Microwave Drying of wool tops

In the finishing process of printing and dyeing, the drying of textiles is an important process in the whole process.Its drying effect has a significant effect on the final product color, intensity, pomeness, etc.Therefore, drying technology has become the research focus of textile printing and dyeing industry.

At present, the drying methods of domestic textile dyeing and finishing industry are mainly divided into hot air drying and radio frequency drying.Hot air drying has the advantages of low cost and low maintenance cost, but the drying time is long, the efficiency is low, the products tend to yellowing and embrittlement after drying.

During the drying process, the temperature is difficult to be accurately controlled and the phenomenon of over-drying is easy to occur, which causes the quality problems such as uneven moisture content after drying, and brings many inconvenience to the subsequent process.Radiofrequency drying is to use high-frequency electric field to convert the electric energy between the plates into the heat energy of drying materials, thus completing the drying process of materials.

Its unit energy consumption is high, the equipment maintenance cost is high in the later period, the main components of the electronic tube service life of 5000h ~ 6000h, the replacement cost is about 65,000 yuan, resulting in the equipment in the later period of operation and maintenance investment is huge, not consistent with the textile printing and dyeing industry energy saving and emission reduction trend.

Microwave heating is a new drying technology which develops rapidly and has great potential in recent years.Microwave heating has the advantages of high efficiency, energy saving, low temperature, even drying and no thermal inertia, and the magnetron has been industrialized production, convenient maintenance and replacement, low maintenance cost, replacement cost is about 20,000 yuan.

Microwave heating has been widely used in chemical industry, fruits and vegetables, building materials, medicinal materials and other industries.Due to the variety of fiber fabrics and the complexity and multi-factor of weaving dyeing and finishing, microwave heating is developing in the textile printing and dyeing industry.

In the worsted industry, top rewashing is a necessary process.The existing methods of drying are steam drying,low efficiency,high temperature and great damage to fiber quality,it becomes the bottleneck factor that restricts the rewashing process of wool tops.

In this paper, microwave drying equipment is used to dry the top, which improves the drying efficiency and product quality of the top, studies the drying efficiency and drying effect of the microwave drying equipment, and provides a certain research basis for the promotion and application of microwave heating in the textile dyeing and finishing industry.

Principle of Microwave Heating:

Microwave is a kind of electromagnetic wave with penetrability in the frequency of 3.0×102 ~ 3.0x105mhz, whose direction and size change periodically at any time.When the electromagnetic wave penetrates the water-bearing material with 915MHz or 2450MHz (only 915MHz and 2450MHz are allowed in China’s industrial heating).Because water molecules are polar molecules, the polarity of water molecules in materials changes at 91.5 billion and 245 billion times per second.

Due to the irregularity of molecular thermal motion and the extremely high regular change of polarized molecules, the heat effect similar to friction is generated between molecules, making the heat evenly and simultaneously occur from all parts of the material in a very short time.The sharp rise in temperature forces the water molecules inside the material to vaporize and diffuse to evaporate, and the material dehydrates quickly to achieve the purpose of drying.

Putting the wool top into the KH-80HPTN10 tunnel-type microwave dryer with the microwave frequency of 2450MHZ and the microwave output power of 80KW.An electronic balance with weighing accuracy of ±0.1g was used to record the changes in the quality of the wool top during the drying process, and to calculate the changes in the moisture content. Then, the drying efficiency of the microwave dryer was calculated according to the water loss of the wool top.

In order to prevent overdrying, we need to adjust drying time and other process parameters, microwave drying is carried out in different stages to improve the drying efficiency of microwave dryer in each stage.In the initial stage of microwave heating, the low drying efficiency is caused by the high moisture content of the material and the low energy state of water molecules that need to absorb part of the microwave energy to make them evaporate.

With the decrease of water content and the increase of water molecular energy state, microwave drying efficiency is improved.The average drying efficiency of the whole drying process is 1.18kg/(kW·h), so the drying efficiency of the microwave dryer can be 1.1 kg/(kW·h) when calculating the drying process parameters.

According to the comparison of many experiments, when the moisture content of the wool top is below 30%, the drying efficiency is the highest and the economic benefit is the best.

The microwave drying principle and mass transfer and heat transfer process of the hair top were analyzed. The microwave drying equipment developed by ourselves was used to dry the hair top of different forms and the experimental results were analyzed. The experimental results showed that:Microwave drying technology can be applied to the drying of wool tops, which can realize the continuous and efficient drying of wool tops, and significantly improve the drying efficiency and quality.

When the whole fleece coil is dried, the internal temperature of fleece coil is higher than 80℃, and the surface condensation is obvious;When loosely dried, the top surface feels fluffy, the temperature is below 40℃, and the energy consumption of multi-layer (6-8 layers) drying is lower than that of single-layer drying.

Microwave dryer has shorter time, lower temperature and better drying effect than traditional drying equipment.This is because in the process of microwave heating,because the surface is easy to dissipate heat, the internal temperature is often higher than the external temperature, and the direction of temperature gradient is the same as that of moisture gradient, and the direction of heat and mass transfer is the same, which promotes the rapid evaporation of internal moisture and the formation of internal pressure gradient, so that the moisture quickly diffuses to the surface and evaporates.

The higher the working frequency of microwave, the more microwave power absorbed by the material per unit time, which can timely compensate the heat taken away by evaporation of moisture and make the energy absorption more effective.The faster the temperature rises, the faster the drying rate, and the shorter the drying time, ensuring the natural quality of the fiber is not damaged to the maximum extent.

Due to the transfer of water vapor from the inside to the outside, the inner layer of the fiber has greater dynamic performance of steam pressure, and the internal transpiration of water is similar to the effect of steaming yarn, so the strength of top fiber is improved. After drying, the color difference before and after the product color is small, feel fluffy, soft, and fiber elasticity is good.

Microwave heating effectively solve the problem of the traditional drying resources consumption, improve the drying efficiency and product quality, realize the greening of textile production, conform to the textile printing and dyeing industry, the development trend of energy saving and emission reduction is steam drying, rf drying technology to upgrade again, will be to transform traditional industries, improve the equipment level of the enterprise, has a broad application prospect.

As a new drying method, our research on microwave heating of materials is not deep enough. To give full play to the advantages of microwave heating, we must do more extensive and in-depth research.

(1)Understanding the influence of material characteristics on microwave heating, and realize quantitative research.

(2)Strengthen the comprehensive research and development of microwave heating technology of various materials and microwave heating equipment to realize the on-line detection and control of microwave heating.