US1927555A - Process of concentrating liquids - Google Patents

Description

Sept. 19, 1933. F. A. OETKEN 1,927,555

PROCESS OF CONCENTRATING LIQUIDS Filed-Dec. 12, 1930 /n venfor:

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Patented Sept. 19, 1933 UNITED STATES.

vPROCESS OF CONCENTRATING LIQUIDS Friedrich August Oetken,

Frankfort-on-the- Main, Germany, assignor to American Lurgi Corporation, New York, N. Y., a corporation of New York Application December 12, 1930, Serial No. 501,935,

and in Germany December 23, 1929 4 Claims.

This invention relates to a process of concentrating liquids.

The economical operation of concentration apparatus is often greatly impaired through the efiect of incrustations on the heating surfaces, resulting in a reduction of the capacity and increased consumption of heat. For this reason, ways and means have long been sought for preventing incrustation on the heating surfaces and thereby improving the emciency of concentration processes.

The partial improvements heretofore effected, however, were still far from constituting a complete solution of the problem, and therefore were in no wise satisfactory. The efllciency of the means employed has been insuflicient and more or less a matter of chance.

The most troublesome of all incrustants are the lime salts (especially gypsum) which occur, not only in saline solutions, but also in many organic liquids. Investigations have shown the prime necessity of preventing the formation of fresh crystals in the heater, or directly in the heating surface of same, and that, in addition, special measures must be adopted to prevent the deposition and adhesion of any floating crystals already present.

The formation of fresh crystal nuclei has its origin in the super-saturation of the solution, whereby according to the laws of crystallization it is necessary for the metastable zone of supersaturation to have already been exceeded. A saturated solution of a substance the solubility of which decreases with increasing temperature, is super-saturated by heating. The upper limit of the metastable super-saturation zone is thereby exceeded at a determined temperature. The difference between this temperature and the temperature of the saturated solution withv the same content of dissolved substance, depends upon the nature of this dissolved substance and amounts to about 1 to 3 centigrade. The discovery of this temperature interval is particularly important in the case of substances whose solubility diminishes on heating, as is the case, for example, with gypsum, when the temperature of the solution rises above about 38 C. A satu rated solution may also become super-saturated by concentration. In consequence, in order to avoid deposition of crystals in the heater, it is necessary to prevent in the heater any even local super-saturation above the metastable zone. This means to prevent in the heater first, all

' concentration or evaporation, second, all local (1) To employ a'sumciently high-jliquid pressure in the heater to keep the boilingpoint above the heating temperature throughout. I

(2) To-control the 'quantity of circulatingliquor so as to keep the increase of'temperature in the heater in definitelimits and to prevent the super-saturation. "developing in the heater from exceeding the upp'erlimit of the metastable zone.

(3) To keep the temperature drop between the a heating medium and the liquid as small as pos sible, in order to obviate the dangerous influence of the higher wall temperature. w

(4) To arrange for the liquid to flow at a high velocity, so that the turbulent flow. may-*ef-ifeet a thorough intermingling ofparticles of liquid which may be of different temperatures.

In addition to the foregoing measures, how-- ever, it is desirable that certain other points should be observed in order to prevent the deposition and adhesion on the heating surface, of

crystals already present in the liquid and circulating through the heater. It is known that the subsidence of coarse floating substancescanbe prevented by increasing the velocity of-flow, and

also by employing vertically disposed heating surfaces.- In the case of very small crystals, however, the adhesive forcemay preponderate,

so that such crystals adhere to the walls, in spite, v

of the employment of the customary velocities,

and especially on the microscopically small irno regularities in the wall material, the structure of which exhibits more or less considerable depress sions and prominences. Experience has shown that this phenomenon is more particularly seen in the case of gypsum crystals. In order to counteract it, the rapid growth of the .crystals must be facilitated, which is effected by relieving the supersaturation of the liquid on the existing crystal nuclei, either completely or at least partially,

before the circulating crystals re-enter the heater.

The essence of the present invention consists in combining, for common action, all the aforesaid factors which have a retarding influence on r the formation of incrustations. Although somel of said measures are already known and have.

