Maximizing Propylene Yield by Two-Stage Riser Catalytic Cracking of

The TSRFCC (two-stage riser FCC) technology was developed at the China ..... This has close relations with the very short residence time of oil vapor ...
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Ind. Eng. Chem. Res. 2007, 46, 4914-4920

Maximizing Propylene Yield by Two-Stage Riser Catalytic Cracking of Heavy Oil Chunyi Li,* Chaohe Yang, and Honghong Shan State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Dongying 257061, P.R. China

The two-stage riser catalytic cracking for maximizing propylene yield has the following characteristics: relative lower temperature with larger catalyst/oil ratio, stratified injections of various feedstocks, and proper contacting time between catalyst and oil vapor. These characteristics can enhance catalytic cracking and minimize thermal cracking, which is very favorable for maximizing propylene yield and lowering dry gas. The experimental results showed that using the special catalyst LTC-2 the yield of propylene is up to 24.11% and the liquid yield approaches 82% when the first stage riser is fed Daqing AR with butenes and the second is fed the light gasoline and heavy oil from the first stage. The final gasoline containing only 26% olefins and nearly 50% aromatics is the desired high octane-number component. The diesel must be hydrogenated due to its higher density. 1. Introduction In recent years, the demand for propylene, commonly used as the feedstock of petrochemical industry, has been growing strongly in China. The naphtha steam pyrolysis process is the world’s most widespread source of light olefins.1 In the process, the pyrolysis of naphtha proceeds at a temperature of up to around 800 °C and the separation of olefins operates at the temperature of lower than -100 °C. Obviously, this is a high energy-consumption process.2 Furthermore, this process is also limited by the shortage of desirable feed. Additionally, the ratio of propylene/ethylene, determined by the reaction mechanism, is lower and difficult to adjust. Nowadays, fluid catalytic cracking (FCC) has been an important source of cheap propylene due to its economic advantages. Compared to naphtha, FCC feeds (atmospheric residue (AR), vacuum gas oil (VGO), coker gas oil (CGO), and vacuum residue (VR)) are cheaper and much more abundant. In addition, FCC operates below 550 °C and the separation of propylene from liquified petroleum gas (LPG) is no need in deep cold. So, the cost of propylene from FCC is much lower than that from steam pyrolysis. To produce more propylene from FCC feed via catalysis, there are two pathways. One is the addition of propylene additives in the conventional FCC reaction-regeneration system, and the yield of propylene rises generally by 30-40% depending on the process and conditions when the additive accounts for 3-5% of the catalyst inventory in the system.3-5 Using the additive is a simple, cheap, convenient, and effective way to improve propylene yield; the disadvantage of this method, however, is that it is impossible to improve propylene yield further by increasing the ratio of the additive with a weak ability to convert heavy oil, otherwise the heavy oil conversion will drop and dry gas will rise markedly due to the dilution of the catalyst inventory. To improve propylene yield in large amounts, another way should be chosen, that is, the special FCC processes for propylene. Nowadays, many processes have been patented or run in commercial scale, such as maximizing gaseous olefins and gasoline with atmospheric residue (ARGG),6 deep catalytic cracking (DCC),7,8 a catalytic cracking process for the production of clean gasoline (MIP-CGP),9 double-riser technology,10-12 and PetroFCC,13 etc. The yield of propylene in these processes * To whom correspondence should be addressed. Tel.: 86-5468396513. Fax: 86-546-8391971. E-mail: [email protected].

