US3273259A

US3273259A - Freeze drying apparatus

Info

Publication number: US3273259A
Application number: US371217A
Authority: US
Inventors: Hackenberg Ulrich
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-06-06
Filing date: 1964-05-29
Publication date: 1966-09-20
Anticipated expiration: 1983-09-20
Also published as: NL6401898A

Description

Sept. 20, 1966 u. HACKENBERG FREEZE DRYING APPARATUS Filed May 29, 1964 United States Patent 3,273,259 FREEZE DRYING APPARATUS Ulrich Hackenberg, Alter Trassweg 9, Bensberg-Refrath, Germany Filed May 29, 1964, Ser. No. 371,217 Claims priority, application Germany, June 6, 1963, L 45,038 Claims. (Cl. 34-92) This invention relates to a method and apparatus for freeze drying and more particularly to a method and apparatus for continuous freeze drying applications.

Freeze drying is a well known process in which a materialto be dried is first frozen and then placed inside an evacuable chamber. Water vapor removal apparatus, such as for example a refrigerated condenser, then absorbs sublimating water vapor produced by heating of the frozen material. A predetermined low pressure of for example 10 mms. of mercury is continuously maintained within the vacuum chamber during the drying process by means of conventional vacuum pumps communicating with the chamber.

The present invention is especially suited for continuous freeze drying applications which utilize a central vacuum drying chamber straddled by isolatable inlet and outlet lock chambers. This arrangement allows product carriers to be periodically loaded into the inlet lock chamber and removed from the outlet lock chamber while continuously maintaining the vacuum drying process in the intermediate drying chamber.

During the freeze drying process high heat quantities per time unit must be conducted to the drying material for both physical and economic reasons. One method of meeting these heating requirements is to transmit heat over small distances from large heating surfaces to the drying material, utilizing the sublimating vapor from the material as the primary heat conductor. In this arrangement the heating surfaces are normally maintained at relatively low temperature such as, for example, 60 C., but the large heating surfaces and small conduction distances permit the transmission of large heat quantities from the heating surfaces to the material. Another heating method entails the use of relatively hot heating elements of, for example, more than 100 C. and transmitting heat by radiation to the material over relatively long distances. This method, however, has the disadvantage of sometimes producing thermal damage to the drying material.

As improved heating method utilizes both above methods by transmitting heat from the heating elements to the drying material in two stages. The first stage consists of radiating heat from relatively hot heating elements of, for example, 200 C. to containers filled with the drying material which containers are designed to also shield the drying material from direct heat radiation. The heat thus absorbed by the material containers is then transmitted to the drying material by both conduction and convection. The two stage heating process was disclosed in US. Patent application 295,617, filed July 15, 1963, by W. Nerge, H. Ehlers, and the applicant.

The above improved heating method while reducing the time required for the drying process exhibits some economic disadvantages. By utilizing hot heating elements, the vapor sublimating from the drying material becomes overheated resulting in increased operating costs for the required evacuation equipment. In conventional freeze drying installations the evacuation equipment typically consists of refrigerated condensers and mechanical vacuum pumps. The condensers and vacuum pumps are connected for gas communication with the drying chamber and are normally positioned so as to provide relatively short conduction paths between the drying material and the evacuation equipment. Since the refrigerated con- 3,273,259 Patented Sept. 20, 1966 denser devices must absorb the heat contained in the condensing vapors, this overheating of the vapors produces an increased load upon the condenser devices. Similarly, the operating costs for the mechanical vacuum pumps whose efficiency is related to volume pumping capacity is also increased by the formation of excessively heated vapor.

The object of this invention, therefore, is to provide a freeze drying method and apparatus which reduces the time required for completion of the drying process and also reduces the operating costs for the evacuation equipment required.

One feature of this invention is the provision of a continuous freeze drying apparatus having an elongated cen tral drying chamber straddled by inlet and outlet lock chambers and wherein all evacuation equipment for the central drying chamber is connected thereto in the vicinity of the outlet lock chamber thereby causing the sublimating vapor to pass through the entire chamber length and to give up excessive heat before reaching the evacuation equipment.

Another feature of this invention is the provision of a freeze drying apparatus of the above featured type wherein the drying chamber contains individually controlled heating elements which allow the maintenance of decreasing temperature level heating plates along the travel path of the drying material.

