Absorption Devices

Device Type

Adsorption devices consist of adsorptive media, either static or mobile, in a containing vessel through which the gas and its contaminants are passed. The contaminants are adsorbed onto and into pores in the adsorbing media.

Typical Applications and Uses

Adsorbers are most commonly used for solvent recovery; control of hydrocarbon emissions from storage tanks, transfer facilities, printing operations; and similar processes where volatile hydrocarbons are present. Activated carbon types are also used to control sulfurous odor, such as that from sewage treatment plants. Special impregnated carbons are used to chemically react with the contaminant once it is adsorbed, thereby extending the carbon life. Where the hydrocarbon has recovery value, absorbers are often used after process vents, evaporators, or distillation columns to polish the emission down to regulatory limits. They are also used on process vents in lieu of thermal oxidizers.

Regenerative adsorbers are generally not used where the contaminant is not economically recoverable, or the desorption process has a low yield. For example, cases where adding steam to desorb the carbon results in an unusable water mixture tend to make adsorption less attractive.

Drum-type units are often attached to process tanks to control hydrocarbon breathing or fill venting losses. The gas flow rates are typically low, and these drum-type units can be applied very economically.

Filter-type units are used in ventilation systems for hospitals, clean rooms, auditoriums, bus stations, loading docks, and other environments where adsorbable hydrocarbons may be present.

Operating Principles

Gas adsorption is the physical capturing of contaminant gas molecules onto or into the surface of a suitable solid adsorbent, such as activate carbon, zeolite, diatomaceous earth, clays, or other porous media. The gas molecule is physically trapped by the pore openings in the medium and accumulates over time until the medium saturates and can hold no more. In some devices, the medium is desorbed in place through the application of a gas such as nitrogen, or steam, to drive the contaminant from the pore openings of the medium. In others, the medium itself is directed to a device where thermal energy (heat) is applied to desorb and recover the medium.

Adsorption is basically a pore surface and size phenomenon. The size of the gas molecule dictates the pore size of the required adsorbent, and the bulk pore area of the adsorbent per unit volume determines the amount of adsorbent required to control the specific pollutant. Adsorbents exhibit certain physical characteristics with respect to pore size. These characteristics are generally called macropores and micropores as shown in Figure 2.1. As defined by the word prefixes, macropores are large pore openings and micropores are small pore openings. In practice, adsorbents exhibit a mixture of both. The volume of adsorbent required is controlled by the contaminant gas rate, and the amount of time allowed before breakthrough is permitted to occur. Breakthrough occurs when the pores are effectively filled with the contaminants or interfering compounds.

The process of activating activated carbon is basically one of opening its pores. The carbon can be acid washed then carefully heated in a reducing atmosphere, or it can be otherwise treated to open the available pores.

Various adsorbents reflect known pore sizes and exhibit specific areas per unit volume. Application engineers have developed adsorption isotherms for various pollutants as they relate to specific adsorbent types. In the family of activated carbons, for example, there are dozens of different carbon types (peanut shell based, coconut shell based, mineral carbon based, etc.), each exemplifying specific pore size and area characteristics. The adsorption isotherms are used to predict the rate of capture of that pollutant in the adsorbent and to therefore anticipate breakthrough.

Figure 2.2 shows a typical adsorption isotherm curve. Adsorption tends to follow the lessons learned earlier about NTUs and driving force. The concentration gradient is important in adsorption processes because a large gradient tends to fill pores quickly, thereby reducing the probability of continued adsorption at a high rate. The designer therefore must allow for a sufficient volume of adsorbent, not only for its ultimate capacity prior to breakthrough but also for the concentration gradient that may exist. If the contaminant exists in high concentration, the volume of adsorbent is increased and the speed at which the gas flows through the adsorbent is decreased.

FIGURE 2.1

Macropores and micropores. (Barnebey Sutcliffe Corp.)

Primary Mechanisms Used

Although the contaminant gas molecule must be fitted to the available pore size of the adsorbent, the mechanism holding the molecule onto the adsorbent is believed to be van der Waals and other weak attractive forces.

FIGURE 2.2

Adsorption isotherm. (Amcec, Inc.)

The adsorption process is more mechanical than chemical. An exception to the latter is chemically treated adsorbents wherein the pores are precharged with a chemical that reacts with the contaminant upon contact.

Given that the contaminant molecules are mechanically attached, they can often be detached or desorbed through the application of steam, heated gases, inert gases, or other processes that force the contaminant out of the pores. In this manner, the adsorbent can be regenerated and reused to some extent until the useful life of the adsorbent is reached.

Design Basics

Adsorbers are usually either of the throwaway or regenerative type. The throwaway type involves the use of a fixed bed of adsorbent in a containing vessel. These vessels can be either periodically emptied of the adsorbent or the entire chamber with adsorbent can be exchanged for a new one. The adsorbent is either regenerated remotely or thrown way. In the regenerative type, the adsorbent is regenerated or desorbed in place. This typically involves two chambers that can be isolated. One chamber is actively adsorbing while the other is being desorbed with either steam, hot air, or an inert gas such as nitrogen.

FIGURE 2.3

Regenerative adsorber. (Barnebey Sutcliffe Corp.)

