MXPA97008818A - Distillation column that uses structured packaging that reduces pa flow - Google Patents

Abstract

The present invention relates to: A heat and / or mass exchange structure including a shell having an external vertical wall with an inner surface, a plurality of first corrugated packing sheets having an outer edge that is spaced away from the surface internal of the shell, and a plurality of corrugated packing seconds placed within the shell and interspersed between the pair of first corrugated packing sheets

Description

DISTILLATION COLUMN THAT USES STRUCTURED PACKAGING THAT REDUCES WALL FLOW FIELD OF THE INVENTION This invention relates to distillation columns and, more particularly, to structured packing for such distillation columns that substantially reduces column wall flow.

BACKGROUND OF THE INVENTION Distillation columns are used for a wide variety of separations in the industry. One area of application of the distillation columns is in the cryogenic air separation systems for the production of oxygen and nitrogen. In a distillation column, as the liquid and vapor flow past each other, both the mass and the heat are exchanged. In a cryogenic air separation facility, the liquid becomes hotter and richer in oxygen as it flows downward, while the gas flowing upward becomes colder and richer in nitrogen. During the past decade, structured packing in such distillation columns had been employed due to its low pressure drop and high mass transfer efficiency when compared to the more traditional internal distillation column components, such as trays. As shown in fig. 1 Many structured packages employ vertically oriented corrugated packing sheets 10 and 12, where the corrugations are positioned at an angle to the vertical. Each packing sheet is positioned so that its corrugation direction is inverted from the corrugation direction of its nearby packaging sheet. A vertical edge of each packing sheet is distinguished by the movement that imparts to the liquid flowing on its respective edge. Thus, the packing sheet 10 includes a vertical edge 14 which is characterized by the fact that the liquid flow directed downwards along the edge 14 is directed inwards and away from a wall surface adjacent thereto. Similarly, the packing sheet 12 includes a vertical edge whose corrugations direct the liquid flow in an external direction towards an adjacent wall surface. The foregoing can be better understood by reference to Fig. 2 where a side view of the packing sheet 12 is shown adjacent to the wall of the distillation column 18. The liquid is introduced in the direction indicated by the arrow 20 and is It infiltrates down like a film on the corrugated sheets with a vapor that flows upwards in channels formed between the sheets. A at a certain fraction of the descending liquid flow reaches the vertical edge 16 of the packing sheet 12 and travels along this edge. The liquid is also transferred to the inner surface of the wall of the distillation column 18 and travels down along this edge. The resulting wall flow deflects the packing which substantially reduces the efficiency of the distillation column and contributes to poor distribution of the liquid within the column. In fig. 3, a packed column section 22 is illustrated without a surrounding shell structure. The packing sheets are installed in packs 24 that are placed in layers, each layer generally between 15.24 and 30.48 cm in height. The adjacent layers 26, 28 and 30 are rotated about a vertical axis to improve the mixing of the downflowing liquid that is being distilled against a vapor flowing upwards. In small columns, each layer 26, 28, 30 can be comprised of an individual "block" of packing sheets formed by attaching the individual packing sheets together using bars that cut all the sheets or using bands that wrap around the circumference of each block. In large columns, each layer can be made from a plurality of layers of packing sheet blocks that fit together to fill the cross section of the containment container. In Fig. 4, a set of blocks comprises a single layer of the packed column section. The corrugated pattern within each block can be sawtooth or sinusoidal or some other repetitive shape. The individual packing sheets touch each other at points of contact along the peaks and valleys of the corrugated ones. The cross-sectional area of a distillation column is determined primarily by the velocities and densities of vapor and liquid flow. Typically, the columns are designed to operate at 80% -90% of the flow velocity at the flood point, for the packaging in question. The flood point