Energy & Fuels 1994,8, 136-140
136
Production of High-Quality Gasoline by Catalytic Cracking over Rare-Earth Metal Exchanged Y-Type Zeolites of Heavy Oil from Waste Plastics Ahmad Rahman Songip,+Takao Masuda, Hiroshi Kuwahara,t and Kenji Hashimoto' Department of Chemical Engineering, Kyoto University, Kyoto 606, Japan Received July 21, 1993. Revised Manuscript Received October 4,1 9 9 P
The effects of reaction conditions and properties of the catalysts used on the product yields and quality of the,gasoline fraction from the catalytic cracking of heavy oil obtained from waste plastics over rare-earth metal exchanged Y-type (REY) zeolites were examined. Gasoline with high contents of isoparaffins and low contents of n-paraffins and aromatics is desired in order to increase the research octane number and to achieve complete burning. A large amount of'gasoline with these favorable components was obtained by the catalytic cracking of heavy oil over a REY zeolite catalyst under the condition of a time factor (ratio of the mass of the catalyst to the mass flow rate of the feed oil) of 0.75-1 kg-cat.kg-oil-l.h, a reaction temperature of 673 K,and a catalyst of small crystal size that has a moderate amount of sites of strong acid strength.
1. Introduction
Increases in amounts of waste plastics have caused serious environmental disruptions. In contrast, waste plastics are regarded as cheap and abundant sources of chemicals and energy; therefore, the recycling of waste plastics is desirable. A chemical recycling method that converts waste plastics to useful monomers or hydrocarbons is considered the most promising of the recycling methods proposed so far.' The usual chemical recycling method used for thermoplastics such as polyethylene consists of pyrolysis and catalytic cracking. The chemical recycling of polyolefins has been studied extensively using combined pyrolysiscatalytic cracking reactors, in which the catalytic cracking zone is directly connected to the pyrolysis zone."1° Waste plastics are cracked thermally in the pyrolysis zone, and the liquid obtained is fed directly to the catalytic cracking zone to produce gasoline. The pyrolysis-catalytic cracking reactor scheme, however, poses the serious engineering and economic problems f On leave from the Department of Chemical Ehgineering, Unirersiti Teknologi Malaysia, 54100 Kuala Lumpur, Malaysia. t Permanent address: Sanwa Kako Co. Ltd., Kyoto 613,Japan. Abstract published in Advance ACS Abstracts, December 1, 1993. (1)Chemical Engineering 1992,July, 30. (2)Ayame, A.; Uemichi, Y.; Yoshida, T.; Kanoh, H. J. Jpn. Petrol. Inst. 1979,22 ( 5 ) , 280-287. (3)Ishihara, Y.; Nanbu, H.; Ikemura, T.; Takusue, T. Fuel 1990,69, 978-984. (4)Ogawa, T.; Kuroki, T.; Ide, S.; Ikemura, T. J.Appl. Polym. Sci. 1981,27,857-869. ( 5 ) Uemichi, Y.; Ayame, A.; Yoshida, T.; Kanoh, H. J.Jpn. Petrol. Inst. 1980.23 (1).35-43. (6)Uemichi; Y:; Ayame, A.; Kashiwaya, Y.; Kanoh, H. J.Chromatogr. 1983.259.69-77. (7) Uemichi, Y.; Kashiwaya, Y.; Tsukidate, M.; Ayame, A.; Kanoh, H. Bull. Chem. SOC.Jpn. 1983,56(9),2768-2773. (8)Vasile, C.; Onu, P.; Barboiu, V.; Sabliovshi, M.; Mori, G. Acta Polymerica 1986,36 (IO), 543-550. (9)Vasile, C.; Onu, P.; Barboiu, V.; Sabliovshi, M.; Mori, G.; Ganju, D.;Florea, M.Ibid. 1988,39 (6),306-310. (IO)Fukuda, T.; Saito, K.; Suzuki, S.; Sato, H.; Hirota, T. U.S. Pat. No. 4 851 601,1989.
