PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [University of Minnesota] On: 8 April 2009 Access details: Access Details: [subscription number 788736612] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Drying Technology Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713597247 Method to Characterize the Air Flow and Water Removal Characteristics During Vacuum Dewatering. Part II—Analysis and Characterization J. Pujara a ; M. A. Siddiqui a ; Z. Liu a ; P. Bjegovic b ; S. S. Takagaki a ; P. Y. Li b ; S. Ramaswamy a a Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota b Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota Online Publication Date: 01 March 2008 To cite this Article Pujara, J., Siddiqui, M. A., Liu, Z., Bjegovic, P., Takagaki, S. S., Li, P. Y. and Ramaswamy, S.(2008)'Method to Characterize the Air Flow and Water Removal Characteristics During Vacuum Dewatering. Part II—Analysis and Characterization',Drying Technology,26:3,341 — 348 To link to this Article: DOI: 10.1080/07373930801898125 URL: http://dx.doi.org/10.1080/07373930801898125 Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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PLEASE SCROLL DOWN FOR ARTICLE
This article was downloaded by: [University of Minnesota]On: 8 April 2009Access details: Access Details: [subscription number 788736612]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Drying TechnologyPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t713597247
Method to Characterize the Air Flow and Water Removal Characteristics DuringVacuum Dewatering. Part II—Analysis and CharacterizationJ. Pujara a; M. A. Siddiqui a; Z. Liu a; P. Bjegovic b; S. S. Takagaki a; P. Y. Li b; S. Ramaswamy a
a Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota b
Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota
Online Publication Date: 01 March 2008
To cite this Article Pujara, J., Siddiqui, M. A., Liu, Z., Bjegovic, P., Takagaki, S. S., Li, P. Y. and Ramaswamy, S.(2008)'Method toCharacterize the Air Flow and Water Removal Characteristics During Vacuum Dewatering. Part II—Analysis andCharacterization',Drying Technology,26:3,341 — 348
To link to this Article: DOI: 10.1080/07373930801898125
URL: http://dx.doi.org/10.1080/07373930801898125
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.
Method to Characterize the Air Flow and WaterRemoval Characteristics During Vacuum Dewatering.Part II—Analysis and Characterization
J. Pujara,1 M. A. Siddiqui,1 Z. Liu,1 P. Bjegovic,2 S. S. Takagaki,1 P. Y. Li,2
and S. Ramaswamy1
1Department of Bioproducts and Biosystems Engineering, University of Minnesota,St. Paul, Minnesota2Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota
This is part II of a study reported earlier on a method tocharacterize the air flow and water removal characteristics duringvacuum dewatering. This article presents experimental data andanalysis of results from the use of a cyclically actuated vacuumdewatering device for removing moisture from wetted porous mate-rials such as paper with the intermittent application of vacuum andaccompanying air flow though the material. Results presentedinclude sheet moisture content as a function of residence time andhence water removal rate under a variety of process conditions.Also, experimental results on air flow through the wet porous struc-ture and hence the role and importance of air flow during vacuumdewatering are presented. Vacuum dewatering process conditionsinclude exit solids content between 11 and 20% solid under appliedvacuum conditions of 13.5 to 67.7 kPa (4 to 20 in. Hg). Regressionanalysis indicated that the exit sheet moisture content exhibited anonlinear relationship with residence time with exit solids reachinga plateau after a certain residence time. Final moisture contentcorrelated linearly with the average overall flow rate of air throughthe paper sample and the basis weight of the material.
Keywords Characterization; Flow measurement; Mass transfer;Paper manufacture; Porous materials; Pressuremeasurement; Rotating disc device; Vacuum dewater-ing; Water removal
INTRODUCTION
The dewatering of open porous bio-based materials suchas tissue and towel, via the application of a downstreamvacuum and the through-flow of drying air, is one of the stepsin the manufacture of these low-density types of paper.[1,8] Incommercial paper-making processes, considerable gains ineconomy and energy efficiency can be achieved by the prior
vacuum removal of water before the subsequent energy-consuming thermal drying process stage.
This is part II of the study reported earlier on themethod to characterize the air flow and water removalcharacteristics during vacuum dewatering.[2] The first studydescribes mainly the experimental apparatus, whichconsisted of a system to dewater the wetted paper withintermittent application of vacuum via a slotted openingin a revolving disc that passes the underside of the sampleduring each revolution. The work constitutes a part of anongoing investigation into the use of air flow rate as asurrogate measure of moisture content for process controlpurposes.[3] Part II presented here gives further experi-mental data and analysis of vacuum dewatering under avariety of process conditions.
