Experimental study of energy performance of grooved feed and grooved plasticating single screw extrusion processes in terms of SEC, theoretical maximum energy efficiency and relative energy efficiency

Journal Pre-proof Experimental study of energy performance of grooved feed and grooved plasticating single screw extrusion processes in terms of SEC, theoretical maximum energy efficiency and relative energy efficiency Omar Estrada, Juan Carlos Ortiz, Alexander Hernández, Iván López, Farid Chejne, María del Pilar Noriega PII:

S0360-5442(19)32574-5

DOI:

https://doi.org/10.1016/j.energy.2019.116879

Reference:

EGY 116879

To appear in:

Energy

Received Date: 13 February 2019 Revised Date:

25 October 2019

Accepted Date: 28 December 2019

Please cite this article as: Estrada O, Ortiz JC, Hernández A, López Ivá, Chejne F, del Pilar Noriega Marí, Experimental study of energy performance of grooved feed and grooved plasticating single screw extrusion processes in terms of SEC, theoretical maximum energy efficiency and relative energy efficiency, Energy (2020), doi: https://doi.org/10.1016/j.energy.2019.116879. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Experimental Study of Energy Performance of Grooved Feed and Grooved Plasticating Single Screw Extrusion processes in terms of SEC, Theoretical Maximum Energy Efficiency and Relative Energy Efficiency. a

([email protected]), Juan Carlos Ortiz a, b ([email protected]), Alexander Hernández ([email protected]), Farid Chejne c ([email protected]), María del Pilar Noriega a ([email protected]) a Instituto de Capacitación e Investigación del Plástico y del Caucho – ICIPC b Universidad EAFIT c Universidad Nacional de Colombia, facultad de Minas, departamento de Procesos y energía Omar Estrada

a, b

([email protected]), Iván López

a

KEY WORDS Polymer extrusion Specific Energy Consumption (SEC) Energy efficiency Grooved Feed Extrusion (GFE) Grooved Plasticating Extrusion (GPE) Energy in Polymer Processing

GFE: GPE: PUT: SEC: SSE:
:
 :
 : LCA:

ABREVIATIONS Grooved Feed Extrusion/Extruder Grooved Plasticating Extrusion/Extruder Plasticating Unit Technology Specific Energy Consumption Single Screw Extrusion/Extruder Energy Efficiency Theoretical Maximum Energy Efficiency Relative Energy Efficiency Life Cycle Analysis

ABSTRACT Experimental results of SEC, energy efficiency, backpressure and melt temperature in GFE and GPE as functions of PUT, rotational screw speed and die restriction are analyzed. For GFE, five screws are analyzed: two conventional, two barrier and one with a double-flighted feed zone. The polymer, motor drive, control system, die, screw diameter and length and temperature profile are the same. The differences in the results can be attributed to the PUT. Results show that the differences in the geometry of the GFE screws affect the extruder operational windows but do not significantly affect the SEC as a function of throughput. GPE shows higher productivity with a lower pressure requirement, SEC and melt temperature compared to GFE configurations. When the SEC of heaters and motor drive is compared between GPE and GFE technology, the behavior of the heater bands is similar but the motor drive energy is more efficiently used in GPE. A novel approach to the
 and
 in polymer extrusion is proposed, obtaining between 54.2% to 84.2% for maximum efficiency and 61.3% to 86.2% for relative efficiency, depending on the PUT and the die restriction.

1 INTRODUCTION In a polymer processing plant, the process itself can represent over 60% of the total plant energy consumption[1]. Additionally, in polymer extrusion, energy efficiency is one of the biggest challenges [2]. In the ceramics industry, the application of heat-pipe based heat exchanger for improving energy efficiency has been researched. The combined theoretical and numerical approach demonstrates that the application of heat pipes to the cooling stack of a ceramic kiln enables to recover more than 863 MWh of thermal energy [3]. In discrete part manufacturing, a life cycle energy analysis integrated process planning (LCEA-PP) method is developed to achieve energy-efficient process for the product design reaching an energy mitigation of over 40% [4]. In aluminum recycling, new processes that avoid the molten state such as Friction Stir Extrusion (FSE) allow a reduction in energy demand up to 74% with respect to Page 1 of 22

conventional routes [5]. These works show the importance of finding new approaches to make different industrial processes more energy efficient. Figure 1 presents an evolution timeline of SSE technology. Technological improvements in SSE are usually related to new screw and barrier designs, and larger and faster plasticating units that yield better homogeneity, productivity and energy efficiency. Menges and his collaborators developed the GFE technology in the 1970s [6], [7]. Eberhard Gruenschloss patented several GFE technologies between 1977 and 2002 claiming that highly productive GFE offers an efficient use of energy, minimum cooling in the grooved zone and a low level of metallic wear [8]. The GPE was patented in 2000. It has a set of helical grooves on the inner barrel surface of the feeding and the plasticizing zones of the screw. The design of the grooves helps achieve a much faster plasticization [9][10]. GFE is the most widely used technology for polyolefin extrusion and GPE is one of the most interesting recent technologies in SSE.

Figure 1 Timeline of SSE evolution.

