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Upgrading of directly liquefied biomass to transportation fuels: catalytic cracking
Biomass 14 (1987) 173-183
Upgrading of Directly Liquefied Biomass to Transportation Fuels: Catalytic Cracking B6rje S. G e v e r t and J a n - E r i k O t t e r s t e d t EngineeringChemistry 1, Chalmers Universityof Technology,S-412 96 Gothenburg, Sweden (Received 28 May 1987; revised version received 23 September 1987; accepted 27 September 1987)
ABSTRACT Hydroprocessed oil derived from biomass via direct liquefication using the I'ERC process was catalytically cracked in a fixed-bed reactor. The ]ruction of the hydroprocessed oil used as feed for fluid catalytic cracking (FCC) had a boiling point range over 350°C. The FCC feed had an oxygen content q["0"8 wt % and a hydrogen to carbon ratio of 1"3:1. Compared with cracking at normal reactor temperatures of 500°C, cracking at 562°C resulted in the prodttction of excessive amounts of coke and gas. Cracking at catalyst to oil ratios lower than the ASTM standard of 4:1.33 reduced the viehls qJ'coke and gas and improved the yields of liquid products. Key words: Fluidized catalytic cracking, FCC, biomass, liquefication, PERC, heavy oil, biomass, residual oil.
INTRODUCTION In a fluid catalytic cracking process (FCC) the feed is mixed with a hot catalyst and cracking is accomplished in a few seconds. After the reaction, catalyst and products are separated and the catalyst, which is deactivated by coke, is transferred to a second reactor where the coke is burnt off at temperatures in the range 700-800°C. The hot regenerated catalyst is used in the next cracking cycle to heat the feed to the reaction temperature and to compensate for the endothermicity of the cracking reaction. Modern FCC units specially designed for heavy oil cracking are provided with facilities to withdraw heat from the regenerator, formed when large amounts of coke, formed when cracking such oils, is burnt 173 Biomass 0144-4565/87/S03.50 - © 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
174
B. S. Gevert, J.-E. Otterstedt
off. Catalytic cracking of heavy oils has recently been reviewed by Otterstedt e t al. ~ and Venuto & Habib. 2 Laboratory investigations on catalytic cracking for feed stock and catalyst evaluation are usually carried out in a standardized fxed-bed reactor which requires only small quantities of catalyst and oil? Synthetic oils, such as directly liquefied biomass or coal liquids, contain much more oxygen and have lower hydrogen to carbon ratios than petroleum oils which make them difficult to crack. In order to use such oils as FCC feed stocks, it is usually necessary to remove oxygen and increase the hydrogen to carbon ratio by hydroprocessing. MATERIALS AND METHODS
Feed oils The feed oil used in this study was produced by direct liquefication of Douglas Fir wood by the PERC process in the Albany liquefication plant (Albany, Oregon, USA). The sample was coded as TR12. This oil was extracted by decahydronaphthalene and the extract hydroprocessed at 370°C and 100 bars (100 Pa) over sulphided 'CoMo' on gammaalumina support? ,-~The hydroprocessed oil (oil A) was vacuum distilled into fractions using a Vegereux column (22 cm long). For comparison, a regular hydroprocessed vacuum gas oil was used (oil B).
Analysis Simulated distillation was carried out on an HP5880 gas chromatograph (GC) equipped with a 0.7 m OV-101 column. ~' This system is limited to distillates in the boiling point range below 540°C and a modified method was used to determine the amount of oil boiling above this temperature by adding known amounts of N-alkanes (C~2, C~ and C~,) and calculating the amount of material boiling above 540°C. The gas samples were analysed on a GC (HP5580A) by splitting the sample into two streams. The first stream went through three columns (bis(2-methoxyl ethyl)adipate, Porapak Q, molecular sieve 13X) with helium as the carrier gas. The second stream was separated in one column (molecular sieve 13X) with nitrogen as carrier gas. The specific surface areas of the catalysts and of alpha-alumina were determined by nitrogen adsorption on a Digisorb 2600, an instrument made by Micromeritics (Norcross, Georgia, USA).
Upgrading of directly liquefied biomass
175
Catalytic cracking equipment and method The cracking experiments were carried out in a fixed-bed reactor (MAT), built and operated in accordance with an ASTM standard method? In this method, 1.33 g of oil is fed to a fixed bed of 4 g of cracking catalyst at a temperature of 483°C. The oil is added during a period of 75 s which corresponds to a WHSV (weight hourly space velocity) of 16. In order to study the effect of W H S V on conversion and product yields, W H S V was changed in some of the experiments. In order to reduce problems with the high viscosity of the feed stock a larger diameter feed line was used than specified by the ASTM method and the feed was also pre-heated to 50-70°C. To receive higher conversions, higher temperatures (500 and 562°C) were used during cracking, than stipulated in the ASTM standard methods. All yields have been expressed as weight percent based on total amount of products.
