Conclusions The qualitative and semiquantitative description of petroleum residuals can be achieved quite rapidly by combining extensive separation data with spectral a n d physical properties. These descriptions include types of functional groups, quantity and types of chain and aromatic ring structures, heteroatom distribution, and molecular size-weight relationships. This approach provides a more meaningful description of the residual, as a feedstock or product, than simple physical, elemental, or spectral experiments on the whole material. These techniques have been found applicable to residuals possessing grossly different properties, origins, maturation and processing histories: for example, Cabinda and Utah residuals rich in waxes and n-paraffins, mid-East and South American residuals rich in sulfur and metals, “asphaltic” residuals, and products from residual processing. These techniques provide excellent “compositional profiles” for process development, modeling, and changes that may be attributed to environmental factors (oil spills, etc.). Acknowledgments Assistance with various separation experiments by R. C.
Query and N M R experiments by R. K. Jensen are grate-
Literature Cited Aczel, T., Lumpkin, H. E., Amer. Chem. Soc.. Div. Petrol. Chem.. Prepr..
17 (41,F66 (1972). Albaugh, E. W., Talarico, P. C., J. Chromatogr.. 74, 233 (1972). Clutter, D. R., Petrakis, L., Stenger, R. L., Jensen, R. K.,Anal. Chem.. 44 (E),1395 (1972). Jewell, D. M., Ruberto, R. G., Davis, B. E., Anal. Chem., 44 (14),2318 (1972a). Jewell, D. M., Weber, J. H., Bunger, J. W., Plancher, H., Latham, D. R., Anal. Chem.. 44 (e),1391 (1972b). McKay, J. F., Jewell, D. M., Latham, D. R., Separ. Sci.. 7 (4),361
(1972). O’Connor, J. G., Burow, F. H., Norris, M. S., Anal. Chem.. 34 (I),82
(1962). Peterson, J. C.,Barbour, R. V., Dorrence, S. M., Barbour. F. A,, Helm, R. V.,Ana/. Chem., 43 (ll),1491 (1971). Talarico, P. C.,Albaugh. E. W., Snyder, R. E . , Anal. Chem.. 40 (14),
2192 (1968.
Receiued for review October 10, 1973 Accepted March 11, 1974
Presented in part at the 164th National Meeting of the American Chemical Society, Division of Petroleum Chemistry, New York, N. Y., Aug 1972.
Chromatographic Studies of Adsorption of Nitric Oxide on Activated Carbon Horn-Ming Chiu,’ Noboru Hashimoto,2 and J. M. Smith* Department of Chemical Engineering, University of California, Davis, California 95676
When pulses of nitric oxide are passed over a bed of activated carbon particles at 180°C reversible adsorption occurs and also some adsorption that is irreversible in the effluent time of the pulse. When the bed is pretreated with nitric oxide, subsequent pulse tests indicate only reversible adsorption. An equation has been derived for the first moment of the effluent pulse when both reversible and irreversible adsorption occurs. Analysis of the NO-activated carbon data with this equation indicates that approximately 41% of the adsorption is irreversible. The techniques used should be useful in measuring the extent of irreversible adsorption on fresh adsorbents and in explaining differences in adsorption characteristics as measured by transient (pulse) and steady-state methods.
