Residence time and conversion in the extrusion of chemically reactive materials

Twin‐screw extruders offer improved control of the residence time distribution (RTD) and mixing in materials such as plastics, rubber and food. Based on the flow and the heat transfer characteristics obtained for a self‐wiping, co‐rotating twinscrew extruder, the residence time and chemical reaction are studied by tracking the particles. For normally starve‐fed twin‐screw extruders, the length of the completely filled section is calculated as function of the process variables using the coupling of the flow with the die. With a model of the solid conveying section, the RTD for the whole extruder is calculated for corn meal at different screw speeds and flow rates. The calculated variation of RTD with the screw speed and the flow rate yields good agreement with observations from many experiments. The variation of the fully filled section length, chemical conversion and mixing effectiveness are also obtained under different operation conditions. Most of the results are in qualitative agreement with experimental results and may be used as guidelines for extruder design and determination of optimal operating conditions.     

Ultrasonic in‐line monitoring of polymer extrusion

In‐line ultrasonic monitoring of polymer co‐extrusion and twin‐screw extrusion are presented. Co‐extrusion of high density polyethylene (HDPE) and a thermoplastic elastomer based on polypropylene‐EPDM (ethylene‐propylene‐diene monomer) has been investigated by ultrasonic sensors consisting of piezoelectric transducers and clad buffer rods. One extremity of the rod (probing end) was installed flush with the die surface so as not to disturb the material flow. The other end was air cooled in order to protect the transducer from excessive heating. This approach has been demonstrated to be quite convenient for monitoring and controlling industrial material processes: first, it can work at temperatures up to 1000°C; second, the clad buffer rod probing end can be machined to the same shape as those of commercial temperature and pressure sensors commonly used in the extrusion process. Therefore, no modifications are required for the installation in the original equipment. The information obtained includes the position of the interface between polymers and the stability of the process. The same ultrasonic probe has also been installed on a barrel of a twin‐screw extruder. This study was performed using polyethylene and polystyrene. It has been verified that the ultrasonic sensor can be successfully operated along the extruder screw and that the ultrasound can give access to the material properties while the polymer is being processed. This means that the technique can be exploited to monitor and control in situ the characteristics of the polymer being transformed in operations typically performed on twinscrew extruders, such as compounding, visbreaking or reactive extrusion.     

Modeling of a cokneader for the manufacturing of composite materials having absorbent properties at ultra‐high‐frequency waves. Part 1: Modeling of flow from residence time distribution investigations

The purpose of this study is to gain better understanding of flow patterns and mixing conditions in a particular single‐screw extruder: the Buss Cokneader. To this end, the residence time distribution (RTD) of the polymer was investigated experimentally for different combinations of the operating variables (i.e. feed rate, screw rotation speed). The measurement of RTD used a standard stimulus‐response technique. Two kinds of tracer were used: free anthracene and anthracene grafted on the polymer. It was shown that only the second could characterize the actual flow of the polymer in the extruder. It does not perturb the flow and has the same rheological behavior as the studied fluid. Thanks to the RTD data, a model of the extruder based on the combination of ideal reactors, such as continuous stirred tank reactors or plug flow reactors, was finally set up. The establishment of relationships between model parameters and extrusion conditions allowed the prediction of RTD with good agreement.     

RESIDENCE TIME DISTRIBUTION IN A SCREW EXTRUDER

It has been studied that the residence time distribution(RTD)function in a screw extruder in relation to that in thescrew and that in the die.For a complex flow field such as that between screw and die,it was supposed that the RTDfunction in the screw and that in the die were almost independent on each other.Therefore,a probabilistic method wasused to predict the RTD function in a complete extruder from that in the screw and that in the die.The experiments fordetermining the RTD were based on a stimulus-response technique.The results predicted,both in single-screw extruderand in twin-screw extruder,were in good agreement with those experimentally obtained.     

Ultrasound‐aided extrusion process for preparation of polypropylene–clay nanocomposites

Two methods for the fabrication of polypropylene/clay nanocomposites using a continuous ultrasound assisted process are compared. In the first approach, a two‐stage process was implemented. The nanocomposites were prepared by using a corotating twin‐screw extruder followed by a single‐screw extruder equipped with an ultrasonic die attachment. In the second method, a single‐stage process was used. The nanocomposites were compounded by using a single‐screw extruder with mixing elements and an ultrasonic die attachment. Two regimens of feeding were realized, namely, starved and flood feeding. The gap size in the ultrasonic treatment zone was varied. Die pressure and power consumption were measured. Similarities and differences of nanocomposites obtained by these two methods are discussed on the basis of their rheological and mechanical properties and their structural characteristics. J. VINYL ADDIT. TECHNOL., 13:40–45, 2007. © 2007 Society of Plastics Engineers.     

Numerical simulation and experimental validation of mixing performance of kneading discs in a twin screw extruder

This work aims at simulation by particle tracking the local residence time distributions (RTDs) of a co‐rotating twin‐screw extruder using computational fluid dynamics. Simulated results follow reasonably well the trend of experimental results obtained by an in‐line measuring instrument for different screw configurations and feed rates. To analyze the distributive mixing performance and overall efficiency of different types of kneading discs (KDs), mixing parameters such as area stretch ratio, instantaneous efficiency, and time‐average efficiency are calculated. Among KDs with stagger angles 45°, 60°, and 90°, the 90/10/64 with disc gaps is most efficient in terms of distributive mixing. The effects of the disc width and disc gap on the local RTD and distributive mixing are also discussed. This provides a numerical tool for assessing point‐by‐point information on the local RTD, flow, and mixing along the screw extruder. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

同向双螺杆挤出机的停留时间分布及填充度

引 言 双螺杆挤出机在高分子材料加工中已被广泛地应用于聚合物共混改性、反应挤出及高分子可控降解等各个方面[1].但先前对挤出过程研究较少,一般仅停留在"黑箱"型经验操作的层面,主要以定性的机械设计为主.     

Mixing ratios obtained using screw disc and screw extruders,and selected properties of mixture of reglanulate and original material

Abstract

Experimental effect of a screw disc clearance size on the homogenisation level and the mechanical properties of extrudate obtained using the mixture of original low density polyethylene (LDPE) and commercial LDPE recyclate were discussed in the present paper. For the purpose of experiments a screw disc extruder was applied. Some tensile tests results and photographs of the extrudate cross-sections obtained at different clearance settings were presented. The results show that the mixing in the conical screw disc zone was carried out more thoroughly than in a helical channel of a single screw extruder. Plasticising in the disc zone of a screw disc extruder makes it possible to obtain extrudate with better properties than in the case of classical extrusion.     

Theoretical analysis of residence time distribution functions and strain distribution functions in plasticating screw extruders

The residence time distribution (RTD) function in a single screw plasticating extruder was theoretically calculated. The calculation is based on the solids conveying, melting, and melt conveying models in extruders. The screw channel is divided into small axial increments and the path of each exiting fluid particle is followed from hopper to die. In addition to the residence times the total shear deformation or strain imposed on the fluid particles was also calculated. This together with the RTD function has led to the definition and calculation of the strain distribution function (SDF). This function is proposed for quantitative characterization of the mixing performance of screw extruders as well as other laminar mixers. Some simple idealized batch and continuous laminar mixers are analyzed in terms of the SDF. Finally, the effect of extruder operating conditions and screw design on the RTD and SDF were investigated by computer simulations.     

Assessing local residence time distributions in screw extruders through a new in‐line measurement instrument

This work aimed at developing a new instrument to measure in real time the residence time distribution (RTD) in screw extruders. The instrument followed the same principle as the one reported in the literature but possessed several important advantages. For example, the detection system had two probes that allowed to simultaneously measure RTDs at any two different locations of an extruder, thus providing the possibility of calculating the local RTD between them by a deconvolution method based on a statistical theory for the RTD. Its performance was evaluated on a corotating twin‐screw extruder using anthracene as tracer and polystyrene as flowing material. The effects of various process parameters such as feed rate and screw speed on the RTDs were investigated. The emphasis was placed, however, on the effect of the staggering angle of kneading discs on local RTDs both in the kneading zone itself and its neighboring upstream and downstream screw zones. This work is in support of an ongoing project on the simulation of flow in corotating twin‐screw extruders. POLYM. ENG. SCI., 46:510–519, 2006. © 2006 Society of Plastics Engineers.     

Residence time distribution in non-intermeshing counter-rotating twin-screw extruders

This paper deals with the residence time distribution (RTD) in a non-intermeshing counter rotating twin screw extruder. The RTDs were measured in three vent zones of the extruder sparately, and in the adjacent zones combined, using a soluble dye as the tracer. Assuming that the RTDs in the adjacent zones are independent of each other, the overall RTD was also calculated using a previously developed statistical theory. The theory has also confirmed the consistency of the present measurements. A predictive RTD model for the non-intermeshing twin screw extruder, based on the flow analysis of the individual screw zones and their statistical superposition, was also developed. The predictions are in good agreement with experiment.     

Relationship between local residence time and distributive mixing in sections of a twin‐screw extruder

Local residence time and distributive mixing were measured in conveying sections and kneading blocks of a twin screw‐extruder. The residence time measurements were completed using carbon black as the tracer and an infrared temperature probe to detect the temperature decrease caused by the changing surface emissivity. The validity of this experimental technique was extensively evaluated. A mixing limited interfacial reaction between polymer tracers was used to directly measure the distributive mixing in the twin‐screw extruder. Possible relationships between mixing and residence time in the sections of the twin‐screw extruder were investigated by combining these two measurements. Distributive mixing in conveying sections was related to the local average residence time and the fill. In contrast, distributive mixing in kneading blocks was related to the local average number of screw revolutions experienced by the polymer. Forward stagger kneading discs achieved the greatest amount of distributive mixing, which was attributed to a combination of local stagnant flow regions and more frequent interfacial reorientation.     

Residence time distribution of a pharmaceutical grade polymer melt in a single screw extrusion process

The residence time distribution (RTD) of a flowing polymer through a single screw extruder was studied. This extruder allows injecting supercritical carbon dioxide (scCO₂) used as physical foaming agent. The tested material is Eudragit E100, a pharmaceutical polymer. RTD was measured at various operating conditions and a model describing RTD has been developed. High screw speed or high temperature implies short residence time, but these parameters do not have the same effect on polymer flow. In the flow rate range studied, scCO₂ has no significant influence. A mathematical model consisting of a plug flow reactor in series with a continuous stirred tank reactor (CSTR) cross-flowing with a dead volume fitted well the experimental data.     

单螺杆挤出机中非牛顿流体层流混合停留时间表征

基于有限差分数值模拟技术,提出了计算非牛顿流体在单螺杆挤出机内停留时间分布的半解析方法,得到了不同操作参数下的停留时间分布,该分布可用来表征聚合物熔体在加工过程中的混合情况.结果表明：半解析方法能够反映由流体的非牛顿性所导致的耦合流场及压力反流对混合的影响,能更真实地反映聚合物熔体在单螺杆挤出机内的混合程度.     

The effects of kneading block design and operating conditions on distributive mixing in twin screw extruders

A mixing limited interfacial reaction between polymer tracers was used to directly measure the distributive mixing performance of a co‐rotating twin screw extruder during melt‐melt blending of polypropylene. The reaction between the polymer tracers, which are low molecular weight succinic anhydride and primary amine terminally functionalized polymer chains, was followed using Fourier‐Transform Infrared Spectroscopy (FT‐IR). Experiments were completed to determine the effects of flow rate, screw speed, and kneading block design on the distributive mixing performance and the residence time distribution (RTD). The only RTD variable that was significantly affected by the experimental factors was the average residence time. Distributive mixing with neutral and reverse kneading blocks was controlled by the average residence time, the fully filled volume, and the shear rate. Conversely, the mixing performance of a forward kneading block did not follow the same trends.     

Effects of screw configurations on the grafting of maleic anhydride grafted low-density polyethylene in reactive extrusion

The effects of screw configurations, that is, the staggering angles and disc widths of the kneading blocks, on grafting reactive extrusion for maleic anhydride grafted low-density polyethylene were investigated in a corotating twin-screw extruder. Samples were collected from three positions along the screw and the die exit. The grafting degree (GD) of the specimens was evaluated by titration. It was found that the kneading block configurations had a significant influence on the grafting reactive extrusion. In addition, another three groups of extrusion experiments were performed to explore the intrinsic relationship between the GD, the degree of fill in the screw channel, the residence time distribution (RTD), and the mixing intensity in various screw configurations. The experimental results indicated that the location of the melting endpoint significantly affected the position at which the reaction began; the degree of fill, RTD, and mixing performance of the screw played important roles in the grafting reaction. The reverse kneading blocks with a narrow disc width, which had a high degree of fill and good mixing capacity, enhanced the increase in GD along the screw during the reactive extrusion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Evaluation of the mixing performance for one novel twin screw kneader with particle tracking

In this article, a novel continuous twin‐screw kneader was proposed. The end‐cross section of the screw rotor consists of convex arcs and cycloidal curves and the rotors profiles were presented. The mixing performance of the novel twin screw kneader was simulated using finite element method (FEM) combined with mesh superimposition technique (MST). Statistical analysis was carried out for flow field using particle tracking technique to research the effect of geometry parameters and working parameters on the mixing performance. To study the dispersive mixing performance, specifically the maximum shear rate, maximum shear stress, maximum mixing index, residence time distribution (RTD) and RTD density function of tracer particles, and dispersive mixing is evaluated using the mixing index in combination with the shear stress. The results show that the changes of centre distance between female and male rotor have little influence on dispersive mixing performance, the lead of rotor has little effect on maximum shear stress and maximum shear rate, while it has an obvious effect on mixing index, cumulative RTD, and RTD density function. The rotor speed has obvious influence on mixing performance, and average residence time of material decreases greatly and the mixing ability is weakened, while the self‐cleaning performance of rotor improved obviously with the increasing of rotor speed. POLYM. ENG. SCI., 54:2407–2419, 2014. © 2013 Society of Plastics Engineers     

Mean residence time analysis for twin screw extruders

This paper presents and experimentally validates a physically motivated model for predicting the mean residence time in twin screw extruders. Accurate estimation of the mean residence time and the propagation delay through a plasticating extruder is critical for implementing feedback control schemes employing sensors mounted along the extruder. Experiments were carried out on a 30 mm Krupp Werner and Pfleiderer co‐rotating twin screw extruder equipped with reflectance optical probes over the melting section and mixing section and at the die. The residence time distributions for twelve operating conditions and two screw geometries are compared. The mean residence times predicted by our model are in good agreement with the experimentally measured mean residence times.     

An optical device to measure in‐line residence time distribution curves during extrusion

This paper proposes a setup for in‐line measurement of residence time distribution (RTD) curves during extrusion. The detection system is composed of a slit die with transparent borosilicate glass windows fixed at the extruder exit, an optical arrangement with an incandescent light microbulb with fixed luminescence, and a Light Dependent Resistor (LDR) photocell of CdS. As the tracer passes through the light path, the reduction in the transmitted light intensity is followed by changes in the voltage induced by the photocell to an electric circuit. The signal is collected by an external interface and sent to a computer. Software acquires the data, does all calculations, and presents all curves and calculated variables to the monitor. The validation of the system was done by checking for data reproducibility and linearity with tracer concentration. The response of the LDR sensor with respect to the concentration of the tracer was calibrated using a set of slight dark film, obtaining a logarithmic relationship. Thus the signal enhances any disturbance, if present, particularly in the tail‐region of the curves. Measurements were taken from a Werner‐Pfleiderer ZSK 30 twin‐screw extruder equipped with K‐Tron gravimetric feeders operating with various screw speeds, feeding rates and screw configurations. In this last case, the presence of kneading elements was taken into account.     

A study of residence time distribution in co‐rotating twin‐screw extruders. Part I: Theoretical modeling

A theoretical model to determine the residence time distribution (RTD) in a co‐rotating twin‐screw extruder is proposed. The method consists of coupling a continuum mechanics approach with a chemical engineering one and allows us to obtain the RTD without any adjustable parameter. The process parameters are obtained using Ludovic® twin‐screw modeling software, and ideal reactors are chosen to depict the screw profile. The influence of screw speed, feed rate and viscosity on RTD are described on a fictive screw profile. The predictions of the model are in qualitative agreement with literature data. The key point of this procedure is obviously the correct association between an ideal reactor and a screw element.