Propane Separation with 4A

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Ind. Eng. Chem. Res. 2001, 40, 5758-5774

Vacuum Swing Adsorption for Propylene/Propane Separation with 4A Zeolite Francisco A. Da Silva† and Alı´rio E. Rodrigues* Laboratory of Separation and Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal

A vacuum swing adsorption process using 4A zeolite pellets with five steps was designed to split an equimolar mixture of propylene/propane. The equimolar feed of C3 is diluted to 50% with nitrogen at 5 bar and 423 K, and the product is obtained when the total pressure is lowered to 0.1 bar. After 15 cycles, cyclic steady-state conditions are reached, a propylene-enriched stream of 97 mol% relative to the propylene/propane mixture, with 4.4% of nitrogen, a recovery of 26% (molar basis), and a productivity of 1.03 mol/(kg h) are obtained. The experimental work is compared with numerical simulations, and the effects of different operating parameters on the VSA process are analyzed. Introduction Propylene/propane separation by distillation to produce high-purity propylene is an expensive, energyconsuming process.1 Hybrid methods combining traditional distillation and adsorption process have been proposed as economical alternatives1-3 based on the idea of separating the paraffins from the olefins by adsorption first and then carrying out the distillation over the paraffins and olefins separately. Vacuum swing adsorption (VSA) has been considered as a valid choice for propylene/propane splitting, with propylene as main product. Propylene is the most adsorbed component in the commercial zeolites3-9 and, more recently, in resins using the “π-complexation” phenomenon.1,10-14 Kumar et al.2 proposed a four-bed five-step VSA process for performing propylene/propane splitting as follows: (I) pressurization, (II) high-pressure feed, (III) cocurrent depressurization to an intermediate pressure, (IV) cocurrent purge with propylene product, and (V) countercurrent blowdown (propylene production). The duration of each step is between 1 and 10 min. Ramachandran et al.15 proposed a two-bed five-step VSA process using 4A zeolite as follows: (I) countercurrent pressurization with nonadsorbed product, (II) highpressure feed, (III) pressure bed equalization, (IV) countercurrent depressurization, and (V) pressure bed equalization. The total cycle time is 100 s, and the system operates between PH ) 1.7 bar and PM ) 0.7 bar for pressure equalization and is then evacuated to between 0.2 and 0.1 bar (PL). With a feed of 88/12 propylene/propane at 90 °C with 4A zeolite, a purity of 96% and a recovery of 97% were obtained, with a productivity of 1.2 mol/(kg h). Rege et al.16 carried out numerical simulations of a four-step VSA process for performing propylene/propane splitting with the following steps: (I) pressurization with feed, (II) high-pressure * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: 351 22 5081671. Fax 351 22 5081674. † Present address: Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal.

Figure 1. Five-step VSA process for separating C3H6/C3H8. Base operating parameters for run 31 with 4A zeolite. Furnace temperature ) 423 K.

feed with fresh feed and recycling of high-pressure purge product, (III) high-pressure purge with high-purity olefin product, and (IV) countercurrent vacuum blowdown (propylene production). The stream exiting during the cocurrent high-pressure purge in this four-step VSA process is recycled and mixed with fresh feed to be processed. This allows the complete saturation of the column in the cocurrent purge step until propylene breakthrough. Da Silva and Rodrigues17 studied a fivestep VSA, as shown in Figure 1, with the following steps: (I) pressurization with feed from 0.1 to 5 bar for 60 s with total volumetric flow of 2 SLPM, 25% propylene, 25% propane, and 50% nitrogen; (II) high-pressure feed for 120 s; (III) high-pressure purge with product

10.1021/ie0008732 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/03/2001

Ind. Eng. Chem. Res., Vol. 40, No. 24, 2001 5759 Table 1. Mathematical Model Solved with SAXS Package to Simulate the VSA Process overall mass balance

∂C

)-

∂(uC)

∂t

n

-

∂z

∑N

Table 3. Adsorption Bed Characteristics and Physical Properties for C3H6/C3H8/N2 System over 4A Rhoˆ ne-Poulenc Zeolite Column bed radius (Rc) bed length (L) bed porosity () bulk density (Fb) wall density (Fw) wall specific heat (C ˆ pw) wall heat film transfer coefficient (hw) overall heat transfer coefficient (U)

i

i)1

(

)

