Adsorption continuous adsorbers

Alternatively, peak asymmetry could arise from thermal effects. During the passage of a solute along the column the heats of adsorption and desorption that are evolved and adsorbed as the solute distributes itself between the phases. At the front of the peak, where the solute is being continually adsorbed, the heat of adsorption will be evolved and thus the front of the peak will be at a temperature above its surroundings. Conversely, at the rear of the peak, where there will be a net desorption of solute, heat will be adsorbed and the temperature or the rear of the peak will fall below its surroundings. 

N diffuses into the structural pores of clinoptilolite 10 to 10 times faster than does CH4. Thus internal surfaces are kinetically selective for adsorption. Some clino samples are more effective at N2/CH4 separation than others and this property was correlated with the zeolite surface cation population. An incompletely exchanged clino containing doubly charged cations appears to be the most selective for N2. Using a computer-controlled pressure swing adsorption apparatus, several process variables were studied in multiple cycle experiments. These included feed composition and rates, and adsorber temperature, pressure and regeneration conditions. N2 diffusive flux reverses after about 60 seconds, but CH4 adsorption continues. This causes a decay in the observed N2/CH4 separation. Therefore, optimum process conditions include rapid adsorber pressurization and short adsorption/desorp-tion/regeneration cycles. 

The latter authors found that a reversible water isothenn was obtained after the low-temperature (i.e. 40°C) evacuation of a carbon cloth, which had been activated by oxidative HNO, treatment. The molecular sieve character of this material was reduced by evacuation at 400°C and this also led to the appearance of hysteresis in the water vapour isothenn. Barton and Koresh (1983) conclude that such hysteresis is mainly due to the concentration of surface oxides which dictate the adsorption value at which the change from cluster adsorption to a continuous adsorbed phase takes place . The relationship between the adsorption of water and the surface concentration of chemisorbed oxygen was first established by Walker and Janov (1968). Bansal et al. (1978a,b) also investigated the influence of the surface oxygen on the adsorption of water they concluded that at p/p° < 0.5 the level of water uptake is determined by the concentration of surface oxygen-containing structures. 

Sorbent (A) theoretical (B) amount of HCl Theoretical Observed adsorption Adsorption time adsorption capacity adsorbed before the consumed capacity capacity until continued until [HCl(g)]/[ads(g)] breakthrough (B)/(A)100 [%] steady state steady state, h [HCl(g)]/[ads(g)] [HCl(g)]/[ads(g)]... 

An example of reversible deactivation is presented in Figure 7, as a variation of the poisoning mode of deactivation shown in Figure 4. The catalyst promoting the reaction A->B involves dynamic adsorption of Component A, wherein molecules of A continually adsorb on and desorb from the surface. While adsorbed, some of the molecules are converted to Component B. The rate of reaction is proportional to the surface coverage of Component A. 

Behenic Ac id-n-Hexadecane Series. The general pattern of the adsorbed acid in this series (Figure 5) can perhaps best be described as a monolayer with well-defined holes, which become smaller as adsorption continues. This structure is clearly shown in BH-3, where the holes at that stage of adsorption range from approximately 200 to 1000 A. in diameter. This mode of adsorption is opposite to that observed for all the preceding acid-solvent pairs, where the polar molecules adsorbed initially in patches which increased in size and perhaps number to form the complete monolayer. The small projections in BH-2 are thought to be from the substrate and not part of the acid structure, because similar projections were occasionally observed on untreated microscope slides. 

Adsorption Kinetics. Figures 2A and 2B show the FN adsorption kinetics on the three surfaces from 0,07 and 0,21 mg/ml FN solutions respectively. Each line is the average of two experiments on a given polymer. At each protein concentration, the initial rate of adsorption is independent of the type of polymer substrate, and adsorption from 0,21 mg/ml is nearly 3 times faster than from 0,07 mg/ml FN, The initial adsorption rates are linear in time until adsorption exceeds 0.06 ug/cm on PEO-PEUU and 0.10 ug/cm on the other polymers (data not shown). This suggests that the adsorption is diffusion controlled up to the above surface concentrations, after which point the adsorption rate decreases and becomes dependent upon the polymer surface chemistry. The amount of FN adsorbed does not reach a plateau within 120 minutes, nor does it reach a plateau when adsorption continues for 18 hours (data not shown). 

