Pressure Swing Adsorption - American Chemical Society

Ind. Eng. Chem. Res. 2001, 40, 3647-3658

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Air Separation by a Small-Scale Two-Bed Medical O2 Pressure Swing Adsorption Jeong-Geun Jee, Jong-Seok Lee, and Chang-Ha Lee* Department of Chemical Engineering, Yonsei University, Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea

A small-scale two-bed six-step pressure swing adsorption (PSA) process using zeolite 13X was performed to provide oxygen-enriched air in the medical system. The binary mixture N2/O2 (79/ 21 vol %) was used for PSA experiments. Cyclic behaviors of the PSA process were investigated from unsteady- to steady-state conditions. Also, effects of various operating parameters on the PSA performance such as the P/F ratio, adsorption pressure, feed flow rate, and adsorption step time were investigated experimentally under the nonisothermal condition. The effect of the P/F ratio was noticeably changed according to the adsorption pressure and feed flow rate conditions. The higher the adsorption pressure, the slower the increasing rate of purity and the higher the decreasing rate of recovery. However, as the adsorption pressure became higher, the effect of the P/F ratio on the O2 purity became smaller. Furthermore, the effect of adsorption pressure on the O2 purity and recovery was diminished gradually to the increase of the P/F ratio. The feed flow rate also had a strong effect on the O2 purity. As for the product purity, the low feed flow rate began to lose its advantage with an increase in the P/F ratio. The recovery and productivity at a high feed flow rate was higher than those at a low feed rate even under the high product purity region. The dominant operating factor to determine the O2 purity was changed from the adsorption pressure to the feed flow rate as the P/F ratio was changed from low to high values. The modified linear driving force (LDF) model similar to a solid-diffusion model predicted the transition behavior of the cyclic process better than the LDF model. Introduction Numerous pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) processes have been designed during the last 30 years for the production of an oxygen-enriched air stream from ambient air.1-3 In the industrial O2 PSA units, the dominant factor to determine the energy requirement of a PSA cycle is the pressure ratio of the high adsorption pressure to the low desorption pressure. A pressure ratio of 4 or higher has been used in industry, which has appeared to be a barrier for PSA air separation.4 The performances of the previous studies on the O2 PSA were about 93-99% purity with 8-44% recovery from the O2/N2 mxiture.5-7 However, to recover the energy consumption by the high adsorption pressure and low recovery, the VSA process8,9 was developed. The O2 VSA showed about 94% purity with 55+% recovery.10 In addition, while the adsorbent productivity of rapid PSA (RPSA) using the short cycle times is much higher at the same recovery and purity conditions than that of conventional PSA, the energy demand is much higher than that of conventional PSA because of the frequent pressure variation.11,12 As another application of PSA, the small-scale units of medical O2 PSA1 with higher than 80% O2 purity were commercialized for patients with chronic pulmonary dysfunction in the mid-1970s. Also, a small-scale application of PSA was extended to provide oxygenenriched air for the crew of a military aircraft.13 Most small-scale units use a two-bed system, operated on a * To whom correspondence should be addressed. Tel: +822-2123-2762. Fax: +82-2-312-6401. E-mail: [email protected].

Skarstrom cycle, sometimes with the addition of a pressure equalization step for a significant improvement in recovery.3 At the scale of domestic medical O2 units, the cost of power is a less significant consideration than process simplicity and reliability. Teague and Edgar13 presented the dynamic response of the bed pressure history and prediction of transient behavior in an on-board oxygen generation (OBOG) system by using a mathematical model including the feed and exhaust valves and purge orifice. They suggested that the magnitude of the temperature swing over a cycle is significantly greater than that observed experimentally in the OBOG system with 10-40 s cycle time. Recently, Mendes et al.5 have focused on the influence on the product purity and recovery of the pressure rising rate during pressurization, pressure lowering rate during the blowdown step, production pressure, and intraparticle viscous flow in an O2 PSA with zeolite 5A. They found that a higher pressurization rate decreases the product purity and recovery, whereas the pressure lowering rate during the blowdown step has almost no effect. Also, they pointed out that the production pressure has a complex effect on the product purity and recovery. However, the influence of the pressure rising rate on the purity and recovery was not observed in the case of O2 PSA using very large pressurization times (30 and 75 s), adsorbent with small diameters, and adsorbent with large average pore diameters.6,14,15 Also, in PSA processes with kinetic separation effects, the pore diffusion model has been used to enhance the simulation accuracy despite the bulky computations.16-18 For cyclic separation processes with short cycle periods, a modification of the linear

