Propylene Separation by Pressure Swing Adsorption Using

Ind. Eng. Chem. Res. 2005, 44, 8815-8829

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SEPARATIONS Propane/Propylene Separation by Pressure Swing Adsorption Using Zeolite 4A Carlos A. Grande 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

In this work we have studied Pressure Swing Adsorption (PSA) process for separation of propane/ propylene mixtures using zeolite 4A extrudates (CECA) as an alternative to distillation. Purified propylene is withdrawn in the blowdown step at 10 kPa because is the most adsorbed compound. The cycle used comprises five steps: pressurization, feed, purge with product, depressurization to intermediate pressure and counter-current blowdown for propylene recovery. Two mixtures with different propylene/propane ratios (54-46 and 85-15 diluted to 50% with nitrogen) corresponding to different sources of propylene production (steam cracking and Fluid Catalytic Cracking) were studied. The effects of the different step times in the performance of the PSA unit were evaluated. PSA performance was also studied using different operating conditions: process temperature and feed pressure. Also, the removal of inert from the process was studied. The simulation of the PSA unit was performed in gPROMS using a bi-LDF approximation for mass transfer. Experimental purities of propylene around 99.0% were obtained. The recovery of propylene in some of the runs reached values higher than 85% which is much higher than 30% obtained in previous studies. The best performance was obtained at 408 K where simulated purity was 99.43% with a recovery of 84.3% and a unit productivity of 2.57 mol/h.kg of zeolite. 1. Introduction The separation of propane/propylene streams is the most intensive separation carried out in petrochemical industry; the polymer-grade propylene requires a purity > 99.5%. Adsorption-based processes may enhance the selectivity of the separation requiring less energy. Such processes have been explored in the last years to carry out the olefin/paraffin separations as an alternative to the traditional distillation.1 To use an adsorption technology like pressure swing adsorption,2,3 a very selective adsorbent has to be used because propylene is the most adsorbed compound, being recovered in the blowdown step simultaneously with partial regeneration of the adsorbent. The propane/propylene separation by PSA is also in research stage, and some processes with specific adsorbents have been patented.5-11 Many available commercial adsorbents were previously evaluated for this separation.4,12-14 From all the commercial adsorbents tested, zeolite 4A presented the highest selectivity toward propylene.12,15,16 In fac,t a PSA process using this zeolite as adsorbent was patented.17 Separation of propane/propylene mixtures with different cycle schemes was discussed in the literature.16,18,19 A brief description of the PSA processes and their performances is shown in Table 1. It can be observed that very different values of purity and recovery of propylene have been obtained in these works. For equimolar mixtures, it was observed * To whom correspondence should be addressed. Tel.: +351 22 508 1671. Fax: +351 22 508 1674. E-mail: [email protected].

that, with purity higher than 99%, propylene recovery was always smaller than 30%. In a previous paper, we have reported experimental data of adsorption equilibrium of propane in zeolite 4A20 much higher than that previously reported in the literature.12,15,16 Even though a very large difference in the crystal diffusivity of both gases was observed, it is the reason that kinetic separation can still be successfully performed using this adsorbent. It is the purpose of this work to evaluate the performance of a PSA unit for propylene purification. We have studied a nearly equimolar C3 mixture with propylene/ propane ratios of 54-46 diluted to 50% with nitrogen (inert gas) found in propylene production from steam cracking of gas oil. Additionally, the mixture with a propylene/propane ratio of 85-15 also diluted to 50% with nitrogen was studied. Mixtures with 80-87% propylene are obtained in the fluidized catalytic cracking (FCC) process. One of the novelties reported here, when compared to previous works developed in our laboratory, is the study performed to eliminate nitrogen from the PSA process. The PSA cycle configuration employed has five steps comprising the following: co-current pressurization, feed, rinse with pure propylene, depressurization to intermediate pressure, and counter-current blowdown for bed regeneration and where propylene product (lowpressure) is withdrawn. The effect of different step times and operating conditions (temperature, inert content, and feed pressure) on process performance was studied to test the validity of the proposed mathematical model under very different conditions and to find operating

10.1021/ie050671b CCC: $30.25 © 2005 American Chemical Society Published on Web 10/15/2005

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Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005

Table 1. Summary of State of the Art for Propane/Propylene Separation by PSA Using Zeolite 4A T [K]

cycle definitions

purity

recovery

ref

373

four steps: pressurization, feed, rinse, blowdown; step times: 400 s (for all); Phigh ) 1.0 bar; Plow ) 0.1 bar; feed: 70 C3H6/30 C3H8b

99.97 99.1

23.6 10.5

13

five steps: pressure equalization (9 s), counter-current pressurization with propane (7 s), feed (34 s), bed equalization (9 s), blowdown (41 s); Phigh ) 1.7 bar; Plow ) 0.26-0.13 bar; feed: 88 C3H6/12 C3H8a

95.6 96.2 96.2 97.6

96.3 97.4 98.2 91.0

25

343 363 383 448

a

T [K]

cycle definitions

step time

purity

recovery

ref

373

four steps: pressurization, feed, rinse, blowdown; all step times have equal duration; Phigh ) 1.0 bar; Plow ) 0.1 bar; feed: 50 C3H6/50 C3H8b

100 400 600 800

99.94 99.01 99.98 99.97

7.95 12.16 27.29 23.59

14

T [K]

cycle definitions

P/F

purity

recovery

ref

423 393 423 423 423

five steps: pressurization (60 s), feed (120 s), rinse (120 s), depresurization to intermediate pressure (60 s), blowdown (120 s); Phigh ) 5.0 bar; Plow ) 0.1 bar; feed: 25 C3H6/25 C3H8/50 N2; different total flow ratesa

0.78 0.45 0.28 0.23 0.13

91.2 97.4 98.6 94.5 87.9

20.0 9.4 17.1 27.3 46.5

26

Experimental values. b Simulated values.

Table 2. Details of Equipment and Adsorbent Properties Used for Propane/Propylene Separation with Zeolite 4A Extrudates (CECA) column parameters column length, m column radius, m column porosity bulk density, kg/m3 wall density, kg/m3 wall specific heat, J/kg‚K wall heat transfer coefficient, W/m2‚K overall heat transfer coefficient, W/m2‚K total flow rate, SLPM

adsorbent parameters (zeolite 4A) 0.87 0.0105 0.43 758 8238 500 60 30 1.10

conditions to produce polymer-grade propylene (purity higher than 99.5%).

crystal radius, m pellet radius, m pellet density, kg/m3 pellet porosity pellet tortuosity solid specific heat, J/kg‚K

1.9 × 10-6 8 × 10-4 1210 0.34 2.0 920

being initially activated at 593 K under constant flow of helium for at least 24 h and kept under inert environment until the finish of all experiments.

