Ind. Eng. Chem. Res. 2004, 43, 7557-7565
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Crystal Size Effect in Vacuum Pressure-Swing Adsorption for Propane/Propylene Separation Carlos A. Grande,† Elena Basaldella,‡ and Alı´rio E. Rodrigues*,† Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal, and CINDECA, National University of La Plata, Calle 47 No. 257, 1900 La Plata, Argentina
The aim of this work is to evaluate the performance parameters of a vacuum pressure swing adsorption (VSA-PSA) unit for propane/propylene separation using zeolite 4A with different crystal sizes. For this purpose, we have synthesized samples of zeolite 4A with different average crystal diameters: 0.3, 1.9, 10.0, and 22.0 µm. We measured the adsorption equilibrium and kinetics of pure propane and propylene on these samples. The adsorption equilibrium has been measured on a manometric equipment from 0 to 100 kPa pressure and at temperatures of 373, 423, and 473 K. The pure gas adsorption kinetics at the same temperatures has been measured by isothermal breakthrough curves using only 1.1% of pure hydrocarbon diluted with helium at atmospheric pressure. Both crystal diffusivity and equilibrium data show only slight differences with previously reported data on commercial extrudates. Experimental data with pellets with 3.8 µm average crystal diameter were measured in a PSA laboratory unit using a five-step cycle (pressurization, feed, rinse, depressurization to intermediate pressure, and low-pressure countercurrent blowdown). With the aid of simulations, we assessed the effect of different crystal sizes on the equipment performance. When a fixed purity is established as the target, the higher recovery and productivity were obtained with the smaller crystals. Introduction Propane/propylene separation is the most difficult separation practiced in the petrochemical industry, and a large effort is being applied in alternative technologies.1 One of the most promising alternatives to cryogenic distillation is pressure-swing adsorption (PSA). Many commercially available adsorbents were already tested for this separation2-4 with relatively poor separation factors. The only exception is zeolite 4A,5,6 which was extensively tested in this process with good results.7 Also, more specific laboratory-made adsorbents for this separation were reported.8-10 A four-step PSA cycle with zeolite 4A was also patented.11 In all of these works, a commercial sample has been directly tested and comments on the low diffusivity of the gases initially exclude zeolite 4A as the adsorbent.3 A nonexplored strategy was to modify the crystal size of the zeolite crystals to have sharper concentration profiles in the column (shorter mass-transfer zones) and also a faster approximation to the steady-state operation. In this work, we have measured adsorption equilibrium and kinetics in four different crystal sizes of zeolite 4A at three temperatures (373, 423, and 473 K), and data have been compared to previously reported values in extrudates.12,13 The adsorption equilibrium has been fitted with the multisite Langmuir isotherm. Kinetic measurements were measured by fitting diluted breakthrough curves of propane and propylene with a model including crystal mass-transfer resistance and axial dispersion (crystal diffusivity was the fitting parameter). The final purpose of this paper is to determine the effects of the zeolite crystal size in the overall perfor* To whom correspondence should be addressed. Tel.: +351 22 508 1671. Fax: +351 22 508 1674. E-mail:
[email protected]. † University of Porto. ‡ National University of La Plata.