is the maximum vapor flow rate at a fixed liquid flow rate at which the column is operable. Physically, the flood point occurs when the vapor load is such that the liquid can no longer flow countercurrent under gravity and against steam. The poor distribution of steam is usually a minor problem in the distillation columns since the vapor is the continuous phase and is capable of equalizing any radial flow variations through pressure equalization. In contrast, maldistribution of the liquid is a major problem since the degree of radial mixing is lower. For this reason, significant efforts have been made to design the liquid distribution structures that are placed on the package and supply the liquid to the distillation column. However, it is known that even with good distribution of initial liquid, a point is reached by moving down in the distillation column where the performance begins to deteriorate. For this reason, packed column sections are generally limited to approximately 15 packing layers. If a given column requires more than 15 layers, the column is inclined within two or more sections, with means for collection and redistribution of the liquid placed between them. Such structures involve additional capital costs and create increased column height. The added height is a severe sanction in the air separation industry, due to the need for a cold box and insulation to enclose the column and therefore reduce the escape of heat. There is a reduction in the yield with the height in such distillation columns and it is due to the maldistribution that induces the packing of the descending liquid flow. This phenomenon is generally attributed to the wall flow which is the liquid that flows down from the inside of the shell of the column, effectively diverting the packing. This is typically contained in the prior art through the use of wall cleaners which are a series of tabs that contact the inner surface of the shell of the column and redirect the wall flow within the packing. As shown in Fig. 5, the prior art has arranged the packing sheets by sandwiching each outwardly directed packing sheet 12 between inwardly directed packing sheets 10. The packaging sheets are positioned so that their vertical edges 14 and 16 are positioned to approximate the curvature of the wall of the distillation column 18, insofar as they are separated from the internal surface thereof. The difficulty with an arrangement as shown in Fig. 5 is that only one side of the outwardly directed packing sheet 12, at its edge, is contacted by an adjacent inwardly directed sheet 10. Therefore, there are no stitches of contact between the packing sheet directed inwards 10 and the packaging sheet directed outwards 12 for a distance A. It varies in size according to the arrangement of the packing sheets and the diameter of the distillation column. The exposed areas exhibit a region where the liquid flow has an unobstructed outlet to the inner surface of the wall 18. Furthermore, this flow represents maldistribution. As can be seen from the expanded areas in Fig. 5, the exposed area is a minimum for packing sheets and a maximum for shorter packing sheets. In addition to wall cleaners, the prior art has described a number of. techniques to try to decrease the maldistribution of the liquid. U.S. Patent 3,599,943 to Munters illustrates the use of transverse corrugated packaging sheets wherein the corrugations of the corrugations have vertical cuts near the lower edges thereof. Such cuts cause the liquid to flow along a fold to change the direction of flow before the liquid reaches an edge. . U.S. Patents 5,262,095 to Bosquain et al and 5,224,351 to Jeanot et al include deformations at the outer edges of each packing sheet to create an obstacle to liquid flow. Such deformations cause the liquid directed outward to be redirected in an inward direction. The innovations described in the aforementioned patents require additional processing of the individual packing sheets with the resulting cost increase. It is therefore an object of this invention to provide structured packing for a distillation column which reduces the rate at which maldistribution of the liquid develops. It is another object of this invention to provide the improved structured packing for a distillation column that requires a minimum additional capital cost and little aggregate manufacturing complexity. It is a further object of this invention to provide the improved structured packing for a distillation column that reduces the wall flow without requiring substantial structural modification for the corrugated packing sheets.

