Processes adsorption

Adsorption processes with molecular sieves as adsorbents are a good tool to separate mixtures of -paraffins, isoparaffins, olefines, and cyclic hydrocarbons according to the PSA mode. 

Principally speaking, separation by adsorption can be based on a steric or a kinetic or an equilibrium effect. With respect to a big variety of adsorbents (pore width, pore size distribution, polar or nonpolar, hydrophilic or hydrophobic behavior, addition of functional groups) and wide ranges of pressure and temperature which can be chosen independently of each other, adsorption processes allow more flexibility in comparison to rectification. 

Desorption can be carried out by displacement of the adsorbate with an appropriate substance. 

Chlorine, ozone, manganese, and iron present in freshwater are eliminated by adsorption. Activated carbon is also used for the purification of condensed steam. Not only dissolved oil constituents but also small emulsified droplets are removed. The purification of aqueous or organic solvents and solutions is often carried out by activated carbon adsorption. 

Besides fixed and fluidized bed adsorbers also stirred vessels filled with an impurity loaded liquid and powdered active carbon are used. The suspension is separated by filtration or centrifugation after a certain residence time necessary for 

The transport of electro active species from the bulk of the solution to the electrode may be governed not only by diffusion but also by adsorption of the species on the electrode surface. When both the mechanisms are operative, the overall electrochemical process may give considerably complicated results. The theoretical treatment is complex and of limited interest to inorganic chemists, therefore, a qualitative approach will be adopted to identify the presence of adsorption phenomena. 

The typical scheme describing the electrode mechanism complicated by adsorption processes is the following  

It represents a reversible electron transfer complicated by adsorption (in equilibrium with diffusive motions) of either Ox or Red. 

This anomaly becomes more evident with the scan rate. In fact, while in the case of a simple reversible electron transfer the current function, ip/v1/2, remains constant with the scan rate, in the case 

The separation between the adsorption peak (pre- or post-peak) and the peak due to the diffusive electron transfer can be taken as a qualitative 

In all cases, porous materials exhibit high surface areas, which maximize the extension of the interface region. 

The surface atoms of a solid, which are coordinatively unsaturated with respect to the bulk atoms, become saturated thanks to the interaction with molecules of the environment. Adsorption is the process whereby molecules from the gas (or liquid) phase are taken up by a solid surface it is distinguished from absorption which refers to molecules entering into the lattice (bulk) of the solid material. The adsorptive is the material in the gas phase capable of being adsorbed, whereas the adsorbate is the material actually adsorbed by the solid. The solid, which exposes the surface sites responsible for the process is called the adsorbent. In Fig. 1.3 the adsorption process at the surface of a solid material is schematically illustrated. 

Adsorption is governed by either physical or chemical forces. In the former case the adsorption is named physical adsorption (physisorption) whereas in the latter case chemical adsorption chemisorption). Details on the nature of these forces will be dealt in Sect. 1.6, through the description of a selection of examples. 

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Adsorption may in principle occur at all surfaces its magnitude is particularly noticeable when porous solids, which have a high surface area, such as silica gel or charcoal are contacted with gases or liquids. Adsorption processes may involve either simple uni-molecular adsorbate layers or multilayers the forces which bind the adsorbate to the surface may be physical or chemical in nature. 

Systems involving an interface are often metastable, that is, essentially in equilibrium in some aspects although in principle evolving slowly to a final state of global equilibrium. The solid-vapor interface is a good example of this. We can have adsorption equilibrium and calculate various thermodynamic quantities for the adsorption process yet the particles of a solid are unstable toward a drift to the final equilibrium condition of a single, perfect crystal. Much of Chapters IX and XVII are thus thermodynamic in content. 

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

It is perhaps fortunate that both versions lead to the same algebraic formulations, but we will imply a preference for the two-dimensional solution picture by expressing surface concentrations in terms of mole fractions. The adsorption process can be written as... 

Derive Eq. XI-IS, assuming a Langmuir adsorption process described in Eq. XI-2, where ka and kd are the adsorption and desorption rate constants. Treat the solution... 

The foregoing is an equilibrium analysis, yet some transient effects are probably important to film resilience. Rayleigh [182] noted that surface freshly formed by some insult to the film would have a greater than equilibrium surface tension (note Fig. 11-15). A recent analysis [222] of the effect of surface elasticity on foam stability relates the nonequilibrium surfactant surface coverage to the foam retention time or time for a bubble to pass through a wet foam. The adsorption process is important in a new means of obtaining a foam by supplying vapor phase surfactants [223]. 

