Dehydration operating practices

Desiccant Materials, 1034 Mechanism of Water Adsorption, 1044 Dehydration System Design Approach, 1048 Operating Practices, 1069... 

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. 

On the other hand, the merits of such insights are obvious. It would become possible to evaluate the relative importance of surface and bulk mechanisms of PT. The transition from high to low proton mobility upon dehydration could be related to molecular parameters that are variable in chemical synthesis. It could become feasible to determine conditions for which high rates of interfacial PT could be attained with a minimal amount of hghtly bound water. As an outcome of great practical value, this understanding could direct the design of membranes that operate well at minimal hydration and T > 100°C. 

From an operator s perspective, if a dehydrator abnormality enables free water to enter a vessel or pipeline, then that free water can readily form hydrates, so that additional saturated vapor (without free water) will cause that initial hydrate mass to grow. Therefore, from a practical standpoint one should require that the hydrocarbon fluid be maintained in the thermodynamic single-phase region if hydrates are to be prevented. 

Once dehydrated, the microfibrils are practically without functionality in ordinary food processing and preparation operations, because the inert microcrystallites are difficult for water to penetrate. The polymorphs, cellulose I and II (Blackwell, 1982 Coffey el al., 1995), are differentiated by their molecular orientation, hydrogen-bonding patterns, and unit-cell structure. Cellulose I is the natural orientation cellulose II results from NaOH treatment under tension of cellulose I with 18-45% alkali (mercerization). The I—II transition is irreversible. Mercerization strengthens the fibers and improves their lustre and affinity for dyes (Sisson, 1943). Sewing thread was relatively pure mercerized cotton until the advent of synthetic polymer fibers. 

TBHP vide supra). The autoxidation of EB is performed at 120-160 °C and 1- bar. MBA and acetophenone (ACP) are formed as by-products via the facile termination of the secondary 1-methylbenzylperoxy radicals. In order to minimize by-product formation by further oxidation of MBA and ACP, the autoxidation is carried out to only low conversions (< 12 %). This solution (ca. 10 %) of EBHP in EB is used in the epoxidation step, i.e., EB is the solvent for the latter step. A high propene/EBHP molar ratio is used and reaction conditions are similar to those of the TBHP process vide supra). The PO selectivity is reported to be 90 % at 92 % EBHP conversion [30] but in practice it may be higher. For comparison the heterogeneous Ti /SiOa catalyst in fixed-bed operation reportedly gives 93-94 % PO selectivity at 96 % EBHP conversion [11]. The products are separated by distillation and MBA is dehydrated to styrene in the vapor phase over a Ti02 catalyst. 

One commercial method to obtain nitric acid concentrations above 68% uses concentrated sulfuric acid to dehydrate the azeotropic composition. Hot nitric acid vapor is passed upward against concentrated sulfuric acid, which moved downward (countercurrent) in a tower packed with chemical stoneware to obtain 90+%HNO3 and a diluted sulfuric acid stream (Fig. 11.6). If this process is practiced on only a small scale, the sulfuric acid may be reconcentrated by addition of oleum, and a portion of the buildup of sulfuric acid in this circuit may be used to make up a commercial nitrating mixture with some of the fuming nitric acid made. Larger scale operation requires the use of a sulfuric acid boiler and a large heat input to reconcentrate the dehydration acid. There is also a noticeable sulfate contamination of the nitric acid product from this process. 

Tsushima et al. (2010) developed an MRI system to investigate the effects of relative humidity (RH) and current density on the transverse water content profile in a membrane under fuel cell operation at a practical PEMFC operating temperature. The MRI visualization revealed that in dry conditions (40% RH), the membrane hydration X number was 3, and the water content profile in the membrane was fiat because the diffusion process in the membrane was dominant in the water transport. In a standard condition (80% RH) the water content in the membrane was 8, and a partial dehydration at the anode was observed at a current density of 0.2 A/cm, indicating that electroosmosis was influential. At the higher RH level of 92%, the water content X within the membrane at 0.2 A/cm was around 22, corresponding to the eqnilibrium state of the membrane in liquid water, and the water content profile with the increase in current density became fiat. This indicates that the liquid water generated in the cathode catalyst layer permeated the membrane, where water transport plays a more dominant role. 

Pressure drop is a critical factor in the design of adsorption systems, as it normally determines the allowable gas velocity and, therefore, the bed cross-sectional area. Although much work has been done on the subject, no completely satisfactory general correlation has been developed that takes into account the shapes of individual particles, size distribution, void fraction, and aging effects, as well as the more readily characterized gas properties and conditions. It is, therefore, common practice to use experimental and operating data and semi-empirical correlations aimed at specific adsorbent types and applications. Typical data and correlations are presented in subsequent sections covering dehydration with solid desiccants and organic vapor adsorption on activated carbon. 

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