Brief Description of the Invention This invention comprises a heat and mass exchange structure that includes a shell having an external vertical wall with an inner surface. A plurality of first corrugated packaging sheets includes angled corrugations to carry a liquid flowing down away from the inner surface of the shell. Each first corrugated packing sheet has an outer edge that is spaced away from the inner surface of the shell.

A plurality of second corrugated packing sheets are placed inside the shell and each is sandwiched between a pair of the first corrugated packing sheets. Each second sheet of corrugated packaging includes angled corrugations to carry the liquid flowing downward to the inner surface of the vertical wall of the shell. Each second corrugated packing sheet has an outer edge that is no closer to the inner surface of the vertical wall of the shell than the outer edge of either pair of first corrugated packing sheets interspersed with the second corrugated packing sheet. BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages will be devised by those skilled in the art from the following description of a preferred embodiment of the invention and the accompanying drawings, in which: Figure 1 illustrates a pair of corrugated packing sheets which exhibit the directions of flow directed both outwardly and inwardly for the passage of liquid therethrough. Figure 2 is a schematic of an outwardly directed corrugated packing sheet and an adjacent distillation column wall. Figure 3 illustrates the structure of a packed section of a distillation column employing three layers of corrugated packing sheets.

Figure 4 illustrates a layer of packing sheets, in cross section, comprising a plurality of packing sheet blocks. Figure 5 illustrates a plan view of a single block / layer in a distillation column of the prior art. Figure 6 illustrates a plan view of a single block / layer embodying the invention. Figure 7 illustrates a plan view of a single block / layer incorporating a further embodiment of the invention. Figure 8 illustrates parameters that are used to arrive at designs for packing sheet arrangements embodying the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The invention provides packing sheets so that on each vertical edge, a packing sheet with an outwardly facing corrugation is surrounded on both sides by packing sheets with corrugations directed inwards. More specifically, the inwardly directed packing sheets are placed around each packing sheet directed outward so that there is no delta distance where the packing sheet is directed outwardly. By this arrangement, the liquid at the vertical edge of any outwardly directed packing sheet has an improved opportunity to be transferred within the package volume. The arrangement described above is more beneficial in circular columns where the adjacent packing sheets are cut to different lengths in order to better approximate the circular cross section. In the preferred embodiment, the outwardly directed packing sheets have their vertical edges recessed a distance of about one-half the height of corrugation folding behind the shortest of the inwardly directed packing sheets. It is understood that any packing sheet directed outwardly at one end, which extends fully through a circular column, is directed inward at its opposite end. A similar inversion occurs for an outwardly directed packing sheet. Accordingly, the edge relationships defined hereinafter are inverted at their terminal extremities if they extend through the circular column. Referring to Fig. 6, a packing sheet arrangement according to the invention is illustrated. The lengths of the outwardly directed packing sheets 12 are shortened with respect to the arrangement of the prior art. Therefore, each outwardly directed packing sheet 12 extends no further than the vertical surface of the adjacent adjacent inwardly oriented sheet of paper 10.

By this arrangement, each outwardly directed packing sheet 12 has contact point along its entire external vertical length for liquid transfer towards the packaging sheets directed inwardly 10. This reduces the amount of liquid that is allowed to pass. in contact with the inner surface of the shell wall 18 or which accumulates on the packing periphery.

Referring to FIG. 7, a further embodiment of the invention is illustrated wherein the outwardly directed packing sheet 12 is recessed inward from each inwardly directed packaging sheet 10 that is adjacent thereto. This arrangement ensures, with an even greater certainty, that the liquid does not accumulate on the outer vertical edge of each outwardly directed packing sheet 12 and also decreases the likelihood of the liquid separating from the outwardly directed packing sheet 12 and flow down the shell wall of the column 18. A preferred starting distance of each outwardly directed packing sheet 12 is approximately half the height of the rebar. The start is measured from the edge of the packing sheet directed inwardly shorter 10. The arrangements shown in Figs. 6 and 7 are particularly attractive in that they are applicable to any type of structured packaging that is manufactured from packaging sheets placed side by side. Such packing sheets do not require special tools or high capital expenditures, they allow even larger beds to be used more effectively. The optimum arrangement lowers the packing sheet directed outward a distance less than the folding height behind the shortest of its packing sheets directed inwardly close. Structured packages are generally textured, folded, then cut to the required lengths before being placed in a form that is that of a required "block". For columns of smaller diameter (generally smaller than 0.91 m), the sheets can extend the length of full string as shown in Fig. 3. For such an arrangement, the length of any given sheet is related to the perpendicular distance from the column axis by the following relationship (see Figure 8 for the nomenclature): L = 2 [(R - g) 2 - X2] 05 where: g is the space between the packing and the wall (in.), x is the perpendicular distance from the center line (in.), R is the internal radius of the shell (in.), L is the length of the sheet (in.). The exposed length, A associated with this sheet located at a distance x from the center line is calculated from; A = [(R - g) 2 - x2] 05 [(R - g) 2 - (x + h) 2] 0 5 where: h is the fold height (in.).

The total length that the blade needs to be shortened is then A plus the recess, r (Fig. 8). For example, given an internal diameter of 1.21 m, a fold height and wall space of 0.508 and 1.27 cm, respectively, some lengths of the sheets in the original layout along with the amount they need to be reduced are given in Table 1 The painting assumes a recess of 0.317 cm.

Table 1 REDUCTION IN LENGTH OF LEAF FOR ELEMENTS OF ROPE For columns of larger diameter, the individual blocks often do not extend through a complete layer of a column but instead are made up of several blocks. In such cases, only those blocks that contain an edge in the column wall need to be modified. Without recess, the invention is easily practiced by simply cutting a packing sheet without corrugation angles directed outwards for the same length as the shortest ones from the near ones. The construction of a block from the individual packing sheets is a little more complicated. Using the prior art arrangement, the packing sheets are usually stacked within the shapes that are in the shape of the block and are held. The total length that the sheet needs to be shortened is then A plus the recess, r (Fig. 8) . For example, given an internal diameter of 1.21 m, a fold height and wall space of 0.508 and 1.27 cm, respectively, some lengths of the sheets in the original layout along with the amount they need to be reduced are given in Table 1 The painting assumes a recess of 0.317 cm.

