Biomedical Engineering Reference
In-Depth Information
25
20
15
10
5
Anionic Resin
Activated Carbon
0
0
5
10
15
20
25
30
35
40
C
bil
(
µ
mol/l)
Figure 4. Typical bilirubin adsorption isotherms. lines: model fitting; circles: experimental
data.
sorption kinetics, breakthrough curves of bilirubin-albumin solutions on the same anionic
resin used by Annesini et al. (submitted) were experimentally determined. Fig. 5 reports
some typical results obtained with different albumin concentrations in the feed. It can be
clearly seen that the bilirubin adsorption rate is low, so that the column effluent contains
a non-negligible bilirubin concentration even at early operating times, when the sorbent is
far from saturation. This finding, also confirmed by some clinical data obtained during a
MARS treatment (unpublished results), has a fundamental relevance in the design of an al-
bumin regeneration system, because it suggests that, in the present configurations of devices
such as MARS, the albumin is never completely regenerated.
Furthermore, the curves reported in Fig. 5 confirm the negative effect of albumin over
bilirubin adsorption.
In order to design the adsorption units of a LSD, a model that provides a quantitative
description of bilirubin adsorption in a fixed bed column is required. Such a model can be
obtained by coupling the differential unsteady bilirubin mass balance in the column, with
mass transfer kinetics from the liquid to the adsorbed phase.
Considering both solute convection and axial dispersion in the liquid phase, bilirubin
mass balance in the column can be written as follows
= D
Z
∂
2
C
BIL
Ε
∂C
BIL
∂T
+ (1−Ε)Ρ
C
∂N
BIL
∂T
∂Z
2
−V
∂C
BIL
(34)
∂Z
were
C
BIL
is the bilirubin concentration in the liquid phase,
N
BIL
the bilirubin adsorbed
amount per unit sorbent mass,
Ε
is the bed porosity,
Ρ
C
is the intrinsic density of the solid
adsorbent,
V
the liquid superficial velocity and
D
Z
the bilirubin axial dispersion coefficient.
Assuming linear driving force (LDF) mass transfer kinetics (Glueckauf and Coates, 1947),
the bilirubin mass balance in the adsorbed phase may be written as
∂N
BIL
∂T
3
R
K
C
(N
∗
BIL
−N
BIL
)
=
(35)
