Membrane-Based Oxygen-Enriched Combustion - Industrial

Article pubs.acs.org/IECR

Membrane-Based Oxygen-Enriched Combustion Haiqing Lin,*,† Meijuan Zhou,† Jennifer Ly,† Jimmy Vu,† Johannes G. Wijmans,† Timothy C. Merkel,† Jianyong Jin,‡,⊥ Adam Haldeman,‡ Earl H. Wagener,‡ and David Rue§ †

Membrane Technology and Research, Inc., 39630 Eureka Drive, Newark, California 94560, United States Tetramer Technologies, LLC, 657 S. Mechanic Street, Pendleton, South Carolina 29670, United States § Gas Technology Institute, 1700 S. Mount Prospect Road, Des Plaines, Illinois 60018, United States ‡

ABSTRACT: The use of oxygen-enriched air, instead of ambient air, can significantly improve the energy efficiency of combustion processes and reduce the cost of CO2 capture from flue gases throughout manufacturing industries. This study examines the overall energy savings and economic benefits that can be obtained using oxygen-enriched combustion based on novel membranes and processes to produce oxygen-enriched air. Membrane processes using low-pressure air as a countercurrent sweep in the permeate were used to minimize the energy cost of producing oxygen-enriched air. High-performance thin film composite membranes based on a series of perfluoropolymers and bench-scale spiral-wound modules were prepared, and showed oxygen permeance as high as 1200 gpu (1 gpu = 10−6 cm3(STP)/cm2·s·cmHg) combined with O2/N2 selectivity of 3.0. The membrane-based oxygen-enriched combustion processes show good energy savings (defined as the fuel savings less the energy consumption of producing oxygen-enriched air) and economic benefits (defined as the value of fuel saved less the operating cost of producing oxygen-enriched air), especially at flue gas temperatures higher than 1090 °C (or 2000 °F). For example, at a flue gas temperature of 1649 °C (or 3000 °F), membrane-based oxygen-enriched combustion shows a net energy savings of 35% and a net economic benefit of 29%, compared to the combustion process with air. The effect of oxygen-enriched air on NOx emissions in a natural gas furnace was also experimentally investigated.

1. INTRODUCTION The benefits of using oxygen-enriched air (air containing more than 21% oxygen), instead of ambient air, have long been recognized in many industrial processes such as catalyst regeneration in refinery fluid catalytic cracking (FCC), partial oxidation of sulfur in Claus plants, wastewater treatment, and combustion applications for glass and foundry operations.1−5 Oxygen-enriched air can lower the capital and operating costs, reduce the CO2 emissions, and increase the process flexibility and reliability.3 The use of oxygen-enriched air would become more attractive for a broader variety and scale of industrial combustion processes, if oxygen-enriched air can be produced in a lower cost and more energy efficient manner.2 Figure 1 illustrates the significant natural gas savings that can be obtained when oxygen-enriched air is used instead of ambient air in natural gas-fired furnaces.1 The use of oxygenenriched air reduces the volume of inert gas (N2); therefore, reducing the heat loss through the furnace exhaust and increasing the energy efficiency.3,6 As shown in Figure 1, the use of oxygen-enriched air can lower natural gas fuel requirements by as much as 40% at a flue gas temperature of 2500 °F (or 1371 °C), which is a typical operating temperature for the production of glass, cement, and steel.2 The actual energy savings may be less than 40% due to waste heat recovery in the furnaces. However, the use of waste heat recovery varies widely, and it is often not used by small producers to reduced capital cost. Therefore, to simplify the discussion, the technoeconomic analysis in the paper assumes that there is no waste heat recovery in the furnace. Figure 1 also shows that the increase in natural gas savings becomes less pronounced at higher oxygen concentrations; © XXXX American Chemical Society

Figure 1. Natural gas savings achieved in furnace operations as a function of the oxygen concentration in combustion air.1 The increase in savings becomes less pronounced at higher oxygen concentrations; consequently, the principal range of interest is 25−35% oxygen. Reprinted from ref 1 with permission. Copyright 1986 Elsevier.

