Why Buyers Ask for Iodine Number
When engineers buy activated carbon for a VOC adsorber or odor control system, one of the first numbers they see is the activated carbon iodine number. It is often written as 800, 900, 1000, or 1100 mg/g. Many buyers treat it as a simple quality grade: higher number means better carbon.
That is only partly true.
Iodine number is useful, but it does not tell the full story of carbon performance. It mainly indicates the amount of small pores, especially micropores, available inside the carbon. These micropores are important for adsorbing small molecules. However, real exhaust gas usually contains mixed VOCs, moisture, dust, acid mist, and temperature changes. In those conditions, iodine number alone cannot predict service life.
For procurement, iodine number should be used as one quality check, not the only buying criterion.
A typical activated carbon iodine number range is:
- 600–800 mg/g: lower activity carbon, sometimes used for simple odor or liquid phase duties
- 800–1000 mg/g: common range for industrial gas adsorption
- 1000–1200 mg/g: high activity carbon, often with higher micropore volume
- Above 1200 mg/g: possible for some specialty carbons, but must be checked with other data
The right value depends on the contaminant, concentration, humidity, gas temperature, required removal efficiency, and bed design.
What the Iodine Number Measures
The iodine number is the mass of iodine adsorbed by one gram of activated carbon under defined test conditions. The unit is usually mg/g.
For example:
`text Iodine number = 950 mg/g `
This means 1 gram of carbon adsorbs about 950 mg of iodine in the test method.
Iodine is a small molecule, so the test mainly reflects adsorption in micropores, usually pores smaller than about 2 nm. These pores give activated carbon a very large internal surface area. A higher iodine number often means higher micropore volume and higher surface area.
As a rough rule of thumb, iodine number is often close to BET surface area in numerical value:
`text Iodine number 900 mg/g ≈ BET surface area around 900 m²/g `
This is only an approximation. It should not be used as a formal conversion because carbon type, ash content, pore size distribution, and test method affect the result.
For gas-phase VOC adsorption, micropores are useful for small compounds such as:
- Light hydrocarbons
- Some solvent vapors
- Low molecular weight organics
- Certain odor molecules
But larger VOC molecules need mesopores, usually 2–50 nm, to enter the carbon structure efficiently. If the carbon has high microporosity but poor mesoporosity, it may test well for iodine but perform poorly with larger organic vapors.
This is why two carbons with the same activated carbon iodine number can give different service lives in the same adsorber.
What Iodine Number Does Not Tell You
The iodine number does not directly measure VOC adsorption capacity in your plant. It also does not show how carbon behaves under moisture, heat, or mixed pollutant conditions.
For buying decisions, these limits are important.
| Property | Does iodine number show it? | Why it matters in VOC adsorbers |
|---|---|---|
| Micropore volume | Yes, indirectly | Important for small molecules |
| Mesopore volume | Poorly | Important for larger VOCs and faster diffusion |
| Butane or CTC adsorption | No | Often more relevant for gas-phase solvent adsorption |
| Moisture resistance | No | Water vapor competes with VOCs for adsorption sites |
| Hardness / abrasion resistance | No | Affects dust generation and pressure drop |
| Ash content | No | High ash can reduce useful carbon content and affect corrosion |
| Pellet diameter | No | Affects pressure drop and mass transfer |
| Breakthrough time | No | Depends on gas flow, concentration, humidity, bed depth, and VOC type |
| Regeneration behavior | No | Important for steam or hot air regeneration systems |
For example, a 1000 mg/g carbon may not be better than a 900 mg/g carbon if the 900 mg/g product has better pore distribution for toluene or xylene. In paint, printing, coating, and chemical exhaust, the VOC mixture often contains medium or large molecules. In these cases, adsorption capacity under the actual VOC is more valuable than iodine number alone.
Humidity is another common problem. Activated carbon adsorbs many organic vapors well when relative humidity is low. But at relative humidity above 60–70%, water adsorption can reduce VOC working capacity, especially for polar VOCs or low inlet concentrations. The iodine number test does not show this effect.
Temperature also matters. Higher gas temperature reduces adsorption capacity. A practical design rule is to keep inlet gas temperature below 40°C when possible for fixed-bed activated carbon adsorption. Some systems can run at higher temperature, but capacity will usually drop and fire risk must be evaluated carefully.
