RETENTION OF FIBERS, FILLERS AND FIBER FINES AT INDIVIDUAL DEWATERING ELEMENTS OF THE GAP FORMER

Transcription

1 RETETIO OF FIBERS, FILLERS AD FIBER FIES AT IDIVIDUAL DEWATERIG ELEMETS OF THE GAP FORMER Mka Kosonen Metso Automaton Jukka Muhonen, Juha S. Knnunen Metso Paper Emal: Emal: PO Box 237 FI Tampere PO Box 587 Fnland FI Jyväskylä Fnland ABSTRACT Ths paper presents a novel method of calculatng the element retenton of ndvdual dewaterng elements. It s based on a mass balance model of the formng and press sectons whch can be constructed when sgnfcant flows and consstences are measured. If dry content of the paper after press s also known, the same model can be used to calculate precse dryness development along the wet end. The proposed method was expermentally verfed n a plot machne producng 52g/m 2 fne paper at a speed of 1200m/mn. Total retenton was vared between 50 75% by adjustng retenton ad dosage. Smultaneously, all the major whte water flows and total, ash, and fnes consstences were measured and element retentons of dfferent stock components were determned. It was found that retenton varaton caused drastc changes n both element retentons and dryness development, whch can also be seen n paper propertes, such as formaton. ITRODUCTIO One of the key varables on the wet end of the paper machne s wre retenton. It has a strong effect on the propertes of the paper produced and machne runnablty as well as on the envronmental load. owadays, the ntroducton of more closed whte water systems smultaneously wth lower grammages and hgher ash contents of paper has made t necessary to pay more attenton to wet end management. [e.g. 3] Conceptually, retenton s wdely understood as the percentage of headbox solds that s conveyed to the press secton. owadays, the most wdely used s the concept of frst-pass retenton, whch can be calculated n a stablzed system from headbox and tray water consstences [4, 13]. owadays, twn-wre formers are used n hgh-qualty prntng paper producton. In a modern roll-blade gap former, the roll secton s responsble for the ntal dewaterng. It gves a unform dewaterng pressure, whch bulds up unformly formed fber layers on the wres, thereby mprovng fnes retenton. On a roll-blade former the proporton of formng roll dewaterng vares between 70 80% of total formng secton dewaterng, dependng on paper grade. [10, 14.] After ntal roll dewaterng, the web s led to the blade secton. Blade dewaterng mproves large scale paper formaton by breakng down fber flocs. Ths effect can be controlled by adjustng the ntensty of shear forces actng on the fber mat. The shear forces act expectedly so far as they exceed the strength of flocs. [10, 15] After the blade secton, water removal s contnued wth varous vacuum dewaterng devces, the vacuum level of whch vares between 20 70kPa [11]. Typcally, web consstency after the formng secton s 16 23%. In ths paper, a new method s proposed for calculatng retenton values for ndvdual dewaterng elements. It s based on a mass balance model of the formng and press sectons, whch can be constructed when sgnfcant water TAPPI Paper Summt 2002

2 flows and ther consstences are known. Ths technque s based on reverse calculaton method, whch provdes a better tolerance on flow measurement errors, especally n bg volumetrc flows. Ths drastcally mproves the precseness of the calculaton and enhances the applcablty of the method. If dry content of the paper after press s also known, the model can also be used to calculate dryness development along the wet end. Ths offers a method to observe water removal at dfferent phases of dewaterng and can be used to present dewaterng element performance n a manner whch s ndependent of bass weght and machne speed. The proposed method was expermentally verfed n a modern plot machne producng 52 g/m 2 fne paper at a speed of 1200m/mn. In the experments, total retenton was vared between 50 75% by adjustng retenton ad dosage. Smultaneously, all the major whte water flows and total, ash, and fnes consstences were measured and element retentons of dfferent stock components,.e. fbers, fllers and fber fnes, were determned. It was found that retenton varaton caused drastc changes n both element retentons and dryness development, whch can also be seen n paper propertes, such as formaton. RETETIO DEFIITIOS In a stablzed system, true total solds retenton s smply the percentage of headbox solds that s conveyed to the press secton. However, the paper ndustry commonly uses dfferent samplng methods to approxmate these values. TAPPI [13] gves three dfferent defntons for retenton: True retenton, Frst-pass retenton, and Comprehensve retenton. In ths secton, the frst two are dscussed n more detal snce the defnton of element retenton s based on these. True retenton The paper machne formng secton dvdes the mass flow dscharged from the headbox nto two fractons: mass flow passng on to the press secton press and flow gong down to the whte water. From the press, the web runs on through the dryers and ends up at the reeler reel. (See Fgure 1.) Formng secton Headbox, A Wre pt R, R A Whte water A Press secton press,apress Dryers ualty measurements reel,areel Fgure 1. Schematc paper machne lay-. TAPPI [13] defnes true paper machne retenton R as follow TAPPI Paper Summt 2002

