Calc of Column Vapor & Liquid Loads

1. Purpose

This procedure provides the basis and methods for calculating column vapor/liquid loadings for tray and packing designs.

2. Appropriate Overage

There is always a degree of uncertainty regarding the design loads of a new column. Actual operation may vary from the design due to feedstock changes, different product quality requirements, or new production needs. Therefore, some overage in column design is reasonable and appropriate. However, per Inflection Point Engineering policy, provide overage in the design reflux rate and the design percentage of flood. Do not provide overage in the load calculations. Use the largest theoretical loads for tray design.

3. Highest Load

The theoretical stage with the highest Vload has the governing vapor load for a column section.

Where:

QV : volumetric flow rate of vapor

V : density of vapor

L : density of liquid

The theoretical stage with the highest volumetric flow rate of liquid has the governing liquid load for a column section.

4. Loadings For Multiple Column Sections

Most columns have more than one section, with each section having its own tray design. This occurs in the following cases:

• The column has a feed point between the overhead and bottoms product; hence, top and bottom sections have different loadings.

• The column has more than one feed point, a sidecut product, or has heat removed or added between top and bottom trays. In this instance, develop additional loadings for each column section with a material or heat balance change.

• Refer to Procedure ” for more details on multiple column sections.

5. Where To Obtain Column Loadings

When available, column loadings are generally obtained from an equilibrium stage model of the column. Care is needed to confirm that the modeling work provides appropriate loads at feed an energy removal or addition areas. The modeling of these areas, when poorly done, may result in erroneous loads being reported.

Thermosyphon reboilers are normally modeled as shown in Figure 1. A thermosyphon reboiler should produce a higher load, compared to a once through reboiler, for the same duty. The difference will be a function of the temperature difference across the reboiler. The larger the temperature difference, the larger the loading variation will be.

Preferential once through reboilers will have loading that are almost identical to an absolute once through reboiler.

Figure 1

Thermosyphon Reboiler

The reboiler return material produces vapor stream 7 and liquid stream 8. Bottom tray liquid 1 and reboiler liquid 8 combine to produce vapor stream 2 and total bottoms liquid 3. The vapor to the bottom tray is then the total of streams 2 and 7.

The third source of column loadings is the heat and weight balance. In some cases the governing loadings will be the vapor from and the liquid to the column top or the liquid from and the vapor to the column bottom. For example, this may be true for absorbers or strippers. Also special start-up and shutdown cases need to be considered. Sometimes downcomers need to be designed to accommodate large amounts of liquid during these special operations.

6. Why Loads Change Going Down From The Top

Figure 2 shows a sketch a column top.

Figure 2

• If the overhead and reflux are fixed, only hL and (hV - hL) affect the vapor rate (V).

• The value of hL increases going down the column which tends to decrease the value of V.

• The value of (hV – hL) generally decreases going down the column which tends to increase the value of V.

• lly, the highest load in the top section of a column is located within the top six trays.

• Columns with large temperature changes per tray, e.g., stabilizers, may have large enough changes in (hV – hL) to cause V to increase going down the column. lly, the stage with the largest V value also has the largest Vload and liquid volumetric flow rate values.

7. Why Loads Change Going Up From The Bottom

Figure 3 shows a sketch of a column bottom.

Figure 3

• If the bottoms rate and reboiler duty are fixed, only hL and (hV–hL) affect V.

• The value of hL decreases going up the column, which tends to decrease the value of V.

• The value of (hV–hL) generally increases going up the column which tends to decrease the value of V.

• lly, the highest load in the bottom section of a column is located within the bottom six trays.

8. Definitions

8.1 Variables

• B - Net bottoms mass flow
• h - Specific Enthalpy
• L - Liquid from tray mass flow
• O - Overhead vapor mass flow
• Q - Reboiler duty
• R - External reflux mass flow
• V - Vapor to tray mass flow

8.2 Subscripts

• B- Bottoms liquid
• L - Liquid from tray
• O - Overhead vapor
• R - External reflux
• V- Vapor to tray