DESORPTION OF NORMAL PARAFFINS FROM MOLECULAR SIEVE 5 A E P H R A I M
K E H A T
A N D
M I C H A E L
H E I N E M A N
Chemical Engineering Department, Technion-Israel Institute of Technology, Haifa, Israel Desorption half times of linear hydrocarbons Cg to CIOat pressures ranging from 2 to 50 mm. of Hg, from a large bed of molecular sieves 5A, designed for low pressure drop, are about 20 seconds. The desorption of the final 10 to 20% of the desorbed material is much slower. The initial desorption rates increase with increased loading and increased desorption pressure and increase slightly with increased temperature and molecular length. A simple picture of molecular motion through a molecular sieve crystal is suggested for the mechanism of desorption.
THEability to exclude molecules above a certain effective diameter, and to withstand temperatures above 450" C., has promoted the commercial use of molecular sieves for two important refinery separations: the increase in octane value of gasoline fractions by reducing the linear hydrocarbons contents (Zigenhaim, 1957), and the separation of normal paraffins in the range of CII to C I ~for use in the production of biodegradable detergents (Scott, 1964). All the separation processes use a pressure swing cycle. The adsorption part of the cycle is conducted under pressure. An intermediate purge by a nonadsorbable gas may follow (Minkoff and Duffett, 1964). Of five possible desorption processes (Cooper et al., 1966), only two are used commercially in the desorption part of the cycle: desorption by pressure reduction (Franz et al., 1959; Griesmer et al., 1965), or by sweeping of a slightly adsorbable low molecular weight hydrocarbon stream through the bed (Sterba, 1965). The adsorption is conducted in the liquid (Sterba, 1965) or the vapor phase (Griesmer et al., 1965). A study of the adsorption isotherms of some normal paraffins can show some of the limitations of these processes. Figure 1 shows the adsorption isotherms of n-hexane at 93" and 300°C. (Barry, 1960), and of n-decane at 300" C. (Peterson and Redlich, 1962), on molecular sieve 5A. Similar curves are available for many other n-paraffins. At high pressures the adsorption isotherms converge a t about 0.10 to 0.11 gram adsorbed per gram of adsorbent. At lower pressures, for a considerable range (saturation range) of pressure, the equilibrium loading decreases slowly with decrease of pressure. At some low pressure (which increases with increased temperature), a sharp decline of the equilibrium loading follows a further decrease of pressure. The effect of increased molecular length is similar to that of decreased temperature. Obviously, a pressure swing cycle should be conducted at the temperature and pressure range where the effect of pressure is considerable for all adsorbed species, which is impossible when a wide range of products is desired. One other limitation is that the time required to approach equilibrium is too great for economic industrial applications and the practical loading is lower than the equilibrium value. Another practical limitation is that a pressure swing below 1 p.s.i.a. is unlikely to be used, at present, in 72
an industrial operation. I t can be seen that n-hexane can be separated at 300" C. and that, in general, operation of about 50' to 100" C. above the critical point is desirable. This rules out simple vacuum desorption for the higher hydrocarbons. The adsorption part of the pressure swing cycle using molecular sieves has been investigated in depth and many design data are available (Allen, 1964; Antonson and Dranoff, 1967; Kehat and Rosenkranz, 1965; Roberts and York, 1967; Schumacher and York, 1967). Very little has been published about the desorption part of the cycle. Flock (1966) has published a limited amount of data on desorption by means of a hydrocarbon carrier gas. Nelson and Walker (1961) studied the desorption of propane from molecular sieve.5A. Peterson and Redlich (1962) made a very thorough study of desorption of n-paraffins a t very low pressures from a bed of about 1 gram of molecular sieve 5A and presented an empirical correlation for the rates of desorption. Some preliminary work in the authors' laboratories (Orbach, 1966) indicated that the rate of desorption by low pressure from a bed of molecular sieve 5A is limited by the pressure drop of the system. The object of the present study was to elimipsi0
001
01
I
10
100
I
I
I
I
1
I
I
I
I
I
IO 100 1000 Adsorption pressure mm Hg
10 00
Figure 1. Adsorption isotherms of n-hexane and n-decane on molecular sieve 5 A Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
nate this effect by the design and operation of a low pressure drop adsorption column, to study the effects of molecular length, temperature, initial loading, and desorption pressure on the rate of desorption, and to investigate the mechanism of desorption from molecular sieves. Experimental