3 R = ( l p reel l ) s 100% (1) where reel = total mass flow at reel, = total mass flow dscharged from the headbox, l p = sheet wdth to the press, and l s = sheet wdth on the wre (= slce wdth). Retenton can also be calculated for ndvdual mass components. For example, true ash retenton R A s defned as: R = ( l p Areel l ) s A 100% (2) where Areel = ash mass flow at reel, and A = ash mass flow dscharged from the headbox. Mass flows [kg/s] at process waters can be calculated from volumetrc flow f [l/s] and consstency C [g/l] f C = (3) 1000 Correspondngly, total mass flow [kg/s] at reel can be calculated by usng paper dry weght OD [g/m²], paper wdth l [m] and web speed v [m/s] at reel r dry reel OD lr v = 1000 (4) Frst-pass wre retenton Frst-pass wre retenton s defned as [4, 13]: where R FP C C = C *100% C = headbox total consstency, and C = whte water total consstency. (5) Usage of ths estmate s based on the dea that the amount of water removed at wre secton f s nearly equal to the amount dscharged from the headbox f,.e. f f (6) Therefore, n a stablzed system the total mass flows from headbox and to whte water are = f C = f C f C. (7) If we also assume, that all mass comng from the headbox goes ether to the press secton or to the whte water,.e. = + press (8) Then, accordng to fundamental defnton for retenton R = press whch we can easly put nto format of (5). f C f C 100% = 100% 100% f C (9) TAPPI Paper Summt 2002

4 Accordngly, frst-pass ash retenton can be calculated from where R AFP A = A A 100% A = headbox ash consstency, and A = whte water ash consstency. (10) Because approxmately 95% of the water s actually removed at the wre secton, assumpton (6) does not exactly hold true. onetheless, equaton (5) can be modfed by assumng that only a proporton K of the water s removed at formng f = K f Whch leads to a modfed defnton for frst-pass retenton [5]: R FPM C = 1 K 100% C (11) (12) RETETIO OF IDIVIDUAL DEWATERIG ELEMETS Defnton The formng secton ncludes several dewaterng devces. These are placed along the formng and press sectons, thus affectng the formng paper web n a seral manner (see Fgure 2). The nput to the frst devce s the headbox slce flow and put from the last s the paper web delvered to the dryng secton. up 1 up 2 up n 1 Slce flow n n 2 1 Element 1 Element Element To dryers down 1 down 2 down Fgure 2. Schematc dagram of serally placed dewaterng elements. Every dewaterng devce affects the paper web by removng a certan amount of materal from t: mostly water, but also some fbers, fllers and other stock components. Generally, we can say that one dewaterng devce forms a n sngle dewaterng element, whch have a nput flow, put flow and whch removes mass flow. Retenton s defned as a rato between the put and nput flows. Therefore retenton of sngle dewaterng element can be defned as R n = 100% (13) Mass flows also both flows n and n can be determned accordng to (3). It s essental that not only consstences C, n f, f are known, because now assumpton (6) does not hold true snce f f. n Total retenton of all dewaterng elements can then be calculated as a product of ndvdual element retentons C but TAPPI Paper Summt 2002

