US3396475A - Freeze drying system - Google Patents

Description

ug. 13, 1968 E. G. SCHEIBf-:L 3,396,475

FREEZE DRYING SYSTEM Filed Jan. 10, 1966 & fief/v ,/9

United States Patent O 3,396,475 FREEZE DRYING SYSTEM Edward George Scheibel, 75 Harrison Ave., Montclair, NJ. 07042 Filed Jan. 10, 1966, Ser. No. 519,602 Claims. (Cl. 34-5) ABSTRACT 0F THE DISCLOSURE A freeze drying system characterized by direct freeze drying contact between liquid feed and a chilled carrier gas at sub-atmospheric pressure. The carrier gas recirculates through a cycle in which the carrier gas and the vapors evolved by the freeze drying are compressed, then lprogressively chilled to a sub-freezing temperature level. The evolved vapors condense during the course of Chilling, either being removed as a liquid condensate or deposited in frozen state in the system. The chilled compressed carrier gas is expanded to further reduce the temperature level, then is employed to chill the compressed carrier gas by indirect heat exchange therewith and to freeze dry the liquid feed by direct Contact therewith. Periodically, the feed is halted, and the system is regenerated by owing relatively hot gas directly from the compresser through the ice laden portions of the system to melt frozen condensate.

The present invention relates to a freeze-drying system of high efficiency with low power requirements.

Conventionally freezedrying procedures are affected by subjecting the starting material to high vacuum conditions and to temperatures sufficiently low to freeze the material. Sublimation reduces the water content of the frozen material to the desired freeze-dried level. The evolved water vapor is compressed and thereafter condensed, usually to solid ice, in a refrigerated chamber.

Heat transfer to the frozen solid is poor largely because the thermal conductivity of the gas is negligible at the relatively low vacuum which is maintained in the freeze dry chamber, and because all heat necessary for sublimation of the ice must be conducted through the frozen solid. In consequence, such freeze dryers ordinarily operates at a small fraction even of the equilibrium vapor pressure of ice, the absolute pressure frequently being as low as tenths or hundreths of a millimeter Hg.

yMaintenance of freeze drying conditions and removal of the evolved water vapor involves high compression ratios (in the thousands).

Characteristically conventional freeze drying features require considerable power, time and labor, making for relatively high cost operation. The principal industrial applications of freeze drying, therefore, are relatively high priced materials, e.g., penicillin.

The principal object of the present invention is to provide an efficient low cost freeze drying technique.

Further objects and the advantages of the present invention will become apparent from the description which follows.

Briey stated the present invention involves freeze drying a pre-chilled liquid feed material by contact with a cold recirculating inert gas. The gas and evolved water vapor is warmed to about ambient temperature, then compressed to 12S-3.0 times the absolute pressure of the freeze drying step. Thereafter the compressed gas is chilled progressively to a suitably low subfreezing ternperature. During the course of chilling, the evolved water vapor content in the recirculating inert gas is condensed (principally to ice). The chilled, now cold, gas is then expanded essentially adiabatically in an expander engine to the operating pressure of the freeze drying step. 'The ice expansion serves to recover an appreciable fraction of the power originally required to compress the gas and to lower the gas temperature still further. The temperature differential achieved by low temperature expansion provides the necessary temperature differential to operate the entire freeze drying system.

The expanded cold gas is warmed to about 10-30 F. by heat exchange against the higher pressure gas being progressively chilled thereby; thereafter the still cold gas directly contacts the material to be freeze dried, removing its water content therefrom as water vapor. Finally the gas and water vapor is again passed in heat exchange with the freshly compressed gas, thereby being warmed to the about ambient temperature of compression.

Employment of the above described inert gas cycle improves thermal conductivity of the gas phase, significantly increasing the sublimation rate of ice. The efuent gas and water vapor leaves the freeze dry chamber at a ternperature near the freezing point with a water vapor partial pressure of several millimeters, approaching even the theoretical of 4.58 millimeter Hg- In addition, a surprisingly low compression ratio, e.g., 1.5, is all that is required to operate the freeze drying system. Moreover, as much as half of the work of compression is recoverable in the expander engine. Overall the power requirements of the present system are almost negligible compared t0 other freeze drying procedures.