10.1021/ie010101l CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001

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Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001

driving force (LDF) equation has been suggested by Nakao and Suzuki (1983)19 and Alpay and Scott (1993).20 Moreover, Kim and co-workers21-23 suggested both the linear and higher approximations based on the biporous diffusion model for short cycle periods, which are functions of the cycle period alone. In this study, a binary mixture that consisted of 21% by volume O2 and 79% by volume N2 was separated by a small-scale two-bed PSA with a small-size zeolite 13X (20-32 mesh) as an adsorbent. The PSA process was performed to produce higher than 80% O2 purity using the traditional Skarstrom cycle with an equalization step to improve the energetic performance. The effects of typical operating parameters on a medical O2 PSA performance, such as the adsorption pressure, P/F ratio, and adsorption step time, were studied experimentally. Also, the effects of both the adsorption pressure and feed flow rate on the purity, recovery, and productivity were investigated at a constant P/F ratio. Because the medical O2 producing system needs to supply oxygen very fast, the PSA cycle time applied for this study was less than 36 s/cycle. The predicted results by a LDF model were compared to those by the modified LDF model based on the pore diffusion model. The dynamic characteristics of the PSA process were also investigated through the values such as the temperature history in the bed, pressure, and concentration variations from unsteady- to steady-state conditions. Mathematical Models To understand the dynamic behaviors of the adsorption bed during the PSA running, the mathematical models were developed by the following assumptions: (i) the gas phase behaves as an ideal gas mixture, (ii) radial concentration and temperature gradients are negligible, (iii) thermal equilibrium between adsorbents and bulk flow is assumed, (iv) the flow pattern is described by the axially dispersed plug-flow model, (v) the mass-transfer rate is represented by the LDF and modified LDF models, and (vi) the pressure drop along the bed was considered by using the Ergun equation. The assumption of neglecting the radial gradient was widely accepted by numerous studies, and the others are also common assumptions in simulating the adsorption process.24,25 The material balance for the bulk phase in the adsorption column is represented by

-DL

∂2Ci ∂z

2

+

∂(uCi) ∂Ci 1 - ∂qi + + Fp )0 ∂z ∂z ∂t

(

)

∂2 P ∂z

2

+

∂P

+P

∂t

()

∂ 1

∂t T

∂u

+u

∂z +u

( )]

∂ 1

∂z T

∂P ∂z

[

+ PT -DL

+ FpRT

∂z

( )∑ 1-

()

∂2 1

n

i)1

2

∂qi ∂t

+

T

) 0 (2)

∂yi

+u

∂z2

+

∂z

∂yi

+

∂z Fp

( )(

RT 1 - ∂qi P

∂t

n

- yi

)

∂qi

∑ i)1 ∂t

) 0 (3)

where DL is the axial dispersion coefficient calculated from the following Wakao equation.26,27

DL 20 + 0.5 ) 2uRp ReSc

(4)

Another characteristic of the adsorption process is the temperature variation caused by the heat of adsorption. In this system, the energy balance for the gas phase includes the heat transfer to the column wall:

-KL

(

∂2T

+ FpCpg u

∂z2

FBCpg)

∂T

+T

∂z

∂z n

∂T

- FB(-∆Hi)

∂t

)

∂u

∂qi

∑ i)1 ∂t

+ (tFgCpg +

+

2hi RBi

(T - Tw) ) 0 (5)

where t is the total void fraction [) + (1 - )p], FB is the bed density, [)(1 - )Fp], and hi is the internal heattransfer coefficient. To consider the heat loss through a wall and the heat accumulation in the wall, another energy balance for the wall of the adsorption bed was used.

FwCpwAw

∂Tw ) 2πRBihi(T - Tw) ∂t 2πRBoho(Tw - Tatm) (6)

where Aw ) π(RBo2 - RBi2). However, in this study, the adsorption bed was constructed by an aluminum pipe for heat dissipation. Therefore, if the system is operated at near isothermal condition, eqs 5 and 6 can be neglected to save the calculation time. To consider the pressure drop effect across the bed, Ergun’s equation was introduced as a momentum balance.28

-

dP ) aµu + bFu|u| dz

(7-1)

2

(1)

When the ideal gas law (ci ) Pyi/RT) is applied to eq 1, the overall and component mass balances can be represented as follows:

-DL

-DL

∂2yi

a)

1- 150 (1 - ) , b ) 1.75 3 2 4Rp 2Rp23

(7-2)

where u is the interstitial velocity. The boundary and initial conditions of mass and energy balances are presented below. The well-known Danckwerts boundary conditions are applied.29

Boundary conditions for feed pressurization and adsorption steps -DL

( )| ∂yi ∂z

z)0

) u(yi|z)0- - yi|z)0+);

( )| ∂yi ∂z

z)L

)0 (8-1)

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3649

-KL

(∂T∂z )|

(∂T∂z )|

) Fg(Cp)gu(T|z)0- - T|z)0+);

z)0

z)L

)0

(8-2)

where yi|z)0- is the feed composition for component i.

