THE EXERGY ANALYSIS OF THE INDUSTRIAL SYSTEMS

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1 THE EXERGY ANALYSIS OF THE INUSTRIAL SYSTEMS 75 THE EXERGY ANALYSIS OF THE INUSTRIAL SYSTEMS Ile SIMINICEANU, Correspondg Member of the Romanan Academy of Techncal Scences, Gh. Asach Techncal Unversty of Iaş Abstract: Ths paper deals wth the exergy analyss of a ntrc acd plant wth ammona oxdaton at normal pressure, and chemcal absorpton at a pressure of 5 bar (/5 process).the exergy balance model establshed prevously for open chemcal systems, and the correspondg exergy effcency have been used, together wth the measured values of operatg parameters (temperatures, pressures, flow rates, and concentratons). The exergy effcency of the /5 ntrc acd plant determed ths work s 3.5%. The total exergy lost and destructed by rreversblty was 556 kw from the 74 kw put. The largest contrbutons had the ammona oxdaton reactor (44. 97%), the absorpton reacton column (. 7%), the steam over heater (9. 5%), the water pre-heater (5.77%), and the turbe (4.5 %).The ways to mprove the effcency of these operatons are also dscussed. Keywords: exergy balance, exergetc effcency, exergy destroyed, exergy lost, ntrc acd plant.. INTROUCTION Tradtonally, most of the dustral systems have been desgned usg the energetc performance crtera based on the frst law of thermodynamcs only. Exergy analyss combes the frst and the second laws of thermodynamcs, and s a powerful tool for analyzg both quantty and the qualty of energy utlzaton. Today, the exergy s consdered the best global energy sustaablty dcator []. The word exergy was troduced by Zoran Rant (94-97), and defed as the maxmum techncal avalable work obtaable whle the system communcates wth the envronment reversbly []. The exergy analyss method has been extended to chemcal and metallurgcal processes due to the method and values of standard chemcal exergy of substances publshed by Kotas [4] and Szargut[5]. Some authors avoded the word exergy by usg avalablty or second law analyss [3, 6]. Fally Amercan school accepted the term exergy whch was consecrated through the ternatonal journal wth the same name. In Romana, the method was frst appled to ammona manufacture processes [7, 8]. In September 4, Scence drect dcated 436 publshed works on exergy analyss, 99 before 995 wth a maxmum of 85 papers 4. A steady crease of number of works reveals the mportance of the subject. Recent works recognze the usefulness of the exergy functon as a global energy sustaablty dcator, as well as for the envronmental mpact estmaton [, 9]. Among the numerous works on exergy analyss, there are a few papers dedcated to the ntrc acd manufacturg plants [,, ]. The frst s due to rofessor enbgh, recognzed as a foundg fathers of Chemcal Reacton Engeerg. Hs poneerg paper [] s cted by Rekert []. enbgh dscussed the mportance of the second law effcency of the chemcal processes, and gave as example the ntrc acd manufacture. He used a smplfed black box of the plant and obtaed a second law effcency of 6. %. Ths means that 94% of exergy, a measure of energy qualty, was destroyed. Ths could be unbelevable because any ntrc acd producton plant s self suffcent energy. Moreover, t s a net exporter of energy. For each tone of ntrc acd, the plant generates.5 to.4 tones of steam wth 4 bar and 673 K. The second who appled the avalablty analyss to a ntrc acd plant was rofessor Rekert []. He used a more detaled analyss, takg to account the prevous synthess of ammona from methane, ar and water. For the ntrc acd manufacture, usg an absorpton column at 5 bar, he obtaed an exergy effcency of 5.53 %. Fally, we must menton a more recent work [] where the exergy effcency of a sobarc hgh pressure / ntrc acd plant was 4.7%. In ths paper, the exergy analyss of a /5 ntrc acd plant s carred out. The measured and calculated quanttes are presented the tables to 3. The equatons used for calculatons are derved and presented the Annex. The overall exergy effcency of the ntrc acd process was of 3.5 %.

