Simulation of catalytic cracking process

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and a system for simulating a catalytic cracking process. More particularly, the present invention is directed to a kinetic computer model for a catalytic cracking process.
2. Description of the Prior Art
In a refinery operation such as a fluid catalytic cracking system, the number of different molecules involved runs into the thousands. Consequently, it is impossible, or at least greatly impractical, to investigate each of the thousands of molecules to determine the kinetics of a system or to characterize feed stocks or products. However, it is known to partition molecules into a number of classes and then to consider each class as an independent entity. For example, it is possible to consider all oxygen molecules as "oxygen", even though the kinetic energies of the individual oxygen molecules are different. Such grouping or lumping is used in a standard petroleum processing analysis known as PONA, in which all species are divided into 4 classes: paraffins, olefins, naphthenes and aromatics.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a method for simulation of a catalytic cracking process for the conversion of the hydrocarbon feed stream wherein the stream is contacted with an active catalyst in a reactor maintained under catalytic conversion conditions to provide reaction products which are removed from the reactor. The catalyst in the reactor becomes contaminated by the deposition of coke thereon. The simulation method comprises programming an automatic processing system to (a) generate rates of change of hydrocarbon reactants in the reactor in accordance with:
da/dt=QK a
where
da/dt=rates of reaction,
Q=catalyst properties and process variables,
K=matrix of reaction rate constants lumped kinetically and according to boiling range, and
a=composition vector of reactants and product species lumped according to molecular type and boiling range, and,
(b) generate the composition vector a as a function of reaction time.
In accordance with another aspect of the present invention, there is provided a system for simulating a catalytic cracking process for the conversion of a hydrocarbon feed stream wherein the stream is contacted with an active catalyst in a reactor maintained under catalytic conversion conditions to provide reaction products which are removed from the reactor. The catalyst in the reactor becomes contaminated by the deposition of coke thereon. The system comprises processing means programmed to generate rates of change of hydrocarbon reactants in the reactor in accordance with:
da/dt=QK a
where
da/dt=rates of reaction,
Q=catalyst properties and process variables,
K=matrix of reaction rate constants lumped kinetically and according to boiling range, and
a=composition vector of reactants and product species lumped according to molecular type and boiling range.
The processing means is further programmed to generate the composition vector a as a function of reaction time.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a catalyst section of a fluid catalytic cracking process;
FIG. 2 shows a kinetic scheme for a specific embodiment of the present invention;
FIG. 3 is a matrix of rate constants for a specific embodiment of the present invention; and
FIGS. 4 through 33 are graphs of computer generated data.

DESCRIPTION OF SPECIFIC EMBODIMENTS
FIG. 1 shows the essentials of a typical catalyst section control system wherein fresh hydrocarbon feed which can include recycle oil from a fractionator (not shown) is applied by a line 35 to the lower end of a riser line 36. Heated regenerated catayst from a standpipe 39 having a control 40 is combined with the oil in the riser line 36 such that an oil-catalyst mixture rises in an ascending dispersed stream to the lower end of a reactor 31. In the reactor 31, there may be further fluidized contacting between the oil and the catalyst particles within a relatively dense fluidized bed diagrammatically represented below the dashed line 42. Generally, a major portion of the necessary cracking and contact of the oil with the catalyst takes place in the riser 36.
At the upper end of the reactor, the catalyst particles are separated from the vaporous cracked reaction products by cyclone separating means (not shown). The reaction products are transferred overhead by a line 37 to a products recovery section which includes at least one fractionator (not shown). A stream of spent or coked catalyst is continuously passed from the reactor 31 to a regenerator 15 by a spent catalyst transfer line 29 having a control valve 28 such that the catalyst is transferred to the regenerator 15 at a controlled rate.
In the regenerator 15, the carbonized or coked catalyst particles are subjected to oxidation and carbon removal in the presence of air being introduced to the regenerator by a line 10. A bypass line 11 having a control valve 38 is connected to the line 10 to vent a portion of the air being introduced into the regenerator 15 and thus regulate the flow rate of air.
In the lower portion of the regenerator 15, a fluidized dense phase bed diagrammatically represented as below the dashed line 19 provides for contact between the coked catalyst particles and the oxidizing air stream. In the upper portion of the regenerator 15, a light phase zone permits the separation of catalyst particles by suitable centrifugal separating means (not shown) from a flue gas stream being discharged from the regenerator 15 by a line 17 having a control valve 24 therein. The line 17 vents the regenerator flue gas or feeds the flue gas to a carbon monoxide boiler (not shown) where the carbon monoxide is converted to carbon dioxide.
A level controller 27 is connected by level indicating taps 25, 26 to the side wall of the reactor 31. A control line 43 from the level controller 27 is connected to the valve 28 in the transfer line 29 to control the flow rate of catalyst through the transfer line 29. Thus, the dense phase bed 42 level and quantity of catalyst in the lower portion of the reactor 31 are maintained at desired values. A temperature controller 32 is connected to a temperature indicating means 30 at the upper portion of the reactor 31, and generates a control signal on a line 33 to control the setting of the valve 40. Thus, a variable quantity of hot regenerated catalyst may be withdrawn from the standpipe 39 to the riser line 36 to maintain a predetermined reactor temperature as defined by the set point of the temperature controller 32.
A pressure sensitive means 22 is positioned in the upper part of the reactor 31, and another pressure sensitive means 20 is positioned in the upper portion of the regenerator 15. The pressure sensitive means 20, 22 are connected to a differential pressure regulator 21 having an adjustable set point to maintain a desired differential pressure between the reactor 31 and the regenerator 15. The differential pressure regulator 21 is connected by a line 23 to the control valve 24 in the line 17 to regulate the flue gas flow through the line 17 and in turn vary the internal pressure within the upper portion of the regenerator 15 to thereby maintain the desired pressure difference between the reactor 31 and the regenerator 15. Generally, the pressure differential between the reactor 31 and the regenerator 15 is relatively low, for example, in the order of about 6 psi, and is necessary to permit the maintenance of suitable pressure differentials across the slide valves 28, 40 in the spent catalyst transfer line 29 and in the standpipe 39 to thus provide for a continuous circulation of catalyst particles between the reactor 31 and the regenerator 15.