The device approximates part of the linearly arrangedFourdrinier paper machine where vacuum dewatering isapplied by successive vacuum boxes on a steadily movingwire carrying the wet porous mat as it is being dewatered.[4]
The wetted paper materials exhibit changes in theirresistance to fluid flow during the dewatering process.Moisture within the interstices of the porous paper canplausibly be removed by compression of the wet porousstructure by the application of vacuum and also due toconvective mass transport.[5] Morphology changes due tomaterial compression can also play a significant part inthe overall water removal. Questions remain regardingthe effects of the downstream vacuum level on the air flowrate and the maximum amount of moisture removed duringthe intermittent application of vacuum during vacuumdewatering. The relationship between sheet moisture contentand air flow rate during vacuum dewatering forms the basisfor using these results as a surrogate measurement for sheetmoisture content for feed-forward adaptive predictivecontrol of the moisture content during paper-making.[3]
Correspondence: S. Ramaswamy, Department of Bioproductsand Biosystems Engineering, 209 Kaufert Laboratory, 2004Folwell Avenue, University of Minnesota, St. Paul, MN 55108;E-mail: [email protected]
Drying Technology, 26: 341–348, 2008
Copyright # 2008 Taylor & Francis Group, LLC
ISSN: 0737-3937 print/1532-2300 online
DOI: 10.1080/07373930801898125
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VACUUM DEWATERING CHARACTERIZATIONS
Details of the experimental setup and procedureare given in the first part of this two-segment series.[2]
The results presented here represent extracts from datasequences of single and multiple exposures of the wetmaterial to vacuum under commercially realistic residencetimes of the order of milliseconds. Experimental measure-ments as a function of residence time include the vacuumtank pressure, the tank temperature, the mass of air inthe tank, moisture content of the sheet before and afterthe exposures, and an accurate characterization of the leak.Experimental measurements are then used to characterizethe water removal as a function of residence time as wellas the average air flow rate as functions of the materialmoisture content and the residence times.
Figure 1 presents downstream tank pressure data for amoist paper sample, exposed to five intermittent appli-cation of vacuum. As shown in Fig. 1, in the period priorto 108.5 s the downstream chamber is opened to the vac-uum tank via the control valve and subsequently becomesequilibrated. This is a very significant rise in the tank pres-sure. After the chamber and tank become equilibrated, andthe slot valve is outside the sample zone, the pressurereaches a short plateau before the slot enters the sampleregion. At this point, the tank pressure rises rapidly untilthe slot disc valve cycles past and exits the sample region.Subsequent to this, the pressure increases slightly, attri-buted to the leak while the slot is going through the restof the revolution. The slight initial rise could also possiblybe due to instrument (pressure gauge response) overshoot.
Figure 2 gives the average temperature of the air-vapormixture in the downstream vacuum tank. Points of first,second, and third exposure to air flow are indicated.Initially the temperature is low and rises with operatingtime to reach a peak around 109.4 s and then decreases.
The temperature increase is about 1�C for each cycle.The increase in temperature is attributed either to the entryof warmer air into the tank or to the correspondingincrease in pressure. Toward the end of the test run, thereis a temperature decrease as the valve between the vacuumtank and the disc is closed and the temperature isapproaching equilibrium.
Based on the pressure and temperature measurementsdescribed above, the mass of air in the tank as a functionof time are shown in Fig. 3. These data follow a similar pat-tern to the step-wise change in tank pressure data. Mass ofair in the tank is an important measurement to characterize theair leaks in the system as well as characterizing air flow duringvacuum dewatering. The figure displays the points corre-sponding to the beginnings of the first and second exposures.
FIG. 1. Experimental test giving tank pressure for 50-g sheet at 67.7 kPa
(20 in. Hg).
FIG. 2. Experimental test giving tank temperature for 50-g sheet at
67.7 kPa (20 in. Hg).
FIG. 3. Air mass in the tank as a function of time for 50-g sheet at
67.7 kPa (20 in. Hg vacuum).
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As discussed in part I of this series, one of the importanttopics being addressed here is the role of air flow. Amethod for accurately characterizing the air flow as a func-tion of residence time for successive exposures to vacuum,precisely synchronizing with the beginning and end of eachexposure, was described in part I. Earlier data indicatedhow data scatter was produced by significant system leaksthat were subsequently reduced. Accurate measurement ofleak effects allowed for correction for such leaks.
Similarly, based on the sheet moisture content measure-ments before and after the exposure to vacuum, waterremoval characteristics under a given vacuum level wasalso calculated. Figure 4 shows the percent solids content(100% minus the percent moisture content) data as afunction of residence time for basis weights (�38 and50 g=m2), at 40.6 kPa (12 in. Hg) vacuum. The waterremoval or the vacuum dewatering rate is essentially theinstantaneous slope of the above curve as a function ofthe residence time.
