Patent Application
20070145611
Not Applicable
Not Applicable
The present invention relates to a gas-liquid contactor baffle used to improve gas-liquid contact for mass transfer in a tray tower.
1. Field of the Invention
The present invention relates to chemical process towers and, more particularly, but not by way of limitation, to a de-entrainment/mass transfer assembly for a tray tower, the trays having baffles, for increasing tray capacity and improving mass transfer efficiency therein.
2. Description of Related Art
It is a continuing goal to improve the efficiency of separation processes and like processes that are conducted using chemical process towers. To this end, many different approaches have been undertaken.
Separation processes that can be performed in chemical process towers include distillation and absorption. The optimum design of a tower having trays ensures maximum throughput (i.e. capacity) and mass transfer efficiency. At high throughput there is a tendency for liquid to be entrained at high gas velocity. This reduces the capacity as well as efficiency, both caused by the liquid blowing to the tray above. It is an objective of the present invention to effectively remove the entrainment.
Distillation and absorption towers are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact towers utilize either trays or packings, and sometimes combinations thereof. In the case of tray towers, any wetted solid surfaces will improve mass transfer through additional intimate contact between liquid and gas phases. There will be further advantage if the solid surface can also serve as a de-entrainment device.
Distillation trays come in two configurations: cross-current (cross-flow) and counter-current (dual-flow). The trays generally consist of a solid tray or deck having a plurality of apertures perforating the deck and are installed on support rings secured within the tower. In cross-current trays, gas ascends through the apertures and contacts the liquid moving across the tray through the “active” area thereof. It is in this area liquid and gas mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is generally referred to as the inlet downcomer. The liquid moves across the tray and exits through a similar channel generally referred to as the exit downcomer. It is the active area of the tray which most directly effects gas liquid contact and thus mass transfer efficiency.
In the case of dual-flow trays, the tray deck covers the entire cross-sectional area of the tower. Gas and liquid flow through the same apertures, and contact in counter-current manner. Thus, there is no need for downcomers.
A problem common in the art is that of entrainment of spray in the gas. When spray is entrained in the rising gas it is carried to higher trays, thus affecting the composition of the mixture at those trays, with the consequence that the efficiency of separation of the components in the process mixture is compromised. The gas flow rate may be reduced in order to reduce the entrainment effect, but a consequence is that the tray throughput is also reduced. Another detrimental effect is that the entrained liquid accumulates on the tray above and there is increased amount of liquid on that tray, thus causing premature flooding or reduction in the capacity.
Improvements have been targeted for the technology of gas-liquid contact trays of the type discussed above to address throughput and mass transfer efficiency issues. Examples of this technology are seen in several prior art patents, which include U.S. Pat. Nos. 3,955,419, 4,604,247 and 4,597,916, each assigned to Glitsch, Inc. and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. Other performance aspects are addressed in the prior art by the use of baffles, plates and de-entrainment devices. For example, U.S. Pat. No. 4,105,723 and U.S. Pat. No. 4,132,761, both assigned to Merricks Corporation, address special baffle and de-entrainment structures which are placed within a process tower.
Chuang et al. in U.S. Pat. No. 5,262,094 teach the utilization of a bed of packing material disposed beneath a fractionation tray for de-entraimnent. Further examples of de-entrainment performed using packing below trays are described by, for example, Monkelbaan et al. in U.S. Pat. Nos. 5,554,329 and 5,707,563, and by Nutter et al. in U.S. Pat. No. 5,975,504. Several other types of de-entrainment devices are described by, for example, Mahar in U.S. Pat. No. 4,274,923, Bentham in U.S. Pat. No. 4,818,346, Stober et al. in U.S. Pat. Nos. 5,837,105 and 6,059,934, and Ross et al. in U.S. Pat. No. 5,972,171. In another example, Lee et al. in U.S. Pat. No. 5,762,668 use a structured packing assembly to reduce entrainment in a chemical process tower and improve mass transfer efficiency.
However, when de-entrainment is performed using chevron types of demisting device or structured packings, the wetted surfaces of these devices are in direct contact with high velocity gas streams, resulting in re-entrainment of the liquid. This reduces the effectiveness of these devices for de-entrainment.
According to the present invention there is provided a gas-liquid contactor baffle which includes a body having a first portion, a second portion and a middle portion, the middle portion being positioned between the first portion and the second portion. The middle portion is a corrugated sheet having a first face and a second face. The corrugated sheet has alternating ridges and open ended channels extending across each of the first face and the second face between the first portion and the second portion. The first portion has a first collection channel adapted to collect liquids from the open ended channels of the middle portion when flow is along the first face in a first direction. The second portion having a second collection channel adapted to collect liquids from the open ended channels of the middle portion when flow is along second face in a second direction.
There will hereinafter be described how this liquid gas contactor baffle improves effectiveness of de-entrainment and provides additional wetted surface area for enhanced mass transfer. As a result, towers equipped with this apparatus have greatly enhanced capacity and efficiency when compared with the prior art.
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:
Exemplary embodiments of the invention, which are non-limiting, will now be described with reference to
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A preferred embodiment of baffle 34 will now be described with reference to
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The method of utilization of apparatus 33 of the present invention will now be described with reference to
Apparatus 33 comprising a plurality of baffles 34 is installed in an array having at least one rank 88 below trays 14 in tower 12. When tower 12 is in operation, liquid 100 spills over weir 28 of one tray 14 and descends downcomer 32 toward another tray 14 immediately below the one tray 14. A head of liquid 100, an extent of which is indicated by curved lines 104, is in downcomer 32, and causes liquid 100 to flow across top surface 16 of tray 14 toward and over weir 28. Gas 102 in zone 36 immediately below tray 14 rises through apertures 20 in tray 14 and bubbles through liquid 100. A froth 106 that is formed has an extent indicated by a curved line. As bubbles 107 exit froth 106 they form a spray 108 that travels upward as indicated by arrows. Baffles 34 intercept spray 108. Referring to
Liquid 100 descending channels 56 is not exposed to any high velocity gas 102, and so is not subject to re-entrainment.
Droplets 110 are much larger than spray 108, and so droplets 110 do not become entrained in gas 102.
In this manner, liquid 100 that has been formed into spray 108 is prevented from being entrained in gas 102 and ascending through apertures 20 in tray 14 above.
When compared with operation of trays 14 having no baffles, the surface area of liquid 100 is increased by having liquid 100 flow along channels 56 on back face 50 of baffles 34 and fall as droplets 110, thereby improving mass transfer between liquid 100 and gas 102, and hence enhancing operation of tower 12.
It has been found through monitoring embodiment 10 shown in
Benefits found to accrue from operation of the present invention include: converting tray action from spray regime to froth regime; deflecting spray by collecting it from the front sides of the baffles and routing it to the back sides of the baffles and subsequently discharging it to the active area below, and increasing liquid surface area for additional mass transfer. These beneficial characteristics of the new invention have been shown to result in greater capacity (up to 20%) and higher tray efficiency.
It has been shown experimentally that a combination of trays 14 with baffles 34 that serve as “spray deflector” can have as much as 15 to 20% more capacity in a weir loading of 0.5 to 5.0 gpm per inch of weir 28 when compared with use of trays alone. The apparatus was tested using different gas rates (C-factor) and liquid weir loadings (WL). When WL was 4.0 gpm/in for standard valve trays separated by 610 mm, the C-factor at capacity increased from 0.39 ft/s to at least 0.46 ft/s when baffles were in place.
In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.
It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiment without departing from the spirit and scope of the invention as hereinafter defined in the Claims.