Energy efficiency is a general concern of the polymer processing industry. In a study that covered 280 medium and small companies in Europe, Fresner et al. showed that energy savings of around 20% can be achieved. The main barriers are the cost of the implementations, the lack of information and the limited internal skills to address the needed improvements [11]. In many cases, the inefficiencies are technological. Equipment obsolescence is an important barrier that prevents the implementation of energy efficiency actions. A characterization of the energy profile of certain ceramic industry enterprises in Colombia found a positive correlation between the potential energy savings and the degree of equipment obsolescence [12], that probably, can be extrapolated to other industrial sectors. Therefore, work that characterizes the energy performance of several technologies is relevant in order to make the best possible decisions [13]. Making a process more energy efficient requires a proper methodology in order to reduce energy consumption, increase productivity, reduce non-compliant products, reduce down times and identify when the technology should be changed or upgraded. Kent showed strategies, targets and tools to reduce the energy consumption in different polymer processes [1]. Spiering et al. proposed a systematic approach for energy efficiency benchmarking in the injection molding process [14]. These approaches allow predicting the energy consumption of polymer production plants. The European commission developed the best practice guide, called RECIPE. This document shows some tools and guides to Page 2 of 22

determine actions that can reduce energy consumption in polymer processing [15]. This author previously developed a strategic decision methodology to increase energy efficiency in industrial processes, called the Energy Gap Method or EGM. The method allows a plant to prioritize interventions based on the energy gaps obtained from different SEC values. Life cycle analysis (LCA) is a tool to evaluate energy improvements in processes. Matarrese et al. developed a method for the design of injection molds striving for an environmentally sustainable process. [16]. Elduque et al. developed a life cycle analysis (LCA) for an HDPE injection molding process where energy consumption is a key factor and an important criterion for the selection of machinery [17]. Dassisti et al. used thermographs of the extrusion process of nets, combined with LCA, to reduce energy consumption and environmental impact [18]. Seow et al. presented such a methodology to design plastic parts to minimize the manufacture energy consumption [19]. All previous examples agree that energy consumption is one of the main factors that impact the analysis of the life cycle of a product or process in the plastics sector. Heat recovery and cogeneration alternatives have also been used to improve the energy efficiency of polymer processing plants. Schlüter and Rosano tested these alternatives in plants in Germany and Australia, with energy savings that can exceed 40% [20]. Dunkelberg et al. carried out a similar study in US, Canadian and German plants, reducing the power demand by up to 34% [21]. In the polymer extrusion process, the productive and energy performances depend on a large number of variables. Some of them are related with hardware (motor drive, heating and cooling systems, insulation, power transmission and control system), technology used (screw design or die geometry), operational conditions (barrel and die temperature profile, screw rotation speed and die restriction) and material properties. Several works are focused on understanding the influence of process variables and technological components in the energy efficiency of single extrusion polymer processing. In 2010, Cantor et al. measured the energy performance in the extrusion process for two polymers and three different screw designs. All were tested at the same operational conditions. Energy consumption of motor drive and heater zones were measured separately. They found that the screw mixing elements reduce the effective melt viscosity and the heaters that are closer to the feed zone consume significantly more energy than those of any other barrel zones. They concluded that the higher the screw rotation speed, the more energy efficient the system is [22]. Rauwendaal showed some software tools to increase energy performance. He concludes the “three M’s”: melt pressure, melt temperature and motor load, are the vital signs of the extrusion process. Data acquisition of these variables is critical to make decisions about productivity and energy behavior in extrusion processes [23]. Later, the same author studied the relationship between the energy consumed by the motor and some geometric parameters of the screw design [24]. He also defines the energy cost as the most relevant factor in extruded products and showed what to consider to improve energy efficiency [25]. In other works he focuses on the process to determine the optimum barreltemperature profile by measuring variables such as die pressure, screw and barrel wear, environmental conditions, resin inlet temperature and moisture level [26]. More recently, EUROMAP, developed technical recommendations in order to determine the energy efficiency of injection molding [27] and blown film extrusion equipment [28] and classified them according to their performance [29]. Potente et al. carried out experiments on a particular short SSE (L/D<5) and they could conclude that the extruder works adiabatically. They supported this experimental analysis with the extruder energy balance calculation [30]. Abeykoon et al. have published several studies about this topic. In [31], they established an energy analysis model to predict the mass temperature in a SSE. This is an empirical model based on the correlation with their experiments; therefore, it was only validated for the operating conditions and the configurations of the PUT used in that work. In [32], they measured the load torque using the motor current, predicting conveying issues with an inferential monitoring and a novel control simulation method. In [33], they carried out a study for polystyrene extrusion, showing power behavior as a function of screw rotation speed and its effect on melt temperature homogeneity. They calculated the SEC and proposed an empirical model to calculate the total power demand. In [34], they included three materials: polystyrene, LDPE and LLDPE, demonstrating that the temperature fluctuations and energy performance are functions of the extruded material, screw design geometry and processing conditions. In [35], they show the control challenges to Page 3 of 22

obtain a better energy performance in a SSE. They talk about the limitations of existing extrusion control because thermal monitoring was based on wall-mounted thermocouples. They conclude that the information provided by these thermocouples is a poor performance indicator. Some of the works mentioned use the same experimental setup testing different screw technologies: a conventional screw with gradual compression without a mixer, a tapered rapid compression screw without a mixer and a barrier-flighted screw with a spiral Maddock mixer. They found that energy performance improves as the screw rotation speed increases, but thermal homogeneity is affected. In [36] a complete review of extrusion control technologies based on energy considerations is presented. Other works related to this experimental setup are given by Vera-Sorroche et al., Abeykoon et al. and Deng et al. The first work used three temperature profiles in polymer extrusion with different screws, they show that the barrier screw has the lowest variations and the best temperature homogeneity, but the largest pressure variations. It also presents minor variations in the SEC measurements according to the temperature profile [37]. The second work is focused on the development of empirical models of energy consumption. Deng et al. showed the complexity of the control variables in an SSE and they used fuzzy logic to enhance response variables like backpressure and melt temperature, when the extruder parameters change. They developed a feedback control system based on fuzzy logic looking for the optimization of energy efficiency and melt temperature homogeneity [38]. A complete review of extrusion control technologies based on energy considerations is presented in [39]. The same author explored the limitations of pressure and temperature sensors to provide the dynamic behavior inside the extrusion process and they tried to predict process thermal stability inferentially. The correlation between backpressure, melt temperature, and screw torque was analyzed, without finding a solid signals correlation. Pressure fluctuations had correlations with melt temperature changes at low screw speeds. However, none of these signals showed sufficiently good performance for them to be used as a tool to monitor the process thermal stability. Estrada et al. showed the SEC differences between single and twin screw extruders operating with the same polymer [40]. In the works presented before, the effect of different die restrictions has not been considered and a different mixing element is used for each screw technology. Therefore, it is not known if the energy performance is related to the screw technology or the mixing element. In addition, one SSE geometry is used for each screw technology or design concept. For that reason, the conclusions are limited to a specific screw. The authors did not report the feed zone type used, but according to the large screw compression ratios, the use of smooth feed zone extruders can be inferred. The contribution of the present study is to provide comparable results in terms of energy efficiency with different PUT´s and screw geometries for GFE and GPE, allowing a better understanding of the plasticating technology’s influence on SEC, and laying the groundwork for future studies. In particular, the present study offers the following contributions with respect to the state of the art: • • • • • •

