Space Velocity & Reactor Calculator

Calculate GHSV, LHSV, contact time, and reactor volume for catalytic processes.

Please check your inputs and try again.

What Space Velocity Controls

Space velocity is feed volume per unit catalyst volume per hour. Its reciprocal is contact time — how long reactants spend over the catalyst — and that is what actually determines conversion.

GHSV = Q / V     contact time = 3600 / GHSV seconds
Space velocityContact timeEffect
Low (500 hr−1)7.2 sHigh conversion, large reactor, risk of over-reaction
Moderate (2,000 hr−1)1.8 sBalanced
High (10,000 hr−1)0.36 sLow conversion per pass, small reactor, better selectivity

The trade-off is between conversion and selectivity. Long contact time drives conversion up but gives desired products time to react further into by-products. Many industrial processes deliberately run at high space velocity with low per-pass conversion and recycle unreacted feed — ammonia synthesis converts only 15–20% per pass for exactly this reason.

MetricBasisWhen preferred
GHSVGas volume per catalyst volumeGas-phase reactions
LHSVLiquid volume per catalyst volumeLiquid feeds
WHSVMass feed per mass catalystComparing catalysts of different density

WHSV is the fairer basis when comparing catalysts, since volume-based metrics are distorted by differences in packing density.

Worked Examples

Example 1: Ammonia synthesis: Q=1000m³/hr gas, V=1m³
GHSV=1000/1=1000 hr⁻¹
Result: Contact time=3.6 seconds
Typical Haber process GHSV range
Example 2: Design: need SV=2000 hr⁻¹, feed=500 m³/hr
V=500/2000=0.25 m³
Result: 125kg catalyst at ρ=500kg/m³
Pilot plant reactor sizing
Example 3: Conversion versus selectivity
Contact time doubled from 1.8 s to 3.6 s
Result: Conversion rises, selectivity often falls
Desired product has longer to undergo secondary reactions. Many processes accept lower per-pass conversion to protect selectivity.
Example 4: Recycle strategy
Ammonia synthesis at 15% conversion per pass
Result: Unreacted gas recycled
High space velocity keeps the reactor small and selectivity high. Separation and recycle are cheaper than the larger reactor low space velocity would require.

Common Mistakes

⚠️
Confusing space velocity with residence time

They are reciprocals. High space velocity means short contact time, which is the opposite of what the word 'velocity' might suggest.

⚠️
Comparing GHSV across catalysts of different density

Volume-based space velocity depends on packing. Use WHSV, based on mass, when comparing different catalyst formulations.

⚠️
Assuming lower space velocity is always better

Longer contact time raises conversion but allows over-reaction to by-products. Selectivity often falls as conversion rises.

⚠️
Not specifying the reference conditions

Gas volumes depend on temperature and pressure. GHSV values must state whether they are at STP or at reaction conditions.

Frequently Asked Questions

Why control space velocity?
SV determines residence time and thus conversion. Higher SV: shorter contact, lower conversion but higher throughput. Lower SV: longer contact, higher conversion but larger reactor. Optimal SV balances cost and conversion.
Weight-hourly space velocity (WHSV)?
WHSV = mass flow / catalyst mass (kg/kg/hr). Used when catalyst density varies. GHSV more common in gas phase, LHSV in liquid phase, WHSV when comparing different catalyst systems.
What is the relationship between space velocity and contact time?
They are reciprocals. Contact time in seconds is 3600 divided by GHSV in reciprocal hours.
Why do industrial processes run at low conversion per pass?
High space velocity protects selectivity and keeps reactors small. Unreacted feed is separated and recycled, which is usually cheaper than a larger reactor.
What is the difference between GHSV and WHSV?
GHSV is gas volume per catalyst volume; WHSV is mass of feed per mass of catalyst. WHSV is fairer for comparing catalysts with different packing densities.
Does lower space velocity always improve conversion?
It usually raises conversion but often reduces selectivity, since products have more time to react further. The optimum balances both.

Formula Explorer connections

Interpretation: This formula connects concentration, time, temperature or transport to the speed of a chemical process. Assumption: The reaction order and mechanism must match the model. Temperature, catalyst, mixing and mass-transfer limitations can alter the observed rate.

Activation Energy Calculator →Activation Energy Graph Calculator →Activation Energy from Two Temperatures →Chemistry Formula Explorer →