Energy Efficiency in Plasticating Screw Extrusion - ACS Symposium

3 Energy Efficiency in Plasticating Screw Extrusion C. I. CHUNG, Ε. M. MOUNT, III, and D. E. McCLELLAND Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 22, 2016 | http://pubs.acs.org Publication Date: August 29, 1979 | doi: 10.1021/bk-1979-0107.ch003

Rensselaer Polytechnic Institute, Troy, NY 12181

The most widely used polymer conversion processes are injec tion molding, extrusion, blow molding, thermoforming, calendering and compression molding. A l l of these processes consist of basi cally three functional steps (Figure l); plastication or melting of a polymer solid, forming of the polymer melt into a desired shape and solidification of the formed polymer article. Forming is accomplished by pressure or other types of stress and requires only a small amount of mechanical energy, insignifi cant in comparison to the energy required for melting. Solidifi cation is accomplished by cooling the polymer melt for thermo plastic polymers, and by curing and subsequent cooling for thermoset polymers. At present, the polymer processing industry does not recover the thermal energy removed from the polymer melt by cooling. The melting step consumes the major portion of energy required for processing and the energy efficiency in this step is regrettably low. Melting is accomplished by the thermal energy from the equipment's heaters and more importantly by the viscous dissipation of mechanical energy from the equipment's motor. Both the heaters and the motor are usually powered by electricity. Since polymers have a very low thermal conductivity, melting by external heating is inevitably a slow process giving a low produc tion rate. However, a large amount of heat can be generated in ternally in polymers by viscous dissipation of mechanical energy because of the high viscosity of polymer melts and thus melting by mechanical energy is a fast process giving a high production rate. Melting is accomplished in a l l modern processes primarily by me chanical energy. We will define in this paper "the mechanical energy efficiency" as the amount of polymer melted per unit me chanical energy consumption of the drive motor. Screw extruders (Figure 2) are used t o melt polymer s o l i d s i n most major processes. Almost a l l t h e r m o p l a s t i c polymers a r e ex truded at l e a s t once, o f t e n twice or more, before t h e i r f i n a l p r o ducts are made. The t r e n d i n the l a s t two decades has been everi n c r e a s i n g speed and s i z e o f extruders i n order t o o b t a i n higher production r a t e s with almost no concern f o r energy e f f i c i e n c y .

0-8412-0509-4/79/47-107-021$05.00/0 © 1979 American Chemical Society

In Energy Conservation in Textile and Polymer Processing; Vigo, Tyrone L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 22, 2016 | http://pubs.acs.org Publication Date: August 29, 1979 | doi: 10.1021/bk-1979-0107.ch003

22

ENERGY

CONSERVATION IN TEXTILE

A N D POLYMER

PROCESSING

Equipment paent Cooling Coolin | Cooling

Losses Enthalpy

» l S oSolidification lidirT5 »tForming 1 - J -=->[ 1

H. Pol/Mr i» Product

Motor Power (Mechanical Energy) Equipment Heating (Thermal Energy) Electric Power Figure 1.

Functional steps and energy flow in a typical polymer process

Figure 2.

Schematic of an extruder



3.

CHUNG E T AL.

