Permeation units

The following derivations and calculations avoid any specific assumptions about units. Instead, the quantities have been described as having consistent units. The degree of separation and recovery, moreover, is independent of the units. If, however, numerical values for permeability, pressure, and flow rate are used, then specific units are required, which in turn determine membrane area, given the membrane thickness, or the overall membrane permeability. 

Commonly used units for the membrane permeability P- for a component i are 

Other units used for the permeability of solids to gases are presented in the International Critical Tables (vol. V, pp. 76-77)  

Henceforth, the symbol P, or, say, P is used for the permeability of an arbitrary component i or /. That is, both i and / denote the key components for the separation, especially for a two-component system but also in a multicomponent system. Inasmuch as the symbol P also is used to denote pressure, as is the common practice, some other symbol could be adopted for permeability, say lower-case, script, or boldface P or Greek or the like. However, the subscripted P- or P- seems self-evident, so pressure is also subscripted to provide its distinguishing feature. 

Therefore, P stands for the higher or upstream pressure (or reject pressure) at the membrane and Py, for the lower or downstream pressure (or permeate pressure), with the difference - P, denoting the pressure-drop across the membrane proper. The analogy is akin to that used for phase separations. Furthermore, the usage in the main appears self-evident in context. 

---

The permeability of gases through membranes is most commonly measured in Barrer, defined as 10-10 cm3(STP)/cm2 s cmHg and named after R.M. Barrer, a pioneer in gas permeability measurements. The term ji/ pio — pit), best called the pressure-normalized flux or permeance, is often measured in terms of gas permeation units (gpu), where 1 gpu is defined as 10 6 cm3(STP)/cm2 s cmHg. Occasional academic purists insist on writing permeability in terms of mol m/m2 s Pa (1 Barrer = 0.33 x 10-15 mol m/m2 s Pa), but fortunately this has not caught on. 

The common unit of P, is Barrer (IBarrer = 10 10cm3 (STP)-cm/ cm2scmHg = 0.76-10 17m3 (STP) m/m2 s Pa). The ratio P,/l is referred to as the permeance, and its common unit is the gas permeation unit (GPU), which is 10 scm3 (STP)/(cm2 s cmHg). If the downstream pressure is negligible compared with the upstream pressure, the selectivity can be expressed as... 

The transport properties of the membrane were measured by using a membrane permeation unit as described in our previous papers.32 42 Two gas mixtures were used as the feed gas for the gas permeation tests one consisting of 20% C02, 40% H2, and 40% N2, and the other consisting of 1% CO, 17% C02, 45% H2, and 37% N2 (both on dry basis). The second composition was used to simulate the composition of the synthesis gas from autothermal reforming (ATR) of gasoline with air. Argon was used as the sweep gas for the ease of gas chromatography analysis. Unless otherwise stated, the feed pressure was maintained at about 2.0 atm, while the permeate pressure was set at approximately 1.0 atm. 

In a membrane permeator unit two important phenomena are encountered transmembrane transport and flow around the membrane. In a membrane reactor a third phenomenon is of importance chemical reaction... 

Fig. 5 Vapor permeation unit. (View this art in color at www.dekker.com.)...

Isopropanol, ethanol. Standard applications for pervaporation, typically dehydrated from their azeotropes to fractions of a percent of water. Many continuous, batch and vapor permeation units are operating around the world. 

The feed of spent solvent is evaporated and the resulting vapor is fed directly to a vapor permeation unit. Water vapor selectively permeates the membrane and is condensed under vacuum. The water free solvent vapor leaving the vapor permeation unit is condensed and is stored for reuse (product). 

Fig. 24 shows a vapor permeation unit installed to remove methanol from the top of a column, treating the boil-off from a transesterification reactor. The column is operated to condense overhead product close to the azeotropic point. Condensed liquid is refluxed through the column. Net overhead vapor is passed through the vapor permeation unit, which is sized to... 

A separation scheme for methanol removal from a methanol rich methanol/ethyl acetate mixture is shown below. The mixture is distilled to the azeotrope, taking out pure methanol as bottom product. The overhead stream is passed directly to a vapor permeation unit, which permeates a methanol rich stream. This stream is condensed and passed back to the methanol column, via the feed buffer (Fig. 27). 

