investigated by Chaffee et al. [147] by improving the cycle ing CO2 such as the capture rate of CO2, volumetric productivity design with good temperature control. The adsorbent was zeolite and specific heat consumption were (+14.5%), (+22%) and (19%), 13X to capture CO from flue gas (simulated by adding pure CO 2 2 respectively, for Zeolite A5 over than 13X. to the air). This adsorbent material was insensitive to moisture. The experimental work for CO2 capture from flue gas of coal Furthermore according to the results, the CO adsorption might 2 fired power plant is studied by Wang et al. [150] using zeolite be increased in the presence of H O; N-containing hybrid material 2 13XAPG by vacuum pressure swing adsorption technique VPSA). adsorbed higher amount of CO than N (contained in feed flue 2 2 The capture plant consisted of two units: dehumidification unit

and CO2 capture unit. The dehumidiﬁcation unit consisted of two cylinders ﬁlled with 156 kg of alumia for removing water vapor and the contaminants amount of SOx and NOx via temperature V7 V8 swing process. The output gases of this unit were CO2 (15.5– 16.5% by volume) and N2 and less than 0.5% of relative humidity. V5 V6 The other unit formed of three column cylinders (adsorbers) occu- pied by 261 kg of zeolite 13XAPG for representing CO2 capture unit by VPSA process. The cycle of the VPSA was quite complicated to consist of eight steps for each adsorber such as pressurization, feed, depressurization, rinse, provided pressure equalization, blowdown, purge, and received equalization. All processes were done auto- matically by programmable logic controller and software. The M1 M2 results showed the beds reached steady state after 100 operating cycles and the adsorption temperature raised to 323 K. The adsorp-

tion isotherms announced the maximum CO2 adsorption was about 4.3 mmol/g comparing with 3 mmol/g with using A5 molecular sieve in their previous work at the same conditions (T = 303 K, P = 100 kPa). For inlet ﬂow rate of ﬂue gas about 32.9–45.9 Nm3/h,

the CO2 recovery and purity were about 85–95% and 37–82%, V3 V4 respectively, with power consumed for blower and vacuum pump about 1.79–2.14 MJ/kgCO2 (two third of the consumed power was by vacuum pump). The maximum CO2 productivity of the unit was V1 V2 3 0.207 molCO2/m adsorbent. Zeolite 13X-APG was utilized by Wang and his co-workers [130] Fig. 6. Schematics design of two-column PSA unit. as the adsorbent for post combustion CO2 capture by VTSA process. Experimental and simulation investigation focused mainly on the type of process such as TSA, VSA and VTSA that was more efﬁcient Table 3 in terms of CO2 recovery and purity. The setup consisted substan- Valve sequencing for different steps in PSA cycle. tially of one bed heated and cooled indirectly by oil passing around the adsorber. The studied ﬂue gas had 15% of carbon dioxide by M1 Feed Blow down Purge Pressurization V1, V7 V3 V5, V3 V1 volume while the complement percentage was nitrogen. The maximum isotherm adsorption was about 4.3 mmol/g of CO at 303 K M2 Purge Pressurization Feed Blow down 2 V4, V6 V2 V2, V8 V4 and 100 kPa. The comparison of results among the three generation methods (TSA, VSA and VTSA) illustrated that the best CO2 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 239

recovery and purity for VSA process were 78.6% and 78.4%, respec- the commercial activated carbon NORIT R2030CO2 to study its tively, at P = 3 kPa for 5 min of evacuation and 0.15 SLPM of N2 CO2 adsorption capability from ﬂue gas (17% CO2, 83% N2 by vol- purging while alone. TSA process without evacuation could achieve ume) and comparing some regeneration methods. The set-up of

78.1% of CO2 recovery and 91.6% of CO2 purity for 443 K of desorp- experimental work consisted of one adsorbent bed receiving a mix- tion. The cooling was at close to ambient conditions during 10 min ture CO2/N2 from two cylinders, each for one gas controlled by to maximize adsorption capability. In the other hand, the com- mass flow controller and then mixing by a helical distributor. bined processes in one process (VTSA) at 403 K of desorption tem- The bed was heating by a coil around it and the outlet of the bed perature and 3 kPa of vacuum pressure could reach 98.2% and 94% was connected by pressure regular and then by dual channel chro- of CO2 recovery and purity, respectively. Furthermore, researchers matograph fitted with thermal conductive detector to calibrate and conducted with the Zeolite 13X as adsorbent for CO2 capture and measure the output concentrations of effluent gases (CO2 and N2). the generation processes correspondingly are shown in Table 4 to The study addressed the comparison between TSA, VSA and VTSA 3 show the ability of this material (Zeolite 13X) of adsorption CO2 for flow rate of 34 cm /min and adsorption pressure of 130 kPa at several conditions. as well as the adsorption temperature was 303 K. The isotherms

showed the maximum CO2 adsorption was about 2.4 at 120 kPa 4.4. Experimental studies on adsorption by carbon-based materials and 303 K and the CO2/N2 selectivity was 7 at the same conditions. TSA announced the smallest values of the CO2 recovery and pro- 4.4.1. Activated carbon ductivity by about 40% and 0.8 mmol/g hr, respectively, at the González et al. [124,120] prepared a cheap activated carbon mentioned adsorption conditions (T = 303 K, P = 303 kPa) since N2 purging for desorption process (at 373 K and 2.7 cm3/min). How- from spent coffee grounds to study the potential of CO2 capture ever, VSA adsorption performed under the vacuum (P = 5 Pa) and by adsorption of flue gas mimicking the post combustion CO2/N2 percentages. Two types of activated carbon obtained from spent temperature about 303 K produced about 1.7 mmol/g hr of CO2 coffee ground were investigated in this study such as NCLK3 and with 87% of recovery. For enhancing the performance, VTSA was NCHA29 at pressure between 0 and 120 kPa and temperature var- applied to produce about 1.9 mmol/g hr and to increase the CO2 ied between 0, 25 and 50 °C by volumetric apparatus. The isother- productivity up to 97% under the vacuum conditions and increasing temperature to 323 K. mal adsorption showed NCLK3 had about 3.5 mmol/g of CO2 as a maximum adsorption at 120 kPa and 25 °C with average heat of adsorption about 27.19 kJ/mol while NCHA29 was less efficient 4.4.2. Carbon fibre composites with CO2 adsorption with about 2.3 mmol/g at the same conditions Carbon fibre composites also promised a better CO2 capture and 36.42 kJ/mol of isosetric adsorption heat. The selectivity and compared to other types of activated carbon, Thiruvenkatachari adsorption working capacity also showed some advantages for et al. [123]. It was synthesized by consolidation as a one brick. NCLK3 over NCHA29 in which the authors claimed that NCLK3 There were some small tubes put inside the material for air and was competitive with zeolite 13X. water heating and cooling during desorption and adsorption pro-

The main properties of the adsorbent affecting CO2 capture by cesses, respectively. Two large beds (2 m) were filled with adsor- adsorption was experimentally investigated by Marco-Lozar et al. bent for investigation the CO2 capture at ambient conditions [121] through comparing the adsorption performance of 17 types (298 K and 1 bar) from flue gases which contained 13% CO2, 5.5% of activated carbon. The different pore size distribution and density O2 and the remaining was N2. The setup controls and monitors of the adsorbent were found to play main roles of selection of included flow mass meter, CO2 analyzer, O2 analyzer and volume adsorbent type at proper pressure. For pressure between 0.1 and meter. The study relied firstly on temperature swing adsorption 1.2 MPa and ambient temperature (post combustion case), it was method for adsorbent regeneration at T = 383 K and ambient pres- observed that the adsorption capacity did not change much by sure (1 bar) without purging any gas and then on vacuum swing increasing microspore volume and it was appropriate to consider adsorption for ambient temperature and 30 kPa of pressure. How- the volume of the microspore less than 0.7 nm. However, in appli- ever, the results showed the two methods were not sufficient for cation that have higher operation pressure (>1.2 MPa: pre combus- efficient recovery CO2 and then suggested vacuum temperature tion and oxy combustion cases), the microspores volume should be swing adsorption for efficient regeneration process. larger to adsorb more amount of carbon dioxide. Regarding to den- The maximum adsorbed CO2 showed by adsorption isotherms sity, the adsorbent bed has a specific volume, so the less adsorbent was 2.51–3.1 mmol/g at ambient condition which added some density means a little amount of the solid material would occupy advances to activated carbon CO2 capture research. Regarding to the size and that significantly reduces the overall amount of desorption techniques, TSA at 398 K and 1 bar had 100% of CO2 adsorbate material. Therefore, the larger density with high concentration and the CO2 recovery was less than 20% while VSA adsorption capacity was preferable. Plaza et al. [122] focused on at 298 K and 30 kPa presented lower than 5% of CO2 recovery with

Table 4

CO2 capture by zeolite 13X.

Process Cycle steps CO2% Ads./des. Ads./des. temperature Recovery Purity Ref.