. been variously employed, they are not, in themselves, capable of preventing incrustations in the heater, in the long run. .Even the employment of two of the different means specifled,such as a no it is necessary'to com,- .00

high liquid pressure in combination with a high velocity, is altogether inadequate. It hasbeen shown, and this is confirmed by practical experience, that, in orderto accomplish the object in view, none of the aforesaid measures must be omitted.

In this connection,refer'ence may be made to" the United States Patent'No. 1,006,823, which,-

on superficial examination, seems to display a.

certain similarity to the process invention.

A fundamental difference, however, existsin that, in carrying ou the, process of the said of the present United States specifi ation, the formation "01 fresh crystals cannot be prevented, but, on the contrary,'means areadopted for separating the crystals freshly formed in the heater, in aspe cial sludge collector between'the heater and the throttle device, in order thereby to obtain a purer salt as end product. Only the first and the fourth of the precautions mentioned above are employed.

In contrast thereto the present invention by also. employing the second and the third of the I precautions mentioned insures that .no formation is also the relief of .the supersaturation "aftersteam has been given off from the liquid. As a matter of fact, it has not been found possible to-prevent incrustations of gypsum and the like 'in'apparatus of this kind, because'local super-.-

heating and development 0f steam bubbles directly at the heating surfaces have not been prevented, and velocities of flow are too low.

The followingpoints may bementioned with regard to the employment of high velocities of flow as a means-for preventing the formation of incrustations a J a The velocity is intendedto act in a dual capacity, on the one hand, to prevent the deposition of solid constituents on the heating surface, and fur-. ther to assure thorough intermingling of the liquid, in order that no local rise in temperature, beyond the permissible limit of the metastable zone, can occur. For this purpose, particularly high velocities must be employed, and it has been ascertained that the velocity must, preferably,

beat least 1 meter per second, if the desired effeet is to be produced. Such high velocities, howver, do not, inthemselves,v ensure the necessary slight heating within the metastable zone, their sole purpose being to render the distribution of temperature more homogeneous in'the current of liquid. The slight heating, in turn; necessitates:

the simultaneous employment of sufficiently large volumes of circulating liquid, and alsolow heating temperatures, or slight temperature drop at the heating surface. Suitable selection of the dimensions of the heating pipes, and of theworking conditions of the circulation pump, will en- I able .the foregoing conditions to be fully metin constructing. the apparatus. i v

The following example serves to explainthe.

' of suitable height, but more simply by throttling conditions once'more, on a numerical basis:--

Sulphite waste liquor, neutralized with lime,

was concentrated in-a circulation evaporator opthe expansion to the pressure of the evaporator;

crating under pressure. The solution was in a 1 state of complete saturation with gypsum. The heater was heated by steam at a temperature of 120 C. The pressure in the heating system was 1.3 atmospheres on the liquid side, and therefore no, steam could be formedin any part of the heater.' The circulation of the liquid was so calculated that, in a single passage through the heater, the rise in temperature amounted only to about 0.8-1 C. At the same time the rate of flow of the' liquor in the heater could be maintained at between 1 and 2 meters per second. On

entering the evaporator, the heated liquor was allowed ,to' expand to atmospheric pressure, thus allowing the liberation of an amount of steam corresponding to the volume in circulation and the preceding heating, while the temperature of the liquor itself fell from about 101 to C. Duringythe further course of the liquid through the evaporator, the supersaturation resulting 95 from the heating in the heater and also from the liberat'on of steam in the evaporator, was relieved on the crystal nuclei in the latter. in circulation with theliquor there, the crystals themselves remaining, for the most part, in suspension. In this 106 condition, the liquor was drawn :out of the evaporator by suction, and again, passed, under where the supersaturation.temperature was again brought by heating up to about 101 C. By this. 5 means, the renewed formation of crystals in the heater, or adhesion of the suspended crystals on to the heating surface was prevented, which, as'

can occupy an entirely different position withinn fl the circulation, according to the special physical properties possessed by the liquid, and it is important that the saturator, in which the liquid is restored from the supersaturated condition to in question. In some cases a temporary displacement of the point of maximum supersaturationin the circulation of the liquid can be effected through the working conditions selected, so thatit seems advantageous to arrange, for example,

turally connected therewith. In this manner a practically sufficient protection against the formation of any incrustation on the heatingsur-- face can also be obtained.

