Spray drying apparatus

United States Patent ^[l11 3,618,655

<td> [72]</td><td> Inventor</td><td> Raymond M. Lockwood Los Altos, Calif.</td>

<td> [21]</td><td> Appl. No.</td><td> 849,589</td>

<td> [22]</td><td> Filed</td><td> Aug. 5,1969</td>

<td> [45]</td><td> Patented</td><td> Nov. 9,1971</td>

<td> [73]</td><td> Assignee</td><td> Marine Technology, Inc.</td>

Beverly Hills, Calif.

Continuation of application Ser. No. 574J02, Aug. 22, 1966, now abandoned.

[54] SPRAY DRYING APPARATUS 7 Claims, 4 Drawing Figs.

[52]

[51]

[50] ^U,S·^Cl....................................................... 159/4 E,

Infri 159/4 AUX

FUM .....u............................................ BOId V16

Field of Search........................................... 99/209

199,200,203,208; 159/4,48,4 A?4 E, 4 B, 1 A

[56 J References Cited

UNITED STATES PATENTS

1,722,175 7/1929 Bowen..........................

2,032,827 3/1936 Andrews......................

159/4 A

241/5

<td> 2,054,441</td><td> 9/1936</td><td> Prebles........................</td><td> 159/4 E</td>

<td> 2,561,394</td><td> 7/1951</td><td> Marshall................</td><td> 159/4 EX</td>

<td> 2,715,390</td><td> 8/1955</td><td> Tenney et al................</td><td> 122/24</td>

<td> 2,738,334</td><td> 3/1956</td><td> Tenney et al................</td><td> 252/359</td>

<td> 2,838,869</td><td> 6/1958</td><td> Desbenoit et al......</td><td> 43/147</td>

<td> 2,887,390</td><td> 5/1959</td><td> Coulter etal..........</td><td> 99/199</td>

<td> 2,926,855</td><td> 3/1960</td><td> Durr et al...................</td><td> 239/129</td>

<td> 3,261,695</td><td> 7/1966</td><td> Sienciewicz et al.......</td><td> 99/207</td>

<td> 1,213,887</td><td> 1/1917</td><td> Krause....................</td><td> 159/4</td>

<td> 1,877,648</td><td> 9/1932</td><td> Douthitt......................</td><td> 159/4 K</td>

Primary Examiner—Norman Yudkoff

Assistant Examiner—J. Sofer

Attorneys- Alan C. Rose, Walter R. Thiel, Alfred B. Levine and Michael L. Wachtell

ABSTRACT: The apparatus and method of this invention pertains to the atomizing of a paste or slurry, and the drying thereof to produce a meal or powder. A paste or slurry of material to be dried is introduced into a pipe, such as the exhaust pipe of a pulse jet engine, in which the cycling reversing flow of gases atomizes the paste or slurry and rapidly dries the resulting particles. The partly dried particles are then dispensed into a tank having vortices of gas at a substantially lower temperature than that found in the pulse jet exhaust.

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PATENTEDnov 9 1971

3,618,655

SHEET 1 OF 2

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Raymond M. lochvood

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PATENTEDnov 9 1971

3,618,655

SHEET 2 OF 2

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3,618,655

SPRAY DRYING APPARATUS

This application is a continuation of Ser. No. 574,202 filed Aug. 22,1966, now abandoned.

This invention relates to drying equipment and processes, and more particularly, to a novel spray drying apparatus and process in which the material to be dried is atomized and dried by the pulsating and oscillating flow of a steam of hot gases.

The general process known as spray drying has been in use for many years. The process has been widely used to remove moisture to obtain or recover a solid material which has been in suspension or solution in a fluid. Typically, the fluid is atomized by a rotating wheel or drum, and the resultant spray is subjected to a flow of hot gases to evaporate the moisture from the spray. The solid particles are then carried from the drying chamber by the flow of the drying gas and are removed from the gas by means such as the cyclone separator.