Catalyst Two commercial catalysts produced by Katalistiks BV, Amsterdam, were used. Catalyst I (EKZ-4) was steam aged at 740°C for 18 h prior to use. Catalyst II (EKZ-2) an equilibrium catalyst from a European refinery, was heated at 300°C in air for 3 h prior to use. Alpha-alumina, heated at 300°C in air for 3 h was used in order to estimate the contribution to conversion due to thermal cracking. The surface area of the catalysts I, II and the alpha-alumina were 157, 81 and 1 m 2 g-~ respectively. The high area of catalyst I compared with II, is mainly due to a higher content of zeolite in the former catalyst. The area stated for catalyst I was measured after steam deactivation. Catalyst II contained 0-016, 0.49 and 0.07% of nickel, sodium and vanadium respectively.
RESULTS A N D DISCUSSION
Characteristics of the feed oils and catalysts Oil A contained approximately 0"8% oxygen, as shown in Table 1. This corresponds to the heavy vacuum gas oils studied by Nilsson e t al. 7 The hydrogen to carbon ratio is a measure of the possibility of cracking the oil catalytically. A low ratio means a high aromatic content which forms more coke during cracking. The boiling curves in Fig. 1 show that oil A
176
B. S. Gevert, J.-E. Otterstedt
TABLE 1 Elemental Composition in wt% and Amount of Residue in Feed Oils Property
Oil A
Oil B
Carbon Hydrogen Nitrogen Sulphur Oxygen
89.7 9"5 0 0 0"8
87.3 l 2-4 0-1 0"3 NAh
Hydrogen to carbon mole ratio Residue"
1.3 11%
1.7 3%
"Defined as the amount of material boiling over 540°C. hNA: Results not available.
9O o
Oil A
70 .c
so 30 lO 300
~00
Temperature ('C)
500
Fig. 1. Boiling point curves for the feed oils as determined by simulated distillation.
contains m o r e large molecules boiling over 540°C. Oils containing large molecules with such a high boiling point will react in the liquid phase and not in the gas phase. T h e large molecules are also restricted in the narrow pores in the catalyst and especially in the zeolite.
The effect of the modification of the A S T M standard to testing catalytic cracking T h e modifications of the M A T unit were tested by running catalyst II with oil B b e f o r e and after modifications and the results are presented in Table 2. T h e modification gave a slightly higher yield of gasoline, 50.5 versus 52.9.
Upgrading of directly liquefied biomass
177
TABLE 2 Testing of MAT Modifications with Oil B at 500°C
Fraction
Coke Gas Gasoline (}as oil Total
Boifing interval
Yield of products (wt %)
¢%')
Before
After
-up to 35 36-215 216-350
4.2 16-0 50-6 16.9
4.2 16-4 52-9 16-9
87.7
90.4
Thermal cracking Oils containing molecules with normal boiling points above about 500°C need higher temperatures in the catalytic cracking process than the normal 500°C to be properly gasified. Furthermore, the effective molecular size of this part is above 10 to 25 A ( 1 to 2.5 nm) which means that they are too large for diffusion into zeolite pores where they can be catalytically cracked. Thermal cracking can reduce the size of the large molecules and the fragments can undergo catalytic cracking in the zeolite pores. On the other hand, thermal cracking can be unselective, leading to high yields of unwanted products such as gas and coke. Finally, oils derived from biomass are sensitive to heating, due to the oxygen content and a poor hydrogen to carbon ratio, and by hydroprocessing,the thermal stability of such oils will improve. The oil used in this study was hydroprocessed. In Table 3 the results of thermal cracking by using alpha-alumina at 5()0 and 562°C are shown. The yield of gas oil was high in both cases and was improved by higher temperature. Gasoline (boiling from 36 to 225°C) yields were also improved by the temperature rise and the formation of coke was small in all cases, showing that the selectivity was good. At the lower temperature oil A gave a higher total yield of cracked products than oil B but at the higher temperature this was reversed. There is a type or types of molecule in oil A which is sensitive to thermal cracking, thus giving a high yield at a low temperature, but when this part of the oil is consumed, it will be more stable and, according to the experiments, of greater stability than oil B at 562°C. There are more aromatics in oil A than in oil B, which can explain the thermal behaviour, since aromatics are generally stable to thermal cracking.