Figure 2 is a schematic drawing of the experimental system. The adsorption column was a brass tube of 56-mm. i.d., 1 mm. thick, and 170 cm. in length. Another tube made of 60-mesh copper wire mesh, 15-mm. o.d., supported by thin brass rods, ran along the center of the brass tube. The space between the tubes was filled with the adsorbent through holes a t the top, while the column was vibrated. Brass plugs were then brazed to the holes. Three iron-constantan thermocouples were brazed t o the outside of the adsorption column a t the bottom, middle, and top. Two %-inch copper tubings led to mercury manometers from the top and bottom of the adsorption column. The vapor line was a brass tube of 28-mm. i.d., and a stainless steel diaphragm valve above the column was used to shut off the vapor flow. The main condenser was an extension of the vapor line 70 cm. long. The secondary condenser was a %-inch copper tubing, 44 cm. long. Both condensers were maintained a t -70°C. by means of acetone and dry ice for n-hexane and at -25" C. by means of carbon tetrachloride and dry ice for n-octane and n-decane. The collection vessel was a standard, closed 100-ml. graduate, maintained at the condensing temperature by the same cooling mixtures, inside an unsilvered Dewar flask. The thermostat was a 6-inch pipe with a circulating liquid oil (PAZ-thermal oil 45), heated by external electric Kanthal resistance wires and heavily insulated. Thermal regulation of the oil was by means of a Fenwal bimetal temperature controller. The temperature was maintained within 2'C., and the system was capable of operation up to 350" C. The vacuum pump was capable of reaching 1 mm. of Hg, and the pressure
to venttjl
I I
i
secondary condenser
feed groduote
vocuurn manometers
absolute pressure manomete
1
sofety trap
adsorption column
was controlled by a leak needle valve. All the valves in the system were diaphragm vacuum valves. Procedure
The molecular sieves were loaded, saturated with water, and dried by heating a t 300°C. for 72 hours a t a pressure of 1 mm. of Hg. The dry weight of the adsorbent bed was 2200 grams. The molecular sieves were Linde Molecular Sieve 5A in the form of extruded cylinders, % 6 inch in diameter. The main vapor valve was initially closed. The feed was adsorbed from the feed graduate, with the column a t the desired temperature and pressure (below 760 mm. of Hg). About 5 to 10 minutes were needed to reach equilibrium a t the loading pressure. The vacuum train was then adjusted to the desired pressure by means of the leak valve. After a constant pressure was obtained, the vapor flow valve was opened, and the height of liquid in the graduate, the time, and the pressure readings of the two manometers were monitored continuously. Since half the loaded feed was usually desorbed under 20 seconds, monitoring was by means of a 8-mm. movie camera, at the rate of 12 frames per second, which was started before the main valve was opened. After 1 minute, single pictures were taken every 30 to 60 seconds for another 10 minutes and one final frame was photographed 20 minutes after the start of the desorption. Color film was used and about 10 cc. of a dyed similar liquid initially placed in the graduate helped to get sharp readings of the interface. After development of the film, the data were read in a 4-inch film editor. After 20 minutes of operation, the main valve was shut, and the column could be reloaded with the same liquid. The readings at 20 minutes of desorption time were taken as the equilibrium loading. Results
Three linear hydrocarbons, n-hexane, n-octane, and n-decane, were used in this study. The range of pperating variables is summarized in Table I. The operating range narrows with increase in molecular length. Figure 3 is a typical record of the amount desorbed and the pressure readings a t the top and bottom of the column as a function of time. The initial rate of desorption is very high and half of the amount that can be desorbed is desorbed in about 20 seconds. This period is followed by a period of slower desorption. The pressure drop calculated from the flow data was only a few millimeters of mercury initially and negligible later. Thy initial radial pressure drop was less than 1 mm. of Hg for all runs. Since the long capillary tubing leading to the lower position manometer was used initially to release 30 cc. of vapor from the manometer, the pressure-time curve for this manometer at the operating pressures was calculated assuming laminar flow, and it matched exactly the experimental pressure-time curves. This indicates that the
Table I. Range of Operating Variables in Experimental Study
w
thermostat
oil
circulation Pump
Linear Parafin
n-Hexane n-Octane n-Decane
Critical Temp,
sC
234 8 296.1 345.5
Loading Range, Mrn Hg
Desorptzon Range, M m Hg
