Editing Step Boundaries

This page is about the workspace controls for a step's boundaries. What the boundary condition types, profiles, valves and pumps mean is documented once, on the engine site: Boundary Conditions.

Edit a boundary on the Top or Bottom tab of the Step Data window (select a step in the Navigator or on the cycle canvas). The first choice is the Boundary Condition Type; the fields below it follow from that choice. The valve, pump and pressure-equalisation options are the Boundary Device Type.

Composition and temperature fields

Each mole fraction of an inlet is an expression like any other field, so it can be a number, a value with a fraction unit (21 [%], 400 [ppm]) or a formula of global variables, such as y_feed and 1 - y_feed. The mole fractions must sum to 1; the sum is checked on the evaluated values.

Profiles

The Profile Type (flow, temperature) or Pressure-Time Profile (pressure) list offers Constant, Linear, Exponential, Log-linear and Double exponential. The equations are on the engine site, under time-varying profiles.

• Under the profile list, the collapsible Profile equation box draws the selected profile's shape, labelled with its start and final values and the end of the ramp, beside its equation. A pressure boundary starts from the bed pressure, which is only known when the step runs, so its sketch rises for an inlet and falls for an outlet.
• Switching from one ramp to another keeps the start and final values and the ramp time. Switching from Constant to a ramp carries the constant value to both ends of the ramp (an inlet temperature ramp starts at the current value with a ramp time equal to the step time); switching back carries the final value to the constant.
• A pressure ramp (a Pressure boundary, or a valve's pressure) has no start value. With Use step time ticked (the default) the ramp lasts the whole step. Untick it to give the ramp its own Ramp Time; unticking starts the ramp time at the step time. A ramp time longer than the step is marked on the field and listed in the Solver panel, and the run is refused.
• For a pressurisation ramp, the approach-to-fluidisation spike shows up as a run warning in the Solver Log.

Valves and pumps

Set a stream's Boundary Device Type to Valve to enter a Valve Flow Coefficient (Cv) and a pressure profile, or to Pump for the Efficiency Model, Polytropic Constant and Energy Model settings. To have the Energy Consumption KPI count a pump's work, tick Calculate pump power on it (see Performance Metrics).

Next: connect steps of a multi-bed process on the Interacting Steps tab (see Building a Cycle).