CA1054553A - Method for controlling the reboiler section of a dual reboiler distillation column - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A control method and system for regulating the heat input to the reboiler section of a distillation column having attendant thereto a first external heat-recovery reboiler. Flow-measuring means, disposed within a substantially liquid-free en-viornment,produces a signal which is representative of the quan-tity of vapor flowing from the reboiler section upwardly into the fractionation section of the column. This signal is transmitted to heat-varying means whereby the heat input to a second external reboiler heater is regulated in response to the quantity of vapor flow.

Description

The control system, encompassed by the present in-ventive conc~pt, ls intended for integration into a fraction-ation, or distillation facility in which external reboiling of a portion of the liquid bottoms material is utilized to supply the heat input necessary to effect the desired sepa-ration of feed stock components. Processes, in both the petroleum and petrochemical industries, utilizing some form of fractionation, or distillation facility, are of a wide variety. In some situations, process design considerations dictate the incorporation of an external heat-recovery re-boiler into the distillation facility. Heat is supplied to the reboiler section of the distillation column by a circu-lating external process stream through the heat-recovery re-; boiler, whereby a portion of the liquid bottoms material is vaporized and returned, in mixed-phase, to the column. The temperature of the circulating process stream returned to other parts of the overall process, is generally fixed with-in relatively narrow limits. As a result of variations in the flow rate, or supply temperature, of the circulating process stream, the quantity of heat supplied to the heat-recovery reboiler necessarily fluctuates. Additionally, ex-., :
perience indicates that the quantity of heat so supplied is not sufficient for the total required distillation column duty. This heat èfficiency necessitates a second "column control" external reboiler heater to supply the additional heat input to the reboiler section of the distillation col-umn. ~-~
Both the heat-recovery reboiler heater, and the column control reboiler heater (which may be either direct- -~

- , . .. . . ~ . . . ..

f ired, or of the more common heat-exchange type), produce a heated, mixed-phase bottoms material which is re-introduced into the column via the reboiler section. Vapors pass up-wardly into the fractionation section while the liquid por-tion is ~enerally withdrawn from the distillation facilityin xesponse to a liquid-level control device. The quantity of vapors actually passing upwardly from the reboiler sec-tion has a direct, pronounced effect upon the ultimately achieved separation. Also, while many factors contribute to the thermal balance, or stability of the fractionation functLon, pernaps the most slgnificant is the effect pro- -duced by the heat input via the reboiling operation. My invention affords a method of controlling such heat input which results in greater stability with respect to both thermal balance and separation efficiency.
For the purpose of providing a clear understand-ing of the present control system, the definition of sever-al terms, as employed herein and in the appended claims, is believed to be warranted. The use of the term "distilla- -tion column" i5 intended to include "fractionation column", "re-run column", "finishing column", "splitter column", "ex-tractive distillation column", etc. Similarly, the "reboil-er section" alludes to that portion of the fractionation column below the lowermost tray, or deck; the "fractiona-tion section" connotes that portion of the column above the " ; ~

reboiler section, and is inclusive of the stripping zone(~elow the feed tray) and the rectification æone (above the feed tray). In short, the present control system enjoys ad-vanta~eous utility in facilities where separation of a feed stream is effected by way of boiling point differential.
Distillation facilities generally fall into one of two categories; the first being characterized by a re-boiler liquid bottoms material having a comparatively wide boiling range. The second category is characterized by a liquid bottoms material which is either a substantially pure compound, or a component mixture having a relatively narrow boiling range, say 10F. or less. Exemplary of pro-,. . .
cesses havlng distillation facilities, which can utilizethe present invention in an advantageous fashion, is cata-lytic reforming, wherein the normally liquid portion of the reaction product effluent is stabilized, or re-run to pro- -vide a motor fuel having a particularly desired boiling range. Another application, to which the present invention may be put, is the separation of ethylbenzene from a mix-ture thereof with various xylene isomers, or the separation of one particular isomer from the mixture. In the separa-tion of an aromatic concentrate from a mixture with non-aro-matics, via solvent extraction, the reboiler liquid bottoms material in the extractive distillation column constitutes -the solvent employed in admixture with the desired aromat-'~;

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ics. In these processes, as well as many others well known --in the art of petroleum and petrochemical operations, an ex-ternal process stream -- e.g. a reaction zone product efflu-ent -- circulates through a heat-recovery reboiler, vapori-zes at least a portion of the liquid bottoms material there-in, and thereby supplies at least a portion of the distilla-tion column heat requirement~ It is to this type of distil-lation technique that the present invention is specifically directed. The principal advantages concern improved stabil-ity with respect to the thermal balance of the column andan enhancement of separation efficiency in achieving the de-sired end product.

