Ammonia Removal Model Bull. Korean Chem. Soc. 2008, Vol. 29, No. 3 555
Ammonia Removal Model Based on the Equilibrium and Mass Transfer Principles
HyeinYoonJJi-Hye Lim, and Hyung-Keun Chung*
'Korean Minjok Leadership Academy, Hoengseong 225-823, Korea
DepartmentofEnvironmentalEngineering, Yonsei University, Wonju 220-710, Korea. *E-mail: hkchung@yonsei.ac.kr Received July 12, 2007
In airstrippingof ammoniafromthe aqueoussolution, a new removalmodelwaspresentedconsidering the equilibriumprinciples for theammonia in aqueoussolution andbetween the aqueousand airphase.The effects of pH,temperatureandairflow rate ontheammoniaremoval wereevaluated with the model. In addition, the saturation degree of ammonia in air wasdefined and used toevaluate the effect of each experimental factoron the removal rate. AspH (8.9 to 11.9) or temperature (20 to 50oC) was increased, the overall removalrate constantsinall caseswereappearedto beincreased. Ourpresented model showsthat thedegrees of saturation wereabout the same (0.45) inall cases when the airflow condition remains the same. Thisresult indicates that the effect of pH and temperatureweredirectly takeninto consideration in themodel equation. As theairflow increases,the overallremoval rate constantswereincreased in all cases as expected. However,the saturation degreewas exponentially decreased with increasingthe airflow rate in the air phase(orabove-surface) aeration.
In the subsurfaceaerationthe saturation degree remains a constantvalue of 0.65 even though theairflowrate was increased. These resultsindicate that the degree of saturation is affectedmainlybythe turbulenceof the aqueoussolutionandremainsthesameabove a certainairflowrate.
Key Words : Ammonia removal model, pH,Temperature, Airflow rate
Introduction
In the aqueous solution, ammoniac nitrogen can be classified into ammonium ion [NH4+] and free ammonia [NH3(aq)]. Of these, only free ammonia can move intothe airphase because ofitsnatureofvolatility. Thisfact can be explained by comparing the Henry's law constants. If the ammoniac nitrogen inthe aqueous solution isdesiredto be removed from the waste waters, two conditions should be considered for theeffective removal. First, thefractionof the freeammonia intheaqueoussolutionshould beincreased by increasing pH and temperature. Second, the transfer rate from the aqueous to air phase should be increased by increasing the temperature and the airflow rate. Ammonia removal method applying these principles is called air contact process that can be classified into the airflow bubbling method and the stripping tower removalmethod.1-6 To estimate the ammonia removal by the air contact processfrom waste waters containing high concentration of ammoniac nitrogen, it is important to understand the behavior of ammonia species in the aqueoussolution and to predict removal rate based onthe experimental conditionsas discussed above.This will provide valuable data for design ing the effective ammonia removal process by aeration.
Numerous researches haveaccomplished bymanyscientists to verify ammonia removal phenomena based on these essential experimental factors. Bayley7 investigated that the chemicalcomposition,the size of bubble,the rising speed of air bubble and the depth ofaeration affectedthe ammonia removal efficiency. Schpirt,8 however, stated thatthe depth ofaeration did not affectthe ammonia removal efficiency.
Srinath andLoehr9 reported the effect of air contact area,
contact time and the concentration of free ammonia. Liao and his coworkers6 compared the ammonia removal effi ciencies of air bubbling method with the stripping tower methodpacked with small plastic rings. Smith andArab10 reported that the air contactarea and surfaceturbulence by air flows had an important role in ammonia removal and they hypothesized that the free ammoniabetween aqueous and gasphase reaches completeequilibriuminstantaneously.
Based on this hypothesis, Cheung11 reported theoretical values of ammonia removal rate and compared with the experimentalvalues.Unfortunately, the experimentalvalues weremuch higherthan thetheoreticalvalues, but theycould not explain these unexpected results clearly. Thishypothesis has not beenproved until today.
Summarizing the above researches,theeffectsofessential parameters ontheammoniaremovalcouldonly be explain edqualitatively aftertheobtained experimental results. The reported ammonia removal rate models were frequently relied onthe1st order reaction kinetics.3,10,12,13 Inthis kindof models, the ammonia concentrationinthe aircontactingthe aqueousphaseassumed tobe"zero”，whichis not correct. In addition,it should be pointedout that the model equation did not include the essential factors of pH, temperature and airflowrate. Although thistypeof modelsmaybe enough to evaluate the obtained experimental results indirectly, the effectsof each experimental factor onthe ammonia removal ratecould not bedirectly evaluated.