5 R = R = 1 In fourdrners, where materal removal happens n one drecton, the above s always true. However, n gap formers dewaterng s two-sded. Ths means that there are dewaterng devces placed on both sdes of the paper web, and also that n some places there can be devces facng each other. Wth facng devces, t s very dffcult to determne retenton of ndvdual devce, snce nput flow to a sngle devce s unclear. Therefore, the only opton s to suppose that these devces form sngle two sded element, whch removes materal on both drectons. Then, the total mass removal of ths two sded element s (14) = + up down (15) where up = materal flow removed by the top sde devce, and down = materal flow removed by the bottom sde devce of the web. Of course, t s relevant to consder the sdedness S of materal removal of two sded element S = up down (16) Calculaton procedure To be able to calculate retenton of every dewaterng element, the nput and put flows of each element should be known. onetheless, we have no method to drectly measure web propertes between elements. However, f the flows and consstences are measured, we can determne the followng: mass flow to the frst element,.e. headbox slce mass flow, mass removal of each dewaterng element, and mass flow to the dryers,.e. put flow from the last element. Thus, we have to ndrectly determne mass flows between the elements. We have two optons: Start wth the headbox flow and calculate put of each element by assumng that n = Start wth the flow to dryers and calculate backwards the nput flow of each element wth n = + In practce, reverse calculaton s the dstnctvely more robust one because t s not senstve to flow measurement errors (see Fgure 3). The reason for ths s that by calculatng backwards we start wth small volumetrc flows and gradually proceed to large volumes. evertheless, n drect calculaton we start wth the bggest volumetrc flow,.e. headbox flow, when small percentual error n t drastcally affects all calculatons. A more detaled presentaton of the effect of measurement errors on the retenton and dryness calculatons can be found n [8]. TAPPI Paper Summt 2002

6 Relatve error [%] Press Press Hgh vac. 2nd np 1st np SB Sucton roll Sucton box Blade secton upper part Blade secton lower part Formng roll Head box Fgure 3. Relatve error n calculated web consstences after dewaterng elements, wth 5% naccuracy n each flow measurements (sold lne = reverse method; dashed lne = forward method). [8] The fnal calculaton procedure of total element retenton goes as follows: dry 1. Determne the dry mass producton rate after dryers [kg/s] accordng to (4). reel 2. It can be assumed that the are no materal losses (except water) at dryng secton. Therefore, we can get the put of the last dewaterng element drectly from = (17) press However, f the dewaterng element s placed at press secton, whch s after trm removal, mass flow must be scaled so that mass removal at each element s relatve to the others and corresponds to slce wdth l s = α (18) where α = l s l. p 3. Calculate nput flow to the element n = + (19) where the amount of removed materal can be calculated as shown n Eq. (3). If the dewaterng element s placed at press secton removed mass flow must be scaled wthα : = α. 4. Calculate retenton of the element n R = (20) 5. Repeat steps 3 and 4 for all elements untl retenton of all dewaterng elements are calculated. The same calculaton procedure can be used to determne element retentons of a sngle mass component, such as fbers, fllers or fber fnes. In that case we can calculate the component specfc producton rate after dryers C wth reel TAPPI Paper Summt 2002

7 where dry C reel C % reel = 100 dry reel = s the total dry mass producton rate [kg/s] after dryers C = component content [%] at paper at reel. % (21) Smlarly, component-specfc mass removal C can be calculated from where C C f C = 1000 f = whte water flow from element [l/s], and C C = component specfc consstency n elements whte water [g/l]. (22) DRYESS DEVELOPMET The procedure presented above can also be used to calculate the dryness development along the formng and press sectons. It dffers from the prevous n that now we are nterested n total mass flows where water s ncluded, not only dry stock. It s also requred that now the shower waters at press secton be taken nto account, snce they play a sgnfcant role n water balance. Addtonal measurements necessary for dryness calculatons are: Mosture percentage of the web after press secton PM [%] shower Amount of shower water f. The dryness calculaton procedure goes then as follows: 1. Determne the total flow (water ncluded) from press secton to dryers f, usng the dry mass press dry producton rate (from Eq. 18) and the mosture content after press reel dry f = ( 1 PM 100) (23) press reel shower 2. Measure the total amount of shower waters f added before element but after element -1. If the dewaterng element s placed at press secton, the flow must be scaled wth α : shower shower f = α f. 3. Calculate the total flow before element n shower f = f + f f (24) where f s the amount of whte water removed at element. If the dewaterng element s placed at press, flow must be scaled wthα : f = α f. 4. Calculate dry solds consstency before element from n n n C = ( f ) 100%. (25) 5. Repeat steps 2 4 untl dry content at all phases of dewaterng s calculated. TAPPI Paper Summt 2002

