Design Considerations for Postcombustion CO2 Capture With Membranes

C H A P T E R 14

Design Considerations for Postcombustion CO2 Capture With Membranes Simona Liguori, Jennifer Wilcox Colorado School of Mines, Golden, CO, United States

1. Introduction The design of low-energy intensive, low-cost, and efficient CO2 capture unit is of extreme importance for the development of carbon capture and storage (CCS) technologies at industrial scale (Davidson and Metz, 2005). This mainly holds for the postcombustion, where the carbon dioxide is diluted in nitrogen with a volume fraction typically between 4% (i.e., gas turbine) and 15% (coal-fired power plant) and the flue gases to be treated are at atmospheric pressure conditions (Table 14.1). These two specificities address a great engineering challenge, especially in terms of the energy requirement of the separation process (Steeneveldt et al., 2006; Herzog, 2001; Wilcox, 2012). Several capture technologies are being developed and evaluated for CO2 capture applications such as absorption, adsorption, cryogenic processes, membranes, and chemical looping (Figueroa et al., 2008). The identification of the most efficient process is subject to controversial debates. The ideal CO2 capture process should show (1) high selectivity of CO2 with respect to the other gases, to reach a concentration above 95%; (2) minimal energy requirement; (3) minimal environmental impact, which can be evaluated from the waste produced by the CO2 capture unit and water footprint; (4) minimal overall cost. By considering these characteristics, the absorption into a chemical solvent (such as amine solution) is currently considered as the best available and most mature technology (Steeneveldt et al., 2006). Amine solvents have been used for decades for natural gas treatment. However, the two main drawbacks of the amine absorption process correspond to a high-energy requirement (Steeneveldt et al., 2006) and waste production. Specifically, almost 4 GJ/t of thermal energy, corresponding to 50% of the steam leaving the intermediate pressure steam turbine module that has to be employed for solvent regeneration, and the degradation of the solvent lead to additional material costs, high disposal costs, and additional environmental pollution. New solvents or alternative separation processes that would not exhibit these disadvantages are explored. In particular, Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-813645-4.00014-3 Copyright © 2018 Elsevier Inc. All rights reserved.

385

386 Chapter 14 Table 14.1: Typical Conditions of a Flue Gas Stream From Various Sources (Metz et al., 2005) Stream Sources

CO2 Concentration (%)

Pressure (bar)

CO2 Partial Pressure (bar)

Gas turbine Fired boiler of oil refinery and petrochemical plant Natural gas fired boiler Oil-fired boilers Coal-fired boilers IGCC after combustion Cement process

3e4 8

1 1

0.03e0.04 0.08

7e10 11e13 12e14 12e14 14e33

1 1 1 1 1

0.07e0.10 0.11e0.13 0.12e0.14 0.12e0.14 0.14e0.33

IGCC, integrated gasification combined cycle.

compared with other capture processes, CO2-selective membranes possess advantages that include lower environmental impact and possible application as add-on equipment with fewer modifications to the power plant (Zhao et al., 2012). Initially, membrane separation processes were considered inadequate for CO2 capture because of the difficulty in developing highly selective materials, high-energy requirement, and prohibitive capital costs (Davidson and Metz, 2005; Figueroa et al., 2008; Favre, 2007). Nevertheless, the situation has, over the past decade, greater attention on membranes as a possible alternative to chemical separation processes because of their ease of application, limited maintenance, ability to perform separation with low-energy penalties, and compatibility with a retrofit strategy without requiring complicated integration (Abetz et al., 2006; Basu et al., 2004; Kohol and Nielsen, 1997; Mazur and Chan, 1982; Perry et al., 2006; Wilcox et al., 2014). However, although membrane processes are conceptually very simple, complicated membrane process configurations are often employed in practice to meet product purity and capture ratio constraints (Roussanaly et al., 2016). In this context, a critical analysis of the benefits and drawbacks of membrane separation processes in a postcombustion capture framework is proposed hereafter. In addition, different types of design possibilities for membrane processes will be discussed in the next sections.

2. Membranes and Postcombustion Carbon Capture Membrane processes are often listed as a potential candidate for CO2 separation for postcombustion capture. Nevertheless, the main issue related to the limited application of the membrane technology is the low CO2 concentration and the pressure of the flue gas, which requires a huge membrane surface area to compensate for low driving force and the use of membranes with high selectivities to fit the specification delivered by the

Design Considerations for Postcombustion CO2 Capture 387 Department of Energy (January 2011), i.e., a target CO2 recovery of 90% with a target product CO2 purity of 95%. In fact, the major drawbacks of the membrane operation concern two different factors: (1) membrane material aspect, which addresses the evaluation of the intrinsic separation performances of the membrane and the (2) process engineering science aspect, which focuses on the determination of the best design and operating conditions for a given membrane material. Combining these two aspects, the installation size and the energy requirement could be obtained (Zolandz and Fleming, 1995). Unfortunately, these two fields are often separated. Specifically, numerous studies report only on material structure and performances, while process design publications sometimes neglect the specificities of membrane materials.

