Bulk freeze drying using spray freezing and agitated drying

Abstract

A freeze dryer processes bulk powder products. The freeze dryer freezes the product by mixing an atomized spray of product with sterile liquid nitrogen. The resultant powder is freeze dried in a vessel, and the vessel contents is agitated to maintain product contact with heated vessel wall and to prevent agglomeration.

Claims

1. A freeze drying system for freeze drying bulk product by removing a liquid, comprising: a freezing chamber; at least one spray nozzle directed to an interior of the freezing chamber, the at least one spray nozzle being connected for spraying the bulk product and a freezing agent, the at least one spray nozzle being configured to spray the bulk product and the freezing agent in the interior of the freezing chamber to create a spray-frozen powder; a vacuum drying chamber; a selectively closeable conduit connecting the freezing chamber with the vacuum drying chamber, the selectively closeable conduit having an open configuration for transferring the spray-frozen powder without using trays and shelves, the selectively closeable conduit further having a closed configuration for conducting a drying operation under vacuum in the vacuum drying chamber while conducting a different stage of freeze drying in the freezing chamber; an agitating mechanism for agitating the spray-frozen powder in the vacuum drying chamber to move particles of the spray-frozen powder into and out of contact with a wall of the drying chamber; a heater for heating the wall of the vacuum drying chamber; a condensing chamber in communication with the vacuum drying chamber and comprising surfaces for condensing a vapor from exhaust gas received from the vacuum drying chamber; and a vacuum pump in communication with the condensing chamber.

2. The system of claim 1, further comprising a controller including memory storing a program that, when executed by the controller, causes the freeze drying system to perform: an aseptic spray freezing cycle wherein the bulk product and the freezing agent are sprayed from the at least one spray nozzle, to produce a spray frozen powder in the freezing chamber at a first pressure; a transfer cycle wherein the selectively closeable conduit is configured in the open configuration and the spray frozen powder is transferred from the freezing chamber to the vacuum drying chamber; and an aseptic vacuum freeze drying cycle wherein the selectively closeable conduit is configured in the closed configuration and the vacuum pump evacuates the condensing chamber and the vacuum drying chamber to a vacuum pressure lower than the first pressure, the heater heats the wall of the vacuum drying chamber and the agitating mechanism agitates the spray frozen powder.

3. The system of claim 2, wherein the aseptic spray freezing cycle and the aseptic vacuum freeze drying cycle are performed concurrently on different bulk product batches.

4. The system of claim 1, wherein the agitating mechanism comprises a vibrating mechanism externally mounted to the vacuum drying chamber.

5. The system of claim 1, wherein the agitating mechanism comprises a vibrating mechanism mounted to a supporting leg of the vacuum drying chamber.

6. The system of claim 1, wherein the agitating mechanism comprises a sterile gas introduction system configured to fluidize the spray-frozen powder.

7. The system of claim 6, wherein the agitating mechanism further comprises a mechanism selected from the group consisting of a vibration mechanism and a stirring mechanism.

8. The system of claim 1, wherein the freezing agent is sterile liquid nitrogen.

9. The system of claim 1, wherein a lower portion of the vacuum drying chamber is conical in shape.

10. The system of claim 1, wherein the heater is an electrical heater.

11. The system of claim 1, wherein the heater is a jacket for circulating a heated fluid.

12. The system of claim 1, further comprising a jacket attached to the freezing chamber for circulating a cooled fluid for cooling the freezing chamber during spraying; and a heat exchanger for cooling the cooled fluid using gas vented from a source of the freezing agent.

13. The system of claim 1, wherein each one of the at least one spray nozzles is connected for spraying both the bulk product and the freezing agent.

14. The system of claim 1, wherein the at least one spray nozzle comprises at least one nozzle connected for spraying only the bulk product and at least one nozzle connected for spraying only the freezing agent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of a prior art freeze drying system.

(2) FIG. 2 is a schematic drawing of a freeze drying system according to one embodiment of the disclosure.

