150211 Magazin Coating Laminating Diffusion Optimized Convection Dryers

7/23/2019 150211 Magazin Coating Laminating Diffusion Optimized Convection Dryers

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Machinery and Processing for Coating and Converting

I n t e r n a t i o n a l

Anlagen und Verfahren zur Beschichtung und Veredelung

Coating1|2015 1

Sonderdruck aus 1-2015www.coating.ch

A DRYING RATE SEVERAL TIMES HIGHER THAN EXISTING RATES.

Thermal drying is a process widely employed in the coating

industry, where thin liquid films are coated on substrates of dif-

ferent materials and are subsequently dried to yield solid coat-

ed layers to cover the substrate. The latter confer specific prop-erties on the coated material that fulfill protective, cosmetic or

functional tasks, depending on the particular application. Various

coating techniques are currently applied and these are described

in the literature, in summary papers and in books [1–6]. These

publications demonstrate that film coating has progressed over

the years, yielding new coating methods and their introduction

into industry. Among these, the slot coating technique has

attracted considerable attention owing to its wide range of appli-

cations and also to the high quality of layers that it can coat.

Most coating processes are nowadays well understood and avail-

able theoretical treatments support coating applications in vari-

ous areas. Similar supports are not available in the field of drying

since theoretical treatments of drying have not been reported.

Therefore, until now, improvements in the drying process of coat-

ed, thin liquid films had to be based on engineering intuition. This

led to the belief that the addition of more heat will yield faster dry-

ing. However, this is not necessarily the case. If the heat is sup-

plied to evaporate the solvent at a certain rate and no correspon-

ding means are in place to remove the vapor at the same rate, the

addition of heat might only increase the temperature of the coat-

ing fluid and it might even decrease the drying rate. Hence a

deeper understanding of the drying process is needed to facilitate

improved drying processes for the coating industry.

FMP Technology GmbH has been working, for some years, to

develop the diffusion-optimized convection drying technique for

the coating industry. Suitable dryers can be laid out, designed,

built and installed that possess drying rates several times higher

than those of dryers currently employed in the coating industry.

High energy savings can be achieved with these new dryers if

their performance is compared with existing dryers for the same

drying rates. Faster coating speeds can be obtained with diffusion-

optimized convection dryers if these dryers possess the same

length as existing dryers.

DIFFUSION-OPTIMIZED CONVECTION DRYERS. Through computa-

tional work, FMP Technology GmbH elucidated the flow effects

that improve the mass transfer performance of airflows applied

in dryers. For this purpose, numerical computations, after some

analytical studies, were carried out for the geometry illustrated

in Fig. 1. For this geometry and for a hot flow passing through

the indicated porous medium, very good heat and mass trans-

fer rates could be achieved. This was computed for the flow

geometry in Fig. 1, which shows not only the computational

domain, but also the boundary conditions employed. The geom-

etry shows that a porous medium plate was used above the wet

film surface to be dried. This arrangement yields a temperature

gradient between the porous plate and the wet film surface, and

also ensures that there is a flow available parallel to the film

that removes the moisture that is evaporated in the drying

process. Through these two processes – production and transport

of moisture – very good mass transfer rates are produced. Values

are reached that are several times higher than those of conven-

tional convection dryers employed in the coating industry.

Diffusion-optimized convection dryers

for the coating industryBy F. Durst, G. Zheng, H. Soltanzadeh und T. Brunner, FMP Technology GmbH

Fig. 1: Schematic of a drying element implemented with a porous plate.



Coating and Laminating

2 Coating 1|2015

Element length, L = 41 [mm];

Inlet, B = 30 [mm];

Lip length, l = 3 [mm];

Porous plate thickness, t = 3.5 [mm];

Permeability coefficient, k = 167x10-12 [m2].

Element height, H = 8 [mm];

Outlet, b = 2.5 [mm];

Gap distance, d = 3 [mm];

Porosity, Ø = 0.55 [-];

Fig. 1 illustrates the boundary conditions for one drying element

inside a diffusion-optimized dryer. These kinds of drying ele-

ments are periodically connected to each other and a number of

drying elements are used to build up a dryer, this number being

dependent on the total length of the dryer.

Simulations were carried out based on the schematic geometry

shown in Fig. 1, where air blows into the region above the wetfilm with a uniform velocity and leaves through outlet slots at the

left- and right-hand side of each drying element. For the compu-

tations, the temperature of the bottom side was fixed at Tb, where

evaporation occurs. The temperature of the inflow and the tem-

perature of the top walls were also taken to be constant, Tt = 200 °C.

