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.)

Interacting Steps: connections and recycle

Connections between steps are managed on the Interacting Steps tab: pick the From step and endpoint and the To step and endpoint, then add the connection. To switch a connection off, remove the pair there. What a connection does, and how it is used for pressure equalisation, light-product pressurisation and light reflux, is documented on the engine site under Connecting Beds. A buffer-tank source is set on the inlet's own boundary fields, and an endpoint uses either a connection or a buffer tank, not both.

KPI Configuration

Set where feed enters and where product is collected on the KPI Configuration tab. Tick a component to track it, then tick Feed, Product and/or Recycle on each endpoint in the table (each stream of a Flow boundary and each Pressure boundary, with its end of the column; closed boundaries have no endpoints). The definitions of purity, recovery, productivity, energy and production rate, and the recycle role, are on the engine site under Performance Metrics. After a run the KPIs are on the Solver panel's Performance Indicators tab (see Running the Solver). The energy KPI is only non-zero if Calculate pump power is on for the boundaries that do work.

Cyclic Steady State tab

The CSS convergence method, its tolerances, Minimum cycles, Consecutive cycles passing the test and the CSS acceleration method are set on the Cyclic Steady State tab. The methods are described on the engine site under Cyclic Steady State. The Number of Cycles still acts as an upper limit, and the solver status becomes Cyclic Steady State when the criterion is met.

Next: define what happens at each end of the column in Editing Step Boundaries.