∂Yi ∂(uCi) ∂Ci ∂ Dzm,iC ) - Ni ∂t ∂z ∂z ∂z

component mass balance

Ergun’s equation

-

2 1.75(1 - )2F ∂P 150µ(1 - ) u+ |u|u ) 3 2 ∂z d 3d p

p

aKm,i Ni ) (1 - ) (C - cji) Bim,i + 1 i

Adsorbent 4A extrudates (Rhoˆne-Poulenc) crystal radius (rc) pellet length pellet radius (Rp) pellet density (Fp) pellet void fraction (p) tortuosity (τ) solid specific heat (C ˆ ps)

hj i Fpwc ∂n h p,i Bim,i mass transfer ∂cji ) 15D (Ci - cji) rate to solid ∂t p ∂t Rp2 Bim,i + 1 ∂n hj i 15D h c,i / (ni - n hj i) ) ∂t r2 c

( )

∂Tg ∂Tg ∂Tg ∂C ∂ λ - uCC ˜p ) + RTg ∂t ∂z ∂z ∂z ∂t 2hw - (1 - )ahf(Tg - Ts) (T - Tw) Rc g

CC ˜v

gas energy balance

n

∑cj C˜

(1 - ){p solid energy balance

i

n

∑nhj C˜

+ F p wc

v,i

i)1

(1 - )pRTs

∂cj ∂t

i

v,ads,i

+ F pC ˆ ps}

i)1

FwC ˆ pw

Toth’s extended equation

n/i )

∂Ts

)

∂t

i

i)1

n

∑(b cj RT ) ]

k 1/k

s

i)1

mass axial dispersion term heat axial dispersion term

k 1/k

i i

s

affinity equilibrium parameter (bi) component propylene propane heterogeneity parameter (k) loading saturation coefficient (m)

bi ) bi,o exp(-∆Hi/RT) -∆Hi (kJ/mol) 29.9 0 0.67 2.03 mol/kg

bi,o (kPa-1) 7.4 × 10-6 7.0 × 10-4

Gas Reference Parameters reference temperature (To) 423 K reference pressure (Po) 1.01 bar gas viscosity (µo) 1.5 × 10-5 kg/(m s) gas thermal conductivity (kgo) 2.9 × 10-2 W/(m K) molecular bulk diffusion (Dm,i,o) propylene 1.8 × 10-5 m2/s propane 1.7 × 10-5 m2/s nitrogen 2.2 × 10-5 m2/s

mbicjiRTs

i i

∑(b cj RT ) ] i)1

∂Tw ) Rwhw(Tg - Tw) - RwlU(Tw -T∞) ∂t

[1 +

n

[1 +

(1 - )ahf(Tg - Ts) wall energy balance

mbicjRTs

n/i )

∑(-∆H ) ∂t +

+ F bw c

1.6 µm 10 mm 1.7 mm 1700 kg/m3 0.34 2.2 920 J/(kg K)

Equilibrium

∂n hj i

n

0.008 m 0.80 m 0.58 700 kg/m3 8238 kg/m3 500 J/(kg K) 60 W/(m2 K) 30 W/(m2 K)

Dzm,i ) 20 + 0.5Sci Re Dm,i

3

gas specific heat

C ˜ pg,i )

∑A T

i

i

i)0

λ ) 7 + 0.5Pr Re kg

mass transfer coefficient

Km,idp ) 2.0 + 1.1Re0.6 Sci1/3 Dm,i

heat transfer coefficient

hfdp ) 2.0 + 1.1Re0.6 Pr1/3 kg

component

A0 A1 [J/(mol K)] [J/(mol K2)] propylene 3.71 2.35 × 10-1 propane -4.22 3.06 × 10-1 nitrogen 31.15 -1.357 × 10-2

A2 A3 [J/(mol K3)] [J/(mol K4)] -1.16 × 10-4 2.21 × 10-8 -1.60 × 10-4 3.22 × 10-8 2.681 × 10-5 -1.168 × 10-8

Crystal Diffusion Parametersa component D h c,i (m2/s) propylene 8.0 × 10-15 propane 1.0 × 10-14 a

Table 2. Main Operating Conditions of Fixed-Bed Experiments Performed with Propylene/Propane over 4A Rhoˆ ne-Poulenc Zeolite run

T (K)

P (bar)

Qpropylene (SCCM)

Qpropane (SCCM)

Qnitrogen (SCCM)

1 2 3

423 423 423

5 5 5

500 0 500

0 500 500

1750 1750 1000

diluted with nitrogen for 120 s; (IV) high-pressure cocurrent blowdown for 60 s; and (V) low-pressure countercurrent blowdown for 120 s, where the propylene product is obtained. Using 13X zeolite pellets as the sorbent, a purity of 98%, a recovery of 19%, and a productivity of 0.785 mol/(kg h) were obtained.