Boundary conditions for purge, pressurizing pressure equalization steps -DL -KL

( )| ∂yi ∂z

(∂T∂z )|

z)L

) u(yi|z)L+ - yi|z)L-);

( )| ∂yi ∂z

z)0

)0 (9-1)

) Fg(Cp)gu(T|z)L+ - T|z)L-);

z)L

(∂T∂z )|

z)0

) 0 (9-2)

where yi|z)L+ is the volume-averaged composition of the effluent stream during the adsorption step for the purge step and a temporal effluent’s composition during a depressurizing pressure equalization step for the pressurizing pressure equalization step, respectively. The fluid velocity is inherently negative during these steps.

Depressurizing pressure equalization and countercurrent depressurization steps

( )| ( )| ∂yi ∂z

)

z)0

∂T ∂z

z)0

) 0;

( )| ∂yi ∂z

) z)L

( )| ∂T ∂z

z)L

)0 (10)

In this study, the pressure history obtained during a PSA experiment at the product end was fitted by polynomials used as a boundary condition for the momentum balance. The sorption rate into an adsorbent pellet is described by the LDF model with a single lumped mass-transfer parameter:

KDe ∂qi ) ωi(q/i - qi), ωi ) ∂t R2

(11)

p

where ωi is a lumped mass-transfer coefficient inside the adsorbent and De is the effective diffusivity defined in a homogeneous solid-diffusion model.30 Also, the below modified LDF model22 was applied for the sorption rate instead of eq 11 because the cycle time in this study was relatively short. The modified LDF model is obtained from the high-order approximation of a pore diffusion model, while the LDF model is the first-order approximation. Therefore, it is reported that the modified LDF model is sufficiently accurate compared to the pore diffusion model and is as easy as the LDF model to use.22

∂qi ) θi(-105qi + z1 + 42q/i ) ∂t ∂z1 ) θi(945)(q/i - qi) ∂t where

θi ) De/Rp2

(12)

Table 1. Characteristics of Adsorbents and Adsorption Bed adsorbent

zeolite 13X

type normal pellet size [mesh] average pellet size, Rp [cm] pellet density, Fp [g/cm3] heat capacity, Cps [cal/g‚K] particle porosity, R bed density, FB [g/cm3]

sphere 20-32 0.07 1.17 0.32 0.21 0.713

Adsorption Bed length, L [cm] inside radius, RBi [cm] outside radius, RBo [cm] heat capacity of the column, Cpw [cal/g‚K] density of the column, Fw [g/cm3] internal heat-transfer coefficient, hi [cal/cm2‚K‚s] external heat-transfer coefficient, ho [cal/cm2‚K‚s]

50 2.5 2.75 0.216 2.7 9.2 × 10-4 3.4 × 10-4

The multicomponent adsorption equilibrium was predicted by the following extended Langmuir-Freundlich model:

qmiBiPini

qi )

n

1+

(13)

BjPnj ∑ j j)1

where qm ) k1 + k2T, B ) k3 exp(k4/T), and ni ) k5 + k6/T. Experimental Section The characteristics of the adsorbent and adsorption bed are shown in Table 1. The adsorbent used in this study was zeolite 13X (20 × 32 mesh, Oxy5). A moisturefree commercial gas mixture (O2/N2, 21/79 vol %) was used as the feed gas. The adsorption isotherms of pure O2 and N2 on zeolite 13X were measured at three different temperatures (293, 303, and 313 K) by using a high-pressure volumetric apparatus up to 650 kPa. This apparatus consists of two stainless steel vessels: a doser cell and an adsorber cell. The pressure variations of these cells were measured by pressure transmitters (Heise 621). The adsorbent was regenerated at 623 K in a furnace after every experiment. The cyclic sequence and step time for the six-step process and a simple flow diagram are illustrated in Figure 1. A typical six-step, two-bed PSA process was applied for the oxygen product from air: (I) feed pressurization (FP), (II) high-pressure adsorption (AD), (III) depressurizing pressure equalization (DPE), (IV) countercurrent depressurization (DP), (V) purge with a light product (PG), and (VI) pressurizing pressure equalization (PPE).31 All of the PSA experiments were performed at the same cyclic step time with 10 s adsorption step time and purge step time. However, to investigate the effect of step time, the two step times were changed from 10 to 5 s only in the effects of the adsorption step-time section. The duration of nonisobaric steps such as FP, PE, and DP steps was determined at 4 s, which was a minimum time to reach a stable pressure at each step. Also, the schematic diagram of the PSA unit used in this study is shown in Figure 2. A two-bed PSA unit constructed in this study was similar in size to that of a medical oxygen generation system. It was possible to

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Figure 2. Flow diagram and cycle sequence of a six-step PSA process. Values in parentheses are step times. Figure 1. Schematic diagram of the apparatus for a two-bed O2 PSA process. Table 2. Operating Conditions for O2 PSA Experiments run no.