is higher than 7% and much more than that in conventional FCC. These processes share the following common characteristics: multireaction zones and/or high operation severity. For the DCC and MIP-CGP processes, there is a diameter-enlarged stage in the middle of the riser; For MAXOFIN and PetroFCC, there are two risers: one feeds FCC feedstock, and the other is specially for cracking FCC gasoline. Because these processes operate under high temperature and long residence time, the dry gas produced, mostly ethylene, is more and diesel is hardly produced. For a refinery, it is difficult to separate ethylene from dry gas economically. Therefore, maximizing propylene yield without producing large amounts of dry gas is a challenge. The TSRFCC (two-stage riser FCC) technology was developed at the China University of Petroleum under the support of the CNPC (China National Petroleum Corporation).14-16 In TSRFCC, fresh feedstock is injected into the first riser and recycling oil from the fractionator is injected into the second one. The two risers share the common disengager and regenerator. Thus, the fresh feedstock and the recycling oil, having different adsorption and reaction properties, all contact the regenerated catalyst with high activity and can react under the most favorable conditions, respectively. Furthermore, in the two risers, the reactions are terminated in time when the yield of light oil reaches the maximum, so the lengths of the two risers are different and much shorter than that of the conventional FCC riser. The fact that TSRFCC can improve liquid yield and reduce dry gas has been evidenced by more than 10 commercial units.17,18 On the platform of TSRFCC technology, maximizing propylene yield without decreasing the yield of liquid products, called TMP for short, was studied. In this paper, the experimental results with Daqing AR as the feedstock were introduced. 2. Experimental 2.1. TSRFCC for Maximizing Propylene (TMP). In TMP, propylene, gasoline, and diesel are the desired products and the yield of the saturated LPG components (generally used as fuel) should be minimized. Moreover, TMP gasoline should contain less olefins and more aromatics so as to present a high octanenumber. It is well-known that small alkane molecules are difficult to crack and olefins are much easier. Thus, we expect that FCC feed had better crack to propylene and olefins with higher than 3 carbon atoms, and the C3+ cut is recycled to produce more propylene and other value-added products.

10.1021/ie061420l CCC: $37.00 © 2007 American Chemical Society Published on Web 06/01/2007

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Practically, FCC can be seen as a carbon-removal process with heavy oil, containing less hydrogen than its products: dry gas, LPG, and gasoline. The lighter the product is, the more hydrogen it contains. That is to say, dry gas, the side product of the process, has the highest hydrogen content. Typically, its hydrogen content may be more than 23%. The hydrogen content of FCC feedstock in China, however, is generally less than 13%. If the hydrogen taken away by dry gas increases by 0.12-0.14% accounting for the feed, then the liquid yield (the sum of the yields of LPG, gasoline, and diesel) will drop by about 1%. Therefore, to maximize propylene yield without decreasing the yield of liquid products, the yield of dry gas must be limited to a minimum; otherwise, from the viewpoint of a hydrogen balance, it is unreasonable. In FCC processes, dry gas is mainly produced by thermal pyrolysis. The thermal pyrolysis of hydrocarbons has higher activation energy and relatively slower reaction rate. To limit its formation, two measures may be taken: the first is lowering the contacting temperature at the mixing instant of the regenerated catalyst and oil vapor, and the second is shortening the residence time of oil vapor in the risers. Shortening the residence time of oil vapor in the risers is easy to realize in practice, and the results of the commercial TSRFCC units have proved that the short riser can reduce the yield of dry gas significantly.18 Lowering the contacting temperature, however, meets some difficulty. The contacting temperature is dependent on the heat balance, and to improve propylene yield and selectivity, it is more favorable to increase the reaction temperature properly. Furthermore, the catalysts with the function of improving propylene yield largely often have lower activity for converting heavy oil. Thus, the unit must be operated at a larger catalyst/oil ratio. Since the reaction temperature is determined by the heat balance, it is almost impossible for the temperature not to rise when the catalyst/oil ratio increases for the conventional FCC processes. It is well-known that long chain hydrocarbons, the main contributor of propylene, are easy to crack either thermally or catalytically. What we should notice is that the bond energy of C-C in a chain hydrocarbon varies with position. The bonds near the ends are stronger, and those in the middle are weaker. Thus, the shorter the hydrocarbon chain, the more difficult to crack.19,20 Small alkane molecules, such as butanes and pentanes are very difficult to crack under the conventional FCC conditions. Small olefin molecules, such as butenes and pentenes are also difficult to crack directly; however, they are easy to crack when proper catalysts are involved in the system due to the variation of the mechanism.21 Therefore, if the catalyst at high temperature first contacts with small hydrocarbon molecules which are difficult to crack directly and, subsequently, the heavy oil is fed and contacts with the temperature-lowered catalyst, then the unit may operate at larger catalyst/heavy oil ratio without increasing the reaction temperature significantly because it is not necessary to preheat the low molecule hydrocarbon stream, thus the dry gas yield may be controlled. So, the proposed scheme is as follows: in the first riser, butenes and the fresh FCC feed are fed with stratified injections; in the second, the light gasoline (mainly composed of C5) and C6)) and the recycling oil are fed with the same manner. According to the characteristics of the above four feed streams, the proper conditions may be compromised. 2.2. Experimental Units. The experimental units include an on-line pulse reaction chromatograph, a microreactor for heavy oil catalytic cracking and a recycling fluid-bed riser unit (we call it a riser unit briefly) with stratified injections.

