Ind. Eng. Chem. Res. 2009, 48, 5439–5444
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Oxygen Separation from Air by Four-Bed Pressure Swing Adsorption Masoud Mofarahi* Chemical Engineering Department, Persian Gulf UniVersity, Bushehr, Iran
Jafar Towfighi and Leila Fathi Chemical Engineering Department, Tarbiat Modares UniVersity, P.O. Box. 14115-111, Tehran, Iran
A study on a four-bed seven-step pressure swing adsorption (PSA) using zeolite 5A was performed experimentally and theoretically for separation of oxygen from air. In this process the steps of feed pressurization, production, blowdown, purge, pressure equalization (two steps), and product pressurization are included in a cycle. The effects of various operating parameters such as adsorption pressure, cycle time, production rate, and purge rate on the product purity and recovery were investigated experimentally. Oxygen purity and recovery both increase when adsorption pressure increases. For most of the experiments at the highest purge rate, a higher purity of oxygen is obtained, but the recovery of oxygen is reduced. Oxygen recovery increases as production rate increases while the purge flow rate remains constant. It was observed that increasing the cycle time increases the performance of the process. An equilibrium based isothermal model in conjunction with an LDF (linear driving force) approximation was employed to simulate process performance. Comparison of the results obtained from the experiments and simulation results shows reasonable agreement. 1. Introduction
2. Process Description
Pressure swing adsorption (PSA) processes are widely used in industries for air and other gas separations. Oxygen and nitrogen are produced from atmospheric air by either of two methods depending upon the volume of production. For highvolume production, cryogenic distillation of liquefied air is employed, whereas, for low to medium volume production, air separations by methods such as pressure swing adsorption are found to be more economical. This mature technology is economical for plants producing up to 250 t/d O2 using zeolites of type A (5A) or X (13X-NaX, LiX, or LiLSX). During the last 30 years, commercial applications for adsorptive O2 generation from ambient air using several molecular sieve zeolites have been expanded.1,2 However, because air contains a small amount of Ar, which has physical properties similar to those of O2, the product generated from the zeolite bed typically contains a substantial amount of Ar impurity. Therefore, in the equilibrium separation process using zeolites, O2 purity is limited to about 94%. For this reason, O2 produced by adsorption technology typically has 90-93% O2, 4-5% Ar, and 2-6% N2 and is generally used in many chemical processes such as the biological treatment of wastewater, steel industries, paper and pulp industries, and glass-melting furnaces.3,4 Although multiple adsorption beds are usually involved in the applications of PSA processes in industry, the detailed operation information is usually retained by each company and seldom revealed. Most of the published studies, therefore, are limited to the processes of single- and dual-bed systems. In the present study a four-bed PSA process using a commercial 5A zeolite is evaluated by experiments and a theoretical model. The PSA performances under the same feed rate, adsorption pressure, and cycle time were compared by variation of the purge flow and production rate. The adsorption dynamics at each step and the experimental PSA results were predicted by the mathematical model.
A four-bed process producing continuously enriched oxygen over 5A zeolite was studied. With the objective of producing high purity oxygen, a seven-step PSA was designed for air separation as follows:5 (1) Production (adsorption), AD (2) Pressure equalization for depressurization, ED (3) Blowdown, BD (4) Purge, PG (5) Pressure equalization for pressurization, EP (6) Pressurization with product, RP (7) Pressurization with feed, PF The sequence time of each step is given in Table 1 Figure 1 shows the first 50 s of the whole cycle, which represents the period of producing product from bed A1. At the start of a cycle, feed air is fed to bed 1 from one end, a product of enriched oxygen is drawn from the other end at a high bed pressure, and, then, some product is used to purge bed 3. At the same time, beds 2 and 4 are connected to equalize the pressure of the two beds. This operation step lasts 20 s. For the next 20 s, bed 1 keeps being fed with air, producing product and purging bed 3, and at the same time, bed 2 is pressurized byproduct. At this step, bed 4 is depressurized by blowdown. For the next 10 s, bed A keeps producing product and purging bed 3, bed 2 is pressurized with feed air, and bed 4 is still at blowdown. The first 50 s are the period of producing product from bed 1. Then, the product is produced from bed 2 during the second 50 s with a phase change. The roles of beds 2, 3, 4, and 1 in the second 50 s replace those of beds 1, 2, 3, and 4 in the first 50 s, respectively. After this phase change,
* To whom correspondence should be addressed. E-mail: Mofarahi@ pgu.ac.ir. Fax: +98 771 4540376.
Table 1. Sequence Time of PSA for Air Separation time (s) adsorber adsorber adsorber adsorber
20
20
10
20
20
10
20
20
10
20
20
10
1 AD ED BD PG EP RP PF 2 EP RP PF AD ED BD PG 3 PG EP RP PF AD ED BD 4 ED BD PG EP RP PF AD
10.1021/ie801805k CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
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intervals with a recorder (Logoscreen 50, Jumo Co.). During measurement, the adsorption cell was immersed in a water bath (MC 12, Julabo Tech.) maintained at (0.02 K via a refrigeration circulator. Prior to each isotherm measurement, zelolite 5A particles were regenerated in an oven overnight (300 °C for 6 h) under a vacuum of less than 0.05 mbar. 3.3. PSA Setup. The layout of the PSA pilot system is shown in Figure 2. It contains four main sections: (a) The feed section is equipped with a mass flow controller which is connected to a compressor. The gaseous feed is pressurized with a two-stage compressor C1. (b) The PSA columns have an ID of 3 cm and a length of 100 cm. They are filled with about 730 gr of 5A molecular sieve adsorbent. A calibrated three resistance temperature detector (RTD; Pt 100 Ω) is installed at one of the four columns (A1) in order to track the temperature front. (c) The bed pressure and pressure drop were measured by the pressure transducers and pressure gauge located at each bed, feed, and product lines. The product and purge flow rates were controlled by mass flow controllers. (d) The analytical section is dedicated to controlling the cycle operation of the system and online analysis of the composition of the mixture exiting the system. The gas chromatograph system, a CP 3800 Varian with a TCD detector, was used for the oxygen/nitrogen mixtures. The GC is coupled with an automatic valve sample system including a 4-valve with an electric actuator and switching valve. The chromatographic analysis is performed with the software provided by Varian. The system was fully automated by a personal computer with a developed control program, and all measurements including pressure, temperature, and mass flow rate were indicated and recorded on the computer. When the cyclic steady state was reached, the collected data were integrated to obtain product recovery, purity, etc. 4. Mathematical Model Figure 1. First 50 s of the operation schedule of the four-bed PSA. Table 2. Physical Properties of Zeolite 5A5 intrusion volume specific surface area particle diameter bed density particle porosity, εp bed inner diameter bed length
0.24 cm3/g 572 m2/g 2-3 mm 670 kg/m3 0.4 3 cm 100 cm
beds 3 and 4 are used to produce enriched oxygen during the third and fourth 50 s periods, respectively, and the overall cycle is completed. 3. Experimental Section 3.1. Materials. The zeolite 5A adsorbant was provided by Linde in the form of pellets (1.6 mm extrudates). Some physical properties of zeolite 5A are shown in Table 2. Adsorbents were regenerated overnight at 300 °C before being used in measurements. 3.2. Equilibrium Measurement. The adsorption isotherms of pure oxygen and nitrogen on zeolite 5A were measured at four different temperatures (273, 283, 303, and 343 K) using a high pressure volumetric apparatus up to 9.5 bar. This apparatus consists of an adsorption cell and a loading cell. The total amount of gas introduced and recovered in the system was determined by appropriate pressure, temperature, and volume measurements. The temperature in each cell was measured by K-type thermocouple operated within (0.01 K accuracy); the pressure was measured with a pressure transducer (JUMO MIDAS 1001). Temperatures and pressures were recorded at constant time
