EQUATION CALCULATES ACTIVATED CARBON'SCAPACITY FOR ADSORBING POLLUTANTS

Carl L. Yaws, Li Bu, Sachin Nijhawan
Lamar University
Beaumont, Tex.

Adsorption on activated carbon is an effective method for removing volatile organic compound (VOC) contaminants from gases.

A new, simple equation has been developed for calculating activated carbon's adsorption capacity as a function of the VOC concentration in the gas.

The correlation shows good agreement with experimental results. Results from the equation are applicable for conditions commonly encountered in air pollution control techniques (25 C., 1 atm).

The only input parameters needed are VOC concentrations and a table of correlation coefficients for 292 C8-C14 compounds. The table is suitable for rapid engineering usage with a personal computer or hand calculator.

BACKGROUND

Physical and thermodynamic properties of organic compounds are especially helpful to engineers and scientists in industry. In particular, capacity data for the adsorption of VOCs on activated carbon are becoming more important in engineering and environmental studies because of increasingly stringent regulations regarding air emissions.

The adsorption capacity results from the new equation can be used to design carbon adsorption systems to remove trace pollutants from gases. For pollution control to very low emissions levels, such systems could include:

Carbon adsorption followed by conventional steam regeneration with solvent recovery, or
Carbon adsorption followed by thermal or catalytic oxidation.

CORRELATION

The correlation for adsorption on activated carbon is based on a logarithmic series expansion of concentration in the gas (Equation 1) (11237 bytes). Correlation constants (A, B, and C) for the first 30 compounds are given in Table 1 (33315 bytes).

The constants in the table were determined from regression of the available data for adsorption on activated carbon. The tabulation is arranged in order of increasing carbon number to provide ease of use in locating data using the chemical formula.

Although the correlation is applicable to hydrocarbons outside the C8-C14 range, these lighter compounds are omitted from the table because of space considerations. Another reason for their omission is that, for compounds that are gases at ambient temperature, the refractive index is needed for the calculations (as shown in Equation 2) (11237 bytes).

A comparison of the correlation results with experimental data is shown in Fig. 1 (13701 bytes) for a representative compound. In this figure, the adsorption capacities are for conditions encountered in air pollution control (concentrations in the ppm range in a gas at 25 C. and 1 atm).

This graph shows favorable agreement between the correlation results and experimental data.

ESTIMATION EQUATION

In preparing the correlation, a literature search was conducted to identify data sources. 1-2 The publications were screened and the appropriate data were keyed into the computer to produce a data base of experimental adsorption capacity values at different concentrations (partial pressures). The data base also served as a basis for checking the accuracy of the correlation.

After the data were collected, adsorption capacities for the remaining compounds were estimated using Equation 2 (11237 bytes). Equation 2, developed by Calgon Carbon Corp., was used to estimate the equilibrium adsorption capacity of activated carbon as a fifth-order polynomial function of activated carbon's adsorption potential.

For Equation 2 (11237 bytes), data for refractive index were taken from literature sources .21-21 Data for vapor pressure and liquid molar volume are from compilations by Yaws. 29 30

EXAMPLE 1

The air from an industrial operation contains 100 ppmv n-octane (C8H18). To estimate the adsorption capacity of activated carbon for removing the pollutant at 25 C. and I atm, substitute the correlation constants for n-octane, and the concentration, into Equation 1 (11237 bytes):

log10Q = 1.06727 + 0.18700[log10(100)] - 0.01249[log10(100)]2 = 1.39131

Q = 24.62 g n-octane/100g carbon

EXAMPLE 2

The air from a spray-painting operation contains 10 ppmy m-xylene (C8H,o), To estimate the adsorption capacity of activated carbon for removing the pollutant at 25 C. and 1 atm, substitute the correlation constants from Table 1 (33315 bytes), and the concentration, into Equation 1 (11237 bytes):

log10Q = 1.31522 + 0.14019 [log10(10)] - 0.01457[log10(10)]2 = 1.44084

Q = 27.60 g m-xylene/100g carbon

OPERATION, DESIGN

In actual operation under plant conditions, an adsorption bed seldom will achieve equilibrium capacity. Copper and Alley suggest a bed capacity of 30-40% of equilibrium for plant operating conditions.6

Similarly, in the U.S. Environmental Protection Agency design manual, Damie and Rogers suggest a working factor of 3 for design of adsorption beds.7 The total carbon requirements for an adsorption system are obtained by determining the carbon required for equilibrium capacity, then multiplying by the working factor.

Factors affecting adsorption bed capacity are discussed in the literature.6 7 25 These factors include:

Loss due to adsorption zone
Loss due to heat wave (adsorption is an exothermic process)
Loss due to moisture in the entering gas
Loss due to residual moisture on the carbon.

Representative adsorption systems for removing organic compounds from gases are shown in Figs. 2-4.

Fig. 2 (13608 bytes) shows an adsorption system with recovery of the organic material, such as a solvent, using steam for regeneration. Fig. 3 (13608 bytes) illustrates an adsorption system using thermal or catalytic oxidation of the organic material that is removed from the gas by carbon adsorption.

In Fig. 4 (13280 bytes), the organic material initially is removed from waste water by air stripping. The air leaving the stripper contains the organic material. This stream subsequently is sent to the adsorption system for recovery of the organic.

The system illustrated in Fig. 4 13280 bytes) can be used to recover organics such as benzene from refinery process waste water.

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