It has become increasingly appreciated in recent years that the surface stmc-ture of the adsorbent may be altered in the adsorption process. Qualitatively,... 

As is made evident in the next section, there is no sharp dividing line between these two types of adsorption, although the extremes are easily distinguishable. It is true that most of the experimental work has tended to cluster at these extremes, but this is more a reflection of practical interests and of human nature than of anything else. At any rate, although this chapter is ostensibly devoted to physical adsorption, much of the material can be applied to chemisorption as well. For the moment, we do assume that the adsorption process is reversible in the sense that equilibrium is reached and that on desorption the adsorbate is recovered unchanged. 

The kinetics of the adsorption process are important in detemiining the value and behaviour of. S for any given system. There are several factors that come into play in detemiining S [25]. 

There are a few other surface-sensitive characterization techniques that also rely on the use of lasers. For instance surface-plasmon resonance (SPR) measurements have been used to follow changes in surface optical properties as a fiinction of time as the sample is modified by, for instance, adsorption processes [ ]. SPR has proven usefiil to image adsorption patterns on surfaces as well [59]. 

Schaaf P and Talbot J 1989 Surface exclusion effects In adsorption processes J. Chem. Phys. 91 4401-9... 

Grand Canonical Monte Carlo Simulations of Adsorption Processe ... 

The BET treatment is based on a kinetic model of the adsorption process put forward more than sixty years ago by Langmuir, in which the surface of the solid was regarded as an array of adsorption sites. A state of dynamic equilibrium was postulated in which the rate at which molecules arriving from the gas phrase and condensing on to bare sites is equal to the rate at which molecules evaporate from occupied sites. 

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

Separation Processes for PX. There are essentially two methods that are currendy used commercially to separate and produce high purity PX (/) crystallization and (2) adsorption. A third method, a hybrid crystallization /adsorption process, has been successfiiUy field-demonstrated and the first commercial unit is expected in the near future. 

Adsorption Processes. Adsorption represents the second and newer method for separating and producing high purity PX. In this process, adsorbents such as molecular sieves are used to produce high purity PX by preferentially removing PX from mixed xylene streams. Separation is accomphshed by exploiting the differences in affinity of the adsorbent for PX, relative to the other Cg isomers. The adsorbed PX is subsequendy removed... 

Currendy, there are three commercially available PX adsorption processes UOP s Parex, IFP s Eluxyl, and Toray s Aromax (not to be confused with Chevron s Aromax process for reforming naphtha into aromatics). In all of these processes, the feed and desorbent inlets and the product oudet ports are moved around the bed, simulating a moving bed. 

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. 

Asahi Chemical Industry Company Ltd. was working to develop an adsorption process in the late 1970s and early 1980s that was to produce high purity EB as well as PX (100—103). In 1981 they reported that pilot plants results were being confirmed in larger equipment. However, this process does not appear to have been commercialized. 

Hybrid Crystallization/Adsorption Process. In 1994, IFP and Chevron announced the development of a hybrid process that reportedly combines the best features of adsorption and crystallization (59,99). In this option of the Eluxyl process, the adsorbent bed is used to initially produce PX of 90—95% purity. The PX product from the adsorption section is then further purified in a small single-stage crystallizer and the filtrate is recycled back to the adsorption section. It is reported that ultrahigh (99.9+%) purity PX can be produced easily and economically with this scheme for both retrofits of existing crystallization units as well as grass-roots units. A demonstration plant was built at Chevron s Pascagoula refinery in 1994. 

The distance requited to approach the constant pattern limit decreases as the mass transfer resistance decreases and the nonlinearity of the equihbrium isotherm increases. However, when the isotherm is highly favorable, as in many adsorption processes, this distance may be very small, a few centimeters to perhaps a meter. 

Favorable and unfavorable equihbrium isotherms are normally defined, as in Figure 11, with respect to an increase in sorbate concentration. This is, of course, appropriate for an adsorption process, but if one is considering regeneration of a saturated column (desorption), the situation is reversed. An isotherm which is favorable for adsorption is unfavorable for desorption and vice versa. In most adsorption processes the adsorbent is selected to provide a favorable adsorption isotherm, so the adsorption step shows constant pattern behavior and proportionate pattern behavior is encountered in the desorption step. 

Fig. 17. The two basic modes of operation for an adsorption process (a) cycHc batch system (b) continuous countercurrent system with adsorbent...

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