The applications for which such drying equipment and methods could be used have been restricted by several factors. First, the liquid being dried must be a liquid having a relatively low viscosity in order that it may be easily atomized so that the moisture may be evaporated by the gas flow. Second, even when the liquid has been properly atomized, it has has been found that the process is not particularly efficient because excessive amounts of heat are required to evaporate the liquid. Investigation has shown that the reason for the inefficient heat ²³transfer is a thin film, or barrier layer, of moist air that is formed around each particle after atomization. This barrier layer inhibits the transfer of moisture from the interior of the particle to the stream of drying gas. For the above reasons, the applications of spray drying techniques had been restricted to those applications where efficiency is less important than precise control of temperature-sensitive products, such as the drying of milk of the like.

Part of the above-mentioned objections to the spray drying process are overcome in a relatively recent development known as jet spray drying. In a typical jet spray drying installation, heated gases are forced at a high but subsonic velocity, typically about 700 feet per second, through a pipe having a relatively small diameter. The fluid to be dried is injected into the pipe in the same direction of travel as the heated gases but at a velocity much less than the velocity of the gases. The velocity difference causes the hot gases to atomize the fluid and to wipe or scrub the surface of the atomized particles to remove the barrier layer as it is formed, thereby promoting the 45 efficient transfer of heat from the gas to the particles and moisture from the particle to the gas. A typical arrangement for such a jet spray drying apparatus is seen in U.S. Pat. No. 2,880,794, issued Apr. 7, 1959 to W. R. Marshall, Jr. for a “Spray Drying Process”.

However this type of drying apparatus is still subject to the first of the above-mentioned limitations as to what substances can be dried in the apparatus. Also, since the particles after atomization are extremely light, the high velocity gases „ _______________________ quickly accelerate the particles to a velocity near that of the 55 P^{arts until} such time as the fuel supply is cut off. It is also seen gas. The wiping or scrubbing action then ceases and the moist barrier layer once again develops around the particles to inhibit further removal of moisture from the particles.

It is accordingly an object of the present invention to provide an improved spray drying process and apparatus.

It is a further object of the present invention to provide an improved spray drying process and apparatus in which the barrier layer around the particles being dried is almost completely eliminated to provide efficient heat transfer and moisture removal.

It is yet another object of the present invention to provide drying process and apparatus which can be used to dry materials which heretofore have not been susceptible to conventional drying processes such as spray drying or any similar drying process.

It is still another objective of the present invention to provide an improved method and apparatus whereby a solid but high moisture content material such as whole fish can be quickly and efficiently converted to a dry meal using simple and relatively inexpensive equipment.

In the prior art the conversion of whole fish to dried fish meal has required an elaborate and expensive plant and a multistep process with considerable handling and conveying of the material during the process. Conventionally the fish are first cooked by steam and the whole cooked fish are then pressed to remove the oil and part of the moisture from the fish bodies. The cooked fishcake is removed from the press and placed in a rotary dryer to remove the rest of the moisture from the cake. The material is then removed from the rotary dryer and ground into meal. The oil and water mixture which was removed at the press is then placed into a centrifuge for separation. The water from the centrifuge, called the stick water, cannot be discarded since it contains much of the nutrients from the fish in the form of water soluble proteins. These water soluble proteins in the stick water can either be returned to the fish meal by returning the stick water to the rotary dryer for drying with the fish cake from the press or can be retained independent of the fish meal for use as fertilizer or the like.

It is a particular object of the present invention to provide an improved method and apparatus for converting fish to fish meal which does not require the expensive equipment and handling of the above-described prior art process.

It is another particular object of the present invention to provide an improved method and apparatus for converting fish to fish meal which efficiently and economically uses spray drying techniques.

