PETROLEUM DISTILLATION METHOD AND SYSTEM

Abstract: In a distillation control system, a nuclear magnetic resonance (NMR) sensor identifies the composition of petroleum distillates from a distillation tower, and preferably also identifies the composition of the input feed method. This information is processed to obtain estimates of values of selected properties of the distillates and the feed. These values are provided to a multivariate controller, together with a setpoint of desired quality factors selected on the basis of competitive market forces, environmental regulations and economic factors including feed and product prices and operating cost. On the basis of the NMR measured values of selected quality properties, the multivariate controller generates values of manipulated operating variables that, when applied to the distillation system, adjust the distillation operation to reduce differences between the controlled variables and their respective setpoints. The ultimate values of the operating variables are determined, preferably in conjunction with an optimizer so that product qualities are maintained and the tower is operated in an economically optimum manner subject to specific characteristics of the refinery.

Description: PETROLEUM DISTILLATION METHOD AND SYSTEM BACKGROUND This invention relates to the field of petroleum refining and in particular to methods and systems for controlling petroleum distillates produced in a distillation tower. Crude oil, also known as petroleum, is a complex mixture of hydrocarbons. The component hydrocarbons of this mixture are separated from one another to produce commercially valuable petroleum products. The physical and chemical processing steps for separating crude oil into component hydrocarbons are collectively referred to as "refining." Because the component hydrocarbons differ in volatility, an effective technique for separating them from one another is fractional distillation. In this technique, heated petroleum is fed to a distillation tower having a temperature gradient that progressively decreases from a high at the base of the tower to a low at the top of the tower. Petroleum vapor, which consists of the component hydrocarbons in their vapor phase, rises through the distillation tower. As it rises, the petroleum vapor encounters progressively lower temperatures. When the petroleum vapor reaches a level at which the temperature in the distillation tower is equal to the condensation temperature of one of the vapor's components, that component condenses. A tray placed at that level of the tower collects that condensed hydrocarbon component. In practice, the distillate collected from any tray within a distillation tower is not a pure form of a particular hydrocarbon. In fact, the distillate, although dominated by a particular hydrocarbon, includes impurities. These impurities affect properties of the distillate. A difficulty associated with the fractional distillation of petroleum is the maintenance of control over selected properties of the distillate. These selected properties include such physical and chemical properties as the distillate's aromaticity, boiling point, flash point, cloud point, viscosity, pour point, API gravity, freeze point, octane, PIONA, and RVP. These properties are commonly measures of a distillate's commercial or market quality as a product and hence are at times refined to herein as product quality factors or simply quality factor. It is possible to control the foregoing quality factors by controlling some or all of the tower operating variables. Examples of tower operating variables include distillates flow rates, stripping steams flow rates, reflux flow rate, feed flow rate, pumparound heat duties, tower pressure, and thermal condition of the tower feed. The exact operating variables available depend the specific configuration of the distillation unit. However, accurate control over quality factors of the distillate requires up-to-date knowledge of the values of selected properties of the constituents of the distillate. One method for obtaining values of selected quality factors of a distillate is to perform laboratory tests on a sample of the distillate. This, however, is costly and time- consuming. As a result, it is difficult to perform laboratory testing frequently enough to maintain up-to-date values of selected quality factors of all the distillates produced by the distillation tower at any one time. Consequently, this method is not suitable for rapid control over the values of selected properties of the distillate. Another method to obtain values of selected quality factors of a distillate recognizes that, to a great extent, these properties are influenced by the temperature distribution in the distillation tower. In this method, sensors distributed in the tower obtain temperature measurements at various heights in the tower. These measurements are delivered to a processor that correlates them with values of quality factors. This correlation can be performed using a look-up table of empirically derived results, or by using a mathematical model that generates values as a function of temperature. Although there is a correlation between the properties of a distillate and the temperature distribution in a distillation tower, this correlation is far from perfect. Accordingly, the foregoing known method relies on the assumption that one can derive values of distillate quality factors solely on the basis of temperature distribution. This assumption is generally erroneous and therefore leads to inaccuracies. It is thus an object of this invention to provide a method and system for obtaining up-to-date information concerning product quality factors of one or more distillates produced by a distillation tower, and for controlling the distillation process to produce distillates having desired product quality factors. Other objects of the invention will be set forth below and will be obvious from the following description. SUMMARY The method of the invention overcomes deficiencies in the prior art by performing on-line nuclear magnetic resonance measurement (NMR) of the values of selected properties of at least one