Membrane Devices

Gas separation modules can be prepared from flat sheets as plate and frame assemblies and spiral-wound elements. Hollow fibres can be fine fibres ( < 1000 |jm OD) or tubular. In general only fine fibres and spiral-wound elements combine

Figure 19 Membrane technology: breakdown by application.

performance and cost parameters providing economic viability. The benefits of the spiral-wound configuration include ease of fabrication and low pressure drop but manufacturing costs are high and the membrane surface area to module volume ratio is low ( + 700 m2 m~3), leading to larger, heavier systems. Hollow fibre modules can achieve very high surface area/volume ratio (>5000 m2 m~3) and provide a higher degree of countercurrent flow but are more difficult to fabricate and can have high pressure drop.

Flat Sheets and Spiral-Wound Elements

A schematic representation of a flat-sheet asymmetric casting device is shown in Figure 15. Flat-sheet membranes are typically produced on a nonwoven cloth which provides support for the nascent membrane. These nonwoven cloths are commercially available on large rolls of 24-48 inches. A thin liquid film of dope solution is metered by a doctor blade onto the nonwoven cloth while the fabric ro tates around a stainless steel roller. The nascent membrane is quenched in the gel tank, washed and collected on a take-up roller. A generic spiral-wound element is shown in Figure 16. The simplest configuration consists of a central collection pipe around which is wound and glued an envelope of flat-sheet membrane. The envelope contains the membrane and feed and permeate spacer channels. The spacer material is an extremely porous, inert material. Feed gas flows parallel to the permeate pipe; permeating gas flows into the permeate spacer and is collected in the pipe. Higher membrane areas can be achieved using the multileaved method in which two to four sheets of membrane are wrapped simultaneously.

Hollow Fibres

A second asymmetric membrane geometry is that of a hollow fibre. The ultrathin skin is present on the external surface of the fibre. Like the flat sheet, the wall of the fibre is microporous and offers low resistance to gas flow. The permeating gases collect in the bore or lumen of the fibre. A generic hollow-fibre spinning device is shown in Figure 17. The apparatus contains a dope reservoir, a bore fluid reservoir, pumps, coagulation baths, wash baths and a take-up winder. The dope solution and bore fluid are co-extruded through a die, resulting in a sheath of polymer around the bore fluid (typically water). This nascent fibre is then coagulated, washed and dried. Hollow-fibre devices contain thousands, even hundreds of thousands, of individual fibres within a single pressure housing (Figure 18). The feed gas is usually fed to the outside of the fibre (shell side) although some processes route the feed gas through the fibre

Table 4 Selected membrane applications

Category

Range ofoperation

Application

Gases separated

Hydrogen recovery

95% H2 <5 MMSCFD

Ammonia synthesis purge gas Syngas ratio adjustment Hydrotreater off gas

H2/N2, Ar h2/co

Nitrogen production

95-97% N2 0.1-2.0 T/D

Inerting: fuel tanks; food transportation Gas and oil drilling

n2/o2, co2 n2/o2, co2, h2o

Oxygen

40% O2

Oxygen-enriched air

O2/N2

Carbon dioxide removal

95 # % CH4 <40 MMSCFD

Natural gas sweetening Enhanced oil recovery Landfill gas

CO2/CH4

Dehydration

Dewpointto -40°C

Instrument air Natural gas dehydration

H2O/air or N2 H2O/CH4

Misc.

Semiconductor process gas

Perfluorocarbon from N2

Figure 20 Triple-orifice die for co-extrusion. 1, bore fluid; 2, substrate polymer dope; 3, coating polymer dope.

bore, especially when well-defined flow characteristics are required.

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