Patent Application
20240369296
The present invention relates generally to freeze drying processes and equipment for removing moisture from a product using sublimation. More specifically, the invention relates to systems and methods for cooling a freeze drying chamber and a freeze drying condenser using a cold thermal energy storage (CTES) system.
Freeze drying is a process that removes a solvent or suspension medium, typically water, from a product. Freeze-drying is a low-pressure, low-temperature condensation pumping process widely used in the manufacture of pharmaceuticals. In a freeze drying process for removing water, the water in the product is frozen to form ice and, under vacuum, the ice is sublimated and the resulting water vapor flows to a condenser. The water vapor is condensed on the condenser as ice and is later removed from the condenser. Freeze drying is particularly useful in the pharmaceutical industry, as the integrity of the product is preserved during the freeze drying process and product stability can be guaranteed over relatively long periods of time. The freeze dried product is ordinarily, but not necessarily, a biological substance.
Typical freeze drying processes used in the pharmaceutical industry may process either bulk product or product contained in vials. In an example of a bulk freeze drying system 100 shown in
The suspended or dissolved product is frozen by removing heat via the heat transfer fluid. Under vacuum, the frozen product 112 is heated, also via the heat transfer fluid, to cause sublimation of ice within the product. Vapor resulting from the sublimation of the ice flows through a passageway 115 into a condensing chamber 120 containing condensing coils or other surfaces 122 maintained below the condensation temperature of the vapor. A heat exchange fluid is passed through the coils 122 to remove heat, causing the vapor to condense as ice on the coils.
Both the freeze drying chamber 110 and the condensing chamber 120 are maintained under vacuum during the process by a vacuum pump 150 connected to the exhaust of the condensing chamber 120. Non-condensable gases contained in the chambers 110, 120 are removed by the vacuum pump 150 and exhausted at a higher pressure outlet 152.
The heat exchange fluids circulating through the condenser 220 and the shelves 223 of the freeze drying chamber 210 may be cooled by the same refrigeration system or by different refrigeration systems.
The present disclosure addresses the needs described above by providing a freeze drying system. The system comprises a freeze dryer chamber including a chamber heat exchanger for cooling and heating a product in the freeze dryer chamber. The system further comprises a freeze dryer condenser connected to the freeze dryer chamber for receiving exhaust gases from the freeze dryer chamber. Condensing surfaces of the freeze dryer condenser are provided to condense the exhaust gases.
A first heat exchange fluid circuit is selectively connected to the condensing surfaces for circulating a first heat exchange fluid to the condensing surfaces. A turbo compressor cooling system is connected for cooling the first heat exchange fluid.
A second heat exchange fluid circuit is connected for circulating a second heat exchange fluid through the chamber heat exchanger. An inter-circuit heat exchanger is connected for exchanging heat energy between the first heat exchange fluid and the second heat exchange fluid.
A cold thermal energy storage system is connected for cooling at least the second heat exchange fluid. The cold thermal energy storage system includes a phase change material for storing cold thermal energy.
Another embodiment of the invention is a method for method for freeze drying a product. The method includes sterilizing a freeze drying chamber using a clean-in-place arrangement; loading the freeze drying chamber with the product; during at least one of the sterilizing and the loading of the freeze drying chamber, recharging a cold thermal energy storage system by cooling a phase change material of the cold thermal energy storage system using a turbo-compressor cooling system; after the recharging the cold thermal energy storage system, chilling an interior of the freeze drying chamber to a process temperature using the turbo-compressor cooling system supplemented by the cold thermal energy storage system; freezing a component of the product in the freeze drying chamber to form a frozen component; sublimating the frozen component in the freeze drying chamber to form a vapor; condensing the vapor in a condenser using the turbo-compressor cooling system; and unloading the product from the freeze drying chamber.
Cooling systems for current commercial freeze dryers often use working fluids that are greenhouse gases. Prior to the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer, cooling system working fluids for freeze dryers were often based on chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which are powerful ozone-depleting agents. That treaty eliminated most use of those working fluids. CFCs and HCFCs were replaced with hydrofluorocarbons (HFCs) such as R-410a and R-507a, which are in wide use today. HFCs, however, are strong greenhouse gases. Various jurisdictions around the world are now restricting or banning the use of HFC-based refrigerants. Some manufacturers, including pharmaceutical manufacturers, are considering proactively eliminating the use of man-made refrigerants in their processing equipment, including freeze dryers.
