Adsorption isotherms, countercurrent

Activation energy, stability in trickle-bed reactors, 76 Activation overpotential, cross-flow monolith fuel cell reactor, 182 Activity balance, deactivation of non-adiabatic packed-bed reactors, 394 Adiabatic reactors stability, 337-58 trickle-bed, safe operation, 61-81 Adsorption equilibrium, countercurrent moving-bed catalytic reactor, 273 Adsorption isotherms, countercurrent moving-bed catalytic reactor, 278,279f... 

A countercurrent moving-bed adsorption column is used to remove benzene from a gaseous emission. Activated carbon is employed as the adsorbent. The flowrate of the gas is 1.2 kg/s and it contains 0.027 wt/wt% of benzene. It is desired to recover 99% of this pollutant. The activated carbon entering the column has 2 X 10 wt/wt% of benzene. Over the operating range, the adsorption isotherm (Yaws et al., 1995) is linearized to... 

Periodic countercurrent systems are widely used in ion exchange and water purification systems for the removal of trace concentrations of organic components with beds of activated carbon.At low sorbate concentrations the adsorption isotherms for many organic pollutants are linear or only slightly favorable so that the mass transfer zone is wide, and in a simple two-bed cyclic batch system the LUB would therefore be uneconomically large. For such systems, in order to obtain an economic process, some form of counter-current operation is necessary in order to reduce the adsorbent inventory. 

Hassan, M.M. Rahman, A.K.M.S., and Loughlin, K.F., Numerical simulation of unsteady continuous countercurrent adsorption system with nonlinear adsorption isotherm, Sep. Technol., 4(1), 15-26 (1994). 

We wish to estimate the height of a continuous countercurrent isothermal adsorber for drying air at 26.7 C, standard atmospheric pressure, from an initial humidity 0.003 to a final humidity 0.0001 kg H20/kg dry air. The entering gel will be dry. Note-. So-called "dry silica gel must contain a minimum of about 5% water if it is to retain its adsorptive capacity. Moisture measurements as ordinarily reported do not include this.) A gel rate 0.680 kg/m s (500 Ib/ft h) and an air rate 1.36 kg/m s (1000 Ib/ft h) will be used. For the moisture contents expected here, the equilibrium adsorption isotherm at 26.7 C, std atm (see Illustration 11.9), can be taken as substantially straight and described by the expression Y 0.018SJf. 

Nitrogen dioxide, N02, produced by a thermal process for fixation of nitrogen, is to be removed from a dilute mixture with air by adsorption on silica gel in a continuous countercurrent adsorber. The mass flow rate of the gas entering the adsorber is 0.50 kg/s it contains 1.5% N02 by volume, and 85% of the N02 is to be removed. Operation is to be isothermal at 298 K and 1 atm. The entering gel will be free of N02. If twice the minimum gel rate is to be used, calculate the gel mass flow rate and the composition of the gel leaving the process. The equilibrium adsorption data at this temperature are (Treybal, 1980) ... 

There is complete formal analogy between continuous countercurrent adsorption and other continuous countercurrent operations such as distillation or gas absorption. Such processes are normally operated at steady state and, for an isothermal system containing only a single adsorbable species, the analysis is straightforward. 

Klaus, R. Aiken, R.C., and Rippin, D.W.T., Simulated binary isothermal adsorption on activated carbon in periodic countercurrent column operation, AlChE J., 23(4), 579-586 (1977). 

As another example, the case of air drying was calculated and compared with the complex calculation result obtained by the method introduced in the previous section (Chihara and Suzuki, I9 3a). The case of isothermal operation (Fig. 11.6, solid and open circles) is compared with the results of the continuous countercurrent flow model (solid and broken lines) in Fig. 11.14. Rigorous calculation was done for a throughput ratio of 0.01 and the change in the profiles of the amount adsorbed after adsorption and desorption steps found to be reasonably small. Thus the simple model simulated quite well the cyclic steady state profile of PSA. 

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