MECHANICAL PULPING REFINER MECHANICAL PULPS

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1 MECHANICAL PULPING REFINER MECHANICAL PULPS Histoy of efine mechanical pulping Fo many yeas all mechanical pulp was made fom stone goundwood (SGW). This equied whole logs. Stating in the 950s, but eally gowing in the 970s, wood chips athe than logs became the pincipal souce of mechanical pulp though the intoduction of chip efines. At fist these wee unpessuized disc efines (RMP), then late high tempeatue, pessuized efines (TMP), followed late by efines with vaious chemical peteatments and post teatments, sometimes with pulp bightening (BCTMP). The combinations of efine type, tempeatue, and chemical teatment ae now so numeous that thei aconyms ae called alphabet pulps. Some of the significant dates of this development ae: 957 Stoa (Sweden) installed a Defibato affinato. Baue soon afte installed one in the U.S. 963 Both companies modified to opeate unde pessue to make Themo-mechanical pulp 970 s Fist 00% TMP newspint 980 s -stage efining and heat ecovey 985 Lage efines 5MW. Chemicals added to futhe soften lignin (CTMP). Mechanical pulps ae eplacing chemical pulps Today: Two companies left: Metso and Anditz. Mechanical pulps ae moving up the value chain, eplacing chemical pulps in many applications. Refine Mechanical Pulp (RMP). Desciption In this type of mechanical pulping, chips ae fed into a efine which consists of two disc plates facing one anothe with bas on each plate. The discs otate elative to one anothe, and the consequent ba cossings ceate the mechanical action of efining. The chips ae fed into the eye of

2 the efine, and pass adially outwad though the gap between the plates, as shown in Fig 3. A efine is shown in coss-section in Fig 4. Duing passage though a efine, the chips ae comminuted into small chunks, then to individual fibes. The fibes ae then futhe developed by emoval of oute pats of the cell wall and by flexibilization. Studies have shown that most of the wood comminution takes place ealy in the efine, nea the beake ba section. The esulting fibes and fibe bundles then dape ove bas as shown in Fig 5. Ba cossings futhe beak them down and develop the individual fibes. To accomplish these tasks, the ba patten on efine plates diffe in the adial diection. Fo example, thee is a coase beake ba section in the inne adius whee most of the comminution takes place. This is followed by an intemediate and fine ba section pattens. Hee most of the fibe development takes place. Figue: Refine coss section, typical efine plate and entie efine. Page

3 Duing a ba cossing, fibes ae held on the ba edge by eaction foces fom the applied compession imposed by the opposing moving ba. In contast, in stone goundwood, the fibes ae held in place by the wood matix as foces ae applied. This diffeing holding mechanism may well account fo the majo diffeences between SGW and RMP pulps. The shote fibes in SGW may esult fom the unyielding estaint of the wood matix when foce is applied. The RMP has moe give. Thus, fibes may be eleased fom the ba edge instead of being uptued. Though epeated impacts of the cossing bas the wall of the fibes stats to beakdown. The oute pat of the wall becomes fibillated as layes of micofibils ae peeled away, significantly inceasing the suface aea of the fibes. The inne wall becomes delaminated which inceases the confomability of the fibes. The inceased aea and confomability inceases the bond aea and stength when foming pape, making a much stonge, highe tensile stength pape. Figue: Intenal and extenal delaminations Page 3

4 A lage amount of enegy is expended in efining (see late). This geneates a vey lage amount of steam, equiing that dilution wate be continually added to maintain the desied consistency, which in the case of efine pulps is about 30%. The steam geneation eaches a maximum at some adial point, as shown in Fig 6. The steam may flow adially inwad and outwad fom this point. If the inwad steam flow is too geat, chips may not be able to ente the efine due to blow back. This epesents a constaint on efine opeation. Figue 6 Page 4

5 . Types of Refines Thee ae essentially thee types of efines used to make mechanical pulps:. Single disc: One of the fist, still used in new installations. Lowest capital cost has smalle capacities than othes.. Double disc: High intensity efining, less enegy to obtain he same feeness. Howeve thee is a cost pulp is moe like SGW than efine pulp. 3. Twin disc: Moe like a lage single disc. Low intensities possible because of lage efining suface. Some opeational poblems like feeding both sides equally. Page 5

