I n d . E n g . C h e m . Res. 1989,28, 920-925
920
Catalytic Cracking under Hydrogen Fluidization Fu-Ming Lee Phillips Research Center, Phillips Petroleum Company, Bartlesuille, Oklahoma 74004
The effects of using hydrogen to fluidize the cracking catalyst in a laboratory fluid-bed reactor have been determined on various oil feeds including vacuum gas oil, an untreated heavy crude, and hydrotreated residual oils. The effects are quite feed dependent. In general, hydrogen fluidization shifts cracking products toward the heavier end, i.e., lower conversion; higher selectivities in gasoline, light cycle oil, and heavy cycle oil; and lower coke yield. We also unexpectedly found that the light olefin to paraffin ratio signficantly increases, while the light isoparaffin to parraffin ratio slightly increases under hydrogen fluidization. Fluidization using residue gas produced from an FCC unit (containing 60 vol % hydrogen) shows effects similar in direction but smaller in magnitude than fluidization with 100% hydrogen. T h e olefin-free residue gas yields less coke. The effect of hydrogen addition to cracking reactor was previously reported by Myers and Busch (1984). They demonstrated that the addition of hydrogen to a steam diluent in the reactor can increase gasoline production and decrease coke production and conversion. The type of lab reactor used was not mentioned. The effect of hydrogen addition was greater with a high-metal catalyst than a low-metal catalyst. Also, higher benefits with hydrogen addition were found with reduced crude than with vacuum gas oil. To substantiate the experimental findings in the lab reactor, they also studied the effects of hydrogen addition in a demonstration unit charging 200 barrels/day. These tests used a high-metal catalyst and a West Texas intermediate resid feed. Again, the gasoline yield was found to increase by 1.2 vol 70 and conversion by 2.4 vol % through hydrogen addition. It should be noted that pretreating the contaminant metals on the catalyst with hydrogen or other reducing gases before cracking can improve the cracking activity and selectivity of the catalyst. The results of numerous investigations have been reported in the literature (Stine and Richter, 1981; Stuntz and Bearden, 1981; Bearden and Stuntz, 1981; Bearden and Stuntz, 1983; Stuntz and Bearden, 1983; Castillo and Hayes, 1983a,b; Hayes and Castillo, 1984; Stuntz and Schucker, 1985; Stuntz and Reid, 1985). From these results, it is conceivable that, at least for high-metal catalysts, a part of the improvements from hydrogen addition to the reactor is due to reduction of the contaminant metals on the catalyst. Besides hydrogen, Hammershaimb and Lomas (Hammershaimb and Lomas, 1984; Lomas and Hammershaimb, 1985a,b) reported that the light-hydrocarbon mixtures containing less than 10 mol 70C3 and heavier hydrocarbons can be added to the riser as catalyst lift gas to improve the cracking product distribution. Their cracking results showed that the heavier components in the lift gas can cause increased dry gas and coke yields. They concluded that the gasoline and light cycle oil yields can be improved without significantly increasing coke or dry gas make by using a lift gas containing less than 10 mol % C3 and heavier hydrocarbons. Rlazek and Ritter (1979) found that the addition of C2-Cs linear olefins to the oil feed can increase the octane rating of gasoline and significantly reduce the coke yield (as much as 50%). They speculated that the increase in gasoline octane might be caused by cyclization and dehydrogenation of the added olefins, thereby producing aromatics. The objectives of this investigation are (1)to confirm the effects of hydrogen addition on catalytic cracking that were reported in the literature, (2) to study the yield effects 0888-5885/89/2628-0920$01.50/0
of contaminant metals on the catalyst, of oil feeds, and of operating variables under hydrogen addition (fluidization), and (3) to determine the effect of the composition of hydrogen-containing gases on cracking.
