Removal of Dissolved Oxygen from Water by Nitrogen Stripping Coupled with Vacuum Degassing in a Rotor–Stator Reactor

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Article Removal of Dissolved Oxygen from Water by Nitrogen Stripping Coupled with Vacuum Degassing in a Rotor–Stator Reactor

Zemeng Zhao 1, Zhibang Liu 2, Yang Xiang 2, Moses Arowo 3 and Lei Shao 2,*

1 Clean Utilization Research Institute, China Coal Energy Research Institute Co., Ltd., Xi’an 710054, China; [email protected] 2 Research Center of the Ministry of Education for High Gravity Engineering and Technology, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China; [email protected] (Z.L.); [email protected] (Y.X.) 3 Department of Chemical & Process Engineering, Moi University, Eldoret 3900, Kenya; [email protected] * Correspondence: [email protected]; Tel.: +86-10-6442-1706

Abstract: Oxygen is a harmful substance in many processes because it can bring out corrosion and oxidation of food. This study aimed to enhance the removal of dissolved oxygen (DO) from water by employing a novel rotor–stator reactor (RSR). The effectiveness of the nitrogen stripping coupled with vacuum degassing technique for the removal of DO from water in the RSR was investigated.

The deoxygenation efficiency (η) and the mass transfer coefficient (KLa) were determined under various operating conditions for the rotational speed, liquid volumetric flow rate, gas volumetric

flow rate, and vacuum degree. The nitrogen stripping coupled with vacuum degassing technique −1 achieved values for η and KLa of 97.34% and 0.0882 s , respectively, which are much higher than −1 Citation: Zhao, Z.; Liu, Z.; Xiang, Y.; those achieved with the vacuum degassing technique alone (η = 89.95% and KLa = 0.0585 s ). Arowo, M.; Shao, L. Removal of A correlation to predict the KLa was established and the predicted KLa values were in agreement Dissolved Oxygen from Water by with the experimental values, with deviations generally within 20%. The results indicate that RSR is Nitrogen Stripping Coupled with a promising deaerator thanks to its intensification of gas–liquid contact. Vacuum Degassing in a Rotor–Stator Reactor. Processes 2021, 9, 1354. Keywords: deoxygenation efficiency; vacuum–N2–H2O–O2 system; rotor–stator reactor; mass trans- https://doi.org/10.3390/pr9081354 fer; correlation

Academic Editors: Elio Santacesaria, Riccardo Tesser and Vincenzo Russo 1. Introduction Received: 8 June 2021 Accepted: 30 July 2021 Oxygen plays a key role in the treatment of wastewater [1,2]. Nevertheless, oxygen is Published: 1 August 2021 a harmful substance and removal of dissolved oxygen (DO) from water is an essential step carried out in power plants in order to prevent corrosion in boilers and pipes, improve

Publisher’s Note: MDPI stays neutral heat transfer and enhance plant efﬁciency [3–5]. This operation is not only adopted in the with regard to jurisdictional claims in power industry but is also necessary in the semiconductor, pharmaceutical, biotechnology published maps and institutional afﬁl- and food industries, which also have stringent requirements for DO levels in water [6,7]. iations. It is thus indispensable to remove DO from water prior to its use in many industries. Various physical techniques, such as thermal degassing, vacuum degassing and nitrogen stripping, are currently employed by industries to remove DO from water. It has been reported that nitrogen stripping is the most effective approach for DO removal among these

Copyright: © 2021 by the authors. techniques [8]. Coupled techniques, such as thermal degassing with vacuum degassing or Licensee MDPI, Basel, Switzerland. thermal degassing with nitrogen stripping, are also promising means for DO removal [8,9]. This article is an open access article However, these techniques are conventionally carried out in packed towers/columns that distributed under the terms and exhibit poor mass transfer performance and, as a result, suffer limitations related to low ef- conditions of the Creative Commons ficiency, bulkiness, high costs and inflexibility in operation [10–12]. It is therefore necessary Attribution (CC BY) license (https:// to develop a simple and efficient technique to remove DO from water, and a gas–liquid creativecommons.org/licenses/by/ contactor capable of good mass transfer performance is desirable. 4.0/).

