Znd. Eng. Chem. Res. 1995,34, 2873-2880
2873
Magnetically Stabilized Fluidized Bed for Gas Separations: Olefin-Paraffin Separations by x-Complexation Vassilios I. Sikavitsas and Ralph T. Yanp Department of Chemical Engineering, State University of New York at Buffalo,Buffalo,New York 14260 Mark A. Burns Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109
Eric J. Langenmayr Corporate Exploratory Research, Rohm and Haas Company, Spring House, Pennsylvania 19477
The feasibility of using magnetically stabilized fluidized beds (MSB) for olefin-paraffin separations by pressure swing adsorption (PSA) is studied by model simulation, and the results are compared directly with that using packed beds. The sorbent used for the separations is a n Agf exchanged resin t h a t selectively forms n-complexation bonds with olefins, with the heat of adsorption of -10 kcal/mol. Superior separations are obtained with MSB for two reasons: high flow rates and small particle sizes that are allowed in MSBs resulting in high diffusion time constants (DJR2)(which are favorable for equilibrium separation). Lower temperature excursions compared to packed bed also favor the separation. The larger axial dispersion in the MSB has only minimal adverse effects on the PSA separation. For example: for a 14/86 ethane/ethylene feed mixture at the same feed throughput (120.8 L(STP)/[h (kg of sorbent)], a n ethylene product at 99.9% purity is obtained at over 50% ethylene recovery from the MSBs whereas only 14% recovery a t the same punty is obtained with the packed beds. Acceptable separation results are also achieved with MSBs for propane-propylene separation, but not with fixed beds.
Introduction A number of workers have studied the influence of magnetization on the dynamics of fluidized solids. Early accounts of this phenomenon were reported by Filippov (1960, 1961) and Kirk0 and Filippov (1960). Katz and Sears (1969) described the effects obtained with electromagnetic fields in fluidized beds. Agbim et al. (1971) observed that for magnetized materials there was an appreciable and uniform bed expansion before bubble formation occurred. In 1978-1979, Exxon was granted two key patents for magnetically stabilized beds (MSB) (Rosensweig, 1978, 1979b). Finally, Rosensweig in his ground-breaking paper (Rosensweig, 1979a) gave the basic description of magnetically stabilized beds. Rosensweig et al. (1981) and Lucchesi et al. (1979) have reported on a number of features of flowable magnetically stabilized fluidized solids and a systematic interpretation of the phenomenon. The radial dispersion and degree of flow uniformity in MSBs is important in determining their applicability as chemical reactors or contactors for physical processes. Radial dispersion data have been reported by Siegell (19821, comparing MSBs to packed and bubbling fluidized beds. Lee (1983) identified the key parameters influencing MSB rheology: magnetic field, bed void space, and bed depth. He also observed that MSB solids exhibit viscoplastic behavior. In a MSB, the fluidholids magnetic system exists in one of three regimes (Figure 1). Below the minimum fluidization velocity, the pressure drop across the bed is less than the bed weight per unit area so the bed is unfluidized. In the stably fluidized region, the bed is
* Author to whom correspondence should be addressed. Present address: Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109.
t i
UNSTABLY FLUIDIZED BED REGION
/’
u*f UNFLUIDIZED BED REGION
APPLIED FIELD (oersteds) Figure 1. Approximate phase diagram showing various states in the magnetic bed.