Plasticating

Screw

23

Extrusion

In f a c t , many modern extruders generate enormous q u a n t i t i e s o f excess heat and they are equipped with high c a p a c i t y c o o l i n g sys tems to remove the excess heat, which would otherwise i n c r e a s e the melt temperature to an u n d e s i r a b l y high value or even degrade the polymer. Consequently, the energy e f f i c i e n c i e s of the modern ex t r u d e r s have decreased s u b s t a n t i a l l y although the p r o d u c t i o n r a t e s have increased. For example, a 2 . 5 i n . ( 6 3 - 5 mm) diameter ex t r u d e r about 20 years ago u s u a l l y had a l e n g t h t o diameter r a t i o of 20 and produced 50-70 kg/hr o p e r a t i n g at 50-100 RPM at the me c h a n i c a l energy e f f i c i e n c y o f 5-7 kg/kw-hr o f the motor power. Today, a 2 . 5 i n . ( 6 3 . 5 mm) diameter extruder u s u a l l y has a l e n g t h to diameter r a t i o of at l e a s t 2 U , o f t e n 30, and produces 7 0 - 1 0 0 kg/hr o p e r a t i n g at 1 0 0 - 1 5 0 RPM at the mechanical energy e f f i c i e n c y o f U-6 kg/kw-hr, a decrease o f about 1 kg/kw-hr from that 20 years ago. Assuming that the e n t i r e energy f o r m e l t i n g i s provided by the motor and no energy i s l o s t , the t h e o r e t i c a l maxi mum mechanical energy e f f i c i e n c y i n e x t r u s i o n processes based on the enthalpy d i f f e r e n c e (H1-H2) between a polymer s o l i d at room temperature (l) i s 6-10 kg/kw-hr or 8 kg/kw-hr on the average f o r most l a r g e volume polymers such as p o l y e t h y l e n e s , polypropylene, p o l y s t y r e n e , p o l y v i n y l c h l o r i d e and nylons. Thus, 5 kg/kw-hr on the average w i t h modern 2 . 5 i n . ( 6 3 . 5 mm) diameter extruders r e presents only 62$ mechanical energy e f f i c i e n c y . The mechanical energy e f f i c i e n c i e s w i t h l a r g e r extruders are even worse. The t o t a l s a l e o f major thermoplastic polymers i n the United States was r e p o r t e d at approximately 10 m i l l i o n metric tons i n 1977 [2JTaking an average 50$ mechanical energy e f f i c i e n c y o f k kg/kw-hr at present and two-path e x t r u s i o n , about 5 x 1 0 kw-hr of e l e c t r i c power should have been consumed t o process these p o l y mers i n 1 9 7 7 . In terms o f f u e l v a l u e , t h i s r e p r e s e n t s about h9 χ 1 0 BTU/year ( t a k i n g t h r e e times the e l e c t r i c f i g u r e ) . At an assumed 75$ mechanical energy e f f i c i e n c y o f 6 kg/kw-hr which should be p o s s i b l e , o n l y about 33 x 1 0 BTU/year o f f u e l would have been used to process these polymers. Thus, a l a r g e amount of f u e l e q u i v a l e n t to 16 χ 1 0 BTU/year could have been saved ac cording to t h i s e s t i m a t i o n . The a c t u a l energy saving by improved energy e f f i c i e n c i e s o f extruders i n the e n t i r e polymer p r o c e s s i n g i n d u s t r y could be many times t h i s amount. The world polymer p r o duction i n 1973 was approximately k3 m i l l i o n m e t r i c tons (3) and thus a tremendous o p p o r t u n i t y e x i s t s worldwide t o save energy i n polymer p r o c e s s i n g . A d d i t i o n a l l y , a b e t t e r understanding o f the conversion processes w i l l l e a d to m a t e r i a l s savings by reducing scrap i n processing. C u r r e n t l y , 2 ^ 3% o f a l l polymers become scrap during p r o c e s s i n g due t o overheating and other adverse process reasons. Energy e f f i c i e n c y has not been a r e a l concern to the polymer p r o c e s s i n g i n d u s t r y u n t i l r e c e n t l y . However, the cost o f energy i s f a s t becoming an important f a c t o r i n polymer p r o c e s s i n g . Also, the p r e s s i n g energy problems i n the United States and around the world have created a growing concern about energy e f f i c i e n c y i n 9

1 2

1 2

1 2


ENERGY

24

CONSERVATION

ΓΝ T E X T I L E

A N DPOLYMER

PROCESSING

the polymer p r o c e s s i n g i n d u s t r y . A number o f a r t i c l e s (h, 5» 6, J) on t h i s subject have appeared r e c e n t l y i n the t r a d e j o u r n a l s .


