Gas and liquid contact apparatus – Contact devices – Porous mass
Reexamination Certificate
2001-05-25
2003-07-01
Chiesa, Richard L. (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Porous mass
C261S084000, C261SDIG007, C366S339000, C422S224000
Reexamination Certificate
active
06585237
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable
BACKGROUND OF THE INVENTION
This invention relates to a contacting device used as packing in mass and heat exchange equipment like distillation column or used as a static mixer to blend fluids. Any commercial chemical process requires mass and heat transfer between fluids. These processing methods can include distillation, absorption, adsorption, desorption, stripping, drying, gas cleaning, humidification, dehumidification and direct contact cooling or heating.
The equipment, which carries out such operation, is required to provide a contacting medium or device for generating effective transfer rates. These contacting media or devices must be capable of providing sufficient interfacial area over which two fluids can interface with one another. Further the area should be configured in a relatively small volume. The higher density of effectively available area provided by a contacting device reduces the size of the equipment and consequently reduce manufacturing cost. In addition, the contacting device should be flexible in its configuration so as to be useful for multitudes of different fluids handled in chemical industry. These contacting devices are generally hollow or solid bodies of predetermined size, shape, configuration and orientation. The “structured” packing has individual members oriented in a predetermined fashion The “random” packing is dumped and has no specific orientation within the equipment.
An ideal contacting device should have three major strengths:
It should provide a wetting film of liquid covering all of provided area.
It should provide least resistance to flow of rising gas.
Each of the contacting phases should remain well mixed at any cross-section of packing providing uniform profile of flow, concentration and temperature.
To some extent, these are contradictory requirements. Provision of higher area for liquid film requires dense structure. This reduces voidage available for gas flow and increases gas pressure drop. Liquids have natural tendency to consolidate. Any aids like structural protrusions to split and spread liquids, become obstacles in the gas path increasing its pressure drop. Prior art describes numerous packing structures based on corrugated sheets layout. These are at best workable compromises—far from perfection. Traditionally known prior art contacting device like structured packing is built with layers of zigzag folded metal sheets (corrugated or pleated sheets) as described in WO 90/10497 and U.S. Pat. No. 4,926,050 (Meier). These sheets are arranged in layers parallel to the direction of the axis of flow but the folders are inclined to the axis. The layers are arranged alternately making multiple channels crossing each other.
The prior art corrugated sheet layout suffers from few deficiencies, which reduce and limit the performance in terms of contacting efficiency. The deficient performance is mostly due to inherent geometry of corrugated sheet layout. Liquids have a natural tendency (due to surface tension) to merge together into a consolidated stream. This “sheeting” of liquid film, leads to under-irrigated portions on the corrugated sheet thus wasting some of the available sheet area. The morphology (inherent spatial geometry of construction) of prior art packing does not particularly avoid this “sheeting” of liquid and consequent lowering of contact efficiency.
Another notable deficiency of corrugated sheets is, limited intermixing of cascading streams at any location. Even the liquid on one side of the corrugated sheet cannot mix with the liquid on the opposite side.
Many inventors have addressed these problems in the past. In one embodiment, the corrugated packing sheets have number of holes drilled for the liquid to cross-over (U.S. Pat. No. 5,876,638, Sunder). In another scheme, “w” and “v” shaped shutter openings are formed on the corrugated sheet to create liquid drip points and improve mixing.(U.S. Pat. No. 4,676,934, Seah). Yet another scheme suggests provision of oblique deflection surfaces that are projected from corrugations to mix liquid flowing on the opposite side of the same sheet (U.S. Pat. No. 5,063,000, Mix and U.S. Pat. No. 5,407,607, Mix).
However, all such holes, shutters or deflections reduce the available sheet area for the liquid film. Further the sudden discontinuity created by these features in the path of the liquid film distorts the already created film. Thus all these improvements become counter-productive by disturbing the very liquid film that one had aimed to stabilize. Further the projections suggested in these improvements add to pressure drop for the rising vapor or gas.
The liquid flowing in one set of channels of the corrugated sheets tends to flow down in the same location without much of a lateral movement. The low level of lateral mixing of the fluids (in the direction perpendicular to the equipment axis) also reduces contact efficiency due to non-uniform concentration gradient.
The low lateral movement of the liquid on the corrugated sheet layout tends to maintain uneven flow profile. The uniformity of liquid spread and in turn exchange efficiency of packing depends totally on initial distribution. This has to be provided by additional devices like liquid distributors located above the corrugated packing. This essentially limits the range of operation of packing, as the distributor normally looses performance much before the efficiency limit of packing is reached. Thus corrugated sheet packing becomes as good as the distributor above it.
The inclined channels at the ends of the bundle of the prior art packing tend to put the liquid towards walls of the equipment. The morphology of the arrangement of corrugated sheets does not provide a way to transport this wall liquid back into the bulk of packing. The severity of this problem was long recognized. One scheme by Billingham (U.S. Pat. No. 5,700,403) suggests provision of recessed vertical edges in the corrugation to divert wall liquid. In another scheme, the folded layers are coupled by redirector elements for the edge seeking liquid to reduce the migration of liquid towards the equipment wall.(U.S. Pat. No. 5,441,793, Suess).
Both the schemes are complicated to fabricate and add few extra steps in manufacture thereby increasing the cost.
Apart from liquid distribution, the uniform distribution of rising vapor or gas and its lateral mixing is also a concern in the corrugated sheet layout. The vapor tends to flow in channels without mixing with the vapor in the adjacent channels in a corrugated sheet bundle.
This problem is addressed in one embodiment by providing fan like vane elements to aid transverse (lateral) mixing (U.S. Pat. No. 5,158,712, Wilhem). In another embodiment, the elementary triangular area in the folding has two cut edges that are deflected in a flap like manner to form vortex packing (U.S. Pat. No. 5,500,160, Suess). However these embodiments still retain legacy of corrugated or pleated sheets morphology. The embodiments suggest deflection of partly cut portions in the corrugated channels to form vanes. These embodiments still achieve limited split and mixing of vapor phase and are undue complex and hence costly. In another scheme, angled pegs are projected at various places from one sheet to adjoining sheet forming a bridge for lateral movement of both liquid and vapor (U.S. Pat. No. 5,975,503, Chuang). All these methods add to cost and complexity of the packing.
In order to spread the liquid and the vapor uniformly across the radial plane, the corrugated sheet bundles are required to be kept short. Further, the prior art bundles have to be stacked so that the channels are rotated by 90 degrees to each other. This feature essential for distribution of liquid is at the cost of additional pressure drop for the rising vapor. The vapor has to abruptly change its direction at each of the numerous junctions between the short bundles. This hig
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