Processes 2021, 9, 1354. https://doi.org/10.3390/pr9081354 https://www.mdpi.com/journal/processes Processes 2021, 9, x FOR PEER REVIEW 2 of 13

limitations related to low efficiency, bulkiness, high costs and inflexibility in operation [10–12]. It is therefore necessary to develop a simple and efficient technique to remove DO Processes 2021, 9, 1354 from water, and a gas–liquid contactor capable of good mass transfer performance2 of 11 is desirable. The rotor–stator reactor (RSR) is a novel multiphase contactor comprising a series of rotorThe rings rotor–stator and stator reactor rings alternately (RSR) is a configured novel multiphase in a radial contactor direction comprising [13]. In an a seriesRSR, a ofhigh rotor-gravity rings and environment stator rings several alternately orders configured of magnitude in a radial greater direction than [ the13]. Earth In an’s RSR,gravitational a high-gravity field can environment be created severalas a result orders of the of magnitudehuge centrifugal greater force than produced the Earth’s by gravitationalrapid rotation field of the can rotor. be created Consequently, as a result the ofliquid the stream huge centrifugal in the RSR forceis broken produced into small by rapidelements rotation and thin of the films rotor. [14– Consequently,16]. There is also the increased liquid stream turbulence in the of RSR the is gas broken and liquid into smallstreams elements, in addit andion thin to films rapid [ 14 renewal–16]. There of the is alsogas– increasedliquid interface turbulence [17,18]. of the All gas of these and liquidproperties streams, can in greatly addition enhance to rapid the renewal mass transfer of the gas–liquid rate in an interface RSR. Thus, [17, 18RSRs]. All have of thesesuccessfully properties been can employed greatly enhance in degradation the mass of transfer organic rate pollutants in an RSR. [19,20], Thus, nanomaterial RSRs have successfullysynthesis [21], been CO employed2 capture [22] in degradationand other applications of organic. pollutants [19,20], nanomaterial synthesisThis [21work], CO therefore2 capture employed [22] and other an RSR applications. to intensify the removal of DO from water throughThis worknitrogen therefore stripping employed coupled an RSR with to vacuum intensify degassing the removal (vacuum of DO– fromN2–H water2O–O2 throughsystem). nitrogen The effect strippings of various coupled operating with vacuum conditions degassing, such (vacuum–N as the rotational2–H2O–O speed2 system). of the TheRSR, effects the liquid of various volumetric operating flow conditions,rate, the gas suchvolumetric as the rotationalflow rate and speed theof vacuum the RSR, degree the liquidof the volumetricRSR, on the flow deoxygenation rate, the gas efficiency volumetric, as flow well rate as on and the the mass vacuum transfer degree coefficient of the , RSR,were on in thevestigated. deoxygenation Separate efficiency, experiments as well involving as on the nitrogen mass transfer stripping coefficient, (N2–H were2O–O2 investigated.system) and Separate vacuum experiments degassing (vacuum involving–H2 nitrogenO–O2 system) stripping were (N also2–H 2 carriedO–O2 system) out for andcomparison. vacuum degassingAdditionally, (vacuum–H a correlation2O–O to2 predictsystem) the were mass also transfer carried coefficient out for comparison. (KLa) in the Additionally,RSR was also aestablished. correlation to predict the mass transfer coefficient (KLa) in the RSR was also established. 2. Materials and Methods 2. Materials and Methods 2.1. Structure of RSR 2.1. Structure of RSR The structure of the RSR is shown in Figure 1a. The RSR consisted primarily of a liquidThe distributor, structure of rotor the RSR seat, is shaft, shown seal, in Figure rotor 1anda. The stator. RSR The consisted rotor was primarily comprised of a liquid of six distributor, rotor seat, shaft, seal, rotor and stator. The rotor was comprised of six circular circular rings (named rotor rings) located on a rotor seat, which was connected to a motor rings (named rotor rings) located on a rotor seat, which was connected to a motor by a shaft, by a shaft, while the stator consisted of five layers of pins (named stator rings) mounted while the stator consisted of five layers of pins (named stator rings) mounted concentrically concentrically on a cover cap. As shown in Figure 1b, the open space between pins in the on a cover cap. As shown in Figure1b, the open space between pins in the same layer and same layer and the perforations in the rotor provide the flow channels for fluids in the the perforations in the rotor provide the flow channels for fluids in the RSR [15]. RSR [15].

FigureFigure 1. 1.Schematic Schematic diagrams diagrams of of an an RSR. RSR. ( a(a)) Structure Structure of of an an RSR; RSR; ( b(b)) 3D 3D diagram diagram of of rotor rotor rings rings and and stator rings: (1) gas inlet; (2) cover cap; (3) stator; (4, 7, 11) bolts; (5) liquid distributor; (6) gas stator rings: (1) gas inlet; (2) cover cap; (3) stator; (4, 7, 11) bolts; (5) liquid distributor; (6) gas outlet; outlet; (8) liquid outlet; (9) seal; (10) shaft; (12) rotor; (13) rotor seat; (14) casing. (8) liquid outlet; (9) seal; (10) shaft; (12) rotor; (13) rotor seat; (14) casing.

AsAs shown shown in in Table Table1 ,1, the the inner inner diameter diameter of of the the rotor rotor rings rings ranged ranged from from 70 70 to to 190 190 mm mm 2 andand thus thus could could provide provide centrifugal centrifugal acceleration acceleration ranging ranging from from 28.48 28.48 to to 1395.68 1395.68 m 2m/s./s. The The statorstator ringsrings (diameter(diameter ranges from 80 80 to to 176 176 mm) mm) allow allow the the redistribution redistribution of of liquid liquid in inan anRSR. RSR.

