Activated carbon adsorption to capture organic vapors from non condensable gases.

System design

The starting point for system design is the study of the isotherms for particular VOCs. Fig. 1 shows the isotherms for three common industrial solvents - toluene, ethyl alcohol and acetone. The amount of VOCs held by the carbon increases as the concentration of VOCs-in-air increases. These isotherms are at 25 degrees C. The 100 degrees C isotherm for Toluene shows a much reduced capacity (Fig. 1). This indicates that the adsorption process is temperature sensitive.

If the amount of carbon needed to effectively capture 500 ppmv of VOCs from a 10,000-cfm air emission is calculated, perhaps a 10% loading on the carbon can be obtained from the isotherm. That calculates to a consumption of 720 lb per hr of carbon, after which the carbon is returned to the kiln for reactivation. Because carbon costs more than $2 per lb, some very large dollar amounts are needed for replacement as the carbon is exhausted.

Thus, one has to consider in-situ regeneration.

Carbon regeneration

As previously mentioned, adsorption is a heat-sensitive process. Therefore, if heat is applied to carbon that has been charged with VOCs, a concentrated stream of solvent will be released. There are two commonly used desorption agents - steam and hot gas (air/nitrogen). Each has its own advantages and limitations. Steam is readily available at most industrial sites. It has a high energy content per pound, and is readily condensed using cooling tower water. Steam is inert.

Although hot gas has a low energy content per pound, it can be much hotter than steam from typical industrial boilers. Air is not inert and thus one must be cautious of the regeneration gas concentration that is employed. Nitrogen is inert and for some applications is the only safe hot gas regeneration agent. As stated previously, one cannot condense low concentrations of VOCs using cooling tower water, so it becomes necessary to recycle the regeneration gas through the bed until sufficient concentration builds up and condensation occurs by means of cooling water. Hot gas provides no wastewater, which can be a significant advantage.

For most systems, steam is the preferred regenerating agent, and it has been used in tens of thousands of carbon adsorption systems.

A simplified flow diagram for a carbon adsorption system shows three adsorber vessels with horizontal static beds of activated carbon (Fig. 2). Volatile Organic Compound Laden Air (VOCLA) is passed through the bed, where the VOCs are captured and the cleansed air is discharged to the atmosphere.

When the bed has been charged with VOCs, it is regenerated by the counterflow passage of live steam. The steam heats the carbon, which releases some of the VOCs, and then is swept away to a condenser and separation. This type of system is called deep static bed technology.

Adsorption system design

Isotherms are static laboratory data. However, plant designers need dynamic information to size a system. Factors affecting the working capacity of carbon include the specific VOC and the inlet/outlet concentrations for which the system is expected to operate, as well as the air temperature and the RH of the air stream (if the RH exceeds 50%, the carbon begins to adsorb moisture and this reduces the VOC capacity of the carbon). The degree of regeneration of the carbon (that is, the amount of Energy put into regeneration) affects the degree of cleansing the bed, which affects subsequent performance.

While the inlet layers of the bed may attain high working saturation (approaching those of the isotherm), the last third of the bed is basically a polishing zone - the objective of which is to provide a very low outlet concentration. The effect of all the above leads to an empirical dynamic working capacity of, at most, 10% to 25% of the value given by the isotherm.

If the VOC concentration is more than 500 ppmv, there is a probability that the solvent value will exceed the operating energy cost, thus providing a return on investment. If a system is used 24 hr a day, five days a week, then a return on investment within 3 years is routinely achieved, which is why many systems are referred to as solvent recovery systems.

Other design considerations

Some VOCs undergo trace hydrolysis during the steam desorption stage. Acetone and other ketones have about 0.5% conversion to acetic acid. Acetates have a similar trace conversion to acetic acid. Chlorinated solvents have trace hydrolysis to hydrochloric acid - again a small amount.

Nevertheless because of the number of regenerations per year, it is very important to select the correct material of construction for the adsorbers and the condensing train. Austenitic stainless steels provide 25 plus years of life expectancy for ketone/acetate recovery systems. Special stainless steels or nickel alloy provide 25 plus years life expectancy when trace hydrochloric acid is the concern. Aromatic or aliphatic hydrocarbons and the alcohols do not produce acidic products so carbon steel is suitable for the adsorbers.

Ketones have trace decomposition due to the catalytic nature of the carbon which is more pronounced at higher temperatures. This is exothermic and care must be taken to ventilate the bed properly to carry away the heat so there is no build up that may cause bed fires.