B. S. Gevert, J.-E. Otterstedt
178
Catalytic cracking with high activity catalyst (I) The two oils were cracked with catalyst I at two different temperatures. The results are presented in Table 4. The total yield of gasoline + gas oil (boiling from 226 to 350°C) is higher at the lower temperature, amounting to 50% by weight for oil A and about 67% for oil B. The higher coke and gas yields at the higher temperature reduced the yields of desired products, which suggest that the optimum temperature for the cracking reaction is below 562°C. The yield of coke is important for the process since the coke must be burnt off in the regenerator. Oil A gave approximately twice as much coke when compared with oil B. Oil A had a hydrogen to carbon ratio of 1"3 while oil B had a ratio of 1.7. If it is assumed that thermal cracking is independent of surface area of the catalyst and type of bed material, the contribution of the catalytic effect can be estimated by subtracting the thermal cracking yields (Table
TABLE 3 Yields of Thermal Effect (wt %) 500 °C
562 °C
Oil A
Oil B
Oil A
Oil B
Coke Gas Gasoline Gas oil
0.4 2.3 3"3 10"7
0.1 1.4 1.9 6.1
1.3 5'6 7.1 15.2
0.4 8"5 13'0 11.3
Total
16.7
9.5
29-2
33.2
TABLE 4 Yields of Catalytic Cracking with Catalyst 1 (wt%) 500 °C
562 °C
Oil A
Oil B
Oil A
Oil B
Coke Gas Gasoline Gas oil
16-0 16.0 30-5 20.1
8-3 22.2 52-6 14.0
19.2 18.5 27.3 17.8
9.2 27.8 46.0 9'6
Total
82.6
97-1
82.8
93.2
Upgradingof directly liquefied biomass
179
3) from the yields of total catalytic cracking (Table 4) and the differences -- pure catalytic cracking -- are presented in Table 5. The amount of gas oil formed by thermal cracking was about equal to the amount formed by pure catalytic cracking at the lower temperature. At the higher temperature a larger amount of products (boiling points below 35 I°C) was produced in the thermal cracking showing that thermal cracking is important in the conversion of the larger molecules. In Table 6 results from Ref. 7 are compared with results obtained in this study. In Ref. 7, the same catalyst was used but it had a slightly differ-
TABLE 5 Yield of Pure Catalytic Cracking with Catalysts I and II (wt%) 500 °C
562 °C
Oil A"
Oil B"
Oil A h
Oil B t'
Oil A"
Oil B"
Coke Gas Gasoline Gas oil
15-6 13.7 27-2 9-4
8.2 20.8 50.7 7.9
12.3 11.0 24.6 9.1
3.8 14.1 49.6 6.2
17.9 12-9 20.2 2.6
9.4 19.3 33 - 1.7
Total
65-9
87.6
55.(/
73.7
53-6
60
a
•
W~th catalyst 1. ~'With catalyst 11.
TABLE 6 Yields (wt%) of Catalytic Cracking and Boiling Points with Catalyst I Compared with Results from the Literature
Boiling intervaP (°C) Coke Gas Gasoline Gas oil SA m-~g - t '
Oil A
Oil B
Oil B"
5"
6"
7"
-16.0 16.0 30.5 20.1
-8.3 22 53 14
-5'9 25 46 14
381-496 7.7 18 36 15
445-541 9-0 19 38 16
483 + (27) 10.0 21 37 18
130
130
157
157
130
130
"Vacuum gas oils from Wilmington California crude] ~'As determined with simulated distillation as 5% by weight at the lower temperature to 95% by weight at the higher temperature. In parenthesis the amount in wt% with boiling points above 548°C. ' Surface area of catalyst used.
B. S. Gevert, J.-E. Otterstedt
180
ent surface area, indicating a different effect of steam treatment, thus giving different activities and yields. The vacuum gas oils 5, 6 and 7 were distilled from Wilmington California crude and their boiling intervals are presented in Table 6. The yield of coke becomes higher with the higher boiling point of the fraction but the coke yield is even higher for oil A. The major reason for oil A to give more coke than the Wilmington vacuum gas oil 7, is the poor hydrogen to carbon ratio (1.3), since the amount of oxygen is about the same and the Wilmington vacuum gas oil 7 contains higher amounts of residue boiling above 540°C. The hydrogen to carbon ratio of the Wilmington vacuum gas oil 7 can be calculated as 1-6.
Catalytic cracking with low activity catalyst (II) In another series of experiments, catalyst II was used at different catalyst to oil ratios. Figure 2 shows that gasoline and gas oil yields increase slightly or are constant at lower catalyst to oil ratios. The yields of coke and gas (boiling below 36°C) in the experiments were reduced with the reduction of the catalyst to oil ratio. Since the catalyst still contains considerable amounts of coke, a fluidized catalytic cracker can maintain its heat balance at the lower catalyst to oil ratio. It should also be noted that
%
30
2s
Q.
z
"" "" " " ~ . . ~
~
2o
A gasoLine V gasoil [] gas o coke
v
e w
:"15
10
j, / / ,I
i 20
Fig. 2.
o"
I
i
2.5 3.0 cotalyst/oit
J 3.5
Yields in w t % as a function of catalyst to oil ratio of catalytic cracking with oil A at 500°C and with catalyst I.
