The freezing and thawing of large amounts of bulk liquids packed in flexible bags and/or rigid containers takes a long time due to limited cooling capacity of the freezing equipment, low overall heat transfer coefficient and the large size of the containers. This will affect the homogeneity of the products being frozen due to changes in product concentration throughout the container as it freezes.
Rapid or shock freezing is intended to alleviate these issues by reducing the freezing time and helping to maintain uniform concentrations, while quick thawing effectively converts the frozen product back to its liquid state so it can be readily used.
Current state-of-the-art fast freezing technologies utilize cabinet freezers and modified freeze dryers where packaged liquids are placed on or between cooled shelves, which
are cooled using mechanical and sometimes cryogenic cooling systems. The rate of freezing is determined by the overall thermal resistance of the systems which in this case quite high due to slow heat transfer through the packaging wall as the heat needs to pass through multiple protective layers designed to prevent damages as a result of contact with cold media as well as ineffective heat transfer on the inside of the package primarily due to natural convection.
The present invention provides a method and apparatus for addressing these shortcomings and improving the freezing of liquids.
In one embodiment of the invention there is disclosed a method for freezing a liquid in a freezer storage bag comprising feeding a coolant to a cooling duct present in the freezer storage bag.
In a different embodiment of the invention, there is disclosed a method for freezing a liquid in a freezer storage bag comprising the steps of feeding a liquid coolant to a heat exchanger thereby forming a gaseous coolant; feeding the gaseous coolant to a manifold; feeding the gaseous coolant from the manifold to at least one cooling duct present in the freezer storage bag; circulating the gaseous coolant through the cooling duct thereby imparting freezing to the liquid in the freezer storage bag; and recovering the gaseous coolant from the cooling duct.
In another embodiment of the invention, there is disclosed a method of freezing a liquid in a freezer storage bag comprising the steps of sucking in a cold gaseous atmosphere inside a freezing cabinet by means of a blower; feeding the cold gas to a manifold; feeding the cold gas from the manifold to at least one cooling duct present in the freezer storage bag; circulating the cold gas through the at least one cooling duct thereby imparting freezing to the liquid in the freezer storage bag; and recovering the cold gas from the cooling duct.
In another embodiment of the invention, there is disclosed a storage unit comprising a freezer storage bag with a cooling duct embedded within the freezer storage bag.
The methods of the present invention are typically applicable to liquids that are water or water soluble substances. However, the liquid that is frozen can be fat-soluble.
For purposes of the present invention, the term freezer storage bag will include freezer storage bags or other containers that are typically used in freezing operations. The freezer storage bags for pharmaceutical use will typically be made of ethylene-vinyl acetate (EVA), thermoplastic elastomers (TPE), polyvinylchloride (PVC) and be cold resistant to −50° C. or below.
The freezer storage bags can range in storage size from 100 milliliters to 50 liters.
Typically, the freezer storage bag will be present in a freezer unit where it will lay flat upon a freezer shelf. Depending upon the size of the freezer unit, more than one freezer bag may be present therein on more than one shelf.
In an alternative embodiment of the invention, the freezer storage bags may be hung or held vertically within the freezer unit. When the freezer bags are in this position, the use of the cooling ducts becomes more pronounced because there are no shelves to assist in providing cooling to the freezer storage bags and their content. Therefore, when held vertically, the heat removal by the cooling ducts improves the efficiency of the freezing operation.
The retaining bracket is part of the freezer storage bag design as it is cut out from the bag material with a circumferential weld seam.
The freezer storage bags are filled with the liquid before they are loaded into the cabinet freezer.
The coolant is typically nitrogen but when air is used in the atmosphere in a freezer cabinet, it will be circulated through the cooling duct. Alternatively, if carbon dioxide is employed in the atmosphere, then it can be used as the coolant. In situations where a liquid coolant is employed in the freezing operation, then these can be employed. Typically, a liquid coolant would be brine or ethanol.
The cooling duct can both assist an external freezer in freezing the freezer storage bag and provide the necessary cooling to freeze the contents of the freezer storage bag on its own.
The cooling duct will typically be fabricated from the same material as the freezer bag. For pharmaceutical use, it must at least meet the legal requirements e.g. of United States Food and Drug Administration (FDA).
The cooling duct is typically sized per the size of the freezer storage bag it is to be inserted into. This size ranges from 5 to 20 millimeters in diameter.
The cooling duct will typically be welded into the freezer storage bag. Such freezer storage bags are already equipped with ducts for filling, emptying and sampling of contents and are likewise welded to the freezer storage bag.