The mathematical models describing the PSA process are developed considering the following assumptions: local equilibrium is established, temperature is constant, axial dispersion is negligible, plug flow conditions are assumed, pressure drop along the column is negligible, and gas behavior is ideal. Mass transfer is represented by the linear driving force (LDF) approximation.6,7 On the basis of these assumptions, the mass balance for each component of the mixture and the total mass balance are written as follows: Overall mass balance ∂(uC) ∂C + + ∂t ∂t
∂qji
n
∑ F ( 1 -ε ε ) ∂t P
)0
(1)
i
Component mass balance ∂ci ∂(uci) ∂ Ci 1 - ε ∂qi -DL 2 + + + Fp )0 ∂z ∂t ε ∂t ∂z 2
(
)
(2)
The initial conditions for fluid flow are the following: u(z, 0) ) 0,
q(z, 0) ) 0
(3)
The boundary conditions of each step are given in Table 3. The sorption rate into an adsorbent is described by the LDF model with a single lumped mass transfer parameter. ∂qji ) Ki(qj*i - qji) ∂t
(4)
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Figure 2. PSA pilot setup. Table 3. Boundary Conditions for Different Steps step adsorption equalization to depressurization blowdown purge
component balance
overall balance
-DL ∂Ci/∂z|z)0+ ) u|z)0(Ci|z)0 - Ci|z)0+) u|z)0 ) uOH ∂Ci/∂z|z)L ) 0 ∂Ci/∂z|z)0 ) 0, ∂Ci/∂z|z)L ) 0 u|z)0 ) 0
∂Ci/∂z|z)0 ) 0, ∂Ci/∂z|z)L ) 0 ∂Ci/∂z|z)0 ) 0 -DL ∂Ci/∂z|z)L- ) u|z)L(Ci|z)L - Ci|z)L-) equalization to -DL ∂Ci/∂z|z)0+ ) u|z)0(Ci|z)0 - Ci|z)0+) ∂Ci/∂z|z)L ) 0 pressurization product pressurization -DL ∂Ci/∂z|z)0+ ) u|z)0(Ci|z)0 - Ci|z)0+) ∂Ci/∂z|z)L ) 0 feed pressurization -DL ∂Ci/∂z|z)0+ ) u|z)0(Ci|z)0 - Ci|z)0+) ∂Ci/∂z|z)L ) 0
u|z)L ) 0 u|z)L ) uOL u|z)L ) 0 u|z)L ) 0 u|z)L ) 0
The individual mass transfer resistance for a biporous adsorbent, in the case of negligible micropore resistance,8 is related to the LDF mass transfer coefficient by the following equation:
(
)( )
RP2 RP ∂qj*i 1 ) + Ki 3kfi 15εPDei ∂ci
Figure 3. Isotherms of pure oxygen on zeolite 5A.
(5)
Using the values kfi and Dei that were determined for the breakthrough and desorption runs, the mass transfer coefficient (Ki) was determined from eq 5. The adsorption equilibrium of oxygen and nitrogen was wellrepresented by a six-parameter Langmuir-Freuendlich isotherm: qi )
qmibiPin
(6)
n
1+
∑bP
nj
j i
All the variables and parameters were made dimensionless. The above set of partial differential equations were then converted to first-order ordinary differential equations (ODEs) of dimensionless time by discretizing all the spatial variables (dimensionless forms) using the orthogonal collocation method. A 12-point collocation was found to be a suitable compromise between minimizing the magnitude of oscillations in the solution of the derivative equations and the overall computation time. Starting from initial bed conditions, normally in equilibrium with the feed mixture at either the high or the low operating pressure, the system of ODEs were solved using Matlab software.
j
5. Results and Discussion Where, the isotherm parameters are functions of temperature: qmi ) ki + k2T,
bi ) k3 exp(k4 /T),
ni ) k5 + k6 /T (7)
5.1. Adsorption Isotherms and Heat of Adsorption. Figures 3 and 4 show the adsorption isotherms of oxygen and nitrogen on zeolite 5A at 273, 283, 303, and 343 K and pressure
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Figure 6. Steady-state experimental temperature profiles at the top of the column (PH ) 5 bar and cycle time ) 200 s).
