I n d . Eng. Chem. RQS.1987, 26, 1540-1545
1540
(3 pages). Ordering information is given on any current masthead page.
Literature Cited Adamek, M.; Scheiber, J. Sb. Ved. Pr. Vys. Sk. Chemickotechnol. Pardubice 1959, 269. Aelion, R. Fibres 1956, 17, 79. 1934, 440. Ashton, R.; Smith, J. C. J . Chem. SOC. Bartlett, P. D.; Nozaki, K. J . Am. Chem. SOC.1946, 68, 1686. Bhagwat, S. P. M. Tech. Dissertation, Indian Institute of Technology, Bombay, 1982. Indian Standards Methods of Sampling and Testing for Oils and Fats; ISI: New Delhi, 1964; IS548 (Part I), p 47. Jones, R. G. J . Am. Chem. SOC.1947, 69, 2352.
Kharasch, M. S.;Mayo, F. R. J. Am. Chem. SOC.1933, 55, 2468. Obrien, S. J.; Bobaleck, E. G. J . Am. Chem. Soc. 1940, 62, 3227. Shapiro, S.; Gunevich, Ya. Analytical Chemistry: Mir: Moscow, 1972; p 355. Smith, J. C. J. Chem. SOC. 1935, 1572. Societe Organic0 Br. Patent 658 758, 1957. Steacie, E. W. R. Atomic and Free Radical Reactions, 2nd ed.; ACS Monograph Series 125; Reinhold: New York, 1954; Vol. 11, pp 728-730. Urshibara, Y.; Takebayashi, M. Bull. Chem. SOC. Jpn. 1938,13,331. Walker, J.; Lumsden, J. S.J . Chem. SOC.1901, 79, 1191.
Received for review May 10, 1985 Revised manuscript received March 24, 1986 Accepted April 18, 1987
Thermal Desorption Behavior of Aliphatic and Aromatic Hydrocarbons Loaded on Activated Carbon Paul K. T. Liu, Steve M. Feltch, and Norman J. Wagner” Calgon Carbon Corporation, Pittsburgh, Pennsylvania 15230-071 7
T h e thermal desorption behavior of organic compounds loaded on granular activated carbon is governed by the thermal stability of the adsorbate and the adsorption energy. This study, with a series of normal alkanes (C4-C20),shows the influence of these two factors on the desorption process. In all thermal desorption processes, the first peak observed is the result of physical desorption of the original compound. Adsorbates greater than C8 exhibit additional peaks due to the thermal decomposition of the adsorbate. The addition of a double bond to these compounds does not have a significant impact on the desorption behavior, while the behavior of substituted aromatics may be predicted from the size of the aliphatic side chain. Suitable regeneration processes and operating conditions may be selected from a knowledge of the thermal desorption patterns. Where desorption without decomposition is expected, low-temperature regeneration with adsorbate recovery may be employed. In those instances where decomposition occurs, high-temperature reactivation may be necessary t o recover sufficient adsorbent activity. Thermal regeneration has been considered as one of the most effective approaches for regenerating activated carbon. In general practice, carbon for vapor-phase applications is regenerated at lower temperatures using steam or nitrogen, while for water and wastewater applications, higher temperatures (>800 “C) with steam, oxygen, and/or carbon dioxide are required to gasify the carbonaceous residue (char) formed during the thermal process. The thermal behavior of adsorbates determines apprapriate operating temperatures and influences the amount of char formed. Therefore, understanding the thermal behavior is critical to economically and effectively regenerate activated carbon. To date, the most complete work on thermal desorption of organic compounds on activated carbon was conducted by Suzuki et al. (1978) using 32 selected compounds. The study divided the compounds into three groups based upon the overall shapes of the thermogravimetricanalysis (TGA) curves. Group I curves were fitted by using a thermodynamic model based on the Langmuir isotherm, while curves for Group I1 compounds could be reproduced by a firstorder kinetic model. Several compounds were listed in Group I11 which formed significant char (at 426 “C in inert atmosphere); however, no clear definition was given to this group. Some compounds listed in Group 11, e.g., decylbenzenesulfonate, formed a higher percentage of char than most of the compounds listed in Group 111. For the purpose of practical applications, a method was given to classify the thermal desorption behavior of organic compounds based upon their boiling point and percentage of aromatic carbon (over total carbon content). In addition 0888-5885/87/2626-1540$01.50/0
