Initial Estimate Generation Methods

Several Initial Estimate Generation methods are provided by PRO/II as described below.

Simple

The column profiles are determined by simple material balances. Temperatures are determined by estimating the product compositions. This model is quick and adequate for simple column configurations. It is the only option available for liquid-liquid extraction columns.

Conventional

This method is very general and designed to provide an adequate estimate for most distillation problems. Shortcut distillation techniques are used to estimate product rates and compositions which are then used to estimate temperatures. Internal tray flows are set up with material balances over each tray, using the product rates and the column reflux estimate. Other supplied temperature estimates are used as appropriate.

For conventional columns that have reboilers and condensers, classic Fenske shortcut distillation calculations are used. In defining performance specifications for the shortcut calculations, the Conventional Estimate Generator first attempts to use the performance specifications which are being applied to the rigorous column solution. When the performance specifications are unsuitable for shortcut calculations, e.g., internal column flows or tray temperatures, the Conventional Estimate Generator must find other specifications. The first alternate specification is always the estimated rate for the column "overhead" or "bottoms" product supplied in the Column Feeds and Products Window. The quality of the initial column estimate is therefore highly dependent on the quality of the estimated "overhead" or "bottoms" rate under these circumstances. It is always good practice to estimate the column "overhead" or "bottoms" rate as accurately as possible.

Refinery

The Refinery method is designed to produce estimates for complex oil refinery distillation columns such as crude and vacuum units, FCC main fractionators, coker bubble towers, etc. These columns have the following distinguishing characteristics: no reboilers, bottom stripping steam, side columns with stripping steam or reboilers, pumparound cooling circuits, and decanted water at the overhead accumulator.

A multi-product shortcut technique developed by SimSci is to generate the estimates for these columns. The estimated product rates are used as specifications for the shortcut model to determine estimated compositions for the products. Tray temperatures are estimated from the products. As appropriate, supplied temperature estimates are also used. The internal tray flows are generated by using the product rates and the column reflux estimate. Adjustments are made in the column profiles for side coolers.

Chemical

Use of this generator should be restricted to highly non-ideal chemical distillation problems in which the Chemical algorithm is used. The method is time consuming and involves the calculation of a series of adiabatic flashes up and down the column trays several times as needed to generate a good composition starting profile.

Electrolytic

Use of this generator should be restricted to highly non-ideal aqueous electrolytic distillation problems in which the Electrolytic algorithm is used. Tray mole fractions are estimated by successive flashes on each tray.

Click RATEFRAC options … to pop up the Column - RATEFRAC Initial Estimates dialog box.

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