Energy & Fuels 2009, 23, 975–978
975
Adsorption Separation and Use of Normal and Iso-hydrocarbons in Hydrogenated Coking Gasoline Ji-Chang Liu* and Ben-Xian Shen Petroleum Processing Research Center, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China ReceiVed NoVember 10, 2008. ReVised Manuscript ReceiVed December 17, 2008
To improve the use efficiency of hydrogenated coking gasoline and to use the hydrocarbon components properly, the AQ hydrogenated coking gasoline was separated into the raffinate oil rich in non-normal hydrocarbons and the desorption oil rich in normal paraffins through the fixed-bed adsorption process using 5A molecular sieves. The adsorption breakthrough curves of normal paraffins were investigated. In comparison to the hydrogenated coking gasoline, the aromatic potential content and the research octane number (RON) of the raffinate oil rose by 14% age points and 30 units, respectively. As a result, the raffinate oil is more suitable to be used as the feed of the catalytic reforming process or the blending component of high-octane gasoline. In similar conditions, the ethylene yield of the steam cracking process using the desorption oil as feedstocks increased 11% age points compared to the hydrogenated coking gasoline feed.
1. Introduction With the increasing proportion of heavy crude oil, more attention is paid to the delayed coking process.1 The coking gasoline is usually used as the blending components of automobile gasoline2 or the feed of the catalytic reforming process3-5 after being hydrogenated. Because of the low octane number and low potential aromatic content, the hydrogenated coking gasoline does not satisfy the following process. The hydrogenated coking gasoline is also used as the feed of the steam cracking process,6 but ethylene is relatively low. According to the molecular scale management, the normal paraffins should be separated from the hydrogenated coking gasoline and used as high-quality feedstock for the cracking process; the rest (mainly iso-paraffins, cyclanes, and aromatics) can be used as high-quality catalytic reforming feed or a blending component of high-octane gasoline. The economical and effective method for separating normal paraffins from hydrogenated coking gasoline is the adsorption process based on the shape-selective adsorption of the 5A molecular sieves.7 In the steam cracking process, the raw * To whom correspondence should be addressed. E-mail: liujc@ ecust.edu.cn. (1) Zhang, A. H. The production and foreground of hydrogenated coking gasoline. Econ. Anal. Pet. Chem. Ind. 2007, 9, 51–55. (2) Liu, D. H.; Hao, D. J. Study on aromatization of hydrotreated coker gasoline for improving octane number. China Pet. Process. Petrochem. Technol. 1999, 30 (4), 21–24. (3) Li, Y. A.; Huang, G. H. Blending hydrotreated coker naphtha in CCR feedstock. China Pet. Process. Petrochem. Technol. 2000, 31 (11), 15–17. (4) Wang, X. L. Commercial test of hydrotreated delayed coking naphtha as reforming feedstock. China Pet. Process. Petrochem. Technol. 2000, 31 (2), 13–16. (5) Zhou, M. P. Blending test of hydrotreated coker naphtha in reforming plant and running analysis. Catal. Reforming Lett. 2001, (3), 26–32. (6) He, H. X. Using hydrotreated coker naphtha as cracking feed. AnQing Petrochem. 1997, 19 (4), 5–6, 40. (7) Silva, J. A. C.; da Silva, F. A.; Rodrigues, A. E. Separation of n/isoparaffins by PSA. Sep. Purif. Technol. 2000, 20, 97–110.
Figure 1. Hydrogenated coking gasoline adsorption/desorption flowchart.
material is about 60-80% of the total cost.8 The ethylene yield is the highest if the separated normal paraffins are used as the cracking feed.9 On the other hand, the decrease in the n-paraffin content of the catalytic reforming feed will be beneficial to
(8) Wang, S. H.; He, X. O. Craft and Technology of Ethylene; Petrochemical Industry Press: Beijing, China, 2000. (9) Yuan, Q. T. Dual-optimization of naphtha feedstock. China Pet. Process. Petrochem. Technol. 1994, 25 (4), 6–10.