Inthis research, a new ammonia removal model equation waspresentedutilizingthechemical equilibrium principles, mass balance and mass transfer theory. The model equation includes the pH, temperature and air flow rate as the essentialparameters and thustheremoval model can explain
the effect ofeach parameter onthe ammonia removal rate quantitatively. The theoretical values for the ammonia removalrate were obtained and compared with the experi mental data. To evaluate the completeness of ammonia equilibrium between the aqueous and air phase, the satura tion factor was also introduced as a new concept and calculated byfitting theexperimental data tothemodel. The presented model shouldalsobeapplicable to theremovalof anyvolatile species in thewastewaters.
ModelDevelopment
AmmoniaRemoval Model. When a container is not fully filled with aqueous solution, the innerpart of the container can be divided into tworegions of aqueous phase and air phase. As theammoniais removed to the atmosphere,there aretwoprocessesof ammonia masstransfer. Firstprocessis the transfer ofammoniafrom the aqueous phase to the air phase in the container and second process is the removal from the air phase in the container to the outside of the container (or to atmosphere). Second process is the final removal process andcan bedivided intotwo mechanism;the removal process caused by gas diffusion and by airflow suppliedintothesystem. In thisprocess, if the gas diffusion can be ignored, theairflowprovidedintothe system should onlybethe mainremoval process. Figure 1 shows that this type of system canbe organized by closing the container with a cover andconnectingbetween the air phase and the external air with a long tube ofsmall inner diameter.In this system, the amount of ammonia removal can be expressed as a functionof ammonia concentration in the air phase and theairflow rate providedintothe aqueous phase. To model the ammoniaremovalrate,thefollowingassumptionshould befirst made.
1. Ammonia concentration in the aqueous phase is uniform.
Advective transport in gaseous diffusion ignored system
2. Ammonia concentration in theair phase is uniform.
3. Ammonia concentration in the air phase is in equili brium with the aqueous phase. In otherwords, ammonia in theaqueous and air phase reaches equilibrium instantly.
Note that each ammonia concentrationintheaqueous and airphase insidethe containercanbe considered because the provided airflow normally makes enough turbulence. If the above all conditions are satisfied, the amount of ammonia removed from the container should be the same as the amount of ammonia contained in the airflow leaving the container. Therefore,the following equationscan bewritten:
△mout= Q- Cg• At= Q - V , M ⑴ 빠뿌=q - m「 (la)
Amout = amount ofammonia removed fromthecontainer mg =ammonia mass in theair phaseinsidethe container Q =airflow rate
Cg = ammonia concentration in the air phase inside the container
Vg =volume of theair phaseinsidethecontainer
At = timeincrement
In theaboveequations, m°ut can beexpressed as a function of mg. Since total amount ofammonia is always constant, thefollowing massbalance equation can bewritten.
mT = mL + mg+ mout (2) mT =total ammonia massofinterest
mL =ammonia massintheaqueousphase mg =ammonia mass in theairinsidethe container mout = ammonia massremoved from the container or in theatmosphere
If the equilibrium between the aqueous and air phase is satisfied(Condition 3), therelationshipbetween two phases canexpressed by usingHenry's law.
C
H=C一느一 (3)
CL . aNH
H =Henry'slaw constant (unitless)
CL =total ammonia concentrationin theaqueousphase Cg = ammonia concentration in the air phase inside the container
Onh3 = fractionof NH3(aq) in theaqueousphase
Head space mm
Mass transfer from water to head space
In the above, the concentration term (Cg andCL) can be changed to mass unit and thus mL can be expressed as a functionofmg and other terms asfollows.
mL 一一一一一一一一一一一一一一一一一一一一一一一一一一一一一一VL - mg aNH . H . Vg
(4) Vg =volumeoftheair phaseinsidethecontainer
VL = volumeoftheaqueousphaseinsidethecontainer
Figure 1. Schematic diagram of the ammonia removal system.
Aftersubstituting equation (4) into equation(2), rearrang
ing the equationformg yields thefollowingequation.
