Gas and liquid contact apparatus – Contact devices – Wet baffle
Reexamination Certificate
1998-04-29
2001-03-27
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Wet baffle
C261SDIG007
Reexamination Certificate
active
06206349
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to fluid—fluid contacting apparatus and, in particular, to structured packings for use in such apparatus. Typically apparatus of the type that the invention relates to is used for operations such as distillation, absorption, scrubbing, stripping, heat exchange etc. in which one fluid (e.g. a liquid) is brought into contact with another fluid (e.g. a gas) with the fluids usually flowing countercurrent relative to each other. In the case of gas (or vapor)/liquid contacting, the gas constitutes the continuous phase.
The invention is especially concerned with fluid—fluid contacting apparatus in which the structured packing comprises a number of packing elements arranged in succession in the direction of fluid flow through the apparatus which is usually in the form of a vertically disposed column or tower. Each packing element comprises a plurality of crimped sheets of material arranged in face-to-face relationship with rectilinear corrugations extending obliquely relative to the direction of fluid flow and successive elements are arranged with the sheets in one element angularly displaced with respect to the sheets of the adjacent element(s). Vendors of commercially available packings of this type recommend angular displacements of 90° (Sulzer Brothers Limited) and 70° (Norton Chemical Company).
In their range of packings, one supplier (Sulzer) produces an ‘X’ range of packings and a ‘Y’ range of packings. The sheet materials used in the two forms of packings are believed to be identical with respect to surface area and surface treatment but differ by the angle of crimp. In the ‘Y’ series of packings, the crimp angle is 45° to the horizontal whereas the ‘X’ series have a crimp angle at 60° to the horizontal.
The ‘Y’ series packing elements have a higher efficiency but lower capacity than the ‘X’ series packing elements. The efficiency of a structured packing is a property of the way vapor and liquid contact each other over the whole surface of the packing. The capacity of the packing is set by the capacity at its most restricted elevation. The ‘X’ series packing elements impose a smaller change in direction on the fluids at the interface due to the larger angle subtended to the horizontal by the crimp angle and therefore have a larger capacity than the equivalent ‘Y’ series packing elements. The pressure drop within the ‘Y’ series packing elements is greater and the use of the surface area for mass transfer is greater, hence the ‘Y’ series packing elements have a higher efficiency.
SUMMARY OF THE INVENTION
Recent indications suggest that the capacity of a structured packing is governed by the behavior of the fluids at the interface between successive packing elements. For instance, where liquid-vapor contact is involved, the pressure drop in the vapor phase is higher at the interface between successive packing elements where the liquid and vapor are forced to move through a change in direction, than in the body of each packing element and, as a result, liquid tends to build-up at the interface. The build up of liquid occurs over a greater range of the operating conditions the higher the liquid load. It is therefore assumed that the widely recognized effect of loss of performance in structured packing at higher pressure is due to a build up of liquid at the interfaces between successive packing elements leading to maldistribution of the liquid into the next packing element in the direction of liquid flow.
According to one aspect of the present invention there is provided fluid—fluid contacting apparatus in which the structured packing comprises a number of packing elements arranged in succession in the designed direction of fluid flow, each packing element comprising a plurality of crimped sheets of material arranged in face-to-face relationship with the corrugations extending obliquely relative to the direction of fluid flow, successive elements being arranged with the sheets in one element angularly-displaced with respect to the sheets of the adjacent element(s), characterized by the provision of means at or in the vicinity of the interface between successive elements for reducing the pressure drop imposed on the continuous phase at the interface.
In this manner, it is possible to secure good efficiency without unduly sacrificing capacity (and vice versa). Said means may have the effect of generally smoothing the rate of change of pressure throughout the packed section of the apparutus without necessarily reducing the overall pressure drop across the packed section (although such overall pressure drop may occur). In particular, said means serves to reduce the rate of change of pressure at and in the immediate vicinity of said interfaces.
Such means may be implemented by configuring the corrugations in the sheets so as to secure reduced pressure drop.
In one embodiment , instead of employing rectilinear corrugations, at least some (preferably the majority if not all) of the sheets of each packing element have at least some (preferably the majority if not all) corrugations whose angle of obliquity varies between opposite faces of the packing element such that the angle of obliquity is greater in the vicinity of at least one (preferably both) of said faces than the greatest angle of obliquity within the body of the packing element.
By “angle of obliquity” at a particular point along the length of a corrugation, we mean the angle between the axis of the corrugation at that point and a plane containing said point and parallel to said opposite faces.
Thus, in a typical implementation of this embodiment, each sheet of a packing element may be provided with corrugations which impart a change in flow direction as fluid flows through the body of the packing element from one face to the opposite face, the corrugations having a terminal portion or portions (depending on whether the particular corrugation extends to one or both of said opposite faces) which intersect said faces at an angle of up to 90° while the intermediate portions of each corrugation over at least part of the length thereof extend at an angle somewhat less, e.g. typically less than 60°.
The angle of obliquity of each such corrugation preferably changes progressively in the lengthwise direction although we do not exclude the possibility of the change being of a discontinuous nature.
By imparting a variable angle of obliquity to the sheets of the packing elements, mass transfer within the heart of each packing element can be maximized and the use of a higher angle of obliquity in the vicinity of the packing element avoids an extreme change in direction as the fluids pass from one packing element to the next.
In another embodiment of the invention, said means at or in the vicinity of the interface between successive elements for reducing pressure drop at the interface may be implemented by producing at least some (preferably the majority if not all) of the corrugations in at least some (preferably the majority if not all) of the sheets of each packing element with a reduced cross-section in the vicinity of a least one (preferably both) of the faces of the packing element thereby reducing the surface area and pressure drop at such location.
The localized reduction in cross-sectional area of the corrugations may be effected by a reduction in depth. The reduction in depth is preferably progressive as the corrugations approach the end faces of the packing elements.
If desired, such localized reduction in the cross-sectional area of the corrugations may be combined with variation in the angle of obliquity as described above or the reduction may be employed with corrugations which are otherwise of conventional configuration. The reduction in cross-sectional area or depth may take place progressively and may be to such an extent that the corrugations terminate short of the edges proper of the sheets, i.e. so that marginal edges of the sheets are flat (non-corrugated). Because a reduction in depth will result in the sheets being out of contact with one another, if desired or necessa
Bushey C. Scott
Sulzer Chemtech AG
Townsend and Townsend / and Crew LLP
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