Indigo drying is mainly divided into raw powder indigo drying and commercial indigo drying. In general, the indigo used for export is mostly the original powder.
The raw powder drying method is mainly the indigo after the synthesis, and after being filtered by a filter press, it enters the dryer in the state of the filter cake. At present, most of the dryers used for drying the original powder indigo are sputum dryers, paddle dryers, rotary quick dryers, and the like. Among the three models, the heat transfer modes of the first two dryers are mainly conduction heat transfer, and the latter one is convection heat transfer. According to the production method, the rake dryer is intermittent drying, and the paddle dryer and rotary quick dryer are continuous drying.
The author believes that when the production scale is small, it should be dried by a drier dryer; when the production scale is large, it should be dried by a continuous dryer such as a paddle dryer or a rotary quick dryer. This is because the one-time investment of the paddle dryer and the rotary quick dryer is relatively high, and it is uneconomical when the scale is not reached. When the output is large, the two models are adopted. Although the investment is high, the production efficiency is high, the working environment is good, the labor intensity is small, the production process is easy to be automated, and the product moisture content is uniform. In addition, one problem that cannot be ignored is that indigo is easily entrained by dry gas because of its relatively low density (at about 200 kg/m3). In the three types of dryers, the order of the amount of entrainment is arranged as follows: a rotary quick dryer, a paddle dryer, and a rake dryer. Therefore, when these dryers are selected, the scale of the gas filtration equipment is different. Especially in the case of a rotary fast dryer based on convection heat transfer, since the particles of indigo can reach about 0.2 micron, the process of gas-solid separation is more important, requiring a larger filtration area of â€‹â€‹the dust collector, and a filter material. More efficient requirements.
A spray dryer is required to produce the product indigo. Pretreatment is required for the production of commercial indigo to achieve the same standard strength. Standardized process treatment refers to high-speed beating in liquid state after adding various additives. The raw material is a flowable liquid with a solid content of 25% to 30%, so it needs to be spray-dried to obtain powder or granular. product. If the pressure spray dryer is used for drying, the product size can be maintained between 80 and 180 mesh. Indigo is a relatively heat-resistant dye, and the dry hot air inlet temperature can be between 230 Â°C and 240 Â°C.
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