Boundary Conditions

Every step specifies what happens at each end of the column through two boundary conditions — one at the Top ($z = L$) and one at the Bottom ($z = 0$). The boundary condition determines whether gas enters, leaves, or is sealed at that face, and with what driving force.

Edit a boundary on the Top or Bottom tab of the Step Data window. The first choice is the Boundary Condition Type.

Boundary condition types

Pressure

Imposes a specified pressure at the face. Use it for steps controlled by a pressure level — pressurisation, blowdown, or a constant-pressure product outlet.

• As an inlet, you also specify the inlet Temperature and Composition; gas enters at the imposed pressure.
• As an outlet, the temperature and composition are taken from the bed (zero-gradient); gas leaves at the imposed pressure.

The pressure follows a Pressure Profile over the step (see Profiles below), so you can ramp the pressure up or down during the step. A pump (compressor or vacuum pump) can be attached to compute compression power and heating — see Pumps.

Flow

Imposes a molar flow at the face, specified through one or more streams. Use it for feed steps and for flow-controlled purge or product steps.

A Flow boundary contains a list of streams (add them with Add Stream); the total flow is the sum of the streams. Each stream has:

• a Boundary Device Type — None (flow specified directly), Valve (pressure-driven), or Pump (power-driven);
• for a None device, a Flow Type and a flow profile;
• as an inlet, its own Temperature and Composition.

Multiple streams on one face let you, for example, mix a fresh feed with a recycle stream.

Closed

A sealed, no-flow end. Nothing enters or leaves; pressure, temperature and composition have zero gradient at the face. Use it for the closed end of a blowdown or equalisation step. A Closed boundary has no further settings.

Flow direction

Each boundary has a Flow direction:

• Inlet — gas flows into the reactor. You specify what enters (temperature, composition, and — for a Flow boundary — the flow rate).
• Outlet — gas flows out of the reactor. The leaving gas takes the bed's local temperature and composition.

A step must have at least one non-inlet end (an outlet or a closed end); the two ends cannot both be inlets.

Specifying inlet flow

For a Flow boundary with a None device, the Flow Type sets how the rate is expressed. All four are equivalent and are converted internally using the boundary-face area and the ideal-gas density:

Inlet composition and temperature

At an inlet you set the Temperature and the Composition (a mole fraction for each component). The mole fractions of an inlet must sum to 1. Outlets ignore these fields because the leaving gas is whatever is in the bed.

Time-varying profiles

Both flow rates and pressures can vary during a step. The Profile Type (for flow) or Pressure-Time Profile (for pressure) offers:

• Constant — a single value held for the whole step.
• Linear — ramps linearly from a Start Value to a Final Value over a Ramp Time.
• Exponential — approaches the Final Value from the Start Value with an exponential (first-order) response over the ramp time.

Ramps are the natural way to model gradual pressurisation or blowdown. The ramp time is clamped to the step duration.

All numeric fields accept expressions and units, e.g. 2 [bar] or a reference to a user variable.

Valves

Set a stream's Boundary Device Type to Valve to make its flow pressure-driven rather than prescribed. You enter a Valve Flow Coefficient (Cv) and a pressure profile; the flow through the valve is driven by the pressure difference between the valve's set pressure and the bed. A larger Cv passes more flow for the same pressure difference.

Valves are the mechanism behind pressure equalisation: put a valve on each bed and connect them, and gas flows from the high-pressure bed to the low-pressure one at a rate set by Cv.

Pumps: compressors and vacuum pumps

A Pump device (or a pump attached to a Pressure boundary) models mechanical compression:

• an inlet uses a compressor (with a Suction Pressure);
• an outlet uses a vacuum pump (with a Discharge Pressure).

The pump settings include an Efficiency Model (constant Isentropic Efficiency, a vacuum-pump curve, or a polynomial), a Polytropic Constant, and an Energy Model (Isothermal or Adiabatic) that determines the outlet-gas temperature rise from compression.

There are two distinct uses:

1. Power-controlled flow — a Pump device on a Flow stream sets the molar flow from a specified power (constant or a polynomial in pressure). This affects the simulation.
2. Power for post-processing — ticking Calculate pump power on a Pressure boundary or a None stream computes compression power and heating for reporting, without changing the flow the boundary imposes. The recorded power feeds the energy-consumption KPI.

Drawing gas from another step (buffer tank)

An inlet can be fed from the gas produced by another step instead of from values you type in. Tick Use buffer tank from another step under Inlet Source, choose the source step and endpoint, and set the percentage of that step's produced moles to draw. This is the basis of averaged light reflux and recycle.

Next, learn how to couple two beds together in Connecting Beds.