Figure 1. Microreactor unit for heavy oil catalytic cracking.

The on-line pulse reaction chromatograph has been described in detail elsewhere.22 The apparatus consists of a minireactor with 2 mm inner diameter and an HP4890 gas chromatography (GC) with a PONA7531 column and a flame ionization detector (FID). The reactor is between the sampling inlet and the column. In the present paper, the conversion of butenes was investigated in very short contacting time (about 10-2 s). In the minireactor, 20 mg of the catalyst is placed in the middle and 5 µL of the reactant pulsed is carried by high purity N2 through the catalyst bed. The effluent from the catalyst bed is fed directly into the chromatograph column, and the composition is detected subsequently by the FID. The microreactor for heavy oil catalytic cracking (Figure 1) is used to investigate the effect of catalyst composition on propylene yield and selectivity. In the experiments, 1 g of Daqing AR is pumped into the reactor containing 5 g of the catalyst to react at 540 °C. The gas composition is analyzed by a Varian GC3800. The contents of gasoline and diesel in the liquid are determined by simulated distillation with an Agilent 6890N GC. The coke yield is calculated by analyzing the carbon content of the spent catalyst. In this paper, most of the experiments were conducted in the riser unit (Figure 2). The unit, similar to the commercial units, includes a riser with stratified injections, a disengager, and a regenerator. The effluent from the top of the disengager goes into the condensing system, and the gas and liquid products are collected and measured, respectively. The gas is analyzed by the Varian GC3800. The liquid is fractionated to gasoline, diesel, and heavy oil by the true boiling-point distillation. The liquid is also analyzed by the Agilent 6890N GC to determine the ratio of gasoline, diesel, and heavy oil. The flue gas is also measured during the reaction and then analyzed by the Varian GC3800 to calculate the coke yield. Practically, the two-stage riser experiments cannot be conducted simultaneously because it is almost impossible to realize on-line separation corresponding to the reaction system for the scale of a 1-2 kg feeding rate. In our laboratory, the two-stage riser results are obtained by the simulated calculation of twotime independent riser experiments. 2.3. Feeds. The feeds used in the paper include mixing C4, pure 1-C4), pure i-C4), and Daqing AR. The mixing C4, pure

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Figure 3. Effect of the ratio of HZSM-5/(HZSM-5 + USY) on Daqing AR cracking and the product distribution. Table 3. Main Physical Properties of LTC-2 Catalyst particle size (vol %) Al2O3 (%)

attrition index

apparent density (kg/m3)

0-40 µm

0-149 µm

mean size (µm)

37.5

2.9

730

5.20

96.8

71.0

Table 4. Catalytic Cracking Results of AR, 1-C4), and Stratifying Injections of 1-C4) and AR in the Riser Unit feeding operation conditions

Figure 2. Schematic of the riser unit with stratified injections. Table 1. Composition of the Mixing C4 (in C4°

percenta) product distribution (%)

C4)

48.53

51.47

i-C4°

n-C4°

trans-2-C4)

1-C4)

i-C4)

cis-2-C4)

40.86

7.67

9.48

15.05

20.37

6.57

a

elements

C (%)

H (%)

S (%)

N (%)

V (µg/g)

Ni (µg/g)

86.54

13.00

0.13

0.26

0.09

5.14

SARA % conradson density carbon % (20 °C) (g/cm3) saturated aromatics resin 1.07

0.8954

58.98

24.57

16.33

asphaltene 0.12

1-C4), and pure i-C4) are all provided by Qilu Petrochemical Company. The composition of the mixing C4 is listed in Table 1. The purities of the pure 1-C4) and i-C4) are all 99.2%. The properties of Daqing AR are shown in Table 2. Daqing AR, having more hydrogen and less heavy metals, is a very good FCC feedstock, especially for producing propylene because it has more long-chain hydrocarbons. 2.4. Catalysts. To significantly improve the yield of propylene, a synergy between process and catalyst must be explored. Generally, HZSM-5 is the optimum zeolite in producing propylene; its ability to crack heavy oil, however, is very weak. Therefore, a certain amount of Y or USY zeolite must be added in the catalyst to ensure the conversion of heavy oil. In the microreactor unit for heavy oil cracking, the effect of the HZSM-5/(HZSM-5 + USY) ratio on the conversion of heavy oil and the product distribution was investigated at the catalyst/Daqing AR ratio of 5 and 540 °C (Figure 3). With the increase of the HZSM-5 ratio, the conversion of heavy oil and the yield of coke drop slowly, and dry gas, LPG, and propylene all rise. What should be noted is that the rising rates of the

temp (°C) catalyst/oil residence time (s) dry gas LPG gasoline diesel heavy oil coke

If there is no special illustration, then it means weight percent.

