Impact of heat recovery and resource diversification in industrial process.
Polyesters or non-cellulose organic fibres i.e. synthetics polymers are heterochain macromolecular essences marked by the attendance of carboxylate ester groups in the resuming units of their main chain (Rodriguez, 1996, Budin & Mihelic-Bogdanic, 2006). These most common polymers find commercial use as fibres, plastics and coating (Stevens, 1999). Because of its very good properties like heat stability and resistance to wrinkling, polyester is known to be one of the main industrial polymers (Edlund & Albertsson, 2003, Bohm et al., 2003). Previously polyester production was based on dimethyl ester polymerization. At present, the most widely used terephtalic acid process is based on cobalt catalyzed air oxidation of paraxylene in nitric acid (Rodriguez, 1996, Stevens, 1999). Continuous polymerization of polyester contains many procedures, where a significant percentage of thermal and electrical energy as well as water is applied (Budin & Mihelic-Bogdanic, 1997, Budin & Mihelic-Bogdanic, 2006).Currently, a large amount of process condensate and hot flue gases are rejected to the surrounding. On a large scale, energy conservation is an important element of energy policy including decreasing the quantity of energy used while achieving a similar outcome of end use (Mihelic-Bogdanic & Budin, 2008).A strong relation exist between energy efficiency and environmental impact, less resource utilization and pollution is normally associated with increased energy efficiency (Budin & Mihelic-Bogdanic, 2006). It is possible to reduce energy cost by using secondary source which could be recovered. One capable method for improving the technological process of polyester production is the implementation of flue gases heat recovery for combustion air preheating (Mihelic-Bogdanic & Budin, 2002, Pulat et al., 2009). Except reuse of waste flue gases the potential substitution of natural gas with solar energy will be also discussed. Solar energy is an intermittent source, most applications require backup energy source to ensure that the requirements of industry are met even on cloudy days and days with insufficient solar radiation. Therefore the solar hybrid system is analyzed because that renewable source can be collected at any reasonable temperature level and utilized in a variety of way (Mihelic-Bogdanic,et al.,1996). The hybrid system with solar collectors and an air preheater is analyzed. This two options will be represented and discussed, one which uses the heat energy from the flue gases and the other together with solar collectors.
2. Data analysis methods
Analyzed plant is designed to manufacture polyester at rate [D.sub.P]=45000 kg/day. The plant use factor is [beta] =83%, or [tau]=7272 h, because the plant is closed two months yearly (Production plant data, 2004).This technology which consists of process operations in autoclave reactor, filters, dryers, esterification reactor, crystallizer, melter, rollers, transesterification reactor, polymerizer, polymer melt, tow drawing, crimper, cutter and baler uses a large amount of electrical energy, steam and water (Fig.1.).The dry saturated steam [t.sub.S]=132[degrees]C (3 bar) in mass of [d.sub.S]=2,3kgS/[kg.sub.P] generated in a natural gas fueled boiler with efficiency [[eta].sub.B] =82% is transferred to the autoclave reactor where terephtalic acid is produced. Water enters the boiler with [t.sub.B] = 15[degrees]C.
[FIGURE 1 OMITTED]
The composition of natural gas by volume is: C[H.sub.4]:[C.sub.2][H.sub.6]:[C.sub.3][H.sub.8]: [C.sub.4][H.sub.10]:[C.sub.5][H.sub.12]:C[O.sub.2]: [N.sub.2]=98,05:0,36:0,12:0,05:0,01:0,85:0,56 and the lower heating value becomes HL=35507 kJ/[m.sup.3] (Potter & Somerton, 1993). Natural gas is burned with excess air coefficient [alpha] =1,2. The hourly steam production is given by the relation:
[D.sub.S]=[d.sub.S]*[D.sub.P]=2,3*45000=103500 [kg.sub.S]/day=4313 [kg.sub.S]/h (1)
and the heat transferred to the boiler is:
[Q.sub.B]=[D.sub.S]([h.sub.S]-[h.sub.B])=4313 (2723-62,8)=11,47*[10.sup.6]kJ/h=275*[10.sup.6]kJ/ day=83,3*[10.sup.9]kJ/year (2)
where the steam and feed water enthalpies are taken from (Budin & Mihelic-Bogdanic, 2002).The volume of natural gas requirement after heat balance is:
[V.sub.F]=[Q.sub.B]/[[eta].sub.B]*[H.sub.L]=11, 47*[10.sup.6]/0,82*35507=349[m.sup.3]/h=9456[m.sup.3]/day (3)
The total fuel consumption, using plant use factor is:
[V.sub.FY]=[V.sub.F]*[tau]=349*7272=2,86* [10.sup.6][m.sup.3]/year (4)
The whole heat condensate from autoclave reactor with temperature tC=820C is withdrawn to the surrounding:
[Q.sub.C]=[D.sub.S]*[h.sub.S2.sup.0]=4313* 343,3=1,48*[10.sup.6] kJ/h (5)
The stoichiometric volume of combustion air (Potter & Somerton, 1993) is:
[V.sub.a]=[[SIGMA][r.sub.m](x+y/4)]/ 21=9,5[m.sup.3.sub.a]/[m.sup.3.sub.F] (6)
where: [r.sup.m]=volume part of each fuel constituent (%), x,y number of C and H atoms respectively in each fuel constituent. So, the actual combustion air flow becomes:
[V.sub.a[alpha]]=[V.sub.a]*[alpha]=9,5*1,2=11, 4 [m.sup.3.sub.a]/[m.sup.3.sub.F] (7)
The total volume of flue gases which consists of C[O.sub.2], [H.sub.2]O, [N.sub.2] and [O.sub.2] is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (8)
while d=13 g/[m.sup.3] is water vapor content in air and [rho]=0,805 kg/[m.sup.3] is density of water vapour in air. In an inefficient process flue gases in amount of:
[V.sub.h]=[V.sub.FG]*[V.sub.F]=12,4* 349=4885,6 [m.sup.3.sub.FG]/h (9)
with temperature [t.sub.FG]=204[degrees]C is rejected to the atmosphere.
