Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Patent
1997-09-19
1999-11-02
Bennett, Henry
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
34122, D21F 500, F26B 1106
Patent
active
059746912
DESCRIPTION:
BRIEF SUMMARY
The invention concerns dewatering, i.e., dehydrating, a cellulose sheet or web, in particular in the field of manufacturing wadding or tissue paper, namely an absorbent paper of comparatively low specific surface weight and generally in creped form and used for household or sanitary purposes: toilet paper, paper towels, etc. In particular, the object of the invention is a dewatering method for a paper web implemented after the forming stage but before final desiccation.
As regards conventional paper manufacture and following web formation and first draining, dewatering is carried out by mechanical pressing before the web is dried. In the case of wadding or tissue paper manufacture, one known means consists in pressing and bonding by an appropriate adhesive the still moist web against a cylinder, which is commonly called a Yankee cylinder, and which is fitted with a drying hood.
A procedure for dewatering and drying by blowing hot air through the web which lays on a permeable fabric, which itself is carried along on a permeable support, is known. The support consists of a porous wall of a rotating drum. A hot-air flow at a pressure slightly higher than atmospheric is made to pass from the drum inside toward the web surface which it then passes through. An enclosure open on its side facing the web and at a shallow vacuum collects the moisture-saturated air which then is evacuated by a suction fan. According to U.S. Pat. No. 3,303,576, a web initially at 20% solids content and having a specific fiber surface weight of 20 g/m.sup.2 is raised to a solids content of 50% using a 250.degree. C. hot-air flow at a rate of approximately 2-3 Nm.sup.3 /sec/m.sup.2 (30-45 lbs/min/ft.sup.2) and at a pressure in the supply enclosure of about 5 to 15 cm water above ambient pressure. The solids content is raised to 80% in a second, through-air dryer. According to the patent and using this apparatus, uniform drying is achieved over the full width of the web without damaging the fibers. The effectiveness of such a system is due in part to the contact evaporation between the moist fibers entering the dryer and the drying air, and further to the air flow dragging the liquid water. Consequently, this kind of drying equipment with one or more cylinders is called a through-air dryer.
Whereas evaporative drying depends on the air volume and on the dry bulb and wet bulb temperatures of this air, the dragging of liquid particles depends on the air speed. U.S. Pat. No. 3,447,247 proposes drying equipment wherein the drying air is projected at high speed onto the web in the form of a plurality of high speed jets of small diameters. As a result, the air, instead of passing through the fiber material by following the zones of least resistance as will be the case if the pressure difference is low, is forced through the web over its full surface. More uniform drying is thus achieved. Moreover, the high jet speeds preclude sideways leaks and the demand for seals is less. Because of the efficacy of such apparatus, the other dryers and/or presses used in combination with the hot-air drying equipment may be eliminated. As regards the technology disclosed in this patent, the speed of the air jets so produced is 40 m/sec. Such a speed is substantially higher than that of conventional through-air dryers. However, it is also observed that the relative negative pressure at the suction box is kept at a low value, 30 cm of water or less.
Even though this kind of drying already was proposed many years ago, it appears not to have been adopted by industry, very likely because of the difficulty in controlling the high-energy air jets disturbing the structure of the paper web and the sealing of the system.
The invention proposes dewatering by simultaneously dragging water in the liquid state and by evaporation caused by a large flow of hot air passing through the moist web moved on a permeable conveying fabric. The procedure of the invention is characterized in that the air flow passing through the web is generated by a deep relative negative pressure of
REFERENCES:
patent: 3432936 (1969-03-01), Cole et al.
patent: 3447247 (1969-06-01), Daane
patent: 3913241 (1975-10-01), Chaikin et al.
patent: 4146361 (1979-03-01), Cirrito
patent: 4662398 (1987-05-01), Wywailowski et al.
patent: 5438765 (1995-08-01), Tyrmi et al.
R.H. Crotogino, "Weiterentwicklung der Trockenpartie; Prallstrom und Durchlufttrocknung," Das Papier, (1975), pp. 127-133.
Kientz Emmanuelle
Lehervet Jean
Lesas Claude
Marchal Paul
Bennett Henry
Gravini Steve
River James
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