adsorption time [s]

1 2 3 4

10

adsorption pressure [atm] 3

5 6 7 8 9 10 11 12 13 14 15 16

4

17 18 19 20 21 22

P/F ratio

feed flow rate [L(STP)/min]

0.67 0.75 0.82 0.90

7.5

0.60 0.63 0.69 0.75 0.84 0.94

9.5

0.63 0.70 0.75 0.76 0.83 0.93 0.60 0.67 0.70 0.76 0.83 0.94

23 24 25 26 27 28 29

5

30 31 32 33 34 35

10

11.5

0.60 0.64 0.69 0.73 0.78 0.85 0.92 5.5

0.66 0.71 0.72 0.77 0.83 0.90

13.1

implement step changes in the following operating variables: feed pressure, feed rate, purge rate, and cycle time. The adsorption beds were made of an aluminum pipe with a 50 cm length, 5 cm i.d., and 0.49 cm wall thickness. The calibrated three resistance temperature detectors (RTD; Pt 100 Ω) were installed at the center

Figure 3. Isotherms of (a) pure O2 and (b) pure N2 at zeolite 13X.

radial positions of 10, 25, and 40 cm from the feed end to measure temperature variations inside the bed. The bed pressure and pressure drop were measured by the two pressure transducers located at the feed and product ends. The feed flow rate was controlled by a metering valve, and the other feed line was used to reach the desired pressure in a feed pressurization step. Also, the surge tank with the same size of adsorption bed was equipped to prevent flow fluctuation. The


Figure 4. Experimental and predicted temperature curves in the middle of the adsorption bed at 4 atm adsorption pressure and 9.5 L(STP)/min feed flow rate.

Figure 6. Experimental and predicted O2 concentration curves at 5.5 atm, 13.1 L(STP)/min, and 0.77 P/F ratio.

Figure 5. Experimental and predicted O2 concentration curves at 4 atm, 9.5 L(STP)/min, and 0.75 P/F ratio. Table 3. Isotherm Parameters, LDF Coefficient ωi ()KDe/Rp2), and Modified LDF Coefficient θi ()De/Rp2) for Zeolite 13X N2 L-F Isotherm Parameter k1 × 103 (mol/g) 12.52 k2 × 105 (mol/g‚K) -1.785 k3 × 105 (1/atm) 2.154 k4 (K) 2333 k5 1.666 k6 (K) -245.2 heat of adsorption, QI (cal/mol) 4390 Mass-Transfer Coefficient LDF coefficient, ωi (s-1) 0.197 modified LDF coefficient, θi (s-1) 0.01203

O2 6.705 -1.435 3.253 1428 -0.3169 387.8 3060 0.62 0.04133

metering valve installed at a pressure equalization line was used to control the flow rate and the step time during a pressure equalization step. The total amounts of feed flow and the flow rate of each step were measured by a wet gas meter. To keep the bed pressure constant, a back-pressure regulator was installed between the adsorption bed and the product line. The concentrations of the influent and effluent flows were analyzed by a portable oxygen analyzer (Teledyne

Figure 7. O2 concentration profiles along the adsorption bed at 4 atm, 9.5 L(STP)/min, and 0.70 P/F ratio (20th cycle) in (a) isothermal and (b) nonisothermal conditions.

Analytical Instruments, 320B/RC-D). The PSA unit and supporting equipment were connected by a 1/4 in. copper tube. The system was fully automated by a personal computer with a developed control program, and all measurements including pressure, temperature, and O2 purity were saved on the computer through an AD converter. After the bed was packed with the adsorbent regenerated at 613 K (during) overnight, the bed was kept at 1.5 atm O2 condition to prevent contamination from the impurities in the atmosphere. Prior to each experimental run, the adsorption bed was vacuumed up to 10-3 mmHg for 2 h. The operating region corresponded to

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Figure 8. Pressure variation history in the cyclic operation at 3 atm adsorption pressure.

Figure 10. O2 concentration profiles along the adsorption bed at (a) the unsteady-state cycle (second cycle) and (b) the steady-state cycle (20th cycle) under 4 atm, 9.5 L(STP)/min, and 0.76 P/F ratio.

The adsorption isotherms of N2 and O2 on zeolite 13X are shown in Figure 3. The adsorption isotherm parameters and the LDF and modified LDF coefficients of N2 and O2 on zeolite 13X are also shown in Table 3. Results and Discussion

Figure 9. Comparisons of concentration variation using the modified LDF model among three different P/F ratios of (a) 0.60, (b) 0.67, and (c) 0.83 at 4 atm and 11.5 L(STP)/min.

an adsorption pressure of 3-5.5 atm, a feed flow rate of 7.5-13.1 L(STP)/min, and a P/F ratio of 0.60-0.94. These operating ranges were investigated in the previous studies related to O2 PSA.4,7,13 The more detailed operating conditions are shown in Table 2. O2 purity was defined as the volume-average amount of the product in the adsorption step. O2 productivity and recovery were defined as follows.3

O2 productivity ) O2 recovery )

O2 [mole] in the product kg of adsorbent‚cycle

O2 amount in the product O2 amount in the feed

Also, the P/F ratio was defined as follows.