The above-mentioned objects of the present invention are achieved through the use of a device known as a pulse jet engine. Such engines have been known in the aircraft industry for many years. These engines consist of a combustion chamber, an inlet pipe, and an exhaust pipe. Though structurally quite simple, the design and operation of these engines depends upon precise control of critical design parameters. These engines, at least in their valveless form, have no moving parts, and once started, continue operating in a resonant manner until their fuel supply is cut off. The engines are started by supplying fuel, compressed air, and a spark to the combustion chamber. The fuel and air mixture explodes, creating hot gases which are driven out both the inlet and exhaust pipes of the engine. The explosion causes an overexpansion in the combustion chamber, which results in a partial vacuum in the combustion chamber after the explosion, This partial vacuum causes a fresh supply of air to be drawn in through the inlet pipe and part of the hot exhaust gases to be drawn back from the exhaust pipe into the combustion chamber. More fuel is injected into the combustion chamber, ^and hot exhaust gases drawn back into the chamber provide the heat to ignite the new mixture of air and fuel. It is thus seen that continued operation requires no spark or compressed air to maintain operation once the operation is started and that the engine will continue operation with no moving that there is an oscillating supply of hot gases in the exhaust pipe which is continuously reversing direction at the resonant or operating frequency of the engine and there is a pulsating supply of hot gases being emitted from the exhaust pipe.

Such pulse jet engines have been used in the past primarily for thrust in jet aircraft, but have also been used in fog generators in which the fog causing substance is injected into the exhaust pipe of the engine to be sprayed out into the atmosphere as fog. Such a device is shown in U.S. Pat. No. 2,857,332, is65 sued Oct. 21, 1958 to W. L. Tenney et al. for a “Machine for Producing Dispersions of Liquids in Air or other Gases for the production of Fogs.”

Briefly stated, and in accordance with one embodiment of the present invention, a system for drying a solid particle bear70 ing fluid is provided which includes a pulse jet engine such as was described above. The particle bearing fluid, which may be, for example, a slurry formed from grinding whole fish, is injected directly into the exhaust pipe of the pulse jet engine. The reversing hot gases in the exhaust pipe atomize the fluid 75 into a fine spray even though the fluid any have been highly

3,618,655 viscous before injection. The hot gases also evaporate the moisture from the spray and the oscillating velocity of the gases causes the hot gases to continuously wipe or scrub the surface of the atomized particles to remove the barrier layer from the particles, thereby efficiently transferring heat into the particles and removing heat from the particles. The spray is injected from the exhaust pipe into a drying zone in which the drying of the fluid by the pulsating hot gases is completed. The dry particles are then collected from the drying zone, either by direct removal from the drying zone or by separation from the gas stream by conventional methods.

The accompanying drawings disclose the presently preferred embodiment of the invention and illustrate other objects and advantages of the invention. In these drawings:

FIG. 1 shows a block diagram of a drying system in accordance with the present invention,

FIG. 2 shows details of one of the pulse jet engines which may be used in the dryer of FIG. 3,

FIG. 3 shows details of a drying system, partially in cross section and partially schematically, using the presently preferred embodiment of the invention, and

FIG. 4 shows a cross-sectional view taken along the lines 4—4 of the dryer of FIG. 3.

FIG. 1 shows a block diagram of a drying system incorporat- 25 ing the present invention. A pulse jet dryer 10 such as is later described in detail is fed with a solid particle bearing fluid from a suitable source 12. In the preferred embodiment of the invention later described the dryer is used to convert whole fish into fish meal so that in that embodiment the fluid would 30 be ground whole fish, which has the consistency of a paste or thick slurry. A suitable fuel such as gasoline or diesel oil is provided to the dryer 10 from a source 14. The solid particle output from the pulse jet dryer 10, in this case a dried fish meal, is collected in a suitable material collector 16.

FIG. 2 shows details of a pulse jet engine 20 which is used in the present invention. The engine includes a combustion chamber 22, inlet pipe 24, an exhaust pipe 26 and an augmenter 28. In the shown engine 20, the exhaust pipe 26 is bent substantially 180° so that the direction of its exhaust is parallel 40 to the direction of the output of augmenter 28.