Received: May 8, 2013 Revised: July 2, 2013 Accepted: July 3, 2013

A

dx.doi.org/10.1021/ie401464z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

gas permeation, and p2 and p1 are upstream (i.e., high) and downstream (i.e., low) pressures (cmHg), respectively. Industrial membranes are often comprised of a thin selective layer governing gas molecular separation. Because the thickness of the selective layer may not be well-characterized, industrial membranes are usually characterized by gas permeance, JA (cm3(STP)/cm2·s·cmHg), which is given as follows:12

consequently, the principal range of interest is 25−35% oxygen. Moreover, if oxygen content is less than 27%, the flame temperature is compatible with most furnace materials. Therefore, existing furnaces can be easily retrofitted.2 Concerns about global climate change provide an additional important incentive for using oxygen-enriched air in combustion.7 Combustion of hydrocarbon fuels generates carbon dioxide, the leading contributor to climate change. As shown in Figure 1, the use of oxygen-enriched combustion saves energy and, therefore, reduces CO2 emissions. The percentage of reduction in CO2 emissions is about the same as the percentage of energy savings. Substantial efforts are underway to develop technologies for capture, utilization, and sequestration of the CO2 contained in combustion flue gases.7 The cost of capturing CO2 from flue gas decreases significantly if the volume of flue gas to be treated decreases and the CO2 concentration in the flue gas increases.8−10 As shown in Figure 2, this is exactly what is

JA =

PA l

(2)

Permeance can be expressed in gpu units, where 1 gpu = 10−6 cm3(STP)/cm2·s·cmHg. Industrial membranes with high permeances are preferred, leading to less membrane area and lower capital cost for the membrane system to treat a given feed stream.13 On the basis of eq 2, membrane permeance can be increased either by selecting a more permeable selective layer material (increasing PA) or by making the selective layer thinner (decreasing l).12 On the basis of the solution-diffusion mechanism governing gas transport in nonporous polymers, permeability of gas A can also be written as follows:11 PA = DA SA

(3)

where DA (cm2/s) is gas diffusivity and SA (cm3(STP)/ cm3·cmHg) is gas solubility in the polymer. A measure of a membrane’s ability to separate two components, A and B, is the membrane selectivity of gas A over gas B, defined as the ratio of gas permeabilities or permeances:11

αA/B =

PA D S = A A PB DB SB

(4)

Diffusivity selectivity, DA/DB, depends on the relative molecular sizes. Oxygen with a kinetic diameter of 3.46 Å is smaller than nitrogen with a kinetic diameter of 3.64 Å, and therefore, diffusivity selectivity always favors oxygen over nitrogen.14,15 Solubility selectivity, SA/SB, is determined by the relative condensabilities of the permeants. Oxygen with a critical temperature of 154.6 K is more condensable than nitrogen with a critical temperature of 126.2 K,16 and therefore, solubility selectivity always favors oxygen over nitrogen. Consequently, polymeric membranes are always selective for oxygen over nitrogen to varying degrees. 2.2. Equivalent Pure Oxygen (EPO2). Equivalent pure oxygen (EPO2) is defined as the amount of pure oxygen that needs to be mixed with atmospheric air to obtain a specified volume, V, of oxygen-enriched air with a specified oxygen concentration, [O2] (in vol %):13,17

Figure 2. Effect of oxygen levels in combustion air on flue gas volume and CO2 concentration of the flue gas from natural gas-based furnaces, without using excess air. The effect is significant even at modest oxygen concentrations.

accomplished by using oxygen-enriched air in combustion processes. As the oxygen concentration in the combustion air increases, the amount of nitrogen in the flue gas decreases, resulting in a reduction in flue gas volume and a corresponding increase in the CO2 concentration. The effect is significant even at modest oxygen concentrations. For example, when the combustion air contains 30% oxygen, the flue gas volume is reduced by 57% and the CO2 concentration in the flue gas is increased by 47%.

EPO2 = V

2. BACKGROUND 2.1. Theory. Permeation of gas A in nonporous polymers is often characterized by permeability, PA (cm3(STP)·cm/ cm2·s·cmHg), which represents the steady state pressure- and thickness-normalized gas flux through a polymer film and is defined as follows:11 NAl PA = A m (p2 − p1 ) (1)

[O2 ] − 20.9 79.1

(5)

This metric provides a rational basis for comparing technologies that produce oxygen at different levels of purity. Later, we will explain production costs in terms of EPO2. 2.3. Technologies for Oxygen Enrichment from Air. At present, most oxygen is produced by cryogenic separation of air and vacuum swing adsorption.4 However, the high cost of oxygen from either of the established processes simply prohibits the use of oxygen-enriched air for many combustion applications.2 Membrane processes for oxygen enrichment were developed to the early commercial stage in the 1980s using silicone rubber

where NA is the gas flux through the polymer (cm3(STP)/s), l is the film thickness (cm), Am is the active film area (cm2) for B



Article

Figure 3. Membrane processes to produce 30% oxygen-enriched air with (a) feed compression and (b) permeate vacuum. Both designs operate with the membrane modules in the cross-flow mode and cannot produce oxygen-enriched air at a cost competitive with conventional technologies.