P/F ratio )

O2 amount in the purge step O2 amount in the adsorption step

Cyclic Steady State of a Small-Scale O2 PSA. Along with the concentration of the effluent stream, the temperature variation inside the adsorption bed represented a cyclic behavior of a PSA process. The temperature variation located at 25 cm from the feed end is presented in Figure 4. The temperature excursion at the initial cycle showed a relatively small extent of temperature swing. This implied that the concentration wave fronts did not proceed much from the feed end because of the vacuum initial condition. However, the temperature excursion of the second cycle was higher than that of the first one because a considerable amount of adsorbates at this location underwent adsorption and desorption during the cycle. Then, the maximum temperature of the temperature swing was decreased with an increase in the cycle. As shown in Figure 4, the temperature variation did not approach a cyclic steady state even after 20 cycles. However, the range of the temperature swing was about 2 K, and the difference between temperature excursions was less than 0.5 K after 15 cycles. The experimental data were compared with the simulated results using the LDF model in eq 11 and the modified LDF model in eq 12. The modified LDF model predicted accurately the experimental results more than the LDF model. Because this modified LDF model is the higher approximation of the pore


Figure 11. Comparisons of O2 (a) purity, (b) recovery, and (c) productivity between 3 atm, 9.5 L(STP)/min and 4 atm, 9.5 L(STP)/ min at various P/F ratios.

diffusion model, this model showed better prediction than the LDF model for the relatively fast cycle process. Also, the predicted temperature curve by the modified LDF model showed a more symmetric form than that of the LDF model, even though the difference between the predicted results by both models was small. This is because the PSA system with a very short cycle time showed a linear increase and a linear decrease in the concentration and temperature, showing almost a symmetric curve.22,32 Figures 5 and 6 show a representative cyclic behavior by comparing the experimental concentration with the predicted concentration variation. Compared to the temperature variation in Figure 4, the composition of the effluent stream approached rapidly to a cyclic steady state. After approximately 15 cycles, the performance difference between the last two cycles was less than 0.05%. As can be expected in Figure 3, the selectivity change in the experimental range using zeolite 13X is almost negligible under the condition of less than 2 K temperature excursion. This implies that the system behaves almost isothermally, as was already observed by Kapoor and Yang (1989)33 and Mendes et al. (2000).5 Therefore, the composition variation was not affected by the temperature variation after certain cycles. Also, the product purity predicted by the modified LDF model was more accurate than that of the LDF model as shown in Figures 5 and 6. In the present study, all of the experimental data were collected at above 15 cycles. Figure 7 shows a comparison between isothermal and nonisothermal oxygen concentration profiles. Because the bed temperature was steeply increased by the adsorption of N2 during a short FP step time (4 s), the concentration profile in the nonisothermal condition

Figure 12. Effects of adsorption pressure on O2 concentration profiles along the adsorption bed in the adsorption step at 9.5 L(STP)/min feed flow rate, (a) low P/F ratio (0.65) and (b) high P/F ratio (0.90) conditions.

proceeded to the product end more than that in the isothermal condition. Except for this step, the concentration profiles at the same step under both conditions showed very similar behaviors. Because of the small heat of adsorption and the aluminum pipe with a high heat-transfer rate, it is implied that this PSA system was operated at near isothermal condition. In cyclic operation, the prediction of cyclic pressure variation is very important because the pressure variation has a great effect to determine adsorption amounts and adsorption rates. Figure 8 shows the pressure profile at the feed end during the cyclic operation. To effectively predict the pressure variation in cyclic operation, an empirical polynomial model obtained from an experimental pressure profile was applied to the simulation. As can be seen in Figure 8, the predicted pressure profile agreed satisfactorily with the experimental value. Furthermore, the predicted pressure history by neglecting Ergun’s equation was almost the same as that of Ergun’s equation even though a short cycle time and a densely packed bed were used. This result implies that the pressure drop in this system can be neglected because of the short length and large diameter of the bed. Figure 9 shows the effect of the P/F ratio on the concentration variation from unsteady to steady state. While the product purity at the first cycle was little affected by the P/F ratio, the effect of the P/F ratio on the purity was very conspicuous with an increase in the cycle. Also, the higher the P/F ratio, the longer the time to reach the steady state. The predicted O2 concentration profiles in the gas phase at the end of each step are presented in Figure

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Figure 13. Comparisons of O2 (a) purity, (b) recovery, and (c) productivity between 7.5 and 9.5 L(STP)/min feed flow rates at 3 atm and various P/F ratios.