2.1 Membrane Material and Design for Power Plant Integration The major hurdle in the engineering analysis of membrane processes in CO2 separation is the large number and range of process variables, which are usually reduced by treating the mixture as a binary mixture neglecting the effects of impurities and assuming a feed mixture temperature, either close to or slightly above ambient temperature (e.g., 40
C). The two main parameters considered for the CO2 capture are the CO2 purity level (y) and the capture ratio (R) (Deschamps and Pilavachi, 2004). The capture ratio of a single-stage membrane unit is defined as R¼q

y xin

(14.1)

where q is the stage cut, corresponding to the ratio between the permeated flow rate (Qp) and the feed flow (Qin), y is the CO2 purity corresponding to mole fraction of CO2 in the permeate, and xin is the mole fraction of CO2 in the feed. Generally, an increase in q is not linear with the capture ratio. For this reason, one of the major challenges is to determine the relationship between the capture ratio (R) and the corresponding permeate composition (y). Any membrane separation process requires a driving force to be effective, classically expressed through the pressure ratio, which corresponds to the ratio between the feed and the permeate pressures. J¼

pf pp

(14.2)

The CO2 purity (y) depends on the feed composition and the material performances, i.e., the membrane selectivity, a, and is defined as the ratio between the CO2 and other gas

388 Chapter 14 permeabilities. In a large majority of cases, only the experimental permeabilities of the pure compounds are known, leading to the so-called ideal selectivity of the material: a¼

PCO2 Pothergases

(14.3)

It is worth noting that the effective purity produced by a membrane unit not only depends on the membrane selectivity but also by the module operating conditions, particularly the pressure ratio (J). In addition, the capture ratio (R) also affects the purity level, due to mass balance constraints. This qualitative analysis has shown how material and process variables are closely interconnected, as discussed in the next sections.

3. Membranes Materials for Carbon Capture A large number of CO2-selective membrane materials for gas applications has been investigated over the years (Powell and Qiao, 2006; Brunetti et al., 2010). Specifically, a great number of studies have been reported on different materials such as polymeric, inorganic, hybrid organiceinorganic, and facilitated transport membranes (FTMs) (Ebner and Ritter, 2009; Luis et al., 2012).

3.1 Polymeric Membrane Polymeric membranes have been widely studied for gas separation applications. On the one hand, application of polymeric membranes to CO2 capture is generally sustained because they present low cost, facile processing, and high packing density, but on the other hand, they are limited due to their low chemical, mechanical, and thermal stabilities and their intrinsic low permeance. The polymeric materials mainly used for gas separation are polyimide, polycarbonates, polyphenyl oxide, polysulfones, cellulose derivatives, and poly (ethylene oxide) (Ebner and Ritter, 2009; Luis et al., 2012). Polyimides are the class of polymers mainly studied. They combine excellent thermal and chemical stability with a very wide range of CO2 permeabilities and show reasonable potential structural variations and ease of membrane formation (Powell and Qiao, 2006). Du et al. (2012) summarized a wide diversity of polyimides, in which those incorporated with the group of 6FDA shows both high permeability and high selectivity. This is mostly due to the presence of the CF3 group, which significantly enhances the stiffness of the chain so that the membrane separates molecules more effectively on the basis of steric bulk. In 1996, Langsam published an extensive review of the gas separation properties of polyimide. Considerable research has been focused on the commercial polyimide, Matrimid 5218. Postsynthesis modification by Hþ ion beam irradiation (Hu et al., 2007) and functionalization by bromination (Stern, 1994) improve the permeability of CO2 and N2 simultaneously, leading to only a small decrease in CO2/N2 selectivity. Poly(ethylene-oxide)

Design Considerations for Postcombustion CO2 Capture 389 Table 14.2: Performance of Dense Polymeric Membranes Membrane

a

aCO2 =N2

CO2 Permeance (GPU)

References

25 40

>12,000 2,000a

Budd et al. (2008) Du et al. (2011)

a

Polybenzodioxanes (PIM-1) Tetrazole-functionalized PIM-1 modified Polaris Poly(ethylene oxide) poly (butylene terephthalate) Poly(ether-block-ester)

50 75

1,000 1,000

Merkel et al. (2010) Metz et al. (2004)

60

1,850

PTT-b-PEO copolymers

58

>500

Yave et al. (2010a) and Liu et al. (2005) Yave et al. (2010b)

The permeance is estimated from permeability considering 1 mm dense layer.

(PEO) membranes are considered attractive materials for CO2 separation because of the polar ether oxygen in the polymer chain, which creates a strong affinity for CO2 (Brunetti et al., 2010). Much effort has been made in designing and synthesizing polymers containing PEO. For example, a composite membrane based on a segmented block copolymer (Polaris), a type of PEO copolymer, has been specifically designed for carbon capture applications up to the industrial scale (spiral wound modules) (Merkel et al., 2010). This material shows a CO2/N2 selectivity of 50 and CO2 permeance of 1000 GPU. Another example of block copolymers is PEOdpoly(butylenes terephthalate) block copolymers with a selectivity of 75 and a CO2 permeance of 1000 GPU (Metz et al., 2004). To date, these performances can be considered as the upper limit for polymeric membranes based on a physical separation mechanism. Table 14.2 shows the performance of polymeric membranes for CO2/N2 separation.

3.2 Inorganic Membrane Inorganic membranes offer higher resistance against high temperature and pressure conditions, and they can potentially show better performances in comparison with polymeric membrane. Classical materials include carbon, alumina, zeolite, and silica (Ebner and Ritter, 2009). Among them, zeolite membranes show interesting performance for CO2 separation applications. In particular, two exceptional results have been obtained by using tailor-made zeolites at the lab scale. Specifically, the NAY zeolite membrane showed a CO2 permeance of 200,000 GPU with a CO2/N2 selectivity of 200 (Krishna and van Baten, 2011). A higher selectivity of 500 and a CO2 permeance of 300 GPU have been reported with another type of zeolite membrane (White et al., 2010). Recent patents (Gobina, 2006; Ku et al., 2007) describe inorganic membranes consisting of a porous separating layer, often silica, deposited on a ceramic support, such as Al2O3, which can be used for CO2 separation applications. Xomeritakis et al. reported in their study a CO2

390 Chapter 14 Table 14.3: Performance of Inorganic and Hybrid OrganiceInorganic Membranes Membrane

aCO2 =N2

Faujasite or zeolite Y membrane on alumina support NAY zeolite membrane (molecular simulation data) Zeolite membranes Microporous silica membrane

>500

300

200

200,000

69 50

2100 900

CO2 Permeance (GPU)

References White et al. (2010) Krishna and van Baten (2012) Ebner and Ritter (2009) Xomeritakis et al. (2007)

permeance of 900 GPU with a CO2/N2 selectivity of 50 by using silica membrane. Table 14.3 reports some performance obtained by using inorganic membranes.

