Molecular sieve desiccant

Of all the desiccants, molecular sieves 4A or 5A are perhaps the best for all-around use. Two obvious advantages are their high affinity for H20 and good capacity. They are fairly inert and, because of their size exclusion of large molecules, are less susceptible than many adsorbents to competitive adsorption of other polar molecules. Molecular sieves are usually compacted with a clay binder into the form of small rods, which minimizes the restriction of gas flowing... 

Molecular sieve desiccants. Molecular sieve, a kind of natmal or synthesized zeolite, is a silica aluminate with tetrahedron as structural rniits. Molecular sieve is a highly efficient and high selective adsorbent due to its huge inner... 

By-product water formed in the methanation reactions is condensed by either refrigeration or compression and cooling. The remaining product gas, principally methane, is compressed to desired pipeline pressures of 3.4—6.9 MPa (500—1000 psi). Einal traces of water are absorbed on siHca gel or molecular sieves, or removed by a drying agent such as sulfuric acid, H2SO4. Other desiccants maybe used, such as activated alumina, diethylene glycol, or concentrated solutions of calcium chloride (see Desiccants). 

The purity of oxygen from chlorate candles before and after gas filtration is indicated in Table 2. A particulate filter is always used. Filter chemicals are HopcaUte, which oxidizes CO to CO2 molecular sieves (qv), which remove chlorine compounds and basic materials, eg, soda lime, which removes CO2 and chlorine compounds. Other than H2O and N2, impurity levels of <1 ppm can be attained. Moisture can be reduced by using a desiccant (see Desiccants). Gas purity is a function of candle packaging as well as composition. A hotter burning unit, eg, one in which steel wool is the binder, generates more impurities. 

Desiccants Used in Static Drying. The most commonly used desiccants are discussed in this section activated alumina, siUca gel, and molecular sieves, which are discussed later under dynamic, sohd drying agents, are also widely used in static or batch-drying situations. 

The efficiency ranking of desiccants in drying air is not always the same as that observed in drying other materials. Other materials may interact with the desiccants to reduce drying effectiveness. From a study of the efficiency of some 25 desiccants for drying several families of laboratory solvents and reagents it was concluded that molecular sieves are the desiccants of choice in most cases (9—17). 

Because of their ordered stmcture, molecular sieves have high capacity at low water concentrations and do not exhibit a capiHary condensation pore-filling mechanism at high water concentrations. The desiccating properties of the material are stiU good at elevated temperatures (Fig. 10). A dew point of —75° C can be obtained in a gas dried at 90°C with a molecular sieve that adsorbs water to the level of 1 wt %. In normal operations at ambient temperature, dew points of < — 100° C have been measured. 

H2O/100 kg of adsorbent. At equilibrium and at a given adsorbed water content, the dew point that can be obtained in the treated fluid is a function only of the adsorbent temperature. The slopes of the isosteres indicate that the capacity of molecular sieves is less temperature sensitive than that of siUca gel or activated alumina. In another type of isostere plot, the natural logarithm of the vapor pressure of water in equiUbrium with the desiccant is plotted against the reciprocal of absolute temperature. The slopes of these isosteres are proportional to the isosteric heats of adsorption of water on the desiccant (see... 

Market Data. The largest U.S. manufacturer of molecular sieves for adsorbent and desiccant use is UOP, which has a production capacity of 18—20 million kg/year. W.R. Grace and Zeochem have about 7 and 2 million kg/year capacity, respectively (55). W.R. Grace is the largest producer of siUca gel desiccants. Activated alumina for use as adsorbent and desiccant is produced by LaRoche Chemicals (formerly Kaiser) and by Aluminum Company of America. About one-third of the U.S. supply of activated alumina adsorbent and desiccant is imported by Rhc ne-Poulenc. 

All desiccants exhibit a decrease in capacity (design loading) with increase in temperature. Molecular sieves tend to be the less severely affected and aluminas the most affected by temperature. 

Aluminas and molecular sieves act as a catalyst with HiS to form COS. When the bed is regenerated, sulfur remains and plugs up the spaces. Liquid hydrocarbons also present a plugging problem to all desiccants, but molecular sieves are less susceptible to contamination with liquid hydrocarbons. 

The structural complexity of the 3D framework aluminosilicates precludes a detailed treatment here, but many of the minerals are of paramount importance. The group includes the feldspars (which are the most abundant of all minerals, and comprise 60% of the earth s crust), the zeolites (which find major applications as molecular sieves, desiccants, ion exchangers and water softeners), and the ultramarines which, as their name implies, often have an intense blue colour. All are constructed from Si04 units in which each O atom is shared by 2 tetrahedra (as in the various forms of Si02 itself), but up to one-half of the Si... 

Adsorption is the property of certain extremely porous materials to hold vapors in the pores until the desiccant is either heated or exposed to a drier gas. The material is a solid at all times and operates alternately through drying and reactivation cycles with no change in composition. Adsorbing materials in principal use are activated Alumina and silica gel. Molecular sieves are also used. Atmospheric dew points of minus 1000°F are readily obtained using adsorption. 

HT and large scale experiments were performed as described in [7], In HT runs, 116 2-ml GC vials (thin film, no stirring) were heated at 100° C for 3 h under 1500 psi of 02/C0 (10% 02 in CO) The presented results are the average of at least two runs (three of four when RSD data presented). Large-scale (60 g) experiments were carried out in 450 mL Hastelloy C Parr reactor under the same conditions in the presence of 3 A molecular sieves (35 g) s a desiccant. 

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

Molecular sieves or magnesium sulfate were activated by heating at 500 °C for 2-14 hours and cooled in a desiccator under vacuum. 

Before each reaction, the molecular sieves (4 A in powder form or 3 A as pellets) were activated by heating for 2 hours at 400 °C, then cooled under vacuum in a desiccator. 

Materials. Reagent grade solvents, dimethyl formamide (DMF), dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO) and methanol were purchased from Baker, stored over molecular sieves once opened, and used without further purification. Aminoethane thiosulfuric acid (AETSA) purchased from Kodak, and Taurine, purchased from Alfa were purified by recrystallization. Each was thrice recrystallized from hot, deionized water. The crystalline precipitate was dried (48 hours at 40 °C) in-vacuo and subsequently stored in a desiccator. Benzophenone (BP) was purchased from Aldrich Chemical Company. QUANTACURE BTC (BTC), (4-benzolybenzyl) trimethylammonium chloride, was used as supplied by Aceto, Inc., Flushing, New York. Phenyl glycidyl ether (PGE) was purchased from MCB, distilled in-vacuo. and stored at -15 °C. Epon 828 was used as supplied bv Shell Chemical Company. The epoxy equivalent weight (EEW) for Epon 828 determined by an appropriate titration, was found to be 187.7. 

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