Drying and gas or vapor contact with solids – Apparatus – Rotary drums or receptacles
Reexamination Certificate
2002-06-06
2004-02-24
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Apparatus
Rotary drums or receptacles
C034S114000, C034S122000, C034S618000
Reexamination Certificate
active
06694639
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sheet material and a production method thereof, having a drying process for a sheet material in a wet condition such as paper or some types of wet process-nonwoven fabric, and an apparatus for drying sheet material.
2. Description of the Related Art
A typical method for drying a paper web on a paper-making machine generally involves transfer of heat from a dryer cylinder having a second-kind pressure vessel structure (Japanese Boiler and Pressure Vessel Code similar to ASME Code in USA, DIN Code in Germany, and ISO Standard) internally heated with medium to low pressure steam, or from a cylinder internally heated by direct combustion or electromagnetic induction heating. These cylinders are surrounded by an open hood having only a roof and a curtain-type wall, or a loosely sealed hood having an opening for many sheets and ropes, a drive shaft and a piping, a duct, and a door moving up and down, having many gaps for moving up and down.
A large volume of medium temperature, low humidity air heated by low pressure steam is blown into respective parts of the hood, mainly into a pocket portion between dryer cylinders. The steam evaporated from the wet sheet material and the air blown mainly into the pocket portion are removed together with leaking air into the hood from many hood openings and gaps, under medium temperature and low humidity conditions with the relative humidity being 30% or less, so that dew condensation does not occur in the hood.
The absolute humidity of the discharge gas is low and the dew-point temperature is also low. Since heat transfer in condensation is quite little, the only way to heat the fresh is to heat by a sensible heat. The heated moist air including a large amount of discharged steam is directly released to the atmosphere. Hence an enormous amount of heat and water are wasted. Moreover, the air supply and discharge blower supplies and discharge large amount of air, thereby wasting a large amount of power.
The dryer cylinder having the second-kind pressure vessel structure consumes a large amount of low pressure steam of about 2 to 4 kg/cm
2
via a rotary joint. The steam condensed inside is extracted and returned to a boiler water supply. During this time, the drain gradually decreases in internal pressure in the tube through each flush tank of the drainage system, and finally returns to the atmospheric pressure in the drain tank of the power station, and re-evaporates and radiates heat.
In Tappi Journal published in May 2000, there is proposed a method of drying a wet sheet material at a high temperature in excess of 100° C., while restraining expansion and contraction thereof, wherein the sheet material is held between two endless steel belts, high pressure cases opened towards the belt surface are provided above and below thereof facing each other, to perform high pressure steam heating and high pressure water cooling, to thereby immediately condense the evaporated steam on the cooling surface, and the condensate is drained onto an endless fabric belt and carried away.
This method requires precious high pressure steam. However, since this involves high temperature steam heating in a closed vessel, the temperature in the vessel is restricted by the saturated temperature of the steam. With the impingement drying method of the present invention, the sheet material can be heated using super-heated steam of 250° C. substantially at atmospheric pressure (needless to say, a direct combustion method is desired for the heating method using a heat exchanger). In the closed vessel, the steam temperature of 250° C. means the steam at a pressure of 40.6 kg/cm
2
. Therefore, steam turbine power generation using the steam pressure difference, which is normally performed in the central power station is not possible. Moreover, this method requires a large quantity of cooling water, which can only be recycled as low temperature warm water.
Furthermore, the above method requires two pressure vessels facing each other and having a strong frame structure mechanically corresponding thereto, enormous investment in plant and equipment involving high pressure heat exchangers and high pressure pumps, and power and steam expenses. It has of course a large effect in improving some aspects of quality, such as paper strength, but on the other hand, the density becomes high due to the high compressive force by the upper and lower steel belts, thereby making the thickness of paper very thin.
The heating surface is smooth like a Yankee glazed surface, but on the other hand, the cooling surface is a corrugated surface with a fabric belt mark, and hence, there is a large difference between the front surface and the back surface. Therefore, there are problems in that it is not suitable except for liner board for parts of cold storage, and there is no heat saving effect.
For drying of thin sheet materials, such as thin paper and toilet and tissue papers, high temperature gas of 350° C. by natural gas or kerosene burner using fresh air or a part recycled moist air are impinging over the exposed sheet material without dryer fabric belt, under a high speed of 70-120 m/sec. from the outer surface of a single large-diameter dryer cylinder (generally called a Yankee dryer, with the sides and the entrance and exit for the sheet material thereof completely opened in the interior space) is heated by medium pressure steam of about 10 kg/cm
2
, and installed so as to open to the outside, which has a canopy hood disposed in a central top half portion of the cylinder. The sheet material has a Yankee glazed surface on one side only, and the other side remains rough. The use of manufactured product is therefore limited somewhat to such uses as a wrapping paper with one glazed side, tissue paper, and crepe paper. Moreover, there are problems related to fire accident by high temperature combustion gas and a diameter of the dryer cylinder becomes a problem with respect to transportation thereof and the diameter cannot be further larger.
Actually, for the paper industry that essentially requires a large amount of pure river water, it is common worldwide for the consolidated area of the paper mills to be in an area blocked by steep ridges, with narrow roads. Therefore, the width of the paper-making machine is restricted by the dryer width capable of truck transportation.
Moreover, there has also been proposed a method of recycling the evaporated saturated steam as a part of the heat supplied to drier cylinders, which are pressurized vessels, without supplying air to the loosely sealed hood. However, in practice, it is difficult to eliminate air completely from the loosely sealed hood of a large volume having covered passageways on the opposite sides thereof.
Furthermore, volumes of air enter into the sealed hood together with the sheet material and the endless fabric belt supplied continuously through an opening from the wet part. When the sheet material breaks due to internal shrinkage and forcible driving, and the low temperature wet paper is absorbed to the dryer cylinder and breaks over and over, it is necessary to stop the machine, open the loosely sealed hood, and after the interior of the hood has been cleared of the breakage, close the hood and re-start. Whenever this happens, the inside of the hood is replaced with the air. Under these circumstances, it is impossible to keep air entering into the hood below 4% as is generally recommended (oxygen: 0.84%, steam partial pressure: 729.6 mmHg) with the related art.
Further, the saturated steam inside the sealed hood enters into the wet zone, when cooled by the wet sheet material as well as external air entering the sealed hood from many openings. Then, moisture condensed on the metal surfaces of the hood and dryer frames may drip onto the sheet material, creating staining defects. Because of such inherent problems, this proposed technique has not been commercialized yet.
As is described in the introduction of “Theory of Drying”, Article 6.2 in Pulp and Paper M
Frishauf Holtz Goodman & Chick P.C.
Lazarus Ira S.
Ragonese Andrea M.
Tokushu Paper Mfg. Co., Ltd.
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