Membranes permeance

Figure 8.16 Block diagrams illustrating the use of C02-selective membranes to separate C02 from power-plant flue gas. These calculations are based on a power plant producing 600 MWe of power. The membrane permeances and size of the membrane units are based on membraneswith a C02 permeance of 1000 gpu and a C02/N2 selectivity of 40.

The synthesis and characterization of permselective membranes are new areas of activities in materials science [1,12,20], Nevertheless, to use these membranes commercially, much work remains to be done [12,20], It is necessary to increase membrane permeance, to resolve brittleness problems, to be capable of scaling-up the process, and to increase the membrane area per unit volume [12,20],... 

Unfortunately due to time limitations, there was no time to coat these steam stable mesoporous membranes with a microporous silica toplayer. Microporous (doped) silica membranes have, however, been applied on conventional mesoporous Y-AI2O3 membranes which were not stable under SASRA conditions. On these membranes permeance measurements have been performed which showed that these membranes could be prepared with a very high selectivity under cleanroom conditions. Stability measurements on (doped) silica bulk material sintered at a high temperature (600-800°C) showed no change in the specific surface area during SASRA treatment. This is an indication that it should be possible to prepare a silica membrane that shows complete stability towards SASRA conditions. 

In an air separation process using a perfect-mixing membrane separator, the feed air flow rate is rip = 1 x 10 cm- (STP)/s, with mole fraction oxygen yp = 0.21 and mole fraction nitrogen yp2 = 0.79. The membrane permeances for oxygen and nitrogen are, respectively, 1.5 x 10" and... 

Their laboratory PVMR consisted of a reservoir in which the reactants were placed together with Nafion pellets, which acted as the catalyst. The liquid in the reservoir was continuously recirculated through the membrane tube, which was placed externally to the reactor. The membrane, itself, was also shown to be catalytic. A flow of inert gas (rather than vacuum) was used to remove the vapors and water from the membrane permeate. For the methanol esterification reaction the improvement in yield was modest (final conversion 77 % vs. 73 % corresponding to equilibrium), because the membrane was not very permselective towards the reaction products. Significant improvements, on the other hand, were observed with the butanol reaction (final conversion 95 % vs. 70 % corresponding to equilibrium), as the membrane is more permselective towards the products of this reaction. Exchanging the acidic protons in the Nafion membranes with cesium ions significantly improved the permselectivity, but also reduced membrane permeance. 

Membrane systems can also be integrated into traditional units. The design of a hybrid membrane separation system is determined by several considerations, such as membrane permeance and selectivity, CO2 concentration of the inlet gas and the target required, the gas value (per ca. 30 N m, the price of gas in 2007 was 6- 7 in the United States, whereas in Nigeria, which is far from being well-developed gas market, it may be as low as 0.50 if the gas can be used at all), and the location of the plant (on an offshore platform, the weight, footprint, and simplicity of operation are critical onshore, the total cost is more significant). ... 

The lUPAC defines a membrane reactor as a device for simultaneously carrying out a reaction and membrane based separation in the same physical enclosure. In fact, designing a cost-effective membrane reactor requires that heat transfer, mass transfer, reaction rate, and membrane permeance are well matched in the reactor design. 

Rate Equations based on Membrane Permeance (Constitutive Relations) ... 

Many studies on systems in the current literature did not consider the Joule-Thompson effect caused by the expansion of permeate gas due to the pressure difference between the high retentate pressure and the low permeate pressure, also known as transmembrane pressure. This expansion leads to a decrease in the permeate temperature, which in turn decreases the membrane permeance. So, ignoring the Joule-Thomson effect may result in a wrong estimation of membrane separation performance and consequently of the reboiler/condenser duties and utility savings obtained from an HMD system. The membrane model employed in the present study takes into account the Joule-Thompson effect by including the following energy balance [Equation (10.2)] ... 

Membrane permeance and selectivity data are based on those of polymeric membranes obtained from Faiz and Li (2012). This is due to a lack of studies on CMS membranes for the separation of olefins/parafifins. The assumption of the use of polymeric membrane data is conservative since CMS membranes have been shown to give better permeance and selectivity than polymeric membranes for olefln/paraffin separation (Xu et al., 2012)... 

