US3629561A

US3629561A - Internal reflux computer for fractionation column control

Info

Publication number: US3629561A
Application number: US50719A
Authority: US
Inventors: Louis D Kleiss
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1970-06-29
Filing date: 1970-06-29
Publication date: 1971-12-21
Anticipated expiration: 1988-12-21

Abstract

Internal reflux in a fractionation column is computed from measurements of the temperatures of overhead vapor and external reflux and the rate of flow of external reflux. The temperatures are sensed by temperature sensitive resistance elements which constitute the feedback and input resistors, respectively, of an amplifier employed to make the computation.

Description

United States Patent [56] References Cited UNITED STATES PATENTS 3,018,229 1/1962 Morgan 3,502,852 3/1970 Lewis 3,517,177 6/1970 Crowell Primary Examiner-Eugene G. Botz AttorneyYoung and Quigg ABSTRACT: internal reflux in a fractionation column is computed from measurements of the temperatures of overhead vapor and external reflux and the rate of flow of external reflux. The temperatures are sensed by temperature sensitive resistance elements which constitute the feedback and input resistors, respectively, of an amplifier employed to make the computation.

AMPLIFIER b. A P i +TRANSDUCER I PATENTED UECZI l8?! 3,629,561

COMPUTER CONTROLLER g5 nigh l FRACTIONATOR I 29 Wail) @552 3a 39 40 u f I A P F A +TRAN5DUCER AMPLIFIER kSET POINT FIG. 3

CONTROLLER INVENTOR. L. D. KLEISS BY ZYMq Q M/ A TTORNEYS INTERNAL REFLUX COMPUTER FOR FRACTIONATION COLUMN CONTROL It is known that fractionation columns can be controlled in an efficient manner in response to a computation of the internal reflux. Internal reflux, which constitutes the external reflux returned to the column plus the vapor which is condensed near the top of the column by subcooled external reflux, can be computed from the following equation:

F A /UAT) where R, is the internal reflux, R is the mass flow of liquid entering the top of the column (external reflux), C, is the specific heat of the external reflux, k is the heat of vaporization of liquid at the top of the column where the external reflux enters, and AT is the difference in temperature between vapor removed from the top of the column and the external reflux returned to the column. While apparatus is known for solving this equation, such apparatus is relatively expensive to construct because it requires the use of an electronic or pneumatic multiplier.

In accordance with this invention, improved apparatus is provided for calculating internal reflux in a fractionation column. This calculation is made without the use of multiplying apparatus. First and second temperature sensitive resistance elements are positioned so as to be exposed to the temperatures of the vapor removed from the top of the column and the external reflux returned to the column, respectively. These two resistors constitute a feedback resistor and an input resistor, respectively, of a conventional operational amplifier. The input signal to the amplifier is representative of the measured rate of flow of external reflux to the column. By proper calibration of the amplifier and associated circuit elements, an output signal is obtained which is representative of the internal reflux. This signal can be employed to control the rate of flow of external reflux to maintain a desired internal reflux in the fractionation column.

In the accompanying drawing, FIG. 1 is a schematic representation of a fractionation column having the internal reflux computer of this invention associated therewith. FIG. 2 is a schematic circuit drawing of an embodiment of the internal reflux computer. FIG. 3 is a more detailed view of the internal reflux computer.

Referring now to the drawing in detail, there is shown a conventional fractionation column which contains a number of vapor-liquid contacting trays. A fluid 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 which adjusts a valve 13. Steam or other heating fluid is circulated through a reboiler 14. This heating fluid is introduced into the reboiler through a conduit 15 at a predetermined rate which is maintained by a flow controller 16 which adjusts a valve 17. A liquid bottoms product is removed from the lower region of column 10 through a conduit 18 which has a control valve therein. A liquid level controller 19 adjusts valve 20 to maintain a predetermined liquid level in the lower region of the column.