Figure 4. Isotherms of pure nitrogen on zeolite 5A. Table 4. Parameters of Extended Langmuir-Freundlich Isotherm and Heats of Adsorption parameter
oxygen
nitrogen
unit
k1 k2 k3 k4 k5 k6 LDF coefficient Ki
14.31 -0.031 0.0005 1126.2 0.52 148.2 0.0098
10.95 -0.028 0.0016 1304.3 -0.74 521.4 0.0032
mol/kg mol/(kg K) 1/bar K K 1/s
0-9.5 bar. As expected, the results showed high selectivity for nitrogen over oxygen at the range of experimental conditions. The adsorption isotherm parameters of extended LangmuirFreundlich model and LDF coefficients of oxygen and nitrogen are also shown in Table 4. 5.2. Experimental and Simulation Results of Sample Runs. On the basis of the experimental and simulation results for a cycle time of 200 s and pressure of 5 bar as a sample of test runs, the general characteristics of the process are discussed. Figure 5 shows the pressure history of the first column during one cycle. As shown in this figure, the adsorption and equalization pressure are 5 and 2.3 bar, respectively. At all steps except the BD step, the pressure changes almost linearly. Temperature profiles in the top of the first column for the cycle time of 200 s and pressure of 5 bar are given in Figure 6. The temperature in the column has an increase in pressurization and adsorption, and a decrease in the depressurization and blowdown steps is observed. This figure shows that the temperature variation in the top of the column is less than 4 °C during whole cycle time.
Figure 5. Steady-state simulated and experimental pressure history at the exit end, for one column and one whole cycle.
Figure 7. Steady-state simulated velocity profile at the exit end, for one column and one entire cycle. Table 5. Operating Conditions of Experimental Runs for Oxygen Separation PH/PL (bar)
cycle time (s)
P/F ratio
product (L/min)
4/1
100/140/200
5/1
100/140/200
6/1
100/140/200
0.1 0.16 0.2 0.24 0.3 0.1 0.16 0.2 0.24 0.3 0.1 0.16 0.2 0.24 0.3
0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5 0.25/0.5/0.75/1/1.5
Considering the pressure drop and temperature variations to be negligible, a typical simulated velocity profile, based on boundary conditions of Table 4, will be similar to Figure 7. This figure shows a simulated velocity profile for the cycle time of 200 s and pressure of 5 bar. As mentioned in this figure, velocity values at the end of the column (Z ) L) are drawn. As shown in Figure 7, the velocity changes from positive to negative values according to flow direction. The first step is the adsorption step, and the other steps are shown in the order as indicated in Figure 7. 5.3. Pressure Swing Adsorption Experiments. A group of PSA experiments were carried out following the adsorption sequence shown in Figure 1. The experimental operating conditions are summarized in Table 5. According to the design specification, the feed rate was fixed at 18-20 L/min, and the cycle time was considered to be a variable parameter to obtain
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high-purity oxygen and determine the performance of the process. Three different pressures for adsorption steps are explored in this study. They include three cycle times each of five purge to feed ratios and five production rates. 5.3.1. Effect of Adsorption Pressure. The study of adsorption pressure influence on oxygen purity and recovery was carried out by varying the adsorption pressure at the same cycle time and production rate. The general conclusion is that, within our experimental range, oxygen purity and recovery both increase when adsorption pressure increases. Figure 8 shows the effect of adsorption pressure on process performance for a cycle time of 150 s, production rate of 0.75 L/min, and different purge to feed ratios. For each pressure, the oxygen purity increases but oxygen recovery decreases as purge to feed ratio increases. Also both oxygen purity and oxygen recovery increases as pressure increases. The reason is that when pressure increases, more nitrogen will be adsorbed by columns. Therefore, more oxygen is produced in the adsorption step and more nitrogen is exhausted during desorption steps. The reasonable agreement between experimental date and simulated results is shown in Figure 8. At a pressure of 6 bar the oxygen purity increases to about 95%, while the oxygen recovery decreases to around 23%. Chou et al.,6 Mendes et al.,9 Farooq et al.,10 and Ruthven et al.11 observed the same trend for both purity and recovery. 5.3.2. Effect of Purge Flow. The purging ratio, P/F, was defined as the ratio of the purge flow to feed flow rate of a cycle. Purging is an important step of PSA operation. Insufficient purging leads to insufficient regeneration of adsorbents and further leads to a decrease of product purity. Overpurging does not help improve the product purity but rather leads to a decrease
Figure 8. Effect of pressure on oxygen purity and recovery.