Table 1. Specifications of the RSR.

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Table 1. SpecificationsItem of the RSR. Unit Value Layer number of rotor rings - 6 Layer number of statorItem rings - Unit5 Value Number of perforations in rotor Layer number of rotor rings- -180, 240, 294, 348, 6 408, 462 rings Layer number of stator rings - 5 Number of pins in stator rings - 12, 16, 20, 24, 24 Number of perforations in rotor rings - 180, 240, 294, 348, 408, 462 Diameter of perforations in rotor Number of pins in stator ringsmm - 12, 16,4 20, 24, 24 rings DiameterDiameter of pins of in perforations stator rings in rotor ringsmm mm5 4 Inner diameterDiameter ofof rotor pins rings instator ringsmm mm70, 94, 118, 142, 5 166, 190 Inner diameterInner diameter of stator rings of rotor ringsmm mm 70,80, 104, 94, 118,128, 152, 142, 176 166, 190 Inner Innerdiameter diameter of the RSR of stator ringsmm mm 80, 104,300 128, 152, 176 Axial depthInner of diameterrotor rings of the RSRmm mm61 300 Axial depthAxial of depthstator rings of rotor ringsmm mm60 61 Axial depthAxial of depth the RSR of stator ringsmm mm65 60 Internal volumeAxial of depth the RSR of the RSRcm3 mm4592 65 Internal volume of the RSR cm3 4592 2.2. Experimental Procedure 2.2. Experimental Procedure Figure 2 shows the experimental setup for the vacuum–N2–H2O–O2 system. The liquid inletFigure and2 showsoutlet theof the experimental RSR were connected setup for theto a vacuum–N centrifugal2 –Hpump2O–O and2 system. sealed tank The, liq- respectively,uid inlet and whereas outlet the of thegas RSRinlet wereand outlet connected were toconnected a centrifugal respectively pump and to a sealed nitrogen tank, gasrespectively, cylinder and whereas a vacuum the system gas inlet, which and outletwas made were up connected of a vacuum respectively pump and to a nitrogenbuffer tank.gas The cylinder buffer and tank a vacuumhelped to system, maintain which the pressure was made balance up of ain vacuum the DO pumpremoval and system. a buffer Taptank. water The was buffer pumped tank helped via the to maintain liquid distributor the pressure into balance the RSR in the vacuumized DO removal below system. atmosphericTap water pressure was pumped and allowed via the liquid to flow distributor radially outward into the RSRthrough vacuumized the rotor belowrings and atmo- statorspheric rings pressure, while nitrogen and allowed gas was to flow introduced radially into outward the RSR through via a thegas rotor inlet ringsand allowed and stator to rings,flow inward while nitrogen through gasthe wasrotor introduced rings and intostator the rings. RSR viaThe a nitrogen gas inlet andgas allowedstream made to flow contactinward in througha counter the-current rotor ringswith the and w statorater stream rings. Thein the nitrogen RSR, leading gas stream to removal made contact of DO in froma counter-current the water by both with vacuum the water degassing stream inand the nitrogen RSR, leading stripping. to removal The gas of and DO fromliquid the streamswater by finally both exitedvacuum the degassing RSR via and the nitrogen gas and stripping. liquid outlet The gass, andrespectively. liquid streams The finally DO exited the RSR via the gas and liquid outlets, respectively. The DO concentration in the tap concentration in the tap water and deoxygenated water was measured using a DO water and deoxygenated water was measured using a DO detector (Rex, SJG-203A, CHN; detector (Rex, SJG-203A, CHN; detection limits: 0~19.99 mg/L). The composition of the gas detection limits: 0~19.99 mg/L). The composition of the gas used in this study was 99.5% used in this study was 99.5% N2 and 0.5% O2. N2 and 0.5% O2.

Figure 2. Experimental set up: (1) Water tank; (2) centrifugal pump; (3, 12) valve; (4) liquid flow meter; (5) RSR; (6) sealed tank; (7) DO detector; (8) vacuum gauge; (9) buffer tank; (10) vacuum Figure 2. Experimental set up: (1) Water tank; (2) centrifugal pump; (3, 12) valve; (4) liquid flow meter;pump; (5) (11)RSR; nitrogen (6) sealed cylinder; tank; (7) (13) DO gas detector; flow meter. (8) vacuum gauge; (9) buffer tank; (10) vacuum pump; (11) nitrogen cylinder; (13) gas flow meter. Experiments involving nitrogen stripping or vacuum degassing were performed in the same way as with the vacuum–N technique. Experiments involving nitrogen stripping2 or vacuum degassing were performed in The operating conditions were set as follows: rotational speed = 200–1400 rpm, liquid the same way as with the vacuum–N2 technique. volumetric flow rate = 0.35–0.6 m3/h, gas volumetric flow rate = 1–7 m3/h, vacuum degree

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= 0.02–0.06 MPa, inlet liquid temperature = 300 ± 2 K (ambient temperature) and initial DO concentration in tap water = 7.85–8.01 mg/L.