of a complicated reaction mechanism, high capital cost, and exorbitant expense for transportation of the bulky waste plastics.ll We have proposed a new process, in which the catalytic cracking reactor is separated from the pyrolysis reactor.1' First, waste plastics are cracked thermally in pyrolysis plants built at waste collection stations. The oil produced isthen transported to a centrally located catalytic cracking plant and converted to gasoline. We have also reported that a rare-earth metal exchanged Y-type (REY) zeolite is an effectivecatalyst for converting the heavy oil obtained from waste plastics to gasoline.1l To increase the gasoline research octane number and to achieve complete burning, the contents of the side-chains and cyclic hydrocarbons must be increased, whereas those of paraffins and aromatics must be reduced.12 We therefore have examined the effects of reaction conditions and catalytic properties on the reaction product yields and on the quality of the gasoline produced. The heavy oil was cracked over REY zeolites of different crystal sizes and acidic properties by varying the temperature and time factor (ratio of the mass of the catalyst to the mass flow rate of the feed oil). The optimum reaction conditions and catalytic properties that produced gasoline of high quality and yield were examined. The fraction of main components and quality of the gasoline obtained under the optimal conditions were compared to those of commercial gasolines. 2. Experimental Section 2.1. Catalyst. Four types of REY zeolites (Si/Al = 4.8) of different crystal sizes and acidic properties were used. The physical and chemical properties of the fresh zeolites are given in Table 1. REY-1 was supplied by Tosoh Co. Ltd., Japan. REY(11)Songip, A. R.;Masuda, T.; Kuwahara H.; Hashimoto, K. Appl. Catal. B 1993,2,153-164. (12)Armor, J. N. Appl. Catal. B 1992,I, 221-256.
0887-0624/94/2508-0136$04.50/0 0 1994 American Chemical Society
Energy &Fuels, Vol. 8, No. 1, 1994 137
Production of High-Quality Gasoline Table 1. Physical and Chemical Properties of the Fresh REY Zeolites catalyst property REY-1 REY-2 REY-3 REY-4 4.8 4.8 4.8 4.8 Si/A1 0.1 0.1 crystal size (pm) 0.1 1.0 2.99 2.78 amount of total acid sites0 2.91 2.44 (mobkg-cat-1) amount of strong acid sites* 0.79 0.57 0.66 0.70 (mol-kg-cat-') Based on the total amount of ammonia desorbed in the TPD experiment. * Based on the amount of ammonia desorbed above 573 K in the TPD experiment. Table 2. Analysis of the Feed Oil. wt%
heavy oil (eClz) gasoline (c5-c11) elemental analysis H C N 0
mol %
95 5 13.7 82.0
66.0
0.0
0.0 1.1
4.3
H/C
32.9 2.00
RON of liquid [-I 4.5. 2, REY-3, and REY-4 were prepared from the Na-Y type zeolite by ion-exchange method using a solution of rare-earth metal chloride. The crystal sizes of the catalysts were measured with a scanning electron microscope (5-510, Hitachi). 2.2. Measurement of Acidic Properties. Catalytic activity decreasedrapidly at the beginning of the reaction but approached aconstant value after about 3 hon 8tream.l' Because the steadystate value for activity was used, the acidic properties of the catalysts used for the reaction during the 3 h on stream were employed. The acidic properties of the catalysts (fresh and used) were measured from the temperature-programmed desorption (TPD) spectra of ammonia. The conventional TPD experiment which utilizes a thermal conductivity detector's could not be used to measure the acidicproperties of the used Catalysts because volatile materials are desorbed and deposited on the detector during the TPD experiment, leading to a rapid reduction in the sensitivity of the detector. A thermal gravimetric analyzer (TGA)therefore was used for the fresh and used catalyta in the TPD experiment. The usual temperature range 373-873 K was used for the fresh catalysts,lS whereas a range of 373-683 K was used for the used catalysts because a large amount of adsorbed ammonia reacted with coke on the catalyst at temperatures higher than 683 K. Hence, the amount of acid sites of the used catalysts was underestimated by about 10% of the amount for the fresh catalysts. The total amount of desorbed ammonia was regarded as representing the total amount of acid sites. The TPD spectra of HZSM-5show two peaks which are usually separated at about 573 K.ll The peak in the higher temperature range corresponds to the ammonia desorbed from the sites of strong acid strength, whereas the peak in the lower temperature range corresponds to the sites of weak acid strength. Therefore, the amount of ammonia desorbed above 573 K was used as the amount of sites with strong acid strength." 2.3. Feed Oil. Polyethylene plastics were first pyrolyzed at 723 K. The respective yields of liquid products, gas, and residual wax were about 74,22, and 4 w t % To ensure homogeneity and to remove lighter hydrocarbons, the liquid product from the pyrolysis was distilled at 473 K. The yield of the residual oil, which was used as the feed oil, was about 70-80 % . Table 2 shows
.
