US1200996A - Method of evaporation, &c. - Google Patents

Description

0. SUDERLUND & T. BOBERG.

METHOD OF EVAPORATION, 8w.

APPLICATION FILED OCT. 14. 19I2.

Patented Oct. 10

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METHOD OF EVAPORATION, 6m.

APPLICATION FILED OCT. 14. 1912.

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APPLICATION FILED OCT. 14. 1912.

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METHOD OF EVAPORATION, &c. APPLICATION FILED OCT. 14. 1912.

Patented Oct-10, 1916.

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METHOD OF EVAPORATION, &c.

APPLICATION FILED OCT. 14. 1912.

Patented Oct. 10, 1916.

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APPLICATION FILED OCT.14. 1912- l,200,996 Patented Oct. 10,1916.

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OLOE SODERLU'ND AND TEOFRON BOBERG, 0F CLAPHAM PARK, LONDON, ENGLAND,

ASSIGNORS- TO TECHNO-CHEMICAL LABORATORIES LIMITED, OF LONDON, ENG- LAND.

METHOD OF EVAPORATION, &o.

Patented Oct. 10, 1916.

Application filed October 14,1912. Serial No. 725,675.

I '0 all whom it may concern:

Be it known that we, OLOF SODERLUND and TEOFRON BoBERe, both subjects of the King of Sweden, and both residing at Fairlaivn, Clarence Road, Clapham Park, London, S. \V., 'England, have invented a certain newa-nd useful Method of Evaporation, &c., of which the following is a specification.

This invention relates to evaporating, distilling and similaroperations.

. A method of and various forms of apparatus for evaporating and distilling have been proposed in which the vapor given off by the liquid undergoing evaporation is compressed so as to raise its temperature and the compressed vapor is returned to heating devices within the evaporating chamber. It has been proposed in this way to evaporate and (listil liquids by employing the vapors arising from them after compression ofthe same. In Patent No. 1,150,- 713 is described an evaporating and distilling method and apparatus of this type. In efl'ecting evaporation in such a device of a solution the boiling point of which increases with the concentration, we have observed that it becomes necessary on account of this fact to vary the working conditions of the compressor as the concentration varies, or else to have such range of compression as will cover the whole of the necessary range of compression. This can best be explained by reference to Figure l of the accompanying drawings,.in which the curve gives the boiling temperatures for varying concentrations of a solution of common salt in water. This curve illustrates the concentration by evaporation of a salt solution from a concentration equivalent to that of normal sea-water to the saturation point. The solution taken for illustration contains 1 kg. of salt, represented by the distance fromthe vertical line on the extreme right to the adjacent scaled line, and the distance from this scaled line to the curve gives the quantities of water present at various concentrations, the numbers just below the axis of abscissae indicating kilos of water. In this scheme of illustration the amount of water which must be removed from the solution in passing from any concentration to any higher concentration is given by the length of abscissa intercepted between the vertical lines through the points of l kilogram of steam to the different temperatures shown on the right-hand scale. Thus to raise the temperature by compression of one kilogram of steam 5 (1 would require an expenditure of energy equivalent to 7 calories. Both ordinate scales have been extended below the zero line for the purpose which hereinafter appears, but these extensions do not represent negative quantities.

The theoretical quantity of work or heat required to be supplied to remove 10 kilograms of water by evaporation from the solution represented in the figure would be equal to the area bounded by the lines a b, b 0, a d, and the portion of the curve intercepted between the points a and d .where unit area on the drawing corresponds to one calorie or 427 kilogram meters The theoretical quantity of heat supplied however, in effecting evaporation according to they system under consideration would be represented by the oblong area a b multiplied by b 0, together with the oblong a b multiplied by b e, or by b f, or b 9, etc., according to whether a temperature difference of about 1 C. 2 C. or 3 (1., etc., was used for efi'ecting the evaporation, the portion of the figure under the horizontal zero line representing the additional amount of heat or Work added by virtue of the temperature difference used to insure heat transmission. It will be seen therefore that the excess of heat required to be supplied in concentrating a solution as imagined in the figure by the system in question, to the degree of saturation would be represented by the oblong a h multiplied by k 2' together with the oblong ,a h multiplied by a further factor dependent upon the temperature difference employed. Under these circumstances the theoretical additional heat supplied exceeds the theoretical amount required by the area bounded by the lines 71 j, j d, and the curve, and this is due to the working conditions requiring to be such as to be capable of effecting the evaporation both under the initial and the final conditions. This extremely large excess of work over the minimum needed is a sourceof ineificiency which can however, be"

very largely eliminated in the manner subsequently described. 7 I

In the patent beforereferred to, it has been shown that in order to carry out evaporation economically by means of "heat-supplied as mechanical work, it is necessary to reduce to the lowest possible limit, the temperature'diflerence between the heat supplying medium and the evaporating liquid.