10.1021/ef800979v CCC: $40.75 2009 American Chemical Society Published on Web 01/27/2009
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Liu and Shen
Figure 2. Flowchart of the steam cracking equipment. Table 1. PONA Composition of AQ Hydrogenated Coking Gasoline hydrocarbons n-paraffins iso-paraffins olefins naphthenes aromatics total content (%)
43.65
29.09
0.00
19.19
8.07
100
Table 2. n-Paraffin Content of AQ Hydrogenated Coking Gasoline n-paraffins
n-C04
n-C05
n-C06
n-C07
n-C08
n-C09
n-C010
∑
content (%)
1.34
10.1
10.64
9.64
7.40
4.07
0.46
43.65
improve the yield of aromatics and reduce the severity of the reaction conditions evidently.10
coking gasoline passed through the bed in the gas phase. The normal paraffins were adsorbed in the channels of 5A molecular sieves, and other components, called raffinate oil, passed through the bed. After the adsorption process, the molecular sieve bed was swept using N2. The adsorbed normal paraffins were desorbed countercurrently, called desorption oil. The flowchart is shown in Figure 1. The retained feed in the bed was collected in the intermediate oil tank. 2.2. Steam Cracking Process. The hydrogenated coking gasoline and the desorption oil from the adsorption process were used as feedstocks for the steam cracking experiments, respectively. The flowchart of the steam cracking equipment is shown in Figure 2. Cracking feeds were vaporized and mixed with the diluting steam. After being heated in the preheater, the oil/steam mixtures cracked to small molecules in the furnace tube. The cracking products were quenched and separated into gas and liquid phases. The gas and liquid were measured and analyzed, respectively. 2.3. Catalytic Reforming Process. The target products of the catalytic reforming process are benzene, toluene, and xylene. From the view of the reaction mechanism, normal paraffins are harder to transfer to the target products than other components in hydrogenated coking gasoline. The optimum feed for the catalytic reforming process is the feed with a high potential of aromatic content. 2.4. Feeds and Reagents. (1) The feedstocks used in adsorption experiments were AQ hydrogenated coking gasoline from Sinopec, China. The paraffins, olefins, naphthalenes, and aromatics (PONA) composition and the normal paraffins in the hydrogenated coking gasoline are listed in Tables 1 and 2, respectively. (2) Adsorbent: 5A zeolite, 4-6 mesh in size, Shanghai UOP Company. (3) Desorbent: N2, 99% in purity, Shanghai 5 Steel Works.
2. Experimental Section 2.1. Adsorption/Desorption Process. A fixed-bed adsorber was adopted in the adsorption/desorption process. The hydrogenated
Figure 3. Adsorption breakthrough curve of total n-paraffins.
Figure 4. Adsorption breakthrough curves of individual n-paraffins.
3. Results and Discussion 3.1. Breakthrough Curves of Normal Paraffins. Hydrogenated coking gasoline containing 43.65% normal paraffins passed the zeolite bed after being vaporized. The normal paraffins were adsorbed in the microchannels of 5A molecular sieves. With an increase in feed time, the adsorption mass transfer front of the normal paraffins gradually moved forward and finally penetrated the zeolite bed. At a temperature of 300 °C and a space velocity of 100 h-1, the breakthrough curves of the total normal paraffins are shown in Figure 3. The breakthrough time is 18 min. The breakthrough curves of the individual normal paraffins are shown in Figure 4. The breakthrough time of long-chain normal paraffins is longer than those of short-chain normal paraffins, indicating that 5A zeolite has a stronger ability to adsorb long-chain normal paraffins. The low-carbon-number normal paraffins occupying the microchannels of 5A zeolite can be partly replaced by normal paraffins with more carbon numbers. The roll-up phenomenon11 can be observed in the adsorption breakthrough curves of the low-carbon-number n-paraffins. It means that the n-paraffin concentration in a certain segment of raffinate oil will be higher than that in the feed because of the displacement effect. To obtain the raffinate oil with low n-paraffin content, (10) Speight, J. G.; Ozum, B. Petroleum Refining Process; Marcel Dekker, Inc.: New York, 2002. (11) Yang, R T. Gas Separation by Adsorption Process; ButterworthHeinemann: Stoneham, MA, 1987; p 173. (12) Zhang, C. C.; Sun, M. Fast determination of gasoline octane number by high resolution gas chromatography. J. Yunnan UniV. 1999, 21 (4), 291– 293.
