[0001] The invention relates to a method and to an arrangement for degassing treatment substance,
especially for degassing a coating color or a sizing agent, of a fiber web. More especially
the invention relates to a method according to the preamble of claim 1 and to an arrangement
according to the preamble of claim 8.
[0002] As known from the prior art in fiber web producing processes typically comprise an
assembly formed by a number of apparatuses arranged consecutively in the process line.
A typical production and treatment line comprises a head box, a wire section and a
press section as well as a subsequent drying section and a reel-up. The production
and treatment line can further comprise other devices and sections for finishing the
fiber web, for example, a sizer, a calender, a coating section. The production and
treatment line also comprises at least one winder for forming customer rolls as well
as a roll packaging apparatus. In this description and the following claims by fiber
webs are meant for example paper and board webs.
[0003] In production of fiber webs, for example of paper or board webs, treatment substances
are added onto the fiber web. Sizing is used to alter the properties of a fiber web
by adding sizing agents (sizing medium), for example glue chemicals onto the surface
of the fiber web at the fiber web machine. In coating onto surface of a fiber web,
is added a layer or layers of coating color (coating medium) at a coating station
followed by drying.
[0004] The coating or the sizing of a fiber web typically utilizes a coating device - a
coater - or a sizing device - a sizer. In connection with the coaters and sizers different
kinds of application technology for application of the coating medium or the sizing
medium on the fiber web are employed, for example curtain technology or blade coating
technology or film transfer technology or rod coating technology or air brush coating
technology or spray coating technology.
[0005] In the process industry, the mixing of gases, such as air, with the liquids and compounds
used in the process typically causes many problems. Particularly in the coating or
sizing of fiber web material, the gas and gas bubbles in the treatment substance cause
unevenness on the surface of the web. Depending on the treatment substance the seriousness
of the problems varies for example due to their different tendency to bind with gases.
In addition, the significance of the problem depends also on the coating or sizing
technology used.
[0006] It is known from prior art to degas treatment substance by vacuum de-aerators comprising
a rotating drum arranged inside a vacuum tank, inside which the treatment substance
is conducted, whereupon due to the centrifugal force, the coating material rises up
the inner wall of the drum and discharges from the drum as a thin film colliding with
the wall of the vacuum tank. One problem with prior art vacuum de-aerators is their
insufificient de-aeration efficiency, especially with high-viscosity treatment substances.
In
WO publication 2006/128963 is disclosed a method and an arrangement to provide an improvement for vacuum-operated
degassing of coating material. In the method the coating substance is arranged to
rise stepwise up the wall of the drum, so that the coating substance will form a thin
veil-like film on at least two different step levels, whereupon the gas bubbles in
the coating substance will break and discharge. In the apparatus the inner surface
of the drum is designed to be stepped, comprising at least two step levels so that
the coating substance rising upwards due to the effect of the rotating motion will
form a thin veil-like film on the said at least two step levels, respectively.
[0007] In
EP publication 0517223 A1 is disclosed manufacturing of a coated paper to be used for a usual printing and
using a curtain type coating method, in which is enabled by de-aerating a coating
substance having a high concentration and a high viscosity is de-aerated under a degree
of vacuum of a saturated vapor pressure or less and under a condition of applying
shear to the coating substance, which is formed as a free-falling curtain in a vertical
direction and a printing base paper is coated with the de-aerated coating liquid so
that the free-falling curtain of the coating substance collides with the coating base
paper running continuously in a direction crossing the free-falling curtain.
[0008] In
DE 102005017952 is disclosed a method for degassing a coating substance, which substance is passed
through a vacuum degassing unit at a temperature close to its boiling point and especially
40-60 °C under a pressure of 20-100 mbar for degassing the treatment substance in
temperatures near boiling temperature. Heat is supplied by a heat exchanger or heated
walls of the vacuum degasser. One disadvantage of this known method is that heating
of the coating substance increases solubility of gases into the substance liquid and
increases amount of steam that must be removed by the vacuum degassing unit from the
system and therefore more powerful and bigger unit is needed and the energy consumption
is increased.
[0009] In some cases heating of the treatment substance might, for example due to chemical
reactions initiated by the heat or due to non-tolerance of heat of some components
of the treatment substance, deteriorate properties required from treatment substance.
For example thermal coatings will lose brightness already at temperatures 50 °C and
some latex coatings will suffer instability in temperatures over 45°C in longer run.
[0010] The heating of the treatment substance, which typically is already in rather, even
too high temperatures, for example 40 - 50 °C, when passed to the degassing, creates
more steam into the substance. In degassing the gases and the steam is typically removed
by a suction pump of the degassing unit from the treatment substance, which might
in some cases be problematic since some suction pump types, for example dry screw
vacuum pumps, which are operable in vacuums over 22 mbar, and liquid ring vacuum pumps,
which are operable in vacuums over 35 mbar, do not function well when steam is present.
[0011] An object of the invention is to create a method and an arrangement for degassing
treatment substance of a fiber web in which the above problems and disadvantages are
eliminated or at least minimized.
[0012] Another object of the invention is to provide a method and an arrangement for degassing
treatment substance of a fiber web in which high temperatures of the treatment substance
are avoided.
[0013] A particular object is to prevent harmful chemical reactions in treatment substance
during degassing.
[0014] A further particular object is to provide a method and an arrangement for degassing
treatment substance of a fiber web in which the temperature of the treatment substance
is controlled during degassing.
[0015] A further particular object is to provide a method and an arrangement for degassing
treatment substance of a fiber web in which the suction pump type of the degassing
unit choice is not limited by steam tolerance.
[0016] In order to achieve the above objects the method for degassing treatment substance
of a fiber web according to the invention is mainly characterized by the features
of claim 1 and the arrangement for degassing treatment substance of a fiber web according
to the invention is mainly characterized by the features of claim 8.
[0017] According to the invention in the method for degassing a treatment substance, in
particular a coating color or a sizing agent, of a fiber web the treatment substance
is degassed in a degassing unit and temperature of the treatment substance is adjusted
to temperature below 35 °C by cooling the treatment substance in a temperature control
unit before degassing of the treatment substance in the degassing unit. According
to the invention the arrangement for degassing treatment substance of a fiber web
comprises a degassing unit and a temperature control unit, advantageously a cooler
or a heat exchanger, before the degassing unit for adjusting temperature of the treatment
substance to temperature below 35 °C before degassing.
[0018] According to an advantageous aspect of the invention temperature changes of the treatment
substance are evened out in the temperature control unit before degassing of the treatment
substance in the degassing unit.
[0019] According to an advantageous feature in the method the temperature control unit maintains
the temperature changes of the treatments substance slow, for example about 1°C/15
min.
[0020] According to an advantageous feature in the method when the treatment substance is
cooled in the temperature control unit, steam in the gas of the treatment substance
is condensed and vacuum is created, since the steam volume decreases due to cooling
and condensing of the steam.
[0021] According to an advantageous feature the treatment substance is cooled to temperatures
20-30 °C before degassing.
[0022] According to an advantageous aspect of the invention the temperature of the treatment
substance, in particular the temperature of a coating or a sizing substance, is controlled
before degassing the substance in the degassing unit by cooling the substance by a
cooler, for example by a heat exchanger. The cooler advantageously maintains the temperature
changes of the treatments substance as slow as possible by which best environment
and best conditions for degassing and for functions of the degassing unit and the
vacuum system are provided. Thus improved degassing effect is achieved.
[0023] According to an advantageous aspect of the arrangement according to the inventions
the temperature of the treatment substance is controlled by a heat exchanger, advantageously
by a cooler.
[0024] According to an advantageous feature of the invention the arrangement comprises a
droplet separator between the degassing unit and the vacuum system. According to an
advantageous feature of the invention the arrangement comprises a dry screw vacuum
pump for providing vacuum 20 - 40 mbar in the vacuum system. Advantageously there
is a droplet separator before the dry screw vacuum pump. In the following the invention
is described in more detail with reference to the accompanying drawing, in which
figure 1 is a schematic process chart of treatment substance flow,
figure 2 is a schematic example of one advantageous aspect of the invention and
figure 3 is an air-saturation vapor pressure diagram.
[0025] According to figure 1 the process of treatment substance flow the treatment substance
is delivered to a feed tank 11. From the feed tank 11 the treatment substance is passed
to temperature control unit 12, which is for example a heat exchanger or advantageously
a cooler. The temperature controlled, advantageously cooled treatment substance is
then passed to degassing unit 13, which comprises a vacuum system 14 with a suction
pump (not shown) for removing gas from the treatment substance. The degassed treatment
substance is the passed to a working tank 15, from which the treatment substance is
passed to a coating device or to a sizing device (not shown). The degassed treatment
substance may also be passed directly to a coating head / an applicator of a coating
device or to a sizing head / an applicator of a sizing device (not shown).
[0026] In the example of figure 2 treatment substance is delivered via feed channel 20 to
the temperature control unit 12 from the feed tank 11 (fig. 1). Temperature of the
treatment substance is measured by a temperature sensor 12A and pressure of the treatment
substance in the feed channel 20 is measured by a pressure sensor 20A. Measurement
information from the temperature sensor 12A and the pressure sensor 20A is transmitted
to a control unit 25. From the temperature control unit 12 the treatment substance
is passed to degassing unit 13, in which the gas is separated from the treatment substance
and removed by the vacuum system 14. The gas flow is passed to the vacuum system via
a droplet separator 16, in which the steam in the substance is cooled to condensate
and moisture is separated from the gas flow. Pressure in the degassing unit 13 is
measured by a pressure sensor 13A and its measurement information is transmitted to
the control unit 25. The control unit 25 controls operation of the system and especially
controls the pressure created by the vacuum system and temperature conditions of the
treatment system by adjusting the operation of the temperature control unit 12. From
the degassing unit the degassed treatment substance is fed via a feed channel 22 to
the working tank 15 (fig. 1). The degassed treatment substance may also be passed
directly to a coating head / an applicator of a coating device or to a sizing head
/ an applicator of a sizing device (not shown).