• •

A novel approach to determine theoretical maximum energy efficiency (
 ) in polymer extrusion processes is proposed. The estimation of relative energy efficiency (
 ) based on measurements and theoretical maximum energy efficiency is discussed. The die restriction variation is included in the energy performance analysis of the extrusion process. A GPE technology is compared in terms of energy efficiency and SEC with GFE technologies, under the same conditions. Several screw types with geometric differences between them were included in the analysis. In the GFE analysis, as the same machine, material and screw mixer components were used, it can be concluded that the differences in energy performance arise exclusively from the geometry of the screw in the feed, compression and metering zones. The introduction of a double flighted screw in the feed zone was included in this analysis. The method of using the extruder operating curve to integrate process control with energy efficiency is introduced. Page 4 of 22

2 METHODOLOGY The experimental approach is designed to offer new insights on plasticating unit energy behavior in SSE. The results can be used by polymer processing plants and machinery manufacturers to increase energy efficiency and reduce the SEC of these technologies. The measurement devices used for the process characterization, the energy balance used for the efficiency analysis and the experimental setups used in the study are presented.

2.1 Measurement devices The extruder is connected to three power measurement devices. The general specifications and the location of each measurement device are listed in Table 1. The power demand of the heating and cooling systems is calculated by subtracting the motor drive from the totalizer power measurements. Table 1: Characteristics and location of energy measurement devices

Element 1.

Totalizer

2.

Motor drive

Measurement Device DENT, Model: Elite-Pro XC Scanning speed: 200 scans/s Data integration: 1 /s Precision: 0.1 W

Variables Measured • • • •

Voltage Current Power Factor Power

A sample of power demand measurements considering different rotation speeds but equal die restriction is presented in Figure 2. During regular operation, the power demand of the motor drive is more stable than the totalizer because heater bands are turned on and off by the temperature profile controller. Measurements start after the power demand of motor drive, the backpressure and the melt temperature present minimum variations. The totalizer and motor drive average power demands are calculated from the recorded data.

Figure 2: Totalizer and motor drive power demand as a function of time and rotation speed, for GFE with PP, using Screw 1 and a “Low” die restriction.

A device called a “Thermocomb” is used to measure the temperature profile of the molten polymer at five different points across the barrel diameter, with an accuracy of 0.1°C. For SEC calculation and melt temperature comparisons, the average temperature value is used. Backpressure is measured with a Dynisco® class 1 pressure transducer with a 0.1-700 bar measurement range and a precision of 0.1bar. For throughput characterization, three samples of each operational point, shown in Table 3, were measured using a chronometer (CASIO HS-10W) and a scale (Mettler BB2400). The maximum standard deviation obtained during the experimentation was 1.5kg/h (5%).

Page 5 of 22

2.2 Energy balance The SEC is a well-known concept in many fields for the evaluation of energy efficiency. In general, it is defined as the ratio between energy consumption and throughput in a given system or process, called an Energy Accounting Center (EAC) [13], [41]. For the purpose of this paper, the EAC is the extruder and SEC is calculated in steady state operation, assuming a production absent of non-compliant products. This SEC is called Stable Production Specific Energy Consumption or  . However, other useful definitions can be considered as discussed by Estrada et al. [13]. In extrusion, the motor drive and heating bands supply all energy required by the process. Most of the energy is used to melt the polymer [24]. The major part of the mechanical energy from the motor is transformed into thermal energy by viscous dissipation [42]. The “black box” of the EAC is shown in Figure 3. Polymer enters the EAC through the hopper in solid form with  temperature and  pressure. The polymer exits the EAC at  temperature and  pressure. These conditions define the polymer energy flux that enters    and leaves    the system. Usually, Pin and Pout are the atmospheric pressure. The motor drive and heaters provide an energy flux to the system  ,  ) as their power demand, and it can be measured for each operational condition, PUT and screw configuration, as shown in Figure 2. Inefficiencies are generated by operational energy losses from the motor drive and the heaters   ,   .

Figure 3: Black box scheme for extruder energy balance calculation.

Based on Figure 3, the extrusion  as a function of measured power demand can be defined as:  =

 +  #

Eq. 1

The macroscopic energy balance makes it possible to obtain an expression for energy efficiency 
), as shown by Estrada et al. [13], given by: Eq. 2   +   #ℎ − ℎ )
= 1 − =  +   + 

where # is the throughput or throughput, ℎ is the polymer specific enthalpy at  and ℎ is the polymer specific enthalpy at  . The thermodynamic SEC or   can be understood as the minimum energy required to achieve the internal energy change per mass unit in an ideal (100% energy efficient) process, when the kinetic and the potential energy changes in the polymer are neglected. If the   = ℎ − ℎ ), then the energy efficiency 
) can be written as: Page 6 of 22


=

ℎ − ℎ )   = 



Eq. 3

The energy efficiency of the extruder is affected by the efficiency of each component. Since the same machine components, polymeric material, L/D ratio and mixing sections are used in the experiments, the variations are only due to the PUTs allowing to compare and conclude about each technology. Quality parameters such as melt homogeneity are not included.

2.3 Experimental setup Three experiments were carried out. The influence of the PUT, the screw design and operational conditions (screw speed and die restriction) on the energy performance were evaluated in terms of the  , the maximum and relative energy efficiencies (defined later).

