US3647635A - Octane number control of distillation column overhead by varying reflux - Google Patents

Abstract

A FRACTIONAL DISTILLATION COLUMN OPERATING AS A GASOLINE SPLITTER IS CONTROLLED BY MEASURING THE OCTANE NUMBER OF THE COLUMN OVERHEAD FRACTION AND ADJUSTING THE REFLUX TO THE COLUMN IN RESPONSE TO THE OCTANE NUMBER. THE OCTANE MEASUREMENT IS EFFECTED BY AN ANALYZER COMPRISING A STABILIZED COOL FLAME GENERATOR WITH SERVO-POSITIONED FLAME FRONT WHICH PROVIDES A REAL TIME OUTPUT SIGNAL INDICATIVE OF SAMPLE OCTANE NUMBER.

Description

/N VEN TONS Walter A. Baja/r By. James H. Mc/ augh/in N I se mmw Vil United States Patent O OCTANE NUMBER CONTROL OF DISTILLATION COLUMN OVERHEAD BY VARYING REFLUX Walter A. Bajek, Lombard, and James H. McLaughlin,

La Grange, Ill., assignors to Universal Oil Products Company, Des Plaines, lll.

Filed Oct. 22, 1969, Ser. No. 868,459 Int. Cl. B01d 3/42; C10g 7/00 U.S. Cl. 196-132 7 Claims ABSTRACT F THE DISCLOSURE BACKGROUND OF THE INVENTION The invention of this application is a process control application of the hydrocarbon analyzer described in United States Patent No. 3,463,613, all the teachings of which, both general and specific, are incorporated by reference herein.

As set forth in application Ser. No. 471,670, the composition of a hydrocarbon sample can be determined by burning the sample in a combustion tube under conditions to generate therein a stabilized cool flame. The position of the flame front is automatically detected and used to develop a control signal which, in turn, is used to vary a combustion parameter, such as combustion pressure, induction zone temperature or air flow, in a manner to immobilize the flame front regardless of changes in composition of the sample. The change in such combustion parameter required to immobilize the flame following a change of sample composition is correlatable with such composition change. An appropriate read-out device connecting therewith may be calibrated in terms of the desired identifying characteristic of the hydrocarbon sample, as, for example, octane number. Such an instrument is conveniently identified as a hydrocarbon analyzer comprising a stabilized cool ame generator with a servo-positioned flame front.

The type of analysis effected thereby is not a compound-by-compound analysis of the type presented by instruments such as mass spectrometers or vapor phase chromatographs. On the contrary, the analysis is represented by a continuous output signal which is responsive to and indicative of hydrocarbon composition and, more specifically, is empirically correlatable with one or more conventional identifications or specications of petroleum products such as Reid vapor pressure, ASTM or Engler distillations or, for motor fuels, knock characteristics such as research octane number, motor octane number or composite yof such octane numbers.

For the purpose of the present application, the hydrocarbon analyzer is further limited to that specific embodiment which is designed to receive a hydrocarbon sample mixture containing predominantly gasoline boiling range components, and the output signal of which analyzer provides a direct measure of Octane number, i.e. research octane, motor octane or a predetermined composite of the two octane ratings. For brevity, the hydrocarbon analyzer will be referred to in the following description and accompanying drawing simply as an octane monitor.

An `octane monitor based on a stabilized cool flame generator possesses numerous advantages over convenice tional octane number instruments such as the CFR engine or automated knock-engine monitoring systems. Among these are: elimination of moving parts with corresponding minimal maintenance and down-time; high accuracy and reproducibility; rapid speed of response providing a continuous, real-time output; compatibility of output signal with computer or controller inuts; ability to receive and rate gasoline samples of high vapor pressure, e.g. up to as high as 500 p.s.i.g., as well as lower vapor pressure samples (.5-250 p.s.i.g.). 'Ihese characteristics make the octane monitor eminently suitable not only for an indicating or recording function, but particularly for a process control function wherein the octane monitor is the primary sensing element of a closed loop control system comprising 0, 1, 2 or more subloops connected in cascade.