The optimal value of membrane feed flowrate (Figure 10.10b) is initially near the lower bound before increasing non-linearly for increasing utility cost savings. The optimal values of membrane area and membrane permeance (Figure 10.10c and lO.lOd) follow the trend observed in Case 1. The optimal values of pressure difference across the membrane... 

The extreme point of the Pareto-optimal point before the spike ( 4.55 Million/yr utility cost savings at 0,462 Million/yr capital cost) is the best choice for rettofitting the PF to an HMD system. This optimal solution corresponds to a membrane feed flowrate of 300 kmol/h, membrane area of 1497 m, membrane permeance of 0.0149 kgmol/m -h-bar, and pressure difference across the membrane of 1560 kPa. It gives a net savings of 4.16 Million/yr (that is, 67% savings on the current utility cost), which is very attfactive with a payback period of just about one year. 

Ahmad, R, Lau, K.K., Shariff, A.M. and Yeong, Y.R (2013) Temperature and pressure dependence of membrane permeance and its effect on process economics of hoUow fiber gas separation system. 

The proposed scheme of Pre-combustion decarbonation based on the use of membranes for hydrogen separation is expected to have some advantages over the conventional and commercially ready technology, such as superior efiiciency and reduction in overall plant capital cost. Membrane-based systems may represent a real advancement in state-of-the-art H2 and CO2 capture in such power plants however, their real application is closely hnked to the realization of a membrane module that is economically built and maintained, as well as to the improvement of membrane permeance and selectivity. 

The effectiveness of the MS module depends on H2 membrane permeance, separation and pressure difference across the membrane (AP). Consequently, to keep AP at its maximum value, the partial pressure of hydrogen on the outside of the membrane should be as low as possible. Hydrogen would, therefore, be swept from the outside surface by employing a flow of a non-reactive gas, such as steam or be pumped away. For a fixed (AP), the dependence of the amount of hydrogen removed by the MS membrane and the total achievable H2S conversion on H2 permeance were calculated and are shown in Figs. 8.7 and 8.8. 

By comparing the expressions in eqn (14.7), this definition can be read in two ways when concentration polarization is negligible, i.e. CPC 0, membrane permeance is, in fact, coincident with that of bulk, or, dually, the permeation driving force between the bulks is the same as that between the membrane surfaces. It should also be noticed that the condition of maximum polarization, i.e. CPC= 1, is an asymptotic conditions where permeating flux tends to zero because the hydrogen partial pressure at membrane surface approaches to zero. This situation is summarized in Table 14.2. 

Hence, according to its definition (eqn (14.7)), CPC can be evaluated from eqns (14.8) to (14.10) (all of them equivalent to each other), where it is underlined that the product between membrane permeance and bulk driving force corresponds to the hydrogen flux evaluated in presence of pure hydrogen on both membrane sides ... 

However, to relate CPC to the membrane permeance according to eqn (14.1), it is necessary to choose an appropriate form of the bulk driving force. Concerning this issue, it should be considered that, when the rate-determining step of the overall permeation process is the diffusion in the Pd-based layer and, also, the atomic hydrogen concentration inside membrane... 

The simulation results obtained were reported in terms of polarization maps, which were developed in order for CPC to be read directly and used under the considered operating conditions to estimate the transmembrane flux once the intrinsic membrane permeance and Sieverts driving force of bulk are known (eqn (14.12)). The results from the model solution have been confirmed by a comparison with the experimental ones in order to validate the analysis ... 

As regards the first case (horizontal dashed lines), it is possible to notice that the membrane permeance is constant. This is because it represents an intrinsic property of the material, which, thus, cannot be influenced by any external fluid-dynamic factor. 

However, the H2 recovery and purity obtained with the membrane operation are lower or comparable with those of the other two systems therefore the mass intensity is higher. It must be noticed that the membrane systems considered in these calculations operate at 50 °C. However, at a higher temperature, the membrane permeance increases therefore higher recovery can be expected. Moreover, the results presented for the PSA and cryogenic systems consider the treatment of a refinery off-gas, where the amount of hydrogen in the feed is 50-75% higher than that used in this work (30%). 

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