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DESIGN AND DEVELOPMENT OF THE SYSTEM -- continued

Target TVOC Adsorbates

Based on an economic and engineering study on VOC control technologies (Dyer and Mulholland, 1994), if the VOCs have a value greater than $0.66/kg, then carbon adsorption and vapor recovery is a profitable process. Regardless of price, if the effluent gas flow rate is greater than 28 m³/min and TVOC concentrations are greater than 500 ppmv, the most cost effective ancillary control technology is carbon adsorption (Dyer and Mulholland, 1994).

Another criteria for selection of carbon adsorption for TVOC recovery is based on the adsorbate retentivity. Adsorption affinity of activated carbons increases as a function of molecular weight of TVOCs. However, the higher the molecular weight, the less volatile the TVOC would be. Both the low volatility and high adsorption affinity increase the retentivity of these compounds and makes their desorption process more difficult.

Considering the above criteria, examples of industrial TVOC emissions that may be recovered efficiently by carbon adsorption technology are listed in Table 3.2.

Suitable Adsorbents

The most commonly used materials for gas separation and purification are activated carbon, silica gel, activated alumina, carbon molecular sieves, and zeolite (crystalline aluminosilicate). Table 3.3 lists some of commercial adsorbents that are used in gas separation and purification processes. From the variety of adsorbents, activated carbon is the most suitable material for separation of TVOCs from industrial gas streams. Carbon selectively adsorbs organic compounds (especially non-polar ones) unlike most inorganic adsorbents. Furthermore, the inorganic adsorbents lack another attractive feature of activated carbons, that is their availability in various pore structures as indicated by total pore volume and pore size distribution. For a given TVOC concentration, the carbon adsorbent that will provide a high adsorption capacity has a high percentage of total surface area and pore volume distributed in optimal pore size ranges.

 Pore size distribution can drastically affect the adsorption characteristics of the adsorbent. The International Union of Pure and Applied Chemistry (IUPAC) classifies pores into three categories based on their diameter, d, as follows: micropores have d < 20 D, mesopores have 20 D < d < 500 D, and macropores have 500 D < d. Micropores are further broken down into ultramicropores with d < 7 D and supermicropores with 7 D < d < 20 D (Kaviany, 1994; Gregg and Sing, 1982). These divisions are mainly based on the different types of forces which control adsorption behavior in different size ranges. For example, adsorption in ultramicropores is generally restricted to a monolayer. In the supermicropores, the superposition of pore wall force fields enhances the physical adsorption by formation of multilayer adsorption and pore filling. The upper limit of 20 D for the micropore classification is introduced because of the disappearance of the adsorption-desorption hysteresis loop at this pore size. This hysteresis is caused by capillary condensation at relative pressures of approximately 0.3-0.4, depending on the nature of the adsorbent and adsorbate (Bhandarkar et al., 1992). For gas purification processes at TVOC concentrations < 10 ppmv, ultramicropores generally provide a higher adsorption capacity. For TVOC concentration > 10 ppmv, pores with diameters in the range of supermicropores and transitional range are preferred. These larger pores are preferred because the adsorbent will have a higher total specific pore volume and the pores are filled at the concentration levels greater then 10 ppmv.

Regeneration

At the end of the adsorption process, when the working capacity of an adsorber is exhausted, it has to be regenerated. The conventional regeneration methods for this task are classified as: 1) thermal swing desorption, 2) pressure swing desorption, 3) elution desorption, and 4) inert purging desorption.

In a thermal swing system, desorption takes place by raising the bed temperature. The bed temperature is raised by passing a hot inert gas through the adsorber or heating the bed by internal or external heating elements. If hot inert gas is used, it is commonly generated from combustion of fuel. In a pressure swing system, desorption takes place by providing a suitable pressure differential between the adsorption and desorption processes. If, for example, the adsorption pressure is atmospheric, vacuum is applied to regenerate the bed. The vacuum pressure is usually generated by refrigeration. In elution desorption, the adsorbate is replaced by an eluate that has more affinity for adsorption. In inert purging desorption, an inert gas is sent through the adsorber and desorption takes place under a mass transfer controlling condition.

Combinations of these methods are also used. One example is thermal swing desorption using saturated low pressure steam. In this process, steam in direct contact with the adsorbent is condensed and replaces the adsorbate molecules in the pores by adsorption or capillary condensation. This elution of water vapor is an exothermic process that provides additional heating. The combination of these three effects provides a higher desorption rate than those of the thermal swing methods that use an inert hot gas.