Da Silva and Rodrigues.9

Table 4. Molar Percentage of Nitrogen in Propylene Product run

% N2

run

% N2

31 32 33 36 38

4.4 2.2 4.9 4.5 4.3

41 47 48 53 54

3.4 4.6 4.4 14.3 5.4

The objective of this work is to assess the application of a five-step VSA cycle to obtain high-purity propylene with 4A zeolites. Fixed-bed breakthrough curves, VSA experiments, and numerical simulations are used in the analysis.

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Ind. Eng. Chem. Res., Vol. 40, No. 24, 2001

Table 5. VSA Experiments Performed for the Separation of Propylene/Propane over 4A Rhoˆ ne-Poulenc Zeolite

run

T (K)

31 32 33 36 38 41 44 45a 46b 47 48 49 50 51 53 54 55 56

423 423 423 423 393 393 423 423 423 423 423 423 423 423 423 423 423 423

Phigh Pmedium Plow pres feed purge H-blow L-blow QC3H6 QN2 (bar) (bar) (bar) (s) (s) (s) (s) (s) (SLPM) (SLPM) 5 5 5 5 5 5 5 5 5 5 5 5 5 3 5.0 5.0 2.5 1.2

0.5 0.5 2.0 0.5 0.5 1.0 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

120 60 180 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120

120 60 120 120 120 120 180 120 120 120 120 120 120 120 120 120 120 120

60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

120 240 180 120 120 120 180 120 120 120 120 120 120 120 120 120 120 120

0.3 0.15 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.4 0.15 0.15 0.075 0.3 0.3 0.3 0.3 0.3

3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6 1.8 0.9 3.6 1.8 0.9 0.0 0.0

P/F 0.45 0.20 0.33 0.61 0.45 0.46 0.67 0.28 0.78 0.61 0.23 0.24 0.13 0.26 0.45 0.45 0.41 0.24

C3H6 purity recovery productivity (%) (%) [mol/(kgzeolite 4A h)] 96.9 86.0 97.0 94.5 97.4 95.8 94.8 98.6 91.2 95.3 94.5 95.6 87.9 95.2 95.9 96.2 91.1 92.3

25.7 77.1 17.6 11.1 9.4 17.6 23.3 17.1 20.0 10.9 27.3 39.5 46.5 22.6 18.8 30.4 30.0 38.0

1.03 1.93 0.772 0.441 0.372 0.693 0.747 1.13 0.45 0.43 1.09 1.57 1.85 0.803 0.756 1.23 1.31 1.47

a Pressurization and feed steps with a stream of 0.8, 0.2, and 1.0 SLPM of propylene, propane, and nitrogen, respectively. b Pressurization and feed steps with a stream of 0.3, 0.7, and 1.0 SLPM of propylene, propane, and nitrogen, respectively.

Figure 2. Flow diagram of the laboratory unit.

Experimental Setup The experimental setup is shown in Figure 2. It contains three main sections: (a) The feed section is made up of three mass flow controllers, MFC1, MFC2, and MFC3, connected to nitrogen, propylene, and propane cylinders. (b) The VSA/PSA column section includes a convective air furnace, five solenoid valves, two check valves, a pressure transducer, three thermocouples, and the column. (c) The pressure regulation section consists of a back-pressure regulator, a vacuum pump, a relief valve, a check valve and two electrovalves. Further details of the experimental setup can be found elsewhere.18 Mathematical Model The mathematical model used in the simulations is shown in Table 1. It is a bidisperse mass transfer control model with a heterogeneous energy balance with local pressure drop following the Ergun’s correlation. The isotherm used is the Toth extended model,19 with the axial mass dispersion Dzm,i, heat conductivity λ, Sher-

wood number Km,idp/Dm,i, and Nusselt number hfdp/kg calculated using the Wakao et al.20 correlations as suggested by Yang.21 All properties in the bulk gas phase, such as viscosity, thermal gas conductivity, and molecular diffusion, are calculated with the reference conditions and then corrected locally by temperature and pressure according to Lu et al.22 During the pressurization/depressurization steps, an exponential valve equation is used following the experimental data.16,22 For fixed-bed runs, the boundary conditions are the same as in the high-pressure feed/purge steps, which are equivalent to Danckwerts’ boundary conditions. As initial conditions, inert gas flowing isothermically through the column is used before the fixed bed and during the first pressurization step in cycle operation. For the cyclic process, the previous conditions at the end of the last cycle are the initial conditions of the new cycle. The set of partial differential equations was solved with a general purpose numerical package for cyclic adsorption processes called SAXS,23 about which further details can be found elsewhere.17,18

