Distribution of Vapor Pressure in the Vacuum Freeze-Drying Equipment

Mathematical Problems in Engineering, Jul 2012

In the big vacuum freeze-drying equipment, the drying rate of materials is uneven at different positions. This phenomenon can be explained by the uneven distribution of vapor pressure in chamber during the freeze-drying process. In this paper, a mathematical model is developed to describe the vapor flow in the passageways either between material plates and in the channel between plate groups. The distribution of vapor pressure along flow passageway is given. Two characteristic factors of passageways are defined to express the effects of structural and process parameters on vapor pressure distribution. The affecting factors and their actions are quantitatively discussed in detail. Two examples are calculated and analyzed. The analysis method and the conclusions are useful to estimate the difference of material drying rate at different parts in equipment and to direct the choice of structural and process parameters.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

http://downloads.hindawi.com/journals/mpe/2012/921254.pdf

Distribution of Vapor Pressure in the Vacuum Freeze-Drying Equipment

Distribution of Vapor Pressure in the Vacuum Freeze-Drying Equipment Shiwei Zhang1 and Jun Liu2 1School of Mechanical Engineering and Automation, Northeastern University, Shenyang 110004, China 2Normal School of Shenyang University, Shenyang 110015, China Received 13 April 2012; Accepted 14 May 2012 Academic Editor: Zhijun Zhang Copyright © 2012 Shiwei Zhang and Jun Liu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract In the big vacuum freeze-drying equipment, the drying rate of materials is uneven at different positions. This phenomenon can be explained by the uneven distribution of vapor pressure in chamber during the freeze-drying process. In this paper, a mathematical model is developed to describe the vapor flow in the passageways either between material plates and in the channel between plate groups. The distribution of vapor pressure along flow passageway is given. Two characteristic factors of passageways are defined to express the effects of structural and process parameters on vapor pressure distribution. The affecting factors and their actions are quantitatively discussed in detail. Two examples are calculated and analyzed. The analysis method and the conclusions are useful to estimate the difference of material drying rate at different parts in equipment and to direct the choice of structural and process parameters. 1. Introduction Generally speaking, the food vacuum freeze-drying equipments of which the freeze-drying area is equivalent to or larger than 50 m2 are classified as big vacuum freeze-drying equipments [1]. In this type of vacuum freeze-drying equipment, there always exists a phenomenon that the drying rates of materials are uneven at different positions [2]. The reason is commonly charged upon the uneven temperature distribution of material plate [3, 4]. But, the uneven distribution of vapor pressure in vacuum chamber is usually ignored. In fact, the big vacuum freeze-drying equipment always adopts the form of multilayer of big area material plates (shelves) in order to improve the production efficiency and output. And in order to enhance the volume utilization rate of the vacuum chamber, the space between material plates is relatively narrow, and the evacuating port of the vacuum chamber is always on one side of it. All these structural characteristics accordingly lead to the result that a flexural, long, and narrow flow transfer passageway is formed from the evaporating surfaces of the materials on the plates to the evacuating port of the vacuum chamber and can produce a flow resistance that cannot be ignored. And in the big vacuum freeze-drying equipment the water vapor flow in the sublimating drying stage is rather large, so in the long and narrow passageway there is a significant water vapor pressure difference, and this finally leads to the result that the drying rate of the materials is different on different plates or even in different positions of the same plate in the big vacuum freeze-drying equipment. Analyzing the distribution of vapor pressure in the big vacuum freeze-drying equipment during the process of vacuum freeze-drying is significant to the correct evaluation of the inhomogeneity of the drying rate and its reduction. The former research on the mass transfer of vapor in the freeze-drying equipment mostly focused on the diffusion process and average drying rate in the materials to be dried [5–7]. But there is less study on the process of the vapor flowing from the surfaces of the materials to the evacuating port of the freeze-drying chamber and the difference of the drying rate in different parts of the materials caused by flowing resistance. This paper develops a mathematic model which describes the flow of the vapor outside the materials in the freeze-drying equipment. The distribution of vapor pressure and the relationship of parameters of each influencing factor are determined. And it also puts forward a quantitative analysis method of drying rate in different positions of the material and the theoretical designing basis of the key structure and processing parameter of the freeze-drying equipment. 2. Structure and Simplified Model As shown in Figure 1, the material plates and pallets in the big freeze-drying equipment are usually arranged as the form of multilayer matrix, several or more than ten layers vertically aligned to form a group. There is a equidistant passageway between each two plates. One group, two groups or even more groups are arranged in parallel in the vacuum chamber. On both sides of each group and between two groups, there is an evacuating channel. The evacuating port of the vacuum chamber is often on the back of it opposite to the evacuating channels between groups (position a in Figure 1) or on the top or the bottom (position b or b′ in Figure 1). It also ca (...truncated)


This is a preview of a remote PDF: http://downloads.hindawi.com/journals/mpe/2012/921254.pdf

Shiwei Zhang, Jun Liu. Distribution of Vapor Pressure in the Vacuum Freeze-Drying Equipment, Mathematical Problems in Engineering, 2012, 2012, DOI: 10.1155/2012/921254