Article pubs.acs.org/EF
Pressure Swing Adsorption Process in Coal to Fischer−Tropsch Fuels with CO2 Capture Ana M. Ribeiro,† Joaõ C. Santos,† Alírio E. Rodrigues,*,† and Sébastien Rifflart‡ †
Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal ‡ Total Gas and Power Division, R&D Department, Immeuble Lafayette, 2 Place des Vosges, 92078 Courbevoie, France ABSTRACT: The carbon dioxide capture by pressure swing adsorption in a coal to Fischer−Tropsch process is addressed. The simulations results show that a (hydrogen + carbon monoxide) product with required composition for Fischer−Tropsch synthesis, H2/CO ratio of 2.22, and inerts content of 3.90% can be obtained. Additionally, the PSA process produces high purity carbon dioxide of 95.16% with high recovery of 91.6%. This CO2 stream is compressed to 110 bar for transportation and storage. The power consumption requirement was calculated to be 311 kW·h/tonCO2 (191 kW·h/tonCO2 for separation and 120 kW·h/ tonCO2 for compression), which is too high for competitive operation of the PSA process when compared to absorbent based processes. A modification of the PSA process operation procedure involving the introduction of a number of tanks to collect fractions of the blowdown stream drastically reduces the power consumption to 129 kW·h/tonCO2 (including separation and compression to 110 bar) without affecting the products specifications.
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INTRODUCTION Environmental concerns, oil price rises, and political instability have prompted the search for alternative energy sources. The chemical conversion of carbon sources, such as natural gas, coal, or biomass, to liquid hydrocarbons allows for an alternative source to the traditional refinery products derived from crude oil.1,2 Fischer−Tropsch synthesis is one of the most important syngas-based liquid synthesis processes. The raw syngas produced from gasification is sent to gas cleanup units to remove contaminants such as H2S, COS, and HCl, which potentially act as catalyst poisons. Carbon dioxide must also be removed before the Fischer−Tropsch reactor. The cleaned syngas reacts over Fischer−Tropsch catalysts, normally cobaltor iron-based, to generate liquid fuels. Those liquid fuels are further upgraded through hydrotreating or hydrocracking reactions. Therefore, the final product fuels from Fischer− Tropsch synthesis usually exhibit exceptional quality in terms of hydrogen content, product molecule uniformity, freeze point, combustion characteristics, and sulfur content.3,4 The types of products obtained in the Fischer−Tropsch synthesis, which include naphtha, diesel, kerosene, lubricants, and waxes, depend on the process conditions, catalyst, and syngas composition (hydrogen to carbon monoxide ratio).2 As mentioned, the pretreatment of the syngas involves a carbon dioxide removal step for capture, which is the focus of this study. The technologies commonly employed for this purpose are absorbent-based systems such as Rectisol and Selexol processes.5,6 Pressure swing adsorption (PSA) may be a viable alternative.7,8 This process is widely used to produce high purity hydrogen (+99.99%) from syngas streams.9,10 However, in this case, the purpose of the PSA unit would be the production of a (H2 + CO) stream adequate for Fischer−Tropsch synthesis and the removal of the carbon dioxide at high concentration. © 2012 American Chemical Society
PROCESS SPECIFICATIONS The objective of this study is to evaluate by simulation the performance of a PSA process to treat syngas produced from coal gasification, after drying, desulfurization, and CO shift, for integration in a Fischer−Tropsch process. The scheme of the global process is shown in Figure 1. The use of a PSA process requires a predrying step of the feed stream, which is not necessary in solvent-based processes. However, in the PSA process, the CO2 product is obtained at conditions ready for capture, while in the solvent-based processes some treatments are then required both for the product and solvent. Under certain circumstances, a water-wash system must be used to reduce the residual content of methanol on the CO2 product. The water fed to the plant is also absorbed by the solvent, and therefore, to keep the water content of the solvent at the desirable level, continuous distilling of a small side stream of the solvent circulation is necessary.11 It was considered that the feed stream to the PSA unit has a flow rate of 1208 kNm3/h at 50 °C and 33 bar with a composition of 47.07% H2, 30.11% CO2, 0.03% CH4, 22.26% CO, and 0.53% N2. A complete removal of H2S prior to the PSA process has been assumed. The PSA process should be designed to obtain two products. A (H2 + CO) rich stream with less than 5% inerts, pressure above 26 bar, and with a H2/CO stoichiometric ratio between 2.0 and 2.4. The constraints on the H2/CO stoichiometric ratio are due to the risk of carbon deposition on the catalyst for values lower than 2.0 and to the promotion of naphtha and LPG production instead of diesel production for values higher than 2.4. The other product is a CO2 rich stream with purity Received: October 25, 2011 Revised: January 17, 2012 Published: January 20, 2012 1246
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Figure 1. Scheme of a coal to Fischer−Tropsch process.
