Energy effective polyester production.

Budin, Rajka ; Mihelic-Bogdanic, Alka

Abstract: Synthetic fibre industry i.e. polyester production is an energy intensive process which unit operations use fuel directly and transformed forms of primary energy like heat and electricity. Important progress could be achieved by the development and the application of the several successful options directed to reducing energy consumption. Therefore the aim of presented analysis is to suggest one of the possibilities for energy conservation implementing combined heat and power production (CHP). The proposed cogeneration process compared with the conventional system results with absolute efficiency increasing of 35,5%. With regard to whole energy demand i.e. fuels directly, heat and electrical energy savings in primary energy source is about 65%.

Key words: polyester, energy consumption, cogeneration, savings

1. Introduction

Polyesters are heterochain macromolecular substances characterized by the presence of carboxylate ester groups in the repeating units of their main chains (Rodriguez, 1996). They were historically the first family of synthetic condensation polymers which comprises all polymers with ester functional groups in the backbone. The most widely used linear polyester is poly(ethylene terephthalate, PET) prepared from terephthalic acid and ethylene glycol. The polymer can be prepared in the bulk or in the solution using an excess of ethylene glycol to increase esterification rate (Stevens, 1999). Polyester like PET is among the more versatile synthetic polymers which take a central position under engineering plastics with widespread commercial use. That could be easily explained by their excellent properties, heat stability and resistance to wrinkling making them superior to cotton and wool (Edlund & Albertsson, 2003, Bohm et al., 2006). With regard to the amount of energy required for unit operations, the polyester production is one of the energy intensive processes with directly fuel, thermal and electrical energy consumption. Therefore the application of methods for reducing energy usage is proposed in presented analyses.

2. Process operation and data

From the industrial process flow sheet (Fig.1.) is visible that the primary energy i.e. fuel directly as well as transformed forms of energy are supplied to the unit operations. Regarding Fig.2. the primary source i.e. fuel oil is used for dryers (3,14), esterification reactor (4), melter (7), polymizer (10) and polymer melt (11). The electric energy is provided for filters (2,6), esterification reactor (4), crystallizer (5), roller (8), transesterification reactor (9), tow drawing (12), crimper (13) and cutter with baler (15). In terms of the energy usage 45,5% (2,7 [MW.sub.e], 9,7 x [10.sup.6] kJ/h) of the input is electricity while 54,5% (3,23 [MW.sub.t], 11,6 x [10.sup.6] kJ/h) is thermal energy (Production plant data, 2004). The heat energy [t.sub.S]= 132[degrees]C (3 bar) in form of the dry saturated steam is produced in the boiler with efficiency [[eta].sub.B]=85% and transferred to the autoclave reactor (1). After process in reactor the steam is rejected as a saturated liquid i.e. process condensate at 132[degrees]C. For presented industrial process the plant use factor is [beta]=84,9% i.e. [tau]=7440 hours per year. From the fuel oil percentage composition by mass: 85,3% C; 11,6% H; 0,6%N; 2,5%[S.sub.v] follows the lower heating value from (Budin & Mihelic-Bogdanic, 2002):

[H.sub.L]=340C + 1035H + 104[S.sub.v]=340 x 85,3 + 1035 x 11,6 + 104 x 2,5=41268 kJ/kg. (1)

The mass rate of dry saturated steam for thermal energy needs [Q.sub.t] = 11,6 x [10.sup.6] kJ/h ([N.sub.t]=3,23 [MW.sub.t]) is:

[D.sub.S]= [Q.sub.t]/([h".sub.132] - [h'.sub.132]) = 11,6 x [10.sup.6]/(2724-560)=5360 kg/h, (2)

with dry saturated steam h" and boiling water h" enthalpies (Budin & Mihelic-Bogdanic, 2002).

The calculated steam mass rate produced in a standard boiler satisfied the whole amount of needed thermal energy. In conventional industrial process the electric energy is usually supplied from grid.