The coated substrate moved with a web speed, Uw, from the left to

the right side of Fig. 1. For the numerical simulations, a zero pres-

sure jump as a boundary condition was imposed for the connect-

ing interfaces between the ends of the elements.

Taking air as an ideal gas, two-dimensional steady laminar flow

simulations were conducted using the commercial CFD software

CCM+ of adapco GmbH [7]. Various parameters of the dryingprocess could be computed in the numerical simulations per-

formed, including velocity, pressure, temperature, density and vis-

cosity of the airflow above the wet film surface. The numerical pre-

dictions carried out yielded the results summarized in Figs. 2–8.

The predictions carried out resulted in mass transfer rates that

were much higher than those of existing dryers. These results are

obvious if one considers the high temperature and density gradi-

ents that can be achieved above the wet film surface. These result

in a high diffusive mass flux at the evaporation interface.

Fig. 4 shows that a constant diffusive mass transport is predict-

ed over the major part of the drying region. It also shows that

there are two negative peaks of the predicted diffusive mass

transport at the ends of the drying element. These «end effects»were introduced by the flow suction through the slots at the two

ends of the drying element. In order to confirm this conclusion

drawn from the numerical predictions, another flow geometry

that excluded the ends, as shown in Fig. 5, was used to carry

out a second set of numerical computations. This figure shows

a porous medium plate set up for the incoming hot air, with the

air leaving in the horizontal direction at the left- and right-hand

sides of the sketched channel. Predictions for this set-up yield-

ed mass transfer results that were constant over the whole dry-

ing region considered due to the exclusion of the «end effect».

This is shown in Fig. 8.

Element length, L = 100 [mm]; Element height, H = 12 [mm];

Gap distance, d = 3 [mm];

Porous plate thickness, t = 3.5 [mm]; Porosity, Ø = 0.55 [-];

Permeability coefficient, k = 167x10-12 [m2]

Utilizing the CFD program CCM+, computations could be carried

out for various conditions and it was found that the mass trans-

fer rates vary strongly with the distance of the porous plate with

Fig. 2: Temperature distribution inside the drying element for the case

of Tw = 20 ˚C, T t = 200 ˚C, Uw = 0 m/s and Vin = 1.0 m/s.

Fig. 3: Density distribution inside the drying element for the case

of Tw = 20 ˚C, T t = 200 ˚C, Uw = 0 m/s and Vin = 1.0 m/s.

Fig. 4: Local diffusive mass flux (red curve) and its mean value (green curve) at

the evaporation interface for the drying element implemented with porous plate

for the case of Tw = 20 ˚C, T t = 200 ˚C, Uw = 0 m/s and Vin = 1.0 m/s.

Fig. 5: Schematic of a drying element implemented with porous plate,

without end effect.

Fig. 6: Temperature distribution inside the drying element for the case of Tw = 20 ˚C,

T t = 200 ˚C, Uw = 0 m/s and Vin = 1.0 m/s.


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respect to the liquid film, the f low velocity through the porous

medium and also the temperature gradient between the liquid

film and the porous plate. It is these parameters that can be used

in practice to yield good drying rates for coated films on different

substrate materials.

APPROACH TO SOLUTION. TEST SECTION AND EQUIPMENT. Experi-

mental investigations of the diffusion-optimized convection dryer

using 10 diffusion-optimized convection dryer elements (dryer

length 1 m) were carried out to confirm the theoretical findings.

Such a dryer is shown in Fig. 9.

Several internal and external runs demonstrated the perform-

ance of individual drying elements, shown in Fig. 10.

RESULTS AND DISCUSSION. To obtain experimental information onthe performance of the FMP dryer, the speed of coating of the roll-

to-roll system was varied until, at the end of the dryer, drying of

the total amount of solvent in the wet film was ensured. Hence it

was possible to determine the drying performance by using the

residence time in the dryer as:

where = Drying rate per unit area, m = Amount of solvent

applied per unit area, = Web speed and L= Length of dryer.

The FMP dryer of 1 m length, shown inFig. 1, was finally integrated into a roll-

to-roll coating test rig of FMP TECHNOL-

OGY GMBH to demonstrate its drying

performance experimentally. At the

lower end of the dryer, a slot die was

installed, to coat the surface of a photo-

graphic paper sheet, as shown in Fig. 11.

With this arrangement, wet films of pre-

established thicknesses were coated on

the paper and subsequently fed into the

dryer, as illustrated in Fig. 12. In Fig. 12,

the coating nozzle is also shown under

coating operating conditions. The noz-

zle was applied in different coating

modes and at various weight ratings.