3.3 Hybrid Membrane Hybrid membranes consist of mixtures of organic and inorganic phases. Classically, inorganic particles (adsorbents) are dispersed into a polymeric matrix. The corresponding material is often called a mixed matrix membrane (MMM). This design offers the possibility to combine the polymer’s easy processability and the superior gas separation performance of inorganic materials (Mahajan et al., 1999). The dispersed inorganic phase may act as a molecular sieve or as a selective surface flow (SSF) material. In other cases, the interactions of the two phases can open interchain distances, thereby improving both selectivity and permeability. The presence of inorganic materials in a polymeric matrix can also improve the physical, thermal, and mechanical properties for aggressive environments as well as stabilize the polymeric membrane against change in permselectivity with temperature. Pera-Titus (2014) presented a review of inorganic materials that may be suitable for use in polymer membranes for CO2 capture. Inorganic fillers have included zeolites (Bastani et al., 2013; Junaidi et al., 2014; Nik et al., 2011; Sublet et al., 2012), carbon nanotubes (Ahmad et al., 2014; Aroon et al., 2013; Rajabi et al., 2013), and metal organic frameworks (MOFs) (Nafisi and Hagg, 2014; Perez et al., 2009). MOFs are organiceinorganic solids showing promise in gas separation and purification (Li et al., 2012). Jiang (2012) has provided a comprehensive review on MOF membranes and their application for carbon capture. According to the author, unlike MOF sorbents, very little work has been carried out on MOF membranes and they are still at their infancy. Recently, molecular simulation studies have reported that it may be possible to obtain MgMOF-74 membranes with a permeance greater than 30,000 and a selectivity of about 30 (Krishna and van Baten, 2012) compared with a MMM based on PEBAX-Silica (81:19) that has a selectivity of about 118 and a CO2 permeance of 205 GPU (Kim and Lee, 2001). Table 14.4 shows some performance data related to the hybrid membranes.

Design Considerations for Postcombustion CO2 Capture 391 Table 14.4: Performance of Hybrid OrganiceInorganic Membranes Membrane MgMOF-74 (Pebax)-silica (90:10) (Pebax)-silica (81:19) (Pebax)-silica (73:27)

aCO2 =N2 30 72 118 79

CO2 Permeance (GPU) >30,000 154 205 277

References Krishna and van Baten (2012) Kim and Lee (2001) Kim and Lee (2001) Kim and Lee (2001)

3.4 Facilitated Transport Membrane FTMs comprises a carrier (typically, metal ions) with a special affinity toward a target gas component, and this interaction controls the rate of transport. In particular, a selective reversible reaction between the gas of interest and a carrier agent incorporated in the membrane takes place. The target gas is readily carried across the membrane, while the diffusion of the other gases is inhibited. The driving force for gas transportation remains the partial pressure difference across the membrane. However, by increasing the CO2 feed partial pressure the CO2 permeance and selectivity subsequently decrease. FTMs can be divided in two categories: fixed-site-carrier membranes (FSCMs), where the carrier is fixed to the polymer by chemical bonding and liquid membranes (LMs), where the carrier can diffuse in the membrane. Both kinds of membrane categories have shown attracting performances for CO2/N2 mixtures. Recently, a permeability above 6000 Barrer and a selectivity of about 500 have been reported for a membrane based on poly(allylamine) as a fixed carrier and 2-aminoisobutyric acid-potassium salt as a mobile carrier in a cross-linked PVA matrix (Huang et al., 2008). The main characteristic of these membranes is represented by the request of water vapor for the selective reaction toward CO2. Specifically, the separation performances of reactive membranes strongly depend on humidity and a certain level of hydration has to be maintained on the two sides of the membrane for correct operation. This is very interesting for the application of CO2 separation from fossil fuel emissions because exhausted gases usually contain saturated water vapor. In terms of design, the carrier in these membranes will become saturated quicker if compression is used on feed side. This specificity logically has a strong impact on the process design features. Table 14.5 reports some performance data related to the FTMs.

3.5 Discussion on Membrane Material Design For a given separation (such as CO2/N2 in postcombustion capture), the membrane should ideally show both high selectivity and permeability (Zolandz and Fleming, 1995). High selectivity is necessary to achieve the purity target, ideally in a single-stage process; the high permeability will allow for minimizing the surface area required for a given application. Moreover, according to Seader and Henley (2006) the membrane should show

392 Chapter 14 Table 14.5: Performance of Facilitated Transport Membranes Membrane

aCO2 =N2

CO2 Permeance (GPU)

References

Polyvinyl amine (PVAm) and polyvinyl alcohol (PVA) blend membrane

150

>300

Fixed carrier in a solvent-swollen membrane and water-swollen chitosan (poly(D) glucosamine) membrane Poly (allylamine) as a fixed carrier 2-aminoisobutyric acid-potassium salt as a mobile carrier in a cross-linked PVA matrix Composite membranes (coating PPO and PSf with high-molecular PVAm) PVAm composite membranes with PH control Amine-modified mesoporous silica membranes DEA supported on poly(vinyl alcohol) membranes Enzyme supported on polymer