M e l t i n g i n Screw Extruders S o l i d polymer i n the form o f p e l l e t or powder dropped i n t o the screw channel from the hopper i s compacted i n t o a s o l i d p l u g due t o the screw r o t a t i o n and the s o l i d p l u g i s f o r c e d t o rub on the heated b a r r e l s u r f a c e . The s o l i d p l u g i s p l a s t i c a t e d o r melted i n screw extruders p r i m a r i l y by the mechanical energy p r o vided by the d r i v e motor r a t h e r than the thermal energy from the heated b a r r e l . An i d e a l i z e d m e l t i n g mechanism i n screw extruders i s shown i n Figure 3. When the s o l i d p l u g i s rubbed on a heated b a r r e l s u r face, a t h i n l a y e r o f melt or melt f i l m develops between the s o l i d p l u g and the b a r r e l s u r f a c e . The melt f i l m exchanges heat w i t h the b a r r e l surface and a l s o generates heat by d i s s i p a t i n g the work done by the motor i n rubbing the s o l i d p l u g on the b a r r e l s u r f a c e . A l a r g e amount o f heat can be generated i n the melt f i l m owing t o the very h i g h v i s c o s i t y o f polymer melts, and i t has been gener a l l y b e l i e v e d that the melt f i l m temperature can become higher than the b a r r e l temperature r e q u i r i n g b a r r e l c o o l i n g i n h i g h speed o p e r a t i o n s . M e l t i n g occurs p r i m a r i l y at the s o l i d plug/melt f i l m i n t e r face due t o the heat f l u x (q) from the melt f i l m i n t o the s o l i d p l u g . The mass r a t e o f m e l t i n g per u n i t i n t e r f a c e area (Ω) w i l l be p r o p o r t i o n a l t o q. (1)

Ω

-Melt scraping force Modern Plastics

Figure 4.

Schematic of screw simuhtor

TABLE I L i s t o f Polymer Samples

Polymer

Source

HDPE

P h i l l i p s Petroleum

132

LDPE

Exxon Chemical

Co.

110

Exxon Chemical

Co.

165

PP POM PS PMMA PC

HDPE LDPE PP POM PS PMMA PC

M e l t i n g Temperature (°C)

Celanese

Glass T r a n s i t i o n Temperature (°C)

l66

Foster-Grant

—

100

Rohm and Has s

—

105

General E l e c t r i c

—

150

= = = = = = =

High Density Polyethylene Low Density Polyethylene Polypropylene Polyoxymethylene Polystyrene Polymethylmethacrylate Polycarbonate


28

ENERGY

CONSERVATION

IN

TEXTILE

AND

POLYMER

PROCESSING

the maximum mechanical energy e f f i c i e n c y ( e ) that can be achieved when the energy from the motor i s used without any l o s s t o heat the polymer from room temperature t o the r o l l s u r f a c e temperature. e depends o n l y on Tb n e g l e c t i n g the small i n f l u e n c e o f pressure on enthalpy. Experimental values o f ε g r e a t e r than e indicate that the polymer melts by the heat conducted from tne heated r o l l surface as w e l l as by the mechanical energy from the motor. In t h i s case, the polymer melt i s not heated above Tb and no energy i s l o s t . Experimental values o f ε l e s s than e i n d i c a t e that the polymer melt i s heated above Tb by the mechanical energy from the motor and thus the r o l l surface must be cooled t o maintain i t s temperature, r e s u l t i n g i n energy l o s s . Assuming t h a t the polymer melt e v e n t u a l l y a t t a i n s the r o l l s u r f a c e temperature by exchang ing heat with the r o l l , the maximum energy e f f i c i e n c y (combining the mechanical energy from the motor and the thermal energy from the heater) w i l l be achieved at the r o l l surface temperature where experimental ε crosses e . R e f e r r i n g t o F i g u r e s 5 - 1 1 , i t i s found that ε i n c r e a s e s w i t h Tb and t h a t ε can be s u b s t a n t i a l l y l e s s than επ! at low p r o c e s s i n g temperatures, ε becomes g r e a t e r than e o n l y at r e l a t i v e l y h i g h p r o c e s s i n g temperatures. From these experimental r e s u l t s , ε i s expected t o i n c r e a s e with i n c r e a s i n g b a r r e l temperature i n a c t u a l e x t r u s i o n operations as f a r as the m e l t i n g mechanism i s concerned. Excessive mechanical energy can be consumed r e s u l t i n g i n over heated melt when the b a r r e l temperature i s kept too low. P o l y carbonate (PC) i n F i g u r e 11 shows a maximum ε at about Tb = 210°C but ε i n c r e a s e s with i n the usual p r o c e s s i n g range above 230°C. This maximum i s a s s o c i a t e d with a t r a n s i t i o n i n the adhesive p r o p e r t y o f PC on the metal s u r f a c e (9). F i g u r e s 12-18 show that ε i n i t i a l l y decreases very s h a r p l y with i n c r e a s i n g Vb to about 30 cm/sec and then decreases s l o w l y beyond Vb = 30 cm/sec. I t i s noted that modern extruders are u s u a l l y operated at high speeds above Vb = 30 cm/sec. ε i s much greater than e at very low Vb i n d i c a t i n g that the energy f o r melting comes mainly from the heated r o l l by conduction, ε becomes almost the same as e at high V^ f o r a l l polymers even at high T-fc. One can expect from t h i s f i n d i n g that the e n t i r e energy for m e l t i n g i s provided by the motor i n a l l high speed e x t r u s i o n operations. In such cases, the energy consumed by the screw f l i g h t t o scrape o f f the molten polymer from the b a r r e l surface and t o pump out the polymer melt c o l l e c t e d i n the melt pool w i l l cause overheating and give a low energy e f f i c i e n c y . Additional mechanical energy o f t e n i s r e q u i r e d i n e x t r u s i o n t o mix v a r i o u s feed components. T h i s a d d i t i o n a l energy consumption, although not wasted, a l s o c o n t r i b u t e s t o overheating. m

