US2747660A - Improvement in the method of operating spray driers for evaporating solutions, emulsions, suspensions, and the like - Google Patents

Description

May 29, 1956 L. o. J. CAMPBELL 2,747,569

IMPROVEMENTS IN THE METHOD OF OPERATING SPRAY DRIERS FOR EVAPORATING SOLUTIONS, EMULSION-S, SUSPENSIONS AND THE LIKE Filed Nov. 29, 1950 In van 2 0 r Cam 012a United States Patent IMPROVEMENT THE METHOD OF OPERATING SPRAY DRIERS FOR EVAPORATING SOLU- Tags, EMULSIONS, SUSPENSIONS, AND THE L Liss Olof Jan Campbell, Jonkoping, Sweden Application November 29, 1950, Serial No. 198,069 Claims priority, application Sweden December 8, 1949 3 Claims. (Cl. 159-48) The present invention relates to a method of evaporating solutions, emulsions, suspensions and the like, and particularly relates to methods wherein the liquid is atomized and spread within a drying chamber having .a gaseous desiccating medium, preferably warm air, flue gases and the like, flowing therethrough. Apparatus for carrying such methods into effect are known, in which the liquid is atomized by means of various spreading means,

such as nozzles or rotating distributing disks, from which.

the liquid particles are hurled out at a very high velocity to become mixed with the warm air flowing through the drying chamber, the water content of the particles being thus caused to evaporate while absorbing the heat of the air. The desiccating medium is preferably introduced into the drying chamber in a manner in which it is compelled to fiow through the chamber in helical paths. In effecting evaporation and drying according to this method it is of very great importance that the liquid particles do not reach the wall of the drying chamber or other solid objects before having dried, inasmuch as the more or less adhesive non-dried particles will otherwise stick to the wall which will be subjected to the warm air, whereby the product will deteriorate considerably and might even become entirely destroyed, as in thecase of milk, for example. Various means and methods are known, which have for their object to prevent the liquid particles from reaching the walls of the drying chamber and depositing thereon. In this connection attempts have been made to prevent the liquid particles primarily from reaching the walls of the actual drying region, that is to say that section of the drying chamber where the particles are hurled out from the generally centrally arrangeddis tributor, and where the percentage of moisture of the particles is the greatest. To this end the drying chamber has been made of a diameter so large that the liquid particles cannot reach the walls through the kinetic energy they possess in the spreading zone, the particlesbeing deviated, through the flow of air, in a direction other than the initial hurling direction generally extending straight against the walls. It has been necessary in this case to make use of diameters in the drying chamber of the order of 5 meters and more. For large working capacities, the drying chamber might under such conditions have a diameter up to meters, when the ejection energy of the particles depends on the out-put, that is to say, the quantity of particles to be spread per unit of time.

Even if by resorting to very great diameters of the drying chamber one would be successful in preventing the particles in their non-dried condition from reaching the wall of the drying chamber, another very great disadvantage from the point of operating economy would be created at the same time. This is so for the reason that at the ordinary drying temperatures the proper quantity of air for every rate of out-put from the point of view of operating economy will be insufiicient to bring about an effective flow through the whole of the cross-sectional area of the drying chamber. Thus the desiccating medium will flow through the drying chamber at a compara- 2,747,660 Patented May 29, 1956 tively low velocity, which in turn involves the drawback that large proportions of the desiccating medium will never have the opportunity to come into contact with the liquid particles. In such plants, it is possible by means of measurements to establish that large regions of the drying chamber are occupied by refiowing eddies and air currents, which are entirely free from liquid or pulverulent particles. These air currents and eddies will, instead, give ofi their heat to the walls of the drying chamber, resulting in very great losses of heat.

The heat requirement of a drying chamber of this type is generally expressed by assigning the quantity of steam required to heat the desiccating air per kg. of water evaporated in the drying chamber. In the drying of socalled whole milk, for example, from measurements in plants in operation values of 3-3.5 kg. of steam supplied by the power plant per kg. of water evaporated in the drying chamber will generally be found. In analyzing the operating conditions, it will be found that of the quantity of heat thus supplied by the power plant approximately 20-30% is wasted as pure heat losses on account of the above-named conditions and the enormous wall surfaces of such drying chambers. Thus the use of drying chambers of large diameter involves a very low operating economy.

The above-mentioned drawbacks are obviated by the present invention which, in addition, involves many novel advantages. It has been found to be possible to characterize the ideal conditions for the obtainment of the advantages aimed at by the invention by fixing a certain relation between the velocity of the desiccating medium in the axial direction of the drying chamber, the temperature of the desiccating medium, and the effective evaporation of water in the drying chamber. According to the invention, the temperature ti of the desiccating medium in degress Centigrade at the entrance to the drying chamber and the temperature ta thereof in degrees centigrade at the exit from the drying chamber, the velocity .V of axial flow of the desiccating medium in meters per second, and the quantity of water G evaporated in the drying chamber in kg./sec. are thus selected so that the expression Zi-tu has a numerical value which is equal to or greater than 5 It is possible in practice to operate with values of this expression of 6, 7, 8, 9, 10 and still higher values.

Withnormal temperatures of the desiccating medium of approximately C., the invention involves a considerable increase in the velocity of flow of the desiccating medium compared with the conditions in the previously known evaporating-apparatus of this kind. Hereby a more intimate and more homogeneous mixture of the air and liquid particles is attained, whereby the evaporation is accelerated, while the heat or" the air can be utilized more effectively at the same time. By reason of the high kinetic energy of the air stream, a more pronounced deviation of the particles from their radial paths of movement and a substantial reduction of the distance traversed by the particles in a radial direction will be obtained. This circumstance, together with the rapid drying, permits the diameter of the drying chamber to be considerably reduced, without incurring the risk of the particles depositing on the walls. Through the reduction of the radial dimensions of the drying chamber, a further reduction of the heat losses due to conduction through the walls will be attained.

In the practical application of the invention, the, mutual relation between the temperature of the desiccating medium, the axial velocity of flow thereof, and the quantity of water evaporated per unit of time maybe varied in different ways. With the same working capacity and the same temperature of theair flowing into the drying chamber, it will thus be possible according to the invention to reduce the diameter of the drying chamber to a fraction of the diameter required in known plants, and at the. same time an improved operating economy and a product of a higher grade will be obtained. Instead of reducing. the diameter of the drying chamber, it is possible to increase the quantity of. desiccating medium, and the greater consumption of desiccating medium is then compensated for by the fact that the evaporation of water increases to a higher degree than does the velocity of desiccating medium. In certain cases it may also be suitable, with the maintenance of the greater diameter and the same consumption of desiccating medium as in the known plant, to increase the temperature of the desiccating medium, the evaporation being thus increased and accelerated.

The advantage of the invention from the point of view of heat economy may be illustrated by specifying the heat requirement of spray drying process according to the invention and that of previously known spray drying processes under the same operating conditions. Thus a steam quantity of the order of 3 kg. of steam to be supplied by the power plant per kg. of water evaporated in the drying chamber is generally required in the known spray drier installations, whereas the corresponding figure for a spray drier installation operating according to the invention is 1.7 kg. of steam to be supplied by the power plant per kg. of evaporated water in the drying chamber. At any rate the heat consumption need not exceed 2 kg. of steam per kg. of water evaporated in the drying chamber.

The inflow of air into the drying chamber may be influenced by means of guide blades provided in an annular inlet around the drying chamber, so that the substantially tangentially inflowing air may be directed more or less inwardly toward the central portions of the chamber. Facility of control will thus be created, in the first place to provide for mixing air and liquid particles as effectively as possible and, secondly, to attain the requisite deflection of the stream of particles with respect to the differing kinetic energy of the particles with a varying specific weight, viscosity and the like.

The invention is illustrated in the accompanying drawing, which shows an embodiment of an evaporating apparatus intended, for instance, for the production of dry milk. Fig. l is a vertical section through the apparatus, and Fig. 2 across section on line A--A in Fig. 1. Fig. 3 shows a cross section on line B-B in Fig. l.