8 EXPERIMETAL RESULTS Experment Desgn The am of the element retenton experments was to exam the senstvty of varous dewaterng elements to retenton ad dosage by usng the method proposed above to calculate the element-specfc retenton values and dryness development. Experments were conducted on a plot paper machne at Metso Technology center at Rautpohja, Fnland. The plot machne was equpped wth Valmet OptFlo headbox, OptFormer wth loadable blade unt and OptPress (see Fgure 4). The headbox was equpped wth turbulence vanes to acheve low MD/CD tensle rato. The formng wres were typcal 2-layer fne paper wres (ar permeablty 5800m 3 /h at pressure drop of 100Pa). Both wres had a tenson of 8.5k/m. Two np press constructon had roll press n the frst np and a shoe np n the second. The dryer secton conssted of two steam heated dryer groups and one mpngement dryng unt Fgure 4. Schematc lays of OptFormer and OptPress. The furnsh was a typcal wood-free fne paper furnsh, consstng of 50/50 hardwood (brch) / softwood (pne) chemcal pulp mxture. Furnsh was refned to 440 CSF. The fller was CaCO 3 and t was added batch-wse to the machne chest to obtan 15% ash content n the paper. A mcropartcle retenton ad system was used durng tests: bentonte clay dosed before the headbox feedng pump and a hgh molecular weght, low catonc charge densty polyacrylamde after the pump. Re-crculated stock was used n the plot machne. It was assumed that pumps and screens cut the long polymer chans so that they dd not contrbute sgnfcantly to flocculaton after re-crculaton. Based on ths assumpton the ntal flocculaton tendency of the crculated furnsh was consdered to be constant and the man effect n retenton was caused by the fresh polymer added before the headbox. As a szng agent catonc starch was added as a batch to the machne chest. Table 1. Key tral parameters. Bass weght 52 g/m 2 Furnsh Chemcal pulp (HW+SW) Fller content 15 % Fller CaCO 3 Machne speed 1200 m/mn Retenton ad 2 component system: Jet/Wre-rato 1.07 C-PAM + Bentonte 4 The most mportant machne parameters are shown n the Table 1. They were kept constant durng all the experments. Paper formaton or other propertes were not optmzed for dfferent retenton levels. The paper samples were reeled from the machne. The dred sheets were then analyzed n an automated paper analyzer (Kajaan PaperLab). Formaton was measured wth the Kajaan formaton tester module, whch determnes TAPPI Paper Summt 2002

9 optcal formaton by measurng lght ntensty varatons through a paper sheet. The measurement method s based on mage analyss. A hgher formaton number means a more unform sheet. Fller content of paper was measured usng TAPPI T211 om-93 standard test method. Table 2. Dewaterng elements for element retenton calculatons. Element number 1. Formng roll 2. Blade secton 3. Sucton dewaterng 4. Press secton To the authors' knowledge, there s no proposed method to measure fnes content of the paper sheet. Therefore, the fnes content of the paper was measured usng TAPPI T261 cm-94 test method. Although the scope of ths method was orgnally to measure the fnes content of stock samples, we attempted to expand the method to measure the fnes content of dred and re-slushed paper samples. Durng the experments, dosage of the retenton polymer was vared. The dosage levels were chosen n such a way that the obtaned retenton levels cover the typcal values used n ndustry. Dosage changes were chosen on the bass of what was presumed to cause sgnfcant changes n total retenton. Bentonte dosage was kept constant durng all trals. Thck stock and fller flow were not altered to compensate for the retenton changes,.e. the level of mechancal retenton of fber network was kept constant. Whte water samples were collected manually from each dewaterng element (see Table 2 and Fgure 4). Total, fller and fnes consstences of whte water samples were analyzed wth standard TAPPI laboratory test methods T240- om-93, T211 om-93 and T261 cm-94, respectvely. Whte water flows were measured wth ether on-lne wer level or magnetc flow measurements. Sngle dewaterng devces were combned as dewaterng elements accordng to Table 2. To measure the statc effect of retenton ad on retenton t s essental that the systems should be n stable condton, durng samplng. Accordng to orman s smplfed short crculaton model [9], the requred tme for system stablzaton depends on the short crculaton volume, whte water flow and retenton level. Components wth lower retenton requre a longer stablzaton tme. Based on prevously publshed results [1, 6] and on experences from the pre-trals the process stablzaton tme was assumed to be mnutes after polymer dosage change. Therefore, the process was allowed to stablze 20 mnutes after each polymer dosage change before sample collecton. Stablzaton was confrmed wth on-lne low consstency analyzers (Kajaan RM). The measured whte water and headbox total consstency responses are shown n Fgure 5. It can be seen that the process was n stable condton after 20 mnutes. Wth the furnsh crculaton tme beng much longer than the short crculaton stablzaton tme, the furnsh composton from the stock preparaton was consdered to be stable durng the tests. TAPPI Paper Summt 2002

10 Whte water total consstency (normalzed) Headbox ash consstency Tme [s] Fgure 5. Retenton ad response dynamcs. Whte Water Consstences To determne fber, fller and fber fnes retenton of ndvdual dewaterng elements, consstences of each mass component n all whte water fracton should be known. Snce the laboratory work needed to fulfll our tral program was relatvely extensve, we decded frst to verfy the need for all tests wth a prelmnary experment. A prelmnary element retenton experment was performed wth same furnsh composton and same machne parameters as n the actual experments. Durng the test, water samples were collected from each dewaterng devce and from headbox stock. Then the total consstency and ash, total fnes, and fber fnes consstences were determned from these samples accordng to TAPPI laboratory test methods T240 om-93, T211 om-93 and T261 cm-94 respectvely. Results from the experment are shown n Fgure 6. It was dscovered, that the fnes content n all whte water fractons was 100%. In other words, whte water nclude no long fbers only fllers and fber fnes. Therefore, to reduce the amount of laboratory work t was concluded that wth reasonable accuracy we can calculate the fnes consstences of the whte water fractons as a dfference between total and ash consstences. However, n determnng fnes content from headbox stock and from paper web, ths assumpton s not vald and T261method should be used. TAPPI Paper Summt 2002

11 100% 90% Whte water composton 80% 70% 60% 50% 40% 30% 20% Total cons. All Fnes Ash Fber Fnes 10% 0% Headbox Formng roll Blade secton Sucton dewaterng Press dewaterng Fgure 6. Mass component composton n the water fractons from dewaterng elements. In the man tral program whte water samples were collected from all dewaterng devces wth fve dfferent retenton levels. Total and ash consstences were measured from the samples and fnes consstences were calculated from these. Results can be seen n Fgures 7 9. It can be seen that all consstences reflect the polymer dosage reasonable well. When the polymer dosage,.e. retenton, s hgh all consstences tend to be hgher than wth lower polymer dosages. It s noteworthy that all consstences react polymer dosage almost dentcally. Although the flow rate n ntal dewaterng s tens of tmes greater than n later phases, the change n the consstency s stll almost the same as at latter elements Average total consstency g/l dosage A dosage B dosage C dosage D dosage E Headbox Formng secton Press secton Fgure 7. Total whte water consstences at dfferent C-PAM dosage levels. TAPPI Paper Summt 2002

12 Average ash consstency g/l Dosage A Dosage B Dosage C Dosage D Dosage E 0.00 Headbox Formng secton Press secton Fgure 8. Whte water ash consstences at dfferent C-PAM dosage levels Fber fnes consstency g/l Dosage A Dosage B Dosage C Dosage D Dosage E 0.00 Headbox Formng secton Press secton Fgure 9. Calculated consstences of fber fnes at dfferent C-PAM dosage levels. Headbox fnes consstency was measured accordng to TAPPI T261 cm-94. Whte Water Flows The amount of water removed by each dewaterng devce s shown n Fgure 10. The greatest changes n flow rates can be seen n ntal dewaterng at the formng roll and at blade secton when fber mat has not yet formed. After ths the flterng resstance s ncreased due to the fber mat formed aganst the wre, and polymer dosage has lttle effect on flow rates. Ths correlates well to prevous experences [2]. Durng the experments headbox flow rate was kept constant at 185 l/s. TAPPI Paper Summt 2002

