The column unit may be used to simulate any vapor/liquid or vapor/liquid/liquid distillation unit. Columns usually consist of at least two trays but Single Tray Columns are allowed. For purposes of this discussion, the term "tray" is used to denote a theoretical tray or equilibrium stage. There is no limit on the Number of Trays that may be considered in a distillation column model.
The trays are considered linked, with the equilibrium vapor from each tray entering the next higher tray and the equilibrium liquid from each tray feeding the next lower tray. The Column Condenser, when present, is always designated as tray number one. The Column Reboiler, when present, is always designated as the highest numbered tray in the model. Every column model must have either a feed or heat duty on the top and bottom trays. The pressure for every tray in the model must be defined in the Column Pressure Profile Window.
Feeds and Products
A column may have any number of feeds, with a feed to any tray. All columns must have at least two products, an overhead product leaving tray one and a bottoms product leaving the highest numbered tray. Optionally, columns may have liquid and vapor phase side products withdrawn from any tray. The Column Feeds and Products Window gives additional information regarding column feeds and products. It is also possible to create Column Pseudoproducts corresponding to column internal liquid and vapor flows, thermosiphon reboiler streams and pumparounds.
Heat Sources and Sinks
Heat sources and sinks may be applied to any tray in a column model. There are several options for Column Condenser, where the condenser is always attached to the top tray of a column model to produce a net liquid stream (reflux) which is returned to the top of the column. The Column Reboiler is always attached to the bottom tray of a column model and produces vapor which is returned to the bottom of the column. Column Side Heaters/Coolers may be attached to any tray to add or remove heat. To simulate non-adiabatic column operation it is also possible to define a Column Heat Leak for any tray in the column.
Performance Specifications
Specifications on column products, internal flows and tray temperatures may be used to determine the values for column operating variables including feed rates, side product draw rates and heat duties. There must be a calculated variable for each specification as described in the Column Specifications and Variables Window. Specifications are discussed in the Generalized Performance Specifications.
Pumparound Streams
Column Pumparounds are used to move liquid or vapor from one tray to another tray, either higher or lower in the column. In practice, there is often a heat source or sink applied to the pumparound return tray to add or remove heat from the column, e.g., a side cooler or heater.
Calculated Phases
Most columns have vapor in equilibrium with a single liquid phase only. Standard procedure for hydrocarbon systems with water is to remove water as a separate liquid phase at the condenser (tray one) only. For this option, water is not allowed to exist as a separate liquid phase on any other column trays. Other options are available in which two liquid phases (where one phase may or may not be liquid water) may be considered in equilibrium with a single vapor phase. See Calculated Phases for more details.
Tray Efficiencies
All tray information that you supply is assumed to correspond to theoretical trays (equilibrium stages) and all distillation calculations are based on theoretical trays. For actual columns this implies that you must apply efficiency factors to the actual number of trays to determine theoretical trays when building a column model. Alternately, may supply the actual tray numbers for the column and use a Column Tray Efficiency model to apply efficiency factors to the actual tray numbers using one of three provided efficiency models. Tray efficiencies may also be selectively applied to only certain trays or components (See the Component Tray Efficiency Window) to match observed separations. All tray efficiency models have limitations and should thoroughly understand these limitations when using this feature.
Side Columns
Complex distillation systems often have side columns linked with a main column, where a main column draw is processed in the side column with some of the draw components withdrawn as a side product and some returned to the main column for further processing. For flowsheet connectivity purposes in PRO/II, side columns are added to the flowsheet as separate unit operations and linked to the main column through the feed and return streams. The side column feeds are set up as main column products and the side column return streams as main column feeds. Column Side Columns may be of two types: side strippers in which the main column draw is stripped with a stripping vapor or by reboiling and side rectifiers in which the side column has a condensing system producing liquid for rectification. The Column Algorithms used for side columns must be identical to that used for the main column.
Column Hydraulics
Column Tray and Packing Hydraulics may be used to rate existing columns and to size new columns at the calculated liquid and vapor loadings. Tray models include valve, sieve and bubble cap. Packings may be random or structured. All column calculations are normally performed at user defined pressures for all trays in the model. The pressure drops determined by the column hydraulic calculations may alternately be used to revise the supplied pressures.
Calculation Algorithms
The Column Algorithms describe the calculation algorithms available. The Inside-Out algorithm is selected as the default algorithm. This method solves a wide variety of vapor/liquid staged column models quickly and efficiently. Other methods are provided for columns with two liquid phases and/or highly non-ideal thermodynamic systems. All column algorithms are iterative in nature starting from Column Initial Estimates which you supply and/or internal estimation techniques. You may supply tuning factors and revised convergence criteria for the algorithm as described in the Column Convergence Data Window and may change the limit on the number of calculated trial solutions. See Number of Iterations for more details.
Reactive Distillation
Chemical reactions can be modeled on column trays for the Chemdist and Liquid-Liquid Column Algorithms. See Reactive Distillation for more details.
Thermodynamic Systems
All the distillation Column Algorithms simulate VLE systems. The Sure and Chemdist algorithms can also model VLLE systems provided that the thermodynamic system selected in the Column Thermodynamic Systems Window is a VLLE Model. It is also possible to define different thermodynamic systems for individual sections of the column.
Single Tray Columns
PRO/II allows the creation of Single Tray Columns, with or without a Reboiler. See Single Tray Columns for additional details. All of the distillation Column Algorithms support this type of column.
Side Column Unit Operation - Technical Information
Liquid-Liquid Extraction Unit Operation - Technical Information