Reduction of Hydrocarbon Emissions from Air through Pressure Swing Regeneration of Activated Carbon
by Jon W. Young, P.E. and W. N. Tuttle, P.E.
(Presented at the AIChE Conference 1997)
Table of Contents
- Process Flow Diagram
For years activated carbon has been widely used in the purification of gases and the decolorization and purification of liquids. In past applications, once the activated carbon had reached equilibrium with the contaminant or became "saturated," it was simply replaced and the saturated or contaminated carbon was sent to an off-site regeneration facility or the carbon was disposed of, usually in a landfill.
Due to increased environmental awareness and costs associated with the disposal of the contaminated activated carbon, processes that can reuse or regenerate the activated carbon in-situ have become more attractive. The re-use of activated carbon after it has become saturated requires the removal or desorption of the contaminant. This desorption process should restore a reasonable percentage of the activated carbon’s working capacity consumed during adsorption. The traditional method for activated carbon desorption is thermal regeneration. Much less common methods are pressure swing regeneration and purge regeneration. As one would expect, each method has advantages and disadvantages. This paper examines a process that has been successful applied in motor fuels vapor recovery for twenty (20) years. This process utilizes a combination of both pressure swing and purge regeneration.
ACTIVATED CARBON REGENERATION METHODS
The first rule in vapor phase applications is a realization that all activated carbons are not the same. Just because a material is a black, granular, and carbonaceous does not mean that it is suitable for vapor phase applications. The physical characteristics of various activated carbons are as unique as the processes used to manufacture them. The base material, pore size and distribution, particle size, activation temperature and activation method will affect the activated carbon’s specific working capacities, retentivities, mechanical strength, reactivity and a variety of other properties. Each of these physical properties, in some way, affects the suitability of an activated carbon for vapor phase applications.
As the physical properties of the carbon dictate its suitability for given vapor phase applications, it is the operating conditions (the applied technology) that ultimately dictates the success of the specific process. As a generalization certain operating conditions can be said to affect the activated carbon’s working capacity. These operational generalizations are:
The working capacity of activated carbon is enhanced with;
increased adsorbate pressure
increased adsorbate concentration and
The working capacity is diminished with the reverse of these conditions
The effective working capacity of an activated carbon is therefore, the difference between the total capacities of condition #1. (when the carbon is subjected to relatively high pressures, high concentrations and/or low temperatures and condition) and #2 (when the carbon is subjected to low pressures, low concentrations and/or high temperatures).
As stated earlier, thermal regeneration has been the traditional method for maintaining the effective working capacity of activated carbon in many vapor phase applications. Typically steam is used as the heat source during regeneration; but hot nitrogen, electric resistance heating and hot flue gases have also been used. One of the disadvantages of thermal regeneration is the time required to heat and cool a large mass of activated carbon, which is highly porous with a low thermal conductivity. In-situ, thermally regenerated systems usually have at least three carbon adsorber vessels, one on-stream receiving the contaminated gases, the second being heated for regeneration, and the third is cooling prior to being put back on-stream. The second and more significant disadvantage is the mechanical damage done to the activated carbon with successive heating and cooling cycles. The damage is due to the uneven thermal expansion and contraction of the carbon particle and manifests itself by reducing the carbon particles to dust. In time, the carbon must be replaced or re-screened creating replacement, downtime and deposal cost. The major advantage of the thermally regenerated system is the large effective working capacity that is created by this regeneration process. Large effective working capacities allow this type of process to be used in applications where the contaminant concentration is low and/or the carrier gas temperature is high. In a number of applications, thermal regeneration is the best available option.
An alternative to fixed bed replacement or in-situ thermal regeneration is in-situ pressure swing technology, commonly referred to as PSA. As inferred above, this regeneration method uses a reduction of system pressure and adsorbate concentrations to achieve the desired effective working capacity of the carbon. By reducing the partial pressure at which the contaminant was adsorbed, a portion of the carbon’s working capacity is restored. The desorbed molecules are swept from the adsorber vessel by a purge gas. Two basic types of PSA systems have been used, the first adsorbs contaminates at an elevated pressure and the pressure of the system is "let down" during the regeneration step. The second basic type is where the contaminates are adsorbed at or near atmospheric pressure and during regeneration the pressure is reduced on the carbon beds by mechanical means, vacuum pumps, vacuum jets, etc.
One of the benefits of PSA, when compared with thermal regeneration, is the longevity of the carbon. Many PSA systems have been operating satisfactorily with the same carbon for up to twelve (12) years. The typically system operates approximately twenty (20) hours per day and over three hundred (300) days per year. In twelve years, that is approximately 76,000 hours of operation without servicing or replacing carbon. A second benefit of a PSA system is simplicity. Unlike most in-situ thermally regenerated systems, the PSA system requires only two adsorber beds, one bed is on-stream receiving contaminated gas while the second bed is being regenerated. A third benefit of a PSA system is its flexibility. Due to the heating requirements of thermally regenerated systems, large quantities of steam are needed. If steam is not readily available, the thermally regenerated system may not be a practical solution. A PSA system requires only electricity to operate the controls and any pumps that may be required. A fourth benefit is that there is there is no contaminated condensate that must be treated.