and poly(phenylene oxide) membranes.13,17 Other types of membranes including polymeric materials,18,19 ceramic membranes,20 and facilitated transport membranes21,22 have also been examined in the past two decades. Various process designs are possible; two typical process designs are shown in Figure 3a and b.1,13,17,23 Feed air containing 21% oxygen is passed across the surface of a membrane that preferentially permeates oxygen. In Figure 3a, the pressure differential across the membrane required to drive the process is provided by compressing the feed gas. An alternative approach is to draw a vacuum on the permeate, as shown in Figure 3b. In both cases, the oxygen concentration of the permeate gas is higher than the desired oxygen concentration (which is 30% in this example), so atmospheric air is mixed with the permeate to achieve the target oxygen concentration. Vacuum operation is shown to be a better choice since it requires much less energy.13,17,23 However, to date, these types of membrane processes are not economically competitive with cryogenic distillation and pressure swing adsorption at production rates of industrial interest, either because the power needed for the feed compression was too high (high operating cost) or the membrane area required was too high due to the lack of high flux membranes and the low driving force for oxygen permeation.12 This study evaluates the overall energy efficiency and economic benefits of oxygen-enriched combustion, where oxygen-enriched air is produced using a new membrane process with high energy efficiency and new high-performance membranes for O2/N2 separation. The thin film composite membranes were fabricated into spiral-wound modules, which were tested at various operation conditions simulating industrial conditions. The effect of oxygen-enriched air on NOx emissions was experimentally investigated, and a technoeconomic analysis of oxygen-enriched combustion based on the newly developed membranes and processes is discussed.

Figure 4. Energy-efficient membrane process to produce oxygenenriched air using countercurrent/sweep membrane operation. The process produces 7.1 m3(STP)/s of 30% oxygen-enriched air, which equals 262 ton O2/day or 100 tons/day EPO2 (defined in eq 5). The membrane has an oxygen permeance of 1200 gpu and an O2/N2 selectivity of 3.0.

atmospheric air is used as a sweep stream and mixes with the permeated gas at the membrane interface, not outside the membrane modules as shown in Figure 3. The sweep design inherently improves energy efficiency because the atmospheric air dilutes the oxygen-enriched permeate flow and increases the driving force for oxygen permeation without requiring more compression energy.8,24−26 A simulation of this design was performed using a commercial process simulation package, ChemCAD 6.3 (ChemStations, Inc., Houston, TX), enhanced with proprietary MTR code for membrane unit operations. The membrane process shown in Figure 4 produces 7.1 m3(STP)/s oxygen-enriched air (containing 30% O2) or 100 tons of EPO2 per day (see eq 5). Atmospheric air is compressed to 60 psia (4.1 bar), using a compressor with an efficiency of 0.85, and passes through a series of countercurrent/sweep modules containing 4240 m2 of membrane area. About 34% of the compressed air permeates the membrane and combines with the sweep air to produce oxygen-enriched air. The membrane residue stream passes through a turboexpander where some of the compression energy is recovered. Assuming an 85% conversion efficiency, the turboexpander will generate 33% of the power required to compress the feed gas. The operating conditions, such as feed pressure and ratio of sweep air to permeate flow rate, are optimized for a membrane with an O2/N2 selectivity of 3.0 and an O2 permeance of 1200 gpu. As shown later, these membrane properties were achieved in the membrane development activities. Table 1 compares the performance of the countercurrent/ sweep design shown in Figure 4 with the conventional feed compression of Figure 3a or permeate vacuum design of Figure 3b. The process simulation was performed using ChemCAD

3. NOVEL MEMBRANE PROCESS WITH COUNTERCURRENT/SWEEP OPERATION Figure 4 shows the new membrane process for oxygen enrichment from air, which has two design features different from the processes shown in Figure 3: 1. a turboexpander that improves energy efficiency and 2. a countercurrent/sweep membrane design that improves separation efficiency. The feed air to the membrane unit is compressed to increase the driving force for permeation. A significant amount of this compression energy can be recovered by sending the pressurized residue stream to a turboexpander as it leaves the membrane unit. On the permeate side of the membrane, C



Article

Table 1. Results of Simulation Comparing Various Membrane System Designs to Produce Oxygen-Enriched Air Containing 30% Oxygena

a The membranes have oxygen permeance of 1200 gpu and O2/N2 selectivity of 3.0. bF = feed; R = residue; S = sweep; S + P = sweep + permeate. cA compressor and turboexpander efficiency of 85% and a vacuum pump efficiency of 75% were assumed in the calculations.

6.3, and the flow rates were varied in the three processes to meet the requirement in oxygen concentration and overall flow rate in the oxygen-enriched air. Countercurrent/sweep operation provides the best combination of power consumption and membrane area. Compared to the feed compression process, the countercurrent/sweep process consumes 29% less energy and uses 33% less membrane area. Compared to the permeate vacuum operation, countercurrent/sweep operation consumes 18% more energy but uses 78% less membrane area. The permeate vacuum process uses 44% less energy and requires about 220% more membrane area than the feed compression process, which is consistent with results from earlier studies.13,17

smooth surface of the gutter layer allows a very thin dense selective layer of perfluoropolymers to be deposited. In general, the microporous support and gutter layer contribute essentially no resistance to gas flow, and therefore, the resistance to gas transport in the thin film composite membranes lies mainly in the selective layer. In this manner, the gas selectivity is determined primarily by the selective layer. However, for high flux membranes with ultrathin perfluoropolymer selective layers, the transport resistance in the gutter layer may not be negligible compared to that in the selective layer, and therefore, the composite membranes could show selectivity slightly lower than that of the perfluoropolymer selective layer alone. 4.2. Module Preparation. The flat sheet thin film composite membranes were fabricated into spiral-wound modules that can operate in countercurrent/sweep mode. As shown in Figure 6, these modules have four ports: feed, residue,

4. EXPERIMENTAL SECTION 4.1. Membrane Preparation. Figure 5 shows a schematic drawing of the flat sheet thin film composite membranes

Figure 6. Exploded view of the membrane envelope for a countercurrent/sweep spiral-wound module, which uses a sweep gas on the permeate side and operates in a partial countercurrent pattern.26,28−31 The membrane envelope has a length of 90−100 cm (in the direction parallel to the product pipe) and a width of 30−60 cm (in the direction perpendicular to the product pipe).

Figure 5. Schematic illustration of the industrial thin film composite membranes prepared in this work.12 The selective layer is a perfluoropolymer with high O2/N2 separation performance.

sweep, and the combined sweep and permeate (sweep + permeate).26,28−31 Feed gas passes down the module parallel to the permeate pipe in the channel created by the feed spacer. A portion of the feed gas permeates the membrane and then enters the permeate channel. The permeate channel can be swept with a sweep gas, allowing the module to operate in a partial countercurrent mode.26,28 Although the module does not have a perfect countercurrent flow pattern between the feed and permeate flow, it has been shown that the module can still have very high sweep efficiency, despite the presence of a nonideal countercurrent flow pattern.26,31,32

prepared in this work.12 The nonwoven polyester paper layer provides mechanical strength to the membrane composite structure. On top of the nonwoven paper, which has a relatively rough surface, a microporous ultrafiltration support layer is formed by wet-phase separation casting of a poly(etherimide) (Ultem 1000, General Electric, Mount Vernon, IN).27 The microporous support has surface porosity of less than 5%, and fine pores with an average pore diameter of less than 100 nm. The proprietary gutter layer and selective layer were formed by a dip-coating process using an industrial-scale coater.12 The D



Article

4.3. Determination of Pure-Gas Permeances. Pure-gas permeances in thin film composite membranes were determined using a constant-pressure/variable-volume apparatus.33 Before each measurement, the entire apparatus is purged using the feed gas to remove any impurities from the system. The permeate gas flow rate is measured using bubble flow meters (Alltech Associates, Inc. Deerfield, IL, USA). The steady state permeance for gas A, can be calculated by33 PA 1 273 ⎛⎜ dV ⎞⎟ = l A m (p2 − p1 ) T ⎝ dT ⎠

difference in eq 7 is analogous to that derived for a countercurrent heat exchanger, in which the logarithmic mean temperature difference is used.26,35 The composite membrane sheets were mounted in a modified Osmonics SEPA CF II Med/High Foulant System cell (GE Osmonics Labstore, Minnetonka, MN). The test cell was modified to allow a near perfect countercurrent flow test.36 The countercurrent/sweep spiral-wound modules were installed in a custom-built module housing, with separate ports to accommodate the four gas streams. Chemical-grade nitrogen, oxygen, and air, each with a purity of 99%, were used as received from Praxair Inc. (Hayward, CA). An air compressor was also used to provide high-pressure air, the composition of which was determined using the GC. 4.5. Evaluation of Oxygen-Enriched Combustion. The effect of oxygen-enriched combustion on NOx emissions was evaluated experimentally at Gas Technology Institute (GTI, Des Plaines, IL). A Bloom Engineering model 1476 hot air baffle burner was selected for testing, because it is a common industrial burner and shows strong dependence of NOx emissions on oxygen levels in the combustion air. Figure 8

(6)

where the downstream pressure, p1, is atmospheric pressure in this case, T is the absolute temperature of the gas (K), and dV/ dt is the steady state volumetric displacement rate of the soap film (cm3/s). Permeance is expressed in gpu units, where 1 gpu = 10−6 cm3(STP)/cm2·s·cmHg. 4.4. Determination of Mixed-Gas Permeances. The composite membranes and membrane modules were tested under countercurrent/sweep conditions using a system as shown in Figure 7. Compressed air flows on the feed side of the

Figure 7. Schematic of countercurrent/sweep test system for flat sheet composite membranes and spiral-wound modules. A rectangular countercurrent test cell (with a membrane area of 150 cm2) was used for membrane testing, and one 4-in.-diameter module housing was used to test countercurrent/sweep spiral-wound modules (with a membrane area of 0.5−3 m2): (MFM) mass flow meter; (GC) gas chromatography.