2. Experimental Section The single-column PSA laboratory unit used in this work was already installed in our laboratory for generic hydrocarbon gas separation.21,22 The operating ranges of the equipment are 313-673 K, 0.1-7 bar, 0.1-6 SLPM (standard liters per minute at 273 K and 1 bar). The gas entering and exiting the column can be analyzed via a multi-port valve system where 11 samples can be stored for subsequent analysis with FID (flame ionization detector) in a Chrompack 9001 gas chromatograph. A full description of the unit (component specifications, instruction for users, and control setup scheme) was given elsewhere.22 In this experimental setup, we recorded pressure at the inlet and outlet of the column, temperature at three different points of the column (0.18, 0.43, and 0.68m from feed inlet), and flow rates of each gas entering the column (independent mass flow meters) and exiting the column (by GC analysis). The experiments reported in this work cover a temperature range of 408-463 K. The total feed pressure was also varied from 250 to 500 kPa according to the content of nitrogen used as inert gas. Equipment details and significant adsorbent properties are presented in Table 2. Air Liquide provided all gases used in this report: propane N35, propylene N24 and nitrogen 50 (purity greater than 99.95, 99.4 and 99.999% respectively). Zeolite 4A extrudates were kindly provided by CECA (France). Around 0.2 kg of extrudates was employed,

3. Theoretical Section Zeolite adsorbents are bidisperse materials consisting of an inert matrix (binder) that contains the zeolite crystals. For this reason, macro- and mesopores of the inert binder may control the mass transfer or this process can be controlled by intracrystalline diffusion in the channels of the zeolite crystals or even by a combination of both resistances. Also, a film mass transfer resistance may exist in the layer surrounding the pellet. The model used in this work takes into account these three resistances that will make the concentration fronts more disperse. The gas-phase dispersion was lumped into a single parameter, the axial dispersion coefficient. As we are working in bulk gas separation, nonisothermal behavior is expected due to the heat of adsorption/desorption; the heat balance has to be included in the model. Also due to bulk adsorption, the velocity of the gas inside the column will not be constant. The assumptions for the model that will be used below are the following: (1) ideal gas; (2) heat, mass, and momentum transport in the radial direction of the column are neglected and only axial coordinate is considered; (3) momentum balance is simplified by using the Ergun equation;

Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8817

(4) meso- and macropore resistances are lumped in a single parameter, the macropore diffusivity that is obtained relating the Knudsen and molecular diffusion via the Bosanquet equation; (5) macro- and micropore diffusion equations are simplified by a bi-LDF (linear driving force) model instead of the mass balances in the macropores and in the zeolite crystals. This assumption has a large impact in computational time; (6) film mass transfer in the layer surrounding the pellets is considered; (7) void fraction is uniform in the entire column. With these assumptions, the fixed-bed model is described by the mass, energy, and momentum balance equations. The component mass balance on the column is22

(

)

∂(uCi) a′kf,i ∂Ci ∂yi ∂ c ) c DaxCT - (1 - c) × ∂t ∂z ∂z ∂z Bii + 1 (Ci - 〈ci〉) (1) where Ci ) yiCT is the gas-phase concentration of component i; ui is the superficial velocity; c is the column porosity; yi is the molar fraction; kf,i is the film mass transfer resistance; Bii ) Rpkf,i/(5pDp,i) is the Biot number; 〈ci〉 is the averaged concentration in the macropores, all valid for component i; while CT is the total gas concentration; a′ is the pellet specific area; and Dax is the axial dispersion coefficient calculated by23

2 1.75(1 - c)Fg ∂P 150µg(1 - c) ) u+ |u|u (3) 3 2 ∂z d 3d p

c

p

where P is the total pressure, µg is the gas viscosity, dp is the pellet diameter, and Fg is the gas density. The LDF model was used to describe the mass transfer rate from the gas phase to the macropores:

p

∂ci ∂〈qi〉 15Dp,i Bii + Fp ) p (Ci - ci) ∂t ∂t R 2 Bii + 1

(4)

p

where Dp,i is the pore diffusivity, Rp is the pellet radius, Fp is the particle density, p is the pellet porosity, and 〈qi〉 is the pellet averaged adsorbed phase concentration. The LDF equation for the crystals averaged over the entire pellet is expressed by

∂〈qi〉 15Dc,i / ) (〈qi 〉 - 〈qi〉) ∂t r2

(5)

c

where Dc,i is the crystal diffusivity, rc is the crystal radius, and 〈q/i 〉 is the adsorbed phase concentration in

( )

∂Tg ∂Tg ∂C ∂ ∂Tg ) + cRgTg λ - uCTC ˜p ∂t ∂z ∂z ∂z ∂t 2hw (1 - c)ahf(Tg - Ts) (T - Tw) (6) Rw g

where C ˜ v is the molar constant volumetric specific heat of the gas mixture, Tg is the temperature of the gas phase, C ˜ p is the molar constant pressure specific heat of the gas mixture, Rg is the universal gas constant, hf is the film heat transfer coefficient between the gas and the solid phase, Ts is the solid (extrudate) temperature, hw is the film heat transfer coefficient between the gas phase and the column wall, Rw is the column radius, Tw is the wall temperature, and λ is the axial heat dispersion calculated by23

λ ) 7 + 0.5PrRe kg

(7)

where kg is the thermal conductivity of the gas and Pr is the Prandtl number. The solid-phase energy balance is expressed by n

(2)

where Dm,i is the molecular diffusivity of component i; Sci ) µg/FgDm,i is the Schmidt number of component i; and Re is the Reynolds number. The momentum balance only considers the terms of pressure drop and velocity changes and relates them by the Ergun equation:

c

cCTC ˜v

(1 - c)[p

cDax ) 20 + 0.5SciRe Dm,i

-

equilibrium with the concentration of component i averaged over the pellet, 〈ci〉. The energy balance will takes into account the three phases present: gas, solid, and column wall. The gasphase energy balance is

∂Ts

n

〈ci〉C ˜ vi + Fp ∑ 〈qi〉C ˜ v,ads,i + FpC ˜ ps] ∑ ∂t i)1 i)1

(1 - c)pRgTs

∂〈ci〉 ∂T

∂〈qi〉

n

+ Fb

∑ i)1

)

+ ∂t (1 - c)a′hf(Tg - Ts) (8) (-∆Hi)

where Fb is the bulk density of the column and (-∆Hi) is the isosteric heat of adsorption of component i not considered a function of adsorbate loading. Finally, the wall energy balance can be expressed by

˜ pw FwC

∂Tw ) Rwhw(Tg - Tw) - RwlU(Tw - T∞) (9) ∂t

with

Rw ) Rwl )

Dw

(10)

e(Dw + e) 1

(

)

