Activated Carbon Applications in Vapor Recovery

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

 

 

Process Description

The vapor to be treated in a VRU is a mixture of air and volatile organic compounds (VOCs), typically containing 20-50% VOCs by volume. VOC molecules consist of lighter compounds evaporated during the loading of gasoline, crude oil or similar products. The vapor passes through a bed of activated carbon, which features an enormous surface area composed of millions of pores. Activated carbon can be visualized as a "molecular sponge", where VOC components adhere to the pore walls via weak intermolecular forces known as van der Waals forces. No chemical changes occur to either the activated carbon or the VOC molecules during this adsorption process. Thanks to the high adsorption capacity of activated carbon, hydrocarbon emissions from VRUs can be reduced to extremely low levels.

 

A VRU comprises two or more activated carbon beds operating in an adsorption/regeneration cycling mode, with each cycle typically lasting 10 to 15 minutes. During the regeneration phase, a vacuum pump reduces the pressure inside the carbon bed to 40-80 millibars. This low pressure shifts the equilibrium, disrupting the weak forces between VOCs and activated carbon, and allowing VOC molecules to exit the carbon bed through the vacuum pump as a high-concentration gas stream (95% VOCs and 5% air by volume). The gas stream flows countercurrently through an absorption tower where it comes into contact with an absorbent (usually fresh gasoline), and VOC components are thereby absorbed into the fresh absorbent. The air that exits the absorption tower, still saturated with trace amounts of VOCs, is recycled by mixing with the incoming VOC-laden feed gas entering the adsorption carbon bed. This creates a small internal loop with zero emissions.

Activated Carbon

The commonly used raw materialsare Yuanli GRP and HEM series. Such activated carbon exhibits excellent adsorption capacity, and its mechanical strength must be ensured to avoid excessive dust generation.

 

Raw materials already possess a certain degree of porosity and a specific surface area of 10–15 square meters per gram. However, during the activation process-typically carried out in an oxidizing steam atmosphere at temperatures ranging from 800°C to 1100°C-their specific surface area increases to over 1500 square meters per gram.

 

The microporous structure of activated carbon provides an effective means for adsorption, while mesopores and macropores are crucial for molecular transport channels. Therefore, it is imperative that activated carbon not only has a highly developed internal surface area but also features a network of pores with varying diameters that enable access to this surface.

 

Morphology – Granular or Pelletized?

Activated carbon is available in either granular form or extruded pelletized form. Granular activated carbon is generally the most cost-effective option, but it tends to pack together and form dense zones. These zones create higher pressure drops and dead spaces within the carbon bed, leading to channeling where gas flows preferentially through high-permeability paths. Uneven adsorption and regeneration across the entire carbon bed will negatively impact the overall performance of the Vapor Recovery Unit (VRU).

 

Pelletized activated carbon is produced by blending activated carbon powder with a binder and extruding the mixture into cylindrical shapes. It is widely regarded as the superior choice for VRUs because it is less prone to packing and forming dead zones. In VRU applications, pelletized activated carbon typically has a diameter of 4 millimeters. Critically, activated carbon of different diameters should not be mixed, as this will result in tighter packing and higher pressure drop across the carbon bed.

 

Capacity and Residual Adsorption

Adsorption capacity refers to the mass of Volatile Organic Compounds (VOCs) that can be additionally adsorbed per unit mass of activated carbon. Fresh activated carbon typically has a capacity of 30% by weight, meaning 1 kilogram of carbon can adsorb 0.3 kilograms of VOCs before reaching full saturation. To fully restore the carbon to its original capacity, reactivation using steam at 1000°C is required. However, this process causes significant carbon attrition, with a substantial proportion of the carbon being lost as dust. Given its impracticality in cyclic systems, a milder vacuum regeneration method is employed instead. Through vacuum regeneration, the long-term capacity (also known as "working capacity") of mineral-based activated carbon is typically around 8% by weight. The difference between the fresh capacity and the working capacity is referred to as "residual adsorption".

 

Heat of Adsorption

The adsorption process is exothermic. During normal cyclic operation of the carbon bed, the heat released raises the temperature by approximately 10–20°C above the ambient temperature. Some hydrocarbons, such as ketones and aldehydes, are more reactive and can cause higher temperature rises within the carbon bed. Certain types of activated carbon-particularly those derived from wood and coconut shells-are more susceptible to "thermal runaway" or "hot spots", which necessitate shutting down the VRU, purging it with inert gas, and cooling it under close supervision. When fresh activated carbon is first exposed to vapors, it exhibits high reactivity and generates significant heat. The temperature inside the carbon bed can rise to around 100°C, so the commissioning of fresh carbon in VRUs (also known as "preloading") must be performed by trained professionals.

 

Key Concern: Dust Avoidance

Dust is a critical issue to be avoided, as it leads to pressure drop losses, reduces adsorption capacity by clogging surface pores, causes excessive wear on vacuum pumps and absorption pumps, blocks filters, and accumulates in gasoline storage tanks.

It is essential that each batch of activated carbon undergoes testing against parameters critical to VRU operation, including dust content, dryness, density, particle size, hardness, and working capacity.

 

Ensuring Optimal Operating Conditions for Activated Carbon

In a properly designed VRU, the service life of activated carbon typically ranges from 10 to 20 years. However, if the VRU design fails to ensure favorable operating conditions for the carbon, its service life may be shortened to as little as 4 to 5 years.

 

Movement Inside the Carbon Bed

In VRUs, the pressure cycles between atmospheric pressure and deep vacuum at least four times per hour. At the start of each regeneration cycle, the vacuum pump operates at full capacity to reduce pressure to the desorption threshold. After regeneration is complete, the pressure must be rapidly equalized to return the carbon bed to a state ready for the next adsorption cycle. Unless the carbon bed is securely fixed, this constant pulling and pushing force will cause carbon particles to rub against each other and wear down into dust. Clearly, low-strength carbon will attrite more quickly, and granular carbon will pack more tightly into clumps. Even hard, mineral-based pelletized carbon will gradually wear down and generate dust over time. Beyond preventing carbon movement, a well-designed VRU should also control and minimize the forces (vacuum and pressure equalization) acting on the carbon bed.

Typically, mass loss of carbon due to dust generation is the single most important factor affecting the service life of activated carbon-and this can be almost entirely avoided through proper VRU design.

 

Long-Term Residual Adsorption

As noted earlier, residual adsorption refers to the portion of the initial adsorption capacity that cannot be recovered through vacuum regeneration. It can be visualized as pores permanently occupied by VOC molecules that cannot be removed by vacuum. In any system, residual adsorption increases slowly over time, resulting in a gradual decline in working capacity. A yearly capacity reduction of a few percentage points should be anticipated. Well-designed VRUs incorporate an initial safety margin to compensate for this decline. As part of a sound VRU maintenance program, carbon samples should also be periodically extracted and analyzed to determine the optimal carbon replacement time well in advance.

 

Parameter	Target Requirement	Optimal Selection
Carbon Bed Flow Distribution	Uniform flow distribution delivers the longest service life, lowest pressure drop, and maximum overall capacity of the VRU.	Pelletized carbon with 4mm diameter
Dust	Low initial dust content	Carbon that has undergone deep cleaning and frequent batch sampling tests
Mechanical Strength	Rigid, high-hardness carbon resists fragmentation into dust and degradation, ensuring long service life	Mineral-based, high-quality carbon with defined hardness/wear index
Working Capacity	Sufficiently high to ensure good long-term efficiency of the VRU, but not excessively high (as this increases the risk of thermal runaway)	Mineral-based carbon with complete long-term working capacity records
Density	Carbon quantity is always specified by weight, but significant density variations make this a key factor when calculating carbon canister dimensions	Working capacity must be correlated with density; dry bulk density of each batch must be verified
Low Pressure Drop	Carbon that does not easily pack into dense clumps	Pelletized carbon with 4mm diameter
Safety/Autoignition Temperature	Higher autoignition temperature provides a larger safety margin against thermal runaway in the carbon bed.	Mineral-based carbon with documented autoignition temperature >450°C
	Dense clumps are more likely to form hot spots	Pelletized carbon is preferred over granular carbon

 

Selecting the Optimal Activated Carbon for Vapor Recovery Units

When choosing the ideal activated carbon for Vapor Recovery Units (VRUs), the most critical criteria include:

Carbon Bed Flooding

Submerging activated carbon in liquid absorbent causes irreversible damage. This should be prevented by installing appropriate high-level detection devices at the vapor inlet and absorption tower.

Aerosol Entrainment into the Carbon Bed

It is critical that the absorption tower is equipped with a mist eliminator, and that the flow rates through the vacuum pump and absorption tower are controlled to avoid aerosol entrainment into the carbon bed-this will cause irreversible damage to the activated carbon.