CHEN Qi, TANG Yongzhi, LU Lin, FENG Qin, HUANG Yichen, DING Zhonggen
(Jingdezhen ceramic university, Department of Energy and Power Engineering, Jingdezhen 333403, Jiangxi, China)
Extended Abstract:[Background and Purpose] As a high energy consuming industry, the ceramic industry generally faces the problem of excess waste heat from the flue gases at medium and low temperatures in the production process. If the waste heat cannot be effectively utilized, it is not only energy waste, but also seriously hinders the low-carbon, energy-saving production and sustainable development of the ceramic industry. It is imperative to explore effective technical solutions for utilizing the kiln waste heat, in order to achieve energy conservation and emission reduction in ceramic production. Ammonium chloride is an important raw material for ceramic production, while the drying process usually requires huge amount of heat energy. Therefore, utilizing kiln waste heat to dry ammonium chloride not only improves energy efficiency but also reduces the production costs, thus having important practical significance. The aim of this study is to construct a kinetic model for the drying of ammonium chloride, focusing on drying characteristics and moisture migration laws, thus providing theoretical and technical supports for optimizing the waste heat utilization in ceramic production processes.[Methods] To achieve the above goals, a technical solution based on kiln waste heat to dry ammonium chloride was proposed, by constructing a kinetic model with user-defined functions (UDF). The water migration law and drying characteristics during the drying process were analyzed through numerical simulation methods. The contents include three aspects. Firstly, through drying experiments, five thin-layer drying models were established, while the most suitable model to describe the moisture change law during the ammonium chloride drying process will be identified. Accordingly, a mathematical model for the drying process of ammonium chloride was constructed. Secondly, the spatial and temporal variations of water migration rate (WMR) during the drying process of ammonium chloride will be analyzed, especially the water migration characteristics in the x- and z-directions. Thirdly, by simulating and analyzing the water migration behavior during the early, middle, and late stages of drying, the phenomenon of water accumulation and the impact on drying efficiency will be revealed.[Results] The two-term model can be used to accurately predict the drying process of ammonium chloride, which can be divided into accelerated drying stage and deceleration drying stage. In the initial stage of drying, the water migration rate (WMR) in ammonium chloride products was rapidly increased from the edge to the center, and then slowly decreased, with slight water accumulation in the central area. As the drying process was progressed, the WMR in the central area was gradually increased, while the overall WMR showed a deline trend. In the x-direction, WMR maintained an upward pattern from the edge to the center. In the z-direction, the variation trend in WMR was similar to that in the x-direction, but its peak shifted from the central region to a slightly higher location. In addition, it was also found that the spatial distribution of water migration rate was closely related to the drying time, so that the drying efficiency could be significantly improved by optimizing the drying process parameters.[Conclusions] In order to reveal the water migration law during the drying process of ammonium chloride, a thin-layer drying kinetics model was proposed, while the user-defined functions (UDF) were combined to numerically simulate the drying process driven by kiln waste heat. The two-term model can be used to accurately predict the drying process of ammonium chloride, with a correlation coefficient R2=0.99891, root mean square error RMSE of 0.006454, and squared error of 0.001532936, indicating high reliability of the model. The drying of ammonium chloride was divided into two stages: acceleration and deceleration. The maximum drying rate was 1.28833×10–5 kg·kg–1·s–1, while it took 14400 seconds to achieve a moisture content of 0.1%. During the drying process, the moisture content was gradually decreased, whereas the moisture content in the edge area was lower than that on the inner side. In the initial stage, the water migration rate (WMR) was increased and then decreased from the edge to the center, whereas water accumulation occurred in the central region. In the later stage, WMR was increased from the edge to the center, while the overall WMR showed a decline trend. In the z-direction, the variation pattern of WMR was similar to that in the x-direction, but the peak shifted from the central region to a higher location and the overall WMR was gradually decreased with drying time.
Key words: ammonium chloride; waste heat drive; drying characteristics; moisture migration law; UDF numerical study