Patent Application
20140202028
The present invention relates to an apparatus and method for drying liquid coatings on a substrate. More particularly, the invention is directed to an inerted plate dryer and its use in drying solvent based coatings.
Drying or curing of liquid coatings on a substrate or web is typically achieved by passing the substrate through a drying chamber, in most cases, a conventional oven (either a floatation or roller supported oven), where the liquid is evaporated and the coating is dried or cured. The oven is heated with heating elements. The heat is passed onto the coating through convection or forced gas flow, typically air. Multiple zones of the oven may be employed to allow flexibility in the temperature adjustment. When organic solvent is used in the coating where explosion and fire can be a potential hazard, regulations impose a maximum concentration of the solvent allowable in the drying chamber to ensure safety of operation. This maximum concentration is defined in terms of a fraction or most often as a percentage (% LEL) of the lower explosive limit (LEL) of the solvent or mixture of solvents removed from the coating undergoing drying or curing. LEL is the lowest concentration where a conflagration or explosion can be propagated from an initially ignited point; LEL is a property of the solvent or solvent mixture, whereas % LEL is just a measure of concentration of a particular solvent or solvent mixture referred to the LEL of that solvent or solvent mixture. The maximum allowed solvent concentration in a given dryer (in terms of % LEL) that a dryer is allowed to safely handle by regulation ultimately does limit substrate or web speed.
The solvent removed from the coating is either condensed into liquid form through a condensation system, or more commonly burned by a thermal oxidation unit (TOX). When a condensation system is used, it often includes a tandem of condensers, typically and needs to operate at a low temperature (below 0° C.) to condensate most of the solvent, with a correspondingly high demand on energy. When a TOX unit is used, there is a maximum amount of solvent per unit time that can go through a TOX due to safety (explosivity, which constrains solvent concentration, and energy release, i.e. maximum operating temperature, which constrains the throughput). This imposes a limit on the solvent content out of the combined streams sent to the TOXs, and therefore ultimately imposes a limit on the maximum substrate speed through the oven.
To make a conventional oven inert, i.e. to use inert gas, rather than the oxygen rich air, could alleviate the potential of explosion and fire, and therefore, increase the solvent throughput a dryer can handle. However, the volumes and flow rates of the inert gas involved in that operation can make it expensive and affect adversely the economic viability of the process.
Another typical issue associated with conventional oven is blistering, i.e., appearance of bubbles in the dried coating. It is caused by rapid bubble growth from gases dissolved or entrained in the coating liquid and from the volatile solvents in the coating, which exhibit a high vapor pressure. To alleviate blistering or to allow the healing of blistering, usually both temperature and speed of the gas (commonly air) is reduced, typically in the first zone(s) of the oven when multiple zone are used.
Another approach to increase the throughput of a conventional oven is to add additional heating zones to that oven. However, the size and volume of conventional ovens makes it difficult to add new zones to retrofit existing installations. Accidents involving explosion or conflagration of solvent laden air above LEL concentration would also involve a greater volume of explosive mixture; Inertization becomes more expensive with the additional zones to inert and may take considerable time (15 to 30 or more minutes). This creates further delays in case of web rupture, which need dryer opening, cleaning, rethreading of the web, and re-inertization.
Another type of dryer that can be used for drying liquid coatings is a plate dryer. They may include heated plates at one or both sides of a moving web. They have been used in pultrusion and other curing processes. In these applications process velocities are low (<30 m/min). Typical flows of the inerted gas-solvent mix are in the range of 1 m/s with low external mass transfer and heat transfer, which makes it not suitable for higher speed coating lines.
U.S. Pat. No. 4,894,927, assigned to Fuji Photo, teaches the benefits of a low volume inerted plate dryer and how the system can include solvent recovery by condensation and how heat can be recovered by placing a heat exchanger between the dryer and the condenser. Furthermore, U.S. Pat. No. 4,926,567, also assigned to Fuji Photo, teaches how the incoming inert stream can be heated by heat exchange with the exhaust of an incinerator where the recovered solvent is burned. Neither patent teaches how the systems are sealed to avoid contamination of the ambient air to the heater and vice versa. Nor do they teach what conditions are needed for the system to be beneficial. Furthermore, both patents consider that the entire dryer exhaust stream undergoes condensation.