(by vol.) pressure (kPa) (°C) CO2 (%) CO2 (%) PSA FP,FD,DP,PUR 15 130/10 50–100 91.8–90 33.3–36.8 Dantas et al. [129] PSA FP,FD, DP,PUR 8.3 303/101.3 25 50 78 Gomes and Yee [151] VPSA (2-stages) 1st-stage: EQ,FP,FD,EQ,DP,PUR 10.5 6.67 30 80 99 Cho et al. [152] 2st-stage: EQ,FP,FD,EQ,DP 15 13.34 30 78.8 99.7 VSA FP,FD,DP 11.2 118/3 30 78.5 69 Li et al. [148] VSA FD,PR1,PR2,EQ,RIN, DP,EQ,PR3,PR4 8–15 130/5–6 40 60–70 90–95 Zhang et al. [153] VSA FP,FD,EQ,RIN1,RIN2,DP,EQ 13 172/5.07 30 69 99.5 Choi et al. [154] TSA 10 101 15/110 56 100 Merel et al. [155] VTSA FP,FD,H,DP,PUR,C 15 101/3 30/90 98.5 94.4 Wang et al. [130]

FP, pressurization with feed; FD, feed; RIN, rinse; EQ, pressure equalization; DP, depressurization; PUR, purge; PR, re-pressurization; H, heating; C, cooling. 240 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 higher energy consumption by vacuum pump. On the other hand, desorption, purging with electrothermal desorption, and cooling utilizing two methods simultaneously (VTSA: T = 398 K and while the other case study expanded the capture cycle to six steps P = 75 kPa) enhanced the performance signiﬁcantly. Besides VTSA, such as feeding, rinsing, electrothermal desorption, purging with

flushing some amount of pure CO2 soon after adsorption process electrothermal desorption, purging and cooling. The flue gas in the- (for remove the amounts of adsorbed N2 and O2 from the bed) ses cycles was about 8.1% (by vol.) of CO2 and the balance is N2. For improving the CO2 recovery up to 97% with 100% of the purity. the same cycle time of the two cases study, the results showed the Two cheap activated carbon adsorbents were made from olive six steps cycle had higher CO2 purity about 46.6% compared to stones and almond shells with single step activation for investigat- 44.8% of four step cycle due to rinse process, whereas the CO2 ing CO2 adsorption separation of flue gas [124]. The study consid- recovery had high percentage for four step cycle with 92.4% and ered the equilibrium isotherms at different conditions (0, 25 and the lower was 81.4% for the six steps cycle. However, the cost of 50 373 K and 2.7 cm3/min of for pressure reached 120 kPa). For both was considered high compared ESA to other process tech- olives stone carbon, the maximum adsorption of CO2 was about niques with about 44.8 GJ/tonCO2 and 33.3 GJ/tonCO2 for four 3.2 mmol/g for 100 kPa and 25 °C while almond shell carbon and sex steps cycles, respectively. Moreover, the hybrid adsorbent showed about 2.5 mmol/g at the same conditions. Simulating the addressed some drawbacks as enlarging the mass transfer zone

flue gas by 14% CO2 and 86% N2 and passing it through adsorbents, due to non-homogeneously. Some increasing in adsorbed amount the obtained breakthrough curves determined that the break- of dioxide carbon and elongating the breakthrough curve were through time of olive stone-based carbon had lower time than due to existing of zeolite 13X itself with good percentage (82%). almond shell-based carbon (by 1 min out of 8 min). But, the CO2 The adsorption behavior of zeolite 13X to Methane, Nitrogen adsorption capacity of olive stone-based carbon expressed a little and Carbon Dioxide were investigated experimentally by Cavenati higher value than that in the almond shell-based carbon et al. [158]. Activation of Zeolite 13X samples was done with (0.61 mmol/g for olives type and 0.58 for almond shell one at Helium, under vacuum through the night at temperature of 120 kPa and 50 °C). The desorption process in this study was done 593 K. The samples were heated at a rate of 2 K/min while Iso- by passing helium gas, because it only focused on adsorption pro- therms were measured at 293, 308 and 323 K at pressure range cess regardless the complete cycle methods. of 0–5 MPa. All of the Isotherms were made completely reversible. A Magnetic Suspension Microbalance (Rubotherm) was employed to perform adsorption equilibrium of the pure gases. The authors’ 4.5. Other experimental studies on adsorption data fitted with the Toth and Multisite Langmuir Model. A strong CO adsorption was recorded, which make them recommend Zeo- 4.5.1. Regeneration process techniques 2 lite 13X as potential material for CO sequestration from flue gas. The regeneration process (desorption) refers to the rejection of 2 Casas et al. [159] performed breakthrough experiment, describing the adsorbed amount and the best measures for its performance pre-combustion CO capture using MOFs (e.g. USO-2-Ni MOF) that is CO recovery and CO purity. The performance of regenera- 2 2 2 and UiO-67/MCM-41 hybrid adsorbents by Pressure Swing Adsorp- tion process techniques for purity and recovery of flue gas was tion (PSA). MOF UiO-67/MCM-41 hybrid was designed jointly with summarized as shown in Table 5, Clausse et al. [156]. It is clear from meso-porous silica, (i.e. MCM-41), of average sized particles: say this table and as mentioned above [122,123,120,156] that the best 1 mm. MCM-41 has a very good adsorption capacity, stabilizing percentages of CO recovery and purity above 90% were obtained 2 effect, and lower Henry’s constant. These are favorable characteris- by combined processes such as pressure temperature swing tics for desorption at high pressure. Furthermore, the 1 mm parti- adsorption (PTSA) and vacuum temperature swing adsorption cles qualify for use at industrial level, for feasible range of resulting (VTSA). Also, the CO recovery and purity reasonable percentages 2 pressure drop. On the other hand, formulation of USO -Ni MOF can be obtained from vacuum pressure swing adsorption process. 2 particles is yet to be up scaled; therefore, only particles of size A hybrid adsorbent consisted of monolithic activated carbon 0.2–0.5 mm were produced. Material and particle densities were and zeolite was investigated for CO capturing performance using 2 characterized by Helium pycnometery and Hg-pycnometery electrical swing adsorption technique (ESA) [157]. The holes in respectively. The material heat capacity of the two materials was consolidated activated carbon were filled by Zeolite 13X to occupy estimated with the use of a Differential Scanning Calorimeter about 82% of the volume of the bed. ESA was designed to desorb (DSC). The authors [159] performed process scale up by first con- the adsorbed amount of CO inside the adsorbent by electrother- 2 ducting a fixed bed experiment, during which the adsorbent was mal regeneration (Joule effect) with temperature reached about packed into column after which three grades of CO /H mixtures 460 °C. Furthermore, ESA was represented by two cases: first case 2 2 were feed through them at temperature of 25 °C and pressure was performed by four steps such as feeding, electrothermal range of 1–25 bar to determine the transfer parameters. In the breakthrough experiment, it was found that the feed flow rate Table 5 had negligible impact on the mass adsorbed and heat transferred Comparison among different regeneration processes in terms of CO2 purity and recovery [156]. under the considered span of conditions. Adsorption Isotherm was measured for pure components with the aid of a Magnetic Sus- Process CO purity (%) CO recovery (%) 2 2 pension Balance (Rubotherm), Langmuir isotherm and correspond- ESA 23.33 29.57 ing isotherm parameters were reported. A comparison was made VPSA 99 53–70 between their results obtained at another result of the same pro- PTSA 99 90 2-bed-2-step PSA 18 90 cess using activated carbon as adsorbent. The comparison showed VPSA 99.5–99.8 34–69 that the selectivity and productivity of the PSA (Pressure Swing PSA 99.5 69 Adsorption) process was increased by the introduction of USO-2- PSA/VSA 58–63 70–75 Ni MOF, compared to activated carbon. UiO-67/MCM-41 hybrid VSA 90 90 PSA/VSA 58 87 showed faintly lower selectivity, but higher specific adsorbent pro- PSA/VSA 82.7 17.4 ductivity compared to activated carbon. 3-bed VSA 90–95 60–70 TSA 95 81 4.5.2. Adsorbent packing processes ESA 89.7 79 Formation of particle is very important; it has a huge effect on PSA 16 89 the adsorbent packing properties, hence, on the process R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 241

performance by Casas et al. [159]. They concluded that existing maximum CO2 adsorption was about 2.4 at 120 kPa and 303 K research on formulated MOFs with average particles size greater and the CO2/N2 selectivity was 7 at the same conditions. The best than 1 mm (that permit scaling up) is adequate to enable their percentages of CO2 recovery and purity above 90% were obtained exploration for industrial scale usage. In addition, bed density by combined processes such as pressure temperature swing and particles, are of great importance in process design. This is adsorption (PTSA) and vacuum temperature swing adsorption because they are responsible for the quantity of adsorbent materi- (VTSA). Also, the CO2 recovery and purity reasonable percentages als that can be packed in enclosed column volume. In this light, the can be obtained from vacuum pressure swing adsorption process. UiO-67/MCM-41 hybrid showed good packing properties, not In the breakthrough experiment, it was found that the feed flow withstanding, further research and improvement is required in rate had negligible impact on the mass adsorbed and heat their mechanical stability in order to make them useable on indus- transferred under the considered span of conditions. The results trial scale. Dantas et al. [128] worked on fixed bed CO2 adsorption showed that the selectivity and productivity of the PSA (Pressure from a gas mixture of 20%CO2/N2. The adsorption medium used Swing Adsorption) process was increased by the introduction of was activated carbon. Helium was used for pre-treatment of the USO-2-Ni MOF, compared to activated carbon. UiO-67/MCM-41 bed. Break through curves were obtained by varying temperatures, hybrid showed faintly lower selectivity, but higher specific adsor- while Linear Driving Force approximation (LDF) was used for the bent productivity compared to activated carbon. The results mass balance, the momentum and energy balance were also showed that the adsorption selectivity was high (>100) in some accounted for in order to reproduce the break through curves. types of MOFs and Zeolites. However, the adsorption capacity val- Investigation of changes in the surface of the activated carbon used ues highlighted the type MgMOF-74 as the best (3.3 mmol/g for due to CO2 accumulation was carried out with Fourier Transport 300 K and 100 kPa). In additions, MgMOF-74 had the longest Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy breakthrough time which got advantages to increase amount of

