Biogas Purification Using Cryogenic Packed-Bed Technology

Mar 15, 2012 - The recovery is carried out with air and when operated in reversed flow mode, the novel CPB technology requires a 22% lower energy duty...
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Biogas Purification Using Cryogenic Packed-Bed Technology Martin J. Tuinier and Martin van Sint Annaland* Multiphase Reactor Group, Chemical Process Intensification, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands ABSTRACT: A novel process concept for biogas treatment based on dynamically operated cryogenic packed beds (CPB) has been proposed and studied with numerical simulations. This work describes the CPB concept and investigates the different process steps involved with simulation results. To demonstrate the potential to treat biogas using the proposed process, the performance is compared to vacuum pressure swing adsorption (VPSA) on the basis of several criteria: purity and recovery of the obtained product, bed dimensions, and energy requirements. Simulation results reveal that the purity and recovery of CH4 are higher for the CPB concept, while also the bed capacity is much higher: the productivity (defined as kgCH4 h−1 mpacking−3) is a factor of 8 higher. The recovery is carried out with air and when operated in reversed flow mode, the novel CPB technology requires a 22% lower energy duty (2.9 MJ/kgCH4 vs 3.7 MJ/kgCH4 for the VPSA process). Furthermore, simultaneous deep H2S removal is possible using the proposed concept, although initial bed temperatures as low as −150 °C are required.

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INTRODUCTION Biogas is formed at, for example, landfill sites and wastewater treatment facilities or is produced by anaerobic fermentation of manure. Biogas mainly consists of CH4 (50−70 vol %) and CO2 (25−45 vol %) and furthermore may contain contaminants such as H2O, H2S, and siloxanes.1 CO2 is a well-known greenhouse gas; however, CH4 has a relative global warming potential 25 times higher than CO2.2 It is reported that greenhouse gas emissions from the agricultural sector account for about 25.5% of the total global anthropogenic emissions.2 It is therefore critical to bring down CH4 emissions. The cheapest option to avoid emissions is to collect biogas and send it to a flare. However, biogas based methane has the potential to serve as a renewable energy resource, to generate power or to be used as a transportation fuel. To convert biogas to commercial grade CH4, several separation and purification steps are required. H2S is formed by the anaerobic fermentation of sulfur containing proteins. H2S removal is necessary, as combustion will lead to the environmentally hazardous SO2 and could form H2SO4, causing corrosion of process equipment. H2S can be removed by oxidation to elementary sulfur or by scrubbing the biogas with an aqueous alkaline solution. The disadvantage of the latter is that CO2 has a higher reactivity with the solvent, causing low selectivities for H2S removal.3 Siloxanes are a group of molecules containing Si, which are found in landfill biogases. The combustion of siloxanes leads to silicates and microcrystalline quartz. These solids will damage engines and turbines and are therefore highly undesirable. Several possible technologies to remove siloxanes are available: (reactive) absorption with liquids, adsorption, and cryogenics.3 CO2 is present in high concentrations in biogas. Removal of CO2 is necessary to increase the energy content of the biogas. The available technologies are scrubbing (using water, a physical or chemical solvent), cryogenic separation, membranes, or pressure swing adsorption (PSA).1,4 Similar technologies are applied or studied for CO2 removal from flue gases. However, differences with flue gas treatment are the higher CO2 content in the feed, the lower temperature at which © 2012 American Chemical Society

the gas is available, and the product requirements. The desired product in flue gas treatment is CO2, while biogas treatment is focused on obtaining CH4. The removed CO2 is normally not considered for sequestration, as the amounts of CO2 produced are lower, making CO2 collection and transportation relatively expensive. All technologies listed above have been commercially applied to remove CO2 from biogas, but energy requirements are normally high due to required pressure differences or elevated temperatures for solvent recovery. This work describes the possibilities of using a novel cryogenic capture technology, using dynamically operated packed beds, and compares it to adsorption technology on the basis of product purity and recovery, bed dimensions, and energy requirements. To be able to compare the performance and requirements of the two processes, a specific detailed study on adsorption from literature has been selected,5 in which both dimensions and properties of the beds as well as energy requirements have been provided. First some details about the selected adsorption process are given. Then the novel cryogenic packed-bed process is described and explained with numerical simulations. Subsequently the two processes are compared for the removal of CO2 from a CH4/CO2 mixture. Finally, additional simulations are presented which show the possibility of simultaneously removing H2S using the proposed process concept.