One type of plate dryers is designed with internal condensing surfaces, which are sometimes referred to as “gap dryers”. In this type of dryer heat is provided by a hot plate or any other suitable source. The carrier web moves over the plate or close by. Condensation occurs inside the dryer, over a cold surface that creates a concentration gradient that drives significant diffusion of the solvent. U.S. Pat. No. 5,581,905 (and sequels) assigned to 3M teaches substantially horizontal configurations of the plates where the cold surface is kept as close as ˜0.5 cm above the drying wet coating. Condensation occurs on the lower surface of the cold top plate which is grooved such that capillarity drives the liquid out towards the edges where it is drained. No significant convective gas flow occurs inside the dryer apart from that induced by web drag. There is the possibility of solvent dripping over the drying coating, as well as water condensation if air enters the system.
Therefore, there is a need for a dryer that can dry solvent based coating with high efficiency, high throughput and more economical than existing dryers.
The embodiments of the present invention described below are not intended to be exhaustive or to limit the invention to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present invention.
The present invention is directed to an inerted plate dryer and the method of using it to dry organic solvents-based coatings.
In one embodiment of the invention, an apparatus for drying a continuous moving web carrying a liquid layer, comprises: a housing enclosing a drying chamber, said housing having entry and exit slots through which said web may be passing through said chamber; said entry and exit slots having a sealing mechanism to prevent leakage of ambient air into the drying chamber, or leaking out of the gas stream from the chamber into the ambient; a bottom heated plate and a top heated plate aligned substantially parallel to each other with a space between them, said space is no more than 10 cm distance apart and preferably less than 5 cm apart, and most preferably between 0.5 to 5 cm apart; at least one inlet for a gas stream to flow into the chamber with a velocity, said velocity is between 2 m/s and 20 m/s, and preferably 6-15 m/s, flowing mainly in either the direction of the substrate movement, i.e. co-current, or against the direction of the substrate movement, i.e. countercurrent; at least one outlet for an exhaust to flow out of the chamber, wherein the carrier web is closer to the bottom plate than to the top plate.
In another embodiment of the invention, a method of drying a continuously moving web carrying a liquid, comprises passing the web through an enclosed dryer via entry and exit slots communicating wherewith; heating the web from both top and bottom using a top and a bottom heated plates, with said web located closer to the bottom heated plates; passing a gas stream from at least one inlet and flowing over the web at a velocity at least 2 m/s to generate an exhaust; discharging the exhaust through at least one outlet; dividing the exhaust into a condensing stream and a by-pass recycled stream; passing the condensing stream through a condenser to generate a liquid condensate and a solvent stripped stream; and mixing the recycled stream with the stripped stream and a make-up inert gas stream to form a inlet gas stream.
In another embodiment of the invention, a method of drying a continuously moving web carrying a liquid, comprises passing the web through an enclosed dryer via entry and exit slots communicating wherewith; heating the web from both top and bottom using a top and a bottom heated plates, with said web located closer to the bottom heated plates; passing a gas stream from inlets located close to each one of the entry and exit slots and flowing over the web, towards the middle of the dryer in co- and counter-current streams, at a velocity at least 2 m/s to generate an exhaust; discharging the exhaust through one outlet situated at the middle of the dryer; passing the condensing stream through a condenser to generate a liquid condensate and a solvent stripped stream; and mixing the recycled stream with the stripped stream and a make-up inert gas stream to form a inlet gas stream.
In yet another embodiment of the invention, a method of drying a continuously moving web carrying a liquid, comprises passing the web through an enclosed dryer via entry and exit slots communicating wherewith; heating the web from both top and bottom using a top and a bottom heated plates, with said web located closer to the bottom heated plates; passing a gas stream from a inlet located close to the middle of the dryer, splitting into co- and countercurrent streams, and flowing over the web, towards each dryer extreme, at least 2 m/s to generate exhausts; discharging the exhausts through outlets situated close to each one of the entry and exit slots; dividing the exhausts into a condensing streams and a by-pass recycled streams; passing the condensing streams through a condenser or condensers to create condensate stream(s) and solvent stripped stream(s); and mixing the recycled streams with the stripped streams and a make-up inert gas stream to form a inlet gas stream.
In yet another embodiment of the invention, a method of drying a continuously moving web carrying a liquid, comprises passing the web through an enclosed dryer via entry and exit slots communicating wherewith; heating the web from both top and bottom using bottom heated plates and at least one special unit atop, with said web located closer to the bottom heated plates; the special units atop can be but are not limited to IR lamps, UV lamps, electron beam emitters, radio frequency emitters, ultrasound sources, etc. which can be used alone, or in combinations between them and also with heated plates; passing a gas stream from at least one inlet, and flowing over the web at a least 2 m/s to generate a exhausts; discharging the exhausts through outlets situated close to each one of the entry and exit slots; dividing the exhausts into a condensing streams and a by-pass recycled streams; passing the condensing streams through a condensation system to create condensate stream(s) and solvent stripped stream(s); and mixing the recycled streams with the stripped streams and a make-up inert gas stream to form a inlet gas stream.
