Gas and liquid contact apparatus – Contact devices – Porous mass
Reexamination Certificate
2002-09-25
2003-12-23
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Porous mass
C261SDIG007
Reexamination Certificate
active
06666436
ABSTRACT:
FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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 resistance—that is, they are relatively “open” 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®, 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 “open” 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 3½ 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 “shallow” 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.
PRIOR ART
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 “wall effect”. 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.
OBJECT OF THE INVENTION
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.
SUMMARY OF THE INVENTION
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 th
Beco Engineering Co.
Bushey C. Scott
Krayer William L.
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