Maximization Of Propylene In FCC Process By Adding ZSM-5 FCC Catalyst
Additive to
Farshi* A., Bazyari A., Borogerdi S.H,., Daneshvar A, Abri H.R. Iran,Tehran,West Side of Azadi sport complex, Research Institute of Petroleum Industry, Development Engineering department, FCC project Manager, Tel/Fax. +9821- 44739792 ,
[email protected]
Abstract: FCC units are present in most refineries for production of gasoline, LPG and middle distillates, representing a conversion capacity above 14 MMBPD. FCC also plays an increasing role in residue conversion with now more than 4 MMBPD of RFCC capacity in the world and accounts for more and more of the propylene market(with more than 25% of propylene world production coming from FCC gas plants)[5].The effects of operating parameters such as reaction temperature, and ZSM-5 as FCC catalyst additive, on the distribution of the product and the yield of propylene were investigated on a bench scale fluidized bed reactor. It is the aim of this work to perform an overall analysis of the yields and selectivity of hydrocarbons obtained in the vacuum gas oil conversion over FCC and ZSM-5 catalysts .The effectiveness of ZSM-5 additive adding in FCC process was investigated. The experimented data of off gas analysis showed that, VGO cracking at high reaction temperatures of 450–550°C increases the yield of propylene. Similar behavior is observed with the addition of 10–25 wt.% ZSM-5 additive. The combination of the two effects (high temperature and ZSM-5 addition) makes the FCC unit an excellent source of light olefins for downstream petrochemical units. Higher FCC reactor temperatures (600650°C) would not have positive effect for increasing propylene yield.
Keywords:FCC,Propylene,Yield,Additive,ZSM-5 Introduction: The process of catalytic cracking of hydrocarbons (FCC) plays a key role in the modern refineries, In Iran; the first FCC unit was made in ABADAN Refinery at 1945. FCC feedstocks are made up of low value hydrocarbons, with high molecular weight, and they are converted into lighter and more valuable products during the cracking process. In FCC, cracking reactions are favored at low pressure and high temperature. However, rapid catalyst deactivation occurs due to coke deposition on the catalyst and catalyst regeneration (controlled combustion of coke)is required. The heat produced during regeneration is used to vaporize the feed and is consumed by the endothermic cracking reactions. FCC is therefore an integrated reaction-regeneration process where heat produced in the regeneration zone and transported by the catalyst is utilized to vaporize the liquid feed and to promote endothermic reactions in the reaction zone (Figure 1)[5]. This very specific characteristic implies that the catalyst flow rate in the reactor does not only depend upon the reaction requirements, but also upon the adiabatic requirements of the process. Therefore, a modification in the thermal balance of the unit can modify the catalyst circulation and there action zone performance. Furthermore, catalyst circulation depends upon a pressure balanced transfer system influenced by: the lay-out of the unit, pressure difference between the vessels, bed levels, standpipe elevation, valve design,Fig-1 shows Reactor-Regeneration integration in FCC process. FCC units typically produce around 3-5%wt. propylene, depending on the feed properties, operating conditions and the nature of the catalysts [1]ZSM-5 based additives, although introduced more than 20 years ago as a way to increase the octane value of FCC gasoline, are now primarily being used to increase light olefins yields from the FCC unit. In fact, the use of these additives is by far the most effective way to increase light olefins yields, as typically only a few weight percent of additive in the inventory improve light olefins yields significantly [2].A bench scale fluidized bed reactor set-up with zone type electrical furnace has been designed and built to study the role of ZSM-5 zeolite additive on propylene production in FCC process.
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Fig 1. Reaction -Regeneration Integration in FCC Process
1-FCC process description
The most common process is FCC, in which the oil is cracked in the presence of a finely divided catalyst which is maintained in an aerated or fluidized state by the oil vapors. The fluid cracker consists of a catalyst section and a fractionating section that operate together as an integrated processing unit(Figure2) The catalyst section contains the reactor and regenerator, which, with the standpipe and riser, forms the catalyst circulation unit. The fluid catalyst is continuously circulated between the reactor and the regenerator using air, oil vapors, and steam as the conveying media. A typical FCC process involves mixing a preheated hydrocarbon charge with hot, regenerated catalyst as it enters the riser leading to the reactor. The charge is combined with a recycle stream within the riser, vaporized, and raised to reactor temperature (900°-1,000° F) by the hot catalyst. As the mixture travels up the riser, the charge is cracked at 10-30 psi. In the more modern FCC units, all cracking takes place in the riser. The "reactor" no longer functions as a reactor; it merely serves as a holding vessel for the cyclones. This cracking continues until the oil vapors are separated from the catalyst in the reactor cyclones. The resultant product stream (cracked product) is then charged to a fractionating column where it is separated into fractions, and some of the heavy oil is recycled to the riser. Spent catalyst is regenerated to get rid of coke that collects on the catalyst during the process. Spent catalyst flows through the catalyst stripper to the regenerator, where most of the coke deposits burn off at the bottom where preheated air and spent catalyst are mixed. Fresh catalyst is added and worn-out catalyst removed to optimize the cracking process.