Dw + e (Dw + e) ln Dw

(11)

where Rw is the ratio of the internal surface area to the volume of the column wall, Rwl is the ratio of the logarithmic mean surface area of the column shell to the volume of the column wall, Dw is the internal diameter of the column, e is the wall thickness, C ˆ pw is the specific heat of the column wall, U is the global external heat transfer coefficient, and T∞ is the oven constant set-point temperature. Adsorption equilibrium of pure components and binary mixtures of propane and propylene on zeolite 4A were determined in a previous work20 and fitted with the multisite Langmuir model. The parameters for

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Table 3. Adsorption Equilibrium and Kinetic Parameters of Propylene and Propane in Zeolite 4A (CECA)

C3H6 C3H8

ai -∆Hi (-) (kJ/mol)

Ki0 (kPa-1)

qmax,i gas (mol/kg)

D°c,i (m2/s)

Ea,i (kJ/mol)

8.44 × 10-6 2.612 28.265 4.66 × 10-14 15.610 2.81 × 10-5 2.189 15.605 2.26 × 10-15 23.670

2.600 3.103

propane and propylene are given in Table 3. Nitrogen is assumed to be a non-adsorbing gas at the temperatures used here. The multisite Langmuir model is expressed by24

q/i qmax,i

(

) K iP 1 -

q/i

∑ qmax,i

)

ai

(12)

where qmax,i is the maximum amount adsorbed, ai is the number of sites occupied per molecule and with an exponential temperature dependence for the adsorption constant Ki described as

Ki ) Ki0 exp

( ) -∆Hi RTs

(13)

where Ki0 is the infinite adsorption constant and (-∆Hi) is the isosteric heat of adsorption, both for component i. The crystal diffusivity of propane and propylene at low hydrocarbon concentration (determined in the linear region of the isotherms) were also reported before25 as a function of temperature and described by ∞ o ) Dc,i exp(-Ea,i/RT) Dc,i

(14)

where D°c,i is the limiting diffusivity at high temperatures and Ea,i is the activation energy. The micropore diffusivity as used in the LDF constant (eq 5) also depends on concentration. In this work, we described this dependency by the Darken’s model:26 ∞ Dc,i ) Dc,i

d ln(Pi)

(15)

d ln(qi)

The diffusivity parameters for propane and propylene in zeolite 4A are also reported in Table 3. The performance of the different experiments and simulations was evaluated according to three basic parameters: purity and recovery of product and unit productivity of the PSA. They are defined by27

purity )

∫0t ∫0t

(

blow

∫0t recovery ) t ∫0

blow

CC3H6u|z)0 dt +

blow

press

∫0t

blow

∫0t

blow

∫0t t CC H u|z)0 dt + ∫0

CC3H6u|z)0 dt 3

6

productivity ) (

CC3H6u|z)0 dt

CC3H6u|z)0 dt -

∫0t

rinse

ttotalwads

CC3H8u|z)0 dt) (16)

rinse

feed

CC3H6u|z)0 dt CC3H6u|z)0 dt (17)

CC3H6u|z)0 dt)Qo

(18)

where wads is the adsorbent mass loaded in the column;

Qo is the inlet flow rate; and tfeed, trinse, and tblow are the times of the feed, rinse. and counter-current blowdown steps, respectively. The model described by eqs 1-15 was solved using gPROMS (PSA Enterprise, UK). The orthogonal collocation method on finite elements (OCFE) was used with 35 finite elements and two interior collocation points in each element of the adsorption bed. 4. Results and Discussion 4.1. Binary Fixed-Bed Experiments. The transport parameters calculated from correlations presented in the literature28-30 have to be checked. Before starting with PSA experiments, binary fixed-bed runs are actually used as “mathematical model verification” experiments, where values of the mass and energy parameters are inserted in the program for future use in PSA simulations. Some fixed-bed experiments were performed with the binary mixture of propane and propylene (diluted with nitrogen): one with a composition of 0.450 C3H6/0.035 C3H8/0.510 N2 (corresponding to a propylene/propane ratio of 85/15) and another with a nearly equimolar C3 composition: 0.250 C3H6/0.240 C3H8/ 0.510 N2. Both experiments were performed at a total pressure of 250 kPa and 423 K with a total flow rate of 1.1 SLPM; results are reported in Figure 1. In Figure 1, the solid lines correspond to the mathematical model described in the Theoretical Section. The parameters used in the model are described in Table 2. It has to be mentioned that the heat transfer coefficient at the wall (hw) was used as a fitting parameter to address the temperature changes in the column. It can be observed that the mathematical model can describe well the behavior of the hydrocarbon mixtures with two different ratios of propylene to propane that correspond to typical industrial streams of propylene production. Temperature increase due to propylene adsorption is also shown. In both curves a stream containing only propane can be taken out of the column without any propylene. Note also that the temperature increase due to propane adsorption is negligible while the one for propylene adsorption is around 12 K for higher propylene content, which is why the process cannot be considered isothermal. 4.2. Description of PSA Cycle and Definition of Operating Conditions. The PSA cycle configuration employed for propane/propylene separation has five steps. These steps are as follows: co-current pressurization with feed stream, feed, co-current rinse with pure propylene, co-current depressurization to intermediate pressure, and counter-current blowdown. The last step is performed using vacuum to withdrawn propylene product and also to partially regenerate the adsorbent. To illustrate the PSA cycle, a schematic diagram of the process is shown in Figure 2. An open-ended design of the five-step cycle to be used for C3 separation in the laboratory unit (with column length and volume already specified) has the following operating and process parameters: Tfeed, Trinse, Qpres, Qfeed, Qrinse, Phigh, Pinter, Plow, tpres, tfeed, trinse, tdepres, tblow, and feed composition. The optimization of a process with so many parameters is very difficult, particularly when they are related by a system of partial differential equations. In the group of parameters mentioned, we can distinguish two general types: the operating parameters and the design parameters. To simplify the study of the process, it is desirable to reduce the number


Figure 1. Propane/propylene breakthrough curves at 423 K and 250 kPa using 1.1 SLPM of total flow rate. (a) and (b) are molar flow rate of gas and temperature profile inside the column (at 0.18, 0.43, and 0.68m from feed inlet) for a mixture 0.250 C3H6/0.240 C3H8/ 0.510 N2 while (c) and (d) correspond to a mixture 0.450 C3H6/0.035 C3H8/0.510 N2. Solid lines are theoretical model predictions, and solid points are experimental values.

Figure 2. Schematic diagram of the PSA cycle used for propane/ propylene separation. Steps description: (1) pressurization with feed; (2) feed; (3) rinse with pure propylene; (4) co-current depressurization to intermediate pressure; (5) counter-current blowdown at low pressure (vacuum) where product is withdrawn and adsorbent is partially regenerated.

of operating conditions. The first assumption is to use the same temperature for the feed (and pressurization) and for the rinse streams, which was initially fixed at 433 K. Other important design parameters are the flow rates. In this work the flow rates of all streams (pressurization, feed, and rinse) assumed a fixed value and will not be varied. Also, hydrocarbon composition in feed was fixed at two different ratios of propylene to propane: 85/15 and 54/46. The change in feed composition studied in this work was the nitrogen removal from the process. Still the system has many design parameters to be defined: step pressures and step times. In this work, the effect of most of these parameters was studied. Initially, the step pressures Phigh ) 500 kPa; Pinter ) 50 kPa; Plow ) 10 kPa were chosen to perform most of the experimental data collection. After studying the effect of the step times in the cycle performance, the operating conditions (inert content, temperature, and feed pressure) will be addressed. 4.3. PSA Experiments with Zeolite 4A: Initial Runs. Before studying the effects of the different

parameters, it is convenient to give an example of an experimental run obtained in a PSA experiment using zeolite 4A (CECA, France). For this purpose an experiment was performed with 60 s of pressurization, 70 s for feed step, 130 s of rinse, 60 s of co-current depressurization, and 220 s for counter-current blowdown step. Operating parameters were kept at the following: T ) 433 K (for all inlet streams and surroundings of the column); Phigh ) 500 kPa (pressurization and feed); Pinter ) 50 kPa (co-current depressurization); Plow ) 10 kPa (counter-current blowdown); pressurization and feed flow rate: 3.16 SLPM; rinse flow rate: 2.70 SLPM; molarfractionofpressurizationandfeedstream: 0.453 C3H6/ 0.074 C3H8/0.473 N2 (ratio of 85/15 propylene/propane); molar fraction of rinse stream: 0.077 C3H6/0.923 N2. The relevant experimental information collected from the PSA unit (pressure and temperature) and GC (molar flow rate of hydrocarbons exiting the column) can be summarized in Figure 3. The pressure at the exit of the column and the temperatures measured by the three thermocouples located at 0.18, 0.43, and 0.68 m from the feed inlet are reported for the first 15 cycles. In this figure we also show the molar flow rate analyzed by GC for the first step and for cycles 39 and 50 where no variation of molar flow rate was experimentally detected. The solid lines in the figures represent the prediction of the model described in the Theoretical Section. The model is in good agreement with the experimental data indicating that the cyclic column behavior is well predicted. Some differences exist in the experimental and simulated molar flow rates in the co-current depressurization step that may be due to dead volumes of the equipment. The experimental purity of this example is 98.7%, even when simulated value is somewhat higher: 99.32%. Differences between experimental and simulated values may be due to dead volumes at the end of the PSA

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Figure 3. Experimental results obtained in the PSA unit for propane/propylene separation using zeolite 4A at 433 K with a mixture 0.450 C3H6/0.035 C3H8/0.510 N2: (a) pressure at the outlet of the column in the pressurization (1), feed (2), rinse (3), depressurization (4), and blowdown (5); temperature histories in the bottom (b), middle (c), and top (d) thermocouples (0.18, 0.43, and 0.68 m from inlet) and molar flow rate of propane and propylene in the second cycle (e) and in cycle 39-50 (f). Solid lines are predictions of the theoretical model, and points are experimental values. Operating conditions are reported in Table 8.

column and/or because the propylene used for the experiments has 0.5% of propane in its composition and this may be critical, particularly in the rinse step. The recovery of propylene in this experiment was 62.7% (see Table 8). This value is higher than previously obtained values using zeolite 4A Rhone-Poulenc.22 This difference may be due to different propane diffusion into the zeolite structure. One possible explanation for the difference in the propane kinetics in different adsorbents can be due to the thermal treatment of the samples (activation protocol) that may dislocate the Na+ ions from the structure producing small differences in the channel aperture as reported for smaller molecules.31 4.4. Improving the Step Times (Design Parameters). Once all the operating parameters are specified, the other parameters to be studied are the step times (i.e., the design parameters, which determine the overall performance of the unit). Although the initial experiment was performed with propylene/propane ratio of 85/ 15, in this section, except when explicitly mentioned, the propylene/propane ratio will be 54/46. It was verified experimentally that with the column used and for a

specific flow rate (Qpres ) 2.785 SLPM), the pressurization step cannot take less than 54 s, and this value was used in almost all the experiments as fixed time for the pressurization step. This cycle has no counter-current purge with light product after the counter-current blowdown step. Light product purification is normally used when the light gas is the purified product.32 Having no counter-current purge allows a straight route to calculate the upper limiting time of the feed step (before breakthrough of propylene). Assuming that the counter-current blowdown (last step of the cycle) is very long, the column will reach adsorption equilibrium at 10 kPa with almost pure propylene. Starting with a column filled with the adsorbent saturated (in equilibrium) with propylene at 10 kPa and with a pressurization step of 54 s, the maximum feed time can be determined. The simulated behavior of the column (molar flow rate exiting the column, temperature and propylene concentration and amount adsorbed at different times) is shown in Figure 4. Propylene is exiting the column since time zero of the feed step, but a major breakthrough


Figure 4. Behavior of the PSA column started with the bed saturated with propylene at 10 kPa and feed with a mixture 0.250 C3H6/ 0.240 C3H8/0.510 N2: (a) molar flow rate of gases exiting the column; (b) temperature profiles at different times 54, 100, 150, 200, 225, and 250 s. Table 4. PSA Performance for Propane/Propylene Separation with Zeolite 4Aa expt no.

pressurization, s

feed, s

rinse, s

depressurization, s

blowdown, s

purity %, simulated

recovery %, experimental

productivity, mol/h‚kg

4

60

60

25

40

220

87.3

1.75

5

54

100

25

40

180

85.6

2.64

7

54

100

25

40

220

86.7

2.36

8

54

100

75

40

220

81.6

2.28

9

54

100

75

40

180

82.6

2.38

10

54

60

25

40

180

86.4

1.97

12

54

60

75

40

180

80.0

1.73

13

54

60

75

40

220

81.3

1.58

16

54

60

15

25

180

84.1

1.64

17

54

85

15

25

220

99.27 (98.7 ( 0.3)b 99.31 (98.6 ( 0.2)b 99.35 (98.7 ( 0.3)b 99.39 (98.8 ( 0.4)b 99.27 (98.7 ( 0.2)b 98.91 (98.4 ( 0.3)b 99.12 (98.6 ( 0.4)b 99.33 (99.0 ( 0.3)b 98.48 (98.2 ( 0.2)b 98.61 (98.3 ( 0.3)b

85.7

2.38

a Operating conditions were as follows: T ) 433 K; P high ) 500 kPa; Pinter ) 50 kPa; Plow ) 10 kPa; pressurization and feed flow rates: 2.785 SLPM; yC3H6 ) 0.27, yC3H8 ) 0.23, yN2 ) 0.50. b Experimental values.