(XPS) analysis. Gas mixture was subjected to different tempera- CO2 adsorbed. tures of 301 K, 323 K, 373 K, and 423 K at a total pressure of 1.02 bar. The adsorption column was located inside a furnace for easy control of the process temperature, the column and furnace; 5. Numerical investigations and mathematical models for fixed which was the adsorption system; were therefore assumed bed column adsorption adiabatic as they were isolated using a fiberglass layer and a non-convective refractory material. However, the breakthrough 5.1. Introduction experiment was treated adiabatically. Siriwardane et al. [38] also observed similar behavior while using 13X zeolite for CO2/N2 gas In order to achieve a suitable and effective design of adsorption mixture adsorption. Dantas et al. [127] suggested that resistances process, there is need for an appropriate model to describe the to internal mass transfer are negligible in the adsorption system. dynamics of the adsorption system [128,38,127]. Most of sug- It was suggested that for turbulent system, mass spread is due to gested models are mathematical models and more recently, Artifi- axial dispersion [129]. cial Neural Network models (ANN) [160] amongst others. The computer simulation tool requires experimental validation for 4.6. Concluding remarks the development of new system. Since experimental setups are quite costly and time consuming, a mixed design approach using Some concluding remarks can be summarized in the following. a well validate simulation tool with reasonable experimental vali- It is indicated that the MOFs types had higher pore volume and dation seems to give the best design results. The simulation tool is surface area than zeolite types. The most issue faces the use of composed of a descriptive mathematical model to predict the MOFs as the adsorbents in separation processes is the decomposi- adsorption system (fixed bed/column) behavior [161]. Such tion under exposure to humid air. The best MOF type (Mg-MOF-74) mathematical models are experimentally verified and make use also has some CO2 adsorption deficiency with existing of moisture, of independent parameters to estimate the required dynamic unlike HKUST-1 type. The experimental work demonstrated that properties of the adsorption system with no extra time and cost the best types could be used for CO2 adsorption were Mg/DOBDC as compared to the experimental procedures. The models also (above 250 mg/g) followed by Ni/DONDC (180 mg/g) and CO/ enable break through curve estimation, temperature profile of con- DOBDC (140 mg/g). On the other hand, the worst types were ZIF- stituent gases at different time and point within the adsorption 8, IRMOF-3, IRMOF-1, UMCM-1 and MOF-177 (all of them less than column. Varieties of materials and their properties could be quickly 10 mg/g). The effect of water vapor was also studied through 50 and easily tested using the mathematical models. In addition, consecutive cycles; it showed that the CO2 capacity of adsorbent variations in compositions and temperatures within the adsorbent is reduced 25% without any deterioration of MOF compared to column, with respect to time and space, and their effect on the dry cases. The CO2 regeneration showed obviously that the TSA is overall performance of the adsorbent system; can be modeled better than VSA. Comparing to other MOFs in the literature in and simulated [162]. terms of adsorption capacity performance, the CPM-5 showed a Mathematical models capable of predicting the dynamics of better CO2 adsorption performance than some kinds of MOFs as adsorption systems are made of coupled partial differential equa- MOF-5 and MOF-177 and in the same adsorption capacity tions representing the flow field, mass and energy transfer within performance of MIL-53(Al), UMCM-150 and Ni-STA-12. Exposing the field (mass, species, momentum and energy balances) [128].

Cu–HF, Zn–NDC and Ni–Nic to liquid water and NO2 decreased The flow field is usually modeled as a fixed bed (with suitable the CO2 adsorption capacity of Zn–NDC by about 30% due to partial boundary condition) in which adsorption takes place. A simultane- decomposition of organic structure, whereas, Cu–HF and Ni–Nic ous solution is required for the system of PDE’s, making the solu- did not suffer from decompositions. Oppositely, Cu–HF and tion to the system involved and complex, hence the need for a

Ni–Nic showed decreases in CO2 adsorption capacity under expos- simplified model with good assumptions for easier computation ing to humidity and SO2 (2 days) while Zn–NDC expressed some and optimization. The study of modeling and optimization of CO2 increasing in adsorption in the same exposed gases. Generally, adsorption on fixed bed has grown over the years and is still of for open pores adsorbent, increasing the gas feed temperature important interest in the field of Carbon Capture and Sequestration decreased the amount of adsorbed CO2 while increasing the feed (CCS). The dynamic behavior of an adsorption chamber system can pressure improved the captured CO2. The isotherms showed the be categorized based on the nature of the relationship between the 242 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 constituent gas species and the solid at equilibrium and the com- 5.2. Some existing mathematical models plexity of the mathematical needed for describing the adsorption mass transfer process [163]. The complexity of the mathematical Mathematical modeling of CO2 adsorption and separation model for describing adsorption process behavior depends on the depends mainly on the mixture from which CO2 is to be separated. level of concentration, the choice of rate equation and the choice It also depends on the type of adsorption process and the adsor- of flow model [163]. bent media. The following are examples of CO2 separation from dif- The fixed bed mathematical models are used to temporarily ferent mixtures such as CO2/CH4,CO2/N2,CO2/H2,CO2/He, CO2/Air, forecast the performance of an adsorption system in terms of CO2/CO and flue gas mixtures as well as pressure swing or vacuum dynamic property variation of the gas and the adsorption bed dur- swing adsorption. ing adsorption e.g. flow rate, temperature, concentration, etc. The description of the pattern of flow within the adsorption column 5.2.1. CO2 in a binary mixture (with CH4, N2,H2 or He) is usually done using the plug flow model or axially dispersed plug Kumar [168] obtained a mathematical model to describe flow model. Some assumptions are usually made but, they differ adsorption separation of CO2 from binary gas mixtures of Carbon from one model to another. E.g. some models account for the dioxide (CO2) and Nitrogen (N2), Carbon dioxide (CO2) and effects of heat generation and heat transfer in the adsorbent bed, methane (CH4), and carbon dioxide and Hydrogen. The model based reasons that it may affect the adsorption rates etc. Some of was made up of a system of coupled partial differential equations. these assumptions include (a) Ideal gas behavior, (b) Negligible The adsorption media (adsorbents) used was 5A zeolite and BPL radial gradient of concentration (and temperature and pressure carbon. The flow pattern was described using plug flow model, where applicable), (c) Negligible heat transfer between gas and while the mass transfer pattern was described using local equilib- solid phase for non-isothermal operation i.e. instantaneous ther- rium model. The mathematical model was solved numerically mal equilibrium and (d) Negligible pressure drop across bed. The using finite difference method after which adiabatic simulation assumption of negligible radial gradient has been made by a num- was carried out. The following assumptions were made: Negligible ber of researchers [164,165]. A lot of existing models are based on radial variation in temperature, concentration, negligible pressure the effects of finite mass transfer rate with mathematical models drop within bed, thermal equilibrium between the gas and solid closely representing real process. Most of the popular existing particles, and non-isothermal heat effects. A Langmuir–Freundlich models use a linear driving force approximation for the description equilibrium isotherm was assumed. It was concluded that isother- of mass transfer mechanism in CO2 adsorption process. After sev- mal assumption was improper for the process design, but it could eral years of research it has been discovered that it is equally be useful for semi-quantitative forecast of adsorption column important to consider the effect of momentum balance and heat behavior. generation and heat transfer in the adsorbent bed. This is Delgado et al. [169,170] described a mathematical model to important because the concentration profile has a dependence on describe the adsorption separation of CO2 from binary gas mixtures temperature variations, may be eminent for high-concentration (CO2–N2,CO2–He and CO2–CH4) on sepiolite, silicate pellets and a feeds, because the heat of adsorption in high concentration feed resin. The flow pattern was described using axial dispersed plug generates thermal waves which travel in axial and radial directions flow model, while the mass transfer pattern was described using [166]. the LDF approximation model. The mass transfer coefficient was Adsorption equilibrium has been mostly represented with non- determined by fitting the experimental data (i.e. lumped). Ergun’s linear isotherms such as the Langmuir isotherm/hybrid Langmuir– equation was employed to describe the momentum balance of the Freundlich isotherm. Linear isotherms have been used but only few system. The PDE’s in the mathematical model were solved numer- cases. The Langmuir model works on the assumption of ideal local- ically using method of orthogonal collection on finite element ized molecular interaction between adsorbate and adsorbent with using PDECOL software. The following assumptions were made: no further interaction on other groups of identical sites. Adsorption Negligible radial variation in temperature and concentration, neg- system hardly adhere strictly to Langmuir model assumptions, ligible pressure drop within bed, thermal equilibrium between the most times, their equilibrium isotherms deviate from the Langmuir gas and the solid particles, and non-isothermal heat effects. An model form. This may be due to the variation in heat of adsorption Extended Langmuir equilibrium isotherm was assumed. The math- which is required to be constant based on Langmuir. From this, it ematical model gave a good description of the breakthrough exper- can be stated that: Since the heat of adsorption changes with con- iment with lower CO2 concentration. However, for the experiments centration, at lower concentration, the Langmuir model can give an with high concentration of CO2 were predicted with higher per- appropriate representation of the system, however, as the concen- centage of error. It was suggested that, introduction of interaction tration of the gas to be tested increases, the accuracy of the model factor into the model boosted the accuracy of the model based on would drop [163]. Due to the limitations of the Langmuir model, the interaction between adsorbed molecules of CO2. Shafeeyan several authors e.g. Freudlich have modified the model e.g. by et al. [162] reviewed different existing mathematical modeling introduction of power law expression (Langmuir–Freundlich equa- methods of the fixed-bed adsorption of carbon dioxide. Shendal- tions), and a host of other authors. The gas phase material balance man and Mitchell [171] obtained a linear mathematical model includes an axial dispersion term, convective term, fluid phase using characteristic method while working on a mathematical accumulation, and the source term due to adsorption of the gas model to describe Pressure Swing Adsorption separation of CO2 molecules (adsorbate) on the solid surface (adsorbent). The equa- from a binary gas mixture of Carbon dioxide and Helium (CO2– tion accounts for: The variation in adsorbate velocity and concen- He). Their adsorption medium (adsorbent) was Silica gel. The flow tration in fluid phase with distance along the bed, the average pattern was described using plug flow model, while the mass concentration of adsorbate components in the solid adsorbent par- transfer pattern was described using local equilibrium model. ticles, while the axial dispersion coefficient represents the effect of The mathematical model was solved analytically, by assuming: axial mixing and the contributing mechanisms. This equation is Negligible radial variation in concentration, negligible pressure used to find the transportation of gas composition along the bed, drop, trace system and isothermal heat effects. A linear equilibrium with an assumption of negligible radial variation in gas concentra- isotherm was assumed. Their model had a limitation of neglecting tion and solid loading [127,128]. Danchkwert’s boundary condi- the mass transfer resistance effect which made their results differ tions are applied here [162,167]. from experimental results. Cen and Yang [172] obtained a mathe- R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 243 matical model to describe Pressure Swing Adsorption separation of concentration in radial direction. A Langmuir equilibrium isotherm