Each of these systems have their own advantages and disadvantages. In direct steam regeneration, working capacity is reduced due to water vapor adsorption, capillary condensation and carbon oxidation. This reduction in working capacity is called heel. The requirement for drying after steam regeneration is another disadvantage of steam regeneration. Furthermore steam regeneration can contribute to polymerization reactions on the surface of carbon adsorbents and cause a smoldering fire. This type of reaction is usually caused by the breakdown of ketones on the carbon surface in the presence of steam, air or transitional metals (Mclnnes, 1995). This mostly happens for higher molecular weight VOCs that require higher steam temperature for desorption. Other disadvantages of steam regeneration are requirements of a post distillation process for separation of the liquid adsorbates from water.

An advantage of a pressure swing or vacuum regeneration is that it cools down the bed instead of heating it up. Disadvantages of vacuum regeneration are requirement of complicated equipment and stronger mechanical design to prevent material collapsing under vacuum.

In this project, electrothermal desorption is used to regenerate the adsorption capacity of adsorbent. Electrothermal desorption is a relatively new method of regeneration that is not yet fully utilized in adsorption systems. In the electrothermal regeneration process, an electric current is passed through the carbon adsorbent. Electrical work due to phonon and defect scattering (Donnet and Bansal, 1990) in activated carbon is directly transformed to thermal energy in the adsorbent and the adsorbed TVOC. By the continuous flow of electric current, the thermal energy of the adsorbed molecules increases to a level that overcomes the surface bonding energy, and the TVOC desorbs from the ACFC. Since electrical work is transformed to desorption energy directly, the carrier gas temperature can be substantially lower than the ACFC temperature.

During electrothermal desorption, the temperature gradient along the radius of adsorbent is negative (or zero, depending on the Biot number) as opposed to conventional thermal desorption methods where the temperature gradient along the adsorbent radius is positive. This negative gradient causes positive contributions to the rate of desorption from heat transfer, the Soret effect and pore effusion. In contrast, these mass transfer contributions are negative for conventional thermal desorption methods. Therefore, electrothermal regeneration should have higher energy efficiency compared to conventional thermal regeneration methods. Another advantage of electrothermal desorption is that the energy transfer rate can be very high and can be controlled easily. This enables a careful control of desorption time and a regenerated TVOC concentration profile for a better cryogenic recovery.

Condensation

Condensation can be used to remove unwanted vapors from gas streams. These constituents can often times be re-used after distillation or can be handled more easily as a liquid for ultimate disposal (Ruhl, 1993). Recovery of vapors is dependent on the type of condensation system selected. Various condenser designs and refrigerants are available for implementation (USEPA^b, 1991).

Condensation Principles

Two general methods of condensation are to increase system pressure or to reduce system temperature (USEPA^c, 1991). Reduction of the gas stream temperature to remove VOCs is more common than direct gas compression (Wilbur, 1985). Condensation by temperature reduction occurs when the temperature of a gas stream is lowered below the saturation concentration temperature of one or more of the vapors in the gas mixture. Further condensation of the vapors will occur as the temperature is reduced. The amount condensed and the gas phase concentration is dependent on the vapors saturation concentrations at the given temperature.

The dependence of the vapor concentration on temperature can be examined using the semi-empirical Wagner equation, given as (Reid et al., 1977):

where

P_VP = Vapor Pressure (bar)

P_c = Critical Pressure (bar)

T_c = Critical Temperature (K)

T = Temperature (K)

VP_A, VP_B, VP_C, & VP_D = Experimental Constants

Using reported experimental values (Table 3.1), the saturation vapor concentration as a function of temperature is evaluated in Figure 3.3 for three commonly emitted TVOCs: acetone, toluene and MEK.

Condensation

Condensation can be used to remove unwanted vapors from gas streams. These constituents can often times be re-used after distillation or can be handled more easily as a liquid for ultimate disposal (Ruhl, 1993). Recovery of vapors is dependent on the type of condensation system selected. Various condenser designs and refrigerants are available for implementation (USEPA^b, 1991).

Condensation Principles

Two general methods of condensation are to increase system pressure or to reduce system temperature (USEPA^c, 1991). Reduction of the gas stream temperature to remove VOCs is more common than direct gas compression (Wilbur, 1985). Condensation by temperature reduction occurs when the temperature of a gas stream is lowered below the saturation concentration temperature of one or more of the vapors in the gas mixture. Further condensation of the vapors will occur as the temperature is reduced. The amount condensed and the gas phase concentration is dependent on the vapors saturation concentrations at the given temperature.