Figure 8. Photos of a (a) baffle burner and (b) modular furnace at Gas Technology Institute (GTI) that were used for oxygen-enriched air combustion tests.

presents the test setup, with the baffle burner mounted on the research modular furnace. The modular furnace is equipped with thermocouples, side view ports, and sampling ports in different locations. Exhaust gas is sampled in the test to monitor the levels of NOx, CO, CO2, and O2 at different test conditions using an HORIBA analyzer (HORIBA Instrument Inc., Illinois, US).

membrane, and low pressure air flows on the permeate side. The test system is equipped with an Agilent MicroGC 3000 portable gas chromatography (GC), to monitor the gas composition of all streams (including the feed, residue, sweep, and “sweep + permeate” stream) as oxygen and nitrogen permeate across the membrane. The flow rates were measured using mass flow meters (Sierra Instruments, Monterey, CA). The experimental data were only considered valid when the mass of each component entering and leaving the permeation cell or module housing was within ±10%. The mixed-gas permeance in countercurrent/sweep operation can be estimated using the following equation:26,34 NA ln(ΔpA,L /ΔpA,0 ) PA = l A m (ΔpA,L /ΔpA,0 )

(7)

ΔpA,0 = pA,F − pA,S + P ,

(8)

and ΔpA,L = pA,R − pA,S

5. RESULTS AND DISCUSSIONS 5.1. Pure-Gas Permeance and Stability in Membranes. We have identified a series of perfluoropolymers (PFPs) with high oxygen permeability and good oxygen/nitrogen selectivity for oxygen enrichment. Thin film composite membranes of the type shown in Figure 5 were fabricated using these perfluoropolymers as the selective layer. Figure 9 shows the effect of perfluoropolymer concentration in the coating solution on pure-gas oxygen permeance and O2/ N2 selectivity. As expected, decreasing perfluoropolymer concentration in the coating solution reduces the selective layer thickness, increasing oxygen permeance.37 The oxygen permeance is inversely proportional to the polymer concentration of the coating solutions. On the basis of the perfluoropolymer O2 permeability of 50 Barrers, the selective layer prepared from the solution with 0.15 wt % polymer has an stimated thickness of 50 nm, using eq 2.

where pA,F, pA,R, pA,S, and pA,S+P are the partial pressures of component A in the feed, residue, sweep and sweep + permeate stream, respectively. NA is the flow rate difference of component A between the sweep and sweep + permeate stream. Equation 7 is similar to eq 1, except that the partial pressure difference is expressed as the logarithmic mean of ΔpA,L and ΔpA,0. The use of logarithmic mean pressure E



Article

provided below. The membrane with highest O2 permeance (980 gpu) exhibits O2/N2 selectivity of 2.7, as shown in Figure 9. On the basis of the resistance layer model and O2 permeance of the gutter layer of 11 000 gpu, the selective layer is estimated to have an O2 permeance of 1080 gpu, which gives an O2/N2 selectivity of 3.0 for the selective layer, consistent with the intrinsic property of the perfluoropolymer. This calculation assumes a minimal effect of the gutter layer on the permeance of the less permeable gas (N2), which is valid here. Based on this understanding, slight adjustments were made to the membrane configuration to allow better performance, as shown later. Thin films of glassy polymers often show aging behavior; i.e., gas permeability declines significantly with time, and the decrease becomes more substantial as the polymer film thickness decreases.39−42 For example, oxygen permeability in poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with a thickness of 400 nm decreased from 20 to 10 Barrers in 40 days.43 The aging behavior in these polymer thin films has been rationalized by the collapse of free volume with time, leading to a decrease in polymer free volume.39 Higher flux polymers with higher free volume may show more dramatic aging behavior, as exemplified in poly[1-(trimethylsilyl)-1-propyne] (PTMSP)44 and polymers of intrinsic microporosity (PIM-1). 45−47 Significant aging behavior creates a great challenge in system design and increases the capital cost of the membrane system, limiting the commercial usefulness of many polymers that demonstrate initially promising (or even spectacular) laboratory performance. The perfluoropolymers in this work have high free volume, and composite membranes with a perfluoropolymer selective layer as thin as 50 nm have been prepared. Investigating the aging behavior in these composite membranes was a necessary next step. Figure 10 shows the stability of various composite membranes based on perfluoropolymers. In these tests, the membrane sheets were stored in air at 20 °C, and different membrane stamps were taken and tested at various times, at a feed pressure of 50 psig. Unlike other high flux glassy polymers (such as a PPO film with a thickness of 400 nm for which oxygen permeability decreased by 50% in 40 days43), perfluoropolymer-based composite membranes showed stable O2 permeance and O2/