2.3.1

Influence of Plasticating Unit Technology (PUT)

Two PUT’s were used: GFE and GPE. For both, the screws have a diameter ') of 45 mm and a length – diameter ratio (⁄') of 30. The feed zone (first 5D of length) barrel’s inner surface on the GFE has eight axial grooves. In GPE, the feed and plasticating zone (first 24D) barrels’ inner surfaces are both helicoidally grooved. These PUT’s are installed in the same hardware (the same motor drive, speed reduction system, control system and die) as shown in Figure 4.

(a)

(b)

Figure 4: Extruder configurations used in the study for (a) GFE and (b) GPE.

The technical specifications are presented in Table 2. Table 2: Technical specs of common machine components used in the experimental setup with GFE and GPE plasticating units

Parameter Motor Drive Speed reduction system and ratio Maximum screw rotation speed Heating Power for GFE Cooling Power for GFE Heating Power for GPE Cooling Power for GPE Control System

Value 41kW–93A–2500rpm Gear box and pulleys-12.5/1 200rpm 20.6kW 0.3kW 22.3kW No cooling. Digital PLC with data acquisition of all variables, except throughput.

Page 7 of 22

2.3.2

Influence of screw type

For the GFE analysis, the performances of two conventional screws, two barrier screws and one double flighted screw are evaluated (see Figure 5). A barrier screw is a technology developed to offer a controlled polymer plasticizing process, reducing the probability of solid bed breakup and improving dispersive mixing. A double flighted feed zone screw is used when the transport rate of solid pellets should be decreased to mitigate overfeeding. The same mixing zone geometry (2D rhomboidal mixer and 4D Maillefer mixer), is used throughout every test. An SSE energy behavior analysis using the same machine and mixing components has not been reported before. SCREW 1 (GFE)

SCREW 2 (GFE)

b) Barrier screw

a) Barrier screw

SCREW 4 (GFE)

SCREW 3 (GFE)

c) Conventional screw

d) Conventional screw SCREW 5 (GFE)

e) Double flighted feed zone screw Figure 5: Schematic diagrams of the screws used in the GFE to study the influence of screw type on the energy performance.

For GPE analysis, the barrel, barrier screw and heater bands with isolation jackets are installed in the same extruder (see Figure 4). This configuration uses a 3D Saxton mixer and a 3D Maillefer mixer. The screw geometry is presented in Figure 6. HELIBAR® BARRIER SCREW (GPE)

Figure 6: Schematic GPE screw design, used to study the influence of screw type on the energy performance

2.3.3

Influence of operational conditions

Three independent operational variables are controlled in the extrusion process: rotational speed of the screw, die restriction and temperature profile. Each point of the operational curve is defined by a die restriction and a rotation speed pair of values as shown in Table 3. For GFE analysis, 15 operational points are tested, with 12 for GPE. For each operational point, melt temperature, backpressure, throughput and energy consumption are measured. The specific throughput (kg/h-rpm) or specific mass flow rate is calculated as the relationship between throughput and rotation speed. The restriction level is controlled using a conical mandrel that acts a valve and uses a (turns/degrees) nomenclature to reproduce a desired die restriction level. The fewer the number of turns and degrees, the higher the die restriction. In the experimental setup, four mandrel die positions are used, defined as: “Low” (4/210), “Medium” (4/120), “High” (4/90) or “Very High” (4/30). The experimental setup parameters are presented in Table 3 and the extruder barrel as well as die temperature profile settings used during the experiments are listed in Table 4. Table 3: Operational points selected to study the influence of operational conditions in GFE and GPE.

Operational point No. 1 2 3 4 5 6

Rotational Screw Speed [rpm] 70 100 130 160 180 70

Die restrictions used with GFE Low Low Low Low Low Medium

Die restrictions used with GPE Low ---Low Low Low Medium

Page 8 of 22

Operational point No. 7 8 9 10 11 12 13 14 15

Rotational Screw Speed [rpm] 100 130 160 180 70 100 130 160 180

Die restrictions used with GFE Medium Medium Medium Medium Very high Very high Very high Very high Very high

Die restrictions used with GPE ---Medium Medium Medium High ---High High High

Table 4: Extruder barrel and die temperature profile settings used for GFE and GPE. Barrel Zones PUT GFE GPE

Adapter

Die

Zone1

Zone2

Zone3

Zone4

Zone5

Zone6

Zone7

Zone8

Zone9

Zone10

Zone11

250°C 240°C

240°C 240°C

230°C 240°C

220°C 240°C

220°C 240°C

220°C 240°C

220°C 240°C

220°C 240°C

220°C 240°C

220°C 240°C

220°C 240°C

Experimental trials were carried out using a polypropylene homopolymer, ESENTTIA 05H82-AV. The material’s properties are listed in Table 5. Table 5: Properties of Polypropylene Homopolymer, ESENTTIA 05H82-AV.

Properties Melt Index Density (solid) Melting Temperature VICAT Softening Temperature Heat Capacity

Units

PP

g/10min g/cm3 °C °C J/g/°C

4.6@(230°C 2.16 kg) 0.906 167.42 145 2.785

3 RESULTS AND DISCUSSION 3.1 Influence of operational conditions Melt temperature, productivity (throughput) and energy efficiency are evaluated to compare the performance of PUT’s. A useful tool to visualize these comparisons is the operational curve, used also to characterize and control the extrusion process [43]. The curve correlates die restriction and rotation speed (independent variables) with throughput and backpressure (dependent variables). This information can be plotted as shown in Figure 7 for GFE (a–e) and GPE (f). Die Restrictions Convention

Note: SECs is given in kWh/kg

Page 9 of 22

Die Restrictions Convention

Note: SECs is given in kWh/kg

a)

GFE - Screw 1

b) GFE - Screw 2

c)

GFE - Screw 3

d) GFE - Screw 4

e)

GFE - Screw 5

f)

GPE – Helibar®

Figure 7: Operational curves obtained for GFE (a – e) and GPE (f).

