Membranes Methane data

The process operates at low thermal level (below 650°C in comparison to 850-950°C needed in tradition plants), whereas membrane modules work at 450°C, a safe temperature for Pd-based membrane stability. Data concerning plant behavior after 720 h are extensively reported in Chap. 10 it has been demonstrated that a final methane conversion after a two-step reaction equal to 60% can be obtained, equivalent to an improvement of 20% over the equilibrium threshold (conversion equal to about 50% at 650°C, 10 barg and a ratio between steam and methane feedstocks of 4.8), moreover recovering a highly pure hydrogen stream. 

Decomposition of methane to H2 and carbon over Ni/Si02 was carried out in a membrane reactor (membrane 90Pd-10Ag) [106]. The use of the membrane reactor allowed increasing the H2 yield by shifting the reaction equilibrium toward the products. An excellent review of the literature data on nonoxidative methane activation over the surface of transition metals was recently published by Choudhary et al. [107]. 

Heavy-atom derivation of an object as large as a ribosomal particle requires the use of extremely dense and ultraheavy compounds. Examples of such compounds are a) tetrakis(acetoxy-mercuri)methane (TAMM) which was the key heavy atom derivative in the structure determination of nucleosomes and the membrane reaction center and b) an undecagold cluster in which the gold core has a diameter of 8.2 A (Fig. 14 and in and ). Several variations of this cluster, modified with different ligands, have been prepared The cluster compounds, in which all the moieties R (Fig. 14) are amine or alcohol, are soluble in the crystallization solution of SOS subunits from H. marismortui. Thus, they could be used for soaking. Crystallographic data (to 18 A resolution) show isomorphous unit cell constants with observable differences in the intensity (Fig. 15). 

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V205/Si02 catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 

A C02-CH4 methane process gas stream, similar to a typical high CO2 natural gas has been under test by SEPAREX for CO2 removal in a 2-in. diameter element pilot plant since September 1981. The feed gas contains 30% CO2 and is delivered to the membrane test unit at 250-450 psig under ambient temperature conditions. The objective of the system is to reduce the CO2 level of the methane to less than 3.5%. The membrane system consists of 5 pressure tubes in series, each tube containing three 40-in. long elements. The gas is conditioned to maintain it at a minimum of 20°F above the dewpoint. The system was operated at a variety of flow rates, pressures, recoveries and temperatures. Selected data are presented in Figures 6 through 8. 

Figure 8.6 The difference between selectivities calculated from pure gas measurements and selectivities measured with gas mixtures can be large. Data of Lee et al. [13] for carbon dioxide/methane with cellulose acetate films. Reprinted from S.Y. Lee, B.S. Minhas and M.D. Donohue, Effect of Gas Composition and Pressure on Permeation through Cellulose Acetate Membranes, in New Membrane Materials and Processes for Separation, K.K. Sirkar and D.R. Lloyd (eds), AIChE Symposium Series Number 261, Vol. 84, p. 93 (1988). Reproduced with permission of the American Institute of Chemical Engineers. Copyright 1988 AIChE. All rights reserved...

Membranes have been suggested for use in the separation of helium from natural gas [Spillman, 1989] which typically contains 85% methane and 10% ethane. While no test data using real or simulate natural gas is available, there is some information on the separation of helium and ethane using alumina and silica membranes [Havredaki and Petropoulos, 1983], Table 7.16. Clearly, Knudsen diffusion is dominant in the these limited tests. Thus no promising separation performance has been demonstrated. 

Geus et al. [75] reports diffusion data at 21 and 145°C for Hj, N2, CH4, CO2 and CF2CI2 in silicalite membranes on a clay support which are obtained with the similar transient permeation technique as used above by Vroon. The diffusion coefficients for methane are about two orders of magnitude smaller than those obtained by PF-NMR methods. Usually this last technique gives relatively large diffusion coefficient values, which in the case of n-butane are of the same order of magnitude as reported for FR techniques and membrane techniques as reported by Kapteyn. 