Another feature of this invention is the provision of a freeze drying apparatus of the above featured types having an outlet lock chamber which extends laterally from the intermediate drying chamber thereby permitting advantageous location of the evacuation equipment and wherein the drying chamber includes a portion of enlarged cross section directly adjacent the outlet lock chamber which enlarged section allows lateral maneuvering of the material carriers into the outlet lock chamber.

Another feature of this invention is the provision of a freeze drying method wherein a material to be dried is conveyed through an elongated evacuated drying chamber, heat is applied to the material during progress through the evacuated chamber, and the resulting sublimating vapor coming from the material is removed along a path extending substantially the entire length of the elongated drying chamber thereby allowing excessive heat contained in the vapor to be removed by contact with the material moving through the drying chamber.

Another feature of this invention is the provision of a freeze drying method of the above featured type wherein the heat applied to the frozen material is applied at reducing temperature levels as the material progresses through the evacuated drying chamber.

These and other features of the instant invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawing which is a schematic showing partly in cross section of a preferred embodiment of the invention.

Referring now to the drawing there is shown a freeze drying installation having a central drying chamber 11 formed by the elongated cylindrical tank 12 of circular cross section and by the housing 13 which is of larger cross section than the elongated tank 12. Along the entire length of and adjacent to the inner Walls of the elongated tank 12 and enlarged housing 13 are a plurality of separate heating plates 14 which are adapted to be independently controlled by suitable heat sources (not shown). An inlet lock chamber 15 having a hinged door 16 is connected to the elongated tank 12 by the gate valve assembly 17. Connected for gas communication with the in terior of the inlet lock chamber 15 through the valve 18 is the cold condenser 19 and the mechanical vacuum pump 21. A compressor 22 is connected to supply a refrigerant to the cold condenser 19. The vent valve 23 3, is positioned between the outer atmosphere and the interior of the inlet lock chamber 15.

The end of the drying chamber 11 adjacent the enlarged housing 13 is bifurcated into a pair of condenser housings 25. The condenser housings 25 are connected to the drying chamber 11 by isolating gate valve assemblies 26 and enclose the cold condenser elements 27. Communicating with each of the condenser housings 25 through gas valves 28 is the mechanical vacuum pump 29. A pair of compressors 31 is connected to supply refrigerant to each of the cold condenser elements 27.

Extending laterally from the enlarged housing 13 is the outlet lock chamber 32 which is connected thereto by the isolating gate valve assembly 33. The outlet lock chamber 32 is provided with a hinged door 34 and an atmospheric vent valve 35. Connected for gas communication with the outlet chamber 32 through the gas valve 36 is the cold condenser 37 and the mechanical vacuum pump 38. A compressor 39 is connected to supply refrigerant to the cold condenser 37.

In the operation of the preferred freeze drying installation shown in the drawing, a suitable product carrier (not shown) is first loaded with a frozen material to be dried and placed inside inlet lock chamber 15 through open door 16 with the isolating gate valve 17 in a closed position. The hinged door 16 is closed and the inlet lock chamber 15 evacuated by the mechanical vacuum pump 21 through open gas valve 18. The isolating gate valve 17 is then opened and the loaded product carrier is automatically transported into the already evacuated drying chamber 11 on the conveyor rail 41 by a suitable motive means (not shown). The isolating gate valve 17 is again closed and the vent valve 23 opened to produce atmospheric conditions in the inlet lock chamber 15. Upon opening of the hinged door 16, another loaded product carrier is moved into the inlet lock chamber 15. After evacuation of the inlet lock chamber 15, the second loaded product carrier is also transported into the drying chamber 11 through the opened isolating valve 17. This procedure is continued until the entire drying chamber 11 is filled with product carriers filled with the material to be dried.

Inside the drying chamber 11 the heating elements 14 apply heat to the frozen material causing sublimation of the moisture therein. This sublimating vapor is pumped through the length of the drying chamber 11 and open isolating gate valves 26 into the condenser housings 25. The condensable portion of this vapor is condensed on the cold surfaces of the cold condensers 27 while the noncondensable portion is pumped out of the housings through open valves 28 by the mechanical vacuum pump 29. The use of the two independent condenser housings is advantageous as it allows one of the units to be isolated from the drying chamber for cleaning and de-icing while the drying process continues with the other unit.