Temperature indicating means 13, 14 within the lower and upper portions of the regenerator 15 are connected to a differential temperature controller 16, which in turn is connected by a line 18 to the valve 38 in the air vent line 11. Thus, when the temperature differential between the lower and the upper portions of the regenerator 15 varies from a predetermined differential as defined by the set point of the differential pressure controller 16, the valve 38 in the vent line 11 is adjusted to control the amount of air flowing in the line 10 to the lower portion of the regenerator 15.
In accordance with an aspect of the present invention, there is provided a lumped invariant kinetic model for catalytic cracking processes. The model contains an invariant kinetic scheme of simultaneous and consecutive reactions to predict the product yields produced in the reactor such as that shown in FIG. 1. The yields predicted in this specific embodiment are gasoline, light fuel oil, and light ends+coke (C lump). Correlation methods based on certain kinetic principles are used to break the C lump into individual light ends and coke.
The lumping scheme groups kinetically similar molecules or components according to boiling range of the molecules or components. The lumping scheme according to a specific embodiment is based on the concentrations of paraffins, naphthenes, aromatic rings, and aromatic substituent groups (paraffinic and naphthenic groups attached to aromatic rings) in the charge stock in line 35 and appears adequate to predict the major product yields in the cracking of widely different charge stocks under a broad range of process conditions. Gas oils of wide boiling range have thousands of compounds of different molecular structures and molecular weights. However, the kinetic behavior of so many different molecules can be reasonably accounted for by such a relatively simple lumping scheme in accordance with this specific embodiment. The product yields of virgin gas oils can be adequately predicted by the simple lumping scheme of paraffins, naphthenes, and aromatics; however, it is necessary to split the aromatics into aromatic rings and aromatic substituent groups to include recycle feedstocks in the model. This is not unexpected, since the molecular compositions of recycle feeds are significantly different from those of virgin gas oils. Recycle feedstocks are generally recycled from the fractionator (not shown) downstream on line 37, and are combined with the fresh charge stock in the line 35.
In addition to the lumping scheme, other factors have been incorporated into the model of the present embodiment to account for process variables and other related phenomena. A catalyst decay term is provided to account for the rapid deactivation of the catalyst which occurs during the catalytic cracking of gas oils in the line 36 and the reactor 31. Other features are an adsorption term for nitrogen poisoning, activation energies, molecular weight, residual carbon on regenerated catalyst in the line 39, and some catalyst effects.
Lumping and Reaction Scheme
The lumped invariant kinetic model for fluid catalytic cracking such as shown in FIG. 1 consists of a kinetic scheme shown in FIG. 2. With reference to FIG. 2, ten lumps are provided to follow the cracking of virgin gas oils and recycle oil charge stocks. The lumps of FIG. 1 are:
P.sub.l =Wt. % paraffinic molecules, (mass spec analysis), 430.degree.-650.degree. F.
N.sub.l =Wt. % naphthenic molecules, (mass spec analysis), 430.degree.-650.degree. F.
C.sub.Al =Wt. % carbon atoms among aromatic rings, (n-d-M method), 430.degree.-650.degree. F.
A.sub.l =Wt. % aromatic substituent groups (430.degree.-650.degree. F.)
P.sub.h =Wt. % paraffinic molecules, (mass spec analysis), 650.degree. F.+
N.sub.h =Wt. % naphthenic molecules, (mass spec analysis), 650.degree. F.+
C.sub.Ah =Wt. % carbon atoms among aromatic rings, n-d-M method, 650.degree. F.+
A.sub.h =Wt. % aromatic substituent groups (650.degree. F.+)
G=G lump (C.sub.5.sup.+ -430.degree. F.)
C=C lump (C.sub.1 -C.sub.4 +coke)
C.sub.Al +P.sub.l +N.sub.l +A.sub.l =LFO (430.degree. F.-650.degree. F.)
C.sub.Ah +P.sub.h +N.sub.h +A.sub.h +HFO (650.degree. F..sup.+)
Adapted Nomenclature for rate constants is detailed in the FIG. 2 for paraffinic molecules. Similar rules apply for the other reaction steps.
This lumping scheme successfully treats gasoline (G lump, C.sub.5 + -430.degree. F.), C lump (H.sub.2, H.sub.2 S, C.sub.1 -C.sub.4,+coke), light fuel oil, LFO, (430.degree.-650.degree. F.) yields resulting from gas oil cracking. It will be noted that the total wt.% conversion is just the sum of the G and C lumps. Detailed composition changes resulting in the light fuel oil, LFO, (430.degree.-650.degree. F.) and heavy fuel oil, HFO, (650.degree. F.+) are obtained by following the concentrations of paraffinic, naphthenic, aromatic rings, and aromatic substituent groups as the gas oil proceeds to crack. The split of aromatics is necessary for the inclusion of recycle charge stocks in the model. This split permits closing of the recycle loop and iterating about a recycle composition until convergence is established.
The kinetic scheme of FIG. 2 shows that a paraffinic molecule in HFO will form paraffinic molecules in LFO (P.sub.h -P.sub.l) and molecules in G lump (P.sub.h .fwdarw.G) and C lump (P.sub.h .fwdarw.C). Paraffinic molecules in LFO can only crack to molecules in G lump (P.sub.l .fwdarw.G) and in C lump (P.sub.l .fwdarw.C).
Likewise a naphthenic molecule in HFO can form a naphthenic molecule in LFO and molecules in the G and C lumps. This is popularly designated as saying there is "no interaction" between the paraffinic, naphthenic, and aromatic groups.