13 160 Whte water flow l/s Dosage A Dosage B Dosage C Dosage D Dosage E Formng roll Blade secton Sucton dewaterng Press dewaterng Fgure 10. Whte water flows at dewaterng elements at dfferent C-PAM dosage levels. Element Retentons Consstences and flow rates presented above were then used to calculate the retenton of ndvdual dewaterng elements by usng reverse calculaton. In ths method mass balance of the formng and press secton s calculated backwards, by startng wth the paper web mass flow after dryers and then addng up all the mass fractons removed by each dewaterng element. The advantage n ths approach s that t tolerates flow measurements errors sgnfcantly better than forward calculaton, n whch the procedure starts from the headbox. Therefore, startng pont here was to determne the mass fractons of fbers, fllers and fber fnes n the paper produced. Hence, paper samples were taken at every tral pont and percentage values of each mass component were determned by usng TAPPI test methods T240 om-93, T211 om-93 and T261 cm-94. evertheless, the paper fber fnes content proposed by T261 was less than 7% n all tral ponts, whch was n dsagreement wth both the estmate proposed by forward calculaton (~30 40%) as well as the fber fnes content measured n the thck stock (27 32%). Presumably, reason for ths was that T261 method s orgnally proposed to measure the fnes content from stock samples and may not be adequate for the re-pulped paper. Most probably, fber fnes partcles are so strongly attached to the long fbers durng dryng that they wll not detach wth mechancal stress. However, ths method may be approprate f the paper samples are taken after press secton where they are not exposed to heat. Because the fnes content measurements of the paper were clearly faulty, forward calculaton was used to estmate fber fnes retenton. However, the senstvty to flow measurement was reduced by calculatng the fnes mass flow to the formng roll from the headbox fnes contents and the total mass flow to the formng roll. Where the total mass flow was orgnally calculated by usng reverse calculaton. Therefore, the error n the headbox flow measurement dd not nfluence the results. TAPPI Paper Summt 2002

14 100 % 90 % Total retenton % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 % 0 % Dosage A Dosage B Dosage C Dosage D Dosage E Formng roll Blade secton Sucton dewaterng Press dewaterng Fgure 11. Total retenton of ndvdual dewaterng elements. Calculated element-specfc total retenton values are shown n Fgure 11. Generally, the results are very near to what was expected. Retenton values reflect polymer dosage very well and ncrease straghtforwardly wth ncreasng polymer dosage. Development of total retenton at typcal operatng pont s shown as the mass flow dagram n Fgure 12. From both Fgures 11 and 12, t s very evdent that man materal losses occur n ntal dewaterng. Ths s qute natural, snce the amount of removed water s by far the largest at the formng roll. Also, the fltratng effect of the fber mat at ths phase s qute neglgble. Formng roll 29 % Blade Secton 7% Sucton dewaterng 2% Press 1% Paper 61 % Fgure 12. Development of total retenton. Ash retenton values at dfferent polymer dosages are presented n Fgure 13. Generally, ash retenton behaves n a smlar way to total retenton. evertheless, t s noteworthy that there are also sgnfcant ash losses n later phases of dewaterng. Generally, ash reacts strongly on polymer dosage. Ths s natural, because fller partcles are very small and therefore ther probablty of retanng mechancally s very low. Thus, fller partcles are retaned almost totally wth the help of retenton ad, and so ther response to polymer dosage should be emphaszed. Development of ash retenton s shown n Fgure 14. It s remarkable how much of the fller s lost at ntal dewaterng. Almost 60% of ash s already lost at formng roll. Anyway, ths s manly caused by the large volumetrc flow. TAPPI Paper Summt 2002