(3) FIG. 3 is a cut-away view of a freeze dryer according to one embodiment of the disclosure.

(4) FIG. 4 is a schematic drawing of a freeze drying system according to one embodiment of the disclosure.

(5) FIG. 5 is a flow chart showing a method in accordance with one aspect of the disclosure.

(6) FIG. 6 is a schematic drawing of a freeze drying system according to one aspect of the disclosure.

(7) FIG. 7 is a flow chart showing a method in accordance with one aspect of the disclosure.

DESCRIPTION

(8) The present disclosure describes systems and methods for freeze drying bulk materials in an efficient manner. In cases where aseptic bulk materials are processed, those materials may be processed without compromising the aseptic qualities of the product. More specifically, the systems and methods of the present disclosure are directed to a bulk powder freeze dryer which is optimized to freeze and dry product in the powder form.

(9) The processes and apparatus may advantageously be used in drying pharmaceutical products that require aseptic or sterile processing, such as injectables. The methods and apparatus may also be used, however, in processing materials that do not require aseptic processing, but require moisture removal while preserving structure, and require that the resulting dried product be in powder form. For example, ceramic/metallic products used as superconductors or for forming nanoparticles or microcircuit heat sinks may be produced using the disclosed techniques.

(10) The systems and methods described herein may be performed in part by an industrial controller and/or computer used in conjunction with the processing equipment described below. The equipment is controlled by a plant logic controller (PLC) that has operating logic for valves, motors, etc. An interface with the PLC is provided via a PC. The PC loads a user-defined recipe or program to the PLC to run. The PLC will upload to the PC historical data from the run for storage. The PC may also be use for manually controlling the devices, operating specific steps such as freezing, defrost, steam in place, etc.

(11) The PLC and the PC include central processing units (CPU) and memory, as well as input/output interfaces connected to the CPU via a bus. The PLC is connected to the processing equipment via the input/output interfaces to receive data from sensors monitoring various conditions of the equipment such as temperature, position, speed, flow, etc. The PLC is also connected to operate devices that are part of the equipment.

(12) The memory may include random access memory (RAM) and read-only memory (ROM). The memory may also include removable media such as a disk drive, tape drive, etc., or a combination thereof. The RAM may function as a data memory that stores data used during execution of programs in the CPU, and is used as a work area. The ROM may function as a program memory for storing a program including the steps executed in the CPU. The program may reside on the ROM, and may be stored on the removable media or on any other non-volatile computer-usable medium in the PLC or the PC, as computer readable instructions stored thereon for execution by the CPU or other processor to perform the methods disclosed herein.

(13) The presently described methods and apparatus utilize spray freezing by combining the atomized liquid product (through spray nozzles) with atomized liquid nitrogen (LN2). In cases where the presently described systems and methods are used in the processing of products requiring sterile or aseptic processing, sterile LN2 is used. One technique for the production of sterile liquid nitrogen is described in PCT International Publication No. WO 2009/029749A1, assigned to Linde, Inc. of Murray Hill, N.J., USA.

(14) An exemplary system 200 in accordance with one disclosed embodiment is shown in FIG. 2. Spray nozzles 212 are connected to a source 211 of liquid product. The nozzles are arranged to atomize the product within a freeze drying vessel 210. The liquid product may be a solution or a suspension of a biological solid in water or another liquid. The atomization of the product results in a dispersion of fine particles within the freeze drying vessel 210.

(15) Both the size of the particles and the distribution of particle sizes are dependent on the spraying technology. For example, nozzle geometry, product flow rate and nozzle placement within the chamber may influence those process outputs. Particle size and size distribution are important to the application of the product. For example, for powder handling, it is preferable to have particle sizes above 100 microns, while for pulmonary applications, particle size should be around 6 microns.