Figure 9. Effect of polymer content in the coating solutions on puregas O2 permeance and O2/N2 selectivity in perfluoropolymer-based composite membranes at 4.4 bar and 20 °C. One gpu = 10−6 cm3(STP)/cm2·s·cmHg. The lines are to guide the eye.

Figure 9 also shows that the pure-gas O2/N2 selectivity decreases slightly at very high oxygen permeances, presumably caused by the gas transport resistance of the gutter layer. For low flux membranes with O2 permeance less than 600 gpu, the transport resistance in the selective layer is much higher than that in the gutter layer (which has a measured O2 permeance of 11 000 gpu). Therefore, resistance to gas transport through the membranes is mainly in the selective layer, and O2/N2 selectivity of the composite membrane is similar to that of the selective layer.38 However, as the selective layer becomes thinner, the transport resistance of the selective layer decreases and becomes more comparable with the resistance of the gutter layer, particularly for the more permeable gas, O2. The resulting selectivity of the composite membranes thus demonstrates a value between that in the selective layer and gutter layer. Because the O2/N2 selectivity in the gutter layer is lower than that in the selective layer (about 3.0), increasing oxygen permeance in the composite membranes tends to slightly lower oxygen/nitrogen selectivity. A more quantitative analysis is

Figure 10. Time dependence of (a) pure-gas O2 permeance and (b) pure-gas O2/N2 selectivity in perfluoropolymer-based composite membranes at 4.4 bar and 20 °C. The thickness of the selective layer in these composite membranes was estimated using eq 2 with a measured oxygen permeability value of 50 Barrers. The lines are to guide the eye. F



Article

N2 selectivity for 60 days, even for the membranes with selective layers thinner than 100 nm. Optimal membranes for O2/N2 separation should have high O2 permeance to reduce the required membrane area and capital cost, and high O2/N2 selectivity to increase the product O2 purity. Figure 11 presents a permeance/selectivity map for

Figure 12. Oxygen flux as a function of sweep/feed flow rate for a perfluoropolymer-based membrane stamp (150 cm2) in the countercurrent/sweep mode at 20 °C. Air was used as the feed gas and sweep gas. The feed gas pressure was 2.0 bar and flow rate was 50 cm3(STP)/ s. The sweep gas was at atmospheric pressure.

pressure. The stage-cuts (defined as the percentage of the feed gas entering the permeate) for all the tests were below 10%. As shown in Figure 12, the use of a sweep gas significantly increased oxygen flux, especially at low values of sweep/feed flow rate. For example, as sweep/feed flow rate increased from 0% to 26%, oxygen flux increased by 53%, from 1.7 to 2.6 cm3(STP)/s. The incremental increase in oxygen flux with increasing sweep flow rate leveled off at higher sweep flow rates. For instance, as the sweep/feed flow rate increased from 26% to 54%, the O2 flux increased by only 7.8%, to 2.8 cm3(STP)/s. Figure 13a and b compares mixed-gas oxygen permeance and oxygen/nitrogen selectivity at different feed pressures and sweep flow rates. The feed flow rate was 8.7 and 26 cm3(STP)/ s for the tests with feed pressure of 2.0 and 4.4 bar, respectively. The sweep gas was at ambient pressure for all the tests. The experimental uncertainty is often within 10% in these measurements. As shown in Figure 13, the mixed-gas O2 permeance and O2/N2 selectivity were independent of feed pressures and sweep/feed flow ratio. 5.3. Module Fabrication and Characterization. The next step in developing a commercially practical membrane technology is to confirm that a larger-scale module package can be prepared and that the separation performance of this module is nearly comparable to that obtained for the membrane in laboratory testing. As a preliminary step in this direction, two bench-scale spiral-wound modules with countercurrent/sweep design (S and L) were fabricated using membranes described previously, produced on a commercial coater. The smaller module (module S) has a membrane area of 0.50 m2, and the larger module (module L) is a full-size semicommercial 4-in.diameter module containing a membrane area of 1.8 m2. Puregas separation properties of these two modules are shown in Table 2. The pure-gas permeances and selectivities of these modules were similar to the properties of the membrane stamps, indicating that the modules were defect-free. Figure 14 shows the effect of sweep flow rate and permeate pressure on the module separation performance. All tests were performed with air as both feed and sweep at 20 °C. For the tests of module S, the feed flow rate was 2000 cm3(STP)/s at sweep pressure of 1.0 bar and 1000 cm3(STP)/s at sweep pressure of 1.7 bar. For the tests of module L, the feed flow rate was 1700 cm3(STP)/s at permeate pressure of 1.0 bar, and 1400 cm3(STP)/s at permeate pressure of 1.3 bar. The stage-