Table 2. Properties of Daqing AR

AR

light oil yield (%) liquid yield (%) conversion (%) olefin yield (%)

ethylene propylene butenes

510

1-C4)

calculateda

510

1-C4) + ARa 510

7 1.38

8 1.45

8.5 1.21

3.70

15.71

6.25

3.85

35.98 25.04 14.4 15.03 5.85 39.44

65.76 14.75 0 0 3.78

30.41 27.44 14.40 15.03 6.46

33.13 27.05 14.52 15.23 6.22 41.57

75.41

74.70

84.97

84.77

2.65

7.35

3.85

2.77

16.44 16.26

28.65 22.94

21.10 3.73

18.63 9.80

a The weight ratio of 1-C /AR ) 16.26/100, and 16.26% is the value 4) of butenes yield of AR cracking alone.

yields of LPG and propylene are first fast and then slow down when the ratio of HZSM-5 exceeds 60%. Thus, the catalyst for maximizing the yield of propylene must contain a proper ratio of HZSM-5/USY (or Y) to ensure the conversion of heavy oil, propylene yield, and selectivity. On the basis of a large amount of experiments, the composition of the catalyst for maximizing propylene, called LTC-2, was determined and prepared by spraydrying. Its main physical properties are listed in Table 3. 3. Results and Discussion 3.1. Reactions of Daqing AR in the Riser Unit. The results of Daqing AR reacting over LTC-2 catalyst, which has been aged with 100% steam at 800 °C for 4 h, in the riser unit at 510 °C (the temperature at the riser outlet) and the catalyst/oil ratio of 7 are shown in Table 4 (data column I). The finding that for a single pass the heavy oil conversion reaches about 85% demonstrates the catalyst ability to convert heavy oil, since

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4917 Table 5. Group Composition of the Gasoline (in percent) hydrocarbons gasolinea alkanes olefins naphthenes aromatics C5) + C6)

light gasoline heavy gasoline light gasoline e70 °C >70 °C reacted

16.89 61.30 4.69 15.90 49.70

16.22 81.03 2.06 0.63 79.00

22.33 24.62 9.79 43.01 5.08

41.50 22.21 9.90 26.37 19.01

a The gasoline is from the independent cracking of AR under the conditions listed in Table 4.

Table 7. Mole Numbers of C2), C3), C5) and Mole Ratio R Calculated Based on the Data in Table 6 butene

T (°C)

C2) moles

C3) moles

C5) moles

R

1-C4)

350 450 500 550 600 700 350 450 500 550 600 700

0 0.04 0.09 0.24 0.28 0.91 0.00 0.10 0.23 0.23 0.50 0.89

0.36 0.49 0.65 0.77 0.78 0.94 0.25 0.54 0.68 0.71 0.87 0.90

0.36 0.30 0.26 0.21 0.17 0.03 0.37 0.30 0.21 0.18 0.12 0.03

1.00 1.33 1.58 1.20 1.12 0.02 0.68 1.11 1.02 1.17 0.61 0.01

i-C4)

Table 6. Product Distribution of 1-C4) and i-C4) Cracking at Various Temperaturesa (in percent) butene T (°C) CH4 1-C4)

i-C4)

350 450 500 550 600 700 350 450 500 550 600 700

C2)

C3)

C4°

C4)

0.00 0.00 15.09 1.83 46.94 0.00 1.03 20.50 5.34 43.32 0.00 2.56 27.37 5.97 41.20 0.00 6.61 32.32 7.93 34.77 0.00 7.70 32.57 6.07 40.05 0.00 25.61 39.32 1.39 31.34 0.00 0.00 10.64 5.04 53.18 0.00 2.80 22.74 6.91 38.39 0.00 6.51 28.51 12.27 31.53 0.00 6.42 29.67 8.00 39.79 0.00 13.96 36.57 9.18 29.96 0.00 24.78 37.62 1.76 33.65

C5)

C6)

