Ind. Eng. Chem. Res. 1994,33,3043-3052
3043
Computer Simulation of the Performance of Fluid Catalytic Cracking Risers and Downers? Yasemin G. Bolkan-Kenny, Todd S. Pugsley, and Franco Berruti’ Department of Chemical & Petroleum Engineering, The University of Calgary, Calgary, Alberta T2N IN4 Canada
Newly developed zeolite catalysts for the fluid catalytic cracking (FCC) process are typically characterized by high activity and rapid deactivation. Modifications of conventional FCC reactors are necessary to avoid severe backmixing which decreases selectivities and yields and to take full advantage of new catalysts. Recent studies suggest that more uniform flow, better reaction control, shorter residence times with narrower residence time distributions (RTD), and higher catalyst/oil ratios can be achieved in downflow systems (downers) compared to upflow systems (risers). In this work, a novel computer simulation of a FCC downer reactor is introduced. Comparison of downer and riser models for the FCC process shows t h a t downers are slightly more beneficial t h a n risers when commercial silica-alumina catalysts are used. In the case of the FCC process using zeolite catalysts, the benefits of downers become more significant. 1.0. Introduction The fluid catalytic cracking (FCC) process is a major source for gasoline production from gas oil and heavy hydrocarbon fractions. Over the past 50 years, there have been significant improvements involving both hardware and catalysts, but conversion rates of heavy fractions to gasoline range around 80 vol %, i.e., only around 35-40 wt % (1988 Refining Handbook). Considering the typically large throughput rates of millions of cubic meters of gas oil per day, any improvement can be highly beneficial. Circulating fluidized beds (CFBs) in which gas and particles flow cocurrently upward (risers) are typically used for the FCC process. Although high throughput rates and good gas-particle contact make them advantageous over bubbling fluidized bed reactors, CFB risers have major problems because the gas phase flows upward without facing major resistance, whereas the particles are pulled down by gravity. This causes severe backmixing leading to nonuniform flow and large variations in particle residence time distributions (RTD).In the FCC process, for which optimal reaction times are typically less than a few seconds, this can result in significant overcracking, reducing gasoline production rates. At the same time, the wide RTD makes reaction control very difficult. Another drawback of risers is the existence of a choking catalystdoil ratio, limiting the catalyst-oil contact. Even a slight increase in the solid volume fraction, which is typically around 1%,can increase conversion rates. In recent years, environmental regulations are becoming strict, requiring major modifications in gasoline formulation. Some of the major concerns are concentrations of aromatics, especially benzene, lead content, and the vapor pressures of products. Recent studies show that gasoline quality can be improved by using newly developed zeolite-based catalysts in the FCC process which enable production of gasoline that contains less aromatics (Gianetto et al., 1994). Newly developed zeolite catalysts for the FCC process are typically characterized by high activity and fast Presented at the symposium on Catalytic Reaction Engineering for Environmentally Benign Processes at the San Diego ACS meeting, March 13-18, 1994.
deactivation rates. To take full advantage of these catalysts, conventional FCC reactors have to be modified to accommodate shorter residence times to make full use of the high activity of the fresh catalyst. Since the allowable limits of undesirable products are very low, any variation in operating conditions can cause their concentrations to exceed these limits. Hence, more uniform flow conditions in the reactor are necessary in order to achieve better reaction control. Recent studies suggest that current disadvantages of risers may be overcome in downflow CFB systems (downers) in which gas and solids both flow in the direction of gravity (Wang et al., 1992; Bai et al., 1991; Gross and Ramage, 1983). Under these circumstances, backmixing is avoided so that more uniform flow and narrower residence time distributions (RTD) can be achieved, leading to better reaction control. At the same time, shorter residence times can be achieved since slip velocities are small relative to risers, virtually eliminating overcracking. Downers can operate at higher catalystdoil ratios, leading t o improved conversions. One advantageous feature of downers is that, after an initial particle acceleration zone, catalyst particles reach a velocity higher than that of the gas allowing a so-called “forward mixing”, so that gas contacts relatively fresher catalyst a t all locations. Only very limited information is available about downers. Gross and Ramage (1983) published experimental results comparing bench scale riser and downer performances for the FCC process. Wang et al. (1992) and Aubert (1993) reported hydrodynamic data and listed basic flow equations for the hydrodynamic characteristics of downers. No hydrodynamic or reactor model is presently available in the literature for CFB downers. A schematic representation of a CFB downer recently built in our laboratory is given in Figure 1. The gas is introduced at the top of the downer column and travels downward cocurrently with the solid particles. The solids are introduced through a unique feeding system, the details of which are described elsewhere (Pugsley et al., 1994). The feeder is of crucial importance in achieving rigorous mixing. The solid-gas mixture flows to the bottom of the downer where it enters the first rapid separation unit, after which the gas is sent to the
Q888-5885/94I2633-3Q43$04.5QlQ 0 1994 American Chemical Society
downflowing gas stream is given as follows: Cyclone
(e, - @JV&( 1 )
K Riser
I
1 I-
SecondaryFluidiung Gas
1
--k+--
I
I
Primary Separator
The inertial force is given as a function of the drag force, gravitational force, and buoyancy force. Depending on the sign of the slip velocity, or difference between the interstitial gas velocity and average particle velocity, the drag force acts opposite to or in the same direction as gravity. The change in sign is accommodated by use of absolute value in eq 1. The drag coefficient, CD,is the only empirical parameter used in this model. It is typically given as a function of particle Reynolds number, Re,. For Re, less than 0.5, Stoke’s law applies and, when it exceeds the value of 1000, Newton’s law is substituted (Bird et al., 1960). For the intermediate region, various correlations are available. In the present work, it has been found that the model predictions do not show any significant variation when four different correlations were used (Bird et al., 1960; Pugsley et al., 1993; Aubert, 1993, Rudinger, 1980). One of the well accepted correlations is given by Bird et al. (1960) as
Secondary Fluidizing Cas Primary Fluidizing Cas
C D = 18.51Ret.6
(2)
where
Figure 1. Schematic representation of the experimental cold unit.