Upgrading of directly liquefied biomass
181
the low activity catalyst (II) only gives slightly lower gasoline + gas oil yield compared with catalyst I. If the unreacted oil (heavy cycle oil boiling between 350 and 540°C) is recirculated over the cracker, total conversion will be higher since the low activity catalyst has a lower total conversion. If the recirculation of unreacted oil is carried out through a hydroprocessing step, the overall yields will be even more improved since the hydrogen to carbon ratio of the recirculate will be increased. Pure cracking was estimated at a catalyst to oil ratio of 3 and is presented in Table 5. Pure catalytic cracking over both low and high activity catalysts gives lower gasoline yields for oil A than for oil B.
Yields of light products The high temperature gives higher amounts of light gases, methane, ethene and ethane (the heavier gases are constant) than the low temperature, which is expected, since the light products have a higher thermal stability (Table 7). Oil A gives more light gases than oil B, but on the other hand, oil B gives more butane and butene which are more valuable gases. The ratio of ethane to ethene products is higher for oil A than for oil B. The main difference between the more active catalyst I and catalyst II is that the higher activity produces more gases of all types: however, catalyst II produces more butene.
CONCLUSIONS Synthetic oil produced from biomass, separated by extraction and hydroprocessed, can, after distillation, be catalytically cracked to give liquid transportation fuels. The hydroprocessed synthetic oil used has shown good thermal stability at the highest temperature used, 562°C, compared with the conventional oil used. The thermal effect in the cracking is more important at 562°C than at 500°C which produced more gas oil (boiling range 226-350°C) instead of gasoline (boiling range 36-225°C). Moderate temperature, 500°C, in catalytic cracking gives a better result than high temperature, 562°C, due to overcracking at the higher temperature. Low catalyst to oil ratios and the use of a low activity catalyst are favourable due to lower coke and gas yields yet still maintaining high yields of gasoline and gas oil. Coke amounts are high, but there are large differences depending on which type of catalyst is used. Either the extracted TR12 must be more
TABLE 7
2.0 1.7 0.4 2.5 0"6
4-0 1.4 1-2
500°C Catalyst I, oil A Catalyst II, oil A Alpha-alumina, oil A Catalyst I, oil B Catalyst 11, oil B
562°C Catalyst I, oil A Alpha-alumina, oil A Catalyst 1, oil B
Methane
1-7 1.1 1-0
1.3 1.0 0.2 1'7 0.7
Ethene
2.4 1-3 0"8
1.8 t.6 0'3 1.7 0-8
Ethane
3-3 0.4 3'5
3"3 2.1 0.7 4.0 1 "0
Propene
2.9 0"8 3"7
2.5 2.7 0-7 5.4 3"9
Propane
2.2 0 7.6
2-6 1-7 0.4 6"3 4.5
lso-butane
Yields of Light Products (wt %) of Total Yield in Catalytic and Thermal Cracking
1.3 0.1 2-4
1-5 0.8 0.3 2.6 1.8
Normal butane
1-2 0-6 3"3
1.0 1.5 0.2 3'3 3.9
Butene C~
1",3
Upgradingof directlyliquefied biomass
183
severely hydroprocessed than was the case with oil A, or specially designed cracking catalysts must be used in order to reduce the amount of coke and improve the selectivity for gasoline. The new cracking catalysts should be designed to give a high performance for oils with a low hydrogen to carbon ratio containing oxygen and molecules boiling above 540°C.
ACKNOWLEDGEMENT The authors would like to express their sincere gratitude to the Swedish National Energy Board which has sponsored this project.
REFERENCES 1. Otterstedt, J.-E., Gevert, S. B., J~irfis, S. G. & Menon, E G. (1986). Fluid catalytic cracking of heavy (residual) oil fractions. Applied Catalysis, 22 (2), 159-80. 2. Venuto, P. B. & Habib, E. T. (1979). Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, New York. 3. ASTM Method D3907-80 Microactivity Test for Fluid Cracking Catalysts. 4. Gevert, S. B. & Otterstedt, J.-E. Upgrading of directly liquefied biomass to transportation fuels -- Hydroprocessing. Biomass, 13,105-15. 5. Gevert, S. B. & Otterstedt, J.-E. Upgrading of directly liquefied biomass to transportation fuels by extraction. IGT Conference on Energy from Biomass and WastesX, 7-10 April, 1986, Washington DC. 6. Upson, L. & Sikkar, E. (1982). Effect of feedstock on catalyst performance. Applied Catalysis, 2, 87-105. 7. Nilsson, E, Massoth, E E. & Otterstedt, J.-E. (1986). Catalytic cracking of heavy vacuum gas oil. Applied Catalysis, 26, 175-89.