* * OBJECTS A~D EMBODIME~TS * *
A principal obje~t of my invention is to provide a control method and system for regulating the heat input to the reboiler section of a distillation column. A corol-lary objective affords a measurement of the actual quantity of vaporous material passing upwardly into the fractiona-tion section of a distillation column from the reboiler sec-tion thereof.
In a dual-reboiler distillation facility, a speci-fic object involves regulating the heat input to one of the external reboilers in response to the total vapor flow into the fractionation section. Ultimate objectives include im-_5_ -,. , , , ... , ~ - .. .. , . ~ . , . :

proved stability in the overall thermal balance of the col-umn, and an enhancement in the steady-state character of the separation e~ficiency.
These objects are achieved by providing a method for controlling heat input to the reboiler section of a dis-tillation column having two external heat-recovery reboiler heaters appurtenant thereto, which method comprises the steps of: (a) withdrawing a liquid bottoms stream from said reboiler section; (b) regulating the quantity of a first portion of said bottoms stream removed from said dis-tillation column as a bottoms product, in response to the level of liquid in said reboiler section; (c) introducing a second portion of said bottoms stream into a first of said external heaters, and therein heating and partially vaporiz-: 15 ing said second portion via indirect contact therein with ~ an external process stream; (d) passing the heated, mixed-; phase second portion of said bottoms stream into a substan-tially liquid-free area of said rcboiler section, and there- -in disengaging liquid from said heated mixed-phase; (e) in- :
troducing a third portion of said bottoms stream into the second of said external heaters, and therein heating and partially vaporizing said third portion; (f) passing the heated, mixed-phase third portion of said bottoms stream into said substantially liquid-free area of the reboiler.
section, and therein disengaging liquid therefrom; ~g) mea-- . - ., , ~

suring, within said reboiler section, the quantity of vapor passing upwardly from the reboiler section into the frac-tionation section of said distiilation column, and, th) regulating the heat input to said second external reboiler S heater in response to a signal representative of the mea- .
sured quantity of vapor passing into said fractionation sec~
tion, thereby regulating the heat input to said reboiler . .
section.
In another specific embodiment, my invention is directed toward a control system for regulating heat input to the reboiler section of a distillation column having a ;
first heat-recovery, external reboiler heater appurtenant thereto, said heater having feed input means through which heat is supplied by a circulating process stream, which con-15 trol system comprises, in cooperative combination: (a) a receiving chamber for liquid bottoms material in sai~ re- ~ :
. boiler section, means to circulate a first portion thereof through said first heater, whereby said first portion is heated and partially vaporized therein; ~b) a second exter-nal reboiler heater having feed input means thereto and means to heat said second heater; (c) means to circulate a second portion of said bottoms material through said second heater, whereby said second portion is heated and partially vaporized therein; (d) heat-varying means for adjusting the heat input to said second reboiler heater; (e) conduit means , ''.

~ _7_ . .

for introducing the partially vaporized first and second portions of said bottoms material from said reboiler heat-ers into said reboiler section; (f) flow-measuring means in-ternally disposed within said reboiler section and respon-sive to the quantity of vapor flowing upwardly from said re-boiler section into the fractionation section of said dis-tillation column; (g) signal-receiving means in communica-tion with said rlow-measuring means for sensing and indica-ting a signal representative of the quantity of vapor pass-ing into said fractionation section, said signal-receiving means being in communication with said heat-varying means to transmit said signal to said heat-varying means, whereby the heat input to said second heater is adjusted in response to the quantity of flowing vapors; and, (h) flow-regulating lS means for withdrawing excess liquid bottoms material from said reboiler section and out of said distillation column.
In another embodiment, the control system is fur-ther characterized in that a level-sensing means receives a signal representative of the level of li~uid in said re-boiler section and transmits said signal to said flow-regu-lating means, whereby the withdrawal of liquid out of said distillation column is adjusted in response to said liquid level. In a specific embodiment, the liquid-sensing means maintains the level of liquid bottoms in said reboiler sec-tion out of contact with said flow-measuring means.