process. In this case, the objective is to obtain two products: a light product with hydrogen and carbon monoxide and a heavy product with high purity carbon dioxide. When a heavy product at a high purity is required, cocurrent depressurization or rinse steps are commonly used.13 However, in this case, another important aspect must be taken into consideration, the constraints in the light product. The H2/CO stoichiometric ratio on the light product must be close to the H2/CO stoichiometric ratio in the feed stream, and the inert content must be less than 5%. As the activated carbon has an intermediate affinity toward the CO, a fraction of the CO present in the feed stream, higher than that of H2, will be adsorbed in the activated carbon during the adsorption step and must be recovered into the light product. The use of a cocurrent depressurization would increase the CO2 content in the bed and recover some CO into the light product. However, to recover the necessary amount of CO, the depressurization would also promote CO2 desorption contaminating the light product to an inert content above 5%. Therefore, a rinse step was selected to desorb the adsorbed CO and increase the heavy product concentration for subsequent production. A cycle with five steps was designed. The steps are adsorption, rinse, blowdown, purge, and pressurization. A scheme of this cycle is presented in Figure 3. The light product is obtained during the adsorption and rinse steps, and the heavy product is obtained during the blowdown and purge steps. The pressurization is made countercurrently with light product. In Figure 3, the extension of the cycle to a 4-bed PSA is also shown. The cycle was designed to have one column in the adsorption step at all times; that is, there is a constant feed consumption. Additionally, there is also continuous production of the two products. The extension to the 4-bed PSA imposes constraints on the steps duration. In this case, the duration of the adsorption must be the same as the blowdown and rinse, as well as the sum of the duration of the purge and pressurization step.
above 95%. The recovery of CO2 should be higher than 90%. The process specifications are summarized in Figure 2. Both feed conditions (composition, flow rate, temperature, and pressure) and product specifications represent a typical case
Figure 2. Process specifications.
study of syngas conditioning obtained from coal gasification to be used in a Fischer−Tropsch process. An activated carbon was selected as a single adsorbent to be used in this process, the activated carbon Norit R2030. Adsorption equilibrium and adsorption kinetics have been determined previously on this adsorbent for all the compounds present in this study.12 Some physical properties of the activated carbon Norit R2030 extrudates are given in Table 1. Taking into account the superficial velocity (typically values in the range of 0.01 to 0.06 m/s) and the space time of the Table 1. Physical Properties of the Activated Carbon Norit R2030 Extrudates shape particle density (kg/m3) pellet porosity pellet radius (m) surface area (m2/g) particle specific heat (J/kg/K)
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cylinders 874 0.60 1.45 × 10−3 700 709
MATHEMATICAL MODEL A mathematical model with mass, energy, and momentum balances that represent the dynamic behavior of a nonisothermal, nondiluted, multicomponent adsorption bed was used to simulate the pressure swing adsorption process. The model was developed on the basis of the following assumptions: ideal gas behavior throughout the column; no mass, heat or velocity gradients in the radial direction; constant porosity along the bed; axial dispersed plug flow; and no temperature gradients inside each particle. Additionally, the model accounts for external mass and heat transfer resistances, expressed with the film model, and it considers that the adsorbent particles are bidispersed with macropore and micropore mass transfer resistances, both expressed with the Linear Driving Force (LDF) model. The momentum balance is
adsorption step (trade-off between the mass transfer resistances effects and process productivity), the design of the unit determined that the feed stream (1208 kNm3/h) should be divided into eight 4-bed PSA units with bed dimensions of 12 m long and 6 m diameter. Under these conditions, the superficial velocity and the space time of the adsorption step are respectively 0.054 m/s and 85 s. The operation of a PSA process involves the definition of a cycle, which must take into account the objective of the 1247
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Figure 3. PSA cycle and extended schedule to a 4-bed process.