2.3. Calculation of KLa and η

The equation for the experimental KLa (Equation (1)) was obtained by referring to literature [23]:

Q x − x (x − x ) = L × in out e in KLa 2 ln /cM (1) HπR (x − xe)in − (x − xe)out (x − xe)out

where xe is the equilibrium molar fraction of O2 in liquid which was obtained using Equation (2): L x = (x − x ) (2) e mG out

The deoxygenation efﬁciency (η) was determined by the molar concentration of O2 at the liquid inlet (cin) and the outlet (cout) (Equation (3)): c − c η = in out × 100% (3) cin

3. Results and Discussion 3.1. Effect of Rotational Speed

The effect of the rotational speed on η and KLa is illustrated in Figure3. Both η and −1 −1 KLa increased from 96.32% and 0.0803 s to 97.34% and 0.0882 s , respectively, with an increase in rotational speed from 200 to 600 rpm, beyond which both η and KLa showed little change with increasing rotational speed. Higher rotational speeds caused a larger centrifugal force, which split the liquid into ﬁner elements and thinner ﬁlms. Consequently, Processes 2021, 9, x FOR PEER REVIEW there was increased turbulence in the gas and liquid streams, a larger gas–liquid5 of interfacial13

area and a faster renewal rate of the gas–liquid interface, resulting in thinner gas and liquid boundary layers. All of these factors led to an enhanced gas–liquid mass transfer rate and thereby a larger KLa and higher η.

100 0.105

0.100 98 0.095 K

96 L 0.090 a (s (%)

η -1

0.085 ) 94

0.080 92 η 0.075 KLa 90 0.070 200 400 600 800 1000 1200 1400 n (rpm)

3 3 Figure 3. Effect of rotational speed on η and KLa (L = 0.4 m /h; G = 3 m /h; P = 0.06 MPa; initial DO 3 3 Figure 3.concentration Effect of rotational = 7.96 speed mg/L). on η and KLa (L = 0.4 m /h; G = 3 m /h; P = 0.06 MPa; initial DO concentration = 7.96 mg/L). Nonetheless, increasing rotational speed brought about increased liquid velocity, 3.2. Effectwhich of Liquid reduced Volumetric liquid Flow residence Rate time in the RSR and liquid holdup declined accordingly, Figure 4 shows the effect of the liquid volumetric flow rate on η and KLa. It was noted that η decreased by 4.22% (from 97.39 to 93.28%) while KLa increased by 27.98% (from 0.0772 to 0.0988 s-1) with an increase in liquid volumetric flow rate from 0.35 to 0.6 m3/h. A higher liquid volumetric flow rate resulted in more liquid droplets and, consequently, a larger gas–liquid interface area, leading to an increase in KLa. Moreover, oxygen is sparingly soluble in water and the removal of DO from water is controlled by a liquid film. Thus, a higher liquid volumetric flow rate can result in a larger KLa. However, at a given gas–liquid equilibrium, vacuum degree and gas volumetric flow rate, a higher liquid volumetric flow rate led to a decline in η. A higher liquid volumetric flow rate led to a shorter liquid residence time in the RSR. This probably caused more liquid to exit the RSR before the maximum DO removal was achieved.

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perhaps leading to a decrease in the gas–liquid interfacial area. The effect of the phenomena at rotational speeds greater than 600 rpm offset the aforementioned benefits of higher rotational speeds in this study, resulting in almost stable η and KLa. According to Henry’s law, the lowest equilibrium oxygen concentration in liquid is 0.2028 mg/L at 25 ◦C. Thus, the highest deoxygenation efficiency is 97.47% for tap water with an initial DO concentration of 8.01 mg/L. The deoxygenation efficiency of 97.34% in this study was close to the highest one, suggesting the good deoxygenation effect of the RSR.

3.2. Effect of Liquid Volumetric Flow Rate

Figure4 shows the effect of the liquid volumetric flow rate on η and KLa. It was noted that η decreased by 4.22% (from 97.39 to 93.28%) while KLa increased by 27.98% (from 0.0772 to 0.0988 s−1) with an increase in liquid volumetric flow rate from 0.35 to 0.6 m3/h. A higher liquid volumetric flow rate resulted in more liquid droplets and, consequently, a larger gas–liquid interface area, leading to an increase in KLa. Moreover, oxygen is sparingly soluble in water and the removal of DO from water is controlled by a liquid film. Thus, a higher liquid volumetric flow rate can result in a larger KLa. However, Processes 2021, 9, x FOR PEER REVIEW at a given gas–liquid equilibrium, vacuum degree and gas volumetric flow rate,6 of 13 a higher

liquid volumetric ﬂow rate led to a decline in η. A higher liquid volumetric ﬂow rate led to a shorter liquid residence time in the RSR. This probably caused more liquid to exit the RSR before the maximum DO removal was achieved.