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These, as well as other objects and embodiments will become evident, by those possessing the requislte ex-pertise in the art, from the following, more detailed de-scription. In further describing my invention, reference will be made to the accompanying diagrammatic illustration which is presented to aid in the clear understanding thereof.
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* * SUMMARY OF I~VE~TIO~ * *
._ .
As hereinbefore set forth, many aspects of the distillation technique constitute contributing factors with respect to the thermal balance and separation efficiency.
In addition to the heat input to the reboiler section by - way of the return of heated, mixed-phase bottoms material, such factors include the rate and temperature or the reflux ; stream; the rate, temperature and composition of the feed stream; and, the various locations of the reflux and feed trays. Th~ effect, however, of the reboiling operation ap-pears to be the more pronounced. Heat input, by way of the - mixed-phase stream return from the external reboiler heater, takes two forms: (1) the sensible heat of the liquid; and,

(2) the latent heat absorbed by the vapors during vaporiza-; tion. Of these two, the greater proportion of heat input is attributed to the latent heat of vaporization. Whether considering a comparatively wide boiling range bottoms ma-terial, a narrow boiling range liquid or a substantially _g_ ' ' . . -lOS4553 pure compound, control of the reboiler section heat input is tantamount to a successful, efficient operation.
Economic considerations in the design of various petroleum and petrochemical processes dictate the utiliza-tion of heat-recovery circuits in order to minimize the overall process utility cost. Therefore, a great many of these processes, having distillation or fractionation fa-cilities integrated therein, will employ a heat-recovery re-boiler which utilizes, as the heating medium, a hotter, ex-ternal process stream. This heat-recovery reboiler gener-ally takes the form of the well known shell and tube heat exchanger. A portion of the liquid bottoms material is withdrawn from the reboiler section of the column, and is introduced into one side of the heat exchanger. The cooled, external process stream is returned to its intended destina-tion within the process. Various other design considera-tions further dictate that the temperature of the external process stream, as it emanates from the heat-recovery re-boiler heater be carefully controlled. Furthermore, due to a varying flow rate, or supply temperature of the circula-ting process stream, the quantity of heat supplied to the reboiler section through this heat-reoovery reboiler wil~
not be constant. It might be added that this particular quantity of heat is also generally insufficient for the to-tal required column duty.

This necessitates the utilization of a second ex-ternal reboiler heater. Although the second reboiler heat-er may be of the direct-fired type, it is, in most instan-ces economically preferable to utilize a second shell and tube heat exchanger, again utilizing a hotter external stream. Heretofore, the quantity of heat supplied by the second reboiler heater would have been maintained by con-trolling the amount of fuel, or heat exchange medium in re-sponse to the temperature of all the heated, mixed-phase bottoms material being returned to the reboiler section.
However, as hereinafter indicated, this controlled tech-nique does not solve the problem of fluctuating heat input to the reboiler section of the distillation column.
Correlations of heat content versus temperature, at varying percentages of vaporization will indicate a rela-tively large temperature differential (delta-T) per unit of heat content, when such correlations are directed toward liquid bottoms material having a comparatively wide boiling range. Therefore, a change in the temperature of the heat- -ed material being returned to the reboiler section can be employed to ad~ust the heat input, thus maintaining some semblance of thermal balance. Where, however, the liquid -~
bottoms material is a substantially pure compound, or a com-ponent mixture having a narrow boiling range -- i.e. 10F., or less -- the-correlations show that very little, if any~

-11- .
'-, , . 1 delta-T is available for vaporization control. That is, the temperature remains virtually the same regardless of the percentage vaporization. In this situation, regulating the heat input by way of temperature measurement of the heated, mixed-phase material serves no useful purpose. It is recognized that, regardless of the character of the re-boiler section bottoms liquid, the appropriate published literature is replete with a multitude of illustrations of control systems designed to maintain either thermal balance, or separation efficiency. No attempt will be made herein to delineate exhaustively the various schemes and techniques. ~ -It will suffice to present a few of the more prevalent exam-ples.