Figure 9. Effect of purge to feed ratio on oxygen recovery and purity.
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Figure 10. Effect of production rate on oxygen recovery and purity.
in product recovery. The recovery and purity of oxygen at a certain feed flow rate depends on the amount of gas used to purge the column. In this study for all groups of test runs, the effect of the purge flow was considered. For most of them at the highest purge rate, a higher purity of oxygen is obtained, but the recovery of the oxygen is reduced. The effect of the P/F ratio on the purity and recovery of oxygen at a pressure of 5 bar, cycle time of 200 s, and production rate of 0.25 L/min is presented in Figure 9. The simulated values of oxygen purity and recovery as a function of P/F ratio also corresponds well with the experimental data (Figure 9). The function of the purge step is to clean the bed by purging the column with part of the product stream produced during adsorption step. The cleaner the bed becomes, the higher the purity of the product. The bed can be more thoroughly cleaned by simply using more purge gas. On the other hand, loss of oxygen increases as purge flow increases. Hence, oxygen recovery is reduced if more purge flow is used. Chou et al.6 and Mendes et al.8 observed the same trend for both purity and recovery. 5.3.3. Effect of Production Rate. The influence of the product flow rate on product purity and recovery was discovered by varying the production rate at a certain feed rate and purge to feed ratio. Figure 10 shows the effect of production rate on process performance for a cycle time of 200 s, 6 bar pressure, and purge to feed ratio of 0.24. When the purge flow remains constant, oxygen recovery increases as the production rate increases, because in this case loss of oxygen in tail gas is reduced. On the other hand, increasing production rate increases the nitrogen amount in purging gas and therefore oxygen purity will be reduced. 4.3.4. Effect of Cycle Time. In the PSA process, adsorption takes place in both the pressurization and adsorption steps, and also, desorption and cleaning take place in the depressurization and purge steps. The choice of each step time has a great effective on process performance. Sufficient cycle time should be provided, so that the desired product purity and recovery can be achieved. The effect of cycle time on process purity, recovery, and productivity are shown in Figures 11 and 12. The main feature of these plots is that, at low cycle time (100 s), no high oxygen purity, recovery, and productivity can be obtained. Among the three cycle times (100, 150, and 200 s) tested, Figure 11 shows that increasing the cycle time increases the performance of the process. The duration of the cycle time in Figure 11 for a short cycle is not large enough to equilibrate the particle with its environment, and thus, depressurization and pressurization after the first cycle time are initialized from a nonequilibrated state. This leads to a decreased process performance. In Figure 12, productivity lines for cycle time of 150 and 200 s cross each other at about 90% purity. This means that at low
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and prediction of process performance have shown a slightly different behavior especially for different cycle times. Acknowledgment The authors are thankful to Linde AG, Linde Engineering Division, Germany, and Gostaresh Co. Ltd., Iran, for supplying zeolite particles. Notation
Figure 11. Effect of cycle time on oxygen purity and recovery.