2 76 EZVOLTAREA URABILĂ FAVORABILĂ INCLUZIUNII. EXERIMENTAL AN COMUTING METHOS The measurements have been done on the full scale ntrc acd plant presented Fg.. Ths mples two chemcal transformatons: ammona oxdaton on a platum- rhodum (5 to % rhodum) gauge as catalyst ( I ), and ntrc oxde absorpton- oxdaton water (II) : NH 3 + 5/4 O = NO + 3/ H O NO + ½ H O + 3/ O = HNO 3 (I) (II) The overall reacton (III= I + II) s: NH 3 + O = HNO 3 + H O (III) The two transformatons can be operated at pressures varyg from to bars. The stochometry shows that equlbrum of (I) needs low pressure, whle (II) s tensfed by hgh pressure. Therefore, the old sobarc (/) plant was replaced by dual /5 plants. ressure was steadly creased both steps to tensfy processes and reduce the volume of equpment. Therefore, plants 5/5 or 5/ are also possble. In the last, absorpton takes place at bar, absorpton s enhanced, and NO x emssons are reduced. The 3 materal fluxes (noted wth crcles Fg ) are presented the table, all mol/s. The block dagram bellow the fgure has only the puts and outputs of the plant. The puts are: ammona gas (), prmary ar for ammona oxdaton (), secondary ar for NO oxdaton to NO (3), process water for ntrc oxde absorpton (4). There s also the electrcty (pure exergy) to drve the compressors C - C 3 and the condense pump. The outlets are: the ntrc acd (5), the 4 bar steam (), and the waste gas (6). The termedate flow rates have been obtaed from the materal balance of each operaton. The frst two columns of the table clude the measured pressure and temperature of each materal flux. Flux T, K, bar Table. Expermental operatg parameters, and calculated exerges. Flow rate mol/s Exergy kw N O NO H O HNO ,

3 THE EXERGY ANALYSIS OF THE INUSTRIAL SYSTEMS 77 Table (contued) Flux T, K, Flow rate Exergy bar mol/s N O NO H O HNO 3 kw Fg.. Bloc dagram of the ntrc acd plant: C ammona blower, C prmary ar compressor, C3 secondary ar compressor, T turbe, condense pump, R ammona oxdaton reactor, R absorpton-reacton column, M mxg devce, S-S condense separators, H-H7 heat exchangers.

4 78 EZVOLTAREA URABILĂ FAVORABILĂ INCLUZIUNII 3. RESULTS The last column of the table contas the calculated exergy for each flux. The standard chemcal exerges tabulated by Szargut et al.[5)] have been used. In the case of ammona, at envronment temperature and pressure, the total exergy equals the chemcal exergy: 9, mol/s x 337, 9 kj/mol = = 6488 kj/s. For the secondary ar () the total exergy s reduced to the physcal component havg the chemcal composton dentcal to standard envronment, but dfferent temperature (54 K) and pressure (5. bar). Table. Overall exergy balance of the ntrc acd plant Flux E, kw % Flux E, kw % Ammona Acd rocess water 8.. Steam Electrcty -Compressor C.3 Lost exergy Compressor C Compressor C η E =.35 Total 74. Total 74. Table 3 presents the exergy destroyed(e ) and the exergy lost (E L ) the unt operatons of the analyzed ntrc acd plant. The equatons derved the Annex have been appled. The hghest effcences have the followg fve operatons: - Ammona oxdaton reactor ( the burner ) 44.97% of the total rreversblty ; - Absorpton- oxdaton reactor ( the column ).7 %; - The super- heater of the steam.5; - The water pre- heater 5.77 % - The turbe 4.5 %. Table 3. Exergy destroyed(e ) and exergy lost (E L ) the unt operatons of the ntrc acd plant Nr. Flux E, E L kw Ψ % Φ % Waste gas Ammona compressor C - Electro-mechanc lost - Internal destructons 3 Ar compressor C - Electro-mechanc lost - Internal destructons 4 Gas compressor C3 - Electro-mechanc lost - Internal destructons 5 Turbe - Electro-mechanc lost - Internal destructons Super heater Oxdaton reactor R - Heat lost ternal rreversblty Water pre- heater H Ar pre- heater H Gas cooler H Mxer M Water pre- heater H Heat recover H Gas cooler H Absorpton- reacton column R TOTAL