The apparatus comprises a preferably cylindrical drum 1, which encloses the drying chamber and is provided with a bottom 2 and a cover 3. The atomization and spreading of the milk is effected by means of the distributing device 4, which is driven by a motor built into a housing 5. The milk is supplied through a pipe 6, which opens into the distributing device 4. The desiccating air is supplied through a chamber 7 enclosing the upper part of the drum 1 and communicating with the interior of the drying chamber through an annular opening 8 in the wall of the drum. Inserted into this annular opening are guide blades 9 adapted to be turned about vertical axes and to be adjusted so as to permit of being setinto varying angular positions relatively to the radii extending outwardly toward the respective guide blades.v Through this adjustability ofthe guide blades, the entering air streams may be directed more or less inwardly toward the central portion of the drying chamber for the purpose of providing for the best possible result under different operating; conditions. At the same time the air streams are caused to undergo a circulating motion about. the axisof the. drying chamber, said motion combining. with the movement in the axial direction so that the air is caused tov flow through the dryingchamber along helical paths. The air escapes through an outlet 11 arranged adjacent to the bottom 2 and connected to an air conduit 10, said outlet' 11 having guide blades- 12 inserted therein, which are adjustable, like the guide blades 9, into different angular positions for the obtainment of an effective separation of the pulverulent substance from the air current, the blades 12 being pivotally mounted about a longitudinal axis. The dried particles collect on the bottom 2 whence they are removed through apertures 13 in the bottom by means of a rotating scraper 14.

The invention will be further described by reference to a few examples.

The quantity of steam required for a certain capacity, that is to say for a certain value of G, is understood to be the quantity of heat which the power plant has to supply as a whole, and which consequently includes both the heat required for the evaporation in the drying chamber and the heat escaping in the form of losses, partly through the walls of the drying chamber, and partly with the escaping desiccating air. The quantity of heat which the power plant has to supply is supplied by the desiccating air flowing through the drying chamber. For a certain value of the factor G, one may obviously vary the quantity of heat to be operated with. Thus is will be possible to operate, as before, with a steam consumption of the order of 3 kg. of steam per kg. of evaporated water or, according to the novel principle, with a steam consumption not exceeding 2 kg. of steam per kg. of evaporated water. In the latter case one thus has to choose the temperature conditions and the quantity of air so that they will result in a steam consumption in the generating plant not exceeding 2 kg. of steam per kg. of evaporated water in the drier. Here, one may proceed in various ways.

1. The diameter of the drying chamber is given. The axial velocity of the desiccating medium will thus be entirely dependent on the velocity of air, and one only has to adapt the temperature conditions and the velocity of airso that the formula will have the desired value. This may be effected by varying either the velocity of air or the temperature conditions. If the diameter of the'drying chamber is relatively great, the velocity of air must become comparatively large, but at the same time ti should be decreased by a corresponding degree, in order that the stipulated value of the steam consumption shall not become too high. As a practical necessity one may assume that In must not be permitted to become lower than 60-80 degrees centigrade, as with lower values the risk of condensation in outgoing conduits and the like is incurred.

It is also possible, however, to make ti relatively great; consequently, while still keeping the steam consumption down, the value of V, that is to say the velocity of air, will have to be reduced by a corresponding degree at the same time.

Ifthe flow rate of air, that is to say the value of V, and the value of ti, are not correspondingly decreased, an excess of heat is supplied and a larger quantity of water can thus be evaporated, that is to say, the value of the factor G will then increase.

2. The temperature ratio is given. It is believed obvious that the temperatures are selected with respect to the prevailing facilities of heating. Normal values are, for instance, ti=approximately 160 degrees centigrade and, as stated above, tu=approximately 80 degrees centigrade. The temperature relation of the formula thus becomes =l, and the factors V and G will be variable. If a certain value of the factor G is taken,.one thus only hasto determine the value of the factor V. The smaller the diameter of the drying chamber chosen, the greater will bethe value of the factor V, since the velocity of air required at the axial temperature relation established to supply the quantity of heat required for the evaporation of the quantity of' water G is set by the value of G.

Further points of departure may obviously present themselves in the designing of a plant within the scope of the invention.

As will be found from the above examples, the temperature ratio may in many cases be reduced to the value 1. The formula according to the invention then distinguishes a plant so designed that the numeral value of the velocity of the desiccating medium is at least 5 times greater than the numeral Value of the quantity of water to be evaporated. The novel efifect thus attained resides in that with said values the quantity of heat required for the evaporation in the plant becomes considerably lower than could be obtained under otherwise similar conditions in previously existing plants.

