US3176756A - Control system for multiple-effect evaporators - Google Patents

Description

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April 6, 1965 s. G. DUKELOW I' CONTROL SYSTEM FOR MULTIPLE-EFFT EVAPORAT Filed Sept. 23. 1960 SAMUEL G. DUKELOW A ORNEY United States Patent O 3,176,756 CONTROL SYSTEM FR MULTlPLE-EFFECT EVAPRATORS Samuel G. Dukelow, San Mateo, Calif., assigner to Bailey Meter Company, a corporation of Delaware Filed Sept. 23, 1960, Ser. No. 53,107 9 Claims. (Cl. 159-44) My invention relates to a control system for multipleeffeot evaporators used in the pulp industry for the concentration of thin black liquor to thick black liquor.

As Well known, if the black liquor was discarded from the process after being washed from the pulp it would be necessary to replace all of the chemicals charged to the digester and the cost of pulp would Ibe prohibitive. Hence, economy dictates the recovery of the chemicals as completely as possible. The first step in the recovery process is the concentration of the thin, or weak as it is sometimes called, fblack liquor to heavy lblack liquor suitable for further processing in a direct contact evaporator and recovery furnace.

Liquor received from the washers usually contains from 12 to `15% solids consisting of spent chemicals and the non-cellulose constituents of the Wood. After concentration in the evaporator system the solids content is ordinarily in the order of 45 to 55%. Essentially the Concentration process in the evaporator system consists in boiling oif a considerable portion of the water found inthe Weak liquor. While such boiling oil can be accomplished in a single evaporator economy dictates the use of several evaporators arranged in series or cascade as it may be called. With this arrangement live stream as theheating medium is admitted to the rst evaporator in the series and the vapor generated in this evaporator then becomes the steam supply for the next evaporator and so on. The thin black liquor, having the lowest boiling point, is fed to the last evaporator in the series and passes in contra flow relation to the steam through the system.` Insuch a multiple-effect arrangement one pound of steam admitted to the first evaporator will evaporate approximately one pound of water and the pound of vapor so formed will evaporate approximately one pound of water in the second evaporator and so on down the line resulting` in a material increase in steam economy over a` single evaporator arrangement.

As evaporators or effects are added a point is reached where a Working temperature difference between liquor and vapor ceases to exist and the liquor in the evaporator in place of boiling merely simmers. For this reason the effects or evaporators are usually limited to between and 7.` In contemporary practice saturated steam is admitted to the lirst efecft at about 15 to 20 p.s.i.g. and the vapor formed in the last effect dischargesto a condenser operating at about 25 to 27" of mercury vacuum; the exact vacuum depending upon the temperature and quantity of cooling water available. p

While the 'multiple-effect evaporator system has materially increased the economy of chemical recovery in the pulp industry it has presented a difficult control problem because of the long time constants and small storage capacities involved. Without proper control, changes in liquor feed iiow, solids concentration, steam conditions, cleanliness of heat exchanger surfaces and the like result in an output of black liquor of non-uniform concentration. Furthermore because of the long time constants, an evaporator system once upset, may require lice hoursV of trial and error adjustment Ibefore it is again stabilized.

ln an application tiled in the United States Patent Oilice on August 17, 1960, Serial No. 50,269, George L. Harman discloses a control system for a multiple-effect evaporator. In the Harman system, steam demand is determined in part from the rate of ilow of thin liquor to the lastefect. Such a control system is satisfactory for a straight through put system. When, however, the how of liquor is detoured through an auxiliary process as by being temporarily detained in a soap separation tank a fixed determination of steam demand from thin liquor flow may result in a non-uniform heavy black liquor. The Harman control in correcting for this nonuniformity may upset the entire evaporator system which may necessitate the system being taken off Automatic and as heretofore mentioned require hours of trial and error adjustment on the part of the operators before stability is restored. My invention is particularly directed to a control `system which compensates for the introduction oi auxiliary processes in multiple-effect evaporators used in the concentration of thin black liquor to heavy black liquor.