How to Compare Carbon Offers Correctly
When comparing activated carbon offers from different suppliers, do not look only at one number. Ask for a full technical data sheet and compare the parameters that affect your process.
For gas-phase VOC adsorbers, useful data include:
- Iodine number, mg/g
- CTC adsorption or butane activity, if available
- BET surface area, m²/g
- Moisture, usually less than 5% for many gas-phase carbons
- Ash content, commonly 5–15%, depending on raw material
- Hardness, often above 90% for pellet or granular carbon used in deep beds
- Bulk density, often 0.38–0.55 g/cm³ for many activated carbons
- Particle size, such as 4 mm pellet, 3 mm pellet, or 4×8 mesh granular carbon
- Pressure drop data, based on bed depth and superficial velocity
- Recommended application, gas phase or liquid phase
Bulk density is especially important for procurement and vessel sizing. Carbon is bought and filled by mass, but adsorber volume is fixed. A lower-density carbon may have a high iodine number but less mass per cubic meter of bed.
You can estimate carbon mass by:
`text Carbon mass (kg) = Bed volume (m³) × Bulk density (kg/m³) `
Example:
`text Bed volume = 2.0 m³ Bulk density = 450 kg/m³ Carbon mass = 2.0 × 450 = 900 kg `
If another carbon has a bulk density of 380 kg/m³, the same vessel holds only:
`text 2.0 × 380 = 760 kg `
Even if the iodine number is higher, the total adsorbent mass is lower. The actual operating life may or may not improve.
For pressure drop, particle size matters. Smaller particles give better mass transfer but higher resistance. For many industrial VOC fixed beds:
- 3–4 mm pellets are common for gas-phase adsorbers
- 4×8 mesh granular carbon is also common
- Typical superficial gas velocity is about 0.2–0.5 m/s
- Typical bed depth is 0.6–1.2 m per adsorption layer
- Pressure drop depends on particle size, bed depth, dust loading, and gas velocity
If the exhaust contains dust, oil mist, acid mist, or sticky aerosol, carbon life can be much shorter. A pre-filter, demister, scrubber, or cooling section may be needed before the carbon bed.
Using Iodine Number in Adsorber Design
The activated carbon iodine number is more useful during material screening than during final design. For final design, the key question is not “What is the iodine number?” but “How long before breakthrough under my gas conditions?”
A simplified mass balance can help buyers understand the scale:
`text VOC load (kg/h) = Air flow (m³/h) × VOC concentration (mg/m³) ÷ 1,000,000 `
Example:
`text Air flow = 10,000 m³/h VOC concentration = 300 mg/m³ VOC load = 10,000 × 300 ÷ 1,000,000 = 3 kg/h `
If the carbon bed contains 1,000 kg of carbon and the working adsorption capacity is assumed to be 10% by weight:
`text Usable VOC capacity = 1,000 × 10% = 100 kg Estimated time = 100 ÷ 3 = 33 hours `
This is only a rough estimate. Real working capacity may be 5–25% by weight, depending on VOC type, inlet concentration, humidity, temperature, carbon type, and allowable outlet concentration. Some strongly adsorbed solvents may give higher capacity; very light or highly volatile compounds may give much lower capacity.
For buying, ask the supplier to base the carbon selection on:
- Exhaust flow rate, normal and maximum
- VOC components and concentration range
- Gas temperature and relative humidity
- Dust, mist, acid gas, or alkaline gas content
- Required outlet concentration or removal efficiency
- Operation hours per day
- Single-use carbon replacement or regeneration design
- Available space and pressure drop limit
A high iodine number cannot compensate for a shallow bed, high gas velocity, poor air distribution, or wet gas. Many poor adsorption results come from system design issues, not carbon quality alone.
Practical Next Step for Buyers
Use iodine number as a first filter, not the final decision. For many industrial VOC applications, an activated carbon iodine number around 900–1100 mg/g is a reasonable starting range, but the correct carbon must match the VOC mixture and adsorber design.
Before ordering carbon or a complete activated carbon adsorber, prepare one sheet with your airflow, VOC list, concentration, temperature, humidity, operating hours, and required outlet target. With these data, the supplier can check carbon type, bed volume, pressure drop, replacement interval, and whether pre-treatment such as a wet scrubber, demister, or filter is needed.