Temp Range,
65-260 50-250 50-150
10-50 2-50 8- 25
150 300 200-300 250-300
O
C
Figure 2. Schematic d r a w i n g of experimental equipment Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
73
I
I
' I4 I
'0,
n .hexane 30OOC 170 t o 2 5 m m
n-nexape 30C"C I70 t o 25mm 0
YO'YB
closed a!tet
I05 seconds
s100
e
i
20
I 01
f
1
0
I
1
I
100 200 Desorption time sec
2
I
3;o
'0
0
,
I
100
200
300
Desorption time sec
1
1
400
500
Figure 3. Amounts desorbed a n d pressure readings as function of time for typical run
Figure 4. Correction of desorption fraction-time curve for desorbed material retained in condensers
actual pressure a t the bottom of the column reached the final pressure within 1 or 2 seconds and the operation was actually isobaric. The only doubtful case is the run at 2 mm., which is close to the operating limit of the vacuum pump. The desorption rate was too fast to maintain isothermal temperatures within the bed. The maximum temperature drop calculated for the experimental runs, assuming that all the heat of desorption was absorbed by the heat capacity of the molecular sieves, was 10°C. Therefore, the experimental results can be taken as approximately isothermal. The maximum error caused by the condensation of the vapor initially present outside the molecular sieve pellets in the determination of the desorption half time was less than 7%. The original experimental data were replotted in the form of desorption fraction defined as:
adsorbent after 20 minutes of desorption time, the operating temperature, and the length of the linear hydrocarbon. The experimental results are summarized in Table 11. Initial Loading. The desorption rate increases with increased initial loading. This effect is smaller for octane and decane, possibly because of the operation in, or close to, the saturation pressure range, where the loading is only slightly affected by the pressure. This was the case for the runs for hexane a t 150"C., for octane a t 200"C., and for all runs with decane. Desorption Pressure. Similarly, the desorption rate increases with increased desorption pressure. I t is therefore not advantageous to operate a t pressures lower than prescribed by the adsorption isotherms. Operation Temperature. Under the experimental conditions, the initial and final loading varied with both pressure and temperature, and the temperature effect is masked by effects of initial and final loading. The effect of temperature is probably much smaller than these effects. An increase in temperature probably causes a slight increase in desorption rate. Length of Linear Hydrocarbon. Contrary to expectations, the desorption rate increases with increase of molecule length. This again may be due to the operation range, as octane was loaded near the saturation range and decane in the saturation range. I n this range the adsorbed molecules are under pressure. I t is probably easier to desorb molecules, and particularly the molecules contained in the intercrystalline voids, by the reduction of pressure under these conditions. The results substantiate the conclusions from the study of the adsorption isotherms that it is desirable to operate in the range below the saturation pressure, and show that all other parameters have only secondary effects. The results also show that desorption times of under 1 minute are practical in a column designed for operation at a low pressure drop.
against desorption time. Figure 4 is such a plot of the data of Figure 3, and includes the data of four additional runs a t the same experimental conditions. The reproducibility is acceptable. One additional source of error was checked in these runs by closing off the main vapor valve a t different times and collecting over a period of 20 minutes the liquid that accumulated in the condenser. The correction for the condenser holdup is shown in the dashed curve. The correction for the initial fast portion of the desorption curve is small, relative to the high initial rates of desorption, and is of the same magnitude for all runs. The effects of the operating parameters on the initial fast rate were analyzed from the experimental data. This rate was represented by the desorption half time, the time needed to desorb half of the difference between initial and final loading. Effects of Operating Parameters on Desorption Half Time
Analysis of Experimental Results
Four experimental parameters were studied: initial equilibrium pressure, equivalent to initial loading, desorption pressure, equivalent to the amount remaining in the
The desorption half times obtained in this work are of the order of 20 seconds, similar to those obtained by Peterson and Redlich (1962) for a few particles and much
74
Ind. Eng. Chem. Process Der. Develop., Vol. 9,No. 1, January 1970
"- I
Table II. Desorption Half Times as Function of Experimental Parameters
Loading Pressure,
Temp.,
c.
