Processes 2021 , 9 , 1117
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Finally, we find that the evaporation heat is related to the drying curve (Figure 1b) and temperature of paper at atmospheric pressure. It can be calculated as following Equation (10), which is obtained by substituting Equation (8) into Equation (6):
·
1 du dt
du dt
Δ H = Δ H lat ·
(10)
u = u ∞
2.2. Advantages of the New Method In this paper, a convenient and rapid method to measure the evaporation heat of paper is proposed. The new method is based on the drying curve, while the old method is based on the sorption isotherm. Because the measurement of the drying curve is more convenient than that of sorption isotherm, the new method has two great advantages. As shown in Table 1, one is the measuring time, in the same laboratory environment, the old method based on the sorption isotherm get each data point generally needs to be in constant temperature and humidity environment for 24 h to balance water, however, the new method based on the drying curve only takes 1–2 h to complete the measurement of the whole drying curve, or even less than 15 min when using the moisture rapid analyzer. During on-line measurement at the production site, in the on-line measurement of production site, it takes 10 min for the new method to measure a data point, but the old method can not measure directly, so it can only use the data measured in the laboratory.
Table1. Comparison of new and old methods.
Item
The New Method
The Old Method
Measuring principle Based on the measurement of drying curve
Based on the measurement of sorption isotherm
(1) Laboratory: The traditional method is 1–2 h, if the moisture rapid analyzer is used, it will take about 15 min (2) On-line measurement: One data point is about 10 min
(1) Laboratory: Each data point generally needs to be in constant temperature and humidity environment for 24 h to balance water, so it usually takes several weeks (2) On-line measurement: Can’t measure (1) Conventional laboratory conditions (2) Constant temperature and humidity environment
Measuring time
Measuring conditions
(1) Conventional laboratory conditions
Another advantage is the measuring conditions. The measurement of the drying curve only needs the conventional experimental environment, while the measurement of the sorption isotherm needs the special experimental environment of constant temperature and humidity. 3. Results and Discussion 3.1. Laboratory Verification In the laboratory, mechanical pulp was used as raw material to make hand paper. The specific steps are as follows: Step1: Through the Wally beater (VB-42F, China pulp and Paper Research Institute Co., Ltd., Beijing, China), the raw material of mechanical pulp was dredged and beat to a specific beating degree (30 ◦ SR ). Step2: The pulp concentration was measured, and a certain amount of pulp was weighed based on the paper with a weight of 60 g/m 2 . Step3: The pulp was dissociated with a fiber standard dissociator (ZY-XW, Shandong Zhongyi Instrument Co., Ltd., Shandong, China), and formed into wet paper on a rapid Kaiser sheet former (ASM-32N2F, China pulp and Paper Research Institute Co., Ltd., Beijing, China). Step4: The wet paper was pressed on the multi-functional extruder (MASP22H803, China pulp and Paper Research Institute Co., Ltd., Beijing, China) by 50 kPa.
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