Such a pulse jet engine arranged as shown is a conventional one well known in the aircraft industry. The augmenter 28 serves to increase the flow of air therethrough by a factor of 15 to 20 over the output from the inlet pipe 24. It is noted that 45 although pipe 24 is called the inlet pipe and pipe 26 the exhaust pipe, substantially equal volumes of hot exhaust gases are emitted through each pipe. However, because inlet pipe 24 is substantially shorter than exhaust pipe 26, during each cycle of operation of the engine the output pulse of exhaust gases completely clears the end of inlet pipe 24 but is still partially contained in the longer exhaust pipe 26. When the over expansion of gases caused by the explosion results in a partial vacuum in combustion chamber 22, fresh air is drawn back into the chamber through inlet pipe 24 and part of the exhaust gases are drawn back which were still in pipe 26. Fuel is injected into combustion chamber 22 by a suitable fuel line not shown in FIG. 2, and this fuel mixes with the fresh air drawn in through inlet pipe 24 and is exploded by the hot gases drawn back through exhaust pipe 26, thereby repeating the operational cycle of the engine.

Continuing with the description of FIG. 2, the fluid to be dried is injected though a pipe 30 into the exhaust pipe 26 of the engine. The pipe 30 penetrates the exhaust pipe at 32 and 65 extends down the axis of exhaust pipe 26, terminating near the end of the exhaust pipe as shown. The fluid is thus preheated during its passage in pipe 30 along exhaust pipe 26. The termination 34 may be a simple termination of pipe 30, or the end of the pipe may be flared as shown if desired. The distance between the termination 34 and the end of the exhaust pipe 26 may be varied depending upon the nature of the substance being dried and the amount of drying which it is desired to effect within the exhaust pipe itself. If the material being dried is not particularly temperature sensitive and if more heat 75 transfer is desired within the exhaust pipe 26, the termination 34 of pipe 30 may be further from the end of exhaust pipe 26.

As has been previously described, when the engine is operating, the exhaust pipe 26 contains hot exhaust gases which flow back and forth within the pipe, with a net movement out of the pipe, of course. In accordance with the present invention, when the fluid is injected into the exhaust pipe 26 through pipe 30, the fluid is quickly broken up and atomized into a spray even if the fluid is very viscous, such as a ground fish slurry. Drying of the spray begins almost instantaneously since the surface of the atomized droplets is continuously scrubbed by the back and forth movement of the hot gases, thereby removing the above described barrier layer and effecting efficient and rapid heat transfer and moisture removal. The partially dried spray leaves the end of exhaust pipe 26 and is injected into a drying zone not shown in FIG. 2. The hot gases output of augmenter 28 is also injected into the drying zone in which the drying of the spray is completed.

The drying zone may be a simple cylinder whose inside diameter is larger than the distance between one edge of the exhaust pipe 26 and the opposite edge of augmenter 28 or may be a more complex and sophisticated arrangement such as is described in detail in FIGS. 3 and 4 below.

The proportion of drying which is done in the exhaust pipe 26 itself and in the drying zone can be regulated by changing the position of termination 34 of pipe 30. In the presently preferred embodiment of the invention, termination 34 is positioned so that 15 to 20 percent of the moisture in the fish slurry is evaporated in the exhaust pipe 26, and the remaining 80 to 85 percent of the moisture to be removed is evaporated in the drying zone. Of course heat transfer is more efficient in the exhaust pipe itself because of the violent scrubbing action caused by the rapidly reversing flow of hot gases; however the temperature of these gases is about 3,000° F. and the dried fish meal would quickly bum if subjected to these temperatures for any substantial period of time. Accordingly, most of the drying is allowed to take place in the lower temperature drying zone. That the temperature is lower can be seen by the fact that the augmenter is supplementing the flow of exhaust gas through it with approximately 15 to 20 times as much fresh air, thereby reducing the temperature of these gases to about 200° to 300° F. Thus the material sprayed into the drying zone is subjected to a high volume flow of pulsating gases having a temperature of about 200° F. In the embodiment to be later described in detail pulse jet engines were used which had a natural resonant frequency of about 150 cycles per second. Thus in this embodiment the particles are subjected to a scrubbing action of hot gases 150 times per second which effectively removes the barrier layer around the particles, but which at the same time never allows the particles to be accelerated to substantially the gas velocity. Instead there is always a substantial and varying difference in the velocity of the particles and the drying gases.