; * * PRIOR ART * *
As hereinabove set forth, temperature control any-where in the reboiler heater circuit falls short of achiev-ing the desired end result. Similarly, a measurement of the flow of mixed-phase material in the return conduit is meaningless since the sensing means does not readily distin-guish between liguid and vapor, and highly inaccurate mea-surements result. Some methods have been proposed which re-ly upon a temperature measurement within the reboiler sec-tion; however, as above-stated, this does not provide an accurate picture of the degree of vaporiæation which has ., ~.. .

lOS4553 been effected in the reboiler heater, and how much vapor flows upwardly from the reboiler section to the fractiona-tion section. These shortcomings are avoided through the utilization of the present invention, the essence of which is the measurement of the quantity of vapor actually pass-ing upwardly from the reboiler section into the fractiona-tion section of the distillation column. A signal, repre-sentative of the vapor flow is appropriately received and transmitted to heat-varying means in the heat exchange me-dium line, or the fuel line, to the reboiler heater. Theinternal configuration of the reboiler section is such that all the vapor passing into the fractionation section passes through th~ flow-measuring means which is disposed in a va-..
por chimney. Furthermore, the measurement of vapor flow is effected in a substantially liquid-free environment. The flow-measuring means is most conveniently either a venturi, or orifice plate, and is disposed within its own vapor ri-ser. A horizontally-disposed, imperforate baffle is loca-ted below the lowermost tray and above the vapor riser con-` 20 taining the flow-measuring means. Thus, liquid flowing ; downwardly from the lowermost tray, into the reboiler sec-tion, is prohibited from entering the vapor riser contain-ing the flow-measuring means. ` Similarly, the level of liq-uid within the reboiler section is maintained out of con-tact with the flow-measuring means. The control system of . .
. , .

~054553 the present invention affords, therefore, the measurement o vapor flow in a substantially liquid-free environment, thereby enhancing the stability of thermal balance as well as separation efficiency.
~ypical of the prior art technique directed to-ward a distillation column having two external reboiler heaters is that found in United States Patent No. 3,309,288 (Cl. 203-1). Here, however, the heat input to one of the reboiler heaters is controlled in response to the level of liquid within the reboiler section, and there exists no rec-ognition of the technique which measures internally the quantity of vapors flowing into the fractionation section from the reboiler section. Furthermore, the present method ~
of control involves regulating the removal of excess reboil- -er liquid bottoms material, from the distillation facility, in response to the liquid level.
3xemplary of other prior art distillation tech-niques are those found in United States Patent No. 3,411,308 (Cl. 62-21) and United States Patent ~o. 3,225,550 ~Cl. 62-~ .
21), which are very ~imilar, and both of which involve frac- -: . .
; tional distillation wherein a portion of the liquid bottoms material is withdrawn and introduced into either an exter-nal reboiler heater, or heat-exchanger, with the heated ma-,...................................................................... . .
- terial being re-introduced into the reboiler section. How-2S ever, in both instances, the Patent~es return a stream . .

... .
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., .

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which is 100-/o vapor, in contrast to a mixed-phase stream.
Were this not the situation, the constant presence of liq-uid, or slugs thereof stemming from a malfunction, would cause the flow-measuring devices utilized by the Patentees, external to the reboiler section, to give false and erra~ic readings, thus rendering their control systems inoperative.
This is precisely the kinds of egregious situations which are avoided through the use of my invention wherein the va-por-measuring device is physically located within the re-boiler section and vapor measurement is effected in a liq-uid-free environment. ~either of the Patentees recognize this technique, nor is the same to be found ir other seg-ments of the prior art. The present invention is a modifi- -~; cation of the re~oiler control system and reboilex section as found in my United States Patents ~o. 3,881,994 (Cl. 202-160) and ~o. 3,888,743 ~Cl. 202-158~.

* * DESCRIPTIO~ OF DR~WING * *
--The accompanying diagrammatic illustration is pre-sented for the sole purpose of affording a clear understand-ing of the control method and system encompassed by thepresent invention. It is not, therefore, considered to have a limiting effect upon the scope and spirit of the present invention as defined by the appended claims. The drawing will be described in conjunction with a commercial-ly-scaled adsorption system designed to recover paraxylene from a mixture thereof with other aromatic hydrocarbons.
The process is designed to recover a paraxylene-rich prod-uct having a purity greater than about 99.0% by volume from a feed stock being charged to the unit at a rate of about 53,700 Bbl./day. The composition of the aromatic feed stream to the process, in moles per hour, is presented in the following Table I:

* * * * * * * * *
TABLE I: Charge Stock Composition ;~
comPonent Mols/Hr.