Retentate from the vapor permeation unit, strongly depleted in methanol, is fed directly to the ethyl acetate column. Pure ethyl acetate leaves this column as bottom product, while overhead azeotrope is sent to the vapor permeation unit. 

Degassing involves chemical, physical and/or electrolytic procedures, as well as the use of gas permeation units [127,128]. Addition of nitrogen and the use of reduced pressure permit a faster and more efficient degassing. These procedures are not however permanent, as there is always a tendency for the solutions to re-equilibrate with the environment. To date, no single efficient and completely safe approach for solution degassing in flow analysis has been reported. 

Even small amounts of carbon monoxide or other contaminants can poison a fuel cell. Sandia National Laboratories (SNL) is working on developing gas-selective thin film membranes to improve and lower the cost for hydrogen purification. Defect-free aluminosilicate and silicalite zeolite thin films supported on commercially available alpha and gamma alumina disk substrates were developed. In tests using SNL s permeation unit, which can test both pure and mixed gases from room temperature to 250°C, excellent separation values for hydrogen were achieved. 

Analyze flux and permeation of gases through membranes on unique in-house permeation unit. 

Designed and built a unique in-house permeation unit that can test both disk and tube membranes, with multiple pure or mixed gases, in a temperature range of 25-500°C. 

Figure 2. Sandia Permeation Unit for Light Gases (including CO), Pure and Mixed (the equipment diagram)...

Built and operated our in-house light gas permeation unit ... 

Hydrogen Flux Measurement in High Pressure Permeation Unit... 

Pig. 7.3 High temperature/high pressure membrane permeation unit... 

Fig. 7.4 Hydrogen fluxes measured from the high pressure permeation unit for the unsupported BCN membrane...

Vapor permeation was found to be a good option for solvent dehydration in cases of impure feed streams containing non-volatile compounds aggressive to the membranes, or when the feedstock contains dissolved solids that can precipitate at the membrane surface, affecting its performance and lifetime. In those cases the liquid feed is evaporated prior to processing by vapor permeation so that the membranes are not exposed to compounds that may affect their performance. The vapor permeation unit also employs hydrophihc polyvinylalcohol (PVA) membranes. Vapor permeation has been also successfully appHed for direct processing of vapor streams from distillation columns. In this way it is not necessary to condense the vapor stream prior to membrane processing. 

Coupling of a distillation column with a vapor permeation unit, the latter treating the vapor from the top of the column, is shown in Fig. 3.18. The distillation column may have to be operated imder pressure in order to allow the membrane system to be run at a temperature of around 100 °C. The permeate can be recycled to the inlet of the distillation column, which wiU result in nearly 100% recovery of the organic component. The only additional energy input in this scheme for the final dehydration of the predistilled product is that for the condensation of the permeate, as the dehydrated product would have to be con-... 

Figure 3.20 depicts a similar arrangement, however, here two organic components, an alcohol and an ester have to be separated and purified from their ternary mixture with water. The ternary vapor mixture from the first column is passed over the membrane of the vapor permeation unit, and nearly all water is removed. The permeate is recycled to the inlet of the first column and all the water is removed from its bottom. The two organic components can now be separated in the second column, with any residual water leaving together with the alcohol. 

In the production of trimethyl borate (TMB) methanol and boric acid are fed to a reactor followed by a reactive distillation column (Fig. 3.24). Methanol is used in a surplus in order to convert all the boric acid and avoid the pollution of the bottom product of the reactive distillation column with residual acid. From the top of this first column an azeotropic mixture of 30% methanol and 70% TMB is obtained under higher than atmospheric pressure. This azeotrope cannot be separated by water wash, as the TMB will immediately hydrolyze when in contact with water. The azeotropic vapor is thus led to a vapor permeation unit, equipped with membranes that permeate methanol, but retain TMB. The concentrated TMB, which contains approximately 3% of methanol, is introduced into a second distillation column, which separates the feed into pure TMB at the bottom and a nearly azeotropic mixture at the top. This mixture is returned into the reflux of the first column, the permeate of the membrane system, mainly methanol, is recycled to the reactor. 

---