Connecting Beds¶
Real PSA/TSA processes use several beds so that one can produce while another regenerates, and beds exchange gas with each other (equalisation, product pressurisation, reflux). Skarstrom captures these multi-bed interactions through connections between steps.
How multi-bed processes are represented¶
Skarstrom solves one bed in detail, marching a single column through its sequence of steps. A multi-bed process is modelled by laying all the relevant steps out on one timeline and connecting the steps that exchange gas. Each connection lets one step's boundary read the gas the other step produces.
Because the interacting steps are connected as a pair, a step that receives gas (say a pressurisation step) is linked to the step that supplies it (a producing step), and the simulator reproduces the coupled behaviour of the two beds without solving both columns simultaneously.
The coupling is lagged by one cycle. A connected boundary uses the gas logged by its partner on the partner's previous run. This is exactly how a real multi-bed process behaves at cyclic steady state — each bed sees what the other bed did a cycle ago — and it converges to the correct coupled solution as the cycles repeat. On the first cycle there is no logged partner data yet, so each side falls back to the values you entered (the first-cycle assumptions). Expect the first cycle or two to be approximate; the solution becomes self-consistent as it approaches cyclic steady state.
Making a connection¶
Connections are stored in the deck with the cycle (each connected endpoint carries a pair id).
- Give each of the two steps a boundary of the right kind (for example a valve stream on each, or a producing stream on one and a matching inlet on the other).
- Connect the From step and endpoint to the To step and endpoint (in C++,
MakeMutualConnection; front ends offer a connection editor;adsim/objects/deck_edit.hhas the helpers that disconnect a pair cleanly). - Press Connect. The pair now appears under Connected Endpoints, where you can Disconnect it.
What passes through the connection depends on the endpoint types:
| Connected endpoint | Reads from its partner |
|---|---|
| Flow stream (no device) | Molar flow, temperature and composition |
| Valve stream | The partner's pressure, used as the driving pressure |
| Pump stream | The partner's pressure (suction/discharge) |
| Pressure boundary | Inlet temperature and composition |
Rules¶
- Interacting steps must have equal step time. Two connected steps represent two beds acting at the same time, so their durations must match.
- A pressure boundary can connect to a flow/valve/pump stream, but two pressure boundaries cannot be connected to each other — one side must set the flow.
- An endpoint cannot connect to itself or to another endpoint of the same step.
- An endpoint can either be part of a live connection or use a buffer-tank source — not both.
Two ways to couple¶
Skarstrom offers two coupling mechanisms, suited to different purposes:
- Interacting-step connections (this page) reproduce the instantaneous profile of the partner stream through the step, using the partner's previous-cycle time series. Use these when the timing of the exchange matters — equalisation, direct product pressurisation, direct reflux.
- Buffer-tank sources feed an inlet with the time-averaged gas (total moles, average temperature and composition) produced by another step's endpoint over its last run. This models a well-mixed surge tank between the two beds. See Light Reflux & Recycle.
The following pages show these tools applied to specific process steps: