Nitrogen membrane modules

Key producers for the nitrogen separation membranes are Air Products, Air Liquide and Ube. They all use either polysulfone or polyimide polymer membranes in hollow fiber configuration. Other nitrogen membrane module producers (IGS Generon systems, Parker Henifiin, Aquillo, etc.) use poly(4-methyl-l-pentene), tetrabromobisphenol polycarbonate and polyphenelyne oxide membranes. 

Capillary membrane modules are not as inexpensive or compact as hollow fine fiber modules, but are still very economical. Their principal drawback is the limited pressure differential the fibers can support, typically not more than 10 to 15 bar. This limitation means capillary modules cannot be used at the high pressures needed for hydrogen or natural-gas processing applications. However, capillary modules are ideally suited to lower-pressure separations, such as nitrogen from air or air dehydration. In these applications, capillary modules have essentially the entire market. 

Capillary membrane modules very similar to those used for nitrogen production are also used to produce dry air. The water molecule is smaller and more condensable than oxygen and nitrogen, so many membrane materials are available with water/air selectivities of several hundred. 

A commercial nitrogen enrichment system is illustrated in Fig. 17. Hollow-fiber membrane modules are connected to a compressed air feed at 70-150 psi. The feed in usually to the bore side of the hollow fibers. Oxygen (and water vapor that may be present) permeate out of the fiber into the shell and exit at low pressure. Dry, nitrogen-enriched air... 

Figure 17 shows a typical arrangement for a hollow fiber membrane module for separation of nitrogen from air. 

A MC module contains thousands of microporous hollow fibres, which are knitted into a fabric that is wound around a distribution tube with a central baffle as shown in Figure 1.15. The baffle ensures the water is distributed across the fibres, and also results in reduced pressure drop across the contactor. The hollow fibres are packed densely in a membrane module with a surfrce area of up to 4000 n / m. The liquid flows outside (shell side) the membrane, while vacuum is appHed on the inside of the fibre (tube side) forming a film across the pores of the membrane. Mass transfer takes place through this film and the pores due to the difference in the gas partial pressure between the shell side and tube side. Since the membranes are hydrophobic, they are not wetted by water, thereby, efiectively blocking the flow of water through the membrane pores. The membrane provides no selectivity. Rather its purpose is to keep the gas phase and the Hquid phase separated. In effect, the membrane acts as an inert support that allows intimate contact between gas and liquid phases without dispersion. Vacuum on the tube side of the membrane increases the mass transfer rate as in a vacuum tower. The efficiency of the process is enhanced with the aid of nitrogen sweep gas flowing on the permeate side of the membrane. 

H3. A countercurrent membrane module with a poly(dimethylsiloxane) membrane operates at 35°C to separate air (assume air is 20.9% oxygen and 79.1% nitrogen). We want a permeate product that is 23.5% ojgrgen (above this limit for safety reasons, stainless steel has to be used in all later equipment). Fjjj = 100,000 cm (STP)/s, Pj. = 1.5 atm, Pp = 1.0 atm, and tjns = 0.00002 cm. 

A porous anodic alumina capillary membrane was coated with [BMIM][NTf2] via impregnation. The as-prepared membrane was placed in a membrane module and connected to a metered feed flow of propene/propane (E/A) and nitrogen sweep... 

Gases like hydrogen, NH3, CO2, and oxygen are able to permeate through the membranes faster than nitrogea Hence, these gases come out from sidewalls exit. A number of such hollow-fibre membrane modules are provided as per flow rate of nitrogen desired. 

In the mid-1990s, MTR Inc. evaluated at laboratory scale (on 1-2 m membrane modules) a wide array of composite membranes which selective membranes were made of various glassy or rubbery polymers (Table 6.5). Intuitively, rubbery polymers proved to be more selective toward methane (due to its higher sorption, induced by its more condensable behavior than nitrogen) while glassy polymers, especially fluorinated ones, proved to be more selective towards nitrogen (due to diffusive selectivity). 

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