mance of a VSA-PSA laboratory unit for propane/ propylene separation. An experiment using zeolite 4A extrudates with 3.8 µm average crystal size diameter (CECA, Paris, France) performed for a mixture containing 27% propylene, 23% propane, and 50% nitrogen (54/ 46 molar relation of propylene to propane) was taken as an example. The cycle consists of pressurization (to 500 kPa), feed, rinse, cocurrent depressurization to intermediate pressure (to 50 kPa), and low-pressure countercurrent blowdown where purified propylene is withdrawn at 10 kPa.14,15 A reference temperature of 433 K was chosen. Also, PSA simulations of the same experiment using different crystal sizes were performed. Experimental Section Zeolite crystals were prepared by batch hydrothermal crystallization. The different batch experiments were carried out in closed polypropylene containers, at 365 ( 1 K, without stirring. The raw materials used were NaOH (Carlo Erba, analytical reagent), commercial sodium aluminate [Al2O3 (36.5%), Na2O (29.6%), and H2O (33.9%)], soluble water/glass with SiO2/Na2O ) 3.18 (w/w) (density equal to 1.36 g/mL), and distilled water. Triethanolamine (TEA; Anedra, analytical reagent) was incorporated in variable quantities to obtain the different crystal sizes. Products were characterized by X-ray diffraction. The diffraction patterns were obtained in a Philips PW 1732/ 10 equipment using Cu KR radiation and a Ni filter, at a rate of 2°/min. The diffraction diagrams of the zeolite samples were identified by comparison with those detailed in the literature.16 The zeolite particles obtained were characterized by scanning electron microscopy (SEM) in a JEOL JSM-6301F (Centro de Materiais da Universidade do Porto, CEMUP), with which an X-ray spectrometer (NORAN-VOYAGER) is coupled to
10.1021/ie049489l CCC: $27.50 © 2004 American Chemical Society Published on Web 10/02/2004
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Figure 1. SEM images of the four different samples: (a) 0.3 µm; (b) 1.9 µm; (c) 10 µm; (d) 22 µm. Table 1. Basic Physical Properties of Zeolite 4A Crystals and Breakthrough Curve Conditions crystal diameter [µm] crystal density [g/cm3]a column length [cm] column diameter [cm] flow rate [cm3/min]b C3 partial pressure [kPa] column porosity
0.3, 1.9, 10.0, 22.0 1.57 3.13 0.69 35.0-38.0 1.1 0.412
a Assumed value from ref 23. b Measured at the laboratory temperature (295 K).
determine the chemical composition. The morphology and crystal sizes of the samples observed by SEM can be seen in Figure 1. The crystal sizes of the samples were determined by averaging crystals from three different SEM micrographs and are 0.3, 1.9, 10.0, and 22.0 µm, although some crystal size distribution was observed as shown in Figure 1. In the larger crystals (10- and 22-µm samples), some coalescence between two or more crystals was also found. All of the adsorption equilibrium measurements were made on a manometric equipment operated in a closed system. Details of the equipment have been given elsewhere.12 For all of the experiments reported in this paper, we used the helium calibration procedure, assuming that this gas is not adsorbed on zeolite 4A at the working temperatures (373, 423, and 473 K). Activation of the samples was made in a vacuum at 673 K overnight. To obtain the breakthrough curves, a small packed column was placed in a gas chromatograph (GC) oven and an inert stream (helium) used to activate the sample was changed to the feed stream via a multiport
Table 2. Experimental Conditions of the VSA-PSA Experiment column length [cm] column diameter [cm] column porosity column density [g/cm3] pellet diameter [cm] pellet porosity crystal radius [µm] heat capacity of zeolite 4A, CP [J/g‚K] temperature [K] feed pressure [kPa] intermediate pressure for step 4 [kPa] blowdown pressure [kPa] pressurization time [s] feed time [s] rinse time [s] depressurization time [s] blowdown time [s] pressurization, feed and rinse flow rate [SLPM]
87 1.05 0.43 0.758 0.21 0.34 1.9 0.920 433 500 50 10 60 100 25 40 180 1.50
valve to measure the breakthrough curve. The hydrocarbon partial pressure was 1.1 kPa (measured in an independent GC) balanced with helium to atmospheric pressure. All of the tubing between the four-way valve and the column is 1/16 in. to minimize dead-volume effects. To activate the samples, a flow of helium of 15 cm3/min at 673 K (1 K/min heating ramp) was used overnight. With that protocol, breakthrough curves of propylene were measured in the four samples at 373, 423, and 473 K and those of propane in the smaller samples (0.3 µm). Details of the breakthrough experiments are listed in Table 1. The single-column VSA-PSA laboratory unit is completely automatic, and manual operation is only required for sampling. The unit allows storage of 11