Table 1 REDUCTION IN LENGTH OF LEAF FOR ELEMENTS OF ROPE For columns of larger diameter, the individual blocks often do not extend through a complete layer of a column but instead are made up of several blocks. In such cases, only those blocks containing an edge in the column wall need to be modified without recess, the invention is easily practiced - simply cutting a packing sheet without corrugation angles directed outwards for the same length as the shorter ones The construction of a block from the individual packing sheets is a little more complicated.Using the prior art arrangement, the packing sheets are usually stacked within the shapes that are in the shape of the block and are held usually together by a metal bar that is driven through the packaging In the arrangement according to the invention, more care is required to ensure that the sheets are positioned correctly, since an edge will not make contact with the shape. Fixing the sheets together at the same time with the correct alignment before placing them inside the form. e be an easily removable nail or spike. Alternatively, for blocks with a flat edge such as in the larger columns, the sheets can be stacked vertically so that they will automatically line up correctly. An alternative is to produce shapes with rippling edges to keep the package in the correct alignment. A disadvantage of the arrangement according to the invention is the reduced coverage of the cross-sectional area of the column by packing due to the cut lengths of its sheets. The effectiveness of the approach to a circle, when using corrugated sheets, improves by decreasing the folding height. The arrangement according to the invention effectively produces an adjustment to the circle equivalent to that of a package with twice the folding height. The fractional loss of area coverage, AA, is equivalent to the loss in the total length of packing sheets. For a layout like the one shown in Fig. 6, without recess r, the fractional effective loss in coverage is approximated by; AA = 2h / pR As an example, air separation packages are typically used with h = 0.508 cm. The fractional loss in coverage is therefore 0.02, 0.01 and 0.007 for a column of 0.608, 1.21 and 2.43 m respectively. This is considered to be an insignificant amount in terms of loss of interfacial area although it may be more important in terms of vapor deviation. This occurs because, compared to the rest of the package, there is less flow resistance in the open space between the edge of the package and the shell of the column resulting from the shortening of the packing sheets. The problem is found in the designs of the prior art due to the presence of a wall space. This is solved by the use of wall cleaners having various designs, although they typically consist of a sheet or layer of wire mesh that wraps around the package and a series of tabs that widen from the package and make contact with the shell. column. Similarly, the problem of vapor deflection resulting from the invention can be eliminated a. through the use of wall cleaners that invade the space emptied by the shortened blade. The magnitude of this can be put into perspective by noting that the column fit of a specific surface area package of 500 m2 / m3 designed in accordance with the invention

Claims (6)

1. A heat and / or mass exchange structure comprising: a shell including an external vertical wall having an internal surface; a plurality of first heat transfer sheets and / or corrugated dough having angled corrugations to carry the liquid flowing downwardly from the inner surface and further having an outer edge separated from the inner surface; and a second sheet of heat transfer and / or corrugated mass sandwiched between each pair of the first heat transfer sheet and / or corrugated mass and having corrugated corrugations to carry the liquid flowing down towards the inner surface, the second heat and / or mass transfer sheet having an outer edge that is separated no closer to the inner surface than the outer co-located edges of the pair of the first heat transfer sheets and / or corrugated mass interspersed with the second sheet of heat transfer and / or corrugated mass. The heat and / or mass exchange structure as set forth in claim 1, wherein each second sheet of heat transfer and / or corrugated mass has a corrugation height h and the outer edge is spaced beyond the surface internal .. that the outer edges of any of the first corrugated sheets interspersed by the second heat transfer sheet and / or corrugated mass, by a distance r where 0 < r < h. 3. The heat exchange and / or mass structure as set forth in claim 2, wherein r is approximately h /

2. 4. A packing arrangement for distillation column comprising: a plurality of first heat transfer sheets and / or corrugated dough having angled corrugations to carry the liquid flowing downward to an inner region of the packaging arrangement and away of an outer periphery thereof and having also an outer edge; and a second sheet of heat transfer and / or corrugated mass sandwiched between each pair of the first sheet of heat transfer and / or corrugated dough and having corrugated corrugations to carry the liquid flowing down towards the outer periphery, the second heat transfer sheet and / or mass having an outer edge that is separated no closer to the outer periphery than the outer edges co-located from the pair of the first heat transfer sheets and / or corrugated mass interspersed with the second sheet of heat transfer and / or corrugated mass. The packaging arrangement as set forth in claim 4, wherein each second sheet of heat transfer and / or corrugated dough has a corrugation height h and the outer edge thereof is separated beyond the outer periphery of either of the pair of first corrugated sheets interspersed with the second sheet of heat transfer and / or corrugated mass, for a distance r where 0 < r < h. 6. The packaging arrangement as set forth in claim 5, wherein r is approximately h / 2. SUMMARY A heat and / or mass exchange structure including a shell having an external vertical wall with an inner surface, a plurality of first corrugated packing sheets having an outer edge that is spaced away from the inner surface of the shell, and a plurality of second corrugated packing sheets placed within the shell and interspersed between the pair of first corrugated packing sheets.