distillate from a distillation tower and, optionally, of the tower feed. The result of this on-line measurement, together with the desired values of the selected quality factors of the distillate, are applied as inputs to an automated controller. On the basis of this information, the controller calculates values for tower operating variables to obtain distillates having the desired quality factors. Because the measurements are performed on-line, a system incorporating the invention can respond rapidly and automatically to variations in the tower input feed and in the output from the distillation tower. Because NMR measurements do not rely on optical or infrared radiation, they are not affected by high opacity of the measured material. Additionally, NMR measurements are relatively stable as a function of temperature. Consequently, reliable NMR measurements can be made across the broad range of temperatures of the various distillates produced in a distillation tower. In most practices of the invention, it is deemed preferable to secure both feedstock measurements and output product measurements, and to manipulate process variables in response to both groups of measurements. However, features of the invention may extend to practices where only the feedstock is measured, or where only one or more output fractions are measured, and one or more process variables are adjusted in response to such measurement. The process variables are adjusted for feed-forward control in response to measurements of feedstock material, and are adjusted for feed-back control in response to measurements of product material. One practice of the invention includes the steps of imposing a steady magnetic field on a hydrocarbon material involved in the distillation process, i.e., on a sample of a distillate or of the feed. With the steady magnetic field in place, an NMR sensor imposes a transient magnetic field on the distillate or feed, and measures its response to this transient magnetic field. An NMR sensor generally does not directly provide values of the selected quality-related properties of the distillate or of the feed. Instead, the NMR sensor provides the chemical composition of the distillate or feed. For this reason, one practice of the invention includes the step of estimating quality factors of the measured material on the basis of its measured composition. These estimated values are then used to control tower operating variables selectively, thereby generating a distillate having desired quality factors. Examples of these quality factors include aromaticity, boiling point, flash point, cloud point, viscosity, pour point, API gravity, freeze point, octane, PIONA, RVP, or other chemical or physical properties. The desired product quality factors are generally dictated by competitive market forces, environmental regulations and economic factors including feed and product prices and operating costs. They may also be constrained by specific characteristics of the refinery, including equipment configuration and availability and operating constraints. The method of the invention can thus provide rapid measurements of the values of selected properties of a variety of products produced by a distillation tower. Because the method of the invention relies on NMR rather than on optical measuring techniques, the accuracy of these measurements can be essentially independent of the opacity or the temperature of the distillates. A system for practice of the invention optionally includes an optimizer for specifying desired long-term values of selected tower operating variables that produce products having the desired quality factors, while optimizing economic operation of the tower considering, among other things, feed and operating cost and product values. The optimizer can also take into account specific characteristics of the refinery, including equipment configuration and availability and operating constraints including product production limits. The system further includes one or more sensors for measuring selected properties of at least one distillate and, optionally, of the feed. These sensed values, together with desired long-term values of selected operating variables specified by the optimizer, are provided to a controller. On the basis of the sensor information, desired quality factors, and desired long-term values, the controller determines current values of the operating variables needed to form distillates having the desired quality factors, while optimizing economic performance of the tower. DESCRIPTION OF THE FIGURES These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying drawings in which: FIG. 1 is a block schematic representation of a multivariate distillation control system embodying features of the invention; and FIG. 2 is a schematic block diagram that shows details of the distillation control system of FIG. 1. DESCRIPTION OF ILLUSTRATED EMBODIMENT FIG. 1 shows a multivariate control system 10 incorporating principles of the invention. The multivariate control system 10 includes a distillation system 11 in communication with a multivariate controller 12. The output of the distillation system 11 is a plurality of hydrocarbon distillates, each characterized by values of selected quality factors. These values, which are represented in FIG. 1 by an output vector y that is fed back to the multivariate controller 12, are the controlled variables for the multivariate control system 10. Measured values of feed quality properties, represented by vector z, are fed forward into the multivariate controller. The multivariate control system 10 further includes an optimizer 13 for generating desired long-term values of selected tower operating variables that produce products of the desired qualities, while optimizing economic operation of the tower. These desired values are represented in FIG. 1 by a setpomt vector r generated by the optimizer 13 and supplied to the multivariate controller 12. Elements of vector r take into account specific characteristics of the refinery, including equipment configuration and availability and operating constraints including product production limits. The optimizer 13 is an