Gas and liquid contact apparatus – Contact devices – Wet baffle
Reexamination Certificate
2000-03-07
2003-07-01
Bushey, C. Scott (Department: 1724)
Gas and liquid contact apparatus
Contact devices
Wet baffle
C261S114500
Reexamination Certificate
active
06585238
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, desorption, 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 columns having large diameter, large height, high cost, high-energy demand, and high-pressure drop. There is a continuing search for higher vapor-liquid contact column capacity (loading), higher efficiency (both tray and column), and lower pressure drop.
Column or tray loading can be increased by any or all of: increasing the active area; decreasing the downcomer area (and especially avoiding the dual downcomer penalty); increasing the weir length; increasing the number of downcomers (and decreasing thereby the liquid path length); and finally, by avoiding the flooding limitation, e.g., by having tray sections which are locally co-current, i.e., locally flooded, with other tray sections relieving the flooding. See, for example, U.S. Pat. No. 5,798,086.
Tray efficiency can be increased by increasing point efficiency, by extending the fluid contact time as froth, and also by horizontal staging of the liquid flow across the tray. Different liquid flow directions and degrees of vapor mixing yield differing tray enhancements, with same direction liquid flow coupled with no vapor mixing yielding the greatest enhancement.
Column pressure drop can be decreased by reducing the total tray count (made possible by increased tray efficiency) and/or by reducing the tray pressure drop, and by reducing liquid height and/or vapor flow restrictions.
Some trays have one downcomer area for arriving liquid, and a completely separate downcomer area for departing liquid. A preferred arrangement is to integrate the arriving downcomer and departing downcomer into the same area, thus avoiding the dual downcomer penalty. Frequently this results in identical downcomer arrangements on adjacent trays, in contrast to the more usual mirror image relationship between adjacent trays. This integrated arrangement is possible in part because the arriving liquid requires much less area than the departing liquid, since the latter also requires phase separation from the froth. The integrated downcomer however would preferably have a positive liquid seal, not a dynamic one, i.e., should preferably have a reverse (inlet) weir extending to approximately below the quiescent tray liquid level. This is because downcomers with dynamic liquid seals have very limited turndown capability.
One problem frequently encountered is that measures which improve one of the above parameters or solve one of the above problems have the undesirable side effect of worsening others. For example, several designs which incorporate multiple tray downcomers to accommodate higher liquid loading also result in very short and irregular liquid flowpaths. This eliminates most of the horizontal staging benefit, and thereby decreases tray efficiency. In effect, column diameter is decreased, but tray count and column height must as a result be increased. Another example of undesirable side effect is provided by some tray designs which incorporate locally co-current sections to increase vapor loading. In one of those (U.S. Pat. No. 5,695,548), the local co-currency causes both loss of all horizontal staging and also the dual downcomer penalty. In three others (U.S. Pat. Nos. 5,110,325 and 2,693,350, plus International Publication WO99/54018), horizontal staging is preserved, but at the expense of very substantial froth-free or spray-free areas on the tray, in addition to the dual downcomer penalty. Any froth-free areas have the effect of reducing residence time of contact between vapor and liquid and hence reducing point efficiency and tray efficiency. Also, tray pressure drop is increased in some by providing only limited area for co-current contact, and by requiring major changes in vapor direction. Finally, only the less desirable opposite direction liquid flow type of horizontal staging is achieved. In summary, prior art teachings of increasing tray vapor capacity via local co-currency have the unintended consequence of reducing active area, point efficiency, and/or tray efficiency.
Additional examples of undesired side consequences arising from measures to increase some performance parameter are provided by the multiple downcomer disclosures, e.g., U.S. Pat. Nos. 5,098,615 and 5,318,732. In the former, as indicated above, multiple downcomers are positioned at right angles on adjacent trays, thereby providing greater liquid loading, but resulting in short and erratic liquid paths and hence loss of all tray efficiency benefit from horizontal staging. The latter disclosure partially corrects this by establishing orderly same direction flow on adjacent trays, and by avoiding the dual downcomer penalty, but with liquid flowpaths of such variable width and length that substantial liquid mixing occurs, and hence only minimal horizontal staging benefit is achieved. Unrestricted vapor mixing above each liquid flowpath further reduces the benefit. The dynamic seals in the downcomers increase active area but restrict turndown, and make startup more difficult.
What is needed, and among the objects of this invention, are process and apparatus for tray crossflow vapor-liquid contact, which simultaneously achieve both higher efficiency and higher capacity, without adverse consequences such as those described above. Horizontal staging should be preserved, preferably the same direction liquid flow type, and the dual downcomer penalty should preferably be avoided. A maximum amount of tray area should be covered with froth.
DISCLOSURE OF THE INVENTION
The above and additional useful objects are obtained by providing an advanced locally co-current vapor liquid contact tray of the liquid crossflow type which is comprised of three advantageous features:
the tray downcomers have both arriving liquid and departing liquid integrated in a common structure, and have a positive liquid seal;
the tray is comprised of horizontally arrayed locally co-current liquid recirculating compartments, with compartment partitions which restrict liquid mixing between adjacent compartments and with non-vertical structure which increases tray surface active vapor injection area; and
the liquid flowpath(s) on the tray have a convergent-divergent (CD) structure: converged flow through a central portion of the tray, and diverged flow back around two approximately mirror image outer portions, with one or more tray downcomers in the liquid path, and with liquid flow the same direction on adjacent trays.
The maximum tray performance advantage is obtained by additionally providing: vapor partitions which restrict vapor mixing between compartments; diagonally locally co-current flow; and for vapor-liquid separation structure in the top portion of each compartment, angled to drain liquid to the compartment downcomer.
Accordingly, the tray achieves increased vapor loading; increased liquid loading (proportional to the number of tray downcomers); high point efficiency due to maximum froth area and volume; even higher tray efficiency due to same direction flow with close approach to plug flow and with little mixing; maximum overall active area by avoidance of the dual downcomer penalty plus full use of both tray center and tray periphery; excellent turndown capability due to positive liquid seals; and little or no additional tray pressure drop. The performance enhancements are achieved without the traditional accompanying negative consequences.
Normally in vapor-liquid contact columns the design fluid loadings vary appreciably in different column sections. Thus, most or all of the above features would be appropriate in the heaviest loaded column sections, but not necessarily elsewhere (unless column diameter is reduced in the lightly loaded sections). Accordingly, not all of the above novel feat
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