given by the Ergun equation. The mass, momentum, and
The values of some transport parameters are required within the model. These were calculated employing frequently used correlations. The axial mass (Dax) and heat dispersion coefficients (λ), as well as the mass transfer (kf) and heat convective coefficients (hf) were estimated using the Wakao and Funazkri correlations.19−21 The system was considered adiabatic (U = 0). The convective heat transfer coefficient between the gas and the column wall (hw) was calculated with the Wasch and Froment correlation.22 The macropore diffusivity (Dp) took into account only the molecular diffusivities, which were calculated with the Chapman−Enskog equation.23 A tortuosity value of 2 was assumed. The micropore diffusivities (Dc) were determined experimentally by Grande et al.12 General properties of the gases, such as density, viscosity, and molar specific heat, were obtained according to Bird et al.23 The molar specific heat of the adsorbed gas was assumed to be equal to the molar specific heat in the gas phase.24 The transport parameter values used in the simulations are presented in Table 3.
energy balance equations of the mathematical model are given in Table 2. A detailed description of the mathematical model is presented elsewhere.14−16 The model has been validated Table 2. Mass, Momentum, and Energy Balance Equations of the Mathematical Model
Table 3. Transport Parameters Values Used in the Simulationsa Dax (m2/s) λ (J/s/m/K) kf (m/s) hf (W/m2/K) hw (W/m2/K) Dp (m2/s)
Dc/rc2 (s−1)
a
2.7 × 10−4 3.5 1.1 × 10−2 550 645 CO2: 5.49 × 10−7 H2: 1.21 × 10−6 CH4: 4.98 × 10−7 CO: 5.24 × 10−7 N2: 4.73 × 10−7 CO2: 5.64 × 10−2 H2: 0.147 CH4: 2.94 × 10−2 CO: 1.96 × 10−1 N2: 1.96 × 10−1
Values at feed conditions.
previously against experimental results, showing very good The mathematical model was solved in gPROMS environment (Process System Enterprise, London, U.K.) using the
predictions.16−18 1248
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Table 4. PSA Simulation Results operating conditions
(H2 + CO) product
CO2 product
run
tads (s)
tpurge (s)
R/ Pra
P/Fb
inerts (%)
H2/CO
productivity (mol/kg/day)
purity (%)
recovery (%)
productivity (mol/kg/day)
power rinse (kW·h/tonCO2)
power CO2 (kW·h/tonCO2)
1 2 3 4 5 6 7
400 400 400 420 440 460 480
100 100 100 105 110 115 120
2.72 2.55 2.41 2.27 2.20 2.10 2.04
0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076
3.84 3.02 2.45 2.61 3.12 3.81 4.89
2.14 2.18 2.23 2.23 2.21 2.21 2.22
151.6 150.4 149.6 149.5 149.9 149.7 149.6
98.39 96.91 95.26 95.03 95.60 95.21 94.87
92.6 94.6 95.9 95.1 94.3 92.6 89.9
61.3 62.7 63.5 63.3 62.4 61.3 59.6
239.3 228.6 219.7 207.0 199.7 191.2 186.4
116.1 118.0 120.1 120.4 119.7 120.2 120.7
a
Rinse to product ratio. bPurge to feed ratio.
