Gas and liquid contact apparatus – Contact devices – Wet baffle
Reexamination Certificate
2001-02-23
2003-10-14
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Wet baffle
C261S114500, C261S148000, C261S155000
Reexamination Certificate
active
06631892
ABSTRACT:
TECHNICAL FIELD
The invention is directed toward multistage vertical tray-type vapor-liquid contactors which find use in a variety of equipment and industrial processes such as fractional distillation, absorption, stripping, mixing, and partial condensation (dephlegmation).
BACKGROUND
Multicomponent fluid vapor-liquid contactors of the tray or plate type have several known limitations which result in large diameter, large height, high cost, high-energy demand, and high-pressure drop. The diameter is determined by the flooding limitation, i.e., the loading. The overall height is determined by the tray efficiency and height of a tray. Tray height is related to the point efficiency and froth height, which are interrelated, and are controlled by vapor injector geometry and weir height. Tray pressure drop is comprised of the vapor injector drop plus the tray liquid head. The energy demand is determined by the reboil requirement for above-ambient contactors, and by the reflux requirement for below-ambient contactors. Incorporating heat exchange in the column (i.e., diabatic distillation) is known to be one method of reducing the external reboil or reflux demand. Examples of diabatic distillation are disclosed in U.S. Pat. Nos. 385,504; 2,330,326; 2,492,932; 2,963,872; and 3,642,452. The limitations enumerated above are interrelated to some extent, with various tradeoffs possible. For example, the energy requirement can be reduced somewhat by adding trays and hence increasing column height, or vice versa. Larger diameter can reduce tray height and overall height, etc.
Vapor-liquid contactors are used in distillation columns, rectifiers, strippers, absorbers, mixing (reverse distillation) columns, absorption cycle apparatus, bioreactors, reactive distillation columns, and similar devices.
Various approaches to overcoming the above limitations have been disclosed in the prior art. Higher point efficiencies have been achieved in spinning cone contactors, and by imparting horizontal velocity to part of the vapor. Higher vapor loadings have been achieved using locally co-current trays plus separators which route the liquid to a twice-lower tray (U.S. Pat. No. 4,361,469). Other approaches to achieving local cocurrency are disclosed in U.S. Pat. Nos. 2,693,350; 3,642;542; 5,766,519; and 5,798,086. Conventional trays have a crosscurrent contact pattern, and cannot be increased to the higher loading characteristic of cocurrency without flooding. Higher liquid loadings have been achieved by multiple downcomers (U.S. Pat. No. 3,410,540). Reduced energy demand has been achieved in the diabatic distillation disclosures. One problem with the prior art approaches to overcoming the various limitations listed above is that they usually address only a single limitation, with modifications which may make it more difficult to overcome the other limitations.
It is known that conventional crosscurrent trays approach a tray efficiency limit of 150% of the point efficiency when the horizontal liquid flow direction is reversed on each successive tray and the vapor is unmixed; 167% with fully mixed vapor; and 200% when the liquid direction is the same on each tray and the vapor is unmixed. This boost in efficiency is owing to the extra boost provided by effectively having multiple horizontal stages on each vertical tray, i.e., due to concentration gradients across the tray.
What is needed, and among the objects of this invention, are apparatus and process for vapor-liquid contact which achieve the thermodynamic advantage of globally counter-current multistage contact, the tray efficiency (reduced column height) advantage of tray crosscurrent contact, and the point efficiency and loading (reduced column diameter) advantages of locally co-current liquid recirculated contact. Preferably, the contactor would achieve further increase in tray efficiency and corresponding decrease in tray count and column height by same-direction horizontal liquid flow on each tray coupled with structure to prevent vapor mixing. Preferably, the contactor would also achieve lower energy demand. Most preferably, all of those desirable objects would be achieved simultaneously by compatible and synergistic measures.
DISCLOSURE OF INVENTION
The above and additional useful advantages are obtained in apparatus and corresponding process for multistage multicomponent fluid mass exchange wherein at least one type of same-direction liquid flow is present on all the trays of the contact section: horizontal same-direction flow, and/or vertical same-direction. More particularly, in the latter case, the contactor is comprised of:
A vapor-liquid contactor comprised of:
a. a multiplicity of vertically stacked trays within a containment;
b. a multiplicity of channel weirs on each tray, each channel weir defining a locally co-current vapor-liquid upflow zone on one side of the channel weir, and a liquid down flow zone on the other side; said channel weirs having a liquid passage opening at or near the bottom for transport of liquid from the downflow zone to the co-current upflow zone;
c. a multiplicity of vapor passages through said trays at the bottom of said co-current upflow zones;
d. a level control liquid weir on each tray, which is at a lower height than said channel weirs;
e. a passage for transport of liquid spillover from said liquid weir to the next lower tray;
f. a vapor supply below said stack of trays; and
g. a liquid supply and vapor withdrawal above said stack of trays
In contrast to conventional trays which have a turbulent froth region wherein the liquid flows in all directions, the above disclosure results in liquid flowing vertically the same direction (downward) in all the downcomers, and upward in all the risers (upflow zones).
More intensified vapor-liquid contact, and hence higher point efficiency or lower froth height, can be obtained by placing enhanced contact media in the co-current upflow channels. The required amount of media is relatively very small, since much of the tray volume is empty space, yet all the fluid traverses the upflow channels, which are highly loaded. The media can also be catalytically active, to support chemically reactive distillation. Thus maximum utilization of a small amount of catalyst is achieved.
The tray loading can be further increased, and/or tray height further decreased, by providing apparatus for vapor-liquid separation above the channel weirs. Thus the vapor-liquid separation is accelerated and made more effective compared to simply relying on open-space separation. It is important that the liquid drainage from the separator be directed to the downflow zone (downcomer channels), as otherwise it can be re-entrained in the upflowing vapor.
Each tray is fitted with a level control liquid weir, much like conventional trays, and the weir overflow is routed to the next lower tray. It is frequently preferred to route the overflow liquid to the same comparative location on each succeeding or adjoining tray—this allows the tray efficiency to approach 200% of the point efficiency provided the vapor is prevented from mixing, e.g., by partitions. When the horizontal liquid flow direction is opposite on adjacent trays, then the tray efficiency only approaches 150% of the point efficiency for unmixed vapor, or 167% for fully mixed vapor.
This gives rise to the other case of same direction liquid flow—horizontal. In that case, the contactor is comprised of:
a. a multiplicity of vertically stacked trays within a containment;
b. a liquid supply area and liquid removal area to and from each tray, wherein each supply and removal area is at the same relative location on each tray, whereby the net horizontal liquid flow on each tray is in the same directional pattern; and
c. a multiplicity of partitions in the vapor space of each tray which are transverse to the flow direction of said liquid, and thereby minimize vapor mixing.
The compartmentalization reduces the horizontal mixing of both the liquid and the vapor, resulting in larger concentration gradients and hence higher tray efficiencies for a given point
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