5 THE EXERGY ANALYSIS OF THE INUSTRIAL SYSTEMS CONCLUSIONS The exergy method s llustrated for a ntrc acd plant as dustral system. The exergy effcency obtaed (3.5 %) s hgher than that obtaed prevously by Rekert (5.53 %)[].The dfference has at least two reasons: (a) Rekert consdered the put ammona lqud state. Therefore hs E cludes the vaporsaton enthalpy of ammona. In our plant, the tal ammona s gaseous; (b) The Rekert plant s not descrbed hs paper. Nevertheless t must be of an older generaton because t generated 3.3 folds less steam (per tone of ntrc acd) than ours. Wth these correctons, the Rekert s effcency would be of over 33. %. The ma exergy destructor of the ntrc acd plant s the ammona oxdaton process, wth a contrbuton of 45 % to the total rreversblty. Ths s a catalytc burng at almost K. The next transformaton step, the absorpton column, requres temperatures as low as possble.such a varaton of temperature always decrease the exergy effcency. A smlar stuaton s the heatg systems of resdental buldgs. Here, the boler has a flame temperature of up to 73 K although the room radators requre only 343 K. Therefore t has very low exergy effcency. That s why, Germany the bolers have been replaced, expermentally, by fuel cells [3]. In fact, the world energy system has an exergy effcency of only few percent (< 5%).The entre strategy of energetcs must be changed for sustaablty. A. Exergy Balance Closed Systems AENIX The equaton of exergy balance s obtaed by couplg the energy balance () wth the entropy balance (). K W U U E E E E Q S K () S Q T Indces and are for tal and fal states of the system, whle F for the fronter of the system. The equaton () s multpled by the temperature of the standard envronment T, then s subtracted from (). The equaton (3) s obtaed: F S U U E E E E T S S K K Q QT W T S T F The exergy varaton ( E E ) s the sum:: E gen U U E E E E T S S V E K K V (4) By troducg (3) (4), results: T E T F E Q W V V gen T S gen (5) Equaton (5) s the exergy balance a closed system. Each term s measured J. In a dscontuous chemcal process, the left sde of (5) s the exergy change due to the reacton mxture () (3)

6 8 EZVOLTAREA URABILĂ FAVORABILĂ INCLUZIUNII troduced the reactor for a batch. The exergy change of ths mxture depends only on the states and. For deal mxtures, the addtve rule may be appled: E n f ch E n f ch (6) f and ch are the physcal and chemcal molar exerges of the substance. The frst term on the rght sde of (5) s assocated wth heat transfer to or from the system durg the process T EQ Q T (7) F The second term on the rght sde s assocated wth the net useful work and can be terpreted as the exergy transfer assocated wth the transfer of energy by work ( useful work): E W = W (V V ) (8) The thrd term on the rght sde accounts for the destructon of exergy due to rreversblty wth the system. The exergy destructon s related to the entropy generaton by (9). Ths s also named avalablty destructon, rreversblty (I) or lost work (W dp ). The equaton (9) s also known as the Gouy- Stodola theorem. E = I = W dp = T S gen (9) A convenent form of the exergy balance for closed systems s the rate equaton (), derved from (5). The terms are exergy fluxes, J/s. A. Exergy Balance Open Systems de T dv Q F W E d T () F d In open systems there are molar flow rates as lets n and outlets n. n n ; n n () To extend () at open systems the terms accountg for exergy transfer assocated to the mass exchange between the system and the envronment. The equaton becomes: de T dv Q F W F n n E d T, J/s () F d The contuous processes ( open systems) are usually operated steady state, wth propertes varable wth tme. The start- up and shut- down are exceptons. The balance equaton of steady de state open systems s obtaed from () puttg = : d Or: T QF W F n n E T (3) F E! EQ E EW E (4) The equaton (4) states that the rate at whch exergy s transferred to the open system E E Q must exceed the rate at whch exergy s transferred out E E W. The dfference s the exergy destroyed wth the system because of the rreversblty. The exergy balance s not a conservatve one.