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Figure 3. Experimental propylene (a) breakthrough and (b) blowdown curves on 4A Rhoˆne-Poulenc zeolite, temperature histories at the bottom, middle, and top of the column (c, e, and g, respectively) and during blowdown (d, f, and h, respectively) for run 1. Solid lines are simulations with model shown in Table 1. Feed at 5 bar and 423 K.

purity Performance Criteria of VSA Process The definition of the performance criteria that characterize the VSA experiments provides a common basis to compare different experiments. The four criteria used are defined as follows

% purity ) (amount of C3H6 obtained during blowdown step) × 100 (2) (amount of C3H6 + amount of C3H8) obtained in blowdown step

recovery purge-to-feed ratio P/F ) amount of C3H6 used during purge step amount of C3H6 fed during pressurization (1) + amount of C3H6 used during feed step

% recovery ) (amount of C3H6 obtained in blowdown step amount of C3H6 used in purge step) × 100 (3) amount of C3H6 fed during pressurization step + amount of C3H6 used in feed step

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Figure 4. Experimental propane (a) breakthrough and (b) blowdown curves on 4A Rhoˆne-Poulenc zeolite, temperature histories at the bottom, middle, and top of the column (c, e, and g, respectively) and during blowdown (d, f, and h, respectively) for run 2. Solid lines are simulations with model shown in Table 1. Feed at 5 bar and 423 K.

productivity productivity ) amount of C3H6 obtained in blowdown step - amount of C3H6 used in purge step (4) total sorbent mass × total cycle time Fixed-Bed Experiments The fixed-bed experiments carried out with the 4A Rhoˆne-Poulenc zeolite consisted of a high-pressure breakthrough followed by countercurrent blowdown. The main operating conditions (temperature, pressure and volumetric flow rates of propane, propylene, and nitrogen) for each experiment are shown in Table 2. The

characteristics of the fixed-bed column, adsorbent particles, and physical system needed for the simulations with SAXS are shown in Table 3. The propylene breakthrough and blowdown steps are shown in Figure 3 with the operating conditions of run 1 (see Table 2). Figure 3a shows a wider and longer breakthrough curve than the experimental curve obtained with 13X zeolite under similar experimental conditions.17 This is expected as a consequence of the higher mass transfer resistance and lower adsorption capacity found for the 4A zeolite compared with 13X zeolite,9 and our results are in qualitative agreement with the breakthrough curves reported by Ja¨rvelin and Fair.7,24


Figure 5. Experimental propylene/propane (a) breakthrough and (b) blowdown curves on 4A Rhoˆne-Poulenc zeolite, temperature histories at the bottom, middle, and top of the column (c, e, and g, respectively) and during blowdown (d, f, and h, respectively) for run 3. Solid lines are simulations with model shown in Table 1. Feed at 5 bar and 423 K.

Figure 4 presents the breakthrough/blowdown experiment with propane for run 2. The results show that propane is practically not adsorbed over the 4A zeolite, which confirms the low adsorption capacity measured with the microbalance experiments.7,9 The multicomponent breakthrough of propylene/ propane with the operating conditions of run 3 is shown in Figure 5. Practically from the beginning of the experiment, propane free of propylene is obtained until 200 s. The temperature histories show a single peak, rather than two as in the case of 13X zeolite, and a much smaller roll-up. This experimental result shows that propane has negligible adsorption over the 4A RhoˆnePoulenc zeolite. Observing the blowdown step curves from Figures 3-5 after 100 s the propylene and propane

concentrations become constant just when the temperature in the blowdown step reaches a minimum. This sets the minimum time to carry the blowdown from 5 to 0.1 bar (cocurrent blowdown time + countercurrent blowdown time). Experiments were also carried out varying the operating temperature (393-453 K), feed volumetric flow rate (1-4 SLPM), and feed pressure (1.5-5 bar) and were simulated with good agreement. Vacuum Swing Adsorption Experiments with C3H6/C3H8 using 4A Zeolite Pellets The VSA cycle with 13X zeolite17 was applied for propylene/propane separation with the 4A RhoˆnePoulenc zeolite. The molar percentage of nitrogen in

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Figure 6. (a) Simulated and experimental pressure history at the exit end of the VSA process (run 31). (b) Simulated and experimental molar flows of propylene and propane at the exit end of the VSA process during cycle 7 (run 31). (c) Simulated and experimental molar flows of propylene and propane at the exit end of the VSA process during cycle 17 (run 31). (d) Simulated and experimental temperature measured at 0.2 m of the feed end during the VSA process (run 31). (e) Simulated and experimental temperature measured at middle of the column during the VSA process (run 31). (f) Simulated and experimental temperatures measured 0.2 m below the top of the column during the VSA process (run 31).