Moreover, in carrying out the process according to the present invention, it is particularly important to take into consideration the fact that: the disengagement of steam, subsequent to the expansion in the evaporator, takes place under substantially different conditions from those to which one has beenaccustomed in the evaporators hitherto in use. These special operative con ditions are due, on the one hand .to the large vol- 149 ume in circulation and slight-difference in the temperature of the liquid before and after the disengagementof the steam, and on the other hand to the highpressure of the liquid prior to The'pressure considered necessary in the heater may be produced by employing columns of liquid the circulation of the liquid. Various appliances pansion of the liquid may be effected.- However,

in the case of rather large pressure: drops the:

formation of correspondingly large amounts of steam causes, certain difliculties in the operation of such apparatus.

The vaporizer is to be operatedsomewhat like a jet condenser. It requires a good distribution.

of the injected liquid, or the creation of the largest possible superficial area of the liquid, in

order that the liberation of the steam may proceed as extensively as possible, the more so when the temperature drop is-very slight. On the other area of the escaping. jets of liquid may not be large enough to enable a rapid and complete disengagement of heat'in the form of steam; When the jet of liquid entering thevaporizer has not a sumciently large surface, the resulting steam tears the Jet apart and there are spray losses, while a part of the liquid may re-enter thecircuit before the whole amount 'of steam is given off.

The pressure drop of the-liquid entering into the vaporizer can be considerably reducedby inserting two throttling stages'between heaterand vaporizer. In the first stage the liquid leaving the heater may expand to a pressure very little above its boiling point. The pressure drop of the second stage must only be great enough to prevent premature evaporation. In consequence cross sectional areas of the second throttling stage-can be large, the superficial area of the jets can be large too, and the steam can be developed without violence. The difference in comparison with the single throttling is particularly large be-' cause. owing to the exceedingly slight elevationof the temperature of the liquid in the heater, only a very small pressure is needed to prevent the formation of steam prior to the-final expansion,

of small diameter, b lonsitudinai, transveree'or circularly disposed slots, or also by sprinklers,

, sieves or perforated sheetmetal of any kind, The

of heating of the liquid, as prescribed by the lnvention and in which the zone of metastable saturation must not be exceeded, necessitates, as

a matter of course, the employment of larger'volumes than usual in circulation, a condition which, in conjunction with the requisite generation ofpressure in the heater, entails a not inconsiderable expenditure of energy for the circulation pump.

The economy of the process can be still fur-- ther improved by employing, instead of electricity, for driving the pump, a steam turbine, the exhaust steam from which is passed into the heater. In a plant comprising a plurality of units, it will be advisable to employ the exhaust steam from eachstage of the turbine for heat--. ing the first stage of the evaporators, in order to obtain a multiple utilization of the exhaust steam in accordance with the number of units.

In order more clearly to understand the nature of the invention, reference is made tothe accompanying drawing 'which illustrates diagrammatically and byway of example, atypical embodiment of apparatus...suitablefor carrying out the. process of the invention; .In said. typical em.-

bodirnent, Ifis. the evaporatorz'and. 2 the heater,

bothjbeing adapted. to." operated under any convenience pressure, orx-alsoundera vacuum.

. The evaporator lisiconnected-"to. the heater?- by "the pipes 3' and '5 on the one-hand 'and the pipe'll ens-the other; The-"circulation-pump. 6, j

.drivenkby the steam turbine-l, is 'connecte'dup in: the. pipe 5. The workingsteam for the turbine admitted; atj8. The .exhaustpipe'9 is connected to the heater-2; The additional heat-- ing' steam requirediis' supplied through pipe- 10;

and may also, desired,'be"whollyorepartially serve'as'heating steam. connecting. pipe- 11 comprises a throttle-device 12,. by means of which the liquid pressure issofarreduce'd, by-comparison with-that m the heater 2, that thetboil-p ingpoint of the liquid'is only slightly higher than the liquid temperature'prevailing at point. The complete expansion to the'absolute pressure in the evaporator i'thentakes place at13. The