Acetone has a very slight decomposition rate, methyl ethyl ketone is about tenfold greater and cyclohexanone is 100 times that of acetone. By careful design and operational monitoring, recovery has been made very effective and many systems are recovering these solvents at fibers and magnetic tape facilities.

Fouling and acidity

Activated carbon can be used for many years provided foulants are not present. Higher boiling point (above 200 degrees C) organics tend to foul the carbon, as will large molecules such as plasticizers, lubricants or silicones. If an exhaust stream is treated by 2 lb of carbon per cfm of air, then 1 ppm for example of foulant amounts to 0.4 lb per year. If only 25% is not desorbed then the adsorptive capacity will be substantially diminished in about a year. In such circumstances it is common to place sacrificial carbon beds upstream of the regenerative beds. These are not in-situ regenerated - thereby avoiding expensive vessels, and valving - but are periodically replaced.

One of the great hopes of hot nitrogen regeneration was that acidic by-products would not be produced. Experience has found that there is trace adsorption of moisture vapor, if present in the exhaust, that vaporizes from the carbon with the solvent in the desorption stage. Due to the higher temperatures of the hot nitrogen (200 degrees C) that are necessary to input sufficient regeneration energy, small quantities of moisture are sufficient to greatly increase the amount of acidic products produced compared to steam desorption at 110 degrees C. This requires the addition of a moisture removal stage, which offsets the savings from cheaper materials of construction for the adsorbers. Similar effects are found with ketones and chlorinated organics.

As mentioned above, the ketone family tends to have trace exothermic decomposition and hot nitrogen would be expected to avoid this concern. The higher temperatures necessary for inert gas desorption greatly increased the ketone breakdown. In the case of cyclohexanone, the by-product is adipic acid, which, because of its high boiling point (more than 300 degrees C) is not desorbed and therefore rapidly fouls the carbon.

Adsorption applications

Chemical species regularly removed by activated carbon include alcohols, CFCs, halogenated hydrocarbons, esters, ethers, glycols, aromatics, olefins, paraffins, ketones and thiols. Some chemical industry applications of carbon adsorption include:

Cellulose acetates . Several products are made from cellulose acetates including fibers. Solvents are an essential part of the production process. The cellulose acetate is dissolved in a solvent to form a dope solution so that it can be pumped. Di-acetates are dissolved into acetone and tri-acetates are dissolved into methylene chloride. The dope solution is pumped into a spinneret that has a number of extremely small holes (usually about 0.1 mm in diameter) through which the dope is extruded. With the release of pressure, the solvent rapidly evaporates from the extrusion, leaving a very fine filament of acetate. Several filaments are spiraled together to form a thread that is wound onto a bobbin for use in manufactured garments, carpet and other fiber products. A typical spinning machine will have 20 to 50 spinnerets, and a major acetate site will have several hundred spinning machines.

Warm ambient air is used to draw the solvent vapors away from the spinnerets - about 50 scfm for each spinneret. To maintain safe operating conditions, the acetone concentration is around 1.5% volume - which is about 75% of its LEL. Because acetone is a valuable chemical, some form of highly effective recovery is needed.

To achieve 99% capture by condensation requires cooling the exhaust air to -80 degrees C (acetone vapor pressure of 0.1 mm Hg) requires very large amounts of deep refrigeration. As spinneret air is ambient, it contains moisture vapor that when cooled below 0 degree C will ice up the exchanger.

Absorption into a scrub fluid such as water is sometimes used. For 99% capture, the absorption has to operate at about 5 degrees C which necessitates substantial refrigeration. The weak liquor only contains about 2.5% acetone, so a substantial distillation system is required and this uses large amounts of energy.

Adsorption onto activated carbon is highly effective. The carbon is regenerated by live-steam desorption. The desorption condensate is a weak liquor containing about 40% acetone. Distillation to more than 99% purity is still required, but the equipment size and energy needs are substantially less than those for the distillation of the absorption method. Because acetone undergoes trace hydrolysis during steam desorption, releasing small amounts of acetic acid, the adsorbers and condensing train are made of austenitic stainless steel.

From the above it is easy to see why carbon adsorption is the primary means for recovery. The complexity of the cellulose acetate fiber production process is such that there are only a few sites, each of very large output. Similarly, the recovery systems are large - there are several facilities in the USA and each recovers more than 50 million gallons per year.