FIGS. 3 and 4 show details of a complete fish drying system in accordance with the present invention. FIG. 3 is a view practically in cross section and partially schematically of such a fish drying section and FIG. 4 is a sectional view along the lines 4—4 of FIG. 3.

The fish which is to be converted into fish meal is placed in hopper 40 and fed into a grinder 42 which grinds the whole fish into a slurry or thick paste consistency. In the preferred embodiment of the invention the fish may be either frozen or thawed when it is placed in the hopper and no additional water or substance is mixed with the fish before grinding. However, since a fish body is over 75 percent water, it contains sufficient moisture to give the ground fish the necessary fluid properties.

Pump 44 pumps the fish slurry through pipe 46 to a pulse jet engine 48 similar to that described in FIG. 2 above. Because of the highly viscous nature of the fish slurry, a fairly powerful pump must be used to move the slurry through the pipe. In one constructed embodiment of the invention the inside dimension of pipe 46 was 1 inch and the pump was located about 10 feet from the pulse jet engine. The pump has an output pres70

3,618,655 sure at the pump of about 200 pounds of pressure per square inch which has dropped to about 20 pounds per square inch by the time it reaches engine 48. If desired, a valve 50 may be provided in pipe 46 to control the rate of flow of the fish slurryPulse jet engine 48 is located in the bottom of a cylindrical tank 52, which tank serves as the drying zone discussed in FIG. 2. Pulse jet engine 48 is mounted on the bottom of the tank and the augmenter and exhaust pipe of the engine extend through the bottom of the tank as shown in the drawing.

As shown in FIGS. 3 and 4, four other pulse jet engines are also employed in connection wit tank 52, with the four other engines being positioned around the bottom of tank 52 spaced equally from each other. Pulse jet engines 54 and 56, shown in FIGS. 3 and 4 are positioned with their axes elevated approximately 30° with respect to the horizon, as shown in FIG. 3 and with their axes pointed approximately midway between the axis of the tank and the sidewall of the tank at the adjacent pulse jet engine, as shown in FIG. 4. Pulse jet engines 58 and 60 are shown only in FIG. 4. These pulse jet engines, in the preferred embodiment, have their axes horizontal and approximately tangential to the walls of the tank at the point where they contact the tank. Each of the engines 54, 56, 58 and 60 has an augmenter similar to augmenter 28 associated with both its inlet and exhaust pipe and in each case, the augmenter terminates substantially flush with the interior surface of tank 52.

Tank 52 also includes a truncated cone 62 and a plate 64 located at the bottom and top of the tank respectively along the axis of the tank. The function of these is later discussed.

Fuel is supplied to all of the pulse jet engines from a tank 66 through a fuel line 68 which is controlled by a valve 70. The fuel may be practically any fluid which is combustible. Pulse jet engines have been known to operate even on powered coal. However, in the preferred embodiment, the fuel is either gasoline, diesel oil or natural gas, because of their ready availability, relative cheapness, and relatively high B.t.u. content per pound.

Consider now the air currents set up within tank 52 when the pulse jet engines 54, 56, 58 and 60 are operating. Although the precise air current paths are not perfectly understood, it is believed that a series of concentric cyclonelike Vortices having alternate upward and downward directions are established in the tank. Two such vortices are shown in FIG. 3, with the inner one being an upward spiral and the outer one being a downward spiral. It has been found that the axes of these vortices tend to be unstable and that the provision of truncated cone 62 on the bottom of tank 52 and plate 64 suspended from the top of tank 52 tends to stabilize, these vortices around the axis of the tank. When all four side engines are operating pulsating vortices are established within the tank.

