Study On The Measurement System And Installation Of Residence Time Distribution

The concept of Residence Time Distribution (RTD) is very important in polymer processing. It represents the experience time of the processing polymer in the extruder. Namely it is the time of every particulate goes through under the fixed heating and shearing conditions. For the continuous mixing machine, the residence time for each particulate is different. That's to say, the material added in the same time would leave the extruder in different time. RTD could characterize a mixing machine property.RTD can be measured by offline or online means. Offline means are time-consuming and yield few data points which are not enough for a detailed analysis at the tail-region of the RTD curves. However, in-line means are faster and can acquire lots of data. According to the searched literatures, there are mainly four type in-line RTD measurements: Ultrasonic type, electrical conductivity type, ultraviolet fluorescence type and light intensity type.Ultrasonic type measures the inorganic tracer's concentration by the variation of the ultrasonic wave's attenuation. Its installation is complex and the measurement cost is correspondingly high. Electrical conductivity type measures the conductive tracer's concentration by the fluidic conductivity. It can only be applied in the water solubility system. Ultraviolet fluorescence type measures the anthracene grafted tracer's fulorescence intensity by micro-photoelectric detector. The anthracene grafted tracer is difficult to prepare. Light intensity type measures the tracer's concentration by the variation of the light intensity. It includes reflection and transmission type. For the reflection type, the correlation between the tracer's concentration and photosignal is difficult to determine for there are much disturbance. In contrast to the reflection type, the correlation can be deduced by the lambert-beer law for transmission type. The available transmission type adopts normal light and photocell to build the measurement system. Light source and photocell are both fixed at the die which has transparent borosilicate glass window. Both of them do not contact the polymer directly and this may lead to the deviation from the lambert-beer law.In this study, we studied the basic principle of the present RTD measurement means and compared their merits and faults. Further more, we developed a new RTD measurement system by light absorption principle. The system was based on the optical fiber sensor which detected the color concentration in the melting polymer. The output of the optical fiber sensor was collected by the computer in the extruding process. RTD curve can then be obtained.In this study, we analysed the feasibility of the new RTD measurement system in theory. On the basis of this, we designed the measurement installation, chose the hardware for the measurement system and developed the software for data acquisition.By performing different experiments, we determined the reproduction quality of the new developed RTD measurement system, studied the contribution of the tracer's quantity and color to the RTD measurement. We also compared the in-line and off-line measurement results. The experiments indicated that the new RTD measurement system can get the polymer RTD curves accurately. The new measurement system had many merits such as simple construction, good reproduction quality, easy to get the tracer, low measurement cost and pollution-free. It was proved to have a high practical value.