PROCESS SHEET
FLUIDISED BED
The operating principle of
the fluidised bed system (otherwise known
as WS system) is very simple.
The ionic exchange resin is loaded in a cylindrical
column in the space between two plates, one
lower and one upper, provided with nozzles.
The space between the two plates is filled
almost completely so that, contrary to conventional
equicurrent systems, only a minimum amount
of space remains unused (approximately 50
mm), thus avoiding compacting of the bed.
The resin is exhausted from the bottom towards
the top, in other words the water, or other
solution to be treated, enters the column
from the bottom and, after crossing the resin
bed, re-emerges at the top. The bed crossing
speed (or the flow speed) must be established
at the start so that a fixed resin bed forms
against the upper nozzle plate. Once the fixed
bed has formed and has become stable, the
(flow) speed can be re-adapted as required,
without affecting the quality of the water
treated.
The fluidised bed that has formed in the
lower section (depending on the outlet speed)
is thus fully used. The final finishing stage
is performed on the fixed bed, in particular
on the layer crossed last by the liquid and
which, consequently, will be supplied with
the highest level of regenerant. A counter
flow process for regeneration of the resins
requires only a minimum excess of regenerant
with respect to the stoichiometric requirement.
A further reduction can be obtained by arranging
the weak and strong resins inside consecutive
cells: the fluid entering the column initially
crosses the lower cell (chamber 1), filled
with weak ionic exchange resin, and then crosses
the upper cell (chamber 2) filled with strong
ionic exchange resin.
This combination of two or more cells inside
one single column is called fluidised bed
compound system (abbreviation VWS). This system
is also used to separate the large quantities
of resin, which would otherwise be the source
of considerable pressure losses, into several
chambers.
With the VWS system it is also possible:
| - |
to reduce the quantity
of regenerant for the resins almost to
the stoichiometric requirement; |
| - |
lto use sulphuric
acid (H2SO4) as a regenerant, fully exploiting
its second dissociation constant for regeneration
of the weak ionic exchange resins; |
| - |
to treat water containing
higher levels of organic impurities than
permitted by the single-chamber columns.
|
Since internal backwashing
of the resins is not possible either in the
WS system or in the VWS system, the resin
is back-washed externally when necessary,
using dedicated tanks.
Diagram:
