Continuous-contact adsorption

Adsorption can be carried out by stagewise or continuous-contacting methods. The stagewise operation of adsorption is called contact filtration because the liquid and the solid are contacted in a mixer and then the solid is separated from the solution by filtration. 

In an isolation step, where yield and concentration are more important than purity, the adsorption mechanism can be considered an on/off process, and several alternative contacting schemes can be used. Ligands have been bound to magnetized particles (137, 138) for continuous countercurrent adsorption in magnetically stabilized fluidized beds. Ligands attached to liquid perfluorocarbons (143), to dextran and related polymers (144), or incorporated into liposomes (145), or reversed micelles (146) may be used for biospecific liquid-liquid extraction or "affinity partitioning". Ligands have also been attached to surfactants and biopolymers for selective precipitation of dilute protein species (147, 148). 

The forced migration of a liquid phase in continuous contact with a stationary phase is encountered in several chromatographic techniques. One of the aspects particular to HPLC is that of the partition mechanisms between analyte, mobile phase and stationary phase. They are based on coefficients of adsorption or partition. 

The adsorption of water vapours on preliminary dehydrated aerosil surface results in the appearing in IR-spectrum firstly a band of 3550 cm then after continuous contact with water vapors at room temperature an absorption is observed near 3660-3680 cm [84, 85]. When explaining this experimental fact, the authors of work [94] introduced a notion on resistance against rehydration and assumed that water molecules diffused into the sample bulk and then interacted with siloxane bonds and became low-movable. The windows can exist on initial silica surface formed by six tetrahedra combination where a penetration is possible of water molecules [93,95,96]. Nevertheless, as it is noted in [81], the questions remain unclear about the nature of the forces pressing H2O molecules to diffuse into solid as well as stabilized forms of these molecules. 

Large-scale adsorptive separation processes may be conveniently divided into two broad classes cyclic batch systems, in which the adsorbent bed is alternately saturated and regenerated in a cyclic manner, and continuous flow systems, generally involving continuous countercurrent contact between feed and adsorbent. The distinction between these two basic modes of operation is shown schematically in Figure 11.1. The present chapter is restricted to processes which operate in the cyclic mode while continuous contacting systems are discussed in Chapter 12. 

The next topics of interest are the operating line for the countercurrent adsorption device and the height of the device or the number of stages needed for the device to achieve a given separation. The device may be of continuous contact type or stagewise contact type. For continuous contact type devices, the approach will follow that in Section 8.1.2.3 employed for gas absorption from a concentrated gas stream. We focus on one particular species i (= A) which is being adsorbed, say, from a gas stream j = g, here the inert (solute-free) gas stream mass flow rate... 

In Figure 8.1.44(a), a countercurrent adsorption device has an upflowing gas stream and a downflowing adsorbent stream in continuous contact Adopting the approach of Section 8.1.1.2, the following solute i balance may be developed between any location z and the column top (z = t) ... 

Industrial-scale adsorption processes can be classified as batch or continuous (53,54). In a batch process, the adsorbent bed is saturated and regenerated in a cychc operation. In a continuous process, a countercurrent staged contact between the adsorbent and the feed and desorbent is estabhshed by either a tme or a simulated recirculation of the adsorbent. 

Continuous Countercurrent Systems Most adsorption systems use fixed-bed adsorbers. However, if the fluid to be separated and that used for desorption can be countercurrently contacted by a moving bed of the adsorbent, there are significant efficiencies to be realized. Because the adsorbent leaves the adsorption section essentially in equilibrium with the feed composition, the inefficiency of the... 

Adsorption-Control Equipment If a gas stream must be treated for a short period, nsnally only one adsorption unit is necessary, provided, of course, that a sufficient time interval is available between adsorption cycles to permit regeneration. However, this is usually not the case. Since an nninternipted flow of treated gas is often required, it is necessary to employ one or more units capable of operating in this fashion. The units are designed to handle gas flows without interruption and are charac terized by their mode of contact, either staged or continuous. By far the most common type of adsorption system used to remove an objectionable pollutant from a gas stream consists of a number of fixed-bed units operating in such a sequence that the gas flow remains nninternipted. A two- or three-bed system is nsn ly... 

Conversion of Earno into an absolute (UHV) scale rests on the values of ff-0 and for Hg used as areference surface. While the accuracy of is indisputable, the experimental value of contact potential difference between Hg and H20, are a subject of continued dispute. Efforts have been made in this chapter to try to highlight the elements of the problem. However, a specialized experimental approach to the measurement of 0 (and A0 upon water adsorption) of Hg would definitely remove any further ambiguity as well as any reasons not to accept certain conclusions. 

It might be thought as a consequence of measurements such as these that leakage factors are the main issues in fuel containment. However, although obviously important, in some cases a leak might occur only at intermittent intervals, and the associated problem might well be easily resolvable by component replacement. In contrast, the relevance of permeation to fluid containment is its continuous nature—its rate may be low, but it occurs all the time that fluid is contacting elastomer. Hence, this phenomenon is now considered in association with related processes absorption, adsorption, and diffusion. 

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