US3203871A

US3203871A - Process control for fractionation column

Info

Publication number: US3203871A
Application number: US60085A
Authority: US
Inventors: Merlin O Clark; Jr William B Wyatt
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1960-10-03
Filing date: 1960-10-03
Publication date: 1965-08-31
Anticipated expiration: 1982-08-31

Description

g- 1965 M. o. CLARK ETAL 3,203,871

PROCESS CONTROL FOR FRACTIONATION COLUMN Filed Oct. 5, 1960 TEMPERATURE TRANSDUCER 23 A CONDENSER 2| M f) w 45 3| 22 27 F TEMPERATURE TRANsDUcER ACCUMULATOR LC 1 32 SET POINT 26 I I0 Q S III SET 30 33 24 POINT CE FRC 2 I FEED Q U SET 55 POINT E :5

I i D I STEAM l4 I6 I |7-| 2 KETTLE PRODUCT W. B. WYATT, JR.

l I) I l I I 1 LL! l I 0: I I 1 1 I I z l I I I 1 LxJ I I I INVENTORS Z T l M. o. CLARK I l 1 1 BY HwmR t FIG. 2 ATTORNEYS United States Patent 3,203,871 PROCESS CGNTRQL FOR FRAC'HONATION COLUMN Merlin 0. Clark and William B. Wyatt, .lra, Berger, Tern, assignors to Phillips Petroleum ompany, a corporation of Delaware Filed Get. '3, 1960, 591'. No. 6%,085 3 Claims. (Cl. 2432-40) This invention relates to the control of fractionation processes.

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

With conventional control equipment, variations in temperature of the reflux of to 30 degrees from the desired value are not uncommon. If the flow of external reflux to the column is maintained at a constant value, the flow or" internal reflux within the column is changed by these temperature fluctuations. A sudden change in internal reflux can upset the operation of the column so that the liquid level in the kettle, the steam supply rate and the overhead vapor Withdrawal rate fluctuate in cycles. Still greater changes in the amount of cooling of the over head vapors must then be made and the cycling can increase in magnitude.

In order to eliminate these difliculties, a computer has recently been developed which calculates the internal reflux in the column from measurements of the flow of external reflux, the temperature of the external reflux re turned to the column and the temperature of the overhead vapor removed from the column. This computer is described in ISA Journal, June 1959, Volume 6, No. 6, pages 34 to 39. The internal reflux R, is defined by the following equation:

where R is the rate of flow of the external reflux, K is the specific heat of the liquid on the top tray divided by the heat of vaporization of the liquid on this tray and AT is the difference between the temperature of the liquid on the tray (overhead vapor temperature) and the temperature of the external reflux returned to the column. In response to this computation, the flow of external reflux can be adjusted so as to maintain the computed internal reflux constant. It has been found that this type of control system greatly improves the stability of operation of fractionation columns.

In accordance with the present invention, a simplified control system is provided which is based on the principles of the internal reflux computer, but which does not "ice require an actual computation of the internal reflux. The flow of external reflux to the column is regulated automatically in response to a measurement of the temperature of external reflux returned to the column. If the temperature of the external reflux should increase, for example, a greater amount of external reflux is returned to the column, whereas the external reflux flow is decreased it the measured temperature decreases. This control system provides improved fractionation column operation where the temperature of the external reflux is likely to fluctuate.

Accordingly, it is an object of this invention to provide improved apparatus for controlling the operation of fractionation columns by adjusting the flow of external reflux to compensate for temperature changes.

Another object is to provide a method of controlling fractionation columns in response to a measurement of the temperature of the external reflux returned to the column.

Other objects, advantages and features of the invention should become apparent from the following detailed description, taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a schematic representation of an embodiment of the control system of this invention.

FIGURE 2 is a graphical representation of the operation of the control method of this invention.

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

Referring now to the drawing in detail and to FIGURE 1 in particular, a feed stream of a fluid mixture to be separated is introduced into a fractionation column 10 through a conduit 11. Conduit 11 is provided with a flow recorder-controller 12 which adjusts a valve 13 to maintain a predetermined rate of flow though conduit 11. Heat is supplied to the lower region of column 13 by means of a conduit 14 which conveys steam or other heating material. A flow recorder-controller 15 adjusts a valve 16 to supply the heating material at a pretedetermined rate. A kettle product stream is withdrawn from the bottom of column 10 through a conduit 17 which has a control valve 18 therein. Valve 18 is regulated by a liquid level controller 19 so as to maintain a predetermined liquid level in the bottom of column 10. Vapors are removed from the top of column 10 through a conduit 21 which communicates with an accumulator 22. through a condenser 23. A portion of the condensate in accumulator 22 is returned to the top of column 10 as external reflux through a conduit 24. The remainder of the condensate is removed as overhead product through a conduit 25 which has a control valve 26 therein. Valve 26 is adjusted by a liquid level controller 27 to maintain a predetermined level in accumulator 22.

in accordance with the present invention, the temperature of the external reflux returned to column 10 is measured by a detecting element 30 which is connected to a temperature transducer 31. Transducer 31 provides an output signal which is representative of the temperature of the external reflux. The flow of external reflux through conduit 24- initiflly is maintained at a predetermined value by means of a flow recorder-controller 32 which adjusts a valve 33. This flow rate is set initially at a preselected value for a preselected external reflux temperature. If the measured reflux temperature should change from this preselected value, the output signal from Flows (gaL/day) Component Feed Overhead Kettle Normal hexane Methyleyclopentnne 2,2-dimethylpentane 2,4dimethylpentane Cyclohexane 1,1-diruethyleyclopentane Under normal operating conditions, the kettle temperature is approximately 250 F. and the temperature at the top of the column is approximately 208 F. The temperature of the external reflux returned to the column is approximately 140 F. The flow of external reflux is approximately 784,400 gallons per day. The pressure at the top of column is approximately p.s.i.g., and the pressure in the kettle is approximately p.s.i.g.