CO2 and other gaseous products of coal gasification. Their adsorp- was assumed. The mathematical model which accounted for a tion medium (adsorbent) was activated carbon. The flow pattern detailed structure of the adsorbent gave a qualitative description was described using plug flow model while the mass transfer pat- of the breakthrough experiment. It gave results of very close match tern was described using local equilibrium model and Linear Driv- to the experimental data used. However, another model based on ing Force approximation model (LDF). An empirical relation was the equivalent channel approach produced wrong results that fore- used for the determination of the mass transfer coefficient for cast higher separation efficiency for the system. Hwang et al. [177]

CO2. The mathematical model was solved using the implicit finite described a mathematical model to describe the adsorption sepa- difference method, by assuming: Negligible radial variation in tem- ration of CO2 on activated carbon using helium as the carrier gas. perature and concentration, thermal equilibrium between gas and The flow pattern was described using plug flow model, while the solid phase, and non-isothermal heat effect. A Langmuir– mass transfer pattern was described using LDF approximation Freundlich isotherm was assumed. Their model differs from exper- model. The mass transfer coefficient was lumped i.e. it was deter- imental data. This was more pronounced in the CO2 concentration. mined by fitting the experimental data. The PDE’s in the mathe- However, the LDF was closer to the experimental data. A mathematical model were solved numerically using method of lines, matical model was developed by Raghavan et al. [173] to describe after which the evolving ODE’s were solved using DIVPAG. The

Pressure Swing Adsorption separation of CO2 from a binary gas remaining algebraic equations were solved using DNEQNF. The fol- mixture of Carbon dioxide and Helium (CO2–He). Their adsorption lowing assumptions were made: Negligible radial velocity, negligi- medium (adsorbent) was Silica gel. The flow pattern was described ble radial variation in temperature and concentration, negligible using axial dispersed plug flow model, while the mass transfer pat- pressure drop within bed, non-adiabatic, and isothermal heat tern was described using Linear Driving Force approximation effects. A Langmuir equilibrium isotherm was assumed. The math- model. The mathematical model was solved by orthogonal collec- ematical model gave a qualitative description of the breakthrough tion and by using finite difference method and by assuming: Neg- experiment and temperature curves. The model had a limitation of ligible radial variation in concentration, negligible pressure drop, how to determine new values of mass transfer coefficient for new traces system inverse dependence of the mass transfer coefficient runs. with pressure, and isothermal heat effects. A linear equilibrium The mathematical modeling of the adsorption separation of CO2 isotherm was assumed. Their model succeeded in making a good from flue gas (20% CO2, 80% N2) on zeolite 13X by Vacuum Swing representation of experimental results. (VSA) was provided by Chou and Chen [178]. The mixture presents A mathematical model that describes Pressure Swing Adsorp- typical dry conditions of flue gas on industrial applications. The tion separation of CO2 from a gas mixture of Carbon dioxide flow pattern was described using axial dispersed plug flow model, (CO2) and methane (CH4) was developed by Kapoor [174]. Both while the mass transfer pattern was described using local equilib- CO2 and CH4 have equal proportion by volume. The adsorption rium model. The PDE’s in the mathematical model were solved medium (adsorbent) was carbon molecular sieve. The flow pattern numerically using method of lines with adaptive grid points, after was described using plug flow model, while the mass transfer pat- which an estimate of the flow rate was done using the cubic spline tern was described using LDF approximation model, with a coeffi- approximation. The evolving ODE’s were solved by integration cient of mass transfer that is cycle time dependent. The with respect to time of flow in adsorption bed using LSODE from mathematical model was solved using implicit backward finite dif- ODEPACK software. The remaining algebraic equations were solved ference method and, by assuming: Negligible radial variation in using DNEQNF. The following assumptions were made: Negligible concentration, negligible pressure drop within bed, and isothermal radial variation in temperature and concentration, negligible pres- heat effects. A Langmuir equilibrium isotherm was assumed. The sure drop within bed, thermal equilibrium between the gas and the results provided by the model is reportedly said to be very close solid particles, and non-isothermal heat effects. An Extended Lang- to the experimental data used within about 3% margin of error muir equilibrium isotherm was assumed. The mathematical model [162]. Cavenati et al. [175] worked on a mathematical model to gave results similar to the experimental data used but with lower describe the adsorption separation of a gas mixture of carbon diox- values than those of the experiment. This discrepancy was sug- ide (CO2) and methane (CH4) on Tekada carbon molecular sieve by gested to be due to the use of non-specific isotherm. Vacuum Swing and Pressure Swing (VSA–PSA). The flow pattern Recently, Dantas et al. [127,128] worked on a mathematical was described using axial dispersed plug flow model, while the model to describe the adsorption separation of binary gas mixtures mass transfer pattern was described using double LDF approxima- of carbon dioxide and Hydrogen (CO2–H2), carbon dioxide and tion model. Pressure variation in the system was described using Helium (CO2–He) on activated carbon and zeolite 13X. The flow Ergun equation. The PDE’s in the mathematical model were solved pattern was described using axial dispersed plug flow model, while numerically using method of orthogonal collection for twenty-five the mass transfer pattern was described using LDF approximation (25) finite elements, with two collection point per element, after model. The mass transfer coefficient was determined by fitting the which the evolving ODE’s were solved using gPROMS. The follow- experimental data (i.e. lumped). The momentum balance in the ing assumptions were made: Negligible transfer of mass, momen- system was described using Ergun equation. The PDE’s in the tum and heat in radial direction, adiabatic and non-isothermal heat mathematical model were solved numerically using method of effects. A multisite Langmuir equilibrium isotherm was assumed. orthogonal collection for six (6) finite elements, with three (3) col- The mathematical model gave a qualitative description of the lection point per element, after which the evolving ODE’s were breakthrough experiment and temperature curves. The model solved using gPROMS. The following assumptions were made: Neg- had a limitation of how to determine new values of mass transfer ligible change in temperature and concentration in radial direction, coefficient for new runs. adiabatic and non-isothermal heat effects. Adiabatic and non- Similarly, Ahn and Brandani [176] predicted the dynamics of adiabatic systems were considered. Toth equilibrium isotherm