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samples taken in the different steps of a single cycle. The equipment is connected to a computer, where the pressure at the inlet and outlet of the column are stored together with the temperature measured at three different points of the column (18, 43, and 68 cm from the inlet). The complete equipment was fully reported elsewhere.7,14 In this work, a five-step cycle comprising pressurization, feed, rinse with product, depressurization, and countercurrent blowdown was used. The full set of experimental conditions used in the experiment is reported in Table 2. Air Liquide provided all gases used in this report: propane N35, propylene N24, nitrogen N50, and helium N50 (purity greater than 99.95, 99.4, 99.999, and 99.999, respectively). Zeolite 4A extrudates used in VSA-PSA experiments were kindly supplied by CECA. Theoretical Section Adsorption equilibrium data of pure gases obtained in this work were fitted with the multisite Langmuir model for homogeneous adsorbents, also called as the Nitta model.17 If we neglect adsorbate-adsorbate interactions, the model can be expressed as
q/i qmax,i
(
) KiP 1 -
q/i qmax,i
)
(1)
Ki ) K0i exp(-∆Hi/RT)
(2)
K0i
is the adsorption constant at the limit of T f where ∞. The advantages of this model are its direct extension to multicomponent prediction, its thermodynamic consistency, and its great ability to fit experimental data. The saturation capacities of propane and propylene are related by the thermodynamic constraint aiqmax,i ) constant.18 The multicomponent extension of the multisite Langmuir model is
qmax,i
(
) KiPyi 1 -
q/i
∑qmax,i
)
ai
(3)
where yi is the molar fraction of component i in the gas phase. For the analysis of the breakthrough curves, we will consider the case of a bed where the flow can be considered as axially dispersed flow. The mass balance in a differential element of the column can be described as19
( )
∂CB 1 - c ∂2CB ∂CB ∂q j Fp -v + ) Dax 2 ∂t c ∂t ∂z ∂z
(4)
with initial condition
CB(0,z) ) 0 and Danckwerts boundary conditions
| | | | ∂CB ∂z
(t,0)
∂CB ∂z
(5)
) v(CB - CB0)
(t,L)
)0
(6) (7)
where CB and CB0 are the time-dependent and initial bulk gas-phase concentrations, Dax is the axial dispersion coefficient, c is the porosity of the column, and ν is the interstitial gas velocity. In these equations we have considered that the amount of gas adsorbed is very small compared to the total flow entering the bed, and thus the velocity is constant in the entire column. In accordance with the “trace system” assumption, the process was also assumed to be isothermal. The Fickian description of diffusion in a spherical zeolite crystal (where micropores are located) is
∂q ∂q 1 ∂ ) D r2 ∂t r2 ∂r c ∂r
[
]
(8)
where Dc is the crystal diffusivity. This equation has to be solved with the following initial and boundary conditions:
ai
where qmax,i is the saturation capacity of component i, P is the equilibrium pressure, and ai is the number of neighboring sites occupied by component i. An Arrhenius law of the following form describes the equilibrium constant K:
q/i
Dax
q(r,0) ) 0
(9)
| |
∂q )0 ∂r (0,t)
(10)
q(rc,t) ) q*
(11)
The crystal diffusivity has an exponential dependence on temperature described by
Dc ) D0c exp(-Ea/RT)
(12)
where D0c is the limiting diffusivity at high temperatures and Ea is the activation energy, although because the system is isothermal, the dependence on temperature was not imposed. The crystal-averaged amount adsorbed q j is defined by
q j)
∫0r qr2 dr
3 rc3
c
(13)
The axial dispersion coefficient has been calculated by19
Dax ) (0.45 + 0.55c)Dm + 2γ2rcv
(14)
where γ2 is constant with a value of 0.5 and Dm is the molecular diffusion coefficient. The VSA-PSA model used in this work was already reported for propane/propylene separation using zeolite 4A.14 The model includes the variations in velocity due to bulk adsorption and also the momentum balance (simplified to the Ergun equation) to account for the pressure drop in the column. Also, the energy balance has to be coupled to the system because isothermal behavior is no longer expected. To describe the resistances inside the particle (macro- and microporous), a bi-LDF (linear driving force) model was used.20 We tested the equivalence of the diffusion equations with the bi-LDF model by measuring single-component breakthrough curves in the VSA-PSA laboratory unit existing in our laboratory. The experimental conditions and
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Figure 2. Adsorption equilibrium of propylene on zeolite 4A crystals of 0.3, 1.9, 10, and 22 µm. Points (×) correspond to the adsorption equilibrium measured in zeolite 4A extrudates (CECA) at the same temperature. Solid lines are the multisite Langmuir model fittings (see Table 3). Symbols: 9, 0.3 µm; [, 1.9 µm; b, 10 µm; 2, 22 µm. Temperatures are 373, 423, and 473 K.
