This invention relates to packed bed assemblies of random-dumped packings used for mass and heat transfer operations between two fluids, typically operations such as gas absorption and desorption, distillation, liquid extraction, and the like.
Extended-surface, random-oriented, packed beds for mass and heat transfer applications are widely used in industry, typically for gas-liquid and liquid-liquid contact. While most packed bed assemblies comprise vertical cylindrical columns employing countercurrent gas-liquid flow, referred to as packed towers, horizontal gas flow units are also known to the art, and are referred to as cross-flow contactors. To save energy and capital costs in either vertical or horizontal flow configurations, industry requires the highest flow capacities and lowest pressure drops for the packed bed contacting media. These objectives have been partially met in recent years primarily through the use of larger packing sizes.
At equal gas and liquid flows or loading rates in random-dumped beds, larger packing sizes of a given shape and design have relatively lower pressure drop and mass transfer performance than the smaller packing sizes. Because of their higher void space, both as individual packing elements, i.e, packing pieces, and as a packed bed, assemblies of the larger packing sizes also have higher limiting liquid and gas flow capacities (loading and flooding) than the smaller sizes of the same design. However, the gain in gas and liquid flow capacity for the larger packing sizes comes at the expense of lower mass transfer performance. The larger packing sizes have lower packing densities, i.e., lower number of packing elements per cubic foot and higher fractional void volumes, than do the smaller size packing elements of the same design. At the higher liquid loadings allowed by the lower packing density, the higher voidage of packed beds of the larger size packings results in increased gas axial back-mixing and loss of the desired countercurrent gas-liquid flow.
Various geometric packing shapes and configurations have been used in attempts to minimize the loss in transfer performance with increasing tower packing size. Nevertheless, for any given packing design, a significant portion of the gain in reducing tower diameter through the use of larger, higher flow capacity, packing sizes is offset by the deeper bed depth required to achieve the desired degree of mass transfer efficiency.
Most random tower packings have shapes that provide anisotropic gas flow resistancexe2x80x94that is, they are relatively xe2x80x9copenxe2x80x9d for gas and liquid flow along one axis, and have a higher amount of planar or filamentary deflection surfaces along the transverse axis. Examples of packings with this property include cylindrical packings such as Raschig and Pall rings, spherical packings such as Jaeger Tri-Packs(copyright), as well as most plastic packings made by injection molding. To facilitate release from the mold, injection-molded packings necessarily have relatively high projected open area in the release direction, and typically, not in the transverse direction. Thus, these packings are highly anisotropic with respect to fluid flow resistance.
Within a packing element, gas and liquid flow will tend to take the path of least resistance, or through the xe2x80x9copenxe2x80x9d or mold release direction. While the random orientation of the elements in the bed is depended on to overcome this adverse property, gas flow macro-direction changes in the bed will be of the order of the packing size. In the smaller packing sizes, gas flow direction changes occur frequently with respect to the bed depth, and gas mixing is adequate. However, for the larger packing sizes the number of flow direction changes per foot of packing depth may be low enough to result in poor gas mixing. For example, a packed bed of 1-inch anisotropic packing elements would theoretically tend to have approximately 10-12 gas flow macro-direction changes per foot of packed depth. On the other hand, a 3xc2xd inch size packing would tend to have only 3 to 4 gas flow direction changes per foot of depth. This contributes to the loss in mass transfer performance with an increase in packing size in anisotropic packing elements. This characteristic has been tacitly acknowledged by the development of xe2x80x9cshallowxe2x80x9d packing shapes that have a relatively short axial depth, such as those disclosed in U.S. Pat. Nos. 3,957,931 and 6,007,915. There are additional parameters, such as the degree of liquid mixing and surface renewal frequency, whose contribution to transfer efficiency decreases with an increase in packing size for a given packing shape and style.
The use of sections or zones of different packing sizes has been taught in the prior art to resolve some specific packed bed problems. For example, because of the typically lower volumetric density of tower packing adjacent to the containing wall, the flow resistance in this area is less than in the center of the packing. Gas and liquid tend to channel along the wall, a phenomenon known as xe2x80x9cwall effectxe2x80x9d. Cameron and Bharga, in U.S. Pat. No. 5,679,290, teach the use of two different sizes of packing in order to overcome wall effect and to obtain uniform gas flow across the column diameter. In ""290, Cameron and Bharga use a plurality of a first packing size in an annulus adjacent to an upper part of the tower wall and a second plurality of a larger packing size in the core of the column in order to obtain uniform gas flow through the tower cross-sectional area.
