(665a) Centrifugal Reverse Osmosis (CRO) - a Novel Process for Achieving Desalination at the Thermodynamic Restriction
AIChE Annual Meeting
2019
2019 AIChE Annual Meeting
Separations Division
Water Treatment, Desalination, and Reuse I
Thursday, November 14, 2019 - 8:00am to 8:21am
For
a specified feed concentration and flowrate, and membrane rejection, the water
recovery for an equilibrium reverse osmosis (RO) stage is uniquely related to
the osmotic pressure differential across the membrane. This relationship
defines the thermodynamic restriction for RO desalination. Fig. 1 shows a plot
of the pressure in bar versus the fractional water recovery for which the
concave-upward line shows the thermodynamic restriction. The area under this
line (i.e., the PdV work) is the minimum energy required
to achieve a desired water recovery; e.g., a 0.65 recovery requires a
transmembrane pressure (TMP) of 80 bar and a minimum energy of 2.46 kWh/m3,
assuming a 100% pump efficiency; this required energy can be reduced if an
Energy-Recovery Device (ERD) is used to recover the pressure energy of the
retentate brine. However, considerably
more energy is required in practice when the feed is pressurized to the maximum
pressure required for the desired water recovery; e.g., a 0.65 recovery
requires 3.43 kWh/m3 corresponding to the shaded red and green
areas. This required energy increase is caused by pumping all the feed to 80 bar. The required energy can be reduced by increasing the
pressure in steps via successive stages. The green-shaded area in Fig. 1 shows
the energy saved by increasing the pressure in steps from 0 to 40, 40 to 60 and
60 to 80 bar. However, this requires interstage
pumping that increases the capital and maintenance costs as well as the process
complexity. RO at the minimum energy dictated by the thermodynamic restriction
would require stepping the pressure in differential increments from ambient to
the maximum using an infinite number of stages. The Closed-Circuit-Desalination
(CCD) process approaches RO at the thermodynamic restriction by recirculating
the feed while gradually increasing its pressure in a closed system. However,
CCD is a batch process that is not amenable for large-scale desalination or for
using an ERD on the retentate brine.
The
novel Centrifugal Reverse Osmosis (CRO) process recently patented by the
Singapore Membrane Technology Center approximates using an infinite number of
stages to increase the pressure, thereby reducing the pumping costs to the
minimum defined by the thermodynamic restriction curve. CRO is advantageous
relative to CCD since it is a continuous process amenable to using an ERD. The
CRO process involves rotating a sandwich of parallel circular flat sheet RO
membranes and associated spacers about the axis-of-symmetry of the CRO module.
The feed enters through ports in a rotating hollow tube, concentric with the
axis-of-symmetry, and flows out radially through the feed channels. Since the
back-pressure at the downstream ends of the feed channels is controlled, the
centrifugal acceleration causes the pressure on feed side to increase with
increasing radial distance from the axis-of-symmetry. Since the permeate
channels vent to the ambient, the rotation does not increase the pressure
within them, thereby allowing water permeation from the high-pressure feed
channels. Using rotation about an axis to create the pressure for RO has been
advanced in prior studies. However, these prior studies used the centrifugal
force only to create the maximum pressure required for the desired water
recovery and did not capitalize on the differential increase in centrifugal
force that permits RO at TMPs only slightly higher than the minimum defined by
the thermodynamic restriction. The retentate brine in CRO is discharged through
a series of jets directed at stationary vanes at the bottom of the module,
thereby serving as an ERD to recover its pressure energy.
A
mathematical model has been developed for the CRO process based on design at
the thermodynamic limit and assuming 100% pump and ERD efficiencies. Fig. 2
compares the predictions for the gross (without an ERD) and net (with an ERD)
specific energy consumption (SEC) as a function of the fractional water
recovery for conventional single-stage reverse osmosis (SSRO) and the novel CRO
process. Table 1 compares the gross and net SEC for 50% and 65% recoveries for
conventional SSRO and the CRO process. The required TMP for 50% recovery is 56
bar and that for 65% recovery is 80 bar, the maximum sustainable pressure for
currently available commercial RO membranes. The CRO process reduces the SECnet relative to SSRO by 31% and 43% for 50%
and 65% water recoveries, respectively. The CRO process not only significantly
reduces the energy required for the RO separation, but also reduces the total
cost of the water production by permitting economic operation at higher water
recoveries, thereby decreasing the normalized pretreatment and brine-disposal
costs.
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