View
215
Download
1
Embed Size (px)
Citation preview
Calijirnia, USA based New Logic International Inc has
developed a membrane system that is said to overcome the
problem offoulina that has long plagued conventional 1
IS
membrane technologies.
lthough membrane-based
systems for the separation
of liquids from solids have
technology has an inherent flaw, namely
membrane fouling. The long-term loss in
grown in popularity over
throughput capacity is due primarily to the
formation of a boundary layer that builds
the last 20 years, the
up naturally on the membrane’s surface
during filtration. In addition to cutting
down on the flux performance of the
membrane, this boundary or gel layer acts
as a secondary membrane reducing the
selectivity of the original one. This
inability to handle the build-up of solids
has tended to limit the use of membranes
to feed streams with low concentrations of
solids.
To help minimise this boundary layer
build- up membrane designers use a
method known as tangential flow or
crossflow filtration that relies on high
velocity fluid flow being pumped across
the membrane surface to reduce boundary
formation. However, it is not economical
to create shear forces greater than 10 OOO-
15 OOO/second, which limits the use of
these systems to low-viscosity fluids. In
addition, increased crossflow velocities
result in a significant pressure drop near
the filter outlet, which leads to premature
fouling that eventually spreads towards the
Now according to New Logic
International, it has developed an
alternative method for producing intense
front of the filter, causing the permeate
shear waves at the face of a membrane that
not only prevents the boundary layer
rates to drop to unacceptably low levels.
build-up, but allows membranes to process
liquids that have a much higher solids
content.
VSEP lbchmdogy The new technology is called vibratory shear
enhanced processing (VSEP), and the shear
cleaning action is created by the vigorous
vibration of leaf elements in a direction
tangent to the faces of the membranes
(Figure l).The shear waves produced by
the membrane’s vibration cause solids and
foulants to be lifted off the membrane
surface and remixed with the bulk material
flowing through the membrane stack. This
high shear processing exposes the
membrane pores to a maximum
throughput that is said to be between three
and ten times that of a conventional
crossflow system.
VOSEP
20 Januarylfeburary ZOOI
TheVSEP membrane filter pack (Figure 2)
consists of leaf elements arrayed as parallel
discs, separated by gaskets, with a membrane
on each side. The disk stack is oscillated
above a torsion spring (Figure 3) that moves
thestackbackandforth(1.9-3.0cmpeakto
peak) at an oscillation frequency of
approximately 53 Hz. The oscillation
produces a shear intensity at the membrane
surface of around 150 OOO/second
(equivalent to a g force of over 200), which
is approximately ten times the shear rate of
the best conventional crossflow systems.
More importantly, the shear is focused at the
membrane surface, where it is most effective
in preventing fouling. At the same time the
bulk fluid between the membrane disks
remains relatively undisturbed as it flows
slowly between the parallel membrane leaf
elements. A further benefit of the system is
that because it does not depend on feed
flow-induced shearing forces, the slurry can
be extremely viscous and still be successfully
dewatered.
The concentrate is extruded between
the vibrating disc elements and exits the
machine once it reaches a desired
concentration level. Thus, the VSEP system
can treat liquids and slurries in a single
pass, eliminating the need for costly
working tanks, ancillary equipment and
associated valving.
The disc pack hold-up volume of a
system with a membrane area of
130 m* is less than 190 lines. As a result,
product recovery in batch processes can be
relatively high, and the volume of the
produce waste after draining the stack is
normally less than 11 litres.
Upon start-up the system is fed with the
slurry, the permeate is produced and the
FIttraWn+Separation
suspended solids in the feed are collected
inside the filter pack. After a programmed
time interval, a valve is opened to release
the accumulated concentrated solids, and
then closed to allow the concentration of
additional feed material. This cycle can be
repeated indefinitely.
Filter Pack Cross Section Clear Permeate Feed
Membrane selection is one of the most
important parameters influencing the
quality of the separation. According to the
company, its VSEP filter packs can
incorporate almost every type of membrane
currently available on the market including
acrylic, nylon, polypropylene,
polysulphone, etc. Other important
parameters that affect system performance
are pressure, temperature, vibration
amplitude and residence time. All of these
elements are optimised during testing and
entered into the programmable logic
controller (PLC) of the system.
I Concentrate
\ l--_- Membrane Permeate Channel
The operating pressure is created by
the feed pump, and aVSEP machine can
routinely operate at pressures as high as
67 bar. Although higher pressures can
produce increased permeate flow rates,
they also use more energy. Therefore, an
operating pressure is set that optimises the
balance between flow rates and energy
consumption. In the majority of cases the
filtration rate can also be improved by
increasing the operating temperature. The
temperature limit on a standard VSEP
system is 92 ‘C, but higher temperature
constructions are also available.
Using nanofiltration membrane
modules in the VSEP systems
enabled approximately 85% of the
feed pressate to be recovered as
clean water and the remaining
recycled as a concentrated
stream.
VOSEP Resonating Drive System
’ Filter Pack Drive
Torsion Spring -
case study
At a facility of a leading manufacturer of
medium density fibre (MDF) board
three VSEP systems incorporating
nanofiltration membrane modules were
installed to help the slurry generated by
the dewatering of the pulp meet
environmental legislation.
The VSEP produced a
concentrated stream at a flow
rate of 38-57 l/mitt, which was
recycled either to the chip belt
dryer or to the pulp refiner,
while the clean water permeate
stream (300-320 l/min) was
discharged to the plant holding
ponds and then recycled as
process water.
The traditional methods for treating
this type of slurry are dissolved air
floatation (DAF) followed by a multimedia
filter, bag filters or traditional reverse
osmosis. However, following an evaluation
of the operating costs the decision was
taken to install theVSEP systems.
Substantial savings in operating costs were
expected primarily because of the
elimination of the need for the addition of
either chemicals or polymers to the slurry.
The feed to the VSEP units
contained between 2000-
8000 mg/l of total suspended
solids (TSS), had a chemical
oxygen demand (COD) of
20 000-70 000 mg/l and 2000-
5000 mg/l of oil and grease. The
permeate concentration was
reduced to approximately 1 mg/l ofTSS,
500 mg/l of COD, and 10 mg/i of oil and
grease, which were all below the necessary
requirements.
electronics industry, and treatment of
effluent produced from paint and pigment
concentration and washing.
The manufacturing facility operated
24 hours a day, 350 days per year. The
maximum pressate flow rate to be treated
in the process was over 350 litres per
minute (l/min).
The VSEP technology has been
successfully used in a number of other
applications including: treatment of
whitewater and bleach plant effluent in
the pulp and paper industry, raw water
treatment for the chemical manufacturing
industry, recovery of heavy metals in the
Forfurther information contact: New Logic
International Inc, 1295 67th Street, Emeryville,
CA 94608, USA.
Tel: +I 510 655 7305;
Fox: +1 655 7307;
E-mail: [email protected];
Website: www, vxep. corn
FHtmtlon+~paratlon January/Feburay 2001 21