secondary separation cyclone and particles are recycled back into the downer via a riser. In a FCC process, spent catalyst particles exiting the downer would undergo regeneration before being reutilized. In this work, the hydrodynamic features of CFB downers are firstly studied to quantify the parameters of the gas-solid flow. A computer simulator is then developed to describe the downer reactor hydrodynamics coupled with the reaction kinetics of the FCC process. The simulator is used to follow the change in hydrodynamic and kinetic parameters throughout the reactor and to predict production performance(yield, selectivity, and conversion). CFB downer and riser models for the FCC process are compared in order to explore possible benefits of downers with respect to the more conventional riser configuration.
2.0. Hydrodynamic Model of the Downer Reactor In order to model the hydrodynamicsof CFB downers, some realistic simplifying assumptions are necessary. The system is assumed to operate at steady state. Due to rapid mixing of gas and particles, high heat and mass transfer rates are assumed; i.e., the operation is isothermal and pseudohomogeneous. Bai et al. (1991) reported that over the cross section of downers the velocity profile is virtually flat and the voidage profile is flat with the exception of an accumulation near the wall region which decreases with increasing the superficial gas velocity. For practical purposes, radial profiles are assumed to be flat and the proposed simulator is based on a one-dimensional model. To provide a relationship between gas and solid streams, the force balance on a single particle in the
(3) At a given superficial gas velocity, average particle velocity and voidage are the unknown parameters in eq 1. The conservation of mass gives the necessary relation between these unknowns:
The average particle velocity, up, is assumed to have an initial value corresponding to the minimum possible velocity, upmf. This velocity is calculated from eq 4 considering the densest particle arrangement, i.e., E = emf, where emf is found to be 0.50 for catalyst particles of 60 pm diameter (Kunii and Levenspiel, 1991). The particles accelerate due to drag and gravity to eventually reach the interstitial gas velocity. The particles then continue accelerating due to gravitational forces until particle acceleration decreases to zero. At this time, the particles flow at the “equivalent terminal settling velocity”, similar to the concept of the terminal settling velocity of a particle in stagnant gas. In the fully developed flow region, the average particle velocity exceeds the interstitial gas velocity. The correlations for the drag coefficient have a limitation when average particle velocity equals the interstitial gas velocity. At this point, the particle Reynolds number becomes zero and eq 2 is not defined. Analyzing eq 1,it can be seen that regardless of the numerical values of Re, and CD, the drag force approaches zero for this case. The singularity is avoided by dropping the drag force term in eq 1, when particles flow at the same speed as the gas. The pressure changes throughout the downer length due to the increase in hydrodynamichead counteracting
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3045 0.12
Gs: 121.7 kgm2s - Stmutab‘
expenment
nr Gs 92 kglm2s
s’mulato‘
X
expenment
Figure 2. Comparison of voidage in the fully developed zone obtained from simulation results with data of Aubert (1993).
Figure 3. Comparison of voidage in the fully developed zone obtained from simulation results with data of Wang et al. (1992).
pressure losses due to acceleration and frictional effects involving gas, particles and the wall. The pressure variation with axial location is given by
-
---+6 P - 6P 62
&head
6P 6zac4
6P
6P
szgas
6zsolid
+-+-
(5)
-
I
6-
-: 8”>” 0
54-
j 3-
where
? 2- i