The side chains and naphthenic rings attached to the aromatic rings react similarly, except for a single "interaction" step which allows A.sub.h .fwdarw.C.sub.Al. This is the only "interaction" reaction step in the model, and is designated by the rate constant K.sub.ahcal in a matrix of rate constants shown in FIG. 3. The aromatic rings in the HFO (C.sub.Ah) and LFO (C.sub.Al) do not form gasoline, but result in the formation of the C lump and are primarily manifested as the coke contribution to the C lump. In the present model, no distinction is made between P, N, A molecules in the gasoline fraction; consequently, all the gasoline molecules are lumped together with a single cracking rate. The matrix of rate constants shown in FIG. 3 is lower triangular and is a consequence of the irreversible nature of the postulated cracking kinetic network. Irreversible reactions lend themselves to stepwise solution and considerable advantage is derived from this fact when determining the rate constants.
Nomenclature for terms used in the present application is listed in Appendix I which forms part of the present specification.
REACTOR MODEL FOR FLUIDIZED DENSE BED
The rate of reaction for a mixture of hydrocarbons is a function of catalyst properties and process variables, and of charge stock composition. In accordance with the present invention, the rate of reaction can be represented as the following equation.
da/dt=QK a
where da/dt=rates of reaction,
Q=catalyst properties and process variables,
K=matrix of reaction rate constants lumped kinetically and according to boiling range, and
a=composition vector of reactants and product species lumped according to molecular type and boiling range.
A specific fluid catalytic cracking reactor model in accordance with the present invention includes non-linear differential equations which describe the behavior of the feedstock composition vector in a plug flow vapor phase, fluid catalyst reactor with time-decaying non-diffusion limited fluid catalyst at atmospheric pressure. Plug flow vapor phase assumes that there is no change in composition across any cross-section of the reactor. In matrix notation these equations are ##EQU1## where
a=composition vector consisting of j lumped species (a.sub.j =moles j/g gas) ##EQU2##
X=dimensionless reactor length.
P=absolute pressure (atmospheres).
R=gas constant (82.05 atm. cm.sup.3 /g-mole .degree.K.).
T=absolute temperature (.degree.K.).
MW=mean molecular weight of the mixture= ##EQU3##
S.sub.WH =true weight hourly space velocity (g feed/g catalyst-hr).
K=matrix of invariant rate constants (g catalyst/cm.sup.3).sup.-1 (hr).sup.-1. a function of T, catalyst type, residual carbon on regenerated catalyst, Basic N poison, pressure, metals, etc. The effects of temperature; Basic N poisoning, catalyst type and residual carbon on regenerated catalyst on the K matrix are detailed in their corresponding sections.
t.sub.c =time from start of run, hr.
.PHI.(t.sub.c)=catalyst decay as a function of catalyst residence time, ##EQU4## where .beta. and .gamma. are constants.
K.sub.Ah =adsorption term associated with the concentration of aromatic rings in the 650.degree. F..sup.+ fraction, (C.sub.Ah).sup.-1
A detailed development of the reactor model is included in Appendix II, and a program listing is in Appendix III of the specification.
Determination of Rate Constants
A pattern search technique was used to determine the rate constants, K, from experimental data. The data supplied to the program consisted of 63 sets of isothermal cracking data at 900.degree. F. in a fluidized dense bed. These were obtained on 15 charge stocks with widely different boiling ranges and compositions. The ranges of charge stock composition, process variables, and resulting yields are given in Table 1. It should be noted that all the experimental data presented are time-averaged data. Further, it should be understood that throughout this application "conversion" or "yields" imply "time-averaged conversion" and "time-averaged yields".
The function used to measure "goodness of fit" is ##EQU5## where
.rho..sub.G.sup.2, .rho..sub.C.sup.2, and .rho..sub.L.sup.2 are the sums of the squares of the deviations over all experimental points for G lump, C lump, and LFO, respectively.
N.sub.D is the number of data points.
N.sub.P is the number of parameters used in the estimation.
Table 1______________________________________Range of Charge Stock Composition, Process Variables,And Resulting Yields Used in Fitting the Model Parameters Range______________________________________Conversion (G lump + C lump) Wt. % 30.5-82.1430.degree. F.) Wt. % 20.0-59.4C lump (H.sub.2, H.sub.2 S, C.sub.1 --C.sub.4, + coke) Wt. 9.1-25.2LFO (430- 650.degree. F.) 14.0-43.0Total Paraffins in Charge Stock (Wt. %) 8.6-51.9Total Naphthenes in Charge Stock (Wt. %) 4.2-68.8Total Aromatic Rings in Charge Stock (Wt. %) 6.1-45.0Total Aromatic Substituent Groups 5.6-23.5Molecular weight of charge stock 206-402Boiling Range (.degree.F.) 430-1000Catalyst Residence Time (Min.) 1.25, 5.0Catalyst/Oil Ratio (Wt.) 1.25-6.0Temperature (.degree.F.) 900Nitrogen Dilution (Mole %) 10Pressure (psig) 0______________________________________
Plots of observed vs. computed yields of gasoline, C lump, and LFO are shown in FIGS. 4, 5 and 6. The best fit occurs where f is a minimum.
The economics of cracking suggest that more importance be attached to the G lump and C lump fit as compared to the fit on LFO. Hence less significance is attached to the sum of the squares of deviations for LFO. This allows the LFO, G lump, and C lump to be fitted simultaneously, yet the deviations on the LFO fit will not excessively sway the G lump and C lump fit. The best set of parameters is shown in Table 2. The reactions have been grouped into four types of reactions to facilitate further discussion. With a weighting of 30% applied to LFO deviations, it may be seen from Table 2 that the average and standard error for gasoline and LFO are comparable. Heavier weighting on LFO will result in a better fit on LFO at the expense of the fit on gasoline and C lump.