100 0.105

0.100 98 0.095 K

96 L 0.090 a (s (%) -1 η ) 94 0.085 0.080 92 η K a 0.075 L 90 0.070 0.35 0.40 0.45 0.50 0.55 0.60 3 L (m /h)

3 Figure 4. Effect of liquid volumetric flow rate on η and KLa (n = 600 rpm; G = 3 m /h; P = 0.06 MPa; 3 Figure 4.initial Effect DO of concentrationliquid volumetric = 7.96 flow mg/L). rate on η and KLa (n = 600 rpm; G = 3 m /h; P = 0.06 MPa; initial DO concentration = 7.96 mg/L). 3.3. Effect of Gas Volumetric Flow Rate 3.3. Effect of Gas Volumetric Flow Rate As shown in Figure5, both η and KLa increased from 95.97 to 97.34% and 0.0791 to −1 3 As0.0882 shown s in, respectively,Figure 5, both with η and an increaseKLa increased in the from gas volumetric 95.97 to 97.34% flow rate and from 0.0791 1 to to 3 m /h, 0.0882 sand−1, respectively, thereafter they with remained an increase almost in stablethe gas with volumetric further increases flow rate in from the gas 1 to volumetric 3 m3/h, flow and thereafterrate. This they was remained because a almost higher stable gas volumetric with further flow increases rate resulted in the in increasedgas volumetric turbulence in flow rate.the gasThis and was liquid because streams, a higher leading gas to avolumetric larger gas–liquid flow rate interfacial resulted area in and increased faster renewal turbulencerate in of the the gas gas–liquid and liquid interface, streams, thereby leading resulting to a larger in gas–liquid the observed interfacial increase area in andη and KLa. faster renewalHowever, rate as of the the gas gas–liquid volumetric interfac flow ratee, thereby further resulting increased in over the 3observed m3/h, the increase oxygen might in η andhave KLa. reachedHowever, equilibrium as the gas betweenvolumetric the flow gas rate and further liquid phases.increased Thus, over the 3 m increase3/h, the in the oxygengas–liquid might have interfacial reached areaequilibrium and the renewalbetween rate the of gas the and gas–liquid liquid interfacephases. Thus, had no the effect on increase in the gas–liquid interfacial area and the renewal rate of the gas–liquid interface had no effect on the oxygen transfer from water to gas. Therefore, both η and KLa remained stable when the gas volumetric flow exceeded 3 m3/h.

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the oxygen transfer from water to gas. Therefore, both η and KLa remained stable when the gas volumetric ﬂow exceeded 3 m3/h.

100 0.105

0.100 98 0.095 K

96 L 0.090 a (s (%) -1 η ) 94 0.085 0.080 92 η 0.075 KLa 90 0.070 1234567 G (m3/h)

3 Figure 5. Effect of gas volumetric ﬂow rate on η and KLa (n = 600 rpm; L = 0.4 m /h; P = 0.06 MPa; Figure initial5. Effect DO of concentrationgas volumetric = 7.96flow mg/L).rate on η and KLa (n = 600 rpm; L = 0.4 m3/h; P = 0.06 MPa; initial DO concentration = 7.96 mg/L). 3.4. Effect of Vacuum Degree

3.4. Effect ofThe Vacuum effect Degree of the vacuum degree on η and KLa is shown in Figure6. Both η and KLa −1 Theincreased effect of from the vacuum 95.03 to 97.06%degree andon η 0.0728and KL toa is 0.0856 shown s in, respectively,Figure 6. Both when η and the K vacuumLa increaseddegree from rose 95.03 from to 0.0297.06% MPa and to0.04 0.0728 MPa. to This0.0856 was s−1 because, respectively, a higher when vacuum the vacuum degree causes degreemore rose from water 0.02 vapor MPa and to lower0.04 MPa. oxygen This partial was because pressure a higher in the vacuum gas phase. degree Therefore, causes a high more watervacuum vapor degree and andlower more oxygen water partial vapor pressure in the gasin the phase gas resultedphase. Therefore, in an increased a high mass Processes 2021, 9, x FOR PEER REVIEW transfer driving force and, consequently, higher η. A higher vacuum degree8 of 13 also results vacuum degree and more water vapor in the gas phase resulted in an increased mass transferin driving a thinner force mass and, transfer consequently, boundary higher layer η [.24 A]. higher Consequently, vacuum degree there was also reduced results mass in a thinnertransfer mass resistance transfer and, boundary hence, K layeLa wasr [24]. larger, Consequently, which is also there conducive was reduced to oxygen mass removal from water. transfer resistance and, hence, KLa was larger, which is also conducive to oxygen removal from water. 100 0.105 However, the DO concentration was low and the mass transfer driving force was small when the vacuum reached a high degree. Furthermore, the oxygen approached 0.100 equilibrium98 between the gas and liquid phases under such conditions. Hence, both η and KLa increased slowly from 97.06 to 97.34% and 0.0856 to 0.0882 s−1, respectively, with an 0.095 increase in the vacuum degree from 0.04 to 0.06 MPa. K