15 Ash retenton % 100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 % 0 % Dosage A Dosage B Dosage C Dosage D Dosage E Formng roll Blade secton Sucton dewaterng Press dewaterng Fgure 13. Ash retenton of ndvdual dewaterng elements. Formng roll 58 % Blade secton 14 % Sucton dewaterng 4 % Press 2% Paper 22.6 % Fgure 14. Development of ash retenton. The calculated fber fnes retenton values are presented n Fgure 15. Although the results are smlar when compared to the results presented above, t s odd that the retenton values do not follow polymer dosage as expected. The probable reason for ths s, that the forward calculaton method s not accurate enough to dstngush the retenton values whch are, however, qute near to each other. Although the nfluence of error n headbox flow rate was excluded, a small error n formng roll flow rates, for example, may deterorate the precson of the whole calculaton. Therefore, these results do not precsely descrbe the retenton of fber fnes at dfferent polymer dosages but nonetheless gve a good estmate of the total amount of fbrous fnes materal n the system. Generally, fber fnes seems to have relatvely good retenton (~80%), whch s lttle affected by polymer dosage. Ths can also be seen on the mass fracton dagram n Fgure 16. Presumably, ths results from the type of fnes partcles, whch n fne paper stock orgn from chemcal pulp. Typcally, chemcal pulp fnes are fbrllar and lamellar partcles, whch are easly entrapped mechancally nto the formed fber mat. Wth mechancal pulp, the stuaton may be totally dfferent, snce the fnes partcles are typcally chunk and flake-lke partcle wth weak bondng potental [7]. TAPPI Paper Summt 2002

16 Fber fnes retenton % 100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 10 % 0 % Dosage A Dosage B Dosage C Dosage D Dosage E Formng roll Blade secton Sucton dewaterng Press dewaterng Fgure 15. Retenton of fber fnes at ndvdual dewaterng elements. Formng roll 19% Blade Secton 4% Sucton dewaterng 1% Press % Paper 76 % Fgure 16. Development of fber fnes retenton. Generally, t can be sad that the retenton of all fbrous materal n fne paper stock, ncludng fber fnes, s relatvely good. And so, the major part of the materal crculaton n whte water s ash. Also, the changes n total retenton are manly caused by the ash changes and the contrbuton of fbrous materal to t s relatvely modest. Dryness Development The reverse calculaton method was used to calculate web dryness along formng and press sectons. Results are shown n Fgure 17, whch shows the absolute dfferences compared to mnmum consstency value at each element. From Fgure 17 t s clear, that hgh retenton resulted hgher dryness after press secton. Low retenton resulted hgh headbox consstency and lowest dryness after blade secton, formng secton and press secton. The dryness levels were measured after the formng secton and corresponded very well to calculated values. Web dryness after press clearly responds to the polymer dosage level and ncrease straghtforwardly as the polymer dosage ncreases. Presumably, ths results from the decreased paper fller content caused by a lower ash retenton. A logcal change n web consstency can be seen at all calculated elements and retenton levels. Based on these results, both the accuracy and repeatablty of the dryness calculatons are excellent and the method can be used further to evaluate formng secton operatons. TAPPI Paper Summt 2002

17 3.0% Outgong web consstency (value-mn) % 2.5% 2.0% 1.5% 1.0% 0.5% Dosage A Dosage B Dosage C Dosage D Dosage E 0.0% Headbox Formng roll Blade secton Sucton dewaterng Press Fgure 17. Dryness dfferences at dfferent polymer dosage levels. Effect on Paper Propertes Durng the trals, paper bass weght and fller content were allowed to change freely accordng to retenton. Total retenton was vared between 50 75%, whch caused the bass weght to change 51 63g/m². Fller content of paper vared smultaneously %. Paper formaton was found to mprove wth decreased retenton. The result s expected, as t s commonly known that ncreased use of brdgng polymer leads to a more flocculated fber suspenson. Swern [12] has stated that the brdgng flocculaton ntroduced by chemcal flocculants also ncrease fber network strength n stock suspenson. Both these factors have a negatve effect on paper formaton % 50 % 55 % 60 % 65 % 70 % 75 % Total retenton, % Fgure 18. Kajaan formaton vs. total retenton. In ths tral the formng parameters were kept constant and wth decreased retenton an mprovement n web formaton was found, as expected. Paper ar permeablty (Bendtsen method) was ab 500ml/mn n all tral ponts and no sgnfcant change was seen n the tral. Tensle ndex rato MD/CD was 2.2 n all tral ponts. Paper tensle ndces were found to vary wth paper fller content. Ol absorpton (Unger method) was found to ncrease slghtly wth ncreasng fller content. TAPPI Paper Summt 2002

