Adsorption break point

As can be understood from Figure 11.5, the amount of adsorbate lost in the effluent and the extent of the adsorption capacity of the fixed bed utilized at the break point depend on the shape of the breakthrough curve and on the selected break point. In most cases, the time required from the start of feeding to the break point is a sufficient index of the performance of a fixed-bed adsorber. A simplified method to predict the break time is discussed in the following section. 

The time required from the start of feeding to the break point can be estimated with the assumption of the constant pattern stated above. Thus, substitution of Equation 11.10 into Equation 11.6 gives the following equation for the rate of adsorption ... 

As shown in Figure 11.10, the operations in AFC are regarded as highly specific adsorption and desorption steps. Thus, the overall performance is much affected by the break point, an estimation of which can be made as described in Section 11.5.2. 

When the height of the adsorbent bed is 50 cm, under the same operating conditions given in Example 11.2, estimate the break point (C =0.1 C g) and the length of the adsorption zone z. ... 

In the downstream processing of bioprocesses, fixed-bed adsorbers are used extensively both for the recovery of a target and for the removal of contaminants. Moreover, their performance can be estimated from the breakthrough curve, as stated in Chapter 11. The break time tg is given by Equation 11.13, and the extent of the adsorption capacity of the fixed bed utilized at the break point and loss of adsorbate can be calculated from the break time and the adsorption equilibrium. Affinity chromatography, as weii as some ion-exchange chromatography, are operated as specific adsorption and desorption steps, and the overall performance is affected by the column capacity available at the break point and the total operation time. 

Calculate the fractional residual capacity at the break point for the fixed-bed of Example 11.2. The fractional residual capacity of the adsorption zone can be approximated as 0.5. 

Two-thirds of the adsorption capacity is not utihzed at the break point in this case. 

One simple way to analyze the performance of a fixed-bed adsorber is to prepare a breakthrough curve (Figure 10.12) by measuring the solute concentration of the effluent as a function of time. As the solution enters the column, most of the solute will be adsorbed in the uppermost layer of solid. The adsorption front will move downward as the adsorption progresses. The solute concentration of the effluent will be virtually free of solute until the adsorption front reaches the bottom of the bed, and then the concentration will start to rise sharply. At this point (tb in Figure 10.12), known as the break point, the whole adsorbent is saturated... 

Before examining the NMR, it is useful to consider some adsorption and desorption studies of this standard catalyst. Hydrogen chemisorption data have been modeled by a Langmuir isotherm for pressures between 1 and 80 Torr (Fig. 26a) (80) but also by a Temkin isotherm at pressures between 0.1 and 100 Torr (Fig. 26b) (48). The Langmuir isotherms in Fig. 26a give H/Pt ratios for monolayer adsorption of 1.17 (circles) and 1.06 (triangles). The break point in the Temkin isotherm of Fig. 26b corresponds to H/Pt = 0.82 (48). (The part of the isotherm below 0.1 Torr has not often... 

The break-point temperature in dehydration (above which the rate was temperature insensitive) matched the maximum temperature for dehydrogenation, suggesting that a common intermediate exists for each reaction, and that the product selectivity is determined by interactions with other molecules and the surface. Above 650 K, the catalytic dehydration channel dominates, but the rate-determining step changes above 700 K. Below 700 K, the reaction rate is nearly independent of the partial pressure of formic acid (ca. 0.2 order). Above 700 K, the rate of the reaction is essentially independent of temperature, implying that reaction is limited by formic acid adsorption and dissociation thus, above 700 K, the rate becomes first-order with respect to the partial pressure of formic acid. Higher pressures of formic acid over the crystal surface should therefore increase the transition temperature - this behavior was observed by Iwasawa and coworkers, and the turnover frequency for catalytic dehydration approached the collision frequency of formic acid at high... 

Another important point can be made from the y-log C curves. At a concentration just before the break point there is a condition of constant slope, which indicates that saturation adsorption has been reached ... 

The packed density of the bed, the void fraction of the particle bed, and the density of the feed solution are 386 kgm-3, 0.5 and 1000 kgm-3, respectively. The averaged overall volumetric coefficient of mass transfer KLa is 9.2 h-1, and a constant pattern of the adsorption zone can be assumed in this case. Estimate the break point at which the concentration of A in the effluent becomes 0.1 CA0. 

Coull, Engel, and Miller17 have developed a method in which an organic vapor-air mixture of any desired composition can be passed through the carbon bed. The exit gases are analyzed continuously by a thermal-conductivity, cell, the accuracy of which has been confirmed by gravimetric methods. The resulting data allow a ready evaluation of the performance of the carbon and yield information as to the adsorption efficiency after the break point is reached. 