120

320

500

>6000

Huang et al. (2008)

500

840

Sandru et al. (2010)

>500

>1800

800

300

Barillas et al. (2011)

200

100

Francisco et al. (2010)

820

52

Zhang et al. (2010)

Deng et al. (2009) Deng and Hagg (2010) Matsuyama et al. (1999) El-Azzami and Grulke (2008)

Kim et al. (2013)

chemical and mechanical compatibility with the process environment: stability, freedom from fouling, reasonable life time, ease of fabrication and packaging, and resistance to high pressures. Membranes used for carbon capture should have all these features. However, most of the studies in the literature mainly analyze the permeability of pure gases leading to an estimation of the CO2/N2 selectivity. Experiments with gas mixtures, humid feeds, and/or other flue gas components, such as O2, SOx, NOx, NH3, to better mimic real flue gas conditions, are ignored with a few exceptions (Hussain and Hagg, 2010; Merkel et al., 2010; Reijerkerk et al., 2011; Sada et al., 1992; Scholes et al., 2010). It is important to understand the role of minor components because they affect the design and operation of membranes for carbon capture. Indeed, as reported by Koros and Fleming (1993), the permeability of a gas at pure versus mixture conditions may vary remarkably due to different molecules competing in both diffusion and sorption processes. The effect of water vapor can be considered as an example. Indeed, it has been hypothesized that water negatively affects the CO2 permeation because of its higher permeability compared with CO2. Nevertheless, Merkel et al. (2010) have reported that the presence of water has a positive effect on CO2 permeation because it can be considered as an internal sweep, which dilutes the permeate by increasing the permeation driving force through the membrane. Similar results have been found by Hussain and Hagg (2010) by using FTMs and by Reijerkerk et al. (2011) with poly(ethylene oxide)-based block copolymers.

Design Considerations for Postcombustion CO2 Capture 393 The opposite result was demonstrated by Scholes et al. (2010) by using a polydimethyl siloxane (PDMS) rubbery membrane. The negative effect of water vapor on the permeability of CO2 was due to the hydrophobic nature of PDMS resulting in very low water sorption. Jiang (2012) noticed that by using MOFs the effect of water may be positive on CO2 separation at a certain pressure range and negative at another pressure range. Therefore, future investigations under realistic flue gas conditions will be crucial for the successful design and development of materials for carbon capture.

4. Membrane Module Configurations for Carbon Capture Apart from the membrane material, the configuration of the membrane module is an additional parameter that must be considered for CO2 separation. In particular, they have to show low production cost and energy consumption, in addition to high packing density. The module configurations described are in reference to only polymeric membranes because polymeric membrane technology is currently applied at the industrial level despite many scientific studies that involve different kinds of membrane applications to CO2 capture. Three types of module configurations are mostly used: hollow fiber (Esposito et al., 2015), spiral wound (Zhao et al., 2011; Nadir, 2016), and envelope (Borsig, 2016). Each module configuration shows different characteristics, which are summarized in Table 14.6. An important parameter to evaluate the membrane module is the packing density, which indicates the surface area of the membrane per unit volume inside the module. A hollow fiber module generally shows the highest packing density (up to 30,000 m2/m3), followed by the spiral wound (between 300 and 1000 m2/m3), and then the envelope, which shows the lowest packing density (less than 500 m2/m3) (Mulder, 1996). Fig. 14.1 illustrates the structures of three types of membrane modules (Wang et al., 2017). As a general comparison, the hollow fiber configuration has advantages over the two other types in terms of cost and packing density. However, fibers may be easily blocked by the particulate matter and must be completely replaced, which may not be very cost-effective for an existing power plant. Table 14.6: Comparison of Membrane Modules

2

3

Packing density (m /m ) Pressure drop Cleaning Manufacturing Cost ($/m2)

Spiral-Wound

Envelope

Hollow Fiber

<1000 High and longer permeate path Hard Easy and cheap 9e44

200e500 Moderate Medium Easy 47e176

<10,000 High in fibers Chemical washing or replaced Cheap 2e9

394 Chapter 14

Figure 14.1 Membrane modules: (A) spiral wound; (B) hollow fiber; (C) envelope (Wang et al., 2017).

4.1 Hollow Fiber Membranes Hollow fiber membranes comprise thin polymeric tubes, with a diameter of 50e200 mm (Baker, 2004). The selective layer is on the outside surface of the fibers, facing the highpressure gas. A hollow fiber membrane module normally contains tens of thousands of parallel fibers sealed at both ends in epoxy tube sheets (Baker, 2004). These membranes are compact, require low energy consumption, and show high flux with moderate selectivity in a full-scale system (Sandru et al., 2010). Moreover, they are cleanable by reversing the permeate flow, they require low area cost and low holdup volume.

Design Considerations for Postcombustion CO2 Capture 395

4.2 Spiral Wound Membranes Spiral wound membranes are compact, which means that high membrane packing density results in more efficient use of the footprint. In addition, they are stable and require minimum energy consumption and low capital and operating costs. Generally, sheets of membranes that are 1e2 m long are cut and folded and subsequently packaged as spiral wound modules (Kashemekat et al., 1991). A single module may contain as many as 30 membranes. Drawbacks of these membranes are that they are not suitable for very viscous flow and are difficult to clean. In addition, if the membrane fails the entire module needs to be replaced.

