Definitions

• This invention relates to chemical process equipment in which a liquid is contacted with a counterflow of gas. This may be for a variety of purposes such as stripping a component from the liquid stream or absorbing a component into a liquid stream. More generically this invention relates to equipment designed to facilitate mass and/or heat transfer between phases.
• the type of equipment to which this invention specifically relates employs cross-flow fractionation trays connected by downcomers .
• a tower is provided with a plurality of fractionation trays arranged generally horizontally within the tower.
• Each tower has a perforated deck and at least one channel, called a downcomer, in which a liquid flowing over the deck may be collected and channeled to the tray below.
• a gas or vapor is introduced at the base of the tower and passes upwards through the perforations in the decks of the fractionation trays. Meanwhile a liquid is introduced at the top of the tower and percolates downward passing over the fractionation trays and down the downcomers to the tray below.
• the downcomer may have a bottom pan where liquid flows around and out through slots in the bottom.
• the liquid should be prevented from passing through the perforations in the decks of the fractionation trays by the pressure of gas passing through the perforations in the upward direction.
• This is a finely balanced process since, if the pressure is too great, the gas will have a shorter transit time within the tower and less efficient contact with the down-flowing liquid.
• the high gas velocity may also cause liquid droplets to be carried up to the tray above, thereby reducing the separation efficiency as a result of back-mixing.
• the gas flow rate is too low the liquid will penetrate through the perforations in the tray decks, (known as "weeping"), and short-circuit the flow patterns which are intended to maximize liquid/gas contacts .
• the gas flow should be slow enough to permit efficient contact with the liquid flow but fast enough to minimize weeping. While a pressure differential between the space above a fractionation tray and the space below is necessary, if this differential is too great gas flow will be accelerated as it passes through the perforations and the efficient bubbling contact will be lost. To maintain the same volume of gas flow but reduce the pressure differential it is necessary to maximize the perforated area of the fractionation tray or provide some other efficient mechanism for the gas to contact the liquid as it passes through the fractionation tray.
• the present invention provides a fractionating column comprising a first perforated fractionation tray with at least one downcomer for channeling an outflow from the first tray to a second perforated fractionation tray directly below the first in which each downcomer channels downflowing liquid from the first tray to a portion of the second tray adjacent the junction of the tray and the column wall.
• the downcomer preferably has a restricted exit area designed to ensure that the outflow therefrom is directed exclusively towards a peripheral, non-perforated portion of the second tray.
• Liquid may, for example, be discharged from the downcomers via radial slots, rectangular slots, castellated weirs or plain weirs specifically designed to direct the flow to a wall/tray junction area from which it can be evenly distributed before contacting the perforated area of the fractionation tray. It is found that if the outflow from the downcomer is spread on the second tray in this manner before it contacts perforations, it is possible to avoid localized heavy flow situations that can lead to weeping. From the wall/tray junction the flow is naturally spread around the wall and enters the perforated area. Because the flow is initially directed towards the wall, a substantial portion of the under-downcomer area can now be perforated either with simple or valved holes, thereby increasing the efficiency of the trays.
• a perforated barrier separates the initial fluid contact area from the perforated portion of the tray such that the flow on to the perforated area is as evenly distributed as possible.
• Figure 1 is cross-section of a portion of a tower containing fractionation trays and a single enclosed downcomer according to the invention.
• Figure 2 is a plan view of the lower of the fractionation trays shown in Figure 1. Perforations are shown in the under- downcomer area.
• the device illustrated in Figures 1 and 2 comprises a vessel, 1, in which two fractionation trays, 2, are located one above the other in generally horizontal positions.
• the fractionation trays are provided with a plurality of perforations, 6, and a downcomer, 7, having entry port, 3, and exit port, 4.
• a downcomer 7, having entry port, 3, and exit port, 4.
• Figure 1 only the downcomer for the upper tray is shown but it is understood that all trays have similar downcomers though the location is usually at the opposed side of the tray to that receiving a flow from the downcomer associated with the tray immediately above. This maximizes the flow path of the liquid and the opportunities for contact with the vapor.
• Liquid exiting the downcomer through exit port, 4, is directed to the under-downcomer zone, 5, which is adjacent the wall of the vessel and is unperforated. From there the liquid spreads in the manner shown by the arrows in Figure 2 such that no part of the tray receives enough liquid to lead to weeping through the perforations in that part.
• the above structure is very advantageous since it ensures that liquid flowing out from the downcomers does not channel towards perforations in the fractionation tray deck in such quantities as to prevent vapor flow through the perforations and cause weeping. Rather by ensuring that the flow first contacts non-perforated areas and is spread evenly in the direction of all perforated areas, a uniformly high degree of liquid/vapor contact is maintained.

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• Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)

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• 1997
  • 1997-01-27 EP EP97902083A patent/EP0954363A1/en not_active Withdrawn
  • 1997-01-27 JP JP10531933A patent/JP2000507878A/ja active Pending
  • 1997-01-27 CA CA002277214A patent/CA2277214A1/en not_active Abandoned

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