Toluene 85.93 P plus ~ * 356.99 Ethylbenzene 812.35 p-Xylene 1076.82 m-Xylene 2783.65 o-Xylene 1236.56 Heavy Aromatics 2.60 * Paraffins plus ~aphthenes * * * * * * * * *
'~
Processes designed for the selective recovery of a particular hydrocarbon from a mixture thereof with its isomers and/or with other classes of hydrocarbons, and ~ which make use of a solid-bed adsorbent, are thoroughly de-; scribed in the published literature. In view of the fact that the solid adsorption process is herein mentioned sole-: -.

ly for illustration purposes, as one of the multitud~ of processes which are ~nhanced through the use of the present invention, it is not necessary to discuss the same in great detail. Descriptions of several such processes may be found in U. S. Patent No. 2,985,589 (Cl. 210-34), U. S.
Patent No. 3,715,409 (Cl. 260-674 SA), U. S. Patent No.

3,734,974 (Cl. 260-674 SA) and U. S. Patent No. 3,558,732.
Briefly, the adsorption process is carried out ~y initially contacting the hydrocarbon feed stream with a solid bed of ion-exchanged crystalline aluminosilicate adsorbent selec-ted from Type X and/or Type Y zeoliti~ material. Th~ de-sired component is adsorbed, and a raffinate is withdrawn from the adsorbent bed. Contact is then made with a suit-able desorbeni which removes the desired component as an extract stream. Since both the extract and the raffinate streams contain desorbent material, they are separately pro-cessed in individual columns to recover the streams substan-tially free from desorbent.
The overhead stream from the extract column con-stitutes the charge stock to fractionation column 1 in the accompanying drawing. A portion of the recovered desorbent in an amount of about 10,325.7 mols/hr., at a temperature of about 369P., is used as the heat exchange medium for heat exchanger 32, the first external heat recovery reboil-er. The temperature of the return stream in line 37, must 1~54553 ~-be controlled at a level of about 350F. A portion of the liquid bottoms material from an associated process column, in an amount of about 1,829.2 mols/hr., at a temperature of about 417F. is employed as the heat exchange medium to heat exchanger 22, the second external "column control" re-boiler heater. The charge stock to fractionation column 1, referred to in the adsorption art as a "finishing" column, introduced by way of inlet port 9 and line 10, has the com-position shown in the following Table II:

* * * * * * * * * ~ ' .
TABLE II: Feed to Finishinq Column Component Mols/Hr.
Toluene 21.58 P plus ~
Ethylbenzene 2.07 p-Xylene 990.67 m-Xylene 2.06 o-Xylene 0.70 * * * * * * * * ~ ,.

Referring now to the drawing, finishing column 1 is shown as having a fractionation section 2 and a reboiler section 3. With respect to the latter, it will be recog-nized, by those skilled in the art, that reboiler section 3 is a "thermosiphon" reboiler, as distinguished from a "once-through" reboiler. An overhead vaporous stream is withdrawn by way of outlet port 4 and line 5. This vapor--" .

-. ........ . : .. - ....... . : , . .

ous fraction containing ~ubstantially all the toluene in the feed and only a trace of p-xylene, is at a tempexature of about 251F.; this material is condensed to a tempera-ture of about 150F., and about 1,261.1 mols/hr. are re-turned to column 1 as reflux via inlet port 6 and line 7.
Column 1 will contain from about 20 to about 200 perfora-ted trays, or decks; in the present illustration, the fin-ishing column contains 60 trays which are alternately dis-posed from uppermost tray 8 to lowermost tray 11. It is understood that the precise number of decks 8 and 11 is not essential to the present inventive concept. A liquid bot-toms portion is withdrawn via outlet port 25 and line 26, at a rate of about 6,479.3 mols/hr. and a temperature of about 326F. Of this amount, 3,319.2 mols/hr. is diverted li via line 31 into heat exchanger 32, wherein the heat con-tent, in terms of 10 BTU/hr. is increased from 42.18 to 58.39, as a result of the return, through line 33 and inle~
port 34, of 1,106.4 mols/hr. of vapor (about 33.3% vaporiza-tion). The hot desorbent in line 35, passes through three way valve 36 in an amount of 7,123.0 mols/hr. The remain-ing 3,202.7 mols/hr. are by-passed through line ~0 into line 37. Temperature Recorder Controller (TRC) 38 senses the temperature of the desorbent in line 37 and transmits the appropriate signal to valve 36 via instrument line 39.
As either the supply temperature, or the rate of desorbent ' ' .~'. :' 1054553 ~:

in line 35 varies, TRC 38 ad~usts valve 36 to the extent necessary to regulate the flow through by-pass 40 in order to maintain the temperature of desorbent in line 37 at 350F.
Thus, the heat content of the mixed-phase bottoms material in line 33 fluctuates.
Another portion of the bottoms material in line 26, 2,164.6 mols/hr., is diverted via line 28 into column control reboiler 22. The remaining quantity, 995.5 mols/
hr. continues through line 26, containing control valve 27, -and is withdrawn as the product stream of the process. A
component analysis of the product stream is presented in - . - . - .
the following Table III:

* * * * * * * * * , ... .......
TABLE III: Paraxylene Product Analvsis Component Mols/Hr.

Toluene P plus N
Ethyl~enzene 2.07 p-Xylene 990.67 m-Xylene 2.06 o-Xylene 0.70 Heavy Aromatics * * * * * * * * * ' . .

.. . .
The heat content of the 2,164.6 mols/hr. of liq-uid bottoms material in line 28 is about 27.51 x 10 BTU/hr. ~
Heating medium from an external source, at a temperature of -~ -.
, , :

~',-'.

417~. and a rate of 1,829.2 mols/hr., is introduced via line 21 into heat exchanger 22. This is sufficient to raise the heat content o~ the bottoms material returning via line 29 and inlet port 30, to about 38.09 x 10 BTU/hr.
(resulting from about 52.1% vaporization in heat exchanger 22). The cooled heating medium is returned, via line 23, containing control valve 24, to its source.
The internal flow-measuring means is shown, in this illustration, as an orifice plate 14, internally dis-posed in vapor riser 15. Vertical baffle 16 and horizontal plate 17 divide the reboiler section so that vapor-liquid mixture return from the reboilers does not contact the liq-uid descending from lowermost tray 11. ~n imperforate tray 12 is located between lowermost tray 11 and vapor riser 15.
Thus, all the liquid material flowing downwardly from tray 11 must seek a pathway to the reboiler section other than the vapor riser itself. Therefore, the vapor riser and ori-fice plate 14 are disposed in a substantially liquid-free environment. The level of liquid bottoms material is main-tained out of contact with orifice plate 14 by Level Indi- -cating Controller (LIC) 41 which functions through the use ~
of column taps 42 and 43. The appropriate signal is trans- ~-mitted from LIC 41, via instrument line 44, to control valve 27 which opens or closes further to regulate the rate -~
of bottoms removal from the column through line 26.

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lOS4553 ~ ~
~ rifice plate ]4 is normal:Ly designed to create a pressure drop of about 20 in. E120, when the desired rate of vapor flows through riser 15 into the fractionation sec-tion of column 1. This pressure difference, representative of the rate of flow of this vapor is received, via conduit 18, by FRC 19. FRC 19 creates a signal that is transmitted to control valve 24 via instrument line 20, and the heat in-: put to reboiler heater 22 is adjusted in response to the rate of vapor flow.

* * ILLUSTRP.TIVE EXAMPhE * *
In presenting this example of the functioning ofthe present dual-reboiler vaporization control system, it will be presumed that column 1 has attained steady-state op-eration and thermal balance while operating in accordance with the values of the variables hereinabove set forth in the description of the drawing. For whatever reason, it will be presumed that the supply temperature of the desor-bent in line 35 decreases, and to the extent that the tem-perature of the returned desorbent in line 37 drops below 350F. TRC 38 will transmit the required signal to three~
way valve 36 which will, in turn, effect an increase in the flow through by-pass 40. The net result is a decrease in the percentage vaporization effected in heat exchanger 32.
~he lowering of the total vapor flow rate in riser 15 will -22- ~

`, .

