Structured packing for liquid-vapor contact column

Abstract: The present invention is structured packing (2) for use in liquid-vapor contact columns. Our approach comprises occluding peripheral openings of structured packings (2) that open into an interspace formed between the column shell (1) and said packings (2) when positioned in non-occlusive opposition to each other with a barrier film (3). This occlusion of peripheral openings reduces vapor flow in said interspace thereby enhancing liquid-vapor contact and also improving column performance.

Description: The present invention is structured packing for use in liquid-vapor contact columns. Our approach comprises occluding peripheral openings of structured packings that open into an interspace formed between the column shell and said packings when positioned in non-occlusive opposition to each other with a barrier film. This occlusion of peripheral openings reduces vapor flow in said interspace thereby enhancing liquid-vapor contact and also improving column performance. Structured packing is increasingly utilized by the chemical processing industry in liquid-vapor contact columns. Structured packing offers unique advantages of increased efficiency, lower pressure drop, greater capacity and net energy savings when compared to conventional heat and mass transfer devices, such as random packed columns or tray columns. Because of these advantages, structured packing is increasingly used to retrofit existing separation columns. Structured packings typically are fairly rigid pre-formed structures comprising ordered elements which form channels opening to the periphery of the packing. The materials of construction for such structured packings include corrugated meshes and sheets fabricated from metals, plastics and ceramics. Therefore, structured packings may not easily conform to the inner wall of a column shell; and in the situation where the inner shell wall is non-uniform, it may be necessary to undersize the pre-formed structured packing to facilitate its placement in the column. In these situations, the channels opening to the periphery of the structured packing provide a means for vapors to enter into the interspace between the column shell and the structured packing, thereby reducing contact of the vapor and liquid phase within the structured packing. This vapor bypassing causes a discontinuity in liquid-vapor contact in the column and also reduces heat and mass transfer between vapors and liquids in the column. This results in reduced column performance. One approach to reduce this discontinuity in liquid-vapor contact has been to fit the structured packing with one or more bands on the outside of the packing elements that serve as "wipers". These bands are intended to provide a seal between the packing elements and the column shell. The use of such bands is further described in U.S. Patents 5,486,318 and 5,224,351. The latter describes a sealing device prepared from a continuous band of an appropriate material of small thickness such as a metal or plastic. We have found that in those situations where an interspace exists between the column shell and the structured packing positioned therein; that such bands are not entirely satisfactory because significant vapor flow can by-pass the structured packing by flowing through the interspace between said bands. The present invention is, therefore, intended to reduce the vapor exiting the structured packings through the laterally orientated peripherally opening channels and thereby minimize the flow of vapors exterior to the bulk of the structured packing. Another method that has been proposed to reduce this prior art discontinuity is to fill the interspace between the column shell and the structured packing with a random packing. Using this method is often difficult in columns having irregular surfaces to assure adequate filling of the space to avoid channeling and the subsequent removal of the packing is more difficult and time consuming. This invention provides an improved liquid-vapor contact column as claimed in claim 1. The term "liquid-vapor contact column" means a contacting column or zone where liquid and vapor phases are contacted on structured packing to effect separation, or reaction and separation, of a fluid mixture feed to said column. The general size, shape, construction, and materials of fabrication of the column are not generally limiting and can be those known in the art for such columns. The term "structured packing" means any liquid-vapor contact media having a regularly repeating pattern of surfaces which facilitates the contact of liquid and vapor phases to effect heat and/or mass transfer between the phases. The structured packing can be formed from elements comprising corrugated sheets, textured sheets, stamped or bent sheets or combination of such sheets in the presence or absence of interspacing flat sheets. The sheets described above may also be perforated. Each of these sheets may further contain a combination of one or more physical modifications, including corrugations, texturing, stampings, bends and perforations. The physical shapes and modifications of the elements forming the structured packing is not limiting to the present invention as long as the elements can be arranged to form laterally orientated channels having peripheral openings. The structured packing has laterally orientated channels. By "laterally orientated" channels, we mean that when the packing is in a desired orientation for placement in a liquid-vapor contact column, the channels open at a side of the structured packing intended to be in opposition to the side wall of a column shell, as opposed to opening on the top or bottom of the structured packing. The material of construction of the structured packing is, for example, metal, plastic or