Optimisation of the Energy Consumption in the Pulp Refining Operation

Transcription

1 Optmsaton o the Energy Consumpton n the Pulp Renng Operaton Jean-Claude Roux*, Jean-Francs Bloch, Patrce Norter Laboratory o Paper Scence and Graphc Arts Grenoble Insttute o Technology Pagora, 461 rue de la Papetere Sant-Martn d Hères France jean-claude.roux@grenoble-np.r In paper ndustry, the pulp renng operaton enables manuacturng a wde range o brous suspensons (pulps) whch nduce derent paper grades. We propose n ths paper a methodology or optmzng a renng nstallaton. A standard dranage test ( ) characterzes n paper ndustry the rened pulp. For the papermakers, the control o the s consequently o prmary mportance snce t s correlated wth the end-use paper propertes. In ths research, renng trals are carred out on bleached sotwood Krat pulp usng a laboratory dsk rener runnng n a batch mode and low consstency (2% to 6% o mass sold racton). The renng knetcs s analyzed by consderng the evoluton o the uncton o the net mass energy consumed. All the curves exhbt an S-shape. At the same net mass energy consumed, the knetcs curves may der consderng the appled net power. Instead o the net power, a more general concept o the renng ntensty s used here the net tangental orce per crossng pont. 4 values o ths ntensty enable derentatng the expermental trals. It has to be noted that 6 ponts o net mass energy consumed are consdered per ntensty. For the covered range o ntensty, between 1.4 and 4.3 N, the quckest renng knetcs s obtaned or (44.5 +/- 1) whch mnmzes the requred energy consumed at 3.1 kwh/(t. ) or a sotwood pulp. Ths numercal value s 5 to 6 tmes less than the ndustral rato kwh/(t. ) classcally encountered n ndustry or prntng and wrtng paper grades. 1. Introducton In paper ndustry, the pulp renng operaton enables manuacturng a wde spectrum o brous suspensons, Page (1989), Peel (1999). These pulps wll lead to manuactured papers whch wll der by ther physcal, optcal and ltraton propertes. The dranage resstance o the pulp ( ) s correlated wth the end-use paper propertes. Thereore, t s o man mportance to analyze the evoluton o the dranage resstance ndex o the pulp, rom ts ntal 0 to ts nal value. Ths evoluton s nluenced by the renng energy E m and the renng ntensty I. The renng energy reveals how much energy s consumed per mass o bers. The renng ntensty reveals how ths energy s transerred to the mass o bers and what are the resultng eects on bers (shortenng, hydraton, and brllaton). The net tangental orce per crossng pont s chosen to descrbe the renng ntensty on bers among derent other ndces (Roux et al., 2009).

2 On a renng nstallaton runnng n a batch mode and n cycles, the mass o pulp M (n dry solds) and the mean resdence tme τ are known quanttes then the renng cumulatve energy, consumed durng tme t, s gven by the equaton: E m (t) = P net.t /M = P net.n.τ /M (1) So, or a gven net power P net appled n a rener, a decrease o the mass energy E m nduces a decrease o the tme t (or a decrease o the number n o cycles). However, we must take nto account the necessary ncrease o the value to reach the requred end-use paper propertes. That s why we ocused our nterest on the determnaton o the rato E m / ( - ), assocated wth the choce o the renng ntensty. In paper ndustry, or a gven paper grade manuactured, the only known quantty s the mass lux o dry solds. The knowledge o both ths rato and o the lux o mass mples the choce o the adequate net power appled n a rener. In the second paragraph, the materals and methods are presented. Then the thrd paragraph s devoted to the expermental results and ther analyss. A last paragraph dealng wth conclusons and perspectves ends up ths paper. 2. Materals and methods 2-1 The renng nstallaton In order to ollow the evoluton o the n the renng operaton, a bleached sotwood Krat pulp was chosen or our laboratory dsk rener nstallaton. The ntal pulp had the ollowng characterstcs: average weghted ber length L 0 = 1.85 mm, water retenton value (water content nsde the bers) WRV 0 = 91 g o water/100g o dry pulp, dranage resstance ndex 13.4, mass racton o ne elements 2.9%. The pulp mass sold racton was constant at 3.5% (low consstency range) durng the trals. The renng plot enables analyzng the eects o the net renng energy and o the renng ntensty ndependently. Hence, when a net power s chosen, the renng knetcs s studed by ncreasng the net cumulatve energy. Ths can be obtaned by ncreasng the tme t (or the number n o cycles) accordng to equaton (1). 5 cycles or 6 ponts o net cumulatve energy are studed per renng ntensty (ncludng the ntal one) and 4 ntenstes are nvestgated. 2-2 Methodology chosen or the expermental trals A dsk rener s a rotor/stator devce where the parallel plates n ront o each other are tted wth metal bars (colored n plan black), accordng to the Fgure 1. Fgure 1: Example o a perodcal angular sector θ on a plate o a dsk rener.

