Skip to main content

Reactor Sizing

All seven reactor types (RCSTR, RPfr, RBatch, RStoic, RYield, REquil, RGibbs) size by residence time. This is the standard preliminary-design approach when reaction kinetics are not part of the flowsheet model: volume follows from the volumetric feed rate and a characteristic time for the reacting phase.

Method​

V = max(Q̇ · τ, 0.1 m³)

Q̇ = inlet volumetric flow (m³/s)
  = (flow_mol [kmol/h] · MW [kg/kmol] / 3600) / ρ [kg/m³]

The residence time τ is selected from the inlet phase:

Inlet conditionτRationale
Vapor fraction ≤ 0.5 (liquid)3600 s (1 h)Conservative CSTR/batch-equivalent holdup for homogeneous liquid-phase reactions
Vapor fraction > 0.5 (gas)10 sGas-phase space velocity ≈ 360 h⁻¹, typical of fast catalytic reactions

Geometry​

From the volume, a cylindrical vessel with aspect ratio L/D = 3 is assumed:

D = ∛(4·V / (3π))
L = 3·D

PFRs report the same diameter/length pair; the cylinder assumption matches the geometry basis of the vessel cost correlations, so sizing and costing stay consistent.

Worked example​

Liquid-phase CSTR, 100 kmol/h aqueous feed (MW ≈ 30 kg/kmol, ρ = 1010 kg/m³):

Q̇ = (100 · 30 / 3600) / 1010 ≈ 0.00082 m³/s
V = Q̇ · τ = 0.00082 · 3600 ≈ 3.0 m³

D = ∛(4·3.0 / (3π)) ≈ 1.08 m,  L ≈ 3.2 m

Overriding the defaults​

Override residence time per reaction

The phase-based defaults are deliberately generic — a fermentation needs hours, a steam-methane reformer fractions of a second. Every reactor size records the assumption residence time τ (phase default — OVERRIDE per reaction) so the default is never mistaken for a kinetic result.

Three ways to override:

  1. Set the reactor volume directly — explicit volume/length+diameter parameters on the node take precedence over sizing.
  2. Custom sizing formula — wrap the reactor as a CustomUnit with a sizing expression (e.g. flow_vol * tau with your own τ); see Custom Sizing.
  3. Ask the agent — the Jasper agent can propose a custom-sized unit with a cited residence time for your chemistry.

What sizing does not do​

Reactor sizing does not solve kinetics, catalyst loading, heat-removal limits, or pressure drop. It produces a Class 4–5 volume estimate that is consistent with the cost correlations downstream. For kinetically-limited designs, size the reactor externally and set the volume explicitly.