6 .3 Pinciples Mechanical pulping is accomplished by the imposition of cyclic foces on wood. The pocess is complex and is still unde study. Given this incomplete knowledge, it is only possible to chaacteize the pocess by some basic quantitative paametes. Specific Enegy The fist and most impotant quantitative paamete to chaacteize efining action is specific enegy, the net enegy expended pe unit mass in poducing the pulp. This paamete is widely used and is vey useful. The specific enegies fo vaious types of mechanical pulp fall in the ange 5-7 GJ/tonne. It is calculated by dividing the powe input, P, by the fibe mass thoughput, F, i.e., E P F P is the powe that gets applied to the pulp and is typically given as the total powe, P t, minus the no load powe, P 0, which is the powe to un the efine with the plates back completely off. The no load powe is suppose to epesent the powe that goes into mechanical losses. P P P t 0 Intensity of Refining - Miles and May theoy Since the pocess is one of cyclic application of foces, the enegy may be divided into a numbe and intensity of loading cycles, with intensity being the enegy impated duing each cycle. These have only ecently been successfully quantified. A key facto in doing so has been the chaacteization is the esidence time of pulp in the efine. At 30% consistency, pulp is a discontinuous, heteogeneous, thee-phase system of fibes, wate, ai, and steam. Thus, the esidence time must be deived fom a foce balance on pulp taking into account centifugal foces, Page 6

7 fictional dag foces, and aeodynamic dag foces fom the steam flow which may flow adially inwad o outwad. This foce balance and the deivation of efining intensity fo mechanical pulp efines is shown in Fig below. Figue: Schematic of pulp flow in efine Conside an element of pulp, dm, at adius,, and conduct a simple foce balance: F C F F + bs Hee, C is the centifugal foce, F ae fiction foces between the pulp and discs and S is the steam dag. The value b is + if the steam is flowing fowad out of the efine o is - if the steam is flowing backwad. S0 at the stagnation point. Centifugal foces: ( ω ) C dm Fiction foces: Page 7

8 ( µ µ ) F F T + + () Whee: T() is the total nomal thust on the pulp in the annulus. m, µ Radial coefficients of fiction between the pulp and discs, espectively. If we assume that the adial components of fiction ae the same, µ, and we know the pessue in the annulus, P m, then F 4πµ P d m Steam Dag: () () () S ρu Cf Ap dm Whee: C f is dag coefficient U is the velocity of steam A p Aeodynamic specific suface of the pulp This esults in the following diffeential equation fo the velocity of the pulp though the efine, dv b dm dm P d U C A dm dt () () ω 4 πµ m() + ρ () f p() () We then elate the wet mass, M( ) to the oven dy weight and the consistency, as Page 8

9 M m c () Then by mulitiplying by dt and dividing by d we get: 4 πµ P ()() c () d v m v dv ω m b c + ρu () Cf Ap() If we assume that the powe dissipated pe unit ae is constant then the powe dissipated in the annullus is P π meda ( ) Whee, - E is the total specific enegy applied to the efine, - and ae the inne and oute adii of the efining zone - da is the aea of the annulus. Powe is assumed to be dissipated though the tangential fiction foce. Then, P () hf ω f t P () hµ P () ω f t m Page 9

10 Whee h fo single disc efine and fo double disc efines. m t is the fiction in the tangential diection. And if we can assume that powe dissipated in feeding the pulp in the adial diection is small then Pf( ) P( ). P () m me ( ) π µ hω t Still need the consistency in the efine. We know it vaies though the efine because we cae continuously geneating steam. We can appoximate it as: C () ( cl ) i ( ) ( ) i L cl Assuming that the latent heat, L, does not change with steam pessue. c i is the initial consistency enteing the efine. Now we can get the esidence time in the efine by integatng the velocity. τ d v This can be done numeically. The aveage numbe of impacts can be calculated as Page 0

11 ( + ) n Nhω τ Whee N is the aveage numbe of bas pe unit length on the efine plate. The aveage specific enegy pe impact (intensity) is: ] e E τ Note: µ 0.5 µ 0.75 t 0.33 Simplified Equations fo Residence time Thee is also a simplified pulp flow equation to pedict esidence time. 4 πµ P ()() c ( ) d v m v dv ω m c ± ρu () Cf Ap() The dag and ush can be assumed to appoximately balance out and thus cancel. Futhe, we can assume that dv / d is small (0) with espect to the othe emaining tems in the above equation, i.e., the centifugal and fiction tems balance. Then, the simplified elation fo esidence time is given by: Page

12 τ µ µ t 3 ω aec i L [ ( ) ] + L ci E ln ln L c E L i And the numbe of impacts: n ( + ) τ Nh ω whee ω is the otational speed (ad/s), c i is the inlet consistency, L is the latent heat of steam, E is the specific enegy, is the inne adius of the efining zone, is the oute adius, and N is the ba density. Use the following additional data fo you calculations: µ /µ t 0.33 L 00 kj/kg a (single-disc efine) h (single disc efine) h (double-disc efine) a (single disc efine) a 4 (double-disc efine) Page