The present work includes calculated  and energy efficiency values to offer a complete representation of the extruder performance. For example, for GFE with Screw 1 (Figure 7a), when the die restriction is “very high” and the Page 10 of 22

rotation speed is 180rpm, the throughput is 66.1kg/h and the backpressure is 357bar, the energy efficiency reaches 48.4% and the  is 0.373kWh/kg. As expected, the operational curves of GFE screws (Figure 7a to Figure 7e) are significantly different. Therefore, their performance at the same screw rotation speed and die restriction, in terms of throughput, backpressure, energy efficiency and SEC are dependent on the screw geometry. This observation is valid even when the screws have the same technological principle. For example, Screw 1 and Screw 2 are GFE barrier screws albeit with different geometries. The specific throughput (0.3652kg/h-rpm vs 0.2845kg/h-rpm) and the ratio between backpressure and throughput (3.36bar-h/kg vs 4.16 bar-h/kg) are significantly different at 180rpm and medium die restriction level, as shown in Table 6. These differences can be observed for other operational points (Figure 7a and Figure 7b). A similar observation can be made for the GFE conventional screws (screw 3 in Figure 7c and screw 4 in Figure 7d). Another notable observation is that GPE reaches a specific throughput almost twice that of GFE. The faster the rotation speed, the better the energy performance. This may be explained because viscous dissipation is increased, reducing the heater energy consumption. The energy efficiency did not exceed 52% in the GFE, and for GPE the energy efficiency is close to 60%. In general, the lower the die restriction and the faster the rotation speed, the lower the SECs value. To determine if the effects of rotation speed and die restriction on the SECs value are statistically relevant, a 22 factorial analysis with a central point is carried out. The two factors are the rotation speed and the die restriction. The levels are 100rpm and 160rpm for rotation speed and “Low” and “Very high” die restrictions. The central point is set at 130rpm with “Medium” die restriction.  values are scaled from 0 to 10.

Figure 8: Standardized effect of screw rotation speed and die restriction level on SECs for GFE with Screw 1.

The standardized effect of the parameters for screw 1 is presented in a Pareto chart (Figure 8). These values are tstatistics that test the null hypothesis of there being no effect at all. A vertical reference line is plotted to indicate which effects are significant (values above 2.5). The main effect on  comes from the rotation speed, as it has been reported in other works. Additionally, the influence of die restriction level on  is also relevant. Previous works did not consider this variable in their analysis. At the same screw rotation speed, a more restrictive die requires more backpressure, as can be seen in Figure 7. A higher backpressure promotes a higher internal energy change in the polymer, explaining the effect of die restriction on  . The interaction between screw speed and die restriction level on  is negligible (AB value). This means that the effect of rotation speed and die restriction on  can be separately studied and analyzed. Similar results are found for the other GFE and GPE screws. As expected, the faster the rotational speed, the better the energy performance (see Figure 7). For all GFE screws, the energy efficiency did not exceed 52% at the maximum screw speed. In contrast, for GPE it reached up to 60%.

3.2 Influence of screw type Figure 9 presents the average power demand of the extruder, motor drive and heater bands as a function of the throughput and die restriction level for each screw design and PUT. The linear relationship between energy Page 11 of 22

consumption and production in industrial processes is well known [41]. This relationship can be expressed in terms of average power demand and throughput in continuous processes like polymer extrusion. In this case, the average power demand can be expressed as:  * =  +  = + # + ,

Eq. 4

where + is the process load, # is throughput and , is the base load. The estimated values of +, , and the coefficient of determination are presented in Table 9.

a)

GFE - Screw 1

c)

GFE - Screw 3

b) GFE - Screw 2

d) GFE - Screw 4

e) GFE - Screw 5 f) GPE – Helibar® Figure 9: Average power demand of the extruder, motor drive and heater bands as a function of throughput and die restriction level for GFE (a– e) and GPE (f).

From Figure 9, the following observations can be made: • In the ranges tested, the average power demands of motor drive, heater bands and total extruder can be modeled as linear functions of the throughput, independently of the screw type and technology used. Page 12 of 22

•

•

The motor energy consumption without motor rotation is very close to zero. When the motor energy consumption is presented as a function of throughput, the intercept is zero and the motor does not exhibit a base load. For this reason, in Figure 10,   is constant. In polymer extrusion, a constant torque motor drive is commonly used, as described by Rauwendaal [24]. The energy measurements are consistent with this motor drive operation. The base load of the extruder (, value), for any screw or PUT shows a fixed average value between 4.5kW and 7.5kW. In both cases, the extruder base load is close to the base load of the heater bands (see dotted circles in Figure 9). This result can be explained because the heater bands consume energy even when the screw is not moving, to compensate for energy losses to the environment and the temperature control system inertia. The extruder control is designed to maintain a constant barrel temperature so energy losses by radiation and convection are mostly constant if the barrel and environmental temperatures remain constant. These losses are compensated by viscous dissipation due to screw rotational speed and the heat coming from the temperature control system. As expected, the faster the screw rotation speed, the lower the contribution of the heaters. For this reason, in all cases, the slope of the line that fits the average power demand of heaters as a function of the throughput is a negative one.

A rectangular hyperbolic function for SECs can be obtained by dividing Eq. 4 by #:  =   +   = + +

, #

Eq. 5

Usually,   is constant in extrusion processes. From Eq. 5 and Eq. 6,  = + # and   = +. This fact had been evidenced in multiple measurements for SSE lines made by the authors (Figure 10). Only the motor moving Screw 5 showed a different tendency, probably because the double flight in the feed zone increases the torque to startup.