As the material is gradually transported through the drying chamber 11, a dry outer layer is formed on the material due to the removal of sublimating water vapor. This dry layer is very susceptible to thermal damage and becomes increasingly thicker as the drying process continues. It is therefore desirable to reduce the level of applied heat as the thickness of the dry layer becomes greater. The reduced heat level can be correlated to the material drying speed (in mms. of water removed per minute) which drying speed decreases as the thickness of the dried material layer increases. For this reason the heating units 14 are controlled so that each successive heating unit, going in direction from the inlet lock chamber 15 toward outlet lock chamber 32, operates at a reduced temperature. For example only, with seven independent heating units the preferred operating temperature for each successive unit would be 280 C., 220 C., 170 0, 130 C., 100 C., 80 C., and 70 C.

The advantage of the present invention is that vapor produced in the initial portions of the drying chamber 11 which vapor is overheated by the higher temperature heating units is forced to travel the entire length of the drying chamber 11 before reaching the cold condensers 27 and the vacuum pump 29. While passing through the chamber 11 a portion of the heat in this vapor will be absorbed by the freeze dry components within the advanced parts of the tank 12 and housing 13 and especially by the frozen material contained in these parts. The heat absorption by the frozen material is particularly beneficial since it assists in providing the desired sublimation of the frozen moisture. The heat lost in this way greatly reduces the average temperature of the vapor reaching the cold condensers 27 and the mechanical vacuum pump 29. This reduced temperature in turn reduces both the energy requirements for maintaining the cold condensers 27 at their desired operating temperature of, for example, 40 C. and the pumping capacity requirements for the mechanical vacuum pump 29.

As each product carrier reaches the enlarged housing 13, the material carried has been completely dried and the carrier is transported into the outlet lock chamber 32 through open isolating gate valve 33. The isolating valve 33 is then closed and the vent valve 35 opened to produce atmospheric conditions within the outlet chamber 32. After removing the dried material, the hinged door 34 and the vent valve 35 are closed and the outlet lock chamber 32 is again evacuated by the mechanical vacuum pump 38 through the open gas valve 36. The isolating valve 33 is then opened preparing the outlet chamber to receive another product carrier containing dried material.

The lateral positioning of the outlet lock chamber 32 permits the advantageous arrangement of the bifurcated condenser housings 25. However, the use of the lateral outlet chamber is made possible by the enlarged housing 13. This enlarged housing permits lateral maneuvering of the product carriers which in the interest of space economy normally conform very closely to the inner wall confines of the cylindrical tank 12.

Thus the invention discloses a freeze drying installation which provides efiicient, safe heating of the drying material and also greatly reduces the operating costs of the evacuation equipment required.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. For example only, the material to be dried could be directly frozen within the inlet lock chamber 15 by evaporative cooling produced by the cold condenser 19. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise and as specifically described.

What is claimed is:

1. A freeze drying apparatus comprising;

(a) an elongated evacuable drying chamber having inlet, outlet and intermediate portions,

conveyor means adapted to transport a material to be dried through said drying chamber, heating means positioned within said drying chamber and adapted to heat the material to be dried; (b) an avacuable inlet lock chamber connected to said drying chamber inlet portion,

inlet lock valve means for isolating said inlet lock chamber from said drying chamber, an inlet door means adapted to provide access into said inlet lock chamber, inlet lock evacuation means adapted to provide evacation of said inlet lock chamber; (c) an evacuable outlet lock chamber connected to said drying machine outlet portion,

outlet lock valve means for isolating said outlet lock chamber from said drying chamber, an outlet door means adapted to provide access into said outlet lock chamber, outlet lock evacuation means adapted to provide evacuation of said outlet lock chamber; and

(d) drying chamber evacuation means directly connected to and adapted to provide evacuation of the entire said drying chamber,

said drying chamber evacuation means being connected to the outlet end of said drying chamber thereby providing a gas evacuation path along the entire length of said drying chamber and wherein no additional evacuation devices are directly connected to either said drying chamber inlet or intermediate portions.

2.'An apparatus according to claim 1 wherein said drying chamber evacuation means includes refrigerated condenser and mechanical vacuum pump means.

3. An apparatus according to claim 2 wherein said drying chamber evacuation means includes at least two refrigerated condenser pumps directly connected to said drying chamber outlet portion and including condenser valve means for independently isolating each of said refrigerated condenser pumps from said drying chamber.