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ADSORPTION Adsorption technology is a widely applied separation technology. Separation of CO2 from gas mixtures by adsorption is based on differences in equilibrium capacities at the adsorbent surface (e.g., zeolite 13X) or on differences in Received: Revised: Accepted: Published: 5552

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the packing surface. A front of desublimating CO2 will slowly move toward the outlet of the bed, until breakthrough is observed. At that point the bed is switched to a recovery step. In the case of CO2 capture, gaseous CO2 is fed to the bed in order to obtain pure CO2 at the outlet. For biogas treatment, CO2 capture is not required, therefore N2 or air can be used in the recovery step. However, when using air, CH4 and O2 might form mixtures which are within explosion limits. This could be solved by flushing the bed with N2 at the end of the capture step (without reducing the pressure). The amount of CH4 present in the gas phase of the bed is only limited, and therefore the amount of N2 required is also minimal. After CH4 is removed from the bed, the recovery could be carried out using air, and the mixing of CH4 and air is avoided. When using conventional cryogenic technologies to freeze out CO2 from gas streams, difficulties could arise due to plugging with solid CO2 or other components (such as H2O). Furthermore, the continuous cooling will result in a growing amount of solid CO2 in time at the heat exchanging surfaces, reducing the heat transfer rate toward the gas phase in time. Additionally, the required temperature and pressure swings to recover the heat exchangers, could result in mechanical stresses. These potential problems are not encountered in the proposed technology. The amount of cold energy stored in the packing is restricted, therefore the amount of CO2 deposition is limited as well and plugging is intrinsically avoided. Moreover, the outlet gas stream is at temperatures equal to the initial bed temperatures, guaranteeing maximal removal of CO2 (zero approach temperature). Furthermore, the beds can be constructed relatively simply, where the packing could consist of low cost material. The evolution of axial temperature, mass deposition, and concentration profiles within the packed beds can be described using a pseudo-homogeneous one-dimensional axially dispersed plug flow model. The main equations are summarized in Table 3 and the effective axial heat and mass transfer coefficients are calculated using the equations in Table 4. This model has been validated with experiments for both N2/CO2 as N2/CO2/H2O mixtures, which has been described in our previous work.10,11

uptake rates (e.g., carbon molecular sieve 3K). Regeneration of a bed is normally obtained by reducing the pressure. An adsorption process therefore consists of several packed beds which are operated simultaneously in different process steps, such as feed, recovery, and pressurization. The removal of CO2 from biogas using adsorption is widely discussed in literature.5−9 Grande and Rodrigues5 analyzed vacuum pressure swing adsorption (VPSA), in which the recovery step is carried out at subatmospheric pressure. The purity and recovery of CH4 and energy consumption by changing pressures and cycle times have been investigated for two different adsorbents, carbon molecular sieve (CMS) 3K and zeolite 13X. The lowest energy requirements have been obtained in a run with CMS as adsorbent. In this run a gas mixture of 16 000 SLPM (0.312 kg/ sec), containing 45 vol % CO2 and 55 vol % CH4 is treated. It is assumed that the biogas is available at 2 bara and 25 °C. Furthermore it is assumed that water and other contaminants have been previously removed. The bed properties and operating conditions are listed in Tables 1 and 2 respectively. Table 1. Bed Properties property

CPB

VPSA

length (m) radius (m) porosity (−) packing material type of packing bulk density (kg m−3) number of beds (−)