The present invention has as its principal objective the direct control of octane number of a gasoline splitter column overhead stream. A typical gasoline splitter is an externally refluxed, multiple tray, fractional distillation column employed to separate the light ends and lower boiling normally liquid gas-oline components from the higher boiling components. The feed to such a column may typically comprise a stabilized reformate from a catalytic naphtha hydroreforming unit. Such a reformate will contain C5 and heavier hydrocarbon constituents, with the end point dependent upon the original end point of the naphtha fraction which was hydroreformed. For example, the reformate which is produced from a naphtha having a 390 F. end point will typically have an end point in the range of about 440 to 450 F. It is normal to fractionate such a reformate to remove the heavier hydrocarbon components. Components boiling at a temperature in excess of about 400 F. have a high octane number, but they are predominantly aromatic hydrocarbons which are precursers to gum formation during gasoline storage and they can cause excessive deposition of carbonaceous material in an automobile engine during combustion. The overhead stream from the gasoline splitter column will thus typically comprise hydrocarbons in the C5 to 400 F. end point boiling range, and the bottoms stream from the column will comprise heavy hydrocarbon constituents boiling above 400 F. (As used herein, the term end point and the temperatures illustrated are those typically dened by laboratory distillation in accordance with ASTM Method D86.)

By and large it has been the practice to operate such a column mostly in the dark so far as the octane number of the product overhead fraction is concerned. That is to say, the column overhead product is manually sampled perhaps once every eight hour shift or perhaps even only once a day. The samples are picked up and taken to the laboratory where the sample is run and the result is then transmitted back to the unit operator who, until then, has not been able to ascertain what change, if any, should have been made at the time the sample was taken. Therefore, to be on the safe side, the unit operator Will usually run the gasoline splitter column with excessive heat input and with corresponding over-reflux whereby the overhead fraction of the stabilized reformate will actually be outside of product specifications with respect to octane number a good part of the time. This method of blind fractionator operation clearly increases the reliners costs.

The control problem is further complicated by the not uncommon practice of using a single fractionation column to process more than one gasoline stream. For example, a single gasoline splitter column will often receive plural or combined feeds which are stabilized reformates from two or more independently operated catalytic naphtha reforming units. yOr the splitter column may be operated on a gasoline feed comprising a mixture of stabilized reformate, cat cracked gasoline, natural gasoline, etc. An upset in the operation of a single such reformer (or other similar gasoline feed source) will carry through to the gasoline splitter and be reflected in olfspecication product since the splitter column overhead product is no longer indicative of only the operation of a single reformer or other gasoline source. Continuously meeting octane number specification is, therefore, an exceedingly diflicult and haphazard task when employing a single splitter column to handle such a plurality of gasoline streams.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved control system for use and in combination with a continuous flow reliuxed fractional distillation column.

It is another object of the present invention to provide such an improved control system for a fractional distillation column operating on one or more stabilized gasoline feed stocks.

It is a further object of the present invention to provide an improved control system for maintaining such a fractional distillation column under operating conditions sufcient to produce an overhead fraction comprising a gasoline product having a substantially constant predetermined octane number.

These and other objectives of the present invention, as well as the advantages thereof, will be more clearly understood as the invention is more particularly disclosed hereinafter.

In accordance with the present invention, the octane monitor comprising a stabilized cool llame generator with servo-positioned ame front is connected to receive a continuous sample of the splitter column overhead product. The output signal of the octane monitor, which can be, and preferably is, calibrated directly in terms of octane number, is utilized-to reset or adjust the rate of ow of reflux to the rectification section of the column so that the octane number of the net overhead fraction is maintained at a substantially constant predetermined level. This control system assures that the overhead gasoline product is always on specification, regardless of upsets or disturbances, and further effects a savings in utility costs in that the splitter column may thereby be operated at minimum heat input and minimum reflux.

Because there is a direct measurement and control of octane rating, this control system is to be distinguished from those prior art control systems wherein some composition property, such as percent aromatics or conductivity or dielectric constant, is measured and controlled, all of these latter properties being merely an indirect indication of octane rating which is only narrowly correlatable therewith. Such indirect correlation becomes invalid for any significant deviation from the design control point.

