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Since very low oxygen levels were required in the
project, the most efficient configuration was select-
ed: the Combo Mode (Figure 3).
This configuration implies both the application of
the vacuum and the supply of Nitrogen as inert gas
Figure 3. Membrane deaeration system in “Combo” mode for the deaeration.
A vacuum level of 67 mbar and a gas sweep rate
for each contactor of 1.6 Nm /h were selected as
3
Unit of
Value operative conditions from lumen side. Sizing show-
Measure
ed that a number of 10 LiquiCel contactors 8x80 (8
inches of diameter and 80 inches of MDA length),
Design Flow rate 92.5 m /h
3
was necessary for the selected separation. Table 2
summarizes the results of MDA system sizing deri-
Nitrogen Purity 99.99 % ving from Liquicel calculation tool.
Quantity of contactors in parallel 10 -
4.Vacuum Tower vs
Operative min temperature, T 4 °C Membrane Deaeration

Vacuum level 67 mbar 4.1 Performance analysis
The comparison between MDA and VT was made
Gas (Nitrogen) sweep rate per 3 considering the same inlet water characteristics
contactor 1.6 Nm /h and treated water requirements. In fact, in both ca-
ses, an inlet water flow rate of 92.5 m /h was con-
3
Inlet dissolved Oxygen (Saturation @ T) 13.8 ppm
sidered and an inlet oxygen concentration of 13.88
mg/L. As inlet oxygen concentration, was conside-
3
Flow rate per contactor 9.2 m /h red that at saturation conditions at minimum opera-
tive temperature (4°C), thus considering the worst
Sizing factor (Filtration > 98% @ 2μm) 2.5 - conditions for the sizing.
As for process specification, the outlet oxygen
concentration in the treated water was required to
Average expected DO 2.7 ppb be lower than 20 ppb. Oxygen removal by vacu-
um towers based system allowed an outlet oxygen
Maximum Dissolved Oxygen (DO) 6.9 ppb concentration of 50 ppb after the two packed sta-
ges (Figure 4). Moreover, the addition of an oxygen
3
Total sweep gas (Nitrogen) 16.1 Nm /h

3
Vacuum Load 321.8 m /h The membranes are the valid alternative
to the vacuum towers in the case that very
Max Pressure drop across membranes 0.8 bar low oxygen concentrations are required
and/or when the adoption of chemicals
Table 2. Membrane deaeration package sizing such as oxygen scavenger is not possible
or not available

scavenger to the outlet water from VT was required
in order to satisfy the outlet specifications.
On the other side, if injection seawater was deaer-
ated by membrane contactors with Combo Mode
(configuration consisting in the use of vacuum plus
Nitrogen stream), an average outlet oxygen con-
centration of 2.7 ppb was guaranteed correspond-
ing to a maximum oxygen concentration of 6.9 ppb
Figure 4. (by considering that a 2µm filtration pretreatment is
Residual oxygen applied before MDA).
concentration in VT The vacuum level that should be applied in order to
vs running time reach the desired outlet oxygen concentration by

68 Impiantistica Italiana - Luglio-Agosto 2017