Figure 14. Comparisons of O2 (a) purity, (b) recovery and (c) productivity between 9.5 and 11.5 L(STP)/min feed flow rates at 4 atm and various P/F ratios.

10a for the unsteady-state cycle and Figure 10b for the steady-state cycle. In Figure 10a, the O2 concentration wave front at the end of a pressurization step proceeded at the end of the bed. The unfavorable wave front had a detrimental effect on the adsorption step because the mass-transfer zone (MTZ) was very wide even at the adsorption step. However, about a 65% O2 purity stream at the end of a DPE step was introduced to the next bed, and the N2-rich stream at the end of the DP step was exhausted at the feed end. In the early cycle, the purge step to increase the product purity affected the concentration profile mostly at the product end, but the concentration profile of a DP step keeps more or less its shape at the feed end. Also, the concentration profiles at all steps showed concave shapes except at a PPE step. At the end of a PPE step, the product end was contaminated again with a convex shape of the O2 concentration profile. This lowers the product purity at the beginning of the adsorption step until the cyclic steady state. However, in a cyclic steady state as shown in Figure 10b, the O2 concentration profile at each step showed an asymptotic shape with a high O2 purity at the product end.10 Especially, the concentration profile at an FP step located behind that of an AD step and the O2 concentration wave fronts at PPE and FP steps became steeper than those in the cyclic unsteady state. This played a very important role in producing a highpurity product because the contamination of the product end by strongly adsorbed components during the desorption steps had bad effects on the adsorption step of the next cycle. Therefore, the product purity was increased from the cyclic unsteady state to the cyclic steady state as shown in Figure 7.

Effects of the Operating Variables on SmallScale O2 PSA. In this study, the effects of the operating variables on a PSA performance such as the P/F ratio, adsorption pressure, feed flow rate, and adsorption step time were studied experimentally and numerically under the cyclic steady-state condition. (i) Effects of the P/F Ratio. The effect of the P/F ratio on the O2 purity, recovery, and productivity is shown in Figures 11, 13, 14, 16, and 18. In all of the figures, as the P/F ratio was increased, the O2 purity was improved and the O2 recovery was decreased linearly.4 However, at each adsorption pressure, the purity was not increased despite an increment in the P/F ratio above the limiting P/F ratio (about 0.8-0.9). However, the O2 productivity was decreased linearly to the increment of the P/F ratio. The purging of the bed with the product allowed for a sharper O2 wave front during the feed step by cleaning the bed of the impurity, thus increasing the product purity. Although the purity of oxygen depicted in all of figures exceeded 99% purity, argon in real air should be considered. Hence, the actual purity of O2 that would be obtained in a real case is about 95-96%. (ii) Effects of the Adsorption Pressure. Figure 11 shows the effect of an adsorption pressure on O2 PSA performance under various P/F ratios. In Figure 11a, the adsorption pressure in the range of low P/F ratio affected significantly the product purity, while the product purity was not affected by the adsorption pressure in the range of high P/F ratio. The difference of the recovery and productivity between both operating conditions decreased with an increase in the P/F ratio. The decline of recovery with an increase in the adsorption pressure is mostly due to the loss of O2 in the feed


Figure 15. Effects of the feed flow rate on O2 concentration profiles along the adsorption bed in the adsorption step at 3 atm, (a) low P/F ratio (0.65), and (b) high P/F ratio (0.90) conditions.

Figure 16. Comparisons of O2 (a) purity, (b) recovery, and (c) productivity among three different adsorption pressure and feed flow rate conditions at various P/F ratios.

end during a DP step. In the low P/F ratio region, the O2 productivity at 4 atm adsorption pressure was larger than that at 3 atm adsorption pressure in Figure 11c because the total capacity for O2 product and the O2 purity was increased with pressure. However, in the high P/F ratio region, the difference of productivity between both operating pressure conditions became almost the same because the difference of product purity and recovery was notably diminished. These results can be clearly explained by Figure 12, which represents the predicted axial profiles of the O2 mole fraction in the gas phase at the end of the adsorption step. Because the slope of the MTZ became steeper at higher pressure, beds can be used more effectively and at the same time the product purity becomes higher. In the case of the low P/F ratio, the difference between concentration profiles becomes smaller as the pressure becomes higher than 4 atm in Figure 12a. Meanwhile, Figure 12b shows that this difference in high P/F ratio becomes small from above 3 atm. Therefore, because the large amount of O2 in the purge step was consumed at the high P/F ratio region and the difference of O2 loss at both adsorption pressure conditions during a DP step was relatively small, the difference of recovery and productivity between both adsorption pressure conditions became smaller with an increase in the P/F ratio as shown in Figure 11. (iii) Effects of the Feed Rate. Figures 13 and 14 show the effects of the feed rate on the product purity, recovery, and productivity at two adsorption pressure conditions. In these two figures, the purity at low feed rate was higher than that at high feed rate because the contact time between feed flow and adsorbent was increased when the feed gas flowed slowly. Also, the

