Inorganic Membranes for Process Intensification: Challenges and

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Inorganic Membranes for Process Intensification: Challenges and Perspective Jerry Y.S. Lin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04539 • Publication Date (Web): 26 Nov 2018 Downloaded from http://pubs.acs.org on November 26, 2018

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Industrial & Engineering Chemistry Research

Inorganic Membranes for Process Intensification: Challenges and Perspective Y.S. Lin School for Engineering of Matter, Transport and Energy Arizona State University Tempe, AZ 85287 Abstract Membrane separators and reactors represent some of the most interesting options for process identification and modularization. In spite of intensive research and significant progress in material development and understanding of fundamentals of membrane properties and membrane processes, commercialization of inorganic membrane separation and reaction processes remains a big challenge. This paper summarizes important separation/permeation properties of several groups of inorganic membranes for molecular separation and discusses key hurdles in commercializing inorganic membrane separators and reactors for process intensification. Future research and development efforts should be focused on addressing these key hurdles including synthesis of membranes with higher permeance and stability, fabrication of low cost and high packing density membrane substrate, and development of cost-effective, scalable methods for synthesis of membrane separation layer. Commercial applications of highly permeable water-selective zeolite membrane separation processes will be expanded to membrane reactors coupling reaction with the removal of water from the membrane reactor.

Keywords: Inorganic membranes, gas separation, liquid separation, membrane reactors, process intensification

Email: [email protected]

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Introduction Inorganic membranes include a large number of macroporous, mesoporous, microporous, and dense membranes made of different inorganic materials ranging from ceramics to metals. Microporous and dense inorganic membranes capable of separating liquid and gas molecules find applications in chemical process intensification as a separator or membrane reactor. Much smaller, more energy efficient and safer chemical processes can be developed by proper integration of a membrane separation unit with a chemical reactor or use of a membrane reactor, which combines a membrane and a reactor into one device. For process intensification applications, the membranes should be operated under conditions comparable to those for chemical reactors, such as high temperatures. Inorganic membranes particularly suit process intensification applications because of their high thermal and chemical stability. Crystalline (such as zeolite) and amorphous (such as silica) membranes make up the majority of microporous inorganic membranes. Recently, microporous crystalline metal-organic framework membranes have received much more attention in the membrane community. Pure metal-organic framework membranes are generally classified within inorganic membranes because their synthesis, structure, and transport properties are similar to crystalline zeolite membranes. Amorphous microporous membranes have been limited to silica and carbon membranes containing mostly ultra-micropores. These microporous inorganic membranes separate gas or liquid molecules by a mechanism that combines solution and activated diffusion for molecules smaller than the pore size of the membranes, or by the molecular sieving mechanism for a mixture containing molecules larger than the membranes pores1. Dense membranes consist of ionic or mixed-conducting metal oxides, metallic membranes, or dualphase membranes. They are perm-selective to oxygen, hydrogen, or carbon dioxide.

These

membranes have a unique crystalline structure allowing oxygen or hydrogen to permeate through the interstitial or lattice defects of the crystalline grains, often in the charged form.2 CO2perm-selective dense membranes are made of an ionic-conducting solid phase for oxygen ion conduction and a molten carbonate phase allowing carbonate ion transport.3-4 These dense membranes only permeate O2, H2, or CO2.


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In general, membrane separation processes are more energy-efficient, cost-effective, and footprint (space)-saving than other separation technologies. Compared to polymer membranes, microporous and dense inorganic membranes offer better separation characteristics in terms of permeability, permeance, and selectivity, and are more chemically and thermally stable. These properties make inorganic membranes particularly attractive for applications in process intensification. In particular, the excellent thermal stability of inorganic membranes enables the integration of a membrane with a catalytic chemical reactor, often operated at high temperatures.

These membrane reactors combine separation with a reaction by either

controlling the feed of a reactant or the removal of a product, increasing the reactant conversion and/or product selectivity, which help reduce the reactor size and increase the reaction efficiency. More importantly, the membrane reactors lower the burden or eliminate the need for the reactant or product separation. All these characteristics and outcomes render process intensification.