Ammonia Removal Model Bull. Korean Chem. Soc. 2008, Vol. 29, No. 3 557 aNH3 -H - V )
m = [
V + ^-
h-
vJ " 屿
-mout) (5)Substituting the above into equation (1), dividing both sidesbyAt, and ifAt isassumed to be approach to zero,the differentialequation (7)can beobtained.
Amout
—---
At
aNH3 'Q ' H
V"%"H・V (mT -mout) (6)
dmout
—:—dt
aNH.Q .H
V+ aNH - H - Vg (mT一mout) (7) Byrearranging equation (7) and integratingthe equation yields the final equation (9) for mout.
air phase. If B is less than 1, it means that instantaneous equilibriumdoes not occur duringthe process. Generally,the Henry’s Law constant can be obtained from a book and the concentration ofammonia inthe aqueous phase can easily be measured. However, since it is difficult to accurately measure the concentrationin the provided air into aqueous phase,thevalueB ofcannot bedirectly obtained.
Ifthe ‘H’ in equations (11) is substituted by ‘BH’，the following modified equation formL can be obtained.
_ ( Vl \( (一쓰h으:BH브N*'
% = mT<VL + aNH--(B-H
) -
vje띠〔外
+ 쓰阳
- B H) - V』 丿
(13) It is noted that allthe parameters except B can be given prior to theexperiment.
mf dm-u = " aNH-QH) dt
0 (mT一mout) 0 [VL+ aNH3 . H . 낑 aNH-. Q-H - t 川 v~+OH~HVg
) \)
dmout
(8)
-1 mout= mT - mT. I exp (9) Theaboveequationshowsthetheoretical maximumof the amount removed fromthecontainerasa function of time. In addition, the amounts ofammonia in the aqueous and air phaseinsidethecontainer can beexpressas.
f aNH- -H - V 丫
m=쩌 v+aHHdexp
F
eS
mL
aNH3 . Q .H. t '
—Vl Fh - H. Vg,
aNH3 . Q .H. t '
—V
+
Fh3 - H - VgI
-1 (10)-1
I
(11)As shown above, thederivedmodel equationscontain all essential parameters of the pH, temperature, and airflow rate. Therefore,thetheoretical amount of removed ammonia from the containercanbe adequately predictedby inputting the experimental parameters prior toexperiment.
Saturationdegree. In theabove section,we assumedthat the instantaneous equilibrium occurs between the aqueous and airphase. If this occurs, the equation (3)can beused to calculate the ammonia concentrationinthe air phaseeasily.
However, if ammonia does not become saturatedin the air phase,theratio of ammonia concentrations in the air to the aqueous phase will be less than Henry's constant. In this research, the saturation degree (仿 was defined toaccount thisdifferenceby modifying theequation(3).
Experiment지
Ammonia Remog Apparatus andRemoval Method.
Figure 2 shows a schematic diagram for the ammonia removal system used in this research. The circular acrylic container(internaldiameterof 15 cm; height of20 cm) was installedinsidethe water bath. Theheight, width and length ofthe water bath were 30 cm, 50 cm, and 30 cm, respec tively. The desired temperature was set and controlledby using the Samwon ENG company'sU-105Theating device, circulatingthewatercontinuously witha submergedpump.
The synthetic wastewater containing certain concentration of ammonia was pouredintothe acrylic container. To keep the constant temperature and uniform ammonia concen tration, a magnetic stirrerwasplaced and operated at 60 rpm during thewholeexperiment.
Theacrylic container wascovered up tightly except for the airsupplyinginlet and theoutlet.Constantairflowregulated by a flow meter (Dwyer Co.'s Rate-Masterair flow meter) was supplied into the aqueous andair phase inthe acrylic containerthrougha 1 cm innerdiametertubing. The outlet was connected to a long tubing (1 cm inner diameter) in order toignorethegasdiffusion effect.
ggg一一C.%一二=B H (Caq . aNH-)
Cg B =
H.( Caq. aNH-) (12)
Magnetic stirrer
Figure 2. Schematic diagram of the system equipped with a submerged aerator, a headspace aerator and an outlet.