Table 2__________________________________________________________________________Model Parameters__________________________________________________________________________G lump (Gasoline Formation Reactions Best Parameters__________________________________________________________________________K.sub.alg (g catalyst/cm.sup.3).sup.-1 (hr).sup.-1 18.50 .times. 10.sup.3K.sub.ahg 63.00 .times. 10.sup.3K.sub.nlg 66.15 .times. 10.sup.3K.sub.nhg 84.70 .times. 10.sup.3K.sub.plg 23.85 .times. 10.sup.3K.sub.phg 55.00 .times. 10.sup.3C Lump Formation ReactionsK.sub.alc 3.63 .times. 10.sup.3K.sub.ahc 34.20 .times. 10.sup.3K.sub.nlc 8.18 .times. 10.sup.3K.sub.nhc 14.87 .times. 10.sup.3K.sub.plc 9.44 .times. 10.sup. 3K.sub.phc 7.85 .times. 10.sup.3K.sub.calc 1.00 .times. 10.sup.3K.sub.cahc 14.63 .times. 10.sup.3Gasoline Crackling ReactionK.sub.gc 4.4 .times. 10.sup.3LFO Formation ReactionsK.sub.ahal 19.00 .times. 10.sup.3K.sub.nhnl 22.50 .times. 10.sup.3K.sub.phpl 20.70 .times. 10.sup.3K.sub.cahcal 5.86 .times. 10.sup.3K.sub.ahcal 50.00 .times. 10.sup.3Heavy Aromatic Ring Adsorption Constant K.sub.Ah, (Wt. % C.sub.Ah).sup.-1 0.128 ##STR1##.beta. (t.sub.c in hours) 162.15.gamma. 0.76Average Absolute Error (G lump), Wt. % 1.26Average Absolute Error (C lump), Wt. % 0.69Average Absolute Error (LFO), Wt. % 1.41 ##STR2## 1.78 ##STR3## 0.95 ##STR4## 1.90__________________________________________________________________________ N.sub.D = No. of data points N.sub.G = No. of parameters associated with the G lump fit N.sub. C = No. of parameters associated with the C lump fit N.sub.L = No. of parameters associated with the LFO fit
It is interesting to compare some of the rate constants listed in Table 2 with the known kinetics of the catalytic cracking of pure hydrocarbons and classes of hydrocarbons. The rate constants for the cracking of the heavy fuel oil fractions of the P, N, and A lumps to gasoline are greater than the respective ones for the light fuel oil fractions. This is quite reasonable as the cracking rates of most paraffins and naphthenes increase with increasing molecular weight.
The aromatic substituent groups in heavy fuel oil (A.sub.h) have the highest rate constant (K.sub.ahc) for C lump formation. This is consistent with the high cracking rate of side chain alkyl groups particularly C.sub.3 and C.sub.4 and the high coking tendency of 3 and 4 membered ring aromatic compounds. Consider the refractory aromatic rings in LFO (C.sub.Al). This lump should exhibit smaller coke forming and cracking tendencies (K.sub.calc) compared to the higher boiling aromatic fractions. The ratios of the respective rate constants for gasoline formation to the corresponding ones for C lump formation are an approximate measure of the selectivity of each lump for gasoline formation. The cracking of gasoline to C lump (K.sub.gc) is considerably smaller than the rate constants for formation as would be expected.
Further, significance of these rate constants may be gleaned from the next section where predicted and experimental yields are discussed for paraffinic, naphthenic, aromatic, and recycle charge stocks.
Comparison of Predicted Product Yields with Experimental Results
Some comparisons of time-averaged predicted versus time-averaged observed product yields for the G lump, C lump, and light fuel oil are shown in FIGS. 4, 5, and 6, respectively. These data were used for the computation of the rate constants given in Table 2. The agreement is extremely good for all 15 widely different charge stocks used in the calculations of the rate constants. The results represent wide ranges of charge stock properties, reaction conditions, and conversion levels.
Plots of gasoline yields versus space velocity are given for four different charge stocks in FIGS. 7 and 8. The catalyst residence times are 5.0 to 1.25 minutes, respectively, in these plots. The points are the experimental data for each charge stock and the solid curves were calculated from the model. N3 is a highly naphthenic charge stock and gives the greatest yields of gasoline. The highly paraffinic charge stock, P3, gives gasoline yields only slightly lower than N3. Both the highly aromatic (PA 33) and recycle (PA 37) charge stocks give much lower gasoline yields. The side chains on aromatic rings crack quite readily, but aromatic rings are very stable and are extremely resistant to cracking reactions. Recycle charge stocks consist largely of refractory aromatic molecules and as expected give very low yields of cracked products.
Some detailed yield data for N3 are given in FIG. 9 which contains plots of gasoline, C lump, and light fuel oil versus space velocity. The yield of gasoline goes through a maximum. The C lump increases with decreasing space velocity and the light fuel oil decreases. The agreements between the calculated and experimental results are very good.
Selectivity curves for N3 are shown in FIG. 10. Yields of gasoline, C lump, and light fuel oil are plotted against total conversion. Gasoline yield goes through a maximum whereas the C lump increases and light fuel oil decreases with increasing conversion. It is particularly significant that the model not only fits the experimental data well, but predicts the proper trends over the entire range of conversion.
Similar data for charge stocks P3, PA33, and PA37 are given in FIGS. 11-16.
Compositional Changes During Reaction
Most importantly, it has been demonstrated that with the model parameters shown in Table 2 the HFO and LFO compositions are accurately traced as conversion proceeds. It must be remembered that these compositional changes were not used in determining the model parameters. Rather, the predictions of compositional change result as a pure prediction from fitting the model to the G lump, C lump, and LFO and as such provide considerable support for the validity of the kinetic scheme.
Detailed experimental analyses of the LFO and HFO are shown for the single highly aromatic charge stock PA33 in FIGS. 17 and 18 as a function of conversion. The solid lines represent the kinetic paths traced by the model for each of the compositional lumps. The model accurately follows the increase and subsequent decrease in the wt. % of the kinetic lumps in LFO, and follows the decrease in the wt. % of the kinetic lumps in HFO.
It is especially important, from the viewpoint of recycle, to be able to predict the polynuclear aromatic rings in the HFO % C.sub.Ah as this lump primarily determines the increased coke production from recycle charge stocks and also reflects its cracking characteristics. At high conversion (60-70 wt. %) the HFO is almost solely composed of polynuclear aromatic rings. Since the lumped composition of these fractions is accurately predicted, recycle situations (recycling HFO or LFO, or both) may now be treated with confidence.