C7

25.05 10.04 1.07 21.23 6.51 2.08 18.45 3.40 1.04 14.69 2.60 1.07 11.95 1.65 0.00 2.34 0.00 0.00 26.11 5.04 0.00 20.93 5.63 2.60 14.48 4.40 2.29 12.53 2.48 1.11 8.09 1.39 0.85 1.88 0.31 0.00

a The data were collected in the on-line pulse reaction chromatograph and normalized.

too high conversions are to be avoided, otherwise there will be not enough heavy oil for the second riser. The yield of LPG approaches 36%, in which 45.69% is propylene and 45.19% is butenes. That is to say, in the LPG, olefins account for about 92%. Apparently, the olefinicity of the LPG is very high and the conventional FCC processes cannot reach this value. In fact, there may be three main factors affecting the LPG olefinicity: catalyst, residence time, and temperature. Catalysts with a high ratio of HZSM-5/USY (or Y), short residence time of oil vapor in the riser, and high reaction temperature are favorable for improving the LPG olefinicity. Propylene is a high value-added product, and butenes, if not used as fuel or chemical feeds, can be recycled to produce extra propylene. Though LPG yield is very high, the yield of dry gas is only 3.7%. This has close relations with the very short residence time of oil vapor in the riser, too. The gasoline yield is 25.04%, in which more than 61% is olefins and C5) + C6) accounts for about 50% (data column I in Table 5). If the gasoline is to be recycled to produce more propylene, it is unnecessary to recycle the full boiling range since the main reactive compounds are concentrated in the light distillate.19 The full range of gasoline has been cut at 70 °C with true boiling distillation, and the light distillate contains 81% olefins, in which C5) + C6) accounts for about 79% (data column II in Table 5). The heavy distillate contains less (data column III in Table 5). Therefore, the cracking results of the light gasoline are dependent on the reaction characteristics of C5) and C6) to a great extent. 3.2. Reactions of Butenes. Though butenes are very small molecules, they may be easily converted into propylene over LTC-2 catalyst in very short contacting times (about 10-2 s) even at very low temperature, and there is no large difference in the cracking results for different butenes due to the very fast isomerization of pure butenes to the equilibrium composition (Table 6). So, the results of a pure butene cracking may represent that of mixed butenes. In the later experiments, pure 1-C4) is used as the feedstock to simulate the reactions of the mixed butenes.

Under the very short contacting time, no CH4 forms even at 700 °C and the ratio of propylene/ethylene is very high as long as the temperature does not exceed 550 °C. Butene conversion into propylene is impossible to proceed by a direct way. Some researchers23,24 have proposed the route of dimerization-cracking, that is, two butene molecule first dimerize to a C8 species which then cracks to a C3) and a C5) or two C3) and a C2). Because there is no CH4 formed, we may think that thermal cracking does not take place in so short a contacting time and the ethylene is from the catalytic cracking of C5). Thus, a C5) cracking must produce a C2) and a C3). Therefore, if the dimerization-cracking route is right, then the results in Table 6 will approximately meet the following relationship,

R)

C3) mole number - C2) mole number ≈1 C5) mole number + C2) mole number

(1)

From the results listed in Table 7, most of the R values below 600 °C approach 1. This may be seen as evidence of the dimerization-cracking route. Since the results from the on-line pulse reaction chromatography show that butenes can be converted into propylene with high selectivity when the contacting time between the reactants and the catalyst is very short, this may also happen in the riser unit, provided that the conditions are suitable. However, if in the riser only butenes are fed, it will be impossible to keep the contacting time very short. The cracking results of 1-C4) alone (data column II in Table 4) show that the dry gas yield is very high, up to 15.71%, though the residence time is 1.45 s, not long for the conventional FCC feedstocks. If the butenes from the AR cracking (data column I in Table 4) are all recycled in the second riser alone, then, based on the data in column I and II of Table 4, the final yields can be calculated by eqs 2-4: For the yield of LPG,

yt,LPG ) yAR,LPG + y1-C4),LPG‚yAR,C4 - yAR,C4

(2)

For the yield of butenes,

yt,C4 ) y1-C4),C4‚yAR,C4

(3)

For the other products,

yt,i ) yAR,i + y1-C4),i‚yAR,C4

(4)

Where, y is the yield in percent, the first subscript means feed and the second one means product, the subscript t indicates the total, and the subscript C4 represents butenes.