The curves shown in Figures 3 and 4 are simulated composition profiles based on experimental data. The calculated trends fit the experimental compositions quite well, and in each case the experimental methane peak is well described. This demonstrates that the basic model for the membrane column can be applied to multicomponent systems as well as to binary mixtures. 

Studies by Chan and Floss et al. [32a] with another kind of MMO, the so-called particulate form pMMO from Methylococcus capsulatus (Bath) (which is membrane bound and contains copper), confirmed that the rate controlling step of methane hydroxylation has a very high KIE, while ethane produces only a moderate KIE. The data obtained in this work point to a mechanism in which C-H bond cleavage is preceded by bond formation at the alkyl carbon, i.e., one proceeding through a pentacoordinated carbon species [32a]. 

Hydrogen sulfide and methane fluxes were measured at ambient conditions for 200 um perfluorosulfonic acid cation exchange membranes containing monoposltlve EDA counterions as carriers. Facilitation factors up to 26.4 and separation factors for H2S/CH up to 1200 were observed. The HjS transport Is diffusion limited. The data are well represented by a simplified reaction equilibrium model. Model predictions Indicate that H S facilitated transport would be diffusion limited even at a membrane thickness of 1 um. 

Figure 12.4 Methane conversion against temperature for membrane reactor. Comparison between experimental data (symbols) and model results (lines) for a 40 SCCM sweep flow rate. Reprinted from G. Barbieri, G. Mar-igliano, E. Drioli, Simulation of steam reforming process in a catalytic membrane reactor, Ind. Eng. Chem. Res., 36, 6, 2001, with permission of American Chemical Society.

Figure 6.4 Pure gas transport data at 25 °C of membranes AF1600 (O), AFl 6 350 30 fD), AF16 80 15 (A), AF16 80 30 (U), AF16 80 40 ( 0), silicalite-1 (O) as derived from literature data (see text), and predictions of the Maxwell model fora AF16/MFI30% membrane ( ) (a) Pure gas steady state permeability vs kinetic diameter of the permeating molecules (b) gas/methane separation factor (c) gas diffusion coefficients from time-lag experiments vs kinetic diameter (d) gas solubility vs the e/k Lennard-Jones parameter...

Table 2.3 Experimental data taken from the open literature concerning the methane steam reforming (MSR) reaction in packed-hed membrane reactors (PBMRs) and fluidized-hed membrane reactors (FBMRs)... 

Figure 6.21. Experimental data for the permeation of CO2 and methane gas through cellulose acetate membranes of different pore sizes. (Reproduced from [233] with permission.)...

GRACE Systems Cellulose acetate membranes are also used in spiral-wound configuration. Their data, reproduced in Figures 10.16,10.17, and 10.18, indicate that the permeability, defined as mol/m s kPa, increases with an increase in transmembrane pressure, for both methane and CO2. However, the... 

A very similar combined process scheme integrating OCM and steam reforming of methane (SRM) was suggested at the same time by Tiemersma et al [44]. OCM is performed in membrane reactor for distributed O2 feeding. The membrane reactor tubes are immersed into a fluidized-bed reforming reactor for optimal heat transfer. Simulation of such process based on kinetic data obtained... 

For the IC combustion application, the feed and permeate gas pressnies are assnmed to be 40 psig and 0 psig, respectively. The feed composition is 35% CH, 35% CO2 and 30% N2 and O2. The processed gas quality that is collected on the netentate side should meet the requirement of 50% CH, 10% CO2 and 40% N2 and O2. Methane yield of the process is 83.9%. The CO CYi selectivity of the membrane is set equal to 10. The same selectivity is assumed for CO2/N2 and COJO. The permeance of CO2 is set equal to 1.47 mVm h bar. Using these data the permeate composition can be calculated to be 15% CH, 72% CO2 and 13% N2 and O2. Mass balance tells us the stagecut required to achieve the above retentate and permeate composition. From the feed and retentate flow rate and the CH concentration in the feed and retentate, the CH yield can finally be obtained. The results of the calculation are summarized in Table 10.6. 

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