Figure 11. Comparison of a perfluoropolymer (PFP) based composite membrane to other membranes in the literature (with assumed 1 μmthick selective layer) for O2/N2 separation. Current commercial membranes for O2/N2 separation have oxygen permeance of 10−100 gpu and oxygen/nitrogen selectivity of 4−6. PPO: poly(phenylene oxide); SR: silicone rubber.

O2/N2 separation in polymeric membranes at 25 °C, where gas transport follows the solution-diffusion mechanism.14,48,49 This type of plot was popularized by Robeson.48,49 Each point represents the selected separation properties for one particular polymer. The upper bound line in the figure gives a rough estimate of the highest selectivity possible for a given permeability in polymer-based materials.14 There is a trade-off between gas selectivity and permeability; that is, materials with higher oxygen permeance have lower oxygen/nitrogen selectivity. Figure 11 assumes that all polymers can be made into stable composite membranes with a selective layer thickness of 1 μm, without further discussions on the feasibility of membrane fabrication and physical aging behavior. The perfluoropolymerbased composite membranes prepared in this study show O2/ N2 separation performance (high and stable oxygen permeance of 1200 gpu and O2/N2 selectivity of 3.0) close to the upper bound. Compared with current commercial membranes (made from polysulfone and polyimides)12 for nitrogen generation from air, these perfluoropolymer-based membranes show orders of magnitude higher oxygen permeance and lower O2/ N2 selectivity. Figure 11 also compares the perfluoropolymer-based membranes with prior membranes developed for oxygen enrichment from air, such as silicone rubber (SR) and poly(phenylene oxide) (PPO) membranes. The perfluoropolymer-based membranes show a good combination of oxygen permeance and O2/N2 selectivity, compared to these polymers. 5.2. Mixed-Gas Permeances in Membranes. Figure 12 shows results for a perfluoropolymer-based membrane stamp tested in the countercurrent/sweep mode with air as feed and sweep gas at 20 °C. The feed gas had a pressure of 2.0 bar and flow rate of 50 cm3(STP)/s. The sweep gas was at ambient G



Article

Figure 13. (a) Mixed-gas O2 permeance and (b) mixed-gas O2/N2 selectivity at 20 °C and different feed pressures and sweep flow rates in a membrane sheet. The feed flow rate was 8.7 and 26 cm3(STP)/s for the tests with feed pressure of 2.0 and 4.4 bar, respectively. The sweep gas was at atmospheric pressure. The lines are to guide the eye.

significant as that in module S. For example, as sweep/feed flow rate increased from 0% to 50% at a permeate pressure of 1.3 bar, oxygen flux increased by only 18%, from 62 to 78 cm3(STP)/s. More work on module scale-up will be needed, to achieve better and more consistent sweep efficiency in future modules. Figure 15a and b compares mixed-gas oxygen permeance and O2/N2 selectivity at different permeate pressures and sweep flow rates for module L. The oxygen permeance and O2/N2 selectivity were independent of permeate pressures and were similar to those of the membrane module (1300 gpu for oxygen permeance and 3.0 for oxygen/nitrogen selectivity, as shown in Table 2). Figure 16 compares the countercurrent/sweep efficiency (defined as the increase in oxygen permeate flow rate caused by the sweep) obtained with a membrane sheet and module S. The test results for the membrane sheet as provided in Figure 12 were obtained at near “perfect” countercurrent conditions. In contrast, the module operates in only a partial countercurrent mode, as shown in Figure 6. Nevertheless, Figure 16

Table 2. Pure-Gas Separation Performance in Two BenchScale Countercurrent/Sweep Perfluoropolymer-Based Membrane Modules (S and L) at 1.7 bar and 20 °C permeance (gpu) bench-scale module

membrane area (m2)

N2

O2

O2/N2 selectivity

S L

0.50 3.0

480 430

1200 1300

2.5 3.0

cuts for all the tests were below 25%. Since the membrane performance is independent of feed pressure (as shown in Figure 13), low feed pressures (1.7 and 2.0 bar) were used in the module testing. As shown in Figure 14, the use of a sweep gas increased oxygen flux in both modules, especially at high permeate pressure. For example, as sweep/feed flow rate increased from 0% to 46% at a sweep pressure of 1.7 bar, oxygen flux in module S increased by 54%, from 14 cm3(STP)/s to 22 cm3(STP)/s. However, the increase in module L was not as