some representative physical parameters used for the simulation of the PSA cycles are shown in Table 2. Performance parameters (purity, recovery, and productivity) were calculated as22
∫0t
purity ) (
∫0t
recovery )
blow
CC3H6u|z)0 dt
∫0t
CC3H6u|z)0 dt +
blow
∫0t
CC3H6u|z)0 dt -
blow
∫0t
blow
∫0t
CC3H8u|z)0 dt) (15)
rinse
CC3H6u|z)0 dt
feed
CC3H6u|z)0 dt (16)
productivity )
∫0t
(
CC3H6u|z)0 dt -
blow
∫0t
rinse
ttotalwads
CC3H6u|z)0 dt)u0
(17)
All of the partial differential equations shown here have been solved using the gPROMS package (PSE Enterprise) using orthogonal collocation of finite elements. For the VSA-PSA model, 35 finite elements with two interior collocation points were used, while for the diluted breakthrough curves, 70 elements have to be used in the smaller crystals (to avoid oscillations in the propane curves) and only 25 in the larger ones, always with two interior collocation points. Adsorption Equilibrium Adsorption of propane and propylene was measured at 373, 423, and 473 K in static-batch experiments. The wet mass of adsorbent used for each experiment was 0.609 g. After activation in a vacuum and at high temperature, the samples lost around 20% in weight (air + preadsorbed moisture). Using an activation temperature of 593 K that was recommended for zeolite,16 the isotherms were nonreproducible and the amount of propylene adsorbed was very low in the initial measurements. This situation was reverted using an activation temperature of 673 K. The propylene isotherms measured in the four samples of zeolite 4A crystals (represented as a dry weight basis) are shown in Figure 2. In the same figure, data on commercial extrudates (expressed as the amount ad-
Figure 3. Adsorption equilibrium of propane on zeolite 4A crystals of 0.3 and 1.9 µm. Points (×) correspond to the adsorption equilibrium measured in zeolite 4A extrudates (CECA) at the same temperature. Solid lines are the multisite Langmuir model fittings (see Table 3). Symbols: 9, 0.3 µm; [, 1.9 µm. Temperatures are 423 and 473 K. Table 3. Adsorption Equilibrium Parameters (Multisite Langmuir) for Propane and Propylene on Zeolite 4A (Amount Adsorbed Expressed as Weight of Pellet (Dry Crystals + 20% Binder Material) gas
qmax,i [mmol/g]
ai
K0i [kPa-1]
-∆H [kJ/mol]
C3H6 C3H8
2.600 3.103
2.612 2.189
8.44 × 10-6 2.81 × 10-5
28.2 15.6
sorbed per gram of dry crystals in the extrudates) fitted with the multisite Langmuir model are presented for comparison,12 showing good agreement. To compare the commercial data with these crystals samples, we assumed that the extrudates have 20% of inert binder that does not contribute to adsorption of propane and propylene. With this assumption, the adsorption equilibrium of propylene was almost the same in all samples. Propane isotherms were measured in the 0.3-µm sample, and only some points were recorded with the 1.9-µm sample. As a result of the lower diffusivity values of propane diffusivity, it takes 3 days to reach a single equilibrium point in a zeolite with 3.8-µm crystal diameter,13 and thus it will take more than 1 week per point in the larger crystals. Because the propylene loading of all of the crystal samples is the same, with some confidence we can assume that the propane adsorption will be also the same in the larger crystals. Results obtained at 423 and 473 K are presented in Figure 3. Results are also compared with previous data obtained in extrudates (also expressed as the amount adsorbed per gram of dry crystals and assuming 20% of inert binder in the commercial sample) fitted with the multisite Langmuir model,12 showing also good agreement. We have compared the adsorption equilibrium of propane and propylene on four samples of zeolite of very different crystal size diameter made in the laboratory with a commercial zeolite sample, assuming that it has 20% of binder. With that comparison, we can conclude that, at least in the range of sizes covered in this work, the adsorption capacity of zeolite 4A for propane and propylene is independent of the crystal size. Indirectly, we can also conclude that the pelletization process did not change the adsorption equilibrium capacity of C3 hydrocarbons on the crystals located in the commercial extrudates used in the previous study. In Table 3, the parameters of the Nitta model used for the fitting of the extrudates12 (crystals + 20% of inert binder) are presented for further use in the PSA model. Note that to reproduce the solid lines in Figures 2 and
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Figure 4. Diluted breakthrough experiments of propylene diffusion in zeolite 4A crystals with a 0.3-µm average diameter at 373, 423, and 473 K. Solid lines are simulations with the model given by eqs 4-13. Symbols: 9, T ) 373 K; [, T ) 423 K; b, T ) 473 K.