CO2 breakthrough on carbon monolith, with different set of was assumed. The mathematical model gave a qualitative descrip- assumptions. The ﬂow pattern was also described using axial dis- tion of the breakthrough experiment for different feed concentra- persed plug ﬂow model, while the mass transfer pattern was tion and temperatures. The Toth model gave adequate results for described using the LDF approximation model. The PDAE’s in the single components but deviations were noticed for multicompo- mathematical model were solved numerically using gPROMS. nent gas mixture. Mulgundmath et al. [179] worked on a mathe- The following assumptions were made: Negligible change in matical model to describe the adsorption separation of binary 244 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

gas mixtures of carbon dioxide and Nitrogen (90% N2–10%CO2), equation. A Langmuir–Freundlich isotherm equilibrium isotherm carbon dioxide and Helium (CO2–He) on zeolite 13X. The flow pat- was assumed. The LDF model successfully predicted the adsorption tern was described using axial dispersed plug flow model, while and desorption steps and gave good simulation results that agreed the mass transfer pattern was described using LDF approximation with experimental data. It has been reported that the experimental model. The PDE’s in the mathematical model were solved numer- data gave higher gas recovery with error range of 4% [162]. ically using method of orthogonal collection for six (6) finite ele- The pressure swing adsorption separation of cracked gas mix- ments, with three (3) collection point per element, after which ture (i.e. CO2, CO, H2, and CH4) on two different adsorbents (Zeolite the evolving ODE’s were solved using gPROMS. The following 5A and activated carbon) was predicted by Park et al. [183]. In their assumptions were made: Negligible change in temperature and mathematical model, the adsorption bed was made in layers. The concentration in radial direction, negligible pressure drop and flow pattern was described using axial disperse plug flow model, non-isothermal heat effects. Adiabatic and non-adiabatic systems while the mass transfer pattern was described using LDF approxi- were considered. Langmuir equilibrium isotherm was assumed. mation model. The mass transfer coefficient was lumped. The The mathematical model gave a qualitative description of the PDE’s in the mathematical model were solved numerically using breakthrough experiment for with good accuracy at the tempera- backward difference method, after which the evolving ODE’s were ture break through point. However, the model gave results of lower solved using GEAR method. The following assumptions were accuracy for the energy balance in the system. made: Negligible radial variation in temperature and concentration, thermal equilibrium between gas and solid phase, negligible

5.2.2. CO2 mixture (with CH4 and H2) pressure drop in axial direction within bed and non-isothermal Doong and Yang [180] described a mathematical model to heat effects. A Langmuir equilibrium isotherm was assumed. The describe Pressure Swing adsorption separation of CO2 from a gas results predicted by the LDF model for a single component system mixture of Carbon dioxide (CO2), methane (CH4) and Hydrogen was close to experimental results of adsorption and desorption (H2); all of equal proportion by volume. Their adsorption medium curves. The model gave a good prediction of the experimental data; (adsorbent) was activated carbon. The flow pattern was described however, the model had a limitation of lower residual gas temper- using plug flow model, while the mass transfer pattern was ature than the one gotten from the experiment. This is due to the described using local equilibrium model and pore diffusion model. neglecting of heat loss to the column end. The mathematical model was solved numerically using finite difference method and, by assuming: Negligible radial variation in 5.2.5. CO2 mixture (with N2 and O2) concentration, negligible pressure drop within bed, and non- Choi et al. [154] worked on a mathematical model to describe isothermal heat effects. A Langmuir–Freundlich equilibrium iso- Pressure Swing Adsorption separation of CO2 from flue gas (83% therm was assumed. It was concluded that Knudsen and surface N2, 13% CO2 and 4% O2) using zeolite 13X. The flow pattern was tension model produced results close to the experimental data described using plug flow model, while the mass transfer pattern used, while the ILE model produce results with lower CO2 concen- was described using LDF approximation model. The set of equa- tration with longer break through. They suggested that the latter tions in the mathematical model was solved by Euler’s method. result may be due to the assumption of infinite rate of pore The following assumptions were made: Negligible radial variation diffusion. in temperature and concentration, negligible pressure drop within bed, and non-isothermal heat effects. An extended Langmuir equi-

5.2.3. CO2 (with Air) librium isotherm was assumed. The mathematical model was Diagne et al. [181] worked on a mathematical model to describe solved using MATLAB function which was operated on the princi-

Pressure Swing Adsorption separation of CO2 from air using molec- ple of Sequential Quadratic Programming (SQP). The model gave ular sieves zeolite (13X, 5X, and 4A). The flow pattern was a close agreement with experimental data, with little differences described using plug flow model, while the mass transfer pattern in the temperature data. Kaguei and Wakao [184] described a was described using LDF approximation model. The set of equa- mathematical model while working on the theoretical and experi- tions in the mathematical model was solved by Euler’s method. mental research on CCS. The adsorption system was a column The following assumptions were made: Negligible radial variation packed with activated carbon. The flow pattern was described in concentration, negligible pressure drop within bed, trace sys- using axial dispersed plug flow model, while the mass transfer pat- tem, and isothermal heat effects. A Langmuir equilibrium isotherm tern was described using pore diffusion model. The mathematical was assumed. The mathematical model gave a qualitative descrip- model was solved analytically using Laplace domain, by assuming: tion of the breakthrough experiment and temperature curves. The semi-infinite column Negligible radial variation in temperature model showed good agreement with experimental data except for and concentration within column, uniform temperature over col- points at which ratio of feed/lean flow rate was less than 2. umn cross section, negligible pressure drop in the axial direction, fixed column wall temperature, and non-isothermal heat effects.

5.2.4. CO2 mixture (CO2, CO, H2, and CH4) A linear equilibrium isotherm was assumed. Their model gave a Lee et al. [182] obtained a mathematical model to predict the good prediction of thermal waves at different axial locations.

Pressure Swing Adsorption separation of coke oven gas mixture In order to predict the adsorption separation of CO2 and CO on (i.e. CO2, CO, N2, and CH4) on two different adsorbents (Zeolite activated carbon, Hwang and Lee [185] obtained a mathematical 5A and activated carbon). The adsorption bed was made in layers. model in which, the flow pattern was described using axial dis- The flow pattern was described using axial disperse plug flow perse plug flow model. The mass transfer pattern was described model, while the mass transfer pattern was described using LDF using LDF approximation model. The mass transfer coefficient approximation model. The mass transfer coefficient was lumped. was made pressure dependent. The PDE’s in the mathematical The PDE’s in the mathematical model were solved numerically model was solved numerically using the method of orthogonal col- using second order finite difference method (for second order lection, after which the evolving ODE’s were solved using DGEAR space derivatives) and second order backward difference method through a Gear’s stiff method in different orders and step size. (for first order space derivatives). The following assumptions were The following assumptions were made: Negligible radial variation made: Negligible radial variation in temperature and concentra- in concentration, negligible pressure drop within bed and isother- tion, thermal equilibrium between gas and solid phase. Effect of mal heat effects. A Langmuir equilibrium isotherm was assumed. pressure drop along bed was taken into account using the Ergun The results predicted by the LDF model for a single component Table 6 Detailed review of adsorption numerical models including mass isotherm type and mass transfer models.

# Authors’ names Application type Model Mass transfer model Isotherm Type Energy model Pressure and velocity model Solution type Dimension 1 Carter and Modelling of adsorption of Carbon dioxide 1-D, From experimental Langmuir Isothermal Negligible pressure drop Numerical solution on Husain [186] and water vapour on molecular sieve transient data isotherm Fortran 2 Kumar [168] Modelling of blow down of adsorption of 1-D, Local equilibrium Langmuir Non-Isothermal Negligible pressure gradient Numerical solution CO2 from gaseous mixture of; CO 2/H2 CO2/ transient model isotherm Adiabatic system across adsorption bed CH4 CO2/N2 on Zeolite 5A and BPL carbon by Negligible radial Flow behaviour: Plug flow Finite difference method PSA temperature gradient with the use of IBM 370/165 3 Hwang and Lee Modelling of adsorption and desorption of 1-D, LDF approximation Langmuir Isothermal Negligible pressure gradient Numerical solution with the [185] gaseous mixture of CO 2 and CO on activated transient model isotherm across adsorption bed use of DGEAR commercial carbon by breakthrough experiment Temperature of column Flow behaviour: Axial code wall, adsorbent and gas dispersed plug flow were all accounted for 4 Chue et al. [40] Modelling of the adsorption of CO 2 from 1-D, Adsorbed Langmuir Non-isothermal Negligible pressure drop in CO2/N2 mixture on Zeolite 13X and activated transient concentration by IAS isotherm bed carbon by PSA model Adiabatic Flow behaviour: Axial Thermal equilibrium dispersed plug flow