If B is "1” in equation (12), theammoniasinthe aqueous and air phase are in equilibrium, indicating that the instantaneous equilibrium occurs between the aqueous and
In theresearch, as thecommonexperimentalcondition for ammonia removal process, ammonia concentration of 700 mg/L, alkalinity of5000 mg/L CaCO3, pH 10, temperature of 30 oC, airflow 2.5 L/min to the aqueous phase were determined. Whennecessary,the only parameter of interest waschanged from thisinitial experimental condition, keep
ingotherparametersthe same as theabove initial condition unlessotherwise specified.
The reagents used in this research are all first grades or greater qualities. Theammoniumchloride(NH4CD was used to prepare ammonia synthetic samples. Sodium hydrogen carbonate (NaHCO3), sodium carbonate (Na2CO3), and sodiumhydroxide (NaOH) were used toregulate alkalinity and pH. Also, in allexperiments performed in thisresearch, alkalinity was fixed to the 5000 mg/L CaCO3 in order to minimizepHchangedue totheintroductionofatmospheric carbon dioxidewhileaeration.
An지ytic지 Method for MeasurementofTotalAmmonia.
Thephenate method14 was applied for the measurement of total ammonia in the aqueous solution. The reaction of ammonia with hypochlorite and phenol resulted in a blue indophenolthat can be detected by a spectrophotometer. The reagents used for the ammoniaanalysiswere 1 Mphenolate solution, 0.35 M NaOCl,and 0.012M sodiumnitroprusside solution as a catalyst. The phenolate solutionwas prepared bydissolving phenol in the 0.8 M NaOH.
In thisresearch, flow injection analysis system(Zellweger Analytics Co.'s QuickChem FIA+ 8000) was used. The system is consisted of a peristaltic pump for transferring reagents, a six-port rotary valve for the sample injection, a mixing coil with heater, and an 8 mmflow-cell for measur
ingabsorbance. Allparts of the system arecontrolled by a personal computer and the measured sample signals were converted to the concentrations by comparing the stored calibrationdata in thecomputer.
The prepared ammonia analysis reagents flowed into the analysis system at 1 mL/min, the samples were passed through reaction coil heated to a 60 oC for2 min, and the absorbance responsible for the reaction products were measured at 630 nm. The applicable concentration range was determined 0.5-10 mg/L total ammonia. A correlation coefficient (R2) for the calibration curves in all measure ments is greaterthan 0.999.In actual analysis, thecollected samples from the container were appropriately diluted with deionized water prior to analysis. All samples were measured more than three times and the relative errors of less than 2 percent were obtained throughout the experi ments.
Results and Discussion
Ev지uation of Ammonia Remov지 Rate Based on 1st- Order Reaction Kinetics. Numerous works have been carried out that free ammonia [NH3(aq)] in the aqueous solution can be removed by aeration and reported that the removal rate can be modeled by the 1st order reaction kinetics.Therefore, wehave firstappliedthesame model to our experimental data and the apparent rate constantswere obtained (Table1).
When the ammonia removal rate was evaluated with varying pH, the apparent ammoniaremoval rate was posi tively increasedwith increasingpH from 8.9 to 10.8,while nofurthersignificant removal ratewasincreasedat pH 11.9.
Thisresultcanbe explainedbythe fact that the fraction of free ammonia is a function of pH. The fraction of free ammonia is exponentiallyincreased until pH 11. Above pH 11, the fraction offreeammoniais reached to almost 100%, and thus no further increase in apparent ammonia removal rate occurs. The increase intemperaturefrom20 to 50 oC in the aqueous solution increased the apparent ammonia removal rate. This finding indicates that the temperature increases Henry's law constant, resulting in the increase of
Table 1. Summary of apparent ammonia removal rate constants in different experimental conditions
Purpose of
experiment pH Temp.
