Process Equipment Design Of Distillation Columns

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Distillation columns are very widely used in the chemical and petroleum industries to separate chemical components into more or less pure product streams. This separation is based on differences in “volatilities” (tendencies to vaporize) among various chemical components. For example, a mixture of methanol and water can be separated by distillation because methanol is more volatile or boils at a lower temperature than water. In a distillation column, the more volatile, or lighter, components are removed from the top of the column, and the less volatile, or heavier components are removed from the lower part of the column.

Nomenclature

Figure below summarizes the nomenclature that is generally used. We have considered only a simple single-feed, two-product column separating a binary (two-component) mixture.

Nomenclature and conventions for distillation column

Feed rate is F moles per minute. Feed composition is zF mol fraction of the more volatile component. The column trays are numbered from the base upward, with feed introduced on the NF tray. The total number of trays in the column is NT. Products removed from the top and bottom of the column are called “distillate” or “top product,” and “bottoms” or “bottom product,’’ respectively, with flow.

Tray hydraulics

The vast majority of industrial distillation columns are equipped with trays or plates (sometimes called “decks” in the petroleum industry) located every 1-3 feet up the column. These trays promote mass transfer of light components into the vapor flowing up the column and of heavy components into the liquid flowing down the column. Vapor-liquid contacting is achieved by a variety of devices. The most widely used trays in recent years have been sieve trays and valve trays because of their simplicity and low cost. Sieve trays are simple flat plates with a large number of small holes. Vapor flows up through the holes, preventing the liquid from falling through. Liquid flows across each tray, passes over a weir, and drops into a downcomer,” which provides liquid for the tray below through an opening at the base of the downcomer. Valve trays are built with a cap that fits over the hole in the tray and that can move up and down, providing more or less effective hole areas as vapor flow rate changes. The plate contractors have been discussed in detail later in the report.

This fairly complex process of flow of vapor up the column and of liquid across each tray and down the column is called tray “hydraulics.” It is important because it imposes very important constraints on the range of permissible liquid and vapor flow rates. If liquid cannot flow down the column, or if vapor-liquid contacting is poor, the separating ability of the column drops drastically. Vapor flows from one tray up through the tray above it because the pressure is lower on the upper tray. Thus there is an increase in pressure from the top of the column to its base. Liquid must flow against this positive pressure gradient. It is able to do so because the liquid phase is denser than the vapor phase. A liquid level is built up in the downcomer to a height sufficient to overcome the difference in static pressure between the tray onto which the liquid is flowing and the tray from which it is coming.

This pressure difference depends on the vapor pressure drop through the tray (which varies with vapor velocity, number and size of holes, vapor density, etc.) and the average liquid height on the tray (which varies with liquid flow rate, outlet weir height, etc.). Tray “flooding” occurs when the liquid height in the downcomer equals or exceeds the height between trays (tray spacing). This is usually due to excessive boil up (vapor rate) but sometimes may be caused by excessive reflux. Therefore, there are maximum vapor and liquid rates.

On the other end of the scale, if vapor rates are reduced too much, the vapor pressure drop through the openings in the tray will be too small to keep the liquid from weeping or dumping down through the holes. If this occurs, vapor-liquid contacting is poor and fractionation suffers. The same thing occurs if liquid rates are so low (as they often are in vacuum columns) that it becomes difficult to hold enough liquid on the tray to get good vapor-liquid contacting.

This pressure difference depends on the vapor pressure drop through the tray (which varies with vapor velocity, number and size of holes, vapor density, etc.) and the average liquid height on the tray (which varies with liquid flow rate, outlet weir height, etc.).

Tray “flooding” occurs when the liquid height in the downcomer equals or exceeds the height between trays (tray spacing). This is usually due to excessive boil up (vapor rate) but sometimes may be caused by excessive reflux. Therefore, there are maximum vapor and liquid rates.

On the other end of the scale, if vapor rates are reduced too much, the vapor pressure drop through the openings in the tray will be too small to keep the liquid from weeping or dumping down through the holes. If this occurs, vapor-liquid contacting is poor and fractionation suffers. The same thing occurs if liquid rates are so low (as they often are in vacuum columns) that it becomes difficult to hold enough liquid on the tray to get good vapor-liquid contacting.

Schematic of Seive tray

Attachments:
Download this file (Process Equipment Design of Distillation Columns .pdf)Process Equipment Design of Distillation Columns[Project Report]2532 Kb

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