The measured surface area includes the entire surface accessible to the gas whether external or internal. Prior to the measurement, the sample is pre-treated at high temperature in vacuum in order to remove any contaminants. To cause sufficient gas to be adsorbed for surface area measurement, the solid must be cooled (normally to the boiling point of the gas). Most often, nitrogen is the adsorbate and the solid is cooled with liquid nitrogen. Adsorption continues until the amount of nitrogen adsorbed is in equilibrium with the concentration in the gas phase. This amount is close to that needed to cover the surface in a monolayer (see Table E9.2). 

Solvent-laden air is exhausted at the three rotogravure printing presses by several fans operating in parallel and is routed in an upward flow through four adsorbers packed with Supersorbon activated carbon. The solvent contained in the air is adsorbed on the activated carbon bed. Adsorption continues until breakthrough, when the full retentive capacity of the adsorbent for solvent vapors is used up. 

When a solid surface is exposed to a gas, the molecules of the gas strike the surface of the solid. Some of the striking molecules stick to the solid surface and become adsorbed while the others rebound. Initially the rate of adsorption is large as the whole surface is bare but as more and more of the surface becomes covered by the molecules of the gas, the available bare surface decreases and so does the rate of adsorption. However, the rate of desorption, which is the rate at which adsorbed molecules rebound from the surface, increases because desorption takes place from the covered surface. As time passes, the rate of adsorption continues to decrease while the rate of desorption increases until an equilibrium is reached between the rate of adsorption and the rate of desorption. At this stage the solid is in adsorption equilibrium with the gas, and the rate of adsorption is equal to the rate of desorption. It is a dynamic equilibrium because the number of molecules sticking to the surface is equal to the number of molecules rebounding from the surface. 

Chromatography— This is a method of separation based upon selective adsorption of components fiom a gas or liquid sample. As the sample is carried through the chromatographic column by the flow of a carrier gas, individual molecules of the sample gas are continuously adsorbed and then... 

Where the solid substrate is not flat, but porous or absorbent, the time taken to reach equilibrium can be much longer. For poly(vinyl acetate) adsorbing on to cellulose filter pulp, initial adsorption is very rapid, with slow but measurable adsorption continuing for as long as 10 days. The initial adsorption on to cellulose by poly(ethyleneimine) was reported to be a second-order kinetics process, though no unequivocal explanation exists for this. Where the substrates consist of colloidal particles, the rate of adsorption has been shown to increase with intensity of shaking. ... 

Dubinin et al. evaluated the amount surface area from the benzene isotherm measured for the nonporous reference adsorbent /7/, we, however estimated the specific surface area Sme of the mesopores from the adsorption isotherm studied /8/, In calculations of the mesopore size distribution and the specific surface area Sme it has been assumed that the parallel - sided slits are rigid and the size distribution does not extend continuously from the mesopore into both the macropore and micropore range. We have used the desorption branch of the hysteresis loop of the isotherm for the computation. The procedure of B.F. Roberts /9/ has been applied. In this computation, which is a rigorous application of the concept of simultaneous capillary condensation and multilayer adsorption, the adsorbed volume is first expressed as a function of pore size then it is converted to pore volume. A standard t - curve /lO/, which represents the benzene adsorption onto nonporous carbon blacks, has been used for correction for multilayer thickness. 

As stated in the introduction to the previous chapter, adsorption is described phenomenologically in terms of an empirical adsorption function n = f(P, T) where n is the amount adsorbed. As a matter of experimental convenience, one usually determines the adsorption isotherm n = fr(P), in a detailed study, this is done for several temperatures. Figure XVII-1 displays some of the extensive data of Drain and Morrison [1]. It is fairly common in physical adsorption systems for the low-pressure data to suggest that a limiting adsorption is being reached, as in Fig. XVII-la, but for continued further adsorption to occur at pressures approaching the saturation or condensation pressure (which would be close to 1 atm for N2 at 75 K), as in Fig. XVII-Ih. 

Surface heterogeneity is difficult to remove from crystalline inorganic substances, such as metal oxides, without causing large loss of surface areas by sintering. Thus in Fig. 2.14 in which the adsorbent was rutile (TiO ) all three adsorbates show a continuous diminution in the heat of adsorption as the surface coverage increases, but with an accelerated rate of fall as monolayer completion is approached. 

Fig. 230 Adsorption of nitrogen at 77 K on a silica powder a) adsorption isotherms b) /-plot. Broken line, uncompacted powder continuous line, power compacted at 2-00 x 10 N m (130 ton in ). (—>—) adsorption (—<-) desorption. / is the ratio of the amount adsorbed on the powder to the amount adsorbed on the compact at the same relative...

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