Application of Activated Carbon in Vapor Recovery

For decades, hydrocarbon vapor recovery has been an indispensable part of fuel loading operations. Since the large-scale installation of vapor recovery units (VRUs) began in the 1990s, the principle of activated carbon adsorption followed by vacuum regeneration has emerged as the best available technique for the vast majority of applications. It is estimated that over 95% of newly built VRUs worldwide adopt this process. When properly designed, vacuum-regenerated activated carbon systems deliver high efficiency, cost-effectiveness, safety, and exceptional reliability-even with minimal maintenance requirements.

 

 

Process Overview

The vapor treated in a VRU is a mixture of air and volatile organic compounds (VOCs), typically containing 20–50% VOC by volume. VOC molecules consist of lighter compounds that evaporate during the loading of gasoline, crude oil, or similar substances. The vapor passes through a bed of activated carbon, which features an enormous surface area composed of millions of pores. Granular activated carbon for VOCs treatment is widely used here as a classic adsorption medium, where VOC components adhere to the pore walls via weak intermolecular forces (i.e., van der Waals forces). No chemical changes occur to either the activated carbon or the VOC molecules during this adsorption process. Owing to its high adsorption capacity, activated carbon can reduce hydrocarbon emissions from VRUs to extremely low levels.

A VRU incorporates two or more activated carbon beds operating in an adsorption/regeneration cycle, with each cycle typically lasting 10–15 minutes. Granular activated carbon for VOCs treatment is commonly deployed in such multi-bed systems for reliable cyclic performance. During the regeneration phase, a vacuum pump reduces the pressure inside the carbon bed to 40–80 mbar. The low pressure shifts the equilibrium, breaking the weak bonds between VOCs and activated carbon. VOC molecules are then removed from the bed as a high-concentration stream (95% VOC by volume, 5% air by volume) via the vacuum pump. This stream flows countercurrently through an absorption tower, where it contacts an absorbent (usually fresh gasoline), and the VOC components are absorbed into the fresh absorbent. The air exiting the absorption tower, still saturated with a small amount of VOCs, is recycled by mixing it with the raw VOC feed gas entering the adsorption bed. This creates a small internal recycle with no emissions.

 

 

Activated Carbon

Common raw materials are coconut shell and coal, which yield activated carbon with excellent adsorption capacity and sufficient mechanical strength to avoid excessive dust generation.

Raw materials inherently have a certain degree of porosity and a specific surface area of 10–15 m²/g. However, during activation-typically carried out at 800–1100°C in an oxidizing steam atmosphere-the specific surface area increases to over 1500 m²/g.

The microporous structure of activated carbon enables effective adsorption, while mesopores and macropores serve as flow channels. Thus, it is critical for activated carbon to not only have a highly developed internal surface area but also provide access to this surface through a network of pores of varying diameters.

 

Form: Granular Activated Carbon vs. Extruded Activated Carbon

Activated carbon is available in either Granular Activated Carbon or Extruded Activated Carbon. Granular Activated carbon is generally more cost-effective but tends to settle and form dense regions. These regions have higher pressure drop and create dead zones with high-flow "channeling" within the carbon bed. Uneven adsorption and regeneration across the bed will negatively impact the overall performance of the VRU.

Extruded Activated Carbon is widely recognized as the superior choice for VRUs because it resists settling into dense dead zones. In VRUs, Extruded Activated Carbon typically has a diameter of 4 mm. Importantly, mixing carbons of different diameters must be avoided, as this leads to denser packing and higher bed pressure drop.

 

Adsorption Capacity and Residual Adsorption Capacity

Adsorption capacity refers to the mass of additional VOCs that can be adsorbed per unit mass of activated carbon. Fresh carbon typically has a capacity of 30 wt%, meaning 1 kg of carbon can adsorb 0.3 kg of VOCs before reaching full saturation. Restoring the carbon to its original capacity requires reactivation with steam at 1000°C, but this process causes significant carbon loss-much of the carbon is lost as dust. This is impractical for cyclic systems, so milder vacuum regeneration is used instead. With vacuum regeneration, the long-term capacity (also called working capacity) of mineral-based activated carbon is typically around 8 wt%. The difference between fresh capacity and working capacity is defined as residual adsorption capacity.

 

Heat of Adsorption

Adsorption is an exothermic process. During normal cyclic operation of the carbon bed, the heat of adsorption raises the temperature by approximately 10–20°C above ambient. More reactive hydrocarbons such as ketones and aldehydes generate significantly higher temperatures in the bed. Certain activated carbons (especially wood-based and coconut shell-based) are more prone to thermal runaway or hot spots, which necessitates shutting down the VRU, purging with inert gas, and cooling under close supervision. Fresh carbon exhibits extremely high reactivity when first exposed to vapor, generating substantial heat. Bed temperatures can rise to around 100°C, so the initial loading (pre-conditioning) of fresh carbon in a VRU must be performed by qualified personnel.

A key concern to avoid is dust, as it causes pressure drop losses, reduces capacity by blocking surface pores, accelerates wear on vacuum and absorption pumps, clogs filters, and accumulates in gasoline storage tanks.

Every batch of activated carbon should be tested for parameters critical to VRU operation: dust content, moisture content, density, particle size, hardness, and working capacity.

 

Maintaining Optimal Activated Carbon Performance

In a properly designed VRU, activated carbon typically has a service life of 10–20 years. However, poor design that fails to maintain favorable operating conditions can shorten the lifespan to as little as 4–5 years.

 

Carbon Bed Movement

In VRUs, pressure cycles between atmospheric pressure and high vacuum at least four times per hour. At the start of each cycle, the vacuum pump pulls strongly to reduce pressure to the desorption threshold. After regeneration, pressure is rapidly equalized to ready the bed for adsorption. Unless securely restrained, this repeated pulling and compaction causes carbon particles to rub against each other, wearing down into dust. Low-strength carbon degrades faster, and Granular Activated Carbon settles into denser clumps more readily. Even high-strength mineral-based Extruded Activated Carbon gradually wears into dust. Proper VRU design must prevent carbon movement and control/minimize forces (vacuum and equalization) acting on the carbon.

Dust-induced mass loss is the single most important factor limiting activated carbon lifespan-and it can be nearly eliminated with proper VRU design.

 

Long-Term Residual Adsorption Capacity

As noted earlier, residual adsorption capacity is the portion of initial adsorption capacity unrecoverable via vacuum regeneration. It represents pores permanently occupied by VOC molecules that cannot be removed by vacuum. Residual adsorption capacity increases slowly over time in any system, gradually reducing working capacity. Carbon capacity typically declines by a few percentage points annually. Well-designed VRUs incorporate an initial safety margin to compensate for this decline. A robust maintenance program includes regular carbon sampling and analysis to determine the optimal replacement time in advance.

 

Selecting the Optimal Activated Carbon for VRUs

The most critical criteria for selecting ideal VRU activated carbon are as follows: 

Key Selection Criteria for VRU Activated Carbon

 

 

Carbon Bed Flooding

Immersion of activated carbon in liquid absorbent causes irreversible damage. Prevent this via high-level detection systems at the vapor inlet and absorption tower.

 

Aerosol Entrainment into Carbon Bed

The absorption tower must be equipped with a mist eliminator, and flow rates through the vacuum pump and absorption tower must be controlled to avoid aerosol entrainment into the carbon bed-this causes irreversible damage to activated carbon.

 

If you are interested in Granular Activated Carbon or Extruded Activated Carbon and want to use them for oil vapor recovery, we can recommend the most suitable activated carbon for you based on your requirements. Please feel free to contact us at any time. We are honored to be able to assist you! 

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