As shown in Fig. 4, initially the gradient of change ofmoisture is high due to the high initial moisture and a fasterwater removal at the beginning. As time progresses, thegradient or water removal rate decreases until at about5 ms, in this case, then it is almost horizontal. This typeof plateau effect on water removal during vacuum dewater-ing has been shown earlier and is repeated well here. Oncethe plateau or maximum water removal is reached under agiven vacuum condition, further application of the samevacuum level does not result in additional water removal.Then applying a higher vacuum level might result inadditional water removal. Commercial paper machinesutilize this process sequence with increasing vacuum levelsin successive vacuum box applications. The maximumexposure time in a given vacuum box (under a given
vacuum condition) should be decided by the point ofdiminishing returns in the above figure.
Combining the air flow versus residence time and sheetsolids content versus residence time data, it is possible toobtain the air flow characteristics under vacuum dewater-ing conditions; i.e., air flow as a function of sheet solidscontent under a given vacuum level. As one would expect,due to the inherent variability in sheet structure one mightexpect some variation in air flow data. Interestingly, asshown in Fig. 5, the air flow through the wet sheet exhibitsa linear relationship to the sheet solids content (or moisturecontent). As indicated earlier, there is a spread of data at agiven moisture content. As the sheet dryness or percentsolids content increases the air flow rate increases. Thismight indicate that as the sheet solids content increasesthere is a gradual change in air passageways through thestructure either through the removal of water from theinter-fiber pores or a uniform change in structure or acombination of both. This is quite intriguing indeed, asunder through-air drying conditions (much higher solidsand lower vacuum levels), the relationship between sheetmoisture content and air flow rate is nonlinear until itreaches a plateau corresponding to a dry sheet.[6,7]
Regression Analysis
As is common with natural bio-based materials such aspaper, there is inherent variability in the basic sheet proper-ties. Despite careful measurements and experiments, due tothe nature of fibers and fiber dispersion it is not avoidableto have minor variability in the basis weight of the sheets.The variability in the sheet basis weight was observed to beapproximately 10–15%. In order to properly take intoaccount the inherent variability in sheet basis weight and
FIG. 4. Moisture content (% solids) as a function of residence time and
sheet basis weight (g=m2) at 40.6 kPa (12 in. Hg vacuum) and regression.
FIG. 5. Air mass flow rate versus moisture content (% solids) and sheet
basis weight (g=m2) at 67.7 kPa (20 in. Hg vacuum) and regression.
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its influence on resulting variability in water removal andair flow, regression analysis was conducted. 3D plotsshowing the interrelationships between some of the keyvariables, i.e., sheet basis weight, air flow rate and sheetsolids content, are given in Figs. 6 to 9. In order to moreaccurately characterize the dewatering and air flowbehavior during vacuum dewatering, it is important toconsider the variability depicted in Fig. 6 and normalizethe results for a given sheet property; i.e., sheet basisweight. Flat 2D regression planes are given in the figures.
Figures 10 to 13 turn the plots to display the edge viewof the regression planes to indicate the amount of scatter inthe data set. These graphs indicate the relationship between
air mass flow rate and the moisture content of the porousmaterials. These data are useful as a surrogate measureused in process control for estimating paper moisturecontent using air flow rate.
Further 3D data plots can be generated showing therelationship between moisture content, basis weight, andresidence time. Examples of these are given in Figs. 14 to17 for four different tank vacuum levels. The relationshipbetween moisture content (in % solid) and residence timeappears to be an exponential function similar to that givenin Fig. 4. The variations due to differences in basisweight appear not to affect this basic relation significantly.Figures 18 to 21 give edge views of the 3D plots giving the
FIG. 6. Moisture content (% solids) as a function of mass flow rate
and sheet basis weight (40–50 g=m2) at 13.5 kPa (4 in. Hg) (3D view).
FIG. 7. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (40–55 g=m2) at 27.1 kPa (8 in. Hg) (3D view).
FIG. 8. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (38–50 g=m2) at 40.6 kPa (12 in. Hg) (3D view).
FIG. 9. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (45–60 g=m2) at 67.7 kPa (20 in. Hg) (3D view).
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moisture content versus residence time (normalizing for thebasis weight).
Following the above approach, after normalizing theresults for a 50 g=m2 sheet basis weight, the relationshipbetween sheet percent solids content and residence time,i.e., dewatering behavior, is shown in Fig. 22 for four dif-ferent vacuum levels. It is interesting that the dewateringcurves show a plateauing behavior as mentioned earlier,reaching a plateau at each of the vacuum levels. However,the percent solids content at which the plateau is reachedand also the residence times at which the plateau is reachedare different for different vacuum levels. Following theformat of the equation reported in the literature, theplateauing effect for 50 g=m2 sheets can be represented
by the exponential equation as shown below:
mm;%solid ¼ d þ nð1� e�t=sÞ ð1Þ
where value d is close to the initial solids content, t is theresidence time, and s is the time constant. (dþ n) representsthe maximum solids content at the applied vacuum level.The above approach can be used to design multistagevacuum boxes with successively increasing vacuumlevels and optimal residence times (i.e., the width of thevacuum slots).