7 THE EXERGY ANALYSIS OF THE INUSTRIAL SYSTEMS 8 A3. Exergetc Effcency To defe the exergetc effcency t s necessary to dentfy both useful products and the puts or resources expended to generate the products. of the system. Both are expressed terms of exergy E, E ).Usg these, the exergy balance (4) become: ( E E E (5) Accordg to the second law, E, and the the rato of products exergy and puts exergy s always less than unty: E E Ths rato depends on the ternal rreversblty of the system. That s why s also named degree of techncal perfecton of the system, or absolute effcency. E E I (6) E (7) E E E Ths s also an overall effcency. If the system s a sequence of unt operatons, the overall effcency (ψ = -η E ) can be dvded a seres : I I I E, or E E E (8) Where I s the rreversblty of the unt operaton. The equaton (8) permts the evaluaton of the relatve contrbuton of each operaton to the overall effcency of the system: I (9) I REFERENCES [] Romero J.C. Exergy as a global energy sustaablty dcator, Renewable. & Sustaable Energy Revews, 33, 4, [] Rant Z., Exerge, e neues Wort fuer technsche Arbetsfaehgket, Forschung auf dem Gebet des geneurwesens, (), 956, [3] Kenney W.F., Energy Conservaton the rocess Industres,Academc ress, Orlando, FL., 984. [4] Kotas T.J., The Exergy Method of Thermal lant Analyss, Butterworths, London, 985. [5] Szargut J., Morrs.R., Steward F.R., Exergy Analyss of Thermal, Chemcal, and Metallurgcal rocesses,sprger Verlag, 988. [6] Moran M.J., Avalablty Analyss: A Gude to Effcent Enery Use, ASME ress, New York, 989. [7] Smceanu I., Exergy analyss of chemcal processes, Mem. Sc. Sect. Romanan Academy, 8() 985, [8] Janu., Smceanu I., Exergy analyss of a chemcal plant, Rev. Chm.(Bucharest), 48(8), 997, [9] Krova- Yordanova Z., Applcaton of the exergy method to the envronmental mpact estmaton, Exergy, 36(6),, [] enbgh K.G., The second law effcency of chemcal processes, Chem. Eng. Sc., 6(), 956, -9. [] Rekert L., The effcency of energy utlzaton chemcal processes, Chem. Eng. Sc., 9, 974, [] Nmkar S., Newada R., Mmzaton of Exergy Losses a Mono- hgh ressure Ntrc Acd rocess, Internatonal Journal of Exergy, 4, ( press). [3] Wter C.-J., Energy effcency no: t s exergy effcency!, Int.Journal of Hydrogen Energy, 3(7), 7, 49-4.

8 8 EZVOLTAREA URABILĂ FAVORABILĂ INCLUZIUNII ANALIZA EXERGETICĂ A SISTEMELOR INUSTRIALE Ile SIMINICEANU, Membru corespondent al Academe de Ştțe Tehnce d Româna Unverstatea Tehncă Gheorghe Asach d Iaş Rezumat: In mod tradtonal, majortatea sstemelor dustrale au fost proectate folosd crterle de performanţă energetce bazate numa pe prma lege a termodamc. Analza exergetcă combă prmul ș al dolea prcpu al termodamc ș este un strument puternc pentru a analza atât canttatea, cât ș caltatea de utlzare a energe. Astăz, exerga este consderată cel ma bun dcator global de sustenabltate energetcă. Exerga a fost deftă de Zoran Rant (94-97) ca maxmă " lucrul mecanc dsponbl tehnc" ce se poate obţe în tmp ce sstemul comuncă cu medul reversbl. rmele aplcaţ ale analze exergetce la procesele chmce au fost realzate de către enbgh ș Rekert. În Româna a fost aplcată la dustra amonaculu. În septembre 4 Scencerec dca un număr de 436 de lucrăr publcate pe subectul "analză exergetcă ", 99 înate de 995, cu un maxm de 85 lucrăr în 4. O creştere constantă a numărulu de lucrăr relevă mportanţa acestu subect. Lucrarea de față se ocupă de analza exergetcă a une stalaţ de acd azotc cu oxdare a amonaculu ș absorbțe, la o presune de 5 bar (5/5 proces). Acesta este un proces nou faţă de cele analzate de către enbgh (/) ș Rekert (/5) ctate ma sus. Modelul blanţulu exergetc stablt anteror pentru sstemele chmce deschse ș randamentul exergetc corespunzător au fost utlzate, împreună cu valorle măsurate ale parametrlor de funcţonare (temperatur, presun, debte ș concentraţ). Randamentul exergetc d stalața de acd azotc 5/5 este de 3.5 %, ma mare decât cel determat de enbgh (. %) ș Rekert (5.53 %), datortă îmbunătățrlor tehnce. Exerga totală perdută ș dstrasa de reversbltate a fost de 556 kw, faţă de trarea de 74 kw. Cele ma mar contrbuţ le-au avut reactorul de oxdare a amonaculu (44.97 %), absorbţa - coloana de reacţe (.7 %), aburul peste încălztor (9.5 %), preîncălztorul de apă (5.77 %) ş turba (4.5 %). Modul de a îmbunătăţ randamentul acestor operaţun este de asemenea dscutat în lucrare.