Multiple gases, adsorption

Modern SMR plants (Figure 2.5b) incorporate a PSA unit for purifying hydrogen from C02, CO, and CH4 impurities (moisture is preliminarily removed from the process gas). The PSA unit consists of multiple (parallel) adsorption beds, most commonly filled with molecular sieves of suitable pore size it operates at the pressure of about 20 atm. The PSA off-gas is composed of (mol%) C02—55, H2—27, CH4—14, CO—3, N2—0.4, and some water vapor [11] and is burned as a fuel in the primary reformer furnace. Generally, SMR plants with PSA need only a HT-WGS stage, which may somewhat simplify the process. 

There are situations in which crystallites are readily visible, especially on supports which do not offer excessive electron scatter. In these cases, metal content can be quantitatively determined for areas which have highly dispersed metal and agglomerated metal. This information in conjunction with the crystallite size distribution provides the microscopist with the information required to make an estimate of metal dispersion (13). These estimates are valuable especially in situations where conventional gas adsorption measurements cannot be made on the metal, i.e., when the crystallites are contaminated, have multiple oxidation states, or are poisoned. 

Fig. 1.30. Mass spectra of platinum clusters Ptn before and after reaction with O2 and D2. In the upper mass spectrum (a) no reaction gas was added to the collision gas cells (cf. Fig. 1.29). Spectrum (b) was obtained with 0.14Pa O2 in reaction cell 1 and no gas in reaction cell 2. Spectrum (c) was sampled after the metal cluster beam passed cell 1 with 0.14 Pa O2 and cell 2 with a deuterium pressure of 0.95 Pa. Note that the mass peaks in (c) are slightly broader and exhibit a small shift with respect to mass spectrum (a). This is due to multiple deuterium adsorption onto the platinum clusters which can, however, not be resolved in the mass spectrum [169]...

The difficulty of gas adsorption methods lies in the fact that purely monomolecular layers are never formed. Already before the adsorbent is completely covered, multiple layers build up locally. Brunauer, Emmett and Teller derived a relation between gas pressure and the amount of gas adsorbed at the surface which is known as the BET isothermal line. They used both a gas kinetic and a statistical model. 

Leckie [14] emphasized the advantage of chemical speciation over overall distribution coefficients in adsorption modeling. On the other hand, in many theoreticar studies of adsorption even the speciation in solution is neglected and only the total concentration of dissolved species is taken into account. One probable reason of paying no attention to well-known experimental facts is that some authors use adsorption equations borrowed from gas adsorption, and obviously these equations are not suitable to deal with multiple solution species involving the adsorbate. 

Considerable recent efforts have been devoted to the investigation of gas separation in both rigid and porous MOF phases. Whilst only a relatively small number of systems have been investigated by selective gas adsorption measurement (as opposed to multiple measurement with various pure gases, which due to cooperative effects provides only a guide to the separation capabilities), a number of distinct separation mechanisms have been evidenced. 

O Shea, J.D., and Liapis, A.I., Evaluation of simple and complex models for mass transfer in the non-isothermal gas adsorption of multiple adsorbates in a single adsorbent particle, Chem. Eng. Res. Des., 68(3), 242-250... 

Single and multiple point instruments are available that operate in static volumetric, continuous flow and gravimetric modes. A brief description of some of these is given below and a listing of commercial gas adsorption instruments is given in Table 2.5. 

The adsorption equilibria measurements of N2 and CO2 on activated carbon were performed using a standard static gravimetric method. Further details of these measurements are reported elsewhere." " The Sips isotherm extended to multi-component adsorption was adopted to fit the experimental equilibrium data (Table 9.2). This model has the Langmuirian form applied to non-uniform surfaces and it has been extensively used to model gas adsorption on micro-porous adsorbents and PSA systems. Optimal parameters were found for the adsorption isotherm model, by fitting simultaneously all the data at multiple temperatures. A global isotherm was obtained for each species as illustrated in Figure 9.15." ... 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Although PSA is a batchwise process, by using multiple beds in a sequential manner the overall process is operated in a continuous fashion. Each bed may contain layers of different adsorbent materials selective for specific contaminants in the hydrogen gas stream to be purified. Each bed undergoes a sequence of four basic steps in a PSA cycle adsorption, depressurization, purge at low pressure, and repressurization. This sequence of cyclic operations for each bed is shown schematically for a four-bed PSA process in Figure 8.4 (Yang, 1987 Cassidy, 1980 Miller and Stocker, 1999). 

Cutlip and Kenney (44) have observed isothermal limit cycles in the oxidation of CO over 0.5% Pt/Al203 in a gradientless reactor only in the presence of added 1-butene. Without butene there were no oscillations although regions of multiple steady states exist. Dwyer (22) has followed the surface CO infrared adsorption band and found that it was in phase with the gas-phase concentration. Kurtanjek et al. (45) have studied hydrogen oxidation over Ni and have also taken the logical step of following the surface concentration. Contact potential difference was used to follow the oxidation state of the nickel surface. Under some conditions, oscillations were observed on the surface when none were detected in the gas phase. Recently, Sheintuch (46) has made additional studies of CO oxidation over Pt foil. 

The commonest multiple step control mechanism in use is that of diffusion to the surface of the catalyst combined with one of the adsorption or surface reaction steps. Mass transfer by diffusion is proportional to the difference between partial pressures in the bulk of the gas and at the catalyst surface,... 

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