In the drawing: y

FIG. 1 shows in diagrammatic form my invention applied to a typical multiple-eifect evaporator system incorporating an auxiliary process. t

Referring to FIG. l I show,.for purposes of illustration, a multiple-elfect system consisting of evaportors L5. inclusive. Thin black liquor is lfed to evaporators 4 and 5 in parallel from a storage tank 6 by means of a pump 7. I have shown thin liquor fed to evaporators 4 and 5 in parallel as this represents a practicefrequently used.

The effluent from evaporator 5 is discharged through a pipe S and forced into evaporator 3 through pipe 9 by pump lil. The effluent from evaporator 4 is likewise fed to evaporator` 3 through pipe 9 by means of a pump 11. The liquor after passing through evaporator 3 is diverted through a soap separation tank 6l) for the intermittent or continuous removal of tall oil. This separation is essentially a settling process where certain ingredients in the liquor found particularly in the so-called Kraft process and known collectively as tall -oil are removed.` `After leaving the tank oil, the liquor is fed by a. pump 12 to evaporator 2, thence fed by a pump 13 .to evaporator 1 whence it is discharged through pipe i4 to a heavy black liquor storage tank l5.

Steam as a heating medium is supplied evaporator 1 through a pipe i6 from any suita'ble source such as the extraction stage of a turbine, pressure reducing valve or the like (not shown) and preferably at constant pressure. As heretofore noted, in contemporary practice steam is supplied the system at about l5 to 20 p.s.i.g. saturated. The condensate of the steam supplied evaporator l is discharged through a pipe 17. This condensate may be returned to a steam generator or wasted.

As well known in :the art the decision =to return or waste is frequently automatically determined from a conductivity `sensitive control, an increase in conductivity indicating contamination of the condensate by the liquor rendering it unfit for recycling through .the steam generator.

The heat of vaporization of the steam fed to evaporator 1 is transferred to the liquor passing therethrough causing the liquor to boil. The vapor so formed is discharged through a pipe 1S to evaporator 2 wherein it forms the snc/aree heating medium for boiling the liquor passing therethrough. This process is repeated in evaporators 3, 4 and 5, the vapor generated in evaporator 2 discharging through a pipe lie -to evaporator 3, the vapor generated therein discharging through a pipe Ztl to evaporator 4, the vapor generated therein discharging through a pipe Z1. to evaporator 5. The vapor discharged from evaporator 5 passes to a condenser 22 supplied with water through a pipe 23. Specific mill conditions determine the type of condenser used, that is, whether it is a jet, or barometric as it is sometimes called, or a surface type.

The condensate from the effects 2, 3, 4 and 5 is dis- V charged through pipes 2d, Z5, 26 and Z7 respectively.

rl'his condensate may be wasted or reused depending upon mill conditions, due care being taken in regard to contamination by the liquor as outlined with respect to evaporator `ll. In some systems the condensate from one effect is flashed to the next succeeding effect improving the economy of operation.

I have purposely illustrated a simplified multiple-effect evaporator system to provide an adequate basis for the description of my invention to follow. As will be apparent to those skilled in the art, ordinarily auxiliary devices are incorporated in a multiple-effect evaporator system such as catchalls in the vapor outiiow pipe from each evaporator which serve to refine the separation of vapor from liquor, condensate traps and the like. As such auxiiiary devices are not pertinent to an understanding of my invention such have not been illustrated.

The control embodying my invention as shown in FIG. 1 is of the type commonly known as, pneumatically operated Vfor the reason that compressed air is used as the operating medium. However, it will be apparent as the description proceeds that my invention may as readily be incorporated in an electric or hydraulic control. i have chosen to show a pneumatically operated control for the reason that the components making up the system are well known in the art and their operation readily understood.

In accordance with my invention the rate of steam flow 1 the to first effect is maintained in desired proportion to the rate of liquor fed the evaporator following the auxiliary process, the proportionality being temporarily modified in accordance with changes in the` total rate of flow of virgin liquor to the system. I have found that such a control maintains the evaporator system stable notwithstanding changes in the virgin feed rate or by the inclusion of an auxiliary process such as the soap separation process. Further, I have found that such a control system produces a uniform heavy black liquor, other factors remaining constant, however, my invention further comprehends automatically readjusting the desired proportionality between liquor flow and steam flow to maintain a predetermined specific gravity of the heavy black liquor thus correcting for the effect changes in such other factors might otherwise have.