m

m


m

m

m

m

m

Conclusions The experimental r e s u l t s w i t h molded polymer samples show that the mechanical energy e f f i c i e n c y .in m e l t i n g s o l i d polymers


by


CHUNG

E T A L .

Phsticating

Screw

Extrusion

e

ROLL SURFACE TEMPERATURE, C Figure 5.

Figure 6.

Mechanical energy efficiency as a function of roll surface temperature for high density polyethylene

Mechanical energy efficiency as a function of roll surface temperature for low density polyethylene


ENERGY

30

-i

20

CONSERVATION IN TEXTILE

1

1 r

—

ι

A N D POLYMER

ο

ι

PROCESSING

•

POLYPROPYLENE V 16

P

= 33 CM/SEC

fa

=

5

7

-

6

3

o KG /CM X = 2.54 CM ο

2

f

υ


-

Ο

12

ο

ο

8

Ο 4-

•

Ο Ο

0

—ιι 160

ι ι 180

ιι 200

•ι 220

1 ι 240

1 1 260

ι 280

ι 300

e

ROLL SURFACE TEMPERATURE, C Figure 7.

Mechanical energy efficiency as a function of roll surface temperature for polypropylene

Figure 8.

Mechanical energy efficiency as a function of roll surface temperature for polyoxymethylene


Plasticating

Screw

Extrusion


CHUNG ET AL.

Figure 10.

Mechanical energy efficiency as a function of roll surface temperature for polymethylmethacrylate


32


ENERGY

CONSERVATION

IN TEXTILE

A N D POLYMER

PROCESSING

HIGH DENSITY POLYETHYLENE T. * 218 C 2 Ρ = 35-40 KG /CM e

b

f

10

Figure 12.

20

30

40 50 60 70 ROLL VELOCITY, CM/SEC

80

90

100

Mechanical energy efficiency as a function of roll velocity for high density polyethylene


CHUNG

E T

A L .

Plasticating

Screw

Extrusion

LOW DENSITY POLYETHYLENE T. = 163°C 2 Ρ = 34-43 KG^/CM


b

ο ο

Ο JSL

10

20

30

40

50

60

70

80

90

100

ROLL VELOCITY, CM/SEC Figure 13.

Mechanical energy efficiency as a function of roll velocity for low density polyethylene

POLYPROPYLENE T. » 233 C e

b

10

20

30

2

P

Q

- 31-37 KG /CiT

X

Q

» 2.54 CM

40

f

50

60

70

80

90

100


Mechanical energy efficiency as a function of roll velocity for poly propylene


ENERGY

200

ι

CONSERVATION

Γ

ι

IN TEXTILE

ι

1

A N D POLYMER

1

POLYOXYMETHYLENE T = 207 C P « 78-84 KG /CM X = 2.54 CM ο

1

PROCESSING

r

e

b


Q

2

f

u 120 Ο

80

40

-

Ο Ο Ο

10

Figure 15.

