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.     

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.     

Measuring the residence time distribution in a twin screw extruder with the use of NIR‐spectroscopy

The mean residence time (MRT) and the residence time distribution (RTD) of polypropylene in a twin‐screw extruder was determined directly in‐line with the help of near infrared (NIR) spectroscopy and the use of an UV‐absorber as tracer. Different experiment alignments such as screw speed, mass throughput, back pressure as well as tracer content and their influence on MRT and RTD have been investigated. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39919.     

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 distributions in a co‐kneader: A chemical engineering approach

In this article, we have studied the residence time distributions (RTD) in a modular co‐kneader. Several papers have already addressed the co‐kneader modeling and operating mode but there is still a lack of experimental data on RTD. To investigate the RTD, we have used a colored tracer dispersed in polypropylene (PP) that was injected in the flow during the compounding of neat PP. The effect of operating parameters such as temperature, feed rate, and screw configuration was investigated, focusing on the influence of mixing and conveying elements in a zone where the polymer is molten. As can be expected, results on various screw configurations show that increasing the number of kneading elements makes the RTD longer. More interestingly, for a defined set of elements, their position does not change the experimental RTD. A chemical engineering approach was used to model the RTD, with an equation derived from a cascade of continuous stirred tank reactors. The model allows to retrieve an elementary RTD for each section of a defined type of elements and to propose a law for their combination in good agreement with experiments. POLYM. ENG. SCI., 55:1237–1245, 2015. © 2015 Society of Plastics Engineers     

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.     

Application of ultrasound in the determination of fundamental extrusion performance: Residence time distribution measurement

By means of new probe design and rapid data acquisition, we have succeeded in in‐line ultrasonic monitoring of residence time distribution (RTD) at the melting, mixing, and pumping zones as well as at the die exit of a Werner & Pfleiderer 30‐mm twin‐screw extruder by mounting the ultrasonic probes on the extruder barrel over the screw elements and at the die. The experimental systems were LDPE, CaCO₃‐filled LDPE, and a Kraton/LDPE blend. The ultrasonic data at each of the extruder functional zones are presented. The ultrasonic results have been used to evaluate an opical RTD measurement method by using an optical sensor side by side with one ultrasonic probe at the mixing zone of the extruder. The comparison of the ultrasonic and optical results has shown that the presented ultrasonic technique could be a good complement to the optical technique in the monitoring and understanding of RTD during polymer extrusion processes.     

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     

Numerical simulation and experimental study of pressure and residence time distribution of triple‐screw extruder

Three‐dimensional flow field of triangle arrayed triple‐screw extruder (TTSE) in different feed rates, screw speeds and screw configurations were simulated by POLYFLOW. Polypropylene processing by TTSE was tested to validate the numerical simulation. The reliability of simulation was tested directly and indirectly by comparing the axial pressure difference and residence time distribution (RTD) separately. The results show that the axial pressure differences from the simulations and experiments were in high agreement. The simulated and experimental RTD curves show the same trend, and so are the simulated and experimental average residence times. These results prove the reliability of simulation and its guidance to the experiment. POLYM. ENG. SCI., 55:156–162, 2015. © 2014 Society of Plastics Engineers     

In‐line near‐infrared spectroscopy: A tool to monitor the preparation of polymer‐clay nanocomposites in extruders

In‐line diffuse reflectance and on‐line transmission near‐infrared spectroscopy (NIR) measurements are performed at the same location of the barrel of a twin screw extruder during the preparation of a polypropylene/clay nanocomposite. Their performance is evaluated by means of a 7‐parameter chemometric model using off‐line rheological and structural (FTIR) data obtained from samples prepared under different screw speed, compatibilizer content and clay loading, as well as a process‐related thermomechanical index. Despite the higher variability of the diffuse reflectance signal, the two models present analogous high quality indices. The aptness of the reflectance measurements is thus validated, which has direct practical advantages, as this probe can be fixed in any typical melt pressure transducer port. The probe is then used for the real‐time in‐line monitoring of the production of the same nanocomposite but now using different throughputs, and the chemometric‐based predictions are compared with experimental off‐line characterization data. The nonlinear effect of throughput is correctly anticipated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Steady and transient flow of a non‐Newtonian chemically reactive fluid in a twin‐screw extruder

The flow of chemically reactive non‐Newtonian materials, such as bio‐polymers and acrylates, in a fully intermeshing, co‐rotating twin‐screw extruder is numerically investigated. A detailed study of the system transient behavior is carried out. The main transient aspects, including response time, variation of system variables, and instability of operation, are studied for both single‐ and twin‐screw extruders, since single‐screw extruder modeling closely approximates the region away from the intermeshing zone in a twin‐screw extruder. The effect of a time‐dependent variation in the boundary conditions is studied. The coupling due to conduction heat transfer in the screw barrel is found to be very important and is taken into account for single‐screw extruders. In the absence of this conjugate coupling, the response time is much shorter. Several other interesting trends are obtained with respect to the dependence of the transient response on the materials and operating conditions. Steady state results are obtained at large time. The calculated velocity distributions in the screw channel are compared with experimental results in the literature for steady state flow and good agreement has been obtained. The calculated results for transient transport agree with the few experimental observations available on this system. Chemical reaction, leading to chemical conversion of the material, is also considered and the resulting effects on the flow and transport determined. These results will be useful in the design, control and optimization of polymer extrusion processes.     

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.     

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.     

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.     

Local residence time,residence revolution,and residence volume distributions in twin‐screw extruders

This work was aimed at studying the overall, partial, and local residence time distributions (RTD); overall, partial and local residence revolution distributions (RRD) and overall, partial and local residence volume distributions (RVD) in a co‐rotating twin screw extruder, on the one hand; and establishing the relationships among them, on the other hand. Emphasis was placed on the effects of the type and geometry of mixing elements (a gear block and various types of kneading elements differing in staggering angle) and process parameters on the RTD, RRD and RVD. The overall and partial RTD were directly measured in‐line during the extrusion process and the local ones were calculated by deconvolution based on a statistical theory. The local RTD allowed comparing the mixing performance of mixing elements. Also it was confirmed both experimentally and theoretically that specific throughput, defined as a ratio of throughput (Q) over screw speed (N), controlled all the above three types of residence distributions, be they local, partial or overall. The RRD and RVD do not provide more information on an extrusion process than the corresponding RTD. Rather they are different ways of representing the same phenomena. POLYM. ENG. SCI., 48:19–28, 2008. © 2007 Society of Plastics Engineers     

In‐line optical detection in the transient state of extrusion polymer blending and reactive processing

Using an opticaldetector we followed the transient state of blends and composites, including a reactive blending during extrusion. The detection system is composed of a slit‐die with transparent windows fixed at the extruder exit, an optical arrangement with a W incandescent light microbulb with fixed luminescence, and a CdS photocell. As the tracer passes though the light path, it absorbs and backscatters part of the light, reducing the total transmitted light intensity. This is followed by changes in the voltage induced by the photocell to an electric circuit. We calibrated the response of the photocell at room temperature using a set of various films with a second phase dispersed, and obtained a logarithmic relationship. The tracers were particulate (phthalocyanine, TiO₂) and polymeric (PS, PA6) phases that absorb and scatter light, producing a residence time distribution (RTD) curvelike trace. Measurements were taken from a twin‐screw extruder Werner‐Pfleiderer ZSK 30 equipped with K‐Tron gravimetric feeders operating at various screw configurations and speeds, and feeding rates. The transient state of PP/PA6 blends can be easily detected optically and recorded using one of the components (either PP or PA6) added as a pulse in a steady‐state flow of the other component. With the simultaneous addition of a compatibilizer (polypropylene grafted with acrylic acid (PP‐g‐AA)) with the PA6, the intensity of the detector signal is substantially increased as a result of the PA6/PP‐g‐AA reaction. Quantitative off‐line infrared spectroscopy of the total amide group corroborated the in‐line measurements. These observations suggest that an in‐line optical detector may be a fast and simple way to study the flow behavior of blends and composites, including reactive processing. POLYM. ENG. SCI. 45:11–19, 2005. © 2004 Society of Plastics Engineers.     

Measurements of residence time distribution for the peroxide degradation of polypropylene in a single-screw plasticating extruder

Measurements of the residence time distribution (RTD) in a single-screw plasticating extruder were carried out during experimental studies of the peroxide-initiated controlled chemical degradation of polypropylene (PP). A radioactive tracer method was employed, and the effect of screw speed, temperature, and reaction on the RTD was examined. An increase of the peroxide concentration resulted in a broader distribution whereas an increase of the extrusion temperature was found to result in a narrower distribution. Use of low screw speeds simply increased the time delay through the extruder without affecting considerably the breadth of distribution. Results obtained from the present experiments were compared with several theoretical models.     

Determination of twin‐screw extruder operational conditions for the preparation of thermoplastic vulcanizates on the basis of batch‐mixer results

In this study, we attempted to prepare a thermoplastic vulcanizate in a twin‐screw extruder by determining the screw configuration on the basis of batch‐mixer results. In this regard, two sets of information were used: (1) the time length, power consumption, and filling factor of different stages of the reactive blending process in the internal mixer and (2) the mean residence time and power consumption of the twin‐screw extruder. Morphological features of the samples taken from the melt‐mixing and dynamic vulcanization zones of the extruder with the selected screw configuration were found to be comparable with corresponding samples taken from an internal mixer reported in our previous study. The rheological and mechanical properties could provide valuable information to support the reliability of this study. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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.     

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.