Because flooding often occurs as a result of the restricted flow area adjacent to the packing support tray in countercurrent flow columns, it is known in the art to provide for higher flooding capacities by using a zone of a larger packing size on the support tray, and then a smaller packing size in the remainder of the column. It is also known in the art to use a zone or layer of smaller packing size on top of a larger column packing size in order to enhance initial liquid distribution. Various assemblies of zones and layers of packing elements are illustrated in the following Weber U.S. Pat. No. 2,055,162, Wible U.S. Pat. No. 2,271,671, Huber U.S. Pat. Nos. 3,285,587, 3,957,931, McKeown U.S. Pat. No. 4,002,705, Hoppe U.S. Pat. No. 4,333,894, Oshima U.S. Pat. No.5,242,626, Nagl U.S. Pat. No. 5,302,361, and Sunder U.S. Pat. No. 6,425,574. The zones of packing elements proposed in these and other prior art disclosures may be random or ordered, or combinations of one or more random or ordered zones or layers or packing elements. In none of the prior art of which I am aware, however, are two different packing sizes mixed with each other or completely co-mingled for use either as the sole packed bed contacting means or as the makeup of a bed, layer, or zone in a combination of beds, layers or zones.
It is an object of the invention to provide a method and apparatus for mass or heat transfer between two fluids in the form of a random-dumped packed bed having improved mass transfer performance combined with high limiting flow capacities.
This invention combines the supplemental surface area and mass and heat transfer capacity of a plurality of a first packing size with the lower pressure drop and increased gas and liquid flow capacity of a plurality of a suitable second larger packing size, by mixing the two sizes to obtain a substantially uniform mixture of the two disparate packing sizes. Mixing of the two packing sizes to a substantially uniformly dispersed mix may be done by any conventional means such as a tumbling drum, or controlled volumetric metering or a combination thereof. The mixed bed of this invention provides for the contribution of the supplemental mass and transfer area of the small packing size fraction while substantially retaining the higher limiting flow capacities of a bed of the larger packing size in which the smaller size packing elements are embedded.
Additional advantages accruing to the mixed-size packed bed of this invention include more uniform fluid flow, including possible alleviations of wall effect, and reduced axial back-mixing. A characteristic of a random-packed bed is that the size of the interstitial void spaces increases as the packing size increases. The interstitial voids are areas of zero flow resistance and zero active mass and heat transfer surface, except for liquid drop free fall therethrough. The interstitial voids between packing elements can also create low-resistance by-pass routes for gas and liquid, causing liquid channeling as well as gas axial back-mixing. These latter effects impair the desired countercurrent gas-liquid contact in a vertical packed column and result in lower contact efficiencies in a cross-flow contactor. Partially filling the interstitial inter-element voids of a packed bed of a larger size packing with smaller packing pieces replaces the empty void spaces with supplemental mass and heat transfer area, and provides for more uniform gas flow resistance and augmented liquid collection and re-distribution.
By studying shapes such as rings and spheres, it is possible to establish the general factors governing selection of the size mixture that will provide a uniformly dispersed and stable mixed bed in the method and apparatus of this invention. A mixture of packing sizes in which the volume of a smaller packing piece is approximately equal to, or greater than, the interstitial inter-element void space between the larger size packing elements provides mixed bed stability and is the preferred mixture. When the smaller packing element volume is larger than the average interstitial inter-element void space of the larger packing size bed, the final mixed bed volume is greater than the sum of the individual bed volumes of the two packing sizes prior to mixing. Because packings are sold on the basis of volume, this property is economically advantageous.
Certain packing shapes or designs, particularly packing elements with external projections or ridges, tend to resist settling and may have unusually high interstitial or non-uniform inter-element void space. In such cases, attempts to mix in a packing size that is substantially smaller than the interstitial void space between packing elements may lead to separation, non-uniform distribution or difficulty in mixing. For example, attempts at mixing 1xc2xc-inch Jaeger Tri-Packs(copyright) with 3xc2xd-inch Jaeger Tri-Packs(copyright) gave beds where the smaller packing pieces fell through the interstitial voids of the larger packing and did not remain dispersed.
While the size and shape of inter-element void spaces will vary with the design and size of the packing elements, void spaces are typically formed between more than two elements; that is, a grouping of elements. The number of the void spaces in a packed bed is therefore significantly less than the number of packing elements comprising the bed. Because the number of packing elements per unit volume increases rapidly with a decrease in nominal size, as is illustrated by the data of Table 1 for spherical and ring packings, relatively small volume percentages of a smaller packing size are required to substantially fill the void spaces of a random bed of a second larger size packing element.
In working with various combinations of packing elements to be used in my invention, the percent by volume in a bed and the percent by number of a given packing element in the bed can be calculated from a table such as Table 1, which can readily be made up for design configurations of packing elements other than those in Table 1. As an example, a 10% by volume mix of a nominal 1xc2xc-inch size Jaeger Tri-Packs(copyright) (1637 pieces per cubic foot) with a nominal 2-inch size Tri-Packs(copyright) (350 pieces per cubic foot) contains 162 1xc2xc-inch size packing elements and 315 2-inch packing elements, or a mix containing 34% by number of the smaller packing size. A 20% volumetric mix of the smaller 1xc2xc-inch size will contain 325 1xc2xc-inch pieces and 280 2-inch pieces, yielding a mix that contains 53.7% by number of the smaller packing size. Similarly, a mixture of only 10% by volume 1-inch Koch Flexirings(copyright) (1464 pieces/cubic foot) and 90% 1xc2xd-inch Koch Flexirings(copyright) (438 pieces/cubic foot) produces a packed bed containing 27% of the 1-inch rings by number.