As previously mentioned, the temperature of the external reflux can fluctuate substantially during operation of the column. The method of control provided by this invention is illustrated schematically in FIGURE 2. This figure shows a plot of the internal reflux as a function of the external reflux for different temperatures of the external reflux. Vertical line 39 shows changes in internal reflux which result from changes in temperature of the external reflux where the external reflux rate is held constant. The horizontal line 40 represents the ideal control which shows changes of flow of external reflux which should be made in response to changes in reflux temperature to give a constant internal reflux rate. In actual practice, however, the response of control system of this invention is not exactly linear, but is more of the form of curve 41. However, curve 41 is so nearly linear within the normal range of operation that satisfactory results are obtained.

The control system of FIGURE 1 can further be improved by adjusting the flow of either the feed to the column or the steam to the column in response at a measurement of the temperature of the overhead vapor. In order to accomplish this result, a temperature detector 45 is disposed in conduit 21. This detector is connected to a temperature transducer 46 which adjusts the set point of either flow recorder-controller 12 or flow recorder-com troller 15. Temperature transducer 46 adjusts the set points of one of these controllers in order to maintain the temperature of the overhead vapors at a constant value. If the temperature of these vapors should decrease, for example, additional steam is supplied to the column or less feed is supplied. Conversely, an increase in temperature results in a decrease in the flow of steam or a decrease in the flow of feed.

Another embodiment of the control system of this invention is illustrated in FIGURE 3.

Temperature detectors

50 and 51 are positioned in

respective conduits

24 and 21. These two detectors, which can be thermocouples connected in opposition, are applied to transducer 31 so that an output signal is provided which is representative of the difference between the two temperatures.

This temperature differential signal is employed to reset flow recorder-controller 32, but in an opposite manner. An increase in the temperature differential, for example, results in a lower reflux rate. Otherwise, the system of FIGURE 3 is identical to that of FIGURE 1.

In view of the foregoing description, it should be evident that a simplified fractionation column control system is provided in accordance with this invention. This system utilizes a minimum number of conventional components which are commercially available and provides stable column operation under many conditions. 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:

1. A fractionation system comprising a fractionation column, first conduit means communicating with said column to introduce a feed mixture, second conduit means communicating with a lower region of said column to withdraw a kettle product, an accumulator, a condenser, third conduit means communicating between an upper region of said column and said accumulator through said condenser, fourth conduit means communicating with said accumulator to remove an overhead product, fifth conduit means communicating between said accumulator and an upper region of said column to return liquid to said column as external reflux, first temperature sensing means disposed in said fifth conduit means adjacent said column, second temperature sensing means disposed in said third conduit means adjacent said column, means responsive to said first temperature sensing means to control the rate of flow through said fifth conduit means, and means responsive to said second temperature sensing means to control the rate of flow through said first conduit means.

2. In a fractionation process wherein a feed mixture of two or more components is directed to a fractionation zone, a vapor stream is removed from the top of said zone, 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 said zone as reflux; a control process which comprises continuously measuring the temperature of said reflux, adjusting the flow of said reflux solely in response to measured temperature changes by increasing the flow when the measured temperature increases and by decreasing the flow when the measured temperature decreases, continuously measuring the temperature of the vapor stream removed from the top of said zone, and adjusting the rate at Which said feed mixture is supplied to said zone responsive to the measured temperature of said vapor stream so as to maintain the temperature of said vapor stream constant.

3. A fractionation system comprising a fractionation column, first conduit means communicating with said column to introduce a feed mixture, second conduit means communicating with a lower region of said column to withdraw a kettle product, an accumulator, a condenser, third conduit means communicating between an upper region of said column and said accumulator through said condenser, fourth conduit means communicating with said accumulator to remove an overhead product, fifth conduit means communicating between said accumulator and an upper region of said column to return liquid to said column as external reflux, first temperature sensing means disposed in said fifth conduit means adjacent said column, second temperature sensing means disposed in said third conduit means adjacent said column, means responsive solely to said first temperature sensing means to control the rate of flow through said fifth conduit means, and means responsive to said second temperature sensing means to control the rate of flow through said first conduit means.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Roelfsema 196132 X Schutte 202-160 X Carney 196132 Fragen et a1. 202-160 Boyd -1 202--160 Salmon 202160 X 6 OTHER REFERENCES ISA Journal, Lufer et 211., June 1959, vol. 6, N0. 6, pages 34-39.

5 NORMAN YUDKOFF, Primary Examiner.

GEORGE D. MITCHELL, RICHARD NEVIUS,

Examiners.