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Drying Technology

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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

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Method to Characterize the Air Flow and Water

Removal 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. Ramaswamy

1

Department of Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, Minnesota

2

Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota

This is part II of a study reported earlier on a method to characterize the air flow and water removal characteristics during vacuum dewatering. This article presents experimental data and analysis of results from the use of a cyclically actuated vacuum dewatering device for removing moisture from wetted porous mate-rials such as paper with the intermittent application of vacuum and accompanying air flow though the material. Results presented include sheet moisture content as a function of residence time and hence 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 vacuum dewatering are presented. Vacuum dewatering process conditions include exit solids content between 11 and 20%solid under applied vacuum conditions of 13.5 to 67.7 kPa (4 to 20 in. Hg). Regression analysis indicated that the exit sheet moisture content exhibited a nonlinear relationship with residence time with exit solids reaching a plateau after a certain residence time. Final moisture content correlated linearly with the average overall flow rate of air through the paper sample and the basis weight of the material.

Keywords Characterization; Flow measurement; Mass transfer; Paper manufacture; Porous materials; Pressure measurement; Rotating disc device; Vacuum dewater-ing; Water removal

INTRODUCTION

The dewatering of open porous bio-based materials such as tissue and towel, via the application of a downstream vacuum and the through-flow of drying air, is one of the steps in the manufacture of these low-density types of paper.[1,8]In commercial paper-making processes, considerable gains in economy 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 the method to characterize the air flow and water removal characteristics during vacuum dewatering.[2]The first study describes mainly the experimental apparatus, which consisted of a system to dewater the wetted paper with intermittent application of vacuum via a slotted opening in a revolving disc that passes the underside of the sample during each revolution. The work constitutes a part of an ongoing investigation into the use of air flow rate as a surrogate measure of moisture content for process control purposes.[3] Part II presented here gives further experi-mental data and analysis of vacuum dewatering under a variety of process conditions.

The device approximates part of the linearly arranged Fourdrinier paper machine where vacuum dewatering is applied by successive vacuum boxes on a steadily moving wire carrying the wet porous mat as it is being dewatered.[4] The wetted paper materials exhibit changes in their resistance to fluid flow during the dewatering process. Moisture within the interstices of the porous paper can plausibly be removed by compression of the wet porous structure by the application of vacuum and also due to convective mass transport.[5]Morphology changes due to material compression can also play a significant part in the overall water removal. Questions remain regarding the effects of the downstream vacuum level on the air flow rate and the maximum amount of moisture removed during the intermittent application of vacuum during vacuum dewatering. The relationship between sheet moisture content and air flow rate during vacuum dewatering forms the basis for using these results as a surrogate measurement for sheet moisture content for feed-forward adaptive predictive control of the moisture content during paper-making.[3]

Correspondence: S. Ramaswamy, Department of Bioproducts and Biosystems Engineering, 209 Kaufert Laboratory, 2004 Folwell Avenue, University of Minnesota, St. Paul, MN 55108; E-mail: [email protected]

ISSN: 0737-3937 print/1532-2300 online DOI: 10.1080/07373930801898125

341

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VACUUM DEWATERING CHARACTERIZATIONS

Details of the experimental setup and procedure are given in the first part of this two-segment series.[2] The results presented here represent extracts from data sequences of single and multiple exposures of the wet material to vacuum under commercially realistic residence times of the order of milliseconds. Experimental measure-ments as a function of residence time include the vacuum tank pressure, the tank temperature, the mass of air in the tank, moisture content of the sheet before and after the exposures, and an accurate characterization of the leak. Experimental measurements are then used to characterize the water removal as a function of residence time as well as the average air flow rate as functions of the material moisture content and the residence times.

Figure 1 presents downstream tank pressure data for a moist paper sample, exposed to five intermittent appli-cation of vacuum. As shown in Fig. 1, in the period prior to 108.5 s the downstream chamber is opened to the vac-uum tank via the control valve and subsequently becomes equilibrated. This is a very significant rise in the tank pres-sure. After the chamber and tank become equilibrated, and the slot valve is outside the sample zone, the pressure reaches a short plateau before the slot enters the sample region. At this point, the tank pressure rises rapidly until the 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 rest of the revolution. The slight initial rise could also possibly be due to instrument (pressure gauge response) overshoot. Figure 2 gives the average temperature of the air-vapor mixture 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 operating time to reach a peak around 109.4 s and then decreases.