If the boiling point of a liquid rises during its concentration and heat supplying medium at a temperature high enough to effect evaporation at the greatest concentration is supplied throughout the entire operation of concentrating, then during the earlier stages there will be maintained. a much-greater temperature difference between the heat supplying medium and the evaporating liquid than is necessary to insure heat transmission.

For every pound ofwater evaporated from the solution approximately one pound of steam must be condensed within the coils in order to supply the necessary heat. The

additional heatsupplied by compression is only for the purpose of causing transmission of the heat and is so small compared with it that an increase of additional heat will have little or no efiect in reducing the quantity of steam which must be coinpressed. The additional heat is naturally" very costly owing to its being derived from mechanical work and totalling not more than some 10% to 25% of the energy of the fuel used to produce it. For this reason it is essential that the additional heat be reduced at all times to the smallest possible amount necessary to cause heat transmission, as any excess over this quantity would "only result in a practically infinitesimal amount of extra evaporation at an inordinate cost.

It will be understood that although the total quantity of heat required for a given evaporation is a definite amount, practical losses from an evaporating plant must increase with the working temperature. If therefore, a great quantity of-the heat required is supplied at a higher temperature than is necessary for the work, the losses will be greatly increased.

"area j m e, and with an increase 1n the pressor,

number of stages the wastage is correspondingly reduced and the area representing the amount of heat theoretically supplied approximates more and more closely to the amount theoretically required. Two dotted lines have been placed on the drawing to indicate the conditions with evaporation in three stages and five stages respectively. It will be seen therefore that the limiting value is reached with an infinite number of stages as in'that case the areas represented by the heat theoretically supplied and required coincide. It is to be observed, however,-that while in effecting the evaporation in a distinct number of stages, it is relatively easy to supply blowers or other compressing devices adapted to effect compression with best efliciency for each stage the evaporation under conditions corresponding to an infinite number of stages, is more difiicult.

This invention consists in a method of evaporation or distillation wherein the evap oration is conducted in accordance with the method described in Patent 1,150,713, but in a limited number of stages, say up to 5, in a rtzorresponding number of separate evaporaors.

Fig. 2 shows a curve calculated for sugar.

solution, a substance which differs in behavior from the behavior from the solution imagined in Fig. l, in that, the curve representing rise of boiling point with increasing concentration is not a simple curve. This of course is a condition which is more common than that imagined in F ig. 1 and in Fig. 2 the discontinuity in the curve occurs at a concentration between 20 and 25% when apparently owing to a change in the hydration of the sugar, the curve changes its direction from its original direction indicated by the chain line portion of the curve 11. p and takes up a new direction originating from the direction 1' n. Figs. 3 to 9 show by way of example various methods of carrying the present invention into effect. Figs. 3 and 3 showing. evaporation eflected in five 5 showing evaporation effected in three' stages by the use of a single multi stage com.-

F igs. 6 and 7 different forms in which evaporation is effected by the use of an e'iector as a compression device, and Figs. 8,. 8 and 9 showing a form of evaporator inwhich the liquid to be evaporated is caused to flow within vertically arranged tube systems in contradistinction to the horizontal tube system over which the liquid is caused to flow in the forms previously illustrated.

InFigs. 3 and 3, 1, 2, 3, 4, 5 represent the separate elementsof the installation. The liquid to be distilled enters by the duct 6 and is pre-heated by the condensed water leaving the chambers l to 5 by the ducts 8 to 12 respectively, flowing through the air separators 13 to 17 respectively, and thus into the common duct 18 and through the heat exchanger to the outlet 19. The liquid preheated in this manner to as near its boiling temperature as possible flows through the duct 20 and enters the element 1 through the ball-cock 21, to be immediately taken by the circulating pump 22, and suppiled to the spray distributing device 23 and caused to flow over the heating tubes 24. The vapor given off is by the blower 25 compressed to a suitable extent, it having if desired, immediately before compression added to it suchan amount of hot water (which may be obtained from the condensed water duct 18 and forced by the pump 26 through the duct 27 and nozzle 28)' as will after compression cause it to be saturated steam. It is of some advantage to saturate the steam on account of the relatively higher heat emissivity of saturated steam with relation to superheated steam. The steam after leaving the compressor is divided into separate portions, the one of which passes into the heater of the first element, and the other portion of which passes to the next compressor. The steam which flows through thet tube system 24 may have a temperature only just enough above the boiling point of the liquid in the element 1 to cause sufficiently rapid 'evaJpo-ration thereof, and in this connection it should be observed that the temperature difference should seldom require to exceed 3 C. while good evaporation can easily be obtained in a device as indicated with a temperature difference of 1 or even less. It is in most instances important to reduce the temperature diflerence as far as possible since although a reduction in temperature difference means a reduction in output of the evaporator, it