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. FIGURE 2: . FLUID CATALYTIC CRACKING
2-Experimental Work : 2.1 Materials 2.1.1 VGO Feed The experiments were carried out in a bench scale fluidized bed reactor. Vacuum gas oil (VGO) that is provided from ABADAN Refinery of Iran was used as the feedstock, and its True boiling point (TBP) analysis are showed in Figure 3. The density of VGO is about 0.91 gr/cc.
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T(oC)
450 400 350 300 250 200 150 100 50 0 0
20
40
60
80
100 Wt%
Fig 3. TBP curve of VGO feedstock in ABADAN Refinery
2.1.2 FCC catalyst and ZSM-5 additives FUTRA FCC catalyst supplied from ABADAN Refinery, was used as a base catalyst in all bench scale reactor tests. Catalyst size distribution was analyzed by Scanning Electron Microscopy( SEM) method. Characterization of catalyst (FCC & ZSM-5) determined by X-Ray diffraction method. Figure 4 shows that FCC catalyst had ZEOLITE base material, the result has been shown that ZSM-5 contained SiO2 (81%w), Na (0.34%w) and Al (3.1%w), the low quantity of sodium shows ZSM-5 has acidity activities .the calculation of ratio of SIO2 to AL2O3 for ZSM-5 shows it is equal to 24 and for FCC catalyst this ratio is equal to 5 .
Fig 4 .The XRD analyzing Result of FUTRA FCC catalyst compared with ZEOLITE
2.2 . Reactor set-up The Experiments Tests were performed in a fluidized bed reactor with three-zone type electrical furnace heater , whose operation has been described previously. Nitrogen gas used as atomizer gas of VGO liquid in fluidized bed reactor while passing through atomizer nozzle .Powder catalyst was measured in laboratory weight scale measurement device, and then it was poured in tubular reactor. In each reaction, the VGO feed was pre-heated prior to injection into the stainless steel reactor. After injecting the feed ( VGO)after atomizing it is vaporized in fluidized bed reactor and then it is cracked to hydrocarbon product including light gas and middle distillate. The heavy product after cooling by chiller system were separated and off gas that is contained of light olefin gas are send to atmosphere. The Process flow
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diagram(PFD)of bench scale set up and Electrical Furnace Heater with temperature controller are showed in figures 5 and 6 respectively.
Fig 5.Process flow diagram of fluidized bed reactor set up
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Fig 6 . Three Zone Type Electrical furnace heater with temperature controllers
3. Catalytic cracking effective parameters The following parameters were affecting to catalytically cracking: Ø Temperature: high temperature would be caused more light gas production, and all cracked product is converted to methane. An optimized temperature is required for producing more valuable products (ethylene and propylene) [2, 4] Ø Catalyst to oil weight ratio (C/O ratio): with effective cat/oil we would be able to control product distribution in FCC process. Further increasing catalyst-to-oil ratio does not lead to a significant increase of propylene and will lead to more undesirable reactions of light olefins[4]. Ø Velocity & gas residency time in reactor: gas velocity in reactor and resident time are important parameters in catalytical cracking. Longer residence time dictates that there is more time for FCC cracking, and therefore, more products would be converted to methane [3].
4. Experimental data Evaluation 4.1. FCC catalyst effect on propylene yield Several tests were conducted in bench scale reactor with FCC catalyst. According to laboratory tests, the outlet-heated zones of furnace were turnoff to prevent cracking of product in high temperature, so with this rules the quantity of propylene was increased to quantity of 15 mole percentage. As with increasing temperature in catalytical cracking
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more light gas(methane) will be produce and at high temperature the cracking product will convert to methane so for producing propylene so high temperature should not be selected.
