Workspace Tutorial: O₂ PSA (Arora 2021)

Reference: Arora, A., Hasan, M.M.F. Flexible oxygen concentrators for medical applications. Scientific Reports 11, 14317 (2021).

Goal. Run a four-step Skarstrom cycle that makes oxygen from air on a LiLSX zeolite, learn how each step is wired, and read purity and recovery correctly when part of the product is sent back into the bed. The deck reproduces the PSA column of Table 4 in the paper (the case with at least 5 L/min and at least 85 % O₂).

What you will see. Purity and net recovery for the production step, which you can compare with the paper's values. The numbers for this deck, and the open disagreement with the paper on recovery, are on the engine site's O₂ PSA tutorial and Validation page. This walkthrough shows where to click and where recovery comes from.

1. Load the deck

In the workspace, open Examples → Arora (2021) → Arora2021_LiLSX_PSA, or go straight to /workspace?example=Arora2021_LiLSX_PSA.json. The companion deck Arora2021_LiLSX_PVSA is the vacuum version of the same cycle (desorption at 0.5 bar instead of 1 bar).

2. Walk through the setup

Fluid Package. Two components: Oxygen and Nitrogen; the feed is air at 21 % O₂.

Adsorbents. The deck carries three materials from Table 2 of the paper: LiX Zeolite, LiLSX Zeolite and Zeolite 5A. Only LiLSX is packed in the column; the other two are there so you can swap them in. Open LiLSX Zeolite to see:

• a non-competitive Dual Site Langmuir isotherm for each component, in partial-pressure form. Nitrogen has the larger site energies, so it is the strongly adsorbed component and oxygen leaves as the light product;
• a Linear Driving Force rate model with constant coefficients, 104.07 s⁻¹ for O₂ and 42.84 s⁻¹ for N₂;
• heats of adsorption of 14.66 kJ/mol (O₂) and 25.45 kJ/mol (N₂).

The same material is also in the curated adsorbent library with its citations.

Column Properties. A cylinder of 0.1 m diameter with one layer, LiLSX Zeolite: 0.127 m long, 30 cells, bed voidage 0.35 (from the paper's bulk density of 672.75 kg/m³ and particle density of 1035 kg/m³) and the particle diameter of 0.035 m as printed in the paper's SI. The Momentum model is Darcy (Kozeny–Carman), because this deck predates the Ergun option. Heat transfer to the wall uses constant coefficients of 70 W m⁻² K⁻¹ inside and out, with an axial thermal conductivity of 0.29 W m⁻¹ K⁻¹ (the paper's SI); the energy balance is non-isothermal with local thermal equilibrium.

Cycle Design → Cycle Data. Four steps, 6.4 s per cycle, 50 cycles. Click a step in the Navigator (or on the cycle canvas) to open its Top and Bottom boundaries:

The purge flow is the paper's purge-to-feed velocity ratio (1.97) converted to a molar flow: 1.97 × 0.23 mol/s × (1 bar / 3.5 bar). The purge and pressurisation gas are typed in at 85 % O₂, the product specification; they are not connected to the production step, so the deck does not check that the product actually reaches that purity.

Initial Conditions. Air at 3.5 bar and 298 K.

Cyclic Steady State. The method is None, so the run always does the full 50 cycles.

KPI Configuration. Oxygen is tracked. Feed is ticked on the air stream of Production and Product on the Production outlet. The Purge and Pressurization inlets are ticked as Recycle: the O₂ they send back into the bed is subtracted from the product for the recovery, which gives the net recovery the paper reports, while the purity stays the purity of the oxygen collected at the Production outlet.

Solver. The deck caps the integrator step at 0.01 s (Solver Settings → Max step), kept from the validation test.

3. Run

Click Solver → Run. The log shows one Cycle n of 50 line per cycle and ends with Simulation completed successfully.

4. Interpret the results

Open the Solver's Performance Indicators tab. The KPIs are computed automatically from the last run, and the cycle selector defaults to the last logged cycle, 49: the solver numbers cycles from 0, so cycle 49 is the 50th. The table lists purity, recovery and production rate for the deck's KPI configuration; the current values and their comparison with the paper are on the engine's tutorial and Validation pages. The purity is that of the oxygen collected at the Production outlet. The recovery and the production rate are net of the recycled oxygen, as the paper defines them.

Why Recycle is its own role. Ticking Product instead of Recycle on the Purge and Pressurization inlets gives the same net recovery, because product endpoints are summed as net outflow. It would also subtract the recycled gas from the purity: more gas goes back into the bed through those inlets than leaves through the Production outlet, so the net total product is negative and the purity is undefined (shown as —; above 100 % in the PVSA deck).

Is it converged? Check the trend: plot Plot type → KPIs, KPI → Recovery to see whether the net recovery is still changing from cycle to cycle, or set a cyclic steady state method and a larger number of cycles.

Product quality inside a step. Set Plot type to Boundary conditions, Step to Production, Stream to Production Pressure Boundary Top, and plot the O₂ Mole Fraction. Look for the product purity rising and then falling as the nitrogen front breaks through towards the end of the step.

Where the recovery goes. Each cycle feeds 0.23 mol/s × 3 s × 21 % ≈ 0.145 mol of O₂, while the purge alone returns 0.129 mol/s × 1.4 s × 85 % ≈ 0.154 mol of O₂, more than the feed brings in, before counting pressurisation. With the purge flow fixed at the value derived from Table 4, almost all of the oxygen produced is spent regenerating the bed, which is why the net recovery is low. For the difference from the paper, see the Validation page.

5. What to try next

• Halve the purge. In the Purge step, change the stream's molar flow from 0.1295 to 0.0647 mol/s and compare purity and recovery with the first run: the classic purity–recovery trade-off. Use a Parametric Study on the purge flow to map the whole curve, or an Optimisation to maximise recovery subject to a purity limit.
• Shorten production to stop before the nitrogen front breaks through, and watch the production-step purity.
• Connect the recycle streams. Replace the typed-in 85 % O₂ purge and pressurisation gas with connections to the production outlet (the light reflux and light product pressurisation pages show how), so the recycled gas has the composition the bed really produces.
• Swap the adsorbent. Change the layer's composition to LiX Zeolite or Zeolite 5A and compare.
• Try the vacuum cycle. Arora2021_LiLSX_PVSA desorbs at 0.5 bar. Its validation numbers are on the Validation page.