The temperature increase is about 1C for each cycle. The increase in temperature is attributed either to the entry of warmer air into the tank or to the corresponding increase in pressure. Toward the end of the test run, there is a temperature decrease as the valve between the vacuum tank and the disc is closed and the temperature is approaching equilibrium.

Based on the pressure and temperature measurements described above, the mass of air in the tank as a function of time are shown in Fig. 3. These data follow a similar pat-tern to the step-wise change in tank pressure data. Mass of air in the tank is an important measurement to characterize the air leaks in the system as well as characterizing air flow during vacuum 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 important topics being addressed here is the role of air flow. A method 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 each exposure, was described in part I. Earlier data indicated how data scatter was produced by significant system leaks that were subsequently reduced. Accurate measurement of leak effects allowed for correction for such leaks.

Similarly, based on the sheet moisture content measure-ments before and after the exposure to vacuum, water removal characteristics under a given vacuum level was also calculated. Figure 4 shows the percent solids content (100% minus the percent moisture content) data as a function of residence time for basis weights (38 and 50 g=m2), at 40.6 kPa (12 in. Hg) vacuum. The water removal or the vacuum dewatering rate is essentially the instantaneous slope of the above curve as a function of the residence time.

As shown in Fig. 4, initially the gradient of change of moisture is high due to the high initial moisture and a faster water removal at the beginning. As time progresses, the gradient or water removal rate decreases until at about 5 ms, in this case, then it is almost horizontal. This type of plateau effect on water removal during vacuum dewater-ing has been shown earlier and is repeated well here. Once the plateau or maximum water removal is reached under a given vacuum condition, further application of the same vacuum level does not result in additional water removal. Then applying a higher vacuum level might result in additional water removal. Commercial paper machines utilize this process sequence with increasing vacuum levels in successive vacuum box applications. The maximum exposure time in a given vacuum box (under a given

vacuum condition) should be decided by the point of diminishing returns in the above figure.

Combining the air flow versus residence time and sheet solids content versus residence time data, it is possible to obtain the air flow characteristics under vacuum dewater-ing conditions; i.e., air flow as a function of sheet solids content under a given vacuum level. As one would expect, due to the inherent variability in sheet structure one might expect some variation in air flow data. Interestingly, as shown in Fig. 5, the air flow through the wet sheet exhibits a linear relationship to the sheet solids content (or moisture content). As indicated earlier, there is a spread of data at a given moisture content. As the sheet dryness or percent solids content increases the air flow rate increases. This might indicate that as the sheet solids content increases there is a gradual change in air passageways through the structure either through the removal of water from the inter-fiber pores or a uniform change in structure or a combination of both. This is quite intriguing indeed, as under through-air drying conditions (much higher solids and lower vacuum levels), the relationship between sheet moisture content and air flow rate is nonlinear until it reaches a plateau corresponding to a dry sheet.[6,7]

Regression Analysis

As is common with natural bio-based materials such as paper, there is inherent variability in the basic sheet proper-ties. Despite careful measurements and experiments, due to the nature of fibers and fiber dispersion it is not avoidable to have minor variability in the basis weight of the sheets. The variability in the sheet basis weight was observed to be approximately 10–15%. In order to properly take into account 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 and air flow, regression analysis was conducted. 3D plots showing the interrelationships between some of the key variables, i.e., sheet basis weight, air flow rate and sheet solids content, are given in Figs. 6 to 9. In order to more accurately characterize the dewatering and air flow behavior during vacuum dewatering, it is important to consider the variability depicted in Fig. 6 and normalize the results for a given sheet property; i.e., sheet basis weight. Flat 2D regression planes are given in the figures. Figures 10 to 13 turn the plots to display the edge view of the regression planes to indicate the amount of scatter in the data set. These graphs indicate the relationship between

air mass flow rate and the moisture content of the porous materials. These data are useful as a surrogate measure used in process control for estimating paper moisture content using air flow rate.

Further 3D data plots can be generated showing the relationship between moisture content, basis weight, and residence time. Examples of these are given in Figs. 14 to 17 for four different tank vacuum levels. The relationship between moisture content (in% solid) and residence time appears to be an exponential function similar to that given in Fig. 4. The variations due to differences in basis weight 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 the basis weight).

Following the above approach, after normalizing the results for a 50 g=m2 sheet basis weight, the relationship between 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 dewatering curves 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 reached and also the residence times at which the plateau is reached are different for different vacuum levels. Following the format of the equation reported in the literature, the plateauing effect for 50 g=m2 sheets can be represented

by the exponential equation as shown below:

mm;%solid ¼dþnð1et=sÞ ð1Þ

where value dis close to the initial solids content, tis the residence time, andsis the time constant. (dþn) represents the maximum solids content at the applied vacuum level. The above approach can be used to design multistage vacuum boxes with successively increasing vacuum levels and optimal residence times (i.e., the width of the vacuum 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. 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).