(°C)
Bubbled Airflow (L/min)
Headspace Airflow (L/min)
Rate Constant
8.93 30 2.5 10 0.0021
pH 9.87 30 2.5 10 0.0040
Effect 10.81 30 2.5 10 0.0055
11.90 30 2.5 10 0.0056
10.0 20 2.5 10 0.0016
Temperature 10.0 30 2.5 10 0.0040
effect 10.0 40 2.5 10 0.0070
10.0 50 2.5 10 0.0097
10.0 30 2.5 0 0.0008
Submerged
aeration 10.0 30 5 0 0.0025
10.0 30 10 0 0.0044
effect
10.0 30 20 0 0.0080
10.0 30 2.5 0 0.0008
Headspace 10.0 30 2.5 10 0.0041
aeration 10.0 30 2.5 20 0.0051
effect 10.0 30 2.5 30 0.0070
10.0 30 2.5 40 0.0073
Table 2. Effect of experimental parameters on the ammonia removal
Change in Experimental Parameter Direct Effect
pH increase Temperature increase Airflow rate increase
-Increase in the fraction of free ammonia in the aqueous solution -Increase in the fraction of free ammonia in the aqueous solution - Increase in the ratio of free ammonia in the air to aqueous phase
- Increase in the transfer rate of free ammonia from the aqueous to air phase - Effect on the air-to-liquid contact area
Ammonia Removal Model Bull. Korean Chem. Soc. 2008, Vol. 29, No. 3 559
Figure 3. Dependence of the ammonia concentration in the aqueous phase on time at various pHs.
•
: experimental data;——:fitted to experimental data;
一
: instantaneous equilibrium model predicted.Time (mm) Time (mm)
Time(min) Time (min)
Figure 5. Dependence of the ammonia concentration in the aqueous phase on time at various submerged aeration rates.
•
:experimental data;——:fitted to experimental data;
一
:instantaneous equilibrium model predicted.
Time (mm Time (min
Time (min) Time (min)
Figure 4. Dependence of the ammonia concentration in the aqueous phase on time at various temperatures.
•
: experimental data;——:fitted to experimental data;一
: instantaneous equilibrium model predicted.Figure 6. Dependence of the ammonia concentration in the aqueous phase on time at various aeration rates. In all cases, submerged aeration rates were 2.5 L/min and additional headspace aerations were varied.
•
: experimental data;——:fitted to experimental data;一
: instantaneous equilibrium model predicted.free ammonia fraction. The increase in the airflow rate increased the apparent ammonia removal rate positively because the mass transfer of ammonia increases as the airflowrateincreases.
The effects of pH,temperature and the airflowrate onthe ammonia removal rate are summarized inTable 2. Asdis
cussed above, since the increase in these essential para meters increasedthe ammonia removal rate, theapplication of the 1st order reaction kinetics appears to be appropriate.
However, we can note that the model of1storder reaction kineticsdoes not includeanyof the experimental parameters and the rate constant obtained from the experimental data canonly be used to explain the difference of removal rate.
Therefore, the 1storder reaction kinetics would not be used
as a prediction model for theammonia removal process.
Ammonia Remov지 Models Based on the Chemic지
EquilibriumPrinciple.From Figure 3to6, thechanges of ammonia concentration as a function oftime were plotted with the experimentally obtained data (symbols) and the proposed model predicted data(solidline). As described in the previous section, the solid lines indicate the maximum removalrate theoretically, meaning that the air leaving the system is completely saturated with ammonia in theaqueous solution.
In all experiments carried out in this research, the ammonia removal rate did not reachthe theoreticalremoval rate. These results indicatethatduringthe bubbling the air,
temperature (°C) 9 10 11 12 13
e 은
O6P
u으
e느
-es
40 50 30 20 6
5 4 3 2 1 o
o.
o.
o.。。
①은 6 CUP U OE U CUS
8
o
pH
① 은 O6P
uow-lnws
Submerged airflow rate (L/min) Total airflow rate (L/min)
Figure 7. Dependence of the saturation degree on various experi
mental conditions.
the instant equilibrium between the air and aqueous phase does not occur.Thisfinding is important becausetheaccept ed hypothesis ofinstantaneous equilibrium10,11between the air bubble and aqueous phaseduringtheaerationwould not becorrect.
SaturationDegree.With our revisedmodel, the extent of equilibrium between the air and aqueous phase can be determined in terms ofsaturation degree as defined in the theoreticalsection. Thesaturationdegreewasdetermined by fitting the experimental data to the revised model equation (equation 13).Dotted lines in Figure3-6 werethefitted data.
Thevaluesof saturationdegrees with differentexperimental parametersweredepictedinFigure7.