Example of Predictive Capabilities of Model
The fluid catalytic cracking reactor model can be used to predict G lump (C.sub.5 +-430.degree. F. gasoline), C lump (H.sub.2, H.sub.2 S, C.sub.1 -C.sub.4, coke), and LFO (430.degree.-650.degree. F.) yields for charge stocks not used in determining the rate constants. Predictions are computed using the kinetic model based on kinetically invariant lumps of paraffins, naphthenes, aromatic rings, and aromatic substituent groups and the model parameters presented in Table 2. The average and standard errors of the predictions are similar to those obtained when the model was fitted to the original data. The model has good prediction capability as demonstrated by the following examples.
Amal gas oil (P3) was run at a catalyst residence time of 10 minutes to test the validity of extrapolating the catalyst deactivation function to longer catalyst residence times. The catalyst deactivation function was previously computed from the cracking results of 15 charge stocks at 1.25 and 5 minutes on-stream periods. FIG. 19 shows the deactivation function adequately predicts the cracking yields of gasoline, C lump, and LFO at longer catalyst residence times (t.sub.c =10 min.).
FIG. 20 is a plot of the yields of gasoline, C lump, and light fuel oil versus space velocity for another gas oil (PA38). This charge stock was not used in the determination of the rate constants given in Table 2. The agreement between the experimental data and the predicted curves is excellent.
A similar plot in FIG. 27 is shown for a wide cut mid-continent gas oil (WCMCGO) a new charge stock not previously used in the model, and again the agreement is very good.
Activation Energies
It is assumed, in the present model, that a single activation energy may be assigned to a group of reactions. However, updated activation energies can be integrated into the model, if necessary. The present model has six activation energies derived from temperature data at 900, 950, and 1000.degree. F. on Amal and WCMCGO. The results of fitting these activation energies to the experimental data are shown in FIGS. 21 and 22 for Amal (P3) and in FIG. 29 for WCMCGO. The activation energies thus obtained are associated with the following groups of reactions:
______________________________________ Activation Energies (cal/g-mole)______________________________________1. Gasoline (G lump) formation reactions from P.sub.h, P.sub.l, N.sub.h, N.sub.l 5,5002. C lump formation reactions from P.sub.h, P.sub.l, N.sub.h, N.sub.l 8,5003. Gasoline (G lump) formation reactions from A.sub.h, A.sub.l 14,5004. C lump formation reactions from A.sub.h, A.sub.l, C.sub.Ah, C.sub.Al 17,5005. C lump formation reactions from Gasoline 20,0006. LFO formation reactions from P.sub.h, N.sub.h, A.sub.h, C.sub.Ah 8,100______________________________________
Nitrogen Poisoning
Basic nitrogen compounds are known to poison acidic cracking catalysts. It has been determined that quinoline added to WCMCGO gives the same effects on conversion and selectivity as the natural occurring nitrogen bases which occur in a typical FCC feedstock.
The effects of nitrogen poisoning have been incorporated into the lumped invariant kinetic model for catalytic cracking by the addition of a catalyst deactivation term related to nitrogen adsorption and the use of a scalar quantity on gasoline formation rate constants.
Catalyst deactivation is accounted for by a deactivation function f(N), given by: ##EQU6## where N=gms of BASIC N to which the catalyst has been exposed at catalyst residence time, t.sub.c. The deactivation function chosen has the form such that at high CATALYST/OIL ratios there are small quantities of Basic N per cracking site and the deactivation is insignificant. .theta. is the normalized catalyst residence time.
A slight increase in selectivity is incorporated amounting to a scalar increase of all gasoline formation reactions.
Fourteen sets of data were fitted to give a SE=1.98 on the G lump and SE=1.16 on the C lump. The Basic N deactivation constant is K.sub.n =3600.0 (gms Basic N/gms of catalyst).sup.-1 and the gasoline formation reaction scalar is such as to increase gasoline formation reactions by 8% for each 0.1 wt. % Basic N in the feed. Basic N effects are neglected, if the concentration is less than 0.04% in the feed.
The deactivation function is such that at the end of an experimental run (.theta.=1) where the Cat/Oil=2.0 and the Basic N in the feedstock is 0.1 wt. % the catalyst activity is reduced by a factor= ##EQU7##
Detailed results for WCMCGO with 0.1 wt. % and 0.2 wt. % addition of nitrogen as quinoline at 1000.degree. F. are indicated in FIGS. 23 and 24.
The model has been successfully tested on a gas oil (TK520) with 0.096% Basic N. The result provides a simultaneous test of lumping scheme, and the Basic N poison term. Comparisons between experimental and predicted yields are shown in FIG. 25 for this charge stock.
CATALYST EFFECTS
Rate constants listed in Table 2 were generated for a 10% rare earth exchanged zeolite Y aluminosilicate on a silica-alumina base. Catalysts will vary in both activity and selectivity. For example, a similar zeolite Y catalyst having a slightly different activity level was determined to require an alteration of the rate constants of Table 2 by increasing the gasoline formation rates by 20%, and by increasing the gasoline cracking rates by 2.5%. FIG. 26 shows comparisons between experimental and predicted yields for the similar zeolite Y catalyst with the altered model.
RESIDUAL CARBON ON CATALYST
The reactor model was prepared for fresh catalyst. However, since residual coke on the regenerated catalyst in the line 39 (FIG. 1) affects the catalytic properties of the catalyst, the effect of such residual coke on catalyst on the rate constants of the model are provided for experimental data for 0 through 0.5 weight % residual coke on a regenerated catalyst. A single matrix scalar cannot be used. Therefore, different factors must be applied to two groups of rate constants. For example, a 0.3 weight % of residual coke on regenerated catalyst requires that the gasoline formulation rate constants be decreased by 43%, and that the C lump formation rate constants be decreased by 35%. The model linearly interpolates these losses in activity between 0.3 wt. % of residual coke on catalyst and a completely regenerated or fresh catalyst.