results were obtained with atmospheric pressure, it was selected as being the most appropriate. It was assumed that cyclic steady state condition was reached when the change in purity of the CO2 product was less than 0.01% between two cycles. The cyclic steady state (concentration and temperature profiles) was, in general, reached after approximately 40 cycles. The power consumption values presented in Table 4 are for the eight 4-bed PSA units. The adsorption duration was set to 400 s, and a rinse flow rate of 1.18 m3/s (correspondent to a rinse to product ratio, R/Pr, of 2.72) was selected (run 1). The results obtained show that the CO2 purity, 98.39%, is above the required value, which indicates that the rinse flow rate can be decreased. This was done in runs 2 and 3, and it can be seen that, for a value of 1.12 m3/s, R/Pr of 2.41, (run 3), the CO2 purity value approaches the required value of 95%. All the process constraints are obeyed in this run. The effect of the adsorption duration was studied next. The adsorption duration was increased in runs 3−7 from 400 to 480 s, with 20 s increments, adjusting the rinse flow rate used to obtain a CO2 product with the required purity. The increase in adsorption duration leads to an increase in inerts content in the light product, a decrease in the CO2 recovery, and, most importantly, a decrease in power consumption. It can be seen that, until an adsorption duration of 460 s, conditions for which all the constraints are obeyed were found. If the adsorption duration is increased to 480 s, the constraints can no longer be obeyed. The CO2 purity is still below 95%, while the recovery is already below 90%. In conclusion, it was found that the best run was run 6. The (H2 + CO) product obtained has an inert content of 3.81% and a H2/CO stoichiometric ratio of 2.21. The CO2 product has a purity of 95.21%, and the CO2 recovery is 92.6%. The power consumption was 311 kW·h/tonCO2 (191 kW·h/tonCO2 for separation and 120 kW·h/tonCO2 for compression). The concentration and temperature profiles at the end of each step at cyclic steady state for run 6 are shown in Figure 4. The (H2 + CO) product is produced in the adsorption and rinse steps. During the adsorption step the CO2 concentration front moves forward in the bed, then comes the rinse, cleaning the bed with CO2 and forcing the CO adsorbed at the inlet of the bed to be desorbed. The rinse step is stopped as soon as the CO2 content in the bed is enough to ensure the 95% purity constraint. The CO2 contained in the bed is produced, first in the blowdown step, in which the pressure in the bed is decreased, and then in the purge step. The pressurization step brings the bed back to the high pressure. As mentioned before, the feed stream should be divided into eight 4-bed PSA units. Figure 5 presents a proposed extension of the cycle schedule to eight units, with a predefined phase lag between each unit. Under these conditions, it is possible to
orthogonal collocation on finite elements as the numerical method. The number of elements used within one bed was 70, with third order polynomials (two interior collocation points). The PSA process, as described in Figure 3, is a 4-bed PSA process that employs in some steps a fraction of the products as feed streams. However, the program implemented in gPROMS simulates a single bed process. To represent the recycling of the 4-bed process, the composition of the recycling streams use the information of the products composition obtained in the previous cycle. Because in cyclic steady state all eight 4-bed units undergo the same changes within one cycle, taking into account the scheduled phase lag, it is possible to extend the information obtained from the simulation of one column to the entire process. As mentioned before, the rinse step uses a portion of the CO2 product obtained at the lower pressure that must be compressed to the higher adsorption pressure. The power consumption of the compressor was calculated considering adiabatic compression, multiple stages with the same pressure ratio, and with a 5 psi pressure drop between stages. It was also considered that, after each stage, the gas is cooled back to the inlet temperature and an efficiency (η) of 85% was assumed. The equation used for the calculation of the power requirements for compressing the rinse stream is
power =
γ− 1/ γ ⎛ ⎞ γ ⎜⎛ P2 ⎞ 1 − 1⎟ nR ̇ g T1 ⎜ ⎟ ⎟ γ − 1 ⎜⎝⎝ P1 ⎠ η ⎠
(1)
where ṅ is the molar flow rate, Rg is the ideal gas constant, T1 is the inlet temperature, P1 and P2 are respectively the inlet and outlet pressure, and γ is the ratio between the heat capacity of the gas mixture at constant pressure and the heat capacity of the gas mixture at constant volume (γ = Cp/Cv).25,26 This equation can be used if average molar flow rate and constant low and high pressure are considered.