Other features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description of the various embodiments and specific examples, while indicating preferred and other embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.
These, as well as other objects and advantages of this subject matter, will be more completely understood and appreciated by referring to the following more detailed description of the presently preferred exemplary embodiments of the subject matter in conjunction with the accompanying drawings, of which:
Unless otherwise indicated, the illustrations in the above figures are not necessarily drawn to scale.
Referring to
Throughout this disclosure, the term top heated plate refers to the heated plate that faces the liquid coated side of the web or substrate. The term bottom heated plate refers to the heated plate that faces the other side of the web or substrate.
The oven can be made inert by saturating it with a proper inert gas, and by maintaining an Oxygen concentration under a critical value, typically at approximately 8% by volume or below. Table 1 lists the maximum oxygen concentration in percentage of volume below which explosion or deflagration or the gaseous mixture containing a solvent can't occur. During operation of the inerted dryer, fresh inert gas would be provided at the entrance seal and additionally in the recirculation system if needed. The system must be equipped with an appropriate number of O2 sensors so to effectively monitor the O2 concentration within the oven chamber and the recirculation ducts. The 90% response time of the monitoring system should be preferably less than 20 s, meaning that the system will signal 90% of the magnitude of a change in concentration within 20 s from its occurrence. The system can be set to alarm at a much lower level, such as 3% of oxygen by volume, and trigger a coater shut-down at 4% oxygen by volume.
The inert gas can be any appropriate gas, such as nitrogen or CO2. Due to low or no oxygen concentration, the risk of explosion and fire is greatly relieved and therefore, the restriction on the amount of solvent in the gas stream is no longer relevant. This leads to one advantage of the inerted plate dryer where higher percentage of solvent can exist in the gas stream inside the drying chamber. Therefore a significant portion of the solvent in the exhaust stream can be recycled back into the dryer. The demand on the downstream condenser is greatly relieved as lesser amount of solvents need to be condensed.
The existence of higher amount of solvent vapor in the gas stream has another benefit: it relieves blistering and delivers defect free coating. The maximum amount of solvent vapor that can be used in the gas stream depends on the type of solvent. The presence of the most volatile solvents in significant concentrations but quite below the saturation concentration, will alleviate blistering by inhibiting the evaporation of the most volatile solvents without a drastic reduction of drying capacity. On the other hand if some of these most volatile solvents are good solvents, they will plastify the upper layer of the coating being dried, avoiding or delaying transfer skinning (formation of a dense layer that exhibits strong resistance to solvent transfer), and therefore will alleviate the blistering consisting of bubbles trapped by the skin layer. In general, as long as the concentration of that solvent in the gas stream is a fraction (for example less than 60%) of the equilibrium concentration, significant mass transfer can still be achieved inside the dryer. For approximate estimation purpose, this would mean that the partial pressure of the solvent in the gas stream over the drying coating is less than the vapor pressure of the solvent at the gas stream temperature times the mol fraction of the solvent in the coating just below the gas stream.
The inerted plate dryer can be also be beneficial when operated as a deaereator. When there is a significant amount of gas dissolved or entrained in the liquid coating, operating the inerted plate dryer with the inerted gas stream saturated or close to saturation with solvent will inhibit solvent evaporation, while heating the liquid coating will facilitate the escape of the dissolved or entrained gas. These conditions are also the better suited for the liquid coating to heal if there were bubble bursting.
To ensure inerted environment inside the heating chamber, the entry and exit slots 160 need to be properly sealed. One embodiment of the invention uses seals with top and bottom faces close enough to the coated substrate to minimize entrainment or convection of ambient air into that oven and the escape of solvent laden gas out of the oven. Referring to
In addition to narrow opening, other methods can also be used to prevent leakage at the entrance or exit slots, such as impinging jets, inerted gas curtains, labyrinths, conditioning chambers, gas extraction slits, etc. For example, still referring to
If the inerted plate dryer is installed ahead of a conventional oven which entrance is at a slightly below atmospheric pressure then the exit seal may use process N2 from the inerted dryer recirculation loop (polluted with organic solvents vapor) provided that the solvent laden gas issued by the exit seal is delivered to the entrance of the conventional oven through a connecting enclosure.
