Membrane Oxygen Exchanging

Hollow-fiber (capillary)-type membrane oxygenators are the most widely used today, and comprise two main types (i) those where blood flow occurs inside the capillaries and (ii) those where there is a cross-flow of blood outside the capillaries. Although in the first type the blood flow is always laminar, the second type has been used more extensively in recent times, as the mass transfer coefficients are higher due to blood turbulence outside capillaries and hence the membrane area can be smaller. Figure 15.3 shows an example of the cross-flow type membrane oxygenator, with a built-in heat exchanger for controlling the blood temperature. 

Bramson ML, Osbom JJ, Main FB, Obrien ME, Wright IS, and Gerbode F. A new disposable membrane oxygenator with integral heat exchange. J. Thorac. Cardiov. Sur. 1965 50(3) 391 00. 

However, a problem remains how to relate the observations (at equilibrium) from isotopic exchange to the conditions met during membrane operation. In chemical relaxation experiments, the oxide is studied after perturbation of the equilibrium state. These methods are thus complementary and probably their combined application, whenever possible together with spectroscopic techniques, such as FT-IR, UV and EPR, has a great capacity to elucidate the kinetics of surface oxygen exchange. 

Oxygen permeation fluxes oxygen exchange and transport models suffer from not considering that the membrane is a polycrystal material in which grain boundaries have different properties than bulk grain. 

A preferred embodiment of an oxygen transport membrane would thus have a thin porous support on the feed side to improve oxygen exchange, a thin dense separation membrane, a fine pore structure interfacial layer to facilitate oxygen transfer out of the membrane and a coarse porous support to maximize product flow and provide the structural support. An example is shown in Fig. 6.4. The coarse porous support material could be made out of inert material because it is not chemically active in the transport of oxygen. This allows the use of less expensive materials which may also have better strength characteristics. 

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