There' is shown in the drawing a constant flow control of the thin liquor feed to the evaporator system with automatic ratioing of the feed among the evaporators, as thin liquor is fed to more than one evaporator. Flow transmitter 2S establishes a pneumatic loading pressure proportional to the total feed of the thin liquor to the system. I have shown this iiow transmitter and other transmitters used in my control schematically as such transmitters may be of any one of several types available. The loading pressure established by transmitter 2S, proportional to the total feed of thin liquor to the system is introdced through pipe 28A into the B chamber of a relay 29 having proportional plus reset action. The relay 29 may, for example, be of the type illustrated and described in United States Patent 2,805,678 issued to Michael Panich on September 10, 1957. I have further shown the mechanism of a similar relay 4t?, to which reference will be made later, in diagrammatic form.

into chamber A of relay Z9 is introduced a manually adjustable loading pressure which establishes Y the set point of the iow control, that is, establishes the demand rate of virgin feed to the evaporator system. When the loading pressures introduced into chambers A and B are equal or stand in predetermined relation to each other the actual rate of feed will be equal to the demand rate of feed. A convenient means of adjusting the set point loading pressure may be provided by a selector station such as shown at 36 being of the type illustrated and described in United States Patent 2,747,595 issued to P. S. Dickey on May Z9, 1956.

The output pressure at D of relay 29 changes in proportion to changes in the difference between the loading pressures in chamber A and B and by virtue of the res ricted connection to chamber C continues to change at a slow rate so long as the pressures in chambers Arand B are unequal or depart from a predetermined relation. Thus the relay 7,59 is spoken of in the art as a proportional plus reset type in that it serves to cause immediate changes proportional to the amount of departure from a desired value and thereafter a continuing change until the actual value is restored to the desired value.

Tie output pressure at D of relay Z9 after passing through selector station 3@ is transmitted through a pipe 31 to a diaphragm operated valve S2 regulating the rate of thin liquor feed to evaporator S.

The loading pressure established by the transmitter 25 is, as shown, also transmitted through a pipe 23B to the chamber A of a relay similar to the relay 29. Into the chamber E of this relay is introduced a loading pressure proportional to the rate of flow of thin liquor to evaporator generated by a iiow transmitter 34. With respect to relay 33 the loading pressure generated by iiow transmitter 23 may be considered as establishing the set point of the constant flow control regulating the liquor feed to evaporator 4, as the output pressure established at D of relay 35, transmitted through selector station 35, is effective for positioning a diaphragm operated valve For any total rate of liquor feed, the ratio betweeri the rates of feed to evaporators 4 and 5 may be adjusted by means of a relay 36A and a manual loader 37. The relay 36A is in general similar to relay 29 but is provided with a bellows 38 for remote adjustment of the proportional band as illustrated and described more particularly in United States Patent 2,743,710 issued to Jack F. Shannon on May 1, 1956. I have further shown the mechanism of a similar relay 49, to which reference will be made later, in diagrammatic form. More particularly the ratio between the input pressure at A'and the output pressure at D of relay 36A is varied in accordance with the loading pressure in bellows 38 established by manual loader 37. Thus it will be evident that a one to one ratio between the flows to evaporators 4 and 5 may be established or any other desired ratio established by adjusting the proportionality between the input and output pressures of'relay 36A.

The selector stations 30 and 3S serve primarily as a means of readily transferring the constant flow control I have described from Automatic to Remote Manual or Hand control as it is frequently called. Each selector station is provided with a hand adjustable knob which when the control is on Remote Manual establishes the loading pressure for positioning the final control element which with respect to selector station 30 is the valve 32 and with respect to the selector station 35 is the valve 36.

ln accordance with my invention the rate of steam flow to the first effect is maintained in desired proportion to the rate of flow of liquor to the evaporator following the auxiliary process. Thus in the embodiment of my invention shown in the drawing the rate of steam liow is maintained in proportion to the rate of flow of liquor to evaporator 2. I have found that this liquor flow more nearly establishes the rate `of steam ilow required to produce a uniform heavy blaekliquor than does the rate of ow of thin liquorto the last etect.