1.65 0.3 0.7 SS monolith 2347 2

4.667 0.4667 0.33 CMS particles 715.4 2

Table 2. Operating Conditions property

CPB

VPSA

feed flow rate (SLPM) feed pressure (bara) recovery pressure (bara) initial bed temperature (°C) purity (CH4%) recovery (CH4%) productivity (kg CH4 h−1 mpacking−3)

16000 5 1.1 −110 99.1 94.3 350.2

16000 8 0.1 20 98.1 79.7 43.1

Table 3. Model Equations for the 1-D Pseudo-homogeneous Model

For further details the reader is referred to their work. Although the most optimal case considered in the work by Grande and Rodrigues was selected for comparison with the novel CPB technology, it should be noted that the economics of the VPSA technology could possibly be further optimized, for example, by using higher blow down pressures.

Component mass balances for the gas phase: nc ∂ωi ,g ∂ωi ,g ∂ωi ,g ⎞ ∂⎛ εg ρg = −ρgvg + ⎜ρgDeff ⎟ − ṁ i″as + ωi ,g ∑ ṁ i″as ∂t ∂z ∂z ⎝ ∂z ⎠ i=1 Component mass balance for the solid phase: ∂mi = ṁ i″as ∂t Total continuity equation for the gas phase: nc ∂(εg ρg) ∂(ρgvg) =− − ∑ ṁ i″as ∂t ∂z i=1

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CRYOGENIC PACKED BEDS The CPB concept has been extensively demonstrated both by simulations and experiments in our previous work.10,11 Although originally developed to capture CO2 from flue gases, it can also be applied to a wide range of other gas separations, such as biogas treatment. The process is based on the dynamic operation of packed beds, which are operated in cooling, capture and recovery steps. First a bed is cooled down to a temperature low enough to condensate/desublimate the contaminants (e.g., CO2 and H2O), while allowing the other components to remain gaseous (e.g., CH4 or N2). When feeding for example a CH4/CO2 mixture to a previously refrigerated packed bed, the gas mixture will cool down and the packing material will heat up until CO2 starts to desublimate at

Energy balance (gas and solid phase): ∂T ∂T ∂ ⎛⎜ ∂T ⎞⎟ (εg ρgCp,g + ρs(1 − εg )Cp,s) = −ρgvgCp,g + λeff ∂t ∂z ∂z ⎝ ∂z ⎠ nc

−

∑ ṁ i″asΔhi i=1

Pressure drop over packing:

∂p f 1 = −4 ρ vg 2 ∂z dh,c 2 g

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with

f=

dh,c 14.9 1 + 0.0445Re Re L

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s), and (5) a pressurization step (70 s). Although their analysis is based on two beds, three beds are required if continuous operation is desired. When CO2 breakthrough is observed, the bed should be recovered. By reducing the pressure to atmospheric pressures and by flushing the system with (dry) air, very fast recovery can be obtained. Therefore the recovery and cooling step can be integrated in one step by using an air flow of 5 kg/s, refrigerated to −110 °C. All CO2 is recovered after 6.5 s, as can be observed from Figure 2a and 2b. In this first 6.5 s, the outlet

Table 4. Heat and Mass Transfer Coefficients for a Monolith Packing Effective axial heat dispersion:13

⎛ ρgvgCp,g ⎞2 1 ⎟⎟ λeff = (1 − εg )λ s + ⎜⎜ ⎝ εg ⎠ αg,sas Gas to solid heat transfer coefficient:14

αg,s =

0.45 ⎛ d ⎞ 2.978⎜1 + 0.095RePr h ⎟ dh Lc ⎠ ⎝

λg

Axial mass dispersion:

Dax = Deff, i +

vg 2dh,c 2 192Deff, i

Re =

with

ρgvgdh ηg εg

15

with

Deff, i =

1 n

yj ,g

∑ j =c 1 D

i ,j

For details on the used numerical solution strategy the reader is referred to Smit et al.12 To compare the CPB concept with VPSA, simulations have been carried out for the cryogenic concept using an equal flow rate and gas composition as used by Grande and Rodrigues.5 The bed is cooled down to −110 °C and the feed is pressurized to 5 bar. Under these conditions the outlet CH4 purity is 99.1%. The dimensions of the beds (see 1) are chosen in such a way that the capture/feed step takes 140 s, similar to the VPSA case. The recovery and cooling step together also take 140 s, therefore the process can be operated continuously when two beds are operated in parallel. Axial temperature and mass deposition profiles during the capture step are shown in Figure 1a and 1b, respectively. It should be noted that the process cycle for the VPSA case consists of a (1) feed (60 s), (2) depressurization (10 s), (3) blow down (140 s), (4) purge (50

Figure 2. Simulated axial temperature (a) and mass deposition (b) profiles during the recovery step in the first seconds and the temperature profiles later in time (c).

flow cannot be recycled to the inlet of the bed via the cooler, due to the presence of CO2. Therefore an amount of 32.5 kg of dry air is required per step (average flow of 0.23 kg/s). After all CO2 is recovered, the entire bed is further cooled down to −110 °C, in order to start a new capture step again (Figure 2c).

Figure 1. Simulated axial temperature (a) and mass deposition (b) profiles during the capture step. 5554

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Figure 3. Simplified process scheme including conditions for the base case.

A simplified process flow diagram including flow compositions and process conditions is shown in 3.

compression and an efficiency of 72%. Results are shown in Table 6.

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ADSORPTION VERSUS CRYOGENIC PACKED-BED CONCEPT The two processes are compared on (i) product purity and recovery of CH4 from the biogas, (ii) required bed dimensions, (iii) energy requirements. Table 2 shows that the product purity in VPSA is 98.1% CH4 against 99.1% in the CPB concept. The recovery of CH4 in VPSA is lower (79.4%), due to losses in purge steps. The losses of CH4 in the CPB concept are limited to the amount of CH4 present in the bed before switching to the recovery. When the pressure is reduced and the system is flushed with air this amount of CH4 is lost: the recovery is 94.3%. The beds required in the cryogenic concept are much smaller (Table 1). The productivity is calculated for both processes: this value is more than eight times higher for the CPB concept (350.2 vs 43.1 kgCH4 h−1 mpacking−3). Grande and Rodrigues5 estimated the energy requirements for the adsorption process, which are summarized in Table 5.

Table 6. Energy Requirements for Cryogenic Packed Beds power (kW)

step

power (kW) 57.7 73.2 28.1 129.8 288.8

base case

reversed flow

37.9 62.6 34.2 256 390.7

37.9 62.6 20.4 142.5 263.4

Furthermore, energy is required by the refrigerator in order to cool down the air. As described earlier, an average amount of 0.23 kg/s of fresh dry air is required to recover the bed. It is assumed that this air is available at 25 °C. After CO2 is removed from the bed, the refrigerated air can be recycled. The temperature of the air at the outlet of the bed is changing during the recovery/cooling step, as shown in Figure 5. The average outlet temperature is −101.7 °C, which is mixed with fresh air of 25 °C, resulting in an average inlet air temperature of −95.8 °C. Because of compression in C-3, the stream heats up and will be cooled down in the refrigerator to −110 °C. The required cooling duty is 102.4 kW. However, the efficiency of a cooler in this power and temperature range is approximately 40%,16 and therefore the power input for the refrigerator is higher. The resulting cooling costs are also listed in Table 6. The total energy requirement for the process then amounts to 4.3 MJ/kg CH4, which is 16% higher than for the VPSA process. The energy requirements are mainly caused by the cooling step. A relatively large air flow of 5 kg/s is required to cool down the bed to −110 °C. When taking a closer look at the temperature profiles in Figure 2, it can be observed that the hot zone at the inlet of the bed (which is at the biogas inlet temperature, 25 °C), is moved through the entire bed. This is also visible in the outlet temperature as a function of the time in Figure 5. Therefore, it is more efficient to reverse the flow direction during the recovery/cooling step, as illustrated in the process flow scheme in Figure 4. Figure 6 shows the resulting axial temperature and mass deposition profiles. In this case a lower total air flow is required (3 kg/s). All CO2 is recovered after 10 s, therefore an average amount of 0.21 kg/s of fresh dry