20

-2_o

30

ρ

_ o _ ^ o - »>. e

40 50 60 70 ROLL VELOCITY, CM/SEC

80

90

100

Mechanical energy efficiency as a function of roll velocity for polyoxymethylene

200

I

I

•

ι -

ι

ι

Γ™

" τ — I

I

POLYSTYRENE Τ, = 233 C e

ο

160

-

b

2 P = 30-39 KG /CM Χο = 2.54 CM Q

υ

f

-

120 ο

80

-

-

ο ο

ο

40

_

ο

Ο

e

m

0 0

ι 10

ο

I 20

ι 30

1 40

I 50

Ο I 60

I 70

I 80

ι 90

•

100


Mechanical energy efficiency as a function of roll velocity for poly styrene



CHUNG

E T

A L .

Plasticating

30

Screw

40

Extrusion

50

60

70


Mechanical energy efficiency as a function of roll velocity for poly methylmethacrylate

POLYCARBONATE T. = 297°C b

P

Q

2 = 34-37 KG^/CM

X„ = 2.54 CM ο

10

20

30

40

50

60

70

80

90

100


Mechanical energy efficiency as a function of roll velocity for poly carbonate



36

ENERGY

CONSERVATION

IN

TEXTILE

A N D POLYMER

PROCESSING

rubbing on a heated metal surface i n c r e a s e s with i n c r e a s i n g metal surface temperature and decreases with i n c r e a s i n g rubbing v e l o c i t y . At low metal surface temperatures and h i g h rubbing v e l o c i t i e s , the melting mechanism by i t s e l f consumes excessive mechanical energy from the motor, much more than necessary t o heat the polymers from room temperature t o the metal surface temperature. The m e l t i n g mechanism at high rubbing v e l o c i t i e s always consumes enough me c h a n i c a l energy t o heat the polymers from room temperature t o the metal surface temperature even at high metal surface temperatures. Therefore, high screw speed e x t r u s i o n operations at low b a r r e l temperatures w i l l overheat the polymers above the b a r r e l surface temperature, r e q u i r i n g b a r r e l c o o l i n g , j u s t by the mechanical en ergy o f the motor consumed i n rubbing the polymer s o l i d on the b a r r e l surface. The polymer melt w i l l be heated even higher by the a d d i t i o n a l mechanical energies consumed by the screw f l i g h t and other flow requirements. T h i s can e x p l a i n why most modern ex t r u d e r s must be operated with b a r r e l c o o l i n g . A l l experimental r e s u l t s r e p o r t e d i n t h i s paper were obtained with molded samples f o r a t h e o r e t i c a l reason. These experiments should be repeated with p e l l e t samples so that the experimental r e s u l t s could be more d i r e c t l y a p p l i e d t o a c t u a l e x t r u s i o n opera t i o n s . The energy e f f i c i e n c y w i l l become an important f a c t o r i n the f u t u r e i n d e s i g n i n g a new extruder f o r a given output r a t e as energy cost i n c r e a s e s . Acknowledgement I t i s g r a t e f u l l y acknowledged that our e x t r u s i o n r e s e a r c h was sponsored by the N a t i o n a l Science Foundation.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Bernhardt, E. C., "Processing of Thermoplastic Materials," Reinhold (1959). Modern Plastics, 55(1), 49 (1978). Platzner, Ν., Chem. Tech., 5, 103 (Feb. 1975). Mack, W. Α., Plastics Engineering, 29(11), 31 (1973). Bauer, L. R., Plastics Technology, 48 (June 1974). Waters, C. Ε., Plastics World, 56 (Dec. 1974). Plastics Technology, 11 (May 1974). Pearson, J.R.A., Imperial College Polymer Science & Engineer ing Group, Report Nos. 4 and 5 (1974); J. Heat and Mass Transfer, 19, 405 (1976). Mount, Ε. Μ., III and Chung, C. I., Poly. Eng. Sci., 18(9), 711 (1978). McClelland, D. Ε., M.S. Thesis, RPI, December 1977.

RECEIVED

February 21, 1979.


Energy Efficiency in Plasticating Screw Extrusion - ACS Symposium

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