(a)

(b)

Figure 10: SECmotor as a function of throughput in (a) one screw of each type in the present work and (b) several industrial SSE lines

Figure 11 presents SEC values as a function of throughput for “low” and “medium” die restrictions. Screws with the same technological concept deliver similar values of  as a function of throughput, despite the geometric differences between them. This conclusion applies to each of the die restrictions used in the experiments. As presented in Figure 11, the  behaviors of Screw 3 and Screw 4 are similar. The differences between the a values for screws 3 and 4 are 5.3%, 11.1% and 4.1% for the “low”, “medium” and “high” die restriction, respectively. The same conclusion applies for Screw 1 and Screw 2, with differences in their a values of 5.8%, 6.9% and 6.1% (see Table 9). It is important to point out that screw pairs with the same technological concept achieve similar  values at the same throughput albeit at different operational conditions (see Figure 7). Screw 5 (The double flighted feed zone Page 13 of 22

screw) has a different energy behavior than the other screws and a dissimilar design concept (Figure 5). It shows the lowest  values at the highest throughput, but the highest  value at low throughput.

a) Low die restriction

b) Low die restriction

c) Medium die restriction

d) Medium die restriction

Figure 11: Behavior of SECs vs throughput plot for GFE (Screw 1, screw 2, screw 3, screw 4 and screw 5) and GPE (Helibar®) at a) “Low” die restriction and b) “Medium” die restriction.

According to Figure 11, conventional screws have lower  values than barrier screws at a given throughput. This behavior can be explained by an additional shear stress appearing in barrier screws due to melted polymer flow through the clearance between the screw barrier flight and the barrel. However, barrier screw technologies usually deliver a more homogenous melt than conventional screws (not measured in this study) [44].

3.3 Influence of Plasticating Unit Technology (PUT) The maximum flow rate delivered by the GPE process is almost twice as much as the one obtained with even the most productive GFE screw evaluated in the present work. This can be partially explained by the fact that GPE has a more efficient polymer melting process. Figure 12 shows the schematic balance of capacities for both GFE and GPE. Here, G1 is the solid polymer transport capacity, G2 is the plasticizing polymer capacity and G3 is the melt polymer transport capacity. A good plasticating unit design ensures the overfeeding regime rule: G3 must be lower than G1 and G2. In this condition, both throughput and backpressure are constant. In GFE, G1 is much larger than G2. For this reason, the metering zone is designed to “limit” polymer melt transport capacity (G3) in order to comply with the overfeeding regime rule. In GPE, the grooves in the plasticating zone increase G2. Therefore, the polymer transport capacity (G3) can be increased by augmenting the transverse flow area on the screw by augmenting the flight height, the flight pitch, or both. This is exactly the case studied in this work. The metering channel depth in GFE varies from 3mm to 4mm, depending on the screw used. Meanwhile, GPE has a metering channel depth of 8mm. The screw design for GPE Page 14 of 22

provides a higher metering zone capacity than GFE, taking advantage of the higher plasticizing capacity, as described by Gruenschloss in [45,46].

Figure 12: Schematic balance of capacities in GFE and GPE processes. A larger box represents a higher capacity

In addition, for GFE, the base load of the heater bands (y-intercept in Figure 9a, b, c and d) is around 5kW, while for GPE, it is close to 10kW (Figure 9e). Since GPE has a higher G3 capacity, the power demand of the heater bands needs to be higher. From Figure 10a, it is clear that GPE technology uses the motor drive’s power better than GFE. In terms of SEC, the   for GPE is approximately 33% lower than GFE, but the   is practically the same for both cases. Table 6 presents the values of specific throughput, backpressure, the ratio between backpressure and throughput, and melt temperature as well as the SECs at “medium” die restriction and maximum screw rotation speed (180rpm). Screws are sorted by SECs value in ascending order. GPE exhibits the lowest SECs (0.237kWh/kg) because this technology has the lowest melt temperature (195°C), requires the lowest backpressure per unit of mass flow (2.30bar-h/kg) and has the highest specific throughput (0.6842kg/h-rpm). In other words, the energy per mass unit required for polymer heating and pressurization is lower. The differences with GFE´s are significant. In GFE, the lowest SECs is 0.329kWh/kg (for Screw 3), the lowest melt temperature is 237°C (for Screw 3), the lowest backpressure per unit of throughput is 3.36bar-h/kg (for Screw 1) and the highest specific throughput is 0.3652kg/h-rpm (for Screw 1). Results from Table 6 show that there is a strong dependence between SECs and polymer melt temperature: the lower the melt temperature, the lower the SECs value. In other words, the SECs in polymer extrusion at high rotational screw speeds is governed by the polymer enthalpy change. Table 6: Specific throughput, backpressure, ratio between backpressure and throughput, melt temperature and SECs at “medium” die restriction and maximum screw rotation speed (180 rpm) for GPE and GFE processes.

Helibar®-GPE Screw 3-GFE Screw 4-GFE Screw 5-GFE Screw 1-GFE Screw 2-GFE

Specific throughput [kg/h-rpm] 0.6842 0.3248 0.3067 0.3138 0.3652 0.2845

Backpressure [Bar] 284.1 240.7 215.5 257.5 221.0 213.0

Backpressure/ throughput [bar-h/kg] 2.30 4.11 3.90 4.56 3.36 4.16

Melt Temperature [°C] 195.0 237.2 242.7 244.0 249.8 240.3

SECs [kWh/kg] 0.237 0.329 0.343 0.347 0.352 0.371

Page 15 of 22

Table 7 presents the values of specific throughput, backpressure, the ratio between backpressure and throughput, melt temperature, and SECs this time at “low” die restriction and minimum screw rotation speed (70 rpm). Comparing the values of Table 6 and Table 7, it is possible to conclude that the specific throughput for GFE and GPE is practically independent of die restriction level and rotation speed. This behavior characterizes both technologies [47]. However, at low screw rotation speeds, the backpressure per unit of mass flow is higher. This means that, more energy is required to pressurize the polymer and overcome the die restriction. Table 7: Specific throughput, backpressure, ratio between backpressure and throughput, melt temperature and SECs at “low” die restriction and minimum rotational screw speed (70 rpm)

Backpressure [Bar]

Helibar®-GPE

Specific throughput [kg/h-rpm] 0.6814

Melt Temperature [°C] 188.0

SECs [kWh/kg]

168.5

Backpressure/ throughput [Bar-h/kg] 3.53

Screw 3-GFE

0.3213

138.6

6.16

236.0

0.463

Screw 4-GFE

0.2999

128.1

6.10

239.4

0.467

Screw 1-GFE

0.3553

140.0

5.63

244.1

0.501

Screw 2-GFE

0.2627

127.4

6.93

228.2

0.521

Screw 5-GFE

0.3027

132.7

6.26

239.4

0.563

0.315

Both Table 6 and Table 7 show that GPE reaches a lower average polymer melt temperature than GFE. The average polymer melt temperature is the average of the melt temperature profile measured using a thermocomb device. This average temperature increases with the rotation speed. In GPE, the average polymer melt temperature lies between 188°C and 195°C, around 40°C to 60°C lower than in GFE. The energy balance in polymeric fluids, where the convective transport of energy and the viscous dissipation are relevant, can explain this behavior. High Brinkman (Br) and Graetz (Gz) numbers characterize polymeric fluid flows. They are defined as: -. =

/0 1 23 −  )

45 =

Eq. 6

6  ℎ1 0 ( 2

Eq. 7

where / is the reference viscosity of the system, 0 is the reference velocity, 2 is the thermal conductivity of the polymer, 3 is the barrel temperature,  is a reference temperature, 6 is the polymer density,  is the polymer heat capacity, ℎ is the flight height and ( is the channel length.

-. is the ratio between viscous dissipation and the heat conduction in the melted polymer. That means, the higher the Brinkman number, the higher conversion of mechanical energy from motor drive into enthalpy via viscous dissipation. 45 is the ratio between the convective transport of energy and heat conduction in the melted polymer. The higher the Graetz number, the more efficient the system is at evacuating heat from the plasticating unit to the die exit through the polymer mass flow. In Table 8, an estimation of Brinkman and Graetz numbers for GFE and GPE at maximum rotation speed is shown. Table 8: Estimation of Brinkman and Graetz numbers for GFE and GPE at the maximum screw speed

Variable Screw Diameter Flight Height Channel Length from plasticizing polymer start

D h L

GFE 0.045 0.004 3.7

GPE Units 0.045 m 0.008 m 3.7 m Page 16 of 22

Variable Rotational Screw Speed Tangential velocity Density Heat Capacity Thermal Conductivity Melting temperature Wall temperature A–Carreau model B–Carreau model C–Carreau model U–Activation Energy - Arrhenius model Tr–Reference temperature - Arrhenius model Average melt temperature Shear rate Viscosity Brinkman number Graetz number

GFE N V ρ Cp k To Tw A B C U Tr Tm γ µ Br Gz

180 0.424 700 2100 0.235 156 240 2100 0.5 0.67 23000 210 245 106.0 127.0 1.2 11.4

GPE 180 0.424 700 2100 0.235 156 240 2100 0.5 0.67 23000 210 195 53.0 243.1 2.2 45.8

Units rpm m/s kg/m3 J/kg-K W/m-K °C °C Pa.s s J/mol °C °C 1/s Pa.s

In Table 8, the viscosity is estimated from the Carreau-Arrhenius model, as follows: /= ;

+7 =  <

=

+7 8 1 + +7 -9 ):

> > F 7? @1AB.>D) 7E @1AB.>D)

Eq. 8

Eq. 9

From Table 8, the Brinkman number for GPE is almost twice that of GFE. This is because GPE has a larger flight height, resulting in lower shear rates when the same rotation speed is considered. Therefore, the representative viscosity is higher. This result also indicates that GPE has a higher mechanical energy conversion into enthalpy than GFE. However, heat produced by viscous dissipation is more quickly transported by the mass flow in GPE, since the GPE Graetz number is around four times larger than that of GFE. A discussion of the effect of Brinkman and Graetz numbers on the melt temperature in shear flows can be found in [48].

3.4 Theoretical maximum energy efficiency 
 ) and relative energy efficiency 
 )

According to the model presented in Eq. 5, the higher the throughput, the lower the influence of the base load (, value) on the  , because the behavior becomes asymptotic to the + value. Therefore, the + value represents the minimum  that is possible to obtain from a given technology, screw, temperature profiles and die restriction used. This represents an ideal value because it can only be reached when the throughput tends to infinity or when the , value is zero (adiabatic condition). Therefore, an estimation of the theoretical maximum efficiency can be carried out using the + value as follows: Eq. 10  
 = + Based on the
 , a relative energy efficiency can be defined as:
 =







Eq. 11

Page 17 of 22

The results of Eq. 10 are presented in Table 9.
 that can be theoretically obtained for the conditions considered in this work varies between 54% and 84%. Apparently,
 depends heavily on the screw type.
 in barrier screws ranges between 54% and 68%. In conventional screws, the expected
 varies between 68% and 74%. For the double flighted screw, we obtain the highest
 (84%). Finally,
 in GPE is around 77%. A hypothesis to explain why the highest
 is found in screw 5 is related to the overfeeding level: the additional flight increases the initial power demand but significantly reduces overfeeding because it reduces the solid transport capacity. This fact decreases torque requirements at high rotational screw speed and therefore the motor drive power demand. Although the GFE – Screw 5 has the highest
 , it has the lowest
 (61% - 66%). This means that GFE – Screw 5 does not take advantage of its
 at the tested rotational speeds. In the GFE case, with barrier screws (screw 1 and screw 2) and operating with the highest die restriction, the highest
 (82% and 86% respectively) was obtained. This is a result of the reduction of
 (from 68% at “low” die restriction to 58% at “very high” die restriction for screw 1 and from 64% at “low” die restriction to 54% at “very high” die restriction for screw 2). In contrast, GPE presents a high and stable
 (77%) and a high
 (around 76%). The lower the , value, the closer to the
 the system is. For this reason, all efforts to reduce the power supplied by heaters are desirable. One option is increasing the energy conversion rate from the motor drive to the polymer enthalpy. Other alternatives include the reduction of heat losses to the environment, decreasing of the heating control system inertia, avoiding the use of oversized heater bands and increasing the heat transfer efficiency from the heater bands to the polymer. Table 9:
 ,
 and minimum SECs expected for each extruder as a function of PUT, screw, and die restriction used.