(13) Hashimoto, K.; Masuda, T.; Mori, T. Stud. Surf. Sci. Catal. 1986, 28, 503-510. (14) Mori, N.; Nishiyama,S.;Tswuya,S.;Masai, M. AppZ. Catal. 1991, 74, 31-52.
1 2 3 4 Time factor, W/F [kg-cat.kg-oil-'.h] Figure 1. Dependencies of the reaction temperature and time factor, W/F, on the conversion of heavy oil: zeolite crystal,size = 0.1 Mm.
0
the analysis of the feed oil. The atomic ratio of hydrogen to carbon (H/C) was measured by elemental analysis with a CHNcorder (MT-3, Yanagimoto Co. Ltd.). The H/C value of the residual oil (H/C = 2) indicates that the feed oil contained neither aromatics nor isoparaffins but was composed mostly of linear paraffins. 2.4. Catalytic Cracking Reaction. Details of the experimental apparatus and procedures, the analytical methods, and the lumping of the reaction products have been given elsewhere.11 The cracking reaction was conducted in a tubular reactor filled with catalyst particles under the following conditions: time factor (WIF is the ratio of the mass of the catalyst, W, to the mass flow rate of the feed oil, F) = 0.2-3.0 kg-cat-kg-oil-l-h and reaction temperature = 573-723 K. All the experiments were terminated after 3 h on stream; therefore, the effect of aging on the catalyst was not examined. The lumping of reaction products was gaseous compounds (carbon numbers 1-4: Cl-C,), gasoline fraction ( C S - C ~heavy ~), oil (above Clz), and coke." The coked catalyst was burned off on a thermal gravimetric microbalance (GT-31, Shimadzu) at 773 K in an air stream. The amount of coke loading was obtained by the weight loss of the catalyst. 2.5. Quality of t h e Gasoline Product. The research octane number (RON), the index of the gasoline quality used, was calculated from the equation proposed by Lovasic et al.15
+
RON = -1.0729YNpz + 0.7875Yp1 0.0976Yp2 + 0.3395Ycp + 0.4049Ym + 69.0306 (1) where Yi is the weight fraction of the i-th component in the gasolihe fraction. Subscript NP2 denotes the n-paraffins without C5, IP1 the total isoparaffins from C5 to C,, IP2 the total isoparaffins without Cs-C,, CP the total cycloparaffins, and AR the total aromatics. Equation 1 suggests that gasoline with a high RON value contains large amounts of isoparaffins (IPl), aromatics (AR), and cycloparaffins(CP) and asmall amount of n-paraffins (NP2). For the complete burning of gasoline, the AR content must be low. Gasoline of high quality therefore contains large amounts of IP1 and CP and small amounts of NP2 and AR.
3. Results and Discussion
3.1. Effects of Reaction Conditions on Product Yields and Gasoline Quality. 3.1.1. Effect of the Reaction Temperature. T h e relationship between the conversion of heavy oil a n d t h e time factor, WIF, at different reaction temperatures is shown in Figure 1. Conversion was defined as the mass fraction of heavy oil (components above CIZ)converted t o gasoline, gas, a n d (15) Lovosic, G. P.; Jambrec, N.; Dew-Siftar, D.; Prostenik, M. V. Fuel 1990,69, 525-528.
Songip et al.