steam thus disengaged. from. theliquid passes .1

away through theibranchj ii-and may, for. ex-

ample, beled to a condenser, a compressoror a second evaporator stage. of 'theQthr ttIes" i2 and.13' may, of course, .be controlled in known manner, and either independently or conj.oint'-' 1y. According to the physical properties of the liquid, the 'saturator Infill-l6 located stone or more points in the liquid circulation, for-example between. the evaporator i and, theci-rculation pump 5, orbetween the two throttling. stages}! -1 and 13-. The saturatorld mayi also 'be connected with the evaporator. '1. as a structural or'also arranged to be disconnected. Thepipe 15. serves 1Q!- evacuating the saturator-4;

when the apparatus is in operation, the liruiid therefore' flows, under effected by thepump 6, for example throughstheflhtater 2 and pipe 11, then through the em throttling stagela and intqthe evaporator 1,, from which,

after the disengagement of steam,,it returns a through the saturator 4 to the circulation'pump 6. In the case ofIother liquids, or other work-" 12. meme throttling mu 1a,; evaporator 1 and to thepumpd. After bein: sumciently the liquid .is-dr'afwns off from the evaporator at 16.-

The'prccess may, of course, be carried on un-.

der pressure or under a vacuum. and also in com- Lunatics-with vapor co pr sso s. av n mtorsz of this kind may also be 'run'with a plurality of units, each evaporator having its own "circulation pump, throttle devices, saturator, 'and so.

forth, as shown in the drawing 'Bearing in mind that the high velocity required, according to the invention, in the heater, is accom'- panied by'a substantial increase in the, transmis-' sion of heat, it will at once be evident that the processaccording'to the present invention allows of the economical concentration of evensuch liquids as were hitherto a source of diflicultie owing to incrustation. Iclaim:'"' 1. A process of concentrating liquids containalso ' 1 liquidsto flow through a heater at a pressure such that the boilingpoint of the liquid is higher than 1 the temperature of the heating medium, controlling thequantity of circulating liquid in such manner that the supersaturation caused by the increase of temperature in. the heater does not exceed the upper limit of the metastable zone,

- relieving the supersaturation of the incrustants releasing thepressure on said liquid during pas-,

in the liquid after it is withdrawn from the heater,

and before'it is returned thereto and releasing the pressure on said liquid upon passing into the evaporator.

2. A process of concentrating liquids contain-- ing calcium sulfate or like substances of negative temperature coeflicient of solubility by circulating the liquids through aheating stage and an evaporating stage which comprises passing theliquid,

in heatexchange relation with a heating medium at a velocity of at least one meter, per second while maintaining the liquid at a pressure such that the boiling point ofthe liquid is higher than the temperature of the heating medium, .cona trolling the heating of the liquid in such manner that the super-saturation caused by the increase I v perature durm the heating stage doesnot ex- 01'- temperature during the heating stage does not exceed the upper limit oi the meta-stable zone,

relieving the super-saturation of the 'incrustants in the liquid after it .is withdrawn from the heatv mg stage and before it is, returned thereto and sage through the evaporating stages g 3. A processof concentrating liquids containing calcium sulfate-or like substances or negative temperature coefficient of solubility by circulate ing the liquids through a heatin8-.:stage and an evaporating, stage which comprises passing thevliquid in heat exchange relation with a I heating medium at a velocity of at least one meter.

per second while maintaining the liquid at a pressure such that the boiling point of the liquid is higher than the temperature of the heating mebetween the liquid flowing through the heating stage and the heating medium below C, re-

. lieving the super-saturation of the incrustants in the liquid after it is withdrawn from the heating stage and before it is returned thereto and releasing thepressure on said liquid during passage through the evaporating stage.

the liquids througha heating stage and an evaporating stage which comprises passing the liquid,

in heat exchange relation with a heating medium at a velocity of at least one meter per second while "maintaining the liquid at a pressure such that the boiling point of the liquid-is higher than the temperature of the heating medium, controlling the heating of the liquid in such manner that the super-saturation caused by'the increase of temceed the upper limit of the meta-stable zone, keep- :ing the difference in temperature between the heating medium below 20C., relieving the supersaturation of the incrustants in the liquid after it is withdrawn from the heating stage and be- -fo re it is returned thereto and releasing the pres- I sure on said liquid during passage through the evaporating stage in two steps in the first step of which the boiling pressure ofthe liquid is nearly but not entirely attained.

v FRIEDRICH AUGUST. OE'I'KEN."

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