4.2 ZSM-5 Adding to FCC catalyst According to Experimental data, 1 gr of VGO was injected to reactor in one minutes, Conversion was varied by changing the catalyst to oil ratio (C/O) in the range 1.0–5.0 gr/gr. This variable was changed by keeping constant the amount of oil fed and hanging the amount of catalyst between 1.0 and 5.0 gr. The experimental temperature was 450550 °C. Reaction products were analyzed by GC chromatography analyzer. In Figure (7) the role of weight of ZSM-5 in product distribution and propylene yield while temperatures was 550 °C and C/O ratio equal to 1 have been shown. As shows in figure7 With increasing weight of ZSM-5, propylene & Ethylene quantity would be increased, as in 50% ZSM-5, the ethylene yield will be reach to 45% and propylene yield would be at 38%. According to following tests, the optimum ZSM-5 quantity for maximization of propylene is about 13- 23 weight percentage. In figure 7-it is obvious that with increasing more ZSM-5 ethylene will be produce more than propylene and olefin quantity(C2=+C3=)will constant for different composition of ZSM-5 with FCC catalyst.
Effect of weight of ZSM-5 in product distribution and propylene yield while temperatures was 550 °C and C/O ratio = 1 80 70 60 50 % 40 30 20 10 0 1gr cat. + 1gr cat. + 0.3 1gr cat. + 0.5 1gr cat. + 1gr 1.16 gr ZSM0.16 ZSM-5 gr ZSM-5 gr ZSM-5 ZSM-5 5
CH4%
C2H4%
C3H6%
Olefin
Figure7. Effect of ZSM-5 adding to FCC catalyst in product distribution and olefin yield in FCC Reactor
4.3 Temperature Effect The effect of temperature on catalytic cracking of VGO over a catalyst composite composed of FUTRA FCC catalyst (1 gr) that physically was mixed with 0.3gr ZSM-5 additive have been shown in figure (8). At figures 8 –the olefin composition in outlet off gas verses different temperature have been shown. The experimental conditions of reaction were given at following lists : Reaction temperature of 450-650 °C, catalyst-to-oil ratio 1, with N2 flow rate 15 cc/sec (gas velocity at reactor is about 11.5 cc/sec, gas resident time in reactor is 1 sec).
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The effect of temperature on catalytic cracking of VGO
70
MOL
60 50 40 30 20 10 0 450
500 CH4
550 C2H4
C3H6
600
650
OLEFINS
Figure8 . Effect of Temperature in product distribution and olefin yield in FCC reactor
At optimized composition of ZSM-5 with FCC catalyst the analyzing and comparison data shows that at temperature 550°c quantity of propylene is more than other temperature. Increasing temperature of reaction cause that more product were cracked to light hydrocarbon like CH4.the detail product distribution shows that olefin(ethylene+propylene) quantity is same between temperature 450 to 650 Oc and at temperature above 550Oc the quantity of ethylene and methane to be increase.
Conclusion It can be concluded that ZSM-5 addition to FCC catalyst is one of the recognized methods in increasing the yields of light olefins (mainly propylene) in FCC process . Increasing reaction temperature in fluidized bed reactor of FCC process from 450 to 550°C has been caused increasing the yield of propylene. But Increasing temperature more than 550°C led to an increase in the yield of methane. On the other hand, combining the effects of high severity and ZSM-5 addition with adding physically ZSM-5 with percentage of 23% wt and temperature of 550°C led to a significant increase in the propylene yields till quantity of 30% mol .so it can be concluded that adding ZSM-5 catalyst with appropriate SIO2/AL2O3 ratio the off gas distribution will be on a way of production propylene and ethylene .The our experimental showed that the olefin composition is constant in off gas due to increasing ZSM-5 and high temperature effect. with adding ZSM-5 Propylene boosting will be more than ethylene and high temperature will be boost ethylene more than propylene .The product distribution in catalyst cracking reaction is on effect of chemical reaction paths that occur in the reactor.
References: [1] M.A. den Hollander, M. Wissink, M. Makkee, J.A. Moulijn, “Synergy effects of ZSM-5 addition in fluid catalytic cracking of hydro treated flashed distillate”, Applied Catalysis A: General 223 (2002) 103–119. [2] A. Aitani, T. Yoshikawa, T. Ino, “Maximization of FCC light olefins by high severity operation and ZSM-5 addition”, Catalysis Today 60 (2000) 111–117. [3] J.S. Buchanan, “The chemistry of olefins production by ZSM-5 addition to Catalytic cracking units”, Catalysis Today 55 (2000) 207–212. [4] L. Xiaohong, L. Chunyi, Z. Jianfang, Y. Chaohe, SH. Honghong, “Effects of Temperature and Catalyst to Oil Weight Ratio on the Catalytic Conversion of Heavy Oil to Propylene Using ZSM-5 and USY Catalysts”, Journal of Natural Gas Chemistry 16(2007) 92-99.
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[5] T. Gauthier, R. Andreux, J. Verstraete, R. Roux, Industrial Development and Operation of an Efficient Riser Separation System for FCC Units, International Journal Of Chemical Reactor Engineering, Volume 3 2005 article A47
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