fluidized yet free of solids mixing. In the unstably fluidized regime, the bed bubbles and mixes even though magnetized. The transitions between these regimes are rather sharp and reproducible (Siegell and Coulaloglou, 1984a). Siegell and Coulaloglou (1984b) studied the transition behavior from the stable to the bubbling regime and the flow properties of MSBs with continuous solids throughput. Siegell (1987) also studied the fluid mechanics of liquid-fluidized magnetically stabilized beds, and determined their operating characteristics and properties. Studies on the same subject have been undertaken by Burns and Graves (1988) as part of an effort to apply the magnetically stabilized fluidized bed to bioseparations. A number of applications, mainly in liquid chromatography, have been proposed (Siegell et
0888-588519512634-2873$09.00/00 1995 American Chemical Society
2874 Ind. Eng. Chem. Res., Vol. 34, No. 8,1995
al., 1985; Burns et al., 1985; Burns and Graves, 1985; Goetz et al., 1991; Chetty and Burns, 1991; Goetz and Graves, 1991; Fleming et al., 1991; Evans and Burns, 1995). Recent experimental studies on bed pressure drop, bed void space, and minimum mass velocity revealed that the bed behavior and structure were controlled by three broad regions of magnetic field intensity: weak, moderate, and strong (Saxena and Shrivastava, 1991). The physical characteristics of the support limit the fluid velocities in magnetically stabilized fluidized bed separators, as well as their magnetic susceptibility. Chetty et al. (1991) showed how MSBs can be used beyond these limitations. The hydrodynamic behavior, as well as heat and mass transfer in MSBs, has been studied by Neff and Rubinsky (19831, Arnaldos et al. (1987), and Casal et al. (1991). Their results showed the superiority of the MSB as a gassolid contactor and in heat transfer, as compared t o packed beds. A comprehensive review on MSBs has been published by Liu et al. (1991). Another review on the early studies in Eastern Europe on MSB has been published by Siege11 (1989). In this work, we study the feasibility of using MSB for gas separation by pressure swing adsorption (PSA). The olefin-parafin separation is used as an example. Olefin-paraffin separations are being performed in industry by cryogenic distillation. This process is costly and energy intensive because of the close relative volatilities. A number of alternatives have been investigated (Eldridge, 1993);the most promising appears t o be x-complexation, in which either a silver or a cuprous ion bonds selectively with the olefin (King, 1987). x-Complexation has been seriously considered for olefinparaffin separation and purification by employing liquid solutions containing silver (Ag+)or cuprous (Cu+)ions (Quinn, 1971; Ho et al., 1988; Keller et al., 1992; Blytas, 1992; Eldridge, 1993). In a recent work of Yang and Kikkinides (1995), two new types of Ag+ and Cu+ ioncarrying substrates have been investigated: cationexchange resins and monolayer salts on alumina. These two sorbents showed superior selectivities, capacities, and reversibilities, which are properties required for cyclic adsorption processes. The sorbent selected for this theoretical work is the Ag+ exchanged resin (Amberlyst 15 from Rohm and Hass Company), which can be readily magnetized. Olefin-paraffin separation with Ag+ exchanged resin is examined since it involves moderately high heats of adsorptioddesorption as well as some intraparticle diffusion resistance, both advantageous for the application of magnetofluidized stabilized beds in PSA. The result of this study defines separations and conditions where MSB is superior to the conventional fured bed for gas separation by PSA. Magnetofluidized Stabilized Bed for Gas Separation. Waghorne et al. (1982) appeared to be the first t o use the MSB in a gas adsorption process (recovery of hydrogen from a feed gas containing hydrogen mixed with non-hydrocarbon components). Figure 2 illustrates how an initially packed bed can be fluidized or magnetofluidized. In the first case, the existence of bubbles in the bed is almost inevitable. In the second case, there is a region of applied magnetic intensities within which the bed is “stably fluidized’’ without the formation of bubbles. The application of MSBs for adsorption, when compared with the conventional packed beds, offers two advantages and two disadvantages:
I I
I I I
. I
I
I I I
I I I
I I
I
‘\-
U > Umf
The bed at minimum fluidizationvelocity
The magnetically stabilized bed (current on)
The bubbling bed (current off)
Figure 2. From left to right: the bed a t minimum fluidization velocity, the magnetically stabilized fluidized bed, the bubbling bed.