Mm. Hg
Desorption Pressure, Mm. Hg
n-Hexane 150 150 150 200 200 200 200 200 250 250 250 250 250 250 300 300 300 300 300 300
150 150 150 160 160 160 90 90 260 260 100 100 65 65 260 260 172 170 80 80
200 200 200 200 250 250 250 250 300 300 300 300
250 200 163 155 150 100 100 50 150 150 103 100
12 12 15 7 10 17 9 21 13 17 17 24 26 40 8 11
10 25 2 10 25 25 6 6 50 25 50 25
8 8 30 12 10 11 14 16
~~
-
0
.e
-
"
-
-
tB o.4k.B,
I -
02
t
I 0
0
0
n
I
I
I
100
200
300
I
400
a
15 27 23
a
13 8 13
Figure 5. Comparison of three experimental runs with correlation of Peterson and Redlich (1 962)
This equation assumes no radial or axial concentration gradients in the bed, and hence a constant gas concentration outside the molecular sieve pellets. For a spherical particle, a constant diffusion coefficient which is different in magnitude for either intercrystalline or intracrystalline diffusion, and the boundary conditions:
t < O
tzo
O < r < a
r=O
w =W" w=o
the solution is given by Crank (1945, p. 87) as:
n-Decane 150 125 105 50 150 100 100 60
I
n - hexane 200°C 160 to 50mm o 160 to 25mm A 160 to 1 0 mm theoretical c u r v e
08
c
50 25 10 50 25 10 25 10 50 25 50 25 25 10 50 25 50 25 25 10
n-Octane
250 250 250 250 300 300 300 300
R
Desorption Half Time, See.
I
I
25 25 8 10
25 25 10 10
7 7 12 10 8 8 8 12
~
smaller than the times of 30 minutes obtained by Orbach (1966) or hours obtained by O'Connor and Norris (1960), for long beds of particles. Therefore, the empirical correlation of Peterson and Redlich (1962), though conducted at much lower pressures, should apply to the data of this study. Figure 5 shows that this correlation does apply t o the experimental data of n-hexane a t 200°C. For the experimental data at, or outside, the temperature limits of the correlation of Peterson and Redlich, the agreement was poorer. The correlation of Peterson and Redlich predicts the increase of desorption rate with increased loading, as was found in this study, but does not take into account the desorption pressure, as their work was conducted a t extremely low pressures. The experimental data were also analyzed by the assumption of either intercrystalline or intracrystalline diffusion as the rate-controlling mechanism. The basic diffusion equation is:
E = div (Dgrad W ) at Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 1, January 1970
(3) Equation 3 is plotted in Figure 6 in the form of the desorption fraction as function of time with D/a2as parameter, together with the experimental data of three runs. The match is good for the initial fast desorption and poor for the later desorption stage. Taking a = 1 micron, the diffusion coefficients for the whole experimental range are of the order of 2 to 3 x lo-" sq. cm. per second. If intercrystalline diffusion is assumed to control, the same equation applies if a is taken as 0.1 cm., the equivalent spherical radius of the pellets. These diffusion coefficients fall in the range of 2 to 3 x IO-' sq. cm. per second. Solutions of Equation 2 for other geometries, and for a flat plate with a linear concentration-dependent diffusion coefficient, did not improve the fit of the data or change the order of magnitude of the diffusion coefficients. Better fit was obtained by the assumption of a flat pellet geometry and an exponential concentration-dependent diffusion coefficient of the form:
D = D oexp ( K W W,)
(4)
For this case a graphical solution is given by Crank (1945). Some solutions for ek = 10 and e' = 25 are reproduced in Figure 7 , together with the experimental data of two runs. This leads to diffusion coefficients for intracrystalline diffusion and for e K = 10 of 5 to 8 x lo-" sq. cm. per 75
os
.K
A n-hexane 150°C o n-hexone 300°C 0 n-decme 300°C -theoretical curves
peratures the number of molecules that can be accommodated increases (Peterson and Redlich, 1962). The molecular sieve pellets are formed by bonding the zeolite crystals with about 20% weight of clay binder. The macropores between crystals occupy about 3 5 5 of the pellet volume. The size of the macropores is about 1 micron (Tsuruizumi, 1961). The kla-inch cylindrical pellets are 1.6 mm. in diameter and 4 mm. in length. The equivalent spherical radius for these pellets was calculated as 0.092 cm. by Roberts and York (1967) and as 0.114 cm. by Antonson and Dranoff (1967).
150 to 25mm 170 to 25rnmlcorrected for condenser holdup1 150 to 25mm
0 ._
06
c
.-5
g
$ a4