optional enhancement of the system 10. Alternative practices for entering setpoint data into the controller 14 include a keyboard device for manual entry and other techniques known in the automated control field. In response to the setpoint vector r and one or more differences between desired product quality factors and the output vector y and/or the feed forward vector z, the multivariate controller 12 generates a vector of manipulated variables x that, when applied to the distillation system 11, adjusts the distillation process to change the values of the elements in the output vector y to approach the corresponding desired values. As shown in FIG. 2, the illustrated distillation system 11 includes an input pipeline 14 leading to a distillation tower 15. Before reaching the tower 15, the pipeline 14 passes through a furnace 16 whose operating temperature is under the control of the multivariate controller 12. A pump 20 coupled to the input pipeline 14 propels crude oil through the furnace 16 and into the distillation tower 15. The distillation tower 15 is of the type commonly found in petroleum refineries. Such a distillation tower 15 typically includes a plurality of tower outputs disposed at different heights along the tower 15. The distillation tower 15 shown in FIG. 2 is merely illustrative and a wide variety of distillation towers can be used. Each output of the distillation tower 15 corresponds to a particular fraction distilled from the petroleum input. Outputs disposed at lower portions of the tower 15 correspond to heavier fractions, such as heating oil or kerosene. Outputs disposed at higher portions of the tower 15 correspond to lighter fractions, such as gasoline or naphtha. The distillation tower can include one or more pumparound streams 50, to remove heat from sections of the tower to adjust internal reflux and to affect the volume distillate produced at a given quality. The number of tower outputs and pumparound stream affects the computational burden on the multivariate controller 12, but not the subject matter of the invention. Hence, for the sake of clarity and ease of exposition. FIG. 2 shows only first and second tower outputs 22, 24 and one pumparound stream 50. A first valve 36 is connected with the first tower output 22 and controls the rate at which a first distillate is withdrawn from the distillation tower 15. Similarly, a second valve 38 is connected with the second tower output 24 and controls the rate at which a second distillate is withdrawn from the distillation tower 15. First and second valve actuators 40, 42 respond to the multivariate controller 12 to control the positions of the first and second valves 36, 38 respectively. A first nuclear magnetic resonance (NMR) sensor 24 is coupled to the first tower output 22 and samples the first distillate, or fraction, through a first shunt tube 26. Similarly, a second NMR sensor 28 is coupled to the second tower output 24 and samples the second distillate, or fraction, through a second shunt tube 30. Similarly, the illustrated control system 10 includes a third NMR sensor 51 coupled to the tower feed through a shunt tube 52. Although the distillation system 11 illustrated in FIG. 2 shows three distinct NMR sensors 24, 28, 51, it will be appreciated that a single NMR sensor can be used, on a time-shared basis, for both the first and second tower outputs 22, 24, and the feed 14. The sensed information output from the NMR sensors 24, 28, 51 are provided to calibrators 32, 34, 53. The calibrators are preferably chemometric modeling units and transform the outputs of the NMR sensors 24, 28, 51 into a format suitable for the multivariate controller 12 to which they are connected. One preferred sensor employs the technology of the I/A Series® Process NMR equipment available from The Foxboro Company of Foxboro, Massachusetts, however, a wide variety of NMR sensors may be used. A chemometric modeling unit for each calibrator 32, 24, 53 and suitable for practice of the invention is preferably implemented by a digital processor executing instructions for estimating values of selected physical properties on the basis of the measured hydrogen chemistry of a sample. These instructions implement procedures well-known to those of ordinary skill in the art. Such procedures include establishing look-up tables, interpolating between values in a look-up table, and implementing mathematical models for estimating values of the selected properties. Each chemometric modeling unit can be local to one NMR sensor as shown in FIG. 2. In one alternative practice of the invention (not shown), the NMR sensors in the distillation system 11 share a common chemometric modelling unit, on a time-sharing basis. Both the optimizer 13 and the multivariate controller 12 preferably are implemented as software instructions executed on a programmable digital processor. In practice, these instructions are executed on a general purpose digital computer. However, particularly to meet demanding performance requirements, the optimizer 13 and the multivariate controller 12 can be implemented with application specific integrated circuits. A suitable optimizer and multivariate controller for practice of the invention are sold by Simulation Sciences of Brea, California under the names ROMeo™ and Connoisseur™, respectively. The specific implementation of the multivariate controller 12 and the optimizer 13 are within the level of skill in the art and do not affect the scope of this invention. In operation of the system shown in FIGS. 1 and 2, the pump 20 propels crude oil through the furnace 16 and into the distillation tower 15, where it separates into a plurality of distillates. These distillates exit from the distillation tower 15 through a plurality of tower outputs, two of which are shown in FIG. 2. The first distillate flows to the first valve 36, and, to the extent that the first valve is open, out of the system. A sample of the first distillate from the first tower output 22 is directed to the first NMR sensor 24 through the first shunt tube 26. The flow rate of the first distillate is thus under the control of the first valve 36. Similarly, the second distillate