Table 5. Energy Requirements for VPSA compression of feed to 8 bar compression of product to 200 bar power for purge step blowdown step total power

step compression of feed to 5 bar compression of product to 200 bar air recycling cooling power total power

The total power required amounted 3.7 MJ/kg CH4. The energy requirements for the CPB concept are based on the process flow scheme shown in Figure 3. Three compressors are required, the first one to pressurize the feed to 5 bar (C-1), the second one to pressurize the product to 200 bar (C-2), and a third one to recycle air through the bed and the cooler (C-3). Simulations showed that the pressure drop in the packed bed is only 0.04 bar, which is explained by the selected nature of the structured packing (monolith). It is assumed that the cooler causes another pressure drop of 0.06 bar, therefore an outlet pressure of 1.1 bar is required for compressor C-3. All compressor duties have been calculated assuming isentropic 5555

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Figure 4. Simplified process scheme including conditions and recovery/cooling step in reversed flow mode.

It can be observed that both CO2 as well as H2S will deposit onto the packing surface, although the zone where H2S deposits also contains CO2. This can be explained by having a closer look at the saturation vapor pressures of both components in Figure 8. When the mixture is cooled down, CO2 starts to deposit at −88 °C, as indicated by the dotted line. The temperature will decrease further, and below −105.1 °C the saturation vapor pressure of H2S becomes lower than its partial pressure in the feed (0.05 bar) and H2S starts to deposit as well. During the cooling of the mixture from −105.1 °C to the initial bed temperature, both components will deposit onto the packing. Therefore, it can be concluded that components with saturation vapor pressures close together will deposit in the same region. Nevertheless, these results show that both components can be efficiently removed from the feed gas.

Figure 5. Outlet temperature as function of time.

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air is required. The resulting outlet temperature as function of the time is also plotted in Figure 5. When comparing the two profiles for the base case and the reversed flow mode, it can be concluded that the average outlet temperature is lower in the reversed flow mode. Furthermore, the waste stream containing CO2 is emitted at a relatively higher temperature, avoiding cold losses. Finally, the lower flow rate causes a lower power consumption by compressor C-3. The required energy for all steps are also included in Table 6. A total amount of 2.9 MJ/ kgCH4 is required, which is 22% lower than for the VPSA process.

DISCUSSION AND CONCLUSIONS

A novel process for biogas treatment using dynamically operated cryogenic packed beds has been proposed and studied with numerical simulations.The cryogenic packed-bed concept shows to be favorable when compared to adsorption technology on several aspects. The purity and the recovery of the obtained CH4 are higher than in VPSA. Higher purity is possible (up to 99.99%) at lower initial bed temperatures. Furthermore, required bed sizes are significantly smaller, resulting in much lower capital investments. However, good insulation of the cryogenic packed beds is required to avoid heat leaks into the system from the surroundings. It should be noted that for a detailed comparison of the required total footprint of both technologies also the equipment of the required utilities in addition to the packed beds need to be considered. Additionally, energy requirements are quite competitive with VPSA, even if a cooler efficiency of 40% is taken into account. Energy requirements are especially low when the recovery/ cooling mode is operated in reversed flow mode. Furthermore, dried air (or nitrogen) is required, which may raise operating costs somewhat. The amount of dried air can be reduced by operating the recovery/cooling step at subatmospheric pressures, at the expense of higher operation costs. Furthermore, it should be noted that the upgraded gas is

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HYDROGEN SULFIDE REMOVAL The H2S content in biogas can range from 0.0001 to 1 vol %.3 When feeding a mixture containing 1 vol % H2S, 45 vol % CO2 and 54 vol % CH4 to a cryogenically refrigerated bed (equal conditions and properties as those listed in Tables 1 and 2), CO2 and H2S can be separated simultaneously. However, with a bed temperature of −110 °C the H2S content can only be reduced to 0.6 vol %. If deep H2S removal is required, the initial bed temperature should be lower. The H2S content can be reduced to 40 ppmv when the initial bed temperature is −150 °C. Figure 7 shows the resulting axial temperature and mass deposition profiles when the mixture is fed to a bed initially cooled to −150 °C. 5556

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Figure 7. Simulated axial temperature and mass deposition profiles after 140 s when a CH4/H2S/CO2 mixture is fed to an initially refrigerated packed bed.