For example, oxygen semi-permeable mixed-conducting ceramic membrane

reactor for partial oxidation of methane for syngas production combines air separation with methane conversion, providing about a 30% capital cost-saving and about a 5% improvement in energy consumption5. It was recently reported that the integration of mixed-conducting ceramic membrane into coal/natural power cycle with carbon dioxide capture could result in a 10-14% increase of plant efficiency and 8%-12% reduction in energy consumption.6 The 1990’s witnessed the beginning of intensive research and development on microporous and dense inorganic membranes for molecular separation and membrane reactor applications. These research and development efforts have continued to this day and have produced a large number of publications and patents, including several monographs7-9 and some commercial applications of these membranes for molecular separation. However, these efforts have not yet resulted in commercial applications of membrane reactors for process intensification. This paper will discuss major characteristics of inorganic membranes with the ability for molecular separation, analyze the challenges that the inorganic membranes face for industrial applications for process intensification, and provide suggestions for future research and development efforts on the inorganic membrane for industrial molecular separation and process intensification applications. 3 ACS Paragon Plus Environment


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Applications of Inorganic Membrane for Process Intensification Macroporous and mesoporous inorganic (mainly alumina, titania, zirconia) membranes have found industrial applications in various filtration separation processes for water and wastewater treatment, pharmaceuticals, food and beverage, chemical processing and biotechnology 7 as well as in membrane bioreactors 10 with an estimated global market size of about $1-2 billion/year ($5 billion/year projected for 2020 11). In addition to the extensive use of inorganic membranes for filtration separation, some inorganic membranes are already used commercially for process intensification. These include the use of ultrafiltration ceramic membranes for the recovery of catalysts from reactors12 and the application of mesoporous ceramic membranes for air conditioning dehumidification.13-15 Ceramic membranes are also used as the electrolyte layer in oxygen-ion16 or proton17 conducting solid oxide fuels for electricity generation or co-generation of electricity and chemicals. Recently, inorganic membranes have also been studied as the separators for batteries to improve the safety of energy storage devices.18-19

These separation processes or devices use either macroporous/mesoporous

membranes or dense ionic-conducting membranes with high transport flux. Furthermore, these applications do not couple the reaction with separation in one device. Industrial membrane separation processes for molecular separation are dominated by polymer membranes. Table 1 summarizes commercial gas separation membrane processes and major separation characteristics of the polymer membranes.20-21

Microporous or mixed-

conducting inorganic membranes offer the ability to separate molecules based on the size and chemical properties of the gas or liquid molecules. These membranes have been studied as a separator for the separation of gas or liquid mixtures, or in a membrane reactor to improve conversion and selectivity of chemical reactions, coupled with the reactant or product separation, as shown in Figure 1. Compared to adsorption or absorption separation processes, membrane separation combines adsorption and desorption in one device, allowing continuous, non-batch-wise operation of the separation process. Though membrane separation processes are more energy efficient and have smaller footprints than the adsorption and absorption separation processes, the efficiency and compactness of the membrane separation units depend mainly on the permeance and selectivity of the membrane. A more permeable membrane 4 ACS Paragon Plus Environment

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provides a higher production rate or requires less membrane area for a given production rate. Perm-selectivity of a membrane determines the product purity. For a membrane with a given selectivity, higher product purity can be achieved through cascade. However, this would result in a larger membrane area (higher membrane cost) and consume more energy for operation.