(16) Another set of spray nozzles 214 is arranged to comingle a spray of an aseptic freezing agent such as sterile LN2 with the atomized liquid product. The atomized liquid product freezes as the sterile LN2 vaporizes and absorbs heat from the liquid product within the freeze drying vessel 210. The spray nozzles 214 are connected to a source 213 of the aseptic freezing agent. In the example shown, sterilized LN2 is used. The use of sterile LN2 as the cold source makes possible the direct contact of aseptic atomized product with the cold source or freezing agent, without contamination. In another embodiment, cold sterile gaseous nitrogen is used in place of LN2.

(17) The dimensions of the freezing chamber are such that a sufficient amount of time is allowed for the product to be in contact with the freezing agent to allow freezing of the product before it reaches the bottom of the chamber. The spray-frozen liquid product collects at the bottom of the freeze drying vessel 210 as a frozen powder, while the gaseous freezing agent is vented from the vessel. Baffles may be used in the freeze drying vessel to allow the particles to settle to the bottom without becoming entrained in the vented gas. The spray freezing process produces small particles of product that are quickly frozen because the smaller particles have much larger surface area to mass ratio and therefore a minimal resistance to heat input. That property also speeds the drying process.

(18) The freeze drying vessel 210 may be pre-cooled to prevent frozen particulates from thawing upon contact with vessel walls or ancillary parts. The freeze drying vessel 210 may also be cooled during the spraying and subsequent steps to maintain the powder frozen as additional product is sprayed and frozen in the vessel. The vessel may be cooled, at least in part, by passing a cooled heat exchange fluid 219 such as oil through heat exchangers 230 positioned to heat or cool the drying vessel 210. The heat exchange fluid is cooled in the heat exchanger 218 by cold N2 exhaust from the condenser 216. The vessel may furthermore have a conical lower section to facilitate handling of the product. The freezing step is complete when a sufficient quantity of liquid product is spray-frozen and has been collected in the lower part of the vessel 210. A vacuum is then pulled on the freeze drying vessel 210. A vacuum pump 260 may be in communication with a condenser 250 that, in turn, may be connected to the freeze drying vessel 210 by opening a valve 256. In that case, the freeze drying vessel 210 is subjected to vacuum pressure by operating the vacuum pump 260 and opening the valve 256 between the condenser 250 and the freeze drying vessel 210.

(19) After the chamber is evacuated, heat is introduced into the vessel walls. The same heat exchangers 230 or different heat exchangers may be positioned at the lower part of the vessel for applying heat through the vessel walls to the frozen powder. In the embodiment shown, the heat transfer fluid 219 passing through the heat exchangers 230 is heated by an oil heater 271. Alternately, the vessel may be directly heated using electrical resistance or other techniques.

(20) To move the particles of the frozen product to the drum walls for heating, while preventing product agglomeration from occurring, the frozen powder is agitated. In one embodiment, a slow speed stirring mechanism includes an agitator 235 in the lower part of the vessel. The slow speed stirring mechanism further includes a motor 236 and a drive shaft 237. The drive shaft passes through a sealed aperture in the vessel 210, permitting the motor to be installed on the outside of the vessel, maintaining the aseptic environment within. In another embodiment, the stirring mechanism is magnetically coupled to an external drive motor, avoiding the use of seals.

(21) Alternatively, a vibration mechanism 339 (FIG. 3) externally mounted to the wall of the vessel 300 induces vibrations in the wall of the vessel, causing the frozen powder to circulate toward and away from the vessel wall. The vibration mechanism may, for example, be a pneumatic piston impact vibrator or may be an offset mass driven by an electric motor. The vibration may alternatively be mounted on a supporting leg (not shown) of the freeze drying vessel. In another embodiment, the vessel is tumbled, inducing the powder to circulate.

(22) Returning to FIG. 2, as frozen liquid in the product sublimates, vapor is carried through the valve 256 into the condensing vessel 250. Cooled condensing surfaces 257 in the condensing vessel collect the condensed vapor. In the case of water vapor, the vapor condenses as ice. The condensed ice must be periodically removed from the condensing vessel.

