US20080078693A1

US20080078693A1 - Method and apparatus for controlling FCC hydrotreating by near-infrared spectroscopy

Info

Publication number: US20080078693A1
Application number: US11/541,337
Authority: US
Inventors: Jeff Sexton; Brian K. Wilt
Original assignee: Marathon Petroleum Co LLC
Current assignee: Marathon Petroleum Co LP
Priority date: 2006-09-29
Filing date: 2006-09-29
Publication date: 2008-04-03
Also published as: WO2008042286A2; WO2008042286A3

Abstract

On-line controlling of FCC hydrotreating is provided which uses near infrared (NIR) analysis to characterize cracking feed for parameters and the resulting characterization thereof. The NIR results can be used in FCC hydroteating software to control on-line unit yields and qualities.

Description

TECHNICAL FIELD

This invention relates to controlling FCC hydrotreating by near infrared spectroscopy. More specifically, the present invention relates to on-line controlling FCC hydrotreating 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.
For modern refineries, the Fluid Catalytic Cracking Unit (FCCU) produces 40 to 60+ % of the gasoline in the gasoline pool. In addition, the FCCU produces a blendstock component for diesel manufacture. Air quality regulations for these transportation fuels will require a further reduction in sulfur content as mandated by the Clean Air Act. For the FCCU process, there are two routes a refiner can utilize to further reduce the sulfur content of these transportation fuels. The first route is via a hydrotreatment process on the feedstock to the FCCU. This hydrotreatment process can by operational severity and design, remove a substantial amount of the feed sulfur to produce a gasoline sulfur content of 100 ppmw or less. The second route a refiner can take involves the use of a specialized catalyst or additive in the FCCU circulating catalyst inventory that can catalytically remove sulfur from the FCCU product distributions. Refiners may elect to use this route for both non-hydrotreated and/or hydrotreated FCCU feedstock derived from various crude sources. In addition, if a refiner utilizes the first route for desired gasoline sulfur content, when the hydrotreater is taken out of service for an outage, this specialized catalyst or additive can be utilized to minimize the increase of gasoline sulfur during the outage period.
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 on-line controlling of FCC hydrotreating with NIR spectroscopy. A Near IR (NIR) spectrophotometer can be used to collect spectra on Fluid Catalytic Cracking (FCC) feedstocks and products. NIR measurement of feed and products from a Cat Feed Hydrotreater (CFH) enables this process to optimize catalyst cycle life, maximize product upgrade value, control environmental emissions and FCC gasoline sulfur.
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

The CFH pretreats the FCC feedstock to remove process FCC contaminants such has sulfur, nitrogen and concarbon. In the process, the CFH adds hydrogen to the FCC feed and saturates some of the aromatic components. This results in improved yield selectivity from the FCC unit and higher product value. Use of on-line NIR characterization of feed and product from the CFH will provide improved control and operation to targets to maximize FCC profitability and ensure FCC environmental emissions and product specifications are maintained. The CFH cycle life can also be more effectively managed to ensure feed to the CFH is controlled to achieve the desired catalyst cycle life. On-line monitoring of feed quality will also assist in pro-active trouble shooting to minimize operating problems from feed quality upsets.
This novel use of NIR is the ability to measure feed and products to and from CFH and FCC unit on-line to serve as a tool to optimize performance.
CFH Optimization provides a good use of on-line operating modes. Of special interest is aromatic saturation. Several units will rely on a CFH to meet gasoline pool sulfur requirements. However, operating in an aromatic saturation mode represents and opportunity to maximize overall efficiency based upon the improvement in FCC yields. Accurate feed and product aromatics and sulfur data is needed on-line to know where to operate on the aromatics equilibrium curve.
Use of NIR to measure feed and products to and from the CFH and FCC will now enable the user to adjust the CFH operating conditions to maximize FCC feed value and adjust FCC operating conditions to maximize FCC product value. This is consistent with the FCC RTO application with the added advantage of being able to pro-actively adjust the FCC feed value with the CFH optimization.
The present invention provides a process for controlling on-line FCC hydrotreating exhibiting absorption in the near infreared (NIR) region. The process steps include:

- a) measuring absorbances 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 the hydrotreating and outputting a processed signal; and
- d) controlling on-line in response to the processed signal, at least one parameter of the FCC hydrotreating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an FCC unit comprising a reactor and a hydrotreater 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.