When a pulse containing an adsorbable gas is passed through a bed of adsorbent particles, the retention time and shape of the effluent peak are functions of the rates of mass transfer and of the adsorption capacity of the bed. If the adsorption rate is linear and reversible in the sense that desorption occurs within the time it takes for the concentration pulse to travel through the bed, the rates and adsorption equilibrium constant can be calculated (Schneider and Smith, 1968) from the measured moments of the effluent concentration peak. In such cases the zeroth moment is unity; that is, the area of the effluent peak is equal to the area of the input pulse. If some of the sites on the adsorbent surface possess very large attractive energies for adsorption, it is expected that desorption would not occur within the retention time of the pulse. Under these circumstances the area of the effluent peak would be less than that of the input pulse, the On leave from Industrial Technology Research Institute (Republic of China). On leave from the Japan Gasoline Co. (Japan). l
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difference corresponding to the extent of this “irreversible” adsorption. After prolonged exposure to adsorbate, it is expected that the irreversible sites would be fully occupied, and subsequent pulse experiments should be a measure of reversible adsorption. Such behavior has been observed for the adsorption of nitric oxide on activated carbon: that is, initial pulses show effluent-peak areas less than that for the inlet pulse, while pulse experiments after prolonged exposure to NO show effluent-peak areas equal to those for the input pulse. The objective of this paper is to extend the reversible chromatographic theory (Schneider and Smith) to include irreversible adsorption and apply the results to experimental data for the NO system. The goal is to determine the magnitudes of reversible and of irreversible adsorption and the ratio of rate ’constants for the two processes. Experimental Section With helium as carrier gas, 0.5-cm3 pulses of 3% NO in helium were introduced into a 5.5-mm i.d. tube packed to
Table I. Characteristics of Packed Column ~~
~
Mesh range of carbon particles Average radius of particles, mm True density of particles, g/cm3 Apparent density of particles, pp, g/cm3 Porosity of particles, 6 Pore volume, cm3/g Packed bed length, z, cm Void fraction of bed, Surface area of particles, m2/g (I
48-60 0.135 1.96-2.12 0.956 0,531 0.55-0.57 20.7 0.438 -450
a length of 20.7 cm with Filtrasorb (Calgon Division of Merck and Co.) activated carbon. The effluent peak was measured with a thermal conductivity cell. All runs were made a t 180°C and at about atmospheric pressure. Above 180°C reaction occurs between N O and activated carbon forming a variety of products (Smith, et al., 1956, 1959). The properties of the carbon particles and the bed are given in Table I. The 0.135-mm particles were prepared by crushing and sieving Filtrasorb 200. The true solid density was measured by helium displacement and the apparent particle density by mercury displacement a t atmospheric pressure. The void fraction of the bed was established from the mass and apparent density of the particles and the total volume of the tube (determined by filling with mercury). Filtrasorb 200 is a standard adsorbent with a broad pore size distribution (pore radii from 10to 20,000 A). Measurements were made over a range of flow rates from 23 to 135 cm3/min (at 24°C and 1 atm). The maximum pressure drop across the column was 4.3 in. Hg. Interparticle velocities in the bed were calculated from the flow rate, converted to 180°C and the average column pressure, and from the measured void fraction. Qualitative Results Initially the bed was purged with helium flow a t 20 to 23 cm3/min for 16 hr a t 280°C. Then to remove the rare, very energetic sites on fresh carbon particles it was necessary to pass a few (4 or 5) pulses of 3% NO through the bed. After this pretreatment it was possible to obtain reproducible effluent peaks in response to pulses passed at 60-min intervals (with helium flow) through the bed. Furthermore, the same results could be obtained after passing helium at 180°C through the same bed for 16 hr. The areas of the effluent peaks for three such reproducibility tests are identified in the lower curve of Figure 1 by the three marks used for the experimental points. Also shown in Figure 1 is a curve of peak area, us. flow rate, obtained when the packed bed was replaced with a short capillary tube; that is, a curve of input peak areas. The difference in the two curves represents the adsorption of N O that is irreversible within the time of passage of the pulse through the bed. The peak areas of the lower curve correspond to reversible adsorption. To test these conclusions, attempts were made to block the sites available for irreversible adsorption. The bed was treated with a continuous flow of 3% N O in helium for 16 hr a t 180°C. Then successive pulses of 3% NO in helium were immediately introduced through the bed. The peak area curve shown in Figure 1 agrees with that for no bed at flow rates greater than about 70 cm3/min. Hence, a t these flow rates the treatment with 3% NO was sufficient to occupy fully the more energetic sites available for irreversible adsorption. It may be noted that the retention time for these pulses was much greater than that for an inert (nonadsorbable) gas passing through the bed, as seen in Figure 2. This increased retention time shows that the NO was not passing unadsorbed through the bed. From such experiments it was concluded ,that only reversible
lSO 140
m I
!
