Molecular Sieve Sizing Calculator

Inputs

Example data table

Sample case for an instrument air drying skid in a plant utility room.

Values are illustrative; always validate with supplier performance curves.

Formula used

1) Moisture removed per volume (converted if needed):

• ΔC = C_in − C_out (g/m³)
• If using ppmv: C (g/m³) = ppmv × 10⁻⁶ × (P/(R·T)) × MW × 1000

2) Water load rate and cycle load:

• ṁ_w = Q × ΔC / 1000 (kg/h)
• m_w,cycle = ṁ_w × t_cycle (kg)

3) Required adsorbent mass (preliminary):

• Capacity_eff = Capacity × η_regen (kg/kg)
• M_sieve = (m_w,cycle / Capacity_eff) × SF (kg)

4) Bed volume and geometry:

• V_bed = (M_sieve / N_beds) / ρ_bulk (m³)
• A = (Q/3600) / v_sup (m²)
• D = √(4A/π) (m)
• H = (V_bed × (1 + Freeboard%)) / A (m)

5) Pressure drop (packed-bed approximation):

• ΔP = [150(1−ε)²/ε³]·(μ·H·v/d_p²) + [1.75(1−ε)/ε³]·(ρ·H·v²/d_p)

Professional article

Molecular sieve dryers are widely used when a project needs very low moisture levels for instrument air, nitrogen, and natural gas services. In construction, these vessels often sit inside packaged utility skids, compressor rooms, or tie‑in areas where reliability and predictable performance matter. A practical sizing method starts by translating the moisture reduction target into an actual water load. Once the water load is known, the required adsorbent mass and bed geometry can be estimated, refined, and reviewed against vendor limits.

This calculator follows a preliminary workflow that is useful during early design and procurement. First, it converts inlet and outlet moisture to a consistent basis. If you enter ppmv, it applies an ideal‑gas conversion using your operating pressure and temperature. Next, it computes moisture removed per cubic meter (ΔC) and multiplies by the operating flow to estimate water removed per hour. The cycle time sets how much water must be captured before a bed is regenerated or switched, producing a per‑cycle water load.

To translate water load into sieve mass, the calculator uses working capacity (kg water per kg sieve) and adjusts it by a regeneration effectiveness factor. This captures the reality that beds rarely return to a perfectly “new” condition after each regeneration. A safety factor then accounts for uncertainties such as seasonal humidity swings, oil or dust fouling, maldistribution, and future throughput increases. With the total mass per bed, bulk density converts mass to bed volume.

Geometry is sized from a chosen superficial velocity. Lower velocities generally reduce pressure loss and improve mass transfer, but they increase vessel diameter and cost. Bed height is obtained from bed volume and cross‑sectional area, including a freeboard allowance for distributors, screens, and settling. Finally, the packed‑bed pressure drop is estimated using an Ergun‑style equation, giving a screening value for blower or compressor margin and helping flag excessive pellet fineness.

Example sizing case

Use these example inputs to reproduce the sample results:

• Flow: 1000 m³/h, moisture: 12 → 1 g/m³, cycle: 8 h
• Working capacity: 0.16 kg/kg, regeneration effectiveness: 0.90, safety factor: 1.20
• Bulk density: 700 kg/m³, superficial velocity: 0.20 m/s, freeboard: 10%

Example case: Flow 1000 m³/h, inlet 12 g/m³, outlet 1 g/m³, cycle 8 h, working capacity 0.16 kg/kg, regeneration effectiveness 0.90, safety factor 1.20, bulk density 700 kg/m³, and superficial velocity 0.20 m/s. The model yields about 458 kg sieve per bed, bed volume near 0.65 m³, a vessel ID around 1.33 m, bed height near 0.52 m including freeboard, and a modest pressure‑drop estimate depending on pellet and gas properties. Always confirm final sizing with vendor isotherms, breakthrough curves, and applicable codes before purchase and installation.

FAQs