FIG. 3 is a plot that illustrates HDS vs. As mode differences.

FIG. 4 is a graph of a catalyst cycle life curve.

FIG. 5 is a graph of an FCC feed upset showing high SO_x.

FIG. 6 is a table of a neural network for on-line control of SO_Xemissions.

FIG. 7 is a graph of the use of NIR on FCC hydrotreating.

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.

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 H₂, in contrast to hydrocracking, in which H₂is 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° 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° F. at atmospheric pressure and then separating by further vacuum distillation from the heavier fractions a cut boiling between 650° F. and 900° to 1025° 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° C., usually 460°-560° 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° C., usually 600°-750° C. Flue gas formed by burning coke in the regenerator is discharged into the atmosphere.
FIG. 1 is a schematic diagram of an FCC unit comprising a reactor and a hydrotreater 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 can be 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 optical 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 cyclone 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 is also 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:


Nor-	New Feed	New Feed	New Feed
mal	with Multi-	with Only	with Only
Oper-	variable	Rate	ROT
ation	Optimization	Varied	Varied

Feed Properties
API	24.6	21.8	21.8	21.8
UOP K	11.69	11.77	11.77	11.77
Concarbon (%)	0.15	0.59	0.59	0.59
Nitrogen (ppm)	1150	162	162	162
Sulfur (%)	0.34	0.55	0.55	0.55
1-ring Aromatics (%)	35	29	29	29
2-ring Aromatics (%)	34	26	26	26
3-ring Aromatics (%)	17	25	25	25
4-ring Aromatics+(%)	14	20	20	20
Process Conditions
Feed Rate (% Capacity)	100	95.3	83.8	100
Reactor Temperature (F.)	1010	992	1006	986
Reactor Pressue (psig)	34.7	33.6	32.3	34.2
Equipment Constraints
Wet Gas Compressor (%)	100	100	100	100
Main Air Blower (%)	100	90	84	94
Yields
Conversion (lv %)	77.55	74.33	76.83	73.59

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

FIG. 3 is a plot that illustrates HDS vs. AS mode differences. The plot shows FCC feed sulfur under different operating philosophies. The feed sulfur is held constant and aromatics, nitrogen or concarbon parameters are varied.

EXAMPLE 4

FIG. 4 is a graph of a catalyst cycle life curve. A critical aspect of managing the CFH is catalyst cycle life. Coke and metals are deposited on the catalyst during the course of the run cycle. This deactivation requires an increase in temperature. End of Run is typically determined when the process is at its maximum inlet temperature capability. At this point the catalyst will need to be changed out with fresh. Monitoring the CFH feed properties will ensure the unit is managed to achieve the desired cycle length and avoid an upset condition where poor feed quality is sent to the unit. This ability to monitor feed provides for greater flexibility and minimizes risk for increased deactivation and catalyst damage. FIG. 4 is a typical catalyst cycle life curve showing the impact of a feed upset. In this case the upset was caused by a leaking heat exchanger. Application of the NIR for on-line feed monitoring would allow better unit monitoring to minimize the risk of this type of upset.

EXAMPLE 5

FIG. 5 is a graph of an FCC feed upset showing high SO_x. All FCC units have environmental emission limits. These are typically SO_x, NOx, CO and particulate matter. Advanced monitoring of FCC feed properties for S and N will enable the refiner to adjust process conditions to ensure a feed change or upset will not cause an environmental exceedence. Operating actions may include decreasing federate, changing feed line-ups, diverting certain feed streams, and adjusting catalyst additive use for Sox and NOx. FIG. 5 is an example of an FCC feed upset resulting in high Sox and opacity. Use of NIR on the FCC feed stream would provide advance notice of the pending problem and enable the operator to take action to mitigate.

EXAMPLE 6

FIG. 6 is a table of a neural network for on-line control of SO_xemissions. The NIR analyzer on the FCC feed can also be used to model FCC emissions. Several refiners have developed either statistical or neural network models to predict FCC emissions either with or without catalyst additives. Use of the NIR to measure feed characterization would be an important new parameter to improve model accuracy. Current models are developed based upon daily feed samples that often result in poor correlations due to variability. FIG. 6 is a summary of a neural network model variables use to predict SO_xemissions on the FCC unit at a refinery. NIR would allow for improved monitoring of the Feed properties and improve the model's capacity.