The first advantage is improved intraparticle mass transfer properties. When intraparticle diffision resistance is dominant in the process, small sorbent particle sizes are necessary. However, for the conventional packed beds, the pressure drop creates a limitation for the allowable particle size. In contrast, in MSBs this limit is removed and much smaller particle sizes are allowed. The use of MSB can therefore improve the separations with strong diffision limitations. Note that, for extremely small particles, the gas velocity would need to be low. The second advantage is improved heat transfer properties (although still inferior to that of the conventional fluidized bed). Heats of adsorptioddesorption are highly detrimental to adsorption separation processes. In MSBs, there is a tendency for the temperature profiles in the beds t o be flattened. Additionally, to operate a packed bed isothermally, substantial and costly heat exchanger configurations must be used whereas MSB isothermal operation can be established with minor equipment modifications (Pirkle et al., 1988). The first disadvantage for MSBs is the need for larger beds, resulting in a decrease of the capacity of the bed due to larger voids. The second disadvantage is the increased axial dispersion (compared with the packed bed). However, it is still approximately 1 order of magnitude lower than the axial dispersion in a conventional fluidized bed (Geuzens and Theones, 1988). It should also be noted that the sorbent particles in MSBs must contain 2%-40% (volume) ferro- or ferrimagnetic material (preferably 10%-15%) (Rosensweig, 1978), thus decreasing the sorbent capacity. As an advantage, MSBs offer easy sorbent loading, removal, and regeneration (if necessary) (Pirkle et al., 1988). PSA Cycle Description. For the proposed separation, the main issue in the design of a proper PSA cycle is the recovery of the strongly adsorbed component (ethylene and propylene) in very high purities. It has been pointed out earlier that a purge step with the strongly adsorbed component can be employed after the adsorption step, resulting in a significant increase in
[
I
7-
I-
Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2875
at
az
IV and the heat balance is
T HIGH-PURITY OLEFIN
-
OLEFIN PARAFFIN FEED Figure 3. Schematic diagram showing the sequence and basic steps of the PSA cycle. Step I is pressurization with feed; step I1 is adsorption step; step I11 is purge with high purity olefin; step IV is countercurrent blowdown.
the purity of that component in the product stream (Sircar and Zondlo, 1977; Cen and Yang, 1986; Yang, 1987; Baksh et al., 1990; Suh and Wankat, 1989; Kikkinides et. al., 1993). A four-step PSA cycle similar to the one used by Cen and Yang (1986) is considered. The four steps have equal time lengths. In this cycle, each bed undergoes the following cycle steps (Figure 3): I, pressurization with feed; 11, high-pressure adsorption; 111, cocurrent high-pressure purge, with high-purity product; IV, countercurrent blowdown (at low pressure). Two types of beds are used and compared: a conventional packed bed and a magnetically stabilized fluidized bed. The later does not operate as MSB during the blowdown step. At that point the bed collapses and operates as a packed bed. The range of interstitial velocities used in the simulation is higher than the minimum fluidization velocity for particles of 0.1 mm size to ensure fluidization and achieve high throughput. The stability of the bed can be controlled with the intensity of the applied magnetic field. Apart from some empirical correlations based on experimental data, bed expansion cannot be accurately estimated. Since expansions between 25% and 70% have been reported in the literature, an expansion of 40% is assumed in our study. The amount of sorbent and the total throughput are the same for the two types of beds t o enable a fair comparison of their performance.
Assuming spherical shape and uniform radius, R , the intraparticle diffusion equation for each species, i, becomes
(4) with boundary conditions:
aqilar = 0 a t r = 0
(5)
qi = qi* at r = R
(6)
where qi* is the equilibrium amount adsorbed at the surface of the particle and can be calculated using the extended Langmuir isotherm:
(7)
1+zbyj j=1
where qm and b are the Langmuir parameters and are functions of temperature: qm = k, - k , T
b = A, exp(k,lT)
(8)
We assume De to be independent of adsorbed amount. The effect of particle size on the diffusion time constant (DJR2)has not been investigated, but is expected to be considerable. The bed boundary conditions are of the following form:
and
Mathematical Model The adiabatic model is used in this study. The mass balance equation for component i in the bed is (Doong and Yang, 1986)
