Building a Cycle

A cycle is the sequence of operating steps a bed repeats — feed, blowdown, purge, pressurisation, and so on. In Skarstrom a cycle is a list of steps, each with its own duration and its own pair of boundary conditions, executed in order and then repeated. Everything is configured in the Cycle Design window.

The Cycle Design window has four tabs:

• Cycle Data — the number of cycles, timing mode, and the list of steps.
• Initial Conditions — the state of the bed at $t = 0$.
• Interacting Steps — connections between steps for multi-bed processes.
• KPI Configuration — feed/product/recycle classification for performance metrics.

Start from a template. The Load Example Cycle button offers ready-made cycles you can adapt: Breakthrough, 4-Step VPSA, 4-Step VPSA with LPP, Skarstrom Cycle, and 6-Step VPSA with Rinse. Loading one is the fastest way to see how a full cycle is wired up.

Steps

Each step in the timeline has a name and two boundary conditions:

• the Top boundary at $z = L$ (the far end of the column), and
• the Bottom boundary at $z = 0$ (the near end).

Select a step to edit it in the Step Data window. There you can rename it, edit its Top and Bottom boundary conditions on their respective tabs, apply an energy source on the Energy Sources tab, and use Swap Boundary Conditions to exchange the two ends.

The row of buttons on the timeline lets you add, delete, and duplicate steps, and move a step left (earlier) or right (later). Duplicating a step copies all of its settings (and clears any connections on the copy), which is handy for building symmetric cycles.

A few rules are enforced when you run:

• A cycle must contain at least one step, and step names must be unique.
• The two boundaries of a step cannot both be inlets — one must be an outlet (or a closed end).
• The Top and Bottom boundaries must be at different ends of the column.

Number of cycles

A PSA/TSA process must run many cycles before it settles into a repeating pattern. Set Number of Cycles on the Cycle Data tab to how many times the whole step sequence repeats. The total simulated time is the cycle time multiplied by the number of cycles.

Rather than guessing the number of cycles, you can let the solver stop automatically once the bed stops changing from one cycle to the next — see cyclic steady state.

Step timing

The Step Time Mode controls how step durations are entered:

• Absolute — each step's duration is entered directly in seconds. The cycle time is the sum of the step times.
• Fraction — you enter a single Total Cycle Time and give each step a fraction of it. The fractions must sum to 1. This makes it easy to rescale a whole cycle by changing one number.

Switching between modes converts the values for you, so no timing information is lost.

Initial conditions

The Initial Conditions tab sets the state of the bed everywhere at $t = 0$, before the first step begins:

• Pressure
• Bed Temperature (the initial gas and solid temperature)
• Wall Temperature
• Velocity
• Initial Mole Fraction for each component — these must sum to 1.

Choose initial conditions that are physically consistent with the first step's boundary conditions; for example, if the first step feeds gas at high pressure into an evacuated bed, start from a low pressure. A poor initial guess can make the first solve harder to converge. (If the model reaches cyclic steady state, the initial condition only affects how many cycles that takes, not the final answer.)

Next: define what happens at each end of the column in Boundary Conditions.