Potentially the present procedure is applicable to all concentration problems where prolonged exposure t0 temperature levels at which the solvent (it need not be Water) in liquid phase has deleterious effect on the concentrated product. In food products, dehydration in the continued presence of water (liquid phase) frequently alters flavor or taste; in essence or perfumes so drying affects the aroma; and in vitamins and pharmaceuticals drying at liquid phase temperatures frequently results in some product decomposition with loss of potency.

For further understanding of the present invention reference is now made to the attached drawing wherein is shown, diagrammatically, a flow sheet embodying a mode of the present procedure and which is described below in terms of a specific example of a preferred ernbodiment of practice thereof. For clarity the exemplary temperature levels have been illustrated on the drawing.

Recirculating inert gas, suitably air (62 F.) is compressed from 0.5 atmospheres absolute to 0.75 atmospheres absolute in compressor 10 (at the rate of 12,100 lbs. per hour). A major portion of the heat of compression is removed (by heat exchange against cooling water) in cooler 12, eg., cooled from F. to 80 F. The recirculating compressed air will be about only 35% saturated (with water vapor) under these conditions. It is cooled to 48 by exchange against the effluent from the spray dryer in exchanger 14 and then cooled further in the upper section 50 of exchanger 18 to 34 F. where it is removed, passed through settling tank 16 to remove liquid water and then returned to exchanger 18 for further cooling in lower section 51 to 20 F. During this stage of the cooling, ice will build up rapidly in heat exchanger 18 since the bulk of the water vapor initially present in the recirculating compressed gas is removed in this section 51 of heat exchanger 18 during the course Of chilling the gas from about 34 F. to the subfreezing level (-20 F.). In the present exemplary instance 10.9 lbs. per hour of water are removed from settler 16, and 61.8 lbs. per hour of ice build up in lower section 51 of heat exchanger 18.

The chilled gas leaving heat exchanger 18 is expanded under load in expander engine 20 to approximately 0.5 atmospheres (absolute). T he work recovered in expander engine 20 is employed to supply some of the load of the 3 compressor 10. The snow formed during the essentially adiabatic expansion in expander engine 20 is collected in settling tank 22 (2.3 lbs. per hour) and the expanded cold 'air (now about 40 F.) passed back into heat exchanger 18 wherein the cold gas is in countercurrent heat exchanger relation to the higher pressure air being chilled. As shown in the drawing, heat exchanger 18 is constructed so that the cold gas passes also in countercurrent direct Contact lwith the freeze dried solids. The freeze dried solids are withdrawn via outlet 19 to collection chamber 21. From collection chamber 21 the freeze dried solids can be removed intermittently without destroying the partial vacuum (1/2 atm.) inside heat exchanger 18.

The cold gas stream passing through heat exchanger 18 is warmed by indirect heat exchange against the higher pressure gas stream and then by direct contact with the freeze dried solids causes the ice to sublime into the gas stream. A small amount of heat is also transferred directly to the solids to assist in the sublimation of the ice. Ultimately, t-he gas stream leaves heat exchanger 18 and enters spray chamber 24 warmed from 40 F. to 27.5 F.

Desirably, the liquid feed solution (eg. 100 lbs. per hour of coffee extract, 75% water, 25% solids) entering spray chamber 24 via line 26 has been pre-chilled, e.g., to about 40 F. The liquid feed spraying into chamber 24 is quickly cooled by evaporation and by direct heat transfer from the low pressure gas stream passing through chamber 24. The air and evolved water vapor leaves spray chamber 24 via line 25 at about 30 F. The frozen solids pass down through heat exchanger 18 in direct contact with the colder gas ultimately leaving the system at the rate of 25 libs. per ihour via outlet 19 in freeze dried state at about the temperature of the incoming cold gas (i.e., -40 F.). Y

The gas 12,100 lbs. per hour air, 76.9 lbs. per hour of water vapor leaving spray chamber 24 via gas outlet 25 passes through heat exchanger 14 wherein it is war-med to about 62 F. by heat exchange against compressed gas, then to compressor to repeat the gas cycle.

As has already been indicated the present system provides for rapidly freeze drying a solution with relatively 'high efliciency. Typically, the gas leaving the spray chamber 14, e.g., air, 90% saturated with water vapor, contains about 0.01 mol fraction of water vapor therein, a partial pressure of several millimeters Hg, a value ywhich Vapproaches the theoretical 4.58 millimeter Hg.