If adsorption were continued beyond the break point, the concentration would rise rapidly to about 0.5 and then more slowly approach 1.0, as shown in Fig. 25.56. This S-shaped curve is similar to those for the internal concentration profiles, and it is often nearly symmetrical. By material balance, it can be shown that the area between the curve and a line at c/cq = 1.0 is proportional to the total solute adsorbed if the entire bed comes to equilibrium with the feed. The amount adsorbed is also proportional to the rectangular area to the left of the dashed line at (, the ideal adsorption time for a vertical breakthrough curve. For... 

FAVORABtE ADSORPTION. For favorable adsorption, the break point occurs between the values predicted for linear adsorption and irreversible adsorption. Solutions are available for certain isotherm shapes and different values of internal and external resistances. These solutions have found use for the design of ion exchangers, where the sohd-fiuid equilibria and the internal diffusivities are more readily characterized than for adsorption. 

Adsorption on activated carbon is a very attractive technique to reduce emissions of VOCs from industrial facilities. Activated carbon has a very high affinity for most VOCs, achieving virtually 100% removal efficiency up to the break point for adsorption on a fixed bed. Not only emissions of the VOCs are avoided, but the adsorbed substances can be recovered and recycled by thermal desorption of the bed. 

Consider the following application of fixed-bed, activated carbon adsorption for the control of VOC emissions. An industrial waste gas consists of 0.5 vol% acetone in air at 300 K and 1 atm. It flows at the rate of 2.3 kg/s through a fixed bed packed with activated carbon. The bed has a cross-sectional area of 5.0 m2 and is packed to a depth of 0.3 m. The external porosity of the bed is 40%, its bulk density is 630 kg/m3, and the average particle size is 6 mm. The average pore size of the activated carbon particles is 20 A, the internal porosity is 60%, and the tortuosity factor is 4.0. A Langmuir-type adsorption isotherm applies with qm = 0.378 kg VOC/kg of carbon, K = 0.867 kPa-1. At the break point, the effluent concentration will be 5% of the feed concentration. Calculate ... 

The second critical ratio corresponds to the disappearance of the long-range connectivity when the free non-perturbed silica surface no longer forms the major component of the silylated silica surface. It can only be detected using a population of molecular probes, as in IGC-FC. Indeed, IGC-FC evidences clearly this second transition a new peak appears, at low energy, in the distribution function of Si2 adsorption energies and, moreover, a break point is observed in the evolution of the BET constant with the TMS coverage ratio. 

Intensive physical and chemical treatment, extended treatment and disinfection e.g. chlorination to break-point, coagulation, flocculation, decantation, filtration, adsorption (activated carbon), disinfection (ozone, final chlorination). 

The effect on the breakthrough capacity of the column is greatest when ij is negative (CfAq/C Ac > 1.0). Under these conditions the natural velocity of the thermal front is higher than the natural velocity of the concentration front so that the temperature profile leads the concentration profile, thus affecting the initial break point. This is by far the commonest situation in adsorption from the vapor phase and it may be seen that a significant loss of dynamic capacity can result from small departures from isothermal behavior. 

As seen in Fig. 12.3-la, the major part of the adsorption at any time takes place in a relatively narrow adsorption or mass-transfer zone. As the solution continues to flow, this mass-transfer zone, which is S-shaped, moves down the column. At a given time /3 in Fig. 12.3-la when almost half of the bed is saturated with solute, the outlet concentration is still approximately zero, as shown in Fig. 12.3-lb. This outlet concentration remains near zero until the mass-transfer zone starts to reach the tower outlet at time t. Then the outlet concentration starts to rise and at the outlet concentration has risen to Cj, which is called the break point. 

EXAMPLE 12.3-1. Scale-Up of Laboratory Adsorption Column A waste stream of alcohol vapor in air from a process was adsorbed by activated carbon particles in a packed bed having a diameter of 4 cm and length of 14 cm containing 79.2 g of carbon. The inlet gas stream having a concentration of 600 ppm and a density of 0.00115 g/cm entered the bed at a flow rate of 754 cm /s. Data in Table 12.3-1 give the concentrations of the breakthrough curve. The break-point concentration is set at dCg = 0.01. Do as follows. 

Scale-Up of Laboratory Adsorption Column Data. Using the break-point time... 

Adsorption Amount at Break Point Theoretical Maximum Capacity... 

Breakthrough Curve-Bed Depth Service Time (BUST) Model. In the operation of a fixed-bed adsorption column, the service time, t, of the bed can be related to the bed depth, Z, for a given set of conditions by a model and equation called the bed depth service time model (BDST). The BDST offers a rapid method of designing fixed-bed columns. The influent solute concentration, Cq, is fed to the column, and it is desired to reduce the solute concentration in the effluent to a value not exceeding Cj. At the beginning of the operation, when the adsorbent is still fresh, the effluent concentration is actually lower than the allowable concentration, Cj, but, as the operation proceeds and the sorbent reaches saturation, the effluent concentration reaches Cj. This condition is called the break point. 

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