4.3 Envelope Membranes In the envelope configuration, sets of two membranes are placed in a sandwich-like fashion with their feed sides facing each other. A suitable spacer is placed in each feed, and permeate side and baffles are introduced to establish a uniform flow distribution and to reduce channeling (Mulder, 1996). The benefits of the envelope configuration are the use of flat membranes without the use of glue and the exchange ability of single membranes. However, this configuration is in need of sealing and shows low packing density. Moreover, pressure drop can take place inside the module.

5. Membrane Design for Carbon Capture Applications The design of a membrane process is aimed at optimizing the entire process system configuration to achieve the required purities at minimum capital and operational expenditures and to enhance the overall performance of the membranes. Modeling and optimization are the main keys to reaching this objective. A reasonable membrane system design based on feasible membranes should take into account the following factors: separation target (CO2 recovery rate and purity); operating conditions such as temperature, pressure, and composition of CO2 in the feed gas; tradeoff balance between material cost and energy consumption. The flue gas of coal-fired power plants has a low CO2 concentration (Herzog, 2001) at ambient pressure, and to achieve the desired CO2 recovery rate of >90% and CO2 purity >95 mol% (Department of Energy, January 2011), which is feasible with the competing technology (chemical absorption) and required for pipeline transport (Conturie, 2006; Hagedoorn, 2007), two strategies should be considered, i.e., a single-stage membrane with high selectivity or optimized multistage membrane systems.

5.1 Single-Stage Membrane There have been numerous efforts in the simulation of membrane-based gas separation over the last six decades. Today, the possibility to accurately predict the separation

396 Chapter 14

Figure 14.2 (A) Parameters for a single-stage membrane process; (B) single-stage membrane module.

performances of a unit through a series of key equations and associated assumptions is abundantly documented (Bounaceur et al., 2006; Kaldis et al., 2000; Chowdhury et al., 2005; Coker et al., 1999; Matson, 1983; Zanderighi, 1996). Fig. 14.2A shows all significant parameters for a single-stage membrane process. Specifically, as in any process design study, a series of variables has to be first defined, and then a fixed number of unknowns can be identified through analytical or numerical resolution. For carbon capture studies, two key limitations are imposed: the CO2 capture ratio (R) (Eq. 14.1) and the CO2 purity produced on the permeate side (y). The separation performance is obtained by considering the CO2 capture ratio and the CO2 purity. For a single-stage, these two parameters are calculated by simulation combining the operating boundary conditions and membrane parameters. Anyhow, membrane separations show a high parametric sensitivity unlike other carbon capture processes, and the following variables play important roles: • • •

composition of CO2 in the feed (xin); driving force, expressed as the pressure ratio across the membrane (Eq. 14.2); ideal selectivity (Eq. 14.3) and the CO2 permeance, which are related to the membrane material.

The influence of these variables will be discussed hereafter. 5.1.1 Relation Between the Pressure Ratio and Ideal Selectivity for a Target CO2 Purity There is a strong interaction among the pressure ratio, the ideal selectivity, and the CO2 purity. Specifically, the required pressure ratio decreases as the desired permeate CO2 purity increases, which induces an increase in the energy requirement. Moreover, for a

Design Considerations for Postcombustion CO2 Capture 397 given pressure ratio, the higher the membrane selectivity, the higher the permeate purity. Also, as reported by Merkel et al. (2010) and Bounaceur et al. (2006), at low pressure ratios (high driving force), the pressure ratio limitation is reduced and the role of membrane selectivity becomes significant. For a given target purity, the pressure ratio depends strongly on the CO2 composition in the feed. In particular, it decreases with increasing CO2 composition. As a consequence, when low CO2 composition has to be treated and when high CO2 purity is desired, the membrane selectivity plays a key role and the energy requirement increases (Belaissaoui et al., 2012a). As an example, for a target CO2 purity of 95% and an inlet CO2 composition of 15%, the ideal selectivity necessary to attain the target is 200 (Zhao et al., 2012). Nevertheless, owing to the trade-off between the membrane permeability and selectivity, the selectivity of the state-of-the-art commercial polymeric membrane is not above 60 (Zhao et al., 2009). Consequently, it is impossible to reach the required CO2 purity using a single-stage membrane, and the use of a multistage process became necessary. However, it should be noted that the possibility of achieving high selectivity with nonpolymeric membranes (Tables 14.3e14.5) could be attractive because it offers the possibility to theoretically use a single stage unit for coal-fired power plants. 5.1.2 Compression Strategy and Energy Requirement The driving force, and consequently the pressure ratio through the membrane, can be obtained by choosing two different configurations: feed compression (Fig. 14.3); vacuum or sweep gas on the permeate side (Fig. 14.4). The feed compression strategy is most often selected for gas separations. However, for carbon capture, it leads to a huge energy requirement due to the compression of the total feed flow rate (Merkel et al., 2010; Bounaceur et al., 2006). The vacuum pumping strategy is recommended when a minimal energy requirement is necessary. In this case, the energy requirement will be in principle lower than the direct compression strategy because a smaller flow rate has to be pumped. However, this option has drawbacks such as the energy efficiency of the vacuum pumps can be much lower than compressors and the vacuum operation can be difficult at large scale. Comparing the two options, it is worth noting that the feed compression strategy leads to a notably lower membrane area than that required for the vacuum pumping strategy. This analysis shows that there is a compromise between the energy requirement and membrane surface area, which means that an optimum low cost/high CO2 recovery exists for each option.

5.2 Multistage Membrane The single-stage system cannot provide the desired product with both high CO2 recovery and purity. This is because the separation process is constrained on one hand by the low

398 Chapter 14

Figure 14.3 Single stage with feed compression configuration.

Figure 14.4 Single stage with vacuum configuration.