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result in a lo~er pressure drop across orifice plate 14.
FRC 19 ~lill, in turn, transmit a different signal via in-strument line 20 to control valve 24. The latter will open further, and the heat input to column control reboiler 22 thus increased by way of increased flow of heating medium, in line 21. A greater percentage of vaporization of the bottoms material in line 28 is effected, there is increased vapor flow in vapor riser 15, and the column once again at-tains thermal balance and a stable separation efficiency.

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Claims (9)

I CLAIM AS MY INVENTION:

1. A method for controlling heat input to the reboil-er section of a distillation column having two external heat-recovery reboiler heaters appurtenant thereto, which method comprises the steps of:
(a) withdrawing a liquid bottoms stream from said reboiler section;
(b) regulating the quantity of a first portion of said bottoms stream removed from said distillation column as a bottoms product, in response to the level of liquid in said reboiler section;
(c) introducing a second portion of said bottoms stream into a first of said external heaters, and therein heating and partially vaporizing said second portion via in-direct contact therein with an external process stream;
(d) passing the heated, mixed-phase second por-tion of said bottoms stream into a substantially liquid-free area of said reboiler section, and therein disengaging liquid from said heated mixed-phase;
(e) introducing a third portion of said bottoms stream into the second of said external heaters, and there-in heating and partially vaporizing said third portion;
(f) passing the heated, mixed-phase third portion of said bottoms stream into said substantially liquid-free area of the reboiler section, and therein disengaging liq-uid therefrom;
(g) measuring, within said reboiler section, the quantity of vapor passing upwardly from the reboiler sec-tion into the fractionation section of said distillation column; and, (h) regulating the heat input to said second ex-ternal reboiler heater in response to a signal representa-tive of the measured quantity of vapor passing into said fractionation section, thereby regulating the heat input to said reboiler section.

2. The method of Claim 1 further characterized in that the quantity of vapor passing into said fractionation section is measured in a substantially liquid-free environ-ment.

3. The method of Claim 1 further characterized in that the heat input to said second external heater is re-duced in response to increased vapor flow and vice versa.

4. A control system for regulating heat input to the reboiler section of a distillation column having a first heat-recovery, external reboiler heater appurtenant thereto, said heater having feed input means through which heat is supplied by a circulating process stream, which control sys-tem comprises, in cooperative combination:
(a) a receiving chamber for liquid bottoms mate-rial in said reboiler section, means to circulate a first portion thereof through said first heater, whereby said first portion is heated and partially vaporized therein;
(b) a second external reboiler heater having feed input means thereto and means to heat said second heater;
(c) means to circulate a second portion of said bottoms material through said second heater, whereby said second portion is heated and partially vaporized therein;
(d) heat-varying means for adjusting the heat in-put to said second reboiler heater;
(e) conduit means for introducing the partially vaporized first and second portions of said bottoms materi-al from said reboiler heaters into said reboiler section;
(f) flow-measuring means internally disposed with-in said reboiler section and responsive to the quantity of vapor flowing upwardly from said reboiler section into the fractionation section of said distillation column;
(g) signal-receiving means in communication with said flow-measuring means for sensing and indicating a sig-nal representative of the quantity of vapor passing into said fractionation section, said signal-receiving means be-ing in communication with said heat-varying means to trans-mit said signal to said heat-varying means, whereby the heat input to said second heater is adjusted in response to the quantity of flowing vapors; and, (h) flow-regulating means for withdrawing excess liquid bottoms material from said reboiler section and out of said distillation column.

5. The control system of Claim 4 further character-ized in that level-sensing means receives a signal repre-sentative of the level of liquid in said reboiler section and transmits said signal to said flow-regulating means, whereby the withdrawal of liquid out of said distillation column is adjusted in response to said liquid level.

6. The control system of Claim 5 further character-ized in that said level-sensing means maintains the level of liquid bottoms in said reboiler section out of contact with said flow-measuring means.

7. The control system of Claim 4 further character-ized in that said flow-measuring means is a venturi.

8. The control system of Claim 4 further character-ized in that said flow-measuring means is an orifice plate.

9. The control system of Claim 4 further character-ized in that said flow-measuring means is disposed within said reboiler section in a substantially liquid-free envi-ronment.