Simulation model

The following section provides a high-level view of both simulation models. See Process Mode for a detailed workflow description.

Conceptual engineering

The conceptual design model (Figure 1) consists of a Source, a Column, and two Sinks that represent the destinations for the distillate and bottoms products. We model the condenser and reboiler as additional top and bottom stage duties that are internal to the column by setting the Condenser and Reboiler parameters to Internal. We set the Internals parameter to Trays so that the column includes basic hydraulic tray sizing and rating calculations according to Fair’s entrainment flooding correlation for 24-inch tray spacing. We initially size the stages by assuming 78% tray flooding and setting the Hydraulics parameter to Sizing. The Dcol variable then reports the maximum stage diameter.

Detailed engineering

The extended simulation for detailed engineering (Figure 2) utilizes additional features, including hydraulic calculations. We construct it by using external condenser and reboiler circuits. We now supply reflux and reboiler return flow externally by setting the Condenser and Reboiler parameters to External. We make similar declarations for the column sump. To ensure that the internal vapor traffic is identical to that of the conceptual model, we quantify the sump’s reboiler draw by using the RoutFrac variable and base it on a 30% vaporized reboiler return.

We also rate the column by setting the Hydraulics parameter to Rating, which uniformly sets the calculated Dcol variable for all stages. The FF array variable reports the individual flooding factors for the trays.

The overhead condenser circuit consists of a heat exchanger model with cooling water on the tube side. We calculate the required flowrate needed to completely condense the overhead product by specifying the process-side vapor fraction as zero and the cooling water return temperature at the design condition of 50°C. AVEVA Process Simulation calculates the required cooling water flow and exchanger duty.

The following table illustrates the specified variables by default and the modified arrangement to meet design conditions.

Table 2: Specifications for overhead condenser and cooling water supply

* A indicates that the variable is calculated and a indicates that the variable is specified

The fully condensed overhead condensate flows to the reflux drum where vapor can flow through a vent valve (XV6) and we pump and split the remaining liquid product into reflux and distillate streams. Since the feed into the reflux drum is all liquid and our feed does not include any non-condensable components, the vent valve flow is zero and we cannot size the control valve Cv. Therefore, we use an arbitrary valve Cv of 10. We specify the valve Cv and calculate the valve position, ManPos.

The following table illustrates the variable specification configuration for valve XV6 in order to achieve this.

Table 3: Specifications for valve XV6

* A indicates that the variable is calculated and a indicates that the variable is specified

In Process mode, we calculate that the position of the valve is closed based on zero flow. However, it is good engineering practice to keep the valve open. After you transition to Fluid Flow mode, you should specify the valve position as slightly opened by setting the ManPos variable to 0.5.

We calculate the thermosiphon reboiler flow to achieve the required reboiler duty. We assume the typical thermosiphon reboiler return is 30% mole fraction vapor and the steam is supplied at 475 kPa and 150°C. To ensure complete condensation of the steam supplied to the thermosiphon reboiler, we specify the steam outlet temperature to be sub-cooled at a design temperature of 120°C instead of specifying the vapor fraction of the exiting steam, which would calculate the dew point temperature.

The following table illustrates the default specifications in the reboiler and the steam supply as well as the modified specifications to meet the design conditions.

Table 4: Specifications for thermosiphon reboiler and steam supply

* A indicates that the variable is calculated and a indicates that the variable is specified