results showed that the difference of purity between two feed rates at 4 atm adsorption pressure was smaller than that at 3 atm adsorption pressure. This implies that the effect of the feed flow rate was gradually diminished with an increase in the adsorption pressure due to the limiting amount of adsorption. In Figures 13b and 14b, O2 recovery was decreased in the low feed flow rate because the large purge amount was needed to remove the strongly adsorbed N2. The recovery difference between the two feed rates decreased with an increase in the adsorption pressure. Also, this difference in the recovery was maintained almost constantly independent of the change of the P/F ratio. This is because the net amount of product at the high feed rate was larger than that at the low feed rate. Figures 13c and 14c show productivity in the range of the experimental P/F ratio. The productivity of the higher feed rate process was higher than that of the lower feed rate independent of the adsorption pressure. Also, although the product purity was changed by the P/F ratio, the productivity showed a parallel decrease in both feed rate processes due to the parallel decrease in the recovery. It is noted that the recovery and productivity at the higher feed rate process was higher than those at the lower feed rate even under the high product purity region, which is different from the effects of the adsorption pressure in Figure 11. However, as for the product purity, the low feed rate process began to lose its advantage with an increase in the P/F ratio. This result was confirmed by the O2 mole fraction curves of adsorption step at various feed flow rates in Figure 15. In the low P/F ratio condition, the MTZ of O2 became wide with an increase in the feed

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Figure 17. Effects of adsorption pressure and feed flow rate on O2 concentration profiles along the adsorption bed in the adsorption step at (a) low P/F ratio (0.65), (b) intermediate P/F ratio (0.75), and (c) high P/F ratio (0.90) conditions.

rate due to the small amount of the purge. Therefore, the product end at the adsorption step was significantly contaminated by the increased feed rate. On the contrary, as shown in Figure 15b, the product end section was kept clean in the high P/F ratio condition by keeping the concentration wave fronts far from the product end until 9.5 L(STP)/min. Also, the MTZ of O2 showed almost the same interval with an increase in the feed rate. Consequently, the need to make the product end section clean by large amounts of purge was reduced at this operating condition. (iv) Simultaneous Effects of the Feed Flow Rate and Adsorption Pressure. Figure 16 shows a change of the process performance with a change of both adsorption pressure and feed flow rate. As can be seen in this figure, the O2 purity in the low P/F ratio was higher at higher adsorption pressure and feed flow rate conditions. However, the purity was crossed over with an increase in the P/F ratio. As mentioned in Figures 11, 13, and 14, the effect of the P/F ratio on the purity was delineated with an increase of the adsorption pressure or feed flow rate. This phenomenon caused the crossover in the O2 purity. Figure 17 shows the O2 mole fraction curves of the adsorption step after 20 cycles. When parts a and b of Figure 17 are compared, the product end at 4 atm operating condition became cleaner than that at 5.5 atm operating condition. Also, at high P/F ratio in Figure 17c, the difference among O2 MTZs at each operating condition was very small. The recovery in Figure 16b was higher as the adsorption pressure was lower. This result implies that the high adsorption pressure still caused low O2 recovery because of large amounts of desorbed O2 at the blowdown and

Figure 18. Comparisons of O2 (a) purity, (b) recovery, and (c) productivity between two different adsorption step times at 4 atm, 11.5 L(STP)/min, and various P/F ratios.

purge steps. However, the recovery was crossed over at the high P/F ratio region. Contrary to the recovery, the productivity was increased with an increase in the feed flow rate and the crossover was not found. This is because high adsorption pressure and high feed flow rate caused enlargement of the adsorption capacity and absolute O2 supply in the system. (v) Effects of the Adsorption Step Time. Figure 18 shows the effect of the adsorption step time on the O2 purity, recovery, and productivity. Only the adsorption step time in the cycle was diminished from 10 to 5 s. As shown in this figure, the purity of O2 was improved but the recovery of O2 was decreased due to the short adsorption step time. Because of the reduced adsorption step time, the N2 amount at the adsorption step in the bed was smaller than that at 10 s adsorption step time. Therefore, compared with the case of 10 s adsorption time, a relatively low purge rate at the reduced adsorption step time led to a high purity. However, the reduced adsorption step time also caused a low product amount. Hence, the recovery and productivity were decreased greatly compared to the operating condition with 10 s adsorption step time under the same adsorption pressure and feed flow rate conditions. To sum up, when the performance of the small-scale medical O2 PSA process was compared with that of the previous O2 PSA studies5-7,10 in the range of 95-99.5 vol % O2 purity; the 25-50% recovery obtained from this study was equal to or slightly higher than the others. Conclusions Dynamic characteristics and performances of the medical O2 PSA process were studied through the