In thel drawing I show"`a constant iow control of the` steam flow, the set point of which is adjusted from liquor flow to evaporator 2. A loading pressure proi portional to the liquor llow to evaporator 2 established `by a ow `transmitter 61 is normally relayed through a relay 62 into the A chamber of relay 40 which as previously mentioned is provided with proportional plus reset action. Into the B chamber of this relay the loading pressure generated by a ilow transmitter- 41, as modified by relay 49, is introduced.` Thus by virtue of relay 40 the rate of steam ow is normally maintained in predetermined ratio to the liquor ow to evaporator 2. The steam ilow constant flow control loop is shown as provided with a selector station 42 and a diaphragm operated flow control valve 43 as the nal control element. Changes in liquor flow will effect immediate and pro portionate changes in steam ow therebyV maintaining the heat input to the system as required to produce an outflow of liquor from the system of uniform concentration. Changes in steam requirement occasioned by changes in liquor flow are made before such changes cause an upset in the system.`

The loading pressure established by iiow transmitter 61 is introduced into the C chamber of relay 62 and is normally reproduced in the D chamber and transmitted to relay 40 through a pipe 63. The loading pressure 'established by flow transmitter 28 is introduced through pipe 2SC directly into the A chamber of relay 62 and through an adjustable restrictor 64 into the B chamber. Changes in yirgin feed to the last evaporator will thus cause immediate and proportionate changes in steam flow as the pressure at D of relay 62 is proportional to the sum of the pressures introduced at A and C. 'I'his change in steam flow gradually decays or is wiped out as the pressure in chamber'B becomes equal to that in chamber A as the pressure 1n these two chambers act in opposition. By proper adjustnient of the restrictor 64 and the proportional band of relay 62 the temporary change in steam ow produced by relay 62 will maintain stability of the evaporator system and the uniformity of the heavy black liquor.

Further in accordance with my invention the constant steam llow control may be trimmed or readjusted automatically in accordance with changes in density or coricentration of the black liquor outflow from the evaporator system. I show by wa'y of example the concentration or density of the black liquordetermined by comparing the boilingtemperature of the black liquor to the temperature of boilingwater at the pressure of the black liquor. The dilterence between these two temperatures is a measure of densityor concentration of the liquor. Referring to the drawing I therein show a condenser 44 supplied with steam from pipe 16 through an adjustable restriction 45. The condenser 44 is maintained at the same pressure as the black liquor by a` vent pipe 46 `connected to the vapor space in evaporator 1.` The arrangement therefore provides a means for maintaining a water sample at saturation temperature at the same pressure as the black liquor in evaporator 1. i

The black liquor leaving evaporator 1 is at boiling temperature,` hence a differential temperature device such as shown `schematically at 47 responsive to the temperature of the water in condenser 44 and to the temperature of the black liquor in pipe 14 may be utilized to generate a loading pressure proportional to the density of the black liquor. This loading pressure after passing through a selector station 48s`erves to adjust the proportional band of relay 49 disposed in the loading line between ow transmitter 41 and relay 40. Changes in liquor density will thereby cause a change in the proportion between the actual rate of steam iiow and loading pressure admitted to chamber B of relay 40. Thus for a given rate of liquor flow a new rate of steam flow will be maintained as re- (t3 quired to hold a given density or concentration of the black liquor leaving the system. The selector station 48 provides a means for manual-ly adjusting the ratio between liquor and steam inputs if desired.

As well known the stability of operation is improved by maintaining a predeterminel absolute pressure in condenser 22, this in some cases may be accomplished by a single element control wherein an absolute pressure meter such as diagrammatically illustrated at S1 establishes a loading pressure proportional to the absolute pressure in the condenser 22 which either directly, or indirectly through a relay to give proportional plus reset action (not shown), controls the rate of cooling water to the condenser by positining a diaphragm valve 52.

Certain types of surface condensers possess undesirable time lag characteristics. Where such a condenser is used it is desirable to provide .la two element control such as I have shown which regulates the flow of cooling water to maintain a predetermined temperature of the cooling water in the condenser. The predetermined temperature is adjusted as required to maintain the desired absolute pressure in the condenser. I have schematically shown such an arrangement in the drawing wherein a temperature transmitter 53 establishes a loading pressure proportional to the temperature of the cooling water in or leaving the condenser and which through a relay 54 .provides an output pressure for positioning the valve 52. The set point of this constant temperature control is adjusted las required to maintain a predetermined absolute pressure in the condenser by introducing the loading pressure generated by transmitter S1 into the B chamber of relay 54, which as shown, is provided with proportional plus reset action.