[FIGURE 1 OMITTED]

3. Fuel consumption for conventional process

The fuel oil consumption (kW, kg/h, kg/year, tOE) for separately heat and electricity

production in an conventional process are calculated:

[D.sub.Ft]=[N.sub.t]/[[eta].sub.B] = 3230/0,85= 3800 kW (3)

[D.sub.Ft] = [[D.sub.Ft] x 3600]/[H.sub.L] = (3800 x 3600)/41268= 331,5 kg/h. (4)

Regarding plant use factor [beta]=84,9% i.e. [tau] = 7440 hours per year, fuel consumption is:

[D.sub.Fty] = [D.sub.Ft] x [tau] = 331,5 x 7440 = 2,5 x [10.sup.6] kg/y. (5)

The yearly thermal energy supply in terms of oil equivalent (tOE):

tO[E.sub.t] = ([Qsub.t] x [tau])/[H.sub.OE] = (11,6 x [10.sup.6] x 7440)/41,868 x [10.sup.6] = 2061,3 (6)

with the heating value of 1 ton oil equivalent [H.sub.OE] = 41,868 x [10.sub.6] kJ.

The presented solution satisfies the whole amount of the thermal needs while electric energy is produced in the power plant with efficiency ?e = 33%. In this case the fuel oil consumption calculated from before used equations (3,4,5,6) is:

[D.sub.Fe]=[N.sub.e]/[[eta].sub.e] = 2700/0,33 = 8182 kW

[D.sub.Fe] = [ [D.sub.Fe] x 3600]/[H.sub.L]= (8182 x 3600)/41268= 713,8 kg/h

[D.sub.Fey] = [D.sub.Fe] x [tau] = 713,8 x 7440 = 5,3 x [10.sup.6] kg/y

tOEe = ([Q.sub.e] x [tau])/[H.sub.OE] = (9,7 x [10.sup.6] x 7440)/41,868 x [10.sup.6] = 1723,7.

The overall efficiency for conventional system with separately heat and power production is:

[[eta].sub.oC] =([N.sub.e] + [N.sub.t])/ * [[D.sub.Fe] + [D.sub.Ft]] = (2700 + 3230)/(8182+3800)=0,495 or 49,5%, (7)

where all values are expressed in kW.

4. Industrial process with cogeneration

Energy technology called cogeneration i.e. combined heat and power production (CHP) cuts energy costs, reduces pollution and green house gas emissions as well as increases energy efficiency. Namely, the conventional electricity generation is inherently inefficient, converting only about a third of the fuels potential energy into usable one, while CHP which produces both electricity and useable heat converts as much as 90 percent of the fuel into utilizable energy (Lemar, 2001; Bonilla et al., 2003). Except thermal efficiency increasing the CHP also has a great potential for the environment as one of most important long term opportunities for reducing carbon emissions and other air pollutants (Bonilla et al., 2003). Therefore, in the presented article the CHP system with fossil fueled boiler and back pressure steam turbine as a topping unit is proposed (Fig. 2). Except before used values some key parameters for proposed CHP configuration are: superheated steam temperature 487[degrees]C and pressure 40 bar; back pressure turbine outlet temperature 132[degrees]C (3 bar). Now, satisfying the process heat requirement ([Q.sub.t] = 11,6 x [10.sup.6] kJ/h or 3230 [kW.sub.t]) the electrical output is: [N.sub.eCHP]=[D.sub.S]([h.sub.487]-h")/3600=5360(3400-2724)/3600=1006,5kW; [Q.sub.eCHP]=3,6 x [10.sup.6] kJ/h. (8)

The fuel consumption is:

[D.sub.FeCHP] = [Q.sub.eCHP]/[[eta].sub.B] = 100,6/0,85= 1184 kW. (9)

The yearly electricity supply in terms of oil equivalent:

tO[E.sub.e] = ( [Q.sub.eCHP] x [tua] )/[H.sub.OE] = (3,6 x [10.sup.6 x 7440)/ 41,868 x [10.sup.6] = 639,7. (10)

[FIGURE 2 OMITTED]

The savings in purchased electric energy compared with conventional shame is:

[S.sub.CHP] = [N.sub.e] - [N.sub.eCHP] = 2700 - 1006,5 = 1693,5 kW or 37,3 %. (11)

In presented option the fuel oil rate is:

[D.sub.FCHP]= [D.sub.FeCHP] + [D.sub.Ft] = ([N.sub.eCHP] + [N.sub.tCHP])/[[eta].sub.B]=(1006,5+3230)/0,85 = 4984 kW. (12)

From (12):

[D.sub.FCHP] =[([D.sub.FeCHP] + [D.sub.Ft]) 3600]/ [H.sub.L]=4984 x 3600/ 41268= 434,7 kg/h (13)

or yearly 3,2 x [10.sup.6] kg.