CO2 partial pressure difference and on the other hand by the trade-off relationship between CO2 recovery and CO2 purity (Zhao et al., 2008; Robeson, 1991). Therefore, multistage or cascade membrane separation becomes a viable option, where a combination of a membrane containing multiple stages, in parallel or in series to reach higher qualities of permeate, may be considered. Unfortunately, such arrangements result in both higher capital and operating costs due to high membrane surface area and high compression costs, respectively. In such scenarios, membranes seem to not be the best available technology, and other separation technologies might be competitive. However, the process synthesis, configuration, and design will play a key role in the success of the membrane system. 5.2.1 Cascade Membrane System The first two-stage system for carbon capture was presented for the first time by Pfefferle (1960), and the system comprised a two-stage membrane with permeate recycling to reach a high-purity permeate. Two decades later, Kakuta et al. (1978) and Ozaki et al. (1978) introduced a cascade membrane system for a binary gas mixture separation, and later Gruzdev et al. (1984) proposed the cascade system for a multicomponent gas mixture. The cascade model shown in Fig. 14.5 represents the most general multistage membrane design. The design idea came from a multistage operation such as a distillation or extraction system (Treybal, 1955). The cascade model comprises both the upstream and downstream. The first stream strips the remaining traces of the gas of interest to desired values, whereas the second one enriches the permeate to higher purities of the gas of interest. However, in many processes only one of the two streams is required.

Design Considerations for Postcombustion CO2 Capture 399

Figure 14.5 Schematic of cascade membrane system with recycle.

For example, in the oxygen separation process from air, the upstream is the only stream required, whereas in the natural gas sweetening, both streams are necessary. However, it is well known that the most technoeconomically optimal configuration is represented by the two- or three-stage membrane system (Ozaki et al., 1978; Pettersen and Lien, 1995), except in case of very low feed concentrations (Li et al., 1990) or lowefficiency membranes (Avgidou et al., 2004). Indeed, it has been shown that the introduction of more stages slightly decreases the membrane surface area and compression energy while subsequently increasing the number of compressors, thereby neutralizing that benefit (Pettersen and Lien, 1995). 5.2.2 Two- and Three-Stage Membrane Systems There have been a number of studies that have focused on the optimal configurations of two- and three-stage membrane systems. Regarding the two-stage systems, Kao et al. (1989) compared two different configurations: the continuous column membrane or CMC (Fig. 14.6) and the two strippers in series permeator system, TSSP (Fig. 14.7). They reported that TSSP shows better performance compared with the CMC configuration except if the aim is to reduce the membrane area or to have high-purity permeate.

Figure 14.6 Two stage with continuous column membrane configuration.

400 Chapter 14

Figure 14.7 Two stage with two strippers in series permeator system.

Qiu et al. (1989) have obtained similar results affirming that TSSP is the best configuration if the purpose is to reach a high-quality retentate, whereas in the case of a high-quality permeate, CMC is more efficient. Bhide and Stern (1993) investigated the minimum cost for different one-, two-, and threestage configurations. They found that the best configuration corresponds to the three-stage system with a single permeation stage in a series with a two-stage permeation cascade with recycle (Fig. 14.8). Pettersen and Lien (1995) analyzed the intrinsic behavior of different single-stage and multistage permeator systems. They found that the upstream section of the cascade (stripping) is the best choice if the aim is to have a retentate stream with the minimum concentration of the gas of interest, whereas the downstream section (enriching) is chosen in the case of a high-purity permeate product. Datta and Sen (2006) evaluated several configurations and conveyed that the best configuration highly depends on the feed quality, separation purposes, and market prices. In summary, it is clear that the selection of the multistage design depends on the separation strategy, which is influenced by the economical input. Specifically, when the gas of interest is in the retentate side and, its partial loss with the permeation is not

Figure 14.8 Three-stage system with single-permeation stage in series with two-stage permeation.

Design Considerations for Postcombustion CO2 Capture 401

Figure 14.9 Three-stage stripping system.

economically significant, then the stripping option is simply used. In this case, the design consists of a two-stage or three-stage membrane system as shown in Figs. 14.7 and Fig. 14.9, respectively. Conversely, when the aim is a high-purity permeate, then the enriching option will be chosen and the design would be the two- or three-stage membrane system shown in Fig. 14.10. However, there are situations where high-quality of both retentate and permeate is desired. In this case, a combined design of Figs. 14.7, 14.9, and 14.10 can be used. According to Seader and Henley (2006), the best approach in designing cascade systems is to select parameters in a way that the composition of the recycled permeate to any stage i is similar to that of the feed entering the same stage. In recent years, there have been numerous efforts to develop the best design for the cascade membrane system, the so-called “superstructures” for membrane process synthesis (Agrawal, 1996; Qi and Henson, 2000; Uppaluri et al., 2004; Saif et al., 2009; Alshehri et al., 2013; Gassner and Marechal, 2010). The first study in this field was made by Vandersluijs et al. (1992) who investigated the qualities of single-stage and two-stage cascade membrane systems for postcombustion capture by using membrane modeling with the assumption of binary CO2/N2 flue gas. They found that for high-purity CO2 product (>80%), the two-stage system could perform

Figure 14.10 Two- and three-stage enriching system.