to these two factors, however, functional groups were found to play an important role in thermal desorption. Amicarelli et al. (1979a,b,c,1980a,b) studied the thermal desorption of phenol and aniline in nitrogen along with their nitro derivatives. They found that the addition of a nitro group on phenol or aniline converted an endothermic peak into exothermic peak in DTA study. This example indicates some functional groups may modify the thermal desorption behavior substantially. More recently, Urano et al. (1982) studied the regeneration rate of activated carbon-containing organic compounds and classified them into three groups, as “vaporization”, “vaporization and carbonization”, and “carbonization”. This classification, based upon thermal desorption mechanisms, appears more logical and is clearly defined. However, this work focused on the rate study, and no method was given for classifying compounds. Instead, they roughly categorized compounds based on the few examples tested. For instance, all hydrocarbon and nitro hydrocarbon compounds were considered as “vaporization” type based on the two model compounds studied: toluene and nitrobenzene. In contrast, some hydrocarbons, as will be discussed later, may undergo “decomposition” and/or “carbonization” and should logically fall into these categories. Thermal desorption kinetics of certain compounds have been investigated in detail in the literature. The desorption kinetics of phenol and benzene in nitrogen were reported by Seewald and Juntgen (1977). A model coupling first-order kinetics with the Dubinin treatment of Polanyi theory was used to interpret the data. The kinetics of 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1541 Table I. Properties of Virgin F-300 iodine no. 977 abrasion % ash 5.99 apparent density % moisture 0.15" MPD molasses no. 271
82.4 0.513 g/cm3 1.69 mm
Table 111. Experimental Conditions a n d Instrumental Parameters for TGA/DSC Runs samples F-300, 100 X 324 mesh initial temp 25 "C size 20-30 mg program rate 10 "C rates 10 "C/min, 200 cm3/min N, final temp 600 "C
"After drying at 150 "C for 3 h.
: VIRGIN 1.2
F300
Table 11. Summary of Model Compounds
compounds alkanes butane hexane octane" decane pentadecane nonadecane alkanes/alkenes 1-decene 1-octadecene 1,7-octadiene aromatic hydrocarbons toluene n-butylbenzene n-hexylbenzene n-decylbenzene
wt%
residue" at 600 "C, wt % of loading
21.4 16.8 25.3 25.7 25.7 7.7
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Experimental Section 1. Materials. All experiments were performed with Virgin F-300 granular activated carbon (Calgon Carbon Corp.), whose physical properties are listed in Table I. The carbon was dried at 150 OC for at least 3 h prior to adsorption. Single organic compounds were loaded by placing the carbon and an amount of the compound in separate alumina pans placed inside a sealed glass container. These were allowed to equilibrate overnight. In order to increase the vapor pressure of the adsorbate to promote adequate loadings, some samples were placed in an oven at temperatures ranging from 50 to 100 OC. Table I1 summarizes physical properties and loading of the model compounds selected for this part of the study. 2. Procedure. TGA and DSC were done on a Du Pont 1090 Thermal Analysis system. Unless otherwise stated, the experimental conditions employed for all runs are
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sucrose and sodium decylbenzenesulfonate desorption in nitrogen were examined by Chihara et al. (1981) and Umehara et al. (1983), respectively. A two-step decomposition, which was first-order in the adsorbates, provided a good description of the observed TGA weight losses. In summary, although two methods have been proposed in the literature to characterize the thermal desorption behavior of organic compounds, they are either incomplete or not well defined. Accordingly, a thorough understanding of the thermal desorption behavior of various organic compounds is needed prior to developing a sound practical classification. Thus, a fundamental study was conducted on a series of compounds to understand their thermal behavior based upon chemical structure and further to categorize and predict their behavior during regeneration. This paper presents thermal analysis results for 13 model compounds representing aliphatic and aromatic compounds. Differential scanning calorimetry (DSC) was used with thermal gravimetric analysis (TGA) to analyze the TGA weight loss associated with the heat change. In addition, the impact of thermal behavior on the regeneration process was investigated and will be discussed.