4. An apparatus according to claim 3 wherein said elongated drying chamber is formed by an elongated cylindrical tank portion of uniform cross section and an end portion of enlarged cross section, and said outlet lock chamber extends laterally from said end portion of enlarged cross section.

5. An apparatus according to claim 1 wherein said elongated drying chamber is formed by an elongated cylindrical tank portion of uniform cross section and an end portion of enlarged cross section, and said outlet lock chamber extends laterally from said end portion of enlarged cross section.

'6. An apparatus according to claim 1 wherein said heating means within said drying chamber comprises a plurality of individually controlled units thereby permitting the application of reducing heating temperatures to the material being dried as the material is transported through said drying chamber.

7. An apparatus according to claim 6 wherein said drying chamber evacuation means includes refrigerated condenser and mechanical vacuum pump means.

8. An apparatus according to claim 7 wherein said drying chamber evacuation means includes at least two refrigerated condenser pumps directly connected to said drying chamber outlet portion and including condenser valve means for independently isolating each of said refrigerated condenser pumps from said drying chamber.

9. An apparatus according to claim 8 wherein said elongated drying chamber is formed by an elongated cylindrical tank portion of uniform cross section and an end portion of enlarged cross section, and said outlet lock chamber extends laterally from said end portion of enlarged cross section.

10. An apparatus according to claim 6 wherein said elongated drying chamber is formed by an elongated cylindrical tank portion of uniform cross section and an end portion of enlarged cross section, and said outlet lock chamber extends laterally from said end portion of enlarged cross section.

References Cited by the Examiner UNITED STATES PATENTS 888,257 5/1908 Passbury 3492 2,523,552 8/1950 Birdseye 345 2,528,476 10/1950 Roos 345 2,751,687 6/1956 Colton 34-92 2,858,795 11/1958 Walker 3492 3,192,645 7/1965 Oetjen 3492 WILLIAM J. WYE, Primary Examiner.

Claims

1. A FREEZE DRYING APPARATUS COMPRISING; (A) AN ELONGATED EVACUABLE DRYING CHAMBER HAVING INLET, OUTLET AND INTERMEDIATE PORTIONS, CONVEYOR MEANS ADAPTED TO TRANSPORT A MATERIAL TO BE DRIED THROUGH SAID DRYING CHAMBER, HEATING MEANS POSITIONED WITHIN SAID DRYING CHAMBER, BER AND ADAPTED TO HEAT THE MATERIAL TO BE DRIED; (B) AN AVACUABLE INLET LOCK CHAMBER CONNECTED TO SAID DRYING CHAMBER INLET PORTION, INLET LOCK VALVE MEANS FOR ISOLATING SAID INLET LOCK CHAMBER FROM SAID DRYING CHAMBER, AN INLET DOOR MEANS ADAPTED TO PROVIDE ACCESS INTO SAID INLET LOCK CHAMBER, INLET LOCK EVACUATION MEANS ADAPTED TO PROVIDE EVACATION OF SAID INLET LOCK CHAMBER; (C) AN EVACUABLE OUTLET LOCK CHAMBER CONNECTED TO SAID DRYING MACHINE OUTLET PORTION, OUTLET LOCK VALVE MEANS FOR ISOLATING SAID OUTLET LOCK CHAMBER FROM SAID DRYING CHAMBER, AN OUTLET DOOR MEANS ADAPTED TO PROVIDE ACCESS INTO SAID OUTLET LOCK CHAMBER, OUTLET LOCK EVACUATION MEANS ADAPTED TO PROVIDE EVACUATION OF SAID OUTLET LOCK CHAMBER; AND (D) DRYING CHAMBER EVACUATION MEANS DIRECTLY CONNECTED TO AND ADAPTED TO PROVIDE EVACUATION OF THE ENTIRE SAID DRYING CHAMBER, SAID DRYING CHAMBER EVACUATION MEANS BEING CONNECTED TO THE OUTLET END OF SAID DRYING CHAMBER THEREBY PROVIDING A GAS EVACUATION PATH ALONG THE ENTIRE LENGTH OF SAID DRYING CHAMBER AND WHEREIN NO ADDITIONAL EVACUATION DEVICES ARE DIRECTLY CONNECTED TO EITHER SAID DRYING CHAMBER INLET OF INTERMEDIATE PORTIONS.