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Table 8. Catalytic Cracking Results of Heavy Oil, Light Gasoline from the First Stage Riser, and Their Stratifying Injections feeding operation conditions product distribution (%)

light oil yield (%) liquid yield (%) conversion (%) olefin yield (%)

a

temperature (°C) catalyst/oil residence time (s) dry gas LPG gasoline diesel heavy oil coke

ethylene propylene butenes

heavy oil from the first riser

light gasoline from the first riser

530 8.5 1.93 4.44 24.75 16.37 16.26 33.43 4.74 32.64 57.39 50.31 2.76 12.39 9.89

530 9.5 1.72 21.76 44.03 30.33 0 0 3.88

12.31 33.51 22.71 8.87 18.24 4.35

16 26.68 12.46

8.77 18.88 11.06

calculated

light gasoline (12.68%) + heavy oil (15.23%)a 530 10.5 1.85 8.30 39.63 23.15 8.57 16.27 4.07 31.72 71.35 83.73 6.84 19.65 15.22

The data in the parentheses are the yields of light gasoline and heavy oil in the first stage riser.

The normalized calculation results are listed in column III of Table 4, which show that the yields of dry gas and propylene are 6.25% and 21.10%, respectively. 3.3. Stratified Injections of 1-C4) and AR. Since when feeding 1-C4) alone in the riser it is difficult to realize the very short contacting time, stratified injections of 1-C4) and Daqing AR are employed, which means that the 1-C4) and AR are fed simultaneously but the injector of 1-C4) is several centimeters below that of AR. Thus, the reactions of 1-C4) may be terminated in time for the AR injections so as to keep the high selectivity to propylene and so that the later can be cracked over the catalyst because there is little coke from butene reactions. The other advantage is that the catalyst temperature has been lowered by the butenes when the AR is fed, so the dry gas from the AR cracking may also drop. The experimental results of the stratified injections of 1-C4) and Daqing AR (the weight ratio of 1-C4)/Daqing AR is 16.26/ 100) are shown in data column IV of Table 4. All the yields are with respect to the AR, and the LPG and butene yields have been deducted by the butene fed. Comparing with the results of AR cracking alone in data column I of Table 4, in the experiment of stratified injections, three conclusions may be made: feeding 1-C4) almost does not affect the cracking of AR; 1-C4) can be converted into propylene with high selectivity without significantly increasing the dry gas yield; partial 1-C4) can form gasoline by oligomerization or other reactions. Comparing with the calculated results (data column III in Table 4) on the basis of the data in columns I and II of Table 4, the stratified injections of 1-C4) and Daqing AR can reduce the dry gas yield by about 40%, though the yield of propylene also drops by 11.7%, obviously, the selectivity to propylene increases largely. 3.4. Reactions of the Heavy Oil, the Light Gasoline from the First Stage Riser Alone, and their Stratified Injections. The remaining heavy oil from the first stage riser of the stratified injections of 1-C4) and AR, whose yield is 15.23%, can still crack to LPG, gasoline, and diesel; however, the conversion is much lower than the fresh feed AR for the easily cracked components have decomposed in the first stage riser (data column I in Table 8). Thus, in the second stage riser, the reaction temperature should be a little higher. For the heavy oil cracking, the yield of propylene, 12.39%, is lower than that of fresh feed and the dry gas yield is somewhat higher. The light gasoline (boiling point lower than 70 °C) from the first stage riser of the stratified injections of 1-C4) and AR, whose yield is 12.68% and whose group composition is very similar to the values of the cracking of AR alone, cracks alone

in the second stage riser to produce 21.76% dry gas and 26.68% propylene (data column II in Table 8), and most of the olefins have cracked (data column IV in Table 5). Obviously, the propylene yield is very high, but the dry gas yield is also very high. The larger the olefin molecule, the faster it reacts.20 So, the contacting time for the light gasoline with the catalyst may be even shorter than that of butenes. To ensure the very short contacting time, the stratified injections should also be used. Furthermore, after the reactions of the light gasoline alone, most of the olefins are converted into small olefin molecules or other hydrocarbons, such as aromatics, alkanes, etc. (data column IV in Table 5). This may be very favorable for reducing the olefin content of gasoline without a loss of octane-number. The stratified injection results of the light gasoline and the heavy oil according to the ratio of their yield in the first riser are listed in data column IV of Table 8. All the yields are also with respect to the AR. The total results of independent injections of the two feeds calculated by eq 5 based on the data in data column I and II of Table 8 are listed in the data column III of Table 8.