To address the problem of using working gases that potentially harm the environment, the authors have utilized a turbo compressor cooling system to perform the primary cooling function in a freeze drying system. A turbo compressor cooling system utilizes air or nitrogen as a working fluid in a Bell-Coleman cycle (also called a reverse Brayton cycle), in which the expansion and compression stages approximate isentropic processes and the cooling and heating stages approximate isobaric processes. The system includes an arrangement in which a turbo-compressor and a turbo-expander share a common shaft with an electric motor. Mechanical energy from the turbo-expander supplements energy from the motor to drive the turbo-compressor, resulting in increased efficiency. The ultra-low temperature working fluid from the expander is used to cool the condenser and the freeze drying chamber of the freeze drying system.
A schematic diagram of an example turbo compressor cooling system 200 is shown in
A freeze dryer return line 220 from the freeze drying system 290 passes through a recuperative heat exchanger 225, where additional heat energy is exchanged between the freeze dryer return line 220 and the compressor output 215, increasing system efficiency. The resulting low temperature, low pressure working fluid in the freeze dryer return line 220 is routed to the inlet of the compressor 214.
Using mechanical energy from the motor 212 and from the expander 210, the compressor 214 compresses the working fluid to produce a high pressure and high temperature working fluid in the compressor output 215. Heat is rejected from that working fluid to atmosphere by an air cooler 230, or by a water chiller or another analogous device. As noted above, additional heat energy is transferred from the compressor output 215 to the freeze dryer return line 220 by the recuperative heat exchanger 225. The resulting high-pressure, low-temperature working fluid is routed to the expander 210 to complete the cycle.
A typical temperature cycle 300 of a commercial freeze drying system is shown schematically in
During the LOAD portion 316 of the cycle, the shelf temperature and condenser coil temperature are maintained near ambient temperature as the vials or bulk materials are loaded into the chamber, or alternatively the shelves may be cooled as demanded by the product recipe. During the TURNAROUND portion 320 of the cycle, the system is unloaded, defrosted, cleaned, sterilized, dried and leak tested. The ice on the condenser coils is melted and drained away. Hot cleaning agents may be used to clean or sterilize the freeze drying components including the freeze drying chamber and the condenser. Clean-in-place (CIP) and sterilize-in-place arrangements that do not require disassembly of the freeze drying equipment may be used. For example, steam nozzles or sterilization sprayers permanently installed to clean the interior of the equipment may be used. The freeze dryer cooling system may be used during the TURNAROUND portion of the cycle to warm the condenser during defrosting and to chill the shelves during loading. Shelf loading temperatures may be between ambient and −50° C., depending on process requirements. The freeze dryer cooling system may also be used to cool the system after drying. During leak testing, the freeze dryer cooling system must be run to evaluate for coolant leaks.
During the FREEZE portion 317 of the cycle, the temperature of product in the freezing chamber must be brought from ambient conditions to −40° C. or lower. The rate of temperature change during the FREEZE portion 317 of the cycle directly affects the overall cycle time of the freeze drying system, because other stages of the cycle may not take place during that stage. The cooling capacity of the cooling system of the freeze dryer therefore directly affects cycle time. Additionally, the freezing rate must be carefully controlled to control critical product quality characteristics within the freezing product. A freeze drying system of sufficient capacity to maintain the desired cooling rate is therefore of critical importance.
During the LYOPHILIZATION portion 318 of the cycle, the freeze drying chamber is maintained at low process temperatures. The freeze dryer cooling system must absorb heat from condensation in the condenser and must maintain the freeze drying chamber at process temperatures. It is further necessary to introduce heat into the system (the shelves) to drive the sublimation process. The sublimation of ice in the product removes energy from the product, and the sublimation would otherwise cause the product to cool down until the sublimation stops. A considerable amount of heat must be added via the shelves to keep the sublimation process going.
During a primary drying phase 318a, crystalline water ice is slowly sublimated to avoid forming liquid water, which might degrade the product. During a secondary drying phase 318b, remaining individual water molecules are removed at a higher temperature because the ice-to-water phase change is no longer of concern. For example, a typical shelf temperature in primary drying might be −10° C. to +10° C., or as high as 20° C., and secondary drying could be 20° C. to 40° C. Little or no cooling is required for the shelves in the secondary phase. In other cases, the product requires a very slow process with shelves at −30° C. In those cases, it might be necessary to bleed in some cooling for control as the pump energy tends to heat the system.
