moved from the space defined between the cylinders. After that, the frozen layer of the material is subjected to a vacuum environment while heated through the surfaces of the cylinders to obtain from the surfaces the heat required for sublimation, so that sublimation of at least one of constituents of the materials occurs. After completion of such sublimation, the layers of the material dried on the surfaces of the cylinders through such sublimation are scraped from the surfaces of the cylinders and collected by a product receiver disposed below the cylinders. The conventional method disclosed in the above U.S. Patents will be hereinafter referred to as the prior art 3. More particularly, in the prior art 3, a heat-transfer medium is circulated through the cylinders so that the liquid material received in the space defined between these cylinders are frozen in positions adjacent to the surfaces of the cylinders to form the desired-thickness frozen layers of the material in such positions. Then, the remaining part of the liquid material still not frozen in the space is removed from the space, and thereafter the frozen layers of the material is subjected to the vacuum environment while gradually heated by means of the heat-transfer medium circulated in the cylinders. The thus obtained product having been dried on the surfaces of the cylinders are scraped from the surfaces of the cylinders by means of a scraper which rests at a position above the cylinders and is driven downward by a threaded rod in such scraping operation. Thus scraped product or dried material is collected by the product receiver.
As described above, in the prior art 3, the layers of material frozen on the surfaces of the cylinders adhere to the surfaces of the cylinders. Consequently, in order to separate the frozen layers of the material from the cylinders, in the prior art 3, there is employed a scraping mechanism comprising a disk-like scraper having a plurality of circular holes each of which has a diameter slightly larger than an outer diame