3 The Fgure 1 dsplays one angular perodcal sector θ, comprsed between the nternal ρ and the external radus ρ e o the dsk plates. Durng ther relatve moton o the rotor n ront o the stator, crossng ponts are generated. We wll only consder ther mean value as they vary wth tme. For an expermental tral at a constant rotaton speed N, a net power s appled on the rener. The renng ntensty chosen n these nvestgatons s the net tangental orce per crossng pont. Consderng the same bars and groove wdths a or both the rotor and the stator, the expresson o the renng ntensty I s gven by the equaton (2): 2 3a.P I = net (2) π 2.N.( ρ 3 ρ 3 e ).sn θ As the rotaton speed N = 25 s -1 and the sector angle θ = 22.5 are kept constant durng the trals, we wll mody the appled net power and the common bar wdth (or groove wdth) n our nvestgatons accordng to Table 1. The net power s equal or the rst and the last tral whereas the bar wdth s the same or the three rst trals. Table 1: Engneerng parameters chosen or the 4 expermental trals. Tral number P net (kw) a (mm) I (N) Fnal E m (kwh/t) Results and analyss The Fgure 2 dsplays the evoluton vs. the net mass energy E m parameterzed by the renng ntensty I I = 3.14 N I = 4.26 N I = 2.06 N I = 1.38 N Em (kwh/t) Fgure 2: Renng knetcs or the 4 expermental trals perormed, each wth a constant renng ntensty I. All the curves exhbt an S-shape wth nlexon ponts. The curves correspondng to trals n 1 and n 2 are nearly superposed and the knetcs o trals n 3 and n 4 are slower than those o trals n 1 and n 2.

4 3.1 Renng knetcs nterpretaton On Fgure 2, the renng ntensty I = 1.38 N (tral n 4) nduces a slow knetcs o the. Furthermore, the net tangental orce per crossng pont s not sucent compared to tral n 1 where I = 3.14 N. Between trals n 1 and n 4, only the bar wdth, a, was moded. Accordng to equaton (2), the bar wdth a, appearng wth an exponent 2, has a hgher nluence on the renng ntensty than the net power. For the tral n 4, the bar wdth a = 3.2 mm s too small and leads to a orce 2.3 tmes less compared to the tral n 1. The orce n tral n 4 s too small or sotwood bers. In the papermakng practce, t s well known that the bar wdth o 4.8 mm s adapted to the ntal length o sotwood bers. Hence, the value o 3.2 mm or the bar wdth s too small or sotwood bers. On the contrary, the three rst trals wth a renng ntensty I comprsed between 2.06 and 4.26 N, enable the pulp to be rened wth an acceptable knetcs. In these cases, the developed orces n the gap clearance o the rener are adapted to sotwood bers. It s nterestng to observe that the 3 nlexon ponts are all obtaned or the same range o values (44.5 +/- 1). However, ther correspondng net renng energes are ncreasng rom 160, 174 to 194 kwh/t, respectvely rom the rst to the thrd tral. The renng ntensty o the thrd tral s 34% less than that o the rst tral (see Table 1) whereas the cumulatve renng energy s ncreasng by 21 % rom the rst to the thrd tral. As the renng knetcs o the thrd tral s slow, the correspondng net cumulatve energy consumed to reach the nlexon pont s the hghest o the three rst trals. The tral n 1 reveals that the optmal value s close to E m = 160 kwh/t wth 43.5 wth an optmal renng ntensty comprsed between I = 3.14 and 4.26 N. 3.2 More general analyss In the precedng paragraph, we were nterested by the most ecent renng knetcs n the vcnty o the nlexon pont. However, or a gven renng ntensty I, at any pont o the S-shape curve, vs. E m, the slope can be determned. For example, an emprcal model leads to a thrd order polynomal uncton or all the our S-shape curves. Ths enables calculatng the slope accordng to equaton (3) where G s a parabolc uncton, n ths case: = G( Em;I) (3) de m A gven net renng energy E m s always lnked wth a unque value o the pulp n an S-shape ncreasng curve, or a gven renng ntensty I. Consequently, another expresson o equaton (3) can be gven by replacng E m by : = F( ;I) (4) de m For the whole set o and renng energes nvestgated n ths expermental campagn, the optmum s reached or the mnmal net renng energy consumed E m to ncrease the value rom ts ntal to ts nal value. Ths optmum s calculated as ollows: Em = 1. de m. (5)