experimental and theoretical approaches. The predicted results from a mathematical model in this study agreed well with the experimental data. Cyclic behaviors of the PSA process were investigated thoroughly from unsteady- to steady-state conditions. The composition variation reached cyclic steady state after approximately 15 cycles while the temperature variation did not even after 20 cycles. Because the performances of cyclic operation were not affected by the temperature variation after certain cycles, the system behaves almost isothermally. The effect of the P/F ratio on purity became conspicuous with an increase in cycles. The O2 concentration wave fronts at the PPE and FP steps in the cyclic steady state became steeper than those in the cyclic unsteady state. Also, the higher the P/F ratio, the longer the time to reach the steady state. The modified LDF model predicted the temperature and concentration variation better than the LDF model. However, the difference between the predicted results by both models at the steady state was almost negligible. As the P/F ratio was increased, the purity was improved and the recovery and productivity were decreased because of a loss of O2 in the product stream. Also, there was limiting P/F ratio (about 0.8-0.9) to have a meaningful effect on the O2 purity. Especially, the effect of the P/F ratio was significantly coupled with the variation of the adsorption pressure, feed rate, and adsorption step time. In the case of low P/F ratio, the difference between concentration profiles at the adsorption step becomes smaller as the pressure becomes higher than 4 atm. However, this difference in the high P/F ratio becomes small from above 3 atm. Therefore, the difference of the process performances by the adsorption pressure was decreased with an increase in the P/F ratio. The effect of the feed flow rate was gradually diminished with an increase in the adsorption pressure due to the limiting amount of adsorption. However, as for the product purity, the low feed rate began to lose its advantage with an increase in the P/F ratio. O2 recovery was increased in the low feed flow rate because the large purge amount was needed to remove the strongly adsorbed N2. The higher the feed flow rate, the higher the productivity. Also, the difference of recovery was maintained almost constantly independent of the P/F ratio. This is because at a high feed flow rate condition, the net amount of product was larger than that at a low feed flow rate. When both the adsorption pressure and feed flow rate changes simultaneously, the product purity was higher at higher adsorption pressure and feed flow rate conditions in the low P/F ratio. However, the purity was crossed over with an increase in the P/F ratio. The adsorption step time also had a considerable effect on the O2 purity, recovery, and productivity. As the adsorption step time was diminished to 5 s, the O2 purity was improved because the N2 amount at the adsorption step in the bed was smaller than that at 10 s adsorption step time. However, the recovery and productivity were greatly decreased in this condition. Hence, the reduced adsorption step time also caused a low product amount. Acknowledgment Financial assistance and support from KOSEF (952-10-01-01-3) and Daesung Sanso Co. are gratefully acknowledged.

Nomenclature Aw ) cross-sectional area of the wall (cm2) B ) equilibrium parameter for Langmuir-Freundlich model (atm-1) ci ) i component concentration in the bulk phase (mol/cm3) Cpg, Cps, Cpw ) gas, pellet, and wall heat capacity, respectively (cal/g‚K) De ) effective diffusivity defined by the solid-diffusion model (cm2/s) DL ) axial dispersion coefficient (cm2/s) -∆H ) average heat of adsorption (cal/mol) k ) parameter for the LRC model K ) proportionality parameter for the LDF model KL ) axial thermal conductivity (cal/cm‚s‚K) L ) bed length (cm) P ) total pressure (atm) PA, PD ) final adsorption and desorption partial pressures, respectively (atm) Pc ) critical pressure (atm) q, q*, q j ) amount adsorbed, equilibrium amount adsorbed, and average amount adsorbed, respectively (mol/g) qm ) equilibrium parameter for the Langmuir-Freundlich model (mol/g) r ) radial distance in the pellet (cm) R ) gas constant (cal/mol‚K) Rp ) radius of the pellet (cm) RBi, RBo ) inside and outside radius of the bed, respectively (cm) t, tst ) time and stoichiometric breakthrough time, respectively (s) TA, TD ) final adsorption and desorption temperatures, respectively (K) Tatm ) temperature of the atmosphere (K) Tc ) critical temperature (K) T, Tw ) pellet or bed temperature and wall temperature, respectively (K) u ) interstitial velocity (cm/s) wc ) wave velocity for concentration front (cm/s) yi ) mole fraction of species i z ) axial distance in the bed from the inlet (cm) Greek Symbols , t ) voidage of the adsorbent bed and total void fraction, respectively Fg, Fp, FB, Fw ) gas density, pellet density, bulk density, and bed wall density, respectively (g/cm3)

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Received for review February 6, 2001 Revised manuscript received May 16, 2001 Accepted May 16, 2001 IE010101L

Pressure Swing Adsorption - American Chemical Society

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