Vacuum drying equipment comes in many varieties, has a wide range of applications, and is developing rapidly. This article only describes the domestic and international development trends of several types of vacuum drying equipment, aiming to facilitate information exchange, identify the challenges that need to be addressed in the development of vacuum drying equipment, and improve the level of vacuum drying equipment in my country.
Keywords: Vacuum drying; Drying equipment; Freeze-drying
Vacuum drying has many advantages: low oxygen content during drying under low pressure prevents oxidation and deterioration of the dried material, and can dry flammable and explosive hazardous materials; it can vaporize the moisture in the material at low temperatures, making it easy to dry heat-sensitive materials; it can recover valuable and useful components from the dried material; and it can prevent the emission of toxic and harmful substances from the dried material, making it an environmentally friendly "green" drying method. Therefore, the application of vacuum drying equipment is becoming increasingly widespread.
The main disadvantages of vacuum drying are the need for a vacuum system capable of pumping out water vapor, resulting in high equipment investment and operating costs; and low equipment production efficiency and small output. Many scientific and technological workers have made great efforts to overcome these disadvantages. At the same time, due to the many advantages of vacuum drying, some products have no choice but to use vacuum drying equipment. Therefore, the development of vacuum drying equipment has a promising future.
1. Uneven Development of Continuous Vacuum Drying Equipment at Home and Abroad
To increase equipment output and ensure product quality, various continuous vacuum drying equipment were developed abroad more than a decade ago. However, in China, development has been relatively slow due to limitations in technology and public awareness.
1) Belt Continuous Vacuum Drying Equipment
The WL-VAO type belt continuous vacuum dryer produced by Nissaka Manufacturing Co., Ltd. in Japan is suitable for drying liquid materials, slurries, pastes, high-concentration materials, and high-viscosity materials. The BV-100.5 type vacuum belt continuous dryer produced by Okawahara Co., Ltd. in Japan uses steam and conduction heating, with adjustable temperature in each section, and adjustable conveyor belt tension and speed. The Swiss company Buch-Gade has developed a series of belt continuous vacuum dryers with automatic cleaning devices. Since 1995, they have been engaged in the design, manufacture, installation, and service of belt continuous vacuum dryers, and their technology is relatively mature. Domestically produced continuous belt vacuum drying equipment is not common. In 2004, the Guangdong Provincial Academy of Agricultural Sciences successfully developed a small-scale experimental device for drying banana powder, with excellent results.
2) Continuous Vacuum Drying Equipment for Grains: Grain drying capacity is large, necessitating continuous drying equipment. Previously, many countries developed grain drying equipment, but vacuum drying of grains was mostly used for seed drying due to its high cost. This is actually a misconception. According to He Xiang, a senior engineer at the Zhengzhou Grain Science Research and Design Institute, their continuous corn vacuum drying equipment has a production capacity of 60t/d, with a fixed investment slightly higher than hot air drying, but operating costs comparable. Considering the quality of the dried product, breakage rate, and cracking rate during the drying process, the total cost of vacuum drying at low temperatures is not higher than hot air drying.
3) Continuous Vacuum Freeze-Drying Equipment: Food raw materials are abundant, and the output of freeze-dried products is large; therefore, continuous food freeze-drying equipment appeared relatively early. In 1985, the Danish company ATLAS produced the CONRAD-800 continuous freeze-drying equipment for freeze-dried coffee production, with a daily capacity of 13 tons. Figure 2 shows a schematic diagram of this equipment, and Figure 3 shows a continuous freeze-drying machine manufactured in Germany.
The first domestically produced continuous vacuum freeze-drying equipment was successfully developed in 2000 by the Shenyang Refrigeration Technology Research Institute. The vacuum unit adopts a rectangular structure, with isolation plates between the feed hopper and the drying chamber. Both the feed hopper and the drying chamber are equipped with automatic weighing systems to determine the drying rate, water output, and final degree of dryness of the frozen food. Two external water traps work alternately to achieve continuous water trapping and ice melting.
There is a significant difference in sales volume between domestic and foreign continuous freeze-drying equipment. From 1985 to 1990, the Danish company ATLAS sold 18 continuous freeze-drying machines, including one purchased in Taiwan. However, no provinces or cities in mainland China have yet to import this equipment, and only one domestically produced machine remains unsold.