Molecular sieves, separation gases with

Hatori, H., H. Takagi, and Y. Yamada, Gas separation properties of molecular sieving carbon membranes with nanopore channels, Carbon, 42, 1169-1173, 2004. 

Figure 9.4 The molecular-sieve composite gas-separation membrane. From Ref. [29] with permission.

To overcome these limitations, mixed matrix membranes (MMM) started to emerge as an alternative approach in membrane technology. In this approach, the superior gas separation properties of the molecular sieve materials combine with the desirable mechanical properties and economical processability of polymers (Moore et al. 2004). A mixed matrix is a blend of inorganic particles (such as nanoparticles) in a polymer matrix, which are well dispersed. The effect of the inorganic dispersed phase on the MMM properties is related to its chemical structure, surface chemistry, and the type of particles. The inorganic materials used... 

These remarkable aluminosilicates can be used as drying agents, ion-exchangers, and molecular sieves for gas separation. Their microporosity provides them with high surface area, and they can be converted into solid acids with superb catalytic activity. 

This type of analysis requires several chromatographic columns and detectors. Hydrocarbons are measured with the aid of a flame ionization detector FID, while the other gases are analyzed using a katharometer. A large number of combinations of columns is possible considering the commutations between columns and, potentially, backflushing of the carrier gas. As an example, the hydrocarbons can be separated by a column packed with silicone or alumina while O2, N2 and CO will require a molecular sieve column. H2S is a special case because this gas is fixed irreversibly on a number of chromatographic supports. Its separation can be achieved on certain kinds of supports such as Porapak which are styrene-divinylbenzene copolymers. This type of phase is also used to analyze CO2 and water. 

However, ia some cases, the answer is not clear. A variety of factors need to be taken iato consideration before a clear choice emerges. Eor example, UOP s Molex and IsoSiv processes are used to separate normal paraffins from non-normals and aromatics ia feedstocks containing C —C2Q hydrocarbons, and both processes use molecular sieve adsorbents. However, Molex operates ia simulated moving-bed mode ia Hquid phase, and IsoSiv operates ia gas phase, with temperature swiag desorption by a displacement fluid. The foUowiag comparison of UOP s Molex and IsoSiv processes iadicates some of the primary factors that are often used ia decision making ... 

Solid-Bed Dehydration. Sihca gel, bauxite, activated alurnina, or molecular sieves can be used for removing dissolved water to meet propane specifications. The soHd-bed dehydrators are used in a cycHc adsorption process. After an adsorption cycle has completed, the bed is heated with a purge gas or a vaporized Hquid-product stream for regeneration. If the latter is used, the Hquid product is condensed, separated from the free water, and returned to the process. After the beds are regenerated, they are cooled and returned to the adsorption cycle. 

An enrichment is defined as a separation process that results in the increase in concentration of one or mote species in one product stream and the depletion of the same species in the other product stream. Neither high purity not high recovery of any components is achieved. Gas enrichment can be accompHshed with a wide variety of separation methods including, for example, physical absorption, molecular sieve adsorption, equiHbrium adsorption, cryogenic distillation, condensation, and membrane permeation. 

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). 

Experience in air separation plant operations and other ciyogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubihty limit. For this reason, the carbon dioxide content of the feed gas sub-jec t to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

The extraction process at BP-Amoco Empress begins with natural gas arriving at the plant at about 15°C and 600 psi pressure. The gas is dehydrated to a -90°C dewpoint by means of molecular sieves. Still at 600 psi, the gas is introduced into heat exchangers and cooled to -70°C, at which point it begins to liquify in a separator. 

The human factors audit was part of a hazard analysis which was used to recommend the degree of automation required in blowdown situations. The results of the human factors audit were mainly in terms of major errors which could affect blowdown success likelihood, and causal factors such as procedures, training, control room design, team communications, and aspects of hardware equipment. The major emphasis of the study was on improving the human interaction with the blowdown system, whether manual or automatic. Two specific platform scenarios were investigated. One was a significant gas release in the molecular sieve module (MSM) on a relatively new platform, and the other a release in the separator module (SM) on an older generation platform. 

A five-column configuration of Such an analyser system is depicted in Figure 14.6. The first event in the process is the analysis of Hj by injection of the contents of sample loop 2 (SL2) onto column 5 (a packed molecular sieve column). Hydrogen is separated from the other compounds and detected by TCD 2, where nitrogen is used as a carrier gas. The next event is the injection of the contents of sample loop 1 (SLl), which is in series with SL2, onto column 1. After the separation of compounds up to and including C5, and backflushing the contents of column 1, all compounds above C5 (Q+) are detected by TCDl. The fraction up to and including C5 is directed to column 2, where air, CO, COj, Cj, and 2= (ethene) are separated from... 

In this work we present results obtained both with batch and continuous flow operation of the gas-recycle reactor-separator utilizing Ag and Ag-Sm203 electrocatalysts and Sr(lwt%) La203 catalysts, in conjunction with Linde molecular sieve 5A as the trapping material, and discuss the synergy between the catalytic and adsorption units in view of the OCM reaction network. 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

Catalytic activity tests have been performed in a quartz microreactor (I.D.=0.8 cm) filled with 0.45 g of fine catalyst powders (dp=0 1 micron). The reactor has been fed with lean fiiel/air mixtures (1.3% of CO, 1.3% of H2 and 1% of CH4 in air resp ively) and has been operated at atmospheric pressure and with GHSV= 54000 Ncc/gcath The inlet and outlet gas compositions were determined by on-line Gas Chromatography. A 4 m column (I D. =5mm) filled with Porapak QS was used to separate CH4, CO2 and H2O with He as carrier gas. Two molecular sieves (5 A) columns (I D.=5 mm) 3m length, with He and Ar as carrier gases, were used for the separation and analysis of CO, N2, O2, CH4, and H2, N2, O2 respectively... 

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