happens after 160 s of feed step (dotted line in Figure 4a). This time is the maximum limiting time because the blowdown step will never be large enough to allow the column to reach equilibrium; therefore, more propylene will stay in the column, and the feed time has to be shortened. Once the maximum feed step is known, some comments on the experiment shown in Figure 3 will be made. In the molar flow rate plots, it can be seen that there is much propylene being lost in the feed step and in the rinse step, even in the second cycle where the column is only partially saturated of hydrocarbons and far away from cyclic steady state. The propylene losses get worse for a large number of cycles. This is an indication that too much propylene is entering to the system. Also, the amount of propane leaving the column in the rinse step is only important in the first moments. A way to reduce propylene losses to improve recovery and also with positive effects on productivity is to reduce the rinse step time. This decrease in the rinse time can be explained in terms of the very small value of the propane diffusivity: the amount of propane adsorbed per cycle is small, but also this amount will not be easily desorbed from the zeolite in the rinse step. For this reason, the rinse step has the only purpose of removing propane from the column and pellets (macropores) voidage. The lower boundary of the rinse step is the

residence time of the gas into the column. Although the purpose of this step is to exclude propane from the column voidage and also from the larger macropores of the adsorbent and since some propylene will adsorb in the zeolite, a reasonable limit of 25 s was assumed as lower boundary. In Table 4, the result of different PSA experiments is summarized. All the operating parameters were kept constant and only step times were changed. The mathematical model predicts well all the collected data. Due to reasons explained above (contamination of propane in the propylene bottle and dead volumes of the experimental setup), a difference of around 0.5 was detected in all the experiments (purity around 98.4-98.8%). For this reason the simulated and experimental values of purity are presented in Table 4 while the recovery and productivity values (calculated from the integral of the propylene molar flow rate) correspond to the experimental values. For a constant value of all other step times, only the rinse time was changed from 25 to 75 s as can be seen in Table 4. In all the experiments when the rinse step is more than 25 s, the recovery drops to values around 80% instead of 85% and changes in purity are only of within 0.1%. Clearly the rinse time of 130 s is excessive for this zeolite sample, and the decrease in the rinse time leads directly to the possibility of a larger feed time,

8822


Figure 5. Experimental results obtained in the PSA unit for propane/propylene separation using zeolite 4A at 433 K with a mixture 0.250 C3H6/0.240 C3H8/0.510 N2: (a) pressure at the outlet of the column; (b) temperature at the bottom thermocouples (0.18 m from inlet) and molar flow rate of propane and propylene in the first cycle (c) and in cycle 41 (d). Solid lines are predictions of the theoretical model and points are experimental values. Operating conditions are reported in Table 4, run 4.

increasing the productivity of the unit. Note that with a decrease in the rinse time, the recovery of product in all the experiments jumped from 63% to more than 80%. The co-current depressurization step can be reduced only up to 30 s when purity begins to decrease. The intermediate cocurrent blowdown was a tradeoff situation between desorption of propane from the extrudates macropores (desired effect to increase propylene purity) and desorption of propylene from zeolite crystals (undesired effect resulting in decrease of product recovery). In this work this step time was kept constant at 40 s, a little less than prior experiments, but still without negative effects on purity. Blowdown step in this case is used to regenerate partially the adsorbent but also to extract the product. The molar flow rate of propane that contaminates the product is mainly taken out of the column in the initial moments of the step. For this reason, shorter steps have lower purity, but a large step produces low productivity of the unit. In Table 4 we show experiments with 180 and 220 s to show the increasing of purity with a larger step. Periods higher than 220 s did not improve purity considerably but decreases the productivity. In almost all experiments performed, increasing the blowdown step also increases the recovery of the product (large amounts of propylene desorb from the zeolite and can be collected). There was a particular case (see expts 8 and 9 from Table 4) where this situation was not valid, but this could be due to some experimental error of a single point that has a large impact in recovery calculations. As an example of the improvement of the PSA performance, we show in Figure 5 a PSA cycle (molar flow rate in the initial cycle and cycle 41 and bottom temperature and pressure history). As the total time of the cycle was diminished (see run 4 in Table 4), the productivity was also improved.

In this work we have used a single-column PSA without reservoir tanks for product storage and recycle. For this reason, the rinse stream consists of purified propylene instead of propylene resulting from the product stream. The results obtained in this section may be different (lower product purity and recovery) if a more impure stream (recycle part of the product) is used in the rinse step. 4.5. Cyclic Steady State (CSS) and Multiplicity of Steady States. Before studying the effect of the operating and design parameters, the cyclic steady state and its multiplicity were investigated. A very small diffusivity coefficient of propane can have detrimental implications at large times. In Figure 3, the temperature and pressure history were plotted for the first 15 cycles. It can be seen that they reached the cyclic steady state. In the case of the molar flow rate, 50 cycles were performed, and no detectable difference was observed after cycle 20. Even though, if we perform simulations for over 500 cycles and we see in detail the evolution of all the variables, we can note that the product purity is continuously decreasing up to a steady state value that is only reached after 250 cycles. This continuous purity decrease is due to a very small increase of the adsorbed phase concentration of propane, which result in a slightly higher release of propane in the blowdown step, contaminating propylene. The evolution of product purity in run 1 (see Table 4) is shown in Figure 6. The experimental points collected in the first 50 cycles are also plotted in this figure. The difference between the simulated values and the experimental ones may be due to dead volumes in the installation or even to the fact that we are feeding propylene 99.4% purity with around 0.5% impurity of propane in the rinse step. The results shown in the dotted line in Figure 6 were obtained starting from an initially clean bed and then passing an inert stream of nitrogen for 48 h. To confirm


Figure 6. Purity evolution for the same experiment started with a clean bed passing a nitrogen flow before experiment started (squares are experimental data, and dotted line is simulation) and a column initially presaturated with propane (triangles are experimental data, and solid line is simulation). Operating conditions are detailed in Table 8, run 1. Table 5. PSA Experiments Reducing the Inert Content in the Feed and Rinse Streams for Propane and Propylene Separation at 433 K Using Zeolite 4Aa flow, SLPM

yC3H6, feed

yC3H8, feed

y N 2, feed

Phigh, kPa

purity %, simulated



5

2.785

0.27

0.23

0.50

500

85.6

2.64

6

2.074

0.37

0.31

0.32

370

86.6

2.61

14

1.700

0.45

0.41

0.14

250

87.9

2.60

S1

1.700

0.55

0.45

0.00

250

99.31 (98.9 ( 0.2)b 99.28 (98.7 ( 0.3)b 99.25 (98.9 ( 0.3)b 99.09

89.5

3.41

expt no.

