Mole sieve

Figure 9-3 shows a typical cryogenic plant where the gas is cooled to -100°F to -150°F by expansion through a turbine or Joule-Thompson (J-T) valve. In this example liquids are separated from the iniei gas at 100 F and 1,000 psig. It is then dehydrated to less than I ppm water vapor to assure that hydrates will not form at the low temperatures encountered in the plant. Typically, a mole sieve dehydrator is used. 

The gas chromatograph (GC) is a Hewlett-Packard 5890 GC with a thermal conductivity detector. A 5A mole sieve column is used with argon carrier gas this gives peaks going in the same direction for both hydrogen and nitrogen. 

Mole sieve. A substance with microscopic pores made of zeolytes or other compounds. The pores or channels in the substance are of atomic dimensions and will attract and allow the entrance of only certain size molecules. Mole sieves are therefore useful in the separation of smaller from larger size molecules of otherwise very similar characteristics such as boiling temperatures. 

Scheme 6. Reagents and conditions (a) mole sieves 3 A (b) BrCF2C02Et, Et2Zn, RhCI (PPh3)3j (c) PdOH)2, H2, then B0C2O (d) DiBAIH then (Et0)2P(0)CH2C02Et, LiCI, /-Pr2NEt (e) Smi2, f-BuOH.

The mole sieve dehydration is a semi-batch process using a solid adsorbent to remove water from a fluid stream. The water adsorbs onto the solid. 

Another advantage of the mole sieve process is that it requires minimal operator intervention the process can be completely automated. This may be a significant advantage in a remote location. 

The biggest disadvantage of the standard mole sieve dehydration unit is the cost. As a rule of thumb, a mole sieve unit costs 1.5 times that of a similar TEG unit (Kohl and Nielsen, 1997). 

Unfortunately, the acid gas components are also readily adsorbed onto most mole sieve materials. Figure 7.5 shows the adsorption isotherms for H2S on three common mole sieve materials at 25°C. Similarly, figure 7.6 show the isotherms for C02 on two mole sieve materials. These plots are based on information provide in Kohl and Nielsen (1997). 

In Plant 2 the project team focused the exposure monitoring effort on personnel involved in the change out of mole sieve desiccant. Plant operators as well as contractors involved in the desiccant change out operation were monitored on a 24-hour basis for several days. In addition, area air sampling stations were set up to monitor mercury concentrations arormd the work area. All personal and area sampling results were below the action limit. Results from the analysis of process samples are presented in the following table ... 

An adsorption site is the area where one solvent molecule may reside In general a micropore (major diameter < 2 nm) will accommodate many solvent molecules. However, a site may also be a volume as more than one solvent molecule may be attached to a single area within a pore It s not absorption when solvent vapor molecules diffuse to the internal volume of activated carbon and attach to internal surfaces Its adsorption because the attachment is to the activated surface. Solvent vapor molecules never penetrate into the carbon matrix (printed in black in the two figures above) - that would be ackorption Information in Chapter 3, Footnote 94 describes use of mole sieves for drying of water from solvents the opening of micropores for adsorption of chlorinated solvents is relatively large. Mole sieves rated at 5 A (0. 

Recall from Chapter 3, Footnote 93 that adsorption of water on mole sieves or solvent on activated carbon is exothermic. Fleat is given off on adsorption of vapor, and must be supplied to desorb those molecules. 

Consequendy, the facilities present in Figure 4.40 must be augmented. The expanded facilities are noted in Figure 4.41 as being within the dashed box, and include (1) the ability to dilute the aqueous stream containing soluble solvent with clean water, and (2) the ability to dry water from the recovered solvent by using another adsorption bed. The second bed has 3 A mole sieves ". ... 

A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

The special case involving the removal of a low (2—3 mol %) mole fraction impurity at high (>99 mol%) recovery is called purification separation. Purification separation typically results in one product of very high purity. It may or may not be desirable to recover the impurity in the other product. The separation methods appHcable to purification separation include equiUbrium adsorption, molecular sieve adsorption, chemical absorption, and catalytic conversion. Physical absorption is not included in this Hst as this method typically caimot achieve extremely high purities. Table 8 presents a Hst of the gas—vapor separation methods with their corresponding characteristic properties. The considerations for gas—vapor methods are as follows (26—44). 