96 L 0.090 a (s -1 (%)

η ) 94 0.085 0.080 92 η 0.075 KLa 90 0.070 0.02 0.03 0.04 0.05 0.06 P (MPa)

3 3 Figure 6. Effect of vacuum degree on η and KLa (n = 600 rpm; L = 0.4 m /h; G = 3 m /h; initial DO 3 3 Figure 6.concentration Effect of vacuum = 7.96 degree mg/L). on η and KLa (n = 600 rpm; L = 0.4 m /h; G = 3 m /h; initial DO concentration = 7.96 mg/L).

3.5. Comparison of Different Deoxygenation Systems in RSR

Figure 7 shows the values for η and KLa achieved by the N2–H2O–O2, vacuum–H2O– O2 and vacuum–N2–H2O–O2 systems in the RSR, respectively. It is evident that the vacuum–N2–H2O–O2 system achieved the highest values for KLa and η among the three systems. This can be attributed to the synergistic effect of nitrogen stripping and vacuum degassing, as oxygen partial pressure in the gas phase of the vacuum–N2–H2O–O2 system at a certain vacuum degree was lower than that of the N2–H2O–O2 system at atmospheric pressure or the vacuum–H2O–O2 system at the same vacuum degree. Thus, according to Henry’s law, the equilibrium concentration of DO in water in the vacuum–N2–H2O–O2 system was lower than that in the N2–H2O–O2 system at atmospheric pressure or the vacuum–H2O–O2 system at the same vacuum degree. In addition, it was also deduced that there was stronger turbulence in the vacuum–N2––H2O–O2 system in the RSR due to a faster gas velocity. This resulted in a larger gas–liquid interfacial area and a higher renewal rate of the gas–liquid interface, thereby leading to higher KLa and η.

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However, the DO concentration was low and the mass transfer driving force was small when the vacuum reached a high degree. Furthermore, the oxygen approached equilibrium between the gas and liquid phases under such conditions. Hence, both η and −1 KLa increased slowly from 97.06 to 97.34% and 0.0856 to 0.0882 s , respectively, with an increase in the vacuum degree from 0.04 to 0.06 MPa.

3.5. Comparison of Different Deoxygenation Systems in RSR

Figure7 shows the values for η and KLa achieved by the N2–H2O–O2, vacuum– H2O–O2 and vacuum–N2–H2O–O2 systems in the RSR, respectively. It is evident that the vacuum–N2–H2O–O2 system achieved the highest values for KLa and η among the three systems. This can be attributed to the synergistic effect of nitrogen stripping and vacuum degassing, as oxygen partial pressure in the gas phase of the vacuum–N2–H2O–O2 system at a certain vacuum degree was lower than that of the N2–H2O–O2 system at atmospheric pressure or the vacuum–H2O–O2 system at the same vacuum degree. Thus, according to Henry’s law, the equilibrium concentration of DO in water in the vacuum–N2–H2O–O2 system was lower than that in the N2–H2O–O2 system at atmospheric pressure or the vacuum–H2O–O2 system at the same vacuum degree. In addition, it was also deduced that there was stronger turbulence in the vacuum–N2—-H2O–O2 system in the RSR due Processes 2021, 9, x FOR PEER REVIEW 9 of 13 to a faster gas velocity. This resulted in a larger gas–liquid interfacial area and a higher renewal rate of the gas–liquid interface, thereby leading to higher KLa and η.

100 (a)

92 (%) η 88

vacuum-N -H O-O 2 2 2 84 N2-H2O-O2 vacuum-H O-O 2 2 80 200 400 600 800 1000 1200 1400 n (rpm) 0.090 (b) 0.081

0.072 ) -1 (s

a 0.063 L K 0.054 vacuum-N -H O-O 2 2 2 N -H O-O 0.045 2 2 2

vacuum-H2O-O2 0.036 200 400 600 800 1000 1200 1400 n (rpm)

