Adsorbates, breakthrough

FIGURE 1. GAC adsorber breakthrough curves of atrazine (filled symbols with dashed lines) and NOM (open symbols) dissolved in lake water (initial concentrations DOC = 12 mg/L, atrazine = 1 mg/L). 

Figure 10.65. Fixed-bed adsorber breakthrough curve of NO in mixed, equimolar oxides at 25°C. Feed conditions [NO] = 500 ppm, [O2] = 10%, He = balance and GHSV = 6000 h (Huang and Yang, 2001 with permission).

Cen, P.L., and Yang, R.T., Analytic solution for adsorber breakthrough curves with bidisperse sorbents (zeolites), AIChEJ., 32(10), 1635-1641 (1986). 

Cooney, D.O., External film and particle phase control of adsorber breakthrough behavior, AlChE J., 36(9). 1430-1432(1990). 

Cooney, D.O., Comparison of simple adsorber breakthrough curve method with exact solution, AIChE J., 39(2), 355-358 (1993). 

FIG. 11 Fixed-bed adsorber breakthrough curve ( ) experimental data (—) model prediction. 

Figure 5.6 (a) shows the breakthrough curve for a single adsorbate from a fixed bed of adsorbent. Breakthrough is deemed to commence at a time fb when the concentration of the adsorbate at the end of the bed increases beyond a certain level, Ce. This may be the level of detection for the adsorbate or it may be the maximum allowable concentration for admission to downstream process units such as catalytic reactors. As breakthrough continues the concentration of the adsorbate in the effluent increases gradually up to the feed value Co. When this has occurred no more adsorption can take place in the adsorption bed. The concentration of the adsorbate on the adsorbent will then be related to the concentration of the adsorbate in the feed by the thermodynamic equilibrium. 

Adsorption Dynamics. An outline of approaches that have been taken to model mass-transfer rates in adsorbents has been given (see Adsorption). Detailed reviews of the extensive Hterature on the interrelated topics of modeling of mass-transfer rate processes in fixed-bed adsorbers, bed concentration profiles, and breakthrough curves include references 16 and 26. The related simple design concepts of WES, WUB, and LUB for constant-pattern adsorption are discussed later. 

Most dynamic adsorption data are obtained in the form of outlet concentrations as a function of time as shown in Figure 18a. The area iebai measures the removal of the adsorbate, as would the stoichiometric area idcai, and is used to calculate equiUbrium loading. For constant pattern adsorption, the breakthrough time and the stoichiometric time ( g), are used to calculate LUB as (1 — (107). This LUB concept is commonly used... 

Mass Transfer and Useful Capacity. The term useful capacity, also referred to earlier as breakthrough capacity, differs from the equihbrium capacity shown on Figures 9a and 9b. The useful capacity is a measure of the total moisture taken up by a packed bed of adsorbent at the point where moisture begins to appear in the effluent. Thus the drying process cycle must be stopped before the adsorbent is fully saturated. The portion of the bed that is not saturated to an equihbrium level is called the mass-transfer 2one. 

The reactor in Fig. 5 operates as follows. A feed solution containing a given concentration of pollutant is pumped to the adsorbent module at a fixed volumetric flow rate. The module is kept isothermal by a temperature control unit, such as a surrounding water bath. Finally, the concentration of the outlet solution is measured as a function of time from when the feed was introduced to the adsorbent module. These measurements are often plotted as breakthrough curves. Example breakthrough curves for an aqueous acetone solution flowing... 

A typical HjS/H, breakthrough plot is shown in Fig. 21 for a gas composition of 5.4% HjS, 14% Ar, with the balance bemg Hj. The Hj (not shown in Fig. 21) is not adsorbed, whereas the HjS is held on the carbon, producing a HjS free H, stream for approximately 18 minutes. In Fig. 21 the HiS concentration can be seen to increase sharply after breakthrough is completed The concentration mcrease... 

Although a proper cycling procedure has been used between alternate beds, an unexpected contamination of the adsorbent would cause a premature breakthrough of the beds resulting in the release of contaminants. A routine shutdown would normally only involve the shut off of the gas flow from the process. 

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

Fixed-bed adsorption may give a higher adsoiption area per unit volume than any other type of adsorber. The point of saturation of the bed is called the breakthrough point. By knowing this point one can determine operation schedules. In designing fixed-bed adsorbers, the... 

Although MIL-47, and especially MIL-53(A1), had been found on many occasions to dynamically respond to adsorption of particular compounds, referred to as breathing [35] in the literature, in these liquid phase conditions, only minor changes of the lattice parameters have been observed. A study of xylene separations in vapor phase on MIL-5 3(A1) shows that breathing profoundly influences the shape of the obtained breakthrough profiles as a function of adsorbate concentration [97]. 

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