Artificial mass transfer device

The chapter itself is organized as follows. In Section 11.1, we discuss experiments in which data can be organized using mass transfer coefficients. Section 11.2 describes blood oxygenators and artificial kidneys, mass transfer devices whose geometry is exactly known. Section 11.3 discusses the role of mass transfer in pharmacokinetics, where the system s geometry is unknown, lumped into other parameters which may include the mass transfer coefficient. Thus the chapter provides an introduction for life scientists to important engineering ideas. 

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. 

Parallel plate mass exchange devices with semi-permeable membranes have been studied in a number of separation techniques including hemodialysis, artificial oxygenation, gas separation, and heavy-metal ion separation C10-18% The accurate prediction of solute separation in these mass exchangers is desirable. For parallel-plate geometry, Grimsrud and Babb C5) and Colton et al. C ) developed series solutions to describe the mass transfer process. Kooljman CB) reviewed the analytical and numerical solutions available in the early 1970 s. Since that time, other... 

Bioactive sorbents represent the simplest form of artificial cells already used in routine clinical applications for humans. Sorbents such as activated charcoal, resins, and immunosorbents could not be used in direct blood perfusion because particulate embolism and blood cells were removed. However, sorbents such as activated charcoal inside artificial cells no longer cause particulate embolism and blood cells removal. This application was developed and used successfully in patients. For example, the hemoperfusion device now used in patients contains 70 g of artificial cells. Each artificial cell is formed by applying an ultrathin coating of collodion membrane or other polymer membranes on each of the 100-p-diameter activated-charcoal microspheres. The mass transfer for this small device is many times higher than that for a standard dialysis machine. 

As an example, let us consider a simple one-compartment model for the prescription of treatment protocols for dialysis by an artificial kidney device (Fig. 1.1). While fire blood irrea concentration (BUN) in the normal individual is usually 15 mg% (mg% = milligrams of the substance per 100 mL of blood), the BUN in irremic patients could teach SO mg%. The purpose of the dialysis is to bring the BUN level closer to the normal. In the artificial kidney, blood flows on one side of the dialyzer membrane and dialysate fluid flows on the other side. Mass transfer across the dialyzer membrane occurs by diffusion due to concentration difference across the membrane. Dilysate fluid is a makeup solution consisting of saline, ions, and the essential nutrients that maintains zero concentration difference for these essential materials across the membrane. However, during the dialysis, some hormones also diffuse out of the dialyzer membrane along with the urea molecule. Too-rapid dialysis often leads to depression in the individual because of the rapid loss of hormones. On the other hand, too-slow dialysis may lead to unreasonable time required at the hospital. 

---