EXAMPLE 7

FIG. 7 is a graph of the use of NIR on FCC hydrotreating. Refiners have had to choose between pre-treat and post-treat options to meet gasoline sulfur requirements. Units that rely on controlling FCC feed sulfur via pre-treating with a CFH will see variation in the feed sulfur to gasoline sulfur ratio with different crude types, CFH operating conditions and degree of hydroprocessing. In order to ensure gasoline product quality, it is important to ensure gasoline sulfur content is controlled. Use of NIR on the FCC feed would allow the unit to adjust processing conditions to maintain product quality and avoid an off-spec product. On-line monitoring of FCC feed quality will allow the operator to adjust the CFH severity, change FCC federate, divert certain feed streams and adjust product fractionation to maintain product quality. FIG. 7 is a typical relationship between feed sulfur, gasoline sulfur and gasoline endpoint. The NIR capability would allow the process to stay at the control point of this curve.

EXAMPLE 8

(Troubleshooting)

FCC Troubleshooting—Use of an on-line NIR would aid in identifying problems with upstream units in advance of current monitoring techniques. This advance notice will aid in pro-active troubleshooting to mitigate the detrimental effects. This may include the following:
a) High salt content indicating potential desalted problem or crude quality change (examples from Texas City sodium with High Island Crude).
b) High metals (Ni+V), co carbon and endpoint resulting from a HEX leak in the crude/vacuum unit or CFH.
c) High co carbon, endpoint and metals resulting from black oil entrainment due to a low wash rate or mechanical problems in crude/vacuum.
d) Poor quality due to stratified tanks or bad line-up from the tank farm.

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.

Claims

1. A process for controlling on line FCC hydro treating (CFH) exhibiting absorption in the near infrared (NIR) region comprising:

a) measuring absorbances using a NIR spectrometer measuring absorbance's at wavelengths within the range of about 780-4000 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 the hydro treating 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.

2. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats of FCC feeds.

3. The process of claim 2 wherein CFH pretreats remove FCC processing contaminants.

4. The process of claim 3 wherein the contaminants are sulfur nitrogen or concarbon.

5. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to control weight percents of each hydrocarbon class.

6. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to hold sulfur content constant and vary aromatics, nitrogen or concarbon parameters.

7. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to provide real time optimization of FCC processing.

8. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to manage CFH catalyst life.

9. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to control SO_x, NO_x, Co or particulate matter in FCC processing.

10. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to adjust the CFH severity, change FCC federate, divert certain feed streams and adjust product fractionation to maintain product quality.

11. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to control feed sulfur, gasoline sulfur or gasoline endpoint.

12. The process of claim 1 including the step of using NIR measuring on line to control CFH pretreats to troubleshoot FCC processing.

13. The process of claim 12 wherein the step of troubleshooting mitigates detrimental effects.

14. The process of claim 13 wherein the detrimental effects are high salt content, high metals, or high deposits.

15. The process of claim 1 wherein said absorbances are measured at wavelengths within the range of about 780-2500 nm.

16. The process of claim 1 wherein said absorbances are measured at wavelengths within the range of 1100-2200 nm.

17. The process of claim 1 wherein said absorbance is measured in at least one wavelength and includes the steps of:

a) periodically or continuously outputting a periodic or continuous signal indicative of the intensity of said absorbance in said wavelength, or wavelengths in said two or more bands or a combination of mathematical functions thereof,

b) mathematically converting the signal to an output signal indicative of the mathematical function; and

controlling the hydro treating on-line in response to the output signal.

18. The process of claim 1 wherein the step of controlling on-line allows for real time optimization processing.

19. The process of claim 1 including the step of:

mathematically converting the signal to an output signal indicative of the parameter.

20. The process of claim 6 including the steps of:

periodically or continuously outputting a periodic or continuous signal indicative of the intensity of the NIR absorbance in the wavelength, or wavelengths in the two or more bands or a combination of mathematical functions thereof,

mathematically converting said signal to an output signal indicative of the mathematical function; and

controlling on-line in response thereto.

21. The process of claim 1 including the step of using the NIR results in FCC hydrotreating simulation software to control on-line unit yields and qualities.

22. The process of claim 1 including the step of using NIR measuring on-line to control FCC pretreats to maximize aromatic saturation by varying temperature to optimize feed upgrading to the FCC.