The quantity of heat supplied by the air may be allotted to three ditferent regions of consumption within the plant.

1. The heat consumption for direct drying, i. e. the water evaporation.

2. The heat consumption for losses by radiation through the walls of the drying chamber.

3. The heat consumption (or loss) corresponding to the higher temperature of the escaping quantity of air. The air sucked into the power plant for heating is understood to possess the temperature of the external atmosphere, but when escaping from the drying chamber it has a temperature higher than that of the external atmosphere, viz the above-named 80 C.

Among these regions of consumption, the regions No. l and No. 3 are of little interest from the point of view of heat economy. No. 2, on the other hand, is of considerable importance. Partly by reducing the dimensions of the drying chamber and partly by conveying and directing the disiccating air by means of adjustable guide blades and the like, a minimum of losses will be obtained, however, under region 2. While unconditionally maintaining the main principle that the value of the formula shall be numerically at least 5, it is the conveyance of the desiccating air which is responsible for the fact that the desired novel effect, i. e. an exceedingly low steam con sumption per kg. of evaporated water, is obtained, which, again, depends in part on the fact that the losses by radiation are practically entirely eliminated, and, in part also on the fact that it will be possible to reduce tu to a certain extent. It will be found that in spray drying plants previously on the market tu is generally approximately 80 (1., whereas within the drying chamber proper, in the so-called drying zone about the actual spreading system, the temperature is only of the order of 60 C. This temperature increase from 60 to 80 C. depends on the tremendously large drying chambers relatively to the quantity of air employed and the quantity of water evaporated and on the lack in these plants of effective facilities for control of the flow of the desiccating air. Consequently, air currents will be produced that pass through the drying chamber without yielding any heat whatsoever for the evaporation of water, while in passing through the drying chamber they will, instead, give oif a portion of their heat to the walls of the drying chamber, the residual quantity of heat of said air currents being only used for the heating of the mixture of the air intended to leave the drying chamber, so that the temperature of the latter becomes higher than that involved in the drying procedure alone.

I claim:

1. A method of operating a spray drier for evaporating aqueous solutions, emulsions, suspensions and the like which comprises atomizing the liquid to be evaporated and dispersing it radially in a drying zone, introducing a dessicating medium at a temperature ti into said drying zone upstream of the liquid admission and in the vicinity of said atomized liquid and causing said medium to flow axially through said zone along helical paths in contact with said liquid, removing said medium at a temperature m from the opposite end of said zone, and removing the residual evaporation product of said liquid from said opposite end of said zone, the axial velocity V of the desiccating medium in flowing axially through said zone being maintained at a high value such that the expression tu G has a numerical value greater than 5 wherein ti and tu are expressed in degrees centigrade, V is expressed as meters per second, and G is the evaporation rate in kilograms per second of water evaporated in the drying zone.

2. A method as defined in claim 1 wherein the desiccating medium is introduced into the drying zone in a plur=ality of coplanar streams spaced circumferentially of the drying zone.

3. A process as defined in claim 1 wherein said liquid is milk.

References Cited in the file of this patent UNITED STATES PATENTS 1,107,784 Gray Aug. 18, 1914 1,157,935 Gray Oct. 26, 1915 2,619,942 Rydberg Dec. 2, 1952 FOREIGN PATENTS 357,205 Germany Aug. 19, 1922

Claims (1)

1. A METHOD OF OPERATING A SPRAY DRIER FOR EVAPORATING AQUEOUS SOLUTIONS, EMULSIONS, SUSPENSIONS AND THE LIKE WHICH COMPRISES ATOMIZING THE LIQUID TO BE EVAPORATED AND DISPERSING IT RADIALLY IN A DRYING ZONE, INTRODUCING A DESICCATING MEDIUM AT A TEMPERATURE TI INTO SAID DRYING ZONE UPSTREAM OF THE LIQUID ADMISSION AND IN THE VICINITY OF SAID ATOMIZED LIQUID AND CAUSING SAID MEDIUM TO FLOW AXIALLY THROUGH SAID ZONE ALONG HELICAL PATHS IN CONTACT

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