Molecular, flow 21, sieve

Porosity, pore size >1 ml/g, 2-50 nm Resistance to diffusion, molecular weight sieving, flow properties, enzyme retention... 

PU is a versatile class of thermoplastic polymer that possesses a range of excellent properties, such as excellent elastomeric property, resistance to abrasion, and high durability [42-44]. Sambaer et al. [45] proposed a new approach to create a simple 3D nanofiber membrane model based on the real structure features of PU membranes. The slip/transition/free molecular flow regime, particle-flber interactions, air/particle slip, sieve, and homogenous flow fleld have been considered in this full 3D particle filtration model (Fig. 12.5a). They suggested that in the force... 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

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. 

In the physical separation process, a molecular sieve adsorbent is used as in the Union Carbide Olefins Siv process (88—90). Linear butenes are selectively adsorbed, and the isobutylene effluent is distilled to obtain a polymer-grade product. The adsorbent is a synthetic 2eohte, Type 5A in the calcium cation exchanged form (91). UOP also offers an adsorption process, the Sorbutene process (92). The UOP process utilizes ahquid B—B stream, and uses a proprietary rotary valve containing multiple ports, which direct the flow of Hquid to various sections of the adsorber (93,94). The cis- and trans-isomers are alkylated and used in the gasoline blending pool. 

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

The conclusion is that for chemisorption measurements in a CSTR, the matter in the empty space must be minimized, which calls for low (atmospheric) pressure, and low concentration of the chemical, in a low flow of carrier gas. Even at low pressure it will work only for very large surface area materials, like molecular sieves or active charcoals. 

Natural gas from MESA s wells flows into a gathering system where pressure is increased to 7 bar (100 psig). Multiple booster stations raise it to 34 bar (500 psig) before gas enters the plant for separation. When gas enters the LNG recovery unit, its pressure must be raised again to 66 bar (950 psig). It is then subjected to a molecular sieve process for moisture removal. A series of heat exchangers lowers the temperature to -34°C (-30°E). 

Normal paraffins in the C,o - C,5 range are recovered from petroleum fractions by adsorption-desorption using molecular sieves. Ammonia can be used to desorb the n-paraffins. By employing two beds of sieves, one on adsorption and one on desorption at all times, a continuous flow of the feed and ammonia is maintained. 

The absorption process usually occurs at moderate pressure, Ionic bonds tend to achieve an optimum performance near 450 psig, but the process can be used for a wide range of pressures. The molecular sieve bed is regenerated by flowing hot sweet gas through the bed. Typical regeneration temperatures are in the range of 300-400°F. 

Tlie cooled gaseous products are dried using an adsorbent such as molecular sieves and compressed to about 500 psig by a multistage compressor. The compressed gas is dien sent to an acetylene converter where acetylene is selectively hydrogenated to ediane. The gaseous mixture dien flows to die purification section of the plant where each component of die gas is recovered by means of cryogenic disdlladon. 

Determination of oxygen. The sample is weighed into a silver container which has been solvent-washed, dried at 400 °C and kept in a closed container to avoid oxidation. It is dropped into a reactor heated at 1060 °C, quantitative conversion of oxygen to carbon monoxide being achieved by a layer of nickel-coated carbon (see Note). The pyrolysis gases then flow into the chromatographic column (1 m long) of molecular sieves (5 x 10-8 cm) heated at 100 °C the CO is separated from N2, CH4, and H2, and is measured by a thermal conductivity detector. 

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. ... 

After cooling, the microspheres were washed by decantation with petroleum ether to give a free-flowing powder. They were then sieved, dried, and stored in a freezer. Size distribution can be controlled by the stirring rate the yield is 70-90%. The process was quite reproducible with respect to yield, size, and loading distribution, if the same molecular weight of polymer was used. Less than 5% error was observed (5). 

The schematic diagram of the experimental setup is shown in Fig. 2 and the experimental conditions are shown in Table 2. Each gas was controlled its flow rate by a mass flow controller and supplied to the module at a pressure sli tly higher than the atmospheric pressure. Absorbent solution was suppUed to the module by a circulation pump. A small amount of absorbent solution, which did not permeate the membrane, overflowed and then it was introduced to the upper part of the permeate side. Permeation and returning liquid fell down to the reservoir and it was recycled to the feed side. The dry gas through condenser was discharged from the vacuum pump, and its flow rate was measured by a digital soap-film flow meter. The gas composition was determined by a gas chromatograph (Yanaco, GC-2800, column Porapak Q for CO2 and (N2+O2) analysis, and molecular sieve 5A for N2 and O2 analysis). The performance of the module was calculated by the same procedure reported in our previous paper [1]. 

To ensure a better separation, molecular sieving will act much better This size exclusion effect will require an ultramicroporous (i.e pore size D < 0.7 nm) membrane Such materials should be of course not only defect-free, but also present a very narrow pore size distribution. Indeed if it is not the case, the large (less separative and even non separative, if Poiseuille flow occurs) pores will play a major role in the transmembrane flux (Poiseuille and Knudsen fluxes vary as and D respectively). The presence of large pores will therefore cancel any sieving effect... 

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. 

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). 

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... 

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