Expressed as the oil equivalent:

tO[E.sub.CHP] = tO[E.sub.e] + tO[E.sub.t] = 639,7 + 2061,3 = 2701. (14)

The CHP plant efficiency:

[[eta].sub.CHP] = ([N.sub.eCHP] + [N.sub.t])/[D.sub.FCHP] = (1006,5 + 3230)/4984 = 0,85 or 85% (15)

with all values in kW.

The superiority of CHP configuration relative to conventional power production results with absolute efficiency increasing of:

[[eta].sub.CHP] - [[eta].sub.oC] = 85 - 49,5 = 35,5%.

5. Conclusion

Presented CHP and conventional system comparison shows an absolute efficiency increasing of [[eta].sub.CHP-[[eta].sub.oC]=35,5% while savings in purchased electricity is 37,3%. Also, from the fuel inputs difference [D.sub.FC]-[D.sub.FCHP]=610,6 the reduction is 58,4%.

In the analyzed process the primary source i.e. fuel oil regarding to the operations are divided into directly consumption and transformed forms expressed as a heat and electricity (Fig.2). From the process data is visible that the directly fuel utilization is [D.sub.F]=722,5 kg/h. In conventional system the fuel consumption for steam production is [D.sub.Ft] = 331,5 kg/h and for electricity [D.sub.Fe] = 713,8 kg/h. The whole amount of fuel for separate steam and electric energy production in conventional process is [summation][D.sub.FC] = 1767,8 kg/h. Compared with CHP system with same mass of directly fuel consumption [D.sub.F]=722,5 kg/h and calculated amount of [D.sub.FCHP] = 434,7 kg/h the whole fuel mass rate is [summation][D.sub.CHP] = 1157,2 kg/h.

Follows the fuel savings:

S = [summation][D.sub.CHP]/[summation][D.sub.FC]= 1157,2/1767,8 = 0,655 or 65,5%. (16)

6. References

Bonilla, D.; Akisawa, A.; Kashhiwagi, T. (2003). Modelling the adoption of industrial cogeneration in Japan using manufacturing plant survey data, Energy Policy,.Vol. 31, No. 9, pp. 895-910, ISSN 0301-4215

Bohm, F, Komber, H. & Jafari, S.H. (2003). Synthesis and characterization of a novel unsaturated polyester based on poly(trimethylene terephthalate). Polymer, Vol.47, No.4, (2003), p.1892-1898, ISSN 0032-3861.

Budin, R. & Mihelic-Bogdanic,A.(2002).Fundamentals of Technical Thermodynamics (in Croatian), Skolska knjiga, ISBN 953-0-31688-7, Zagreb

Edlund, U. & Albertsson, A. C. (2003). Polyesters based on diacid monomers. Advanced Drug Delivery Reviews, Vol. 55, No.6, pp. 585-609, ISSN 0169-409.

Lemar, P.L. (2001). The Potential impact of Policies to Promote Combined Heat and Power in US Industry, Energy Policy, Vol. 29, No.14., pp.1243-1254, ISSN 0301-4215

Production plant data. Private communication, 2004. Zagreb

Rodriguez, F. (1996). Principles of polymer systems, Taylor & Frances, ISBN 1560323256, Washington

Stevens, M.P. (1999). Polymer chemistry, Oxford University Press, ISBN 0195124448, Oxford

Authors' data: Prof. Dr. Budin R.[ajka] *, Prof. Dr. Mihelic-Bogdanic A.[lka] **, * Faculty of chemical engineering and technology Zagreb, Croatia, ** Faculty of textile technology, Zagreb, Croatia, rbudin@marie.fkit.hr, amihel@marie. fkit.hr

This Publication has to be referred as: Budin, R. & Mihelic-Bogdanic, A. (2006). Energy Effective Polyester Production, Chapter 09 in DAAAM International Scientific Book 2006, B. Katalinic (Ed.), Published by DAAAM International, ISBN 3-901509-47-X, ISSN 1726-9687, Vienna, Austria

DOI: 10.2507/daaam.scibook.2006.09