yt,i )

yHeavyoil,i‚yHeavyoil + yLightgasoline,i‚yLightgasoline yHeavyoil + yLightgasoline

(5)

Where, the first subscript means feed and the second one means product; yHeavyoil, the yield of heavy oil in data column IV of Table 4, is equal to 15.23%; yLightgasoline, the yield of light gasoline in the data column IV of Table 4, is equal to12.68%. Comparing with the calculated results, the propylene and butene yields in stratified injections are improved markedly, and what should be noted particularly is that the dry gas yield drops by about 32% and the heavy oil conversion rises significantly because in the stratified injections the catalyst/heavy oil ratio will be enhanced due to the cold injection of light gasoline when the outlet temperature of the riser does not change. 3.5. Results of Two-Staged Riser Catalytic Cracking. In our laboratory, real two-staged riser catalytic cracking cannot proceed due to a lack of a continuous separation system. The results of two-staged riser catalytic cracking are obtained by simulated calculation based on the data from independent experiments. The feedstocks fed in the second stage riser are separated from the products of the first stage riser by the true boiling-point distillation. In the first stage riser experiment, Daqing AR and 1-C4) are fed and the feeding amounts of 1-C4) are determined by the yield of butene in the cracking of the AR alone. The results have been discussed in section 3.3. In the second stage riser

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4919 Table 9. Product Distribution of Two-Staged Riser Catalytic Cracking for Maximizing the Yield of Propylene first riser + second riser

products product distribution (%)

dry gas LPG gasoline diesel heavy oil coke

light oil yield (%) liquid yield (%) conversion (%) olefin yield (%)

6.17 44.19 20.83 16.91 4.54 7.36 37.74 81.93 95.46 4.68 24.11 14.05

ethylene propylene butenes

Table 10. Group Composition of the Final Gasoline of the Two-Staged Riser Catalytic Cracking (the Heavy Gasoline Produced in the First Riser + the Gasoline Produced in the Second Riser; in percent) hydrocarbons

n-alkanes

i-alkanes

naphthenes

aromatics

olefins

7.09

10.85

7.51

48.58

25.97

content (%)

experiment, the light gasoline and heavy oil are separated from the liquid produced in the first stage riser experiment by the true boiling-point distillation. The distillation ranges of the light gasoline and heavy oil are lower than 70 °C and higher than 360 °C, respectively. The ratio of the two feedstocks is equal to that of their yields. The results have been introduced in section 3.4. The results of the two-staged riser catalytic cracking are calculated according to the following equations. For the yield of gasoline,

yt,gasoline ) yf,gasoline + ys,gasoline‚(yf,lightgasoline + yf,heavyoil) - yf,lightgasoline (6) For the yield of LPG,

yt,LPG ) yf,LPG + ys,LPG‚(yf,lightgasoline + yf,heavyoil) - yf,butenes (7) For the yield of butenes,

yt,butenes ) ys,butenes‚(yf,lightgasoline + yf,heavyoil)

(8)

For the yield of heavy oil,

yt,heavyoil ) ys,heavyoil‚(yf,lightgasoline + yf,heavyoil)

(9)

For the other products,

yt,i ) yf,i + ys,i‚(yf,lightgasoline + yf,heavyoil)

(10)

Where, the subscript t, f, and s are for the two-stage total, first stage, and second stage, respectively. Obviously, here, we do not consider the recycling of the butenes and light gasoline produced in the second stage riser. The results calculated by the above formulas are shown in Table 9. The total propylene yield is up to 24.11%; however, the dry gas yield is only 6.17%, in which ethylene accounts for 75.85%. Furthermore, the yields of gasoline and diesel are 20.83% and 16.91%, respectively. The total liquid yield approaches 82%. The final gasoline, containing more aromatics and less olefins (Table 10), may be used to produce high octanenumber gasoline. The diesel, whose density is higher, up to 890 kg/m3, should be hydrogenated. 4. Conclusions On the basis of the conventional two-staged riser catalytic cracking technology, the possibility to maximize the propylene