Figure 5. Diluted breakthrough experiments of propylene diffusion in zeolite 4A crystals with a 10-µm average diameter at 373, 423, and 473 K. Solid lines are simulations with the model given by eqs 4-13. Symbols: 9, T ) 373 K; [, T ) 423 K; b, T ) 473 K.
Table 4. Crystal Diffusivity Parameters, Dc [cm2/s], Obtained from the Fitting of Breakthrough Curves diameter [µm] gas
T [K]
C3H6
373 423 473 373 423 473
C3H8
0.3
1.9
10.0
22.0
3.1 × 10-12 3.3 × 10-12 6.4 × 10-12 6.8 × 10-12 7.4 × 10-12 7.1 × 10-12 9.8 × 10-12 9.4 × 10-12 3.6 × 10-14 6.8 × 10-14
3 (expressed as a dry basis of pure crystals), the saturation capacities of both gases have to be increased by 20%; i.e., qmax,C3H6 ) 3.120 mmol/g and qmax,C3H6 ) 3.724 mmol/g. Adsorption Kinetics The same crystals used as those used for equilibrium determination were employed for breakthrough curve measurements. Only the smaller crystals were used for propane measurements. Adsorption experiments of propylene in the 0.3-µm sample at 373, 423, and 473 K are shown in Figure 4 (for conditions see Table 1). The solid lines represent the model described by eqs 4-13. The experiments were performed using such a flow rate to ensure isothermal behavior (experimentally confirmed at the outlet of the column). Because the samples were crystals, the only possible resistances are crystal diffusivity, film mass transfer, and axial dispersion. Because the flow rate was high, the film mass-transfer resistance was neglected. The variations of crystal diffusivity affect only slightly the trend of the curve, confirming that the parameter Dc/rc2 is high and thus the control of the curve is shared by the crystal diffusivity and the axial dispersion. The values used for the fitting of the curves in Figure 4 are reported in Table 4. Breakthrough experiments under the same conditions (mass of adsorbent and flow rate) at 373, 423, and 473 K were performed for the crystals of 10 µm. The experimental data together with the fitting of the model described by eqs 4-13 are shown in Figure 5. In this case the outlet concentration profile is much more dispersed. The diffusivities used for fitting the simulation to the experimental results are also reported in Table 4. Note that at 423 K propane goes out of the column faster than it does at 473 K. The smaller diffusivity (at lower temperature) makes propane break through faster than at high temperature (with higher diffusivity). To satisfy the adsorption equilibrium capacity, these lines cross each other at longer times and at 423 K the time to reach equilibrium is larger than that at 473 K.
Figure 6. Diluted breakthrough experiments of propylene diffusion in zeolite 4A crystals at 423 K with 0.3-, 1.9-, 10-, and 22-µm average diameters. Solid lines are simulations with the model given by eqs 4-13. Symbols: 9, 0.3 µm; [, 1.9 µm; b, 10 and 522 µm.
Breakthrough experiments on the other crystals (1.9 and 20 µm) were performed only at 423 K. These breakthrough curves are shown in Figure 6. Again, in the larger crystals, the effect of the crystal size distribution is pronounced. The diffusivity values used for the fitting are also reported in Table 4. As can be seen, the values reported for all of the crystals are within 15% and the difference is not large when compared with the results previously obtained in extrudates.13 One of the main advantages of having very small crystals of zeolite 4A is that it allows more accurate measurements of the propane diffusivity. Even with the very small crystal size of the samples, extremely fast curves are expected for zero length column (ZLC) measurements. Breakthrough curves of propane at 423 and 473 K measured in the 0.3-µm sample are shown in Figure 7. In this case also, the process was isothermal. The thick solid line corresponds to the model described by eqs 4-13. The diffusivity values used to fit the experimental data to the model are reported in Table 4. Note that the experiment at 423 K comes out of the column faster than that at 473 K. The smaller diffusivity (at lower temperature) makes propane break through faster than at high temperature (with higher diffusivity). To satisfy the adsorption equilibrium capacity, these lines cross each other at longer times and at 423 K the time to reach equilibrium is larger than that at 473 K. The adsorption diffusivity of propylene was measured in four different crystal sizes of laboratory-prepared zeolite 4A, while propane experiments were performed only in the smaller crystals. The diffusivity values obtained from the fitting of the diffusion model confirm
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Figure 7. Diluted breakthrough experiments of propane diffusion in zeolite 4A crystals with a 0.5-µm average diameter at 423 and 473 K. Solid lines are simulations with the model given by eqs 4-13. Symbols: 9, T ) 423 K; b, T ) 473 K.