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listed in Table 111. A relatively slow heating rate of 10 "C/min was selected to minimize the potential for masstransfer resistance during desorption that might have occurred at higher heating rates. Particles sized to 100 X 325 US. mesh were used to minimize the intraparticle mass-transfer resistance while maintaining sufficient mass-transfer efficiency within the sample bed. Flow rate and bed depth were controlled to minimize the overall mass-transfer resistance. The former determines the interphase mass transfer between the carbon bed and its surrounding purge gas, while the latter defines the interparticle mass-transfer resistance. TGA and DSC curves for the base carbon are presented in Figure 1. A typical carbon, loaded with 25.6% octane, was selected to determine the proper flow rate and bed depth. Figure 2 shows the influence of flow rate on the weight loss curve. A slight change in weight loss vs. temperature was noticed as the flow rate increased from 20 to 2000 cm3/min, indicating that trace interphase masstransfer resistance exists in this flow rate range. The overall weight loss curve at 200 cm3/min approached that with 2000 cm3/min, indicating that resistance for the entire range of temperature was insignificant. Therefore, a 200 cm3/min nitrogen purge flow rate was selected for this study to achieve negligible interphase mass-transfer resistance. Figure 3 presents the weight loss curves for three different bed depths ranging from 5 to 60 mg. A significant change on weight loss vs. temperature was found as the bed depth increased by doubling the weight of the sample
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 SAMPLE BED DEPTH
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from 30 to 60 mg, while negligible differences were observed in thin beds as the sample weight was varied from 5 to 30 mg. Therefore, a sample weight of 30 mg was chosen to minimize interparticle mass-transfer resistance while maintaining the maximum accuracy from the TGA. In summary, the operating conditions were selected to achieve the intrinisic thermal desorption behavior exclusive of mass-transfer resistance. The DSC requires that the sample be packed in a very small closed cell to maintain a uniform sample temperature. In order to accelerate the release of desorbed material during heating, small vents were installed in the cell lid. The DSC profiles for all the compounds appear to be shifted slightly and consistently to the higher temperatures when compared with the corresponding TGA curves. This may be due to the higher mass-transfer resistance inside the DSC cell, resulting in a slower release of the desorbed material.
Thermal Desorption Behavior of Alkanes The thermal desorption behavior of simple, low molecular weight alkanes (e.g., butane, hexane, and octane) is characterized by a single broad derivative weight loss and a corresponding broad endothermic peak as shown in Figure 2. The adsorption capacities of these compounds could be predicted by using a generalized correlation curve for physical adsorption (Grant et al., 1962), suggesting that the single peak is due to physical desorption. Other alkanes, e.g., pentane and heptane, studied by Suzuki et al. (1978), showed similar desorption patterns. These DSC results indicate that the thermal desorption behavior of simple, low molecular weight alkanes (up to C,) is simple physical desorption. Additionally, no residue was found on the carbon surface after desorption, as shown in Table 11. Higher molecular weight alkanes, i.e., decane and pentadecane, exhibit a three-stage desorption curve in both the DSC and differential thermal gravimetry (DTG) studies, as shown in Figure 4. The first peak is believed to be due to simple desorption. Beyond this first peak, both compounds undergo a constant weight loss until a third peak occurs around 500 "C. Apparently, after simple desorption at the lower temperatures, higher molecular alkanes undergo cracking, possibly through chain cleavage followed by the subsequent desorption of the new compounds. The DSC study, shown in Figure 5, exhibits a corresponding trend of endothermic heat, suggesting that cracking is occurring in this temperature range. In previous work (Suzuki et al., 19781, Clo was classified as a compound having simple desorption similar to the lower molecular weight alkanes, although trace amounts of char (-5%) were found near 500 "C. This char formation provides strong evidence of cracking in addition to desorption. The thermal desorption behavior of C15 was classified as a combination of Groups I and I1 by Suzuki
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et al. (1978). This appears to agree with the behavior obtained on this compound as shown in Figure 5. However, the first desorption peak for the CIScompound did not occur in their study, possibly due to differences in sample loading and preparation. Liquid-phase immersion and drying overnight at room temperature was used by Suzuki et al. (1978) to prepare their samples. Some adsorbate, loaded at the lower energy sites, may vaporize in drying, eliminating the initial desorption peak that was found in our study. In addition, the higher loading (about 26%) used in our work vs. the 22% used in their study, suggests that their sample has less adsorbate available in the lower energy sites for low-temperature desorption. The highest molecular weight alkane tested, nonadecane (Figure 6), shows a single sharp peak beginning at 350 "C. Since the DSC study does not show any change due to
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1543
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cracking until 350 "C, the nonadecane may undergo cracking and then desorb at this temperature range. At higher adsorbate loadings, a desorption peak prior to 350 "C might be expected similar to that found for the CI5 compound. The thermal behavior of all three compounds (ClotCI5,CI9)suggests that an endothermic reaction around 500 OC is typical for alkanes involved in thermal decomposition.
Thermal Desorption Behavior of Alkenes Figure 7 shows that decene desorption is similar to decane, while octadecane's behavior is similar to the higher molecular weight alkanes, pentadecane and nonadecene, more closely resembling nonadecane as might be expected.