yield was studied. New conditions were adopted: a relative lower temperature with larger catalyst/oil ratio, stratified injections of various feedstocks, and proper contacting time with the catalyst for various feedstocks. In the fluid catalytic cracking processes, the reaction temperature is determined by the thermal balance. Here, by the stratified injections of the cold light feedstock and the preheated heavy feedstock, the operation under high catalyst/oil ratio can be realized without a large temperature increase to enhance catalytic cracking and minimize thermal cracking, and the proper contacting time for different feedstocks to convert to propylene with high selectivity can also be met. The experimental results have proved the above ideas. When Daqing AR is as the feedstock with butenes and light gasoline recycling, as well as the special developed catalyst LTC-2 is used, the yield of propylene is up to 24.11% and the liquid yield still approaches 82%. Furthermore, the gasoline, containing nearly 50% aromatics and only 26% olefins, presents a high octane-number. The diesel must be hydrogenated due to its higher density. Acknowledgment Thanks are due for the financial support given by CNPC (040401-02-04) and Shandong Independence Creation Projects (2006zz08). Literature Cited (1) Pinho, A.; Furtado, J. G.; Neto, P. P.; Moreno, J. A. Double riser FCC: an opportunity for the petrochemical industry. Presented at the NPRA annual meeting, Salt Lake City, UT, 2006; AM-06-13. (2) Wang, S. H. Ethylene Unit Technology; Sino Petrochemical Publishing: Beijing, 1994. (3) Sun, W.; Li, X. H.; Chang, Z. M.; Li, C. Y. Application of catalyst LTB-2 for maximizing propylene production in two-stage riser FCC unit. Pet. Refinery Eng. 2006, 36, 5. (4) Zhao, X. J.; Roberie, T. G. ZSM-5 Additive in Fluid Catalytic Cracking. 1. Effect of Additive Level and Temperature on Light Olefins and Gasoline Olefins. Ind. Eng. Chem. Res. 1999, 38, 3847. (5) Zhao, X. J.; Roberie, T. G. ZSM-5 Additive in Fluid Catalytic Cracking. 2. Effect of Hydrogen Transfer Characteristics of the Base Cracking Catalysts and Feedstocks. Ind. Eng. Chem. Res. 1999, 38, 3854. (6) Zou, L. Q.; Gu, X. W.; Zhang, H. Z. Application of ARGG in Fluid Catalytic Cracking. Pet. Technol. 1998, 27, 756. (7) Xie, C. G. Commercial Application of Deep Catalytic Cracking Catalysts for Production of Light Olefins. Pet. Technol. 1997, 26, 825. (8) Yang, Y. G.; Luo, Y. Commercial application of DCC-II and its flexibility on production. Pet. Process. Petrochem. 2000, 31, 1. (9) Han, W. D.; Huang, R. K.; Gong, J. H. Commercial application of new FCC processsMIP-CGP. Pet. Refinery Eng. 2006, 36, 1. (10) Davls F. E., Jr.; Graven, R. G.; Lee, W. Catalytic cracking of FCC gasoline and virgin naphtha. US patent 3,928,172, 1975. (11) Henry, B. E.; Wachter, W. A.; Swan, G. A. Fluid cat cracking with high olefins production. US patent 0,189,973, 2002. (12) Winter, W. E. Cycle oil conversion process incorporating shapeselective zeolite catalysts. US patent 6,569,316, 2003. (13) Wang, L. Y.; Wang, G. L.; Wei, J. L. New FCC process minimizes gasoline olefin, increases propylene. Oil Gas J. 2003, 101, 52. (14) Shan, H. H.; Dong, H. J.; Zhang, J. F.; Niu, G. L. Experimental study of two-stage riser FCC reactions. Fuel 2001, 80, 1179. (15) Shan, H. H.; Zhang, J. F.; Duan, A. J.; Wang, Z. J.; Niu, G. L. Study on two-staged riser catalytic cracking technology. J. UniV. Pet., China 1997, 21, 55. (16) Li, Z.; Zhang, J. F.; Shan, H. H.; Han, Z. X.; Du, F. Development of two-staged riser FCC technology. II. Increase of the light fraction yield and decrease of the olefin content in gasoline. Acta Pet. Sin. (Pet. Process. Section) 2001, 17, 26. (17) Shan, H. H.; Li, C. Y.; Niu, G. L.; Yang, C. H.; Zhang, J. F. Research progress in fluid catalytic cracking technology. J. UniV. Pet., China 2005, 29, 135. (18) Xiang, Y. D.; Zhang, J. L.; Shan, J. W.; Wang, L. Optimizing the production of two-stage riser fluid catalytic cracking. Pet. Process. Petrochem. 2006, 37, 16.

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ReceiVed for reView November 6, 2006 ReVised manuscript receiVed April 25, 2007 Accepted April 26, 2007 IE061420L