at least the order of magnitude of previously reported values for pellets with a crystal size diameter of 3.8 µm.13 These results show that the crystal diffusivity for each hydrocarbon was approximately the same in all of the samples and, furthermore, the pelletization process does not affect seriously the diffusion of these gases;13 large differences (Dcrystals /Dpellets ) 6-8) were reported c c for ethane in crystals and pellets of zeolite 4A, indicating some damage of the crystals during pelletization.21 With these results, we can have the possibility of “adsorbent design” for a targeted separation; in this case, propane and propylene vary in crystal size according to convenient results in the VSA-PSA unit. VSA-PSA Once the adsorption equilibrium and kinetics of a sample are determined, modeling of any adsorptionbased process is possible. Some idealized theoretical rules can be applied to the gas mixture, and most of the time, results can only be confirmed with experiments. In this case, the adsorption equilibrium behavior of the full set of crystals tested in this work is in good agreement with previous values reported for extrudates.12 In this previous paper, the Nitta model successfully described the binary adsorption. The VSA-PSA cycle definition as well as compositions and operative variables (temperature and pressure) was also previously used for this separation in zeolite 4A pellets,7,14 and the same conditions were used in this work as a comparison. In this case, the feed is a propylene/propane mixture with a molar ratio of 54/46 and diluted with a nitrogen molar fraction of 50% of the total. The cycle consists of a pressurization step (up to 500 kPa), a feed step at constant pressure, a rinse step with only propylene (7.7% diluted in nitrogen), a cocurrent depressurization to intermediate pressure (to 50 kPa), and a countercurrent blowdown (to 10 kPa) where purified propylene is withdrawn from the column, partially regenerating the column for a new cycle. The main difference between the example shown here and the experiments performed in other works7,14 is the rinse time, which has been drastically diminished to improve recovery without serious effects in purity. A schematic diagram of the cycle together with the pressure varia-
Figure 8. (a) Schematic diagram of the VSA-PSA cycle used for propane/propylene separation. (1) Pressurization with a feed from Plow ) 10 kPa to Phigh ) 500 kPa. (2) Feed: Phigh ) 500 kPa. (3) Rinse: Phigh ) 500 kPa. Composition: 7.7% of propylene diluted with nitrogen. (4) Depressurization to intermediate pressure. Pinter ) 50 kPa. (5) Blowdown. C3H6 recovery. Plow ) 10 kPa. (b) Pressure at the exit of the column in cycle 200 (CSS): points are experimental data, and the solid line is a simulation of the experiment.
tions at the exit of the column during a whole cycle is shown in Figure 8. Starting conditions were the column filled with nitrogen at 120 kPa. A total of 40 cycles were performed using the scheduling described in Table 2 using the commercial sample zeolite 4A (CECA). The collected data were the temperature at three different points of the column, inlet and outlet pressures, and pure gas flow rates, and in a GC, 11 samples can be stored to analyze the concentration of a cycle. Experimental results of the VSA-PSA example experiment are shown in Figure 9, where the temperature at three different points of the column (18, 43, and 68 cm), pressure history at the end of the column, and molar flow rates of the initial cycle and in cycle 39 are reported. The solid lines are the fitting of the model used to simulate the process.14 Propylene was obtained at high purity in the
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Figure 9. Experimental pressure at the outlet of the column (a), temperature in the bottom (b), middle (c), and top (d) thermocouples (0.18, 0.43, and 0.68 m from the inlet), and molar flow rate of propane and propylene in the initial cycle (e) and in cycle 39 (f) for VSAPSA for propane/propylene separation.