means in proportion a far greater decrease in power absorbed. p

The tube system of the evaporator is as indicated in the drawing preferably of decreasing total cross sectional area through the apparatus so that notwithstanding progressive diminution in volume of the steam on account of condensation, the rate of flow may be maintained as far as possible and the diminution of heat transmission as far as possible avoided. That portion of the steam leaving the compressor 25 which has been passed to the next compressor 29 is by the latter (after the addition to it, if desired, of a suitable quantity of hot water, by the nozzle 30) compressed to a further degree sufficient to bring about eflicient evaporation of the concentrated liquid which has by the duct 31passed into the element 2 and has. by the pump 32 been supplied to the distributer 23 and thereby distributed over the'heating system 34.

It should be observed that by a seal 35 proi'ided in the lower portion of the element 1, intermingling of the concentrated liquor and the freshly entering liquid can be avoided without at the same time impeding the repeated circulation of the liquid further increased'and so on throughout the series of elements. The evaporating spaces of all the elements of the installation are interconnected by the ducts 35, 36, 37, and 38 so that the first compressor 25 of the series is supplied with the steam generated in all the elements of the series. The liquor after having passed through the series of vessels leaves through a float controlled outlet 39.

Fig. 4 shows a modification of the above form in which all the steam after compression by the compressor is passed through the heating device of the elementto which the blower corresponds, while the uncondensed portion of the steam is passed to the next compressor. Thus, the steam given ofl in the elements of the series is by the compressor 25 sufliciently compressed to effect evaporation in the first element and passes through the heating system 24 thereof, the uncondensed portion leaving and passing by the duct 40 to the compressor 29 after having been drained of its condensed water which passes to the air separator ,l3vand the duct 18 as before, the residue of the compressed steam leaving the heating system 34 of the element 2 and passing to thecompressor of the element 3 and so on throughout the system.

In Fig. 5 is shown a modification in which an evaporating installation of three elements is combined to form one single evaporator with a result ofvery considerable decrease in radiation losses and costof construction, etc. This is accomplished by providing a multi-stage compressor adapted to take the whole of the vapor givenoff by the evaporation of the liquid in the chamber 41, compress it in the intial stage or stages to a sufficient degree to effect evaporation of the liquid flowing over the heating system 24 when a sufficient portion of its steam 1s passed to the heating system 24 Fig. 6 shows an evaporatinginstallation according to the present invention in which, in order to avoid as far as possible moving parts the, compressor is effected by high pressure steam ejectors and the circulation of the liquid is effected by steam jet pipes. It should be observed ,that, in devices such as ha e been described with reference to the preceding drawings, it is in most cases de- 10 sirabletto add to the system a small amount of live steam, this being introduced into the lower-portion of theevaporating chamber to clear away the stagnant air layer which maytend to form and .to interfere with the heat transmission, and at the same time acting to make up radiation losses and to balance to some extent or entirely the heat loss represented by the residual heat inevitably carried away by the condensed water. Un- (ler these circumstances, as this steam can be added in the form of high pressure steam, the energy contained by the latter can be caused to perform the Work of compression, and this has been aimed at in the installation indicated in Fig. 6.

