Last Updated on Fri, 22 Mar 2013 AS A CHECK, (QC4-Q'C4)=(QVD4-1 + QVI D4. QO'4-Q VOHIJFigure 3.13. Heat and material balance—top sidestream product draw.tower overhead stream comprises the inerts. Convert the atmospheric dew point of D4 to this hydrocarbon partial pressure and check the assumed temperature.2. The temperature of the liquid on Tray D4 is found by converting the atmospheric bubble point of the product liquid to the hydrocarbon partial pressure existing above Tray D4.3.

• Ballast Tray Design Manual Bulletin No 4900 1

Calculate the duty of the cooler as Qc4 by making an overall system heat balance, keeping in mind that the outlet temperature from this exchanger has already been defined,4. Calculate the portion of required to cool the liquid (D4 + Lj)4) from the draw tray temperature to the exit temperature. Designate this quantity as Q'c4-The remaining portion of is that required to cool the pumparound reflux, Lp^.5. Check the calculation of Lp^ by making a heat balance around Envelope VI,6.

Check the value of Q^4 by calculating the heat requirements for cooling (LpA + D4 + Lj^) over the required temperature range. This must check the value calculated in Step 4 above. Vapor-Liquid TrafficAt all the trays which were calculated in the design, vapor and liquid flow rates were developed. In this section, these pieces of information will be assembled to develop an internal traffic di agram.

At this point, the following items are known.1. Vapor leaving the flash zone including total distillate products, overflash, steam, air and hydrocarbon decomposition gas.2. Product streams leaving their respective draw trays.3. Material balance around all product strippers.4. Pumpback and pumparound reflux rates.5. Internal liquid flow rates to draw trays. Induced reflux where appropriate.Using this information, developing the diagram reduces to a matter of arithmetic.

The only place where caution must be used is in handling induced reflux. Consider a section of the tower containing the tray below a draw down to the tray above the next lowest draw. The known liquid rates in the section are pumpback reflux to the upper tray, its induced reflux and reflux from the lower tray. There is almost always a difference between the sum of the first two quantities and the third. In the analysis, this difference is assumed to be split equally between the trays in the see. tion. This information is required for tray design or for analysis of the trays in existing towers.

Discussion of tray design techniques is outside the scope of this work, and the interested reader is referred to the previously cited vendor literature. Fractionation AnalysisThere are no correlations known to the for analyzing fractionation in vacuum towers. The Packie analysis can be employed but only to show that a proper trays-reflux balance exists in the various sections.

Estimates of gap-overlap may be made as a matter of interest, but it must be remembered that Packie's curves were intended to apply only to atmospheric crude towers.Heat and Material Balance Calculations for Fuels-Type TowersThis section presents the remainder of the procedures required to calculate the heat and material balance around a fuels-type vacuum tower. Instructions for making the calculations at the flash zone and at the overflash liquid condensing section were presented in the previous section. As a quick refresher, the following items must be accomplished to this point.1. Set the overall material balance, including air leakage and hydrocarbon decomposition gas.2. Establish a pressure profile across the tower using previously recommended guidelines.3. Make flash zone calculations of steam requirements and heat input to the tower in accordance with earlier sections. Temperature ProfileSet the temperature of the vapor and liquid streams at key points in the tower.

Unlike an atmospheric tower or a lube-type tower, these temperatures can be established analytically and do not require a triai-and-error approach. In this type of tower, the entire vapor charge to the tower is generated in the flash zone, and, with the exception of the overflash condensing section, there is no internal equilibrium reflux. All the heat is removed by pumparound reflux. Since the operation of the sidestream product condensing sections can be described closely as equilibrium condensations, the vapor temperatures can be estimated from the reduced crude EFV curves at the appropriate degree of vaporization and hydrocarbon partial pressure.

Liquid temperatures are set by converting the atmospheric bubble points of the products to the appropriate partial pressures existing above the draw trays. This analysis also applies to the overflash liquid condensing section because the reflux required to condense the overflash is generally the same relative number of moles as the overflash.The temperatures of the exit products and the pumparound reflux streams from their coolers are established taking into account the temperature-viscosity relationships of these very heavy oils.