Figure 7. Comparison of the (a) η and (b) KLa of the N2–H2O–O2, vacuum–H2O–O2 and vacuum– Figure 7. Comparison of the (a) η and (b) K a of the N –H O–O , vacuum–H O–O and vacuum– N2–H2O–O2. L = 0.4 m3/h, G = 3 m3/h, P = 0.06 MPa andL initial DO concentration2 2 2 = 7.96 mg/L in the2 2 3 3 Nvacuum–N2–H2O–O2–H2. 2LO–O= 0.42 system; m /h, L =G 0.4= m 33 m/h, G/h, = 3P m=3/h 0.06 and MPainitial andDO concentration initial DOconcentration = 7.85 mg/L in = 7.96 mg/L in the the N2–H2O–O2 system; and L = 0.4 m3/h, P = 0.063 MPa and initial3 DO concentration = 8.01 mg/L in vacuum–N2–H2O–O2 system; L = 0.4 m /h, G = 3 m /h and initial DO concentration = 7.85 mg/L in the vacuum–H2O–O2 system). 3 the N2–H2O–O2 system; and L = 0.4 m /h, P = 0.06 MPa and initial DO concentration = 8.01 mg/L in3.6. the Correlation vacuum–H for 2KO–OLa 2 system). It was assumed that the mass transfer coefficient KLa in the RSR was influenced by 14 major factors, as shown in Table 2. The values of these factors were based on the experimental conditions and physical properties of the gas and liquid.

Table 2. Major influence factors of KLa.

Factor Symbol Value Unit Dimension Liquid inlet velocity uL 3.1~5.3 m/s [LT−1] Angular velocity of rotor ω 21~147 s−1 [T−1] Geometric radius of rotor R 0.115 m [L] Liquid density ρL 997 kg/m3 [ML−3] Liquid viscosity μL 8.94 × 10−4 kg/(m⋅s) [ML−1T−1] Liquid surface tension σ 71.97 kg/s2 [MT−2] Gravitational acceleration g 9.81 m/s2 [LT−2]

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3.6. Correlation for KLa

It was assumed that the mass transfer coefﬁcient KLa in the RSR was inﬂuenced by 14 major factors, as shown in Table2. The values of these factors were based on the experimental conditions and physical properties of the gas and liquid.

Table 2. Major inﬂuence factors of KLa.

Factor Symbol Value Unit Dimension −1 Liquid inlet velocity uL 3.1~5.3 m/s [LT ] Angular velocity of rotor ω 21~147 s−1 [T−1] Geometric radius of rotor R 0.115 m [L] 3 −3 Liquid density ρL 997 kg/m [ML ] −4 −1 −1 Liquid viscosity µL 8.94 × 10 kg/(m·s) [ML T ] Liquid surface tension σ 71.97 kg/s2 [MT−2] Gravitational acceleration g 9.81 m/s2 [LT−2] Temperature T 25~29 ◦C[Θ] Inlet air humidity ϕ -%- Processes 2021, 9, x FOR PEER REVIEW 10 of 13 −1 Gas inlet velocity uG 8.85~61.95 m/s [LT ] 3 −3 Gas density ρG 0.5~1.25 kg/m [ML ] −5 −1 −1 Gas viscosity µG 1.78 × 10 kg/(m·s) [ML T ] Temperature T 25~29 −9 ℃ 2 [Θ] −2 Oxygen diffusivity in water DAB 2.41 × 10 m /s [L T] VacuumInlet air humidity degree φ P - 0~0.06% MPa - [L−1MT2] Gas inlet velocity uG 8.85~61.95 m/s [LT−1] Gas density ρG 0.5~1.25 kg/m3 [ML−3] Gas viscosity μG 1.78 × 10−5 kg/(ms) [ML−1T−1] OxygenThe basicdiffusivity dimensions in water of theDAB variables2.41 were × 10−9 [T], [L] andm2/s [M]. According[L−2T] to the Bucking- −1 2 ham pi theoremVacuum degree [25 ], KLa can beP expressed0~0.06 as follows (EquationMPa (4)):[L MT ]

The basic dimensions of the 2variables were [T], [L] and [M]. According to the KLaR β γ ε θ ξ Buckingham pi theoremSh [25],= KLa can =be αexpressed(ReL) ( Reas followsG) (Sc (EqG) uation(EuL )(4())Fr: L) (4) DAB ℎ = = α() () () () () (4) The coefﬁcients of the correlation were obtained by ﬁtting the experimental data to giveThe the coefficients following of equation the correlation (Equation were (5)):obtained by fitting the experimental data to give the following equation (Equation (5)): Sh = (Re )0.69(Re )−1.62(Sc )−1.41(Eu )−0.88(Fr )−0.14 0.99 L. G. .G . L . L (5) ℎ = 0.99() () () () () (5)

As shown in in Figure Figure 8,8 , the the predicted predicted KLKa Lvaluesa values were were in agreement in agreement with with the the experi- mentalexperimentalKLa values, KLa values, with with deviations deviations generally generally within 20%. 20%. Thus, Thus, the the correlation correlation can can be used tobe predictused to predictKLa in K anLa in RSR. an RSR.

Figure 8. Diagonal graph of experimental and predicted KLa values. Figure 8. Diagonal graph of experimental and predicted KLa values. Table 3 shows that the accuracy of the correlation obtained in this study was similar to that of other studies in the literature. Furthermore, the correlation could predict KLa at atmospheric pressure as well as at reduced pressure.

Table 3. Comparison of different deoxygenation systems in various high gravity devices.