between gas and solid 225–255 (2016) 161 Energy Applied / al. et Ben-Mansour R. phase 5 Hwang et al. Modelling of adsorption of gaseous mixture 1-D, LDF approximation Extended Isothermal Negligible pressure gradient Numerical solution [177] of CO2 and CO on activated carbon by transient model Langmuir across adsorption bed breakthrough experiment Lumped mass transfer isotherm Non-adiabatic and adiabatic Flow behaviour: Plug flow Linear algebras were solved coefficient systems using DIVPAG commercial Temperature of column Negligible radial velocity code while non-linear wall, adsorbent and gas algebra equations were were all accounted for solved using DNEQNF Negligible radial commercial code temperature gradient 6 Diagne et al. Modelling of adsorption of CO 2 from air by 1-D, LDF approximation Langmuir Isothermal Negligible pressure drop Euler’s method [181] PSA on Zeolite (5A, 13X and 4A) transient model isotherm Flow behaviour: Ideal plug flow 7 Ding and Alpay Modelling of adsorption and desorption of 1-D, LDF model based on Langmuir Non-isothermal. Negligible Pressure distribution by Numerical solution with the [187] CO2 on hydrotalcite at high temperature transient pore diffusion isotherm radial temperature gradient Ergun’s equation use of gPROMS commercial Thermal equilibrium Flow behaviour: Axial code between fluid and particles dispersed plug flow 8 Takamura et al. Modelling of CO 2 adsorption from gaseous 1-D, LDF approximation Langmuir Isothermal Negligible pressure drop Discretisation of coupled [188] mixture of CO2 and N2 on Zeolites (Na–X and transient model isotherm Flow behaviour: Plug flow PDEA equations in space Na–A) Flow behaviour: Plug flow and time. Final solution of ODE with variable time step 9 Choi et al. [154] Modelling of CO2 adsorption from flue gas 1-D, LDF approximation Extended Non-isothermal Negligible pressure drop in Numerical solution with the mixture containing 13% CO 2, 83% N 2 and 4% transient. model Langmuir radial direction use of MATLAB function O2 on zeolite 13X by break through isotherm Adiabatic system Flow behaviour: Plug flow experiment and PSA operation Negligible temperature Gas flow rate in bed is gradient in radial direction mainly affected by bed height 10 Chou and Chen Modelling of CO 2 adsorption from flue gas 1-D, Local equilibrium Extended Non-isothermal. Negligible Negligible pressure gradient Analytical + numerical [178] mixture containing 20% CO 2 and 80% N 2 on transient model Langmuir radial temperature gradient solution zeolite 13X by VSA. isotherm Thermal equilibrium Flow behaviour: Axial Solution of spatial between fluid and particles dispersed plug flow derivatives by upwind difference Solution of flow rates by cubic spline Solution of temperature, concentration and adsorbed mass by integration with the use of LSODE from 245 ODEPACK commercial code (continued on next page) 246

Table 6 (continued)

# Authors’ names Application type Model Mass transfer model Isotherm Type Energy model Pressure and velocity model Solution type Dimension

11 Cavenati et al. Modelling of fixed bed adsorption of CO 2, Experimental Toth Isotherm Isothermal Experimental measurement Numerical solution to solve [158] CH4 and N2 on Zeolite 13X at high pressure measurement and for mass deposited in by breakthrough experiment Multisite adsorbent using MATLAB Langmuir commercial code isotherm 12 Cavenati et al. Modelling of fixed bed adsorption of CO 2 1-D, A double LDF Multisite Non-isothermal. Negligible Pressure distribution by Numerical solution with the [175] from a gaseous mixture of 45% CO 2 and 55% transient approximation model Langmuir radial temperature gradient Ergun’s equation use of gPROMS commercial CH4on carbon molecular sieve 3 K by PSA isotherm Flow behaviour: Axial code dispersed plug flow 13 Ahn and Modelling of fixed bed adsorption and 1-D, LDF approximation Langmuir Isothermal Relationship between Numerical solution with the Brandani [176] desorption of CO 2 on Carbon Monoliths by transient model isotherm average velocity and use of gPROMS commercial break through experiment average pressure drop was code estimated with the use of equation by Cornish 1928

Flow behaviour: Axial 225–255 (2016) 161 Energy Applied / al. et Ben-Mansour R. dispersed plug flow 14 Cavenati et al. Modelling of fixed bed adsorption of CO 2 1-D, Bi-LDF model Multicomponent Non-Isothermal Pressure distribution by Numerical solution with the [189] from a gaseous mixture of 20% CO 2/60% CH 4/ transient extension of Ergun’s equation use of gPROMS and 20% N2 on zeolite 13X by Layered multisite Temperature of column Flow behaviour: Axial Pressure Swing Adsorption (LPS) Langmuir wall, adsorbent and gas dispersed plug flow were all accounted for Negligible radial temperature gradient 15 Moreira et al. Modelling of fixed bed adsorption of Helium 1-D LDF approximation Langmuir Isothermal Negligible pressure drop Numerical with the use of [190] diluted CO2 on hydrotalcite (Al–Mg) transient model isotherm PDECOL in FORTRAN Calculation of mass Flow behaviour: Axial commercial code transfer coefficient by dispersed plug flow theoretical correlations 16 Delgado et al. Modelling of fixed bed adsorption of CO2 1-D LDF approximation Extended Non-isothermal. Negligible Pressure distribution by Numerical solution by [169,170] from gaseous mixture of; CO 2/He CO 2/CH4 transient model Langmuir radial temperature gradient Ergun’s equation PDECOL commercial code CO2/N2 on Silicalite pellets, sepiolite, and Lumped mass transfer isotherm Pressure variation in time resin using break through experiment coefficient and space Flow behaviour: Axial dispersed plug flow 17 Dantas et al. Fixed bed adsorption of gaseous mixture of; 1-D, LDF approximation Toth Isotherm Non-Isothermal Pressure distribution by Numerical solution using [127–129] CO2/N2 and CO2/He on zeolites 13X and transient model Ergun’s equation gPROMS commercial code activated carbon by break through experiment and PSA Lumped mass transfer Adiabatic and non-adiabatic Axial dispersed plug flow coefficient system Model accounted for Heat transfer in gas, solid and wall 18 Biswas et al. Modelling of adsorption separation of 1-D, LDF model Multisite Isothermal Pressure distribution by Discretisation by Newton [191] gaseous mixture of CO, CH 4,H2,CO2 on transient Langmuir model Ergun’s equation based approach Zeolite 5A and activated carbon Lumped mass transfer Assuming temperature of Flow behaviour: Axial Algebraic solution coefficient wall, gas phase and dispersed plug flow adsorbent are equal 19 Agarwal [192] Fixed bed adsorption of CO2 from gaseous 1-D, LDF approximation Dual site Temperature equilibrium Pressure distribution by Numerical solution with the mixture of CO2/N2, 45% CO 2/55% H2 by PSA transient model Langmuir between gas phase Ergun’s equation use of interior point NPL isotherm adsorbent solver Lumped mass transfer Constant column wall Flow behaviour: Axial coefficient temperature dispersed plug flow 20 Krishna and Modelling of PSA performance and break 1-D, Isotherm Negligible pressure drop Molecular simulation with van Baten [22] through characteristics of zeolites (MFI, transient Assumed flow behaviour: the use of Configuration- JBW, AFX, NaX) and MOFs (MgMOF-74, Plug flow Bias Monte Carlo (CBMS) MOF-177, CuBTTri-mmen) for gaseous mixture of CO2/N2 Table 6 (continued)

# Authors’ names Application type Model Mass transfer model Isotherm Type Energy model Pressure and velocity model Solution type Dimension