Figure 1 Schematic illustration of membrane separator and membrane reactors. The shaded area in (b) and (c) indicates catalyst. The majority of membrane reactors are operated in the following three modes (Figure 1, b-c): In the first two modes, the reactor wall is made of a perm-selective membrane, and a conventional catalyst is packed in the membrane reactor. The most commonly studied mode of membrane reactor is coupling a reaction with the removal of a final (or intermediate product) from the reactor (Figure 1, b). A typical example is the hydrogen-selective membrane reactor for dehydrogenation reaction22-23. Removing a product enhances the conversion of a reaction limited by thermodynamic equilibrium, increasing the yield of the desired product. The reactor also provides product/reactant separation. Thus, this type of membrane reactors requires a 5 ACS Paragon Plus Environment


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smaller amount of catalyst and eliminates (or reduces the burden of) separation process. The key requirements for the membrane for use in these membrane reactors are similar to those for the membrane separators, i.e., high permeance and selectivity. Also, the operation conditions, catalyst, and membrane should be so selected that the reaction rate matches the permeation flow rate.

Table 1 Current Membrane Gas Separation Industrial Applications Gas

Membranes

Selectivity

Permeance (GPU)#

N2/Air (Nitrogen production)

   

Polysulfone (Air Products) Polyimide (Air Liquid) Polycarbonate (Generon) Poly(phenylene oxide) (Aquilo)

O2/N2 =3-6

5-50

CO2/CH4 (Natural gas processing)



Cellulose Acetate (UOP)

CO2/CH4 = 10-15

100-200

H2/N2 or NH3 (Hydrogen separation)

 

Polysulfone (Air Products) Polyimide (Air Liquid)

H2/N2 = 50-150

80-200

Hydrocarbons (C3/N2) (Organic vapor recovery)



Silicon rubber (polydimethylsiloxane) (MTR)

C3/N2 =5-20

500-5000

-7

2

# 10 mol/m .s.Pa=300 GPU @ Total gas separation membrane market is estimated at 1-1.5 $billion /year, first two applications accounting for ~70% market share * Data selected from 20

The second commonly studied mode of membrane reactors is that the reactor combines reaction with separation of reactant (Figure 1, c). These reactors include oxygen-ion transport membrane for syngas production with in-situ air separation to obtain oxygen as the reactant 24 and a ceramic-carbonate dual-phase membrane for dry reforming of methane with in-situ separation of flue gas to capture CO2 as the reactant.25 The main advantages of these membrane reactors are the elimination of the additional separation process to obtain the reactant, intensifying the overall reaction process. Another advantage of this type of membrane reactors 6 ACS Paragon Plus Environment

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is that the membranes can be used to control the distribution of a reactant (such as oxygen) along the reactor flow direction to optimize overall selectivity and reactivity, which affects the distribution of heat of reaction and hence temperature profile in the reactor. The requirements for this type of membrane are similar to the first group of membrane reactors. The third group of membrane reactors relies on the membrane to manipulate reaction mechanism, thus improving reaction selectivity and the yield for the desired product (Figure 1, d). A good example is the membrane reactor made of bismuth oxide doped with yttrium and samarium (BYS) which is oxygen permeable and also catalytically active and selective for the formation of methyl radicals for oxidative coupling of methane to ethane and ethylene.26-27 Such a membrane reactor can enhance the product selectivity and yield, eliminating follow-up separation process for recovering the reactant or separation of by-product from the product stream. This results in the improvement of the overall process efficiency. For microporous and dense inorganic membranes capable of molecular separation, the commercial applications have been limited to the application of organic solvent dehydrogenation by hydrophilic, water-selective zeolite (mainly LTA type) membranes,28-29 with over 400 plants estimated in operation in the world, and the production of ultrahigh purity hydrogen by metallic membranes for integrated circuit manufacture.30 The major technical hurdles that have limited industrial applications of inorganic membranes for separation and reaction include (1) moderate permeance, (2) low membrane module packing density, (3) high membrane costs, and (4) insufficient thermal and chemical stability.

