Carbon canister design

As initially discussed in Section 3, carbon canisters are used in the automotive emission control system to temporarily store hydrocarbon vapors. The vapors are later purged into the air charge stream of the air induction system, thus regenerating the carbon canister. Carbon canister design is dependent on the characteristics of the vapors sent to the canister and the amount of purge air available. In the following section, factors that affect the performance of the evaporative emission control system will be discussed. 

Gasoline working capacity (GWC) also shows a strong relationship with the pore volume in the mesopores. Similar to BWC, GWC is a measure of adsorption capacity in which actual gasoline vapors are used as the adsorbate. The relationship between the BWC and GWC is shown in Fig. 12. The data shows a strong relationship between the BWC and GWC. The relationship would be expected since both the BWC and GWC have excellent linear correlations with the pore volume in the small mesopores. 

Canister working eapaeity was studied for a 1.27 mm mean diameter granular versus 2.1 mm mean diameter pellet earbons of equal BWC. Under the ORVR loading eonditions the granular earbon had a 6-12% higher GWC. However, the high pressure drop of the granular earbon would make it very diffieult to use in an ORVR system. 

Another way to examine the effeet of earbon partiele size on kineties is to look at the bleed emissions from a earbon eanister [20,35]. Bleed emissions are those emissions that oecur prior to break through. They are the result of the diffusion of gasoline vapor components that ean develop during extended soak times between purge and adsorption events. 

The plot in Fig. 13 shows the bleed emissions that were measured after a 24 hour soak. Two eanisters were tested, one loaded with a wood granular carbon with a mean particle diameter of 1.27 mm, the second with a wood pellet carbon with a mean particle diameter of 2.10 mm. Both carbon samples had equal BWC of 11.4 g/lOOml. Although both earbons had the same BWC, the larger pellet earbon had lower bleed emissions. These diffusion results are expected in light of Pick s Law. 

The pressure drop of a one liter canister during an ORVR event was studied. Canisters were filled with a pelletized carbon with a mean particle diameter of 2.1 mm and a coal granular carbon with a mean diameter of 1.3 mm. During the canister loading of 50 grams/min the canister with the 2.1 mm pellet had a pressure drop that increased from 0.35 to 0.55 kPa (1.4 to 2.2 inches of water). The granular carbon experienced a pressure drop of 0.98 to 1.5 kPa (3.9 to 6.0 inches of water) under the same conditions. 

---

The key components in the fuel vapor control system include the fuel tank, vapor vent valves, vapor control valve, vapor tubing, the activated carbon canister, and the engine vapor management valve (VMV) [25,26], During normal vehicle operation, fuel tank vapor pressure is relieved through the use of vapor vent valves installed in the vapor dome of the fuel tank. The vent valves are designed to allow for the flow of fuel vapor from the tank, and to assure that liquid fuel does not pass through the valve. 

There are many other factors that can affect the performance and on-vehicle reliability of activated carbon canisters. The following items shown in Table 6 represent some of the more important factors that must be taken into consideration when designing an evaporative emission canister. 

The design of activated carbon canisters for evaporative emission control is... 

The rate of vapor generation during refueling is a major parameter affecting the design of carbon canisters to meet ORVR requirements. 

A typical simulation result is shown in Fig. 3. Under the given conditions, the concentration of fuel gas in bulk phase at the exit (Fig. 3a) is zero and the concentration of evaporative fuel gas at solid phase (Fig. 3b) at the exit did not reach the equilibrium concentration of activated carbon during adsorption. These results indicate that the canister of ORVR system is properly designed to adsorb the evaporative fuel gas. The temperature changes in canister (Fig. 3 c) during the operation remains in the acceptable range. The test results for different weather conditions showed that the canister design in this study can fulfill the required performance. 

The carbon canisters lack instrumentation and controls therefore, the carbon could become overloaded unless it is carefully monitored. Fluctuating or less-than-design airflow rates could cause the carbon to form channels, reducing the effective capacity of the canister. 

In air conditioning (qv) of closed spaces, a wider latitude in design features can be exercised (23,24). Blowers are used to pass room or cabin air through arrays of granules or plates. Efficiencies usuaHy are 95% or better. The primary limiting factor is the decreased rate of absorption of carbon dioxide. However, an auxHiary smaH CO2 sorption canister can be used. Control of moisture entering the KO2 canister extends the life of the chemical and helps maintain the RQ at 0.82. 

In this example, the one liter canister is designed as a cylinder with a length-to-diameter (L/D) ratio of five. The vapor feed stream to the canister is a 50/50 mixture of n-butane and air, and the inlet flow rate is set at 40 grams per hour of n-butane. The curves in the Fig.9 show that break through occurs shortly after the 100 minute point in the load. Up to break-through, the activated carbon bed has adsorbed about 65 grams of HC. 

Table 6. Other parameters/operating conditions affecting canister performance and design Recirculating Fuel System Non-Recirculating Fuel System Single vs. Multiple Carbon Beds Gasoline vs. Alcohol-based Fuels Liquid Fuel Ingestion into Carbon Bed Water Ingestion into Carbon Bed Dispensed Fuel Temperature...

---