The Column unit operation may be used to simulate any distillation or liquid-liquid extraction process. Distillation columns are described in the Distillation Column section of this chapter. Although liquid-liquid extraction (llex) columns are
generally not trayed, the distillation column nomenclature is used and the term tray denotes an equilibrium stage. A Liquid–Liquid Extraction Column must contain at least two trays.
The trays are considered to be linked with the light-liquid phase moving up the column and the heavy liquid moving down. There is no limit on the number of trays in a liquid-liquid extraction column model. Any tray may have a feed, product draw, or duty. There must be a feed to the top and bottom trays.
Note: Side columns may not be used with liquid-liquid extraction columns.
The following distillation column features are not applicable to LLEX columns and will be disabled:
• Condenser and reboiler
• Pumparounds
• Tray hydraulics
• Tray efficiencies.
Feeds and Products
Column feeds and products are added as part of the flowsheet construction in the PFD. They may be accessed from the Column Feeds and Products window accessible via the Feeds and Products… icon on the Column main data entry window.
Feed tray numbers may be added or changed in this window. There is no limit on the number of feeds a column may have.
For products, the product type, phase, tray number, and flow rate are supplied in this window. There is no limit on the number of products a liquid-liquid extraction column may have and products may be withdrawn from any tray of the column.
Product types include:
Overhead, Bottoms, Fixed Rate Draw, and Pseudo-product. Every column must have an overhead product leaving tray one and a bottoms product leaving the highest numbered tray. The product phase may be Light Liquid (Liquid 1) or Heavy Liquid (Liquid 2).
Product rates must be supplied for all draw products. Rates may be supplied in molar, mass, or liquid volume units. An estimated value must also be provided for either the overhead or bottoms product. The estimated value for the overhead or bottoms rate should be as accurate as possible to enhance convergence. It is necessary to use a Performance Specification to set a desired flow for the overhead or bottoms product.
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Pseudo-products
Pseudo-products are used to create streams corresponding to column internal streams, making them available for flowsheet calculations. Pseudo-products are defined in the Column products window accessible via the Pseudo-products… button on the Column Feeds and Products window. The following types of pseudo-products are available:
Net tray light or heavy liquid flow
Total tray light or heavy liquid flow
Column Algorithm
The solution algorithm is selected in the drop-down list box on the Column main data entry window. The Inside-Out (default), Sure, and Chemdist algorithms are for distillation columns. To specify a liquid-liquid extraction column, select the Liquid-Liquid option.
Calculated Phases
When the Liquid-Liquid algorithm is selected, the phase system will automatically be set to liquid/liquid.
Number of Trays
The number of trays in the model is entered in the data entry field provided on the Column main data entry window. Every Column must have at least two trays.
There is no limit on the number of trays in a Column.
Number of Iterations
The maximum number of trial solutions is supplied in the data entry field provided on the Column main data entry window. The default value is 30 for the Liquid-Liquid algorithm.
Pressure Profile
The pressure for every tray in a column model must be defined. All calculations are performed at the defined tray pressures. The tray pressures are defined in the Column Pressure Profile window, which is reached by clicking Pressure Profile… on the Column main data entry window. Tray pressures may be supplied on an overall or tray by tray mode by choosing a radio button in this window.
For the overall mode, the top tray pressure must be supplied and either the pressure drop per tray or the total pressure drop across the column. A default value of zero is supplied for the pressure drop per tray and the column pressure
drop. All tray pressures are derived by linear application of the supplied pressure drop.
Individual tray pressures are supplied for the tray by tray mode. Note that the top and bottom trays must be included when supplying a table of individual tray pressures. Missing pressures are determined by linear interpolation of supplied values.
Heaters and Coolers
Side heaters and side coolers may be configured in the Column Side
Heaters/Coolers window that is accessed using the Heaters and Coolers… icon on the Column main data entry window. A negative duty indicates cooling; a positive duty is used for heating. There are no limits on the number of side heaters/coolers. For each side heater/cooler, the following information must be provided: tray number, a reference name, and the duty, with the appropriate algebraic sign.
Initial Estimates
The Liquid-Liquid algorithm uses an iterative solution technique, starting from an initial estimate of the tray temperature, flow and composition profiles. By default, the initial estimate is produced internally using the initial estimate generator.
User-supplied profiles may be used to replace some or all of the values produced by the estimate generator.
Click Initial Estimates… on the Column main data entry window to enter the Column Initial Estimates window.
When using the initial estimate generator, profiles are determined by a simple material balance. Temperatures are determined from estimated product
compositions. You may optionally provide temperature estimates for the top and bottom trays which replace values predicted by the estimate generator, as well as an estimate of the ratio of the liquid flows on tray 1.
When the initial estimate generator is not used, the data which must be supplied as input profiles are tray temperatures and flows, either light or heavy liquid, or a combination thereof. Note that the minimum data which may be supplied are the temperatures and flows for the top and bottom trays for the column. While these are the minimum data required, they are rarely adequate to produce an
acceptable initial estimate. It may also be desirable to provide solution profiles from a converged solution to speed future calculations with a column model.
Initial profiles are entered in tables accessed by clicking the following buttons on the Column Initial Estimates window: Net Vapor Rate…, Vapor Composition…, Tray Temperature…, Liquid Composition…, and Net Liquid Rate… Composition estimates are rarely needed for most problems.
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