The solvent for tri-acetate fibers and photographic film base is usually methylene chloride. Although not flammable, it does have narcotic and toxic properties that require adequate ventilation. The spinnerets operate in similar fashion to those for cellulose di-acetate with around 1.5% methylene chloride in the exhaust air.

Ninety-nine percent recovery by condensation involves cooling to below -80 degrees C plus the ambient moisture vapor freeze problems mentioned above. Absorption into water is not possible as methylene chloride has a low solubility with water. Carbon adsorption is the primary recovery process. The desorbate is then distilled to remove the small quantities of water. During steam desorption, there is trace hydrolysis that releases small quantities of hydrochloric acid - therefore the adsorbers and condensing train are of special stainless steels or nickel alloys that give an operational life expectancy that exceeds 25 five years.

Continuous reactors. There are several processes that have continuous oxidation reactors such as, for instance, in the manufacture of hydrogen peroxide. The reactor contains a catalyst bed that enhances the combination of hydrogen with oxygen to form the peroxide.

Several solvents may be involved as a source for the hydrogen such as trimethyl benzene. The oxygen source is ambient air, that, because the reactor operates at a pressure of several atmospheres, has to be compressed to enter the reactor. As air contains about 80% nitrogen, it is necessary to continuously vent the reactor to maintain the correct reactor oxygen level. The vent gas carries with it significant amounts of solvent - trimethyl benzene has a saturation level in air at 20 degrees C of about 1500 ppmv. The vented solvent is valuable as well as being a VOC. Use of vapor phase carbon adsorption returns the solvent to the process and provides VOC control.

Another continuous reaction is in the production of ethylene dichloride. This is a major building block chemical that is used in the production of polyvinylchloride. The chlor-oxygen reaction gets its oxygen from ambient air. So it is necessary to continuously vent the reactor of surplus nitrogen. The vent stream also contains ethylene dichloride, a toxic and flammable narcotic that must be controlled.

If oxidation is employed for VOC control, then the oxidizer requires an after scrubber to remove the HCl and the scrub liquor requires alkali input for neutralization. Carbon bed recovery provides VOC control, returns valuable raw material to the process, and avoids the high energy costs of oxidation and chemical input to the neutralization step. Several ethylene dichloride producers have taken the carbon bed method of control.

Batch Reactors. Venting of batch reactors often involves the discharge of VOC, which requires emissions control. One example is a hydrogenation process operating at about 100 atmospheres.

Before the batch of granules could be unloaded from this large vessel, it was necessary to vent the hydrogen to the atmosphere. One of the reaction components was monochlorobenzene, which, at 30 degrees C, has a saturation level of 20,000 ppmv. It is a narcotic as well as being flammable. The preferred control method was carbon adsorption which cleansed the hydrogen carrier gas such that it could then be vented to the atmosphere. After steam desorption the chlorobenzene is returned to the process for reuse.

Solids Drying. Pharmaceutical facilities often use solvents to convey fine chemicals. At some stage the solvent is dried by evaporating it into an air stream. The vent stream then requires emissions control.

One example is in the production of penicillin. In the early stages of manufacture the antibiotic is in an acetone mother liquor. Eventually the acetone is removed as a vapor and captured from the dryer exhaust air by carbon adsorption. After distillation the acetone is returned to the process.

Coating. There are many processes that involve coating a product where the coating is dissolved in a solvent and the resultant dope is applied to a solid, after which the solvent's task is completed and the solvent is evaporated away.

Pharmaceutical tablets receive their outer protective layer often in a methylene chloride solvent. After coating, the solvent is evaporated and the emission must be controlled by carbon or oxidation.

Hydrogen sulfide. The highly odorous, flammable, poisonous gas (34 molecular weight and: -60 degrees C boiling point) is often a by-product of food processing and industrial processes. It is also found in natural gas, oil refining waste streams and at sewage works. Its low molecular weight and low boiling point suggest that activated carbon will have a low capture capacity.

The isotherm shows less than 2% capacity if the carrier gas is an inert such as nitrogen. However, if the carrier gas is air or another oxygen-containing gas, then the capture capacity increases tenfold.

Concurrent with the adsorption onto the carbon is a reaction between the hydrogen sulfide and oxygen that produces water vapor and elemental sulfur. This form of adsorption involving a chemical reaction is called chemisorption. The elemental sulfur remains on the carbon. The carbon catalyzes the reaction.

This effect can be further enhanced by impregnating the carbon with iron, which leads to several opportunities for carbon.