Figure 14. Relative oxygen flux for (a) module S and (b) module L as a function of sweep/feed flow rate in the countercurrent/sweep mode at 20 °C. Air was used as the feed and sweep gas. The feed gas pressure was 2.0 bar for module S and 1.7 bar for module L. H



Article

Figure 15. (a) Mixed-gas oxygen permeance and (b) mixed-gas O2/N2 selectivity at different sweep pressures and flow rates in countercurrent/ sweep module L at 20 °C. Air was used as the feed gas and sweep gas. The feed gas pressure was 1.7 bar. The lines are to guide the eye.

Table 3. Effect of Oxygen Levels in the Combustion Air on NOx Emissions for an Unmodified Bloom Baffle Burner with a Natural Gas Firing Rate of 500 scf/h NOx emissions (ppm) at different O2 levels in the combustion air excess air (%)

21%

25%

29%

0 5 10

64 79 80

94 142 156

207 274 304

The NOx level increased with increasing excess air and oxygen enrichment level in the air, as expected.2 NOx levels can be effectively reduced by oxygen lancing, where a portion of the oxidant (air or enriched air) is injected through a lance above the main flame during combustion.50 Figure 17 compares the effect of lance percentage on NOx concentration in the flue gas for combustion air containing 21% O2, 25% O2, and 29% O2. When firing with 25% enriched air, the NOx decreased from 115 ppm with no lancing to 55 ppm

Figure 16. Comparison of countercurrent/sweep efficiency (indicated by the relative oxygen permeate flux) in a membrane sheet and in module S at 20 °C. Test condition details are provided in the captions for Figures 12 and 14. The lines are to guide the eye.

shows that the efficiency of countercurrent/sweep in the module is very similar to that in the membrane sheet. However, as shown in Figure 14, the larger module (module L) does not show sweep efficiency as good as that of the smaller module (module S), indicating that further optimization of the module configuration to improve sweep efficiency will be needed. 5.4. Effect of Oxygen-Enriched Air on NOx Emissions. Combustion with oxygen-enriched air often leads to higher flame temperature, resulting in higher NOx content in the flue gas.2 Higher NOx content in flue gases is undesirable because emissions of this pollutant are subject to various environmental regulations. Therefore, the benefits of oxygen-enriched air combustion in terms of process efficiency must be weighed against potential negative changes to the flue gas emissions profile. In this study, the effect of oxygen-enriched air combustion on NOx emissions was evaluated experimentally using a natural gas-fired burner at Gas Technology Institute. Firing rate was found to affect NOx levels, so all comparative tests were conducted at a constant natural gas firing rate of 500 scf/h (standard cubic foot per hour). Table 3 shows the effect of excess air on NOx level in the burner without modifications.

Figure 17. Effect of lance percentage on NOx concentration in the flue gas using combustion air containing 21% O2, 25% O2, and 29% O2. The furnace was operated at a firing rate of 500 scf/h natural gas with 5% excess combustion air. I



Article

with 10% lancing of the enriched air above the flame. At roughly 6% of the enriched air sent to the lance, the burner operating with 25% oxygen-enriched air generates the same NOx as the burner operating with normal air and no lancing. These results confirm that lancing is an effective method to reduce NOx emissions and to avoid excess NOx production when oxygen-enriched air is used for combustion. The amount of lancing needed to meet this goal increases as the oxygen enrichment level is increased. It should be noted that the effect of excess air level and lancing on the NOx level also depends sensitively on the type of industrial burners.

Table 5. Calculation of Energy Savings for Membrane-Based Oxygen-Enriched Combustion. With 30% Oxygen in the Combustion Air, the Burner Can Save 35% Energy at a Flue Gas Temperature of 1649 °C two cases with a firing rate of 1 MMBtua

6. ENERGY ANALYSIS OF MEMBRANE-BASED OXYGEN-ENRICHED COMBUSTION 6.1. Energy Consumption of Membrane-Based Oxygen Enrichment. The membrane process shown in Figure 4 produces 30% oxygen-enriched air at 100 tons of EPO2 per day using perfluoropolymer-based composite membranes with oxygen permeance of 1200 gpu and oxygen/nitrogen selectivity of 3.0. One performance metric often used to compare oxygen production technologies is the energy required to generate a given volume of oxygen. Table 4 compares this energy

production energy (MMBtu/ton EPO2)

production volume (ton EPO2/day)

cryogenic51

0.84−1.36

>50

VSA51

2.08

20−90

PSA51

2.60

Membrane-Based Oxygen-Enriched Combustion - Industrial

Recommend Documents