Light End and Coke Yields
Correlations are provided in the model to predict the yields of light ends from catalytic cracking. The correlation is based on gasoline and C lump yield and the lumped composition of the charge stock, and is in the following form.
L.sup.i =(a.sup.i G+b.sup.i C) (a.sub.pl.sup.i P.sub.lo +a.sub.nl.sup.i N.sub.lo +a.sub.al.sup.i A.sub.lo +a.sub.cal.sup.i C.sub.alo +a.sub.ph.sup.i P.sub.ho +a.sub.nh.sup.i N.sub.ho +a.sub.ah.sup.i A.sub.ho +a.sub.cah.sup.i C.sub.aho)
L.sup.i =light end i (wt. %)
i=C.sub.1, C.sub.2, C.sub.2 ", C.sub.3, C.sub.3 ", nC.sub.4, iC.sub.4, C.sub.4 ", nC.sub.5, iC.sub.5, C.sub.5 "
P.sub.lo, N.sub.lo . . . , C.sub.aho =Wt. % composition of the charge stock
G, C=Wt. % G lump and Wt. % C lump
a.sup.i, b.sup.i, a.sub.p.sup.i . . . a.sub.cah.sup.i =correlation constants used to fit 95 sets of data on each light end i.
The results are summarized in Table 3, and some typical results for the individual light end yields are shown in FIGS. 32 and 33. Computed yields for C.sub.1 -C.sub.4 are generally within 10% or less of the observed values.
Table 3__________________________________________________________________________Light End Correlation Constants and Results Stan- Aver- dard age Error Error Abso- Abso- Average Range of lute lute Values Valuesa.sup.i b.sup.i a.sub.ph.sup.i a.sub.nh.sup.i a.sub.ah.sup.i a.sub.cah.sup.i a.sub.pl.sup.i a.sub.nl.sup.i a.sub.al.sup.i a.sub.cal.sup.i Wt. % Wt. % Wt. Wt.__________________________________________________________________________ %C.sub.1 -0.0551 0.3270 0.0659 0.1522 0.2005 0.2263 0.0294 0.0817 0.0737 0.3570 0.0738 0.056 0.4 0.06-1.0C.sub.2 -0.0551 0.3270 0.0659 0.1522 0.2005 0.2263 0.0294 0.0817 0.0737 0.3570 0.0738 0.056 0.4 0.06-1.0C.sub.2 = -0.0258 0.2060 0.1622 0.2911 0.2683 0.0785 0.0415 0.2828 0.4451 0.0772 0.0445 0.032 0.5 0.19-1.0C.sub.3 -0.1673 1.0780 0.1033 0.2066 0.1356 0.0069 0.1199 0.2655 0.1921 0.0087 0.1339 0.104 1.5 0.40-3.3C.sub.3 = 0.2540 0.3564 0.2424 0.0846 0.1690 0.1299 0.1727 0.0798 0.1597 0.1602 0.1618 0.105 2.4 1.00-3.9nC.sub.4 -0.0394 0.3620 0.3199 0.2871 0.1333 0.0031 0.2687 0.3804 0.4823 0.0091 0.0862 0.065 1.1 0.20-2.2iC.sub.4 0.0012 1.3950 0.1774 0.2893 0.1297 0.0041 0.2329 0.3271 0.2382 0.0065 0.2450 0.181 4.5 1.0-9.0C.sub.4 - 0.1288 -0.0063 0.7972 0.3455 0.5440 0.5866 0.5726 0.2469 0.5466 0.4226 0.1720 0.122 2.6 1.2-3.8nC.sub.5 0.0510 -0.0011 0.2114 0.1973 0.1379 0.1608 0.3557 0.0831 0.1380 0.4561 0.1080 0.075 0.4 0.09-0.73iC.sub.5 0.1803 -0.0013 0.7797 0.6949 0.3467 0.2749 0.8779 0.5228 0.9388 0.3287 0.875 0.667 4.5 0.96-7.84C.sub.5 = 0.0896 -0.0670 1.1540 0.0437 0.6362 1.0965 0.2829 0.0499 0.7844 0.5349 0.204 0.148 1.5 0.6-2.4__________________________________________________________________________
Carbon on catalyst is treated using the coking relation, C=at.sub.c.sup.n
where
C is wt. % carbon on catalyst
a is a function of charge stock
t.sub.c is the catalyst residence time
n is an exponent which is a function of catalyst.
The equation below is a relation that is charge stock independent with a standard error SE of 0.24 (absolute wt. %) for wt. % coke produced on charge. Computed coke yields are generally within 6% or less of the observed values. ##EQU8## where a=0.631 P.sub.lo +0.110 N.sub.lo +1.475 A.sub.lo +0.0727 C.sub.Alo +0.631 P.sub.ho +0.297 N.sub.ho +0.773 A.sub.ho +2.225 C.sub.Aho
t.sub.c =catalyst residence time in minutes
P.sub.lo, N.sub.lo, A.sub.lo, C.sub.Alo =Wt. % paraffins, naphthenes, aromatic substituent groups and aromatic rings in LFO of charge
P.sub.ho, N.sub.ho, A.sub.ho, C.sub.Aho =Wt. % paraffins, naphthenes, aromatic substituent groups and aromatic rings in HFO of charge.
The coke yield in wt. % may then be calculated from
Coke Yield (wt. %)=1.1 C (cat/oil)
where the factor 1.1 accounts for the carbon hydrogen ratio in the coke.
COMPUTER PROGRAM
The computer program of Appendix III facilitates the rapid treatment of experimental data. The program performs the following functions:
1. Searches for the best fit to the data (G lump, C lump, LFO) by means of a pattern search on the parameters of the system.
2. Goes into an output routine which prints the pertinent process variables for each run and then calculates the light end and coke yields.
3. The program then proceeds to produce plots of
(i) observed vs. computed yields for G lump, C lump, and LFO.
(ii) observed and computed yields vs. space velocity.
(iii) selectivity plots.
4. The program also allows for different reactor types to be called, (this is specified by the user in the input). The program is capable of treating data obtained from the following reactor types
(i) time-averaged fluidized dense bed data.
