What is meant by vapor and liquid loading? What is its significance?
Answers
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24/04/2011
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Frank Rukovena Jr, Rukovena Consulting: FRI-Retired, Rukovenaengr@gmail.com
Building upon Ralph’s answer, as he says, loading refers to the vapor and liquid traffic passing through a packed bed or tray. When considering the effect of them on the capacity of either of these devices it is necessary to consider their ration to one another as well as their independent flow rates. As a general statement for counter-current devices the higher the liquid load the lower the allowable vapor loads and vice versa.
For trays, it is possible to blow all of the liquid off of the tray deck if the liquid rate is low and the vapor velocity high. This is known as tray blowing. At higher liquid rates with high velocity vapor, the liquid plus froth will build up on the tray until its level will fill the tray space and reach the tray above. This is typically called flooding. Also at high pressure drop across tray it is possible to back the liquid level in the downcomer and flood the tray above. This is called downcomer backup flooding. At high enough liquid rates and a small enough downcomer inlet open area not all of the liquid leaving a tray will not have downcomer open area to get into the top of the downcomer, this is called downcomer choking. This will also flood the tray which it is suppose to be leaving. In all of these types of flooding the separation efficiency will decline and the pressure drop vary widely.
For packing operating in counter current mode similar phenomena happen. At high enough vapor velocities (loading) the liquid flowing down the packing will backup and not be able to leave the packed bed. This is also known as flooding. For low pressure drop packing this can happen with pressure drops per foot of packing in the range of 0.75 in to 1 in of fluid per foot of packing. The liquid holdup in a packing is mostly independent of vapor rated until the loading region is reached at this point the liquid hold up in the packing will start to increase until the packing is filled with liquid and is in “flood”. When the flood point is reached the column separation efficiency will decline and the pressured drop start to fluctuate widely.
For packing and trays, flooding does not always start first at the top of the column but it starts in the section of the column that has the highest vapor and liquid traffic or some combination thereof and the most restrictive equipment design. You may want to consult with “Perry’s Chemical Engineering Handbook” or book’s by Henry Kister “Distillation Design” and “Distillation Operation”.
Other Considerations:
For trays how the liquid is feed on to a tray does effect the operation of that tray and maybe even the tray below it if it is done very poorly. Vapor entering a tray tower if done incorrectly can upset the tray above but it a tray tower is not as sensitive to this is as a packed tower because the higher pressured drop of the tray compared to a packed tower. Also for a tray tower make sure that the liquid leaving the downcomer is not interfered with by the entering vapor. For large towers with multiple flow paths, vapor and liquid misdistribution per pass can occur and reduce the towers performance.
For packed towers, the specific attention needs to be paid to the design of the liquid distributors. A poorly designed liquid distributor will lower the efficiency of the packed bed below it. Because of the low pressured of packing one must pay more attention to the vapor velocities entering the tower and in some cases a vapor distributor may be required to make the vapor enter the packed bed evenly and/or to prevent liquid being carried back up the tower by the entering vapor.
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03/01/2010
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
It means the actual flow quantities up and down through the equipment. The "loadings" are then compared with the maximum allowable quantities as determined by the physical size of the equipment as well as the operating P and T and properties of the flowing fluids. For example, through a section of fractionating trays, that comparison would be expressed as "percent of flood". Typical design is 80 to 85 percent of flood for a fractionator.
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