Upgrading of Directly Liquefied Biomass to Transportation Fuels: Catalytic Cracking B6rje S. G e v e r t and J a n - E r i k O t t e r s t e d t EngineeringChemistry 1, Chalmers Universityof Technology,S-412 96 Gothenburg, Sweden (Received 28 May 1987; revised version received 23 September 1987; accepted 27 September 1987)
ABSTRACT Hydroprocessed oil derived from biomass via direct liquefication using the I'ERC process was catalytically cracked in a fixed-bed reactor. The ]ruction of the hydroprocessed oil used as feed for fluid catalytic cracking (FCC) had a boiling point range over 350°C. The FCC feed had an oxygen content q["0"8 wt % and a hydrogen to carbon ratio of 1"3:1. Compared with cracking at normal reactor temperatures of 500°C, cracking at 562°C resulted in the prodttction of excessive amounts of coke and gas. Cracking at catalyst to oil ratios lower than the ASTM standard of 4:1.33 reduced the viehls qJ'coke and gas and improved the yields of liquid products. Key words: Fluidized catalytic cracking, FCC, biomass, liquefication, PERC, heavy oil, biomass, residual oil.
INTRODUCTION In a fluid catalytic cracking process (FCC) the feed is mixed with a hot catalyst and cracking is accomplished in a few seconds. After the reaction, catalyst and products are separated and the catalyst, which is deactivated by coke, is transferred to a second reactor where the coke is burnt off at temperatures in the range 700-800°C. The hot regenerated catalyst is used in the next cracking cycle to heat the feed to the reaction temperature and to compensate for the endothermicity of the cracking reaction. Modern FCC units specially designed for heavy oil cracking are provided with facilities to withdraw heat from the regenerator, formed when large amounts of coke, formed when cracking such oils, is burnt 173 Biomass 0144-4565/87/S03.50 - © 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
174
B. S. Gevert, J.-E. Otterstedt
off. Catalytic cracking of heavy oils has recently been reviewed by Otterstedt e t al. ~ and Venuto & Habib. 2 Laboratory investigations on catalytic cracking for feed stock and catalyst evaluation are usually carried out in a standardized fxed-bed reactor which requires only small quantities of catalyst and oil? Synthetic oils, such as directly liquefied biomass or coal liquids, contain much more oxygen and have lower hydrogen to carbon ratios than petroleum oils which make them difficult to crack. In order to use such oils as FCC feed stocks, it is usually necessary to remove oxygen and increase the hydrogen to carbon ratio by hydroprocessing. MATERIALS AND METHODS
Feed oils The feed oil used in this study was produced by direct liquefication of Douglas Fir wood by the PERC process in the Albany liquefication plant (Albany, Oregon, USA). The sample was coded as TR12. This oil was extracted by decahydronaphthalene and the extract hydroprocessed at 370°C and 100 bars (100 Pa) over sulphided 'CoMo' on gammaalumina support? ,-~The hydroprocessed oil (oil A) was vacuum distilled into fractions using a Vegereux column (22 cm long). For comparison, a regular hydroprocessed vacuum gas oil was used (oil B).
Analysis Simulated distillation was carried out on an HP5880 gas chromatograph (GC) equipped with a 0.7 m OV-101 column. ~' This system is limited to distillates in the boiling point range below 540°C and a modified method was used to determine the amount of oil boiling above this temperature by adding known amounts of N-alkanes (C~2, C~ and C~,) and calculating the amount of material boiling above 540°C. The gas samples were analysed on a GC (HP5580A) by splitting the sample into two streams. The first stream went through three columns (bis(2-methoxyl ethyl)adipate, Porapak Q, molecular sieve 13X) with helium as the carrier gas. The second stream was separated in one column (molecular sieve 13X) with nitrogen as carrier gas. The specific surface areas of the catalysts and of alpha-alumina were determined by nitrogen adsorption on a Digisorb 2600, an instrument made by Micromeritics (Norcross, Georgia, USA).
Upgrading of directly liquefied biomass
175
Catalytic cracking equipment and method The cracking experiments were carried out in a fixed-bed reactor (MAT), built and operated in accordance with an ASTM standard method? In this method, 1.33 g of oil is fed to a fixed bed of 4 g of cracking catalyst at a temperature of 483°C. The oil is added during a period of 75 s which corresponds to a WHSV (weight hourly space velocity) of 16. In order to study the effect of W H S V on conversion and product yields, W H S V was changed in some of the experiments. In order to reduce problems with the high viscosity of the feed stock a larger diameter feed line was used than specified by the ASTM method and the feed was also pre-heated to 50-70°C. To receive higher conversions, higher temperatures (500 and 562°C) were used during cracking, than stipulated in the ASTM standard methods. All yields have been expressed as weight percent based on total amount of products.