Vapors are withdrawn from the top of the column through a conduit 21 which has a condenser 22 therein. The resulting condensate is delivered to an accumulator 23. A first portion of the condensate is withdrawn through a conduit 24 as an overhead product stream. The flow through conduit 24 is regulated by a level controller 25 which adjusts a valve 26 to maintain a predetermined liquid level in accumulator 23. A second portion of the condensate is passed through a conduit 27 to the upper region of column 10 to fonn the external reflux stream. The rate of flow through conduit 27 is controlled by a valve 28 which is regulated by a flow controller 29. The input signal to controller 29 is obtained from an internal reflux computer 30. A flow transducer, such as an orifice meter 31, transmits a signal to computer 30. A first temperature sensitive resistance element 32 is positioned in thermal contact with vapors removed from the top of column 10 through conduit 21. A second temperature-sensitive resistance element 33 is positioned in thermal contact with the external reflux introduced into column 10. These two resistance elements are connected to computer 30. The apparatus thus far described, with the exception of the specific internal reflux computer 30 of this invention, constitutes a conventional control system for a fractionation column wherein the rate of flow of external reflux is adjusted in response to computation of the internal reflux in the column. This type of control is described in US. Pat. No. 3,018,229, for example.

The internal reflux computer of this invention is illustrated schematically in FIG. 2 A flow transducer 35 receives a signal from meter 31 and establishes an output signal R which is proportional to the rate of flow of external reflux through conduit 27. This signal is applied through input resistor 33 to the input of a summing amplifier 36. Resistor 32 is a feedback resistor of amplifier 36. The output of amplifier 36 is applied to the input of a summing device 37. Signal R, is attenuated by attenuator 47, and the output of attenuator 47 is the second input to summing device 37. The gain of amplifier 36 is such that the output signal therefrom is representative of the quantity p/ where K is a constant.

When this quantity is reinforced by adding a fraction of the input signal, R,( 1-K), from attenuator 47, the output R, becomes The gain of an operational amplifier is proportional to the ratio of the feedback resistor to the input resistor. Since the resistances of

elements

32 and 33 vary linearly with respect to the temperatures sensed by the elements, the gain of amplifier 36 varies linearly with respect to the differential temperature (AT) sensed by

elements

32 and 33. By further adjustment of the gain of the amplifier, the output is representative of e(( p/ as above.

A specific embodiment of the internal reflux computer of FIG. 2 is shown in the circuit diagram of FIG. 3. This embodiment is designed for use with milliampere signal systems. Other embodiments may be used with voltage signal systems.

In FIG. 3, element 38 is a transducer which establishes an output signal that is representative of the pressure differential across orifice 31 in conduit 27. As is well known, such a signal is proportional to the square of the rate of flow through the conduit. The output signal from transducer 38 is applied to the input of a network 39 which establishes an output signal representative of the square root of the input signal. The signal is thus proportional to the rate of flow of the external reflux. A diode function generator network can be employed as network 39, for example. The milliampere signal from network 39 flows through resistor 40, and the IR drop across this resistor is a voltage proportional to R The signal from network 39 also flows through the input terminals of transducer 41 and through a current limiting resistor 42. Transducer 41 has output isolated from input, and it attenuates the input signal so that the current output from this device produces an IR drop proportional to R,(l K) across adjustable resistor 45. Resistor 46 is a current limiting resistor. A positive voltage resulting from the IR drop across resistor 40 flows through temperature-sensitive resistance element 33 to the positive input of amplifier 36. Upon receiving this positive signal, the current output of amplifier 36 must increase until the IR drop across adjustable resistor 45 overcomes the IR drop produced by current flow from transducer 41 and also produces an additional 1R drop so that negative feedback through temperature-sensitive resistance element 32 exactly balances the amplifier input from element 33. In this particular embodiment, resistor 45 serves a dual function. The IR drop across this resistor produces an adjustable feedback voltage to amplifier 36. Also, since output currents from transducer 41 and amplifier 36 both flow through resistor 45, the resulting IR drop from these separate current sources represents a combination of these two output signals. Resistor 45 thus performs the function of the summing device 37 in FIG. 2. A capacitor 43 is connected as shown to stabilize amplifier 36. The output current from amplifier 36 is the computed internal reflux signal, and goes to controller 29.

in one specific embodiment of this invention, transducer 41 can be a Model SC-302 Signal Converter of Rochester instrument Systems, lnc., Rochester, NY. Amplifier 36 can be a Model 31 l-WM Transmitter of Acromag, incorporated, Wixom, Mich. Temperature-

sensitive resistance elements

32 and 33 can be 1,000 ohm resistance temperature detectors.