Most commercial pharmaceutical freeze dryer applications have maximum floor space requirements for installation in a lab or manufacturing facility, and any new freeze dryer cooling system must occupy an area comparable to the area occupied by a traditional HFC-based system.
The author has found that a reasonably-sized turbo compressor cooling system does not have the peak capacity needed to keep the cool-down/freezing cycle of a commercial freeze dryer within an acceptable cycle time. To take advantage of the environmental benefits of a turbo compressor cooling system in a commercial freeze drying system, while still meeting the peak cooling requirements and maximum floor space specifications of the system, the authors have supplemented a turbo compressor cooling system with a cold thermal energy storage (CTES) system.
A CTES system stores and retrieves cold thermal energy for low temperature applications. CTES uses the latent heat storage properties of a material. The technique stores heat at a differential temperature or different phase that allows energy to be stored for later use.
Two approaches/types of energy storage material may be used: sensible heat and latent heat. A sensible heat approach uses large volumes of a cold fluid, and relies on the latent heat storage of that fluid. Thermal energy is gradually transferred to the fluid as the temperature of the fluid rises.
Latent heat systems use the phase change energy of a phase change material (PCM) to provide a near constant temperature energy sink. Latent heat systems use a smaller volume for larger energy storage.
Examples of PCM's for cold applications include paraffin (organic) (to −37° C.), petroleum-derived materials, plant derived materials, eutectic salts (to −65° C.) and alcohol/glycols (to −100° C.). In one embodiment of the presently described system a eutectic salt-based PCM is used to supplement a turbo compressor cooling system of a freeze dryer.
A freeze dryer cooling system 400 according to one embodiment of the present disclosure is shown schematically in
Freeze drying components of the system include a freeze drying chamber 410 with cooled shelves 423, and a condenser 420, each of which must be cooled during a freeze drying cycle. The cooling system 400 includes two separate circuits, each containing a heat transfer fluid: a first circuit 491 containing the condenser 420, and a second circuit 490 containing the freeze drying chamber 410. Both circuits 490, 491 pass through an inter-circuit heat exchanger 450 such as a brazed plate heat exchanger for transferring heat energy between the two circuits. Preferably, heat exchange between fluids in the two circuits is performed without the use of an intervening or intermediate heat transfer fluid. The heat transfer fluid in each of the first and second circuits 491, 490 may be a liquid heat transfer oil.
The first circuit 491 includes a turbo compressor cooling system 440 connected for cooling the heat transfer fluid in the first circuit. The heat transfer fluid in the first circuit is circulated by the circulation pumps 430. Adjustable valves 441, 442 control the proportion of heat transfer fluid circulated from the turbo compressor cooling system 440 through the condenser 420 versus through the inter-circuit heat exchanger 450.
The turbo compressor cooling system includes a heat exchanger for transferring heat energy between the working fluid of the turbo compressor and the heat transfer fluid in the first circuit. Preferably, heat exchange between the working fluid of the turbo compressor and the heat transfer fluid in the first circuit is performed without the use of an intervening or intermediate heat transfer fluid.
The second circuit 490 of the freeze dryer cooling system 400 cools the shelves 423 or other heat transfer elements of the freeze drying chamber 410. Heat is removed from the heat transfer fluid in the second circuit 490 by an inter-circuit heat exchanger 450 and transferred to the heat transfer fluid in the first circuit 491. The heat transfer fluid in the second circuit 490 is circulated by the shelf/CTES circulation pumps 470.
The CTES system 460 is selectively included in (or excluded from) the second circuit 490 using a bypass valve 463 and valves 461, 462. As explained in more detail below, heat may be transferred from the CTES 460 to the heat transfer fluid in the first circuit 491 (to refreeze the CTES), or stored cold thermal energy may be transferred from the CTES to the heat transfer fluid (to supplement the turbo compressor cooling system in cooling the shelves). Preferably, heat exchange between the CTES 460 and the fluid in the second circuit 490 is performed without the use of an intervening or intermediate heat transfer fluid; i.e., without the use of another heat transfer fluid to transfer heat between the CTES 460 and the heat transfer fluid in the first circuit 491.