η PUT - Screw GFE-Screw 1

GFE-Screw 2

GFE-Screw 3

@maximum rotation speed 49.50%

ηmax

ηr

a=SECmin

b

r2

68.50%

72.26%

0.257

5.745

0.9837

Medium Restriction

50.10%

67.50%

74.22%

0.261

5.923

0.9987

Very High Restriction

48.40%

58.90%

82.17%

0.294

5.265

0.998

Low Restriction

47.80%

63.60%

75.16%

0.272

4.834

0.9939

Medium Restriction

48.10%

62.30%

77.21%

0.279

4.897

0.9912

Very High Restriction

46.70%

54.20%

86.16%

0.312

4.333

0.9921

Low Restriction

52.20%

73.30%

71.21%

0.227

5.269

0.9991

Medium Restriction

51.00%

74.10%

68.83%

0.226

5.947

0.9956

-

69.80%

-

0.243

6.081

0.9909

Low Restriction

50.60%

71.30%

70.97%

0.239

5.036

0.9952

Medium Restriction

49.80%

68.40%

72.81%

0.251

5.261

0.9974

-

67.90%

-

0.253

5.596

0.9998

Low Restriction

51.60%

84.20%

61.28%

0.202

7.397

0.9883

Medium Restriction

50.10%

81.10%

61.78%

0.213

7.421

0.9953

Die Restriction Low Restriction

Very High Restriction GFE-Screw 4

Very High Restriction GFE-Screw 5

GPE-Helibar

Very High Restriction

49.40%

73.80%

66.94%

0.235

7.118

0.9923

Low Restriction

59.60%

77.80%

76.61%

0.175

7.21

0.9813

Medium Restriction

58.30%

77.00%

75.71%

0.179

7.526

0.9782

High Restriction

57.80%

77.90%

74.20%

0.175

7.629

0.997

GPE shows the lowest level of  and the lowest + value. However, this technology did not yield the maximum theoretical energy efficiency from a thermodynamic point of view, since the melt temperature is much lower than the Page 18 of 22

ones obtained with GFE thus, the material leaves the system with lower enthalpy. In GPE, the high transport of energy by convection affects the melt temperature profile, reducing the average melt temperature, as shown in Table 8. Higher thermodynamic energy efficiencies require longer plasticating units. This conclusion may change in practice, if the EAC includes cooling and other post-extrusion processes. In that case, unnecessary high melt temperatures at the die exit may represent more work for the cooling system, affecting the expanded EAC energy efficiency. 3. CONCLUSIONS A study of the energy performance of two different extrusion technologies (GFE and GPE) was carried out. GFE was tested with five different screws, and GPE with one screw. Energy performance was analyzed as a function of die restriction and rotational screw speed. The melt temperature, productivity and energy performances at different operational conditions were discussed for each case. It was statistically demonstrated that SECs depends on rotational screw speed and die restriction. The interaction effect of these variables is negligible. For this reason, the impact of these variables on SECs should always be considered, but they can be studied separately. A relationship between SECs and screw type used was found and a model to estimate the
 and
 of the plasticating unit is proposed. These efficiencies are evaluated in each extruder configuration considering several operational conditions. The results show that the GPE, the newest technology studied and recognized as a technology significantly more productive than GFE, has a lower  , a higher and more stable maximum energy efficiency as well as a high relative energy efficiency compared to the GFE process. This energy behavior in GPE can be explained as the technology better converts mechanical energy from the motor into enthalpy, and it has a better balance between solid transport, plasticizing and fluid transport capacities compared to GFE. The results suggest that an energy efficient SSE process requires high productivity, low SECs, high
 and high
 . In future work, the homogeneity of the polymer melt as a quality parameter should be considered. The effect of barrier and die temperature profile on the energy performance should be studied and the potential effects of the use of an amorphous material included in the analysis. 4. ACKNOWLEDGMENTS This work was supported by the Department of Science and Technology of Colombia (COLCIENCIAS), ICIPC and EAFIT University. We thank our colleagues from ICIPC who provided insight and expertise that greatly assisted the research. We would also like to show our gratitude to HELIX GmbH who provided the HELIBAR® plasticating unit. We thank ESENTTIA, the company that donated the material required for this research. Farid Chejne wishes to thank to the project "Strategy of transformation of the Colombian energy sector in the horizon 2030" funded by the call 788 of Colciencias Scientific Ecosystem. Contract number FP44842-210-2018 5. BIBLIOGRAPHY [1] Kent R. ENERGY MANAGEMENT IN PLASTICS PROCESSING: strategies, targets, techniques, and tools. 3rd Editio. United Kingdom: ELSEVIER THE LANCET; 2018. [2]

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Page 22 of 22

Experimental Study of Energy Performance of Grooved Feed and Grooved Plasticating Single Screw Extrusion processes in terms of SEC, Theoretical Maximum Energy Efficiency and Relative Energy Efficiency. Omar Estrada a ([email protected]), Juan Carlos Ortiz a, b ([email protected]), Alexander Hernández a, b ([email protected]), Iván López a ([email protected]), Farid Chejne c ([email protected]), María del Pilar Noriega a ([email protected]), a Instituto de Capacitación e Investigación del Plástico y del Caucho – ICIPC b Universidad EAFIT c Universidad Nacional de Colombia, facultad de Minas, departamento de Procesos y energía

Highlights • • • • •

A novel approach to determine de theoretical maximum energy efficiency is proposed An approach to evaluate the relative energy efficiency is proposed GFE with five different screws and GPE are evaluated under de same conditions The influence of rotational screw speed and die restriction is considered Operational curve to integrate process control and energy efficiency is introduced