(23) After completion of the drying step, the freeze drying vessel 210 is returned to atmospheric pressure and a valve 245 at the bottom of the drying chamber opens to allow the dried product to move through a collection valve or plate to a removable collection canister 240. Unlike a traditional tray freeze dryer system, handling of the freeze dried product is minimized, and transfer from the vessel to the collection canister may take place in a controlled, aseptic environment.

(24) The freeze drying system 200 provides a bulk freeze dryer having a larger throughput and easier product collection than previous freeze drying solutions such as tray dryers. The technique permits the spray-freezing of product in a sterile freeze drying operation. No known prior sterile freeze drying methods utilize spray freezing.

(25) A freeze drying vessel 300, shown in FIG. 3, includes several exemplary features discussed above. The vessel includes an upper vessel wall 302 having a cylindrical shape and a lower vessel wall 301 having, in the embodiment shown, a conical shape. A top plate 303 is sealed to the upper vessel wall and is removed only for assembly and repair procedures, and not during normal processing or maintenance.

(26) In the embodiment wherein the product is agitated by stirring, the top plate 303 may support a motor 336 and drive train 337 for driving an agitator comprising a spiral blade 335. The blade 335 is shaped to move product that is proximate both the upper vessel wall 302 and the lower vessel wall 301. The blade rotates in close proximity with the walls, minimizing dead space between the blade and the walls. The agitator is supported from above, obviating the need for a bearing assembly at the bottom of the vessel where the freeze dried product is discharged at the end of a cycle.

(27) A rotational washing nozzle 340 directs a liquid sanitizer on the inside vessel walls and top plate as the nozzle rotates. The complete assembly may be sterilized via steam, vaporized hydrogen peroxide (VHP), or another sterilant. Because all components that contact the product are enclosed within the freeze drying vessel, and the vessel need not be opened after each cycle, sterilization may not be necessary after each cycle.

(28) Also mounted to the top plate 303 are nozzles 212 (FIG. 2) for spraying the liquid product and nozzles 214 for spraying the sterile freezing agent. The nozzles 212, 214 may be mounted flush with, or slightly recessed in, the inner surface of the top plate 303, to clear a top portion of the spiral blade 335 when that blade is rotating. Alternatively, nozzles 212, 214 may extend into the interior of the vessel 300, and the spiral blade 335 may be configured to provide clearance for the nozzles. In yet another embodiment, the spray freezing process takes place in a separate vessel, and the frozen powder is transferred to the vessel 300.

(29) A discharge plate or valve 345 at the lower end of the vessel is opened after each cycle to discharge the freeze dried product. When closed, the discharge plate or valve is in close proximity with the rotational path of the spiral blade 335 to eliminate any dead space that would otherwise be created. Similarly, an inspection door (not shown) may be provided in an opening of the upper vessel wall 302 and may be configured to provide an inner surface that is flush with the inner surface of the upper vessel wall, also reducing dead space.

(30) Another embodiment 400 of the disclosed freeze dryer, shown in FIG. 4, includes a separate freezing vessel 410 that feeds several drying vessels 480a, 480b, 480c arranged in parallel. The freezing vessel 410 operates in a manner similar to that described above with reference to FIG. 2. Spray nozzles 412 are connected to a source 411 of liquid product. The nozzles 412 are arranged to atomize the product within the freezing vessel 410. Another set of spray nozzles 414 is arranged to comingle a spray of an aseptic freezing agent such as sterile LN2 with the atomized liquid product. Liquid in the atomized product freezes as the sterile LN2 vaporizes and absorbs heat from the product, before the product reaches the floor of the freeze drying vessel 410. The spray nozzles 412 are connected to a source 413 of the aseptic freezing agent.

(31) Each drying vessel 480a, 480b, 480c is selectively interconnected with the freezing vessel 410 by respective passageways 481a, 481b, 481c. The drying vessels may be selected for receiving frozen product from the freezing vessel 410 by opening valves at each end of the corresponding passageways. For example, drying vessel 480a is selected by opening the valves 482, 483 at each end of the passageway 481a. Valves in the remaining passageways 481b, 481c remain closed as the drying vessel 480a receives product from the freezing vessel 410. The other drying vessels 480b, 480c are selected to receive product in a manner similar to that described for drying vessel 480a.

