Permeance, membrane separation

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.

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... 

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. 

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)... 

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. 

Recycle procedures for spent membranes have been developed to improve membrane economics by recycled membranes to function in the same manner as the originals. The permeances of the membranes vary in a range of 30-70 N m m h bar ° and are affected by the support and process parameters. The CRI membranes operate in a pressure range of 30 5 bar dP and at temperatures of up to 550 °C. Typical examples of the performance of CRI-Criterion Pd/Stainless Steel H2 separation membrane are given in Table 11.7. 

The two most important characteristics of inorganic membranes are permeance and separation factor. Permeance is a measure of the gas flow rate per unit area per unit pressure difference. A more fundamental unit is permeability, which is the permeance multiplied by the thickness of the membrane. In most cases, the thickness of the membrane is not known very accurately and so permeance is a more practical unit. 

The most commonly investigated performance characteristics of gas separation membranes are flux, permeability coefficient/permeance and selectivity. The flux is the amount (mass or moles) of gas that permeates through the membrane per unit time and unit surface area the permeability coefficient is the quantitative expression of a specific measure of gas moving through a membrane and the selectivity is the separating ability of a given membrane (Ockwig and Nenoff, 2007). 

A variety of syngas compositions, ready to be fed to the membrane modules, can be produced by acting on the steam/carbon ratio and on the reformer outlet temperature through the variation of the firing of two hot gas generators. This makes it possible to study the role played by partial pressures of the main components in separation performance. The effect of temperature on membrane permeance at fixed reformer working conditions may be studied by varying the temperature set point of two air coolers placed at the inlet of the membrane modules. 

The driving force of a membrane for gas separation is the pressure difference across the membrane. The yield of the separated gas can be expressed in terms of membrane permeance, which can be characterised by the amount of permeated gas that passes through a certain membrane area in a given time at a definite pressure difference. The values of permeability are often quoted in Barter (1 Barter=10 cm s cm cm Hg = 3.35 x 10 mol m m s Pa STP, standard temperature and pressure). Gas permeation phenomena can be described by a simple solution diffusion model, which involves (l) sorption or dissolution of the permeating gas in the membrane at the higher pressure side, (2) diffusion through the membrane and (3) desorption or dissolution at the lower pressure side. Thus, the permeability coefficient P can be determined by the product of the solubility coefficient S and the mutual diffusion coefficient D [eqn (5.1)] ... 

The manufacture of DPMs based on infiltrated molten carbonates in the porosity of perovskite membranes constitutes the most recent and innovative application of perovskites for CO2 capture. Since the first studies reported by Wade and coworkers [33] and Lin and coworkers [34], many efforts have been made to develop stable high-flux membranes for CO2 separation. Table 39.5 collects the most remarkable results, whereas Figvue 39.12 plots the permeance versus permeability plots for dual-phase perovskite-carbonate materials compared with low-temperature CO2 separating membranes (zeolites and MOFs/ZIFs). 

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