1. Field of the Invention
This application relates to improved freeze-drying systems and methods. More particularly, it concerns such systems and methods whereby a wide variety of liquid substances may be dried at lower cost and with higher quality than previously possible.
2. Description of the Prior Art
Two drying techniques related to the present invention are spray-drying and freeze-drying. Spray-drying requires that the product to be dried is a liquid or can be made into a liquid which is sprayed through a nozzle under pressure to produce a mist or stream of fine droplets. These fine particles are exposed to a hot gas which evaporates the moisture almost instantaneously, leaving a dry powder which can later be reconstituted to its liquid state by addition of the original solvent (hereafter assumed to be water for purposes of discussion, but not as a limitation on the invention).
Freeze-drying is of more recent origin than spray-drying, depending, as its name implies, on technology to freeze a substance while drying it. In this type of process, the substance to be dried (which need not be a liquid) is first frozen so that the water contained in it is turned to ice. The ice is then removed by sublimation, which is the direct evaporation of the gas (water vapor) from the solid state (ice) without ever becoming liquid. When the process is complete, the original product is cold, but dry; it may then be warmed and packaged for storage. Like the spray-dried product, it can be reconstituted by the addition of water.
Freeze-drying has advantages over spray-drying for the dehydration of many substances. Thus, the low temperatures used in the process prevent deterioration of the product caused by heat. Also, some substances are sticky at the higher temperatures used in other methods of drying and cannot therefore be successfully dried without using additives or drying aids. Further, once frozen, the substance is not susceptible to deterioration during the drying process by exposure to liquid water. Additionally, freeze-dried products are very easily reconstituted by the addition of water and such process is faster and more complete than with products dried by other methods.
The result of these advantages is that the final, reconstituted product of freeze-drying is of a higher quality than can be achieved by other drying procedures. In particular, the taste and texture of reconstituted freeze-dried foods are often indistinguishable from the original, even after prolonged storage at ambient conditions.
Given these clear advantages, the question may arise why freeze-drying has not replaced other dehydration methods. The answer is that, unfortunately, freeze-drying has been typically more expensive than any of the other means of dehydration and has, therefore, been limited to high-value goods or situations where high quality and reduction of bulk are very important.
A combination of spray-drying and freeze-drying is possible. This is achieved by spraying a liquid substance into a freezing environment, and then freeze-drying the resulting accumulation of small frozen particles, which combination can be called spray-freeze-drying (SFD) (see U.S. Pat. Nos. 2,471,035; 3,300,868; 3,362,835 & 3,396,475). A particularly attractive means of drying the accumulated particles in SFD operations is by passing a gas through them in such a way as to a fluidized bed (see U.S. Pat. No. 3,313,032). However, operations disclosed in such prior art involve features that limit their commercial applicability in one or more ways, namely, (a) inefficiencies in heat and mass transfer, (b) lack of flexibility and control to allow processing of a wide variety of substances, including highly concentrated or sticky substances and (c) require system components which are difficult or expensive to fabricate or operate.