The disadvantage of the PSA system is that its applications are limited. Adsorbate concentrations must be relatively high to help compensate for the less severe regeneration conditions than experienced with thermally regenerated systems.
HYDROCARBON VAPOR RECOVERY SYSTEM
The United States EPA, acting in response to the Clean Air Act of 1970 and its amendments of 1977, promulgated rules for the control of volatile organic compounds (VOCs) emitted into the atmosphere. VOCs are precursors to the development of photochemical oxidants, ozone, in the troposphere.
The EPA identified the transfer of refined petroleum products (mostly motor fuels) as one of the major sources of VOC emissions. Fuel loading into transport vehicles such as tank trucks, railcars and marine vessels emitted large quantities of VOCs into the atmosphere, even the fueling of automobiles was identified as a significant emission source. Today, virtually every fuel-loading terminal in the United States has some method of controlling emissions, at efficiencies greater than 95% with most activated carbon PSA systems exceed 98% efficiency.
At this time the predominant recovery technology used in fuel loading terminals, worldwide, is the activated carbon PSA system. This technology has been installed and used in approximately 1000 locations in North America with approximately another 500 systems utilized in other parts of the world.
The typical activated carbon PSA system utilizes two identical adsorber vessels filled with an activated carbon specifically selected for vapor phase applications. The two adsorption vessels are required to ensure continuous operation, one adsorber is on-stream receiving the fuel vapors (a mixture of VOCs and air) while the second is under regeneration. Operating at slightly above atmospheric pressure, the activated carbon preferentially adsorbs the non-methane and non-ethane VOCs from the vapor mixture and vents the cleaned air directly to the atmosphere.
The regeneration process is accomplished by reducing the pressure of the activated carbon bed and introducing a small quantity of inert purge gas to the bed while under vacuum. Designs vary, but the typical pressure swing required for proper regeneration is from near atmospheric down to a range of 30 to 100 mbara. Towards the end of the regeneration cycle a small amount of purge gas is introduced to the outlet side of the bed, pulled through the bed and into to the vacuum system. The purge gas, normally air, provides the motive force to remove additional quantities of VOCs out of the adsorber. The system automatically cycles the adsorbers every 10 to 15 minutes, depending upon the design, and places the freshly regenerated bed on-stream to receive and recover VOCs from the vapor stream.
Once removed from the carbon, the VOCs are much more concentrated than the original inlet vapor stream and can be treated with traditional liquefaction method such as absorption or condensation. Typically, a simple randomly packed absorber column is utilized with liquid fuel from storage as the absorbent.
Although of primary importance, safety considerations will not be discussed in this paper. As with any process, there are many safety and operational considerations that should be built into the design and because the recovery device processes hydrocarbon vapors and air that could very likely be in the explosive range, safety of the design should be thoroughly reviewed.
The hydrocarbon components and the concentration must be known in order to select the appropriate carbon and the regeneration conditions (vacuum level and purge gas volumes) for the application. The ability of an activated carbon to adsorb and retain a particular molecule is dependent on the pore size and distribution of the carbon. The concentration, or more correctly, the partial pressure of the component which is to be adsorbed will affect the required desorption or regeneration conditions. As the concentration of the adsorbate decreases, the regeneration pressure must also decrease.
Activated Carbon Quantity
Apart from the physical properties, including the working capacity of the activated carbon, the quantity of activated carbon required for each adsorber is dependent upon two factors. These factors are (1) the volumetric flow from the vapor source and (2) the recycle loading from the liquefaction process downstream of the vacuum system. Since the adsorber vessels cycle every 10 to 15 minutes, they must be sized to handle relatively high instantaneous loading rates from the vapor source. Although the average daily loading rate may be only 4.0 m3/min., the average 15-minute rate may be as high as 10.0 m3/min. The efficiency of the liquefaction process and the average displacement capacity of the vacuum system determine the amount of vapor loading from the recycle.
In most fuel loading applications, the pressure available to move the hydrocarbon vapors through the recovery device is limited. The liquid filling of the transport vehicle is usually the only motive force available to move the vapors from the tank to the recovery device. Tank trucks and marine vessels usually have a maximum pressure rating of less than 60 mbarg. In order to accommodate this limitation, the configuration of the adsorber vessel must limit the superficial velocity through the activated carbon to 0.05 to 0.10 m/s or some form of vapor blower must be incorporated.
Unlike the adsorber vessels, the vacuum system can be sized for a loading rate average over a longer period. Depending upon the design methods used by the manufacturer the typically averages are based on 3, 4, or 8 hours. As inferred above, most fuel loading operations have extremely heavy loading periods followed by light loading periods. The size of the vacuum system can be minimized by taking advantage of the light loading periods to "catch up" on the loading that occurred during the heavy periods. Minimizing the vacuum system also minimizes the size and cost of the downstream liquefaction process since the equipment associated with the regeneration skid is sized based on the vacuum system.