Effects of pH and Temperature. In all experiments varying pH and temperature in the aqueous solution, the saturation degreeswereestimatedat about0.45regardless of givenexperimental conditions. Thereason why the degrees of saturationwere measured the same is because all dataare normalized. As noted in the introduction, ammoniac nitro gen inthe aqueous solution is consistedofammonium ion [NH4+] and free ammonia [NH3(aq)], and equilibriumoccurs between NH3(aq) and NH4+. According to the equilibrium theory,theconcentration fraction ofNH3(aq) is a function of pH and temperature. Of these two species, only free ammonia is possible to transferfrom the aqueousto the air phase. In our model, theparameters ofpH and temperature are included tocalculate the amountof freeammonia. And thisfree ammonia ratherthan total ammonia isconsidered as anonly transferring species from the aqueous to air phase.
Therefore, the pH and temperature would not affect the ammonia removal rate. Likewise, equilibrium between NH3(air) and NH3(aq) occurs, whose equilibrium can be explained by Henry's law. In this case, only temperature affects the equilibrium and the model takes this factor into consideration. Therefore, the saturation degrees would be thesame for thegiven temperaturesin this study.
Effect of Airflow. Two methods of aeration into the removal system wereconsidered in this study. One method
wasthesolelysubmergedaerationintothe aqueous solution and theother wasthe aerationabovethesurface of aqueous solution.
Whenthe subsurface airflow rate wasincreasedfrom2.5 to 20 L/min without the headspace aeration, the saturation degree has shown to be about the same value of 0.6. This result is somewhatinteresting because the extentofequili brium between the aqueous and air phase are the same regardless of airflow rate. The reason maybe explained by two ways. First, the extent of equilibrium between the air and aqueous phase would be affected by the liquid-to-air contact area due to the turbulence of aqueous solution.
Second, the liquid-to-air bubble contact area was the same for thegiven airflow rate of 2.5 to 20L/min,indicating that the apparent turbulence was not increasedas the subsurface airflow ratewasincreased.
In order to investigate the effect of aerationto the head space, the airflows were varied from 0to 40 L/min, while subsurfaceaeration rate wasadjusted to a constantvalue of 2.5 L/min. In thisexperiment,thesaturationdegreewas 0.60 atthe headspace airflow rate of0 L/min, and decreasedto 0.25 as the airflow rateincreased to 40 L/min (Figure 7.D).
This result can be expected because instantaneous equili brium does not occurbetweenthe aqueousand air phase as stated in the above discussion. Thus, as increasing the headspace airflow caused less saturation before leavingthe removal system. Considering that the saturationdegree was decreased with increasing the air phase aeration, the sub surfaceaerationwas shown to bemore effectiveto remove ammonia than the air phaseaeration.
Conclusions
In this work, an ammonia removal model was presented utilizingthe chemical equilibriumprinciples, mass balance and masstransfertheory. In addition,the saturationdegrees were determined to evaluate the completeness of equili
brium after the modification ofthe initial model. The pre
sented model was successfully used to predict the theore
tically maximum ammonia removal rate by inputting the essential parameters of pH, temperature, and the airflow rate. The ammonia removal model based on the 1st order reaction kinetics was re-evaluated. Being consistent with other researchworks,the ammonia removalrate is increased with increasing the pH, temperature, and the airflow rate.
However, we concludethat this model would not beusedto explain the effect of above parameters systematically, because noneof such parametersareincluded inthemodel equation. In most of air stripping ofammonia from the aqueous solution, the ammonia removal rate did not reach thetheoreticalremovalrate.Thisresultindicatesthatduring the bubbling the air, theinstantequilibrium between the air and aqueous phasedoes notoccur. The experimentvarying pH and temperature in the aqueous solution, the saturation degrees were estimated at about the same value (0.45) regardless ofgivenexperiment conditions. This result indi catesthat thepresentedmodel takes thepH and temperature
Ammonia Removal Model
intoconsideration for the calculationof NHs(aq)inaqueous solution and the equilibrium of free ammoniabetween the aqueous and airphase. The saturationdegreewas decreased with increasing the airflow rate to the airphase above the aqueous solution, while thesaturationdegreeremains about the samewhen theairflowwasprovided tothe subsurface of the aqueous solution. This results show that the extent of equilibrium between the air to aqueous phase would be affected by the liquid-to-air contact area caused by the turbulence ofaqueous solution. In addition, the subsurface aerationwas shown tobe more effective method to remove ammonia than the air phaseaeration.
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