402 Chapter 14 remarkably better than the single-stage process. Analogous results were reported by Carapellucci and Milazzo (2003) who pointed out that the best option for enriching the CO2 stream is the two-stage design while the addition of the third stage increases the complexity of the process without particularly improving the CO2 purity. Ho et al. (2008) studied the single-stage and two-stage cascades with and without retentate recycle. In addition, they investigated two different configurations: feed compression and vacuum. They reported that the two-stage system operated under vacuum and with the recycle shows the best performance in terms of CO2 purity. However, they found that the vacuum strategy requires larger membrane surface area but could save 35% of the capture cost. Another membrane strategy for carbon capture is the design of a two-stage, two-step membrane system with CO2 recycling. In comparison with the two-stage design, the retentate stream exiting the first stage is sent to a second stage where a sweep gas is used in a countercurrent flow configuration to further remove CO2. Combustion air is used as a sweep gas, which carries the permeated CO2 back to the boiler along with the feed air. The retentate stream is vented while the permeated CO2 product is compressed for transport and storage. Merkel et al. (2010) investigated the two-stage membrane system with and without sweep gas. For their case study, they studied a 600-MW power plant with 90% CO2 capture using membrane technology, and they reported that the membrane area needed to decrease from 3.0 to 1.3 million square meters and the power from 145 to 97 MW by adopting the sweep gas technology. Alshehri et al. (2013) developed a superstructure considering n-stage membranes to find the optimal configuration for a 300MW coal-fired power plant. Specifically, in their model they considered a multicomponent gas model, and they solved an objective function that minimized the costs associated with operating and capital expenses. The model was able to identify two best configurations depending on the CO2/N2 selectivity. In particular, if the membrane had a CO2/N2 selectivity of 100, then the two-stage membrane system was the best, whereas for a selectivity of 50, the three-stage membrane system showed the best performance.

5.3 Hybrid Membrane Design The development of CO2 capture systems has principally focused on the study of a single technology. Only few studies have investigated a hybrid capture system by combining multiple separation technologies (Yuan et al., 2017). For example, Membrane Technology and Research, Inc. (MTR) and the University of Texas at Austin (UT Austin) developed a hybrid membrane absorption capture technology, consisting of the MTR Polaris membrane contactor combined with UT Austin’s piperazine advanced flash stripper capture technology (Freeman et al., 2014). They designed two different configurations, where the chemical absorption and the membrane are in series and in parallel, respectively. In the first configuration, the chemical adsorption unit is followed by the membrane system and

Design Considerations for Postcombustion CO2 Capture 403 the total CO2 removal reaches 90%. In the second arrangement, the flue gas stream is split and directed to the absorption column and membrane system, with the benefit of having only half of the absorber size. It can be concluded that in the series arrangement the energy required for solvent regeneration is less and in the parallel configuration the capital costs decrease compared with a conventional chemical absorption process. Zhao et al. (2014) and Belaissaoui et al. (2012) investigated via modeling analysis the hybrid process consisting of the cryogenic and membrane technologies. In both studies, the process arrangement is in series where the membrane is installed before the cryogenic unit to concentrate CO2. However, the main difference between the two studies is the specific configurations of the cryogenic unit. Specifically, 3 GJ/ton of CO2 are consumed by the process simulated by Belaissaoui et al. (2012) with CO2 capture above 85% and CO2 purity higher than 89%, whereas the hybrid process of Zhao et al. (2014) possesses lower efficiency loss than amine absorption and cascade membrane system when the CO2 separation is less than 90%. By considering these few studies, the hybrid system appears to be more energy saving and economical. This design can be a possible future direction for advanced carbon capture technology, although there are not enough data to prove that the hybrid membrane is truly feasible.

6. Cost Consideration and Membrane System Design Several technoeconomic analyses have been carried out that evaluate the feasibility of membrane systems to remove CO2 from flue gases and improve the viability of membrane technology for carbon capture. One of the first technoeconomic studies was made by Vandersluijs et al. (1992), who analyzed the technical feasibility and mitigation costs of polymeric membranes for carbon capture. They used a model for cross-flow permeation to establish the CO2 reduction costs that depend on the separation targets. Specifically, for CO2 recovery of 75% with purity of 50%, the minimum possible cost was estimated to be $48=tCO2 avoided. By increasing the recovery and purity at 90% and 95%, respectively, the cost increases up to $71=tCO2 avoided. The authors report that in order for the membrane to become economically competitive, it should show a CO2/N2 selectivity higher than 200 in addition to a high permeability. The selectivity value was cited by other membrane studies (Aresta, 2003; Favre, 2007; Feron et al., 1992; Wolsky et al., 1994). Kazama et al. (2004) performed an economic analysis to evaluate the cardo polyimide hollow fiber membrane, which they developed. The membrane showed a high CO2 permeance of 1000 GPUs and CO2/N2 selectivity of 40. They reported that the membranes can become economically advantageous and present an alternative to the existing aminebased capture systems if the CO2 concentration is around 25% or more. Matsumiya et al. (2005) evaluated the energy consumption of a novel ultrafiltration hollow fiber module for CO2 separation from flue gas and studied the effects of the permeate-side pressure,