138 Energy &Fuels, Vol. 8,No. 1, 1994 60
g-
I
50
3 b)
E
b)
2
0
20
$
2
10 0
0
20
40
60
80
Conversion of heavy oil [%I 100
-
'
80
2
40
3 6)
.-
G
I
-
+ 573K
I
I
I
100 1
0 623 K
0 673K A 723K
60
20
0
20
0
40
60
80
100
Conversion of heavy oil [wt%] 1.2
F
z
0.9
.-3
0.6
Y
x
3
3
t
A 723K
30
'5,
RON
I
0 623K 0 673K
40
Y
I
Y
0.3 0
20
0
40
60
80
100
Conversion of heavy oil [To] Figure 2. Effect of the reactiontemperature on the relationship of product yields and the conversion of heavy oil (zeolite crystal size = 0.1 pm): (a, top) gasoline yield, (b, middle) gas yield, and (c, bottom) coke yield. Table 3. Effect of Temperature on Product Yields at WIF = 0.76 kg-cat-kg-oil-1.h temp heavy oil gasoline gas coke
(K)
(wt %)
(wt %)
(wt%)
(wt%)
573 623 673 723
55.45 30.77 15.84 11.43
37.22 48.04 51.93 39.61
6.96 20.76 31.71 48.42
0.37 0.43 0.52 0.54
coke. It was calculated from the following equation: conversion = 1- mass of heavy oil at the outlet (2) mass of heavy oil at the inlet The higher the temperature, the faster the reaction proceeded. According to the results reported by Nace,lG the increase in the conversion of heavy oil with the increase in the reaction temperature mainly was caused by enhancement of the rate of scission of the carbon-carbon bonds. Although conversion was greater as the temperature increased, there was no significant difference between the conversions at 673 and 723 K. The effects of temperature on the yields of the reaction products are shown in Table 3. As the temperature (16) Nace, D. M. Ind. Eng. Chem. Process Des. Deu. 1969,8 (l), 24.
:s .? E
30 n
V
"
iri
I
AR
2 550
600
650
700
750
Temperature [K] Figure 3. Effects of the reaction temperatureon the RON value of the gasoline and the composition of ita main components (zeolite crystal size = 0.1 pm, WIF = 0.75 kg-catakg-oil-l-h):NP2 = n-paraffins without CS, IP1 = C& isoparaffins, AR = aromatics. increased, the amount of unreacted heavy oil decreased and the yields of gas and coke increased. The gasoline yield reached the maximum value at 673 K and then decreased with further increases in temperature. Figure 2a shows the effect of the reaction temperature on the relationship between gasoline yield and the conversion of heavy oil. The gasoline yield increased with increasing conversion to a maximum value and then decreased significantly. This suggests that the gasoline fraction formed by the cracking of the heavy oil subsequently underwent further cracking that yielded gaseous products and coke. Thus, gasoline was an intermediate product. The maximum gasoline yield, related to the rates of gasoline formation and cracking, appeared at about 673 K. The same optimum temperature has been found for the catalytic cracking of gas0i1.l~ Figure 2b shows the relationship between the gas yield and the conversion of heavy oil at various reaction temperatures. Because almost all the data lie on a single curve, the reaction temperature had no significant effect on the gas yield at constant conversion level. The yield of gas products increased as the reaction progressed. Figure 2c shows the effect of different reaction temperatures and conversion levels of heavy oil on the coke yield. At the same conversion level, high temperatures reduced coke formation. The difference between the coke yields at 673 and 723 K was small. Similar findings have been reported for a catalytic cracking reaction of gasoil in which coke formation proceeded well at reaction temperatures below 673 K." The effects of the reaction temperature on gasoline quality, i.e., the RONvalue of gasoline,and the composition of its main components are shown in Figure 3. Below 673 K, the RON value increased with temperature because of enhancements of the formation rate of the IP1 (isoparaffins) and the cracking rate of the NP2 (n-paraffins without Cg) fractions. Above 673 K, however, the cracking of IP1 (isoparaffins) proceeded and reduced the yield of IP1, leading to the decrease in the RON value. On the basis of the gasoline, coke, and gas yields as well as the RON value, Figures 2 and 3 and Table 3 indicate that the most favorable reaction temperature was about 673 K. 3.1.2. Effect of theTimeFactor, W/F.Table4shows the effects of the time factor, WlF,on the reaction products (17) Corma, A.; Wojciechowski, B. W. Catal. Rev. Sci. Eng. 1986,27 (l), 29-150.