Product temperatures may also be set by heat balance considerations in downstream units. Overflash Liquid Condensing SectionThis section of the tower is calculated in the same manner as the lube-type tower. The procedure was discussed earlier and illustrated by Figures 3.19 and 3.10.Sidestream Products (DI and D2) Condensing SectionsThe calculation of a fuels-type tower is much simpler than a lube-type. The material balance and heat balance relationships are analytical rather than trial-and-error in nature. Figure 3.14 shows the complete heat and material balance relationships for such a tower.

The expressions and equations on this figure are self-explanatory. Note also that this sketch contains all the vapor-liquid internal traffic data.References1. Wheeler, 'Design Criteria for Chimney Trays,' Hydrocarbon Processing 47, no.

7 (July, 1968), pp. 'Ballast Tray Design Manual,' Bulletin No. 4900, Fritz W. Glitsch & Sons, Inc., Dallas, Texas. Heat and material balance summary—fuels type vacuum tower with pumparound heat removal.3.

Koch Flex it ray Design Manual, Koch Engineering Co. Inc., Wichita, Kansas.4. Nelson, Petroleum Refinery Engineering. (New York: McGraw-Hill Book Company, Inc., 1958).5.

'Glitsch Grid-A New Design for Column Internals,' Bulletin No. 7070, Fritz W. Glitsch & Sons, Inc., Dallas, Texas.6. Ludwig, Applied 'Process Design for Chemical and Petrochemical Plants, Vol. I (Houston: Gulf Publishing Company, 1964).7. Packie, 'Distillation Equipment in the Oil Refining Industry,' AJChE Transactions 37 (1941), pp. Edmister, Applied Hydrocarbon Thermodynamics (Houston: Gulf Publishing Company, 1964).9.

Maxwell, Data Book on Hydrocarbons (Princeton, N.J.: D. Van Nostrand Co., 1965).10. Winn, 'Physical Properties by Nomogram,' Petroleum Refiner 36, no. 2 (February, 1957), p, 157.Was this article helpful?

A downcomer tray assembly for vapor liquid contact towers. The region of the tray beneath an upper downcomer is constructed with a raised, perforated region for facilitating vapor passage therethrough and improving mass transfer efficiency. The downcomer includes a series of grouped discharge orifices disposed above either the perforated inlet or, in some cages, covered inlet areas which comprise momentum barriers. The covered areas in conjunction with the grouped orifices break the momentum of the liquid impacting upon the inlet area to reduce weeping.

The vapor rising through the open inlet area sections is also selectively directed into the liquid discharged from the grouped downcomer orifices to promote uniform aeration of the liquid. A splash deflector is disposed outwardly of the raised inlet area to reduce any liquid maldistribution flowing from the inlet area and to deflect liquid splashed outwardly therefrom. What is claimed is:1. FIELD OF THE INVENTIONThe present invention pertains to gas-liquid contacting trays and, more particularly, an improved downcomer-tray assembly incorporating a grouped orifice downcomer and tray construction for higher efficiency operation.HISTORY OF THE PRIOR ARTDistillation columns are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact columns utilize either trays, packing or combinations thereof. In recent years the trend has been to replace the so-called 'bubble caps', by sieve and valve trays in most tray column designs, and the popularity of packed columns, either random (dumped) or structured packing have been utilized in combination with the trays in order to effect improved separation of the components in the stream.Successful fractionation in the column is dependent upon intimate contact between liquid and vapor phases. Some vapor and liquid contact devices, such as trays, are characterized by relatively high pressure drop and relatively high liquid hold-up.

Another type of vapor and liquid contact apparatus, namely structured high efficiency packing, has also become popular for certain applications. Such packing is energy efficient because it has low pressure drop and low liquid hold-up.

However, these very properties at times make columns equipped with structured packing difficult to operate in a stable, consistent manner. Moreover, many applications simply require the use of trays.Fractionation column trays come in two configurations: cross-flow and counter flow. The trays generally consist of a solid tray or deck having a plurality of apertures and are installed on support rings within the tower. In cross-flow trays, vapor ascends through the apertures and contacts the liquid moving across the tray, through the 'active' area thereof. In this area, liquid and vapor mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is referred to as the Inlet Downcomer.

The liquid moves across the tray and exits through a similar channel referred to as the Exit Downcomer. The location of the downcomers determines the flow pattern of the liquid. If there are two Inlet Downcomers and the liquid is split into two streams over each tray, it is called a two pass tray. If there is only one Inlet and one outlet Downcomer on opposite sides of the tray, it is called a single pass tray. For two or more passes, the tray is often referred to as a Multipass Tray. The number of passes generally increases as the required (design) liquid rate increases.Not all areas of a tray are active for vapor-liquid contact. For example, the area under the Inlet Downcomer is generally a solid region.