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RESULTS The PSA simulation study was performed through the evaluation of the influence of the process parameters on the PSA performance. The results of seven simulations are presented in Table 4. A blowdown pressure of 1 bar was employed. The use of blowdown pressures higher than 1 bar was evaluated, but the results showed that the performance of the process decreased significantly due to poorer regeneration of the bed. On the other hand, the use of subatmospheric pressure for blowdown would imply the use of vacuum pumps with deleterious consequences on costs and power consumption. As good 1249
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Figure 4. Concentration and temperature profiles at the end of each step at cyclic steady state obtained for run 6.
Figure 5. Cycle schedule for the eight 4-bed PSA. 1250
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Figure 6. Evolution of the (H2 + CO) product along one cycle at cyclic steady state obtained for run 6: (a) product flow rate, (b) H2/CO stoichiometric ratio, and (c) inerts content.
fractions of the outlet stream of the blowdown step. The outlet stream of each tank is compressed and introduced in the tank with the pressure immediately higher, up to the high pressure of the process. A fraction of this high pressure stream is used as rinse inlet stream, and the remaining is withdrawn as product and compressed to the required storage pressure. As the PSA process has eight 4-bed units, the blowdown step was subdivided into eight steps: B1−B8. Eight tanks are then used, each to collect one of the sub-blowdown steps. With this arrangement, the inlet stream to the tanks coming from the blowdown step is constant, as there is at all times one, and only one, column at each of the sub-blowdown steps. The outlet stream of the purge step is also CO2 product. Therefore, the tank at lower pressure receives not only the outlet stream of the last sub-blowdown step but also the purge outlet stream. This process was denoted “tank in series process”. A scheme of the cycle schedule for the eight 4-bed PSA is given in Figure 5, and a scheme of the tanks arrangement for the tank in series process is shown in Figure 8. As the CO2 product has to be compressed to be used in the rinse step and for sequestration, the collection of the blowdown stream at intermediate pressures (instead of complete depressurization to 1 bar) allows for the preservation of energy in the form of pressure and consequent reduction of the power consumption. The pressure decrease in the blowdown step employed in the results shown in Table 4 was a linear one from 33 to 1 bar. If a different scheme of pressure decrease is employed, an even higher reduction on the power consumption could be achieved. From the results of run 6, an approximate correlation between the outlet flow rate of the column and the column pressure can be obtained. Using this correlation, the pressure of the tanks that minimize the power consumption can be determined. Values of 25.00, 18.60, 13.60, 9.54, 6.42, 4.06, 2.35, and 1.00 bar were obtained for each of the eight tanks, as represented in the scheme of Figure 8. The detailed results obtained with this new process are compared in Table 5 with the results obtained with the single tank process (run 6). It can be seen that the new process arrangement does not affect significantly the performance and that all the process constraints continue to be obeyed. The power consumption, which includes the power required to compress the CO2 product to 110 bar for storage, is significantly reduced from 311 kW·h/tonCO2 to 129 kW·h/tonCO2.
minimize the variations in the flow rates and composition of the two products. Figure 6 and Figure 7 show respectively the
Figure 7. Evolution of the CO2 product along one cycle at cyclic steady state obtained for run 6: (a) product flow rate and (b) composition.