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No significant char is formed from these two compounds. In contrast, 177-octadienebehaves quite differently, as shown in Figure 7. Although its TGA behavior is similar to that of decane and decene, its DSC curve shows a significant exothermic reaction at 327 OC, indicating that this compound undergoes a different reaction during desorption. This compound also forms a significant amount of char in the process. Thermal Desorption Behavior of Aromatics Aromatics with an alkyl side chain, such as toluene and butylbenzene, exhibit DTG/DSC curves characteristic of a single-step physical desorption, as shown in Figure 8. This observation is similar to that made by Suzuki et al. (1978),who found simple desorption patterns for benzene and toluene. Aromatics with an alkyl side chain greater
1544 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1.6
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than C5 desorb in a fashion similar to higher molecular weight alkanes with a DTG/DSC peak around 500 "C (Figure 9). For example, the behavior of decylbenzene and hexabenzene is similar to those of pentadecane and decane, respectively.
Table IV. Desorption Temperatures of the Selected Alkanes approx end desorption temp, "C bp, "C
Thermal Desorption Mechanism The thermal desorption behavior of aliphatic and aromatic hydrocarbons studied indicates that the first effect observed in heating an adsorbed compound is a shift in the adsorption equilibrium to favor desorption of the original material. The adsorption energy and the thermal stability of the compound determine whether it will be removed completely in this manner before other reactions occur. Polanyi Adsorption Potential theory suggests that the adsorbate is loaded on the high-energy sites first in the adsorption process followed by successively lower energy sites until equilibrium is achieved. At high loadings, some of the adsorbate will reside on the lower energy sites where desorption can occur at lower temperatures. Therefore, if a compound classified according to the simple desorption pattern is loaded to successively higher loadings in activated carbon, it will simply result in more desorption at lower temperatures. As the temperature is increased, a number of additional chemical reaction steps are observed, usually one or two additional steps. These reactions are most likely thermal fragmentation, occurring at progressively higher temperatures depending upon the nature of the compound. The results suggest that simple alkanes (up to Cg) undergo physical desorption. On the basis of the Potential theory, the adsorption energy is proportioned to the log of P,, the saturation vapor pressure, and P, is
inversely proportional to the molecular weight of the compound at a given temperature. Consequently, the lower the molecular weight, the lower the temperature required to desorb the compound. Table IV lists the trends for the three compounds tested in this study. Alkanes of higher molecular weight (XI,) are not stable over the entire desorption range. They undergo thermal decomposition followed by desorption. In heavily loaded carbons, a simple desorption peak may occur prior to the thermal decomposition peak as shown in Figure 5. Our limited data suggest that the presence of a double bond does not change the thermal desorption behavior significantly. These compounds' behavior may be predicted from the corresponding simple alkanes. However, the presence of conjugated double bonds appears to influence the behavior dramatically. Thermal behavior of the aromatic compounds with simple alkyl side chains appears to correlate well with the behavior of simple alkanes. Toluene and butylbenzene have simple desorption patterns similar to hexane and octane. Decylbenzene behaves in a manner very similar to pentadecane, desorbing in three distinct stages. Hexylbenzene behaves as a transitional simple alkane compound with a decomposition peak adjacent to the desorption peak. In summary, aromatic compounds with alkyl side chains follow the same desorption trend as the
butane hexane octane
220 360 425
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Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1545 Table V. Classification of Compounds According to Thermal Desorption Behavior type 1, vavorization
type 2, decomvosition
type 3, char formation
Aromatic with Alkyl Side Chain Cs (226) C4 (183) Ci2 (293)
c1 (111)
Numbers in parentheses refer to the boiling point of the compounds in OC. b A t 15 mmHg.
simple alkanes. Their behavior appears to be determined by the number of carbons in the side chain and may be predicted qualitatively from the behavior of simple alkanes. The thermal desorption behavior of the compounds investigated can be classified as type 1,vaporization; type 2, decomposition; and type 3, decomposition followed by char formation. Compounds belonging to types 2 and 3 may have a vaporization peak prior to a decomposition peak, depending upon the loading. Type 3 differs from type 2 in that a carbonaceous residue forms after decomposition. This residue affects the degree to which the original capacity is restored and can be quantified easily by using TGA analysis. The compounds investigated in this study are summarized in Table V using this classification scheme. For a given homologous series, such as alkanes, alkenes, or aromatics, boiling point appears to be a reliable indicator in classifying compounds with type 1 behavior. However, this criterion appears to be inadequate in distinguishing between type 2 and type 3 behavior. Rather, the compound structure appears to play a dominant role, as demonstrated by the effect of conjugated double bonds in 1,7-octadiene. Other examples of this type of behavior have been documented in the literature by Amicarelli et al. (1979a, 1980b).