The main cause of carbon attrition on PSA vapor recovery systems is improper repressurization of the adsorber vessel after regeneration. Vapor phase activated carbons have relatively low densities, usually in the range of 225 to 350 kg/m3. The low density in combination with the large adsorber vessel size facilitates bed fluidization during the repressurization step. If the bed fluidizes, the contact with the vessel wall and other carbon particles will cause the particle to fracture and dust.
The operation of a hydrocarbon vapor recovery system is almost continuous in many gasoline distribution terminals. It is not uncommon for a vapor recovery system to be required to operate 20 to 24 hours per day for over 300 days per year. In many locations, due to the environmental regulations, the terminal is not allowed to load fuel if the vapor recovery system is not functioning properly. Therefore, all components selected for use on the PSA package should be rated for continuous duty and easily repaired or replaced should they malfunction. The proper selection of two critical components is paramount to the design of PSA vapor recovery systems, the vacuum pumps and the activated carbon.
As mentioned earlier, the design regeneration vacuum level for the recovery system is in the range from 30 to 100 mbara. Most systems supplied in fuel terminal applications are designed for approximately 100 mbara. In these applications a two-stage liquid ring pump is utilized with an ethylene glycol type seal fluid. Although a single stage pump may be able to achieve a 100 mbara level, its volumetric capacity at this pressure is less than 25% of the nominal capacity of the pump. The volumetric capacity of a two-stage liquid ring vacuum pump does not decay until the suction pressure is lower than required, usually around 50 mbara. Since most of the hydrocarbon is desorbed from the carbon at these vacuum levels, the volumetric capacity at the design pressure is crucial for removing the hydrocarbon from the adsorber and establishing proper carbon regeneration.
The currently proven section of activated carbons suitable for PSA systems is very limited. In excess of 150 different activated carbons have been tested by the various PSA equipment manufactures. To date, only four (4) activated carbons have proven suitable for the fuel terminal applications, two (2) are wood based and two (2) are coal based. Most carbons are not suitable for this application because of their retentivity of the hydrocarbons present in the vapor stream. If the retentivity is too low then the bleed-through level of hydrocarbon from the adsorber will be too high and the legislated emissions rate exceeded. If the retentivity is too high then the carbon can not be adequately regenerated by the mild PSA vacuum regeneration method and after a few cycles break-through will be experienced.
The second typical shortfall of most activated carbons is the gasoline vapor working capacity, (GWC). Since liquid gasoline is a mixture of various hydrocarbons, the vapor from that liquid is also a mixture of different hydrocarbons. A true and accurate analysis of gasoline vapor will vary based on which refinery the product was produced and seasonal changes. Because of the impreciseness definition/analysis of "gasoline vapor," GWC is not a suitable quality control parameter. An easier control parameter to use for screening a possible activated carbon for this application is the butane working capacity (BWC). The BWC can be used to indicate which carbon may be suitable, but this test, by itself, is not a definitive indicator of the overall suitability of an activated carbon. If the BWC of a sample activated carbon is a minimum of 0.065gm/cc then further testing with "gasoline vapor" in a test designed to simulate field conditions may be warranted.
In the United States, the U.S. EPA ultimately sets the required performance for vapor recovery technology. The most recent EPA regulations require that a vapor control system used in a "major source" fuel loading terminal may emit no more than 10 mg of hydrocarbon per liter of gasoline loaded (10 mg/l). This calculates to a control efficiency of approximately 98 to 99% averaged over a six-hour period.
Worldwide, the performance regulations are similar to the U.S. EPA regulations with a few exceptions. A few countries in Europe have adopted a regulation that requires the hydrocarbon vent concentration to be no higher that 150 mg/Nm3. This requirement is approximately two orders of magnitude more stringent than current U.S. regulations. This stringent requirement can be attained by treating the vent gas from the conventional PSA system or by regenerating the activated carbon at lower pressures thus achieving a greater working capacity each regeneration cycle.
The vapor recovery technologies that have been successfully installed in fuel transfer operations and are capable of meeting the current emission standards are very limited. Even though other technologies such as lean oil absorption, and refrigeration are available, activated carbon PSA systems have proven to be the preferred technology. The activated carbon PSA technology has been applied successfully at approximately 1500 locations worldwide at tank terminals and loading operations for tank trucks, railcars and marine vessels. Over 90% of all gasoline vapor recovery systems in operation around the world are activated carbon PSA systems.
The reason for the market dominance of the PSA technology is based on the economics and the performance of these systems. The capital equipment cost and the operating/maintenance costs are generally lower than with competing technologies. The process and therefore the equipment is relatively simple and easily understood and the emissions performance of the system can meet the most stringent worldwide environmental regulations.
Return to Technical Paper List