Figure 6. Simulated axial temperature (a) and mass deposition (b) profiles during the recovery step in the first seconds and the temperature profiles later in time (c). The bed is operated in reversed flow mode. Figure 8. Saturation vapor pressures of CO2 and H2S as a function of the temperature.

compressed to 200 bar in this study. However, if the cleaned product would be liquefied, the CPB technology shows the advantage that the product gas leaving the packed beds is already at a low temperature of −105 °C, reducing further cooling costs. In this work a relatively small biogas treatment flow rate was considered (16 000 SLPM, or 0.312 kg/s). In future work the performance of the CPB technology will be compared with competing technologies at different production capacities. This study focused on the CH4/CO2 separation and showed the possibility to simultaneously remove H2S. Results in our previous work 11 showed that H 2 O can be separated simultaneously using the CPB concept. Furthermore, siloxanes could also be separated in the same or a separate bed. Therefore, it can be concluded that the proposed CPB technology is a very promising process for biogas treatment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Shell Global Solutions International for their financial support.

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NOTATION αs = specific solid surface area per unit bed volume, m2/m3 Cp = heat capacity, J/kg/K dx.doi.org/10.1021/ie202606g | Ind. Eng. Chem. Res. 2012, 51, 5552−5558

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Deff = effective diffusion coefficient, m2/s Dh,c = hydraulic diameter of the monolithic channels, m f = Fanning friction factor for the monolithic channels (-) L = bed length, m mi = mass deposition of component i per unit bed volume, kg/m3 ṁ i″ = mass deposition rate per unit surface area for component i, kg/m2/s nc = number of components, p = pressure, Pa Re = Reynolds number (ρgvgdp/ηg) t = time, s T = temperature, K, °C v = superficial velocity, m/s z = axial coordinate, m

(11) Tuinier, M.; van Sint Annaland, M.; Kuipers, J. A novel process for cryogenic CO2 capture using dynamically operated packed beds An experimental and numerical study. Int. J. Greenhouse Gas Control 2011, 5, 694−701. (12) Smit, J.; van Sint Annaland, M.; Kuipers, J. Grid adaptation with WENO schemes for nonuniform grids to solve convection-dominated partial differential equations. Chem. Eng. Sci. 2005, 60, 2609−2619. (13) Vortmeyer, D.; Schaefer, R. Equivalence of one- and two-phase models for heat transfer processes in packed beds: one dimensional theory. Chem. Eng. Sci. 1974, 29, 485−491. (14) Hawthorn, R. Afterburner catalysts effects of heat and mass transfer between gas and catalyst surface [recent advances in air pollution control]. Amer. Inst. Chem. Eng. Symposium Series 1974, 70, 428−438. (15) Cybulski, A.; Moulijn, J. Monoliths in heterogeneous catalysis. Catal. Rev.: Sci. Eng. 1994, 36, 179−270. (16) Timmerhaus, K.; Flynn, T. Cryogenic Process Engineering; Plenum Press: New York, 1989.

Greek Letters

Δhi = enthalpy change related to the phase change of component i, J/kg εg = bed void fraction, η = viscosity, kg/m/s λeff = effective conductivity, W/m/K ρ = density, kg/m3 ω = mass fraction, kg/kg

Subscripts

g = gas phase i = component i s = solid phase Abbreviations

CMS = carbon molecular sieve CPB = cryogenic packed bed PSA = pressure swing adsorption VPSA = vacuum pressure swing adsorption

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REFERENCES

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