Adsorption concentration profiles

The Aromax process was developed in the early 1970s by Toray Industries, Inc. in Japan (95—98). The adsorption column consists of a horizontal series of independent chambers containing fixed beds of adsorbent. Instead of a rotary valve, a sequence of specially designed on—off valves under computer control is used to move inlet and withdrawal ports around the bed. Adsorption is carried out in the Hquid phase at 140°C, 785—980 kPA, and 5—13 L/h. PX yields per pass is reported to exceed 90% with a typical purity of 99.5%. The first Aromax unit was installed at Toray s Kawasaki plant in March 1973. In 1994, IFP introduced the Eluxyl adsorption process (59,99). The proprietary adsorbent used is designated SPX 3000. Individual on-off valves controlled by a microprocessor are used. Raman spectroscopy to used to measure concentration profiles in the column. A 10,000 t/yr demonstration plant was started and successfully operated at Chevron s Pascagoula plant from 1995—96. IFP has Hcensed two hybrid units. 

Fig. 12. (a) Development of the physically unreasonable overbanging concentration profile and the corresponding shock profile for adsorption with a favorable isotherm and (b) development of the dispersive (proportionate pattern) concentration profile for adsorption with an unfavorable isotherm (or for... 

Adsorption Dynamics. An outline of approaches that have been taken to model mass-transfer rates in adsorbents has been given (see Adsorption). Detailed reviews of the extensive Hterature on the interrelated topics of modeling of mass-transfer rate processes in fixed-bed adsorbers, bed concentration profiles, and breakthrough curves include references 16 and 26. The related simple design concepts of WES, WUB, and LUB for constant-pattern adsorption are discussed later. 

A one-dimensional isothermal plug-flow model is used because the inner diameter of the reactor is 4 mm. Although the apparent gas flow rate is small, axial dispersion can be neglected because the catalj st is closely compacted and the concentration profile is placid. With the assumption of Langmuir adsorption, the reactor model can be formulated as. 

Figure 3.31. The influence of coadsorbing ions (citrate) on the Pt concentration profile (adsorption of chloroplatinic acid, H2PtCl6) a.-c. increasing amount of citric acid (Geus and Van Veen, 1999).

Use of densitometric detection provides an insight into the concentration profiles of chromatographic bands, thus furnishing an indispensable prerequisite, needed for proper assessment of the retention mechanisms in the preparative adsorption TLC. Figure 2.4 shows three types of the band eoncentration profiles. The Gaussian peak (a) in this figure represents the linear isotherm of adsorption of a given species, peak... 

From the asymmetrical concentration profile with front tailing (see Figure 2.4b), it can correctly be deduced that (1) the adsorbent layer is already overloaded by the analyte (i.e., the analysis is being run in the nonlinear range of the adsorption isotherm) and (2) the lateral interactions (i.e., those of the self-associative type) among the analyte molecules take place. The easiest way to approximate this type of concentration profile is by using the anti-Langmuir isotherm (which has no physicochemical explanation yet models the cases with lateral interactions in a fairly accurate manner). 

The main difference between the chromatographic process carried out in the linear and the nonlinear range of the adsorption isotherm is the fact that in the latter case, due to the skewed shapes of the concentration profiles of the analytes involved, separation performance of a chromatographic system considerably drops, i.e., the number of theoretical plates (N) of a chromatographic system indisputably lowers. In these circumstances, all quantitative models, along with semiquantitative and nonquantitative rules, successfully applied to optimization of the linear adsorption TLC show a considerably worse applicability. 

Figure 10.9 Concentration profiles through an adsorption bed exhibit a moving font.

For each of the model compounds, some material will have leached deeper Into the soil than Is shown in the table. The model calculates only the position of maximum concentration. For a compound like DBCP, which has a very weak adsorption interaction with the soil, the concentration profile will be spread out. DBCP would probably be found at low concentrations at the 1017 cm level. For the strongly adsorbed compounds, such as toxaphene and methoxychlor, the concentration peak will be narrow, and the depth of maximum concentration is the depth where most of the material is. 

Figure 8 shows how the intrapellet concentration profiles vary with time during the course of CO desorption. Both the gas-phase (solid lines) and surface (dotted lines) CO concentration profiles exhibit relatively mild gradients inside the pellet, in contrast to the steep profiles established during the adsorption process. This can be attributed to the fact that the intrinsic rate of desorption is slower than that of adsorption. 

Figure 7. Computed time variation of intrapellet concentration profiles during CO adsorption. Key ---------------------, gas phase and----> surface.

Fig. 5.1.7. Concentration profile of C12-LAS on FBBR during adsorption and degradation experiments at a surfactant concentration of 100 mgL. ...

Figure 14. Simple model demonstrating how adsorption and surface diffusion can co-Urnit overall reaction kinetics, as explained in the text, (a) A semi-infinite surface establishes a uniform surface coverage Cao of adsorbate A via equilibrium of surface diffusion and adsorption/desorption of A from/to the surrounding gas. (b) Concentration profile of adsorbed species following a step (drop) in surface coverage at the origin, (c) Surface flux of species at the origin (A 4i(t)) as a function of time. Points marked with a solid circle ( ) correspond to the concentration profiles in b. (d) Surface flux of species at the origin (A 4i(ft>)) resulting from a steady periodic sinusoidal oscillation at frequency 0) of the concentration at the origin.

Figure 10.1b shows spatial concentration profiles within the column, at different snapshots in time. Note that the profile spreads with increasing travel distance (and thus with increasing time). The positions (distances) noted by the points and correspond to the times and shown in Fig. 10.1a. The effect of retardation, caused by the additional mechanism of chemical adsorption, is shown in Fig. 10.1c both the average velocity of the contaminant (corresponding to the point dc = 0.5) and the degree of spreading around this value are reduced. This behavior is discussed further in Sect. 11.1.

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