US3158556A - Fractionation control - Google Patents

Description

Nov. 24, 1964 E. E. HOPPER FRACTIONATION CONTROL 3 Sheets-Sheet 1 Filed Sept. 24, 1959 (59% mumoh.

INVENTOR. E. E. H O PFER A TTORNE V5 E. E. HUPPER FRACTIONATION CONTROL Nov. 24, 1964 5 Sheets-Sheet 2 Filed Sept. 24, 1959 M MA TIME-- F/G. 2b

` INVENTOR.

E.E.HoPP|-:R

BY l

Arron/vn:

3 Sheets-Sheet 3 ACCUMUL ATOR E` E. HOPPER FRACTIONATION CONTROL.

PSET PONT INVENTOR. 5.5. HOPF'ER H WQ/w+ www A TTUR/VEVS SET POINT 86 RATIO RELAY Nov. 24, 1964 Filed Sept. 24, 1959 COMPUTER INTERNAL REFLUX United States Patent Otice 3,153,556 FRAC'HNATN CNTRL E. Hopper, Borger, Tex., assigner to Phillips Petroleum Cempany, a corporation of Delaware Filed Sept. 24, 1959, Ser. No.842,085 2 Claims. (tCl. EQ2- 160) This invention relates to the computation of internal reflux in a fractionation column. In another aspect it relates to control systems for fractionation columns which are based on computations of internal reflux.

In recent years an increasing use has been made of fan coolers for condensing overhead vapors from fractionation columns. However, use of this type of cooler has resulted in a rather serious operating problem because it is difficult to control the exact amount of cooling provided. Such schemes as fan speed control, variable pitch fan blade control and hot vapor by-pass control have been employed in an attempt to solve the problem, but have not been entirely satisfactory. Sudden atmospheric temperature changes, such as occur during a rainstorm, for example, result in a lowering of the reflux temperature. This causes an increase in the ilow of liquid leaving the top tray because more of the vapor which enters this tray is condensed. The net result is an increase in overhead product purity at the expense of a decreased overhead product rate.

In accordance with the present invention there is provided a novel system for computing the amount of internal reux in a fractionation column. Internal reilux constitutes the external redux returned to the column plus the vapor which is condensed near the top of the fractionation column by the sub-cooled external reilux. This computation is made from a measurement of the rate of flow of the external reflux and a measurement of the differential between a fixed reference temperature and the temperature or the external redux returned to the column. Signals representative of these two measurements are combined to provide a measurement of the internal rellux in the column. A signal representative of this internal reflux canV be employed to control the operation of the column to maintain desired steady state operating conditions. This control can be made automatically or by an operator from the information provided. n

In another aspect of this invention, a fractionation column control system is provided which combines the output sinal from a process analyzer with the output signal from the internal redux computer. In still another aspect, a fractionation column is controlled so as to maintain a preselected ratio between the column feed and the computed internal reflux.

Accordingly, it is an object of a system for computing tion column.

Another object is to provide control systems for fracthis invention to provide the internal rellux in a fractionationation columns which utilize computations of the in ternal relux in the column.

Other objects, advantages and features of the invention` should become apparent from the 'ollowingdetailed det scription which is taken in conjunction with the accompanying drawing in which: Y FIGURE l is a schematic representation of a fractionation column having the computer and control system of this invention incorporated therein.

3,158,556 Patented Nov. 24, 1964 FIGURES 2a and 2b are graphical representations of the operation of a fractionation column without and with the control system of FIGURE 1, respectively.

FIGURE 3 is a schematic representation of a second embodiment of the control system of this invention.

FIGURE 4 is a schematic representation of a third embodiment of the control system of this invention.

Referring now to the drawing in detail and to FIGURE 1 in particular, there is shown a conventional fractionation column 10 which is provided with a number of vaporliquid contacting trays. A lluid mixture to be separated is introduced into column 10 through a conduit 11 at a predetermined rate which is maintained by a flow controller 12 that adjusts a valve 13. Conduit 11 extends through rst and second heat exchangers 14 and 15. Steam, or other heating medium, is directed through heat exchanger 15 by means of a conduit 16 which has a control valve 17 therein. A temperature controller 18 ad-l justs valve 17 in response to a measurement of temperature in conduit lldownstream from heat exchanger 15 so as to tend to maintain the temperature of the feed stream constant.

Steam, or other heating medium, is circulated through the lower region of column 10 through a conduit 20 at a predetermined rate which is maintained by a ilow controller 21 that adjusts a valve 22. A kettle product stream is withdrawn'from the .bottom of column 10 through a conduit 23 which communicates with heat exchanger 14. The rate of product withdrawal through conduit 23 s regulated by a liquid level controller 24 which resets a flow controller 25. Controller 25 adjusts a valve 26 in conduit 23 so as to tend to maintain a constant level in the bottom of column 10.

