Mass transfer model
4.1. Model hypothesis
In order to describe the andrographolide transfer, the following
hypotheses were used:
1. The solid particle has two shapes: plane shape corresponding
to leaves and cylindrical shape corresponding
to stems. The solute diffusion takes place in the transverse
direction and is monodirectional.
2. Every particle is symmetrical and homogeneous.
3. The diffusion coefficient is constant in all experiment.
The andrographolide concentration in a particle, C1, depends
only on position and time.
4. The solvent in the batch reactor is perfectly mixed. The
transfer resistance in the liquid phase is negligible and
the andrographolide concentration in the solvent depends
only on time.
5. The transport of the andrographolide particles is a diffusion
phenomenon. It is described by a diffusion coefficient
that relates to D1 (or D) and independent of the
time.
6. At the interface, the concentrations of every species in
solution between the internal liquid (in pores) and external
to particles are supposed to be equal.
7. Initial concentration in each experiment is calculated using
the experiment at 150 min because the ability of extraction
for each solvent is different.
8. The mass percentage of leaves in all samples is supposed
to be about 80%.