3. Air preheating with flue gases
The high temperature of boiler exhaust flue gases [t.sub.B] = 204[degrees]C present a problem of clean energy generation. Flue gases temperature of industrial natural gas fired boiler can be lowered using an air preheater system (Fig.2.).The heat of flue gases will be transferred to preheat the incoming ambient air that is essential for fuel combustion (Budin & Mihelic-Bogdanic, 1997, Mihelic-Bogdanic & Budin, 2008, Pulat et al.,2009).
Ambient air with [t.sub.aAi] = 20[degrees]C enters the air preheater with efficiency [[eta].sub.A] = 90%, where is heated with flue gases in amount of VFG = 12,4 [m.sup.3.sub.FG]/ [m.sup.3.sub.F] that inlet temperature is [t.sub.FGi] = 204[degrees]C. The specific heat of air is [c.sub.pa] = 1,29 kJ/[m.sup.3]-deg and for flue gases [c.sub.pFG] = 1,379 kJ/[m.sup.3]-deg.
[FIGURE 2 OMITTED]
The outlet air temperature [t.sub.ao] and flue gases outlet temperature [t.sub.FGo], from the air preheater balance:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (10)
become: [t.sub.ao]=185,6[degrees]C; [t.sub.FGo]=61,58[degrees]C.
So, the heat transferred to air preheater from flue gases to ambient air respecting fuel consumption is:
[Q.sub.FGA]= [V.sub.F]*[V.sub.FG]* [C.sub.pFG]([t.sub.FGAi]-[t.sub.FGAo])=394* 12,4*1,379(204-61,58)=959,52*[10.sup.3]kJ/h (11)
or conveyed as natural gas savings:
[F.sub.FS]=[Q.sub.FGA]/[H.sub.L]=959,52*[10.sup.3]/ 35507=27,02 [m.sup.3.sub.F]/h (12)
The fuel consumption is now:
[V.sub.FA]=[V.sub.F]-[V.sub.FS] =394-27, 02=366,98 [m.sup.3.sub.F]/h (13)
The heat consumption for dry saturated steam production is:
[Q.sub.BA] = [V.sub.FA]*[[eta].sub.B]* [H.sub.L]=366,98*0,82*35507=10,68*[10.sup.6]kJ/ h=256,3*[10.sup.6]kJ/day (14)
The natural gas savings in comparison with the process without flue gases heat recovery is:
[S.sub.FG]=([V.sub.F]-[V.sub.FA])/[V.sub.F]=(394-366,98)/ 394=0,0686-0,07 i.e. 7% (15)
The volume of flue gases in this case is:
[V.sub.FGA]=[V.sub.FG]*[V.sub.FA]=12,4* 366,98=4550,5 [m.sup.3.sub.FG]h (16)
The volume of flue gases becomes lower by about 7 % and at the same time the gases are cooled from 204[degrees]C to 61,58[degrees]C.
4. Application of solar energy
The natural gas consumption in polyester production could be reduced applying solar energy (Mihelic-Bogdanic, et al., 2000). The implementation of this friendly available and environmental benign source as a part of substitution for natural gas in using air preheater is presented. For that reason it is essential to know the potential location of the plant and the appropriate meteorological data. This hybrid system is assumed to be located near Zagreb ([phi] = 45[degrees]49'N), Croatia.
Ground data for monthly average daily radiation on horizontal surface were computed from the daily hourly radiation measurements and are shown in Fig.3.(Croatian Meteorological Bulletin, 2006).
[FIGURE 3 OMITTED]
The useful solar radiation is computed (Tab. 1.) by the equation:
where [Q.sub.r] is the average daily solar radiation, [[eta].sub.C] is collector efficiency which varies during the year from 25% in January, February Mart, October, November December to 65% in June, July, August while in April, May and September this value becomes 50% (ASHRAE, 1996).Collector area is calculated as:
A=F*[Q.sub.BA]/[Q.sub.U]=1,25 * 256,3*[10.sup.6]/ [Q.sub.U]= 320,4*[10.sup.6]/[Q.sub.U] (18)
where F=1,25 is the security factor for collector installation and QBA is daily heat consumption in plant with air preheater. using useful daily radiation, minimum flat plate collector area of 23,13*[10.sup.3][m.sup.2], number of days in a month, monthly useful solar energy becomes (Tab.1.):
Two months January and December are omitted while the plant doesn't work in this period. The total solar yearly useful radiation is Qy = 49,77 *[10.sup.9] kJ.
Solarized system with air preheater in polyester production is presented in Fig.4.
[FIGURE 4 OMITTED]
This concept includes collector fields, heat exchanger, boiler, waste heat from flue gases. Steam boiler is heated by energy from solar collectors and air for combustion is preheated with waste flue gases. Such approach makes possible to reduce natural gas consumption. using solar heat together with flue gases heat recovery the natural gas consumption is:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (20)
Comparison with process without flue gases heat recovery and solar radiation shows savings of:
[S.sub.FGS]=([V.sub.F]-[V.sub.FAS])/ [V.sub.F]= (394-158,4)/394=0,598 i.e. 59,8% (21)
The volume of flue gases is this case is:
[V.sub.FGAS]=[V.sub.FG]*[V.sub.FAS]= 12,4*158, 4=1964,16 [m.sup.3.sub.FG]/h (22)
The implementation of flue gases heat recovery for combustion air preheating in common with solar energy in polyester manufacturing has been recognized as a potential option to improve energy efficiency. From the calculation results it is visible that the increasing efficiency in the proposed hybrid system is remarkable. To attain sustainable development much effort must be devoted not only to discovering satisfying energy resources but also to increasing the efficiencies of processes utilization these resources. For that reason successfully recovered waste heat directly substitutes purchased energy and therefore reduces the fuel consumption. Also, except improved energy efficiency heat recovery provides one of the greatest opportunities for cost effective reduction in pollution and improvement in energy security. Energy management i.e. efficiency, economy and environmental protection are the most important issues in any industrial process. In presented optimization procedure the impact energy sources diversification which results with fuel consumption lowering in polyester manufacturing is curried out. The results indicate that energy consumption after applying process with flue gases heat recovery saves natural gas by about 7% while the thermal air pollution is lowered from 4885,6[m.sup.3.sub.FG]/h to 4550,5 [m.sup.3.sub.FG]/h. Results of the evaluated study where in solarized process using minimum collector area of 23,13*[10.sup.3][m.sup.2] and air preheater are implemented show natural gas savings of about 60%. Also the volume of exhaust flue gases is reduced to 1964,1 [m.sup.3.sub.FG]/h. In any case, the worthwhile energy management using waste heat recovery and solar heat is often the most economical solution to energy shortages and is a more environmentally benign alternative to increased energy production and also an important part of lessening climate change.
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This Publication has to be referred as: Mihelic-Bogdanic, A[lka] & Budin, R[ajka] (2009).Impact of Heat Recovery and Resource Diversification in Industrial Process, Chapter 12 in DAAAM International Scientific Book 2009, pp. 107-116, B. Katalinic (Ed.), Published by DAAAM International, ISBN 978-3-901509-69-8, ISSN 1726-9687, Vienna, Austria
Authors' data: Univ.Prof. Dipl.-Ing.Dr.techn. Mihelic-Bogdanic, A[lka] *; Univ.Prof. Dipl.-Ing.Dr.techn. Budin, R[ajka] **, * Faculty of Textile Technology, University of Zagreb, Savska 16, 10000, Zagreb, Croatia, ** Faculty of Chemical Engineering and Technology, University of Zagreb, Savska 16, 10000, Zagreb, Croatia, email@example.com, firstname.lastname@example.org
Tab. 1. Solarized process values Month [Q.sup.U] * [10.sup.3] A * [10.sup.3] [d.sub.m] kJ/[m.sup.2]day [m.sup.2] days February 1,63 196,6 28 Mart 2,43 131,85 31 April 7,4 43,3 30 May 9,65 33,3 31 June 13,40 23,91 30 July 13,85 23,13 31 August 12,6 26,35 31 September 7,00 45,8 30 October 2,1 152,6 31 November 0,90 356,0 30 Month [Q.sub.m] * [10.sup.9] kJ/month February 1,06 Mart 1,74 April 5,13 May 6,90 June 9,30 July 9,93 August 8,72 September 4,86 October 1,50 November 0,63 [Q.sub.y] = 49,77 * [10.sup.9] kJ/year
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|Title Annotation:||Chapter 12|
|Author:||Mihelic-Bogdanic, A.; Budin, R.|
|Publication:||DAAAM International Scientific Book|
|Date:||Jan 1, 2009|
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