Note that the index k corresponds to the step number in the PSA cycle and for each step we have the following boundary conditions: Step 11. High-pressure adsorption: ZII
The overall mass balance is
=2,
YI1,i
= Yf$
%I
=Uf, p =p,
where yf,iis the average mole fraction of species i in the feed and the recycled effluent from step 111. Step 111. High-pressure purge with concentrated olefin:
2876 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995 2111 = 2 ,
YIII,I
= ypIV,i,
UIII
= ut3
=pH
where yp1v,iis the time-averaged effluent mole fraction of species i during step IV from the previous cycle. For the first cycle a stream of pure olefin was used to initiate the process. Step IV. Countercurrent blowdown: ZIV
= L - 2 , UIV = 0,
P =P(t)
Step I. Pressurization with feed:
Table 1. Adsorption Bed Characteristics and Operating Conditions for the Standard PSA Simulation Adsorbent Bed 100 cm bed radius 500 cm expanded bed height packed bed height 350 cm expanded bed density 0.475 g/cm3 packed bed density 0.67 g/cm3 expanded bed void fraction 0.58 packed bed void fraction 0.40 0.28 caY(g K) heat capacity of the bed sorbent particle size (in packed bed) 0.6 mm 0.1 mm sorbent particle size (in MSB)
21 = 2
Operating Conditions ethandethylene feed compositiona C2HdC2H4 = 14/86 C3H$C& = 58/42 propane/propylene feed composition" ethandethylene feed throughput 120.8 L(STP)/ [h (kg of sorbent)] propane/propylene feed throughput 96.6 L(STP)I [h (kg of sorbent)] PH 1atm PL 0.03 atm 10.92 cal/(mol K) heat capacity of CzHdC2H4 mixture 16.65 caY(mo1K) heat capacity of C3H$C3Hs mixture feed temperature, T 298 K
a t zI = 0, yi = yi,f atz,=L, u=O
P = P(t) The initial conditions of each step are the conditions at the end of the preceding step. Note that the pressure history is an input in PSA and, in the present work, is represented by an exponential change with time from PH to PL during blowdown and from PL to PH during pressurization:
dPIdt = ahPe-at where AF' is the difference between PH and PL in the blowdown step and also in the pressurization step. In the above equations the basic assumptions are ideal gas behavior, constant viscosity and heat capacity of the gas phase, negligible radial dispersion, and instant thermal equilibrium between the gas and the solid particles. Pressure drop for the packed bed is also assumed to be negligible. (The actual pressure drop for the conditions of this study is 0.08 atm. If we use the smaller particles that are used in the MSB in the packed bed, then the pressure drop would rise to 0.75 atm, prohibiting their use in a conventional PSA packed bed.) The dimensionless Peclet numbers for mass and heat axial dispersion, Pem and Peh, are both set equal t o 500, since in an industrial adsorber the effect of axial dispersion is negligible. This is not true for the magnetically stabilized fluidized bed. Geuzens and Theones (1988) observed that the axial dispersion coefficients in a MSB decreased by a factor of 10 or more. They also presented detailed experimental results and used similar particle sizes as used in our simulation. On the basis of their results, we set Pem = 80 for the MSB. Due to the lack of information on the heat axial dispersion, we also set Peh = 80. Numerical Solution of the Model. A numerical scheme with Galerkin finite elements for discretizing the equations in the bed in the axial direction, and with orthogonal collocation for discretizing the intraparticle diffusion equation in the radial direction inside the particle, is used. Twenty quadratic elements are used in the axial direction of the bed. Four t o six collocation points are enough for descretizing the intraparticle diffusion equation. The model equations are written in dimensionless form. They are discretized in space, and the resulting system of ordinary differential equations is solved by the semi-implicit &method, with a variable time step. The fully implicit Euler method is used in the initial parts of integration, during each cycle step, to ensure stability and avoid numerical oscillations. The Crank-
a The feed compositions are typical of steam cracking products (Keller et al., 1992).
Table 2. Langmuir Parameters and Heats of Adsorption of C&, C2&, C3H6, and C3&, on Ag+ Exchanged Amberlyst 15 Resin (Yang and Kikkinides, 1995) ki (mmoYe) ~~~~
C2H4 CzH6 C3H6 C3Ha
4.40 0.212 2.25 1.55
k2
k3
(mmol/(aK)) (atm-l) 0.208 0.012 0 0.0022 1.28 0.0037 0.268 0.0043
k4
(K) 1200 1870 886 397
AH (kcal/mol) 10.0
4.8 10.3 5.1
Nicholson method is used in later stages, in order to keep the same accuracy with fewer time steps. A VAX 600 Model 520 is used for all the computations. It takes 4-5 min CPU time for each cycle and, in general, 300-500 cycles for the cyclic steady state to be reached.