[0027] As shown in figures 1 and 2 temperature of the treatment substance is adjusted by
the temperature control unit 12 and temperature changes of the treatment substance
are evened out before degassing of the treatment substance in the degassing unit 13.
Temperature control unit 12 is advantageously a cooler or a heat exchanger, which
maintains the temperature changes of the treatments substance as slow as possible
and thus best environment and best conditions for degassing and for functions of the
degassing unit 13 and the vacuum system 14 are provided.
[0028] In figure 3 is an air-saturation vapor pressure diagram, which provides basic information
for the control unit 25 (fig. 2) for creating required pressure level in view of the
temperature of the treatment substance for desired degassing conditions. When the
temperature of the treatment substance is controlled in the temperature control unit
12, a vacuum is created as the steam volume in the gas flow decreases due to cooling
and condensing of the steam. The steam condensate is removed in the droplet separator
16.
Reference signs used
11 |
feed tank |
12 |
temperature control unit |
12A |
temperature sensor |
13 |
degassing unit |
13A |
pressure sensor |
14 |
vacuum system |
15 |
working tank |
16 |
droplet separator |
20 |
feed channel from feed tank |
20A |
pressure sensor |
22 |
feed channel to working tank |
25 |
control unit |
1. Method for degassing a treatment substance of a fiber web, in which method the treatment
substance is degassed in a degassing unit (15), characterized in that temperature of the treatment substance is adjusted to temperature below 35 °C by
cooling the treatment substance in a temperature control unit (12) before degassing
of the treatment substance in the degassing unit (15).
2. Method according to claim 1, characterized in that temperature changes of the treatment substance are evened out in the temperature
control unit (12) before degassing of the treatment substance in the degassing unit
(15).
3. Method according to claim 1 or 2, characterized in that in the method the temperature control unit (12) maintains the temperature changes
of the treatments substance slow below about 1 °/15 min.
4. Method according to any of claims 1-3, characterized in that in the method as the temperature of the treatment substance is cooled in the temperature
control unit (12) steam in the gas of the treatment substance is condensed and vacuum
is created the steam volume decreases due to cooling and condensing of the steam.
5. Method according to any of claims 1-4, characterized in that the treatment substance is cooled to temperatures 20-30 °C before degassing.
6. Method according to any of claims 1-5, characterized in that the treatment substance is a coating color or a sizing agent.
7. Method according to any of claims 1 - 6, characterized in that the treatment substance is delivered from a feed tank (11) via a feed channel (20)
passed to the temperature control unit (12), that the temperature controlled, advantageously
cooled treatment substance is then passed to the degassing unit (13) for removing
gas from the treatment substance and that the degassed treatment substance is the
passed to a working tank (15) from which the treatment substance is passed to a treatment
device, in particular to a coating device or to a sizing device.
8. Arrangement for degassing treatment substance of a fiber web, which arrangement comprises
a degassing unit (15), characterized in that the arrangement comprises a temperature control unit (12) before the degassing unit
(15) for adjusting temperature of the treatment substance to temperature below 35
°C before degassing.
9. Arrangement according to claim 8, characterized in that the temperature control unit (12) is a cooler or a heat exchanger.
10. Arrangement according to claim 8 or 9, characterized in that the arrangement further comprises a feed tank (11) and a feed channel (20) for passing
the treatment substance to the temperature control unit (12) and another feed channel
for passing the treatment substance from the degassing unit (13) to a working tank
(15) from which the treatment substance is passed to a treatment device, in particular
to a coating device or to a sizing device.
11. Arrangement according to any of claims 8-10, characterized in that the arrangement comprises a control unit (25) receiving measurement information from
a temperature sensor (12A), from a pressure sensor (20A) in the feed channel (20)
and from a pressure sensor (13A) in the degassing unit for controlling the temperature
of treatment substance in the temperature control unit (12) and the pressure of the
degassing unit (13).
12. Arrangement according to any of claims 8-11, characterized in that the arrangement comprises a droplet separator (16) and a vacuum system (14) connected
to the degassing unit (15).
13. Arrangement according to any of claims 8-12, characterized in that the treatment substance is a coating color or a sizing agent.
14. Arrangement according to any of claims 8-13, characterized in that the arrangement comprises a dry screw vacuum pump for providing vacuum 20 - 40 mbar
in the vacuum system (14).
15. Arrangement according to any of claim 14, characterized in that the arrangement comprises a droplet separator (16) before the dry screw vacuum pump.