flows to the second valve 38 and, to the extent the second valve is open, out of the system. A sample of the second distillate from the second tower output 24 is directed to the second NMR sensor 28 through the second shunt tube 30. The illustrated system 10 embodies the optional feature whereby a sample of the input feed stock is directed to the feed NMR sensor, 51, through shunt tube 52, and is returned to the feed stream. The NMR sensor 24 imposes a steady magnetic field on the sample from the first distillate to align the magnetic dipole moments associated with the molecules in the sample. With the steady magnetic field in place, the NMR sensor 24 imposes a transient magnetic field having a direction different from, and preferably orthogonal to, that of a steady magnetic field. This transient magnetic field temporarily aligns the magnetic dipoles of the sample in a direction other than that in which the static magnetic field aligns them. When the transient magnetic field is turned off, the dipoles in the sample spring back into the alignment imposed upon them by the steady magnetic field. As they do so, the dipoles generate an RF signal. The rate at which a particular dipole springs back to alignment with the steady magnetic field, and hence the frequency of the resulting RF signal, is characteristic of the sample's molecular structure. The RF spectrum thus generated, and which the NMR sensor 24 detects, provides a way of determining the chemical composition of the sample. The NMR sensor 24 thus provides information on the chemical composition of the distillate from the first tower output 22. It is known in the art to predict the corresponding values of selected properties from this measured composition of the sample. This operation of converting the measured sample composition into values of selected properties is carried out by a first calibrator 32, preferably a chemometric modeling unit 32 in communication with both the multivariate controller 12 and the NMR sensor 24. The input information to the first chemometric modelling unit 32 is the chemical composition of the sample as measured by the NMR sensor 24. The output of the first chemometric modelling unit 32 is a corresponding set of measured values of the selected properties. The second and third NMR sensors 28 and 51 and the associated calibrator 34, 53 operate in an identical manner to supply, to the multivariate controller 12, information concerning the properties of the second distillate and of the feed. In response to the data provided by the NMR sensors, 24, 28, 51, and typically together with other conventional data from other sensors (not shown) of pressure, temperature, time, and flow rate as conventional and known in the art, the multivariate controller 12 determines the values of manipulated operating variables of the illustrated distillation system 11. These determined values minimize the differences between the desired values of the product quality factors and actual values of the corresponding selected properties, as provided by the first and second NMR sensors, 24, 28 operating in conjunction with the first and second chemometric modelling units or other calibrators 32 and 34. Desired long-term values of the operating variables, as determined by the optimize 13, are also taken into account by the controller 12. The feed NMR 51, operating in conjunction with its chemometric modeling unit, 53, provides the controller 12 information on feed quality, so that adjustments to tower operation can be made before the effect of feed variation becomes evident in the product. This feed forward capability is important in managing feed stock changes from one crude type to another. The controller 12 determines values of the manipulated variables using methods known in the art. These methods typically include reference to a look-up table and the implementation of empirically derived dynamic mathematical models. Having determined the values of the manipulated variables, the multivariate controller 12 transmits control signals to the first and second actuators 40, 42. These actuators 40, 42 then selectively adjust the first and second valves 36, 38 to adjust the flow rates of the distillates from the first and second outputs 22, 24 of the distillation tower 15. In addition, the multivariate controller 12 can provide a control signal to the furnace 16, for controlling the temperature at which crude oil enters the distillation tower 15. The controller 12 can, in like manner, adjust any of numerous other operating variables, as known in the art. Although the invention is disclosed herein as it applies to the control of selected properties of two distillates, it will be apparent from the description that the invention is readily extendible to control selected properties of more than two distillates. Similarly, the invention can be practiced to measure and respond to each of the multiple feed stock materials Having described the invention, and a preferred embodiment thereof, what is claimed as new and secured by Letters Patent is:

Claims: 1. In a petroleum refining system, a method of controlling selected properties of a petroleum distillate, said method comprising the steps of obtaining an NMR spectrum of the petroleum distillate, on the basis of said NMR spectrum, ascertaining values of the selected distillate quality factors, and on the basis of said ascertained values, determining values of manipulated operating variables to obtain desired values of said selected quality factors 2. The method of claim 1 wherein said step of obtaining an NMR spectrum comprises the steps of imposing a steady magnetic field on at least a sample of the petroleum distillate, superimposing a transient magnetic field on said steady magnetic field, and measuring a magnetic-dipole response of said petroleum distillate to said transient magnetic field. 3. The method of claim 1 wherein said selected quality factors are selected from a group consisting of aromaticity, boiling point, flash point, cloud point, viscosity, pour point, API gravity, freeze point, octane, PIONA, and RVP. 4. The method of claim 1 further comprising the step of specifying said desired values of said manipulated operating variables on the basis of market conditions. 