Resistors

40, 42, 45, and 46 can be 50, 270, 50, and 270 ohms respectively. Capacitor 43 can be 2 microfarads. In calibrating this specific embodiment, network 39 is adjusted for a true zero output, which is to say that zero flow through conduit 27 produces zero-current flow. Amplifier 36 is set for maximum gain, and is adjusted so that zero input produced midscale output. During calibration, the zero of transducer 41, the span of transducer 41, and resistor 45 are adjusted sequentially until the computer produces the correct output signal for all combinations of flow through conduit 27 and temperature differences between

elements

32 and 33.

The input signal to controller 29 is thus representative of the calculated internal reflux. Controller 29 compares this value with a desired set point signal and manipulates valve 28 to maintain the calculated value equal to the set point value. As an alternative, the computed internal reflux can be recorded to provide a visual indication of the internal reflux. An operator can manually adjust the rate of flow of external reflux in response to this recorded value.

The calibration of the apparatus of FIG. 3 is simplified if

elements

32 and 33 are selected so as to have approximately the same initial resistance and the same temperature coefficient of resistivity. If this is not possible, the resistances of each element can be adjusted by connecting small variable resistors in parallel with the temperature-sensitivity resistors. ln place of the differential pressure transducer 38 and the square root network 39, any other type of flow transducer can be employed which provides a reasonably linear output signal representative of the rate of flow through conduit 27.

While this invention has been described in conjunction with a presently preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

1. in a fractionation system wherein a feed mixture to be separated is introduced into a fractionation column, a vapor stream is removed from the top of the column, said vapor stream is cooled to condense at least a part thereof, and at least a part of the resulting condensate is returned to the column as external reflux; apparatus to compute the internal reflux in the column comprising:

means to measure the rate of flow of the external reflux and establish a first electrical signal representative thereof;

a first temperature-sensitive resistance element positioned to sense the temperature of vapor withdrawn from the top of the column;

a second temperature-sensitive resistance element positioned to sense the temperature of the external reflux returned to the column;

means applying said first signal to the input of said amplifier through said second resistor;

means connecting said first resistor between the output and the input of said amplifier as a feedback resistor; and

means to sum a fraction of said first signal and the output signal from said amplifier.

2. The apparatus of claim 1, further comprising means to adjust the gain of said amplifier so that the output signal from the amplifier is representative of the quantity R,((C,/)t)A T+K), where R is representative of said first signal, C, is the specific heat of the external reflux, A is the heat of vaporization of liquid in the column where the external reflux enters, AT is representative of the temperature difference between said first resistance element and said second resistance element,andK isaconstant.

3. The apparatus of claim 1, further comprising means responsive to said means to sum to regulate the rate of flow of external reflux to the column to tend to maintain the sum of said first signal and said output signal constant.

Claims

1. In a fractionation system wherein a feed mixture to be separated is introduced into a fractionation column, a vapor stream is removed from the top of the column, said vapor stream is cooled to condense at least a part thereof, and at least a part of the resulting condensate is returned to the column as external reflux; apparatus to compute the internal reflux in the column comprising: means to measure the rate of flow of the external reflux and establish a first electrical signal representative thereof; a first temperature-sensitive resistance element positioned to sense the temperature of vapor withdrawn from the top of the column; a second temperature-sensitive resistance element positioned to sense the temperature of the external reflux returned to the column; an amplifier; means applying said first signal to the input of said amplifier through said second resistor; means connecting said first resistor between the output and the input of said amplifier as a feedback resistor; and means to sum a fraction of said first signal and the output signal from said amplifier.

2. The apparatus of claim 1, further comprising means to adjust the gain of said amplifier so that the output signal from the amplifier is representative of the quantity Re((Cp/ lambda ) Delta T+ K), where Re is representative of said first signal, Cp is the specific heat of the external reflux, lambda is the heat of vaporization of liquid in the column where the external reflux enters, Delta T is representative of the temperature difference between said first resistance element and said second resistance element, and K is a constant.