Gas and liquid contact apparatus – Contact devices – Wet baffle
Reexamination Certificate
2001-06-13
2003-06-10
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Wet baffle
C261S114500
Reexamination Certificate
active
06575438
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a downcomer apparatus and to a vapor-liquid contact apparatus provided with the downcomer apparatus, and preferably to a chemical processing vapor-liquid contact apparatus in which a vessel contains a plurality of substantially horizontal trays which support a vapor-liquid mixture. Liquid is introduced at the upper end of the vessel and it flows down from tray-to-tray, via a plurality of stepped downcomer apparatus, and the trays are apertured to provide bubbling areas through which ascending vapors can rise to contact liquid and/or vapor-liquid mixtures which are supported on and flowing across the trays.
BACKGROUND OF THE INVENTION
It has been recognized in the art that the performance of a contact tray apparatus can be enhanced if the liquid flow on the tray is uniform in the respect that the flow in the lateral areas of the tray is substantially the same as the flow along the central flow axis thereof. Heretofore, shaped downcomer tips, directional vapor outlets and other means have been utilized for this purpose.
It has also been recognized in the art that the performance of a contact tray apparatus can be enhanced in some situations if the inside wall of the downcomer is sloped from vertical in order to create a decreasing downcomer cross-sectional area from the top to bottom. This design effectively maximizes the downcomer inlet opening and minimizes the area occupied by the bottom of the downcomer, thus maximizing the bubbling area for additional vapor flow. As a downcomer on the side portion of a tray is sloped to decrease the cross-sectional area, the length of the chordal liquid release decreases proportionally. Typically, the degree of downcomer sloping is often limited in order to maintain a chordal liquid release length no less than 60% of the tower diameter in order to ensure adequate liquid distribution onto the tray.
There has also been utilized in the art truncated downcomers which are trough like and contain an integral floor that is elevated above the deck of the tray below and extends outward from a supporting downcomer wall. The truncated downcomer floor has apertures designed to control the liquid flow to the tray below and also forms a liquid seal to prevent vapors from flowing upward through the downcomer. The liquid release from a truncated downcomer is vertically downward as opposed to a conventional downcomer which releases the liquid horizontally onto the tray as a moving fluid body past a vertical spacing defined by a downcomer lower edge. The elevated apertured floors of a truncated downcomer are designed to physically separate the downcomer from the tray deck in order to allow the deck area underneath the downcomer to be perforated for vapor flow thus increasing the effective bubbling area of the tray. The elevated apertured floor also provides a downcomer discharge location separate from the hydraulic head and turbulence of the frothy mixture on the tray below. However truncated downcomer designs have some inherent limitations. The design requires a dynamic seal where the pressure drop of the liquid leaving the floor apertures prevents vapor from flowing up the downcomer from the tray below adversely affecting the tray performance. The truncated downcomer, by definition, shortens the vertical downcomer length. With some liquid/gas mixtures, the truncated design may not provide sufficient downcomer length causing the fluid in the downcomer to back up onto the tray above, thus limiting the tray capacity. The floor orifices in truncated downcomers are susceptible to plugging from particulate matter in systems where solids are present in the operating fluids. Also, the downward release of liquid from the apertured floor tends to cause the liquid to be released unevenly onto the tray deck below.
When the target area of an element of liquid downflow from a truncated downcomer is near an obstruction such as the inner wall of a vessel, the liquid capillary wave celerity emanating radially from the liquid impact point in the target area will strike and rebound from the vessel wall. Due to the concave curvature of the wall of a cylindrical vessel, some of the rebounding capillary wave liquid will be directed toward the central flow axis of the tray, thus causing a focusing effect which results in a higher flow rate at the central flow axis of the tray than at the sides thereof. This effect creates a liquid peak at the central flow axis and causes liquid recirculation eddy tendencies along the sides of the central flow. Both of these occurrences tend to reduce tray mass transfer efficiency and decrease the effective capacity of the tray. Heretofore, specialized floor orifice patterns and directional vapor outlets on the tray decks other means have been utilized to mitigate this effect.
U.S. Pat. No. 6,003,847 describes a prior art downcomer embodiment that utilizes a downcomer with a highly sloped, semi-conical wall with an outlet formed by a lower edge of the semi-conical wall and the inner surface of the tower wall to control the release of the liquid traveling between that lower edge and the tower wall which liquid then travels down to the tray below. The outlet opening defined by the lower edge of the semi-conical wall and tower wall has a central outlet portion and outer outlet portions with the outer portions increased in size as compared to the center portion. This arrangement is described as providing more liquid flow through the opposing end portions than through the center of the downcomer outlet and also is indicated as being considered to provide a more uniform flow across the tray. In conjunction with the above noted outlet opening, U.S. Pat. No. 6,003,847 describes the use of a multi-chordal inlet weir offset horizontally inward on the tray receiving liquid from the flow controlling semi-conical outlet above to control the liquid flow traveling out onto the tray deck. From this device, the volumetric flow will be proportional to the length of the slot or inlet weir. The concave weir, by definition, releases a disproportionate amount of liquid to the center of the flow path and also directs the liquid towards a focal point located on the flow path centerline, thus creating uneven liquid distribution on the tray deck. Heretofore, devices of this type have used directional vapor apertures on the tray deck to redistribute the liquid. However, these devices have only limited effectiveness within a limited range of equipment operation.
Also in prior art, sloped or small downcomer tray designs in large diameters may often do not have sufficient mechanical strength and require separate trusses or beams that would likely impede the flow of the gas-liquid mixture on the tray deck and limit capacity.
SUMMARY OF THE INVENTION
Under the present invention, a downcomer is provided that is designed to maximize the active bubbling area available and maintain the maximum downcomer length available for separation of the vapor-liquid mixture in the downcomer, while also providing uniform liquid flow distribution at the inlet edge of the active bubbling area. The downcomer of the present invention is also designed to enhance tray and downcomer structural support.
By providing a more uniform liquid flow distribution at the inlet edge of the active bubbling area, the present invention is also directed at avoiding the aforementioned problem of having the liquid flow non-uniform due to a greater amount of centralized flow and recirculation eddy tendencies along the sides of the central flow.
Thus, according to the present invention, the flow of fluid into the bubbling area of a tray is made more uniform across the width of the bubbling area by providing a novel shape and location of a liquid passageway defined by a downcomer outlet which feeds liquid evenly onto the tray. In a preferred embodiment, for example, a downcomer is provided that has a unique chordal shape and means for controlling the vertical column height of liquid flow traveling between the downcomer lower edge and tray below, which arrangement helps av
Campbell Christina J.
Nutter Dale E.
Pilling Mark W.
Bushey C. Scott
Sulzer Chemtech USA, Inc.
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