5. The method of claim 1 further comprising the step of selecting said manipulated operating variables to include the flow rate of said petroleum distillate. 6. The method of claim 1 further comprising the step of selecting said manipulated variables to include the temperature range at which said distillate is formed. 7. The method of claim 1 further comprising the step of selecting said manipulated operating variables from the group of variables consisting of distillate flow rate, stripping steam flow rates, reflux flow rate, feed rate, pump around heat duties, tower pressure, and thermal condition of tower feed. 8. A system for controlling selected quality factors of a petroleum distillate, said system comprising an NMR sensor in fluid communication with a source of the petroleum distillate, said NMR sensor configured to obtain an NMR spectrum of at least a sample of the petroleum distillate, and a multivariate controller in communication with said NMR sensor, said multivariate controller generating, on the basis of spectrum-responsive data supplied by said NMR sensor, instructions for obtaining desired values of said selected quality factors of the petroleum distillate. 9. The system of claim 8 wherein said NMR sensor comprises means for imposing a steady magnetic field on the sample, means for superimposing a transient magnetic field on said steady magnetic field, and means for measuring a magnetic-dipole response of the petroleum distillate to said transient magnetic field. 10. The system of claim 8 further comprising means for selecting said selected property factors from a group consisting of aromaticity, boiling point, flash point, cloud point, viscosity, API gravity, freeze point, octane, PIONA, and RVP. 11. The system of claim 8 further comprising an optimizer in communication with said multivariate controller for determining said desired values of tower operating variables. 12. The system of claim 11 wherein said optimizer determines tower operating variables on the basis of market conditions. 13. The system of claim 8 wherein said multivariate controller comprises means for specifying tower operating variables selected from the group consisting of flow rate of a petroleum distillate, stripping steam flow rates, reflux flow rate, feed flow rate, pumparound heat duties, tower pressure and thermal condition of the tower feed, said operating variables being specified to control said selected properties of the petroleum distillate. 14. The system of claim 8 wherein said multivariate controller comprises means for specifying a temperature range in which said petroleum distillate is formed, said temperature range being specified to control said selected properties of the petroleum distillate. 15. A method for controlling a petrochemical refining system having a distillation tower, said method comprising the steps of performing an on-line NMR measurement of at least one material that the refining system processes to ascertain measured values of selected properties of that measured material, specifying desired values of said selected properties of the measured material, and determining, on the basis of said measured values of said selected properties, values of manipulated variables of the refining system for controlling selected properties of at least one output fraction from the distillation tower. 16. A method according to claim 15 wherein said step of performing an NMR measurement measures at least a feedstock material of the refining system, and wherein said determining step determines values of manipulated variables for at least feed-forward control of the refining system. 17. A method according to claim 15 wherein said step of performing an NMR measurement measures at least an output product material of the refining system, and wherein said determining step determines values of manipulated variables for at least feed-back control of the refining system. 18. Apparatus for controlling a petrochemical refining system having a distillation tower, said apparatus comprising an NMR sensor configured to measure properties of at least one material that the refining system processes, and a multivariate controller in communication with said NMR sensor, said multivariate controller generating, on the basis of measurement-responsive information supplied by said NMR sensor, control signals for adjusting the operation of the refining system to obtain desired values of selected properties of at least one output fraction. 19. Apparatus according to claim 18 wherein said NMR sensor measures properties of at least one feedstock material of the refining system, and wherein said controller generates control signals at least for feedforward control of the refining system. 20. Apparatus according to claim 18 wherein said NMR sensor measures properties of at least one output product material of the refining system, and wherein said controller generates control signals at least for feedback control of the refining system. 21. A method for controlling a petrochemical process having a distillation tower for producing a petrochemical fraction, said process including the steps of performing an on-line NMR measurement of parameters regarding the composition of at least one feedstock material input to the petrochemical process, performing an on-line NMR measurement of parameters regarding the composition of at least one petrochemical fraction produced with the distillation tower, applying the measured information to a controller of the petrochemical process, applying to the controller target information identifying desired parameters of at least one fraction of the petrochemical process, and producing with the controller, in response to the measured information and the target information, control signals for adjusting the process to attain a selected value of the measured information relative to the target information.

Inventors: CLINKSCALES THOMAS A, KARG RANDAL W, CLINKSCALES, THOMAS, A., KARG, RANDAL, W.
Applicants: FOXBORO CO, THE FOXBORO COMPANY
Application year: 2001
Main classification: C10G7
IPC codes: B01J19, C10G7