Efficiency of Hydrogenated Coking Gasoline
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Table 3. Potential Aromatic Contents of AKS Hydrogenated Coking Gasoline and Raffinate Oil oil sample
C6 naphthenes
C7 naphthenes
C8 naphthenes
C6H6
toluene
xylene
potential aromatic content
hydrogenated coking gasoline (%) raffinate oil (%)
4.55
5.01
5.63
0.26
1.46
2.69
18.70
8.03
8.79
10.01
0.45
2.56
4.77
33.02
Table 4. Calculated Octane Numbers of AQ Hydrogenated Coking Gasoline and Raffinate Oil RON MON
hydrogenated coking gasoline
desorption oil
increment
61.13 56.54
91.97 83.59
30.84 27.05
the feed should be stopped when n-butane breakthrough appears. The adsorption process will then switch to the desorption process. 3.2. Raffinate Oil as Feedstocks of the Catalytic Reforming Process. The adsorption process was carried out in a well-desorbed bed. At a temperature of 300 °C and a space velocity of 100 h-1, the yield of raffinate oil is 56.1% and the normal paraffin content in the raffinate oil is zero. The potential aromatic contents of AQ hydrogenated coking gasoline and the raffinate oil from the adsorption separation process are presented in Table 3. The potential aromatic content of raffinate oil increases 14% age points compared to that of hydrogenated coking gasoline. It is expected that the aromatic yields from the catalytic reforming unit using raffinate oil instead of hydrogenated coking gasoline as the feedstock can increase over 30% without modifying the existing equipment and operating conditions. 3.3. Raffinate Oil as a Blending Component of High Octane Gasoline. The calculated octane numbers for hydrogenated coking gasoline and raffinate oil using the simulation method12 according to the compositions are listed in Table 4. In comparison to hydrogenated coking gasoline, the octane number of the raffinate oil with the removal of n-paraffins improved evidently. The research octane number (RON) of raffinate oil increased over 30 units and reached 91. The densities of the AQ hydrogenated coking gasoline and
raffinate oil are 704.7 and 712.3 kg/m3, respectively. The American Society for Testing and Materials (ASTM) distillation curves of AQ hydrogenated coking gasoline and raffinate oil are shown in Figure 5. The distillation range of the raffinate oil corresponds to that of AQ hydrogenated coking gasoline. When the octane number, density, and distillate range are summarized, the raffinate oil is more suitable for the blending component of high-octane gasoline compared to the hydrogenated coking gasoline. 3.4. Desorption Oil as a Feedstock of the Steam Cracking Process. In the laboratory-scale adsorption bed, the yield of desorption oil rich in normal paraffins was over 40.5% and the total yield of the raffinate and desorption oils is 96.6%. The major loss was normal paraffins that had not been condensed in the desorbent nitrogen. The product yield could be further increased if nitrogen was recycled for repeated use in the commercial unit. The n-paraffin content in desorption oil was over 98%. Under the experimental conditions (850 °C of furnace outlet temperature, 0.4 s of residence time, and 0.6 of the steam/oil ratio), which were close to those of commercial unit, the laboratory cracking unit running on naphtha gave the similar ethylene, propylene, and butadiene yields to the commercial units. The similarities in olefin yields indicate that the experimental unit is reliable to evaluate the cracking performance of the hydrogenated coking gasoline and desorption oil. It can be seen from Table 5 that the ethylene yield increased from 29.91% using the hydrogenated coking gasoline as feed to 41.03% using the desorption oil as feed. The corresponding yield of three olefins (i.e., ethylene, propylene, and butadiene) increased from 55.72 to 66.35%. It is evident that the higher ethylene, propylene, and butadiene yields will be obtained when using the desorption oil instead of hydrogenated coking gasoline as the steam cracker feed. 4. Conclusions
Figure 5. ASTM distillation curves of AQ hydrogenated coking gasoline and raffinate oil.
(1) AQ hydrogenated coking gasoline can be separated into the raffinate oil rich in non-normal hydrocarbons and the desorption oil rich in normal paraffins through the adsorption process using 5A molecular sieves. The normal paraffin content in the raffinate oil is 0%, and the normal paraffin content in the desorption oil could reach over 98%. (2) The raffinate oil showed the potential aromatic content of 14% age points higher than the hydrogenated coking gasoline feed and, therefore, could serve as the premium catalytic reformer feedstock. Meanwhile, the RON of the raffinate oil increased over 30 units and reached 91 and, therefore, could be used as the blending component of high-octane gasoline. (3) Using the desorption oil as feedstocks, the ethylene yield of the
Table 5. Cracking Olefin Yields of AKS Hydrogenated Coking Gasoline and Desorption Oil yield (%) hydrogenated coking gasoline desorption oil increment
gas
ethylene
propylene
93.04
29.91
20.04
butadiene 5.77
three olefins 55.72
97.11 4.07
41.03 11.12
19.97 -0.07
5.36 -0.41
66.35 10.63
978 Energy & Fuels, Vol. 23, 2009
steam cracking process increased 11% age points compared to the hydrogenated coking gasoline feed in the similar conditions. (4) If the normal paraffins are separated from the non-normal hydrocarbons contained in hydrogenated coking gasoline, the desorption oil rich in normal paraffins can be used as a steam cracker feed and the raffinate oil without
Liu and Shen
normal paraffins can be used as a catalytic reformer feed or a blending component for high-octane gasoline. As a result, the use efficiency of hydrogenated coking gasoline can be dramatically improved. EF800979V