There are a variety of packing shapes and geometries, including cylinders, spheres, saddles, etc., and the configuration and size of the interstitial inter-element voidage varies with the packing geometry and packing size. In addition to the well-known Jaeger Tri-Packs(copyright) and Koch Flexirings(copyright) mentioned above, see the variously configured packing elements disclosed in the following Ellis U.S. Pat. No. Ellis 3,957,931, Leva U.S. Pat. No.4,203,934, Hackenjos U.S. Pat. No. 4,203,935, Leva U.S. Pat. No. 4,316,863, Glen et al U.S. Pat. No. 4,554,114, Lang U.S. Pat. No. 4,724,593, Halbirt U.S. Pat. No. 5,543,088, Southam U.S. Pat. No. 5,670,095, Rukovena U.S. Pat. No. 6,007,915, Fan U.S. Pat. No. 6,182,950, Shojaie U.S. Pat. No. 6,371,452, and Sunder U.S. Pat. No. 6,425,574. My invention may be used with any of these or any other commercially available packing elements. This diversity of packing elements, however, suggests that preliminary testing of mixtures of a given packing design may be useful in order to suitable or optimum size mixtures for the application of this invention.
In the method and apparatus of this invention, the volume percent of the smaller packing size of the total packing volume is from 5 to 50%, and preferably from 5 to 30%. The appropriate volume and number mixture of sizes may be estimated from the piece count per unit volume data for each selected packing type and size combination by anyone skilled in the art of packed bed design. Alternatively, interstitial void volumes may be visually measured or estimated by mixing test quantities of the different packing sizes in a suitable container.
Precise calculations of the volumes of either the packing elements or the void spaces are difficult to make, particularly in view of the many different designs. Accordingly, as used herein, the term xe2x80x9csizexe2x80x9d is not an absolute, but a relative, term. The relative size of a packing element may be arbitrarily taken as equal to the volume occupied by a given number of randomly dumped packing elements divided by that number. For example, if 1000 packing elements A fill a cubic foot when randomly dumped, then the relative size of packing element A may be taken as equal to 0.001 cubic feet, or 1.73 cubic inches. Referring to Table 1, although they are both nominally designated as xe2x80x9c1-inchxe2x80x9d sizes, using the foregoing definition, the 1-inch Jaeger Tri-Packs(copyright) have a relative sized of 0.75 cubic inches as against 1.18 cubic inches for the Koch Flexrings(copyright).
As used herein, the term xe2x80x9cinterstitial inter-element void spacexe2x80x9d is also a relative termxe2x80x94that is, it represents an average of the volumes of the voids between contacting or nearly contacting packing elements. The average void volume between groups or assemblies of the packing elements is related to the relative size of the packing elements for a given configurationxe2x80x94that is, the larger the packing element size, the larger the void space between assemblies of the packing. Therefore, the relative interstitial inter-element voids may be taken as directly proportional to the relative size of the packing element as determined above. As used herein, when I refer to a packing element B which is of a size substantially equal to the average size of the interstitial void spaces of a random packed bed of a given type and size of packing element, I mean it has a size which, when distributed substantially evenly throughout a bed of the elements A, will occupy the void spaces in the bed of larger elements A while neither increasing the total volume of the bed nor falling through the void spaces.
Preferred mixture percentages may also be based on the relative cost increase of the mixture compared to the base price of the larger packing size. For example, the prices of polypropylene Jaeger Tri-Packs(copyright), in the nominal 2-inch and 3xc2xd-inch sizes, are currently $14.24 and $9.36/cubic foot, respectively. From Table I, 5% by volume of the 2-inch size would have 18 elements, representing a cost of $0.72. The 95% of the 3xc2xd inch packing, or 46 pieces, would cost $8.97, giving a total cost for the mixture of $9.69. This is only $0.33 more per cubic foot than the $9.36/CF of the larger packing size, or a 3.1% increase, assuming volume additivity. However, tests show that, for this combination of packing sizes, the volumes are more than additive (the 2-inch polypropylene Jaeger Tri-Packs(copyright) packing is larger than the average interstitial voids of the bed of the 3xc2xd inch packing size) so that the percent cost increase would be even less than 3%. For this size mixture of polypropylene Jaeger Tri-Packs(copyright), there would be 18 of the 2-inch elements mixed with 46 of the 3xc2xd inch elements, giving a number percentage of 28% of the smaller packing size for a nominal 5% volumetric mix.