evolution of the (H2 + CO) and CO2 products along one cycle at cyclic steady state obtained for run 6. In the case of the (H2 + CO) product, it can be seen (Figure 6) that the oscillations in flow rate are of around ±4%, while the oscillations in the product H2/CO stoichiometric ratio and inerts content are ±1%. The observed oscillation in CO2 product flow rate (Figure 7) is of ±3%, with the CO2 molar fraction changing between 95.15% and 95.33%. A Rectisol process requires around 260 kW·h/tonCO2 at raw gas pressure of 25 bar for recovering 90% of CO2.27,28 As can be seen from Table 4, the power consumption obtained is significantly higher, and for the PSA process to be competitive with the currently used solvent processes, this value should be drastically reduced. With the objective of decreasing the power consumption, an alternative process was designed.29 This process integrates a series of tanks of decreasing pressure that are used to collect 1251
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Figure 8. Scheme of the tank arrangement for the tank in series process.
and the production of a (H2 + CO) product with a H2/CO stoichiometric ratio close to the feed value. The activated carbon Norit R2030 was selected as the adsorbent. The designed PSA cycle has five steps: adsorption, rinse, blowdown, purge, and pressurization. For a feed flow rate of 1208 kNm3/h and using eight 4-bed PSA units with bed dimensions of 12 m length and 6 m diameter, the process was able to produce the two products with the required specifications. (H2 + CO) inerts content of 3.90%, (H2 + CO) stoichiometric ratio of 2.22, and CO2 purity and recovery respectively of 95.16% and 91.6%. However, the power consumption required for the compression of the rinse stream was very high, 191 kW·h/tonCO2, which added to the 120 kW·h/tonCO2 needed for the CO2 compression, makes a total power consumption of 311 kW·h/tonCO2. With the purpose of reducing the power consumption, a new process that integrates a series of tanks of increasing pressure to collect the CO2 product stream was designed. This change in the operation procedure of the blowdown step has only a very small effect on the performance of the process in terms of products specifications but affects significantly the power consumption requirements. Including the compression of CO2 product to 110 bar for storage, an enormous reduction is achieved from 311 kW·h/tonCO2 to only 129 kW·h/tonCO2.
Table 5. Comparison between the Results Obtained with a Single CO2 Tank Process and the Tank in Series Process bed length (m)
bed diameter (m)
12 6 flow rates (m3/s) (at operating conditions) feed
rinse (R/Pr)
1.524
0.940 (2.10)
purge (P/F) 1.524 (0.0076) Plow (bar)
Phigh (bar) 33
1 step times (s)
adsorption, rinse, blowdown 460
purge
345 results with single tank
(H2 + CO) product composition (%)
pressurization
115
CO2 product
H2/ CO
composition (%)
recovery (%)
productivity (mol/kg/day)
2.21 CO2: 3.17 H2: 66.26 CH4: 0.02 CO: 29.93 N2: 0.62 inerts (%)
CO2: 95.21 H2: 0.70 CH4: 0.05 CO: 3.72 N2: 0.32
92.6
149.7 (H2 + CO) 61.3 (CO2)
3.81
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191.2 + 120.2 results with tank in series process
(H2 + CO) product composition (%)
power consumption (kW·h/tonCO2)a
CO2 product
Corresponding Author
H2/ CO
composition (%)
recovery (%)
productivity (mol/kg/day)
2.22 CO2: 2.54 H2: 66.77 CH4: 0.02 CO: 30.05 N2: 0.62 inerts (%)
CO2: 95.09 H2: 0.65 CH4: 0.05 CO: 3.89 N2: 0.31
94.1
149.5 (H2 + CO) 62.4 (CO2)
3.18
*Phone: + 351 22 508 1671. Fax: + 351 22 508 1674. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank LSRE financing by FEDER/POCI/2010.
power consumption (kW·h/tonCO2)a 129.0
a
The power consumptions include the compression of the CO2 to 110 bar for storage.
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AUTHOR INFORMATION
REFERENCES
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CONCLUSIONS
The study of a pressure swing adsorption process to treat a syngas stream obtained from coal gasification for integration on a Fischer−Tropsch process was conducted. The PSA cycle to be used was selected based on the requirements of the process: the production of a high purity heavy product, carbon dioxide, 1252
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