In the shown embodiment of the invention, the temperature varies from about 400° F. at the bottom of the tank to about 200° F. at the top of the tank. These temperatures can be varied by both varying the rate of flow of fuel to the engines and by varying the rate that exhaust gases leave the tank, thereby varying the internal pressure in the tank. By so varying the internal pressure in the tank, the amount of cooler secondary air pumped through the augmenters is varied to vary the temperature of the output gases from the augmenters.

Consider next what happens when the four side engines are operating as described, and pulse jet engine 48 is started and supplied with the fish slurry. As was described in connection with FIG. 2, the slurry is atomized and partially dried in the exhaust pipe of engine 48 and is injected into the interior of tank 52 upwardly as shown into the established vortices. In this zone the atomized fish slurry is subjected to a pulsating flow of hot gases which inhibits the development of a moisture barrier layer around the atomized particles. The hot scrubbing action of the gases quickly and efficiently transfers heat into the particles and removes moisture from the particles. As the fish meal dries, it is circulated around the interior of tank 52 by the circulating gases, with the lighter particles of fish meal being carried out exhaust pipe 72 with the used gases and the heavier particles settling to the floor of tank 52. In the preferred embodiment it has been found that approximately one-third of the fishmeal is carried out pipe 72 and approximately twothirds settles to the bottom of the tank. This proportion may be varied by varying the placement and angles of the side pulse jet engines.

Since there is a slight positive air pressure in the tank, the heavier fish meal particles may be continuously removed from the bottom of the tank by openings at the bottom of the tank, shown schematically at 74. The lighter particles may be removed from the exhaust gas stream by a conventional cyclone separator 76 whose output falls into bin 78.

There is an additional factor which it is believed contributes to the efficiency and speed of drying, but whose effect is not fully understood. This is the extremely high level noise output of the pulse jet engines. In the interior of tank 52, the noise level is about 150 db. relative to the standard level of 0.0002 dynes per square centimeter when all five pulse jet engines are operating. It is believed that this high noise level in the form of pressure waves whose fundamental frequency of operation of the engines contributes further to the relative velocity of the particles and the drying gases to provide the scrubbing action which inhibits the build up of the moisture barrier layer around the particles.

In a pilot plant constructed in accordance with the described preferred embodiment of the invention, the diameter of tank 52 was about 8 feet and its height about 16 feet. The four side pulse jet engines each had a heat capacity of 1,000,000 B.t.u. per hour and the injector pulse jet engine 48 had a heat capacity of 2,000,000 B.t.u. per hour. Gasoline was used as the fuel and has a specific heat capacity of 20,000 B.t.u. per pound. Since the pulse jet engines are about 98 to 99 percent efficient in converting the fuel to heat and kinetic energy, each side pulse jet engine required about 50 pounds of fuel per hour and the injector pulse jet engine required about 100 pounds of fuel per hour. The side engines operated at a frequency of about 150 cycles per second and the injector engine at a frequency of about 130 cycles per second. The plant was capable of processing about 4,000 pounds of raw fish per hour into about 1,000 pounds of fish meal.

The above description of the preferred embodiment of the invention describes its operation in connection with fish meal drying. The same apparatus has been used to dry other fluids such as tomato paste, crushed bananas, and milk. In each case, the operation has been similar and has been found to be quick and efficient. By proper control of the above described variables, it has been found possible to dry these substances without any temperature damage to the dried product.

While the invention is thus described and the presently preferred embodiment discussed in detail, numerous modifications of this embodiment will be obvious to those skilled in the art which still lie within the spirit and scope of the invention. Thus, for example, additional pulse jet engines may be provided to increase the capacity of the drying system or a drying zone having a different configuration to fit a specific application may be used instead of the disclosed arrangement. It is thus intended that the invention be limited only by the scope of the appended claims.