Hemodialyzer mass transfer

Leypoldt, J. K., Cheung, A. K., Agodoa, L. Y., Daugitdas, J. T., Greene, T., and Keshaviah, R R. (1997). Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates. The Hemodialysis (HEMO) Study. Kidney Int. 51, 2013. 

In the situation where the effect of filtration - that is, water movement across the membrane due to the difference in hydrostatic pressure and/or osmolarity - can be neglected, the overall resistance for mass transfer in hemodialyzers with flat membranes is given as... 

Calculate the overall mass transfer coefficient ffp (based on the hollow-fiber inside diameter) and the dialysance of the hemodialyzer for urea, neglecting the effect of water permeation. 

In a hoUow-fiber-type hemodialyzer of the total membrane area (based on o.d.) A = 1 m , 200 cm min of blood (inside fibers) and 500 cm min of dialysate (outside fibers) flow countercurrently. The overall mass transfer coefficient for urea (based on the outside diameter of the hollow fiber) is 0.030 cm min . Estimate the dialysance for urea. 

This chapter will focus on three types of membrane extracorporeal devices, hemodialyzers, plasma filters for fractionating blood components, and artificial liver systems. These applications share the same physical principles of mass transfer by diffusion and convection across a microfiltration or ultrafiltration membrane (Figure 18.1). A considerable amount of research and development has been undertaken by membrane and modules manufacturers for producing more biocompatible and permeable membranes, while improving modules performance by optimizing their internal fluid mechanics and their geometry. 

Figure 18.3 Schematic of fluid and mass transfer between blood and dialysate compartments in a hemodialyzer.

Researchers at Oregon State University have demonstrated the advantages of microchannel architecture in improving the hemodialysis process. Using microchannel architectare, they were able to show 70-80% reductions in the necessary transfer area relative to commercial hollow fiber systems for the clearance of creatinin (Fig. 7.23) and urea (Fig. 7.24) from a simulated blood stream [285]. The microchannel advantage, as has been seen in other applications, comes in the form of well-defined and narrow channels that facilitate rapid mass transfer into and out of the fluid media. This approach is expected to change the current paradigm in hemodialysis from clinical treatment to at-home use, and may allow for the creation of a wearable hemodialyzer [286]. 

Thus, in 4 hr, which is a typical treatment time, the urea concentrate has been reduced by 45%. It is left as an exercise at the end of this chapter for a study of the effect on the rate of urea removal of changing the hemodialyzer geometry, the blood and dialysate flow rates to the dialyzer, the rate of waste withdrawal, the volume of the dialysate tank, and the sensitivity of the rate of urea mass transfer to the mass-transfer coefficient. In particular, the above estimate of the coefficient on the shell side may be low because the entry to and exit fi om the hemodialyzer of the dialysate is normal to, rather than parallel to the fibers. This should enhance the shell-side coefficient. ... 

In Section 4.3.1, we were introduced to a hemodialyzer with blood on one side of the membrane and the dialyzing solution on the other side. Solutes (metabolic waste products) from blood diffused through the liquid filled pores of the membrane to the dialysate side. Using a simple lumped analysis based on the overall solute mass-transfer coefficient Ku, we will develop an expression for the solute removal efficiency of a hemodialyzer in which blood as well as the dialyzing solution are in steady cocurrent flow (Section 8.1.7 treated countercurrent dia-lyzers). The analysis is valid for any other system, not just hemodialysis. 

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