Abstract: It has been widely recognized that the mixing process has significant impacts on the performance of low-density polyethylene (LDPE) reactors due to the rapid radical polymerization occurred in the reactors, but how the macro- and micro-mixing affect the reactor performance was still controversial in publications. In this work, a cold-flow LDPE autoclave with multi-feedings was scaled down (1/2) from an industrial reactor and built to systematically investigate the macro- and micro-mixing characteristics of fluid by experiments. Furthermore, the effects of macro- and micro-mixing on the polymerization were comprehensively analyzed. The results showed that according to the delay time t_d and macro-mixing times t_M calculated from residence time distribution (RTD) curves, the macro-mixing states are significantly different at various axial positions (h/H), especially at lower agitation Reynolds number Re. But with the increase of Re, since the circulation flow in the reactor is strengthened, the t_d for each feed gradually decreases to 0, and the t_M at different axial positions tend to be identical. For micro-mixing, the qualities of micro-mixing at different axial positions are similar, and the average micro-mixing time t_m in the reactor decreases exponentially with the increase of Re. Moreover, a fitting model was established. Through the comparison of the characteristic times of macro-mixing (t_d, t_M), micro-mixing (t_m) and elementary reactions within the industrial range of Re, it can be concluded that the properties of LDPE products are dominated by the macro-mixing behavior, and the consumption of initiators is affected by both the macro- and micro-mixing behaviors. This conclusion is of great significance for the design, optimization and operation of LDPE reactors.

Key words: Low-density polyethylene (LDPE), Autoclave reactor, Macro-mixing, Micro-mixing, Characteristic time

摘要： It has been widely recognized that the mixing process has significant impacts on the performance of low-density polyethylene (LDPE) reactors due to the rapid radical polymerization occurred in the reactors, but how the macro- and micro-mixing affect the reactor performance was still controversial in publications. In this work, a cold-flow LDPE autoclave with multi-feedings was scaled down (1/2) from an industrial reactor and built to systematically investigate the macro- and micro-mixing characteristics of fluid by experiments. Furthermore, the effects of macro- and micro-mixing on the polymerization were comprehensively analyzed. The results showed that according to the delay time t_d and macro-mixing times t_M calculated from residence time distribution (RTD) curves, the macro-mixing states are significantly different at various axial positions (h/H), especially at lower agitation Reynolds number Re. But with the increase of Re, since the circulation flow in the reactor is strengthened, the t_d for each feed gradually decreases to 0, and the t_M at different axial positions tend to be identical. For micro-mixing, the qualities of micro-mixing at different axial positions are similar, and the average micro-mixing time t_m in the reactor decreases exponentially with the increase of Re. Moreover, a fitting model was established. Through the comparison of the characteristic times of macro-mixing (t_d, t_M), micro-mixing (t_m) and elementary reactions within the industrial range of Re, it can be concluded that the properties of LDPE products are dominated by the macro-mixing behavior, and the consumption of initiators is affected by both the macro- and micro-mixing behaviors. This conclusion is of great significance for the design, optimization and operation of LDPE reactors.

关键词: Low-density polyethylene (LDPE), Autoclave reactor, Macro-mixing, Micro-mixing, Characteristic time