Desorption continued

Directly on admittance of H2S, CO2 desorbed from the solution into the gas-phase and thus immediately resulted in a positive overall (absorption) driving force, but desorption continued. After some 20-30 minutes the H2S flow was stopped to enable re-equilibration. It was observed that CO2 was again absorbed into the solution to almost the initial equilibrium (see figure 7 for a typical example). This unambiguously proves that the recorded concentration curves in the gas-phase are due to reaction processes in the penetration zone and have nothing to do with bulk equilibrium which would not have lead to re-absorption of CO2. The total amount of absorbed H2S was negligible (0.02 mole/mole DIPA) and did not affect the bulk equilibrium. 

In desorption, the result is very different. Now, the direction is from the upper right to the lower left portion of the curve. The slope Aq/AY has the smallest value (F is fastest) during the initial portion and V becomes slower as desorption continues. This causes the desorption wave to spread and an elongated breakthrough curve results. 

When a solid surface is exposed to a gas, the molecules of the gas strike the surface of the solid when some of these striking molecules stick to the solid surface and become adsorbed, while some others rebound back. Initially the rate of adsorption is large because the whole surface is bare, but the rate of adsorption continues to decrease as more and more of the solid surface becomes covered by the adsorbate molecules. However, the rate of desorption, which is the rate at which the adsorbed molecules rebound from the surface, increases because desorption takes place from the covered surface. With the passage of time, the rate of adsorption continues to decrease, while the rate of desorption continues to increase, until an equilibrium is reached, where the rate of adsorption is equal to the rate of desorption. At this point the solid is in adsorption equilibrium with the gas. It is a dynamic equihbrium... 

When the molecules of the adsorbate approach those of the adsorbent surface, the above forces of attraction are opposed by forces of repulsion. Adsorption and desorption continue till an equilibrium is reached. A molecule being adsorbed is not associated with a single centre. Rather it interacts with many neighbouring centres (see Figure 7.5). 

Results for the completely sealed system are given in Fig. 11. The first step of the TG appears to be a rapid decreasing reaction which corresponds to the loss of 3/2 moles of H2O and the formation of CaS04 /2H20. The DTA results indicate endothermic effects at 129, 133, 162, and 219°C, respectively. The two low temperature peaks (129°, 133°C) occur without an apparent decrease in TG. These peaks are designated and Og (in Fig. 11). The rapid descent of the first step of the TG is considered to be due to the desorption of water owing to the hydration atthe and Og steps. Desorption continues gradually after these steps, and... 

Irreversible adsorption discussed in Section XI-3 poses a paradox. Consider, for example, curve 1 of Fig. XI-8, and for a particular system let the equilibrium concentration be 0.025 g/lOO cm, corresponding to a coverage, 6 of about 0.5. If the adsorption is irreversible, no desorption would occur on a small dilution on the other hand, more adsorption would occur if the concentration were increased. If adsorption is possible but not desorption, why does the adsorption stop at 6 = 0.5 instead of continuing up to 0 = 1 Comment on this paradox and on possible explanations. 

Figure Bl.26.8. Adsorption/desorption peaks for nitrogen obtained with the continuous flow metiiod (Nelsen F M and Eggertsen F T 1958 Anal. Chem. 30 1387-90).

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...

The variant of the cylindrical model which has played a prominent part in the development of the subject is the ink-bottle , composed of a cylindrical pore closed one end and with a narrow neck at the other (Fig. 3.12(a)). The course of events is different according as the core radius r of the body is greater or less than twice the core radius r of the neck. Nucleation to give a hemispherical meniscus, can occur at the base B at the relative pressure p/p°)i = exp( —2K/r ) but a meniscus originating in the neck is necessarily cylindrical so that its formation would need the pressure (P/P°)n = exp(-K/r ). If now r /r, < 2, (p/p ), is lower than p/p°)n, so that condensation will commence at the base B and will All the whole pore, neck as well as body, at the relative pressure exp( —2K/r ). Evaporation from the full pore will commence from the hemispherical meniscus in the neck at the relative pressure p/p°) = cxp(-2K/r ) and will continue till the core of the body is also empty, since the pressure is already lower than the equilibrium value (p/p°)i) for evaporation from the body. Thus the adsorption branch of the loop leads to values of the core radius of the body, and the desorption branch to values of the core radius of the neck. 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

Bombardment of a liquid surface by a beam of fast atoms (or fast ions) causes continuous desorption of ions that are characteristic of the liquid. Where the liquid is a solution of a sample substance dissolved in a solvent of low volatility (often referred to as a matrix), both positive and negative ions characteristic of the solvent and the sample itself leave the surface. The choice of whether to examine the positive or the negative ions is effected simply by the sign of an electrical potential applied to an extraction plate held above the surface being bombarded. Usually, few fragment ions are observed, and a sample of mass M in a solvent of mass S will give mostly [M + H] (or [M - H] ) and [S -I- H]+ (or [S - H] ) ions. Therefore, the technique is particularly good for measurement of relative molecular mass. 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

Although the above has considered only the use of a continuous main ion beam, which is then pulsed, it is not necessary for the initial beam to be continuous it too can be pulsed. For example, laser desorption uses pulses of laser light to effect ionization, and the main ion beam already... 

During Stages II and III the average concentration of radicals within the particle determines the rate of polymerization. To solve for n, the fate of a given radical was balanced across the possible adsorption, desorption, and termination events. Initially a solution was provided for three physically limiting cases. Subsequentiy, n was solved for expHcitiy without limitation using a generating function to solve the Smith-Ewart recursion formula (29). This analysis for the case of very slow rates of radical desorption was improved on (30), and later radical readsorption was accounted for and the Smith-Ewart recursion formula solved via the method of continuous fractions (31). 

Adsorbent drying systems are typicaHy operated in a regenerative mode with an adsorption half-cycle to remove water from the process stream and a desorption half-cycle to remove water from the adsorbent and to prepare it for another adsorption half-cycle (8,30,31). UsuaHy, two beds are employed to aHow for continuous processing. In most cases, some residual water remains on the adsorbent after the desorption half-cycle because complete removal is not economically practical. The difference between the amount of water removed during the adsorption and desorption half-cycle is termed the differential loading, which is the working capacity available for dehydration. 

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... 

VACUUM RADIATING DESORPTION AND INFRARED SPECTROMETRY (VRDIR) FOR CONTINUOUS MONITORING OF SUSPENDED PARTICULATE ORGANIC MATTERS IN ATMOSPHERE... 

Consider a local concentration of solute migrating down a column. During this migration, adsorption and desorption steps will continuously and frequently occur. In addition, each occurrence will be a random event. Now a desorption step will be a random movement forward as it releases a molecule into the mobile phase, where it can move forward. Conversely, an adsorption step is a step backward, as it results in a period of immobility for the molecule while the rest of the zone moves forward. The total number of random steps taken as the solute mean position moves a distance (l) along the column is the number of forward steps plus the number of backward... 

Normal paraffins in the C,o - C,5 range are recovered from petroleum fractions by adsorption-desorption using molecular sieves. Ammonia can be used to desorb the n-paraffins. By employing two beds of sieves, one on adsorption and one on desorption at all times, a continuous flow of the feed and ammonia is maintained. 

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