For high-temperature separation and reaction

processes, engineering scale-up of inorganic membrane separators or reactors remains a big challenge. In the next section, we will discuss the first three hurdles which are related to each other. Permeance, membrane module packing density, and membrane costs. Figure 2 shows a cross-flow membrane separator for separation of the gas mixture. For this membrane separator, the production rate of the product (in the permeate) is related to the membrane permeance (Fi), membrane area (A) and driving force as 31 (1)

𝑄𝑖 = 𝐹𝑖𝐴(𝑃ℎ𝑥𝑖 ― 𝑃𝑙𝑦𝑖) 7 ACS Paragon Plus Environment


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where xi is the average molar fraction in the feed and yi is the molar fraction in the permeate. The product purity (in the permeate) is related to the membrane selectivity (αij) as:

with

   ij Ph yi  k   k 2  xi  ( ij  1) Pl    

0.5

(2) (3)

 Ph P  k  0.51   h xi   ( ij  1) Pl Pl 

The product purity depends on the membrane selectivity and operation conditions. Under a given operation condition, the product production rate is determined by the membrane permeance and membrane area. For a given membrane area, the volume or the footprint of the membrane unit is controlled by the membrane module packing density, and the cost of the membrane unit is dictated by the membrane price.

Figure 2 A cross-flow membrane separator for gas separation Table 2 lists the typical selectivity and permeance of three groups of inorganic membranes for molecular separation. Microporous inorganic (e.g., zeolite) membranes operate in 25-500oC, and, for non-adsorbing gas (or at high temperatures), the gas permeation flux is approximately proportional to the transmembrane pressure drop.

As shown, typical H2/CO2

selectivity and H2 permeance for a zeolite membrane listed in Table 2 is respectively about 100 and 600 GPU32. Though the permeance of the zeolite membrane is 5 to 10 times that of polymer membranes, it is still moderate if one considers the packing density of the zeolite membrane module as to be discussed next. However, metal and mixed-conducting ceramic membranes offer significantly higher selectivity than polymer membranes (both groups of membranes theoretically only allow H2 or O2 to permeate, excluding other molecules). The permeation flux of these membranes, in general, does not depend linearly on the transmembrane pressure drop. 8 ACS Paragon Plus Environment

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For example, for thick metal or mixed-conducting perovskite membranes with bulk diffusion ratelimiting, the hydrogen or oxygen permeation flux is related to the hydrogen or oxygen partial pressures in feed stream or permeate stream (P’, P”) as 33 𝐽𝐻2 =

𝐾.𝐷

𝐽𝑂2 =

𝐿

1

1

(4)

2 2 (𝑃′𝐻2 ― 𝑃"𝐻2 )

𝑅𝑇𝜎𝑜𝑖 2

(

16𝐹 𝐿

1 𝑃"𝑛𝑂2

―

1

)

(5)

𝑃′𝑂𝑛2

where K and D are hydrogen solubility and diffusivity in the metal, 𝜎𝑜𝑖is the oxygen ionic conductivity for the mixed-conducting perovskite membrane at PO2=1 atm, and n= 0-1. Thus, the definitions of permeability or permeance commonly used for microporous inorganic or polymer membranes are not suitable for the metal and mixed-conducting ceramic membranes. Table 2 Gas Separation Properties of Inorganic Membranes Membrane (operation temperature)

Gas mixture

Typical Selectivity

Typical Permennce

Flux at 1/0.5 atm (cc/cm2.min)

Flux at 20/0.5 atm (cc/cm2.min)

Microporous (e.g. zeolite) membranes (25-500oC)

H2/CO2

100

2.0 x10-7 (mol/m2.s.Pa) or 600 GPU

1.5

30

Metal membranes (300-500 oC)

H2/CO2

>1500

1.1 x10-3 (mol/m2.s.Pa1/2)

15

188

Mixed-conducting oxide membranes (700-1000 oC)

O2/N2

>5000

1.3x10-2 (mol/m2.s.Pa1/4)