0 0
20
40
I
IO
1W
FLOW RATE, m3 min ( H O C
120
140
I aim)
Figure 1. Effect of adsorption on effluent peak areas; 3% NO-He pulses.
adsorption occurred after the treatment with 3% NO for 16 hr. The deviation of the two upper curves at flow rates less than 70 cm3/min is apparently due to some desorption occurring from the "irreversible" sites during the long effluent time of the pulse. In subsequent pulses, these irreversible sites are reoccupied leading to a reduced peak area. These results indicated that it was possible to separate irreversible and reversible adsorption, thus warranting a quantitative analysis of the data. Moment Analysis of D a t a From the measured effluent peaks C ( t , t ) first moments were calculated from the equation Jn
t C(z,t ) dt
These moments were then corrected for the dead volumes, between the injection valve and column and between the column outlet and the thermal conductivity cell, using the data obtained when the bed was replaced with the capillary tube. Such corrected moments ( p l ) e x p t l- ( P 1 ) d could then be compared with the equation relating the first moment to the adsorption characteristics of the system. Such an equation has been derived in detail by Schneider and Smith (1968) for reversible adsorption. The procedure is based upon writing differential, conservation equations for the mass of adsorbable component in the bed and relating the solution of these equations in the Laplace domain to the first moment. The result is
A requirement for this solution to be valid is that the adsorption rate be linear. For the reversible case the linear rate equation is Q,
I 4 (k,C, -
hdn)
(3)
where the forward and reverse rate constants are related by the equilibrium constant K = k , / k d . Ind. Eng. Chem., Fundam., Vol. 13, No. 3,1974
283
Table 11. Adsomtion Rate Constant Ratios
+
ki/hd = 7.47 (cm3 pore vol)/g; from upper line, Figure 2 kJkd = K = 4.35 (cm* pore vol)/g; from middle line, Figure 2 k , / & = 3.12 (cma pore vol)/g k,/kd
Table 111. Reversible and Irreversible Adsorption"
Flow rate, ml/min
Fraction adsorbed on irreversible sites
Fraction adsorbed on reversible sites
40 60 80 100 120 Average
0.420 0.410 0.415 0.407 0.419 0.41
0.580 0.590 0.585 0.593 0.581 0.59
nt mom1 la nm-
0
001
Om
OW
OM
005
min
1IY
Figure 2. Effect of adsorption on first moments of effluent peaks.
a Site concentrations: for reversible adsorption, N , = 3.79 X lo1*sites/(cm3 pore vol); for irreversible adsorption, Ni = 2.63 X 10lssites/(cma of pore vol).
0 0.4
The middle line in Figure 2 corresponds to reversible adsorption and its slope, according to eq 2, establishes k,/kd to be 4.35 (cm3 pore vol)/g. By difference, the ratio ki/kd for irreversible adsorption is 3.12 cm3/g and the ratio of irreversible to reversible rate constants is
0.3
ki kr
ki/kd 3.12 - 0.72 ki/kd 4.35 The lower line in Figure 2 was calculated for an inert (nonadsorbable) pulse; that is, calculated from eq 2 with k,/kd = 0. As stated earlier, the applicability of eq 2 and 5 depends upon the rate of adsorption being linear. To test this at the 3% NO concentration level, first moments were measured for 3 and 5% NO pulses a t 180°C. Figure 3 shows that nearly the same first moments were obtained. That is, essentially the same values of ki/kd + k,/kd were obtained for both concentrations. The rate constant ratios so determined from the first-moment data a summarized in Table 11. - E - = - -
P i -
. I
0.1
0.1
o
001
om
003 1 V
no(
om
os
mln
Figure 3. Effect of concentration of N O on first moments.