18 COCLUSIOS Ths paper presented a method of calculatng retenton values for ndvdual dewaterng elements along the wet end. The method s based on reverse calculaton procedure and can be appled when all the major whte water flows and ther consstences are measured. The same procedure can be appled to a sngle stock component, such as fbers, fllers and fber fnes, f the components' content n the paper produced are analyzed. However, t should be emphaszed that care must be taken when ths method s appled for ndvdual components, especally for those whch are dffcult to determne, such as fber fnes. The same approach can also be used to calculate precse dryness development when the dry content of paper after the press secton s known. The method was tested n a modern plot paper machne producng 52g/m 2 ash contanng fne paper at a speed of 1200m/mn. In the tral total retenton level was vared between 50 75% by adjustng retenton ad dosage. Calculated element retenton as well as web dryness responded consstently and accurately to retenton changes. In general, the presented model can be used to evaluate the operatons at formng and press sectons. Even though no drect connecton from element retenton values to paper qualty varables was not establshed, ths knd of study helps to understand the fundamentals of wet end processes and provdes a tool for system optmzaton. REFERECES 1. CHO, B.-U., GARIER, G., PARADIS, J., PERRIER, M., "The Process Dynamcs of Fller Retenton n Paper usng PAM/Bentonte Retenton Ad System ", Preprnts of 87'th PAPTAC Annual Meetng, (Montreal, Canada, January 31 February ). 2. GESS, J.M. ed., Retenton of Fnes and Fllers Durng Papermakng, Atlanta, Georga, USA, TAPPI Press, (1998). 3. GULLICHSE, J., PAULAPURO, H., Papermakng Part 1, Stock Preparaton and Wet End, Papermakng Scence and Technology, Helsnk, Fnland, Fapet Oy, (2000). 4. KAITZ, R.A., "Retenton Defntons", TAPPI 1989 Papermakers Conference, (Washngton DC, USA, Aprl, 1998). 5. KAUOE, A., Studes on On-lne Retenton and Catonc Demand Measurement and Ther Utlzaton on a Paper Machne, Doctoral thess, Tampere Unversty of Technology, Tampere, Fnland, (1988). 6. KOSOE, M.,"Multvarable MD-controls wth Adaptve Process Models", To be publshed n the 88 th Annual Meetng of PAPTAC, (Montreal, Canada, January 28 31, 2002). 7. LUUKKO, K., Characterzaton and Propertes of Mechancal Pulp Fnes, Doctoral thess, Helsnk Unversty of Technology, Espoo, Fnland, (1999). 8. MUHOE, J., A formng secton control concept study, MSc thess, Helsnk Unversty of Technology, Espoo, Fnland, (To be publshed 2002). 9. ORMA, B., "The Water and Fbre Flow System n the Paper and Board Mll", Proceedngs of 24 th EUCEPA Conference, (Stockholm, Sweden, May ). 10. ODELL, M., "Customzng Roll and Blade Formng to Control Paper Structure", Valmet Paper Machne Days 1996, (Jyväskylä, Fnland, June, 1996). 11. RAISAE, K., Water Removal by Flat Boxes and a Couch Roll on a Paper Machne Wre Secton, Doctoral thess, Helsnk Unversty of Technology, Espoo, Fnland, (1998). 12. SWERI, A., Flocculaton and Fbre etwork Strength n Papermakng Suspensons Flocculated by Retenton Ad Systems, Doctoral thess, Royal Insttute of Technology, Stockholm, Sweden, (1995). TAPPI Paper Summt 2002

19 13. TAPPI, "TIP Retenton Defntons", In: TAPPI Techncal Informaton Papers 01/02, TAPPI Press, (2001). 14. VERKASALO, L., "Effcent Formng at Hgh Speeds", XI Valmet Paper Technology Days, (Jyväskylä, Fnland, June, 1998). 15. ZHAO, R. H., KEREKES, R. J., "The Effect of Consstency on Pressure Pulses n Blade Gap Formers", Pap. Puu 78(1 2):36 (1996). TAPPI Paper Summt 2002