The experiments were performed in

such a way that the coating speed was

adjusted until, for a certain weight of

a liquid film, complete drying was

achieved at the end of the 1 m modular

drying element. Leaving the mass flow through the coating die

constant, the coating speed could be changed and for all speeds

the liquid film could be dried in the experiments. This is a nice

feature of the diffusion-optimized convection dryer introduced in

this paper. Initial experimental investigations on the drying of

pure water were performed. The drying rate measurements are

shown in Fig. 13, illustrating that the diffusion-optimized convec-

tion dryer allows drying rates that are 4–5 times existing drying

rates known for convection dryers from several manufacturers.

Fig. 10: One element of the diffusion-optimized convection dryer showing the

inlet and outlet openings for the airflow.

Fig. 11: Slot die under coating operation and vertical FMP dryer

Air inflow

Air e xhaust

Fig. 9: Photograph of a 1 mr FMP dryer set-up for the verification experiments

Fig. 8: Local diffusive mass flux (green curve) and its mean value (red curve) at

the evaporation interface for the drying element implemented with a porous

plate, without end effect, for the case of T w = 20 ˚C, T t = 200 ˚C, Uw = 0 m/s and

Vin = 1.0 m/s.

Fig. 7: Density distribution inside the drying element for the case of Tw = 20 ˚C, T t =

200 ˚C, Uw = 0 m/s and Vin = 1.0 m/s.

Fig. 12: Test rig at FMP

Technology GMBH for

coating and drying experi-

ments




4 Coating 2|2015

CONCLUSION. The convection dryers nowadays employed in thecoating industry possess low drying rates, poor energy efficien-

cies and use high f low rates of air to dry thin layers of aqueous flu-

ids. To improve dryers of this kind requires the diffusive transport

of the evaporated solvent to be enhanced. In optimized diffusion

dryers, it is necessary to ensure that the rates of vapor production

and vapor transport are made equal.

Because of the above criterion, diffusion-optimized convection

dryers are needed in the coating industry and this paper summa-

rizes research and development work to make such dryers avail-

able. This work resulted in dryers that can operate with an effi-

ciency several times higher than that achievable with existingdryers, as outlined in the paper. It is stressed that diffusion-opti-

mized convection dryers can save energy costs not practically

achievable in any other way. Very short dryers can be built and

limitations of coating speeds, due to dryer length, can be eliminat-

ed. Diffusion-optimized convection dryers permit the energy costs

for drying of thin liquid films to be drastically reduced. The hot air

flow velocity, the distance of the porous plate to the wet film sur-

face and the airflow temperature used can be employed to control

the drying rate.

FMP Technology GmbH can deliver drying elements up to 2 m in

width. For wider substrates, shorter pieces can be connected to

each other or elements of special design need to be considered.

The introduced diffusion optimized convection dryer elements

can easily be combined to dryer-units as indicated in Fig. 9. How-

ever, is also possible to combine the elements with conventional

dryers/heaters, already employed in coating lines. Hence, the

introduced dryer elements can be inserted, piece-by-piece, into

existing dryers of the coating industry.

REFERENCES

[1] N. Dongari, R. Sambasivam & F. Durst, Slot coaters operating

in their bead mode, Coating, Vol. 11: 1–6, 2007.

[2] N. E. Bixler, Stability of a coating flow, PhD thesis submitted

to the faculty of the Graduate School of the University of Min-

nesota, 1982.

[3] K. J. Ruschak, Coating flows, Annual Review of Fluid Mechan-

ics, Vol. 17: 65-89, 1985.

[4] S. F. Kistler & P. M. Schweizer, Liquid Film Coating; Scientific

Principles and Their Technological Implications, Chapman &

Hall, 1997.[5] A. Goldschmidt & H. J. Streitbeger, BASF Handbook on Basics

of Coating Technology (American coating literature), Elsevier,

2003.

[6] A. A. Tracton, Coating Technology Handbook, Taylor and

Francis, 2005.

[7] Documentation of STAR-CCM+ V 8.04, cd-adapco.

FMP Technology GMBH

D-91058 Erlangen

www.fmp-technology.com

At ICE USA: Hall A6, Booth 566

Fig. 14: Combinations of diffusion optimized convection dryer elements with infra-

red heaters. IR-Modul = IR-module; FMP Trocknungsmodul = FMP drying module

Fig. 13: Results for drying rate measured for pure water coated

on a photographic paper substrate

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150211 Magazin Coating Laminating Diffusion Optimized Convection Dryers