Experimental Equipment and Procedures Experimental work was performed in a laboratory-scale quartz reactor called the Microconfined bed unit (MCBU). The MCBU utilized a fixed fluidized bed, as shown in Figure 1. In the reactor, the fluid (catalyst) bed section was 32 mm in diameter and 160 mm in height where the catalyst bed was structurely supported by a fritted quartz disk which allowed the fluidizing gas to pass through from the bottom of the disk. The cracking product vapor expansion section, which measured 46 mm in diameter and 160 mm in height, was on top of the fluid bed section. About 35 g of catalyst was charged to the quartz reactor and was fluidized with nitrogen in cracking and stripping cycles for base operation. For hydrogen fluidization experiments, the 160 cm3/min of nitrogen was replaced by 400 cm3/min of hydrogen in the cracking cycle. Higher hydrogen flow rates were required to achieve the same degree of catalyst fluidization. For some experiments, the catalyst was pretreated with pure hydrogen at 1250 O F for 20 min to reduce the metals on the catalyst before the cracking cycle. Air was used as the fluidizing gas to burn coke during the regeneration cycle. The oil feed was injected over 30 s through a syringe located about 1-in. above the fluidized catalyst bed. Cracked product vapor ascending through the vapor expansion section was withdrawn from the top of the reactor and collected in an ice trap followed by a gas receiver. Regardless of the fact that oil was fed from the top of the catalyst bed, catalyst and oil contact was found to be excellent due to the following reasons: (1)Liquid oil feed dripped out of the syringe tube at only 1 in. above the fluidized bed and quickly contacted the catalyst. (2) The oil feeds were basically high boiling residual oils which could not be easily stripped from the reactor (by the upflowing fluidizing gas) without contacting the catalyst bed. Experiments were conducted to verify the fact that catalyst and oil contact is not affected by the upflowing fluidizing gas. As demonstrated in Table XIV, changes in the fluidization rate in the MCBU reactor by using different inert gases at different flow rates did not change cracking conversion and product distribution. Liquid and gas products were analyzed by gas/liquid chromatography (GC). The gasoline end point was set at 430 O F . The fraction between 430 and 650 O F was con0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989 921 AIR
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PISTON FOR FEED INJECTION TUBE
-INJECTION TUBE QUARTZ WOOL
PROCESS GAS RECENER
I
U WET ICE CONDENSER-RECENER
-L WATER
FURNACE
Figure 1. Microconfined bed unit (MCBU).
sidered to be the light cycle oil, while the 650+ O F fraction was considered to be the heavy cycle oil. Coke was determined by weighing the reactor plus catalyst before and after regeneration. The cracking temperature for all the runs was 950 O F and the catalyst regeneration temperature was 1250 O F . The material balance for the run to be accepted had to be between 95% and 105%. Cracking conversion was calculated by using the following formula: conversion (vol %) = (mL of oil feed)[l - volume fraction (LCO + HCO)]/(mL of oil feed)100% (volume fraction (LCO analysis).
+ HCO) was obtained from GC
Experimental Results In this investigation, the effects of hydrogen fluidization on catalytic cracking were determined on various oil feeds ranging from vacuum gas oil to untreated heavy crude as well as hydrotreated residual oils. The properties of these feeds are summarized in Table I. Six different catalysts were used to crack the various oil feeds. The physical properties of these cracking catalysts are presented in Table 11. Effect of Oil Feeds. (1) Effect of Hydrogen Fluidization on Cracking Untreated Heavy Crude: Monagas Pipeline Crude (MPC). MCBU cracking experiments on MPC, with nitrogen (base case) or hydrogen as the fluidizing gas, were conducted using several different catalysts. These catalysts included fresh amorphous silica-alumina catalyst (catalyst A), a commercial equilibrium catalyst (catalyst B), and a high-metal-containing catalyst (catalyst C-catalyst B impregnated with 3.64 wt % vanadium and 0.82 w t '70nickel). The product distributions and operating conditions of the MCBU runs are given in Table 111. From these results, it was found that hydrogen fluidization caused an increase in light cycle and heavy cycle oil while gasoline and coke yields decreased.
Table I. Physical Properties of Oil Feeds North Slope oil feed VGO" MPCb H T residC API 20.8 12.2 18.5 c , wt 70 86.4 83.9 87.0 12.0 10.8 11.8 H , wt % N, wt 70 0.32 0.45 0.16 s, wt % 1.14 2.80 0.41 Ni, ppm