propylene product is shown in Table 4 for some runs. The group of experiments carried out is shown in Table 5. Experiments 31-33 were performed to explore operating points of the process. The propylene purity obtained with 4A zeolite (96.9%) was lower than that with 13X zeolite (98%), but the recovery and productivity were higher, being 25.7% and 1.03 mol/(kgzeolite 4A h) versus 19.2% and 0.785 mol/(kgzeolite 13X h). In run 32, the feed and purge times were reduced from 120 to 60 s while the blowdown step was carried out for 240 s so that the total cycle time remained 480 s. The highest recovery and productivity were obtained with this strategy, namely, 77.1% and 1.93 mol/ (kgzeolite 4A h), respectively; however, the propylene purity was only 86%. The duration of the feed step was fixed at 180 s and the purge step at 120 s in run 33, while the blowdown step was reduced from 240 to 180 s, with

an intermediate pressure fixed at 2.0 bar. The purity was improved to 97%, but the recovery and productivity were reduced to 17.6% and 0.772 mol/(kgzeolite 4A h), respectively. Selecting run 31 as the base case, the effects of column temperature, pressure feed, intermediate pressure, and purge flow were investigated. Evolution of Vacuum Swing Adsorption Process Using 4A Zeolite. Figure 6a-f compares the experimental pressure, molar flow, and temperature histories with the results obtained for simulations using the model shown in Table 1 with SAXS. Figure 6a shows the experimental pressure and the simulated curve. Once the experiment is started, no changes occur from cycle to cycle for the pressure history, as also noticed for the other variables. This result is expected because the pressure level is an imposed variable and it is the faster variable propagating in the column.25 Figure 6b


Figure 7. Propylene and propane mole fractions versus axial distance z at the end of each step until the cyclic steady state is reached (run 31).

and c shows the experimental molar flows of propylene and propane exiting the column and the simulated predictions. The propylene throughput exiting the VSA unit is followed by the simulations, but the propane history is only roughly described by the simulations. However, the experimental and predicted areas below the curve along a complete cycle are approximately the same, confirming that the total mass balance is correct. The instantaneous difference in the two responses is due

to the dynamic effect of the extra column volumes that are part of the VSA unit. Whereas SAXS predicts the flow exiting directly from column, the flow exiting at the very end of the unit is measured (i.e., after the backpressure valve and vacuum pump, which are not included in the SAXS model). During single breakthrough/blowdown experiments each step is carried out for a longer period of time until a final equilibrium state is achieved. In consequence, all equipment is stabilized,

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Figure 8. Evolution of propylene and propane adsorbed-phase concentrations (per kilogram of 4A zeolite) versus axial distance z at the end of each step until the cyclic steady state is reached (run 31).

and the predictions from SAXS agree better with the experimental results as the effect of the complementary equipment is minimized. During a VSA cycle, the operating conditions vary from one step to another, and the dynamics of the other equipment becomes more important. The experimental temperature histories and simulated values obtained at 0.2, 0.4, and 0.6 cm from the feed end are shown in Figure 6d-f, respectively. The

simulations carried out capture the temperature evolution fairly well. It can be seen from this figure that, after the 5th or 6th cycle, the temperature evolution around a complete cycle is practically the same for the 9th or higher cycle. Furthermore, the experimental cyclic steady state (CSS) was checked by calculating the average product composition over a full cycle. The evolution of the VSA process up to the cyclic steady-state conditions can be further understood in


Figure 9. Temperature evolution at the end of each step versus axial distance z until the cyclic steady state (run 31).

Figures 7-9. In Figure 7, the mole fractions of propylene and propane at the end of each step are displayed for cycles 1-20. From the 5th cycle onward, it is very difficult to distinguish between different cycles. The same is found for the adsorbed amount in the solid phase and the temperature evolution (see Figures 8 and 9, respectively). The VSA process investigated using the 4A zeolite reaches cyclic steady-state conditions in 1520 cycles. Another result to be mentioned is that propane disappears from the column after the purge step, and so, a single purge step is sufficient to clean the column from propane. However, the cocurrent blowdown step is required to eliminate the nitrogen from the product. Numerical CSS was obtained by running the dynamic process simulator SAXS and performing mass balances around a full cycle until the difference between two consecutive cycles was lower than a given error. At this point, no direct determination of the CSS was used by inserting the periodicity condition numerically.26 The evolutions over a complete steady-state cycle of the mole fraction, amount adsorbed in the solid phase, temperature, and superficial gas velocity are shown in Figures 10-12, respectively. From Figure 10, it is observed that the column is pressurized with fresh feed after the vacuum countercurrent blowdown when the column was filled with practically pure propylene. The mole fraction is reduced because nitrogen enters the column during the pressurization step (the feed is 25%