The embodiments of the invention in which an eX- clusive property or privilege is claimed are denedas follows:

l. In a control system for a multiple-effect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled olf the liquor in one evaporator Iacts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the first evaporator and ythin black liquor to the last evaporator whereby the black liquor and heating medium pass through the system in contra flow relation, the combination comprising; `control means maintaining a constant rate of flow of the steam admitted to the first evaporator having an adjustable set point, means responsive to the flow of liquor -at a point in the flow stream between the first and last evaporators adjusting the set point to maintain a predetermined constant ratio between the rate of o'w of liquor at said point and the rate of flow of steam to the first evaporator and means responsive to changes in the rate of ow of thin black liquor to the last evaporator temporarily modifying the predetermined constant ratio in accordance witlr changes in the rate of ilow of thin black liquor to the last evaporator.

2. In a control system for a multiple-etect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled ott the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the first evaporator and thin Y black liquor to the last evaporator whereby the black liquor and heat-ing medium pass through the system in contra how relation, thecombination comprising; control means maintaining a constant rate of flow of the steam admitted to the first evaporator having an adjustable set point, means responsive to the ilow of liquor at a point in the flow stream between the rst and last evaporators adjusting the set point to maintain a predetermined oonstantratio between the ow `of steam to the first evaporator and the flow of liquor at said point in the flow stream, means measuring riow of Vthin liquor to the last evaporator', means operated by said last named means producing a control eiiect proportional to the rate oi change in the ilow of thin liquor and means operated by said control eiiect modifying said predetermined constant ratio in substantially direct proportion to the rate of change in the iiow of thin black liquor.

3. In a control system tor a multiple-effect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled `oil the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the iirst evaporator and to admit thin black liquor to the last evaporator whereby the black liquor and heating medium pass through the system in contra tiow reaction and wherein the liquor is diverted through a soap separation tank in passing from one evaporator to another, the combination comprising; control means maintaining a constant rate of flow of the steam admitted to the iirst evaporator having an adjustable set point, means responsive to the tiow of liquor leaving the soap separation tank adjusting the set point to maintain a predetermined ratio between the flow of steam to the first evaporator and the How of liquor leaving the soap separation tank, means measuring the flow or thin liquor to the last evaporator, means operated by said last named means producing a control effect proportional to the rate of change in the iiow of thin liquor and means operated by said control effect modifying said predetermined ratio to produce a temporary change in the 'low of steam to the rst evaporator in substantially direct proportion to the rate of change in flow of thin 'liquor to the last evaporator.

tem in contra iiow relation, the combination comprising;

means producing a control eiieet corresponding to changes in the iiow of thin liquor to the system and means operated by said control elect temporarily changing the iiow of steam to the rst evaporator in substantially direct proportion to changes in the ow of thin liquor to the last evaporator.

5. VIn a control system for a multiple-eiiect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled oil"r the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the first evaporator and thin black liquor to the last evaporator whereby the black liquor' and heating medium pass through the system in contra liow relation, the combination comprising; constant iiow control means of the steam admitted to the iirst evaporator having an adjustable set point and means eiieoting a temporary adjustment of said set point in substantially direct proportion with changes in the rate of ilow of thin liquor to the last eiect.

6. In a control system fora multiple-eiiect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled off the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the first evaporator and thin black liquor to the last evaporator whereby the black liquor and heating medium pass through the system incontra ow relation, the combination comprisun ing; constant flow control means of the steam admitted to the iirst evaporator having an adjustable set point, means measuring the flow of thin liquor to the last evaporator, means operated by said last named means producing a control etiect proportional to the rate of change in the iiow of thin liquor and means operated by said control adjusting said set point to produce a temporary change in the ow of steam to the first evaporator in direct proportion to the rate of change in the iiow of thin liquor to the last evaporator.

