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DESIGN AND DEVELOPMENT OF THE SYSTEM

A flow diagram of the ACFC adsorption/electrothermal regeneration/LN₂ condensation system for TVOC recovery is given in Figure 3.1. The system can utilize one or more fixed-beds. The system can be classified as an electrothermal-swing adsorption (ETSA) process. The industrial TVOC effluent passes through an ACFC fixed-bed where ACFC separates the TVOC by adsorption. The bed exhaust (cleaned gas) is then recycled for other process needs or vented to the atmosphere. After breakthrough of TVOC from the fixed bed, the untreated TVOC stream can be directed to the second bed for a continuous process operation while the exhausted bed is switched to a regeneration step. During the regeneration process an order of magnitude lower flow rate pure N₂ gas is passed through the adsorption bed and electrical power is supplied to the ACFC. Electrothermal energy regenerates the ACFC and provides a N₂ gas stream containing a concentrated desorbed TVOC.

TVOC concentration in the N₂ carrier gas is controlled by the amount of electrical power supplied and the flow rate and temperature of the carrier gas. The concentrated vapor stream is then directed to a LN₂ cryogenic condenser where the TVOC is condensed and separated from the noncondensible carrier gas. The cold nitrogen gas from the condenser exhaust can be used to cool the adsorber after the desorption step, and then it can be used for other process needs.

Adsorption

Physical adsorption is a process in which attraction force fields at the solid surfaces pull molecules or ions from the gas phase and bind them reversibly to the surface. The strength of adsorption forces depend on the nature and state of the solid and of the individual gas molecules. The difference in strength of interaction between a solid surface and different gas molecules is the basis of adsorption separation. Intimate binding by physical adsorption is most pronounced in a monomolecular layer at the solid surface, although the adsorption layer can persist to three or four molecular layers at some locations of the surface (Ruthven, 1984). The thickness of the adsorption layer depends on the strength of the local force field, the nature of the adsorbate, and the gas phase concentration of adsorbate. In general, the amount of adsorbate that can be attracted by an adsorbent is a function of available surface area and the strength of the force fields at the surface. Therefore, increasing the surface area in combination with increasing the attraction force fields increases the adsorption capacity. One way to increase the force fields is by generating small pores in the adsorbent materials. The overlap of the force fields from the opposite pore walls strongly enhances the adsorption in the micropores. Another significance of small pores is that condensable vapors can be liquefied in sufficiently narrow pores at a pressure lower than the saturation pressure of the vapors. This mechanism of binding vapors in pores is called capillary condensation.

The requirement for an adequate adsorptive capacity restricts the choice of adsorbents for practical separation processes to microporous materials with pore diameters ranging from a few Angstroms to a few tens of an Angstrom (Ruthven, 1984). Discovery of naturally microporous adsorbents, such as zeolite, and development of the new adsorbents, such as activated carbon and silica gel, provided the opportunity to efficiently utilize adsorption phenomena in industrial separation processes. Some industrial separation processes that have successfully utilized physical adsorption phenomenon are mentioned in Table 3.1.

Industrial adsorption systems are classified either by their method of regeneration or by their type and configuration of adsorber. By regeneration method, they are either thermal-swing adsorber (TSA), pressure-swing adsorber (PSA), purge-swing, and elution systems. Classified by the type of adsorber, they are either fluidized-beds, moving-beds, or fixed-beds. Fluidized beds are mainly used when waste hot gas is available for use during regeneration. In fluidized beds, the adsorbent is contacted counter-currently with the gas stream on perforated trays in relatively shallow beds. The gas uniformly distributes over the bottom cross section of the bed. Due to momentum transfer, the bed expands and the solid particles move freely and circulate through two adsorption and desorption sections. In moving-beds, both adsorbent and gas mixture move through the adsorber in a continuous manner. The adsorbent is moved from an adsorption chamber to a regeneration chamber. Numerous moving-bed adsorbers have been designed and built, but relatively few are in large-scale use (Vermeulen, 1975). Combination of fluidized-bed adsorption and moving-bed desorption has also been used. However, both fluidized-and-moving beds are unpopular for their complexity of design, attrition of adsorbent particles that can lead to excessive adsorbent losses, and operation difficulties.