The dependence of the vapor concentration on temperature can be examined using the semi-empirical Wagner equation, given as (Reid et al., 1977):

where

P_VP = Vapor Pressure (bar)

P_c = Critical Pressure (bar)

T_c = Critical Temperature (K)

T = Temperature (K)

VP_A, VP_B, VP_C, & VP_D = Experimental Constants

Using reported experimental values (Table 3.1), the saturation vapor concentration as a function of temperature is evaluated in Figure 3.3 for three commonly emitted TVOCs: acetone, toluene and MEK.

Condensation

Condensation can be used to remove unwanted vapors from gas streams. These constituents can often times be re-used after distillation or can be handled more easily as a liquid for ultimate disposal (Ruhl, 1993). Recovery of vapors is dependent on the type of condensation system selected. Various condenser designs and refrigerants are available for implementation (USEPA^b, 1991).

Condensation Principles

Two general methods of condensation are to increase system pressure or to reduce system temperature (USEPA^c, 1991). Reduction of the gas stream temperature to remove VOCs is more common than direct gas compression (Wilbur, 1985). Condensation by temperature reduction occurs when the temperature of a gas stream is lowered below the saturation concentration temperature of one or more of the vapors in the gas mixture. Further condensation of the vapors will occur as the temperature is reduced. The amount condensed and the gas phase concentration is dependent on the vapors saturation concentrations at the given temperature.

The dependence of the vapor concentration on temperature can be examined using the semi-empirical Wagner equation, given as (Reid et al., 1977):

where

P_VP = Vapor Pressure (bar)

P_c = Critical Pressure (bar)

T_c = Critical Temperature (K)

T = Temperature (K)

VP_A, VP_B, VP_C, & VP_D = Experimental Constants

Using reported experimental values (Table 3.1), the saturation vapor concentration as a function of temperature is evaluated in Figure 3.3 for three commonly emitted TVOCs: acetone, toluene and MEK.

Dependence of acetone’s and toluene’s saturation vapor concentrations on temperature that were observed from the experiments (Figure 3.3; Vargaftik, 1975) are consistent with the calculated vapor pressures from the Wagner equation (Table 3.4.)

At equilibrium, the concentration of vapor at a given temperature can be determined from eq. (3.2). Also, by knowing the initial concentration of vapor and the final temperature (thus equilibrium saturation concentration), the amount of vapor condensed can be determined assuming equilibrium conditions at the condenser outlet.

Heat Exchange Condenser Principles

Heat exchange condensers operate on the principle of lowering the temperature of a gas stream containing a condensable gas to a temperature corresponding to a saturation vapor concentration below the entering saturation vapor concentration. The equilibrium saturation vapor concentration is dependent on the temperature of the condenser gas stream. Lowering the temperature of the condenser will result in lower effluent vapor concentrations as discussed above (Figure 3.3). The temperature of the vapor laden gas stream is dependent on the transfer of heat from the refrigerant to the vapor.

Two general types of heat exchange condensers are commercially available, direct contact and indirect contact (Buonicore and Davis, 1992). The heat exchange condensation process can also be enhanced by compressing the gas stream in tandem with temperature reduction (USEPA^b,c , 1991). Gas stream compression for VOC condensation is not as widely utilized either individually or combined with direct and indirect contact methods (Buonicore and Davis, 1992).

Direct contact condensers mix the refrigerant with the process gas stream. Heat is more efficiently exchanged due to the intimate contact between the refrigerant and VOC. Direct contact condensation is typically simpler, less expensive to install and requires less auxiliary equipment (Buonicore and Davis, 1992). However, the refrigerant is mixed with the process stream. This may prevent refrigerant recycling and/or cause contamination of the refrigerant.

Indirect contact condensers utilize a physical barrier across which only heat is exchanged between the refrigerant and the process gas stream. Heat exchange is therefore less efficient in indirect methods. Keeping the refrigerant separate from the process gas stream allows for refrigerant re-use. This is beneficial if the refrigerant undergoes a cyclic mechanical refrigeration process. Indirect contact condensers typically cost more and are more complicated to design and operate (Buonicore and Davis, 1992). Because of the indirect contactors advantages, however, it is the most common type of condenser in air pollution control applications (USEPA^b, 1991).

Dependence of acetone’s and toluene’s saturation vapor concentrations on temperature that were observed from the experiments (Figure 3.3; Vargaftik, 1975) are consistent with the calculated vapor pressures from the Wagner equation (Table 3.4.) At equilibrium, the concentration of vapor at a given temperature can be determined from eq. (3.2). Also, by knowing the initial concentration of vapor and the final temperature (thus equilibrium saturation concentration), the amount of vapor condensed can be determined assuming equilibrium conditions at the condenser outlet.

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