The gas stream can be fed into the heating chamber in concurrent or counter-current directions with respect to the moving coated substrate. In the co-current feeding, the gas stream comes in through inlets near the substrate entrance, and the exhaust is discharged through the outlets near the substrate exit. In the countercurrent feeding, the gas stream comes in through inlets near the exit, and is discharged through the outlets near the entrance. An initial co-current followed by counter current feeding can be achieved through feeding through inlets near both entrance and exit slots, and discharging through an outlet in the middle of the chamber. Similarly, an initial counter-current followed by co-current feeding can be achieved through feeding through an inlet at the middle of the dryer and discharging through outlets near both entrance and exit slots. Also, a single inerted plate dryer can include a multiplicity of co-current and countercurrent sections, with the adequate placement of inlets and exhausts for the gas stream. The direction of feeding does impact the interaction between the hot inerted gas stream and the coated liquid layer and therefore the drying history of the coated layer and, in consequence, the efficiency of drying.
Now referring to
It should be stressed that a single inerted plate dryer unit can comprise one or more co-current and countercurrent inerted gas stream section, with the placement of multiple feed and exhaust ports for the inerted gas stream.
The top heated plate and the bottom heated plate can be heated through any suitable mechanisms known by person skilled in the art. The plates can be single units or arrays of smaller plates, as may be required to accommodate a curved path, and also to allow flexible temperature control. Each heated plate or plate array can have one or multiple heating zones. The temperature of each zone can be adjusted independently, so that, for example, temperatures of plates atop the web can be different from that of the plate under the web, or one of these plates can be heated and the other set at ambient temperature. The temperature can also vary from one zone to another for the same plate or plate array along the web path. Also, top plates may be substituted totally or partially or be intercalated by special units such as IR lamps, UV lamps, electron beam emitters, radio frequency emitters, and ultrasound sources. The gap h between the two plates is kept at a small number to ensure efficient heat transfer, and high enough gas velocity inside the chamber. Such inter-plate gap is preferred to be no more than 10 cm apart. It is even more preferred to be less than 5 cm apart and it is most preferable when it is between 0.5 and 3.5 cm. The plate spacing near the extremes can be larger than in the rest of the dryer to accommodate the feed and exhaust assemblies as well as the mounting of the seals. The moving web is positioned in between the two heated plates, being closer to one of the plates, preferably the bottom plate. The distance h2 between the bottom heated plate and the moving web should be kept as small as possible. It is preferred to be less than a 20 mm. It is even more preferred to be less than 10 mm. The distance h1 between the top heated plate and the moving web should also be kept to be no greater than a few cm. It is preferred to be less than 5 cm. The plates can have a mechanism that allows adjustment of the distance between the corresponding top and bottom plates. The distance from the bottom plate to the web, which can move over rollers, can be set by adjusting the bottom plate.
The top and bottom heated plates may be angled with respect to each other. The angle between the top and bottom plates can vary along the path of the web for best drying effect or accommodating other accessories. These sections where the top plates and the bottom plates are at an angle to each other may be used to control the pressure along the inerted gas stream path as kinetic and pressure energy are exchanged, with certain losses, in the converging & diverging passages that the plates create. Thus adequate placement these of converging and diverging zones near the inlet and exit seals for pressure regulation may facilitate seal action. Also, greater velocity in the narrower sections would enhance heat transfer and the associated mass transfer.
The surface of each heated plate can be smooth, or textured. Textures can be designed to enhance mixing (likely turbulent atop the substrate and laminar between the substrate and the bottom plate) to enhance mass and heat transfer rates inside the chamber. The textures on the bottom plate can also be designed to create a laminar layer between the bottom plate and the web such that the web can move as close as possible to the heated plate without actually touching the plate. Eddies in cavities on the lower plate can also be used to increase or maintain high enough heat transfer rates. The simplest texturing is shallow slots running across the plates' width. Also, localized depressions can be produced on the plate surface, in a staggered pattern with respect to the direction of flow of the inerted gas stream (or machine direction). Alternatively, fixtures can be mounted over the plates, as thin strips running across the plates' width. Also other shapes such as but not limited to thin discs, ovals or tear shaped flats could be mounted in a staggered pattern, in the machine direction, to enhance secondary flows. If these fixtures are made of soft material, they can be used to support the substrate, which would slide over the fixtures, instead or in addition to rollers
In another embodiment of the invention, multiple inerted plate dryers can be used along the web moving direction. This can be effective when just one inerted plate dryer would not be able to dry the coating satisfactorily, even with optimized operating conditions. The design length of a single inerted dryer zone is ultimately limited by pressure drop through the drying chamber, which may reduce the effectiveness of the seal. Also when an oven length is too long high solvent concentration can accumulate in the gas stream, which would deteriorate mass transfer on the remaining length of the oven and make the oven ineffective.