To a solution of 33 g. (O.S mole) of potassium hydroxide (Note 1) in 1.5 1. of distilled water in a 5-1. flask or other appropriate container fitted with a mechanical stirrer is added 80 g. (0.5 mole) of methyl hydrogen adipate (Note 2). With continuous stirring a solution of 85 g. (0.5 mole) of silver nitrate in 1 1. of distilled water is added rapidly (about IS minutes). The precipitated methyl silver adipate is collected on a Buchner funnel, washed with methanol, and dried in an oven at 50-60°. For the next step the dried silver salt is finely powdered and sieved through a 40-mesh screen. The combined yield from two such runs is, 213 g. (80%). 

A solution of bromine (24.9 g, 2.1 mole) in acetic acid (100 ml) is added over a period of 4 min at 16° to a rapidly stirred fine suspension (60 mesh sieve) of... 

A Welsbach T-816 Ozonator purchased from the Welsbach Corporation, Philadelphia, Pennsylvania, was used. The oxygen stream was dried by passage through dry silica gel and molecular sieves and then introduced into the ozonator with the operating voltage set at 115 V., the gas pressure at 8 p.s.i.g., and the gas flow rate at 1 1. per minute. The resulting ozone flow rate was 0.00245 mole per minute, as determined by titration of a potassium iodide trap. ... 

Syntheses. Isotactic poly(methyl methacrylate) was synthesized by the method of Tsuruta et al. (9 ). Under a nitrogen atmosphere, a quantity of 6 mL (0.056 mole) of methyl methacrylate (MMA) dried over 4A molecular sieve was dissolved in 24 mL of similarly dried toluene. To the glass vial containing the reaction was added 0.65 mL of 1.6 M n-butyllithium, and the reaction was kept at -78°C in a dry ice/isopropanol bath. The polymerization was halted 24 hr later with the addition of hydrochloric acid and methanol (methanol/water 4.1 by volume). The polymer was dried in vacuo at 50°C, redissolved in methylene chloride, precipitated by being poured into water-containing methanol, and dried in vacuo at 50°C. Tacticlty and composition were verified with % NMR. Yield 47%. 

In an attempt to quantify the relationship between the TiOOH groups and the yield of propene oxide from the extinction coefficients of the latter s 1409-and 1493-cm-1 bands, it was determined that 0.6 mol of the epoxide formed per mole of framework Ti center in the molecular sieve. That is, at least 60% of all framework Ti (80% of the surface-exposed Ti) is converted to TiOOH upon reaction with H202. The consumption of the TiOOH species during the oxygen insertion into propene was also independently confirmed by the loss in intensity of its LMCT band at 360 nm when the catalyst was brought in contact with propene at room temperature (Fig. 50). 

Catalytic asymmetric epaxidation (13, 51-53). Complete experimental details are available for this reaction, carried out in the presence of heat-activated crushed 3A or powdered 4A molecular sieves. A further improvement, both in the rate and enantioselectivity, is use of anhydrous oxidant in isoctane rather than in CH2C12. The titanium-tartrate catalyst is not stable at 25°, and should be prepared prior to use at -20°. Either the oxidant or the substrate is then added and the mixture of three components should be allowed to stand at this temperature for 20-30 min. before addition of the fourth component. This aging period is essential for high enantioselectivity. Epoxidations with 5-10 mole % of Ti(0-/-Pr)4 and 6-12% of the tartrate generally proceed in high conversion and high enantioselectivity (90-95% ee). Some increase in the amount of catalyst can increase the enantioselectivity by 1-5%, but can complicate workup and lower the yield. Increase of Ti(0-i-Pr)4 to 50-100 mole % can even lower the enantioselectivity. 

In a i-l. round-bottomed flask, equipped with a mechanical stirrer and cooled in an ice-salt bath, is placed 275 g. (250 cc., approximately 1 mole) of 15 per cent sodium hydroxide solution. This is cooled to —10° (Note 1), and 115 g. (105 cc., approximately 1 mole) of 30 per cent hydrogen peroxide which has been similarly cooled is added in one portion. The heat of reaction causes the temperature to rise markedly. When the temperature has again dropped to — io°, 75 g. (0.5 mole) of phthalic anhydride which has been pulverized to pass a 40-mesh sieve is added as quickly as possible while the contents are stirred vigorously in the freezing mixture (Notes 2 and 3). As soon as all the anhydride has dissolved, 250 cc. (0.5 mole) of 20 per cent sulfuric acid which has been previously cooled to — xo°, but not frozen (Note 4), is added. 

The reager t chosen for this study was 1, 1-dime-thoxyethene prepared by dehydroch1orination of chloroace-taldehyde dimethyl acetal according to Met] vain (4.4 dry reagent can be stored for several weeks or months in small vials, with mole.r u)ar sieves, in a refrigerator. 

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