(ii) time-averaged fixed bed data using a scalar to account for more efficient catalyst utilization.
(iii) instantaneous data - pilot plant fluidized dense bed.
With reference to the program of Appendix III, PROGRAM MAIN reads in the input data and the initial guess for the rate constants associated with the kinetic scheme and proceeds to determine the best set of rate constants that fits the experimental data.
Beginning with SET ISEARCH, read in input data (1) yields from cracking operation, (2) charge stock properties, and (3) reactor conditions.
Beginning with READ 3, read in initial guess for rate constants.
Beginning with 70 OBJSTR, the program determines the best set of rate constants to fit the experimental data.
Beginning with C COMPUTE AVERAGE ERRORS AND SE, the program computes standard errors of the model fit for gasoline, conversion and light fuel oil.
SUBROUTINE REACTR primarily sets up the fluidized dense bed FCC reactor model and proceeds with the integration of the differential equations through the reactor bed. Outlet concentrations are time-averaged to account for catalyst deactivation. The time-averaged computer values for yields of gasoline, conversion and light fuel oil are then compared to the experimental data to determine how closely the model predicts the bed behaviour. The reactor model may be of three forms, (1) time-averaged fluidized dense bed, (2) instantaneous riser, (3) instantaneous fluidized dense bed. The reactor model is specified by the user in the input.
Beginning with Y(1)=F(J,16), set up initial conditions for the kinetics scheme.
Beginning with H=TIM(K), for the time-averaged fluidized dense bed, integrate the differential equations through the reactor bed, and beginning with COMPUTE AVERAGED YIELDS, YBAR (J,L) compute time-averaged yields.
Beginning with RISER CALCULATION, the same integration scheme may be applied to a riser reactor model.
Beginning with INSTANTANEOUS FLUIDIZED BED REACTOR, the same integration scheme may be applied to an instantaneous fluidized bed reactor.
Beginning with 202 CONTINUE, the program computes the standard error for all the sets of experimental data provided.
SUBROUTINE GAUSS 6 allows the model yield spectrum to be time-averaged for the case where the time-averaged fluidized dense bed data is obtained with catalyst deactivation.
SUBROUTINE CONVERT takes the input data read in the main program and converts it to a more suitable format for computation and printout.
SUBROUTINE FOXY represents the differential equations describing the main kinetic framework in the reactor model. These equations describe the rate of change of each of the ten lumped species in the kinetic scheme shown in FIG. 2. It also computes the rate of formation of gasoline, conversion and light fuel oil. Furthermore, it computes the composition of paraffins, naphthenes, aromatic substituent groups and aromatic rings in the light fuel oil heavy fuel oil fractions.
SUBROUTINE OUTPUT uses correlations to predict light hydrocarbon yields (C.sub.1 -C.sub.5), and coke. These predictions together with the gasoline conversion (C lump+G lump) and LFO are printed-out in a suitable format and compared to the experimental yields.
Beginning with LIGHT END AND COKE CORRELATION on page 16, light ends correlative prediction is generated.
Beginning with CARBON ON CATALYST, the coke prediction is generated.
Beginning with 10 FORMAT, format statements for output are provided.
The program of Appendix III is written in FORTRAN and is suitable for a Control Data Corporation CDC 1604 computer.
The model and program are readily adaptable to any catalytic cracking operation such as a moving bed (e.g., thermofor catalytic cracking), and a fluid riser of a fluid catalytic cracking process for either lab system or a commercial unit.
Appendix IV shows by way of example a comparison between predicted and observed yields for two feed stocks identified as WCMCGO and T-K520. Further Appendix V shows by way of example under PARAMETERS a K which gives a minimum error (SE).
Appendix V shows by way of an example a printout of a best fit of yields for a WCMCGO charge stock in a fluidized dense bed or fixed bed reactor under the conditions stated thereon.
APPENDIX I__________________________________________________________________________NOMENCLATURE__________________________________________________________________________Romana Coking constant for Voorhies equation, C = at.sub.c.sup. n .about.a Composition vector consisting of j lumped species (a.sub.j = moles j/gm gas)a.sub.j Concentration of lump j (moles j/gm gas)A.sub.h Wt. % aromatic substituent groups in HFO (650.degree. F..sup.+)A.sub.ho Wt. % aromatic substituent groups in HFO of chargeA.sub.l Wt. % aromatic substituent groups in LFO (430.degree.-650.degree. F.)A.sub.lo Wt. % aromatic substituent groups in LFO of chargeC "C lump", Wt. % H.sub.2, H.sub.2 S, C.sub.1 -C.sub.4 + cokeC.sub.Ah Wt. % aromatic rings in HFO (650.degree. F..sup.+)C.sub.Aho Wt. % aromatic rings in HFO of chargeC.sub.Al Wt. % aromatic rings in LFO (430.degree.-650.degree. F.)C.sub.Alo Wt. % aromatic rings in LFO of chargeG "G lump", Wt. % gasoline (C.sub.5.sup.+ -430.degree. F.)K Rate constant matrixK.sub.Ah Heavy aromatic ring adsorption coefficient (Wt. % C.sub.Ah).sup.-1 ##STR5## ##STR6##N.sub.h Wt. % naphthenic molecules in HFO (650.degree. F..sup.+)N.sub.ho Wt. % naphthenic molecules in HFO of chargeN.sub.l Wt. % naphthenic molecules in LFO (430.degree.-650.degree. F.)N.sub.lo Wt. % naphthenic molecules in LFO of chargeP Absolute pressure (atmospheres)P.sub.h Wt. % paraffin molecules in HFO (650.degree. F..sup.+)P.sub.ho Wt. % paraffin molecules in HFO of chargeP.sub.l Wt. % paraffin molecules in LFO (430.degree.-650.degree. F.)P.sub.lo Wt. % paraffin molecules in LFO of chargeR Gas constant (82.05 atm. cm.sup.3 /g-mole .degree.K.)S.sub.WH True weight hourly space velocity (g feed/g catalyst-hr)t.sub.c Time from start of run, hrT Absolute temperature (.degree.K.)X Dimensionless reactor lengthGreek.beta. Catalyst deactivation constant.UPSILON. Catalyst deactivation constant.PHI.(t.sub.c) Catalyst decay as a function of catalyst residence time, ##STR7##.sigma..sub.C.sup.2 Sum of the square of the deviations for C lump.sigma..sub. G.sup.2 Sum of the square of the deviations for G lump.sigma..sub. L.sup.2 Sum of the square of the deviations for LFO__________________________________________________________________________
APPENDIX II
Development of Reactor Model
When gaseous chemical reactions occur which produce a change in the molecular weight of the reacting mixture (e.g., cracking reactions), the gas density changes accordingly. If these reactions take place in a tubular flow reactor, then this density variation produces a corresponding change in the linear velocity of the flowing gas. This needs to be modeled into the reactor description.