ceramic. The structured packing is thus formed from stainless steel, polytetrafluoroethylene, polyethylene, ceramic or a combination of such materials. The material of construction may be, for example, in the form of a solid sheet, with or without physical modifications, or of a mesh with or without physical modifications. Examples of structured packings useful in the present invention are generally described in the following U.S. Patents: 2,047,444; 3,402,105; 4,186,159; 4,296,050; 4,455,339; 4,497,752 and 4,497,753. In this invention, the structured packing must have laterally orientated channels with peripheral openings which can be occluded with a barrier film. The barrier film of our invention is constructed from any material which can generally be conformed to the surface of the structured packing having the peripheral openings to thereby effect occlusion of said openings and to prevent vapor exiting therefrom. This barrier film can be formed from, for example, metal, plastic, rubber or foam. The barrier film can also be in the form of a sheet or foil. The barrier film can further be a flexible polymeric sheet formed from a plastic, such as polytetrafluoroethylene, polypropylene and polyethylene, or a rubber such as a silicone elastomer. The barrier film may also be formed from a thin metal sheet or foil or a curable silicone elastomer or foam composition which is cured in place. The thickness of the barrier film can be any thickness which provides a barrier for reducing vapors exiting the peripheral openings and which totally fills the interspace between the structured packing and column shell. In those situations where the barrier film is formed insitu around the structured packing by applying a curable viscous liquid or foam, our barrier film may also extend partially into the laterally orientated channels having peripheral openings. The method of retaining the barrier film in occlusive contact with the peripheral openings is not critical to this invention. The barrier film can be retained by attachment to the exterior surface of the elements forming the laterally orientated channels by, for example, welding, fusing, gluing, interdigitating, banding, or a combination thereof. The barrier film may also be retained in place by compressive fit against the column shell. The present invention comprises substantially occluding the peripheral openings with a barrier film. By the term "substantially", we mean that all or a portion of the peripheral openings of the laterally orientated channels are partially or totally occluded by the barrier film in a manner to restrict the exit of vapor from the peripheral openings. By use of the term "substantially", we intend to exclude those situations where minor portions of such peripheral openings are occluded by close compression against a column shell or by incidental compressive occlusion of the peripheral openings during manufacture, handling, and installation processes. In a preferred embodiment of this invention, at least 90 percent of the peripheral openings are partially or totally occluded by our barrier film. Figure 1 is a cross-section of structured packing comprising laterally orientated channels having peripheral openings occluded with a barrier film according to the present invention. Figure 2 is a cross-section of a column shell having positioned therein an array of structured packing elements having a barrier film occluding those peripheral openings at the surface of the structured packing elements in non-occlusive opposition with the column shell of this invention. Figure 3 illustrates an embodiment of our invention where multiple segments of structured packing are vertically positioned in a column shell. Figure 4 depicts a preferred embodiment of our invention where multiple segments of structured packing are vertically positioned in a column shell. When a structured packing is sized for use as a single element in a column, or as a vertically stacked segment, then the circumference of said element is partially or entirely occluded with a barrier film as illustrated in Figure 1. When the structured packing is sized such that an array of elements are necessary to pack the column, as illustrated in Figure 2, then it is desirable to only occlude all or a portion of the peripheral openings of the laterally orientated channels which are at the surface of the structured packing intended to be in opposition to the column shell. The advantage of our invention of structured packing is particularly realized when said packing comprises an outer surface in non-occlusive proximation to a column shell and laterally orientated channels exist having peripheral openings through the outer surface. By the term "non-occlusive proximation", we mean that a space exists between at least a portion of the outer surface of the structured packing and the column shell. The barrier film may occupy all or a portion of such space. Further depiction of our invention is provided by reference to Figures 1-4. Figure 1 is a cross-section of structured packing 2 comprising laterally orientated channels having peripheral openings occluded with barrier film 3 according to our invention. In Figure 1, barrier film 3 also totally encompasses the circumference of structured packing 2. Figure 2 is a cross-section of column shell 1 containing an array of structured packing elements 2 having barrier film 3 occluding those peripheral openings at the surface of structured packing elements 2 in non-occlusive opposition with column shell 1 as in our invention. Figure 3 illustrates an embodiment of our invention where multiple structured packing elements comprising laterally orientated channels having peripheral openings which are vertically stacked within a column shell 1. This shell has positioned therein multiple segments of structured packing 2; and each segment of structured packing 2 is sealed at the top against the column shell by wiper band 4. Furthermore, barrier film 3 is applied against the surface of structured packing 2, that is in non-occlusive opposition to column shell 1, to occlude the peripheral openings of the laterally orientated channels of structured packing 2. Finally, barrier film 3 essentially covers the total surface of structured packing 2 that is in non-occlusive opposition to column shell 1. Figure 4 illustrates a preferred embodiment of this invention where multiple structured packing elements, comprising laterally orientated channels having peripheral openings, are vertically stacked within a column shell 1. This shell has positioned therein segments of structured packing 2; and each segment of structured packing 2 is sealed near the top against the column shell by wiper band 4. Furthermore, barrier film 3 is applied against the surface of structured packing 2, that is in non-occlusive opposition to column shell 1, to occlude the peripheral openings of the laterally orientated channels of structured packing 2. Finally, barrier film 3 is sized and positioned such that a top portion of structured packing 2 is not occluded by barrier material 3. This configuration allows for any liquid which may accumulate in the space between column shell 1 and structured packing 2 to drain down the column. The following examples are provided to further illustrate the present invention as claimed herein. Example 1 Vapor distribution in a column containing a ceramic structured packing and having a 16 mm gap between the column shell and the structured packing was assessed. The column comprised a 1 m length metal cylinder of 32.4 cm in diameter. Positioned in the column at 25 cm from the top was a ceramic structured packing 29.2 cm in diameter and 15.2 cm in height (Flexeramic® 28, Koch Engineering Company, Inc., Wichita, Kansas). The 16 mm gap between the structured packing and the column was sealed by wrapping the structured packing with a 16 mm polypropylene rope at either the top (T), bottom (B), or both top and bottom (T/B), as described in Table 1 in the row labelled "Rope Wrap". In run 3, the structured packing was further wrapped with paper to occlude the peripheral openings ("Paper Wrap"). Air was fed to the bottom of the column through a distributor and airflow was measured by use of a probe inserted into the structured packing and into the interspace between said packing and the column shell at 10 cm from the bottom of the packing. The results of these runs are reported as the ratio of air flow in the interspace between the structured packing and the column shell to the air flow within the structured packing ("Ratio"). Effect of Barrier Material on Vapor Flow Run No. 1 2 3 Paper Wrap no no yes Rope Wrap B T/B T Ratio 2.07 1.45 0.33 Example 2 The effect of occluding the peripheral openings of structured packing on liquid-vapor contact was evaluated in a column. The column comprised a 0.6 m length of rectangular metal channel 9.5 cm by 10.8 cm in cross section. Positioned in the column was a ceramic structured packing 6.4 cm by 7.9 cm in cross section and 8 cm high (Flexeramic® 28). The approximate 16 mm gap between the structured packing and the column shell was sealed by wrapping the top edge of said packing with a 16 mm polypropylene rope. Water was fed to the top of the reactor through a distributor and air was fed to the bottom of the reactor also through a distributor. The relative humidity of the air exiting the top of the reactor was measured as an indication of liquid-vapor contact. In a control run in which the structured packing was not wrapped with a barrier material, the relative humidity of the air exiting the top of the reactor was from 70 to 85%. In a second run, the structured packing was wrapped with a 0.8 mm thick sheet of polytetrafluoroethylene to occlude the peripheral openings. In said run, the relative humidity of the air exiting the top of the reactor was 90 to 100%.

Claims: A liquid-vapor contact column comprising a column shell having positioned therein a structured packing comprising an outer surface in non-occlusive proximation to the column shell and laterally orientated channels having peripheral openings through said outer surface, characterized by substantially occluding said peripheral openings with a barrier film, wherein the upper portion of the structured packing is sealed against the column shell by a wiper band. A Liquid-vapor contact column according to claim 1 where the structured packing comprises corrugated sheets. A Liquid-vapor contact column according to claim 1 where the structured packing comprises corrugated ceramic sheets. A Liquid-vapor contact column according to claim 1 where the barrier film is a flexible polymeric sheet. A Liquid-vapor contact column according to claim 4 where the flexible polymeric sheet is a plastic selected from the group consisting of polytetrafluoroethylene, polypropylene and polyethylene. A Liquid-vapor contact column according to claim 4 where the flexible polymeric sheet is a silicone elastomer. A Liquid-vapor contact column according to claim 1 where the barrier film is formed from a metal sheet or metal foil. A Liquid-vapor contact column according to claim 1 where the barrier film is a foam. A liquid-vapor contact column according to claim 1 comprising multiple structured packing elements and wherein the peripheral openings are at least 90 percent partially or totally occluded by a barrier film comprising a flexible polymeric sheet.

Inventors: BISCHOFF TODD F, HALM ROLAND LEE, VAN KOEVERING DENNIS GENE, BISCHOFF, TODD F., HALM, ROLAND LEE, VAN KOEVERING, DENNIS GENE
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1998
Main classification: B01J19
IPC codes: F16J15, C09K3, B01J10, B01J19