Energy & Fuels, Vol. 8, No. 1, 1994 139
Production of High-Quality Gasoline 60
-
I
I
u
4 I
Coke
0
0
2
1
3
Time factor, W/F [kg-catakg-oil-'.h] Figure 4. Effect of the time factor, WIF, on the RON value of the gasoline and the composition of ita main components (zeolite crystal size = 0.1 pm,reaction temperature = 673 K): NP2 = n-paraffinswithoutCS,IP1= c6-C~isoparaffins, AR = aromatics. Table 4. Effect of the Time Factor, W/F, on Product Yields at T = 673 K WIF
(kg-cat. kg-oil-1.h)
heavy oil
gasoline
(wt % )
(wt%)
0.18 0.29 0.75 1.72 2.53
44.46 35.13 15.84 8.46 7.53
46.60 52.44 51.93 41.08 25.41
0.2
gas (wt%)
8.82 12.24 31.71 49.66 66.07
0.25
*
-
Table 5. Effect of Crystal Size on Reaction Products Yields and Gasoline Quality: Reaction Temperature = 673 K and W/F = 0.75 kg-cat.kg-oil-*-h catalyst
coke
yields. The conversion of heavy oil, as well as the yields of gas and coke, increased with the time factor. The maximum gasoline yield was obtained at a low time factor and then decreased. The effects of the WIFvalue on the RON value and the composition of the main components in the gasoline obtained at 673 K are shown in Figure 4. Below a WIF value of about 0.75 kg-cat*kg-oil-l-h,the increase in the RON value was due to the significant increase in the IP1(isoparaffins) and the large reduction in the NP2 (nparaffins without Cg) fractions. Above a WIF value of 1 kg-cat-kg-oil-l-h,only the reaction of IP1 to AR (aromatics) took place, producing a slight decrease in the RON value. These results suggest that the optimum WIFvalue for the production of gasoline of the highest quality was in the range 0.75-1 kg-cat-kg-oil-l.h. The same figure suggests the reaction pathway for the main components of gasoline: Initially the NP2 (nparaffins without CS)fraction is formed by the cracking of heavy oil. NP2 fraction is then isomerized to yield the IP1 (isoparaffins) fraction. The NP2 and IP1 fractions undergo cyclization and aromatization to produce AR compounds.18 This type of reaction pathway proceeds by the carbonium ion mechanism.8 3.2. Effects of Catalytic Properties on Product Yields and Gasoline Quality. 3.2.1. Effect of Catalyst Crystal Size. Table 5 shows the effect of crystal size on the yields of reaction products and on gasoline quality. The smaller the crystal size of the catalyst, the greater is the conversion of the heavy oil. The ratio of the crystal sizes of the two catalysts is O.l:l.O, whereas the difference in conversion at the same reaction time is not large. This indicates that the reaction proceeds under transient condition between the reaction and intraparticle diffusion controls. The effect of diffusion on the reaction rate should be discussed using a kinetic model and was analyzed by Songip et al.19 (18) Groten, W. A.; Wojciechowski, B. W. J. Catal. 1990, 122, 362.
0.35
Amount of strong acid sites [mol.kg"] Figure 5. Effect of the amount of strong acid sites of the used catalyst on product distributions: zeolite crystal size = 0.1 pm, reaction temperature = 673 K, and WIF = 0.75 kg-cat-kg-oil-1.h.
(wt%)
0.12 0.19 0.52 0.80 0.99
.