Reference This Study Lin et al., Guan et al., System Vacuum–N2–H2O–O2 N2–H2O–O2 Vacuum–H2O–O2 High gravity device RSR RSR Rotating packed bed Liquid flow rate, m3/h 0.35~0.6 0.75~2.0 0.02~0.10 Gas flow rate, m3/h 1~7 1.5~4.0 - Rotational speed, rpm 200~1400 300~1450 800~1400 Vacuum degree, MPa 0.02~0.06 0 0.08~0.1 Range of mass transfer coefficient, s-1 0.073~0.098 0.077~0.34 0.031~0.202 Reference - [26] [27]

Processes 2021, 9, 1354 9 of 11

Table3 shows that the accuracy of the correlation obtained in this study was similar to that of other studies in the literature. Furthermore, the correlation could predict KLa at atmospheric pressure as well as at reduced pressure.

Table 3. Comparison of different deoxygenation systems in various high gravity devices.

Reference This Study Lin et al. Guan et al.

System Vacuum–N2–H2O–O2 N2–H2O–O2 Vacuum–H2O–O2 Rotating packed High gravity device RSR RSR bed Liquid flow rate, m3/h 0.35~0.6 0.75~2.0 0.02~0.10 Gas flow rate, m3/h 1~7 1.5~4.0 - Rotational speed, rpm 200~1400 300~1450 800~1400 Vacuum degree, MPa 0.02~0.06 0 0.08~0.1 Range of mass transfer 0.073~0.098 0.077~0.34 0.031~0.202 coefficient, s−1 Reference - [26][27]

4. Conclusions

This study employed a coupled vacuum–N2 technique to remove DO from water in an RSR. The effects of various operating conditions, including the rotational speed, the liquid volumetric flow rate, the gas volumetric flow rate and the vacuum degree, on the values of η and KLa were investigated. The optimum operating conditions in terms of deoxygenation efficiency and energy consumption were determined as a rotational speed of 600 rpm, a liquid volumetric flow rate of 0.4 m3/h, a gas volumetric flow rate of 3 m3/h and a vacuum degree of 0.06 MPa. Under these conditions, the vacuum–N2–H2O–O2 −1 system achieved values for η and KLa of 97.34% and 0.0882 s , respectively. Additionally, a correlation to predict the KLa in the RSR was established, and the results show that the predicted values were in agreement with the experimental values, with deviations generally within 20%. These results indicate that the coupled technique carried out in an RSR is a feasible approach to remove DO from water under reasonable operating conditions thanks to the enhanced synergistic effect of nitrogen stripping and vacuum degassing under a high gravity environment.

Author Contributions: Conceptualization, L.S.; methodology, Z.Z.; validation, Z.L.; formal analysis, Y.X.; investigation, Z.Z.; writing—original draft preparation, Z.Z.; writing—review and editing, M.A.; supervision, L.S.; project administration, L.S.; funding acquisition, L.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number 22078009. Data Availability Statement: Not applicable. Acknowledgments: The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 22078009). Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

cM Total mole concentration of mixture, mol/L cin Molar concentration of O2 at the liquid inlet of RSR, mol/L cout Molar concentration of O2 at the liquid outlet of RSR, mol/L 2 DAB Oxygen diffusivity in water, m /s p EuL Liquid Euler number, 2 ρLuL 2 uL FrL Liquid Froude number, ω2 R2 Processes 2021, 9, 1354 10 of 11

G Liquid volumetric flow rate, m3/h g Gravitational acceleration, m/s2 H Height of RSR, m −1 KLa Overall volumetric mass transfer coefficient, s L Liquid volumetric flow rate, m3/h m Phase equilibrium constant of oxygen n Rotational speed of RSR, rpm P Vacuum degree, MPa QL Liquid molar flow rate, mol/s R Radius of RSR, m Re Gas Reynolds number, RuGρG G µG Re Liquid Reynolds number, RuLρL L µL Sc Gas Schmidt number, µG G ρG DAB 2 Sh Sherwood number, KL aR DAB T Temperature, ◦C 2 uG Gas inlet velocity, m/s, uG = G/πrG 2 uL Liquid inlet velocity, m/s, uL = L/πrL x Molar fraction of O2 in liquid xe Equilibrium molar fraction of O2 in liquid xin Molar fraction of O2 at the liquid inlet of RSR xout Molar fraction of O2 at the liquid outlet of RSR α, β, γ, ε, θ, ξ Fitting coefficients σ Liquid surface tension, kg/s2 η Deoxygenation efficiency, % µG Gas viscosity, kg/(m·s) µL Liquid viscosity, kg/(m·s) 3 ρG Gas density, kg/m 3 ρL Liquid density, kg/m ϕ Inlet air humidity, % ω Angular velocity of rotor, s−1 [L] Length dimension [M] Mass dimension [T] Time dimension [Θ] Temperature dimension

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