5 Introducng equaton (4) n equaton (5) leads to: E m = 1. F (,I) (6) Ths curve s dsplayed n Fgure 3. Thereore, a graphcal nterpretaton o the optmum can be gven by the mathematcal average o the uncton 1/F(,I), between the ntal and nal values o the o the pulp durng the renng operaton. We may also consder a smple crteron based on the length o the plateau or the mnmum value. dem/ vs. parameterzed by I(N) = 3.0-trangle, 3.3-square, 3.6-cross] dem/ - kwh/(t. ) Fgure 3: Recprocal o the uncton F(, I) vs. n the vcnty o the nlexon pont or 3 renng ntenstes I=3.0 N (trangle); I=3.3 N (square); I=3.6 N (cross). The arrows represent the doman o stablty or the mnmum value. At the end o the precedng paragraph, the optmal renng ntensty was supposed to be ound between I = 3.14 N and I = 4.26 N. Thereore, we decded to nvestgate the range o the renng ntenstes comprsed between 3.0 N to 3.6 N. In ths range, all the curves exhbt a U-shape. The mnmum o the curves s located at values o whch correspond to the nlexon pont and are not dependant on the renng ntensty. The range o values o or whch the rato de m / s stable s a uncton o the renng ntensty I. We can observe that the curves show a non lnear eect. For the same varaton o the renng ntensty o 0.3N, the curves or I = 3.0 N and I = 3.3 N are close rom each other whereas the curves or I = 3.3 N and I = 3.6 N are more separated. The parabolc-shape s the result o the emprcal model (order 3) o the renng knetcs and o ts dervaton. However, the tendency s ndependent o the chosen model. For a gven ntensty, we are nterested by the mnmum value o the rato de m / and by ts stablty around the nlexon pont. The value o the optmal renng ntensty obtaned here s thereore I = 3.6 N. At the nlexon pont, or ths optmal ntensty, the mnmum value o the rato de m / reaches 3.1 kwh/(t. ). From equaton (6), or the optmal ntensty, on the nterval speced (45 +/ 10), the rato de m / has a value o 3.2 kwh/(t. ). Now, one extends the nterval o values by (45 +/- 20), the global energy needed ncreases. In ths case, the optmal

6 soluton s ound to be 3.5 kwh/(t. ). Ths soluton s roughly 5 to 6 tmes less than the rato ound n the paper lterature or prntng and paper grades, or example, whch s ound to be between 15 and 20 kwh/(t. ). 4. Conclusons and perspectves In the low consstency renng o ber suspensons, the knowledge o the knetcs s o prmary mportance or the papermaker as t allows the ndustral control o the renng operaton. Indeed, the nal dranage ndex s assocated to the end-use paper propertes. The requred energy consumpton per mass o bers can be determned. Moreover, ths renng energy s overestmated, t can lead to a waste o energy n a renng nstallaton. In order to determne the value o the optmal renng energy, expermental nvestgatons must be perormed on the studed rener nstallaton runnng n batch mode. For a gven renng ntensty, the knetcs s analyzed or derent renng energes. By plottng the rato de m / vs, parameterzed by the renng ntensty I, the optmal renng ntensty was determned at 3.6 N leadng to the mnmum o the rato de m / at 3.1 kwh/(t. ). Then, consderng a varaton o (45 +/- 20), the average rato was ound at 3.5 kwh/(t. ) or the sotwood pulp. Ths value s 5 to 6 tmes less than the rato n the paper lterature whch s ound to be between 15 and 20 kwh/(t. ) or the prntng and wrtng paper grades or example. The expermentaton presented here can be extended to any knd o bers n order to determne the best adapted renng ntensty (or net power) or the most ecent renng knetcs assocated wth the smallest net renng energy. Reerences Page D.H., 1989, Fundamentals o Papermakng, Vol. 1: The Beatng o Chemcal Pulps the Acton and the Eect, Eds. Baker C.F. and Punton V.W., Mechancal Engneerng Publcatons Lmted, Cambrdge, England, 1-38 Peel J.D., Eds., 1999, Paper Scence and Paper Manuacture. Angus Wlde Publcatons, Vancouver, Canada. Roux J.C., Bloch J.F., Bordn R. and Norter P., 2009, Advances n Pulp and Paper Research, Vol. 1: The Net Normal Force per Crossng Pont: a Uned Concept or the Low Consstency Renng o Pulp Suspensons, Eds. S.J. l Anson, Oxord, England, 51-83