Performance Metrics

Skarstrom computes the standard PSA/TSA key performance indicators (KPIs) — purity, recovery, productivity, specific energy, and production rate — for a chosen product component over a chosen cycle. These are what you use to compare designs and to drive optimisation.

KPIs are only meaningful at cyclic steady state. Evaluate them on a converged cycle.

Configuring KPIs

Before KPIs can be computed, the solver needs to know which steps feed the bed, which collect product, and which return recycled product. Set this on the Cycle Design → KPI Configuration tab:

Select the component's sub-tab and tick Track this component. Only tracked components appear in the results.
In the Step / Feed / Product / Recycled table, mark each step's role for this component, and set the Top/Bottom location — the end of the column where that stream crosses the boundary.

Feed steps are where fresh gas enters the bed (flux into the bed).
Product steps are where product is collected (flux out of the bed).
Recycled steps are where product is sent back into the bed (e.g. reflux or LPP); these are subtracted so recycled gas is not double-counted as net product.

The metrics

For a product component, integrating the molar fluxes over the relevant steps of a cycle gives:

Purity — the fraction of the collected product that is the product component:

\text{Purity} = \frac{n_{\text{product component, out}}}{n_{\text{total, out}}}

Recovery — the net product component collected as a fraction of that component fed:

\text{Recovery} = \frac{n_{\text{product, out}} - n_{\text{product, recycled}}}{n_{\text{product, fed}}}

Recovery can be negative if more product is recycled than is collected — the interface warns you when this happens.

Productivity (mass) — net product mass produced per unit mass of adsorbent per unit time:

\text{Productivity}_{\text{mass}} = \frac{n_{\text{product}}\, M}{t_{\text{cycle}}\; m_{\text{adsorbent}}} \quad \left[\frac{\text{kg}}{\text{kg}\cdot\text{s}}\right]

Productivity (volume) — the same but per unit bed volume:

\text{Productivity}_{\text{vol}} = \frac{n_{\text{product}}\, M}{t_{\text{cycle}}\; V_{\text{bed}}} \quad \left[\frac{\text{kg}}{\text{m}^3\cdot\text{s}}\right]

Energy Consumption — the specific energy: total compression work over the cycle per unit mass of product:

\text{Energy Consumption} = \frac{E_{\text{total}}}{n_{\text{product}}\, M} \quad \left[\frac{\text{J}}{\text{kg}}\right]

where $E_{\text{total}}$ is the sum over all steps of the time-integrated compressor/pump power.

Production Rate — the net molar rate of product:

\text{Production Rate} = \frac{n_{\text{product, out}} - n_{\text{product, recycled}}}{t_{\text{cycle}}} \quad \left[\frac{\text{mol}}{\text{s}}\right]

where $M$ is the product molar mass, $t_{\text{cycle}}$ the cycle time, $m_{\text{adsorbent}}$ the total adsorbent mass, and $V_{\text{bed}}$ the total bed volume.

Energy Consumption requires compression power. For the energy KPI to be non-zero, enable Calculate pump power (or use a pump device) on the boundary conditions that do work — pressurisation, blowdown, evacuation — and set their efficiency and energy models.

Reading the results

After a run, open the Performance Indicators tab in the Solver window. Each tracked component has a tab showing Purity, Recovery, Productivity (mass), Productivity (volume), Energy Consumption, Production Rate and Total Mass of Solid, plus a Step Energy Consumption breakdown per step. Use the Cycle Index selector to read the KPIs at a particular cycle.

To see how a KPI evolves as the process approaches cyclic steady state, plot it against cycle number: in the plotting options, set the Object Type to KPI and choose the metric — this produces a KPI vs Cycle curve.