(32) The drying vessels 480a, 480b, 480c function as described above with reference to FIG. 2. For example, regarding drying vessel 480a, one or more heating jackets 430 are positioned at the lower part of the vessel for applying heat through the vessel walls to the frozen powder. A heat transfer fluid 419 is pumped through the heating jackets 430 to provide heat energy. A slow speed stirring mechanism including an agitator 435 in the lower part of the vessel moves particles of the frozen product to the drum walls for heating, while preventing product agglomeration from occurring. The slow speed stirring mechanism further includes a motor 436 and a drive shaft 437.

(33) Upon completion of the drying cycle, the product may be released through passageways 484a, 484b, 484c to a common collection vessel 440. Each passageway has valves 485, 486 at the ends for selectively connecting the collection vessel 440 with a particular drying vessel. Alternatively, each drying vessel 480a, 480b, 480c may have a dedicated collection vessel (not shown).

(34) Because drying is a more time consuming step than freezing, individual batches being processed by the freeze drying system 400 would be in different stages of drying. For example, as a batch of frozen product is being transferred from the freezing vessel 410 to the drying vessel 480a, another batch of product that had earlier been transferred to drying vessel 480b might be undergoing heating/sublimation in the drying vessel, while yet another batch that had been transferred even earlier to drying vessel 480c might have completed drying and repressurization, and be in the process of transfer to the collection vessel 440. In that way, the freezing vessel output is processed in staggered batches, allowing full utilization of both the freezing vessel and the drying vessel.

(35) One or more condensing vessels 490 are in communication with the drying vessels through conduits 491a, 491b, 491c. A vacuum pump (not shown) is connected to the condensing vessel and maintains the freeze drying system at vacuum pressure during processing. In a preferred embodiment of the disclosed system, at least two parallel condensing vessels 490 are used in the system, with each drying vessel 480a, 480b, 480c being alternatively connectable to more than one condensing vessel. That arrangement permits a condensing vessel to be taken off line for defrosting while continuing to direct effluent from the drying vessels to an alternate condensing vessel.

(36) The freeze drying system 400 permits the freeze drying process to run semi-continuously, with the spray freezing process operating continuously and the drying process being divided into parallel vessels that process successive, staggered batches, resulting in continuously filling the collection vessel. Condensing vessels may be taken off line and defrosted without interrupting the continuous process.

(37) Also presently disclosed and shown schematically in FIG. 5 is a unique freeze drying method 500 for use in drying a bulk product containing a liquid solvent, under aseptic conditions. The liquid solvent may be water, alcohol or another solvent. The bulk product is sprayed, in step 510, into an aseptic freezing vessel. Concurrently, an aseptic freezing agent, such as sterile LN2, is sprayed, in step 520, into the aseptic freezing vessel and intermingled with the sprayed bulk product. The liquid freezing agent quickly evaporates, absorbing heat from the sprayed bulk product and causing the solvent in the bulk product to freeze. A frozen powder is formed before the bulk product reaches a lower portion of the freeze drying vessel.

(38) The frozen powder may be transferred to a separate drying vessel for performing the subsequent steps, or may remain in the freezing vessel. In either case, the frozen powder is subjected, in step 530, to vacuum, and is agitated, in step 540, with an aseptic low speed stirring mechanism, a vibrator or another agitation mechanism. At the same time, the frozen powder is heated slightly, in step 550, to cause sublimation of the frozen solvent in the bulk product to form a freeze dried product. The heat may be transferred to the frozen powder from the walls of the vessel.

(39) Vapor from the sublimation of the solvent from the product may be collected by condensing the vapor on a cooled surface in a condensation vessel. The condensed solvent must be removed periodically from the cooled surface. In the case where water is used as the solvent, solid ice is collected in the condensation vessel, which must be periodically defrosted.

(40) The freeze dried product is then returned, in step 560, to atmospheric pressure and transferred to a canister.

(41) In the case where the frozen powder is transferred to a separate drying vessel, several drying vessels may be use to service a single freezing vessel, thereby creating a semi-continuous process. A batch portion of frozen powder is produced and transferred from the aseptic freezing vessel to a first aseptic drying vessel, and, in the first aseptic drying vessel, the frozen powder is subjected to vacuum, stirred and heated. A second batch of the frozen powder is produced and transferred from the aseptic freezing vessel to a second aseptic drying vessel, and, in the second aseptic drying vessel, is subjected to vacuum, stirred and heated. The processing in the first and second drying vessels is staggered to sequentially draw from the freezing vessel. A sufficient number of additional drying vessels may be used to keep the freezing vessel operating continuously.

(42) An exemplary system 600 in accordance with embodiments of the disclosure is shown in FIG. 6. The system 600 utilizes components and arrangements as disclosed above with reference to FIGS. 2 and 4, and in addition may utilize a sterile gas introduction system 690 to cause fluidization of a bed of frozen powder in a drying chamber 680.

(43) Spray nozzles 612 are arranged to atomize the product together with an aseptic freezing agent such as sterile LN2. The product from a source 611 and the freezing agent from a source 613 may be sprayed together from each of one or more nozzles 612 as shown, or each of the freezing agent and product may be sprayed from separate nozzles such as nozzles 212, 214 shown in FIG. 2. The atomized liquid product freezes in the freezing chamber 610. The spray-frozen liquid product falls to the bottom of the freezing chamber as a frozen powder.

(44) A selectively closeable passageway 681 interconnects the freezing chamber 610 with the drying chamber 680. The passageway 681 is selectively closeable by means of a valve 682 or other means that, when closed, is capable of maintaining a pressure differential between the freezing chamber 610 and the drying chamber 680. When closed, the selectively closeable conduit permits different, parallel operations to be performed in the two chambers 610, 680. For example, while spraying or sterilizing takes place in the freezing chamber 610, a drying operation may take place in the drying chamber 680.

(45) The frozen powder is transferred from the freezing chamber 610 to the drying chamber 680. The transfer may take place continuously as the product is frozen, the spray-frozen powder accumulating in the drying chamber. Alternatively, the product may accumulate in a lower portion of the freezing chamber 610, with the conduit 681 remaining closed during the freezing process. The conduit is then opened to allow product transfer to the drying chamber 680 after a sufficient quantity of product is spray-frozen. The transferring may take place by gravity using an arrangement similar to that shown in FIG. 6, or may take place via an active product transfer device such as an auger or a vibratory conveyor.

(46) Once the spray-frozen powder is accumulated in the drying chamber 680, the passageway 681 is closed and the drying chamber is subjected to vacuum. A vacuum pump 660 may be in communication with a condenser 650 that, in turn, may be connected to the drying chamber 680.

(47) Heat is applied to the frozen powder by heating the drying chamber walls using heaters 630 such as fluid flow heat exchangers or electric heaters. To move particles of the spray-frozen powder into and out of contact with the chamber wall for heating, while preventing product agglomeration from occurring, the spray-frozen powder is agitated, as described above with reference to FIG. 2. In one embodiment, a vibration mechanism 639 induces vibrations in the wall of the drying chamber 680, causing the spray-frozen powder to circulate toward and away from the chamber wall. Alternatively, a slow speed stirring mechanism may be used, or the vessel may be tumbled.

(48) The spray-frozen powder may alternatively be brought into and out of contact with the wall of the drying vessel 680 by introducing a flow of sterilized gas such as nitrogen into the spray-frozen product bed through nozzles of a sterile gas introduction system 690. The gas flow fluidizes the bed of spray-frozen product. The gas flow may be sufficiently strong to induce a circulation of the spray-frozen powder into and out of contact with the chamber wall, promoting heat transfer from the heaters 630. Alternatively, the fluidizing gas flow may be used in combination with a non-gas-flow agitation mechanism such as vibratory agitation, stirring or tumbling. The fluidized bed of spray-frozen powder responds readily to the additional agitation, circulating into and out of contact with the chamber wall.