A distinct advantage of the present system is its relative lack of sensitivity to minor unbalances. Thus, for example, in small scale units Where appreciable heat may leak into the system the refrigeration obtained by expansion of the chilled gas in expander engine will create a smaller temperature differential (than 20 F.) between the high and the low pressure gas streams. However, since the minimum temperature differential theoretically required for operation of the present freeze dry system is only 6 F., the heat leak will not seriously affect operation. As a practical matter, the affect of heat leakage is to change the operational period before regeneration is required to remove ice deposited in the high pressure side of heat exchanger 18.

Mention has already been made of that much of the water vapor taken up in the gas cycle will be deposited out as ice on the high pressure side of heat exchanger 18. As ice builds up there the outlet pressure and temperature of the gas being chilled are affected until freeze dry operations must cease, if only because ultimately ice deposition would block gas flow altogether. Practice of the present invention includes Ihalting freeze dry operation periodically for regeneration purposes. Regeneration is affected by shutting off the liquid or slurry feed line 26, then sending the hot compressed gases via by-pass lines 35 directly from compressor 10 to the lower section of the heat exchanger 18 (by passing the cooler 12, heat exchanger 14 and the upper section of heat exchanger 18). Within a few minutes, e.g., l0 minutes, the hot gas will melt all of the ice in exchanger 18 andy the snow collected in settler 22 so it can be drawn off via line 36 to settler 22 as liquid water. The cold gas from settler 22 is Iheated and recycled to the compressor via line 37, heater 38. The system may be started up by once again sending the circulating gas through cooler 12 and heat exchangers 14, 18 and expander engine 20 and then when temperatures have reached the desired level once again introducing the feed material from inlet line 26. Practice of the current invention on an industrial scale specifically contemplates having fve or six units in parallel (not shown) so that continuous operation can be obtained with one of the units always on regeneration. Also a vacuum pump (not shown) is needed to create the desired sub-atmospheric pressure levels in the system.

VThe heat transfer surfaces provided in heat exchangers 14 and 18 should be of the extended surface type with linning virtually essential in section 51 on the high pressure `side of heat exchanger 18. Fins provide a sufficiently large surface area for substantial deposition of snow or ice formed during cooling of the high pressure stream before the surface becomes so insulated by the ice as to reduce heat transfer rates to uneconomic limits. If small fin tubes are employed, lower section 51 of exchanger 18 will become plugged in a relatively short time and automatically shut down the system by reducing the pressure differential on the expander engine 20. By way of example, fin tubes about 4 inches in diameter can operate for about an hour before ice plugging will halt operation. It should also be recognized that the exclhange surface provided can be in excess of that normally required for noncondensing service since, as the surface becomes coated with solid it becomes insulated and the condensation automatically progresses to the following area of surface.

Ordinarily the recirculating carrier gas will be air but when the material to .be spray dried is susceptible to air oxidation, employment of a more inert gas may be advisable. Such gases include, for example, nitrogen, the noble gases (helium, neon, argon, krypton, xenon), or common gaseous refrigerants such as the well known Freons. Each have their special advantages. The noble gases have a lower specific lheat than the Freons or nitrogen, but in the case of helium this is partially offset by the increased diffusivity therein of water vapor. On the other hand the higher molecular weights of Freons give more efficient operation in a centrifugal compressor, requit-ing less stages, and this feature is the major advantage of the Freons, assuming of course that they are nonreactive with the freeze dried product. Nitrogen is the least expensive gas, but is intermediate in its other properties between helium and the Freons. The prime requirement for the carrier gas is, of course, that it be inert to the freeze dried product.

In specific application the procedure of the present invention may be employed in the freeze drying of food products, which can be made to retain their natural flavor thereby, e.g., coffee, milk, lorange juice, other citrus fruits. The present procedure is contemplated also for the drying of fruits, vegetables and eggs.

The present procedure is not limited to freeze drying of aqueous solutions only. Other volatile solvent solutions may be dried in the same manner, even with solvents which do not solidify to ice at the desired minimum operating temperatures. Where freezing is involved, the heat exchanger 18 is appropriately divided as illustrated to remove liquid condensate (eg. water) from its upper section 51 at just above the freezing point. If the solvent does not freeze at all, the heat exchanger 18 need not be divided and periodic regeneration unnecessary. The drying system could then be a continuous operation.

As has already been indicated, a principal advantage of the present procedure is its low power requirements.