21 Casas et al. Fixed bed adsorption of CO 2 from gaseous 1-D, LDF model Langmuir and Sip Thermal equilibrium Pressure distribution by Finite volume method and [193] mixture of CO2/H2 on activated carbon by transient isotherms between gas stream and Ergun’s equation time integration on IMSL break through experiment adsorbent DIVPAG commercial Lumped mass transfer Column wall temperature is Flow behaviour: Plug flow package using Gear’s coefficient accounted for separately method 22 Mulgundmath Fixed bed adsorption of CO 2 from gaseous 1-D, LDF approximation Langmuir Non-Isothermal Negligible pressure drop et al. [179] mixture of 10% CO2/90% N2 on Ceca 13X by transient model for external fluid isotherm Temperature of column Flow behaviour: Axial break through experiment film mass transfer wall, adsorbent and gas dispersed plug flow were all accounted for 23 Casas et al. Mathematical modelling of CO2 adsorption 1-D, Mass transfer Langmuir Non-Isothermal Pressure distribution by Integration via Gear’s [159] from CO2/H2 mixture in MOF and UiO-67/ transient coefficient determined isotherm Adiabatic Ergun’s equation method with the use of MCM-41 by PSA and break through by fitting of Model accounted for Heat IMSL DIVPAG (Fortran) experiment experimental data transfer in gas, solid and commercial code measured in the range wall of interest Isosteric heat of adsorption and heat capacities of the fluid and the solid phase 225–255 (2016) 161 Energy Applied / al. et Ben-Mansour R. 24 Sabouni [194] Modelling of adsorption of CO 2 From in 1-D, Mass transfer Langmuir– Isothermal Negligible pressure drop Numerical solution with the CPM-5 by breakthrough experiment transient coefficient determined Freundlich through column use of COMSOL by fitting of isotherm Constant gas velocity experimental data through column 25 Ribeiro et al. Modelling of CO2 adsorption from flue gas 1-D, Two different LDF Multisite Temperature equilibrium Pressure distribution by Numerical solution with the [157] by a mixture of Activated carbon transient models; one for micro Langmuir model between the solid phases Ergun’s equation use of gPROMS commercial honeycomb monolith and Zeolite 13X pores and the other for code hybrid system by Electrical Swing macro pores Adsorption (ESA) Lumped mass transfer Negligible temperature Assumed flow behaviour: parameter for meso gradient in adsorbent Axial plug flow pores and micro pores; obtained from Bosanquet equation 26 Krishnamurthy Modelling of CO2 adsorption from dry flue 1-D, LDF approximation Extended dual Non-Isothermal Non Isobaric Numerical solution by stiff et al. [195] gas in Zeochem zeolite 13X by break transient model site Langmuir Pressure distribution by ODE solver; ode23s in through experiment and VSA model Darcy’s equation MATLAB commercial code 247 248 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 system was close to experimental results of adsorption and des- maximum adsorption rates are determined from proper isotherms orption curves. The mass transfer coefficient and the assumptions which describe the adsorbed amount as a function of pressure gave good results, close the experimental data for adsorption and given a certain temperature. In the fixed bed adsorption models, desorption for multi-component sorption system. Table 6 provides the ideal gas behavior is normally considered with constant fluid detailed review of Adsorption Numerical models including mass properties and constant bed porosity. It is also assumed that the isotherm type and mass transfer models. The table gives a detailed process is adiabatic. The fixed bed adsorption model is described account of the mathematical models used in previous studies. The by the following equations which are derived from, mass, momen- table presents the type of the two most important properties used tum and energy equations: in the models; namely the adsorption isotherm model and the mass transfer model. Other important consideration such as the 5.3.2. Governing equations heat/energy transfer as well as the pressure drop models are also Species (mass balance) conservation for CO2 and N2, Dantas reviewed up to 2014. et al. [128,129] Most of the modelling studies indicated that the gases flow ! @Cj @uCj @2Cj @q through the bed are treated as one dimensional flow (1D) and e þ ¼ eDax ð1 eÞq j ð1Þ @ @ @ 2 p @ the effect of radial direction or 3D simulation still need modelling t z z t and performance optimizations investigations. Moreover, the ideal As discussed above, the LDF mass transfer model for each compo- gas behaviour assumption for gases dominated most of the CO 2 nent is given by: separation numerical investigations. Another point is that the available data obtained by experimental work as adsorption and @qj ¼ K ; q q ð2Þ thermal properties of adsorbent and adsorbate materials could @t L j j j only be used in the modelling to validate the simulation and inves- where the maximum adsorption is determined from the following tigate the adsorption process behaviour and its performance opti- isotherm equation: mization. Therefore, the modelling is restricted by what has been hi 1=n performed by experimentation. ¼ þ n ð Þ qj qm;jKeq;jPj 1 Keq;jPj 2a

5.3. Modeling of adsorption of CO2 for carbon capture and the equilibrium adsorption coefﬁcient is given by:

ðD = Þ Hj RTg In this section we present the mathematical model and some Keq;j ¼ Ko;je ð2bÞ sample simulations results of the adsorption fixed bed (see Mass Conservation Fig. 5) for CO separation. The fixed bed represents high aspect 2 X ratio column or cylinder often used industry for separating gases q @u ð1 eÞ @q g ¼ q j ð3Þ using the PSA or TSA process. A similar geometry to that used in @z e p @t j experimental adsorption studies to measure the capacity of tested materials is used in the break-through setup. Because of the high The momentum equation is simplified for the porous media case aspect ratio of such system, the concentration, temperature, pres- under very slow flow rate to the Darcy model equation: sure and velocity gradients are mainly along the axis of the cylin- 2 @p l ðÞ1 e ð1 eÞ drical bed. These axial gradients are much larger than the radial ¼ g þ : q 2 ð Þ 150 2 u 1 75 3 g u 4 @z e3 e dp gradients; hence the one-dimensional (1D) assumption is made dp as done in all the studies listed in Table 6 above. After validation The energy equation for the gases can be written as: of the numerical model with experimental data, we use it to sim- ! ulate different materials including the commonly used ones such 2 @Tg u@Tg @ Tg @Ts eq Cv; þ q C ; ¼ ek C ð1 eÞq as activated carbon and the novel material such as MOF-5, MOF- g g @t g p g @z L @z2 s p @t 74 and MOF-177 which have been recently developed and have X @q very good potential to become the adsorption materials of the þð1 eÞq DH j p j @t future. In addition, we have simulated three different operation j modes of the fixed bed namely (i) the break-through test simula- 4h tion, (ii) the storage simulation and (iii) the PSA simulation. The w ðT T Þð5Þ d g w thermo-physical properties of the bed materials, the geometric int details and the operating parameters including temperatures, pres- While the energy conservation for the solid part of the bed can be sures, gas mixture inlet compositions are given for each case. written as: X @Ts @qj 6hf 5.3.1. Fixed bed adsorption model q C ¼ q DH þ ðT T Þð6Þ p s @t p j @t d g s The conservation of mass, species, momentum and energy j p equations are developed to describe the fixed bed adsorption sys- The bed wall temperature is solved from the following equation: tem (Fig. 5). Since the bed has a large aspect ratio, the gradient in @ radial direction are ignored and hence the 1D approximation. The q Tw ¼ a ð Þ ð Þ wCp;w whw Tg Tw and 7 flow behavior is characterized with axially dispersed plug flow @t model and the mass transfer rate is assumed to follow the Linear a ¼ = ð þ ÞðÞ Driving Force (LDF) model. The LDF model for mass transfer was w dint l dint l 7a initially developed by Glueckauf and Coates [196]. They suggested The properties of the flue gas within the adsorption operating win- that the uptake rate of a species into adsorbent solid particles is dow were modelled through the gas mixture concepts and are given proportional to the difference between the concentration of that as follows: species at the outer surface of the particle, denoted as qj (equilib- Density of mixture rium adsorption amount) and its average concentration within the P P y M particle (volume-averaged adsorption amount) denoted as q . This q ¼ i i i ð Þ j g 8 model is expressed in Eq. (2) below. In addition, the equilibrium RuT R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 249 where M is the molecular weight.

Ru is the gas constant (8.314 J/mol K). Thermal conductivity can be obtained using Wilke’s approach

Xn ¼ Pyiki ð Þ kg n 9 y U i¼1 j¼1 i ij and 0:5 0:25 l M 1 þ li i j Mj U ¼ hi ð Þ ij pffiffiffi 0:5 10 8 1 þ Mi Mj The boundary conditions for the above equations are written as follows: Fig. 7. Validation breakthrough curve for CO2 &N2 on activated carbon for @Cj Tgfeed = 373 K. Experimental data [128]. z ¼ 0 : eDax ¼uCj Cj @z z zþ zþ

@Cj z ¼ L : ¼ 0 @z z @ Tg z ¼ 0 : ekL ¼ucCs Tg Tg @z z zþ zþ @ Tg z ¼ L : ¼ 0 @z z ¼ : ¼ z 0 uCj z uCj zþ The initial conditions t ¼ 0 : P ¼ Pinlet; Tw ¼ Tg ¼ Ts ¼ Tg;inlet and Cjðz; 0Þ¼qjðz; 0Þ¼0

The system properties that describe the heat and mass parameters modelled as follows: Fig. 8. Validation breakthrough curve for CO2 &N2 on activated carbon for Tgfeed = 423 K. Experimental data [128]. kL ¼ kg ð10 þ 0:5 Pr ReÞ

hw ¼ kg ð12:5 þ 0:048 ReÞ

hf ;j ¼ Nu kg=dp concentration of N2 at the outlet becomes greater than the feed U L concentration [8,9], which is a common behaviour in multi compo- D ¼ inlet ax Pe nent gaseous mixture adsorption. The quick breakthrough of the LDF model compared to the experimental model may be due the Dimensionless Numbers: some differences in binary mixture properties used, constant fluid ðq Þ properties (e.g. density, viscosity etc.) and the ideal gas law g Uinlet dp Re ¼ l assumption. g As shown from Figs. 7 and 8, the present model captures the ð l Þ Cp;g g Pr ¼ changes in CO2 and N2 concentrations with time quite well from kg the qualitative point of view. The peak in the nitrogen concentra- : = Nu ¼ 2:0 þ 1:1 ðRe0 6 Pr1 3Þ tion is well captured by the model. The model shows under- : prediction of the saturation time. The figures also indicate that 0:508 ðRe0 020 LÞ Pe ¼ the model provides better agreement as the temperature becomes dd high. The breakthrough time gets shorter as the temperature increases. This is attributed to the fact that nitrogen has higher dif- 5.4. Overview of results of numerical simulations of adsorptive carbon fusion at higher temperatures, thus, nitrogen adsorption becomes capture faster. After implementing the LDF model for different situations including breakthrough experiment or pressure swing operation; 5.4.1. A comparison of breakthrough simulation results using Linear and different materials (AC, Zeolite X13, MOF5 and MOF74) we Driving Force Model (LDF) with breakthrough experimental result can conclude that this model gave good agreement between the The work of Dantas et al. [128] presents breakthrough experi- experimental measurement and model predictions. ments for a temperature range of 28–150 °C (301–423 K) on activated carbon. The adsorption bed used was 0.171 m 0.02 mØ 5.4.2. Simulated results of the breakthrough behaviour of Mg-MOF-74 in size and feed flow rate was 30 mL/min. The data provides vari- The adsorption breakthrough curves CO2 and N2 on Mg-MOF-74 ation of CO2 and N2 concentrations with time at the exit section. for the separation of CO2 from a binary gas mixture of 15% CO2 wt Figs. 7 and 8 show a comparison of experimental data and LDF N2 which is as shown in Fig. 9. The adsorption bed used was model simulation for the break through curves for the adsorption 0.171 m 0.02 mØ in size and feed flow rate was 30 mL/min. of CO2 from binary gas mixture of 20% CO2wt N2 for 100 °C& Fig 9 portrays that Mg-MOF-74 has very high selectivity for CO2 150 °C (273 K & 423 K) respectively. These figures show the ratio which conforms to existing reports. The plots also show that the of species concentrations at bed exit to the feed concentration. break through time for CO2 in the described mixture decreased The total feed gas flow rate in each case is 30 mL/min. The with temperature. At a feed gas temperature of 301 K, CO2 adsorp- roll-up behaviour of N2 remains as explained before i.e. the tion took well above 500 min before breakthrough which conforms 250 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