The manifold will typically be designed such that the same amount of coolant gas enters each individual coolant duct. This flow can be controlled by the supply pressure of the coolant by way of the back pressure of a blower, or pressure control valve. In practice, typically 2 to 30 lines can be employed from the manifold.
The lines are typically made from the same material as the freezer storage bag. If there is no direct contact with the material that is sought to be frozen, particularly pharmaceutical, then other materials can be used such as flexible steel lines.
A spacer or strap can be mounted or fixed to the cooling ducts in a manner to avoid rotation of the cooling duct. The spacers are typically fashioned from stainless steel or plastic materials like EVA. The spacers should be made from material that approved for pharmaceutical use as well as being resistant to cold. The spacers are typically fashioned in a rectangular cross section.
An operator would typically employ 2 or 3 spacers per cooling duct so a typical freezer storage bag may have 2 to 6 spacers present therein.
The spacers will be attached to the cooling duct by clamping. This mechanism relies on the clamp, in a relaxed position, being smaller than the outer diameter of the duct such that when the clamp expands, the spacer is securely held in place in the cooling duct.
When two or more cooling ducts are employed in a freezer storage bag, they are spaced apart by using the spacers having different leg lengths.
The longer the time it takes to freeze the contents of a freezer storage bag, the more expensive and inefficient the operation of freezing. The solution to this problem is to minimize the distance d thereby making the overall process of freezing the contents of a freezer bag quicker and more efficient.
The cooling duct 22 can be mounted or embedded in the freezer storage bag 20 in the same manner as the fill line 21 used for filling and removing goods from the freezer storage bag 20. One means for embedding the cooling duct as well as the fill lines in general is by plastic welding, particularly when the freezer storage bag is made of a soft plastic material.
A side view of the freezer storage bag 20 is shown in
It can be seen then that it is important to embed the appropriate cooling duct into the freezer storage bag in a manner that will reduce the distance from one wall of the freezer storage bag from its opposite wall thereby to reduce freezing time. Ideally, the cooling duct will be located within the middle of the freezer storage bag thereby ensuring that the distance to each wall of the bag is about the same. This will help to achieve a more uniform freezing process.
As seen in
This positioning is also shown by
In an alternative embodiment of the freezer storage bag 40 shown in
A second cooling duct 55 is mounted as well in the freezer storage bag 50. Like the first internal cooling duct 56, it too hangs vertically. This second cooling duct is fed coolant through input 55A and discharges the coolant through output 55B. This embodiment is also good when a larger freezer storage bag is employed.
The gaseous nitrogen will flow through the inputs 70, 71 and 72 into the cooling ducts (not shown) of each of the three freezer storage bags 64, 65 and 66 and provide cooling to the contents therein. The cooling ducts may be held in place with spacers (not shown) to ensure that the cooling ducts are present in the middle of the bag, thereby providing optimum cooling and freezing to the contents of the freezer storage bags.
The flow of the gaseous nitrogen once it enters the freezer storage bags 64, 65 and 66 will be through individual valves, V1, V2 and V3 respectively. The nitrogen gas will flow through the output lines 73, 75 and 77 to output lines 74, 76 and 78 respectively and flow through the valves V1, V2 and V3 respectively where the nitrogen gas will be discharged in an environmentally conscious manner to the atmosphere.
Typical temperatures in freezing cabinets are −5° C. to −70° C. The flow rate depends upon the duct diameter. Typical velocities are 5 to 15 meters/second, The pressure in the ducts is approximately ambient, e.g., 1000+100 mbar (pressed) and 1000−100 mbar (sucked). At a duct diameter of 12 mm, the corresponding flow rate is in the range of 3 to 9 kilograms/hour.
A blower B receives cold gas through line 86 and feeds it to a manifold 97. The manifold 97 connects through lines 87, 89, and 91 to the input connections 88, 90 and 92 of the three freezer storage bags 84, 85 and 86 respectively. The cold gas flows into the cooling ducts which are not shown and which may be supported by one or more spaces (also not show) such that the cooling duct is positioned approximately in the middle of each of the three freezer storage bags 84, 85 and 86. The cold gas is discharged from each of the freezer storage bags through lines 93, 94 and 95 respectively.
Alternatively, the cold gas can be sucked through the cooling ducts by means of blower B rather than pressed through the ducts. The advantage is that a blower warms up the gas. When sucking the gas through the ducts, it has a lower temperature and thus a better cooling potential.
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.