200.0 DEFINITION AND DESCRIPTION OF TERMS

201.0 Flow Quantities

(Refer to Figure 1)

Figure 1. Graphic Description of Terms

201.1 Feed is the material to be separated, including multiple feed streams.

201.2 Bottoms describes the high-boiling product leaving the bottom of the column (or the reboiler).

201.3 Distillate is the product distilled overhead. It may leave the distillation system as a vapor, liquid, or a combination of both.

201.4 Side-stream Product is product withdrawn from an intermediate section of the column.

201.5 Overhead Vapor designates the vapor from the top of the column and includes material to be condensed for reflux. It is the combined distillate and external reflux.

201.6 Reflux is used to designate the quantity of liquid returned to the column.

201.6.1 External (Overhead) Reflux is the quantity of liquid returned to the top of the column. External reflux may be subcooled, which can result in increased internal reflux.

201.6.2 Internal Reflux is the calculated quantity of liquid leaving the top theoretical stage inside the tower. The internal reflux is different from the external reflux in that it is in thermal equilibrium with the top theoretical stage inside the tower.

201.6.3 Pumparound is the quantity of liquid withdrawn from, and returned to, the column after being cooled. A pumparound can be subcooled, and then returned to the tower at a location other than the top. A pumparound is sometimes called Circulating Reflux.

201.6.4 Reflux Ratio is the ratio of the external reflux flow to the distillate. Some applications may use the ratio of external reflux flow to feed to represent the reflux ratio.

201.7 Throughput refers to the combined liquid and vapor traffic passing through a cross section of the column.

201.7.1 Internal Liquid is the calculated quantity of liquid flowing from point to point in the column.

201.7.2 Internal Vapor is the calculated quantity of vapor passing from point to point in the column.

201.7.3 Entrainment is the liquid carried upward by the vapor stream from one point to another.

201.7.4 Weeping is the liquid that flows downward through the deck openings in trayed towers.

202.0 Key Components

A key component is a component of interest in a column. In a multi-component mixture, separation is based upon the lower boiling “light key” and the higher boiling “heavy key.” At times components with boiling temperatures between those of the light and heavy key may also be present. Such a component may be called a “middle boiler,” an “intermediate,” or a “distributed key.”

203.0 Mass Transfer Efficiency

The efficiencies used in describing the performance of fractionation columns are briefly described below [1].

203.1 Theoretical Trays or Plates or Stages are stages on which the vapor and liquid streams leaving the stage have reached thermodynamic equilibrium.

203.2 Overall Column Efficiency refers to the performance of the column as a whole. For trayed columns, this efficiency is the ratio of the number of theoretical trays or stages that would be required for the observed separation to the number of actual trays in the column. This number is dimensionless and is usually expressed as a percentage.

For packed columns, the HETP (Height Equivalent to a Theoretical Plate) for the column as a whole is the ratio of the overall height of packing in the column to the number of theoretical stages in the column. This number has units of length per theoretical plate (or stage).

For calculation of overall efficiency, the stages are those developed by the trays and/or packing. Any stages attributed to a reboiler or a partial condenser should not be included in this calculation. Overall efficiency is useful in comparing one test with another, or in comparing the test with design. The overall efficiency of sections of a column may also be of interest. For example, the overall efficiency below the feed point may be different from the overall efficiency above the feed.

203.3 Apparent Murphree Tray Efficiency can be measured by taking samples around a single tray. It is the actual change in composition accomplished by the tray divided by the change that would occur on a theoretical tray based on exit liquid composition. It accounts for the effects of entrainment, weeping, liquid mixing, maldistribution, and other factors.

203.4 Ideal Murphree Tray Efficiency is the performance of a single tray, exclusive of the deleterious effects of entrainment, weeping, and liquid backmixing. The ideal Murphree tray efficiency can be predicted from the Murphree point efficiency.

203.5 Murphree Point Efficiency is the Murphree efficiency at a single point on the tray.

203.6 HETP (Height Equivalent to a Theoretical Plate) is the height of packing required to perform as one theoretical plate or stage of separation.

203.7 HTU (Height of a Transfer Unit) is the height of packing required to obtain one transfer unit and is a measure of the mass transfer efficiency. As such, it incorporates mass transfer coefficients. The more efficient the mass transfer (i.e., larger mass transfer coefficient and/or larger vapor-liquid contacting area), the smaller the value of HTU. This value can be estimated from empirical correlations, or measured directly from pilot plant tests (See Section 605.2.2).

203.8 NTU (Number of Transfer Units) is a measure of the difficulty of the separation. A single transfer unit gives the change of composition of one of the phases equal to the average driving force producing the change. The NTU is similar to the number of theoretical stages or plates required for trayed columns. A larger number of transfer units is required for a higher purity product (See Section 605.2.2).

204.0 Operating Lines

Operating Lines are the material-balance lines on a McCabe-Thiele type of diagram for a binary system [2]. The use of operating lines has been extended to multi-component systems by distributing the non-key components to the key binary components [3].

205.0 Pinch

Pinch is a term that describes a local condition within the column under which no appreciable change in composition of the liquid or vapor from stage to stage occurs due to a lack of mass transfer driving force, and not to column malfunctioning, flooding, high entrainment, or a dry tray or packing. For a binary system, a pinch is graphically depicted when an operating line is approaching or intersects the equilibrium curve on a McCabe-Thiele diagram.

206.0 Maximum Throughput

206.1 Maximum Hydraulic Throughput is the highest loading at which a column can operate without flooding. Since the loading is affected by both liquid and vapor rates, many combinations of those rates may define a maximum hydraulic throughput curve. In practice, this maximum may be limited by the trays, or the packed bed, or one of the internals.

206.1.1 Flooding describes the condition of the column when the hydraulic capacity is exceeded. At loadings achieving or reaching the flooding point, liquid accumulates uncontrollably and continued operation becomes impossible. The flooding point depends on both vapor and liquid velocities, system properties, and tray or packing geometry [4]. The flooding point can be recognized by the existence of one or more of the situations described in Section 502.1.

206.2 Maximum Operational Capacity is the highest loading at which stable operation leading to acceptable overall column efficiency is achieved. This point may occur well below the column’s maximum hydraulic capacity. Since loading involves both liquid and vapor, a maximum operational capacity curve can be defined on a plot of liquid vs. vapor rates.

206.3 Maximum Efficient Capacity is a term applied most commonly to random packing, where HETP is typically flat with respect to capacity, and then decreases as flood is approached before increasing again. The maximum efficient capacity is the last point at which the HETP of the packing is equivalent to its value in the flat part of the curve.

207.0 Minimum Operating Rate

Minimum Operating Rate is the smallest loading at which a column performance is acceptable. The separation may become unacceptable because of loss of efficiency due to excess weeping of the tray, incomplete wetting of the packing, liquid or vapor maldistribution, or column instability.

208.0 Operating Section

An Operating Section of a column is a part to which no feed is added, no product is removed, and no external heat is added or removed. It may consist of one or more beds of packing, or multiple trays.

209.0 Hardware

209.1 Components of a Trayed Column

A trayed column consists of one or more sections of trays, each separated by an inlet and/or outlet stream that can be liquid, vapor or a mixture of both.

209.1.1 Trays are generally horizontal plates that enable vapor and liquid to contact each other. Trays are composed of decks with (or without) downcomers.

209.1.2 Downcomers are conduits through which liquid passes from one tray to another.

209.1.3...