Catalyst Two commercial catalysts produced by Katalistiks BV, Amsterdam, were used. Catalyst I (EKZ-4) was steam aged at 740°C for 18 h prior to use. Catalyst II (EKZ-2) an equilibrium catalyst from a European refinery, was heated at 300°C in air for 3 h prior to use. Alpha-alumina, heated at 300°C in air for 3 h was used in order to estimate the contribution to conversion due to thermal cracking. The surface area of the catalysts I, II and the alpha-alumina were 157, 81 and 1 m 2 g-~ respectively. The high area of catalyst I compared with II, is mainly due to a higher content of zeolite in the former catalyst. The area stated for catalyst I was measured after steam deactivation. Catalyst II contained 0-016, 0.49 and 0.07% of nickel, sodium and vanadium respectively.
RESULTS A N D DISCUSSION
Characteristics of the feed oils and catalysts Oil A contained approximately 0"8% oxygen, as shown in Table 1. This corresponds to the heavy vacuum gas oils studied by Nilsson e t al. 7 The hydrogen to carbon ratio is a measure of the possibility of cracking the oil catalytically. A low ratio means a high aromatic content which forms more coke during cracking. The boiling curves in Fig. 1 show that oil A
176
B. S. Gevert, J.-E. Otterstedt
TABLE 1 Elemental Composition in wt% and Amount of Residue in Feed Oils Property
Oil A
Oil B
Carbon Hydrogen Nitrogen Sulphur Oxygen
89.7 9"5 0 0 0"8
87.3 l 2-4 0-1 0"3 NAh
Hydrogen to carbon mole ratio Residue"
1.3 11%
1.7 3%
"Defined as the amount of material boiling over 540°C. hNA: Results not available.
9O o
Oil A
70 .c
so 30 lO 300
~00
Temperature ('C)
500
Fig. 1. Boiling point curves for the feed oils as determined by simulated distillation.
contains m o r e large molecules boiling over 540°C. Oils containing large molecules with such a high boiling point will react in the liquid phase and not in the gas phase. T h e large molecules are also restricted in the narrow pores in the catalyst and especially in the zeolite.
The effect of the modification of the A S T M standard to testing catalytic cracking T h e modifications of the M A T unit were tested by running catalyst II with oil B b e f o r e and after modifications and the results are presented in Table 2. T h e modification gave a slightly higher yield of gasoline, 50.5 versus 52.9.
Upgrading of directly liquefied biomass
177
TABLE 2 Testing of MAT Modifications with Oil B at 500°C
Fraction
Coke Gas Gasoline (}as oil Total
Boifing interval
Yield of products (wt %)
¢%')
Before
After
-up to 35 36-215 216-350
4.2 16-0 50-6 16.9
4.2 16-4 52-9 16-9
87.7
90.4
Thermal cracking Oils containing molecules with normal boiling points above about 500°C need higher temperatures in the catalytic cracking process than the normal 500°C to be properly gasified. Furthermore, the effective molecular size of this part is above 10 to 25 A ( 1 to 2.5 nm) which means that they are too large for diffusion into zeolite pores where they can be catalytically cracked. Thermal cracking can reduce the size of the large molecules and the fragments can undergo catalytic cracking in the zeolite pores. On the other hand, thermal cracking can be unselective, leading to high yields of unwanted products such as gas and coke. Finally, oils derived from biomass are sensitive to heating, due to the oxygen content and a poor hydrogen to carbon ratio, and by hydroprocessing,the thermal stability of such oils will improve. The oil used in this study was hydroprocessed. In Table 3 the results of thermal cracking by using alpha-alumina at 5()0 and 562°C are shown. The yield of gas oil was high in both cases and was improved by higher temperature. Gasoline (boiling from 36 to 225°C) yields were also improved by the temperature rise and the formation of coke was small in all cases, showing that the selectivity was good. At the lower temperature oil A gave a higher total yield of cracked products than oil B but at the higher temperature this was reversed. There is a type or types of molecule in oil A which is sensitive to thermal cracking, thus giving a high yield at a low temperature, but when this part of the oil is consumed, it will be more stable and, according to the experiments, of greater stability than oil B at 562°C. There are more aromatics in oil A than in oil B, which can explain the thermal behaviour, since aromatics are generally stable to thermal cracking.