Results and Discussion The experimental isotherm data of Yang and Kikkinides (1995) on C2H6, C2H4, C3H8, and C3H6 on Ag+ exchanged resin at two different temperatures are fitted by the Langmuir isotherm (eq 7). The fitting parameters are given in Table 2. Comparisons between the experimental points and the Langmuir isotherms resulting from these fitting parameters are shown in Figures 4 and 5. The diffusion time constants (DJR2) are assumed to be independent of the amounts adsorbed. Experimental values of the diffusion time constants for particles with 20-30 US mesh at 25 "C are listed in Table 3. A high purity olefin product (99.9% for the case of ethylene) using Agf exchanged resin as a sorbent in a PSA cycle is the first objective of this work. The second objective is to examine the feasibility and theoretical limits of the magnetically stabilized fluidized beds (MSB) in gas separations. The performance of a MSB and a conventional packed bed are tested and compared in an identical PSA cycle. The total amount of the sorbent and the throughput are kept constant. Product purity and recovery are the parameters that will determine the performance of each type of bed. For each case, the optimal operating conditions are first identified. Optimization is done by changing the
Ind. Eng. Chem. Res., Vol. 34,No. 8,1995 2877 I
I
1 .o
-P
2 0.8
1
,/
-n
Table 4. Cyclic Steady State Results of PSA Separation under Optimum Conditions (See Table 1 for Bed Size and PSA Conditions)
E
w
0.6
a Q
U
0 ethylene , T=25 C 0 ethylene , T=60 c
0.4
i-
z 2
ethane , T=25 c 0 ethane , T=60 c
0
I
4 0.2
-
I
m Y
0.o
0.0
Y
0.8
0.2
PR&JR~~atm)
.
I
1.0
I
1
x
ti>
8 E
-
0 3
Q
a C a
5 w
2z 0.4 5 0.2
propane , T=25 C , T=60 C
0 propane
Hi
i
-
n u
r l u
0.0
0.2
0.8
PR &U
Ri f a t m )
T
1 .o
Figure 6. Equilibrium isotherms of propane-propylene on Ag+ ion-exchanged resin fitted by the Langmuir isotherm. Table 3. Overall Diffusion Time Constant (D$R2) for Diffusion in Ag+ Exchanged Amberlyst 15 Resin at 26 "C with Average Particle Size 20-30 Mesha D$R2,l/s CZH4 1 x 10-3 CZH6 7 x 10-4 C3H6 1.2 10-5 C3Hs 4 x 10-6 ~~
a
Data from Yang and Kikkinides (1995).
cycle time and comparing product purity with product recovery. Results for each case are given in Table 4. Under the term "feed", the relative amount of olefin/ paraffin mixture is presented. The product is the effluent from the blowdown step minus the necessary amount for the purge step. The relative concentrations of the adsorption step from the effluent are also reported in Table IV. The length of the cycle time is an important factor since intraparticle diffision resistance is significant. The stronger intraparticle diffusion resistance for the C3 molecules makes the propane-propylene separation with packed beds very disadvantageous when a high propylene product purity is required. Axial dispersion in MSB, although expected to significantly affect the recovery of the olefin product, seems
99.9 99.8 99.87 99.74 99.9 99.0 99.1
1
0.15 -
0.10
-
4
Adsorption step time E--.
0.05
-
0.00 0,990
0propylene , T=60 C
C3H6
t
'
I-
C3H6
(vel%)
0.20 -
-x
0 propylene , T=25 C
CZH4
0.25
3
1.2
C2H4
product
With the diffusion time constant used for packed beds. Isothermal conditions.
7
Figure 4. Equilibrium isotherms of ethane-ethylene on Agf ionexchanged resin fitted by the Langmuir isotherm. 1.4
packed bed packed bed MSB MSB" MSBb MSB MSBb
component CzH4 C3H6 C2H4
feed effluent from (L(STP)/ adsorption step (h kg) (vel%) 103.9 83.6 40.6 41.9 103.9 67.0 103.9 83.7 103.9 9.3 40.6 37.7 40.6 20.7
9oosec
*-01200sec - 1500 s e ~
+
0.992
0.994
4
0.996
0.998
1
1.000
Ethylene product purity
Figure 6. Performance curves for ethandethylene separation with a conventional packed bed PSA cycle and effect of time of the adsorption step (at fmed feeacycle and amount of sorbent). (See Table 1 for bed and operating conditions.)