The control system of this invention is also to be distinguished from those prior art systems employing automated knock-engines as the octane measuring device. The instant octane monitor is compact in size, can be totally enclosed by an explosion-proof housing and therefore can be used in hazardous locations. In fact it is normally fieldinstalled immediately adjacent to the gasoline splitter column. A knock-engine, however, cannot be employed in hazardous locations and must therefore be situated remote from the sample point.

'Ihe sample transport lag or dead time of a closecoupled octane monitor lis typically of the order of two minutes, and its 90% response time is another two minutes. This is a very good approach to an essentially instantaneous or real time output. By way of contrast, the transport lag alone of a knock-engine may be of the orderof thirty minutes or more, which those skilled in the control system art will recognize to |be a substantial departure from real time output. With that much dead time built into a closed loop, it is extremely diflicult to achieve and maintain stability. The injection of an outside disturbance of any appreciable magnitude, in such a potentially unstable system, will often result in undampened cycling with the consequence that the system will have to be put on manual control.

In a broad embodiment, the present invention is directed to a control system for use and in combination with a continuous ilow, fractional distillation column, the feed to which is a gasoline fraction, the overhead from which comprises the lower boiling components of said fraction and the bottoms from which comprises the higher boiling components of said fraction, said column including a rectilication zone having a reflux conduit means in communication therewith at a iirst locus and meansv to supply reflux to said reilux conduit means, said control system for said column comprising: (a) means operatively associated with said reux conduit means to vary the flow of reflux to said rectification zone; (b) a hydrocarbon analyzer comprising a stabilized cool flame generator with a servo-positioned flame front continuously receiving a sample of said column overhead and developing an output signal which in turn provides a measure of sample octane number; and, (c) means transmitting said analyzer output signal to said reflux ow varying means (a) whereby the flow of reflux to said column is regulated responsive to octane number of said column overhead and said octane number is thereby maintained at a substantially constant predetermined level.

Preferred specific embodiments will incorporate one or more cascaded subloops which more immediately control the reflux liow to the column. For example, there may be a flow control loop on the reliux line to the rectification section of the column, the octane monitor output then being cascaded to the flow controller setpoint. Alternately, rectification section temperature control may reset the ow controller and the octane monitor output will reset such temperature controller setpoint. Other embodiments will become apparent in light of the detailed description of the invention.

The invention may now be more clearly understood by reference to the accompanying drawing which illustrates a typical splitter column together with one mode of controlling the flow of reflux thereto in a manner sufficient to maintain constant octane number on the overhead product.

DESCRIPTION OF THE DRAWING With reference now to the drawing, there is shown a gasoline splitter column 4 receiving a plurality of stabilized gasoline feeds. Splitter column 4 is a conventional continuous ow externally reuxed fractional distillation column containing from l0 to 50 or more vertically spaced vapor-liquid contacting stages as, for example, bubble decks, sieve decks, preforated trays or the like. Line 1 carries a Feed No. 1 comprising stabilized reformate from the stabilizer column of a naphtha reforming unit No. 1. Line 2 carries Feed No. 2 comprising stabilized reformate from the stabilizer column of a naphtha reforming unit No. 2. The combined reformates are charged to the column 4 via line 3 which connects with the column at a locus approximately midway in the height thereof. A plurality of vapor-liquid contact stages above this locus comprises the rectification zone S and a plurality of contact stages below the locus comprises the stripping zone'6 of the column.

The two reforming units are separate, independently operated catalytic naphtha reforming units; the details thereof form no part of the present invention, being conventional and well known in the art. A typical catalytic naphtha hydroreforming unit is described in U.S. Patent 3,296,118 (Class 208-) to which reference may be had for specific information concerning ow arrangement, catalyst, conditions etc. The feed to column 4 is generally under stabilizer reboiler level control from the preceding reforming units rather than direct ow control. Accordingly, the feed rate is usually, but not always, relatively constant, but it may be subject to some variation due to changes in naphtha feed composition, catalyst and/or operating conditions in either or both of the catalytic reforming unit reaction zones, or due to changes in operating conditions of the reforming unit stabilizer columns.