The inerted plate dryer of the current invention can be used as a stand-alone drying unit, or as an add-on to an existing installation, due to its smaller volume and slender size. For example, it can be positioned before a conventional oven. The inerted plate dryer can be used to flash out a significant amount of solvent from the initially solvent-rich coating to deliver a partially dried coating to the conventional oven, and therefore relieve the amount of solvent to be handled by the downstream conventional oven and the installed TOX. Therefore, the use of the inerted plate dryer as a first drying zone can increase overall drying efficiency for a given total length of oven. This can be advantageous when used to increase capacity of older assets.
Given targets of line speed and final residual solvent concentration, there is a minimum length of the inerted plate dryer needed to deliver a defect free dried coating. If the inerted plate dryer is shorter that this minimum length, it is not beneficial because either a) the dried coating at the end of the dryers retains a high solvent concentration, at low temperatures of the inerted gas stream and heated plates, and/or b) blistering is induced inside the inerted plate dryer and/or solvent concentration is exceeded in the following conventional dryer as the temperatures of the inerted gas stream and/or heated plate are increased. Even though an IPD is well suited as a first drying zone preceding a conventional dryer, the drying efficiency, as measured by the maximum speed at which a given coating can be dried, can reach a maximum as the ratio of inerted plate dryer length to overall oven length is increased; this would happen when the conventional dryers exhibits better heat and mass transfer efficiencies, as modern high speed floatation ovens are bound to have, despite their LEL limitation, which in any case becomes irrelevant at the latter stages of drying. Therefore, there is an optimal window for design and operation of an inerted plate dryer.
Table 2 is an example of operating parameters to demonstrate the concept of a minimum length of the plates. The inerted plate dryer is placed before a conventional oven with multiple zones. The amount of solvent in the coating upon entering zone 1 of the conventional oven is calculated as a percentage to the lower explosivity limit (LEL) allowed at the operating condition in the ensuing conventional zone 1. Starting from a case with marginal % LEL (45% LEL) in that zone and keeping the plates at 2.2 m long, increasing or decreasing the temperature of the plates and of the inert gas does not decrease the amount of solvent in the gas of the ensuing conventional zone 1. Therefore the inerted dryer cannot be beneficial for line speed increase. When the plate length is increased to 3.2 m, and the gas and plate temperatures to 120 C the amount of solvent in the coating entering zone 1 is significantly lower than that without the inert plate dryer. Therefore the line speed can be increased until the LEL in the conventional zone reaches the limit of 45% LEL again. The inerted plate dryer is now beneficial.
In operating the inerted plate dryer, finding the window for design and operation require the optimization of the design and operation parameters, such as plate number, plate lengths, plate temperature, gas velocities, fraction of the gas stream that undergoes condensation, condenser conditions, etc. In general, optimal design and operating conditions are dependent on the solvent compositions of the coating. Both the design and operation optimization entail the analysis of the mass, energy and momentum balances for each drying zone, together with size and operational constrains. Given the size and complexity of this analysis and optimization, full analysis requires the use of modern numerical process simulation, as is obvious to a knowledgeable practitioner of the art.
In the processes described above, other ancillary equipments, such as fans, de-misting equipment, water separators, valves (controlled or not), etc., can be also used, as is evident to a knowledgeable practitioner of the art.
The foregoing detailed description of the present invention is provided for purposes of illustration, and it is not intended to be exhaustive or to limit the invention to the particular embodiments disclosed. The embodiments may provide different capabilities and benefits, depending on the configuration used to implement the key features of the invention. Accordingly, the scope of the invention is defined by the following claims.
All patents, published applications, and articles noted herein are hereby incorporated by reference in their entirety.
While the subject matter has been described in connection with what is presently considered to be the most practical and preferred embodiments, it will be apparent to those of ordinary skill in the art that the subject matter is not to be limited to the disclosed embodiments, and that many modifications and equivalent arrangements may be made thereof within the scope of the subject matter, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and products. It is particularly contemplated that one or more features or aspects of any one or more embodiments described herein, can be combined with one or more other features or aspects of other embodiments.
The present application claims the benefit of U.S. Provisional Application No. 61/522,547 filed Aug. 11, 2011, which is incorporated herein by reference in its entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US12/50145 | 8/9/2012 | WO | 00 | 2/10/2014 |
| Number | Date | Country | |
|---|---|---|---|
| 61522547 | Aug 2011 | US |