When irreversible adsorption also occurs, a rate equation for the total adsorption may be written flt =
2
(K,C, - kdn
+ h,C,)
(4)
where k , C, represents the additional irreversible adsorption that occurs on different sites from those for reversible adsorption. Solution of the conservation equations for this case leads to the first-moment expression (/-(l)exptl
- (/-(l)d - 2to = ?{1+ U
(+)a[1
+
(2+ $)I}
(5)
Since the method of solution is similar to that for the reversible case, the mathematical details need not be given. Figure 2 shows the first moments as calculated from eq 1 for pulses before and after treatment with the continuous stream of 3% NO in helium. The larger moments (upper line) for the pulses before treatment correspond to the combination of reversible and irreversible adsorption. According to eq 5 the slope of the upper line is a measure of the sum, k , / k d k i / k d . The value of this sum as determined from the slope of the line is 7.47 (cm3 pore vol)/g.
+
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Ind. Eng. Chem., Fundarn., Vol. 13, No. 3,1974
Extent of Reversible Adsorption The fractional peak areas for irreversible and reversible adsorption as read from Figure 1 are shown in Table 111. Approximately 41% of the total adsorption is irreversible. If it is assumed that one NO molecule occupies one site and that all the NO molecules are either reversibly or irreversibly adsorbed, the information in Table 111 can be used to calculate site concentrations, Ni and N , , for irreversible and reversible adsorption. The total molecules of NO in the input pulse are known and the fractional absorptions are given in Table 111. The resulting site concentrations are given in the lower part of Table 111. The ratio of rate constants shown in Table I1 is on the basis of cm3 of pore volume. With the site concentrations known, these results can be converted to the more intrinsic ratio of rates per site. Equations for the forward rates of adsorption, per site, may be written
Taking the ratio of irreversible to reversible rates
Using the results in Table III and eq 6. ai'/%'
3.79 2.63
-(0.72) =
1.04
This result indicates that, while the rate per cm3 of pore volume is less for irreversible adsorption, the reduced concentration of irreversible sites means that the rate per site is about the same for the reversible and irreversible processes. The second conclusion is that with this system about 41% of the total adsorption is irreversible in the sense that desorption does not occur within the time of passage of the pulse through the bed. These results are significant in demonstrating that chromatographic adsorption experiments are not indicative of results of steady-state experiments when the adsorption is not fully reversible in the time interval of the chromatograph.
Acknowledgment Fellowship grants from the Industrial Technology Research Institute (Republic of China) and the Japan Gasoline Co. are gratefully acknowledged. The Calgon Corporation provided the activated carbon. Nomenclature C = concentration of adsorbable component in the interparticle space in the bed, mol/cm3; Ci = concentration in the intraparticle pore volume K = reversible adsorption equilibrium constant, kr/kd, (cm3 pore vol)/g kd = desorption rate constant, (sec)-l
ki = irreversible adsorption rate constant, (cm3 pore vol)/ (g)(sec) R r = reversible adsorption rate constant, (cm3 pore vol)/ (g)(sec) Ni = number of sites for irreversible adsorption per cm3 of pore volume; N , = number of sites for-reversible adsorption per cm3 of pore volume n = concentration of adsorbable component on adsorbent, mol/g t = time, sec; to = pulse injection time u = interparticle velocity, cm/sec z = bed length, cm Greek Letters a = void fraction in bed fl = intraparticle void fraction Q r = net rate of reversible adsorption, mol/(cm3 pore vol)(sec); Qt = rate of reversible plus irreversible adsorption Qi' = irreversible adsorption rate per site, mol/ (site)(sec); fir' = forward reversible adsorption rate per site (&xotl = first moment from experimental effluent peak, sec (p& = first moment of dead volumes, sec p p = apparent density of particle, g/cm3 Literature Cited Schneider. P., Smith, J. M . , A./.Ch.E. J . , 14, 762 (1968). Smith, N . R . , Lesnini, D., Mooi, J., J . Phys. Chern., 60, 1063 (1956). Smith, N. R.. Swinehart, J., Lesnini, D., J. Phys. Chern.. 63, 544 (1959)
Receiuedfor reuieu: January 23, 1974 Accepted April 24, 1974
Ind. Eng. Chem., Fundam., Vol. 13, No. 3, 1974
285