Gasoline splitter column 4 is maintained under operating conditions sufficient to separate the combined reformate feed stock into an overhead gasoline fraction having an end boiling point of about 400 F. and a bottoms fraction comprising heavy hydrocarbon constituents of the combined reformate feed having a boiling range of from about 400 F. to about 550 F., or even higher. While the refiner will typically set control of splitter column 4 to produce an overhead fraction having an end point of about 400 F., this is only a secondary consideration. The primary consideration is normally to produce an overhead fraction having an octane number of predetermined value, and this octane number is the primary control for operation of the column 4. Any deviation of octane number will require a compensating deviation of endpoint in order to produce an overhead product of constant octane number.

In order to accomplish the desired separation, the gasoline splitter column 4 will contain the rectification zone 5 and the stripping zone 6, as indicated hereinabove, in order that the most effective and efficient separation of hydrocarbon constituents may be accomplished within the fractionating column. In addition to the rectification and stripping zones, the column is provided with a reboiling section for heat input, and an overhead section which provides reflux liquid in a manner which shall be set forth hereinafter.

The reboiler section of fractionating column 4 comprises a reboiler liquid line 7, a reboiler heat exchanger 8, and a reboiler vapor return line 9 which are of conventional construction and design. Conventional instrumentation, not shown, is provided to control the heat input to the reboiler system. In addition, gasoline splitter column 4 is provided with a bottoms fraction draw-off line 10, whereby the heavy gasoline product may be withdrawn and sent to storage or to other processing.

The desired gasoline constituents of the combined reformate feed which is introduced into splitter column 4, are withdrawn in a vapor phase from column 4 via line 11 and passed to a heat exchanger 12 wherein they are condensed and cooled to about 100 F. or less. The condensed and cooled gasoline fraction passes from the heat exchanger 12 via line 13 into a fractionator overhead receiver 14 which is typically maintained at a pressure of from about 5 to 100 p.s.i.g., or more, in order to maintain low boiling constituents within the liquid phase. The liquid accumulated in the overhead receiver 14 is separated into two portions. A first portion is withdrawn via line 15 as a light gasoline product and sent to storage facilities, not shown. This light gasoline product typically will have a boiling range of from about C5 to about 400 -F. as indicated by ASTM Method D-86.

The second portion of the condensed overhead is withdrawn from the overhead receiver 14 via line 16 as the reflux which is returned to the column 4 in order to maintain the proper degree of vapor rectification within zone 5. The refiux conduit 16 also contains therein a flow measuring means such as an orifice 17 and a flow controlling means such as control valve 18. The reflux iiow rate is regulated by a flow control loop comprising the orifice 17, a flow signal line 19, a flow controller 20, a controller output line 21, and the control valve 18. The set point of fiow controller 20 is automatically adjustable.

A temperature controller 23, also provided with an automatically adjustable set point, senses and controls the rectification zone temperature as detected by a thermocouple or other sensing means 24 located within the rectification zone at a locus below the reux inlet of the column. The resulting temperature output signal is transmitted from the temperature controller 23 via controller output line 25 to adjust or reset the setpoint of flow controller 20.

Octane monitor 26, utilizing a stabilized cool flame generator with servo-positioned flame front, is `fieldinstalled adjacent column 4. In a preferred embodiment, the flows of oxidizer (air) and fuel (gasoline sample) are fixed as is the induction zone temperature. Combustion pressure is the parameter which is varied in a manner to immobilize the stabilized cool flame front. Upon a change in sample octane number, the change in pressure required to immobilize the flame front provides a direct indication of the change in octane number. Typical operating conditions for the octane monitor are:

Air flow-3500 cc./min. (STP) Fuel flow-l cc./min.

Induction zone temperature-700 F. (Research octane), 800 F. (motor octane) Combustion pressure-4-20 psig;

Octane range (maX.)--102 1 1 The actual calibrated span of the octane monitor as here utilized will, ln general, be considerably narrower. For example, 1f the target octane is 95 clear (research method), a suitable span may be 92-98 research octane. When a relatively narrow span is employed, the change in octane number is essentially directly proportional to the ohlange in combustron pressure.