propylene, 25% propane, and 50% nitrogen). The nitrogen penetrates the column more than the other two components. Then, the high-pressure feed begins where incipient breakthrough is observed while the solid phase is partially saturated with propylene (see Figure 11). The reduction in temperature variations during the cyclic steady state compared to those observed during the initial cycles is also explained by this fact. Initially, the column is not filled with propylene, the amount of propylene being adsorbed is higher, and the temperatures variations are more important. After the feed step follows the high-pressure purge step, where the propane is forced to exit the column. During cocurrent blowdown, the nitrogen is eliminated from the column as can be seen in Figure 11 where the mole fraction of propylene reaches over 80%. At the end of this step, the column is prepared to produce high-purity propylene product. During vacuum countercurrent blowdown, the total pressure is reduced from 0.5 to 0.1 bar, and propylene achieves the >99.9% purity. The temperature and superficial gas velocity profiles shown in Figure 12 supply additional information about the VSA cycle being investigated. During the first two steps, the column is loaded with fresh feed, and the maximum temperature is achieved. On the last three steps, the column is regenerated, and the temperature falls, especially when countercurrent blowdown is performed. Figure 13 shows the evolution of the axial pressure during the countercurrent/cocurrent blowdown

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Figure 10. Propylene and propane mole fraction evolution during each step versus axial distance z along a complete steady-state cycle (run 31).

and pressurization steps indicating that we can neglect the pressure drop along the column. Effect of Temperature on the Performance of VSA with 4A Zeolite. Figure 14a shows the effect of operating temperature on VSA performance, using 4A zeolite as the sorbent with the flow rates and adsorption step times of run 31 at 393, 423, and 453 K. This figure clearly shows a maximum of recovery at around 420 K

and a reduction in the experimental purity (from 97.4 to 95.8%) when the temperature rises from 393 to 453 K. From these results, it can be concluded that the optimum operating temperature for the 4A sorbent is between 393 and 423 K. At lower temperatures, the isotherm is more unfavorable for desorption, and the mass transfer resistance effect is more important, whereas at higher temperatures, the amount of propy-


Figure 11. Evolution of axial profiles of propylene and propane adsorbed-phase concentrations in 4A zeolite for each step along a complete steady-state cycle (run 31).

lene produced from the countercurrent blowdown is not sufficient to maintain the VSA cycle, because of the substantial reduction of the loading capacity of the sorbent. Effect of Intermediate Blowdown Pressure on the Performance of VSA with 4A Zeolite. Figure 14b shows the effect of the intermediate pressure, PM, on the performance of the VSA cycle using the 4A zeolite with the operating conditions of runs 38, 41, and 42 (see

Table 3). When the pressure is increased from 0.3 to 1 bar, the purity is maintained over 95% while the recovery rises to nearly 20%. From these results, one can conclude that an intermediate pressure in the range of 0.5-1 bar is appropriate for performing the cocurrent blowdown from the high-pressure feed at 5 bar. At pressures lower than 0.5 bar, much of the propylene is wasted and is not available for the following countercurrent blowdown.

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Figure 12. Evolution of temperature and superficial velocity during each step along a complete steady-state cycle (run 31).

Effect of High-Pressure Feed on the Performance of VSA with 4A Zeolite. Figure 14c shows the influence of the total pressure, PH, on the performance of the VSA unit as obtained during runs 31, 51, and 52. When the total pressure is reduced from 5 to 1 bar, both the experimental recovery and purity decrease. The observed theoretical propylene purity is always higher than the experimental values (99.8-99.9%), whereas the propylene recovery is fairly well described by the

simulations. The simulated high values of purity always over 99% are not surprising, based on the characteristics of the C3H6/C3H8 adsorption over the 4A zeolite. The proposed VSA cycle completely depletes propane from the column during the purge and countercurrent blowdown steps, as shown in Figures 10 and 11, because propane is essentially not adsorbed by 4A zeolite. Similar simulated results with the 4A zeolite were obtained by Rege et al.16 with a four-step VSA process.


Figure 13. Pressure evolution during the pressurization, cocurrent, and countercurrent blowdown steps during the steady-state cycle (run 31).