To attempt to gain more area of the tray for vapor/liquid contact, the downcomers are often sloped. The maximum vapor/liquid handling capacity of the tray generally increases with an increase in the active or Bubbling Area. There is, however, a limit as to how far one can slope the downcomer(s) in order to increase the Bubbling Area otherwise the channel will become too small. This can restrict the flow of the liquid and/or restrict the disengagement of vapor retained in the liquid, cause liquid to back up in the downcomer, and thus prematurely limit the normal maximum vapor/liquid handling capacity of the tray. The present invention specifically addresses the problem of restricted disengagement of vapor retained in the liquid.A variation for increasing the Bubbling Area and hence vapor/liquid handling capacity is a Multiple Downcomer (MD) tray. There is usually a plurality of box shaped vertical channels installed in a symmetrical pattern across the tray to direct liquid onto and off of the tray.

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The downcomers do not extend all the way to the tray below but stop short of the tray by a predetermined distance which is limited by a sufficient space to permit disengagement of any vapor retained in the liquid entering the Exit Downcomer. The downcomer pattern is rotated 90 degrees between successive trays. The bottom of the boxes is solid except for slots that direct the liquid onto the Bubbling Area of the tray below, in between the outlet downcomers of the tray. The MD tray falls into the category of Multipass Trays and is usually used for high liquid rates.Addressing now select cross flow plate designs, a particularly effective tray in process columns is the sieve tray. This tray is constructed with a large number of apertures formed in the bottom surface. The apertures permit the ascending vapor to flow into direct engagement with the liquid that is flowing across the tray from the downcomer described above.

When there is sufficient vapor flow upwardly through the tray, the liquid is prevented from running downwardly through the apertures (referred to as 'weeping'). A small degree of weeping is normal in trays while a larger degree of weeping is detrimental to the capacity and efficiency of a tray.The capacity of a tray is a function of the open area of holes and of tray spacing. When spacing is fixed, capacity may be increased by increasing the percent open area. This practice is limited, however, by decreased turndown due to weeping at low vapor rates. To overcome such weeping, a tray has been developed which is constructed from closely spaced rods of trapezoidal cross-section. They are manufactured and sold by the assignee of the present invention under the trademark SCREEN TRAY.The trapezoidal wire members of the SCREEN TRAY are tapered upwardly, and this creates a Venturi effect to ascending vapor. Surface tension effects become pronounced with such close wire spacing.

Combined with the Venturi effect produced by vapor rising through the tapered throats between the wires, surface tension phenomena reduce weeping significantly at low liquid rates and keep spray height low. The upward taper also defines a larger surface area for liquid flowing across the tray. For additional discussions of SCREEN TRAYS and another invention related thereto which improve gas-liquid contact, reference is made to co-pending, U.S. Patent application Ser. 07/304,942 (now abandoned) filed on Jan.

31, 1989 and assigned to the assignee of the present invention.Tray efficiency is also known to be improved in sieve type trays by increasing the froth height of the liquid and reducing the backflow of the liquid flowing across the tray. Froth is created when vapor bubbles percolate upwardly through the liquid flowing across the tray. The suspension of the vapor in the liquid prolongs the vapor liquid contact which enhances the efficiency of the process. The longer the froth is maintained and the higher the froth is established, the greater the vapor liquid retention. Higher froth requires smaller vapor bubbles and the formation of the bubbles at a sufficiently slow rate.

Likewise, backflow occurs beneath the froth when circulating currents of liquid are established during the liquid flow across the plate. This generally forms along the lateral portions thereof. These currents carry liquid back across the tray in a manner that reduces the concentration-difference driving force for mass transfer.

It is the concentration-difference between the vapor and the liquid which enhances the effectiveness of the vapor-liquid contact.The concentration-difference between the vapor and the liquid can be effected in many ways; some reducing efficiency. For example, as operating pressure increases, descending liquid begins to absorb vapor an it moves across a tray.

This is above that normally associated as dissolved gas as governed by Henry's Law and represents much larger amounts of vapor bubbles that are commingled or 'entrained' with the liquid. This vapor is not firmly held and is released within the downcomer, and, in fact, the majority of said vapor must be released otherwise the downcomer can not accommodate the liquid/vapor mixture and will flood, thus preventing successful tower operation. This phenomena is generally deemed to occur when operating pressure is such as to produce a vapor density above about 1. And typically amounts to about 10 to 20% of the vapor by volume. For conventional trays, as shown below, the released vapor must oppose the descending frothy vapor/liquid mixture flowing over the weir into the downcomer.