- The high pressure steam is supplied by the pipe 43 and issuing from the ejector 44 draws the steam given off in the element 1, and in the element 2 therefrom, and compresses it to the required extent forcing it through the heating system 24 whence afterremoval of the condensed water the residue after further compression by live steam issuing from the ejector 45 passes through the heating system 34 of the element 2, any uncondensed residue entering the condenser 46 in which it is condensed by the inflowing liquor to be concentrated, the inflowing liquor enters by the duct 6, passes through theheat exchanger 7 thence by the duct 47 through the condenser and therefrom by the pipe 20 to the ball-cock feed 21 in the element 1, while the condensed water from the element 1, after passing through the duct 8, air separator 13, and duct '18, meets in the further air separator 48 with the condensed water entering the same from the condenser through the pipe 49. Circulation of the liquid is brought about by steam jet 50. pumps 50 and 51 operated by live steam supplied from the duct 43, while provision is made at the point 52 for the introduction if desired, of an additional quantity of live steam for the purposes before referred to. 55 The flow of liquor to be concentrated between the vessels 1 and 2 may as in the form shown in Fig. 5 be controlled by the difference in specific gravity between the liquor in the two elements. Irif ig.,7 is shown a modification of the evaporating installation described in Fig. 6 in which the necessary temperature difierence is obtained in the element 2 by maintaining'reduced pressure in the evaporating space of this element 2 with relation to the pressure existant in the evaporating space of the element 1, so that instead of having an ejector 44 (Fig. (3) acting to raise the pressure for the first stage, and the second ejector 45 (Fig.- 6) raising the pressure in the second stage, the one ejector 44 (Fig. 7) creates the necessary compression of the steam for both elements 1 and 2,'while the injector 53 creates a partial vacuum in the element 2. Otherwise the arrangement of the various details of the installation is the same as that in Fig. 6. As it is a matter requiring careful adjustment of the parts, etc., to insure that the liquid shall flow in an unbroken thin film over the whole of the 30 surface of the heating tubes, and as such condition is a requisite to the best evaporative efliciency it may in many cases be desirable to arrange the tubes vertically and to cause the liquid to flow vertically down 35 the surfaces as in this way it is relatively easy to insure uniform distribution. Certain difliculties may also present themselves in effecting removal of deposit from the outside of the heating tubes when such is unavoidably formed, and this favors an arrangement in which the tubes are vertical and the liquid is caused to flow down the inside of the same. Furthermore, a vertical arrangement of the tubes permits of readily forming eachelement of the evaporator as a cylinder to the form best fitted to withstand pressure or suction. An evaporating installation constructed on these lines is indicated in Figs. 8, 8, and 9. Fig. 8 being a sectional elevation, and Fig.

9, a sectional plan thereof, while Fig. 8 shows in detail the device adopted for distributing the liquid upon the inside of the tubes. Referring to these figures, it will be seen that the tubes 24 are arranged vertically between tube plates 54 and 55. The liquor raised by the circulating pump 22 flows from the orifices 55 (Fig. 8) of the distributer placed at the top of each tube and thus forms a thin film over the whole of the inside of each tube. Each distributing device is provided with an open ended tube 56 which allows communication between the upper chamber and the lower so 1 that the vapor generated in all parts of the vessel. can be withdrawn by the ejector 44 and forced into the space between the tube plates. It will be observed that this space is by bafiles 57 caused gradually to diminish 1 in cross sectional area so that the same effect as is obtained in the forms already described by diminishing a number of tubes is here obtained. It is, however, preferable to em ploy instead of the bafile arrangement shown, one in which the baflies extend wholly across the vessel and are alternately pierced .at the top and bottom so that the heating medium is caused to flow substantially parallel with the tubes through the system. In

this way, considerably less friction arises than when employing the bafiie system shown on the drawing.

Distributing devices such as are shown in Fig. 8 are practically necessary in constructions in which the steam is withdrawn from the upper portion of the vessel as by the tubes 56 the vapor given off Within the tubes 24 can be withdrawn.

The device shown in Fig. 8 operates in the same way as that described with reference to Fig. 7, but it will be understood that not only may the ejectors shown be replaced by rotary or other compressors, but the arrangement of the parts and the various details may be the same as in any other of the forms already indicated. 1

At 58 and 59 are shown respectively diaphragm regulators having for their object to maintain a constant pressure within the evaporating space of the elements 1 and 2, this being effected by controlling by means of the diaphragm valves 60 and 61 adapted to bypass on a fall of pressure a portion of the compressed steam from the heating chamber. The duct 62 shown is for the supply of steam to the vessels for heating them up when starting the apparatus from the cold condition.