If inert gases are present in the reaction mixture, they too will influence this linear velocity and the reactant concentrations.
To formulate a reactor model, several assumptions must be made concerning the flow in the reactor, both of gas and solids.
Assumptions in Reactor Model
1. Reactor cross section is uniform.
2. Void fraction is uniform.
3. Mass flow rate through reactor is steady and in plug flow.
From 1, 2, and 3 and the equation of continuity (i.e., mass balance) G, the mass velocity, is constant throughout the bed. That is,
G=.rho.u=constant
where
G=Mass velocity, g/(cm.sup.3 free cross section) (hr)
u=Gas velocity in the bed, cm/hr
.rho.=Gas density, g/cm.sup.3
A component material balance on a differential section of the reactor gives ##EQU9## where a.sub.j =Concentration of component, j, moles j/g gas
r.sub.j =Rate of formation of component, j, moles j/(cm.sup.3 gas) (hr)
t.sub.c =Time from start of run, hr
x=Distance into reactor from inlet, cm
No assumptions have been made to this point about the reaction kinetics so the model is still perfectly general.
Rate of Reaction
It is assumed that the rate of disappearance of a chemical species, j, in a single reaction is proportional to the molar concentration of species j (i.e., .rho.a.sub.j), and the mass density of catalyst relative to the gas volume (i.e., C.sub.c /.epsilon.). (NOTE: C.sub.c is defined as g catalyst/cm.sup.3 bed; .epsilon. is bed void fraction). It is further assumed that the adsorption of heavy inert aromatic rings on the catalyst surface will influence the availability of active sites and consequently the rate of reaction, thus ##EQU10## The rate constant, k.sub.j ', has units of (g catalyst/cm.sup.3).sup.-1 (hr).sup.-1. Combining the rate and material balance equation, ##EQU11## The rate constant need not be constant but can decay with time.
Conversion to Laboratory Units
Experimental data are not usually reported in the form used by the model equation. Mass fractions usually replace moles/g gas, space velocity replaces mass velocity and so on. To make this model more readily useful, therefore, we have changed it to accept usual laboratory data.
Let X=x/L=dimensionless distance into bed
S.sub.WH =g feed (oil+inerts)/(hr) (g catalyst)
NOTE: S.sub.WH is not the same as the weight hourly space velocity generally reported, i.e., g oil/hr g catalyst, which neglects the effect of inerts. In this discussion S.sub.WH will be used exclusively; it is the True Weight Hourly Space Velocity.
From the definitions of G and S.sub.WH ##EQU12##
Assuming that the rate of concentration change with time, ##EQU13## is small relative to the rate of change with position in a fluidized dense bed this is tantamount to saying that the oil molecules traverse the bed so fast that they see catalyst of essentially the same age then our model becomes ##EQU14## Now introduce catalyst decay as a function of catalyst residence time, t.sub.c. Assume, too, that the decay is non-selective:
k.sub.j '=k.sub.j .PHI.(t.sub.c)
where k.sub.j are invariant rate constants With the ideal gas assumption ##EQU15##
This system is not linear because MW is not constant. It changes with distance into the bed. Note that ##EQU16## since the units for a.sub.j is moles j/gm of gas. The computer program solves this system of ordinary differential equations numerically using an extrapolation to zero routine.
Coordinate Transformation in Fixed and Fluidized Dense Beds
Experimental runs using fluid and fixed beds often obtain products collected over the duration of a run. If catalyst decay is present, then this collected material represents the mixed average reactor effluent. To account for time-averaging it is necessary to integrate the model equations from bed inlet to outlet (X=0 to X=1) and then integrate the reactor effluent over the duration of the run (t.sub.c =0 to t.sub.c =t.sub.run).
To simplify greatly the calculational effort, the following coordinate transformation is performed.
Let ##EQU17##
This transformation of the reaction coordinate, X, yields a "crazy clock time" W which incorporates into its definition the effect of S.sub.WH and t.sub.c and is given by: ##EQU18## Note that this transformation holds only for fixed or fluidized dense beds (for a riser .PHI.(t.sub.c)=f(X)). The model becomes simply ##EQU19##
To see that this single result can be quite useful, determine the mixed average concentration for a particular run.
From the initial conditions (the specific feedstock) integrate the model equation to give a as a function of W.
Next evaluate W for X=1 (reactor outlet) ##EQU20## where P, R, T, S.sub.WH are known from the run.
Next choose six times from 0 to t.sub.c according to a 6-point Gaussian quadrature integration formula. Using the equation above this specifies the six transformed coordinate values at which a(W) is evaluated and supplied to the Gaussian formula. This together with the appropriate weighting factors gives the time-averaged composition.
For any given feed composition, only one evaluation of a.sub.j vs. W is required, thus computation time is substantially reduced.
It is important that the significance of this coordinate change not be overlooked. With one set of solutions a vs. W, we know the reactor effluent for all S.sub.WH and t.sub.c for both fixed and fluid beds. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5##

Number	Name	Date
3591783	Zumwalt	Jul 1971
3702003	Ramirez, Jr. et al.	Oct 1972
3725653	Carr et al.	Apr 1973

Simulation of catalytic cracking process

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