B. S. Gevert, J.-E. Otterstedt
178
Catalytic cracking with high activity catalyst (I) The two oils were cracked with catalyst I at two different temperatures. The results are presented in Table 4. The total yield of gasoline + gas oil (boiling from 226 to 350°C) is higher at the lower temperature, amounting to 50% by weight for oil A and about 67% for oil B. The higher coke and gas yields at the higher temperature reduced the yields of desired products, which suggest that the optimum temperature for the cracking reaction is below 562°C. The yield of coke is important for the process since the coke must be burnt off in the regenerator. Oil A gave approximately twice as much coke when compared with oil B. Oil A had a hydrogen to carbon ratio of 1"3 while oil B had a ratio of 1.7. If it is assumed that thermal cracking is independent of surface area of the catalyst and type of bed material, the contribution of the catalytic effect can be estimated by subtracting the thermal cracking yields (Table
TABLE 3 Yields of Thermal Effect (wt %) 500 °C
562 °C
Oil A
Oil B
Oil A
Oil B
Coke Gas Gasoline Gas oil
0.4 2.3 3"3 10"7
0.1 1.4 1.9 6.1
1.3 5'6 7.1 15.2
0.4 8"5 13'0 11.3
Total
16.7
9.5
29-2
33.2
TABLE 4 Yields of Catalytic Cracking with Catalyst 1 (wt%) 500 °C
562 °C
Oil A
Oil B
Oil A
Oil B
Coke Gas Gasoline Gas oil
16-0 16.0 30-5 20.1
8-3 22.2 52-6 14.0
19.2 18.5 27.3 17.8
9.2 27.8 46.0 9'6
Total
82.6
97-1
82.8
93.2
Upgradingof directly liquefied biomass
179
3) from the yields of total catalytic cracking (Table 4) and the differences -- pure catalytic cracking -- are presented in Table 5. The amount of gas oil formed by thermal cracking was about equal to the amount formed by pure catalytic cracking at the lower temperature. At the higher temperature a larger amount of products (boiling points below 35 I°C) was produced in the thermal cracking showing that thermal cracking is important in the conversion of the larger molecules. In Table 6 results from Ref. 7 are compared with results obtained in this study. In Ref. 7, the same catalyst was used but it had a slightly differ-
TABLE 5 Yield of Pure Catalytic Cracking with Catalysts I and II (wt%) 500 °C
562 °C
Oil A"
Oil B"
Oil A h
Oil B t'
Oil A"
Oil B"
Coke Gas Gasoline Gas oil
15-6 13.7 27-2 9-4
8.2 20.8 50.7 7.9
12.3 11.0 24.6 9.1
3.8 14.1 49.6 6.2
17.9 12-9 20.2 2.6
9.4 19.3 33 - 1.7
Total
65-9
87.6
55.(/
73.7
53-6
60
a
•
W~th catalyst 1. ~'With catalyst 11.
TABLE 6 Yields (wt%) of Catalytic Cracking and Boiling Points with Catalyst I Compared with Results from the Literature
Boiling intervaP (°C) Coke Gas Gasoline Gas oil SA m-~g - t '
Oil A
Oil B
Oil B"
5"
6"
7"
-16.0 16.0 30.5 20.1
-8.3 22 53 14
-5'9 25 46 14
381-496 7.7 18 36 15
445-541 9-0 19 38 16
483 + (27) 10.0 21 37 18
130
130
157
157
130
130
"Vacuum gas oils from Wilmington California crude] ~'As determined with simulated distillation as 5% by weight at the lower temperature to 95% by weight at the higher temperature. In parenthesis the amount in wt% with boiling points above 548°C. ' Surface area of catalyst used.
B. S. Gevert, J.-E. Otterstedt
180
ent surface area, indicating a different effect of steam treatment, thus giving different activities and yields. The vacuum gas oils 5, 6 and 7 were distilled from Wilmington California crude and their boiling intervals are presented in Table 6. The yield of coke becomes higher with the higher boiling point of the fraction but the coke yield is even higher for oil A. The major reason for oil A to give more coke than the Wilmington vacuum gas oil 7, is the poor hydrogen to carbon ratio (1.3), since the amount of oxygen is about the same and the Wilmington vacuum gas oil 7 contains higher amounts of residue boiling above 540°C. The hydrogen to carbon ratio of the Wilmington vacuum gas oil 7 can be calculated as 1-6.
Catalytic cracking with low activity catalyst (II) In another series of experiments, catalyst II was used at different catalyst to oil ratios. Figure 2 shows that gasoline and gas oil yields increase slightly or are constant at lower catalyst to oil ratios. The yields of coke and gas (boiling below 36°C) in the experiments were reduced with the reduction of the catalyst to oil ratio. Since the catalyst still contains considerable amounts of coke, a fluidized catalytic cracker can maintain its heat balance at the lower catalyst to oil ratio. It should also be noted that
%
30
2s
Q.
z
"" "" " " ~ . . ~
~
2o
A gasoLine V gasoil [] gas o coke
v
e w
:"15
10
j, / / ,I
i 20
Fig. 2.
o"
I
i
2.5 3.0 cotalyst/oit
J 3.5
Yields in w t % as a function of catalyst to oil ratio of catalytic cracking with oil A at 500°C and with catalyst I.