blowdown step: 98.4% ( 0.3 experimental and 99.4% simulated with 86.6% recovery. Even when the temperature, pressure, and even the molar flow rate exiting each step seem to reach the cyclic steady state (CSS) in the first 15 cycles, the purity of propylene is continuously and very slowly decreasing in the first 300 cycles. This small and continuous decrease is directly related to the very small value of propane diffusivity. Because the time constant of the process is related to the value of Dc/rc2, diminishing the crystal size, the CSS will be achieved faster. If we look at the simulated concentration profiles of propylene inside the column, we can see that they are rather disperse because of its low diffusivity. The concentration profiles of propylene at the end of each step in the CSS are shown in Figure 10a. If the aperture of the crystals is opened more (exchanging with Ca2+, for example), the selectivity toward propylene will decrease, so the only solution to diminish mass-transfer resistance is to decrease the crystal size of the adsorbent. For comparison, in Figure 10b we show the concentration profiles for the same experiment but with crystals of 1-µm diameter. In this case, the length of the unused bed can be further diminished (by increasing
the feed time) without increasing propylene losses. In this case, intuitively, we notice that the feed time can be increased (increasing productivity) without significant propylene losses. Using smaller crystals in a micropore-controlled process, we have sharper concentration fronts, thus enhancing the productivity of the unit. Even though using smaller crystals, the propylene purity is smaller. As the crystal size decreases, the time constant (crystal radius2/diffusivity) decreases, the reason being that breakthrough curves get sharper (as shown for propylene in Figure 6). The sharpness of the pure-component breakthrough curves is important not only for propylene but for propane as well. In a VSAPSA cycle, the binary mixture is introduced. In the case of smaller crystals, the amount of propylene adsorbed in the bed is higher than that in the larger crystals. The concentration of propylene in the gas phase (directly related to the amount adsorbed) is shown in Figure 10 for two different crystal sizes, showing that more propylene is adsorbed in the bed with smaller crystals. However, also with smaller crystals, the propane adsorbed in the bed is higher, as shown in Figure 11, especially at the end of the column, where less propylene is there to displace propane. When the crystal size
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Figure 12. Effect of average crystal size diameters on the purity (9), recovery (b), and productivity ([) of the VSA-PSA experiment.
Figure 10. Simulated concentration profiles of propylene at the end of each step (1, pressurization; 2, feed; 3, rinse; 4, depressurization to intermediate pressure; 5, blowdown) for a commercial sample with a crystal diameter of 3.8 µm (a) and for 1.0-µm diameter (b).
Figure 11. Simulated adsorbed-phase concentration profiles of propane at the end of each step (0, pressurization; ×, feed; 4, rinse; ], depressurization to intermediate pressure; +, blowdown) for a commercial sample with a crystal diameter of 3.8 µm (a) and for 1.0-µm diameter (b).
decreases, the time constant is smaller and so more propane will diffuse into and also out of the crystals. For this reason, during blowdown, higher amounts of propane are desorbed from the column, contaminating the propylene and thus obtaining propylene with less purity than when decreasing the crystal size. The difference between the amount of propane adsorbed in the bed at the end of the depressurization step and that at the end of the blowdown step (see Figure 11) corre-
Figure 13. CSS approach with different zeolite 4A crystal sizes.