Unlike moving-and-fluidized beds, fixed-bed systems are very simple in design and operation, having relatively few moving parts. In the fixed-beds, the gas mixture passes through a stationary bed of adsorbent material. Figure 3.2 shows a schematic illustration of a typical fixed-bed and the method of flow distribution through it.

The most common problem of conventional fixed-beds is channeling. In channeling, some of the adsorbate passes through open channels in the bed without coming into contact with adsorbent. This can happen, for example, with bed leakage. Even small leaks produce channeling in the bed. Channeling reduces the adsorption efficiency rapidly and severely.

Usually a typical processing system uses more than one fixed-bed adsorber. The gas mixture passes through some of the fixed-beds while the others are being regenerated. A typical TSA fixed-bed adsorption system for solvent vapor recovery that consists of three adsorbers in parallel is also shown in Figure 3.2. One bed will undergo adsorption, one regeneration and one will cool after it is regenerated. The number of adsorbers are selected based on optimization of overall process operation and cost benefits. However, the most common processing scheme is a pair of adsorbers alternating between the adsorption step and the regeneration step. In situations where the effluent stream is discharged intermittently, it is possible to use one fixed-bed. However, in general, one fixed-bed provides interrupted flow while multiple beds can ensure continuous flow for the vapor recovery unit.

As an adsorbate laden gas stream passes through a fixed-bed, it is selectively adsorbed. This causes a concentration change at the solid phase and in the gas phase. At the beginning, the highest mass transfer takes place at the locations where the fluid first contacts the adsorbent. Thus the gas concentration drops exponentially from the inlet to the end of the bed. As time passes, the adsorbent at the bed inlet becomes saturated and the mass transfer zone (MTZ) moves from the inlet toward the end of the bed. By definition, the MTZ refers to the region in which the adsorbate gas concentration changes from 95% to 5% of its inlet value. This is the region where most of the mass transfer takes place. With respect to time, a S-shaped concentration profile develops for the MTZ which travels down the bed. The sharpness of the MTZ primarily depends on the adsorption capacity and shape of the equilibrium isotherms, the parameters that affect transport processes in the bed (such as velocity, size of adsorbent particles, packing density and diffusion resistances) and secondary effects from transport processes within the adsorbent particles. In order to utilize the adsorbent efficiently and reduce the amount of energy during regeneration, the MTZ should be narrow.

The concentration of contaminant at the outlet of the adsorber is negligible from the beginning of the process to the time that the MTZ reaches the outlet. From then on, the concentration level at the outlet rises until it reaches the inlet concentration value. A plot of the outlet concentration with respect to time is called the breakthrough curve (BTC). The break point or breakthrough time is often used to refer to the time at which the outlet concentration reaches a specific permissible value. For this project, this value is taken to be 5% of the inlet concentration. After the break point, the concentration at the bed outlet rises rapidly to about 50% and then it approaches 100% more slowly. The breakthrough curve takes a S-shaped form for a favorable isotherm with the symmetric point at the 50% concentration level. For a symmetric concentration profile, it can be shown that the adsorbed amount from the start to the completion of breakthrough is equal to the amount of adsorbate that has been passed through the bed until the time of 50% outlet concentration. Furthermore, this amount would be very close to the value adsorbed if the entire bed was at equilibrium with gas inlet concentration in a quiescent condition. Due to this property, the time of 50% concentration level is called stoichiometric time. In the hypothetical situation of a bed outlet concentration suddenly jumping from the zero value to the inlet value, stoichiometric time would designate the breakthrough time, and it could be estimated by the following relation where

M_a = total mass of adsorbent, (g)

W_c = working capacity of bed = (W_e-W_o), (cm³/g)

W_e = equilibrium capacity, (cm³/g)

W_o = initial retained capacity, (cm³/g)

U_s = superficial gas velocity, (cm/s)

C_i = inlet gas concentration, (cm³/cm³)

A = bed intersection area, (cm²)

In realistic situations, breakpoint time t_b is shorter than t_s, recognizing that the breakthrough curve will not be a step function due to mass transfer limitations. Given a set break point concentration, the shorter the time interval between t_b and t_s, the sharper the breakthrough curve and the higher the bed capacity would be at breakpoint. The shape and sharpness of the breakthrough curve for a given adsorbent depend on the type of equilibrium isotherm, the mass and heat transfer rates, and hydrodynamic factors such as mixing and contact time.

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.

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