to have only a moderate negative effect in the process. When the axial dispersion term is neglected from the MSB-PSA model, only a slight increase in the product recovery is observed (in the order of 3%-5%). Ethane-Ethylene Separation. In the conventional (i.e., packed bed) PSA cycle, the optimum product recovery is 14%,for an ethylene product purity of 99.9% (Figure 6). In the PSA cycle with the magnetofluidized stabilized bed, for the same product purity (99.9%),the optimum product recovery is higher than 50% (Figure 7). To understand the reasons for this large difference from the performance of the two types of bed, we will discuss the effects of the different factors separately. Diffusivity is the first important difference between the two beds. In the packed beds, 0.6 mm sorbent particles are used (20-30 US mesh). In the fluidized beds, particles as small as 0.1 mm can be used without increasing the bed pressure drop. Therefore the MSB's diffusion time constant is 36 times higher than the packed bed's diffusion time constant. If one uses the larger particles in the MSB, the optimum product recovery, for the same high purity, will drop t o 12.5% (Figure 8). This seems to be the major factor that leads to the superiority of MSBs against packed beds for the olefin-paraffin separation. In the absence of diffision limitations, the performance of the MSB is slightly worse due to the higher axial dispersion. The considerably high heats of adsorptioddesorption of the olefins, which are the strongly adsorbed components, increase the temperature during the pressurization, adsorption, and purge steps considerably. At the
2878 Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 0.90
5
P
0.80 0.70
r
I :
c
1 .oo
1
=-. .'._ . e----
u
Adsorption step time
6> 0.95
k - N 150 sec 0- - 0 225 sec
'.
.,,,'+ - + 3 0 0 ~ ~
-8..
8 2
'5
0.90
3
3
IC)
D 0.60
P
2
P
-xa,
0.50
5
0.40
a,
0.85 Adsorption step time
C
-x Q)
5
c
*
0.30 0.985
0.990
0.995
1 .ooo
w
0.80
m- - 480 sec - 0540 sec + - + 600 sec
0.75 0.990
Figure 7. Performance curves for ethane-ethylene separation with a MSB-PSA cycle and effect of time of the adsorption step (at fixed feedcycle and amount of sorbent). (See Table 1 for bed and operating conditions.) 0.25
0.20 L
2
+ 0.15 0 3
-
i
D
2 Q
E -%
5 w
i
0.10 L
!
9)
0.05
0.00
Adsorption step time k - a450 SBC
* - 0525 sec +-+~OOSBC
0.992
0.994
0.996
0.998
Ethylene product purity
Ethylene product purity
L. a, > u O
\
4.
1.000
Figure 9. Performance curves for ethane-ethylene separation with a MSB-PSA cycle under isothermal conditions, and effect of time of the adsorption step (at fixed feedcycle and amount of sorbent). (See Table 1 for bed and operating conditions.)
valuable product is wasted to purge the bed from the paraffin which remains in the gas phase, in the bed, after the adsorption step. Overall a larger void space has a negative effect in the performance of the MSB, which explains the slightly inferior performance of the MSB when particles of the same size for the packed bed are used. If the required purity of the ethylene is lower (e.g., 99%),then the packed bed recovery increases to almost 20%. At the same time the MSB recovery increases to 80%. For the conventional PSA cycle, the diffusion time limitation requires a long cycle time (60-80 min). The MSB-PSA cycle operates a t much shorter cycle times (10-15 min). This means that the amount of the purge gas compared to the amount of feed in each cycle is larger in the short cycle time PSA process, resulting in a steep decrease in product recovery with product purity (Figures 8 and 9). In other words, in a long cycle time PSA process, the product puritylproduct recovery line is almost horizontal (Figure 6)whereas, in a short cycle time PSA process, the product puritylproduct recovery line is steeper (Figure 7). For the same reason, in the MSB-PSA cycle for product purities less than 99.7%, a 10 min cycle time is superior, when for product purities above 99.7%, a 15 min cycle yields higher product recoveries. Propane-Propylene Separation. Due to the lower diffusivities of propane and propylene, the conventional packed-bed PSA cycle suffers from severe diffusion limitations. For propylene product purities >99%, recoveries lower than 1%are obtained. The use of MSB allows lower diffusion resistances, and consequently shows a product recovery higher than 17% (Figure 10). Even for product purities higher than 99.5%, product recoveries around 15% are achieved. For the same separation, under isothermal conditions, superior product recoveries (64% a t 99% product purity) are obtained (Figure 11). For a 99.5% product purity, the product recovery drops to 45%. Higher product purities can be obtained at higher cycle times.