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increases (i.e., the moisture level decreases), the dewater-ing rate decreases, startdewater-ing from a maximum value at the initial solids content. Also, as would be expected, increasing the vacuum level results in an increased rate of dewatering.

Effect of Dewatering History at Different Pressures

The effects of prior history on dewatering at different pressures were evaluated by conducting experiments on the 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 then further exposed to 27.1 kPa (8 in. Hg) for the next

exposure. After this, the same sheet is further exposed to one exposure at 40.6 kPa (12 in. Hg). Between each exposure, the sheet moisture content and the average air flow rate are measured. One of the reasons for exploring this is to study the effect of prior application of vacuum as is commonly practiced in multi stage vacuum dewater-ing in commercial paper machines. If there is an effect of history on the dewatering, then this has to be appropri-ately taken into account in predicting the air flow rates at a given vacuum dewatering stage during the commer-cial operation.

The results obtained here showed that the air flow rates obtained 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).

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predicted air flow rates gained from the regression equations. Hence, it was concluded that the effect of his-tory on the behavior of the sheet dewatering characteristics is minimal at the different vacuum levels.

CONCLUSION

A rotating disc experimental apparatus has been developed and is used for characterizing vacuum dewater-ing of permeable porous materials such as tissue and towel. The experimental setup involves the intermittent application of vacuum, under commercially realistic residence times of the order of milliseconds. In addition

to water removal as a function of residence time, accurate characterization of the air flow during vacuum dewatering has also been conducted.

Water removal characteristics during vacuum dewater-ing exhibit exponential behavior reachdewater-ing an eventual steady-state or plateau. The maximum solids content is a function of the vacuum level applied. The residence time for reaching the maximum solids content is a function of vacuum level applied. Both of the above are important con-siderations for vacuum system design in commercial paper machines. Accurate measurement of air flow characteristics during 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).

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time. It is interesting that air flow rates under vacuum dewatering process conditions exhibit a linear relationship to percent solids content. Dewatering behavior was found

to be unaffected by prior history of vacuum applications in the study.

NOMENCLATURE

d Fitting parameter for moisture content in

%solid

mm,%solid Moisture content in%solid

n Fitting parameter

t Time

X 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 thanks to Dr. Gary Worry for help and funding support.

REFERENCES

1. Peel, J.D. Paper Science and Paper Manufacture; Angus Wilde Publications Inc.: Vancouver, 1999.

2. Pujara, J.; Siddiqui, M.A.; Liu, Z.; Bjegovic, P.; Takagaki, S.S.; Li, P.Y.; Ramaswamy, S. Method to characterize the air flow and water removal characteristics during vacuum dewatering. Part I—Experi-mental method. Drying Technology 2008,26(3), (accepted).

3. Li, P.Y.; Ramaswamy, S.; Bjegovic, P. Pre-emptive control of moisture content in paper manufacturing using surrogate measurements. Trans-actions of the Institute of Measurement and Control2003,25(1), 36–56. 4. Attwood, B.W. A laboratory investigation of dynamic drainage at vacuum boxes. Pulp and Paper Magazine of Canada 1960, 61, T97–T103.

5. Ramaswamy, S. Vacuum dewatering during paper manufacturing. Dry-ing Technology2003,21(4), 685–717.

6. Ryan, M.; Modak, A.; Zuo, H.; Ramaswamy, S.; Worry, G. Through air drying, progress in drying technologies special issue. Drying Technology2003,21(4), 719–733.

7. Ryan, M.; Zhang, J.; Ramaswamy, S. Experimental investigation of through air drying of tissue and towel under commercial conditions. Drying Technology2007,25(1), 195–204.

8. Polat, O.; Crotogino, R.H.; Douglas, W.J.M. Through-drying of paper: A review. Advances in Drying1991,5, 263–299.

FIG. 22. Regression curves for moisture content (%solids) as a func-tion of residence time for sheet basis weight at 50 g=m2at four vacuum levels (13.5, 27.1, 40.6, 54.2 kPa).

FIG. 23. Dewatering rate versus initial moisture for 50 g=m2sheets at different vacuum pressures (13.5, 27.1, 40.6, 67.7 kPa).

348 PUJARA ET AL.