Vapors are withdrawn from the top of column 10 through a conduit 28 which communicates with the inlet of a condenser 29 through a control valve 32. Valve 32 is set by a pressure controller 33 so as -to tend to maintain a constant pressure in the top of column 10. The outlet of condenser 29 communicates with an accumulator 3i) through a conduit 31. A conduit 34, which has a control valve 35 therein, is connected .between conduit 28 and accumulator 30. The amount of vapor which bypasses condenser 29 through conduit 34 s adjusted by a pressure regulator 36 which controls valve 35 to tend to maintain a constant pressure in accumulator 3G. Liquid in accumulator 3d is withdrawnthrough a conduit 38 which has a pump 39 therein.V A conduit 40 extends from the discharge end of pump 39 to the top of column 10 to return liquid to the column as reflux. The remainder of the liquid removed from accumulator 30 is removed from the system through a conduit 42 as the overhead product. The rate of withdrawal of liquid through conduit42 is adjusted by a liquid level controller 43 which resets a iiow controller 44 thatA regulates a .valve 4S in conduit 42. so .as to tendrto maintain a constant liquid level in accumulator 3i). iy I `In order to explain the operation ofthe internal reilux computer and control system of this invention, an equa'-` tion which is representative of the internal retlux in a fractionation column will be derived.

Y The material balance at the toptray of the fractionator can be'expressed: f

Rei Vir-Rfi- Vo where;

Re=mass flow of liquid entering top tray (external reflux) Vi=mass flow of vapor entering top tray Ri=mass How of liquid leaving top tray (internal reflux) Vozmass ilow of vapor leaving top tray.

The heat balance at the top tray can be expressed:

Rehei-ViHzRlh-ivoH (2) where:

Ize=enthalpy of external rellux h1=enthalpy of internal reflux H :enthalpy of vapor streams (assumed to be equal).

of the reux at the fractionator pressure and the actual external reux temperature.

Equation 3 can be substituted into Equation 2 to eliminate H and rewritten:

Equation 4 can be substituted into Equation 5 to eliminate he and rewritten:

(hel-) (V1-Vo) :NRt-Re) -l-ReCpAT (6) From Equation 1 it is known:

V1 Vo=Rl-Re (7 Equation 7 can be substituted into Equation 6 and reduced to obtain:

The computer of this invention solves Equation 8. The term Re is measured by a flow transducer which establishes a signal representative of the rate of ilow through conduit 40. This ow transducer can advtantageously be a differential pressure transmitter 50 which establishes an output pressure representative of the pressure differential across an orifice in conduit 40. This pressure is transmitted to the inlet of a square root torce bridge 51 which establishes an output pneumatic pressure that is representative of the square root of the input pressure. The output pressure of bridge 51 is thus directly proportional to the flow through conduit 40.

In accordance with this invention, the term AT is not measured directly. Instead, a signal is established which is representative of the dilerence between a xed reference temperature and the temperature of the external reflux. A thermocouple 54 is positioned in reiiux conduit 40 to establish a first voltagerepresentative of the temperature of the external reflux. A voltage source S2 is connected across the end terminals of a potentiometer S3. The negative end terminal of potentiometer 53 is connected'to the negative terminal ot thermocouple 54. The positive terminal of thermocouple 54 and the con- 4 CP T The output pressures from transducer 55 and force bridge 51 are applied to the respective inputs of a multiplying force bridge 60 which establishes an output pressure that is proportional to the product Re(1-i-KAT). Ihe output signal from force bridge 60 thus represents the computed internal reux of the fractionation column.

The output signal from bridge 60 is applied through a conventional ow recorder-controller 61 to adjust a vaive 63 in redux conduit 4G in such a manner as to tend to maintain the computed internal reliux constant at a preselected value. It the computed internal redux should increase, valve 63 is adjusted to reduce the tlow through conduit 4U. Conversely, if the computed internal reflux should decrease, valve 63 is adjusted to increase the ilow through conduit 40.

- As a specific example of the computer and control system of this invention, reference is made to a uid separation wherein a feed stream containing primarily isopentane and normal pentane is separated to provide an overhead product stream containing primarily isopentane and a kettle product stream comprising primarily normal pentane. Typical compositions (percentages) and iiow rates (barrels per hour) of the three streams are set forth in the following table:

tactor of potentiometer 53 are connected to the respective input terminals of a pneumatic transducer 55.Y The contactor of potentiometer 53 can bevadjusted to vary the reference voltage supplied by the potentiometer. ThisA can be changed as required toY approximate the bubble point temperature of the external relux at the pressure within column lil. The resulting signal applied to the input of transducer 55 is thus representative of AT. Transducer 55 is calibrated so that the output signal is proportional to (l-i-KAT). The terrn'K is equal toY The reux to feed ratio is of the order of tive to one. AT normally is of the order of 55 F.