Process Design Considerations One of the major objectives in regenerating activated carbon is to restore its a'dsorbtion capacity for reuse. In some applications, for example, solvent stripping, recovery of the adsorbate is important to process economics. Accordingly, the following discussion will focus on capacity restoration of the adsorbent as well as adsorbate recovery for reuse. The selection of regeneration conditions is not critical for compounds with a single desorption peak since intact desorption prevails over the entire desorption range. However, higher regeneration temperatures accelerate desorption by shifting gas/solid equilibrium rather than relying on mass transfer at lower operating temperatures. Therefore, selecting the highest practical temperature without causing adsorbate cracking is recommended for regenerating carbons loaded with this group of compounds. Naturally, other factors, such as the ignition point of the bed, may also influence the temperature selection. Unlike the previous group, the operating temperature may have an influence on the regeneration effectiveness for compounds with multiple desorption peaks. Where adsorbate recovery is required, the operating temperature
should be maintained near the temperature for the desorption peak to minimize thermal decomposition at elevated temperatures. However, the desorption rate may also be too slow to be practical. Attempts to improve mass transfer, such as using a smaller particle size, may enhance the total amount desorbed slightly but may not have a significant impact on the process. The ideal adsorbent for this group of compounds should have minimal high-energy micropores whose adsorption energies are comparable or higher than the bond breaking energy for the compound in question. Consequently, less adsorbate will be retained in the high-energy sites to undergo chemical decomposition at elevated temperatures. Other strategies, such as solvent displacement and steam regeneration may be considered. They may remove more adsorbates below the cracking temperature by displacement, thus minimizing or eliminating the problem associated with thermal decomposition. The efficiency of recovering adsorbate may play a role in deciding on the method of regeneration. If adsorbate recovery is not a prime consideration, higher operating temperatures will result in a more efficient regeneration as discussed previously. However, for highly char-forming compounds such as 1,7-octadiene, maximizing desorption at a lower temperature may be advantageous in reducing the amount of char formed. Once the char has formed, much higher temperatures (>600 "C) and oxidation environments are required to gasify the char and restore adsorption capacity (Liu and Wagner, 1984). Design considerations discussed in this work are based on the thermal desorption behavior of adsorbed compounds in a nitrogen atmosphere. Practical applications may be far more complicated, requiring an overall economic evaluation of the process along with the aforementioned technical considerations.
Acknowledgment We acknowledge Dr. Robert V. Carrubba of Calgon Carbon Corp. for his support and encouragement in this publication. Registry No. C, 7440-44-0; butane, 106-97-8 hexane, 110-54-3; octane, 111-65-9; decane, 124-18-5; pentadecane, 629-62-9; nonadecane, 629-92-5; l-decene, 872-05-9; l-octadecene, 112-88-9; 1,7-octadiene, 3710-30-3; toluene, 108-88-3;butylbenzene, 104-51-8; hexylbenzene, 1077-16-3; decylbenzene, 104-72-3.
Literature Cited Amicmelli, v.; Baldassarre, G.; Liberti, L. Thermochim. Acta 1979a, 30, 247. Amicarelli, V.; Baldassaree, G.; Liberti, L. Thermochim. Acta 1979b, 30, 255. Amicarelli, V.; Baldassarre, G.; Liberti, L. Thermochim. Acta 1979c, 30, 259. Amicarelli, V.; Baldassarre, G.; Liberti, L. J . Thermal Anal. 1980a, 18, 155. Amicarelli, V.% Baladssarre, G.; Liberti, L. Thermal Anal. ICTA 1980b, 80,433. Chihara, K.; Smith, J. M.; Suzuki, M. AIChE J. 1981, 21, 213. Grant, R. J.; Manes, M.; Smith, S. B. AlChE J . 1962, 8, 403. Liu, P. K.T.; Wagner, N. J. Summer National Meeting of AICHE, Philadelphia, Aug 19-21, 1984. Seewald, H.; Juntgen, H. Ber. Bunsenges. Phys. Chem. 1977,81,638. Suzuki, M.; Misic, D. W.; Koyama, 0.;Kawazoe, K. Chem. Eng. Sci. 1978, 33, 271. Umehara, T.; Harriott, P.; Smith, J. AIChE J. 1983, 29, 732. Urano, K.; Yamamoto, E.; Takeda, H. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 180. Received for review September 13, 1985 Revised manuscript received March 2, 1987 Accepted April 10, 1987