Dashed line 27 represents a suitable sampling system to provide a continuous sample of column overhead to the octane monitor. For example, the sampling system 27 may comprise a sample loop taking the light gasoline product at a rate of cc. per minute from a point upstream of a control valve and returning it to a point downstream from the control valve, the sample itself ibeing drawn off from an intermediate portion of the sample loop and injected at a controlled rate by a metering pump to the combustion tube of the octane monitor. The octane monitor output signal is transmitted via line 28 to the setpoint of temperature controller 23. This may be a direct field connection, but preferably the octane monitor out-put will first be sent to an octane controllerrecorder located in the refinery control house, with the control signal therefrom then being set to reset the setpoint of temperature controller 23 which may be a ternperature recording controller also located in the control house.

PREFERRED EMBODIMENTS As indicated above, one preferred embodiment of the present invention consists of the application of the inventive control system in the splitting of reformate gasolines. As previously noted, the heavy ends of such reformate gasolines are high in octane number due to the fact that high boiling aromatic constituents are concentrated in the heavy end of the reformate. In splitting reformates to make various boiling range fractions, it is generally found that the octane number of the heavy gasoline product which is withdrawn via line 10 is consistently higher than the octane number of the light gasoline product which is withdrawn via line 15. This correlation of octane number with gasoline fraction is found to occur even when as little as 5 volume percent or as much as 60 volume percent of the reformate gasoline is removed as a bottoms product via line 10.

Thus, when operating column 4 on a reformate feed stock, any decrease in the measured octane number of the ovrhead product indicates that an insufficient amount of heavy boiling components is being withdrawn as a portion of the overhead product. In order to compensate for this condition, the octane monitor 26 will call for an increase in the rectification zone temperature in order to include a greater portion of the high octane number heavy ends in the overhead vapor which leaves column 4 via line 11. Temperature controller 23, being reset by the octane monitor, will then call for a decrease in reflux flow which in turn will be effected by fiow controller 20 and control valve 18.

7 Again, when operating column 4 on a reformate feed stock, an increase in the measured octane number of the overhead product is an indication that an excess of high octane number heavy ends is being withdrawn from column 4 in the overhead fraction. The octane monitor 26 therefore will call for a decrease in the rectification zone temperature in order to eliminate a greater portion of the heavy ends-from the overhead Vapor. Temperature controller 23 being reset by the octane monitor will call for an increase in the reflux flow which in turn will be effected by flow controller 20 and control valve 18.

'I'hose skilled in the art realize, of course, that a gasoline splitter column such as column 4 does not always operate on a feed stock comprising reformate gasoline. In many instances splitter column 4 may operate to separate an overhead and a bottoms fraction from a gasoline feed stock which may comprise one or more gasolines such as cracked gasoline, natural gasoline, alkylate gasoline, etc., and the feed stock may comprise stabilized and unstabilized gasolines which may include debutanized, depentanized, and dehexanized gasolines. Thus, it is possible that there will be embodiments of operation wherein the heavy :gasoline product withdrawn via line 10 will have an octane number which is consistently lower than the octane number of the light gasoline product Withdrawn via line 15. In those instances, the octane monitor 26 will call for overall corrective action which will be the reverse of that which has been indicated hereinabove for operations on reformate feed stocks. That is to say, if the overhead product of line ,15 indicates a decrease in themeasured octane number, this would be an indication that an excessive amount of low octane hea-Vy ends is being withdrawn overhead via line 1\1, and the control system would function to increase the amount of reflux in order to eliminate a greater portion of the heavy ends from the overhead vapor. On the other hand, if an increase in the measured octane number of the overhead product is indicated, then the octane monitor 26 would compensate by calling for a decrease in the amount of reflux to column 4 in order to allow a greater portion of the low octane heavy ends in the overhead vapor leaving via line 11.

Those skilled in the art will readily ascertain the proper direction of corrective action which is to be taken in the inventive control system for any specific gasoline feed stock composition and any specific fractionation cut-point from the teachings which have now been presented hereinabove.

Those skilled in the art realize, of course, that thermocouple 24 could be placed in locations other than that shown as, for example, in vapor outlet line 11. The drawing, however, illustrates a preferred embodiment wherein the temperature controller 23 senses and controls not the overhead vapor as it emerges directly from column 4, but rather the liquid or vapor temperature obtaining within the rectification zone at a point some distance below the reflux inlet of line 16 and above the feed inlet of line 3. In this preferred embodiment, the thermocouple 24 is typically located several trays (for example 2-6 trays) below the reflux inlet of line 16. This arrangement will afford a more immediate detection of changing heavy ends concentration, at least several minutes before such heavy ends reach the overhead vapor line 11 to cause a change ln the octane number of the overhead product.

