Molecular sieve traps

The recycle reactor is shown schematically in Figure 1. It consists of a catalytic or electrocatalytic reactor unit with a bypass loop, a recycle pump and a molecular sieve trap unit. The latter comprises one or two packed bed columns in parallel each containing 2-10 g of Linde 5A molecular sieve pellets. On line gas chromatography (Shimadzu 14A) was used for the analysis of CH4, O2, CO, CO2, C2H4 and C2H6 in the reactants and products. 

In this work we present results obtained with the YSZ reactor operated in the hatch mode with electrochemical oxygen addition, and with the quartz plug flow reactor operated in the continuous-flow steady-state mode. In the case of continuous flow operation, the molecular sieve trap comprised two packed bed units in parallel in a swing-bed arrangement (Fig. 1), that is, one unit was maintained at low temperature (<70°C) to continuously trap the reactor products while the other was heated for -30 min to 300°C to release the products in a slow stream of He. 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

Figure 5. Effect of adsorbent mass in the molecular sieve trap on the ethylene, ethane and total C2 selectivity at a fixed methane conversion of 15%. Recirculation flowrate 220 cm3 STP/min...

Gases were supplied to the infrared cell from a gas manifold. 4.99% NO in He and 2.14% CH4 in He were obtained from Matheson. Oxygen and helium were obtained on-site. The He, NO, and CH4 cylinders were passed through an oxysorb trap, an ascarite trap, and a molecular sieve trap, in that order, for additional purification. The O2 was passed through an ascarite and a molecular sieve trap. 

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the roughing line, which may occur at low pressure. Contamination by back-streaming oil can be drastically reduced by using proper traps like molecular sieve traps with zeolite (see Section 1.6.4). 

The — 78 °C trap and molecular sieve trap are designed to remove CH3CN. Since traces of moisture are inevitably present, this means that HC1 is also likely to be produced the molecular sieve trap will also remove it. An alternate... 

The HC1 dissolves quickly in the water but the PF3 hydrolysis is slow. The product is then given a final purification from water and other materials by a vacuum distillation from a methylcyclohexane slush bath (—128 °C). However, the molecular sieve trap works better and avoids the 10 to 20% loss that occurs from PF3 hydrolysis. The product should give a negative chloride test7 indicating absence of HC1 and/or by-products PC1F2 or PC12 F. 

The carrier gas was chemically pure helium which was dried by passing through a molecular sieve trap (as were the feed gases methane, ethane,... 

Carrier gases are normally obtained in bottled form at about 2500 psi (150-160 atm). A two stage regulator is recommended at the cylinder. The second stage pressure regulator is usually set at 40-100 psi. After the gas leaves the cylinder it should pass through a molecular sieve trap (grade 13X). The trap will remove... 

Spurious peaks could occur due to volatile compounds emitting from the flow controller diaphragm. This problem can be solved by use of metal diaphragms. Some manufacturers have used short molecular sieve traps between the flow controller and the inlet system, but, if a poor inlet system is used, the sample can flash-back upon injection and condense on the trap, later causing spurious peaks or noise and drift as the sample slowly comes back off the trap. 

Hydrogen (Takachiho Co., 99.999%) and He (Takachiho Co., 99.999%) were dried by passing them through a Deoxo unit (SUPELCO Co. Oxysorb) and a Linde 13X molecular sieve trap prior to use. NH3 (Takachiho Co., 99.999%) was used without further purification. The alumina-supported molybdena was prepared using a mixture of hexa-ammonium molybdate and y-alumina (Nikki Chemicals Co.) and calcined in air at 823 K for 3 h. 

The decision of what type of trap to use is typically more likely made by what one is familiar with, what is available, and/or what is economically viable. For example, you may be inclined to use liquid nitrogen in a situation where dry ice would be adequate if you re in a facility that has a lot of liquid nitrogen available. On the other hand, you might try to get by with dry ice when you don t have access to liquid nitrogen. Alternatively, perhaps there is a situation where a molecular sieve trap would be optimum. If you are not familiar with that type of trap, it is less likely to be used. 

Oxytraps, 8—Molecular sieve trap, 9—Line to sample cavity, 10—Line to reference cavity, 11—N2 purge streams through plastic shrouds, 12—Draft shield, 13—Aluminum block. 

Infrared spectra were recorded on a Perkin-Elmer I80 grating spectrometer. Self-supporting wafers of the catalyst powder were placed in an situ quartz IR cell (19) which allowed pretreatments at various conditions. Before the NO adsorption experiments, N2 (purified by passage through Cu turnings at 523 K and a molecular sieve trap (Linde 5A) kept at 195 K) was passed over the catalyst at 673 K for 16 hr. This was followed by cooling to ambient temperature. Nitric oxide (99% purity) was further purified by freeze-thaw cycles. Further details have been given previously (20). 

Molecular Sieve Trap including All items fabricated 108.00 3/8 in. Swagelok Union by Glass Tech. Service... 

Fig. 1. Schematic diagram of the apparatus. N, needle valve M, molecular sieve trap Ti T4, liquid nitrogen traps and Bz, buffer volumes Bi B3, capillary tubes B, reactor F, furnace D, detector ML, McLeod gauge.

Samples were taken from the top, the middle and the bottom of each reactor and their coke content was determined by means of a LECO analyzer. In this determination, the sample is combusted in a high- frequency induction furnace and the carbon dioxide produced is selectively adsorbed in a molecular sieve trap. The C02 is later released by heating the trap and quantified by thermal conductivity. 

Xenon is next eluted in Area 2. At the end of the sampling period, the main trap is valved off from the system and heated to 200° C to release the xenon. The outflow from the main trap is carried by ultrapure nitrogen (no CO2) through a MFC at 0.24 1/min. Desorption is timed to release most xenon flow is diverted when the remaining radon is expected to desorb. This slow flow takes the desorbed xenon gas with N2 carrier gas though one of a series of disposable chemical traps (NaOH + Al) to remove most CO2. The now purified gas is collected on a 0.2-15 A molecular sieve trap at -40 C, while the carrier N2 escapes. The trap is then heated to 200°C to desorb xenon and transferred to a tiny cold trap that is cooled to -120°C to sorb xenon in preparation for loading the counting cell. This step is necessary to transfer the gas into a scintillation cell, as shown in Area 3 of Fig. 15.7 and also in Fig. 15.8. 

Temperature-programmed reduction (TPR) experiments were carried out in a quartz-made microreactor connected to a thermal conductivity detector (TCD) equipped with active charcoal column, using 0.2 g calcined catalysts from 373 K to 1073 K. The gas stream, 5 % H2 diluted by nitrogen as reducing gas, was fed via a mass flow controller. After the reactor, the effluent gas was led via a 3 A molecular sieve trap to remove the produced water. 

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