The ancillary equipment includes dampers to swing the contaminant gas stream from one chamber to the other and isolation valves and controls to administer steam to desorb in situ. Some of these designs use an inert gas such as nitrogen for desorption purposes. The desorbed vapors are often condensed and collected or are directed to a thermal oxidizer for destruction. Figure 2.3 shows a multiple chamber adsorber schematic for capture and recovery of solvent-laden air and regeneration in situ using steam.

Sometimes, the designer creates a deep bed of adsorbent and installs it in a modular housing. These are popular for point-of-use volatile organic compound (VOC) control. Equipped with its own fan and pressure-drop monitor, the packaged unit is simple to install and operate. When the adsorbent is consumed (breakthrough occurs), the adsorbent housing can be shipped for regeneration off-site. Figure 2.4 shows a packaged, deep bed-type adsorption unit.

Adsorber gas velocities are usually extremely low to reduce the pressure drop of the system. Because the adsorbent particles are close together, their

FIGURE 2.4

Packaged adsorption unit. (Barnebey Sutcliffe Corp.)

resistance to gas flow is quite high. Gas velocities of 1-3 ft/sec or less are common. The bed depth is dictated by the calculated volume of adsorbent needed to operate before breakthrough based upon the adsorption isotherm(s) for the contaminants) to be removed. To avoid channeling of gases, multiple beds are sometimes used. Each bed may be 1-2 ft thick followed by a vapor space to permit gas redistribution. This low gas velocity means that adsorbers are generally large devices.

A throwaway-type (drum) adsorber is shown in Figure 2.5. The adsorbent is precharged in the drum and the drum is designed for off-site regeneration or disposal.

These designs are often used for tank vent emission control for volatile hydrocarbons where the gas flow rate is 50-150 actual cubic feet per minute. Upon achieving breakthrough or scheduled replacement, the canister is removed from service, sealed, and shipped to the supplier for off-site regeneration or replacement.

Unfortunately, water and water vapor can be adsorbed as well on most adsorbents (exception: zeolites). The water vapor becomes, in effect, an unwanted contaminant because it takes away adsorbent area that would be better used to collect the real contaminant. To reduce water's effect on the adsorbent, humid gas streams are sometimes reduced in water vapor content

FIGURE 2.5

Canister-type adsorbers. (Carbtrol Corp.)

by first cooling the gas stream to condense water vapor, then reheating the stream to be well above the water dewpoint. The adsorber housing is then insulated to prevent the water from cooling and reforming a vapor. In low humidity applications, the gas stream is sometimes sent through a bed of gravel or rocks to remove entrained water vapor. Sending the gases through a strong acid scrubber can also dry the gases so that the adsorption process is maximized.

The canister-type systems often include a bed of gravel or a separate water trap canister to reduce the carryover of water to the adsorption canister. Others are band heated to keep the gas humidity below the dewpoint. Sometimes, heated air is bled into the system to reduce the gas moisture content. The most effective method, however, involves cooling the gases to condense water followed by indirect reheat.

If the contaminant gas easily desorbs and can exceed the lower explosive limit (LEL), the adsorber vessel must be designed for explosion-proof operation. The adsorption process is one of methods for concentrating a dilute gaseous stream, so LEL considerations must be considered.

The activated carbon-type adsorbers are generally used in applications of less than 150°F. For higher temperatures, zeolites are often used. Zeolites are mineral-based adsorbents that are less affected by water vapor and temperature. Zeolites have been effectively used in rotating wheel-type

FIGURE 2.6

Zeolite-type adsorption concentrator (Munters Zeol).

devices as shown in Figure 2.6 and as mentioned in Chapter 1. They are used ahead of thermal oxidizers to concentrate the contaminants in a dilute gas stream to a point where they can economically be destroyed thermally. This concentrator-type service reduces the size of the required thermal oxidizer.

Panel-type air filters are also available precharged with activated carbon or another suitable adsorbent. Figure 2.7 shows such a panel filter wherein the finely divided carbon is mixed with the filter medium itself. In other designs, pelletized carbon fills the space between filter medium panels, thereby providing some VOC control. These designs are used in room ventilation systems. The adsorbent, the filter medium, or both can be pretreated with a biocide to kill bacteria that may also be found in the gas stream. Highly specialized filters such as these are used to protect military personnel who handle mobile vehicles such as tanks and personnel carriers from gaseous weaponry and deadly battlefield smoke particulate.

Operating Suggestions

As previously mentioned, water and water vapor should be removed prior to nonzeolite-type adsorbers. If regenerative type adsorbers are contemplated, the vendor should be consulted regarding the integration of the adsorber

FIGURE 2.7

Panel-type adsorption filter. (Barnebey Sutcliffe Corp.)

into the process and a thorough economic analysis should be performed. On many applications, the use of a regenerative type adsorber can provide significant savings in recovered solvent or chemical.

Except for the rotating wheel-type adsorber, the capacity of any adsorber slowly decreases from the moment of initial operation. As the adsorption gradually moves to the point of breakthrough, the adsorption efficiency stays relatively constant. For this reason, time, or a breakthrough sensor (hydrocarbon analyzer) must be used to determine breakthrough. If batch-type adsorbers are used, one must carefully monitor the time between regeneration or replacement or invest in monitoring equipment that indicates when regeneration or replacement is required.

 
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