In many cases, such opposition leads to poor tower operation and premature flooding.Another serious problem which manifests itself in such operational applications is maldistribution of flow. If the tray design in such that liquid flow volumes can build up in particular areas such as the center of a tray the liquid buildups can be passed from tray to tray, resulting in reduced efficiency and localized flowing. It is thus a major design consideration to regulate liquid flow from the downcomer as well as across the active tray area.The technology of gas-liquid contact addresses many performance issues. Certain performance and design issues are seen in the publication 'Ballast Tray Design Manual', Bulletin No. 4900-Fifth Edition, by Glitsch, Inc., assignee of the present invention. Other examples are seen in several prior art patents, which include U.S. 3,959,419, 4,604,247 and 4,597,916, each assigned to the assignee of the present invention and U.S.

4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. A particularly relevant reference is seen in U.S. 4,499,035 assigned to Union Carbide Corporation that teaches a gas-liquid contacting tray with improved inlet bubbling means. A cross-flow tray of the type described above is therein shown with improved means for initiating bubble activity at the tray inlet comprising spaced apart, imperforate wall members extending substantially vertically upwardly and transverse to the liquid flow path. The structural configuration is said to promote activity over a larger tray surface than that afforded by simple perforated tray assemblies. This is accomplished in part by providing a raised region adjacent the downcomer area for facilitating vapor ascension therethrough.U.S.

4,550,000 assigned to Shell Oil Company teaches apparatus for contacting a liquid with a gas in a relationship between vertically stacked trays in a tower. The apertures in a given tray are provided for the passage of gas in a manner less hampered by liquid coming from a discharge means of the next upper tray. This is provided by perforated housings secured to the tray deck beneath the downcomers for breaking up the descending liquid flow. Such advances improve tray efficiency within the confines of prior art structures.

4,543,219 assigned to Nippon Kayaku Kabushiki Kaisha of Tokyo, Japan teaches a baffle tray tower. The operational parameters of high gas-liquid contact efficiency and the need for low pressure lose are set forth. Such references are useful in illustrating the need for high efficiency vapor liquid contact in tray process towers. 4,504,426 issued to Karl T. And assigned to Atomic Energy of Canada Limited is yet another example of gas-liquid contacting apparatus. This reference likewise teaches the multitude of advantages in improving efficiency in fractionation and modifications in downcomer-tray designs. The perforated area of the tray is extended beneath the downcomer with between 0 to 25% less perforation area.Yet another reference is seen in U.S.

3,410,540 issued to W. Bruckert in 1968. A downcomer outlet baffle is therein shown to control the discharge of liquid therefrom. The baffle may include either a static seal or dynamic seal. In this regard the openings from the downcomer are sufficiently small to control discharge and may be larger than the tray perforations and of circular or rectangular shape. The transient forces which may disrupt the operation of a downcomer are also more fully elaborated therein. These forces and related vapor-liquid flow problems must be considered for each application in which a downcomer feeds an underlying tray.

The location of the downcomer discharge orifice is one of the issues addressed by the present invention.It would be an advantage to provide a method of and apparatus for enhanced downcomer-tray vapor flow manifesting increased efficiency. Such a downcomer-tray assembly is provided by the present invention wherein a slash deflector is disposed outwardly of the downcomer, which downcomer area may include a uniformly raised, active inlet area panel secured there-beneath. The downcomer is constructed with grouped discharge orifices for select discharge upon the inlet area. The inlet area panel has a plurality of vapor discharge apertures, some of which are covered by momentum barriers, disposed beneath the grouped discharge orifices of the downcomer. The momentum barriers may also be replaced with an active area having non-perforated regions.

Ballast Tray Design Manual Bulletin No 4900 1

Such designs prevent the liquid from being driven through the apertures of the raised panel inlet area, particularly when there are low vapor flow rates. The momentum of the falling liquid from the downcomer is thus selectively controlled to promote uniform aeration of the liquid, reduce flow maldistributions and reduce weeping of liquid through the tray.SUMMARY OF THE INVENTIONThe present invention relates to gas-liquid contacting trays and improvements in the downcomer-tray assembly.