Operating between atm. and 1/2 atm. expander engine 20 recovers about 40% of the power input to the process, and in consequence only about 0.8 kwh. is required per pound of water removed from the feed solution. Operation with the low pressure side at atmospheric pressure and the high pressure side at 1.5 atmospheres absolute, about doubles the power requirement. On the other hand, operating at lower pressures, i.e., 0.25 atmosphere at the low pressure side and 0.375 atmosphere on the high pressure side reduces the power requirement to about half the above value. Accordingly, even with equipment available on an o-the-shelf basis, freeze drying according to practice of this invention costs far less than conventional spray drying. 1f equipment can be designed to operate at sufficiently reduced pressures operational costs may be reduced virtually to that of boiling water in a single effect evaporator.

It will be obvious to those skilled in the art that various changes may be made without departing from the spirit of the invention and therefore the invention is not limited to what is shown in the drawings and described in the specification, but only as speciiied in the appended claims.

What is claimed is:

1. A regenerative freeze dry process which comprises:

(l) recirculating a carrier gas through a cooling-warming cycle wherein compressed -gas is progressively chilled in the cooling part of the cycle from about ambient temperature to a sub-freezing temperature in heat exchanger means by heat exchange against expanded gas, then expanded to further cool the gas, the expanded gas being progressively warmed in the warming part of the cycle by heat exchange against the compressed gas to about ambient temperature, then compressed for recycle;

(2) introducing liquid feed directly into the expanded gas, said gas being at a ysub-freezing temperature level, the feed being freeze dried by direct contact with the cold expanded gas, the vapor evolved from the feed thereby being carried along in the expanded gas through the warming part of the cycle, compression and the cooling part of the cycle to ultimate condensation during the course of said progressive chilling of the compressed gas;

(3) periodically halting introduction of the feed and warming the sub-freezing portion of the heat exchanger means to melt condensate ice deposited therein and removing the liquid condensate.

2. The process of claim 1 wherein liquid condensate is removed from the compressed gas being cooled at a temperature level close to the freezing point thereof, just prior to further chilling of the compressed gas to subfreezing temperatures.

3. The process of claim 1 wherein the warming-cooling heat exchange is effected in stages, the rst of the stages chilling the compressed gas not below the freezing point of the liquid in the feed and wherein the feed is introduced between stages.

4. The process of claim 1 wherein the compression ratio is about 1.5.

5. The process of claim 1 wherein the feed material is pre-chilled close to the freezing point of the liquid in the feed. l

6. The process of claim 1 wherein the heat of compression of the compressed gas is removed by an external source of cooling and wherein the regenerative warming is effected by passing hot compressed gases directly to the normally sub-freezing portion of the heat exchanger.

7. The process of claim 1 wherein the feed is sprayed into the expanded gas.

8. The process of claim 1 wherein water is the liquid in the feed.

9. The process of claim 1 wherein air is the carrier gas.

10. A drying process which comprises:

(1) recirculating a carrier vgas through a coolingwarming cycle wherein compressed gas is progressively chilled in the cooling part of the cycle from about ambient temperature to a low temperature by heat exchange against expanded gas, then expanded to further cool the gas, the expanded gas being progressively warmed in the warming part of the cycle by heat exchange against the compressed gas to about ambient temperature, then compressed for recycle;

(2) introducing liquid feed directly into the expanded gas, said gas being at a low temperature level, the feed being dried by direct contact with the cold expanded gas, the vapor evolved from the feed thereby being carried along in the expanded gas through the Warnung part of the cycle, compression and the cooling part of the cycle to ultimate condensation during the course of said progressive chilling of the compressed gas; and

(3) removing the vapor condensate.

11. The process of claim 10 wherein the compression ratio is in the range of about 12S-3.00.

12. The process of claim 1 wherein expansion is effected under load.

13. The process of claim 1 wherein the pressure level of both compressed and expanded gases is sub-atmospheric.

14. The process of claim 3 wherein liquid condensate is removed from the compressed gases in the second o1 the stages of said warming-chilling heat exchange jusl prior to the chilling of the compressed gas from just above freezing to sub-freezing temperature levels.

15. The process of claim 1 wherein directly after expansion thereof the expanded gas is partially warmed to a still subfreezing temperature level prior to introduction of the liquid feed thereinto.

References Cited UNITED STATES PATENTS 3,263,335 8/1966 Kan 34-5 3,266,169 8/1966 Smith 34-5 3,290,788 12/1966 Seelandt 34-5 3,313,032 4/1967 Malecki 34-5 WILLIAM I. WYE, Primary Examiner.

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