0.3
crystal size (pm) conversion of heavy oil (%) yield (wt %) gaso1ine gas
coke
gasoline component (wt %) NP2 IP1
AR RON (-)
REY-1
REY-2
0.1 84.17
1.0 69.83
51.93 31.71 0.53
48.39 20.99 0.45
15.38 40.89 29.04 97.84
15.7 42.87 24.58 97.54
Although a small crystal size produced a higher gasoline yield than did a large size, the difference in the quality of the gasoline yields was insignificant. Hence, the cracking reaction was favored by a catalyst of small crystal size. 3.2.2. Effect of the Acidic Properties of the Used Catalysts. In reactions that proceed by the carbonium ion mechanism, the reaction rate is accelerated by strong acid sites. Effects of the amount of strong acid sites of the used catalysts on the distributions of the reaction products at about 80% conversion of the heavy oil, at which the maximum gasoline yield was obtained (Figure 2a), are shown in Figure 5. The amount of strong acid sites corresponds to the amount of ammonia desorbed above 573 K in the TPD experiment. The amount of unreacted heavy oil decreased whereas that of the gas product showed an increment with increasing amount of strong acid sites of the used catalysts. The amount of coke also showed a small increment with increasing amount of strong acid sites. Because gasoline is an intermediate product, its yield gradually increased to a maximum value and then decreased with the increase in the amount of strong acid sites. The amounts of the products and unreacted heavy oil, however, were not correlated to the amounts of total acid sites or to the weak acid sites of the used catalysts, and the data were largely scattered. Figure 6 shows that the amount of strong acid sites of the used catalysts had no significant effect on the RON value or the compositions of the main components of the gasoline fraction. The above results show that enhancement of the conversion of heavy oil by an increase in the amount of (19) Songip, A. R.; Masuda, T.; Kuwahara,H.; Hashimoto, K. Energy Fuels, in press.
Songip et al.
140 Energy & Fuels, Vol. 8,No. 1, 1994
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120
I
I
Y
z
2 e s
4
80
-
40
.
.
RON
Y
%
.-.-
Y
8
F
-
IP1 AR NP2
t I
I
gasolines with those of the gasoline obtained under the optimal conditions: temperature = 673 K, time factor = 0.75-1 kg-cat-kg-oil-l-h,crystal size = 0.1 pm, and amount of strong acid sites of the used catalyst = 0.28 mol-kg’. The gasoline obtained contains a larger amount of IP1 (isoparaffins) and a smaller amount of AR (aromatics) than the commercial gasolines. The amount of NP2 (nparaffins without CS)and the RON value of the gasoline obtained in this work are between those of the regular and high- octane gasolines. 4. Conclusions
component
IPl (wt % ) AR (wt % ) NP2 (wt %) RON (-)
gasoline obtained (optimized)
regular gasoline
high-octane gasoline
40.44 29.03 12.36
29.41 33.23 15.41
36.18 50.53 9.96
100.92
90.44
108.01
strong acid sites of the used catalysts mainly accelerates the cracking of the heavy oil and gasoline, therefore producing lighter hydrocarbons. To obtain alarge amount of high-quality gasoline, the used zeolite catalyst should have a strong acid sites value of about 0.28 mol-kg-1 (Figures 5 and 6). 3.3. Comparison withcommercial Gasolines. Table 6 shows a comparison of the contents of the main components and RON values of regular and high-octane
1. The effects of reaction conditions on the activity and selectivity of REY zeolite catalysts were examined in the catalytic cracking of heavy oil obtained from waste plastics. Areactiontemperature of 673 K and atimefactor of 0.75-1 kg-cat-kg-oil-1.h were the optimum reaction conditions. Under these conditions, the highest gasoline yield was obtained, and the gasoline had the highest quality. 2. Catalytic cracking of the heavy oil over the REY zeolite proceeded predominantly via the carbonium ion mechanism. 3. A catalyst with a crystal size of 0.1 pm was best for the catalytic cracking of the heavy oil. 4. An increase in the amount of strong acid sites of the used catalysts enhanced the cracking rates of the heavy oil and gasolineand yielded lighter hydrocarbons. Because gasoline is an intermediate product, a proper amount of acid sites of the used catalysts is required to mbimize both the gasoline yield and quality. 5. The gasoline produced from the heavy oil obtained from waste plastics under the optimal reaction conditions and catalytic properties has more environmentally favorable qualities, i.e., more isoparaffins and fewer aromatics, than do commercial gasolines.