It will be appreciated that as the evaporation progresses the bulk of material to be treated and the volume of vapor given ofi? diminishes, and for this reason each succeeding element of the installation can be made of a smaller volume than'that preceding it as is indicated in the forms of apparatus described, and in this connection it may be noted that a curve such as that shown in Fig. 2 will readily show the relative volumes of the vessels as these will be proportional to the bulks evaporated in the respective vessels, that is to say, proportional to the lengths q n, s t, u o. In fact, it being known what materials and appliances are available, a curve of the type shown in Fig. 2 can be used with advantage in determining for the purpose of design of the installation the work to be done by the relative parts, and so forth. So far, installations in which the separate stages are conducted in separate vessels have been described, but as indicated, provided a compressing device having a sufiiciently wide range of fairly high efiiciency can be obtained, it may be advantageous to extend the principle above referred to and to conduct the evaporation in a number of stages in a single vessel by allowing the power taken by the compressor gradually or by steps to increase with the increase in concentration of the material under treatment, and where it is immaterial if the substance under treatment is heated for a relatively prolonged period, then a plurality of the devices of the character last referred to may be emunit device is to be preferred since the ma-. -ter1al can be caused to flow more or less rapidly through the same and concentration quickly effected, the bulk of material present in the elements of the whole installation being relatively small. It is to be observed however, that owing to the liquid being distributed as a very thin film over the heating surface and moving rapidly thereon while only a small temperature difference between the treated material and the heating medium is employed, concentration of solution of the most sensitive materials can be'carried out without resorting to reduced pressure, and it becomes for example readily possible to carry out the concentration of sugar solution under normal pressure.

' It will be observed that by conducting evaporation according to the present inven tion, the use of cooling water is entirely avoided in the only case where a condenser becomes really necessary or desirable, the cooling water being replaced by the feed liquor to be evaporated (see Fig. 6). It should be noted, however, that with an installation such as that indicated in Figs. 6 and 7 if the ejector is of such low efliciency as not to be capable of supplying the necessary energy with the small amount of steam corresponding to the losses to be made up and where a vacuum is employed a small condenser may need to'be added and in this case some cooling water may be required.

It will be-understood that it is important that such element should be itself as eflicient an evaporator as is possible, and this is apparently attained by insuring an extremely rapid flow of the material to be evaporated over the heating surface, it flowing thereon in as thin a film as possible, and the temperature difference being at the same time as small as possible as already indicated.

Having now described our invention, what we claim is 1. The process of evaporating solutions in a plurality of stages in which each stage of the evaporation is effected by a separate heating element, said heating elements operating at progressively increasing temperatures imparted thereto by vapor which for each successive element has been heated to a higher degree of compression.

2. A method of concentrating solutions in which the solution is first concentrated to a certain degree of heating it in one vessel, col.- lecting vapor evolved therefrom, compressing it and returning it to heating devices of the vessel after which the solution is passed successivelyrthrough other vessels for like devices of successive vessels.

3. A method of concentrating solutions in which the solution is concentrated to a certain degree in one vessel by spreading it as a film over a heating surface, collecting vapor evolved from the film, compressing it to raise its temperature at most about 3 C. above the boiling point of the solution upon the heating surface and passingthe compressed v-apor against the opposite side of said heating surface after which the partially concentrated solution is passed consecutively to further similar vessels, the vapor suppliedto the heating elements of which is compressed to the higher compressions necessary to maintain heat transmission despite the progressive rise in boiling point of the gradually concentrating solution.

4. A method of'concentrating solutions consisting in successively passing the solution over the surfaces of heating elements in each of a plurality of evaporating vessels in the form of a rapidly moving film, 'collecting the vapor from the various parts of said film and introducing said vapor into the heating elements of the various vessels the vapor fed to each heating element being raised by compression to a temperature at most some 3 C. above the boiling point of the solution in contact with that heating element. 1

5. A method of concentrating a solution consisting in successively spreading the solution as a film over the surface of heating elements in each of a plurality of evaporating vessels, collecting the vapor from said film, subjecting portions of said vapor to compresslon until they respectively attain temperatures somewhat above the boiling point of the solution upon the various heating elements, and delivering said portion of "apor so compressed to the corresponding elements.

6. Theprocess of evaporating a solution in a plurality of stages in which each succeeding stage of the evaporation is effected at an increased degree of concentration and a higher temperature corresponding there to and the progressively increasing temperatures are imparted to the evaporating solution by the vapor thereof which has been heated by compression to an increased extent for each succeeding stage.

7 A method of concentrating solutions in stagesconsisting in positively actuating irrespective of the ebullition thereof a stream of solution to trickle as a film down a heating surface, compressing the vapor. evolved therefrom until heated just sufliciently to act as heating medium for said surface and passing on the solution as its concentration increases tosucceeding heating surfaces which are heated by vapor compressed to a' greater extent in each succeeding case. In testimony whereof we affix our signatures in presence of two witnesses.

- OLOF soDERLUND. TEOFRON BOBERG. Witnesses:

F. ASLUND, .BERTRAM H. MATTHEWS.

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