A heater circuit 425 is provided to selectively heat the heat transfer fluid flowing to the freeze drying chamber 410 during the sublimation portion of the freeze drying cycle. A valve 426, together with a bypass valve 411, regulate flow of heat transfer fluid either through the shelves 423 or bypassing the shelf circuit during refreezing of the CTES.
The bolded circuit lines of the exemplary cooling system 400 shown in
In the configuration shown in
As further shown in
The bolded flow path shown in
The bolded flow paths of the exemplary cooling system 500 shown in
Heat transfer fluid in the second circuit 490, as shown in
The exemplary cooling system 600 as shown in
The heater circuit 425, which is not activated during the FREEZE portion of the cycle, is used to add heat energy to the shelves to cause sublimation under vacuum during the LYOPHILIZATION portion of the cycle.
While both the condenser 420 and the shelves 423 are cooled by the turbo-compressor cooling system 440, those two freeze drying system components have different cooling requirements. The condenser 420 generally has lower temperature requirements than the shelves, but does not require a high cooling capacity to achieve and maintain those low temperatures. In contrast, the shelves 423 do not require chilling to temperatures as low as those required by the condenser, but require greater cooling capacity than the condenser 420. In the freeze drying cooling system shown in
An exemplary cooling system 700, shown in
By placing the CTES 760 in a separate circuit 791 from the circuit 790 containing the freeze drying chamber 710, the shelf circuit 723 of the freeze drying chamber may be operated in heating simultaneously with recharging the CTES. During the freeze drying process, the heater 725 is typically activated and the heat transfer fluid in the circuit 790 is at temperatures above the refreeze temperature of the CTES. Those conditions prevent recharging of a CTES placed in the same circuit 790 as the shelves 723. In the cooling system 700, shown in
Should the set point of the condenser 720 be above the set point of the primary loop 791, valve 742 will open in proportion to the amount of cooling required to maintain the condenser at its set point. The valve 741 may be closed and the valve 743 may be opened to bypass the inter-circuit heat exchanger 750, the second circuit 790 and shelves 723, in which case a primary cooling loop is created. That primary cooling loop is independent of shelf or condenser cooling, and includes the pumps 730, the turbo compressor 740 and the CTES 760, for direct and efficient recharging of the CTES.
A method 800 in accordance with embodiments of the disclosure is shown in
The product is then loaded (operation 820) into the freeze drying chamber by placing the product on the shelves. The product may be in bulk form or may be in vials having partially opened stoppers allowing water vapor to escape rom the vial during the lyophilization process. Both the product and the freeze drying chamber shelves are at ambient temperature or are prechilled during the loading process. In one embodiment, the CTES system is used to speed the prechilling process. In another embodiment, the CTES system is bypassed during the prechilling process, preserving the cold thermal energy of the CTES for use in cooling product loaded shelf by shelf.
The CTES system is recharged (operation 830) during one or more operations of the TURNAROUND portion 320 of the freeze drying cycle (
Because the recharging operation 830 is performed in parallel with the sterilizing and loading operations 810, 820, the use of the CTES system to supplement the turbo-compressor cooling system does not unduly lengthen the overall freeze drying cycle time.
Once the CTES system is recharged, the freeze drying chamber is cooled (operation 840) and the product is frozen using the turbo-compressor cooling system supplemented by the CTES. The length of time required to perform this operation is reduced by using the CTES to supplement the turbo-compressor cooling system. In the embodiment including a CTES system based on a phase change material, the phase change material remains at a substantially constant temperature as heat is transferred from the shelves of the freeze drying chamber by the heat transfer fluid. During this operation, the condenser may also be chilled using heat transfer fluid cooled by the turbo-compressor cooling system, in preparation for the freeze drying operation.
After the product is frozen, the product is freeze dried (lyophilized) (operation 850) in the freeze drying chamber. The freeze drying operation typically includes maintaining the product in a frozen condition while subjecting the product to a vacuum pressure. A small amount of heat is added to the product to initiate sublimation of the frozen solvent or suspension medium.
Finally, the chamber is allowed to return to substantially ambient pressure and temperature, and the freeze-dried product is unloaded (operation 860) from the chamber. Because neither the freeze drying chamber nor the condenser is chilled during this operation, the recharging operation 830 in preparation for the next freeze drying cycle may be started during unloading.
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2021/027579 | 4/16/2021 | WO |