Upgrading of directly liquefied biomass
181
the low activity catalyst (II) only gives slightly lower gasoline + gas oil yield compared with catalyst I. If the unreacted oil (heavy cycle oil boiling between 350 and 540°C) is recirculated over the cracker, total conversion will be higher since the low activity catalyst has a lower total conversion. If the recirculation of unreacted oil is carried out through a hydroprocessing step, the overall yields will be even more improved since the hydrogen to carbon ratio of the recirculate will be increased. Pure cracking was estimated at a catalyst to oil ratio of 3 and is presented in Table 5. Pure catalytic cracking over both low and high activity catalysts gives lower gasoline yields for oil A than for oil B.
Yields of light products The high temperature gives higher amounts of light gases, methane, ethene and ethane (the heavier gases are constant) than the low temperature, which is expected, since the light products have a higher thermal stability (Table 7). Oil A gives more light gases than oil B, but on the other hand, oil B gives more butane and butene which are more valuable gases. The ratio of ethane to ethene products is higher for oil A than for oil B. The main difference between the more active catalyst I and catalyst II is that the higher activity produces more gases of all types: however, catalyst II produces more butene.
CONCLUSIONS Synthetic oil produced from biomass, separated by extraction and hydroprocessed, can, after distillation, be catalytically cracked to give liquid transportation fuels. The hydroprocessed synthetic oil used has shown good thermal stability at the highest temperature used, 562°C, compared with the conventional oil used. The thermal effect in the cracking is more important at 562°C than at 500°C which produced more gas oil (boiling range 226-350°C) instead of gasoline (boiling range 36-225°C). Moderate temperature, 500°C, in catalytic cracking gives a better result than high temperature, 562°C, due to overcracking at the higher temperature. Low catalyst to oil ratios and the use of a low activity catalyst are favourable due to lower coke and gas yields yet still maintaining high yields of gasoline and gas oil. Coke amounts are high, but there are large differences depending on which type of catalyst is used. Either the extracted TR12 must be more
TABLE 7
2.0 1.7 0.4 2.5 0"6
4-0 1.4 1-2
500°C Catalyst I, oil A Catalyst II, oil A Alpha-alumina, oil A Catalyst I, oil B Catalyst 11, oil B
562°C Catalyst I, oil A Alpha-alumina, oil A Catalyst 1, oil B
Methane
1-7 1.1 1-0
1.3 1.0 0.2 1'7 0.7
Ethene
2.4 1-3 0"8
1.8 t.6 0'3 1.7 0-8
Ethane
3-3 0.4 3'5
3"3 2.1 0.7 4.0 1 "0
Propene
2.9 0"8 3"7
2.5 2.7 0-7 5.4 3"9
Propane
2.2 0 7.6
2-6 1-7 0.4 6"3 4.5
lso-butane
Yields of Light Products (wt %) of Total Yield in Catalytic and Thermal Cracking
1.3 0.1 2-4
1-5 0.8 0.3 2.6 1.8
Normal butane
1-2 0-6 3"3
1.0 1.5 0.2 3'3 3.9
Butene C~
1",3
Upgradingof directlyliquefied biomass
183
severely hydroprocessed than was the case with oil A, or specially designed cracking catalysts must be used in order to reduce the amount of coke and improve the selectivity for gasoline. The new cracking catalysts should be designed to give a high performance for oils with a low hydrogen to carbon ratio containing oxygen and molecules boiling above 540°C.
ACKNOWLEDGEMENT The authors would like to express their sincere gratitude to the Swedish National Energy Board which has sponsored this project.
REFERENCES 1. Otterstedt, J.-E., Gevert, S. B., J~irfis, S. G. & Menon, E G. (1986). Fluid catalytic cracking of heavy (residual) oil fractions. Applied Catalysis, 22 (2), 159-80. 2. Venuto, P. B. & Habib, E. T. (1979). Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, New York. 3. ASTM Method D3907-80 Microactivity Test for Fluid Cracking Catalysts. 4. Gevert, S. B. & Otterstedt, J.-E. Upgrading of directly liquefied biomass to transportation fuels -- Hydroprocessing. Biomass, 13,105-15. 5. Gevert, S. B. & Otterstedt, J.-E. Upgrading of directly liquefied biomass to transportation fuels by extraction. IGT Conference on Energy from Biomass and WastesX, 7-10 April, 1986, Washington DC. 6. Upson, L. & Sikkar, E. (1982). Effect of feedstock on catalyst performance. Applied Catalysis, 2, 87-105. 7. Nilsson, E, Massoth, E E. & Otterstedt, J.-E. (1986). Catalytic cracking of heavy vacuum gas oil. Applied Catalysis, 26, 175-89.