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IMAGES OF DISTILLATION COLUMNS
(GOOGLE “DISTILLATION COLUMN IMAGES”)
No one drew a satisfactory picture of a distillation column. I understand to some extent why—most of the pictures you have been exposed to do not show the details that are needed for a tray to work. You all need to go by D-107 and spend some time looking at the tray cartridge there. The downcomers are “pipes” in the cartridge. A typical larger column uses the side of the tower as part of the downcomer most of the time.
Notice how the trays physically separate the column into many different stills. The downcomers and weirs seal the separation (and notice how separate is spelled) while allowing the liquid to flow down the column. The vapor goes up through the holes in the trays and passes through the liquid that is held up on the trays.
This is another sketch that has the parts well shown. The axial flow example the flow goes from the middle to the outside (with circular or radial flow and not a common design) and then from the outside to the middle. You can also split a large tray into two trays. This would be like putting two of the “cross flow” systems “back to back.”
The downcomers are written in here rather than drawn in. This is a popular drawing and is reused several times in various Wikipedia topics. At the bottom of this file is an “expanded” view of this same picture that shows the downcomers.
This is a bubble cap tray. Most of the chimneys don’t have the caps on them. The vapor comes up the chimney and then down the outside of the chimney and into contact with the liquid on the tray. You can see the weir on the right side of the tray that keeps the caps covered in liquid. This design has a very high turndown ratio. It can’t weep liquid and will hold a level under almost all situations. There is a hole (one or more) in the tray somewhere to drain it if the column is shut down. Otherwise you could not “clear” the tower for maintenance.
This is a stylized picture of a bubble cap column. They did not draw the downcomers that are needed to keep the vapor from bypassing the tray. This is a common short cut in drawing columns but there are always downcomers in the most common tray designs. There are some designs where the liquid simply falls from one tray to another but these are very inefficient because the liquid and vapor do not have enough contact to equilibrate.
Distillation columns use this principle effectively repeating the separation process at individual trays within a column. Such columns may have over a hundred trays on which VLE (vapor liquid equilibrium) is occurring. Depicted below is a cartoon cut-away of a small section of a tray-type distillation column. Two sieve trays are shown. Hot vapor (shown in white) flows up through the holes in the sieve tray and vigorously bubbles through the liquid (shown in blue). The vapor and liquid mix on this tray and the new mixture boils. The composition of the liquid, x, leaving the will be enriched in the less volatile component while the composition of the vapor, y, will be enriched in the more volatile component. The vapor rises up to the next tray where it will contact a liquid of larger x and mixing and re-equilibration will again occur. The liquid leaving the tray will flow over the weir and down the downcomer to the next tray where it will mix with the vapor on that tray. In this way, x tends to become smaller at each lower tray, while y tends to increase at each tray higher in the column. VLE refers to the relationship between the x and y values on each tray.
At equilibrium, the temperature and pressure of the liquid and vapor phases will be equal, but the compositions will not. Why is this? There are two main factors that make the vapor and liquid compositions different at equilibrium: the pure component vapor pressures and the nonidealities in the liquid phase. These two factors are discussed more fully in the next section on the Thermodynamics of VLE. However, to understand fully these factors, one needs to understand the molecular nature of the liquid and vapor phases and how the properties of the fluids arise from the molecular interactions. The section on Molecular Interations will help understand this relationship of molecular interactions and the resultant phase behavior. Both of these sections should be studied before running the simulator.
www.et.byu.edu The picture and text above were from this site. We have sieve trays in T-100. If you don’t have enough vapor flow to keep the liquid from running through the holes, sieve trays don’t keep much liquid on the trays. This reduces their turndown ratio dramatically.
http://www.answers.com/topic/continuous-distillation is a web site with some very good text (and one of the pictures above) on distillation.
The pressure under which a distillation is performed is a matter of choice, although operation at pressures more removed from atmospheric becomes more costly. Altering the pressure of a distillation can serve to alter the vapor-liquid equilibrium relationship, a feature that can be used to advantage.
Because of the relationship between vapor pressure and temperature, the temperatures within a distillation column are lower for lower pressures. Vacuum distillation is an effective means of maintaining lower temperatures for separations involving heat-sensitive materials.
Steam distillation is an alternative to vacuum distillation for separations of organic substances. In this process, steam is fed directly to the bottom of a column and passes upward, composing a substantial fraction of the vapor phase. The combined partial pressures of the organic substances being distilled are thereby lessened, giving the lower temperatures characteristic of a vacuum distillation. http://www.answers.com/topic/distillation
Industrial distillation is typically performed in large, vertical cylindrical columns (as shown in image 2) known as "distillation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more.
Image 3 (on the left): Chemical engineering schematic
of a continuous fractionating column. Typical of many drawings, it fails to show the downcomers. Please remember that the downcomers are there in almost every column design that involves trays. The only exception I have seen are some designs for handling very dirty materials and they will look a lot like the sketch on the left.. Image 3 depicts an industrial fractionating column separating a feed stream into one distillate fraction and one bottoms fraction. However, many industrial fractionating columns have outlets at intervals up the column so that multiple products having different boiling ranges may be withdrawn from a column distilling a multi-component feed stream. The "lightest" products with the lowest boiling points exit from the top of the columns and the "heaviest" products with the highest boiling points exit from the bottom.
Image 4 (on the left): Chemical engineering schematic of typical bubble-cap trays in a fractionating column. As mentioned above, the downcomers are “written in” here and not drawn in. They extend below the “puddle” of fluid at a side of the column to seal the flow of vapor and force it through the trays and into intimate contact with the liquid on the trays.
Industrial distillation towers are usually operated at a continuous steady state. Unless disturbed by changes in feed, heat, ambient temperature, or condensing, the amount of feed being added normally equals the amount of product being removed.
It should also be noted that the amount of heat entering the column from the reboiler and with the feed must equal the amount heat removed by the overhead condenser and with the products.
Industrial fractionating columns use external reflux to achieve better separation of products. Reflux refers to the portion of the condensed overhead liquid product that returns to the upper part of the fractionating column as shown in Image 3.
Inside the column, the down flowing reflux liquid provides cooling and condensation of up flowing vapors thereby increasing the efficacy of the distillation tower. The more reflux and/or more trays provided, the better is the tower's separation of lower boiling materials from higher boiling materials.
The design and operation of a fractionating column depends on the composition of the feed and as well as the composition of the desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method or the Fenske equation can be used. For a multi-component feed, simulation models are used both for design, operation and construction.
Bubble-cap "trays" or "plates" are one of the types of physical devices which are used to provide good contact between the up flowing vapor and the down flowing liquid inside an industrial fractionating column. Such trays are shown in Images 4 and 5.
The efficiency of a tray or plate is typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a fractionating column almost always needs more actual, physical plates than the required number of theoretical vapor-liquid equilibrium stages.
This is an expanded view of image 4 (above). Here they drew the downcomers. Notice that the pressure drop through the tray above holds the liquid in the downcomer above the level of the liquid on the tray. There is usually an “overflow” weir between the downcomer and the active part of the tray that is not shown.
lorien.ncl.ac.uk/ming/distil/distilint.htm This is a very good web site for distillation information. The author of this site also wrote some of the information above. You will have to paste it into your browser’s address box to access it.