a

Step times: pressurization (54 s), feed (100 s), rinse (25 s), depressurization (40 s), and blowdown (180 s) at a pressure of 10 kPa. b Experimental values.

the cyclic steady state and to check if multiplicity of steady states exists, the same experiment was repeated but starting from a column saturated with propane (125 kPa partial pressure) for 3 h. Multiplicity of steady states in PSA applications was previously reported in the separation of propane/propylene mixtures 33 and can also exist in this system that has non-isothermal behavior and non-linear isotherms.34 The experiment started with the column initially filled with propane; pressure and temperature histories and molar flow rates of gases were also well described by the model. The evolution of the purity profile instead of starting from a very high value in the initial cycles and then decrease as shown before, in the case of the column previously saturated with propane, the purity started from lower values and improves with time until a constant value around 98.6%. The purity evolution with time (simulated and experimental values) of this experiment is shown in solid line in Figure 6. The same cyclic steady state line was obtained (although in this case it starts from a lower value of purity passing through a maximum slightly higher than 99.5% and decreasing to 99.32% where it stays constant) indicating no multiplicity of steady states in runs starting with different saturation conditions of the column. 4.6. Decrease in the Nitrogen Content. Another item studied here was the decrease of inert in the process. The fact of having an added inert in the process makes it undesirable for industrial applications because additional units are required for separation of the olefin from the inert and for recycling-recompressing the inert that is not always available in a petrochemical plant

(carrying consequent additional costs to the overall process). As we had 50% of nitrogen in the total flow rate, the strategy used to diminish the nitrogen was to run two other experiments reducing the molar fraction of inert up to 14% in the feed and pressurization steps. Table 5shows the performance and conditions of the different experiments where the partial pressures of propane and propylene are kept almost constant but reducing the content of nitrogen continuously. In Figure 7, we show the molar flow rate of propane, propylene, and nitrogen (nitrogen results corresponds only to simulated values because we were not detecting nitrogen in the PSA unit) of the three different experimental runs and the simulated one without nitrogen (S1). All the cycles shown were collected after reaching the thermal cyclic steady state. The most important aspect to mention is that only a difference in (3 K in the temperature oscillation was verified. This increase is due to a lower flow rate of gas to remove heat generated by propylene adsorption. As it can be seen from Figure 6, the molar flow rate out of the column remains constant, indicating that nitrogen was an inert that in this case is not required to improve the system performance. Even though using the same step times, the recovery of product is higher when nitrogen content is decreased, the purity also decreases so new conditions (increase the rinse step time) should be established to improve purity without nitrogen. When nitrogen content is reduced, the recovery is increased slightly due to small reduction in the axial dispersion (lower flow rate of gas). Even when the nitrogen content was seriously decreased up to a molar

8824


Figure 7. Molar flow rate (when thermal cyclic steady state was achieved) at the end of the PSA column for propane/propylene ratio 54/46 with different nitrogen molar fractions: (a) yN2 ) 0.50; (b) yN2 ) 0.32; (c) yN2 ) 0.14; (d) yN2 ) 0.00 (simulated results). Solid lines are predictions of the theoretical model, and points are experimental values. Experimental conditions and performance parameters are reported in Table 5. Table 6. Effect of Temperature in Propane/Propylene Separation by PSA Using Zeolite 4A Extrudates (CECA)a

Figure 8. Performance parameter of PSA runs for different molar fraction of nitrogen in feed and pressurization steps with a propylene/propane ratio of 54/46. Experimental conditions are presented in Table 4.

fraction of 0.14, the amount of inert in the blowdown stream exiting the column together with the product remains near 5%, which is very similar with the product contamination when using 50% inert in the feed stream. The simulated performance parameters as a function of the nitrogen molar fraction in pressurization and feed streams are shown in Figure 8. As the nitrogen reduction did not show significant detrimental changes in the PSA performance for the study of other conditions (temperature and feed pressure), the experiment with 14% of nitrogen will be used as the reference experiment at 433 K. 4.7. Effect of Temperature. In the experiments performed before, the set of operating values was constant and the effect of different step times was studied. It is also interesting to know the effect that the operating conditions have in the process. As the model was able to fit experiments in a wide range of conditions for a single set of operating parameters, if we use other

expt no.

T, K

feed flow, SLPM

purity %, simulated

20

408

1.700

14

433

1.700

S3 21

448 463

1.700 1.700

99.43 (99.0 ( 0.2)b 99.25 (98.9 ( 0.3)b 99.18 99.06 (98.7 ( 0.3)b

recovery %, productivity, experimental mol/h‚kg 84.3

2.57

87.9

2.60

88.3 88.9

2.66 2.67

a Step times: pressurization (54 s), feed (100 s) at 250 kPa, rinse (25 s), depressurization (40 s), and blowdown (180 s) at a pressure of 10 kPa; yC3H6 ) 0.45; yC3H8 ) 0.41; yN2 ) 0.14. Rinse flow rate: 1.005 SLPM. b Experimental values

conditions and the model still works, there is a certain confidence to say that the simulations will be valid in a wider region. The first operating condition to study is the temperature. All experiments presented above were performed at 433 K, chosen as the tradeoff condition between kinetic selectivity and productivity based on calculations of the kinetic selectivity factor with the explanation that the productivity of the column decreases with temperature (less adsorption at higher temperatures). In this case we will explore two other temperatures: 408 and 463 K. Simulations were also performed at 448 K, but no experiment was done. As mentioned before, the pressurization and feed streams will have only 14% of nitrogen. Experimental conditions at the different temperatures as well as the performance parameters (simulated purity and experimental recovery and productivity) are reported in Table 6. According to the performance parameters shown in Table 6, we can see that the recovery increases with temperature while purity of propylene decreases. The


Figure 9. PSA results for propane/propylene separation at 408 K using zeolite 4A with a feed mixture of 0.250 C3H6/0.240 C3H8/0.510 N2: (a) experimental pressure at the outlet of the column; (b) temperature in the bottom, middle, and top thermocouples (0.18, 0.43, and 0.68 m from inlet); (c) molar flow rate of propane and propylene in cycle 35 (for experimental conditions see Table 6, run 20); and (d) simulated profiles of propane adsorption in different positions of the column. Solid lines are predictions of the theoretical model, and points are experimental values.