They always obtained over 99% purity in all simulations despite total cycle times ranging from 400 to 3600 s and interstitial feed and purge velocities n the ranges 0.080.80 and 0.065-0.13 m/s, respectively, with recoveries of between 8 and 27%. In contrast, with the other sorbent (AgNO3/SiO2), based on π-complexation adsorption (both propylene and propane are adsorbed), they obtained purity variations between 97.6 and 99% by changing the total cycle time between 240 and 1600 s and the feed and purge velocities in the ranges 0.2-1.4 and 0.1-0.90 m/s, respectively, with recoveries of between 18 and 44%. Ramachandran et al.15 stated that 4A zeolite is a better adsorbent than 5A and 13X zeolites for propylene/propane separation at high temperature because of its higher propane rejection. The experimental purity they obtained for temperatures in the range 30-175 °C is between 92 and 97% as found here, but the recovery was much higher (91-96%). The higher recovery they reported is mainly due to the introduction of an equalization step. Also, we can improve the recovery by taking part of the product of the highpressure purge step as feed in the high-pressure feed.16 It is important to note that Ramachandran et al.15 did not report a 99.9% experimental purity possibly because they did not introduce a rinse step in their cycle. This explains the lower-purity propylene product obtained in their experiments. The difference between the purities obtained by simulations and experiments with the 4A zeolite was

not observed with the 13X zeolite using the same experimental setup. A possible explanation for this fact is the following: After the high-pressure feed step, the externals of the column are filled with propane. Then, during the high-pressure purge and cocurrent blowdown, the nitrogen exiting the column cleans the pipes connected to the column before the countercurrent vacuum blowdown but does not clean the other ancillary equipment (multiport valve and GC internals). During the vacuum countercurrent blowdown step, the average purity of the propylene produced depends on the total amount that is withdrawn from the column. Assuming that the column is practically clean of propane, as indicated by the simulations in Figures 7 and 8, the propane that is contaminating the product stands outside (valves, back-pressure regulator, sample valve, GC). As the total feed pressure is reduced, the amount of propylene loaded is also reduced, as during the production step, the propylene obtained from the column is not sufficient to displace the contaminant propane in the pipe lines and internals. With the 13X zeolite, this result was not detected because the adsorption loading was higher, 1.2 versus 0.88 mol/kg, and the total amount of propylene produced was enough to displace the propane present in the pipes and internals. Effect of Purge-to-Feed Ratio on the Performance of VSA. Figure 14d shows the effect of the P/F mole ratio on the performance of the VSA cycle investigated. With an increment in P/F from 0.13 to 0.45 (runs 31, 49, and 50; see Table 5) the purity increases from 87.9 to 96.9%, and the recovery is reduced from 46.5 to 25.7%. The results follow a similar pattern for 13X zeolite with increasing P/F ratio.17 The model results agree with the experimental recovery, but again, the simulated and experimental purities differ, especially at lower P/F mole ratios. The propane contaminant is not fully diluted with the propylene obtained at low P/F ratio. At higher P/F values, the amount loaded in the column before the two blowdown steps is higher, as is the amount of propylene obtained from the column. Effect of Nitrogen and Propylene Feed Flow during High-Pressure Purge on the Performance of VSA with 4A Zeolite. Figure 14e and f shows the effects of varying the amount of nitrogen at fixed propylene purge flow (0.3 SLPM; see runs 31, 53, and 54, Table 5) and varying the amount of propylene at fixed nitrogen flow (3.6 SLPM; see runs 31, 47, and 48, Table 5), respectively. At higher nitrogen flows, better purity is obtained (96.2 vs 96.9%), but the recovery is reduced from 30 to 25%. A similar result is found with the propylene purge flow: at constant nitrogen flow, higher propylene flows result in better purities (94.5 vs 95-96%) but reduced recoveries (27 vs 11%). Comparing the two effects, higher sensitivity for the propylene purge flow over the nitrogen flow is obtained. Conclusions Experimental fixed-bed and vacuum swing adsorption (VSA) experiments were conducted for propylene/ propane separation using 4A Rhoˆne-Poulenc zeolite pellets. The fixed bed was loaded in 400-600 s at 5 bar and 423 K, with a volumetric flow rate of 2 SLPM of an equimolar propylene/propane mixture diluted at 50% with nitrogen, while countercurrent blowdown from 5 to 0.1 bar took between 120 and 200 s. On the basis of the breakthrough/blowdown experiments, a five-step VSA process was designed for pro-

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Figure 14. Effects of operating parameters on VSA performance using 4A Rhoˆne-Poulenc zeolite as the sorbent.