(49) As frozen liquid in the product sublimates, the resulting vapor is carried into the condensing vessel 650 where condensed vapor is resolidified and collected.

(50) After completion of the drying step, the drying vessel 680 is returned to atmospheric pressure and a valve 645 at the bottom of the drying chamber is opened to allow the dried product to be removed. Handling of the freeze dried product is minimized, with the entire process taking place in a controlled, aseptic environment.

(51) The use of a single drying vessel 680 that is separate from the freezing vessel 610 permits the two vessels to be designed specifically for their particular purposes, without compromise. Further, both vessels may be used in parallel, substantially increasing efficiency of the process. Moreover, such a design allows easier scale-up for larger batches with uniform product characteristics and easy product handling.

(52) A method for freeze drying a bulk product containing a liquid, in accordance with embodiments of the invention, is illustrated by a flow chart 700 shown in FIG. 7. The bulk product and the freezing agent are sprayed together into the freezing chamber, as shown at block 710. The freezing agent is intermingled with the sprayed bulk product to freeze the liquid contained in the bulk product to form a spray-frozen powder. During the spraying process, the freezing chamber is maintained at a first pressure.

(53) The freezing agent is a sterile material such as sterile liquid nitrogen that quickly evaporates during spraying, absorbing heat from the bulk product. The bulk product and the freezing agent may be sprayed from separate nozzles into the freezing chamber, where they intermingle. Alternatively, both the bulk product and the freezing agent are sprayed from one or more common nozzles into the freezing chamber; i.e., the bulk product and the freezing agent are mixed before entering the freezing chamber, and each nozzle sprays both materials.

(54) During the spraying operation, heat may be removed from a wall of the freezing chamber using a heat transfer fluid cooled using vented gas from production of the freezing agent.

(55) The spray-frozen powder is then transferred, at block 720, from the freezing chamber to a vacuum drying chamber. The transferring may, for example, comprise configuring the selectively closeable conduit connecting the freezing chamber with the vacuum drying chamber from a closed configuration to an open configuration. The spray-frozen powder may be moved by gravity from one chamber to the other, by a mechanical means such as an auger, by conveyor, by entraining in a gas stream, or by a combination of those or other techniques.

(56) The vacuum drying chamber is then subjected to a vacuum pressure lower than the first pressure, as shown in block 730. Vacuum may be applied by a vacuum pump in communication with a condensation chamber that, in turn, communicates with the drying chamber. Vapor from sublimation of the frozen liquid may be condensed in the condensing chamber.

(57) The spray-frozen powder is agitated at block 740 under the vacuum pressure in the vacuum drying chamber using an agitating mechanism to bring particles of the spray-frozen powder into and out of contact with the chamber wall. The agitating may comprise vibrating a wall of the vacuum drying chamber. The agitating may alternatively or in addition comprise introducing a sterile gas into the spray-frozen product to form a fluidized bed. The agitating may comprise stirring, tumbling or another technique.

(58) While agitating the spray-frozen powder under the vacuum pressure in the vacuum drying chamber, the spray-frozen powder is heated to cause sublimation of frozen liquid to form a freeze dried product, as shown in block 750. To effect the sublimation, heat may be transferred to a wall of the vacuum drying chamber using a heat transfer fluid, electric heating or another means.

(59) The operations performed in the freezing chamber, including spraying, may be performed concurrently with operations performed in the vacuum drying chamber, including subjecting to vacuum, agitating and heating. When performing different, parallel operations in the two chambers, the selectively closeable conduit connecting the freezing chamber with the vacuum drying chamber is maintained in a closed configuration, and different batches of the bulk product are present in the two chambers.

(60) The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Description of the Invention, but rather from the Claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.