404 Chapter 14 temperature, and the inner diameter of the hollow fiber membranes. Regarding the latter parameter, the author reported that the energy consumption was in the range of 0.072 kWh/kg-CO2 (0.259 GJ/tonne-CO2) to 0.211 kWh/kg-CO2 (0.796 GJ/tonne-CO2) when the hollow fiber inner diameter varied from 1.4 to 0.8 mm. In addition, they studied two different configurations: feed compression and vacuum. They found that by using a vacuum, the energy required to create a given driving force is less than the one required by feed compression. A similar result was reported by Zhai and Rubin (2013) by considering a two-stage membrane system. Ho et al. (2006) compared the viability of a single-stage membrane process for postcombustion carbon capture with respect to the amine-based system, and they studied the effects of membrane characteristics, operating parameters, and system design on capture costs. In particular, they considered three membranes: poly(phenylene oxide) (PPO), polyimide (PI), and PEO. The total capture cost was in the range of US$55 to 61=tCO2 avoided. Their study confirmed that the membrane cannot economically compete with the amine process. In another study the same authors, Ho et al. (2008) compared the feed compression and the vacuum approaches. They found that the vacuum strategy required relatively high membrane area, although it could realize 35% less capture cost per tonne of CO2 avoided. Merkel et al. (2010) synthesized a new membrane, which shows higher permeance than the commercial CO2 membrane. In their study, they reported that the enhancement of the CO2 permeance plays a more important role than the CO2/N2 selectivity to reduce the overall cost. In a recent study, Zhai and Rubin (2013) studied the performance and the costs of single- and multistage membrane configurations. The latter have allowed them to reach high recovery and purity, i.e., 90% and 95%, respectively. In addition, they found that the multistage membrane can be competitive with the amine-based capture process. Maas et al. (2016) made an energetic and economic analysis for a membrane-based separation process for carbon capture, and they found that the cascade membrane system design achieves the lowest energy consumption. From their research it emerges that the cost of CO2 allowances has to exceed $44=tCO2 to make membrane separation technology economically advantageous. Table 14.7 shows some energetic and economic estimates of membrane-based processes and some of the results are compared with chemical adsorption. It can be seen that a membrane-based separation process does not have apparent advantages with respect to the chemical absorption technology (Table 14.8), in terms of energy and cost at 90% CO2 recovery. Membrane technology is less competitive than initially expected because of the large membrane surface area and mechanical work necessary to do the CO2 separation.

Table 14.7: Energetic and Economic Evaluations of Membrane-Based Separation Process

T (
C)

Permeance (Nm3/m2 h bar)

CO2 Recovery (%)

600

30

0.5

450

30

3 1000 (gpu)

50 70 70 90

600

25 30 50

3 4.3 5

Without sweep gas. With air as sweep gas.

b

90

Membrane Area (Mm2) FirsteSecond

CO2 Avoidance Cost ($/tCO2 )

6.62e0.24 13.92e0.34 2.44

CO2 of Electricity ($/MWh)

References Zhao et al. (2010)

85a 55b 0.40e0.07 0.29e0.04 0.24e0.03

CO2 Capture Cost ($/tCO2 )

53

36 49a 36b

125a 105b

107

Zhai and Rubin (2013) Maas et al. (2016)

Design Considerations for Postcombustion CO2 Capture 405

a

Output of Power Plant (MW)

406 Chapter 14 Table 14.8: Energetic and Economic Evaluations of Chemical Absorption Process Using MEA Output of Power Plant (MW)

CO2 Recovery (%)

CO2 Avoidance Cost ($/tCO2 )

CO2 Capture Cost ($/tCO2 )

CO2 of Electricity ($/MWh)

600

90

e

43

66

450

90

62

93

73

References Abu-Zahra et al. (2007a,b) Massood Ramezan (2007)

7. Conclusion The application of membrane separations for postcombustion carbon capture has attracted attention in recent years. Significant improvements have been made in the development of membrane materials, membrane performance, and process design. The research on polymeric membranes for carbon capture is addressed toward the chemical and physical modification of the membrane to greatly enhance the separation performance. Specifically, agentsdorganic or inorganicdare added into the matrix of the membrane and, currently, the FTMs, which incorporate a carrier molecule, seem to be promising for carbon capture. Selectivity and permeability are the key factors for a good membrane. In addition, the membrane must exhibit chemical and mechanical compatibility with the process environment, stability, long life time, easiness to fabrication and packaging, and resistance to high pressures. However, most of the present studies on CO2 capture membrane materials are focused on improving the performance in terms of selectivity and permeability without paying attention to other important requirements, i.e., the impacts of minor gas components such as water vapor, SOx, NOx, O2. Indeed, the membrane use at the industrial level might not be possible without analyzing these critical technical and operational issues in various perspectives. Therefore, despite numerous efforts and remarkable improvements in the performances of membrane materials, the best application and role of membrane units remain still unclear. Regarding the design of membrane, it can be concluded that the single-stage membrane configuration is not a feasible solution for carbon capture because of the low CO2 content of flue gas, even for membranes with high permselectivities. In addition, it has been shown in several studies that the vacuum configuration has a positive impact in reducing the system energy penalty. Multistage or hybrid systems offer the most promising solutions for the use of membrane systems for carbon capture, although these configurations are unexplored up to now. Only few studies have used the membrane-based process to treat realistic flue gas compositions, obtaining promising experimental results (Merkel et al., 2010; Hagg et al., 2012; Sandru et al., 2012).

Design Considerations for Postcombustion CO2 Capture 407 In summary, the priority of the research has to be addressed to support (1) design studies for multistage and/or hybrid systems, (2) membrane material, and (3) economic analysis. However, a deep and complete analysis is needed to help investigate trade-offs between performance and cost objectives and recognize the most promising system design and targets of material properties for competitive membrane capture technologies.

List of Acronyms CCS CMC FSCMs FTMs LMs MMM MOFs MTR PEO SSF TSSP

CO2 capture and storage Continuous column membrane Fixed-site-carrier membranes Facilitated transport membranes Liquid membranes Mixed matrix membrane Metal organic frameworks Membrane Technology and Research, Inc. Poly(ethylene-oxide) Selective surface flow Two strippers in series permeator system

List of Symbols R q y xin J pf pp a P

Capture ratio Stage cut CO2 purity corresponding to mole fraction of CO2 in the permeate Mole fraction of CO2 in the feed Driving force Feed pressure Permeate pressure Ideal selectivity Permeability

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