pylene/propane separation with the objective of producing high-purity propylene. The five-step vacuum swing adsorption process proposed to separate propylene/ propane was investigated by experimentation and simulation. The column packed with 4A Rhoˆne-Poulenc pellets produced an enriched propylene stream with 97% purity, 26% recovery, and a productivity of 1.03 mol/ (kg h), and the cyclic steady-state condition was achieved in no more than 15 cycles. The bidisperse mass transfer model implemented in a SAXS simulator adequately described the main behavior of the fixed-bed and VSA experiments. The effects of different operating parameters on the performance of the VSA process, including temperature, intermediate pressure, high-pressure feed, P/F mole ratio, and high-pressure purge flow, were shown. In the temperature range between 393 and 453 K, better results in terms of purity/recovery were obtained at 423 K. The performance of the VSA cycle investigated was better at a higher total pressure of 5 bar and an intermediate pressure between 0.5 and 1 bar. A P/F

ratio of between 0.3 and 0.4 guaranteed purities higher than 95% when the nitrogen and propylene were kept in the ranges 1-3.6 and 0.15-0.4 SLPM, respectively. Considering simultaneously the experimental results obtained with 13X and 4A zeolites, purities over 95% and recoveries of between 20 and 40%, with productivities in the range of 0.75-1.2 mol/(kg h), are easily achieved with both sorbents. In terms of the experimental purities obtained, the 13X zeolite produces better values of 97-98%, whereas with the 4A zeolite, the experimental values are in the 95-96% range. However, the theoretical values of purity for propylene/ propane with the 4A zeolite are in the 99.8-99.9% range, the experimental recovery is in agreement with theoretical values. Acknowledgment Financial support from PRAXIS XXI /3/3.1/CEG/2644/ 95 is gratefully acknowledged. F. A. Da Silva acknowledges a research fellowship from PRAXIS XXI/BD5772/ 95.


Notation a ) pellet specific area, Bim,i ) mass Biot number () RpKm,i/5pD h p,i) cji ) average concentration of component i in the pellet, mol/ m3 Ci ) concentration of component i in the bulk, mol/m3 C ˆ ps ) solid heat capacity, J/(kg K) C ˆ pw ) wall heat capacity, J/(kg K) C ˜ p, C ˜ v ) heat capacity at constant pressure and volume of the gas mixture, J/(mol K) dp ) pellet diameter, m D h c,i ) crystal diffusion coefficient of component i, m2/s Dm,i ) molecular bulk diffusivity of component i, m2/s D h p,i ) pore diffusivity of component i, m2/s Dzm,i ) mass axial dispersion coefficient of component i, m2/s hf ) film heat transfer coefficient between gas and solid, W/(m2 K) hw ) film heat transfer coefficient between gas and the wall, W/(m2 K) -∆Hi ) isosteric heat adsorption of the component i, J/mol k ) equilibrium heterogeneity parameter kg ) gas thermal conductivity, W/(m K) Km,i ) external mass transfer coefficient for component i, m/s L ) length of fixed bed, m m ) loading saturation parameter, mol/kg n j i ) average adsorbed concentration of component i in the pellet, mol/kg n/i ) adsorbed-phase concentration in the crystal in equilibrium with the gas inside the particle, mol/kg P ) gas pressure, kPa PH ) high feed/purge pressure, kPa PL ) low vacuum pressure, kPa PM ) intermediate pressure, kPa Pr ) Prandtl number Rc ) column radius, m R ) ideal gas constant, 8.3144 J/(mol K) Re ) Reynolds particle number Sc ) Schmidt number t ) time, s T ) temperature, K u ) superficial gas velocity, m/s U ) overall heat transfer coefficient, W/(m2 K) Yi ) mole fraction of component i in the bulk z ) axial position, m m-1

Greek Letters Rw ) ratio of the internal surface area to the volume of the column wall, m-1 Rwl ) ratio of the log mean surface to the volume of column wall, m-1 λ ) heat axial conductivity coefficient, W/m2K ) interparticle void fraction of bed p ) pellet void fraction Fp ) pellet density, kg/m3 Fw ) wall density, kg/m3 Superscripts * ) equilibrium o ) pure component - ) volumetric average ) ) double volumetric average ∼ ) per mol ˆ ) per kilogram Subscripts c ) crystal g ) gas

h ) heat i, j ) component m ) mass p ) pellet; constant pressure s ) solid v ) constant volume w ) wall z ) axial coordinate ∞ ) ambient

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Received for review October 10, 2000 Accepted August 1, 2001 IE0008732

Propane Separation with 4A

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