The regression data can be recast in terms of the dewa-tering rate versus the initial moisture content (see Fig. 23).The figure indicates that as the initial percent solid
FIG. 10. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (45–50 g=m2) at 13.5 kPa (4 in. Hg) (Edge view).
FIG. 11. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (45–55 g=m2) at 27.1 kPa (8 in. Hg) (Edge view).
FIG. 12. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (40–50 g=m2) at 40.6 kPa (12 in. Hg) (Edge view).
FIG. 13. Moisture content (% solids) as a function of air mass flow rate
and sheet basis weight (45–60 g=m2) at 67.7 kPa (20 in. Hg) (Edge view).
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increases (i.e., the moisture level decreases), the dewater-ing rate decreases, starting from a maximum value atthe initial solids content. Also, as would be expected,increasing the vacuum level results in an increased rateof dewatering.
Effect of Dewatering History at Different Pressures
The effects of prior history on dewatering at differentpressures were evaluated by conducting experiments onthe same sheet at different levels of vacuum pressure.Initially, a wet sheet is exposed to a vacuum of 13.5 kPa(4 in. Hg) for one exposure. The same sheet was thenfurther exposed to 27.1 kPa (8 in. Hg) for the next
exposure. After this, the same sheet is further exposedto one exposure at 40.6 kPa (12 in. Hg). Between eachexposure, the sheet moisture content and the average airflow rate are measured. One of the reasons for exploringthis is to study the effect of prior application of vacuumas is commonly practiced in multi stage vacuum dewater-ing in commercial paper machines. If there is an effect ofhistory on the dewatering, then this has to be appropri-ately taken into account in predicting the air flow ratesat a given vacuum dewatering stage during the commer-cial operation.
The results obtained here showed that the air flow ratesobtained from these experiments were close to the
FIG. 14. Moisture content (% solids) as a function of residence time
and sheet basis weight at 13.5 kPa (4 in. Hg) (3D view).
FIG. 15. Moisture content (% solids) as a function of residence time
and sheet basis weight at 27.1 kPa (8 in. Hg) (3D view).
FIG. 16. Moisture content (% solids) as a function of residence time
and sheet basis weight at 40.6 kPa (12 in. Hg) (3D view).
FIG. 17. Moisture content (% solids) as a function of residence time
and sheet basis weight at 57.6 kPa (20 in. Hg) (3D view).
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predicted air flow rates gained from the regressionequations. Hence, it was concluded that the effect of his-tory on the behavior of the sheet dewatering characteristicsis minimal at the different vacuum levels.
CONCLUSION
A rotating disc experimental apparatus has beendeveloped and is used for characterizing vacuum dewater-ing of permeable porous materials such as tissue andtowel. The experimental setup involves the intermittentapplication of vacuum, under commercially realisticresidence times of the order of milliseconds. In addition
to water removal as a function of residence time, accuratecharacterization of the air flow during vacuum dewateringhas also been conducted.
Water removal characteristics during vacuum dewater-ing exhibit exponential behavior reaching an eventualsteady-state or plateau. The maximum solids content is afunction of the vacuum level applied. The residence timefor reaching the maximum solids content is a function ofvacuum level applied. Both of the above are important con-siderations for vacuum system design in commercial papermachines. Accurate measurement of air flow characteristicsduring vacuum dewatering has been reported for the first
FIG. 18. Moisture content (% solids) as a function of residence time
and sheet basis weight at 13.5 kPa (4 in. Hg) (Edge view).
FIG. 19. Moisture content (% solids) as a function of residence time
and sheet basis weight at 27.1 kPa (8 in. Hg) (Edge view).
FIG. 20. Moisture content (% solids) as a function of residence time
and sheet basis weight at 40.6 kPa (12 in. Hg) (Edge view).
FIG. 21. Moisture content (% solids) as a function of residence time
and sheet basis weight at 67.7 kPa (20 in. Hg) (Edge view).
AIR FLOW AND WATER REMOVAL DURING VACUUM DEWATERING—PART II 347
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time. It is interesting that air flow rates under vacuumdewatering process conditions exhibit a linear relationshipto percent solids content. Dewatering behavior was found
to be unaffected by prior history of vacuum applicationsin the study.
NOMENCLATURE
d Fitting parameter for moisture content in% solid
mm,%solid Moisture content in % solidn Fitting parametert TimeX Moisture content (fraction solid, g dry=g final)
Greek Symbols
s Time constant
ACKNOWLEDGEMENTS
The authors thank the National Science Foundation(NSF=DMI 0085230) for research funding. Special thanksto Dr. Gary Worry for help and funding support.
REFERENCES
1. Peel, J.D. Paper Science and Paper Manufacture; Angus Wilde