Concentration at exit 1500 min Concentration at exit 1500 min

1.2 1.2

1 1

0.8 0.8

0.6 0.6 Tgfeed = 301K Tgfeed = 323K C/Cinlet C/Cinlet 0.4 0.4 Pfeed = 1.2bar Pfeed = 1.2bar 0.2 0.2

0 conc of CO2 at exit 0 conc of CO2 at exit conc of N2 at exit conc of N2 at exit -0.2 -0.2 0 500 1000 1500 0 500 1000 1500 Time (min) Time (min)

Concentration at exit 500 min Concentration at exit 500 min

1.2 1.2

1 1

0.8 0.8 Tgfeed = 423K 0.6 Tgfeed = 373K 0.6

C/Cinlet 0.4 C/Cinlet 0.4 Pfeed = 1.2bar Pfeed = 1.2bar 0.2 0.2

0 conc of CO2 at exit 0 conc of CO2 at exit conc of N2 at exit conc of N2 at exit -0.2 -0.2 0 100 200 300 400 500 0 100 200 300 400 500 Time (min) Time (min)

Fig. 9. Breakthrough curves for CO2 &N2 adsorption on Mg-MOF-74 at various feed temperatures.

CO2 by the material which leads to a steep rise in the concentration of Nitrogen at bed exit. Pfeed = 50bar

40bar 5.4.3. Simulated results for adsorptive storage of CO2 on MOF-5 & 30bar MOF-177

20bar Simulation of the adsorbed mass of CO2 on MOF-5 & MOF-177 10bar show increase in the amount of CO2 adsorbed on bed with the 5bar increase in gas feed pressure which is as shown in Figs. 10 and 4bar 3bar 2bar 11 respectively for pressure values from 1 to 50 bar, respectively. 1bar The adsorption bed used was 0.171 m 0.02 mØ in size and feed

ﬂow rate was 30 mL/min. As the pressure increases, CO2 molecules are pressed against the surface of the solid. This increases the avail-

Fig. 10. Proﬁle of amount of CO2 stored on MOF-5 for 50 min for various feed pressures. Pfeed = 50bar

40bar quite closely to existing reports for similar conditions [24,197]. This breakthrough time decreases as the feed gas temperature 30bar increases, which may be due to reduction in the value of the Lang- 20bar muir adsorption equilibrium parameter with temperature, which turn decreases the retention time and leads to longer break- 10bar through. The continuous adsorption of some quantity of CO2 at 5bar bed exit after breakthrough might be due to existing suggestion 3bar 4bar 2bar that the single component single site Langmuir model inade- 1bar quately predicts CO2 adsorption in Mg-MOF-74 even at loading below 8 mol/kg [24]. The roll-up exhibited by Nitrogen in all four cases conforms to existing reports for multicomponent adsorption [8,9]. This phenomenon is due to the displacement effect of CO on 2 Fig. 11. Proﬁle of amount of CO2 stored on MOF-177 for 30 min for varying feed Nitrogen which happens during initial continuous adsorption of pressures. R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255 251

able surface area for CO2 adsorption within existing adsorption adsorbed in similar cycles of PSA run when the purge time was sites. The increase in the available surface area for adsorption in set to 25s. About 0.26 g of CO2 adsorbed during the ninth cycle, turn leads to an increase in the maximum possible adsorption of after which the rate of deterioration of the material reduced.

CO2, hence, the amount of adsorbed CO2 increases. The highest Between the ninth and twenty ﬁfth cycles, the rate of deterioration adsorbed amount for MOF-5 after 50 min of adsorption is achieved of the material slowed down as the material achieved cyclic steady with 50 bar feed pressure which is about 7.4 g. This matched with state. The amount CO2 adsorbed in the ﬁfteenth cycle is about the work of Zhao et al. [111]. The highest adsorbed amount for 0.24 g. MOF-177 after 30 min of adsorption is achieved with 50 bar feed pressure which is about 11.3 g. This matched with the work of Saha and Bao [198]. 6. Conclusions

A review on the separation of carbon dioxide from typical 5.4.4. Simulated results of PSA of CO2 on Mg-MOF-74 power plant exhaust gases using the adsorption process is pre- The simulated pressure curve and total amount of CO2 adsorbed in bed exit for PSA with 10s of counter-current pressurization, 50s sented. This method is believed to be one of the most economic of feed, 25s of counter-current depressurization and 50s of and least interfering ways for post-combustion carbon capture as counter-current purge are as shown in Figs. 12 and 13 respectively. it can accomplish the objective with small energy penalty and very The adsorption bed used was 0.2 m 0.02 mØ in size, feed and few modifications to existing power plants. The review focused on purge pressures are 1.3 bar & 0.5 bar respectively while feed and the candidate materials that can be used to adsorb carbon dioxide, purge flow rates are 5e5m3/s & 8.3e6m3/s respectively. For the experimental investigations that have been carried out to study these cycles, the feed gas temperature and concentration for each the process of separation using adsorption and the numerical models developed to simulate this separation process and serve as a cycle were maintained at 373 K and 15% CO2 wt 85% N2 respectively. Twenty-five (25) PSA cycles were simulated. From the curve, tool to optimize systems to be built for the purpose of CO2 adsorp- it can be seen that increase in the duration of purge has effect on tion. The review pointed out some of the remaining challenges for the adsorption capacity of the bed. For the same bed size and feed post combustion carbon-capture. In particular, to handle typical temperature and similar operating condition, increase in the dura- CO2 mixtures in exhaust gases (78 N2, 13% CO2,9%H2O), new tion of purge from 25s to 50s helped to slow down the rate of bed materials of high selectivity and high adsorption of carbon dioxide deterioration during the first nine cycles with about 0.61 g, 0.51 g, high stability with water vapor, good thermal stability, good thermal conductivity, high specific heat, good corrosion resistance as 0.42 g and 0.36 g of CO2 adsorbed in the 2nd, 3rd and 4th cycles respectively as compared to 0.58 g, 0.45 g, 0.36 g and 0.32 g well as sufficient mechanical strength to endure repeated cycling are required. It is indicated that there is a need for evolution of a contemporary class of more effective, comparatively cheap, and industrially applicable materials for carbon capture and storage applications in order to minimize the uncontrolled emissions of greenhouse gases into the atmosphere, which is necessary on a national and international scale. In terms of experimental investigations, the present work done on physical adsorption experiments relied on small amount of adsorbents (in few grams). Adsorption beds with larger mass should be studied to reflect the capability for utilizing such systems in the actual applications. In addition, the number of adsorption/desorption cycles that the adsorbent can handle without deterioration lacks a long time operations and recordings. As well, more configurations of the bed (other than the tubular beds) are also important in this field. The knowledge gap related to the modelling is that the present simulation consid-

Fig. 12. Pressure curve at bed exit for 25 cycles of four-step PSA of CO2 on Mg-MOF- ered only one dimensional ﬂow and ignored the radial or 3D ther-

74 from gas mixture of 15%CO2, 85%N2 at 373 K for PSA run with tpurge = 50s. mal and adsorption behaviours. The mass transfer rate in the majority of simulations is currently represented by a linear driving force (LDF) model. More physically realistic approaches should be implemented and comparison between them is needed to achieve signiﬁcant accuracy and more importantly good agreement with experimental results. Finally we suggest that more investigations are carried out on the thermodynamic analyses, using the ﬁrst

and the second law efﬁciencies; of physical adsorption of CO2 capture.

Acknowledgments

The authors wish to acknowledge the support received from King Abdulaziz City for Science and Technology (KACST) Carbon Capture and Sequestration Technology Innovation Center (CCS- TIC #32-753) at King Fahd University of Petroleum and Minerals (KFUPM) for funding this work through Project No. CCS10. The sup- Fig. 13. Total amount of CO2 adsorbed in bed for 25 cycles of four-step PSA of CO2 port of KFUPM through the Research Institute and the Deanship of on Mg-MOF-74 from gas mixture of 15%CO2, 85%N2 at 373 K for PSA run with tpurge = 50s. Scientiﬁc Research is greatly appreciated. 252 R. Ben-Mansour et al. / Applied Energy 161 (2016) 225–255

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