5

7

Table 2 compares the permeation fluxes of these three membranes under two specified feed and permeate pressures. As shown, the metal membranes offer H2 permeation flux 5 to 10 times higher than the zeolite membrane. The mixed-conducting ceramic membrane also has a higher permeation flux than the zeolite membrane (at low feed pressure). It should be noted that the downstream pressure has a limited effect on permeation flux for zeolite membranes (for weakly adsorbing permeating molecules or at high temperatures) and for metal membranes (for hydrogen), but has much larger effect on mixed-conducting membranes, as shown by Eq.(5). For mixed-conducting ceramic membranes, oxygen permeation flux is much higher when a reducing 9 ACS Paragon Plus Environment


gas reacting with oxygen at a fast kinetic rate is used as the sweep in the permeate side.34 On the contrary, the upstream pressure has a strong effect on the permeation flux of zeolite membranes (for weakly adsorbing permeating molecules), and minimum effect on mixedconducting ceramic membranes. However, for strongly adsorbing permeating molecules the downstream pressure has a strong effect on the permeation flux of zeolite or other microporous inorganic membranes.

Figure 3 Appearance of (left figure) a polymer hollow fiber gas separation module (module diameter 3.5 cm) and a multi-channel alumina membrane element (module diameter 3 cm) respectively from two membrane manufacturers, and (right figure) porous alumina (about 1 mm in diameter) and dense ionic transport membrane (1.5 mm in diameter) hollow fiber membranes.

The above data show that overall inorganic membranes have selectivity significantly higher than polymer membranes. Their permeance (or flux under a specified transmembrane pressure) is also about 5 to 10 times higher than polymer gas separation membranes. For a membrane separation unit, the product production rate is determined by both permeance and membrane surface area (eq.(1)). Figure 3 compares the appearance of a polymer hollow fiber membrane module and a multi-channel ceramic membrane element from the commercial manufacturers. The physical characteristics of the fibers and channels are estimated and given in Table 3. For the two module/element shown in Figure 3, the polymer module has a packing density of more than ten times that of the ceramic membrane element. Most commonly used ceramic membrane elements are made of single or multiple tubes, whose packing density is compared with polymer membrane modules of various geometries in Table 4. In general, the 10 ACS Paragon Plus Environment

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packing density of polymer membrane modules for gas separation is about 10-100 times that of the ceramic membrane elements (modules). Table 3 Estimates of Polymer and Ceramic Membrane Module (Element) Packing Density Module

Polymer Hollow Fiber

Fiber or Channel OD (mm)

Ceramic membrane multichannel

0.333

2

3.0

2.0

Number of Fiber or Channels

4000

63

Fiber or Channel Cross-sectional Area%

50%

50%

~ 6000

~ 500

Element OD (cm)

2

3

Area/Volume  (m /m ) (based on element)

The multi-channel ceramic element shown in Figure 3 represents industrial inorganic membrane modules with the highest packing density.

Recently, many laboratories and

membrane manufacturers have produced inorganic (mainly alumina) hollow fibers (also shown in Figure 3). However, the fiber diameters are in the range of 0.6-1.0 mm (ID) or 1.1-1.5 mm OD3536, slightly smaller than the channel size of the ceramic element shown in Figure 3, but significantly larger than that for the polymer hollow fiber membranes. Modules made of these ceramic hollow fiber membranes could provide a higher packing density than the multi-channel ceramic elements listed in Table 3. However, the rigidity and brittleness of ceramic hollow fibers make module packing and potting very difficult, especially for high-temperature applications. Figure 4 shows a polymer membrane gas separation plant for the concentration of hydrogen from refinery H2/hydrocarbon mixture at the capacity of 15,000 M3/hr. The photo shows 8 Prism hollow fiber polymer membrane modules. Table 5 lists the estimated hollow fiber size, packing density, and hydrogen permeance and selectivity of the polymer hollow fiber membranes. Based on the hydrogen permeance, hydrogen concentration in the feed, permeate, and total feed flow rate, a membrane area of about 13,400 m2 was estimated, consistent with the number of modules assuming a packing density of 6000 m2/m3. Inorganic carbon molecular sieve (CMS) membranes on tubular alumina support produced by a membrane manufacturer 37 11 ACS Paragon Plus Environment