FIGURE 2a of the drawing is a typical record of the kettle product now rate obtained when the column is provided with the control system thus far described except for the internal reiiux computer. FIGURE 2b illustrates the kettle product flow when the internal reflux computer override control is provided. It should be evident that a substantially smoother operation of the column is obtained by use of this control system.

The computer components illustrated in FIGURE l can be conventional pneumatic equipment well known in the art. For example, bridges 51 and 60 can be of the type described in U.S. Patent 2,643,055. However, the computer can also be constructed of electrical components Well known in the art. Multiplication can be accomplished by potentiometers actuated by servo motors, for example.

The fractionation column control system can be improved still urther by overriding the internal reux ow controller 61 set point by a signal from an analyzer which measures a property of the uid mixture Within the column. A sample stream is Withdrawn from the upper region of column 10 through a conduit '70 which communicates with the inlet of an analyzer 7l. This analyzer can advantageously be a dilterential retractometer which fractive index decreases, the reilux rate is decreased. It

should be evident that other types ot analyzers can be employed for this purpose. Examples of such analyzers include infrared analyzers, ultraviolet analyzers, mass spectrometers and chromatographic analyzers. The choice of the particular analyzer employed depends to a large extent upon the constituents being separated in the fractionation column.

An important feature of this invention resides in the use of a reference temperature representative of the desired vapor temperature at the top of the column. This can prevent errors which might result from an actual temperature measurement at this point. For example, a column upset or change in feed could result in an increase in the higher boiling constituent of the feed mixture in the column overhead. This would increase AT so that the controller would decrease external reilux. However, this is opposite the proper'control step that should be applied. The higher boiling constituent would increase still further to compound the error. However, the iixed reference temperature prevents this error from being made.

Heretofore, it has beeen common practic in fractionation control systems to maintain the reflux flow at a predetermined value to give a desired reflux to feed ratio. 1f the feed rate should change, the reflux llow must be reset manually to maintain the desired ratio. It is generally desirable to operatea fractionation column with a minimum amount of reux in order to conserve the heat supplied to the column. In accordance with another aspect of this invention, an improved method of control is provided wherein the internal reilux to feed ratio is maintained constant at a preselected value. FIGURE 3 is a schematic diagram of such a control system.

The controls associated with the kettle and feed streams of column 1t) are identical to those shown in FIGURE 1 except that ilow controller 12. is omitted. However, controller 12 can be employed, and reflux ow controls are substantially identical to those in FIGURE 1. Condenser 29 is an air fan cooler which is provided with an auxiliary temperature recorder-controller 86 that responds to the temperature of condensed vapors introduced into accumulator Si). This controller adjusts either the speed of the fan or the pitch of the blades so as to reduce temperature fluctuations in the liquid directed to accumultor 30. As previously discussed, such a control system is helpful but does not eliminate all temperature fluctuations.

The control system of FIGURE 3 is provided with an internal reflux computer 8l which includes elements 5S, 50, 51 and 60 0f FIGURE l. The output signal from the reilux computer is applied to flow recorder-controller 61. A differential pressure transmitter 82 establishes a signal representative of the pressure differential across an orifice in feed conduit 11. The output signal of transmitter 82, which is representative of the square of the ilow rate in conduit 11, is applied to the input of a square root forcebridge 83; The output signal of bridge 83, which is representative of the ow, is applied through a delay means 85 to the first input of a ratio controller 84. The second input signal to ratio controller 84 is the output signal of computer 81. Ratio controller 84 provides an output signal which is representative of a desired ratio between the two input signals. This signal is applied to the set point of tlow controller 61. The desired ratio to be provided by controller 84 can be adjusted manually by manipulating the set point of the controller. The control system of FIGURE 3 thus provides a method of maintainmg the ratio of the internal reflux to the feed rate constant at a preselected value. Thisis a decided improvement over control systems previously known.

Delay means 85 is providedso that the reilux will be adjusted some time after the feed flow rate changes. Otherwise, a sudden increase in feed, for example, would result in a proportional increase in external reflux. Such an increase in external reux would decrease the overhead product llow, perhaps to zero. This type of operation isobviously undesirable. The delay, on the Vother hand, permits the column to become stabilized at the increased feed llow rate before the ratio is reset. If a pneu- Ymatic control system is employed, delay means SS can if desired. The overhead internal reflux in fourth signal comprise a restriction followed by a storage tank, or a series of such restrictions and storage tanks. If an electrical control system is employed, delay means can comprise a conventional active type RC delay network.