Figure 12. Effect of cycle time on oxygen purity and productivity.
purity (less than 90%) process productivity for a cycle time of 150 s is higher than that for cycle time of 200 s. The comparison between experimental data with results obtained from simulation can be seen in Figures 11 and 12. Model predictions in terms of cycle time are quite far from experimental results especially for cycles of 150 and 200 s. 6. Conclusions Oxygen production from air in a four-bed pressure swing adsorption setup on a commercial type zeolite 5A has been studied both experimentally and numerically over a wide range of operating conditions. A PSA unit using seven steps allowed the production of high-purity oxygen directly from air. Influence of adsorption pressure, purge to feed ratio, production rate, and cycle time on process performance were investigated experimentally and were compared with simulation results. For our experimental range, oxygen recovery and purity increases when adsorption pressure increases from 4 to 6 bar. For a cycle time of 200 s, the P/F ratio between 0.2 and 0.3 guarantees purities higher than 90%, while pressure and production rate are kept at 5 bar and 0.25 L/min, respectively. The cycle time has a considerable effect on the oxygen purity and recovery. The performance of the PSA process investigated was better at a higher cycle time. Furthermore, the oxygen recovery increases and oxygen purity decreases when the production rate increases. Although a considerable agreement has been found between the experimental data and simulated results of oxygen purity and recovery for various operating parameters, results of experiments
bi ) equilibrium parameter for Langmuir-Freuendlich model C ) gas phase concentration (mol/m3) ci ) concentration of ith component in the gas phase (mol/m3) Dei ) effective macropore diffiusivity (cm2/s) dp ) particle diameter (cm) k ) parameter for Langmuir-Freuendlich model K ) proportionality parameter for LDF model (1/s) kfi ) effective mass transfer coefficient of ith component n ) parameter for Langmuir-Freuendlich model P ) pressure (bar) q*i ) equilibrium amount adsorbed (mol/kg) qi ) amount adsorbed of ith component (mol/kg) qmi ) saturation loading (mol/kg) Rp ) radius of the particle (cm) t ) time (s) u ) interstitial velocity (m/s) z ) distance along the length of the column (m) Greek Letter ε ) bed void fraction εp ) particle porosity Fp ) density of particles (kg/m3)
Literature Cited (1) Rege, S. U.; Yang, R. T.; Buzanowski, M. A. Sorbents for air prepurification in air separation. Chem. Eng. Sci. 2000, 55 (21), 4827. (2) Jee, J. G.; Lee, S. J.; Kim, M. B.; Lee, C, H. Three-Bed PVSA Process for High-Purity O2 Generation from Ambient Air. AIChE J. 2005, 51 (11), 2988. (3) Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Boston, MA, 1987. (4) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: New York, 1994. (5) Silva, J. A. C.; Rodrigues, A. E. Sorption and Diffusion of n-Pentane in Pellets of 5A Zeolite. Ind. Eng. Chem. Res. 1997, 36, 493. (6) Chou, C. T.; Huang, W. C. Simulation of a Four Bed Pressure Swing Adsorption Process for Oxygen Enrichment. Ind. Chem. Eng. Res. 1994, 33, 1250. (7) Jain, S.; Moharir, A. S.; Li, P.; Wozny, G. Heuristic Design of Pressure Swing Adsorption: Preliminary Study. Sep. Purif. Technol. 2003, 33, 25. (8) Iyuke, S. E.; Daud, W. R. W.; Mohamad, A. B.; Kadhum, A. A. H.; Fisal, Z.; Shariff, A. M. Application of Sn-activated carbon in pressure swing adsorption for purification of H2. Chem. Eng. Sci. 2000, 55 (20), 4745. (9) Raghavan, N. S.; Ruthven, D. M. Numerical simulation of a fixedbed adsorption column by the method of orthogonal collocation. AIChE J. 1983, 29 (6), 922. (10) Mendes, A. M. M.; Costa, C. A. V.; Rodrigues, A. E. Oxygen Separation from Air by PSA: Modelling and Experimental Results Part I: Isothermal Operation. Sep. Purif. Technol. 2001, 24, 173. (11) Farooq, S.; Ruthven, D. M.; Boniface, H. A. Numerical simulation of a pressure swing adsorption oxygen unit. Chem. Eng. Sci. 1989, 44, 2809. (12) Ruthven, D. M.; Farooq, S. Air separation by pressure swing adsorption. Gas Sep. Purif. 1990, 4, 141.
ReceiVed for reView November 25, 2008 ReVised manuscript receiVed April 5, 2009 Accepted April 29, 2009 IE801805K