show much higher hydrogen permeance and comparable hydrogen perm-selectivity as compared to the polymer membranes, as listed in Table 5. The multi-tube module of alumina supported CMS membranes has a relatively high packing density for inorganic membranes (estimated at 300 m2/m3). Because of a significantly higher H2 permeance, the CMS membrane area required is much smaller than for polymer membranes. However, due to lower packing density (compared to the polymer hollow fiber membrane modules), a large number of ceramic membrane modules is needed. This means that the CMS membrane system would occupy a footprint 10-50 times larger than that of polymer membrane system for hydrogen concentration. Table 4 Membrane Module (Element) Packing Density Membrane

Module packing density (m2/m3)

Ceramic membrane tubes

10-300

Polymer membrane plate frame

300-500

Polymer membrane spiral wound

300-1200

Polymer hollow fiber membrane

6000-12,000

# data for polymer membranes from reference 38

Figure 4 A Membrane Hydrogen Separation/Concentration Plant, Sinopec (Yanshan) Refinery, Beijing, China


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Table 5 Estimated Parameters for Polymer vs. Inorganic Membrane Separation Units for Hydrogen Concentration Polymer membranes (hollow fiber module) Fiber or tube dimension

0.2 mm OD, 4000 mm L

Hydrogen permeance

100 GPU

H2/N2 selectivity

100

Module sizes and volume

30 cm (D) x 400 cm (L) 0.28 m3 (V)

Element packing density

6000 (m2/ m3)

Membrane area required

13,400 m2

Number of modules

8

Carbon molecular sieve membranes (on ceramic support)# ~ 7 mm OD, 760 mm L 600 GPU 100 7.6 cm (D) x 76 cm (L) 0.0034 m3 (V) 300 (m2/ m3) 2,200 m2 1500

# Membrane dimensions and selectivity/permeance data from 37 Polymer hollow fiber membranes can be produced in a highly cost-effective manner resulting in low membrane prices ($20-50/m2). The tubular multiple-layer ceramic supports used as the substrate for the separation layer for molecular separation are generally produced batchwise by a multiple-step production method including energy-intensive drying and hightemperature sintering steps.39 Common supports for gas separation membranes are microfiltration ceramic membranes consisting of a submicron-sized pore top-layer (on which the gas separation layer is deposited) and a micron-sized pore base support with a possible intermediate layer. The low-end price of such microfiltration ceramic membrane support would be about $200/m2. However, commercial microfiltration ceramic membranes available in the market are priced in $400-$2,000/m2. Coating a separation layer (such as CMS or zeolite) would add more costs. Such inorganic membranes with a separation layer for molecular separation are priced in $1,000-5,000/m2, which is much higher than for polymer membranes for gas separations. Table 6 shows an estimate of the costs of polymer and CMS membranes for a hydrogen concentration plant. The estimates show that the inorganic membrane unit could cost 5-10 times that of the polymer membrane unit for this application. 13 ACS Paragon Plus Environment


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Table 6 Membrane Cost Estimate for Hydrogen Separation Polymer membranes (hollow fiber module) Membrane costs

$20-50/m

Membrane area required for treating the refinery gas Costs of membranes

Carbon molecular sieve membranes (on ceramic support)

2

$1000-2000/m

2

13,400 m

2

2,200 m

$268K - $670K

$2,200 - $4,400K

2

o

Figure 5 Hydrogen permeation flux for Pd-Cu membrane at 450 C with syngas feed with pre-treatment to reduce H S content from 300 ppm to less than 1 ppm40 2

Membrane Stability Palladium alloy metal membranes are the most attractive membranes for industrial applications as they offer the highest hydrogen selectivity/permeance among all hydrogenselective membranes. Reducing the thickness of metal membranes (to below 1 m) substantially lowers amount of the noble metal needed for each membrane, and thus the membrane costs. However, the metal membranes are known to suffer from chemical and thermal stability issues41. The last decade has seen many efforts focused on improving the stability, including alloying palladium membrane with Cu or Au. Neverthless, the stability issues still have not been solved. These membranes are poisoned by exposure to a trace amount of sulfur-containing compounds 14 ACS Paragon Plus Environment

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(e.g. H2S, even down to 1 ppm), causing a reduction in hydrogen permeance, as shown in Figure 5.40, 42-43

Figure 6 Results of stability study of oxygen permeation flux and permeate oxygen purity for a mixed-conducting ceramic membrane in a Technology Demonstration Unit (TDU) (12 wafers) of Air Products and Chemicals, with air feed at about 16 atm and permeate side in a vacuum.44 The temperature and absolute flux are not disclosed but are estimated in the range of 790940oC and 5-6 ml/cm2. min, respectively Oxygen or hydrogen permeable mixed-conducting ceramic membranes are normally operated at high temperatures (>700oC). They also experience chemical stability problems due to the chemical reaction of the metal oxide with gas species such as CO2, H2S, or even water vapor.2

For

example,

the

highly

oxygen-permeable

perovskite

membrane

of

Ba0.5Sr0.5Co0.8Fe0.2O3− (BSCF) shows a reduction in oxygen permeation flux once exposure to a CO2 containing gas. 45 For oxygen permeable membranes, the stability is determined mainly by the Gibbs energy for the chemical reaction between the metal oxide of the membrane and the gas molecules. Proton-conducting perovskite-type metal oxide membranes are inherently unstable at high temperature for hydrogen permeation/separation.46

Most studies on the

synthesis and properties of mixed-conducting membranes performed by academic research laboratories did not focus on the long-term stability of these membranes. Some efforts were made by the industry to develop stable perovskite membranes for oxygen separation. Figure 6 shows the result of a 2-year stability study of a perovskite membrane (composition not disclosed) in a technology demonstration unit for separation of air to oxygen and nitrogen. The oxygen 15 ACS Paragon Plus Environment


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permeation flux of this membrane decreases by about 10% with no change in selectivity for 625 days.44 This membrane does not have the same stability in the reducing gas. Amorphous microporous silica membranes prepared by sol-gel or CVD methods contain ultra-micropores ( 50

Zeolite membranes show high selectivity (up to 1000), and moderate permeance (>1000), with high stability

H2/CO2

Glassy polymer (such as PBI), permeance 1-10 GPU, selectivity 10.

Zeolite membranes appear promising, stable, high permeance, and good stability

Oxygen from air

O2 perm-selective polymer membranes with higher permeance are required (PIM, 6003000 GPU)

Mixed-conducting ceramic membranes with high selectivity and permeance. High-temperature operation and membrane costs are an issue

H2O/ ethanol etc.

Alcohol perm-selective membranes are desired, but selectivity and stability remain an issue

Hydrophobic zeolite membranes show promises, but reproducibility and flux are low.

