METHOD AND APPARATUS FOR CONTROLLING CATALYTIC CRACKING BY NEAR-INFRARED SPECTROSCOPY

TECHNICAL FIELD This invention relates to controlling a catalyzed cracking unit (FCC) and feedstock selection by near infrared spectroscopy. More specifically, the present invention relates to the controlling on-line of catalytic cracking processes for producing lower molecular weight products from hydrocarbon feeds by NIR spectroscopy.

BACKGROUND OF THE INVENTION

Near IR spectroscopy has been used in the past to determine physical properties of petroleum hydrocarbon mixtures. This includes using the NIR results to control refinery processes including gasoline blenders and catalytic reforming units. It is a quick, non-destructive analytical technique that is correlated to primary test methods using a multivariate regression analysis algorithm such as partial least squares or multiple linear regression. It has been used in a laboratory to predict properties of refinery blender streams and finished gasoline and diesel fuel.

Optimization, design and control of catalytic cracking process units all benefit from kinetic models which describe the conversion of feeds to products. In order to properly describe the effects of changes in feed composition, such models require descriptions of the feed in terms of constituents which undergo similar chemical reactions in the cracking unit. For design and optimization studies, a protocol which involves off-line feed analysis taking weeks or even months to provide a feed description.

Kinetic models are commonly used to predict process yields as a function of feedstock quality, catalyst and processing conditions. An optimizer can be used within the model to determine the optimum combination of rate, processing conditions and catalyst properties. However, this practice requires

detailed feed and product yield and analytical data. Current analytical techniques require a long lead time to generate the needed input to the model.

In the search for improved petroleum refining, we have developed online controlling of FCC processes with NIR spectroscopy. This involves careful and precise measurement of FCC yields as well as key process parameters including feed quality; feed rate, FCC operating conditions and FCC product properties.

Other objects and advantages of the present invention will become apparent to those skilled in the art upon a review of the following detailed description of the preferred embodiments and the accompanying drawings.

SUMMARY OF THE INVENTION

Real Time Optimization concept, for the refining industry to utilize direct measurement of feed and processing conditions to manipulate the process. We have developed a practice utilizing Advance Process Control (APC) technology to maximize operation against constraints. RTO allows the process to trade yield, recovery, capacity and efficiency among different process variables and equipment constraints. An on-line measurement of feed and product qualities via NIR enables the RTO process to work. This novel use of NIR is the ability to measure feed and products on-line as input to a RTO application on an FCC unit.

A near IR (NIR) spectrophotometer can be used to collect spectra on fluid catalytic cracking (FCC) feed stocks. The collected NIR spectral data was correlated to traditional laboratory tests including HPLC Heavy Distillate Analyzer (HDA) results of aromatic core type (1-ring core, 2-ring core, 3-ring core, 4-ring core and polars), ASTM D2887 high temperature simulated distillation, basic nitrogen, total nitrogen, API gravity, total sulfur, MCRT, and percent of Coker gas oil in Vacuum Gas Oil (VGO). The NIR can be used to monitor the FCC feed stocks quality more quickly and efficiently than performing the lab tests. The NIR results can be used to monitor the FCC feed stocks quality more quickly and efficiently than perform the lab tests.

Furthermore, certain critical wavelengths have been found to be of special value in determining the optimum operation of a catalytic cracking unit (FCCU).

The present invention provides a process for controlling on-line catalytic cracking hydrocarbon feeds, intermediates and products exhibiting absorpition in the near infreared (NIR) region. The process steps include: a) measuring absorbances of the feed using a spectrometer measuring absorbances at wavelengths within the range of about 780- 4000 nm, e.g., 780-2500 nm, and outputting an emitted signal indicative of said absorbance; b) subjecting the NIR spectrometer signal to a mathematical treatment (e.g. derivative, smooth, baseline correction) of the emitted signal. c) processing the emitted signal or the mathematical treatment using a defined model to determine the chemical or physical properties of feeds, intermediates or products and outputting a processed signal; and d) controlling on-line in response to the processed signal, at least one parameter of the catalytic cracking feed, intermediate or product.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of an FCC unit comprising a reactor and a regenerator showing the control system of the present invention in place for operating that FCC unit.

Fig. 2 is a Table which shows samples, including hydrotreater charges and products and FCC feeds used to control on-line weight percents of each hydrocarbon class.

Figs. 3 and 4 illustrate typical NIR absorption spectra of FCC feed samples and the resulting 2 ^nd derivative spectra.

Fig. 5 is a table showing a summary of the NIR regression statistics for each of the parameters.

Figs. 6, 7 and 8 are graphs showing NIR predicted results versus Lab results for sulfur, API gravity, and % Coker Gas Oil. Figs. 9 and 10 are graphics showing controlling plots for feedstock quality and product yields.

DETAILED DESCRIPTION OF THE INVENTION Petroleum refining is a never-ending quest for higher throughputs, better yields, higher onstream factors, improved reliability, cheaper feedstocks and cleaner fuels. At the heart of this effort is the fluid catalytic cracking or FCC process. The FCC process is undergoing waves of evolutionary change with improvements in feed injection, riser termination, catalyst stripping, spent catalyst distribution, cracking catalyst and additive performance, emissions reduction and FCC naphtha sulfur reduction technology.

When seeking to optimize the performance of the FCC unit, it is critical to accurately define the operation as it exists before decisions regarding significant process or equipment changes are finalized. This involves careful and precise measurement of FCC yields as well as key process parameters including feed quality; feed rate, FCC operating conditions and FCC product properties.

We use a Near IR (NIR) spectrophotometer to collect spectra on Fluid Catalytic Cracking (FCC) feedstocks and products. Improved FCC kinetic models and computer simulations have resulted in use of an optimizer program to select operating parameters of the FCC unit to maximize processing against unit constraints. This is typically done off-line using a discrete set of data. NIR measurement of feed and products enables this process to be done on-line allowing the process to operate at maximum efficiency.

Over 300 FCC feed stocks from multiple refineries were analyzed on a Near Infrared instrument operatin between 1100-2500 nm while primary lab results were obtained from the MPC Refining Analytical and Development Laboratory. Collected spectra were imported into FOSS Vision software to perform math functions and multivariate regression analysis. Partial Least Squares regression equations were generated for 20 properties. These properties include core aromatics, distillation points, total and basic nitrogen, sulfur, API gravity, and % Coker Gas Oil. The NIR results can be used in FCC simulation software to predict unit yields and qualities. The NIR provides a 2Ox savings in labor costs over the conventional lab methods and a significant reduction in analysis time. This time savings would allow for quicker characterization of purchased gas oil.

According to the invention, infrared (preferably NIR) analysis surprisingly has been found capable of predicting the product slate resulting from a particular FCC feed under specified conditions, e.g., cracking severity conditions. Moreover, it has been found that infrared analysis of FCC feed can be made and compared with a model feed and the difference therebetween correlated with catalytic cracking process parameters, e.g., cracking severity, in order to provide economical operation and desired product slates. Variations in the feedstock can affect conversion, product distributions, product properties, operating conditions of the unit, and refinery economics.

FCC PROCESS Catalytic cracking is the backbone of many refineries. It converts heavy feeds (600° - 1050° F) such as atmospheric gas oil, vacuum gas oil, coker gas oil, lube extracts, and slop streams, into lighter products such as light gases, olefins, gasoline, distillate and coke, by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures (15 to 30 psig), in the absence of externally supplied H ₂ , in contrast to hydrocracking, in which H ₂ is added during the cracking step.

Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.

FCC feedstocks include that fraction of crude oil which boils at 650° to 1000 ⁰ F, such fractions being relatively free of coke precursors and heavy metal contamination. Such feedstock, known as "vacuum gas oil" (VGO) is generally prepared from crude oil by distilling off the fractions boiling below 650 ⁰ F at atmospheric pressure and then separating by further vacuum distillation from the heavier fractions a cut boiling between 650 ⁰ F and 900° to 1025 ⁰ F. The fractions boiling above 900° to 1025°F are normally employed for a variety of other purposes, such as asphalt, residual fuel oil, #6 fuel oil, or marine Bunker C fuel oil. However, some of these higher boiling cuts can be used as feedstocks in conjunction with FCC processes which utilize carbo-metallic oils by Reduced Crude Conversion (RCC) using a progressive flow type reactor having an elongated reaction chamber. The FCC process may be controlled by selecting a feedstock of specified characteristics to the unit as well as controlling process parameters.

Varying process conditions can affect the product slate. Operating under more severe cracking conditions by increasing process temperatures can provide a gasoline product of higher octane rating, while increasing conversion can provide more olefins for alkylate production, as well as more gasoline and potential alkylate. Catalytic cracking can also be affected by inhibitors, which can be naturally present in the feed or added separately. Generally, as boiling range of the feed increases, so does the concentration of inhibitors naturally therein. Inhibition effect can be temporary or permanent depending on the type of inhibitor present. Nitrogen inhibitors generally provide temporary effects while heavy metals such as nickel, vanadium, iron, copper, etc., which can quantitatively transfer from the feed to the catalyst provide more permanent inhibition. Metals poisoning results in higher dry gas yields, higher hydrogen factor, higher coke yields as a percent of conversion, and lower gasoline yields. Coke precursors such as

asphaltenes tend to break down into coke during cracking which deposits on the catalyst, reducing its activity.

In catalytic cracking, an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425°- 600 ⁰ C, usually 460°- 560 ⁰ C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen-containing gas, usually air. Coke burns off, restoring catalyst activity and heating the catalyst to, e.g., 500°- 900 ⁰ C, usually 600° - 75O ⁰ C. Flue gas formed by burning coke in the regenerator is discharged into the atmosphere. Many FCC units now use zeolite-containing catalyst having high activity and selectivity. These catalysts are generally believed to work best when the coke on catalyst after regeneration is relatively low, say, less than 0.1 wt%, and preferably less than 0.05 wt%.

To regenerate FCC catalysts to these low residual carbon levels, and to burn CO completely to CO2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators have turned to high efficiency regenerators and to CO combustion promoters. Many FCC units operate in complete CO combustion mode, i.e., the mole ratio of CO2/CO is at least 10. Refiners burn CO within the regenerator to conserve heat and minimize air pollution. The preferred way to burn CO in the regenerator is to add platinum catalyst.

Various methods of practicing the present invention can be carried out. In one embodiment, a desired product slate can be determined based upon or an IR analysis of cracking product. This data can then be used for unit monitoring.

Alternatively, the feed may be characterized based upon an IR analysis of the feed. This data can then be used for unit monitoring.

Feed variables which may be used in the present invention are sleeted from the group consisting of wt.% or vol.% monoaromatics, diaromatics, triaromatics, benzothiophenes and dibenzothiophenes, paraffins/naphthenes, aromatics, nitrogen content, and the like.

Product variables which may be used in the present invention are selected from the group consisting of C4 free gasoline (vol) total C4's (vol) dry gas (wt), coke (wt), gasoline octaine, LFO, HFO, H2S, sulfur in LFO, aniline point of LFO and the like.

Fig. 1 is a schematic diagram of an FCC unit comprising a reactor and a regenerator showing the control system of the present invention in place for operating that FCC unit.

Fig. 1 shows feed 20 is heated by fired heater 22 which is heated by gas burner 24, fuel to which is controlled by automatic valve 26. Just before the fuel enters the fired heater 22, a sample 30 is withdrawn and conducted by tubing into NIR unit 32. In an alternate embodiment (not shown), a fiber optic probe inserted directly into the feed line before fired heater 22 can obviate the need for withdrawing sample. NIR unit 32 is located on-line and can include a sample conditioning means for controlling the temperature, and for extracting bubbles and dirt from the sample. The NIR unit also comprises a spectrometer means which may be a spectrometer of the NIR, Fourier Transform Near Infrared (FTNIR), Fourier Transform Infrared (FTIR), or Infrared (IR) type, ruggedized for process service and operated in a temperature-controlled, explosion- proof cabinet. A photometer with present potical filters moving successively into position, can be used as a special type of spectrometer.

NIR spectrometer 32 outputs a signal to computer 40 which preferably takes a derivative of the signal from the spectrometer, and subjects it to a defined model to generate the properties of interest. The

model is optionally derived from signals obtained from NIR measurement of cracking products.

In operation, the FCCU operates conventionally with feed being fired in heater 22 entering riser 50, together with catalyst descending through the catalyst return line 52 and entering riser 50. The vaporized products ascend riser 50 and are recovered in the reactor by cyclone 54 with product vapors 58 exiting to the main column for fractionation and recovery of various products. Naphtha product can be recycled through line 60. Spent catalyst descends from the reactor through lines 64 into the regenerator 68 and contacts air to burn off carbon and produce flue gas which exits through flue cyclones 70 and flue gas line 72. Various other components are shown, but not described. For example, computer 40 controls catalyst cooler 76 through catalyst temperature line 78. Injection water line 80 also is shown.

Optionally or alternatively, a second sample taken from the reactor product vapors 58 can be input through line 74 to NIR 32, permitting the spectrometer to analyze the products so that computer 40 can compare the group type analysis of the products against the optimum products slate desired for maximum economy.

Example 1

Different feedstocks will result in different yields from the FCC process. If the unit is operating against a constraint , the process will need to adjust to avoid exceeding an equipment limitation. Typical process variables include feed rate, reactor temperature, feed preheat and pressure. The process response from each of the variables is non-linear. The optimum set of conditions to maximize profitability to unit constraints will typically vary depending upon the feed quality. The following is an example of different operating conditions required to maximize profitability for a change in feed:

The results show the application of RTO using NIR allows the FCC process to automatically adjust processing conditions to maximize processing as feedstock quality changes. Without the feedstock quality via NIR and RTO, the process will operate at a non-optimum condition until a model optimizer can be run and the results implemented. Conventional practice is limited to use of APC where typically only 1 variable an be manipulated to push the unit against constraints. On-line RTO chooses a set of operating conditions to maximize value.

Example 2

Fig. 2 is a Table which shows samples, including hydrotreater chargers and products and FCC feeds used to control weight percents of each hydrocarbon class.

Two hundred fifty samples, including hydrotreater charges and products and FCC feeds were used to create a PLS model for predicting weight percents of each hydrocarbon class. The samples were analyzed using the online NIR. Wavelengths were chosen for each group and a summary appears in Fig. 2.

Example 3

Figs. 3 and 4 illustrate typical NIR absorption spectra of FCC feed samples and the resulting 2 ^nd derivative spectra.

Example 4

Fig. 5 is a table showing a summary of the NIR regression statistics for each of the parameters.

Example 5 Figs. 6, 7 and 8 are graphs showing NIR predicted results versus Lab results for sulfur, API gravity, and % Coker Gas Oil.

The results show that it has been found possible to be able to predict feed properties from a detailed description of the catalytic cracking feed composition by hydrocarbon group types (HGT), which can affect catalytic cracking operations and products. The properties can be related to the weighting of certain components in the composition of the cracking feed,

e.g., monoaromatics, diaromatics, triaromatics, benzothiophenes and dibenzothiophenes. HGT is determined by the techniques illustrated in the Table in Fig. 2 and from changes in specific near infrared absorption bands. Correlating the content of these components in the feed to FCC product properties can be accomplished using the process of this invention for unit monitoring.

Example 6 Figs. 9 and 10 show graphs of typical unit monitoring plots used to track feedstock quality and product yields. Use of the NIR will allow this be done on-line or in the lab and provide better resolution. Fig. 9 shows weight percent data for Feed Sulfur, Feed Gravity and Contradson Carbon. Fig. 10 shows weight percent data for Gasoline Conversion, Gasoline Yield, LCO yield, and slurry LV%. Figs. 9 and 10 show the conventional method of off-line analysis.

The graphs reflect weekly test runs and unit monthly reports. The graphs are not real time optimization of the process. They merely show weekly or monthly trends. They do not allow for RTO adjusting the process as the results are "after the fact". For example, these long term trends may be the result of a temporary upset that does not need correction long after the disturbance.

Modifications

Specific compositions, methods, or embodiments discussed are intended to be only illustrative of the invention disclosed by this specification. Variation on these compositions, methods, or embodiments are readily apparent to a person of skill in the art based upon the teachings of this specification and are therefore intended to be included as part of the inventions disclosed herein. The above detailed description of the present invention is given for explanatory purposes. It will be apparent to those skilled in the art that

numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.