7. In a control system for a multiple-effect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled 01T the liquor in one evaporator acts as the-heating medium in the following evaporator and arranged to admit steam as the heating medium to the first evaporator and to admit thin black liquor to the last evaporator whereby the black liquor and heating medium pass through the system in contra iiow relation, the combination comprising; regulating means of the rate of iiow of steam supplied the system, means measuring the low of liquor at a point in the flow stream between the lirst and last evaporators in the system,k and means operated by said last named means operating said regulating means to produce changes in the rate of iiow of steam in substantially direct proportion to changes in the How of liquor at said point.

8. ln a control system for a multiple-effect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled off the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the iirst evaporator and thin black liquor to the last evaporator whereby the black liquor and heating medium pass through the system in contraliow relation, the combination comprising; `conrol means having an adjustable set point maintaining a constant rate of iiow of steam admitted to the first evaporator, means measuring the ow of liquor at a point in the flow stream between the first and last evaporators, means operated by said iiow measuring means generating a control effect corresponding to the flow of liquor at said point, and means operated by said control eiect adjusting the set point of said control means to maintain a constant ratio between the rate of flow of liquor at said point and the rate of ow of steam to the first evaporator.

9. In a control system for a multiple-effect evaporator system for the concentration of thin black liquor to heavy black liquor composed of several evaporators arranged in cascade by vapor piping in which the water boiled oirr the liquor in one evaporator acts as the heating medium in the following evaporator and arranged to admit steam as the heating medium to the first evaporator and to admit thin black liquor to the last evaporator whereby the black liquor and heating medium pass through the system in contraow relation and wherein the liquor is detoured through an auxiliary process, in passing `from a given evaporator to the next downstream evaporator with regard to liquor iow, the combination comprising; means measuring the flow of liquor leaving the auxiliary process, regulatingmeans of the rate of flow of steam to the system, and means operated by said measuring means operating said regulating means to produce changes-.in the rate of tiow of steam in substantially direct proportion to changes in the llow of liquor leaving the auxiliary process.

References Cited bythe Examiner UNTED STATES PATENTS 1,996,526 4/35 Serpas 159-31 X 2,040,284 5/36 Tell 15s- 44 2,073,825 3/37 Beck et al. 159-44 {ther references on following page) 9 10 UNITED STATES PATENTS FOREIGN PATENTS 11/38 Holven 159--44 1,223,627 6/60 France. 11/39 Hinckley 159-20 d s/so Stinson 159-44 OTHER REFERENCES 1/55 Reynolds 261 114 5 Integrated Control System, A. C. Camp pubhcatlon 5/56 Shannon. Sugar,pp. 26, 27, Iune 1950. 5/56 D- k 7/57 fniegyet al 159 44 X NORMAN YUDKOFF, Primm Examiner. 9/ 57 PaliCh- GEORGE D. MITCHELL, CHARLES OCONNELL, 6/58 Lankenau 159-20 10 BENJAMIN BENDETT, Examiners. 5/61 Bellinger 262-160

Claims (1)

1. 3. IN A CONTROL SYSTEM FOR A MULTIPLE-EFFECT EVAPORATOR SYSTEM FOR THE CONCENTRATION OF THIN BLACK LIQUOR TO HEAVY BLACK LIQUOR COMPOSED OF SEVERAL EVAPORATORS ARRANGED IN CASCADE BY VAPOR PIPING IN WHICH THE WATER BOILED OFF THE LIQUOR IN ONE EVAPORATOR ACTS AS THE HEATING MEDIUM IN THE FOLLOWING EVAPORATOR AND ARRANGED TO ADMIT STEAM AS THE HEATING MEDIUM TO THE FIRST EVAPORATOR AND TO ADMIT THIN BLACK LIQUOR TO THE LAST EVAPORATOR WHEREBY THE BLACK LIQUOR AND HEATING MEDIUM PASS THROUGH THE SYSTEM IN CONTRA FLOW REACTION AND WHEREIN THE LIQUOR IS DIVERTED THROUGH A SOAP SEPARATION TANK IN PASSING FROM ONE EVAPORATOR TO ANOTHER, THE COMBINATION COMPRISING CONTROL MEANS MAINTAINING A CONSTANT RATE OF FLOW OF THE STEAM ADMITTED TO THE FIRST EVAPORATOR HAVING AN ADJUSTABLE SET POINT, MEANS RESPONSIVE TO THE FLOW OF LIQUOR

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