# Modified from 20 19 ACS Paragon Plus Environment


As mentioned above, water-selective zeolite membranes are used in commercial processes for organic solvent dehydration including ethanol/water separation. Table 7 lists other emerging applications of inorganic membranes for molecular separation in comparison with polymer membranes. The inorganic membranes listed in the table have demonstrated sufficient selectivity for the separation of these gas or liquid mixtures. At the current module packing density, the permeance of these membranes listed in Table 7 is too low. To put them in perspective, the commercialized water-selective zeolite membranes under normal conditions offer a water flux of about 5-50 kg/m2.hr (with H2O/ethanol selectivity larger than 2000). Typical inorganic membranes for hydrogen separation under normal operating conditions would have a H2 flux of about 0.1kg/m2.hr. Thus, to commercialize inorganic membranes for these separation applications, the major effort on material and membrane development should focus on improving gas permeance of these membranes, by 2 to 10 times, through improving membrane permeability and reducing membrane thickness. The packing density of a membrane module and permeance of the membrane determine the footprint of the membrane unit. For a membrane of a given price (dollar/m2), the permeance and packing density also affect the costs of the membrane unit. The traditional single-or multichannel, multi-layer support tubes are costly, difficult to grow a separation layer, and have a lower module packing density. The hollow fiber ceramic substrates would give higher module packing density, but they still have the same problems associated with the tubular substrates. Also, potting the hollow fiber ceramic membrane into modules is a big challenge, especially for high temperature and membrane reactor applications. One attractive direction is to use planner substrates, such as thin, low cost (price down to $50/m2) porous metal sheets57. Crystalline membranes of metal organic materials, which would not work at high temperatures and thus do not justify the use of ceramic substrates, should be prepared on low cost, flexible polymer substrates.58 These planner substrates allow growth of the metal-organic framework separation layer in the scalable fashion, significantly lowering the costs of final separation membranes. Furthermore, they can readily be packed into plate-stacked or spiral-wound modules with high packing density for commercial applications. Developing new, scalable, and cost-effective methods for coating the separation layer, such as vapor phase formation of ZIF membranes on 20 ACS Paragon Plus Environment

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ceramic substrates 59 or possibly 3-D printing method, 60 will be critical to the reduction of the separation membrane costs. Many academic studies have been reported on membrane reactors that couple reactant or product separation with chemical reactions.. There have been large industrial efforts on developing mixed-conducting ceramic membrane reactors for converting methane to syngas with air separation, in parallel to the development of these membranes for large-scale air separation. These efforts have resulted in the development of small-scale oxygen generators for commercial applications in health service and steel or glass industries. However, commercializing membrane reactor processes using the mixed-conducting ceramic membranes remain a big challenge, due to the problems and hurdles related to the membranes discussed above. Since water-selective zeolite membranes have been used in industrial separation processes, one area of industrial applications of inorganic membranes would be membrane reactors with in-situ water removal61, using the water-selective zeolite membranes. These applications include zeolite membrane reactors for esterification reactions with pervaporation water removal using water-selective membranes such as T, A, and ZSM-5 zeolite membranes.62 High siliceous zeolite membranes show a high permeation flux at high feed pressure and have excellent chemical and thermal stability (see Table 2 and Figure 8)53. Thus, these membranes offer potential industrial applications in a membrane reactor operated at high pressures favoring removal of product at a high flux.63 Conclusions Significant progress has been made in material development and understanding of fundamentals of inorganic membrane properties and membrane processes. In spite of the progress and success in commercial applications of inorganic membranes in filtration and hydrocarbon-water separation, the commercialization of inorganic membrane for molecular separation and reaction processes remains a difficult challenge.

The key hurdles in

commercializing inorganic membrane separators and reactors for process intensification include the high costs and large footprint of membrane units due to moderate permeance, high membrane price, and low membrane module packing density. Most inorganic membranes are 21 ACS Paragon Plus Environment


not sufficiently stable for industrial applications. Future research and development efforts should be focused on addressing these key hurdles including synthesis of membranes with higher permeance and stability, fabrication of low cost and high packing density membrane substrates, and the development of cost-effective, scalable methods for the synthesis of the separation layer. It is expected that commercial applications of highly permeable water-selective zeolite membrane separation processes will be expanded to membrane reactors coupling reaction with the removal of water from the membrane reactor.

Highly stable high siliceous zeolite

membranes may also find industrial applications in membrane reactors operated in high pressures. Acknowledgment The author acknowledges the support of the Department of Energy ( DE-FE0026435) and National Science Foundation (CBET-1511005, CBET-1604700) on inorganic membrane and membrane reactor research conducted in his laboratory. Corresponding Author *E-mail: [email protected]. ORCID Jerry Y.S. Lin: 0000-0001-5905-8336 Notes The authors declare no competing financial interest. References 1. 2. 3. 4. 5.

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Graphical Abstract


Inorganic Membranes for Process Intensification: Challenges and

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