A second embodiment of the ratio control system is illustrated in FIGURE 4. The output signal from delay means 85 is applied to the first input of a ratio relay 86. The set point constitutes the second input of relay 86. The output signal Vof relay 86 resets ilow controller 61 which adjusts valve 63 in the reflux conduit.

In View of the foregoing description it should be evident that there is provided in accordance with this invention a novel computer for measuring the internal reflux in a fractionation column. Control systems utilizing such a computer to regulate the rate of external reilux stabilize the operation of fractionation columns substantially. Furthermore, such a control system permits a fractionator to v operate at a higher load than is otherwise possible. The computer of this invention utilizes a minimum number of standard components which are available commercially and which are of a type that is generally familiar to process operators. The computer of this invention can readily be adapted for use in existing columns without extensive modifications.

While the invention has been described in conjunction With present preferred embodiments, it should be evident that it is not limited thereto.

What is claimed is: K

l. In a fractionation system wherein a fluidmixture of twoor more components is directed to a fractionation column, a vapor stream is removed from said column, said vapor stream is cooled to condense at least a part of same, and at least a part of the resulting condensate isreturned to the column as external rellux, acontrol system comprisingmeans to establish a first signal representative of the ilow of external reflux to said column, a -thermocouple positioned to establish a second signal representative of the temperature of said external reflux', v

where Cp is the specific heat of said external reflux, A is the heat of vaporization of liquid in the top of said co1- umn, and AT is the difference between said third and second signals, means to multiply said first signal by said fourth signal to establish a fifth signal representative of said column, and means responsive to said fifth signal to control the operation of said column by regulating the llow of said external reflux.

2. In a fractionation system wherein a fluid mixture of two or more components is directed to a fractionation column, a vapor stream is removed from said column, said vapor stream is cooled to condense at least a part ofv same, and at least a part of the resulting condensate is sponsive to said second and third signals to establish a *t1-etw) pressure within said column, means re-' 7 the heat of vaporization of liquid in the top of said columns, and AT is the diierence between said third and second signals, and means to multiply said rst signal by said fourth signal to establish a fth signal representative of internal reflux in said column.

References Cited in the le of this patent UNITED STATES PATENTS 1,735,470 Noel Nov. 12, 1929 2,180,512 Fenske Nov. 21, 1939 2,202,218 Mallory May 28, 1940 2,236,035 Luhrs Mar. 25, 1941 2,459,404 Anderson Ian. 18, 1949 2,580,651 Boyd Jan. 1, 1952 2,709,678 Berger May 31, 1955 8 Clay Apr. 21, 1959 Kron Mar. 28, 1961 Bellinger May 23, 1961 Morgan Ian. 23, 1962 Lupfer Feb. 6, 1962 OTHER REFERENCES Instruments and Process Control, published by NY. State Vocational and Practical Arts Association, 1945,

10 pages 155-185 Automatic Control of Rectification Processes, Anisimov, published in Moscow, 1957, by State Scientic- Technical Press of Petroleum and Mineral Fuel Literature, pages 26-29 of translation, translated in 1959 by 15 Consultants Bureau, Inc., New York, book division.

Claims (1)

1. IN A FRACTIONATION SYSTEM WHEREIN A FLUID MIXTURE OF TWO OR MORE COMPONENTS IS DIRECTED TO A FRACTIONATION COLUMN, A VAPOR STREAM IS REMOVED FROM SAID COLUMN, SAID VAPOR STREAM IS COOLED TO CONDENSE AT LEAST A PART OF SAME, AND AT LEAST A PART OF THE RESULTING CONDENSATE IS RETURNED TO THE COLUMN AS EXTERNAL REFLUX, A CONTROL SYSTEM COMPRISING MEANS TO ESTABLISH A FIRST SIGNAL REPRESENTATIVE OF THE FLOW OF EXTERNAL REFLUX TO SAID COLUMN, A THERMOCOUPLE POSITIONED TO ESTABLISH A SECOND SIGNAL REPRESENTATIVE OF THE TEMPERATURE OF SAID EXTERNAL REFLUX, AN ADJUSTABLE VOLTAGE SOURCE FOR ESTABLISHING A THIRD SIGNAL REPRESENTATIVE OF A PREDETERMINED DESIRED FIXED REFERENCE TEMPERATURE WHICH APPROXIMATES THE BUBBLE POINT TEMPERATURE OF THE EXTERNAL REFLUX AT THE PRESSURE WITHIN SAID COLUMN, MEANS RESPONSIVE TO SAID SECOND AND THIRD SIGNALS TO ESTABLISH A FOURTH SIGNAL

Images