sponds to the contamination of the product: in the case of crystals with 3.8 µm, this difference is extremely small, while for the smaller crystals, larger differences are obtained. Simulations of the same experiment, with crystal diameters varying from 1 to 5 µm, were done. The performance results shown in Figure 12 only mean that it will be very difficult to reach very high purity of propylene with small crystals while keeping high recovery and productivity. Results obtained here have higher productivity, recovery, and purity than those previously obtained.7 Also, the CSS approach is different according to crystal size. The most critical parameter to achieve CSS is the purity. The propylene purity obtained in the example experiment, with crystal diameters varying from 1 to 5 µm, is shown in Figure 13. When commercial samples are used, 300 cycles are needed for CSS, while only 25 are required in the case of crystals with 1 µm. Although this experiment was performed using cycle times to obtain high purity and recovery in the commercial extrudates and conditions were not optimized for smaller crystals, in the VSA-PSA application for polymer-grade propylene production (target of >99.5% purity), crystals from 4 to 5 µm should be used with acceptable values of recovery and productivity, while for chemical-grade propylene (target of >96% purity), smaller crystals, having almost the same recovery but much higher productivity, can be used. Because this example was far away from being optimized for smaller crystals (see Figure 10b), in the case of the 1-µm crystals, we performed a simulation using the same pressure and temperature conditions and the same flow rates but changing the step times (tpress ) 55 s; tfeed ) 170 s; trinse) 25 s; tinterblow) 30 s; tblow ) 170 s), obtaining a purity of 96.5%, a recovery of 87.2%, and a productivity of 3.5 mol of C3H6/kg‚h, much higher than the one obtained with larger crystals. Conclusions Four samples of zeolite 4A with different crystal sizes were synthesized. The adsorption equilibrium and
Ind. Eng. Chem. Res., Vol. 43, No. 23, 2004 7565
kinetics of propane and propylene have been experimentally measured at 373, 423, and 473 K in all of the samples. The adsorption equilibria of both hydrocarbons were in very good agreement with data previously reported in extrudates. Adsorption kinetics of both gases also confirm the order of magnitude of the diffusivities of propane and propylene also previously reported (values within (15%), although in this case, the measurements of propane diffusion and equilibrium with the smaller crystals have been much simpler and also more specific analyses, such as breakthrough curves, could be performed. As an indirect conclusion, we can say that the pelletization process does not affect the extrudates previously used in our laboratory. An experiment using zeolite 4A was performed, obtaining a propylene purity of 98.4 ( 0.3% experimental and 99.4% simulated, a recovery of 86.6%, and a productivity of 2.33 mol/kg‚h. By simulations of the same experiment with smaller crystals in the zeolite, we obtained smaller purity of the product with much higher productivity and recovery (for 1-µm crystals, the purity was 94.1%, the recovery 92.7%, and the productivity 2.42 mol/kg‚h. Although this experiment was performed using cycle times to obtain high purity and recovery in the extrudates and conditions were not optimized for smaller crystals, the lower purity using smaller crystals will remain using this cycle. In VSAPSA simulations with the five-step cycle employed in this work, polymer-grade propylene was obtained only with crystals between 4 and 5 µm, while smaller crystals can be used for chemical-grade propylene, increasing product recovery and unit productivity. Acknowledgment Authors are thankful for financial support from the Foundation for Science and Technology (FCT), Project POCTI/1999/EQU/32654. C.A.G. acknowledges FCT Grant SFRH/BD/11398/2002. The work is also part of the CYTED Project V.8 (“Clean Technology for the Separation of Light Olefins”). Notation ai ) number of neighboring sites occupied by component i CB ) bulk molar concentration of the sorbate in the gas phase, mmol/cm3 CB0 ) initial bulk molar concentration of the sorbate in the gas phase, mmol/cm3 Dax ) axial dispersion, cm2/s Dc ) crystal diffusivity, cm2/s D0c ) crystal diffusivity at limit T f ∞, cm2/s Ea ) activation energy for crystal diffusion, J/mol Dm ) molecular diffusivity, cm2/s Ki ) adsorption constant of component i, kPa-1 K0i ) adsorption constant at limit T f ∞ for component i, kPa-1 P ) total pressure, kPa qi ) adsorbed-phase concentration of component i, mmol/g q/i ) equilibrium adsorbed-phase concentration of component i, mmol/g qmax,i ) saturation capacity of component i, mmol/g R ) ideal gas constant, 8.314 kJ/mol‚K rc ) crystal radius, cm tblow ) blowdown step time, s trinse ) rinse step time, s ttotal ) total cycle time, s T ) temperature, K u ) superficial velocity, cm/s yi ) molar fraction of component i in the gas phase
Greek Letters Fp ) particle density, g/cm3 c ) column porosity ∆Hi ) heat of adsorption of component i, J/mol ν ) interstitial velocity, cm/s γ2 ) constant for calculation of the axial dispersion coefficient wads ) adsorbent weight, kg
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Received for review June 11, 2004 Revised manuscript received July 23, 2004 Accepted August 25, 2004 IE049489L