While the double cascade arrangement illustrated in the drawing represents a preferred embodiment, it is within the scope of this invention to omit the temperature controller 23 and to reset iiow controller 20 directly by the octane monitor output signal transmitted via line 28. Alternatively, the flow controller 20 could also be omitted, in which case octane monitor output signal line 28 would connect directly with valve 18. It may be expected, however, that elimination of either or both of the subloops will result in somewhat poor overall control because rectification zone temperature and reflux ow variations will become a source of additional upsets, and also because the relatively large time constant of the stabilizer column itself tends to make single loop control unstable.

Although the inventive control system has been disclosed relative to the separation of a gasoline fraction to produce an overhead fraction having an end boiling point of about 400 F. and a bottoms fraction containing heavier hydrocarbon constituents, the invention is not so limited. The control system is clearly applicable to any distillation wherein a gasoline fraction is separated into an overhead containing the lower boiling components of the fraction and a bottoms containing the higher boiling components of the fraction, regardless of the distillation cut-point between the fractions. As used herein, the term higher boiling components refers to those hydrocarbon constituents which boil at a temperature above the distillation cutpoint for the overhead fraction. Thus, if the fractional distillation is undertaken to produce an overhead gasoline having an endpoint of, say, 380 F., the higher boiling components will comprise the bottoms fraction of the distillation. And if the distillation is undertaken to dehexanize the gasoline feed, the higher boiling components comprise hydrocarbons having seven or more carbon atoms per molecule. Similarly, the term lower boiling components refers to those hydrocarbon constituents which boil at a temperature below the distillation cut-point.

The invention claimed:

1. In combination with a continuous ow fractional distillation column, the feed to which comprises a gasoline fraction, the overhead from which comprises the lower boiling components of said fraction and the bottoms from which comprises the higher boiling components of said fraction, said column including a rectification zone having a reflux conduit means in communication therewith at a first locus and means to supply reflux to said reflux conduit means, a control system for said column comprising:

(a) means operatively associated with said reflux conduit means to vary the flow of reflux to said rectification zone;

(b) a hydrocarbon analyzer comprising a stabilized cool flame generator with a servo-positioned ame front continuously receiving a sample of said column overhead and developing an output signal which in turi provides a measure of sample octane number; an

(c) means transmitting said analyzer output signal to said reflux flow varying means (a) whereby the flow of reux to said rectification zone is regulated responsive to octane number of said column overhead and said octane number is thereby maintained at a substantially constant predetermined level.

2. The system of claim 1 wherein the feed to said column comprises at least one stabilized gasoline fraction.

3. The system of claim 1 wherein said reflux flow varying means comprises a flow control loop including a flow controller having an adjustable setpoint regulating the rate of flow of reiiux through said reflux condition means, said setpoint being adjusted in response to said analyzer output signal.

4. The system of claim 3 further characterized in the provision of means to sense the temperature in said column at a second locus, temperature control means having an adjustable set point connecting with said temperature sensing means and developing a temperature output signal, said means transmitting the last-mentioned output signal to the setpoint of said flow controller, said means (c) transmitting said analyzer output signal to the temperature controller setpoint whereby the latter is adjusted responsive to overhead octane number.

5. The system of claim 4 wherein said temperature sensing means is located in said rectification zone.

6. The system of claim 5 wherein said second locus is below said -iirst locus.

7. T he system of claim 6 wherein said distillation column contains a plurality of fractionation trays and said temperature sensing means is located several trays below said rst locus.

10 3,463,725 8/ 1969 Macfarlane et al. 203--3 X 3,475,288 10/ 1969 Ezzell 203-3 X References Cited UNITED NORMAN YUDKOFF, Primary Examiner 5 D. EDWARDS, Assistant Examiner STATES PATENTS Berger 203,--3 X Lupfer et al 203-3 X -Rijnsdorp 203-3 x U'S' Cl' X'R' Rijnsdorp et al. 203--3 X 196100; 202-160; 203-3, Dig. 18, 2; 20S-Dig. 1,

Fenske et al. 23-253 X 10 358

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