I
0.985
0.990
0.995
1.000
Ethylene product purity Figure 8. Performance curves for ethane-ethylene separation with a MSB-PSA cycle with the same diffusion time constant used for packed bed, and effect of time of the adsorption step (at fixed feedcycle and amount of sorbent). (See Table 1 for bed and operating conditions.)
end of the adsorption step, the temperature in the packed bed rises 60 K, whereas in the MSB the temperature rise is 54 K. These strong heat effects decrease considerably the capacity of the sorbent particles, and as a result decrease the efficiency of the process. It must be noted that the simulation is done with an adiabatic model, which means that no heat loses from the wall are considered. Any heat exchange with the surroundings will improve the separation (Yang, 1987). It is well-known that the maximum performance of a PSA cycle is achieved when the process is isothermal. Isothermal conditions can be obtained easily in a MSB by using internal heat exchangers, compared to a packed bed which require more complicated modifications. When the MSB operates isothermally, the optimum product recovery is more than 90% (Figure 9). This is another important advantage of the MSB in a PSA process. The void space is considerably larger in MSBs. This means that a larger amount of gas is needed to pressurize the bed. It also means a larger amount of
Conclusions It is shown that the use of magnetically stabilized fluidized beds in PSA processes can significantly improve the performance of the traditional packed bed PSA
Ind. Eng. Chem. Res., Vol. 34,No. 8, 1995 2879 0.200
2 a > 0 0
t 0.175
2
c
0
3
3
0.150
P
a
t
Adsorption step time
C -2 0.125 Q,
C-
+-
E
a 0.100
3000 sec
0- - 0 3600 S W
4200 sec
L . 1 .ooo
0.996
0.984
0.988
0.992
Propylene product purity Figure 10. Performance curves for propane-propylene separation with a MSB-PSA cycle and effect of time of the adsorption step (at fixed feedcycle and amount of sorbent). (See Table 1 for bed and operating conditions.)
2-
u
0.60
2
c 0
3
0.50
h Q)
I
Superscript * = equilibrium
t
Literature Cited
C
2 0.40 Q,
Adsorption step time E-
600 sec
+ - + 900 sec
0--01200sec . . .~~ 0.30 0.94
Greek Letters E = fractional void in the bed LL = mass axial dispersion constant, cm2/s @b = density of the bed, g/cm3 Subscripts b = bed f = feed H = high i = species i j = speciesj k = step number in PSA cycle L = low p = product
0.70
2!0
L = length of the bed, cm N = total number of components in the mixture P = pressure, atm q = adsorbed amount, moVg Q = volume-averaged adsorbed amount, moVg q m = saturated amount adsorbed, defined by eq 8, moVg Q = (-AH) = heat of adsorption, caVmol r = radial distance from the center of the spherical particle, cm R = radius of the particle, cm R, = gas constant, atm cm3/(molK) 7'0 = initial and ambient temperature, K t = time, s u = interstitial velocity, c d s umf= minimum fluidization velocity, cm/s y = mole fraction in the gas phase z = axial position in the bed, cm
'
'
'
0.95
'
0.96
0.97
0.98
0.99
1.00
Propylene product purity Figure 11. Performance curves for propane-propylene separation with a MSB-PSA cycle under isothermal conditions, and effect of time of the adsorption step (at fixed feedcycle and amount of sorbent). (See Table 1 for bed and operating conditions.)
cycle. The MSB allows the use of much smaller sorbent particles, and the resulting decrease in diffusion limitations is the main reason for the improvement in separation. Moreover, nearly isothermal conditions can be more easily accomplished in a MSB than in a packed bed, permitting the separation t o reach its maximum performance. In the absence of the above advantages, MSB would yield slightly inferior results compared to the conventional PSA due to a higher axial dispersion coefficient and larger bed voids.
Acknowledgment This work was supported by NSF Grant CTS9212279.
Nomenclature a = parameter in the pressure history curve, l/s b = Langmuir parameter, defined by eq 8, atm-l C P ,=~ heat capacity of the gas phase, caV(g K) Cp,, = heat capacity of the solid phase, cal/(g K) De = effective diffusivity, cm2/s DL= mass axial dispersion constant, cm2/s
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Abstract published in Advance A C S Abstracts, July 15, 1995. @