purity of product recovered in the blowdown step is lower because propane diffusivity increases with temperature. On the other side, if we decrease the temperature, the diffusivity of both gases is smaller and thus propylene purity is higher (less adsorption/desorption of propane), but also the cycling of propylene adsorption/ desorption is more difficult (smaller diffusivity coefficient) leading to smaller recovery. Also, the productivity of the bed increases continuously with temperature. In this case, the ∆qC3H6 between the feed and blowdown steps together with the higher diffusivity of propylene leads to the increase of productivity with temperature. It has to be noticed that even when product recovery and unit productivity increases with temperature, the requirement of a higher purity of product must prevail in the final set of operating conditions of the process. The product purity obtained experimentally at 408 K was 99.0 ( 0.2%, even when this value is lower that the simulated value, it is higher than in the experiments at 433 K where purity of 89.4-89.8% was obtained. Again the difference can be explained by dead volumes of the unit and the propane contamination in the propylene stream used particularly in the rinse step. The plot of the experimental values obtained for the experiment at 408 K (run 20) where propylene was obtained with higher purity (99.43% propylene simulated against 99.0% obtained experimentally) is shown in Figure 9. In this figure the time variation of the amount adsorbed of propane in different positions of the column is shown until cyclic steady state is reached. Note that in these lines it is not possible to distinguish between adsorption and desorption of propane on a single cycle. As a brief summary of the results obtained when changing the temperature of the process, Figure 10

Figure 10. Temperature dependence of PSA performance parameters for propane and propylene separation using zeolite 4A (CECA) for a feed mixture of 0.250 C3H6/0.240 C3H8/0.510 N2. The empty point at 448 K represents simulated results for recovery. Points for purity correspond to simulated values.

shows the performance parameters (recovery and purity) for the runs at 408, 433, 448, and 463 K. Open symbols correspond to simulated run at 448 K. The purity of product has a linear decrease with process temperature while recovery decreases faster at temperatures lower than 433 K. The higher purity at lower temperatures can be explained by the faster decrease of diffusion of propane (higher activation energy) when compared to propylene. Unfortunately, a decrease in propylene diffusion results in a smaller recovery of product and unit productivity. 4.8. Effect of Step Pressures. In this section we have chosen run 14 (see Table 6) as a reference. Note that the temperature in run 14 is 408 K where better results were obtained. The results obtained in this experiment (with feed pressure of 250 kPa) were compared with simulated results using higher feed pressures: 300 and 400 kPa. Unfortunately, if the feed

8826


Table 7. Simulated Results for Propane and Propylene Separation Using PSA with Different Feed Pressures and Step Timesa expt pres, feed, rinse, Phigh, purity %, recovery %, productivity, kPa simulated simulated mol/h‚kg no. s s s 20

54

100

25

250

S4 S5 S6 S7 S8

65 80 80 80 80

80 60 80 100 120

25 20 20 20 20

300 400 400 400 400

99.43 (99.0 ( 0.2)b 99.33 99.12 99.17 99.21 99.24

84.3b

2.57b

86.6 89.3 88.4 87.3 85.8

2.49 2.16 2.47 2.73 2.93

a Temperature ) 408 K; step times: depressurization (40 s) and blowdown (180 s) at a pressure of 10 kPa; yC3H6 ) 0.45; yC3H8 ) 0.41; yN2 ) 0.14; feed flow rate: 1.700 SLPM; rinse flow rate: 1.005 SLPM. b Experimental value.

Figure 11. Effect of feed pressure in performance of a PSA unit for propane and propylene for similar number of moles treated using zeolite 4A at 408 K with a feed mixture of 0.250 C3H6/ 0.240 C3H8/0.510 N2. Operating conditions are specified in Table 7. Empty point corresponds to simulated values while solid point correspond to experimental value of run 20 (see Table 7).

pressure is further increased over 250 kPa, the FID detector of the gas chromatograph saturates in some of the steps of the PSA cycle. For this reason only, simulations were carried out for comparison. Low pressure (blowdown) and intermediate pressure (depressurization) were kept constant at 10 and 50 kPa, respectively. The feed pressures used for this parametric study were 250, 300, and 400 kPa. When the pressure of the pressurization and feed step is changed, the times of these steps also have to be changed. Experimental studies were performed at 300 and 400 kPa, and the pressurization time corresponds to the real time that is necessary to pressurize the column. For the feed step, the time was initially selected to introduce approximately the same number of moles as in the cases with feed at 250 kPa using the ideal law of gases. Also at 400 kPa the feed step was increased to study this parameter. The complete set of operating parameters for all the simulations is detailed in Table 7. Simulated performance parameters (purity, recovery, and unit productivity) are detailed in this table. The first thing to notice is that the purity of propylene obtained as product decreases when feed pressure is increased. The opposite trend holds for recovery. To help in understanding these results, purity and recovery versus feed pressure are presented in Figure 11. The explanation of this trend is the following: when pressure is increased, the capacity of the column also increases (favorable isotherm), so the number of moles of propylene initially distributed in the entire column now will be more concentrated near the entrance of the column. For this reason less propylene is lost in the feed and rinse steps, increasing the recovery. Also because

Figure 12. Effect of feed step time in performance of a PSA unit using zeolite 4A for propane and propylene using feed stream of 0.250 C3H6/0.240 C3H8/0.510 N2 at 400 kPa and 408 K (simulated results). Operating conditions are specified in Table 7.

propylene is not occupying the last portion of the column, more propane can be adsorbed, and desorbed in the blowdown step, decreasing purity. An alternative to improve purity at higher pressures is to make a more effective use of the column. A possible option can be the increase of the feed step time. Results of simulations with the same conditions but increasing the feed step time are presented for the case of feed pressure at 400 kPa in Figure 12 and also in Table 7. The number of moles of propane adsorbed in the last portion of the column decreases when more propylene is adsorbed, and thus purity increases at the expense of decreasing recovery (more propylene is lost in feed and rinse steps). 4.9 Mixtures with Ratio 85% Propylene/15% Propane. Finally, a mixture having much more propylene than propane was studied. The mixture with 85/ 15 propylene/propane ratio may be obtained in the fluid catalytic cracking (FCC) process optimized for propylene production. This characteristic composition is available only when economic constraints are favorable to this operation. The set of experiments performed is shown in Table 8, together with the performance values obtained by simulation of the process. These experiments were also performed at 433 K and using nitrogen to dilute the mixture almost to 50%. All the experiments reported here were well-predicted by the model; the relation between experimental and simulated values of recovery and productivity are coincident. The experimental purity obtained ranged from 98.6 to 99.0%. These values were oscillating with (0.2% in the same experiment (all around 98.8%); as no effective trend was observed in the experiments, the simulated value is being reported. As productivity is expressed in moles of propylene treated per hour per kilogram of zeolite, the values of productivity are higher than in the 54/46 mixtures because more moles of propylene per second are introduced to the column. In these experiments, the general trend is the same as previously observed for the 54/46 (propylene/propane ratio): if the rinse step is decreased to a minimum value, the recovery also increases while purity is only slightly affected; when the blowdown time is reduced to 180 s instead of 220 s, the purity also decreases. As an example of the experiments shown in Table 8 we report in Figure 13 the temperature and pressure profiles as well as the molar flow rates of propane and propylene leaving the column in cycles 1 and 72 obtained in run 16. In the flow rate plot the nitrogen flow is also reported. It can be seen that when compared

Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8827 Table 8. PSA Results for Propane/Propylene Separation by PSA Using Zeolite 4A (Mixture 85/15 Propylene/Propane Ratio Diluted to 50% with Nitrogen)a expt no.

pressurization, s

feed, s

rinse, s

depressurization, s

blowdown, s

purity %, simulated



1

60

70

130

60

220

62.7

2.69

3

60

60

25

40

220

83.1

3.08

16

54

60

15

25

180

92.2

4.10

17

54

85

15

25

220

89.6

4.34

18

54

60

40

25

180

99.32 (99.0 ( 0.2)b 99.18 (98.9 ( 0.2)b 99.12 (98.6 ( 0.3)b 99.23 (98.8 ( 0.2)b 99.31 (98.5 ( 0.4)b

90.2

3.90

a Temperature ) 433 K; feed at 500 kPa; blowdown at 10 kPa; y C3H6 ) 0.453; yC3H8 ) 0.074; yN2 ) 0.473; feed flow rate: 3.160 SLPM; rinse flow rate: 2.700 SLPM. b Experimental values.

Figure 13. PSA results for propane/propylene separation for 85/15 propylene/propane ratio using zeolite 4A: (a) experimental pressure at the outlet of the column; (b) molar flow rate of propane, propylene, and nitrogen (simulated) in cycle 72 (for experimental conditions see Table 8, run 16); (c and d) temperature in the bottom and top thermocouples (0.18 and 0.68 m from inlet). Solid lines are predictions of the theoretical model, and points are experimental values.

with experiments performed with 54/46 propylene/ propane ratio, the amount of nitrogen contaminating the propylene stream in the blowdown step is much less than before, even though a process without nitrogen is desired. 5. Conclusions An alternative process to cryogenic distillation for propane and propylene is presented in this work. Vacuum pressure swing adsorption process with five steps (pressurization, feed, rinse, depressurization to intermediate pressure, and counter-current blowdown) was used. We focused on two mixtures with different propylene/propane ratio: 54/46 and 85/15 corresponding to steam cracking of gasoil and FCC sources of propylene production. Initial dilution with 50% nitrogen was used, but further experiments showed that the use of nitrogen does not offer any advantages, only has slight detrimental effects dispersing the hydrocarbon concentration profiles (higher axial dispersion due to higher flow rates), and can be removed from the process.

The mathematical model proposed in this work could predict the PSA behavior, which was confirmed experimentally in many different conditions. Purity higher than 99% with recovery above 85% was obtained in many simulations performed in this work. Single-column experiments produced a purity of propylene between 98.4 and 99.1%. The difference can be due to 99.4% propylene purity used in the rinse step and also due to dead volumes of the unit. The cycle times were improved to have much higher recovery of propylene than the 30% previously reported in the literature. According to the experiments and the simulations performed, it was noted that appropriate time for the rinse step results from a tradeoff situation between displacement of propane from the column and decrease of propylene recovery. Also, it is clear from experiments that a longer blowdown step produces better product purity but with the cost of lower process productivity. The intermediate blowdown was also reduced to improve the productivity and with almost no effect in purity and recovery. This step has to be long

8828


enough to remove propane but short enough not to desorb propylene. Different process temperatures and feed pressures were evaluated. It was verified that better purities of propylene were obtained at lower temperature (408 K) and lower feed pressure (250 kPa).

U ) global external heat transfer coefficient, W/m2‚K ui ) superficial velocity, m/s xi ) adsorbed phase molar fraction of component i yi ) molar fraction of component i in the gas phase

Acknowledgment

Rw ) ratio of internal surface area to volume of column wall, m-1 Rwl ) ratio of logarithmic mean surface area of column shell to volume of column wall, m-1 βeq ) equilibrium selectivity factor βsel ) kinetic selectivity factor calculated by eq 7.1 Fg ) gas density, kg/m3 Fp ) adsorbent density, kg/m3 λ ) axial heat dispersion, W/m2‚K (-∆Hi) ) heat of adsorption of component i, J/mol c ) column void fraction p ) porosity of the pellet τp ) pore tortuosity µg ) gas viscosity, Pa‚s

The 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. This work is also part of the CYTED Project V.8 “Clean Technology for the Separation of Light Olefins”. Nomenclature a′ ) area-to-volume ratio, m-1 ai ) number of neighboring sites occupied by component i Bii ) Biot number of component i Ci ) bulk molar concentration of component i in the gas phase, mol/m3 〈ci〉 ) averaged concentration in the macropores for component i, mol/m3 C ˜ p ) molar constant pressure specific heat of the gas mixture, J/mol‚K C ˜ ps ) constant pressure specific heat of the adsorbent, J/kg‚ K C ˆ pw ) specific heat of the column wall, J/kg‚K CT ) total gas concentration, mol/m3 C ˜ v ) molar constant volumetric specific heat of the gas mixture, J/mol‚K C ˜ vi ) molar constant volumetric specific heat of component i, J/mol‚K Dax,i ) axial dispersion coefficient of component i, m2/s Dp,i ) pore diffusivity of component i, m2/s Dc,i ) crystal diffusivity of component i, m2/s ∞ Dc,i ) infinite dilution crystal diffusivity of component i, m2/s ∞ ) infinite dilution crystal diffusivity of component i Dc,io at infinite temperature, m2/s dp ) pellet diameter, m Keq,i ) adsorption constant of component i, 1/kPa K°eq,i ) adsorption constant at limit T f ∞ for component i, 1/kPa hf ) film heat transfer coefficient between gas and solid phase, W/m2‚K hw ) film heat transfer coefficient between gas phase and column wall, W/m2‚K kf,i ) film mass transfer resistance, m/s kg ) gas thermal conductivity, W/m‚K kg,ex ) thermal conductivity of gas surrounding the column, W/m‚K P ) total pressure, kPa qi ) adsorbed phase concentration of component i, mol/kg q/i ) equilibrium adsorbed phase concentration of component i, mol/kg 〈qi〉 ) extrudate averaged adsorbed phase concentration, mol/kg qi ) crystal averaged adsorbed phase concentration, mol/ kg qmax,i ) saturation capacity of component i, mol/kg Rg ) ideal gas constant, 8.314 kJ/mol‚K rc ) crystal radius, m tblow ) blowdown step time, s trinse ) rinse step time, s ttotal ) total cycle time, s Tg ) temperature of the gas phase, K Ts ) solid (extrudate) temperature, K Tw ) wall temperature, K

Greek Letters

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Received for review June 9, 2005 Revised manuscript received August 30, 2005 Accepted September 15, 2005 IE050671B

Propylene Separation by Pressure Swing Adsorption Using

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